Feb-2024

Biofilm: A hidden threat

A new approach to the costly problem of biofilm formation in refinery and petrochemical operations.

Brian Martin, Marathon Petroleum Corporation
Tim Duncan and Gordon Johnson, Solenis LLC

Article Summary

Refineries and petrochemical operations rely on water-cooled heat exchangers in many areas of their facilities. These heat exchangers provide the heat removal from refining processes required for the production of various products and intermediates. The efficient transfer of heat in these exchangers often determines production rates. Fouling of the heat exchanger surfaces or flow restriction resulting from biofilm, scale, or debris may limit production and result in downtime for cleaning. Additionally, corrosion of the heat exchangers because of microbiological deposits may result in failures that require downtime, maintenance, and capital expenses.

Expenses can run into millions of dollars, particularly if they include unscheduled downtime and heat exchanger replacement. Proper management of heat exchanger performance includes analysis of heat transfer data and understanding failure mechanisms. Data management tools can assist in the development of preventative maintenance guidelines and in the optimisation of chemical treatment programmes that minimise these expenses. Many refineries and petrochemical plants struggle with heat exchanger bundle failures and efficiency losses between turnarounds. Inspections of failed bundles often reveal under deposit corrosion (UDC) with biofilm as the culprit.

Traditional monitoring and control techniques
Warm cooling tower water containing microorganisms and nutrients fosters ideal conditions for microbial growth and biofilm formation. Microorganisms and nutrients enter the cooling system through multiple paths. They enter the system in the make-up water – even though it may have been treated for microorganisms, the treatment only renders the water sanitary, not sterile. As the water flows over the tower during the evaporation process, microorganisms and nutrients enter the system through the scrubbing process. Nutrients enter the cooling system from hydrocarbon leaks on the process side of heat exchangers, and they enter in the form of phosphate corrosion inhibitors applied to protect the carbon steel piping and heat exchangers from corrosion.

Figure 1 depicts various stages of biofilm formation on a surface as microorganisms and nutrients continually inoculate the cooling water system. In the first stage, the cooling water transports these microorganisms to the surface. In the second stage, the microorganisms begin to attach themselves to the surface and within 20-30 minutes of system inoculation begin colonisation. In the third stage, because microorganisms reproduce through cell division at a geometric rate, within one to two days significant growth can occur.
Part of this growth involves the production of extracellular polymeric substances (EPS). The composition of the EPS includes polysaccharides, proteins, extracellular DNA (eDNA), and lipids. The EPS from various microorganisms interact with each other and form a slime matrix that encompasses and protects the microorganisms. In the fourth stage, within three days to three weeks, the thickness of the biofilm matures. In the fifth and final stage, at maturation, detachment occurs because of turbulence or ecological conditions. This detached biofilm can then populate other regions of the cooling water system.

Most microbiological control programmes using strong oxidising biocides, such as bleach or chlorine gas, even when used in combination with non-oxidising biocides, can only control biofilm up to a point. The matrix formed by the EPS encapsulates the microorganisms and provides a level of protection from these biocides. The EPS creates a demand for strong oxidisers, which generally cannot be exceeded at typical dosages. Dosages that can exceed the demand have a negative impact on the corrosion rates of metal surfaces themselves and degrade dispersants that are used to provide protection from inorganic deposition. Non-oxidising biocides similarly have difficulty penetrating the EPS’s protective slime matrix without reacting with the EPS. Economics do not favour traditional approaches to biofilm control.

Underappreciated and underestimated aspects of industrial cooling water treatment include the effect of biofilm on heat transfer and the resultant heat exchanger failure from microbiologically induced corrosion (MIC). As shown in Figure 2, thinner biofilms, as compared with mineral scale, exhibit a more severe resistance to heat transfer. Microbiological fouling inhibits heat transfer up to four times that of calcium carbonate fouling. Additionally, once the biofilm exceeds 50 microns, approximately the thickness of adhesive tape, the resulting anaerobic conditions support the growth of acid-producing bacteria. The acidic waste products from anaerobic bacteria often aggressively pit heat exchanger tubes and eventually cause leaks, requiring repair or replacement.

Traditional techniques for monitoring microbial growth and biofilm formation cannot measure biofilm. Biofilm forms when planktonic (free-floating) microorganisms begin to adhere on surfaces, such as pipe walls, heat exchangers, and cooling tower fill. Traditional approaches to monitoring microbial activity include measuring halogen residuals, heterotrophic plate counts, and adenosine triphosphate (ATP) levels in the bulk water. Unfortunately, no correlation exists between any of the results of these monitoring techniques and the attached, sessile microorganism levels that cause biofilm.

Since traditional approaches to monitoring neither predict nor indicate biofilm, mechanical approaches have been employed to monitor the efficiency of heat exchangers to determine if biofilm fouling is present. However, detecting biofilm by measuring heat exchanger approach temperatures, unfortunately, only indicates the presence of biofilm after the fact. Similarly, flow studies only show restrictions and loss of velocity after biofilm has formed.

New approach
Solenis’ proprietary ClearPoint biofilm detection and control programme provides a new approach to the costly problem of biofilm fouling. This programme comprises three components: a novel biofilm analyser, proprietary chlorine stabiliser chemistry, and expert service. Employing the biofilm analyser, the programme provides early detection and accurate measurement of biofilm growth in real-time. The chlorine stabiliser chemistry is used to produce a patented, in situ stabilised active chlorine solution. The solution significantly reduces microbiological activity without the adverse side effects associated with strong oxidising biocides. Field service personnel provide the expertise required to maintain clean and efficient heat exchangers.

The proprietary OnGuard 3B analyser uses a patented ultrasonic sensor, shown in Figure 3, to accurately measure the thickness of biofilm that accumulates on a heated target assembly, shown in Figure 4. The sensor detects biofilm with a measurement accuracy of approximately 10 μm and at a resolution of ±5 μm. The analyser mimics critical heat exchanger conditions in real-time by duplicating the shear stress on a surface while also simulating the local surface temperature to provide continuous fouling factor measurements that inform the adjustment of chemical feed when required. The analyser can also differentiate between soft deposits (organic and microbiological fouling) and hard deposits (scaling). The early detection capability of the analyser allows corrective actions to be taken before biofilm can cause heat transfer loss or equipment damage.