HK1180357B - Polymer recovery process in the filtration of polyether polyols - Google Patents

Abstract

A filtration method is disclosed for recovering purified polyether polyol comprising the steps of providing an aqueous solution of a polyether polyol containing an alkali metal catalyst residual formed from a transesterification process utilizing an alkali metal catalyst, contacting the aqueous solution 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. Water is removed from the second aqueous solution at a temperature above a set limit of said polyether polyol to produce a dehydrated slurry containing a polyether polyol phase substantially free of residual alkali metal and a precipitated solid phase comprising sulfate and/or sulfite salts of the alkali metal catalyst, magnesium hydroxide, and excess magnesium sulfate and/or sulfide, 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. The dehydrated slurry is then passed through a filtration system to separate the polyether polyol phase from the precipitated solid phase.

Description

Method for recovering polymer in polyether polyol filtration

Technical Field

The present disclosure relates to a process for producing polyether polyols. More particularly, it relates to a process for improving filtration of residual metal catalyst to recover a purified polyether polyol product.

Background

Homopolymers of THF, also known as polytetramethylene ether glycol (PTMEG), are well known for spandex, polyurethanes, and other elastomers. These homopolymers impart superior mechanical and dynamic properties to polyurethane elastomers, fibers, and other forms of end products. As discussed in U.S. patent No. 4,120,903, a polymerization process for making polytetramethylene ether glycol (PTMEG) using Tetrahydrofuran (THF) via an intermediate PTMEA (i.e., PTMEG diacetate) has been commercially practiced since about 1997. The process involves THF ring opening using perfluorosulfonic acid ionomer resin as the first step in PTMEA production. The most commonly known method for converting PTMEA to PTMEG is by conventional transesterification using an alkali metal catalyst such as sodium methoxide. This process produces residual catalyst that needs to be removed from the PTMEG product.

There are many known methods for removing residual alkali metal catalyst from PTMEG product after the transesterification step. Some of these known methods are disclosed in U.S. patent nos. 4,137,396, 4,985,551, 4,460,796, 4,306,943, and 6,037,381. U.S. patent No. 5,410,093, which is incorporated herein by reference in its entirety, relates to a process in which an alkali metal catalyst is neutralized in an aqueous medium in the presence of excess magnesium sulfate. The inorganic by-products of the neutralization may include sodium sulfate and magnesium hydroxide. The various inorganic solids present in the PTMEG were then separated in a chamber plate filter press (chamberplate filter) operation. Prior to filtration, water must be removed from the solution containing the residual catalyst.

It is desirable to increase the filtration feed rate and filtration throughput between shut down and clean up to improve the catalyst removal process. Moreover, maintaining a pressure drop across the filtration system is a common problem affecting both filtration feed rate and filtration throughput between shutdowns. It is well known that increased pressure drop can result in a decrease in the feed rate into the filtration system. This will also cause a reduction in throughput between shut down and cleaning of the filtration system.

Thus, in processes for recovering polyether polyol products, there is a need for a method of maintaining a desired pressure drop across a filtration system to increase filtration feed rate and increase throughput between shut down and cleaning of the filtration system.

Disclosure of Invention

A method of maintaining a desired pressure drop across a filtration system to increase filtration feed rate and increase throughput between shut down and cleaning of the filtration system in a process for recovering polyether polyol product is disclosed.

The pressure drop across the filter is maintained at a desired level by controlling the particle size distribution of the particles exiting the dryer system before feeding them into the filtration system. In one embodiment of the invention, the particle size distribution is controlled by adjusting the recirculation valve position of a dryer recirculation valve that controls the pressure drop in the dryer system and allows for recirculation of particles in the dryer system.

Embodiments include the following steps:

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

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

(c) removing water from the second aqueous solution of step (b) at a temperature in the range of from about 125 ℃ to about 145 ℃ to produce a dehydrated slurry containing a polyether polyol phase having a residual alkali metal content of less than 1ppm and a precipitated solid phase comprising sulfate and/or sulfite salts of an alkali metal catalyst, magnesium hydroxide, and excess magnesium sulfate and/or sulfite, wherein the particle size distribution of the precipitated solid phase is controlled to minimize the amount of particles therein of less than 3 microns;

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

(e) polyether polyol is recovered from the separated polyether polyol phase.

In another embodiment, removing water from said second aqueous solution of step (b) is accomplished in a dryer recirculation system, wherein said dryer recirculation system comprises a primary dryer and a primary dryer heater, and optionally a secondary dryer and a secondary dryer heater.

In another embodiment, 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.

In another embodiment, the pressure drop in the dryer recirculation system is optimized to minimize the amount of particles smaller than 3 microns in the precipitated solid phase.

In another embodiment, the amount of particles smaller than 3 microns in the precipitated solid phase is less than 15% by volume.

In another embodiment, the dryer recirculation system includes a recirculation valve that is adjustable in position to control a pressure drop in the dryer recirculation system.

In another embodiment, the recirculation valve position is determined based on the molecular weight of the polyether polyol from step (a).

In another embodiment, the alkali metal catalyst is selected from the group consisting of: an alkali metal hydroxide, an alkali metal alkoxide, an alkaline earth metal hydroxide, an alkaline earth metal alkoxide, and combinations thereof, and the precipitated solid phase comprises magnesium hydroxide, a sulfate salt of the alkali metal catalyst and excess magnesium sulfate, magnesium sulfite, or mixtures thereof.

In another embodiment, the alkali metal catalyst is sodium methoxide and the precipitated solid phase comprises magnesium hydroxide and sodium sulfate.

In another embodiment, the polyether polyol has a molecular weight in the range of 650 to 3000 daltons.

In another embodiment, the polyether polyol is a poly (tetramethylene ether) glycol or a copolymer thereof.

Drawings

Fig. 1 is a process diagram depicting the prior art.

Figure 2 is a process diagram of one embodiment of the present invention.

FIG. 3 is a graph showing particle size distributions at different recirculation valve positions.

Figure 4 is a graph showing filter press throughput at different recirculation valve positions.

Detailed Description

A method of maintaining a desired pressure drop across a filtration system by controlling the particle size distribution of particles exiting a dryer system prior to feeding into the filtration system is disclosed. In one embodiment of the invention, the particle size distribution is controlled by adjusting the valve position of a dryer recycle valve that controls the recycle of particles in the dryer.

All patents, patent applications, test procedures, priority documents, articles, publications, manuals, and other documents cited herein are fully incorporated by reference to the extent that they are: such disclosure is not inconsistent with the present invention and where such incorporation is permitted for all jurisdictions.

The term "polymerization" as used herein includes within its meaning the term "copolymerization" unless otherwise stated.

The term "PTMEG" as used herein, unless otherwise stated, refers to poly (tetramethylene ether glycol). PTMEG is also known as polyoxybutylene glycol.

The term "THF" as used herein, unless otherwise indicated, refers to tetrahydrofuran and includes within its meaning alkyl-substituted tetrahydrofurans copolymerizable with THF, such as 2-methyltetrahydrofuran, 3-methyltetrahydrofuran and 3-ethyltetrahydrofuran.

The process from U.S. Pat. No. 5,410,093 is shown in FIG. 1 with fixedA recirculation valve (160). In a PTMEG manufacturing process, a polymer stream (not shown) containing alkali metal catalyst residues formed from a transesterification process employing an alkali metal catalyst is contacted with a stoichiometric excess of magnesium sulfate, magnesium sulfite, or a combination thereof, and water in a mixer (not shown) to form an aqueous polymer stream (80). The aqueous polymer stream (80) carries with it solid Mg (OH)₂Dissolved MgSO₄And Na₂SO₄Which is fed to the primary dryer (100) recycle stream. These inorganic salts are formed in the neutralization reaction in the previous step in the manufacturing method.

These inorganic salts must be sufficiently removed from the product to meet customer requirements prior to storage as a finished product. Drying and filtration of these salts was achieved using a 2-stage dryer system (100 and 200) followed by a plate and frame cloth filter (280).

Drying is achieved using a primary dryer (100), a heater (140) under vacuum, and recycling back to the primary dryer allows the stream to reach steady state before it is fed to the filter press (280). Further drying was achieved using a secondary dryer (200) and a vacuum condenser (120) to remove traces of water prior to filtration. Typically, the dryer (100, 200) operates at a temperature in the range of 125 ℃ to 145 ℃ and a pressure in the range of about 40 to about 100mmHg absolute vacuum. In one embodiment of the invention, the target temperature for the process is 130 ℃.

As the heated recycle stream enters the primary dryer through a fixed recycle valve (160), water is flashed off and evaporated from the bulk recycle stream. The dissolved salts in the stream come out of solution to form solid crystals of known distribution, which are then filtered off in a filter press (280). The present inventors have found that crystal formation and solids distribution in the polymer have a significant impact on the efficiency of the filter press.

As shown in fig. 2, a specific embodiment of the present invention is shown and will be described herein. The process for filtering out inorganic solids from the product consists of the following processing equipment:

will contain solid Mg (OH)₂Dissolved MgSO 2₄And Na₂SO₄The product of dissolved inorganic solids is passed through a primary dryer (100) in a circulation loop containing a circulation pump (180), a heater (140), and a restriction orifice. Part of the product stream is withdrawn to a secondary dryer (200). This 2-stage drying process ensures complete removal of moisture prior to entering the filter press (280). Vacuum is drawn from the dryer through a condenser (120) to a storage tank (holduptank) (220). A similar circulation loop with its own pump (240) and heater (260) is installed around the secondary dryer to remove residual moisture. The secondary system is also connected to the same vacuum system with a condenser to collect the condensate in the tank (220). A partial stream is withdrawn from the circuit as a forward feed to the filter press (280).

The inventors have found that filter press (280) operation is very sensitive to recycle valve (165) position for products with molecular weights in the range above 1600 daltons, as measured by crystal formation, as opposed to lower molecular weight grades which are more tolerant of valve position. For higher molecular weight grades, it has also been found that lowering the recycle valve (165) position results in less formation of small particle size crystals in the dryer. This has been found to significantly alter the particle size distribution and reduce the pressure drop across the filter press (280) at a given flow rate. It has been found that keeping the particle size above 3 microns has resulted in an optimal filtration operation. At optimum operating conditions, reduced filtration pressure drop allows for higher filtration feed rates, thus increasing throughput. This also increases the total cumulative throughput between shut down and cleaning of the filtration system.

The position of the recycle valve helps maintain the particle size distribution by controlling the pressure drop through the dryer system. The rheology of the molten polymer circulated through the dryer system experiences extreme shear as it passes through the recycle valve. This results in a change in the particle size of the suspended salt as it comes out of solution. The resulting particle size distribution is dependent on shear energy as a function of pressure drop in the dryer recycle loop, which can be adjusted by varying the recycle valve (165) position as a function of polymer grade. In one embodiment of the invention, the optimum filtration operation is achieved when the particle size distribution of the crystals leaving the dryer is maintained so that less than 15% by volume of the particles are less than 3 microns in size.

The adjustable recycle valve (165) also allows for the production of different grades of polymer with molecular weights in the range of 650 to 3,000 daltons in the same manufacturing facility. Various recirculation valve (165) positions were tried for different levels to find the optimal setting. The standard equation for predicting filter cake filtration pressure drop is:

ΔP＝k_(Mc)μM_cu/A

wherein: Δ P ═ pressure drop across filter cake

Flow rate of polymer

Mu. = polymer viscosity (function of temperature and molecular weight)

M_cFilter cake mass (estimated by cumulative pounds to press)

_(Mc)Porosity as a function of cake mass, particle size, etc

A is the filtration area

u/A ═ flow-through rate

k is a constant which is a function of filtrate particle size, density and volume

Combination term k_(Mc)Is considered to be the specific resistance of the filter cake (specific cake resistance). This value will depend on the solids produced during manufacture and the filter aid selected. The molecular weight and temperature of the polymer are taken into account in the viscosity term. At the same temperature and flow rate, increasing the molecular weight will increase the viscosity and thus increase the pressure drop. This increase in pressure drop is independent of the type of filter aid. The polymer flow rate and the filtration area dictate the total flux. The filtration run is typically terminated when the pressure drop reaches the maximum limit of the feed pump (240) or the process limit setting based on safety considerations.

Examples

The following examples disclose the method and its ability to be used. The invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the scope and spirit of the present invention. Accordingly, the embodiments are to be construed as illustrative and not restrictive in nature.

Example 1

Example 1 consider the use of a 1000-2000 scale from the INVISTALPORTE fieldData obtained for PTMEG samples. The dryer (100) operates at a temperature target of 130 ℃ and a vacuum pressure range of 40-100mmHg absolute. Fig. 3 shows the particle size distribution from the dryer (100) with the recycle valve (165) position set at 80% and 32% (the recycle valve position can be set between 0-100% of the valve Output (OP). The particle size distribution was determined in a small volume mode using a Beckman-Coulter LS230 particle size analyzer. The particle size distribution pattern was found to be different as the circulation valve position was changed. At the 32% lowered valve (165) position, the amount of particles smaller than 3 microns is reduced from about 25% to about 15% by volume. Thus, it is shown that by varying the cycling valve position, the amount of particles larger than 3 microns can be increased.

Example 2

The data for this example is obtained from an invitalaporate site, where the desiccator (100) was operated at a temperature target of 130 ℃ and a pressure range of 40-100mmHg absolute vacuum. FIG. 4 shows the "normalized pressure drop" (MW > 1600 daltons) for 55% and 35% respectively for the recycle valve position for high molecular weight polymer (MW > 1600 daltons)"versus" cumulative press throughput ". When the circulation valve position was set at 35%, the maximum filtration feed rate increased from about 7,200pph to about 9,000pph, or a 25% performance increase. Thus, by varying the circulation valve position, the cumulative total filter press throughput increases from about 500,000 pounds to about 1,100,000 pounds, or more than twice the life of the filter press.

Example 3

The pressure drop across the filtration system used to filter the residual metal catalyst from the polyether polyol product is achieved by the following method. First, an aqueous solution containing a polyether polyol and an alkali metal catalyst residue formed by a transesterification method using an alkali metal catalyst is provided. The aqueous solution is then contacted with a stoichiometric excess of magnesium sulfate, magnesium sulfite, or a combination thereof, wherein the stoichiometric excess is based on the amount of the alkali metal catalyst residue, to form a second aqueous solution. Water is then removed from the second aqueous solution at a temperature in the range of from about 125 ℃ to about 145 ℃ to produce a dehydrated slurry containing a polyether polyol phase substantially free of residual alkali metal and a precipitated solid phase comprising sulfate and/or sulfite salts of an alkali metal catalyst, magnesium hydroxide, and excess magnesium sulfate and/or sulfite, wherein the particle size distribution of the precipitated solid phase is controlled to minimize the amount of particles therein that are less than 3 microns. The dewatered slurry is then passed through a filtration system to separate the polyether polyol phase from the precipitated solid phase, and the polyether polyol is recovered from the separated polyether polyol phase.

Example 4

The method of example 3 was repeated with additional steps. This embodiment further comprises drying the aqueous solution in a dryer recirculation system, wherein the dryer recirculation system comprises a primary dryer and a primary dryer heater, and optionally a secondary dryer and a secondary dryer heater. In this example, the particle size distribution of the precipitated solid phase was controlled by adjusting the pressure drop in the dryer recirculation system. The feed rate across the filtration system is determined by the particle size distribution.

Example 5

The method of example 4 is repeated with additional steps. In this embodiment, the pressure drop in the dryer recirculation system is optimized to minimize the amount of particles in the precipitated solid phase that are smaller than 3 microns.

Example 6

The method of example 5 was repeated with additional steps. In this example, the amount of particles smaller than 3 microns in the precipitated solid phase is less than 15% by volume.

Example 7

The method of example 6 was repeated with additional steps. In this embodiment, the dryer recirculation system includes a recirculation valve that is adjustable in position to control a pressure drop in the dryer recirculation system.

Example 8

The method of example 7 was repeated with additional steps. In this embodiment, the position of the recycle valve is determined based on the molecular weight of the polyether polyol from step (a).

Example 9

The method of example 8 is repeated with additional steps. In this embodiment, the alkali metal catalyst is selected from the group consisting of: an alkali metal hydroxide, an alkali metal alkoxide, an alkaline earth metal hydroxide, an alkaline earth metal alkoxide, and combinations thereof, and the precipitated solid phase comprises magnesium hydroxide, a sulfate salt of the alkali metal catalyst and excess magnesium sulfate, magnesium sulfite, or mixtures thereof.

Example 10

The method of example 9 was repeated with additional steps. In this example, the alkali metal catalyst is sodium methoxide and the precipitated solid phase comprises magnesium hydroxide and sodium sulfate.

Example 11

The method of example 10 is repeated with additional steps. In this embodiment, the polyether polyol has a molecular weight in the range of 650 to 3000 daltons.

Example 12

The method of example 11 is repeated with additional steps. In this embodiment, the polyether polyol is a poly (tetramethylene ether) glycol or a copolymer thereof.

It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For purposes of illustration, a concentration range of "about 0.1% to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1% to about 5% by weight, but also the individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term "about" can include ± 1%, ± 2%, ± 3%, ± 4%, ± 5%, ± 8% or ± 10% of the modified value or values. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about 'y'".

While illustrative embodiments of the invention have been described with particularity, it should be understood that the invention is capable of other and different embodiments and that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present disclosure, including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains.

Claims (7)

1. A filtration process for recovering a purified polyether polyol, the process comprising the steps of:

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

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

(c) removing water from the second aqueous solution of step (b) at a temperature in the range of 125 ℃ to 145 ℃ to produce a dehydrated slurry containing a polyether polyol phase having a residual alkali metal content of less than 1ppm 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 the precipitated solid phase is controlled to minimize the amount of particles therein of less than 3 microns, wherein the amount of particles therein of less than 3 microns is less than 15 volume percent;

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

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

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

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 smaller than 3 microns in the precipitated solid phase.

3. The method of claim 2, wherein the dryer recirculation system includes a recirculation valve that is adjustable in position to control a pressure drop in the dryer recirculation system.

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

5. The process of claim 1, wherein the alkali metal catalyst is selected from the group consisting of: an alkali metal hydroxide, an alkali metal alkoxide, an alkaline earth metal hydroxide, an alkaline earth metal alkoxide, and combinations thereof, and the precipitated solid phase comprises magnesium hydroxide, a sulfate salt of the alkali metal catalyst and excess magnesium sulfate, magnesium sulfite, or mixtures thereof.

6. The process of claim 1, wherein the alkali metal catalyst is sodium methoxide and the precipitated solid phase comprises magnesium hydroxide and sodium sulfate.

7. The method of claim 1, wherein the polyether polyol has a molecular weight in the range of 650 to 3000 daltons.