How Much Catalyst is needed for Synthesizing Ammonia

How Much Catalyst is needed for
Synthesizing Ammonia
S. SINGH, A. NAHAR
KBR Inc.
Houston, TX, USA
While comparing operating performance of the ammonia synthesis loops of similar production capacity, plant owners compare the total catalyst volume and the size and number of converters being used in their synthesis loops. Ammonia converter is one of the major equipment items that decide installed cost of a synthesis loop besides syngas compressor and refrigeration compressor. Per pass ammonia conversion, total compression power and heat recovery are key process parameters often compared by the owners.
KBR recently concluded a study and compared performance of the ammonia synthesis loops of 2200 MTPD capacity based on make-up synthesis gas produced from (a) Partial Oxidation Process post nitrogen wash (b) Steam Natural Gas Reforming Process based on Purifier technology and (c) Steam Natural Gas Reforming Process based on conventional technology.
Catalyst volume, per pass conversion, compression power and steam make are all compared for these three cases. Capacity utilization of the synthesis loop and catalyst life over the plant life cycle is also compared.
The study concludes that the inert level in make-up gas, stability and purity of the make-up gas composition, are the key factors that demarcate the synthesis loops based on three types of make-up gases into the three distinct classes that determine capital and operating cost of these synthesis loops.
INTRODUCTION
Owners of various existing ammonia plants compare the synthesis loop in their plants with each other to reconcile their opinion on cost effectiveness of specific synthesis loop considering the installed and operating
costs. Some of these loops may be unchanged from their original design but others would have gone through
one or more revamps through the life cycle.
The findings of this KBR study assist the owners of various existing ammonia plants who may compare the synthesis loops in their plants with each other to reconcile their opinion on cost effectiveness of specific synthesis loop considering the installed and operating costs. The study concludes that
(1) Inert level of make-up gas,
(2) Stability of the make-up gas composition, and
(3) Sustained purity of the make-up gas (i.e. oxides)
are the three most important factors that demarcate performance of synthesis loops based on the three types of make-up gases depending upon type of their frontend design used.
Synthesis loop based on a frontend using partial oxidation (POX) process with nitrogen wash has the minimum catalyst requirement compared to the other two cases due to negligible inert in the makeup gas. This case has all the advantages over the rest with lower capital and operating costs.
While the Purifier and conventional processes produce synthesis gas from natural gas in frontend by steam reforming, synthesis loops based on Purifier process based frontend have significant capital and operating cost advantages over synthesis loops with conventional process frontends, i.e.
o Catalyst volume is significantly lower in case of Purifier process, due to lower inert in makeup gas
o Compression power hence energy consumption is lower as well in Purifier case
o Utilization of the design production capacity on life cycle basis is higher for Purifier case due to stable make-up gas composition
 Catalyst life is longer in spite of starting with a lower catalyst volume in case of Purifier and POX due to better make-up gas purity
Thus clearly the ammonia synthesis loops receiving make-up gas from POX and Purifier process based frontends have lower capital and operating costs than the conventional cases.

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