PTQ Q2 2024 Issue

the much higher absolute BD content with respect to the acetylene species in the feed. These experiments neatly show the importance of precise temperature control, espe- cially when operating larger reactors. Hydrogen/VA molar ratio variation As depicted in Figures 4 and 5 , the molar hydrogen to VA ratio was systematically varied between 0.3 and 7.2. With the feed containing 1.6 mol% of acetylenes and PD, the stoichiometric H 2 /VA ratio for quantitative hydrogenation is 2.6. With an increasing H 2 /VA ratio, an increase in acet- ylene conversion could be observed. However, 1,3-buta- diene also began to be increasingly hydrogenated. At the highest H 2 /VA ratio of 7.2, VA was nearly quantitatively consumed, while MA and EA conversion reached around 90 wt%. Higher exothermicity is expected, as indicated by slightly higher temperatures in the catalyst bed. With the heater/cooler system, the exothermicity could be controlled at a maximum of 1°C at the highest H 2 /VA ratio. Influence of variable process conditions Operating the unit under standard conditions as previously described, the upflow configuration of the unit was tested against a downflow configuration. In downflow operation, a significant loss was observed in overall acetylenes con - version (see Figure 3 ). The most probable reason for this is that in the downflow regime, some gas-filled voids remain between the catalyst particles, preventing the formation of a uniform feed phase in the catalyst bed. By reducing the reactor pressure to 0.6 barg, all hydro- carbons could be evaporated, and the reaction could be run in the gaseous phase (see Figure 4 ). As for the liquid state experiments, the H 2 /VA ratio was systematically changed at constant a hydrocarbon-based LHSV of 22 h -1 . At equal H 2 /VA ratios, much higher acetylene derivatives conversions could be observed compared to liquid phase

VA conversion MA conversion EA conversion PD conversion

operation at a significantly shorter contact time with the catalyst. Correspondingly, the BD loss was significantly lower (see Figure 5 ). It is noteworthy that at the lowest H2 /VA ratio, VA is selectively hydrogenated to 1,3-butadiene, thus caus- ing a negative butadiene conversion. This high selectiv- ity is also targeted in commercial processes to maximise 1,3-butadiene yield. The most likely reasons for the higher selectivity of gas phase operation are less restricted mass transfer in the gas phase, plus the lower tendency for dou- ble bonds to adsorb on the Pd active sites at lower pres- sure. Furthermore, in a liquid state, H 2 first needs to be solved in liquid hydrocarbon; then, an equilibrium H 2 con- centration must be established in the hydrocarbon phase, permitting the H 2 concentration available at the active site to be limited. Figure 3 Acetylene conversion at standard conditions, upflow vs downflow, p = 24 barg, T = 35°C, H2 :VA = 2 mol/mol, LHSV = 22 h -1

Conversions vs. H/VA ratio, liquid phase upow

Conversions vs. H/VA ratio, gas phase

36.8

100

34.6

36.4

36.2

35.8

35.6

35.4

H/VA Ratio in feed (mol/mol)

VA conversion

EA conversion

MA conversion

PD conversion

BD conversion

Temperature

Figure 4 Acetylene conversion vs H 2 to vinylacetylene ratio (left axis), catalyst bed temperature (right axis); left: p = 24 barg, LHSV = 22 h -1 (liq., upflow), right: p = 0.6 barg, LHSV = 22 h -1 (gas phase)

PTQ Q2 2024