Faculteit Ingenieurswetenschappen Vakgroep Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. G. B. Marin Simulation of a Steam Cracker using Grey and Non-Grey Radiation Models Wouter Foubert Promoter: Prof. Dr. Ir. G. J. Heynderickx, Prof. Dr. Ir. G. B. Marin Coach: Ir. G. Stefanidis Scriptie ingediend tot het behalen van de academische graad van burgerlijk scheikundig ingenieur Academiejaar 2005 – 2006 Preface Little in this work would have been possible without the support and advice of my coach George. I would like to take this opportunity to sincerely thank him and wish him the best, both professional and personal. Hopefully we’ll keep in touch. Graag zou ik professor Heynderickx en professor Marin bedanken omdat ze mij de mogelijkheid geboden hebben om dit werk tot stand te brengen. Het studentenleven is in vele opzichten een beetje een strijd, daarom een woordje van dank voor mijn strijdmakkers en, hopelijk, vrienden voor het leven Bart, Filip en Tom. Jullie zijn altijd welkom! Ik zou ook graag al mijn medestudenten en dan vooral deze van de Sterre bedanken voor de leuke momenten en aangename babbels. Astrid en Myriam, zonder jullie was alles anders verlopen. Heel erg bedankt voor jullie steun en advies. Tenslotte een speciaal dankwoord voor mijn moeder, vader, broers en zus. Wouter 13 juni 2006 FACULTEIT INGENIEURSWETENSCHAPPEN Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. Guy B. Marin Opleidingscommissie Scheikunde Verklaring in verband met de toegankelijkheid van de scriptie Ondergetekende, Wouter Foubert afgestudeerd aan de UGent in het academiejaar 2005 - 2006 en auteur van de scriptie met als titel: Simulation of a Steam Cracker using Grey and Non-Grey Radiation Models verklaart hierbij: 1. dat hij geopteerd heeft voor de hierna aangestipte mogelijkheid in verband met de consultatie van zijn/haar scriptie: o de scriptie mag steeds ter beschikking gesteld worden van elke aanvrager o de scriptie mag enkel ter beschikking gesteld worden met uitdrukkelijke, schriftelijke goedkeuring van de auteur of de promotoren o de scriptie mag ter beschikking gesteld worden van een aanvrager na een wachttijd van jaar o de scriptie mag nooit ter beschikking gesteld worden van een aanvrager 2. dat elke gebruiker te allen tijde gehouden is aan een correcte en volledige bronverwijzing Gent, (datum) (Handtekening) Krijgslaan 281 S5, B-9000 Gent (Belgium) tel. +32 (0)9 264 45 16 • fax +32 (0)9 264 49 99 • GSM +32 (0)475 83 91 11 • e-mail: [email protected] http://www.tw12.ugent.be/ UNIVERSITEIT GENT Faculteit Ingenieurswetenschappen Vakgroep Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. G. B. Marin Simulation of a Steam Cracker using Grey and Non-Grey Radiation Models Wouter Foubert Promotor: Prof. Dr. Ir. G. J. Heynderickx, Prof. Dr. Ir. G. B. Marin Coach: Ir. G. Stefanidis Academiejaar 2005-2006 Outline of the thesis The first objective of this work is to make a comparative study between a grey and non-grey gas radiation model when applied in a simulation of an industrial steam cracking furnace segment with radiation burners. Results show a 6.3% difference in thermal efficiency, the grey gas model results being the highest. A second objective is a study on the influence of the emissivity of the furnace wall on the thermal efficiency of a steam cracking furnace. Therefore two coupled furnace-reactor simulations are performed with different wall emissivities. Results show an increase of thermal efficiency, naphtha conversion and ethylene yield when a high-emmisivity coating is applied to the wall. In Chapter 1, an introduction on steam cracking and the simulation tools is presented. The objectives of the thesis are also defined. Chapter 2 gives an overview of the most common approximate radiative heat transfer models described in the literature. In Chapter 3 an overview is given of the most widely used models for estimation of radiative properties of gases. In Chapter 4 simulation results of grey and non-grey gas CFD simulations of an industrial naphta cracking furnace are presented. This is done by quantifying the differences in results of grey and non-grey simulations concerning important predicted variable profiles, like the flue gas flow and temperature profile, as well as the heat fluxes to the tubes. Chapter 5 investigates the influence of high-emissivity coatings in steam cracking furnaces. The efficiency of the application of high-emissivity coatings on the furnace walls in steam cracking technology can only be evaluated on the basis of a description of radiative heat transfer in frequency bands. To this end, a non-grey gas radiation model based on the Exponential Wide Band Model (EWBM) is developed and applied in the context of three- dimensional CFD simulations of an industrial naphtha cracking furnace with side-wall radiation burners. General conclusions are presented in Chapter 6. Simulation of a Steam Cracker using Grey and Non-Grey Radiation Models Wouter Foubert Promotoren: Prof. Dr. Ir. G. J. Heynderickx, Prof. Dr. Ir. G. B. Marin Abstract In this work, three-dimensional CFD radiation the latter is of paramount importance that it simulations of a steam cracking furnace with radiation is modelled in a sufficiently accurate manner. burners are performed. To execute these simulations the The first aim of this work is a comparative study commercial Computational Fluid Dynamics (CFD) between grey and non-grey radiation models applied software package FLUENT is used. The focus is on to an industrial scale steam cracking furnace. modelling of radiative heat transfer since this is the most important mode of heat transport in the radiative section Emission and absorption of radiation by gases show of a steam cracking furnace. While radiation property different behaviour from emission and absorption of variations for opaque solids are fairly smooth, gas radiation by surfaces. While the radiation property properties exhibit very irregular wavelength variations for opaque solids are fairly smooth, gas dependencies. As a result, two general types of models properties exhibit very irregular wavelength have been developed to describe the radiative properties dependencies. As a result, two general types of of a gas: grey and non-grey gas models. In this work, models have been developed to describe the radiative these two types of models are compared for a steam properties of a gas: grey and non-grey gas models. cracking furnace segment. Secondly, the non-grey gas Non-grey gas models take into account the wavelength radiation model is used in the context of coupled furnace- reactor simulations to investigate the effect of applying dependencies of the radiative properties of gases. high-emissivity wall coatings on the furnace thermal Most non-grey models, applicable for practical efficiency and the cracking results. simulations, divide the complete wavelength interval into distinct bands which are either absorbing or non- Keywords: Steam cracking, high emissivity coatings, absorbing and determine average values per band. In grey vs. non-grey radiation modelling. contrast to the non-grey models, grey gas models provide total radiative property values independent of the wavelength interval. Because this grey gas I. INTRODUCTION modelling requires only one solution of the Radiative Steam cracking of hydrocarbons is a petrochemical Transfer Equation (RTE) over the entire wavelength process in which saturated hydrocarbons are broken spectrum using an average absorption coefficient, it is down into smaller, often unsaturated, hydrocarbons. It by far the most widely used approach in practical is the principal industrial method for producing the radiative systems. The obvious drawback of the grey lighter alkenes (or commonly olefins), including model is the ill-correspondence between the model ethylene and propylene. The steam cracking process and the physical reality, i.e. the band-absorbing real takes place in reactor coils that are suspended in a gas compared to the grey gas. In this work a grey gas furnace with two main sections: a convection section and a non-grey gas radiation model are developed and and a radiant section. A hydrocarbon feed stream applied in the context of an industrial scale steam enters the furnace at the top of the convection section cracking furnace segment. Both are based on the and is initially heated by heat exchange with flue gas, Exponential Wide Band Model of Edwards. These mixed with steam and further heated to incipient models are implemented into FLUENT through the cracking temperature (500-680°C, depending on the use of User Defined Functions (UDF). The target is to feedstock). The stream then enters a fired tubular quantify the effect of the grey gas approximation on reactor (suspended in the radiant section or fire box) important predicted variable profiles like the one of where, under controlled residence time, temperature flue gas flow and temperature as well as on the reactor profile, and partial pressure, it is heated from 500- tube heat flux. These are important parameters for the 650°C to 750-875°C in 0.1-0.5s. During this short optimal design and operation of the furnace. reaction time hydrocarbons in the feedstock are The second aim of this work is to perform a cracked into smaller molecules. The firebox itself is coupled full furnace-reactor simulation. This is done heated by gas-fired wall-mounted radiant burners by linking FLUENT with the in-house reactor and/or floor mounted long-flame burners to a simulation program COILSIM. These coupled temperature of 1000°C to 1200°C. simulations which make use of a non-grey gas radiation model are used to investigate the influence of II. OBJECTIVES the furnace wall emissivity on the heat fluxes to the The research in this work will focus on modelling reactor tubes and on the thermal efficiency of the of radiative heat transfer in steam cracking furnaces.