Interfadal surface area

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

Yeong et al. [88, 92] used a microstructured film reactor for the hydrogenation of nitrobenzene to aniline in ethanol at 60 °C, 0.1-0.4MPa hydrogen pressure and a residence time of 9-17 s. Palladium catalyst was deposited as films or particles on the microstmctured plate. Confocal microscopy was used to measure the liquid film thickness. With increasing flow rate between 0.5 and 1.0 cm min , thicker liquid films between 67 and 92 pm were observed. The kia of this system was estimated to be 3-8 s with an interfadal surface area per reaction volume of 9000-15 000 m m. Conversion was found to be affected by both liquid flow rate and hydrogen pressure and the reactor operated between the kinetic- and mass transfer-controlled regimes. 

OBRs can exhibit enhanced mass transfer between gases and liquids. The main mechanisms for the enhancement are increased hold-up and increased breakage of bubbles (reducing size, thereby increasing interfadal surface area and reducing rise velocity, which increases bubble residence time further). Figure 5.16 compares the mass transfer coefficient for an OBR with that of an STR for an air-water system on the basis of power density (Ni and Gao, 1996). 

The value of heat transfer coefficient of a single particle in a fluidized bed system is generally not high. It is in the range of I to 700 W/(m K). However, due to the large interfadal surface area, in the order of 3,000 to 45,000 m /m, extremely high rates of heat transfer are achieved in this system. The heat capacity is in the order of 10 J/(m K). As a result, thermal equilibrium is reached quickly. In designing fluidized bed dryers, an isothermal condition is often assumed. 

Figure 14.16. Flooding velocities in liquid-liquid packed towers [J.S. Eckert, Encycl. Chem. Process. Des. 21, 149-165 19S4)]. V = ft/hr (superficial velocity) C = continuous phase D = disperse phase a = sqft area of packing/cuft A = difference e = void fraction in packing n = viscosity centipoise continuous phase p = Ib/cuft a = (dynes/cm) interfadal surface tension F = packing factor.

Surface Tension The contracting force per unit length around the perimeter of a surface. Usually referred to as surface tension if the surface separates gas from liquid or solid phases, and interfadal tension if the surface separates two nongaseous phases. Although not strictly defined the same way, surface tension can be expressed in units of energy per unit surface area. For practical purposes, surface tension is frequently taken to reflect the change in surface free energy per unit increase in surface area. See also Surface Work. 

The making of an emulsion involves many nonequilihrium features, at least from the mechanical point of view. Actually the product of the interfadal tension by the produced surface area y AA. which is the inlerfacial energy, is always much smaller than the mechanical energy put into the system by the stirring device. A signiheani characteristic is the way and the efficiency in which the energy is provided to the drop so that breaking is favored over coalescence. This has to do not only with the device but with formulation and eventual transient events. 

When a drop of one immiscible fluid is inunersed in another fluid and comes to rest on a solid surface, the sur-faee area of the drop will take a minimum value due to the forces acting at the fluid-fluid and rock-fluid interfaces. The forces per unit length acting at the fluid-fluid and roek-fluid interfaees are referred to as interfacial tensions. The interfacial tension between two fluids represents the amount of work required to create a new unit of surface area at the interface. The interfadal tension can also be thought of as a measure of the immiseibility of two fluids. Typieal values of oil-brine interfadal tensions are on the order of20-30 dyn/crrc When eertain ehemical agents are added to an oil-brine system, it is possible to reduce the interfadal tension by several orders of magnitude. 

Erne and coworkers measured the interfacial capacitance of macroporous GaP electrodes as a function of the electrode potential [138, 139). It was found that the capacitance is large for sufficiently small band-bending (interfacial layer in the porous solid) and decreases to the value of a nonporous interface at larger band-bending. Similar effects have been found with macroporous SiC and Si electrodes [18, 141). In fact, the interfadal capacitance is a measure for the surface area of the macroporous network, with the width of the depletion layer, VTsc. as a measuring stick. 

From the above, it is clear that the specific surface area of the filler, which is closely related to filler shape, is of fundamental importance in the xmderstanding of the structtue-property relationship of nanocomposites. The change in particle diameter, layer thickness, or fibrous material diameter from micrometer to nanometer, changes the ratio by three orders in magnitude. At this scale, there is often distinct size dependence of the material properties. Furthermore, with the drastic increase in interfadal area, the properties of the composite are dominated more by the properties of the interface or interphase [2] and they play a much more important role in enhancing the mechanical properties of nanocomposites than in conventional composites or bulk materials [ 1 ]. 

In fiuid-fiuid (gas-liquid and liquid-liquid) systems, a solute usually difluses from one fluid phase into the other, in which the reaction then takes place. The product may pass back into the first non-reactive phase. In liquid-liquid and some gas-Uquid systems, the reaction may actually occur in both phases (e.g. oximation of cyclohexanone [4]). As with heat transfer, rapid reaction kinetics trend to exacerbate mass transfer limitations, especially when only small specific interfadal areas are available. In contrast to the specific surface area for heat transfer, fluid-fluid interfadal areas are dependent on physical properties and operating conditions. 

Surface and Double-layer Properties Valette [19] has analyzed earlier experimental data on the inner-layer capacity at PZC for Ag(lll), Ag(lOO), and Ag(llO) surfaces in order to estimate the surface area and capacitance contributions of superficial defects for real electrodes, as compared to ideal faces. Considering the apphcation of surface spectroscopy techniques to single-crystal Ag electrodes, one should note that anisotropy of the SHG response for metal electrode allows one to analyze and correlate its pattern with interfadal symmetries and its variations by changing nonlinear susceptibility and the surface structure. Early studies on Ag(lll) single-crystal electrodes have... 

In Interval II, the system is composed of monomer droplets and polymer par-tides (Figure 6.2(c)). The monomer consumed by polymerization in the polymer particles is replaced by monomer that diffuses from the monomer droplets through the aqueous phase. The mass-transfer rate of monomers with water solubility equal or greater than that of styrene (0.045 g/100 g of water) is substantially higher than the polymerization rate, and hence monomer partitions between the different phases of the system according to the thermodynamic equilibrium. Therefore, in the presence of monomer droplets, the concentration of the monomer in the polymer particles reaches a maximum value. As discussed below (see Section 6.4.1), this saturation value arises from the energy (interfadal tension) needed to increase the surface area of the polymer partides upon swelling. Conse-... 

Equation (2.5) explains the temperature dependency of the interfadal tension as the change in entropy during the formation of surface. Increasing temperatures are combined with rising mobility of molecules. Thus, the work of forming new surface area becomes lower and the interfadal tension is decreased at elevated process temperatures. 

Ctotai is the total surfactant concentration in the emulsion, Vw is the volume of the continuous water phase, Cb the concentration of free surfactant molecules in the bulk phase. At the total interfadal area in the emulsion and Hthe adsorption (surfactant molecules per surface area). Combining Eq. (2) with the Langmuir adsorption isotherm Eq. (3) leads to Eq. (4) with b being the adsorption coefficient. 

A special example of secondary bonding is the repair of an existing structure. Once the damaged material has been removed, the structure s surface area to be bonded is clean (if proper precautions are taken), activation is desired to promote chemical bonds. Silane coupling agents (discussed above) are one successful means to form interfadal chemical bonds. 

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