Effect of catalyst loading on gasliquid mass transfer in a slurry reactor: A statistical experimental approach

A statistical experimental design was employed to study the effects of pressure, temperature, catalyst loading, and mixing speed on the solubilities (C*) and volumetric gas/liquid mass transfer coefficients (k_L^a) for H₂, N₂, CO, CH₄ and C₂H₄ in a liquid mixture of hexanes containing iron oxide catalyst in a 4-litre agitated autoclave. Statistical correlations for k_L^a values for the gases used were developed. Mixing speed and solid concentration showed the strongest effects on k_L^a. At low catalyst concentrations, a maximum in k_L^a was observed and at concentrations > 37 mass%, k_L^a decreased by more than one order of magnitude.     

Gas-liquid mass transfer in two and three-phase upflows through a vertical tube

Oxygen absorption rates were measured to determine volumetric coefficients of gas-liquid mass transfer coefficients k_La in gas-liquid and gas-liquid-solid upward flows through a vertical tube. The liquid was deionized water or aqueous glycerol solution, and the solids were glass beads or polystyrene beads. The dependencies of k_La on gas velocity, liquid velocity, temperature, solid material, and solid concentration were examined. The experimental results were correlated with empirical equations. The mechanisms of the solid loading effect are discussed.     

Mass Transfer Characteristics of Syngas Components in Slurry System at Industrial Conditions

The gas‐liquid mass transfer behavior of syngas components, H₂ and CO, has been studied in a three‐phase bubble column reactor at industrial conditions. The influences of the main operating conditions, such as temperature, pressure, superficial gas velocity and solid concentration, have been studied systematically. The volumetric liquid‐side mass transfer coefficient k_La is obtained by measuring the dissolution rate of H₂ and CO. The gas holdup and the bubble size distribution in the reactor are measured by an optical fiber technique, the specific gas‐liquid interfacial area aand the liquid‐side mass transfer coefficient k_L are calculated based on the experimental measurements. Empirical correlations are proposed to predict k_L and a values for H₂ and CO in liquid paraffin/solid particles slurry bubble column reactors.     

A contribution to the measurement of oxygen mass transfer in a laboratory pressure reactor

A method was used to measure the liquid‐side volumetric coefficient of oxygen mass transfer (k_La) in closed, semi‐batch pressure reactors used in hydrometallurgical laboratories. In this method, the oxygen pressure was monitored as oxygen was continuously sparged into a pressure vessel containing a sodium sulfite solution. A material balance equation was derived for oxygen in the vessel and the experimental data were fitted to this equation. From the constant parameters of the equation, k_La was calculated. The solution in the vessel also contained an appropriate amount of cobalt catalyst so that oxygen was consumed rapidly by oxidation of sulfite to sulfate. Under these conditions, the oxygen concentration in the bulk liquid phase could be assumed to be equal to zero. Values of k_La determined by the method under various conditions were reproduced within 12% deviation from the average values. k_La was found to increase moderately with temperature in the range of 25 to 75 °C, with an activation energy of 33.09 ± 1.33 kJ mol⁻¹. The presence of hydrophobic or hydrophilic solids was found to have a deleterious effect on k_La. © 2000 Society of Chemical Industry     

Effect of water on the solubilities and mass transfer coefficients of gases in a heavy fraction of fischer-tropsch products

The effects of water on the solubilities, C^*, and volumetric mass transfer coefficients, k_La, for CO, H₂, CH₄ and CO₂ in a heavy fraction of Fischer-Tropsch liquid were examined at elevated pressures and temperatures at different mixing power inputs. For these gases, higher solubilities were measured in the hydrocarbon mixture saturated with water than those obtained in the hydrocarbon free of water. The k_La values for the four gases were slightly affected by the presence of dissolved water in the hydrocarbon mixture; and they were strongly dependent on the power input per unit liquid volume. Two empirical correlations for k_La as a function of turbine speed and pressure are proposed.     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

Volumetric mass transfer coefficients in an internal loop airlift reactor with low‐density particles

The influence of organic additives (propanol, benzoic acid, isoamyl alcohol and carboxymethylcellulose) on the volumetric mass transfer coefficient, k_La, in an internal loop airlift reactor with low‐density particles (nylon‐6 and polystyrene) was investigated. The k_La values increased with increase in superficial gas velocity, U_sg, and decreased with increase in solid loading. A draft tube to reactor diameter ratio, D_R/D, of 0.4 gave maximum k_La values. The addition of benzoic acid and propanol increased the k_La values owing to their coalescence inhibiting characteristics. The addition of isoamyl alcohol decreased k_La, owing to the formation of rigid bubbles and recirculation of small bubbles having a low oxygen content. The k_La values decreased with increase in the concentration of the non‐Newtonian fluid carboxymethylcellulose (CMC). The proposed correlations predicted the experimental data well. Copyright © 2006 Society of Chemical Industry     

Instantaneous Mass Transfer under Gas‐Liquid Taylor Flow in Circular Capillaries

The mass transfer process under CO₂‐water Taylor flow was experimentally investigated in circular capillaries with different lengths. The measured volumetric mass transfer coefficient k_La was found to reduce with the increase of mass transfer time under the same operational conditions. With computational fluid dynamics simulations, the instantaneous k_La values decreased sharply at the initial stage of the mass transfer process. The effects of numerous experimental parameters on separated contribution of k_La were investigated for different dimensionless mass transfer times. The instantaneous k_La values for arbitary transfer times could be calculated and agreed well with experimental data.     

Gas Holdup and Volumetric Mass Transfer Coefficient in a Slurry Bubble Column

The gas holdup, ?, and volumetric mass transfer coefficient, k_La, were measured in a 0.051 m diameter glass column with ethanol as the liquid phase and cobalt catalyst as the solid phase in concentrations of 1.0 and 3.8 vol.‐%. The superficial gas velocity U was varied in the range from 0 to 0.11 m/s, spanning both the homogeneous and heterogeneous flow regimes. Experimental results show that increasing catalyst concentration decreases the gas holdup to a significant extent. The volumetric mass transfer coefficient, k_La, closely follows the trend in gas holdup. Above a superficial gas velocity of 0.04 m/s the value of k_La/? was found to be practically independent of slurry concentration and the gas velocity U; the value of this parameter is found to be about 0.45 s^–1. Our studies provide a simple method for the estimation of k_La in industrial‐size bubble column slurry reactors.     

Gas hold-up and oxygen transfer in three-phase external-loop airlift bioreactors: Non-Newtonian fermentation broths

The effects of solids loading on gas hold-up and oxygen transfer in external-loop airlift bioreactors with non-Newtonian fermentation media are discussed. Experiments were performed in two model external-loop airlift bioreactors with aqueous solutions of carboxymethyl cellulose (CMC) and xanthan gum representing non-Newtonian flows. Low-density plastic particles of 1030 and 1300 kg m⁻³ were used and the solids loading was varied in the range 0–20% (v/v). For the inelastic non-Newtonian CMC aqueous solutions, the presence of low-density solid particles slightly increased the riser gas hold-up, ϕ_gr, but decreased the volumetric mass transfer coefficient, k_La. On the other hand, ϕ_gr decreased but k_La increased with solids loading in the viscoelastic non-Newtonian xanthan gum aqueous solution. The extent of these effects depended on non-Newtonian flow behavior. Theoretical models of riser gas hold-up and volumetric mass transfer coefficient have been developed. The capability of the proposed models was examined using the present experimental data obtained in the model external-loop airlift bioreactors and the available data in the literature. The data were successfully correlated by the proposed correlations except the results for k_La coefficient in the xanthan gum solution.     

Interfacial Area and Liquid‐Side Volumetric Mass Transfer Coefficient in a Downflow Bubble Column

An experimental investigation was made to measure interfacial area, a, and liquid‐side volumetric mass transfer coefficient, k_La, in a downflow bubble column by chemical methods viz., absorbing CO₂ in aqueous sodium hydroxide and sodium carbonate/bicarbonate buffer solution respectively. The effect of gas and liquid flowrate and nozzle sizes on a and k_La were investigated. The experimental data obtained in the present system were analyzed and correlations were developed to predict a and k_La in terms of superficial gas velocity. The variation of a and k_La with specific power input were shown in graphical plot and compared with other gas‐liquid systems.     

Gas‐Liquid Mass Transfer in Pulp Retention Towers

The volumetric gas‐liquid mass transfer rate, k_La, was measured under batch conditions in a 0.28 m diameter laboratory‐scale retention column. Tests on water, and on unbleached kraft (UBK) pulp suspensions (mass fractions, C_m from 0.013 to 0.09) were made with air or nitrogen sparged through the column at superficial gas velocities between 0.0015 to 0.05 m/s. k_La varied with suspension mass concentration and superficial gas velocity, initially decreasing with increasing mass concentration, reaching a minimum between C_m = 0.03 and 0.06, and then increasing. The minimum in k_La coincided with a change in hydrodynamics within the column, from bubble column behaviour below C_m = 0.03 to porous solid behaviour above C_m = 0.06.     

Influence of recycling on mass transfer and reaction in a G-L jet loop reactor with variable interfacial area

The effect of recycling on mass transfer, characterized by the volumetric mass transfer coefficient k_La, was investigated by absorbing CO₂ into water in a laboratory jet loop reactor. Based on a mathematical model, which includes the correlation function k_La = k_La(r_L), a first order chemical reaction and the variation of interfacial area, a = a(ε_G), caused by strong absorption effects, the interaction ‘recycling – mass transfer – reaction’ was simulated and its influence on the reactor performance is discussed.     

Mass transfer properties of bubble columns suspending immobilized glucose oxidase gel beads for gluconic acid production

The volumetric gas-liquid oxygen transfer coefficient, k_L a, and the liquid–solid coefficient, k_S, were measured in a 6.7 L external loop airlift bubble column (ELBC), a 2.5 L internal loop airlift (ILBC) and a 2.5 L normal bubble column (NBC) by the steady state method proposed previously using the oxidation of glucose with air catalyzed by glucose oxidase, GO. For an improved and simultaneous determination of k_L a and k_S, GO was entrapped in calcium alginate gel beads together with fine palladium particles instead of catalase to decompose the hydrogen peroxide produced. The gas holdup, ?_G, in each type of bubble column and the liquid circulation velocity, u_L, governing ?_G in the ELBC were also measured to correlate the data on k_La according to the previous correlations proposed for a larger scale of the ELBC, ILBC and NBC. The data on k_L a, k_S, ?_G and u_L (only for the ELBC) in the reaction system were compared to each other for the three types of bubble columns. The results are well predicted by the previous correlations.     

Mass transfer characteristics in a gas-liquid reciprocating plate column. I. Liquid phase volumetric mass transfer coefficient

In a series of two papers the results of investigating the mass transfer characteristics of two gas-liquid reciprocating plate columns of the Karr type by different methods are presented. The subject of the first part is a study of the liquid phase volumetric mass transfer coefficient, k_La, while the second part deals with the interfacial area. The volumetric coefficient k_La was investigated using the sulphite method, the pure physical absorption of oxygen, and a dynamic method under culture conditions, the second of these three methods being the most favorable. Very good agreement among these methods was found. Generally, k_La increased with increasing vibration intensity, superficial gas velocity, and the number of perforated plates. Liquid-phase properties appeared to affect k_La only slightly. The coefficient k_La was correlated in terms of the maximum power consumption and the superficial gas velocity:      

A computational method for determination of the mass-transfer coefficient in packed-bed immobilized enzyme reactors

A computational method was developed that determined the mass-transfer coefficient k_L or the volumetric mass-transfer coefficient k_La in packed-bed immobilized enzyme (IME) reactors. To study the performance of this method, two experimental systems were considered where an enzyme was immobilized on a non-porous support surface (surface-IME system) or within a porous support (pore-IME system). The values of k_L and k_La determined in these packed-bed IME reactor systems were successfully expressed in terms of the substrate concentration at the reactor inlet and the liquid flow rate. Furthermore, the correlations obtained for k_L and k_La were used to calculate the unconverted fractions of substrate at the reactor outlet. Comparison showed that the calculated results were in satisfactory agreement with the experimental values.     

Oxygen Mass Transfer Rate in a Pressurized Lab‐Scale Stirred Bioreactor

Oxygen mass transfer from air to the liquid phase in bioreactors with aerobic cultures has long been a serious impairment to the productivity of various bioprocesses. An increase of the oxygen mass transfer rate (OTR) can be the key to overcome oxygen limitation. The influence of higher air pressure on OTR was measured and a significantly enhanced OTR could be obtained. The oxygen volumetric mass transfer coefficient (k_La) was described by a function of the air pressure in a stirred lab‐scale pressurized bioreactor. The correlation obtained proved that k_La slightly decreased with higher air pressure, following a power function.     

Revisiting Basics of kLa Dependency on Aeration in Bubble Columns: a Is Surprisingly Stable

A comprehensive experimental characterization of a small-scale bubble column bioreactor (60 mL) is presented. Bubble size distribution (BSD), gas holdup, and k_La were determined for different types of liquids, relevant fermentation conditions and superficial gas velocities u_G. The specific interfacial area a and liquid mass transfer coefficient k_L have been identified independent of each other to unravel their individual impact on k_La. Results show that increasing u_G leads to larger bubbles and higher gas holdup. As both parameters influence a in opposite ways, no increase of a with u_G is found. Furthermore, k_L increases with increasing bubble size outlining that improved oxygen transfer is not the result of higher a but of risen k_L instead. The results build the foundation for further simulative investigations.     

Gas-liquid mass transfer in “dead-end” autoclave reactors

Gas-liquid volumetric mass transfer coefficients, (k_La), have been obtained for “dead-end” autoclave reactors operated in two different modes: (a) gas introduced into the gas phase, and (b) gas introduced through a dip-tube in the liquid. Three different methods of k_La determination have been compared. Effects of agitation speed, impeller diameter, gas to liquid volume ratio (V_g/V_L), position of the impeller and reactor size on k_La have been investigated. The k_La data were found to be correlated as: k_La = 1.48 × 10^?3 (N)^2.18 (V_g/V_L)^1.88 (d_I/d_T)^2.16 (h₁/h₂)^1.16 The critical speed of surface breakage, at which transition from the surface convection to the surface entrainment regime occurs, was also determined for different impeller positions, impeller diameters and gas to liquid volume ratios.     

Mass transfer characteristics of horizontal agitated contactors

The values of effective interfacial area, a, and liquid side mass transfer coefficient, k_La, were obtained in 0.385, 0.57 and 1.0 m i.d. horizontal agitated contactors by using chemical methods. Propeller type of impellers were employed for agitation. Two modes of operation were studied: (i) surface aeration and (ii) sparging of gas. The impeller speed was varied from 50 to 700 rev/min, and the nozzle gas velocity was varied from 40 to 160 m/s. The effects of impeller spacing, liquid submergence and nozzle gas velocity on the values of a and k_La were investigated. A scale-up criterion is discussed.