On some thermodynamic and kinetic aspects of desorption from zeolites under microwave irradiation

Thermochimicu Acta, 152 (1989) 43-51 Elsevier Science Publishers B.V., Amsterdam

ON SOME THERMODYNAMIC MICROWAVE

43 -

AND KINETIC

Printed

in The Netherlands

ASPECTS

OF DESORPTION

FROM ZEOLITES

UNDER

IRRADIATION

M. BENCRANAAl, 1Laboratoire de Marrakech

M. LALLEMANT,

M.H. SIMONOT-GRANGE

de Chimie Physique, (Maroc).

Universite

et G. BERTRAND2

Caddi Ayyad,

Facultk

des Sciences

2Laboratoire de Recherches sur la Rdactivit4 des Solides C.N.R.S U.A.23, FacultC des Sciences Mirande - B.P 138 - 21004 DIJON CEDEX (France)

SUMMARY

The desorption of 13x xeolite is studied under controlled pressure and temperature conditions (T = 42OC) in an apparatus combining microthermogravimetry with a microwave guide ; the experimental study leads to the kinetic and thermodynamic characteristics of the zeolite in the microwave field. The lack of any modifications to the kinetic regime due to the field and some experimental results (evolution of the diffusion coefficient with the microwave power) tends to indicate that the irradiation effect may be reduced in this case to calorifit effect.

INTRODUCTION The first oil-crisis investigation

lead. If this solar of problems. capture

of 1973 has produced

for the use of renewable

First,

it is necessary

it. Finally,

reserve,

it is necessary

of

in the

its use faces a lot

to set up the techniques intermittent

a vast effort

with the solar energy

energy makes up an enormous

this dilute, uncertainand

for storing

in all the world

energies,

which allow

to

energy and then to transform

to be able to restore

this energy

it

in

order to use it. For all these problems, cal compounds chemical tricity

chemists

"(ref. 1)" which

heat pumps "(refs. the enormous

Their applications household

efficiency

1) The formation adsorbent.

conventional difficult

proceedings

heating

scale. But some problems

chemi-

in so-called

present

on elec-

energy

at

or solid adsorption.

and energy

storage

need to be solved

on a

for obtai-

:

problem

heating which

0040-6031/S9/$03.50

for cooling,

or more precisely

which

present

which always

0 1989 Elsevier

the hydration

stage of chemical

of regeneration

operation

storage

cycles of liquid absorption

It's an essential

2) The problem

They propose

of being able to store and restore

are designed

or industrial

ning a better

to offer.

can store and carry solar energy

2-3)". These

advantage

will. They use thermodynamic

have solution

requires

Science

cycling.

is often performed

a lot of major

by pumping

drawbacks.

high temperatures.

Publishers

of an anhydrous

B.V.

or by

It's a long and

44

In both cases, the knowledge

* A basic or fundamental mechanismsin applied

the hydration

to a material

** Or applied

the regeneration

of

research

of these systems. For example, cycle effects, pressure

(specific

of microwave

as the power of the field or

gas.

field effect applied

to only one of these points

to a material

on the microwave

action,

new and fruitful means of investigation

lyophilisation,

dissipating

thawing

or dehydration.

energy everywhere,

Indeed

in volume

because

"(refs.

:the

regeneration.

during

it's a means

at present,

4-10)"

they

such as drying,

of carrying

and

or at interfaces.

The aim of this study is to supply the relevant lowingquestions

of the real working

the dehydration-hydration

the mass effects,

We will focus our attention constitute

field effect

effect or heating).

on key parameters

In this paper, we will devote our attention research

:

either

of reaction

of microwave

the delay time or other parameters

of adsorbent

requires

the knowledge

case or the research

during

study

of the transformation

study, for example,

elements

to answer

the fol-

:

(i) Is microwave

interaction

a simple conventional

heat

or is there a spe-

cific action of the field ? (ii) How do of microwave

the kinetics

of the regeneration

stage evolve

in the presence

power ?

(iii) What are the working vi) of an affinity

parameters

thermal machine

using

(Pressure

P, temperature

13x zeolites

as adsorbent

T and power ?

EXPERIMENTAL The apparatus

is composed

of three essential

parts

:

. The microwave guide . The thermobalance . The reactor. Microwave The microwave at a frequency

part (Fig. la) is made up of a wave generator

of 2.450 MHz and protected

ted power is diverted

towards

by a wave circulator

an adapted water

plies a rectangular

wave guide

wave is propagated.

The transmitted

Cl), operating (2). The reflec-

load (3). The generator

(4) (type RG 112/U) inside which power is absorbed

sup-

a progressive

by a second water

load

(5). Thermobalance The coupling

of both methods-microthermogravimetry

carried out in laboratory sure and temperature,

allows

to record continuously

the variations

and microwaves-(Fig. under controlled

of mass of irradiated

samples

using

la) pres-

45

Microoven

Wave

load

PS 50

w 4

i

Vacuum

Fig. la. Experimental a microbalance sensitivity

(6) of zero SETARAM

used through

its possibilities It is positioned

tor by a metallic

circuit.

type PS 50. It's a dissymetric

This apparatus

g-

mass

scoop

: microwave

Apparatus

is l/SO

sample.

or pressure

of maximum

of electromagnetic under

balance,

its

load 50 g is currently

taring

if we often vary the

the wave guide and connected

tube (7) in order to avoid overpressures

with

the reac-

at the level of the

(8).

Reactor The reactor mities

(Fig. lb) consists

by two teflon-plates

by means

(9) which

of a furnace ribbon

piloted

hole at the centre of its larger is made up of a quarts-stick beam branches

and allows

ness of the reactor The reactor

are 4 mm thick,

to position

from a heating to vacuum

tight crossing mounted in a cylindrical

its temperature

diameter the sample

by toric joints

and supported

(10). of a cylindrical

element with cold extremities

and to pressure

tube. This microoven

pass. This one by one of the

in a wave guide axle. The tight-

(11) consisting

is provided

on a brass block, which

is fixed

The guide has a

faces to let the sample-stand

of 2 mm

is provided

closed at its extre-

in power by a rheostat.

is topped by a microoven

ling (12) made The tightness

of a part of wave guide,

by the utilization

slides

is necessary

spoo-

type TRRRMOCOAX.

via

of

toric joints

for the previous

activa-

tion of 13X seolites. Procedure After activation cuum, conditions

of 13X seolite

during

point, a water vapour temperature

in temperature

24 hours, we rehydrate

pressure

corresponding

(T = 42°C). From this hydration

T = 35O'C and pressure

it by imposing

to the saturation

through

the cold

of seolite

state, the dehydration

= va-

at

is achieved

46

Thermolok

Microoven @) (thermocoax withyold

(1

Teflon-p] .ate

t13X

Sample-standand SCOOD

Fig. lb. Exphrimental for a given pressure

zeolite

I

@-n

Apparatus

IB

Microbalance

llr

: microwave-thermogravimetry

in a 0.1 - 4.5 kPa range in accordance

power "i. It must be noted ted again, always

0b

that after each dehydration,

in the same conditions

PS 50

coupling. with incidental

the sample

is rehydra-

(T = 42OC, P = 5.6 kPa and xi = OW).

RESULTS The desorption

curves

The action of the microwave shown

by the thermopravimetric

field on the dehydration curves

__-

5oow

-.--

7oow

______-

3sou

--w .--0-9-w-

Fig. 2. Example

of loss-mass

of 13X zeolite

(Fig. 2) representing

curves m versus

is

the evolution

190w 1oow ow

time, T = 42°C and variable

powers.

of the desorbed for different

water mass versus

1) At null field and whatever parabolic

shape. In the presence

of the shape of curves which (i) First,

time, at a constant

powers "i. From these curves,

an initial

the pressure, of microwave

then presents

pressure

observations

can be made.

the m(t)

two parts

and

curves have a quasian evolution

:

to a loss of mass with a

since the power is high. This first part lines up

expression

tl=KA

water vapour

field, we observe

fast rise, corresponding

speed all the more important with the following

different

: (Fig. 3a)

"(ref.

11)"

A

500

0 300 l 190 v 100

P= 3.16kPa

Fig. 3a. Linear time).

transform

of experimantal

ni conditions.

This one lines up with the following = K' + K"t

(Fig. 3b)

0

to a continuously

a stage in the chosen pressure

Ln(l-o)

30

: CL=K&

curves m(t)

(ii) The second part of the curves corresponds speed until reaching

7 0

P, temperature expression

"(ref.

law (short decreasing T and power

:

11)"

Time (pin)

Fig. 3b.Linear transform law (long time).

of experimental

curves m(t)

_ ln(l-cC) = K' + K"t

48 Then, we can notice terpreted

in this case, that the curves

obtained

in field are in-

with the same laws as those in null field. There are no alterations

of kinetic 2) For

system through

the application

higher

(ri > 500 W) and in particular

sure values,

powers

other curves are obtained

ter a first part which

for the weaker

pres-

as you can see in Fig. 2 (curve B). Af-

a stage which

than that previously

at slight pressure

field.

seems to be the same, the loss of mass reaches

mum, and then goes through tly smaller

of an electric

hoped

represents

for the given power, while

and high powers are obtained

a maxi-

a rate of hydration

a microwave

plasma

sligh-

such curves is produced

in the reactor. 3) The general sent~amaximum

shape of the curves, with continuously

speed of desorption

ches a decomposition

step cm,). We notice

(Vi = 500 W), the curves v.

The desorption

equilibrium

(“i)p

reaof

and me ('ITi)are linear.

under field

of the field over the desorption

, which represents

figure 4

slope, pre-

and it always

that as far as the limit value

the experiment

The influence

decreasing

(vo) at the time origin

the network

equilibrium

of the isotherm

is shown

in

curves M = f(P)T obtai1

0.2 M(%) :F

30

0.85

1.5 2.15 2.8 3.45 T, 42’C Z = anhydrous zeoiite mass -58 gm

4.1 P(kR

25 20 15 10

5 C Fig. 4. Network curves.

graph

"(refs.

and values

. It

isotherms

12-13)".

of the power

is experimentally

for Pl and nil values

20

15

of adsorption

ned by calculation pressures

10

5

0

30

"(refs.

The compositions

(isopower

checked

of pressure

oalc*

25

curves)

11-12)" and isopower

obtained

with various

can be plotted

that the equilibrium

and microwave

315

power

on the same

desorption

obtained

(point A in the figure 4)

49

is the same as the equilibrium rature T defined which

therm

obtained

by the junction goes through

The desorption

under

obtained

in field appears of the steady

of the latter. The quantity

the application

depends

isopower

Pl at a tempe-

curve and the iso-

point A.

equilibrium

ting of the zeolite by the field, because

equilibrium

the same pressure

of the considered

of adsorbed

on the power of the field through

as a result

of the hea-

temperature

obtained

water

by

at the desorption

the material

temperature

imposed by the field.

Evolution

of the diffusion

The appropriate diffusion

coefficients

transformation

coefficient

tion of the microwave

the power of the field

f(t) (Fig. 3) enable

a=

Their evolution

in the zeolite. power

with

in (figure 5a). This figure

l o.lEkPa

to reach the water

is represented invites

as a func-

two remarks

:

@

ro.WkPa o j.16kPo

Fig. 5a. Variation coefficients. (i) We notice

as function

of the incident

that De coefficient

in near conditions

which

to the thermodynamic

power of

: the diffusion

characterizes

equilibrium

the diffusion

varies

of water

little with

the power

of the field. (ii) We observe water

that Do, the coefficient

in conditions

field, at constant diffusion

far removed pressure.

coefficient

with

characterizing

the diffusion

from this same equilibrium,

increases

This result can lead to infer an evolution

the field and the opportunity

of a specific

of

with

the

of the action

of the field on the material. Infact,

the comparison

ned for the temperature

between

the curves of the figure Sa, and those obtai-

(Fig. 5b) shows that both offer

ding to three domains of power. (iii) A first part I, isatvi

< 30 W power

in which

three parts corresponthe effect

of the field

is negligible. (iv)

For 30 < vi < 100 (part II), the temperature

tion depending

on the power,

the slope depends

follows

on the pressure.

a linear varia-

50

0b

v 0.18kPa o 0.86kPa 0 1.93kPa A 3.16kPa m 4.24kPa

vi I

200

100

300

400

Fig. 5b. Variation as function of the incident ture at the desorption equilibrium. If "i > 100 W, the zeolite

(v)

power, but the slope keeps

“O

temperature

independent

(wa$

: the material

power of

varies

of the pressure

tempera-

proportionally

to the

(part III).

INTFXPRETATION These results incident

can be interpreted

power is divided

by taking into account

(i) The power used for the heating se of the temperature

(iii) Finally,

of the zeolite which

which

of the zeolite. We notice

1 to 1000 ratio in accordance Thus, at a given pressure

Then, if we consider

1) At low power

maximum

this division curves

speeds

Of

is

surfaces.

a steady state of the irra-

leading

by the heating

of the incident

to a possible

cooling

term.

power, we will understand

(Fig. 5). the desorption,

the total power applied

increase with

the temperaof the reactor.

on the material

the power of the field

: speed

of energy and not by the pressure.

(part III), we notice

that the seolite

is then no longer the factor limiting

speed of desorption

consequence

the latter in a

the same as the temperature

uses nearly

by the bringing

2) At high powers of energy

if the processes

are compensated

is practically

phenomenon

Initial and instantaneous

bringing

power,

(part I of the curves) during

ture of the seolite The desorption

is regulated

and microwave

of the obtained

with

with that of the sample-guide

can be reached

down, chiefly desorption,

is a

that this power

below the incident power and adopts a linear variation

the variation

an increa-

for the losses after a heat transfer.

the power used for the desorption

of the temperature

diated material

justifies

of this material.

(ii) The power used to compensate

decrease

the fact that the

into three parts,

is then fixed by a chemical

gets heated.

the desorption restriction

The ; the

imposed

by

51

local conditions

of temperature

and pressure.

which has not been used by the desorption material

and then possibly

3) At intermediate

to initiate

powers,

the same participation

kinetics

The surplus

process,

secondary

energy,

is first used to heat the phenomena

of desorption

in the global desorption

of microwave

(Plasma).

and thermic

exchanges

have

process.

CONCLUSION From an experimental guide, we have carried

apparatus,

field, through a preliminary

activation

tigation

has allowed

pointing

out the characteristics

cerned

explainedby

the thermic

acting essentially

evolution

of the material

experimental

some precisions

of the desorption

could be used in parallel

The experimental

invesway,

in field. In the con-

are mostly

under a microwave

field,

energy.

study which

concerning

of 13X zeolite

connection

obtained

depends

thermodynamic

on pressure and kinetic

in field. This information

with that already

published

and used favou-

rably in the elaboration/implementation

of a microwave

mable

in a machine producing cooling

to the stage of seolite

an adsorbate-adsorbent

in a

the study does not seem to take out any spe-

with the supply of calorific

and power has provided

with a wave desorption

in a reproducible

of the field. The results

In this case, the systematic

characteristics

curves

of such transformation

domain,

mechanisms

the 13X seolite

of the zeolite.

to draw the progression

field and pressure

cific interaction

linking microthermogravimetry

out a study concerning

regeneration

applicator,

conforfrom

cycle.

REFERENCES 1 Proceeding from the "International on thermochemical Energy Storage", Stockholm, January 7-8, 1980, edited by G. WETTEMARX (1980). 2 E.A.Brumberg, Tepidus Storage System, German-Swedish workshop on thermochemical energy storage and absorption heat technology, Stockholm, 1980. 3 H. BjurstrGm and W. Raldow, Energy Res., 1981, 5, p. 43. 4 F. Henry and M. Devillard, Application des microondes aux mesures precises d'humiditk, Communication Journee de 1'Association Fran$aise des Thermiciens, Paris, Avril 1984. 5 P. Bailleul, Circulation de l'eau dans les milieux : application des microondes, Rapport interne 1984, Centre d'Etudes et de Recherches de 1'Industrie des liants Hydrauliques. 6 A.J. Berteaud, P. Leprince et A. Priou, Rapport de la mission au Japon de repr&entant du Club Microondes E.D.F., Avril 1985. 7 A. Germain, Sechage des encres heliogravures par microondes, Diplome d'IngCnieur C.N.A.M., Paris 1983. 8 P.E. Huble, Chem. Eng. 1982, 4, p. 125. 9 L. Quemeneur, J. Choisnet et B. Raveau, Mater. Chem. Phys., 1983, 8, p. 293. 10 H. Julien et H. Valot, Polymer, 1985, 26, p. 506. 11 R.M. Barrer, J. Cem. Sot., 1948, 49, p. 133. 12 F. Belhamidi-Elhannoui, These de 3&me cycle, Dijon, 1982. 13 M.H. Simonot-Grange, F. Belhamidi-Elhannoui, Thermochim. Acta., 1984, 77, p.311.