Steady Under Pressure: Distillation Pressure Control - Method 25

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Method 25: 3 :  Control bypass rate.

Method: Does not work.
Process: Net vapor rate zero.
Advantages: None.
Disadvantages: Does not work.
Application: None.
Variants: Combine the condensate and vapor bypass before it enters the drum.
Configuration notes: This is an incorrectly configured hot vapor bypass design. Instead of maintaining a liquid level in the condenser, the condenser is allowed to fully drain into the condensate drum.
Operation: Bypass rate is controlled in an attempt to control pressure.
Warnings: The key here is no liquid level in the condenser and no vapor product. With no liquid level in the condenser, liquid leaving the condenser is only slightly subcooled, or not subcooled at all. No heat removal is available to condense the vapor that bypasses the condenser. With no subcooling in the liquid to condense the bypassed vapor, it is impossible for this system to maintain pressure. Pressure will float on the heat removal capacity of the condenser regardless of control valve position.
Special Note: This method has some controversy. Some instances have been reported as successes and others as failures . The author's experience supports the 'does not work' conclusion and recommends avoiding it.

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This page updated 03 April 2000.
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