Recycle and separation modes

All reactor modes sometimes can be operated advantageously with recycle of part of the product or intermediate streams. When the recycle is heated or cooled appropriately it can serve to moderate undesirable temperature travel. This function is well served with pumparound from a stirred tank through an external heat exchanger. Recycle streams also can be processed for changes in composition before return. 

As the recycle ratio through a PFR is increased, changes in temperature and composition across the reactor itself become smaller. Eventually it can be regarded as a differential reactor with approximately constant temperature. Between the fresh inlet to the system and the product withdrawal, substantial differences will develop. The differential operation at virtually constant temperature thus eliminates the main objection to the PFR as a device for obtaining data from which a rate equation can be determined. 

Notation and material balances for recycle operation are stated in problems P4.06.01 and P4.06.02. An adiabatic operation is studied in P4.06.08. 

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Both types of units have generally been operated in trace mode that is, background or elutant electrolyte is fed to the unit along with the mixture to be separated. A desirable and possible means of operation for preparative applications is in bulk mode, in which one separated component follows the other without background electrolyte being present, except that other ions may be required to bracket the separated zones. Overlap regions between components should be recycled, and pure components collected as products. 

A separate continuous DAF operation conducted by Krofta and Wang59 under 33.3% recycle flow pressurization mode demonstrated that aluminum sulfate, sodium aluminate, and polyelectrolyte combination at pH 6.2 also effectively recovered both fibers and titanium dioxide from the same white water containing 500mg/L of titanium dioxide and 1000 mg/L of cotton fibers. 

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