Experimental investigation of temperature profiles, parametric sensitivity and multiple steady states in deactivated fixed bed reactors

C+emrcal Engtneenng Scaence Vol 35, pp 258-263 Pergamon Press Ltd , 1980, Pnnted m Great Bntam

33

EXPE-AL INVESTIGATION OF TEMPERATURE PAR&METRIC SENSITIVITY AND MULTIPLE STEADY l3ED REACTORS DEACTIVATED FIXED

V.,HlavCIEek+,

0.

MikuY+,

E.

J&a+

and

of Chemioal Engineering, Inst. +Dept. Technology, Pra&ue, Czechoslovakia

PROFILES, STATES

V.

Pour++

of

Chemical

++Inst. of Inorganic Chemistry, Czechoslovak Pragzle, CzeohoslovaEda of Soielloes,

IN

Academy

ABSTMCT

An experimental study of oxidation of oarbon monoxide in a tubular Pt/Al 0 reactor paoked with commercial oatalyut was performed. The effect of the poisoning of the catalys"3 on the exit conversion was investigated, Tetraethyllead (TEL) and CS, were used as catalyst poisons. It was shown that on a fresh catalyst and for heigher concentrations of CO two steady state operations may exist, i.e. hysteresis phenomena For a weakly deactivated catalyst by TEL the operation charaocour. cteristics of the bed may be improved. A strong deactivation by TEL results in short hysteresis loop which finally disappears. It was shown experimentally that deactivation by CS2 may result in excessive hot This deactivation is reversible. After ceasing the poison in spots. the feed and on temporary inoreasing the inlet temperature the aotivity of the bed packed with Pt/A1203 catalyst resters.

KEYWORDS

Deactivation;

multiple

steady

states;

temperature

waves;

hysteresis

phenomena. INTRODUCTION

Many effluent Sases contain small amounts of undesired impurities which give rise to a deactivation of catalyst. For instance, it has been well established that a fully leaded gasoline may deactivate catalysts based on Pt or Pd quickly (1). Tetraethyllead (TEL) causes permanent loss of activity (irreversible poisoning) (2), on the other hand certain catalyst POisOnS result only in a reversible deactivation, i.e. for a poisonless effluent gas the activity of the catalyst bed restors (3). The purpose of this investigation was to obtain data on behavior of Pt/Al 0 tubular reactors packed with catalyst where a deactivation the "3e feot of different catalyst poiprocess oucura. To demonstrate sons, CS2 and TEL were seleoted which give rise to reversible and irreversible poisoning respeotivelyPt/Al 0 As a model reaction oxidation of CO was chosen. Commercial 2 3,s catalyst (sperical partioles) was used. Experimental apparatus tubular reactor provided with an evacuated jacdescribed in (4). The ket covered by a silver coating was plaoed vertically and the feed of the reactinS ear was sent to the bottom or to the top of the reactor, Two catalyst poisons were ini.e. an up-flow or a down-flow resulted. vestigated: CS2 and TEL. The f’ormer poison was injected ab a liquid in TEL was fed to the reactor by a preheated gas stream (CO, 0 ) while making use of the tension met & ad, i.e. a mixture of CO + 0 bubbled through liquid TEL at 40°C. The reactor was placed in a ba z h which 258

Temperature profiles, parametnc sensltivlty and multiple steady states

F-33

259

The degree of deactivakept at the temperature of the inlet gas. tion was followed by measuring the axirl temperature profiles in the The temperaturewas meabed and exit oonversion of the reaoting gas. sured by an iron-conutantan thermooouple placed in an axial thermowell. The composition of the exit gas was analyxed by a therma 1 conductivity cell. The charaoteriatios of the reaotor and catalyst u6ed are presented in Tabs. 1 and 2.

was

TABLE

1

Reactor Reaotor Reactor Catalyst

Operatinx

Parameters

length internaJ. diameteexternal diameter bed length

2

Characteristics

Catalyst Diameter Specific surfaoe Average pore diameter Real density Apparent density Average Pt concentration Poroaity REVERSIBLE

the

Reactor

0.56 m 0.026 m 0.060 m 0.16 m (reversible Mtion) 0.14 m (irreversible activation)

rate Volumetric flow (a tandard conditions) Linear flow rate (interstitial, standard conditions) Inlet CO concentration Inlet cs2 conoentration Inlet TEL oonoetration

TAE3LB

of

3

. 10-5

0.166

m

deaotide-

m3s'= s-=

3 or 5 mol $ 61006 0.04 moJ_ $ 0.04 mgl % of

Cetalvst

spherical particles, 0.0034 m 2.05 . LO5 m2kg" 25 A 3 050 kg mB3 1250 kg me3 o 5 _ . 0.6 wt.%

Pt/A1203

0.4

DEACTIVATION

For reversible deactivation of the bed CS2 was used. The effects of continuous and discontinuous deactivation were investigated. The reactor behavior for a oontinuous deaotivation may be inferred from Fig. 1. It is obvious that the velocity of the deaotivation wave depends on the inlet poison aoncenfrafion; the higher the feed poison concentration the higher the speed of the moving zone. It was observed experimentally that during the deactivation process excessive hot spots occured which moved down-stream. Evidently, as a result higher inlet poison concentration may give rise to a transient run-away process which may damaged the catalyst. For a discontinuous deactivation the bed may restore. Such a situation is displayed also in Fig. 2. The deactivation prooess is stopped after 3060 sec. However, the reaction zone moves further downstream and an extinction prooess occurs. This is apparently the result of CS adsorbed in deaativation zone whioh slowly desorbes after the fee a poilroning was rtopped. After a certain period (7000 s) the adsorbed poison is washed-out and after increasing the inlet temperature an ignition process at the reactor inlet occurs. From Fig. 2 two different transition proceases between two steady states may be inferred. For a fresh aatalyst the effect of a decrease of inlet temperature results in a travelling temperature wave which moves towards reactor exit. The transient hot a ot temperature exceeds the steady state value (the"wrow-way" effeot 7 - cf. the dashed line 3. For a transition from the lower to the upper steady state (see dashed line 4) the temperature wave originate6 at the reactor outlet and moves toward reaotor inlet. For a deaotivated bed (solid line) the curve 1 is analogous to curve 3, however the temperature wave arising from the transition of the lower to the upper steady state

Dynamrcs.

260

Stab&y

and Control

120

60

Fig.

(curve

part

2)

of

is

the

1.

e-If&f

F-33

180

AxfaX transient temperature and temporal conversion grofiles (3 Z CO, 0.0023 $5 CS2, inlet temperature 1400 C)_

strorrgly reaotor-

IRRlWERSfBLE

of Chemical Reuctors

influenced

by

the

lower

aotivity

in

the

outlet

DEACTIVATION

For reversible deactivation of the bed TEL was used. This poison was selected in order to get engineer&q data on partially deactivated Pt The experiments illustrate the behavior of a car catalysts with TEL. afterburner if a misfueling of gasoline occurred; i-e, if accidentally 3 showa the course of a the car was filled by leaded gasoline. Fig. hysteresis loop for 5 $ CO and for different aotivity of the catalyst bed. For a weakly deactivated bed the reaction temperature moved to

Temperature

F-33

profiles, parametnc

261

sensltlvlty and multiple steady states

400,

t [“cl

300

200

IOC 0 Fig,

2.

5

IO

Trajectory of the hot spot between two steady states.

L-d[m] for

a

l5

transition

is very stve obuervation: a deactiLower values (uee Fig. 3). This vation of the catalyst improvea the operation of the catalytio bed. We For a iltronger deactivation have observed thfr effeat also for 3 4’p CO. the ignition point moves again towards higher temperatures and the exit conversion decreases. A weak deaotivation of the catalyst by TEL resuikts of hysteresis loop is enlarged in an unexpected phenomenon - the size and the whole hysteresis loop movea to lower temperatures. For a higthe hyrtereais loop moves to higher inlet temperatuher deactivation res. From Fig, 3 we may see that the hysteresis loop for a fresh oatalyst is very similar to that for 7 ml of TEL added. For these two oasee the ign%tion and extinction temperaturea are the same. For a fiigh deactivation of the bed the hysteresis loop shrinks and moves to higher inlet temperatures For a ve high degree of deactivation hysteresis effects disappear (18 ml TEL7 . The two steady state temperature profidegree of deacles for 5 9 CO lnay be found elsewhere (5)- For higher tivation (e.g. 16 ml TEL) the hyrteressis ouz-ve deforms and for certain values of inlet temperature two '%.lpper" steady states are possible. There profiles are shown in Fig. 4.

"inlet temperature - ezit eonThe strarqge behavior of the dependence veraIon" depends on two parameter-adfabatic taaperrture riue and activation energy. For low values of the aotivation energy, even if the the hystereaia loop ir very 6mall adiabatic temperature rise is high, or does not exist (6). This behroior is typical for metal oxide oataOn the other hand, for catalysts baaed on noblysts (Mn02, CuO etc.). le metals, the size of hysteresis ir large and for strongly polluted exhaust gas the oxidation may ooour at very low valuer of the inlet

262

Dynamrcs,

F-33

7

l-

Y(l)

:

i

I 0.5 o -

Fig.

Stabdrty and Control of Chemzcal Reactors

;

:

I

I

50

100

3.

4

>i

0

k 150to['c]

1 200

I

I?+-‘I

50

100

Dependence of erit converlrion temperature t for 5 ss co and lues of inlet0 temperature 1 - fresh catalyst z-33mlTEL - 7 mlTEL 2 - 12 ml TEL

3,

I

L 78

-

I 350 t,

y (1) on different

:z 18 30

:: ml ml

p3200

inlet M-

:z TEL TEL

tempersture, *upporing that the reaotor is camied out in the upper ignited state, Since au appropriately designed afterburuer convertor should operate In then upper steady state, it ir etidant that it is not easy to replace the pt or Pd catalyst. This study hms &own the effeot of poisouLng on t&e behaviour of an afterl~urner prcked with Pt catalyrt. Both the reversible and irreversible poisoning MIB expemimentally ti It was shown that a we4ak poironfng of the Pt ortmlyst by vertigated. TEL improves the operating oharaoteristicr of the bed,

Temperature profiles, parametnc sensltivlty and multiple steady states

F-33

250-

200-

10

Fig.

4.

2

L?102[rn~

state tgmpera$ure profileS. Two "tlpperW steady Inlet temperature to i 150 C, 5 P CO, TEL.. deaotivation 16 ml

REFERENCES 1.

2. 2: 5. 6.

CLS

Shiller J.W., &nd Piken A.G. [l%‘b). PerfO-nUe of catalysta in a vehiole field teat. 6th AICHE Meetina. -Washington, D.C. (~~76). Enviromental Soi. Tech~ologu, C.N. Otto K., and Montreuil 154. 10, M,ikuH O., Pour V., and Hlav&Oek V. (1977). Jour. Cakl., 48, 98. Hla,v&bek V, et al. (1979). platinum Metal1 Re'Piew, in press. Mikug O., Pour V., and Hla&sek V. (1979). Jour. Catrl., in press. Experiment;: s$asz.ofs multiple j. (1974). HlatiEek V., and Votruba p . on steady etates in adiabatic catalytic SyStenr. 3 Cheunioal Reaction Ensirieeriw, mston9 Illinois. #mver

no1ithj.o

35-l/2--~

E,E,,

263