Mass Transfer in a closed stirred gas/liquid contactor: Part 1: The mass transfer rate kLS

Liquid phase mass transfer rates kLS for the absorption of oxygen in tap water and in aqueous ionic solutions have been determined in two closed stirred tank contactors for a power input between 3 and 70 W/kg and (impeller diameter)f(tank diameter) ratios DifT of 0.3, 0.35 and 0.4. The contactors had diameters\ud of 0. I9 and 0.6 m. The dispersed phase fraction was 1% by volume in all experiments. The values of kLS were determined using two different techniques:\ud (1) a transient physical absorption method; and (2) the sulphite method

Gas-liquid mass transfer : influence of sparger location

The performance of three sparger diameters (DS = 0.6D, DS = D, DS = 1.6D) in combination with three positions (below, above or level with the impeller) for gas-liquid dispersion and mass transfer were evaluated in the case of the Rushton turbine and the A315 propeller in up- or down-pumping mode. The results show that the best results in terms of gas handling and mass transfer capacities are obtained for all impellers with the sparger placed below it and with a diameter at least equal to the impeller diameter. For the sparger position below the agitator, the kLa values of the Rushton turbine are greater than those of the A315 propeller, whatever the pumping mode. The A315 propeller in up-pumping mode is, however, more economically efficient in terms of mass transfer. In all cases, the up-pumping mode gives better results than the down-pumping one

Effect of radial impeller size in the presence and absence of baffles on the copper exchange on zeolite NaX

Effect of hydrodynamics on ion exchange in a batch reactor is still not appropriately studied even though proper mixing parameters may considerably affect the process of solid suspension and its costs. In this work, hydrodynamic conditions generated by straight blade turbine (SBT) impellers on suspension in the batch reactor with and without baffles were investigated. The aim of this work was to analyze influence of impeller diameter and zeolite mass on just suspended impeller speed, (NJS) power consumption, maximum amount of copper exchanged onto zeolite NaX and copper exchange kinetics as well. All experiments were conducted at the same temperature, initial concentration of the copper solution and zeolite particle size. The obtained results showed that just suspended impeller speed decreases as impeller diameter increases in the reactor with and without baffles but this trend is considerably more pronounced in the reactor with baffles. The increase in zeolite mass causes a slight increase of NJS in the both reactor. In the reactor with the baffles this increment became noticeably higher as impeller diameter decrease. Power consumption, at the state of complete zeolite suspension, decreases as impeller diameter increases and its values in the reactor without baffles are considerably lower as well. Kinetics results indicated that the amount of copper ion increases significantly in the initial stage and then gradually until the equilibrium is reached for all hydrodynamics conditions and mass of zeolite examined

Effect of mixing on enzymatic liquefaction of sago starch

The effect of mixing as a function of agitation speed and impeller diameter on the rate and degree of enzymatic liquefaction of sago starch was carried out using a stirred tank reactor with a single Rushton turbine impeller. The performance of the reactor as a mixing device was first examined using different concentrations of carboxymethylcellulose, which exhibited pseudoplastic behaviour similar to that of the solution during the sago starch liquefaction process. A correlation between mixing time (tm) and Reynolds number (Re) in the form of tm = bRec is presented; the constants for the correlation depended on viscosity of the fluid. For the two ratios of impeller diameter (Di) to tank diameter (Dt) used, 0.407 and 0.542, agitation speed gave significant influence on both overall rate and degree of liquefaction of sago starch. Mixing time (tm) was independent of impeller diameter used, and correlated well with the overall rate of liquefaction (P) (calculated as the reducing sugar produced divided by time of liquefaction) and expressed as P = 1.95tm-0.362

Mass Transfer in a closed stirred gas/liquid contactor: Part 2: The liquid phase mass transfer coefficient kL

The liquid phase mass transfer coefficient kL for the absorption of oxygen in tap water and in ionic solutions has been calculated from the quotien\ud \ud It is concluded that the liquid phase mass transfer coefficient is roughly proportional to the stirrer speed. The gas fraction e apparently has littl

CFD analysis of industrial multi-staged stirred vessels

This paper presents tools for analysis of CFD results adapted for flows in multi-stage stirred vessels through out two industrial cases. Those tanks fitted with double-flow impellers are used first to cool down highly viscous resins and subsequently for indirect emulsification. Since the simulation of these processes in their whole complexity would be unrealistic, it considers single-phase flows without heat transfer. The result analysis in order to prove that the mixing and the circulation are effective is not usual; in these cases, the circulation and impeller numbers are not adapted. The average axial flow numbers are relevant of the circulation in the whole tank and of the connection between the flows produced by the propellers in the given configuration. The velocity profiles give relevant results, but are not sufficient whereas the particle tracking validates that the propellers do not work together in one case and do work together in a second one

A novel reactor for determination of kinetics for solid catalyzed gas reactions

A novel perfectly mixed laboratory reactor for determining kinetics of heterogeneously catalyzed gas-phase reactions has been developed. Perfect mixing is achieved by circulating the gas in the reactor using an axial flow impeller in a well streamlined enclosure. Pellets are fixed in a rectangular opening in the blades of the impeller. They rotate with the impeller, thus realizing high particle velocities in the reactor. Interparticle mass transfer was studied experimentally by vaporization of naphthalene pellets. The mass-transfer coefficient in the novel reactor was found to depend on the velocity of a particle in the reactor. Mass-transfer coefficients in an internal recycle reactor at equal impeller tip speeds are 4-6 times lower than those in the novel reactor, and conditions can be chosen easily where at higher rotational speeds the mass- and heat-transfer rates are 8-10 times higher than in classical recycle reactors. The recycle flow rate in a recycle reactor was found to depend strongly on the resistance to flow caused by the catalyst bed itself. The novel reactor was tested under reacting conditions using the hydrogenation of ethene

Gas-liquid mass transfer : a comparison of down-and up-pumping axial flow impellers with radial impellers

The performance of a down- and up-pumping pitched blade turbine and A315 for gas-liquid dispersion and mass transfer was evaluated and then compared with that of Rushton and Scaba turbines in a small laboratory scale vessel. The results show that when the axial flow impellers are operated in the up-pumping mode, the overall performance is largely improved compared with the down-pumping configuration. Compared with the radial turbines, the up-pumping A315 has a high gas handling capacity, equivalent to the Scaba turbine and is economically much more efficient in terms of mass transfer than both turbines. On the other hand, the uppumping pitched blade turbine is not as well adapted to such applications. Finally, the axial flow impellers in the down-pumping mode have the lowest performance of all the impellers studied, although the A315 is preferred of the pitched blade turbine

Numerical Investigation of Mixing Time in a Torus Reactor

In this work, the mixing performance in a batch torus reactor was investigated using computational fluid dynamics (CFD). To validate the numerical model, the CFD results for the mean bulk velocity and power number were compared with the experimental data reported in the literature. Next, by solving numerical dispersion of a passive tracer in unsteady state, the effects of important parameters such as the impeller type, impeller diameter, impeller blade angle and impeller rotation speed on the mixing time were explored. The analysis of numerical results allows us to suggest simple correlations for the prediction of the mixing time as a function of the total specific power consumption P/V, impeller diameter to reactor diameter ratio D/d and impeller blade angle sinφ

Numerical studies of the velocity distribution within the volute of a centrifugal pump

Centrifugal pumps play an essential role in engineering systems since they are widely used in the process and power industries. The performance of a centrifugal pump needs to be maximised due to its importance and this depends on the flow structure within the pump. The flow structure within a pump is very complex due to the presence of a rotating impeller and its interaction with the volute casing. In this paper, a numerical investigation using CFD analysis has been carried out to determine the effect of volute geometry on the flow field within a centrifugal pump. The results obtained from the numerical investigation have been validated with the experimental data. Further analyses have been carried out to investigate the effect of volute cross-sectional area on the velocity distribution. The overall results indicate that the head increases as the volute cross-sectional area increases