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CDU overhead system
To provide a CDU with top reflux there are several configurations. The two main ones are as follows
Configuration 1: Double condensation
Configuration 2: Top pumparound
In configuration 1, total overhead vapors are condensed in two condensers in series. In the first condenser part of vapor equal to the required top reflux is condensed, water is separated and hydrocarbon liquid is returned to column as top reflux. The remained vapors (this includes the top product plus non condensable vapors) is routed to the second condenser.
In configuration 2, the top reflux is provided by a top pumparound and the overhead vapors ( (this includes ONLY the top product plus non condensable vapors) are condensed in a condenser and no liquid is returned to the CDU.
For a refinery with capacity above 100,000 bpd what configuration is recommended? Considering in both case energy is used in top/feed exchangers network. Can Heat Integration cause to use one of these configurations? We know in configuration 2 some more stages are needed as we have added one more pumparound!

Additional:
Thank you for the answers! Don't you think, the double condensation configuration results in lower flowrate in top section? When I can remove water in the first condenser, why not to return the reflux in lower temperature! furthermore I think thermodynamically, configuration 1 is better than configuration 2. As Ralph stated, the latter also needs at least two/three more trays for top pumparound! What I am not sure is energy saving! it is believed that configuration 2 results in a better heat integration.
 
Answers
07/06/2011 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
The first configuration that you mention is used to reduce water condensation inside the column, which will lead to corrosion, also it reduces the probability of water condensation in the overhead, all this allow to use less expensive metallurgy in the mentioned areas. You have to make sure that water condensation happens at the outlet of the first condenser so chlorides can be removed.
For the second configuration, pumparound return temperature will be lower, which could lead to corrosion, specially that pumparound return will probably have free water (due to water solubility being reduce as temperature is drop in the exchangers)
06/06/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Both designs are effective in reducing corrosion in the top section of the column, compared with a design that uses a single overhead accumulator. In any of the 3 cases, resist the desire to yield light naphtha overhead and heavy naphtha as the first side draw product. In other words, in any case, the overhead vapor temperature needs to be well above the temperature at which water is a liquid.
An advantage of the top pumparound (PAR) only design is that the crude exchanger does not need to be elevated for “free draining”. This savings can directionally offset the cost of additional trays and column height associated with the top PAR only design.
One thing to compare: For the general power failure case, and using the “hot firebox, tube boilout criterion”, there can be a significant difference in the size of the relief system. It has to do with the “Cox Chart” effect on LMTD of the air and water cooling portion of the heat removal configuration when calculated at relieving pressure.