The origins and fates of chlorides in hydroprocessing units
A step-by-step roadmap to identifying and managing the negative effects of chlorides in hydroprocessing units.
STEVEN A TREESE
Viewed : 2255
This article in three parts explores the impacts chlorides may have on hydroprocessing units (hydrotreaters and hydrocrackers). It will provide a methodical approach to identifying the typical effects that point toward chlorides, the sources of chlorides in process feed streams, chloride induced failure mechanisms, methods for identifying chlorides, strategies for chloride control, and a step-by-step process outline for dealing with a problem. Some of the approaches and impacts here can also be applied to other halogens in hydrotreaters, such as fluoride.
The first part of the article will focus on recognising a chloride problem in a hydroprocessing unit. In the second part, we will tackle how to identify the magnitude and source of the problem.
Part three of the article will focus on ways to address the chloride issues.
Problems caused by chlorides are often missed or misdiagnosed in a refinery. They impact not only the hydroprocessing units, but other units as well. Sometimes the methods used to manage chlorides in upstream units, such as corrosion inhibitors, merely move the problem on downstream. Partial solutions in hydroprocessing units may, in turn, just pass problems on to other units. Comprehensive solutions require a wider understanding of the problem.
Locating a chloride root source is made more difficult if a problem has gone unrecognised or has been allowed to persist for a few months. The chlorides will propagate throughout a refinery in multiple streams and in multiple forms to obscure the original source. There may be multiple sources. Someone may have introduced the chlorides into a system without realising it or without realising the impacts.
The amount of material required to create a significant chloride problem is often very small. To get 1 wppm chloride from perchloroethylene (PERC) contamination in a 50000 barrels of naphtha hydrotreater feed, you only need about 1 gallon of PERC in the feed (0.0055 vol% of the stream). Introducing a barrel of PERC into the naphtha stream can contaminate it for several days.
So how can you approach a chloride problem? Methodical application of the steps below is suggested. The balance of this article provides the background to execute the steps:
1. Recognise the problem Recognise the chloride problem from the impacts observed in the plant. Where is the problem? Are other units seeing problems?
2. How big is the problem? How much material are you looking for? Calculate or estimate the amount.
3. Identify the source(s) Identify potential sources for the chlorides, both organic and inorganic. Look especially at reformers and isomerisation units where concentrated organic chlorides are present. Use analyses to narrow down the possible sources. For organic chlorides, determine specifically what chloride compound(s) you are looking for (speciation). If all you find is PERC, for instance, then you need to suspect the reformer, the isomerisation unit, or solvent dumping.
4. Manage the chlorides This may include physical changes or correcting practices and procedures. You may need to use higher metallurgy in some equipment. You may need to adopt a coping strategy rather than a complete solution.
Chlorides and/or their effects can be successfully controlled, once they are identified and understood.
We will begin by looking at how to recognise a problem rooted in chlorides.
Step 1: Recognise the problem
Chloride as a possible issue can be identified from its typical effects on hydroprocessing units. There will likely be impacts in other units also which can serve to support your identification.
Figure 1 illustrates several areas to look for indications of chloride impacts in a hydroprocessing unit. Impacts are sometimes seen in other units of the refinery.
Referring to Figure 1, the most common issues indicative of chlorides include:
- Deposition of salts in reactor preheat exchangers and charge heater (A) Deposits of white salt in exchangers when opened often indicate a chloride problem. This is seen in cokers and crude units, as well as hydroprocessing units. If there is any entrained water in the hydroprocessing unit feed or chloride salts above saturation are present, there will be fouling of the feed preheat exchangers. The salts will simply lie down on the exchanger tubes as their solubility dictates. If the tubes are austenitic stainless steel, stress corrosion cracking may occur. Under-deposit corrosion is also a likely result. In any event, there would be a loss of heat transfer and, eventually, high pressure drop.
- Reactor fouling (C) Chloride containing salts that reach the reactors will decompose or hydrolyse at reaction conditions, releasing HCl and leaving metals fouling the catalyst. If, somehow, sodium chloride is present in the feed, it will deposit directly in the catalyst bed without decomposing, forming a hard rind and causing high pressure drop. Fortunately, most catalysts today are fairly resistant to poisoning, so the metals may not hurt much. The HCl probably does not harm the catalyst and, in the case of hydrocracking, may even help catalyst activity a little. More importantly, the HCl moves into the effluent train.
- Reactor effluent fouling and corrosion (B, E) Heat exchanger tube and shell thinning or pitting, especially in reactor effluent exchangers, is often seen. The most common corrosion location of concern is the reactor effluent side of the feed/effluent exchangers where ammonium chloride salts (NH4Cl) deposit in the exchangers, especially when wash water practices are inadequate. If austenitic stainless steel is present, chloride stress corrosion cracking presents an additional metallurgical challenge.
Fouling or high pressure drop in effluent exchangers at higher temperatures than ammonium bisulphide laydown occurs (say over 250°F, 120°C) also points toward ammonium chloride deposition.
In the reactor effluent train, ammonium chloride will deposit below the temperature indicated by:
TDep = 523 * EXP(0.0507 * Ln(Ksp)).
Ksp = PNH3 * PHCl
Where TDep = deposition temperature, °F
PNH3 = ammonia partial pressure, psi
PHCl = Hydrochloric acid partial pressure, psi.4
Note that the feed nitrogen is just as important in this equation as the chlorides. The use of amines upstream to control corrosion or scavenge H2S will aggravate a chloride problem. Once deposited, the ammonium chloride increases pressure drop, reduces heat transfer, and causes tube damage by under-deposit corrosion. This effect is intensified as water begins to condense in the reactor effluent. The first drop of water will be rich in acid gas and very corrosive. Ammonia generated from feed nitrogen or injected with wash water can help reduce the pH impacts, but ammonia is not as soluble at high temperatures as HCl, so the HCl tends to control the pH.
- Stripper/fractionator feed preheat (A) Some of the most difficult exchanger conditions are presented when fractionator or stripper feed is preheated by high pressure reactor effluent. The fractionator feed has a small amount of residual free water that is carried into the fractionator preheat exchangers. This water contains dissolved ammonium chloride. As the stream is heated, the water eventually evaporates, leaving ammonium chloride salt deposits on the tubes where it evaporates. The exchanger where you can expect trouble can be identified using flash calculations, if you can estimate the water slip out of the upstream separators.
The deposit insulates the tubes, raising the temperature on the underside of the deposit until the ammonium chloride breaks down into ammonia and hydrochloric acid. The presence of trace amounts of water in the hygroscopic ammonium chloride deposit promotes acid attack of the tubes in the form of pitting.
Add your rating:
Current Rating: 3