Mechanically agitated processes parameters

The mechanism of particle incorporation is treated extensively in the next section, but a generalized mechanism is given here to better comprehend the effects of the process parameters. Particle incorporation in a metal matrix is a two step process, involving particle mass transfer from the bulk of the suspension to the electrode surface followed by a particle-electrode interaction leading to particle incorporation. It can easily be understood that electrolyte agitation, viscosity, particle bath concentration, particle density etc affect particle mass transfer. The particle-electrode interaction depends on the particle surface properties, which are determined by the particle type and bath composition, pH etc., and the metal surface composition, which depends on the electroplating process parameters, like pH, current density and bath constituents. The particle-electrode interaction is in competition with particle removal from the electrode surface by the suspension hydrodynamics. 

The deposition variables are the process parameters most suited to regulate the particle composite content within the limits set by the particle properties and plating bath composition. Particle bath concentration is the most obvious process variable to control particle codeposition. Within the limits set by the metal plating process and the practical feasibility also current density, bath agitation and temperature can be used to obtain a particular composite. Consequently the deposition process variables are the most extensively investigated parameters in composite plating. The models and mechanisms discussed in Section IV almost exclusively try to explain and model the relation between these process parameters and the particle codeposition rate. 

Despite these successes, important process parameters, like bath agitation, bath constituents and particle type are disregarded. The constants k, 0 and B inherently account for these constants, but they have to be determined separately for every set of process parameters. Moreover, the postulated current density dependence of the particle deposition rate, that is Eq. (2), is not correct. A peak in the current density against the particle composite content curve, as often observed (Section III.3.H), can not be described. The fact that the peak is often accompanied by a kink in the polarization curve indicates that also the metal deposition behavior can not be accounted for by the Tafel equation (Eq. 4). Likewise, the (1-0 term in this equation signifies a polarization of the metal deposition reaction, whereas frequently the opposite is observed (Section 111.3,(0 It can be concluded that Guglielmi s mechanism... 

Following emulsification, the emulsion is dispersed by mechanical agitation into the external feed phase containing the solute to be extracted. The efficiency of this extraction process is dependent on several parameters as discussed below. 

Changing the process parameters. Temperature, pressure and work (agitators, mechanical action) all affect the end product in some way. Temperature is the most visible parameter. Agitation can be critical in both breaking up a product in a lump form or it may cause the imdesirable effect of churning the product into a very difficult paste. Products which are thixotropic or dilatant are most affected by agitation. 

BC suspension obtained from an agitation culture system was mixed with PVA powder to fabricate PVA-BC composite by considering PVA as a matrix and BC as a reinforcement material. The composite was proposed for cardiovascular soft tissue replacement application [78], The composite could be adjusted its mechanical properties to get close to those of specific tissue [aorta and heart valve] by changing the composition of the composite and the processing parameters. 

In the future, novel developments of liquid membranes for biochemical processes should arise. There are several opportunities in the area of fermentation or cell culture, for the in situ recovery of inhibitory products, for example. Another exciting research direction is the use of liquid membrane for enzyme encapsulation so that enzymatic reaction and separation can be combined in a single step. Chapter 6 by Simmons ial- (49) is devoted to this technique. The elucidation of fundamental mechanisms behind the liquid membrane stability is essential, and models should be developed for the leakage rate in various flow conditions. Such models will be useful to address the effect of parameters such as flow regime, agitation rate, and microdroplet volume... 

Direct Emulsification. Polymer colloids called "artificial latexes" can be prepared by dispersion of bulk polymers or polymer solutions into an aqueous medium. Direct emulsification processes are reviewed by ElAasser (23). The preparation procedures involve mechanical dispersion that may be followed by removal of solvent. According to ElAasser "the efficiency of emulsification," and hence the particle size characteristics of the latex, "is determined by the efficiency of formation of fine droplets and the efficiency of stabilization of the formed droplets." Important parameters in the process include the source of energy or agitation, its intensity, and duration type and concentration of emulsifiers mode of addition of emulsifier and the two phases density ratio of the two phases temperature and the rheology of the two phases. 

Some viscosity may be expected to develop during the solution polymer stage of the polymerization. While the extent and duration of the viscosity that can be tolerated is a fimction of the agitation efficiency and heat removal capability of the reactor, the salt concentration is the primary parameter to be adjusted in order to obtain the optimum in-process viscosity. If the salt concentration has been properly adjusted, then at the onset of the precipitation event the viscosity of the system ops markedly and the polymerization is finished as a dispersion. If the salt concentration is too low during the polymerization, the precipitation event and transition to a heterogeneous system never occurs, and effective mixing and heat transfer will eventually be lost. If, on the other hand, the salt concentration is too high, the dispersion will have little thermodynamic stability in the absence of mechanical shear. 

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