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We have tried several times to continue inject wash water @ 80-100l/h at the upstream of air cooler of NHT separator drum but every time recycle gas flow becomes low remarkably due to suction valve get choked which seemed some entrained water and salt carried over and blocked the suction load valve of Compressor. Therefore, please advise us what could go wrong and how to solve the problem? Moreover, we have increased the temperature after Separator aircooler (A-201) to avoid the tube corrosion which reduced the DP fluctuation across the reactor R-201 but frequency of salt formation at the suction strainer was increased and it seemed de-sublimation point of salt attained at the downstream of A-201 which is close to the suction of recycle gas compressor. However, last couple of days we did not inject the wash water and monitored the rate of salt formation at suction strainer and found very insignificant amount of salt that could be the reason the de-sublimation point shift and not coming close to the suction of compressor for the time being. But, Today again get problem just after injecting only 50 litre wash water but no salt is found at strainer which seemed water carry over and choke suction valve again. For your info, we have drained water from the separator boot continuously- is there any problem with the water injection point? So, should we shut down the plant and need to do steam cleaning whole NHT Circuit to eliminate all the salt from the system?
 
Answers
18/03/2017 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
I don't understand how compressor strainer pressure drop increases because of wash water addition at aircooler inlet. It may be a coincidence. Consider following points in mind while operating the unit.
1. Make sure wash water is continuous addition at the upstream of Air cooler. Using intermittent wash water injection may cause more problems as salts accumulated in separator vapor line (Compressor suction line) and corroded Fe and Salts increases when HC is wetted with water intermittently. Continuous injection of water will continuously remove salts from HC and gas.
2. Water carryover is the last step before HC carryover to compressor as water density is higher and it is the first to separate as per 3 phase separator sizing calculations.
3. I don't think Chloride in feed will be a problem as you have not mentioned any problems in Air cooler upstream exchanger and Aircooler. Also if you observe no problems with Reactor effluent air cooler, water injection point would also be fine.
4. Do you have scrubber in place at the compressor suction? If there is no scrubber, check H2S concentration in recycle gas.
5. What is the NH4HS sublimation temperature? If it is less than aircooler outlet temperature, may need to change operating pressure and temperature of the unit.
03/03/2017 A: NS Murthy, Suez, murthy.ns.ext@suez.com
Your problem needs three pronged approach. As the hydrotreater is NHT, the feed naphtha should be relatively free of impurities viz. organic chloride, phenols, amines, free water, oxygenates, etc. Additionally even the make up hydrogen if it is from downstream reformer or platformer, it can also bring in chlorides. All these elements / compounds do form salts of Ammonia in hydrotreaters. Hence, the first step is to determine the impurities ingress and prevent or minimize these.
The second line of defense can be chloride guard on the feed line. Talk to process licensors or adsorbents manufacturers to design such guard beds which can again remove such chlorides and in turn prevent need for water wash.
The last in the chain of guarding operational continuity will be decision of continuous or intermittent water wash. Experience say continuous water wash is beneficial and it shall be aided by good spray distribution arrangement. Salts on ammonia viz. NH4HS and NH4Cl will deposit at location conducive based on product kp. As this can 'shift' depending upon concentration and temperature (anywhere from 30 to 50deg C) having adequate water will be critical to wash off these salts. There will be a penalty owing to LMTD drop with continuous water wash. It may be still worth rather than losing thruput. GE Water and Process Technology team can assist you with right additives to mitigate this issue along with other design aspects.
Good luck.

03/03/2017 A: Sandesh Bhisekar, ADNOC REFINING, sandeshbhisekar@gmail.com
Increase Separator Air Cooler Inlet temperature, maintain well above 100°c. outlet temperature same. Increase separator pressure slightly to help separation. Also check feed quality for any water content. Check CO Analyser may be faulty.
02/03/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Sounds to me like you have entrained water droplets getting through the separator to the Compressor. The water evaporates and leaves behind salt deposits. Confirming that is the problem and analyzing the cause is would require more data than has been reported here.