Figure 48. Viscosity at constant shear rates for various days in the fermentation cycle.

RELATIVE VISCOSITY Figure 49. Decrease in mass transfer rate with viscosity for a given mixer and air rate.

Table 4. Cost of Mixing for Production of Antibiotics

(Based on Oxygen Transfer Rate)

Cost of electrical power 0.70/MJ

Equipment Cost (expressed as power cost) 0.80/MJ Efficiency of oxygen mass transfer

Dilute system, lOg//* 10 mols 02/MJ

More concentrated, 20g//* 6.4. mols 02/MJ

Power and Equipment cost 1.5 0/MJ Cost of dissolved oxygen

Production cost of antibiotics 460/kg Fractional cost for mixing

(antibiotics production) 0.6-1.6%**

Production Yield 1 kg/200 mols 02

*Cell concentration.

**Of production cost.

Electrical power is assumed at 0.70/MJ (to obtain 0/kW-hour multiply by 3.6). The equipment is amortized, using present worth, over a 5-year period, which results in a figure of 0.80/MJ. Total cost of the equipment and operation is therefore 1.50/MJ. The cost of dissolving oxygen is 0.15e/mol 02 dissolved at 10 g//. At 20 gII, it is 0.230/mol 02.

Assuming that it takes 200 mols of oxygen to produce 1 kg of product and that there is a production cost of $60 total per kg of product, the percent cost of oxygen in the dilute system is approximately 0.7% of the total production cost. There is also assumed in this example that there is a fixed cost of $30/kg which does not change with the agitator, and that the variable fermentation cost goes down as the productivity of the particular tank in the process increased.

This is listed in Column A of Table 5. Column D gives the results from the paper by Ryu and Oldshue, which described the use of a 500 hp mixer operating at 20gII. While the percent of cost due to the mixer has increased, the total production cost per kg of product has gone down 25% to a value of approximately $45.2/kg.

Table 5. Comparison of New Mixer to Original Mixer


Low Power Mixer . .New High Power Mixer.. (A) (D) (E) (F) (G)


Low Power Mixer . .New High Power Mixer.. (A) (D) (E) (F) (G)

Agitation Power, kW

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