Pressure correction vacuum distillation

Two heat-sensitive organic liquids of an average molecular mass of 155 kg/kmol are to be separated by vacuum distillation in a 100 mm diameter column packed with 6 mm stoneware Raschig rings. The number of theoretical plates required is 16 and it has been found that the HETP is 150 mm. If the product rate is 5 g/s at a reflux ratio of 8, calculate the pressure in the condenser so that the temperature in the still does not exceed 395 K (equivalent to a pressure of 8 kN/m2). It may be assumed that a = 800 m2/m3, /x = 0.02 mN s/m2, e = 0.72 and that the temperature changes and the correction for liquid flow may be neglected. 

The boiling point is one of the most important physical constants of a liquid. It is also easily determined with sufficient accuracy for most purposes if a reasonable quantity of a pure substance is available, for then it is only necessar to carefully distill the sample, noting both the vapor temperature and the barometric pressure. A particularly convenient apparatus for distillation of small quantities is shown in Fig. 1-10. The shape of the flask here is a desirable one because it confines the liquid to a smaller area than does a round-bottom flask. Thermometers with standard taper joints (1-in. immersion) are very convenient and reduce the possibility of contamination, and in vacuum distillation, of leakage. They are also calibrated for partial immersion, thus making emergent stem corrections (page 83) unnecessary. They... 

The measurement of pressure at the top of the distillation column is critical to valid distillation results because the observed vapor temperature must be corrected to the atmospheric equivalent temperature at standard pressure conditions (760mmHg). There is a general belief that the minimum pressure should be 2mmHg or greater for reasonably accurate measurements and correction to the atmospheric equivalent temperature. At pressures below 2mmHg, the pressure measurement is too inaccurate and a discontinuity can arise in the atmospheric equivalent temperature distillation curve from atmospheric to vacuum distillation. 

Calculating pressure drop is of considerable importance in atmospheric absorbers, heat transfer services, and vacuum distillations. Although pressure drop plots are available for most commercial types and sizes of random dumped tower packings, these data usually have been collected on air/water systems. While the air flow rate can be corrected for changes in gas density, no adequate method exists for handling the effect of liquid properties. 

The ethylmagnesium bromide is prepared in dry tetrahydrofuran and stored no longer than 1 week in a 250-ml. tube fitted with a 3-way vacuum stopcock and a dropping buret. The solution is decanted into the buret, and the correct volume is transferred to the reaction flask with positive nitrogen pressure. The tetrahydrofuran is purified by distillation from lithium aluminium hydride. See Org. Syn., 46, 105 (1966), for warning regarding the purification of tetrahydrofuran. 

The barometer is an 800-mm or longer tube which is sealed at the upper end, filled with mercury (Fig. 3-la), and inserted into a pool of mercury. If it has been filled in such a way as to avoid having air trapped in the upper end (e.g., by distilling in the mercury under a good vacuum), the height of the mercury column above the pool gives the atmospheric pressure. A temperature correction is usually applied, so that the readings correspond to that of a mercury column at 0°C. The open-end manometer (Fig. 3-1/ ) is closely related to the... 

With pipe diameters of this size, the molecules do not collide with each other, but solely with the walls of equipment and tubes. Therefore, different flow laws apply here than at higher pressures where the molecules collide mostly with each other. With vapours that are sensitive to heat, the collision numbers at distillation temperatures are in the hundreds of thousands. This has a negative impact on the gentle handling of the substance to be distilled. All these factors have to be taken into account for the correct evaluation of the type and size of the vacuum pumps, the tubes and the other distillation equipment. 

There have been many theoretical determinations of the rates of distillation under vacuum, but none that appeared to be applicable to vacuum dezincing when its development was commenced at Port Pirie in 1946. Thus it was felt to be desirable to develop the theoretical side (37) simultaneously with the practical development, as a guide to understanding and possible later application to optimising of the process. Figure 7 illustrates the concept of the distillation process which was developed. It was necessary to discard some faulty ideas or misconceptions, which derived fix>m implicit notions associated with equilibrium, but not kinetic conditions. Carman (38), for example, assumed that the partial pressure of the vapour of the condensing species is equal to its partial pressure in the condenser. It is not, unless the condensation rate is zero. Richardson (6) assumed that the measured vacuum is equal to the distilling species, which it is not, but is instead the partial pressure of the inert atmosphere. Warner (40) assumed the partial pressure of zinc to be constant across the distillation space, which is not correct unless the distillation rate is zero. 

Benzoxazolinone (2-hydroxybenzoxazole) [59-49-4] M 135.1, m 137-139 , 142-143"(corrected), b 121-213°/17mm, 335-337°/760mm. Benzoxazolinone is purified by reciystallisation from aqueous Mc2CO followed by distillation at atmospheric pressure, then in a vacuum. The methyl mercury salt reciystallises from aqueous EtOH and has m 156-158°. [Bywater et al. JAm Chem Soc 67 905 1945, Beilstein 27 III/IV 2677 ]... 

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