Staged Distillation

Distillation stages One Less than one One Less than one... 

The main disadvantage of precipitation with a polar (water-soluble) solvent is the need for a costly distillation stage to recover the relatively laige volumes of solvent used. Another disadvantage is the precipitation of proteins, salts and, in some cases, pigments which reduces the purity and leads to discoloration of the product. To overcome these problems, precipitation using less polar solvents, such as methylethylketone, has been proposed. Only 23% (w/v) methylethylketone is sufficient to saturate the aqueous phase and precipitate exopolysaccharides quantitatively. 

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. 

Only two distillation stages are needed for water and propanoic acid removal. This results in lower energy consumption which contributes to the overall reduction in CO2 emissions to 0.31 t/t product compared to 0.48 t/t product with the Monsanto process. 

Having a relative volatility, one can estimate theoretical distillation stages. There are a number of modem computer programs for distillation. These give excellent results, however, these must be applied by someone with a sound understanding of distillation in general and the accuracy of the input data in particular. 

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. 

The procedure described here is characterized by good yields, mild conditions, and easy synthesis of a pure form from readily available starting materials. Since tertiary aliphatic acetylenes do not form readily under these conditions, the excess of alkyllithium used is not particularly critical. The small amount of by-products that also form is similarly readily removed at the distillation stage. 

The aqueous phase must be neutral. Acidic impurities catalyze trimerization of the anhydrous aldehyde7 in the distillation stage. 

In the early alchemical texts (and in particular the Rosarium philosophorum ), this picture of soul alchemy is developed in parallel with descriptions of what was a sore physical process. Thus it was that the soul development of the alchemist went hand in hand with an actual physical operation, and this operation, the details of which have not been wholly lost, involved colour and form changes within a sealed flask, isomorphic to the inner changes of soul alchemy, described by these bird symbols. Thus we had a physical process which involved a blackening, a whitening a rapid iridescence of colours, a circular distillation stage, and a final sublimation. Part of the task of modern alchemy must be to rediscover this physical process, and explore its further ramifications. 

It is essential to have an efficient condenser during the reflux and distillation stages because the product, trimethylsilylacetylene, is extremely volatile. 

Liquid-liquid extraction, using one or two solvents respectively, is widely used when distillation is impractical, especially when the mixture to be separated is temperature-sensitive and/or more than 100 distillation stages would be required. When one solvent is used, it selectively dissolves only one or a fraction of the components in the feed mixture. In a two-solvent extraction system, each solvent has its own specific selectivity for dissolving the components of the feed mixture. Additional separation operations are generally required to recover, for recycling, solvent from streams leaving the extraction operation. 

The ideal distillation stage is a device that meets these criteria ... 

Examples. Figure 2.1 illustrates the first criterion. The system in Fig, 2.1a has a vapor product and a liquid product and therefore obeys this criterion, The systems in Fig, 2,16 and c have no vapor products and therefore are not equilibrium stages. Generating a vapor phase in these systems (Fig, 2,1 d and e) renders them equilibrium stages. Figure 2.1c and e depict a total and a partial condenser, respectively. The total condenser is not a distillation stage, whereas the partial condenser is. 

Ffgure 2.1 The distillation stage concept, to) General presentation of an ideal distillation stage (b) and (e) single-phase product (these are not ideal distillation stages) (tf) and phase product (these are ideal distillation stages). 

Vapor leaving a distillation stage is richer than the feed in the more-volatile components. Liqnid leaving the stage is richer than the feed in the less-volatile components. In order to improve the separation, multiple stages are used. 

For a single binary distillation stage following equations apply ... 

Distillation stage calculations are usually performed with ideal stages, The number of ideal stages required for the separation is divided by the overall column efficiency (Sec, 7,1,1) to obtain the required number of trays. In packed towers, the number of stages in the column is multiplied by the HETP (Height Equivalent of a Theoretical Plate, see Sec. 9.1,2) to obtain the packed height. 

The reactor effluent is quenched by water injection, and then by passage through a series of heat exchangers in which steam is produced. It is then cooled by a second water quench or by means of a heavy hydrocarbon. The condensates are separated, and the gases are compressed and sent to a train of simple or extractive distillation stages to... 

---