Structured Catalysts
and Reactors
K.Rajalakshmi
CH09M003
An inherent feature of conventional packed-bed reactors is their
random and structural maldistributions.
Random maldistributions result in:
(1) Non uniform access of reactants to
the catalytic surface, worsening
the overall process performance
(2) Unexpected hot spots and thermal
runaways of exothermic reactions.
(3) Fouling and attrition
Effect of liquid channeling on column efficiency for a system with a
relative volatility of 1.07. Total number of theoretical plates N of 10, 20,
40, and 100 at top liquid composition X of 90 and 60 mole percent.
Manning and Cannon, Ind. Eng. Chem., 49, 347 (1957)
Structured catalysts (reactors) are promising as far as the elimination of
these drawbacks of fixed beds is concerned.
Structured Catalysts and Reactors (2nd edition) – 2006 Andrzej Cybulski and Jacob A. Moulijn
Monoliths
Monoliths are structures that contain various types of interconnected or
separated channels in a single block of material.
Ceramics: Cordierite, alumina,titania,silica
Metal
Monolithic reactors are those filled with monoliths that are either made
of porous catalytic material or the catalytic material is deposited
(‘washcoated’) in the channels of an inert monolithic support.
In both arrangements, the channel walls function as catalyst and the
channels provide space for flow of gas and/or liquid.
Advantages
• No filtering of catalyst necessary
• No attrition of catalyst
• Low pressure drop
• High geometric surface area
• Efficient mass-transfer
• In the case of internal diffusion limitations:
more efficient use of catalyst due
to thin catalytic layer
• Easy scale up
Disadvantages
• Relatively high manufacturing cost
• Little practical experience in multi-phase applications
History
Automotive Catalytic Converters
pellet filled catalytic converter
Monolith Key Features:
– no attrition
– high surface area
– low pressure drop
– rapid light-off
monolith catalytic converter
Hydrodynamics and mass transfer
For co-current gas–liquid flow, several flow
regimes can occur. The preferred one usually
is the so-called Taylor or slug flow. This type
consists of gas bubbles and liquid slugs flowing
consecutively through the small monolith
channels. The gas bubble fills up the whole
space of the channel and only a thin liquid
film separates the gas from the catalyst.
The rate of mass transfer in taylor flow is large due to the following
reasons.
First the liquid layer between bubble and catalyst coating is thin,
increasing mass transfer.
Secondly, the liquid slugs show an internal recirculation during their
travel through a channel. Because of this, radial transfer of
mass is increased.
The gas bubbles push the liquid slugs forward as a piston and a type of
plug flow is created.
Taylor flow can be induced in single-phase liquid phase reactions over
monoliths by adding an inert gas component.
Gas-liquid-solid system
Selectivity Improvement
Benzaldehyde hydrogenation
Batch
– slurry , monoliths or extrudates
– slurry < 50 μm, monolith 4 cm Ø,
extrudates 1.7x 5 mm
Pilot
– monoliths 1 cm Ø - variation cell density
– trickle bed 4.7 cm Ø, extrudates 1.7x 5 mm
T. A. Nijhuis et al. Chemical Engineering Science 56(2001) 823-829
Benzaldehyde hydrogenation - selectivities
at 50 % conversion
T. A. Nijhuis et al. Chemical Engineering Science 56(2001) 823-829
Catalytic hydrogenation of anthraquinone
SiO2 - used as the monolith support material
Palladium - active catalyst component.
Comparison of monolithic, slurry and packed-bed reactors
R. Edvinsson Albers et al. Catalysis Today 69 (2001) 247–252
Membrane reactors
Catalyst-membrane systems are promising structured catalysts.
The combination of reaction and membrane separation can result in
increase in the reaction yield beyond what the reaction equilibrium
allows and/or modifying the process selectivity.
Classification of membrane reactor configurations according
to membrane function and location
Structured Catalysts and Reactors” (2nd edition) – 2006 Andrzej Cybulski and Jacob A. Moulijn
Based on material consideration
Membrane Reactors
Organic membrane
reactors
Inorganic membrane
reactors
Dense (metal)
membrane
Porous
membrane
Potential applications of Inorganic membrane reactors
Conversion enhancement of
equilibrium limited reactions
Coupling of reactions
Controlled addition of reactant
Selectivity increase of
intermediate products
Dense membranes (e.g. Pd alloys or solid electrolyte) can supply one
of the reactants in a monatomic form, particularly active towards, for
instance, partial oxidations or partial hydrogenations
Porous membranes such as γ -alumina, modifies in an advantageous
way the residence time and the concentration profile of the reactants
in the catalytically active zone .
Esterification processes in a H-ZSM-5 membrane reactor
A catalytically active zeolite membrane has been used to displace equilibrium by
selective water permeation during ethanol esterification. The acidic membrane both
catalyzed the reaction and selectively permeated the water product, while reactants
were fed at the other side. The catalytic performance was better than that in a
packed bed with the same amount of zeolite material.
M. Pilar Bernal et al. Chemical Engineering Science 57 (2002) 1557–1562
Methanol to olefin conversion
H-ZSM-5 membranes was used for the conversion of methanol to
olefin.Olefins easily react further to aromatic products (MTG-process)
over this catalyst, but with proper balancing of the reaction rate and
the membrane permeation rate, olefin selectivities of 80 to 90% at
methanol conversion levels of 60 to 98% were achieved.
T. Masuda et al. Chemical Engineering Science 58 (2003) 649 – 656
N2 was flowed backward to sweep out molecules permeating
from the feed side to the permeate side of the membrane.
T. Masuda et al. Chemical Engineering Science 58 (2003) 649 – 656
Pressure drop was expected to enhance the diffusion rates of molecules
through the membrane and to realize the direction of the diffusion from the
feed side to the permeate side of the membrane.
Membrane assisted fluidized bed reactor (MAFBR)
An MAFBR is a special type of reactor that
combines the advantages of a fluidized bed
and a membrane reactor. This setup allows
the coupling of the most typical properties
of fluidized-bed reactors (good degree of
mixing, high heat transfer coefficients
allowing close-to-isothermal operation, etc.)
with the separation properties of the membrane.
Steam reforming of methane
The Pd membranes, permselective towards hydrogen, are immersed in
a fluidized bed of catalyst pellets.
Perm-selective membranes are intended to
break the thermodynamic barrier and shift
the equilibrium forward to enhance hydrogen
production while also purifying the product.
Vacuum is applied to extract the permeating
compound throughout the membrane.
Parallel-Passage and lateral-Flow Reactors
Particulate catalysts can be arranged in arrays of any geometric
configuration. In such arrays, three levels of porosity (TLP) can
be distinguished.
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The fraction of the reaction zone that is free to the gas flow is the
first level of porosity.
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The void fraction within the arrays is the second level of porosity.
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The fraction of pores within the catalyst pellets is referred to as
the third level of porosity.
Parallel-passage and lateral-flow reactors are examples of TLP reactors.
Parallel-Passage and lateral-Flow Reactors
The parallel-passage reactor (PPR) and the lateral-flow reactor (LFR)
are fixed-bed reactors suitable for the treatment of large volumes of
gas at relatively low pressure.
Since the PPR and LFR can use catalysts in the shape and size as used
in conventional fixed beds, no dedicated catalyst manufacturing plants
are generally required to fulfill the catalyst needs, and there are no
special requirements for catalyst handling beyond those for traditional
fixed-bed catalysts.
The gas flows through these passages along the catalyst layers,
instead of through the bed as in a traditional fixed-bed reactor.
The straightness of the gas passages also prevents particulates present in
the gas from being caught by impingement upon obstacles, and thus the
PPR can be used for treating dust-containing gases, similarly to
monolithic (honeycomb)-type reactors, which are also applied in treating
flue gas.
In contrast to the PPR, where all the gas passages connect
the inlet directly with the outlet by being open at both ends, the gas
passages of the LFR are each closed off at one end, neighboring
passages being open and closed at different ends.
The gas is forced to flow through the layers of catalyst, instead of
alongside them as in the PPR
Industrial Applications
The Shell flue gas desulfurization
The Shell flue gas desulfurization removes sulfur oxides from flue gas
in a PPR using a regenerable solid adsorbent (acceptor) containing
finely dispersed copper oxide.
The essential elements in the development of the SFGD process are
the development of a mechanically and chemically stable active
acceptor that can withstand thousands of acceptance/regeneration
cycles and the Parallel flow reactor as a dust-tolerant system.
Flow scheme of the SFGD process as applied for sulfur
oxides removal from refinery furnace off-gas.
Groenendaal, W. et al.., AIChE Symp. Ser., 72, 12–22, 1976
The PPR and LFR are also applied in the process for NOx removal from
off-gases. The Shell low- temperature NOx reduction process is based
on the reaction of nitrogen oxides with ammonia catalyzed by a highly
active and selective catalyst, consisting of vanadium and titania on a
silica carrier.
The low pressure drop and dust tolerance of the PPR and LFR are of
potential interest in many end-of-pipe treatments of waste gases to
reduce emissions that meet with increasing environmental concern.
Structured Packings for reactive distillation
The combination of chemical reaction with distillation of reactants in a
single piece of process equipment is called reactive distillation.
Since in a reactive distillation process the reaction products are
continuously removed from the reaction mixture, chemical equilibrium
limitations can be overcome and high reaction rates are maintained.
Reactive distillation columns consist of three sections: a reactive
section located between an upper enriching and a lower stripping
section.
The reactive distillation column can be regarded as a countercurrent
gas–liquid catalytic trickle-bed reactor operating at the boiling point.
The column internals need to fulfill various functions:
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Immobilize catalyst of particle sizes typically 0.2 to 3 mm.
Efficient liquid contacting of the catalyst.
High capacity in countercurrent operating mode.
Efficient gas–liquid mass transfer for high separation efficiency.
Adjustable residence time.
Mechanical stability and resistance to catalyst swelling.
CDTech (Catalytic Distillation Technologies) has developed the socalled ‘‘catalyst bales’’ . This is a structure containing the catalyst
within layers of fiberglass cloth, being rolled up into bales together with
a layer of stainless steel demister wire mesh. Bales are stacked in the
column to form the reaction zone.
Sulzer Chemtech and Koch Engineering have developed similar reactive
distillation packing technologies: KATAPAK-S and KATAMAX ,
respectively.
In these structures the catalyst is
immobilized between two sheets of metal
wire gauze forming ‘‘pockets.’’
Each of the wire gauze sheets is corrugated,
resulting in a structure with flow channels of
a defined angle and hydraulic diameter.
The ‘‘pockets’’ are assembled with the flow
channels in opposed orientation, so that the
resulting combination is characterized by an
open cross-flow structure pattern.
Structured catalyst-sandwiches. (a) Catalyst sandwiched between two
corrugated wire gauze sheets. (b) The wire gauze sheets are joined
together and sewn on all four sides. (c) The sandwich elements arranged
into a cubical collection. (d) The sandwich elements arranged in a round
collection.
Applications of structured packings in reactive distillation
Hydrolysis of methyl acetate
Large quantities of methyl acetate are formed as a side product in the
production of polyvinyl alcohol (PVA). By the hydrolysis of the methyl
acetate, methanol and acetic acid are recovered and recycled back to
the PVA production.
CH3COOCH3+H2O
CH3COOH+CH3OH
Because of the small equilibrium constant of the reaction,
conventional hydrolysis processes can only reach low conversion of
methyl acetate per pass (around 30%) and require large recycle
streams.
Sulzer Chemtech (Switzerland) has developed together with WackerChemie (Germany) a new process for the hydrolysis of methyl acetate.
The new methyl acetate hydrolysis process combines a reactor and a
reactive distillation column containing Katapak-SP structured packing.
The reactor outlet product is fed to the reactive distillation column,
where reaction conversion is increased up to 97%.
Structured Reactors
Driving forces
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Pressure drop
Mass transfer
Surface area
Catalyst Efficiency
Fluid distribution
Catalyst Separation
Equilibrium limiting reactions
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