Modelling of Sorption Enhanced Chemical Looping Steam Reforming (SE-CLSR) of Methane in a Packed Bed Reactor

摘要

In the sorption enhanced steam methane reforming (SE-SMR) process, hydrogen (H2) can be produced in concentration up to 98 vol. % (dry basis) in a single reactor packed with a mixture of reforming catalyst and carbon dioxide (CO2) sorbent. This is defined as pre-combustion capturing of CO2 and the high purity H2 produced can be used as a fuel for electricity generation, synthesis of ammonia-derived fertilisers, or hydrotreating of naphtha and other heavy gas oil in petroleum refinery. A cyclic operation between the production of H2 and regeneration of CO2 sorbent is required, but the energy demand for the sorbent regeneration is high. A proposed method to decrease this energy demand is to couple SE-SMR with chemical looping (CL), which naturally separates the nitrogen (N2) from the syngas via the highly exothermic cyclic oxidation with air of a metallic material, which acts as the reforming catalyst when reduced (oxygen transfer material or ‘OTM’). The combination of SE-SMR and CL makes the process energy efficient and eliminates the need for (i) high temperature as compared to the conventional steam methane reforming (SMR) process (typical temperature range is 750- 950°C), (ii) the water gas shift (WGS) reactors downstream of the reformer, and (iii) external heating using the natural gas fuel in the reformer. However the H2 generation of a high purity from one reactor operation is intermittent, as part of a cyclic operation, with the reactor alternately operating in Fuel Reactor mode (FR), with fuel and steam feed or Air Reactor mode (AR), with air feed. Adsorption of CO2 shifts the equilibrium of reaction towards more H2 production and ultimately increases the efficiency of the process towards H2 production. Production of H2, CH4 conversion and overall efficiency of the process depend upon many operating parameters. The effects of inlet temperature, reactor pressure, molar steam to carbon ratio (S/C) in the feed, and gas mass velocity on the SE-SMR and the sorption enhanced chemical looping steam reforming (SE-CLSR) of methane processes is reported in this thesis.udThe formulation of the SE-CLSR process model requires the modelling of packed bed reactors. This mathematical modelling covers various individual models (sub-models) for; SMR, SE-SMR, OTM reduction and oxidation of reduced OTM. The gPROMS model builder 4.1.0® is used to solve the model equations. In this work, an experimentaludIVudkinetics study and model of SMR process over 18 wt. % NiO/α-Al2O3 catalyst are presented for an adiabatic fixed bed reactor in the temperature range of 300-700°C at 1 bar pressure. The model is validated by comparing the results with the experimental data obtained as part of this work. The simulation results are in excellent agreement with the experimental results. The equilibrium results are generated using Chemical Equilibrium with Applications (CEA) software. The effect of various operating parameters (temperature, pressure and S/C) on the CH4 and water conversion (%) is modelled and compared with the equilibrium values. The mathematical model of SE-SMR was developed based on the industrial operating conditions of temperature and pressure. The 873-973 K was found to be the optimum range of temperature, under the high pressure (30 bar) conditions, for the production of H2 of purity exceeding 85%. The developed model of SE-SMR was validated against the literature data.udThe mathematical model of SE-CLSR process was developed under adiabatic conditions. This model is the combination of reduction of catalyst followed by oxidation of the reduced catalyst. The individual models of reduction and oxidation are developed by using kinetic data available in the literature and later on validated with experimental results proposed in the literature. The already developed model of SE-SMR process is combined with the OTM reduction model to mimic the dynamic process occurring in the fuel reactor (FR) system. This FR is combined with air reactor (AR) and the combined model is run for 10 cycles. The sensitivity of the process is studied under the various operating conditions of temperature (873-1023 K), pressure (1-30 bar), molar S/C (2-6) and mass flux of the gas phase (Gs = 2-7 kg m-2 s-1). In this work, the operating conditions used for the production of H2 represent realistic industrial production conditions. The sensitivity analysis demonstrates that the developed model of SE-CLSR process has the flexibility to simulate a wide range of operating conditions of temperature, pressure, S/C and Gs.