Volumetric liquid side mass transfer coefficient

The Volumetric Liquid-Side Mass Transfer Coefficient at the Gas-Liquid Interface... 

Abstract—Gas-liquid interfacial areas a and volumetric liquid-side mass-transfer coefficients kLa are experimentally determined in a high pressure trickle-bed reactor up to 3.2 MPa. Fast and slow absorption of carbon dioxide in aqueous and organic diethanolamine solutions are employed as model reactions for the evaluation of a and kLa at high pressure, and various liquid viscosities and packing characteristics. A simple model to estimate a and kLa for the low interaction regime in high pressure trickle-bed reactors is proposed. 

Gas-liquid mass transfer can have a strong effect on TBR overall performance therefore its accurate evaluation is essential for achieving successful design and scale-up. In spite of the vast information available on gas-liquid mass transfer characteristics of atmospheric TBRs [1,2] only a few researchers have studied how interfacial areas, a, and volumetric liquid-side mass transfer coefficients, kLa, evolve at elevated pressures. For example, it has been reported that both a and kLa increase as gas density is rised while the gas superficial velocity is kept constant [3-5], Similar observations regarding gas hold-up and two-phase pressure drop, as well as the delay in the onset of pulsing have also been reported [6],... 

VOLUMETRIC LIQUID-SIDE MASS TRANSFER COEFFICIENT... 

Gas-liquid interfacial areas, a, and volumetric liquid-side mass transfer coefficients, kLa, are measured in a high pressure trickle-bed reactor. Increase of a and kLa with pressure is explained by the formation of tiny bubbles in the trickling liquid film. By applying Taylor s theory, a model relating the increase in a with the increase in gas hold-up, is developed. The model accounts satisfactorily for the available experimental data. To estimate kLa, contribution due to bubbles in the liquid film has to be added to the corresponding value measured at atmospheric pressure. The mass transfer coefficient from the bubbles to the liquid is calculated as if the bubbles were in a stagnant medium. 

Similarly, for bubble columns (two-phase sparged reactors), Sharma and Mashelkar (1968) proposed Equation 6.2 for the volumetric liquid-side mass transfer coefficient, > based on measurements by the chemical method ... 

Volumetric liquid-side mass transfer coefficient (1/s)... 

There is considerable information available in the hterature on the design of ejectors (steam jet ejectors, water jet pumps, air injectors, etc.) supported by extensive experimental data. Most of this information deals with its use as an evacuator and the focus is on ejector optimization for maximizing the gas pumping efficiency. The major advantage of the venturi loop reactor is its relatively very high mass transfer coefficient due to the excellent gas-liquid contact achieved in the ejector section. Therefore, the ejector section needs careful consideration to achieve this aim. The major mass transfer parameter is the volumetric liquid side mass transfer coefficient, k a. The variables that decide k a are (i) the effective gas-hquid interfacial area, a, that is related to the gas holdup, e. The gas induction rate and the shear field generated in the ejector determine the vine of and, consequently, the value of a. (ii) the trae liquid side mass transfer coefficient, k. The mass ratio of the secondary to primary fluid in turn decides both k and a. For the venturi loop reactor the volumetric induction efficiency parameter is more relevant. This definition has a built in energy... 

Sobieszuk P, CygaAski P, Pohorecki R. Volumetric liquid side mass transfer coefficient in a gas-liquid microreactor. Chem. Process Eng. (Inz. Chem. Procesowa) 2008 29(3) 651-661. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

As the Henry coefficient He is known (64), K a can be calculated from the measured CO conversion X by eq. (11) With respect to eq. (12) the plot of 1/Koa vs. 1/Ccat should give a straight line for a constant gas velocity ( l is constant). The intercept of this line gives the reciprocal of the volumetric mass transfer coefficient l/kj a. An excunple of such plots is shown in Fig. 11 for two gas velocities. From the volumetric mass transfer coefficients kra which are in the reasonable range of 0.01 to 0.02 s the liquid side mass transfer coefficient kjj can be calculated as the interfaclal area a is known. The mean value of k is about 0.01 cm/s. The k value can reasonably be described by the correlations of Hughmark (65)... 

The scant information regarding the influence of internal draft tubes on the liquid side mass transfer coefficient is of conflicting nature [67,70]. The following correlation, due to Bello et al. (71 ], may be used to estimate the volumetric mass transfer coefficient in air lift reactors with a downcomer and riser ... 

Lemoine R, Morsi BI. Hydrodynamic and mass transfer parameters in agitated reactors. Part II gas-holdup, Sauter mean bubble diameters, volumetric mass transfer coefficients, gas-liquid interfa-dal areas, and liquid-side mass transfer coefficients. InL J. Chem. React. Eng. 2005 3 A20. 

Mass transfer across the gas-liquid interface is the ratecontrolling step in most of the applications of three-phase fluidization and,in particular, the volumetric mass transfer coefficient, k a, has a profound effect on bed performance. Knowledge of the individual liquid side mass transfer coefficient, kL, and the interfacial area, a, and the effect of various parameters on these terms, rather than the volumetric mass transfer... 

Onda et d. [141] calculate the liquid-side mass transfer coefficient fixnn its volumetric value and the wetted surface area of the packing, determined by their own equation (93). 

In many practical applications, gas-liquid mass transfer plays a significant role in the overall chemical reaction rate. It is, therefore, necessary to know the values of effective interfacial area (aL) and the volumetric or intrinsic gas-liquid mass transfer coefficients such as kLah, kL, ktaL, kg, etc. As shown in Section IX, the effective interfacial area is measured by either physical e.g., photography, light reflection, or light scattering) or chemical methods. The liquid-side or gas-side mass-transfer coefficients are also measured by either physical (e.g., absorption or desorption of gas under unsteady-state conditions) or chemical methods. A summary of some of the experimental details and the correlations for aL and kLaL reported in the literature are given by Joshi et al. (1982). In most practical situations, kgaL does not play an important role. 

The liquid-side volumetric mass transfer coefficient in microreactors is one to two orders of magnitude larger as compared to conventional three-phase reactors. This is mainly due to higher specific interfacial area in microreactors. The values of the Uq-uid-side and gas-side mass transfer coefficients in falUng fihn microreactors (FFMRs) are in the ranges of Aq. from 1 x 10 to 1 X 10 m/s and kc from 10 to 10 m/s, respectively. 

Liquid-side volumetric mass transfer coefficient kLa = 0.078ug (s ). 

The physical absorption technique (manometric method) is suitable to determine the liquid side volumetric mass transfer coefficient as well as the gas-side one. Results show that kLa is independant of pressure and depends mainly on the system s hydrodynamics and secondly, that koa is inversely proportional to the total pressure and can be related to the liquid Reynolds number. 

The reported study on gas-liquid interphase mass transfer for upward cocurrent gas-liquid flow is fairly extensive. Mashelkar and Sharma19 examined the gas-liquid mass-transfer coefficient (both gas side and liquid side) and effective interfacial area for cocurrent upflow through 6.6-, 10-, and 20-cm columns packed with a variety of packings. The absorption of carbon dioxide in a variety of electrolytic and ronelectrolytic solutions was measured. The results showed that the introduction of gas at high nozzle velocities (>20,000 cm s ) resulted in a substantial increase in the overall mass-transfer coefficient. Packed bubble-columns gave some improvement in the mass-transfer characteristics over those in an unpacked bubble-column, particularly at lower superficial gas velocities. The value of the effective interfacial area decreased very significantly when there was a substantial decrease in the superficial gas velocity as the gas traversed the column. The volumetric gas-liquid mass-transfer coefficient increased with the superficial gas velocity. 

Cho, J.S., and Wakao, N. (1988), Determination of liquid-side and gas-side volumetric mass transfer coefficients in a bubble column, Journal of Chemical Engineering of Japan, 21(6) 576-581. 

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