Stavrinides, Steven; (1995) Breakage of protein precipitates in mechanically agitated vessels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Protein break-up in stirred reactors plays a key role during the formation, processing and subsequent recovery of protein precipitates. This is largely due to their characteristically delicate nature and high sensitivity to shear forces. The experimental observations are presented for the breakage of Isoelectric Soya protein precipitate in suspension in mechanically agitated vessels under turbulent flow conditions. The experiments were performed in a 0.29 m diameter vessel equipped with four standard baffles and a six bladed, 45° angle turbine impeller positioned centrally and driven from the top of the vessel by a 1.1 kW motor with infinitely variable speed control. Impeller speed and power input were monitored using a shaft mounted torque transducer. The parameters investigated were size of impeller (ranging between 120 mm and 205 mm), speed of impeller (ranging between 60 rpm and 800 rpm) and total protein concentration (ranging between 0.35 kg/m3 and 35 kg/m3). Reynolds numbers ranged from 36,000 to 560,333 corresponding to a range in the mean energy dissipation rate of 37.5 W/m3 to 5683 W/m3 equivalent to a range in the mean velocity gradient of 194 s-1 to 2385 s-1. Experiments were carried out in order to measure the effect of these parameters on the rate of protein precipitate break-up and to determine their final equilibrium size distributions. The basis used for the analysis of the results was to maintain a constant energy input per unit volume of suspension within the various impeller-vessel geometrical configurations. A standard procedure was used for the preparation and conditioning of the protein precipitates for every run. At the end of the conditioning period a step change in impeller speed was carried out and from then on samples were removed periodically for up to five hours for particle size analysis using a modified Coulter Counter technique. The analysis of the results suggests that the break-up of these particles is sensitive to even the minor changes in operating conditions. It appears from the experiments that most of the protein aggregate breakage occurs within the first half an hour of exposure to shear, the modal particle size distribution decreasing from a value of approximately 10 μm to about 1.0 μm. The rate of protein aggregate breakage as well as the final protein aggregate size at the end of the five hour period of exposure to shear are strongly dependent on both the speed of agitation and protein concentration. The effects of impeller speed on the breakage of the protein aggregates becomes progressively greater as the speed of agitation is increased and similar trends are obtained when varying protein concentration. The results indicate that for a given protein concentration, the initial rate of aggregate breakage is uniquely dependent on the impeller speed and impeller diameter, both parameters being related to the mean energy dissipation rate inside the vessel. The concept of a constant mean energy dissipation rate provides a rational scale-up basis for the design of industrial mechanically agitated reactors for precipitation. Moreover it provides a common basis upon which it ought to be possible to assess and compare breakage of protein precipitates in seemingly different types of processing equipment, such as pumps, pipelines and centrifuges. Additionally, a model has been established to describe the influence of these different hydrodynamic conditions on the breakage of protein precipitates in turbulent suspension. The proposed model accurately describes the influence of the mean energy dissipation rate and also the protein concentration on the initial breakage frequency of the protein aggregates.

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