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In a hydroprocessing Unit, what factors determine location for adding washwater? I have seen two locations in my unit - one upstream of the air fin fan and one at the inlet of the last CFE. Where should I inject wash water?
Vishnu Prasad, Hindustan Petroleum Corporation Ltd, email@example.com
Feed nitrogen & chloride content are most important factors in determining the location of a water injection point. The ammonium chloride salts formed in the hydrotreator can be deposited due to sublimation at temperatures of 200 deg C+. Also, if your feed chloride content is high ammonium bisulphide deposition occurs at lower temperatures, typically 65 degrees C and below. Wash water is injected to wash away these salts formed during the operation.
If your feed is free of chloride, water injection to the air fin cooler inlet would be sufficient. You can refer to API 932-B for the details.
Dipankar Phukan, Indian Oil Corporation, firstname.lastname@example.org
It depends upon the feed you are processing and amount of chlorides your feed has, and the temperature and pressure maintained. Mostly, connection at the last CFE is for intermittent use and at fin fan cooler inlet for continuous use. At the injection point at least 25% of water should be in the liquid state. I have seen salt deposition in the last CFE, which has led to an increase in delta P across the exchanger. As a practice in my previous plant we used to inject wash water upstream of the last CFE once a fortnight for a period of 2-4 hours.
Jake Gotham, InSite Technical Services, email@example.com
Most units are designed with a water injection point upstream of the air-cooler, and another upstream of the last heat exchanger. I’ll describe the function of the two injection points separately:
1. Water injection upstream of the air-cooler. This is provided to remove ammonium bisulphide which forms in the reactor effluent from the ammonia and H2S generated in the reactor. In most units processing diesel, VGO or residue feed, there is sufficient nitrogen in the feed to necessitate operating this water wash continuously. In naphtha and kerosene hydrotreaters, clean 2nd stage hydrocrackers and some lube units there may be insufficient nitrogen present to require this water wash to operate continuously.
2. Water injection upstream of the last heat exchanger. If there is any chloride in the feed or hydrogen supply, ammonium chloride can also form in the reactor effluent. This tends to start at a higher temperature, typically in the last heat exchanger before the air-cooler. It is good practice to monitor the pressure drop in the effluent system to look for this problem. If the system is well designed, water injection can be used to remove the ammonium chloride and reduce the pressure drop. This should only ever be an intermittent injection. If it becomes necessary to use this injection point continuously, there is a serious chloride issue which needs urgent attention.
Both water injection points are provided to remove fouling and reduce corrosion. However, poor design or poor operating practices can cause other corrosion problems. Please get in touch if you’d like any help reviewing or improving your water wash systems.
Eric Vetters, ProCorr Consulting Services, firstname.lastname@example.org
Water washes in hydrotreaters are designed to prevent solid salts from forming and to dilute those salts to a low enough concentration that the salt solution as is not corrosive. The water wash needs to be injected ahead of the point where salts will first form. The salts of concern are ammonium bisulfide and ammonium chloride.
Most often these salts do not form ahead of the REAC (reactor effluent air cooler), but they can. Two different factors or a combination of the two can move this salt point upstream into the last exchanger of the feed effluent train. At start of run with no fouling the feed/effluent heat transfer is much more efficient and reactor temperature is also lower at SOR. This means that reactor effluent temperature going into the REAC will be coolest at SOR. For diesel and lighter hydrotreaters this cooler temperature also means that additional hydrocarbon will condense, which also increases the vapor phase concentration of the salt forming compounds. If that temperature is low enough salts can start to form in the last exchanger instead of the REAC. The other thing is if the concentrations of salt forming constituents are higher.
One thing that can catch a refinery by surprise in this area is if ammonia removal is higher than anticipated. This situation can occur with a new catalyst or a new/redesign of a unit. In one revamp I was involved with the catalyst vendor wanted to be conservative and not overpredict the amount of nitrogen removal in a diesel hydrotreater. When the unit started up the amount of ammonium being formed was significantly higher than anticipated based on the predictec nitrogen removal efficiency, and the feed/effluent heat transfer was better than anticipated. The result was that the amount of water wash was inadequate (ABS concentration too high), and the location at the REAC inlet was wrong (combination of higher salt point and lower process temperature than anticipated).
If the ydrogen source is from a catalytic reformer problems with the chloride balance can put extra chlorides in the make up hydrogen and increase the salt point. High chlorides can come from excessive moisture in the reformer stripping chlorides off of the catalyst and/or over injection of chloriding agent (semi-regen or cyclic units).
Injecting into the last CFE exchanger often requires an increase in the wash water rate and reduces the overall heat recovery by quenching the effluent temperature in the CFE, but if salts are forming there, it is a necessary step.
Morgan Rodwell, Fluor Canada Limited, email@example.com
Salt deposition temperature: using the curves in API 932-B you can predict the temperatures at which ammonium salts will form. You should perform water wash upstream of the point where the process drops to the temperature in question. If you are 100% certain there are no halides in the systems then right in front of the air cooler can work if you can ensure good distribution to the passes, because the salt deposition temperature of NH4SH is quite low. But if you have chlorides, bromides or fluorides present you will need to do the water water wash at higher temperatures.
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