DE970432C - Process for separating mixtures of substances by distillation - Google Patents

Description

(WiGBl. P. 175)

ISSUED SEPTEMBER 18, 1958

p 27731 IFc / 12 a D

It is known that the separation quality, which is achieved in distillations, to a large extent by the Use of fractionation columns can be improved. It is also known that, especially for Distillations in the laboratory, fractionation columns, which consist of a vertically positioned cylindrical, Outwardly thermally insulated pipe exist, have special advantages. The advantages are there among other things in the fact that the aerodynamic resistance such pillars give to the rising vapor offer, is small that also the operating content, that is between the bubble and the condensation point amount of substance located is low and then with these advantages also a high quality of separation can be connected.

Arrangements are also known in which the fractionating column consists of two concentric tubes exists between which the steam rises; they offer advantages similar to those of the first mentioned pillars, which are formed from vertically positioned tubes with a circular cross-section.

In the relevant literature, many attempts are described which deal with the connection between Separation quality and steam upflow velocity at

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deal with such fractionators. On the basis of such information, it was generally of the opinion that in fractionating columns with a circular cross-section and a tube diameter of 0.3 cm, for example, the achievable separation quality increases if the speed u at which the steam rises in the column decreases for so long until u untern a limit of about 3 cm / sec was reached and then an improvement in the separation quality was no longer so that u of a distillation at all will rapidly impossible at higher degradation by further lowering the vapor velocity u possible. Theoretically, a steam upflow velocity has already been discussed which is below the stated value of 3 cm / sec; but no practical importance was attached to it because difficulties were seen in generating and stabilizing such low vapor velocities in a column.

The invention consists in that the return is generated at the upper end of the column with the exclusion of air will. This makes it possible to use considerably lower steam upstream speeds Achieve and maintain a value of 3 cm / sec in the column or in a column. Is namely on Air is present at the top of the distillation column, as was the case with the previous tubes If distilling columns is the case, the air penetrates from the environment due to its great diffusivity the condensation point into the deeper parts of the fractionation device and interferes with it the process of distillation. The exclusion of air from the vicinity of the condensation point at which the reflux is generated, can be achieved that the fractionating column above the said Condensation point is sealed gas-tight towards the top and that the separating material to be removed sucked out of this space with the help of an overheated probe at the desired speed will. The absence of air in the vicinity of the one intended for the generation of the return flow The condensation point in turn has the consequence that the temperature is adjacent to the vapor space The surface of the cooler is only slightly below the boiling point, which the mixture to be distilled is at the pressure established during the distillation. This is a measure resulting from the mentioned requirement that the generation of the return is to be carried out with exclusion of air, inevitably results. In contrast, in known distillation processes without exclusion of air is in the range of the condenser, the temperature of the condensing distillate is significantly below the saturation temperature at the externally set distillation pressure, indicating the presence of non-condensable Gas, e.g. B. Air, requires.

The need to exclude air is evident from an experiment in which a mixture of benzene and carbon tetrachloride was distilled at the rate U ₀ in a tube with a length of L = 30 cm and a radius of R = 0.3 cm. If the return flow was generated with the exclusion of air, the result, as already mentioned, was a separation quality corresponding to an ideal number of plates η = 100; the return was generated by cooling the upper tube end at 25 ⁰ C with admission of air under otherwise identical conditions, corresponded to the separation quality only an ideal plate number twentieth

It has now also been shown that distillations at lower steam upflow speeds than 3 cm / sec with a suitable choice of u result in a significant increase in the separation quality compared to the values previously considered to be optimal. In particular, it has been found that the speed u at which the vapor rises in the column provides a maximum of separation quality when

is made. D is the diffusion constant in the vapor mixture, α the radius of the circular cylindrical distillation tube.

For a mixture of benzene and carbon tetrachloride, for example, at atmospheric pressure D = 2 x 10 ^-2 cm ² / sec, so that ₀ = 0.13 cm / in a tube of radius a = 0.3 cm for the optimal speed, a value of M sec results. With a fractionation attachment 30 cm in length and a radius of 0.3 cm, an ideal plate number η- 100 was achieved _{at the speed W 0} with a mixture of benzene and carbon tetrachloride. At the speed u = 3 cm / sec previously considered to be optimal, however, only a tray number η = 17.5 was obtained (cf. e.g. Rose, Ind. Eng. Chem. JE, 28 [1936], 1210). With u = 0.25 cm / sec, the number of trays was 95, i.e. an increase of 5.4 times as compared to the maximum achievable number up to now. An actual decrease in the separation quality was only observed when the steam speed fell below the value specified for U _0.

From what has been said, it can be seen that the steam upflow speed achieved so far is about 23 times greater than the value u ₀ , which provides the best separation quality, and it can be seen that the number of trays achievable with a fractionating column is above the value previously considered achievable can be increased considerably if the steam speed is significantly reduced compared to the value previously feasible on the basis of the invention.

The diffusion constant mentioned in equation (1) can be taken from tables or in a known manner can be calculated from the molecular weights and the gas-kinetic collision cross-sections of the molecules.

The generation of the steam speed u is best carried out in a manner known per se by regulating the supply of heat to the still. To in the aforementioned example, a steam velocity u _{0 to} 0.13 cm / sec in änem tube of 0.3 cm radius to produce, 0.037 ^cm8 steam per second are necessary, which at atmospheric pressure and 80 ⁰ C a vapor amount of i, 3 · Io- ⁸ mol / sec and accordingly with a molar heat of vaporization of the mixture of 7ooocal / mol means a heat quantity of 90 ~ ³ cal / sec.

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So that this vapor velocity is maintained in the whole fractionating column, it is necessary that the The distillation still itself and the column are thermally well insulated from the outside, which in turn is known Means such as vacuum jacket or jacket with boiling liquid can be achieved.

Used as a fractionation column instead of a vertical tube with a circular cross-section, a pair of plane-parallel plates, which are at a distance of 2 "

ίο are set up from each other, used, or consists of the fractionating column, which practically comes out on the same thing, of a cylindrical cavity with a circular cross-section, as it was created by two concentric tubes, the radii of which differ from each other by the amount 2 α found that a maximum of separation quality is achieved when the speed

1/2 "· Ώ
speed of the rising steam U ₀ . _P = - ¹ ge

ao will do. A practically satisfactory result is achieved when the flow velocity in a range between one to ten times the

Amount

lies.

Claims (3)

Patent claims: 1. A method for separating mixtures of substances in a fractionation column, in which the the space available for the rising steam and the falling condensate heat-insulating walls is enclosed, characterized in that the return on upper end of the column is generated in the absence of air. 2. The method according to claim 1, characterized in that the steam upflow rate to be regulated by the supply of heat to the distillation vessel is made equal to one to ten times the amount - ^ - (D = diffusion constant of the steam, a = radius of the column). 3. The method according to claim 1 or 2, characterized in that the column to be removed The material to be separated is sucked out with the help of an overheated probe. Considered publications:
German patent specification No. 699 767. © 809 615/41 9.54