Isobaric vapour–liquid equilibrium of (tert-butanol + water) system with biological buffer TRIS at 101.3 kPa

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Highlights

  • The isobaric VLE data of tert-butanol + water + TRIS were measured at 101.3 kPa.

  • The effect of TRIS on the azeotropic behaviour of tert-butanol + water was investigated.

  • The experimental VLE data were well correlated with the NRTL-HOC model.

Abstract

Isobaric vapour–liquid equilibrium (VLE) data were measured for the binary system of {tert-butanol (1) + water (2)} and the ternary system of {tert-butanol (1) + water (2) + tris(hydroxymethyl)aminomethane (TRIS) (3)} at 101.3 kPa by using a modified recirculating type of Othmer equilibrium still. The concentration range of the biological buffer TRIS in the feed mixtures is from 0.05 to 0.20 in mass fraction. The experimental results show that the presence of TRIS induces significant shift of azeotropic composition and increases the relative volatility of tert-butanol to water. TRIS can be a promising mass separating agent for the separation of (tert-butanol + water) mixtures. The experimental VLE values were correlated with the NRTL-HOC model to determine the binary interaction parameters. Generally, the NRTL-HOC model well correlates the VLE data for the systems investigated in this study. The model parameters obtained from this study can be applied to design a new green process for separating -tert-butanol from its aqueous solution.

Introduction

Separation of aqueous alcohol mixtures is often encountered in industrial applications. Several of those mixtures form azeotrope, which makes the difficulty of recovery of alcohol from its aqueous solution through ordinary distillation method. Therefore, it requires alternative methods to overcome the separation barrier. For example, one may use an inorganic salt as a mass separating agent via extractive distillation. Many studies on the separation of alcohols from their aqueous solutions by adding inorganic salts have been extensively reported [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. One of effects of adding inorganic salts into the mixtures is known as “salting-out”. It can increase the relative volatility of alcohol to water. This shifting effect of salts is often applied to improve the efficiency of separation for azeotropic mixtures. Unfortunately, the use of salt may cause corrosion of the equipment.

A new green alternative entrainer, which is eco-friendly, appears to be essentially needed to facilitate the separation process in minimising the drawbacks of using inorganic salts. Biological buffer is an organic molecule that may be used as a new entrainer in azeotropic mixture separation due to its buffering-out effect. Moreover, it is also a biological compatible agent, which will not cause corrosion in the equipment. Inducing liquid–liquid phase-splitting by adding biological buffers in aqueous solution of cyclic ether, alcohols, acetonitrile, or acetone has been investigated by Taha and Lee [26], [27], [28], Taha et al. [29], [30], [31], [32], and Altway et al. [33]. In a buffer solution, it is composed of a weak acid (proton donor) and its conjugate base (proton acceptor). Buffer solutions are commonly used to maintain pH value at a nearly constant level in a wide variety of chemical applications. Tris(hydroxymethyl)aminomethane (TRIS) is probably the most frequently used buffer substance in biological experiments because of highly soluble in water, inert in many enzymatic systems, and high buffer capacity [34]. It has potential to be a promising biological buffer which can be used as a new green entrainer to shift the azeotrope point in alcohol–water system.tert-Butanol is widely used as a solvent in industries, as a gasoline anti-nock agent, and as a neat fuel and/or oxygenate fuel constituent in gasoline, diesel, jet fuel, aviation gasoline, heating oil, and bunker oil in the mixed butanol composition. On the other hand, tert-butanol has a minimum boiling azeotrope with water which is difficult to be separated with conventional distillation. The use of biological buffer in azeotrope separation could effectively reduce unit energy consumption. As our best knowledge, the studies on the VLE behaviour for the ternary system of (tert-butanol + water + biological buffers) are still unavailable in public literatures.

The present study provides the VLE for tert-butanol aqueous solutions containing TRIS which is used as a novel material mass separating agent in the separation process. The thermodynamic consistency of the experimental values was tested by Herington method [35]. Reliable thermodynamic models and their parameter values are needed for design and optimisation of separation process purposes. The NRTL-HOC model [36], [37] was used in this study to correlate the experimental VLE results.

Section snippets

Materials

tert-Butanol and the biological buffer (TRIS) are analytical grade and purchased from commercial sources. The deionized water was prepared from our laboratory. The purity levels of the compounds were checked by gas chromatography (GC) and reported in Table 1. Due to none detectable impurities being found from the chemicals, all materials were used as received.

Apparatus and experimental procedures

A modified Othmer-type recirculating still was used in this study to determine isobaric VLE data. The schematic diagram of apparatus and

Experimental results

The isobaric VLE values for binary system {tert-butanol (1) + water (2)} were measured and compared with the data from literatures [38] and [39] in order to validate the experimental method. Table 2 lists the experimental values (T, x1, and y1) for {tert-butanol (1) + water (2)} binary system and graphical comparison with the literature values is made in Fig. 1, which show that our results agree well with those of Ref. [38] in water-rich region and also satisfactory with those of Ref. [39] in

Conclusions

In the present study, isobaric VLE has been measured for the binary system of {tert-butanol (1) + water (2)} and the ternary system of tert-butanol (1) + water (2) + TRIS (3) with different concentrations of TRIS (w3 = 0.05, 0.10, 0.20) at 101.3 kPa by using a modified recirculating type of Othmer equilibrium still. All the experimental data were passed the thermodynamic consistency test. The equilibrium temperature increases as TRIS was added in the aqueous tert-butanol mixtures. The vapour phase

Funding sources

The authors gratefully acknowledge the financial support from the Ministry of Science & Technology, Taiwan, through Grant No. NSC99-2811-E-011-023, and scholarship from National Taiwan University of Science & Technology.

Acknowledgement

The authors thank to Dr. Ho-mu Lin for valuable discussions.

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