Model-Based Optimization of an Acetylene Hydrogenation Reactor To Improve Overall Ethylene Plant Economics

The steam cracking process is a common technology for producing ethylene from naphtha. However, one of the major contaminants in the ethylene product stream is acetylene, which poisons catalysts used in downstream polymerization processes and must be converted to ethylene by selective catalytic hydrogenation in order to upgrade product quality and increase overall ethylene yield. The selective hydrogenation process is usually carried out in a multistage fixed-bed catalytic reactor with internal cooling between stages to ensure that the outlet concentration of acetylene does not exceed 1 ppm. Nevertheless, side reactions also occur, including total hydrogenation to ethane, which increases energy consumption at the recycle furnace and decreases overall plant productivity. Moreover, in a tail-end hydrogenation process, the catalyst surface often becomes covered by so-called green oil generated from acetylene oligomerization, which causes catalyst deactivation and lowers the selectivity to ethylene. A key challenge of the tail-end acetylene hydrogenation process is to maximize the selectivity to ethylene while maintaining a full conversion of acetylene and maximizing the run length between cataly stregenerations. In this work, a model of the tail-end three-stage fixed-bed catalytic selective hydrogenation reactor was developed and validated to accurately predict the reactor outlet composition and other important variables. To achieve the optimum operating policy, the model-based dynamic optimization was applied to maximize overall process economics through enhancement of selectivity to production of ethylene. Implementation of the optimal operating policy on a commercial acetylene hydrogenation reactor resulted in a 13% improvement of ethylene selectivity and 10% increase of overall process economics while simultaneously decreasing the rate of catalyst deactivation. This modeling and optimization approach should be applicable to other fixed-bed hydrogenation processes, such as hydrogenation of methyl acetylene and propadiene in propylene product stream.