Most distillation control systems, either conventional or advanced, assume that the column operates at a constant pressure. Maintaining constant operation supports stable operation and unit profits. Pressure fluctuations make control more difficult and reduce unit performance. Pressure variations swing column vapor loads and temperature profiles. Using temperature control as a substitute for composition control requires using pressure compensation to maintain desired compositions. Varying pressure changes relative volatilities and affects fractionation performance. Vacuum columns are especially susceptible to this problem. Other, less commonly expected problems, can arise from pressure fluctuations. Pressure drops may also result in a normally single-phase feed to turn into a flashing feed. Having two-phase feed when expecting single phase feed can flood a column .
Effective pressure control prevents column flooding and minimizes
compensation requirements for temperature substitution in composition
control. Effective pressure control improves advanced control
and unit optimization by making running close to the unit's maximum
capacity more reliable. While important, few sources have thoroughly
examined distillation pressure control systems. After reviewing
major process factors involved in selecting pressure control schemes,
we will examine the major types of column pressure control, their
characteristics, and application.
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