This invention relates to a process for making olefins which combines the thermal and/or catalytic cracking of a hydrocarbon feedstock with the coupling of methane and allows separation of an enhanced amount of C.sub.2 + products, and more particularly, to a process for making olefins in which a hydrocarbon feed is thermally cracked to form olefins in parallel with the catalytic coupling of methane to form largely C.sub.2 hydrocarbons using an oxygen-affording gas, the methane coupling and cracking processes so arranged that the heat produced in the exothermic coupling reaction is effectively transferred to the endothermic cracking process, and in which the refrigeration required to liquify air to produce oxygen for the methane coupling process is used to effect the cryogenic separation of the products contained in the effluent from the cracking process.
The commercial production of olefins including importantly ethylene, propylene and smaller amounts of butadiene is generally accomplished by the thermal cracking using steam of ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. In a typical ethylene plant the cracking furnaces represent about 25% of the cost of the unit while the compression, heating, dehydration, recovery and refrigeration sections represent the remaining about 75% of the total. This endothermic process is carried out in large pyrolysis furnaces with the expenditure of large quantities of heat which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large and ways to reduce it are of substantial commercial interest. In addition, it is of interest to reduce the amount of methane produced in the cracking process, or to utilize it other than for its fuel value.
More recently, because of the supply side pressure to find non-petroleum sources for industrial chemicals and the environmental need to reduce methane flaring from producing oil wells, natural gas, a source which is relatively abundant in the United States and other locations elsewhere in the world, has been investigated as a source of hydrocarbons and oxygenates. Various methods to convert the methane in natural gas to hydrocarbons have been suggested and some commercialized. Projects in New Zealand and at Sasol in S. Africa are examples in which methane is converted to useful products. In New Zealand, methane is converted to methanol and then to hydrocarbons, and in Sasol, methane is first converted to syn gas and then to other products.
The direct conversion of methane to major industrial intermediates such as ethylene and propylene has been the subject of much research in the past 10 years. While a number of catalysts and processes have been suggested for the conversion none has yet been commercialized. One process which has been intensely researched is the high temperature methane coupling process using an oxygen-affording gas and a solid, metal oxide catalyst to form largely ethane and ethylene. Carbon dioxide formation which is favored thermodynamically is an undesired product in methane coupling as its formation uses carbon which is not readily available to form the desired hydrocarbons.
More recently in U.S. Pat. No. 5,025,108, Institut Francais du Petrole (IFP) has taught a process for producing olefins from natural gas which involves pre-separation of the C.sub.2 + components from the methane, catalytic oxidation of the methane to primarily ethane and ethylene, and the return of the C.sub.2 + components to the effluent side of of the methane coupling reaction where the saturated C.sub.2 + components are then cracked to olefins. The process is said to effectively utilize the heat produced in the exothermic methane coupling to carry out the endothermic cracking process. The process has several drawbacks however including importantly the mixing of the carbon oxides and water components produced by the substantial amount of hydrocarbon burning in the methane coupling process into the cracking process stream. Such components require an expensive separation from the hydrocarbons downstream in the IFP process.
Now a way has been found to produce olefins in a process not having many of the past disincentives. The process integrates the production of the olefins by hydrocarbon cracking with the methane coupling in a manner in which the individual processes synergistically fit together. Realized objectives of the new process are:
1. Thermal integration of the endothermic olefin cracking process with the exothermic methane coupling process; PA0 2. Thermal integration of the refrigeration processes for enriched or purified oxygen production, olefins recovery, and optionally, natural gas liquids processing; PA0 3. High overall yield of olefins and other products with near complete feedstock utilization; and PA0 4. Substantial reduction of NO.sub.x as a result of process heat being generated by the coupling of methane with oxygen rather than the combustion of fuel with air.