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With the global biodiesel production growing as never seen before, encouraged by government policies, fiscal incentives, and emissions laws to control air pollution, there has been the collateral effect of generating massive amounts of crude glycerol, a by-product from the biodiesel industry. The positive effect of minimizing CO2 emissions using biofuels is jeopardized by the fact that the waste generated by this industry represents an enormous environmental disadvantage. The strategy of viewing “waste as a resource” led the scientific community to propose numerous processes that use glycerol as raw material. Solketal, the product of the reaction of glycerol and acetone, stands out as a promising fuel additive capable of enhancing fuel octane number and oxidation stability, diminishing particle emissions and gum formation, and enhancing properties at low temperatures. The production of this chemical can rely on several of the Green Chemistry principles, besides fitting the Circular Economy Model, once it can be reinserted in the biofuel production chain. This paper reviews the recent advances in solketal production, focusing on continuous production processes and on Process Intensification strategies. The performance of different catalysts under various operational conditions is summarized and the proposed industrial solketal production processes are compared.

Thiosulfate (Ts) is an inorganic ligand capable of immobilizing heavy metals and partially activating Cd in different media by forming soluble cadmium-thiosulfate (CdTs) complex ions. We investigated the feasibility of process intensification for the selective removal of cadmium from thiosulfate complex solutions via ultraviolet (UV) irradiation. A Cd removal rate of 50% was achieved after 2 h of UV irradiation at a Cd concentration of 80 mg/L, a thiosulfate concentration of 0.1 mol/L, and an initial pH of 10.00 ± 0.05. Additionally, we applied various characterization techniques to confirm that the photoproduct comprised CdS. Evaluation of the ultraviolet photolysis revealed that it followed first-order reaction kinetics and sequentially underwent primary and secondary photochemical reactions. The results demonstrated that Cd can be recovered as a CdS nanocomposite from a CdTs complex solution according to a removal mechanism involving primary and secondary reactions of the photochemical process. These findings offer several environmental benefits based on their potential application to address the problem of Cd pollution.

Soil restoration by exploiting the principles and basics of electrokinetic (EK) has been extended to involve several categories, such as electrokinetic remediation in soil (SEKR), soil consolidation, the prevention of soil pollution, reclaiming salt-affected soil, the dewatering/dryness of wet soils, water reuse, seed germination, sedimentation, etc. As an extension of our recently published review articles on the soil electrokinetic (SEK) process intensification/optimization, the present review illustrates the effect of a reverse-polarity mode (RPM) on the efficiency of the SEK. Based on several searches of six database search engines, we did not find any relevant reviews focused on SEK improvements using the RPM. The influences of the RPM are described by various features, including (a) pollutant removal (organic, inorganic, and mixed pollutants) and (b) integration with other processes (phyto/bioremediation and Fenton oxidation), geosynthetics (consolidation, stabilization, and sedimentation), SEK operation conditions, and soil properties. Most of the RPM studies have focused on the remediation of organic pollutants. Several benefits can be gained from applying the RPM, such as (a) controlling the soil’s temperature, pH, and moisture values at desirable levels, (b) reducing a large number of chemical additives, (c) high remediation efficiency, (d) maintaining the indigenous fungal community’s appropriate diversity and abundance, (e) a stable and higher electric current, (f) enhancing microbial growth, etc. However, the hindrances to applying the RPM are (a) reducing the electroosmosis flow, (b) relatively high energy consumption, (c) reducing the diversity of soil microbes with a prolonged experiment period, (d) providing oxygen for a microbial community that may not be desirable for anaerobic bacteria, etc. Finally, the RPM is considered an important process for improving the performance of the SEK, according to experimental endeavors.

To overcome the challenges of the increasing global energy and to solve the global energy & environment problems, process intensification is one way to develop new efficient production pathways for the chemical industry. Process intensification plays an important role in the gas-liquid mass transfer processes. This review provides an overview of the developments in gas-liquid mass transfer enhancement. A major enhancement method, namely introducing additives (including nanoparticles, oil, electrolyte, and surfactant) summarized and discussed here, includes the most recent accomplishments in gas-liquid mass transfer engineering. This review is expected to inspire new research for future developments and potential applications in scientific research and industry regarding gas-liquid mass transfer engineering. Finally, it presents conclusions and perspectives on enhancing gas-liquid mass transfer.

Solid-state fermentation, featured by water-saving, eco-friendly and high concentration product, is a promising technology in lignocellulosic ethanol industry. However, in solid-state fermentation system, large gas content inside the substrate directly leads to high oxygen partial pressure and inhibits ethanol fermentation. Z. mobilis can produce ethanol from glucose near the theoretical maximum value, but this ethanol yield would be greatly decreased by high oxygen partial pressure during solid-state fermentation. In this study, we applied N2 periodic pulsation process intensification (NPPPI) to ethanol solid-state fermentation, which displaced air with N2 and provided a proper anaerobic environment for Z. mobilis. Based on the water state distribution, the promotion effects of NPPPI on low solid loading and solid-state fermentation were analyzed to confirm the different degrees of oxygen inhibition in ethanol solid-state fermentation. During the simultaneous saccharification solid-state fermentation, the NPPPI group achieved 45.29% ethanol yield improvement and 30.38% concentration improvement compared with the control group. NPPPI also effectively decreased 58.47% of glycerol and 84.24% of acetic acid production and increased the biomass of Z. mobilis. By coupling the peristaltic enzymatic hydrolysis and fed-batch culture, NPPPI made the ethanol yield and concentration reach 80.11% and 55.06 g/L, respectively, in solid-state fermentation.

Process intensification methods are used to increase the reaction conversion in a short duration. The application of microchannel reactors is one of the methods widely used for enzymatic catalytic reactions. In this study, biodiesel is produced from waste cottonseed oil (WCO) in a microchannel reactor. The feedstock is reacted with ethanol in the presence of immobilised Rhizopus oryzae lipase packed in the reactor. The microchannel reactor comprises a polytetrafluoroethylene tube with an inner diameter of 0.68 mm packed with immobilised Rhizopus oryzae lipase. The reactants were fed into the reactor through a T-mixer. The effects of different parameters such as ethanol-to-oil molar ratio, temperature and residence time were optimised. Further, the conversion obtained in the microchannel reactor was compared with a conventional batch process to identify the effectiveness. An optimum conversion of 92.5% was achieved at a 4:1 ethanol-to-oil molar ratio, 45 °C, and a residence time of 120 min was obtained for the microchannel process, whereas 90.7% conversion was achieved at the highest reaction time of 18 h for a batch process under the same condition. The increase in conversion and reduction in reaction time was due to the use of microchannel reactors increasing the mass transfer rate. In addition to this, the physiochemical properties of the product biodiesel were determined using standard methods and the results were compared with the ASTM D6751 standard.

From the viewpoint of resource and energy-saving, the high extraction rate of alternating liquid–liquid flow (slug flow) is important given that it enables its novel use in extraction. Additionally, a specific extraction rate must be maintained for the practical application of slug flow to chemical extraction. Although slug flow is easily generated, controlling the slug length is difficult. In this study, two diaphragm pumps were interlocked to generate a slug flow. By linking the movement of the diaphragms of the two pumps, we could successfully and efficiently control the slug length, and the interlocking diaphragms could easily control the length of the aqueous and oil phase segments of the slug flow. The lengths of the aqueous and oil phases of the slug flow, which could not be quantitatively controlled, could be expressed in terms of the linear velocity of the liquid, the kinematic viscosity, and the tube diameter using the Reynolds number. This relation aids the extraction equipment design using slug flow. Furthermore, the mass transfer coefficient of extraction obtained using the slug flow generated by the developed device was similar to that obtained by the conventional method of a syringe pump. These results indicate that slug flow can be successfully applied to extraction processes.

Since the late 1980s, the scientific community has been attracted to microwave energy as an alternative method of heating, due to the advantages that this technology offers over conventional heating technologies. In fact, differently from these, the microwave heating mechanism is a volumetric process in which heat is generated within the material itself, and, consequently, it can be very rapid and selective. In this way, the microwave-susceptible material can absorb the energy embodied in the microwaves. Application of the microwave heating technique to a chemical process can lead to both a reduction in processing time as well as an increase in the production rate, which is obtained by enhancing the chemical reactions and results in energy saving. The synthesis and sintering of materials by means of microwave radiation has been used for more than 20 years, while, future challenges will be, among others, the development of processes that achieve lower greenhouse gas (e.g., CO2) emissions and discover novel energy-saving catalyzed reactions. A natural choice in such efforts would be the combination of catalysis and microwave radiation. The main aim of this review is to give an overview of microwave applications in the heterogeneous catalysis, including the preparation of catalysts, as well as explore some selected microwave assisted catalytic reactions. The review is divided into three principal topics: (i) introduction to microwave chemistry and microwave materials processing; (ii) description of the loss mechanisms and microwave-specific effects in heterogeneous catalysis; and (iii) applications of microwaves in some selected chemical processes, including the preparation of heterogeneous catalysts.

Enzyme cascades are a powerful technology to develop environmentally friendly and cost-effective synthetic processes to manufacture drugs, as they couple different biotransformations in sequential reactions to synthesize the product. These biocatalytic tools can address two key parameters for the pharmaceutical industry: an improved selectivity of synthetic reactions and a reduction of potential hazards by using biocompatible catalysts, which can be produced from sustainable sources, which are biodegradable and, generally, non-toxic. Here we outline a broad variety of enzyme cascades used either in vivo (whole cells) or in vitro (purified enzymes) to specifically target pharmaceutically relevant molecules, from simple building blocks to complex drugs. We also discuss the advantages and requirements of multistep enzyme cascades and their combination with chemical catalysts through a series of reported examples. Finally, we examine the efficiency of enzyme cascades and how they can be further improved by enzyme engineering, process intensification in flow reactors and/or enzyme immobilization to meet all the industrial requirements. Enzymes, either purified or as whole-cell biocatalysts, can be concatenated into catalytic cascades and used to produce pharmaceutically relevant molecules. This Review discusses the advantages and requirements of multistep enzyme cascades and also highlights how they can be harnessed to achieve highly sustainable and cost-efficient syntheses.

Biocatalysis has widened its scope and relevance since new molecular tools, including improved expression systems for proteins, protein and metabolic engineering, and rational techniques for immobilization, have become available. However, applications are still sometimes hampered by low productivity and difficulties in scaling up. A practical and reasonable step to improve the performances of biocatalysts (including both enzymes and whole-cell systems) is to use them in flow reactors. This review describes the state of the art on the design and use of biocatalysis in flow reactors. The encouraging successes of this enabling technology are critically discussed, highlighting new opportunities, problems to be solved and technological advances. Biocatalyzed reactions with different classes of enzymes can be implemented with the integration of flow reactor technology, potentially leading to sustainable and highly productive continuous processes. The combination of biocatalysis and flow chemistry opens the door to extensive application in cascade reactions. Biocatalyzed flow reactions can occur either in monophasic flow or in segmented (slug) flow, where two or more immiscible phases are present. Limitation of substrate/product inhibition effects, in-line purification with easy recovery of the product, and no mechanical mixing are among the most distinctive advantages of flow-based biocatalysis. Automated machines and devices for in-line product recovery are now available at relatively low prices, making flow-based biocatalysis an easy-to-use technology.