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

PHYSICAL PROPERTIESCOMPONENT

PROPERTY Toluene Acetic Anhydride

Molecular Weight 92.0 102.0Boiling Point, at 760 torr, oC 110.8 139.6Antoine Constants: A 16.0137 16.3982LN(VP)=A-B/(T+C): B 3,096.52 3,287.56VP(=)mmHg, T(=)K: C -53.67 -75.11Water Solubility, g/100 g H2O 0.05 12.0Liquid Density, g/cm3 0.866 1.082Melting Point, oC -95.0 -73.0

Table 2

Table 1

PROCESS OBJECTIVES

COMPONENT FEEDRecovered Toluene

ProductRecovered Acetic

Anhydride Product

Toluene , wt % 40 99.99% 1%

Acetic Anhydride, wt % 60< 100 ppm acetic

anhydride or acetic acid

99%

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 pressurevs. temperature data can be used to predict VLE. This is generally acceptablewhen the compounds are closely related, such as members of a homologous series.Examples include linear alcohols, paraffinic hydrocarbons, aliphatic substitutedbenzene (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 thepair, 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 topredict VLE for benzene and propanol.

3. Develop VLE data for key pairs of components. Set up a VLE apparatus to testeach 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 DataThe literature was then searched for VLE data for an analogous system. Datafor benzene-acetic anhydride (Figure 1A) and cyclohexane-acetic anhydride(Figure 1B) were found in Dechema. These data clearly indicate non-ideality.

Figure 1BFigure 1A

Conclusions Drawn From Analogous System Data

After analyzing the analogous system data, an engineer should expect that the toluene-acetic anhydride system will exhibit similar but more severe non-ideality because tolueneboils closer to acetic anhydride than either cyclohexane or benzene.

Two initial predictions of the VLE curve for toluene-acetic anhydride were made by usingthe 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.

Predicted Toluene/Acetic Anhydride VLE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Mole Fraction Toluene in the Liquid

Y=X

Benzene/AA NRTL's

Cyclohexane/AA NRTL's

Since the plotted data showed significant non-ideality it was decided that generating VLE data wasthe preferred alternative.

Figure 2

Figure 3

VLE ApparatusTo generate the VLE data, a simple, inexpensive apparatus was constructed of glass andpolytetrafluoroethylene components, similar to the design shown in Figure 3. It is criticalthat the apparatus yield exactly one theoretical stage.

ETHANOL-WATER SYSTEMEXPERIMENTAL REFERENCE EXPERIMENTAL REFERENCE

Ethanol in

Liquid m.f.

Ethanol in Vapor

m.f.

Ethanol in Vapor

m.f.K Value K Value

0.1414 0.5095 0.4911 3.60 3.470.1598 0.5049 0.5055 3.16 3.160.2414 0.5627 0.5466 2.33 2.26

Calibration of VLE Appartus with Known System

After assembly of the apparatus shown in Figure 3, a known system was checked to ensurethat the apparatus will yield exactly one theoretical stage. The known system should boilin 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.

Table 3

Table 4

Experimental Toluene-Acetic Anhydride DataUsing the calibrated experimental setup, VLE datafor toluene-acetic anhydride was generated over arange of compositions. The data is shown inTable 4. More data was collected at the toluenerich end of the curve due to predictions from theanalogous systems that there may be an azeotropeor possibly an asymptote in the VLE curve.

TOLUENE-ACETIC ANHYDRIDE SYSTEMMOLE FRACTIONS

LIQUID VAPOR

Temp.

(oC)

Acetic

AnhydrideToluene

Acetic

AnhydrideToluene

Relative

Volatility

107.5 0.02835 0.97165 0.03010 0.96990 0.94030107.5 0.05673 0.94327 0.05278 0.94722 1.07926108.0 0.08502 0.91498 0.07973 0.92027 1.07253108.1 0.12518 0.87482 0.10000 0.90000 1.28777108.5 0.19692 0.80308 0.13114 0.86886 1.62467111.0 0.37074 0.62926 0.18605 0.81395 2.57750

Figure 4

The data was then regressed and NRTL coefficients were derived. A smooth curve was thendeveloped for the system using theNRTL coefficients as shown in Figure 4.

Toluene/Acetic Anhydride VLE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Mole Fraction Toluene in the Liquid

measured data

calculated data

Y=X

Figure 5

Toluene-Acetic Anhydride VLE

0.950

0.955

0.960

0.965

0.970

0.975

0.980

0.985

0.990

0.995

1.000

0.950 0.955 0.960 0.965 0.970 0.975 0.980 0.985 0.990 0.995 1.000

Mole Fraction Toluene in the Liquid

Azeotrope FoundA 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.

Because the components form an azeotrope, it is not possible using simple distillation toseparate the components into pure acetic anhydride and pure toluene.

Process DesignGiven the feed composition, a single distillation column is adequate to recover relatively pureacetic anhydride as a bottoms product and a mixture which approaches the azeotropic composition as a distillate. Approximately 93% of the acetic anhydride was recovered inone pass through this distillation column. Some design parameters for the distillation column are:

Theoretical stages 19Packing Type Flexipac® 2YPacked Height 33 ft.Reflux Ratio 1.4

Distillate Composition 91 wt% TolueneBottoms Composition 99 wt% Acetic Anhydride

The toluene in the distillate was recovered by water extraction to remove the small amountof 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