EXPERIMENT 2
STUDY OF DISTILLATION IN A PACKED COLUMN There are many types of distillation columns, each designed to perform specific types of separations and each design design differs in terms of configuration. In batch operation, the feed to the column is introduced batch-wise - the column is charged with a 'batch' of a feed mixture after which distillation distillation is carried out. When the desired task is achieved, achieved, the next batch of feed is introduced. Continuous columns on the other hand process a continuous feed stream. No interruptions interruptions occur unless unless there is is a problem problem with the column column or surrounding surrounding process units. They are capable of of handling high high throughputs throughputs and are the more common of the two types. Columns can be further classified according to the nature of the feed that they are processing: binary column in which the feed contains only two components and multicomponent column column where the feed contains contains more than two components. components. Columns can also be categorized categorized according according to the number number of product streams they have. Distillation columns can also be classified according to the type of internals in the column: a. Tray column - where trays of various designs are used to hold up the liquid to provide better contact between vapour and liquid, hence better separation b. Packed column column – where, instead instead of of trays, packings packings are used used to enhance enhance contact between vapour and liquid. This experiment is designed to study the distillation of a binary mixture of ethanol and water in a packed distillation column. 2.1 OBJECTIVES 1. To make use use of a packed bed for the distillation of a binary mixture of ethanol and water with varying reflux ratios (including total reflux) in order to determine the Height Equivalent to a Theoretical Plate (HETP) for the column. 2. To study the material balance across the column. column.
FIGURE 2.1 x-y Diagram for Distillation
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EXPERIMENT 2
2.2 THEORY Figure 2.1 shows the vapour-liquid equilibrium curve (x-y diagram) and the operating line for the distillation process under general reflux. The general equation of the operating line can be expressed as: x d R y= x+ R + 1 R + 1 where R is the reflux ratio and x d is the mole fraction of the liquid distillate collected at the top. As the reflux ratio is increased, the gradient of the operating line for the rectification section moves towards a maximum value of 1. Physically, what this means is that more and more liquid that is rich in the more volatile components are being recycled back into the column. If the distillation is carried out in a plate column, it is possible to calculate the number of theoretical plates that are required for the distillation. This makes it possible to define a height of the packed column which effects separation of a mixture equivalent to a theoretical plate. The height so calculated is known as the Height Equivalent to a Theoretical Plate (HETP). 2.3 APPARATUS A view of the unit is shown in Figure 2.2
FIGURE 2.2 A view of the Packed Bed Distillation Unit The rig consists of a glass column (A) packed with glass Raschig Rings and a thermosyphon reboiler assembly (B) at the bottom of the column. The thermosyphon reboiler assembly consists of a glass flask (F) connected to an electrically heated vessel (G) as shown in the diagram. A thermometer (T) measures the temperature of liquid inside the glass flask (F). A vapour condenser (C) is located at the top of the column 14
EXPERIMENT 2
which makes use of water as the coolant. The condensate from the condenser is drawn out and separated into two streams P and Q as desired. The stream P is returned to the column through a valve V1 and a flow meter R1. The stream Q is sent to a second condenser (D) through a valve V2 and a flow meter R2. The liquid from the condenser is sent to the distillate receiver (E) through a valve V4 or to the reboiler through a valve V5. Two thermocouples T1 and T2 measure the temperatures of vapour at the top of the column and the liquid at the reboiler. A valve V6 located at the bottom of the glass vessel of the reboiler assembly enables an operator to withdraw spent liquid for cleaning and other purposes at the end of the experiment. The layout of the equipment is shown in Figure 2.3.
FIGURE 2.3 Packed Column Distillation Layout The distillation unit has instrumentation consisting of: 1. A watt meter for the reboiler heater 2. Thermocouples to measure the temperatures of the liquid in the reboiler and the vapour at the top of the column 3. Flow meters R1 and R2 The unit can be operated as follows: 1. All the distillate is collected as the top product (zero reflux) 2. Some of the condensed top product is collected and the rest is returned to the column (with reflux) 3. All condensed top product is returned to the column (total reflux) 15
EXPERIMENT 2
2.4 SERVICES AND MATERIALS REQUIRED 1. Water supply for the condensers 2. Power supply 220V AC 3. 18 Liters of 50% Ethanol-Water mixture 2.5 PRE-EXPERIMENTAL QUESTIONS a. b. c.
In terms of physical properties, what types of components in a mixture cannot be separated by distillation Explain how a packed bed assists mass transfer in a distillation process Identify the following units in the rig i. Packed column ii. The reboiler and heater assembly iii. Condensers iv. Valves v. Flow meters vi. Thermocouples vii. Cooling water paths viii. Vapour path ix. Distillate paths
2.6 PROCEDURE Safety Caution 1. You must familiarize yourself with the flow paths and apparatus before beginning the experiment. Distillation operation usually deals with very hot, highly flammable materials. Extreme care must be taken around the apparatus, in taking readings and collecting samples 2. The flow meters should be operated smoothly in order to avoid pressure surges within the equipment 3. The flow rates of liquids should not exceed the maximum rates of the flow meter 4. The water supply pressure should not exceed 3 bar gauge 5. Inspect the equipment visually for any damaged components or glass breakage. If any such defects are noticed, please report to your supervisor 6. Safety glasses and safety helmet must be worn at all times in the laboratory 7. Appropriate rubber gloves must be worn when organic solvents are handled Instructions for Smooth Functioning 1. Once adjustments are made on parameters such as flow rates etc. sufficient time should be allowed for the process to attain steady state. [Allow at least 15 minutes for this ] 2. Check zero errors of the manometers and adjust the reading to zero if necessary
Calibration Curve 1. Prepare a 5 % ethanol-water solution in a test tube 2. Using the refractometer, obtain the refractive index reading for the solution 3. Using increments of 5 %, obtain refractive index readings for ethanol-water mixtures up to 100 % 4. Plot a refractive index vs. % concentration graph
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EXPERIMENT 2
Start-up Procedure 1. Turn on the cooling water supply to the condensers and make sure water is indeed flowing through them. Also check water flow to the drain from the cooling water outlet. Do not start the experiment until the cooling water flow is visible in the outlet to the drain 2. Close all valves (V1, V2, V3, V4, V5, V6 and V7) 3. Ensure that the reboiler flask is charged with the ethanol-water mixture and the liquid level is satisfactory. Otherwise the vessel should be topped up through the opening (P). The reboiler flask should never be filled with liquid above the maximum level. The level should also never recede below the minimum level during operation 4. Obtain a sample of the ethanol-water mixture from the reboiler through valve V7 and test its composition with the refractometer 5. Set the reboiler temperature to 89 oC 6. Switch on the heaters of the reboiler unit at the control panel and wait until the mixture is seen boiling and vapour is seen to reach the top of the column Note: The heat supply may be adjusted by manipulating the set points if necessary Operation with Total Reflux 1. Open valve V1 (Ensure that all the other valves are closed) 2. Wait until the first drop of reflux is returned to the column 3. Collect samples from the reboiler and the overhead through valves V7 and V3 respectively 4. Observe the flow rate at R1 and temperatures readings of T1 and T2 Note: Under steady state conditions, the flow rate and the temperature readings at the top and bottom of the columns should be steady 5. Measure the concentration of the samples drawn using the refractive index method 6. Collect samples from the reboiler and overhead every 10 minutes Operation with Reflux 1. Open valve V1 2. Before the first drop of reflux is returned to the column, open valves V2 and V4 3. Prior to the collection of the first drop of distillate in the receiver, ensure that there is a flow of reflux to the column. If there is no flow, close valves V2 and V4 4. Once the onset of reflux flow and distillate collection have occurred simultaneously, adjust the flow rates on R1 and R2 to 6 and 2 liters/hour respectively Note: This will ensure a reflux ratio of 3 for the operation 5. Collect samples from the reboiler and the overhead through valves V7 and V3 respectively 6. Take temperature readings of T1 and T2 Note: Under steady state conditions the flow rates indicated by R1 and R2 and the temperature readings at the top and bottom of the columns should be steady 7. Measure the concentration of the samples drawn using the refractive index method 8. Collect samples from the reboiler and overhead every 10 minutes General Measurement Measure the volume of the mixture in the reboiler flask at the end of the experiment [Please read the shut-down procedure first] Shut-down Procedure 1. Turn off the reboiler heaters power switch 2. Turn off the main power control switch 3. When condensate no longer collects, shut off the cooling water supply to the condensers
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Note: Do not drain hot liquid from the reboiler. If necessary, liquid within the system could be drained through valves V7 and V3 only when the liquid is cool 2.7 EXPERIMENT LOG SHEET Height of Packing (z) Ambient Temperature (T A ) Type of Packing Volume of Liquid Mixture in Flask/Still
: : : :
m o C Liters
a. Experiment with Total Reflux TIME (minutes) Rotameter Reading R1 (L/hr) Temperature of Thermocouple T1 ( oC) Temperature of Thermocouple T2 ( oC) REBOILER SAMPLE TOP PRODUCT SAMPLE
0
10
20
30
…
0
10
20
30
…
Refractive Index Concentration (%) Refractive Index Concentration (%)
c. Experiment with Reflux TIME (minutes) Rotameter Reading R1 (L/hr) Rotameter Reading R2 (L/hr) Reflux Ratio Temperature of Thermocouple T1 ( oC) Temperature of Thermocouple T2 ( oC) REBOILER SAMPLE TOP PRODUCT SAMPLE
Refractive Index Concentration (%) Refractive Index Concentration (%)
Volume of liquid mixture remaining in still = Liters.
2.8 POST-LAB QUESTIONS i. ii.
Plot the equilibrium curve on a x-y chart. Draw the operating line for the case of total reflux and obtain the number of theoretical stages required and the HETP value. iii. Draw the operating line for the case of partial reflux and obtain the number of theoretical stages required and the HETP value. iv. Calculate the material balance for ethanol at the end of the experiment. v. Discuss any possible errors in the experiment.
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EXPERIMENT 2
Data 1. Ethanol-water binary mixture has the following equilibrium distribution: Mole Fraction of Ethanol in Liquid 0.019 0.072 0.100 0.124 0.166 0.234 0.261 0.327 0.397 0.508 0.520 0.573 0.676 0.747 0.890
Mole Fraction of Ethanol in Vapour 0.170 0.389 0.440 0.470 0.510 0.545 0.558 0.583 0.612 0.656 0.660 0.684 0.739 0.782 0.890
REFERENCES 1. Coulson, J.M. and Richardson, J.F., Chemical Engineering, Volume2, Pergamon Press, 1991 2. Treybal,R.E, Mass Transfer Operations, McGraw Hill, 1990 3. McCabe, W.L., Smith, J.C. and Hariott, P., Unit Operations in Chemical Engineering, McGraw Hill, 2000
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