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©c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10.1002/14356007.b04 087
Reactor Types and Their Industrial Applications
Klaus-Dieter Henkel, Buna AG, Schkopau, Federal Republic of Germany
1. Introduction............... 1
2. Basic Types of Reactors........ 2
3. Survey of Real Reactors and Their
Uses..................... 4
3.1. Reactors for Gas-Phase Reactions. 4
3.2. Reactors for Liquid-Phase Reac-
tions..................... 8
3.3. Reactors for Gas – Liquid Reactions 8
3.4. Reactors for Solid-Catalyzed Reac-
tions..................... 13
3.4.1. Reactors for Heterogeneous Gas Catal-
ysis..................... 13
3.4.2. Reactors for Liquid-Phase and Gas –
Liquid Reactions over Solid Catalysts 13
3.5. Reactors for Noncatalytic Reactions
Involving Solids............. 19
3.5.1. Reactors for Noncatalytic Gas – Solid
Reactions.................. 19
3.5.2. Reactors for Noncatalytic Liquid –
Solid Reactions.............. 21
3.5.3. Reactors for Noncatalytic Solid-Phase
Reactions.................. 21
3.6. Electrothermal Reactors....... 21
3.7. Reactors for Electrochemical Pro-
cesses.................... 24
3.8. Reactors for Biochemical Processes 27
3.9. Reactors for Photochemical and Ra-
diochemical Processes......... 28
3.9.1. Photochemical Reactors......... 28
3.9.2. Radiochemical Reactors........ 32
4. References................. 33
1. Introduction
The reactor in which the chemical reaction takes
place occupies a central position in the chemi-
cal process. Grouped around the reactor are the
process steps involving physical treatment of the
material streams, such as conveyance, heat trans-
fer, and separation and mixing operations. The
reactor provides the volume necessary for the re-
action and holds the amount of catalyst required
for the reaction. The energy required to over-
come the activation threshold of each partial re-
action is also supplied in the reactor, and the
proper temperature and concentration are main-
tained.
The most important reaction-related factors
for the design of a reactor are
1) The activation principle selected, together
with the states of aggregation of the reactants
and the resulting number and types of phases
involved
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 a
reaction mixture include
1) Activation by addition of heat
2) Catalytic activation
3) Activation by decomposition of an initiator
4) Electrochemical activation
5) Biochemical activation
Less important options for activation are visible
or 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 can
be present, or be produced, in various states of
aggregation. Furthermore, inert diluents or heat-
transfer media can be present in different phases.
Finally, the catalyst, which is generally in the
solid or liquid phase, often has to be taken into
consideration.
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 frequently
require supply or removal of large quantities of
heat. Thermally neutral reactions involve con-
siderably less technical sophistication.
The concentration and temperature depen-
dences of a chemical reaction are described by
the reaction rate. In practice most reaction sys-
tems are complex and include parallel, sequen-
tial, and equilibrium reactions. To obtain the
highest possible yield of desired product under
these conditions, the temperature and pressure must be held within certain ranges, the tempera- ture must be controlled along the reaction path, and a definite residence-time distribution in the reactor must be achieved. If, in addition, sub- stances or energy have to be transferred from one phase to another, appropriate transport con- ditions have to be implemented. When catalysts are used, catalyst loss due to aging and poison- ing must be considered. These factors impose a complex of requirements that must be kept in mind when designing a reactor. Against the requirements established by the process, the designer must balance costs of fab- rication, consumption of materials, and opera- tional reliability. In practice, many possibilities are often available for realizing a chemical pro- cess, and in such cases the decision must depend on an assessment of the overall process as well as commercial constraints on the plant.
2. Basic Types of Reactors (→ Model
Reactors and Their Design Equations)
A variety of reactor designs are used in indus- try, but all of them can be assigned to certain basic types or combinations of these. The basic types are as follows (see → Principles of Chem- ical Reaction Engineering, Chap. 4.2.):
Batch stirred-tank reactor
Continuous stirred-tank reactor
Tubular reactor
Given certain flow and thermal conditions, these types are also referred to as “ideal” reactors. With respect to flow conditions the ideal stirred- tank batch reactor is characterized by complete mixing 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 residence time. 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 is based on the assumption of a reaction system that is homogeneous as regards the phase. Thus transport resistance between phases does not oc- cur.
The thermally ideal operating states are the isothermal and adiabatic states, i. e., either very intensive heat exchange with the surroundings or no exchange at all is assumed. In practical operation, the ideal states are achieved only approximately. Examples of typ- ical nonidealities include
- The formation of real flow patterns, such as dead zones, short-circuit flows, and channel- ing
- Transport processes in the individual phases, such as axial backmixing
- The formation of concentration and temper- ature profiles as a result of transport resis- tances in and between phases
- Segregation processes
- Incomplete mixing of reactants
The essential advantages and disadvantages of the three basic reactor types are discussed in what follows.
Batch Stirred Tank (→ Stirred-Tank and Loop Reactors) Principal Applications:
- Liquid-phase reactions
- Liquid – solid reactions
Advantages:
- Quick production changeover possible; use for substances produced on a small scale
- Process steps upstream or downstream of the reaction can also be performed in the reactor
- Better process control than in continuous op- eration when solid or highly viscous phases form or are present
- Well-defined residence time
Disadvantages :
- Relatively high operating costs due to long downtimes and high manpower requirements
- Quality differences between charges because reaction conditions are only partly repro- ducible
- Limited temperature control capabilities, es- pecially with highly endothermic or exother- mic reactions
3.1. Reactors for Gas-Phase Reactions
Homogeneous gas-phase reactions utilized in
industry are generally characterized by large
positive or negative enthalpies of reaction and
high reaction temperatures. To obtain the desired
product spectrum, residence times must usually
be very short. The high reaction temperature can
be maintained or the requisite heat supplied by
burning part of the feed.
Tables 1 and 2 and Figures 1 and 2 summa-
rize the reactors used for such reactions as well
as their applications.
Figure 1. Reactors for exothermic gas-phase reactions
A) Burner; B) Tubular reactor; C) Reactor with recycle; D)
Fluidized-bed reactor
a) Gaseous reaction mixture; a 1 , a 2 ) Gaseous feed compo-
nents; b) Gaseous product; c) Coolant; d) Partial stream of
product; e) Catalyst
Figure 2. Reactors for endothermic gas-phase reactions
A) Burner; B) Reformer; C) Fluidized-bed reactor; D)
Moving-bed reactor; E) Reactor with fixed bed of inerts;
F) Regenerative furnaces
a) 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-Phase
Reactions
In general, liquid-phase reactions are exother- mic. In the case of multiphase systems, inten- sive mass and heat transfer must be provided for; this is possible only in reactors with compulsory mixing, such as stirred tanks. Along with a num-
Figure 3. Reactors for liquid-phase reactions
A) Tubular reactor; B) Reformer; C) Sulzer mixer – reactor; D) Reactor with external recirculation; E) Reactor with internal
recirculation (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 reactor
a) Liquid reaction mixture; a 1 , a 2 ) 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 preceding
reaction 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◦
Table 1. Reactors for exothermic gas-phase reactions
Reactor type Features Examples of applications
Burner for high reaction rates combustion of H 2 S to SO 2 (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 methane nitration of propane Tubular reactor well-defined residence time (tubes up to 1000 m long) chlorination of methane intermediate injection possible of propene to allyl chloride pressure drops of butadiene to dichlorobutane good temperature control capability chlorolysis of chlorinated hydrocarbons Reactor with recycle suitable for low reaction rates chlorination of methane good mixing cooling inside or outside reactor Fluidized-bed reactor nearly isothermal conditions because heat transport is very efficient
chlorination
of methane intensive mixing of 1,2-dichloroethane to tri- and perchloroethylene chlorolysis of chlorinated hydrocarbons
Table 2. Reactors for endothermic gas-phase reactions
Reactor type Features Examples of applications
Burner very high reaction temperatures attainable by partial combustion of reactants
Sachsse – Bartholom´e process for acetylene production
short residence times high-pressure gasification for synthesis gas production (Texaco, Shell) Reformer high reaction temperatures attainable mainly by radiation
steam cracking of naphtha and other hydrocarbons to ethylene well-defined residence times vinyl chloride production by cleavage of dichloroethane pyrolysis of acetic acid to ketene of 2-methyl-2-pentene to isoprene (in presence of HBr) of chlorodifluoromethane to tetrafluoroethylene Fluidized-bed reactor heat supplied along with solids Lurgi Sandcracker Moving-bed reactor heat supplied along with solids Langer – Mond process for production of ultrapure nickel continuous removal of solid products Reactor with fixed bed of inerts
fixed bed ensures heat storage and intensive mixing Kureha process for acetylene and ethylene production
production of CS 2 from CH 4 and sulfur vapor Regenerative furnaces battery operation gas generation from heavy crudes no dilution by heat-transfer medium
ber of other reaction types, nearly all industrially
important polymerization reactions take place
in 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 classification
principle stated above. These are, in particular,
“gas-phase polymerization” reactions, some of
which take place over solid complex catalysts
of the Ziegler – Natta type (high-density poly-
ethylene, linear low-density polyethylene, and polypropylene). The essential feature of polymerization re- actions is that, in contrast to other liquid-phase reactions, the viscosity increases rapidly during the course of reaction and causes difficulties in heat and mass transport. In industry, this prob- lem is countered by (1) the use of special stirring and kneading devices; (2) running the process in several stages; (3) raising the temperature as the conversion increases; and (4) carrying out poly- merization in thin films.
Table 3. Reactors for liquid-phase reactions (one or more phases present)
Reactor type Features Examples of applications
Tubular reactor well-defined residence time polymerization reactions good temperature control capabilities bulk polymerization to LDPE ∗ polycondensation to PA 66 ∗ (2nd stage) hydrolysis reactions of ethylene oxide and propylene oxide to glycols of chlorobenzene to phenol and chlorotoluene to cresol of allyl chloride production of ethyl acetate from acetaldehyde production of isopropanolamine dehydrochlorination of 1,1,2-trichloroethane to vinylidene chloride Reformer high reaction temperature visbreaking well-defined residence time delayed coking pyrolytic dehydrochlorination of tetrachloroethane to trichloroethylene high-pressure gasification of heavy crudes Multitubular reactor large heat-transfer area bulk polymerization to PS ∗, HIPS ∗, and SAN ∗ multistage design with stirring elements between stages is possible Sulzer mixer – reactor (plug-flow configuration)
mixing elements consist of tubes carrying heat-transfer medium
bulk polymerization to PS ∗ and polyacrylates
large heat-transfer area temperature-controlled starch conversion suitable for processes in which viscosity increases intensive radial mixing with little axial backmixing very narrow residence-time distribution Reactor with external recirculation
good mixing and heat-removal conditions cleavage of cumene hydroperoxide to phenol and acetone (2nd stage of Hock process) no moving parts Beckmann rearrangement of cyclohexanone oxime to caprolactam 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 chloride bulk polymerization to PS ∗, HIPS ∗, SAN ∗, and PMMA ∗ Reactor with internal recirculation
very intensive mixing production of melamine from molten urea (high-pressure process) production of aromatic nitro compounds production of adipic acid from cyclohexanol and nitric acid Bulk polymerization to PS ∗, HIPS ∗, and SAN ∗ Loop reactor for slurry polymerization polymerization reactions suspension is circulated at high velocity to prevent buildup
slurry polymerization to PP ∗
production of HDPE ∗ and LLDPE ∗ Powder-bed reactor liquid monomers supported on already polymerized granules
polymerization reactions
polymerization to HDPE ∗ and PP ∗ block copolymerization to PE – PP ∗ for high conversion evaporating and condensing monomer acts as heat-transfer agent (boiling, cooling) vertical and horizontal designs precipitation polymerization to PAN ∗, IIR ∗, PE ∗, PP ∗
Table 3. (Continued)
Reactor type Features Examples of applications
Reaction column reaction and separation in a single apparatus aldol condensation of n -butyraldehyde to 2-ethylhexenal equilibrium can be modified by removing one or more components from reaction space
saponification
of chloropropanol with milk of lime of fatty acids esterification of acetic acid with butanol of phthalic anhydride with alcohols decomposition of amalgam of ammonium carbamate to urea and water Multichamber tank virtually identical to cascade of stirred tanks polymerization to LDPE ∗ (ICI) requires little space chamber-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 possible little backmixing at high viscosity also 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 trioxane final 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 injection mold
special design for bringing several liquid reactants together
production of PUR ∗
Belt reactor with mixing head for fabrication of sheets and films production of PIB ∗, PMMA ∗, PUR ∗, PVAL ∗ Spinning jet (with coagulating bath)
for production of strands viscose spinning
Spray reactor direct heating in hot stream of gas thermal H 2 SO 4 cleavage production of MgO from MgCl 2 (spray calci- nation) Falling-film reactor gentle temperature control due to large heat-transfer area
sulfation of fatty alcohols
diazotization of aromatic amines diazo coupling
∗ The following abbreviations are used: ABS = acrylonitrile – butadiene – styrene copolymer; BR = butadiene rubber; CR = chloroprene rubber; 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 = isoprene rubber (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(vinyl alcohol); 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 = toluene diisocyanate; UF = urea – formaldehyde; UP = unsaturated polyester.
Table 3 and Figures 3) and 4 summarize the
types of reactors used in industry for liquid-
phase reactions. Figure 4 shows special reactor
designs for polymerization reactions.
3.3. Reactors for Gas – Liquid Reactions
Gas – liquid reactions include many industrially
important processes, such as oxidation, alkyl-
ation, chlorination, and flue-gas scrubbing. The
prerequisite for an efficient reaction is rapid mass transport between gas and liquid. Impor- tant criteria for assessment include
- The interfacial area
- The mass or volume ratio of gas to liquid
- The energy required to mix the phases
Other important factors are temperature control, heat removal, and residence time (especially that of the liquid phase).
Reactor design is dictated largely by the way in which the interface is generated. The follow- ing methods are possible:
- Reactors with continuous liquid-phase and fixed gas distribution devices [bubble columns (→ Bubble Columns), packed and tray reactors (→ Reaction Columns)]
- Reactors with mechanical gas dispersion (sparged stirred tanks)
Table 4. Reactors for gas – liquid reactions
- Reactors with continuous gas phase and liq- uid dispersing devices (spray reactors, liq- uid-ring pumps)
- Thin-film reactors (→ Thin-Film Reactors)
Figure 5 illustrates reactor types for gas – liquid reactions. Important applications are listed in Ta- ble 4.
3.4. Reactors for Solid-Catalyzed
Reactions
Heterogeneous catalytic processes play a major role in chemical technology, because many key products and intermediates can be manufactured in this way. Fluid reactants react in the presence of a solid catalyst, the mechanism as a whole consisting of the reaction proper and a series of upstream and downstream transport steps.
3.4.1. Reactors for Heterogeneous Gas Catalysis
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 reactor with fixed catalyst are the pressure drop of the flow- ing gas in the catalyst bed and the danger of un- stable operation points, especially with strongly exothermic reactions, when flow 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 the type of temperature control:
- Reactors with no special temperature control
features (adiabatic operation)
- Reactor systems with stagewise temperature
control (chiefly for equilibrium reactions)
- Reactors with continuous heat exchange
along the flow path (polytropic operation)
Fixed-bed reactors without equipment for tem- perature control are marked by a particularly simple construction and low flow resistance, which makes them suitable for high gas through- puts. A summary of these reactors appears in Table 5 and Figure 6. Reactor systems with stagewise temperature control are used primarily for equilibrium reac- tions. Such a reactor consists of simple adiabatic reactor elements connected in series and takes the form of several units or a system housed in a common reactor shell. Temperature control is accomplished by heat transfer between reactor stages 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 type along 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 such a reactor is the multitubular reactor , which is frequently used in the chemical industry. The drawbacks relative to other fixed-bed reactors include the much more complicated design and the limitation on throughput due to the smaller cross-sectional area available for flow. Temperature control is achieved by the use of gaseous and liquid heat-transfer media. One highly effective approach is the use of boil- ing liquids (e.g., pressurized-water and evapo- ratively cooled reactors). A special case is the autothermal process regime, in which the reac- tion mixture itself is used as a temperature con- trol medium before it flows through the catalyst bed. Fixed-bed reactors with 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 by gravity and the drag force exerted by the flow- ing reaction fluid on the catalyst particles. The regime in the reactor can vary widely, depend- ing on the ratio of these forces. The fol-lowing features must be taken into consideration when using reactors of this type:
- The possibility of continuous catalyst regen- eration
- Increased mechanical loads on the catalyst and reactor materials
- The favorable conditions for heat and mass transport, resulting from rapid movement of solids and small catalyst grain size
Table 8 and Figure 9 list reactor types and appli- cations.
3.4.2. Reactors for Liquid-Phase and Gas – Liquid Reactions over Solid Catalysts
Fixed-bed reactors (trickle-flow reactors and packed bubble columns) are used for liquid- phase reactions, as well as gas – liquid reactions over solid catalysts. The presence of a liquid
Figure 5. Reactors for gas – liquid reactions
A) 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 tanks
a) 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) Sulzer
mixer – reactor (see Fig. 3C for detail of a single reactor); n) Static mixer
Table 5. Fixed-bed catalytic reactors for gas-phase reactions with no special provisions for temperature control
Reactor type Features Examples of applications
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 methylamines desulfurization and methanation in synthesis-gas path upstream of primary reformer hydrogenation of nitrobenzene to aniline (Allied, Bayer) production of vinyl propionates from acetylene and propionic acid isomerization of n -alkanes dehydrogenation of ethylbenzene to styrene disproportionation of toluene to benzene and xylene Fixed-bed reactor with direct heating by combustion methane cleavage in secondary reformer combustion zone of part of hydrocarbon feed Radial-flow reactor much lower pressure drop than ammonia synthesis (Topsoe, Kellogg) axial-flow reactor dehydrogenation of ethylbenzene to styrene multistage configuration possible (Dow) enhanced backmixing due to small reforming thickness of bed uniformity of flow requires exact sizing of 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 formaldehyde very short residence time catalyst 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 be regenerated
dehydrogenation of butane to butadiene (Houdry process) by burning off reaction heat can be supplied by catalyst regeneration
SO 2 reduction with methane (Andrussow process)
battery operation
Table 6. Fixed-bed catalytic reactors for gas-phase reactions with stagewise temperature control
Reactor type Features Examples of applications
Cascade of fixed-bed reactors large pressure and temperature differences are possible
reforming of heavy gasoline
hydrocracking conversion of H 2 S and SO 2 to elemental sulfur (Claus process) isomerization of five-to-six-ring naphthenes Multibed reactor with cold-gas injection
used for exothermic equilibrium reactions ammonia synthesis
injection of reaction mixture leads to lower methanol synthesis conversion and thus increased number hydrocracking of stages hydrogenation of benzene injection of water lowers concentration at constant conversion
desulfurization of vacuum gas oil
adaptation of bed depth to progress of reaction Multibed reactor with interstage cooling
used for exothermic equilibrium reactions ammonia synthesis ( ¨OSW, Fauser, Montecatini)
internal or external heat exchangers SO 2 oxidation (with interstage adsorption) no dilution effects hydrodealkylation of alkyl aromatics adaptation of bed depth to progress of reaction Multibed reactor with heat supply
used for endothermic equilibrium reactions dehydrogenation of ethylbenzene to styrene (Dow)
interstage heating or interstage injection of superheated steam
Table 7. Fixed-bed catalytic reactors for gas-phase reactions with continuous temperature control
phase, however, leads to much greater drag and
friction forces on the catalyst. If the reaction in-
volves both gas and liquid phases, maintenance
of uniform flow conditions through the catalyst
bed and intensive mixing of the phases can be
difficult. The crucial factor for the efficiency of
catalytic processes is the wetting of the catalyst
by the liquid. Since reactors of this type are usu-
ally operated adiabatically, local overheating is a 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 of the liquid (stirred tanks and bubble columns
Table 8. Moving-bed catalytic reactors for gas-phase reactions
Reactor type Features Examples of applications
Moving-bed reactor gravity transport of catalyst cracking (TCC, Houdry flow process) reaction conditions largely similar to those in fixed-bed reactor
dehydrogenation of butane
advantageous when catalyst can be regenerated by burning off residues Fluidized-bed reactor catalyst agitated by gravity and resistance force of gas flow
cracking (Kellogg, FFC, Flexicracking)
almost isothermal conditions can be achieved in fluidized bed
hydrocracking
pressure drop independent of gas throughput over a wide range
reforming
form of fluidized bed can be varied as a function of geometric and hydraulic conditions
ammoxidation
strong backmixing internals to improve mass transport and heat transfer are common
of propene to acrylonitrile (Sohio process)
catalysts must have high abrasion resistance of o -xylene to o -phthalodinitrile production of adiponitrile from adipic acid and ammonia oxychlorination of ethylene to 1,2-dichloroethane (Goodrich) production of melamine from urea (BASF) hydrogenation of nitrobenzene to aniline (BASF, Cyanamid) of ethylene oxidation of o -xylene or naphthalene to phthalic anhyride of butane to MA∗ (Du Pont) of SO 2 to SO 3 of ethylene to ethylene oxide of NH 3 to NO of HCl to chlorine dehyrogenation of isopropanol of n -butane to n -butene production of chloromethylsilanes from chloromethane (catalytic gas – solid reaction) production of vinyl chloride (Cloe process) chlorination of methane and ethylene production of butadiene from ethanol isomerization of n -butane production of isoprene postchlorination of PVC∗ combustion Entrained-flow reactor uses very fine-grained catalyst Fischer – Tropsch process (Synthol process) whole quantity of catalyst circulates continuously between reaction section and tempering or regeneration unit
∗ For abbreviations, see footnote to Table 3
3.5. Reactors for Noncatalytic Reactions
Involving Solids
A variety of specialized reactors are available
for noncatalytic reactions involving solids. The
discussion that follows deals only with the in-
dustrially important types.
3.5.1. Reactors for Noncatalytic Gas – Solid Reactions
In general, noncatalytic gas – solid reactions are characterized by low overall reaction rates and
Table 9. Fixed-bed catalytic reactors for liquid-phase and gas – liquid reactions
Reactor type Features Examples of applications
Trickle-flow reactor can operate in cocurrent or countercurrent desulfurization and refining of petroleum products temperature control by intermediate injection or recirculation
hydrocracking
danger of uneven liquid distribution and incomplete wetting of catalyst
production of butynediol from acetylene and formaldehyde narrow residence-time distribution direct hydration of propene to 2-propanol (Texaco) hydrogenation of organic intermediates (butynediol, adiponitrile, ethylhexenal) of aldehydes, esters, and carboxylic acids to alcohols of natural fats to fatty acids of residues (low-temperature hydrogenation of tars) posthydrogenation Packed bubble column danger of flooding limit throughput capacity amination of alcohols catalyst subject to greater mechanical stress (retention necessary)
cobaltizer and decobaltizer in oxo synthesis
high liquid proportion promotes heat removal disproportionation of toluene to benzene and xylene large amount of backmixing in liquid phase
Table 10. Suspended-bed and fluidized-bed reactors for liquid-phase and gas – liquid reactions over solid catalysts
Reactor type Features Examples of applications
Bubble column with simple design hydrogenation suspended 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 cyclohexane be prevented hydrodesulfurization suitable if product drops out as solid Reactor with external recirculation
heat-exchange and mixing devices in external loop hydrogenation of organic intermediates (nitrobenzenes, nitriles, nitronaphthalenes, etc.) for continuous and batch operation catalyst separation outside reactor Sparged stirred tank with suspended catalyst
can also be operated in semicontinuous and batch modes
hydrogenation of organic intermediates (nitro compounds, aromatics, butynediol) ensures intensive mixing of all phases fat hydrogenation increased cost for sealing and maintaining stirrer drive
catalytic refining
Cascade of sparged stirred tanks with suspended catalyst
higher final conversions than in single stirred tank hydrogenation of NO to hydroxylamine
suitable for slow reaction rates continuous hydrogenation of fats adaptable to intermediate injection and other interconnections
hydrolysis of fats to fatty acids and glycerol production of toluenediamine from dinitrotoluene Fluidized-bed reactor small pressure drop catalyst must have very high mechanical strength
hydrocracking and desulfurization of heavy petroleum fractions and still residues (H-Oil process; three-phase fluidized bed)
high process temperatures; in addition, the struc- ture and geometry of the solid can change during the reaction. Reactors for this service can essentially be grouped into those for semicontinuous opera- tion, that is, with no solids transport (vertical shaft kilns and rotary drums), and those for con- tinuous operation, that is, with continuous solids transport. The second type, in turn, can be di- vided into
Reactors with gravity transport of solids
Reactors with mechanical transport of solids
Reactors with pneumatic transport of solids
These three groups differ widely with respect to residence time, conditions of mass and heat transport between gas and solid phases, and heat-input capabilities. The first group includes moving-bed reactors. Since the gas has to flow through 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 endothermic reactions in the same reactor.
