1.  Field of the Invention
This invention relates to chemical conversions that involve processes wherein solids and fluids are contacted to effect the desired conversion.  More particularly, this invention relates to chemical conversion of gaseous hydrocarbons over fixed beds of solids.  One particular embodiment of this invention relates to the synthesis of hydrocarbons from a methane source.  Another particular embodiment of this invention relates to dehydrogenation of hydrocarbons.  This invention also relates to a novel, multi-reactor, fixed bed, gas-solid contacting system. 
2.  Fixed Bed Reactor Systems
A wide variety of chemical conversions are known wherein fluid reactants are contacted with solids, the solids either functioning as catalysts to promote the conversion of reactants to desired products or functioning as a reactant (or both).  Typically, the solids in such processes, whether functioning as catalysts or as reactants, require periodic replenishment or regeneration. 
A preferred means for conducting many such industrial chemical processes involves the use fluidized beds of solids with circulation of solids between reaction and regeneration zones.  While ideally suited for many chemical conversions, circulating solids systems have limitations restricting their usefulness in particular applications.  For example, as the period between successive replenishment/regeneration and reaction steps decreases, increasingly larger amounts of solids must be moved within the system, vastly increasing the cost and complexity of the reaction system.  As a further example, chemical conversions requiring relatively severe operating conditions (especially those requiring high temperatures) may adversely effect the physical or other properties of the solid, which in turn adversely effect the solid's fluidization characteristics. 
Thus, despite the widespread use of fluidized bed or other moving bed systems in fluid-solid contacting, chemical conversion processes, fixed bed systems remain important to a substantial number of chemical processes.  See, for example, U.S. Pat.  No.  4,406,777 which describes a number of problems encountered with fixed bed reactor systems involving gradually deactivating solids and solutions proposed therefore. 
In addition, when used for exothermic chemical conversions, removal of heat from fixed beds of solids is difficult without causing prohibitive temperature gradients in the reactor beds.  Use of tubular reactors may overcome this difficutly, but improved systems are desirable. 
3.  Synthesis of Hydrocarbons from a Methane Source
A major source of methane is natural gas.  Other sources of methane have been considered for fuel supply, e.g., the methane present in coal deposits or formed during mining operations.  Relatively small amounts of methane are also produced in various petroleum processes. 
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane.  For example, the methane content of natural gas may vary within the range from about 40 to about 95 volume percent.  Other constituents of natural gas include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen. 
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it.  Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons carbons although some ethane may be included.  Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields.  Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane. 
Large scale use of natural gas often requires a sophisticated and extensive pipeline system.  Liquefaction has also been employed as a transportation means, but processes for liquefying, transporting, and revaporizing natural gas are complex, energy-intensive and require extensive safety precautions.  Transport of natural gas has been a continuing problem in the exploitation of natural gas resources.  It would be extremely valuable to be able to convert methane (e.g., natural gas) to more readily handleable or transportable products.  Moreover, direct conversion of olefins such as ethylene or propylene would be extremely valuable to the chemical industry. 
Recently, it has been discovered that methane may be converted to higher hydrocarbons (e.g., ethane, ethylene and higher homologs) by contacting methane with a reducible metal oxide as a selective oxygen source.  As the methane is converted to hydrocarbon products and coproduct water, the active oxygen of the metal oxide is depleted, resulting in a reduced metal oxide.  The reduced metal oxide is relatively inactive for the oxidative conversion of methane but active oxygen may be replaced by regenerating of a reducible metal oxide.  Such regeneration is accomplished by reoxidation of the reduced metal oxide. 
Reducible oxides of several metals have been identified which are capable of converting methane to higher hydrocarbons.  Oxides of manganese, tin, indium, germanium, lead, antimony and bismuth are particularly useful.  See commonly-assigned U.S. Pat.  Nos.  4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; 4,443,649; and 4,443,984, the entire contents of each being incorporated herein by reference. 
Commonly-assigned U.S. patent application Ser.  No.  522,935, filed Aug. 12, 1983, discloses and claims a process which comprises contacting methane with an oxidative synthesizing agent under elevated pressure (e.g., 2-100 atmospheres) to produce greater amounts of C.sub.3 +hydrocarbon products. 
Commonly-assigned U.S. patent application Ser.  No.  522,938, filed Aug. 12, 1983, discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with particles comprising an oxidative synthesizing agent which particles continuously recirculate between two physically separate zones--a methane contact zone and an oxygen contact zone. 
Commonly-assigned U.S. patent application Ser.  No.  522,937, filed Aug. 12, 1983, discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with an oxidative synthesizing agent containing a promoting amount of alkali metal and/or compounds thereof. 
Commonly-assigned U.S. Pat.  No.  4,495,374 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with an oxidative synthesizing agent containing a promoting amount of alkaline earth metal and/or compounds thereof. 
Commonly-assigned U.S. patent application Ser.  No.  600,665, filed Apr. 16, 1984 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of praseodymium and at least one member of the group consisting of alkali metals, alkaline earth metals, and compounds thereof. 
Commonly-assigned U.S. patent application Ser.  No.  600,918, filed Apr. 16, 1984, now abandoned discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of terbium and at least one member of the group consisting of alkali metals, alkaline earth metals, and compounds thereof. 
Commonly-assigned U.S. patent application Ser.  No.  600,917, filed Apr. 16, 1984 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of cerium and at least one member of the group consisting of alkali metals, alkaline earth metals, and compounds thereof. 
4.  Dehydrogenation of Hydrocarbons
Various processes for the dehydrogenation of hydrocarbons are known.  Such processes seek to produce olefins and/or dienes from alkanes or to produce dienes from olefins.  More conventional dehydrogenation processes include thermal or catalytic dehydrogenation.  More recently, oxidative dehydrogenation processes have been introduced.  See generally McKetta, J. J., et al.,  Encyclopedia of Chemical Processing and Design, Vol.  5, pages 127-139 (Marcel Dekker, N.Y., N.Y. 1977) (discusses various dehydrogenation processes in the context of butadiene manufacturing). 
In the oxidative dehydrogenation process, hydrogen is removed from a hydrocarbon by oxygen, forming water.  Oxydehydrogenation catalysts have been made from a variety from metal oxides and salts. 
5.  Objects of the Invention
It is an object of this invention to provide a fixed bed reactor system, especially one having enhanced capabilities for the removal and utilization of heat generated by exothermic chemical conversions. 
It is another object of this invention to provide an improved process for the chemical conversion of hydrocarbons. 
It is a further object of this invention to provide a fixed-bed process for the synthesis of hydrocarbons from a methane source. 
It is still a further object of this invention to provide a fixed-bed process for the dehydrogenation of hydrocarbons. 
Other aspects, objects and the several advantages of this invention will become apparent to those skilled in the art upon reading this disclosure and the appended claims.