Pressure in distillation

The pressure in distillation column 1 has been increased to allow feed vaporization by heat recovery (from the distillation column condenser). Inspection of the new curves in Fig. 14.9a raises further possibilities. With the proposed modification, the overheads from the... 

The raw ethylsilicate formed is continually fed through an overflow pipe into one of distillation tanks 8. Usually there are several tanks while some are used for distillation, others are filled with etherification products. After the tanks are filled, the temperature is raised to 78-80 °C and during approximately 3 hours the alcohol vapours condensed in cooler 9 are directed through phase separator 10 back into the tank i.e. the tank operates in the self-serving mode. This makes the etherification more complete. After that the temperature is gradually (at the speed of 5-10 grad/h) raised to 140 °C. The excess pressure in distillation tanks should not exceed 0.02 MPa. 

CgB is practically independent of pressure in distillation systems (21,25 see also cyclohexane-n-heptane curves on Fig. 6.9a), This suggests that CSB is, at the most, only a very weak function of physical properties. Further, it suggests that any physical property variations that accompany a change in distillation pressure (and, therefore, also in the equilibrium temperature), do not affect CSB. This contradicts trends predicted by some of the earlier flood correlations (18-20). 

This type of pump is widely used in laboratories as an aid to filtration, for the partial evacuation of desiccators and for reducing the pressure in distillation apparatus. Such pumps are all too frequently connected directly to the apparatus to be evacuated and while this procedure is effective it must be under constant supervision. There are, however, many and varied demands on any laboratory water supply, so that the pressure may fluctuate and fall below the safe pressure for the pump, in which case the water will flow into the evacuated space to spoil the work in hand. 

Vapour pressure curves Optimum pressure in distillation 4.4... 

The basic advantage associated with the use of reduced pressure in distillation is the lowering of the boiling point and the consequent possibility of separating compounds below the temperature at which they decompose or undergo chemical alteration, such as polymerization. Examples from industry that may be quoted are the di.stillation of lubricating oils, the fractionation of crude phenols (which is performed... 

Another motive for using reduced pressure in distillation — especially on an industrial scale — may be that low-pressure steam is to be used as heating agent and that its temperature would be too low for distillation at normal pressure. In practice, furthermore, the corrosion of a metal still can be an important consideration, and on this account it may be necessary not to exceed a certain temperature. 

Be able to adjust the pressure in distillation colunms to achieve heat integration and to consider the usage of multiple-effect distillation and the usage of compression to achieve designs that are more profitable. 

Distillation. There is a large inventory of boiling liquid, sometimes under pressure, in a distillation column, both in the base and held up in the column. If a sequence of columns is involved, then, as discussed in Chap. 5, the sequence can be chosen to minimize the inventory of hazardous material. If all materials are equally hazardous, then choosing the sequence that tends to minimize the flow rate of nonkey components also will tend to minimize the inventory. Use of the dividing-wall column shown in Fig. 5.17c will reduce considerably the inventory relative to two simple columns. Dividing-wall columns are inherently safer than conventional arrangements because they lower not only the inventory but also the number of items of equipment and hence lower the potential for leaks. 

It is often advisable to lubricate ground-glass joint surfaces with an extremely thin film of vaseline. This applies particularly to joints employed in assemblies for distillation under reduced pressure. For distillations under greatly reduced pressures or at very high temperatures it is essential to employ a special lubricant, e.g., silicone grease. 

During this process some water will have condensed in the steam-trap D and also in the distillation bulb F. If at the end of the steaming-out process, the Bunsen burner is removed from the generator A, the pressure in A will be reduced owing to steam condensation, and the liquid in F will be sucked back into D provided that the benL-over tube is carefully adjusted, the bulb F may be almost completely emptied of liquid as desired. Finally the condensed water in the steam-trap D may be run out by op ing the tap Tj. 

The maintenance of a constant pressure in a system during distillation under diminished pressure is of great practical importance if trustworthy boiling points are desired. Devices which maintain a constant pressure in a system that is higher than the minimum pressure that the pump will give are termed manostats. A simple manostat, due to M. S. Newman, is illustrated in Fig. II, 23, 4. 

Wider passages are provided for vapours and the comparatively narrow tubes, which are usually fitted through holes bored in cork or rubber stoppers, are absent this considerably diminishes danger in violent reactions and also tends to give better results in distillation under reduced pressure as well as diminishing the hazard of choking. ... 

Vinylacetic acid. Place 134 g. (161 ml.) of allyl cyanide (3) and 200 ml. of concentrated hydrochloric acid in a 1-htre round-bottomed flask attached to a reflux condenser. Warm the mixture cautiously with a small flame and shake from time to time. After 7-10 minutes, a vigorous reaction sets in and the mixture refluxes remove the flame and cool the flask, if necessary, in cold water. Ammonium chloride crystallises out. When the reaction subsides, reflux the mixture for 15 minutes. Then add 200 ml. of water, cool and separate the upper layer of acid. Extract the aqueous layer with three 100 ml. portions of ether. Combine the acid and the ether extracts, and remove the ether under atmospheric pressure in a 250 ml. Claisen flask with fractionating side arm (compare Fig. II, 13, 4) continue the heating on a water bath until the temperature of the vapour reaches 70°. Allow the apparatus to cool and distil under diminished pressure (compare Fig. II, 20, 1) , collect the fraction (a) distilling up to 71°/14 mm. and (6) at 72-74°/14 mm. (chiefly at 72 5°/ 14 mm.). A dark residue (about 10 ml.) and some white sohd ( crotonio acid) remains in the flask. Fraction (6) weighs 100 g. and is analytically pure vinylacetic acid. Fraction (a) weighs about 50 g. and separates into two layers remove the water layer, dry with anhydrous sodium sulphate and distil from a 50 ml. Claisen flask with fractionating side arm a further 15 g. of reasonably pure acid, b.p. 69-70°/12 mm., is obtained. 

Heat a suspension of 22 g. of the diacetate in a mixture of 120 ml. of concentrated hydrochloric acid, 190 ml. of water and 35 ml. of alcohol under reflux for 45 minutes. Cool the mixture to 0°, filter the solid with suction, and wash with water. Purify the crude aldehyde by rapid steam distillation (Fig. II, 41, 3) collect about 1500 ml. of distillate during 15 minutes, cool, filter, and dry in a vacuum desiccator over calcium chloride. The yield of pure o-nitrobenzaldehyde, m.p. 44—45°, is 10 g. The crude solid may also be purified after drying either by distillation under reduced pressure (the distillate of rather wide b.p., e.g., 120-144°/3-6 mm., is quite pure) or by dissolution in toluene (2-2-5 ml. per gram) and precipitation with light petroleum, b.p. 40°-60° (7 ml. per ml. of solution). 

Another method of purihcation consists in distillation under reduced pressure. The fraction of b.p. I40-I50°/20 mm. is collected separately, - it solidihes on standing, melts at 76-76-5°, and is practically pure-... 

Introduce a solution of 100 g. of sodium bisulphite in 200 ml. of water and continue the stirring, preferably for 10 hours with exclusion of air. A thick precipitate separates after a few minutes. Collect the bisulphite compound by suction filtration, wash it with ether until colourless, and then decompose it in a flask with a lukewarm solution of 125 g. of sodium carbonate in 150 ml. of water. Separate the ketone layer, extract the aqueous layer with four 30 ml. portions of ether, dry the combined organic layers over anhydrous magnesium sulphate, remove the ether at atmospheric pressure, and distil the residual oil under reduced pressure from a Qaisen flask with fractionating side arm (Fig. II, 24, 5). Collect the cyclo-heptanone at 64r-65°/12 mm. the yield is 23 g. 

Note 2. In order to minimize the hold-up. a partial vacuum (ea. 100 mmHg) may be applied during the last stage of the distillation the fraction obtained in this way can be redistilled at normal pressure in a small distillation apparatus. 

The product of this reaction can be removed as an azeotrope (84.1% amide, 15.9% acetic acid) which boils at 170.8—170.9°C. Acid present in the azeotrope can be removed by the addition of soHd caustic soda [1310-73-2] followed by distillation (2). The reaction can also take place in a solution having a DMAC-acetic acid ratio higher than the azeotropic composition, so that an azeotrope does not form. For this purpose, dimethylamine is added in excess of the stoichiometric proportion (3). If a substantial excess of dimethylamine reacts with acetic acid under conditions of elevated temperature and pressure, a reduced amount of azeotrope is formed. Optimum temperatures are between 250—325°C, and pressures in excess of 6200 kPa (900 psi) are requited (4). DMAC can also be made by the reaction of acetic anhydride [108-24-7] and dimethylamine ... 

The purification of Hquid nitro alcohols by distillation should be avoided because violent decompositions and detonation have occurred when distillation was attempted. However, if the distillation of a nitro alcohol cannot be avoided, the utmost caution should be exercised. Reduced pressure should be utilised, ie, ca 0.1 kPa (<1 mm Hg). The temperature of the Hquid should not exceed 100°C hot water should be used as the heating bath. A suitable explosion-proof shield should be placed in front of the apparatus. At any rise in pressure, the distillation should be stopped immediately. The only commercially produced Hquid nitro alcohol, 2-nitro-1-butanol, is not distilled because of the danger of decomposition. Instead, it is isolated as a residue after the low boiling impurities have been removed by vacuum treatment at a relatively low temperature. 

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