GB2351085A

GB2351085A - Process for the preparation of polyether polyols

Info

Publication number: GB2351085A
Application number: GB0013788A
Authority: GB
Inventors: Have Marie Antoinete Ten; Martine Kathrin Stegeman
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1999-06-08
Filing date: 2000-06-06
Publication date: 2000-12-20
Also published as: GB0013788D0

Abstract

Process for the preparation of a polyether polyol comprising the steps of <SL> <LI>(a) reacting an initiator having at least two active hydrogen atoms with at least one alkylene oxide in the presence of a catalyst comprising potassium hydroxide; <LI>(b) neutralising the polyether polyol reaction product obtained in step (a) by adding phosphoric acid to this reaction product; <LI>(c) simultaneously with or subsequently to step (b) adding water to the polyether polyol reaction product thereby forming a two phase system comprising a potassium phosphate-containing aqueous layer and an organic layer containing the polyether polyol; <LI>(d) separating both layers yielding a polyether polyol product and an aqueous phosphate-containing product (brine); <LI>(e) treating the brine to form in water insoluble phosphate salts which precipitate; and <LI>(f) removing the phosphate salts thus prepared yielding a purified aqueous product. </SL> Step d) may be effected by centrifugation or settling.

Description

2351085 PROCESS FOR THE PREPARATION OF POLYETHER POLYOLS The present

invention relates to a process for the preparation of polyether polyols yielding a polyether polyol product and a purified waste water stream.

Methods for preparing polyether polyols, also sometimes referred to as poly(alkylene oxide) polyols, are well known in the art. Typically, such methods involve reacting a starting compound having a plurality of active hydrogen atoms with one or more alkylene oxides, such as ethylene oxide, propylene oxide, butylene oxide or mixtures of two or more of these. Suitable starting compounds include polyfunctional alcohols, generally containing 2 to 6 hydroxyl groups. Examples of such alcohols are glycol, such as diethylene glycol, dipropylene glycol, glycerol, di- and polyglycerols, pentaerythritol, trimethylolpropane, triethanolamine, sorbitol, mannitol, etc. Usually a strong base like potassium hydroxide is used as a catalyst in this type of reaction.

After the polymerization reaction has completed the potassium has to be removed from the polymerization product (neutralization). Several ways for achieving this are known in the art. For instance, removal of the potassium ions by ion exchange can be applied. However, additional solvent is needed to reduce the viscosity of the polyol product sufficiently to enable an effective ion exchange. The use of such additional solvent introduces the risk of leakage, which is undesired from an environmental viewpoint, and moreover the method is expensive while the costs are even further increased by regeneration facilities in order to enable a solvent recycle. Another method to remove potassium from the polyether polyol product is by using absorbents like magnesium silicate. The disadvantages of this method are the high cost and the high amount of solid waste created.

Yet another common for removing potassium is by adding one or more anions to the polyol product, which form insoluble salts with the potassium ions. Phosphoric acid is often used as the anion source. The insoluble potassium phosphates formed are subsequently removed, for instance by filtration. Main disadvantages of filtration, however, are the fact that it is a laborious method because of the high viscosity of the polyol and the fact that the filtercake needs to be disposed of while it contains salt, filter aid and polyol product, thereby losing product.

It was found that the above method for preparing polyether polyols and in particular the techniques used for removing potassium residues from the polyether polyol product are not optimal and still leave room for improvement. The present invention aims to provide a process for preparing polyether polyols, wherein the potassium-containing catalyst residues are effectively removed in an environmentally and economically attractive manner.

Accordingly, the present invention relates to a process for the preparation of a polyether polyol comprising the steps of (a) reacting an initiator having at least two active hydrogen atoms with at least one alkylene oxide in the presence of a catalyst comprising potassium hydroxide; (b) neutralising the polyether polyol reaction product obtained in step (a) by adding phosphoric acid to this reaction product; (c) simultaneously with or subsequently to step (b) adding water to the polyether polyol reaction product thereby forming a two phase system comprising a potassium phosphate-containing aqueous layer and an organic layer containing the polyether polyol; (d) separating both layers yielding a polyether polyol product and an aqueous potassium phosphate-containing product (brine); (e) treating the brine to form in water insoluble phosphate salts which precipitate; and (f) removing the phosphate salts thus prepared yielding a purified aqueous product.

In step (a) of the process the polyether polyols are typically prepared by reacting a mixture of initiator compound and potassium hydroxide catalyst at a temperature of from 80 to 150 OC, more particularly from 90 to 130 OC, with at least one alkylene oxide at such a rate that the alkylene oxide is taken up by the reaction mixture in approximately 2 to 30 hours, preferably 3 to hours, at atmospheric pressure. Higher pressures may also be applied, but the pressure will usually not exceed 20 bar and preferably is from 1 to 5 bar. The alkylene oxide may be diluted with inert gas, such as nitrogen, and normally the alkylene oxide is added to the reaction mixture in the course of the reaction. When mixed alkylene oxides are used, such as mixture of propylene oxide and ethylene oxide, random polyether polyols will be formed. Successive addition of different alkylene oxides will result in block copolymeric polyether polyols. Preferred alkylene oxides are propylene oxide and ethylene oxide.

In step (b) of the present process the actual neutralisation of the polyether polyol reaction product formed in step (a) takes place by adding phosphoric acid to this reaction product. As a result, the potassium ions of the catalyst which are still present in the polyether polyol product react into potassium phosphate. The polyol product react into potassium phosphate. The expression "potassium phosphate" as used throughout this specification refers to salts comprising both potassium and phosphate and hence include potassium dihydrogen phosphate, dipotassium monohydrogen phosphate and tripotassium phosphate.

In step (c) of the present process water is added to the polyether polyol reaction product. The potassium phosphate formed in step (b) will dissolve in the water and hence the amount of water added should preferably be sufficient to dissolve essentially all potassium phosphate present in the product of step (b). At the same time the amount of water should be such that a two-phase system can be formed, i.e. the amount of water added should exceed the solubility of water in the particular polyol to be treated under the process conditions applied. A two-phase system is formed consisting of a potassium phosphate-containing aqueous layer (brine) and an organic layer containing the polyether polyol product.

As indicated herein before steps (b) and (c) can be carried out consecutively or simultaneously.

If step (b) is carried out before step (c), the phosphoric acid is suitably added in the form of a concentrated solution in water, for instance in a concentration of 50-90 wt%, suitably 65 to 90 wt%. The amount of phosphoric acid added should be sufficient to neutralise all potassium present in the polyether polyol reaction product. As a result of the addition of phosphoric acid potassium phosphate is formed. Potassium phosphate is insoluble in polyether polyol and hence will precipitate. In the subsequent step (c), wherein water is added, the potassium phosphate will then dissolve in water.

Both concentrated phosphoric acid and water suitably have a temperature of from 10 to 35 'C and most suitably are at room temperature. The polyether polyol product of step (a), though, has a higher temperature, which will normally be between 80 and 120 OC when the phosphoric acid is added. When the water is added the temperature of the neutralised polyether polyol product should suitably have decreased to below 100 'C to prevent boiling of the water added. The pressure applied during steps (b) and (c) is arbitrary and may suitably range from essentially 0 bar up to 15 bar, preferably from 0.1 to 5 bar. If step (b) is carried out in vacuo, then the vacuum should be broken once step (b) is finished. The time necessary to carry out the reaction in step (b) may also vary within wide limits. Depending on the amount of phosphoric acid used and the temperatures applied, the length of step (b) will normally vary from several minutes to several hours, suitably from 10 minutes to 5 hours. It is preferred that while step (b) is carried out continuous mixing of the components takes place to ensure optimum contact between the reactants. In subsequent step (c) stirring suitably takes place until all precipitate formed in step (b) has dissolved. Then stirring is stopped, so that the two-phase system can be formed.

If step (b) is carried out simultaneously with step (c), the potassium phosphate formed will immediately dissolve in the water. In this embodiment an aqueous solution of phosphoric acid is suitably used, which has a phosphoric acid concentration as low as 1 to 25 wt%, more suitably 5 to 20 wt%, even more suitably 10 to 15 wt%.

Because the volume of the phosphoric acid solution in this case is larger than in the case where steps (b) and (c) are carried out consecutively, this embodiment can suitably be effected by pumping the polyether polyol product of step (a) into a neutralisation vessel containing the aqueous solution of phosphoric acid.

The conditions applied in simultaneous steps (b) and (c) are suitably such that the temperature of the aqueous solution of phosphoric acid is 10 to 35 'C, more suitably room temperature, while the temperature of the polyether polyol product with which it is combined is 80 and 'C. For the pressure the same applies as indicated above for steps (b) and (c).

After step (c) a two-phase system is obtained containing two liquid layers. These liquid layers are subsequently separated in step (d). Such separation can be effected in ways known in the art e.g. in a coalescer settler or by centrifugation. As a result a brine stream containing the phosphate salts and a polyether polyol product is obtained. The conditions under which step (d) is carried out depends on the separation technique used, but in general the temperature may range from 25 to 'C and the pressure from essentially 0 bar to 15 bar.

It is within the common skills of the person skilled in the art to select the precise conditions for the specific separation technique used.

The brine stream contains a relatively high concentration of phosphates and for that reason it is undesirable to dispose this stream into the environment without further purification. Therefore, the brine stream is further treated in step (e) to form in water insoluble phosphate salts which precipitate.

Such formation of water insoluble phosphate salts can suitably be achieved by the addition of a reagent comprising one or more cations which form crystalline, in water insoluble salts with the phosphate. Suitable cations in this connection include calcium, magnesium and ammonium ions as well as combinations thereof. The reagent typically comprises one or more salts of one or more suitable cations and is soluble in water. Such salts are well known to the skilled person. The in water insoluble salts formed may be calcium hydrogen phosphate, calcium ammonium phosphate, calcium phosphate, potassium magnesium phosphate, magnesium ammonium phosphate and the like. A reagent comprising calcium ions as cation, i.e. a water soluble calcium salt, is preferred.

In a preferred embodiment of the present invention step (e) additionally comprises contacting the brine with a seed material which promotes the formation of crystalline, in water insoluble salts with the phosphate.

This is preferably effected by contacting the brine with a fluidized bed of grains of the seed material, which bed is kept in fluidization by the brine stream, and wherein the reagent is added in such way that the insoluble phosphate salts are formed on the seed material. A very suitable method to remove the phosphates from the brine using a fluidized bed of grains of seed material to effect crystallisation and precipitation of phosphate salts is described in US-4,389,317. A major advantage of applying the method described in this U.S. patent in the present process is the fact that the phosphate salts formed have a large crystal size and are not part of a bulky sludge. Thus, these salts are easy to handle and make utilisation of the phosphate as fertiliser feasible.

Finally, in step (f) of the present process the precipitate formed in step (e) is removed, thereby yielding a purified water stream and a solid phosphate containing product, which after drying can for instance be used as fertiliser. Removal of the precipitate can be effected by any suitable solid-liquid separation technique known in the art, for instance by filtration.

The aqueous phase obtained now is essentially free of phosphates and hence can be discharged into surface water without any problems.

Claims

C L A I M S

1. Process for the preparation of a polyether polyol comprising the steps of (a) reacting an initiator having at least two active hydrogen atoms with at least one alkylene oxide in the presence of a catalyst comprising potassium hydroxide; (b) neutralising the polyether polyol reaction product obtained in step (a) by adding phosphoric acid to this reaction product; (C) simultaneously with or subsequently to step (b) adding water to the polyether polyol reaction product thereby forming a two phase system comprising a potassium phosphate-containing aqueous layer and an organic layer containing the polyether polyol; (d) separating both layers yielding a polyether polyol product and an aqueous phosphate-containing product (brine); (e) treating the brine to form in water insoluble phosphate salts which precipitate; and (f) removing the phosphate salts thus prepared yielding a purified aqueous product.

2. Process as claimed in claim 1, wherein steps (b) and (C) are carried out consecutively.

3. Process as claimed in claim 1, wherein steps (b) and (C) are carried out simultaneously.

4. Process as claimed in claim 2 or 3, wherein sufficient water is added to dissolve essentially all the potassium phosphate formed and to form a two-phase system.

5. Process as claimed in any one of claims 1-4, wherein step (d) is effected by centrifugation or settling.

6. Process as claimed in any one of claims 1-5, wherein step (e) comprises the addition of a reagent comprising one or more cations which form crystalline, in water insoluble salts with the phosphate.

7. Process as claimed in claim 6, wherein the reagent comprises calcium, magnesium and/or ammonium ions as cation.

8. Process as claimed in claim 6 or 7, wherein step (e) additionally comprises contacting the brine with a seed material which promotes the formation of crystalline, in water insoluble salts with the phosphate.

9. Process as claimed in claim 6 or 7 and in claim 8, wherein the brine is contacted with a fluidized bed of grains of the seed material, which bed is kept in fluidization by the brine stream, and wherein the reagent is added in such way that the insoluble phosphate salts are formed on the seed material.