Home >Documents >Zeolite Deactivation During Hydrocarbon Reactions ... · PDF file ZEOLITE DEACTIVATION DURING...

Zeolite Deactivation During Hydrocarbon Reactions ... · PDF file ZEOLITE DEACTIVATION DURING...

Date post:29-Feb-2020
Category:
View:16 times
Download:3 times
Share this document with a friend
Transcript:
  • ZEOLITE DEACTIVATION DURING

    HYDROCARBON REACTIONS:

    CHARACTERISATION OF COKE

    PRECURSORS AND ACIDITY, PRODUCT

    DISTRIBUTION

    BAODONG WANG

    A Thesis submitted for the degree of Doctor of Philosophy of the University College London

    Department of Chemical Engineering

    University College London

    London, WC1E 7JE

    December 2007

  • 1

    ABSTRACT

    The catalytic conversion of hydrocarbons over zeolites has been applied in large

    scale petroleum-refining processes. However, there is always formation and

    retention of heavy by-products, called coke, which causes catalyst deactivation.

    This deactivation is due to the poisoning of the acid sites and/or pore blockage.

    The formation of coke on hydrocarbon processing catalysts is of considerable

    technological and economic importance and a great deal of work has been carried

    out to this study.

    The main aim of this work is to understand the deactivation of zeolite catalysts as

    a result of coke deposition. The deactivation by coke of USHY zeolite was

    investigated during catalytic conversion of hydrocarbons – 1-pentene, n-heptane

    and ethylbenzene – as representatives of olefins, paraffins and aromatics

    respectively, at different reaction temperatures, time-on-streams and composition.

    Three novel techniques, coke classification, thermogravimetric method for

    characterising coke precursors and indirect temperature programmed desorption

    (TPD) for catalyst acid sites characterisation were developed to further study

    catalyst deactivation mechanism. Product distribution, coke formation,

    characterisation of coke precursors, as well as the role of strong acid sites on

    hydrocarbon reactions are presented and discussed.

    During catalytic reactions of 1-pentene over USHY zeolite, cracking and hydride

    transfer were the predominant reactions in initial stage which deactivated rapidly

  • 2

    allowing isomerisation to become the main reaction afterwards. Deactivation

    studies showed that coke formation was very strong initially which is in good

    correlation with the initial rapid deactivation. The hydrogen freed during this

    initial time from the formation of high C/H ratio coke components contributed to

    the formation of hydride transfer products. The amount of coke precursors

    decrease with increasing reaction temperature due to the higher desorption of coke

    precursors into gas phase while hard coke amount increased with temperature as

    expected from an activated process. The coke amount formed was not

    proportional to the reactant feed composition, because of a strong pseudo-zeroth-

    order initial coking on strong acidic sites. The thermogravimetric method provides

    insight into the chemical character of coke precursor components in terms of the

    mode of their removal and allows further classification of coke precursors into

    small and large coke precursors. The concentration and strength of acid sites of

    coked catalysts were studied by the TPD methodology. Besides, characterisation

    of coke precursors was also revealed. The initial deactivation preferentially on

    strong acid sites is very fast. The concentration of free acid sites is inversely

    correlated well with the total concentration of coke rather than individual coke

    groups. Coke precursors tend to be more stable at higher reaction temperatures.

    Furthermore, by selectively poisoning strong acid sites of USHY zeolite, it shows

    conclusively that strong acid sites are responsible for cracking and hydride

    transfer reactions as well as strong coke formation while weak acid sites can only

    catalyse double bond isomerisation.

  • 3

    ACKNOWLEDGEMENTS

    I am extremely grateful to my supervisor Dr. George Manos for his guidance, his

    continual support and encouragement. I have benefited from his many words of

    wisdom, his high standard of accomplishment and for instilling in me his high

    ethical standards.

    I would like to thank my colleagues, Panos, Seyed and Nnamso for all their help

    and encouragement. I especially thank Dr Enhong Cao for his assistance of GC set

    up.

    I am greatly appreciative of my parents for their unwavering love and support

    during my PhD.

    The financial support of the Overseas Research Students Awards Scheme and K.

    C. Wong Scholarship is gratefully acknowledged. The conference grants funded

    by UCL graduate school and the Royal Academy of Engineering are appreciated.

  • 4

    ABSTRACT........................................................................................................... 1

    ACKNOWLEDGEMENTS.................................................................................. 3

    LIST OF FIGURES .............................................................................................. 8

    LIST OF TABLES .............................................................................................. 14

    1 INTRODUCTION....................................................................................... 15

    2 LITERATURE SURVEY........................................................................... 20

    2.1 ZEOLITES ............................................................................................ 20

    2.1.1 History of Zeolites ....................................................................................... 21

    2.1.2 Zeolite Composition and Structure ............................................................. 24

    2.1.3 Aluminum Content and Acidity ................................................................... 28

    2.1.4 Zeolites X and Y (Faujasites) ...................................................................... 31

    2.2 ADSORPTION AND DIFFUSION ...................................................... 33

    2.3 SHAPE SELECTIVITY........................................................................ 38

    2.4 HYDROCARBON REACTIONS OVER SOLID ACIDIC

    CATALYSTS.................................................................................................... 40

    2.4.1 Foundations of Catalytic Cracking ............................................................. 41

    2.4.2 Cracking of Alkenes .................................................................................... 43

    2.4.3 Cracking of Alkanes .................................................................................... 50

    2.4.4 Cracking of Alkylbenzenes .......................................................................... 58

    2.5 COKING AND DEACTIVITION ........................................................ 59

    2.5.1 Coke Characterisation ................................................................................ 60

    2.5.2 Effects on Coking ........................................................................................ 62

    2.5.2.1 Pore Structure Effect .......................................................................... 62

    2.5.2.2 Active Sites Effect ............................................................................... 64

  • 5

    2.5.2.3 Operating Condition Effect ................................................................ 65

    2.5.2.4 Nature of the Feed Effect.................................................................... 68

    2.5.3 Modes of Deactivation ................................................................................ 68

    2.5.3.1 Active Sites Poisoning ........................................................................ 69

    2.5.3.2 Pore Blockage .................................................................................... 71

    3 EXPERIMENTAL WORK........................................................................ 72

    3.1 EQUIPMENT........................................................................................ 72

    3.1.1 Reactor ........................................................................................................ 72

    3.1.2 Saturator ..................................................................................................... 74

    3.1.3 Ten-way Sampling Valve............................................................................. 77

    3.1.4 Gas Chromatograph.................................................................................... 78

    3.1.5 Thermogravimetric Analysis ....................................................................... 82

    3.1.6 Temperature Programmed Desorption ....................................................... 82

    3.2 EQUIPMENT PROCEDURES............................................................. 85

    3.3 CATALYST PREPARATION ............................................................. 87

    3.4 CALCULATIONS ................................................................................ 88

    3.4.1 Components Mole Fraction Calculation..................................................... 88

    3.4.2 Conversion .................................................................................................. 88

    3.4.3 Novel Methods for Coke Characterisation.................................................. 90

    3.4.3.1 Coke Classification............................................................................. 90

    3.4.3.2 Coke Precursors Characterisation...........

Click here to load reader

Reader Image
Embed Size (px)
Recommended