1.  NATURAL GAS BASED HYDROGEN PRODUCTION (Manousiouthakis)
           

Natural gas reforming based hydrogen production is one of the most economical routes for hydrogen production today. It involves the incomplete endothermic transformation of natural gas and water to hydrogen, carbon dioxide and carbon monoxide.  In our preliminary research, we have compared the conventional methane reforming/pressure swing adsorption hydrogen production process, with a novel process based on methane reforming and phase equilibrium separations. In the conventional production process, hot methane and steam are fed to the steam methane reformer (SMR) where the reversible reactions r₁, r₂ and r₃ take place:

CH4 + H2O = CO + 3H2             (r1)                         ΔHo1 : 206.1  kJ/mol          

CO + H2O = CO2 + H2               (r2)                         ΔHo2 : -41.15 kJ/mol          

CH4 + 2H2O = CO2 + 4H2          (r3)                        ΔHo3 : 164.9  kJ/mol          

Overall reactor operation requires that heat be provided to the reformer, and this is done through combustion of methane (fuel) and PSA waste gas. Hydrogen is produced together with all the other species and its generation is further increased in the water gas shift (WGS) reactor(s) where only the exothermic reaction r2 is catalyzed at temperatures lower than that of the reformer. Most of the water is separated by condensation as the gas stream is cooled down to almost ambient temperatures before entering the pressure swing adsorption (PSA) unit where hydrogen can be purified to 99.999+%. Species other than hydrogen are selectively adsorbed on a solid adsorbent, and are then desorbed generating the PSA waste gas whose combustion provides heat for the reformer, for the preheating of feeds and the generation of export steam. Recovery of the waste heat from the still-hot gases leaving the reformer is also used to the same end.

The aforementioned conventional process is not very efficient, nor does it allow for sequestration of carbon dioxide.  We have already carried out heat and power integration studies for the conventional process and have found that its utility cost (19 cents/kgH2) can be reduced by 36 % through heat integration, and that a small utility profit can even be generated through heat and power integration. 

In our preliminary research, presented at the 2004 AIChE Meeting, we also proposed a novel process for hydrogen production utilizing reforming and phase equilibrium based separation. This novel process generates both pure (99.9999% H₂) hydrogen and dry ice (98.5% CO₂). If the produced dry ice is sold for as low as 6 cents/kg CO₂, or if an equivalent carbon tax credit for CO2 sequestration is provided, then this alternative hydrogen purification process results in lower operating cost than the heat and power integrated conventional PSA process.  CO₂ sequestration in the form of dry ice involves simple release of water ice covered dry ice projectiles in the ocean. The large density of dry ice, as compared to water, makes these projectiles sink and imbed themselves in the deep ocean floor, where they transform into CO₂ hydrates called clathrates that are solid at these conditions.

We propose the continued investigation of the above novel process for hydrogen production, as well as alternative hydrogen production methods, using a variety of process optimization methods.  One of the design tools we will employ is the Infinite DimEnsionAl State-space (IDEAS) conceptual framework to synthesize optimal designs for a hydrogen production plant. The IDEAS framework considers all possible process networks for an a priori given set of process technology types and gives rise to infinitely-sized mathematical programs with a linear feasible region.