Corrosion Refinery

Refinery Reduces Crude Overhead Corrosion through Analysis and Online Control

N.P. Hilton, N Williams, E.W. Vetters, S. Ronnie Nalco Champion

7705 Hwy 90-A, Sugar Land Texas, USA

Phillips 66 UK Ltd South Killingholme, North Lincolnshire, DN40 3DW

ProCorr Consulting Services LLC

ABSTRACT

Crude overhead corrosion control is a major mechanical integrity concern within the refinery industry with several reported industry failures. Traditional corrosion monitoring and control revolves around periodic laboratory sampling analysis of the crude overhead sour water to determine the corrosive nature of the process (i.e. pH & Chlorides) and an indication of corrosion rates by the measurement of iron in the sour water.

A Refinery, in the United Kingdom, is a typical refinery processing a wide variety of challenging crudes. As a result, controlling corrosion in their crude overhead system remains one of their biggest challenges. The refinery decided to trial a new automated analyzer for Crude Overhead Systems. The new analyzer is a patented approach, combining equipment and process control, allowing the refinery to continuously measure the boot water pH and automatically adjusting the neutralizer injection to achieve a control range, whilst also measuring chlorides and irons hourly. This system has allowed the refinery to control their neutralizer addition, gaining control of the pH and iron in the overhead system, thus controlling the crude overhead corrosive environment in real time. The instrument has also allowed in-depth understanding of how the process conditions can change (i.e. slate changes) and the effects these changes can have on the overhead corrosion control.

INTRODUCTION

Controlling corrosion in the overhead systems of atmospheric distillation units is a major ongoing issue at many refineries. Crude oil coming into the refinery contains chloride salts and other contaminants. The desalting process removes the bulk of the salt, but residual salts left in the crude will hydrolyze to form hydrochloric acid (HCl) in the atmospheric furnace. The HCl and other contaminants, such as amines and organic acids, all end up in the overhead system of the atmospheric distillation column.2 Without corrosion control measures, the acids in the overhead system will cause a very low pH at the water dew point, resulting in very aggressive corrosion. Hydrogen sulfide (H2S) present in all

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Paper No.

4347

©2014 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

atmospheric overhead systems will also form iron sulfide corrosion products that can either add to corrosion or inhibit corrosion, depending on how well the system is controlled.

A variety of techniques are employed to control corrosion in the overhead system. Common control measures include a water wash to force the water dew point and dilute corrosive species, use of neutralizing amines or ammonia to control pH (typically between 5.0 and 6.5), and corrosion inhibitors to create a protective barrier on metal surfaces susceptible to corrosion. Neutralizer rates are typically manually adjusted based on pH measurements by either an operator or the chemical supplier. Corrosion inhibitor is typically kept at constant ppm injection rate based on the volume of overhead liquid.

Caustic is also frequently added to the desalted crude to lower the effective salt hydrolysis rate, which reduces the amount of hydrochloric acid produced.2 Caustic can lead to increased furnace fouling as well as reduce catalyst activity if a resid is fed to an FCC or hydroprocessing unit. While caustic use can be beneficial, it is desirable to keep the injection rate at a minimum. The injection rate is typically held fairly constant and only adjusted if there are large changes up or down in overhead chlorides. Refineries are often quicker to increase caustic rates when chlorides spike and slower to decrease the caustic rate when chloride levels return to normal. As a result an excess of caustic is often used and the refinery decides to just live with the problems it causes downstream.

Corrosion is difficult to control because feedstock and operating conditions that drive corrosion are constantly changing. The amount of chlorides in the overhead system is directly related to the amount of salts leaving the desalter. Constantly changing crude slates mean that the amount of salt leaving the desalter is prone to swing routinely, resulting in constantly varying amounts of chlorides. If not enough neutralizer is added the pH of the overhead water will be too low, which will destroy protective iron sulfide scales and cause aggressive corrosion.

Besides increasing cost, if excessive neutralizer is added the water pH goes too high, which can change crystalline structure of the protective iron sulfide scale and cause it to slough off of the metal surfaces. When pH subsequently returns to the normal pH range, elevated corrosion rates will occur until a protective scale is regenerated. Also, excessive neutralizer usage can cause a number of other problems, such as the formation of corrosive salts in the overhead system and the recycle of amines back to the atmospheric column via atmospheric overhead water commonly used as desalter wash water. Excessive amines in the atmospheric column cause corrosive amine hydrochloride salts to form in the tower.

The issue is further complicated because several key measurements needed to control corrosion require pulling a sample and analyzing it for parameters such as pH, chlorides, and iron. Key measurements such as chlorides and pH are often only measured two or three times per week and at most daily. In the best-controlled systems, operators may take local pH measurements as often as once or twice a shift. Even then that means pH is being measured at most 4-6 times per day and chlorides are being measured no more than once per day.

Trying to control corrosion in a system that is constantly changing with such infrequent measurements is very difficult, especially since minor changes in process conditions can result in an exponential rise in corrosivity. Corrosion is often an event-driven process. It has been reported that 90% of the corrosion occurs during just 10%1 of the time.1 A corrosion event can occur between measurements and the refinery never even knows that it happened. When overhead corrosion cannot be controlled, expensive upgrades to alloys such as UNS N10276 and UNS R50400 are often required.

In an attempt to create a step-change improvement in the way crude unit overhead corrosion is controlled, an online corrosion control skid has been developed that continuously measures pH, chlorides, and iron in the crude unit overhead boot water. With regular, automated measurements of key corrosion variables, it is possible to automatically adjust chemical injections to optimize the corrosion control program on a continuous basis. The system has two modes for controlling the

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injection of neutralizer: control based on the pH measurement and control based on the chlorides present in the overhead boot water. The chloride measurement can also control caustic injection. The iron measurement is used to control corrosion-inhibitor injection. This paper discusses the issues and benefits associated with the application of this system at a particular Refinery.

THE TECHNOLOGY

From many years servicing the global refining industry, it has been understood that many of the current corrosion best practices were no longer adequate enough to protect crude overhead systems in refineries processing an ever-changing and challenging slate of crudes. Although this group of refiners does not represent the majority of the industry, there is an ever-growing segment of the industry pushing the envelope in order to stay competitive in today’s market. Realizing there had to be a better way to improve corrosion performance for these refiners, a path was set to develop a family of sensors capable of detecting specific corrosive ions in crude overhead systems.

The system needed to determine the concentration and variability of the corrosive ions during normal operations, interpret the results, and automatically deliver the right amount of chemical additive to address the corrosive condition. It was also realized that, if specific sensors could be developed to measure and track performance, over time a fundamental shift in understanding of how operations and operational upsets impact corrosion and reliability could be developed. The hope was that a new approach and technology could significantly improve crude unit reliability for the refining industry.

A system needed to be developed that could continuously interrogate the crude overhead on a predetermined interval and turn around the test results fairly rapidly. This system also needed to operate seven days a week, three hundred and sixty five days a year. The issue was that the gap between sampling and action could be many hours. It was determined that a time interval of up to one hour between sampling and action was probably sufficient to generate an accurate measurement, take action, and wait for the corresponding change.

We realized early in the development process that proven, accurate, and reliable tests methods already in use would be the fastest route to prove this concept, get into the field, and to market. Developing new novel sensors was ruled out as too costly and time consuming.

As with any automated system, a means of managing the vast quantities of data generated would be important. The data generated needed to be continuously available, instantly accessible, with the ability to plots trends, and generate alarm notifications. The data needed to be available in the customers DCS and process historian.

DEVELOPMENT OF THE PROTOTYPE

The project was formally kicked off in 2008 with the goal of being in field trials within a year, leading to a viable commercial offering inside of three years. It was decided that the first phase would all be monitoring trials. The sensors and analyzer were only going to monitor the process and report back. Once satisfied and confident in the results being generated, the second phase would be closing the loop and controlling chemical additives based on the analytical results.

Once a technology was identified a period of rigorous laboratory simulations were conducted. The goal of the laboratory phase of testing was to build familiarity with the equipment and develop the test methods required for field use. The first weeks were spent calibrating the sensors and developing the appropriate range and slope for each test. Time was also invested in simulating the refinery conditions (quality of water) the sensors could encounter in the field. Various known interferences were introduced and the sensors responses to these interferences were tested. Through adaptation of the test methods and the use of various reagents, satisfactory results were achieved.

As with most off-the-shelf technology, the sensors and analyzer housing was not specifically designed for use in the refining environment. Considerable time and effort was taken making sure the cabinet

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met refinery electrical classification requirements. Since the analyzer would be in the field 24/7/365, thought had to be taken with potential issues that could occur, such as loss of power and potential sour water leaks. Since the instrument would have a constant water stream flowing to it, we had to understand the ramifications if a leak developed. Questions had to be asked, such as, “If power to the analyzer was lost, how is the water supply shut off and the analyzer shut down safely? “

After six months of laboratory testing, the instrument was ready to go to the field. An understanding of how well the sensors and analyzer would hold up to the variability and harsher conditions of the refining processes was needed. Straight-line correlations and accurate results had been achieved on the laboratory bench, but would these hold up in the field? How often would calibration be required? Could this potentially be a critical variable? Would hydrocarbon impact the sensors and test methods? Would solids present an issue? How would the analyzers handle significant levels of H2S in the boot water? Lastly, what sort of cleaning procedure would be needed?

The initial phases of testing were conducted at three refineries selected for their unique variety of processing conditions. The initial trials took place over a period of three years with the goal being to give these units the best possible test. What was learned was that each refinery presented its own set of unique conditions and challenges. The biggest take away from the three trials was that conditioning of the water sample was the key to success. The quality of the incoming water had by far the biggest impact on the accuracy and reliability of the analyzers and corresponding results; no water flow means no results.

REFINERY APPLICATION OF KEY LEARNING’S

The Refinery initially became interested in this technology as a means to improve pH control and, thus, reduce corrosion in their No. 2 crude unit overhead system. The Refinery ran an aggressive slate of crudes that presented some unique challenges. The crudes processed required some fairly unique operating practices and demanded constant attention to the conditions and operations of the crude overhead system.

The crude overhead analyzer was installed and commissioned in June 2012. As with the initial trial locations, the unit was initially set up to monitor the process. Following an initial commissioning and testing period, the analyzer unit was commissioned in pH control in July 2012.

The analyzer unit takes a continuous sample of the crude unit overhead accumulator boot water and measures pH every 2 minutes and chlorides and iron every 60 minutes. The boot water pH is used to automatically control the addition of neutralizer to the crude overheads system to a desired pH set-point. This is achieved through controlling the air supply to the chemical injection pump. A simplified schematic of the system is displayed in Figure 1.

With modifications the analyzer unit is capable of using the chlorides results to control the caustic addition rate, and iron results to control the inhibitor rate, these functions have not been employed at this particular refinery, as the greatest benefit in this specific application was thought to be pH control.

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Figure 1: Analyzer unit and crude overheads system schematic

From the outset, it was clear that the conditions present at this particular Refinery were an order of magnitude more difficult than the conditions observed during the first field trials, and first few commercial applications. The water quality at this Refinery was far more severe than anything encountered during previous field tests, with a considerably higher solids content (corrosion byproducts), warmer boot water temperatures, and higher hydrocarbon content.

To deal with the various challenges, several modifications had to be made to the analyzer during the months following initial commissioning. To address the increased solids in the boot water, an additional filter had to be installed upstream of the analyzer. Various mesh sizes were trialed to determine the optimum size, which minimized plugging of the filter while ensuring that adequate quality water reached the analyzer.

Oil contamination of the boot water was another problem encountered. During certain crude runs the naphtha entrapment of the sample stream resulted in blinding of the pH probes and, depending on the amount of naphtha present, giving erroneous results for iron and chloride. The introduction of a second hydrocarbon filter, switching filter elements, and the introduction of a planned maintenance regime addressed this issue.

One of the key improvements for the Refinery was the change in pH probes. Online pH measurement in the oil and gas industry is frequently extremely difficult. Operating in elevated temperatures with highly reactive contaminants, traditional pH-references electrodes can in some cases last only days. After initial success with the original probes, a change in crude slate resulted in the need to investigate a new approach. With such a challenging environment of acid crudes, caustic, and neutralizer addition, it was necessary to develop a probe which could handle this environment.

Working with a key partner a new solid-state probe was designed specifically for the crude overhead analyzer. These probes proved to have a considerably longer life and were much less susceptible to deposition on the probes surfaces.

In addition to the improvements made in pH measurements, the pH control has been tuned in order to provide acceptable response to changes in process conditions whilst minimizing the error between steady state operation and the pH set-point.

Periodically during warmer weather, unstable pH readings have been experienced, which can result in the analyzer reporting no result. It was identified that, over the months since initial commissioning, the sample cooler had scaled on the cooling water side of the heat exchanger, resulting in an increase in temperature of process water to the analyzer. De-scaling the cooler resolved this issue, significantly

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improving the performance of the analyzer. A program has now been put in place to de-scale the cooler on a quarterly basis.

The result of these field modifications is that the crude overhead analyzer can now operate as designed with only weekly routine maintenance. These improvements have allowed the refiner to gain confidence in the analyzer and--as can be seen in Figures 2 & 3--over the months following commissioning, as the various improvements to the pre-conditioning of the process water and maintenance and operation of the crude overhead analyzer have been made, both the pH control and crude overheads corrosion have significantly improved.

RESULTS

Historically, the overheads accumulator boot water has been sampled on a daily basis (five times per week) for laboratory analysis of pH, chlorides, and iron. Prior to the installation and automation of the analyzer unit, the neutralizer injection rate would be adjusted based upon the spot laboratory pH result. Trials of sampling the boot water on a shift-by-shift basis have also been conducted in the past, but without significant improvements in crude overhead corrosion control.

The purpose of installing the analyzer unit was to significantly improve the pH control and to reduce the corrosion rates of the crude unit overheads system.

Figure 2: Laboratory boot water pH analysis

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Figure 3: Laboratory boot water iron analysis

Figure 2 shows the daily laboratory boot water pH result, both before and after the installation and commissioning of the analyzer unit. It is clear from Figure 2 that the variability of the pH has reduced since the analyzer unit was automated, especially as various improvements and modifications have been made to enhance the reliability of the analyzer and quality of the process water sample.

Figure 3 displays the daily laboratory boot water iron result over the same time period. It is clear that the control of the crude unit overheads corrosion has significantly improved as a result of tighter pH control.

Crude unit overhead systems are typically operated in the pH range 5.0 – 6.5. Figure 4 shows the analyzer unit iron and pH data. From this chart it can be concluded that, for the system at this particular Refinery, a pH aim range of 6.0 – 7.0 may be more appropriate. A pH of >7 could also be employed, however, higher pHs could lead to salting issues and also require increasing amounts of neutralizer, resulting in increased operating costs for little incremental benefit to corrosion rates.

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Figure 4: ANALYZER unit boot water pH vs. iron

The application of the new analyzer unit technology has allowed this Refinery to meet its corporate Key Performance Indicator (KPI) conformance target of ≥90%. This is further illustrated in Figure 5, which shows all iron results from the analyzer unit between June 2012 and July 2013 (“no result” data has been removed).

Figure 5: ANALYZER unit boot water iron data

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ANALYZER – CASE STUDIES

As discussed earlier, corrosion is often an event-driven process. A corrosion event can occur between daily measurements, and the refinery never even knows that it happened. The following are examples that this particular Refinery has experienced since the analyzer unit was installed and commissioned. In each of the examples the analyzer unit has both highlighted and minimized the impact of each event.

The analyzer has highlighted that, with non-automated neutralizer addition, that it is not uncommon for this Refinery to see large swings in both pH and iron in the boot water during upset conditions and or crude tank switches.

Wet Crude Tank

On the 27th August 2012, crude feed to No 2 Crude unit was commissioned from Tank 1, as Tank 2 had reached minimum gauge.

The crude in Tank 1 had been reported as containing a higher BS&W than Tank 2, and was settled and water drawn as per standard operating procedure prior to being commissioned as feed to the crude unit.

As Tank 1 was brought on feed, the desalter amps and interface levels rose rapidly, indicating the creation of an emulsion layer in the desalter. In response, the operators increased the demulsifier injection and reduced the wash water flow to the desalter.

Figure 6 shows that, as a result of Tank 1 being brought on feed and the significant reduction in desalter wash water flow, the overhead accumulator boot water pH reduced significantly from its set-point of 6.5 to 4.5. In response to the reduction in pH, the analyzer unit increased the flow of neutralizer to the crude unit overheads system. Owing to the fact that a significant increase in neutralizer flow was required, the stroke length of the chemical dosing pump had to be manually adjusted. This manual operation accounts for the stepped increase in neutralizer displayed in Figure 6.

In this scenario, the analyzer unit with real-time pH analysis immediately highlighted the low pH of the crude overheads system. Without the analyzer unit, it is likely that the process upset could have resulted in low pH conditions in the crude overheads system for 24-48 hrs, as any corrective action would have been based upon a daily boot water spot sample. The real time analyzer unit data allowed for prompt corrective action to be taken through increasing the neutralizer flow, minimizing the duration of the corrosive environment. Furthermore, once a sustained period of target pH conditions had been experienced, the neutralizer flow was optimized.

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Figure 6: Wet crude tank

Layered Crude Tank

On the evening of 1st July 2013, a layered crude tank was brought on feed to No 2 Crude unit. It was apparent that, as the crude tank was brought on-feed, the crude overheads system boot water pH started to drop. Figure 7 shows that as the pH dropped, the neutralizer flow to the crude overhead system was increased. There was also a clear relationship between the falling boot water pH and increasing boot water iron, immediately highlighting the impact of the more corrosive environment.

As with any control loop, there is a trade-off between speed of response and system stability. This incident highlighted that there was a potential opportunity to further improve the response of the pH control; however, without the analyzer unit and automated neutralizer addition, the pH would have fallen further, resulting in significant increases in corrosion. Without the real-time data of the analyzer unit, it is likely that this event may have gone unnoticed.

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Figure 7: Layered crude tank

Spiking Crude Processed at High Percentages

On 7th February 2013, crude ‘X’, which was normally processed in a blend with other crude feed stocks, was processed at 100%. Figure 8 shows that as crude ‘X’ was being processed, the crude overheads system boot water pH began to drop and boot water irons began to increase. In order to compensate for the falling pH, the analyzer unit continued to increase the neutralizer addition; however, the neutralizer pump had insufficient capacity to meet the required dosing rate (illustrated by the neutralizer pump output reaching 100%).

In order to maintain control of the crude overheads system pH, following the site’s Management of Change procedures, a larger neutralizer pump with suitable turndown for normal operation was installed. The installation of the pump can be seen on Figure 8 as the pH steps back up and the neutralizer pump output steps down to around 50%. Although processing 100% crude ‘X’ is an unusual event, the analyzer unit highlighted that corrosion rates whilst processing this crude with insufficient neutralizer addition were significantly higher than normal operation. With only daily or three times per week laboratory analysis, the fact that the neutralizer pump was undersized for 100% crude ‘X’ may not have been identified.

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CONCLUSION

Installation of an online analyzer that regularly monitors key overhead corrosion parameters has allowed an individual Refinery to make a step-change improvement in its ability to control pH and corrosion in a very challenging overhead system. The increased amount of data generated by the crude overhead analyzer allows the refinery to see and respond to corrosion events much more effectively. The increased data also make it easier to understand what is driving corrosion thus potentially allowing further improvements in corrosion control through changes in process operations. The key to obtaining the operating reliability of the analyzer that is necessary to provide good control is in providing a robust sample-conditioning system that allows the analyzer to operate with a high stream factor.

REFERENCES

1. N.P. Hilton “Mitigate corrosion in your crude unit” Hydrocarbon Processing, September 2010, Pages 75-79

2. NACE International Publication 34109 (2009 Edition), “Crude Distillation Unit—Distillation Tower

Overhead System Corrosion,” Technical Committee Report by NACE International Task Group TG342 (NACE International, December 2009).

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Corrosion Refinery