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.
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.
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.
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.