which it will not be possible to remove enough heat to maintain constant temperature, unless the cooling capacity is increased by incorporating internal cooling coils or by using external heat exchangers. These modifications may prove costly or else interfere with mixing in the vessels. There is possibly the alternative of using micro-organisms with higher optimum temperatures. The oxygen requirements of a process may limit the size of vessel which can be operated successfully. In acetic acid (vinegar) production, where very efficient aeration is critical, because Acetobacter spp. are very sensitive to oxygen depletion in the broth, the maximum acetator capacity which could be used was about 50,000 dm3 (Pape, 1977).
There is an upper size limit for a custom-built fermenter that can be transported to a site, while much larger vessels may be built on site. Most countries limit the maximum size of unit that can be transported on the road. In 1979 the first ICI pic production vessel for single-cell protein was installed; this had a volume of 1.5 x 106 dm3, was 80-m tall and cost £6 X 106 (Sharp, 1989). It was constructed in France, floated on a barge to the River Tees, and transported a very short distance on land. At the time of erection it was the largest fabricated fermenter ever to be transported.
Because of the capital investment and operational costs there is now a trend to consider unconventional fermenter designs of simple construction with very efficient oxygen-transfer to be used for specific purposes, particularly single-cell protein. In this context, Schugerl et al. (1978) have considered the case of a single-cell protein plant for 100,000 tonnes annum"1 using methanol as the main substrate. The plant investment was only 20% of the production cost. In this investment cost only 20 to 25% was due to the fermenter. Of this, the vessel accounted for 5 to 10%, the stirrer and aerator 5 to 10% and cooling 10 to 15%. Energy, water, aeration and auxiliaries were estimated to be a further 10% of the production costs. Since the fermenter investment costs are approximately 4 to 5% and the operating costs are 10%, it was concluded that the type of vessel could influence only 15% of the total production costs. If a tower fermenter were used, the main advantage would appear to be lower operating costs, particularly reduced energy and cooling-water costs, as the removal of mechanical stirring would diminish the cooling water requirements.
A useful guide has been prepared by the Institution of Chemical Engineers (1977) which outlined, in reasonably quantitative terms, many factors making up a check list which must be incorporated into the final capital costing of a chemical-plant project. It is possible from a knowledge of the proposed plant location, a sketch of the chemical plant proccss flow sheet, the size of the major items of equipment and the service requirements, to estimate capital and operating costs to ±15% (Backhurst and Harker, 1973). Unfortunately, comparable literature for fermentation plants is not available.
Some details of the cost of equipment used in fermentation processes has been discussed by Whitaker (1973), Humphrey (1975), Mateles (1975), Rolz (1975), Nyiri and Charles (1977), Maiorella el al (1984), Hacking (1986), Kalk and Langlykke (1986), Reisman (1988) and Atkinson and Mavituna (1991). It is essential to remember that most of the data are estimates for proposed processes. Some equipment-cost breakdowns are quoted in Table 12.5.
Kalk and Landlykke (1986) quote 1985 costs of fer-
menters. In the unit size range of 1.0 to 45 m3, costs for modular units with control and recording ranged from $90,000 to $350,000. The cost of 70 to 250 m3 vessels, which needed site erection, ranged from $800,000 to $2,000,000.
If a vessel is to be used to produce genetically engineered products, the costs will increase significantly because of the extra containment provisions which must be incorporated during construction (see also Chapter 7). It has been estimated that the basic fermenter costs will increase by 10-30% for each increase in Containment level (Hambleton et al, 1991). At level 2 or B3 (see Chapter 7 for definition) the provision of containment of an instrumented 20-dm3 fermenter may increase the cost from £45,000 to £100,000. The cheaper alternative might be to enclose an ordinary fermenter in a suitable containment cabinet. Hambleton et al (1991) have reported the use of a
Table 12.5. Capital cost breakdown for fermentation plant
% of total
(a) Penicillin plant, estimated for five 225,000 dm3 fermenters with ancillary equipment (Swartz, 1979)
Laboratory equipment Spare parts
(b) Norprotein plant (Mogren, 1979)
Raw materials storage
Media preparation and utilities
Cell recovery and drying
(c) ICI pic. Single-cell protein plant (Smith, 1980)
Raw materials Storage and packing Off-site services On-site services Fermentation Compression Dewatering Drying
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