Reactor Types and Their Industrial Applications

c© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.b04 087

Reactor Types and Their Industrial Applications 1

Reactor Types and Their Industrial Applications

Klaus-Dieter Henkel, Buna AG, Schkopau, Federal Republic of Germany

1. Introduction . . . . . . . . . . . . . . . 12. Basic Types of Reactors . . . . . . . . 23. Survey of Real Reactors and Their

Uses . . . . . . . . . . . . . . . . . . . . . 43.1. Reactors for Gas-Phase Reactions . 43.2. Reactors for Liquid-Phase Reac-

tions . . . . . . . . . . . . . . . . . . . . . 83.3. Reactors for Gas –Liquid Reactions 83.4. Reactors for Solid-Catalyzed Reac-

tions . . . . . . . . . . . . . . . . . . . . . 133.4.1. Reactors forHeterogeneousGasCatal-

ysis . . . . . . . . . . . . . . . . . . . . . 133.4.2. Reactors for Liquid-Phase and Gas –

Liquid Reactions over Solid Catalysts 133.5. Reactors for Noncatalytic Reactions

Involving Solids . . . . . . . . . . . . . 19

3.5.1. Reactors for Noncatalytic Gas – SolidReactions . . . . . . . . . . . . . . . . . . 19

3.5.2. Reactors for Noncatalytic Liquid –Solid Reactions . . . . . . . . . . . . . . 21

3.5.3. Reactors for Noncatalytic Solid-PhaseReactions . . . . . . . . . . . . . . . . . . 21

3.6. Electrothermal Reactors . . . . . . . 213.7. Reactors for Electrochemical Pro-

cesses . . . . . . . . . . . . . . . . . . . . 243.8. Reactors for Biochemical Processes 273.9. Reactors for Photochemical and Ra-

diochemical Processes . . . . . . . . . 283.9.1. Photochemical Reactors . . . . . . . . . 283.9.2. Radiochemical Reactors . . . . . . . . 324. References . . . . . . . . . . . . . . . . . 33

1. Introduction

The reactor in which the chemical reaction takesplace occupies a central position in the chemi-cal process. Grouped around the reactor are theprocess steps involving physical treatment of thematerial streams, such as conveyance, heat trans-fer, and separation and mixing operations. Thereactor provides the volume necessary for the re-action and holds the amount of catalyst requiredfor the reaction. The energy required to over-come the activation threshold of each partial re-action is also supplied in the reactor, and theproper temperature and concentration are main-tained.

The most important reaction-related factorsfor the design of a reactor are

1) The activation principle selected, togetherwith the states of aggregation of the reactantsand the resulting number and types of phasesinvolved

2) The concentration and temperature depen-dence of the chemical reactions

3) The heat of the reactions taking place

The most important activation principles for areaction mixture include

1) Activation by addition of heat

2) Catalytic activation3) Activation by decomposition of an initiator4) Electrochemical activation5) Biochemical activation

Less important options for activation are visibleor ultraviolet light and radioactive radiation.

With regard to phase relationships in the re-action space, a number of combinations are pos-sible. The reactants and reaction products canbe present, or be produced, in various states ofaggregation. Furthermore, inert diluents or heat-transfermedia can be present in different phases.Finally, the catalyst, which is generally in thesolid or liquid phase, often has to be taken intoconsideration.

The (negative or positive) heat of the reac-tions taking place in a reactor influences the ex-tent and nature of provisions for heat transfer.Exothermic or endothermic reactions frequentlyrequire supply or removal of large quantities ofheat. Thermally neutral reactions involve con-siderably less technical sophistication.

The concentration and temperature depen-dences of a chemical reaction are described bythe reaction rate. In practice most reaction sys-tems are complex and include parallel, sequen-tial, and equilibrium reactions. To obtain thehighest possible yield of desired product under

2 Reactor Types and Their Industrial Applications

these conditions, the temperature and pressuremust be held within certain ranges, the tempera-ture must be controlled along the reaction path,and a definite residence-time distribution in thereactor must be achieved. If, in addition, sub-stances or energy have to be transferred fromone phase to another, appropriate transport con-ditions have to be implemented. When catalystsare used, catalyst loss due to aging and poison-ing must be considered. These factors imposea complex of requirements that must be kept inmind when designing a reactor.

Against the requirements established by theprocess, the designer must balance costs of fab-rication, consumption of materials, and opera-tional reliability. In practice, many possibilitiesare often available for realizing a chemical pro-cess, and in such cases the decision must dependon an assessment of the overall process as wellas commercial constraints on the plant.

2. Basic Types of Reactors (→ModelReactors and Their Design Equations)

A variety of reactor designs are used in indus-try, but all of them can be assigned to certainbasic types or combinations of these. The basictypes are as follows (see →Principles of Chem-ical Reaction Engineering, Chap. 4.2.):

1) Batch stirred-tank reactor2) Continuous stirred-tank reactor3) Tubular reactor

Given certain flow and thermal conditions, thesetypes are also referred to as “ideal” reactors.With respect to flow conditions the ideal stirred-tank batch reactor is characterized by completemixing on microscopic and macroscopic scales.In the ideal tubular reactor, plug flow is assumed,i. e., no mixing occurs in axial (flow) direction,but ideal mixing takes place in the ra-dial direc-tion. Thus, as in the batch stirred-tank reactor,all particles experience a well-defined residencetime. In contrast, the continuous stirred-tank re-actor has a very broad residence-time distribu-tion (→Principles of Chemical Reaction En-gineering, Chap. 4.2.1.). The ideal analysis isbased on the assumption of a reaction systemthat is homogeneous as regards the phase. Thustransport resistance between phases does not oc-cur.

The thermally ideal operating states are theisothermal and adiabatic states, i. e., either veryintensive heat exchange with the surroundingsor no exchange at all is assumed.

In practical operation, the ideal states areachieved only approximately. Examples of typ-ical nonidealities include

1) The formation of real flow patterns, such asdead zones, short-circuit flows, and channel-ing

2) Transport processes in the individual phases,such as axial backmixing

3) The formation of concentration and temper-ature profiles as a result of transport resis-tances in and between phases

4) Segregation processes5) Incomplete mixing of reactants

The essential advantages and disadvantages ofthe three basic reactor types are discussed inwhat follows.

Batch Stirred Tank (→Stirred-Tank andLoop Reactors)

Principal Applications:

1) Liquid-phase reactions2) Liquid – solid reactions

Advantages:

1) Quick production changeover possible; usefor substances produced on a small scale

2) Process steps upstream or downstream of thereaction can also be performed in the reactor

3) Better process control than in continuous op-eration when solid or highly viscous phasesform or are present

4) Well-defined residence time

Disadvantages :

1) Relatively high operating costs due to longdowntimes and highmanpower requirements

2) Quality differences between charges becausereaction conditions are only partly repro-ducible

3) Limited temperature control capabilities, es-pecially with highly endothermic or exother-mic reactions


Continuous Stirred TankPrincipal Applications:

1) Liquid-phase reactions2) Gas – liquid reactions3) Gas – liquid reactions over suspended cata-

lysts

Advantages:

1) Low operating costs, especially at highthroughputs

2) Consistent product quality due to repro-ducible process control

3) Wide range of throughput

Disadvantages:

1) Final conversions lower than in other basicreactor types because of complete mixing(i.e., unreacted starting materials can get intothe product stream)

2) High investment costs to implement contin-uous operation

3) Changeover to other products generallycomplex and time-consuming because ofreaction-specific design

Tubular Reactor (→Tubular Reactors)Principal Applications:

1) Homogeneous gas-phase reactions2) Liquid-phase reactions3) Gas- and liquid-phase reactions over solid

catalysts (→Fixed-Bed Reactors)4) Gas – liquid reactions

Advantages:

1) Favorable conditions for temperature controlby heat supply or removal

2) No moving mechanical parts, hence espe-cially suitable for high-pressure service

Disadvantages:

1) Very high degree of specialization, oftenwith complicated design and high investmentcosts

2) Relatively large pressure drops

Reactors are interconnected to make up for thedrawbacks of a single reactor, especially to adaptreaction conditions during scale-up capacity, aswell as to optimize conversion and yield. Partialreactors can be combined in a single apparatus orconnected in a system of reactors; these partialreactors may differ in shape and size.

Types of interconnections are series, parallel,and recycle.

Series Connection:

1) Multibed reactors2) Tower reactors, reaction columns3) Cascades of stirred tanks (→Stirred-Tank

and Loop Reactors)4) Multiple-hearth reactors (→Metallurgical

Furnaces, Chap. 2.)5) Different reactor types connected in series

(e.g., stirred tank and tubular reactor)

Parallel Connection: Multitubular reactorsRecycle Connection: Loop reactors

(→Stirred-Tank and Loop Reactors)Complicated reactor designs result, espe-

cially when different reactor types are combinedin a single apparatus. At the same time, such acombination offers maximum adaptability to therequirements of a given reaction process. Thedesigner must, of course, examine every caseindividually to ensure that the results justify thevery high development and investment costs forsuch special reactors. The following survey ofreal reactors includes these special types of re-actor designs only when their utility extends be-yond a single case.

3. Survey of Real Reactors and TheirUses

The phase relationships in the reaction space arecrucial in the design of reactors for catalytic,thermal, and polymerization processes and ac-cordingly form the top-level classification fea-ture for such reactors. Sincemany different com-binations of phases are possible, the survey isbased only on the state of the reactants at the in-let to the reactor or the beginning of the reactionand the phase of the reaction site (catalyst phase,liquid phase with dissolved reactant). Reactionproducts that form additional phases and inertsubstances of all types (except for solvents, asjust noted) are ignored.

Reactors used in electrothermal, electro-chemical, biochemical, photochemical, and ra-diochemical processes are treated separately.Reactor types for which no industrial applica-tion is currently known are not listed.


3.1. Reactors for Gas-Phase Reactions

Homogeneous gas-phase reactions utilized inindustry are generally characterized by largepositive or negative enthalpies of reaction andhigh reaction temperatures. To obtain the desiredproduct spectrum, residence times must usuallybe very short. The high reaction temperature canbe maintained or the requisite heat supplied byburning part of the feed.

Tables 1 and 2 and Figures 1 and 2 summa-rize the reactors used for such reactions as wellas their applications.

Figure 1. Reactors for exothermic gas-phase reactionsA) Burner; B) Tubular reactor; C) Reactor with recycle; D)Fluidized-bed reactora) Gaseous reaction mixture; a1, a2) Gaseous feed compo-nents; b) Gaseous product; c) Coolant; d) Partial stream ofproduct; e) Catalyst

Figure 2. Reactors for endothermic gas-phase reactionsA) Burner; B) Reformer; C) Fluidized-bed reactor; D)Moving-bed reactor; E) Reactor with fixed bed of inerts;F) Regenerative furnacesa) Oxygen or air; b) Hydrocarbon; c) Fuel gas; d) Prod-uct; e) Heat-transfer medium; f) Steam; g) Flue gas; h) Air;i) Quench; j) Reaction section; k) Regeneration section;l) Catalyst; m) Convection zone

3.2. Reactors for Liquid-PhaseReactions

In general, liquid-phase reactions are exother-mic. In the case of multiphase systems, inten-sivemass and heat transfermust be provided for;this is possible only in reactors with compulsorymixing, such as stirred tanks. Along with a num-


Figure 3. Reactors for liquid-phase reactionsA) Tubular reactor; B) Reformer; C) Sulzer mixer – reactor; D) Reactor with external recirculation; E) Reactor with internalrecirculation (draft tube); F) Stirred tank; G) Cascade of stirred tanks; H) Column reactor; I) Multichamber tank; J) Fluidized-bed reactor; K) Spray reactor; L) Falling-film reactora) Liquid reaction mixture; a1, a2) Liquid feed components; b) Liquid product; c) Coolant; d) Heating agent; e) Water; f) Or-ganic phase and water; g) Baffle; h) Organic phase; i) Partial stream of product; j) Catalyst; k) Reaction mixture from precedingreaction stage; l) Water from preceding stage; m) Packing; n) Off-gas; o) Fuel gas for burners; p) Quench; q) Convection zone;r) Mixing element consisting of tubes carrying heat-transfer medium; s) Mixing elements rotated 90◦


Figure 4. Special reactor designs for polymerization reactionsA) Multitubular reactor; B) Multistage multitubular reactor with interstage stirring; C) Reactor with external recycle (multi-tubular or screw-conveyor type); D) Reactor with external recycle (annular); E) Reactor with internal recirculation; F) Sulzerloop reactor (see Fig. 3C for detail of a single reactor); G) Loop reactor; H) Tower reactor; I) Ring-and-disk reactor; J) Extruderreactor; K) Powder-bed reactor; L) Mixing head; M) Belt reactor with mixing head; N) Spinning jet with coagulating batha) Polymerizationmixture; a1, a2) Feed components; b) Polymerization product; c) Coolant; d) Staticmixer; e) Pump; f) Screw-conveyor design for viscous media; g) Sulzer mixer – reactor; h) Sulzer mixer – reactors in plug-flow configuration; i) Air;j) Plunger; k) Nozzle; l) Mixing head; m) Belt reactor; n) Spinning bath; o) Packed bed of polymer granules


Table 1. Reactors for exothermic gas-phase reactions

Reactor type Features Examples of applications

Burner for high reaction rates combustion of H2S to SO2 (Claus vessel)very high reaction temperatures carbon black production (furnace, gas, thermal carbon

black processes)explosion limits must be taken into consideration chlorine – hydrogen reaction

chlorination of methanenitration of propane

Tubular reactor well-defined residence time (tubes up to 1000m long) chlorinationof methane

intermediate injection possible of propene to allyl chloridepressure drops of butadiene to dichlorobutanegood temperature control capability chlorolysis of chlorinated hydrocarbons

Reactor with recycle suitable for low reaction rates chlorination of methanegood mixingcooling inside or outside reactor

Fluidized-bed reactor nearly isothermal conditions because heat transport isvery efficient

chlorination

of methaneintensive mixing of 1,2-dichloroethane to tri- and perchloroethylene

chlorolysis of chlorinated hydrocarbons

Table 2. Reactors for endothermic gas-phase reactions


Burner very high reaction temperatures attainable by partialcombustion of reactants

Sachsse –Bartholome process for acetylene production

short residence times high-pressure gasification for synthesis gas production(Texaco, Shell)

Reformer high reaction temperatures attainable mainly byradiation

steam cracking of naphtha and other hydrocarbons toethylene

well-defined residence times vinyl chloride production by cleavage of dichloroethanepyrolysisof acetic acid to keteneof 2-methyl-2-penteneto isoprene (in presence of HBr)of chlorodifluoromethaneto tetrafluoroethylene

Fluidized-bed reactor heat supplied along with solids Lurgi SandcrackerMoving-bed reactor heat supplied along with solids Langer –Mond process for production of ultrapure

nickelcontinuous removal of solid products

Reactor with fixed bed ofinerts

fixed bed ensures heat storage and intensive mixing Kureha process for acetylene and ethylene production

production of CS2 from CH4 and sulfur vaporRegenerative furnaces battery operation gas generation from heavy crudes

no dilution by heat-transfer medium

ber of other reaction types, nearly all industriallyimportant polymerization reactions take placein the liquid phase. For the sake of complete-ness,a few important exceptions among poly-merization reactions are included in this sec-tion, even though they do not fall under liquid-phase reactions according to the classificationprinciple stated above. These are, in particular,“gas-phase polymerization” reactions, some ofwhich take place over solid complex catalystsof the Ziegler –Natta type (high-density poly-

ethylene, linear low-density polyethylene, andpolypropylene).

The essential feature of polymerization re-actions is that, in contrast to other liquid-phasereactions, the viscosity increases rapidly duringthe course of reaction and causes difficulties inheat and mass transport. In industry, this prob-lem is countered by (1) the use of special stirringand kneading devices; (2) running the process inseveral stages; (3) raising the temperature as theconversion increases; and (4) carrying out poly-merization in thin films.


Table 3. Reactors for liquid-phase reactions (one or more phases present)


Tubular reactor well-defined residence time polymerization reactionsgood temperature control capabilities bulk polymerization to LDPE∗

polycondensation to PA 66∗ (2nd stage)hydrolysis reactionsof ethylene oxide and propylene oxide to glycolsof chlorobenzene to phenol and chlorotoluene to

cresolof allyl chloride

production of ethyl acetate from acetaldehydeproduction of isopropanolaminedehydrochlorination of 1,1,2-trichloroethane tovinylidene chloride

Reformer high reaction temperature visbreakingwell-defined residence time delayed coking

pyrolytic dehydrochlorination of tetrachloroethane totrichloroethylenehigh-pressure gasification of heavy crudes

Multitubular reactor large heat-transfer area bulk polymerization to PS∗, HIPS∗, and SAN∗multistage design with stirring elements betweenstages is possible

Sulzer mixer – reactor(plug-flow configuration)

mixing elements consist of tubes carryingheat-transfer medium

bulk polymerization to PS∗ and polyacrylates

large heat-transfer area temperature-controlled starch conversionsuitable for processes in which viscosity increasesintensive radial mixing with little axial backmixingvery narrow residence-time distribution

Reactor with externalrecirculation

good mixing and heat-removal conditions cleavage of cumene hydroperoxide to phenol andacetone (2nd stage of Hock process)

no moving parts Beckmann rearrangement of cyclohexanone oxime tocaprolactam

suitable for low reaction rates production of hydroxylamine sulfate (Raschig process)heat exchanger can be placed outside reactor production of phosphoric acid (wet process)

saponification of allyl chloridebulk polymerization to PS∗, HIPS∗, SAN∗, andPMMA∗

Reactor with internalrecirculation

very intensive mixing production of melamine from molten urea(high-pressure process)production of aromatic nitro compoundsproduction of adipic acid from cyclohexanol and nitricacidBulk polymerization to PS∗, HIPS∗, and SAN∗

Loop reactor for slurry polymerization polymerization reactionssuspension is circulated at high velocity to preventbuildup

slurry polymerization to PP∗

production of HDPE∗ and LLDPE∗Powder-bed reactor liquid monomers supported on already

polymerized granulespolymerization reactions

polymerization to HDPE∗ and PP∗block copolymerization to PE – PP∗

for high conversionevaporating and condensing monomer acts asheat-transfer agent (boiling, cooling)vertical and horizontal designs

precipitation polymerization to PAN∗, IIR∗, PE∗,PP∗


Table 3. Continued


Stirred tank, batch orsemicontinuous

limited heat-transport capability polymerization reactions

mechanical stirring means bulk polymerization to PS∗, PMMA∗,suitable for slow reactions HIPS∗, ABS∗ (1st stage of each process)

polycondensation to PA 66∗solution polymerization to PVAC∗, PAN∗, PE∗,

PP∗, EPM∗, EPDM∗, SB∗, SB – S∗, EO – PO∗polycondensation to UF∗, MF∗, PF∗ resinsprecipitation polymerization to PVC∗, PAN∗, PE∗,

PP∗, EPM∗, EPDM∗ suspension polymerization toPVC∗, EPS∗, PMMA∗, PVAC∗, and ion-exchangeresins based on PS∗, HIPS∗, ABS∗ (2nd stage)emulsion polymerization to numerous polymer

dispersionsproduction of aromatic nitro compoundssulfonation of benzeneesterification of PA∗ and alcohol to diphthalatesmany other syntheses of dyes and pharmaceuticals

Stirred tank, continuous suitable for fast reactions with large negative orpositive heat of reaction approximately completemixing conversion generally not completemechanical stirring means

polymerization reactions bulk and solutionpolymerization to PS∗, PMMA∗, HIPS∗, and ABS∗(1st stage in each case); copolymers with nonazeotropicmonomer ratiosprecipitation polymerization to PAN∗, IIR∗, PE∗,

PP∗emulsion polymerization to PVC∗ and SAN∗

esterificationof acrylic acid with alcoholof acetic acid with ethanol

dehydrationof 1,4-butanediol to tetrahydrofuranof ethanol to diethyl ether

saponificationof benzyl chlorideof fatty acids

dehydrochlorinationof 3,4-dichloro-1-butene to chloropreneof 1,1,2-trichloroethane to vinylidene chloride

cyclization of glycols to 1,4-dioxanenitration of aliphatic hydrocarbonsalkylation of isobutane with n-butenesproduction of melamine from molten urea (Montecatini)oxidationof cyclohexanone/ol with HNO3 to adipic acidof mono- to dicarboxylic acidsof allyl alcohol with H2O2 to glycerol

Cascade of stirred tanks suitable for slow reactions adaptable to neededreaction conditions stage by stage residence-timedistribution close to that of tubular reactor

polymerization reactions transesterification of DMT∗ toDGT∗ polycondensation to PETP∗ and PBT∗ solutionpolymerization to BR∗, IR∗, UP∗, UF∗, MF∗, PF∗resinssolution or precipitation polymerization to PE∗, PP∗,

EPM∗, EPDM∗emulsion polymerization to SBR∗, CR∗, NBR∗

production of hydroxylamine sulfate (Raschig process)production of cyclohexanone oxime from cyclohexanoland hydroxylammonium sulfatenitration of aromatic hydrocarbonsdecomposition of ammonium carbamate to ureaproduction of plasticizers from phthalic anhydride andalcoholproduction of MDA∗ in conjunction with downstreamtubular reactorproduction of methacrylamide from acetocyanohydrinproduction of MDI∗ from MDA∗ and TDI∗ fromTDA∗


Table 3. (Continued)


Reaction column reaction and separation in a single apparatus aldol condensation of n-butyraldehyde to 2-ethylhexenalequilibrium can be modified by removing one ormore components from reaction space

saponification

of chloropropanol with milk of limeof fatty acids

esterificationof acetic acid with butanolof phthalic anhydride with alcohols

decompositionof amalgamof ammonium carbamate to urea and water

Multichamber tank virtually identical to cascade of stirred tanks polymerization to LDPE∗ (ICI)requires little spacechamber-by-chamber feed injection possible alkylation of isoparaffins with olefins (Kellogg)

Tower reactor for continuous processes bulk and solution polymerization of PS∗, HIPS∗,ABS∗, SAN∗, PA 6∗

section-by-section temperature control possiblelittle backmixing at high viscosityalso in cascade or with upstream stirred tank

Ring-and-disk reactor narrow residence-time distribution final stage in production of PETP∗ and PBT∗Extruder for highly viscous media polymerization reactions

production of POM∗ from trioxanefinal stage in production of PA 66∗

Fluidized-bed reactor very good heat- and mass-transport conditions polymerization to HDPE∗, LLDPE∗, PP∗fluid coking of heavy residual oils (Exxon)melamine production from molten urea

Mixing head with injectionmold

special design for bringing several liquid reactantstogether

production of PUR∗

Belt reactor with mixing head for fabrication of sheets and films production of PIB∗, PMMA∗, PUR∗, PVAL∗Spinning jet (with coagulatingbath)

for production of strands viscose spinning

Spray reactor direct heating in hot stream of gas thermal H2SO4 cleavageproduction of MgO from MgCl2 (spray calci-

nation)Falling-film reactor gentle temperature control due to large

heat-transfer areasulfation of fatty alcohols

diazotization of aromatic aminesdiazo coupling

∗The following abbreviations are used: ABS= acrylonitrile – butadiene – styrene copolymer; BR= butadiene rubber; CR= chloroprenerubber; DGT= diglycyl terephthalate; DMT= dimethyl terephthalate; EO – PO= ethylene oxide –propylene oxide block copolymer;EPDM= ethylene – (propene – diene) copolymer; EPM= ethylene – propene copolymer; EPS = expandable polystyrene;HDPE= high-density polyethylene; HIPS = high-impact polystyrene; IIR = isobutylene – isoprene rubber (butyl rubber); IR = isoprenerubber (synthetic); LDPE= low-density polyethylene; LLDPE= linear low-density polyethylene; MA=maleic anhydride;MDA=4,4′-diaminodiphenyl methane; MDI = methylene diphenylene isocyanate; MF=melamine – formaldehyde;NBR= butadiene – acrylonitrile copolymer (nitrile rubber); PA= polyamide; PAN= polyacrylonitrile; PBT= poly(butylene terephthalate);PE = polyethylene; PE – PP = polyethylene – polypropylene copolymer; PETP= poly(ethylene terephthalate);PF = phenol – formaldehyde; PIB = polyisobutylene; PMMA=poly(methyl methacrylate); PO= poly(propylene oxide);POM=polyoxymethylene; PP = polypropylene; PS = polystyrene; PUR= polyurethane; PVAC=poly(vinyl acetate); PVAL= poly(vinylalcohol); PVC= poly(vinyl chloride); SAN= styrene – acrylonitrile copolymer; SBR= styrene – butadiene rubber;SB= styrene –butadiene block copolymer; SB – S = styrene – butadiene – styrene block copolymer; TDA= toluene diamine; TDI = toluenediisocyanate; UF = urea – formaldehyde; UP= unsaturated polyester.

Table 3 and Figures 3) and 4 summarize thetypes of reactors used in industry for liquid-phase reactions. Figure 4 shows special reactordesigns for polymerization reactions.

3.3. Reactors for Gas –Liquid Reactions

Gas – liquid reactions include many industriallyimportant processes, such as oxidation, alkyl-ation, chlorination, and flue-gas scrubbing. The

prerequisite for an efficient reaction is rapidmass transport between gas and liquid. Impor-tant criteria for assessment include

1) The interfacial area2) The mass or volume ratio of gas to liquid3) The energy required to mix the phases

Other important factors are temperature control,heat removal, and residence time (especially thatof the liquid phase).


Reactor design is dictated largely by the wayin which the interface is generated. The follow-ing methods are possible:

1) Reactors with continuous liquid-phase andfixed gas distribution devices [bubblecolumns (→Bubble Columns), packed andtray reactors (→Reaction Columns)]

2) Reactors with mechanical gas dispersion(sparged stirred tanks)

Table 4. Reactors for gas – liquid reactions

3) Reactors with continuous gas phase and liq-uid dispersing devices (spray reactors, liq-uid-ring pumps)

4) Thin-film reactors (→Thin-Film Reactors)

Figure 5 illustrates reactor types for gas – liquidreactions. Important applications are listed inTa-ble 4.


Table 4. Continued


3.4. Reactors for Solid-CatalyzedReactions

Heterogeneous catalytic processes play a majorrole in chemical technology, because many keyproducts and intermediates can bemanufacturedin this way. Fluid reactants react in the presenceof a solid catalyst, the mechanism as a wholeconsisting of the reaction proper and a series ofupstream and downstream transport steps.

3.4.1. Reactors for Heterogeneous GasCatalysis

Reactors with a fixed catalyst bed are distin-guished from those with moving catalyst.

Fixed-Bed Reactors (→Fixed-Bed Reac-tors). The characteristic features of a reactorwithfixed catalyst are the pressure drop of the flow-ing gas in the catalyst bed and the danger of un-stable operation points, especially with stronglyexothermic reactions,whenflow through the cat-alyst bed becomes nonuniform. Fixed-bed reac-tors must be shut down after a certain time on-stream to regenerate or replace the catalyst.

Fixed-bed reactors can be classified by thetype of temperature control:

1) Reactors with no special temperature controlfeatures (adiabatic operation)

2) Reactor systems with stagewise temperaturecontrol (chiefly for equilibrium reactions)

3) Reactors with continuous heat exchangealong the flow path (polytropic operation)

Fixed-bed reactors without equipment for tem-perature control are marked by a particularlysimple construction and low flow resistance,whichmakes them suitable for high gas through-puts. A summary of these reactors appears inTable 5 and Figure 6.

Reactor systems with stagewise temperaturecontrol are used primarily for equilibrium reac-tions. Such a reactor consists of simple adiabaticreactor elements connected in series and takesthe form of several units or a system housed ina common reactor shell. Temperature control isaccomplished by heat transfer between reactorstages or by the injection of tempered gas or va-por streams at points along the flow path. Table 6

and Figure 7 present reactor systems of this typealong with applications.

If the reaction process imposes special re-quirements on temperature control, heat-trans-fer surfaces must be located throughout the re-actor volume. The best-known design for sucha reactor is the multitubular reactor, which isfrequently used in the chemical industry. Thedrawbacks relative to other fixed-bed reactorsinclude the much more complicated design andthe limitation on throughput due to the smallercross-sectional area available for flow.

Temperature control is achieved by the useof gaseous and liquid heat-transfer media. Onehighly effective approach is the use of boil-ing liquids (e.g., pressurized-water and evapo-ratively cooled reactors). A special case is theautothermal process regime, in which the reac-tion mixture itself is used as a temperature con-trol medium before it flows through the catalystbed. Fixed-bed reactorswith continuous heat ex-change are described in Table 7 and Figure 8,along with applications.

Moving-Bed and Fluidized-Bed Reactors(→Fluidized-Bed Reactors). In moving-bed re-actors, transport of the catalyst is influenced bygravity and the drag force exerted by the flow-ing reaction fluid on the catalyst particles. Theregime in the reactor can vary widely, depend-ing on the ratio of these forces. The fol-lowingfeatures must be taken into consideration whenusing reactors of this type:

1) The possibility of continuous catalyst regen-eration

2) Increased mechanical loads on the catalystand reactor materials

3) The favorable conditions for heat and masstransport, resulting from rapid movement ofsolids and small catalyst grain size

Table 8 and Figure 9 list reactor types and appli-cations.

3.4.2. Reactors for Liquid-Phase andGas –Liquid Reactions over Solid Catalysts

Fixed-bed reactors (trickle-flow reactors andpacked bubble columns) are used for liquid-phase reactions, as well as gas – liquid reactionsover solid catalysts. The presence of a liquid


Figure 5. Reactors for gas – liquid reactionsA) Tubular reactor with injector; B) Bubble column; C) Liquid-ring pump; D) Sparged stirred tank; E) Buss loop reactor; F)Sulzer mixer – reactor in loop configuration; G) Reaction column; H) Spray reactor; I) Falling-film reactor; J) Rotary kiln; K)Cascade of stirred tanksa) Liquid feed component; b) Gaseous feed component; c) Liquid product; d) Off-gas; e) Packing; f) Heating agent or coolant;g) Drive unit; h) Catalyst; i) Reaction mixer with mixing nozzle; j) Pump; k) Heat exchanger; l) Gas separator; m) Sulzermixer – reactor (see Fig. 3C for detail of a single reactor); n) Static mixer


Figure 6. Fixed-bed catalytic reactors for gas-phase reac-tions with no special provisions for temperature controlA) Simple fixed-bed reactor; B) Fixed-bed reactor with com-bustion zone; C) Radial-flow reactor; D) Shallow-bed reac-tor; E) Regenerative furnacea) Gaseous reaction mixture; b) Gaseous product; c) Cata-lyst; d) Air; e) Hydrocarbon; f) Flue gas; g) Reaction section;h) Regeneration section; i) Condensate; j) Steam; k) Steamgenerator; l) Burner; m) Inert guard bed

Figure 7. Fixed-bed catalytic reactors for gas-phase reac-tions with stagewise temperature controlA) Cascade of simple fixed-bed reactors; B) Multibed re-actor with cold-gas or steam injection; C) Multibed reactorwith intercooling (internal); D) Multibed reactor with inter-cooling (external)a) Gaseous reaction mixture; b) Gaseous product; c) Cata-lyst; d) Heating agent; e) Cold gas; f) Coolant


Table 5. Fixed-bed catalytic reactors for gas-phase reactions with no special provisions for temperature control


Simple fixed-bed very simple design reforming (Platforming, Rheniforming, etc.)reactor not suitable for reactions with large hydrotreating(axial flow) positive or negative heat of reaction CO converting

and high temperature sensitivity amination of methanol to methylaminesdesulfurization and methanation in synthesis-gaspath upstream of primary reformer

hydrogenation of nitrobenzene to aniline (Allied, Bayer)production of vinyl propionates from acetylene andpropionic acid

isomerization of n-alkanesdehydrogenation of ethylbenzene to styrenedisproportionation of toluene to benzene and xylene

Fixed-bed reactor with direct heating by combustion methane cleavage in secondary reformercombustion zone of part of hydrocarbon feedRadial-flow reactor much lower pressure drop than ammonia synthesis (Topsoe, Kellogg)

axial-flow reactor dehydrogenation of ethylbenzene to styrenemultistage configuration possible (Dow)enhanced backmixing due to small reformingthickness of beduniformity of flow requires exact sizingof distributing and collecting ducts

Shallow-bed reactor used for high reaction rates and unstable oxidation of ammonia to NOx

products oxidative dehydrogenation of methanol to formaldehydevery short residence timecatalyst can also be in gauze form production of hydrocyanic acid from ammonia, methane,

and air (Andrussow process)suitable for autothermal operation

Regenerative furnace suitable when catalyst ages rapidly and can beregenerated

dehydrogenation of butane to butadiene (Houdryprocess)

by burning offreaction heat can be supplied by catalystregeneration

SO2 reduction with methane (Andrussow process)

battery operation

Table 6. Fixed-bed catalytic reactors for gas-phase reactions with stagewise temperature control


Cascade of fixed-bed reactors large pressure and temperature differences arepossible

reforming of heavy gasoline

hydrocrackingconversion of H2S and SO2 to elemental sulfur (Clausprocess)isomerization of five-to-six-ring naphthenes

Multibed reactor withcold-gas injection

used for exothermic equilibrium reactions ammonia synthesis

injection of reaction mixture leads to lower methanol synthesisconversion and thus increased number hydrocrackingof stages hydrogenation of benzene

injection of water lowers concentration at constantconversion

desulfurization of vacuum gas oil

adaptation of bed depth to progress of reactionMultibed reactor withinterstage cooling

used for exothermic equilibrium reactions ammonia synthesis (OSW, Fauser, Montecatini)

internal or external heat exchangers SO2 oxidation (with interstage adsorption)no dilution effects hydrodealkylation of alkyl aromaticsadaptation of bed depth to progress of reaction

Multibed reactor with heatsupply

used for endothermic equilibrium reactions dehydrogenation of ethylbenzene to styrene (Dow)

interstage heating or interstage injection ofsuperheated steam


Table 7. Fixed-bed catalytic reactors for gas-phase reactions with continuous temperature control

phase, however, leads to much greater drag andfriction forces on the catalyst. If the reaction in-volves both gas and liquid phases, maintenanceof uniform flow conditions through the catalystbed and intensive mixing of the phases can bedifficult. The crucial factor for the efficiency ofcatalytic processes is the wetting of the catalystby the liquid. Since reactors of this type are usu-

ally operated adiabatically, local overheating isa danger, especially with exothermic reactions.Fixed-bed reactors are well suited to high-pres-sure processes by virtue of their simple design.

A second important group includes suspen-sion reactors, in which very fine catalyst par-ticles are distributed throughout the volume ofthe liquid (stirred tanks and bubble columns


Figure 8. Fixed-bed catalytic reactors for gas-phase reac-tions with continuous temperature controlA) Multitubular reactor; B) Tubular reformer; C) Fixed-bedreactor with heating or cooling elementsa) Gaseous reactionmixture; b) Gaseous product; c) Heatingagent or coolant; d) Catalyst; e) Cooling tubes; f) Circulat-ing water; g) Steam; h) Tube sheet; i) Fuel gas for burners;j) Off-gas

Figure 9.Moving-bed catalytic reactors for gas-phase reac-tionsA) Moving-bed reactor; B) Fluidized-bed reactor; C)Entrained-flow reactora) Reactionmixture; b) Gaseous product; c) Catalyst; d) Air;e) Flue gas; f) Blocking steam; g) Reaction section; h) Re-generation section

with suspended catalyst). Because transport re-sistances are reduced, these reactors offer a closeapproach to isothermal operating conditions anda favorable utilization of the catalyst volume.

Sophisticated techniques are required to sep-arate the finely divided catalyst from the liquid.Equipment for this purpose can be installed in-side or outside the reactor. At the same time,this arrangement permits continuous catalyst re-placement. All suspension reactors have the dis-advantage of increased backmixing, especially

of the liquid phase, which affects product distri-bution.

The fluidized-bed reactor differs from thesuspension reactor in the use of coarser catalystparticles and the formation of a well-defined ag-itated catalyst bed below the liquid level.

Industrially important reactors for liquid-phase and gas – liquid reactions over solid cata-lysts are listed, together with their applications,in Tables 9 and 10 and Figures 10 and 11.


Table 8.Moving-bed catalytic reactors for gas-phase reactions


Moving-bed reactor gravity transport of catalyst cracking (TCC, Houdry flow process)reaction conditions largely similar to those infixed-bed reactor

dehydrogenation of butane

advantageous when catalyst can beregenerated by burning off residues

Fluidized-bed reactor catalyst agitated by gravity and resistanceforce of gas flow

cracking (Kellogg, FFC, Flexicracking)

almost isothermal conditions can be achievedin fluidized bed

hydrocracking

pressure drop independent of gas throughputover a wide range

reforming

form of fluidized bed can be varied as afunction of geometric and hydraulicconditions

ammoxidation

strong backmixing internals to improve masstransport and heat transfer are common

of propene to acrylonitrile (Sohio process)

catalysts must have high abrasion resistance of o-xylene to o-phthalodinitrileproduction of adiponitrile from adipic acidand ammoniaoxychlorination of ethylene to1,2-dichloroethane (Goodrich)production of melamine from urea (BASF)hydrogenationof nitrobenzene to aniline (BASF,

Cyanamid)of ethylene

oxidationof o-xylene or naphthalene to phthalic

anhyrideof butane to MA∗ (Du Pont)of SO2 to SO3

of ethylene to ethylene oxideof NH3 to NOof HCl to chlorine

dehyrogenationof isopropanolof n-butane to n-butene

production of chloromethylsilanes fromchloromethane (catalytic gas – solid reaction)production of vinyl chloride (Cloe process)chlorination of methane and ethyleneproduction of butadiene from ethanolisomerization of n-butaneproduction of isoprenepostchlorination of PVC∗combustion

Entrained-flow reactor uses very fine-grained catalyst Fischer – Tropsch process (Synthol process)whole quantity of catalyst circulatescontinuously between reaction section andtempering or regeneration unit

∗ For abbreviations, see footnote to Table 3

3.5. Reactors for Noncatalytic ReactionsInvolving Solids

A variety of specialized reactors are availablefor noncatalytic reactions involving solids. Thediscussion that follows deals only with the in-dustrially important types.

3.5.1. Reactors for Noncatalytic Gas – SolidReactions

In general, noncatalytic gas – solid reactions arecharacterized by low overall reaction rates and


Table 9. Fixed-bed catalytic reactors for liquid-phase and gas – liquid reactions


Trickle-flow reactor can operate in cocurrent or countercurrent desulfurization and refining of petroleum productstemperature control by intermediate injection orrecirculation

hydrocracking

danger of uneven liquid distribution and incompletewetting of catalyst

production of butynediol from acetylene andformaldehyde

narrow residence-time distribution direct hydration of propene to 2-propanol (Texaco)hydrogenationof organic intermediates (butynediol, adiponitrile,

ethylhexenal)of aldehydes, esters, and carboxylic acids to

alcoholsof natural fats to fatty acidsof residues (low-temperature hydrogenation of tars)

posthydrogenationPacked bubble column danger of flooding limit throughput capacity amination of alcohols

catalyst subject to greater mechanical stress (retentionnecessary)

cobaltizer and decobaltizer in oxo synthesis

high liquid proportion promotes heat removal disproportionation of toluene to benzene and xylenelarge amount of backmixing in liquid phase

Table 10. Suspended-bed and fluidized-bed reactors for liquid-phase and gas – liquid reactions over solid catalysts


Bubble column with simple design hydrogenationsuspended catalyst small pressure drop of CO (Fischer – Tropsch synthesis)

danger of undesired liquid-phase reactions of tars and coals (bottom phase)inhomogeneous catalyst distribution must of benzene to cyclohexanebe prevented hydrodesulfurizationsuitable if product drops out as solid

Reactor with externalrecirculation

heat-exchange and mixing devices in external loop hydrogenation of organic intermediates (nitrobenzenes,nitriles, nitronaphthalenes, etc.)

for continuous and batch operationcatalyst separation outside reactor

Sparged stirred tank withsuspended catalyst

can also be operated in semicontinuous and batchmodes

hydrogenation of organic intermediates (nitrocompounds, aromatics, butynediol)

ensures intensive mixing of all phases fat hydrogenationincreased cost for sealing and maintaining stirrerdrive

catalytic refining

Cascade of sparged stirredtanks with suspended catalyst

higher final conversions than in single stirred tank hydrogenation of NO to hydroxylamine

suitable for slow reaction rates continuous hydrogenation of fatsadaptable to intermediate injection and otherinterconnections

hydrolysis of fats to fatty acids and glycerol productionof toluenediamine from dinitrotoluene

Fluidized-bed reactor small pressure drop catalyst must have very highmechanical strength

hydrocracking and desulfurization of heavy petroleumfractions and still residues (H-Oil process; three-phasefluidized bed)

high process temperatures; in addition, the struc-ture and geometry of the solid can change duringthe reaction.

Reactors for this service can essentially begrouped into those for semicontinuous opera-tion, that is, with no solids transport (verticalshaft kilns and rotary drums), and those for con-tinuous operation, that is, with continuous solidstransport. The second type, in turn, can be di-vided into

1) Reactors with gravity transport of solids2) Reactors with mechanical transport of solids

3) Reactors with pneumatic transport of solids

These three groups differ widely with respectto residence time, conditions of mass and heattransport between gas and solid phases, andheat-input capabilities. The first group includesmoving-bed reactors. Since the gas has to flowthrough the bed of solids, mass and heat trans-port between the phases is relatively good. Tem-perature control can be effected by simultane-ously carrying out exothermic and endothermicreactions in the same reactor.


Reactors with mechanical transport of solidsinclude rotary kilns and multiple-hearth fur-naces (→Metallurgical Furnaces, Chap. 1.,→Metallurgical Furnaces, Chap. 2.). Transportof gas and solid phases through the reactorlargely occurs separately. Intensive heat andmass transfer occurs only at the surface of thebed of solids. Complete involvement of the solidphase in the reaction process depends on con-tinuous, intensive mixing of the solids. Heat isoften supplied directly by burners. More thanone unit can be in operation in a single appara-tus (e.g., drying, heating, cooling, and variousreaction steps).

Figure 10. Fixed-bed catalytic reactors for liquid-phase andgas – liquid reactionsA) Trickle-flow reactor (countercurrent); B) Trickle-flow re-actor (cocurrent); C) Packed bubble columna) Liquid reactants; b) Gaseous reactants; c) Liquid product;d) Off-gas; e) Catalyst; f) Rupture disk

Solids transport by the gas stream is possibleonly with small particle sizes and the narrow-est possible grain-size distribution. This groupincludes fluidized-bed and entrained-flow reac-tors, dust roasters, and suspension furnaces. Be-cause of the favorable conditions for heat andmass transport, these reactors offer shorter res-idence times and thus higher throughputs thanother types. The installation of heat-transfer sur-faces, supplementary solid heat-transfer media,and direct heating is possible.

Industrially important reactor types for non-catalytic gas – solid reactions are listed in Ta-ble 11 and Figure 12 along with applications..

3.5.2. Reactors for NoncatalyticLiquid – Solid Reactions

Reactors used for noncatalytic liquid – solid re-actions must be designed for the transport andmixing of phases, sometimes at high solids con-tents. Batch and semicontinuous designs aretherefore dominant. Table 12 and Figure 13present a survey of important reactor types fornoncatalytic liquid – solid reactions and sampleapplications.

3.5.3. Reactors for Noncatalytic Solid-PhaseReactions

Reactors used for noncatalytic solid-phase reac-tions are similar to those used for noncatalyticgas – solid reactions. Long residence times andhigh reaction temperatures are necessary, espe-cially for reactions between different solids, be-cause of the low transport rates therein. Heatcan be supplied by indirect or direct heating orby burning solid fuels.

Inert gases are employed for heat transportand agitation of the solids. Important applica-tions are listed in Table 13.

3.6. Electrothermal Reactors

A variety of electrical heating schemes are usedfor some important noncatalytic reactions bet-ween gases and solids when very high reactiontemperatures and large quantities of heat are re-quired. In the simplest case, heating elements


Table 11. Reactors for noncatalytic gas – solid reactions

(rods, strips, etc.) are used for this purpose. Amuch more efficient method, however, is direct, electric heating. Options here include arc, re-sistance, and induction heating.

The very high temperatures produced bythe arc cause ionization in gases and thusactivate the reactants; this feature is uti-lized in plasma processes for high-tempera-


Table 12. Reactors for noncatalytic liquid – solid reactions


Stirred tank batch or semicontinuous operation predominant production of alkali cellulose and nitrocellulosesolids content limited by power of stirring apparatus reduction of nitrobenzene with metals to aniline or

hydrazobenzenebauxite digestionproduction of salicylic acid from dry sodium phenolate(Kolbe – Schmitt process)hydrolysis of calcium cyanamide to cyanamideproduction of BF3 from B2O3, CaF2, and H2SO4

production of alkylaluminums from aluminum, olefin,and hydrogenproduction of tetraethyllead

Cascade of stirred tanks for low reaction rates and high final conversions apatite digestionTank with liquid recirculation semicontinuous operation with solids fixed in tank

and liquid recirculatingcellulose digestion

production of ammonium sulfate from ammoniumcarbonate and gypsum

Rotary drum for batch operation, high solids content production of cellulose acetate and cellulose ethersproduction of AlF3 by wet process

Fluidized-bed reactor Semicontinuous operation water treatmentintensive liquid circulation

Steeping press combination of reaction and liquid separation production of cellulose etherbatch operation

Kneader used for highly viscous media production of nitrocellulose, cellulose ether, andcellulose acetate

for batch operation production of celluloid from nitrocelluloseproduction of superphosphate

Screw-conveyor reactor used for highly viscous media digestion of rutile or ilmenite with H2SO4

batch operationMultiple-hearth reactor continuous operation production of acetylene from carbide (dry gas generator)

long solids residence timeRotary kiln direct heating for high reaction temperatures digestion of fluorspar or phosphate with H2SO4reducing

decomposition of H2SO4 in presence of carbon

Table 13. Reactors for noncatalytic solid-phase reactions


Shaft reactor see Table 11 metallurgical processes, e.g.,powder boriding of iron-based materialsdirect reduction of iron ores with carbon (Kinglor –Metor

process)Multiple-hearth reactor see Table 11 calcinationRotary kiln see Table 11 cement production

burning of lime, dolomite, gypsum, and magnesitecalcinationthermal decomposition of FeSO4 and BaCO3

reduction of barite with carbon to BaSreduction of ores with carbon (e.g., to ZnO)

Fluidized-bed reactor see Table 11 burning of lime (multistage)

ture pyrolysis (→Plasma Reactions, Chap. 2.1.;→Metallurgical Furnaces, Chap. 5.5.).

Equipment used for solid reactions includesarc and resistance-heated reduction furnacesand the Acheson furnace (→Metallurgical Fur-naces, Chap. 5.2., →Metallurgical Furnaces,Chap. 5.3.). TheAcheson furnace is a resistance-heated device for pure solid – solid reactions;

that is, in contrast to other processes, no melt-ing of the solid charge occurs. All electrothermalprocesses are characterized by very high equip-ment cost and high electric power consumption.The prerequisite for their economical operationis a low unit price for energy.

This group of reactors and their applicationsare summarized in Table 14 and Figure 14.


Figure 11. Suspended-bed and fluidized-bed reactors for liquid-phase and gas – liquid reactions over solid catalystsA) Bubble column with suspended catalyst; B) Fluidized-bed reactor; C) Buss loop reactor; D) Sparged stirred tank withsuspended catalyst; E) Cascade of sparged stirred tanks with suspended catalysta) Liquid feed components; b) Gaseous feed components; c) Liquid product; d) Catalyst; e) Off-gas; f) Heating agent orcoolant; g) Heat exchanger; h) Pump; i) Reaction mixer with mixing nozzle

3.7. Reactors for ElectrochemicalProcesses (→Electrochemistry;→Metallurgical Furnaces, Chap. 5.7.)

In electrochemical reactions, electrons are sup-plied to a reactant in the electrolyte or re-movedfrom it with the aid of an electric current. Amin-imum voltage called the decomposition voltagemust be applied to the electrodes for this pur-pose. In addition to the electrochemical reac-tions occurring on the electrode surface, trans-port processes and chemical reactions in theelectrolyte bath are important.

Electrochemical processes have the follow-ing advantages:

1) High product purity (no secondary reactions)

2) Low reaction temperature (except for fused-salt electrolysis)

3) Easy control of reaction rate through varia-tion of electrode voltage

They have the following disadvantages:

1) High energy losses in the system2) Large space requirements3) High investment costs

For these reasons, electrochemical processes areused only when no available thermal or catalyticprocess can accomplish the samepurpose,whichis especially true in the production of chlorine,aluminum, and copper. A survey of importantapplications for electrolytic processes is givenin the following:


Figure 12. Reactors for noncatalytic gas – solid reactionsA) Shaft kiln; B) Moving-bed reactor; C) Multiple-hearth reactor; D) Rotary kiln; E) Fluidized-bed reactor; F) Spray reactor;G) Entrained-flow reactora) Solid feed components; b) Gaseous feed components; c) Solid product; d) Off-gas; e) Air; f) Cyclone; g) Drive unit


Figure 13. Reactors for noncatalytic liquid – solid reactionsA) Stirred tank; B) Cascade of stirred tanks; C) Tank with liquid recirculation; D) Rotary drum; E) Fluidized-bed reactor; F)Steeping press; G) Kneader; H) Screw-conveyor reactor; I) Multiple-hearth reactor; J) Rotary kilna) Liquid feed components; b) Solid feed components; c) Liquid product; d) Solid product; e) Drive unit


Table 14. Electrothermal reactors

Chlorine production by chlor – alkali elec-trolysis

– Mercury amalgam process– Diaphragm-cell process– Membrane process

Metal winning by fused-salt electrolysis– Aluminum– Magnesium– Sodium

Metal refining– Copper– Nickel

Electrolysis of inorganic materials– Electrolysis of water– Fluorine production by electrolysis of hydro-gen fluoride

– Production of sodium chlorate by electroly-sis of sodium chloride

– Electrochemical oxidation of sodium chlo-rate to perchlorate

– Recovery of persulfuric acid– Production of ozone

Electrolysis of organic materials– Production of adiponitrile from acrylonitrile– Production of dimethyl sebacate– Reduction of nitrobenzene to aniline

– Production of perfluorocaprylic acid– Production of dihydrostreptomycin

The design of the reaction system (i.e., cell ge-ometry and flow configuration), the electrode ar-rangement and material, and control of phasesand concentrations are highly process specific.

Typical designs are illustrated in Figure 15.

3.8. Reactors for Biochemical Processes(→Biochemical Engineering;→Biotechnology)

Some important biochemical processes, such asthose used in making beer, wine, alcohol, andbaker’s yeast, have been known for centuries.Typical of these reactions is their use of enzymesas biocatalysts. The enzymes can be present ascell constituents of living microorganisms, orthey can be isolated in dissolved form or boundto inert supports (→ Immobilized Biocatalysts).The prerequisite for the use of live microorgan-isms is the provision of favorable living con-ditions. Such conditions include the presenceof optimal amounts of nutrients and oxygen (inaerobic processes); maintenance of the temper-ature, pressure, maintenance of pH in certainranges, and sterile conditions.


Figure 14. Reactors for electrothermal processesA) Plasma torch; B) Fluohm reactor; C) Arc-heated reduc-tion furnace; D) Resistance-heated reduction furnace; E)Acheson furnace; F) Reactor with indirect electric heatinga) Solids; b) Molten product; c) Gaseous reaction mixture;d)Gaseous product; e)Catalyst; f) Carrier gas; g) Electrodes;h) Plasma; i) Slag; j) Resistive charge; k) Off-gas

In addition to these factors,metabolism is im-portant for reactor design. Aerobic processes re-quire an adequate supply of oxygen. In anaero-bic processes, the admission of gas from outsidemust be prevented; gases and solvent vapors re-sulting from the reaction must also be removedfrom the reactor.

Reactors for these processes can be classifiedas follows:

1) Reactorswith dissolved or suspended biocat-alysts (submerged processes) for aerobic oranaerobic conditions

2) Reactors with immobilized biocatalysts foraerobic or anaerobic conditions

Reactors for use in submerged aerobic pro-cesses have provisions for efficient aeration andintensive liquid circulation. Aeration is accom-plished with fixed or moving distributors, noz-zles, or submerged or rotating jets. Liquid circu-lation is ensured by various stirring systems orby forced or natural convection.

A summary of the most important reactortypes and their applications is given in Table 15and Figure 16.

Reactors for anaerobic conditions do nothave aeration equipment. Usually, sealed vesselswith or without stirrers are used (fermenters).Applications of these reactor types include fer-mentation processes (e.g., lactic acid fermenta-tion, alcohol production, mash fermentation).

The immobilization of enzymes on suitablesupports enables the use of reactor designs sim-ilar to those for heterogeneous catalytic process-es. If the enzymes are supported on semiperme-able membranes, separation and reaction can becombined in membrane reactors.

Reactors with immobilized biocatalysts, to-gether with their applications, are listed in Ta-ble 16 and Figure 17.

3.9. Reactors for Photochemical andRadiochemical Processes

The photochemical and radiochemical princi-ples are used to a very limited extent in indus-try because conditions for economical operation(e.g., high quantum efficiency) are seldom met.

3.9.1. Photochemical Reactors(→Photochemistry, Chap. 3.)

The rate of a photochemical reaction is deter-mined by the concentration of reactants and bythe intensity, quantity, and wavelength of lightsupplied. Light in the wavelength range that isabsorbedby the reactionmixture canbe formally


Figure 15. Reactors for electrochemical processesA) Metal winning by fused-salt electrolysis; B) Electrolytic metal refining; C) Electrolysis of inorganic material; D) Electrol-ysis of organic material; E) Mercury amalgam process; F) Diaphragm-cell process; G) Membrane processa)Water; b) Chlorine; c) Sodium chloride; d) Hydrogen; e) Sodium; f) Sodium hydroxide; g) Anode; h) Cathode; i) Membrane;j) Product; k) Amalgam; l) Recycle brine + chlorine; m) Mercury; n) Graphite; o) Diaphragm; p) Electrolytic salt solution ofmetal to be refined; q) Anode slime; r) Electrolyte removal; s) Organic feed solution; t) Oxygen

treated as a reactant. As a consequence, pho-tochemical reactions exhibit a position depen-dence of the reaction rate, even with completemixing, because the flux density of light quantadecreaseswith increasing distance from the light

source. The feasible thickness of the reactionspace, and thus the type and size of reactor thatcan be used, depend not only on the power ofthe emitter, but also on the optical properties ofthe reactormaterial and the reactionmedium. In-


Figure 16. Reactors for submerged aerobic processesA) Sparged stirred tank; B) Bubble column with forced circulation; C) Jet reactor with forced circulation; D) Submerged-jetreactor with forced circulation; E) Bubble column with natural circulation; F) Loop reactor; G) Sieve-tray tower; H) Trickle-bed reactor; I) Reactor with rotating internalsa) Gas; b) Fermentation medium; c) Product; d) Off-gas; e) Recycle stream


Table 15. Reactors for submerged aerobic processes


Sparged stirred tank various stirring and circulation apparatus suitable for higherviscosities

production of antibiotics amino acids yeast

Reactors with forced circulationBubble column very broad residence-time distribution production of yeast

good dispersion properties aerobic wastewater treatmentJet reactor free jet, jet nozzle, or central tube designs possible

for low viscositieshigh gas velocities, good mass transfer

Submerged-jet reactor very broad residence-time distribution processing of spent sulfite liquorgood mass transfer fermentation of waste substratesdanger of slime settling out

Reactors with naturalcirculationBubble column much backmixing, broad residence-time distribution production of biomass citric acid

for low viscositiessimple construction

Loop reactor for low viscositieslittle dispersive action

Sieve-tray tower good mass transfer due to fine bubble structuresSurface reactorsTrickle-bed reactor low mass-transfer coefficients and negligible dispersive action production of acetic acid aerobic

wastewater treatmentReactor with rotating internals use of paddles, cylinders, etc. suitable for viscous media aerobic wastewater treatment

Table 16. Reactors for biochemical processes over immobilized biocatalysts (for aerobic and anaerobic conditions)

tensive mixing must be ensured, especially forthick beds. Light can be supplied from outside(through the reactor wall) or by submerged lightsources.Whenhigh-power light sources are used

a large amount of heat is evolved and supplemen-tal cooling devices must be employed.

A survey of reactor types and their industrialapplications appears in Table 17 and Figure 18.


Figure 17. Reactors for biochemical processes over immo-bilized biocatalysts (for aerobic and anaerobic conditions)A) Stirred tank with suspended catalyst; B) Fixed-bed reac-tor; C) Fluidized-bed reactor; D) Membrane reactora) Biocatalyst; b) Fermentation medium; c) Product; d) Off-gas; e) Permeate; f) Membrane tube; g) Retentate

Figure 18. Reactors for photochemical processesA) Tubular reactor; B) Bubble column; C) Stirred tank; D)Falling-film reactor; E) Belt reactora) Gaseous feed components; b) Liquid feed components;c) Product; d) Emitter; e) Coolant; f) Off-gas; g) Externalreflector; h) Falling film; i) Belt

3.9.2. Radiochemical Reactors (→RadiationChemistry)

Radiochemical reactions are induced by the ac-tion of ionizing radiation. In addition to highenergy consumption, the extremely complex de-sign of radiation sources and shielding worksagainst the wider use of this reaction principle.

The following are known applications:

1) Production of ethyl bromide (Dow process,Fig. 19)

2) Radiative cross-linking of poly(vinyl chlo-ride) and polyethylene

3) Production of alkyltin compounds4) Degradation of polymers

Some reactions, such as chlorinations, can beimplemented in either photochemical or radio-chemical form.


Table 17. Reactors for photochemical processes


Tubular reactor for homogeneous gas- and liquid-phase reactions chlorination of benzene to hexachlorocyclohexanesulfochlorinationchlorination of methane to dichloromethane

Bubble column requires favorable optical conditions and low viscosity sulfochlorination of paraffins (cascade)also used in cascades and with central tube side-chain chlorination of aromatics

production of dodecanethiol from 1-dodecene andH2S

Stirred tank optically induced differences in reaction rate equalizedby intensive stirring

oximation of cyclohexane with nitrosyl chloride

production of provitamin D3

Falling-film reactor suitable for poor optical conditions because film is verythin

production of vitamin D2

Belt reactor especially for highly viscous media polymerization to PAN, PAC, PVC, PVAC∗∗ PAN=polyacrylonitrile; PAC= polyacrylate; PVC= poly(vinyl chloride); PVAC= poly(vinyl acetate).

Figure 19. A reactor for a radiochemical process (produc-tion of ethyl bromide by the Dow process)a) Gaseous reactionmixture; b) Liquid product; c) Shielding

4. References

1. “Chemische Reaktoren-Ausrustungen und ihreBerechnung,” VerfahrenstechnischeBerechnungsmethoden, part 5, VEBDeutscher Verlag fur Grundstoffindustrie,Leipzig 1981.

2. H. Gerrens: “Uber die Auswahl vonPolymerisationsreaktoren,” Chem. Ing. Tech.52 (1980) 477 – 488.

3. K.H. Reichert, W. Geiseler (eds.): PolymerReaction Engineering, VCHVerlagsgesellschaft, Weinheim 1989.

4. W.-D. Deckwer: “Bioreaktoren,” Chem. Ing.Tech. 60 (1988) 583 – 590.

5. K. Schugerl: “Characteristic Features ofBioreactors,” Bioreaction Engineering, vol. 2,John Wiley and Sons, New York 1990.

6. A. Heger: Technologie der Strahlenchemie vonPolymeren, Carl Hanser Verlag, Munchen1990.

Reactor Types and Their Industrial Applications

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