Medium-solid systems

It thus appears that the enantioselectivity of CALB for this reaction in the solid/ gas bioreactor is similar to that in an organic liquid medium. Solid/gas biocatalysis therefore offers important potential for production of enantiomerically pure compounds, provided that these transformations involve components having a degree of volatility. Furthermore, as the addition of solvents is avoided in this system, separation and purification during downstream processing are simplified, and side reactions are suppressed. 

Dispersed System Disperse Phase Dispersing Agents and Dispersion Medium. Dispersed system is an apparently homogeneous, system which consists of a microscopically heterogeneous mixture of two or more finely divided phases, eg liquid-liquid (emulsions such as milk) liquid-solid (suspension, such as clay in water) gas-liquid (aerosols, such as fogs, clouds, mists) and gas-solid (such as smoke, dusts)... 

When the particles aggregate to form a continuous network structure which extends throughout the available volume and immobilises the dispersion medium, the resulting semi-solid system is called a gel. The rigidity of a gel depends on the number and the strength of the interparticle links in this continuous structure. 

When heat transfer occurs in a moving medium, it is usually called convection. As an example, heat energy can be transferred from a solid plane surface at one temperature to an adjacent moving fluid at another temperature. Consider the case shown in Fig. 1.3. Heat energy is conducted from the solid to the moving fluid, where energy is carried away by the combined effects of conduction within the fluid and the bulk motion of the fluid. The heat transfer from the solid system to the fluid can be expressed by the empirical equation... 

The power required for a given stirrer type and associated vessel configuration depends on the speed of rotation N, the stirrer diameter du the density p, and the kinematic viscosity v of the medium. In vessels without baffles, the liquid vortex, and therefore the acceleration due to gravity, g, is immaterial, as long as no gas is entrained in the liquid. Thus, P = f(N, dt,p, v), and in the dimensionless form, Ne = /(Re), a relationship generally known as the power characteristics of the stirrer. Here, Ne = P/(pN3df) is the Newton or Power number, and Re s Ndf/v the Reynolds number. This relationship was described in Sections II and III for gas-liquid and gas-liquid-solid systems. 

Figure 8-41 Effect of Increasing the Concentration of the Disperse Phase on the Flow Behavior of a Disperse System. 1—continuous phase, 2— low solids content, 3—medium solids content, 4—high solids content.

An attempt was made to measure the particle local velocity at 1.5 pipe diameters downstream of the elbow by using a particle velocity probe (56). However, the technique failed, presumably because the strong secondary fiow prevented the velocity probe from being aligned with the velocity vector. For this reason, velocities obtained at 22 pipe diameters downstream of the elbow had to be used to estimate the concentrations at this level (1.5 pipe diameters). Figure 20 shows the estimated solids concentration normalized by the discharge concentration CJC ) for fine and medium sand particles 1.5 pipe diameters downstream of the elbow. Most of the relative concentrations are lower than unity, and consequently the mean concentration based on these measurements would be lower than the true value. Similar findings were obtained by Sansone (57) in gas-solid systems downstream of a 90 elbow. This phenomenon occurs because the velocity vector and the probe axis are not colinear, so that the concentration results are only of qualitative value. 

Fluidization refers to the state of solid particles in a suspended condition owing to the flow of fluid, gas, and/or liquid. Contact schemes of fluidized bed systems can be classified on the basis of the states of solid motion. For a batch-solids system, the fluid at a low velocity merely percolates through the voids between packed particles, while the particles remain motionless. The solids in this case are in the fixed bed state. With an increase in the fluid velocity, particles move apart and become suspended the bed then enters the fluidization state. The fluidization characteristics vary, depending on whether gas, liquid, or gas-liquid is the fluidizing medium. 

In liquid-solid systems, the impaction or interception mechanism is referred to as filtration. Filtration can be classified in several ways vacuum, pressure, belch, continuous, cake, or depth. Filtration is generally considered to be a separation involving passage of liquid through a porous medium that retains most of the panicalates contained in the liquid. The porous medium mey be a wire screen, cloth, paper, or a bed oF... 

The increasing importance of environmental considerations places new requirements on paint resins and has broadened the range of paint systems. Paints are now required that have a low solvent content (medium-solids, high-solids coatings) or are solvent-free (powder coatings), that can be adjusted by dilution with water (waterborne paints), and that are thermoplastic or capable of undergoing cross-linking. All of these properties must be obtained via the polymer structure of the binders. Important parameters are described below. 

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