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.
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.
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-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 ProductThe 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-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 CycleVapor 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 vaporThe 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 = 270ºFIn 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 = 26.24A 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 minimizeand 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 column.
A column operating at refrigerated temperatures permits special uses for the auxiliary equipment. If excess cooling is available from the vapor recompression cycle, then it may be possible to use this valuable cooling to chill water or cool some other item. Excess cooling is not expected in most cases, but excess heat needs to be removed from the column. A conventional heat pump refrigeration system may be used to provide the cooling, but there are a number of other possibilities. The bottoms vapor can be compressed by an auxiliary compressor to where it condenses at ambient temperatures, using the bottoms liquid as the heat transfer fluid just as a heat pump refrigeration system would. In this situation it is desirable to have the compressor in the overhead. This leaves the condenser-reboiler operating at a higher temperature than the other case, and the bottom vapors leaving the reboiler are then in a good position to be compressed by an auxiliary compressor. Note that the entire condensing and reboiling loads are carried by the condenser-reboiler, and the auxiliary equipment are not parallel to the condenser-reboiler as in the case of higher temperature operations.
4-D-6.3 Compressor Drives and Their Energy Costs
The compressor of the vapor recompression cycle will require a driving work source. The compressor work requirement is expected to be sizeable in most cases. The compressor and its driver will be located near the column to avoid long pipelines for the vapor stream entering and leaving the compressor. Thus, we can have one of a number of local power sources. Among the possible driver sources are:
(1) An electric motor.
(2) A steam driven turbine, letting high pressure steam down to a lower pressure (for example, 600# steam to 40#).
(3) A gas turbine, running on natural gas.
(4) A diesel engine.
(5) A process expansion turbine (gas or liquid).To obtain a true picture of the overall energy savings by a vapor recompression cycle, the energy source's original generation efficiency must be included. For example, if an electric motor is used, the costs and efficiencies of the electric plant should be included. For example, if the vapor recompression cycle has a heat/work ratio of 10, after accounting for the electric generation efficiency of 33%, we get an overall energy savings of 3.3. The column run conventionally would use 3.3 times as much energy as the vapor recompression cycle, not 10 times as much.
Likewise the fuel cost and efficiencies for the gas turbine or diesel drive should be considered, as well as the equipment and maintenance cost of gas turbine or diesel.
A steam turbine, say 75% efficient, will probably be the most efficient driver overall, where the excess high pressure steam exists and the low pressure steam is needed. A part of the plant's steam plant costs should be charged to the turbine.
4-D-6.4 Insulation of Columns Using Vapor Recompression or Heat Pumps
The insulation required for the column and its associated equipment will vary with the situation for a vapor recompression system. For the case of hot operation of the column where the condenser duty is in excess, no insulation is needed for stopping heat flow, although legal requirements and other needs may require a small amount of insulation on exposed surfaces. Where the column is hot and extra reboiler duty is needed, the column should be insulated like a conventional distillation column.
For a column operated at cold temperatures, insulation will probably be required to prevent moisture freezing on the lines and heat flows into the column. The amount of insulation required will again be related to whether excess cooling is available or extra cooling is needed. The refrigeration cost will determine the optimum thickness.
In general a small amount of insulation is expected to be needed, to protect against ambient conditions, but that the thickness used is expected to be somewhat less than that of a conventional column.
4-D-6.5 Vapor Recompression for Interreboilers, Other Columns
Vapor recompression, transferring heat from a colder source to a hotter sink can be used at any point. For example, if a column has intercondensers and interreboilers, these items can be used as part of a vapor recompression cycle just like an overall column recompression cycle can be used. The heat duties will need careful balancing when unusual points are used. Note that the temperature difference between the inner units will be lower than that across the overall column, and vapor recompression may be favorable for inner units even though the top to bottom difference is too large to be favorable for the overall vapor recompression.
The inner units may be joined in any fashion. An interreboiler can be interfaced with the overhead condensing vapor; an inter-condenser to an interreboiler or the bottoms product reboiler.
The vapor recompression cycle can be used for other columns, the overhead vapors of one column compressed and condensed to boil the bottoms of another column. The total heat duties will probably not be very close in these cases, meaning lots of auxiliary heat transfer equipment will remain.
4-D-7. Reasons For Conversion of an Existing Column
There can be a number of reasons why it is desired to switch an existing conventional column to a vapor recompression cycle.
A. The new vapor-recompression cycle could allow the column to be operated at a lower pressure, if desired, where the lower pressure increases the relative volatility. This would allow less reflux to be needed, therefore less heat duties, increased production capacity, or increased product purity. As the column vapor load capacity drops with the square root of the density, a loss of capacity could also occur.
B. The vapor recompression system can be designed to provide a different (more or less) amount of reflux compared to the current system. It may be that the current column's condenser and reboiler is undersized for the desired load, and would need changing in any case.
C. The high potential efficiency of a vapor-recompression cycle may be desired, the heat/work ratio of this case being favorable as compared with a conventional steam driven reboiler.
D. The heat source (i.e. steam) used by the current column may be needed elsewhere, or excess high pressure steam is available for driving turbines. The vapor recompression cycle replacing the old system may improve the overall integration of utilities in the entire plant.
E. The existing column may be currently operated with refrigeration or a heat pump cycle, and conversion to a vapor recompression cycle is not difficult and more efficient than the previous unit.4-D-8. Conversion of an Existing Column
In the case of a conventional column currently equipped with a normal reboiler and condenser, conversion of the column to a vapor recompression operation will prove expensive and difficult. A number of new items will be needed and old items replaced. Some of the these items are:
A. A new compressor, its driver, and associated control equipment will be needed.
B. The control system for the existing column may prove usable for the vapor recompression column, and complete replacement needed.
C. Sufficient space for all new equipment (condenser-reboiler, compressor, driver, + auxiliaries) must be present.
D. The existing reboiler and condenser will most probably not be usable as part of the needed condenser-reboiler. A new condenser-reboiler much larger than the present heat exchange equipment will probably be needed.
E. Auxiliary condensers or reboilers will be needed for the column heat balance and control. However, the existing condensers and reboilers can be kept and used for this auxiliary function.For the case of the existing column, it may be best to put only part (say 50%) of the overall condenser and reboiler's duty on the vapor recompression cycle. This allows much heat to be saved, but leaves the existing reboiler and condenser in place and operating the column. Note that an all new condenser-reboiler, compressor, and driver, will be needed for the partial conversion use.
4-D-9 Advantages of Vapor Recompression
The advantages of vapor recompression are:
(1) A large amount of heat can be transferred with little work. For example, suppose we had a heat/work ratio of 10. This means that for every Btu of work added, we replace 10 Btu's of heat that would otherwise be required. This can lead to overall savings also. Assume we generate one Btu of electricity from three Btu's of fuel heat (33% efficiency). Then overall we save 10/3 = 3.33 Btu of heat for every Btu fuel used by the vapor recompression system over that of the conventional system.
(2) The heat flow to supply the condenser or reboiler may be of particular value. For example if the column is cold, the cooling must be accomplished by expensive refrigeration. This is also true at high temperatures 500ºF + where energy added to the reboiler can no longer be supplied simply by steam. The vapor recompression cycle, by being balanced, is affected only to a limited effect by the relation of the actual temperature to the ambient. The pressure in a vapor recompression column can be set where desired to achieve maximum separation.
(3) By freeing the condenser and reboiler of the desire to hold temperature between a minimum of about lO0ºF to a maximum of about 500ºF, the points easily reached by cooling water and condensing steam, we can set the temperature, and therefore the pressure, at any point we wish. This effect is of particular importance where changing the pressure effects the relative volatility. By operating at more favorable conditions we can reduce the reflux requirement and therefore the heat duties. The effect of the pressure change on the column will be to change the wall thickness, and column diameter, the diameter falling as the reflux is reduced, but increasing as the pressure drops. Do not neglect the effects of pressure changes on the overall column.
(4) Work energy from excess high pressure steam which is generated for low pressure steam requirements may be available in the overall plant balance. This energy would be very cheap if the alternative or current practice is to wastefully let down the steam across a valve. A steam turbine driver for the vapor compressor can be used, giving a low work energy cost.
(5) Electricity can be brought in from the outside to run the compressor driver, so that large amounts of steam are not needed. Thus the steam plant can be smaller.
(6) The vapor recompression system uses little cooling water or steam flow. Possible utility savings can occur as less cooling water is discharged, and the steam, condensate, and cooling water lines can be made smaller.
(7) Swings in the ambient temperature and weather will have little effect on the operation of vapor recompression.4-D-10. Disadvantages of Vapor Recompression
The disadvantages of vapor recompression are:
(1) Premium electrical or steam pressure work energy is used for driving the compressor, and no advantage can be taken of possible existing low value sources of waste heat to run the reboiler.
(2) The additional cost for the compressor and its driver are required.
(3) The condenser-reboiler of vapor recompression has the overhead and bottoms product on either side, leading to savings by having one half of the total area for the separate condenser and reboiler of a conventional column. This advantage is lost and more heat exchanger surface is required for the condenser-reboiler as compared to the conventional condenser and reboiler because:a. The fluid used (cooling water and steam) in the reboiler and condenser will have a much higher heat transfer coefficient than the column fluids.
b. Thedriving force for both the reboiler and condenser will be much higher than that allowed across the condenser-rebolier as its heat/work efficiency drops with increasing
across the system.
c. Theacross the condenser-reboiler causes a loss in efficiency, therefore requiring a larger compressor, driver, and work input. The
will be minimized to allow a smaller compressor and driver.
These items add up to the fact that the exchanger surface required for the condenser-reboiler will probably be significantly larger than the combined surface on the condenser and reboiler in a conventional column.(4) Auxiliary units such as extra reboilers, condensers will be required in order to balance the heat duty and control the column.
(5) The mechanical complexity of the vapor compressor is high, the system requiring more maintenance and suffering more breakdowns.
(6) More instrumentation will be required to control the compressor and the other auxiliary items of the vapor recompression system.
(7) The control of the vapor recompression system is different than that faced in an ordinary column. New methods will have to be learned by the operating personnel.
(8) Flexibility is lost as the column has only limited ability to function at other than design conditions. The compressor will be sized to be most efficient at one operating rate. Increasing the column reflux over design will be very hard.
(9) Altering the column for reuse to operate for a new situation will be more difficult than with a conventional column, as the vapor recompression system will probably need to be replaced.
(10) Continuous auxiliary refrigeration may be required for a low pressure column with vapor recompression that would be run at higher temperature by a conventional distillation system.Overall, by swtiching to vapor recompression one gains energy efficiency at the cost of greater mechanical complexity and flexibility loss. Capital costs for vapor recompression will probably be greater than a conventional case, but it depends on the situation and what all is taken into account in economic changes.
4-D-11. Advantanges and Disadvantages of the Heat Pump
Heat pump driven systems are similar to vapor recompression systems. Most of the advantages and disadvantages are the same as compared to a vapor recompression system. Disadvantages and advantages as compared to a vapor recompression column are:
(1) Two separate vessels are required for the condenser and reboiler. Also each vessel has two
's. This could lead to as much as four times the exchanger area being needed. Reducing this somewhat is the fact that the heat transfer fluid used can have a higher heat transfer coefficient. Also a better refrigeration fluid can be used which can allow a larger
across the exchangers and a smaller compressor.
(2) The advantage of the heat pump is that the compressor cannot contaminate the distillation products. Also a reciprocating compressor can be used. Better control and flexibility of the column is obtained by the heat pump, similar to that of a conventional column.Overall, the key to use of a heat pump will be the opportunity to use a better and isolated heat transfer fluid, against the cost of the extra heat exchanger surface.
4-D-12. Guidelines for Considering Vapor Recompression
The following guides indicate when to consider a vapor recompression system in comparison with a conventional system for a distillation column. To actually compare the systems, see 4-D-13.
(1) When the column is to operate wholly in refrigeration temperatures for both the condenser and reboiler, then vapor recompression should always be considered.
(2) Where the condenser temperature is expected or desired to be below l00ºF, some consideration of vapor recompression should be given if the condenser-reboiler temperatures are close, say less than 40ºF.
(3) Where the reboiler temperature is very high, +500ºF, and the temperature difference is moderate (<l00ºF).
(4) Cases where the condenser and reboiler temperatures are nearly the same (<20ºF) so that high efficiencies can be projected.
(5) Where some problem with supplying steam exists for a conventional column and reasonably good efficiencies can be expected, say 3, 4 to 1 heat/work ratios.
(6) Where water is the main fluid, with trace quantities of contaminates being concentrated so that only a small temperature difference is expected. Water is an excellent fluid for vapor recompression as it is a good refrigerant and has a very high heat transfer coefficient.Two cases for not using vapor recompression are as follows:
(1) When larger temperature differences are expected between the condenser and reboiler.
(2) When the column can be operated with cooling water for the condenser and waste heat (1 ATM or less steam) for the reboiler.4-D-13. Procedure for Vapor Recompression Evaluation
After the design and economics are developed for a conventional column, the guidelines for considering vapor recompression are reviewed. If vapor recompression appears feasible, then:
(1) Study the conventional design and find its basic utility costs. See if refrigeration is needed, or if a high value heat source is used. Compare this heat cost with the cost of work energy (electricity or high pressure steam). The resulting work cost/heat cost ratio will give a minimum heat/work ratio aimpoint.
(2) With the temperature and pressures used for the conventional column, work out the ideal heat/work ratio, assuming a small extrathe reboiler-condenser, say 10ºF.
The actual obtainable heat/work ratio should be about 50-80% of the ideal heat/work ratio, for the given
. An ideal heat/work ratio of less than 10 at this point would be inauspicious, unless the heat flow is expensive.
(3) Study the effects of pressure on capacity and separation in the column, and decide on a new column operating pressure from this. The new column needs to be sized, and a new reflux ratio assigned. Rework the ideal heat/work ratio of Part (2) remembering to take into account the changed heat duties for the condenser and reboiler.
(4) Study the heat duties of the condenser and reboiler, and design a vapor recompression system to work for this system. Place the compressor in the overhead or bottom vapor line, and put in an auxiliary unit for heat balancing.
(5) Assume the actual heat to work ratios are 70% less than the ideal heat to work ratio. In symbols, this is
(6) Find the heat transfer coefficient of the vapors and liquids involved and size the heat exchanger for a 100F difference.
(7) Find the costs of heat exchanger surface, and compare this with the capital and utility charges for the compressor and its driver. Using the heat/work ratio of 70% ideal find the optimumto minimize the combined exchanger and compressor costs.
(8) Add the costs for the vapor recompression unit, columns + reboiler-condenser + compressor drive + auxilliary and using the capital and operating costs compare these with the costs of the conventional column with reboiler and condenser. At this point, we should have a good idea to the expected capital and operating costs verse a conventional system. If nearly equal or less, a detailed study is in order: guessing the previously specified column and.
(9) Study the heat duties of the condenser and reboiler, and see how possible changes in the feed and product states effect the balancing. Attempt to minimize the addition of an expensive head flow such as refrigeration or steam.
(10) Reevaluate the design of the system made in part 4 above. Study the auxiliary system, especially if extra refrigeration is required.
(11) Work out the entire system, using the actual fluids, equipment efficiencies, and heat flows. Fix the auxiliary requirements.
(12) With the actual system, work out its true economics. At this point, the superior system, vapor recompression or conventional column should clear.
Now we can design throughly to obtain the optimum system.
(13) Reexamine the column pressure, which sets the absolute temperatures in the overhead and bottoms liquid. Arrange so the pressure is optimized between column costs, and auxiliary costs. Too cold temperatures may be avoided to avoid too large diameters, to prevent the requirement of special materials of construction, and to prevent large use of inefficient auxiliary refrigeration.
(14) Optimize the heat exchanger surface area with the resulting compressor driver size, using the actual heat/work ratios. A standard compression ratio and gas flow may be desired, if so, adjust the exchanger surface accordingly.
(15) The desired flexibility of the column should be examined, and the auxiliary reboilers and condenser supplied accordingly. The desirability of operating with other than specified conditions should be evaluated.
(16) The control scheme and its necessary instrumentation should be designed.
(17) The plant personnel should be contacted for their opinions on the proposed system.
(18) Final calculations should be made and the economics presented to management.4-D-14. Example Propane-Propylene Splitter
The following example is presented to demonstrate the mathematics involved in working a vapor recompression column. The distillation column, feed, splits, and reflux are all assumed and should not be considered to reflect an actual column.
4-D-14.1. Situation Statement
A propane-propylene splitter is assumed. The overhaul product is assumed 99% + pure propylene, the bottoms product is 50% propylene, 50% propane. The feed, on a 100 mole basis, is 80 moles propylene, 20 moles propane. This results in a 75% recovery. The bottoms product will probably be recycled back to another splitter, but this is unimportant. The distillation column was arbitrarily assumed to have a reflux rate of 6.67, or 400 moles liquid per 100 moles feed. The number of column trays is of no importance. The column pressure was set at 40 psia top, 45 psia bottom. The feed is a saturated liquid, the overhead will be taken as vapor, bottom product as liquid. The compressor efficiency is assumed 85%, electric drive motor of 90% efficiency. In the first example, the overheads will be assumed to be compressed. A
of 20ºF was set for the condenser-reboiler.
A number of minor assumptions were made to simplify the problem. The overhead will be treated as 100% propylene. The bottoms vapor pressure and enthalpies were assumed to be ideal additions of the two components. Equimolar overflow was assumed from the distillation column top to bottom. Pressure drops through pipes and exchanger equipment was ignored. Subcooling the liquid leaving the condenser, and superheating the vapor leaving the reboiler side were ignored. Heat flows to the ambient surroundings were ignored.
Note the low pressure of the column (40 psia) was set because the relative volatility of the propylene-propane set increases from approximately 1.09 at 300 psia to 1.18 at 40 psia. Slightly higher pressures might be more favorable on overall economics.
The vapor recompression example can be seen in Figure 4-13. The numbers shown in Table 4-1 are for a basis of 100 lb. moles. These numbers represent the assumed conditions in the problem statement.
4-D-14.2 Solution
Since the pressure at the top is 40 psia, the bubble point of pure propylene corresponds to a temperature of - 9ºF. The pressure at the bottom is 45 psia for the 50 - 50 mixture. After solving by iteration using the average of propylene and propane pure pressures, we get a bubble point temperature of 2.8ºF at the bottom of the column. The enthalpies of the various streams (pure propylene and mixed propylene-propane) at these temperatures and pressures are found from an enthalpy chart. The enthalpies of the propylene-propane mixture are found by assuming each component is separate and multiplying by its mole fraction.
Much information about the streams can be seen by inspection. The top of the column is at - 9ºF, 40 psia, 100% propylene, so the reflux liquid to this (stream 4) is at this temperature, pressure. The overhead vapor stream 2 splits into streams 12 and stream 6. Stream 10, leaving the flash tank, is in equilibrium with stream 4 so it is also at -9ºF, 40 psia. The bottom of the column is at 2.8ºF, 45 psia, 50-50 propylene-propane, this bottoms liquid is stream 3. Streams 9, 13, and 7 come directly from the bottoms stream therefore they have the same temperature-pressure data. The vapor reflux at the bottom is stream 5, which is also at 2.8ºF, 45 psia, 50-50 mixture. This stream is a combination of streams 11 and 15. Note the bottom reboilers merely change the bottoms liquid to vapor, without changing the temperature or pressure.
At this point the key items missing are the pressure of the vapor leaving the compressor, the mass flow of the flash vapor, and the mass flow to the bottoms liquid to the condenser-reboiler. From our problem statement we know that
to be 20ºF. The reboiler side temperature is 2.8ºF. Therefore, the condenser side temperature of condensation must be 2.8ºF + 20ºF = 22.8ºF. For this temperature we receive a condensing pressure of 72.4 psia. As the liquid leaves the condenser-reboiler at the 22.8ºF we know that stream 8 is at 72.4 psia, 22.8ºF.
Stopping at this point to work out the ideal heat/work quantities we find:
,
Ideally, extra heat input will be needed for qc.
Accounting for the known 85% efficiency compressor, 90% motor the best that can be achieved is,
The refrigeration cycle is less efficient than the Carnot cycle, so more work than this will be used.
Returning to the problem, we need to perform the flash tank calculation. We know the pressures and temperature of the involved streams 4, 8, & 10, and the flowrate (400) of stream 4.
Let M = flowrate of stream l0 so;
400 + M: flow of stream 8
enthalpy in = enthalpy out
(400 + M) (4507) = M (11,311) + 400 (3918)
M = 34.63 moles
Therefore stream 10 is 34.63 lb moles, stream 8 is 434.63
Stream 12 = Stream 2 + Stream 10 - Stream 6
Stream 12 = 460 + 34.63 - 60 = 434.63At this point we know the flows in the overhead stream. Now we need to find the compressor work and energy added.
Work Calculation
For propylene gas at about 0°F, cp = 8.8 Btu/lb mole
for -9ºF, 40 psia, Z, the compressibility factor = 0.96
Ideal Work Equation for Compression
Work done by compressor at 85% eff.
Electrical Work supplied to motor
Note that the compressor inefficiencies are counted in the exit gas enthalpy, the motor loss is not.
Enthalpy of gas leaving compressor (Stream 14)
Enthalpy in + compressor work = 11311 + 641.8 = 11,953 Btu/lb mole
For a pressure of 72.4 psia this enthalpy corresponds to a temperature of 51ºF.
Total work electrical energy
overall : 434.63 X 713.12 =
309,946 Btu heat work ratios, actual
9.54 Btu heat/Btu energy
Extra losses, over compressor & motor inefficiencies
9.54/10.85 = 87.9%, 100 - 87.9 = 12.1%
The load across the condenser-reboiler, is
qc' = flow (enthalpy out-enthalpy in)
= 434.63 (4507-11,953) = -3,236,255 Btu
qh' = -qc' =
3,236,255 Btu
The reboiler side duty must match the condenser side, so
qh' = flow (enthaply out- enthaply in)
3,236,255 = flow (11757 - 4296)
flow (stream 9 and 11) = 433.76
The flow of stream 15 is equal to stream 5 - stream 11
Stream 15 = 460 - 433.76 = 26.24 lb mole
Flow of 13 = Flow of 15.At this point we should note that qh for the total reboiling = 3,432,060 Btu, qh' = 3,236,255 Btu so that we have to add energy in the small auxiliary reboiler. The difference will be made up in the small reboiler as is expected, flow (13,15) (enthaply out-enthaply in) =
26.24 (11757-4296) =
195,776 Btu Auxiliary
Note: 3,432,060 - 3,236,255 = 195,800 Btu or the same.
Figure 4-14 and Table 4-2 show the results of the complete example.
The energy costs of the column are essentially 0 for heat and
= 3,099 Btu/lb mole feed split for electric power.4-D-15. Work Problem Propane-Propylene Splitter with BottomsVapor Compression.
In our example of the propane-propylene splitter we worked on the case of compression of the overhead bottom. Work the same column using the same assumptions but with bottoms vapor compression. Note that a desuperheater is added in the vapor return. This is used merely for mathematical purposes, in a real column the superheated vapors would be returned directly to the column. The starting point is shown by figure 4-15 and Table 4-3.
To save time, the obvious relationships and enthalpies have been included on Figure 4-16 and Table 4-4. These values are the simple result of stream equalities, and the enthalpies which are read from the chart.
The problem will be to find the work required for this case, and compare overall results with that of the overhead recompression example. The solution is found in Appendix 7-D.
Data needed k = 1.22, Z = 0.96
Figures 4-17, 4-18, 4-19 and 4-20.
A column may be equipped with simple conventional instruments or equipped with instruments that are computer controlled. The additional cost of sophisticated computers control systems must be economically justified to management for installation on new columns or retrofitted on existing columns. Savings may be from lower energy costs, higher production rates, lower capital requirements for intermediate storage between columns, less off specification product, etc.
Conventional instruments cannot control at optimum conditions because they cannot take corrective action until the variable being controlled has moved from its setpoint. Also, the optimum operating conditions depend upon the feed rate and feed composition. Since the control points must be changed to new optimum conditions, the operator needs assistance in deciding these changes. A computer can calculate the new setpoints and adjust the controllers automatically. Management is generally reluctant to make major expenditures to retrofit process units with these process control systems, based upon economic guesstimates by engineers on the pay back.
An alternate to this reluctance is to use the Distributed Process Control Systems (DPCS). The distributed control concept means that a single failure of a control cannot affect more than a limited area of the process. Thus, it is possible to install a DPCS by installing the system in steps, each step being justified economically.
The benefits of microprocessor control of columns are discussed in a recent article by M.R. Skrokov (Appendix 7-C). In addition to design engineering savings in manpower, the operating costs of the plants are reduced by the better control. However, the availability of control systems using microprocessors is claimed to be currently limited and appears aimed at total plant operation. Thus, microprocessing equipment for controlling single column may not be commercially available. Mr. Skrokov recommends that the instrument manufacturers supplement their large multiprocessing systems with small dedicated microprocessors. This observation could be the reason why small companies will be unable to use the economic benefits of microprocessors until small systems are available.
Let us assume a distillation column with distillates and bottom products. Component A has a value of $.30 per pound in the distillate, but no value in bottom product. Similarly, component B has a value of $.20 per pound in the bottoms, but no value in the distillate. How do you operate the column assuming a restraint of minimum purity? Dr. Latour, in his paper presented at the ISA Conference in Houston, May 23, 1978, developed this problem. Figure 4-21 shows a plot of the value of the products versus column reflux ratio. The maximum profit occurs at a reflux flow rate where the net recovered value peaks on 24. The lowest energy cost is not at this point, but at the specification restraint point a reflux flow of 13. When considering both profit and energy usage, the column should be controlled at a reflux flow rate between the maximum profit point and the specification restraint. Exceeding the reflux flow at the maximum profit point wastes energy.
If the value of the products from the column is fixed, the only restraint being the minimum specification, then the maximum profit and minimum energy usage are both at the reflux flow rate where the specification restraint is located.
At the ISA meeting in Houston, in May 1978, Mr. D.E. Lupfer presented a paper on manipulating the distillation column pressure to increase production and save energy. Operation of columns at the lowest pressure without flooding the column or overloading the condenser, had been practiced by Mr. Lupfer on hundreds of columns. Shinskey (Appendix 7-C), Skrokov (Appendix 7-C), and Fauth and Shinskey (Appendix 7-C) have also discussed the benefits of operating the columns pressure as low as feasible. In the Fauth article, the floating pressure control was part of an advanced control system for a typical gas plant depropanizer. Of the total cost reduction of $1269 per day by using the control system, $345 was attributed to energy savings by the floating pressure control systems.
The floating control systems operation is discussed in the Shinskey and Fauth article. Although the cost of the instrumentation for floating the pressure is low, column temperatures can no longer be used for control because they will vary with pressure. Thus, other control devices such as analyzers, or pressure compensated temperature measurements are required. Shinskey made the following comment in his article on "Control Systems Can Save Energy":
"At first, operators are skeptical of floating-pressure control -they feel more comfortable with constant pressures and temperatures. When its contribution to energy savings is pointed out, they are generally willing to try it. After a brief trial period, they learn that it does not interfere with quality control, and even increases production capacity; soon it becomes accepted. Yet at each installation and with each new application, the concept of floating specifications needs to be sold again."
Benefits as high as a 30% reduction in energy usage are reported by Shinskey, so the floating pressure control system deserves serious consideration.
The desired amount of insulation on a distillation column depends on the individual situation and varies at parts of the column. For example, suppose we have a distillation column with a top temperature 180ºF, bottoms 230ºF, and are using cooling water for the condenser and 40 psig steam for the reboiler. Then the insulation required on the reboiler and bottom section should be based on the value of 40 psig steam, as a Btu lost will have to be replaced by more steam. On the other hand, insulation on the condenser will save no energy, and in fact cost money as the Btu's saved by insulation must be removed by cooling water.
A different situation could occur for a column that operates at 400ºF bottom, 300ºF top, using 600 psig steam in the reboiler, producing 25 psig steam in the condenser. For this case, insulating the column bottoms and reboiler will save valuable 600 psig steam so much is needed. Insulation on the upper section of the column is also valuable as saved heat generates useful 25 psig steam in the condenser. So this entire column needs insulation.
In the case of a column operating in the refrigerated condition, insulation must be used on the condenser and top portion of the column to prevent heat flowing into the column, which would then have to be removed by expensive refrigeration. If the reboiler and bottom sections of the column are also cold insulation will also be required as these sections are part of the coolant cycle.
Insulation may be required for reasons other than energy savings. Insulation on the column will prevent the column from being affected by swings in the weather, changing the heat transfer rate at the tower surface. For cold columns, prevention of ice condensation may be desired. There are OSHA limits on the maximum permissible bare metal temperature for personnel protection. Also, if located indoors in a small specialty operation, insulation could improve the general workplace conditions.
