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STEADY UNDER PRESSURE

Process Considerations In Pressure Control

Overview

The two major earlier reviews of pressure control took different approaches to categorizing methods of tower pressure control. Boyd broke down the control systems into groups based on distillation pressure, presence of inert gas, and product type (vapor or gas). Chin's excellent review broke down the methods by product type then by general method (mass or energy flow control). However, some of the decisions on grouping the energy flow methods into sub-groups appear somewhat arbitrary.

Here, we will continue Chin's method, but without straining to over-categorize the energy removal control methods. Instead, we will reuse Chin's major groupings (and general approach) but modify the subgroup organization.

Major groups:
1. Vapor product always present.
2. Vapor product greater than or equal to zero on steady-state, negative vapor flow rate transients possible.
3. Vapor product rate zero on steady-state (total condenser), negative vapor flow rate transients possible.

Subgroups:
A. Mass flow control (vapor rate control)
B. Energy flow control: variable surface area, process side control
C. Energy flow control: variable heat flux, process side control
D. Energy flow control: variable surface area, utility side control
E. Energy flow control: variable heat flux, utility side

These subgroups concentrate our attention on the method of control (surface area or heat flux) and the controlled medium (process or cooling utility).

Additional Process Considerations

Additional process considerations will be noted in the explanatory text for each method. Process considerations that affect the selection of method include breadth of boiling range of process stream, presence and relative quantity of non-condensable gas.


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This page updated 17 March 2000.
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