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May-2019

Real-time total chlorine monitoring to ensure optimal desalter performance

Chlorides are always present in raw crude oils and their concentrations vary greatly. Depending on origin, transportation methods, and process conditions, chloride concentrations can spike in a very short period and lead to devastating corrosion events across the refinery.

Evan Thomas
XOS
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Article Summary
Some estimate that global corrosion damage is around 15   billion USD annually, and this doesn’t take into account the profit and production time loss. There are a variety of factors that contribute to this corrosion, and it is of the utmost importance to have as much data for as many parameters as possible to effectively combat corrosion before it can cause serious problems. XOS’ Clora® Online, X-ray fluorescence industrial process analyser, provides a total chlorine measurement in real-time for use in desalter management and corrosion reduction plans.

Background
When present in the furnaces of atmospheric and vacuum distillation units, NaCl, MgCl2, and CaCl2 will hydrolyse to form hydrogen chloride vapours. When these vapours mix with condensed water in the cooler sections of the tower, they form highly corrosive hydrochloric acid. This leads  to serious corrosion events and turnarounds, or worst case, fires and explosions. Chlorides that don’t hydrolyse can lead to fouling, heat exchanger efficiency loss, and extreme local corrosion which decreases the life span of the heat exchanger.

The crude desalter is the first line of defence and a critical piece of equipment tasked with removing chlorides by dilution, emulsification, and coalescence. Ideally, the desalter achieves 90% removal efficiency, and even with second stage desalters, the chloride removal is at best 99%. However, during upsets, far more chlorides can pass through and enter the atmospheric tower. This can lead to catastrophic corrosion damage if not detected.

There is a direct correlation between unit life and desalter performance. Changing crude slates makes effective desalting more difficult due to the variable crude quality and unintended side effects of incompatible blends. This  in turn leads to higher and variable desalted crude salt content, further affecting unit reliability. In addition, the optimal desalter operating conditions are not static values, and real-time changes must be made to reduce risk to the atmospheric tower and other valuable units.

The vast majority of chlorides entering the desalter are inorganic chlorides of the sodium, magnesium, and calcium variety and will mostly be removed through  proper desalter operation. Some chlorides, termed non- extractable chlorides, cannot be removed by the desalter and end up in the atmospheric or vacuum distillation units, or naphtha hydrotreater units. There is not one location that these originate from, but they are typically either organic in nature or inorganic chlorides enshrouded in paraffins or asphaltenes.

In events where organic chlorides make it to the naphtha hydrotreater, they can be subjected to conditions whereby they form HCl. Under normal conditions, NH3 generated from the denitrification process in the presence of HCl form dry NH4Cl salts which can deposit on the heat exchangers. While these dry salts are not corrosive, they are extremely hygroscopic, and in the presence of water can lead to extreme local corrosion issues. The deposits on the heat exchangers are typically removed by periodic injections of neutralised wash water. An effective chloride measurement is critical here as a ratio of nitrogen to organic chloride in the feed dictates the necessity of either periodic or continuous water washing. The neutralisation required by this wash water is also determined by the measured chloride content.

Temperature adjustments of up to 25° C may be necessary to avoid asphaltene precipitation and allow for the removal of formerly non-extractable chlorides. Organic chlorides have been found to originate from chemical cleaning solvents used in the production fields. Such solvents have historically also been used to clean industrial equipment to dissolve deposits composed of waxes, paraffins, and tar. Finally, chloride-based solvents used in fracturing fluids have been used for well stimulation. If these solvents contaminate the crude oil, they are typically oleophilic, meaning they do not easily separate from crude oil, and rarely get removed in the desalter. To make matters worse, at low levels these chlorides will not be detected by most analytical techniques, and most salt in crude analytical techniques will not measure them at all. Therefore, refiners are unaware of their presence even with a good monitoring program.

In one case, a refinery had as little as 1% of the non-extractable chlorides hydrolysing in the atmospheric distillation unit, which led to new overhead condenser tubes failing in less than 14 hours. In another case, non-extractable chlorides that made it to the naphtha hydrotreater reduced the service life of the heat exchangers to 33 days.1 While these are extreme cases likely caused by poor material selection choices, chlorine corrosion can still cause failures in as little as a few months in installations where appropriate materials are used.
A variety of methods to deal with unit corrosion have been used, such as source control, crude blending, monitoring of the overhead and use of neutraliser, monitoring of the crude and use of caustic, and other chemical additions. In extreme cases, some refiners upgraded metallurgy in all condensers and associated piping at great cost. These responses may be appropriate for normal operation but will not be adequate for upset conditions or for different crude slates.

Occasionally, more problems develop in an attempt to solve corrosion issues. For example, use of too much caustic can lead to fouling and stress corrosion cracking in the coker unit. A proper monitoring schedule is critical to develop and maintain effective corrosion reduction plans.

Typical chloride inspection occurs by grab samples and lab measurement as little as once a week. While these measurements may look good and indicate proper performance, they usually miss potentially catastrophic chloride spikes. Industry leader Nalco Champion has conducted studies that show 90% of corrosion damage occurs in as little as 10% of the time. These spikes  occur during crude tank switches, slop oil processing, processing of opportunity crudes, and other unstable periods of operation. Untreated, these spikes cause the greatest amount of corrosion damage over the unit’s lifetime. In addition, many measurement techniques and lab protocols fail to take sample settling into account, leading to a misleading measurement. Process control based on this type of monitoring program is impossible.

The key to effective corrosion control begins with an effective chloride measurement regimen. By knowing the desalter inlet and outlet crude, as well as overhead chloride content, changes can be made to optimise the desalter in real-time. The ability to track the effects of other changes, such as caustic injection in the desalted crude, neutraliser in the overhead, or institution of a water wash, enables the most effective corrosion mitigation program. By monitoring the naphtha hydrotreater chlorides, process control is enabled and catastrophic corrosion events can be prevented. Online chloride monitoring helps to optimise a plant’s profit, performance, and safety.
 
Introduction

Over the last 20 years, XOS has proven itself as a leader in online sulphur analysis using Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF®) technology with over 250 analysers in refineries worldwide. Initially, online chlorine analysis in crude presented more challenges due to low-level quantification requirements and sampling complications. Typical measurement techniques that involve separating the oil from water, mixing, and measuring have traditionally run into problems where chlorides settle out of solution leading to different measurements over time. In addition to this, limited testing frequency and extended time between when the sample was grabbed and when it was measured often make these measurements useless in upset identification and control. To this end, a MWDXRF chlorine in crude analyser was developed.
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