Your Descriptive Title
Report Type:Planning
Author:Ming Ni and Dennis Vajradhirata
Submitted:1/9/00
ChemE 437 Section:1/10/00
Team:D
Leader:Ming Ni
Evaluator:Dennis Vajradhirata
Introduction
Objectives
The objectives of this experiment are
 To study the characteristics of a bubble-cap column operating under total reflux with a binary mixture.
 To run steady state tests on the UO lab’s distillation column.
 To determine the effect of boil-up rate on Murphree vapor efficiency (EMV) of the two bubble-cap
trays.
Background
Distillation is known as a separation process to separate a liquid mixture into all or a part of its constituents
by heating the mixture and recovering the vapors by condensation. The degree of separation is mainly
depending on the properties of its constituents, such as the boiling points and vapor pressures. A
separation cannot be achieved when relative volatility, 1,2=1.0, whereas 1,2=PS1/PS2. For a binary system,
a more volatile component is called light key, LK, and a less volatile component is called heavy key, HK.
There are two kinds of distillation column: trayed and packed column. In a tray column, the essential parts
of this distillation system are: 1) total condenser, 2) trayed column and 3) partial reboiler. Each tray is
considered as an equilibrium stage. The partial reboiler also acts as an equilibrium stage. The vapor rate
leaving the reboiler is the boilup[1]. The total condenser is not considered as an equilibrium stage. After
vapor condensed to liquid in the total condenser, the liquid will enter the top stage with the external reflux
rate L0. The reflux ratio R is defined as L0/D, where D is distillate rate. For total reflux, D = 0. Therefore,
reflux ratio R = .
Murphree vapor efficiency (E MV) is about the efficiency of an individual plate. It is defined as the ratio, in
per cent, of the actual change in vapor composition to the change that would occur if complete vaporliquid equilibrium were reaches. (adapted from Zimmerman O.T and Lavine I. [2])
The Murphree vapor efficiency (EMV) can be expressed as:
E MV 
y n  y n1
 100
y n*  y n1
where EMV = Murphree vapor efficiency.
yn = mole fraction of the more volatile constituent in the vapor leaving plate n.
yn+1 = mole fraction of the more volatile constituent in the vapor rising to plate n.
yn* = mole fraction of the more volatile constituent in the vapor leaving plate n if the vapor were in
equilibrium with the liquid leaving that plate.
No relevant data exist. The most commonly used binary mixtures are: (1) benzene - toluene, (2) ethanol water, (3) acetic acid-water, (4) chloroform-toluene, and (5) carbon tetrachloride- benzene. [2]
I used SciFinder and Web of Science for the literature searching.
Materials and Methods
Equipment
This distillation unit in the lab consists of a four-plate bubble-cap column, a partial reboiler, a total
condenser and accessories. All the four plates need to be analyzed to get Murphree vapor efficiency (E MV)
of each individual plate. Under the condition of total reflux, there are no feed, no distillate or bottoms
product. Therefore, it is easy to achieve a steady-state operation.
Assembly of bubble-cap column is shown schematically as Figure-1. The column is fairly safe with the
liquid level above the critical line, which is also meaning that the liquid level in the condensate drum is not
too high. The bottoms tank pressure should remain below 35 psi, otherwise the column will be extremely
hot.
The composition of liquid and vapor samples at each plate is measured by refractometry.
Materials
The materials used in this experiment are:
 Iso-propanol
Density
0.789 g/cm3
Boiling Point
82.5  C
 Isobutanol (2-methyl propanol)
Density
0.805 g/cm3
Boiling Point
107.5  C
Procedures
This procedure is copied from UO lab.
Start-up procedures for distillation column
1.
2.
3.
4.
5.
6.
7.
8.
Turn on condenser water including opening the Main Water Supply valve and the Product Cooler
Water valve.
Open Heat Exchanger Bypass, while three-way valve on the water/stream exit pipe to the drain is
closed.
After the condenser water is on, open Heat Exchanger Steam valve completely. Slowly open the main
Steam Line until it is fully open. Slowly turn the steam pressure regulator handle clockwise to deliver
about 10 lbs. Of steam to reboiler heat exchanger.
After most of the condensate has been removed from the steam coils, close the Bypass valve. Also
turn on the cooling water so that the steam condenses to water before entering the drain.
When running at total reflux, keep the distillate rotameter closed.
Close the reflux valve, start the reflux pump; and open the reflux valve slowly or else the float will get
stuck in the top of the rotameter and make flow measurements difficult.
Turn on the temperature recorder.
Adjust steam pressure using the steam pressure regulator. Set the pressure to be 20  25 lbs.
Shut Down Procedure (allow 30 minute for column to cool)
1.
2.
3.
4.
5.
6.
Gradually decrease pressure to 0 lbs. By slowly turnings steam pressure regulator handle counter –
clockwise. Stop at 0 lbs. In order to prevent spring inside the regulator from uncoiling too much.
Turn off the Main Steam and Heat Exchanger Steam.
Close the blue cooling water valve that was used to cool the exit steam condensate.
Close the reflux and turn off the reflux pump.
Turn off the temperature recorder.
Leave the condenser water on until the reflux pipe entering the condenser heat exchanger is feels cool.
Discussion
Composition Data
After running under the condition of steady state, samples will be taken from the top to the bottom of the
column, first vapor sample, then the liquid sample. The compositions of samples will be determined using a
refractometry. A calibration curve can be obtained by varying composition of isopropanol / isobutanol
mixture. Thus, we can determine composition by using the calibration curve. In addition, we can use an
approximate equation as followed[4]:
xa 
n  nb
na  nb
where xa = volume composition of a;
na= refractive index of pure a;
nb= refractive index of pure b;
n= refractive index of sample.
The equilibrium vapor mole fraction, y*i,n, can be read directly from x-y diagram by using NRTL equation.
The diagram is given in the lab, we can plot the diagram using the given data and fit the data using EXCEL.
Errors
Errors associated with this experiment are:
 Taking sample at different time.
 There were some fluctuations of temperature (+0.7F).
 The percentage reflux fluctuated around +0.03% during the experiment.
 Condensate level was too high causing non uniform composition of the condensate
 There is an instrumental error when taking measurements of refractive index
 This unit operation should run as an adiabatic process. There is a heat loss during the process and this
may affect the EMV.
 Subcooled Reflux
Rint = R ( 1 + ( Cp,liq Tsub /  Hvap ) )
If you forget to use Rint, you get too small of a slope for both Rectifying and Stripping Operation-line
and you predict too many stages. If you are trying to predict stage efficiencies, you will over estimate
EMV.
Alternative Approach
Alternative approach we considered is:


Hydrocarbons : Drickamer-Bradford EO = 13.3 - 66.8 Log10 [(cP)]
In general : O'Connell EO = 50.3 ( LK,HK (cP) )-0.226
When the equilibrium and operating lines are straight, but not necessarily parallel, overall efficiency can be
related to Murphree Efficiency by:
E0 
log 1  E MV (λ  1)
logλ
Literature Cited
[1] Seader, J.D. and Henley, E.J. Separation Process Principles. John Wiley & Sons, Inc., New York.
1998.
[2] Zimmerman O.T. and Lavine I. Chemical Engineering Laboratory Equipment, Dover, N.H., Industrial
Research Service, 1955
[3] Perry, Robert H. Perry's Chemical Engineers' Handbook, 7th Edition. McGraw-Hill, New York. 1997.
[4] Meites, Louis Handbook of analytical chemistry New York, McGraw-Hill, 1963