Elizabeth Towle Batch Distillation of Ethanol Design
1. Batch Distillation Project
A Project Report for CHE 2014
Submitted to the Faculty of the
Department of Chemical Engineering
Worcester Polytechnic Institute
Worcester, MA 01609
April 14, 2016
Coal Rocks!
“What’s that rock? It’s coal!”
Benjamin Drury
Weiran Gao
Natalie Thompson
Elizabeth Towle
2. Introduction:
Batch distillation has fulfilled many requirements in human life and throughout our history. Whether it be
making alcoholic beverages or producing a few hundred kilomoles with a varying molar concentration,
batch distillation has thoroughly incorporated itself as a necessity for human productivity. This project
required us to utilize batch distillation to separate ethanol from water as a model for a palm oil purification
process to create ethanol fuel in Cameroon. While both batch distillation and continuous distillation have
both advantages and disadvantages, batch distillation was selected for this report as continuous distillation
was already shown to be impractical in previous reports. Furthermore, batch distillation will be useful if
the palm oil has varying ethanol composition or if the process is to be run sporadically. This report
endeavors to analyze the batch distillation as conducted in a test pilot column, using both theoretical and
experimental data.
Methods:
To conduct theoretical analysis, the Rayleigh Equation was first used to analyze the validity of the
experimental data. Several top stage vapor temperatures and their corresponding reboiler temperatures
from the experimental data were chosen as data points for the analysis. These data points were then
converted to compositions in the still pot (xw) and their corresponding distillate compositions (xD) using a
T-x,y graph for an ethanol/water binary system found from a VLE database (Appendix, Figure 1). This
was followed by calculations of the final pot volume (Wfinal) using the Rayleigh Equation:
.nl F
Wfinal
= − ∫
xF
xw, final
dxW
xD−xW
Total distillate volume (Dtot) and average distillate composition, xD, avg were then calculated using
equations:
Dtotal = F - WF
xD, avg= F−Wfinal
F xF−Wfinal xw, final* *
respectively. These calculated data were then compared to the data collected from the column.
The second part of theoretical analysis was conducted using the software Aspen Batch Distillation Version
7.2. This allowed for the analysis of ideal enthalpic condition. This data was run on the basis of an initial
3. charge and composition and a final still pot composition matching the experimental data. The IDEAL
model was used to create an ideal set of data from Aspen, though this is highly unrealistics. Two other
models, the Polymer Soave-Redlich-Kwong Equation-of-State Property Method (POLYSRK) and the
non-random two-liquid (NRTL) model were used to calculate this data. This is a possible source of error,
as neither method will fit the data perfectly. However, this report worked upon the premise that POLYSRK
and NRTL would be more accurate models than the ideal, and the most accurate models available. Varied
reflex ratios were tested to analyze their impact upon the data collected, within the NRTL model. Varied
distillation column diameters were tested within the POLYSRK model. The data calculated was compared
with the experimental data and the data calculated using the Rayleigh equation.
Figure 1: Professional Drawing of Pilot Distillation Column
The figure above represents a multistage batch distillation column. An initial amount of material is fed
into the pot at the bottom and then heated up such that some of the material escapes the pot as a vapor.
The vapor then rises up the column through the stages until it exits through the top of the column in the V
line. From there, the vapor enters a total condenser which converts it to a liquid. The liquid gets divided
into two pipes; some goes back into the column to increase the quality of the product and some gets
extracted as the final product.
4. Results and Discussion:
Calculated and Experimental Data
Table 1: Comparisons of Data Methods
Value Exp. Data Aspen Ideal (D= 1.64 ft) Aspen Nonideal (D= 1.64 ft) Rayleigh
F (kmoles) 2.62988 2.62988 2.62988 2.62988
Wfinal (kmoles) 2.54322 2.55486 2.54875 2.6068
Dtotal (kmoles) 0.04072 0.021017 0.018134 0.023
xD, avg 0.4862 0.523 0.607 0.4744
xF 0.0063 0.0063 0.0063 0.0063
xw,final 0.002183 0.002183 0.002183 0.0022
Time (hrs) 5.55 4.866 5.2
In the above data, the experimental data is compared to the various methods of calculating it, including the
Rayleigh equation, Aspen IDEAL and Aspen POLYSRK. Rayleigh seems to be the closest in both xDavg
and Dtotal. Aspen Ideal is closer in xDavg,but POLYSRK is more accurate in time. NRTL was not included
here as the times calculated were significantly different. However, it will be used later.
In this table WF and F are estimates based on the densities of ethanol and water and the assumption that
they form an ideal mixture. In addition, xF and xw,final used for Aspen analysis were estimated to be equal to
those given by the experimental data. For the Rayleigh calculations, several assumptions were necessary.
First, the pressure was approximated at 1 atm. Values for xF and xw, final were found using T-xy graph with
the initial and final temperatures given in the experimental data. To calculate the integral of the Rayleigh
equation, it was necessary to use the trapezoidal method, an imprecise method. Another source of error
may be the estimated initial charge, as the increased operational temperatures may mean that the densities
used were imperfect. Another point of difference between the Raleigh calculated values and the Aspen DT
values is due to the fact that Aspen accounts for holdup in the column while the Rayleigh equation
assumes that the amount of distillate received is equal to the amount of initial charge lost in the distillation
process. Finally the T-x-y graph is read by hand, which is an imprecise method.
5. Table 2: Comparisons of Experimental and POLYSRK Aspen Model
Temperature Value Aspen Calculated (o
C) Experimental (o
C)
Top Stage Initial 81 79
Top Stage Final 94.6 93
Still Pot Initial 102.3 98
Still Pot Final 102.55 99.7
It was interesting, though not unexpected, to see that the theoretical and experimental temperatures varied
for each section. Primarily, Aspen is showing higher temperatures, most likely because it does not account
for heat loss to the environment, as the pilot plant is not likely to be perfectly adiabatic. Other contributing
factors include assuming no interaction occurred between the water and ethanol molecules, measuring
inaccurate height and diameter of the column, and assuming an inaccurate reflux ratio. The Aspen data also
represents a total change in temperature of 0.002ºC which is not accurate as the temperature should change
drastically as the process goes on. This could be because our chosen property method, POLYSRK, was
not accurate for our system, because there was pressure buildup in the column, or that any other
assumptions we made are not accurate. However, it was the most accurate calculator of time. Therefore,
the model POLYSRK was used for non ideal Aspen calculations. Below, diameter was varied to
demonstrate the effects of adjusting this variable on other variables, especially xDavg and time.
Table 3: Variations in Diameter in the Non-ideal POLYSRK Aspen Model
Value D = 1.64 ft D = 4 ft D = 6 ft
F (kmoles) 2.62988 2.62988 2.62988
Wfinal (kmoles) 2.54875 2.53332 2.52471
Dtotal (kmoles) 0.018134 0.042563 0.05117
xD, avg 0.607 0.259 0.216
Time (hrs) 5.2 4.7 3.5
6. Table 4: Variations in Reflux Ration in the Non-ideal NRTL Aspen Model
Reflux Ratio Time (hrs) xDavg Still Pot Final (o
C)
1 0.291 0.524 102.673
2 0.459 0.501 102.674
3 0.610 0.493 102.674
The data above were taken from another attempt of Aspen analysis using the NRTL model. Using the
NRTL Method, a similar temperature compared to both the IDEAL and POLYSRK models, though no
closer to the experimental data. The xDavg was slightly closer to the xDavg found experimentally, however the
time obtained was significantly different. However, in a further attempt to approximate the xDavg obtained
experimentally, the reflux ratio was modified. As is visible in the table above, increasing the reflux ratio
from the given value of 2 did bring the xDavg to nearly the experimental value. However, this only extended
the time to 0.610 hours, still an order of magnitude away from the experimental time.
Conclusions and Recommendations:
The analysis provided in this report suggests that the process can be modeled successfully. It was found
that the data from Aspen, the experimental data, and the Rayleigh Equations were all within an order of
magnitude. The POLYSRK Aspen Model was the closest to the experimental data in some regards, and the
calculated Rayleigh values were more accurate in other respects. Based on this similarity in values and
considering the fact that these methods of calculating the required data each used different assumptions,
this process is recommended as a basis for the final column design.
10. Aspen Calculations
The following data is from the POLYSRK (R=2, D=1.64ft) Model as this was deemed the most accurate.
Input Values
Set up:
Configuration
Number of stages : 11
Valid phases : Vapor-Liquid
Pot Geometry
Orientation : Vertical
Top head type : Hemispherical
Bottom head type : Hemispherical
Diameter : 1.64 ft
Height : 8 ft
Volume : 0.544 m3
Pot Heat Transfer
Jacket heating
Jacket covers bottom: True
Jacket bottom height : 0.0 ft
Jacket top height : 0.05 ft
Jacket Heating
Heating option : Steam Heating
Steam Specification: Pressure
Heat Transfer Coefficient: 0.5152 kW/m/K
Steam Pressure: 7.5 psig
Limiting Method: Latent Heat
Steam Mass Flow Rate: 0.0028 kg/s
Condenser
Condenser type : Total
Reflux
Reflux specification : Reflux ratio
Reflux ratio : 2
Reflux drum present : False
Pressure Holdups:
Pressure
Pressure profile and holdups : Fixed
Condenser pressure : 1.01325 bar
Column pressure drop : 0.1 bar
11. Holdups
Start stage: 2
End stage: 10
Holdup: 0.006 kmol
Receivers:
Distillate
Number of distillate receivers : 1
Receiver taking liquid distillate : 1
Receiver taking vapor distillate : 1
Initial Conditions:
Main
Initial condition : Total reflux
Initial Charge
Composition basis : Mole-frac
Total initial charge : 2.62988 kmol
Component Fresh Charge Composition
ETHANOL: 0.0063
WATER: 0.9937
Operating Steps:
Operating Step distil Status : Complete Active : True
Changed Parameters
Location: Reflux splitter; Reflux ratio
Reflux splitter; Reflux ratio; 2
Jacket Heating; Steam Pressure; 7.5 psig
End Condition
Step end condition : Trigger value
Variable location: Pot
Variable: Liquid mole fraction
Component: Ethanol
Approach from: Approach from below
Value: 0.002 kmol/kmol
12. Table 7: Temperature Profile History
Table 8: Distillate Composition (Ethanol) History
