Ammonia synthesis is operated in loop mode, since the conversion per pass is only 20-30%. The synthesis gas contains small quantities of methane and argon (inerts), which build up in the loop. In order to achieve an optimum conversion it is necessary to purge a certain quantity of gas from the synthesis loop to reduce the concentration of these inerts. This purge gas is sometimes used as fuel in the primary reformer, after recovery of ammonia in the purge gas absorber. This purge gas contains also hydrogen, which can be recovered by installing a purge gas recovery unit (hydrogen recovery unit). The recovered hydrogen is sent back to the synthesis loop to increase poduction or save energy, as the quantity of hydrogen produced by steam reforming can be reduced (Nand and Goswami, 2009).
Membrane, cryogenics and pressure swing adsorption (PSA) processes have all been commercially applied for the recovery of hydrogen from the purge gas. The choice of separation technology is driven by the desired purity, degree of recovery, pressure and temperature. Membrane separators are widely accepted to recover hydrogen from purge gas and recover typically 85-90% hydrogen, with a hydrogen purity of 87-90%. The cryogenic process operates at high pressures (7 MPa) and can achieve a hydrogen recovery of 90-98%. Pretreatment of the purge gas to remove ammonia and water is required for the cryogenic process. Pressure swing adsorption has been used for ultra-high-purity hydrogen from the purge gas and has a somewhat lower hydrogen recovery (70-85%) than the membrane and cryogenic processes. No pretreatment is required and water and ammonia are removed by PSA in addition to argon and methane (UNIDO, 1979 p.175).
When comparing membrane separators with cryogenic technology it is noted that recovery based on cryogenic technology is more energy efficient, whereas the recovery based on membrane technology requires a lower investment. Which technology to choose will have to be decided on a case-to-case basis (Christensen, 2001a). Various manufacturers supply different types of hydrogen recovery technologies, including Air Products, Air Liquide, Linde, and UOP.
This measure is applicable to both retrofit and new high pressure synthesis loop steam reforming plants (IPTS/EC, 2007 p.85).
In 1980, an Indian plant installed a hydrogen separation and recovery unit based on the cryogenic process. By recycling the hydrogen contained in the purge gas, about 60%, to the synthesis loop, the ammonia throughput increased by about 40-50 tpd; a 5% increase in plant production. In addition to the increased production, the energy use decreased by 0.96 GJ/t NH3. The tail gas from the purge gas recovery unit containing 30% methane and 15% hydrogen, was used as fuel in the primary reformer (Nand and Goswami, 2009).