Patent Publication Number: US-2023147479-A1

Title: Processes and production plants for producing polyols

Description:
FIELD 
     This disclosure relates to processes for preparing polyols, such as polyether polyols, as well as to production plants configured to carry out such processes. 
     BACKGROUND 
     Polyether polyols having a relatively high content of primary hydroxyl (OH) groups are desired in many polyurethane applications. Conventionally, these polyether polyols are produced in two-steps. In a first step, all propylene oxide (or a mixture of propylene oxide and ethylene oxide) is polymerized, using a basic catalyst, such as potassium hydroxide, in the presence of a starter compound having active hydrogen atoms. This results in an intermediate polyether polyol having mainly secondary OH groups. In a second step, sometimes referred to as an “EO tip”, ethylene oxide is then added to the intermediate polyether polyol, thereby converting the majority of the secondary OH groups into primary OH groups. In this process, the same basic catalyst (for example, KOH) is often used for the propoxylation reaction and for the ethoxylation reaction. Following production of the polyether polyol, the basic catalyst is often neutralized using an acid and the resulting salts are removed from the polyol, such as by filtration. 
     In many cases it is desirable to produce polyether polyols using double-metal cyanide (DMC) catalysts. This is often because, compared with the conventional production of polyether polyols by means of basic catalysts, use of DMC catalysts can result in a decrease in the content of monofunctional polyethers with terminal double bonds, so-called monols. The polyether polyols thus obtained can be processed to form high-quality polyurethanes (for example, elastomers, foams, coatings). In addition, DMC catalysts can possess an exceptionally high activity, thereby rendering it possible to produce polyether polyols at very low catalyst concentrations so that a separation of the catalyst from the polyol is no longer necessary. 
     One drawback of using DMC catalysts for the production of polyether polyols has been that with these catalysts, unlike basic catalysts, a direct EO tip can be difficult. This is because when ethylene oxide is added to a poly(oxypropylene) polyol containing a DMC catalyst, the result can be a heterogeneous mixture which consists for the most part of unreacted poly(oxypropylene) polyol (having mainly secondary OH groups) and a small extent of highly ethoxylated poly(oxypropylene) polyol and/or polyethylene oxide. As a result, in many cases, DMC catalyzed polyether polyols having a high content of primary OH groups are produced using a two-step process in which the EO tip is carried out in a second, separate step by means of conventional base catalysis. 
     One disadvantage of this two-step process is the expensive, energy intensive and time consuming removal of water from the DMC polyether polyol/aqueous basic catalyst mixture (since the basic catalyst is introduced in the form of an aqueous solution of the catalyst, such as a 45% KOH solution). In addition, storage is required for the intermediate polymer, which involves significant capital and maintenance expense. Further, in order to fully react the water away a separate propylene oxide “drying step” is often needed prior to carrying out the EO tip, otherwise low functionality monol and/or glycol is produced. Such an additional step is, however, time consuming and energy intensive. 
     As a result, it would be desirable to provide processes and production plants capable of producing DMC-catalyzed polyether polyols having a high content of primary OH groups, in which the efficiency of the water removal process is improved. It would also be desirable to provide such a process that does not require the capacity to store the intermediate DMC-catalyzed polyether polyol prior to producing the final polyether polyol having a high content of primary OH groups. It would also be desirable that the process does not require use of a propylene oxide “drying step” to remove water from the reaction mixture prior to carrying out the EO tip. 
     SUMMARY 
     In some respects, this disclosure relates to processes for preparing a polyol. The processes comprise: (a) continuously producing an intermediate polyol in a first reactor by a process comprising: (1) introducing into the first reactor a mixture comprising a DMC catalyst and an initial starter, wherein the mixture is added in an amount sufficient to initiate polyoxyalkylation of the initial starter after introduction of alkylene oxide into the first reactor; (2) introducing alkylene oxide to the first reactor; (3) continuously introducing a continuously added starter into the first reactor; and (4) continuously introducing fresh DMC catalyst and/or further DMC catalyst/starter mixture to the first reactor such that catalytic activity of the DMC catalyst is maintained; (b) continuously discharging the intermediate polyol from the first reactor; (c) continuously mixing the intermediate polyol with an aqueous solutions of alkali metal to provide a mixture comprising the intermediate polyol, alkali metal and water; (d) continuously dehydrating the mixture comprising intermediate polyol, alkali metal and water, thereby continuously producing a dehydrated mixture comprising the intermediate polyol and the alkali metal; (e) transferring the dehydrated mixture to a second reactor; and (f) producing the polyol in the second reactor by feeding an alkylene oxide to the second reactor to thereby react the intermediate polyol with the alkylene oxide in the presence of the alkali metal. 
     In other respect, this specification relates to production plants for preparing a polyol. These production plants comprise: (a) a first reactor comprising: (1) an inlet in fluid communication with a source of alkylene oxide; (2) an inlet in fluid communication with a source of starter; (3) an inlet in fluid communication with a source of DMC catalyst; and (4) an outlet configured to continuously discharge an intermediate polyol from the first reactor; (b) a source of an aqueous solution of alkali metal in fluid communication with the outlet of the first reactor and configured to continuously add the aqueous solution of alkali metal to the intermediate polyol as it is continuously discharged from the first reactor, thereby producing a mixture comprising the intermediate polymer, the alkali metal and water; (c) a packed column comprising a polyol inlet and a polyol outlet, wherein the polyol inlet is in fluid communication with the outlet of the first reactor, wherein the packed column is configured to continuously remove water from the mixture comprising the intermediate polymer, the alkali metal and water, thereby producing a dehydrated mixture comprising the intermediate polyol and the alkali metal; (d) a second reactor comprising: (1) an inlet that is in fluid communication with the outlet of the packed column and configured to receive the dehydrated mixture comprising the intermediate polyol and the alkali metal; (2) an inlet in fluid communication with a source of alkylene oxide; and (3) an outlet configured to discharge the polyether polyol from the second reactor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically represents a production plant in accordance with embodiments of the inventions described in this specification. 
     
    
    
     DETAILED DESCRIPTION 
     Various implementations are described and illustrated in this specification to provide an overall understanding of the structure, function, properties, and use of the disclosed inventions. It is understood that the various implementations described and illustrated in this specification are non-limiting and non-exhaustive. Thus, the inventions are not limited by the description of the various non-limiting and non-exhaustive implementations disclosed in this specification. The features and characteristics described in connection with various implementations may be combined with the features and characteristics of other implementations. Such modifications and variations are intended to be included within the scope of this specification. As such, the claims may be amended to recite any features or characteristics expressly or inherently described in, or otherwise expressly or inherently supported by, this specification. Further, Applicant(s) reserve the right to amend the claims to affirmatively disclaim features or characteristics that may be present in the prior art. Therefore, any such amendments comply with the requirements of 35 U.S.C. § 112 and 35 U.S.C. § 132(a). The various implementations disclosed and described in this specification can comprise, consist of, or consist essentially of the features and characteristics as variously described herein. 
     Any patent, publication, or other disclosure material identified herein is incorporated by reference into this specification in its entirety unless otherwise indicated, but only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material expressly set forth in this specification. As such, and to the extent necessary, the express disclosure as set forth in this specification supersedes any conflicting material incorporated by reference herein. Any material, or portion thereof, that is said to be incorporated by reference into this specification, but which conflicts with existing definitions, statements, or other disclosure material set forth herein, is only incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material. Applicant(s) reserves the right to amend this specification to expressly recite any subject matter, or portion thereof, incorporated by reference herein. 
     In this specification, other than where otherwise indicated, all numerical parameters are to be understood as being prefaced and modified in all instances by the term “about”, in which the numerical parameters possess the inherent variability characteristic of the underlying measurement techniques used to determine the numerical value of the parameter. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter described in the present description should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. 
     Also, any numerical range recited in this specification is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” is intended to include all sub-ranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited in this specification is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. Accordingly, Applicant(s) reserves the right to amend this specification, including the claims, to expressly recite any sub-range subsumed within the ranges expressly recited herein. All such ranges are intended to be inherently described in this specification such that amending to expressly recite any such sub-ranges would comply with the requirements of 35 U.S.C. § 112 and 35 U.S.C. § 132(a). 
     The grammatical articles “one”, “a”, “an”, and “the”, as used in this specification, are intended to include “at least one” or “one or more”, unless otherwise expressly indicated. Thus, the articles are used in this specification to refer to one or more than one (i.e., to “at least one”) of the grammatical objects of the article. By way of example, “a component” means one or more components, and thus, possibly, more than one component is contemplated and may be employed or used in an implementation of the described implementations. Further, the use of a singular noun includes the plural, and the use of a plural noun includes the singular, unless the context of the usage requires otherwise. 
     As used herein, the term “functionality” refers to the average number of reactive hydroxyl groups, —OH, present per molecule of the —OH functional material that is being described. In the production of polyurethane foams, the hydroxyl groups react with isocyanate groups, —NCO, that are attached to the isocyanate compound. The term “hydroxyl number” refers to the number of reactive hydroxyl groups available for reaction, and is expressed as the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of one gram of the polyol (ASTM D4274-16). The term “equivalent weight” refers to the weight of a compound divided by its valence. For a polyol, the equivalent weight is the weight of the polyol that will combine with an isocyanate group, and may be calculated by dividing the molecular weight of the polyol by its functionality. The equivalent weight of a polyol may also be calculated by dividing 56,100 by the hydroxyl number of the polyol—Equivalent Weight (g/eq)=(56.1×1000)/OH number. 
     The processes and production plants of this specification will now be described with reference to  FIG.  1   . As indicated earlier, some embodiments of this specification relate to processes for preparing a polyol, such as a polyether polyol. These processes comprise continuously producing an intermediate polyol in a first reactor. The intermediate polyol can have, for example, a functionality of 2 to 8, such as 2 to 6 or 2 to 4, and a hydroxyl number of 10 to 500 mg KOH/g, such as 10 to 200 mg KOH/g, 10 to 100 mg KOH/g, or, in some cases, 20 to 50 mg KOH/g. 
     In the processes of this specification, the intermediate polyol is produced continuously. As used herein, the term “continuous” refers to a mode of addition of a relevant catalyst or reactant that maintains an effective concentration of the catalyst or reactant substantially continuously. Catalyst input, for example, may be truly continuous, or may be in relatively closely spaced increments. Likewise, continuous starter addition may be truly continuous, or may be incremental. Thus, it is possible to incrementally add a catalyst or reactant in such a manner that the added materials concentration decreases to essentially zero for some time prior to the next incremental addition. In some implementations, however, catalyst concentration is maintained at substantially the same level during the majority of the course of the continuous reaction and low molecular weight starter is present during the majority of the process. Incremental addition of catalyst and/or reactant which does not substantially affect the nature of the product is still “continuous” as that term is used herein. It is feasible, for example, to provide a recycle loop where a portion of the reacting mixture is back fed to a prior point in the process, thus smoothing out any discontinuities brought about by incremental additions. 
     Such continuous intermediate polyol production can be conducted using any of a variety of continuous reactors. For example, in some implementations, continuous intermediate polyol production can take place using a single stage continuous stirred tank reactor (“CSTR”). 
     In particular, as shown in  FIG.  1   , production plant  10  may include a first reactor  20  that is a single stage CSTR. In this implementation, an inlet of CSTR  20  is in fluid communication, via line  24 , with a source of alkylene oxide  26 , an inlet of CSTR  20  is in fluid communication, via line  30 , with a source of H-functional starter  32 , and an inlet of CSTR  20  is in fluid communication, via line  36 , with a source of DMC catalyst  38 . The various afore-mentioned inlets to CSTR  20  may be the same inlet or they may be different inlets (such as is depicted in  FIG.  1   ). CSTR  20  is configured to continuously discharge the intermediate polyol from CSTR  20 . As is apparent, an outlet of CSTR  20  is in fluid communication, via line  42 , with an inlet of a dehydration apparatus  50 , such as packed columns  50   a  and  50   b  shown in  FIG.  1   . Also, in this implementation, an outlet of CSTR  20  is in fluid communication, via line  46 , with intermediate polyol storage vessel  48 , which, in turn, is also in fluid communication, via line  42 , with an inlet of dehydration apparatus  50 . The presence of intermediate polyol storage vessel  48  may be particularly desirable in cases where second reactor  60  is a batch (or semi-batch) reactor. Further, an inlet of dehydration apparatus  50  is in fluid communication with a source of an aqueous solution of basic catalyst  55 . In the particular implementation depicted in  FIG.  1   , source of aqueous solution of basic catalyst  55  is in fluid communication with line  42 , thereby allowing the intermediate polyol being continuously discharged from CSTR  20  and/or intermediate polyol being discharged from intermediate polyol storage vessel  48  to mix with the aqueous solution of basic catalyst prior to the intermediate polyol entering dehydration apparatus  50 . An inlet of dehydration apparatus  50  is thus configured to continuously receive a mixture of aqueous solution of basic catalyst and intermediate polyol as intermediately polyol is continuously discharged from CSTR  20 . In addition, in this implementation, an inlet of dehydration apparatus  50  is configured to continuously receive intermediate polyol from intermediate polyol storage vessel  48 . As a result, in operation, dehydration apparatus  50  may continuously receive intermediate polyol from CSTR  20 , continuously receive intermediate polyol from intermediate polyol storage vessel  48 , or may continuously receive intermediate polyol from both CSTR  20  and intermediate polyol storage vessel  48  simultaneously. In any of these cases, the intermediate polyol can be mixed with aqueous solution of basic catalyst prior to entering dehydration apparatus  50 . 
     Aside from the single stage CSTR depicted in  FIG.  1   , the first reactor may comprise another type of continuous reactor, such as a two stage CSTR, a plug flow reactor, or a loop reactor (i.e., a reactor with internal and/or external recycling of substances, optionally with a heat exchanger arranged in the circulation), such as a stream loop reactor, a jet loop reactor, a Venturi loop reactor, a tube reactors configured in loop form with suitable devices for circulating the reaction mixture, or a loop of several tube reactors connected in series or several stirred tanks connected in series. 
     Regardless of the specific type of first reactor employed, the processes of this specification comprise continuously producing the intermediate polyol in the first reactor by a process comprising: (1) introducing into the first reactor a mixture comprising a DMC catalyst and an initial starter, wherein the mixture is added in an amount sufficient to initiate polyoxyalkylation of the initial starter after introduction of alkylene oxide into the first reactor; (2) introducing the alkylene oxide to the first polyol reactor; (3) continuously introducing a continuously added starter into the first reactor; and (4) continuously introducing fresh DMC catalyst and/or further DMC catalyst/further starter mixture to the first reactor such that catalytic activity of the DMC catalyst is maintained. 
     The starter(s) employed may be any compound having active hydrogen atoms. Suitable starters include, but are not limited to, compounds having a number average molecular weight of 18 to 2,000, such as 62 to 2,000, and having 1 to 8 hydroxyl groups. Specific examples of suitable starters include, but are not limited to, polyoxypropylene polyols, polyoxyethylene polyols, polytetatramethylene ether glycols, glycerol, propoxylated glycerols, propylene glycol, ethylene glycol, tripropylene glycol, trimethylol propane alkoxylated allylic alcohols, bisphenol A, pentaerythritol, sorbitol, sucrose, degraded starch, water and mixtures thereof. 
     In certain embodiments, the starter used to prepare the DMC catalyst/starter mixture introduced in the above-mentioned step (1) is an oligomeric starter, such as an oxyalkylated oligomer based on the same low molecular weight starter whose continuous addition is to be used in the above-mentioned step (3). For example, where propylene glycol is to be continuously added to the reactor in step (3), a suitable oligomeric starter useful in preparing the activated catalyst/starter mixture may be a 300 Da to 1,000 Da molecular weight polyoxypropylene glycol. The same oligomeric starter would also be suitable for use where dipropylene glycol and/or water are continuously added starters. In another example, where glycerin is a continuously added starter, an oxypropylated glycerine polyol having a molecular weight of 400 Da to 1,500 Da may advantageously be used in the above-mentioned step (1). In some implementations, however, a monomeric starter, such as ethylene glycol, propylene glycol, and the like, may be used. Thus, in some implementations, the starter used to prepare the catalyst/starter mixture in the above-mentioned step (1) may be the same as the continuously added starter used in the above-mentioned step (3). 
     In certain implementations, the continuously added starter may comprise water, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,2-, 1,3-, and/or 1,4-butylene glycol, neopentyl glycol, glycerin, trimethylolpropane, triethylolpropane, pentaerythritol, a-methylglucoside, hydroxymethyl-, hydroxyethyl-, and/or hydroxypropylglucoside, sorbitol, mannitol, sucrose, tetrakis [2 hydroxyethyl and/or 2-hydroxypropyl]ethylene diamine, as well as mixtures of any two or more thereof. Also suitable are monofunctional starters such as methanol, ethanol, 1-propanol, 2-propanol, n-butanol, 2-butanol, 2 ethylhexanol, and the like, as well as phenol, catechol, 4,4′ dihydroxybiphenyl, and 4,4′-dihydroxydiphenylmethane, including mixtures of any two or more of the foregoing. 
     In some implementations, the continuously added starter comprises a polyoxyalkylene polymer or copolymer or suitable initiator for the production thereof, which has a molecular weight less than the desired product weight. Thus, the molecular weight of the continuously added starter may vary from 18 Da (water) to 45,000 Da (high molecular weight polyoxyalkylene polyol). In some implementations, the continuously added starter may comprise a starter having a molecular weight less than 1,000 Da, such as less than 500 Da, or less than 300 Da. 
     Alkylene oxides suitable for introduction in the afore-mentioned step (2) include, but are not limited to, ethylene oxide, propylene oxide, oxetane, 1,2- and 2,3-butylene oxide, isobutylene oxide, epichlorohydrin, cyclohexene oxide, styrene oxide, and the higher alkylene oxides such as the C 5 -C 30  α-alkylene oxides. In some implementations, a mixture of propylene oxide and ethylene oxide may be used, such as those with high ethylene oxide content, i.e., up to 85 mol percent. In some implementations, propylene oxide alone or a mixture of propylene oxide with ethylene oxide or another alkylene oxide is used. Other polymerizable monomers may be used as well, such as anhydrides and carbon dioxide. 
     The process for producing the intermediate polyol may employ any double metal cyanide (DMC) catalyst. DMC catalysts are non-stoichiometric complexes of a low molecular weight organic complexing agent and optionally other complexing agents with a double metal cyanide salt, such as zinc hexacyanocobaltate. Exemplary suitable DMC catalysts include those suitable for preparation of low unsaturation polyoxyalkylene polyether polyols, such as are disclosed in U.S. Pat. Nos. 3,427,256; 3,427,334; 3,427,335; 3,829,505; 4,472,560; 4,477,589; and 5,158,922, each of which being incorporated herein by reference. In some implementations, the DMC catalyst comprises one that is capable of preparing “ultra-low” unsaturation polyether polyols, such as are disclosed in U.S. Pat. Nos. 5,470,813, 5,482,908, and 5,545,601, each of which being incorporated by reference thereto. 
     The DMC catalyst concentration is desirably chosen to provide adequate control of the polyoxyalkylation reaction under the given reaction conditions. In some implementations, the DMC catalyst is used in an amount of 0.0005 to 1% by weight, such as 0.001 to 0.1% by weight, or, in some cases, 0.001 to 0.01% by weight, based on the amount of polyether polyol to be produced. 
     An organic complexing ligand may be included with the DMC catalyst. Any organic complexing ligand may be part of the DMC catalyst, such as those described in U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849, 5,158,922 and 5,470,813, EP 700 949, EP 761 708, EP 743 093, WO 97/40086 and JP 4145123. Such organic complexing ligands include water-soluble organic compounds with heteroatoms, such as oxygen, nitrogen, phosphorus or sulfur, which can form complexes with the DMC compound. In some implementations, the organic complexing ligand comprises an alcohol, aldehyde, ketone, ether, ester, amide, urea, nitrile, sulfide, or a mixture of any two or more thereof. In some implementations, the organic complexing ligands comprises a water-soluble aliphatic alcohol, such as, for example, ethanol, isopropanol, n-butanol, iso-butanol, sec-butanol, tert-butanol, or a mixture of any two or more thereof. 
     The DMC catalyst may contain a functionalized polymer. As used herein, the term “functionalized polymer” refers to a polymer or its salt that contains a functional group, such as oxygen, nitrogen, sulfur, phosphorus, halogen, or a mixture of any two or more thereof. Specific examples of suitable functionalized polymer include, but are not limited to, polyethers, polyesters, polycarbonates, polyalkylene glycol sorbitan esters, polyalkylene glycol glycidyl ethers, polyacrylamides, poly(acrylamide-co-acrylic acids), polyacrylic acids, poly(acrylic acid-co-maleic acids), poly(N-vinylpyrrolidone-co-acrylic acids), poly(acrylic acid-co-styrenes) and the salts thereof, maleic acids, styrenes and maleic anhydride copolymers and the salts thereof, block copolymers composed of branched chain ethoxylated alcohols, alkoxylated alcohols, polyether, polyacrylonitriles, polyalkyl acrylates, polyalkyl methacrylates, polyvinyl methyl ethers, polyvinyl ethyl ethers, polyvinyl acetates, polyvinyl alcohols, poly-N-vinylpyrrolidones, polyvinyl methyl ketones, poly(4-vinylphenols), oxazoline polymers, polyalkyleneimines, hydroxyethylcelluloses, polyacetals, glycidyl ethers, glycosides, carboxylic acid esters of polyhydric alcohols, bile acids and their salts, esters or amides, cyclodextrins, phosphorus compounds, unsaturated carboxylic acid esters and ionic surface- or interface-active compounds. 
     In some implementations, where used, functionalized polymer is present in the DMC catalyst in an amount of 2 to 80% by weight, 5 to 70% by weight, or, in some cases, 10 to 60% by weight, based on the total weight of DMC catalyst. 
     The DMC catalyst may or may not be activated prior to use in the process of preparing the intermediate polyol. Activation, when desired, involves mixing the catalyst with a starter molecule having a desired number of oxyalkylatable hydrogen atoms, and adding alkylene oxide, preferably propylene oxide or other higher alkylene oxide under pressure and monitoring the reactor pressure. The reactor may be maintained at a temperature of, for example, 90° C. to 150° C., 100° C. to 140° C., or, in some cases, 110° C. to 130° C. A noticeable pressure drop in the reactor indicates that the catalyst has been activated. The same alkylene oxide(s) as is to be employed in to produce the intermediate polyol may be used to prepare activated catalyst, or a different alkylene oxide may be employed. With higher alkylene oxides having low vapor pressure, a volatile alkylene oxide such as ethylene oxide, oxetane, 1,2-butylene oxide, 2,3-butylene oxide, or isobutylene oxide may be employed in lieu of or in conjunction with the higher alkylene oxide to facilitate pressure monitoring. Alternatively, other methods of measuring alkylene oxide concentration (GC, GC/MS, HPLC, etc.) may be used. A noticeable reduction in free alkylene oxide concentration indicates activation. 
     In some cases, however, “fresh” DMC catalyst may be employed without activation. “Fresh” catalyst as used herein is freshly prepared, non-activated DMC catalyst, i.e., non-activated DMC catalyst in solid form or in the form of a slurry in low molecular weight starter, polyoxyalkylated low molecular weight starter, or a non-starter liquid. In some implementations, all or a substantial portion of the liquid phase of a fresh DMC catalyst mixture will include the same low molecular weight starter used for continuous starter addition, a polyoxyalkylated low molecular weight starter. 
     In some implementations, a portion of intermediate polyol may be cycled back to a catalyst activation reactor and employed for catalyst activation. 
     In preparing the intermediate polyol, according to some embodiments, the addition of starter is continuous in the sense that a concentration of low molecular weight starter and/or its low molecular weight oxyalkylated oligomers is maintained for a substantial portion of the total oxyalkylation. In a tube reactor, for example, starter may be introduced separately at numerous points along the reactor, or dissolved in alkylene oxide and introduced along the length of the reactor. In a CSTR, starter may be added to alkylene oxide, and may be added at numerous locations within the reactor. Low molecular weight starter need not even be present in the catalyst/starter mixture, which may employ a much higher molecular weight starter. By whatever method added, low molecular weight starter should be present for a substantial portion of oxyalkylation, such as 50% of oxyalkylation, 70% of the alkoxylation, or more. In some implementations. a low molecular weight starter concentration is maintained for a portion of the oxyalkylation which is effective to reduce the proportion of high molecular weight tail in the intermediate polyol product as compared what would be produced in a batch process where all starter is added at once. 
     The amount of continuously added starter may be increased to very high levels without unduly broadening molecular weight distribution. The continuously added starter may represent in excess of 90 equivalent percent of total starter, such as where the percentage of continuously added starter is 98 to 99+%. Despite the continuous addition of starter, polydispersity is generally below 1.7, such as below 1.3 to 1.4, or 1.05 to 1.20. 
     In preparing the intermediate polyol, it may desirable to have a small concentration of starter present in the reaction mixture at all times, although a final “cook out” to facilitate complete reaction of alkylene oxide may be performed without starter present. Continuous addition of as little as 1-2 equivalent percent of starter relative to total product weight may be effective to substantially eliminate the high molecular weight tail. However, despite the continuous addition of a very significant, and in some cases, major amount of low molecular weight starter, the molecular weight distribution is usually not significantly broadened and products of very low polydispersity are obtained. 
     As indicated, in some implementations, the continuous process of preparing the intermediate polyol involves establishing oxyalkylation conditions in a continuous reactor. Thus, when it is stated herein that a mixture comprising a DMC catalyst and an initial starter is introduced into the first reactor “in an amount sufficient to initiate polyoxyalkylation of the initial starter after introduction of alkylene oxide into the intermediate polyol reactor” it merely means that oxyalkylation conditions are established at some point in time. For example, an initial establishing of oxyalkylation conditions does not need repeating. Following establishment of oxyalkylation conditions, only the addition of alkylene oxide, continuously added starter, and further catalyst need be maintained. 
     Moreover, the term “starter” as employed in the phrase “DMC catalyst/initial starter” refers to an oxyalkylatable molecule of any molecular weight. This oxyalkylatable molecule may be a low molecular weight starter molecule having a molecular weight below about 300 Da, such as propylene glycol, dipropylene glycol, glycerin, a three mole oxypropylate of glycerin, etc., or may be a much higher molecular weight molecule, for example the product of desired product molecular weight. 
     Suitable processes and equipment for continuously producing the intermediate polyol are described in U.S. Pat. No. 5,689,012 at col. 5, line 55 to col. 17, line 16, the cited portion of which being incorporated herein by reference. 
     As previously mentioned, the processes of this specification comprise continuously discharging the intermediate polyol from the first reactor and continuously mixing the intermediate polyol with an aqueous solution of an alkali metal alkoxide and/or an alkali metal hydroxide to provide a mixture comprising the intermediate polyol, an alkali metal, and water. Suitable alkali metal alkoxides include, for example, those that contain 1 to 4 carbon atoms in the alkyl radical. Specific examples of suitable alkali metal alkoxides are, without limitation, sodium methylate, sodium and potassium ethylate, potassium isopropylate and sodium butylate. Suitable alkali metal hydroxides include, for example, sodium hydroxide, cesium hydroxide, and potassium hydroxide. In some implementations, the amount of alkali metal alkoxide and/or an alkali metal hydroxide in the aqueous solution is 2 to 60% by weight, such as 10 to 60% by weight, 20 to 55% by weight, or 30 to 50% by weight, based on the total weight of the aqueous solution, with the remainder of the solution consisting essentially of water. 
     In some implementations, the aqueous solutions of an alkali metal alkoxide and/or an alkali metal hydroxide catalyst is used in an amount such that alkali metal is present in an amount of 0.01 to 5% by weight, 0.2 to 3% by weight, or, in some cases, 0.1 to 1.0% by weight, based on the total weight of the polyol produced by the processes of this specification. 
     As indicated, the intermediate polyol is continuously mixed with the aqueous solution of an alkali metal alkoxide and/or an alkali metal hydroxide to provide a mixture comprising the intermediate polyol, an alkali metal, and water. In some implementations, such as the implementation depicted in  FIG.  1   , such mixing can be achieved by inline mixing of the aqueous solution of an alkali metal alkoxide and/or an alkali metal hydroxide with the intermediate polyol as it is continuously discharged from the first reactor. Such inline mixing may, if desired, be enhanced by the use of a mixing device, such as a static mixer or a jet mixer, that may be present at the injection point of the aqueous solution or downstream therefrom. 
     The processes of this specification further comprise continuously dehydrating the mixture comprising intermediate polyol, alkali metal, and water, thereby continuously producing a dehydrated mixture comprising the intermediate polyol and the alkali metal, though it should be understood that, while the dehydrated mixture will contain less water than the mixture prior to dehydration, such dehydration may not be complete, so that some water still remains in the mixture following the dehydration process. In some implementations, however, the water content of the dehydrated mixture is no more than 400 ppm, sometimes no more than 200 ppm. 
     In some implementations, the foregoing continuous dehydration of the mixture comprising intermediate polyol, alkali metal, and water may be accomplished by continuously passing the mixture through one or more packed columns, such as the two packed columns  50   a  and  50   b  arranged in series that is depicted in  FIG.  1   . In addition to packed columns or trayed columns, other dehydration apparatus&#39; suitable for use in the processes of this specification can be readily envisaged, such as falling film evaporator, wiped film evaporator, kettle evaporator, flash tank, etc. 
     Thus, in some implementation, the dehydration is accomplished by passing a stripping gas through the mixture comprising intermediate polyol, alkali metal, and water, such that water is transferred to the stripping gas. In some implementations, a nitrogen-containing gas, such as nitrogen gas, is a suitable inert stripping gas. As a result, in some implementations, such as the implementation depicted in  FIG.  1   ., an inlet of dehydration apparatus  50  is in fluid communication with a source of stripping gas  58 , such as a source of N 2  gas. 
     In some implementations, the foregoing dehydration may be accomplished by a desorption process in which water passes into the inert stripping gas because of partition equilibria between gas phase and liquid phase. Thus, in some cases, such desorption involves expelling water in an inert stripping gas stream, such as an N 2  gas stream. The stripping gas can, such as is depicted in  FIG.  1   , be fed countercurrent to the mixture comprising intermediate polyol, alkali metal, and water. Water migrates from the liquid phase into the gas phase. 
     Thus, in some cases, the dehydration of the mixture comprising intermediate polyol, alkali metal, and water is executed by passing the mixture countercurrent, i.e., against the direction of flow of an inert stripping gas through one or more packed columns at, for example, reduced pressure and elevated temperatures. More specifically, in some implementations, the dehydration may be carried out at a temperature of 100 to 160° C., such as 130 to 150° C. In some implementations, the column(s) is operated at a pressure of from 1 to 100 mmHg (absolute), such as 1 to 5 mmHg (absolute). 
     The stripping gas may be fed to the packed column in any suitable amount to accomplish the desired level of dehydration. For example, in some implementations, the stripping gas is fed to the packed column in an amount of 0.002 kg to 0.006 kg of stripping gas per kg of the mixture comprising intermediate polyol, alkali metal, and water, such as 0.003 kg to 0.004 kg of stripping gas per kg of the mixture comprising intermediate polyol, alkali metal, and water. 
     Suitable packed columns for use in embodiments of the processes of this specification include any columns having internals with separation activity, such as trays, random packings and structured packings. Specific examples of trays include, but are not limited to, bubble trays, tunnel trays, valve trays, sieve trays, dual flow trays and grid trays. 
     Random packings include packing elements constructed of, for example, steel, stainless steel, copper, carbon, earthenware, porcelain, glass, plastic, or a combination thereof. Specific examples of suitable random packing structures are Raschig® rings in which small pieces of tube to make a packing bed, Pall® rings which are similar to Raschig® rings but also include support structures and external surfacing texture within the ring walls, saddle rings, such as Berl® saddles and Intalox® saddles that are shaped like saddles, Lessing® rings, which are made of ceramic and have internal partitions to increase surface area and enhance efficiency, and, Tri-Packs that has a spherical shape and interior ribs to maximize surface area and wetting. 
     As will be appreciated by the ordinary skilled artisan, structured packing is a type of packing that channels a liquid into a specific shape. Structured packing utilizes discs made of, for example, metal, plastic or porcelain, with the discs having an internal structure arranged into a type of honeycombed shape. Unlike random packing, structured packing are constructed of large pieces of material that contains holes, grooves, corrugation and other textured elements. Specific types of structured packing, which are suitable for use in the inventions of this specification include, but are not limited to, knitted wire structured packing, fabric packings, and corrugated sheet metal structured packing. 
     In some implementations, the foregoing dehydration may further comprise passing the mixture through an in-line molecular sieve that is arranged downstream of the packing column(s). As will be appreciated, molecular sieves are often constructed of zeolite, i.e., microporous aluminosilicates. 
     As indicated earlier, the processes of this specification further comprise transferring the dehydrated mixture to a second reactor and producing the polyether polyol in the second reactor by feeding an alkylene oxide to the second reactor to thereby react the intermediate polyol with the alkylene oxide in the presence of the alkali metal. 
     As shown in  FIG.  1   , in some implementation, an outlet of dehydration apparatus  50  is in fluid communication, via line  61 , with dehydrated mixture storage vessel  59 , which, in turn, is also in fluid communication, via line  66 , with an inlet of second reactor  60 . The presence of dehydrated mixture storage vessel  59  may be particularly desirable in cases where second reactor  60  is a batch (or semi-batch) reactor. In the particular implementation depicted in  FIG.  1   , therefore, dehydrated mixture discharged from dehydration apparatus  50  and/or dehydrated mixture from dehydrated mixture storage vessel  59  may be fed to second reactor  60 . As a result, in operation, second reactor  60  may receive dehydrated mixture continuously from dehydration apparatus  50 , receive dehydrated mixture from dehydrated mixture storage vessel  59 , or may receive dehydrated mixture from both dehydration apparatus  50  and dehydrated mixture storage vessel  59  simultaneously. 
     As a result, according to the processes of this specification, the intermediate polyol is used as a starter in preparing the polyol produced in the second reactor. In some implementations, the polyol produced in the second reactor is a “long chain” polyol that has a functionality of 2 to 6 and an equivalent weight of 1000 to 2000 Da, such as a functionality of 2 to 4 and an equivalent weight of 1500 to 2000 Da. 
     If desired, in addition to the intermediate polyol, other starters may be used to prepare the polyol produced in the second reactor. Such other starters may include, without limitation, low molecular weight starters such as glycerin, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, tripropylene glycol, trimethylolpropane, 1,3-butanediol, 1,4-butanediol, pentaerythritol, sorbitol, sucrose, ethylenediamine, and toluene diamine, among others, as well as combinations of two or more of the foregoing. 
     Suitable alkylene oxides that may fed to the second reactor according to implementations of the processes of this specification include, but are not limited to, ethylene oxide, propylene oxide, oxetane, 1,2- and 2,3-butylene oxide, isobutylene oxide, epichlorohydrin, cyclohexene oxide, styrene oxide, and the higher alkylene oxides such as the C 5 -C 30  α-alkylene oxides. In some implementations, propylene oxide, ethylene oxide, or a mixture of propylene oxide with ethylene oxide is used as the alkylene oxide fed to the second reactor. Further, in some cases, sufficient ethylene oxide is fed to the second reactor to provide the resulting polyol with an ethylene oxide “cap” in which up to 20% of ethylene oxide, such as 15 to 20% of ethylene oxide is added as a cap, such weight percents being based on the total weight of the final polyol produced in the second reactor. 
     In some implementations, the processes of this specification do not include a propylene oxide drying step to remove water from the reaction mixture prior to making the polyol in the second reactor. 
     The processes conditions used to make the polyol in the second reactor can vary. In some implementations, the dehydrated mixture described above is added to the second reactor and is heated to the desired reaction temperature, such as a temperature of 105° C. to 130° C., and the alkylene oxide is added to the second reactor. In some implementations, the alkylene oxide is fed over 2 to 10 hours depending on the configuration and heat removal capabilities of the second reactor. After the total amount of alkylene oxide is fed, the reactor contents may be allowed to react further until the pressure in the reactor is level indicating no further change in the amount of oxide present. The final polyol is then refined to remove the alkali metal, such as by acid neutralization followed by filtration, treatment with solid adsorbents, treatment with solid inorganic compounds and treatment with ion exchanges resins. 
     The second reactor may be of any configuration, such as batch, semi-batch, or a continuous reactor. In some implementations, however, the second reactor is, as is illustrated in  FIG.  1   , a batch. 
     In particular, as shown in  FIG.  1   , production plant  10  may include a second reactor  60  that is a batch reactor. In this implementation, an inlet of reactor  60  is in fluid communication, via line  62 , with a source of alkylene oxide  64 , an inlet of reactor  60  is in fluid communication, via line  66 , with an outlet of dehydration apparatus  50 , and, in some cases, an inlet of reactor  60  is in communication with a source of H-functional starter  68  (the source of H-functional starter  68  may be the same as source of H-functional starter  32  or it may be a different source of H-functional starter). The various afore-mentioned inlets to reactor  60  may be the same inlet or they may be different inlets (such as is depicted in  FIG.  1   ). reactor  60  is configured to discharge polyol from reactor  60 . As is apparent, an outlet of reactor  60  is in fluid communication, via line  70 , with an inlet of a polyol work-up system  72 . Polyol work-up system  72  includes means for removing alkali metal from the polyol exiting reactor  60 , such means may include, for example, treatment with an ion-exchange resin, liquid-liquid extraction, or treatment with an absorbent, such as magnesium silicate. Suitable methods for working-up the polyol exiting reactor  60  are described in U.S. Pat. Nos. 3,715,402; 3,823,145; 4,721,818; 4,355,188 and 5,563,221, which are incorporated herein by reference. Polyol work-up system  72  can be in fluid communication with polyol storage  80 . 
     The processes and production plants of this specification are currently believed to provide several advantages. First, it is believe that they are capable of producing DMC-catalyzed polyether polyols having a high content of primary OH groups, in which the efficiency (in terms of energy reduction and reduced time) and consistency of the water removal in the process is improved. Second, they do not require the capacity to store the intermediate DMC-catalyzed polyether polyol prior to producing the final polyether polyol having a high content of primary OH groups. Third, it is believed that they can eliminate the need for a propylene oxide drying step to remove water from the reaction mixture prior to carrying out the EO tip. 
     Various aspects of the subject matter described herein are set out in the following numbered clauses: 
     Clause 1. A process for preparing a polyol, comprising: a)continuously producing an intermediate polyol in a first reactor by a process comprising: (1) introducing into the first reactor a mixture comprising a DMC catalyst and an initial starter, wherein the mixture is added in an amount sufficient to initiate polyoxyalkylation of the initial starter after introduction of alkylene oxide into the first reactor; (2) introducing alkylene oxide to the first reactor; (3) continuously introducing a continuously added starter into the first reactor; and (4) continuously introducing fresh DMC catalyst and/or further DMC catalyst/further starter mixture to the first reactor such that catalytic activity of the DMC catalyst is maintained; b) continuously discharging the intermediate polyol from the first reactor; c) continuously mixing the intermediate polyol with an aqueous solutions of alkali metal to provide a mixture comprising the intermediate polyol, alkali metal, and water; d) continuously dehydrating the mixture comprising intermediate polyol, alkali metal, and water, thereby continuously producing a dehydrated mixture comprising the intermediate polyol and the alkali metal; e) transferring the dehydrated mixture to a second reactor; and f) producing the polyether polyol in the second reactor by feeding an alkylene oxide to the second reactor to thereby react the intermediate polyol with the alkylene oxide in the presence of the alkali metal. 
     Clause 2. The process of clause 1, wherein the first reactor comprises a single stage continuous stirred tank reactor (“CSTR”), a two stage CSTR, a plug flow reactor, or a loop reactor, such as a stream loop reactor, a jet loop reactor, a Venturi loop reactor, a tube reactors configured in loop form with suitable devices for circulating the reaction mixture, or a loop of several tube reactors connected in series or several stirred tanks connected in series. 
     Clause 3. The process of clause 1 or clause 2, wherein the initial starter comprises a compound having a number average molecular weight of 18 to 2,000, such as 62 to 2,000, and having 1 to 8 hydroxyl groups, such as a polyoxypropylene polyol, a polyoxyethylene polyol, a polytetatramethylene ether glycol, glycerol, a propoxylated glycerol, propylene glycol, ethylene glycol, tripropylene glycol, trimethylol propane, an alkoxylated allylic alcohol, bisphenol A, pentaerythritol, sorbitol, sucrose, degraded starch, water, or a mixture thereof. 
     Clause 4. The process of any one of clause 1 to clause 3, wherein the initial starter comprises an oxyalkylated oligomer based on the same low molecular weight starter used as the continuously added starter, such as where the continuously added starter comprises propylene glycol and the initial starter comprises a 300 Da to 1,000 Da molecular weight polyoxypropylene glycol. 
     Clause 5. The process of any one of clause 1 to clause 3, wherein the starter used to prepare the catalyst/starter mixture is the same as the continuously added starter. 
     Clause 6. The process of any one of clause 1 to clause 5, wherein the continuously added starter comprises water, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,2-, 1,3-, and/or 1,4-butylene glycol, neopentyl glycol, glycerin, trimethylolpropane, triethylolpropane, pentaerythritol, α-methylglucoside, hydroxymethyl-, hydroxyethyl-, and/or hydroxypropylgluco side, sorbitol, mannitol, sucrose, tetrakis [2 hydroxyethyl and/or 2-hydroxypropyl]ethylene diamine, or a mixture of any two or more thereof. 
     Clause 7. The process of any one of clause 1 to clause 6, wherein the molecular weight of the continuously added starter is from 18 Da to 45,000 Da, such as 18 Da to less than 1,000 Da, 18 Da to less than 500 Da, or 18 Da to less than 300 Da. 
     Clause 8. The process of any one of clause 1 to clause 7, wherein the alkylene oxides introduced to the first reactor comprises ethylene oxide, propylene oxide, oxetane, 1,2- and 2,3-butylene oxide, isobutylene oxide, epichlorohydrin, cyclohexene oxide, styrene oxide, a C5-C30 α-alkylene oxides, or a mixture of any two or more thereof, such as where the alkylene oxide introduced to the first reactor comprises a mixture of propylene oxide and ethylene oxide or propylene oxide alone. 
     Clause 9. The process of any one of clause 1 to clause 8, wherein the continuously added starter represents more than 90 equivalent percent of the total starter added to the first reactor, such as where the percentage of continuously added starter is 98 to 99+% of the total starter added to the first reactor. 
     Clause 10. The process of any one of clause 1 to clause 9, wherein the aqueous solutions of alkali metal comprises an alkali metal alkoxide, such as sodium methylate, sodium and potassium ethylate, potassium isopropylate and sodium butylate, and/or an alkali metal hydroxides, such as sodium hydroxide, cesium hydroxide, and potassium hydroxide, such as where the amount of alkali metal alkoxide and/or an alkali metal hydroxide in the aqueous solution is 2 to 60% by weight, 10 to 60% by weight, 20 to 55% by weight, or 30 to 50% by weight, based on the total weight of the aqueous solution, with the remainder of the solution consisting essentially of water. 
     Clause 11. The process of clause 10, wherein the aqueous solution of an alkali metal alkoxide and/or an alkali metal hydroxide is used in an amount such that alkali metal is present in an amount of 0.01 to 5% by weight, 0.2 to 3% by weight, or, in some cases, 0.1 to 1.0% by weight, based on the total weight of the polyol produced by the process. 
     Clause 12. The process of any one of clause 1 to clause 11, wherein the intermediate polyol is continuously mixed with the aqueous solution of an alkali metal by inline mixing of the aqueous solution of an alkali metal with the intermediate polyol as it is continuously discharged from the first reactor. 
     Clause 13. The process of any one of clause 1 to clause 12, wherein the water content of the dehydrated mixture comprising the intermediate polyol and the alkali metal is no more than 400 ppm or no more than 200 ppm. 
     Clause 14. The process of any one of clause 1 to clause 13, wherein the continuous dehydration of the mixture comprising intermediate polyol, alkali metal, and water comprises continuously passing the mixture through one or more packed columns, such as the two packed columns. 
     Clause 15. The process of clause 14, wherein the dehydration comprises passing a stripping gas through the mixture comprising intermediate polyol, alkali metal, and water, such that water is transferred to the stripping gas. 
     Clause 16. The process of clause 15, wherein the stripping gas comprises nitrogen gas. 
     Clause 17. The process of any one of clause 1 to clause 16, wherein the dehydration comprises passing the mixture comprising intermediate polyol, alkali metal, and water countercurrent to the direction of flow of an inert stripping gas through one or more packed columns at reduced pressure and an elevated temperature, such as at a temperature of 100 to 160° C. or 130 to 150° C. and a pressure of 1 to 100 mmHg (absolute) or 1 to 5 mmHg (absolute). 
     Clause 18. The process of clause 17, wherein the stripping gas is fed to the packed column in an amount of 0.002 kg to 0.006 kg or 0.003 to 0.004 kg of stripping gas per kg of the mixture comprising intermediate polyol, alkali metal, and water. 
     Clause 19. The process of clause 14 to clause 18, wherein the packed column comprises trays, random packings or structured packings. 
     Clause 20. The process of any one of clause 14 to clause 19, further comprising passing the dehydrated mixture through an in-line molecular sieve arranged downstream of the packing column. 
     Clause 21. The process of any one of clause 1 to clause 20, wherein the polyol produced in the second reactor has a functionality of 2 to 6 and an equivalent weight of 1000 to 2000 Da, such as a functionality of 2 to 4 and an equivalent weight of 1500 to 2000 Da. 
     Clause 22. The process of any one of clause 1 to clause 21, wherein the alkylene oxide fed to the second reactor comprises ethylene oxide, propylene oxide, oxetane, 1,2- and 2,3-butylene oxide, isobutylene oxide, epichlorohydrin, cyclohexene oxide, styrene oxide, a C5-C30 α-alkylene oxides, or a mixture of any two or more thereof, such as where the alkylene oxide fed to the second reactor comprises sufficient ethylene oxide to provide the polyol with an ethylene oxide cap in which up to 20% of ethylene oxide, such as 15 to 20% of ethylene oxide is added as a cap, such weight percent being based on the total weight of the final polyol produced in the second reactor. 
     Clause 23. The process of any one of clause 1 to clause 22, wherein the process does not include a propylene oxide drying step to remove water from the reaction mixture prior to making the polyol in the second reactor. 
     Clause 24. The process of any one of clause 1 to clause 23, wherein the polyol is prepared in the second reactor by a process comprising: (1) adding the dehydrated mixture to the second reactor; (2) heating the dehydrated to the desired reaction temperature, such as a temperature of 105° C. to 130° C., (3) adding the alkylene oxide to the second reactor over a period of 2 to 10 hours, and (4) after the total amount of alkylene oxide is fed, allowing the reactor contents to react further until the pressure in the reactor is level. 
     Clause 25. The process of any one of clause 1 to clause 24, further comprising transferring the polyol from the second reactor and refining the polyol to remove the alkali metal, such as by acid neutralization followed by filtration, treatment with solid adsorbents, treatment with solid inorganic compounds and treatment with ion exchanges resins. 
     Clause 26. The process of any one of clause 1 to clause 25, wherein the second reactor comprises a batch reactor or a continuous reactor, such as a single stage CSTR, a two stage CSTR, a plug flow reactor, or a loop reactor. 
     Clause 27. A production plant for preparing a polyol, comprising: (a) a first reactor comprising: (1) an inlet in fluid communication with a source of alkylene oxide; (2) an inlet in fluid communication with a source of starter; (3) an inlet in fluid communication with a source of DMC catalyst; and (4) an outlet configured to continuously discharge an intermediate polyol from the first reactor; (b) a source of an aqueous solution of alkali metal in fluid communication with the outlet of the first reactor and configured to continuously add the aqueous solution of alkali metal to the intermediate polyol as it is continuously discharged from the first reactor, thereby producing a mixture comprising the intermediate polymer, the alkali metal and water; (c) a packed column comprising a polyol inlet and a polyol outlet, wherein the polyol inlet is in fluid communication with the outlet of the first reactor, wherein the packed column is configured to continuously remove water from the mixture comprising the intermediate polymer, the alkali metal and water, thereby producing a dehydrated mixture comprising the intermediate polyol and the alkali metal; (d) a second reactor comprising: (1) an inlet that is in fluid communication with the outlet of the packed column and configured to receive the dehydrated mixture comprising the intermediate polyol and the alkali metal; (2) an inlet in fluid communication with a source of alkylene oxide; and (3) an outlet configured to discharge the polyether polyol from the second reactor. 
     Clause 28. The production plant of clause 27, wherein the first reactor comprises a single stage continuous stirred tank reactor (“CSTR”), a two stage CSTR, a plug flow reactor, or a loop reactor, such as a stream loop reactor, a jet loop reactor, a Venturi loop reactor, a tube reactors configured in loop form with suitable devices for circulating the reaction mixture, or a loop of several tube reactors connected in series or several stirred tanks connected in series. 
     Clause 29. The production plant of clause 27 or clause 28, wherein the first reactor is in fluid communication with a source of a starter comprising a compound having a number average molecular weight of 18 to 2,000, such as 62 to 2,000, and having 1 to 8 hydroxyl groups, such as a polyoxypropylene polyol, a polyoxyethylene polyol, a polytetatramethylene ether glycol, glycerol, a propoxylated glycerol, propylene glycol, ethylene glycol, tripropylene glycol, trimethylol propane, an alkoxylated allylic alcohol, bisphenol A, pentaerythritol, sorbitol, sucrose, degraded starch, water, or a mixture thereof. 
     Clause 30. The production plant of any one of clause 27 to clause 29, wherein the first reactor is in fluid communication with a source of alkylene oxide comprising ethylene oxide, propylene oxide, oxetane, 1,2- and 2,3-butylene oxide, isobutylene oxide, epichlorohydrin, cyclohexene oxide, styrene oxide, a C5-C30 α-alkylene oxides, or a mixture of any two or more thereof, such as a source of propylene oxide and a source of ethylene oxide. 
     Clause 31. The production plant of any one of clause 27 to clause 30, wherein the outlet of the first reactor is in fluid communication with a source of an aqueous solution of alkali metal comprising an alkali metal alkoxide, such as sodium methylate, sodium and potassium ethylate, potassium isopropylate and sodium butylate, and/or an alkali metal hydroxides, such as sodium hydroxide, cesium hydroxide, and potassium hydroxide, such as where the amount of alkali metal alkoxide and/or an alkali metal hydroxide in the aqueous solution is 2 to 60% by weight, 10 to 60% by weight, 20 to 55% by weight, or 30 to 50% by weight, based on the total weight of the aqueous solution, with the remainder of the solution consisting essentially of water. 
     Clause 32. The production plant of any one of clause 27 to clause 31, wherein the packed column comprises trays, random packings or structured packings. 
     Clause 33. The production plant of any one of clause 27 to clause 32, further comprising an in-line molecular sieve arranged downstream of the packing column. 
     Clause 34. The production plant of any one of clause 27 to clause 33, wherein the second reactor is in fluid communication with a source of alkylene oxide comprising ethylene oxide, propylene oxide, oxetane, 1,2- and 2,3-butylene oxide, isobutylene oxide, epichlorohydrin, cyclohexene oxide, styrene oxide, a C5-C30 α-alkylene oxides, or a mixture of any two or more thereof. 
     Clause 35. The production plant of any one of clause 27 to clause 34, wherein the second reactor comprises a batch reactor or a continuous reactor, such as a single stage CSTR, a two stage CSTR, a plug flow reactor, or a loop reactor. 
     Clause 36. The production plant of any one of clause 27 to clause 35, wherein an outlet of the second reactor is in fluid communication with an inlet of a polyol work-up system. 
     Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.