What is the impact of chlorides on hydrotreating catalyst and unit?
Answers
|
14/05/2020
|
A:
|
Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
The general rule with chlorides is that they are harmless as a hot dry vapour, but problematic if they cool down in the presence of ammonia or water. In normal operation, hydrotreating catalyst and the reactor are not harmed by chlorides, but as the reactor effluent cools through the heat exchangers and air-cooler, the problems start. If there is moisture in the feed, small amounts of liquid water can condense in the reactor effluent, forming a low pH hydrochloric acid. This is very corrosive to most materials. The second problem is desublimation of ammonium chloride salts which create a fouling and corrosion problem.
Water wash is frequently used to mitigate both these problems. Adding a large amount of water dilutes the hydrochloric acid, bringing the pH back up. It also dissolves ammonium chloride salts. Most units are designed with a water wash injection point upstream of the air-cooler. In many units this operates continuously to remove ammonium bisulphide, but will also remove ammonium chloride if the chloride contamination level is low. If the chloride contamination is worse, ammonium chloride will deposit in the heat exchanger upstream of the air-cooler. Often a water wash is provided here to be used in response to a problem. Water wash systems are very effective at dealing with these issues, but can also create their own problems if not designed well and operated correctly.
Get in touch with me directly if you’d like to discuss further. I can help you identify the source of the chlorides, how to eliminate the source, operational changes to minimise the impact and, if necessary, design changes to make the unit less vulnerable.
Regards
Jake
|
|
13/05/2020
|
A:
|
Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
The biggest issue with chlorides (and other halides) in hydroprocessing units is that they are converted to HCl (or HF or HBr) in the process due to the high partial pressures of hydrogen. These acid vapours then will react with NH3 in the effluent train as the temperature is reduced. API RP 932-B has clear guidance on this and curves to calculate the deposition temperature of the ammonium halide salts. You must maintain the operating temperatures ABOVE the deposition temperatures to avoid salt formation. If salts form, they are hygroscopic and will absorb water from the effluent, potentially causing underdeposit corrosion as aqueous acids form under the salt. Further, wash water injection must be very carefully designed because the absorption of the acids into the wash water is rate limited and requires very intimate contact. Many new designs use static mixers with the wash water injection to improve the contact area and maximize the transfer of the halide acids into the aqueous phase. There have been many examples of failures (loss of containment) in the effluent trains of hydroprocessing units due to ammonium chloride corrosion.
|
|
13/05/2020
|
A:
|
Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
HDS catalyst is not chlorided and chlorides don't impact the catalyst. Chlorides will combine with ammonia in the reactor effluent to form ammonium chloride salts if there is not a water wash ahead of the salt formation temperature. The salts are both a corrosion and fouling issue.
|
|
13/05/2020
|
A:
|
Jon Isley, Fluor, jon.isley@fluor.com
Chlorides have minimal to no impact on the catalyst. Organically bound chlorides will react rapidly and be broken down into HCl or equivalent ions.
Chlorides will sublime as NH4Cl salts in the reactor effluent cooling train and potentially cause corrosion in downstream equipment. Namely in the reactor effluent coolers or alternatively in the fractionator overhead system. NH4Cl salts coat and plugs equipment, and in the presence of water is highly corrosive. Refer to API 932 and other references/publications for further guidelines.
|
|
13/05/2020
|
A:
|
Mohan Shyam Pathak, UOP A Honeywell Company, mohanspathak@gmail.com
Chloride is a very difficult unwanted compound encountered in the refinery. The negative impact of chloride on hydrotreating catalyst and unit are mainly related to its salt forming ability when it comes into contact with NH3. The other impact is related to the product formed after hydrolysis of chloride salt at hydrotreating reactor conditions. Some of the specific issues are noted below.
1. Fouling by salts in preheat exchanger at sublimation temperature.
2. Fouling charge heater during any trip
3. Reactor fouling - This covers the impact of chloride salts on catalyst. The chloride salts hydrolyse and produce HCl in some cases. If NaCl salts are present then it is deposited on the top of the catalyst bed thus increasing the dP.
4. Compressor issues - the recycle gas may contain some chloride and thus deposition and corrosion can be observed.
5. Adverse impact on downstream sections/units - reformer, amine system and fuel gas handling units will also be adversely impacted by chloride.
|