Mass transfer properties required

Complexity in multiphase processes arises predominantly from the coupling of chemical reaction rates to mass transfer rates. Only in special circumstances does the overall reaction rate bear a simple relationship to the limiting chemical reaction rate. Thus, for studies of the chemical reaction mechanism, for which true chemical rates are required allied to known reactant concentrations at the reaction site, the study technique must properly differentiate the mass transfer and chemical reaction components of the overall rate. The coupling can be influenced by several physical factors, and may differently affect the desired process and undesired competing processes. Process selectivities, which are determined by relative chemical reaction rates (see Chapter 2), can thenbe modulated by the physical characteristics of the reaction system. These physical characteristics can be equilibrium related, in particular to reactant and product solubilities or distribution coefficients, or maybe related to the mass transfer properties imposed on the reaction by the flow properties of the system. 

In principle, TPD can also be applied to high-surface area catalysts in a reactor, and this may yield useful qualitative information. Deriving quantitative information from TPD on supported catalysts is also possible, but requires that mass transfer properties such as intraparticle diffusion are properly taken into account. For details of this approach, the reader is referred to an interesting discussion by Kanervo et al. [38],... 

The development of mass transfer models require knowledge of three properties the diffusion coefficient of the solute, the viscosity of the SCF, and the density of the SCF phase. These properties can be used to correlate mass transfer coefficients. At 35 C and pressures lower than the critical pressure (72.83 atm for CO2) we use the diffusivity interpolated from literature diffusivity data (2,3). However, a linear relationship between log Dv and p at constant temperature has been presented by several researchers U>5) who correlated diffusivities in supercritical fluids. For pressures higher than the critical, we determined an analytical relationship using the diffusivity data obtained for the C02 naphthalene system by lomtev and Tsekhanskaya (6), at 35 C. 

During the extraction phase the whole mass transfer may be regarded as a quasi-stationary process. Scale-up rules therefore take account of external mass transfer from the solid surface to the supercritical fluid only. If pilot and production plants are required to display the same mass transfer properties, then... 

However, it is clear that attempts to correlate antibiotic production with the effect of temperature changes during fermentation should be carried out with an understanding of the correlation between the rheological properties of fermentation fluids and mass transfer, nutrient requirements, and the rate of production at each stage of fermentation. 

In biodiesel production, methanol and lipid reaction products are immiscible and form two phases at room conditions. This results in low reaction effidency and lipase deactivation. Hydrophobic solvents can minimize this effect, however, they are toxic and require a separation unit, which further increases the overall production cost Supercritical carbon dioxide (SC-COj) has frequently been used to replace organic solvents in various chemical processes. Due to its properties— including the easy and complete removal of the solvent, an ability to manipulate the physical properties of the solvent by simply changing the pressure or tanperature, nontoxidty, nonflammability, and enhancement of substrate mass transfer properties— it was suggested as a green solvent in biocatalyst reactions. A chemical feature of SC-COj is its low critical temperature (below the denaturation temperature of lipase). This feature combines the good solubility of nonpolar compounds, such as lipids, and makes SC-CO2 the perfect medium for biodiesel production. 

As in the case of temperature control, the process within a composition loop may be extremely complex. In fact, most mass transfer operations require multiple control loops to cope with the number of variables which affect product quality. But for the moment it is important to examine the properties of a composition loop apart from the intrigues of mass transfer. Therefore a simple blending system will be analyzed. 

Engineering factors include (a) contaminant characteristics such as physical and chemical properties - concentration, particulate shape, size distribution, chemical reactivity, corrosivity, abrasiveness, and toxicity (b) gas stream characteristics such as volume flow rate, dust loading, temperature, pressure, humidity, composition, viscosity, density, reactivity, combustibility, corrosivity, and toxicity and (c) design and performance characteristics of the control system such as pressure drop, reliability, dependability, compliance with utility and maintenance requirements, and temperature limitations, as well as size, weight, and fractional efficiency curves for particulates and mass transfer or contaminant destruction capability for gases or vapors. 

This form is partieularly appropriate when the gas is of low solubility in the liquid and "liquid film resistanee" eontrols the rate of transfer. More eomplex forms whieh use an overall mass transfer eoeffieient whieh ineludes the effeets of gas film resistanee must be used otherwise. Also, if ehemieal reaetions are involved, they are not rate limiting. The approaeh given here, however, illustrates the required ealeulation steps. The nature of the mixing or agitation primarily affeets the interfaeial area per unit volume, a. The liquid phase mass transfer eoeffieient, kL, is primarily a funetion of the physieal properties of the fluid. The interfaeial area is determined by the size of the gas bubbles formed and how long they remain in the mixing vessel. The size of the bubbles is normally expressed in terms of their Sauter mean diameter, dj, whieh is defined below. How long the bubbles remain is expressed in terms of gas hold-up, H, the fraetion of the total fluid volume (gas plus liquid) whieh is oeeupied by gas bubbles. 

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