Sulfur poisoning of nickel methanation catalysts

Applied Catalysis. 5 (1983) 323-336 Elsevier Scientific Publishing Company,

SULFUR

POISONING

EFFECTS

II.

OF NICKEL METHANATION

OF H2S CONCENTRATION,

ON DEACTIVATIOM

- Printed in The Netherlands

CATALYSTS

CO AND H20 PARTIAL

PRESSURES

AND TEMPERATURE

RATES

Erek J. EREKSON2 1 Department

323 Amsterdam

and Calvin H. BARTHOLOMEW1'3

of Chemical

Brigham

Engineering,

Young University,

Provo, Utah 84602,

USA. 2

Gulf Research and Development Co., Box 2039, Pittsburgh, 3 To whom correspondence should be addressed.

(Received

14 June 1982, accepted

19 November

Penn. 15230, USA.

1982)

ABSTRACT Poisoning of unsupported and supported nickel methanation catalysts was investigated in Pyrex, fixed-bed and quartz, mixed-flow microreactors over a range of reaction conditions and H2S concentrations. The results indicate that poisoning rates are complex functions of H2S concentration, temperature and partial pressures of CO and H20. In general, deactivation rates decrease with increasing H2S concentration, due to formation of multilayer sulfides, and increase with increasing CO and H20 partial pressures. The application of deactivation models to poisoning of nickel by sulfur is discussed.

INTRODUCTION Poisoning

of nickel hydrogenation

feed in trace quantities

catalysts

is an industrial

only a few ppm of H2S can reduce catalyst Accordingly,

there is an important

which enable

prediction

impurities

are present.

Wentrcek

beds, based on the deactivation

rd = $

where

of deactivation processes

there have been relatively

rates and the parameters

et al. C53 developed

a model

in the Indeed,

life to weeks or even days Cl-IO].

life in hydrogenation

Nevertheless,

present

of serious consequence.

need for development

of catalyst

[3-91 of sulfur deactivation

by sulfur compounds

problem

for sulfur

which

few investigations

influence

poisoning

models

in which sulfur

these rates.

of catalysts

in fixed

rate equation

= kdns

(1)

rd is the rate of deactivation,

concentration and co-workers behavior:

of H2S and s the density [4,8] modified

kd is the deactivation of sulfur adsorption

this model

to allow modelling

rate constant, sites.

n the

Bartholomew

of activity-time

324 rd = $

= kdna

(2)

where a = normalized

activity

based on the assumption

(specific

activity

that a = s/so, where

at time t divided

so is the initial

by fresh activity)

site density

for

sulfur adsorption. However,

studies

by Bartholomew

rates in CO hydrogenation catalyst

surface

co-workers

et al. [3,4,8]

were dependent

indicated

not only on sulfur concentration

area, but also on H2/C0 ratio and temperature.

[1,2] found that the extent of deactivation

concentration methanation

in the feed. Moreover,

Fitzharris

on nickel at high temperatures

sites were deactivated Bartholomew

per adsorbed

and

Oalla Betta and

by H2S depended

upon water

et al. [7,9] found that during

(e.g. 673 K) approximately

sulfur atom, contrary

two nickel

to the assumption

of

et al. [3,4,8].

The present

study was undertaken

ion, of partial

pressures

and on deactivation assumption

that sulfur poisoning

rate constants

a = s/so under reaction

amount of H2S adsorbed to determine

to (i) determine

the effects

of CO and H20 and of temperature and to (ii) determine conditions

if the sulfur adsorption

the validity

representative

on nickel was also measured

of H2S concentrat-

on rates of deactivation of the

of methanation.

The

over a wide range of temperature

stoichiometry

is temperature

dependent.

EXPERIMENTAL Pure nickel powder

(INCONickel #287) was used in H2S adsorption measurements

and in most fi/ed bed deactivation alumina

support,

details).

ESCA analysis

impurities

of the

to reported

cordierite

monolith

Works) monolith Hydrogen

support

followed

hydrogen

(99.99%, Whitaore)

These premixed

study.

of the nickel powder and drying

was purified

using a Nix-Ox

were used in preparation

formed

through

slowly.

Pd catalyst

from Matheson.

in the mixture

(Union

tank. Hydrogen

and gases. sieve

sulfide

from a 66 ppm high purity

specifically

before mixing with purified

reactant

(450 K) molecular

All three H2S/H2 mixtures

designed

to obtain the desired

of compressed

a heated

of 2.1 and 10 ppm were prepared

further

on a

(Corning Glass

Sieve 5A (Linde) trap. CO (99.99%, Matheson)

or Teflon lined cylinders

above)

by impregnation

a 7 mm length of CELCOR

gases were passed

obtained

were not purified

were prepared

surface

[II]. Coated monolithic

Pure nickel powder was supported

suspension

any iron carbonyl

mixtures

H2S/H2 standard

described

by a Molecular

reactant

trap to decompose

aluminium

by dipping

into an aqueous

carbon and oxygen

catalysts

[3,12].

gas (99.99%, Whitmore)

Carbide)

in hydrogen

nickel

techniques

on the

below 650 K (see [6] for

INCO nickel powder revealed no significant

other than the usual ubiquitous

and po,/dered A1203 supported according

runs in order to avoid H2S adsorption

which was found to be significant

reactant

H2S/H2 or CO/H,S/H2

were stored

for H2S gas service

in and

or H2 gas (as

streams

for use in this

325 Total surface

areas were measured

face areas were determined Adsorption

capacities

previously

reduced

by argon BET at 77 K, and nickel metal

from total H2 chemisorption

of H2S were determined

(FPD)). P2S adsorption

was reached capacities

nickel

for pure powdered

in H2 ai 775 K for 18 h by flowing

sample until saturation

(as measured

samples

0.2 ppm H2S in H2 over each

by a flame photometric

were determined

sur-

at 298 K Cll-141.

uptakes

detector

at 525, 575, 625, 675, 725 and

775 K.

FIGURE

1

Pyrex microreactor

The procedure

for fixed-bed

reduced

in flowing

the reduction

hydrogen,

temperature

the run temperature,

creasing

the hydrogen

a six port Teflon

TO determine

was 673 K. The cell was cooled

h-' , for 2 h.

flow and replacing

sampling

reactor

described

in flowing

to the reactant

H2S was subsequently

hydrogen

to

mixture,

typically

introduced

by de-

it with an H2S/H2 mixture.

310 (Supelco). containing

the relationship

deactivation

elsewhere

H2S, or

The sample was

were measured

Methane

pro-

by FPD, using

valve with a 5.0 x 10W6m3 loop and a 1.2 m Teflon

for streams

of sulfur adsorbed,

l),

, at 573 K. For runs above 573 K

by FID, and H2S concentrations

directly

sample.

A 0.4

(see Figure

was 0.14 micromole

of the catalyst

GHSV = 10,000 h-'

column packed with Chromasil after the reactor

capacity

and the sample was exposed

1% CO in H2, GHSV = 40,000

duction was measured

into a Pyrex microreactor

on the Pyrex microreactor

about 3% of the H2S adsorption

experiments.

tests was as follows:

H2S deactivation

to 0.6 g sample of INCO powder was loaded The total h2S adsorption

and poisoning

used in adsorption

Teflon

tubing was used before and

H2S. between

deactivation

tests were also performed

C15,161.

Monolithic

supported

rate and amount

in a quartz, catalysts

mixed-flow

were used to

326 avoid pressure-drop

problems

[16]. The procedure

the fixed bed tests involving followed

a 2 h reduction

by an H2S deactivation

in these tests was similar

and 2 h steady-state

activity

to test,

test.

RESULTS Bulk compositions, activities supported

surface areas,

for fresh catalysts nickel catalysts.

stoichionetries Methanation

hydrogen

are listed

The specific

adsorption

activities

were relatively

unaffected

orders for CO and H2 were -0.3 and 0.9 before,

uptakes

in Table

by H2S poisoning;

1.

i.e. the reaction

and -0.4 and 0.8 after poisoning

50% of the nickel sites.

H2S adsorption

stoichiometries,

expressed

in the form of sulfur atoms adsorbed

per surface nickel atom are shown in Table 2 as a function

of temperature

575 to 775 K. Values of S/Nis are the same at all temperatures, within

and alumina-

(Table 1) and H2S adsorption

(Table 2) were based on the total hydrogen

kinetics

aporoximately

uptakes and specific

in Table 1 for unsupported

experimental

accuracy

estimated

from

i.e. 0.5-0.7

f 0.2,

at +15-20%.

TABLE 1 Composition,

Surface Areas and Specific Composition

Catalyst

/umoles

/wt% Ni 100

INCO Ni Powder

Ni/Al,O,

Powder

3

aTotal uptake measured bArgon multipoint 'Molecules

of methane

5.1d

0.4gd

2.ge

0.34e

18 h reduction

areab

Methane turnoverC freq. x IO3 at 500 K 0.7f

1.5 g

at 298 K

produced

CO, 4% H2. Rates measured

act

g-'

BET surface 2 -1 m g

at 77

K

per catalytic

site (based on H2 uptake)

less than 5% using a reactant

dAfter 2 h reduction

fE

uptakea

of Nickel Catalysts

25

BET measured

at CO conversions

eAfter

Hydrogen

Activities

after

mixture

containing

per second

95% N2, 1%

IO-15 min. on stream.

at 573 K, GHSV = 10,000 h-' in H, L

in H2 at 775 K

= 113 kJ/mole

gE

= 96 kJ/mole; NCH act (see footnote c above)4

Visible micrographs reduction

= kPCO

-"'3P

H2

O",

where

N

CH4

(160X) of INCO nickel powder

at 775 K (Figure 2a) and after further

isthemethaneturnovernumber

samples were obtainedtafter

exposure

over a period of about

6 h to 0.2 ppm H2S/H2 at 725 and 775 K (Figures 2b and 2~). This exposure was sufficient obtained.

in the latter two samples that saturation

The microaraph

for the freshly

reduced

catalyst

coverage

to H2S

with sulfur was

(Fioure 2a) indicated

327 TABLE 2 H2S Adsorption

stoichiometry

Temp. of Adsorption

(0.2 ppm H2S/H2)

as a function S/Nisa

/K

0.01

575

0.49

625

0.75 * 0.08

675

0.70 + 0.06

725

0.53 f 0.13

775

0.50 f 0.05

aAtoms of sulfur adsorbed

per surface

f

nickel atom measured

H/Ni, = 1 (see Ref. [13]. The standard

298 K and assuming of the experimental

of temperature

precision.

Experimental

accuracy

by H2 adsorption deviations

is estimated

at

are a measure

to be +15-20%.

C

FIGURE 2 (b)

(a)

INCO nickel

INCO nickel

powder

to 0.2 ppm H2S at 725 K.

powder

(160X) after

(160X) after 18 h reduction (c)

H2 at 775 K and 6 h exposure

INCO nickel powder

of numerous

sample

small spherical

as well as large irregularly

in H2 at 775 K.

in H2 at 775 K and 6 h exposure (160X) after 18 h reduction

to 0.2 ppm H2S at 775 K (rod-shaped

pieces of glass wool used to support

the presence

18 h reduction

particles

in

are

in reactor).

particles

shaped particles

having diameters

having

of 2-6 urn,

outer dimensions

of 50-150

urn.

328

In the samples treated are significantly

in 0.2 ppm P2S at 725 and 775 K (Figures 2b and 2c), there

fewer small, spherical

(loo-250 urn) irrepular-shaped temperatures

(e.g. 525-625

particles

particles.

and significantly

For samples

K) there was no evidence

treated

more,

larger

in H2S/H2 at lower

from similar micrographs

of

sintering. Figure 3 shows normalized poisonino

divided

molecules

adsorbed

deactivation nonlinear

per initial nickel

in the presence

decline

increasing

methanation

activity

(the mass-based

by the rate of the fresh catalyst)

surface atom, during

of eS, H2S

in situ, fixed-bed

of 0.2 and 1.0 ppm H2S. There is an initial rapid

at low exposures

exposure,

rate during

as a function

followed

by a linear decrease

albeit a more rapid decrease

in activity

with

in the case of the O.P-ppm

test

1.0

0.9

0.6 0.7 : 5 F

0.6 0.6

0 a

0.4 0.3 0.2 0.1 0 8s(H,S

FIGURE 3

Activity

deactivation

1

2

MOLECULES

/ INITIAL

vs. es (H2S molecules

Values of kd obtained

adsorbed/initial

0

from fixed-bed

Ni site) during

Reactor

0.2 ppm,A

deactivation

conditions

Generally

ration or PH s/P H

and PH s/PH

However,

/P

than2 H2S2concentration;

runs using the model of

ratios,

/P

ratio was decreased

kd appears

to be more a function

was held constant

per adsorbed

molecule

at 1 ppm, while the

of H2S, calculated

that less than one site is deactivated

sorbed. At 0.2 ppm D is nearly unity, molecule

of

in kd for

by a factor of 5. Table 3 also shows 0, the number from the slope of

the data in Figure 3. The value of CI is less than 1 at relatively meaning

over a

in two cases by adding

e.g. note the factor of two increase

'R2S H2 the two runs in which H2S concentration

centrations,

were 525 K,

the trend is towar 8 big 4 er kd values as the inlet concent-

decreases.

'H2S H2 of sites deactivated

in situ

1 ppm.

et al. [4,8] are listed in Table 3. These data were obtained

range of inlet H2S concentrations N2 diluent.

Ni SITE)

in a fixed bed of INCO nickel powder.

100 kPa, 2% CO in H2 and 500 cm 3 min -I.

Bartholomew

3

indicating

high H2S con-

per H2S molecule

one site deactivated

ad-

per H2S

adsorbed.

Values of kd and d obtained

in fixed-bed

runs over a range of temperature

at

329 0.2 and 1 ppm are listed stoichiometries

in Table 4. Deactivation

were only moderately

or slightly

1 ppm H2S, values of kd were nearly constant

rate constants influenced

and deactivation

by temperature.

At

over the 100 K range of temperature

(Eact = 8.2 kJ mol-'). In the presence of 0.2 ppm H2S, deactivation rate constants increased by a factor of two with increasing temperature (Eact = 23 kJ mol-I), while

the number of sites deactivated

over the 75 K temperature

per H2S molecule

adsorbed was about

1.0

range.

TABLE 3 Deactivation

Rate Constants

for INCO-nickel

at several

H2S concentrationsa

'H S"H 2 2

H2S Concentration

x lo6

/ppm 6.6

33

0.07

0.2

1.0

5.0

0.14

0.3

1.0 0.5

1.0 0.5

0.26 0.30

0.3 0.5

0.2

0.2

0.65

0.9

aAll runs were made on INCO nickel ratios (CO concentrations

were

i'

powder at 525 K, 100 kPa, and large H2/CO

l-2%). Space velocity

was between 40,000

and 80,000

h-'. b

kd is the deactivation constant -1 h-1 units of ppm .

from the deactivation

model,

rd = kdna, where

kdhas

'0 is the number of nickel

sites deactivated

Effects of CO concentration tests with different

per adsorbed

on H2S deactivation

CO concentrations

while

H2S molecule.

were studied

keeping

by making

stant. Table 5 lists the kd and 0 values.

During these tests COS appeared

the exit stream when the CO concentration

was 5% or above.

be H2S or COS, the latter distinguish quarter

is suspected.

chromatographically

of the reactant

as COS to the product to the quantity

between

H2S adsorbed

stream.

on the catalyst

while

CO run (I was much greater

however,

that the observed

less than at 5% CO because,

of the sulfur passed

the reactor

the effective

concentration

to

the rest passed through

in Table 5 were calculated

vation at 20% CO is actually through

in

this could

H2S and COS). For the 20% CO run only a

For the 20%

at 2-10X CO. It should be emphasized,

(Although

con-

At sub ppm levels it was difficult

The 0 values

of H2S adsorbed.

four

the H2S concentration

without

used to calculate

than runs

rate of deacti-

in the former case, 80%

adsorbing

deactivation

according

on the catalyst.

Hence,

rate is 0.04 ppm for

330 TABLE 4 Effects of temperature

on deactivation

rate constants b

Temperature

H2S Concentration

C

cl

kd

/K

/ppm 1

0.2

aAll runs were performed

525

0.26

575

0.32

0.3 0.4

625

0.35

0.6

525

0.65

0.9

575

0.99

1.0

600

1.27

1.0

on INCO nickel powder at 100 kPa and 1 or 2% CO with

hydrogendiluent.

Thus, the ppm H2S and the PH S/PH ratio were the same. 2 2 kd is the deactivation rate constant from the equation rd = kdna and has units -1 -1 ofppm h .

b

‘CT is the number of nickel sites deactivated

20% CO, compared

per adsorbed

H2S molecule.

to 0.18 ppm for 5% CO.

Tests were made with H20 vapor added to the feed for supported nickel.

These tests are compared

Table 6. The addition catalyst. alumina

In the case of Ni/A1203,

adsorbed

large amounts

than in its absence). adsorbed

order to determine temperatures

the stoichiometry

under conditions

of unsupported

sulfided

surface

near the origin. of unsulfided

because

of water

not all the

in a quartz mixed-flow

no gradients

nickel/monolith

reactor

of temperature,

CO concentration

similar to Fitzharris

was plotted against

various

functions

Figures 4 and 5 show plots of

at 525 and 575 K plotted against against

data in these figures

in

of nickel over a range of

[S-lo]. Methods

of the catalyst

area and at 625 K plotted

The activity

in

was larger since the

less in the presence

of H2S poisoning

surface area of the catalyst.

activity

of water

for the unsupported

surface.

could be present

[7,9,10] were used. Activity

1.5 power.

where

capacity

for Ni/A1203

runs were also performed

H2S concentration

of the unsulfided

of H2S (although

the metal

test in the absence

kd significantly

H2S adsorption

Thus, kd was smaller

H2S deactivated

H2S deactivation

and/or

with a similar

of H20 vapor increased

and unsupported

the unsulfided

fit straight

the un-

area to the

lines that pass very

Plots of the data in Figure 4 using other than the first power

surface area, and of the data in Figure 5 using other than the 1.5

331 TABLE 5 Deactivation

rate constantsa

Mole % CO

for a range of CO concentrations

H*/CO Ratio

0

C

Outletd

2

50

0.2

0

0.65

0.9

5

20

0.2

0.02

1.5

1.2

10

10

0.2

0.08

2.0

1.6

20

5

0.2

0.16

4.3

4.4

aAll runs were performed hydrogendiluent. b

x lo6

'H S"H 2 2 Inlet

on INCO nickel

The space velocity

kd is the deactivation -1 -1 h .

powder at 525 K, 100 kPa, and with a

was 80,000

rate constant

h-l, PH2S = 0.2 ppm.

from the equation

rd = kdna and has units

ofppm

'0 is the number of nickel d

sites deactivated

Could be H2S or COS; latter is suspected.

distinguishchromatographically

between

per adsorbed At sub-ppm

H S molecule. 2

levels it was difficult

to

H2S and COS.

TABLE 6 Effects of H20 vapor on H2S deactivation

Catalyst

rate constantsa b

Mole %

C

CJ

kd

H20 0.65

0.9

1

1.17

0.8

1

0.17

0.8

INCO Powder

0

INCO Powder 3% Ni/AlP03

aAll runs were made at 525 K, 100 kPa, 2% CO in hydrogen b

kd is the deactivation -1 h-1 afppm .

rate constant

'0 is the number of nickel

sites deactivated

power, were found to have slopes cepts significantly

removed

from the equation

and 0.2 ppm H2S. rd = kdna and has units

per adsorbed

significantly

different

H2S molecule.

from unity and inter-

from the origin.

DISCUSSION The data of this study provide by sulfur

under conditions

evidence

relevant

that the poisoning

to commercial

operations

of nickel catalysts (i.e. 0.2 to 30 ppm

1.0

I

I

I

I

I

I

I

O.Q0.80.70.60.50.40.30.2O.l0 0.2

0 FIGURE 4

0.4 (1 -8)

Activity

0.6

0.8

0.2

0.4

1.0

0.8

1.0

(1 _ *)I.5

versus sulfur-free

525 K and 575 K. Test conditions:

0.6

area for INCO nickel powder/monolith

quartz

at

CSTR, 150 cm3 min -', 0.2 ppm H2S. 2% CO

in H 2, 100 kPa and 625 K.

FIGURE 5

Activity

on a monolith;

versus sulfur-free

test conditions:

area to the 1.5 power for INCO nickel powder

quartz

CFSTR,

150 cm3 min -I, 0.2 ppm H2S, 2% CO

in H2, 100 kPa and 625 K.

H2S/H2 and 500-800 supposed

[4,5,8].

K) is more complex

and in H2S concentration, changes

(measured

In addition,

directly

temperature,

to be first order

rate constant

is discussed

is apparently in some detail

in activity

is found to vary with

CO concentration,

the number of sites deactivated

in a CSTR reactor)

Each of these phenomena

to model than previously

rate is assumed

the deactivation

in H2S concentration,

centration.

and difficult

If the deactivation

and H20 vapor con-

per adsorbing

a function

H2S molecule

of temperature.

below.

Effects of R2S Concentration Deactivation apparently adsorbing

rate constants

since progressively H2S molecule

a temperature of equilibrium

at 525 K decrease

fewer nickel surface

of 525 K, formation thermodynamic

the activity

formation

H2S concentration,

sites are deactivated

per

(see Table 3). For values of PH2s/PH2 of 5 and greater at of a bulk sulfide

data [9,10,17].

layered nickel sulfide at the entrance extend

with increasing

of surface

of a bulk sulfide

is predicted

Thus, formation

to the catalyst

of a bulk or multi-

bed can very significantly

nickel sites in the remainder

is not predicted

from extrapolation

of the bed. Although

for PH s/PH < 1 ppm at 525 K [9,10,17],

= 0.5 and 1.0, shoiing &ly 0.3 and 0.5 sites the data in Table 3 for P H2S'PH2 deactivated per adsorbed H2S molecule, are consistent with the formation of multilayer sulfides which formation

likewise

of multilayer

fides are not expected

extend the life of the catalyst.

or subsurface

sulfides

has been recently

under conditions

reported

[18,19].

Evidence

for the

where bulk sul-

The fact that the de-

333 activation

rate constant

centration

is reduced

practical

implications.

stantial

benefit

increases

by more than a factor of 2 as the H2S con-

by a corresponding It means

factor

from 0.5 to 0.2 ppm has important

that under these conditions

in terms of catalyst

life in reducing

there is no sub-

the H2S concentration

by

a factor of 2.

Effects

of Temoerature

The observation deactivation

of only moderate

rate constant

0.2-I ppm suggests However,

or slight dependence

(Eact = 8.2-23

that film diffusion

calculated

maximum

on temperature

might be limiting

mass-transfer

dependence

powder catalyst

of kd is apparently

diffusional

influences,

rates are 1000 times faster than the

equilibrium

adsorption

the catalyst

is apparently

0.5-O-7

atoms/site)

in CO hydrogenation concentration

over the entire

due to H2S adsorption

of H2S apparently as evidenced

of the surface

accelerates

by the significant

Figure Z), although

of H2S-assisted

in the self-diffusion

reasonably

constant

molecule

is increased

two nickel

at 675 K. These changes

'ture may result from changes With temperature

previously

(e.g. dissociation

stoichiometry

is

the data in Figfrom

adsorbed

sites to be deactivated

1 nickel

as temperature

for each H2 mole-

stoichiometry

with tempera-

step for CO hydrogenation

temperatures,

two nickel

system [21].

with the data of Fitz-

(525-575

may involve effectively

CO) while

of ad-

stoichiometry

increases

molecule

in deactivation

C22]. Thus at low reaction

of adsorbed

at 775 K for

in the iron-sulfur

temperatures,

in the rate-determining

step in CO hydrogenation

range. The presence

to be a result of in-

that the H2S adsorption

to 1.5 nickel sites/H2S

harris et al. [7] indicating

form

at 725 and 775 K,

of a metal due to the presence

that the deactivation

adsorbed

an additional

stabilized

is thought

from 525 to 625 K. This trend is consistent

cule adsorbed

determining

thermally

over a wide range of reaction

ures 4 and 5 suggest site/H2S

of nickel particles

sintering

coefficient

the data in Table 2 suggest

is apparently

growth over a period of 5-6 h. (see

sorbed H2S [lo,201 and has been observed While

low

sulfide formation.

the sample was originally

18 h. This phenomenon

(S/Nis =

of interest

at a sufficiently

in the higher temperature

particle

Indeed, the

range of temperature

there

the sintering

sulfide

of the nickel surface

(575 to 775 K) when H2S is adsorbed

to poisoning

of deactivation

creases

coverage

(0.2 ppm) to avoid multilayer

In addition

on how fast H2S

per sulfur atom adsorbed.

that a saturation

is observed

dependent

fast

to the bed. Thus,

of the surface or multilayer

formed, and how many sites are deactivated data in Table 2 confirm

nor pore-

of extremely

at the entrance

essentially

bed, the stoichiometry

1 and 0.2 ppm. small temperature,

of film-diffusional

a manifestation

coverage

of

the relatively

an effect

but more probably to saturation

the rate of deactivation enters

was nonporous,

neither

at

the rates of deactivation.

rates at which H2S was fed to the bed at H2S concentrations Since the nickel

of the

kJ) during fixed-bed deactivation

K) the rate-

only one nickel site

sites may be required

(e.g.

334 for carbon hydrogenation)

at high temperature

(675 K).

Effects of CO Concentration The data in this study (Table 5) show that (i) the amount of H2S adsorbed nickel during methanation creases with increasing The breakthrough formation

decreases

in the presence

of COS which is not readily

The increase

the reaction

of CO hydrogenation

of H2S, Gardner

deactivation

deactivation

and Bartholomew

CO dissociation

effect

[23].

(2%) chosen for this study [24] in the absence sulfur apparently

by both sulfur and inactive carbon

Moreover,

rate constants

the previous

at high CO concentrations

are both consistent

pre-

extent than from

and larger

observations

[26] that four nickel sites are deactivated

sorbed sulfur during methanation

when viewed

in

CO concentration

carbon to a greater

al. [7] that only two nickel sites are deactivated concentration

involving

both CO and H2S. Although

due to carbon

the larger deactivation

and Pederson

conditions

in this laboratory

with increasing

of the active

of CO. Thus poisoning may explain

experiments

involving

by the

carbon formed on nickel during methanation,

hydrogenation

values of d at high CO concentrations. Rostrup-Nielsen

on in-

of COS have been de-

[25] have shown that adsorbed

the amount of inactive

the dissociation

amounts

(525 K) and CO concentration

low to obviate

sumably by poisoning

under these reaction

catalysts

rate constant

a synergistic

temperature

were sufficiently

adsorbed

in recent sulfur poisoning

in deactivation

(Table 5) suggests

of CO can be explained

[23]. Indeed, equilibrium

tected chromatographically situ H2S poisoning

rate constant

CO concentration.

of sulfur

in short beds of catalyst

increases

and (ii) the deactivation

by per ad-

and by Fitzharris

et

by each sulfur atom at low CO

with each other and with the data of this study

in terms of the carbon/sulfur

synergism,

rather than an ensemble

C7,261.

Effects of Water Vapor Dalla Betta and Shelef more detrimental

[2] showed that H2S in combination

to methanation

vapor. They surmised

activity

thus freeing more of the nickel surface study showing are consistent

than in the absence

that a carbonaceous

increased

deactivation

material for attack

by sulfur.

rates for H2S combination

with this point of view. However,

catalysts

water vapor,

The results of this with water

vapor

it should be noted that water deactivate

nickel meth-

[22].

In the case of the alumina-supported effect.

of reactant

was removed by the water

by itself has been shown to inhibit and even irreversibly anation

with water vapor was

It was observed

hinders the adsorption

in this study

nickel catalysts,

of H2S on the alumina

in the gas phase available

there is an additional

(see Ref. [6] for details)

for adsorption

support,

thereby

on the nickel metal.

that water vapor

increasing

the amount

335 CONCLUSIONS The results

1.

for nickel

of this study show that deactivation

in CO hydrogenation

temperature,

CO concentration

deactivation

behavior

vary in complex

for modelling

deactivation desired

deactivation

of sulfur poisoning

rate constants

reaction

conditions

process.

rate expression.

model

thermodynamic multilayer

typical

data for the nickel-sulfur

nickel sulfides

lower H2S concentrations At sufficiently

H2S adsorption (e.g. 500-775 nickel

may prolong

to synergistic

deactivation

Sulfur poisoning

at low reaction

vapor added to the reactant

mixture

at

of

range

i.e. the number of changes

may be due to changes and reactant

carbon and sulfur.

of the metal.

in

concentrations At suffici

At 525 K and

per adsorbed

sulfur atom, con-

et al. [4,8].

of alumina-supported

on the support

thereby

sintering

site is deactivated

'H2S/'H2 sistent with the model of Bartholomew

to operation

during methanation,

in temperature

of

applicat-

are formed.

of deactivation,

by both inactive

H2S enhances

= 0.2 ppm, one nickel

the formation

Ni atom over a wide temperature

These changes

are

of equilibrium

in commercial

life, compared sulfides

sulfides

(e.g. 0.2 ppm) the stoichiometry

the stoichiometry

step with changes

ently high temperatures,

support,

catalyst

per sulfur atom adsorbed

and CO concentration.

the rate-determining

4.

as

to other

( i.e. 525 K) at

[17]. However,

undesirable

at which only surface

K). However,

or bulk nickel

with extrapolation

system

low concentrations

sites deactivated

adsorption

of extrapolating

inlet conditions

is not necessarily

is 0.5 atoms S/surface

with temperature

and/or

of methanator

as low as 1 ppm, consistent

ions, since this behavior

3.

in which experimental

must be emphasized.

formed under conditions H2S concentrations

Nevertheless,

under the same conditions

The danger

The data of this study show that multilayer

2.

the

conditions

[4,5; Eq. 1 and 23 is recom-

for situations

have been determined

for use in the modelling

Accordingly,

over a wide range of reaction

by a single deactivation

the use of the Wise/Bartholomew mended

fashion with H2S concentration,

and water vapor concentration.

cannot be modelled

and poison concentrations

rates and stoichiometries

nickel catalysts

temperatures,

diminishes

causing more severe poisoning

is complicated

i.e. 525-625

the adsorption

by

K. Water

of H2S on the

of the metal.

ACKNOWLEDGEMENTS The authors Energy

gratefully

(Contract

acknowledge

EF-77-S-01-2729)

financial

and technical

support

from the Department

assistance

of

by Clair F. James.

REFERENCES : 3

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336 5 6 7

a 9 10 11 12

21 22 23

:z 26

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