The hydrogen economy is a possible component of an energy future based on use of alternative and renewable energy sources, deemed desirable from the general consensus of the worldwide community that we do not want to further exacerbate the climate problems that we have introduced over the last two centuries from burning fossil fuels. The burning of fossil fuels emits toxic pollutants into the air, such as sulfur compounds and oxidized forms of nitrogen (NOx) but also emit copious amounts of the inert carbon dioxide. The latter is widely recognized as the major cause of the global warming phenomenon.
For a hydrogen economy to develop, efficient means of hydrogen generation are required. Thermochemical cycles were conceived in the 1960s but only one operating pilot plant and no commercial installations based on the processes have been built. In the present work the use of a membrane reactor to enable the newly conceived Sulfur-Sulfur cycle, based on equations 1 - 3 is modeled.
4H₂O+4SO₂ -> H₂S + 3H₂SO₄ Eq. 1
H₂SO₄ -> SO₂ + H₂O + 1/2O₂ Eq. 2
H₂S + 2H₂O -> SO₂ + 3H₂ Eq. 3
The rationale for the use of a membrane reactor to enable the cycle is based on enhancing extent of reaction beyond its predicted equilibrium point due to the severely unfavorable thermochemical parameters for the steam reforming of hydrogen sulfide reaction (Eq. 3 above) which has a low equilibrium concentration of products. The membrane reactor will employ a molybdenum sulfide catalyst driving the steam reformation of hydrogen sulfide reaction and simultaneous extraction of hydrogen (one of the products) will allowing for the reaction to occur to higher extent.
A computational model of a catalytic membrane reactor was constructed using the well-known finite element model package Comsol v4.1 in which a catalytic microchannel reactor separated from a sweep gas by a thin hydrogen permeable membrane is built and parametric sweeps to evaluate the effect of membrane transport parameters, pressure and gas feed velocities are calculated. Though the steam reforming of hydrogen sulfide reaction has a competing thermal cracking reaction, the present work focuses on modeling one reaction only (the steam reformation reaction) for simplicity. Fully dense metallic membranes with chemselective permeability to hydrogen are modeled with transport parameters derived from reported literature values for similar applications.
The results show that employing a membrane reactor does significantly affect the completeness of the reaction by product extraction (if you do run the model with membrane transport set to zero, compare the extent at zero with extent at 3.6x10⁻⁶ mol.s⁻¹.m⁻²). The effect of changing sweep gas velocity is contingent on membrane properties, and membranes with small diffusion coefficients severely limit the ability of extraction of hydrogen from the feed. Therefore, it is more important that membranes with very high hydrogen permeability be employed in designing a reactor to implement this process, allowing for effective hydrogen separation and high conversion of the reactants in the process. Reactor pressure has minimal effect on the extent of reaction and therefore reactors designed to implement the process may be designed to operate at close to ambient pressure.
