**1. Introduction**

Efficient chemical conversion and utilization of CO2 is envisioned as an important vector in future carbon-neutral and carbon-negative human society [1,2]. Researchers are in hot pursuit of disposing the energy- and industry-related CO2 emissions by renewable electricity or discarded H2 to produce feedstock or commodity chemicals [3–5]. In particular, thermo-/electro- chemical CO2 reduction to oxygenates, for instance, aldehydes, alcohols, and carboxyl acids, exerts great prospects in high-atom-economy CO2-based chemical manufacture by reducing the by-production of H2O [6–10]. Direct hydrogenation of CO2 into methanol (MeOH) has attracted intense research attention in recent years in that MeOH serves as a commodity chemical as well as a crucial platform to various downstream products [11–15]. However, this process is inherently limited by the kinetic inertness and thermodynamic stability of the CO2 molecule, resulting in extremely low per-pass CO2 conversion (~10~20%), and thereby a poor atom economy [16–18].

A feasible solution to by-pass the thermodynamic limitation is to "bridge" the lowenergy CO2 reactant and the high-energy MeOH product with a CO2-derivable, mediumenergy intermediate, such as ethylene carbonate (EC), dimethyl carbonate, methyl formate,

**Citation:** Huang, H.; Cao, C.; Wang, Y.; Yang, Y.; Lv, J.; Xu, J. Model-Based Analysis for Ethylene Carbonate Hydrogenation Operation in Industrial-Type Tubular Reactors. *Processes* **2022**, *10*, 688. https:// doi.org/10.3390/pr10040688

Academic Editor: Blaž Likozar

Received: 21 March 2022 Accepted: 28 March 2022 Published: 31 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and so on [19–23]. Such indirect green methanol production processes are advantageous in milder reaction conditions as well as higher per-pass CO2 conversion (over 95%) and overall methanol yield. Amongst these processes, EC hydrogenation as illustrated in Figure 1 is of great interest:

$$\underbrace{\alpha \text{ \textquotedblleft} \text{ }}\_{\text{OH}^{\text{-}} \text{ }} \text{-} \text{ }} \underbrace{\text{\textquotedblleft} \text{ }}\_{\text{OH}^{\text{-}} \text{ }} \text{-} \text{ }} + \underbrace{\text{\textquotedblleft} \text{-} \text{ }}\_{\text{OH}^{\text{-}} \text{ }} \text{-} \text{ }} \overset{\text{\textquotedblleft} \text{H}^{\text{-}}}{} \text{-} \text{ }}\_{\text{OH}^{\text{-}} \text{-} \text{ }} \text{-} \text{ }} \Delta H\_{\text{I}} = -87.3 \text{ kJ} \cdot \text{mol}^{-1}$$

**Figure 1.** EC hydrogenation reaction.

This process co-produces methanol and ethylene glycol (EG), another value-added bulk chemical, with theoretically 100% atom economy. In addition, EC production using CO2 and ethylene oxide has already been demonstrated [24,25], making the subsequent EC hydrogenation process a promising enabler for industrially relevant green alcohols production.

Development of catalyst systems for EC hydrogenation has gained considerable progress. Since Han et al. [26] first proposed a Ru II PNP catalyst for homogeneous EC hydrogenation, the catalyst system has evolved from homogeneous nobel metal complex [27] to heterogenous supported copper (Cu) based catalysts [28–33], in view of difficult separation and recovery of the former and, in contrast, convenient deployment of the latter. As the early-version Cu-based catalysts are inferior to the homogeneous catalysts in performance, continuous efforts are being paid to develop more active and selective candidates suitable for industrial application [34–36]. Song et al. [37] prepared MoOx-promoted Cu/SiO2 catalyst, which achieved 89% MeOH yield and 99% EG yield within 150 h timeon-stream at a H2/EC ratio of 20 and a WLHSVEC (weight liquid hourly space velocity of EC) of 0.64 gEC·gcat−1·h−1. Carbon modified Cu catalyst proposed by Chen et al. [38] displayed good activity and stability after using for 264 h, owing to improved dispersion of Cu particles, showing EC conversion of 100%, and EG and MeOH selectivities up to 99.9% and 85.8%, respectively. Appropriate synergy between Cu<sup>0</sup> and Cu+ species are believed to play an important role in the activation of H2, providing adsorption sites for the carbonyl groups of EC, and stabilizing the surface acyl and methoxy species, leading to improved EC conversion and alcohol yields [39,40].

Despite the progress in catalyst investigation, to date, the heterogeneous EC hiydrogenation process has not been studied at a pilot scale. Tailoring the transport characteristics of industrial-scale reactors in accordance with the intrinsic reaction kinetics is crucial for commercial operation of catalytic processes [41,42]. In this respect, industrial methanol synthesis serves as a reference given a similar reaction enthalpy change and the employment of similar Cu-based catalysts. Multi-stage adiabatic fixed bed reactors and boiling watercooled multi-tubular reactors are the most commonly applied types of reactors in methanol synthesis [42]. The design and operation optimization of these reactors has been well aided by model-based analysis accounting for intrinsic reaction kinetics, internal/external mass transfer at the scale of catalyst particles, and heat and mass transfer at the reactor-scale [43]. Recently, Samimi et al. [44] compared water cooled, gas cooled, and doubled cooled reactors for direct CO2 hydrogenation to MeOH by one-dimensional reactor models focusing on the phase stability. The gas cooled reactor exhibits the lowest possibility of methanol condensation and the highest methanol yield. Cui et al. [45] assessed the potentials of adiabatic, water cooled and gas cooled reactors for direct CO2 hydrogenation to MeOH using coupled two-dimensional (2D) computational fluid dynamics (CFD) models for the reactor and single catalyst particles. The water-cooled reactors demonstrate outstanding temperature control at the expense of a higher capital cost. It is apparent that, to promote the process towards commercialization, understanding of the operational behaviors of EC hydrogenation in industrial-type reactors is urgently needed.

Herein, we investigated the operational characteristics of industrial-type adiabatic, water-cooled and oil-cooled tubular reactors for a 3 × <sup>10</sup><sup>4</sup> t/a EC hydrogenation process

by model-based comparative analysis. Two-dimensional (2D) pseudo-homogeneous CFD models were established with engineering kinetic models developed for heterogeneous EC hydrogenation over an industrial Cu-based catalyst, and validated by bench-scale and pilotscale reaction data. The operation windows of key reactor operating variables, including reactant/coolant inlet temperature, total pressure and space velocity, were delineated for the representative industrial-type reactors, considering both the reactor performance and the EC phase change boundaries. The presented results will pave the way to future industrial design and optimization of reactors and process for the green alcohols production.
