In the realm of wet-process phosphoric acid plant construction, selecting the right materials is paramount, especially for tanks, filter sections, and concentration units. Today, we delve into the realm of stainless steels and their role in the evaporator heat exchangers for phosphoric acid concentration.
Why metallic tubes trump graphite tubes?
While graphite has long been the go-to material for phosphoric acid evaporator tubes due to its superior thermal conductivity, it comes with some drawbacks. Over time, scale build-up hampers heat transfer efficiency, negating its benefit. To combat this, it's crucial to minimize scaling through regular cleaning or boosting acid velocity. However, the brittleness of graphite poses a risk of breakage during scale removal, even with just high-pressure washing, and higher acid velocity can lead to substantial erosion and wear problems in the inlet of graphite evaporators.
This is where metallic tubes come into play. Their superior mechanical strength offers several key benefits. Metal tubes are easier to clean and require less frequent maintenance, they can withstand higher acid velocities and have good strength and resistance to erosion wear.
But does this mean all types of metallic tubes could be used?
The corrosion resistance of each stainless steel is attributed to its capacity to form a protective oxide layer, primarily composed of chromium. Consequently, materials with higher chromium content exhibit an enhanced ability to create this protective oxide layer. The effect of chromium is two-fold; reducing corrosion rates both in passive and active states, which is important in preventing corrosion.
Pure phosphoric acid is not very corrosive, however, the presence of impurities makes the wet-process phosphoric acid extremely corrosive, and standard stainless steels like 316L is unable to handle such an extremely corrosive environment. This is where specialized high-alloy stainless steels and nickel-chromium-molybdenum alloys become indispensable for their higher chromium content.
Material solutions for phosphoric acid applications
Enter Sanicro® 28, meticulously designed for the rigors of phosphoric acid production, particularly in the high-stress environment of heat exchangers within concentration units. This marvel of engineering serves as a formidable substitute for graphite tubes in heat exchangers tasked with handling concentrated phosphoric acid. The durability of metallic heaters typically spans a decade, yet Sanicro® 28 defies expectations with proven success stories extending up to twenty years.
Building on the success of Sanicro® 28, we introduced Sanicro® 35 in 2020 after years of extensive research and development, including general corrosion laboratory tests. This innovative alloy boasts similar chromium content to its predecessor but features higher nickel and molybdenum content. Notably, the elevated molybdenum content significantly enhances its resistance to localized corrosion caused by halides, as evidenced by its impressive Pitting Resistance Equivalent number of 52, compared to ~40 for Sanicro® 28.
While Sanicro® 28 offers a significant improvement over graphite tubes in phosphoric acid evaporators, Sanicro® 35 might be an even more promising choice, particularly in environments with severe corrosion from pitting and halides. While more specific data is needed, its higher PREN (Pitting Resistance Equivalent Number) indicates better performance than Sanicro® 28 in chloride-containing phosphoric acid.
To sum it up, these innovative alloys represent the culmination of extensive research and development efforts, to provide enhanced performance and longevity, even in the harshest environments. This is crucial in today's era of advancing industries focused on greater efficiency and sustainability. Prioritizing the use of advanced materials enables businesses to position themselves for long-term success in a competitive, environmentally conscious market.