Circulating fluidized bed reforming process for hydrogen production from methane.

摘要

Hydrogen finds use in diverse industries such as chemical, refining, metallurgy, electronics and food processing. In recent years there has been a tremendous thrust towards the development of a hydrogen based energy economy. This dissertation is an effort to propose and evaluate a new configuration for efficient production of hydrogen from methane.; The prevalent design for methane steam reforming is a fixed bed with the catalyst placed in hundreds of parallel tubes enclosed in a furnace. This configuration has some limitations such as low effectiveness factor of the catalyst, thermodynamic equilibrium, catalyst deactivation by carbon formation necessitating the use of high steam to methane ratios, need for a huge furnace for providing the heat for the endothermic reactions and requirement of additional steps to obtain pure hydrogen from the reactor products.; A bubbling fluidized bed (BFB) reformer has earlier been investigated for overcoming the shortcomings of the fixed bed reformer. Though these have been quite successful, the BFB reformers suffer from a hydrodynamic limitation on the flowrate. The proposed configuration builds on the advantages of the BFB reformer and uses the following features: (i) Powdered catalyst particles in a circulating fluidized bed (CFB) design to overcome the diffusion limitations and increase the effectiveness factor to ∼1 while also overcoming the hydrodynamic limitations of the bubbling fluidized bed reformers; (ii) Aides for breaking the thermodynamic equilibrium such as hydrogen permeable membranes to remove the product hydrogen from the reaction mixture and obtain pure hydrogen, and calcium oxide to remove the product carbon dioxide; (iii) Introduction of oxygen to carry out autothermal oxidative reforming; (iv) Use carbon formation and combustion in a reactor-regenerator configuration to attain autothermal operation.; The above points have been investigated in detail in this dissertation. It is shown that the use of a fluidized bed leads to an increase in hydrogen productivity of the reformer. The use of calcium oxide together with membranes is shown to further enhance hydrogen productivity. The use of binary residence times for the catalyst and calcium oxide particles is shown to increase the hydrogen yield. The effect of oxygen addition is studied and an optimum in the oxygen to methane feed ratio is obtained. The operation of a reactor-regenerator is also studied wherein some carbon is allowed to form on the catalyst in the riser through the use of low steam to methane ratios and the carbon thus formed is burned in the regenerator along with other combustible reformer products, using the catalyst as a carrier for the heat required for the reforming reactions. It is shown that it is possible to feed liquid water at room temperature to the process directly or through a feed pre-heater, instead of using a boiler with completely external heat supply to produce steam. Operation of the reactor-regenerator configuration leads to complex phenomena like multiplicity of steady states and bifurcation.