Simulation of a membrane reactor for ammonia decomposition

by Kraisuwansarn, Nichakorn

The purpose of this research is to study the

feasibility of combining reaction and separation in a

membrane separation unit. The asymmetric ceramic membrane

reactor applied to the gas-phase catalytic decomposition

reaction of ammonia from an IGCC (Integrated Gasification

Combined Cycle Processes) gas mixture was simulated in the

temperature range of 810-1366 K and over the pressure range

of 18.248E5-35.482E5 Pa. The assumptions for the development

of the model equations were plug flow on both sides of the

membrane, negligible reverse reaction, and negligible heat

and mass transfer resistance in the catalyst. A mass balance

over a differential volume of the reactor gives eight

simultaneous ordinary differential equations for four gas

components. These equations were solved simultaneously as an

initial value problem using the DIVPAG subroutine(Gear's

method) from the IMSL Math Library. The conversion for

ammonia decomposition was successfully increased beyond the

value obtained in a plug flow reactor by removing the

product from the reaction zone via Knudsen diffusion through

reactor walls. The general behavior of the membrane reactor

and the plug flow reactor are compared from the viewpoint of

equilibrium conversion shift. Decreasing the pressure ratio

and increasing the total flow rate of the sweep gas in the

separation side contributed to the higher conversion shift.

The optimum thickness of the ceramic membrane selective

layer was found to be in the range of 3-9 pm. The fractional

conversion of membrane reactor is always greater than plug

flow reactor.

This work was supported by a subcontract from the U.S.

Department of Energy Morgantown Energy Technology Center

(contract #DE-AC21-89MC26313).