CONTINUOUS DISTILLATIONCOLUMN - UOP3CCA. INTRODUCTION TO THE E.docx
- 1. CONTINUOUS DISTILLATIONCOLUMN - UOP3CC A. INTRODUCTION TO THE EQUIPMENT Distillation has always been and will continue to be one of the most important industrial processes for separating the different components of a liquid mixture. Laboratory scale distillation columns are needed to provide adequate practical training for student engineers and plant operators in a safe environment. They may also be used to acquire process separation data, of use in full-scale plant design. The UOP3CC allows the study of both batch and continuous distillation, packed or plate column operation, operation under atmospheric pressure or under vacuum, azeotropic distillation, and manual, PID, PLC or computer control of the process. Data logging of the process is also possible. UOP3CC Continuous Distillation Column with computer control Figure 1. Schematic Diagram of Apparatus Figure 2. Diagram of Distillation Column Figure 3. Diagram of UOP3CC Process Unit Figure 4. Diagram of UOP3CC Console B. DESCRIPTION Where necessary, refer to the drawings on pages 19 to 22. 1. Overview The UOP3CCContinuous Distillation Column is a self-contained distillation facility consisting of two interconnected units: a
- 2. floor standing process unit and a bench mounted control console. 2. Floor-Standing Frame The distillation column is mounted on a floor standing, welded tubular steel framework (1) fitted with four adjustable feet (2). The frame is designed to allow the use of a fork lift or pallet truck to manoeuvre the unit into position initially. 3. Distillation Column The 50mm diameter sieve plate column is made up of two glass sections (3) and (4) each containing four sieve plates. The columns are separated by a central feed section and arranged vertically for counter-current vapor/liquid flow. The column is insulated to minimize heat loss. The glass column incorporates a total of eight sieve plates in two sections (3) and (4) each containing four plates. Each plate (D) is located by a central support rod (E) and incorporates a weir (F) and downcomer (G) to create a liquid seal between successive stages. The liquid seal on the final plate in each section is achieved by U-tube (H). Feed mixture from either of the feed tanks is pumped by pump (7) to the base, centre or top of the distillation column at connections (A), (B) or (C) respectively. The feed pump incorporates a length of Viton rubber tubing. This tubing is suitable for all of the recommended test mixtures (see “Operational Procedures”). Where other test mixtures are being used, the suitability of this material must be checked. 4. Reboiler The reboiler (13) situated at the base of the column is manufactured from 316 stainless steel and incorporates a flame- proof immersion type heating element. Either batch or continuous distillation can be carried out using this reboiler. In continuous operation, valve (V1) is open and bottom product flows from the reboiler through the bottom product cooler (15) to the bottom product tank (9). It is possible to preheat the feed
- 3. to the column by directing the feed through a spiral coil in the bottom product cooler where heat is transferred from product leaving the reboiler at the boiling point. When feeding cold feed directly to the column, the product from the reboiler is cooled in the bottom product cooler by circulating cold water through the spiral coil. For batch operation, valve (V1) remains closed so that the reboiler can be filled with the initial charge (10 to 12 litres) of binary mixture. The column and reboiler are both insulated to minimize heat loss. A level sensor (17) inside the reboiler protects the heating element from overheating due to low operating level and a sight glass (18) allows the level in the reboiler to be observed. 5. Condenser Vapor from the top of the column passes to a water-cooled, coil-in-shell condenser (8), which may be fitted with an insulated jacket to allow heat balances to be carried out. The shell of the condenser incorporates a pressure relief valve (PRV1) to protect the system in the event of a blocked vent and cooling water failure. Cooling water enters the condenser at a regulated rate through a rotameter (FI1) and the flow rate is controlled by diaphragm valve (V5). A cooling water supply is connected to the inlet nozzle (19) and serves also to operate the vacuum pump (20) when operation at reduced pressure is required. Water supply to the vacuum pump is controlled by valve (V14), which must only be operated when valve (V5) is open. 6. Decanter Condensate is collected in a glass decanter (11) (phase separator) which is bypassed for normal distillation experiments by opening valve (V10). When the decanter is in use (separation of two immiscible liquids as condensate), valve (V10) is closed so that the overflow (25) and underflow (26) pipes inside the
- 4. vessel, can take effect. With valve (V10) open, condensate from the condenser outlet passes directly through the decanter to the inlet of the reflux ratio control valve (12) which is a 3-way solenoid operated valve. Depending on the setting of the reflux timers, condensate is directed by the reflux valve either back to the top of the column or to the top product collecting vessel (10). When directed to the column, the reflux passes through a U-seal where a valve (V3) can be used for measuring boil-up rate or for draining the U-seal. The contents of the top product tank (10) can be drained into the reboiler (13) for re-use viavalve (V12). 7. Thermocouples Temperatures within the system are monitored by fourteen thermocouple sensors (T1 to T14) located at strategic positions in the system. T1 to T8 are located in the column and measure the temperature of the liquid on each sieve plate. There are seventeen locations for the temperature sensors, three of which do not have sensors installed but which can be fitted with sensors moved from other, less relevant locations when necessary. 8. Manometer The total pressure drop across the column is indicated on a U- tube manometer (P1) via appropriate tappings in the column fitted with isolating valves (V6) and (V7). 9. Product Receiver All of the vessels in the system are connected to a common vent on the top product receiver. This vent is normally connected through a 4.0 m length of tubing to a fume cupboard or safe atmospheric vent outlet. 10. Vacuum Pump Operation at reduced system pressures is achieved using the water powered vacuum pump (20). When in use, the flexible
- 5. vent pipe from the common connection on the top product receiver is attached to the inlet of this vacuum pump at (23), and motive water admitted via valve (V14), which must only be operated when valve (V5) is open. The level of vacuum is adjusted using needle valve (V15) and is indicated on pressuregauge (P1). 11. Control Console The console is attached to the process unit by an umbilical cable which is of adequate length to allow the console to be positioned at least 2.0 m away (outside the “Zone 2” area). See the Safety section at the front of this manual, and the Specifications section on page 15. The following pages provide a description of the console controls and connections. Figure 5a. Console Controls and Connections Figure 5b. Console Controls and Connections The rear of the UOP3CC Console houses all cable connection glands, mains connections and DC connections. Two data ports are provided, a USB port for direct connection to a PC with Armfieldsoftware, and a 20-way signal output port, which provides voltage outputs for each of thesensor readings. Figure 6b. Rear of UOP3CC Console The rear panel also houses zero and span potentiometers for each of the thermocouples, inorder that the displays on the front of the console can be adjusted to read correctly. C. OPERATION Where necessary, refer to the drawings on pages 19 to 22 and to
- 6. the console description beginning on page 25. 1. Warning! The vacuum pump must never be started before opening valve (V5) to allow cooling water to the condenser (8). Failure to observe this will cause solvent to be discharged to the drain with the vacuum pump motive water. 2. Reflux Ratio Control The reflux ratio timer on the control console is used to set the quantity and frequency of condensate returning to the distillation column. With the timer switched off, all of the condensate will be directed to the column (total reflux). 2.1. Typical reflux ratio examples If the reflux ratio required is 2:1 and the total cycle time required is 21 seconds: Condensate will be directed by the reflux ratio valve to the column for 14 seconds then to the top product receiver for 7 seconds. This cycle will then be repeated continuously until different values are inserted to the controller or until the reflux control is switched off. If a ratio of 4:1 is required over the same cycle time: Condensate will be directed to the column for 16.8 seconds and to the top product receiver for 4.2 seconds. The calculation is as follows: 4 + 1 = 5; 21/5 = 4.2; 4 x 4.2 = 16.8; (21 - 16.8 = 4.2) 2.2. Setting the controller The time range and mode of the controller can only be set when the electrical supply to the controller from the control console is switched off. Switch off the reflux controller switch on the control console. This switches off the power supply to the controller. The controller is now not controlling reflux flow. Because the controller has an internal battery, the display is still illuminated and the controller settings may be adjusted. Press the SET button on the reflux ratio timer. The controller should be set to Immediate Cycle (CY) mode. If CY is not
- 7. already set, use the D button to cycle through the modes until CY is displayed. Press the SET key again. Select the time range required. Seconds × 10 is suggested as the most suitable time range, allowing cycle times of between 00.1 and 99.9 seconds to be set. Use the D button to cycle through the modes until sec × 10 is displayed. The set time may then be adjusted at any time (even when the controller is switched on), as follows (this assumes that secx10 mode has been selected): Set the time interval during which condensate should be directed to the top product receiver (CY+ on the controller): Press SET to select the 10 digit Press D to set the 10 digit Press SET to select the 1 digit Press D to set the 1 digit Press SET to select 0.1 digit Press D to set the 0.1 digit Set the time interval during which condensate should be directed back to the column (CY- on the controller): Press SET to select the 10 digit Press D to set the 10 digit Press SET to select the 1 digit Press D to set the 1 digit Press SET to select 0.1 digit Press D to set the 0.1 digit Press SET to end time adjustment. To begin controller operation, switch on power to the reflux controller using the reflux controller on/off switch. The CY- digit in the bottom right of the display now indicates flow of condensate back to the column. The CY+ digit indicates flow of condensate to the top product tank. To display the set time during operation, press SET once to display set time CY+,twice to display set time CY-. 3. Measuring Temperatures There are thirteen temperature sensing stations on the
- 8. equipment, which are designated as follows: T1 = Top tray of distillation column T2 = 2nd tray T3 = 3rd tray T4 = 4th tray T5 = 5th tray T6 = 6th tray T7 = 7th tray T8 = 8th tray T9 = Temp of liquid in reboiler T10 = Temp of vapor leaving the column above tray 1 T11 = Temp of cooling water entering condenser T12 = Temp of cooling water leaving condenser T13 = Temp of condensate as reflux/top product T14 = Temp of feed liquid from feed tank It is intended that both thermocouples T11 and T12, can be moved to any of the three positions marked T on the flow diagram. This will be necessary when carrying out a feed preheat experiment or an azeotropic distillation experiment. In fact all of the thermocouples are identical so any can be moved to different locations but T11 and T12 are the recommended “movable” sensors as their connecting cables will not require any special re-routing. When moving sensors, always ensure that the blank fitting removed from the new sensor location is used to blank off the fitting from which the sensor was removed. Temperatures may be viewed on the display on the right-hand panel of the control console. To display any temperature from T1 to T8, set the upper selector dial to the corresponding station designation. To display temperatures T9 to T14, turn the upper selector dial fully clockwise (to the furthest right-hand setting) and then set the lower selector dial to the station designation required. 4. Measuring Column Pressure Drop The overall pressure drop over the column can be measured
- 9. using the manometer P1. Always open V6 before V7, take the pressure reading then immediately close both valves. This will reduce the risk of contamination of the manometer water by the hydrocarbons. Also to prevent contamination, never open valves V6 or V7 when flooding is occurring on the sieve plates (boil-up rate too high). 5. Taking Samples for Analysis Samples for analysis can be taken from pertinent points in the system as follows: Feed liquid - From feed tank Liquid in reboiler - V2 (WARNING! Liquid at boiling point!) Condensate from condenser - V3 (reflux/top product) Top product receiver - V4 Bottom product receiver - V11 Note: When using valve V3 to obtain a sample of top product or to measure boil up rate the valve should not be fully opened, to prevent vapor from escaping. Gradually open valve V3 until flow of reflux into the column stops but liquid is retained in the flexible connecting pipe. Small adjustments of the valve position can be applied to maintain the desired level in the pipe. Provided that the same level in the pipe is maintained at the start and finish of the timing operation then the boil up rate measured will be accurate. 6. Feed Pump Calibration The peristaltic feed pump is designed to give approximately 1 ml/min per revolution of the drive shaft. As the variable speed motor is capable of speeds varying from 0 to 300 RPM, the pump will be able to deliver approximately 0 to 300 ml/min of feed to the column. In order to achieve greater accuracy, it is necessary to produce a calibration graph of the actual flow rate against the position of the variable speed dial on the control console. The dial has ten full turns, each full turn marked in one hundredth segments. To produce the graph it will only be necessary to measure the flow at ten settings over the full range. Disconnect the feed
- 10. tubing to the distillation column. Ensure the feed tank with the pump suction pipe inserted has sufficient water for the calibration. Using a 250 ml graduated cylinder and a stop watch, simply determine the flow rate at the ten settings and construct a graph which can be subsequently located near the control console. Note: The distillation column has been designed for feed rates between 50 and 200 ml/min depending on the chemicals being used so a slight inaccuracy at the minimum and maximum settings of the pump speed will not affect the process. 7. Operation of the Decanter (Phase Separator) The decanter in normal operation is used with valve (V10) open which allows condensate entering the vessel to flow directly to the reflux valve. When carrying out an experiment which utilizes a third liquid component, valve (V10) is closed and the decanter comes into operation. The condensate entering the decanter will be made up of the miscible binary mixture plus an immiscible component. The heavier component will separate and collect at the base of the decanter and its’ level will begin to rise. Eventually the lighter phase will overflow the fixed overflow and, when the level is sufficiently high, the heavier phase will overflow the adjustable overflow. The adjustable overflow will always be below the level of the fixed overflow and when adjusted will determine the height of the interface between the light and heavy components. Figure 7. Schematic of Decanter As a guide, begin the process with the adjustable overflow 1cm below the level of the fixed overflow. 8. Operation of the Reboiler Heating of the liquid in the reboiler is achieved by an electrical heating element. The maximum power of the element is 2.0 kW and this is adjustable at the control console. Due to the various flux requirements of the liquids which can be used in the reboiler, the heater must always be switched on at zero power
- 11. (adjustment fully anti-clockwise). The power can then be increased carefully until boiling is achieved and fine adjustment is carried out to cause the required activity on the sieve plates (observed on Trays 1 and 5). Excessive power to the reboiler may cause vapor to escape from the vent pipe due to overloading of the condenser. NOTE: The reboiler heater maximum power is 2.0 kW rated at a supply voltage of 240V (120V). 2.0 kW will not be achieved if the supply voltage is low but this will not affect the process as maximum power is rarely, if ever, required. 9. Using the UOP3CC with Supplied Software The UOP3CC is supplied with educational software on CD- ROM. Ensure that the software is installed as described in the Installation Guide. To run the software, open the ‘Start’ menu, and choose ‘UOP3cc Distillation Column’ from the ‘Armfield Unit Operations Software’ group. The initial screen will load displaying the first page of the presentation screen. The toolbar at the top of the screen contains four buttons which are used to navigate the software: · View Diagram – displays a mimic diagram of the apparatus, with sensor readings displayed in real time. Data values can be recorded by clicking the · ‘GO’ button. · View Graph – displays a graph of selected recorded values. · View Table – displays a table of recorded data. · View Presentation – displays the presentation screens. Help texts are available within the software explaining how to use the software, and detailing experimental theory and procedures. The software can be used with any of the Teaching Exercises listed in this manual to aid with data recording, and to provide online help for the student. D. SPECIFICATIONS 1. Overall Dimensions (process module)
- 12. Height: - 2.25m Width - 0.85m Depth - 0.80m 2. 4.2 Overall Dimensions (console) Height: - 0.30m Width - 0.33m Depth - 0.40m 3. Electrical Supply UOP3CC-A UOP3CC-B Green/yellow lead Earth (Ground) Earth (Ground) Brown lead Live (Hot) Live (Hot) Blue lead Neutral Neutral Fuse rating 10A 20A Voltage 220-240V 110-120V Frequency 50Hz 60Hz 4. Cold Water Supply The equipment requires connection to a clean water supply with a pressure of 2 bar and a flow rate of 15 litres/minute. The equipment must be connected to the water supply using 12mm ID flexible hose(not supplied).
- 13. In hot climates the cold water supply temperature may be too warm to completely condense the test mixture under evaluation. Vapor or condensate will be visible in the flexible vent pipework if either the cooling water flow rate is too low or the cooling water inlet temperature is too high. Under these circumstances a chilled water supply may be required. If the vacuum pump is not in use then this can be limited to 4.4 litres/minute at 2 bar pressure. 5. Connection to Drain Water exiting the condenser should be directed to a suitable drain capable of accepting warm water at up to 15 liters/minute. The equipment must be connected to drain using 12mm ID flexible hose (not supplied). Water exiting the vacuum pump may contain traces of solvent. If local regulations do not allow discharge of water containing even small amounts of solvent to drain, then the water exiting the vacuum pump must be passed through a separator vessel to remove any solvent traces. 6. Solvent Vapor Extraction (ventilation requirements) All solvent vessels and pipework are connected to a common vent pipe at the right hand rear of the equipment (attached to the common connection on the top product receiver). It is recommended that, for operation under atmospheric pressure, the flexible PTFE tubing provided be connected to this and routed to a safe place outside the building or to a fume cupboard so that any vapors produced by abnormal conditions will be dispersed safely. For operation under reduced pressure conditions, the vent pipe must be connected directly to the inlet of the vacuum pump. 7. Bund (spillage containment) A stainless steel bund tray is provided which must be placed directly beneath the process unit on the floor, between the four support legs. This is designed to contain any accidental spillage
- 14. within the confines of the framework and thus within the Zone 1 area. E. Distillation Experiment 1. Nomenclature Name Symbol Units Temperature at top of column T1 °C Temperature in second tray T2 °C Temperature in third tray T3 °C Temperature in fourth tray T4 °C Temperature in fifth tray T5 °C Temperature in sixth tray T6 °C Temperature in seventh tray T7 °C Temperature at bottom of column T8 °C Temperature in reboiler T9 °C
- 15. Temperature of column exit vapor T10 °C Cooling water entry temperature T11 °C Cooling water exit temperature T12 °C Condensate/top product temperature T13 °C Cooling water flow rate FI1 l/h Total pressure drop across column DP1 mm H2O Vacuum pressure P1 bar Vacuum pump pressure relief valve PRV1 - Boiler heater power kW Boil-up rate l/h Condensate volume collected Time to collect
- 16. Name Symbol Units Continuous feed valve V1 Reboiler sample extraction valve V2 Condenser sample extraction valve V3 Top product receiver sample valve V4 Cooling water flow control valve V5 Bottom manometer connection V6 Top manometer connection V7 Dosing vessel floe control valve V8 Condensate decanter bypass valve V10 Drain valve V11 Product recycling valve V12
- 17. Vacuum pump water supply control V14 Vacuum pump flow control valve V15 2. Experiment 2.1. Objective · To determine the effects of operating variables in batch distillation and continuous distillation. · To compare the theoretical calculations and experimental results. 2.2. Procedure Total reflux 1. Make up 10 liters of a mixture of 12 wt% ethanol-water mixture. 1. Set the equipment to operate at total reflux by switching off the reflux ratio timer on the console. 1. Ensure all valves on the equipment are closed. 1. Open valve V10 on the reflux pipe. 1. Load the 10 liter charge of prepared feed mixture (the liquid to be distilled) directly into the reboiler through the filler cap provided. Make sure the filler cap on the top of the reboiler is firmly replaced. 1. Turn on the power to the control panel. 1. Set the temperature selector switch to T9 (the temperature in the reboiler). 1. Open valve V5 until the cooling water flow rate FI1 to the condenser is approximately 3 liters/min. 1. On the control panel, turn the power controller for the reboiler heating element on full (fully anti-clockwise) and switch the switch turning on the power to the heating element to “power on” position. Another red lamp will illuminate indicating the heating element is on.
- 18. 1. Turn the power controller clockwise until a reading of approximately 0.75kW is obtained on the digital wattmeter. The contents of the reboiler will begin to warm up and this can be observed on the temperature readout meter. 1. Eventually, vapor will begin to rise up the column and the progress of this can be clearly observed as well as detected by the increasing temperatures when switching the temperature selector on T8, T7, T6, T5, T4, T3, T2 and T1. Vapor will enter the condenser and reappear as droplets into the glass walled distillate receiver vessel (11). 1. The distillate will build up a small level in the receiver and eventually overflow to the reflux regulator valve (12). Since the valve is not on (on the control panel), and it will not be necessary to have it on in this experiment, which is run under total reflux, the condensed vapor will return to the column. 1. The cool distillate is then returning on the top of the column, and will cascade down the trays forming a liquid level on the trays and bubbling of vapor passing through the liquid. The system will have reached an equilibrium condition when the temperatures T1, T2, T3, T4, T5, T6, T7 and T8 are constant. 1. Measure the boil-up rate by performing a timed volume collection: Operate valve V3 so that all the condensate is diverted into a measuring cylinder. First take a small discarding sample of 5 to 10 ml into an alternative vessel, then divert the condensate into the measuring cylinder and use the stopwatch to measure the time required to collect a set quantity. This will not disrupt the equilibrium conditions in the column provided a liquid level is maintained in the condensate feeding pipe. When taking a sample, partially open valve V3 and drain the condensate (in a separate measuring cylinder) from the reflux system until a steady flow is obtained. (Ensure that liquid remains in the flexible connecting tube to prevent vapor from escaping.) Start sample collection and timing at the same time, and collect a sizeable amount. Repeat the measurement three times and take an average value to determine the boil-up rate in liters/hr at the corresponding power input, kW.
- 19. Figure 8. Schematic of system 1. Take a sample of the overheads through valve V3. When doing so be careful never to drain the condensate return line, i.e. partially open valve V3 to leave a small amount of liquid in the line all the time. Generally, when taking samples, drain a “discarding” sample of approximately 5 to 10 ml before taking the representative sample in a sample glass. Do not drain too much of the “discarding” sample because of the disturbance of the mass balance. Discard the “discarding” sample in a safe way. After the representative sample has been taken, keep the sample glasses in an upright position and tightly seal them with Parafilm. 1. In a similar manner andpreferably at the same time take a sample of the bottom through valve V2. 1. After the samples cool down, record the refractive index for the taken overhead sample. 1. Control the boil-up rate by changing the power input and repeat (14)-(16). 1. Record Power input (kW), boil-up rate, and concentrations. Partial reflux 1. Make sure the system is in equilibrium condition at a pre- determined power input. 1. Set the reflux controller to the desired values e.g. start with 5:1 by setting 20 sec back to column and 4 sec to top product receiver. The setting is like the setting of a digital watch. C- means back to column and C+ means to top product receiver. When this is done, switch the reflux valve on (on the control panel). You should also see condensed vapor flowing to the top product receiver. 1. Take a sample of the overheads through valve V3. When doing so be careful never to drain the condensate return line, i.e. partially open valve V3 to leave a small amount of liquid in the line all the time. Generally, when taking samples, drain a “discarding” sample of approximately 5 to 10 ml before taking
- 20. the representative sample in a sample glass. Do not drain too much of the “discarding” sample because of the disturbance of the mass balance. Discard the “discarding” sample in a safe way. After the representative sample has been taken, keep the sample glasses in an upright position and tightly seal them with Parafilm. 1. In a similar manner andpreferably at the same time take a sample of the bottom through valve V2. 1. After the samples cool down, record the refractive index for the taken overhead sample. 1. Repeat this procedure every ten minutes until five samples of both overhead and bottom are obtained. Record the temperatures T8 and T1 to calculate the average column temperature. 1. Repeat this procedure for different reflux ratios. 1. Repeat the whole procedure for several different boil-up rates to cover over the operating range of the column. 3. Theory Total reflux Figure 9. Simple Schematic of Stages in Total Reflux Since there is no feed, no bottom product or no top product, the liquid flow in the column is equal to the vapour flow in the column: V = L A material balance over the M.V.C. (most volatile component) gives V yn = L xn+1 Since V = L this gives yn = xn+1 Sample Calculation: Top product required: 90 mol% ethanol. Bottom product required: 5 mol% ethanol
- 21. From the equilibrium curve, composition on the top sieve plate is yt = 0.88, xt = 0.8 Page 26 of 26 and yt-1 = xt = 0.75 xt-1 = 0.72 yt-2= xt-1 = 0.72 xt-2= 0.69 yt-3 = xt-2 = 0.69 xt-3= 0.65 yt-4= xt-3 = 0.65 xt-4 = 0.60 yt-5= xt-4 = 0.60 xt-5= 0.54 yt-6= xt-5 = 0.54 xt-6 = 0.47 yt-7 = xt-6 = 0.47 xt-7 = 0.40 yt-8 = xt-7 = 0.40 xt-8 = 0.37 This composition is close to that required and can be withdrawn from the bottom product. Theoretically, therefore, the distillation column containing eight sieve plates plus the boiler will give the compositions calculated above. However, as the experiment will show this is not in fact correct. Relate the theory and the results of the experiment with the column efficiency. The previous calculation can be illustrated in a so-called McCabe-Thiele diagram: Figure 10.McCabe-Thiele diagram Total condensate flow rate a) b) c) d)
- 22. e) Average flow rate = Top Product Composition molpercent ethanol Bottom Product Composition molpercent ethanol Refer back to the Theory section of this experiment. Did the results obtained in thisexperiment correlate with the McCabe- Thiele diagram presented in the theory section? Partial reflux Calculation of Number of Plates using the LEWIS-SOREL method
- 23. Figure 11. Material balance of top of column Vn=Ln+1+D With respect to M.V.C. this becomes ynVn =Ln+1xn+1+D xd Since the liquid overflow is constant Ln = Ln+1 (1) Figure 12. Material balance of bottom of column Vm =Lm+1-W With respect to M.V.C. this becomes ymVm =Lm+1xm+1-Wxw Since the liquid overflow is constant Lm=Lm+1 (2) Equations (1) and (2) combined with the equilibrium curve can be used to calculate the composition on the various plates working from the condenser down to the still. The plate which has a composition nearest to that of the feed should be used as the feed plate. Consequently the number of theoretical plates and position of entry for the feed can be calculated.
- 24. Sample Calculation: Using a feed of a binary mixture of 60 mol percent methylcyclohexane and 40 mol percent toluene. Top product required: 75 mol percent methylcyclohexane Bottom product required: 44 mol percent methylcyclohexane Reflux ratio 5:1 A material balance on the M.V.C. (most volatile component), methylcyclohexane gives, Feed = Top product + Bottom product 100 × 0.60 = 0.75D + 0.44W and D = 100-W 100 × 0.60 = 0.75(100-W) + 0.44W W = 48.39 and D = 51.61 Also Ln = 5D (reflux ratio = 5) Ln = 258.05 andVn= Ln + D Vn = 309.66 from equation (1) yn = 0.83xn+1 + 0.13 for the top section Lm = 258.05 + 100 = 358.05 Vm = Lm - W = 358.05 - 48.39 Vm = 309.66 = Vn from equation (2) ym = 1.16xm+1 - 0.07 for the bottom section From the equilibrium curve,
- 25. Composition on the top sieve plate is yt = 0.75, xt = 0.72 and yt-1 = 0.83 x 0.72 + 0.13 = 0.73 xt-1 = 0.69 yt-2 = 0.83 x 0.67 + 0.13 = 0.69 xt-2= 0.65 yt-3= 0.83 x 0.65 + 0.13 = 0.67 xt-3= 0.62 yt-4= 0.83 x 0.62 + 0.13 = 0.65 xt-4 = 0.60 Since the feed composition is 0.60, it can be introduced on this tray (t-4). yt-5 = 1.16 x 0.60 - 0.07 = 0.63 xt-5 = 0.58 yt-6= 1.16 x 0.58 - 0.07 = 0.60 xt-6= 0.54 yt-7 = 1.16 x 0.54 - 0.07 = 0.56 xt-7= 0.50 yt-8 = 1.16 x 0.50 - 0.07 = 0.51 xt-8 = 0.44 This composition is close to that required and can be withdrawn from the bottom product. Theoretically, therefore, the distillation column containing eight sieve plates plus the boiler will give the compositions calculated above. However, as the experiment will show this is not in fact correct. Relate the theory and the results of the experiment with the previous experiment to determine column efficiency. 4. Issues to discuss The discussion points you may want to include, but not limited
- 26. to, are: · Calculate the number of theoretical plates for a given separation at total reflux using Fenske equation. What significance this number has? · Estimate the number of theoretical plates for a given reflux ratio using McCabe-Thiele Method. · Estimate the minimum reflux ratio using the McCabe-Thiele Chart. · Calculate the efficiency of the column using the following equation: · Knowing the composition of distillate and bottom and the corresponding volatilities, the column efficiency can be determined. · Is there any obvious relationship between boil-up rate and column efficiency? How would you increase the efficiency? · Discuss the differences between the theoretical and experimental values. d n n 1 n n x V D x V L y 1 n + =