Autothermal process for the production of olefins

#The present invention relates to the field of catalytic oxidation of hydrocarbons. More particularly, the present invention relates to the catalytic partial oxidation of paraffinic hydrocarbons, such as ethane, propane, and naphtha, to produce olefins, such as ethylene and propylene.

#Olefins find widespread utility in industrial organic chemistry. Ethylene is needed for the preparation of important polymers, such as polyethylene, vinyl plastics, and ethylene-propylene rubbers, and important basic chemicals, such as ethylene oxide, styrene, acetaldehyde, ethyl acetate, and dichloroethane. Propylene is needed for the preparation of polypropylene plastics, ethylene-propylene rubbers, and important basic chemicals, such as propylene oxide, cumene, and acrolein. Isobutylene is needed for the preparation of methyl tertiary butyl ether. Long chain mono-olefins find utility in the manufacture of linear alkylated benzene sulfonates, which are used in the detergent industry.

#Low molecular weight olefins, such as ethylene, propylene, and butylene, are produced almost exclusively by thermal cracking (pyrolysis/steam cracking) of alkanes at elevated temperatures. An ethylene plant, for example, typically achieves an ethylene selectivity of about 85 percent calculated on a carbon atom basis at an ethane conversion of about 60 mole percent. Undesired coproducts are recycled on the shell side of the cracking furnace to be burned, so as to produce the heat necessary for the process. Disadvantageously, thermal cracking processes for olefin production are highly endothermic. Accordingly, these processes require the construction and maintenance of large, capital intensive, and complex cracking furnaces. The heat required to operate these furnaces at a process temperature of about 900° C. is frequently obtained from the combustion of methane which disadvantageously produces undesirable quantities of carbon dioxide. As a further disadvantage, the crackers must be shut down periodically to remove coke deposits on the inside of the cracking coils.

#A catalyst comprising platinum and tin supported on an alumina monolith was prepared by the following method. Platinum and tin were codeposited on a foam monolith (92 weight percent alpha alumina, 8 weight percent silica; 1.8 cm diameter×1 cm thick, 45 ppi (18 ppcm)) by impregnation with an aqueous solution of platinum and tin in a Pt:Sn atomic ratio of 1:5. The impregnation solution was prepared from a stock aqueous solution of hexachloroplatinic acid (0.193 M H2PtCl6) and a stock aqueous solution of stannous chloride (0.372 M SnCl2) acidified with 5 weight percent hydrochloric acid. Sufficient impregnation solution was used to obtain a platinum loading of 1.3 weight percent. The impregnated monolith was dried in ambient air and then reduced under flowing hydrogen (5 volume percent in nitrogen) at a flow rate of 1 cubic foot per hour (cfh) (473 cm3/min) using the following temperature profile: 1 h from ambient to 125° C., then 1 h from 125° C. to 300° C.; 1 h from 300° C. to 450° C., held for 30 min at 450° C., and then cooled to room temperature.

#The catalyst was sandwiched between two inert alpha alumina monoliths which acted as radiation shields. The monoliths were sealed in a quartz tube using FiberFrax™ brand alumina-silica cloth (FiberFrax is a trademark of and available from Unifrax Corporation), and the reactor was insulated by wrapping the quartz tube with high temperature insulation. A feed comprising ethane (2.6 standard liters per minute (slpm)), oxygen (1.3 slpm), hydrogen (2.6 slpm) and nitrogen (1.147 slpm) was fed to the reactor. Total flow was 7.647 slpm (GHSV 180,305 h−1) at 15 volume percent dilution with nitrogen. The ethane to oxygen molar ratio was 2:1; the hydrogen to oxygen molar ratio was 2:1.

#The catalyst was operated autothermally and the heat generated by the reaction was sufficient to sustain the process. However, heat was needed initially to ignite the process. The procedure for light-off involved establishing the flows of nitrogen, ethane, and hydrogen, then adding the oxygen flow; and then heating the feed to 200° C. until ignition. This procedure ensured a fuel-rich feed for safety considerations. The light-off conditions were 7 slpm total gas flow, 2.24 slpm of ethane, 2.24 slpm of hydrogen, 1.12 slpm of oxygen, 1.40 slpm of nitrogen, 20 percent dilution with nitrogen, an ethane to oxygen molar ratio of 2:1, a hydrogen to oxygen molar ratio of 2:1, and a pressure of 1.34 atm abs (136 kPa abs). After light-off, the external heat source was removed, and the gas flow rates and pressure were adjusted to the desired conditions, as shown in Table 1. Pressure was maintained at 1.34 atm abs (136 kPa abs). Shutdown of the reactor was accomplished by turning off oxygen prior to alkane and hydrogen.