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In the Design of a DHDT unit, what are the criterion for selection of a HHPS (Hot High Press. Separator), along with a CHPS (Cold High Press. Separator) or with CHPS only unit?
Also, in some configurations of DHDT, we can see a HHPS + CHPS + CLPS (Cold Low Press. Separator / i.e. Flash Drum)
What are the criterion for such selection ? Also what are the advantages and disadvantages of the 3 options?
 
Answers
05/04/2015 A: Sarath Konda, DuPont India, sharathsatya@gmail.com
Apart from energy savings, one more advantage of having a Hot Separator is Reactor Effluent Air Cooler fouling will be less as most of the heavy hydrocarbon (or HPNA's) will pass thru hot separator to stripper. This will lower the possibilities of Wash Water maldistribution due to fouling of tubes and lower some corrosion concerns
17/07/2014 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
Two main disadvantages if we add Hot High pressure Separator.
1. Around 10% of the makeup H2 wastage can be observed because of high H2 solubility at higher temperatures. Since HHPS is at high temperatures, more H2 goes through HHPS liquid line and end up in stripper offgas which goes to fuel gas header. It is un recoverable.
2. H2 purity reduces little bit in the Recycle Gas from CHPS. The main reason is since most of the hydrocarbon stream is routed to HHPS liquid line, no hydrocarbon stream available to get a sponging affect and absorb all lighter materials (C1, C2 and C3) which are generated in reactor. These lighter materials circulate in the recycle loop and hence reduces the purity a little bit.
Advantage if we provide HHPS
1. Obviously the energy savings because of not rejecting the whole stream to air condenser before CHPS.
2. It is very good advantage and required for VGOHT mainly to avoid HPNA condensation in air cooler.
3. For VGO type of materials, water separation is easy because of high density difference between water and lighter material if there is a hot separator is provided.
11/06/2014 A: Robert Larsson, Technip, robban8@yahoo.se
The advantages by adding a Hot High Pressure Spearator (HHPS) would be:
-Energy savings as the oil from the HHPS can be sent directly to the stripper witout reheating
-Smaller Reactor Effluent Air Cooler and Cold High Pressure Separator (CHPS)
-Better separation between water and HC in CHPS (as the density difference is greater), which means lower residence time (smaller vessel) and less risk for emulsions.
-The HC stream from the CHPS can be reheated against the stripper bottoms so no need for a high pressure exchanger to reheat againt the reactor effluent or a fired heater.
The advantages for having a CHPS only would be that the Recycle Gas purity would be higher as the hydrogen solubility losses increases with a HHPS. This can be an issue if low purity make up gas is used. Also more of the H2S gets dissolved in the liquid if using CHPS only which could be an advantage in some situations. Most of the time the CAPEX would be slightly lower with a CHPS only but it is not always the case.
For the diesel and heavier oils the HHPS and CHPS layout will often be preferred one, but as mentioned before an analysis for each unit needs to be done as it depends on other factors as well as size of unit, make up gas purity and product specifications etc.
Adding low pressure separators are mainly done for two reasons. One is avoiding having to deal with gas break through case from the high pressure separators in the stripper or adding a PSV on the line itself. This is especially the case for high pressure units. By adding a Cold Low Pressure Separator (CLPS) an additional stage of separation of water from HC is provided and hence this would minimze the water carry over to the downstream the stripper and corrosion issues in the stripper.
28/04/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The short answer is “energy savings.” As you said, there is wide variation in the number of drums in hydroprocessing unit designs. In their designs, capital cost and energy consumption were balanced to achieve an optimum design. At least, that is what should have been done. For example, it is obvious that the recycle compressor suction should be from a high pressure drum, and not a low pressure drum. Yet, one of the resid hydrocracking licensors' designs has the compressor taking suction from the low pressure cold drum.
The largest number of drums I have seen in a simple diesel hydrotreater was designed by Imperial Oil Company circa 1980.