Cascades of bioreactors

In this thesis a common phenomenon in bioprocess engineering is described : the execution of a certain bioprocess in more than one bioreactor. Chapter 1, a review, classifies bioprocesses by means of a number of characteristics :i) processes with a variable stoichiometry ,ii) processes with a constant stoichiometry using biocatalysts ,iii) processes with a constant stoichiometry that are autocatalytic .This chapter also offers a method to decide in which cases it can be worthwhile to use more than one bioreactor. The possible advantage is gained by a possible reduction in the total residence time needed to accomplish a certain degree of conversion. The shorter that residence time, the smaller the bioreactor(s) can be, and with that the capital investment reduces. The minimalresidence time is attained if the bioreactors all have a different volume. In general the volume of each bioreactor decreases along the series. Moreover, the total volume of the series decreases if an increasing number of bioreactors are used in the series, although that decrease becomes increasingly less. The largest decrease in total residence time occurs by using two bioreactors in series instead of one single bioreactor, whereas the use of more than three bioreactors in series usually offers little advantage since the extra costs for pumps and similar additional parts is getting too high.Chapter 2 describes the optimum design of a series of bioreactors for the case that the biocatalysts are immobilized, and chapter 3 describes this for immobilized autocatalytic systems. In chapter 3 some rather straightforward assumptions are made for the behaviour of immobilized growing cells, which may not be true in reality. Chapters 4 and 5 show the dynamic behaviour of the cells, including an experimental evaluation of such a system. As a model system Nitrobacter agilis cells were used. These cells perform the conversion of nitrite in nitrate, which is of importance for waste-water treatment, more precisely the removal of ammonia. Hereby ammonia is first converted to nitrite, nitrite to nitrate, and nitrate finally is converted to nitrogen gas. The model that is derived in chapter 4 however, has a more general applicability.Chapters 6, 7 and 8 describe a system with a variable stoichiometry. In a first bioreactor insect cells are produced, which are infected with a baculovirus in one or two subsequent bioreactors. The infected cells then will produce polyhedra, which have a use as bioinsecticide. According to the current knowledge, insects cannot develop resistency against baculoviruses. Moreover, baculoviruses are extremely specific for an insect species, which means that useful insects are not affected. In these chapters it is shown that the production in continuously operated series of bioreactors is not unlimited in time, and that that is caused by the so-called passage effect : if the viruses have infected a cell a number of times, their infectivity decreases and the reaction stops. The model described in chapter 7 can predict what should be the optimal reactor configuration, and in chapter 8 this is experimentally shown : the cells must be grown in a bioreactor with a feed of medium, and if that bioreactor is filled part of its contents are pumped to a second bioreactor in which infection with baculovirus occurs. During the time that the bioreactor in which the cells are grown is filled, the infected cells in the infection bioreactors produce polyhedra. After that the infection bioreactor is largely emptied, so that some virus remains in the reactor, and new cells are added, after which the infection proceeds, and so on. In this manner the time that the production process runs can prolongate fourfold as compared to a fully continuous process.The number of applications of series of bioreactors is limited. An important cause for this is that, in practice, the for most bioprocesses required sterility is not easily maintained if the process is executed in more than one bioreactor. Chapter 9 shows a possible solution to that problem : in the presented Multiple Air-Lift Loopreactor up to three air-lift loopreactors in series are incorporated into one bioreactor. List of explained words.Air-Lift Loopreactor : A bioreactor without stirrer that consists of two compartments : a riser and a downcomer. Mixing andoxygen transfer are accomplished by sparging air at the bottom of the riser.Autocatalytic :A reaction where the biocatalyst itself is produced.Baculovirus :A rod-shaped virus occuring in insects.Biocatalysts : Compounds, usually enzymes, that accelerate a reaction but do not take part in the reaction.Bioprocess Engineering : The application-oriented science of the integration of one or more biological disciplines and processengineering. Chapter 7 is a good example of the integration of virology and process engineering.Immobilized : In this thesis this means the inclusion of cells in a carrier, to retain them in a bioreactor.Insect cells : Cells of an insect (in this case Spodoptera frugiperda ), capable of growth in suspension.Polyhedra : The form of baculoviruses occurring in nature. To protect the virions against environmental influences they are packed in protein matrices, the polyhedra. Once arrived in the gastro-intestinental tract, the polyhedra are dissolved and the virus particles are released, after which the insect is infected.Stoichiometry : The ratio, on a molar basis, between the substrate offered and the product formed.Residence time : The average time spent in a bioreactor.