Question

The purge stream is an economic loss. Discuss the effects of reducing the purge flow rate on the process. Could the purge be eliminated? If so how? if not why not?

Answer 1

Purge stream is taken out from the system to avoid accumulation of inert components is system. Purge stream is taken out from recycle stream. So if purge stream flow rate is reduced, higher flow rate of inerts(which do not take part in reaction) also gets back to system. Since, inert does not convert in reactor, with time concentration of inert shall keep on increasing. This building up of inert in system shall increase the pressure of system. Effect of this pressure increase is that either higher energy shall be required for flow of feed. However, in real plant operations upstream pressure can not be increased therefore feed flow rate shall decrease leading to loss of production. Therefore, it is desirable to maintain proper flow of purge stream to avoid any loss of production.

Theoretically speaking there can be two scenarios when purge stream can be removed: First, Reactor operates in once through mode i.e. recycle of stream is not required as single pass conversion of reactor is high enough to avoid recycle so purge is also not required. Secondly, an inert separation facility upstream of reactor is installed which removes inert completely from feed.

However, in real application recycle of reactor feed is always required as conversion in single pass is not high. Similarly, it is difficult to install a separation facility which completely removes inert from feed. Hence, a small amount of inert is always present in feed and if purge stream is not used, concentration of inerts shall keep on building up. Therefore, in both scenarios it is not possible to remove purge stream completely in a process.

In fact in real applications, purge stream is taken to recovery section where reactive components lost with purge stream are recovered and are returned to process.