Slurry Flows

The flow of solids suspended in a liquid stream is encountered in some evaporator applications. The characteristics of the flow depend upon the physical properties and relative amounts of the solid and liquid and upon the shape and particle size distribution of the solid being transported. 

Flow of solid-liquid mixtures (slurries) in pipes differs from the flow of homogeneous liquid in several ways. With liquids, the flow regime is defined by the physical properties of the fluid and piping the complete range of velocities is possible. With slurries, two additional distinct flow regimes and physical properties are superimposed on the liquid system. 

Critical velocity of a slurry in horizontal pipes is the fluid velocity below which the solids begin to deposit at the bottom of the pipe. In vertical pipes, solids that would be deposited if the pipe were horizontal are easily transported because their settling velocity is usually much lower than normal flow velocities. In general  

The concentration of solids in a slurry system is usually expressed as the ratio of the weight of solids per weight of liquid (also weight percent). Concentration may also be expressed as volume percent, the two concentrations being related by solid and liquid specific gravities. Solids concentration in the slurry is an important parameter it may affect the critical velocity and pressure drop. 

The abrasive nature of slurries is a major consideration in design and selection of equipment for the slurry system. Proper selection and layout of major components of the system — pumps, pipe, valves and fittings, and instrumentation — are important for the proper functioning of the system. 

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The two procedures primarily used for continuous nitration are the semicontinuous method developed by Bofors-Nobel Chematur of Sweden and the continuous method of Hercules Powder Co. in the United States. The latter process, which uses a multiple cascade system for nitration and a continuous wringing operation, increases safety, reduces the personnel involved, provides a substantial reduction in pollutants, and increases the uniformity of the product. The cellulose is automatically and continuously fed into the first of a series of pots at a controlled rate. It falls into the slurry of acid and nitrocellulose and is submerged immediately by a turbine-type agitator. The acid is deflvered to the pots from tanks at a rate controlled by appropriate instmmentation based on the desired acid to cellulose ratio. The slurry flows successively by gravity from the first to the last of the nitration vessels through under- and overflow weirs to ensure adequate retention time during nitration. The overflow from the last pot is fully nitrated cellulose. 

In addition to encompassing all of the unit operations in the plant, the plant flow sheets may also include materials handling operations associated with the transport and storage of materials in and around the mill. Typically, flow sheets provide quantitative information regarding water and slurry flows, toimages, and assays. 

Fig. 24. Flow meters for on-line measurement of slurry flow rates (a) magnetic and (b) ultrasonic (6).

FIG. 6-32 Flow pattern regimes and pressure gradients in horizontal slurry flow. (Ftom Govier and Aziz, The Flow of Complex Mixtures in Pipes, Van Nostrand Reinhold, New York, 1972.)... 

Continuous Cake Filters Continuous cake filters are apphcable when cake formation is fairly rapid, as in situations in which slurry flow is greater than about 5 L/min (1 to 2 gal/min), shiny concentration is greater than 1 percent, and particles are greater than 0.5 [Lm in diameter. Liquid viscosity below 0.1 Pa s (100 cP) is usually required for maintaining rapid liquid flow through the cake. Some designs of continuous filters can compromise some of these guidelines by sacrificial use of filter aid when the cake is not the desired product. 

Example 6 Slurry Sampling by Rotary Traverse of Gravity Flow Increment volume, quantity of slurry extracted hy one cutter rotation, is S from hulk slurry flow B expressed in voliime-per-imit time. R is cutter rotation per minute. D is cutter angle opening, with D/360 extraction ratio for continuous cutter rotation. 

Particle diameter is a primary variable important to many chemical engineering calculations, including settling, slurry flow, fluidized beds, packed reactors, and packed distillation towers. Unfortunately, this dimension is usually difficult or impossible to measure, because the particles are small or irregular. Consequently, chemical engineers have become familiar with the notion of equivalent diameter of a partiele, which is the diameter of a sphere that has a volume equal to that of the particle. 

Figure 2-49. Slurry flow regime (heterogeneous, homogeneous) is a function of solid s size and specific gravity. By permission, Der-annelaere, R. H. and Wasp, E. J., "Fluid Flow, Slurry Systems and Pipelines," Encyclopedia of Chemical Processing and Design, J. Mc-Ketta, Ed., M. Dekker, vol. 22,1985 [25].

Figure 2-50. Representative plot of pressure drop for slurry flow. By permission, Turian, R. M. and Yuan, T. F., Flow of Slurries in Pipelines, AI.Ch.E. Journal, vol. 23, 1977, p. 232-243.

Capacity the rate of liquid or slurry flow through a pump. This is usually expressed as gallons per minute (GPM) by pump manufacturers and design engineers in the chemical and petrochemical industries. A few convenient conversions are ... 

When the bottom wiper plug reaches the float collar there is a brief sharp rise in pump pressure. The pump pressure increase breaks the diaphragm in the lower wiper plug. Once the diaphragm is broken the spacer and the cement slurry flow through the float collar into the final joint or two of the casing... 

Sizing, safety relief, 436, 437-441 API liquid valve, 444 Balanced valves, 441 Conventional valves, 438 Critical back pressure, 440 Effects of two-phase flow, 437 Hydraulic expansion, 441 Rupture disks, 434 Sub-critical flow, 449 Slurry flow, process pipe, 142-147 Regimes, 143... 

Shook, C. A. and Roco, M. C. Slurry Flow. Principles and Practice (Butterworth-Heinemann, Oxford, 1991). 

Wilson, K. C. and Puoh, F. ]. Can. Ji. Chem. Eng. 66 (1988) 721. Dispersive-force modelling of turbulent suspensions in heterogeneous slurry flow. 

During the first period, the slurry flow is eroding the filter-cake as it is growing thus a steady state, in which the filtration occurs through a cake of constant thickness, is rapidly reached. At the same time, because the slurry is losing water but no solid particles, its density is increasing in line with the fluid loss rate. 

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