Microstructured mass transfer coefficient

In falling film microstructured reactors, Zhang et al. [50] proposed an empirical relation to estimate the mass transfer coefficient as shown in Equation 7 (Table 7.6). 

In a vertically placed 20 parallel channel microstructured falling film reactor of 60 mm length, 1 mm width, and 0.3 mm depth, gas and liquid flows with 46 ml min and 3.6 ml min , respectively, estimate (1) thickness ofthe wall film, (2) mean velocity of liquid film, (3) Fourier number, (4) Reynolds number, and (5) mass transfer coefficient, k. ... 

The mass transfer coefficients obtained in microstructured devices and in conventional gas - hquid contactors are hsted in Table 7.11. The liquid-side k a and interfacial area in microstructured devices are at least 1 order of magnitude higher than those in conventional contactors such as bubble columns and packed columns, being up to21s . ... 

The volumetric mass transfer coefficients found in the liquid-liquid microstructured devices at various flow rates were compared with those for conventional equipment in Table 7.12. Identical to gas-hquid devices, the mass transfer coefficients found in liquid-hquid microstructured devices are well above those of conventional contactors. 

Besides the mass transfer coefficient, pressure drop plays an important role in the design of microstructured devices. The discussion here is focused on Taylor flow and annular flow. 

Figure 15.4 Volumetric mass transfer coefficient as a function of the hydraulic diameter in microstructured channels (D = 10-=m s-Y...

Several workers have used fast chemical reactions to determine the global mass transfer coefficient (kgi) in microstructured liquid-liquid reactors (75, 76]. The neutralizations of trichloroacetic add [76] [Equation (15.72)] and acetic acid [75] were used as model reactions. The reactions are instantaneous and, therefore, are controlled by mass transfer. 

Comparison of a single-tube packed-bed reactor with a traditional batch reactor was also published in the case of o-nitroanisole hydrogenation, not for productivity purposes but rather as laboratory tools for kinetic studies (Scheme 9.11) [46]. It was shown that the better efficiency of mass transfer enables the microreactor to obtain intrinsic kinetic data for fast reactions with characteristic times in the range 1-100 s, under isothermal conditions, which is difficult to achieve with a stirred tank reactor. However, the batch reactor used in this study was not very well designed since a maximum mass transfer coefficient (kia) of only 0.06 s was measured at 800 rpm, whereas kia values of up to 2 s are easily achieved in small stirred tank reactors equipped with baffles and mechanically driven impellers [25]. This questions the reference used when comparing microstructured components with traditional equipment, with the conclusion that comparison holds only when the hest traditional technology is used. 

This approach enables analysis of the KirkendaU effect in the contact zone. As a result of 2D computer simulations, different types of diffusion zone microstructure were obtained. The influence of KirkendaU effect on the behavior of the zigzag diffusion path is treated, and the possible type of the resulting diffusion zone between two-phase alloys without particles of the other phase is obtained. We concluded that the change of the rotation angle of the principle value vector of the effective diffusion coefHcients matrix and the concentration dependence of mass transfer coefficients lead to deviations of the averaged concentrations in the two-phase zone from the directions of the principle value vector obtained in the models developed earlier [12, 13, 48]. 

Turbulent mass transfer near a wall can be represented by various physical models. In one such model the turbulent flow is assumed to be composed of a succession of short, steady, laminar motions along a plate. The length scale of the laminar path is denoted by x0 and the velocity of the liquid element just arrived at the wall by u0. Along each path of length x0, the motion is approximated by the quasi-steady laminar flow of a semiinfinite fluid along a plate. This implies that the hydrodynamic and diffusion boundary layers which develop in each of the paths are assumed to be smaller than the thickness of the fluid elements brought to the wall by turbulent fluctuations. Since the diffusion coefficient is small in liquids, the depth of penetration by diffusion in the liquid element is also small. Therefore one can use the first terms in the Taylor expansion of the Blasius expressions for the velocity components. The rate of mass transfer in the laminar microstructure can be obtained by solving the equation... 

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