Electrification of Chemical Engineering: A New Way to Intensify Chemical Processes

The increasing use of fossil fuels as an energy source has resulted in a serious problem regarding two of their main drawbacks: (i) the exhaustion of these resources and (ii) the greenhouse gas (GHG) emissions associated with their use [1]. The member states of the United Nations (UN) set the so-called Paris Agreement in 2015, in which they reached an agreement towards the limitation of the global warming to 2 °C versus pre-industrial levels [2]. This target may be reached by reducing GHG emissions from 80 to 95% of 1990 levels by 2050, requiring a complete transformation of industry [3]. Different strategies may be used, including the gradual elimination of nonrenewable energy resources; the transition to a circular economy; the production of greener electricity; and the electrification of heating, transportation, and industrial processes [4,5]. The increasing availability of cheap renewable electricity provides an opportunity to decarbonize energy-intensive processes. As part of this decarbonization effort, the commodity chemical industry is an important target due to its large energy requirements and high GHG emissions [6].

In particular, many efforts towards the electrification of chemical process industries (CPI) have been made. In fact, more energy-efficient and more selective and compact processes have been made possible with the adoption of, for example, intensified separation processes, plasma technology, ultrasound microwaves, and electric arc furnaces. In this way, both capital costs and waste generation have been hugely reduced [7]. Process intensification (PI) innovations in combination with a greener electric grid can help to establish cleaner and more sustainable production processes across the CPI.

Another approach is to replace the energy produced by burning carbon-based fuels with energy from “green” sources, such as electricity produced from renewable sources [6]. In particular, the transfer of thermal energy into electrified reactors can be achieved in various ways, for example through microwaves (MW). In recent years, many researchers have focused their attention on the use of MW in different fields, ranging from the preparation of catalysts to their use in chemical processes.

Tedesco et al. [8] investigated the MW-assisted conversion of lignocellulosic biomass to levulinic acid using sulphated zirconia and trace hydrochloric acid. The results of the experimental lab-scale tests showed that an optimum yield of about 63%wt can be achieved at 160 °C over 80 min and with a 2:1 catalyst-to-biomass ratio with 10 mM of diluted HCl. In particular, the authors showed how the synergistic performance of the sulphated zirconia and hydrochloric acid significantly improved levulinic acid yields compared to each individual catalyst alone.

Hou et al. [9] investigated the possibility of producing biodiesel through the MW-assisted transesterification of high-acid-value waste cooking oil. The results of the experimental tests showed that by using an NaOH catalyst of 0.8%wt, a 12:1 molar ratio of methanol to oil, a reaction time of 2 min, a reaction temperature of 65 °C, and microwave power of 600 W, a biodiesel yield meeting the Taiwan CNS 15072 biodiesel standard could be obtained.

Most of the current efforts in the field of biodiesel production are devoted to coupling MW heating technology and heterogeneous catalysts for biodiesel synthesis to improve the efficiency of the process [10]. Studies in these fields have shown that the application of MW technology for biodiesel generation using heterogeneous catalysts appears to be quite efficient with respect to biodiesel yield, catalytic activity, reaction time, and energy efficiency. In particular, experimental tests have suggested that the application of MW technology in the heterogeneously catalyzed transesterification process significantly reduced the reaction times and demonstrated better catalytic activity compared to conventional heating. However, there are some drawbacks that have been raised regarding the application of MW in biodiesel production, most of which center around their lower penetration depth, which is a few centimeters. This drawback results in an instantaneous loss in microwave intensity, and this problem may be amplified while scaling up the batch process. This issue needs to be rectified, or the continuous mode can be adopted using a film-type reactor to overcome this problem. Examples of other drawbacks include spark ignition, high-temperature zones at the edges, and microwave leakage, which must be overcome for the large-scale adoption of this technology [10].

Limousy et al. [11] prepared different binary metal oxide catalysts containing Ni, Cu, or Co oxides by means of an MW-assisted Solution Combustion Method. The prepared catalysts were tested in CO oxidation, and the experimental tests demonstrated that the Cu-containing catalysts had the best catalytic activity.

Meloni et al. [12] proposed the use of microwaves for the regeneration of a 13X zeolite bed for N₂O capture from tail gases. The consecutive adsorption–desorption cycles performed at N₂O concentrations of 10, 20, and 40%vol highlighted that the MW did not damage the zeolite’s structure, allowing perfectly repeatable steps in terms of both the adsorbed and desorbed amount of N₂O. Moreover, the MW-assisted TSA assured an energy and purge gas savings of up to 63% and 82.5%, respectively, compared to traditional regeneration processes, resulting in effective PI.