For heat transfer applications involving sulfuric acid, materials of construction vary depending on the temperature, concentration, impurities, required service life and cost effectiveness. Common cost-effective materials include SiC ceramic, graphite, polyphenylene sulfide (PPS-GR), stainless steels and nickel alloys.
SiC Ceramic is the premium heat transfer material because it is 100% corrosion resistance for all concentrations of sulfuric acid. CG Thermal’s Umax® Advanced Ceramic tube is composed of alpha sintered silicon carbide (SiC) which has no fillers or free silicon, making it universally chemically inert. All metallic material options come with a life expectancy limited by a corrosion rate whereas the Umax® advanced ceramic will not corrode.
SiC is 50% harder than tungsten carbide which allows for increased velocities of 9-15 ft/s without limiting tube life due to erosion. The increased flowrate will result in a higher heat transfer rate and reduced fouling rate, thereby reducing the surface area required. Because of SiC’s smooth, impervious surface fouling material will remain entrained in the process fluid as it passes through the tube with little resistance.
Overall, these advantages result in higher thermal efficiency and less required surface area. Though the SiC material is more costly, longevity and reliability make it the material of choice for smaller systems. For large heat exchangers, the operational benefits should be weighed against the cost benefits of other materials.
For sulfuric acid concentrations up to 75% and 345°F, Impervite® graphite is the preferred material of choice with optimal CAPEX and OPEX values. It can be considered for concentrations up to 90% under special consideration and with a more limited operating life.
Impervite® graphite is a fully graphitized, more ductile material impregnated with phenolic resin. Its characteristics result from raw material specification, high graphitization temperature which results in a very crystalline graphite structure, and the highly controlled phenolic impregnation process making it a very resilient material with high toughness that will withstand system stresses well beyond the capabilities of carbon-graphite or resin bonded “graphite” tubes.
With metal tubes, corrosion results in thinning of the tube wall. With graphite tubes, this is not the case. However, with impervious graphite tubes in certain conditions (typically higher temperatures and higher acid concentrations), the phenolic resin used to make the tube impervious can be attacked over time. This results in resin loss in the tube and lower tube strength. Such loss of strength can lead to eventual tube breakage. In sulfuric acid concentrations above 85%, the acid can cause the graphite base to oxidize and lead to failure. Still, graphite remains an excellent heat exchanger material for many process applications requiring heating or cooling of sulfuric acid.
The PPS-GR tube combines the benefits of graphite and polymer materials to provide corrosion resistance, efficiency, and reliability in sulfuric acid processes. This tube material provides an excellent combination of resilience to operating stresses, corrosion resistance, thermal efficiency, low fouling, and maintainability. This is a newer, promising material with success up to 60% concentration at 290°F. CG Thermal is testing this at higher concentrations and continues to see extraordinary results. Even at its current state of verification, this material is a very cost-effective alternative to cathodically protected stainless steel heat exchangers.
In a properly designed unit, SST construction can be used effectively for high-temperature, gas-to-gas recuperators. Recuperators are used in applications such as catalytic operations, high-temperature energy storage, thermal oxidizers, steel mills, and steel foundries. These recuperators are capable of handling temperatures well over 1400°F. However, at the higher temperatures, it is important to understand the capabilities and limitations of the various grades of SST when determining which alloy will maintain the required strength at the design temperatures.
When it comes to liquid process streams, carbon steel and SST are acceptable options for heat transfer materials for concentrations between 93%-99% at room temperature. However, for other concentrations or slightly elevated temperatures, only a handful of specialty alloys can be used if a corrosion rate of less than the customary 5 mpy for heat exchange media is to be maintained. For example, when concentrations exceed 70% at elevated temperatures, tantalum remains the only metallic option.
There are many more considerations to take when working with sulfuric acid, especially in the design, transport, and storage stages. To learn more, download this white paper.