Streamlining the Simulation of a Propane Dehydrogenation Plant with Steady Catalyst Deactivation

With the current shale gas market, on-purpose dehydrogenation of propane has become a viable option for the production of propylene. The software ProMax® was used to simulate a commercial propane dehydrogenation plant that accounts for deactivation of the catalytic active sites along the reactor length. The plant includes four catalytic moving-bed dehydrogenation reactors with inter-stage heating, and a selective dehydrogenation reactor to saturate undesired species. Propylene is obtained by cryogenic separation and fractionation, and the unreacted propane is returned to the reactor section.

Mass and energy balance equations are applied for a reactor modeled with the reacting fluid flowing radially while the catalyst activity varies in the axial direction. A network was developed to include a complete set of reactions, including dehydrogenation/hydrogenation, and hydrogenolysis on the active metal sites, as well as cracking, cyclization, and oligomerization transformations on the active acid sites.

Since the catalyst is sent to a continuous catalyst regeneration section before being returned to the dehydrogenation reactors, the physical implication is that the deactivation profile for each type of site varies only along the length of each reactor and remains constant in the time domain. As a result, the deactivation of the catalytic sites was modeled by employing a recently developed empirical function dependent on the axial velocity of the catalyst and parameters that are intrinsic to the type of catalytic site.

The results of the simulation show that selectivity to propylene and per pass conversion obtained are comparable with commercial units, in the range of 85-92 wt.% and 29-35 % respectively. In addition, optimum conditions for the dehydrogenation reactors were determined to maximize the conversion of propane, and its selectivity to propylene. The impact on operating conditions and energy requirements of units downstream of the dehydrogenation reactors were also studied.