Between flooded condenser pressure control and hot vapor bypass control in a distillation column which one is more preferable and under what circumstances?
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13/08/2013
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keith bowers, B and B Consulting, kebowers47@gmail.com
Hot Vapor Bypass is more reliable for operations with known non-condensables of widely varying flow rates.
If the column overhead does not have any non-condensables, flooded condensers can work smoothly.
In both cases, hydrate formation can plug exchanger tubes if the cooling medium (air or water) is too cold and the pressure is high enough.
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28/11/2012
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Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
At the risk of being very negative, "flooded condensers" are always ill-advised and always disruptive to distillation. This is especially so if P/Pc,sm ratio or rho,vapor density is higher. Resultant chronic cycling of absolute tower pressure causes chronic cycling of heat of evaporation and this "confuses" the trays and theoretical stages inside the tower itself. Computer models and physical models of distillation towers ASSUME 'gravity flow' overhead condensers.
Flooded condensers have been specified in the design phase either out of lack of understanding or ill-advised intent to "cheap down" the capital cost of the distillation tower. Performance penalties will ensue.
If heat sink is cooling water, lack of adequate cooling water pressure or plant prohibitions on CW booster pumps favors keeping the condenser down near grade.
It is difficult to achieve smooth flow through any type of hot vapor bypass due to low deltaP available and then there is difficult in the real world [not the computer world] for interfacing and mixing to occur to reach envisioned steady state thermodynamic equilibration.
With or without hot vapor bypass, flooded condensers operate by filling up with liquid [even if there is some gas co-product] and then "blowing out" liquid slugs to the reflux drum. There is really no other way for the condensed liquids to be pumped forward against a static head.
If there is substantial gas co-product, two phase fluid flow will mitigate the "slurping". But hydraulic design methods for two phase fluid flow in practical piping really can not ensure an approach to "steady state two phase fluid flow".
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28/11/2012
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
During flow sheet reviews, you will find experienced engineers and operators with firm convictions that one or the other applications does not work properly. I have found that there are two reasons for that. In the case of flooded condensers, there were noncondensables present or the elevation of the drum was wrong. In the case of the hot vapor bypass, the design was faulty. Either system will operate properly if designed properly and if there are no noncondensables in the case of a flooded condenser.
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