Recovery first separation split

Table 3.6 Recovery matrix for the first separation split.

For example, the separations d,h, and d2h2 are sloppy splits with different amounts of A in the distillate, and accordingly with different recoveries. The separation of pure A at the top is represented by the split d h, which corresponds to a direct sequence . Accordingly, the separation of B/C in a second column is represented by the edge BC, on which h is the feed. Similarly, the first split in an indirect sequence , in which C is separated in bottoms and A/B at the top, is shown by the segment d"b". The locus of all splits between the above limit cases allows the regions of attainable products to be defined. 

The considerations developed so far allows setting up the final conceptual flowsheet, as displayed in Figure 11.9. After reaction and quench the off-gas is submitted to a first separation of acrylonitrile by low-temperature cooling, at 10 °C. In the decanter the liquid splits into two phases. If the acetonitrile concentration is negligible, the organic phase containing acrylonitrile can be sent directly to the first purification column (Heads). The aqueous phase is sent to the acrylonitrile recovery. The off-gas from flash is compressed at 4.5 bar and submitted to absorption in cold water of 5 °C. In this way higher acrylonitrile recovery may be achieved (over 99.8%) with reduced water consumption. 

The inspection of the superstructure illustrated by the Fig. 7.17 emphasises again the role of the first-separation step. Sharp recovery will minimise the interactions between subsystems. On the other hand, the selection of the separation techniques and the design of units should bring flexibility to prevent bottlenecks. For example, adsorption or membranes should not be used in the first split, but could be certainly considered in the synthesis of the subsystem for gas separations. 

As a second alternative, we may assume that the first split ensures the recovery of the whole aromatics fraction. By inspecting Table 3.9 three separation methods may be envisaged 1) azeotropic distillation, 2) extractive distillation and 3) liquid-liquid extraction. They are commented briefly ... 

A fraction of Ce, La. Nd and Pr derived from bastnasite or monazite is a typical feedstock in the recovery process of cerium on a commercial scale. Separation of the rare-earth elements may be achieved by splitting the mixed rare-earth elements into a cerium/lanthanum and didymium (Nd/Pr) fraction first. The cerium/lanthanum fraction may be used as a further feedstock in a second extraction stage and will yield high pure cerium and lanthanum solution respectively. Cerium can then be precipitated as. for example, an oxalate or a carbonate which may be used as precursor for cerium derivatives. 

After mixing with the entrainer C, the new feed becomes the point m,. The mixing operation is represented by the segment f,-C, the position of m, depending on the ratio entrainer/initial mixture given by the lever rule. Suppose that we would like to separate A of good purity by a first split. The distillate is described by the point d, close to the vertex of A. The bottom product is marked by the point b, collinear with d, and m,. Both d, and b, will be situated on the same residue curve. In addition, the position of b, must obey a hard constraint imposed by the distillation boundary it cannot go beyond. Thus, high purity A can be obtained, but the maximum recovery is dictated by the distillation boundary. 

In Figure 1 a simplified process scheme of the antisolvent crystallization of sodium chloride is displayed. The process is divided into three steps the crystallization, the solid-liquid separation and the antisolvent recovery or liquid-liquid separation. In the first step sodium chloride is crystallized by mixing the feed brine with an antisolvent. The crystallization is carried out at temperatures below the liquid-liquid equilibrium line in the single liquid phase area (see Figure 2). In the second step the crystals are separated from their mother liquor, e.g. by filtration or in a centrifuge. In the third and final step the antisolvent is separated from the water phase at a temperature above the liquid-liquid equilibrium line in the two liquid phase area, in which the ternary amine-water-salt system splits up into an amine and an aqueous phase. The recovered antisolvent is recycled within the process and most ideally the water phase is reused for the dissolution of crude sodium chloride. In this paper the crystallization and the liquid-liquid separation steps will be treated. 

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