Conference papers and presentations
Nitrogen + Syngas Conference (The Hague, Netherlands, February 17-19, 2020)
Applying a digital twin of a primary reformer to determine economic routes for monetising high CO2 gas fields via GTL
Zbigniew Urban, PSE – a Siemens business, London, UK
Monetising offshore natural gas fields with high CO2 content presents a significant economic challenge. An example is the large (4 tcf) Malaysian K5 field, which has 70% CO2 content. Reducing CO2 content from 70% to a maximum of 6% involves high capital costs and complex installations, and there are risks of solid CO2 formation within the equipment. The brute force approach of simply separating the CO2 from methane using cryogenic distillation is thus not economically viable, so innovative approaches are necessary to overcome the constraints of the gas composition. Often such processing needs to take place offshore, adding to the complication.
PSE, Siemens’ process business, has developed high-fidelity digital twins for use in process synthesis of various gas processing solutions. The Steam Methane Reforming (SMR) package, validated against industrial-scale literature references and subsequently applied on a number of industrial projects, has been employed together with its counterpart GTL modelling suite, to assess the use of high CO2 content natural gas as a feedstock for syncrude production in Fischer-Tropsch (FT) reactors. The syngas feed to the FT reactor is generated in the SMR instead of the oxygen-blown autothermal reformer typically used in GTL technology.
The SMR digital twin is effective because of the fidelity of the underlying physics and chemistry model formulations, which incorporate rate-based approach; single pellets of catalyst are modelled in a distributed manner with boundary conditions dependent on pellet axial and radial location within the reformer tube. The combination of first-principles physics and chemistry kinetic parameters that can be easily correlated to a specific catalyst give the SMR digital twin a high degree of predictive accuracy, essential for the process synthesis described in this paper.
It runs out that the presence of CO2 can be turned into an advantage to resolve the well-known dilemma of achieving the correct Hydrogen-Carbon Monoxide ratio in the syngas for FT production. Operating an SMR at a typical S-C ratio of 2.5, which is sufficient to control coke formation, produces syngas that is too Hydrogen-rich for FT. There are reforming catalysts that can operate at much lower S-C ratios and which deliver syngas ready for FT, but these are typically super-active novel catalysts that are much more expensive than standard offerings and are often not fully reliable. In addition, low S-C ratio results in higher methane slippage.
The preliminary synthesis analysis indicates that the 70% CO2 content gas from the K5 field needs only to be reduced to 46% to become usable in syncrude production, a much easier and more economically-feasible task than reducing it to 6%. A benchmark study shows that 12.9 tonnes/h of 46 mol % CO2 gas delivering 3686 kg/h of Methane yields 1680 kg/h of syncrude. This compares favourably with producing 2000 kg/h of syncrude from 4500 kg/h, or 5 MMCF/day, of typical lean associated gas (3686 kg/h of Methane at 92.3 mol %) in a process where the syngas needs the additional complication of pressure-swing adsorption or membrane separation or catalytic reverse WGS reactor for Hydrogen reduction before directing it to FT reactors.