Distillation
Introduction Distillation is the thermal process used to separate or purify liquid mixtures. There are two big categories of distillation: simple distillation based on one-stage equilibrium and the fractional distillation (fractionation) based on multiple-stage equilibrium. In our laboratory exercise, we will be doing a multiple-stage equilibrium distillation of biofuel-water mixture. The objective here is to distill ethanol from the mixture to produce concentrated ethanol, which can be further processed to be used as biofuel or biochemical. A schematic diagram of a multiple-stage fractional distillation is shown in Figure 1. Here the feed F
(mol/h) at a known initial concentration of ethanol (mole fraction) (xF) enters the distillation column at some intermediate tray (stage). The boiled vapor and the liquid are in equilibrium on each tray in the column, the vapor moving up the column and the liquid down the column through the sieve trays ensuring good contact between the two reaching equilibrium. The distilled vapor is condensed (using cold water in heat exchanger) in a condenser in the top, part of it is recycled back to the column (L (mol/h)) and the distillate product D (mol/hr) at a higher concentration of ethanol (xD) is removed from the top. The majority liquid that is flowing down the column is heated in a reboiler and the vapor returned back to the column while removing some portion as bottom product W (mol/h) at an ethanol concentration (xw).
Figure 1. Multiple-stage fractional distillation (single) column with reflux
Figure 2 is another schematic of the fractional multiple-stage distillation showing the trays inside and the vapor and liquid flows. Here, the section above the feeding position is call enriching section while the section below the feeding position is call stripping section. There are several sieve trays installed inside. The vapor and the liquid from each stage/tray flow countercurrent to each other. The liquid in a stage flows to the stage below and the vapor from a stage flows through the pores of sieve upward to the stage above. Hence, in each stage a vapor stream and a liquid stream are mixed, and a vapor and a liquid stream leave in equilibrium.
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Figure 2. Fractional distillation with sieve trays or stages Some of the key assumptions in distillation are that there is equilibrium between the vapor and liquid leaving the tray; for ideal mixtures this can be described by Raoult’s law. The latent heats of vaporization of the components being distilled are approximately the same. So, the energy needed for vaporizing 1 mol of component A is the same as energy released from condensing 1 mol of component B. This results in a constant equimolar counter flow assumption in distillation. Figure 3 shows the vapor liquid equilibrium diagram for ethanol-vapor mixture.
Figure 3. Vapor Liquid Equilibrium (VLE) diagram for ethanol-water system. x is the mole fraction of ethanol in the liquid phase and y is the mole fraction of ethanol in the vapor phase – they both are in equilibrium. Blue is the equilibrium line and Red is the 45◦ line.
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As you can see in Figure 3 above about 0.8 mole fraction of ethanol in the liquid mixture (x) there is an azeotrope formed; beyond which the mole fractions of ethanol in the liquid and vapor phases are the same. Hence, they cannot be further purified or concentrated. For an ideal system, using Raoult’s law the relationship between mole fractions and partial pressures for a binary system can be given as,
yA = xA PA*/P
yB = xB PB*/P
Where xA, xB are the mole fractions of components A and B in the liquid phase in a binary mixture Where yA, yB are the mole fractions of components A and B in the vapor phase in a binary mixture PA* and PB* are the saturation vapor pressures of pure component A and B at a given temperature P is the total pressure in the vapor phase. General format of above equations:
* yi =Pi /P= xiPi /P
where yi is the mole fraction of component i (A or B) in the vapor phase.
For a binary mixture of A and B, the relative volatility of component A with respect to B is defined as:
/ / / 1 / 1
∗ ∗ So, /
1 1 where (For simplicity, the subscript “AB” is usually omitted for a binary system).
Separation factor
The separation factor of ethanol (over water) for the distillation is defined as
/ /
where WEtOH and WH2O are concentrations of ethanol and water (in wt%) in the top product (D) and feed (F), respectively.
Overall Material Balance
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Using the nomenclature used in Figure 1, the overall material balance and component balance for the whole distillation column can be given as follows: F = D + W (overall total material balance)
FxF = DxD + WxW (overall material balance on A)
The overall material balance equations can be used to determine two unknown variables of D, xD , W, xW (F and xF are usually given)for designing distillation columns. For the distillation laboratory experiment, D, xD, W, xW are measured, (F and xF are known, can also be measured) and the overall material balance and component balance can be used to verify the accuracy of your measurements.
An important operating parameter Reflux Ratio (R) is defined as R = L/D where, L= amount of the top condensate that is recycled back to the column (mol/h) D= amount of the top condensate that is removed from the top (mol/h) For more detailed description on the theory and operation of distillation column please see the more detailed notes as well as the reference book. Calculations and Reporting 1) In our laboratory exercise, we will be conducting multiple-stage distillation of ethanol-water mixture at a known initial concentration in the feed xF (approximate initial concentration is known, but still need to analyze its accurate concentration), constant feed flow rate F (mol/h), reboiler temperature (T) and at a reflux ratio (R). After reaching equilibrium, you will be measuring the temperature profile across the various stages including the top and bottom, pressure drop across the column and the amount of energy supplied to the heater in the reboiler Q (J/h). We will be taking samples of the feed, top product (distillate), bottoms product and measuring their ethanol concentration using specific gravity and Gas Chromatography measurements. We will also be measuring the flow rate of the top product and the bottom product. 2) To check concentrations and top and bottom product flow rate, conduct an overall material balance of the system and a component balance for ethanol and water separately and show that they are in balance. If not, explain why there may be a discrepancy. 3) Based on the temperature profile and pressure data measured and using the Txy data of the ethanol- water system (data of the boiling point diagram, given in the file “TxyEthanolWaterSystem.xlsx”) for ethanol-water mixture, calculate and plot the equilibrium concentrations of ethanol and water at various stages of the distillation column including the top condenser and the bottom. Compare the concentrations of the top condenser and the bottom against the experimentally determined values (measured by GC etc.). 4) Determine the relative volatility of the ethanol-water system at various stages of the distillation column using the vapor and liquid mole fractions calculated in 3). 5) Calculate the Separation Factor for the distillation column after reaching equilibrium under these experimental conditions.
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5) Another complete experimental data set obtained at reflux ratio different from yours will be provided to you. Conduct the above calculations at two different reflux ratios (R), compare the performance, and determine the effect of reflux ratio on the overall distillation column performance. Some of the parameters used to compare performance include concentrations (mole fractions) in the top and bottom products, heat duty supplied to the reboiler and top and bottom product flow rate. NOTE that the reflux percent (R%) shown on the display panel and in the data acquisition software of our distillation system is the total condensate of the top condenser, i.e., R% = L/(L + D) * 100. You need to convert all R% values into R (= L/D), which is the real reflux ratio (here on mass basis).
Files to submit include:
(1) Your Full report in the name of YourGroupName_Distillation_YourName.pdf, e.g., Group3_Distillation_John.pdf
(2) Your excel file in the name of, e.g., Group3_Distillation_John.xls
(3) Raw data in the name of, e.g., Group3_Distillation_Rawdata.xls (one group only need to submit one raw data file, but each of you can always submit it and I’ll keep one copy only)
References [1] Distillation Laboratory Experiment manual, GUNT [2] Christie J. Geankoplis, Transport Processes and Separation Process Principles 4th Edition [3] Distillation Theory and Operation class notes
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