Underflow entrainment

In the production of sodium hydroxide by the lime soda process, a slurry of calcium carbonate particles in a dilute sodium hydroxide solution results. A four-stage countercurrent washing system is used. The underflow entrains approximately 3 kg liquid/kg dry calcium carbonate solids. The inlet water is pure water. If 8 kg wash water/kg dry calcium carbonate solids is used, predict the recovery of NaOH in the wash liquor. 

With reference to Fig. 18-81 (both graphs), EF represents the locus of overflow compositions for the case in which the overflow stream contains no inert sohds. E F represents the overflow streams containing some inert sohds, either by entrainment or by partial solubihty in the overflow solution. Lines GF, GL, and GM represent the loci of underflow compositions for the three different conditions indicated on the diagram. In Fig. 18-81 7, the constant underflow line GM is par-aUel to EF, the hypotenuse of the triangle, whereas GF passes through... 

Leaching is the removal of solutes from admixture with a solid by contracting it with a solvent. The solution phase sometimes is called the overflow, but here it will be called extract. The term underflow or raffinate is applied to the solid phase plus its entrained or occluded solution. 

Downcomer vapor underflow ("vapor entrainment" or "gas recycle1 ) is analogous to liquid entrainment. It reduces both tray capacity and efficiency (17,44,45). In low- and medium-pressure distillation systems, where gas density is significantly lower than liquid density, it takes only a small quantity of gas to generate volumes comparable to the liquid volumetric flow rate. The quantity of gas recycle is therefore small, and it has little effect on tray performance. At high pressures, the quantity of gas recycled is significant. An analysis of some FRI data (44) for iC4-nC4 distillation showed vapor entrainment increases from about 7 percent at 165 psia to about 50 to 60 percent at 400 psia on a molar basis. 

Equation (6.59) was derived from the orifice equation with an orifice coefficient of 0.6 (3), and assuming pure liquid is passing under the downcomer. Tests by Lockett and Gharani (43) showed that Eq. (6.59 gives conservative predictions, even under conditions of significant vapor entrainment in the downcomer underflow (Sec. 6.2.3). 

Equilibrium relations in leaching usually are simpler than in liquid-liquid equilibria, or perhaps only appear so because few measurements have been published. The solution phase normally contains no entrained solids so its composition appears on the hypotenuse of a triangular diagram like that of Example 14.9. Data for the raffinate phase may be measured as the holdup of solution by the solid, K lb solution/lb dry (oil-free) solid, as a function of the concentration of the solution, y lb oMb solution. The corresponding weight fraction of oil in the raffinate or underflow is... 

As a final step, the binary ethanol-water underflow from the second column may be opportunistically separated to produce a pure water underflow and a composition close to the original feed to which it also may be recycled. The amount of extractive entrainer must now be readjusted because of the increased amount of feed to the first column, but iteration shows that the flowsheet structure generated remains unchanged and has converged. The tasks specified accomplish the composition goals, producing pure water and pure ethanol. The entrainer remains totally within the system. The subsequent integration into equipment involves three columns and one decanter (Fig. 29). 

Boom failure — The failure of a containment boom to contain oil due to excessive winds, waves, or currents or improper deployment. Boom failure may be manifested in oil underflow, oil splashover, submergence or planing of the boom, or structural breakage. (See also Critical Velocity, Entrainment Failure.)... 

Drop settles and coalesces but is re-entrained] faulty location of exit nozzles for liquid phases/distance between exit nozzle and interface is < 0.2 m/overflow baffle corroded and faUure/interface level at the wrong location/faulty control of interface/liquid exit velocities too high/vortex breaker missing or faulty on underflow line/no syphon break on underflow line/liquid exit velocities too high. [Drop settles but doesn t coalesce [phase inversion] /pH far from zpc/surfactants, particulates or polymers present/electrolyte concentration in the continuous phase < expected/[coalescer pads ineffective] /[drop size decrease] /[secondary haze forms] /[stable emulsion formation] /[interfacial tension too low] /[Maran-goni effect]. ... 

Drop settles and coalesces but is re-entrained] faulty location of exit nozzles for liquid phases/distance between exit nozzle and interface is < 0.2 m/overflow baffle corroded and failure/interface level at the wrong location/faulty control of inter-face/liquid exit velodties too high/vortex breaker missing or faulty on underflow line/no syphon break on underflow line/liquid exit velodties too high. 

Often the specifications will give the flow rate of dry solids or the flow rate of wet solids. The underflow liquid flow rate can be calculated from the volume of liquid entrained with the solids. Let e be the porosity (void fraction) of the solids in the underflow. That is,... 

Note that we could have just assumed that overflow and underflow rates are constant and derived Eq. (14-6). However, it is much more informative to show the three assunptions required to make overflow and underflow rates constant. These assunptions show that this analysis for washing is likely to be invalid if the settlers are not removing all tire solid, if for some reason the amount of liquid entrained changes, or if the fluid density changes markedly. The first two problems will not occur in well-designed systems. The third is easy to check with density data. 

A countercurrent cascade for leaching is shown in Figure 14-3A. We will consider the (idealized) case where entrainment of liquid with the solid underflow can be ignored. The assumptions are ... 

In the previous analysis, assunptions 4 and 7 are often faulty. There is always entrainment of liquid in the underflow (for the same reason that there is an underflow liquid in washing). Since diffusion in solids is... 

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