Steady Under Pressure: Distillation Pressure Control - Method 15

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STEADY UNDER PRESSURE

Method 15: 3B :  Control bypass flow to condensate receiver. 'Hot vapor bypass.'

Method: Vary condensing surface area versus subcooling surface area.
Process: Net vapor rate zero.
Advantages: Condenser may be mounted below condensate drum.
Disadvantages: Concept is not straightforward. Requires subcooling area in condenser.
Application: Often used with very large and heavy condensers or with equipment requiring recurring cleaning or maintenance (exchangers at grade).
Variants: Condensate drum pressure instead of tower overhead pressure may be controlled. General method often referred to as 'hot vapor bypass.'
Configuration notes: A liquid level must be maintained in the exchanger at all times. Careful attention to piping is required. Liquid must enter the drum without mixing with the drum's vapor space . Either have the liquid enter the drum from the bottom or have an internal pipe down from the top. The author prefers the bottom entry as this creates less liquid turbulence. The control valve and bypass line must be sized so that the bypass flow rate changes allow for a DP change that corresponds to the liquid level range available in the condenser.
Operation: To maintain pressure, the control valve DP varies. As control valve DP changes, pressure balance between the bypass and the condenser flows varies the liquid level in the condenser. This changes the allocation of condensing versus subcooling surface available. The liquid and vapor are not in equilibrium. A thin layer of hot liquid separates the bulk liquid pool from the vapor. Essentially, this allows for maintaining drum pressure with a blanketing layer of tower overhead vapor instead of externally supplied vapor.
Warnings: Some units have worked very well with this scheme. Others have failed. Selecting bypass rates and exchanger surface required is mostly empirical. Some general problem areas for this method:
High purity products: This method does not work well with high purity products that have narrow boiling ranges. The liquid insulating layer between the bulk condensate pool and the vapor space fails to adequately insulate the liquid. Control is erratic.
Self refluxing condensers: Heavy material in the overhead vapor condenses first. Some liquid falls to the bottom of the exchanger and runs along it to the outlet. This may change the composition of the vapor sufficiently that at the outlet of the condenser the vapor is no longer fully condensable.
Corrosion of internal pipe: If a top entrance of liquid into the drum is used, the internal pipe must no corrode through. A hole corroded in the internal pipe above the liquid level can lead to mixing of the topmost hot liquid layer and unstable operation. Keep in mind that the internal pipe may corrode from both sides.

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This page updated 03 April 2000.
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