A study of the use of computer simulations for the design of integrated downstream processes

Abstract

Simulation may be used as a powerful tool for accelerating bioprocess design. This paper demonstrates the use of simulations in exploring the nature and impact of the interactions that exist in a typical bioprocess for the recovery of an intracellular protein. The study shows that an integrated approach to design must be adopted in order to achieve acceptable process designs. Data from a fedbatch fermentation, with verified models for cell harvesting, cell disruption and cell debris removal have been integrated to demonstrate the consequence of process design and operating decisions on the resulting process performance. The trade-offs between product recovery and the extent of cell debris removal for a range of operating conditions have been represented through a series of windows of operation which show how process conditions must be altered in order for given process performance levels to be realised. The capacity to account for process performance including the impact of interactions is seen as a pre-requisite for rigorous bioprocess sequence design and optimisation. IntroductionDownstream processes for protein production and purification of biomolecules are often complex and may consist of many unit operations. The sequence for the recovery of a typical intracellular enzyme may require as many as eight steps to yield a crude enzyme [29]. Subsequent purification based on chromatography may necessitate a significant number of additional stages and hence the overall yield is often low. To increase the overall process efficiency it is desirable to operate each stage at a high yield, or to reduce the number of steps [12]. The latter approach may require substitution of new operations to enable such a truncation in the sequence without loss in performance. However in both cases a systematic approach to process design and operation is needed which takes account of any interactions existing between steps since these can severely influence subsequent performance. Previous workers have examined such process interactions. Changes in the morphology of P. chrysogenum that occur during fermentation have been shown to result in very different filtration rates being achieved in subsequent downstream processing [27] while the conditions of bead milling S. cerevisiae impact the efficiency of subsequent depth filtration [24]. Here a clear compromise between the extent of milling to achieve good product release and the consequent blockage of the filter medium by fines was apparent. These results suggest that it is necessary to analyse bioprocesses in an integrated fashion such that changes in the performance due to altered specifications in one step can be predicted. Without such a capacity the process chain cannot be operated to yield its potential since a high yield in one step may lead to lower recovery for the next step. A clear example of this is the trade-off between homogenisation conditions chosen for protein release and the issue of subsequent cell debris separation by centrifugation. Here a high yield at the homogenisa...

A methodology for centrifuge selection for the separation of high solids density cell broths by visualisation of performance using windows of operation

“…Factors of significance for centrifugation have been discussed previously (Boychyn et al, 2000;Zhou et al, 1997) and relate to feed material propertiessuch as size and shape of particles and concentration of solids in the feed, operating parameters such as flowrate and processing time, and parameters related to the centrifuge type and design, for example solids holding capacity.…”

“…Windows of Operation provide one such tool, which will help in design and also enable the rapid comparison of alternative operating strategies. Previous work has demonstrated the use of Windows of Operation as a visualisation tool within bioprocessing (King et al, 2004;Nealon et al, 2005;Woodley and Titchener-Hooker, 1996;Zhou and Titchener-Hooker, 1999;Zhou et al, 1997). Such Windows have not only been shown to be useful in the selection of suitable operating conditions for a given processing objective but also to indicate where a particular item of equipment is not suitable for the chosen process target.…”

Modelling biopharmaceutical manufacture: Design and implementation of SimBiopharma

“…The second has developed detailed models of individual unit operations verified using experimental data (e.g. Gritsis, & Titchener-Hooker, 1989;Zhou, Holwill, & Titchener-Hooker, 1997;Varga, TitchenerHooker, & Dunnill, 2001). In addition there has been work specifically targeted at batch processes but primarily developed for chemical process development (e.g.…”

Use of scale-down methods to rapidly apply natural yeast homogenisation models to a recombinant strain

“…This information is required for full integration of the unit operation with process stages for debris removal such as centrifugation or ®ltration [14]. Choosing operating conditions for the homogeniser then involves a trade off between the extent of product release and subsequent ef®ciency of the debris removal stage to prevent excessive solid loadings on up-stream operations such as chromatography [11,26].…”