PTQ Q1 2024 Issue

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

R efineries 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 produc- tion 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 per- formance includes analysis of heat transfer data and under- standing 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 refiner- ies 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 sys- tem 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 inoc- ulate the cooling water system. In the first stage, the cool- ing 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 extra- cellular polymeric substances (EPS). The composition of the EPS includes polysaccharides, proteins, extracellular DNA (eDNA), and lipids. The EPS from various microor- ganisms 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 thick- ness 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 popu- late 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

Stage 1 Conditioning layer

Stage 2 Bacterial attachment

Stage 3 Biolm formation/ EPS production

Stage 4 Biolm maturation

Stage 5 Detachment

Figure 1 Five stages of biofilm formation

PTQ Q1 2024