010138 Column Pressure

010138 Column Pressure - Effectively Control Column...

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Unformatted text preview: Effectively Control Column Pressure Effectively Control Column Pressure Andrew W. Sloley, The Distillation Group, Inc. Maintaining proper pressure can play a crucial role in the stable operation of a distillation column. Heres how to select the most appropriate pressure-control scheme . CEP January 2001 www.aiche.org/cep/ 39 Distillation ost distillation-column control systems, either conventional or advanced, assume that the tower operates at a constant pressure. Pressure fluctuations make control more difficult and reduce unit performance. Pressure variations alter column vapor loads and temperature profiles. So, when using temperature control as a substitute for composition control, pressure compensation is essential to maintain desired compositions (1) . Pressure variations change relative volatilities and affect fractionation performance. Vacuum columns are especially susceptible to this prob- lem. Other, less common problems can arise from pressure fluctuations. Pressure drops also may turn a normally single-phase feed into a flashing feed. Two-phase feed in a column de- signed for single-phase feed can cause flooding (2) . Effective pressure control minimizes com- pensation requirements for temperature control, and prevents column flooding. It also improves advanced control and unit optimization by en- abling more reliable operation close to the units maximum capacity. While distillation pressure-control systems are important, few sources (3,4) have thoroughly examined them. So, in this article, after reviewing major process factors involved in selecting pressure control schemes, we will examine the major types of column pressure control for both vacuum and pressure systems, their characteristics, and most suitable application. M Distillation The bases for control Pressure control involves adjusting mass or energy bal- ances by manipulating the amount of mass or heat flow into or out of the tower. Mass flow methods typically control the tower vapor inventory either directly by throttling the vapor rate out of the system or indirectly by manipulating downstream equipment that evacuates gas from the system. Energy methods control the heat flux in the overhead con- denser via temperature (on either side of the condenser) or effective surface area. In general, it is best to use a method that manipulates a variable physically as close as possible to the controlled vari- able. Because the control objective usually is tower overhead pressure or condensate drum pressure, our control configura- tions will look at different ways to control pressure by varying condenser duty and overhead product rates. The literature in- cludes systems that control overhead pressure by reboiler duty Method: Vary net vapor rate, no makeup gas....
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This note was uploaded on 12/29/2011 for the course CHE 128 taught by Professor Scott,s during the Fall '08 term at UCSB.

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010138 Column Pressure - Effectively Control Column...

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