Material of Construction

Stainless steel is the more commonly employed material for the fabrication of biotechnology equipment. The selection of the right steel quality in biotechnology is based on a compromise between material costs, availability, and the physical and chemical requirements of the process. Based on the crystal structure, standard chrome steels at room temperature are ferrite based (alpha or a iron). When ferric steels are heated to 910°C, the crystal structure is changed to the nonmagnetic austenite (gamma or y iron). The stabilization of the austenite structure at room temperature is enhanced by the addition of austenite formers such as nickel, molybdenum, or carbon. Austenitic steels are the material of choice used as shell plate for fabrication of fermentors. Compared to the nonaustenites, these steels show improved corrosion and heat resistance, and are nonmagnetic, apart from having good tenacity and workability (195).

Iron is the most abundant element in steel and very much prone to corrosion attack. Corrosion in the form of rust can be prevented by the addition of up to 12.5% chromium, which reacts with ambient oxygen to form a passive surface layer of chromium oxide. This chromium oxide layer protects the steel from corrosion. Stainless steels are defined with a maximum corrosion loss of 0.1 mm per year. Carbon in the stainless steel is required for stabilization of the austenite structure. The low carbon steels, SS 304L and SS 316L, are known worldwide as standard steels. Generally, vessels used in biological processes are fabricated with 316 or 316L steel. The vessels widely used in food technology or harvest storage tanks are fabricated with a cheaper and less corrosion resistant steel of grade 304 or 304L. For all other vessel parts not in contact with the product broth, such as jackets, cheaper, nonstainless steel grades are used (196). The commonly used steels in fermentation industry are shown in Table 3.7 (197).

The selection of a vessel material for fabrication should take into consideration:

1. Sensitivity of the organism, particularly eukaryotic cells

2. Extent of vessel corrosion on exposure to fermentation media and utilities (mostly acidic pH)

3. Aseptic operation requires use of SS316, SS316L, SS304, or SS304L, with specification of "L" contributing to an additional 15% vessel cost

Although the above factors govern the choice of material, testing of materials with actual fermentation broth or a growing culture is seldom reported.

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