Parameters that Influence Mass Transfer

Many parameters affect the mass transfer between two phases. As we discussed above, the concentration gradient between the two phases is the driving force for the transfer and this, together with the over-all mass transfer coefficient, determines the mass transfer rate. The influence of process parameters (e. g. flow rates, energy input) and physical parameters (e. g. density, viscosity, surface tension) as well as reactor geometry are summed up in the mass transfer coefficient. The important parameters for Kta in stirred tank reactors are  

However, the mass transfer rate can be influenced not only by physical properties, but by chemical reactions as well. Depending on the relative rates of reaction and mass transfer, a chemical reaction can change the ozone concentration gradient that develops in the laminar film, normally increasing the mass transfer coefficient, which in turn increases the mass transfer rate. 

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It is noteworthy that the best results could be obtained only with very pure ionic liquids and by use of an optimized reactor set-up. The contents of halide ions and water in the ionic liquid were found to be crucial parameters, since both impurities poisoned the cationic catalyst. Furthermore, the catalytic results proved to be highly dependent on all modifications influencing mass transfer of ethylene into the ionic catalyst layer. A 150 ml autoclave stirred from the top with a special stirrer... 

Diffusion in the macro-pores of a formed parhcle is generally speaking a very important mechanism. If we speak in terms of resistances to mass transfer macropore resistance is often the largest of the resistances to mass transfer. For transport in the macro-pores we must introduce two parameters that influence the transport. 

This chapter will first provide some basics on ozone mass transfer, including theoretical background on the (two-) film theory of gas absorption and the definition of over-all mass transfer coefficients KLa (Section B 3.1) as well as an overview of the main parameters of influence (Section B 3.2). Empirical correction factors for mass transfer coefficients will also be presented in Section B 3.2. These basics will be followed by a description of the common methods for the determination of ozone mass transfer coefficients (Section B 3.3) including practical advice for the performance of the appropriate experiments. Emphasis is laid on the design of the experiments so that true mass transfer coefficients are obtained. 

Most of the parameters that influence the rates of mass transfer and chemical reaction, and therefore the efficiency of the system, have already been discussed in Chapter B 3, however, in addition to the resistances to mass transfer found in gas/water systems, two more resistances can be found in three-phase systems ... 

To carry out the experiments in a meaningful and systematic way, it will be necessary, first, to consider one of the parameters as a variable while keeping the others constant and then to measure the corresponding pressure drop. The same type of experiment is carried out for the measurement of the heat transfer coefficient. Contrary to the mass transport pressure drop, which could be measured directly, the heat transfer coefficient is obtained indirectly by measuring the temperature of the wall and of the fluid at the entrance and exit of the pipe. The determination of the functional relationship between Ap/1, a and the various parameters that influence the process is illustrated in Fig. 6.1. 

Here k is the external mass-transfer coefficient, ( ) is the humidity-potential coefficient (corrects for the humidity not being a strictly true representation of the driving force close to unity most of the time), Ytv is the humidity above a fully wetted surface, and Yq is the bulk-gas humidity. Equation (12-52) has been used extensively as the basis for understanding the behavior of industrial drying plants owing to its simplicity and the separation of the parameters that influence the drying process the material itself /, the design of the dryer k, and the process conditions (j) (yw - Tc)/. 

The main parameters influencing mass transference through membranes in flow analysis have been discussed in specialised texts that generally deal with both gas diffusion and dialysis [254—257],... 

In essence, Sh, a and 0 are the inportant membrane-fluid and facilitation parameters that influence the mass transfer of the solute from the fluid side to the dialyzate fluid. 

The concentration of reactant A at the external surface of the catalyst particle is very close to zero. Moreover, Eqn. (9-45) shows that the rate of reaction does not depend on either the rate constant (ky) or the effective diffusion coefficient (f)A,eff). The only rate or transport parameter that influences the reaction rate is the external mass-transfer coefficient, kc-For this situation, the reaction is said to be controlled by external mass tranter. The concentration profile of reactant A is shown in Figure 9-13. 

Typical Values. Table 11.3 shows typical parameter values for mechanically agitated tanks and other gas-liquid contacting devices. Not shown are values for kgAj since these are usually so large that they have no influence on the mass transfer rate. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

The efficiency of extraction is mainly dependent on temperature as it influences physical properties of the sample and its interaction with the liquid phase. The extraction is influenced by the surface tension of the solvent and its penetration into the sample (i.e. its viscosity) and by the diffusion rate and solubility of the analytes all parameters that are normally improved by a temperature increase. High temperature increases the rate of extraction. Lou et al. [122] studied the kinetics of mass transfer in PFE of polymeric samples considering that the extraction process in PFE consists of three steps ... 

Steps 1 and 7 are highly dependent on the fluid flow characteristics of the system. The mass velocity of the fluid stream, the particle size, and the diffusional characteristics of the various molecular species are the pertinent parameters on which the rates of these steps depend. These steps limit the observed rate only when the catalytic reaction is very rapid and the mass transfer is slow. Anything that tends to increase mass transfer coefficients will enhance the rates of these processes. Since the rates of these steps are only slightly influenced by temperature, the influence of these processes... 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

In this case it is assumed that a pure gas A is being absorbed in a solvent eontaining a chemically inert component B. Both the solvent and B are not volatile and the fraction of A in the liquid bulk equals zero. The binary mass transfer coefficient Kij between A and the solvent in eq. (4) is given a typical value of 1 X lO" m/s, whereas the total concentration of the liquid Cr is set to 1 x 10 mol/m, also a typical value. Parameters to be chosen are the solubility of A, x i, the fraction of B in the solvent Xg, the mass transfer coefficient between A and B, K/ g and the mass transfer coefficient between B and the solvent, Kg. The results of the calculations are presented in Table 1. Since both the solvent and component B possess a zero flux. Kgs has no influence on the mass transfer process and has therefore been omitted. The computed absorption rate has been compared with the absorption rate obtained from analytical solutions for the following cases. 

The results confirm that the adsorption of ammonia is very fast and that ammonia is strongly adsorbed on the catalyst surface. The data were analyzed by a dynamic isothermal plug flow reactor model and estimates of the relevant kinetic parameters were obtained by global nonlinear regression over the entire set of runs. The influences of both intra-particle and external mass transfer limitations were estimated to be negligible, on the basis of theoretical diagnostic criteria. 

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