Anticorrosion protection of fermentation apparatus

Abstract

ne amounted to 0.2-0.4 mm per year, which, according to the standard classification, corresponds to a low resistance of the material (group IV, 6 points). Experiments and the practical use of steel fermenters have shown that with the lapse of time in the course of the working of the apparatus corrosion slows down because of the formation on the surfaces in contact with the culture liquid of protective passivating films. In a study of a film about 2 mm thick taken from the walls of an industrial fermenter for the production of penicillin, it was found to have a layer structure [2]. This film, which had deposited after 1.5 years of working, consisted mainly of calcium carbonate cemented by caramelized sugars, pectins, etc. The film formed after some years on the working surface of a steel fermenter for the production of streptomycin contained about 74% of iron oxide and approximately 18% of organic substances [3]. The formation of passivating films is of great importance both from the point of view of the anticorrosion protection of the metal and for the normal conditions of growth of the culture and the production of the antibiotic, since an excess of iron in the medium due to corrosion has an adverse effect on biosynthesis [4-6]. This is the case particularly when fats with high iodine numbers forming unsaturared fatty acids are used as foam suppressors, since iron ions stimulate the formation of peroxide compounds which are toxic to the producing agent [7]. A marked fall in the productivity of fermentation in the absence of a protective film on a steel surface in contact with the culture liquid has been observed in experiments [I]. The antibiotic activity of the liquid fell in the case of penicillin by a factor of 1.5, for streptomycin by almost 2, and in the case of ehlorotetracyeli ne almost completely. When a protective film is present there is no appreciable fall in activity, and this makes it possible to use carbon steel fermenters. However, this film does not prevent corrosion completely but only retards it. Also important is the fact that the protective film is not uniform in the different parts of the apparatus. Its formation and the strength of its adherence to the metal surface are affected by the nature of the flows of aerated liquid created by the mixer, the temperature stresses, and other factors. In some parts of the apparatus the film is particularly strongly subjected to erosion, sharp changes in temperature on sterilization and cooling, mechanical effects in the cleansing of the fermenter, etc. In view of this, such parts of steel apparatus as coils, stirrers, and stairs are put out of commission by corrosion in the course of a few years' work [8]. As a result of the incomplete adhesion of the film to the surface of the metal and its local lamination, etc., so-called pitting corrosion takes place in various parts of the apparatus, leading to the formation below the film of deep (up to 8 ram) cavities and bubbles, since the thickness of the wall of the body of industrial fermenters with a capacity of 50 m 3 is 12-14 ram, such corrosion represents a threat to the mechanical strength of the apparatus when pressure is created in it. These reasons explain the tendency observed in the world practice of the industrial production of antibiotics to make fermenters from alloy stainless steels or from a bimetal (carbon steel-stainless steel). Such apparatus is suitable for working and has a long life, but its manufacture is affected by the shortage of stainless steel and is associated with large capital charges, which, in a number of cases, make it impracticable (for example, in the organization of the production of fodder antibiotics in agricultural undertakings) [9]. For these cases it is necessary to find an anticorrosion protection of the internal surface of steel fermenters by means of special coatings.