WO2017089080A1

WO2017089080A1 - Process for purifying propene oxide

Info

Publication number: WO2017089080A1
Application number: PCT/EP2016/076321
Authority: WO
Inventors: Bärbel Kolbe; Manfred BÄRZ; Marc Brendel; Kidon Won; Hojin Yang; Soohyun PARK; Maik Bernhard
Original assignee: Evonik Degussa GmbH; ThyssenKrupp Industrial Solutions AG
Current assignee: ThyssenKrupp Industrial Solutions AG; Evonik Operations GmbH
Priority date: 2015-11-26
Filing date: 2016-11-02
Publication date: 2017-06-01
Anticipated expiration: 2018-05-26
Also published as: KR102022226B1; AR106813A1; TW201730170A; KR20180069835A

Abstract

Propene oxide is purified by extractive distillation, feeding crude propene oxide comprising from 15 to 97 % by weight of propene oxide, from 2 to 84 % by weight of methanol, and acetaldehyde to a feed point A in the middle section of an extractive distillation column; feeding an aqueous extraction solvent to a feed point B located above feed point A; and feeding a reactive compound of structure R¹-Y-NH₂, where Y is oxygen or NR² and R¹ and R² are independently of one another hydrogen, an alkyl group or an aryl group, or a salt of such reactive compound, to feed point B, feed point A or between feed points A and B; withdrawing a stream S1 from the top of the extractive distillation column to provide a first propene oxide product containing less than 200 ppm water; and withdrawing a stream S2 at a withdrawal point 5 to 25 theoretical separation stages above feed point B to provide a second propene oxide product containing 500 to 10000 ppm water.

Description

Process for purifying propene oxide

Field of the invention

The present invention relates to a process for purifying propene oxide which provides two propene oxide products of different purity, a first one suitable for making polyether polyols and a second one suitable for making 1 ,2-propanediol or propylene glycol monoethers.

Background of the invention

Making propene oxide by reacting propene with hydrogen peroxide in the presence of a titanium zeolite catalyst has the advantage of providing propene oxide without co-production of other compounds that have to be marketed and produces less waste than the classical chlorohydrin process. The reaction of propene with hydrogen peroxide in the presence of a titanium zeolite catalyst is usually carried out in a methanol solvent to achieve high reaction rates and product selectivity. The crude propene oxide provided by this reaction contains residual methanol solvent and volatile by-products formed in the reaction and the work-up, in particular the aldehydes formaldehyde, acetaldehyde and propionaldehyde, and their acetals with methanol and

1 ,2-propanediol, as well as methyl formate.

Most of the propene oxide is further reacted to polyether polyols which are used as monomers for making polyurethanes. For this purpose, the propene oxide has to be of high purity, containing less than 200 ppm water, less than 100 ppm methanol and less than 50 ppm acetaldehyde.

A significant part of the propene oxide is further reacted with water to 1 ,2-propanediol. For this purpose, the purity requirements are less stringent and water content is not critical.

Methanol, acetaldehyde and methyl formate are difficult to remove from propene oxide by simple distillation. In mixtures containing more than 98 mol-% propene oxide these compounds have essentially the same volatility than propene oxide. Therefore distillative purification to low levels of methanol, acetaldehyde and methyl formate is not feasible via conventional distillation.

Several methods for removing methanol, acetaldehyde and methyl formate from propene oxide by extractive distillation have been developed, but most of them have the disadvantage of requiring several distillation columns for providing high purity propene oxide.

WO 2004/048355 discloses a method for removing methanol and acetaldehyde from a crude propene oxide in a single distillation column by an extractive distillation where a compound containing an unsubstituted NH2 group is additionally fed at or above the feeding point of the crude propene oxide. The method provides propene oxide of high purity suitable for making polyether polyols. Summary of the invention

The inventors of the present invention have now found that modifying the method of

WO 2004/048355 by using water as the extraction solvent and withdrawing a second propene oxide product from the extractive distillation column above the feed point of extraction solvent provides a second propene oxide product suitable for making 1 ,2-propanediol and reduces energy consumption and equipment size of the propene oxide purification.

Subject of the invention is therefore a process for purifying propene oxide, comprising

a) feeding a crude propene oxide comprising from 15 to 97 % by weight of propene oxide, from 2 to 84 % by weight of methanol, and acetaldehyde to a feed point A in the middle section of an extractive distillation column;

b) feeding an aqueous extraction solvent to a feed point B of said extractive distillation column, said feed point B being located above feed point A;

c) feeding a reactive compound of structure (I)

(I) R -Y-NH₂

where Y is oxygen or NR² and R and R² independently of one another are hydrogen, an alkyl group or an aryl group, or a salt of a compound of structure (I)

to said extractive distillation column

i) at feed point B,

ii) at feed point A, or

iii) at a feed point C between feed points A and B;

d) withdrawing a stream S1 from the top of said extractive distillation column to provide a first propene oxide product; and

e) withdrawing a stream S2 from said extractive distillation column at a withdrawal point 5 to 25 theoretical separation stages above feed point B to provide a second propene oxide product. Brief description of drawings

Fig. 1 shows an embodiment of the invention where the reactive compound is fed to the extractive distillation column admixed with the aqueous extraction solvent.

Detailed description of the invention

The process of the invention for purifying propene oxide is carried out in an extractive distillation column. The extractive distillation column may be a tray column containing discrete trays such as sieve trays or bubble cap trays. The extractive distillation column may also be a packed column and both random packings as well as structured packings, such as metal gauze packings may be used. The extractive distillation column may also combine sections with discrete trays and sections with packings. The extractive distillation column will in general also comprise at least one overhead condenser and at least one column reboiler. The extractive distillation column has at least two feed points, a feed point A for feeding a crude propene oxide in the middle section of the extractive distillation column and a feed point B for feeding an aqueous extraction solvent located above feed point A. The feed points define three sections of the extractive distillation column, a stripping section between the column bottoms and feed point A, an extraction section between feed point A and feed point B and a rectifying section between feed point B and the top of the extractive distillation column. Preferably a distillation column is used that has a separation efficiency of 10 to 30 theoretical stages in the stripping section, a separation efficiency of 15 to 40 theoretical stages in the extraction section and a separation efficiency of 20 to 60 theoretical stages in the rectifying section, i.e. feed point B is preferably located from 15 to 40 theoretical separation stages above feed point A and from 20 to 60 theoretical separation stages below the top of the extractive distillation column.

A crude propene oxide comprising from 15 to 97 % by weight of propene oxide, from 2 to 84 % by weight of methanol, and acetaldehyde is fed to a feed point A of the extractive distillation column. The crude propene oxide preferably comprises from 4 to 4000 ppm by weight acetaldehyde. The crude propene oxide may also comprise other aldehydes and ketones, such as formaldehyde, propionaldehyde and acetone, as well as acetals of these aldehydes with methanol or

1 ,2-propanediol, such as dimethoxymethane, 1 , 1 -dimethoxyethane, 1 , 1 -dimethoxypropane, 4-methyl-1 ,3-dioxolane, 2,4-dimethyl-1 ,3-dioxolane and 2-ethyl-4-methyl-1 ,3-dioxolane. The crude propene oxide may also comprise further solvents, such as ethanol or water.

The crude propene oxide is preferably provided by an epoxidation reaction where propene is epoxidized in a methanol solvent using a titanium zeolite containing titanium atoms on silicon lattice positions. Preferably, a titanium silicalite catalyst is used, preferably with an MFI or MEL crystal structure. Most preferably a titanium silicalite 1 catalyst with MFI structure as known from

EP 0 100 1 19 A1 , is used . The titanium silicalite catalyst is preferably employed as a shaped catalyst in the form of granules, extrudates or shaped bodies. For the shaping process the catalyst may contain 1 to 99% of a binder or carrier material, all binders and carrier materials being suitable that do not react with hydrogen peroxide or with propene oxide under the reaction conditions employed for the epoxidation, silica being preferred as binder. Extrudates with a diameter of 1 to 5 mm are preferably used as fixed bed catalysts. The methanol solvent can be a technical grade methanol, a solvent stream recovered in the work-up of the epoxidation reaction mixture or a mixture of both.

The epoxidation reaction is preferably carried out at a temperature of 20 to 80°C, more preferably at 25 to 60°C, and at a pressure of from 1 .9 to 5.0 MPa, more preferably 2.1 to 3.6 MPa and most preferably 2.4 to 2.8 MPa. Propene is preferably used in excess to hydrogen peroxide, preferably with an initial molar ratio of propene to hydrogen peroxide of from 1 .1 : 1 to 30: 1 , more preferably 2: 1 to 10: 1 and most preferably 3: 1 to 5: 1 . Propene is preferably used in an excess sufficient to maintain an additional liquid phase rich in propene throughout the reaction. Using an excess of propene provides high reaction rate and hydrogen peroxide conversion and at the same time high selectivity for propene oxide. The propene may comprise propane, preferably with a mass ratio of propane to the combined amount of propene and propane of from 0.07 to 0.20, more preferably from 0.10 to 0.15.

Hydrogen peroxide can be used as an aqueous solution, preferably containing from 30 to 75 % by weight hydrogen peroxide and most preferably from 40 to 70 % by weight.

The epoxidation reaction is preferably carried out with addition of ammonia to improve propene oxide selectivity as described in EP 0 230 949 A2. Ammonia is preferably added in an amount of from 100 to 3000 ppm based on the weight of hydrogen peroxide.

The epoxidation reaction is preferably carried out continuously in a fixed bed reactor by passing a mixture comprising propene, hydrogen peroxide and methanol over a fixed bed comprising a shaped titanium silicalite catalyst. The fixed bed reactor is preferably equipped with cooling means and cooled with a liquid cooling medium. The temperature profile within this reactor is preferably maintained such that the cooling medium temperature of the cooling means is at least 40°C and the maximum temperature within the catalyst bed is 60°C at the most, preferably 55°C. The epoxidation reaction mixture is preferably passed through the catalyst bed in down flow mode, preferably with a superficial velocity from 1 to 100 m/h, more preferably 5 to 50 m/h, most preferred 5 to 30 m/h. The superficial velocity is defined as the ratio of volume flow rate/cross section of the catalyst bed . Additionally it is preferred to pass the reaction mixture through the catalyst bed with a liquid hourly space velocity (LHSV) from 1 to 20 h \ preferably 1 .3 to 15 h^~ . It is particularly preferred to maintain the catalyst bed in a trickle bed state during the epoxidation reaction. Suitable conditions for maintaining the trickle bed state during the epoxidation reaction are disclosed in

WO 02/085873 on page 8 line 23 to page 9 line 15. The methanol solvent is preferably used in the epoxidation in a weight ratio of 0.5 to 20 relative to the amount of aqueous hydrogen peroxide solution. The amount of catalyst employed may be varied within wide limits and is preferably chosen so that a hydrogen peroxide consumption of more than 90%, preferably more than 95%, is achieved within 1 minute to 5 hours under the employed epoxidation reaction conditions. Most preferably, the epoxidation reaction is carried out with a catalyst fixed bed maintained in a trickle bed state at a pressure close to the vapor pressure of propene at the reaction temperature, using an excess of propene that provides a reaction mixture comprising two liquid phases, a methanol rich phase and a propene rich liquid phase. Two or more fixed bed reactors may be operated in parallel or in series in order to be able to operate the epoxidation process continuously when regenerating the epoxidation catalyst. Regeneration of the epoxidation catalyst can be carried out by calcination, by treatment with a heated gas, preferably an oxygen containing gas or by a solvent wash, preferably by the periodic regeneration described in WO 2005/000827.

Non-reacted propene can be separated from the reaction mixture of the epoxidation reaction by distillation or by depressurization, preferably in a flash evaporator. Preferably, non-reacted propene is separated by depressurization to a pressure of from 0.16 to 0.30 MPa.

The liquid mixture remaining after depressurization is preferably separated by distillation in a pre- separation column to provide an overhead product comprising propene oxide, methanol and residual propene and a bottoms product comprising methanol, water and non-reacted hydrogen peroxide. The pre-separation column is preferably operated to provide an overhead product comprising from 20 to 60 % of the methanol contained in the liquid phase after depressurization. The pre-separation column preferably has from 5 to 20 theoretical separation stages in a stripping section and less than 3 theoretical separation stages in a rectifying section and is most preferably operated without reflux and without a rectifying section to minimize the residence time of propene oxide in the pre-separation column. The pre-separation column is preferably operated at a pressure of from 0.16 to 0.3 MPa. Propene oxide and methanol are condensed from the overhead product of the pre-separation column and propene is preferably stripped from the resulting condensate in a propene stripping column which provides the crude propene oxide as the bottom stream.

In the process of the invention for purifying propene oxide, an aqueous extraction solvent is fed to feed point B of the extractive distillation column. The aqueous extraction solvent preferably comprises more than 80 % by weight water, more preferably more than 90 % by weight water. Preferably, the aqueous extraction solvent comprises no further solvent in addition to water. The extraction solvent is preferably fed in an amount providing a mass ratio of the extraction solvent relative to the amount of methanol contained in the crude propene oxide feed of from 0.01 to 1 , more preferably from 0.03 to 0.2. The use of such an amount of aqueous extraction solvent provides effective extraction of methanol and a propene oxide product with a low content of methanol and at the same time avoids hydrolysis of propene oxide in the extractive distillation column.

In addition to the aqueous extraction solvent, a reactive compound of structure (I)

(I) R -Y-NH₂

where Y is oxygen or NR² and R and R² independently of one another are hydrogen, an alkyl group or an aryl group, is fed to the extractive distillation column. Alternatively, a salt of a compound of structure (I) can be fed . The reactive compound is preferably hydrazine, hydrazine hydrate or a hydrazinium salt. Reactive compounds of structure (I) react with carbonyl compounds at the conditions of the extractive distillation to give high boiling hydrazones, oximes or oxime ethers. The amount of the reactive compound fed to the distillation column is preferably chosen so that the molar ratio of the reactive compound relative to acetaldehyde is in the range of from 0.5 to 2. The use of such an amount of a reactive compound of structure (I) provides effective conversion of carbonyl compounds to high boiling compounds and provides a propene oxide product with a low content of acetaldehyde and other carbonyl compounds. At the same time, by-product formation by reactions of the reactive compound with propene oxide can be kept at a low level.

The reactive compound may be fed to the extractive distillation column at feed point B, at feed point A or at an additional feed point C between feed points A and B. Preferably, the reactive compound is fed to the extractive distillation column at feed point B admixed with the aqueous extraction solvent.

In the process of the invention for purifying propene oxide, a stream S1 is withdrawn from the top of the extractive distillation column to provide a first propene oxide product and a stream S2 is withdrawn at a withdrawal point 5 to 25 theoretical separation stages above feed point B to provide a second propene oxide product. The first propene oxide product withdrawn as stream S1 from the top of the extractive distillation column has a low content of water and typically comprises less than 100 ppm by weight of water. A propene oxide with such a low content of water is suitable for the manufacture of polyether polyols which can be used to make polyurethanes. The second propene oxide product withdrawn as stream S2 has a higher content of water which is typically in the range of from 500 to 10000 ppm by weight. A propene oxide with a content of water in this range is suitable for the manufacture of 1 ,2-propylene glycol and of propylene glycol ethers. Both propene oxide products have a low content of carbonyl compounds and typically contain less than 50 ppm by weight acetaldehyde. The mass ratio of stream S1 to stream S2 is preferably from 0.5 to 5.0. Stream S2 may be withdrawn as a liquid stream or as a vapor stream and is preferably withdrawn as a vapor stream, which provides a higher reflux ratio in the column section above the withdrawal point for stream S2 and thereby improves the purity of stream S1 . Withdrawing a part of the propene oxide product as a side stream S2 with a higher content of water considerably reduces the energy consumption of the extractive distillation compared to the prior art process where all of the propene oxide product is withdrawn at or near the column top.

In a further embodiment of the invention, the crude propene oxide is mixed with an aqueous alkaline solution and the resulting mixture is reacted for 1 to 60 minutes at a temperature of from 20 to 100°C before the mixture is fed to feed point A. The aqueous alkaline solution is preferably an aqueous solution of sodium hydroxide, potassium hydroxide, or sodium carbonate. Most preferred are aqueous sodium hydroxide solutions containing from 0.1 to 56 % by weight sodium hydroxide. The amount of the aqueous alkaline solution is preferably chosen so that the molar ratio of hydroxide ions introduced with the aqueous alkaline solution relative to the amount of methyl formate contained in the crude propene oxide is in the range from 1 .1 to 4. Reacting the crude propene oxide with an aqueous alkaline solution converts methyl formate contained in the crude propene oxide by hydrolyzing it to methanol and formate. The purified propene oxide obtained with this embodiment of the invention has a reduced content of methyl formate. Preferably the amount of aqueous alkaline solution is chosen to obtain a purified propene oxide having a content of methyl formate of less than 100 ppm by weight.

Fig. 1 shows an embodiment of the invention where the reactive compound is fed to the extractive distillation column admixed with the aqueous extraction solvent and stream S2 is withdrawn as a vapor stream. The extractive distillation column (1 ) has a feed point A (3) where the crude propene oxide (2) is fed and a feed point B (5) where the reactive compound is fed admixed with the aqueous extraction solvent (4). The extraction section between feed points A and B contains trays, whereas the stripping section and the rectifying section contain column packings. A stream S1 (6) is withdrawn from the top of the extractive distillation column and provides a first propene oxide product with a low content of water and a stream S2 (7) is withdrawn from the extractive distillation column at a withdrawal point (8) 5 to 25 theoretical separation stages above feed point B to provide a second propene oxide product with a higher content of water. Stream S2 is withdrawn as a vapor stream and is condensed outside the column in a condenser not shown.

List of reference signs:

1 Extractive distillation column

2 Crude propene oxide

3 Feed point A

4 Reactive compound admixed with the aqueous extraction solvent

5 Feed point B

6 Stream S1

7 Stream S2

Examples

The energy consumption of the column reboiler is calculated with the simulation software

AspenPlus for purifying a stream of 25830 kg/h of a crude propene oxide comprising 49 % by weight of propene oxide, 46 % by weight of methanol, and 400 ppm by weight acetaldehyde, using a stream of 2000 kg/h of water containing 0.5 % by weight of hydrazine as aqueous extraction solvent and reactive compound. The calculation is carried out for an extractive distillation column operated at 0.13 MPa and having 14 theoretical stages in the stripping section, 25 theoretical stages in the extraction section and 42 theoretical stages in the rectifying section with a withdrawal point for stream S2 15 theoretical stages above feed point B.

In example 1 , not according to the invention, no stream S2 is withdrawn and all propene oxide product is withdrawn at the column top. A stream S1 of 12500 kg/h propene oxide comprising 80 ppm by weight water is obtained at the column top. The energy consumption of the column reboiler is 7.32 MW.

In example 2, according to the invention, a stream S2 of 5000 kg/h propene oxide comprising 2000 ppm by weight water is withdrawn at the withdrawal point for stream S2 and a stream S1 of 7500 kg/h propene oxide comprising 80 ppm by weight water is withdrawn at the column top. In this case, the energy consumption of the column reboiler is 5.23 MW, i.e. 29 % lower than in example 1 .

Claims

Claims:

1 . A process for purifying propene oxide, comprising

a) feeding a crude propene oxide (2) comprising from 15 to 97 % by weight of propene oxide, from 2 to 84 % by weight of methanol, and acetaldehyde to a feed point A (3) in the middle section of an extractive distillation column (1 );

b) feeding an aqueous extraction solvent (4) to a feed point B (5) of said extractive distillation column, said feed point B being located above feed point A;

c) feeding a reactive compound of structure (I)

(I) R -Y-NH₂

to said extractive distillation column

i) at feed point B,

ii) at feed point A, or

iii) at a feed point C between feed points A and B;

d) withdrawing a stream S1 (6) from the top of said extractive distillation column to provide a first propene oxide product; and

e) withdrawing a stream S2 (7) from said extractive distillation column at a withdrawal point (8) 5 to 25 theoretical separation stages above feed point B to provide a second propene oxide product.

2. The process of claim 1 , wherein the reactive compound is hydrazine, hydrazine hydrate or a hydrazinium salt.

3. The process of claim 1 or 2, wherein the reactive compound is fed to said extractive distillation column at feed point B admixed with the aqueous extraction solvent.

4. The process of any one of claims 1 to 3, wherein the crude propene oxide comprises from 4 to 4000 ppm by weight acetaldehyde.

5. The process of any one of claims 1 to 4, wherein the molar ratio of the reactive compound to acetaldehyde is from 0.5 to 2.

6. The process of any one of claims 1 to 5, wherein stream S2 is withdrawn as a vapor stream.

7. The process of any one of claims 1 to 6, wherein the mass ratio of stream S1 to stream S2 is from 0.5 to 5.0.

8. The process of any one of claims 1 to 7, wherein the mass ratio of the extraction solvent fed relative to the amount of methanol contained in the crude propene oxide fed is from 0.01 to 1 .

9. The process of any one of claims 1 to 8, wherein feed point B is located from 15 to 40

theoretical separation stages above feed point A.

10. The process of any one of claims 1 to 9, wherein feed point B is located from 20 to 60

theoretical separation stages below the top of the extractive distillation column.

1. The process of any one of claims 1 to 10, wherein the crude propene oxide is mixed with an aqueous alkaline solution and the resulting mixture is reacted for 1 to 60 minutes at a temperature of from 20 to 100°C before said mixture is fed to feed point A.