Fouling mitigation

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  11. Fouling
  12. Cooling technology


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Fouling Mitigation describes the ways to eliminate fouling that reduces the efficiency of cooling towers, heat exchangers, pipelines and control equipment.

Several means and methods have been developed to remove the dirt and sediments that cause fouling. They are either offline methods that rely o a periodical cleaning, or online devices that operate continuously.

Concept

The accumulation of sediment and deposits on heat transfer surfaces in cooling towers, heat exchangers, pipelines and control equipment decreases performance and productivity, and in advanced cases causes equipment damage. This phenomenon is known as fouling and is an unavoidable part of many industrial processes. Fouling mitigation seeks to correct or prevent these problems.

Industry challenges

Fouling slows down industrial production and results in higher operating and maintenance costs due to restricted fluid flow, increased tube pressure, loss of throughput, unnecessary energy consumption. In advanced cases, it may be necessary to replace equipment.

Aqueous fouling

Water is the most readily available and effective cooling medium. The main sources of fouling in water cycles are sediments (scale, silt, algae) and sludge (corrosion and bacterial activity). Fouling on the water side of heat exchangers typically necessitates offline cleaning every several months. Though generally not requiring a shutdown of the entire production line, this still requires a substantial maintenance effort and cost burden.

Nonaqueous fouling

One example is the Crude Preheat Train in crude oil distillation, which accounts for a large percentage of the energy used in oil refining. In the USA alone, fouling in the preheat trains alone is estimated to cost around $1.2 billion per annum. With Phosphoric Acid (P2O5), there is an enormous fouling problem in the evaporators, requiring shutdown every 2 weeks. Typically, a single day of shutdown in a large phosphoric acid plant causes a loss of nearly 1000 metric tons of produced P2O5 solution. In a typical Bauxite Refinery (Alumina Production) in the US, Australia, Jamaica, Morocco, Brazil, and South Africa, aluminosilicate scaling requires shutdown of 10 heat exchangers (1000 tubes, 7 meters length each) every 200 to 500 operating hours for chemical cleaning. To avoid a production halt, two redundant series of 10 heat exchangers each (total of 20) must be used during maintenance periods.

Methods

There are currently two forms of fouling mitigation: offline, which requires the complete shutdown of key processes for cleaning; and online, which cleans the tubes while the equipment is running.

Offline techniques clean heat exchangers by mechanical and/or chemical means while the system is down, which results in lost production time. They are labor-intensive and expensive; as much as 8% of the maintenance costs in a typical industrial plant are due to fouling mitigation in heat exchangers. When offline mitigation requires the use of aggressive chemicals, the company is saddled with additional operating costs, and new problems relating to the increased safety hazards for company personnel and disposal of toxic waste. Moreover, the fouling process starts again immediately after cleaning, and the gradual accumulation of deposits reduces performance until the next cleaning treatment is initiated. These drawbacks make heat exchanger operation with regular offline cleaning expensive, cumbersome and inefficient for the entire industrial process.

The online method is an ongoing process which uses mechanical means to keep the heat exchanger clean while it operates. Some cleaning systems also use chemicals (which must be carefully adapted to each process), but online mechanical-only cleaning is both environmentally responsible and highly cost-effective.

Online mechanical cleaning boosts performance in two ways: it does not require system shutdown with temporary loss of operation; and it keeps performance optimal and energy-efficient through continuous cleaning, which does not allow any fouling to occur. In addition, online mechanical cleaning does not only eliminate staff time for cleaning services, but also purchase of chemicals and waste disposal.

In terms of efficiency, savings and environmental safety, online mechanical heat exchanger cleaning is the smartest option available. But conventional online cleaning has several remaining disadvantages that reduce its reliability: Inconsistent tube cleaning - Complex installation requiring modification of heat exchanger inlet nozzle - Short ball service life (average 1 month), necessitating constant electronic monitoring and replacement - Pump friction on balls further reduces ball life - Ball material cannot endure high temperatures or harsh chemicals - Complex trapping unit structure needed, with an integrated backwash mechanism to release clogging.

The conventional approach to fouling control combines the “blind” application of biocides and anti-scale chemicals with periodic lab testing. This often results in the excessive use of chemicals with the inherent side effects of accelerating system corrosion and increasing toxic waste- not to mention the incremental cost of unnecessary treatments. There are however solutions for continuous fouling monitoring In liquid environments, such as the Neosens FS sensor, measuring both fouling thickness and temperature, allowing to optimize the use of chemicals and control the efficiency of cleanings.

See also

References

  • Pritchard, A.M., The Economics of Fouling. in Fouling Science and Technology, eds. L.F. Melo, T.R. Bott, and C.A. Bernardo, NATO ASI Series E, vol. 145, Kluwer Academic Publishers, 1987
  • Garrett-Price, B.A et al.: Fouling of Heat Exchangers – Characteristics, Costs, Prevention, Control and Removal. Noyes Publications, Park Ridge, New Jersey (1985)
  • Thackery, P.A. The Cost of Fouling in Heat Exchanger Plant. Effluent and Water Treatment Journal (March 1908)


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