Miscellaneous Pressure Control Methods

The advantages of the system are fast response and the ability to run the condenser and reflux drum at a lower pressure than the column. The latter may be a distinct advantage when the colunrn is revamped to operate at higher pressure. Because of its fast response, at least one designer (258) feels that this is the best method when tight pressure control is required with total condensers. Others (e.g., 77, 164) do not favor this scheme because of its high costs and its potential problems during low-rate operation. 

This method is costly and incurs a loss of inerts. If the inerts are recovered, it adds the cost of a recovery system. Sometimes, some of the heavy inerts may dissolve in the liquid and create problems downstream. The method has the advantage of fast response, but should generally be avoided because of its high cost. 

One drawback of this system is that the distillate control valve may at times dump liquid out of the reflux drum faster than the reboiler can produce vapor to make up for this (234). Using a reflux drum low-level override which cuts back distillate or reflux flow was advocated (234) as a guard against draining the reflux drum. Another drawback of this system is a possible interaction between the pressure and temperature controllers. 

Split range control. Often, a split-range pressure control is used. With this method, pressure may be normally controlled by venting a small vapor product stream. If the pressure falls and vent closure is insufficient to reinstate it, one of the other control methods automatically cuts in. 

Floating pressure control. While tight pressure control is mandatory in distillation columns, it may be desirable to slowly manipulate the set point of the pressure controller. This is the essence of Shinskey s floating pressure control (362). The method minimizes column pressure without violating constraints such as maximum condenser or column capacity, or the ability to send column products to downstream units. 

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Figure 17.7 Miscellaneous pressure control methods, (a) Recirculation temperature control (b) Column overhead control (c) inerts pressure control (d) drained condenser bypass (e) pressure-reboil control (/) floating pressure control, combined with coolant flow manipulation.

In this section, miscellaneous methods of pressure control are reviewed. Figure 17.7a shows a scheme where pressure is controlled by liquid recirculation. Either the inlet or the outlet coolant temperature can be controlled. Inlet temperature control is better if the coolant outlet temperature approaches the distillate temperature. 

There will also be a pressure drop due to the valves used to isolate equipment and control the fluid flow. The pressure drop due to these miscellaneous losses can be estimated using either of two methods ... 

Importance of solvent viscosity or free volume in the TICT phenomenon was discussed in the previous section. There are a number of ways to control viscosity of the medium. The easiest way is to change solvent, however, this brings about the complicated problem of influencing miscellaneous solvent effects. Use of mixed solvents also causes the ambiguity of selective solvation. The best method presumably is the study of pressure effects [20], By applying hydrostatic pressure to a solution, the solvent reduces its free volume without much affecting other solvent properties. 

Control techniques. Pressure and condensation control techniques are classified into four categories vapor flow variations, flooded condensers, coolant flow variations, and miscellaneous methods. These techniques are described below. 

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