Natural gas is found in many locations around the world. However in many locations transportation by conventional pipeline to markets is possible. The natural gas must be converted to a form that can be transported. Typical conversion processes include liquefaction to make LNG, synthesis gas generation followed by a synthesis gas conversion process and combinations. The liquefaction of natural gas requires significant energy to compress the gas during the liquefaction process. Likewise in synthesis gas production, the synthesis gas is made by partial oxidation of the natural gas with oxygen. The preparation of the oxygen from air takes significant amounts of energy. Typically the energy for these processes is provided from the natural gas itself, but this reduces the amount of natural gas that can be transported to markets.
Natural gas also is frequently contaminated, usually with sulfur containing compounds such as hydrogen sulfide (H2S). Prior to conversion, the natural gas must be purified and this process yields a H2S-rich gas by-product stream. Hydrogen sulfide is a highly toxic gas and it cannot be disposed of as such. The H2S-rich gas stream is typically converted to sulfur by a H2S conversion process.
An excellent reference to the purification of natural gas and conversion of H2S into sulfur is found in Kirk Othmer.
H2S conversion processes, such as the Claus process, a portion (approximately one-third) of the H2S is oxidized in an exothermic reaction to SO2; with energy as a by-product. The energy is typically in the form of steam.
2H2S+3O2→2SO2+2H2O
The SO2 and the unreacted H2S are reacted in a series of reactors to form elemental sulfur which is condensed and converted to a solid form for disposal.
2H2S+SO2→3S+2H2O
The Claus process by itself is not 100% effective in converting all H2S into elemental sulfur. Typical recoveries up to about 97% can be achieved. The remainder of the H2S and SO2 are present in the Claus plant tail-gas. Often the concentrations of these species in the tail-gas are too high for direct disposal or by disposal in a flare. Rather additional processing steps must be used.
Typical improvements to the Claus process include the following tail-gas processing processes:
Alternatively, the H2S in the second H2S-rich gas stream can be processed in a Stretford where it is adsorbed into an aqueous solution of sodium carbonate, sodium vanadate, and an oxidation catalyst. The H2S reacts to form sulfur, which is recovered, and a solution of a reduced vanadium species. The reduced vanadium is oxidized back to sodium vanadate. In U.S. Filter Company's Lo-Cat process the vanadium used in the Stretford process is replace with an aqueous iron compound.
In each of these H2S conversion and tail gas cleanup processes oxygen is needed for oxidation of H2S or to regenerate catalysts. Likewise a reducing agent is needed in the SCOT and Beavon processes to convert SO2 back to H2S. Likewise in the Superclaus and Hi-Activity processes, reduction of SO2 back to H2S will assist in sulfur conversion. While the oxygen used in their Claus, Superclaus, Hi-Activity, Stretford and Lo-Cat processes can be supplied by air, enriched air or essentially pure oxygen itself have been claimed to benefit the operations. A source of the oxygen (at a concentration greater than air) and the reducing reagent are desired.
Synthesis gas is a mixture comprising hydrogen and carbon monoxide and optionally other gases such as water and carbon dioxide.
Fischer-Tropsch include both High Temperature: (HTFT) and Low Temperature Fischer-Tropsch (LTFT) processes, but the preferred Fischer-Tropsch process is a flow Temperature Fischer-Tropsch process, most preferably operated in a slurry bed. The HTFT processes operate at temperatures of 250° C. and above, while the LTFT process operates at below 250° C.
Waxy as in Waxy Fischer-Tropsch product means containing greater than 20% normal hydrocarbonaceous compounds (paraffins, olefins alcohols) of carbon number equal to or greater than 5, preferably greater than 50%, most preferably greater than 75%.
LNG (natural gas liquefaction) and Air Separation are described in Kirk Othmer, Vol. 8, pages 40-65 entitled Cryogenic Technology, incorporated herein by reference. More specifically, these processes are described in Kirk Othmer reference sections discussing LNG is on page 49, section 3.3. Air separation starts on page 43, section 3.1. the preferred air separation process is the “pumped LOX” process which supplies oxygen at the pressure needed for use in the synthesis gas production process.
Hydrogen Production and H2S Recovery are described in Kirk Othmer, Vol. 13, pages 759-808, entitled Hydrogen, incorporated herein by reference. More specifically, these processes are described in Kirk Othmer reference sections discussing hydrogen production is preferably obtained by a Steam Methane Reforming (SMR) process as defined on pages 775-780. The hydrogen recovery process can be done by either a Pressure Swing Adsorption (PSA) or membrane separation processes as defined on pages 794-796.
The invention comprises integrating processes for H2S conversion and natural gas conversion processes such as Fischer-Tropsch, LNG, and the like to achieve overall integration process improvements.
The purified natural gas is then processed in either or both of the following natural gas conversion processes: liquefaction (45) and/or synthesis gas production (65). The product from the liquefaction process is liquefied natural gas (200) also known as LNG. Oxygen (50) needed for the synthesis gas production is prepared in an air separation process (55).
Energy is needed for the liquefaction and air separations processes. At least a portion of the energy needed for these processes is provided by the energy recovered in the H2S conversion process. Energy for the liquefaction and air separations processes and not provided by the H2S conversion process is provided from the purified natural gas. The proportion of energy provided from the H2S conversion process is between 0.1 and 50%, preferably between 1 and 25%, and most preferably between 2 and 10%.
The product from the synthesis gas production is synthesis gas (90) which is processed in either or both of a Fischer-Tropsch process (75) or a methanol synthesis process (1105). The product from the Fischer-Tropsch process is a waxy product (110) which is upgraded in an upgrader (85) to produce upgraded products (300) which can consist of fuels (jet, diesel, kerosene), solvents, chemicals, lubricant base oils, waxes and combinations. The upgrading process consumes hydrogen (120) which is produced in a hydrogen production process (95) using purified natural gas (30) supplied by a line not shown. The hydrogen supplied to the upgrader is not completely consumed, and excess hydrogen (220) is produced in the upgrading reactor.
The product from the methanol synthesis process is methanol (400). The methanol can be further reacted in a methanol to gasoline process (115) to make aromatics (500) consisting of benzene, toluene, zylenes, C9 aromatics and C10 aromatics and combinations. These aromatics can be used as aromatic chemicals or in gasoline. Alternatively the methanol can be reacted in a methanol to olefins process (125) to yield an olefinic product (600) consisting of ethylene, propylene, butanes and combinations. Ethylene is the preferred product. Optionally the olefins can be reacted in polymerization processes (135) to yield polymers (700) consisting of polyethylene and polypropylene.
In this embodiment illustrated in
In this embodiment illustrated in
The synthesis gas used in the H2 recovery process can be obtained from either of two locations or both: directly from the synthesis gas process (65) and recovered from the effluent from the Fischer-Tropsch process (75). The Fischer-Tropsch process does not convert all of the synthesis gas fed to the unit. The remaining unconverted-synthesis gas is referred to as a Fischer-Tropsch tail gas. This material it typically used as fuel. If hydrogen is supplied to the sulfur plant tail gas process by the H2 recovery process using synthesis gas, the preferred source of the synthesis gas is the tail gas from the Fischer-Tropsch process.
The invention is claimed hereinafter. Modifications obvious to the ordinary skilled artisan are intended to be within the scope and interpretation of the claims. For example sulfurous biomass can be a source to make synthesis gas.
Number | Date | Country | |
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60871491 | Dec 2006 | US |