Laminar contactor

So, for laminar contactors we predict that the mass transfer is a much weaker function of the amount of energy dissipated than for turbulent contactors. The fact that energy is not required for interfacial area generation is in perfect agreement with this scaling analysis. Of course, there still is an impact of velocity on... 

The stagnant-film model discussed previously assumes a steady state in which the local flux across each element of area is constant i.e., there is no accumulation of the diffusing species within the film. Higbie [Trans. Am. Jn.st. Chem. Eng., 31,365 (1935)] pointed out that industrial contactors often operate with repeated brief contacts between phases in which the contact times are too short for the steady state to be achieved. For example, Higbie advanced the theory that in a packed tower the liquid flows across each packing piece in laminar flow and is remixed at the points of discontinuity between the packing elements. Thus, a fresh liquid surface is formed at the top of each piece, and as it moves downward, it absorbs gas at a decreasing rate until it is mixed at the next discontinuity. This is the basis of penetration theoiy. 

Unsteady state phenomena have been stated to be of greater importance for non-Newtonian than for Newtonian materials and therefore warrant experimental investigation. The prediction of pressure drop for two-phase flow of a gas and a non-Newtonian fluid seems to be in a well-perfected state but requires extension to situations in which the liquid flow is laminar. Apparently no information is yet available on the problems of mixing, entrainment, and other similar relationships which are of importance if such contactors are to be designed for chemical rather than mechanical purposes. 

Fig. 4.11. Laminar-film contactors (a) wetted-wall and (b) wetted-sphere...

In the laminar-film contactor shown in Fig. 4.116, the supporting surface has the form of a sphere held in place by a wire<3>. This arrangement has the advantage that, in the regions where the liquid runs on to the sphere, and where it leaves the sphere, the surface areas exposed to the gas are relatively small so that, even if the hydrodynamics of the liquid flow in these regions is not ideal, the effect on the rate of absorption of the gas will be small. As in the case of the cylindrical tube, the contact time for each element of liquid as it flows around the sphere can be calculated from the liquid flowrate, although the mathematics of the analytical treatment is somewhat more complicated than in the case of the tube. 

Both phases in a continuous form packed columns thin-film contactors wetted-wall columns contactors with flat surface laminar jet absorber disc (sphere) columns... 

There are several other types of apparata used for reactive absorption, though these are less widespread. In mechanically agitated bubble columns it can be assumed that both phases are ideally mixed, whereas in the jet absorber, the gas stream breaks on a liquid surface and is dispersed in the liquid. In spray towers and venturi scrubbers, liquid is sprayed as fine droplets. Thin-jilm contactors, in which liquid film is scrapped from the walls by impellers, are applied for reactive absorption in viscous liquids. Some units, such as wetted-wall columns, contactor, laminar jet absorber and disc (sphere) column are used mainly in laboratory. 

The major difference between this reactor and other gas-liquid reactors such as the wetted-wall column, the laminar-jet absorber, the disk contactor, and the stirred cell is that the experimenter has independent control of the physical factors, such as individual film resistances and interfacial area. 

Various absorbers used for the measurements of absorption rates in gas-liquid reaction processes can also be used to make similar measurements for the gas-liquid-solid reaction processes. Commonly-used absorbers are the laminar-jet absorber (Fig. 5-19). the wetted-wall column absorber (Fig. 5-20), the rotary-drum absorber, the disk column absorber (Fig. 5-21), the single-pellet absorber (Fig. 5-22). and the gradientless contactor (Fig. 5-23). The key features of these absorbers... 

FIGURE 11.14 The stirred cell for obtaining precise kinetic data for fluid-fluid reactions (a) stirred cell, (b) rotating drum, (c) cylindrical wetted wall, (d) wetted sphere, (e) laminar jet, and (f) stirred contactor. (From Danckwerts, P. V., Gas-Liquid Reactions, New York McGraw-Hill, 1970.)... 

In addition to the stirred cell, other laboratory reactors commonly used include rotating drum contactor, wetted wall column, wetted sphere column, laminar jet, and stirred contactor. These equipments are shown schematically in Figures 11.14b-f. AU have several common features, the principal one being a weU defined gas-liquid interfacial area and the ability to vary the area per unit reactor volume a). In the stirred cell, it is achieved by varying the liquid height. As an alternative way, a solid circular baffle is placed at the gas-liquid interface. Holes are drilled on the baffle plate so that the hole opening area becomes the interfacial area. For varying a, baffle plates are made with different free (hole) areas. 

Most work in microchannel extraction focuses on improving the extraction efficiency or the phase separation or system development through adding multiple imit operations on a single chip or by scaling up. Because laminar flow exists in the microchannel devices, the intimate mixing of turbulent flow in traditional contactors is not present. Most studies have shown that the dissimilar phases flow parallel to each other with movement of solute molecules caused by molecular diffusion only. Thus, extraction is governed by contact time between phases [202]. 

It must be clear from the various equations developed above that the gas-liquid interfacial area is a very important parameter in determining the rate of mass transfer. Any precise measurement of the mass transfer coefficient is possible only if the area is correctly known. This is best accomplished by using a stirred cell with a fixed gas-liquid interfacial area, although other experimental reactors such as the wetted wall column, laminar jet, and disk contactor can also be used (see Danckwerts, 1970 Doraiswamy and Sharma, 1984). The two commonly used cell designs are those of Danckwerts (1970) and Levenspiel and Godfrey (1974). 

Shirazian, S., Moghadassi, A., Moradi, S. 2009. Numerical simulation of mass transfer in gas-liquid hollow fiber membrane contactors for laminar flow conditions. Simul. Model. Pract. Theory 17 708-718. 

Yoshizuka et al. [105] studied the extraction kinetics and mechanism of metal extraction in a hollow-fiber contactor, by using a diffusion-based model with interfacial reaction and by considering the laminar flow of the aqueous and organic solutions through the hollow fiber. The rate constants for various steps were calculated by the experimental kinetic data. 

Using a simple scaling analysis, involving (1) viscous pressure drop, (2) hydrostatic pressure drop, (3) interfacial pressure drop and (4) penetration theory for mass transfer, it has been demonstrated that two-phase laminar bubble-train flow in small channels can exhibit better mass transfer for a given power input than turbulent contactors. 

Hosseinalipour SM. Transport processes in laminar and turbulent opposing jet contactors. Ph.D. thesis, McGiU University. Montreal, 1996. 

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

Subsequently, Chiang and Toor (3) in considering the absorption of pure anmonia into water in a laminar jet contactor, performed a slightly more complex analysis of physical absorption to take account of the volume change of the liquid phase in addition to the heat release effect. Their equations for the physical absorption of ammonia into water were... 

This type of reaction is relatively simple for laboratory investigation. A number of inexpensive model contactors are available for these investigations. These include stirred cell and laminar jet apparatus (Danckwerts 1970). The gradient-less contactor... 

The reactor was a rotating disk contactor for making bisphenol A (BPA) polycarbonate. The reactor model uses a staged approach, and the crux is the prediction of the mass transfer rate of by-product (phenol). The two relationships expressed in Eqs. (44) for the volume of a gas bubble Vj, as a function of the gas flow rate Qg [69] and of its rising velocity uj, in a laminar regime [70] were the key data. 

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