Gas absorption with catalytic reaction

Shorter Commurucations

970 Chemzcal Engmeenng Science

1977 Vol 32 pp 970-972

Pergamon Press

Pnnted I” Great Bntam

Gas absorption with catalytic reaction (Recewed

15 September

1976, accepted

In the previous work[l], the process of gas absorption with a single reaction into a slurry contaming fine particles was numerically analyzed and discussed m terms of the enhancement factor when the average diameter of the sohd particles was conslderably less than the tluckness of the hqmd film For that case the solid dissolution and the chemical reaction become parallel steps The sohd dlssolutlon and chenucal reactlon affect each other and enhance the rate of absorption For the case of gas absorption with catalytic reaction in a solution contammg fine suspended catalyst particles, analogous situations may be encountered That is, for this case, mterparticle dtiuslon m the hqmd film for gas absorption, mtraparticle dtiusion and chenucal reaction on active sites wlthm the catalyst particle become parallel steps The problem to be consldered here IS the process of the gas absorption accompanied by a smgle catalytic reaction m a solution contammg fine suspended catalyst particles whose average diameter IS conslderably less than the thickness of the hqtud film for the gas absorption I3lssolved gas dtiuses m a hqmd phase, simultaneously reactmg within fine catalyst particles Before undergoing the catalytic reaction, the dissolved gaseous species must dtiuse mto hqurd-filled pores of the catalyst particle Thus, the elementary steps of interparticle diffusion m a hquid film for gas absorption, mtrapartlcle ddTusion and chemical reaction are in parallel Then, conservation equations for gaseous species m the (bulk) hquld fihn and m the particles are D

30 - E)L R

d*C,

AT=

dc, dr I r_R

(1)

=k,c*” with the boundary

conditions

CA =

z = 0, z=z,,

c*,

(3)

c,=o

r = 0,

(4)

dc,ldr

r=R,

=0

CA = CA

(6)

Instead of solvmg non-linear chfferentlal eqn (2), introduction a catalytic effectiveness factor, El. reduces eqn (1) to = (1 -

D$$ Exactly

the effectiveness

l)LE,C,”

factor E,

(5)

of

(7)

1s moddied

of C_.,(r) except for the first-order factor cannot be analytIcally esapproximate solution has been

Thiele modulus

Equation (9) IS the same form as the effectiveness factor for the first-order reaction Equation (7) may be wntten in the dimensionless form d2Y, _ -M&,(Y,)YA” dx2

(11)

sublect to x = 0, x= 1,

Y* = 1 Y,=O

(12) (13)

The solution of eqn (11) with eqns (12) and (13) gwes the enhancement factor I$~ as 9=-~l._,=J(2M.I.‘E,(Y,)Y,“dY,+K) Here K following

(14)

is an integration constant, which 1s determmed

by the

equation

dK E,(Y,)Y,”

=l dY,+

K

05)

)

So the well-known relationshIp between enhancement factor N$ and reaction-diffusion modulus t/M0 depends on the order of chemrcai reachon n and the modtied Thlele modulus q/m, Figures 1 and 2 indicate typical examples of the relations between $ and I//M. for various values of the parameter, d/m, Both the figures are relevant to non&near kmetics such as n = 2 and 0 5 The curves for the relationship between ~5and VI&, are m parallel each other at large values of v’MO This unpbes that the value of the mtegration constant K m eqn (14) approaches zero,

that IS, the absorption sltuatpn approaches the fast-reaction reame Then, by designatmg M to be ti=2M,,

‘E,(Y,)Y,“dY, I0

(16)

and plottmg di$ agamst s$, the effect of modtied Tiuele modulus and order of reaction of the enhancement n hqurd film The relationship can be given by 4 = VMltanh d/M, which IS plotted by a broken curve Engrneenng

E SADA H KUMAZAWA M A BUTT

NOTATION

(9)

fi=&(ii&G-&) where qrn

197)

Department of Chemwal Nagoya University Nagoya, Japan

is defined by

E = 4~R2D,,(dcAldtL, I $rR’k,,C,n and hence E, is a function kmetlcs The effectiveness tunated, but an excellent proposed [21 as

5 January

defined by

concentration m liquid phase, mole/cm3 concentration m hqlud m catalyst pores, mole/cm3 molecular dlffusivity in liquid phase, cm’/sec effective dtiusivity m porous particles, cm’/sec catalytic effectiveness factor integmtlon constant rate reaction constant for nth order reactlon, (ems/mole)“-‘lsec generalized reaction-dtiuslon modulus defined by eqn (16) reaction-dlff uaon modulus, zLd/I( 1 - l )k,,C:; ‘/D,]

modified Thiele modulus defined by eqn (10) modified Thiele modulus, Rt/[(n f I)/2 k.Cz;‘/D,] order of reaction relative to absorbing

component

971

Shorter Commumcatlons

Fig 1 Relation of enhancement

factor to reactlon-dfiuslon modulus as a parameter of moddkd the second-order reactlon

Tluele modulus for

factor to reacUon-ddfusron modulus as a parameter of modtied the half-order reactlon

Tluele modulus for

n=05 50

-

20

-

B IO -

5-

2-

Fig 2 Relation of enhancement

Fig 3 Relatton of enhancement

factor to generahzed

reaction-dtiuslon

modulus

Shorter

972

R r x Y* z z,

radius of catalyst parhcles, cm radial coordmate of catalyst particles, cm dlmenslonless distance, z/z, dlmenslonless concentration m hqmd phase, C’JC,. distance rnto the liquid from the gas-hqmd mterface, Uuckness of hquld fihn, cm

Cornmumcatlons

Subscripts A absorbmg I

gas-hquld

component Interface

Clll

I(EFERENCEs Sada E , Kumazawa H and Butt M A, Chem Engng Sci , to be published [2] Petersen E E , Chemcal Reactton Analyszs, Prentrce Hall, CldTs, New York 1965 Englewood

[l]

Greek e

$

symbols

volde fraction enhancement

m suspended factor

phase

Removal of dilute sulfur dloxlde by aqueous slurries of magnesium hydroxide particles (Recerved 13 October 1976, accepted 22 February 1977) In recent years, the process analysis of chemical absorption mto a slurry has become Important m the ratlonal design and development of wet scrubbing processes for the removal of sulfur dloxlde from flue gases The elementary steps encountered in wet scrubbing by slurries are diffusion and reaction of gaseous specles and sohd dlssolutlon m hquld film[ i-41 From such a standpomt, m previous papers[l, 21, a film theory model was developed which describes the smgle and simultaneous chemical absorption by slurries of fine particles The model was favourably compared with the experimentally observed data for smgle and simultaneous absorption of sulfur dloxlde and carbon dloxlde mto aqueous slurries of hme usmg a stured tank absorber At present, most of wet scrubbing processes for the removal of sulfur dloxlde are by hme and hmestone slurry solutions, but It 1s expected that the usage of magnesmm hydroxide as suspended solid may yield high scrubbmg capacity as a result of the presence of more soluble reaction product, magnesium sulfi e, than the correspondmg calcmm salt Therefore, further work on the removal of sulfur dioxide by aqueous slurries of magnesmm hydroxide was undertaken using the stured tank absorber It 1s the purpose of this paper to present the absorption data for the removal of dilute sulfur dloxlde by magnesmm hydroxide slurries and to compare w&h the theoretical predIctIon accordmg to the previously proposed model[l] The experimentally observed deviation of absorption rates from the theoretrcal predlctions was Interpreted to be responsible for the higher solublhty of the reaction product, magnesium sulfite In water EXPJiXlMENTAL

All the experiments were carried out in a semi-batch stirred tank absorber with a plain gas-hqtud interface The absorber was batch-wise with respect to hquld phase A schematic drawing of the absorption vessel and stirrers IS shown m Rg 1 The ab sorption vessel was 80 mm dla with four symmetrically located baffles each one eighth of the vessel diameter The effective gas-hqmd contact area was SOcm* Two stirrers were used for the agitation of liquid phase and for gas phase These were made of PVC and were driven by two separate motors The hqtud phase sturer was fan turbine with eight flat blades and was placed at half of the liquid depth The gas phase stirrer was also fan turbme wjth four flat blades The absorption vessel was fixed m a water Jacket at a constant temperature The hquld phase was slurry solution of fine particles of magnesium hydroxide The slurry concentration was varied from saturated solution to 20% magnesmm hydroxide The volume of slurry solution in the absorption vessel was always 450 cm’ The hquld sturer speed was varied from 80 to 270rpm for the evaluation of physical mass transfer coefficient, whereas, m all chemical absorption runs, It was kept constant at 175 rpm

55

0

l-1 ==+

Fig

1 Agitated vessel and sturers (l), Gas inlet, (2), Gas outlet, (3), Gas phase stirrer, (4), Liquid phase bturer (5), BdWe

The gas phase was a mixture of sulfur dloxlde and mtrogen saturated with water vapour Before each experimental run, the empty absorption vessel was purged with dilute sulfur dioxide The gas flow rate was mamtamed 20cm3/sec The gas sturer speed was kept constant at 500rpm The gas phase composltlon was varied up to 5% sulfur dioxide The inlet and outlet gas streams composltlons were analyzed by gas chromatograph Porapak Q w;as used as a column packing in gas chromatograph The Porapak column length was 1 50 meter and temperature was 60°C The chemical absorption rates for sulfur dloxlde were determmed from the inlet and outlet gas composltions and the total gas flow rate In all the experlmental runs, measurements were made at atmospheric pressure and a constant temperature of 25°C RESULTS AND DISCUSSION

To express the chemical absorption results m more convenient form of enhancement factor (t/k,“), it required a knowledge of physical mass transfer coefficient (kL”) The physlcal mass transfer coefficient was determmed_from the measurements of rates of absorption of pure rutrous oxide into various concentration slurry solutlons The hquld sturer speed was varied from 80 to 270rpm Rates of absorption of nitrous oxide at atmospheric pressure into batch of slurry solution were measured by soap film meter Experimental results for the physlcal absorption of nitrous oxide were plotted m Fig 2 as k,” vs n with slurry concentration as a parameter A series of experlmental points fall on stratght