Packed Distillation Colunm
ChE 1007 8-1-01
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Location
Benedum SB-92, Manual is available
Introduction
The purpose of the lab is to introduce batch distillation using a packed column. Batch distillation is a technique used for separating two or more miscible liquid or vapor mixtures that are separated into their component fractions of desired purity. This is accomplished by the application and removal of heat. The separation is based on the boiling points of the mixture components. You will operate this column at total reflux, periodically taking small distillate samples, and determine the number of theoretical trays in the column.
Distillation is the most common separation technique, however it does suffer from some disadvantages (Refer to Table 1 for applications). It usually takes a large amount of heat, both in terms of heating and cooling, to run a distillation apparatus. This heat requirement can contribute to more than 50% of plant operating cost. The best way to cut down on operating costs is to improve the distillation unit’s efficiency and operation via process control and optimization.
Table 1. Industrial applications of distillation columns.
Industry Food Petrochemical Chemical Chemical Chemical Chemical
Application Concentrating alcoholic content of beverages Separation of crude oil into various products. Hydrocarbon Processing Solvent Recycling Radioactive Waste Reduction Monomer Purification
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Theory
Distillation is a process in which miscible liquids are separated based on their physical properties, specifically, relative volatilities. A liquid can be classified as volatile when it is readily vaporized at a relatively low temperature. The boiling of the more volatile components of the mixture drives the distillation process. When the vapor is cooled, the more volatile material condenses in a greater proportion than the less volatile material.
The two types of distillations utilized in industry are batch and continuous. Batch distillation is desirable when small quantities of high valued chemicals need to be separated. The biggest advantage to using a batch column is its flexibility. This allows one to deal with unknowns in the feed or product specifications. In a batch system, the column can handle different mixtures by simply changing its operating conditions. The main disadvantage to using a batch system is that the longer the components are exposed to high temperatures, the better the chances that the components are broken down via thermal degradation. Along with this, the energy requirements are usually higher for a batch system. A column is built for separating a specific mixture in continuous distillation. Therefore, the distillation column apparatus needs to be modified for each new mixture that is to be separated.
A batch distillation apparatus consists of a distillation column, a condenser, and a reboiler. The distillation column provides an environment where the gas and liquid phases of each component can approach equilibrium. A column can contain either packing or trays. In both types of columns, an increase in surface area allows for better contact between the liquid and vapor phases. In a column containing trays there is a discrete distribution of surface area, whereas in a packed column the distribution of area is continuous. The continuous distribution found in a packed column maximizes the surface area available for mass transfer, therefore allowing for a more efficient separation. In order to provide the highest contact area, a column is filled with packing that has a large volumetric area and that has high porosity. The liquid trickles down the column and through the packing as small droplets. The gas is sent through the column in
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the upward direction. This countercurrent flow of liquid and vapor exists only in a packed column. Ideally, the porosity of the packing should not hinder the gas flow through the column. In this lab, the packing in the column is glass sand. The distillation column also contains a condenser, which cools and condenses the vapor leaving the top of the column. A reboiler is connected to the bottom of the distillation apparatus and it provides the reboil heat that is necessary for distillation.
Condenser
Packing
Reboiler
Figure 1. Schematic of a simple, packed distillation unit.
A useful way to determine a column’s effectiveness is to limit its operating conditions. One way to accomplish this is to run the column at total reflux. In total reflux, all of the overhead vapor (reflux) and all of the bottoms liquid (boilup) is returned to the column. Total reflux conditions allow for assumptions that make calculations easier and allow the student an easy way to graphically evaluate the column.
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Temperature vs. MeOH concentration 105 100 95 e r 90 u t a r 85 e p 80 m 75 e T 70 65 60
Dew Point Curve Bubble Point Curve
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Figure 2. Boiling Point Diagram of a Binary Mixture at a Specific Pressure.
The Tx diagram shows how the equilibrium compositions of the components in a mixture vary with temperature at the column pressure. The dew point is the temperature at which the saturated vapor starts to condense. The bubble point is the temperature in which the liquid starts to boil. The Txy diagram for methanol in water can be produced using the UNIFAC method in Aspen. Data for this system is in the Appendix.
Boiling point diagrams can help aid in the construction of a vapor-liquidequilibrium (VLE) curve, Figure 3. The VLE plot shows the bubble point and dew point at constant pressure. The equilibrium line describes the compositions of the liquid and vapor in equilibrium at some fixed pressure.
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Figure 3. Representation of Vapor-Liquid Equilibrium Curve at a Specific Pressure.
A packed distillation column allows for continuous contact of liquid and vapor, however it is convenient to analyze the column as if it were discontinuous (a staged tower). The packing in the column can be divided up into segments that are of equal height. Each of these segments can be looked at as a “stage”. It can be assumed that each stage allows the vapor and liquid to leave the stage in equilibrium with each other. This method of assuming that the packing can be broken down into stages is not physically accurate, but can be used for calculations. The following equation relates column height and the number of equilibrium stages to the height equivalent to a theoretical plate (HETP).
HETP is defined as the height of packing needed to obtain the change in composition obtained with one theoretical contact. HETP is measured experimentally and usually can range from one to four feet. A small HETP indicates a small column and more efficient packing. To measure the HETP, the compositions of the top and bottoms streams must be found at total reflux and the number of equilibrium stages must be calculated. The following equation shows the relationship between the HETP, the height of packing and the number of theoretical stages.
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There are two methods to find the number of theoretical stages in the column. Both techniques use the McCabe-Thiele analysis, which is a graph of vapor composition (mole fraction) verses liquid composition (mole fraction). The system operating line and the equilibrium line for the system are plotted. The operating line for a batch system with total reflux is y=x. This is true because the flow rate of liquid must be the same as the
Methanol x-y diagram 1 0.9 0.8
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r o p a 0.6 v n i l 0.5 o n a h t 0.4 e M
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Reboiler
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Methanol in liquid
Figure 4. Total Reflux McCabe-Thiele with Theoretical Number of Stages.
flow rate of vapor. McCabe-Thiele analysis can be performed in two ways. One method calculates theoretical stages numerically by expressing the equilibrium and operating lines mathematically and finding the number of vertical and horizontal intersections (stages) required to reach the desired separation. The other method allows the student to manually draw the stages on the VLE graph. The first method is more accurate because of the numerical calculations, however it is much more time consuming. Regardless of
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the method used, the graph obtained will look similar to Figure 4, which can be produced using Aspen.
The HETP is dependant upon the packing type and size, the gas flow rate and the chemicals being separated. The higher the HETP the lower the efficiency of the packing. If the gas flow rate is low, the HETP will generally be higher because the packing is not completely wet. The HETP can be calculated as explained in the above method, however literature values are usually much more accurate. One approximation that can be used is to set the HETP equal to the column diameter. Lab Objectives • •
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The student should understand the theory behind a distillation column. The student should gain a working knowledge of liquid separation using a packed batch distillation column. The student will carefully measure the amount and composition of the material charged to the vessel. The student should be able to quantify the composition of distillate samples (the column should be run at total reflux) using mixture density at a specified temperature (typically, cool the samples to ambient temperature). The student should be able to understand the principles of batch distillation at total reflux. The student should understand the principles involved in vapor/liquid equilibrium. The student should be able to determine the number of theoretical plates along with HETP in the packed column operating at total reflux. The lab result can be represented on a figure such as Figure 4. The student should be able to use analytical methods such as density (using pychnometers and/or densimeters) to determine the composition of a sample of distillate.
Lab Tasks
The task associated with this lab is to separate methanol from water using a packed batch column operating at total reflux. The separation should be measured by the purity of the distillate sample. From the initial concentration of the feed and the concentration of the distillate sample the number of theoretical stages and HETP for the column can be found. Composition-density and/or composition refractive index calibration curves can be generated to assist in determining the composition of the distillate sample.
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Equipment Schematic 8
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4 Height of Packing
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Equipment Description
1) Heat Source 2) Rebolier: vaporizes the liquid that is sent to it 3) Feed Line: Transports the feed to the distillation unit 4) Packing (Glass Sand) : Provides maximum surface area for mass transfer 5) Reflux Tube: Place where overhead vapor is returned to column 6) Condenser: Cools and condenses the vapor leaving the top of the column 7) Water Line 8) Distillate Release Valve: allows sample to be taken from column. Also controls the reflux ratio.
Example Procedure
1) Open the water line valve. 2) Fill the still pot with the desired concentration of methanol and water mixture. A concentration of 5 mole % is recommended. 3) Turn heat source on. 4) Let the column run for approximately one to two hours to reach steady state. 5) After the column has reached steady state, collect a distillate sample by opening the distillate release valve. 6) Turn the heat source and the water line off. 7) Allow the sample to cool to room temperature. 8) Analyze the sample using a densimeter, a refractometer, or a pychnometer in order to determine the distillate composition. Take multiple measurements on the same sample (if possible) in order to determine the precision of the measurments. Be very careful to make sure the densimeter tube is clean and dry before loading the sample. 9) Allow the still pot and contents to cool. Drain the excess liquid from the still pot. 10) Calibration curves must be created for each analytical instrument.
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Data Analysis
The student should be reminded that the following must be recorded to analyze the results: § Feed composition and amount § Composition of the distillate Reflux ratio § It is advisable to determine the precision of the composition measurement technique. This means that one should take multiple readings of both the reference mixtures (the calibration standards) and the distillate sample. You should compute the population standard deviation to insure that the expected error is not large compared with the composition that you are trying to measure. Report error bars on the measurement and on the computed mole fractions (if possible). The number of measurements you need depends on the desired precision of the quantity being measured. The larger the sample size the higher the precision. References
Diwekar, Urmila. Batch Distillation. Taylor and Francis, United States, (1995) Kister, Henry, Distillation Design. McGraw-Hill, New York, NY (1992) Perry, Robert and Green, Don. Perry’s Chemical Engineering Handbook, Seventh Edition, McGraw-Hill, New York, NY (1997) Stichlmair, Johann and Fair, James. Distillation: Principles and Practice. Wiley-VCH, New York, NY (1998) Wankat, Phillip., Equilibrium Staged Separations. Prentice Hall, Upper Saddle River, New Jersey (1988) http://www.distillationgroup.com/distill.htm http://lorien.ncl.ac.uk/ming/distil/distil0.htm
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Txy data, x and y in mol fractions, T in oC T oC 100 92.39 87.53 84.01 81.48 79.48 77.90 76.56 75.36 74.19 73.16 72.20 71.29 70.45 69.58 68.69 67.83 66.97 66.14 65.31 64.50
x 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
y 0 0.2797 0.4277 0.5233 0.5870 0.6352 0.6723 0.7036 0.7317 0.7592 0.7834 0.8065 0.8287 0.8496 0.8718 0.8946 0.9167 0.9387 0.9597 0.9806 1.00
Kojima, Tochigi, Seki, Watase; Kagaku Kogaku 32, 149 (1969)
With Updates: 1/9/02
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