Concentrated Staged Distillation

The acidic crude alkylbenzene flows over the head of the separator LSI (Fig. 17) through a mixer where alkali of a suitable concentration is fed into a separator (ST5) in which the high specific density alkali settles out (Fig. 18). The alkali is again fed into the mixing process. The crude LAB is pumped into an intermediate tank (T) and from there over a sodium hydroxide-containing column (DC) where it is dried before proceeding to the distillation stage. 

Automation of the manual UV method using an AutoAnalyzer method that incorporates a flash distillation stage has been described [S] and has been used routinely. This technique is unsuitable for unattended automatic operation because the flash distillation stage requires constant supervision. Attention was therefore concentrated on alternative chemical methods of measuring total nicotine alkaloids. By far the most promising of these is an AutoAnalyzer method based on the Konig reaction [6]. The mechanism of this reaction has been discussed by Roy [7] and is illustrated in Fig. 3.S. 

III rectification stage. Concentrated PMS-lb is separated in the rectification apparatus similar to the one described above. It is distilled under atmospheric pressure into two fractions the head fraction (153-193 °C) and PMS-l,5b (193-195 °C). The head fraction is added to concentrated PMS-lb for further rectification, the tank residue is added to the tank residue of stage I. 

Molar vapour flow-rate of component from stage n Mol fraction of component in liquid phase Mol fraction of component A in binary mixture Mol fraction of component B in binary mixture Mol fraction of component in bottom product Mol fraction of component in distillate Equilibrium concentration Mol fraction of component i... 

The optimal feed concentration of the pervaporation unit depends on carrier flow rate, reflux ratio and number of theoretical trays of the extractive distillation. Retentate concentration and cut rate of the pervaporation stage follow from the requested product quality xlt + x31 <0.008. For the design of the pervaporation stage, the worst case has been assumed that only benzene and no furfural (3) will pervaporate. The major factor for the cost reduction is the much lower energy consumption of the hybrid process of 1.18 t/h heating steam against 1.7 t/h for the conventional process. 

The McCabe-Thiele constructions described in Chapter 8 embody rather restrictive tenets. The assumptions of constant molal overflow in distillation and of interphase transfer of solute only in extraction seriously curtail the general utility of the method. Continued use of McCabe-Thiele procedures can be ascribed to the fact that (a) they often represent a fairly good engineering approximation and (b) sufficient thermodynamic data to justify a more accurate approach is often lacking. In the case of distillation, enthalpy-concentration data needed for making stage-to-stage enthalpy balances are often unavailable, while, in the Case of absorption or extraction, complete phase equilibrium data may not be at hand. 

Fig. 14.4-1. An idealized staged eountereurrent extraction. The feed H and extractant L flow at constant rates in the limit of the dilute solutions considered here. As in distillation, the concentrations in these streams are identified by the stage where each originates.

Sodium hydroxide solution cannot be used at this stage since it may produce benzoic acid by the Cannizzaro reaction (Section IV,123) from any unchanged benzaldehyde. If, however, the reaction mixture is diluted with 3-4 volumes of water, steam distilled to remove the unreacted benzaldehyde, the residue may then be rendered alkaline with sodium hydroxide solution. A few grams of decolourising carbon are added, the mixture boiled for several minutes, and filtered through a fluted filter paper. Upon acidifying carefully with concentrated hydrochloric acid, cinnamic acid is precipitated. This is collected, washed and purified as above. 

Water is continuously added to the last extraction bath and flows countercurrenfly to filament travel from bath to bath. Maximum solvent concentration of 15—30% is reached in the coagulation bath and maintained constant by continuously removing the solvent—water mixture for solvent recovery. Spinning solvent is generally recovered by a two-stage process in which the excess water is initially removed by distillation followed by transfer of cmde solvent to a second column where it is distilled and transferred for reuse in polymer manufacture. 

In typical processes, the gaseous effluent from the second-stage oxidation is cooled and fed to an absorber to isolate the MAA as a 20—40% aqueous solution. The MAA may then be concentrated by extraction into a suitable organic solvent such as butyl acetate, toluene, or dibutyl ketone. Azeotropic dehydration and solvent recovery, followed by fractional distillation, is used to obtain the pure product. Water, solvent, and low boiling by-products are removed in a first-stage column. The column bottoms are then fed to a second column where MAA is taken overhead. Esterification to MMA or other esters is readily achieved using acid catalysis. 

Heavy water [11105-15-0] 1 2 produced by a combination of electrolysis and catalytic exchange reactions. Some nuclear reactors (qv) require heavy water as a moderator of neutrons. Plants for the production of heavy water were built by the U.S. government during World War II. These plants, located at Trad, British Columbia, Morgantown, West Virginia, and Savaimah River, South Carolina, have been shut down except for a portion of the Savaimah River plant, which produces heavy water by a three-stage process (see Deuterium and tritium) an H2S/H2O exchange process produces 15% D2O a vacuum distillation increases the concentration to 90% D2O an electrolysis system produces 99.75% D2O (58). 

The (x, i )), values in Eq. (13-37) are minimum-reflux values, i.e., the overhead concentration that would be produced by the column operating at the minimum reflux with an infinite number of stages. When the light key and the heavy key are adjacent in relative volatihty and the specified spht between them is sharp or the relative volatilities of the other components are not close to those of the two keys, only the two keys will distribute at minimum reflux and the Xi D),n values are easily determined. This is often the case and is the only one considered here. Other cases in which some or all of the nonkey components distribute between distillate and bottom products are discussed in detail by Henley and Seader (op. cit.). 

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. 

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