Tight crude oil can cause fouling in shell and tube heat exchangers

The U.S. leads the market in tight crude oil, but the refining process proves to be hard on shell and tube heat exchangers.

The U.S. leads the market in tight crude oil, but the refining process proves to be hard on shell and tube heat exchangers.

MicroMarket Monitor released a report stating that much of the shell and tube heat exchanger industry in North America is fuelled by the chemical and petrochemical industries. According to the report, 28.8 percent of the market share of heat exchangers in 2014 went to the chemical industry. Heat exchangers are often used for oil refining and the shell and tube variety are the most commonly used.

The American Institute of Chemical Engineers explained only four countries are currently extracting crude oil from shale formations: Argentina, China, Canada and the U.S. Crude oil from shale formations is more difficult to retrieve and hydraulic fracturing, or fracking, is the method by which it is extracted. The U.S. is the leading producer of this type of oil, commonly called tight oil.

While customized shell and tube heat exchangers are great machines to use for processing, there are some things manufacturers and operators should be aware of. According to Emerson Process Management, the refining process is highly prone to fouling. Unexpected fouling is an especially harmful problem that is becoming more common among facilities refining light tight crude oil.

"Crude oil fouling is one of the main causes of energy inefficiency in refineries."

Fouling can be harmful to a refinery because it causes lost production time, increased costs and, as a result, reduced profits. Unexpected fouling means operations need to cease production for cleaning. Plus, the fouling causes excess energy consumption and decreased throughput by the crude unit fired heater. According to a report from the Department of Chemical Engineering at the Imperial College London, crude oil fouling in pre-heat trains is one of the main causes of energy inefficiency in heat exchangers. Because of the strain it causes across multiple aspects of the refinery, the cost of crude oil fouling is high. Crude oil fouling was found to cost the U.S. approximately $1.2 billion a year.

Asphaltene precipitation leads to accelerated fouling
Different industries may see varying causes of fouling. In processing tight crude oil, fouling is commonly due to asphaltene precipitation buildup. This is primarily due to the blending of tight oil with other crude oil types. When tight oil alone goes into an exchanger, it tends to bottleneck in the naphtha processing and crude overhead units. This is because most refineries are designed to process oil of a specific composition, the AIChE explained. Blending them will reduce the risk of limiting the efficiency of processing units in the bottom of the barrel.

While blending these types of oil is necessary to allow the machinery to operate correctly, this practice is also the one that contributes to asphaltene precipitation causing fouling. When incompatible crude types are blended, asphaltenes are not stable in the solution, resulting in the precipitate. AIChE noted that certain ratios of incompatible oils result in less precipitate than others. For instance, a blend of 20 percent tight oil with 80 percent of another type will create fewer asphaltenes than a 30-70 mixture.

Tight oil also contains levels of naphtha that are higher than other types of crude oil, which can also contribute to the rapid production of asphaltene precipitation.

Prevent fouling through monitoring
To prevent fouling, manufacturers have inspected machines periodically and taken notes manually and recorded information in spreadsheets. While this has largely been effective in the past to respond appropriately to signs of fouling, increased production of tight crude oil, which fouls more quickly than most products, manual note-taking and periodic observation is no longer a practical form of prevention.

"Traditional forms of fouling prevention are less effective with accelerated fouling."

Instead, Emerson Process Management suggested manufacturers use online capabilities to constantly monitor the state of exchangers' performance. Wireless temperature and differential pressure measuring devices are one way to constantly monitor the performance and efficiency of a heat exchanger. By monitoring the health of the exchanger, refiners are able to carefully choose when to schedule a turnaround, rather than waiting until the fouling problem is no longer avoidable, leading to an unforeseen cleaning day. By being able to choose when to clean the tubes, refiners are able to calculate which day is best, so as to minimize the financial harm a day without production will cause.

Not only will monitoring the condition of the equipment using online devices keep refiners current about potential fouling, but it can also aid in preparing maintenance staff of what is to come when it's time for a turnaround. It can also help indicate if additional parts need to be purchased ahead of time. Some exchangers are equipped with a bypass, so cleaning can be done without losing a day of production. However, for those that don't have this capability, as is often the case, advance knowledge of the kind of maintenance that should occur may reduce the time the turnaround takes and mitigate some unforeseen obstacles.

Posted in Shell and Tube Heat Exchanger Solutions

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