ENHANCED AMMONIA SYNTHESIS IN MULTIFUNCTIONAL
REACTOR WITH IN SITU ADSORPTION
Nikola Nikačević, Mina Jovanović, Menka Petkovska
Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, 11000 Beograd, Serbia
nikacevic@tmf.bg.ac.yu
Conventional ammonia synthesis, performed in catalytic packed bed reactor, is a
highly demanding process concerning energy consumption. Furthermore, the conversion of
reactants (per single pass) is very low, due to thermodynamic limitations of chemical
equilibrium. The unconverted reactants have to be separated from the product outside of the
reactor and recycled back in the loop with large energy consumption. The PI concept is to
introduce a third phase in the reaction zone – flowing solids which are selective adsorbents
for ammonia. Removal of the product from the system shifts equilibrium towards more
production of ammonia, as the driving force for forward reaction remains high in the reactor.
Higher conversions result in minimization of the recycle loop which leads to energy and
investments savings.
In this work a mathematical model for hybrid reactor-separator is developed. The
model integrates ammonia catalytic reaction with ammonia adsorption in counter-current gas
– flowing solids – fixed bed reactor. The reaction kinetics and equilibrium constants, the
adsorption equilibrium constants, and the mass transfer coefficients are calculated by
commonly used models from the literature. Heat effects and temperature dependences of
properties are taken into account. Our previous models for the prediction of flow dynamics
properties (solids holdup and pressure drop) in gas – flowing solids – fixed bed contactors are
integrated with the overall reactor model. One-dimensional, steady-state model assumes plug
flow of both gas and flowing solids phases, which is a reasonable assumption (especially for
the gas phase). The reactor is adiabatic; there is no need for internal cooling, because flowing
particles carry out some of the heat generated from exothermal reaction and adsorption (heat
capacity of particles is higher than gas capacity).
Model simulations of an industrial-scale multifunctional reactor were performed. The
operating conditions: input temperature, input total pressure, solids flux and gas feed rate
were varied in order to examine reactor performance. The operating windows for reactant
conversion, in situ product adsorption, and reactor temperature are presented. The
investigation demonstrates that ammonia synthesis process can be improved considerably if
an integrated reactor-separator is used. The analysis also emphasizes potential problems in
operation and design.