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Perfusion culture : investigation of temperature distribution in an acoustic separator Drouin, Hans


Mammalian cell perfusion cultures require practical and efficient cell retention devices that maintain high performance while minimizing negative influences on the culture. The acoustic separator is a mechanically simple device that provides high separation efficiency for months of continuous operation. The cells are retained in the chamber by acoustic forces whose magnitude is limited because the acoustic energy is ultimately dissipated as heat. On the other hand, the cell suspension, pumped into the acoustic separator through a recirculation line, is cooled somewhat by heat transfer to the ambient environment. Thus, the thermal control of the acoustic filter is essential to avoid negative effects on the cell culture while maximizing efficient separation. The purpose of this work was to investigate the thermal aspects of acoustic separations. Cell culture experiments demonstrated that CHO cells could be exposed to a cyclic temperature variation from 31.5 to 38.5°C, in a simulated acoustic separator environment, without significant effects on their growth rate, glucose consumption or t-PA production. Following an investigation of the acoustic separator recommended settings, a minimal recirculation flow rate of 15 L day⁻¹, at an ambient temperature of 22°C with a 45 s run time was found to provide efficient operation with limited environmental influences on the cells. Nonetheless, for a reactor cell concentration of 107 cells mL⁻¹ and a 5L day⁻¹ harvest flow rate, the separation efficiency was greater than 95% for ambient temperatures from 19 to 26°C. Air cooling flow rates from 0 to 16 L min⁻¹ did not perturb the separation efficiency of the system though air cooling was required to limit the temperature increase. A central composite factorial design experiment was used to obtain surface response models of the inlet and outlet temperatures as well as the inlet to outlet temperature change. These empirical models provided a tool to help optimize acoustic separator operation (i.e., selecting conditions that ensure temperatures are maintained in the acceptable range). Also, a theoretical model of the acoustic separator was developed, based on energy conservation, which provided an estimate of the 3- dimensional temperature distribution in the device. Once all of the unknown parameters had been determined by fitting the model to measured temperature data, it was able to predict the outlet temperatures to within 1°C. It was estimated that 56% of the power input was transformed into heat in the liquid compared with 6.5% in the transducer wall and 2% in the reflector wall. It was assumed that the remainder was lost due to the conversion of electric to acoustic energy and to conduction and eventual dissipation to the surroundings through the other solid components of the separator. Temperature profiles generated by the model as well as experimental measurements confirmed that the air cooling device was essential to control the temperature in the acceptable range.

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