This series of three articles explores the impacts chlorides may have on hydroprocessing units (hydrotreaters and hydrocrackers). It provides 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.
This first article of the series focuses on recognizing a chloride problem in a hydroprocessing unit. In the second article, we tackle how to identify the magnitude and source(s) of the problem. The third article presents ways to address the chloride issues.
Read Part 2 of this blog
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 unrecognized or has been allowed to persist for a few months. The chlorides will propagate throughout a refinery in multiple streams and in multiple forms which can obscure the original source. There may be multiple sources. Someone may have introduced the chlorides into a system without realizing it or without realizing 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 50,000 barrels of naphtha hydrotreater feed, you only need about 1 gallon of PERC in the feed (0.0055 v% 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.
Chlorides and/or their effects can be successfully controlled, once they are identified and understood.
We begin by looking at how to recognize a problem rooted in chlorides.
Chlorides 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 below illustrates several areas to look for indications of chloride impacts in a hydroprocessing unit. Impacts are sometimes also seen in other units of the refinery.
Fouling or high pressure drop in effluent exchangers at higher temperatures than ammonium bisulfide laydown occurs (say over 250OF) 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)) (Ref. 4)
Ksp = PNH3 * PHCl where
TDep = Deposition Temperature, OFPNH3 = Ammonia Partial Pressure, psiPHCl = Hydrochloric Acid Partial Pressure, psi.
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.
As a general comment, chloride issues are often missed because evidence may come in the form of iron sulfide (FeS) deposits in equipment, which may be attributed to sulfidic corrosion. The FeS may actually come from wet NH4Cl under-deposit corrosion via the following reaction route:
Once you realize you have a chloride issue, you need to determine the magnitude and find the source(s) of the chlorides. These are the subjects addressed in the second article of this series. Read Part 2 of this blog
If you have a question relating to this blog, you may post a comment for the author at the bottom of this page. If you would like to submit an Information Request please click below:
Request Info from Becht
Steven Treese began his professional career with Union Oil Company of California in 1973 as a Research Engineer with a BS in Chemical Engineering from Washington State University. He remained with essentially the same company through several “heritages” before retiring from Phillips 66 in 2013 after over 40 years in industry. He continues to provide process consulting serrvices by request. His range of experience includes operations, hydroprocessing, hydrogen plants, catalyst development, design, process safety, utilities, sulfur recovery, geothermal, shale oil, nitrogen fertilizers, procurement, and process licensing.
Steve has a handful of publications and was on the 1994 NPRA Q&A Panel. He has been an inventor on patents in diverse areas, including vessel internals, enhanced oil recovery, and hydroprocessing. He was lead editor for the “Handbook of Petroleum Processing, 2nd Edition” (Springer, 2015) and is the author of an upcoming book on measurement history.
Steve is a licensed Professional Engineer, a senior member of the American Institute of Chemical Engineers, and a mentor for FIRST Robotics Team 3049 from Bremerton, WA.
Click to View Becht’s Current Technical Training Public Course Calendar