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in our C5/C6 Isomerization unit, the reactor effluent after feed/effluent heat exchanger has 43% liquid at 1040C. this effluent is being cooled to 400C using air finfan cooler and water trim cooler. Hydrogen rich gas is separated from liquid hydrocarbon in a separator at 18 kg/cm2 and used as recycle gas. We normally maintain H2/HC = 1.5 mole ratio.
We want to install a hot separator at the outlet of feed effluent exchanger to separate the condensed hydrocarbon and the remaining gases and uncondensed vapour will be further cooled in air fin fan cooler and water trim cooler. Liquid hydrocarbon from hot separator and cold separator will be mixed and sent to stabilizer to remove C1-C4 components.
1) I want to know whether this type of hot separator is feasible and did any refinery has this type of system.
2) How much will it affect recycle gas purity.
 
Answers
28/05/2017 A: Ganesh Maturu, Reliance Industries Limitied, maturu.ganesh@gmail.com
Nowadays many refiners are considering heat recovery in unconventional approach. Providing hot separator will save some energy but economically it should be justifiable. However you need to have detailed review. Following are some points to be considered.
1. Providing hot separator reduces recycle gas purity by 1 - 2 %. because of low hydrocarbon at cold separator.
2. H2 losses will be increases as hydrogen solubility high at high temperatures. You should calculate these losses while checking energy benefits.
3. Providing hot separator is economical if you have streams which can absorb 150...200deg c heat.
17/05/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I have never heard of such a system in this service and would be surprised if it were economically viable as a project. Normally a hot separator is used on a unit like a cat feed hydrotreater, where the reactor effluent mostly condenses at a much higher temperature. In that case the liquid recovered in the hot separator is much much higher than 50% and the temperature is much hotter.
In your case the stabilizer feed may only get ~27C warmer (104*.43 + 40*.57 vs. 40C now). Because heat put in the feed is less efficient than heat put in at the reboiler, the reboiler duty reduction will be less than the amount of heat added to the feed.
I would expect hydrogen recover to go up slightly (less H2 absorbed into cold separator liquid because the liquid volume goes down) and purity to decrease (less light ends also absorbed, which has the net effect of reducing purity.