Designing a Separations Process Without VLE Data
by Thomas Schafer - Koch Modular Process Systems, LLC
This presentation utilizes as it’s example a problem presented to
KMPS by a pharmaceutical client who was incinerating a valuable
solvent stream.
Components to be separated - Toluene from Acetic Anhydride
Problem Definition - Customer Objectives & Physical Properties
PROCESS OBJECTIVES
COMPONENT
Table 1
FEED
Toluene , wt %
40
Acetic Anhydride, wt %
60
Recovered Toluene
Product
Recovered Acetic
Anhydride Product
99.99%
< 100 ppm acetic
anhydride or acetic
acid
1%
99%
PHYSICAL PROPERTIES
PROPERTY
Table 2
Molecular Weight
Boiling Point, at 760 torr, oC
Antoine Constants: A
LN(VP)=A-B/(T+C): B
VP(=)mmHg, T(=)K: C
Water Solubility, g/100 g H2O
Liquid Density, g/cm3
Melting Point, oC
COMPONENT
Toluene
Acetic Anhydride
92.0
110.8
16.0137
3,096.52
-53.67
0.05
0.866
-95.0
102.0
139.6
16.3982
3,287.56
-75.11
12.0
1.082
-73.0
Approaches That Should Be Considered When VLE Data is Not Available
Engineers may choose from the following alternatives:
1. Assume the compounds form an ideal solution, which means vapor pressure
vs. temperature data can be used to predict VLE. This is generally acceptable
when the compounds are closely related, such as members of a homologous series.
Examples include linear alcohols, paraffinic hydrocarbons, aliphatic substituted
benzene (benzene, toluene, xylene), polymeric glycols.
2. Find VLE data for an analogous system, one that contains one of the compounds of
the pair. The 2nd compound should be closely related to the other compound of the
pair, containing the same or similar structure and functional groups. An example of
this technique would be to use liquid activity coefficients of benzene and ethanol to
predict VLE for benzene and propanol.
3. Develop VLE data for key pairs of components. Set up a VLE apparatus to test
each component pair. Data developed this way can be regressed to provide
interaction coefficients which can then be used in a process simulator to
explore a range of design alternatives.
Analogous Systems Data
The literature was then searched for VLE data for an analogous system. Data
for benzene-acetic anhydride (Figure 1A) and cyclohexane-acetic anhydride
(Figure 1B) were found in Dechema. These data clearly indicate non-ideality.
Figure 1A
Figure 1B
Conclusions Drawn From Analogous System Data
After analyzing the analogous system data, an engineer should expect that the tolueneacetic anhydride system will exhibit similar but more severe non-ideality because toluene
boils closer to acetic anhydride than either cyclohexane or benzene.
Two initial predictions of the VLE curve for toluene-acetic anhydride were made by using
the benzene-acetic anhydride and cyclohexane-acetic anhydride NRTL coefficients, with toluene
vapor pressure data. The predicted curves are plotted in Figure 2. The data indicate that azeotropic
behavior is probable.
Figure 2
Since the plotted data showed significant non-ideality it was decided that generating VLE data was
the preferred alternative.
VLE Apparatus
To generate the VLE data, a simple, inexpensive apparatus was constructed of glass and
polytetrafluoroethylene components, similar to the design shown in Figure 3. It is critical
that the apparatus yield exactly one theoretical stage.
Figure 3
Calibration of VLE Appartus with Known System
After assembly of the apparatus shown in Figure 3, a known system was checked to ensure
that the apparatus will yield exactly one theoretical stage. The known system should boil
in a similar temperature range to the experimental system. Internal condensation must be
avoided as it can result in up to two theoretical stages in the test apparatus. Data for
ethanol-water was then compared to literature data as shown in Table 3. As can be seen,
the test system results in almost exactly one theoretical stage.
ETHANOL-WATER SYSTEM
EXPERIMENTAL
REFERENCE EXPERIMENTAL
Ethanol in
Ethanol in Vapor Ethanol in Vapor
K Value
Liquid m.f.
m.f.
m.f.
0.1414
0.1598
0.2414
0.5095
0.5049
0.5627
0.4911
0.5055
0.5466
Table 3
3.60
3.16
2.33
REFERENCE
K Value
3.47
3.16
2.26
Experimental Toluene-Acetic Anhydride Data
Using the calibrated experimental setup, VLE data
for toluene-acetic anhydride was generated over a
range of compositions. The data is shown in
Table 4. More data was collected at the toluene
rich end of the curve due to predictions from the
analogous systems that there may be an azeotrope
or possibly an asymptote in the VLE curve.
TOLUENE-ACETIC ANHYDRIDE SYSTEM
MOLE FRACTIONS
LIQUID
Temp.
(oC)
Acetic
Anhydride
Toluene
107.5
107.5
108.0
108.1
108.5
111.0
0.02835
0.05673
0.08502
0.12518
0.19692
0.37074
0.97165
0.94327
0.91498
0.87482
0.80308
0.62926
VAPOR
Acetic
Toluene
Anhydride
Relative
Volatility
0.03010
0.05278
0.07973
0.10000
0.13114
0.18605
0.94030
1.07926
1.07253
1.28777
1.62467
2.57750
0.96990
0.94722
0.92027
0.90000
0.86886
0.81395
Table 4
The data was then regressed and
NRTL coefficients were derived.
A smooth curve was then
developed for the system using the
NRTL coefficients as shown
in Figure 4.
Figure 4
Azeotrope Found
A minimum boiling azeotrope was calculated at 96 mole% (95.6 wt%) toluene and 4 mole%
acetic anhydride. Figure 5 is an enlarged plot of the toluene-acetic anhydride VLE curve in
the range of 95-100 mole% toluene.
Figure 5
Because the components form an azeotrope, it is not possible using simple distillation to
separate the components into pure acetic anhydride and pure toluene.
Process Design
Given the feed composition, a single distillation column is adequate to recover relatively pure
acetic anhydride as a bottoms product and a mixture which approaches the azeotropic
composition as a distillate. Approximately 93% of the acetic anhydride was recovered in
one pass through this distillation column. Some design parameters for the distillation
column are:
Theoretical stages
Packing Type
Packed Height
Reflux Ratio
19
Flexipac® 2Y
33 ft.
1.4
Distillate Composition
Bottoms Composition
91 wt% Toluene
99 wt% Acetic Anhydride
The toluene in the distillate was recovered by water extraction to remove the small amount
of acetic anhydride. Figure 6 is a photo of a modular process system that was built to
perform the separation described. The resulting process recovers 92% of the acetic
anhydride and 99% of the toluene from a stream that was previously incinerated.
Figure 6
Modular Separation System
for Recovery of
Toluene & Acetic Anhydride