Mass transfer internal circulation

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

Boger et al. [50] analyzed the performance of MLRs with internal density-driven circulation (IMLR). They found the gas-hquid mass transfer superior and the overall mass transfer performance comparable with those for slurry reactors. 

Mass transfer in a gas-liquid or a liquid-liquid reactor is mainly determined by the size of the fluid particles and the interfacial area. The diffusivity in gas phase is high, and usually no concentration gradients are observed in a bubble, whereas large concentration gradients are observed in drops. An internal circulation enhances the mass transfer in a drop, but it is still the molecular diffusion in the drop that limits the mass transfer. An estimation, from the time constant, of the time it wiU take to empty a 5-mm drop is given by Td = d /4D = (10 ) /4 x 10 = 6000s. The diffusion timescale varies with the square of the diameter of the drop, so... 

For the sake of developing commercial reactors with high performance for direct synthesis of DME process, a novel circulating slurry bed reactor was developed. The reactor consists of a riser, down-comer, gas-liquid separator, gas distributor and specially designed internals for mass transfer and heat removal intensification [3], Due to density difference between the riser and down-comer, the slurry phase is eirculated in the reactor. A fairly good flow structure can be obtained and the heat and mass transfer can be intensified even at a relatively low superficial gas velocity. 

Smaller bore diameters naturally produce slugs of smaller diameter [31,97]. Typically, a smaller length can also be generated thereby. As a consequence, internal circulation in the slug and specific interface between the slugs are increased. It is assumed that the impact of the increase in internal circulation on mass transfer/ reaction processing is generally more dominant. 

Thom, J.R.S., Prediction of pressure drop during forced circulation boiling of water, International Journal of Heat and Mass Transfer, 7, pp. 709-24 (1964). 

Interfacial tension is the parameter in equations influencing the drop size, as discussed in preceding sections. The smaller the value of a, the smaller are the resulting drops, if all the other conditions are the same, and the larger is the transfer area per unit volume. On the other hand, small drops may show little or no internal circulation, which implies equivalent consequences for the mass transfer coefficient and a lower rising velocity and, accordingly, a lower flow rate at the flooding point. 

The surface viscosity effect on terminal velocity results in a calculated drag curve that is closer to the one for rigid spheres (K5). The deep dip exhibited by the drag curve for drops in pure liquid fields is replaced by a smooth transition without a deep valley. The damping of internal circulation reduces the rate of mass transfer. Even a few parts per million of the surfactant are sometimes sufficient to cause a very radical change. 

Mass transfer during formation of drops or bubbles at an orifice can be a very significant fraction of the total mass transfer in industrial extraction or absorption operations. Transfer tends to be particularly favorable because of the exposure of fresh surface and because of vigorous internal circulation during the formation period. In discussing mass transfer in extraction, it has become conventional (H12) to distinguish four steps (1) formation, (2) release, (3) free rise or fall, (4) coalescence. Free rise or fall has been treated in previous chapters. Steps 1 and 2 are considered here. 

As noted above, small bubbles, a uniform gas holdup radial distribution and an appropriate liquid circulating velocity can intensify mass transfer between the gas phase and the continuous phase and improve the production efficiency in EL-ALRs. In order to reduce the bubble size and obtain a more uniform radial distribution of the local gas holdup and the liquid and bubble rise velocities, and regulate the liquid circulating velocity appropriately as well, in this work, a novel internal is used and mounted in the riser column to improve hydrodynamics and mass transfer. The hydrodynamic behavior and mass transfer characteristics of an EL-ALR with the new designed internal are investigated. 

Diminicis, D. P. (1973). An experimental investigation of near critical and supercritical burning with internal circulation. Inter. J. Heat Mass Transfer, 21 585-595. 

Subbarao, D. and Gambhir, S., Gas to particle mass transfer in risers, in "Proceedings of 7th International Circulating Fluidized Beds Conference", pp. 97-104, Canadian Society for Chemical Engineering, Niagara Falls. 

Surface-active contaminants, even in trace concentrations, tend to be adsorbed on the droplet surface and reduce or totally prevent internal circulation. This is particularly the case for smaller (< 1 mm) droplets. For internally stagnant droplets, the droplet phase mass transfer coefficient is given by the approximate expression... 

Both types of convection stimulate a vivid internal fluid circulation within the porous plate, thereby reducing the liquid phase mass transfer resistance almost down to zero. This circulation becomes suppressed by friction force if the pore... 

In many multiphase (gas-liquid, gas-solid, liquid-liquid and gas-liquid-solid) contactors, a large degree of circulation of both discrete and continuous phases occurs. This circulation causes a good degree of mixing and enhances heat and mass transfer between fluid and walls. The degree of circulation depends on a number of parameters such as the size of equipment, the nature of the phases involved, velocities of various phases, nature of the internals within the equipment and many others. 

The gas-side mass-transfer coefficients kefl and ko increase with liquid feed rate or with gas velocity at each given position in the venturi scrubber and decrease at constant liquid rate and gas velocity with increasing distance from the point of liquid injection (J7, VI1). The values ofkifl generally increase with increasing liquid flow rate or gas velocity (often referred to as the velocity at the throat). However, ki,a will sometimes exhibit a maximum when the gas velocity increases the explanation is that, at higher gas velocities, an increase in turbulence in the throat of the venturi results in the formation of droplets smaller than the thin filaments first formed at lower gas velocities. Internal circulation is reduced in these smaller droplets, and there is also a reduction in the size of the zone of intense turbulence. These two phenomena lead to a maximum for the values of/cL. as found experimentally by Kuznetsov and Oratovskii (K15) and Virkar and Sharma (VI1). The values of the effective interfacial area a increase with both gas and liquid flow rates. 

The mass-transfer coefficient in each film is expected to depend upon molecular diffusivity, and this behavior often is represented by a power-law function k . For two-film theory, n = 1 as discussed above [(Eq. (15-62)]. Subsequent theories introduced by Higbie [Trans. AIChE, 31, p. 365 (1935)] and by Dankwerts [Ind. Eng. Chem., 43, pp. 1460-1467 (1951)] allow for surface renewal or penetration of the stagnant film. These theories indicate a 0.5 power-law relationship. Numerous models have been developed since then where 0.5 < n < 1.0 the results depend upon such things as whether the dispersed drop is treated as a rigid sphere, as a sphere with internal circulation, or as oscillating drops. These theories are discussed by Skelland [ Tnterphase Mass Transfer, Chap. 2 in Science and Practice of Liquid-Liquid Extraction, vol. 1, Thornton, ed. (Oxford, 1992)]. 

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