PTQ Q2 2024 Issue

Feed composition

Compound

1,3-

4-vinylcyclo- n-butane 1-butene c-2-butene Vinyl-

Methyl-

Ethyl-

Propa- Propylene

butadiene

hexene

acetylene

acetylene acetylene diene

wt%

37.6

7.25

5.90

4.59

0.088

0.56

0.34

0.075

0.29

Balance

Table 1

pressure was controlled using a backpressure regulator sit- uated after (liquid phase operation) or before (gas phase operation) the reactor. The resulting product mixture was diluted with N 2 downstream, whereas any greenoil formed was kept liquid and thus separated from the product stream. The temperature of the catalyst bed was controlled by a heating/cooling mantle connected to a thermostat equipped with a parallel electrical heating circuit. The accessible tem- peratures ranged from sub-ambient to 600°C. With this flexible set-up, it was possible to run the reac - tion at various pressures and temperatures in both liquid and gas phase. Although most commercial selective hydro- genations of these C 3 /C 4 species are performed in the liquid phase, the experiment additionally compared liquid and gas phase performance to demonstrate the unit’s flexibility to accommodate different light olefin selective hydrogenation reactions. In liquid phase operation, the impact of upflow vs downflow configuration was investigated. The hydrocarbon feedstock simulates a C 3 -C 4 olefin stream. It should be noted that the feedstock contains 7.25 wt% 4-vinylcyclohexane as a dimerisation product of 1,3-butadiene, extending the educt spectrum up to C 8 . The effluent stream was analysed with an Agilent gas chromatograph (GC) equipped with a flame ionisation detec - tor (FID) and an RTX-Alumina BOND column. Conversion was calculated based on the weight and molar educt, and product flow rates, with the educt flow rates defined by repeated bypass measurements between the experiments. Results and discussion Operation under standard conditions A commercial Lindlar catalyst was used (5 wt% Pd,

selectively poisoned by lead (Pb) on a calcium carbonate carrier, Merck). 5 The reactor with an inner diameter of 8 mm was filled with a 2:3 (by volume) mixture of the Lindlar cat - alyst and silicon carbide as inert material. The total catalyst zone length was 77 mm. Reference conditions for testing were a reactor temperature of 35°C, a pressure of 24 barg, a molar hydrogen/VA ratio of 2, a hydrocarbon feed-based liquid hourly space velocity (LHSV) of 22 h -1 , and an upflow configuration of the reactor system. This approximates a typical operating window of com- mercial units with a hydrogen dosage just below the sto- ichiometric amount with respect to all acetylenes and PD. These parameters were systematically varied during the testing program, as will be further discussed. Under these bespoke standard conditions, the following conversions were observed: 78.6 wt% for VA, 49.4 wt% for EA, and 54.1 wt% for MA. The higher conversion of VA compared to MA and EA is characteristic of Pd-based catalysts. Hydrogen was quantitatively consumed, which is also typical for a tail-end selective hydrogenation process. However, containing 5 wt% Pd, the commercially available Lindlar catalyst provided a fivefold higher active metal load than a typical industrially available catalyst for this reaction and, hence, was significantly more active. This explains the high BD loss of 9.53 wt%. LHSV variation Starting from the standard conditions, the hydrocar- bon-based LHSV varied in a broad range between 7.2 h -1 and 82.4 h -1 . Due to the high activity of the Lindlar cat- alyst used, the hydrogen conversion remained at nearly 100 mol% over the whole LHSV range. At higher temper- atures, a temperature increase of the catalyst zone could be detected, with the zone temperature ranging from 35.5°C at LHSV = 7.2 h -1 to 36.7°C at LHSV = 82.4 h -1 . The heater/cooler system was able to keep the catalyst zone temperature constant within a narrow temperature range also at high rates of heat production by the hydrogenation reaction. Temperature variation Temperature was set to 15, 35 and 50°C, while the other parameters were kept at their standard values (see Figure 2 ). The highest acetylene conversion was observed at 15°C, with a corresponding minimised butadiene (BD) loss. When raising the temperature to 35 and 50°C, the acetylene con- version decreases, with slightly rising butadiene conver- sion. Quantitative hydrogen consumption was observed at all temperatures, limiting the overall hydrogenation activity. It is noteworthy that the relative shifts in BD conversion are much smaller than the shifts in acetylene conversion due to

T = 16.7 ˚C

T = 35.8 ˚C

T = 50.1 ˚C

10.5

9.5

VA conversion

MA conversion

EA conversion

BD loss

Figure 2 Acetylene conversion (right axis) and butadiene loss (left axis) vs reactor temperature (upflow, liq.), p = 24 barg, H 2:VA = 2 mol/mol, LHSV = 22 h -1

PTQ Q2 2024