Abstract:
A particularly useful process for continuous production of a source of formaldehyde which is an un-purified liquid stream derived from a mixture formed by oxidative dehydrogenation (oxy-dehydrogenation) of at least dimethyl ether using a catalyst containing silver as an essential catalyst component is disclosed. Advantageously, the source of formaldehyde according to the invention is provided to an integrated, continuous process for catalytic production of oxygenated organic compounds, particularly polyoxymethylene dimethyl ethers, which are suitable components for blending into fuel having improved qualities for use in compression ignition internal combustion engines (diesel engines).

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 09/544,907 filed Apr. 7, 2000, which is a continuation-in-part of U.S. application Ser. No. 09/190,699 filed Nov. 12, 1998, now U.S. Pat. No. 6,265,528 and a continuation-in-part of U.S. application Ser. No. 09/310,790 filed May 12, 1999, now abandoned, which is a division of U.S. application Ser. No. 09/150,359 filed Nov. 12, 1998, now U.S. Pat. No. 5,959,156 and claims the benefit of the referenced applications which are specifically incorporated herein, in their entirety. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to production of organic compounds, particularly polyoxymethylene dimethyl ethers. More specifically, it relates to processes for continuous production of a source of formaldehyde which is an un-purified liquid stream derived from a mixture formed by oxidative dehydrogenation (oxy-dehydrogenation) of at least dimethyl ether using a catalyst containing silver as an essential catalyst component. Advantageously, sources of formaldehyde according to this invention are provided to an integrated, continuous process for catalytic production of oxygenated organic compounds, particularly polyoxymethylene dimethyl ethers, which are suitable components for blending into fuel having improved qualities for use in compression ignition internal combustion engines (diesel engines).  
         BACKGROUND OF THE INVENTION  
         [0003]    Conversion of low molecular weight alkanes such as methane to synthetic fuels or chemicals has received increasing attention because low molecular weight alkanes are generally available from secure and reliable sources. For example, natural gas wells and oil wells currently produce vast quantities of methane. Reported methods for converting low molecular weight alkanes to more easily transportable liquid fuels and chemical feedstocks can be conveniently categorized as direct oxidative routes and/or as indirect syngas routes. Direct oxidative routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, indirect syngas routes typically involve production of synthesis gas as an intermediate product.  
           [0004]    Routes are known for converting methane to dimethyl ether. For example, methane is steam reformed to produce synthesis gas. Thereafter, dimethyl ether and methanol can be manufactured simultaneously from the synthesis gas, as described in U.S. Pat. No. 4,341,069 issued to Bell et al. They recommend a dimethyl ether synthesis catalyst having copper, zinc, and chromium co-precipitated on a gamma-alumina base. Alternatively, methane is converted to methanol, and dimethyl ether is subsequently manufactured from methanol by passing a mixed vapor containing methanol and water over an alumina catalyst, as described in an article by Hutchings in New Scientist (Jul. 3, 1986) 35.  
           [0005]    Formaldehyde is a very important intermediate compound in the chemical industry. The extreme reactivity of the formaldehyde carbonyl group and the nature of the molecule as a basic building block has made formaldehyde an important feedstock for a wide variety of industrially important chemical compounds. Historically, formaldehyde has found its largest volume of application in the manufacture of phenol-formaldehyde resins, urea-formaldehyde resins and other polymers. Pure formaldehyde is quite uncommon since it polymerizes readily. It was usually obtained as an aqueous solution such as formalin, which contains only about 40 percent formaldehyde. However, more recently, formaldehyde is usually transported as an item of commerce in concentrations of 37 to 50 percent by weight. A solid source of formaldehyde called paraformaldehyde is also commercially available.  
           [0006]    Because of the reactivity of formaldehyde, its handling and separation require special attention. It is a gas above −19° C. and is flammable or explosive in air at concentrations of about 7 to about 12 mol percent. Formaldehyde polymerizes with itself at temperatures below 100° C. and more rapidly when water vapor or impurities are present. Since formaldehyde is usually transported in aqueous solutions of 50 percent by weight or lower concentration, producers have tended to locate close to markets and to ship the methanol raw material, which has a smaller volume.  
           [0007]    It is known that some reactions may be carried out by means of catalytic distillation. In catalytic distillation, reaction and separation are carried out simultaneously in a distillation column with internal and/or external stages of contact with catalyst.  
           [0008]    In U.S. Pat. No. 4,215,011, Smith, Jr. discloses a method for the separation of an isoolefin, preferably having four to six carbon atoms, from streams containing mixtures thereof with the corresponding normal olefin, wherein the mixture is fed into a reaction-distillation column containing a fixed-bed, acidic cation exchange resin and contacted with the acidic cation exchange resin to react the isoolefin with itself to form a dimer and the dimer is separated from the normal olefin, the particulate catalytic material, i.e., the acidic cation exchange resin, being contained in a plurality of closed cloth pockets, which pockets are arranged and supported in the column by wire mesh.  
           [0009]    In U.S. Pat. No. 4,443,559, Smith, Jr. discloses a catalytic distillation structure which comprises a catalyst component associated intimately with or surrounded by a resilient component, which component is comprised of at least 70 vol. percent open space for providing a matrix of substantially open space. Examples of such resilient component are open-mesh, knitted, stainless wire (demister wire or an expanded aluminum); open-mesh, knitted, polymeric filaments of nylon, Teflon, etc.; and highly-open structure foamed material (reticulated polyurethane).  
           [0010]    In U.S. Pat. No. 5,113,015, David A. Palmer, K. D. Hansen and K. A. Fjare disclose to a process for recovering acetic acid from methyl acetate wherein the methyl acetate is hydrolyzed to methanol and acetic acid via catalytic distillation.  
           [0011]    In German Democratic Republic DD 245 868 A1 published May 20, 1987 in the text submitted by the applicant, preparation of methylal is carried out by reaction of methanol with trioxane, formalin or paraformaldehyde in the presence of a specific zeolite. Autoclave reactions of 1 to 8 hours are described using a zeolite of the “LZ40 type” with a ratio of silicon dioxide to alumina ratio of 78 at temperatures from 493 to 543 K. Methylal content of the product as high as 99.8 percent (without methanol) is reported for trioxane at 523 K for 3 hours. Reaction pressures did not exceed 5 MPa in the autoclave. Neither conversions nor selectivity are reported.  
           [0012]    In U.S. Pat. No. 4,967,014, Junzo Masamoto, Junzo Ohtake and Mamoru Kawamura describe a process for formaldehyde production by reacting methanol with formaldehyde to form methylal, CH 3 OCH 2 OCH 3 , and then oxidizing the resulting methylal to obtain formaldehyde. In the methylal formation step, a solution containing methanol, formaldehyde and water was brought into solid-liquid contact with a solid acid catalyst, and a methylal-rich component was recovered as a distillate. This process employs reactive distillation performed using a distillation column and multireaction units. The middle portion of the distillation column was furnished with stages from which the liquid components were withdrawn and pumped to the reactor units, which contained solid acid catalyst. The reactive solutions containing the resulting methylal were fed to the distillation column, where methylal was distilled as the overhead product.  
           [0013]    Polyoxymethylene dimethyl ethers are the best known members of the dialkyl ether polymers of formaldehyde. While diethyl and dipropyl polyoxymethylene ethers have been prepared, major attention has been given to the dimethyl ether polymers. Polyoxymethylene dimethyl ethers make up a homologous series of polyoxymethylene glycol derivatives having the structure represented by use of the type formula indicated below:  
           CH 3 O(CH 2 O) n CH 3    
           [0014]    Chemically, they are acetals closely related to methylal, CH 3 OCH 2 OCH 3 , which may be regarded as the parent member of the group in which n of the type formula equals 1. They are synthesized by the action of methanol on aqueous formaldehyde or polyoxymethylene glycols in the presence of an acidic catalyst just as methylal is produced. On hydrolysis they are converted to formaldehyde and methanol. Like other acetals, they possess a high degree of chemical stability. They are not readily hydrolyzed under neutral or alkaline conditions, but are attacked by even relatively dilute acids. They are more stable than the polyoxymethylene diacetates.  
           [0015]    Due to the relatively small differences in the physical properties (melting points, boiling points, and solubility) of adjacent members in this series, individual homologs are not readily separated. However, fractions having various average molecular weight values have been isolated. The normal boiling point temperature of a fraction having average n of 2 in the type formula is reported as 91° to 93° C. Boiling points at atmospheric pressure calculated from partial pressure equations range from 105.0° C. for n of 2, to 242.3° C. for n of 5. (Walker, Joseph Frederic, “Formaldehyde”, Robert E. Krieger Publishing Co., issued as No. 159 of American Chemical Society Monograph series (1975), pages 167-169)  
           [0016]    Polyoxymethylene dimethyl ethers are prepared in laboratory scale by heating polyoxymethylene glycols or paraformaldehyde with methanol in the presence of a trace of sulfuric or hydrochloric acid in a sealed tube for 15 hours at 150° C., or for a shorter time (12 hours) at 165° to 180° C. Considerable pressure is caused by decomposition reactions, which produce carbon oxides, and by formation of some dimethyl ether. The average molecular weight of the ether products increases with the ratio of paraformaldehyde or polyoxymethylene to methanol in the charge. A high polymer is obtained with a 6 to 1 ratio of formaldehyde (as polymer) to methanol. In these polymers, the n value of the type formula CH 3 O(CH 2 O) n CH 3  is greater than 100, generally in the range of 300 to 500. The products are purified by washing with sodium sulfite solution, which does not dissolve the true dimethyl ethers, and may then be fractionated by fractional crystallization from various solvents.  
           [0017]    U.S. Pat. No. 2,449,469 in the names of W. F. Gresham and R. E. Brooks reported obtaining good yields of polyoxymethylene dimethyl ethers containing 2 to 4 formaldehyde units per molecule. This procedure is carried out by heating methylal with paraformaldehyde or concentrated formaldehyde solutions in the presence of sulfuric acid.  
           [0018]    It is well known that internal combustion engines have revolutionized transportation following their invention during the last decades of the 19th century. While others, including Benz and Gottleib Wilhelm Daimler, invented and developed engines using electric ignition of fuel such as gasoline, Rudolf C. K. Diesel invented and built the engine named for him which employs compression for autoignition of the fuel in order to utilize low-cost organic fuels. Development of improved diesel engines for use in automobiles has proceeded hand-in-hand with improvements in diesel fuel compositions, which today are typically derived from petroleum. Modern high performance diesel engines demand ever more advanced specification of fuel compositions, but cost remains an important consideration.  
           [0019]    Even in newer, high, performance diesel engines combustion of conventional fuel produces smoke in the exhaust. Oxygenated compounds and compounds containing few or no carbon-to-carbon chemical bonds, such as methanol and dimethyl ether are known to reduce smoke and engine exhaust emissions. However, most such compounds have high vapor pressure and/or are nearly insoluble in diesel fuel, and they have poor ignition quality, as indicated by their cetane numbers. Furthermore other methods of improving diesel fuels by chemical hydrogenation to reduce their sulfur and aromatics contents, also causes a reduction in fuel lubricity. Diesel fuels of low lubricity may cause excessive wear of fuel injectors and other moving( parts which come in contact with the fuel under high pressures.  
           [0020]    Recently, U.S. Pat. No. 5,746,785 in the names of David S. Moulton and David W. Naegeli reported blending a mixture of alkoxy-terminated poly-oxymethylenes, having a varied mixture of molecular weights, with diesel fuel to form an improved fuel for autoignition engines. Two mixtures were produced by reacting paraformaldehyde with (i) methanol or (ii) methylal in a closed system for up to 7 hours and at a temperatures of 1500 to 240° C. and pressures of 300 psi to 1,000 psi to form a product containing methoxy-terminated poly-oxymethylenes having a molecular weight of from about 80 to about 350 (polyoxymethylene dimethyl ethers). More specifically, a 1.6 liter cylindrical reactor was loaded with a mixture of methanol and paraformaldehyde, in molar ratio of about I mole methanol to 3 moles paraformaldehyde, and in a second preparation, methylal (dimethoxymethane) and paraformaldehyde were combined in a molar ratio of about 1 mole methylal to about 5 moles paraformaldehyde. In the second procedure, a small amount of formic acid, about 0.1 percent by weight of the total reactants, was added as a catalyst. The same temperatures, pressures and reaction times are maintained as in the first. Disadvantages of these products include the presence of formic acid and thermal instability of methoxy-terminated poly-oxymethylenes under ambient pressure and acidic conditions.  
           [0021]    There is, therefore, a present need for catalytic processes to prepare oxygenated organic compounds, particularly polyoxymethylene dimethyl ethers, which do not have the above disadvantages. An improved process should be carried out advantageously in the liquid phase using a suitable condensation-promoting catalyst system, preferably a molecular sieve based catalyst which provides improved conversion and yield. Such an improved process which converts lower value compounds to higher polyoxymethylene dimethyl ethers would be particularly advantageous. Dimethyl ether is, for example, less expensive to produce than methanol on a methanol equivalent basis, and its condensation to polyoxymethylene dimethyl ethers does not produce water as a co-product.  
           [0022]    The base diesel fuel, when blended with such mixtures in a volume ratio of from about 2 to about 5 parts diesel fuel to 1 part of the total mixture, is said to provide a higher quality fuel having significantly improved lubricity and reduced smoke formation without degradation of the cetane number or smoke formation characteristics as compared to the base diesel fuel.  
           [0023]    This invention is directed to overcoming the problems set forth above in order to provide Diesel fuel having improved qualities. It is desirable to have a method of producing a high quality diesel fuel that has better fuel lubricity than conventional low-sulfur, low-aromatics diesel fuels, yet has comparable ignition quality and smoke generation characteristics. It is also desirable to have a method of producing such fuel which contains an additional blended component that is soluble in diesel fuel and has no carbon-to-carbon bonds. Furthermore, it is desirable to have such a fuel wherein the concentration of gums and other undesirable products is reduced.  
         SUMMARY OF THE INVENTION  
         [0024]    Economical processes are disclosed, for production of a source of formaldehyde which is an un-purified liquid stream derived from a mixture formed by oxidative dehydrogenation (oxy-dehydrogenation) of dimethyl ether using a catalyst containing silver as an essential catalyst component. Advantageously, sources of formaldehyde according to this invention are provided to an integrated, continuous process for catalytic production of oxygenated organic compounds particularly polyoxymethylene dimethyl ethers, which are suitable components for blending into fuel having improved qualities for use in compression ignition internal combustion engines (diesel engines).  
           [0025]    According to the present invention, there is now provided a continuous process for oxidative dehydrogenation of dimethyl ether to form a source of formaldehyde by a process comprising continuously contacting a gaseous feedstream comprising dimethyl ether, dioxygen and diluent gas with a catalytically effective amount of an oxidative dehydrogenation promoting catalyst comprising silver as an essential catalyst component at elevated temperatures in a range from about 550° to about 750° C. to form a gaseous mixture comprising formaldehyde, methanol, dioxygen, diluent gas, carbon dioxide and water vapor; cooling the gaseous dehydrogenation mixture with a liquid to take up and hold formaldehyde therein; and separating the resulting liquid source of formaldehyde from a mixture of gases comprising dioxygen, diluent, carbon monoxide, carbon dioxide and water vapor wherein the dioxygen content of the gaseous feedstream is no more than 10 mole percent.  
           [0026]    Advantageously, the separation of the liquid source of formaldehyde from the mixture of gases is carried out by means of a multi-stage column. Preferably, the liquid source of formaldehyde is recovered as an aqueous solution containing less than 25 percent water by using at least one continuous adsorption column with cooling to temperatures in a range downward from about 100° C. to 25° C.  
           [0027]    In a preferred embodiment of the invention the process further comprises admixing at least a portion of the gases from the multi-stage column into the gaseous feedstream with the, proviso that the amount of formaldehyde in the portion is less than about 500 ppm, more preferably less than about 100 ppm, most preferably less than about 50 ppm.  
           [0028]    In one aspect the invention is a process for oxidative dehydrogenation of dimethyl ether to form a source of formaldehyde by a process comprising continuously contacting a gaseous feedstream comprising dimethyl ether, dioxygen, dinitrogen and steam, with a catalytically effective amount of an oxidative dehydrogenation promoting catalyst comprising silver as an essential catalyst component at elevated temperatures thereby forming a gaseous mixture comprising formaldehyde, methanol, dioxygen, diluent gas, carbon dioxide and water vapor; cooling the gaseous dehydrogenation mixture with a liquid to take up and hold formaldehyde therein; and separating the resulting liquid source of formaldehyde from a mixture of gases comprising dioxygen, diluent, carbon monoxide, carbon dioxide and water vapor, wherein the ratio of dioxygen to total ether and, if present, alkanol in the gaseous feedstream is in a range from 1:1 to 1:100 moles.  
           [0029]    Suitable levels of alkanol having a normal boiling point of less than about 100° C., may be present in the gaseous feedstream. If present, preferred the alkanol is a member of the group consisting of methanol, ethanol and mixtures thereof. More preferably, if present, the alkanol is methanol.  
           [0030]    According to another aspect of the present invention, there is now provided a process for oxidative dehydrogenation of dimethyl ether to form a source of formaldehyde by a process comprising continuously contacting a gaseous feedstream comprising dimethyl ether, methanol, dioxygen, dinitrogen and steam, with a catalytically effective amount of an oxidative dehydrogenation promoting catalyst comprising silver as an essential catalyst component at temperatures in a range from about 550° to about 750° C. thereby forming a gaseous mixture comprising formaldehyde, methanol, dioxygen, diluent gas, carbon dioxide and water vapor; cooling the gaseous dehydrogenation mixture with a liquid to take up and hold formaldehyde therein, and separating the resulting liquid source of formaldehyde from a mixture of gases comprising dioxygen diluent, carbon monoxide, carbon dioxide and water vapor, wherein the ratio of dioxygen to total ether and methanol in the gaseous feedstream is in a range from 1:1 to 1:10 moles.  
           [0031]    In another aspect the present invention provides a continuous process for catalytic production of oxygenated organic compounds, particularly polyoxymethylene dimethyl ethers. More specifically, continuous processes of this invention comprise providing a feedstream comprising methanol, formaldehyde and a soluble condensation promoting component capable of activating a heterogeneous acidic catalyst, and heating the feedstream with the heterogeneous acidic catalyst under conditions of reaction sufficient to form an effluent of condensation comprising water, methanol and one or more polyoxymethylene dimethyl ethers having a structure represented by the formula  
           CH 3 O(CH 2 O) n CH 3    
           [0032]    in which formula n is a number from 1 to about 10. Advantageously, at least a liquid of the effluent containing polyoxymethylene dimethyl ethers is contacted with an anion-exchange resin to form an essentially acid-free mixture.  
           [0033]    Another aspect this invention is an integrated process which further comprises formation of the feedstream by a process comprising continuously contacting a gaseous feedstream comprising dimethyl ether, dioxygen and diluent gas with a catalytically effective amount of an oxidative dehydrogenation promoting catalyst comprising silver as an essential catalyst component at elevated temperatures to form a gaseous mixture comprising formaldehyde, methanol, dioxygen, diluent gas, carbon dioxide and water vapor, cooling the gaseous dehydrogenation mixture with an adsorption liquid and adsorbing formaldehyde therein; and separating the resulting liquid source of formaldehyde from a mixture of gases comprising dioxygen, diluent, carbon monoxide, carbon dioxide and water vapor.  
           [0034]    At least a portion of the mixture of gases separated from the resulting aqueous source of formaldehyde is recycled into the gaseous feedstream.  
           [0035]    Preferably the formaldehyde formed is recovered as an aqueous solution containing less than about 60 percent water, preferably less than about 50 percent water and more preferably less than about 25 percent water by using at least one continuous adsorption column with cooling to temperatures in a range downward from about 100° C. to 25° C.  
           [0036]    Suitable sources of dioxygen are air or a dioxygen-enriched gas stream obtained by physically separating a gaseous mixture containing at least about 10 volume percent dioxygen into a dioxygen-depleted stream and a dioxygen-enriched gas stream. Preferably the gaseous mixture contains at least 60 volume percent dinitrogen and wherein the dioxygen-enriched gas stream comprises a volume ratio of dinitrogen to dioxygen of less than 2.5 to 1.  
           [0037]    For this aspect of the invention, suitable oxidative dehydrogenation promoting catalyst comprises silver and optionally up to about 8 percent by weight of a compound selected from the group consisting of oxides of boron, phosphorous, vanadium, selenium, molybdenum and bismuth, phosphoric acid, ammonium phosphate and ammonium chloride.  
           [0038]    For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawing and described below by way of examples of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    [0039]FIG. 1 is a schematic flow diagram depicting a preferred aspect of the present invention for continuous catalytic production of polyoxymethylene dimethyl ethers by chemical conversion of dimethyl ether and formaldehyde in which unreacted dimethyl ether is recovered from the effluent for recycling, and a resulting dimethyl ether-free liquid mixture is heated in a catalytic distillation column with internal stages of contact to convert formaldehyde and methanol present to methylal and higher polyoxymethylene dimethyl ethers. This reaction mixture is contacted with an anion exchange resin to form an essentially acid-free product mixture and fractionated to provide suitable components for blending into diesel fuel. The source of formaldehyde in the integrated process depicted in FIG. 1 is a stream of aqueous formaldehyde in methanol derived from oxidative dehydrogenation of dimethyl ether. In this aspect of invention, recycle gas from the formaldehyde absorber is combined with air, mixed with dimethyl ether, preheated against reactor product, and then fed to the formaldehyde reactor.  
         [0040]    [0040]FIG. 2 is a schematic flow diagram depicting another preferred aspect of the present invention for continuous catalytic production of polyoxymethylene dimethyl ethers by chemical conversion of methanol and formaldehyde in which a feedstream comprising a soluble condensation promoting component capable of activating a heterogeneous acidic catalyst is heated with the heterogeneous acidic catalyst in a catalytic distillation column with internal stages of contact. A liquid portion of the effluent of condensation, containing polyoxymethylene dimethyl ethers, is contacted with an anion exchange resin disposed within a section of the distillation column below the stages of contact with the acidic catalyst to form an essentially acid-free mixture, and fractionated to provide suitable components for blending into diesel fuel. The feedstream in the integrated process depicted in FIG. 2 is again a stream of aqueous formaldehyde in methanol derived from oxidative dehydrogenation of dimethyl ether. In this aspect of invention, recycle gas from the formaldehyde absorber is combined with air, mixed with dimethyl ether, preheated against reactor product, and then fed to the formaldehyde reactor.  
     
    
     GENERAL DESCRIPTION  
       [0041]    According to improved processes of the present invention a source of formaldehyde is formed by subjecting dimethyl ether in the vapor phase to oxidative dehydrogenation in the presence of a catalytically effective amount of an oxidative dehydrogenation promoting catalyst comprising silver as an essential catalyst component at elevated temperatures to form a gaseous mixture comprising formaldehyde, methanol, dioxygen, diluent gas, carbon dioxide and water vapor; cooling the gaseous dehydrogenation mixture with an adsorption liquid and adsorbing formaldehyde therein; and separating the resulting liquid source of formaldehyde from a mixture of gases comprising dioxygen, diluent, carbon monoxide, carbon dioxide and water vapor.  
         [0042]    According to this aspect of the present invention, the ratio of dioxygen to total dimethyl ether is any mole ratio which results in the production of the desired source of formaldehyde. The ratio of dioxygen to total ether and, if present, alkanol is preferably between about 1:1 and about 1:1000 moles. More preferably, the ratio of dioxygen to dimethyl ether is between about 1:1 and about 1:100 moles. Most preferably, the ratio of dioxygen to dimethyl ether is between about 1:1 and about 1:10 moles.  
         [0043]    Operating conditions of reaction usually fall in the following ranges:  
         [0044]    Dimethyl ether: 1 mol percent up to 17.4 mol percent  
         [0045]    Air: 99 mol percent to 82.6 mol percent  
         [0046]    Reaction temperature: 200° to 450° C.  
         [0047]    Space velocity: 1,000-20,000 hr −1    
         [0048]    Preferable conditions are as follows:  
         [0049]    Dimethyl ether: 3 mol percent to 12 mol percent  
         [0050]    Reaction temperature: 250° to 400° C.  
         [0051]    Space velocity: 1,000-10,000 hr −1    
         [0052]    Sources of dimethyl ether useful herein are predominantly dimethyl ether, preferably at least about 80 percent dimethyl ether by weight, and more preferably about 90 percent dimethyl ether by weight. Suitable dimethyl ether sources may contain other oxygen containing compounds such as alkanol and/or water, preferably not more than about 20 percent methanol and/or water by weight, and more preferably not more than about 15 percent methanol and/or water by weight.  
         [0053]    The dioxygen can be added to the reaction mixture as pure molecular oxygen, or diluted with an inert gas such as nitrogen or argon. It is preferred to keep the dioxygen at no more than 10 mole percent of the entire reaction feed so as to avoid the formation of explosive mixtures.  
         [0054]    The Karl Fischer process, developed in the early 1900s by Karl Fischer Apparate and Rohrleitungsbau of Germany, is probably the most widely used silver catalyst process.  
         [0055]    Several features distinguish this type of process from those catalyzed by metal oxides. Whereas the metal oxide processes achieve an overall yield of about 93 percent, commercial silver catalyzed units are generally believed to achieve an overall yield of about 88 percent. Per-pass methanol conversion is about 80 percent, with 90 percent selectivity to formaldehyde. The silver-catalyzed process operates with an excess of methanol and requires a specialized piece of equipment, such as a packed tower or a methanol boiler equipped with a special air mixer, to generate the methanol-air feed mixture. Since less air is used, however, equipment sizes are smaller and power consumption is less than for the metal oxide process.  
         [0056]    The silver-catalyzed reaction is endothermic; but depending on the amount of feed preheat and air added, the exothermic reaction of oxygen with hydrogen sustains the reaction at temperatures in excess of 550° C. Steam can then be generated by cooling the reactor effluent before it is fed to the absorber. However, the steam has a lower pressure (about 30 psig), and the amount produced is insufficient to satisfy process steam requirements. This steam deficit is offset by the fuel value of the substantial amount of hydrogen (more than 18 vol percent) that remains in the absorber off-gas. If the absorber offgas is processed in a thermal oxidizer, a blower for additional combustion air is required, but a substantial amount of 200 psig steam may be produced.  
         [0057]    The dioxygen can be added to the reaction mixture as pure molecular oxygen, or diluted with an inert gas such as nitrogen or argon. It is preferred to keep the dioxygen at no more than 10 mole percent of the entire reaction feed so as to avoid the formation of explosive mixtures.  
         [0058]    According to the present invention, within the oxidation reaction zone methanol is oxidized with a source of dioxygen in the presence of an oxy-dehydrogenation catalytic composition containing, as an essential ingredient, silver with or without up to about 10 percent of a supplemental inorganic compound based upon the total weight of metal oxide and supplemental inorganic compounds. Suitable oxy-dehydrogenation catalysts have been developed for converting methanol with a source of dioxygen to produce formaldehyde, as described in U.S. Pat. No. 5,401,884, U.S. Pat. No. 5,102,838, U.S. Pat. No. 4,786,743, U.S. Pat. No. 4,521,618, U.S. Pat. No. 4,474,996 and U.S. Pat. No. 4,359,587 which patents are specifically incorporated herein in their entirety by reference.  
         [0059]    The starting materials are fed through a silver-containing fixed-bed catalyst installed in a vertical tubular reactor. The catalyst preferably comprises silver crystals having a particle size of from 0.1 to 3 mm, in particular from 0.2 to 2.5 mm. The fixed-bed catalyst can have a multi-layer structure through arrangement of the silver crystals in layers of different particle size.  
         [0060]    The starting mixture of methanol vapor, oxygen-containing gas, and, if used, steam and inert gas is preferably passed through the tubular reactor from top to bottom.  
         [0061]    Otherwise, the process is carried out in one step by passing the starting mixture through the fixed catalyst bed at from 550° to 750° C., in particular from 590° to 720° C., particularly advantageously at from 660° to 700° C. The process is preferably carried out continuously at from 0.5 to 3 bar, in particular at from 0.8 to 2 bar, preferably at from 1 to 1.5 bar. The residence times in the catalyst zone are from 0.001 to 1 second, preferably from 0.002 to 0.1 second. The reaction gases leaving the catalyst zone are advantageously cooled within a short time, for example to below 350° C. The cooled gas mixture can expediently be fed to an adsorption tower, in which the formaldehyde is washed out of the gas mixture by means of water.  
         [0062]    In the present method, dimethyl ether may be used alone, or methanol and dimethyl ether can be used in admixture with each other to produce formaldehyde.  
         [0063]    In another aspect, processes of the present invention employ a heterogeneous, condensation promoting catalyst capable of hydrating dimethyl ether in conversion of dimethyl ether and formaldehyde to form a condensation effluent. In general, after the feedstream is passed over the catalyst it will contain a mixture of organic oxygenates at least one of which is of higher molecular weight than the starting dimethyl ether. For example, effluent mixtures can comprise water, methanol, formaldehyde, dimethyl ether, methylal and other polyoxymethylene dimethyl ethers having a structure represented by the formula  
         CH 3 O(CH 2 O) n CH 3    
         [0064]    in which formula n is a number ranging between 1 and about 15, preferably between 1 and about 10. More preferably the mixture contains a plurality of polyoxymethylene dimethyl ethers having values of n in a range from 2 to about 7. Conditions of reaction include temperatures in a range from about 50° to about 300° C., preferably in a range from about 150° to about 250° C.  
         [0065]    Stoichiometry of this condensation may be expressed by the following equations;  
         CH 3 OCH 3   +n CH 2 O→CH 3 O(CH 2 O) n CH 3    
         2CH 2 OH+ m CH 2 O         CH 3 O(CH 2 O) m CH 3 +H 2 O  
         [0066]    which may be combined as in the following equation when n is equal to m;  
         CH 3 OCH 3 +2CH 3 OH+2 n CH 2 O         2CH 3 O(CH 2 O) n CH 3 +H 2 O  
         [0067]    As shown above, the synthesis of methylal and higher polyoxymethylene dimethyl ethers from dimethyl ether, methanol, and formaldehyde is a reversible reaction that yields water as a co-product. Under certain conditions at least a portion of the water may be consumed in a dehydrogenation reaction expressed by the following equations;  
         CH 3 OCH 3 +CH 3 OH+H 2 O→3CH 2 O+3H 2    
         [0068]    and  
         CH 3 OCH 3 +H 2 O         2CH 3 OH  
         [0069]    After recovering a dimethyl ether-free mixture form the condensation effluent, the mixture is heated in a catalytic distillation column with an acidic catalyst, which is heterogeneous to the feedstream, under conditions of reaction sufficient to convert formaldehyde and methanol present to methylal and higher polyoxymethylene dimethyl ethers. Examples of the solid acidic catalyst for use in the present invention include cation exchange resins, sulfonated fluoroalkylene resin derivatives, and crystalline aluminosilicates.  
         [0070]    The feed stream is heated in a catalytic distillation column with an acidic catalyst, which is heterogeneous to the feedstream, under conditions of reaction sufficient to convert formaldehyde and methanol present to methylal and higher polyoxymethylene dimethyl ethers. Examples of the solid acidic catalyst for use in the present invention include cation exchange resins, sulfonated fluoroalkylene resin derivatives, and crystalline aluminosilicates.  
         [0071]    Cation exchange resins that can be used in the present invention may be carboxylated or sulfonated derivatives, but sulfonated derivatives are preferred because of the high reaction yield that can be attained. Ion exchange resins that can be used may be gel-type cation exchange resins or macroporous (macroreticular) cation-exchange resins, but the latter as exemplified by Amberlite 200C of Organc Co, Ltd. and Lewalit SP112 of Bayer A. G. are preferred because of the high reaction yield that can be attained. Specific examples of useful ion exchange resins include a styrene-divinylbenzene copolymer, an acrylic acid-divinylbenzene copolymer, a methacrylic acid-divinylbenzene copolymer, etc. A sulfonated tetrafluoroethylene resin derivative (trade name, Naflon H) is preferably used as a sulfonated fluoroalkylene resin derivative. The most desirable of these solid acidic catalysts are macroreticular cation exchange resins having sulfonate groups.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0072]    In order to better communicate the present invention, a preferred aspect of the invention is depicted schematically in FIG. 1. Referring now to FIG. 1, a mixture containing dimethyl ether in substantially liquid form is unloaded, for example from a road tanker (not shown), into dimethyl ether storage vessel  12  which supplies charge pump  14  through conduit  13 . Charge pump  14  transfers the liquid dimethyl ether from storage vessel  12  into catalytic reactor  20  through conduit  16  and manifold  22 . Aqueous formaldehyde is supplied to manifold  22  through conduit  18 , and into catalytic reactor  20  which contains a condensation-promoting catalyst based upon a suitable molecular sieve.  
         [0073]    It should be apparent that effluent from the catalytic reactor is a valuable product in itself A portion of the stream can optionally be diverted from catalytic reactor  20  for delivery to a destination (not shown) where stream may subsequently be separated to recover, for example, dimethyl ether, formaldehyde, methylal and/or other polyoxymethylene dimethyl ethers. The stream can alternatively be utilized as a source of feed stock for chemical manufacturing.  
         [0074]    The effluent stream from catalytic reactor  20  is transferred through conduits  23  and  26 , by means of pump  24 , and into ether recovery column  30 , where unreacted dimethyl ether is separated from the effluent stream to form a resulting liquid mixture of condensation products containing any unreacted formaldehyde. A dimethyl ether fraction is taken overhead through conduit  32  and into condenser  34  where a liquid condensate is formed. A suitable portion of the liquid condensate is refluxed into column  30  through conduits  35  and  36  while another portion of the condensate is supplied to manifold  22  through conduits  37  and  39 , by means of pump  38 , and into catalytic reactor  20 .  
         [0075]    Conduit  28  supplies pump  40  with liquid from the bottom of column  30 . A suitable portion of the liquid stream from the bottom of column  30  is transferred through conduits  41  and  42 , by means of pump  40 , and into reboiler  43  which is in flow communication with the bottom of the column through conduit  44 . A liquid stream from the bottom of column  30  is transferred through conduit  45  into reactive distillation column  50 , where simultaneous chemical reaction and multicomponent distillation are carried out coextensively in the same high efficiency, continuous separation apparatus. Optionally, a stream containing methanol from storage vessel  46  may by fed into the reactive distillation column  50 . Charge pump  48  transfers methanol in substantially liquid form into the reactive distillation column  50  through conduits  47  and  49 .  
         [0076]    Solid acidic catalyst is present in the reactive distillation column  50  to allow solutions containing water, methanol, formaldehyde, methylal and one or more other polyoxymethylene dimethyl ethers to be brought into solid-liquid contact counter-currently with the catalyst to form products including methylal and higher molecular weight polyoxymethylene dimethyl ethers. More volatile reaction products are taken overhead from the high efficiency separation apparatus, whereas water and less volatile reaction products are carried down the high efficiency separation apparatus.  
         [0077]    The overhead vapor stream from reactive distillation column  50  is transferred through conduit  52  into condenser  54 . A suitable portion of condensate from condenser  54  is refluxed into reactive distillation column  50  through conduits  55  and  56 . A product stream containing methylal is transferred through conduit  58  to product storage (not shown). Conduit  59  supplies pump  60  with liquid containing higher molecular weight polyoxymethylene dimethyl ethers from the bottom of column  50 . A suitable portion of liquid from the bottom of column  50  is transferred, by means of pump  60 , through conduits  62  and  63  into reboiler  64  which is in flow communication with the bottom of the column by means of conduit  66 . A product stream containing higher molecular weight polyoxymethylene dimethyl ethers is transferred through conduit  68  to product storage (not shown). Preferably, an anion exchange resin is disposed within a section of the distillation column below the stages of contact with the acidic catalyst to form an essentially acid-free mixture.  
         [0078]    An aqueous side stream containing low levels of unreacted formaldehyde and/or methanol is discharged from column  50  through conduit  72 .  
         [0079]    In this aspect of invention, recycle gas from the formaldehyde absorber is combined with air, mixed with dimethyl ether, preheated against reactor product, and then fed to the formaldehyde reactor.  
         [0080]    Referring now to the upper portion of FIG. 1, a mixture containing dimethyl ether in substantially liquid form is supplied from dimethyl ether storage vessel  12  to pump  14  through conduit  13 . Dimethyl ether is transferred through conduit  15  into feed manifold  92 . A recycle stream of wet gas is transferred into feed manifold  92  by means of blower  88 . A gaseous stream containing dioxygen and dinitrogen from a source (not shown) is supplied to compressor  94  through conduit  93 . Compressed gas is transferred through conduit  95 , combined in feed manifold  92  with the recycle stream of wet gas from the formaldehyde adsorber and the dimethyl ether. The resulting mixture is heated against reactor effluent in heat exchanger  80 , and transferred into oxidation reactor  90  through conduit  82  and feed manifold  84   
         [0081]    Formaldehyde reactor  90  contains an oxidative dehydrogenation catalyst disposed in thin layer directly above a vertical heat exchanger where effluent from the catalyst layer is promptly cooled. Boiler feed water at about 110° to 130° C. is supplied through conduit  85  to the heat exchanger for generation of low pressure steam in the lower section of the formaldehyde reactor. The steam is transferred through conduit  86 , mixed with the preheated mixture of dimethyl ether, wet recycle gas and air stream in feed manifold  84 , and transferred into formaldehyde reactor  90 . Steam is metered into the preheated methanol-air mixture to control the reactor outlet temperature. The mole ratio of fresh air feed to methanol is between 0.5 and 2.0, preferably about 1.25 and typically the mole ratio of dimethyl ether to steam is about 3. The pressure is only slightly above atmospheric. Since the catalyst layers are less than one inch in thickness, the pressure drop is negligible.  
         [0082]    In this embodiment of the invention, metallic silver catalyzes the conversion of dimethyl ether to formaldehyde by a reversible dehydrogenation reaction at temperatures from about 500° to 700° C.:  
         CH 3 OCH 3 +½O 2 →2CH 2 O+H 2    
         [0083]    The oxidative dehydrogenation catalyst is generally silver crystals supported on a stainless steel mesh, or a shallow bed of silver crystals, spherical particles, or granules. The reaction is endothermic, and theoretical equilibrium is approximately 50 percent yield at 400° C., 90 percent at 500° C., and 99 percent at 700° C. To conveniently sustain elevated reaction temperatures required to obtain high yields, a portion of the hydrogen formed is oxidized to water. Formation of water is exothermic and provides heat to maintain the endothermic hydrogenation reaction. Heat is also provided by the direct oxidation of methanol:  
         CH 3 OH+½O 2 →CH 2 O+H 2 O  
         [0084]    These reactions are rapid and therefore the process is essentially adiabatic. At 650° C., the reaction is substantially complete with contact times of less than 0.01 second. Methanol conversion in the reactor is typically between 65 percent and 80 percent, depending largely on the amount of steam introduced at the methanol vaporization step. Formaldehyde is lost by several side reactions, including those producing co-products including carbon monoxide, carbon dioxide, methane, formic acid, and methyl formate.  
         [0085]    To minimize side reactions, it is important to avoid excess oxygen and to operate with exposure time of products and reactants to the catalyst at high temperatures as short as possible. An excess of methanol or methanol and steam is also important, serving to avoid an explosive feed composition. A mixture containing between 6.7 mol percent and 36.5 mol percent methanol in air at 1 atm constitutes a severe explosion hazard.  
         [0086]    Gaseous effluent from oxidation reactor  90  is transferred through conduit  96 , cooled against the reactor feedstream in exchanger  80  and then passed to an absorption system, where methanol and formaldehyde are absorbed. The effluent gases flow through conduit  98  into spray column  100  where a solution of formaldehyde is formed. Formaldehyde solution from the bottom of spray column  100 , at temperatures in a range downward from about 100° C. to about 75° C., is supplied to pump  104  through conduit  103 . As previously described, a portion of the aqueous formaldehyde is transferred through conduit  18  and manifold  22  into catalytic reactor  20 . Formaldehyde solution from the spray column is generally about 55 percent by weight formaldehyde, about 43 methanol weight percent about 2 weight percent water and less than 350 ppm of formic acid. Another portion of the formaldehyde solution is combined with crude formalin solution, supplied from the bottom adsorption column  110  through conduits  113  and  108 , by means of pump  114 , and circulated to the top of spray column  100  through conduit  106 . It is important to maintain the temperature of the pump-around stream above about 70° C. to prevent paraformaldehyde formation. Optionally, a portion of the cooling required in spray column  100  may be obtained by including a heat exchanger in the flow through conduit  106 .  
         [0087]    A gaseous overhead stream from spray column  100  is transferred through conduit  102  into adsorption column  110 , which contains a high efficiency packing or other means for contacting counter-currently the gaseous stream with aqueous adsorption liquid. A dilute aqueous formaldehyde from the bottom of adsorption column  110  is circulated in a pump-around to the bottom section of the column through conduits  113  and  115 , cooler  116 , and conduit  117  by means of pump  114 .  
         [0088]    Further up the column pump-arounds are be cooled to successively lower temperatures. In this embodiment of the invention a liquid side stream is supplied to pump  124  through conduit  125 , transferred through manifold  127 , cooled in cooler  126 , and returned to adsorption column  110  through conduit  128 .  
         [0089]    In some configurations, the lower pump-around stream is not cooled at all. Caustic solution may be added to the chilled water to improve absorber performance, but it leaves traces of sodium as a contaminant in the product.  
         [0090]    A vapor side draw from adsorption column  110  is transferred through conduit  112  and mixed with fresh air as previously described. The amount of formaldehyde in the vapor side draw from adsorption column  110  is less than about 500 ppm, preferably less than about 100 ppm, more preferably less than about 50 ppm, and most preferably less than about 30 ppm.  
         [0091]    The adsorber overhead passes through conduit  18  into condenser  122 . An appropriate amount of condensate is formed and refluxed to the top section of the adsorber column through conduit  123 . Overhead temperatures in adsorption column  110  are in a range of about 15° to about 55° C., preferably about 30° to about 40° C. Gases are vented from condenser  122  through conduit  120  to disposal, typically, in a thermal oxidation unit (not shown).  
         [0092]    The absorber overhead, which contains trace amounts of formaldehyde (about 10-30 ppm), is treated in several ways by catalytic or thermal converter to oxidize hydrocarbons and recuperative heat exchange. Typically, 170 psig to 200 psig steam is generated to improve overall economics of preferred embodiments of the invention.  
         [0093]    In order to better communicate the present invention, still another preferred aspect of the invention is depicted schematically in FIG. 2. In this integrated process a feedstream comprising methanol, formaldehyde and a soluble condensation promoting component capable of activating a heterogeneous acidic catalyst is provided by contacting dimethyl ether in the vapor phase with a catalytically effective amount of an oxidative dehydrogenation promoting catalyst comprising silver as an essential catalyst component at elevated temperatures to form a gaseous mixture comprising formaldehyde, methanol, dioxygen, diluent gas, carbon dioxide and water vapor; cooling the gaseous dehydrogenation mixture with an adsorption liquid and adsorbing formaldehyde therein; and separating the resulting liquid source of formaldehyde from a mixture of gases comprising dioxygen, diluent, carbon monoxide, carbon dioxide and water vapor. In this aspect of invention, recycle gas from the formaldehyde absorber is combined with air, mixed with dimethyl ether, preheated against reactor product, and then fed to the formaldehyde reactor.  
         [0094]    Referring now to the upper portion of FIG. 2, a mixture containing dimethyl ether in substantially liquid form is supplied from dimethyl ether storage vessel  12  to pump  14  through conduit  13 . Dimethyl ether is transferred through conduit  15  into feed manifold  92 . A recycle stream of wet gas is transferred into feed manifold  92  by means of blower  88 . A gaseous stream containing dioxygen and dinitrogen from a source (not shown) is supplied to compressor  94  through conduit  93 . Compressed gas is transferred through conduit  95 , combined in feed manifold  92  with the recycle stream of wet gas from the formaldehyde adsorber and the dimethyl ether. The resulting mixture is heated against reactor effluent in heat exchanger  80 , and transferred into oxidation reactor  90  through conduit  82  and feed manifold  84   
         [0095]    Formaldehyde reactor  90  contains an oxidative dehydrogenation catalyst disposed in a thin layer directly above a vertical heat exchanger where effluent from the catalyst layer is promptly cooled. Boiler feed water at about 110° to 130° C. is supplied through conduit  85  to the heat exchanger for generation of low pressure steam in the lower section of the formaldehyde reactor. The steam is transferred through conduit  86 , mixed with the preheated mixture of dimethyl ether, wet recycle gas and air stream in feed manifold  84 , and transferred into formaldehyde reactor  90 . Steam is metered into the preheated methanol-air mixture to control the reactor outlet temperature. The mole ratio of fresh air feed to methanol is between 0.5 and 2.0, preferably about 1.25 and typically the mole ratio of dimethyl ether to steam is about 3. The pressure is only slightly above atmospheric. Since the catalyst layers are less than one inch in thickness, the pressure drop is negligible.  
         [0096]    Gaseous effluent from oxidation reactor  90  is transferred through conduit  96 , cooled against the reactor feedstream in exchanger  80  and then passed to an absorption system, where aqueous formaldehyde is absorbed in methanol. The effluent gases flow through conduit  98  into spray column  100  where a solution of formaldehyde is formed. Formaldehyde solution from the bottom of spray column  100 , at temperatures in a range downward from about 100° C. to about 75° C., is supplied to pump  104  through conduit  103 . A portion of the aqueous formaldehyde is transferred through conduit  45  into reactive distillation column  50 . Formaldehyde solution from the spray column is generally about 55 percent by weight formaldehyde, about 43 methanol weight percent about 2, weight percent water and less than 350 ppm of formic acid.  
         [0097]    It should be apparent that effluent from the spray column is a valuable product in itself A portion of the stream can optionally be diverted from adsorption tower  100  for delivery to a destination (not shown) where the stream may subsequently be separated to recover, for example, formaldehyde and methanol and/or dimethyl ether. The stream can alternatively be utilized as a source of feed stock for chemical manufacturing.  
         [0098]    Formaldehyde solution is combined with a solution of formaldehyde supplied from the bottom of adsorption column  110  through conduits  113  and  108 , by means of pump  114 , and circulated to the top of spray column  100  through conduit  106 . It is important to maintain the temperature of the pump-around stream above about 70° C. to prevent paraformaldehyde formation. Optionally, a portion of the cooling required in spray column  100  may be obtained by including a heat exchanger in the flow through conduit  106 .  
         [0099]    A gaseous overhead stream from spray column  100  is transferred through conduit  102  into adsorption column  110 , which contains a high efficiency packing or other means for contacting counter-currently the gaseous stream with aqueous adsorption liquid. A dilute aqueous formaldehyde from the bottom of adsorption column  10  is circulated in a pump-around to the bottom section of the column through conduits  113  and  115 , cooler  116 , and conduit  117  by means of pump  114 .  
         [0100]    Further up the column pump-arounds are be cooled to successively lower temperatures. In this embodiment of the invention a liquid side stream is supplied to pump  124  through conduit  125 , transferred through manifold  127 , cooled in cooler  126 , and returned to adsorption column  110  through conduit  128 .  
         [0101]    In some configurations, the lower pump-around stream is not cooled at all. Caustic solution may be added to the chilled water to improve absorber performance, but it leaves traces of sodium as a contaminant in the product.  
         [0102]    A vapor side draw from adsorption column  110  is transferred through conduit  112  and mixed with fresh air as previously described. The amount of formaldehyde in the vapor side draw from adsorption column  110  is less than about 500 ppm, preferably less than about 100 ppm, more preferably less than about 50 ppm, and most preferably less than about 30 ppm.  
         [0103]    The adsorber overhead passes through conduit  118  into condenser  122 . An appropriate amount of condensate is formed and refluxed to the top section of the adsorber column through conduit  123 . Overhead temperatures in adsorption column  110  are in a range of about 15° to about 55° C., preferably about 30° to about 40° C. Gases are vented from condenser  122  through conduit  120  to disposal, typically, in a thermal oxidation unit (not shown).  
         [0104]    The absorber overhead, which contains trace amounts of formaldehyde (about 10-30 ppm), is treated in several ways by catalytic or thermal converter to oxidize hydrocarbons and recuperative heat exchange. Typically, 170 psig to 200 psig steam is generated to improve overall economics of preferred embodiments of the invention.  
         [0105]    A portion of the solution of formaldehyde in methanol is diverted from pump  104  into reactive distillation column  150  through conduit  145 . In reactive distillation column  150  simultaneous chemical reaction and multicomponent distillation are carried out coextensively in the same high efficiency, continuous separation apparatus. Optionally, a stream containing methanol from storage vessel  146  may by fed into the reactive distillation column  150 . Charge pump  148  transfers methanol in substantially liquid form into the reactive distillation column  150  through conduits  147  and  149 .  
         [0106]    Solid acidic catalyst is present in the reactive distillation column  150  to allow solutions containing water, methanol, formaldehyde, methylal and one or more other polyoxymethylene dimethyl ethers to be brought into solid-liquid contact counter-currently with the catalyst to form products including methylal and higher molecular weight polyoxymethylene dimethyl ethers. More volatile reaction products are taken overhead from the high efficiency separation apparatus, whereas water and less volatile reaction products are carried down the high efficiency separation apparatus.  
         [0107]    The overhead vapor stream from reactive distillation column  150  is transferred through conduit  152  into condenser  154 . A suitable portion of condensate from condenser  154  is refluxed into reactive distillation column  150  through conduit  156 . A product stream containing methylal is transferred through conduit  158  to product storage (not shown). Conduit  159  supplies pump  160  with liquid containing higher molecular weight polyoxymethylene dimethyl ethers from the bottom of column  150 . A suitable portion of liquid from the bottom of column  150  is transferred, by means of pump  160 , through conduits  162  and  163  into reboiler  164  which is in flow communication with the bottom of the column by means of conduit  166 . A product stream containing higher molecular weight polyoxymethylene dimethyl ethers is transferred through conduit  168  to product storage (not shown). Preferably, an anion exchange resin is disposed within a section of the distillation column below the stages of contact with the acidic catalyst to form an essentially acid-free mixture.  
         [0108]    An aqueous side stream containing low levels of unreacted formaldehyde and/or methanol is discharged from column  150  through conduit  172 .  
         [0109]    In view of the features and advantages of the continuous catalytic processes for direct condensation of formaldehyde and dimethyl ether to form a mixture containing one or more polyoxymethylene dimethyl ethers in accordance with this invention, as compared to the known methanol condensation systems previously used, the following examples are given.  
       EXAMPLES 1 to 3  
       [0110]    In Examples 1, 2 and 3 a crystalline borosilicate catalyst exhibiting the MFI crystal structure was used to convert a predominately dimethyl ether feedstream and a liquid feedstream of aqueous formaldehyde in methanol. Effluent of the condensation reactor comprised water, methanol, formaldehyde, dimethyl ether, methylal and higher polyoxymethylene dimethyl ethers having a structure represented by the type formula  
         CH 3 O(CH 2 O) n CH 3    
         [0111]    in which formula n is a number from 1 to about 7.  
         [0112]    Crystalline borosilicate molecular sieve in the form of an extrudate ({fraction (1/16)} inch) was calcined overnight at 500° C. The calcined extrudate was crushed and sieved to 18-40 mesh. A tubular quartz reactor was charged with 3.27 grams (5 cc) of the sieved particles. The tubular quartz reactor (approx. 10 mm inside diameter) was equipped with a quartz thermowell terminating at about the midpoint of the catalyst bed.  
         [0113]    A liquid feed solution was prepared in a pressurized 50 mL autoclave using 11.13 grams of paraformaldehyde (95%), 15.94 grams of methanol, and 1.80 grams of water. Contents of the autoclave were stirred and heated to temperatures of 130° to 140° C. for 1 hour, and then cooled. The resulting solution was fed by a syringe pump into a preheat zone above the catalyst bed. Using mass flow controllers, a gas feed mixture of dimethyl ether and nitrogen was also fed to the top of the reactor.  
         [0114]    Liquid products from the reactor were collected in a cool (0° C.) 25 mL flask for subsequent weighing and GC analysis. Gases exiting the collection flask were analyzed by on-line GC using both TCD and FID detectors. Samples of liquid products were collected during sampling intervals of 2 hours over an approximately 16 hour period of operation. Gas analyses were obtained by GC during each sampling interval.  
         [0115]    Two samples were collected while temperature of the catalyst bed was controlled to three progressively higher temperatures. Each sample was about 7 grams. Operating conditions and results are summarized in Tables I, II and III.  
         [0116]    Net conversion of the methoxy moiety (Net MeO, percent) is an indication of the conversion of groups regardless of origin i,e., both methanol (MeOH)which has one MeOper mole and dimethyl ether (DME) which has two MeO per mol. Net MeO may be expressed as follows:  
         Net                 MeO     =     100   ×           (     MeOH   +     2      DME       )     feed     -       (     MeOH   +     2      DME       )     effluent           (     MeOH   +     2      DME       )     feed                               
 
       EXAMPLE 4  
       [0117]    In this example an acidic catalyst was used to convert a liquid feedstream of formaldehyde in methanol under conditions which allowed gas-liquid contacting of the solid catalyst (trickle bed operation). Effluent of the condensation comprised water, methanol, formaldehyde, dimethyl ether, methylal and higher polyoxymethylene dimethyl ethers.  
         [0118]    The acidic catalyst was a proton exchanged sulfonic acid based ion exchange resin. This polymeric material is a Bronstead (protic) acid. A tubular quartz reactor was charged with 5 cc of acidic catalyst particles. The tubular quartz reactor (approx. 10 mm inside diameter) was equipped with a quartz thermowell terminating at about the midpoint of the catalyst bed.  
         [0119]    A liquid feed solution was prepared in a pressurized 50 mL autoclave using 7.42 grams of paraformaldehyde (95%) and 15.93 grams of methanol. Contents of the autoclave were stirred and heated to temperatures of 130° to 140° C. for 1 hour, and then cooled. The resulting solution was fed by a syringe pump into a preheat zone above the catalyst bed. Using mass flow controllers, a gas feed mixture of dimethyl ether and nitrogen was also fed to the top of the reactor.  
         [0120]    Liquid products from the reactor were collected in a cool (0° C.) 25 mL flask for subsequent weighing and GC analysis. Gases exiting the collection flask were analyzed by on-line GC using both TCD and FID detectors. Operating conditions and results are summarized in Table IV.  
       EXAMPLE 5  
       [0121]    In this example an acidic catalyst was used to convert a mixture of formaldehyde in methanol under conditions which allowed liquid contacting of the solid catalyst. A liquid feed solution was prepared in a pressurized 50 mL autoclave using 7.4 grams of paraformaldehyde (95%) and 15.9 grams of methanol. Contents of the autoclave were stirred and heated to temperatures of 130° to 140° C. for 1 hour, and then cooled. The autoclave was opened and charged with  1 . 0  gram of catalyst. Contents of the autoclave were heated to reaction temperature for 2 to 3 hours with stirring. After cooling to ambient temperature and settling, the supernatant liquid was sampled for GC analysis and formaldehyde titration analysis. Results are summarized in Table V.  
       EXAMPLE 6  
       [0122]    Products of several condensation runs were composited, and the composite vacuum filtered through a medium glass frit. A 90 gram aliquot of filtrate was shaken with 20 grams of basic ion-exchange resin beads (DOWEX 66) which were then allowed to settle for one hour. The resulting supernatant liquid was then gravity filtered through a medium paper filter. A suitable amount (54 grams) of molecular sieve type 3A, which had been activated by calcination at about 538° C., was mixed into the filtrate, and the mixture allowed to stand overnight at ambient temperatures. Liquid was separated from the sieve by vacuum filtration through a medium glass flit. A 45.97 gram aliquot of this acid-free, dry filtrate was charged to a small distillation apparatus consisting of a 100 mL 3-neck flask, a fractionating column and condenser. The charge was distilled into eight overhead fractions which were collected at temperature cuts according to the following schedule.  
         [0123]    Schedule of Overhead and Bottom Temperatures  
                                                     Fraction   Temperatures, ° C.                Number   Overhead   Bottom               1   42 to 46    70 to 94       2   47 to 76     95 to 109       3   77 to 94    110 to 118       4   95 to 100   119 to 127       5   101 to 107    128 to 136       6   108 to 112    137 to 146       7   113 to 123    147 to 162       8   124 to 150    163 to 174                  
 
         [0124]    White solids (possibly paraformaldehyde) were observed in the column and condenser during cuts  2  through  4 , but not thereafter. Composition of the distilled fraction and bottoms are given in Table VI.  
       EXAMPLE 7  
       [0125]    In this Example a silver catalyst in the form of needles was used at several elevated temperatures to provide a source of formaldehyde by oxidative dehydrogenation of dimethyl ether, steam and methanol. A tubular quartz reactor was charged with 3.83 grams (1 cc) of the silver needles. The tubular quartz reactor (approx. 10 mm inside diameter) was equipped with a quartz thermowell terminating at about the midpoint of the catalyst bed. Quartz wool was placed above the catalyst zone to assist in vaporizing liquid feed. The liquid feed solution containing 18.6 percent methanol and 81.4 percent by weight water was fed by a syringe pump into the preheat zone above the catalyst bed. Using mass flow controllers, a gaseous feedstream of 59.93 percent by volume dimethyl ether, 31.59 volume percent nitrogen and  8 . 48  percent by volume dioxygen was also fed to the top of the reactor. Samples were collected while temperature of the catalyst bed was controlled to temperatures in a range from about 400° to about 650° C. Operating conditions and results are summarized in Table VII.  
       EXAMPLE 8  
       [0126]    In this Example a silver catalyst in the form of needles was used at several elevated temperatures to provide a source of formaldehyde by nonoxidative dehydrogenation of dimethyl ether and steam. The liquid feed of water was fed by a syringe pump into the preheat zone above the catalyst bed. Using mass flow controllers, a gaseous feedstream of 89.1 percent by volume dimethyl ether and 10.9 volume percent nitrogen was also fed to the top of the reactor. Samples were collected while temperature of the catalyst bed was controlled to temperatures in a range from about 400° to about 650° C. Operating conditions and results are summarized in Table VIII.  
         [0127]    For the purposes of the present invention, “predominantly” is defined as more than about fifty percent. “Substantially” is defined as occurring with sufficient frequency or being present in such proportions as to measurably affect macroscopic properties of an associated compound or system. Where the frequency or proportion for such impact is not clear, substantially is to be regarded as about twenty per cent or more. The term “essentially” is defined as absolutely except that small variations which have no more than a negligible effect on macroscopic qualities and final outcome are permitted, typically up to about one percent.  
                                                                         TABLE I                       Conversion of Feedstreams at about 100° C. Using a Crystalline       Borosilicate Catalyst Exhibiting the MFI Crystal Structure                                    Temperature, ° C.   100   101           Run Time, min   95   155           Gas Feed, mol percent           Nitrogen   32.925   32.925           DME   67.075   67.075           Liquid Feed, weight percent           Methanol   55.20   55.20           Formaldehyde   38.55   38.55           Water   6.25   6.25           Feed Rates           Gas scc/min   34.1   34.1           Liquid mL/min   0.00756   0.00756           Conversions, mole percent           Methanol   67.15   66.96           DME   4.36   2.71           Net MeO   28.20   27.10           Formaldehyde   78.84   78.84           Selectivities, percent                Gases   CO   0   0               CO 2     0   0           Liquids   Methylal   80.548   78.269               HPE   0.750   0.751                DME/MeOH   5.38   5.44           Carbon Balance   92.57   93.39                                  
 
         [0128]    Table II  
                                                                         TABLE II                       Conversion of Feedstreams at about 130° C. Using a Crystalline       Borosilicate Catalyst Exhibiting the MFI Crystal Structure                                    Temperature, ° C.   132   131           Run Time, min   245   305           Gas Feed, mol percent           Nitrogen   32.925   32.925           DME   67.075   67.075           Liquid Feed, weight percent           Methanol   55.20   55.20           Formaldehyde   38.55   38.55           Water   6.25   6.25           Feed Rates           Gas scc/min   34.1   34.1           Liquid mL/min   0.00756   0.00756           Conversions, mole percent           Methanol   53.59   53.68           DME   5.12   4.75           Net MeO   23.52   23.33           Formaldehyde   86.71   86.71           Selectivities, percent                Gases   CO   0   0               CO 2     0.095   0.086           Liquids   Methylal   64.480   64.699               HPE   0.323   0.326                DME/MeOH   3.48   3.50           Carbon Balance   91.63   91.53                                  
 
         [0129]    [0129]                                                                         TABLE III                       Conversion of Feedstreams at about 160° C. Using a Crystalline       Borosilicate Catalyst Exhibiting the MFI Crystal Structure                                    Temperature, ° C.   164   160           Run Time, min   345   400           Gas Feed, mol percent           Nitrogen   32.925   32.925           DME   67.075   67.075           Liquid Feed, weight percent           Methanol   55.20   55.20           Formaldehyde   38.55   38.55           Water   6.25   6.25           Feed Rates           Gas scc/min   34.1   34.1           Liquid mL/min   0.00756   0.00756           Conversions, mole percent           Methanol   34.82   35.19           DME   7.45   1.12           Net MeO   17.84   14.05           Formaldehyde   90.59   90.59           Selectivities, percent                Gases   CO   0   0               CO 2     0.370   0.317           Liquids   Methylal   42.970   43.410               HPE   0.094   0.096                DME/MeOH   2.37   2.54           Carbon Balance   92.40   94.76                                    
         [0130]    [0130]                             TABLE IV                       Trickle Bed Conversion of Feedstreams Using an Ion Exchange       Resin Based Catalyst Exhibiting Bronstead Acid Sites                                    Temperature, ° C.   71           Feed Rates           Gas scc/min   10           Liquid mL/min   0.0756           Conversions, mole percent           Methanol   87.04           Formaldehyde   92.27           Selectivities, percent           Methylal   97.78           HPE   1.77                                    
         [0131]    [0131]                             TABLE V                       Liquid Phase Conversion Using an Ion Exchange Resin       Based Catalyst Exhibiting Bronstead Acid Sites                                    Temperature, ° C.   67           Conversions, mole percent           Methanol   73.38           Formaldehyde   77.91           Selectivities, percent           Methylal   88.20           HPE   6.03                                    
         [0132]    [0132]                                                                                                                             TABLE VI                           COMPOSITION OF OVERHEAD FRACTIONS AND BOTTOMS                Compound       CH 3 O(CH 2 O) n CH 3  where the value of n is:            Fraction   Methylal   Methanol   Hemiacetals   Trioxane   2   3   4   5   6   7                    Starting   49.95   0.0   0.69   2.42   22.60   12.42   6.40   3.15   1.45   0.61       1   97.21   0.95   0.05   0.0   0.46   0   0   0   0   0       2   93.83   2.52   0.38   0.0   2.84   0   0   0   0   0       3   20.81   12.92   8.85   2.39   54.80   0.17   0   0   0   0       4   3.24   11.12   6.40   4.49   74.19   0.57   0   0   0   0       5   0.56   8.47   2.29   5.83   82.07   0.78   0   0   0   0       6   0.40   3.10   0.16   7.21   88.05   1.08   0   0   0   0       7   0.43   0.99   0.0   9.38   86.60   2.55   0.05   0   0   0       8   0.32   0.47   0.0   11.77   82.98   4.37   0.08   0   0   0       Bottoms   0.29   0.02   0.0   0.54   1.10   49.49   26.19   13.05   6.34   2.96                    
         [0133]    [0133]                                                                                                                                                                                 TABLE VII                       Oxidative Dehydrogenation of Dimethyl Ether       Using a Catalyst of Silver Needles                                Temperature, ° C.   397   508   614   660       Run Time, min   60   100   150   195            Gas Feed, mol percent            Nitrogen   31.59   31.69   31.59   31.69       DME   59.93   59.93   59.93   59.93       Dioxygen   8.48   8.48   8.48   8.48            Liquid Feed, weight percent            Methanol   18.6   18.6   18.6   18.6       Water   81.4   81.4   81.4   81.4            Feed Rates            Gas scc/min   146   146   146   146       Liquid mL/min   0.4125   0.4125   0.4125   0.4125            Conversions, mole percent            Methanol   9.21   10.55   13.53   15.30       DME   15.44   16.41   17.18   23.94       Net MeO   14.01   15.08   16.34   21.96       Dioxygen   94.40   93.85   94.28   94.53            Selectivities, percent            Light Alkanes   1.37   4.07   19.47   28.59       CO   18.19   20.15   25.41   20.15       CO 2     9.06   7.92   5.29   3.39       Formaldehyde   56.57   59.16   45.79   37.70       Methylal   0   0   0   0       HPE   0   0   0   0       DME/Methanol   3.23   3.20   3.30   3.10                            
         [0134]    Table VIII  
                                                                                                                                                                                 TABLE VIII                       Nonoxidative Dehydrogenation of Dimethyl       Ether Using a Catalyst of Silver Needles                                Temperature, ° C.   408   511   612   650       Run Time, min   115   175   290   465            Gas Feed, mol percent            Nitrogen   10.9   10.9   10.9   10.9       DME   89.1   89.1   89.1   89.1            Liquid Feed, weight percent            Water   100   100   100   100            Feed Rates            Gas scc/min   102   102   102   102       Liquid mL/min   0.07563   0.07563   0.07563   0.07563            Conversions, mole percent            DME   3.63   2.56   9.38   23.39       Net MeO   3.63   2.56   9.36   23.28            Selectivities, percent            Light Alkanes   46.21   46.48   53.06   52.51       CO   0   0   5.51   14.96       CO 2     0   0   0.34   0.25       Formaldehyde   53.79   53.52   41.10   32.29       Methylal   0   0   0   0       HPE   0   0   0   0