Elsevier

Fluid Phase Equilibria

Volume 454, 25 December 2017, Pages 22-34
Fluid Phase Equilibria

Experimental studies and thermodynamic analysis of isobaric vapor-liquid-liquid equilibria of 2-propanol + water system using n-propyl acetate and isopropyl acetate as entrainers

https://doi.org/10.1016/j.fluid.2017.09.010Get rights and content

Highlights

  • The separation of 2-propanol + water azeotropic mixture is studied.

  • The entrainers selected are n-propyl acetate and isopropyl acetate.

  • Isobaric VLLE data is measured for 2-propanol + water system with the two entrainers.

  • UNIFAC model predicts the experimental results better than NRTL and UNIQUAC.

  • The feasibility of the entrainers is studied using the residue curve map.

Abstract

Heterogeneous azeotropic distillation is a widely used technique in chemical industries for the purification of alcohols from their aqueous mixtures. This study aims at exploring the possibility of using n-propyl acetate and isopropyl acetate as entrainers for heterogeneous azeotropic distillation to effect the separation of 2-propanol + water azeotropic mixture. A novel yet simple still was constructed to measure the vapor-liquid equilibrium (VLE) and vapor-liquid-liquid equilibrium (VLLE) data for the binary and ternary systems. The isobaric VLE data for the binary systems 2-propanol + water, 2-propanol + n-propyl acetate and 2-propanol + isopropyl acetate and isobaric VLLE data for the ternary systems 2-propanol + water + n-propyl acetate and 2-propanol + water + isopropyl acetate were measured at atmospheric pressure. Subsequently, the experimental data was compared with predictions by thermodynamic models such as UNIFAC, UNIQUAC and NRTL. The feasibility of the entrainers n-propyl acetate and isopropyl acetate in separating the azeotropic mixture 2-propanol + water was studied using the Residue Curve Map (RCM). The experimental data showed the existence of a ternary homogeneous azeotrope in the 2-propanol + water + n-propyl acetate system and a heterogeneous ternary azeotrope in the 2-propanol + water + isopropyl acetate system. Since the heterogeneous azeotropic distillation process depends on the presence of ternary heterogeneous azeotrope, n-propyl acetate is not considered a suitable entrainer for the separation of 2-propanol + water mixture. Finally, a separation sequence for the heterogeneous azeotropic distillation of 2-propanol + water system using isopropyl acetate as entrainer has been proposed.

Introduction

2-Propanol or Isopropyl alcohol is a widely used organic compound in chemical industries because of its extensive use as industrial solvents or as fuel additives. The addition of 2-propanol increases the octane level of the fuels and reduces harmful exhaust pipe emissions. If water is present in these fuel additives, it may lead to phase-splitting, resulting in engine damage. Unfortunately, 2-propanol forms a minimum boiling azeotrope with water due to the presence of unfavorable interaction between alcohol and water. Thus, the separation of 2-propanol from water is a challenging problem and in recent years, many different processes have been studied for the recovery of pure 2-propanol to achieve an efficient lower economic cost process [1], [2], [3], [4], [5], [6].

Distillation is the most widely used technique for the separation of mixtures in chemical industries. But in the case of azeotropic mixtures, separation beyond the azeotropic point by ordinary distillation is not possible as there will be no vapor enrichment beyond the azeotropic point [3]. Hence azeotropic mixtures require special techniques such as utilizing a third component called “entrainer” that can alter or improve the relative volatility of the original more volatile component resulting in their separation. Azeotropic distillation is the most commonly used technique to facilitate the effective separation of such mixtures. If the added entrainer is completely miscible with the original components, it results in a single liquid phase mixture with a homogeneous azeotrope and the process is called homogeneous azeotropic distillation. If the added entrainer is partially miscible with the original components and forms heterogeneous azeotrope preferably with one of the components then the process is called heterogeneous azeotropic distillation. The latter case which combines the presence of azeotropes and liquid-liquid immiscibilities is used extensively in industries to aid difficult separations due to the easy recovery of entrainer used. For the design of heterogeneous azeotropic distillation columns, the experimental vapor-liquid-liquid equilibrium (VLLE) data is required to plan the distillation sequence. The presence of such VLLE data in the literature is limited due to the highly complex experimentation required [3], [7]. In an earlier work [3], n-propyl acetate was identified as an efficient entrainer for heterogeneous azeotropic distillation to separate the azeotropic mixture n-propanol + water. A novel yet simple still was constructed to measure the vapor-liquid equilibrium (VLE) and vapor-liquid-liquid equilibrium (VLLE) data for the binary and ternary systems.

The selection of a suitable entrainer is important in heterogeneous azeotropic distillation to achieve an efficient and cost-effective column design. The added entrainer should induce liquid-liquid split in the system and form a minimum boiling heterogeneous azeotrope preferably with one of the components [3]. n-Propyl acetate and isopropyl acetate are clear and colorless esters formed by the esterification reaction of acetic acid with n-propanol and 2-propanol respectively. The possibility of these esters being an entrainer for heterogeneous azeotropic distillation is very high as they are partially miscible with water. Moreover, these esters have low toxicity and are highly bio-degradable which makes them eco-friendly solvents.

To this end, the present study aims at exploring the possibility of using n-propyl acetate and isopropyl acetate as entrainers for heterogeneous azeotropic distillation to effect the separation of pure 2-propanol from aqueous mixtures. In the first stage, measurements were made for the isobaric VLE data for the binary systems 2-propanol (1) + water (2), 2-propanol (1) + n-propyl acetate (3a) and 2-propanol (1) + isopropyl acetate (3b) and isobaric VLLE data for the ternary systems 2-propanol (1) + water (2) + n-propyl acetate (3a) and 2-propanol (1) + water (2) + isopropyl acetate (3b) at atmospheric pressure. The isobaric VLE data were reported earlier for 2-propanol + water [2], [4], [8], 2-propanol + n-propyl acetate [9] and 2-propanol + isopropyl acetate [10], [11]. There is no VLLE data reported in the literature for the ternary systems 2-propanol (1) + water (2) + n-propyl acetate (3a) and 2-propanol (1) + water (2) + isopropyl acetate (3b) systems.

In the next stage, the experimental data for all the three binary systems measured in the present work were used to correlate the thermodynamic models NRTL and UNIQUAC using the Maximum Likelihood Method (Aspen plus V8.6) to obtain the binary interaction parameters. These binary interaction parameters obtained for NRTL and UNIQUAC models were then used to predict the isobaric VLLE data of the ternary systems 2-propanol + water + n-propyl acetate and 2-propanol + water + isopropyl acetate. Along with these models, VLE and VLLE for the binary and ternary systems were predicted by UNIFAC model using Aspen Plus built-in parameters and compared with the equilibrium data. These model studies indicate the ability and reliability of such models in predicting the VLLE data. Then, the suitability of the entrainers n-propyl acetate and isopropyl acetate in separating the azeotropic mixture 2-propanol + water were studied using residue curve map (RCM). Finally, a separation sequence for the separation of 2-propanol + water mixture using isopropyl acetate was proposed.

Section snippets

Chemicals used

The organic solvents used were 2-Propanol (>0.990 mass fraction, for analysis, Merck), n-Propyl Acetate (>0.990 mass fraction, for analysis, Sigma Aldrich), Isopropyl Acetate (>0.990 mass fraction, for analysis, Sigma Aldrich) and Acetonitrile (HPLC grade, > 0.998, Merck). The reagents were used without purification after gas chromatography (GC) failed to show any impurities. The water used was Millipore (Milli-Q Ultrapure System, Merck). The water content was measured by Karl Fischer

Vapor-liquid phase equilibria

The isobaric vapor-liquid equilibrium (VLE) data of the binary systems 2-propanol (1) + water (2), 2-propanol (1) + n-propyl acetate (3a) and 2-propanol (1) + isopropyl acetate (3b) were measured at the atmospheric pressure and are given in Table 2, Table 3, Table 4 respectively. These tables contain equilibrium temperature (T) in Kelvin, mole fractions of 2-propanol in the liquid (x1) and vapor (y1) phases, activity coefficients of component i and component j (γi, γj), excess Gibbs energy (gE

Conclusions

Heterogeneous azeotropic distillation is a widely used process in the chemical industries for the purification of alcohols from their aqueous mixtures. The isobaric VLE data for the three binary systems 2-propanol + water, 2-propanol + n-propyl acetate and 2-propanol + isopropyl acetate and isobaric VLLE data for the two ternary systems 2-propanol + water + n-propyl acetate and 2-propanol + water + isopropyl acetate were measured at atmospheric pressure. The experimental data showed the

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not – for – profit sectors.

Nomenclature

NT
number of experimental points
GE
excess Gibbs free energy of the mixture (J mol−1)
T
boiling point (K)
P
total pressure of the system (Pa)
R
universal gas constant (J mol−1 K−1)
xk
liquid phase composition of pure component k
yk
vapor phase composition of pure component k

Greek letters

γ
activity coefficient
α
relative volatility

Subscript

i, j, k
components i, j, and k

Superscript

I
organic rich phase
II
aqueous rich phase

References (25)

  • L. Berg, P.E. Bascomb, United States Patent [19], 5,085,739,...
  • J. Pla-Franco et al.

    Isobaric vapor–liquid–liquid equilibria for the ternary systems ethanol + water + propyl acetate and 1-propanol + water + propyl acetate

    J. Chem. Eng. Data

    (2014)
  • Cited by (11)

    • Isobaric vapor – liquid – liquid equilibrium for water + MTBE + alcohol (ethanol or 1-butanol) mixtures

      2020, Fluid Phase Equilibria
      Citation Excerpt :

      The first entrainer used for heterogeneous azeotropic distillation was benzene [10], but it was replaced by other less carcinogenic and more gasoline compatible chemicals being cyclohexane one of the most used entrainer around the world. In order to increase the number of alternative entrainers, some alkanes (e.g., hexane [11–13], heptane [13–15], isooctane [13,16,17], cyclohexane [18,19]), aromatics (e.g., toluene [20–23], 1,4-dimethylbenzene [24]), ethers (di-n-propyl ether [13], and diisopropyl ethers [13,25]), alcohols (e.g., isoamyl alcohol [26], 2-pentanol [26], 2-octanol [27], isobutyl alcohol [25], 2-butanol [28]), acetates (e.g n-propyl acetate [29,30], isopropyl acetate [31]), p-xylene [32], and 2‐methoxyethanol [33] have been tested. An excellent account of the explored entrainers can be found in Refs. [34,35]

    View all citing articles on Scopus
    View full text