The oxidative coupling methane to ethylene process faced problems of kinetic restrictions on selectivity and by-product utilization. Combining the oxidative methane coupling process with conventional ethane pyrolysis and a joint gas separation system looks like the most economically efficient.
Methane conversion according to the energy and process flow scheme as designed by the authors is performed in a cascade of reactors with intermediate cooling and high-pressure steam generation, CH,/O, ratio being approx. 3, temperature ranging 650–900 dg. C. At 30–35% methane conversions, selectivity to C, hydrocarbons accounted for 55–57%. A gas-separation system, incorporating absorption and low-temperature units ensures use of CO and a portion of unreacted methane by way of fuel for process furnaces, with recovery of product ethylene and recycle ethane sent to thermal pyrolysis. Consideration is given to comparative technoeconomic data of conventional pyrolysis process with the energy and process flow scheme for catalytic oxidative coupling of methane under development.
In the last five years special attention of researchers has been paid to the process of catalytic oxidative coupling of methane. A large number of various catalysts have been examined'. However, even the best investigated samples did not provide the yield of C,-hydrocarbons in excess of 20–22%2*3, if methane-oxygen mixture was not diluted by plenty of inert gas4. Process selectivity drops with increase of methane conversion which is connected with the greater reactivity of ethane in comparison with methane. Running the process at low conversions of methane does not enable the gaseous products to be recovered in a form suitable for fractionation. When methane conversion increases, process selectivity to C,-hydrocarbons drops at the expense of carbon oxides build-up. Simultaneously there is a growth of heat liberation in the reactor unit. The reaction heat recuperation and efficient use of by-products allows an improvement in the techno-economical indices of the process and enables the production of ethylene from natural gas to be competitive with the production of ethylene by ethane pyrolysis.
For the development and assessment of the process flow scheme use was made of experimental data obtained on 1% La, O,/MgO catalyst. At CH,/O, ratio being equal to 3 and maximum catalyst bed temperature ranged 860–880°C the methane conversion amounted to 30-35% and selectivity to C,-hydrocarbons was 55-57% '. For technological calculations the yield of C,-hydrocarbons was adopted equal to 19% out of which 213 is ethylene.
The reactor unit is arranged by way of a cascade of adiabatic reactors with intermediate cooling of the stream down to 650°C with generation of highpressure steam. After cooling and compression the reactio