Model investigation of initial fouling rates of protein solutions in heat transfer equipment
Creators: name: Rose, Ian C.
Publisher: name: University of British Columbia
Year: 2009

description: As protein solutions are heated, denaturation and aggregation processes give rise to deposition on the heated surface. The present study undertakes the problem of understanding how process variables such as fluid velocity and temperature affect the initial fouling rate. Previous studies of chemical reaction fouling for other systems have demonstrated contrasting behaviour of the initial fouling rate with respect to fluid velocity: some investigations report an increase, others a decrease, and still others both an increase followed by a decrease of initial fouling rate with increasing fluid velocity. In this work a theoretical model for initial chemical reaction fouling in turbulent flow, where attachment is treated as a physico-chemical rate process in series with mass transfer (Epstein, 1994), was examined. According to the model, mass transfer is directly proportional to the friction velocity, and attachment is inversely proportional to the square of this velocity. Therefore, at a given wall temperature, it follows that if the initial fouling rate is mass transfer controlled (low fluid velocity), the deposition flux increases as the fluid velocity increases. If, however, the initial fouling rate is attachment controlled (high fluid velocity), the deposition flux will decrease as the fluid velocity increases. Therefore as the velocity is lowered the initial fouling rate goes through a maximum at a given wall temperature. In addition, this maximum initial fouling rate can be expected to increase and to move towards higher critical velocities as the wall temperature increases. Two separate experimental studies were performed, one using a 1 wt. % whey protein solution at pH 6, and the other a 1 wt. % lysozyme solution at pH 8. These experiments were performed over film Reynolds numbers of 2950 - 22730, clean inside wall temperatures of 59 - 102°C and bulk temperatures of 30 - 57°C in a 9.017 mm i.d. electrically heated, stainless steel tube. The above features of the model were qualitatively demonstrated with both protein solutions, i.e. a maximum in experimental initial fouling rate at a given wall temperature over a range of fluid velocities, and an increase in the maximum rate and in the corresponding critical velocity as the wall temperature increased. Lysozyme fouling results showed that as the mass flux increased from 200 kg/m²s to 1101 kg/m²s, the fouling activation energy, ΔEf, increased from 29 kJ/mol to 118 kJ/mol. This observation was consistent with the model, since the optimum model prediction, with an average absolute percent deviation of 23.3 %, was obtained with a kinetic reaction order of 0.75 and chemical activation energy, ΔE, of 161 kJ/mol. Thus at low velocity, when mass transfer dominated, ΔEf was low, but as the velocity was increased and chemical attachment became increasingly more important, ΔEf increased, but never to the value for the pure chemical reaction, ΔE, since mass transfer could never be entirely neglected. Whey protein modeling results, after rejection of renegade data points, showed an optimum solution, with an average absolute percent deviation of 24.5 % from the fit of the model, with a kinetic reaction order of 0.99 and a chemical activation energy, ΔE, of 201 kJ/mol. These values were compatible with the kinetic parameters for whey protein denaturation in the literature. Using estimates of the deposit physical properties from the whey protein fouling experiments, the dimensionless mass transfer constant (k') for both protein fouling experiments were found to lie between the isothermal value of Metzner and Friend (1958), and the non-isothermal value of Vasak and Epstein (1996), where all physical properties had been evaluated at the wall temperature. In contrast, the present work evaluated the physical properties in the mass transfer term at the film temperature, and therefore an intermediate estimate of k' was to be expected. In general, both protein fouling studies show results that conform in the mass transfer control region with the Epstein (1994) model, but in the attachment control region, the inverse dependence of the initial fouling rate on the friction velocity is even greater than the second power dependence predicted by that model. [Scientific formulae used in this abstract could not be reproduced.]