Structured Catalysts and Reactors

K.RajalakshmiCH09M003

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

MonolithsMonoliths are structures that contain various types of

interconnected or separated channels in a single block of material.

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.

Ceramics: Cordierite, alumina,titania,silicaMetal

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

HistoryAutomotive Catalytic Converters

pellet filled catalytic converter

monolith catalytic converter

Monolith Key Features:– no attrition– high surface area– low pressure drop– rapid light-off

Hydrodynamics and mass transfer

For co-current gas–liquid flow, several flow For co-current gas–liquid flow, several flow

regimes can occur. The preferred one usuallyregimes can occur. The preferred one usually

is the so-called Taylor or slug flow. This type is the so-called Taylor or slug flow. This type

consists of gas bubbles and liquid slugs flowing consists of gas bubbles and liquid slugs flowing

consecutively through the small monolithconsecutively through the small monolith

channels. The gas bubble fills up the wholechannels. The gas bubble fills up the whole

space of the channel and only a thin liquidspace of the channel and only a thin liquid

film separates the gas from the catalyst.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 ofmass 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 ImprovementBenzaldehyde hydrogenation

Batch– slurry , monoliths or extrudates– slurry < 50 μm, monolith 4 cm Ø, extrudates 1.7x 5 mmPilot– 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

Inorganic membrane reactors

Organic membrane reactors

Dense (metal)membrane

Porousmembrane

Potential applications of Inorganic membrane reactors

Conversion enhancement of equilibrium limited reactions Controlled addition of reactant

Coupling of reactionsSelectivity 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 reactantsin 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.

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.

Steam reforming of methane

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.

The fraction of the reaction zone that is free to the gas flow is

the

first level of porosity.

The void fraction within the arrays is the second level of porosity.

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: 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 so-

called ‘‘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 joinedtogether and sewn on all four sides. (c) The sandwich elements arranged into a cubical collection. (d) The sandwich elements arranged in a roundcollection.

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 Wacker-Chemie (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 ReactorsDriving forces

• Pressure drop• Mass transfer• Surface area• Catalyst Efficiency• Fluid distribution• Catalyst Separation• Equilibrium limiting reactions

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