Suspensions, flow

Wet screening is being replaced in many laboratories by instmments that use a photosensor to record the diameter of each particle in a water suspension as that suspension flows past the sensor. 

Reverse Osmosis. In reverse osmosis (qv), a solution or suspension flows under pressure through a membrane the product is withdrawn on the other side. This process can treat dissolved soHds concentrations ranging from 1 mg/L to 35 g/L (14). The principal constraint is the requirement that the waste material be relatively nonfouling. Recent advances have been mosdy in membrane development, and pilot studies are required (15). Energy costs can be significant, and it is frequently necessary to pretreat influent in order to minimize fouhng. Reverse osmosis can deal with particles < 1 to 600 nm in size. 

Shirato, Gotoh, Osasa, and Usami [J. Chem. Eng. Japan, 1, 164— 167 (January 1968)] present a method for determining the mass flow rate of suspended sohds in a liqiiid stream wherein the liquid velocity is measured By an electromagnetic flowmeter and the flow of sohds is calculated from the pressure drops across each of two vertical sections of pipe of different diameter through which the suspension flows in series. 

In the case of multiparticle blockage, as the suspension flows through the medium, the capillary walls of the pores are gradually covered by a uniform layer of particles. This particle layer continues to build up due to mechanical impaction, particle interception and physical adsorption of particles. As the process continues, the available flow area of the pores decreases. Denoting as the ratio of accumulated cake on the inside pore walls to the volume of filtrate recovered, and applying the Hagen-Poiseuille equation, the rate of filtration (per unit area of filter medium) at the start of the process is ... 

Air injection may sometimes be an alternative to deflocculation. In general, defloccu-lated suspensions flow more readily but they tend to give much more highly consolidated sediments which can be difficult to resuspend on starting up following a shutdown. Furthermore, deflocculants are expensive and may adversely affect the suitability of the solids for subsequent use. 

For a given suspension rheology and flow rate there is a critical permeability of the filter, below which no cake will be formed. The model also suggests that the equilibrium cake thickness can be precisely controlled by an appropriate choice of suspension flow rate and filter permeability. 

For the general purpose of minimizing air flow, transport velocity, wear and power, the fluidized dense-phase mode of flow is preferred for long-distance applications. Efficient blow tank feeders, rotary-screw compressors, refrigerated dryers and stepped-diameter pipelines also are recommended. For products that are not suited to fluidized dense-phase, the possible modes of flow include dilute-phase (suspension flow) or bypass conveying (Wypych, 1995a). 

Raczynski [9] replaced the beaters for pulping nitrocellulose by disk mills (Fig. 157). A nitrocellulose water suspension flows down from a tank (7) through... 

Dilute-phase (suspension flow) solids mass flow = 6th-1, air mass flow =0.260kgs-1, solids loading = 6, pipeline pressure drop = 30 kPa, transport velocity = 19.3-25.0 ms-1. 

Thus, the sub-total pressure drop due to the suspension flow passing through the accelerating tube is written as... 

Kitron, A., Elperin, T. and Tamir, A. (1990). Montte-Carlo simulation of gas-solid suspension flows in impinging streams. Inter. J. Multiphase Flow, 16 1-17. 

Wu, Gao an and Wu, Yuan (1997). Resistance of impinging stream contactor to solid-air suspension flow. Chinese J. Chem. Eng., 5(3) 270-279. 

In general, at least six mechanisms, which are not independent of each other, contribute to turbulence modulation in gas-solid suspension flows ... 

To estimate the particle migration velocity, it is assumed that (1) particles are spherical and have the same size (2) all particles are charged to the same extent (3) the particle motion is governed by the Coulomb force and the Stokes drag only and (4) the direction of the applied electric field is perpendicular to the direction of the suspension flow. 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

The collection of particles is achieved in a countercurrent flow between the water droplets and the particulates. In a cyclonic scrubber, water is injected into the cyclone chamber from sprayers located along the central axis, as shown in Fig. 7.19. The water droplets capture particles mainly in the cross-flow motion and are thrown to the wall by centrifugal force, forming a layer of slurry flow moving downward to the outlet at the bottom of the cyclone. Another type of scrubber employs a venturi, as shown in Fig. 7.20. The velocity of the gas-solid suspension flow is accelerated to a maximum value at the venturi throat. The inlet of the water spray is located just before the venturi throat so that the maximum difference in velocity between droplets and particles is obtained to achieve higher collection efficiency by inertial impaction. A venturi scrubber is usually operated with a particle collector such as a settling chamber or cyclone for slurry collection. 

A model for wet scrubbing in a cross-flow is illustrated in Fig. 7.21. Consider a rectangular scrubbing domain of length L, height H, and width of unity in Cartesian coordinates. Assume that the gas-solid suspension flow is moving horizontally, and that the solid particles are spherical and of uniform size. The particle concentration across any plane perpendicular to the flow is assumed to be uniform. The water droplets fall vertically and are uniformly distributed in the flow system. 

For the vertical countercurrent scrubbing system shown in Fig. 7.22, the gas-solid suspension flows upward from the bottom of the chamber and the water droplets fall downward. It is assumed that both the gas-solid suspension velocity and droplet velocity are unchanged in the process of scrubbing. It is also assumed that the no-slip condition exists between the particles and gas. The particle concentration across any horizontal plane is assumed to be uniform. 

Solids can flow in either moving bed mode or dilute suspension mode. Solids stresses among particles and between particle and pipe wall are considered in the moving bed flows but neglected in the dilute suspension flows. 

Equation (8.66) is directly applicable to the suspension flow mode of solids transport. For a moving bed flow, Eq. (8.66) can be simplified to... 

As mentioned, the flow rate in a standpipe depends on the solid feed device as well as the flow control valve. In this section, we discuss the gas-solid flows in a simple standpipe system where the feed device is a mass flow hopper and the solid flow regulator is a discharge orifice [Chen et al., 1984]. As shown in Fig. 8.15, the entrance of the vertical standpipe is connected to a conical hopper feeder of half angle solids flow patterns are considered. One is a dilute suspension flow, and the other is a solid moving bed. In this case, the following additional assumptions are needed ... 

The momentum equation of solids for suspension flow is given by... 

The pressure drop in the overflow and underflow standpipes can be predicted using Eqs. (8.64) through (8.67) considering either the moving bed flow or the suspension flow. The pressure drops across the fluidized bed and grid or distributor can be evaluated, respectively, by Eqs. (9.7) and (P9.7) given in Chapter 9. 

Figure 11.2. Flow regimes for horizontal conveying of solids (a) Dilute suspension flow ...

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