Mass transport coefficients

The mass transport coefficient in tlris example can be related to the properties of the flowing gas by the equation... 

Coh ) = concentration (or activity) at the surface which may be obtained from the solubility product of Mg(OH)j, and Aqh- = the mass transport coefficient of hydroxyl ions without chemical reaction and is obtained from ... 

The rate of extraction depends on the mass transport coefficient (f), the phase contact area (F) and the difference between the equilibrium concentration and the initial concentration of the dissolved component, which is usually expressed as the driving force of the process (a). The rate of extraction (V) can be calculated as shown in Equation (135) ... 

Km Michaelis-Menten constant mass transport coefficient... 

The rate of mass transport is the product of these two factors, the density of atoms and the diffusion coefficient per atom, as shown in Fig. 6. Over a large temperature interval up to the mass transport coefficient is almost perfectly Arrhenius in nature. The enhanced adatom concentrations at high temperatures are offset by the lower mobility of the interacting atoms. Thus, surface roughening does not appear to cause anomalies in the... 

The side-by-side diffusion cell has also been calibrated for drug delivery mass transport studies using polymeric membranes [12], The mass transport coefficient, D/h, was evaluated with diffusion data for benzoic acid in aqueous solutions of polyethylene glycol 400 at 37°C. By varying the polyethylene glycol 400 content incrementally from 0 to 40%, the kinematic viscosity of the diffusion medium, saturation solubility for benzoic acid, and diffusivity of benzoic acid could be varied. The resulting mass transport coefficients, D/h, were correlated with the Sherwood number (Sh), Reynolds number (Re), and Schmidt number (Sc) according to the relationships... 

The Higuchi-Hiestand model [43] permits the a priori estimation of the mass transport coefficient for the dissolution of finely divided drug particles. The model relates the particle radius, a(t), with time according to... 

In this chapter the theory and practice of limiting-current technique for the measurement of mass-transport coefficients have been described. The selective discussion and tabular compilation of results of investigations that used limiting-current measurements should be indicative of the widespread use of this relatively novel method. 

Ag heat conductivity of the frozen product (ice) blid mass transport coefficient... 

Important hints on the reaction site can be gained by the Hatta numbers (Ha) of mass transport at the G/L- and L/L-phase boundaries. These numbers are also essential in order to estimate mass transport rates and concentration profiles within the boundary layer. Since the main resistance of mass transport is in the aqueous phase, mass transport coefficients and Ha numbers mentioned in the text are related to the aqueous phase. 

The resulting values are shown in Table 4. As expected, the diffusion coefficients of prenal and citral are smaller in water than in n-hexane. Since the mass transport coefficients in each boundary layer directly correlate with the diffusion coefficient (Eq. 3), this result confirms the assumption that the overall mass transport resistance can be predominantly referred to the aqueous catalyst phase ... 

The overall mass transport coefficient for mass transport from the dispersed organic phase into the continuous aqueous phase was then calculated according to the Calderbank equation (Eq. 4) ... 

Table 5 Overall mass transport coefficients kn and kna values for the mass transport of prenal and citral in the Diphasic system n-hexane/water...

As expected, the overall reaction rate increases with increasing catalyst volume rate. The effect can be explained in terms of the dependency of the mass transport coefficient ll on the Re munber (Eq. 12). Due to the increase of the volume hold-up of the aqueous phase, the residence time of the organic phase decreases, so that the observed conversion degrees do not change within the limits of the investigated regime. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

Average body weight of the treated rats in the critical study (Paulet and Desbrousses 1972) was 0.225 kg. Since the overall mass transport coefficient (Kg) is not available, the value has been assumed to equal 1. 

The variable mo is the hitherto unspecified mass transport coefficient of species O, which for the time being is considered constant. As E becomes more negative, the current increases (Fig. 5.1, curve B) until it reaches the plateau limiting current /l- There it is limited by the mass transport of species O to the surface of the electrode. At this point, every molecule of O that reaches the surface of the electrode is immediately reduced to R. Therefore, the surface concentration of O is zero. 

Note that the mass transport coefficients r and mo are different and that the concentration gradient is reversed, hence the minus sign. As we have specified above, the electrochemical reaction is very fast, which means that the Nernst equation (5.20b) is satisfied for all values of the surface concentrations of O and R. Thus,... 

We may substitute it into (7.10), obtaining a mass transport coefficient as defined by... 

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