Extraction pinch

Pinched regions decanter Hquid—Hquid extraction... 

Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively nonvolatile component, the solvent, that forms no azeotropes with the other components in the mixture (23). It is widely used in the chemical and petrochemical industries for separating azeotropic, close-boiling, and other low relative volatiUty mixtures, including those forming severe tangent pinches. 

In normal applications of extractive distillation (i.e., pinched, closeboiling, or azeotropic systems), the relative volatilities between the light and heavy key components will be unity or close to unity. Assuming an ideal vapor phase and subcritical components, the relative volatility between the light and heavy keys of the desired separation can be written as the produc t of the ratios of the pure-component vapor pressures and activity-coefficient ratios whether the solvent is present or not ... 

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatihty is largely determined by the solvent concentration and hence the S/F ratio. The relative volatility tends to increase as the S/F ratio increases. Thus, a given separation can be accomplished in fewer equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

If an extended tie-line passes through the pole point P, an infinite number of stages will be needed. This condition sets the minimum flow of extraction-solvent required. It is analogous to a pinch point in distillation. 

Drain off the alcohol. Take a small pinch of the powder from the basket and allow it to dry thoroughly. When dry, taste a tiny amount of the powder. If it numbs the tongue, repeat the extraction. If not, discard the powder and proceed to stop 5. (Tasting a small amountofthe powder will not harm you, but be sure not to swallow it and rinse your mouth out several times afterwards). 

The reaction mixture is poured, with stirring, into 1 kg of a saturated sodium chloride solution (Note 5) The mixture is extracted with four 100-ml portions of low-boiling petroleum ether. The extracts are combined and dried over anhydrous potassium carbonate The liquid is decanted, the potassium carbonate is washed with 25 ml of low-boiling petroleum ether, and the solution is added to the main fraction A pinch of anhydrous potassium carbonate is added to the petroleum ether solution, and the solution is then distilled from a Claisen flask to give a colorless liquid, yield 151-169 g (80-90%) b.p. 122— 125°/25 mm, n% 1 4260, df 0 932 (Note 6). 

In a 500 ml round-bottomed flask equipped with a magnetic stirrer are placed 22 5 g (0 105 mole) of powdered sodium metaperiodate and 210 ml of water The mixture is stirred and cooled m an ice bath (Note 1), and 12 4 g (0 100 mole) of thio-anisole (Note 2) is added The reaction mixture is stirred for 15 hours at ice-bath temperature and is then filtered through a Buchner funnel The filter cake of sodium iodate is washed with three 30-ml portions of methylene chloride The wrater methylene chloride filtrate is transferred to a separatory funnel, the lower methylene chloride layer is removed, and the water layer is extracted with three 100-ml portions of methylene chloride The combined methylene chloride extracts are treated with activated carbon (Note 3) and dried over anhydrous sodium sulfate (Note 4) The solvent is removed at reduced pressure to yield 13 6-13 9 g of a slightly yellow oil (Note 5) which crystallizes on cooling The crude sulfoxide is transferred to a 25 ml distillation flask with the aid of a small amount of methylene chloride After removal of the solvent, a pinch of activated carbon is added to the distillation flask (Note 6) Simple vacuum distillation (Note 7) of the crude product through a short path still affords 12.7-12 8 g (91%) of pure methyl phenyl sulfoxide, b p 78-79° (0 1 mm ), m p. 33-34° (Notes 8 and 9)... 

Add the vanilla extract and lemon juice, pulsing on and off until the dough becomes wet and sticks together when you pinch it. 

Figure 2.14 illustrates the overall approach by pinch-point analysis. The first step is extraction of stream data from the process synthesis. This step involves the simulation of the material-balance envelope by using appropriate models for the accurate computation of enthalpy. On this basis composite curves are obtained by plotting the temperature T against the cumulative enthalpy H of streams selected for analysis, hot and cold, respectively. Two aspects should be taken into account ... 

Improvement of one outlet purity (usually the extract) Even if there is theoretically no need to add a fifth zone for binary separation, in practice, we must deal with real systems and two effects may prevent the system from reaching extreme purities (i.e., greater than 99.5%). The first effect is related to competitive adsorption In some cases, for low concentration of one of the two species compared to the other, the selectivity disappears or may even be reversed this is equivalent to a pinch or an azeotrope in distillation. The... 

As previously mentioned, the SFE pump should produce a constant pressure of supercritical fluid with a rate controlled by a flow restrictor after the extraction vessel. There are a number of types of flow control devices, including a capillary made from fused silica, a pinched stainless steel tube, or a variable orifice allowing for electronic control of the pressure. 

As a consequence of these restrictions, separation of binaiy mixtures by extractive distillation corresponds to onfy two possible three-component distillation region diagrams, depending on whether the binary mixture is pinched or close-boiling (DRD 001), or forms a minimumboiling azeotrope (DRD 003). The addition of high-boiling solvents... 

Acetic acid-water-vinyl acetate Pinched, azeotropic system Self-entraining Element of recoveiy system for alternative to production of methyl acetate by reactive distillation alternative to extractive pressureswing distillation... 

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