Fluid solids unit

Fig. 4. Multiphase fluid and fluid—solids reactors (a) bubble column, (b) spray column, (c) slurry reactor and auxiUaries, (d) fluidization unit, (e) gas—bquid—sobd fluidized reactor, (f) rotary kiln, and (g) traveling grate or belt drier.

Multiple Phases Reaclions between gas/liquid, liquid/liquid, and fluid/solid phases are often tested in CSTRs. Other laboratoiy types are suggested by the commercial units depicted in appropriate sketches in Sec. 23. Liquids can be reacted with gases of low solubili-... 

Filtration is the separation of a fluid-solids mixture involving passage of most of the fluidthrough a porous barrier which retains most of the solid particulates contained in the mixture. This subsec tion deals only with the filtration of solids from liquids gas filtration is treated in Sec. 17. Filtration is the term for the unit operation. A filter is a piece of unit-operations equipment by which filtration is performed. The filter medium or septum is the barrier that lets the liquid pass while retaining most of the solids it may be a screen, cloth, paper, or bed of solids. The hquid that passes through the filter medium is called the filtrate. 

G, G> Mass flowrate per unit cross sectional area of fluid, solid kg/m2s ML-2 -1... 

Some test data are shown in Table 3.2 where alumina is compared to PTFE and stainless steel 316. Since corrosion is predominantly initiated at the fluid-solid interfaee, it seems plausible to compare the corrosion results on a per unit surface area basis. In the above example, from the data of weight change per unit area per day or % weight change per unit area per day. 

Based on unit mass of solid in fluid-solid systems,... 

Axial dispersion. An axial (longitudinal) dispersion coefficient may be defined by analogy with Boussinesq s concept of eddy viscosity ". Thus both molecular diffusion and eddy diffusion due to local turbulence contribute to the overall dispersion coefficient or effective diffusivity in the direction of flow for the bed of solid. The moles of fluid per unit area and unit time an element of length 8z entering by longitudinal diffusion will be - D L (dY/dz)t, where D L is now the dispersion coefficient in the axial direction and has units ML T- (since the concentration gradient has units NM L ). The amount leaving the element will be -D l (dY/dz)2 + S2. The material balance equation will therefore be ... 

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiliary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular solids. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the solids become suspended in the air and move freely through the bed. 

He considered, in his own words, somewhat in detail, Boussinesq s problem of the steady passage of heat from a good conductor immersed in a stream of fluid moving (at a distance from the solid) with velocity v. The fluid is treated as incompressible and, for the present as inviscid, while the solid has always the same shape and presentation to the stream. In these circumstances the total heat, Q, passing in unit time is a function of the linear dimension of the solid, 1, the temperature difference, AT the stream velocity, v, the capacity for heat of the fluid per unit volume,... 

Many reactors fall in the classification of fluid-solid catalytic units where the catalyst may be retained in a fixed-bed position in the reactor with the reactant flowing through the catalyst bed, or the unit may be operated as a fluidized-bed reactor with the catalyst particles being suspended in the flowing fluid due to motion of the fluid. A third type of reactor is one in which the catalyst particles fall slowly through the fluid by gravity in the form of a so-called moving bed. 

The question remains as to when the various diffusion effects really influence the conversion rate in fluid-solid reactions. Many criteria have been developed in the past for the determination of the absence of diffusion resistance. In using the many criteria no more information is required than the diffusion coefficient DA for fluid phase diffusion and for internal diffusion in a porous pellet, the heat of reaction and the physical properties of the gas and the solid or catalyst, together with an experimental value of the observed global reaction rate (R ) per unit volume or weight of solid or catalyst. For the time being the following criteria are recommended. Note that intraparticle criteria are discussed in much greater detail in Chapter 6. 

By n ,j is denoted the unit vector normal to the fluid-solid interface pointing from fluid to solid while A s reperesents the area between the fluid-solid phases. 

Judging from its units, the convection heat transfer coefficient li can be defined as the rate of heat Iran.ifer bet veen a solid surface and a fluid per unit siitface area per unit temperature difference. 

The weight of catalyst in a vessel is determined by measuring the pressure differential between taps installed at the top and bottom. Density of the fluidized catalyst is determined in a similar manner from the differential pressure between taps located a measured distance apart in the dense phase. Location of the catalyst level can be determined from the combination of the density and the total weight of catalyst, or by the use of a series of pressure taps placed at intervals along the height of the vessel. A hot-wire probe has been used to locate the level in laboratory fluidized beds (250), but this technique has not been adopted for fluid cracking units. The method depends upon the fact that heat-transfer rate from the heated wire is much higher when immersed in the dense phase of fluidized solids than when in the dilute phase. 

Among the unit operations, adsorption may be considered a prototype for all fluid-solid separation operations. When it is conducted under countercurrent conditions, the calculation methods required are entirely analogous to those for countercurrent absorption or extraction (H3). Often, however, it is most economical to conduct adsorption in a semi continuous arrangement, in which the solid phase is present as a fixed bed of granular particles. The fluid phase passes through the interstices of this bed at a constant flow rate and for an extended period of time. The concentration gradients in the fluid and solid phases display a transient or unsteady-state behavior, and their evolution depends upon the pertinent material balances, rates, and equilibria. 

The bounce-back collision typically employed at fluid-solid boundaries, where fluid particles are turned back in the direction they came from following collision with a solid wall, causes the effective wall position to extend one half lattice unit into the fluid from the solid surface (Stockman et al., 1997). This is not a serious problem for velocity computations in slow flows, but has the potential to be a significant problem for tracer/dispersion simulations. Increasing the number of lattice points inside a flow channel can reduce this error, but is computationally very expensive. 

The case of = 1 is a reasonable approximation for a great variety of cases while = 0 covers another common situation where the reaction rate is limited by the disengagement of molecules from the surface. The rate kRp, has its usual interpretation as moles formed per unit volume of reactor per unit time when A, is the surface area of the fluid-solid interface per unit volume of reactor. For single-particle experiments, Ai will be the surface area and will be in moles reacted per unit time. 

A porous medium consists of a packed bed of solid particles in which the fluid in the pores between particles is free to move. The superficial fluid velocity V is defined as the volumetric flow rate of the fluid per unit of cross-sectional area normal to the motion. It is the imbalance between the pressure gradient (VP) and the hydrostatic pressure gradient (pg) that drives the fluid motion. The relation that includes both viscous and inertial effects is the Forscheimer equation [47]... 

Fig. 6.2-30 Two vacuum batch fluid-bed units with closed loop and solvent recovery systems (courtesy Glatt, Binzen, Germany) Fig. 6.2-31 Diagrams of the principle and outline of a fluid-bed granulator with solid rotating bottom plate (courtesy Glatt, Binzen, Germany)...

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