Simulation of a Natural Gas Steam Reforming Reactor at Different Scales

The concept of sustainable energy is often associated to the so-called hydrogen economy. However, hydrogen cannot be regarded as an energy source, since it is not present in nature as free H2. Therefore, it must be produced using chemical processes. Among them, natural gas steam reforming (NGSR) is the most widespread and economically feasible process. Natural gas (NG) is a mixture with no well-defined and constant composition. However, methane is the prevailing component (around 85- 90%), but also higher hydrocarbons (i.e. ethane, propane, butane...) can be found. NGSR involves the proper endothermic reaction of reforming which produces syngas (a mixture of H2, CO and CO2). Then, the slight exothermic reaction of water gas shift, further converts CO in CO2 producing more hydrogen. The overall process is highly endothermic, so requires a large amount of heat. Therefore, the reactors are tubes placed in a furnace which provides direct heat to the tubes. Even though this process implements a well-established technology, it still presents some issues, such as carbon formation and deposition. Usually, a high steam to carbon (S/C) ratio allows to reduce carbon formation and its deposition: a value of S/C higher than 2.5 is generally believed to be safe for coke-free operation, nevertheless the problem of carbon formation and deposition is still not solved. The aim of the first part, is the development of an accurate model for these reactors. The mathematical model underlying the chemical and physical system is made of the mass and energy balances. The constitutive equations are then coupled with the kinetic equations for all the reaction involved in the process. The kinetic equations for the NGSR process are retrieved from the literature (i.e. Xu and Froment) but, with a simplified approach, they are adapted to our specific case. The overall set constitutes a partial differential and algebraic equation (PDAE) system which and requires boundary conditions which generally are chosen to be flowrate and composition of the feeding mixture. The resolution of the PDAE system needs the implementation of a numerical method through a finite element method (FEM), implemented through COMSOL Multiphysics\uae. The major problem which has shown up is the numerical method convergence. However, at the end of the simulation it is possible to obtain plots and maps of the main physical and chemical quantities of interest. Furthermore, an experimental analysis of end-of-life commercial catalyst coming from a full-scale industrial SMR reactor is carried out. This experimental analysis provides interesting results regarding catalyst structure and the eventual carbon deposition. Therefore, a possible qualitative explanation for the carbon formation can be given

Fouling in a steam cracker convection section part 1 : a hybrid CFD-1D model to obtain accurate tube wall temperature profiles

To study fouling in steam cracker convection section tubes, accurate tube wall temperature profiles are needed. In this work, tube wall temperature profiles are calculated using a hybrid model, combining a one-dimensional (1D) process gas side model and a computational fluid dynamics (CFD) flue gas side model. The CFD flue gas side model assures the flue gas side accuracy, accounting for local temperatures, while the 1D process gas side model limits the computational cost. Flow separation in the flue gas side at the upper circumference of each tube suggests the need for a compartmentalized 1D approach. A considerable effect is observed. The hybrid CFD-1D model provides accurate tube wall temperature profiles in a reasonable simulation time, a first step towards simulation-based design of more efficient steam cracker convection sections

Large Eddy Simulation of the steam cracking process in refinery furnaces

There has been a tremendous increase in the production of ethylene over the last few decades, which has put tremendous focus on steam cracking processes and with an objective of improving its efficiency. This study, performed as a part of IMPROOF (Integrated guided Model PROcess Optimization of steam cracking Furnaces) project, is intended towards attaining that goal. Large Eddy Simulation (LES)- is at a stage of being mature enough to be used in design and optimization of processes in industrial equipments. However, the application of LES to study large flow equipments is still a technical challenge due to the high computational cost arising from numerical stiffness. In this study, a novel, chimera-based, local time stepping scheme is developed to speed up explicit time integration based LES solvers and applied (for the first time) to study the reactive flow inside a steam cracking furnace. This new numerical technique is studied for its numerical properties using Global Spectral Analysis (GSA) and the impact of local time stepping on the accuracy and resolution of the baseline numerical scheme is analyzed. The speed up obtained using this method is also ascertained with the help of canonical 2D and 3D nonreactive as well as reactive flow simulations. Numerical prediction of combustion inside a steam cracker comes with its own challenges. While detailed chemical mechanisms are ruled out from being used in simulations due to its high cost, simple global chemical mechanisms are not accurate enough to predict the flame structure and flame properties accurately. In this study, species transport equations are used with an analytically reduced chemical (ARC) mechanism. These chemical mechanisms are reduced from an up-to-date detailed mechanism using Directed Relational Graph with Error Propagation (DRGEP) technique and quasi-steady state (QSS) assumptions. The reduced mechanism is validated with respect to the detailed mechanism and experimental measurements is found to be in excellent agreement for all the flame properties of interest in this study. Radiative heat transfer is the predominant mode of heat transfer in steam cracking furnaces and hence cannot be avoided in realistic furnace simulations. In this study, the LES solver (along with the newly developed acceleration technique) is coupled with a radiative transfer equation (RTE) solver to carry out a coupled LES-RTE simulation. The approach is validated with experimental data from an axisymmetric jet diffusion flame and the experimental and numerical data is observed to be in good agreement with each other. Finally, all these methodologies are simultaneously applied to study the reactive flow occurring in the fire side of a steam cracking furnace. The LES acceleration technique speeds up the computations while ARC mechanism assists in predicting combustion reactions in an accurate manner. The radiative heat transfer effects are included by coupling the LES solver with the RTE solver as mentioned previously. Unsteady LES simulations of the combustion occurring inside the firebox is carried out. The computed and measured data for temperature and heat flux is found to be in close agreement with each other. LES of such a furnace demonstrates revealing information on the flame stabilization mechanism and the mean flame properties such as its shape and length and are discussed in this thesis. This study is intended to be a technology demonstrator by addressing three of the core challenges in the numerical modeling of steam cracking furnaces. By addressing these challenges, it is hoped that the petrochemical community is taking one step closer to using LES for their design and analysis processes in the near future

Best Available Techniques (BAT) Reference Document for the Production of Large Volume Organic Chemicals. Industrial Emissions Directive 2010/75/EU (Integrated Pollution Prevention and Control)

The Best Available Techniques (BAT) Reference Document (BREF) for the Production of Large Volume Organic Chemicals is part of series of documents presenting the results of an exchange of information between EU Member States, the industries concerned, non-governmental organisations promoting environmental protection, and the Commission, to draw up, review and – where necessary – update BAT reference documents as required by Article 13(1) of Directive 2010/78/EU on Industrial Emissions (the Directive). This document is published by the European Commission pursuant to Article 13(6) of the Directive. The BREF for the production of Large Volume Organic Chemicals concerns the production of the following organic chemicals, as specified in Section 4.1 of Annex I to Directive 2010/75/EU: a. simple hydrocarbons (linear or cyclic, saturated or unsaturated, aliphatic or aromatic); b. oxygen-containing hydrocarbons such as alcohols, aldehydes, ketones, carboxylic acids, esters and mixtures of esters, acetates, ethers, peroxides and epoxy resins; c. sulphurous hydrocarbons; d. nitrogenous hydrocarbons such as amines, amides, nitrous compounds, nitro compounds or nitrate compounds, nitriles, cyanates, isocyanates; e. phosphorus-containing hydrocarbons; f. halogenic hydrocarbons; g. organometallic compounds; k. surface-active agents and surfactants. This document also covers the production of hydrogen peroxide as specified in Section 4.2 (e) of Annex I to Directive 2010/75/EU; and the combustion of fuels in process furnaces/heaters, where this is part of the abovementioned activities. The production of the aforementioned chemicals is covered by this document when it is done in continuous processes where the total production capacity of those chemicals exceeds 20 kt/yr. While the main aim of the LVOC BREF is to facilitate reduction of emissions from chemical processes, other environmental issues - like energy efficiency, resource efficiency, wastes and residues - are also covered. This BREF contains 14 Chapters. Chapters 1 and 2 provide general information on the Large Volume Organics industrial sector and on generic industrial production processes used in this sector. Chapters 3 to 12 provide general information , applied processes and techniques, current emission and consumption levels, techniques to consider in determination of BAT and emerging techniques for various illustrative processes: lower olefins, aromatics, ethylbenzene and styrene, formaldehyde, ethylene oxide and ethylene glycols, phenol, ethanolamines, toluene diisocyanate and methylene diphenyl diisocyanate, ethylene dichloride and vinyl chloride monomer and hydrogen peroxide. Chapter 13 presents BAT conclusions as defined in Article 3(12) of the Directive. Concluding remarks and recommendations for future work are presented in Chapter 14.JRC.B.5-Circular Economy and Industrial Leadershi