Application of wavefront analysis for kinetic investigations of watergas shif reaction

Chemrcal Engrneermg Scrence Vol 35 pp 1021 1030 Prmted m Great Br~tam Q Pergamon Press Ltd 1980

APPLICATION OF WAVEFRONT ANALYSIS INVESTIGATIONS OF WATERGAS SHIFT E Instltut

FIOLITAKIS,

fur Techmsche

Chemle

U

HOFFMAN

and H

I, Umversltat Erlangen-Nurnberg, West Germany (Recemd

17 May

FOR KINETIC REACTION

HOFMANN D-8520

Erlangen,

Egerlandstr

3,

1979)

Abstract-It

has been demonstrated that durmg dynamic operation of the low temperature watergas shift reactlon on the wavefront the CO,-production IS faster than the H,-productlon For a catalyst pretreated for several hours with a mixture of CO/N, its re-oxldatlon by a mixture of H,O/CO/N, IS a slow process, whereas on the wavefront the H,-concentration IS twofold the CO,-concentration The quantitative analysis of CO,-wavefront data for a catalyst pretreated for several hours with a H,O/N,-mixture has given It is shown a reactron order nc, = 0 625 and an activation energy E = 46 27 kJ/mole for the CO oxldatlon For a catalyst pretreated for a long time period with a H,O/N-mixture that H,O mhlblts the CO oxldatlon and then reduced m a short tnne period by a H,O/CO/N-mixture the reoxldatlon by a I-%,0/N,-mixture 1s a slow process The wavefront analysis has also been applied to clear-up the H,O-sorption on the low temperature catalyst, suggestmg that a physisorptlon ofLangmulr type 1s combined with a second mechamsm saturatmg the H,O-capacitance for low values of PHz,

INTRODUCTION

In two previous papers [l, 21, the wavefront analysis has been described for kmetlc mvestigatlons of homogeneous reactions 111packed beds and for the study of heterogeneous reactions with several elementary steps This paper deals with the application of this technique for kmetlc mvestlgatlons of the watergas shift reactlon on a low temperature shift catalyst CO + H,O

+ CO,

+ H, (-AH,

= 42 7 kJ/mole) (1)

For an oxide-type catalyst, such as used m this mvestlgatlon, the composltlon m the bulk of catalyst, 1e the thermodynamic activity of oxygen atoms, will be changed slowly after a change in the ambient gas composltlon until a thermodynamic equlllbrmm between atoms m the surface and atoms m the bulk has been established [lo], 1e until the thermodynamic activity m the bulk becomes equal to the activity m the adsorption layer on the catalyst surface That means for a flow reactor m the steady state, an actlvlty profile m the flow direction can be established which nnphes that kinetic data obtamed from steady state measurements correspond to any average catalyst actlvlty and cannot be attributed to a unique activity Furthermore, one has to expect that m general the steady state kmetlc data cannot sufficiently describe the reactor dynamics (see e g [ 111) Moreover, it 1s necessary that the kinetic mvestlgatlons for weIl-defined catalyst activities can be performed in order to obtain the functional relationship between kmetlc parameters and catalyst actlvtty EXPERIMENTAL

Theoretical conslderatlons on wavefront analysis [ 1, 21 have shown the convenience of a step disturbance m

one of the inlet concentrations m order to get a clearly defined wavefront Therefore, the signal generator consists of a unit which enables the sudden exchange of two inlet gas streams, having &fferent composltlon but equal flow rates Figure 1 shows the scheme of the whole equipment used for the investigation It consists of a dosing section, the reactor itself, and the analysis section The dosing sectlon consists of two identical feedhnes m order to avoid pressure pulses m the evaporator during swltchmg The reactor operates nearly adiabatically, the heat capacrty of the reactor wall can be neglected If only the wavefront of a concentration disturbance 1s analysed The axial temperature dlstrlbutlon could be measured by a movable thermocouple m the axis of the catalyst bed The concentration transient at the entrance and the outlet of the reactor was followed by a quadrupol mass spectrometer (Balzers QMG 101) connected with the reactor by a capillary inlet system For further details see [3-53 In order to measure snnultaneously the partial pressures of the four reactants, the mass spectrometer has been coupled with an analog computer EAI TR48, the mass spectrometer was operated m repeat scan mode with maxnnal scan speed (1 ms/amu) The concentration transients have been regstered on recorders, the C02and Hz-signals have been addltlonally connceted with an XY EAI Vanplotter 1110 in order to obtain the reaction trajectories m a concentration phase plane during the transient operation of the reactor Figure 2(a) shows the outlet response repeated for a step inlet disturbance of an Inert component (He), mdrcatmg that dispersion Figure 2(b) effect- m the catalyst bed are neghgble indicates the traJectorles m the phase plane for inlet and outlet responses after a step disturbance of H, and CO, m N, measured at about 150°C The relatively

1021

1022

E FIOLITAKIS, U HOFFMANN

and H HOFMANN

small deviation of the traJectory from the ideal diagonal can be attributed to a non-linear transient behavior of the used turbo-molecular pumpmg umt and of the signal generator, It brmgs a systematic error m the XY-recordings which has to be taken mto account m the analysrs of the expernnental results The catalyst used was the Glrdler CuO/ZnO-low temperature shift catalyst (with Al,O, and Cr,O,) type G-66 B and G-66 E urlth a particle diameter of 0 25-O 40 and 0 40-O 50 mm The catalyst bed height was 0 045 to 0 062 m, the reactor diameter was 0 026 m The shift reaction had been studied m the of 160-26O”C, the sorption temperature range experiments were carried out also at lower temperatures, e g at 80-150°C [4] The total gas flow rates used, 005 to 0 3Nm3/h, correspondmg to mean residence times of 0 l&O 2 set (hydrodynamzc time constant), cause a very small pressure drop along the bed (some cm H,O), therefore the pressure m the reactor had been regarded as constant and equal to the atmosphenc pressure With the catalyst dnnenslons and flow rates applied, any influence of film dlffuslon and pore dlffuslon could be excluded [3,93 In this paper only the forward reaction of eqn (1) has been studied The step height of the H,O-concentration step varied between 0 and 50 vol %, the step height of the CO-concentration step between 0 and 40~01 %

RESULTS Fig 1 Experlmental equipment, WPl, WP2, water dosmg pump, VDl, VD2, evaporator, MXl, MX2, dry gas mixer, VHl, VH2, preheater, SG, slgnal generator, R, reactor, K,, K,, condenser, KAP, gas inlet capllary, GEV, gas Inlet valve, QMS, quadrupole mass spectrometer, MS, mass spectrometer contro1 umt, HVP, high vaccum pumpmg umt, VPl, VP2, vacuum pump, VCM, vacuum measurmg device, AR, Analog computer, REC, recorder

TIME

Fig 2(a)

#

(1) Preltmmary sorptzon experiments wathout chemzcal reactzon In order to clear-up whether the reactants are measurably adsorbed on the catalyst surface, dynamic sorption expernnents had been performed for each reactant separately in an inert gas stream ofnitrogen at

P

Step Inlet disturbance and outlet response for Hehum

A&cation

OIBAR

Fig 2(b) TraJeCtOrleS m the phase plane for inlet (left) and outlet (nght) responses after a step lApH,I = 925 bar, IApco,l = 0,25 bar, IApN,I = 0,5Obar

decreases considerably, e g the sorption capacity for CO, at temperatures higher than 100°C IS practically zero For posltlve H,O-mlet concentration steps as well as CO,-mlet concentration steps a shock wave results, whereas for negative steps a snnple wave appears (see Figs 3 and 4), that means the sorption isotherm of H,O as well as CO, on this catalyst must be convex According to the theory, one should expect that the non-ideal mlet signal (curve 1 of Figs 3 or 4) becoties steeper by the shock In contrast to that, Figs 3 and 4 show that all outlet signals are nearly parallel to the inlet signal, an mdlcatlon that the mlet and outlet slgnes are Ideal, but the devlatlon from ldeahty results from the measurmg device, as suggested also from Fig 2 (2) Experiments

at 150°C

The

time

different temperatures For higher temperatures, e g over lOO”C, H, and CO remove oxygen from the catalyst m the form of H,O and CO,, for lower temperatures, e g 25”C, no measurable effect has been observed with the used flow rates On the other hand, Figs 3 and 4 show that CO, and H,O are measurably, reversibly adsorbed With mcreasmg temperature the sorption capacity of the catalyst for H,O and CO,

4

ko,

Adsorptaon -~

BAR

1

input Slgnol

2

” placed

” Dlsby r,

3

output

SIgnal

with chemud

constant

determined

independently

Input

pH20

had been

experiments

10

25-C.

0092

20

30

Nm3/h

40

50

IsJ

m N, at 25”C, catalyst bed

- SIgnal

Output-Sagnol

ltlO°C 15ooc 1lOOC

3 I

’

kinetics

by stationary

Desorptron

I

Posltlve and negative step Inlet disturbance and outlet response for CO, length 0 045 m, catalyst G-66B

1 2

reactron

of the global

to be m the order of magnitude of 1 set [3], therefore by wavefront analysis with a hydrodynamic tnne constant of 0 1-O 2 set at least the rate-determmmg step of the reaction should be detectable According to the theory of wavefront analysis [2], the state wlthout chemical reaction had been chosen as mltlal stationary state of the catalyst bed, as this state IS well defined, provided the pretreatment of the

0

Fig 3

1023

of wavefront analysis for kinetic mvestigations of watergas shift reactlon

[ bar1 Adsorption

0 20

-

0

-

10

0

/,< 0

Fig 4

10

‘, 20

,

, 30

,

Desorptlon

, LO

,

I

1 50

0

10

8-T~. 20

I 30

Positive and negative step mlet disturbance and outlet responses for H,O same comments as m Fig 3

1

I 10

m N,

Is1 0,184Nm3/h,

else

E FIOLITAKIS. U HOFFMANN and H HOFMANN

1024

catalyst was the same This way one can guarantee that (a) the catalyst bed and the flowing gas phase have an ldentlcal temperature, (b) defined mltlal states, that means defined acttvltles, of the catalyst surface can be reached e g by pretreatmg of the catalyst with a mutture of H,, H,O or CO and N, until constancy has been reached [ 3 1, and (c) the axial temperature profile 1sconstant and nearly isotherm (vanatlons are smaller than 1 to 2°C) Points (a) and (b) are essential for this type of wavefront analysis, point (c) is more for convenience Qualztatwe analyszs If one 1s interested m the oxldatlon of CO to CO, as one of the elementary steps, one stimulates this step by a CO-Inlet disturbance, keeping constant the H,O-concentration m an N,-carrier stream Typical responses for this dlsturbance are shown m Fig 5 The CO,-response IS spontaneous and some overshoot on the wavefront can be observed Whereas the wavefront corresponds to the isothermal and welldefined n-utlal state of the catalyst bed, the further transient zs also dependent on secondary effects The exothermlty of the reactlon IS mamly to be seen m case of higher mltlal temperatures and for higher concentration steps The overshooting reflects a change of the mltlal state of the catalyst surface, probably consumption of oxygen from the surface The absence of a dead time or a delay indicates that under these reactlon condltlons the catalyst has no measurable sorption capacity for CO, To distinguish whether a chemlsorptlon mechanism and/or a redox mechanism partlclpates m the COoxldatlon m further experiments, the catalyst had been

pretreated for several hours (e g 12 hr) with a H,O/N,-mixture in order to obtain a high oxldatlon level of the catalyst, followed by periodic expenments which have been performed as follows 1 mm reduction phase with a CO/N,-mucture, 60 minutes reoxldatlon phase with a H,O/N,-mixture, 15 mm stripping phase with purified and dried nitrogen In the reduction phase CO, but no H, 1s produced and the CO,- and CO-responses are reproducible, even after several cycles (see Fig 6a) After several cycles these expenments are continued with a CO/H,O/N,mixture with a one minute reaction phase (see Fig 6a) and without any stripping phase In Fig 6(b) one sees

T&l gas flow mte 0137 Nn3/h Total gas flow mte 0183 t&d/h

T,=L75K T,=L95 K

I’

t

/’

008

,0’

*), 0 2L9 bar

,’ /’

0

60

//’

i

123

180 -

tome Is1

Fig 5 CO,-responses in a H,O/N,-mixture (0 40 bar H,O) after pretreatment of the catalyst with a NJH,O-mixture (0,40 bar H,O), catalyst bed length 0 045 m, catalyst G-66B

Ill

pen lb& 004

003 002 001 0 I Fig 6(a) CO- and CO,-responses after pretreatment of the catalyst with a H,O/‘N,-mixture (0 30 bar HZO, T = 16O”C, total gas flow rate 0 092Nm’/h, here and in all followmg figures catalyst G-66E, bed length 0062m)

~

responses for a posltlve CO-step (0,3 bar CO) m N *, ---step (0,3 bar CO) m a N,/H,O-kmlxture (0,30bar

responses for a positive COHzO)

Application

001

000

002

003

mg temperature response was measured m the middle of the catalyst bed The H,O-response slgnal shows the damped shock wave, delayed m comparison to a shock wave without chemical reaction (see Fig 7(c)), mdlcatmg a sorption capacity of the catalyst for water, even at 2OO”C, as well as a consumption of H,O by the reactlon The wavefront velocity of this shock wave IS smaller than the hydrodynamic wave velocity (78 set delay time m Fig 7(c) mstead of 0 14 2 set hydrodynamic delay time) but faster than the front of the temperature wave (e g measured at the mflectlon point in Fig 7(a)) which guarantees the lsothermlty of the H,O-shock front Obviously, the H,O-shock wave, which had mltlated the reactlon, has been passed by the CO,- and Hz-waves, showmg typlcal overshootmg effects m the CO, CO, and H, response (Fig 7(a)) until the H,Owave arrives at the exit (see also Fig 6 m [2]) Analysmg the phase plane Fig 7(b) additIona mterestmg facts can be stated (a) At the wavefront (neglecting the overshootmg) the H,-concentration IS twofold the CO,-concentratlon (see also Fig 7(a)) mdlcatmg that a part of the H,O must have been consumed for changing the oxldatlon level of the catalyst (b) The overshootmg of the CO, at the wavefront (Fig 7(a)) IS higher than the overshootmg of H, indicating a higher production rate of CO, either by the same reasons as m Fig 6(b) or by an addltlonal strlppmg effect of H,O on adsorbed CO,

Pco2 BAR

Fig 6(b) Reactlon tryectory m the CO,/H,-phase plan correspondmg to the second penod of Fig 6(a) for a CO-step m a N,/H,O-mtxture

the

of CO, and H, transients, to the second period of Fig 6(a) for a CO-step m a N,/‘H,O-mixture Figure 6(b) shows that both CO, and H, are produced, but the CO, producton rate at the wavefront IS larger than the H,-production rate demonstrated by devlatlon of the traJectory from the stolchlometrlc straight line Comparmg the responses m Fig 6(a) It IS to be seen that the CO,-wavefront concentration for a CO-step m N,/H,O IS lower by a factor of about 2 than the correspondmg concentratlon for a CO-step m N,, mdlcatmg that H,O mhlblts the CO-oxldatlon (see also [6]) Figure 7(a)-(c) show responses for a posltlve H,Oinput step m a CO/H,O/N,-mixture after pretreatment with CO/purified and dried N, The correspondphase

traJectory

corresponding

b

1025

of wavefront analysis for kmetlc mvestlgatlons of watergas shift reactlon

10

15

20

25

30

35

40

45

50 irnermlnl

Fig 7(a) H,, H,O, CO, CO, and temperature responses for a posltlve H,O mlet step (0,3 bar H,O) m a CO/N,-mixture after pretreatment of the catalyst wth a CO/N,-mixture (0,3 bar CO) Total gas flow rate 0 092 Nm3/h, lmtlal temperature 200°C The temperature response ISmeasured at x = 0 5, other responses at x=10 CES 3515-B

1026

E

FIOLITAKIS, U HOFFMANN

and H HOFMANN

well m agreement with recently published results [12-l The fractional reaction order contradicts the assumption of a pure redox mechanism for the shift reaction, as for a redox mechanism an order of n = 1 IS to be expected [2] This mdlcates that the oxygen consumption from the catalyst observed m Fig 6(a) under condltlons without H,O IS either not relevant m the shift reaction mechanism in the presence of H,O or must be combined with a second mechanism under shift condltlons Furthermore, this second mechanism cannot be of the Eley-Rldeal-type for CO, as also m this case one should expect n = 1 [2] Another aspect of the oxidation reaction 1sshown m

Fig 7(b)

Reactlon trajectory correspondmg

m the CO,/H, to Fig 7(a)

phase

plane

(c) The CO,/H,-traJectory m the phase plane (after passing the wavefront) approaches only assymptotltally the stolchtometrlc line (1 1 ), mdlcatmg that, the reoxldatlon process of the catalyst must be slow (see also [71)

Quantrtatwe analysts The quantitative analysis by the help of the Marquardt routme [8] of 94C0, responses on a CO-inlet step of the type of Fig 5 at a constant H,O-level (440 bar H,O), I e under shift conditions on the catalyst G-66B, according the rate equation r = Aexp

[-E/RT

]c&,

(2)

resulted m the followmg effective kmetlc parameters (on the 95% srgmficance level) for the shift reaction n = 0625 f 0007

E = 46 270 + 0 147 kJ/mole

Fig Wa) Here the CO,, Hz-responses are depicted on CO inlet steps for a perlodlc operation of the reactor at a constant H,O-level (0 3 bar H,O, T = 160°C) The CO-exposure time was about 6Osec, followed each tnne by a 12 mm recuperation tnne during which only H,O/N, had been passed over the catalyst bed The decreasing CO,-concentrations at the wavefront (from curve 1 to curve 5) indicate that the recuperation time was msufficlent for regaining the oxidation capacity of the catalyst obtainable by a recuperation time of one hour, therefore, the oxldatlon of the catalyst with H,O up to the state given by curve 1 1sa slow step also m the absence of CO, snnllar to the statement given above for the shift reaction mixture, with a time constant ofmore than 12 mm, compared to a tune constant of about 1 set, calculated by wavefront analysis for the stationary operation under shift condltlons (curve 4 and 5) Figure S(b) shows clearly, that the reaction trajectory approaches the stolchlometrlc line If the catalyst reaches Its stationary oxldatlon level for the shift condltlons, I e the deviations have Its ongm m the change of the oxldatlon level of the catalyst Furthermore the intermediate stationary end point depends on the mltlal oxldatlon level of the catalyst As the actlvlty of the catalyst for CO-oxldatlon IS obwously influenced by the oxldatlon level of the catalyst, a detailed study of the oxidation of the catalyst by H,O-vapor seemed to be interesting

without dlemlaI1 reoctnn ______________________--___-_______________ with chem~nl reactmn

0

25

,,,,

50

75

100

125

)

time Cs 1

Fig 7(c) H,O-response with and wlthout reactlon (same condltlons as m Fig 7(a) using enlarged time scale Switch time at the slgnal generator (zero pomt of the time axis) is about 2 set earher than appearance of the Inlet signal (dead time of measuring device and entrance Ime)

Apphcatlon

of wavefront analysis for kinetic mvestlgatlons of watergas shift reaction

LO

0

Fig S(a)

H,- and CO,-responses

Fig S(b) Reaction traJectorres m the CO,/H,-phase plane for penodlc operation correspondmg to Fig 8(a) Sorption experiments wrth H,O As mentioned above, for H,O-mlet concentration steps shock- and sunple waves result, suggestmg that the kinetic time constants of stunulated H,O-sorption

Fig 9

60

70

tlmelsl

for a perlodlc operation of the reactor (operatmg condltlons as III Fig 6)

-001--) BAR

(3)

50

1027

steps are smaller than the hydrodynamic tune constants For the detailed study of the H,O-sorption by wavefront analysis posltrve and negative mlet steps of different step height have been applied at two temperature levels (85 and 150°C) [4] For all experiments isothermal mltlal condltlons at a maximum oxldatlon level of the catalyst (reached by long range pretreatment with H,O/N,) had been established and the step height was chosen so that the wavefront lines are located in the subregion S, (see [Z]) m order to guarantee the lsothermlty of the wavefront Figure 9 shows the transients of the outlet concentration of H,O for posltlve and negatwe entrance steps of H,O Different mlgratlon tunes for a posltlve and a negative disturbance (9 0 seconds agamst 19 6sec) are clearly to be seen Here the wavefront 1s defined as the mtersectron of the tangent at the Inflection pomt of the response with the mltlal stationary concentration hne This way, the total error for the wavefront mlgratlon tune, mcludmg all other errors of the measuring device, has been estimated to be less than 0 5 set Figure 10 shows the correspondmg temperature transients, taken at the center of the packing at about x = 0 5 It can be seen that considerable heat effects are connected with the sorption of H,O and that-except for the wavefront-the transient corresponds to nonisothermal condltlons

Transients of the outlet H,O-concentration for a positive and negatrve H,O pretreated with H,O/N,

mlet step, catalyst

E

1028

FIOLITAKIS,

U

HOFFMANN

and H

HOFMANN

and m case of an “up step” from zero concentration level for the shockwave

155-

r‘:::6oo .

4

P c

150-

Fig 10

Time

Temperature

sac

transients correspondmg x =0,5

Assuming an eqmhbrmm Langmulr isotherm

n

4

Wsh =

KI H20 =

PHzO

sorption, governed

11 at

by the

[ kmole/m3 catalyst bed],

K,pHzo

1 +

to Fig

4 t1 - PH,0/p)2

w=

i,

+p

l--E &

R+l

0

Fig

11

U KI

+ K2P"*0)2

010

020

(4)

U

KI

+y R'Ttl

@a)

+K,P,~,)

For given values of pHzO (mlet step height) and T the Langmmr parameters K, and K, can be determined according to eqns (4) and (5) from the measured wavefront velocities w Mean values and mdlvldual confidence hmlts for the two parameters from repeated experunents at varymg p,lo-level are depicted m Fig 11 The same procedure has been applied to fit the above data to the BET, Freundhch and Temkm Isotherms (see [4]), but It was unpossible to obtain a fitting like Fig 11 As the parameters depend not only on the sorption temperature, but also on the p,lo-level, there exists a lack of fit for the Langmmr isotherm As shown m [2], the eqn (5a) for the shockfront velocity IS nothing else but a matenal balance at the wavefront, I e wS,,must be independent of the sorption mechanism,

(3) one obtains for the wavefront velocity (see [2] m case of a downstep to zero concentration level for the simple wave

l--E

wsh

‘1

=

PHIO

l--E

U

WI

'1 PHzO + --RgT’&o &

Therefore, by mean of eqn (5b) nHZO(the total amount of H,O adsorbed at the catalyst) can be determined from the measured values of wThalone The crosses m Fig 12 show n,,o-values as function of pHIOO, agam It was unposslble to fit this data, obtained from the shockwave-velocity slgmficantly with the Langmmr

030

-

0.40

050 PHpaT1

LangmuIr-sorption-Isotherm parameters K,, K, obtained at different pH2,-levelsby combmmg velocltles of the simple wavefront and the shock wavefront

the

Application of wavefront analysis for kmetrc mvestlgatlons of watergas shift reactlon

l54o-=_ “P

@s

“np

--I

Lmde 3 10

1029

1 = 85OC- @

IX B

-

8

so

- 5 10-3

t

T=lSO%-•

I I 010

0

I

I

I

I I Ox)

I,

1

I1 030

I

I

I

-P,

I I OLO

I

I

-

0

I

Q50 2

O[bar3

Fig 12 Adsorbed amount of H,O determined by the velocity of the shock wavefront (crosses) and Langmulr sorption Isotherm obtalned from the analysis of the velocity of the simple wavefront (solld lines) model eqn (3) On the other hand, the wavefront velocttles of the simple waves which arlse for a negattve step disturbance (desorptlon) fit slgmficantly at the 95% level the eqn (4) m which the Langmuir model was supposed The calculated parameters are at 85°C

K,

= 1731 +0116-

kmole m3, bar K,

at

150°C K,

= 1934 &- 0 350bar-’

= 0 418 + 0051 g K,

1

=0342+0326bar-’

The above-given values for K, and K, result in sorption isotherms as depicted as heavy lines m Fig 12 It 1s obvious that these curves are located below the nHZo-values from the shockfront analysis The differences between these curves and the shockfront points are statlstlcally distributed around a constant distance to the Langmulr lsotherme as Indicated by the dashed lines, which 1s only dependent on the temperature according to n H20 = k,,exp

CAHJR,Tl

(6)

with k,, = 2 7 10e3 [mole/m3 catalyst bed]

and AH,, = 36 [kJ/mole] From the K,-values at 85°C and 150°C an adsorptlon enthalpy of the Langmulr type sorption of H,O on the catalyst be calculated as can - AH,,, = 33,54 kJ/mole, which 1s close to the evaporation enthalpy of water (-AH,‘:;) = 39,65 kJ/mole, suggestmg that this part of Hz0 might be physically adsorbed

The contradlctlon that the shockwave velocltles do not fit the Langmulr sorption isotherm though the simple wave velocltles do, can be understood If one considers that the second sorption mecharusm saturates already the H,O-capacitance of the catalyst used Alternatwely, for low PH20r I e for all H,O-levels It 1s possible that m desorptlon experunents at the wavefront only a Langmulr type eqmhbrmm sorption mechanism 1s stnnulated whose time constants are smaller than the hydrodynamic time constant, whereas m adsorption expenments a second non-eqmhbrmm sorption mechanism 1s also stimulated It 1s an open question whether this second mechamsm IS a capillary condensation m micropores 10 A or a chemlsorptlon The catalyst used m these sorption mvestlgatlons (G 66-E) has a BET surface of 35 m’/g and a pore size distribution with the greater part m the micropore region (d < lOA) Together with the amount of H,O adsorbed It can be calculated that a 40% surface coverage was reached at the maximum m the sorption experiments CONCLUSIONS

It has been demonstrated that the wavefront analysis is a powerful tool for reactlon analysis In case of the low temperature watergas shift reaction at least two different sorption mechanisms for H,O on the catalyst have been detected Furthermore the dependence of the oxldatlon level of the Cu-catalyst from the oxldatlon potential of the reactlon mixture had been shown The catalyst reoxldatlon 1s a slow step compared to other steps of the shift reaction H,O mhlblts the CO oxldatlon and probably cleans the catalyst surface free of CO, Further results on the kinetic mvestlgatlons of the low temperature watergas shift reaction will be published m future NOTATION A ‘CO?

‘Hz0

preexponential factor concentrations of CO and H,O m the gas phase equal to P,,/R,T, P,&R,T kmol/m’/gas

E FIOLITAKIS,~

1030

HOFFMANN

actlvatlon energy, kJ/mole rate constant defined as A exp (- E/R,T) eqmhbrmm constants of Langmulr sorK,,K, ption Isotherms according eqn (3) n reactlon order 0, nHzO concentration of H,O in the solid phase, kmol/m3 solid specfic maxnnum capacity of the sohd N, phase, kmol I/m3 solid partial pressure of the component Z, bar step height m feed concentration, bar total pressure, bar reaction rate kmol/m3 s gas constant, kJ/mol K tnne, s absolute temperature, K fluid velocity, m/s volume of the catalyst bed used, m3 dnnenslonless space coordmate defined as reactor space coordinate [m ]/reactor length [m] propagation velocltles of a sunple and a shock wave, m/s

and H HOFMANN

a.1 2

E k

Greek

symbols E porosity

e H2,,

of the packmg, about 0 5 coverage of the solid phase by physlsorbed H,O

f3

coefficients

defined by eqns (3 and 4) m

Cl1

concentration

m the stationary

phase

Induces CS

s

chemlsorptlon stationary REFERENCES

rll Flohtalcis E. Hoffman U Engng Scr 1979 34 677 [2] Flohtakis E, Hoffman U

Chem

and Hofmann

H,

and Hofmann

H , Chem

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