CHL Tut 3

CLL 780 Tutorial 3- Bioreactor mixing and scale up Basics to revise before solving: A) Bioreactor mixing and power requirement B) Power number, Aeration number C) Bioreactor scale up Q1. Calculate the power requirement with and without aeration, of a 1.5 m diameter stirred tank, containing water 1.5 m deep, equipped with a six blade Rushton turbine that is 0.5 m in diameter d, with 0.25 d long and 0.25 d wide, operating at an operational speed of 180 rpm. Air is suspended from the tank bottom at a rate of 0.6 m3/min. Operation is at room temperature. Values of water viscosity is 0.001 kg/m s and water density is 1000 kg/m3. (Note: Use the plot between Re and power number as provided below in Q2)

Empirical relation between P gassed (PG) and P non gassed (Po) is given as: Log(PG/Po) = -192 (Di/Dt)4.38 (Re)0.115 (Fr)0.653 (Na) where Di is the impeller diameter and Dt is the tank diameter. Re, Fr and Na are the Reynolds number, Froude number and the aeration number respectively. Also find out the KLa for oxygen absorption into water for the case of aerated stirred tank. Empirical relation for KLa is given as: KLa = 0.026 (PG/V)0.4 (Vs)0.5 where PG/V is the power to volume ratio and Vs is the superficial gas velocity. Q2. A bacterial fermentation was carried out in a reactor containing broth with average density 1200 kg/m3 and viscosity 0.02 Ns/m2. The broth was agitated at 90 rpm and air was introduced through the sparger at a flow rate of 0.4 vvm. The fermenter was equipped with two sets of flat

blade turbine impellers and four baffles. The dimensions of vessel, impellers and baffle width were: Tank diameter, Dt = 4 m; impeller diameter, Di = 2 m; baffle width, Wb = 0.4 m; Liquid depth, H = 6.5 m.

Determine a) ungassed power, P ; b) gassed power, Pg ; c) kLa

Q3. A bioreactor with internal diameter of 2 m is equipped with baffles and two sets of flat blade impellers and four baffles. The impeller has a diameter of 1.25 m, and the baffle width is 20 cm. The viscosity of the liquid broth is 0.025 Nsm-2 and the density is 1220 Kg/m3. Determine the power number and the power required for the two sets of impellers if the rotational speed of the impeller is 60 rpm. Q4. A vessel is to be designed to contain a liquid slurry of suspended particulates using a mechanical stirrer. To ensure that the particulate remain in suspension, scale tests are carried out under fully turbulent conditions in a tank with a diameter of 0.6 m. The test tank has baffles and a flat blade impeller for which the tank diameter to impeller ration is 3. The suspension is considered to be successful for a mixing speed of 3Hz for which the power consumption is 0.18 kW and Reynolds number of 145000. Following the success of the test mixing, determine the rotor speed that would ensure that the same level of mixing is achieved at full scale if a geometrically similar vessel is to be scaled up by a linear scale of 6. Also find out the Reynolds number for the scaled up vessel assuming constant physical properties and no change in operating conditions. Q5. Consider the scale up of a fermentation from a 10 L to 10,000 L vessel. The small fermenter has a height to diameter ratio of 3. The impeller diameter is 30% of the tank diameter. Agitator speed is 500 rpm and three Rushton impellers are used. Determine the dimensions of the large fermenter and the agitation speed for: a) Constant P/V b) Constant impeller tip speed c) Constant Reynolds number