The present development is a method that can be useful in purification of raw gas or offgas streams from steam crackers or fluid catalytic crackers (FCC). By the method of the present development, acetylene, methylacetylene, NO, and O2 are simultaneously removed from a raw gas feedstream that comprises ethylene, hydrogen, and CO without significant loss of ethylene, using a supported Ru-based catalyst. The catalyst comprises between 0.01 wt % to 5 wt % ruthenium distributed on a support selected from alumina or other commonly known catalyst support materials.
Catalytic cracking processes, such as fluid catalytic cracking (FCC) and deep catalytic cracking (DCC), have been widely used in industry for many years to produce transportation fuels, such as gasoline and diesel. The off-gases from the FCC and DCC processes contain valuable products such as ethylene and propylene. However, these off-gas streams contain relatively dilute concentrations of olefins and it is generally perceived as not being economically feasible to recover the olefins by conventional means, such as fractionation. Thus, most refineries use the off-gas as fuel-gas.
Recently, the recovery of these relatively high value olefins from off gas streams has gained increasing interest. For example, U.S. Pat. No. 5,981,818 describes a process for recovery of dilute olefins from off-gases. Besides valuable olefins, FCC/DCC off-gases also contain detrimental impurities such as acetylenes and di-olefins. These impurities need to be removed from the off-gas streams in order to utilize the high value olefins in downstream processes. Typically, acetylenes and dienes found in olefin streams are commercially removed by a selective hydrogenation process.
Most selective acetylene hydrogenation operations at the commercial scale use Pd-based catalysts. In addition to hydrocarbons, an off-gas stream often contains nitric oxides, oxygen, sulfur, and other impurities. The Pd-based catalysts have high activity and selectivity for selective hydrogenation of acetylene and dienes; but they are very sensitive to sulfur and some other poisons. Moreover, the Pd-based catalysts are not known to be particularly effective for removal of nitric oxides and/or oxygen.
Nickel catalysts have also been used in selective hydrogenation of acetylene and dienes. Nickel catalysts are resistant to sulfur poisoning, but are not selective toward hydrogenation of acetylene. Most commonly, while acetylene is removed, significant amounts of olefins are also hydrogenated to saturated hydrocarbons. Nickel-based catalysts also tend to form nickel carbonyl when the carbon monoxide level is high in the feed gas stream, particularly at low temperatures. Nickel carbonyl is a highly volatile, highly toxic substance which can deposit in downstream equipment and pose a significant safety hazard to workers in the area.
U.S. Pat. No. 2,747,970 teaches and claims a process of removing carbon monoxide and carbon dioxide from a gas stream using a catalyst consisting of 0.01% to 2.0% ruthenium on an activated earth metal oxide, such as activated alumina. The process comprises directly contacting the gas stream with the supported catalyst while maintaining a reaction temperature of at least 120° C. until the carbon content of the CO and CO2 is substantially completely converted to methane. However, the process does not teach that the same catalyst and method can be used to remove acetylene, methylacetylene, butadiene, NO, and O2 from an ethylene gas stream without risk of loss of ethylene. The prior art which does teach the use of ruthenium catalysts for purification of ethylene streams typically cites the ruthenium catalysts as examples of ineffective catalysts for such applications. For example, in U.S. Pat. No. 4,299,800, a catalyst comprising 0.5 wt % ruthenium on alumina was evaluated for oxygen removal from an ethylene comprising feedstream. At low temperatures (50° C.), oxygen removal was low and ethylene conversion was essentially non-detectable. However, at higher temperatures (200° C.), oxygen removal reached 99.4%, but with concomitant ethylene conversion (loss) of 11.2%, as compared to less than 5% ethylene conversion when using silver, gold or vanadium on alumina.
Thus, there is a need for a process for removing oxygen, acetylenes, and nitric oxides from off gas streams wherein the ethylene is not converted to lower value hydrocarbons during the purification process and wherein the purified ethylene-containing gas stream comprises less than about 1 ppm of each of acetylenes, nitric oxides and oxygen.