Heat-Integrated Reactor Concepts

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Heat-integrated reactor concepts for catalyticreforming and automotive exhaust purification

Grigorios Kolios 1, Achim Gritsch, Arstides Morillo, Ute Tuttlies,Jens Bernnat, Frank Opferkuch 2, Gerhart Eigenberger *

Institut fur Chemische Verfahrenstechnik, Universitat Stuttgart, Boblingerstr. 72, D 70199 Stuttgart, Germany

Available online 30 June 2006

Abstract

Optimal solutions in environmental catalysis require a well-coordinated development of catalysts and of process design. This contribution is

devoted to energy integrated design concepts for fuel reforming and for automotive exhaust purification. The examples presented demonstrate the

importance of an innovative process design for optimal utilization of existing catalysts and show the potential of future developments.

New concepts for steam reforming through the efficient coupling of the endothermic reforming reaction with an exothermic combustion

reaction are discussed in the first part. These concepts have been implemented for methanol steam reforming in a counter-current reactor with

distributed side feed of burner gas and for methane steam reforming in a modular reactor with a co-current reaction section for the endothermic and

the combustion reaction and attached counter-current heat exchangers. Both applications employ the so-called folded sheet reactor design, which

ensures an excellent heat transfer between the reforming and combustion channels and efficient heat recovery.

A similar design solution is introduced for the apparently different case of automotive exhaust purification. The proposed concept aims at

decoupling exhaust after-treatment from engine control. Its main component is a counter-current heat exchanger with integrated purification stages

for HC-oxidation, NOXstorage and reduction and soot filtering. A small catalytic burner at the hot end of the heat exchanger provides both heat and

oxidizing or reducing agents on demand. A new soot filter design allows for safe soot filter regeneration.

# 2006 Published by Elsevier B.V.

Keywords: Autothermal reactors; Heat-integrated reactors; Process integration; Steam reforming; Automotive exhaust purification; Diesel soot filtering

1. Introduction

Environmental catalysis aims at the efficient conversion of

raw materials into chemical products. This implies high yield of

the desired products, minimal noxious side products and high

energy efficiency. Particularly the last point requires a close

cooperation between catalyst development and process design.

In this contribution, the focus is set on process design concepts

for energy efficient catalytic processes in two rather differentapplication areas, namely for high temperature endothermic

reactions like steam reforming and for automotive exhaust

purification. In both applications, the aim is to provide optimal

temperatures for the required reactions as well as high thermal

efficiency. Several novel multifunctional heat-exchanger

reactor concepts will be discussed both through simulations

and experimental results. They accomplish optimal energy

utilization by recovering the heat from the treated hot stream to

heat up the cold feed.

The respective principle of autothermal reactors is well

established for weakly exothermic reactions like combustion of

pollutants in exhaust air [13]. Fig. 1 (top left) shows thegeneral concept with regenerative heat exchange, leading to the

so-called reverse-flow reactor. The reactants are periodically

fed to the fixed-bed reactor from opposite reactor ends. To start

the reaction, the active catalyst section needs to be heated up

above ignition temperature Tign of the reaction. The end

sections of the reactor can be inert and serve as regenerative

heat exchangers where the respective cold feed is heated up by

the hot packing and the hot gas leaving the reaction section is

cooled down. Fig. 1 (right) shows the typical temperature and

conversion profiles for a weakly exothermic reaction in an

www.elsevier.com/locate/apcatbApplied Catalysis B: Environmental 70 (2007) 1630

* Corresponding author. Tel.: +49 711 685 85257; fax: +49 711 685 85242.

E-mail address: [email protected] (G. Eigenberger).1 Present address: Christ AG, Hauptstr. 192, CH 4147 Aesch, Switzerland.2 Present address: Modine Europe GmbH, Modinestr. 1, D 70794 Filderstadt,

Germany.

0926-3373/$ see front matter # 2006 Published by Elsevier B.V.

doi:10.1016/j.apcatb.2006.01.030
mailto:[email protected]://dx.doi.org/10.1016/j.apcatb.2006.01.030http://dx.doi.org/10.1016/j.apcatb.2006.01.030mailto:[email protected]


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adiabatic reactor in the ignited periodic steady state for the limit

of fast flow reversal. In case of catalytic combustion, the

reaction zone is usually confined to a small region of the

catalytic section where the reaction runs to completion. From

an overall heat balance for the adiabatic reactor it follows that

the difference between entrance and exit temperature and hence

the driving temperature difference for the counter-current heat

exchange is identical to the adiabatic temperature rise DTad of

the reaction mixture. Regenerative heat exchange allows to

increase the temperature in the center of the reactor by a value

DTmax of 1020 times the adiabatic temperature rise, dependingon the flow rate and the characteristics of the heat exchange.

This corresponds to a heat exchanger efficiency of

hHEX DTmax DTad

DTmax 9095%:

It can be shown [4] that the behavior of the reverse-flow reactor

in the limit of fast cycling is equivalent to a counter-current

fixed-bed reactor with indirect (recuperative) heat exchange

(Fig. 1, left: middle). Here, two gas streams of equal heat

capacity enter the reactor simultaneously at the opposite ends

and are separated through heat transferring walls. The two end

sections serve as counter-current heat exchangers. Since thebehavior in one flow direction is just the mirror image of that in

the other direction, a counter-current reactor for weakly

exothermic reactions can be simplified to a reactor with only

one common feed and effluent side if the gas flow is turned

around in the hot center of the reactor (Fig. 1, left: bottom).

Compared to the reverse flow reactor the counter-current

fixed-bed reactor has the advantage of continuous operation

without flow switches. However, it requires excellent heat

transfer between adjacent channels in order to compete with the

direct heat transfer between gas and packed bed in regenerative

heat exchangers. This requires narrow channels in the order of

1 mm and (preferably) catalytically coated walls for ensuring

direct transfer of the heat of combustion. Simple operation

control of counter-current reactors but a more complex design

make them suited for small to medium scale applications,

whereas reverse-flow reactors can easily be scaled up to large

units. The following discussion is restricted to recuperative

concepts with indirect heat exchange. However, it should be

mentioned that powerful regenerative concepts with similar

functional properties have been developed as well [57].

A suitable recuperative design uses parallel plate channels

with a channel width in the order of millimeters and a length in

therange of half a meter. A foldedsheet reactor designhas provenparticularly useful for this purpose [8]. It consists of a high-

temperature-resistant stainless steel foil of 0.20.4 mm thick-

ness, which is folded to form many parallel flow pockets and

fixed in a housing as shown in Fig. 2 (left). The folded sheet

separates two reaction compartments from each other. Feed/exit

ports at the two ends distribute/collect the gas streams into/from

the respective pockets. The thin folded sheet is supported by

spacers in order to keep the intended channel geometry. Different

spacer designs with improved mixing or heat transfer properties

can be used. Moreover, they canbe catalytically coated(cat 1 and

2). Corrugated spacers as shown in Fig. 2 (left) have a low

pressure drop, serve as (replaceable) catalyst carriers and

improve heat transfer through their close contact with the foldedsheet. Several counter-current reactors with heat exchange areas

up to 10 m2 have been built and successfully tested. One of such

reactors is shown in the middle ofFig. 2. It has an inert entrance

section with the active catalyst starting at 0.4 m. On the right-

hand side, measured and simulated temperature profiles are

shown for air flow with different propene feed concentrations.

Thetemperature drop after themain reaction zone at theentrance

of the active catalyst is due to heat losses.

Decisive for an efficient heat recovery in an autothermal

reactor is a proper design of the heat exchange sections. It

requires counter-current heat exchange with equal heat capacity

fluxes in both directions and low axial heat conductivity. This is

G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 17

Fig. 1. Left: reverse flow reactor (top); counter-current fixed-bed reactor, with similar flow and heat transfer pattern as the reverse flow reactor (middle); counter-

current fixed-bed reactor, exploiting profile symmetry(bottom). Right: temperature (T) and conversion profiles (X) of the reverse flow reactor overthe reactor lengthz

in the cyclic steady state for fast flow reversal, similar to counter-current reactor profiles.


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exemplified in Fig. 3. In the middle the temperature profiles of

an optimally designed counter-current folded sheet heat

exchanger are given. A metal foil of 0.18 mm thickness isused; the channels of 1 mm width contain inert spacers. The

resulting axial Peclet number of 125 indicates that convective

heat transfer dominates the behavior of the device. It provides

an excellent heat exchange efficiency of 98%. If the same heat

exchanger is operated with twice the heat capacity flux

downstream as compared to upstream, the profiles of Fig. 3

(left) result. Obviously, the smaller hot stream can heat up the

larger cold stream only to half the total temperature difference,

reducing the heat exchange efficiency from 98 to 67%.

Considering a micro-heat exchanger with 0.1 mm channel

diameter, the same heat transfer area as above requires only one

tenth of the original length but the wall thickness (0.18 mm)will stay the same. Now the axial Peclet number is reduced to 2,

indicating that axial heat conduction through the channel walls

dominates over convection. This changes the shape of the

resulting temperature profiles as shown in Fig. 3 (right) and

reduces the heat recovery to 65%. This shows that micro-

reactors with micro heat exchange sections usually do not

provide sufficient heat recovery, in spite of their excellent heat

transfer properties.

2. Coupling of endothermic and exothermic reactions

Efficient energy integration is even more important for

endothermic high temperature reactions as compared with

weakly exothermic reactions. Here, the additional requirement is

to efficiently supply the heat of reaction, e.g., by an exothermic

reaction (usually a waste-gas combustion), in order to achieve an

overall slightly exothermic process. Then the above-mentioned

autothermal concepts should be in principle applicable.

However, it turns out that major problems may result from axial

separation of the two reactionzones, resultingin excessively high

combustion temperatures. This is already the case in large-scalesteam reformers equipped with packed-bed reformer tubes of

several centimeters internal diameter and natural gas burners at

the side or top walls [9]. Chemical equilibrium requires exit

reformate temperatures closeto 900 8C for sufficient equilibrium

conversion. The burners provide the heat of reaction with flame

temperatures in the order of 2000 8C. This heat is transferred to

G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163018

Fig. 2. Counter-current heat exchanger reactor for waste air purification through catalytic combustion. Left: folded sheet reactor model with corrugated spacers as

catalyst carriers (cat 1 and 2). Middle: design sketch of a respective reactor. Right: measured and simulated temperature profiles for different propene feed

concentrations in air.

Fig. 3. Temperature profiles in counter-current heat transfer for a folded sheet heat exchanger with 0.18 mm wall thickness. Left and middle: channel width of 1 mm

and channel length 25 cm, with heat capacity flow ratio of 1 (middle), and 0.5 (left). Right: micro-heat exchanger with 100 mm channel diameter and 2.5 cm channel

length.


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the reforming catalyst through radiation and convection. The

whole process is strongly heat transfer limited, since the

reforming catalyst is very active in the mentioned temperature

range. Only about 50% of the heat of combustion can be

transferred to thereforming reaction.The rest hasto be recovered

through a complex network of heat exchangers. This is the

reason, why such reformers are only effective and profitable, if

they are integrated into the heat transfer network of a large

chemical production site.

Recent developments aim at a higher energy integration and

efficiency. As an example a sketch of the heat-integrated Haldor

Topsoe Convection Reformer is shown in Fig. 4. The tube-in-

tube reformer allows for internal backflow of the produced

syngas using the sensible heat of the process gas to heat up the

cold feed. In addition, the heat of the burner flue gas is taken-up.

This may increase the thermal efficiency to 7080%. Never-

theless, the reformer reaction remains strongly heat transfer

limited and large temperature differences are required to transfer

the required heat of combustion to the reforming catalyst pellets.

For optimal heat integration it would be intriguing to couplethe flow of the reforming gas counter-currently with that of the

combustion gas and to adjust the flow rates such that about

equal heat capacity flows run in both directions. Such a design

was studied for the case of methane steam reforming combined

with methane combustion in [10] using the flow scheme as

shown in Fig. 5 (top). The simulation results shown in Fig. 5

represent the behavior of a ceramic monolith reactor of 2 mm

channel diameter where the channels are divided in a chess

board-like pattern to the combustion and the reforming gas. The

bars in the two lower diagrams of Fig. 5 show the extension of

the respective combustion (bottom) and reforming catalysts

(above), which were assumed to be coated at the monolith

walls.

The results represent an optimal configuration with respect

to the flow rates on combustion and reforming side as well as

catalyst distribution. Nevertheless, the maximum temperature

exceeds 1500 8C. This is the result of the axial separation of the

exothermic and endothermic reaction as indicated by the two

methane conversion profiles XCH4 in Fig. 5. A more detailed

analysis [11] shows that under pure counter-current operation it

is indeed impossible to achieve the desired overlapping of the

reaction zones, if full conversion of the reforming reaction is

required.

One way to enforce overlapping is a distribution of one feed

of the combustion reaction (air or fuel) over the length of theactive catalyst zone. This is shown in Fig. 6, again for the case

of methane steam reforming. A uniform burner gas distribution

at sufficient temperatures results in a uniform heat supply,

which nicely corresponds to the heat request Qendo of the

reforming reaction and leads to its almost linear conversion


Fig. 4. Sketch of the Haldor Topsoe Convection Reformer with improved heat recovery as compared to standard reformer design.


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profile. Details of an appropriate design of the combustion

relative to the reforming zone are given in [11,12].

A counter-current fixed-bed reactor with distributed burner

gas feed has been realized experimentally for methanol steam

reforming with a production capacity of hydrogen with 10 kW

lower heating value (LHV) [13,14]. In addition to the reforming

stage, the reactor contains a section for the evaporation of the

methanol/water feed mixture and a water-gas shift section. The

reactor design, based on the folded sheet concept, is shown in

Fig. 7. On the combustion side air flows from top to bottom. The

anode off-gas from a fuel cell stack or the purge gas of a PSA

hydrogen purification unit is used as a fuel and is distributed

individually to every pocket of the burner side at five levels.

Fig. 7(a) shows the folded sheet concept, Fig. 7(b) the locations

of the combustion catalyst sections, the reforming catalyst and

the water-gas shift catalyst section. Fig. 7(c) shows a picture of

the reactor from the burner gas side, Fig. 7(d) shows one fuel

gas distributor element during a test run. The flamelets illustrate

the achieved uniformity of gas distribution into the combustion

side pockets.

Fig. 8 shows simulated temperature and conversion profiles.

These predictions have been completely confirmed by the

experiments, yielding methanol conversions higher than 90%

and CO concentrations below 1.6%. A remarkable feature ofthe reactor is the fast dynamic response upon load changes as

shown in the bottom graph of Fig. 8.

In spite of its successful implementation for methanol steam

reforming the filigree side feed distributor was considered too

sensitive to be used at the substantially higher temperatures

occurring during methane steam reforming. Instead, a modular

reactor concept was conceived where the heat exchange

between the reforming and the combustion reaction takes place

in co-current mode. As shown in Fig. 9 (top), this concept has

the additional advantage that the reforming product gas

exchanges heat with its feed in the left heat exchange section

while the combustion gas does the same in the right heatexchanger. This allows choosing the throughput on the

reforming and the combustion side independently from each

other, yielding improved design flexibility.

Fig. 9 shows simulated temperature profiles for the coupling

of methane steam reforming and methane combustion in a

catalytic wall reactor (left) and a packed-bed catalytic reactor

(right). As discussed before (Fig. 2), the temperature profiles in

the counter-current heatexchangesections are linear due to equal

heat capacity fluxes, and are therefore not shown. The discussion

focuses on the reaction section. In both cases the same specific

heatexchange area has beenassumed. Nevertheless, the behavior

of the two configurations is completely different. In the catalytic

wall reactor combustion and reforming catalyst is deposited onopposite sides of the same wall, leading to an excellent heat

transfer with almost the same temperatures for the two catalysts.

This allows for the overlapping of the combustion and reforming

conversion profile and leads to a smooth temperature profile

where the rapid reforming reaction even tends to quench the

combustion. The packed-bed reactor on the other hand features

run-away of the combustion reaction as shown by the severe

temperature excursion at theinlet cross-section. This is dueto the

substantial heat transfer resistance between the catalytic

packings in both channels. Systematic parametric studies with

the above model revealed a considerable sensitivity with

respect to the kinetics of the combustion reaction [24].


Fig. 5. Temperature (T) and methane conversion profiles XCH4 for methanesteam reforming and methane combustion in a counter-current monolith rector

of 2 mm channel diameter [10]. The location of the respective catalysts is given

by the bars in the conversion profiles.

Fig. 6. Counter-current fixed-bed reactor for methane steam reforming coupled

with methane combustion and evenly distributed methane side feed [12]. The

color bars in thetop drawing mark thelocationof thereforming andcombustion

catalysts. Xrefand Qendo are the methane conversion and the heat requirement of

the reforming reaction. (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of the article.)


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In the above simulations simple kinetic expressions have

been adopted for catalytic and homogeneous methane

combustion [15,16]. In reality, the combustion reaction depends

in a complex way on the gas composition and the flow and back

mixing characteristics. To study this behavior in the small

channels considered, a series of combustion experiments have

been performed in a single channel device as shown in Fig. 10.

If premixed fuel gas and air enter the reactor, back-ignition mayoccur up to the mixing point. Therefore, mixing of fuel gas and

air took place at the entrance of the hot combustion channel as

shown in Fig. 10 (top right). Mixing was enhanced by a static

mixer before entering the catalyst section. The photos ofFig. 10

(middle and bottom) show the heat evolution via the color

patterns (brightness) of the glowing metal housing. With

hydrogen in the feed (Fig. 10, middle left) a back-ignition up to

the mixing point with homogeneous combustion always

resulted. Even with pure methane the danger of back-ignition

into homogeneous combustion was big, if the flow velocity was

low and the methane concentration was high (Fig. 10, bottom

left vs. middle right). However, if CO2 or water vapor was

added as a mediator and radical scavenger, an extendedcombustion zone without homogeneous pre-ignition could be

established. It was shown recently [17,18] that a pre-ignition of

hydrogen containing combustion gas could be safely prevented,

if the catalyst coated channel dimensions are in the range of

100 mm. This would, however, lead to a micro-reactor design

with the restrictions mentioned in connection with Fig. 2.

Based upon the above results and additional considerations

and experiments reported in [19,24], the prototype of a methane

steam reformer for a production capacity of 5 m3/h (STP)

hydrogen (14 kW LHV) has been designed and set-up. Fig. 11

shows a sketch of the set-up along with simulations on its

operating behavior. An additional fuel injection port has been

provided in the catalytic section. According to the design

calculations the specific productivity of the reactor stage is

estimated to 7 Nm3 H2/(l h) (STP) and the thermal efficiency of

the system is expected to be as high as 90%. Meanwhile,

experimental results have fully confirmed the design simula-

tions, showing full conversion for a wide load range and a fast

and smooth response to load changes [24].

Summarizing Section 2, it has been shown that close heatintegration between catalytic reforming and catalytic combus-

tion allows for the design of compact, small-scale reformers for

decentralized hydrogen production for fuel cells with excellent

fuel economy and a rapid start-up and load-change behavior.

The so-called folded sheet reactor concept proved to be both

sufficiently simple and efficient for this purpose.

3. Automotive exhaust purification

The problems connected with automotive exhaust purifica-

tion seem to be substantially different from the problems and

solutions discussed previously. However, it will be shown that

several of the above concepts may be helpful in futureautomotive exhaust purification systems as their requirements

are dictated by more and more stringent emission regulations.

First, the present state and problems of automotive exhaust

purification for passenger cars with combustion engines will be

briefly reviewed. Table 1 gives a respective summary. Only the

three main types of combustion engines are considered here.

The three-way catalyst is a well-established standard for l-

controlled Otto-engines. The task of NOX-reduction and soot

filtering in diesel cars is much more challenging.

It is obvious from Table 1 that the different purification steps

require different operation temperatures and exhaust composi-

tions. The present dogma of automotive exhaust purification is


Fig. 7. A 10 kW methanol steam reformer, integrating feed evaporation (Evap.), reforming (Ref.) and water-gas shift reaction (WGS) on side 2 with distributed

hydrogen combustion on side 1 in a folded sheet counter-current reactor design [13,14]. (a) Folded sheet concept and (b) burner gas feed and distribution ofcombustioncatalyst (left) with respect to thesections forevaporation, reformingand water-gasshift. (c) Burner gassideof thereactor and(d) test runof one of thefive

gas distributors to verify uniform burner gas distribution into all pockets of the burner gas side.


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to provide these conditions through elaborate computer control

of the engine combustion conditions. But it becomes more and

more obvious, that this tends to pose insurmountable difficulties

particularly in view of the more stringent future emission limits.

3.1. Three-way catalyst with l-control

This well established technology is based upon the

observation that all three main noxious exhaust components

of Otto-engines, CO, unburned hydrocarbons (HC) and nitrous

oxides (NOX) can be simultaneously converted to CO2, water

and N2 on a noble metal catalyst, if the oxygen content of the

exhaust gas (the l-value) is rigorously controlled to

stoichiometric conditions (l = 1). Since, in practice, the l-

value oscillates around l = 1 with a considerable amplitude, an

oxygen storage component is an essential ingredient of current

three-way exhaust catalysts.

One yet unsolved problem concerns the cold start emissions,

since the catalyst has to be heated up to its operating

temperature through the engine exhaust. Cold start emissions

are currently minimized by placing a pre-catalyst as close as

possible to the engine and the main catalyst further down-

stream. The fuel-rich hot exhaust gases enable a rapid ignition

of the pre-catalyst and in sequence of the main catalyst.


Fig. 8. Simulated profiles of the 10 kW methanol reformer under design conditions over the length of the reforming section for the reforming side (left) and the

combustion side(right). qloc gives the local heat input through the combustion reaction (positive values) and the heat demand of the reformingreaction (negative). The

bottom graph shows experimental results of the reformate flow produced (also given in kW thermal power) upon step changes of the liquid feed.


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A second problem is connected with engine operation under

full power. Then, the exhaust temperature may exceed 1000 8C

which thermally deactivates the catalyst. This is why the main

catalyst is placed further downstream and occasionally

provided with optional cooling of the exhaust line. A third

problem may results from HC peak emissions, causing

temporary temperature excursions at the catalyst entrance.

Summarizing, three-way catalysts are well established but

require a rigorous design and an efficient operation manage-

ment through engine control.

3.2. NOX storage catalysts

Lean-burn Otto-engines or diesel engines require a NOXreduction technology, dedicated for operating under oxygen

excess. In the following, only the so-called NOX storage

catalyst will be considered, since it is the presently favored

solution for passenger cars. In lean-burn Otto-engines the

storage catalyst is usually positioned downstream a conven-

tional three-way catalyst, since in a number of situations (like

cold start and full load) these engines also run under

stoichiometric conditions (l = 1). Under fuel lean conditions

the exhaust-NOX is incorporated into the storage material in

form of nitrates until the storage capacity is exhausted. In

periodic intervals a short regeneration step under fuel rich

conditions is applied, in which NOX is released and reduced to

nitrogen and CO2 or water.

The detailed analysis and appropriate modeling of NOXstorage catalysts is a topic of ongoing research [20], which shall

not be discussed here. It is clear that operation of the NOX-

storage catalyst requires additional sensors and a complex

control strategy which is presently accomplished entirely

through engine control. Accordingly, engine design and control

are more and more adjusted to emission regulations rather than

with respect to optimal performance and efficiency.

3.3. Diesel soot filtering

In addition to NOX removal, diesel engines will soon require

soot filtering, since improvements in the engine combustion

process will no longer suffice to meet the upcoming fine particle

emission limits. Presently, several filter techniques are under

development including fiber and sinter-metal filters. The most

commonly applied soot filters, however, consist of ceramic

(cordierite or SiC) monoliths, where every second channel is

closed on either side. Fig. 12 shows the basic design. Soot filters

are regenerated periodically by burning-off the soot deposited.

The major problem in particulate filter regeneration is the

mismatch between the exhaust gas temperature and the required

regeneration temperature. Diesel exhaust is seldom hotter than


Fig. 9. Co-/counter-current reactor concept for methane steam reforming (top). Co-current reaction section profiles for (left) a catalytic wall reactor with equilibrium

controlled reforming reaction and right for a fixed-bed reactor with reasonable values for the reforming kinetics; both cases with equal specific heat transfer area.


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350 8C, in city stop-and-go traffic often below 200 8C.

Generally, diesel soot requires ignition temperatures above

600 8C in air. Therefore, several concepts have been proposed

for reducing the soot ignition temperature. If the exhaust

contains reasonable amounts of NO2, the ignition temperatureis reduced to about 350400 8C. This fact is exploited in the so-

called CRT (continuous regeneration trap) design, where the

engine is tuned for producing large amounts of NO, which is

oxidized to NO2 over a noble metal catalyst prior to or at the

soot filter. It has to be noted, however, that during CRT soot

combustion NO2 is only reduced to NO and a subsequent De-NOX stage is still required.


Fig. 10. Burner channel experiments. Top: sketch of the experimental set-up. Middle and bottom: photos of the glowing patterns at the conditions indicated.

Fig. 11. Designand simulation results of a co-/counter-current methane steam reformer forthe production of 5 m3 STP hydrogen/h. Left: flowconfiguration (top) and

simulated temperature and conversion profiles. Right: design sketch and flow structure of the assembly.


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Using catalytically impregnated soot filters for reducing the

ignition temperature is not very effective due to the poor contact

between the soot and the catalyst particles. More efficient is the

addition of a catalyst precursor to the diesel fuel which ensuresthat the catalyst is finely dispersed in the soot particles (PSA

system). This reduces the soot ignition temperature to about

450 8C but the resulting catalyst dust gradually blocks the filter

and requires its early replacement.

In any case, soot ignition requires increased fuel consump-

tion to produce a sufficiently hot exhaust gas for an extended

period of time. In addition, soot combustion may either die out

after ignition, leading to incomplete soot removal, or run away

into excessive combustion temperatures, which may destroy the

filter and occasionally burn down the whole car. These are

reasons why the car industry is still somewhat reluctant to

introduce soot filters as equipment standard.The problematic of filter regeneration shall be discussed

with a simplified model case. Fig. 13 shows simulation results

of the regeneration behavior of a uniformly loaded soot filter.

The soot combustion kinetics has been obtained with old

almost graphitized soot [21]. This is the reason why the ignition

temperature in the simulation is about 100200 K higher than in

reality. Soot removal is initiated through increasing the feed

temperature. At 700 8C feed temperature the soot combustion

takes place uniformly over the entire filter volume causing a

moderate temporary temperature rise towards its rear end.

Approximately 10 min are necessary for complete soot

removal. Increasing the feed temperature to 900 8C, the

ignition behavior changes substantially. Now a thermalcombustion front develops where the peak temperature

increases exponentially and most of the soot is burnt off

within 30 s. The filter peak temperature would locally exceed

1300 8C, which is near the melting temperature of cordierite.

The damage potential of runaway of gassolid combustion

reactions has been confirmed experimentally both in engine andcar tests. Typical damages result in destruction of soot filters

due to thermal stress (breakage) or even melting of the filter.

Fig. 13 (right) shows a comparable damage of a partially molten

diesel oxi-cat made of cordierite. It is normally placed in front

of the soot filter and creates the required ignition temperature

through combustion of fuel, injected in the exhaust pipe.

The reason for thermal runaway during soot filter

regeneration can be explained based on a simplified wave

model [22,23]. The velocity WT of a pure thermal wave in a

packed bed is described by the well-known relation:

WT

VZerGcPG

erGcPG 1 erScS ;

e being the void fraction of the bed, rGcPG and rScS the heat

capacities of gas and packing, respectively, and VZ the gas

interstitial velocity.

The propagation velocity of a combustion zone is given from

an integral mass balance assuming total combustion:

WR VZecO2qB

;

cO2 being the oxygen feed concentration and qB is the initial

soot loading of the packed-bed (in mol C/m3). At low oxygen

concentration or high soot loading the combustion zone will lagbehind the temperature wave. Hence, the heat of combustion

will be carried downstream, limiting the maximum temperature


Table 1

Types of exhaust pollutants (potential) solutions and required purification temperature range for different types of passenger car engines

Passenger car engines and exhaust temperatures Types of pollutants and (potential) solutions Required purification

temperatures (8C)

Conventional Otto engines: air/fuel

ratio l around 1 (3001000 8C)

Three types of pollutants (CO, HC, NOX)

l engine control and three-way catalyst 300700

Lean Otto engines: l) 1 at low power (2004508

C),l 1 at max. power (3001100 8C) Pollutants: NOX, CO, HCNOX storage catalyst 250450

Three-way catalyst 300700

NOX storage catalyst, sulphur regeneration 750

Diesel engines: l) 1 (150350 8C)(up to 700 8C at high load)

Main pollutants: soot, NOX, HC

Soot trap regeneration 450900

NOX storage catalyst 250450

NOX storage catalyst sulphur regeneration 750

Diesel oxidation catalyst 250700

Fig. 12. Ceramic soot filter monolith. Left: photo, right: sketch of the flow through the channel walls.


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of the packing. If, conversely, the oxygen concentration is high

or the soot loading is low, the combustion zone will rash in front

of the thermal wave. Again, the heat of combustion will be

distributed over a larger portion of the packed-bed. However, in

the limiting case of equal propagation velocities of combustion

and thermal front, the heat of reaction accumulates within the

front, leading to excessive temperatures.

The above considerations can be summarized in two simplerelations for the maximum temperature increase in the

combustion front:

DTF WR

WR WTqBDhR

rscsforWR >WT and

DTF WT

WT WRcO2DhR

erGcpGforWT >WR;

where DhR is the enthalpy of the combustion reaction. Fig. 14

shows the graph ofDTF for a given value ofWT. For WR = WT

the relation features a singularity, where the simplified model

predicts an infinite front temperature. Moderate front tempera-

tures are only possible for WR) WT, implying low soot load-ings and high oxygen concentration (the limit is the adiabatic

temperature rise with respect to the soot loading, DTSad qBDhR=rScS or for W

R $ 0, implying low oxygen con-centration and high soot loadings (with the limit of adiabatic

temperature rise with respect to the gas concentration

DTGad cO2DhR=erGcpG.Interestingly, the model implies that lower front tempera-

tures would occur for negative values of WR. Accordingly,

initiating soot regeneration at the rear end of the filter (back-

ignition) and propagation of the regeneration zone in counter

flow direction would result in an inherently save operation. This

conclusion will be taken up in Section 4.2.

If the necessity to add a total oxidation catalyst and a NOXconversion device to the exhaust train is taken into account, the

diesel exhaust purification is even more complicated and

challenging than the examples discussed before. Summarizing,

forthcoming engine exhaust purification regulations make it

more and more difficult to fulfill the legal requirements with

add-on solutions and elaborate engine control measures without

sacrificing engines performance and fuel economy.A new conceptual approach seems to be necessary where

exhaust purification should be decoupled from engine control.

Integrated reactor concepts, developed in chemical engineering

during the last decades, in combination with a targeted

development and improvement of appropriate catalyst systems,

could provide appropriate solutions. Such an approach will be

outlined in the next section.


Fig. 13. Simulation results for the regeneration of a uniformly loaded soot filter at two different feed temperatures, using soot combustion kinetics [21] of old

(graphitized) soot (left) and photo of a diesel oxidation catalyst, positioned in front of the soot filter and molten during the initiation of the filter regeneration.

Fig. 14. Combustion front temperature increase over reaction front velocity for

decoking of fixed beds or for soot filter regeneration.


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4. A new, integrated engine emission control concept

The requirements for an optimized engine emission control

concept which overcomes the present add-on solutions can be

specified as follows:

lower total energy consumption as presently, equal or reduced pressure loss of the exhaust train as

presently,

no additional fuels or additives for De-NOX or soot filterregeneration,

thermally safe soot filter regeneration concept, lower cost and space requirements than present add-on

solutions.

Following, it will be demonstrated that the above goals are

indeed attainable. The basic idea is to combine all emission

relevant components into one unit which may even contain the

muffler and devices for heating the passenger compartment.

The key element of the proposed solution is again an optimalenergy integration concept, comprising of an efficient counter-

current heat exchanger reactor. It will minimize the additional

heat consumption, if the engine exhaust is too cold and may

also be used to protect the exhaust catalysts from excessive

temperatures if the exhaust is too hot. The purification stages

requiring strongly differing temperature levels can be

incorporated at appropriate positions in the reactor.

4.1. An example for diesel exhaust purification

Fig. 15 shows the proposed concept for the case of diesel

exhaust purification. The engine exhaust gas enters the counter-current heat exchanger section axially, is led through the up-

flow channels to the top, turned around into the down-flow

channels and leaves the device laterally close to the inlet. Heat

can be added into the top chamber via an electrically ignited

catalytic burner, which is fed with the engine fuel and air (or, in

case of lean burn engines, with oxygen containing exhaust). It is

used for starting-up and maintaining the required temperature

level. The catalytic burner can be operated under fuel lean

conditions as a conventional catalytic burner, or under fuel-rich

conditions, if generation of reducing agents (CO, H2) is

required. The purification stages are integrated in the heat

exchanger channels at positions which correspond with their

optimal operation temperature window. A suitable set-up for

diesel exhaust purification consists of a NO oxidation catalyst

in the upstream channels with a soot filter at the top in order to

utilize the CRT effect. In the down-flow channels a NOXstorage

catalyst can follow, which may be regenerated either through

engine control or, preferably, through the CO/H2-mixture

created in the catalytic burner under partial oxidation

conditions. An oxidation catalyst with oxygen storage capacity

may follow to prevent CO-leakage.

A modification for lean-burn Otto-engines is quite straight-forward. Here the oxi-cat in the entrance channels may be

replaced by a three-way cat and the soot filter can be omitted. In

a further step, it would also be possible to integrate muffler

components into the unit and to use (part of) the hot exhaust at

the units top for heating of the passenger compartment, even

without engine running.

For the heat exchanger section the parallel metal plate or

folded sheet reactor concept discussed in Sections 1 and 2 can

be used. A recent modification allows for an axial inflow with

low pressure drop and prevents blocking through particulates

deposition which could be a problem in alternative designs

with flow deflection at the entrance. In addition, appropriatecatalysts can be placed at different positions in the inflow

and outflow channels if they are deposited on the spacers.


Fig. 15. Sketch of the proposed integrated emission control concept. Left: detail of the counter-current parallel plate design; middle: cross-sectional cut through the

up- and down-flow channels; right: catalyst placement in the down-flow channels.


13/15

The design is comparatively light weight because of the thin

metal walls and provides an excellent heat recovery. This

allows maintaining the optimal catalyst temperatures with a

minimum of additional fuel consumption in case the exhaust

is too cold. In case the exhaust is too hot, the catalysts are

prevented from overheating through cold air injection at the

top. Due to the optimal temperature control a considerably

reduced catalyst volume and an extended catalyst lifetime can

be expected (present exhaust catalysts are largely oversized

for normal operation conditions to provide sufficient capacity

for low temperature operation and catalyst deactivation).

Concerning space requirements, the exhaust catalyst needs

no longer be squeezed into the engine compartment, but can be

placed under the car floor since its temperature is controlled

independently from the engine exhaust.

4.2. Soot filtering options

The new design concept also opens new alternatives for soot

filtering and its regeneration. As explained in Fig. 15, the sootfilter will be positioned at the top, either before the exit of the

entrance channels or at the beginning of the exit channels. The

latter option has the advantage to also prevent possible soot

emissions from the catalytic burner. The filter temperature can

either be maintained continuously at a temperature sufficient

for a continuous combustion of the soot deposited (above

350 8C if the CRT-effect of NO2 oxidation is exploited). This

option has been examined in detail in [21]. The necessary fuel

input through the catalytic burner is small, if the heat exchanger

is as effective as the designs discussed in Sections 1 and 2.

Then, under steady state conditions, an adiabatic temperature

rise of 50 8C at the top of the unit increases the filter

temperature by about 300 8C.

The second option is the conventional periodic regeneration

of the filter once the soot loading increased the filter pressure

drop beyond an acceptable threshold. The filter is attached to

the exit channels at the top as shown in Fig. 16 (left). This

enforces a counter-current flow at the feed and the permeate

side of the filter. The internal feedback of counter-current heat

exchange leads to a substantially different regeneration

characteristic as compared with the behavior described in

Figs. 12 and 13. Fig. 16 (right) shows the simulation of the

regeneration process. If initiating regeneration through

increasing filter feed temperature to 900 8C a combustion

front is rapidly carried into the filter, but the maximum

temperature stays at a moderate level except for the final stage(t> 120 s), when it rises to about 1500 8C (Fig. 16, top right).This is obviously a much more favorable behavior than

observed in Fig. 13.

In addition, the new design provides the option for initiating

a safer regeneration procedure by raising the rear end

temperature with the catalytic burner. Fig. 16(c) shows the


Fig. 16. Integratedsoot filter: (a) flowstructure and (b) regeneration profiles for ignitionvia exhaust gas temperature increase and (c) regeneration profiles for ignition

via burner gas temperature increase. In case (c) the shaded soot loading is not burnt off.


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simulated temperature and soot loading profiles for rear end

ignition. The temperature overshoot completely disappears.

However, soot combustion dies out, being quenched by the cold

feed. A steady state is reached where approximately 60% of the

filter area is free of soot. However, this mode is susceptible to

blocking the entrance cross section. A reasonable alternative

would therefore consist in combining the advantages of both

regeneration strategies shown in Fig. 16. This can be achieved, if

the catalytic burner during soot filter regeneration is operated in

the partial oxidation mode, producing a CO- and H2-containing

gas, which is oxidized with the lean engine exhaust at a catalyst

positioned somewhere in the middle of the downstream filter

channels. The heat of combustion will then be released at an

optimal location in the filter itself, causing a milder soot

combustion without substantial temperature overshoots.

These examples indicate, that the integrated exhaust

purification concept offers a multitude of options for optimizing

automotive exhaust purification. Besides improved controll-

ability of the process it provides favorable, well-defined

conditions to different catalytic stages. This should enablecatalyst developers to optimize catalysts with respect to their

specific purpose, free of secondary constraints imposed by

inappropriate operating conditions.

Presently, a prototype of the heat-integrated after-treatment

unit is under test (Fig. 17). In this prototype, the NOX storage

catalyst and the catalytic burner are not yet included. The unit is

suited for the exhaust treatment of a 2.2 l diesel engine.

5. Summary and conclusions

In the first part of this contribution several new concepts for

the efficient coupling of endothermic and exothermic reactions

have been discussed both through simulations and experimental

results. The examples included methanol steam reforming in a

counter-current flow reactor with distributed side feed for the

combustion reaction and methane steam reforming in a co-

current reaction section with counter-current heat exchangers

attached. Both concepts require an excellent heat transfer

between the reforming and combustion channels, leading to a

small channel width but sufficient channel length for a good

heat recovery. The so-called folded sheet reactor concept

proved to be both sufficiently simple and efficient for this

purpose. It is well suited for compact, small-scale reformers for

decentralized hydrogen production for fuel cells, since it shows

a rapid start-up and load change behavior.

The second part has been devoted to automotive exhaust

purification. In automotive exhaust purification three-way

catalysts for l-controlled engines are a well-established

standard. Lean-burn Otto as well as diesel engines, however,

require new concepts, in which different purification steps have

to be combined. They include total oxidation of combustibles,

NOX reduction and soot filtering with subsequent sootcombustion. For NOX reduction the presently favored option

for passenger cars is the NOX-storage catalyst which requires a

frequent regeneration under fuel rich (reducing) conditions and

an occasional high temperature desulphurization. Since the

above steps have their favorable operation windows at strongly

different temperatures, it becomes more and more difficult to

operate the purification system through engine control.

Therefore, a new approach to automotive exhaust purifica-

tion has been proposed, where the exhaust purification unit can

be operated independently from engine control. Its main

ingredient is an efficient heat exchange section for which the

design concepts from the first part of the paper can be used.


Fig. 17. Designsketch andphoto of a soot filter/heat exchanger. The sketchis a cutthroughthe up-goingchannels (left side) andthe down-goingchannels (right side).


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The unit contains a counter-current heat exchanger, where the

different components are situated at positions at which the

necessary operation temperatures can be provided. A small

catalytic fuel burner at the hot end of the heat exchanger can be

controlled independently to provide the required temperature

level and (reducing or oxidizing) gas composition. An

integrated new soot filter design allows for a safe regeneration

without excessive temperatures. This design will enable to

operate the different exhaust purification catalysts under well

defined, optimal temperature conditions and will, therefore,

facilitate exhaust catalyst development.

Acknowledgments

Support of the first parts of this work through the Deutsche

Forschungsgemeinschaft and through Adam Opel AG is

gratefully acknowledged.

References

[1] G. Kolios, J. Frauhammer, G. Eigenberger, Chem. Eng. Sci. 55 (2000)

59455967.

[2] Yu.Sh. Matros, G.A. Bunimovich, Cat. Rev.-Sci. Eng. 38 (1996) 168.

[3] B. Van de Beld, K.R. Westerterp, Chem. Eng. Technol. 17 (1994) 217

226.

[4] U. Nieken, G. Kolios, G. Eigenberger, AIChE J. 41 (1995) 19151925.

[5] G. Kolios, G. Eigenberger, Chem. Eng. Sci. 54 (1999) 26372646.

[6] B. Glockler,G. Kolios, G. Eigenberger,Chem.Eng.Sci. 58 (2003)593601.

[7] M. van Sint Annaland, PhD Thesis, Twente University, Enschede, The

Netherlands, 2000.

[8] G. Friedrich, G. Gaiser, G. Eigenberger, F. Opferkuch, G. Kolios, Eur-

opean Patent EP 0885653B1 (2003).

[9] P. Haussinger, Hydrogen, vol. A13, VCH Verlagsgesellschaft, Weinheim,

1989, 311 ff.

[10] J. Frauhammer, G. Eigenberger, L.V. Hippel, D. Arntz, Chem.Eng. Sci. 54

(1999) 36613670.

[11] G. Kolios, J. Frauhammer, G. Eigenberger, Chem. Eng. Sci. 57 (2002)15051510.

[12] J. Frauhammer, PhD Thesis, University of Stuttgart, Germany, VDI

Verlag, Dusseldorf, 2003.

[13] B. Glockler, A. Gritsch, A. Morillo, G. Kolios, G. Eigenberger, Chem.

Eng. Res. Des. 82 (A2) (2004) 148159.

[14] A. Morillo, A. Freund, C. Merten, Ind. Eng. Chem. Res. 43 (2004) 4624

4634.

[15] L. Ma, D.L. Trimm, C. Jiang, Appl. Catal. A: Gen. 138 (1996) 275283.

[16] C.K. Westbrook, F.L. Dryer, Combust. Sci. Technol. 27 (1981) 31.

[17] G. Veser, Habilitation Thesis, University of Stuttgart, Germany, 2001.

[18] D.G. Norton, E.D. Wetzel, D.G. Vlachos, Ind. Eng. Chem. Res. 43 (2004)

48334840.

[19] A. Gritsch, G. Kolios, G. Eigenberger, Chem. Ing. Technol. 76 (2004)

722725.

[20] U. Tuttlies, V. Schmeier, G. Eigenberger, Chem. Eng. Sci. 59 (2004)47314738.

[21] F. Opferkuch, PhD Thesis, University of Stuttgart, Logos Verlag, Berlin,

2004.

[22] A. Salden, PhD Thesis, University of Stuttgart, Germany, Logos Verlag,

Berlin, 2002.

[23] K.R. Westerterp, H.J. Fontein, F.P.H. van Beckum, Chem. Eng. Technol.

11 (1988) 367375.

[24] A. Gritsch, PhD Thesis, University of Stuttgart, 2006, in preparation.


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