Fluids, bulk transport

Solution We suppose that the mass transfer and diffusion steps are fast compared with bulk transport by convection. This is the design intent for ion-exchange columns. The reaction front moves through the bed at a speed dependent only on the supply of fluid-phase reactants. Assuming piston... 

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

The conveying of coal by fluid media through the pipeline has been proposed as a potential method of bulk transport. 

The assessment of the role of kf during protein adsorption in a fluidized bed may be performed with the help of a dimensionless transport number. Slater used the correlations provided by Rodrigues to simulate film transport limited adsorption of small ions to fluidized resins [54], In this study dimensionless groups were used to describe the influence of the system parameters particle side transport, liquid dispersion, and fluid side transport. Dispersion was accounted for by the column Peclet number analogous to Bo as introduced above and mass transport from the bulk solution to the resin was summarized in a fluid side transport number NL. 

In the two-film model (Fig. 9.4), it is assumed that all of the resistance to mass transfer is concentrated in thin films adjacent to the phase interface, and that transfer occurs within these films by steady-state molecular diffusion alone. Outside the films, in the fluid bulk phases, the level of mixing is so high that there is no composition gradient at all. This means that in the film region we have one-dimensional diffusion transport normal to the interface. 

Advection is the process by which material contained in a flowing fluid is transported by bulk motion of the fluid. An important example is blood flow, which delivers oxygen and nutrients to the tissues of the body. Maintaining blood flow is essential to maintaining life in higher organisms. 

Since there is no radial bulk transport of fluid between the monolith channels, each channel acts basically as a separate reactor. This may be a disadvantage for exothermic reactions. The radial heat transfer occurs only by conduction through the solid walls. Ceramic monoliths are operated at nearly adiabatic conditions due to their low thermal conductivities. However, in gas-liquid reactions, due to the high heat capacity of the liquid, an external heat exchanger will be sufficient to control the reactor temperature. Also, metallic monoliths with high heal conduction in the solid material can exhibit higher radial heat transfer. 

In catalytic gas-solid reactions, the reaction takes place at catalytic sites on the surface of the solid. To obtain appreciable reaction rates, porous solids are used and the reactions take place on the surface of the pores in the interior of the particle. Hence, catalytic gas-solid reactions involve seven steps (1) transport of the reactant from the fluid bulk to the mouth of the pore, (2) diffusion of the reactant to the interior of the pore, (3) adsorption of the reactant to the surface of the solid,... 

This brief overview of sohd catalysts addresses only the nature, class, and catalytic properties of a variety of catalysts. Modeling the kinetics of a reaction occurring on a solid surface is a challenging task and is firmly rooted in the principles of surface science. As this is still an evolving area, empirical shortcuts are often invoked. Furthermore, for sohd catalysts, the reactant(s) must first diffuse into the solid, and product(s) must diffuse out of it. Also, the heat evolved or required must be transported between the solid and the fluid bulk. Hence, diffusion accompanied by reaction becomes a major consideration. These microenvironmental aspects of sohd catalysts are briefly described below. 

The dynamic behavior of ionic liquids is important for both practical and theoretical reasons. From a practical standpoint, bulk transport properties such as the viscosity, self-diffUsivity, thermal conductivity and electrical conductivity govern the effectiveness of these liquids in any application. For example, mass transfer of reactants and products is critical to the performance of ionic liquid solvents, and is highly correlated with the self-difiiisivity and viscosity. Viscosity also plays a role in the cost of pumping the liquid and its performance as a lubricant. Thermal conductivity is a key parameter for thermal fluid applications, and electrical conductivity is obviously important in electrochemical applications. 

In addition, it is important to recognize that catalysis by solids involves diffusion of reactants from fluid bulk to the catalyst surface, as well as diffusion within the solid matrix. The latter invariably occurs simultaneously with the reaction, and the former is usually (but not necessarily rigorously) treated as an independent precursor to it. Thus any analysis of catalysis by solids is based on understanding its action under the physical influence of the microenvironment in which it functions. Catalysis by solids actually occurs on the catalyst surface and constitutes the surface field problem, which is the core of its action. Diffusion of reactant within its matrix is an internal or intraparticle field problem, and its transport from bulk to surface is an external or interphase field problem. 

Solid catalysts by their very nature involve diffusion of reactant fluids within their matrix. These fluids react even as they diffuse. Thus the problem of internal diffusion accompanied by reaction becomes important. Another problem of equal importance is the transport of the reactant from the fluid bulk to the catalyst surface—often referred to as external diffusion. Together these constitute the microenvironment of the catalyst pellet and form the subject matter of this chapter. 

The conventional mode of surface transportation of fluids before the advent of CCP was by sea, rail, or road transport. These modes are still in use for petrol, diesel, LPG, and several hydrocarbons however, wherever bulk transportation on a continuous basis is required between two fixed locations, use of CCP is the most economical solution. The modes of transport other than CCP have the following limitations nonavailability of sufficient roads, rail tracks, and port-harbor facilities to take up the traffic load procedures and control involved in the transport operation (permits/licenses/OctroiAoll/ regional transport office, etc.) and logistics such as manpower requirement, maintenance, fuel cost, availability, weather and climate, pollution generated, safety, insurance, and security. 

They are transported back into fluid bulk. 

In Figures 23-14 and 23-16, electroosmosis is from left to right because cations in the double layer are attracted to the cathode. Superimposed on electroosmosis of the bulk fluid, electrophoresis transports cations to the right and anions to the left. At neutral or high pH, electroosmosis is faster than electrophoresis and the net flow of anions is to the right. At low pH, electroosmosis is weak and anions may flow to the left and never reach the detector. To separate anions at low pH, you can reverse the polarity to make the sample side negative and the detector side positive. 

Conduction through the pore medium (interstitial fluid, e.g., air, water) Convective transport by the pore medium Radiative transport from solid surfaces through the pore fluid Radiative transport from the solid through the solid network or bulk... 

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