Bioreactors mass transfer effects

This chapter describes the different types of batch and continuous bioreactors. The basic reactor concepts are described as well as the respective basic bioreactors design equations. The comparison of enzyme reactors is performed taking into account the enzyme kinetics. The modelhng and design of real reactors is discussed based on the several factors which influence their performance the immobilized biocatalyst kinetics, the external and internal mass transfer effects, the axial dispersion effects, and the operational stabihty of the immobilized biocatalyst. 

In the absence of viable cells in the bioreactor, an effective mass transfer coefficient can be obtained from... 

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

Understanding the effect of reactor diameter on the volumetric mass transfer coefficient is critical to successful scale up. In studies of a three-phase fluidized bed bioreactor using soft polyurethane particles, Karamanev et al. (1992) found that for a classical fluidized bed bioreactor, kxa could either increase or decrease with a change in reactor diameter, depending on solids holdup, but for a draft tube fluidized bed bioreactor, kxa always increased with increased reactor diameter. 

Intraparticle Mass Transfer. One way biofilm growth alters bioreactor performance is by changing the effectiveness factor, defined as the actual substrate conversion divided by the maximum possible conversion in the volume occupied by the particle without mass transfer limitation. An optimal biofilm thickness exists for a given particle, above or below which the particle effectiveness factor and reactor productivity decrease. As the particle size increases, the maximum effectiveness factor possible decreases (Andrews and Przezdziecki, 1986). If sufficient kinetic and physical data are available, the optimal biofilm thickness for optimal effectiveness can be determined through various models for a given particle size (Andrews, 1988 Ruggeri et al., 1994), and biofilm erosion can be controlled to maintain this thickness. The determination of the effectiveness factor for various sized particles with changing biofilm thickness is well-described in the literature (Fan, 1989 Andrews, 1988)... 

There have been misconceptions among some researchers regarding the influence of the dissolved oxygen tension (DO) in baculovirus infection and product expression, as often no distinction is made between dissolved oxygen tension and oxygen limitations in mass transfer. The analysis of the effects of DO levels can only be studied effectively in bioreactors with controlled DO levels [56]. 

Consider an idealized simple case of a Michaelis-Menten type bioreaction taking place in a vertical cylindrical packed-bed bioreactor containing immobilized enzyme particles. The effects of mass transfer within and outside the enzyme particles are assumed to be negligible. The reaction rate per dilfcrential packed height (m) and per unit horizontal cross-sectional area of the bed (m ) is given as (cf. Equation 3.28) ... 

Kilonzo, P.M. Margaritis, A. The effect of non-Newtonian fermentation broth viscosity and small bubble segregation on oxygen mass transfer in gas-lift bioreactors a critical review. Biochem. Eng. J. 2004, 17, 27-40. 

The rate of this reaction is independent of the concentration of sodium sulfite (within the range of 0.04 to 1 N) and is also much faster than the oxygen transfer rate. Therefore, the rate of oxidation is controlled by the rate of mass transfer alone. The sodium sulfite solution, however, has quite different physical and chemical properties. This technique is helpful though in comparing the performance of bioreactors and studying the effect of scale-up. 

In this study, two types of bioreactors were tested for H2 production by purple nonsulliir bacterium using a shift reaction. The main idea was to improve mass transport of gaseous CO into an aqueous bacterial suspension. A simple method of using hoUow fiber membrane technology to enhance mass transfer of CO has proven effective, but is likely too expensive for commercial applications at the present time. Different types of membrane-based... 

When the substrate in a bioreactor is a water-soluble material (e.g. cellulose, oils), the effects of intraparticle mass transfer may be important. In such systems, extracellular enzymes can break down substrate eventually into water soluble components, as products, or as intermediates for consumption by micro-organisms (Okazaki and Moo-Young, 1978). 

Mass transfer limitations can be relevant in heterogeneous biocatalysis. If the enzyme is immobilized in the surface or inside a solid matrix, external (EDR) or internal (IDR) diffusional restrictions may be significant and have to be considered for proper bioreactor design. As shown in Fig. 3.1, this effect can be conveniently incorporated into the model that describes enzyme reactor operation in terms of the effectiveness factor, defined as the ratio between the effective (or observed) and inherent (in the absence of diffusional restrictions) reaction rates. Expressions for the effectiveness factor (rj), in the case of EDR, and the global effectiveness factor (t ) for different particle geometries, in the case of IDR, were developed in sections 4.4.1 and 4.4.2 (see Eqs. 4.39-4.42,4.53,4.54,4.71 and 4.72). Such functions can be generically written as ... 

In the following sections, a few examples of the effect of antifoam agents on the properties of cultivation mediiun and foam are considered. Most of the authors evaluated the effect of antifoam agents on the voliunetric mass transfer coefficients in bioreactors. 

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