One of the critical aspects in the design of a hydro-processing unit is the analysis of the reactor and separator loop depressurization. Depressurization helps in lowering the reaction and separator loop pressure by venting the loop contents to the flare. This results in safe shutdown and prevents possibilities of thermal runaway. Depressurization can often be the deciding factor for equipment design downstream of the reactor. Conventional relief load sizing calculations do not account for the transient changes in temperature and pressure during a de-pressuring event. A dynamic process model for the unit is an accurate and powerful tool to predict temperature and pressure profiles for all exchangers and equipment that fall in the flow path between the reactor and the separator.

This paper discusses a dynamic simulation study to determine the relief loads for several hydro processing units of a major grass-root refinery. From the simulations, increasing temperature and decreasing pressure for each item of equipment was estimated as a function of time. These data were then analyzed to determine the peak mechanical stresses on the piping and exchangers during the entire event. Mechanical design conditions of the equipment were increased where necessary.

The second objective was to size the depressurization system which was designed to reduce the unit pressure to 50% of design pressure (or 100psig, whichever is lower) over a period of 15 minutes. The sizing took account of several cases such as power failure, reactor runaway etc.

Finally, the study estimated the stripper feed condition at different time points during the depressurization scenario. This was conducted to evaluate the column relief load in a situation where temperature is increasing, but flowrate is decreasing. The possibility of vapor breakthrough to the stripper from the separator during depressurization was also analyzed. In some cases this can be the controlling relief load for the downstream column, as the feed stream gets significantly more pre-heat when the upstream heat-sinks, such as the unit feed liquid, are lost.