Quantitative comparison of alternative control schemes for air-cooled condensers

Authors

  • William L. Luyben

    Corresponding author
    1. Process Modeling and Control Center, Dept. of Chemical Engineering, Lehigh University, Bethlehem, PA 18015
    • Process Modeling and Control Center, Dept. of Chemical Engineering, Lehigh University, Bethlehem, PA 18015
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Abstract

Air-cooled heat exchangers present unique control problems because they are more sensitive to ambient disturbances than water-cooled heat exchangers. Each of the several alternative methods for manipulating heat removal has some disadvantages. The obvious choice of manipulating the flow rate of the air is mechanically difficult. This investigation compares three alternative process and control structures for controlling pressure in a distillation column with an air-cooled condenser: manipulating air flow (fan speed, louver opening, or blade pitch), throttling a control valve in the overhead vapor line, or bypassing vapor around the condenser. Results show that direct manipulation of air flow rate is slower than other methods, and the deviations in column pressure are larger. Overhead vapor throttling gives good control of column pressure, provided the nonlinearity of the butterfly valve is taken into account and the dynamics of the large valve are not too slow. However, reflux-drum pressure and temperature vary drastically. This necessitates the use of a high-head reflux pump and results in varying degrees of subcooling, which affect the separation in the column. Hot-vapor bypassing, using two control valves (one in a vapor bypass line controlling reflux drum pressure and one in the vapor line to the condenser controlling column pressure), provides effective pressure control and lessens the variability in reflux-drum temperature. However, more condenser area is required and the plumbing must be correct so that hydraulic problems do not occur. © 2005 American Institute of Chemical Engineers AIChE J, 2006

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