Evolution of processes for synthesis gas production: Recent developments in an old technology

The manufacture of gas mixtures of carbon monoxide and hydrogen has been a vitally important part of chemical technology for about a century. Originally, such mixtures were obtained by the reaction of steam with incandescent coke and were known as "water gas". Used first as a fuel, water gas soon attracted attention as a source of hydrogen and carbon monoxide for the production of chemicals, at which time it gradually became known as synthesis gas. Eventually, steam reforming processes, in which steam is reacted with natural gas (methane) or a petroleum naphtha over a nickel catalyst, found wide application for the production of synthesis gas. A modified version of steam reforming known as autothermal reforming, which is a combination of partial oxidation near the reactor inlet with conventional steam reforming further along the reactor, improves the overall reactor efficiency and increases the flexibility of the process. Noncatalytic partial oxidation processes using oxygen instead of steam also found wide application for synthesis gas manufacture, with the special feature that they could utilize low value feedstocks such as heavy petroleum residua. In recent years, catalytic partial oxidation employing very short reaction times (milliseconds) at high temperatures (850-1000 degreesC) is providing still another approach to synthesis gas manufacture, as Professor Lanny Schmidt and his students have shown in their pioneering exploratory research in this area. (See, for example, Hickman and Schmidt Science 1993, 259, 343.) Here, we consider some crucial issues in catalytic partial oxidation and report some new data with a bearing on these issues. Nearly complete conversion of methane, with close to 100% selectivity to H-2 and CO2 can be obtained with a Rh monolith under well-controlled conditions. Experiments on the catalytic partial oxidation of n-hexane conducted with added steam give much higher yields of H-2 than can be obtained in experiments without steam, a result of much interest in obtaining hydrogen-rich streams for fuel cell applications.