94 Fermentation and Biochemical Engineering Handbook 5.0 FERMENTER COOLING When designing a fermenter, one primary consideration is the removal ofheat. There is a practical limit to the square feet of cooling surface that can be achieved from a tank jacket and the amount of coils that can be placed inside the tank. The three sources ofheat to be removed are from the cooling of media after batch sterilization, from the exothermic fermentation process and the mechanical agitation The preceding topic about the design of a continuous sterilizer empha sized reduced turnaround time, easier media sterilization, higher yields and one speed agitator motors. The reduced turnaround time is realized because the heat removal after broth sterilization is two to four times faster in a continuous sterilizer than from a fermenter after batch sterilization. The cooling section of a continuous sterilizer is a true countercurrent design Cooling a fermenter after batch sterilization is more similar to a cocurrent heat exchang Assuming that all modern large scale industrial fermentation plants sterilize media through a continuous sterilizer, the heat transfer design of the fermenter is only concerned with the removal of heat caused by the mechani- cal agitator(if there is one)and the heat of fermentatio ese data can be obtained while running a full scale fermenter. The steps are as follows 1. Heat Loss by Convection and radiation a. Perry's Handbook: [14 ∪=1.8 Btu/hr/F/ft No insulation; if tank is insulated determine proper 6. Calculate tank surface area =A C. Temp. of Broth=T1 d. Ambient Air Temp =T2 Q1=UA(T1·T2)=Btu/hr
94 Fermentation and Biochemical Engineering Handbook 5.0 FERMENTER COOLING When designing a fermenter, one primary consideration is the removal of heat. There is a practical limit to the square feet of cooling surface that can be achieved from a tank jacket and the amount of coils that can be placed inside the tank. The three sources of heat to be removed are from the cooling of media after batch sterilization, from the exothermic fermentation process, and the mechanical agitation. The preceding topic about the design of a continuous sterilizer emphasized reduced turnaround time, easier media sterilization, higher yields and one speed agitator motors. The reduced turnaround time is realized because the heat removal after broth sterilization is two to four times faster in a continuous sterilizer than from a fermenter after batch sterilization. The cooling section of a continuous sterilizer is a true countercurrent design. Cooling a fermenter after batch sterilization is more similar to a cocurrent heat exchanger. Assuming that all modern large scale industrial fermentation plants sterilize media through a continuous sterilizer, the heat transfer design of the fermenter is only concerned with the removal of heat caused by the mechanical agitator (if there is one) and the heat of fermentation. These data can be obtained while running a full scale fermenter. The steps are as follows: 1. Heat Loss by Convection and Radiation a. Perry's Handbook:[14] U = 1.8 Btu/hr/"F/ft2 (No insulation; if tank is insulated determine proper constant .) b. Calculate tank surface area =A c. Temp. of Broth = Tl d. Ambient Air Temp. = T, Q, = uA (Tl - T2) = Btukr
Fermentation Design 95 Convection and radiation depend upon whether the tanks are insulated or not, and the ambient air temperatu especially during the winter. Measurements of convection and radiation heat losses are, on average, 5% or less of total heat of fermentation(winter and uninsulated tanks) 2. Heat Loss by Evaporatio a. If fermenters have level indicators, theaverage evapo- ation per hour is easily determined b. Calculate pounds of water/hour evaporated from psychometric charts based on the inlet volume and humidity of air used, and at the broth temperature The exhaust air will be saturated, Determine heat of vaporization from steam tables at the temperature of the broth= HEy= Btu/lb Q2=HEy X(b water evap/hr)=Btw/hr Evaporation depends upon the relative humidity of the compressed air, temperature of the fermentation broth and the aeration rate. It is not uncommon that the loss ofheat by evaporation is 15 to 25%ofthe heat of fermentation. Modern plants first cool the com- pressed air then reheat it to 70-80%relative humidity based on summertime air intake conditions. Conse- quently, in winter the air temperature and absolute humidity of raw air are very low and the sterile supply will be much lower in relative humidity than summer conditions. Therefore in the winter more water is evaporated from the fermenters than in the summer. ( Water can be added to the fermenter or feeds can be made more dilute to keep the running volume equal to summer conditions and productivity in summer and winter equal) 3. Heat Removed by refrigerant a. This is determined by cooling the broth as rapidly as possible 5%F below the normal running temperature
Fermentation Design 95 Convection and radiation depend upon whether the tanks are insulated or not, and the ambient air temperature, especially during the winter. Measurements of convection and radiation heat losses are, on average, 5% or less of total heat of fermentation (winter and uninsulated tanks). 2. Heat Loss by Evaporation a. Iffermenters have level indicators, the average evaporation per hour is easily determined. b. Calculate pounds of waterhour evaporated from psychometric charts based on the inlet volume and humidity of air used, and at the broth temperature. The exhaust air will be saturated. Determine heat of vaporization from steam tables at the temperature of the broth = HEV = Btu/lb. Q2 = HEV x (lb water evaphr) = Btuh Evaporation depends upon the relative humidity of the compressed air, temperature of the fermentation broth and the aeration rate. It is not uncommon that the loss of heat by evaporation is 15 to 25% ofthe heat of fermentation. Modern plants first cool the compressed air then reheat it to 70-80% relative humidity based on summertime air intake conditions. Consequently, in winter the air temperature and absolute humidity of raw air are very low and the sterile air supply will be much lower in relative humidity than summer conditions. Therefore, in the winter more water is evaporated from the fermenters than in the summer. (Water can be added to the fermenter or feeds can be made more dilute to keep the running volume equal to summer conditions and productivity in summer and winter equal.) 3. Heat Removed by Refrigerant a. This is determined by cooling the broth as rapidly as possible 5°F below the normal running temperature
96 Fermentation and Biochemical Engineering Handbook and then shutting off all cooling. The time interval is then very carefully measured for the broth to heat up to re(△ T and time) b. Assume specific heat of broth= 1.0 Btu/lb-F c. Volume of broth by level indicator (or best estimate) Q3=SpHt. x broth vol.×8345×△T÷ time(hr) 23= Btu/hr 4. Heat Added by Mechanical Agitation a. Determine or assume motor and gear box efficiency about 0.92) 6. Measure kw of motor Q4=kW×3415× efficiency=Btu/hr 5. Heat of Fermentation=AH, g1+Q2+g3-4=MH The heat of fermentation is not constant during the course of the fermentation. Peaks occur simultaneously with high metabolic activity Commercial fermentation is not constant during the course of the fermenta tion. Commercial fermentations with a carbohydrate substrate may have peak loads of 120 Btu/hr/gal. The average AH, for typical commercial fermentations is about 60 Btu/hr/gal. The average loss of heat due to evaporation from aeration is in the range of 10 to 25 Btu/hr/gal. Fermenta tions with a hydrocarbon substrate usually have a much higher AHf than carbohydrate fermentations. Naturally, most c determine the△Hr for each product, especially after each major medium revision. (Typically, data are collected every eight hours throughout a run to observe the growth phase and production phase. Three batches can be averaged for a reliable AHf.) In this manner, the production department can give reliable data to the engineering department for plant expansions
96 Fermentation and Biochemical Engineering Handbook and then shutting off all cooling. The time interval is then very carefully measured for the broth to heat up to running temperature (AT and time). b. Assume specific heat of broth = 1 .O BtuAb-OF c. Volume of broth by level indicator (or best estimate) = gal Q3 = Sp.Ht. x broth vol. x 8.345 x AT + time (hr) 4. Heat Added by Mechanical Agitation a. Determine or assume motor and gear box efficiency b. Measure kW of motor (about 0.92) Q4 = kW x 3415 x efficiency = Btuihr 5. Heat of Fermentation = AHf e, + Q2 + Q3 - Q4 = AHf The heat of fermentation is not constant during the course of the fermentation. Peaks occur simultaneously with high metabolic activity. Commercial fermentation is not constant during the course of the fermentation. Commercial fermentations with a carbohydrate substrate may have peak loads of 120 Btu/hr/gal. The average AHf for typical commercial fermentations is about 60 Btu/hr/gal. The average loss of heat due to evaporation from aeration is in the range of 10 to 25 BtuMgal. Fermentations with a hydrocarbon substrate usually have a much higher Mf than carbohydrate fermentations. Naturally, most companies determine the AHr for each product, especially after each major medium revision. (Typically, data are collected every eight hours throughout a run to observe the growth phase and production phase. Three batches can be averaged for a reliable AHf.) In this manner, the production department can give reliable data to the engineering department for plant expansions
ermentation Desigh The following is how the heat transfer surface area could be designed for a small fermenter. The minimum heat transfer surface area has been alculated(based on the data below)and presented in Table 2 Assume S/S fermenter capacity 30,000gal Agitator 15hp/1,000gal Heat of fermentation(peak) 100 Btu/hr/gal Heat of agitation 38 Btu/hr/gal Heat transfer. U coils 120 Btu/hr/sq ft Heat transfer, u jacket 80 tw/hr sq ft No Btu lost in evaporation Chilled water supply 50°F Chilled water return 60°F Broth temperature(28C) Table 2. The heat Transfer Surface Area(ft2)Required for Tank with Coils onl Jacket Only Mechanical agitation Air agitation onl 150 After the heat transfer surface area requirements are known, various shaped(height to diameter)tanks should be considered. Table 3 illustrates parameters of 30,000 gallon vessels of various h/d ratios
Fermentation Design 9 7 The following is how the heat transfer surface area could be designed for a small fermenter. The minimum heat transfer surface area has been calculated (based on the data below) and presented in Table 2. Assume: S/S fermenter capacity Agitator Heat of fermentation (peak) Heat of agitation Heat transfer, U coils Heat transfer, U jacket Safety factor Chilled water supply Chilled water return Broth temperature (28°C) Table 2. The Heat Transfer 30,000 gal 15 hp/ 1,000 gal 100 Btu/hr/gal 38 Btu/hr/gal 120 Btu/hr/sq A 80 Btu/hr sq A No Btu lost in evaporation 50°F 60°F 82°F Surface Area (A2) Required for Tank with: Coils Only Jacket Only Mechanical agitation 200 Air agitation only 150 5 6 After the heat transfer surface area requirements are known, various shaped (height to diameter) tanks should be considered. Table 3 illustrates parameters of 30,000 gallon vessels of various WD ratios
98 Fermentation and Biochemical Engineering Handbook Table 3. Maximum Heat Transfer Surface Area Available(ft?)on 80% of the Straight Side H/d F(it) D (ft) Jacket Coils* 27313.79381,2 2.5 31.7 12.71,0101,340 35811.91,0701,400 3.5 1,455 43.4 10.81,1801,150 *Coil area is based on 3. 5 inch o d, 3. 5 inch spacing between helical coils and 12 inches between the tank wall and the center line of the coil It can be seen by comparing Tables I and 2 that if mechanical agitation is used and a jacket is desired, then additional internal coils are required. the intermal coils can be vertical, like baffles, or helical. Agitation experts state that helical coils can be used with radial turbines if the spaces between the coil loops are I to 1.5 pipe diameters. Once helical coils are accepted, Why use a jacket at all? Reasons in favor of coils (in addition to the better heat transfer coefficient)are 1. Should stress corrosion cracking occur(due to chlorides in the cooling water), the replacement of coils is cheaper than the tank wall and jacket 2. The cost of a fermenter with helical coils is cheaper than a jacketed tank with internal coils 3. Structurally, internal coils present no problems with continuous sterilization However ifbatch sterilization is insisted upon, vertical coils are one solution to avoiding the stress between the coil supports and tank wall created when cooling water enters the coils while the broth and tank wall are at 120C. notice that the method of medi sterilization batch or continuous. is related to the fer menter design and the capital cost
98 Fermentation and Biochemical Engineering Handbook Table 3. Maximum Heat Transfer Surface Area Available (ft2) on 80% of the Straight Side H/D F (ft) D (ft) Jacket Coils* 2 27.3 13.7 938 1,245 2.5 31.7 12.7 1,010 1,340 3 35.8 11.9 1,070 1,400 3.5 39.7 11.3 1,130 1,455 4 43.4 10.8 1,180 1,150 *Coil area is based on 3.5 inch o.d., 3.5 inch spacing between helical coils and 12 inches between the tank wall and the center line of the coil. It can be seen by comparing Tables 1 and 2 that if mechanical agitation is used and ajacket is desired, then additional internal coils are required. The internal coils can be vertical, like baffles, or helical. Agitation experts state that helical coils can be used with radial turbines if the spaces between the coil loops are 1 to 1.5 pipe diameters. Once helical coils are accepted, Why use ajacket at all? Reasons in favor of coils (in addition to the better heat transfer coefficient) are: 1. Should stress corrosion cracking occur (due to chlorides in the cooling water), the replacement of coils is cheaper than the tank wall and jacket. 2. The cost of a fermenter with helical coils is cheaper than a jacketed tank with internal coils. 3. Structurally, internal coils present no problems with continuous sterilization. However, if batch sterilization is insisted upon, vertical coils are one solution to avoiding the stress between the coil supports and tank wall created when cooling water enters the coils while the broth and tank wall are at 120°C. Notice that the method of media sterilization, batch or continuous, is related to the fermenter design and the capital cost