This invention relates generally to carbon-to-liquids systems, and more specifically to methods and systems for minimizing liquid product variation from a reactor portion of a system.
The terms C5+ and “liquid hydrocarbons” are used synonymously to refer to hydrocarbons or oxygenated compounds having five (5) or greater number of carbons, including for example pentane, hexane, heptane, pentanol, pentene, and which are liquid at normal atmospheric conditions. The terms C4− and “gaseous hydrocarbons” are used synonymously to refer to hydrocarbons or oxygenated compounds having four (4) or fewer number of carbons, including for example methane, ethane, propane, butane, butanol, butene, propene, and which are gaseous at normal atmospheric conditions.
At least some known Fischer-Tropsch (FT) units have been optimized to produce synthesis gas (syngas) from natural gas, also known as Gas-to-Liquids process (GTL). Typically, syngas refers to a mixture of H2, CO and some CO2 at various proportions. To improve C5+ selectivity and minimize selectivity to C4−, i.e. natural gas and liquefied petroleum gas (LPG) production in known units, a FT reactor is operated with relatively high residence times, with relatively high per pass conversion, and with hydrogen to carbon monoxide (H2/CO) ratios below the consumption ratio. The remote location of most carbon-to-liquids plants makes natural gas and LPG co-production economically unattractive because of the relatively high transportation costs.
Minimizing natural gas and LPG production generally results in a significant fraction (30-40%) of the FT liquids being over-converted to wax. The wax formed must then be converted back to a diesel range, typically C10-C20 hydrocarbons, using a separate hydrocracking reactor. Also, the relatively high per pass conversion that is used to increase C5+ production generally adversely limits the pressure of the FT reactor, and the byproduct water partial pressure increases with conversion and total pressure. As the water partial pressure is increased the catalyst can be generally deactivated through oxidation of the active catalyst sites. Low water partial pressure may cause competitive adsorption of water, CO, and H2 molecules on the catalyst active site, thus reducing syngas conversion. Iron-based FT catalysts in particular can be greatly affected by water. Cobalt-based FT catalysts are generally more resistant to oxidation by water. Other carbonaceous fuels may also be used to provide the syngas input to the FT process. However, undesirable product variations may be caused by the operating characteristics of the known FT gas-to-liquids systems described above.