Temperature Control How to Compensate Pressure Changes in Temperature Control

Author / Editor: YUVRAJ LAWATE & RUSHIKESH JANAI / Dominik Stephan

Temperature control is an essential conditions of many industrial processes. Pressure changes, nevertheless can quite drastically interface with temperature monitoring. Here’s a look at some controls that when used correctly give the desired results...

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Controlling process parameters is vital to satisfactory operation of critical fractionators
Controlling process parameters is vital to satisfactory operation of critical fractionators
(Source: Aker Solutions)

In the operation of distillation columns, feedback controllers controlling compositions of product streams are employed to operate columns at desired product specifications. The process variable for these feedback composition controllers, are actual stream compositions measured either on-line or in laboratories. Main drawback of this technique is high dynamic lag and large time constant of analyser system loop.

Alternatively, Tray temperature is often used as process variable to calculate stream compositions rather than complex analyser systems. Temperature control is an easy and inexpensive way of composition control as it uses high reliability and low maintenance measuring element. However, temperature control can suffer from pressure variations in the column, as column temperature can change due to variations in column pressure at fixed composition.

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Changing Pressure Affects Temperature Control

Temperature controller may interpret change in column pressure as change in composition and would send a corrective signal. This false signal could lead to disturbances in the column operation. Generally with columns operating under high pressures, change in pressure doesn’t have considerable impact on temperature control. Effect of change of column pressure on temperature control is more prominent in low pressure columns and especially in columns operating under vacuum.

Also, effect of pressure change in case of close separation columns is more pronounced as temperature variation with composition is small and even small effect of pressure change on temperature can appear

relatively large. Thus, in such cases pressure compensation is provided to temperature control. The article illustrates pressure compensated temperature calculations with industrial examples of Deisobutanizer and Debutanizer columns in refinery applications.

When pressure compensation is provided, it is required to generate an equation which can give satisfactory correlation between column pressure and control temperature i.e. vapour pressure and saturation temperature. Vapour pressure is, in general, a nonlinear function of temperature and composition. Most vapour-pressure equations are empirical equations derived from integration of the Clausius-Clapeyron equation. These equations are for pure components and coefficients are obtained by experimental procedures.