2 replies to this topic

[Riaz01]

Can any body explain in detail why CO conversion in Ammonia synthesis plant is done 02 steps High temperature and Low temperature shift converter with different catalyst??

 

Thanks in advance

[Said Salim]

Dear Riaz,
As I know the shift reaction is favored by the low temperature i.e. more conversion will take place. However, at low temperature the reaction rate is slow and hence to get both features two steps are used.

[Saml]

Several reasons.

 

1) For low temperature shift if the CO level in the inlet is high, the exit temperature will be also high (is an exothermic reaction) and you will have higher unconverted CO. So the bulk of the conversion is done before you get to the lower temperature. Any unreacted CO has to be later methanated with an impact on Hydrogen and synthesis loop purge (unless you have a separation unit and remove Argon and Methane before)

 

2) You always want to recover heat at the higher possible temperature for thermodynamic efficiency. So converting in the HDS is better since you get the exotherm at  high temperature.

 

3) Low temperaure shift catalyst is more expensive than HDS. Also more sensitive to temperature.