Texas Energy Conservation Program

Distillation Column Operations

Cover

Preface

Table of Contents

Abstract

List of Tables

List of Figures

Section 1. Introduction

Section 2. Design review, audit of energy and material balance

Section 3. Energy saving improvements with minimal capital investments

Section 4. Energy saving improvements with capital investments

Section 5. Economics

Section 6. Bibliography with abstracts

Section 7. Appendices

Conversion Notes

Cover

Energy Conservation Seminars
For Industry

presented by
The Energy Utilization Department
of the
Texas Industrial Commission
410 East Fifth Street
Austin, Texas
(512) 472-5059
Gerald R. Brown, Executive Director
Lance E. dePlante, Manager
Energy Utilization Department

Preface

The information presented herein is intended to enhance knowledge of industrial energy conservation and to provide the necessary tools to implement an energy conservation program in an industrial plant. References to specific products or ideas should not be considered endorsements of said products or ideas by the Texas Industrial Commission. This workbook and other projects of the Industrial Energy Utilization Department are funded through a U.S. Department of Energy grant administered by the Governor's Office of Energy Resources.

TEXAS ENERGY CONSERVATION PROGRAM
DISTILLATION COLUMN OPERATIONS

These materials were prepared as a result of work sponsored by the Governor's Office of Energy Resources through funds provided by the Department Energy. Neither the Texas Industrial Commission, nor the sponsoring agencies, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, expressed or implied, or assumes any legal liability for the successfulness of the implementation of energy conservation techniques described. References to specific ideas, products, and services should not be construed as endorsements. It is hoped that the information provided through these materials will be useful in your efforts to explore opportunities available for energy conservation.

Table of Contents

Energy Conservation Manual

Distillation Column Operations

Title

Disclaimer

Table of Contents

Abstract

List of Tables

List of Figures

Section 1 - Introduction 1 - 1

Section 2 - Design review, audit of energy and material balance 2 - 1

A. Review of plant design 2 - 1

B. Audit of actual plant operation 2 - 1

C. Data collection during plant operation 2 - 3

Section 3 - Energy saving improvements with minimal capital investements 3 - 1

A. Operating procedure revisions

(1) Reducing the reflux ratio of columns 3 - 1

(2) Lowering product specifications 3 - 3

(3) Lowering pumping costs 3 - 4

(4) Lowering steam usage 3 - 9

(5) Process heaters 3 -11

B. Scheduling shutdowns to maximize energy recovery and profits 3 -13

Section 4 - Energy savingins improvements with capital investements 4 - 1

A. Optimization of heat recovery - heat exchangers 4 - 1

B. Column revisions 4 - 5

(l) Additional or more Efficient trays 4 - 5

(2) Additional column draw 4 - 7

C. Optimization of recovery and use of energy 4 - 7

(1) Introduction 4 - 7

(2) Column Heat Utilization 4 - 9

2.1 Bottoms product 4 -10

2.2 Distillate product 4 -11

2.3 Condenser duty 4 -11

2.4 Reboiler duty 4 -11

2.5 Feed preheating 4 -12

(3) Changing the column's temperature 4 -12

(4) Two-stage condensation 4 -12

(5) Waste heat boilers 4 -13

(6) Multiple effect heat cascading for distillation columns 4 -13

(7) Split tower 4 -15

(8) Interreboilers, intercondensers, and feed preheating 4 -19

(9) Feed preheating 4 -21

(10) Interreboiler 4 -21

(11) Intercoolers and feed precoolers 4 -21

(12) Circulating refluxes 4 -22

D. Use of vapor recompression and heat pumps for distillation 4 -24

(1) Introduction 4 -24

(2) Distillation column's reflux and heat balance 4 -25

(3) Vapor recompression 4 -26

(4) Heat pump 4 -27

(5) Theory behind vapor-recompression and heat pumps 4 -28

5.1 The Carnot cycle 4 -28

5.2 The refrigeration cycle 4 -33

(6) Vapor recompression 4 -37

6.1 Situations 4 -37

6.2 Auxiliary heat transfer equipment 4 -38

6.3 Compressor drives and their energy costs 4 -40

6.4 Insulation of columns using vapor recompression or heat pumps 4 -41

6.5 Vapor recompression for interreboilers, other columns 4 -41

(7) Reasons for conversion of an existing column 4 -42

(8) Conversion of an existing column 4 -43

(9) Advantages of vapor recompression 4 -44

(10) Disadvantages of vapor recompression 4 -46

(11) Advantages and disadvantages of the heat pump 4 -49

(12) Guidelines for considering vapor recompression 4 -50

(13) Procedure for vapor recompression evaluation 4 -51

(14) Example, propane-propylene splitter 4 -54

14.1 Situation statement 4 -54

14.2 Solution 4 -55

(15) Work problem propane-propylene splitter with bottoms vapor recompression 4 -60

E. Improving control of distillation columns 4 -61

F. Reducing heat losses using insulation 4 -64

Section 5 - Economics 5 - 1

A. Definition of economic terms 5 - 2

(1) Profit 5 - 2

(2) Net back 5 - 2

(3) Depreciation 5 - 3

(4) Investment tax credit 5 - 5

(5) Fixed costs 5 - 5

(6) Variable costs 5 - 6

(7) Cash flow 5 - 6

(8) Discounted cash flow 5 - 6

(9) Return on investment (R.O.I.) 5 - 7

B. Concept of investment equivalence to save energy 5 - 8

C. Economic interpretations for energy savings 5 - 9

D. Steam economics 5 -11

E. Cooling water 5 -13

F. Compressed air 5 -14

G. Vacuum pumps and steam ejectors 5 -14

H. Exchangers used for heat recovery 5 -15

I. Conclusion 5 -15

Section 6 - Bibliography with abstracts 6 - 1

Section 7 - Appendices 7 - 1

A. Energy savings checklist - General 7 - 1

B. Process energy checklist 7 - 6

C. References - Technical articles 7 -10

D. Solution to work problem 4-F-15 7 -14

Abstract

Distillation operations have been branded as high energy users. An estimate 3% of the total energy used in the United States in 1976 was for distillation. Energy conservation is indicated. This manual is addressed to the small or medium sized chemical or refining company. It is structured to guide these people on how to analyze and reduce energy requirements. The criteria of no reduction or increased profitability of the process are stressed in analyzing any energy saving proposals.

Information for writing the sections came from technical articles, design and operating experience, and seminars on energy conservation.

This manual is divided into seven sections. The contents of the sections are discussed in the following paragraphs.
Before any energy conservation steps can be logically taken, a knowledge of energy usage of the existing facility must be known. Section 2 of this manual describes a procedure for reviewing the original plant design, auditing the energy usage as presently operated, and collecting plant data if required for the audit.

After the distillation process is analyzed for energy usage, the first step is to study energy saving improvements needing minimal capital investments and quickly implementable. Section 3 covers this, giving ideas on changing the operating procedure and scheduling shutdowns to maximize profits and minimize energy usage.

Capital investments to save energy are generally longer term projects. These projects include the optimization of heat recovery and revisions of the column. Capital intensive and complex systems using vapor recompression or heat pumps are possible energy savers. These are covered in Section 4 along with heat losses and column control.

For distillation processes, the energy used per pound of product is a simple ratio for evaluating the performance of the program to reduce energy usage. Similarly, an economic guideline is helpful in requesting management to make decisions concerning capital investments. In Section 5, the concept of investment equivalence to save a unit of energy is developed for use as an economic guideline. The economic interpretations of several energy savings proposals are discussed. Potential conflicts in placing a cost value on various steam pressures by accountants compared to its value from a thermo-dynamic or energy level viewpoint are discussed.

The appendices include reprints of technical articles pertinent to distillation columns, a general energy savings checklist, a process energy checklist, and the results of a sample work problem on vapor recommendation.

List of Tables

List of Figures

Section 1. Introduction

Many words and phrases may have more than one meaning. In energy discussions, the expression "energy conservation" is presently spoken with two meanings. The original meaning is related to the first law of thermodynamics, which states that energy is always conserved, never destroyed, but changes from one form and level to another. Now that the United States is no longer endowed with new sources of low cost energy fuels, energy con-servation has taken on the meaning of reducing the amount of energy used either increasing the efficiency of performing a certain task, or using a substitute requiring less energy. Examples of conservation are the use of higher efficiency air conditioning units, lighter weight automobiles, and handwashing dishes.

In the chemical industry, the meaning of energy conservation includes conserving the temperature level of the energy and in consequent the availa-bility of the energy to produce work. Since distillation processes require large amounts of work and heat energy to perform the required separations, these processes are prime areas for better energy utilization.

Many Americans are skeptical about the United States being in an energy crisis. They say that energy is plentiful, but have they considered the cost to produce it? Russell E. Train, formerly administrator of the EPA, made the following comment in an address upon receiving the $150,000 Tyler Ecology Award:

"...the artificially low prices for more conventional energy maintained by subsidy and regulation. In 1976 the average weighted price of the industrial use of energy per million Btu was $2.55, whereas the average replacement cost---the cost of finding and producing new energy resources---was $3.74. Thus, the replace-ment cost of natural gas is now more than 70% above the average price, that of oil about 45% above, and that of electricity nearly 40% above. Only in the case of coal did replacement cost approx-imate actual price. Since our political processes have so far proven unequal to the task of achieving more economically realistic prices for energy, whether by taxes, pricing policy, or by deregula-tion, or any combination of these, ..."

If his costs are realistic, then the United States is living on previously developed resources. When they are depleted, the cost of energy will soar.

If the decision is made by management to reduce the energy requirements of the processes, it implies that long term profits or return on the company's investment must not decrease. This economic viewpoint is a prerequisite to the writing of any energy conservation manual.

This manual is divided into seven sections, it is assumed that the reader has sufficient technical knowledge to understand the principles of heat transfer, separation operations, and thermodynamics. After information is presented on how to conduct an energy audit of the distillation process, energy saving ideas that require minimal capital investments are given. Similarly, ideas for long term capital investments are discussed. Finally, economics and the concept of investment equivalence to save a unit of energy are detailed.

The appendices include copies of technical articles pertinent to distillation processes. It also lists ideas on energy savings in general and specific to distillation operations. It is the purpose of this manual to aid the chemical company in reducing the energy requirements of the distillation units without a reduction in profitability of the process.

Section 2. Design review, audit of energy and material balance

Before proceeding with a detailed energy analysis on your distillation unit as presently operated, you should find out the energy consumption of the same type of separation by the industry. Your sources of information are: (1) similar distillation columns within the company, (2) contact with the original engineering design company, (3) contact with technical people from your professional groups or college or professional friends, and (4) the technical literature. For example, Mix, Dweck, and Weinberg estimated and reported specific consumptions in Btu/lb of product for various product separations in the CEP April, 1978 issue (see Appendix 7-C). They believe that a large percentage of the columns in operation can be retrofitted for energy conservation with attractive economic benefits.

2-A. Review of Plant Design

Your plant engineering files should contain all the design information for the process. If it is not available, this information should be requested from the original design company. In particular, process flow sheets, design calculations, piping and instrumentation drawings, specifications of the equipment purchased, performance characteristics of the equipment, utility usage tabulations, and revisions since the original installation are very valuable for the analysis. Examples of process flow sheets are found in Figures 2-1 and 2-2.

Design values for fuel, steam, and electrical usage should be found on the utility summary forms. Calculated values for specific operating conditions should be in the process calculations. Values for fuel and steam usage should be indicated on the process flow sheet. For example, if the design values showed 30,000 lbs per hr. of 75 psig saturated steam to produce 6000 lbs per hr of product, the ratio of the pounds of 75 psig saturated steam to pounds of product is 5. If the condensate is not recovered, the energy usage is (1185 - 48)5 or 5685 Btu per lb. If a competitor operated with the same ratio of steam to product, but recovered the condensate at 200º F, his energy usage is (1185 - 188)5 = 4985 Btu per lb. This is an energy saving of 12%.

Specifications of purchased equipment and their performance are valuable for any plant study. They must be used with caution because revisions may have been made since the original installation. If the changes were not documented (not uncommon in small plants) or simply given verbally to the present unit supervisor, you may not know that revisions occurred.

2-B. Audit of Actual Plant Operation

After the background information is compiled and the energy information extracted, the present energy usage of the unit should be determined. Plant accounting records should be checked for present and past usage of steam, fuel, electricity, etc. This information may be reported on a monthly basis on "value added" sheets or "production cost" sheets. All values reported by accounting should be considered questionable until they can be verified for accuracy. Instruments may be broken. Flow meters may measure usage for more than one unit, and the flow split guesstimated. If the guess was wrong, the estimated values recorded by accounting are in error and could incorrectly bias your decision on a proposed energy conservation project.

Plant inspections should be made of the measuring instruments. An orifice meter may have been calibrated for 100 psig line pressure, but the actual gas pressure found in the plant is 150. The meter's conversion factor and reported usage will be incorrect.

Production rates reported by accounting should be confirmed. Production figures are based upon meter readings and/or product shipments plus storage tank content changes. A level indicator on a storage tank may be based upon a 0.800 gravity liquid, but the actual gravity is 0.750. The production figure is not correct.

A heat and material balance can be made of the existing operation after the plant instrumentation has been corrected. This information will be compared with the original design balance and other energy figures found.

2-C. Data Collection During Plant Operation

When developing a heat and material balance for the existing operation, you may have insufficient information recorded on daily operating and laboratory logs to compile the balance. Since distillation units are generally well instrumented, the only expense burdens for a plant data collection test are the manpower to collect the data and laboratory charges to perform the analyses on the special samples. Of course, if one flow meter measures steam usage to two different units, an additional meter must be added to separate the units.

The degree of success of a plant data collection test is influenced by the preparation and planning stages. Step one is to list the data required for calculating the heat and material balance. Measuring locations are marked on the engineering flow diagram. Step two requires a tour of the unit, confirming and having calibration checks made of critical measuring instruments. Dial thermometers, pressure gauges, and dp cells are examples of these instruments. Table 2-1 is an example of a data collection sheet for electric motors in the unit. When reading pressure drops across an exchanger, it is preferable to use the same pressure gauge to read up stream and downstream pressures. A three way selector valve such as made by D/A Manufacturing Co., Tulia, Texas is a very convenient option for making two readings with the same pressure gauge. A more expensive option is to use a pressure differential transmitter.

The accuracy of flow meters can be checked by the use of a prover, if the necessary piping manifold is in place or installed. Otherwise, the meter design calculations and test results made by the instrument department should be studied and checked. If an orifice meter is in use, you can visually confirm that the upstream side of the orifice plate is inserted in the line correctly and that the orifice size stamped on the plate agrees with specifications. The condition of the orifice opening cannot be checked unless it is removed.

After all instruments are checked, you can take one data set of readings, noting time to make readings, and problems in collecting readings or samples. A heat and material balance can be calculated and inconsistencies noted. For example, in making an energy balance across an exchanger, the heat transferred to the colder stream is found higher than the cold stream. An incorrect temperature reading or flow rate may be the reason. When this "dry run" is completed and changes made, the plant test and evaluation are performed.

A data collection run for the electrical usage is determined by reading amperage loads on each motor and reading the wattmeter for the unit over the test period. Electric motors connected to instrument air and plant air com-pressors should be included in the energy audit.

Section 3. Energy saving improvements with minimal capital investments

Process units built prior to 1973, the year of the drastic rise in energy costs, were generally designed on a low capital cost investment basis for maximum rates of return. Energy saving equipment was included in the investment if it obviously improved the return on investment. No extensive engineering was directed at energy in the design phase.

In the current period of high energy costs, economics still dictates how much energy a new plant design can conserve. But the incentive to expend more engineer-ing time in the design phase to optimize the process with maximum energy conserva-tion has increased. Likewise, there is the economic incentive to return to older operating plants and retrofit them with additional energy saving equipment.

Similarly, years ago, plant operators had been instructed to minimize off specification production. They achieved this and reduced the amount of scrutiny and effort needed to operate the unit by producing a purer product than necessary. This results in an increase in energy usage. This section of the manual will cover changes in plant operation with minimal capital investments to reduce the energy required to produce one pound of product.

3-A. Operating Procedure Revisions

Your operating procedures were probably written before the large increase in energy cost drew attention to energy conservation as one primary objective. In addition, the operators are probably using the procedures only as a guide and have developed their own procedures based upon ease of operation.

3-A-1. Reducing the Reflux Ratio of Columns

The optimization of the reflux ratio of the distillation column can produce significant energy savings. The investigation can start by checking the operating manual and column performance specifications for the design conditions, including the reflux ratio. If the design conditions are no longer valid due to changes in feed composition or product requirements, it is recommended that a vigorous distillation calculation be made. If the calculations are very difficult, you can make use of commercial computer programs made available through various computing service bureaus (see section 4-B). The design reflux should be compared with the actual ratios controlled by each shift operator. The daily laboratory analyses of the column products are compiled and compared with the design specifications. If the column is operated at a reduced production rate, the design reflux rate should be calculated for this reduced rate.

It is extremely difficult to change people, even more difficult when it requires more work effort without visually seeing the results. If one operator was found who operated the column at a lower reflux ratio than the others, you might get the confidence of the operators by getting all the operators to maintain this ratio. If you merely write a note in the unit's operating log leaving instructions, you will probably not be successful in lowering the reflux ratio. You must work closely with the superintendent, foremen, and operators instilling confidence as you show the energy savings resulting from their efforts. If the operating depart-ment has monthly meetings for the supervisory people, you can use it as a forum to present your objectives, how you plan to approach them, and request their support and assistance. Later you can report progress and discuss problem areas.

Steam or fuel usage per pound of product can be tabulated daily along with reflux ratio, product purity, etc. and compared with column performance before the change. The savings in energy can be converted to a monetary value and reported to the operating people. As an alternate you might represent the energy savings as barrels of imported oil per year.

As the reflux ratio is reduced, a point will be reached at which the operators are overworked and having difficulty in maintaining product pur-ity. This is the opportunity to show your concern to the operators by backing off on the ratio.

3-A-2. Lowering Product Specifications

Sometimes, product specifications can be lowered. Who decided on the present product specifications? Are they justifiable? For example, the sales group may have had the product purity increased to justify selling more product and beating the competitors. The buyer may require a purity in excess of his real needs. Higher purity product requires more energy to be consumed per pound of product. Since the sales depart-ment has probably expressed an optimistic opinion as to the value of higher product specifications in the market place, an economic analysis based upon their opinions would most likely say to make no specification changes. A better approach may be to analyze the specification requirements for each type of user of the chemicals and determine if the higher specification is required. A different selling technique may retain the customer even if product specifications are lowered to save energy.

If the product from the column is feed to another unit in the plant, then the effect of lowering the purity on the other unit must be determined. Thus, the energy conservation project requires the additional collection and tabulation of operating data. A statistical approach may be required to fully interpret the results of changes due to the variability of the processes by changes in other parameters.

3-A-3. Lowering Pumping Costs.

When making an inspection of the unit for an energy audit, you should note any operation of two centrifugal pumps in parallel. Within the distill-ation unit, you can have reflux pumps, product pumps, feed pumps, pumpa-rounds, etc. with spares. Other examples are cooling water pumps in the water cooling tower and cooling pond systems.

If the pumping system was designed for one pump and the operator places the spare pump in service, too, he has not doubled the flow rate. Instead, each pump provides one half of the developed system flow rate and each operates at the identical head. To understand this, let us assume a centrifugal pump characteristic curve as shown in Figure 3-1. At 100 gpm of flow, one pump produces 130 ft of head. If identical pumps are on stream, the flow is 100 + 100 or 200 gpm at 130 ft of head. The characteristic curve for two pumps was developed this way and is also shown in Figure 3-1. The actual flow rate through the piping system is set by the intersection of the pump curve with the system head curve. Referring to Figure 3-1, the flow rate is 160 gpm with one pump operating and 172 gpm with two pumps on stream. In the latter case, each pump is handling one half the flow or 86 gpm.

The efficiency of centrifugal pumps varies with flow rate. Thus, pumps are selected in the design phase to operate at or near their highest efficiency. As seen in Figure 3-1, the pumping efficiency decreased from 46.5% at 160 gpm to 34% at 86 gpm. Assuming an electric motor efficiency of 95%, the energy used in both cases is determined as follows:

For one pump operating

For two pumps in parallel

By increasing the flow 7.5%, the energy requirements increased 60%. As an alternate to two pumps, the size of the impellers could be increased to handle the 172 gpm of flow with one pump. Assuming an efficiency of 47%, the energy required is:

Thus, 17.4 - 12.6 or 4.8 Hp was conserved. In section 5 of this report, the concept of investment equivalent for energy savings is developed. This is the amount of capital that can be invested to save a unit of energy. If new impellers were placed in the two pumps (one pump is the spare), the impellers would likely be expensed (if the motors were changed, the new motors would probably be capitalized). How long would it take to recover the expense of purchase and installation of the two impellers if the pump operated at 172 gpm with 0.95 on stream time? Assuming the cost of electricity at 3.0 cents per KWHr and the replacement expense of $800, the payout is:

(X) (.95)(4.8)(.746)(.030)= $800

where X = hrs

X = 7839 hrs or 0.9 years

Management should be receptive to this expenditure.

As chemical plants expand by adding more process units, additional cooling water is probably required. Usually, the existing cooling water lines are not replaced with larger lines, but additional pumps are added to handle the increased flow requirements. Suppose a new pump was purchased with an impeller that gives a higher head to compensate for the higher system pressure drop. The impellers of the existing pumps are replaced with larger diameter impellers. This is a minimal capital cost pumping install-ation, but what about energy usage?

As an example, Figure 3-2 shows the pump characteristics and system curves for a cooling water pumping system before and after expansion. Flow was increased from 1500 gpm to 2250 gpm. At the original flowrate, pumps A and B operated at 750 gpm each at 70 ft of head and probably at the best efficiency for these pumps. With the expansion, flowrate is at 2250 gpm at a head of 108 ft. At 108 ft of head, pumps A and B handle 1150 gpm or 575 gpm each. The efficiency of the two pumps probably dropped. Frictional energy increased 38 ft. The following calculations assume a $0.03 per KWHr of electricity:

Operating cost before change

Pumping cost = (55.8)(.746)(0.03)(24) = $29.98/day

Pumping cost per day per gpm = $.020

Operating cost after change

Pumping cost = (136.5)(.746)(.03)(24) = $73.33

Pumping cost per day per gpm = $.033

The pumping cost per gpm has increased 65% in addition to the capital costs, not a very efficient modification. Before making the pumping change, it may be possible to reduce frictional energy losses. The existing distribution system should be traced and pressure drop calculations made for sections of the system that appear to have high pressure drops. Maybe a short section of pipe could be replaced with a larger size. Maybe the proposed tie-in point for the cooling water to the new process could be moved closer with a small increase in piping costs, but a significant lowering of frictional energy losses.

Another possible way to cut energy usage is to limit cooling water flow through the exchangers. It is doubtful that the operating procedure covered this aspect. If flow is not throttled, the flow through an exchanger is determined by the DP available from the pumping system and the frictional energy losses in the exchanger and piping. For example, an unthrottled flow showed 8 psi across the exchanger. Design flow was for 800 gpm with a 5 psi
drop across the exchanger. Since flow is approximately proportional to the square root of the pressure drops, the flow rate is or 1000 gpm. An inexpensive type butterfly valve with a manual lock positioner could be in-stalled to throttle the flow to 800 gpm, saving 200 gpm of cooling water.

If a cooling water system operated at 6000 gpm and 50 psig before the exchanger flows were throttled and 5000 gpm at 55 psig after the throttling, how much energy was saved? Let us assume there is an improvement in efficiency from 0.50 to 0.52.

Horespower before change

Horespower after change

Electrical savings

Savings = (43) (.746) (24) (365) (.95) (.03)

= $8000 per year

Even better savings may be gained by changing impellers, etc. to give 5000 gpm at 50 psig or less. If a process fluid is being cooled by cooling water to l00ºF, but a fluid temperature of 120ºF is acceptable, it may be possible to use less cooling water or cooling water preheated by another source, thereby reducing cooling water flow.

Flow of liquids through piping transfer lines is generally controlled by the use of throttling valves. Past design practice has been to design the control valve to take from 25% to 50% of the system pressure drop. This gives the control valve a rangeability of approximately 50 to 1. The valve has converted work energy derived from electricity into frictional heat. Most processes don't require this much rangeability so a larger control valve with less pressure drop could replace the original valve, the rangeability being reduced say to ll to 1. Of course, energy savings can only occur if the pressure in the line is reduced, possibly by reducing the diameter of the pump impeller. The electric motor should also be replaced with one of lower horsepower that meets power requirements. Just installing a new control valve will be useless as the valve will throttle down until flow is controlled to the original point. Shinskey, in the "Control Systems Can Save Energy" article graphically discusses this energy saving idea.

3-A-4. Lowering Steam Usage

One of the most talked about energy wasters is steam leakage from "bad" steam traps and leaking fittings. Steam traps are blamed for being inefficient or worn out and causing as much as 10% of the generated heat from steam to be lost. Is this true or just a sales method to sell more traps? It turns out that steam leaks cause a significant energy loss.

Mr. Goyette, in his article "Estimating the Costs of Steam Leaks", (see appendix 7-C) shows the cost effect of steam leakage from various size holes (1/8", 1/16", and 1/32") in a 150 psig steam system. The cost was based upon incremented steam costs. An example showed that a 1-inch union was found leaking at a loss of $3000 per year. The repair cost was $50 or a six day payout. Of all the energy savings steps that the Tenneco plant did, Mr Goyette said the single largest contributor was steam-leak repairs. Steam traps will wear out. Armstrong Machine Works claim that the inexpensive disk type steam trap wears out in 6 months and should be replaced that frequently. If condensate is recovered, leaking traps can cause an excessive return temperature and cause failure of the condensate return pumps. Severe water hammer can occur as hot steam contacts conden-sate that has cooled below the temperature of the steam.

The following steps are recommended for saving energy in your steam condensate distribution system and starting an effective steam energy management program:

1. Develop an estimate of the cost of steam leaks based upon your plant costs similar to the Goyette article described above. A method for demonstrating visually to plant people what these losses are can be made.
2. Run a survey, recording all leaks, size, cost, and location.
3. Check the operation of all installed disc traps used for drips and steam tracing. If found leaking, consider replacing with a more efficient type trap. Before replacing, check installation design and confirm trap size (not over or undersized).
4. Check installation and operation of steam traps used on equipment using the sound detection method, the pyrometer method, or the glove method. The installation should be checked for proper trapping. Items checked include strainer, check valve, back pressure, orifice, and inert gas venting. Improper venting can cause a severe reduction in heat transfer rate.
5. Check vent valves on steam jacketed equipment and kettles for proper operation (removal of inerts without steam loss).
6. Start a preventative maintenance program to maintain the steam distribution system in excellent condition. If manpower is not available in maintenance, you can have the operating people maintain a simple log for their area of responsibility.
7. Steam trap manufacturers will be happy to furnish information to assist in your energy saving program to reduce steam losses, but use your own economic costs to decide whether to replace, repair, or redesign the system.

There is insufficient published information to say that 10% of the steam is wasted by steam traps, but some major chemical companies have invested large amounts of manpower and money to replace or revise steam trapping systems in their plants.

3-A-5. Process Heaters

The Texas Industrial Commission has developed a manual specific to boiler and process heater efficiency. Consequently, our discussion of process heaters will be very limited, briefly covering the reduction of excess combustion air and reduction of stack temperature with small capital investment.

Control of Excess Air

According to Mr. A. M. Woodard, (see article, "Reduce Process Heater Fuel", in appendix 7-C), over half the total fuel consumption for refineries is for process heaters, the remaining for steam generation. These fired heaters can be improved from an energy efficiency viewpoint by reducing the amount of heat in the stack discharge. With the advent of the more accurate and simpler oxygen analyzers, the control of excess air in a fired heater can be automatically or manually controlled by the operator. Mr. Woodard's article details a method of sampling the flue gas, monitoring and controlling the system. Four systems are described, but system 3 is recommended. This consists of locating the draft and oxygen analyzer readouts in the control room, too. The operator can then monitor and control the operation of the heater or heaters with ease and comfort. Two safeguards are built into the system. stop installed to prevent full closure. failure, the positioner opens the damper. The damper has a mechanical If there is an instrument air failure, the positioner opens the damper. A simple stepwise procedure for heater adjustment is given on the last page of the article.

A target excess oxygen for the oxygen recorder with remote manual damper control was given in the article as 4.0% for gas and 4.5% for oil firing. More recently, manufacturers are indicating the oxygen can be controlled at 2%. The decision to go this low must be based upon the risk of temporarily going below stoichiometric conditions with possible explosion when the heater returns to excess oxygen conditions. Based upon figure 1 of Mr. Woodard's article, substantial reductions in heat input are accomplished by this approach. This modification will probably cost less than $5000, yet show considerable savings.

Recovery of Heat from Stack Gases

The amount of heat extracted from burning a fuel can be related to the flue gas or stack temperature. The extracted heat is defined as the heat absorbed by the process stream being heated and the losses from the furnace casing (generally around 2%). Thus the percent heat extracted is:

Heat available in Btu/lb of fuel at the Flue Gas temperature (FGI), divided by the Heat Content of the Combustion Fuel in Btu/lb times 100.

The lower heating value (LHV) of the fuel is used for efficiency calculations. The flue gas temperature depends upon the design condition of the convection section of the heater and the physical condition of the convective tubes. A reasonable FGT is the inlet process fluid temperature plus approximately 150º F. If your inlet fluid is at 300º F, the FGT is approximately 450º. A check of the FGT for your heater may show 500º F. Thus, your convective tube section may have lost some of its heat transfer ability by loss of fins on the tubes. This becomes a replacement expense.

3-B. Scheduling Shutdowns to Maximize Energy Recovery or Profits

If an exchanger (or reboiler or condenser) used to recover heat from a hot stream is slowly losing the amount of heat recovered because of fouling, when do you shutdown? This decision can be based upon maximizing heat recovered or minimizing the loss in profits. Three cases are described below:

Case 1---Decision based upon energy conservation

Given: An exchanger used to recover waste heat is rated at 11,000,000 Btu/Hr when clean before fouling. This exchanger slowly loses its heat transfer capability and the loss is estimated to be 10,000 Btu/Hr per day. A 12 hour shutdown is required to replace the tube bundle.

Find: Frequency of shutdown to maximize the energy recovery. Assume a 3500 day period of time.

A) At start of day l, heat transfer rate = At the end of day l, heat transfer rate is or 10,990,000 Btu/Hr

B) Let C = number of repairs during the 3500 day period. The heat recovered for any given day, X of the cycle is

The heat recovered for an entire cycle is

For the 3500 day period, total heat recovered is

Case 2---Decision based upon maximum profit, production rate not affected.

Given: Same conditions as Case 1

Each Btu is worth $2

Each shutdown costs $10,000 in maintenance and $20,000 in profits.

Find: Frequency of shutdowns to maximize dollar savings

A) Savings =

B)

C)

Case 3---Decision based upon maximum profit, production rate affected by loss of heat transfer.

Given: Same conditions as Case 1 and 2, but production capacity is reduced by .05% per day. Each .05% loss in rate is $20 per day (20,000 x x .0005) in profits.

Find: Frequency of shutdowns to maximize dollar savings.

A) Savings =

B)

or

C)

Summary

	Case 1 Max Energy	Case 2 Max Profit	Case 3 Max Profit
Btu's total
No. of cycles	105.5	9.9	24.4
Days per cycle	33	355	143

It is doubtful that management would agree to shutdowns every 33 days to maximize energy savings when the maximum profit occurs at 355 days (Case 2), or 143 days (Case 3). However, as the energy cost increases, the frequency of exchanger cleaning will increase for Cases 2 and 3. In a real plant, the assumptions of linear losses of heat transfer and production may not be true, but the principles of handling the decision making are still valid.

Section 4. Energy saving improvements with capital investments

Energy consumption for all distillation processes in the United States in 1976 was estimated at 3% of the entire national energy usage. Since distillation is considered a low efficiency process, it should be possible to improve efficiency with investments of capital and still receive a reasonable return on investment.

Investments may be made in additional exchangers for heat recovery, column revisions, better insulation, or column control. In contrast to these simple changes not requiring capital investments, the more complicated vapor recompression or heat pump changes are reviewed.

4-A. Optimization of Heat Recovery - Heat Exchangers

The basis for optimizing heat recovery involves the first and second laws of thermodynamics. The first law covers the energy balance, the conservation of energy and the energy equivalence of work and heat. The second law develops the concept of energy level, the irreversible process, and the conversion of heat to work energy.

If one process stream must be heated and can be heated using another process stream without using energy from steam or electricity, the heat recovered saves fossil fuels. The cost savings in energy must exceed the capital investment equivalence of energy for the heat exchangers and ancillary equipment to be worthy of installation.

It is easier to design a new facility with the objective of optimizing energy use than an existing plant. According to Steinmeyer (Seminar on energy conservation in the AIChE today Series),

"----the existing plant cannot economically achieve the same low (energy) usage as a new plant. The cost to return to an existing plant and reinsulate a vessel, add heat exchangers, or increase the number of distillation trays on the basis of energy conservation alone is much higher than starting out in the design phase of a new plant. Thus, any proposed changes in an existing unit must be carefully analyzed so that no expenditure for making the change is overlooked. Changes that reduce profit because certain expenditures were overlooked will be remembered by management when additional changes are recommended."

The amount of heat that can be exchanged depends upon the fluid's temperature level and the amount available. The optimization of heat recovery involves exchanging Btu's at as high a temperature as possible. For example, a vapor product stream is condensing at 350ºF in an exchanger using cooling water to remove the heat. The cooling water temperature discharges at 110ºF. At this temperature level, the energy in the cooling water has no use and is totally wasted.

To give an example of the amount of heat available, assume liquid stream A is flowing at 10,000 lbs/hr at 400ºF, liquid stream B is flowing at 600 lbs/hr, and 400ºF too. If both streams must be cooled to 300°F, stream A has the greater availability of heat. If liquid stream C is flowing at 100,000 lbs/hr at 300ºF, the heat available above 300ºF for transfer is zero. Stream C could be used to heat up a cooler stream, D, to 280ºF and then stream A could heat up stream D to 380°F. The method for optimizing heat recovery is described in the technical article by Huang and Elshout (see Appendix 7-C).

A heat availability diagram is shown as Figure 2 in their technical article, "Optimizing the Heat Recovery of Crude Units", by Huang and Elshout. Four streams, the overhead reflux, kerosene pump around, gas oil product, and the residuum are available for exchanging heat with the crude in a 130,000 bbl per stream day crude unit. Each exchange stream has restrictions as to the temperature range that heat can be removed, and the rate of flow. Huang and Elshout had plotted the heat available in "Enthalpy Times Mass Rate", as millions of Btu's per hour for each stream using 0 enthalpy as the lower restriction temperature for the stream available for heat exchange. Figure 4-1 is the same drawing as found in Figure 2 of the Huang and Elshout except the total heat availability curve was returned to its unshifted position.

The total heat availability curve is determined by summing the enthalpy rate for each stream at a given selected temperature. For example, at 300º, the enthalpy rate is 0 (kerosine PA) + 37 (G 0 Product) + 55 (residuum) + 320 (OVH) : 412 million Btu/hr. At 400ºF, the enthalpy rate is 55 (kerosine) + 60 (G 0 Product) + 106 (residuum) + 320 (OVHD) = 541 million Btu/hr.

The total heat exchange curve as plotted is right of the crude oil heat requirement curve. At first, this would indicate that the crude can be heated to 645ºF and have an excess of 60 million Btu/hr excess (675 x 106 Btu/hr at 645ºF - 515 x 106 Btu/hr crude requirement). This is not true because heat must be available at the required temperature level. Below 370°F, the slope of the total heat availability curve is less than the crude requirement curve. This means that sufficient heat is available at the proper temperature to heat up the crude. Above 370°F, the slope is greater than the crude curve and insufficient heat is available. Even with infinite heat transfer, the final crude heat exchange temperature must be below 645ºF.

Haung and Elshout shifted the total heat availability curve to the left until the two curves touched. They said this represented the maximum amount of heat that can be exchanged with infinite heat transfer. Below the pinch point, we have already concluded that more than enough heat is available at the proper temperature to heat up the crude. Thus, the maximum amount must be represented by the end point of the total available with the shifted curve or 420 million Btu's per hr. The maximum crude temperature is 530ºF. When Huang and Elshout studied the heat optimization of this unit, they studied four cases and the maximum temperature reached was 480ºF. (See Case D, their Figure 4).

Bannon and Marple of Shell Oil Company presented a paper on "Heat Recovery In Hydrocarbon Distillation" (see Appendix 7-C for paper), in November 1977. They show two ways to improve the thermal efficiency of distillation columns based upon the concepts just discussed. If the overhead vapor from a column is at a temperature high enough to be useful and produces a boiling range top product, the overhead can be condensed into two stages. First, heat is removed to condense only enough of the overhead vapors to produce column reflux. The temperature of the condensation stage is at a higher level than if the entire overhead vapors were condensed in one step. Then, the remaining vapors are condensed and cooled to product conditions. Bannon and Marple described a crude oil distilling column at one of their manufacturing complexes. This column used the two stage condensation approach and transferred 203 million Btu/hr to the crude oil feed. If one stage operation, the heat recovered would only be 122 million Btu/hr, a loss of 81 million Btu/hr.

If heat can be withdrawn from a column to balance column vapor loads and improve separation, the temperature level of the heat removed and made available for exchange can be increased by designing at high circulating rates. The three factors for designing circulating reflux systems are the number of systems, the placement of the systems, and the circulation rate. These factors are described in the Bannon and Marple article.

The heat recovery efficiency of your distillation columns can be checked for possible improvements. This can be done by using the Elshout "Heat Exchanger Network Simulator" program available on the computing service bureau, United Computing Systems (UCS) or other similar programs. You can also develop your own available heat curves. Using the exchangers available in the plant as well as new exchangers, you may be able to hand calculate a fairly good heat recovery system that is economically feasible.

4-B. Column Revisions

Many options are available for conserving energy in distillation processes. Mix, et al have outlined and also placed in tabular form guidelines for selecting energy saving options. The more attractive options found in their table and article are discussed below.

4-B-1. Additional or More Efficient Trays

According to Mix, et al, tray changes are economically feasible if:

4-B-1)

Where N = Number of trays in the column

= Murphie Plate efficiency

= Relative volatility (light to heavy)

S = Separation Factor

D = Distillate

B = Bottoms

LK = Light Key

HK = Heavy Key

x = Concentration, mole fraction

P = Column pressure in ATM

K = Reflux ratio R/RM

Before one proceeds, it is recommended that a rigorous distillation calculation be performed on the existing column using the actual temperature, pressures, compositions, etc. of the column. Distillation programs that have been developed by Chemshare, Simulation Sciences, Phillips 66, and others for simulating your column are available through various computing service bureaus.

A plot can be made of the distribution of the various components tray by tray. This plot may indicate the feed tray may be changed or additional trays may be beneficial if entirely in the rectification or stripping section of the column.

If equation 4-B-1 shows the column may benefit from more trays, you can run several cases with reflux as the variable (heat load changes) and determine the saving in energy.Y ou have the option of adding more trays or replacing existing trays with more efficient type trays. For example, Kirpatrick, in his article, "M D Trays Can Provide Savings In Propylene Purification", (see Appendix 7-C), describes the design of propylene-propane splitters and the application of trays with 13" spacings compared to the usual 18 to 24" spacing. With the shorter spacing and more efficient design, a single column 13.25 ft in diameter and 265 ft tall, using 196 M D trays was installed and producing polymer grade propylene.

4-B-2. Additional Column Draw

Three possible column draw options are pasteurization, intermediate product and intermediate impurity. Pasteurization means the removal of light ends from the distillate by venting off the accumulator and removing the distillate product several trays below the top. Six criteria are listed by Mix, et al.

The intermediate product is considered when the temperature difference between bottom and top exceeds l00°F, and when the split of one key between two products is desired. The intermediate impurity drawoff is useful for removing impurity buildup under high reflux operations. The impurity flow rate must be less than .01 times the feed rate in lb moles per hr, and the relative volatility between the light and heavy key less than 1.5.

4-C. Optimization of Recovery and Use of Energy

4-C-1. Introduction

The maximizing of the overall plant energy efficiency is our purpose in utilizing waste heat possibilities and energy conservation methods. Distillation columns consume and reject large amounts of heat. Much of this heat is lost and not recovered. By a proper reevaluation it might be possible to greatly increase the recovered heat and reduce the input requirements. Several items will be of major importance in this reevaluation.

(1) The temperature and heat flows within the column.
(2) The changes that can be made within the column, including changed upstream and downstream requirements.
(3) The plant utilities, heat and cooling sources.
(4) The needs of the nearby surrounding processes.

The relation of the efficiency of the distillation column to the overall energy efficiency of a plant cannot be optimized without knowledge of the requirements of the other processes of the plant. Integration of the overall plant is the key to maximum energy savings. To evaluate the available options for this purpose, the following needs to be known about the plant.

(1) The process streams that require heating. The beginning and ending temperature, total heat capacity, and the current heating methods of each stream is required.
(2) The process streams that can be cooled. The beginning high temperature, any low temperature bound, the total heat capacity, and current cooling method of each stream is required. Remember that every Btu that can be usefully recovered replaces a Btu that would otherwise have to come from a fuel. Any part of heat recovered from a cooled stream is useful.
(3) Any reboilers or evaporators on neighboring units are of interest where a potential use of the distillation columns condensing vapor exists. For this, the temperature, duty, continuity of operating parameters, and the current heating method of the nearby reboilers needs to be known.
(4) The overall plants steam system. The steam header pressures, capacities, flows, and overall stream balance (amount letdown, excesses, etc.) is needed.
(5) Any units requiring large amounts of low pressure steam or low quality heat for some purpose. The requirements of duty and temperature is needed, in addition to the distances from the column to the unit. As low pressure steam requires large lines, long distance transport is costly.
(6) All heat sources from nearby equipment, condensers, etc. that can be used by the distillation column for its reboiler and feed preheating duties. Note that some or all of the distillation column reboiler duty can be supplied by a high temperature liquid stream.

4-C-2. Column Heat Utilization.

A distillation column has three basic sources of reject heat, the bottoms product, the condensing overhead vapor, and the distillate product. The two basic heat inputs are the reboiler and the feed.

4-C-2.1 Bottoms Product

The bottoms product liquid is the hottest source of heat and the obvious heat source. Due to its temperature and liquid form, the bottoms stream will probably already have some use on an existing column, such as feed preheating. The most efficient use of the bottom product is made by maximizing the temperature at which heat is recovered, and by maximizing the total heat recovery. This situation is reflected by perfect countercurrent exchange with equal heat capacity on both sides, where there are only a few degrees driving force throughout the exchanger. As heat is more valuable at higher temperatures, we must try to recover the heat at as high a temperature as possible. As even low temperature heat can be valuable, our aim must also be to recover as much heat as possible. Use a number of exchangers in series, instead of a single exchanger, is also useful. For example, assume we have a 700º stream. (and assume heat capacity = 1,000 Btu/ºF, and all steam at 1000 Btu/lb). We could use a single waste heat boiler to produce 50 psig steam, causing the stream to cool 700 - 350 = 350 lb of 50 psig steam. A better system would be to use a series of waste heat boilers. We could produce 200 lb of 400 psig steam (700 - 500), 100 lb of 150 psig steam, (500 - 400), 50 lb of 50 psig steam (400 - 350), and 100 lb of 1ATM steam (350 -250). By using a series we have recovered higher value steam (400 and 150 psig), and more steam overall (450 lb versus 350 lb).

The ability to use the heat in the bottoms product will depend on its requirements for downstream processing. If the product is required hot downstream, it is impractical to cool it and then to reheat the bottoms stream. If the stream does not need to remain hot, the following represent possible uses of the bottoms liquid heat.

(1) Preheating the column feed.
(2) Use to run all or a portion of another column's reboiler.
(3) Exchange with another process liquid stream.
(4) Steam generation and boiler feedwater heating.

4-C-2.2 Distillate Product

The options that apply to recovering heat apply equally well to the distillate product. The opportunities of heat recovery differ as the distillate product is at a lower temperature than the bottom, and the distillate product may be a vapor, therefore containing a large amount of heat in its vaporized condition.

4-C-2.3 Condenser Duty

The largest potential reject heat source of the distillation column is the condenser. All this heat is available at essentially a single temperature, and all the heat duty must be removed. Possible uses of the condenser duty could be to supply heat to a neighboring column's reboiler, to produce waste steam, or to heat large liquid streams at low levels, such as supplying hot water for a building.

4-C-2.4 Reboiler Duty

The reboiler represents the largest heat input to the distillation column. The reboiler requires heat at a single high temperature. It is desirable to minimize the steam consumption if possible by using condensing vapors from other columns, hot process streams, or special very low pressure steam.

4-C-2.5 Feed Preheating

The bottoms product or another hot liquid stream is often used to preheat the feed. Whatever source used should cause the maximum overall energy efficiently for the plant.

4-C-3. Changing the Columns Temperature

The existing or proposed column does not necessarily have to operate on the design conditions. (Do not operate existing columns over the allowable pressure). By changing the temperature in the column a small amount, we may be able to obtain a valuable energy recovery. Lowering the temperature might allow a less valuable steam to be used. Raising the temperature may allow a waste heat boiler to be used, or the vapor used to provide reboiling in another column. Note that changing the temperature will effect the column's operation (different pressure) and raise or lower both the reboiler and condenser temperatures.

4-C-4. Two-Stage Condensation

For some multicomponent distillation columns there is a broad range over which the overhead vapors condense (dew point to bubble point). By using more than one condenser instead of a single total condenser, we have the opportunity to recover some of the heat at a higher temperature. For example, we could have two condensers, the first condenser condensing part of the overhead to provide reflux, the second condensing the distillate product. This situation is effectively a partial condenser with vapors later condensed. The items to be emphasized on a multistage condensation column are to avoid subcooling as much as possible, and the recovery of the waste heat by steam generation in the high temperature condenser.

4-C-5. Waste Heat Boilers

The use of a condenser as a waste heat boiler is simple. The condenser is operated in a partially flooded situation, where the level changes as the heat duty is changed. The water is boiled at constant pressure in the tube side, and all steam produced sent to steam headers or its ultimate use. The temperature at which the condenser operates is important. For a temperature of less than 200ºF, no sort of steam can be produced. For higher temperatures the steam produced is determined by the required pressure (1ATM, 40#, 150#, etc.). The condenser will have to be larger as the temperature driving force goes down, so the economics should be looked at. It may be necessary, in a case where low pressure steam could be produced but no use exists at this low pressure, although one does at a slightly higher pressure, to mechanically compress the low pressure steam to a higher pressure, say from 25 psig to 40 psig. Remember the true values given to the different steam pressures during the evaluation of different waste heat steam generator possibilities.

4-C-6. Multiple Effect Heat Cascading for Distillation Columns

The condensing overhead vapors of one distillation column can be used to provide the reboiling duty of another column, where the condensing temperature is higher than the reboiling temperature. This creates in effect the equivalent of a multieffect evaporator system, except that the distillation columns is used, rather than the direct evaporation. Any number of distillation columns can be placed in series, such as the three column example of Figure 4-2. Note that different materials are being split, and the columns are disimilar, except for the heat duties.

The columns run by using the overhead of one column to provide the reboiling of the other will probably not have the same heat duties, therefore any excess duty can be carried by an auxiliary system. Where the hotter column is smaller than the cooler column, an auxiliary reboiler will be needed for the cooler column. In the other case where the hotter column is larger, an auxiliary condenser on the hot column will be used, with all the cooler columns duty carried by the hot column's vapor. With the proper auxiliaries, the heat cascaded columns can be operated almost independently, therefore little control problems will be met. The heat cascaded distillation columns are different from a split tower arrangement, because the split tower has the same feed and products.

The heat savings by use of heat cascading are obvious as each reboiler run by the overhead vapor of another column removes that much of an external heat input. The costs are for a slightly more complex system, and the piping and extra heat exchanger surface for the condenser reboiler. The heat cascaded system work best where nearby columns exist, these columns having different temperatures, and each column has a fairly narrow range of temperature between the top to bottom of the column. In some cases, it may be desirable to operate the hot column at a higher pressure and the cool column at a lower pressure than optimum in order to increase the temperature difference between them. The use of heat cascading will interfere with other possible uses of the hot condenser duty, such as in producing waste heat steam, so that the various cases must be evaluated for the optimum case.

4-C-7 Split Tower

The use of a split tower can afford significant energy savings over a conventional distillation column. A split tower arrangement consists of splitting the feed into two equivalent streams and distilling in two smaller columns. The two columns operate at different pressures, one higher than the other, resulting in its overhead vapor having a condensing temperature high enough to be able to use the condensing vapor to provide the reboiling duty in the lower pressure column. The bubble point temperature of the overhead vapor must be high enough over the bubble point of the lower pressure reboiling bottoms to provide a sufficient delta T for the condenser-reboiler. The feed stream will be split so that the condenser duties of the high pressure column approximately matches the required reboiler duty of the low pressure column. (See Figure 4-3 for an example split tower arrangement).

The heat, input to the reboiler, of the high pressure column rises to the condenser where it then provides the reboiling duty of the other column. By use of the split tower arrangement, we have cut our energy use almost in half. Note that instead of two columns, any number of columns can be used in the split tower fashion. However, For each additional tower, an extra delta T must be supplied, plus the temperature drop across the column. In addition, the energy savings drops as each column is added. The two tower system saves 50% of the energy. Another tower saves (50 - 33) or only 17%. A fourth tower will save only 8.3%. So our writeup will deal with only the two column arrangement.

The split tower system has a single reboiler and single condenser. The temperature difference between the reboiler and condenser will be much greater than that of an ordinary column. This occurs because the two columns each have their own temperature diffence to be met from the top to bottom, and the driving force for the condenser-reboiler must be supplied. As a result of this for the split tower arrangement to work, the following factors must be present:

(1) The temperature and pressure in the high pressure column must be below the critical points.
(2) The pressure must not be so large as to require too heavy column walls.
(3) The low pressure column must not be too low, so low a vacuum as to cause trouble.
(4) The products must not be degraded by the highest temperature or frozen, or too viscous at the lowest temperature.
(5) The heat source must be able to supply heat at a temperature above that of the reboiler.
(6) The condenser temperature must not be below that obtainable by conventional air and water cooling. Refrigeration cannot be tolerated, unless the conventional column would also need refrigeration.

The split tower arrangement has a large temperature difference between the reboiler and condenser, thus it will probably be desirable to minimize this by using small delta T's across the reboiler, condenser-reboiler, and condenser. This will mean a large heat exchanger surface being required. Even so, it is likely a higher temperature heat source will be needed for the reboiler. As it is at a higher temperature, the heat will be more expensive, such as a higher pressure steam. This means we are saving energy, but using a more costly source.

The feed to a single tower will be split in two for the two column arrangement. Therefore, the individual columns will be about one-half the size of the single column. However, the relative volatility and the mass flowrate/area through the columns will change with the pressure, resulting in a differently sized tower than just one-half the size.

From an economical viewpoint a split tower arrangement will require two columns, instead of one larger one. Each column will require its own instrumentation, causing twice the instrument costs. The higher pressure column will need thicker walls, and its size may be larger than expected. (See preceding paragraph). A larger exchange surface is needed for the various exchangers. Various auxiliary exchangers may be required for column control. The savings of the split tower arrangement come from the reduced heat requirement. However, the value of the heat used should be higher per Btu used than in the case of a single column. In many respects, a split tower will be similar in economic desirability to a vapor recompression column. The key is to have a low temperature difference from the top to the bottom of the column.

In designing the split tower arrangment, the low pressure column should be set by the achievable condenser temperature. Then the split tower should be worked backwards from this point, a reasonable temperature drive given for the condenser-reboiler, then the high pressure column found, finally resulting in a temperature for the reboiler. With this temperature the available heat sources should be examined, for example, the various steam pressures, and one chosen. The delta T available should then be distributed between the reboiler, condenser-reboiler, and condenser to obtain the minimum required heat exchanger surface area. The feed between the towers should be split in order to approximately give equal duties for the high pressure condensation and low pressure reboiling under design operating conditions.

The control of a split column will be more complex than that of a single column. The object of the control system will be to decouple the two towers to a certain extent. The use of an auxiliary condenser on the high pressure column and a auxiliary reboiler on the low pressure column will give energy efficiency and good control. Control can also be had by having only one auxiliary exchanger, and by having one of the columns run at a higher duty than the other. The feed split ratio between the columns can be used as part of the control. Note that the bottoms of the high pressure tower can be mixed with the low pressure bottoms and flashed in the low pressure tower. This would result in a uniform bottoms composition.

The split tower design offers a good possibility of energy savings with a new installation. Where an existing column exists already, it would be possible to increase capacity by adding another tower next to the existing one and installing a new condenser-reboiler so that the existing column will become one-half of a split tower arrangement. In cases where no capacity increase is desired, but the column original size was such that two towers were used, it may be possible to convert it to a split tower operation by installing a more efficient column internal trays and by adding a condenser-reboiler, new piping, and new instrumentation. The savings that can result from a split tower design are very much afftected by the cost of energy to the reboiler, so the true energy cost should be evaluated before using a split tower.

4-C-8. Interreboilers, Intercondensers, and Feed Preheating

The reboiler is at the highest temperature of any part of the distillation column, therefore it is the worst place to add heat as a high temperature (and therefore more valuable) heat source must be used. Likewise, the condenser represents the worst place to remove heat as its temperature is the lowest, and any recovered heat will be of low value. If heat can be added at another part of the column in place of heat added at the bottom, we can use a less valuable heat source (i.e. lower pressure steam) or have a smaller heat exchanger surface area due to the increased delta T available. In the case of the condenser heat rejection being replaced by rejection at another part of the column, a smaller heat exchange surface could be used, or the heat recovered (example waste steam generation), or a refrigeration requirement for the condenser reduced. Thus if we can shift some of the reboiler or condenser duty to another part of the column, we may be able to save money.

The reboiler duty can be reduced by using one or more inter-reboilers and feed preheating. The condenser duty can be reduced by use of intercondensers and feed precooling (i.e. condensation of a vapor feed). Note that if we hold the total heat duties constant, and use interreboilers and intercondensers, then the number of trays in the column will have to be increased at the top and bottom sections, although the column cross-sectional area can be reduced. The key to proper use of feed preheating and interreboilers is to make sure the reboiler duty goes down correspondingly with the increased auxilliary duty, hold overall energy use constant while less valuable energy can be used.

The effects of using interreboilers, feed preheating, and inter-condenersers on separation can be seen easily on the simple McCabe-Thiele diagram of Figure 4-4. The different liquid-to-vapor ratios are found for the use of an interreboiler and plotted on the diagram. The extra theoretical trays can be counted. The location of the new trays in the column is obvious. The new vapor rates in the different column sections can be used to find the required cross-sectional areas of the sections of the column.

4-C-9. Feed Preheating

The object of feed preheating is to replace a portion of the reboiling duty with that of the feed preheating. This method is particularly valuable when most of the feed is removed as the distillate product. This situation results in reboiler duty being greatly reduced by preheating. Preheating of the feed is of special benefit as, the feed may be at a low initial temperature, so it can be heated to high temperature by a simple countercurrent exchange with another stream, such as the bottoms and distillate product. Having a partially vaporized feed is easily handled by any of the distillation column design methods. Another possibility is that the feed may be already taken as a vapor product from some previous column. By feeding the feed as partially or wholly as a vapor, the vapor flow (and therefore the required diameter) will be larger in the rectifying stage than the stripping stage.

4-C-10. Interreboiler

The use of an interreboiler effects the column in a manner similar to that of the feed preheating. In this case we are attempting to use a less valuable heat source (i.e. low pressure steam) to replace the more valuable heat at the reboiler. The use of an interreboiler will be most effective in cases where the upper section of the column requires more reflux than the lower stages. The stripper section will have a lower vapor flowrate, but will require more stages than a conventional column.

4-C-11. Intercoolers and Feed Precoolers

Other than in this case of a refrigerated column, where any savings are valuable, the use of intercondensers and feed precoolers will be more difficult to justify. The use of an intercondenser can be effective when waste steam (or heat) can be produced by an intercooler, but not by the lower temperature condenser. This is most effective when the stripping section requires more reflux, or more bottom product is taken, but that the rectifying section needs little. An objective in using an intercondenser or feed precooler is not to have to increase the required reboiler duty. Feed precooling would be used in the case where the initial feed is a vapor.

A special situation exists where a multicomponent distillation is being used with light products predominating toward the top of the column. An intercondenser may be used in order to reduce the vapor rate in the top of the column, allowing a thinner section to be used on the top.

4-C-12. Circulating Refluxes

One commonly encountered situation is where a multicomponent separation column exists which has several sidedraws. On a column of this type we can take liquid from one tray, cool it in an external exchanger, and return the liquid to a tray a few trays higher. This can be called a pumparound, a circulating reflux, or an intermediate condenser.

Circulating reflux systems are used for two reasons, recovering energy and balancing vapor loads. The internal recycling system causes a hot liquid stream to be drawn off. The heat that is removed from the hot liquid can be recovered. For example, it can be used to heat a stream, or to produce steam. As the stream is at a higher temperature than the overhead condenser, the heat can be recovered at the more valuable higher temperatures. As the stream is a liquid, it can be easily handled in conventional heat exchange equipment. The other reason for using a circulating reflux is to reduce the vapor load in the upper column. As lighter lower molecular weight components predominant towards the top without a circulating reflux the gas volume will increase, resulting in the need for a larger cross-section.

R. P. Bannon and S. Marple in their paper (See Appendix 7-C) suggest that there be at least as many circulating refluxes as side draws (See Figure 4-5). The objective in this case is to minimize the draw tray liquid spillover down the column. Spillover flow needs to be 10% or so to allow for control variations. If spillover occurs, it means that we are removing heat at a lower temperature than is necessary. Two situations are suggested for the placement of the pumparound's removal and return trays. One is that the liquid reflux and the side draw are taken from the same tray, with the returning cooled liquid being added several trays higher, under the next sidedraw. The other is to have the liquid removed at a lower tray, and then returned to the tray just below the sidedraw. For the second case the temperature is maximized as a low tray is used. As we are attempting to remove heat at as high as temperature as possible, the refllux flowrate should be large so that its return temperature is as high as possible. Both the initial removal and final return temperature are important in determining the temperature at which we can recover heat.

By using a circulating reflux we reduced the amount of reflux in the upper trays. This results in a requirement for more trays to maintain the same separation. Since the vapor flowrate is also reduced in the upper section, we will have as a result a slimmer, taller column.

4-D. Use of Vapor Recompression and Heat Pumps for Distillation

4-D-1. Introduction

Vapor recompression consists of taking the overhead vapors of a column, condensing the vapor to liquid, and using the heat liberated by the condensation to reboil the bottoms liquid from the same column. The temperature driving force needed to force heat to flow from the cooler overhead vapors to the hotter bottoms product liquid is set up by either compressing the overhead vapor so it condenses at a higher temperature, or lowering the pressure on the reboiler liquid so it boils at a lower temperature, then compressing the bottoms vapor back to the column pressure. A conventional column has a separate condenser and reboiler, each with its own heat transfer fluid such as cooling water and steam. The vapor recompression column has a combined condenser-reboiler, with no external heat transfer fluids.

The vapor recompression cycle has a set ratio between available condenser side to reboiler duty. The reboiler heat flow obtained will be equal to the sum of condenser duty plus the work added to the gas stream and its inefficiencies. In all cases where the column reboiting and condenser duties do not match in the manner stated an auxiliary system will be needed to supply the excess column condenser or reboiler duty.

The advantage of vapor recompression lies in its ability to move large quantities of heat between the condenser and reboiler of the column with a small work input. This results from cases where there is only a small difference between the overhead and bottoms temperature. A conventional column with steam heating and water cooling may use ten times the Btu's of a column running with vapor recompression.

4-D-2. Distillation Columns Reflux and Heat Balance

In distillation, vapors move up trays and liquids down to separate the feed into lighter and heavier components. A reflux of liquid from the top and vapor from the bottom provide the needed mass flows. A pressure gradient created by increased temperatures as we move down the trays provides the force to move vapors up the column. As you go from the top to the bottom of the column, the temperature increases, caused both by the increased concentration of heavier components and the increasing pressure required.

Let TOV be the bubble point temperature at which the overhead vapor from the top of the column condenses for the liquid reflux. Let TBL be the bubble point temperature that the bottoms liquid boils for the vapor flux. It is clear that most of the heat used in the distillation column is to provide the liquid and vapor refluxes. This heat is added to vaporize the bottoms liquid at TBL. The heat is removed to condense the overhead vapor at TOV.

For the overhead vapors, a condenser is needed to provide reflux and its desired distillate product state. The condenser will condense overhead vapors to provide reflux. The reflux stream can be subcooled to reduce the mass flowrate, but the heat duty required for reflux is constant, . The condenser can be operated as a partial or complete condenser. For a partial condenser some or all of the desired product may be taken as a gaseous product, resulting in an extra stage. For the condenser where the product is taken as a liquid, this duty is also added to the overall condenser duty, so that we get approximately that the heat duty of the condenser, .Definitions of the nomenclature are found on Table 4-5, "Nomenclature of Section 4-B".

The bottoms liquid needs a reboiler to provide vapor reflux (RV). The reboiler duty for vapor reflux needed is equal to vapor reflux RV times the heat of vaporization for the bottom liquid, so heat duty = . Normally the bottoms product will be removed as a liquid, but if needed the bottom product can be vaporized also, giving the total heat duty as .

As can be seen most of the heat used in distillation is added, qh at TBL and removed, qc, at TOV. The reflux requirement results in qc and qh being reasonably close, but as TBL > TOV the column needs external input to operate.

An ordinary column has a separate condenser and reboiler. The condenser running on the overhead vapors rejects the heat into an external heat transfer fluid. The fluid can be cooling water, air, chilled brine, boiling refrigerant, or boiling water producing waste steam. The conventional case would be using cooling water to absorb the heat given off in the condenser. In this case the heat is lost. The reboiler boiling the bottoms liquid requires an external heat input from a heat transfer fluid. Possible fluids include steam (condensing anywhere from 0 to 600 psig), condensing dowtherm vapor, molten salts, burning natural gas, a hot liquid stream, or hot water. A conventional column will use valuable steam to supply the needed heat. An example of a conventional column can be seen in Figure 4-6.

4-D-3 - Vapor Recompression

A distillation column driven by a vapor recompression cycle differs. In a vapor recompression cycle, the condensing and reboiling are supplied in a single exchanger (the condenser-reboiler). Work is input to the system to transfer heat from the condensing overhead vapor to supply heat for the reboiling bottoms liquid. A driving force to override the fact that TBL > TOV is arranged by changing the system's pressure, so that the temperature at which the overhead vapor now condenses at is higher than the temperature the boiling liquid boils at. All the heat qc removed from the condenser side goes to heat the reboiler. The reboiler also receives the work energy added to the system, plus its inefficiencies. So the qh = qc + W.

Where qh and qc do not match the overall column requirements of qh and qc the difference is made up by auxiliary exchanger such as a normal condenser or reboiler. Thus, in a vapor recompression, the external inputs of heat transfer fluids are greatly reduced, while some work is needed.

The temperature driving force necessary to transfer heat from the overhead vapor to the bottom liquid, and to supply the necessary driving force across the exchanger surface () can be supplied in one of two ways;

(1) The overhead vapor can be compressed, so that its bubble point is at the higher temperature over TBL needed to give the wanted across the condenser-reboiler.
(2) The bottom liquid pressure can be reduced in the condenser-reboiler, so that bubble point is at a low enough temperature below TOV to provide the wanted . Then the bottom vapor is compressed to a pressure high enough to allow it to enter the column.

These two situations are shown in Figure 4-7.

Note that a flash tank is added when the overhead vapors are compressed to avoid returning superheated liquid to the top of the column where it will flash. In the other case where bottoms are compressed, the Superheated vapors from the bottoms liquid are of no importance.

4-D-4. - Heat Pump

A heat pump uses a separate condenser and reboiler as does an ordinary column. However, heat is transferred from the condenser to the reboiler in a manner similar to that of vapor recompression. An isolated heat transfer fluid is boiled in the condenser, compressed, and condensed in the reboiler. (See Figure 4-8). The characteristics of the vapor recompression cycle are retained by the heat pump cycle, but an additional exchanger and an extra across it is needed.

4-D-5. - Theory Behind Vapor - Recompression and Heat Pumps

4-D-5.1 - The Carnot Cycle

The Carnot cycle is defined as an ideal reversible process for the extraction of work from a heat flow between two different levels. A Carnot cycle consists of the following four steps:

(1) A reversible isothermal expansion of a gas at Th from P1 to P2. qh heat is gained and some work is produced.
(2) A reversible adiabatic expansion of a gas from P2 to P3 at which the temperature drops from Th to Tc. No heat is transferred (adiabatic) and some work is produced.
(3) A reversible isothermal compression of the gas at Tc from P3
to P4. qc heat is lost and some work is used.
(4) A reversible adiabatic compression of the gas from P4 to P1, where the temperature increases Tc to Th. No heat flows, but some work is used. This overall cycle results in a number of associated equations.

This equation is the famous Carnot equation for efficiency. It is known that the maximum amount of work that can be done by any process for a given amount of heat at Th is the Carnot efficiency . Derivative equations include:

Carnot efficiency 0 < < 1

The Carnot cycle is totally reversible. Therefore the work can be input to transfer heat from the cold source to the hot source. Rearranging the equations results in:

This means heat can be moved by work. As in the Carnot cycle for work, it turns out that the maximum heat, qc, that can be transferred between Tc and Th by a given amount of work. Wideal is given by the Carnot cycle.

From now on we will refer to all q/w, heat/work ratios in the absolute value terms, or:

The work imput will be done to the system by an external source, heat flow qc will be removed at Tc and heat flow qh will be supplied to the higher temperature Th.

For a real process there will be efficiency losses. For this case of heat movement by work, there are two efficiency losses to be worried about.

= The efficiency within the process cycle. These inefficiencies result in heat being added to qh. These items are caused by such things as refrigeration cycle inefficiencies, the compressor inefficiencies, the fluid losses, etc.

= The efficiency losses that do not cause heat to the cycle. This is best exemplified by the driver losses, for example, an electric motor loss or a steam turbine loss do not add heat to the process, so we get the following equations:

(Note that Wideal will be expressed for a flowrate qc)

Potentially large heat/work ratios are possible which means much heat can be transferred for a little work. For the case of a constant heat difference (Th - Tc) of l00ºF the heat/work ratios will increase with increasing temperatures.

At Tc = 40ºF,

                       

At Tc = 540ºF,

At Tc = 1040ºF,

At Tc = 1540ºF,

                           

Even more important than the absolute temperature is the temperature difference Th - Tc. For a constant Tc of 40ºF we get:

At Th = 140ºF, Th - Tc = 100ºF,

At Th = 90ºF, Th - Tc = 50ºF,

At Th = 60ºF, Th - Tc = 20ºF,

At Th = 45ºF, Th - Tc = 5ºF,

For Vapor Recompression, we will count the heat exchanger as part of the process. So for the ideal work equations the temperature Th - Tc is represented by , the top to bottom of the column temperature difference plus the delta T across the condenser-reboiler. The Tc = TOV where the overhead vapor is compressed, where the bottom liquid is expanded.

4-D-5.2 - The Refrigeration Cycle

Vapor recompression uses a refrigeration cycle rather than a Carnot cycle for heat transfer be tween the two sources at Tc and Th. A refrigeration cycle consists of 4 steps.

(1) A vapor stream condenses at pressure Ph, representing saturation point for Th. This gives of qh heat, no work.
(2) The liquid stream is flashed from Ph to Pc. The Pc is the saturation pressure for temperature Tc. No heat or work flow occurs.
(3) The remaining liquid boils at Pc, Tc removing qc of heat. No work is used.
(4) The vapors at Pc , Tc are compressed adiabatically to Ph, the temperature rising above Ph (back to Step 1). No heat flows but W work is used.

The refrigeration cycle therefor absorbs heat at Tc, releasing the heat at Th, only two pressure levels Pc and Ph are met (other than within the compressor). The refrigeration cycle is not reversible and is less efficient than a Carnot cycle between the two temperatures. Figure 4-9 demonstrates temperature-enthalpy processes for the two cycles.

Let us work out a simple example of a refrigeration cycle. Water is the working fluid. The water will boil at 212ºF absorbing heat and reject heat at 230ºF. The pressure of saturated water at 212ºF = 14.698 psia, at 230ºF the pressure is 20.78 psia. Assume no inefficiencies other than that characteristic of the fluid and refrigeration cycle.

Water boils at 212ºF forming vapor. Assume we have 1 lb of vapor. The work needed to compress it from 14.698 psia to 20.78 psia is:

k = 1.2857, P1 =14.698, V1 = 26.8 ft3/lb
w = 26.24 Btu/lbm vapor

The temperature of the gas after compression is

h (water vapor 212ºF) = 1150.5 Btu
h (after compression) = h212 + W = 1176.74 Btu at 20.78 psia
Temperature = 270ºF

In the condenser operating at 20.78 psia, saturated liquid leaves so

hout - hin = 198.32 - 1176.74 = - 978.42 Btu/lb

The liquid leaving the condenser is flashed from 230ºF, 20.78 psia to 14.698 psia, 212º.

0.018715 lbm steam + 0.98128 lbm water

The water is boiled at 212ºF for the heat absorption

hout - hin = 1150.5 - 180.16 = 970.34 Btu/lbm
(.98128 lb) x (970.34) = 952.17 Btu

qc = 952.17 BTU qh = 970.34 BTU W = 26.24

A Carnot Cycle will give:

The Refrigeration Cycle gives:

This shows 3% efficiency is lost by the refrigeration cycle for this case.

The characteristic of the refrigerant heat transfer fluid used is important on efficiency. The following list gives the desired characteristics listed in rough order of importance.

(1) The vapor pressure of the fluid must be in a reasonable range for the desired temperatures. A vapor pressure of 0.1 psia or 5000 psia would make the fluid undesirable.
(2) The liquid cannot freeze at the lowest temperature met in the cycle.
(3) A high heat transfer coefficient is desirable to minimize and exchanger surface.
(4) A high heat of vaporization per mole is desired. This maximizes heat transfer for a given gas flow through the compressor.
(5) The Ph/Pc for the temperatures Th, Tc need to be minimized, both for power saving and possible compressor problems.
(6) If the refrigerant is near its critical point, the above two quantities (4,5) turn very unfavorable.
(7) The heat capacity of the liquid to be low as to minimize flashing vapor.
(8) k, the heat capacity ratio, should be as low (near 1) as possible.

4-D-6. Vapor Recompression

4-D-6.1 Situations

A number of different situations can exist for a vapor recompression setup. The location of the main compressor in the overhead or bottoms gas stream is only one of the problems. The status of the feed, distillate, and bottoms product as to whether they are vapor or liquid makes a difference. For a vapor recompression system with overhead vapor compression, this is a need to avoid the hot liquid from the condenser-reboiler from flashing upon entering the distillation column. This can be done either with a flash tank returning vapor to the overhead stream, or an exchanger can be used to cool the liquid below the flash temperature. Every column will require either an auxiliary reboiler or condenser to correctly match its heat balance. Both these auxiliaries may be added and oversized, so as to control the equipment. In the case of a refrigerated column the method of refrigeration is of interest.

Figure 4-10, 4-11, and 4-12 will give an idea as to the number of possibilities of different setups. Figure 4-10 is for a heat pump. Figure 4-11 and 4-12 show four individual cases, I A and B, and II A and B that indicate a desirable or optimum setup for the given situation. Cases I A and B deal with the situations met in a hot column where heat is costly (steam supplied for example) and cooling is done by water or air. Cases II A and B deal with a different situation where there is a cold column which operates at refrigerated conditions, making cooling valuable.

4-D-6.2 Auxiliary Heat Transfer Equipment

A vapor recompression column will need auxiliary equipment to perform the necessary balancing of the condensing and reboiling duties of the distillation column. In addition, the column is controlled by these auxiliaries. The following general situations of heat duty could be met.

(1) The vapor recompression cycle is used for only part of the heat duty for either the condenser or reboiler, say 50%. For this case a large conventional condenser and reboiler will be used as the auxiliary. Essentially we would have a conventional distillation column, with great control over it, but with a significant portion of the heat load carried out by the high efficiency vapor recompression cycle. This hybrid column will have most of the advantages of both a conventional column and a vapor recompression column.

(2) One of the duties, either the condensing or reboiling duties, will be in great excess. In this case only one item, a large auxiliary condenser or reboiler will be needed. The auxiliary item will control the column and balance its heat duties.

(3) The condensing and reboiling duty requirements are close and the great portion of the heat load is carried by the vapor recompression cycle. A separate small auxiliary reboiler and auxiliary condenser would both be added. This will allow control of the column and some flexibility.

A system with a separate heat pump cycle will experience similar problems, although the auxiliary equipment will be interfaces with the heat pump fluid, not directly to the colum