Chemical Reactors

5/25/2018 Chemical Reactors

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Chemical Reactors and their Applications

Norges teknisk-naturvitenskapelige universitet

Chemical Reactors

and theirApplications


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Outline

Reactorconcepts

Natural gas reforming concepts

Downstream processes


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Outline

Reactorconcepts




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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

Fixed bed reactors




Bubble columns


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Fixed Bed Reactors

Concept

Collection of fixed solidparticles.

The particles may serve as acatalyst 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


Temperature control

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


Applicable for exo-/endothermic

reactions


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Fixed Bed ReactorsSO3reactor

NH3reactor

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Fixed Bed Reactors

Multi-Tube Reactor

Several tubes of small

diameter filled with particles.


Expensive

High surface area for heatexchangeVery good verytemperature 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


Temperature control

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 drophigh 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 Reactors

Porous catalyst particle The particles are porous to

increase the surface area of thecatalyst.

Reactants are transported insidethe pores by means of moleculardiffusion and adsorb to the activesites where the reaction occurs.

Products desorb and diffuse backto the bulk.

Heat is transported by conduction.

Intra-particle diffusion/conduction

may be rate determining for large

particles (egg-shell particles).

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Fixed Bed Reactors


Temperature control

Pressure drop


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Fixed Bed Reactors


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

Fixed bed reactors




Bubble columns

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Fluidized Bed Reactors

Concept

Collection of solid particles dispersed

in a continuous phase.

The particles may serve as acatalyst, adsorbent or a heat carrier.

Continuous flow of gas or liquid

Applications

Catalytic cracking processes


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

Fixed bed reactors




Bubble columns

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Stirred tank ReactorsConcept

Forced mixing by use of impeller.

Applied in reactive systems whenmixing 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 phasemixing

Fermentation reactor

Food and paper industry

Natural gasconversion/polymerization

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Stirred tank Reactors

The 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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Impellers Radial flow impellers are suitable

for dispersion of gas in liquid.

Axial flow impellers are suitable to

blend liquids and suspend solids inliquids.

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Uniform temperature


Exception: slurries with high concentrations of large particles

(difficult mixing).

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Reactor Concepts

Fixed bed reactors




Bubble columns

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Slurry loop Reactors

Concept

Collection of solid catalystparticles dispersed in a liquidphase (slurry).

The slurry is circulating at a highvelocity impelled by an axialpump.

The mixing pattern is veryintensive and well defined.

Typical application

Polymerization

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Slurry loop Reactors


Uniform temperature

Very efficient heat-exchange

Can operate at high polymer concentrations

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Reactor Concepts

Fixed bed reactors




Bubble columns

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Bubble Columns

Concept

Gas dispersed in a continuousliquid 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

Uniform temperature


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Outline

Reactorconcepts



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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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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 reformer

CH4+ H2OCO + 3H2 Hr=206 kJ/mol

CO + H2OCO2+ H2 Hr= -41 kJ/mol (Water gas shift)

Overall heat of reaction is endothermicmulti-tube reformer

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

Pressure 15-30 bar

H2/CO >3

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Steam reforming

Burner 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 formation

2COC + CO2 Hr=-173 kJ/mol (The Boudouard reaction)

CH4C + 2H2 Hr=75 kJ/mol (Decomposition of methane)

CO + H2C + H2O Hr=-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+ nH2OnCO + (n+m/2)H2 Hr>0

CO + 3H2CH4+ H2O Hr= -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 compoundsdesulfurization

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


No need for expensive oxygen plant.

Material limitations on temperature

limited conversion.

High H2/CO ratio, suitable for hydrogen production with CO2capture, not for methanol- or FT-synthesis.

Carbon formation

Steam corrosion problems.

Costs in handling excess H2O.

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Partial oxidation

CH4+ O2CO + 2H2 Hr= -36 kJ/mol

CH4+ 2O2CO2+ 2H2O Hr=-803 kJ/mol

CO + O2CO2 Hr=-284 kJ/mol

H2+ O2H2O Hr= -242 kJ/mol

Overall heat of reaction is slightly exothermic.

No catalyst (burners)

Temperature 1600-1900K

Pressure 150 bar

H2/CO


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Catalytic partial oxidation

Reactions are catalyzed to:

improve selectivities

eliminate the need for burners

eliminate soot formation

lower reaction temperatures

Drawbacks

CH4/O2mixtures can be explosive.

Problems with selectivities at high pressures

(above 20 bars).

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Partial oxidation


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 1200-1400KPressure 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


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 byintroduce oxygen permeable membranes.

Remove H2in the reactor by using membranes and thereby avoidequilibrium 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 CO2in 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

Reactorconcepts



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Ammonia synthesis

Methanol 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 CaNO3from air.

1913; The Haber/Bosch-process was developed.

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Ammonia synthesis

N2+ 3H22NH3 Hr= -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 limitedHigh pressure (100-250 bar) and low

temperature (675-770K).

Low single-pass conversionRecycling necessary.

CO and CO2has 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 Topse 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 synthesis

CO + 2H2CH3OH Hr= -90.8 kJ/mol

CO2+ 3H2CH3OH + H2O Hr= -49.6 kJ/mol

CO + H2OCO2+ H2 Hr= -41 kJ/mol

Ideal H2/CO-ratio is 2.

Low single-pass conversionRecycling necessary.

Equilibrium limitedHigh pressure (50-100 bar) and low

temperature (500-550K). T < 570K due to catalyst sintering.

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

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Methanol synthesis

Distillation

Column 1: Gases and lightimpurities 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 Topse 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+ O2CH3OH Hr= -126 kJ/mol

Significant efficiency increase.

No CO2production.

Low yields.

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

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CO + 2H2-CH2- + H2O Hr= -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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T570 K to avoid heavy wax formation T

Chemical Reactors

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