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Advanced modelling of vinyl chloride monomer productionvia thermal cracking of ethylene dichlorideRenato do Carmo Claro Yih WongThesis to obtain the Master of Science Degree inChemical EngineeringSupervisors:Professor Henrique Aníbal Santos de MatosDr. Štěpán ŠpatenkaExamination CommitteeChairperson:Professor Maria Fátima Costa FareloSupervisor:Professor Henrique Aníbal Santos de MatosMembers of the Committee:Professor Maria Joana Assis Teixeira Neiva CorreiaNovember 2014

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“Wit beyond measure is man’s greatest treasure.”– J.K. Rowling“Valeu a pena? Tudo vale a penaSe a alma não é pequena.”– Fernando Pessoaiii

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AcknowledgmentsFirstly, I would like to thank professors Carla Pinheiro and Henrique Matos, who made available this oneof a time experience to take this internship at PSE. I also want to thank professor Costas Pantelides andeverybody at PSE for making this opportunity available, and welcoming me these past seven months.I would also like to thank my supervisors, both from IST and PSE. To Stepan Spatenka, thank youfor sharing your knowledge and know–how during this last seven months. To professor Henrique Matos,for his guidance and assistance whenever we met.I would also like to thank everybody at PSE who helped me with my thesis, namely Maarten andCharles, for the training course in gPROMS, and Trung, for helping me with the Macro used in the LSKMimplementation.I would be amiss if I didn’t mention the great friends I found while at London, all of whom helped mebuild a home away from home. To all of you, my biggest gratitude, and I can’t wait to see you all again.A special acknowledgement to the interns at PSE from FEUP, Catarina and Rubina, for your supportand help throughout our internship. Thank you for listening to all my problems, and entertaining me withyours.To my house mates, Artur and Mariana, thank you for the great times. It was great living with you,and I would do it again in a heartbeat.I would like to thank all the friends I made during these last five years at IST. You have been amazing,and are fortunately too many to mention here, but to all of you thanks.To Ana, Duarte, and Rita, words cannot express what I feel for you. These last five years have beenamazing, and I am proud to call you my friends.To Bernardo, Leonor, and Rúben, my biggest gratitude for your friendship and hard work. With you,being friends and work partners isn’t mutually exclusive, and I have enjoyed all the time spent with you.To Margarida, Rita, Teresa, Bernardo, and Henrique, my friendship with you has only grown throughout these last five years, and for that I am eternally grateful.Finally, to my most wonderful family, my parents and brother, for always being there for me. NothingI do would be possible without you.v

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ResumoO cloreto de vinilo (VCM) é uma das maiores comodidades, sendo a matéria-prima principal na produçãodo policloreto de vinilo (PVC), e é produzido através do craqueamento térmico do dicloroetano, EDC,numa fornalha. No presente trabalho, um modelo desta fornalha foi desenvolvido em linguagem gPROMS R .Este é composto por um modelo da serpentina, onde é descrita a reacção de craqueamento, e ummodelo da câmara de combustão, onde a transferência de calor pela combustão do combustı́vel foiconsiderada. Vários mecanismos cinéticos, tanto moleculares como radicalares, foram validados comdados disponı́veis na literatura. Para reduzir o tempo de computação da simulação quando são utilizados mecanismos radicalares, a matriz dos coeficientes estequiométricos foi comprimida, o que permitiudiminuir o tempo de computação em metade. Relativamente ao modelo da fornalha, foram consideradas diferentes correlações para estimar a emissividade dos gases de combustão concluindo-se queos resultados obtidos entre são muito semelhantes. Foi ainda usado um modelo onde a fornalha édividida em zonas, tendo-se concluı́do que a temperatura da fornalha não varia significativamente como aumento de zonas na câmara de combustão. Finalmente, foi realizada uma análise de sensibilidadeà quantidade de combustı́vel consumido, e verificou-se um mı́nimo no consumo especı́fco de combustı́vel (87.6 kg combustı́vel/t VCM). O uso de iniciadores foi também testado, e verificou-se que como mecanismo utilizado, o cloro reduz a temperatura necessária na serpentina, e o tetraclorocarbono ooposto.Palavras-chave: Modelação, VCM, craqueamento, mecanismo radicalar, gPROMSvii

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AbstractVinyl chloride (VCM) is one of the most important commodity materials, being the main raw materialin the production of polyvinyl chloride (PVC), and is mainly produced through the thermal cracking ofdichloroethane, EDC, in a pyrolysis furnace. In the present work, a model of this furnace was developedin gPROMS R . This is composed of a model of the coil, where the cracking reaction is described, and amodel of the firebox, where the heat transfer by the combustion of the fuel was modelled. Several kineticmechanisms present in the literature, both molecular and radical, were implemented and validated withavailable data. To reduce the computing time of the simulation when using radical mechanisms, thestoichiometric matrix was compressed, which was able to the computing time in half. Regarding thefirebox model, different correlations for emissivity estimation were compared using a single zone model,and it was found that the results obtained with them were very similar. A zone model was also used,where the firebox was divided in several zones, and it was concluded that the temperature profile didnot change significatively with the increase of zones in the firebox. Finally, a sensitivity analysis wasperformed on the fuel consumption, and a minimum in specific fuel consumption (87.6 kg fuel/t VCM)was found. The use of initiators was also tested, and it was shown that with the used mechanismchlorine greatly reduces the temperature needed in the coil, and using carbon tetrachloride the oppositewas observed.Keywords: Modelling, VCM, cracking, radical mechanism, gPROMSix

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ContentsAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vResumo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .viiAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ixList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xiiiList of Figures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviiNomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xxGlossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi1 Introduction11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.2 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 Background32.1 The PVC market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32.2 VCM production combined process [6], [7] . . . . . . . . . . . . . . . . . . . . . . . . . . .42.2.1 Other routes for VCM production . . . . . . . . . . . . . . . . . . . . . . . . . . . .62.3 The pyrolysis furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72.4 Firebox models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82.4.1 Well-stired furnace model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82.4.2 Calculation of the gas emissivity . . . . . . . . . . . . . . . . . . . . . . . . . . . .92.4.3 Non–grey gas effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102.4.4 Zone model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112.5 Cracking kinetic mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122.5.1 Molecular mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122.5.2 Radical mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .132.6 Coke formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .132.7 Reaction initiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .143 Materials and Methods173.1 The gPROMS Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .173.2 The Multiflash Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .173.3 Implementation of Large Scale Kinetic Mechanisms . . . . . . . . . . . . . . . . . . . . .18xi

3.3.1 Sparse matrice treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .183.3.2 LSKM preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193.3.3 LSKM output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .203.4 The ReadData Foreign Object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214 Reactor model234.1 Source and sink models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .234.2 Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244.2.1 gML Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244.2.2 Distributed Thermal Contact. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244.3 Coil model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244.3.1 One–dimensional tube model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244.3.2 gML to LSKM converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .284.3.3 LSKM to gML converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .284.4 Heat transfer models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .294.4.1 Advanced energy input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .294.4.2 Firebox model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .295 Results315.1 Implementation of the molecular mechanism . . . . . . . . . . . . . . . . . . . . . . . . .315.2 Firebox simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .355.3 Implementation of the radical mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . .385.3.1 LSKM performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .385.3.2 Model performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .405.4 Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .425.4.1 Analysis of the fluid properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . .425.4.2 Analysis on the operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . .445.4.3 Promoters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .476 Conclusions516.1 Achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .526.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52Bibliography56A Kinetics57A.1 Molecular kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57A.2 Radical kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58B Heat Capacity95C Emissivity estimation99xii

List of Tables5.1 Geometry parameters of the coil in the studied cases . . . . . . . . . . . . . . . . . . . . .315.2 Simulation results using the geometry in Li et al. [16] and assigning a COT of 756 K . . .325.3 Simulation results using the geometry in Li et al. [16] and assigning a conversion of 55% .345.4 Inputs used for the firebox model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .355.5 Composition of the fuel used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .355.6 Firebox results for the well–stirred furnace model . . . . . . . . . . . . . . . . . . . . . . .375.7 Firebox results using different number of zones . . . . . . . . . . . . . . . . . . . . . . . .385.8 Simulation results for the radical mechanisms using the geometry in Li et al. [16] andassigning a conversion of 55% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .415.9 Coil outlet compositions using the radical mechanisms . . . . . . . . . . . . . . . . . . . .425.10 Results for the sensitivity analysis on the thermal conductivity and viscosity . . . . . . . .43A.1 Kinetic parameters for the mechanism by Kaggerud [13] . . . . . . . . . . . . . . . . . . .57A.2 Kinetic parameters for the mechanism by Li et al. [16] . . . . . . . . . . . . . . . . . . . .57A.3 Kinetic parameters for the mechanism by Dimian and Bildea [6] . . . . . . . . . . . . . . .57B.1 Heat capacities for the molecular species . . . . . . . . . . . . . . . . . . . . . . . . . . .95B.2 Heat capacities for the molecular species (cont.) . . . . . . . . . . . . . . . . . . . . . . .96B.3 Heat capacities for the radical species . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96B.4 Heat capacities for the radical species (cont.) . . . . . . . . . . . . . . . . . . . . . . . . .97C.1 Values for the parameter b for correlation 2.10 . . . . . . . . . . . . . . . . . . . . . . . . .99C.2 Values for the parameter n for correlation 2.10 . . . . . . . . . . . . . . . . . . . . . . . . .99C.3 Values for the parameter a0 for correlation 2.11 . . . . . . . . . . . . . . . . . . . . . . . .99C.4 Values for the parameter a1 for correlation 2.11 . . . . . . . . . . . . . . . . . . . . . . . . 100C.5 Values for the parameter a2 for correlation 2.11 . . . . . . . . . . . . . . . . . . . . . . . . 100C.6 Values for the parameter a3 for correlation 2.11 . . . . . . . . . . . . . . . . . . . . . . . . 100C.7 Constants for the degree of emission of the pure gas phase . . . . . . . . . . . . . . . . . 100xiii

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List of Figures2.1 Utilisation of PVC production capacities in various regions, 2009 . . . . . . . . . . . . . .42.2 The vinyl chloride production process . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.3 Scheme of the VCM pyrolysis furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73.1 LSKM excel input sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193.2 LSKM excel species sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .203.3 Format of the .txt file used on the ReadData FO . . . . . . . . . . . . . . . . . . . . . . . .224.1 Schematic of the models and connections used to simulate the pyrolysis furnace . . . . .234.2 Schematic of the coil model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .285.1 Temperature profile in the coil for different molecular mechanisms at a fixed COT . . . . .325.2 Conversion profile in the coil for different molecular mechanisms at a fixed COT . . . . . .325.3 Heat flux profile in the coil for different molecular mechanisms at a fixed COT . . . . . . .335.4 Temperature profile in the coil for different molecular mechanisms at a fixed conversion .345.5 Heat flux profile in the coil for different molecular mechanisms at a fixed conversion . . . .345.6 Temperature profile in the coil for different emissivity correlations . . . . . . . . . . . . . .365.7 Heat flux in the coil for different emissivity correlations . . . . . . . . . . . . . . . . . . . .365.8 Temperature profile in the coil for different emissivity correlations . . . . . . . . . . . . . .375.9 Heat flux in the coil for different emissivity correlations . . . . . . . . . . . . . . . . . . . .385.10 Comparison of the number of parameters and variables using the mechanism by Borsawith and without compressing the stoichiometric matrix . . . . . . . . . . . . . . . . . . . .395.11 Comparison of the run time of the initialisation procedure with and without compressingthe kinetic scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .395.12 Comparison of the run time of the simulation using a saved variable set with and withoutcompressing the kinetic scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .395.13 Comparison of the run time of a simulation with a 10% variation of the assigned COT fromthe saved variable set with and without compressing the kinetic scheme . . . . . . . . . .405.14 Temperature profiles using the geometry by Li and the radical mechanisms . . . . . . . .405.15 Conversion profiles using the geometry by Li et al. [16] and the radical mechanisms . . .415.16 Heat flux profiles using the geometry by Li et al. [16] and the radical mechanisms . . . . .415.17 Estimation of the heat capacity for EDC . . . . . . . . . . . . . . . . . . . . . . . . . . . .43xv

5.18 Estimation of the heat capacity for VCM . . . . . . . . . . . . . . . . . . . . . . . . . . . .435.19 Estimation of the heat capacity for HCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . .445.20 Influence of fuel flowrate on EDC conversion . . . . . . . . . . . . . . . . . . . . . . . . .445.21 Relation between EDC and COT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .455.22 Variation of selectivity with conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .455.23 Variation of specific fuel consumption with conversion . . . . . . . . . . . . . . . . . . . .465.24 Variation of conversion with feed flowrate . . . . . . . . . . . . . . . . . . . . . . . . . . .465.25 Variation of COT with feed flowrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .465.26 Variation of selectivity with feed flowrate . . . . . . . . . . . . . . . . . . . . . . . . . . . .475.27 Variation of specific fuel consumption with feed flowrate . . . . . . . . . . . . . . . . . . .475.28 Variation of conversion with CIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .485.29 Variation of selectivity with CIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .485.30 Variation of specific fuel consumption with CIT . . . . . . . . . . . . . . . . . . . . . . . .485.31 Influence of CCl4 and Cl2 on COT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .495.32 Influence of CCl4 and Cl2 on conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . .495.33 Influence of CCl4 and Cl2 on selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . .50A.1 List of species used on the mechanism by Schirmeister et al. [25] . . . . . . . . . . . . . .58A.2 List of species used on the mechanism by Schirmeister et al. [25] . . . . . . . . . . . . . .58A.3 List of reactions used on the mechanism by Borsa [2] . . . . . . . . . . . . . . . . . . . .59A.4 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .60A.5 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .61A.6 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .62A.7 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .63A.8 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .64A.9 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .65A.10 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .66A.11 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .67A.12 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .68A.13 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .69A.14 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .70A.15 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .71A.16 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .72A.17 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .73A.18 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .74A.19 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .75A.20 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .76A.21 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .77A.22 List of reactions used on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . . .78xvi

A.23 List of reactions species on the mechanism by Borsa [2] . . . . . . . . . . . . . . . . . . .79A.24 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .80A.25 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .81A.26 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .82A.27 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .83A.28 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .84A.29 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .85A.30 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .86A.31 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .87A.32 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .88A.33 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .89A.34 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .90A.35 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .91A.36 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .92A.37 List of reactions species on the mechanism by Borsa [2] (cont.) . . . . . . . . . . . . . . .93xvii

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NomenclatureGreek symbolsαAbsorptivity. HVariation in enthalpy. Hf298.15K,IG Standard enthalpy of formation of the ideal gas. SVariation in entropy. Emissivity.λThermal conductivity.( )G Correction factor to a water vapour–carbon dioxide mixture.νViscosity.σStefan–Boltzmann constant.Roman symbolsGS1 RTotal exchange area between gas ans sink in radiative equilibrium.AArea.CFraction to the total furnace area.cpHeat capacity.DDiameter.EaActivation energy.FFlowrate.HFluid’s heat flux.hHeat transfer coefficient.hSpecific enthalpy.kRate constant.xix

KcEquilibrium constant.LMean beam length, or coil length.NMass flux.nReaction order.pPressure.QHeat exchanged.RGas constant.rReaction rate, or radius.TTemperature.VVolume of the firebox.vVelocity.wMass fraction.zAxial direction of the coil.Subscripts1Sink.aAir.CConvection.fFuel, forward.GFlue iation.rReverse.xx

GlossaryCFD Computational Fluid Dynamics.CIT Coil inlet temperature.COT Coil outlet temperature.EDC Ethylene dichloride, or dichloroethane.FO Foreign object.HTC High temperature chlorination.LSKM Large scale kinetic mechanism.LTC Low temperature chlorination.NC Number of components.NR Number of reactions.PVC Ployvinyl chloride polymer.TMT Tube metal temperature.VCM Vinyl chloride monomer, or chloroethene.xxi

xxii

Chapter 1IntroductionVinyl chloride monomer (VCM) is currently, in addition to ethylene and NaOH, one of the most importantcommodity materials [7]. About 95% of VCM is used for the production of polyvinyl chloride (PVC)[7], which is currently the second most abundant plastic in the world, behind only polyethylene, with aworldwide production capacity in 2009 of 30 million tonnes a year [18].Currently, the main production process of VCM is the chlorination of ethylene to dichloroethane(also known as ethylene dichloride, or EDC), followed by its dehydrochlorination to VCM by thermalcracking. The dehydroclorination of EDC is currently performed by its pyrolysis in cracking furnacesat temperatures about 500-550 C [7]. This occurs via a first-order free radical mechanism [7], witha conversion of about 50-60% per pass. This is done in order to limit by–product formation, obtainingyields of about 95-99%.Despite the high yields, a small fraction of by-products are formed in this process which, due tothe large material through-put, create severe inefficiencies. A solid carbonaceous material, coke, is deposited inside the reactor coils which requires periodic shut-downs of the entire plant for its removal. Alsogas phase by-products such as chloroprene and butadiene cause downstream difficulties in distillationcolumns.Thus, the need arises for a model which can accurately predict by–product formation to allow for amodel based optimisation of the whole process.1.1MotivationThe EDC cracking process presents many difficulties in modelling, the main being the complexity ofmodelling a large scale radical mechanism, with currently over 800 reported equations by Borsa [2].Although these models have been implemented in sequential modelling, EDC cracking has never beenimplemented in an equation oriented process modelling tool such as gPROMS, eventually allowing thewhole plant optimisation.The main objective of this work is to build a model which can rigorously describe the EDC crackingprocess, dealing with the challenges of implementing a large radical kinetic scheme. For this, different1

kinetic mechanisms are tested, in order to analyse which one better fits the experimental data.1.2OutlineFirstly, a review of the literature present on this topic is shown in chapter 2. Afterwards, the main toolsused in the modelling of this process are present in chapter 3.In chapter 4 the main equations and models used to describe the EDC cracking furnace are presented.Chapter 5 describes the main results from the simulations.Finally, chapter 6 presents the main conclusions from this work as well as suggestions for futurework.2

Chapter 2BackgroundIn this chapter, a brief analysis on the market for polyvinyl chloride PVC, the main application for vinylchloride, is carried out. Afterwards, a description of the processes for VCM production is presented.Finally, the pyrolysis furnace is described, as well as the main mechanisms used to simulate EDCcracking, and the main models used for simulating a firebox.2.1The PVC marketPolyvinyl chloride (PVC) is currently the second most abundant plastic in the world, behind only polyethylene. Although PVC was first synthesised in 1830-1834, industrial production of this polymer startedonly in the 1930s, through the catalytic hydrogenation of acetylene. Currently most production of PVC isthrough VCM, in plants use a balanced process where ethylene is chlorinated through direct chlorinationand oxychlorination, the latter using the HCl produced in the thermal cracking of EDC to VCM.As mentioned in chapter 1, currently the great majority of vinyl chloride (over 95%) is used in theproduction of PVC.Currently, Asia and Europe are the leading regions in terms of PVC production capacities. However,in 2009 North America’s production was higher than Europe due to its higher utilisation level, as canbe seen in figure 2.1. China is clearly the largest producer of PVC, with over 7 million tonnes per year(corresponding to 26% of the market share worldwide). However, up to 81% of this production is notwith VCM produced by EDC cracking, but with the hydrochlorination of acetylene.PVC produced from the rest of Asia is mainly produced in Japan by Asahimas Chemical and Shin–Etsu, Taiwan by Formosa Plastics, and South Korea by LG Chemical Ltd.Regarding production in Europe, production of PVC is mainly from ethylene (chlorination to EDC andcracking to VCM accounts for 98% of Europe’s PVC market). PVC produced in Europe tends to integratethe full production process from chlorine to PVC, due to the difficulties of transporting chlorine.3

Figure 2.1: Utilisation of PVC production capacities in various regions, 2009 [18]2.2VCM production combined process [6], [7]VCM production through ethylene is currently a balanced process, meaning all by–products are recycledin a way which ensure a closure of the material balance having only VCM as the final product, startingfrom ethylene, chlorine and oxygen. This is done through three main units:1. Direct chlorination of ethylene to EDC:0C2 H4 Cl2 C2 H4 Cl2 (EDC) HR 71kJ/mol(2.1)2. Oxychlorination of ethylene to EDC:C2 H4 2 HCl 120O2 EDC H2 O HR 238kJ/mol(2.2)3. Cracking of EDC to produce VCM:0C2 H4 Cl2 (EDC) C2 H3 Cl (VCM) HCl HR 218kJ/mol(2.3)The balanced process can therefore be described by the overall equation:C2 H 4 12Cl2 14O2 VCM 12H2 O(2.4)A schematic representation of this process can be found in Figure 2.2.The direct chlorination of ethylene to EDC is an exothermic reaction, which is most commonly performed in the liquid phase of ethylene dichloride for better temperature control. A Lewis catalyst isemployed, ordinarily iron (III) chloride. The chlorination can either be at low (LTC) or high temperatures4

Figure 2.2: The vinyl chloride production process [14](HTC).In the LTC process, ethylene and chlorine react dissolved in EDC, which acts as a solvent, at temperatures below the boiling point, around 50–70 C. This enables a higher selectivity (over 99 %), however,steam is needed for the rectification of EDC, thus rejecting the heat of reaction.The HTC process is c

Vinyl chloride (VCM) is one of the most important commodity materials, being the main raw material in the production of polyvinyl chloride (PVC), and is mainly produced through the thermal cracking of . 5.3 Simulation results using the geometry in Li et al. [16] and assigning a conversion of 55%.34 .

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