schmauch & singleton (1964) - technical aspects of ortho-parahydrogen conversion

7/21/2019 Schmauch & Singleton (1964) - Technical Aspects of Ortho-Parahydrogen Conversion

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~ wa modification. These two different orientations

of

energy leve only by

nuclear spins in the hydrogen molecule are responsible molecules and add rotational quantum numbers (1, 3,

for the difference in magnetic, optical, and thermal

5 . .

.

)

refer to levels which can be occupied only by

properrieS of the two modifications. The resultant ) molecules.

nuclear spin of the molecule contributes to the total :equilibrium, the molecules are distributed through-

rotational energy

of

the molecule which can exist only out these various energy levels as a function

of

tempera-

in certain

discrete quantites

or

energy levels.

ture. The ortho-para ratio

is

thus determined by the

rotational energy levels ace designated

by

rotational number of molecules

occupying

odd

or

cum energy states,

quantum numbers, J. which can have integral values respectively. If the emperature ischanged, the equilib-

such

as

0,

1,

2,

3,

4 .

.

.corresponding to increasing rium energy dutr ibut ion

is

also changed. However,

discrete amounts of rotational energy. It happens that. under normal conditions, the transition probability

cum rotational quantum numbers (0, 2, 4 between ortho and para states is practically zero.

These

.

1

..

...,..

.

.~

, ;,..:-

jr.

r.

.,

.

oicupied

.

. ..

5 :

. : '

;

.

.

:..

..

. .



concluded, as a result of their work on p

surfaces, that the mechanism must involve

of hydrogen at the catalyst surface.

nt

of

the catalyst upon th

section, there are two

hydrogm

-

@ one boitnd~.atom

.aud Saeege 20).

was

Eon-

to



able velocity in the presence of molecular oxygen.

Other paramagnetic molecules such as nitric oxide were

found to be effective. The conversion

is

caused by the

idu en ce of the inhomogeneous magnetic field of the

paramagnetic molecule upon the nuclear magnetic

field of the hydrogen molecule during co ll ion.

I t

is

a

simple bimolecular mechmism in the gas phase.

The

conversion velocity

is

independent of the hydrogen

pressure, is proportional

to the

oxygen pressure, and is a

direct function of the temperature.

A theoretical treatment of

the

paramagnetic conversion

which was developed by Wigner 60) and improved by

Kalckar and Teller

(32)

showed agreement with experi-

mental data

on

the magnetic conversion. The catalytic

activity is proportional to the square of the magnetic

moment

of

the paramagnetic molecule.

This

was sub-

stantiated by Far& and S a c k

20)

who studied the

effect of paramagnetic ions in solution and dissolved

oxygen on the conversion of hydrogen. Wigner’s

theory included a factor involving the distance of closest

approach of the hydrogen to the paramagnetic molecule.

The theoretical absolute rate of conversion depends on

the inverse 6th power of this factor. By assuming an

approach factor

betweem

one and two angstrom units,

experimental convenion rates were in agreement with

theory.

The heterogeneous catalysis of the ortho-parahydrogen

conversion by a magnetic mechanism was studied by

such early workers as Bonhoeffer and

Harteck

5),

Bonhoeffer and Farkas 6), and Rummel 38).

They

shoived that the rate of conversion of para- to

or iho-

hydrogen on charcoal, as a function of temperature,

passed through

a

definite minimum. This,suggested

that two mechanisms were operating, one increasingly

effective as the temperature was lowered, and responsi-

ble for a negative temperature coefficient between liquid

air and mom temperatures; the other increasingly

effectiveas the temperature \ as raised, and responsible

for the positive temperature coefficient above 20’ C.

Taylor 47)howed that two types of adsorption occurs

in these temperature ranges. He showed that van der

Waals or physical adsorption occurs at low temperatures

and decreases as the temperature increases while acti-

vated

or

chemisorption takes place above room tem-

peratures and increases as the temperature increases.

The low temperature conversion was believed to involve

the magnetic effect of impurities in the charcoal.

Rummel(38) demonstrated that the

low

temperature

(molecular) adsorption of oxygen on the charcoal

strongly enhanced the conversion of parahydrogen.

I t was also shown that the adsorption of oxy-gen at high

temperatures (activated adsorption) failed to enhance

the conversion, but in fact hindered it by covering part

of the active surface. Gould, Bleakney, and Taylor

(24)

showed that outgassed charcoals at liquid

air

tem-

peratures catalyzed hydrogen conversion but not hydro-

gendeuterium exchange. This

is

indicative of a

magnetic mechanism. In 1933, Taylor and Diamond

(449)

howed that paramagnetic subtancen are better

low-temperature conversion catalysts th ndiamagnetics.

They also showed that the catalytic activity was

a

direct

function of the magnetic properties of the catalyst.

Farkas and Sandler (27) and Turkevich and Selwood

50) showed that low-temperature heterogeneous catal-

ysis

of the para- t o orthohydrogen conversion could be

ascribed to the influence of the inhomogeneous magnetic

field of the paramagnetic comp-ent of the catalyst

The

work

of Harrison and McJhwell (25,

26)

showed that dia-

magnetic zinc oxide physically adsorbed hydrogen but

was not very effective

as

a conversion catalyst.

They

also showed that para-magnetic m-diphenyl-&picryl

hydrazyl,

a

solid free radical, physically

adsorbed

hydro-

gen very slightly and was only slightly effective as

a con-

version catalyst. However, a mixture of the two

ma-

terials proved to be more effective

a8

a conversion

catalyst than expected by their individual characteris-

tics.

In view of

thii,

an effective low-temperature catalyst

bas

three

primary requirements. I t must have a high

physical adsorptive capacity for hydrogen, a high con-

centration of “active”

or

magnetic

specie, and a high

paramagnetic susceptibility. However, the conversion

rate

is

also influenced by such variables as pressure,

temperature, feed composition, and space velocity

of

the hydrogen. The effectsof these-parameters idu en ce

the kinetics of the conversion and perhaps determine

whether the conversion is controlled by adsorption,

desorption, diffusion,

or

the surface

reaction.

Detailed

studies of

the

kinetics of the heterogeneous catalytic

para-orthohydrogen conversion a t low temperatures on

zinc oxide, aa-diphenyl-&picryl hydrazyl and a mixture

of the two were performed by Harrison and McDowell

25,26).

These studies show that the conversion follows

fimt-order kinetics since the expression

-upon the physically adsorbed hydrogen.

u

-

u - k t

I -

V O L 5 6

NO.

5 M A Y

1 9 6 4

23



heat transfer between the catalyst pellets and the walls of the chamber

purities in the hydrogen stream. They showed that

certain impurities such as methane, ethylene, a nd carbon

monoxide caused temporary poisoning which could be

remedied by reactivation, but others such as hydrogen

sulfide, butyl mercaptan, and chlorine caused permanent

poisoning.

It

is believed that both the permanent and

the temporary poisons initially reduced the activity by

merely decreasing the hydrogen adsorptive capacity,

but upon reactivation, the permanent poisons actually

react chemically with the catalyst and permanently

alter the catalytic sites while the temporary poisons do

not.

The importance of efficient heat transfer between the

catalyst and the walls of the catalyst chamber is illus-

trated in a paper by Weitzel (56). Since the para-

orthohydrogen conversion at low temperature is endo-

thermic, it is desirable to supply heat to the system so

that the driving force for conversion remains constant.

Otherwise, the catalyst temperature decreases as con-

version proceeds and the equilibrium parahydrogen con-

centration, as determined by the catalyst temperature,

approaches that of the feed. At extremely high con-

version rates, this heat transfer problem may influence

the conversion rates.

Weitzel (56) also presents some experimental da ta on

the effect of pressure on catalyst efficiency. These

data show that, over the pressure range of 20 to

400

p.s.i.g., there is a small but definite increase in the

conversion with increase in pressure. For example,

the rate of conversion of normal hydrogen (25 para)

to

48

para at

76' K.

was achieved with an ST P space

velocity

of

790

per min. at

20

p.s.i.g. and 1200 per min.

at 400 p.s.i.g. This effect has been attributed to the

increased residence time and/or increased physical

adsorption at higher pressures. These data are con-

sistent with the theory of Harrison and McDowell

25, 26)

which predicts that the conversion rate con-

stant will vary inversely with pressure under these condi-

tions. Even though the rate constant decreases with

increases in pressure, the conversion rate can actually

increase since it

is

the product of the concentration and

the rate constant. This same effect can be seen in the

work of Chapin and Johnston (73). They show that

the conversion rate constant decreases as the pressure

increases but further analysis of the data indicates that

the quantity of hydrogen converted per unit time (con-

version rate) actually increases as a function of pressure.

This effect may be better illustrated by an examination

AUTHORS George E. Schmauch is Manager of Contract Re-

search and Alan H . Singleton is Project Manager, both in the

Research and Development Department

of

Air Products and

Chemicals, Inc ., Allentown, P a. T he work reported in

this paper was sponsored by the Research and Technology

Division of thc U. S. Air Force, Wright-Patterson Air Force

Base, Ohio.

of the mathematical expression for a first order reaction

rate.

dc

- - kc

=

moles/liter/min.

dt

According to this expression, the reaction rate at any

instan t is equal to the product of the rate constant, which

is constant a t a definite temperature for a given reaction,

and the concentration of the reactant a t tha t instant (23).

Therefore, a n increase in pressure

of

the system decreases

the reaction rate constant but also increases the con-

centration of reactants per unit volume. Th e net

effect is an actual increase in reaction rate.

Two papers by Weitzel (57, 58) are concerned with

the ac tual kinetics of the flow conversion of ortho- and

parahydrogen. They follow the suggestions by Hougen

and Watson (28) and Yang and Hougen

(61)

for dealing

with the various mechanisms which may be encountered

in gaseous reactions catalyzed by solid surfaces in flow

reactors. In heterogeneous reactions, the mechanism

consists of several consecutive steps. For the ortho-para

conversion of hydrogen, these steps are:

Di usion of the hydrogen from the main stream to the surface

of

the catalyst.

Di usion fr om the surface into the pores

o

the catalyst.

Adsorption

o

the hydrogen on the catalyst surface.

Mag netic interaction resulting in the otherwise forbidden par a-

ortho transition.

Desorption of the hydrogen.

Dz usio nfro m the pores to the surface

o

the catalyst.

Diffusion from the surface to the main stream.

One of these steps will normally be slower than any of

the others and will therefore control the rate of conver-

sion. Weitzel (57) studied the effect of diffusion on the

rate of conversion by following a procedure suggested

by Corrigan (75) which consists of varying the linear

velocity while holding a constant space velocity or

contact time. Th e efficiency of diffusion between the

main gas stream and the catalyst surface is favored by a

high linear velocity because high velocity results in a

greater turbulence and a thinner laminar film around

the particles than is present at low linear velocity.

For a given space velocity, the linear velocity varies

directly with the length of the catalyst bed. Weitzel

(57) found that for catalyst chambers of the same

diameter but different length, there is no significant

difference in conversion rate for various linear velocities

a t constant space velocity. This is evidence that diffu-

sion is not rate-determining under the conditions studied.

However, for catalyst chambers of different diameter and

length, there is a significant difference in conversion

rate for various linear velocities at constant space

velocity. Th e smaller diameter catalyst chamber tube

(l/d-inch 0.d. as opposed to a/,-inch 0.d.) produced the

VOL. 5 6 NO.

5

M A Y

1 9 6 4

2 5



high-

conversion

rate. A RXMOM& explanation

is that the d e r ube

has

a higher heat transfer effi-

ciency so that

the

catalyst

is

more nearly an isothermal

dollvcrz~r. If the convm&n a nut isothcmal, the

equilibrium ortho-para ratio changes as the convertex

temperaturechanges and tends to reduce the conversion

interval

or

driving

force

of

theconversion.

The importance of pore diffusion

was

assessed by

developed by .We& 57,

58)

etics df the ov&-aU conversion

Tress ion . for the ortha-parahydqen

.

n e a v i s o t h d cxmditiom.,

. .

. ThisdeparihKefcomh e a i ~ ~ , e o u l c l

d t rom

a

rate

controlling unimolecillar

surface

reaction in which one

component is adsorbed more stm&y,.than the other.

1 ' .

98

M I D

U

ST R M A N.0

NO

kcEE

L 111N G CH EM I S T R Y

any decrease in k by increase in pressur, YI

GUUWZ-

acted by the increase in the concentration of c which

is

a

function of pressure. Thus, although the conversion

rate constant is a function of p-e the rate of

wn-

version isessentially independent

df

prassure.

Thc convemion of

normal hydrogen

to he equilibrium

f rma t -195O C. ovm y-FerOa, arFe& and a series

of iron

qick-zinc

&e mixtwrcs heated at several

a m p a a h l n a to

produce

varying

degrees

of

rmation to

iameinc~iteiareported~Gva~ellPLa2pdSeotttrU).

The m a l b show that -pFetOI is the most efficient of

thcaeaataly~ar da-Fe&is uext. Themixedcatalysts

€ d a w with moderate dv it ie s. From a study

uf

t h e

ma p d c

propaties of

these mamia4 it

is

concluded

that furomagnetic materials

are

more effective than

aatiterrotnagn@ticmaterials for inducing an +para

emwemion

and

am mon,&ective than para-

nraepaic materials. Since little

io

Locayn abotlt the

ffsctsof atolaagnct ism on catalpiq theaedart,Nggast

orthohgdrogcn

converaion ca fal ya Also, the wonk of

J d

nd

Veith

(30)

m d

Veith

52)

illuahate thar

as

e d

tic

field

appeared capable of increasiag

the activity of

fmomagnetic catalpa for

the conversion

of

para-

to orthohydrogen at ambient tempexaturn.

The e f k t of

a

magno& n the law temperature

conversion is not known and presents an area for futwe

inves t igah .

Several d e s ealing with the hydrogen

shift

have

a p e d &Iy recently. In a s of papers (9-

77).

Jiiuyarnm reportshis shldiesd othgel# and oxides

that

fiazpemagnetic aratlrials ata0 b@ 4 d m d

s

pa~i-

I

I

i

i .



Improved catalyst for ortho-parahydrogen conversion is available

at a higher temperature than the storage temperature,

it is desirable to utilize the refrigeration capabili ty of the

hydrogen.

In general, where relatively large quanti ties of hydro-

gen are required, it will be stored as a liquid with a

composition close to 100 para. If the hydrogen is

used in a refrigeration circuit which does not contain a

catalyst, the temperature-enthalpy path will follow the

para- curve of Figure

2.

However, if a catalyst is in-

cluded in the system, causing the temperature-enthalpy

path to follow the equilibrium curve of Figure 2 , a

significantly greater quantity of refrigeration czn be

provided by the hydrogen a t low temperature .

EXPERIMENTAL PROGRAM

Catalyst Properties

As pointed out in the literature review, the physical

approach utilizing heterogeneous catalysis appeared to

be the most promising approach to speeding up the con-

version reaction. I t was concluded on the basis of theo-

retical considerations that the following properties are im-

portant : high physical adsorptive capacity for hydrogen ;

high concentration of

a

para-magnetic component on the

surface of the catalyst; and a large magnetic moment

associated with the para-magnetic component. Th e re-

sulting catalyst which has been developed, AP AC HI -1 ~

is the successful combination of these properties.

Measurement of Catalyst Activi ty

Catalyst activity is measured in a flow process at

constant temperature. An equation describing the

conversion has been derived 74)and is shown below

:

Th e factor

k t F / ( l - C,

s a function of temperature and

pressure. Thus, if the quant ity converted,

F ( C i - Co),

is plotted against the log mean driving force,

c, - e,

a straight line with slope k t F / ( I - C,) is obtained. The

activity of the catalyst for hydrogen conversion is directly

proportional to this slope.

The experimental apparatus for measuring conversion

activity has been described in detail elsewhere

14,

39).

The basic approach involves measuring

C,

and

C

at

various flow rates while maintaining catalyst tempera-

ture and the hydrogen pressure constant.

Factors Affecting Catalyst Activity

In addition to the basic measurements of catalyst

activity, a number of measurements were made of

factors related to the mechanism of the reaction. Experi-

ments were made to determine the importance

of

the

following on catalyst activity : hydrogen dissociation ;

catalyst particle size; catalyst surface area; and con-

taminant nitrogen adsorption.

Hydrogen Dissociation

As

pointed out in the literature review, dissocia-

tion of a nonequilibrium mixture of hydrogen will

cause it to recombine in an equilibrium condition. I n

order to determine whether the APACHI-1 catalyst was

capable of dissociating hydrogen, the hydrogen-

deuter ium exchange reaction on the catalyst surface

was studied. If the catalyst was effective in dissociating

molecular hydrogen and molecular deuterium, the

products of the recombination would include hydrogen

deuteride according to the reaction shown below :

H z + D z a 2 H D

If the dissociation reaction were not catalyzed, no

hydrogen deuteride would be detected. Thus, the

results of this exchange reaction study were to indicate

whether the low-temperature conversion was effected

by the physical (magnetic) mechanism or a combination

of the physical and chemical (dissociation) mechanisms.

This study required an analytical system for the

detection and estimation of the components, hydrogen,

deuterium, and hydrogen deuteride. This was accom-

plished by using a gas-solid chromatographic technique

as suggested by Moore and Ward

(35).

This technique

permits the separation

of

hydrogen, deuterium, and

hydrogen deuteride on a specially treated alumina

column at liquid nitrogen temperature using helium as

a carrier gas.

A

thermal conductivity detector was

used to indicate the separated components.

Th e experiments were performed by flowing a mixture

of

80yoHz

and

20

Dz at a constant

flow

rate through a

catalyst chamber containing the APACHI catalyst at

various temperatures. These experiments indicated

that the exchange reaction was catalyzed by this im-

proved catalyst even at liquid nitrogen temperature.

Th e exchange reaction was observed at the following

temperatures,

- 320”

F.,

-297’

F., and 212” F. The

exchange experiments showed that the exchange reaction

exhibited a positive temperature coefficient-the exchange

rate increased as the temperature increased. This was

illustrated by experimental data Jvhich showed an

increase in the HD concentration in the effluent stream

as the temperature was increased while maintaining a

constant H2-Dz low rate. For example, a flow rate of

450 cc./min. of an

8 0 7 G

H z and

2070

Dz mix through a

28

I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y



0.49-gram sample of catalyst produced 3.3y0

HD

in the

effluent a t -320' F. and 29.5% H D in the effluent a t

212' F.

These experiments indicate that the low temperature

ortho-para conversion of hydrogen as effected by this

improved catalyst is the result of both a physical and a

chemical mechanism. However, the contr ibution of the

chemical mechanism to the over-all conversion activity

is considered to be quite small. This conclusion was

reached as a result of the ease with which the exchange

activity of the catalyst at cryogenic temperatures could

be poisoned. Exposure to trace quantities of moisture

as contaminant in the hydrogen gas completely inhibited

the ability of the catalyst to exchange hydrogen and

ortho-para conversion activity a t this temperature.

This suggests tha t a t -320' F., only a small fraction of

the active sites are capable of effecting conversion by

are converting as a result of magnetic interactions with

the molecular hydrogen.

Similar exchange studies were performed with a

hydrous ferric oxide gel catalyst and there was no in-

dication of hydrogen-deuterium exchange even a t 21 2'

F. This suggests that the iron gel catalysts are not

capable of dissociating molecular hydrogen at tempera-

tures up to 212' F. and that the ortho-parahydrogen

conversion effected by this material is strictly the result

of a physical (magnetic) mechanism. These conclusions

are compatible with the fact that the iron gel catalysts

are not effective conversion catalysts or normalizers at

ambient temperature while the APACHI catalysts are

effective normalizers.

deuterium at -320'

F.

but did not seriously alter the

hydrogen dissociation and that the majority of the sites

Particle Size

Range (Microns )

Effect

o f

Catalyst Particle Size on Conversion Activity

A study of the effect of catalyst particle size on the

conversion activity for the APACHI catalyst was per-

formed. Th e catalyst was separated into various mesh

size ranges by sieving.

A U. S .

Standard Sieve Series

was used to accomplish this sizing. Samples of each

size were activated by purging with high-purity, dry

nitrogen at 150' C. for two hours, and the conversion

activity was determined at -320'

F.

and 100 p.s.i.g.

Th e results are tabulated in Table I .

These data indicate that the activity increases as the

particle size decreases and this effect is attributed to

internal or pore diffusion limitations in the catalyst.

By reducing the particle size, the average pore length is

reduced and the diffusion characteristics are improved

until a point is reached beyond which no further im-

provement is observed. This particle size is considered

to be that which exhibits no limitation to conversion

activity as a result of diffusional restrictions. In effect,

the total surface is available for conversion, whereas in

larger particles, the interior surface cannot function

efficiently because of mass transfer limitations.

Activity

Relative to

Iron

Gel (70-

to

80-Mesh)

Effect of Surface Area

on

Conversion Activi ty

A special technique for increasing the surface area,

and therefore the activity of a catalyst, has been qualita-

tively examined. Th e effects of this special technique

are illustrated in Table I1 where the B.E.T. surface

area and the activity, relative to the standard iron gel,

are presented for both specially treated and untreated

samples for a given activation temperature. Th e special

samples ar e designated as

(-

2) while the control samples

are marked

(-

1).

TABLE

I.

EFFECT

O F

PARTICLE SIZE

ON

A C T I V I T Y

Mesh S i re

Range

25-30

30-50

50-70

70-80

7TO

to

59 3 . 2 5

59 to 297 3.58

297 to 210 4.11

210 to 177 4.08

yo ncrease

in Activ i ty

10

4-

5

-0

TABLE

I I .

EFFECT

OF

SURFACE AREA

ON

A C T I V I T Y

Catalyst

Sample No.

52-

1

52-2

19-A-

1

19-A-2

26- 1

26-2

26-

1

26-2

28- 1

28- 2

30-

1

30-2

Surface

Area,

m 2 / G m .

479

542

32

1

273

256

239

256

239

309

376

555

. . .

Activation

T emp . , C.

145

145

150

150

130

130

155

155

250

250

150

150

Relative

Activity

4 .60

5 . 7 3

1.90

1 .56

4 .59

5 .13

5 .60

6 . 5 3

4.44

5 . 2 7

2 .19

4 .26

yo

Change

in Activity

+ 2 4 . 6

- 1 7 . 9

1 1 .8

+ 1 6 . 6

+ i 8 . 7

+94 .5

This tabulation represents four samples of supported

catalysts with different concentrations of promoter and

one unsupported catalyst. The catalyst systems repre-

sented by numbers 52 and 28 show both an increase in

surface area and an increase in activity as a result of this

special treatment. Samples of number 26 exhibit an

increase in activity but a paradoxical decrease in surface

area. In the case of 19-A which was the unsupported

sample, the treatment resulted in a decrease in both

surface area and activity. Number 30 exhibited

a

very significant increase in activity as a result of the

special technique but the surface area of the treated

sample is not yet available for comparison with the

untreated sample.

Although the catalyst data resulting from special

treatment do not show a clear-cut correlation between

VOL.

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M A Y

1 9 6 4 9



uusian

surface area and activity, only the unsupported catalyst

(19-A) experienced a decrease in both. The four sup-

ported catalysts showed definite increases in activity

after

treatment, although the surface area did not

d e c t

this

gain in every case. It appears tha t

this

special preparation technique influences the pore size

distribution aswell as the surface area. Further exami-

nation of this procedure is expected to broaden our

understanding of

this

effect and ultimately provide

additional improvements in catalyst activity.

ERM of Nih0g.n Adsorption

Knowledge of the influence of adsorbed impurities

upon catalyst el€ectiveness

is

of theoretical and practical

interest. The ortho-parahydrogen conversion mecha-

nism in

heterogeneous catalysis involves the adsorption

of hydrogen. Any adsorbate which competes with

hydrogen may be expected to reduce the catalyst effec-

tiveness unless

this

competing adsorbate is also a catalyst.

The extent and character of the reduction due to adsorp-

tion of

an

impurity may illuminate the catalyst behavior.

Experiments were carried out in which hydrogen con-

taining trace quantities of nitrogen were passed through

a catalyst

bed.

Simultaneous measurements were

made of outlet hydrogen composition and nitrogen

concentration. The experimental method and the

etailed mults are reported in (33). Typical results

re

shown

in

Figure

4.

Outlet parahydrogen concen-

ation at constantflow ate isplotted as a function of the

quantity of hydrogen which

has

passed through

the

Plotted on the same graph

is

the outlet nitrogen

tion. The outlet parahydrogen concentration

from the initial equilibrium concentration

the catalyst is saturated with nitrogen, as ndicated

the sudden increase in nitrogen concentration in the

at six cubic feet of hydrogen. At this point,

is

achieved, and the performance of the

lyxt becomes cons tantat the reduced level of activity.

The relationship between the portion

of

the original

E.T. surface not covered with adsorbed

the

residual catalytic activity

is

presented

Figure 5. If the reduction

in

orbparahydrogen

I N D U S T R I A L AND ENGINEERING C H E M I S T R Y

conversion activity were diroctly related to the reduction

in

B.E.T.

surface due to nitrogen adsorption, the diagonal

straight line would represent the nitrogen adsorption

effect.

It

is seen that the falloff

in

activity is much

greater than such a linear effect.

A coverage of

10

of the catalyst with

nitmgen

reduces the activity to

about

3 y

of the original; a coverage of 25 reduces

the activity to 20 of the original;

a

coverage of 50

of the surface reduces the activity to about 10 of the

original. It may

also

be noted that the curve of Figure

5

empirically correlates the effect

of

adsorbed nitrogen

upon three different catalysts which vary greatly in

B.E.T. surface area and in intrinsic activity.

These experiments show that a relatively

small

portion

of the catalyst surface area

is

responsible

for

most of the

catalytic activity.

The “active sites” which bring about

most of the ortho-parahydrogen conversion are

apparently also the sites which preferentially adsorb

nitrogen. Such adsorbed nitrogen drastically reduces

the catalyst activity.

The line of Figure

5,

drawn

through the data points and extended, rep-ts the

viewpoint that the loss in catalytic activity is exponen-

tially

related

to the fraction of the catalyst surface which

is covered with adsorbed nitrogen.

It appears

that

the

loss in catalytic activity as a function of surface coverage

by nitrogen

is

similar to the decrease in differential

heats of adsorption as a function of coverage, both

phenomena being related to site energy distribution.

Further theoretical and experimental work s in progress

for the examination of these relationships.

PRESENT STATE

OF

THF ART

Through the experimental program herein described,

it has been p s i b l e to develop an understanding

of

some

or

the important variables of a hydrogen conversion

catalyst. Through proper attention to the significant

variables, a catalyst has been fabricated which is about

10

times more active than the

iron

gel catalyst on a

weight basis. Table

If1

shows the relative activities of

chromium oxide, iron gel, and APACHI catalyst as

determined in our laboratory.



r

TABLE

111. ORTHO-P4RA

CONVERSION

CATALYST

ACTIVITIES

APACHI 2.4

100 160

kon

ge l

0.27

IO0

160

Chromio on

alumina 0.054

IO0 160

Figure 6 shows the outlet concenhation of an iso

thermal

bed as

a function of .catalyst weight for

three

catalysts: APACHI, iron

gel,

and chromia on alumina.

,

. .

vc

literan&

reV&

has

provided a thorough

omenon and has indicated that heterogened.

is the most promising technique

?#he rate a t which ths conversion can be effec

t

C

t

=

static reaction

rate

w m m t

(fin.-')

= male

fraction

of

parahydmgen.

= residence

h e

in catalyst chambcr(min.)

hbseripti:

t

o

e, m = equilibrium d u e

i

f

=

condition

at'eatalyst

outkt

=

DFC+

at t ime t.

= accuringat time ecm

=

condition at catalyst

inlet

-

REFERENCES , .

(1)

Hark-

Dem , J . .Llk

Sr,

I r ,

2850

(1932).

(2)

Aui. Air h m ~

& Qb'mkds,

I=.. I h f N m e ,

cb

(3) Budct P.L..Wcitnl D.W

o d y ,

. W..

Pnr

8. Wr. f.,

1.

285

(4) Ilamhoc8cr,K.F.. Fktek , P.. N d . , 7,181 1929).

(5) Bomhodcr,K. .,H m d , P. E.w hm.

M,

113 (1929).

(6)

~ , K K . F . F u t . * A . W , D I Z l l (1931).

3

~ ~ ~ , R F . , F u t u . A . , T n v . ~ ~ S r . , U , U 2 . 5 6 1 ( 1 9 3 2 ) .

(8)

~ ~ , S . , E ~ ~ t , P . H . , T ~ , K . J . h . ~ ~ . , ~ ~ (

(9) BU~MDS,

A,

X&i& i X&, 1, No. 2, SO6 (1960).

(lo)

Iu.,.

No.

3 (IWO).

(11)

IM., 1. No. I ( 1960 ) :

(12)

C.pmn.P.C.,63.Ss=.~.Bn;d*r.~, ~(1995).

(13)

Chapin,

D. ,.a.. I*. J .

h. s*,

S,

2406

(1957).

(14)

CW

.

G..

Kuei;t.. I.F.,

196

(1954).

A

&hmawh, 0.E.," I n ~ t b a t i w

of

tbc

Panonbo hif t dH-

. IT

oducad .I=.,

Sum

&e+l applicationqof hydrogen.coryzsion

are

e a t e r e d k d

ortho

toparaconves8ion

f& the

purpose

d roducing

a product with high para concentration

in liquefaction plants. Military and Fc sp ac e applica-

:.,,

tions prescndy the

of

low-

etas which

iud

WOMEMCUTURE

.X

= fraction parahydmgcn

c

= gmerpl WncentratiOIl

ofrcacting+

E

= hydrogen

flow

ntc (g./min./g.

of catslyrt)

V O L

6 6 NO. 5

M A Y

1 9 6 4

31

schmauch & singleton (1964) - technical aspects of ortho-parahydrogen conversion

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