METTLER TOLEDO: Battling Corrosion in Refineries with In-Line Process Analytics
It is estimated that the global costs of refinery corrosion are in the order of 15 billion USD annually. Even though the problem may never be fully solved, advances in process analytics with a new generation of intelligent pH sensors offers huge cost savings.
Complex problem
The main difficulty in combating refinery corrosion is that the problem has not one single source, but many. In addition, some of the corrodents involved might interact and increase or inhibit each other’s corrosivity. Physical process conditions also play a role; so temperature, flow, etc. have to be taken into account, as does the refinery infrastructure itself. Given the number of variables then, it becomes clear that corrosion is a complex
issue.
Sulfur
Because it provides a lower feedstock cost, sour crude is preferred by refineries. In sour crude sulfur is present in the form of mercaptans, H2S, sulfide salts, elemental sulfur and so on. Many of these species are reactive and cause sulfide stress cracking and sulfuric acid corrosion throughout the refining process.
Acidity
Besides sulfur, crude contains many species that are quantified by the total acid number (TAN) of the oil. Typically found are naphthenic acids, which are organic, but also mineral acids (H2S, HCN, CO2, etc.) can be present, all of which can contribute significantly to equipment corrosion. Even materials suitable for sour service do not escape damage under such an onslaught of aggressive compounds. Again, because of cost considerations, a trend towards a preference for crudes with a higher TAN is noticeable.
Desalting
Desalting of the crude is the first step in refining that has a direct effect on corrosion and fouling. By mixing and washing the crude with water, salts and solids transfer to the water phase which settles out in a tank. An electrostatic field is induced to speed up the separation of oil and water. In this way, inorganic salts that could cause fouling or hydrolyze and form corrosive acids, are largely removed. Often, chemicals are added in the form of demulsifiers to break the oil / water emulsion. Other chemicals such as caustic soda are also introduced to neutralize acidic components. Uncontrolled feeding of caustic can, however, have a detrimental effect and result in a significant quantity of contaminants ending up in downstream processes, which may have serious consequences.
Distilling
Even with an efficient desalting operation, an abundance of corrodents can still appear during downstream processing. A good example of this is the sour water corrosion that occurs in the crude distiller. During the process lots of acid gases are formed of which hydrogen sulfide is
notorious.
Steam, which is injected into the crude tower to improve fractionation, condenses in the upper part of the unit. The hydrogen sulfide dissolves in the condensate and forms a weak acid which is known to cause stress corrosion cracking in the top section of the tower and in the overhead condenser. This may lead to frequent retubing of the condenser and in severe cases to replacement of the entire crude tower top.
In-line analysis
All the problems mentioned above, and the costs associated with them, can be significantly reduced by reacting to the measurements obtained from accurate in-line analysis of pH values.
In respect of desalting, constant in-line monitoring of pH in the desalter water effluent allows for efficient dosing of caustic or acid which can result in significant cost savings. The stability of the oil / water emulsion depends partly on pH. Maintaining the pH of the mixture within a certain range helps the demulsifier chemicals in breaking the emulsion by interacting directly with the water droplets. The speed and quality of the separation process can thus be improved which leads to less water carry-over, which in turn can result in a significant reduction in downstream corrosion and fouling.
Even though corrosion in the disitiller is well known to refinery operators, effective counter measures are not always in place. Typically, corrosion inhibitors and lots of neutralizers such as caustic soda or ammonia are injected with the aim of increasing the pH of the sour water. Although this is an obvious response to the problem, the cure can be worse than the disease. The presence of various acid gases and ammonia can result in solid salts depositing including ammonium bisulfide, one of the main causes of alkaline sour water corrosion. pH levels higher than 7.6 dramatically increase ammonium bisulfide corrosion and by overdosing caustic that level is easily achieved. Proper neutralizer dosing through pH measurement of the overhead condenser boot water will not only reduce corrosion but also reduce chemical consumption.
Most pH electrodes are not specifically designed for refinery applications and hence suffer from poor performance in the presence of high sulfide levels and the substantial hydrocarbon load. METTLER TOLEDO’s understanding of the conditions in petroleum refineries has led to the development of pH electrodes specifically designed to cope with high temperatures, extremes of pH value and solid contaminants.
Xerolyt Extra
The most common cause of sensor failure in refineries is poisoning and fouling of the reference electrode system. This results in drift of the pH measurement. Oily substances and solids may coat or clog the diaphragm of the reference junction which obviously has a negative impact on sensor performance as well.
METTLER TOLEDO’s InPro 4260 i pH electrode features Xerolyt Extra solid polymer electrolyte. The InPro 4260 i has an open junction, which is in fact a hole that brings the solid electrolyte in to direct contact with the process medium. Contrary to the miniscule capillaries of any other type of diaphragm in conventional pH electrodes, the diameter of the open junction is extremely large and much less
susceptible to clogging or fouling. Another significant difference is in the choice of polymer electrolyte. Xerolyt Extra has been designed specifically for service in hydrocarbon environments and provides a strong and lasting barrier against sulfide poisoning.
Automated cleaning and calibration
Despite innovative design, even the best pH electrode needs frequent maintenance. Of any process instrument, the pH electrode probably has the widest rangeability and an extraordinary sensitivity that covers it. Depending on the process application, after a certain time in operation a calibration is required to make sure the instrument will maintain its reliability and accuracy. When very sour crudes are refined, or when the overhead hydrocarbon / water separation is not optimal, sensor maintenance may be required to an extent where it becomes problematic and the value of the measurement is no longer considered an asset. In such applications the implementation of an automated cleaning and calibration system is the answer.
Proven to be of great value in numerous “dirty” applications is METTLER TOLEDO’s EasyClean 400 system. Fully certified for use in hazardous areas, this unit enables the most difficult pH measurement tasks. When necessary, EasyClean 400 pneumatically retracts the electrode and gives it a thorough cleaning followed by a two-point calibration, without any human intervention. The standalone unit carries cleaning and buffer solutions on board and requires no more than instrument air and water supplies to carry out its tasks. An extra valve is available in case an additional cleaning liquid is needed.
Intelligent Sensor Management
In the end, all pH electrodes will show signs of aging and performance will start to fall below the reliability required. METTLER TOLEDO pH electrodes with Intelligent Sensor Management (ISM) communicate their remaining operating life through a digital signal that also carries measurement values, temperature and enhanced sensor diagnostics. Based on actual process conditions and history, the Dynamic Lifetime Indicator will predict when sensor replacement is due. This is one of many interesting features of the ISM platform that truly enables predictive maintenance.
Lower costs and more uptime
Advances in sensor technology and intelligent automation of the measurement system enable pH measurement in the most challenging refinery sour water environments. When the correct equipment is chosen, in-line pH measurement and management facilities have proven to be of great value in reducing plant-wide corrosion and the consumption of chemicals such as pH control reagents and corrosion inhibitors. This not only results in significant cost savings but also in increased earnings through longer process
uptime.
More information: www.mt.com/pro-pH
- www.mt.com/ISM
Mettler Toledo AG
Process Analytics
Im
Haackacker 15
8902 Urdorf, Switzerland
Posted: March 3, 2010
|
+
+ Media Links + +
For more information, media details or
sample copies please contact
wilhelms@pennwell.com
www.sicking.de
|