PROCESS DESIGN OF HIGH PRESSURE REFORMING FOR AMMONIA SYNTHESIS
Publication Date: 2012-Sep-18
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These design and economic factors shouM' be considered when selecting reforming pressure and flowsheet for steam-hydrocarbon ammonia synthesis
Orlando J. Quartulli, The M. W. Kellogg Company, New York City
DEVELOPMENTS IN CATALYSTS and in reformer furnace design have permitted sizable increases in the pressure level of catalytic reformers in more recent ammonia plants. While there are many advantages associated with high pressure reforming, many process factors must be considered before a suitable reformer pressur~ level can be selected. Moreover, there is no specific optimum reformer pressure level which applies to all process se- quences aud hydrocarbon feeds. Actually, if an optimum exists, it probably falls in a range and can shift from one level to the next depending on many factors. It will be shown that a satisfactory working range with respect to refo~Tner pressure is 350 to 450 psig. The actual level selected depends on price structure, plant capacity, com- pressor size and type, and plant payout considerations.
Wtfile operation at high refortnlng pressure reduces synthesis gas compression cost and increases unit thermal efficiency, other associated factors have unfavorable ef- fects on both operating costs and investment. As reformer pressure is increased, the unfavorable methane-steam equilibrium requires more steam or higher operating tem- peratures for a specific design. Extreme increases in reformer pressure also raise operating pressure of down- stream equipment which is reflected in higher capital costs. Higher reforming pressure requires more horse- power for feed and air compression equipment and major pumps, all of which increase operating costs.
Even general consideration of these broad statements reveals the causes for disagreement on optimum reform- ing pressure and the best flowsheet for ammonia synthesis.
Tiffs author feels that there is no single optimum pressure applicable to all reforming systems, situations, and hydrocarbon feeds, Probably an optimum pressure fails in a range and depends on many process factors. To support this position, some aspects of high pressure reforming that relate to performance and operating costs will be discussed. Also included are several populm, am- monia process flowshects and some of the major factors to be considered before selecting a process design. Such factors as steam-carbon ratio, residual methane, hydro- gen-nitrogen ratio, inests purge, heat recovery, power requirements, compressor selection, feed and utility price structure are discussed with particular emphasis on a 200-500 prig refomfing system.
Only a natural gas feed will be considered although the same principles would apply to other hydrocarbon feeds. A medium pressure synthesis system (4,700 psig)
will be used in developing process and economic informa- tion for the over-all plant. This is done without prejudice towards operation at higher or lower synthesis pressu...