(263a) Hydrogen Production From Natural Gas Via Sorption-Enhanced Variable Volume Batch-Membrane Reactors

Anderson, D. M. - Presenter, Georgia Institute of Technology
Kottke, P. A., Georgia Institute of Technology
Fedorov, A. G., Georgia Institute of Technology

Natural gas has become increasingly abundant with recent advances in extraction techniques,1 motivating the development of new methods for efficient hydrogen production from natural gas in a small-scale, distributed fashion to allow for collocated production/usage and avoid the cost/technical challenges of implementing a hydrogen distribution infrastructure.2 Steam-methane reforming (SMR) is a mature technology that is the primary route for commercial scale hydrogen production from natural gas.3 The SMR process consists of three parallel reactions:4

(1)    CH4 + H2O ↔ 3H2 + CO, ΔH298K = 206.1 kJ/mol

(2)    CO + H2O ↔ H2 + CO2, ΔH298K = -41.2 kJ/mol

(3)    CH4 + 2H2O ↔ 4H2 + CO2, ΔH298K = 164.9 kJ/mol

Owing to the strongly endothermic nature of the primary steam reforming reaction, industrial MSR processes typically operate at a highly elevated temperature (800-900°C), followed by low temperature, exothermic water gas shift (WGS) reaction to produce additional H2.5 The high temperatures of the industrial MSR reactors are neither practical nor economical for small-scale distributed applications, and the need for subsequent WGS reactors further inhibits straightforward scale-down of the traditional industrial approach. Combined reaction-separation processes, wherein products of the reversible reactions are selectively removed to circumvent equilibrium thermodynamic limitations and enhance forward reaction rates, are an attractive means to reduce the required MSR operating temperature, thus reducing the equipment cost/maintenance expenses and increasing the volumetric yield of hydrogen, both of which are appealing for on-site power generation.6

This work explores the potential of the CO2/H2 Active Membrane Piston (CHAMP) reactor7,8 for low-temperature, distributed hydrogen production from natural gas. The CHAMP is a variable volume batch-membrane reactor that operates similar to an Internal Combustion (IC) engine cycle, and is designed to dynamically maintain the optimal conditions (pressure, temperature, and residence time) for transport of reactants to the catalyst, reaction kinetics at the catalyst, and permeation of H2 through a selective Pd/Ag membrane, as it proceeds through a multi-step reaction-separation cycle. During the reaction step of the CHAMP cycle, judicial compression of the reactor volume is utilized to increase the concentration of the remaining fuel and hydrogen, thereby providing additional driving force for reaction and permeation in order to counteract the reduction in pressure due to permeation and in reaction rate due to fuel depletion.

Herein, thermodynamic analysis is used to show that in order to achieve CH4 conversion of greater than 90% at temperatures as low as 400°C and at low steam-to-carbon ratios (which are desired for high thermal efficiency and H2 yield density), a combination of selective CO2 sorption and H2 membrane permeation is required. Operational aspects of the sorption-enhanced CHAMP reactor, termed CHAMP-SORB, are discussed, and the thermal efficiency of the cyclic process is reported as a function of the key process variables (pressure, temperature, steam-to-carbon ratio, and sorbent characteristics). The results of the analysis are reported in the form of regime maps for the CHAMP-SORB reactor which can be utilized as design tools to match the reactor’s capabilities to desired application requirements. Lastly, the implications of the reaction, permeation and sorption kinetics are also discussed to identify the key timescales governing each step of the CHAMP-SORB cycle and to assess the impact on hydrogen yield/power density of the proposed reactor concept.


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