Head fraction

Distillation. Most fatty acids are distilled to produce high quaHty products having exceUent color and a low level of impurities. Distillation removes odor bodies and low boiling unsaponifiable material in a light ends or heads fraction, and higher boiling material such as polymerized material, triglycerides, color bodies, and heavy decomposition products are removed as a bottoms or pitch fraction. The middle fractions sometimes can be used as is, or they can be fractionated (separated) into relatively pure materials such as lauric, myristic, palmitic, and stearic acids. 

The heads fraction may be highly acidic and corrosive if sulfur dioxide or bisulfite is used in the production or preservation of the wine. The reversible reactions involved are ... 

Ethyl acetate is the major low-boiling impurity of heads fractions from continuous columns if bisulfites are absent in the distilling material. A heads fraction from a typical brandy column usually contains less than 1% of these volatile impurities, although the concentrated heads from an aldehyde-concentrating column may contain as much as 10-15% aldehydes. In either case, ethyl alcohol is the major component of the heads cut, and its recovery in usable form has been a troublesome processing problem. 

The crystalline fractions are now systematically recrystallized from methanol. The head crop is recrystallized from about 600 cc. of this solvent, and the remaining crops are crystallized in order from the successive mother liquors. At each stage the volume of the solution is adjusted by addition or distillation of solvent so that from one-third to one-half of the salt crystallizes. A series of large Erlenmeyer flasks, fitted with a reflux condenser or distilling head as required, is convenient for the purpose. The recrystallization is continued, using fresh or recovered solvent, until the head fraction has [a]i> — 2.8° or less (c = 4, methanol). It is unnecessary to take rotation values until the crystalline appearance and solubility behavior of the head fraction become approximately constant. Usually five to seven recrystallizations are necessary for purification. When pure, the head fraction is removed from the series and the recrystallization of the remaining crops is continued as long as it appears profitable. The final mother liquor from each series is reserved. 

The first rectification stage. From collector 10 the mixture of methyl-chlorosilanes is periodically fed into pressure container 11, from where at 50-65 °C it is sent through heater 12 (by self-flow) onto the feeding plate of rectification tower 13. From the tower the tank liquid (methyltrichloro-silane, dimethyldichlorosilane and tank residue) flows into tank 14, where the temperature of 80-90 °C is maintained, and from there is continously poured into collector 22. After the tower, vapours of the head fraction at a temperature below 58 °C, consisting of the rest of methylchloride, di- and trichlorosilane, dimethylchlorosilane, methyldichlorosilane and the azeotropic mixture of silicon tetrachloride and trimethylchlorosilane are sent into refluxer 15, cooled with water, and into refluxer 16, cooled with salt solution (-15 °C). After that, through cooler 17 the condensate is gathered in receptacle 19. Volatile products, which did not condense in reflux-ers 15 and 16, are sent into condenser 18 cooled with Freon (-50 °C). There they condense and also flow into receptacle 19. As soon as it is accumulated, the condensate is sent from receptacle 19 into collector 20. 

Separation of head fractions. The head fraction, which is obtained at the first stage of continuous rectification of methylchlorosilanes, from collector 20 self-flows into tank 44. The temperature in the tank in the beginning of the process is maintained at 60-70 °C, and at the end it should be from 90 to 95 °C. Vapours from the tank rise up tower 40 and enter reflux-ers 39, cooled with water and salt solution (-15 °C) from there, part of condensate is returned to reflux tower 40, and the rest is sent through cooler 38 into receptacles 41 and fed into collectors 43. 

The head fraction, separated below 67 °C, is rectified to obtain the following fractions ... 

Tank residues after the distillation of the head fraction can be sent back into the tank of the rectification tower for repeated distillation. 

The condensate is rectified in two stages stage I is the distillation of the head fraction, which contains chlorosilanes, at atmospheric pressure stage... 

Head fraction I, which is a mixture of methyldichlorosilane, methyltri-chlorosilane, dimethyldichlorosilane and a small amount of benzene, is separated in the 36-78 °C range and collected in receptacle 16. Then this fraction can enter batch box 4. Fraction II (benzene) is distilled in the 78-82 °C range and collected in receptacle 11, and then poured into receptacle 18. It can be re-used in the synthesis (in this case benzene from collector 18 is sent into batch box 3). Tank residue, which after the distillation of the first two fractions is a concentrate with 50% of methylphenyldichlorosilane, is sent from tank 12 into collector 19 and from there into tank 20, heated with vapour (1.4 MPa). 

The distillate in tower 11 is periodically sampled. At first the analysis is carried out until the density reaches 1.002 g/cm3, after that one determines the main substance content. When the main substance content is at least 93% and that of the head fraction does not exceed 1-2%, the separation of the intermediate fraction is completed. 

The head fraction is sent into the tank of tower 20, the intermediate fraction is added to concentrated PMS-lb for further rectification, and the tank residue is added to concentrated PMS-l,5b. 

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. 

Acetals are equilibrium products between aldehydes and alcohols. As discussed by Williams and Strauss (30) acetals generally have less intense aromas than the corresponding alcohols and aldehydes. 1,1,3-Triethoxypropane and diethoxybutan-2-one (derived from acrolein and diacetyl, respectively) are common acetals in the heads fractions from continuous stills acetals from other aldehydes including acetaldehyde, propanal, isobutanal, and isovaleraldeyde are also common (30). The equilibrium between the aldehyde and the acetal is highly dependent on alcohol concentration and pH, again m ng accurate quantitation of either the aldehyde or the acetal dependent on the analytical conditions (e.g., sample dilution, solvent extraction, etc.) (30). 

This procedure is very time-consuming and may be speeded up materially if a small amount of one of the pure components (a seed ), preferably the less soluble one, is available. If a slightly supersaturated solution that is reasonably free from dust and other particles is prepared, nucleation usually occurs slowly. If a seed is now added, the rate at which the component which is seeded will crystallize wDl be markedly greater than that of the other component and if the solution is filtered soon after most of the initial crystallization has occurred, it will often be found that the solid contains mainly the component which was seeded. A good example of this is found in the separation of the isomers of methvlethylisobutylcarbinyl acid phthalate as the brucine salt. In the first separation, 20 recrystallizations of the head fraction were required in order to obtain optically pure material. By using a small amount of this material as a seed, optically pure material was later obtained after about seven cr stallizations. 

The head fraction of column 1 is mostly a mixture of various low boilers, with methanol as the principal component, but also contains some furfural and some water. This fraction is introduced into a randomly packed column 3 where the furfural and most of the water are separated, to be fed into the decanter 2. 

It was found that when the given mixture (the raw solvent ) is submitted to a simple distillation, neither the head fraction nor the sump fraction shows any significant increase in diketone concentration. This is synonymous with saying that the diketone concentration in the vapor is roughly equal to the diketone concentration in the liquid, very much as in the case of distilling an azeotropic mixture. This is not surprising as the starting mixture, rep-... 

The head fraction of the extractive distillation described above is called extractive distillate . Historically, the easiest way of getting diacetyl out of this mixture was found to be cystallization as diacetyl has an unusually high freezing point of-2.4 °C. On the other hand, 2,3-pentanedione cannot be conveniently recovered in this fashion as its freezing point is -52 °C. Thus, when crystallization is adopted as the mode of recovery, it is accepted that 2,3-pentanedione is lost completely. 

In 1992, a rather unusual furfural plant was built. With a front end according to the AGRIFURANE process described in chapter 10.2, the back end was designed as shown in Figure 112. The filtered reactor condensate containing 5 % furfural, 1.7 % acetic acid, 0.17 % formic acid, and various low boilers was introduced into an extraction tower 1 fed with chloroform at the top. On the way downwards, the heavy chloroform (density 1.498 g/cc at room temperature) picked up the furfural, and in view of the poor solubility of chloroform in water, it formed a chloroform/furfural extract at the bottom. This extract entered a distillation column 2 removing the chloroform as the head fraction. From a buffer tank 3, this chloroform was recycled to the extraction tower 1. The sump fraction of the distillation column 2 consisted of furfural, polymers, waxes, and some low boilers. This fraction was introduced into a distillation column 4, which yielded a head fraction of low boilers, a side stream of furfural, and a sump fraction of polymers and waxes. 

The extract of tower 6 entered a distillation column 7 recovering the amine/hydro-carbon mixture as the sump fraction, to be recycled into the extractor 6, white the head fraction of column 7, a mixture of acetic acid and formic acid, was separated in the distillation column 8. 

The head stream (570 kg/h) containing 38 % by weight of furfural goes to an auxiliary column (not shown) where the furfural is recovered as the sump fraction. This sump fraction is the stream of 429 kg/h entering the decanter from the right-hand side. The head fraction of the auxiliary colunm is called raw solvent . Specified in chapter 16.6 (page 129), it contains the low boilers as well as the diacetyl and 2,3-pentanedione by-products. 

The product of condenser 10, collected in decanter 14, forms two liquid phases, a heavy furfural phase and a light aqueous phase. The heavy furfural phase, consisting of approximately 92 % furfural and 8 % water, by weight, enters a dehydration column 15 energized by a steam coil in the bottom. The sump fraction, representing pure furfural freed of water, is withdrawn by pump 16 and fed into tank 7. The head fraction of column 15, having a composition close to the furfural/water azeotrope, joins the vapor stream of column 9 in condenser 10. 

The light phase of decanter 14 enters the head of a distillation column 17 energized by steam injection. The sump fraction of this column is water, while the head fraction is again almost azeotropic and thus suitable for being introduced into condenser 10. 

The remaining furoyl chloride (b.p. 173 °C) is transferred into a vacuum distillation column 15 energized by a steam coil, where the furoyl chloride is obtained as the head fraction liquefied in condenser 16. The vacuum pump 17 maintains this system at a pressure of 7 mm Hg., A part of the condensate is used as reflux, while the product is collected in tank 18. The yield of furoyl chloride is in excess of 89 percent, the losses being a small fore-run collected in tank 19, a small after-run collected in tank 20, and a small quantity of a carbonaceous residue. 

The mixture of benzene and thionyl chloride in tank 14 is separated in a distillation column 21 heated by a steam coil. The sump fraction is benzene which is recycled back into tank 5 by means of pump 22. The head fraction of column 22 is thionyl chloride. It is liquefied in condenser 23 cooled by a chiller 24. A part of the condensate is used as reflux while the remainder is fed back into tank 6 by means of pump 25. 

After approx. 48 h of fermentation, the fermented mash shows an alcohol content of approx. 6-10% by vol. When distilled in continuous stills with 3 or more colunms, the alcohol level in the distillate can be enriched up to 96.6% by vol. The purification of the raw distillate is normally carried out in continuous rectification equipment, for which the raw alcohol is usually diluted to approx. 15% by vol. before rectification in order to better separate fusel oil components. The first fractions of the rectified distillate - the so-called head fraction - contain significant quantities of acetaldehyde, methyl alcohol and low-boiling esters the middle part - heart fraction - represents so-called neutral spirits with an alcohol content of approx. 96.6% by vol. The tail fractions contain higher alcohols and higher esters etc. Since head and tail fractions contain organoleptic properties undesirable for neutral alcohol, they usually are employed as technical alcohol or have lately also been isolated and used as starting raw materials for the production of natural aroma components. 

Figure 6-16. Distillation head, fraction cutter. (Courtesy - Ace Glass Co., Vineland, NJ)...

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