BY PRATIK SUDRA

Recycle operations The recycling operation with chemical reactions is

common in industrial processes.

This is mainly performed to utilize the valuable reactants to their maximum so that the loss of the reactants minimized.

Recycling is returning back a portion of the steam leaving a process unit to the entrance of the process unit for further processing.

Recycling operations, commonly encountered in unit operations and unit processes are performed for:

Maximum utilization of the valuable reactants.

Improvement of the performance of the operation.

Utilization of the heat being lost in the exit stream.

Better operating conditions of system.

Improvement of the selectivity of a product.

Maintaing process rate at a high value.

Enrichment of a product.

In distillation operations, a part of the distillate is fed back to column to enrich the product, so that almost pure product can be obtained.

In drying operations, a major portion of the hot air leaving the dryer is recirculated to conserve heat-to utilize the heat being lost in the exit air.

In absorption towers, better wetting of tower packings is done by recycling part of the exit solvent stream.

In chemical reactions, exit catalyst or spent catalyst is returned to the reactor for reuse after being regenerated.

For achieving complete conversion, keeping the yield of byproduct to minimum and effecting reactions at a high rate.

A Recycle stream is a process stream that returns material leaving a process unit back to the entrance of the same unit

Recycle operations are carried out under steady-rate conditions.

In these operations these is no build-up or material takes place inside the process or in the recycle stream.

About the entire process including the recycle stream –where in the fresh feed is equated with the net product.

About the junction point 1 at which the fresh feed combines with the recycle stream.

About the junction the gross product is divided into recycle and net product.

Involving only process feed and gross product streams.

Purging operation In this operation, a fraction of the recycle stream is

continuously bled-of order to limit the concentration of inerts to a desired level in the mixed stream.

In the synthesis of ammonia, some of the gas stream must be purged to prevent build of argon and methane.

The recycling of air/recirculation of air in the drying of solids is shown diagrammatic.

In a recycling operation: combined feed=Fresh feed +Recycle feed

The combined feed is a mixture of the fresh feed and the recycle feed/recycle stock

Gross product= Recycle feed + Net Product

The gross product is a mixture of the net product and the recycle feed.

Recycle ratio: It is the ratio of the quantity of recycle feed to the quantity of fresh feed.

Combined feed ratio: It is the ratio of quantity of mixed feed to the quantity of fresh feed.

Purge Ratio: It is the of ratio of the quantity of purge stream to the quantity of recycle feed.

In manufacture of ammonia, built up of CH4 in recycle loop a certain tolerance limit is avoided by purging.

Bypassing operation In these operations, a fraction of the feed stream to a

process unit is diverted around and combined with the output.