(81c) Improving Overall Ethylene Plant Performance By Optimising C2 Hydrogenation

The steam cracking process in an ethylene plant produces a certain amount of acetylene, which needs to be reacted with hydrogen to maintain ethylene product quality as well as to increase ethylene yield. The reaction is typically carried out in 2 or 3-stage fixed-bed catalytic reactor with inter-stage cooling. Side reactions include the production of ethane, which needs to be recycled to the cracker, reducing overall plant throughput and increasing energy use. In tail-end C2 hydrogenation units, acetylene oligomerisation produces green oil, which deposits on the catalyst surface and causes catalyst de-activation and loss of selectivity. The challenge for C2 reactor operators is to maximise overall selectivity to ethylene while maintaining product quality.

This paper describes the application of advanced process modelling to a typical C2 hydrogenation tail-end process. The approach uses high-fidelity models of the catalyst bed to predict composition, temperatures and other important attributes to a high degree of accuracy.

Having validated the model against experimental data, it was used to determine optimal operating conditions. The objective of the optimisation was to maximise economic gain by maximising ethylene production. The decision variables used were the time-varying inlet temperature to each bed, the hydrogen flow to each bed and the run length, with constraints including the maximum bed temperatures and outlet C₂H₂ concentration. By altering the conversion in each of the three beds by altering the inlet conditions it was possible to realise a 13% increase in ethylene gain and a 10% improvement in process economics, and at the same time reduce production of unwanted ethane and green oil. There is now potential to implement the model on line for real-time monitoring of catalyst activity and green oil accumulation. A similar approach can be applied to C3 hydrogenation.