Multiphasic operation parameters

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]. 

Figure 1731. Fluidized bed reactor processes for the conversion of petroleum fractions, (a) Exxon Model IV fluid catalytic cracking (FCC) unit sketch and operating parameters. (Hetsroni, Handbook of Multiphase Systems, McGraw-Hill, New York, 1982). (b) A modem FCC unit utilizing active zeolite catalysts the reaction occurs primarily in the riser which can be as high as 45 m. (c) Fluidized bed hydroformer in which straight chain molecules are converted into branched ones in the presence of hydrogen at a pressure of 1500 atm. The process has been largely superseded by fixed bed units employing precious metal catalysts (Hetsroni, loc. cit.). (d) A fluidized bed coking process units have been built with capacities of 400-12,000 tons/day.

In direct contrast to intrinsic kinetics, the transport processes (mass/heat transfer coefficient) depend on the type of multiphase reactor, its size, and operating parameters. Thus, one can have an order or two of magnitude changes in the gas-Uquid mass transfer coefficient, k a, when shifting over from packed columns to stirred... 

Operating MSR under novel process windows, the key performance parameters can be increased by a few orders of magnitude. A few examples are presented here. In the case of esterification of phthalic anhydride with methanol 53-fold higher reaction rate between 1 and 110 bar for a fixed temperature of 333 K was observed [14]. A multiphase (gas/liquid) explosive reaction of oxidation of cyclohexane under pure oxygen at elevated pressure and temperature (>200 C and 25 bar) in a transparent silicon/glass MSR increased the productivity fourfold. This reaction under conventional conditions is carried out with air [15]. Another example is for the synthesis of 3-chloro-2-hydroxypropyl pivaloate a capillary tube of 1/8 in. operated at 533 K and 35 bar, superheated pressurized processing much above the boiling point, allowedto decrease reaction time 5760-fold as compared to standard batch operation [16]. The condensation of o-phenylenediamine with acetic acid to 2-methylbenzimidazole in an MSR is an impressive example of the reduced reaction time from 9 weeks at room temperature to 30 s at 543 K and 130 bar [17]. 

As mentioned in Section 2.2, the main issue relating to scale-up of a multiphase reactor is the values of the transport coefficients, and Once these values are determined, comparison of the parameters in the parentheses of the right-hand side of Equation 2.19 allows the rate-controlling step for a given set of operating conditions in a given type of multiphase reactor. The worked examples in Chapters 7A, 7B, 8, and 9 illustrate this procedure. 

Gas-liquid mixed tanks are used for various operations in industrial practise. The design of gas-liquid mixing units and reactors is still done by empirical correlations, which are usually valid for specific components, mixing conditions and geometries. Computational Fluid Dynamic (CFD) techniques have been used successfully for single-phase flow, but gas-liquid flow calculations are still tedious for computers. Therefore, simpler and more accurate multiphase models are needed. In order to verify multiphase CFD calculations and to fit unknown parameters in the multiphase models, experimental local bubble size distributions and flow patterns are needed. 

Characterization of the physical and chenucal parameters of multiphase systems with complex reactants and interfacial phenomena is extremely difficult and may limit the usefulness of the correlations mentioned above. Achievement of a scalable microenvironment is also difficult but may be crucial to successful scale-up. These factors, combined with the multiplicity of uses for batch reactors, argues for maximizing the versatility of both pilot and production scale equipment to encompass a range of operating conditions for specific reactions, as well as to maximize the number of different reactions that can be run successfully. Methods of achieving this versatility are discussed iu later sections and in Chapter 13. However, as wide as this range may be, there will be many reactions that cannot be scaled-up successfully without iucorporatiou of reactor design alternatives. We discuss some of these in the next section. 

New material by name resin was created with specified density (1140 kg/m ) and viscosity (0.60 kg/m.s) along with the existing air (density 1.225 kg/cc, viscosity (1.7894e-5 kg/m.s). Multiphase VOF option was selected under the model option, air was defined as the primary phase and resin was set as secondary phase. Gravity (-9.81 m/s ) was activated in the operating conditions panel in the z direction, density of air (1.225 kg/cc) was specified under variable density parameter for better convergence of solution. Mixed mode (both for air and resin) boundary conditions for the inlet (pressure inlet, 0 pascal) and outlet (pressure outlet, -97325 pascals in z direction) were set. Mixed mode fabric permeability (viscous resistance, 1/m ) and fluid porosity (1-fabric porosity) were defined for the fabric. No slip boundary conditions were set (default) for the walls for both the phases. For resin phase, 1 was set under the volume fraction for inlet and 0 was set for back-flow volume fraction for outlet boundary conditions. 

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