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Introduction Batch Distillation is used widely within the chemical industry to purify small quantities of chemicals. A batch distillation column consists of a tank that is charged with the mixture to be separated and then heated to produce a vapour. The vapour leaves the pot and passes through the column where separation takes place. At the top of the column the vapour is condensed and part of the liquid produced (the reflux) flows down the column in counter current flow with the vapour rising up the column. The other part of the condensate is drawn off as the overhead product. In effect the batch distillation column is acting as the rectification section of a continuous distillation column. Quite often the distillation column will be installed above a heated tank in which a reaction is taking place, so that separation can take place during the reaction process or after the reaction has been completed. Objectives The overall objective of this lab project is to study the separation of at water-methanol mixture in a pilot scale batch distillation column. The specific objectives are
The first two objectives can and should be completed before the lab session.
Batch Distillation Experiment - Equipment Description Description of the column The batch distillation column consists of a round bottomed flask at the base, which is charged with a methanol - water mixture at the start of the experiment, a glass column packed with small glass cylinders, a condenser positioned above the column, a reflux valve, a product cooler and a collection cylinder (see the diagram on page 2). Heaters The round bottomed flask sits in an electric heater, which is controlled from the heater control panel. There are three heating elements and their power output is determined by the dials labelled VT1, VT2 and VT3. The dials should all be set to the same value, in which case the power output of the heaters (in Watts) is given by the equation W = 0.15 S ^2.18 where S is the dial setting The heaters are turned on and off using the switches next to the dials on the heater control panel. There are no temperature measuring instruments connected to the heater control so the dial will read zero. Thermocouple Display Thermocouples are used to measure the temperature at various points in the column. The thermocouples that are connected to the display have blue and white wires. They are numbered from 1-9. Numbers 1-6 measure temperatures in the column, with 6 being the temperature of the top product vapour. Numbers 7-9 measure the cooling water temperature at the inlet to the product cooler, the inlet to the condenser and the outlet of the condenser respectively. The dial is used to select which thermocouple outlet is displayed. Reflux Valve Control The reflux ratio is controlled using a solenoid valve which has two positions;
The reflux ratio is determined by the ratio of the time the valve is open to the time the valve is closed. If the valve is open for twice as long as it is closed then the reflux ratio is 2. The dial on the reflux valve control box is used to set ratio of the time the valve is open to the time the valve is closed and therefore the numbers on the dial represent the actual reflux ratio. When the dial is at zero the valve will be open all the time. The switch on the control box turns the reflux valve on and off. Sample Valves There are 10 sample valves. Number 1 is a syringe which allows a sample to be taken from the round bottomed flask. Numbers 2 - 9 give samples of the liquid flow within the packing. Number 10 samples the overhead liquid product. SAFETY NOTE: Samples coming from the column will be at temperatures of up to 100°C. Make sure that you hold the sample vial using tongs or heat resistant gloves. Do not hold the sample vial with you bear hands whilst taking a liquid sample from the column. Batch Distillation Experiment - Experimental Procedure
Methanol Concentration Measurement After you have taken a liquid sample make sure you screw a lid onto the sample vial immediately or else you will loose some of the liquid sample as vapour and change its composition. After the sample has cooled down to room temperature measure its refractive index using the refractometer set up in the Chemical Engineering Lab on the 3rd floor. Using the attached calibration curve convert these refractive index readings into mass fractions of methanol. Batch Distillation Experiment - Start Up and Shut Down Procedure for starting up the batch distillation column
Procedure for shutting down the batch distillation column
Batch Distillation Experiment - Treatment of Results You should produce plots of column position against temperature and column position against methanol concentration as a function of time, for each reflux ratio used. Sketch graphs are shown below.
Use the measured cooling water flow rate, cooling water inlet and outlet temperatures to the condenser, the power supplied by the heater and the flowrate of the distillate to work out the energy used per kilogramme of product, to effect the separation of methanol and water. Compare this to the minimum energy required for the separation (Reference 3) and suggest reasons for the difference in values. If the top product composition has maintained an approximately constant value (see Reference 2) then use a McCabe-Thiele plot to measure the number of stages and the number of transfer units. The equilibrium data for the methanol water system can be calculated using the data given at the end of this sheet or experimental data can be used (see Reference 1). You should also check to see if the liquid concentration samples taken at the end of one hour lie on the operating line on the McCabe Thiele plot, used to calculate the number of stages.
Calibration Curve for the Refractometer
Data
References
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Laboratory
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