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REACTORS AND CATALYSTS

Reactors and Catalysts

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  • REACTORS AND

    CATALYSTS

  • Reactors

    Where a reaction takes place

    Ideal reactors:Batch reactor

    Continuous reactors CSTR

    PFR & PBR

    Industrial reactors: Liquid phase reactions

    Gas phase reactions

  • Liquid phase reactions

    CSTR, semi-batch/ batch reactor, slurry reactor

    Semi-batch reactor:

    Good temperature control by regulating feed rate

    Capability of minimizing unwanted side reactions (through maintaining low concentration of one of the reactants)

    Two-phase reactions like gas is bubbled through liquid

    CSTR:

    Intense agitation

    Good temperature distribution because it is well agitated

    Conversion of reactant per volume is very small; requires large volume reactors (disadvantage)

    Cascade of CSTRs provide high conversion

    Most of homogeneous liquid phase flow reactors are CSTRs

    Eg. Manufacture of Nitrobenzene from Benzene requires a cascade of CSTRs

  • CSTR

  • Gas phase reactions

    Tubular reactors (PFR, PBR)

    Tubular reactors

    Easy to maintain (no moving parts)

    High conversion per reactor volume (in PFR) / per catalysts weight (in PBR)

    Difficult to control temperature within the reactor, hot spots for exothermic reaction

    Most of homogeneous Gas phase flow reactors are PFRs

    Fixed bed reactor (PBR) packed with solid catalysts

  • Common catalytic reactors Fluidized bed reactors

    Heterogeneous reactions

    Analogous to the CSTR (well mixed, so good temperature distribution)

    Handle large amounts of feed and solids

    Good temperature control

    Temperature is uniformly throughout (no hot spots)

    Ease of catalyst replacement or catalyst regeneration (by sending catalyst to nearby regenerating equipment)

  • Circulating fluidized bed reactor for FTS

  • Fixed bed reactors

    Plug flow for gases

    Hot spots in exothermic reactions (can ruin the catalysts)

    Eg: Hydrodemethylation of toluene to produce benzne

    Plugging if small catalyst particles are used which create pressure drop

    Staged adiabatic packed bed reactor (proper interchange of heat and proper gas flow

    Staged packed bed with intercooling

  • Fixed bed reactor for FTS

  • Slurry reactor

    Multiphase reactor (reaction between gas and liquid takes place on a solid catalyst)

    Catalyst is suspended in the liquid and gas is bubbled through the liquid

    Ideal situations: liquid phase is well mixed, catalysts are uniformly distributed, gas phase is in plug flow

    Liquid phase may be a reactant (hydrogenation of methyl linoleate) or inert (FTS)

    Liquid phase act as a sink for exothermic reaction

    Good temperature control

    Heat recovery is possible

    Constant overall catalytic activity maintained easily by addition of small amount of catalyst

    Useful for catalysts that can't be pelletized

    Large heat capacity of reactor acts as a safety feature against explosions

    Disadvantages

    Reactor may plug up

    Uncertainties in design process

    Finding suitable liquids may be difficult

    Higher ratio of liquid to catalyst than in other reactors

  • http://encyclopedia.che.engin.umich.edu/Pages/Reactors/Slurry/Slurry.html

  • Trickle bed reactor

    Multiphase reactor

    Gas and liquid flow co-currently on a packed bed of catalyst particles

  • Catalysis

    Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst.

    A catalyst is not consumed by the reaction itself A catalyst may participate in multiple chemical

    transformations

    Catalysts that speed the reaction are called positive catalysts and that slow a reaction is called negative catalysts

    Though the catalysts speed up a reaction, it never determines the equilibrium or endpoint of a reaction. This is governed by thermodynamics alone

    According to transition theory, the catalysts reduces the potential energy barrier over which the reactants must pass to form products

  • Action of solid catalysts: the reactant molecules are

    changed, energized or affected to form intermediates in the

    regions close to the catalyst surface

  • Typical catalysts materials

    The chemical nature of catalysts is as diverse as catalysis itself

    Proton acids are probably the most widely used catalysts, especially for the many reactions involving water, including hydrolysis and its reverse

    Multifunctional solids often are catalytically active, e.g.zeolite, alumina, higher-order oxides, graphitic carbon, nanoparticles,

    Transition metals are often used to catalyze redox reactions (oxidation, hydrogenation). Examples are Ni, Co, V.

    Many catalytic processes, especially those used in organic synthesis,requires noble metals such as Pt, Pd, Rh, Ru, Au

  • Typical petrochemical catalysts

    Supported noble metals: Pt, Pd, Rh, Ru, Re, Pt-Re

    Supported transition metals: Ni, Co, Fe, Cu, Mo

    Catalyst supports: Al2O3, SiO2, TiO2, Activated Carbon, zeolites,

    Raney type metal catalyst: Ni, Cu-Ni

    Oxide catalysts:Cr2O3, Fe2O3, Al2O3-Cr2O3, Fe2O3-K2CO3-Cr2O3, Ca3Ni(PO4)3,Bi2O3MoO3

    Sulfides catalysts: MoS2/Al2O3, WS2/Al2O3, NiS/Al2O3, CoS/Al2O3

    Micro- and mesoporous materials

  • Catalyst classification

    Metal catalyst on supported systems

    Molecular sieve catalyst

  • Preparation of metals on support

    For the effective utilization of the metal

    The principal catalyst-preparation technique involves two stages. First, rendering a metal-salt component into a finely divided form on a

    support (dispersion) and secondly; conversion of the supported metal

    salt to a metallic or oxide state (thermal treatment)

    Dispersion techniques may be impregnation, adsorption from solution, co-precipation, or deposition

    Thermal treatment may be calcination (inert atmosphere) or reduction (active atmosphere)

  • Impregnation is achieved by filling the pores of a support with asolution of the metal salt from which the solvent is subsequently

    evaporated. The catalyst is prepared either by spraying the support with a

    solution of the metal compound or by adding the support material to a

    solution of a suitable metal salt, such that the required weight of the

    active component is incorporated into the support without the use of

    excess of solution. This is then followed by drying and subsequent

    decomposition of the salt at an elevated temperature. This technique has

    been widely used for the preparation of small amounts of catalyst for

    basic studies.

  • Adsorption is defined as the selective removal of metal salts or metalion species from their solution by a process of either physisorption or

    chemical bonding with active sites on the support. Depending upon the

    strength of adsorption of the adsorbing species, the concentration of the

    active material through the catalyst particle may be varied and

    controlled. This technique is widely used in the preparation of industrial

    catalysts as it permits a greater degree of control over the dispersion and

    distribution of the active species on the support.

  • Co-precipitation: The preparation of supported catalysts by the co-precipitation of metal ions with the support ions usually produces an

    intimate mixing of catalysts and support. An example of this technique is

    the co-precipitation of metal ions with aluminium ions to produce a

    precipitated alumina gel containing the metal hydroxide. This precipitate

    when calcined produces a refractory support with active component

    dispersed throughout the bulk as well as at the surface.

  • Chemical Vapour Deposition (CVD): It is the vapour plating of thesupport with a volatile inorganic or organometallic compound. The

    process requires only a moderate vacuum and is currently one of the

    methods under research in industry as a means of preparing catalysts

    with a purely surface deposition.

  • Thermal treatment:

    Calcination :

    To get metal oxides as catalyst

    In the presence of inert gases such as nitrogen, helium

    Reduction :

    To get metal as catalyst

    In the presence of reducing gases such as Hydrogen

  • Multifunctional solids/Porous

    solids/Molecular sieve catalysts

    Porous solids with pores of the size of molecular dimensions 0.3 to 2 nm

    Eg: zeolites (crystalline), carbon (amorphous), glasses, oxides, aluminosilicates

    Nowadays mesoporous materials (2 to 50 nm pore size) also use as catalysts

  • Zeolite catalysts

    Crystalline and uniform pore size

    Most commercial molecular sieve catalyst

    The high concentration of ionic hydrogen atoms (H+) attached to oxygen atom framework is another key feature for zeolite catalyst

    Different types of zeolites - named according to the framework eg: ZSM-5 medium pore size (0.45 to 0.6 nm dia) formed by ten ring, zeolite X, Y

    large pores (~ 0.8 nm) by 12 ring

  • ZSM-5 catalysts

    ZSM-5 has some unique features for its catalyst activity towards

    cracking and aromatization

    Pore structure

    Well defined three dimensional intersecting channel system, medium

    pore size, and high diffusivity for hydrocarbons

    Acidity

    strong acid sites, the easiness for their availability (acid sites lie on the

    intercrystalline surface), high silica-alumina ratio (Si/Al 10 to 100)

    Crystal structure of zeolite ZSM-5 (a)

    building unit, (b) chain, (c) sheets, (d) three

    dimensional channel structure

  • Catalyst deactivation Catalyst loose its activity due to:

    Sintering or crystal growth of the active material

    Fouling of the active surface with involatile reaction by-products

    Poisoning of the active surface by feed impurities

    Blockage of the support pore structure

    Sintering (aging)

    Structural modification

    Loss of catalyst activity or loss of active surface area

    Resulted from the prolonged exposure to high temperature

    Eg. Reforming of heptane over Pt/Al2O3 Catalyst deactivation due to sintering

  • Fouling

    Coke deposition on the surface of the catalyst

    Common for reactions involving hydrocarbons

    Coking can be reduced by running the reaction at elevated pressure and hydrogen rich streams

    Usually regenerated by burning off the carbon

  • Poisoning

    Poisoning molecules irreversibly chemisorbed to active sites, reducing the number of active sites available for reaction

    Poisoning molecule may be reactant, product or any other impurity in the feed

    Blockage

    Molecules having size larger than the pore diameter block the entry of smaller molecules into the pores

    Larger molecules may be reactant or product

    Eg: formation of PAH inside the pores of ZSM-5 during aromatization reactions