Q & A > Question Details
Concern: Trade off between purity and recovery of H2 from PSA
Obj: Increasing recovery of H2 on slightly compromise on purity (99.99 to 99.90)
Present Status: In our one of Refineries, the recovery of H2 through PSA is around 89.5%. Purity obtained in 99.99% whereas 99.90% is sufficient. Any changes to the operation still gives 99.99% purity thereby reducing the recovery to almost 89-89.5%
 
Answers
01/11/2014 A: Rajesh Nandanwar, Bharat Oman Refineries Limited, Bina, rajesh.nandanwar@borl.co.in
Recovery and purity are two important parameters and definitely by reducing purity you can increase the recovery if you have the recovery factor margins. Only thing you should remember is that by bringing down the purity, you are not increasing the impurities above design.
Normally Steam methane reformer PSA are designed for CO+CO2 impurity of <20 ppm. For all practical purposes it should be considered that CO <20 ppm and CO2=0 ppm level are to be maintained. CO is the first component that would travel with product and if the product CO slip has gone up from this it means that CO & CO2 is getting adsorbed on upper level molecular sieves. CO, CO2 being high in molecular weight would get strongly adsobed in lower molecular size sieves which are designed for adsorption of CH4 with mol wt of 16 only. Once adsorbed, CO & CO2 would take more number of purge cycles to get completely desorbed and thus that mol sieve is not available for adsorption of CH4.
If this operation is prolonged for many days, desorption is not possible and you could see that you are loosing the adsorption capacity and lower recovery factor need to be applied for correcting the same. In evantual case of not noticing this and maintaining higher recovery factor there is chance of permanantly deactivating the mol sieves which you can understand the cost and other impact. Be careful.