HRP960601A2 - Process for separating medium boiling substances from a mixture of low, medium and high boiling substances - Google Patents

Abstract

A process is disclosed for producing an aqueous solution of free hydroxylamine. A solution obtained by treating an hydroxylammonium salt with a base is treated with water or water vapour at a temperature >/= 80 DEG C, so as to separate the solution into an aqueous hydroxylamine fraction and a salt fraction. This process is soft and easy to implement on a large scale. The risk of decomposition is minimised because of the low thermal stresses, the low hydroxylamine concentration and the short dwelling time during implementation of the process.

Description

The proposed invention relates to the process of separating the medium boiling point fraction from a mixture of low, medium and high boiling point fractions, which is separated into a fraction containing low and medium boiling points and a fraction containing low and high boiling points.

A problem, which is often encountered in the chemical industry, is the separation of intermediate boiling points in pure form or with only traces of low boiling points from a liquid mixture, which contains more liquid substances, and which consists of fractions of low (L), medium (M) and high boiling point (H).

To do this, known distillation methods can be applied, for example those described in Ullmann's Encyclopedia of Industrial Chemistry, vol. B3, p. 4-46 et seq. A general feature of known distillation methods is that high boiling points are extracted at the bottom in pure form, or possibly with residual traces of the medium boiling point, and that the medium boiling point component is separated at the top of the column at temperatures determined mainly by the concentration of the high boiling point component and its boiling point. Furthermore, with known methods, it is not possible to separate a mixture of low and medium boiling point together, because a mixture of low and high boiling point, which is without a medium boiling point, is simultaneously separated. However, this is desirable in many cases, especially if the low and high boiling points are intended for later joint use (sale, recovery, disposal).

Pages 4-48 of the aforementioned publication describe the use of side columns to separate medium boiling point mixtures from low, medium and high boiling point mixtures (L, M, H mixtures). In this case, low and medium boiling points are always separated as well. The same applies to the directly or indirectly related columns described on pages 4-62 and 4-63 of the above publication. In all these cases, it is absolutely necessary to separate the medium-boiling component from the high-boiling component by distillation, which in any case requires boiling points that are at least equal to that of the medium-boiling component and, in extreme cases, close to the boiling point of the high-boiling component, and therefore very high. This is especially the case if the medium-boiling component must be completely separated from the high-boiling component. Such high temperatures, even in the case of thermally relatively stable substances, can rise enough to cause decomposition or chemical conversion (polymerization, etc.) of the substance involved. For this reason, complex distillation procedures are often necessary for the separation of this type, for example mild distillations carried out under conditions of reduced pressure (thin-layer evaporators, molecular jet distillation, etc.). The disadvantage of such distillation processes is their extremely low efficiency. This leads to high capital investment and production costs, which then mean that separation by distillation, which is beneficial in itself, can be uneconomical to implement.

Special procedures for separating difficult-to-dissolve liquid mixtures are also known. Special procedures come into consideration only if they are more cost-effective or when other, general procedures are not satisfactory. They are often used for substances with limited thermal stress endurance properties, i.e. if their boiling point is above or close to the decomposition temperature. A well-known method of separating low-volatility components from a mixture containing immiscible components is distillation using a carrier gas. The method is based on the fact that in a mixture of immiscible substances, each substance behaves as if the other is not there; in other words, at a given temperature, each substance has a partial pressure that is - regardless of the composition of the mixture - one to the vapor pressure of the respective substance r. The result is that the pressure of such a mixture is equal to the sum of the vapor pressures of the individual components. A well-known example of such a system is the water/bromobenzene system. The boiling point of the mixture is at 95°C, while the pure substances have a boiling point at 100°C (water) and 156°C (bromobenzene). The carrier gas for distillation is particularly suitable for the separation of immiscible components with a relatively high boiling point (e.g. glycerol), for the separation of substances that polymerize or decompose even before reaching the boiling point (fatty acids), and for the separation of substances that are very difficult to handle, and for substances whose direct heating to boiling point can be dangerous (eg turpentine).

The best-known example of distillation with a carrier gas is distillation with steam, i.e. where steam is the carrier gas. This is widely used, for example, in the oil refining industry, to remove light hydrocarbons from absorption oils; in the coal industry, for steam distillation of hydrocarbon fractions during coal distillation; for separating turpentine from resin in the rubber industry; and in the preparative chemical industry. Steam distillation is a special form of azeotropic or extractive distillation, as described in the above publication on pages 4-50 to 4-52. The technical effect of this procedure is based on the knowledge that with the addition of a substitute substance (extracting agent), the azeotropic point is exceeded and, therefore, the desired concentration is achieved above the azeotropic point.

The disadvantage of all these processes is that an additive (extracting agent) is introduced into the system that is to be distilled, and this extracting agent must be separated from the system again by an additional process.

Another known method of removing relatively high-boiling substances from a mixture of substances is extraction. The disadvantage of extraction is that it always yields only highly dilute solutions of the high-boiling component or the medium-boiling component in the extraction medium. In general, the process is only economical if the product can be separated by phase separation, i.e. if the mixture of substances shows a lack of miscibility. The object of the proposed invention is therefore to provide a simple and mild process for separating the medium boiling point component or fraction containing low and medium boiling points from a mixture including low, medium and high boiling points.

We have found that this goal can be achieved, surprisingly, by treating the above-mentioned mixture in a column at the bottom with low-boiling steam.

The proposed invention therefore provides a process for separating a fraction containing low and medium boiling points (L, M fraction) and a fraction containing low and high boiling points (L, H fraction) from a homogeneous mixture containing low, medium and high boiling points (L , M, H mixture), which process involves treating the L, M, H mixture in a low-boiling steam column and separating it into the L, M fraction and the L, H fraction. The medium-boiling component is collected in the low-boiling vapor, so that the L, M fraction can be recovered above the feed side of the mixture, and the L, H fraction is recovered in the liquid phase.

The mixture to be separated generally goes directly to the top of the column. Processing of the mixture with low-boiling steam is primarily carried out in a countercurrent, and especially by passing the low-boiling steam into the bottom of the column or introducing the low-boiling liquid component and boiling it at the bottom. The low-boiling component fed to the column is usually the same one present in the mixture.

It has been found particularly useful to carry out the treatment with low-boiling steam in the extraction column. It may be a conventional plate column, for example a fractional plate or mesh plate column, or a column equipped with conventional packing, for example Raschig rings, Pall rings, bent bases, etc., preferably having a theoretical number of plates in the range of 5 to 100. Depending on the separation task, the number of plates can be even higher than 100.

As a result of the low-boiling vapor passing to the bottom of the column, the medium-boiling component collects the low-boiling vapor. The L,M fraction is usefully obtained at the height of the feed plate or above it. First, the L,M fraction is extracted from the top of the column.

The L, M fraction generally includes a low-boiling component, with large to very large excess. This is why it is particularly useful to concentrate the L,M fraction to enrich it with the medium boiling point component. This can be done, for example, by passing the L, M fraction through a separate, multiphase rectification column, in which the low-boiling component is separated and the L, M fraction is enriched with the intermediate-boiling component, or even the pure intermediate-boiling component is obtained. .

It is particularly useful to place the rectification column as a separate distillation column or to mount it directly on a low-boiling steam treatment column and distill off the low-boiling component near the top. The enriched L, M fraction, or mid-boiling component, can be removed by stripping with a side stream from the column return stream. In this context, it is particularly useful to use a mostly vertical dividing wall. In this case, the mixture to be separated is fed approximately to the center of the extraction/rectification column. At the height of that supply point, a partition wall is installed in the column so that it extends in height generally from 1 to 10, preferably from 1 to 5 theoretical plates, so that it divides the column into two vertical separate spaces, whereby the supply is made approximately in the center dividing wall. In this way, the fraction enriched with the intermediate boiling point component can be subtracted on the side opposite the feed side, in the region of the dividing wall. A dividing wall separates the drain side from the supply side. Equal concentrations of the medium-boiling component are present on both sides of the partition wall, but the high-boiling components are present in the mixture only on the inlet side. The fraction enriched with the medium-boiling component is preferably drained at the feed level or, if appropriate, slightly below that point.

In an alternative embodiment, which includes a partition wall, a side column can also be mounted opposite the extraction/rectification column, in such a way that the side column is in contact with the gaseous and liquid phases of the extraction/rectification column, in each case by one or more degrees separations above or below the feed side, and the fraction richer in the medium-boiling component is removed via the side column. The side column is arranged so that the high-boiling component cannot pass over the drain side of the side column. Measures suitable for this purpose are known to those skilled in the art.

Optionally, a droplet separator (dehumidifier or other conventional device) can be installed above the feed plate, or in the steam drain, in such a way as to prevent re-entry of the high-boiling component with droplets.

The L, M fraction enriched with the medium-boiling component, by means of the above-mentioned rectification column, is optionally separated, or concentrated in the following column with a rectification section and an extraction section.

A further useful embodiment of the novel process involves passing vapor from the stripper, or stripper/distillation column, preferably after conventional compression, as a low-boiling component, or low-boiling vapor, back to the bottom of the processing column.

Since the new process heats directly with a low-boiling component, or with low-boiling steam, and steam compression only needs to overcome the differential pressure across the column, a significant reduction in energy consumption is possible, and, at the same time, the consumption required for cooling.

The processing column and/or the rectification column or the distillation column can operate under atmospheric, sub-atmospheric or supra-atmospheric pressure and continuously or batchwise. Against this basic fact, the conditions depend of course on the mixture to be separated, and can be determined by the expert in the usual way. The critical factor is the temperature of the low-boiling steam, which must be high enough to distil the L,M fraction and to obtain the L,H fraction at the bottom of the column.

The advantage of the new procedure is that it can be easily carried out and that it is carried out without the addition of any other substance. The concentration of the intermediate boiling point component is low in the inlet region of the process. The retention time in the process, i.e. in the column, is relatively short. Due to the simplicity of the process, the required capital investment is low. Even more, the procedure is almost endless in its possibilities of extension.

The new process enables the implementation of an extremely mild separation of the L,M fraction or the medium-boiling component from a mixture containing low, medium and high boiling points, at the level of the boiling temperature of the low-boiling component. The process is therefore particularly useful if it is necessary, under as mild conditions as possible, to separate the heat-sensitive component of the medium boiling point, which is prone, for example, to decomposition or polymerization, from the L, M, H mixture. The process is particularly useful if the high-boiling component is present in the raw mixture, in pure or highly enriched form, or if it is otherwise of high viscosity, as a precipitate or as a solid, or if at a relatively high concentration it is prone to chemical reaction, for example polymerization . The new process actually ensures the possibility of extracting the high-boiling component dissolved in the low-boiling component. As a result, it is only necessary to manage the solution; in other words, problems with viscosity, solids, etc. are avoided.

The new procedure is particularly suitable for obtaining heat-sensitive products. Examples of this application are:

- obtaining an aqueous solution of hydroxylamine from an aqueous solution of a salt of hydroxylamine,

- obtaining compounds that can polymerize, for example recovering polystyrene from the mixture obtained in the production of styrene,

- recovery of chlorinated hydrocarbons, for example, recovery of dichloroethane from the mixture obtained in the production of dichloroethane,

- recovery of carboxylic acids and aldehydes from acids for the extraction of cyclohexane oxidation with air or from the production of adipic acid,

- decomposition of organic acids and aldehydes, such as acetic acid, acrylic acid, methacrolein or methacrylic acid, which due to production influences may still contain high boiling points, organic compounds, salts (catalysts), etc., and

- separation of amines from mixtures containing ammonia and high boiling points.

The new procedure is explained in more detail by the layout shown in Figure 1.

Figure 1 shows a column for separating the L, M, H mixture, which consists of an extraction column 1, on which a rectification column is mounted. The mixture, which is separated, is fed directly to the top of the extraction column 1. The low-boiling steam L goes to the bottom of the extraction column l in countercurrent to the mixture. At the bottom of the column, the L,H fraction is removed, while at the top of the column, the L,M fraction is obtained, which is mostly free of high-boiling components. This fraction is concentrated, i.e. enriched with the medium boiling point component, in a rectification column. The enriched L,M fraction is discharged approximately above the feed point of the mixture to be separated. The low-boiling component is obtained at the top of the rectification column, and, if desired, can be condensed, or it can be used further. Alternatively, the low-boiling component can be returned, directly or after compression, to the bottom of the recovery column 1.

The following examples illustrate the invention but do not limit it.

Example 1

Obtaining an aqueous solution of hydroxylamine (HA) from a solution of hydroxylamine (HA)/ammonium sulfate (AS) using an extraction column.

An aqueous solution containing 218 g HA/l and 680 g AS/l is fed at a rate of 300 ml/h into the highest part of the extraction column. The extraction column was made of glass, had a height of 2 m and a diameter of 35 mm and was packed to a height of 1.8 m with 3 mm glass Raschig rings. 1000 ml/h of distilled water was brought to the bottom of the column. The column was under a pressure of 40 kPa.

The temperature at the bottom was is 84°C. 1000 ml/h of salt-free HA aqueous solution distilled at a rate of 39.0 g HA/h from the top of the column, which corresponded to 59.6% of the input HA at the feed. 300 ml/h of ammonium sulfate solution, with an HA content of 86% HA/l, was taken from the bottom of the column. This corresponds to 39.4% of the total HA at the inlet.

The maximum concentration of HA in the column was 100 g/l. The amount of liquid in the column, which depends on the individual filling, was 20-225 ml. The retention time in the column was therefore only 1.5-10 minutes. At such a low concentration and for such a short time, the rate of degradation is low.

Further experiments are shown in the table below.

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Example 2

Separation of aqueous HA solution from aqueous HA/Na2SO4 solution using an extraction column

The aqueous solution from example 3, which contained 11 wt. % HA and 23.6 wt. % Na2SO4, was fed at a rate of 978 g/l to the highest stage of the extraction column. The extraction column was made of enamel, had a height of 2 m and a diameter of 50 mm, and was packed with 5 mm Raschig glass rings. The column was under atmospheric pressure. Steam under 2.5 bar absolutely entered the bottom of the column. The steam/supply ratio was 2.9:1. 985 g/h of sodium sulfate solution with an HA content of 1.7 g HA/l was removed from the bottom of the column. This corresponds to 1% at the inlet HA in the feed. 3 tops of the column distilled 3593 g/h of salt-free HA aqueous solution, which contained 36.8 g HA/l, which corresponded to 99.2% of the input HA at the feed.

Further examples are set out in the table below.

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Example 3

Obtaining an aqueous HA solution from an aqueous HA/sodium sulfate solution using an extraction-distillation column

An aqueous solution containing 221 g HA/l and 540 g AS/l was fed at a rate of 202 ml/h to the eleventh plate of a glass fractional plate column with a diameter of 35 mm, a total height of 1.6 m and with 21 plates (lowest plate = plate 1) . 1300 ml/h of steam (at a temperature of approx. 125°C) was fed to the bottom of the column. The pressure in the column was 99 kPa. At the top of the column, 180 ml/h of mostly HA-free water (0.6 g HA/l) was taken at a column head temperature of 99.8°C and a reflux ratio of 1:3 (return stream: re-feed). Aqueous solution of HA (product solution), concentration 44 g/l, was removed at a speed of 1180 ml/h with a side flow from plate 12. 400 ml/h of salt solution was removed from the bottom of the column.

Example 4

Obtaining an aqueous HA solution from an aqueous HA/sodium sulfate solution using a strip-distillation column with side-drain concentration

Aqueous solution as in example 3, which contained 11 wt. % HA and 23.6 wt. % Na2SO4, was fed to the eleventh theoretical plate of a glass column with fractional plates, where the diameter of the column was 50 mm (the number of plates corresponds to 30 theoretical plates). Steam under a pressure of 2.5 bar and at approx. 125°C was brought to the bottom of the column. The pressure in the column was 101 kPa. Water mostly without HA (0.05 g HA/l) was removed from the top of the column. Aqueous solution of HA without salt (product solution) was extracted in liquid form, with a concentration of 8.3 wt. %, via the side current from plate 12. The salt solution was removed at the bottom of the column with a residual HA content of 0.2 wt.%.

Example 5

Concentration of an aqueous solution of hydroxylamine without salt by distillation

In a glass column with a fractional bottom with a diameter of 50 mm and 30 fractional plates, a stabilized solution of hydroxylamine without salt was continuously supplied to the eighth plate. Additionally, on the highest plate, plate no. 30, a small amount of stabilizer dissolved in the hydroxylamine solution was measured into the column. The reflux ratio was set to 0.5. Water distilled from the top of the column. The distillate still contained a residual amount of hydroxylamine of 0.07 wt. %. Approx. 240 ml/h of hydroxylamine solution with a concentration of 50 wt. % was extracted using a pump from the bottom of the column.

Claims (12)

1. Procedure for separating the fraction, which contains a component of low and medium boiling points (L, M fractions) from a homogeneous mixture containing components of low, medium and high boiling points (L, M, H mixture), characterized by the fact that L, M , H mixture works at the bottom; column with low boiling point vapors and is separated into L, M fraction and L, H fraction. 2. The process according to claim 1, characterized in that the L, M, H mixture is processed by introducing low-boiling steam into the bottom of the column. 3. The method according to claim 1 or 2, characterized in that the processing takes place in countercurrent. 4. The process according to any of the preceding claims, characterized in that an extraction column is used as the column. 5. The process according to any of the previous claims, characterized in that the L, M fraction is extracted at the height of the feed plate or above it, especially over the top of the column. 6. The process according to any of the previous claims, characterized by the fact that the L, M fraction leads to a rectification column, in which the low-boiling component is separated, so as to obtain the L, M fraction enriched with the medium-boiling component. 7. Process: according to claim 6, characterized in that the rectification column is mounted on the treatment column, the low-boiling component is separated by overhead distillation, and the L, M fraction enriched with the medium-boiling component is removed by means of a side stream. 8. The method according to claim 7, characterized in that the treatment column is connected to the bottom of the side column and that the rectification column is connected to the top of the side column. 9. The process according to claim 1, indicated by the fact that the extraction/rectification column, at the height of the L, M, H mixture supply point, is equipped with a mostly vertical dividing wall. 10. The method according to any of the preceding claims, characterized in that the L,M fraction is separated. or concentrates on the medium boiling point fraction in a further column with a rectification section and an extraction section. 11. The process according to any of the preceding claims, characterized in that the low-boiling component, which is removed from the column, or the rectification column, if necessary after compression, goes at least partially back to the bottom of the column. 12. The process according to any of the preceding claims, characterized in that it is applied to obtain heat-sensitive products of medium boiling point.