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We are treating Heavy Naphtha to produce DSN in Naphtha Hydro-treating Unit. The effluent stream from reactor bed flows to a Separator Drum after cooling through an air-cooler. As per the recommendation at the up stream of air cooler, we are giving 350 litres/ hr wash water after each 15 days interval. Even though, instead of NH4Cl salt we are getting Iron Chloride salt at the suction of Compressor strainer. Therefore, frequency of changeover of compressor has increased remarkably. In this case, what should be the preventive measure for the air cooler as corrosion product is generating. This problem has been observed after 3 years of operation life. Can you share your experience please?
 
Answers
11/02/2017 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
This is the typical issue with any hydrotreater when feed contains high amounts of chloride and nitrogen. I suggest doing following checks will solve your problem.
1. You have mentioned that water is added just at the upstream of Aircooler. Find out air-cooler upstream temperature before wash water is injected.
2. Calculate NH4Cl and NH4HS desublimation temperatures using API correlations. Typically NH4Cl desublimation temperature is much higher than NH4HS desublimation temperature. If your unit actual aircooler inlet temperature is lower than NH4Cl desublimation temperature, water injection point in your unit may not be at correct location. Water injection point needs to be moved to the reactor effluent line where the temperature is above NH4Cl desublimation temperature. This is very important to find out the location of water injection. Typically designers take care of this. However actual feed Chloride and nitrogen comes out to be higher than design values and hence may change its location. Some times water injection locations comes out tobe at the upstream of process exchangers.
3. Use any steady state simulators like Aspen Plus, Pro/II or Hysys, generate simulation file at the upstream of aircooler conditions. Use pressure and temperature same as actual air cooler inlet temperature. Flow and composition can be obtained by back blending products and recycle gas. Mix actual water at this conditions and see if there is any free water observed in liquid phase. If no free water in liquid phase, it is necessary to increase wash water flow until some free water observed in liquid phase at the upstream of air cooler.
4. Other last option could be to upgrade the metallurgy of aircooler and piping with Inconel metallurgy which is the costly solution.
5. If you don't want to do above exercise, change the intermittent wash water injection facility to continuous wash water facility and monitor separator boot water properties. Having intermittent wash water injection is more dangerous as Chlorides or NH4HS accumulates over the period of time and when we provide temporary wash water injection, NH4HS and NH4Cl concentration increases drastically at air cooler inlet and increases corrosion.
10/02/2017 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
If the chloride adsorbent on the Cat Reformer net gas has become saturated with HCl, you may see organic chloride breakthrough to the hydrogen system including the makeup gas supply to the Naphtha Hydrotreater (NHT) unit. Organic chlorides would thermally decompose at reactor conditions, releasing HCl which will be highly corrosive at the dew point location in the reactor effluent system and would generate quite a lot of iron chloride scale. The same applies if the naphtha feed to the NHT has been contaminated with chlorinated solvents for degreasing. These chlorinated solvents are sometimes used by crude oil producers to remove heavy fouling deposits in production wells and by refinery maintenance crews to degrease tanks at tank turnarounds.
09/02/2017 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
In HDS NH3 is formed that combines with H2S to give ammonium bisulfide that is washed with proper amount of water to protect the equipment from corrosion. Ammonium chloride is formed when ammonia reacts with HCl. Does your feed contains chloride compounds or you are using make up gas from CRU high in HCl content. There is limitation of HCl to be less than 3ppm or so in make up gas. So u need to take up steps to minimise HCl in make up gas & modify washing procedure. Be sure that DSN going to CRU meets water specs in charge.
09/02/2017 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com
Fouling or corrosion caused by ammonium salts such as NH4Cl or NH4HS are often observed at hydrotreaters, hydrocrackers or reformer units. These salts have a very high corrosion potential, why operational problems can be observed after a period of time. The presence of iron chlorides at the suction of your compressor strainer is a clear indication, that unwanted ammonium salt moisture is transported with the gas phase. It can react with the metal surface of the equipment to form the iron chloride deposits in the strainer or even more worse to foul the compressor blades. If you observe corrosion deposits in the strainer I would also expect severe corrosion attack in the heat exchangers after the reactor.
In your process unit there is more humidity than free water present. A water wash procedure with 350 litres/hour after 15 days interval, which you described will not help at all. Kurita´s patented ACF technology is a chemical program, which can be installed easily to avoid or significantly minimize the risk of ammonium salt fouling or corrosion. Our ACF programs use a very strong organic base, which immediately reacts with strong acids such as HCl or ammonium salts. Depending how much ammonium salts are present at hydrotreaters, Kurita sometimes uses special formulation of ACF combined with a special film-forming component to provide a perfect protection of the metal surface. The ACF reaction products have a neutral pH and a very low corrosivity. Please also search on our home page (www.kurita.eu/en/) to find more information about the ACF technology.
09/02/2017 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
That does not sound frequent enough with the water wash. Let's see what others say.
08/02/2017 A: Muhammad Akhtar, Orpic, mbadgk@gmail.com
I agree, what is the feed chloride content (check desalter operation) and nitrogen content. The water injection rate is very low for washing. In fact it will create even more corrosive corrosive atmosphere. I have seen 3 to 5 m3/hr of water wash at any point of time. It can be continuous or intermittent based on the design of the unit. In my case I have worked in two different unit with different feed composition, both used 5 m3/hr of water wash rate. One unit was continuous wash while other is every fifteen days. So important thing is water wash flow.
08/02/2017 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
If you are running a feed high in chlorides, I would recommend first cleaning up your feed (either improving desalting in crude unit or water washing the coker overhead (if you are running coker naphtha). Second, I would recommend continuous water wash to the reactor effluent air cooler. Iron chloride salts are direct evidence your plant is dissolving.