Energy and exergy analyses of an integrated membrane reactor and CO2 capture system to generate decarbonized hydrogen

Abstract

At present, most of the world hydrogen production comes from the steam reforming of natural gas, carried out in conventional reactors, which show some disadvantages such as thermodynamic equilibrium limitation, harsh operating conditions, and are responsible for huge CO2 emission in the atmosphere. To overcome these disadvantages and meet the pressing requests of the hydrogen energy decarbonisation, the interest in membrane reactors, able to provide both hydrogen generation and separation in the same unit and at milder conditions, is increasing. Indeed, in these devices, the produced hydrogen in the reaction side is removed for permeation through a membrane continuously, and it is collected as a decarbonized stream in the permeate side, meanwhile overcoming the thermodynamic equilibrium limitation of the conventional reactors, and contributing to the intensification of the process and to the CO2 capture and storage. In this study, firstly, a one-dimensional model of a membrane reactor is developed and validated. The changes in the molar flow rates of each species and the temperature through the membrane reactor length are investigated. Furthermore, the effects of some operating parameters (the operating temperature, the steam-to-carbon ratio, and the inlet pressure on the reaction side of the membrane reactor) on the system performance (methane conversion, hydrogen yield, CO2 yield, and thermal efficiency based on lower heating value) are examined. Successively, the membrane reactor model is integrated into a system-level model, taking into account further a CO2 capture unit and the plant components (compressor, boiler, burner, pump, blower, and two mixers) in order to analyze theoretically the potentiality of this integrated membrane reactor based system to generate decarbonized hydrogen (high grade hydrogen production combined to the full CO2 capture to avoid its release in the environment). The novelty of this work deals with the application for the first time of a system-level modeling is based on the principles of electrochemistry and thermodynamics to develop a detailed energy and exergy analysis, calculating further the rate of exergy destruction of each component of the proposed membrane reactor based integrated system. At the baseline simulation conditions (773 K, 9 bar, and S/C = 3/1), thermal efficiency (based on lower heating value), methane conversion of the system, hydrogen yield, CO2 yield are equal to 51 %, 67 %, 22 %, and 66 %, respectively. Furthemore, the exergy destructions distribution is 49 % for the burner, 36 % for the boiler, 14 % for the membrane reactor, and 1 % others.