Spillback suction pressure

But, suppose we must maintain a constant pressure in the wet-gas drum. The pressure in this drum may be controlling the pressure in an upstream distillation column. To hold a constant pressure in the drum, we will have to resort to spillback suction pressure control, illustrated in Fig. 28.5. 

If the molecular weight of the gas increases, the gas density will increase. The AP will increase. The pressure in the wet-gas drum will drop. The new suction throttle PC valve will start to close. This will restore the pressure in the wet-gas drum, without increasing the flow of gas through the spillback valve. But what happens to Px How does closing the suction throttle valve affect the actual compressor suction pressure ... 

Both the suction throttle pressure control (PC) valve and the spillback [flow-control (FC)] valve are in a nice operating position. All we wish to do is save electricity. 

This simple and direct method (Fig. 17.4a) is usually the best choice when the column has a vapor product (77). The controller directly manipulates vapor inventory and, therefore, coliunn pressure. The equivalent method commonly used to control vacuum columns (Fig. 17.46) has the pressure controller varying the quantity of spillback to the ejector suction. The spillback control method can also be applied to pressure columns where the vapor product is compressed. 

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