Abstract:
A method for preparing oxygenated hydrocarbons includes steps of: reacting a first heated hydrocarbon-containing gas stream with an oxygen-containing gas stream in a reactor for form a first product blend, recovering the energy generated in the reactor in order to preheat incoming hydrocarbon feed to the reactor and/or to drive endothermic reactions that generate synthesis gas, separating and condensing one or more liquid oxygenated hydrocarbons from the product stream, separating a reject stream from a recycle stream, mixing remaining gaseous hydrocarbon product from the recycle stream with the first hydrocarbon-containing gas stream after one reaction cycle, converting the first reject stream to a synthesis gas mixture, and converting the synthesis gas mixture to light alkanes to be blended with one or with oxygenates in an output stream to optionally form higher molecular weight oxygenates.

Description:
TECHNICAL FIELD 
     In at least one aspect, the present invention relates to methods and equipment for partially oxidizing a hydrocarbon feed gas. 
     BACKGROUND 
     Steam reforming of natural gas is currently the most cost effective method of producing hydrogen and carbon oxides. The gaseous mixture of hydrogen and carbon oxides (carbon monoxide and/or carbon dioxide) is hereinafter referred to as “synthetic gas” or “syngas”. Syngas is useful as an intermediate for the manufacture of products such as ammonia, methanol or synthetic petroleum. Currently, commercial methanol production is almost entirely based on reforming light hydrocarbons, especially methane, first to syngas, followed by syngas clean up, methanol synthesis, and methanol separation. This process has been the dominant route of methanol production since the 1920&#39;s. The entire process, however, is cumbersome with a high degree of complexity and associated costs. Therefore, a direct method has been developed using direct homogenous partial oxidation of methane to methanol (the “DHPO” method). 
     The DHPO method is however generally limited by the need to choose between high conversions and high selectivity to obtain economic yields of methanol. In both catalytic and non-catalytic DHPO methods, higher conversions tend to create the co-products hydrogen, carbon oxides, and water, whereas higher selectivity leads to lower conversion rates which has traditionally made the process uneconomic. 
     U.S. Pat. Nos. 8,293,186; 8,202,916; 8,193,254; 7,910,787; 7,687,669; 7,642,293; 7,879,296; 7,456,327; and 7,578,981 overcome some of the known DHPO system limitations by using a reactor quench step and a high volume recycle system with integrated separations and low pressure drop. These patents describe these DHPO system improvements in detail and are incorporated herein by reference. However, despite the improved efficiency of our DHPO process, relative and comparable to that of the syngas-based methanol synthesis, carbon oxides and hydrogen are produced in our DHPO system process. This can limit the overall carbon efficiency to less than 100%. Furthermore, to limit the buildup of such gases and nitrogen, the process requires a reject gas stream such as a purge. Said reject gas often contains some alkane content, lowering carbon efficiency. 
     Furthermore, the DHPO process reactor as described in our aforementioned patents and patent applications is unable to process synthesis gas. Because of this, the process excludes a wide range of carbonaceous materials from being advantageously utilized. 
     Accordingly, there is a need for methods and apparatuses that can produce synthesis gas from the reject gasses from the recycle loop as well as utilize synthesis gas produced gas from a variety of carbonaceous materials for enhanced carbon efficiency and process yields, as well as that can utilize the waste heat generated by the exothermic DHPO reaction 
     SUMMARY 
     The present invention solves one or more problems of the prior art by providing in at least one aspect a method and apparatus for more efficiently synthesizing oxygenated hydrocarbons, e.g. methanol, ethanol, formaldehyde, acetaldehyde, etc. The present embodiment combines the benefits of a direct homogeneous partial oxidation (DHPO) system and the flexibility of synthesis gas such as to increase the molecular carbon content of the DHPO product and utilize the waste heat of the DHPO process. In our improved DHPO system, synthesis gas already created by over-oxidation inherent with the DHPO reactor, the alkane content of the reject gasses, and heat surplus recoverable from direct partial oxidation, are utilized to enhance the carbon efficiency of the overall process as well as create chemicals of an increased molecular carbon content which traditionally have higher value than the components of the original oxygenated products. Furthermore, the reject streams which are used to create synthesis gas cannot be accepted as feed by the DHPO reactor yet are more fully scrubbed of impurities such as sulfur that are known to poison catalysts which transform synthesis gas into oxygenates or feed alkanes. 
     In the present invention, these materials and energy streams of the former DHPO systems which formerly been wasted in the prior art are used herein to provide a DHPO system which more efficiently and cost-effectively produces oxygenated hydrocarbons. This novel ability to more fully utilize the carbon content of the gasses rejected by the recycle loop eliminates the traditionally mutually exclusive choice between reactor conversion and selectivity in regards to overall process yield. In addition, the use of higher nitrogen content in feed oxygen traditionally necessitated higher purge rates and lower carbon efficiency. The present invention allows for the usage of higher nitrogen content in the feed oxygen for lowered capital costs again without the sacrifice of overall process yield. 
     In an embodiment, a method for preparing oxygenated hydrocarbons includes steps of: 
     a) reacting a first heated hydrocarbon-containing gas stream with an oxygen-containing gas stream in a reactor forming a first product blend, 
     b) recovering the energy generated in the reactor in order to preheat incoming hydrocarbon feed to the reactor and/or to drive endothermic reactions that generate synthesis gas, 
     c) separating and condensing one or more liquid oxygenated hydrocarbons from the product stream; 
     d) separating a reject stream from a recycle stream, 
     e) mixing remaining gaseous hydrocarbon products from the recycle stream with the first hydrocarbon-containing gas stream after one reaction cycle; 
     f) converting the first reject stream to a synthesis gas mixture; and 
     g) converting the synthesis gas mixture to light alkanes to be blended with the DHPO feed gas or with oxygenates in an output stream to optionally form higher molecular weight oxygenates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a system for removing formaldehyde from a partially oxidized hydrocarbon. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
     It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way. 
     It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components. 
     Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains. 
     The term “synthetic gas” or “syngas” as used herein refers to gaseous mixture of hydrogen and/or carbon oxides (carbon monoxide and/or carbon dioxide). 
     With reference to  FIG. 1 , a schematic illustration of an apparatus a process for converting a carbonaceous gas to oxidized products in a gas-to-chemicals (GTL) process is provided. In a refinement, the apparatus functions in a continuous manner when in operation. Homogeneous direct partial oxidation is performed in a reactor  101  which is supplied with a hydrocarbon-containing gas  10  and an oxygen-containing gas  11 . In a refinement, the reaction is operated at pressures from about 450 to 1250 psia and temperatures from about 350 to 450° C. In particular, hydrocarbon-containing gas  10  and an oxygen-containing gas  11  react in a vessel to form a first product blend which is a blend (i.e., a mixture) of partially oxygenated compounds that include formaldehyde. In a refinement, the first product blend and/or output streams  31 ,  32  include C 1-10  alcohols and/or C 1-5  aldehydes. In another refinement, the first product blend and/or output streams  31 ,  32  include an alcohol selected from the group consisting of methanol, ethanol, propanols, butanols, pentanols and combinations thereof, and/or aldehyde selected from the group consisting formaldehyde, acetaldehyde, propionaldehyde and combinations thereof. In another refinement, the first product blend and/or output streams  31 ,  32  include an alcohol selected from the group consisting of methanol, ethanol, and combinations thereof, and aldehyde selected from the group consisting formaldehyde, acetaldehyde, and combinations thereof. Examples of systems and methods of performing the partial oxidation as set forth in U.S. Pat. Nos. 8,293,186; 8,202,916; 8,193,254; 7,910,787; 7,687,669; 7,642,293; 7,879,296; 7,456,327; and 7,578,981; the entire disclosures of which are hereby incorporated by reference. In a refinement, the hydrocarbon-containing gas includes C 1-10  alkanes. In another refinement, the hydrocarbon-containing gas includes an alkane selected from the group consisting of methane, ethane, propanes, butanes, pentanes and combinations thereof. In another refinement, the hydrocarbon-containing gas includes an alkane selected from the group consisting of methane, ethane, and combinations thereof. Examples of oxygen containing gas include molecular oxygen which may be in the form of concentrated oxygen or air. In a refinement, the oxygen-containing gas stream is made oxygen rich (e.g., by passing air through a membrane to increase oxygen content). The low conversion and selectivity of homogeneous direct partial oxidation requires that a recycle loop is utilized to increase the overall carbon efficiency. 
     Following partial oxidation reaction the reactant stream is rapidly cooled in a series of heat exchangers  103  and  104  to prevent decomposition of the produced oxygenates. The heat energy transferred by exchanger  104  might optionally be used to provide energy which may be used in the creation of synthesis gas. After cooling the liquids are separated from the gas stream as station  102 . The gas stream is then submitted to a separation process for removal of non-hydrocarbon fractions a station  105  which may be performed via scrubbing, membrane separation, adsorption processes, cryogenic separations, or by purging a small gas fraction. If station  105  is a liquid scrubbing system, liquid products are sent to a flash drum  107  where dissolved gases are removed. Non-hydrocarbon gases  14  are removed from the recycle loop, and the hydrocarbon gases  2  are then recycled to combine with fresh methane gas  1  which has been pressurized to the pressure of the loop by compressor  200 . The stream composed of recycled hydrocarbons plus fresh methane gas is pressurized to make up for pressure losses in the recycle loop, preheated via the cross exchanger  103  and further by the preheater  108 , when necessary, to meet the desired reaction conditions. 
     Liquids generated by the gas-to-chemicals process are composed predominantly of alcohols and aldehydes (e.g., methanol, ethanol and formaldehyde) as set forth above. The raw liquid stream  22  generated by the GTL process is generally composed of 50-70% alcohols and 5-20% aldehydes 15-30% water. Downstream processing of these liquids may include a number of different synthesis routes to higher-value chemicals and fuels, but simple distillation of alcohols from aldehydes is performed in a simple fractional distillation column  106  in which alcohols are recovered in the distillate  31  and the aqueous aldehyde solution from the column bottoms  32 . 
     The compositions of the streams  14  obtained from separation of non-hydrocarbon gases from the recycle loop and from degassing the liquid mixture  15  may vary significantly depending on the separation methods employed in station  105 . Stream  15  would be typically be needed to regenerate a scrubbing fluid by liberating dissolved gasses such as carbon dioxide or carbon monoxide, which would be enriched in this stream. Stream  15  is composed predominantly of lighter hydrocarbons and carbon oxides (e.g., CO 2  and/or) which are soluble in the liquid solution, but are vaporized when decreasing the pressure. 
     Stream  15  may or may not be blended with stream  14  depending on the needs of the synthesis gas reactor  108 . Stream  14  is a separated gas stream form station  105  such might be separated from a purge stream, membrane, cryogenic, or adsorption process. Although stream  14  would be enriched in non-hydrocarbon gasses, there would be some light alkanes present as well. A simple purge method in station  105  results in hydrocarbon fractions that may reach up to 70%, while selective removal techniques tend to preserve hydrocarbons in the recycle loop  2 . Stream  14  and  15  are blended to form stream  16 , which is rich in synthesis gas. 
     Stream  16  goes through a reactor  108 , which converts the hydrocarbon portion to synthesis gas in stream  17 . Stream  17  then goes on to react with liquid streams in reactor  109  (for example output streams  31  or  32 ). Stream  32  is the bottoms product of distillation column  106  and would contain low volatility, high boiling components such as formalin, heavy alcohols, and some acids. Stream  31  is the overhead from distillation column  106  and would be rich in the higher volatility low boiling components such as light alcohols. Streams  17  and said liquid product streams would then react to form oxygenates of a carbon number greater than that in the liquid reactant stream. Such oxygenates produced by reactor  109  might include esters such as formates and DMC, or carboxylic acids from a CO rich synthesis gas in stream  17 . Higher alcohols and aldehydes from mixed alcohol synthesis, alcohol homologation, and aldehyde synthesis can form from a relatively hydrogen rich synthesis gas in stream  17 . As mentioned, stream  32  contains aqueous formaldehyde, which is known to react with synthesis gas to form glycolic acid and glycol aldehyde. In another refinement, the synthetic gas is generated by a pyrolysis reaction or generated externally and blended with stream  17 . In a further refinement, the pyrolysis reaction generates light alkanes in addition to synthetic gas. 
     Alternatively, stream  17  may react with itself in reactor  109  and form light alkanes (e.g., C 1-4  alkanes) for use as a feed gas to be blended with stream  1 . The light alkane product of this reaction would typically be rich in C 2 + hydrocarbons, which are known to produce a distribution of alcohols with a higher molecular weight when compared to methane under homogenous partial oxidation conditions. Certain catalysts are also known to produce both alcohols and light alkanes. In addition, stream  17  may be blended with externally produced synthesis gas to produce a gas mixture in reactor  109  which can be utilized by reactor  101 . This feature allows for feedstock flexibility in the direct homogenous partial oxidation process. In another variation, the synthesis gas is generated in a reactor  108  by implementing a steam, dry, or tri-reforming reaction. In a refinement, the tri-reforming reaction is assisted by energy (e.g., it uses the heat) recovered from a heat exchanger  104   
     In one embodiment, DHPO gas rejected by a DHPO recycle loop is used to produce syngas in reactor  108 . The syngas further reacts to produce both oxygenates and light alkanes in reactor  109 . The conversion may be effected using a suitable catalyst, for example, an actinide/lanthanide modified catalyst as described in U.S. Pat. No. 4,762,588. DHPO Oxygenate products may be separated from light alkanes using any simple liquid separation system well-known in the art. The separated alkanes may then be blended with the feed gas in stream  1  following nitrogen removal, if necessary. 
     In another embodiment, in a DHPO system comprising a synthesis gas, the gas may be separated in the recycle system using one or more membranes alkanes such as might be found with station  105 . Many membrane materials lack sufficient selectivity to completely separate non-hydrocarbon such as nitrogen and carbon dioxide from hydrocarbon streams. In this configuration, the light alkanes can be present in the permeate or retentate streams of the membrane. Using well known techniques, this stream would be converted into synthesis gas. Hydrogen and carbon dioxide may optionally be separated from this synthesis gas in stream  17  by a membrane or scrubbing system prior to reactor  109  to make a stream rich in CO which could then be used in carbonylation and carbon insertion reactions in reactor  109 . The hydrogen may optionally be used further reduce the carbonylated species. Alternatively, syngas is known to react directly with alcohols and form higher alcohols, esters, or aldehydes. 
     In another embodiment, some of the light alkanes present in stream  16  may be thermally decomposed to provide hydrogen and carbon black in reactor  108 . This thermal decomposition may be assisted by heat exchanger  104 . The carbon black could either be partially combusted in oxygen to yield pure carbon monoxide or reacted with the carbon dioxide to yield carbon monoxide. This pure carbon monoxide can then be used as a reactant in carbonylation or carbon insertion reactions in reactor  109 . The hydrogen may optionally be used further reduce the carbonylated species present in stream  18  after reactor  109 . 
     Further to the previous embodiment, an external carbon source may be utilized to react with carbon dioxide to yield carbon monoxide in either a catalytic or non-catalytic process assisted by heat recovered by heat exchanger  104 . The carbon monoxide may then be reacted with oxygenates in carbon insertion or carbonylation reactions in a manner consistent with the previous embodiment. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.