Ammonia Recovery from the Synthesis Loop

The liquid ammonia of the high-pressure separator is flashed to about 20 bar, whereby the majority of the dissolved gases are released in the let-down vessel. This gas is normally used as a fuel, preferably after removal of ammonia vapor to avoid NOx formation in the combustion furnace. 

The ammonia from the let-down vessel may be sent directly to downstream users or flashed further to atmospheric pressure for storage in a cold tank. All ammonia vapors removed from flash gases and from purge gas by water scrubbing can be recovered as pure liquid product by distillation if there is no direct use for the aqueous ammonia. 

Absorption refrigeration with aqueous ammonia instead of a mechanical refrigeration system [932]-[937] did not find widespread application. 

Although ammonia condensation was already used in Haber s lab-scale ammonia plant and in early pilot plants of Bosch, the first commercial units of BASF used ammonia absorption in water to remove the product from the cool synthesis loop gas, because various technical problems were encountered with refrigeration at that time. It was only in 1926 that ammonia condensation was introduced in the Haber-Bosch 

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. 

---