Tech Tip #3 - Handling a Flooded Condenser with Hot Vapor Bypass

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: December’s Tech Tip – Handling a Flooded Condenser with Hot Vapor Bypass.

: In most distillation control applications, it is most economical to operate the tower at minimum pressure.  For most systems, relative volatility increases as pressure is reduced, making separation easier.  The reflux ratio can then be reduced to achieve the same separation allowing for a reduction in energy.  This separation energy benefit is usually counteracted by an increase in latent heat and a slight reduction in tray efficiency (tray efficiency may improve if the tower was weeping or dumping at the higher pressure) as the tower pressure is lowered.  In general, the effect of pressure on relative volatility dominates and distillation column separation energy can be minimized by operating at the lowest practicable pressure.  The extent to which pressure can be reduced is determined by the spare capacity available in the condenser, notwithstanding other constraints.

: Pressure control with a flooded condenser can utilize a hot vapor bypass, a control valve in the condenser outlet or a flooded reflux drum where the distillate flow is controlled.  This Tech Tip deals with the case where the tower overhead condenser is a flooded condenser with a hot vapor bypass.  Closing the hot vapor bypass valve causes the vapor in the reflux drum to condense against the sub-cooled reflux and thereby lower tower pressure.  This draws liquid from the condenser, exposing more tube surface and thereby increasing the rate of condensation.  Opening the valve allows liquid to flow back into the condenser, covering tubes and reducing heat transfer and thus raising pressure.  Most flooded condensers are the horizontal type with vapor on the shell side and coolant on the tube side. 

: The position of the bypass valve is an indication of condenser loading.  The objective would be to keep the valve at a minimum, typically at 5-10 percent open.  However, due to valve non-linearities, it is often difficult to control against valve position.  Another indication of condenser loading is delta T.  The lower the temperature difference (i.e. the less sub-cooling) the closer the condenser is to maximum capacity.  Column pressure is minimized subject to this overhead delta T constraint.  The delta T is the difference between the tower overhead temperature and the condenser outlet temperature.  The minimum overhead delta T constraint indicates the minimum tower pressure that can be achieved while still condensing all the overhead product.  As the delta temperature approaches its minimum, the condensing ability is approaching its maximum available.  As the cooling water temperature increases, it will be necessary to raise the tower pressure to maintain sufficient delta T.  When the tower is not cooling limited, other constraints may limit the minimum tower pressure such as column flooding or hydraulic limits.

: When tower pressure is an manipulated variable in a controller, it should be moved slowly.  If the pressure is moved too quickly, it could cause a significant disturbance to the tower.  The liquid on the trays is at its bubble point.  As pressure falls, some of the liquids sensible heat is converted to latent heat.  A sudden decrease in pressure could cause a rapid boil-up on the trays potentially leading to tower flooding.

: Sometimes it is advantageous to break a level loop in a multivariable control application.  In this case, the manipulated variable would then become the distillate flow out of the reflux drum.  In the case of flooded condensers, the level loop on the drum should remain closed.  The key to successful pressure control in using a hot-vapor bypass is in the sizing of the control valve.  The pressure drop across the valve is relatively constant, being essentially the head of liquid between the reflux drum level and the condenser level.  Allowing the drum level to fluctuate will cause the level in the condenser to fluctuate which in turn will cause the tower pressure to fluctuate.  In installations where the volume of liquid contained in the condenser is significant compared to that in the drum, there may be pronounced interaction between the pressure and drum level control loops.  The pressure controller should be adjusted as tightly as possible with the drum level loop open and then the level controller adjusted with the pressure loop closed.  This procedure gives preference to pressure control, which is the more important function.


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