Choice of an entrainer

The choice of an entrainer used to make a desired separation in an azeotropic distillation depends on the binary mixture being separated and the nonidealities of these components with the added entrainer. While several different entrainers might be used to provide a separation, the final selection may depend on the required purity of the product. If several entrainers can produce a product of desired purity, the final choice may depend on an economic evaluation of the several schemes. 

In addition, for a minimum AB azeotrope the entrainer may be 1) low boiler that forms medium-boiling maximum azeotrope with A, and 2) maximum boiler. The last case is quite special, and it is known as extractive distillation. For maximum AB azeotrope, the entrainer may be high-boiler that forms medium-boiling minimum azeotrope with B. It should be noted that is difficult to find entrainers giving opposite azeotropes with A or B with respect to the original AB azeotrope. Hence, the choice of an entrainer generating separations in one distillation field is in practice limited. 

Azeotropic distillation. The choice of an entrainer for this mixture is very limited because of the number of azeotropes formed by ethyl acetate with low-boiling hydrocarbons. 

The selection of an entrainer with boundary crossing is based on the observation that in a RCM both A and B must be nodes, stable or unstable. By consequence the pure components are separated either as overhead or bottom products. Table 3.17 gives a list of recommended heuristics [13, 14]. In all cases, tbe distillation boundary must be highly curved, although how curved cannot be specified theoretically. In this case the simplest entrainer choice is a low-boiler entrainer for minimum AB azeotrope, and a high-boiler entrainer for maximum AB azeotrope, again not easy to meet in practice. 

Data of Azeotropes. The choice of an azeotropic entrainer for a desired separation is much more restricted than that of solvents for extractive distillation, although many azeotropic data are known. The most modern and extensive compilation is that of Gmehling et al. (1994), a two-volume set that includes many ternary and some quaternary azeotropes. Probably the most available... 

Extractive distillation is based on the ability of an entrainer to increase selectively the relative volatility of components. The choice of entrainer is based on its chemical affinity with one of the components to be separated. Extractive distillation can be used to separate both zeotropic and azeotropic mixtures. Table 7.31 presents a list of industrial applications. 

The diffusion coefficient as defined by Fick s law, Eqn. (3.4-3), is a molecular parameter and is usually reported as an infinite-dilution, binary-diffusion coefficient. In mass-transfer work, it appears in the Schmidt- and in the Sherwood numbers. These two quantities, Sc and Sh, are strongly affected by pressure and whether the conditions are near the critical state of the solvent or not. As we saw before, the Schmidt and Prandtl numbers theoretically take large values as the critical point of the solvent is approached. Mass-transfer in high-pressure operations is done by extraction or leaching with a dense gas, neat or modified with an entrainer. In dense-gas extraction, the fluid of choice is carbon dioxide, hence many diffusional data relate to carbon dioxide at conditions above its critical point (73.8 bar, 31°C) In general, the order of magnitude of the diffusivity depends on the type of solvent in which diffusion occurs. Middleman [18] reports some of the following data for diffusion. 

A liquid level is maintained with an overflow weir while the vapor comes up through the perforated floor at sufficient velocity to keep most of the liquid from weeping through. Hole sizes may range from 1/8 to 1 in., but are mostly l/4-l/2in. Hole area as a percentage of the active cross section is 5-15%, commonly 10%. The precise choice of these measurements is based on considerations of pressure drop, entrainment, weeping, and mass transfer efficiency. The range of conditions over which tray operation is... 

However, increasing surfactant concentration has the drawback of reducing globule drop size and increasing the interfacial area available for mass transfer of both solute and water. This effect is enhanced in the case where the surfactant molecules themselves have an affinity for water [95,96]. If the surfactant concentration exceeds the critical micelle concentration (cmc), water transport in W/O/W systems by reversed micelles can occur [89,97]. An increase in concentration of some surfactants such as SPAN 80 also leads to an increase in the entrainment of the external phase during permeation promoted by an excess of surfactant molecules [71,98]. Miesiac et al. [99] found that in the case of penicillin G separation, the choice of surfactant could control not only the extraction rate, but also the back transfer rates of the hydrolysis products. 

Flow direction. This must be examined from two points of view. First we consider the flow direction of the reactants in relation to one another. In an entrained-bed process the reactants flow by definition in co-current flow, but in a moving-bed reactor one has a choice. The most well-known moving-bed reactor, the Lurgi-Sasol dry bottom gasifier, operates in counter-current flow, but there are a number of smaller gasifier designs that use cocurrent flow to reduce the tar make. 

Application of Equation (12.48) with the Antoine equation [Equation (12.13)] shows that steam distillation can be used to distill an organic compound at much lower temperatures than would otherwise be possible. Considering the system pressure P as constant, then the more steam introduced, the lower will be the partial and vapor pressures of the organic and thus the lower its boiling point. The other key element of the choice of water as the entrainer is that it is easily separated from the product (organic) by simple condensation followed by decanting. 

It will be seen that the boiling point of the azeotrope of water and the entrainer is lower, the lower the boiling point of the entrainer. The ideal choice will be an entrainer combining a sufficiently low azeotropic boiling point, an economically low proportion in the azeotrope and a small solubility in water. 

Extractive distillation usually is preferred over azeotropic distillation, if both methods can be used to separate the feed components. The bulk of the solvent in extractive distillation is not vaporized in each cycle, as compared to azeotropic distillation where the entrainer is recovered from the overhead vapor stream. The energy input necessary to effect separation usually is lower for extractive distillation than for azeotropic distillation. Also, an extractive distillation column can operate over a wider range of pressures than an azeotropic distillation column, because the azeotropic composition is a function of pressure. Finally, there usually is a wider choice of solvents than entrainers, thus enabling the designer to minimize added component cost. 

It is generally true that adding an extraneous substance such as an entrainer or solvent to a process is undesirable. Since it can never be completely removed, it adds an unexpected impurity to the products. There are inevitable losses (ordinarily of the order of 0.1 percent of the solvent-circulation rate), and they may be large since solvent/feed ratios must frequently be greater than 3 or 4 to be effective. An inventory and source of supply must be maintained. Solvent recovery costs can be large, and new problems in choices of materials of construction are introduced. It follows that these processes can be considered only if, despite these drawbacks, the resulting process is less costly than conventional distillation. 

Suitable entrainers from the above list are toluene, di-propyl-ether and n-propyl acetate. The first one may be skipped as it is an undesired impurity. Di-propyl-ether is suitable practically insoluble in water and low reciprocal solubility, of the order of 4%. Moreover, it is a byproduct of the reaction. The choice is n-propyl-acetate is convenient too, such as sharing the same alcohol with the fatty ester. 

The feed system is an important choice for entrained-flow gasifiers. GE and E-Gas use a coal-water slurry compared with the dry-feed systems of Shell and Future Energy, which require lock hoppering. 

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