Chemistry Engineering Lab Report

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Experiment DC: Distillation Column Overall Column Efficiency OBJECTIVE To determine the overall column efficiency. METHOD To operate the column over a selected boil-up rate, taking samples in order to measure the refractive index (and hence to determine the concentration of the components). DESCRIPTION The UOP3CC Continuous Distillation Column is a self-contained distillation facility (see Figure 1) consisting of a 50mm diameter sieve plate column made up of two glass sections, each containing four sieve plates. The plates are arranged vertically for counter-current vapour/liquid flow. The column is insulated to minimise heat loss. A reboiler is situated at the base of the column and incorporates a flameproof immersion type heating element. Because the experiment is in batch operation, valve (V1) remains closed. A level sensor inside the reboiler protects the heating element from overheating due to low operating level and a plastic tube allows the level in the reboiler to be observed. Vapour from the top of the column passes to a water-cooled, coil-in-shell condenser. 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 variable area flowmeter (FI1) and the flowrate is controlled by diaphragm valve (V5). A cooling water supply is connected to the inlet nozzle and serves also to operate the vacuum pump if operation at reduced pressure was required. Water supply to the vacuum pump is controlled by valve (V14), which must only be operated when valve (V5) is open. Condensate is collected in a glass decanter (phase separator) which is by-passed for normal distillation experiments by opening valve (V10). With valve (V10) open, condensate from the condenser outlet passes directly through the decanter to the inlet of the reflux ratio control valve 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. 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 1 can be drained into the reboiler for re-use via valve (V12). 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. 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 4.0m length of tubing to a fume cupboard or safe atmospheric vent outlet. Console and computer control/display The console is attached to the process unit for display and control of the various operating parameters of the distillation column (see Figs. 2-4). Optionally the process can be fully controlled from the connected computer (see Fig. 5). Tabular Display To view the Table screen click the View Table icon from the View dropdown menu as shown: from the main tool bar or click Table The data is displayed in a tabular format, similar to the screen as shown: 2 As the data is sampled, it is stored in spreadsheet format, updated each time the data is sampled. The table also contains columns for the calculated values. THEORY To calculate the number of theoretical plates for a given separation at total reflux, Fenske developed the following formula: where n = number of theoretical plates xA = mole fraction of more volatile component xB = mole fraction of least volatile component av = average relative volatility Subscripts d, b indicate distillate and bottom respectively The efficiency is given by: Knowing the composition of distillate and bottom and the corresponding volatilities, the column efficiency can be determined. 3 PROCEDURE The reboiler is already filled with the liquid to be distilled (15 mole% ethanol, 85 mole% water). 1. Switch on the computer. 2. At the big control panel on the wall, set the “UOP AREA EXTRACT FAN” (selector on the left), at “Hand position”. 3. Switch on the Armfield console by the computer. Load the software in the computer. Go to “View” and select “Diagram”. 4. Identify all components in Figure 1. All valves must be closed except for V5 (cooling water) and V10 (below the decanter). 5. Set the equipment to operate at total reflux: on the reflux controller of the console, put the selector on Manual position and make sure the ON/OFF button is not on (no green light). 6. In the small room where the rig is, open the red valve on the wall to get cooling water to the condenser. Set the water rotameter (above valve V5) to 3000 ml/min. 7. Choose options A or B and leave the apparatus for at least 30 minutes so that the system can reach equilibrium conditions. A. Automatic control using the software (PID1). On the console, press the red button to activate the heater. Then put the selector on I/O Port position to have it controlled from the computer. Press PID1 on the screen’s diagram and set the following parameters: Mode of operation: Set point (T7): Proportional band: Integral time: Derivative time: Automatic 77 oC 50% 15 s 0 s B. Manual control using the console. On the console, press the red button to activate the heater. Then put the selector on Manual position. For about 5 minutes set the heat controller at high rate, 1.75 kW. Reduce to 0.85 kW once liquid starts flowing from the decanter back to the 4 column. 8. Start recording the data by clicking on . 9. Check that temperatures increase so that T1 reaches approximately 77 oC. 10. While the system is heating up, mark in Figure 1 the flow of products (cooling water in blue and ethanol/water in red). The experiment takes place at total reflux, so there will be no feed to the top product receiver or bottom product receiver. It is a batch operation and takes place at normal pressure. 11. Take a sample of the distillate through valve V3 and a sample of the bottom through valve V2. Use googles and gloves for this. CAUTION! SAMPLES ARE HOT! Keep the vials in an upright position. Repeat this procedure every 10 min until at least 3 samples of both distillate and bottom are obtained. Label them carefully as D1, D2, D3,…, B1, B2, B3,…. Take the samples to the lab with your supervisor, to learn how to use the refractometer. Measure and register the refractive index for the different samples. 12. Record the temperatures T1 to T8 to calculate the average top and bottom column temperature. 13. Click on to stop recording the data and save the file as Excel 5.xls. 14. Cool down by pressing the red button in the console to turn off the heater. Check that temperature is dropping down. 15. Switch off the cooling water (red valve on the wall). 16. Shut down everything when temperatures have dropped down: 16.1. Turn off the console and the computer. 16.2. On the wall panel (UOP Area extract fan), set it “Automatic”. 16.3. Dispose the samples in a safe manner, using googles and gloves: pour them in the plastic jerry can available in the rig’s room, labelled as “WATER AND ETHANOL”. 5 Fig. 1. Schematic diagram of apparatus 6 Fig.2. Display and control of reflux ratio Fig. 3. Display and control of power to rebolier heating element 7 Fig. 4. Display of distillation column temperature Fig. 5. Display of distillation column temperature 8 Experiment DC: Distillation Column Data Sheet Group Number: Group Members: Date: NB Wait 10 minutes before taking a fresh set of samples. Distillate composition- ethanol Reading 1 2 3 Average Composition Volatility Refractive index Bottom Composition- ethanol Reading 1 2 3 Average Composition Volatility Refractive index Average top column temperature Average bottom column temperature Average column temperature = = = °C °C °C 9 T [°C] y 100  - 10.575 x 0 0.019 95.5 0.0721 89 0.0966 86.7 0.1238 85.3 0.1661 84.1 0.2337 82.7 0.2608 82.3 0.3273 81.5 0.3965 80.7 0.5079 79.8 0.5198 79.7 0.5732 79.3 0.6763 78.7 0.7472 78.4 0.8943 78.1 x = mole fraction of ethanol in liquid y = mole fraction of ethanol in vapour 0 0.17 0.3891 8.197 0.4375 7.274 0.4704 6.286 0.5089 5.202 0.5445 3.92 0.558 3.578 0.5826 2.869 0.6122 2.403 0.6564 1.851 0.6599 1.792 0.6841 1.612 0.7385 1.352 0.7815 1.21 0.8943 1 = relative volatility 10