Quaternary solubility of acetic acid, diacetin and triacetin in supercritical carbon dioxide

https://doi.org/10.1016/j.supflu.2016.09.005Get rights and content

Highlights

  • Acetic acid has the highest solubility in scCO2.

  • The presence of diacetin and triacetin decreases the solubility of acetic acid in the mixture.

  • The hydrogen-bond forming affect the solubility in scCO2.

  • Selective extraction of acetic acid is feasible using scCO2.

Abstract

By a continuous-flow apparatus, quaternary solubility of acetic acid, diacetin, and triacetin in supercritical CO2 was measured at constant flow rate of 180 mL min−1 and temperatures of 313 K, 333 K and 348 K in a pressure range of 70–180 bar. Acetic acid had the highest solubility in scCO2. The binary solubility of acetic acid was also investigated at temperature of 333 k in the pressure range of 70–180 bar. With going from binary to quaternary, acetic acid solubility decreased in the range of 85–90% at different pressures. The quaternary solubility of acetic acid was decreased in the presence of diacetin and triacetin, due to more effective hydrogen bonds with acetic acid and supercritical CO2. In addition, it was observed that the solubility of polar diacetin molecules in non-polar CO2 solvent increased by increasing temperature.

Graphical abstract

The selectivity of acetic acid extraction versus pressure from the mixture of acetic acid, DA and TA with scCO2 at different temperatures.

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Introduction

Over the past decade, it has been imperious to find new applications for glycerol (the major by-product of biodiesel) or convert it to the more applicable or expensive chemicals, due to the introduction of large amounts of it. Esterification of glycerol with acetic acid using catalyst to get mono, di and tri esters of glycerol acetates is one of the proposed solutions for conversion of relatively low-priced glycerol to valuable products. Glycerol acetates named monoacetin (MA), diacetin (DA) and triacetin (TA) have great industrial applications. They can be used as food additive, in the manufacturing of explosives and smokeless powder, as a cocoa butter blooming, as an intermediate in the synthesis of structural lipids, for plasticizer coating and foodstuffs, in cryogenics and biodegradable polyesters, chemical products in the food and cosmetic industries, in cigarette filters, as gelatinizing agent, as fuel additive, solvents for printing inks and dyestuffs, and softening agents [1].

These derivatives of glycerol are synthesized via esterification of glycerol with acetic acid or acetic anhydride with or without a homogeneous or heterogeneous catalyst using an organic solvent in both batch and continuous processes [2]. The three consecutive reversible steps of glycerol esterification with acetic acid is shown in Fig. 1. As it can be seen, all three-glycerol acetates are synthesized simultaneously through the reaction process. Whereas the reaction products is usually accompanied with some by-products that cause a change in the color or the odor. To this end, a purification process is needed that would be difficult and costly. As a result, selective synthesis of esters with high purity has been a great challenge for some researchers [3], [4], [5], [6], [7], [8], [9].

It should be noted that the boiling points of MA (282 °C), DA (260 °C) and TA (259 °C) are close to each other [10]. Especially the close boiling points of DA and TA, make their separation very difficult and costly, using conventional separation methods [11]. Also excess acetic acid remains in the mixture of the products. Therefore, separation of acetic acid for reusing and making the process economical is necessary. Supercritical fluid extraction, compared to conventional extraction methods, has a great potential as an efficient and clean alternative. Supercritical carbon dioxide (scCO2) is known as the most intensively used solvent due to its low critical point (Pc = 73 bar, Tc = 304 K), non-toxicity, inertness, and non-flammability, while remaining an inexpensive and environmentally acceptable substance [12]. The solubility information for the chemicals can provide data applicable for possible tailoring of the abovementioned reaction toward a specific product or higher efficiency including possible separation of mixture components of the products during the continuous synthesis, in scCO2 [13]. Most of the experimental solubility measurements in scCO2 deal with binary systems [14], [15], [16]. However, to the best of our knowledge, quaternary solubility measurements for acetic acid/DA/TA mixture in scCO2 is not reported yet. The main goal of this study was to find proper pressure and temperature in which scCO2 dissolves acetic acid selectively and extract it from the mixture of acetic acid/DA/TA. In this way, it is possible to develop a new process for selective extraction of excess acetic acid during the continuous synthesis of the products.

Section snippets

Materials

CO2 (purity > 99.95%) and ethanol (purity > 99%) were obtained from Zamzam Co. Ltd. (Isfahan, Iran) and Temad Co. (Tehran, Iran), respectively. Acetic acid (purity > 99%) and Pyrex wool were purchased from Merck Co. (Germany), DA (purity = 50% verified by GC-FID), TA (purity > 99%) were from Fluka Co. (Germany), 1-hexanol (purity > 98%) and 1-decanol (purity > 99%) were from Riedel-deHaën Co. (Germany). It must be noted that all the substances was used without further purification.

Procedure

The solubility measurements

Quaternary solubility of acetic acid/DA/TA versus sample weight

The quaternary solubility of acetic acid, DA and TA in scCO2 were measured for the four sample weights of 0.8, 1.5, 3.0 and 6.0 g at 313 K and 100 bar. In each experiment, after passing 1 L of CO2 gas through the trap at constant flow rate, at average laboratory temperature (300 K) and pressure (0.89 bar), samples were taken to measure the amount of extracted out of the cell. The results are shown in Fig. 3, Fig. 4, Fig. 5 . The trend of the solubility versus expanded CO2 volume for the four samples

Conclusions

By using a continuous-flow apparatus, the quaternary solubility of acetic acid, DA and TA in scCO2 were experimentally determined at temperatures of 313, 333 and 348 K from 70 to 180 bar at a constant expanded gas flow rate of 180 mL min−1. Acetic acid solubility increased by increasing density via increasing pressure. In the range of studied temperatures and pressures, in four points, scCO2 can extract acetic acid selectively from the mixture of acetic acid, DA and TA (313 K and 70 bar, 333 K and 70 

Acknowledgement

The partial financial support by Isfahan University of Technology is acknowledged.

References (21)

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