Advanced Process Control and Optimization of Acetylene Hydrogenation Reactors

摘要

Polymer grade Ethylene has stringent specifications of acetylene impurity, around 1 PPM maximum. Since acetylene stays with ethylene-ethane fraction of the cracked gas all through the separation process, typical ethylene processes rely on catalytic hydrogenation of acetylene to meet product specification. In addition to the desired reaction of acetylene hydrogenation, ethylene hydrogenation also takes place. Obviously, ethylene hydrogenation must be minimized as it contributes directly to product loss. In addition to these two main reactions, a number of other side reactions also take place that lead to the formation of low order polymers (green oil) that build up on the catalyst surface and slowly reduce its activity. For this reason reactors must be regenerated on a frequent basis. Reactor run-length varies, typically between 60 to 90 days, depending on the operating conditions encountered by the reactor. Typically 3 catalytic reactors are employed with two reactors in service at any given time. For each reactor there typically are two manipulated variables (MV): hydrogen flow to the reactor and reactor inlet temperature. Carbon Monoxide, which acts as an inhibitor, is sometimes added to improve reaction selectivity and could be an additional MV. During normal operation there are degrees of freedom in the operation of the reactor system; that is there are moreMVs than needed for control, and that presents us with an opportunity for economic optimization. Control of acetylene reactors however is a challenging problem because of the inherent difficulty in controlling to an impurity level that is nearly zero. Similarly, economic optimization is also challenging as it requires a careful tradeoff between reactor activity, selectivity and run-length. For Petromont, these challenging problems have been solved by using Honeywell's robust multi -variable predictive controller that works in conjunction with and gets guidance from a rigorous, on-line, non-linear, multi-time period, kinetic based optimizer. These advanced applications were first commissioned in October 2001. These applications were modified in May 2002 after the process turn around in which some instrumentation changes were made. Use of these advanced applications has resulted in a much smoother operation of the process and a net improvement in the ethylene yield by about half a percent.