(398b) Replacing Steam with Methane in Thermal Cracking Processes

With a continued rise in its availability, shale gas represents a cheap raw material source for olefin production. Its major components include methane, ethane and propane, with methane making up over 60 mole% of all the components in the gas. Meanwhile, methane is mostly unreactive in thermal cracking reactions until temperatures of about 1000^oC at ambient pressure. This particular feature of methane makes it particularly suitable to replace steam as a diluent in a thermal cracking process to produce olefins (such as ethylene) from paraffins (such as ethane and propane). Replacing steam with methane has a potential for cost savings in the construction and operation of a thermal cracking plant. Whereas liquid water has to be converted to steam before it can be used as a diluent in the steam cracking process and has high latent heat that must be supplied, methane need not undergo any such phase change before it can be similarly used in the present novel process.

Appropriate kinetics for methane, ethane and propane pyrolysis were selected from different sources in the literature to analyze the process for different operating temperatures ranging from 800 ^oC to 900 ^oC; and pressures of 2 Bar to 5 Bar using ANSYS Chemkin Pro. Two feed cases were used in the analyses. The first feed case has composition ratio of 0.795 mole methane, 0.147 mole ethane and 0.058 mole propane. The second feed case has composition ratio of 0.649 mole methane, 0.224 mole ethane and 0.127 mole propane. The compositions of these feed cases are typical of what are obtained from certain shale gas field regions in the United States.

Product distribution results show that methane is essentially an inert in the cracking process and basically serves to reduce the partial pressure of the ethane-propane components of the feed mixtures; thus, validating the novel idea of replacing steam with methane as a diluent. Also, ethylene is the major product obtained from the cracking process for all the cases and sets of operating conditions considered. This is similar to what would be obtained from a typical steam cracker. Required residence times for the process are also in the order of the ones required for the conventional steam cracker. Meanwhile, the process with methane as a diluent is simpler and has the potential for considerable savings in capital and operating costs of a thermal cracking plant compared to steam cracking.