Purge recovery unit

Ammonia flash and purge gases are treated in a scrubbing system and a hydrogen recovery unit (not shown), and the remains are used as fuel. 

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. 

If the make-up gas contains inerts, purge gas is drawn from the loop after the second cold exchanger just before addition of the make-up gas. At this point the gas has the maximum content of inerts. The purge gas is usually transferred to a purge gas recovery unit for recovery of hydrogen. Recovered hydrogen is recycled to the process and introduced at a relevant stage of the synthesis gas compressor. 

The steam reformer model can handle feeds from methane to naphtha, with all the t5q)ical components that are present in natural gas, as well as recycled S5mthesis purge gas, or hydrogen recovery unit tail gases. Naphtha feed is characterized as about 30 chemical species, some of which are pure components, and some are hydrocarbon fractions (pseudo components). Each hydrocarbon species participates in a reaction that includes adsorption onto the catalyst, reforming, and desorption. The model includes diffusion effects within the catalyst, as well as heat transfer resistance from the bulk gas to the catalyst surface. 

In common with other processes, the overall efficiency can be improved by recovery of hydrogen from the purge gas instead of using it as fuel. Either of the two general types of hydrogen recovery unit may be used cryogenic recovery " or membrane separation. ... 

For the same production capacity, the oxygen-based process requires fewer reactors, all of which operate in parallel and are exposed to reaction gas of the same composition. However, the use of purge reactors in series for an air-based process in conjunction with the associated energy recovery system increases the overall complexity of the unit. Given the same degree of automation, the operation of an oxygen-based unit is simpler and easier if the air-separation plant is outside the battery limits of the ethylene oxide process (97). 

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