Jyeshtharaj B. Joshi

Computational flow modelling and design of bubble column reactors

Abstract The present design practice of the bubble column reactors is still closer to an art than science because of the complexity of the fluid mechanics. In view of this, there have been continuous attempts to understand the complex three-dimensional turbulent two-phase flow. The present paper reviews the modelling efforts on the flow patterns published in the last 30 years with relatively more focus on the last 10 years. Over this period, there have been sustained efforts to improve our understanding of the governing equations of the change (equations of continuity and motion) for two-phase flows. Both Eulerian and Lagrangian approaches have been extensively used. The development has been mainly on three fronts: (i) formulation of interfacial forces (ii) closure problem for the eddy viscosity and (iii) modelling of the correlations arising out of Reynolds averaging procedure. As regards to interface force terms, the published literature has been critically analysed. The present status of our understanding of the drag force, virtual mass force and lift force has been presented. The physical significance of the various formulations has been brought out. The mechanism of the energy transfer from gas to liquid phase has been explained. The developments in closure problem have been most dramatic. The progress of the past 30 years has been reviewed with a focus on the past 10 years. The published literature has been critically analysed and chronology of development has been presented. The effort has been concentrated on cylindrical bubble columns where results on flow pattern could be extended to the design. The studies on transient flow pattern in two-dimensional columns have not been covered because the subject is still under development and the results cannot be extended to the design objective. The closure problem is intimately linked with the physics of turbulence. An attempt has been made to develop a complete correspondence between an operation of real column and the model simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. The effects of the superficial gas velocity, column diameter and bubble slip velocity on the flow pattern have been examined. Extensive comparison has been presented between the predicted and the experimental velocity profiles. For the design of the bubble columns, the knowledge of various design parameters (such as pressure drop, rate of mixing, residence time distribution of both the phases, heat and mass transfer coefficients) is needed. For the estimation of these parameters, the prevailing procedures are largely empirical. The fundamental basis for the estimations is possible through the understanding of the detailed macro- and micro-flow patterns. This basic direction has been the subject of several publications, particularly during the last 5 years. All these studies have been critically analysed in the present review paper. A coherent and holistic approach has been presented on the modelling of fluid mechanics and design of bubble column reactors. Recommendations have been made for the future research in this area.

Mechanically agitated gas-liquid reactors

The hydrodynamic, heat and mass transfer characteristics of mechanically agitated contactors have been critically reviewed. The mixing time (θmix) can be correlated well by a model based on circulation path and average circulation velocity. Flow patterns in mechanically agitated contactors (single phase) provided with disk turbine, pitched blade turbine and propeller can be characterised. The minimum impeller speed required for the gas induction (Ncr) in hollow shaft impellers can be predicted; the minimum impeller speed required for surface aeration (Ns) can also be predicted. Different strategies of operating mechanically agitated contactors have been examined. The advantages of multistage contactors over single contactors have been stressed. Recommendations have been made for correlations which can be used for design purpose; lacunae in the available literature have been delineated and recommendations for further work have been made.

Kinetics of wet air oxidation of phenol and substituted phenols

Abstract Wet air oxidation (WAO) of aqueous solutions of phenol and substituted phenols namely, o -, m - and p -chlorophenols, o -, m -cresols, o - and p -methoxyphenols, o -ethylphenol and 2,6-dimethylphenol, were carried out. The process was studied in a 1 litre stainless steel autoclave at temperatures in the range of 150–180°C. The oxygen partial pressure was varied from 0.3 to 1.5 MPa and the initial phenol concentration was 200 mg/l. The oxidation of phenols in water involves a free-radical mechanism and proceeds in two steps. The oxidation reaction was found to be first order in oxygen and also first order with respect to phenolic substrates in both steps. The values of activation energy were found to be in the range of 12.4 x 10 3 -201 x 10 3 kJ/kmol. The conditions have been found under which the overall oxidation reaction becomes reaction controlled or mass transfer controlled. The values of mass transfer coefficient have been obtained. The data based on bench scale shows wet air oxidation of phenols can achieve destruction efficiencies exceeding 99.9%. The reduction of COD during oxidation of all phenols was also measured. Greater than 90% COD reduction was achieved. Some aspects of the process design of the oxidation reactor have been discussed.

Mixing in Mechanically Agitated Gas-Liquid Contactors, Bubble Columns and Modified Bubble Columns

Abstract Mixing time measurements were made in 300 and 1000 mm i.d. mechanically agitated contactors with different types of impellers, located at different heights from the bottom of the vessel. Mixing time measurements were also made in 150, 200, 385 and 1000 mm i.d. bubble columns with varying liquid heights. Transient pH measurement and conductivity measurement were used to measure the mixing times. Impeller speed was varied in the range of 3.33–20 r/sec in the case of mechanically agitated contactors and gas superficial velocity was varied in the range of 10–250 mm/sec in bubble columns. Effect of physical properties of the fluid (surface tension, ionic strength, liquid viscosity) and that of the non-Newtonian behavior on mixing time was studied. Mixing time in the presence of drag reducing agents was also investigated. In the range of variables covered in this work mixing time in mechanically agitated contactors and bubble columns was found to be in the range of 4–6 Mixing time predictions based on the longest loop length and circulation velocity are made in the presence and absence of a gas for mechanically agitat A procedure is given for the prediction of the critical impeller speed for gas phase dispersion in mechanically agitated contactors.

Lipase-Catalyzed Esterification

Lipases are versatile catalysts. In addition to their natural reaction of fat hydrolysis, lipases catalyze a plethora of other reactions such as esterification, amidation, and transesterification of esters as well as organic carbonates. Moreover, lipases accept a wide variety of substrates while maintaining their regioselectivity and stereoselectivity. Lipases are highly stable even under adverse conditions such as organic solvents, high temperatures, and so forth. Applications of lipases include production of food additives, chiral intermediates, and pharmaceutical products. Among these, synthesis of various chiral intermediates in pharmaceutical industry and cocoa butter substitutes is being commercially exploited currently. Lipase-catalyzed esterification and transesterification in anhydrous media (e.g., organic solvents and supercritical fluids) has been an area of major research activity in the past decade or so. Absence of water eliminates the competing hydrolysis reaction. Moreover, substrate specificity, regioselectivity, and stereoselectivity of the enzyme can be controlled by varying the reaction medium. Although organic solvents, which are generally used for lipase-catalyzed reactions, are nearly anhydrous; they contain water in trace quantities. This water content can be controlled over a range and has a profound effect on the activity of lipases. Water not only affects the enzyme but also acts as a competing nucleophile. Enzyme activity has been correlated with thermodynamic activity of water in the medium rather than with the concentration of water. Because lipases are not soluble in most organic solvents, the method of preparation of the enzyme has a strong influence on the enzymatic activity. The major factors are the pH of the aqueous solution in which the enzyme last existed, additives used during preparation, and method of removal of water (e.g., freeze-drying, evaporation, extraction of enzyme into solvent, etc.). Immobilization of lipases allows easy recovery and reuse of the enzyme. Various immobilization techniques have been studied for lipases and some of them have been shown to enhance the activity and stability of the enzyme. Enzyme stability is an important parameter determining the commercial feasibility of the enzymatic process. Various factors, such as temperature, reaction medium, water concentration, as well as the method of preparation, affect the stability of the lipases. This review deals with fundamental as well as practical aspects of lipase catalysis. A discussion has been presented on various factors affecting lipase activity and stability. Moreover, a brief account of current and potential applications of lipases has been given.

Role of hydrodynamic shear in the cultivation of animal, plant and microbial cells

The rapid developments in biotechnology have resulted in the identification and use of a large variety of biologically active substances produced from microbial, plant and animal origin. These range from enzymes and antibiotics to highly complex molecules such as immunoglobulins, growth factors and hormones. The advances in bioprocess technology have enabled the cultivation of different micro-organisms, namely bacteria, yeast and fungi, on a large scale. The potential use of plant and animal cells has, however, not yet been completely realized. One of the major limitations in the scale-up of plant and animal cell culture is the shear sensitivity of these cells. This arises owing to the large size of the cells. In addition, animal cells are especially fragile because of the lack of cell walls.

Coherent flow structures in bubble column reactors

Abstract This paper reviews the flow patterns in bubble columns with a focus on transient flow structures. The subject of mean flow pattern has not been covered in view of the earlier publication (Chemical Engineering Science 56 (2001) 5893–5933), which also deals with the subject of the formulation of governing equations and the description of interface force terms for gas–liquid dispersions. The published literature in the last ten years has been analyzed on a coherent basis and the present status has been brought out for (i) Euler–Euler versus Euler–Lagrange approach, (ii) closure formulation for eddy diffusivity in gas–liquid dispersions, (iii) numerical issues such as grid size, discretization scheme and the solution algorithms and (iv) two-dimensional versus three-dimensional formulation. The following geometries of bubble columns have been investigated in the past: (i) two-dimensional column with off-center sparging, (ii) two-dimensional column with central sparging, (iii) two-dimensional column with uniform sparging and (iv) cylindrical columns with uniform sparging. All these cases have been covered in the present paper. A critical account of the predictive capability of all the models for transient flow structures has been presented. This paper also includes the application of multiresolution analysis of velocity–time series for the identification of coherent flow structures.

Fluid mechanics and blending in agitated tanks

A flow and mixing model can provide a sound fundamental basis for the quantitative and optimum design of impeller and tank geometries. This paper describes the results of detailed numerical simulations using such a model for the flow generated by a downflow-pitched blade turbine in a fully baffled cylindrical vessel and the subsequent bulk mixing. Comparisons of predicted flow characteristics with experimental data (measured in a 0.3 m i.d. vessel using a laser Doppler anemometer) show good agreement. The results of this flow model were then used to simulate the mixing of an inert tracer (introduced in pulse mode) in agitated tanks. Various mixing time definitions were studied and compared with published correlations. Some numerical experiments were performed to generate guidelines for the development of new impellers.

FLOW GENERATED BY PITCHED BLADE TURBINES II: SIMULATION USING κ-ε MODEL

Experimental data on average velocity and turbulence intensity generated by pitched blade downflow turbines (PTD) were presented in Part I of this paper. Part II presents the results of the simulation of flow generated by PTD The standard κ-e model along with the boundary conditions developed in the Part 1 have been employed to predict the flow generated by PTD in cylindrical baffled vessel. This part describes the new software FIAT (Flow In Agitated Tanks) for the prediction of three dimensional flow in stirred tanks. The basis of this software has been described adequately. The influence of grid size, impeller boundary conditions and values of model parameters on the predicted flow have been analysed. The model predictions successfully reproduce the three dimensionality and the other essential characteristics of the flow. The model can be used to improve the overall understanding about the relative distribution of turbulence by PTD in the agitated tank

Microbial cell disruption: role of cavitation

Abstract A novel technique of using hydrodynamic cavitation for the large-scale disruption of yeast cells is described. Bakers yeast and brewers yeast cells in a pressed yeast form were used. Cell disruption was monitored in the form of increase in soluble protein content. Disruption by hydrodynamic cavitation is compared with that obtained by established techniques such as blade blender and acoustic cavitation (ultrasonication). The effect of cell concentration, time of treatment and number of passes in the flow loop system on the extent of cell disruption is reported. The energy efficiency of the hydrodynamic cavitation setup is compared with that of established techniques. Hydrodynamic cavitation was found to be at least an order of magnitude more energy efficient than established techniques such as ultrasonication or blade blender (mixer).