Polymer recovery process in the filtration of polyether polyols

1. A filtration method for recovering purified polyether polyol comprising the steps of:

(a) providing an aqueous solution of a polyether polyol containing an alkali metal catalyst residual formed from a transesterification process utilizing an alkali metal catalyst;

(b) contacting the aqueous solution of a polyether polyol of step (a) with a stoichiometric excess of magnesium sulfate, magnesium sulfite or a combination thereof to form a second aqueous solution, wherein said stoichiometric excess is based on the amount of said alkali metal catalyst residual;

(c) removing water from said second aqueous solution of step (b) at a temperature in range from 125° C. to 145° C. to produce a dehydrated slurry containing a polyether polyol phase with a residual alkali metal content below 1 ppm and a precipitated solid phase comprising sulfate and/or sulfite salts of the alkali metal catalyst, magnesium hydroxide, and excess magnesium sulfate and/or sulfite, wherein the particle size distribution of said precipitated solid phase is controlled to minimize the amount of particles therein that are smaller than 3 microns, wherein the amount of particles in said precipitated solid phase that are smaller than 3 microns is below 15% by volume;

(d) passing the dehydrated slurry of step (c) through a filtration system to separate the polyether polyol phase from the precipitated solid phase; and

(e) recovering polyether polyol from the separated polyether polyol phase, wherein said polyether polyol is poly(tetramethylene ether) glycol or a copolymer thereof;

wherein removing water from said second aqueous solution of step (b) is accomplished in a dryer recirculation system; and

wherein controlling the particle size distribution of the precipitated solid phase of step (c) is accomplished by adjusting the pressure drop in the dryer recirculation system, and wherein the feed rate across the filtration system of step (d) is determined by the particle size distribution.

2. The method of claim 1 wherein the pressure drop in the dryer recirculation system is optimized to minimize the amount of particles in said precipitated solid phase to be smaller than 3 microns.

3. A filtration method for recovering purified polyether polyol comprising the steps of:

(a) providing an aqueous solution of a polyether polyol containing an alkali metal catalyst residual formed from a transesterification process utilizing an alkali metal catalyst;

(b) contacting the aqueous solution of a polyether polyol of step (a) with a stoichiometric excess of magnesium sulfate, magnesium sulfite or a combination thereof to form a second aqueous solution, wherein said stoichiometric excess is based on the amount of said alkali metal catalyst residual;

(c) removing water from said second aqueous solution of step (b) at a temperature in range from 125° C. to 145° C. to produce a dehydrated slurry containing a polyether polyol phase with a residual alkali metal content below 1 ppm and a precipitated solid phase comprising sulfate and/or sulfite salts of the alkali metal catalyst, magnesium hydroxide, and excess magnesium sulfate and/or sulfite, wherein the particle size distribution of said precipitated solid phase is controlled to minimize the amount of particles therein that are smaller than 3 microns, wherein the amount of particles in said precipitated solid phase that are smaller than 3 microns is below 15% by volume;

(d) passing the dehydrated slurry of step (c) through a filtration system to separate the polyether polyol phase from the precipitated solid phase; and

(e) recovering polyether polyol from the separated polyether polyol phase, wherein said polyether polyol is poly(tetramethylene ether) glycol or a copolymer thereof;

wherein removing water from said second aqueous solution of step (b) is accomplished in a dryer recirculation system; and

wherein controlling the particle size distribution of the precipitated solid phase of step (c) is accomplished by adjusting the pressure drop in the dryer recirculation system, and wherein the feed rate across the filtration system of step (d) is determined by the particle size distribution;

wherein the pressure drop in the dryer recirculation system is optimized to minimize the amount of particles in said precipitated solid phase to be smaller than 3 microns; and wherein said dryer recirculation system includes a recirculation valve, said recirculation valve position adjustable to control the pressure drop in the dryer recirculation system.

4. The method of claim 3 wherein the position of said recirculation valve is determined based on the molecular weight of the polyether polyol from step (a).

5. The method of claim 1 wherein said alkali metal catalyst is selected from the group consisting of alkali metal hydroxide, alkali metal alkoxide, alkaline earth metal hydroxide, alkaline earth metal alkoxide, and combinations thereof, and said precipitated solid phase comprises magnesium hydroxide, sulfate salts of said alkali metal catalyst, and excess magnesium sulfate, magnesium sulfite or mixture thereof.

6. The method of claim 1 wherein said alkali metal catalyst is sodium methylate and said precipitated solid phase comprises magnesium hydroxide and sodium sulfate.

7. The method of claim 1 wherein said polyether polyol has a molecular weight ranging from 650 to 3000 Daltons.