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Absorption removal of sulfur dioxide by falling water droplets in the presence of inert solid particles
Pergamon
Atmospheric Enmronment Vol 28, No 21, pp 3409-3415, 1994 Copyright © 1994Elsevier Science Ltd Pnuted m Great Bntam All rights reserved 1352-2310/94 $7 00 + 0 0 0
1 3 5 2 - 2 3 1 0 (94) 0 0 1 6 1 - 8
ABSORPTION REMOVAL OF SULFUR DIOXIDE BY FALLING WATER DROPLETS IN THE PRESENCE OF INERT SOLID PARTICLES I - H u N G L I U , * C H I N G - Y U A N CHANG,*~r S u - C H I N L I U , * I - C H E N G C H A N G * a n d S H I N - M I N SHIH~ *Graduate Institute of Environmental Engineering and ~Department of Chemical Engmeenng, National Talwan University, Taipel 106, Taiwan, R O.C
(First recewed 21 November 1993 and retinal form 14 Aprd 1994) A b s t r a c t - - A n experimental analysis of the absorption removal of sulfur &oxide by the free falling water droplets containing the inert solid particles is presented_ The wheat flour powder is introduced as the inert solid particles Tests with and without the flour powder m the water droplets are examined. The mass fluxes and mass transfer coefficients of SO 2 for the cases with and without the flour powder are compared to elucidate the effects of the inert solid particles contained in the water droplets on the gas absorption The results indicate a slgmficant difference between the two cases for the concentrations of the flour powder in the absorbent droplets (Cs) within the ranges of the experimental conditions, namely 0_1 to 10 wt% flour powder In the absorbent droplets_ In general, the inert solid particles of the flour powder as the impurities m the water droplets tend to decrease the SO 2 absorption rate for the experimental absorption system under lnvestlgahon Various values of C s cause various levels of the Interracial resistance and affect the gas absorption rate The mterfaoal resistance is recognized by l n t r o d u o n g an interracial mass transfer coefficient kSw~th its reoprocal being proportional to the magnitude of the tnterfacial resistance The values of 1/~ m a y be computed by the use of the equation 1/k~ = (1/KoL , -- I/KoL), where KOL, and KOL are the overall hquld-phase mass transfer coefficients with and without the inert solid particles, respectively. The values of k. with C s of 0 1 to 10 wt% are about 0.295-0 032 cm s - * for absorbing 1000-3000 ppmv SO 2 with the water droplets_ This kind o f i n f o r m a h o n is useful for the SO 2 removal and the formation of acid rain that the Impurities of the inert solid particles contaminate the water droplets
Key word index_ Sulfur dioxide removal, gaseous pollutant absorptmn, interracial resistance, solid particles, acid rain
NOMENCLATURE
DA(s) o r
Do DA or
A Ae
D of SO 2 m gas phase
species A or SO 2 DA(I), surface area of a droplet, 7zd2 Dp, DQ D of A( = 5 0 2 ) , P( = HSO~-), Q( = H +) in water coefficients as given in CTos=al(tc) b aDb Ot effective dlffuslvlty of P ( = HSO3) in water, as CA1, CA,~ hquld-phase concentrations of solute A at hquld given in equation (5) droplet surface In equilibrium with PAp PAjs,CAts d diameter of a droplet =CA, for C s = 0 dpj &ameter of parUcle in 3th interval concentrations of solute A in bulk Iiqmd for cases En CAo, CAos enhancement factor for gas absorption due to without and with solid particles, CAos= CAo for C s chemical reaction as given m equaUon (3) =0 F-r, FT~ as defined by equations (9), (11) actual hquld-phase concentration of solute A at He CAs Henry's law constant for bulk liquid (water), PA, hqmd droplet surface in presence of solid particles = HeCA, or PAls = HeCAI. drag coefficient Kc CD dissociation e q m h b r i u m constant for A ~ 2 P , K c weight percent concentration of inert solid parCs = 4Kc~ tlcles in absorbent liquid droplet Kc ~ dlssooatlon equdibrmm constant for hydrolysis CTo, CTos accumulated concentrahons of SO 2 absorbed in reacUon of SO2t~l or A ~ P + Q absorbent droplet for cases without and with solid Kc 2 dissociation e q u d l b n u m constant for &ssoclatlon particles, CTos= CTo for Cs = 0 reaction of HSO]Cro at tc of 0 478 s with Cs of zero (CTo)~r KOL, KOLs overall hqmd-phase mass transfer coefficients for D molecular dlffUSlVRy cases without and with solid particles, KOL~= KOL kc~~, kc~r t To w h o m correspondence should be addressed 3409
for C s = 0 rate constants of forward and reverse reactions of reversible hydrolysis reaction of SO2~l
3410
1-HUNG LIU et al
ke2f. kt2r rate constants of forward and reverse reactions of reversible dissociation reaction of HSO~ individual gas- and hqmd-phase mass transfer ko, kL coefficients /,, gas-liquid lnterfaclal mass transfer coefficient m mass of a droplet, rcd3pt/'6 N~ mass flux of species A transferred into hqmd droplet across interface N A with dissociation reaction for cases without NAr, and with solid particles, NA~, = NA~ for Cs = 0 NAr~ species P or HSO~P partial pressure of solute in bulk gas PAG PA,, PA,~ partial pressures of gas-phase solute at gas-hqmd interface ['or cases without and with solid particles, PA~,--PA, for Cs=O total pressure of gas phase PT Q species Q or H + weight fraction of particles in jth interval, also qj frequency of occurrence of particles m jth interval universal gas constant R RE relative error, standard deviauon divided by mean value of data Reynolds number, p~,ud, #c , Re G Schmidt number, #c,/Ip~,Dc,) SCG Sherwood number, kGRTd/D~, ShG failing distance of a droplet absolute temperature T falling time of a droplet t contact time of a droplet with SO z during absorption u falling velocity of a droplet v~ volume of a droplet, nd~/6 contact distance of a droplet with SO 2 z Greek letters
:to, ~t, fl #G, ,UL PG, PL a
coeflicmnts as given m (/,, %) - : t t C s ~j vlscosmes of gas, hquld droplet densities of gas, liquid droplet surface tension of liquid
Subscript
A c G, g l J L,I o r ref S, s T
dissolved gas component A or SO2 contact gas phase gas liqutd interface particle size interval hqmd phase bulk phase with reaction reference value solid particle or case m the presence of sohd particles total
INTRODUCTION The a b s o r p t i o n removal of sulfur dloxtde from the polluted atmospheres by the falhng water droplets is one of the t m p o r t a n t scavenging mechanlsrns in rain a n d the wet scrubbers (Altwtcker and C h a p m a n , 1981, Kohl and Rtesenfeld, 1985) W h e n the atr Is polluted with the oxides of sulfur and mtrogen, the rain fall produces the strong sulfuric and m t n c acids The precipttation that passes t h r o u g h such c o n t a m i n a t e d mr may become more acidic than n o r m a l The actd Is harmful to the aquattc wfldhfe because xt increases the acidity m the aquatic h a b t t a t s a n d accelerates the weathermg rates of the building matertals ( M o r a n et al., 1980) To reduce the f o r m a t t o n of actd ram, the
reduction oi SO 2 emission by sorne proper control technologms is of i m p o r t a n c e One of the possible m e t h o d s ts to use the hquld droplet a b s o r p t i o n to remove the SO 2 from the flue gas and the exhaust pipes (Flagan and Semfeld, 1988) The absorption of SO 2 by the liquid absorbents has been employed by industries for m a n y years (Danckwerts, 1970, A s t a n t a et al, 1983, Kohl and Rtesenfeld. 1985. Flagan and Semfeld. 1988) Some of the main advantages of the hqutd a b s o r p t i o n are the high treatment capactty, low pressure drop, low investment costs, and the posstbthty of h a n d h n g three-phase system (Pmtlla et u l . 1984) The processes of the a b s o r p t i o n of SO,. generally use either the chemical or the phystcal a b s o r b e n t s of high capacity However, due to the solubihty of SO2 m water being high enough to provide a satisfactory a b s o r p t i o n capacity, the use of water as an a b s o r b e n t has remained to be of considerable i m p o r t a n c e for the removal of SO 2 (Hiktta et a l , 1978, Altwmker and C h a p m a n , 1981. C h a n g and Rochelle, 1981. Kajt et a l , 1985) The a b s o r p t t o n m e c h a m s m of the gaseo-s SO2 with the hqutd water and the water droplets for an unc o n t a m i n a t e d gas hquld interface has been well k n o w n (Danckwerts, 1970, Hlklta et a l , 1978, A s t a n t a et a l , 1983, Kajl et a l . 1985) W h e n the a b s o r p t i o n has some particulate i m p u r m e s c o n t a m i n a t e d on Ihe gas hqutd mterface of the liqmd water or the water droplets, an mterfaclal resistance may be formed and affect the a b s o r p t i o n rate A q u a n t l t a h v e value to the interracial resistance ~s usually recogmzed b,, m t r o d u c l n g an mterfactal mass transfer coeffioent /,, ( G o o d n d g e and Bncknell, 1962) The value of 14,, is p r o p o r t i o n a l to the m a g n i t u d e of the interracial resistance The c o n t a m i n a t i o n situation may occur, for example, d u r m g the wet spray s c r u b b m g when both gaseous species and particulates are s~multaneously removed by the h q m d droplets F o r the physical absorptton system of COz'H_,O, the interracial resistance caused by the presence of the surface actwe agents has been noted by m a n y investigators (Cullen and Dawdson, 1956. G o o d n d g e and Brtcknell, 1962, G o o d r l d g e and Robb, 1965, Sada and Htmmelblau, 1967, R a l m o n d t and Toor, 1969. Nguyen Ly et a l , 1979) As for the chemical a b s o r p t i o n system of S O z / H 2 0 , few researchers did study the effect of the mterfaclal reststance of surfactant film on the rate of gas a b s o r p t m n into a qumscent llqutd with plane interface (Plevan and Q u m n . 1966, Nguyen Ly et a l , 1979) However, for SO2/HzO with droplet, the interfacml resistance caused by the presence of the particulate t m p u r m e s has not yet been paid m u c h attentton Thts study thus performs the experimental work of the a b s o r p t i o n of SO 2 into the falling water droplets to elucidate the effects of the mert sohd particles on the gas a b s o r p t i o n Recently, Holstvoogd et al (1986) studmd the gas absorption with reactmn m a slurry c o n t a l n m g the fine insoluble particles wlth a fimte r e a c t t o n / a d s o r p t t o n c a p a o t y They noted that. during absorptton, a grow-
Removal of sulfur dmxlde ing layer of slurry with the non-reacttve particles can be formed starting at the interface Neglecting this effect leads to an overestimation of the overall reaction rate in the film Quicker et al (1989) investigated the rate of the CO2 absorption in a stirred cell with the plane interface in the presence of the fine solids (actwated carboq, kieselguhr and alumina). They reported that, for t~e case of the activated carbon with an adsorbing cal~acity, the gas absorpuon rate increases in the linear adsorption region at low sohd Ioadmgs This is d~e to the adsorption of the dissolved CO2 by the activated carbon. But, at solid loadings above 10 vol%, the~gas absorption rate decreases for all three particulatt~ matters. The retarding effect of sohd parUcles on th/~ gas absorpuon rate may be due to the mterfacial resistance However, in thear studies (Holstvoogd et al., ~ 8 6 ; Quicker et al, 1989), the possible interfacml resistance has not been recognized and distingmshed from the overall mass transfer resistance Thus, it as also the aim of this work to reveal the anterfacml resistance caused by the anert sohd partacles. The accumulated concentration m liqmd, mass flux, and anterfacial resistance of the gas absorption of SO2 using the water droplets for the cases with and without the Impurities of the inert sohd particles of the flour powder are determined. EXPERIMENTAL
The system of sulfur dioxide absorption by the falling water droplets ts studied. The flour powder IS introduced as the inert solid particles to form some mterfacml resistance m the system_ An absorption column with the free falhng
3411
absorbent hquid droplets is used The system ~s schematically descnbed m Ftg 1 and dlustrated below About three runs have been performed for each expenment_ The data presented here are the average values of the results The average of the relative errors {RE, [square root of (sum of squares of dewatmns/no, of data)I/mean value} Is about 5% (1) Properties of flour powder The inert sohd powder used m this study is the wheat flour of low strength made by Llang-Yu Co The contents of C, H, N, O, S and CI of the flour powder are about 40 3, 6 82, 2.39, 49 41, 0.175 and <0 1%, respectively, as determined by the elemental analysis The parUcle density of about 1 48 gem -3 is determined by an Accupyc 1330 V1 02 Pycnometer The speafic surface area of 0 05 m 2 g- 1 is measured by a model 2100D surface-area pore-volume analyzer from Mlcrometncs Instrument Corporation according to the BET method The structure of the flour powder particles is not porous m nature. The solubdlty of the sohd flour powder m water is measured to be negligible_ Using the flour slurry at the same concentration as that m the usual expenments to adsorb the sulphurlc acid solution (200 mg dm- 3) for 2 h, one observes that the absorption of the sulfate ion is measured to be neghglble_ The wscoslty of the filtered hqmd absorbent from the flour slurry Is determined to be only about 2% higher than that of the pure water by a Haake Rv 20 Rotovisco analyzer All these charactenstlcs may thus support that the flour powder has httle adsorpUon capacity for SO, and has little effect on the inherent properties of the liqmd absorbent_ The measurement of the particle stze d~stnbutton of the flour powder Is performed by a Granulometer 715 F008 analyzer The data may be represented as the weight fraction (qj) vs the diameter (d~,,) of particles m the jth interval. For dv of 1, 1.5, 2, 3, 4, 6, JS, 12, 16, 24, 32, 48, 64, 96, 128 and 1(5'2/am, the corresponding values of qj are 1 2, 0.4, 20, 1.8, 2 1, 42, 3 6, 3.0, 7.0, 17.0, 14 1, 13.9, 5.9, 10.2, 10.0 and 3 6 wt%. The mass medmn s~ze ts about 30/am. Most partlcles have sizes m the range of about 20-50 pan
_
16
17
7
13
10
I1
Fig. 1 Schemattc dtagram ofexpenmental apparatus (1) absorption column; (2) recetver; (3) receiver stand, (4) needle, (5) flow meter, (6) slurry bottle, (7) metering pump, (8) stirrer; (9) filter, (10) compressor; (11) humidifier; (12) bubble meter, (13) NaOH soluUon bottle, (14) SO2 cylinder; (15) N 2 cyhnder, (16) mixing tank; (17) vent to fume hood
3412
I-HuNG Ltu et al
(2) Absorption removal of gaseous pollutant by hqmd and slurry droplets The gaseous pollutant of this system is slmulately produced by the mixture of SO 2 and air The SO 2 gas supplied from a cylinder (San-Fu C o , with purity of 99 99%) is controlled by a stainless steel flowmeter (Shm-Chuan C o , with m a x i m u m reading of 0 1 dm3/60 s) and precisely measured by a bubble gas meter The carrier gas of air produced from an air compressor is filtered by a filter (with filter paper of 0_45 #m) and controlled by a stainless steel flowmeter (Shin-Chuan C o , with m a x i m u m reading of 30 dma/60 s) which has been previously calibrated by a rotameter Before the carrier gas of air is fed to a mixing tank, it Is saturated with water vapor by passing through a humidifier to avoid the evaporation of the water droplets in the absorption column About 60 cma/60 s SO 2 and 20 dm3/60 s air are mixed Ill the mixing tank (with volume of about 18 d m a, retention time of about 54 s)_ By precisely adjusting the volumetric flow rate ratio of SO 2 and mr, a mixture of SO 2 and air with SUe concentration of 3000 (or 1000) ppmv is obtained The mixed gaseous pollutant at 5 dm3/60 s is sent to the absorption column The excess gas is bypassed through the waste gas absorption bottle filled with N a O H solution before it is vented to a fume hood The flour powder slurry Is prepared by the mixture of the flour powder and the distllled-deionlzed water (with Mllhpore M d h - Q system) The concentrations of the flour powder slurry (Cs) are 001, 0 1, 0 5, 2 5, 5, and 10 wt%_ A stirrer at 5 rpm is used to maintmn the uniform concentration of the flour powder slurry This stlrnng speed is appropriate to prevent the formation ofthe little air bubble in the slurry The flour powder slurry is passed through a stainless mesh of 70 mesh (ASTM E11-58T) to avoid the blockade of pipehne. It is then transported by a micro tube p u m p (EYELA, MP-2, Tokyo Rlkaklkal Co ) to the hypodermic needle (type 22G with diameter of 0 6 m m ID) in the absorption column By precisely adjusting the speed of the mlcropump, the slurry droplets with the constant volume and pumping frequency ((90_+2)/60s) are generated from the needle The droplet formation section Is purged with a small a m o u n t of nitrogen to prevent the absorption of SO 2 d u n n g the droplet formatlon The droplet diameter is determined by counting, collecting and weighing the droplets with the assumption that they are spheres The flour powder slurry droplets and the mixture of SO2/alr contact in an absorption column (plexiglass, 15 cm ID, 220cm length) with cocurrent flow type The droplet generator is composed of a glass tube, a rubber tube and a treated hypodermic needle_ The formation of the slurry droplets at the beginning is separated from the SO 2 gas mixture by an inner tube (4 cm ID) to prevent the Interference of the SO 2 gas flow Before the SO 2 gas mixture contacts with the slurry droplets, it passes through a gas distributor to equally distribute the SO 2 gas mixture The slurry droplets begin to absorb SO 2 at 4 5 cm below the needle tip The droplets are collected in a glass bottle receiver (4 cm ID, 15 cm length), which IS filled with 10 cm 3 water so as to ddute the SO 2 concentration of droplets, and placed on the top of a stand (PVC tube) Using the stand with different length, one can vary the contact distance of absorption from 19 to 144 cm, which corresponds to the contact time of absorption of about 0 125-0 479 s A kerosene forming a layer of 9 crn thick above the fresh water is added to the receiver before each absorption experiment so as to sMeld the collected slurry droplets from the SO 2 gas stream Test experiments have indmated that the kerosene layer is effective for the prevention of the undeswed absorption of the collected droplets with the SO 2 gas stream for at least 480s The collecting time of droplets in the receiver m each experiment is only 60 s which is very m u c h less than 480 s During the 60 s collecting time, about 90 q- 4 droplets are collected in 10cm 3 water in the receiver. The concentration of the
dissolved SO 2 in water is measured by using an ion chromatograph (Dlonex QIC, Model QIC-2) with the deduction of the eontnbutlon of the blank run without the absorption of SO 2 gas. It is then used to compute the mass flux. The enhancement or reduction effects of the SO2 absorption due to the presence of the inert solid particles can then be examined_ The temperature of the absorption experiments is at about 294-296 K
T H E O R Y A N D D A T A ANALYSIS
T h e a b s o r p t i o n o f SUE into w a t e r i n v o l v e s p h y s i c a l a b s o r p t i o n a n d h y d r o l y s i s r e a c t i o n s ( E r t k s e n , 1969) T h e e q m h b r m m at the g a s - h q m d interface is des c r i b e d by H e n r y ' s law. T h e rate c o n s t a n t s o f h y d r o l y sis r e a c t i o n o f S O 2 ( l ) (with K c l = kciJkctr) a n d d i s s o c i a t i o n r e a c t i o n o f H S O 3 (with K c 2 = kc2Jk~2r) h a v e been r e p o r t e d by p r e v i o u s i n v e s t i g a t o r s (for e x a m p l e , E t g e n et al, 1961, E r t k s e n , 1969, Y a t e s a n d Best, 1976, H i k i t a et al, 1978). k~l r (at 293 K) a n d k~l r (at 2 9 8 K ) are of 34x106s-1 and 2xl0 s (mol d m - 3)- 1 s 1, a n d k e n a n d kc2 r are of 104 s - i a n d 1011 (mol d m 3) - 1 s - J, respectively k~2r is so s m a l l as c o m p a r e d to k~l r t h a t the s e c o n d ~onlzatlon r e a c t i o n (with e q u i l i b r i u m c o n s t a n t Kc2) m a y be safely neglected ( M o r g a n , 1931). F o r the first i o n i z a t i o n r e a c t i o n (with e q u l h b r t u m c o n s t a n t K c i ) , the v a l u e s o f k ~ i f a n d k~l , are b o t h l a r g e t h a t Jt c a n be r e g a r d e d as a n i n s t a n t a n e o u s reversible r e a c t i o n H e n c e , t h e a b s o r p tion o f S O 2 i n t o w a t e r c a n be r e g a r d e d as the m a s s t r a n s f e r a c c o m p a n i e d by a n i n s t a n t a n e o u s reversible KC t
reaction
o f the f o r m
A~,-~- P + Q, w h e r e
Kc~ =
[P][Q]/[A]=[HSO3] [H*]/[SO2j In a d d m o n , d u e to t h e electrical n e u t r a h t y in t h e p u r e water, t h e c o n c e n t r a t i o n s o f H + a n d H S O 3 are e q u a l everyw h e r e m w a t e r T h u s , t h e a b s o r p t i o n o f S O 2 into w a t e r c a n also be s i m u l a t e d b y the r e a c t i o n o f t h e f o r m K¢
A~2P,
where Kc = [P]2/[A] = 4Key
In t h e case of a b s o r p t i o n with t h e p a r t i c u l a t e m a t t e r c o n t a m i n a t e d in t h e a b s o r b e n t h q u t d d r o p l e t s , a n interracial r e s i s t a n c e m a y be f o r m e d A q u a n t i t a t i v e v a l u e to d e s c r i b e the interracial r e s i s t a n c e m a y be r e c o g n i z e d by i n t r o d u c i n g a n interracial m a s s t r a n s f e r coefficient k, as ( G o o d r l d g e a n d B n c k n e l l , 1962; G o o d r l d g e a n d R o b b , 1965) N A = N~r~ =
]~G ( P A G
-- P~.,s) = ks (CAls -- C A j
= EnkL(Cxs - - CAos} = (1/Hek~ + 1 / E n k L + l / k 0 = KOL~(PAG/He -- CA J ,
~ (PAG/He -- CAGe)
for f i m t e k,
(1)
For the case with no mterfacml resistance, one has k~oz T h i s t h e n gives NA = NAt = kG (PAG -- PA. ) = EnkL(CA, -- C~o) = ( 1/Hek G + 1 ,'Enk L) i(PAG/He _ CAo ) = K o L ( P A G / H e - - CAo), for kc-* rr_,
(2)
Removal of sulfur dmxlde The enhancement factor due to chemical reaction (En) m equatmns (1) and (2) for the SOz/water system may be expressed as COlander, 1960; Kajl et al, 1985) En = 1 + [(De/DA(I))KcI/(CA, * o.s o s + C°~)]
(3)
K m = ['HSO3]['H+]/[SO2(I)] -- [ P ] [ Q ] / [ A j
(4)
D* = 2DpDQ/(Dp + DO)
(5)
where
The concentration of unreacted sulfur dioxide, CA°,, is related to the total or combined concentration of unreacted and reacted sulfur dioxide, Cro~, as CTo~= [SO,0)] + [HSO3 ] + [SO32 - ] = [-802(1) ] q*-[ H S O 3 ] -- CAos + (KclCAos)o 5.
(6)
F r o m equations (1) and (2), the interracial mass transfer coefficient k, is given as follows ks = ( 1 / K o g s _ 1 / K o L ) - 1 = H e ( I / K o G ~_ 1/KoG)- 1.
(7)
The value of 1/k, is proportional to the magnitude of the Interracial resistance A higher value of 1/k~ will result in a lower value of mass flux At any gas-hquid contact time tc, a mass balance over a spheric droplet gives the following equatmns for the concentration of the total dissolved SO 2 (CTo~) m the bulk phase of droplet: for the case with interfacial resistance, dCro, = (AdVp) N ^,, dt~ = (6/d) KOLs(PAG/He -- CAo,) dt¢
(8)
Fr, = f dCroJ(PAG/He -- C^,~) = (6fl0KoL . t~
(9)
for the case without interfacial resistance, dCro = (6/d) NA~ dto = (6/d) KOL ( P A J H e -- C^.) dt,
(10)
F T = [dCTo/(PAG/He -- CA. ) = (6/d) KOL t~. J
(11)
The slopes of plots of FT, vs t¢ and F r vs to can be used to compute the global values of Kot ~ and KOL, respectively. An alternative way to evaluate KoL, and KOL iS to apply the mass balance equations (8) and (10) of differential form. The global values of KOL. and KOL may be obtained from the slopes of plots of d C r ~ d t ~ vs ( P ^ J H e - - C A o , ) and dCr./dt ~ vs ( P ^ J H e - C A . ) , respecuvely A direct computation of d C r ~ / d t ~ and dCxo/dtc also gaves the local values of KOL0 and KOL at any t~ A comparison of the global and local values of KoL, or KOL gives an indication of the validity of the use of global values. SubstRuting the values of KOL,
3413
and KOL obtained into equation (7) then yields the value of k~ The equations describing the falling velocity u, time t and distance s of a droplet were gaven by Lappel and Shepherd (1940) For the values of drag coefficient Ca, the equations gaven by Berry and Pranger (1974) may be employed. Since the falhng droplet begins contacting with SO 2 gas at s = 4 5 cm ( t = 0 0 9 6 s ) in the experiments, the contact distance (z) and contact time (to) of gas and liquid droplet are: z = s - 4 5 c m , tc = t 0 . 0 9 6 s. Employing the above analysis, one can relate z to t c and convert the experimental data of CTos vs z to Cro s vs to. The data of Cro~ vs t c are then correlated by CTos = al(tc) b_ The individual gas-phase mass transfer coeffioent k6 may be obtained by the equation S h G = 2 + 0 552Sc~;/aRe ° 23 (Geankoplls, 1972). Once the values of KOL~,KOL, ks, and k C have been obtained, one can also compute the value of Enk L using equations (1) or (2) The value of CA. for the use In computing En is obtained by applying Henry's law, and equations (2) and (10). Equation (3) then yields the value of En, which cooperates with the obtained Enk L to give the individual liquid-phase physical mass transfer coefficient kL. The values of physical and chemical properties at 2 9 5 K used m this work are: D A ( , ) = 1 2 9 x I 0 -~ cm2s -1, DA(II= 1.63x 10-s cm2s-1, Dp = 1.299 x 10 -5 cm 2s-1 (p = HSO3), Do = 8.19x 10 -5 cm 2 s -1 (Q = H+), D* = 2 2 4 x 10 -5 cm2s -1 (from equation (5)), d = 0.281-0300cm, He = 72,773 K P a ( m o l c m - a ) -~ or 718.2atm ( m o l c m - 3 ) - I , Kc~ = 1.42 x 10- 2 mol d m - 3, p~ = 0.304 K P a or 0 003 atm, #~ = 1.822x 1 0 - 4 g (cms) - I , #L = 9.6X 1 0 - 3 g (cms) -1, PG = 1.199 X 10-3 gem -3, Pl = 0.996 -1.016 gcm -3, a = 72.4 dyne c m -
RESULTS AND DISCUSSION
The experimental data of the accumulated total concentration of the unreacted dissolved and reacted sulfur dioxide (Cros) absorbed into the slurry droplets at different contact distance (z) are presented,~n Fig. 2 as CTo. vs contact time (re). The contact distance has been related to the contact time according to the equations of the drop motion (Lappel and Shepherd, 1940) and of the drag coefficmnt (Berry and Pranger, 1974) The value of Cro~ increases with t c simply because more SO z has been absorbed. A higher value of the concentration of the inert solid particles in the absorbent droplets (C~) results in the lower values of Cro, as illustrated in Fig. 2. The presence of the inert solid particles in the absorbent hquid droplets reduces an mterfacial resistance for the SO 2 absorption. The interracial resistance becomes slgmficant as C~ reaches about 0.1 wt% and retards the absorption rate. This interfacial resistance caused by the inert solid particles increases with Cs within the ranges of the experimental c o n d m o n s (C~ of 0 1 to 10 wt%)
3414
I-HUNG LIU et al kL , (cm/s)
C TOS/(CTO)refh 12~ I
q
11! 1'
"
0.05
J 0.04
o~
~---e
_-~i
kL,HB 0 03
kL,HS
--
o.,
::1
0 02
i
1 001 i I
-
kt p
i
0
005
01
015
02
(a)
025
03
035
04
045
OS
0~ 0
055
005
01
015
02
025
Ic , (s) Fig
::[
~
:
~
S
o 0
OOS 0 1
-~°
,
015
02
025
03
045
05
OS5
0,35
04
0,45
05
o19t
mass transfer coefficient k L
-
_
_
osl
°if
055
0
t~, (s)
0
Fig 2. Variation of CT, with to under various C, (A, ~) C~ = 0 wt%; (B, A) 0_01%, (C, ,{~) 0 1%, (D, V) 0.5%, (E, O) 25%; (F, IS]) 5%, (G, o) 10% d = 0281 - 0 300 cm, PL = 0.996-1 016 gcm -3_ Temperature (To)=295+ 1K (CTo).f = C r o ~ at t¢ of0478s (z of 144 cm) with C, of zero. Symbols are experimental data Solid hnes across symbols are regression hnes (a) 3000ppmv SO2ts), (Cro).t~= 1551mmoldm -a (b) 1000 ppmv SO2(w), (CTo).n = 1 078 mmol dm 3
Figure 3 presents the values of k L of this analysis obtained by dividing Enk L by En along with those computed by the model equations of Hlgbie (1935), Handlos and Baron (1957) and Hsu and Shin (1993), respectively (equations (12)-(14)) These equations are kL, e = ( 4D A/rrtc) ° s
Liquid-phase p h y s i c a l
3
ks , (cm/s)
f,:
0
(b)
04
of experimental values of this work (with symbols), and theoretical results (with solid lines) of Hlgble (k L p), Handols and Baron (kL. rlB), and Hsu and Shlh (kL,]ts) Other notations are the same as in Fig 2
J
~
035
tc , (s)
Cros /(CTo),.,.
0~i
03
(12)
kL, na=OOO375u/(1 + #L//aG)
(13)
kL, HS=0 88[(8o/3~m) ° 5DA]° s.
(14)
Equation (12) of Hlgbie was developed for the absorption of a gas into a stdl hqmd during short periods of exposure. Equation (13) of Handols and Baron deals with a model assummg turbulent internal flow wtthm a drop. Equation (14) of Hsu and Shih is a semtempirical equation with a coefficient of correlation of 0 9 based on the surface stretch model for osciUatmg drops proposed by Angelo et al (1966). The model postulated that d u n n g drop oscallation the fresh surface area was formed and returned to the bulk, whtch
04
0
0
1
2
3
4
__ _ - - _ _ r 5 6
7
8
9
10
Cs, (wt.%) Fig 4 Plot of global k~vs C, Symbols: experimental, (~, _-,) 3000 and 1000 ppmv SO,(R), (. . . . . ) regression hne
was assumed to be well mixed. The values of k L of this work are between those of others employing various mixing models This indicates that the internal mixing extent m the hquid droplet of this study is between still and turbulent mixing condmons There may be some extent of the internal circulation m the pure liquid droplet (C, = 0), which has been noted by the previous mvesttgators (Kronig and Brink, 1950)_ F]gure 4 shows the variation of the global values of k~ with C~. An mcrease of Cs tends to retard the gas absorption rate as noted m the preceding discussion The retarding effect is vigorous for Cs vaned from about 001 to 2.5 wt% For Cs higher than 2.5 wt%, a further increase of C, only shghtly reduces the gas absorption rate The variaUon of ks with C s is not of the linear form. A correlation equation using (k~-0to) = ~tI Cs # seems satisfactorily fitting the results of this work. The values of,, 0, al, fl are obtained to be 0.041,
Removal of sulfur dioxide 0 062, and 1 207, respectively. The value of R 2 of the correlatton e q u a t i o n ts a b o u t 0.89 It ts n o t e d that the o b t a i n e d correlation e q u a t i o n of k s with C~ is based o n two SO2t~) c o n c e n t r a t i o n s (with PAc/PT of 1000 a n d 3000 ppmv) a n d one dropstze (with diameter d of a b o u t 0_281-0_300cm). The influence of P^o/PT is mainly o n En, which is found to decrease with P^o/PT, b u t not o n k~ (Sh C = 2 +0.552Sc~/3Re ° 53), kL (equations (12)-(14)), a n d k s Since Reo, u a n d m are dependent o n d, k C a n d k L would also be d e p e n d e n t on d according to Sh o = 2 + 0.552Sc~/S Reg 53 a n d equations (13) a n d (14), respectively. As to the role o f d o n E n a n d k~, further study is called for elucidating tts possible effects
CONCLUDING REMARKS The presence of the inert solid particles m the water droplets tends to reduce a n lnterfacial resistance, which is p r o p o r t i o n a l to the reciprocal of the mterfacml mass transfer coefficient ks, and reduce the gas a b s o r p t i o n rate_ In general, a h~gher value of the c o n c e n t r a h o n of the inert sohd particles m the water droplets (Cs) results in a higher interfacml reststance F o r Cs of 0.1 to 10 w t % , the values ofk~ of S O 2 / H 2 0 system are a b o u t 0.295-0.032 cm s - ~ It ~s r e c o m m e n ded t h a t the effects of m a j o r solid l m p u n t t e s o n the gas a b s o r p t i o n should be examined for the a b s o r p t i o n system u n d e r consideration Acknowledgemem--Thls study was supported by the National Science Council of the Republic of China on Talwan, under projects NSC 79-0421-E002-61Z and 80-0410-E00269_ The technical assistances provided by the Particulate Technology and the Chemical Engineering Laboratones of the Department of Chemical Engineering of National Talwan Umverslty are appreciated
REFERENCES
Altwlcker E R and Chapman E (1981) Mass transfer of sulfur dioxide into falling drops a comparison of experimental data with absorption models. Atmospheric Environment 15, 297-300 Angelo J B, Lightfoot E N and Howard D W. (1966) Generalization of the penetration theory for surface stretch: application to forming and oscillating drops A I C h E J 12, 751 Astarlta G., Savage D W_ and Bislo A (1983) Gas Treatmo wzth Chemzcal Solvents Wiley, New York Berry E X and Pranger M. R (1974) Equation for calculating the terminal velocities of water drops J appl_ Meteor 13, 108-111 Chang C S and Rochelle G T (1981) SO 2 absorphon into aqueous solutions_ A_I Ch E J 27, 292-297 Cullen E T and Davldson J F (1956) The effect of surface active agents on the rate of absorption of carbon dioxide by water Chem Enong Sct 6, 49-55 Danckwerts P V (1970) Gas-Llquld Reaction McGraw-Hill, New York
3415
Elgen M , Kustm K_ and Maass G (1961) Die Geschwindlgkelt tier Hydration yon SO2 in wassriger Losung. Z Phys Chem. Frankfurt 30, 130 Eriksen T E (1969) Diffusion studies in aqueous solutions of sulfur dioxide Chem Engng Scz 24, 273-278_ Flagan R C and Seinfeld J H (1988) Fundamentals of Air Pollution Engineering. Prentice-Hall, Englewood Chffs, NJ Geankophs C_ J (1972) Mass Transport Phenomena, p 291 Holt, Rinehart and Winston, New York Goodrldge F and Bricknell D. J_ (1962) Inteffaclal resistance in the carbon dioxide-water system. Trans_ lnstn. Chem Enong 40, 54-60. Goodndge F and Robb I. D (1965) Mechanism of interracial resistance In gas absorption Ind. Engng Chem Fundam 4, 49-55 Handlos A E and Baron T (1957) Mass and heat transfer from drops in hquld-hquid extraction A I Ch E. J 3, 127-136. Hlgble R (1935) The rate of absorption of a pure gas into a still liquid during short periods of exposure Trans Am lnst Chem_ Engrs 35, 365 Hlklta H., Asal S and Nose H (1978) Absorption of sulfur &oxide into water A l.Ch E J 24, 147-149 Holstvoogd D, Ptasmskl K J and van Swaaij W_ P M. (1986) Penetration model for gas absorption with reaction in a slurry containing fine insoluble particles Chem Enong Sct 41, 867-873. Hsu C. T and Shlh S M. (1993) Semlemplrlcal equation for hqmd-phase mass-transfer coefficient for drops A l.Ch E J_ 39, 1090-1092 Kajl R, Hishmuma Y and Kuroda H. (1985) SO2 absorption by water droplets J Chem Enong Japan 18, 169-172 Kohl A_ L and Rlesenfeld F C. (1985) Gas Pur~catwn, 4th edmon. Gulf Pubhshmg Co, Houston, TX Kromg R and Brink J_ C (1950) On the theory of extraction from falling droplets. Appl. ScJ Res A2, 142-154. Lapple C. E_ and Shepherd C B (1940) Calculation of particle trajectories_ Ind Engng Chem 32, 605-617 Moran J_ M , Morgan M D and Wiersma J. H. (1980) Introduction to Environmental Science_ University of W~sconsm, Green Bay, WI Morgan O_ M and Maass O (1931) An investigation of the equilibria existing in gas-water systems forming electrolytes Can J. Res_ 5, 162-199 Nguyen Ly L A, Carbonell R G and McCoy B J (1979) Diffusion of gases through surfactant films mterfaoal resistance to mass transfer A l.Ch.E J 25, 1015-1024 Olander D R (1960) Simultaneous mass transfer and equlhbhum chermeal reaction A I.Ch.E.J. 6, 233-238 Olander R O (1966) The Handlos-Baron drop extraction model_ A_I Ch.E J 12, 1018-1019. Pinilla E. A, Dmz J M and Coca J. (1984) Mass transfer and axial dispersion In a spray tower for gas-liquid contacting, Can. J. Chem. Engno 62, 617-622 Plevan R_ E. and Qumn J A (1966) The effect of monomolecular films on the rate of gas absorption into a quiescent liquid. A I Ch E J. 12, 894-902 Quicker G , Alper E and Deckwer W. D (1989) Gas absorption rates in a stirred cell with plane interface in the presence of fine particles, Can_ J Chem Engng 67, 32-38 Ralmondl P and Toor H. L (1969) Interfaclal resistance in gas absorption A I Ch E J_ 5, 86-92 Sada E and Hlmmelblau D M (1967) Transport of gases through insoluble monolayers, A I Ch E J 13, 860-865_ Yates J G. and Best R J (1976) Kmehcs of the reaction between sulfur dioxide, oxygen, and cupric oxide In a tubular, packed bed reactor Ind En#n# Chem. Process Des_ Dev. 15, 239-243
Atmospheric Enmronment Vol 28, No 21, pp 3409-3415, 1994 Copyright © 1994Elsevier Science Ltd Pnuted m Great Bntam All rights reserved 1352-2310/94 $7 00 + 0 0 0
1 3 5 2 - 2 3 1 0 (94) 0 0 1 6 1 - 8
ABSORPTION REMOVAL OF SULFUR DIOXIDE BY FALLING WATER DROPLETS IN THE PRESENCE OF INERT SOLID PARTICLES I - H u N G L I U , * C H I N G - Y U A N CHANG,*~r S u - C H I N L I U , * I - C H E N G C H A N G * a n d S H I N - M I N SHIH~ *Graduate Institute of Environmental Engineering and ~Department of Chemical Engmeenng, National Talwan University, Taipel 106, Taiwan, R O.C
(First recewed 21 November 1993 and retinal form 14 Aprd 1994) A b s t r a c t - - A n experimental analysis of the absorption removal of sulfur &oxide by the free falling water droplets containing the inert solid particles is presented_ The wheat flour powder is introduced as the inert solid particles Tests with and without the flour powder m the water droplets are examined. The mass fluxes and mass transfer coefficients of SO 2 for the cases with and without the flour powder are compared to elucidate the effects of the inert solid particles contained in the water droplets on the gas absorption The results indicate a slgmficant difference between the two cases for the concentrations of the flour powder in the absorbent droplets (Cs) within the ranges of the experimental conditions, namely 0_1 to 10 wt% flour powder In the absorbent droplets_ In general, the inert solid particles of the flour powder as the impurities m the water droplets tend to decrease the SO 2 absorption rate for the experimental absorption system under lnvestlgahon Various values of C s cause various levels of the Interracial resistance and affect the gas absorption rate The mterfaoal resistance is recognized by l n t r o d u o n g an interracial mass transfer coefficient kSw~th its reoprocal being proportional to the magnitude of the tnterfacial resistance The values of 1/~ m a y be computed by the use of the equation 1/k~ = (1/KoL , -- I/KoL), where KOL, and KOL are the overall hquld-phase mass transfer coefficients with and without the inert solid particles, respectively. The values of k. with C s of 0 1 to 10 wt% are about 0.295-0 032 cm s - * for absorbing 1000-3000 ppmv SO 2 with the water droplets_ This kind o f i n f o r m a h o n is useful for the SO 2 removal and the formation of acid rain that the Impurities of the inert solid particles contaminate the water droplets
Key word index_ Sulfur dioxide removal, gaseous pollutant absorptmn, interracial resistance, solid particles, acid rain
NOMENCLATURE
DA(s) o r
Do DA or
A Ae
D of SO 2 m gas phase
species A or SO 2 DA(I), surface area of a droplet, 7zd2 Dp, DQ D of A( = 5 0 2 ) , P( = HSO~-), Q( = H +) in water coefficients as given in CTos=al(tc) b aDb Ot effective dlffuslvlty of P ( = HSO3) in water, as CA1, CA,~ hquld-phase concentrations of solute A at hquld given in equation (5) droplet surface In equilibrium with PAp PAjs,CAts d diameter of a droplet =CA, for C s = 0 dpj &ameter of parUcle in 3th interval concentrations of solute A in bulk Iiqmd for cases En CAo, CAos enhancement factor for gas absorption due to without and with solid particles, CAos= CAo for C s chemical reaction as given m equaUon (3) =0 F-r, FT~ as defined by equations (9), (11) actual hquld-phase concentration of solute A at He CAs Henry's law constant for bulk liquid (water), PA, hqmd droplet surface in presence of solid particles = HeCA, or PAls = HeCAI. drag coefficient Kc CD dissociation e q m h b r i u m constant for A ~ 2 P , K c weight percent concentration of inert solid parCs = 4Kc~ tlcles in absorbent liquid droplet Kc ~ dlssooatlon equdibrmm constant for hydrolysis CTo, CTos accumulated concentrahons of SO 2 absorbed in reacUon of SO2t~l or A ~ P + Q absorbent droplet for cases without and with solid Kc 2 dissociation e q u d l b n u m constant for &ssoclatlon particles, CTos= CTo for Cs = 0 reaction of HSO]Cro at tc of 0 478 s with Cs of zero (CTo)~r KOL, KOLs overall hqmd-phase mass transfer coefficients for D molecular dlffUSlVRy cases without and with solid particles, KOL~= KOL kc~~, kc~r t To w h o m correspondence should be addressed 3409
for C s = 0 rate constants of forward and reverse reactions of reversible hydrolysis reaction of SO2~l
3410
1-HUNG LIU et al
ke2f. kt2r rate constants of forward and reverse reactions of reversible dissociation reaction of HSO~ individual gas- and hqmd-phase mass transfer ko, kL coefficients /,, gas-liquid lnterfaclal mass transfer coefficient m mass of a droplet, rcd3pt/'6 N~ mass flux of species A transferred into hqmd droplet across interface N A with dissociation reaction for cases without NAr, and with solid particles, NA~, = NA~ for Cs = 0 NAr~ species P or HSO~P partial pressure of solute in bulk gas PAG PA,, PA,~ partial pressures of gas-phase solute at gas-hqmd interface ['or cases without and with solid particles, PA~,--PA, for Cs=O total pressure of gas phase PT Q species Q or H + weight fraction of particles in jth interval, also qj frequency of occurrence of particles m jth interval universal gas constant R RE relative error, standard deviauon divided by mean value of data Reynolds number, p~,ud, #c , Re G Schmidt number, #c,/Ip~,Dc,) SCG Sherwood number, kGRTd/D~, ShG failing distance of a droplet absolute temperature T falling time of a droplet t contact time of a droplet with SO z during absorption u falling velocity of a droplet v~ volume of a droplet, nd~/6 contact distance of a droplet with SO 2 z Greek letters
:to, ~t, fl #G, ,UL PG, PL a
coeflicmnts as given m (/,, %) - : t t C s ~j vlscosmes of gas, hquld droplet densities of gas, liquid droplet surface tension of liquid
Subscript
A c G, g l J L,I o r ref S, s T
dissolved gas component A or SO2 contact gas phase gas liqutd interface particle size interval hqmd phase bulk phase with reaction reference value solid particle or case m the presence of sohd particles total
INTRODUCTION The a b s o r p t i o n removal of sulfur dloxtde from the polluted atmospheres by the falhng water droplets is one of the t m p o r t a n t scavenging mechanlsrns in rain a n d the wet scrubbers (Altwtcker and C h a p m a n , 1981, Kohl and Rtesenfeld, 1985) W h e n the atr Is polluted with the oxides of sulfur and mtrogen, the rain fall produces the strong sulfuric and m t n c acids The precipttation that passes t h r o u g h such c o n t a m i n a t e d mr may become more acidic than n o r m a l The actd Is harmful to the aquattc wfldhfe because xt increases the acidity m the aquatic h a b t t a t s a n d accelerates the weathermg rates of the building matertals ( M o r a n et al., 1980) To reduce the f o r m a t t o n of actd ram, the
reduction oi SO 2 emission by sorne proper control technologms is of i m p o r t a n c e One of the possible m e t h o d s ts to use the hquld droplet a b s o r p t i o n to remove the SO 2 from the flue gas and the exhaust pipes (Flagan and Semfeld, 1988) The absorption of SO 2 by the liquid absorbents has been employed by industries for m a n y years (Danckwerts, 1970, A s t a n t a et al, 1983, Kohl and Rtesenfeld. 1985. Flagan and Semfeld. 1988) Some of the main advantages of the hqutd a b s o r p t i o n are the high treatment capactty, low pressure drop, low investment costs, and the posstbthty of h a n d h n g three-phase system (Pmtlla et u l . 1984) The processes of the a b s o r p t i o n of SO,. generally use either the chemical or the phystcal a b s o r b e n t s of high capacity However, due to the solubihty of SO2 m water being high enough to provide a satisfactory a b s o r p t i o n capacity, the use of water as an a b s o r b e n t has remained to be of considerable i m p o r t a n c e for the removal of SO 2 (Hiktta et a l , 1978, Altwmker and C h a p m a n , 1981. C h a n g and Rochelle, 1981. Kajt et a l , 1985) The a b s o r p t t o n m e c h a m s m of the gaseo-s SO2 with the hqutd water and the water droplets for an unc o n t a m i n a t e d gas hquld interface has been well k n o w n (Danckwerts, 1970, Hlklta et a l , 1978, A s t a n t a et a l , 1983, Kajl et a l . 1985) W h e n the a b s o r p t i o n has some particulate i m p u r m e s c o n t a m i n a t e d on Ihe gas hqutd mterface of the liqmd water or the water droplets, an mterfaclal resistance may be formed and affect the a b s o r p t i o n rate A q u a n t l t a h v e value to the interracial resistance ~s usually recogmzed b,, m t r o d u c l n g an mterfactal mass transfer coeffioent /,, ( G o o d n d g e and Bncknell, 1962) The value of 14,, is p r o p o r t i o n a l to the m a g n i t u d e of the interracial resistance The c o n t a m i n a t i o n situation may occur, for example, d u r m g the wet spray s c r u b b m g when both gaseous species and particulates are s~multaneously removed by the h q m d droplets F o r the physical absorptton system of COz'H_,O, the interracial resistance caused by the presence of the surface actwe agents has been noted by m a n y investigators (Cullen and Dawdson, 1956. G o o d n d g e and Brtcknell, 1962, G o o d r l d g e and Robb, 1965, Sada and Htmmelblau, 1967, R a l m o n d t and Toor, 1969. Nguyen Ly et a l , 1979) As for the chemical a b s o r p t i o n system of S O z / H 2 0 , few researchers did study the effect of the mterfaclal reststance of surfactant film on the rate of gas a b s o r p t m n into a qumscent llqutd with plane interface (Plevan and Q u m n . 1966, Nguyen Ly et a l , 1979) However, for SO2/HzO with droplet, the interfacml resistance caused by the presence of the particulate t m p u r m e s has not yet been paid m u c h attentton Thts study thus performs the experimental work of the a b s o r p t i o n of SO 2 into the falling water droplets to elucidate the effects of the mert sohd particles on the gas a b s o r p t i o n Recently, Holstvoogd et al (1986) studmd the gas absorption with reactmn m a slurry c o n t a l n m g the fine insoluble particles wlth a fimte r e a c t t o n / a d s o r p t t o n c a p a o t y They noted that. during absorptton, a grow-
Removal of sulfur dmxlde ing layer of slurry with the non-reacttve particles can be formed starting at the interface Neglecting this effect leads to an overestimation of the overall reaction rate in the film Quicker et al (1989) investigated the rate of the CO2 absorption in a stirred cell with the plane interface in the presence of the fine solids (actwated carboq, kieselguhr and alumina). They reported that, for t~e case of the activated carbon with an adsorbing cal~acity, the gas absorpuon rate increases in the linear adsorption region at low sohd Ioadmgs This is d~e to the adsorption of the dissolved CO2 by the activated carbon. But, at solid loadings above 10 vol%, the~gas absorption rate decreases for all three particulatt~ matters. The retarding effect of sohd parUcles on th/~ gas absorpuon rate may be due to the mterfacial resistance However, in thear studies (Holstvoogd et al., ~ 8 6 ; Quicker et al, 1989), the possible interfacml resistance has not been recognized and distingmshed from the overall mass transfer resistance Thus, it as also the aim of this work to reveal the anterfacml resistance caused by the anert sohd partacles. The accumulated concentration m liqmd, mass flux, and anterfacial resistance of the gas absorption of SO2 using the water droplets for the cases with and without the Impurities of the inert sohd particles of the flour powder are determined. EXPERIMENTAL
The system of sulfur dioxide absorption by the falling water droplets ts studied. The flour powder IS introduced as the inert solid particles to form some mterfacml resistance m the system_ An absorption column with the free falhng
3411
absorbent hquid droplets is used The system ~s schematically descnbed m Ftg 1 and dlustrated below About three runs have been performed for each expenment_ The data presented here are the average values of the results The average of the relative errors {RE, [square root of (sum of squares of dewatmns/no, of data)I/mean value} Is about 5% (1) Properties of flour powder The inert sohd powder used m this study is the wheat flour of low strength made by Llang-Yu Co The contents of C, H, N, O, S and CI of the flour powder are about 40 3, 6 82, 2.39, 49 41, 0.175 and <0 1%, respectively, as determined by the elemental analysis The parUcle density of about 1 48 gem -3 is determined by an Accupyc 1330 V1 02 Pycnometer The speafic surface area of 0 05 m 2 g- 1 is measured by a model 2100D surface-area pore-volume analyzer from Mlcrometncs Instrument Corporation according to the BET method The structure of the flour powder particles is not porous m nature. The solubdlty of the sohd flour powder m water is measured to be negligible_ Using the flour slurry at the same concentration as that m the usual expenments to adsorb the sulphurlc acid solution (200 mg dm- 3) for 2 h, one observes that the absorption of the sulfate ion is measured to be neghglble_ The wscoslty of the filtered hqmd absorbent from the flour slurry Is determined to be only about 2% higher than that of the pure water by a Haake Rv 20 Rotovisco analyzer All these charactenstlcs may thus support that the flour powder has httle adsorpUon capacity for SO, and has little effect on the inherent properties of the liqmd absorbent_ The measurement of the particle stze d~stnbutton of the flour powder Is performed by a Granulometer 715 F008 analyzer The data may be represented as the weight fraction (qj) vs the diameter (d~,,) of particles m the jth interval. For dv of 1, 1.5, 2, 3, 4, 6, JS, 12, 16, 24, 32, 48, 64, 96, 128 and 1(5'2/am, the corresponding values of qj are 1 2, 0.4, 20, 1.8, 2 1, 42, 3 6, 3.0, 7.0, 17.0, 14 1, 13.9, 5.9, 10.2, 10.0 and 3 6 wt%. The mass medmn s~ze ts about 30/am. Most partlcles have sizes m the range of about 20-50 pan
_
16
17
7
13
10
I1
Fig. 1 Schemattc dtagram ofexpenmental apparatus (1) absorption column; (2) recetver; (3) receiver stand, (4) needle, (5) flow meter, (6) slurry bottle, (7) metering pump, (8) stirrer; (9) filter, (10) compressor; (11) humidifier; (12) bubble meter, (13) NaOH soluUon bottle, (14) SO2 cylinder; (15) N 2 cyhnder, (16) mixing tank; (17) vent to fume hood
3412
I-HuNG Ltu et al
(2) Absorption removal of gaseous pollutant by hqmd and slurry droplets The gaseous pollutant of this system is slmulately produced by the mixture of SO 2 and air The SO 2 gas supplied from a cylinder (San-Fu C o , with purity of 99 99%) is controlled by a stainless steel flowmeter (Shm-Chuan C o , with m a x i m u m reading of 0 1 dm3/60 s) and precisely measured by a bubble gas meter The carrier gas of air produced from an air compressor is filtered by a filter (with filter paper of 0_45 #m) and controlled by a stainless steel flowmeter (Shin-Chuan C o , with m a x i m u m reading of 30 dma/60 s) which has been previously calibrated by a rotameter Before the carrier gas of air is fed to a mixing tank, it Is saturated with water vapor by passing through a humidifier to avoid the evaporation of the water droplets in the absorption column About 60 cma/60 s SO 2 and 20 dm3/60 s air are mixed Ill the mixing tank (with volume of about 18 d m a, retention time of about 54 s)_ By precisely adjusting the volumetric flow rate ratio of SO 2 and mr, a mixture of SO 2 and air with SUe concentration of 3000 (or 1000) ppmv is obtained The mixed gaseous pollutant at 5 dm3/60 s is sent to the absorption column The excess gas is bypassed through the waste gas absorption bottle filled with N a O H solution before it is vented to a fume hood The flour powder slurry Is prepared by the mixture of the flour powder and the distllled-deionlzed water (with Mllhpore M d h - Q system) The concentrations of the flour powder slurry (Cs) are 001, 0 1, 0 5, 2 5, 5, and 10 wt%_ A stirrer at 5 rpm is used to maintmn the uniform concentration of the flour powder slurry This stlrnng speed is appropriate to prevent the formation ofthe little air bubble in the slurry The flour powder slurry is passed through a stainless mesh of 70 mesh (ASTM E11-58T) to avoid the blockade of pipehne. It is then transported by a micro tube p u m p (EYELA, MP-2, Tokyo Rlkaklkal Co ) to the hypodermic needle (type 22G with diameter of 0 6 m m ID) in the absorption column By precisely adjusting the speed of the mlcropump, the slurry droplets with the constant volume and pumping frequency ((90_+2)/60s) are generated from the needle The droplet formation section Is purged with a small a m o u n t of nitrogen to prevent the absorption of SO 2 d u n n g the droplet formatlon The droplet diameter is determined by counting, collecting and weighing the droplets with the assumption that they are spheres The flour powder slurry droplets and the mixture of SO2/alr contact in an absorption column (plexiglass, 15 cm ID, 220cm length) with cocurrent flow type The droplet generator is composed of a glass tube, a rubber tube and a treated hypodermic needle_ The formation of the slurry droplets at the beginning is separated from the SO 2 gas mixture by an inner tube (4 cm ID) to prevent the Interference of the SO 2 gas flow Before the SO 2 gas mixture contacts with the slurry droplets, it passes through a gas distributor to equally distribute the SO 2 gas mixture The slurry droplets begin to absorb SO 2 at 4 5 cm below the needle tip The droplets are collected in a glass bottle receiver (4 cm ID, 15 cm length), which IS filled with 10 cm 3 water so as to ddute the SO 2 concentration of droplets, and placed on the top of a stand (PVC tube) Using the stand with different length, one can vary the contact distance of absorption from 19 to 144 cm, which corresponds to the contact time of absorption of about 0 125-0 479 s A kerosene forming a layer of 9 crn thick above the fresh water is added to the receiver before each absorption experiment so as to sMeld the collected slurry droplets from the SO 2 gas stream Test experiments have indmated that the kerosene layer is effective for the prevention of the undeswed absorption of the collected droplets with the SO 2 gas stream for at least 480s The collecting time of droplets in the receiver m each experiment is only 60 s which is very m u c h less than 480 s During the 60 s collecting time, about 90 q- 4 droplets are collected in 10cm 3 water in the receiver. The concentration of the
dissolved SO 2 in water is measured by using an ion chromatograph (Dlonex QIC, Model QIC-2) with the deduction of the eontnbutlon of the blank run without the absorption of SO 2 gas. It is then used to compute the mass flux. The enhancement or reduction effects of the SO2 absorption due to the presence of the inert solid particles can then be examined_ The temperature of the absorption experiments is at about 294-296 K
T H E O R Y A N D D A T A ANALYSIS
T h e a b s o r p t i o n o f SUE into w a t e r i n v o l v e s p h y s i c a l a b s o r p t i o n a n d h y d r o l y s i s r e a c t i o n s ( E r t k s e n , 1969) T h e e q m h b r m m at the g a s - h q m d interface is des c r i b e d by H e n r y ' s law. T h e rate c o n s t a n t s o f h y d r o l y sis r e a c t i o n o f S O 2 ( l ) (with K c l = kciJkctr) a n d d i s s o c i a t i o n r e a c t i o n o f H S O 3 (with K c 2 = kc2Jk~2r) h a v e been r e p o r t e d by p r e v i o u s i n v e s t i g a t o r s (for e x a m p l e , E t g e n et al, 1961, E r t k s e n , 1969, Y a t e s a n d Best, 1976, H i k i t a et al, 1978). k~l r (at 293 K) a n d k~l r (at 2 9 8 K ) are of 34x106s-1 and 2xl0 s (mol d m - 3)- 1 s 1, a n d k e n a n d kc2 r are of 104 s - i a n d 1011 (mol d m 3) - 1 s - J, respectively k~2r is so s m a l l as c o m p a r e d to k~l r t h a t the s e c o n d ~onlzatlon r e a c t i o n (with e q u i l i b r i u m c o n s t a n t Kc2) m a y be safely neglected ( M o r g a n , 1931). F o r the first i o n i z a t i o n r e a c t i o n (with e q u l h b r t u m c o n s t a n t K c i ) , the v a l u e s o f k ~ i f a n d k~l , are b o t h l a r g e t h a t Jt c a n be r e g a r d e d as a n i n s t a n t a n e o u s reversible r e a c t i o n H e n c e , t h e a b s o r p tion o f S O 2 i n t o w a t e r c a n be r e g a r d e d as the m a s s t r a n s f e r a c c o m p a n i e d by a n i n s t a n t a n e o u s reversible KC t
reaction
o f the f o r m
A~,-~- P + Q, w h e r e
Kc~ =
[P][Q]/[A]=[HSO3] [H*]/[SO2j In a d d m o n , d u e to t h e electrical n e u t r a h t y in t h e p u r e water, t h e c o n c e n t r a t i o n s o f H + a n d H S O 3 are e q u a l everyw h e r e m w a t e r T h u s , t h e a b s o r p t i o n o f S O 2 into w a t e r c a n also be s i m u l a t e d b y the r e a c t i o n o f t h e f o r m K¢
A~2P,
where Kc = [P]2/[A] = 4Key
In t h e case of a b s o r p t i o n with t h e p a r t i c u l a t e m a t t e r c o n t a m i n a t e d in t h e a b s o r b e n t h q u t d d r o p l e t s , a n interracial r e s i s t a n c e m a y be f o r m e d A q u a n t i t a t i v e v a l u e to d e s c r i b e the interracial r e s i s t a n c e m a y be r e c o g n i z e d by i n t r o d u c i n g a n interracial m a s s t r a n s f e r coefficient k, as ( G o o d r l d g e a n d B n c k n e l l , 1962; G o o d r l d g e a n d R o b b , 1965) N A = N~r~ =
]~G ( P A G
-- P~.,s) = ks (CAls -- C A j
= EnkL(Cxs - - CAos} = (1/Hek~ + 1 / E n k L + l / k 0 = KOL~(PAG/He -- CA J ,
~ (PAG/He -- CAGe)
for f i m t e k,
(1)
For the case with no mterfacml resistance, one has k~oz T h i s t h e n gives NA = NAt = kG (PAG -- PA. ) = EnkL(CA, -- C~o) = ( 1/Hek G + 1 ,'Enk L) i(PAG/He _ CAo ) = K o L ( P A G / H e - - CAo), for kc-* rr_,
(2)
Removal of sulfur dmxlde The enhancement factor due to chemical reaction (En) m equatmns (1) and (2) for the SOz/water system may be expressed as COlander, 1960; Kajl et al, 1985) En = 1 + [(De/DA(I))KcI/(CA, * o.s o s + C°~)]
(3)
K m = ['HSO3]['H+]/[SO2(I)] -- [ P ] [ Q ] / [ A j
(4)
D* = 2DpDQ/(Dp + DO)
(5)
where
The concentration of unreacted sulfur dioxide, CA°,, is related to the total or combined concentration of unreacted and reacted sulfur dioxide, Cro~, as CTo~= [SO,0)] + [HSO3 ] + [SO32 - ] = [-802(1) ] q*-[ H S O 3 ] -- CAos + (KclCAos)o 5.
(6)
F r o m equations (1) and (2), the interracial mass transfer coefficient k, is given as follows ks = ( 1 / K o g s _ 1 / K o L ) - 1 = H e ( I / K o G ~_ 1/KoG)- 1.
(7)
The value of 1/k, is proportional to the magnitude of the Interracial resistance A higher value of 1/k~ will result in a lower value of mass flux At any gas-hquid contact time tc, a mass balance over a spheric droplet gives the following equatmns for the concentration of the total dissolved SO 2 (CTo~) m the bulk phase of droplet: for the case with interfacial resistance, dCro, = (AdVp) N ^,, dt~ = (6/d) KOLs(PAG/He -- CAo,) dt¢
(8)
Fr, = f dCroJ(PAG/He -- C^,~) = (6fl0KoL . t~
(9)
for the case without interfacial resistance, dCro = (6/d) NA~ dto = (6/d) KOL ( P A J H e -- C^.) dt,
(10)
F T = [dCTo/(PAG/He -- CA. ) = (6/d) KOL t~. J
(11)
The slopes of plots of FT, vs t¢ and F r vs to can be used to compute the global values of Kot ~ and KOL, respectively. An alternative way to evaluate KoL, and KOL iS to apply the mass balance equations (8) and (10) of differential form. The global values of KOL. and KOL may be obtained from the slopes of plots of d C r ~ d t ~ vs ( P ^ J H e - - C A o , ) and dCr./dt ~ vs ( P ^ J H e - C A . ) , respecuvely A direct computation of d C r ~ / d t ~ and dCxo/dtc also gaves the local values of KOL0 and KOL at any t~ A comparison of the global and local values of KoL, or KOL gives an indication of the validity of the use of global values. SubstRuting the values of KOL,
3413
and KOL obtained into equation (7) then yields the value of k~ The equations describing the falling velocity u, time t and distance s of a droplet were gaven by Lappel and Shepherd (1940) For the values of drag coefficient Ca, the equations gaven by Berry and Pranger (1974) may be employed. Since the falhng droplet begins contacting with SO 2 gas at s = 4 5 cm ( t = 0 0 9 6 s ) in the experiments, the contact distance (z) and contact time (to) of gas and liquid droplet are: z = s - 4 5 c m , tc = t 0 . 0 9 6 s. Employing the above analysis, one can relate z to t c and convert the experimental data of CTos vs z to Cro s vs to. The data of Cro~ vs t c are then correlated by CTos = al(tc) b_ The individual gas-phase mass transfer coeffioent k6 may be obtained by the equation S h G = 2 + 0 552Sc~;/aRe ° 23 (Geankoplls, 1972). Once the values of KOL~,KOL, ks, and k C have been obtained, one can also compute the value of Enk L using equations (1) or (2) The value of CA. for the use In computing En is obtained by applying Henry's law, and equations (2) and (10). Equation (3) then yields the value of En, which cooperates with the obtained Enk L to give the individual liquid-phase physical mass transfer coefficient kL. The values of physical and chemical properties at 2 9 5 K used m this work are: D A ( , ) = 1 2 9 x I 0 -~ cm2s -1, DA(II= 1.63x 10-s cm2s-1, Dp = 1.299 x 10 -5 cm 2s-1 (p = HSO3), Do = 8.19x 10 -5 cm 2 s -1 (Q = H+), D* = 2 2 4 x 10 -5 cm2s -1 (from equation (5)), d = 0.281-0300cm, He = 72,773 K P a ( m o l c m - a ) -~ or 718.2atm ( m o l c m - 3 ) - I , Kc~ = 1.42 x 10- 2 mol d m - 3, p~ = 0.304 K P a or 0 003 atm, #~ = 1.822x 1 0 - 4 g (cms) - I , #L = 9.6X 1 0 - 3 g (cms) -1, PG = 1.199 X 10-3 gem -3, Pl = 0.996 -1.016 gcm -3, a = 72.4 dyne c m -
RESULTS AND DISCUSSION
The experimental data of the accumulated total concentration of the unreacted dissolved and reacted sulfur dioxide (Cros) absorbed into the slurry droplets at different contact distance (z) are presented,~n Fig. 2 as CTo. vs contact time (re). The contact distance has been related to the contact time according to the equations of the drop motion (Lappel and Shepherd, 1940) and of the drag coefficmnt (Berry and Pranger, 1974) The value of Cro~ increases with t c simply because more SO z has been absorbed. A higher value of the concentration of the inert solid particles in the absorbent droplets (C~) results in the lower values of Cro, as illustrated in Fig. 2. The presence of the inert solid particles in the absorbent hquid droplets reduces an mterfacial resistance for the SO 2 absorption. The interracial resistance becomes slgmficant as C~ reaches about 0.1 wt% and retards the absorption rate. This interfacial resistance caused by the inert solid particles increases with Cs within the ranges of the experimental c o n d m o n s (C~ of 0 1 to 10 wt%)
3414
I-HUNG LIU et al kL , (cm/s)
C TOS/(CTO)refh 12~ I
q
11! 1'
"
0.05
J 0.04
o~
~---e
_-~i
kL,HB 0 03
kL,HS
--
o.,
::1
0 02
i
1 001 i I
-
kt p
i
0
005
01
015
02
(a)
025
03
035
04
045
OS
0~ 0
055
005
01
015
02
025
Ic , (s) Fig
::[
~
:
~
S
o 0
OOS 0 1
-~°
,
015
02
025
03
045
05
OS5
0,35
04
0,45
05
o19t
mass transfer coefficient k L
-
_
_
osl
°if
055
0
t~, (s)
0
Fig 2. Variation of CT, with to under various C, (A, ~) C~ = 0 wt%; (B, A) 0_01%, (C, ,{~) 0 1%, (D, V) 0.5%, (E, O) 25%; (F, IS]) 5%, (G, o) 10% d = 0281 - 0 300 cm, PL = 0.996-1 016 gcm -3_ Temperature (To)=295+ 1K (CTo).f = C r o ~ at t¢ of0478s (z of 144 cm) with C, of zero. Symbols are experimental data Solid hnes across symbols are regression hnes (a) 3000ppmv SO2ts), (Cro).t~= 1551mmoldm -a (b) 1000 ppmv SO2(w), (CTo).n = 1 078 mmol dm 3
Figure 3 presents the values of k L of this analysis obtained by dividing Enk L by En along with those computed by the model equations of Hlgbie (1935), Handlos and Baron (1957) and Hsu and Shin (1993), respectively (equations (12)-(14)) These equations are kL, e = ( 4D A/rrtc) ° s
Liquid-phase p h y s i c a l
3
ks , (cm/s)
f,:
0
(b)
04
of experimental values of this work (with symbols), and theoretical results (with solid lines) of Hlgble (k L p), Handols and Baron (kL. rlB), and Hsu and Shlh (kL,]ts) Other notations are the same as in Fig 2
J
~
035
tc , (s)
Cros /(CTo),.,.
0~i
03
(12)
kL, na=OOO375u/(1 + #L//aG)
(13)
kL, HS=0 88[(8o/3~m) ° 5DA]° s.
(14)
Equation (12) of Hlgbie was developed for the absorption of a gas into a stdl hqmd during short periods of exposure. Equation (13) of Handols and Baron deals with a model assummg turbulent internal flow wtthm a drop. Equation (14) of Hsu and Shih is a semtempirical equation with a coefficient of correlation of 0 9 based on the surface stretch model for osciUatmg drops proposed by Angelo et al (1966). The model postulated that d u n n g drop oscallation the fresh surface area was formed and returned to the bulk, whtch
04
0
0
1
2
3
4
__ _ - - _ _ r 5 6
7
8
9
10
Cs, (wt.%) Fig 4 Plot of global k~vs C, Symbols: experimental, (~, _-,) 3000 and 1000 ppmv SO,(R), (. . . . . ) regression hne
was assumed to be well mixed. The values of k L of this work are between those of others employing various mixing models This indicates that the internal mixing extent m the hquid droplet of this study is between still and turbulent mixing condmons There may be some extent of the internal circulation m the pure liquid droplet (C, = 0), which has been noted by the previous mvesttgators (Kronig and Brink, 1950)_ F]gure 4 shows the variation of the global values of k~ with C~. An mcrease of Cs tends to retard the gas absorption rate as noted m the preceding discussion The retarding effect is vigorous for Cs vaned from about 001 to 2.5 wt% For Cs higher than 2.5 wt%, a further increase of C, only shghtly reduces the gas absorption rate The variaUon of ks with C s is not of the linear form. A correlation equation using (k~-0to) = ~tI Cs # seems satisfactorily fitting the results of this work. The values of,, 0, al, fl are obtained to be 0.041,
Removal of sulfur dioxide 0 062, and 1 207, respectively. The value of R 2 of the correlatton e q u a t i o n ts a b o u t 0.89 It ts n o t e d that the o b t a i n e d correlation e q u a t i o n of k s with C~ is based o n two SO2t~) c o n c e n t r a t i o n s (with PAc/PT of 1000 a n d 3000 ppmv) a n d one dropstze (with diameter d of a b o u t 0_281-0_300cm). The influence of P^o/PT is mainly o n En, which is found to decrease with P^o/PT, b u t not o n k~ (Sh C = 2 +0.552Sc~/3Re ° 53), kL (equations (12)-(14)), a n d k s Since Reo, u a n d m are dependent o n d, k C a n d k L would also be d e p e n d e n t on d according to Sh o = 2 + 0.552Sc~/S Reg 53 a n d equations (13) a n d (14), respectively. As to the role o f d o n E n a n d k~, further study is called for elucidating tts possible effects
CONCLUDING REMARKS The presence of the inert solid particles m the water droplets tends to reduce a n lnterfacial resistance, which is p r o p o r t i o n a l to the reciprocal of the mterfacml mass transfer coefficient ks, and reduce the gas a b s o r p t i o n rate_ In general, a h~gher value of the c o n c e n t r a h o n of the inert sohd particles m the water droplets (Cs) results in a higher interfacml reststance F o r Cs of 0.1 to 10 w t % , the values ofk~ of S O 2 / H 2 0 system are a b o u t 0.295-0.032 cm s - ~ It ~s r e c o m m e n ded t h a t the effects of m a j o r solid l m p u n t t e s o n the gas a b s o r p t i o n should be examined for the a b s o r p t i o n system u n d e r consideration Acknowledgemem--Thls study was supported by the National Science Council of the Republic of China on Talwan, under projects NSC 79-0421-E002-61Z and 80-0410-E00269_ The technical assistances provided by the Particulate Technology and the Chemical Engineering Laboratones of the Department of Chemical Engineering of National Talwan Umverslty are appreciated
REFERENCES
Altwlcker E R and Chapman E (1981) Mass transfer of sulfur dioxide into falling drops a comparison of experimental data with absorption models. Atmospheric Environment 15, 297-300 Angelo J B, Lightfoot E N and Howard D W. (1966) Generalization of the penetration theory for surface stretch: application to forming and oscillating drops A I C h E J 12, 751 Astarlta G., Savage D W_ and Bislo A (1983) Gas Treatmo wzth Chemzcal Solvents Wiley, New York Berry E X and Pranger M. R (1974) Equation for calculating the terminal velocities of water drops J appl_ Meteor 13, 108-111 Chang C S and Rochelle G T (1981) SO 2 absorphon into aqueous solutions_ A_I Ch E J 27, 292-297 Cullen E T and Davldson J F (1956) The effect of surface active agents on the rate of absorption of carbon dioxide by water Chem Enong Sct 6, 49-55 Danckwerts P V (1970) Gas-Llquld Reaction McGraw-Hill, New York
3415
Elgen M , Kustm K_ and Maass G (1961) Die Geschwindlgkelt tier Hydration yon SO2 in wassriger Losung. Z Phys Chem. Frankfurt 30, 130 Eriksen T E (1969) Diffusion studies in aqueous solutions of sulfur dioxide Chem Engng Scz 24, 273-278_ Flagan R C and Seinfeld J H (1988) Fundamentals of Air Pollution Engineering. Prentice-Hall, Englewood Chffs, NJ Geankophs C_ J (1972) Mass Transport Phenomena, p 291 Holt, Rinehart and Winston, New York Goodrldge F and Bricknell D. J_ (1962) Inteffaclal resistance in the carbon dioxide-water system. Trans_ lnstn. Chem Enong 40, 54-60. Goodndge F and Robb I. D (1965) Mechanism of interracial resistance In gas absorption Ind. Engng Chem Fundam 4, 49-55 Handlos A E and Baron T (1957) Mass and heat transfer from drops in hquld-hquid extraction A I Ch E. J 3, 127-136. Hlgble R (1935) The rate of absorption of a pure gas into a still liquid during short periods of exposure Trans Am lnst Chem_ Engrs 35, 365 Hlklta H., Asal S and Nose H (1978) Absorption of sulfur &oxide into water A l.Ch E J 24, 147-149 Holstvoogd D, Ptasmskl K J and van Swaaij W_ P M. (1986) Penetration model for gas absorption with reaction in a slurry containing fine insoluble particles Chem Enong Sct 41, 867-873. Hsu C. T and Shlh S M. (1993) Semlemplrlcal equation for hqmd-phase mass-transfer coefficient for drops A l.Ch E J_ 39, 1090-1092 Kajl R, Hishmuma Y and Kuroda H. (1985) SO2 absorption by water droplets J Chem Enong Japan 18, 169-172 Kohl A_ L and Rlesenfeld F C. (1985) Gas Pur~catwn, 4th edmon. Gulf Pubhshmg Co, Houston, TX Kromg R and Brink J_ C (1950) On the theory of extraction from falling droplets. Appl. ScJ Res A2, 142-154. Lapple C. E_ and Shepherd C B (1940) Calculation of particle trajectories_ Ind Engng Chem 32, 605-617 Moran J_ M , Morgan M D and Wiersma J. H. (1980) Introduction to Environmental Science_ University of W~sconsm, Green Bay, WI Morgan O_ M and Maass O (1931) An investigation of the equilibria existing in gas-water systems forming electrolytes Can J. Res_ 5, 162-199 Nguyen Ly L A, Carbonell R G and McCoy B J (1979) Diffusion of gases through surfactant films mterfaoal resistance to mass transfer A l.Ch.E J 25, 1015-1024 Olander D R (1960) Simultaneous mass transfer and equlhbhum chermeal reaction A I.Ch.E.J. 6, 233-238 Olander R O (1966) The Handlos-Baron drop extraction model_ A_I Ch.E J 12, 1018-1019. Pinilla E. A, Dmz J M and Coca J. (1984) Mass transfer and axial dispersion In a spray tower for gas-liquid contacting, Can. J. Chem. Engno 62, 617-622 Plevan R_ E. and Qumn J A (1966) The effect of monomolecular films on the rate of gas absorption into a quiescent liquid. A I Ch E J. 12, 894-902 Quicker G , Alper E and Deckwer W. D (1989) Gas absorption rates in a stirred cell with plane interface in the presence of fine particles, Can_ J Chem Engng 67, 32-38 Ralmondl P and Toor H. L (1969) Interfaclal resistance in gas absorption A I Ch E J_ 5, 86-92 Sada E and Hlmmelblau D M (1967) Transport of gases through insoluble monolayers, A I Ch E J 13, 860-865_ Yates J G. and Best R J (1976) Kmehcs of the reaction between sulfur dioxide, oxygen, and cupric oxide In a tubular, packed bed reactor Ind En#n# Chem. Process Des_ Dev. 15, 239-243