Modelling and simulation in conventional fixed-bed and fixed-bed membrane reactors for the steam reforming of methane

https://doi.org/10.1016/j.ijhydene.2016.01.083Get rights and content

Highlights

  • A detailed mathematical pseudo-homogeneous model to the FBMR.

  • Operation of Low gas superficial velocity (i.e. NRe < 1.0).

  • Operation of membrane reactor as a way to move the progress of the reaction.

  • The conversion level in the operation with hydrogen permeation.

  • The pressure reduction in the permeation zone.

Abstract

This paper presents a dynamics mathematical model to simulate the steam reforming of methane that take place in conventional fixed bed reactor (FBR) as well in fixed bed membrane reactor (FBMR) with steam added both with co-current mode. The model covers all aspects of main chemical kinetics, heat and mass phenomena in the membrane reactor with hydrogen permeation in radial direction across a Pd-based membrane. Firstly, a dynamics study was made for describing that temperatures of gaseous and solid phases reach to steady-state as well as molar flow rates. The effect several parameters including the axial position (z) divided by the reactor length Lz, reaction temperature and hydrogen partial pressure (PH2=Ppz) in permeation side were investigated. The conversion of methane is significantly enhanced by the partial removal of hydrogen from the reaction zone as a result of diffusion through the Pd-based membrane. Simulation results showed that a conversion from 99.85% could be achieved in a FBMR at reaction temperature of 600 °C relative to a conversion from 88.87% to 950 °C in a FBR. Besides, results showed that the yield of H2 reached to level from 1.548 (dynamics-state) and 1.626 (steady-state) in a FBMR at reaction temperature of 550 °C while the yield of H2 achieved to level from 1.261 (dynamics-state) and 1.445 (steady-state) in a FBR at reaction temperature of 725 °C.

Introduction

The process intensification strategy using membrane reactors can lead to the development and the re-design of more compact and efficient new processes that allow better exploitation of raw materials, lower energy consumption and plant volume reduction. Catalytic steam reforming of methane (CSRM) can be carried out in a fixed-bed membrane reactor (FBMR) as a way to intensify the hydrogen production [1].

The world consumption of hydrogen per year (137 million kg of hydrogen per day) is about 50 million ton and its request is rapidly increasing [2]. CSRM is currently a well-established technology and has been the most important industrial process for the production of hydrogen and/or synthesis gas (syngas) [3]. As the main production route of hydrogen on the industrial scale, their different aspects have been investigated by many authors [4], [5], [6].

In the CRSM process, methane reacts with steam to produce a mixture of hydrogen, carbon monoxide and carbon dioxide [7], [8]. The production of hydrogen in a FBMR is highlighted by changing the reaction equilibrium toward the product side through effective pressure difference induced by the continuous removal of hydrogen over the membrane [4], [6], [9]. Hydrogen was produced in a FBMR from biogas based on the evidences of the dry methane reforming process conducted with a nickel catalyst at 700 K under atmospheric pressure [10]. The complexity of optimizing the operating conditions of a FBMR is significant due to the subjacent phenomena occurring within this type of reactor [11], [12]. Reforming reactions are highly endothermic and can be performed with active catalysts (Ni, Rh) at high temperature, high pressure and steam-carbon ratio varying between 1.4 and 4.

Several studies have been carried out with CSRM process to separate H2 from a gaseous mixture by using a Pd-based FBMR [13]. The commercial separation of hydrogen through metallic membranes is mainly focused on palladium alloys membrane. The production of hydrogen in the reaction zone of the FBMR permits the hydrogen permeation through hydrogen-selective membrane [14], [15], [16].

The purpose of this paper is to evaluate the performance of the conventional fixed-bed reactor (FBR) and fixed-bed membrane reactor (FBMR) used to simulate the CSRM. Based on the energy and mass balances of chemical species, a system of partial differential equation (PDE) was formulated for describing governing equations of the energy and mass balances. Simulations presented from the model equations provide the evaluation of the CSRM process in FBR and FBMR reactors with a nickel catalyst. The performance of above reactors was studied in terms of temperature profiles, methane conversion at different temperatures, hydrogen production and the selectivity of H2, CO, CO2 and CH4.

Section snippets

Physical model

A schematic configuration of the proposed system used in the development of the mathematical model is shown in Fig. 1. The system consists of two concentric tubes (including the conventional FBR and FBMR reactor), where the external tube is constructed in steel and the internal tube constitutes the thin Pd-based membrane. The catalyst is placed in the annular space between the two tubes, forming the fixed bed (external reaction zone). During the operation, the reaction zone (fixed bed) is

Chemical components of the reforming process

The catalytic steam reforming process includes as reaction PSRM, WGS and OSRM [3], equating as follow:CH4+H2OCO+3H2;ΔH298K0=205.8kJmol1(PSRM)CO+H2OCO2+H2;ΔH298K0=41.1kJmol1(WGS)CH4+2H2OCO2+4H2;ΔH298K0=164.9kJmol1(OSRM)

The above process demands an efficient heat supply to the system due to the heat integration of endothermic (1), (3) and exothermic reaction (2). Chemical components included in the reforming reactions have their stoichiometric coefficients listed in Table 2.

Governing equations

A detailed one-dimensional pseudo-homogeneous model is formulated regarding the mass and energy balance equations in the reaction zone of the FBMR with the consideration of hydrogen flow across the membrane. The model is based on the following assumptions: (i) the system operates under non-isothermal conditions and in non-stationary regime in the reaction zone, (ii) model equations in the reaction zone are of plug-flow type with axial dispersion, while the model equation in the permeation zone

Results and discussion

As a result, CSRM was first tested in a conventional FBRfor the simulations of this work; a hot inert initial state was used where a steady flow of N2 at 500 °C and 450 kPa is fed under adiabatic conditions (no heat gain or loss through the reactor walls). The model for CSRM in the FBR and FBMR reactors sketched in Fig. 1 consists of a sets of governing equations for describing chemical species i (i = CH4, H2O, H2, CO and CO2) as well as the energy balances of the gaseous phase and solid phase

Conclusions

Conducted from the point view of steam reforming of methane in presence of a nickel catalyst (Ni (8.6%wt.)/γ-Al2O3), a numerical simulation was accomplished to report predictions for the operations in FBR and FBMR reactors. A computer code to process and analyze the behavior of the operating variables allowed the following conclusions:

  • 1.

    CSRM reactions have great effects on temperature distribution of FBR and FBMR. The WGS reaction happens in reverse, absorbing heat and reducing the temperature of

Acknowledgment

The authors of this paper would like to thank CNPq (National Council of Scientific and Technological Development) (grant number: 483541/2009-9) for the financial support given. (Process 48354/2012).

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