M. Moo-Young

Liquid circulation in airlift reactors

Energy balance over an airlift loop is used to obtain a theoretical equation [eq. (16)] for the prediction of liquid circulation velocity in those devices. The resulting equation is shown to satisfactorily describe most of the available liquid circulation data (13 different airlift reactors) for a large range of reactor operating scales (0.06–1.06 m3 liquid volumes) including measurements on two pilot scale vessels. The equation applies to both external- and internal-loop types of airlifts over almost two decades of liquid circulation velocities (0.027–1.05 m s−1), reactor height range of 1.36–8.5 m, and riser-to-downcomer cross-sectional area ratios in the range 0.5–9.1. The liquid circulation rate is predicted to increase with the square root of the reactor height. The influence of wall-drag on liquid circulation is shown to be negligible relative to the impact of the frictional losses in the top and bottom connecting sections between the riser and the downcomer where abrupt changes of flow direction take place. Energy dissipation due to wakes behind rising bubbles is most significant.

AIRLIFT REACTORS: CHARACTERISTICS, APPLICATIONS AND DESIGN CONSIDERATIONS

Bioreactors of the airlift type are a promising design for aerobic fermentations. The basic knowledge required for understanding and predicting the performance of these reactors is only now beginning to emerge. In this review we present our observations and those of other investigators in an attempt to build up a coherent picture of airlift devices. All the major aspects—mixing and hydrodynamics, mass and heat transfer—in these reactors are considered. Comparisons between bubble columns and airlift systems are made where analogies, similarities and/or differences between them provide insight into airlift systems. Throughout, the areas of particular concern and those in need of further research in this field are mentioned. Extensive work on all forms of airlift reactors, particularly in non-Newtonian media—homogeneous and suspensions—remains to be done. Current knowledge does not permit airlift reactor design with a high degree of confidence. However, the technical feasibility of all types of fermentations...

Metabolic pathways of clostridia for producing butanol

Worldwide demand for energy has been the impetus for research to produce alcohol biofuels from renewable resources. This review focuses on the biosynthesis of butanol, which is regarded to be superior to ethanol as a fuel. Although acetone/butanol fermentation is one of the oldest large-scale fermentation processes, butanol yield by anaerobic fermentation remains sub-optimal. Metabolic engineering provides a means for fermentation improvements. Consequently, a comprehensive assessment of the intermediary enzymes involved in butanol formation from carbohydrates by the saccharolytic bacterium, Clostridium acetobutylicum and other closely allied clostridia was performed to provide guidelines for potentially enhancing butanol productivity. The activity of the enzymes, their regulation and contribution to the metabolic pathways was reviewed. Published kinetic data for each important enzymatic reaction were assessed. For most enzymatic reactions, the systematic investigation of the kinetic data and the properties of the enzymes led to the development of rate equations that were able to describe activity as the function of the substrates, products, and allosteric effectors.

Theoretical prediction of volumetric mass transfer coefficients in bubble columns for Newtonian and non-Newtonian fluids

Abstract A theoretical model for volumetric mass transfer coefficients in bubble columns has been developed. It is based on Higbies penetration theory and Kolmogoroffs theory of isotropic turbulence. The capability of the proposed model is discussed in the light of experimental data obtained from a 40-L bubble column and a 1000-L pilot plant fermenter using Newtonian and non-Newtonian fluids, and available data in the literature. Good agreement with these data over a wide range of conditions suggests a general applicability of the proposed model.

Plasmid stability in recombinant Saccharomyces cerevisiae

Because of many advantages, the yeast Saccharomyces cerevisiae is increasingly being employed for expression of recombinant proteins. Usually, hybrid plasmids (shuttle vectors) are employed as carriers to introduce the foreign DNA into the yeast host. Unfortunately, the transformed host often suffers from some kind of instability, tending to lose or alter the foreign plasmid. Construction of stable plasmids, and maintenance of stable expression during extended culture, are some of the major challenges facing commercial production of recombinant proteins. This review examines the factors that affect plasmid stability at the gene, cell, and engineering levels. Strategies for overcoming plasmid loss, and the models for predicting plasmid instability, are discussed. The focus is on S. cerevisiae, but where relevant, examples from the better studied Escherichia coli system are discussed. Compared to free suspension culture, immobilization of cells is particularly effective in improving plasmid retention, hence, immobilized systems are examined in some detail. Immobilized cell systems combine high cell concentrations with enhanced productivity of the recombinant product, thereby offering a potentially attractive production method, particularly when nonselective media are used. Understanding of the stabilizing mechanisms is a prerequisite to any substantial commercial exploitation and improvement of immobilized cell systems.

INFLUENCE OF NON-NEWTONIAN FLOW BEHAVIOUR ON MASS TRANSFER INBUBBLE COLUMNS WITH AND WITHOUT DRAFT TUBES†

Gas hold-up and mass transfer were examined in a column with and without a draft tube. It was found that the introduction of a draft tube increases the gas hold-up but decreases the volumetric mass transfer coefficient in Newtonian fluid systems. For non-Newtonian fluid systems, both parameters were increased by the presence of the draft tube. Empirical correlations are proposed for the gas hold-up and the volumetric mass transfer coefficient in the bubble column with Newtonian and non-Newtonian fluid systems. The correlations are in general agreement with the data in this work and in the literature. They should be useful for design and scale-up purposes. It was also found that introduction of an ancillary impeller improves the mass transfer in non-Newtonian fluids due to the break-up of large bubbles.

Tissue-type plasminogen activator: Characteristics, applications and production technology

Plasminogen activators have immense clinical significance as thrombolytic agents for management of stroke and myocardial infarction. Tissue-type plasminogen activator (tPA) is generally preferred as being effective and safer than either urokinase or streptokinase type activators. Large-scale production of tPA became possible through groundbreaking developments in cell lines and bioprocess technology. Nevertheless, at thousands of dollars per treatment, tPA remains expensive. Enhancing cellular productivity and downstream product recovery through new approaches continue to be major challenges as discussed in this review. Recent clinical experience suggests the need for yet better fibrinolytic agents and attempts are underway to modify the tPA molecule to second generation products. Emerging trends in this field are outlined.

Prediction of the volumetric mass transfer coefficient in pneumatic contactors

Semi-theoretical expressions are developed for the prediction of the volumetric mass transfer coefficient, (KLaD)T, in pneumatic contactors, using the correlation of Calderbank and Moo-Young Chem. Engng Sci.16, 39 (1961) for the mass transfer coefficient and the local isotropic turbulence theory for predicting bubble diameter. A direct proportionality of (KLaD)T to the gas hold-up is predicted, with an exponent of 1.2 on eG, while, in terms of physical parameters, (KLaD)T is predicted to be proportional to the 0.8 power of the superficial gas velocity for both bubble column and airlift contactors, and proportional to (1 + Ad/Ar)−2 for airlift contactors, where Ad/Ar is the downcomer-to-riser cross-sectional area ratio. Experimental results obtained in bubble column and airlift contactors (external-loop and concentric-tube) of pilot-plant scale (ca. 50L liquid capacity), with water and 0.15 kmol m−3 NaCl solution as liquid media, were used to test the proposed expressions.

Liquid phase mixing in bubble columns with Newtonian and non-Newtonian fluids

Abstract A hydrodynamic model for the liquid phase in bubble columns is developed. The proposed model for fully developed turbulent Newtonian and non-Newtonian fluids is based on an energy balance and the mixing length theory. The predictions of the model are in reasonably good agreement with data on liquid velocity at the column axis and the axial dispersion coefficient. The liquid velocity data in an inverted conical bottom gas—liquid column contactor have also been measured. They are correlated by the proposed model reasonably well.

The effect of antifoam agents on mass transfer in bioreactors

The influence of antifoam agents on the liquid-phase mass transfer coefficient in stirred tank and bubble column bioreactors is studied. A physical model based on a surface-renewal concept and additional data in 40-dm3 bubble column bioreactor are presented. Comparisons between the physical model and the data indicate that the model predicts the maximum influence of antifoam agents on the liquid-phase mass transfer coefficient.