chemical reactors

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Chemical Reactors and their Applications

Norges teknisk-naturvitenskapelige universitet

Chemical Reactorsand their

Applications

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Outline

– Reactor concepts

– Natural gas reforming concepts

– Downstream processes

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Outline




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Reactor Concepts

– Fixed bed reactors

– Fluidized bed reactors

– Stirred tank reactors

– Slurry loop reactors

– Bubble columns

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Reactor Concepts





– Bubble columns

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Fixed Bed ReactorsConcept– Collection of fixed solid

particles.– The particles may serve as a

catalyst or an adsorbent.– Continuous gas flow– (Trickling liquid)

Applications– Synthesis gas production

– Methanol synthesis

– Ammonia synthesis

– Fischer-Tropsch synthesis

– Gas cleaning (adsorption)

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Fixed Bed Reactors

Challenges/Limitations

– Temperature control

– Pressure drop

– Catalyst deactivation

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Fixed Bed Reactors



– Pressure drop


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Fixed Bed Reactors

Temperature control

– Endothermic reactions may die out

– Exothermic reactions may damage the reactor

– Selectivity control

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Fixed Bed Reactors

Single-Bed Reactor– All the particles are located

in a single vessel

Advantages/Disadvantages– Easy to construct

– Inexpensive

– Applicable when the reactions are not very exo-/endothermic

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Fixed Bed Reactors

Multi-Bed Reactor– Several serial beds with

intermediate cooling/heating stages

Advantages/Disadvantages– Applicable for exo-/endothermic

reactions

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Fixed Bed ReactorsSO3 reactor

NH3 reactor

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Fixed Bed Reactors

Multi-Tube Reactor– Several tubes of small

diameter filled with particles.

Advantages/Disadvantages– Expensive

– High surface area for heat exchange Very good very temperature control

– Applicable for very exo-/endothermic reactions

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Fixed Bed ReactorsSteam reformer

Reactor height: 30 m

Number of tubes: 40-10000

Tube length: 6-12 m

Tube diameter: 70-160 mm

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Fixed Bed Reactors



– Pressure drop


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Fixed Bed Reactors

Pressure drop– Friction between the gas and particle phase results in a

pressure drop.

– High pressure drop high compression costs

– Some systems have low tolerance for pressure drop.

– The pressure drop is mainly dependent on reactor length, particle diameter, void fraction and gas velocity.

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Fixed Bed Reactors

Large particles has to be used (dp>1mm).

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Fixed Bed ReactorsPorous catalyst particle– The particles are porous to increase

the surface area of the catalyst.– Reactants are transported inside

the pores by means of molecular diffusion and adsorb to the active sites where the reaction occurs.

– Products desorb and diffuse back to the bulk.

– Heat is transported by conduction.

Intra-particle diffusion/conductionmay be rate determining for largeparticles ( egg-shell particles).

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Fixed Bed Reactors



– Pressure drop


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Fixed Bed Reactors

Catalyst deactivation– The catalyst gets deactivated if the active sites get

contaminated.

– Sulfur compounds deactivate Ni-catalysts– Desulfurization is often necessary prior to reforming.

– Formation of carbon deposits deactivate the catalysts.– Large carbon deposits may clog the tubes, causing hot-

spots that damage the reactor.

– Catalyst regeneration is necessary.

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Fixed Bed Reactors

Summary Advantages/Disadvantages

– High conversion is possible

– Large temperature gradients may occur

– Inefficient heat-exchange

– Suitable for slow- or non-deactivating processes

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Reactor Concepts





– Bubble columns

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Fluidized Bed Reactors

Concept– Collection of solid particles dispersed

in a continuous phase.– The particles may serve as a

catalyst, adsorbent or a heat carrier.– Continuous flow of gas or liquid

Applications– Catalytic cracking processes– Fischer-Tropsch synthesis– Polymerization– Waste combustion– Drying

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A fluidized bed exhibits liquidlike behavior

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Continuous regeneration

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– Conversion may be poor if gas is bypassing.

– Erosion of vessel and pipe lines.

– Uniform temperature

– Efficient heat-exchange

– Can handle rapid deactivating processes.

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Reactor Concepts





– Bubble columns

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Stirred tank ReactorsConcept– Forced mixing by use of impeller.– Applied in reactive systems when

mixing is the rate determining step.– Single phase: liquid mixing.– Two phases: liquid/gas, liquid/particle– Three phases: liquid/particle/gas

Typical applications– Chemical component and phase

mixing– Fermentation reactor– Food and paper industry– Natural gas

conversion/polymerization

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Stirred tank ReactorsThe mixing is influenced by:

– stirring rate and pumping capacity

– liquid height

– baffle design– (baffles reduces solid body rotation)

– size and geometry of the tank

– size and geometry of heat equipment

– size and type of impeller

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Stirred tank ReactorsImpellers– Radial flow impellers are suitable

for dispersion of gas in liquid.

– Axial flow impellers are suitable to blend liquids and suspend solids in liquids.

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Stirred tank Reactors



– Efficient heat-exchange– Exception: slurries with high concentrations of large particles

(difficult mixing).

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Reactor Concepts





– Bubble columns

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Slurry loop ReactorsConcept– Collection of solid catalyst particles

dispersed in a liquid phase (slurry).– The slurry is circulating at a high

velocity impelled by an axial pump.– The mixing pattern is very intensive

and well defined.

Typical application– Polymerization

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Slurry loop Reactors



– Very efficient heat-exchange

– Can operate at high polymer concentrations

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Reactor Concepts





– Bubble columns

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Bubble ColumnsConcept– Gas dispersed in a continuous

liquid phase.– Two phases: liquid/gas.– Three phases: slurry/gas

Typical applications– Natural gas conversion

– Waste water treatment

– Bio-processes

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Bubble Columns

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Bubble Columns

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Bubble Columns

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Bubble Columns


– Non-uniform product if bubble size distribution is heterogeneous


– Efficient heat-exchange

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Outline




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Natural gas

– Vital component of the world's supply of energy (approx. 20%).

– Fuel

– Most common feedstock for hydrogen production or synthesis gas production.

– Production of base chemicals (methanol, ammonia)

Typical composition

CH4 70-90%

C2H6-C4H10 0-20%

CO2 0-8%

N2 0-5%

H2S 0-5%

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Natural gas reforming concepts

– Steam reforming (SR)

– Partial oxidation (POX)

– Autothermal reforming (ATR)

– New reforming concepts

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Steam reforming

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Steam reforming

Primary reformerCH4 + H2O CO + 3H2 ΔHr=206 kJ/mol

CO + H2O CO2 + H2 ΔHr= -41 kJ/mol (Water gas shift)

– Overall heat of reaction is endothermic multi-tube reformer

– Reactions are catalyzed over Ni-catalyst. Temperature 1100-1200K

Pressure 15-30 bar

H2/CO >3

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Steam reformingBurner configurations

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Steam reforming

Better temperature control with side fired burners

Catalyst deactivates

Retaining productivity by increasing temperature

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Steam reforming

Carbon formation2CO C + CO2 ΔHr= -173 kJ/mol (The Boudouard reaction)

CH4 C + 2H2 ΔHr= 75 kJ/mol (Decomposition of methane)

CO + H2 C + H2O ΔHr= -132 kJ/mol (Heterogeneous water gas reaction)

– Carbon deposits deactivates the catalyst.

Actions to reduce carbon formation– High steam/carbon (S/C) ratio reduces carbon formation.

– Expensive to produce steam.

– Addition of CO2 reduces carbon formation

– Pre-reformer if higher hydrocarbons are present– Common S/C-ratio is 2.5–4.5

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Steam reforming

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Steam reforming

Adiabatic Pre-reformer

CnHm + nH2O → nCO + (n+m/2)H2 ΔHr>0

CO + 3H2 CH4 + H2O ΔHr= -206 kJ/mol

– Overall heat of reaction is exothermic or thermoneutral.

– Reactions are catalyzed over Ni-catalyst.

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Steam reforming

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Steam reforming

Hydro-desulfurizer (HDS)– Sulfur compounds are present in practically all gas feedstocks.– Ni-catalysts are poisoned by sulfur compounds desulfurization

– Cyclic organic sulfur compounds are hydrogenated to H2S over Co-Mo or Ni-Mo catalysts.

– H2S and other sulfur species are adsorbed over a bed of zinc-oxide.

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Steam reforming

Advantages/Disadvantages– No need for expensive oxygen plant.

– Material limitations on temperature limited conversion.

– High H2/CO ratio, suitable for hydrogen production with CO2 capture, not for methanol- or FT-synthesis.

– Carbon formation

– Steam corrosion problems.

– Costs in handling excess H2O.

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Partial oxidation

CH4 + ½O2 → CO + 2H2 ΔHr= -36 kJ/mol

CH4 + 2O2 → CO2 + 2H2O ΔHr= -803 kJ/mol

CO + ½O2 → CO2 ΔHr= -284 kJ/mol

H2 + ½O2 → H2O ΔHr= -242 kJ/mol

– Overall heat of reaction is slightly exothermic.

– No catalyst (burners)

Temperature 1600-1900K

Pressure →150 bar

H2/CO <2

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Catalytic partial oxidationReactions are catalyzed to:– improve selectivities

– eliminate the need for burners

– eliminate soot formation

– lower reaction temperatures

Drawbacks– CH4/O2 mixtures can be explosive.

– Problems with selectivities at high pressures(above 20 bars).

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Partial oxidation

Advantages/Disadvantages– Less expensive than SR-plants.

– H2/CO ratio suitable for methanol- or FT-synthesis

– Soot problems (POX)

– Needs expensive oxygen plant.– (dependent on downstream process)

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Autothermal reforming

Temperature 1200-1400K

Pressure 20-100 bar

H2/CO 2-3

Catalytic/non-catalytic partial oxidation provides heat for steam reforming

More energy efficient

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Autothermal reforming

Advantages/Disadvantages– Less expensive than SR-plants.

– Higher conversion than SR (higher operating temperature).

– No soot problems

– Needs expensive oxygen plant.– (dependent on downstream process)

– Often used as a secondary reformer downstream an SR.

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Multifunctional reactors

Membrane reactors– Combine air separation and partial oxidation in one unit by

introduce oxygen permeable membranes.

– Remove H2 in the reactor by using membranes and thereby avoid equilibrium limitations

– Lower reaction temperatures can be used.

Chemical looping reforming– Continuous circulation of metal particles which serve as oxygen-

and heat carrier (metal oxide) for partial oxidation of methane. Two reactors are required: Air reactor and fuel reactor.

– Simple separation of oxygen.– No explosive mixtures.

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Multifunctional reactors

Sorption enhanced reaction process (SERP)

– Remove CO2 in the SR-process by using adsorbents mixed with the catalyst particles and thereby avoid equilibrium limitations. The adsorbent is regenerated by either increasing the temperature or reducing the pressure (temperature- or pressure swing).

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Outline




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Downstream processes

– Ammonia synthesis

– Methanol synthesis

– Fischer-Tropsch synthesis

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Ammonia synthesis

Ammonia– Base chemical for:

– Nitrogen fertilizers (CaNO3,KNO3)

– Explosive industry

Production history

– 1905; Birkeland/Eyde succeeded in producing CaNO3 from air.

– 1913; The Haber/Bosch-process was developed.

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Ammonia synthesis

N2 + 3H2 2NH3 ΔHr = -91.4 kJ/mol

– Ideal H2/N2-ratio is 3.

– Steam reforming is suitable reforming process due to high H2/CO-ratio. It is combined with an air-blown ATR that introduces N2.

– Equilibrium limited High pressure (100-250 bar) and low temperature (675-770K).

– Low single-pass conversion Recycling necessary.

– CO and CO2 has to be removed prior to the

ammonia synthesis several extra process units.

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Ammonia synthesis

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Ammonia synthesis

ICI quench reactor

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Ammonia synthesis

Haldor Topsøe radial flow reactor Kellogg vertical reactor Kellogg horizontal reactor

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Ammonia synthesis

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Methanol synthesis

Methanol– Base chemical for:

– Formaldehyde– Acetic acid

– Automobile fuel and fuel additive (MTBE)

Production history– 1923; BASF was the first to synthesize methanol from syngas.– 1960s; New catalysts were developed for low-pressure

production.

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Methanol synthesisCO + 2H2 CH3OH ΔHr = -90.8 kJ/mol

CO2 + 3H2 CH3OH + H2O ΔHr = -49.6 kJ/mol

CO + H2O CO2 + H2 ΔHr = -41 kJ/mol

– Ideal H2/CO-ratio is 2.– Low single-pass conversion Recycling necessary.

– Equilibrium limited High pressure (50-100 bar) and low temperature (500-550K).

– T < 570K due to catalyst sintering.

– The catalyst has to be very selective since methanol is thermodynamically less stabile than i.e. CH4. Cu/ZnO/Al2O3

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Methanol synthesis

Distillation– Column 1: Gases and light

impurities are removed.

– Column 2: Methanol is separated from heavy alcohols and water.

Reactor (ICI)– 40% of the feed enters the reactor– 60% of the feed is used as quench.

Separator– Gas and liquid are separated

after several cooling steps.

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Methanol synthesis

Lurgi reactor Haldor Topsøe reactor concept

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Methanol synthesis

Slurry reactor (fluidized bed)– Inert hydrocarbon liquid (absorbs heat, uniform temp.)– Solid catalyst.– Higher single-pass conversion less compression costs.

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Methanol synthesis

Direct conversion of methane

CH4+ ½O2 CH3OH ΔHr = -126 kJ/mol

– Significant efficiency increase.

– No CO2 production.

– Low yields.

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Fischer-Tropsch synthesis

Applicability– Fuels– Waxes

History– 1923; Fischer/Tropsch converted synthesis gas into a wide range

of hydrocarbons and/or alcohols.– WW II; Germany applied FT-synthesis to make fuels.– 1950s→; South Africa started to make fuels and base chemicals

in FT-plants to reduce the dependence on imported oil.

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CO + 2H2 -CH2- + H2O ΔHr = -165 kJ/mol

– Chain growth.

– High exothermicity.– Effective heat removal is a major consideration in reactor design.

– Converted over Fe- or Co-based catalysts.

– Selective productivity is not possible product ranges.

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T<530 K due to carbon deposition T>570 K to avoid heavy wax formation T<570 K due to hydrocracking

chemical reactors

Documents

fixed bed

outline reactor

fluidized

bed reactor

reforming

bubble columns

steam reforming

large particles