Removal of dilute sulfur dioxide by aqueous slurries of magnesium hydroxide particles

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, Englewood CldTs, New York 1965

[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

Shorter I

I

I

OH- is instantaneous The ordinate, 4/& represents the enhancement due to the presence of suspended particles of magnesium hydroxide in the liquid phase For the sake of corn parison, the theoretical prediction according to Model II[l] with different correspondence of the parameter N to solid concentration w is presented by solid lines 1, 2 and 3 Lmes 1, 2 and 3 are descrtbed using the correspondence that the parameter N is 2 35, 1 20 and 1 30, respectively for 10 wt% slurry The correspondence with line 1 was determined by fitting the experimental data lying in higher solid concentrations Line 2 was taken from the previous work[2] Line 3 is drawn by fitting the data after a correction of the extra reaction of SO2 with SO,*-, which is discussed below in detail All experimental points fall above the theoretIcal lines Especially in low solid concentrations remarkable enhancement is observed In the system of SOr Ca(OH)= slurry, such a lngh enhancement was not obtained and the experimental data could be satisfactorily predicted according to Model IT It is not believed that observed enhancement in SOrMg(OH)* slurry is lust caused by the presence of fine solid particles in the liquid film as well as the bulk liquid Then, the causes for high enhancement will be investigated in what follows In the present experiment, dilute sulfur dioxide is absorbed in a batch-type stirred tank absorber with a plain gas-liquid interface The Ion SO,‘- to be produced by the reaction between SO, and OH- IS accumulated as the process proceeds The solutnhty of MgSO, in water is much higher than those of Mg(OH),, Ca(OH), and CaSO, The species MgSO, formed exlsts in a dissolved state As the ion SO,‘- or the species MgSO, LS accumulated in the liquid phase, dissolved SO, is consumed by the reaction with SO,*- as well as OH- in the schemes of

I

0

W

0

:/

0

a

5

0

JO

&I I

I

100

50

200

“> Rg

2

I

20 I

500

rPm

Liqmd side mass transfer coefficient of nitrous oxide magnesium hydroxide slurry in water at 25°C

973

Commumcatlons

into

line with a slope of 0 8, irrespective of slurry concentration This shows close agreement with the previously reported results for such type of agitated absorption vessel The physical mass transfer coefficient for sulfpr dioxide into chfferent slurry concentration solution was evaluated by the following correlation k ,A” = k,“(D,/D,,,Y’3

SO, + 20HThe gas-side mass transfer coefficient can be evaluated by the empirical correlation presented by Hikita et al [5] This correlation has been obtained by a similar type of apparatus which is used m the present work For the present experimental condltions, the gas-side mass transfer coefficient was calculated to be 7 0 X 10e4 molelsec cm’ atm This value suggests that for the present system studied, the gas-side resistance can be neglected (less thdn 10%) in comparison with the total resistance To compare the experimental results with theoretical predictions, the value of r, 4, M and N should be known For the evaluation of r and 4, the physical solutnlities and dtiusivities for nitrous oxide and sulfur dioxide were taken the same as in our previous work[l, 21 The value of k,” at 175 rpm was evaluated to be 191 x 10m3cm/sec The solubility of magnesium hydroxide into water was taken to be 0 00046 mole/l[6] The reaction between sulfur dioxide and OH- was assumed to be instantaneous Figure 3 gives the variation of 4/&, with w as open circles for dilute sulfur dioxide absorption into a slurry of magnesium hydroxide The value of & is evaluated from the equation &, = 1 + l/rq assuming that the reaction of sulfur dioxide wrth 24

(

1

I

I

Fig 3 Experimental enhancement factor as a function of solid concentration for dilute sulfur dioxide absorption into slurries of magnesium hydroxide

The ion HSO,-

= SO,*- + H,O

(I)

SO, + SO,*- + H,O = 2HSO,-

(2)

further HSO,-

reacts + OH-

with OH-

to form

= SO,*- + H,O

SO,*-

as (3)

In the process of sulfur dioxide absorption in alkahne solutions with no suspended particles, HSO,- cannot coexist with OH-, so that reaction (1) never tikes place directly The above consideration shows that reactions (2) and (3) take place at two differently located planes in two reaction planes model Whereas, in a slurry process to be considered here, both dissolved SOz and HSO,- to be produced by reaction (2) can react with OH- which is fed by the dissolution of solid particles in the liquid film, so that dissolved SO, IS consumed according to reactions (I) and (2) simultaneously In view of the above consideration=+, observed enhancement in Fig 3 is considered to be attributed to the presence of solid particles (reactant of reactions 1 and 3) in the liquid film and the extra reactIon of SO, with S032- Though the concentration of S03*- in the hqmd phase is not exactly constant, but increases as the absorption process proceeds, the liquid phase IS assumed to be regarded as a pseudo-steady state in high solid concentrahons The enhancement caused by the reaction between SO1 and SO,‘formed was assessed by extrapolatmg the observed enhancement for high solid concentrations to that for a saturated solution in The solid dissolution promotes reaction Fig 3, that 1s (+/&),+, (2) through reaction (3) as well as reachon (1) So the enhancement due to the solid dissolution may be expressed by (d/40) - K4/&),4 - l] The values of the quantity obtained thus are plotted as full circles in the same figure After such corrections, experimental data fall on the theoretical prediction according to Model 11[ l] The parameter N for 10 wt % slurry is estimated to be 1 30 and is close to the previous value, 1 20[2] In the system of SO,Ca(OH), slurry the concentration of SOs2- to be produced by the reaction of SOz wdh OH- is extremely low, because the solubihty of CaSO, in water IS about 25 times as low as that of Ca(OH), Consequently, the reactIon between drssolved SOz and S03*- can be neglected, so that the experimental results in the previous work [l, 21 are compatible with the Model II

Shorter

974

Commumcatlons

CONCLUSION

The absorption of ddute sulfur dloxlde mto aqueous slurries contammg fine suspended particles of magnesmm hydroxide was performed usmg a shrred tank absorber with plane gas-llquld Interface Experlmental data on sulfur dioxide absorption rates were compared with the theoretical predIctIons accordmg to the proposed model m a previous paper[l] The devlatlon between measured and theoretical absorption rates, which was not observed III a previous system, sulfur dloxlde-lime slurry[l], may be due to higher solublhty of the reacuon product, magnesium sulfite, m water than that of the correspondmg calcium salt Department of Chemical Nagoya Unrverslty Nagoya, Japan Tsuyakm Kogyo Co, Brsar, Axhi, Japan

Engmeermg H

E SADA KUMAZAWA M A BUTT T

Ltd

distance thickness

into hquid phase from gas-liquid of hquld film for gas absorption,

Greek symbols Y stolchlometrlc coefficient p density of solid parncles, factor Q enhancement

Superscnpt 0 without

chemical

22 June

28 February

d(u,CrC,,T) dz

=

dC”k (_->4

InsIde

the particles d2C,, 2 d&r 2fzg==7i;; dt

A

d2C, L 2dC,k_ 7 +sdZd’C,

--T+--dZ

L

2dC,,_ d&

--

f

(7) P* -zrH

A

DH

rH--

(8) rcD

(9)

JrcdMD

ac, at = rcB + rcD

(10)

where n,

-aaDD

- [arh,(T

reactlon

1977)

(1)

dW%)

(N = 0)

REFERENCES

1976, accepted

In a recent paper Dumez and Froment [ 1] reported on the kmetlcs of the dehydrogenatlon of I-butene mto butadrene on a chromlaJumma catalyst Theu study also tncluded the kmetlcs of cokmg from both I-butene and butadlene and the influence of particle size on the rates of reaction Further, they simulated the pseudo-steady state behavior of an mdustnal adlabatrc reactor sublect to cokmg by means of the followmg system of dlfferentlal equations For the fluid phase

dr=

particles

of mtrapartwle mass transfer bmitations in reactor design. A simplified approach (Recewed

_arDzr

cm

[l] Sada E , Kumazawa H and Butt M A , Chem Engng Scr , under pubhcatlon [2] Sada E , Kumazawa H and Butt M A , Chem Engng Sci , under pubhcatlon [3] Ramachandran P A and Sharma M M , Chem Engng Scr I%9 24 1681 [4] Uchlda S , Kolde K and Shrndo M , Chem Engng Scr 1975 30644 [5] Hlklta H , Asal S , Ishlkawa H and Salto Y , Chem Engng Scr 1975 30 607 [6] Seldell A and Lmke W F, Solubrhtles of Inorgamc and Metal Organic Compounds American Chemical Society, Washmgton D C (1965)

NOTATION

d(u,C,) -= dz

interface, cm

g/cm’

Subscripts A absorbed gas A (SO,) B dissolved solid species B I gas-hqmd interface 0 m the absence of suspended

SUM1

surface area of sohd particles, 6wlpd,, cm2/cm3-dlsperslon concentration m liquid phase, mole/l diffusivity in liquid phase, cm’/sec hquld-side mass transfer coefficlent, cmlsec mass transfer coefficient for sohd dlssolutlon, cm/set k.A~z,21D, JJC.4.lC,, D.JD, amount of sohd, g/cm3-dlsperslon dlmenslonless distance into hquld phase from gas-hquld interface, z/z,

The problem

L z,

(3)

- TiJ + a,h.(T-

r)l

(4)

with

&

For the sohd phase r, = r,,

+ a,(-AH)D,(*)Re

(5)

-I- r,,

= exp (-UC, d =

o” kc,apPe [l

W)

+ kcDp,’ as + 1 695d/@,)]’ ‘=

(13)

with QL=&

(6)

Most of the terms m these equations

are famlhar

The right hand