1、Structured Catalysts and ReactorsK.RajalakshmiCH09M003An 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)Unex
2、pected hot spots and thermal runaways of exothermic reactions.(3)Fouling and attritionEffect 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.Mannin
3、g 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)2019 Andrzej Cybulski and Jacob A.MoulijnMonolithsMonoliths are structures that contain various typ
4、es 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 cha
5、nnel walls function as catalyst and the channels provide space for flow of gas and/or liquid.Ceramics:Cordierite,alumina,titania,silicaMetalAdvantagesnNo filtering of catalyst necessarynNo attrition of catalystnLow pressure dropnHigh geometric surface areanEfficient mass-transfernIn the case of inte
6、rnal diffusion limitations:more efficient use of catalyst due to thin catalytic layernEasy scale upDisadvantagesnRelatively high manufacturing costnLittle practical experience in multi-phase applications HistoryAutomotive Catalytic Converterspellet filled catalytic convertermonolith catalytic conver
7、terMonolith Key Features:no attrition high surface area low pressure drop rapid light-offHydrodynamics and mass transfer 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
8、,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 ov
9、er monoliths by adding an inert gas component.Gas-liquid-solid systemSelectivity ImprovementBenzaldehyde hydrogenationBatch slurry,monoliths or extrudates slurry 50 m,monolith 4 cm,extrudates 1.7x 5 mmPilot monoliths 1 cm -variation cell density trickle bed 4.7 cm,extrudates 1.7x 5 mmT.A.Nijhuis et
10、al.Chemical Engineering Science 56(2019)823-829Benzaldehyde hydrogenation-selectivitiesat 50%conversionT.A.Nijhuis et al.Chemical Engineering Science 56(2019)823-829Catalytic hydrogenation of anthraquinone SiO2-used as the monolith support material Palladium-active catalyst component.Comparison of m
11、onolithic,slurry and packed-bed reactorsR.Edvinsson Albers et al.Catalysis Today 69(2019)247252Membrane reactorsCatalyst-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 equil
12、ibrium allows and/or modifying the process selectivity.Classification of membrane reactor configurations according to membrane function and locationStructured Catalysts and Reactors”(2nd edition)2019 Andrzej Cybulski and Jacob A.MoulijnBased on material consideration Membrane ReactorsInorganic membr
13、ane reactorsOrganic membrane reactorsDense(metal)membranePorousmembranePotential applications of Inorganic membrane reactorsConversion enhancement of equilibrium limited reactionsControlled addition of reactantCoupling of reactionsSelectivity increase of intermediate productsDense membranes (e.g.Pd
14、alloys or solid electrolyte)can supply one of the reactants in a monatomic form,particularly active towards,for instance,partial oxidations or partial hydrogenationsPorous membranes such as -alumina,modifies in an advantageous way the residence time and the concentration profile of the reactantsin t
15、he catalytically active zone.Esterification processes in a H-ZSM-5 membrane reactorA 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 w
16、ater 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(2019)15571562Methanol to olefin conversionH-ZSM-5 membranes was used for the conversion
17、 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 Eng
18、ineering Science 58(2019)649 656N2 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(2019)649 656Pressure drop was expected to enhance the diffusion rates of molecules through the membrane and
19、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 typi
20、cal 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
21、 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 permeatingcompound throughout the membrane.Steam reforming of methaneParallel-Passage and lateral-Flow Reactor
22、sParticulate catalysts can be arranged in arrays of any geometric configuration.In such arrays,three levels of porosity(TLP)can be distinguished.nThe fraction of the reaction zone that is free to the gas flow is the first level of porosity.nThe void fraction within the arrays is the second level of
23、porosity.nThe 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 ReactorsThe parallel-passage reactor(PPR)and the lateral-flow reactor(LFR)are fixed-bed
24、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 spe
25、cial 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
26、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
27、 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 PPRIndustrial Applications The Shell flue gas desulfurization
28、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 accep
29、tor 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.,72,1222,1976The PPR and LFR are also appl
30、ied 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 toleran
31、ce 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 distillationThe combination of chemical reaction with distillation of reactants in a single piece of proc
32、ess 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 secti
33、ons:a reactive section located between an upper enriching and a lower stripping section.The reactive distillation column can be regarded as a countercurrent gasliquid catalytic trickle-bed reactor operating at the boiling point.The column internals need to fulfill various functions:nImmobilize catal
34、yst of particle sizes typically 0.2 to 3 mm.n Efficient liquid contacting of the catalyst.n High capacity in countercurrent operating mode.n Efficient gasliquid mass transfer for high separation efficiency.n Adjustable residence time.n Mechanical stability and resistance to catalyst swelling.CDTech(
35、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 zon
36、e.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
37、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 tw
38、o 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 distillationHydrolysis of methyl ac
39、etateLarge 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 constan
40、t 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 ac
41、etate 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 SeparationnEquilibrium limiting reactions Thank You
