Sol-Gel Coatings Investigated for Shell-and-Tube Heat Exchangers

A study to investigate the use of hybrid sol-gel coatings for fouling mitigation of shell-and-tube heat exchangers was conducted by NACE International members Seth T. Taylor with Chevron Energy Technology Co. (Richmond, California) and Ed Curran with Curran International (Dickinson, Texas), and their colleagues. They note that efficient, safe, and reliable operation of heat exchangers requires regular maintenance and cleaning to avoid fouling and corrosion, which can cause significant challenges for oil and gas operators. Unwanted deposits of corrosion products, inorganic salts, and assorted organic foulants on metallic heat transfer surfaces can significantly reduce energy efficiency and process reliability. Fouling can also lead to new damage mechanisms such as underdeposit corrosion and trigger temperature excursions that, in extreme cases, can cause process safety incidents.

In recent years, a new class of fouling-resistant coatings derived from sol-gel processing has been developed. These coatings combine the chemical resistance and thin profile of inorganic coatings with the low cost, flexibility, and spray-application of polymer coatings. The hybrid organic/inorganic sol-gel coating features a ceramic silicon oxide (SiOx) backbone and organic components dispersed throughout, which result in low surface energy that makes it highly fluid repellant with little adhesion of solids. 

Plate-and-frame heat exchangers coated with this hybrid sol-gel material were tested at the low temperatures found in laboratory and field environments, which demonstrated their resistance to fouling on titanium plates in crude oil. Shell-and-tube heat exchangers for crude oil processing, however, have very different substrates and operational environments. Carbon steel (CS) or stainless steel (SS) are the typical tube materials, and the operating conditions often have higher temperatures and pressures than those experienced by plate-and-frame heat exchangers.

The study evaluated variations in metal surface pretreatment, the application process, and coating composition on CS and SS tubes. Coating performance was assessed using various thermal, mechanical, and fouling/immersion tests to determine efficacy and commercial viability. Two sol-gel coatings were investigated: one for low-temperature use (up to 400 °F [204 °C]) and one for high-temperature use (up to 650 °F [343 °C]). Microstructure characterization was performed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) in the SEM. Crude oil immersion tests were conducted at varying temperatures to assess the chemical stability of different coatings, and to compare them against uncoated (bare) metallurgy. To simulate steam-out conditions during heat exchanger maintenance, coated samples were exposed to steam in an autoclave at a pressure of 365 °F (185 °C) and 150 psig (1 MPa). Coating surface energy was measured using a series of test inks with known surface tensions ranging from 28 to 72 mN/m. Fouling resistance was measured in a test rig where process fluid flowed in the annular space between a standard 0.75 in (19 mm) diameter tube and outside pipe. Sol-gel coating adhesion was assessed using the test methodologies in ASTM D3359, “Standard Test Methods for Measuring Adhesion by Tape Test” and ASTM D4541, “Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.” 

These studies indicated that strong adhesion of the sol-gel coating can be achieved on CS and SS surfaces in a variety of fluids and over a broad range of temperatures. Sol-gel coating stability was demonstrated at temperatures exceeding 600 °F (316 °C) in crude oil, and in a steam environment corresponding to a saturation temperature of 365 °F (185 °C). Crude oil fouling rates for sol-gel-coated tubes were significantly lower than those for uncoated tubes, and accumulated deposits were removed at shear stress values that correspond to typical operating conditions in commercial heat exchangers. A reduction in mineral-scale fouling on the sol-gel coated samples due to hard water immersion was also observed.