(189e) Reactor Exotherms Due to Hydrogen Ingress

Hydrotreating reactors are commonplace throughout the steam cracking recovery process. These reactors are required to meet product specifications and, in some cases, to satisfy downstream unit operational requirements. These recovery sequences often include C2, C3, and C4 acetylene converter reactors. In addition, there are additional hydrotreaters for diolefin/olefin saturation (e.g. C4, SCN) as well as desulfurization. Most of these reactors require controlled hydrogen injection to regulate the reaction, with the exception of front end acetylene converters where hydrogen is present in excess.

The reactants in these hydrotreaters are primarily hydrogen and acetylene(s) and to some degree olefins/diolefins. All of these hydrotreating reactions are exothermic and these reactors have the potential to runaway due to the increasing reaction rate with increasing temperature. A runaway is usually coincident with the onset of unselective reactions which are favored by high temperature, reactives concentration, and/or residence time. An example of high reactives concentration would be having too high an injection of hydrogen, e.g. a control failure. Another concern is maldistribution which can lead to high residence time. Therefore, these reactor systems often include trip systems based on detection of high temperature or residence time (e.g. low flow limits). The action of these trip systems stops the reaction by removing one or both of the reactants (e.g. isolate/depressure or remove hydrogen injection).

This presentation will share examples where hydrogen continued to these reactors despite the trip systems activating. These examples illustrate the potential for hydrogen to be introduced unintentionally into the reactor vessels. This presentation will also include potential options to mitigate these examples or similar scenarios from happening in the future.