(63d) Thermodynamic Analysis of Adsorption Enhanced Reforming Processes

Different from conventional high-temperature steam reforming for hydrogen production, adsorption enhanced reforming (AER) is a novel process that occurs at moderate temperatures, in which the yield and purity of hydrogen are enhanced through simultaneous CO2 removal. The aim of thermodynamic analysis in this work is to provide insight into the AER process and guide its design and operation.

In this work, a generic algorithm of chemical equilibrium calculation in a gaseous reactive system with simultaneous single or multiple species adsorption is developed by minimizing the total Gibbs free energy. In the problem formulation, it is assumed that: (1) the reactive system is maintained at isothermal and isobaric conditions; (2) the gaseous phase obeys the ideal gas law; (3) there is no heterogeneous reaction on the surface of the adsorbent; and (4) the Langmuir isotherm is valid for all adsorbed species. With these assumptions, the formulated optimization problem is converted to a set of nonlinear algebraic equations solved using Newton-Raphson scheme. The so-called "Reduced Gibbs Iteration" approach employed in the NASA Chemical Equilibrium Application code is incorporated in the current algorithm to effectively track a large number of species simultaneously.

An example of steam reforming of ethanol with simultaneous CO2 adsorption is used to illustrate the proposed approach. It is shown that at T = 500 °C and P = 5 bar, the hydrogen yield is only 15.4% if there is no CO2 adsorption. When CO2 is removed by adsorption, both CO2 and CO drop in the gas phase, resulting in an enhancement in both hydrogen yield and purity. However, the removal of carbon element in the form of CO2 should exceed 40% in order to achieve a decent improvement in the conversion rate of ethanol to hydrogen. Moreover, due to the coupling of multiple equilibrium phenomena, a significant removal of CO2 is challenging as the required amount of adsorbent and reactor size increase exponentially. A comprise between the absorbent amount and the hydrogen yield is essential such that the heat required for the reforming process is covered by the waste heat provided by the combustion of off-gases downstream of the hydrogen separation unit. The thermodynamic analysis of such an integrated process indicates an 86.3% theoretical maximum overall conversion rate of ethanol to hydrogen (the corresponding H2 purity out of the reformer is 89.4% on wet basis or 96.2% on dry basis) with little external heat supply if the feed is available at T = 500 °C. An engineering calculation that accounts for preheating of the feed, reforming with CO2 adsorption, cooling of the product, combustion of off-gases as well as CO2 regeneration will be discussed at the end of the presentation.