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Sustain. Chem. 2021, 2 293 activity [48]. Nonetheless, Faria et al. studied the effect of the particle size of Amberlyst-15 for a similar reactive system and concluded that the reaction occurs under diffusion- controlled regime; therefore, the internal mass transfer resistances effectively influence the catalytic activity, which makes particle size a relevant parameter [62]. The limited miscibility between acetone and glycerol is one of the obstacles of this reactive system [48]. To overcome this challenge, the most common solutions are using acetone in great excess and applying auxiliary solvents that enhance the miscibility of the system. For the solvent selection, Faria et al. developed a methodology to determine the most appropriate solvent for reactive-adsorptive processes considering properties that directly affect the system as miscibility, reactivity, adsorption, and kinetics. The method also evaluates environmental aspects as lethal dose (LD50), NFPA health hazard classification, octanol−water partition coefficient, and persistence time [63]. According to this methodology, solvents that are reactive to any of the reagents at the reaction operating conditions cannot be used and the interaction of the solvent with the catalyst is a key factor [10]. Different production processes were proposed for the synthesis of solketal in batch and continuous modes of operation (most of the studies about batch processes), especially in what concerns surpassing the thermodynamic limitation. This can be done by either removing one of the products from the reaction medium, typically water, or by adding ace- tone in excess. In batch processes, a major issue is reaction rate drop when the equilibrium is nearly reached, being necessary to stop the synthesis to collect and purify the products, then clean and refill the reactor to restart the process. Depending on the catalyst, some sort of regeneration may be required after each run [48]. When operating in batch, water removal can be accomplished by the use of entrainers, as previously mentioned [50]. The open literature reports the use of substances, such as benzene, petroleum ethers, and chloroform; however, the problem faced in this operation is the relatively low boiling point of acetone, which often was removed from the reaction medium even before the entrainers [48,64]. Another alternative explored was the use of desiccants with a role in catalysis as well (sodium sulphate and phosphorus pentoxide). Experiments confirmed the economic unfeasibility of this option due to the high solid con- sumption, together with some environmental concerns, due to the waste generation [48,64]. Furthermore, technologies as membrane sieves, membrane batch reactors, and modified batch operations were experimented and achieved good conversion yields, but in all cases, acetone in great excess was necessary [48]. Because of the already mentioned miscibility issues, Royon et al. tested the use of supercritical acetone to improve the contact of the reagents, yet the low conversion yields reached were discouraging [48,65]. The results reported for the continuous processes are encouraging, showing much higher efficiency, with solketal being produced in less time [48]. Among the numerous advantages of studying continuous processes, the one that needs to be highlighted is the possibility of scaling up the process to the industrial level [60]. The most explored technology for the large-scale production of basic chemicals and intermediates is the catalytic fixed-bed reactor; thus, this operation is the most applied in studies concerning the continuous ketalization of glycerol [66]. Most of the research on the field is recent and more details are provided in Section 4.3. One of the first continuous operations proposed for the solketal synthesis was actu- ally a semi-continuous counter-current distillation reaction, investigated by Clarkson et al., where glycerol was fed in batch and acetone in continuous, and the reaction was catalyzed by Amberlyst-DPT. Using acetone to glycerol molar ratio of 2, the authors registered a con- version value of 98%, but the temperature of the process had to be carefully monitored since providing much heat removes acetone from the reaction medium and low temperatures affect the conversion due to glycerol’s miscibility [67]. The studies on continuous operations also considered the use of homogeneous catalysts, such as the synthesis in a Continuous Microwave Reactor proposed by Cablewski et al. with pTSA as catalyst (84% conversion yield) [68] and in a module of several continuous glass flow reactors connected in series with sulfuric acid asPDF Image | Continuous Valorization of Glycerol into Solketal
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