Ammonia recovery from synthesis loop

Application The ICIAMV process produces ammonia from hydrocarbon feedstocks. The AMV process concept offers excellent energy efficiency together with simplicity and reduced capital cost for plant capacities between 1,000 tpd and 1,750 tpd. Key features include reduced primary reformer duty, low-pressure synthesis loop and hydrogen recovery at synthesis loop pressure. 

Inerts and excess nitrogen from the ammonia synthesis loop are removed by a purge from the circulator delivery and treated in a hydrogen recovery unit. Recovered hydrogen is recycled to the circulator suction. 

Recovery of ammonia by water scrubbing offers the advantage of achieving a very low residual ammonia content, but the drawback is that the whole recycle gas has to be dried afterwards and in addition distillation of aqueous ammonia is necessary to yield liquid ammonia. Nevertheless the scrubbing route was again proposed for a synthesis loop to be operated under extremely low pressure (around 40 bar) [938]. Snam Progetti [280], [939], [940] has proposed removing the ammonia from the loop gas at ambient temperature down to 0.5 mol% by absorption in dilute aqueous ammonia. 

Figure 6.37 Flow sheet for the recovery of H2 from the synthesis loop of an ammonia plant.

The material efficiency of modern synthesis loops is very high hydrogen efficiencies of 98% can be achieved when hydrogen recovery from purge gas is included in the process. The remaining 2% of the feed hydrogen which is not converted to ammonia is utilized as fuel. 

Product Recovery from Ammonia Synthesis Loops.253... 

M. W. Kellogg has also suggested a process (the KAAP process) based on a new catalyst [928, 949]. This technology has been used in a revamp project where a new reactor was installed downstream of the existing reactor in an ammonia synthesis loop (see Sect. 6.4.3.4). In addition, more radically new process schemes deviating from the traditional route have been described. An example is a scheme based on so-called parallel reforming [30,32,120,121] and a low pressure loop with ammonia recovery by water absorption [769]. None of these new developments have been implemented in practice. 

Figure 10.5. Ammonia synthesis loop. This layout is typical of a loop for a large capacity (1000 tonne day ) plant. Preheat of the converter feed gas to 150 C allows high-grade heat recovery from the converter exit gas. Reprinted from Catalyst Handbook, 2 ed., Ed. by M. Twigg, Copyright (1989) by kind permission of M. Twigg.

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