Ultrasound-induced emulsification of subcritical carbon dioxide/water with and without surfactant as a strategy for enhanced mass transport

Abstract

Pulsed ultrasound was used to disperse a biphasic mixture of CO2/H2O in a 1 dm(3) high-pressure reactor at 30 °C/80 bar. A view cell positioned in-line with the sonic vessel allowed observation of a turbid emulsion which lasted approximately 30 min after ceasing sonication. Within the ultrasound reactor, simultaneous CO2-continuous and H2O-continuous environments were identified. The hydrolysis of benzoyl chloride was employed to show that at similar power intensities, comparable initial rates (1.6±0.3×10(-3) s(-1) at 95 W cm(-2)) were obtained with those reported for a 87 cm(3) reactor (1.8±0.2×10(-3) s(-1) at 105 W cm(-2)), demonstrating the conservation of the physical effects of ultrasound in high-pressure systems (emulsification induced by the action of acoustic forces near an interface). A comparison of benzoyl chloride hydrolysis rates and benzaldehyde mass transport relative to the non-sonicated, 'silent' cases confirmed that the application of ultrasound achieved reaction rates which were over 200 times faster, by reducing the mass transport resistance between CO2 and H2O. The versatility of the system was further demonstrated by ultrasound-induced hydrolysis in the presence of the polysorbate surfactant, Tween, which formed a more uniform CO2/H2O emulsion that significantly increased benzoyl chloride hydrolysis rates. Finally, pulse rate was employed as a means of slowing down the rate of hydrolysis, further illustrating how ultrasound can be used as a valuable tool for controlling reactions in CO2/H2O solvent mixtures.

A biphase H2O/CO2system as a versatile reaction medium for organic synthesis

“…Cenci et al also conrmed that with application of ultrasound the reaction rates are over 200 times faster for the hydrolysis of benzoyl chloride in biphase CO 2 /H 2 O system (CO 2 8 MPa, 30 C). 134 Chatterjee et al 135 studied cleavage of C-O bond of diphenyl ether by hydrolysis and hydrogenolysis in biphase CO 2 /H 2 O system in the presence of hydrogen (CO 2 5-15 MPa, H 2 0.5-4 MPa, 80 C, 5 h). They used Rh@C catalyst and revealed combined synergetic effect of CO 2 and H 2 O, which was evident from the product distribution (cyclohexane, cyclohexanol, dicyclohexyl ether) taking into account further hydrogenation (due to H 2 presence) of intermediates (benzene, phenol, cyclohexylphenyl ether).…”

Reduction of aerobic and lactic acid bacteria in dairy desludge using an integrated compressed CO2 and ultrasonic process

“…An ultrasound probe (Sonics and Materials, USA) was integrated into the vessel head and sealed to maintain the pressure at the operating conditions. Although not mechanically agitated, the ultrasound probe allowed sufficient dispersion of the gas with the desludge, as validated by Cenci et al ( 2014 ). Then, 700 mL of desludge was charged into the vessel, which resulted in the probe tip being immersed in the desludge to a depth of 2 mm, and the headspace and ancillaries were evacuated using the gas of choice (CO 2 or N 2 ).…”

Formation and characterization of emulsions consisting of dense carbon dioxide and water: Ultrasound

“…When CO 2 is selected as the SCF in conjunction with water, both solvents in the biphasic system are environmentally benign, non-toxic, inexpensive, and abundant. In fact, scCO 2 -water mixtures as both emulsions [22][23][24][25][26][27][28][29][30][31] and microemulsions [32] have been used for a wide variety of chemical reactions including Diels-Alder cycloaddition [33], olefin epoxidation [34], hydrolysis reactions [35][36][37], polymerization reactions [38], Barbier reaction [39], oxidation [40], hydrogenation [41,42], hydroformylation [43], biomass pretreatment [44], hydrogen peroxide synthesis [45,46], and particle and materials synthesis [47]. Beyond the chemical process industries, scCO 2 /water mixtures -including emulsions -are likely important in geological systems and potentially of interest for geological CO 2 sequestration and CO 2 enhanced oil recovery [48].…”

“…Timko et al [36] found that short-lived, surfactant-free emulsions could be generated consisting of scCO 2 and water and used them for hydrolysis [35] and Diels-Alder reactions [33]. More recently, Cenci et al [37,39] used pulsed ultrasound to generate water-scCO 2 emulsions, studied their stability, and used them for a hydrolysis [37] and Barbier reactions [39]. In addition to surfactant-free emulsions, Cenci et al investigated the use of an inexpensive hydrocarbon surfactant, Tween 80, to increase the volume fraction of scCO 2 dispersed in the water phase from approximately 5% to 35% and to extend the apparent breaking time from 15 min to 60 min.…”

“…These two sets of ultrasound studies [18,33,[35][36][37]39,51] indicate the potential for ultrasound to generate emulsions consisting of water and scCO 2 phases. Unfortunately, characteristics of emulsions consisting of water and dense CO 2 phases have been incompletely published, the mechanism of ultrasound formation of water-scCO 2 emulsions is not fully understood, and the energy and scale-up requirements are poorly understood.…”