autothermal fixed bed reactor concept

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5946

2. Autothermal operation for weakly exothermic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59462.1. Basic principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5946

2.1.1. The countercurrent "xed-bed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59462.1.2. Periodic #ow reversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59462.1.3. The analogy between fast #ow reversal and countercurrent operation . . . . . . . 5948

2.2. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59502.2.1. Autothermal reactors for total oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59502.2.2. Autothermal reactors for equilibrium limited reactions. . . . . . . . . . . . . . . . . . . 5951

2.3. Autothermal reactor stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59522.3.1. Circulation loop reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59532.3.2. The cooled reverse #ow reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5953

2.4. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5954

3. Autothermal combination of exo- and endothermal reactions . . . . . . . . . . . . . . . . . . . . . . . . 59553.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59553.2. Simultaneous operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59573.3. Asymmetric operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59583.4. Symmetric operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59603.5. A simpli"ed picture of the autothermal coupling of endo- and exothermic reactions . . 5962

3.5.1. Simultaneous operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59623.5.2. Asymmetric operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59633.5.3. Symmetric operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59633.5.4. Regenerative heat transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5964

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5965

Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5965

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5965

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5965

*Corresponding author. Tel.: #49-711-641-2229; fax: #49-711-641-2242.E-mail address: [email protected] (G. Eigenberger).

Chemical Engineering Science 55 (2000) 5945}5967

Review

Autothermal "xed-bed reactor concepts

G. Kolios, J. Frauhammer, G. Eigenberger*Institut fu( r Chemische Verfahrenstechnik, Universita( t Stuttgart, Bo( blinger Str.72, D-70199 Stuttgart, Germany

Received 22 October 1999; accepted 24 March 2000

Abstract

The principles, properties and applications of autothermal "xed-bed reactor concepts are presented. First we focus on di!erentreactor types for weakly exothermic reactions and discuss their basic behavior, their stability and nonlinear dynamic features. Thesecond part is devoted to the autothermal coupling of endothermic and exothermic reactions. A systematic classi"cation is proposedfor the process alternatives developed so far and a simpli"ed model is developed from which basic features of an optimal design can bededuced. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Autothermal operation; Reverse #ow operation; Circulation loop reactor; Countercurrent reactor stability; Coupling of exothermic andendothermic reactions

0009-2509/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 1 8 3 - 4

Fig. 1. Two types of autothermal "xed-bed reactors with typical tem-perature pro"les. The dotted pro"les correspond to a doubling of theadiabatic temperature rise. (a) Standard design with adiabatic "xed bedand separate countercurrent heat exchange, (b) `countercurrent "xed-bed reactora.

1. Introduction

The unit operation concept of Chemical Engineeringrequires that chemical processes are set up as a sequenceof separate operations for mixing, heat transfer, reaction,separation, etc. where each unit operation is usuallyrepresented by a separate device. In contrast, the ex-tended or multifunctional reactor concept, explicitly for-mulated by Agar and Ruppel (1988a,b) suggests to takeadvantage of the combination of chemical reaction withselective reactant addition or product removal and theintended and speci"c addition or removal of heat in oneapparatus. Such concepts have meanwhile attracted con-siderable attention in our community (Blumenberg, 1992;Westerterp, 1992; Ho!mann & Sundmacher, 1997). Oneof the simplest and most common examples of multifunc-tional reactors which have been in use in Chemical Reac-tion Engineering for many decades are autothermalreactors for weakly to moderately exothermic reactions,where the cold reactor feed is heated up to the requiredreaction temperature by the hot reactor e%uent. This canbe done in a separate heat exchanger or by partial ortotal integration of the heat exchange into the reactor.Typical well-known examples are the integration ofa countercurrent heat exchanger into the high-pressurehousing of a methanol or ammonia synthesis reactor(Hochgesand et al., 1989; Bakemeier et al., 1985). But it isnot too long ago when it became obvious that the integ-ration of either regenerative or recuperative heat ex-change into the catalyst packing of a catalytic "xed-bedreactor could have some speci"c advantages overa simple combination of an adiabatic reactor with a sep-arate heat exchanger. Even more recent is the systematicexploitation of the autothermal coupling of exo- andendothermic reactions.

2. Autothermal operation for weakly exothermicreactions

2.1. Basic principles

The standard reactor design for an autothermal "xed-bed reactor consists of an adiabatic "xed bed of catalystwhich is connected with a heat exchanger so that the hotreactor e%uent heats up the cold feed. If we consider thecountercurrent heat exchanger of Fig. 1a, the slope of thetemperature pro"le can be approximated by Eq. (1)(Nieken, Kolios & Eigenberger, 1995).

d¹

dz"

m5zcp

2j%&&

*¹!$

with j%&&

"jw(1!ew)#

(m5zcp)2

hwav

, (1)

where m5z

is the mass #ux density, (1!e) is the wallvolume fraction, h

wis the heat transfer coe$cient (as-

sumed equal for up- and down#ow) and av

is the speci"cheat transfer area.

2.1.1. The countercurrent xxed-bed reactorIt is obvious that the countercurrent heat exchange

can be integrated into the catalyst "xed bed or a counter-current heat exchanger could be "lled with catalyst insideand outside its tubes (Fig. 1b). If the heat transfer condi-tions in both designs of Fig. 1 are similar, the temperaturepro"les in the heat exchange sections will be the samesince the temperature slope is given by Eq. (1). In thefollowing, such a reactor will be called a `countercurrent"xed-bed reactora. The advantages of the countercurrentreactor design are twofold: The packing allows for a bet-ter heat transfer to the walls and hence for a morecompact and possibly less expensive design. The secondadvantage is a considerably reduced temperature sensi-tivity which will be discussed in more detail later.

2.1.2. Periodic yow reversalA second approach to autothermal operation is closely

connected with the phenomenon of a moving or creeping

5946 G. Kolios et al. / Chemical Engineering Science 55 (2000) 5945}5967

Fig. 2. Principles of reverse-#ow operation: (a) Temperature (¹) andconversion (X) pro"les of a moving reaction front in an adiabatic"xed-bed reactor; (b) temperature and conversion pro"les at the end ofthe two "rst semicycles; (c) transient to the periodic steady state.

Fig. 3. Adiabatic "xed-bed reactor with periodic #ow reversal: (a)reactor scheme with switching valves; (b) temperature and concentra-tion pro"les in the periodic steady state (feed values: ¹0, c0); (c) exittemperature ¹l vs. time t.

reaction front, a topic with a long and prominent historyin Chemical Reaction Engineering (Wagner, 1945; Wicke& Vortmeyer, 1959; Padberg & Wicke, 1967; Gilles, 1974;Pinjala, Chen & Luss, 1988; Chen & Luss 1989; Bur-ghardt, Berezowski & Jacosen, 1999). If an adiabaticreactor is started by heating the catalyst bed above theignition temperature of the exothermic reaction con-sidered, the feed temperature can subsequently belowered to ambient. As a consequence a moving reactionfront develops where the bed is cooled by the cold feedand the feed is heated up by the hot bed (Fig. 2a).

The lateral displacement of the temperature front caneasily be calculated from the well-known formula of thevelocity w of a moving reaction front (Wicke & Vor-tmeyer, 1959):

w"

(1!(*¹!$

/*¹)) eocp

(1!(*¹!$

/*¹)) eocp#(1!e)o

scs

. (2)

As long as the moving front is still inside the catalyst bed,full conversion will be obtained but if it moves out, thereactor will extinguish. Several ways have been conceived

of how to prevent the extinction and to perpetuate thereactor operation. One way would be to reignite thereaction at the entrance as soon as the front leavesthe reactor. This leads to the circulation loop reactordiscussed in Section 2.3.1. Certainly the best-known wayis to reverse the #ow direction after the front has movedto a certain position into the bed and to repeat this #owreversal periodically until a periodic steady state hasbeen established. Fig. 2b shows the pro"les of an irrevers-ible reaction at the end of the "rst and the second semi-cycle and Fig. 2c shows the evolution to the periodicsteady state for the pro"les at the end of the half-periodwhere the #ow comes from the left.

Fig. 3 gives a schematic of an adiabatic "xed-bedreactor with periodic #ow reversal showing the temper-ature and concentration pro"les in the periodic steadystate and the course of the reactor exit temperature¹l with time t. In the periodic steady state the temper-ature (and conversion) pro"les for two successivesemicycles are just mirror images of each other. It followsfrom an overall heat balance that the integral mean of thereactor exit temperature ¹l will exceed the feed temper-ature ¹

0just by *¹

!$. Under periodic #ow reversal both

ends of the "xed bed are obviously used as regenerativeheat exchangers. Since regenerative heat exchange is gen-erally considered simpler and more e$cient than recu-perative heat exchange, the reverse #ow reactor hasfound considerable industrial application primarily forthe catalytic or homogenous combustion of organic pol-lutants in exhaust air.

It is not too surprising that the autothermal operationof "xed-bed reactors with regenerative heat exchange hasa long history of periodic reinventions. Khinast, Jeongand Luss (1999) report a "rst patent issued to Cottrell(1938) for exhaust puri"cation, a subsequent mentioningin Frank-Kamenetski (1955) for selective oxidation,a patent for SO

2oxidation issued to Watson (1975) and

a patent for catalytic puri"cation of Wojciechowski

G. Kolios et al. / Chemical Engineering Science 55 (2000) 5945}5967 5947

Fig. 4. Analogy between fast #ow reversal and countercurrent opera-tion: (a) scheme of a reverse-#ow reactor. The catalyst is the shadedarea; (b) temperature pro"les in the gasphase (**) and in the solidphase (!!!) in the reverse-#ow reactor for short switching periods(`sliding regimea); (c) scheme of a countercurrent reactor with geometricand catalytic properties equivalent to (a); (d) temperature and conver-sion pro"les in the countercurrent reactor.

(1980). But the credit has certainly to be given to theNovosibirsk school of Boreskov and Matros and theircolleagues for elaborating the scienti"c understanding,for demonstrating the potential of reverse #ow reactorsup to the industrial scale and for communicatingthis principal all around the world (Matros, 1989; Matros& Bunimovich, 1996 and references therein).

In the reverse #ow reactor both ends of the packed bedare used as regenerative heat exchangers in much thesame way as recuperative heat exchange is used in thecountercurrent "xed-bed reactor. And the question mayarise of how the two di!erent concepts of autothermaloperation compare with each other.

2.1.3. The analogy between fast yow reversal andcountercurrent operation

If we consider an adiabatic "xed-bed reactor with veryfast #ow reversals (Fig. 4a) the temperature of the catalystbed can hardly follow the gas temperature changes andstays almost constant (Fig. 4b). If the cold gas comesfrom the left it is heated up by the hot packing, if the hotgas comes from the right out of the hot center part itdelivers its heat to the packing.

The limit of fast #ow reversal leads to a direct corre-spondence with countercurrent operation (Eigenberger& Nieken, 1994; Nieken et al., 1995). If we consider similaroperating conditions as above for a countercurrent "xed-bed reactor where the catalyst is deposited at the separat-ing wall (Fig. 4c), we get the same temperature and com-position pro"les in the two channels which we obtainunder fast #ow reversal in successive semicycles, providedthe heat transfer parameters to the catalyst are the sameand the heat resistance of the wall is negligible. As anylimiting case the limit of fast #ow reversal is somewhatacademic since it assumes quasisteady gas balances, whichmeans that the residence time of the gas has to be con-siderably shorter than the switching period. Neverthelessthis analogy provides a simple basis for short-cut calcu-lations since the steady-state pro"le of a countercurrentreactor can be computed much easier than the periodicsteady state of a reverse #ow reactor. This has been shownby Matros and co-workers with what they call the `slidingregimea (Matros (1989) as well as by Bhatia (1991)).

In case of a single irreversible reaction the analogy canalso be used for drawing a simpli"ed plot of the resulting(periodic) steady state pro"les. This is explained in Fig.4d. Eq. (1) can be used to calculate the slope of thetemperature pro"le for the part of the bed where onlyheat is exchanged. As soon as the ignition temperature¹*'/ is exceeded, the conversion starts and countercur-rent heat exchange and reaction take place simulta-neously. In the reaction zone the temperature rises by atleast *¹

!$over ¹*'/. As a lower limit approximation

for the maximum temperature we therefore get¹.!9'¹*'/#*¹

!$and the temperature and conversion

pro"les as given in Fig. 4d result.

The simple graphical design of Fig. 4d allows for a veryeasy interpretation of the in#uence of design and operat-ing parameters on the temperature and conversion pro-"les for countercurrent or reverse #ow conditions (Fig. 5):If two reactors run under identical operating conditionsbut with catalysts of di!erent activity, the slope of thetemperature pro"le remains constant. The ignition tem-perature of the less active catalyst, ¹*'/

2will be higher,

leading to the higher maximum temperature pro"le2 (Fig. 5a).

Autothermal catalytic reactors are in a certain rangeself-adaptive with respect to a slow catalyst deactivation: Ifthe activity decreases, the ignition temperature rises and aslong as it stays below ¹45!" (Fig. 5a) a longer portion of thereactor will be used for heat exchange. This self-regulationmechanism can however be detrimental if a catalyst witha low thermal stability is used. Then a vicious circle resultswhere catalyst decay causes an increased maximum tem-perature which in turn accelerates decay.

Inert packings at the reactor ends, frequently used forreducing costs, destroy this self-regulation mechanismsince the minimum length of the heat exchanging part of


Fig. 5. Simpli"ed graphical design for an interpretation of the in#uence of design and operating parameters on the temperature and conversion pro"lesfor countercurrent or reverse #ow conditions: (a) in#uence of catalyst activity or inert sections; (b) in#uence of feed concentration or adiabatictemperature rise; (c) in#uence of the period length q

.!9.

the reactor is "xed by the extension of the inert zones. InFig. 5a the temperature pro"le 2 would also result fora very active catalyst if inert front and end sections wouldextend over a length z

2.

If the feed concentration (or the adiabatic temperaturerise) is increased, steeper gradients and a higher temper-ature rise above the ignition temperature result (Fig. 5b).The extent of the `plateaua of the temperature pro"le is


Fig. 6. Periodic steady-state temperature pro"les for the oxidation ofsimilar amounts (same *¹

!$) of propane and of propene under periodic

#ow reversal, experimental results from (Nieken, 1993).

therefore a measure for the robustness of the steady stateagainst extinction.

Compared to the conventional design of Fig. 1a theintegrated autothermal reactor is less sensitive to concen-tration changes. This can be seen qualitatively from thedotted temperature pro"les in Fig. 1. They correspond toa doubling of the feed concentration for a simple irrevers-ible reaction like the total catalytic combustion of or-ganic pollutants in exhaust air. The doubling of the feedconcentration corresponds to a doubling of the adiabatictemperature rise *¹

!$and, according to Eq. (1) to

a doubling of the temperature gradient. For the conven-tional design this results in a doubling of the temperatureincrease (¹

.!9!¹0). In the countercurrent "xed-bed

reactor or the reverse #ow reactor the maximum temper-ature will be considerably lower since the catalytic reac-tion will ignite at about the same ignition temperature¹*'/. This means that the section of countercurrent heatexchange without reaction is reduced and so is the sub-sequent temperature increase.

The lateral displacement of the temperature-front ina reverse-#ow rector can be calculated from Eq. (2). Thisallows for the estimation of the maximum possible dura-tion q

.!9of one semicycle which can be calculated from

the equations given in Fig. 5c as q.!9

"*l.!9

/w.The simpli"ed results of Fig. 5 can be readily used for

a "rst short-cut design or an interpretation of experi-ments. This has been shown for a number of examples byNieken (1993), Nieken et al. (1995) van de Beld andWesterterp (1994) and ZuK #e and Turek (1997). Fig. 6gives an example for the oxidation of similar amounts(same *¹

!$) of propane and of propene in a reverse #ow

reactor. The temperature gradient in the end section isthe same for both gases but di!erent maximum temper-atures result because the ignition temperatures of pro-pane is about 4003C as opposed to 2003C for propene.

2.2. Applications

The application of the reverse #ow reactor concept forweakly exothermic reactions has been studied in numer-ous contributions in recent years. Most of them arementioned in a comprehensive review by Matros andBunimovich (1996).

2.2.1. Autothermal reactors for total oxidationTotal oxidation of combustible components in exhaust

air has so far been the prime industrial application. Bothhomogeneous combustion in an inert packing and cata-lytic oxidation is used. The catalytic oxidation has theadvantage of considerably lower maximum temperature,resulting in less fuel consumption if fuel has to be addedto sustain the autothermal operation. NO

xproduction is

generally no problem since the maximum temperatureseldom exceeds 5003C. Catalytic combustion is withinlimits adaptive with respect to gradual catalyst decay as

explained in connection with Fig. 5a. Homogeneouscombustion is preferred where catalyst poisoning couldbe a problem. Here temperature is often controlled toabove 9003C by co-feeding of natural gas.

One practical problem to be solved in periodic #owreversal is the prevention of a back#ush of untreated gasinto the exit line immediately after #ow reversal (see Fig.3a). The most common solution in o!-gas puri"cation isthe use of a three-bed design where the beds switch fromreverse #ow operation to a purge step and back toreverse #ow operation (Fig. 7). An elegant possibility ofoperating a regenerative "xed-bed reactor in a continu-ous mode with integrated purge steps without usingvalves is to use a LjungstroK m-type rotating regeneratordesign (Fig. 8) (Eigenberger & Nieken, 1991, Eigenberger,1992). Since rotating seals are only necessary on the coldgas side they present no major problem.

In large-scale operation catalytic combustion with anadiabatic temperature rise as low as 103C (correspondingto a feed concentration in the order of 0.025wt% hydro-carbons) can be run autothermally in reverse #ow reac-tors. If the feed concentration is less, combustibles haveto be added to the feed to prevent extinction. Care mustbe taken that the added combustibles have a similarignition temperature as the exhaust components, other-wise extinction or only partially ignited steady states mayresult (Nieken, Kolios & Eigenberger, 1994). Alterna-tively hot exhaust gases from an external combustionchamber can be injected into the center of the reactor(Nieken, 1993). The more severe problem consists in theprevention of overheating if the feed is temporarily toorich in combustibles which can hardly be excluded inindustrial exhaust puri"cation. Due to the e$cient heatexchange a rapid temperature increase would resultwhich could deteriorate catalyst activity or even damage


Fig. 7. Three-bed design for o!-gas puri"cation with the middle bedbeing back#ushed by a clean purge.

Fig. 8. Schematic of a rotating "xed-bed reactor.

the construction. Several methods have been proposedincluding internal cooling, cold gas addition, hot gaswithdrawal, and structuring of the packed-bed activityand heat conductivity but no single measure is able tosolve the problem without reducing the puri"cation e$-ciency (Nieken, 1993). In addition internal cooling maygive rise to an instability of the periodic steady statewhich will be treated in Section 2.3.

A countercurrent "xed-bed reactor requires a veryhigh speci"c heat exchange surface to compete with thee$ciency of regenerative packed-bed heat exchange.A standard shell and multitube design would be insu$-cient. Possible concepts can be deduced from compactheat exchangers, e.g. parallel plate heat exchangers wherethe catalyst is deposited at the plate walls (Gaiser, 1993).

An alternative solution is shown in Fig. 9 (Friedrich,Gaiser, Eigenberger, Opferkuch & Kolios, 1997). Herethe in- and outcoming gas #ow is separated by a multiplyfolded wall and the catalyst is deposited on an undulatedcarrier which is inserted into the folds. This constructionallows for a simple sealing at the cold feed/exit sides andeasy replacement of the catalyst. With a channel width inthe order of 2mm a countercurrent reactor will reacha similar thermal e$ciency as a design using regenerativeheat exchange. Fig. 10 shows experimental results ob-tained for the catalytic combustion of propene. The min-imum adiabatic temperature rise for autothermaloperation of the pilot plant unit was 20 K. The compactcountercurrent reactor design is particularly suited forsmaller gas #ows up to 2000 m3

N/h.

2.2.2. Autothermal reactors for equilibrium limitedreactions

The autothermal oxidation of SO2

to SO3

belongs tothe "rst reactions with periodic #ow reversal studied andpublished by the Novosibirsk group of Boreskov, Matrosand Kiselev (1979), and Boreskov and Matros (1983).Subsequent examples comprise the methanol and theammonia synthesis. These reactions belong to the class ofmoderately exothermic equilibrium limited reactions.Also the countercurrent "xed-bed reactor concept of Fig.1b has been proposed for this class of reactions (Anony-mous, 1994). Both the bene"ts and the limitations ofautothermal operation for equilibrium limited exother-mic reactions have been nicely brought out in a paper byYoung, Hildebrandt and Glasser (1992). An example oftheir simulations of the methanol synthesis is given inFig. 11. The temperature pro"le resembles the familiarbell shape of a catalytic combustion, but contrary to theprevious examples the reaction is not completed whenthe temperature maximum has been reached. It is onlysubdued by approaching equilibrium as can be seen fromthe conversion pro"les. With decreasing temperature to-wards the exit section the conversion exceeds the equilib-rium conversion of adiabatic operation. This can also beseen from the temperature/conversion plot of Fig. 11(bottom) where the equilibrium line and the line of opti-mal temperature (maximum reaction rate) are given.Autothermal operation is superior to one adiabatic reac-tor since its operation line is closer to the optimal tem-perature, but a two-stage adiabatic reactor withinterstage cooling (thick line) will result in a considerablybetter conversion plus the production of process steam.About the same "ndings can be drawn from the study ofVan den Bussche and Froment (1996) of the so-calledSTAR-con"guration of reverse #ow reactors for meth-anol synthesis. They showed that in order to obtainreasonable conversion and to avoid excess temperaturesit is necessary to place a separate heat exchanger/steamgenerator at the high-temperature node of their STAR


Fig. 9. Autothermal parallel plate countercurrent reactor: Schematic of the folded plate countercurrent reactor (left) and measured temperaturepro"les for the noncatalytic combustion of traces of propene (right).

Fig. 10. Experimental results obtained for the catalytic combustion ofpropene in the parallel plate countercurrent reactor of Fig. 9 (feedconcentration propene: 380ppm).

arrangement which limits the maximum temperature toabout 2303C.

The general conclusion to be drawn is that autother-mal operation could be of advantage for the processing oflow concentration feeds like SO

2containing e%uents

from the metallurgical industry (Xiao, Wang & Yuan,1999) or of purge gas from a methanol synthesis loop but

that it is of limited value for exothermic equilibrium-limited synthesis reactions.

2.3. Autothermal reactor stability

It is obvious that autothermal operation always takesplace in the region of multiple steady states. Fig. 12ashows the region of multiplicity of an autothermal reac-tor con"guration of Fig. 1a with the feed temperature¹

0as parameter. The ignited steady state is the desired

solution for normal operation. The other stable steadystate lies at the extinguished branch with (almost) noconversion. The intermediate steady state is the unstablesolution. It separates the temperature region in which thereactor would ignite from the region in which the reac-tion will extinguish to the lower steady state. Similarresults can be obtained for the adiabatic "xed-bed reac-tor with periodic #ow reversal.

Whereas the stability analysis of a countercurrent"xed-bed reactor is fairly straightforward, special consid-erations are required for reactors with periodic #owreversal. Here the truely stationary steady state of thecountercurrent reactor has its equivalence in the oscillat-ing `periodic steady statea. Methods are required whichallow for the direct computation of the periodic steadystates and it is not until recently that e$cient methods forthis task have been developed. It has been shown byseveral groups that a single exothermic reaction in anautothermal reactor of the countercurrent or theadiabatic reverse #ow reactor type has only the above


Fig. 11. Reverse #ow reactor for an exothermic equilibrium reaction(Young et al., 1992). Temperature and conversion pro"les during onehalf-period (top and middle) and the course of the reaction in a temper-ature/conversion plot (bottom). The reverse #ow reactor characteristic(thin lines) is comapred to a two-stage adiabatic reactor with interstagecooling (thick line).

three symmetrical solutions (Chumakova & Zolotarskii,1990; Salinger & Eigenberger, 1996a,b; Khinast & Luss,1997). If multiple exothermic reactions can take place,more than three steady states are possible depending onhow many reactions may ignite independently (Nieken,1993; Salinger & Eigenberger, 1996a,b; Khinast & Luss,1997) but dynamic instabilities can be excluded as long ascountercurrent or reverse #ow reactors with negligibleheat losses are considered.

This result is not too obvious since it was alreadyobserved by Ruppel (1980), that a conventional autother-mal reactor of the type shown in Fig. 1a can exhibitautonomous periodic oscillations. Fig. 13 is a reprint ofthe original work from which a physical explanation canbe drawn: The oscillations move through the packed bedin the form of temperature peaks in which total conver-sion takes place. As soon as a peak moves from theadiabatic reactor into the heat exchanger it creates a newtemperature peak which reignites a new combustionfront. The bifurcation diagram (Fig. 13c) shows a similarhysteresis loop like in Fig. 12a which is marked by theirmaximum peak temperature. In addition a branch ofperiodic oscillations occurs.

2.3.1. Circulation loop reactorThis dynamic instability of autothermal reactors

has in the sequence been studied in detail byGilles and coworkers (Mangold & Gilles, 1996; Hua,Mangold & Gilles, 1998). They found that the dynamicinstability is most pronounced if a cocurrent heatexchanger is used, since in this case the leaving temper-ature front has the longest time to create a new front atthe reactor entrance. A respective reactor has beennamed a circulation loop reactor and is shown inFig. 14a. The cocurrent heat exchange section and a partof the reactor loop are "lled with catalyst. The entranceheater is needed to start the reactor and to support itsoperation. The bifurcation diagram (Fig. 14b) showsa largely instable hysteresis loop and a broad region ofautonomous oscillations, an example of which is given inFig. 14c.

2.3.2. The cooled reverse yow reactorThe reverse #ow reactor also can produce dynamic

instabilities but so far this has only been observed if heatlosses from the packed bed are taken into account. Thishas "rst been shown in a pioneering paper by R[ ehac\ ek,Kubic\ ek and Marek (1992) and was later con"rmedin a "rst detailed stability analysis by Salinger & Eigen-berger (1996a,b). The most comprehensive analysis sofar has just appeared in two papers by Khinast,Gurumoorthy and Luss (1998) and Khinast et al. (1999).Only a few basic points shall be discussed in the followingwhile the reader is referred to the original sources fordetails.

We assume a reverse #ow reactor with one moderatelyexothermic reaction which is cooled over its entire length.Fig. 15 a shows the in#uence of increasing coolingintensity on the periodic steady-state temperature pro-"les. The adiabatic pro"le (D"0) exhibits the well-known shape. If cooling is applied (D'0) a typicaldouble hump in the temperature plateau develops. It iscaused by the fact that the heat of reaction is only set freein a small portion of the catalyst bed after the ignitiontemperature has been exceeded. With increasing cooling


Fig. 12. Multiplicity features of an autothermal reactor: Maximum temperature versus inlet temperature (a); and conversion versus inlet temper-ature (b).

Fig. 13. Autonomous oscillations of adiabatic "xed-bed reactors withheat recycle (Ruppel, 1980). (a) Reactor con"guration. (b) Periodicallytravelling reaction front with reignition inside the heat exchanger. (c)Branch of stable steady state (**) and branch of stable oscillatingsolutions (2).

intensity D the temperatures of the two maxima decreaseand they get closer together until they merge to a singlemaximum. This situation is reached immediately beforeextinction, which occurs because of a too strong coolingintensity.

The solutions of Fig. 15a are symmetric since thepro"le at the end of a period is just a mirror image of thepro"le one or two or N periods before. It turned out thatin addition there can also be asymmetric solutions, pre-ferably for short periods where the pro"les move betweentwo positions which are both shifted either to the left orthe right of the center. The most interesting feature how-ever is the appearance of aperiodic solutions where thepro"les never come back exactly into the same position.This is shown in some detail in Fig. 16. The temperatureand weight fraction pro"les at the end of two successivesemicycles are given in each of the pictures for a sequenceof periods. The solid lines mark a semicycle with #owfrom the left, the dashed lines with #ow from the right.

The sequence of temperature pro"les in successive cyclesis marked by a creation of new temperature maxima,their subsequent movement and growth and their "nalshrinkage and disappearance. Conversion will be com-pleted in the dominating temperature maximum as longas it stays in the center part of the reactor but as soon asit approaches one boundary, residence time may be tooshort for total conversion and a new maximum develops.An example is the left temperature maximum which "rstappears in the 5th period, is dominating between the 31stand the 35th period while it slowly moves to the left andis later on replaced by a new maximum created to theright of it. This right maximum will increase in height andmove to the right in subsequent cycles. In total we havea slow autonomous movement of subsequent temper-ature maxima to the left or the right and the periodicswitches of the #ow direction. And since two periodicprocesses are in general not synchronized, aquasiperiodic behavior results. It may even turn intodeterministic chaos as was already observed in R[ ehac\ eket al. (1992) and R[ ehac\ ek, Kubic\ ek and Marek (1998) andrecently con"rmed by Khinast et al. (1999). A new publi-cation of Garg, Khinast and Luss (2000) shows thata rather similar oscillatory instability can also be ob-served in a cooled countercurrent reactor.

2.4. Summary

Multifunctional autothermal reactors with integratedheat recovery are used for weakly exothermic high-tem-perature reactions. Main applications so far are in the"eld of waste gas puri"cation. Two alternative designsexist which are based on di!erent mechanisms for heatrecovery, the countercurrent reactor using recuperativeheat exchange and the reverse #ow reactor using regen-erative heat exchange. Both designs have comparableproperties with respect to design parameters and operat-ing conditions. They show a certain adaptivity againstdeviations in operating conditions due to the integratedheat exchange. Autothermal reactors are usually oper-ated in the region of multiple steady states, where theignited branch is the desired one. It has been shown


Fig. 14. Circulation loop reactor (Mangold & Gilles, 1996): Schematic of the reactor (a); bifurcation diagram (b); and temperature pro"les at di!erenttimes in periodic steady state (c).

through detailed stability analysis that the countercur-rent "xed-bed reactor and the reverse #ow reactor witha single reaction have only three steady states if heatlosses are excluded, whereas the autothermal reactorwith separated heat exchanger can also exhibit dynamicinstabilities through autonomous reignitions. In addi-tion, a variety of instabilities has been discovered forreverse #ow reactors if heat losses from the packed bedare taken into account. They include aperiodic frontmovement in subsequent cycles and deterministic chaos.

3. Autothermal combination of exo- and endothermalreactions

3.1. Introduction

Endothermic reactions can only be carried out if theheat of reaction is supplied at the required reactiontemperature. Since the necessary heat is usually providedby an auxiliary combustion reaction, many conceptshave been conceived and realized industrially of how the


Fig. 15. Stability of the symmetric solution in a reverse #ow reactor with heat losses over the wall (Khinast et al., 1998): (a) The in#uence of coolingintensity D on the shape of the symmetric solution pro"les (dimensionless temperature H

.!9over reactor length m); (b) the corresponding stability

diagram.

Fig. 16. Aperiodic oscillations of temperature (¹) and concentration (c) pro"les in a cooled reverse #ow reactor. The solid lines mark the last pro"lesbefore #ow reversal with #ow from the left, the broken lines the respective pro"les with #ow from the right.

endo- and exothermic reactions can be reasonably com-bined. Such a combination has often been called`autothermala if it is realized inside of one reactor, irre-spective of the fact that the feed and exit temperatures ofthis reactor could be pretty high. Well-known examplesare autothermal steam reforming (Hochgesand et al.,1989), the Cato"n Process (Ercan & Gartside, 1996) orseveral propositions for the use of periodic #ow reversalfor endothermic reactions (Haynes, Georgakis & Caram,1992; Kulkarni & DudukovicH , 1996a).

In the following we will restrict ourselves to the morenarrow de"nition of autothermal operation of Section 1,where the feed and the exit temperatures are considered

to be close to ambient or slightly above the evaporationor condensation temperatures of the reactants involved.This ensures optimal heat recovery in the reactor. Sincethis kind of autothermal operation is only possible if theoverall reaction is weakly-to-moderately exothermic itposes a "rst constraint on the coupling of the two reac-tions. Again we can in principle use either indirect heatexchange in an appropriate countercurrent reactor set-up or regenerative heat exchange under reverse #owoperation. In both cases the heat from the hot e%uent isrecovered in order to heat up the feed so that the feedenters and the products leave the reactor with low tem-perature. In their analysis Kolios and Eigenberger (1997)


Fig. 17. Simultaneous, symmetric and asymmetric operation for thecoupling of exothermic and endothermic reactions (methane combus-tion and methane steam reforming).

Fig. 18. Syngas generation with simultaneous methane combustion ina reverse-#ow reactor (Blanks et al., 1990).

have found three di!erent ways to achieve this goal,which we call

f the simultaneous operation,f the asymmetric operation and,f the symmetric operation.

The di!erences are shown in Fig. 17 for the example ofmethane steam reforming coupled with methane com-bustion. Under simultaneous operation the reactants forboth (endo- and exothermic) reactions are mixed and allreactions run more or less in parallel. The simultaneousmode is thus rather similar to the reactor operationdiscussed in Section 2. Under asymmetric and undersymmetric conditions both reactions are separated intime or in space. Under asymmetric conditions the feedfor the endothermic reaction always comes from one side(in the regenerative mode during one portion of thewhole period) and the feed for the exothermic reactioncomes from the opposite side (during the rest of theperiod). Under symmetric conditions the feed for the

endothermic reaction comes from both sides as in simul-taneous operation and the necessary heat of reactionfrom the exothermic reaction is supplied directly or in-directly in the middle of the reactor.

3.2. Simultaneous operation

A pioneering paper on simultaneous operation waspublished by Blanks, Wittrig and Peterson (1990). Theystudied methane steam reforming together with methanecombustion in a reverse #ow reactor consisting of twoinert end portions and the active center part. Successfuloperation of several laboratory and pilot plant reactorsup to 60 cm ID and 4 m length with CO yields up to 90%has been reported. The periodic steady-state pro"les aresymmetric in the sense that they are mirror images ofeach other in successive semicycles (Fig. 18), but theydi!er from the steady-state pro"les discussed in Section2. The rapid methane combustion results in a distincttemperature peak which is followed by the temperaturedecrease caused by the endothermic steam reforming.The length of the active catalyst zone and the cycle timehad to be properly adjusted to prevent excess temper-atures, back reaction with decreasing temperatures orcoke formation at temperatures below 7503C. This


Fig. 19. Gas temperature (¹g) and concentration (y) pro"les for asymmetric operation of methane steam reforming (RE-GAS process, Kulkarni and

DudukovicH , 1998); (a,b) `wrong way processa; (c,d) `normala process. The odd semicycle denotes methane combustion, the even semicycle methanesteam reforming (reverse reactions excluded).

sensitivity obviously led Amoco to stop the further devel-opment of this process.

3.3. Asymmetric operation

One of the "rst mentions of autothermal asymmetricoperation can be found in Levenspiels Dankwerts Mem-orial Lecture in 1988 (Levenspiel, 1988). Among otherintegrated reactor concepts he proposed and discussedthe RE-GAS process for coal gasi"cation. Using periodic#ow reversal as depicted in Fig. 17b he assumed a #ow ofpowdered coal in air for one semicycle to heat up an inertpacked bed and an opposite #ow of suspended coal withsteam for coal gasi"cation for the second half-period.Compared to alternative options with simultaneous feedof coal, air and steam the RE-GAS process allows toobtain a nitrogen-free syngas during the steaming period.This scheme was theoretically studied for catalytic "xed-bed processes in a series of papers by Kulkarni andDudukovicH (1996b,1998). They came to the conclusionthat the asymmetric process with `colda feeds and e%u-ents, which they called the wrong-way process (after thewrong way behavior of a moving temperature front (Pin-jala et al., 1988)) was the wrong way to combine exo- and

endothermic reactions. Their "ndings were based onsimulations of methane steam reforming where no reas-onably concentrated fuel mixtures could be processedwithout exceeding peak temperatures of 1500 K. Thiswas due to the fact that with cold feed from the combus-tion side a high and moving temperature peak with steepslopes was formed in which not enough heat for thesubsequent endothermic reaction could be stored(Fig. 19, left). They proposed a `normal RE-GAS pro-cessa instead which was fed from the combustion sidewith feed temperatures above the ignition temperature ofmethane (1000 K). This ensured the establishment ofa broader, not moving temperature zone which accumu-lated enough heat for the subsequent steam reforming(Fig. 19, right), but the goal to incorporate the full heatexchange between feeds and e%uents in the reactor couldnot be met.

Independent work of Kolios (1997) and Kolios andEigenberger (1997,1999) on asymmetric operation con-cerned the coupling of dehydrogenation of ethylbenzeneto styrene with the combustion of the hydrogen produc-ed. One result of these studies was the big in#uence of anaxial structuring of the catalyst bed into front and endsections and an active center part. Without inert sections


Fig. 20. E!ect of inert zones for ethylbenzene dehydrogenation coupledwith hydrogen combustion (asymmetric operation mode) in a reverse#ow reactor (Kolios, 1997). First line: Temperature pro"les during theethylbenzene dehydrogenation (production phase); Second line: Ethyl-benzene conversion pro"les during the production phase; Third line:Temperature pro"les during hydrogen combustion (regenerationphase); Fourth line: Hydrogen mass-fraction pro"les during the regen-eration phase; Reactor with active catalyst only (left column) andreactor with inert end sections (right). The heat production throughhydrogen combustion is the same in both cases.

the maximum temperature reached during hydrogencombustion was too low to start styrene synthesis, Fig. 20(left column). The reason is the low ignition temperatureof hydrogen combustion on styrene catalyst, the result ofwhich had been discussed in Fig. 5a. Using inert frontand end sections brought the bed temperature up. Koliosshowed that in this case a larger portion of the "xed bedcould be used for high-temperature heat storage duringthe combustion stage (Fig. 20, right column). At thebeginning of the dehydrogenation period surprisinglyhigh conversions could be obtained because of the favor-able increasing temperature pro"le. Unfortunately theheat consumption led to a rapid drop of the temperaturepro"le, causing a drop in conversion so that the mean

conversion over the half-cycle was only moderate. Modi-"cations e.g. by using latent storage packings in thereaction zone gave some improvements but since inasymmetric operation styrene production takes placeonly during one semicycle, this mode was considered lessattractive.

Since short cycle times are obviously fortunate for theasymmetric operation mode it seems interesting to lookat the alternative for fast cycling, countercurrent opera-tion. This option has been studied in some detail byFrauhammer, Eigenberger, von Hippel and Arntz (1998)and Frauhammer, Eigenberger, von Hippel & Arntz(1999) using again the example of methane steam reform-ing. They considered asymmetric countercurrent opera-tion as shown in Fig. 17b (left) in adjacent channelsof a ceramic monolith. Fig. 21 shows simulation resultsfor temperature and conversion. If active catalyst extendsover the full length of both channels (a) the incomingfuel gas (methane and air) ignites readily and a temper-ature maximum develops close to the right border ofthe monolith reactor. The process gas coming fromthe opposite side is continuously heated up leading tototal conversion before the maximum temperature isreached. Afterwards, the dropping temperature pro"lecauses a decrease in the equilibrium conversion and someback-reaction which reduces the exit conversion to about70%.

An obvious measure to avoid back-reaction is to limitthe reforming catalyst to the part of the reactor wheremaximum conversion is obtained (Fig. 21b). Heat recov-ery could be improved further by providing a larger heatexchanger zone at the right entrance. This can be done byreducing the length of the active catalyst section in thecombustion channels (Fig. 21c). Compared to thetwo previous examples now the gas streams enter andleave the monolith at lower temperatures and the energye$ciency (the ratio between the actual heat consumptionof the endothermic reaction and the actual heat releaseof the combustion) increases from 48% in example (a)over 69% in (b) to 76% in (c). Obviously the properposition of the active catalyst inside the reactor is impor-tant for the optimization of the reactor's performanceand e$ciency. Nevertheless the adjustment of the posi-tion of the reaction zones is a nontrivial task sincea methane}air mixture tends to ignite homogeneouslyabove about 7503C.

A second problem of practical realization is the ther-mal stability of the reactor material. An appropriateceramic monolith should be able to generally withstandthe expected maximum temperatures in the combustionchannels of up to 15003C but so far experimental veri"ca-tion was only possible for diluted feed with maximumtemperatures below 10003C (Fig. 22). Higher temper-atures caused cracks in the ceramic material which wereattributed to thermal stress in the peak temperatureregion.


Fig. 21. Steam reforming of methane in a countercurrent monolith reactor (asymmetric operation mode) from (Frauhammer et al., 1999). Simulationresults for di!erent positions of active catalyst (black bars).

Fig. 22. Steam reforming of methane in a countercurrent monolithreactor- experimental results (Frauhammer et al., 1999).

3.4. Symmetric operation

First simulation results on symmetric operation havebeen published by Snyder and Subramaniam (1994) forthe reverse #ow operation of styrene synthesis throughethylbenzene dehydrogenation. Similar to Fig. 17c (right)ethylbenzene and steam were fed alternatingly from left

to right and vice versa while superheated steam, gener-ated by the combustion of the produced hydrogen wascontinuously fed into the center of the reactor. Severaloptions with central feed or distributed steam feed werestudied which showed the general applicability of theconcept. Compared to the industrially established multi-tubular isothermal or the adiabatic multibed process thesimulated results were however not too encouragingsince only conversions below 25% were predicted. In anindependent study Kolios (1997) and Kolios and Eigen-berger (1997,1999) showed that these moderate predic-tions had to be attributed to the use of somewhatoutdated kinetics and too conservative estimates aboutthe maximum temperature allowed.

Fig. 23 shows these simulation results for a base casecon"guration. As in the study of Snyder and Sub-ramaniam inert front and end portions and an activecatalyst in the center are assumed. Fig. 23a shows tem-perature pro"les in the periodic steady state. After the#ow switch the synthesis gas enters from left and isheated up by the packing according to pro"le 1. In thecenter of the reactor superheated steam of 12003C isadded and assumed to mix perfectly with the passing gas.The favorable increasing temperature pro"le causes anethylbenzene conversion of above 50% (Fig. 23b). Duringthe course of the semicycle the bed temperature of thereactor decreases in the left part due to endothermicreaction but it increases in the right part due to the hotsteam feed. This causes the maximum temperature andthe conversion to stay at su$ciently high values over thewhole cycle (Fig. 23c and d). Compared to the asymmet-ric operation of Fig. 20 which was simulated under com-parable operating conditions, the reactor behavior is


Fig. 23. Symmetric operation mode for ethylbenzene dehydrogenation with hot gas injection in the midle of the reactor: Temperature pro"les (a) andconversion pro"les (b) during one semicycle in periodic steady state with #ow entrance from the left, maximum and exit temperature (c) and conversionand selectivity during the cyclic steady state (d).

Fig. 24. Experimental results for the ethylbenzene dehydrogenation in an autothermal reverse #ow reactor. (a) Schematics of the lab-scale reactor andthe catalytic burner (Kolios, 1997; Kolios & Eigenberger, 1999). (b) Temperature pro"les in cyclic steady state at the end of semicycle when #ow comesfrom the right: Simulation (**) and experiment (}# }).

much more favorable. It could be further improved byincreasing the amount of side stream. Conversions up to80% with good selectivities were predicted. In most casesthe hydrogen e$ciency (the amount of hydrogen produc-ed over the amount of hydrogen used for the generationof the hot side stream) was above 1, ensuring operationwithout external thermal energy consumption.

The predictions could be veri"ed in a sequence ofexperiments in a lab scale reactor shown in Fig. 24a.A catalytic burner for hydrogen and air with steam ascarrier gas was placed in the center of the packed bed.Fig. 24b shows measured temperature pro"les at the endof the period where #ow comes from the right. Alsoshown are simulation results where axial heat conduction

in the walls and the internals was taken into account.A reasonable agreement can be observed except for thecenter section at and behind the hot side feed. Heremixing of the two gas streams is obviously incompleteand the thermocouples are more in#uenced by the hotside stream than by the colder synthesis gas. Conversionsof about 65% with selectivities corresponding to the bestvalues obtained with the same catalyst under isothermaloperation could be obtained. The results show that sym-metric operation seems to be a prospective method forthe e$cient coupling of endo- and exothermic reactionswithout the danger of too high maximum temperatures.This will be further substantiated with the followingconsiderations.


3.5. A simplixed picture of the autothermal couplingof endo- and exothermic reactions

It should be obvious from the preceding discussionthat there are several interesting options for an e$cientautothermal coupling of endo- and exothermic reactions.But so far most of the options published still have draw-backs or problems which need to be solved before anindustrial realization can be considered promising. Ingeneral these problems are related to the temperaurecontrol of the process, i.e. to the question of how theexothermic and the endothermic reaction can be e$-ciently coupled without the occurrence of excessivelyhigh temperatures. To answer this question it seemsreasonable to start with some basic constraints and spec-ify some general features which need to be incorporatedinto an e$cient autothermal coupling.

One starting point is that the feed and exit streams ofthe autothermal reactor should be `colda, that meansconsiderably lower than the required reaction temper-ature. Since inside the reactor temperatures well abovethe ignition temperatures of both the exothermic and theendothermic reaction are required, one should try toincorporate the most e$cient heat exchange availablewhich is countercurrent recuperative heat exchange of#ows with equal thermal capacity (or a respective regen-erative heat exchange with short cycle times).

The basic energy constraint is that the heat generationof the exothermic reaction has to exceed the heat con-sumption of the endothermic reaction. Assuming #ows ofequal thermal capacity this means that the adiabatictemperature rise of the exothermic reaction *¹%90

!$has to

exceed the adiabatic temperature drop of the endother-mic reaction !*¹%/$0

!$and that the sum of both,

*¹)%9"(*¹%90!$

#*¹%/$0!$

) (3)

will be available for countercurrent heat exchange for thetwo #ows. The countercurrent heat exchangers must bebig enough to heat the combustion feed both above theignition temperature of the combustion reaction ¹*'/ andthe syngas feed to a su$ciently high reactor entrancetemperature. In case that the following reactor isadiabatic, `su$ciently higha means that the reactor en-trance temperature has to exceed the required reactorexit temperature ¹%26 by the expected adiabatic temper-ature drop *¹%/$0

!$. ¹%26 is a lower limit temperature

which guarantees that the rate of the endothermic reac-tion at the required exit conversion is still su$cientlyrapid. *¹%90

!$, *¹%/$0

!$, ¹*'/ and ¹%26 together with the

temperature slope of equal capacity countercurrent heatexchange are the "ve important quantities which in thefollowing will be used to de"ne base cases of autothermalcoupling of exo- and endothermic reactions. Note that inFig. 5 we needed only *¹%90

!$, ¹*'/ and d¹/dz (Eq. (1)) to

get a full picture of the autothermal behavior with anexothermic reaction.

Before discussing integrated reactor concepts it seemsreasonable to "rst go back to the unit operation ap-proach mentioned in Section 2.1 and to consider combi-nations of countercurrent heat exchangers with adiabaticreactors which ful"ll the above requirements. In the fol-lowing we will do this for each of the three con"gurationsgiven in Fig. 17a}c.

As a "rst example we will try to combine an exother-mic reaction with a thermally neutral reaction(*H

R"0), such that the reactor feed temperatures well

exceed the ignition limits of both reactions ¹*'/. Theresult is given in Fig. 25a. The appropriate sequence ofunit operations consists of two countercurrent heat ex-changers connecting an adiabatic reactor for the exother-mic reaction and an adiabatic reactor for the thermallyneutral reaction. Assuming temperature-independentheat transport parameters as in Fig. 5, a similar graphicalconstruction can be used for the temperature pro"les ofthe heat exchange sections. From Eq. (1) the slope in bothheat exchangers is given. To obtain the highest maximumtemperatures or optimal energy utilization requires equaltemperature di!erences in both countercurrent heat ex-changers, hence *¹

1"*¹

2"*¹)%9/2"*¹%90

!$/2. If the

available heat exchange area would have been dividedsuch that the left heat exchanger gets 4/5 and theright one 1/5 of it, the temperature pro"les, as shownin Fig. 25b would result. The position of the new temper-ature maximum is given by the length of the respectiveheat exchangers, hence the temperature slopes in therespective heat exchange sections (Eq. (1)) correspondto 1 : 4. This means that *¹

1"*¹%90

!$/5 and

*¹2"4*¹%90

!$/5. This is about the result obtained in

Fig. 21b, where the length of the right heat transfersection was limited by the rapid ignition of the combus-tion reaction. It is obvious that this design is inferior toFig. 25a if a maximum area of the catalyst bed shouldstay above an assumed ignition temperature ¹

*'/in or-

der to obtain maximum conversion with minimum en-ergy consumption.

3.5.1. Simultaneous operationA unit operation analogue of the simultaneous opera-

tion concept of Fig. 17a (left) which ful"lls all of the aboverequirements is given in Fig. 26. The (simultaneous) exo-and endothermic reaction is now considered to takeplace sequentially in two consecutive adiabatic reactors.It is obvious that the sequence of exothermic and en-dothermic reaction requires less heat transfer area tobring the feed to a required reaction temperature thanthe oppposite sequence. In the adiabatic reactor arrange-ment of Fig. 26 the maximum temperature is givenby ¹%26#(!*¹%/$0

!$), assuming ¹*'/(¹%26. Since

*¹%90!$

'!*¹%/$0!$

an estimate for the steam reformingof methane with undiluted feed (¹*'/"7003C,*¹%/$0

!$"!10503C) in Fig. 26 would lead to a max-

imum temperature of ¹.!9'17503C.


Fig. 25. Asymetric operation between a thermally neutral and an exothermic reaction. Heat exchangers of equal (a) and of di!erent size (b).

Fig. 26. A unit operation analogue of simultaneous operation.

If, contrary to the assumptions of Fig. 26 the tworeactions could be run in the same catalyst bed withcomparable rates the heat generation and consumptionwould compensate and the temperature would remainalmost constant at ¹%26. In the example of Fig. 18 theexothermic reaction is obviously considerably faster thanthe endothermic one leading to the pronounced temper-ature peak. A possible remedy in this case would be toslow down the combustion reaction and * since thisseems impossible* to either use multiple feeds for oxy-gen or to care for an excellent axial heat transfer in thereaction zone. Since the behavior of both #ow directionsof the design in Fig. 26 is obviously similar, the reactor

design can be simpli"ed to the part depicted by solid lineswith an internal recycle of the #ow between the exother-mic and the endothermic stage.

3.5.2. Asymmetric operationAn energetically optimal unit operation analogue for

the asymmetric operation of Fig. 17b is given in Fig. 27(top). It can be seen that the left heat exchanger has to beconsiderably longer than the right one since it has to heatup the endothermic feed to a maximum temperaturewhich is again given by ¹%26#*¹%/$0

!$. Since the thermal

capacity of both streams should be equal, a drivingtemperature di!erence of (*¹%90

!$#*¹%/$0

!$)/2 is available

at each heat exchanger and the temperature slopes areequal. If heat exchange would be allowed between thetwo adiabatic reactors the maximum temperature couldbe reduced. If the heat production and the heat consump-tion rates could be made equal for both reactions, themaximum temperature would even be limited to about¹%26. Since this seems impossible, similar remedies asdiscussed for simultaneous operation should be con-sidered.

3.5.3. Symmetric operationA unit operation analogue for symmetric operation is

shown in Fig. 28. It is su$cient for countercurrent opera-tion and could be extended as in Fig. 26 by a symmetriccounterpart at its right-hand side in order to map theprinciple of the reverse #ow process in Section 3.4. Sev-eral con"gurations are possible. Since the necessary heatof reaction shall be supplied by heat exchange with a fuel


Fig. 27. A unit operation analogue of asymmetric operation.

Fig. 28. A unit operation analogue of symetric operation. Con"guration with heat supply by heat exchange (a) and heat supply by hot gas addition (b).

combustion circuit or by direct hot gas addition from it,two interconnected circuits, the (endothermic) syngasloop and the (exothermic) combustion circuit have to beconsidered. As before, optimal energy utilization requiresthat both streams have equal thermal capacity and equalspeci"c heat exchange area. This leads to the con"gura-tions of Fig. 28 where the temperature pro"les in the twomain countercurrent heat exchangers are identical. InFig. 28a the heat exchange between the two circuits takesplace via a third countercurrent heat exchanger, whereas

in Fig. 28b the hot gas stream is added to the syngas #owbefore and an equivalent amount is withdrawn after theactive catalyst bed. If this addition is distributed over thelength of the syngas reactor the maximum temperatureon the syngas side can be considerably reduced. As a re-sult, simple addition of a hot side stream as suggested inFig. 17 and utilized in Figs. 23 and 24 turns out to be anenergetically suboptimal solution because the require-ment of equal heat capacity streams in the heat exchan-gers is violated.

Comparing Figs. 28a and b it might be surprising thatthe countercurrent solution of Fig. 28a is equivalent tothe mixed stream solution Fig. 28b. The reason is simplethat the feed temperature of the (endothermic) syngasreactor is of no importance as long as its exit temperatureat the required conversion is above ¹%26. Of course thedesign of Fig. 28b will need a bigger syngas reactorbecause of the lower feed temperature and the dilutedfeed concentration. The latter however can be of advant-age for equilibrium-limited reactions with volume in-crease like dehydrogenations.

3.5.4. Regenerative heat transferSo far an optimized design has primarily been dis-

cussed for countercurrent operation. The easiest transferto regenerative operation with periodic #ow reversal isobviously possible for symmetrical operation. This wasalready shown for the examples of Kolios and Eigenber-ger (1997) and Snyder and Subramaniam (1994). Theonly required addition is the above-mentioned provision


of a countercurrent heat exchanger for the burner feedgas. Here it might be of advantage to switch the positionsof the hot gas feed ports from half-period to half-periodas already suggested by Snyder and Subramaniam (1994).Under simultaneous as well as under asymmetricoperation there is always the danger of a high and narrowtemperature maximum if the rate of the exothermic reac-tion could not be slowed down to that of the endothermicone. This is well demonstrated by the simulations of Kul-karni and DudukovicH (1998), where Fig. 19a has a closeshape similarity with the idealized temperature pro"les ofFig. 27. For periodic #ow reversal the pro"les are even lessadvantageous than for countercurrent operation becauseonly few thermal energy can be stored in the regenerativereaction zone without reaching excessive peak temper-atures. The only remedies recommended are again anincreased axial heat transport, possibly combined witha latent heat storage (Kolios & Eigenberger, 1997). But ingeneral it seems that recuperative heat exchange is ofadvantage for the simultaneous or the asymmetric coup-ling of endo- and exothermic reactions, if the problem ofmaterial stability for high-temperature heat exchange canbe su$ciently solved.

4. Conclusions

Autothermal multifunctional reactors have gaineda considerable interest in the last decades. The integrationof reaction and heat exchange within one piece of equip-ment has basic advantages over the conventional unitoperation design: Heat losses can be minimized and thereactor is less sensitive to perturbations because of itsinherent adaptivity. Autothermal reactors are widely spreadin industrial applications in the "eld of waste air puri"ca-tion. Nevertheless the design of autothermal reactors isa nontrivial task because the intrinsic feedback of heat givesrise to a complex parametric and dynamic behavior.

A new and very promissing application of autothermalmultifunctional reactors is the coupling of endothermicand exothermic reactions where the product of the en-dothermic reaction is the desired one. Several processschemes have been proposed which can be subdivided inthree categories: the simultaneous, the asymmetric andthe symmetric operation mode. So far the potential of thenew processes has been demonstrated, but also the needfor optimization became obvious. The development ofnew reactor designs (monolith reactor, folded-plate reac-tor) and the application of advanced materials have beentriggered by these studies in order to enhance the processe$ciency. The distribution of the catalytic activity indi!erent zones along the axis has been shown to havea strong impact on the reactor performance. For furtherdevelopment the following startegy seems to be adequate:The basic features of di!erent design variants can beexplored using the presented simpli"ed unit-operation

sequence substitutes of the integrated process. Detailedsimulation and analysis is necessary for further optimiza-tion of the structure and of the operating conditions ofthese reactor concepts.

Notation

av

surface to volume ratiocp

speci"c heat capacity of the gas mixturecs

speci"c heat capacity of the solid packingc0 feed concentrationhw

wall or catalyst heat transfer coe!cientmR

zmass #ux density

¹ temperature¹0 feed temperature¹%26 required reaction zone exit temperature for en-

dothermic equilibrium reaction¹*'/ (approximate) ignition temperature of reaction¹.!9 maximum reactor temperaturev e!ective velocity of gas mixturew front moving velocity (Eq. (2))w weight fractionX conversion> mole fractionz axial reactor coordinate

Greek letters

*HR

enthalpy of reaction*¹ total temperature rise (maximum minus feed

temperature)*¹

!$adiabatic temperature rise of the reaction

*¹)%9 driving heat exchange temperature di!erence(see Eq. (3))

e void fraction of catalystj%&&

e!ective axial heat conductivity of reactor orheat exchanger

jw

heat conductivity of wallo density of the gas mixtureos

solid density (of packing)

Acknowledgements

Our results presented in this review have been madepossible through grants of the German Science Founda-tion (DFG) and of the German Bundesministerium fu( rBildung und Forschung (BMBF).

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