If you have any questions, feel free to contact me or one of my team members, David Jolley or Chevales Ward.

Transcription

1 October 7, 2002 Bryan Williams Blue Team - Distillation Contractors College of Engineering and Computer Science University of Tennessee at Chattanooga 615 McCallie Avenue Chattanooga, TN To: Dr. Jim Henry, P.E. Professor of Engineering University of Tennessee at Chattanooga 615 McCallie Avenue Chattanooga, TN Dear Dr. Henry: The following report detailing the investigation into the status and performance of the University s distillation column is being submitted for your consideration. The submission of this report marks the first milestone for the Blue Team in the Engineering 435 course at the University of Tennessee at Chattanooga. This report details the objectives, theory, procedures, equipment analysis, findings, and conclusions obtained during the preliminary investigation of the distillation column. Maintenance, calibrations, heat transfer analysis, and batch distillation performance testing results are discussed herein. References to data available on the Internet are given as an alternative to tabulating all collected data within the report. If you have any questions, feel free to contact me or one of my team members, David Jolley or Chevales Ward. Sincerely, Robert Bryan Williams Senior, Chemical Engineering: University of Tennessee at Chattanooga 615 McCallie Avenue Chattanooga, TN (423) thefauxme@hotmail.com Bryan Williams October 7, 2002 Page 1 of 35

2 Distillation Column Maintenance, Calibration and Performance Testing Author: Co-Workers: Bryan Williams David Jolley Chevales Ward Submitted To: Dr. Jim Henry Dr. Frank Jones Date of Submission: Bryan Williams October 7, 2002 Page 2 of 35

3 I. Abstract The objective of this investigation was to study the batch distillation process of a binary system by performing column maintenance, calibration, and performance testing. Calibration of the feed pump, reboiler volume, and computer wattmeter were performed followed by an analysis of the heat transfer in the reboiler, carried out by studying the temperature changes and heat transfer coefficients at constant heat input. Two batch methanol-water distillations were performed to analyze the correlation of temperature to mixture composition and the connection between reflux and distillate collection. Upon conclusion of the investigation, it was concluded that the distillation column apparatus was functioning properly and was accurately tuned to produce reliable experimental results. Furthermore, it was found that the heat lost by the reboiler increased as the temperature in the reboiler increased. Additionally, it was observed that the temperature at which a liquid mixture boils details the composition of the mixture by weighting itself toward the boiling point of the component present in the largest amount. It was also observed that the distillate rate increased with decreasing reflux percentage. Specifically, changing from 80% reflux to 40% reflux decreased the time in which 1 L of distillate was collected from 20 minutes to 15 minutes and at 0% reflux, 1 L of distillate was collected in 26 minutes, all with the same heat going into the reboiler. Bryan Williams October 7, 2002 Page 3 of 35

4 II. Table of Contents Page Number Letter of Transmittal...1 Title Page...2 I. Abstract...3 II. Table of Contents...4 III. Introduction...5 IV. Theory V. Equipment VI. Procedure VII. Results VIII. Discussion of Results IX. Conclusions X. Recommendations XI. References XII. Appendices Figures 1 - Energy Diagram for the Reboiler Distillation Column Schematic Diagram Cooling Water Flow Valves Prior to Maintenance Column Maintenance of Reboiler Surface Column Maintenance of Tray Exterior Reboiler Volume and Feed Pump Calibration Process Diagram Feed Pump Calibration Curve Computer Watt Meter Calibration Heat and Energy Flows Used In Reboiler Analysis Heat and Energy Flows Used In Condenser Analysis Process Temperature Data for Distillation Col umn Heat Transfer Analysis Process Temperature Data for Distillation Column During Water Boiling Reboiler Temperature vs. Time UA vs. Reboiler Temperature Heat Loss vs. Reboiler Temperature Heat Absorbed by Cooling Water During Boiling Process Temperature Data for 0% Reflux Distillation Performance Test Reboiler Boiling Temperature Data for 0% Reflux Distillation Performance Test Distillate Volume Collected for 0% Reflux Performance Test Distillate Rate vs. Time for 0% Reflux Performance Test Distillate Rate vs. Distillate Volume for 0% Reflux Performance Test Heat Absorbed by Cooling Water During Boiling at 0% Reflux Process Temperature Data for 80% / 40% Reflux Performance Test Distillate Volume Collected for 80% / 40% Reflux Performance Test Distillate Rate vs. Time for 80% / 40% Reflux Performance Test Distillate Rate vs. Distillate Volume for 80% / 40% Reflux Performance Test Heat Absorbed by Cooling Water During Boiling at 80% and 40% Reflux Tables I. Feed Pump Calibration Data II. Computer Wattmeter Calibration Data III. Temperature Change Over Time Analysis Data IV. Heat Absorbed by Cooling Water V. Distillation Mixture Composition Bryan Williams October 7, 2002 Page 4 of 35

5 III. Introduction The objective of this investigation was to study the batch distillation process of a binary system by performing column maintenance, calibration, and performance testing. The first objective was to perform necessary column maintenance to assure that the equipment was functioning at its optimum capacity prior to any calibrations or experimental runs. Following these maintenance steps, the column s feed pump, computer wattmeter, and reboiler volume were calibrated to assure that the controls were operating correctly and acceptably, and also to make the experimental procedures easier to perform. After completion of these calibrations, an analysis of the heat loss from the reboiler was performed. This was followed by two methanol-water distillations at different reflux percentages. The University of Tennessee at Chattanooga s distillation column was used to carry out this investigation. The distillation column primarily consists of a liquid storage tank with heating elements, called the reboiler, and a column of distillation trays which serve to allow the liquid and vapor phases to come into contact and into equilibrium with one another. The vapor that rises from the reboiler is condensed by cooling water circulating atop the column in a condenser. This condensed vapor can be collected as distillate, sent back through the column to the reboiler as reflux, or a fractional combination of the preceding alternatives [1]. This report documents the investigation performed between August 20, 2002 and September 17, 2002 and reports the key findings. This report continues in Section IV with a detailed theoretical background which describes the distillation process and presents the fundamental physical and mathematical relationships governing the separation process. Section V provides extensive detail of the distillation column and supporting equipment, detailing each of the components and depicting a schematic diagram of the unit. The following Section, VI, outlines the procedures followed during the maintenance, calibration, and experimentation phases so the reader can grasp a full understanding of exactly how the study was performed. The results obtained during the course of the investigation follow with tabulated data and multiple figures depicting the experimental findings. A discussion of these results follows in Section VIII, with conclusions drawn from the objectives in Section IX. Recommendations for experimental improvements follow in Section X, while a list of references, and a detailed collection of appendices conclude the report. Bryan Williams October 7, 2002 Page 5 of 35

6 IV. Theory A. Overview Distillation involves the separation of the components of a liquid mixture by boiling the liquid to produce a vapor that has a higher composition of the more volatile species, identified by a lower boiling point [2]. This vapor is then condensed and returned to a receiver as the distillate. Distillation exploits differences in the boiling points of the liquid constituents in order to drive the separation process. As the liquid mixture is heated, the more volatile species evaporates, rises through the trays of the column, and reaches the condenser where it is cooled, condensed, and collected in a receiver. The still, or reboiler, consequently has a composition that is more pure in the heavier, or less volatile components. One important detail to remember before examining distillation is that the separation is not perfect. Small amounts of the less volatile species will evaporate as the liquid mixture boils, producing a distillate composition that is not pure in the light component. Successive distillations will improve the purity of the distillate composition [1]. The distillation experiments documented in this report are batch distillations of a binary system. Batch distillation is simply the distillation of a fixed volume of liquid mixture, where there is no feed stream entering the process [1]. A binary system consists of only two components, commonly referred to as the light or volatile component (A) and the heavy or less volatile component (B) [1]. A methanol-water system was studied in compliance with management s direction. The performance tests were also run with a specified percentage of reflux. Reflux is the name of the liquid that is returned through the top trays down through the column to the reboiler after being condensed [1]. For example, a reflux of 80% translates to mean that 80% of the liquid that evaporates is condensed and returned to the column in and attempt to increase the purity of the distillate. The remaining 20% is collected in the distillate receiver. B. Reboiler Energy Balance Initially, the focus of the investigation was determining the heat transfer relationships for the reboiler. Selecting a control volume around the reboiler, it can be found that there are principally three energy terms to consider. First, there is heat input, provided by the heating elements. Second, there is the change in temperature of the water due to heating by the elements. Finally, there is a certain amount of heat loss from the system to the surroundings due to the fact that the system is hotter than the surroundings and the two shift toward equilibrium with one another. This is illustrated in Figure 1. Reboiler Heat Loss, Liquid Mixture Water Heating Q absorbed Calrod Heaters Q L Heat Input, Q in Figure 1 - Energy Diagram for the Reboiler Performing an energy balance on the reboiler will reveal Equation 1, and a more detailed derivation will yield the more useful form of Equation 2. The full derivation is included in Appendix C. dt ( m H O C p, H O * TB ) Q in Q Loss d.. * 2 2 = (1) Bryan Williams October 7, 2002 Page 6 of 35

7 dt dt B. ( m C ) = Q UA( T T ) [ ] H O * 2 p, H 2O in B (2) The energy balance states that the change in temperature of the reboiler contents is equal to the heat input to the system less the heat losses from the system to the surroundings. Since the mass of water is constant for a batch distillation process with no distillate collection (100% reflux) and the heat capacity of water can be reasonably assumed constant over the range of operating temperatures, these values can be removed from the derivative term [3]. The change in reboiler temperature over time, prior to reaching the boiling temperature, can be determined from the slope of the line on a reboiler temperature vs. time plot. In theory, the temperature of the solution will remain constant, at the boiling point of the solution, during the boiling process, as all energy taken in is used to vaporize the liquid [3]. So, theoretically, the slope would be zero at these points. Equation 3 allows for the calculation of an instantaneous slope across successive data points 1 and 2. dt b dt ( T2 T1 ) ( t t ) = (3) 2 1 The heat input is constant for these performance tests, and the heat loss can be calculated using Newton s Law of cooling where U is the overall heat transfer coefficient, A is the heat transfer surface area, and T is the ambient room temperature [4]. q = UA( T T ) b (4) Equation 4 states that as the temperature difference increases, then so does the heat loss. In effect, the temperature difference drives the heat loss. Measuring the reboiler surface area allows for the calculation of the overall heat transfer coefficient U. The overall heat transfer coefficient can be estimated at between 2 and 25 W/(m 2 K) for this system with free convection of a gas [4]. Anything above 100 W/(m 2 K) would not make physical sense for the type of heat loss experienced by the system. The heat input to the system was read from a Valhalla multimeter and the computer voltage signal, V s, was calibrated from this reading. It was desired to find the proportionality constant, K, between the true heat input and the voltage signal read by the computer. This voltage signal is produced by an AAC Watt Transducer (Model S217) manufactured by American Aerospace Controls. This K is related to the slope of a plot of V s versus Watts, and this relationship can be expressed as C. Condenser Energy Balance W = K V s. (5) Another source of heat transfer in the system is the absorption of heat by the condenser cooling water during the boiling phase. This heat can be expressed as the product of the volumetric flow rate (V) times the density of water (ρ) (which is the mass flow rate cooling water) times the constant pressure heat capacity (C P ) times the change in cooling water temperature ( T CW ). Mathematically, this is expressed as. CW Q. = V CW * * CP * ρ T. (6) CW D. Distillation Mixture Composition In the performance testing phase of the investigation, it was necessary to calculate molar percentages of the two components in a binary system. Knowing the volumes (V i ) of each pure substance in the mixture, the densities (ρ ι ) of each component can be used to determine the mass (m i ) of each component in the mixture [5]. Bryan Williams October 7, 2002 Page 7 of 35

8 m = * ρ (7) i V i i Then, the number of moles of each species (n i ) can be found by dividing the mass by the molecular weight (MW i ) of each substance. m i n i = (8) MWi Finally, the mole percentage of each component can be computed as outlined in Equation 9. n (%) = ni n i *100 (9) In analyzing the temperature data collected during the distillations, it is important to note the boiling point of the solution and its difference from the individual species pure boiling points. Theoretically, if a solution has a higher composition of one component, then the boiling point of the solution should be closer to the pure species boiling point of that component. Furthermore, as the more volatile component boils, it should be seen that the reboiler temperature will rise and approach the boiling point of the pure heavy component, assuming that the light component is in the vapor phase or collected as distillate. In analyzing the distillate volume collected, it is important to look for any impact of distillate rate caused by changes in reflux percentage. Bryan Williams October 7, 2002 Page 8 of 35

9 V. Equipment The distillation column used in this experiment is depicted in Figure 2. It consists of a reboiler, which is a clear glass cylinder with calrod heaters partially occupying the bottom half of the container. The approximate area of the reboiler s external surface is 0.42 m 2. The liquid mixture is housed in the reboiler where the calrod heaters apply heat input to bring the mixture to a boil. Temperature, pressure, and level sensors monitor the conditions in the reboiler, and a reboiler pump allows for draining the heavy liquid product at the conclusion of an experiment. This product is typically pumped to a large storage tank for proper use afterward. Feed is supplied to the reboiler through a feed pump, which draws from a feed line that can be connected to graduated cylinders, the feed tank, or any other liquid container. The feed line has a volume of approximately 0.5 L, and this volume must be taken into account when filling the reboiler. The feed enters the column in the feed section shown in Figure 2, and falls through the lower trays into the reboiler. A measurement of the feed temperature is collected where shown. As the liquid in the reboiler boils and vapor is produced, it ascends the column, through each tray from twelve to one. Temperature measurements are made at each level. An electromagnetic reflux control is situated atop the distillation column trays, where condensed liquid may be redirected down the column if the reflux is turned on. As the vapor rises through the condenser, the cooling water absorbs heat from the vapor and condenses the vapor to a liquid. Temperature indicators record the temperature of the cooling water supply and return lines, as well as the condenser vapor. The cooling water flow rate can be adjusted by adjusting the solenoid valves or manual valves in the water supply line and is measured via a paddlewheel flow meter. The distillate that is not sent back through the column is directed through a cooler and into the distillate receiver. The level indicator at the distillate receiver does not function correctly, so volume and time readings must be taken to measure the amount of distillate collected and calculate the rate. When the receiver becomes full, it may be drained using the distillate pump, which pumps the collected distillate to the product tank or other collection vessel. The entire process is controlled using a LabView control interface on the computer. Bryan Williams October 7, 2002 Page 9 of 35

10 Cooling Water TI Return Supply Condenser TI Figure 2 - Distillation Column Schematic Diagram F TI Cooling Water Reflux TI TI Tray 1 Tray 2 Cooler Distillate Receiver TI TI TI Tray 3 Tray 4 Tray 5 LI Distillate Pump Cooling TI Tray 6 Water Flow Valves Feed TI Feed Pump Computer Controller TI Tray 7 Heat Loss vs. Reboiler Temperature TI Tray 8 Feed / Heat Loss (W) Reboiler Temperature ( C) TI TI Tray 9 Tray 10 Product Tank TI Tray 11 TI Tray 12 TI PI Reboiler LI Liquid Mixture Calrod Heaters Reboiler Pump Bryan Williams October 7, 2002 Page 10 of 35

11 VI. Procedure A. Distillation Column Maintenance and Calibration First, column maintenance and calibrations were performed to assure that the equipment was prepared for the experimental tests. First, the solenoid cooling water supply valves and the manual cooling water supply valves were reversed so the solenoid valves would function properly. The solenoid valve must have a certain amount of pressure to operate correctly, and if the manual valves were partially closed, then the solenoid valve would not have the necessary pressure to function. Therefore, using some pipe wrenches, square wrenches, a vice, and Teflon tape, we interchanged the two valves so that the solenoid valve could function properly. Figure 3 shows the two valves in the incorrect position, since water enters the system from below. No picture is available showing the valves in the reversed positions. Figure 3 - Cooling Water Flow Valves Prior to Maintenance Another column problem that was inherited was a burned and baked reboiler, the exterior of which was charred from burned insulation and overheating. The outside of the container was scraped with a screwdriver to remove the residue. Figure 4 depicts this process. Figure 4 - Column Maintenance of Reboiler Surface Finally, the column itself, specifically the outside of the trays, was cleaned for good measure. Figure 5 depicts this cleaning process. Bryan Williams October 7, 2002 Page 11 of 35

12 Figure 5 - Column Maintenance of Tray Exterior In the next step of the process, the reboiler volume, feed pump flow, and computer wattmeter were calibrated. The reboiler volume was calibrated by measuring out increments of 1 L of water and pumping them into the reboiler via the feed pump. Once one liter was pumped into the system, a graduated mark was made on the reboiler surface to provide a tool to visually measure the reboiler volume. A total of 15 L were pumped into the reboiler, and 1 L graduations were made on the reboiler up to a 15 L mark. Careful attention was paid to not run air in the feed lines to avoid any error in liquid volume that was fed. Next, the feed pump calibration was performed. To perform this calibration, the feed pump was set at variable speeds (2, 4, 6, 8, and 10) and the volume of water that was pumped from the feed cylinder in one minute was recorded. Therefore, this calibration produced a chart of volumetric flow rates corresponding to computer pump settings. Figure 6 depicts this experimental setup for both calibrations. Figure 6 - Reboiler Volume and Feed Pump Calibration Process Diagram Bryan Williams October 7, 2002 Page 12 of 35

13 Finally, the computer wattmeter was calibrated. This was accomplished by connecting a Valhalla multimeter to the distillation column to measure the true wattage and amps supplied to the reboiler heaters. At discrete values of amperage (5, 10, and 15 Amps) the wattage was read from the multimeter and the computer voltage signal was read from the LabView display. The voltage signal is from the powertransmitter that is located on the column. It has a 0-5 kw rating on 220V AC and is connected alongside the main power supply box. A plot of computer voltage signal versus true wattage was plotted, and the proportionality constant, or the slope of the line given by the plotted points was calculated. Equation 5 was used to find this constant, K. This calibration produced a constant by which the voltage signal can be multiplied to yield the true wattage input to the system. B. Performance Testing and Energy Analysis Upon completion of the maintenance and calibration steps, performance tests were run on the column to assure that it was working correctly and to report upon the batch distillation of the binary methanol-water system. The first test run was run with 14 liters of water and was aimed at determining the heat transfer relationships in the column and reboiler. First, 14 liters of pure water were pumped into the reboiler by setting the feed pump to 10 and connecting the supply line to a series of graduated cylinders with precisely measured volumes of water in them, totaling 14 liters. With the water in the reboiler, the amperage was set to 15 amps, and 2487 Watts of constant power were supplied to the system via the calrod heaters in the reboiler. Reflux was set to 100% such that no distillate would be collected and all evaporated vapor would be returned to the column. The process temperatures and pressures were measured as the water heated toward its boiling point. When vigorous boiling occurred, the heat was left on for an additional 10 minutes and then the heat input was set to 0 Amps and, accordingly, 0 Watts. Additional temperature data was recorded as the system gave off heat in order to reach equilibrium with its surroundings. Following the data collection, the energy balance and analysis prescribed in the theory of this report, Equations 1-4 and 6, was performed on the raw data to find the change in temperature over time, the overall heat transfer coefficient, the heat losses from the system, and the heat absorbed by the cooling water during condensation. Upon analysis of the heat losses from the system, a methanol-water distillation was performed at 0% reflux. A solution consisting of 14 L of water and 1 L of methanol was pumped into the reboiler as prescribed above, and 2487 W of power at 15 amps were supplied to the system. Mole percentages of each component were calculated from the volumes using Equations 7-9. Again, process temperatures were recorded throughout the run time. As the solution reached the mixture s boiling point, distillate was collected in the receiver. Due to the fact that the distillate level indicator does not function properly, the change in distillate receiver volume over time was measured and recorded. A plot of the distillate volume collected versus time was constructed so as to interpret the rate at which the distillate was collected and to observe any discrepancies therein. Because the distillate receiver has a maximum volume of 1 L and 3 L of distillate were collected, the distillate pump was turned on and set to 10 during times when the receiver was almost full. This took approximately 2 to 3 minutes to drain the distillate receiver, and as a consequence three data points were lost due to this necessity. In the next performance test, an identical liquid mixture was used, except that the reflux was set to 80% and later changed to 40%. Upon heating and vaporizing part of the liquid mixture, the distillate receiver began to collect product. The reflux was kept on 80% until exactly 0.5 L of distillate was collected. At that point, the reflux was reset to 40% and the corresponding change in the distillate flow rate was observed. After an appreciable amount of time the heat input was removed and the system was slowly cooled to ambient temperature. Bryan Williams October 7, 2002 Page 13 of 35

14 VII. Results A. Distillation Column Maintenance and Calibration The feed pump calibration was performed at settings 2, 4, 6, 8, and 10. The recorded change in volume over the one-minute interval at each of these settings is given as Table I. Figure 7 presents the calibration curve for the feed pump. Table I. Feed Pump Calibration Data Pump Setting Feed Flow Rate (ml/min) Feed Pump Calibrations 700 Flow Rate (ml/min) Pump Setting Figure 7 - Feed Pump Calibration Curve The computer Wattmeter was calibrated by comparing the wattage and amps recorded by the computer to the standard displayed on the Valhalla multimeter and the control box meter. The computer receives a voltage signal from the system that it represents as a wattage reading. The voltage signal read by the computer and the wattage measurement from the multimeter at three distinct amperage settings are reported in Table II. Table II. Computer Wattmeter Calibration Data Control Box Meter (A) Valhalla (A) Valhalla (W) Computer V s (W) Bryan Williams October 7, 2002 Page 14 of 35

15 Graphing the computer voltage signal versus the Valhalla wattage reading and determining the slope of the linear line produced proportionality constant, or calibration parameter, between the two readings. Figure 8 shows the calibration curve for the computer wattmeter and reports the proportionality constant as Therefore, the CPU meter accurately measures kilowatts directly. Alternately, the Voltage signal (V s ) multiplied by 1000 produces the true reboiler wattage (W), or W = 1000V s. (10) Computer Watt Meter Calibration 3.0 CPU Signal (Vs) Valhalla (W) Reboiler Watts = 1000V s V s = 0.001W Figure 8 - Computer Watt Meter Calibration B. Energy Analysis Armed with the true wattage input at 15 amps, analysis of heat transfer in the reboiler was performed. A volume of 14 L of water was used for this analysis with 100% reflux. This volume is accurate in that the volume of the feed line, which is important in terms of delay time for feed composition changes, was also taken into account. The volume of the feed line was determined to be approximately 500 ml, which consists of 15 feet of inch radius feed line and 375 ml of feed in the pieces of equipment in the feed pathway. For heat loss calculations, a reboiler volume of 0.42 m 2 was used, approximating the reboiler as a cylinder with a 20 inch length, and two 4.3 inch radius circular end caps. Figures 9 and 10 show the heat and energy flows used in the analysis. Figure 9 shows a constant heat input to the reboiler and the corresponding heat losses to the surroundings and heat absorbed by the reboiler mixture. Figure 10 shows the heat transferred from the inlet vapor to the cooling water (which leaves with an increased temperature relative to supply) and the corresponding condensed liquid which re-enters the column. Bryan Williams October 7, 2002 Page 15 of 35

16 Reboiler Heat Loss, Liquid Mixture Water Heating Q absorbed Calrod Heaters Q L Heat Input, Q in Figure 9 - Heat and Energy Flows Used In Reboiler Analysis Return - Hot Condenser Cooling Water Q Transfer Supply - Cool Inlet Gas Condensed Vapor - Cool Figure 10 - Heat and Energy Flows Used In Condenser Analysis Process data was recorded using the LabView interface. This data is available on the Internet as detailed in reference [6]. Figure 11 presents all of the process temperature data for this experimental run, with particular interest being paid to the heating range (0min-40min), boiling range (40min-55min), and cooling range (t>55min). It is important to note that the reboiler temperature is not uniform, particularly during heating. The water below the heaters is relatively cool while the water in direct contact with the heaters is very hot. Then, during the boiling phase, it was observed that the rigorous mixing helped to more evenly distribute the temperature within the reboiler. The pure water mixture begins to boil at approximately 99.5 C, as literature references would attest to [5]. It is also interesting to note that the reboiler temperature remains constant during the boiling phase. This phenomenon is characteristic of a pure species, but a distinction will be made later on in this report regarding this pure species behavior and the behavior of a mixture. Temperatures above 100 C were recorded in some of the trays, which is topic for controversy, but they are due to instrumental errors in the measurements. Figure 12 depicts a zoomed in region of the graph during the boiling process, so that the constant reboiler temperature can be noted. Bryan Williams October 7, 2002 Page 16 of 35

17 Process Temperature Data Temperature ( C) Time (min) Tray 1 T (C) Tray 2 T (C) Tray 3 T (C) Tray 4 T (C) Tray 5 T (C) Tray 6 T (C) Tray 7 T (C) Tray 8 T (C) Tray 9 T (C) Tray 10 T (C) Tray 11 T (C) Tray 12 T (C) Reflux T (C) Reboiler T (C) Figure 11 - Process Temperature Data for Distillation Column Heat Transfer Analysis Process Temperature Data Temperature ( C) Time (min) Constant Reboiler Temperature Tray 1 T (C) Tray 2 T (C) Tray 3 T (C) Tray 4 T (C) Tray 5 T (C) Tray 6 T (C) Tray 7 T (C) Tray 8 T (C) Tray 9 T (C) Tray 10 T (C) Tray 11 T (C) Tray 12 T (C) Reflux T (C) Reboiler T (C) Figure 12 - Process Temperature Data for Distillation Column During Water Boiling Specifically, the change in reboiler temperature over time was of particular interest. Analysis of this temperature change was performed as the reboiler liquid was heated to a boil and then after the heat input was removed and the system slowly reached thermal equilibrium with its surroundings. This is depicted in Figure 13. Bryan Williams October 7, 2002 Page 17 of 35

18 Reboiler Temperature vs. Time Temperature ( C) Time (min) Figure 13 - Reboiler Temperature vs. Time An instantaneous slope across each data pair collected was calculated using Equation 3 (A sample calculation appears in Appendix II of this report). Five-minute averages of the temperature and slope were calculated to be used along with the unsteady-state energy balance to analyze the heat transfer in the reboiler. From this energy balance, the overall heat transfer coefficient was determined by solving Equation 1 for the product UA and then dividing by the reboiler area, 0.42 m 2. Finally, the total heat loss was calculated from Newton s Law of Cooling. Table III presents this energy balance data. Table III. Temperature Change Over Time Analysis Data T ( C) dt/dt ( C/min) UA (W/ C) U (W/(m 2 C)) Q loss (W) Bryan Williams October 7, 2002 Page 18 of 35

19 At room temperature, the slope (dt/dt) was found to be 2.7 C/min. Using Equation 6 to solve for the heat input at room temperature, where losses should be negligible, the value found was approximately 2.6 kw, very close to the true 2.5 kw input detected from the wattmeter. Figure 14 depicts a plot of UA vs. Temperature for periods of reboiler heating and cooling. Since the area is constant, the trends shown in these figures are also for the heat transfer coefficient. Namely, with heat input, UA increases with increasing temperature. Without heat input, UA remains constant with temperature. Figure 14 clearly shows how much heat is lost at a given reboiler temperature, for heating and cooling. Heat loss with constant heat input increases at an increasing rate as the system temperature rises. Heat loss without heat input increases at a steady rate of 50 W/ C as the temperature rises. The heat losses incurred for a system with heat input are much larger than those without heat input, as Figure 15 shows. Bryan Williams October 7, 2002 Page 19 of 35

20 UA vs. Reboiler Temperature UA (W/ C) No Heat Input Constant Heat Input Reboiler Temperature ( C) Figure 14 - UA vs. Reboiler Temperature Heat Loss vs. Reboiler Temperature Heat Loss (W) Constant Heat Input 0 No Heat Input Reboiler Temperature ( C) Figure 15 - Heat Loss vs. Reboiler Temperature Another method of heat transfer in the system is the heat absorbed by the condenser cooling water during the boiling and vaporization phase. The heat absorbed is shown graphically as a function of the reflux temperature in Figure 16. The heat-absorbed data is presented as Table IV. Bryan Williams October 7, 2002 Page 20 of 35

21 Heat Absorbed by Cooling Water During Boiling Heat Absorbed by Water (W) Reflux Temperature ( C) Time (min) Reflux T ( C) Figure 16 - Heat Absorbed by Cooling Water During Boiling Table IV. Heat Absorbed by Cooling Water Cooling Water Flow (gpm) Cooling Water Supply T ( C) Cooling Water Return T ( C) Heat Absorbed by Cooling Water (W) At constant reboiler temperature, the heat into the system should be approximately equal to the heat removed by the condenser. From the last points in Table IV at 104 degrees C, the heat removed is approximately 1.8 kw, comparable to the 2.5 kw supplied less some losses to the surroundings at this temperature. C. Performance Testing After analysis of the heat transfer relationships in the distillation column, two performance tests were run on the methanol-water binary system. In the first experiment, a solution consisting of 14 L of Bryan Williams October 7, 2002 Page 21 of 35

22 water and 1 L of methanol was prepared for batch distillation with 0% reflux. Mole percentages of the mixture constituents were calculated using Equation 10 and are tabulated in Table V. Table V. Distillation Mixture Composition Volume Methanol (L) Volume Water (L) Mole % Methanol Mole % Water As before, process temperature data was recorded during the distillation process. Notably, the temperature at which the liquid boils is around 95 C, lower than that of pure water. This data is available on the Internet and is presented as Figure 17 [7]. Process Temperature Data Temperature ( C) Time (min) Tray 1 T (C) Tray 2 T (C) Tray 3 T (C) Tray 4 T (C) Tray 5 T (C) Tray 6 T (C) Tray 7 T (C) Tray 8 T (C) Tray 9 T (C) Tray 10 T (C) Tray 11 T (C) Tray 12 T (C) Reflux T (C) Reboiler T (C) Figure 17 - Process Temperature Data for 0% Reflux Distillation Performance Test It is interesting to note that the reflux temperature was approximately 95 C when the solution came to a rigorous boil. The reflux temperature then increased as boiling and evaporation continued. Figure 18 details this area. This will be addressed in the discussion of results section to follow. Bryan Williams October 7, 2002 Page 22 of 35

23 Process Temperature Data Temperature ( C) Increasing Reboiler Temperature Tray 1 T (C) Tray 2 T (C) Tray 3 T (C) Tray 4 T (C) Tray 5 T (C) Tray 6 T (C) Tray 7 T (C) Tray 8 T (C) Tray 9 T (C) Tray 10 T (C) Tray 11 T (C) Tray 12 T (C) Reflux T (C) Reboiler T (C) Time (min) Figure 18 - Reboiler Boiling Temperature Data for 0% Reflux Distillation Performance Test The volume of distillate collected over time was also monitored, and Figure 19 depicts this data. Distillate Volume vs. Time Distillate Volume (ml) Time (minutes) Figure 19 - Distillate Volume Collected for 0% Reflux Performance Test Figures 20 and 21 depict plots of distillate rate versus time and distillate rate versus distillate volume. Bryan Williams October 7, 2002 Page 23 of 35

24 Distillate Rate vs. Time Distillate Rate (ml/min) Time (min) Figure 20 - Distillate Rate vs. Time for 0% Reflux Performance Test Distillate Rate vs. Distillate Volume Distillate Rate (ml/min) Distillate Volume (ml) Figure 21 - Distillate Rate vs. Distillate Volume for 0% Reflux Performance Test Also during this time, heat was removed by the condenser, as Figure 22 shows. During the boiling process, the amount of heat removed was fairly constant, as the plotted points show. Bryan Williams October 7, 2002 Page 24 of 35

25 Heat Absorbed by Cooling Water During Boiling 1800 Heat Absorbed by Water (W) Reflux Temperature ( C) Figure 22 - Heat Absorbed by Cooling Water During Boiling at 0% Reflux In the second performance test, the same composition of methanol-water solution was used but the reflux was set at 80% and then changed to 40% during the experiment. The process temperatures were again monitored and are presented in Figure 23. The data is available on the Internet [8]. Process Temperature Data Temperature ( C) Time (min) Tray 1 T (C) Tray 2 T (C) Tray 3 T (C) Tray 4 T (C) Tray 5 T (C) Tray 6 T (C) Tray 7 T (C) Tray 8 T (C) Tray 9 T (C) Tray 10 T (C) Tray 11 T (C) Tray 12 T (C) Reflux T (C) Reboiler T (C) Figure 23 - Process Temperature Data for 80% / 40% Reflux Performance Test As before, the distillate volume was recorded over time and this data is presented as Figure 24. Also, additional plots of distillate rate versus time and distillate rate versus distillate volume are presented as Figures 25 and 26, respectively. Bryan Williams October 7, 2002 Page 25 of 35

26 Distillate Volume vs. Time % Reflux 40% Reflux Distillate Volume (ml) Time (min) Figure 24 - Distillate Volume Collected for 80% / 40% Reflux Performance Test Distillate Rate (ml/min) % Reflux Distillate Rate vs. Time 40% Reflux Time (min) Figure 25 - Distillate Rate vs. Time for 80% / 40% Reflux Performance Test Bryan Williams October 7, 2002 Page 26 of 35

27 Distillate Rate (ml/min) Distillate Rate vs. Distillate Volume 80% Reflux 40% Reflux Distillate Volume (ml) Figure 26 - Distillate Rate vs. Distillate Volume for 80% / 40% Reflux Performance Test Figure 27 details the constant heat absorbed by cooling water during boiling at 80% and 40% reflux. Heat Absorbed by Cooling Water During Boiling 2500 Heat Absorbed by Water (W) Reflux Temperature ( C) Figure 27 - Heat Absorbed by Cooling Water During Boiling at 80% and 40% Reflux Bryan Williams October 7, 2002 Page 27 of 35

28 VIII. Discussion of Results A. Maintenance and Calibration Results The maintenance performed on the column served its purpose and assured that the system was in peak operating conditions. The cleaning and cooling water valve switching prepared the unit to function appropriately for the test runs. The feed flow meter calibration produced a curve translating the feed pump settings into feed flow rates. For continuous distillation processes, this calibration curve allows the user to adjust the pump setting to produce the desired flow rate. The pump flow rate levels out from setting 8 to 10, only increasing in flow rate by about 30 ml/min between the settings, unlike the more noticeable increase seen in the lower settings, for example 175 ml/min going from 2 to 4. The computer wattmeter calibration provided a proportionality constant of 1000, which is the factor by which the voltage signal can be multiplied to produce the true wattage measured. With this factor and the linear relationship it describes, a formula can be created in LabView to display the correct wattage on the screen for the benefit of future users. Similarly, the graduations marked on the reboiler offer a quick and convenient way for future users to determine the volume of liquid in the reboiler with a simple glance. B. Heat Transfer Analysis Results The heat transfer analysis performed on the distillation column offered valuable insight into the thermal behavior of the system. Figure 13 shows that as the water was heated at constant power equal to 2487 W, the reboiler temperature increased at a decreasing rate. Initial slopes were as high as 2.5 C/min and faded to 1.1 C/min as the boiling point was reached. The temperature of the water was constant during the boiling process, as shown in Figure 11 during the 40 min to 55 min time range, as all the energy was being used to vaporize the liquid, thus validating the theory of this report. When the supply of heat was turned off, boiling ceased, and the liquid began to cool, coming to equilibrium with its surroundings. It cooled faster, some 0.3 C/min, when the temperature was in the higher ranges of approximately greater than 80 C, while this rate diminished to 0.1 C/min as the mixture began to reach room temperature. Again, Figure 13 depicts this reported trend. The overall heat transfer coefficient was found to be approximately 10 W/(m 2 K) during the cooling of the reboiler back to room temperature. The heat transfer coefficient ranged from about 10 to 45 W/(m 2 K) during the heating process, with higher heat transfer coefficients seen during boiling. These low heat transfer coefficients agreed with the theoretical range of U values for the free convection of gases [4]. These heat transfer coefficient trends for heating and cooling are shown in Figure 14. Figure 15 depicts a general trend for heat loss as a function of temperature. Both graphs show that as the temperature is increased, the heat losses from the system increase. It should be noted that the losses are higher with heat input to the system than they are without heat input to the system. At 80 C, the heat loss in the heated system is 675 W, while in the cooling system it is only 250 W at the same temperature. This temperature dependence of heat loss was expected from theory, as Equation 4 states mathematically that as the temperature difference or driving force increases, then the heat loss will also increase. Figure 16 revealed a final interesting point about the heat losses from the system. As the reflux temperature increased, the amount of heat absorbed by the cooling water increased. In one case, the heat absorbed by the cooling water was 1865 W, 75% of the entire 2476 W input to the system. The quantity of heat absorbed by the cooling water in the boiling process offered another additional conclusion. From Equation 6, if the volumetric flow rate of the water was lessened then the temperature increase seen in the cooling water would have been greater. However, the quantity of heat absorbed would have been the same as long as sufficient flow was provided to carry away 1865 W of heat. C. Performance Testing In the next phase of the investigation, the two distillations produced some noteworthy results. In the first distillation, at 0% reflux, the process temperature graph, Figure 17, offered some insight into the purity of the mixture. Pure methanol boils at 64.7 C, while pure water boils at C [5]. For the distillation of 3.1 mol % methanol and 96.9 mol % water, the reboiler temperature was 95 C when boiling began to occur. The reflux temperature spiked to 95 C almost instantly when the boiling process began. This temperature lies between the boiling points of the two liquid constituents, more heavily weighted Bryan Williams October 7, 2002 Page 28 of 35

29 toward water, the component present in larger proportion. Likewise, reviewing the data for the pure water run and the heat transfer analysis, the reboiler temperature reached 99.5 C before rigorous boiling began to occur. So the boiling point of the mixture offers some insight as to the relative composition of each species in the mixture. Also, we see in Figure 17 that as boiling continues the reboiler temperature increases. This suggests that the lower boiling methanol is being removed from the mixture, and the temperature increases to boil the now almost pure water. The distillate volume collected, Figure 19, showed that the volume collected was approximately equal to 2000 ml. Clearly since only 1 L of methanol was present in the mixture, water was present in the distillate. After about 2 L had been removed, the rate at which the distillate was being collected diminished, as the graph shows, from about 28 minutes for 1 liter for the first 2 liters to about 40 minutes for 1 liter following. Here it is suggested that the water is being heated to its pure boiling point since most of the methanol is assumed to be removed from the liquid. The next distillation at 80% and 40% reflux also validated the boiling temperature and purity correlation, but the change in reflux after 0.5 L of distillate was collected was particularly noticed in the distillate volume plot, Figure 24. The first half-liter of distillate was removed in about 20 minutes, while the next half-liter of distillate was removed in about 15 minutes. The increase in this distillate collection rate is directly attributable to the reduction in the reflux rate. As less condensed vapor is redirected through the column at the lower reflux rate, distillate is collected in the receiver at a faster rate as is expected from theory. The experimental results validated this theoretical behavior. In both distillation trials, the heat removed by the condenser cooling water was constant. Bryan Williams October 7, 2002 Page 29 of 35

30 IX. Conclusions Upon completion of the experiment, it was largely concluded that the objectives set forth were met. Column maintenance performed during this investigation corrected an operational problem with the solenoid flow valves in the cooling water line and beautified the work area. The calibration of the computer wattmeter, reboiler tank volume, and feed flow meter provided basis measurements for experiments to follow. Analysis of the heat transfer in the reboiler revealed the trend that as the temperature of the reboiler increases then the heat lost to the surroundings increases. Quantitative measurements of the heat transfer coefficient describing the heat loss to the surroundings were made, and reliable logical values were calculated. It was determined that the condenser cooling water can absorb a large percentage of the heat input during boiling, up to around 75%. Distillation of the methanol-water system revealed that the initial boiling point of the mixture could offer insight into the composition of the mixture by weighting itself toward the boiling point of the liquid present in the largest amount. Furthermore, the reboiler temperature increases during the boiling phase as the light component is vaporized and the mixture heats up to the boiling point of the heavy component. Finally, it was observed that a higher reflux percentage produced a lower distillate flow rate. Bryan Williams October 7, 2002 Page 30 of 35