Ind. Eng. Chem. Process Des. Dev. 1902, 21, 771-774
771
COMMUNICATIONS Simultaneous Absorption of Dilute NO and SO2 into Aqueous Slurries of Mg(OH), with Added Fe"-EDTA Chelate
The simultaneous absorption of NO and SO, into aqueous slurries of Mg(OH)* with added Fe"-EDTA was carried out using a stirred tank with a plane gas-liquid interface at 298 K and 0.1013 MPa. The M (OH), fine particles suspending have a negligible Influence on the complexing reaction of dissolved NO with Fer -EDTA chelate. As the concentration of coexisting SO2 increases from 0 to 2500 ppm, the rate of absorption of NO fist decreases, then increases, and finally approaches that in the absence of SO,. The absorption rate of NO during the simultaneous absorption is controlled by the concentration of Sot- at the interface. I t was deduced that the bulk slurry phase is not saturated with MgSO,(s) because it reduces the Fe**(NO)(EDTA)complex.
B
Introduction I t has been well known that the chelate of EDTA (ethylenediamine tetraacetate) coordinating to ferrous or ferric ion (Fe'LEDTA or FeILEDTA) is an effective absorbent appropriate for removal of NO (Hattori et al., 1977). It has also been reported that some reducing agents such as 530;-can reduce NO coordinated to Fe-EDTA to regenerate Fe-EDTA and a high degree of removal of NO can be maintained. The wet scrubbing method seems to have a possibility for simultaneous treatment of nitrogen and sulfur oxides. In our previous papers (Sada et al., 1980, 1981), the kinetics of the complexing reaction of NO with FeLEDTA and Fe"LEDTA chelates (NO Fe"-EDTA = Fe"(NO)(EDTA) and NO F e c E D T A = Fem(NO)(EDTA)) were analyzed by chemical absorption theory. In addition, the absorption of NO with coexistence of SO2 into Fe"EDTA and Fe"LEDTA solutions with added aqueous MgS03 slurry as the reducing agent was carried out using a stirred tank with a plane gas-liquid interface, and the effect of coexisting SO2on the absorption rate of NO and the role of MgS03 slurry were clarified. The present work was undertaken on the basis of the concept that the reducing agent is the SO;- ion which is not previously added to the absorbent (Fe"-EDTA) but is produced through the reaction with absorbed SO2. The absorbent used for simultaneous treatment of NO and SO2 was an aqueous slurry of Mg(OH), with added FeILEDTA. The significance of the usage of Mg(OH), slurry as an absorbent for SO, lies in its high absorption capacity. Furthermore, the reaction product MgS03 functioning as the reducing agent has relatively low solubility in water and high capacity. Accordingly, the interaction between SO3,- and Fen-EDTA chelate (Fe'I-EDTA SO:- = Fer1(S032-)(EDTA))can be suppressed, and this fact is favorable to the absorption of NO.
+
+
+
Experimental Section The experimental apparatus and procedures are similar to those in the previous work (Sada et al., 1980, 1981). Experimental Results and Discussion The experimental results on absorption of NO into aqueous mixed solutions of Fe"-EDTA and Na2S03are 0196-4305/82/1121-0771$01.25/0
shown in Figure 1. The solid line only represents a line around which experimental points on the rate of absorption into aqueous FeILEDTA solutions without added Na2S03are scattering. It is proved that the absorption rates with added Na2S03 above 0.022 M reduces by a constant rate in comparison with those without added Na#Og. The rates of the absorption i n t ~ an aqueous slurry of Mg(OH), with F&EDTA alone coincide well with those into aqueous clear solution of FeeEDTA. The Mg(OH)2 fiie particles suspending apparently have no influence on the absorption rates. Figures 2 and 3 indicate the experimental results on the simultaneous absorption of NO and SO2 into aqueous slurries of Mg(OH), with added FeILEDTA, where the absorption rates of NO are plotted against the interfacial concentrations with the inlet SO2concentration as a parameter. The dobdash lines represent the absorption rate of NO in the absence of SOz. As the concentration of SO, in the influent stream increases from 0 to 2500 ppm, the absorption rate of NO first decreases, then increases, and finally approaches the absorption rate in the absence of
so,.
It has been experimentally confirmed that the process of absorption of dilute SO, into aqueous slurries of Mg(OH), is completely controlled by diffusion in the gas film within the present experimental conditions (Sada et ai., 1980). The reaction product MgS03 is accumulated in the liquid phase and the SO;- species takes part in the surface reaction with SOz. Therefore, when the size of Mg(OH), particles is considerably smaller than the thickness of the liquid film, the concentration profiles of the relevant species near the interface can be sketched as Figures 4a and b, depending on whether the concentration of S032in the bulk liquid phase is equal to or less than the saturation concentration of MgS03. At the gas-liquid interface, i.e., primary reaction plane
SO, + S032-+ H 2 0
-
2HS03-
(i)
takes place, and at the secondary reaction plane HS03- + OH-
-
S032-+ H 2 0
(ii)
takes place. Both reactions can be regarded as being instantaneous. @ 1982 American Chemlcal Society
772
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982
I E,
"1
'
x2
0 0044 00 0 2 25
/:
1
5 IO1 2
tions can be written as follows. In the region of 0 I x I
/
key [Na,SOJ, M
0
.**
/ I
0
key
pH
a
68 88
/r
5L
(4) .
(5)
99 (Mg(OH1,
,
added1 M
2 ft/ ' [ 1 1 ~ l l ~ - E ~ ] = O 0 3
In the region of x 2 Ix I 1
When the concentration of 502- in the bulk liquid phase is assumed to be equal to the saturation concentration of MgSO, (i.e., YE0 = YEs), YE = YEo holds in the region of x2 Ix I 1. For YE0 < YEs, the mass balance equation for SOsZ-is given by d2YE/dX2 = 0
~
The boundary conditions to be imposed are
I
,
(7)
& [Fe( I I I-E DTA: 1002 M
5 wt%
Mg10H12
1 I
1
2
C,, x
20
IO
5 108,
50
mol/L
Figure 2. Absorption rates of NO during simultaneous absorption of NO and SOz into aqueous slurries of Mg(OH)? with 0.02 M Fen-EDTA. >
50 key
ys0,
-P
z
g
/"
-
P
at x = 1:
&
1500
2500
/
,,'
Bd
& 2 . q '
YB
= 1;
YE
=
(12)
YE0
The dimensionless concentration of species E and F at the interface are derived for YEo = YESas
2-
rc
5-
4
z
/ -
500
-
10-
/
ppm
1000
20w
f,
[Fe(lll-EDTAl =005 M Mg(OHl2 5 w t % .
I
and Thus, the process of absorption of dilute SOz is controlled by the following mass balances. In the region of 0 I z I z 2
YFj
= sinh N1I2x2 rF
1
tanh
w/2(1- x,)
+
1
1-i
tanh w/2x2 rF (14)
respectively. The dimensionless position of the secondary reaction plane x 2 can be determined by the following relation which is derived from the boundary condition of eq 8. In the region of z2 Iz IzL (3)
In the dimensionless form, the above mass balance equa-
(rFYFi + l)N1/' N1/2 + tanh N%, sinh N 1 / 2 ~ 2 2rE(yEO -
YEi) - 2 r F y F i x2
= 0 (15)
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982 773
Table I. Calculated Concentrations of SO,l-and HS0,-at the Interface during the Simultaneous Absorption of NO and SO, Baaed on Figure 4a YSO,.f, YSO,,09 Nso x IO8, CEO - CEir CFi, YFi g-ion/L PPm PPm molls cm* x, YE0 - Y p g-ion/L 0.918 1.63 2.86 5.00
180
500 -..
~~
~
320 560 980
1000 1500 2500
0.856 0.919 0.953 0.973
~
2.4 4.3 7.7 13.5
0.0022 0.0040 0.0071 0.0124
7.6 14.2 25.6 45.6
0.0070 0.0131 0.0236 0.0420
Table 11. Calculated Concentrations of SO,z-at the Interface and the Secondary Reaction Plane during the Simultaneous Absorption of NO and SO, B a d on Figure 4b Y*E
YSO,,fl
yEi
CEi,
g-ion/L
(YEn = 35)
m m
500 1000 1500 2500
31.0 33.3 34.6 35.3
28.7 29.0 26.9 21.8
0.0264 0.0267 0.0247 0.0201
Y*E (YE0 =
18.2 19.5 20.3 20.7
Liquid phase
Gas phase
yEi
CEi,
g-ion/L
20) 15.8 15.2 12.6 7.2
0.0145 0.0140 0.0116 0.0066
Y *E
yEi
(YE0 =
15)
13.9 14.9 15.5 15.8
11.5 10.6 7.8 2.3
CEi
g-ionjL 0.0106 0.0098 0.0072 0.0021
The dimensionless position of the secondary reaction plane here can be determined by the following equation. (rFYFi
+ 1) Nlf2-
tanh I P / 2 x 2 a
N/'
-
sinh N / 2 x 2 2rE( YE* - YEi) - 2rFYFi
= 0 (19)
x2
2.0
22 ZL
Liquid phase
Gas phase
2.0
22 ZL
Figure 4. Conceptual concentration profiles during absorption of dilute SO2 into aqueous slurries of Mg(OH)>
For Ym < YES,solution of eq 4 through 7 gives the dimensionless concentrations of species E and F at the interface as
The difference between SO:- concentrations in the bulk of liquid and at the interface, CEO- CEi, and the concentration of HSO, at the interface, CFi,are shown in Table I. They were calculated from eq 13 and 14 for Ym = YES, where the concentration of 503- in the bulk liquid phase is equal to 0.0618 g-ion/L (Seidel and Linke, 1965). Table I shows that the concentration of S032-at the interface is varied from 0.0596 to 0.0494 g-ion/L while the concentration of SO2 in the influent stream increases from 500 to 2500 ppm. The concentrations of S032- at the interface and the secondary reaction plane for Ym < YESare shown in Table 11, where the concentration of S032-at the interface was decreased by increasing the concentration of SO2 in the influent stream and decreasing the concentration of 50-: in the bulk liquid phase. It is essential to estimate the interfacial concentration of SO:- because it affects the rate of absorption of NO as stated below. The process of absorption of NO with the coexistence of SO2 into aqueous slurries of Mg(OH)2 with added FeeEDTA chelate is considered to be a single absorption of NO into an aqueous solution of Fe"-EDTA chelate containing HS03-, SO3%,and OH- distributing as sketched in Figures 4a and b. The 50:- species formed by the reaction of dissolved SO2with Mg(OHI2,on one hand, may coordinate to Fe'LEDTA as Fe'LEDTA
and YFi = sinh I W 2 x 2
1 1 1-1 tanh W2(l- x Z ) tanh N 1 f 2 x 2 rF (17) respectively. Here, YE* represents the dimensionless concentration of species E a t x2, and is given by rF
+ SO:-
= Fe11(S032-)(EDTA)
and makes the absorption ability to NO lower. The dissolved SO2, on the other hand, may regenerate the Fe"EDTA chelate through the reaction with Fe"(S0,2-)(EDTA) and, accordingly, may function as promoter for absorption of NO. When the concentration of SO2 in the influent stream is 2500 ppm, the concentration of SO:- at the interface is estimated less than ca. 0.005 g-ion/L after judging from Figure 1. If the concentration of SO:- in the bulk liquid phase is assumed to be equal to the saturation concentration of MgS03(s),the interfacial concentration of S032for ySod = 2500 ppm never recovers the the absorption rate
Ind. Eng. Chem. Process Des. Dev. 1982, 21 774-776
774
~
in the absence of SO,, which is depicted by the dot-dash line. In order that the absorption rate of NO during the simultaneous absorption for ysoZf = 2500 ppm may recover to that in the absence of SO,; that is, the interfacial concentration of SO3,- may be less than about 0.005 g-ion/L, and the dimensionless concentration of SOZ- in the bulk of liquid must be 15-20 according to Table 11. In the region of z > z2, there exists SO3,- alone, and there may occur reductions such as 2Fe"(NO)(EDTA)
+ SO3,-
-
2Fe"-EDTA 2Fe"(NO)(EDTA) 2Fe'LEDTA
+ S042-+ N 2 0 (iii)
+
+ 5So3,- + 3H20
+ 2NH(S03)22-
SO4,- + 40H- (iv)
It is deduced that the concentration of SO3,- in the bulk liquid phase is less than the saturation concentration of MgS02(s) because of existence of such reductions. Conclusion
The Mg(OH)2fine particles suspending in Fe'I-EDTA solutions apparently have no influence on the complexing reaction between dissolved NO and FeLEDTA. The absorption rate of NO into aqueous slurries of Mg(OH)2with added Fe'LEDTA first decreased, then increased, and finally approached that in the absence of SO2as the concentration of coexisting SO2increased from 0 to 2500 ppm. The absorption rate of NO during the simultaneous absorption was controlled by the concentration of SO-: at the interface. Nomenclature
A, = surface area of solid particles, cm2/cm3of slurry C = concentration in liquid phase, mol/L or mol/cm3 D = diffusivity in liquid phase, cm2/s jsoz= dimensionless absorption rate of SO2,N?02~L/DBCBs k = second-order forward rate constant of complexlng reaction, L/mol s k, = mass transfer coefficient for solid dissolution, cm/s
N = k,A,zL2/DB N A = absorption rate of NO, mol/s cm2 rI = DI/DB (I = A, E, and F) x = dimensionless distance into liquid phase from gas-liquid interface, z/zL x 2 = dimensionlessposition of secondary reaction plane, z2/zL y = gas-phase concentration, ppm YI = dimensionless concentration in liquid phase relative to that of species A at gas-liquid interface or that of species B at solid surfaces z = distance into liquid phase from gas-liquid interface, cm z2 = position of secondary reaction plane, cm zL = thickness of liquid film, cm Subscripts A = NO
B = OH-
E = SO:-
F = HS03f = feed stream i = gas-liquid interface o = effluent stream s = surface of solid particle 0 = bulk liquid phase I1 = region of 0 Iz Iz2 depicted in Figure 4 I11 = region of z 5 z 5 z L depicted in Figure 4 Literature Cited Hattori, H.; Kawai, M.; MlyaJlma, K.; Sakano, T.; Kan. F.; Saito, A.; Ishikawa, T.; Kanno, K. Kogai 1077, 12, 27. Sa&, E.; Kumazawa, H.; Kudo, I.; Kondo, T. Ind. Eng. Chem. Process D e s . D e v . 1980, 19, 377. Sada,E.; Kumazawa, H.; Kudo, I.; Kondo, T. Ind. f n g . Che" Process Des. D e v . 1981, 20, 46. SeMel, A.; Linke, W. F. "Sdubilkies of Inorganic and Metal Organic Compounds"; American Chemical Society: Washington, DC, 1965: p 524.
Department of Chemical Engineering Eizo Sada* Kyoto University Hidehiro Kumazawa Kyoto, 606, Japan Yasushi Sawada
Takashi Kondo Received for review March 30, 1981 Revised manuscript received March 8, 1982 Accepted April 29, 1982
Froth to Spray Transition on Sieve Trays A correlation is developed to predict the froth to spray transition on industrial scale sieve trays. The correlation is expressed directly in terms of tray loadings, tray geometry, and system physical properties and therefore does not require an a priati knowiedge of Quid holdup on the tray. The correlation is compared with data for the air-water system and limited data for hydrocarbon systems under actual distillation conditions which have recently become available. The correlation shows encouraging agreement with these experimental measurements.
Introduction
The two dominant flow regimes on industrial scale sieve trays are froth (liquid continuous) and spray (gas continuous). The type of regime can strongly influence the hydraulic and mass transfer performance of a tray (Pinczewski and Fell, 1977) and it is therefore important for the tray designer to have a means of determining the type of flow regime which will predominate on a given tray in terms of tray loading, tray geometry, and system physical properties. A number of studies of the froth to spray transition on sieve trays has been reported and these have been recently reviewed by Lockett (1981) and Hofhuis and Zuiderweg (1979). The most comprehensive experimental studies are those of Porter and Wong (1969), who investigated a number of gaslliquid systems on a tray with no liquid 0196-4305/82/1121-0774$01.25/0
cross-flow and Pinczewski and Fell (1972) (see also Loon et al., 1973),who examined the effect of liquid cross-flow using the air-water system. Together these data cover a wide range of tray loadings, tray geometry, and system physical properties. Both sets of data are well correlated by the equation (Hofhuis and Zuiderweg, 1979)
which shows that gas and liquid densities are the major physical properties affecting the transition with surface tension and fluid viscosities having little or no effect. This influence of system physical properties has recently been confirmed for actual distillation systems (Hofhuis and Zuiderweg, 1979; Prince et al., 1979). 0 1982 American Chemical Society
