Storey, S. A.; (2000) Physical property indices to guide bioprocess synthesis and design. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This study focuses on characterising two features that often complicate the selection and design of downstream processes: chromatographic matrix fouling and problematic solid-liquid separation steps. The method of frontal analysis was used to assess the fouling properties of bioprocess streams. Breakthrough curves, measured using this procedure, provide a snapshot of adsorption characteristics for chromatography columns. Conducting the analysis for anion exchange columns (Q-Sepharose), both before and after exposure to a constant volume of potentially fouling process streams, revealed adsorption differences linked to fouling. Mathematical modelling studies allowed the calculation of mass transfer coefficients and other kinetic parameters, to provide a precise delineation of which adsorptive mechanisms are impaired by exposure to the process streams of interest. Foulant exposure was shown to significantly reduce the number of available adsorption sites in the matrix phase, and to increase the external fluid film resistance to mass transfer. The frontal analysis test is also a useful means of rapidly evaluating different process alternatives, from a process synthesis perspective. Streams from several alcohol dehydrogenase (ADH) purification options were assessed using the method, yielding measurements that correlated well with chromatographic performance. A factorial experiment systematically altered the fouling properties of a bacterial cell lysate stream, producing a series of materials that approximated a continuous scale of fouling strengths. These streams were analysed for their fouling properties and chromatographic performance, reaffirming the strength of the correlations observed earlier. The results quantitatively relate the fouling measurements to chromatography system design, and provide the basis of a predictive model. A centrifugal clarification index is developed which extends the classical use of Sigma theory by graphically depicting the trade-off between performance, flowrate and installed separation area. A practical application is described, comparing two streams from similar sources: periplasmic extracts from different strains of Escherichia coli. An increase in process performance results in very different design and cost requirements for the two streams, despite their apparent similarities. The example demonstrates the utility of the method, identifying suspensions that require large amounts of additional separation area to achieve only marginal gains in performance. A hierarchical method for bioprocess flowsheeting is described, which draws upon this information, in addition to other physical property measurements and ultra scale-down tools.

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