MANUFACTURE OF ORGANIC LIQUIDS

Abstract: An improved process for the manufacture of organic liquids from gases containing hydrogen and carbon monoxide which avoids the build-up of nitrogen content through the use of a series of organic liquid reactors.

Description: This invention relates to the manufacture of organic liquids. In particular this invention relates to the manufacture of organic liquids selected from methanol, ethanol and higher alcohols, Fischer-Tropsch products and olefins from feed gases containing hydrogen, nitrogen, and carbon monoxide produced by the partial oxidation of hydrocarbons and other carbonaceous materials in air. Methods of producing organic liquids from mixtures of hydrogen and carbon monoxide (commonly called synthesis gas) are well known in the art. Methanol, for instance, is normally manufactured by reacting carbon oxides (mainly carbon monoxide) with hydrogen over a copper containing catalyst, whereby the carbon oxides and hydrogen are converted to methanol. Another well-known process that uses synthesis gas is the Fischer-Tropsch system, which is used to manufacture synthetic transportation fuels (gasoline and diesel). Higher alcohols and olefins are among other products that may be manufactured from synthesis gas. To achieve a more complete conversion of the synthesis gas into products, the effluent gas from the organic synthesis reactor is usually recycled to the reactor inlet, thus forming the well-known 'synthesis loop'. Examples of processes incorporating such a synthesis loop are disclosed in British Patent Publication No. 2047249 and French Patent Specification No. 1046822. If the synthesis gas used in such processes is produced by the partial combustion of a hydrocarbon or other carbonaceous material with air, the nitrogen present in the air will build-up to undesirable levels in the synthesis loop. In order to synthesize organic liquids without an undesirable build-up of nitrogen in the synthesis loop, the nitrogen has to either be removed from the synthesis gas or the partial combustion process has to be accomplished in the absence of air, i.e., with oxygen. The present invention seeks to provide a process for synthesizing certain organic liquids, such as methanol, from gases derived from the partial combustion of hydrocarbons or other carbonaceous materials with air without the undesirable build-up of nitrogen occurring within the system. According to the present invention there is provided a process for the synthesis of organic liquids selected from methanol, ethanol and higher alcohols, Fischer-Tropsch products and olefins from a feed gas containing hydrogen, nitrogen and carbon monoxide produced by the partial oxidation of hydrocarbons and other carbonaceous materials with air, which process comprises: (a) passing the feed gas through a series of two or more synthesis reactors; and (b) removing the organic liquid from each reactors. The feed gas may be produced by the partial combustion of hydrocarbons, such as natural gas or petroleum, or other carbonaceous material, such as coal or coke, with air. Steam can also be present during the combustion process. Any process can be used for the partial combustion. An example of a process for the partial combustion of hydrocarbons is the Texaco partial oxidation process which operates at around 1371°C (2500°F) and between 1.01 x 10⁶ and 1.01 x 10⁷ Nm⁻² (10 and 100 atmospheres). An example of a process for the partial combustion (i.e., gasification) of coal is the U-gas fluid bed process, which operates at around 982°C (1800°F) and at 5.05 x 10⁵ and 1.52 x 10⁶ Nm⁻² (5 to 15 atmospheres). Whichever process is used for the partial combustion operation, the synthesis gas produced will have as its main constituents hydrogen, carbon monoxide and nitrogen. For the efficient operation of the organic synthesis, it may be necessary for the synthesis gas to be essentially free of sulfur compounds, carbon dioxide and/or water vapor. Sulfur compounds, if present in the feedstock, can be removed either from the feedstock before partial combustion or afterwards. In the case of a hydrocarbon gas feedstock containing a few parts per million of sulfur compounds, this would be best accomplished upstream of the partial combustion using an absorbent, such as zinc oxide, operating at around 371°C (700°F). In the case of a hydrocarbon gas with a large sulfur content (over 500 volume parts per million (vpm), or a coal, coke, or petroleum oil, the removal of sulfur is best accomplished by washing the partial combustion product gas with a solvent, such as monoethanolamine solution at 37.8°C (100°F). Carbon dioxide can be removed from the partial combustion product gas by a similar monoethanolamine process or other established washing process. Water vapor can be removed from the partial combustion product gas by cooling and condensation. The resulting purified gas stream comprising mainly hydrogen, carbon monoxide and nitrogen is passed through a series of two or more reactors wherein the organic synthesis desired is performed. Any number of synthesis reactors can be used within the scope of this invention. The number of reactors is used in the organic liquid synthesis generally ranges from two to four, depending on the liquid produced. Any catalyst known in the prior art for synthesizing the organic liquids can be used in this invention. Typical catalysts include copper and zinc-containing catalysts for methanol manufacture and iron oxide-based catalysts for Fischer-Tropsch processing. The organic liquid synthesis reactors generally operate at temperatures and pressures suitable to the catalyst being used. Typically, the synthesis reactor will operate at temperatures ranging from 93.3 to 260°C (200 to 500°F) and pressures ranging from 1.01 x 10⁵ to 1.01 x 10⁷ Nm⁻² (atmospheric to 100 atmospheres). The organic product is removed from the effluent stream from each reactor. This separation is generally carried out by cooling and condensing the organic product from the effluent. The condensation process takes place at a temperature ranging from -128.9 to 148.9°C (-200°F to 300°F) and a pressure ranging from 1.01 x 10⁵ to 1.01 x 10⁷ Nm⁻² (atmospheric pressure to 100 atmospheres). Cooling and condensing can be supplemented or replaced, if desirable, by another separation process, such as washing with a suitable liquid or adsorption on a solid such as with molecular sieves. The reactor effluent gas, after product separation, is then passed to the next reactor inlet. The gas leaving the product separator following the final reactor is a waste gas stream that can be expanded to low pressure for use as a fuel. Optionally the waste gas stream can be processed to recover its hydrogen and carbon monoxide content for recycle to a point upstream. Examples of such recovery include: (1) Adding steam to the waste gas stream, passing it over to a CO shift catalyst, such as iron oxide, so that the steam oxidizes most of the carbon monoxide to carbon dioxide with release of an equal volume of additional hydrogen, washing out the carbon dioxide present with a solvent such as monoethanolamine, and finally condensing out most of the nitrogen content of the gas to leave a gas stream consisting mainly of hydrogen. This may then be recycled to the synthesis operation. (2) Passing the waste gas through an adsorbent, such as a molecular sieve, so as to substantially separate its hydrogen content so that the hydrogen may be recycled to the synthesis operation. (3) Washing carbon monoxide from the waste gas using a solvent such as "COSORB" such that the carbon monoxide removed may be recycled to the synthesis operation. The invention will now be described by way of example with reference to the accompanying drawing, in which: the figure is a flow diagram of the preferred embodiment of the present invention. Referring to the figure, in a preferred embodiment of this invention, hydrocarbons or other carbonaceous materials are fed via line 1 to an optional desulfurization step and then to a partial combustion step, where the hydrocarbons or other carbonaceous materials are combined with air via line 2 and optionally steam line 3 to partially combust the hydrocarbons or other carbonaceous materials to form a synthesis gas. The synthesis gas leaves the partial combustion steam via line 4 (where it may be subject to an optional purification step) and is then passed to an organic liquid synthesis zone 5. In the organic liquid synthesis zone, the synthesis gas is passed through a series of reactors, 6, 7 and 8 which convert the synthesis gas to the desired fluid product. After passing through each reactor, the unreacted synthesis gas and organic liquid are fed to separators 9, 10 and 11 via lines 12, 13 and 14 for separation of the organic liquid from the untreated synthesis gas. The organic liquid is then recovered via lines 15, 16, 17 and 18. The unreacted synthesis gas is passed to the next reactor via line 19 and 20. After a final separation step, a waste gas is recovered via line 21. The waste gas can be treated to recover H₂ and CO and recycled via line 22 or it can be passed via line 23 for further use, such as a fuel. COSORB is a trade name.

Claims: A process for the synthesis of organic liquids selected from methanol, ethanol and higher alcohols, Fischer-Tropsch products and olefins from a feed gas containing hydrogen, nitrogen and carbon monoxide produced by the partial oxidation of hydrocarbons and other carbonaceous materials with air, which process comprises: (a) passing the feed gas through a series of two or more synthesis reactors; and (b) removing the organic liquid from each reactor. A process as claimed in Claim 1 in which the organic liquid is methanol. A process as claimed in Claim 1 or Claim 2, where two to four organic liquid synthesis reactors are present. A process as claimed in any one of Claims 1 to 3 in which waste gas from the final synthesis reactor is expanded to low pressure for use as a fuel. A process as claimed in any preceding Claim in which waste gas from the final synthesis reactor is treated to recover the hydrogen or carbon monoxide content for recycle to an earlier stage in the process. A process as claimed in any preceding Claim in which the synthesis reactors operate at a temperature ranging from 93.3 to 260°C (200 to 500°F) and a pressure ranging from 1.01 x 10⁵ to 1.01 x 10⁷ Nm⁻² (atmospheric pressure to 100 atmospheres) in the presence of a catalyst comprising copper, iron and zinc compounds as the principal active constituents. A process as claimed in any preceding Claim in which the effluent stream from each reactor is cooled to condense and separate the organic liquid therefrom. A process as claimed in Claim 7 in which the condensation process takes place at a temperature ranging from -128.9 to 148.9°C (-200 to 300°F) and a pressure ranging from 1.01 x 10⁵ to 1.01 x 10⁷ Nm⁻² (atmospheric pressure to 100 atmospheres).

Inventors: KOWAL, WIESLAW MARK, SKINNER, GEOFFREY FREDERICK
Applicants: FOSTER WHEELER ENERGY LIMITED
Application year: 1987
Main classification: C07C29
IPC codes: C07C27, C07C29, C07B61, C07C67, C07C31, C07C1, C10G2