Suspending power requirement

Slurry reactors. For three-phase systems the definition of conditions at which (catalyst) particles are in motion is important. Two limiting states with respect to particle behaviour can be distinguished (1) complete suspension, i.e. all particles just move, and (2) uniform suspension, i.e. the particles are evenly distributed over the whole reaction zone. The power required to reach the second state is much higher, while uniform suspension is not often necessary. Circulation of the liquid with the dissolved gas is usually sufficiently fast to provide reactants to the surface of catalyst particles if they are suspended at all. 

The properties of the fluid. The density and the viscosity of the fluid influence the power requirement for a given set of operating conditions corrosive properties of the fluid determine the acceptable materials of construction. If solid particles are suspended in the fluid, this factor dictates the amount of clearance necessary and may eliminate the possibility of using certain types of pumps. 

The just-suspended state is defined as the condition where no particle remains on the bottom of the vessel (or upper surface of the liquid) for longer than 1 to 2 s. At just-suspended conditions, all solids are in motion, but their concentration in the vessel is not uniform. There is no solid buildup in comers or behind baffles. This condition is ideal for many mass- and heat-transfer operations, including chemical reactions and dissolution of solids. At jnst-snspended conditions, the slip velocity is high, and this leads to good mass/heat-transfer rates. The precise definition of the just-suspended condition coupled with the ability to observe movement using glass or transparent tank bottoms has enabled consistent data to be collected. These data have helped with the development of reliable, semi-empirical models for predicting the just-suspended speed. Complete suspension refers to nearly complete nniformity. Power requirement for the just-suspended condition is mnch lower than for complete snspension. 

In discussing scaleup procedures, all workers report that lower stirrer speeds are needed to suspend solids in larger tanks if geometrical similarity is maintained. However, small differences in the exponents for D in the equation for can lead to large differences in power requirements, since the power per unit volume PjV varies with Some data for suspending sand in water using 45° pitched-... 

One of the earlier studies was carried out in 1971 by Charles and Charleswho investigated the feasibility of transporting coarse materials in heavy media (sand in flocculated suspensions of clay in water). They concluded that the power requirement for the transport of one million tonnes/year of solids in a 200 mm diameter pipe could be reduced by a factor of about 6 by using heavy media in place of water. KenchingtoN who also studied the transport of coarse particles in china clay suspensions, concluded that a significant proportion of the coarse particles may be suspended in the dense medium without a noticeable increase in pressure drop. Duckworth et showed that it is... 

It is possible to extend the procedure developed above to certain multiphase applications in which the power requirement has to be estimated. In the case of low-viscosity liquid/liquid systems, as encountered in solvent extraction, and for coarse solids suspended in low-viscosity liquids at low concentrations, the operation is likely to be carried out in the turbulent region. The single-phase power curves can be used in such instances with the mean density being used in both the power number and Reynolds number. However, such an approach must not be used for gas/liquid systems where predictions based on average density values can lead to gross over-estimates of the power requirement. This is considered in detail in Chapter IS. 

In the case of large stirred vessels the avoidance of settiing (index AS) is the decisive process of crystal suspending. This requires the mean specific power irrput... 

The impeller speed recommended will in general be higher than Njs, the speed required for the just suspended state estimated by the Zwietering correlation. The speed required should be based on experimental data. For quick estimates of the speed and power requirements for complete uniformity, the ratios in Table 10-2 may be applied to the estimated value of Njs. 

In hair coloring a light ash blond shade may require as Httie as 0.5—1% of intermediates, whereas a tme black may require up to about 5%. In principle, the formulator blends precursors that yield red, blue, and yellow dyes. The base in which the components are dissolved or suspended is similar to that used in simple bleaches and may include alkanolamides, various types of surfactants, thickening agents, and solvents. Removal of undesirable dyes is achieved by treating the discolored hair with a powerful reductant of the sulfite family. 

Pnettmatically Actuated Diaphragm Pumps (Fig. 10-53) These pumps require no power source other than plant compressed air. They must have a flooded suction, and the pressure is, of course, limited to the available air pressure. Because of their slow speed and large valves, they are well suited to the gentle handling of liquids for which degradation of suspended solids should be avoided. 

Much of the variation in Njs can be reduced by using Pjs, which is the power in the just-suspended state. This also gives a Better feel for the comparison of various impellers based on the energy requirement rather than speed, which has no economic relevance. 

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

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