Liquid side mass transfer coefficient

When the reaction in the porous catalyst is very rapid, the conversion rate will be determined by gas/liquid or liquid/solid mass transfer. Particularly volumetric gas/liquid mass transfer coefficients (liquid side) are not very much different in slurry-reactors and in three phase packed beds (aU under optimum conditions). 

Mass-transfer coefficient gas-liquid in liquid side, cm VCcm/ s) Overall mass-transfer coefficient gas-liquid, cmp/(cm, s) Mass-transfer coefficient liquid-solid, cmp/icmi s)... 

Here, kg and ki are the gas-side and liquid-side mass transfer coefficients. Their units are identical to those for Kg and Ki, m/s. Like the overall coefficients, they are usually measured and reported as the composite quantities kgAj and kiAi with SI units of s. ... 

The measurement of liquid side gas - liquid mass transfer coefficient kia, showed that the value of kia increase with increasing rotation speed (V) and gas flow rate (Qg). hi the present research, the effect of impeller rotation on mass transfer coefficient was more significant than the effect of gas flow rate. The following correlation was obtained kia =1.7 x 10 ... 

A phenomenon that is particularly important in the design of reverse osmosis units is that of concentration polarization. This occurs on the feed-side (concentrated side) of the reverse osmosis membrane. Because the solute cannot permeate through the membrane, the concentration of the solute in the liquid adjacent to the surface of the membrane is greater than that in the bulk of the fluid. This difference causes mass transfer of solute by diffusion from the membrane surface back to the bulk liquid. The rate of diffusion back into the bulk fluid depends on the mass transfer coefficient for the boundary layer on feed-side. Concentration polarization is the ratio of the solute concentration at the membrane surface to the solute concentration in the bulk stream. Concentration polarization causes the flux of solvent to decrease since the osmotic pressure increases as the boundary layer concentration increases and the overall driving force (AP - An) decreases. 

SAHAY, B. N. and SHARMA, M. M. Chem.Eng. Sci. 28 (1973) 41. Effective interfacial areas and liquid and gas side mass transfer coefficients in a packed column. 

The values of the mass transfer coefficient will be different on each side of the boundary. For example, when a gas dissolves in a liquid, feg in the gas film will be different from k[ in the liquid film. However, the concentration at the interface is not always known and this leads to the use of overall mass transfer coefficients in conjunction with overall driving forces. The following argument shows how these are related to the individual film coefficients. 

The first term on the right-hand side of Eq. (257) is provided by the quasisteady model, whereas the second represents the contribution of the transient process. Measurements of the mass transfer coefficients from the dissolution of the wall of a tube into a turbulent liquid having Schmidt numbers as large as 10s could be correlated with the expression [56]... 

A buffer solution containing urea flows along one side of a flat membrane and the same buffer solution without urea flows along the other side of the membrane, at an equal flow rate. At different flow rates the overall mass transfer coefficients were obtained as shown in Table P8.1. When the liquid film mass transfer coefficients of both sides increase by one-third power ofthe averaged flow rate, estimate the diffusive membrane permeability. 

In cases where the major resistance is in the liquid phase, the ratio RL/ RT= 1 and the simplification can be made that the over-all coefficient is equal to the liquid film coefficient. Which resistance dominates has to be determined from the ratio kLa / (kGa Hc) (Table 3-3). For compounds with a low Hc such as semi-volatile organic compounds, both resistances can be important (Libra, 1993). In oxygen transfer the liquid-side resistance dominates and KLa = kLa. This is also true for most of the cases in ozone mass transfer, unless there is strong mass transfer enhancement by very fast or instantaneous reactions of... 

Fortunately changes in k,a due to mass transfer enhancement from ozone decay can be neglected, as Huang et al. (1998) showed by example of cyanide ozonation in strongly basic solutions (pH = 12-14) in a system where the value of the purely physical liquid side mass transfer coefficient was not too low (kL° > 0.03 cm s l). This is supported further by the results from several ozonation experiments, which showed that no ozone decay occurs in the liquid film at lower pH values (phenol, pH = 10 (Metha et ah, 1989) 4-nitrophenol, pH = 8.5 (Beltran et ah, 1992 a)). 

Finally, in Fig. 3.4-12 [24], a comparison is given for the overall, gas-based, mass transfer coefficient for several liquid-to-gas and solid-to-gas packed beds and column systems. In Fig. 3.4-12, for a given data point, the vertical distance up to the Tan et al. [27] correlation (which is for a solid-to-fluid boundary layer) would provide a measure of the liquid-side mass-transfer resistance associated with the liquid. This is so because amount of the large gas... 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

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. 

There is practically nothing about the high-pressure liquid-side-mass transfer coefficient, ku in TBR in the open literature. The only paper published was that of Lara-Marquez et al. [57], The values of kia are determined by using the following chemical absorption systems in the slow reaction regime ... 

N. Midoux, B.I. Morsi, M. Purwasasmita, A. Laurent and J.C. Charpentier, Interfacial area and liquid-side mass transfer coefficient in trickle-bed reactors operating with organic liquids, Chem. Engng. Science, 39 (1984) 781-794. 

The design of packed column reactors is very similar to the design of packed columns without reaction (Volume 2, Chapter 12). Usually plug flow is assumed for both gas and liquid phases. Because packed columns are used for fast chemical reactions, often the gas-side mass transfer resistance is significant and needs to be taken into account. The calculation starts on the liquid side of the gas-liquid interface where the chemical reaction rate constant is compounded with the liquid side mass transfer coefficient to give a reaction-enhanced liquid-film mass transfer... 

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

Gas-liquid interfacial area per unit volume of dispersion a = 195 m2/m3 Liquid-side mass transfer coefficient kL = 2.5 x 10 4 m/s... 

From Fig. 4.3 it may be seen that this value, although smaller than that in Example 4.2, is still sufficiently large for all the reaction to occur in the film. Also the concentration of dissolved C02 at the interface in contact with the incoming gas containing C02 at a partial pressure of 0.001 bar will be only 0.001/., i.e. 0.001/25 4 x 10"5 kmoi/m3 which is much less than 0.05 kmol/m3, the concentration of the OH- ion, so that the reaction will still behave as nseudo first-order. The enhanced liquid-side mass transfer coefficient kl will thus be k[ V(9.5 x 103 x 0.05 x 1.8 x 10 9) 0.925 x 10 3 m/s. 

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