Decantation Phase Splitting

One of the most cmcial problems of extraction processes is the splitting of the phases after completion of the inteifacial mass transfer. Phase splitting is very diffi- 

In particular, the mechanisms effective during coalescence are not known sufficiently yet. Hence, small-scale experiments are often required. As even small concentrations of contaminants have a dominant effect on drop coalescence those experiments have to be conducted with the original liquids. 

Two experimental results of phase splitting by decantation are presented in Fig. 6.3-7. In the system butanol/water with small interfacial tension the process of 

The influence of phase ratio on phase sphtting is shown in Fig. 6.3-8 with the systems butanol/water and tetralin/water. Obviously, the phase ratio has little influence on phase sphtting in the system tetralin/water (intermediate interfacial tension). In the system butanol/water, however, the time required for complete phase sphtting 

The influence of traces of contaminants on phase splitting is demonstrated in Fig. 6.3-9. The pretreatment of the water, either by a fresh or a used deionization agent, changes the phase splitting time by two orders of magnitude. The effect of surface active contaminants (sodium lauryl sulfate) is even larger as the experiments with MIBK/water clearly prove. 

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One extremely powerful feature of heterogeneous distillation is the ability to cross distillation boundaries. It was noted previously that distillation boundaries divide the compositions into two regions that cannot be accessed from each other. Decanters allow distillation boundaries to be crossed, as illustrated in Figure 12.32. The feed to the decanter at F is on one side of the distillation boundary. This splits in the decanter to two-liquid phases E and R. These two-liquid phases are now on opposite sides of the distillation boundary. Phase splitting in this way is not constrained by a distillation boundary, and exploiting a two-phase separation in this way is an extremely effective way to cross distillation boundaries. 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

For the synthesis of heterogeneous batch distillation the liquid-liquid envelope at the decanter temperature is considered in addition to the residue curve map. Therefore, the binary interaction parameters used in predicting liquid-liquid equilibrium are estimated from binary heterogeneous azeotrope or liquid-liquid equilibrium data [8,10], Table 3 shows the calculated purity of original components in each phase split at 25 °C for all heterogeneous azeotropes reported in Table 1. The thermodynamic models and binary coefficients used in the calculation of the liquid-liquid-vapour equilibrium, liquid-liquid equilibrium at 25 °C and the separatrices are reported in Table 2. 

As stated by Rodriguez-Donis et al. [6], the reflux policy to be used is strongly influenced by the split ratio liquid reflux needed at the top of the column is lower than Zr, then the distillation can be performed by using only the reflux of entrainer-rich phase. Otherwise, the separation of original components requires the reflux of a combination of both decanted phases. 

MESH) equations which are solved for the whole column, decanter included and taking into account the liquid-liquid phase split. Numerical treatment of the Differential Algebraic Equation (DAE) system and discrete events handling is performed with DISCo, a numerical package for hybrid systems with a DAE solver based on Gear s method. The column technical features and operating conditions are shown in Table 4. A sequence of two operational batch steps, namely... 

After heating at about 65 °C the esterification starts in the reactor R-l, which can be a CSTR, but preferably a PFR or a combination PFR/CSTR. The first reactor should ensure a conversion slightly above 90%. Intermediate removal of glycerol takes place to shift the equilibrium and get lower content of monoglyceride. A simple phase split by decanting can be applied at temperatures of 40 to 60 °C. The decanting time could be very variable, between a few minutes and 1 h. The presence of soaps and monoglycerides hinders the phase separation, while more neutral pH and lower methanol content helps. Modern coalescence separators can ensure a relatively smooth separation if the amount of soap is not excessive. 

Since the decanter forms part of the reflux loop it is important to keep the distillate s residence time in it as small as possible in batch distillation. As Fig. 7.1 shows, the droplet size generated by azeotropic distillation including condensation and subsequent cooling processes is very small and without accelerated coalescing an undesirably large decanter is needed to get the maximum phase split. 

The first tower in Figure 11.44 gives the ternary azeotrope as an overhead vapour, and nearly pure ethanol as bottom product. The ternary azeotrope is condensed and splits into two liquid phases in the decanter. The benzene-rich phase from the decanter serves as reflux, while the water-ethanol-rich phase passes to two towers, one for benzene recovery and the other for water removal. The azeotropic overheads from these successive towers are returned to appropriate points in the primary tower. 

One of the key thermodynamic parameter in heterogeneous batch distillation is the decanter split ratio liquid-liquid tie line at the decanter temperature or alternatively by the mole ratio of the entrainer-rich phase ZR to the overall liquid phase Z° into the decanter as follows ... 

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. 

Flash2 - rigorous vapor-liquid split or vapor liquid liquid split FlashS - rigorous vapor-liquid-liquid split Decanter - separate two liquid phases Sep - use split fractions... 

It may be observed that the ternary azeotrope m falls Inside the heterogeneous region. Thus, an overhead vapour of this composition splits by decantation in two phases, one rich in entrainer o other in water w the ethanol being distributed in both. Moreover, o, and w, are in different distillation regions. By clever mixing with other streams, these streams can produce feasible feeds for ethanol and water recovery columns, by overcoming the constraints of the distillation boundaries. Hence, liquid-liquid decantation creates opportunities for the separation of an azeotropic mixture. 

The liquid streams from the separator and the bottom of the absorber are combined and fed into a distillation column. The bottoms from the column is split into two streams absorber lean oil and recycle acetic acid. The overhead vapor condenses into two liquid phases because of the nonideality of the phase equilibrium. The aqueous phase from the decanter is removed as product and sent to further processing, which we do not consider here. Some of the organic phase (mostly vinyl acetate) is refluxed back to the column, and some is removed for further processing. 

Because the split of the condensate from the tops of both columns is, if anything, improved by being done at a high temperature, no cooler is needed between the condenser and the decanter. The density difference between the butanol-rich phase at 0.85 and the water-rich phase at 0.99 is large enough to allow operations with a modest-sized decanter. 

From the ternary feed pure acetone is recovered as overhead fraction. Below the feed point the hquid within the column enriches in 1-butanol and, in turn, splits into two liquid phases. The aqueous phase with very low 1-butanol content is removed in a decanter, which is externally arranged. The organic phase is recycled into the column to recover pure 1-butanol as bottoms. 

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