Bulk Transport of Fluids

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. 

Convection involves the transfer of heat by means of a fluid, including gases and liquids. Typically, convection describes heat transfer from a solid surface to an adjacent fluid, but it can also describe the bulk movement of fluid and the associate transport of heat energy, as in the case of a hot, rising gas. Recall that there are two general types of convection forced convection and natural (free) convection. In the former, fluid is forced past an object by mechanical means, such as a pump or a fan, whereas the latter describes the free motion of fluid elements due primarily to density differences. It is common for both types of convection to occur simultaneously in what is termed mixed convection. In such instance, a modified form of Fourier s Law is applied, called Newton s Law of Cooling, where the thermal conductivity is replaced with what is called the heat transfer coefficient, h ... 

The bulk motion of fluid is common throughout the environment this advective motion is described mathematically by the direction and the magnitude of its velocity. If a chemical is introduced into flowing air or water, the chemical is transported at the same velocity as the fluid. While spreading due to Fickian transport may occur at the same time, as described in the next section, the center of mass of the chemical moves by advection at the average fluid velocity. 

The movement of solute molecules across the capillary wall occurs primarily by two mechanisms diffusion and solvent drag. Diffusion is the passive mechanism of transport that rapidly and efficiently transports small solutes over the small distances (tens of microns) between the blood supply (capillaries) and tissue cells. Solvent drag refers to the movement of solute that is entrained in the bulk flow of fluid across the capillary wall and is generally neghgible, except in cases of large molecules with small diffusivities and high transcapiUary fluid flow. 

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. 

In the concrete-like composites, the stresses are transferred from one phase to another through the interface and it should be accepted that the ITZ may be weaker or stronger, and that its structure is always different from that of the bulk matrix. The flow along and across an interface is more intensive than in other phases, because the interface layers have a lower density and may be more penetrable by fluids and gases. It is believed that the ITZ participates in the determination of the overall permeability of the material. Therefore, the influence of the ITZ properties on the mechanical behaviour of the material and on the transport of fluids and gases was studied by many researchers, even though its importance was estimated differently. The problem is, to what extent is the ITZ different from the bulk matrix and how large it is. 

This approach was continued in a series of internal reports one of which,issued in 1973 (38), explored the conditions governing the formation of the two types of boundary separating the full film from the cavitated regions within a hydro-dynamic or a hydrostatic bearing. These were known as the reformation and cavitation boundaries respectively. The former has both form and position and depends on the transportation of fluid in bulk. Cavitation boundaries on the other hand are essentially special, are dependent on pressure and have no intrinsic velocity. 

Standard textbooks on transport phenomena (Bird et oL, 2002) and fluid mechanics provide the derivation of this equation and a detailed interpretation of each term. Certain types of conditions are commonly encountered during bulk motion of fluids in separators these in turn allow some assumptions to be made that simpliiy the equation of motion. The three most common assumptions are ... 

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 general, the overall reaction process may comprise several individual steps, as shown in Figure 3.24. It could be seen that these steps pertain to (i) mass transfers of reactants and the products between the bulk of the fluid and the external surface of the solids (ii) transport of reactants and the products within the pores of the solid and (iii) chemical reaction between the reactants in the fluid and those in the solid. In order to be able to determine the rate-controlling step and to ascertain whether more than a single step should be consid-... 

Biofilms adhere to surfaces, hence in nearly all systems of interest, whether a medical device or geological media, transport of mass from bulk fluid to the biofilm-fluid interface is impacted by the velocity field [24, 25]. Coupling of the velocity field to mass transport is a fundamental aspect of mass conservation [2]. The concentration of a species c(r,t) satisfies the advection diffusion equation... 

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