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Design and Application of a Catalytic Distillation Column

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Design and Application of a Catalytic Distillation Column CATALYTIC DISTILLATION DESIGN AND APPLICATION OF A CATALYTIC DISTILLATION COLUMN by JOSIAS JAKOBUS (JAKO) NIEUWOUDT Thesis presented in partial ful…lment of the requirement for the Degree of MASTER OF SCIENCE IN ENGINEERING (CHEMICAL ENGINEERING) in the Department of Process Engineering at the UNIVERSITY OF STELLENBOSCH in co-operation with the UNIVERSITY OF CAPE TOWN CATALYSIS RESEARCH UNIT in the Department of Chemical Engineering SUPERVISORS University of Stellenbosch Dr. L.H. CALLANAN University of Cape Town A/Prof. K.P. MöLLER GRADUATION DATE DECEMBER 2005 i DECLARATION I, the undersigned, hereby declare that the work contained in this thesis is my own original work and that I have not previously in its entirety or in part submitted it at any university for a degree. __________________ 16th May 2005 J.J. Nieuwoudt Date ii iii SUMMARY Catalytic Distillation (CD) is a hybrid technology that utilizes the dynamics of si- multaneous reaction and separation in a single process unit to achieve a more compact, economical, e¢ cient and optimized process design when compared to the traditional multi-unit designs. The project goal (and key question) is (how) to design a cost-e¤ective, simple and accurate laboratory-scale continuous CD system that will su¢ ciently and accurately supply useful data for model validation. The system to be investigated is the continuous hydrogenation of an -ole…n C6 (1-hexene) feed stream to the corresponding alkane (n-hexane) product with simultaneous reactant/product separation. Hypothetically, a system can be constructured to determine whether hydrogenation will bene…t from the heat and mass transfer integration observed under CD conditions in terms of energy usage, temperature control and the catalyst’ssurface hydrogen concentration. System convergence with commercial distillation simulation packages were not at- tained and a dynamic design approach was followed based on the converged non-reactive solution. A simpli…ed McCabe-Thiele approach approximates the system’sbehavioural operational trends. The continuous computer control, monitoring, logging and emer- gency shutdown system is routed through LabVIEW 7.1 Express. Software and/or electronics is used to maintain constant feed ‡ow rates, reboiler heat duty, level and re‡ux ratio set-points and to generate transient temperature pro…les. A modi…ed push- pull system achieves pressure control and liquid compositional sampling is currently manual. It is concluded that the modularly commissioned system can supply much needed transiently monitored mass balances, energy balances and concentrations as required for non-equilibrium (NEQ) CD computer model validation. Its design addresses separation of close-boiling components, the system’s laboratory-scale size and issues introduced by a non-condensable gas. Hydrodynamic e¤ects must still be considered. There is additional capacity for on-line concentration measurements and for a side reactor. The designed system is a powerful, ‡exible tool to experimentally explore the potential of CD. iv v OPSOMMING Katalitiese distillasie (KD) gebruik gelyktydige reaksie-skeidingsdinamika in ’nenkele proseseenheid om ’nmerkbaar meer kompakte, ekonomiese, energie e¤ektiewe en prak- tiese sisteem daar te stel vergeleke met tradisionele multi-eenheid ontwerpsbenaderings. Die projekdoel (en gepaardgaande sleutelvraag) is (hoe) om ’n ekonomiese, een- voudige en akkurate laboratorium-grootte kontinue KD sisteem te ontwerp wat vol- doende en akkurate data sal verskaf vir die geldigheidstoetsing van sisteemmodelle d.m.v. gevallestudies. Die sisteem wat ondersoek sal word behels die kontinue hidrogenering van ’n C6 -ole…ene (1-hekseen) voerstroom na die ooreenstemmende alkaanproduk- stroom (n-heksaan) terwyl reagens/produk skeiding gelyktydig plaasvind. Hipoteties gesproke kan ’n sisteeem gebou word wat kan toest of hidrogenering in terme van en- ergieverbruik, temperatuurbeheer en waterstof katalis-oppervlakkonsentrasie baatvind by die hitte- en massaoordrag integrasie wat onder KD toestande waargeneem word. Sisteemkonvergensie is nie bereik met ’nkommersiële distillasie simulasie pakket nie en ’ndinamiese ontwerpsbenadering, gebaseer op die konvergente oplossing vir die nie- reaktiewe sisteeem, is dus gevolg. ’nVereenvoudigde McCabe-Thiele model poog om die sisteem se operasionele neigings te antisipeer. Die kontinue rekenaarbeheer, -monitering, -leggering en -noodstop sisteem werk deur LabVIEW 7.1 Express. Sagteware en/of elektronika volhou ’nkonstante vloeitempo, opkokerdrywing, vlak en re‡uksverhouding setpunte and genereer ook tydafhanklike temperatuurpro…ele. ’n Trek-druk sisteem word vir drukbeheer toegepas en vloeistof komposisionele toetsing word met die hand gedoen. Die uiteindelike gevoltrekking is dat die deelsgewys inbedryfgestelde sisteem nodige, bruikbare tyd-afhanklik gemoniteerde massa-, energiebalanse en konsentrasies vir mod- elveri…ëring van nie-ekwilibrium KD rekenaarmodelle kan verskaf. Ontwerpskwessies met betrekking tot naby-kokende komponente, die klein skaal van die sisteem en die aanwesigheid van ’nsuperkritiese gas word aangespreek. Hidrodinamiese e¤ekte moet steeds oorweeg word. Daar is addisionele kapasiteit vir aanlyn konsentrasiemetings en die toevoeging van ’n kantreaktor. Die ontwerpte sisteem is dus ’n kragtige, buigbare instrument om die potensiaal van KD eksperimenteel te ondersoek. vi [Watterson, 1993] May this NOT be true of this thesis! vii ACKNOWLEDGEMENTS The …rst two words of thanks must go to my supervisors Dr. Linda H. Callanan (University of Stellenbosch) and A/Prof. Klaus P. Möller (University of Cape Town, UCT) who helped and guided me through this project from grass roots to commissioning. I also gratefully acknowledge the additional classes, support, information and fa- cilities made available to me by the Catalysis Research Unit of the Chemical Engineering Department at the University of Cape Town where the system was constructed and where it now stands. Such collaboration between the Chemical Engineering Departments of the Universities of Stellenbosch and Cape Town is to be applauded and encouraged. The timely completion of the project would also not have been possible without the necessary support services. In this regard Marc Wüst (UCT catalysis technical o¢ cer), Bill Randall and Granville de la Cruz (UCT electronic workshop) and Jannie Barnard and Anton Cordier at the Stellenbosch mechanical workshop deserve special mention. The UCT mechanical workshop must also be included for the signi…cant amount of hours that they put into the system. Last, but certainly not least, a word of gratitude must also be given to Sasol who made this project possible with a generous post-graduate sponsorship. viii Contents Contents ix Nomenclature xv List of Figures xix List of Tables xxi I LITERATURE REVIEW 1 1 INTRODUCTION 3 1.1 History and Background . 3 1.2 Advantages and criteria . 4 1.3 Existing and possible applications . 6 1.3.1 TheMTBEprocess.......................... 6 1.3.2 Selective hydrogenation . 7 1.4 Project goals, hypothesis, key questions and de…nitions . 8 2 REACTION KINETICS AND PROCESS CONDITIONS 11 2.1 Mechanism .................................. 11 2.2 Reactionkinetics ............................... 13 2.3 Processconditions .............................. 15 2.3.1 Possible range for process conditions . 15 3 MODELLING THEORY 19 3.1 Residuecurvemaps ............................. 20 3.2 Equilibrium (EQ) models . 22 3.3 Non-equilibrium (NEQ) models . 23 3.3.1 MESH equations for the non-reactive distillation section . 24 3.3.2 MESH equations for the reactive section . 26 3.4 Non-condensable gases . 27 ix x CONTENTS 4 MASS TRANSFER EQUATIONS 29 4.1 Phase hydrodynamics . 29 4.2 Fick’sLaw................................... 30 4.2.1 Filmtheory .............................. 32 4.2.2 Two-…lm theory . 33 4.3 The Stefan-Maxwell approach to mass transfer . 34 4.3.1 Interphase mass transfer . 35 4.3.2 Mass transfer within pores: the dusty ‡uid model . 36 4.4 Modellingtools ................................ 38 5 LITERATURE REVIEW: CONCLUDING REMARKS 39 II PROCESS DESIGN 41 6 PROCESS DESIGN METHODOLOGY 43 7 EXISTING SYSTEM DESCRIPTION 45 7.1 Reaction investigated: Hydrogenation . 45 7.2 Experimental apparatus . 46 7.3 Previous column internals . 47 7.3.1 Catalyst speci…cations and reactive packing . 47 7.3.2 Non-reactive packing . 48 8 MASS AND ENERGY BALANCES 51 8.1 Simulation using distillation packages . 51 8.1.1 Non-reactive distillation . 51 8.1.2 Reactive distillation: adding a reaction . 51 8.2 Spreadsheet calculations and ProII veri…cations . 53 8.2.1 Revised design approach . 53 8.2.2 Excel spreadsheet . 53 8.3 McCabe-Thiele system trends . 54 9 MECHANICAL DESIGN 61 9.1 Materials of construction (MOC) . 61 9.2 Pipesystem .................................. 63 9.3 Maincolumnbody .............................. 63 9.4 Column internals and liquid distribution . 63 9.4.1 Column internals (non-reactive sections) . 64 9.4.2 Column internals (reactive section) . 64 9.4.3 Liquid distribution and redistribution . 66 CONTENTS xi 9.5 Reboiler .................................... 67 9.6 Coolingloops ................................. 69 9.6.1 Two-stage partial condenser . 69 9.6.2 Bottomscooler ............................ 71 10 PROCESS CONTROL 73 10.1 Isobaric pressure control . 73 10.2 Gaseous and liquid feeds . 74 10.3 Re‡ux line con…guration . 76 10.3.1 Levelcontrol ............................. 78 10.3.2 Re‡ux ratio (R = V_L=V_D)...................... 81 10.4Reboiler ...................................
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