DE102018219557A1 - Process for the production and purification of propylene glycol - Google Patents

Abstract

The present invention relates to a process for the preparation of propylene glycol by reaction of propylene oxide with water, the process comprising a distillative work-up (D) of the propylene glycol after synthesis to separate off by-products and water, and according to the invention the work-up of at least one of the distillative work-up (D. ) downstream separation step using a thin film evaporator or a falling film evaporator (12). During the distillation process, color-forming components can form through the reaction, which color very strongly and are present in very low concentrations, so that they cannot be detected using analytical methods that are standard for quality control. In addition, these coloring components boil very high compared to the other components in the product. In the context of the present invention, however, it was found that, due to the comparatively very high boiling point / boiling range, these coloring components can be removed in a subsequent one-stage evaporation step, so that a color-free end product is obtained.

Description

The present invention relates to a process for the preparation of propylene glycol by reaction of propylene oxide with water, the process comprising working up the propylene glycol by distillation after the synthesis in order to separate off by-products and water.

According to the prior art, propylene glycol is mainly produced by the reaction of propylene oxide and water. The propylene oxide used can be obtained by the so-called HPPO process (hydrogen peroxide propylene oxide).

A process for the epoxidation of olefinic compounds by means of hydrogen peroxide in the presence of synthetic zeolites is already in the EP 0 100 119 B1 described. The EP 1 377 563 B1 describes the catalytic oxidation of propene with hydrogen peroxide in a multi-phase reaction, using a titanium-containing zeolite as a catalyst.

From the WO 2017/089075 A1 A process for the preparation of propylene glycol (1,2-propanediol) is known, in which propene is first epoxidized with hydrogen peroxide in the presence of a catalyst mixture and the propylene oxide obtained is then reacted with water to give propylene glycol.

The epoxidation of propene to propylene oxide with hydrogen peroxide follows the reaction equation (1) shown below: C ₃ H ₆ + H ₂ O ₂ → C ₃ H ₆ O + H ₂ O (1)

The subsequent hydrolytic conversion of the propylene oxide obtained in reaction (1) to propylene glycol is usually carried out catalytically by ring opening the epoxide in the presence of an ion exchange resin or a smaller amount of an acid, in accordance with the reaction equation (2) shown below: C ₃ H ₆ O + H ₂ O → CH ₃ CHOHCH ₂ OH (C ₃ H ₈ O ₂ ) (2)

In the EP 2 389 347 B1 describes a process for the production of deodorized propylene glycol (1,2-propanediol) by working up by distillation of the hydrogenation product obtained in the hydrogenation of glycerol, in which the propylene glycol is purified from impurities by distillation, the contaminated propylene glycol first being fed into a low-boiling column and The low boilers, in particular methanol, 1-propanol, 2-propanol, water and low-boiling odorous substances are separated off at the top of the column, while a propylene glycol stream which still contains impurities and consists of at least 97% propylene glycol is drawn off in the bottom of the column. In a second step, a further column for purifying distillation is then used, from which purified, low-odor propylene glycol is withdrawn as a liquid side draw, while a preferably small stream of still contaminated, odor-containing propylene glycol is withdrawn via the top of the second column. High boilers such as ethylene glycol and glycerin are separated off at the bottom. From this document, the purification of propylene glycol by a multi-stage distillation is known, although in this process the propylene glycol is not produced by reacting propylene oxide with water, but by another process.

The EP 1 973 620 B1 describes a process for the separation of propylene glycol from aqueous compositions, in which an at least two-stage distillation with decreasing working pressures is proposed for the separation of the propylene glycol. It is stated that basically any type of distillation column can be used for the distillation, with falling film evaporators also being mentioned. According to the concrete examples in this document, this known process starts from residual material streams from the production of propylene glycol, which to a large extent (more than 97% by weight) consist of water and contain only about 2% propylene glycol. In a total of four successive distillations, streams are obtained at the top of the distillation columns, which contain purified water with more than 99% purity and are then subjected to reverse osmosis. A mixture obtained after the water has been separated off and which contains less than 30% by weight of water is then subjected to a further distillation step in which the propylene glycol is separated off from the mixture. A dividing wall column is used for this further step. This known process is therefore primarily concerned with recovering propylene glycol contained in small amounts in the predominantly water-containing residues streams in order to increase the yield of the propylene glycol synthesis. Further steps for the preparation of the propylene glycol, which serve to remove color and / or odor components, are not described in this document.

When working up propylene glycol, which is produced from propylene oxide and water, the propylene oxide being produced, for example, by means of the HPPO process mentioned at the outset, a conventional multi-stage evaporation is usually carried out in the conventional processes and then this is done Propylene glycol is purified by a distillation sequence, whereby it is separated from by-products and by-products as well as the remaining water. It is problematic that when the propylene glycol is prepared in pure form using this process, odor and color components occur which have a severe adverse effect on the product quality. In some cases, distillation attempts succeed in removing the odor components, but the product still has a slightly yellowish color which is not within the specifications of the propylene glycol specification. The type and concentration of the coloring components occurring here could not be determined in tests by the applicant in the context of the present invention, and components other than the propylene glycol itself could not be detected analytically. Nevertheless, these coloring components and possibly odor components are impurities which cannot be accepted in the high quality product aimed for within the scope of the invention.

The object of the present invention is to provide a process for the preparation of propylene glycol with distillative work-up of the propylene glycol with the features of the type mentioned at the outset, which is suitable for removing the odor-forming and coloring components mentioned.

The solution to the aforementioned problem is provided by a method of the type mentioned at the outset with the features of claim 1 and claim 2.

According to the invention, according to a first alternative solution variant, the working up comprises at least one separation step downstream of the working up by distillation using a thin film evaporator.

In experiments within the scope of the present invention, it was found that the color-imparting components accumulate in the sump during distillation, while the condensate was color-free. A clearly colored bottom product could also be decolorized by evaporation. This leads to the conclusion that the coloring components are extremely non-volatile substances. Therefore, their removal in a distillation sequence is not successful, because these coloring components appear to be created during the distillation.

In the present application, a thin-film evaporator is understood to mean an evaporator that evaporates solutions in a thin film. A thin-film evaporator can be operated in particular in a vacuum and is therefore suitable for the gentle separation of substance mixtures. For example, the liquid is directed from above onto a vertical evaporator surface, which is heated from the outside.

Using a suitable device, for example a rotating system with stirring blades, a thin product film is generated, thus optimizing the heat transfer and mass transfer. The lower-boiling fraction evaporates from the product film within a short time.

An alternative solution variant within the scope of the invention provides that the working up comprises at least one separation step downstream of the working up by distillation using a falling film evaporator.

In the present application, a falling-film evaporator or falling-stream evaporator means a device in which the liquid to be evaporated flows, for example, in a tube as a coherent liquid film. The heat transfer is largely determined by the film thickness and the degree of turbulence of the film. Since there is no static liquid column in a falling film evaporator, the evaporation can take place with small driving temperature differences between the heating medium and the liquid to be evaporated. Evaporation can take place at low absolute pressures and thus at low temperatures, so that falling film evaporators can be used particularly for the evaporation of temperature-sensitive liquids.

A preferred development of the method according to the invention provides that the propylene glycol is worked up by distillation in a distillation unit in which low boilers and / or higher glycols and / or further high boilers and / or water are separated from the propylene glycol to be obtained before the subsequent separation step connects.

This distillation unit can comprise, for example, at least two distillation columns connected in series. In this way, it is possible to remove the water, higher glycols such as diethylene glycol, triethylene glycol and higher, and other high-boiling substances from the reaction mixture, but the applicant suspects that the color-imparting and / or odor-forming components in the distillation unit during this distillative workup only arise, and as a result these components cannot be removed from the propylene glycol with the aid of this distillation process, even when using several distillation columns connected in series. Therefore, according to the teaching of the present invention, there is a further downstream separation step which follows the already multiple distillation and which is selected in an evaporator Falling film evaporators and thin film evaporators is carried out. In this separation step, which follows the distillative workup, color components and / or odor components are then removed from the propylene glycol.

As part of a further development of the method according to the invention, it is advantageous if pre-evaporation in an evaporation plant is also provided for the removal of preferably the predominant proportion of water from the reaction mixture before the distillative workup takes place. This predominant removal of the water reduces the volume of the mixture to be subsequently distilled and reduces the effort involved in the distillation and the subsequent separation step. This evaporation plant can comprise, for example, one or more evaporators connected in series.

A preferred development of the method provides that water separated from the reaction mixture in the evaporation plant and / or in the distillation unit is returned to the reaction step for the production of propylene glycol by reaction of propylene oxide with water. This measure reduces the amount of fresh water that has to be supplied to the reactor from outside during the hydrolysis of the propylene oxide.

According to a preferred development of the method according to the invention, the procedure in the separation step following the distillative workup is such that the propylene glycol previously worked up by distillation is fed to the top area of a falling film evaporator or thin film evaporator, with vaporous propylene glycol being removed in the lower area from the falling film evaporator or thin film evaporator and this being vaporized Propylene glycol is then condensed by means of a cooling device. The higher-boiling impurities, that is to say in particular also the coloring and odor-forming components, can accumulate in the evaporator in the liquid phase which forms in the sump and can then be removed from the system.

A preferred development of the task solution according to the invention provides that the heat of condensation generated during the workup is at least partially coupled back into the manufacturing process. This enables an energetic optimization of the process.

For example, according to a preferred development of the invention, it is advantageous in this context if the heat of condensation generated in the process is used for preheating the water which is used in the reactor when converting propylene oxide to propylene glycol.

According to a preferred development of the method according to the invention, vapor compression can be provided, for example, to recover the heat of condensation.

In the context of the present application, vapor compression is understood to mean a method for heating and cooling a distillation column which is known per se from the prior art. Heat is added to the distillation column in the sump in order to vaporize one or more substances from a mixture of substances. Heat is dissipated in the head to condense the vapors. Since the vapor has a lower dew point than the boiling point of the sump due to the different composition, the heat drawn off at the top of the column cannot be used directly to heat the sump. The vapor is therefore compressed during vapor compression. As a result, the temperature rises above the boiling point in the sump (driving temperature difference) and the heat of condensation can be used for evaporation.

For example, propylene glycol, preferably propylene glycol, can be vaporized with a high degree of purity in the case of vapor compression in the context of the invention.

Compression of the vapor pressure by a factor of at least approximately 1.5, preferably at least approximately 1.6, is preferably provided.

According to a preferred development of the invention, for example, at least two fans / vapor compressors connected in series are provided for vapor compression. This achieves the degree of vapor compression provided in the process.

In the context of the present invention, it is advantageous if the vapor compression comprises an evaporator which is operated with its own vapors as a heating medium. This measure enables an energetic optimization of the method, since in this case the supply of hot steam for heating the evaporator can be completely or at least partially eliminated.

According to a preferred development of the invention, the heat of condensation arises in a temperature range from approximately 120 ° C. to approximately 135 ° C.

The present invention furthermore relates to a method for producing and Purification of propylene glycol of the type described above, in which the propylene oxide used as starting material was previously obtained by reacting propylene with a peroxide. This is only one possible variant of the production of the propylene oxide, in particular according to the HPPO process mentioned at the beginning. However, the present invention is by no means restricted to this method for producing the propylene oxide as a precursor of the propylene glycol.

• 1 eine schematisch vereinfachte Darstellung der wesentlichen Komponenten einer erfindungsgemäßen Anlage zur Herstellung und Reinigung von Propylenglykol.

The present invention is explained in more detail below on the basis of exemplary embodiments with reference to the accompanying drawing. It shows:

• 1 a schematic simplified representation of the essential components of a plant according to the invention for the production and purification of propylene glycol.

Below is with reference to the 1 the basic structure of a plant according to the invention for the production and subsequent purification of propylene glycol is explained. Only the components of the system which are essential for understanding the invention are shown here. Some units of the plant are optional for the method according to the invention and are therefore in the 1 shown in dashed lines.

These are the parts of the system A to D in 1 and the associated line systems, which are first explained below. These system parts are only optional for the method according to the invention and essentially relate to steps upstream of the method. Nevertheless, the plant parts A to D at this point in order to provide a better understanding of the relationships that led the applicants to the development on which the present invention is based.

This in 1 shown module A symbolizes a plant for the production of propylene oxide, which is produced, for example, from propylene and hydrogen peroxide, other methods known to those skilled in the art for the production of propylene oxide also being considered. Which method is used here for the production of propylene oxide is therefore not relevant to the invention.

This in 1 shown system module B represents a reactor for the production of propylene glycol, here for example from propylene oxide and fresh water without the use of catalysts. Alternatively, catalysts can be used and / or water from the downstream evaporation / distillation units into the reactor B to be led back.

With the letter C. is in 1 An evaporation system for pre-evaporation or separation of the majority of water from the reaction mixture is shown, usually consisting of several evaporators connected in series.

This in 1 with the letter D Designated module relates to a distillation unit, usually consisting of several distillation columns connected in series. Here, the remaining portion of water from the reaction mixture, low boilers, higher glycols (eg diethylene glycol, triethylene glycol and higher), and further high boilers are separated from the propylene glycol to be obtained as a pure product. In this distillation unit, color-forming components can form through reactions during the distillation process, which color very strongly and are present in a very low concentration, so that they cannot be detected using analytical methods used as standard for quality control. In addition, these coloring components boil very high compared to the other components in the product. In extensive tests within the scope of the present invention, however, it was found that, due to the comparatively very high boiling point / boiling range, these coloring components can be separated off in the downstream, in particular one-stage, evaporation step according to the invention, so that a color-free end product is obtained.

The representation of the previously mentioned system parts A to D in 1 is simplified in a highly schematic manner and the description below is limited to a few aspects relevant to the understanding of the present invention. Via a first line 1 propylene becomes the first educt of the module A fed. Via a second line 2nd hydrogen peroxide is the second educt of the module A fed. In the module A the conversion of propylene with hydrogen peroxide to propylene oxide takes place. That in module A Product generated by this reaction is propylene oxide via the output line 3rd from the module A dissipated and the system module B fed, that is, the reactor for the production of propylene glycol. For this the system module B as a further educt via the line 4th Water supplied, optionally the supply of a catalyst at 22 to the plant module B can be provided if the reaction in the reactor B provides for a catalytic conversion. In this reaction, hydrolysis from propylene oxide, an epoxy, produces 1,2-propanediol with ring opening, which is also referred to as propylene glycol.

Via the output line 5 becomes from the reactor B a mixture of the product propylene glycol and unreacted water and higher glycols and the evaporation system C. fed. In the evaporation plant C. there is a pre-evaporation and removal of most of the water from the reaction mixture. Optionally, a return from in the evaporation system C. separated water via the line 6 to the reactor B be provided for a new implementation in the hydrolysis reaction. From the evaporation plant C. is about the line 7 a mixture of propylene glycol, higher glycols and optionally the residual water and the distillation unit D fed. In the distillation unit D the low boilers are separated off and sent via the line 9 dissipated. Furthermore, in the distillation unit D the water is separated and optionally via the pipe 10th returned to the evaporation plant C. . Higher glycols and other high boilers can also be separated and piped 8th be dissipated.

That after the separation of the low boilers and the water in the distillation unit D Pure propylene glycol obtained is then via the outlet line 11 from the distillation unit D dissipated and an evaporator downstream of the distillation 12 supplied, which can in particular be a falling film evaporator. Alternatively, comes for the evaporator 12 also a thin film evaporator, for example. That on the line 11 flowing propylene glycol is by means of the circulation pump 17th conveyed and then reaches the falling film evaporator in the upper area 12 and flows inside the falling film evaporator 12 for example in tubes due to gravitation downwards. The medium to be evaporated is heated via heating steam, which flows, for example, in the evaporator in cocurrent to the liquid to be evaporated, via the line 13 fed and also in the upper area of the evaporator 12 is fed. The medium to be evaporated can be a thin liquid film inside the tubes of the evaporator 12 Flow from top to bottom and the resulting two-phase mixture is then in the lower area of the evaporator 12 Cut. The heating steam condenses on the outside of the pipes in which the medium to be evaporated flows.

The condensed propylene glycol from the two-phase mixture, which may also contain impurities, can be collected in the bottom of the evaporator and via the circulation pump 17th to the evaporator head again 12 to be led back. Pure vaporous propylene glycol is passed through the outlet line 23 from the evaporator 12 dissipated. About the line 18th can downstream of the circulation pump 17th liquid medium can be partially removed from the circuit. There is also a condensate drain through the line 14 at the bottom of the evaporator's falling film section 12 possible. If necessary, a relief valve 24th for condensed propylene glycol downstream of the branch line 14 be provided in the line which supplies the condensed propylene glycol to the bottom of the evaporator after the expansion.

According to an optional variant of the invention, the evaporator can be heated 12 vapor compression can be provided, which represents an energetic optimization of the method. In principle, the evaporator 12 can also be easily heated with steam. If vapor compression is provided, one or more vapor compressors are connected in series 15 present, the vapors from the bottom of the evaporator 12 compress and feed to the evaporator at 16 in the upper area. Alternatively or simultaneously, 16 takes place in the upper region of the evaporator 13 the supply of hot steam from outside the system.

The color component-free vaporous propylene glycol cleaned in the falling film evaporator is removed from the evaporator 12 dissipated and over the line 23 a heat exchanger 25th supplied, which for cooling the propylene glycol via lines 19th a cooling medium is supplied and discharged from it. Gaseous inerts contained in the propylene glycol can be supplied via the line 20th from the heat exchanger 25th be dissipated. The withdrawal of colorless and odorless propylene glycol from the heat exchanger 25th and from the system takes place via the line 21st .

Reference symbol list

1

Propylene

2nd

Hydrogen peroxide

3rd

Propylene oxide

4th

water

5

Mixture of propylene glycol / higher glycols / water

6

Separated water (recycling optional)

7

Propylene glycol / higher glycols / residual water

8th

Higher glycols and other high boilers

9

Low boilers

10th

(Rest) water (return optional)

11

Propylene glycol (pure, after distillation)

12

Evaporator (here: downflow evaporator, optional / alternative thin evaporator)

13

Heating steam supply

14

Condensate drainage

15

Vapor compressor (optional / alternative)

16

Feed the brothers

17th

Evaporator circulation pump

18th

Ejection

19th

Cooling medium supply and discharge

20th

Deduction for gaseous inert

21st

Deduction for colorless / odorless propylene glycol

22

Catalyst feed (alternative / optional)

23

Vapor-shaped propylene glycol free of color components

24th

Expansion valve for condensed propylene glycol (optional / alternative)

25th

Heat exchanger

A

Plant module

B

Plant module / reactor

C.

Evaporation plant

D

Distillation unit

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 0100119 B1 [0003]
• EP 1377563 B1 [0003]
• WO 2017/089075 A1 [0004]
• EP 2389347 B1 [0007]
• EP 1973620 B1 [0008]

Claims (17)

A process for the preparation of propylene glycol by reaction of propylene oxide with water, the process comprising a distillative workup of the propylene glycol after the synthesis to separate by-products and water, characterized in that the workup uses at least one separation step downstream of the distillative workup (D) Includes thin film evaporator. A process for the preparation of propylene glycol by reaction of propylene oxide with water, the process comprising a distillative workup of the propylene glycol after the synthesis to separate by-products and water, characterized in that the workup uses at least one separation step downstream of the distillative workup (D) Falling film evaporator (12). Procedure according to Claim 1 or 2nd , characterized in that the distillative workup of the propylene glycol takes place in a distillation unit (D) in which low boilers and / or higher glycols and / or further high boilers and / or water are separated from the propylene glycol to be obtained before the subsequent separation step follows. Procedure according to one of the Claims 1 to 3rd , characterized in that a pre-evaporation in an evaporation plant (C) is provided for the removal of preferably the predominant proportion of water from the reaction mixture before the distillative workup takes place. Procedure according to one of the Claims 3 or 4th , characterized in that the distillation unit (D) comprises at least two distillation columns connected in series. Procedure according to one of the Claims 1 to 5 , characterized in that removal of color components and / or odor components from the propylene glycol takes place in the separation step following the distillative work-up (D). Procedure according to one of the Claims 3 or 4th , characterized in that in the evaporation plant (C) and / or in the distillation unit (D), water separated from the reaction mixture is returned to reaction step (B) for the production of propylene glycol by reaction of propylene oxide with water. Procedure according to one of the Claims 1 to 7 , characterized in that propylene glycol worked up by distillation is fed to the top area of a falling film evaporator (12) or thin film evaporator, vaporous propylene glycol is removed in the lower area from the falling film evaporator (12) or thin film evaporator and this is then condensed by means of a cooling device (19). Procedure according to one of the Claims 1 to 8th , characterized in that during the work-up in the subsequent separation step in the thin-film evaporator or in the falling film evaporator (12), the heat of condensation is at least partially coupled back into the manufacturing process. Procedure according to Claim 9 , characterized in that in the downstream separation step accumulating heat of condensation is used for preheating the water which is used in the conversion of propylene oxide to propylene glycol. Procedure according to one of the Claims 9 or 10th , characterized in that a vapor compression (15) is provided for the recovery of condensation heat in the downstream separation step. Procedure according to Claim 11 , characterized in that during vapor compression (15) propylene glycol, preferably propylene glycol, is evaporated with a high degree of purity. Procedure according to Claim 11 or 12 , characterized in that a compression of the vapor pressure by a factor of at least about 1.5, preferably at least about 1.6 is provided. Procedure according to one of the Claims 11 to 13 , characterized in that at least two fans / compressors (15) connected in series are provided for vapor compression. Procedure according to one of the Claims 11 to 14 , characterized in that the vapor compression (15) comprises an evaporator which is operated with its own vapors as a heating medium. Procedure according to one of the Claims 9 to 15 , characterized in that the heat of condensation occurs in a temperature range from about 120 ° C to about 135 ° C. Procedure according to one of the Claims 1 to 16 , characterized in that the propylene oxide used as starting material was previously obtained by reacting propylene with a peroxide.

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