Advanced Process Control and Simulation for Chemical Engineers

Some Aspects of Phase

Equilibria Behavior and

Verification

10

CONTENTS

10.1 Introduction ..................................................................................................... 113

10.2 Discussion and Results .................................................................................... 114

10.3 Conclusion ....................................................................................................... 118

Keywords ................................................................................................................. 119

References ................................................................................................................ 119

10.1 INTRODUCTION

Liquid-liquid equilibrium data are essential for the comprehension of extraction pro-

cess, and solvent capacity, solubility, selectivity, and other extraction variable can be

estimated from these data. The efficient separation of organic acids and alcohols from

aqueous solution is an important subject in chemical fermentation industries and many

solvent have been used to improve the recovery processes [1, 2].

The phase behavior experimental data for (water + carboxylic acid + 1-octanol)

and (water + carboxylic acid or alcohol +1-hexanol) systems were reported by Senol

[3, 4]. Letcher and Redhi [5] studied the phase behavior of (butanenitrile + a carbox-

ylic acid + water) systems and Bilgin et al. [6] tested the ternary system behavior of

(water + propionic acid + alcohol) systems.

Certain liquid-liquid equilibrium data of the ternary systems were also investigated

by non-random two liquid (NRTL) [7], universal quasi chemical theory (UNIQUAC)

[8], universal functional group activity coeffi cient (UNIFAC) [9], Dortmund modifi ed

UNIFAC [10] models. The experimental data [3, 4] were used in order to analysis the

liquid-liquid equilibrium behavior in this chapter. The experimental data [3, 4] were

compared with Aspen Plus 11.1.

The main purpose of this chapter is to verify the phase equilibria behavior for

ternary liquid systems, using different models. The NRTL [7] models for strongly

non-ideal mixture, and especially for partially immiscible systems, the NRTL equation

often provides a good representation of experimental data if the adjustable parameters

obtained by enough attention using experimental data. The UNIQUAC [8] equation

is applicable to a wide variety of non-electrolyte liquid mixture containing non-polar

114 Advanced Process Control and Simulation for Chemical Engineers

or polar fl uids such as hydrocarbons, alcohols, nitriles, ketones, aldehydes, organic

acid, and water including partially miscible mixture. With only two adjustable binary

parameters, it cannot always represent high quality data with high accuracy, but for

many typical mixtures encountered in chemical practice, UNIQUAC provides a satis-

factory description.

The UNIFAC [9] activity coeffi cient model is an extension of the UNIQUAC

model. It applies the same theory to functional groups that UNIQUAC uses for mol-

ecules. A limited number of functional groups are suffi cient to form an infi nite num-

ber of different molecules. Group-group interactions determined from a limited, well

chosen set of experimental data are suffi

cient to predict activity coeffi cients between

almost any pair of components. The UNIFAC (Fredenslund et al., 1975, 1977) can be

used to predict activity coeffi cients for VLE. For LLE a different dataset must be used.

Mixture enthalpies, derived from the activity coeffi cients are not accurate. This model

can be applied to VLE, LLE, and enthalpies (Larsen et al., 1987). Another UNIFAC

modifi cation comes from the University of Dortmund (Germany). This modifi cation

is similar to Lyngby modifi ed UNIFAC, but it can also predict activity coeffi cients at

infi nite dilution (Weidlich and Gmehling, 1987). The UNIFAC modifi cation by Gmehling

and coworkers (Gmehling et al., 1993, Weidlich and Gmehling, 1987), is slightly dif-

ferent in the combinatorial part [10].

10.2 DISCUSSION AND RESULTS

The compositions of experimental data for the ternary mixtures of (water + acetic

acid +1-hexanol) and (water + acetic acid + 1-octanol) obtained at T = 293.15K,

atmospheric pressure are shown in Tables 1 and 2 [3, 4]. The corresponding tri-

angular diagrams for the ternary mixtures of (water + acetic acid +1-hexanol) and

(water + acetic acid + 1-octanol) are presented in Figures 1, 2, 3, and 4. The NRTL,

UNIQUAC, UNIFAC, and Dortmund modified UNIFAC equations of state were

used in order to compare with the experimental data. The comparison between

calculated and experimental compositions (water + acetic acid +1-hexanol) can be

seen in Figures 1 and 2.

TABLE 1 Experimental data compositions (mass fraction) at T = 293.15K

[3].

Water-rich 1-hexanol-rich

w

1

w

2

w

1

w

2

Water (1) + Acetic acid (2) + 1-hexanol (3)

0.9922 0 0.0305 0

0.9624 0.0296 0.0383 0.0198

0.9022 0.0889 0.0514 0.0648

0.8553 0.1345 0.0705 0.1032