*3.2. Reactor Profiles*

#### 3.2.1. Adiabatic

As the simplest form of fixed bed reactors, adiabatic reactors are often designed with multi-stage feeding in order to control the temperature rise of the catalytic bed by interstage cooling. For the EC hydrogenation reaction, however, the adiabatic temperature rise is estimated as only 10~20 K under typical operating conditions because of the high H2/EC ratio, despite a considerable reaction enthalpy change (−87.3 kJ/mol). Therefore, a one-stage adiabatic reactor is considered herein in view of a facile temperature control that could be envisioned.

For the adiabatic reactor loaded with sphere catalysts (5 mm), Figure 4 shows the contours of bed temperature and partial pressures of EC and EG under three different inlet temperatures. The adiabatic reactor exhibits a plug-flow behavior with minimal radial gradients of velocity, temperature and species concentrations. In the axial (flow) direction, a mild adiabatic temperature rise of c.a. 12 K is observed, which is scarcely affected by the variation of inlet temperatures from 443 to 483 K (Figure 4a−c). With the increase of the inlet temperature, the average bed temperature increases, thus accelerating EC transformation as reflected by the decrease of average *P*EC inside the catalyst bed. Consistently, in the front part of the catalyst bed, *P*EG, is found to increase with the inlet temperature. *P*EG near the reactor outlet, however, is lower for higher inlet temperatures as a result of the intensified secondary EG hydrogenation reaction (R3). This side reaction has a high activation energy of 114 kJ/mol (Table S1) and is sensitive to temperature rise than the main reaction (R1). The consumption of EG in the rear part of the catalyst bed is remarkable when the local bed temperature rises to above 473 K (Figure S3), corresponding to inlet temperatures above 463 K. Therefore, maintaining a moderate bed temperature is necessary to maximize the selectivity to desired alcohol products.

**Figure 4.** Contours of *T*, *P*EC and *P*EG in the adiabatic reactor with different reactant inlet temperatures of (**a**) 443 K, (**b**) 463 K and (**c**) 483 K under the conditions of 3 MPa, 0.3 gEC·gcat−1·h−<sup>1</sup> and H2/EC = 200.

Another necessary practical consideration for reactor operation is that EC is a high boiling-point organic liquid with a dew-point temperature of c.a. 450 K under the inlet condition (Figure S2), which overlaps with the range of operating temperatures. If EC liquefies within the catalyst bed, liquid film would cover the catalyst particle, not only blocking the active sites but also causing coking in the longer term. At the inlet temperature of 443 K, EC condensation is predicted to occur in the first c.a. 20% of the catalyst bed from the inlet (marked by a vertical line in Figure 4a) as judged by the local partial pressure of EC. From the condensation boundary downstream to the reactor outlet, EC remains in the gas phase as a consequence of both the increasing temperature profile of the adiabatic reactor and the decreasing EC partial pressure with the reaction proceeding.

## 3.2.2. Boiling Water Cooling

Boiling water-cooled fixed-bed reactors often adopt a shell-and-tube structure in which multiple parallel reaction tubes are immersed in flowing boiling water in a shell. To enhance heat transfer from the tube to the coolant, a small tube diameter is chosen. Figure 5 shows the contours of bed temperature and partial pressures of EC and EG within a single reaction tube of the boiling-water cooled reactor under three different coolant temperatures. Within the first 10% of the catalyst bed, the bed temperature approaches the coolant temperature with the temperature gradients in the radial direction reducing from 20 K to zero (Figures 5a–c and S7a,c). No exotherm is observed within the entire catalyst bed in the axial direction for the three coolant temperatures. Because of the relatively low heat release from the reaction and the high convective heat transfer coefficient of boiling water, the coolant temperature rather than the reactant inlet temperature is the determining factor for the bed temperature (Figure S6a). When the coolant temperature increases, the average *P*EC decreases with the accelerated EC hydrogenation reaction. Different from the adiabatic reactor, an upward trend is observed in *P*EG with an increasing coolant temperature from 423 K to 463 K because EC conversion to EG is promoted whilst the EG hydrogenation side-reaction remains negligible in the temperature range. For the inlet temperature of 463 K, it is remarkable that the boiling water-cooled reactor gives rise to an EG yield ~10% higher than the adiabatic reactor, with only 12 K difference in the bed temperature.

**Figure 5.** Contours of *T*, *P*EC and *P*EG in boiling water-cooled reactor with different coolant temperatures of (**a**) 423 K, (**b**) 443 K and (**c**) 463 K under the conditions of 3 MPa, 0.3 gEC·gcat−1·h−1, H2/EC = 200 and reactant inlet temperature of 463 K.

The lower bed temperatures relative to the adiabatic reactor increase the risk of EC condensation. Figure 5a,b illustrate that the condensation zones are more extended in the near-wall region than around the reactor centerline. Due to an oscillatorily increasing bed porosity towards the reactor wall (Table S5), the EC flowrate increase towards the wall (Figures S4 and S5), contributing to higher local partial pressures of EC. Such a behavior is prominent for the multi-tubular heat-exchange reactors with high tube-to-particle diameter ratios and thus considerable channeling flows near the wall.

#### 3.2.3. Conduction Oil Cooling

In the pilot plant reactor, conduction oil is selected as the coolant in consideration of easiness of control and its satisfactory performance of heat removal. The heat transfer coefficients of conduction oils are significantly lower than the boiling water and depend on geometrical and operational parameters such as the oil flow rates, the distance between adjacent baffle plates, the type of baffle, etc. The specific relationships are given in the Supporting Information (Section S3.2). In general, conduction oil is not as effective as boiling water in controlling the reactor temperature, especially under low oil flow rates. Taking 25% open baffle, 1 m distance between adjacent baffle plates and the 200,000 kg·h−<sup>1</sup> mass flow rate as the reference case, the resulting heat transfer coefficient is calculated to be 619 W·m−2·K<sup>−</sup>1. These values will be used in the following study.

Figure 6 shows that the bed temperature drops quickly close to the inlet of the oilcooled reactor as a consequence of the low inlet temperature of the conduction oil. Then, with the oil being heated up continuously by the exothermic reaction, the bed temperature gradually rises towards the reactor outlet (Figure 6a−c and Figure S7a). The radial temperature gradient in the catalyst bed reduces from 9 K at the inlet to zero after 10% of the catalyst bed. Distinct from that of the boiling water-cooled reactor, the bed temperature of the oil-cooled reactor is also affected by the inlet temperature of reactants: both the increase of reactant inlet temperature and the coolant inlet temperature decrease the EG selectivity (Figure S6b). Different also from both the adiabatic and the boiling water-cooled reactors is that *P*EG increases slightly with the bed temperature for oil inlet temperatures below 463 K, but decreases slightly with the bed temperature when the oil inlet temperature exceeds 463 K. The outlet flow rates of EG are the highest among the studied reactor types for reactant inlet temperatures between 423 and 463 K thanks to the moderate temperature profile, which balances between promoting EC conversion and preventing secondary EG hydrogenation. Meanwhile, the predicted condensation region of EC is significantly smaller than that of the boiling-water cooled reactor, and the condensation only appears under the lowest coolant temperature of 423 K.

**Figure 6.** Contours of *T*, *P*EC and *P*EG in oil-cooled reactor with different coolant temperatures of (**a**) 423 K, (**b**) 443 K and (**c**) 463 K under the conditions of 3 MPa, 0.3 gEC·gcat−1·h−1, H2/EC = 200 and reactant inlet temperature of 463 K.

#### *3.3. Effects of Key Operating Variables*

To elaborate the reactor's operation behaviors, the effects of major operation variables on the reactor performance are further depicted. Five operation variables are studied, including the inlet temperature *T*in, the coolant temperature *T*c, the operating pressure *P*op, the

space velocity *SV*, and the inlet H2/EC. The chosen performance metrics are EC conversion *X*EC, EG selectivity *S*EG, MeOH selectivity *S*MeOH and total alcohols selectivity *S*Alcohol.

## 3.3.1. Temperature

For all reactor types, Figure 7 shows that *X*EC and *S*MeOH increase while *S*EG and *S*Alcohol decrease with the increase of inlet temperature. At higher temperatures, the faster reaction results in a greater amount of EC being converted (*X*EC rises). Given that the activation energies of Reaction 1, 2 and 3 (R1)–(R3) are 30.1, 28.1 and 113.8 kJ·mol−1, respectively (Table S1), higher temperatures promote hydrogenation of EG to by-products (*S*EG decreases) but inhibits EC conversion to CO instead of MeOH (*S*MeOH increases). Comparison among different reactor types (Figure 7a–d) show slight differences in the EC conversion. *X*EC of the adiabatic reactor is higher than that of the other two types of reactor owing to its high average temperature of the catalyst bed. *X*EC of the boiling water-cooled reactor is the lowest among the three types of reactor, but the difference from that of the oil-cooled reactor diminishes as the coolant temperature is raised above 453 K. Since the reactant inlet temperature also affects the performance of the oil-cooled reactor, Figure 7d additionally illustrates the effect of reactant inlet temperature on *X*EC, *S*EG, *S*MeOH and *S*Alcohol. The variations in the reactant inlet temperature and the oil temperature exhibit similar influences on the reactor performance.

**Figure 7.** Reactor performance under different reactant/coolant inlet temperatures: (**a**) adiabatic reactor with *T*in = 433–483 K, (**b**) boiling water-cooled reactor with *T*in = 463 K and *T*<sup>c</sup> = 423–463 K, (**c**) conduction oil-cooled reactor with *T*in = 463 K and *T*<sup>c</sup> = 423–463 K, (**d**) conduction oil-cooled reactor with *T*<sup>c</sup> = 463 K and *<sup>T</sup>*in = 433–483 K. Operating conditions: 3 MPa, 0.3 gEC·gcat−1·h<sup>−</sup>1, H2/EC = 200.

EC condensation occurs in the boiling water-cooled reactor when the boiling water temperature is 448 K (Figure 7b). This critical temperature is consistent with the reactant inlet temperature leading to condensation in the adiabatic and oil-cooled reactors (Figure 7a,d) because their inlet regions are the most prone to condensation. For the oil-cooled reactor, moderate heat removal by the conduction oil combined with its relatively low heat capacity yields a bed temperature higher than the initial oil temperature. Therefore, condensation with a fixed reactant inlet temperature of 463 K does not occur until the oil temperature is lowered to 428 K (Figure 7c).
