大気汚染物質除去に関する研究 : (6) Ca(OH)_2,NaCl_2 と Mg(OH)_2,NaClO_2 スラリーによる希薄 NO の吸収

89

Removal o

f

Atm

⑪

8

1

曲 目

i

cP

o

H

u

t

a

n

t

s

(

6)

A

b

s

o

r

p

t

i

o

n

o

f

Lean NO i

n

Aqueous S

l

u

r

r

i

e

s

o

f

Ca (

O

H

)

2

w

i

t

h

N

aCI02

o

r

Mg(OH)2 w

i

t

h

N

aCI02

I

c

h

i

b

e

i

KUDO

，

Takashi KONDO

，

E

i

z

o

SADA'

，''

and H

i

d

e

h

i

r

o

KUMAZA

W A

本

大気汚染物質除去に関する研究

(6)

Ca(OH)2

，

NaC

1

z

と

Mg(OH)2

，

NaCI02

ス ラ リ ー に よ る 希 薄

NO

の吸収

工藤市兵衛@近藤高司@佐田栄三本。熊沢英博キ

Th芭absorptionoI lean NO in an旦queousslurry of Ca(OH)

，

or Mg(OH)

，

with NaCIO

，

was C丘rriedout using旦stirrεdtank absorber with a plane gas-Iiquid interface at 25'C andlatm. The rates of absorption of NO and the accompanying desorption of NO

，

for the Ca(OH)

，

slurry were in clesεagreement with those for the aqueous mixed solution of N aCI02 and N aOH with highεr OH-concentration， wher巴asfor the Mg(OH)2 slurry， the absorptin rate of NO noticeably exceeded that for the former systems. Furthermore， the ratio of the NO

，

desorption rate to the NO absorption rate considerably exceeded the theoretical perediction for gas且bsorptionwith

出econsecutive reaction (maximum deviation attainned 117%) . AIso， chlorin日dioxidewas detected in the gas phase. It was deduc巴dfrom these experimεntal evidences that there occur both desorption of the decomposition product CI02 and gas-phase oxidation of NO with CIO

，

to produce NO

，

INTRODUCTION

In the previous work [1，2J， the absorption of lean No in aqueous mixed solutions of NaCI02 and NaOH was carried out using a stirred vessel with a plane gas-Iiguid intεrface， and the chemicai absorption kinetics was analyzed. As a result， the following information was derived. In the high NO concentra tlon r昭iongreater than 2000 ppm， the order of reac tion with respet to NO was estimated as 2， whereas for the NO concentration less than several hundred ppm， the re呂ctionorder varied from 2 to 1 with decreasing the NO concentration. The order of reac -tion of，NaCIO

，

was determined as unity only for the

NaCIO

，

concentration greater than 0.8 molar.For th巴 concentration 1巴ssthan 0.8 molar， the dependence on the conc邑ntrationb巴comesmarkedly and a simple relationship was not extracted. when the concent -ration of N aCIO

，

exceeds 0.3 molar and the partial pressur巴ofNO exceeds 0.002 atm， the reaction beι

ween NO and NaCIO

，

in an aqueous solution can be regarded as second-order in NO and first-order in

NaCI02. The巴ffectof the NaOH conc巴ntrationon the third-order rat巴constantk was expressed by k = 3.80x1012exp(-3.73 [NaOHJ ) in the range of 0.5<

[N aOHJ < 0.5 molar.In considering this result， the rate of the reaction increas巴swith decreasing the NaOH concentratiom. but too low concentration of OH- results in decomposition of NaCIO

.

，

Then a decomposition product CIO

，

is evolved in the gas phase and the gasphase oxidation of NO takes place. Hence in order to suppress the decomposition of NaCIO

，

and give the absorbent a stable oxidation ability， it is necessary to keep the OH-concentration to a value. However， if a depletion of OH-due to the reaction can be prevented， then the decomposition of NaCI02 may b巴suppressedeven when the concentra -tion of OH-is kept low. Such且 situationis esta blished by using sparingly soluble alkaline 巴旦rth hydroxide as an alkali source. An aqueous slurry of Ca(OH)2 or Mg(OH)2 has low concentration of OH but high alkaline capacity. Thus， in the present work，

the absorption rate of NO by an aqueous slurry of Ca(OHi

，

with NaCI02 or Mg(OH)

，

with NaCIO

，

in a This paper has been published in Chem. Eng.Sci.， 719，34 (1979)

90 工藤市兵衛・近藤高司・佐田栄三・熊沢英博

stirred vessel was measured and the absorption mechanism was analyzed in terms of the chemical absorption theory.

EXPERIMENTAL

All the absorption runs were mad巴usinga stirred tank absorber with a plane gas-liquid interface at 25 "C and 1 atm . The absorber used(I.D.=80 m m，

Liquid volume=500 cm') is the same as in the pre -vious work [1，2J . The absorber was operated centi -nuously with respect to the gas phase and batchwise with respect to the liquid phase. Two stirrers driven by two separate motors were used to agitate the gas and liquid phases. The stirring speeds of the liquid phase and gas phase stirrers were maintained at 162 and 500 rpm， respectively. The concentration of NO in the feed stream was varied from 50 to 800 ppm.The concentrations of NO and N02 in the gas phase were determined by UV derivative spectrophotometer (Ya -naco U0-1). Absorption rates of NO were calculated from the diff巴rence between inlet and outlet concentrations of NO and the total gas flow rate Also， desorption rate of N02 were calculated from the outlet concentration of N02 and the total gas flow rate.

EXPERIMENTAL RESULTS AND DISCUSSION

1.Absorption of NO in an aqueous slurry of Ca(OH)2 with NaCI02 寸一一←-，----1 パ~ノ d三戸ノ'

ノ

ヂ

ノ

ロロf _0'

ノ 。

1♂卜 x叫x ぴ〆 門

:

5

4

i

J

////ぺノ 三 ￨ ///

{

刷酬" 白

e

I

右 血 / ロ

γ-z~

I

ず。 w • wt% ，o"f.-'M.C叩2J~ 1.50 mol/

い月

1p 0'; ロ イp" 〆必』。 /0 /も 一一 【 出 叩 o 10

ペ

5 167 2 5 I

♂;

CA11• mol/I Fig.1. Absorption rates of NO for the NaCIO，jCa (OH)2 slurry system. Figure 1 shows absorption rates of NO in aqueous slurries of Ca(OH)2 with NaCI02under various NO concentrations. The liquid phase contains 2

ム

7.5and 10 wt% of fine Ca(OH)2 particles in an aqueous NaCI02 solution. The OH-concentration in the agu eous solution saturated with Ca(OH)2 amounts to

0.046 g-ion/l.It is apparent that experimental absorp -tion rates are not influenced by the solid concentra -tion. The dot-dash line represents the absorption rate of NO by an aqueons mixed solution of1.5 molar NaCI02 and 0.1 molar NaOH [2J . In the range of CAll <5 x 10-7 mol/1， the slope of the dot-dash line approximately equals unity，which implies that the reaction betwe巴n NO and NaCI02 in an alkaline solution can be expressed by first-order with respect to NO. When using Ca(OH)2 as an alkaline sources， the OHωconcentration in a slurry (0.046 g-ion/1) corresponds to about half the value. for the dot-dash line. These， the reaction rate constant for the slurry solution is increased by c.a. 22 percent， but the ab -sorption rate increases only by c.a.ll percent. ln this way， experimental absorption rates fall closely on the dot必dashline within the experimental error.The experimental result that the absorption rate is not infiuenced by the slurry concentration may be ex -pected from the theoretical prediction that the ab -sorption process lies under th巴fast-reactionregime. 100 国ト re'" 20 10 、 弘、 c le=旧O 5 10 20 田 旧O .JM Fig. 2. Dependence of the parameter N on the enhancement factor for the slurry system. Figure 2 indicates the relationship between en hancement factor and reaction-diffusion modulus as a parameter of N for gas absorption with a second -order reaction in slurry (A(g)→AB(aq)， B(s)→(aq)， A(aω+νB(aq)→products). The parameter N is de命led by ksApZ2L/D8， where Ap is equal to 6w/pdp， and hence is proportional to solid concentration w. This figure clearly shows that the relatinn ofφvsゾJV[is almost independent of N or w in a fast-reaction regime. The rates of NO absorption in a slurry of Ca(OH)2 with NaCI02 may be expected from the previous work [2J ，and it is deduced that NaCI02 in absorbent is not excessively decomposed.

2. Absorption of NO in an aqueous slurry of Mg(OH). with NaCI02

In an aqueous solution saturated with Mg(OH).， the OH-concentration is equal ot 0.00092 g-ion/1.

91 suggξsts the pres邑nceof CIO，.On the oth巴rhand， it is considered that the absorption spectra for chlo -rine ovεrlap on thos邑 fornitrogen dioxide， but chlorine could not be detεcted because the mole -cular己xtinctioncoefficient for chlorine is much lower than that for nitrogen dioxid邑 (Thus，inthe analyzer used， the low巴d，limit of detection for chlorine was about 5000 ppm.) Figure 6 shows a typical example of variation of the absorption rate (NA1) and the degr巴eof removal of NO with the process time. The time required to reach th邑steady-statein the absorption process， is increasing with an increase in solid concentration， but the absorption rate at steady-state is indepen -dent of the solid concentration. The steady-state 旦bsorptionrate， which is plotted against the inter -f旦Cl呂1concentration in Fig. 1， is enough high not to be expected from the results for the aqueous slurry of Ca(OH)

，

with N旦CIO

，

as well as the aqueous mixed solution of NaCIO

，

and N呂OH.It is reduced

from detected components in the e伍uentgas that such high absorption r旦tesare attributed to the existence of the gas-phase oxidation of NO with CIO

・

，

InFig. 6 is also shown the v呂riationof the 大 気 汚 物 質 除 去 に 関 す る 研 究 As the absorption of NO proceeds， the pH of the absorbent near the gas-liquid interface may shift above 7， because reaction products through re呂C

tion (iii) described later， HNO

，

and HNO" are accumulat邑d.In an acidic solution， the absorbent NaCIO

，

decomposes to form CIO

:

，

The absorption spectra of thεchlorine dioxide evolved from an aqueous solution of NaCIO

，

with 1

1 350 4CI0

，

-+2H+ー.2CI0

十

，

CIO

，

十

CI ω 間 口 o n 凶 凶 ω 肖 ^ &マロ A Fig. 3. Absorption spectra of CIO

，

evoled from aqueous NaCIO

，

solution by adding H

，

SO

・

，

0.6 百 〉 0.4 E ω 』 恥。 ω 0.2 _。l:'_ 即 刀 0.1 A-A ム ロ マ

。

ふ 企 時 A姐@ 圏 v • N~ λ _z W， wtラも ワ， C 担工 5 mol/1 BO - 0 i1I YAlf~450 ppm 企 企 u vtニ25.5 CC/5 0 o 10 20 30 40 50 60 time， min D A n マ 同 ム 。 マ ロ ム マ

。

o o v o v ム E E IO8ト -:; O ^【 i ; ( 吉 o ロ マω〕 凶 民 L ム占 Y -g '"I 0 0 dct. <'!I l ムマ。 O~ v-D f ド

l

h

仙

[

l

州

﹁

[

ハ

仏

し

。

101' Fig.4.

川

J

350 400 450 500 i1avelength I nm IO9 」一一一一一一一」 400 450 illave length I nm Absorption spectra of e侃uentgas 70 Absorption rate of NO and desorption rate of NO

，

for the N aCIO

，

/Mg(OH)

，

slurry syst巴m.E旺巴ctof solid concentration 10 5 0 Fig.6 ω 悶 口 O 仏 ω ω 出 desorption rate of NO

，

(including the productioIl rate of NO

，

due to the gas-phase oxidation of NO with CIO，)with the process time. From the figure， the ratio N'A，jNA1 at stεady-stat巴attainsOA3~0.52 AIso， for y Alf二215ppm， N'A

，

/NA1 attained0.28~0.

41.For the NaCIO

，

/Ca(OH)

，

slurry system， on the other hand， the ratio for YAlf = 205 ppm reduced to

0.16~0.19. This magnitude can be predicted from

the simulation of gas absorption with the consecu tive reaction whose mechanism and kinetics are well defined in the previous work [1，2J Figure 7 shows the e百ectof N旦CIO

，

concentra -tion on the rate of NO absorption NA1 and the rate of NO

，

desorption (including gas-phase production) N'A

，

.The NaCIO

，

concentration has negligible effect Absorption spectra of NO

，

diluted with N 2・ 5 molar by adding small amount of H

，

SO

，

are shown in Fig. 3. The absorption spectra here are th巴second-orderderivative of the direct (conven -tional) absorption spectra. A seriεs of characteris -tic peaks identifying CI0

，

app己arsin the rang巴of

wave length of 350 nm-450nm. Figure 4 indicates the measurements of absorption spectra of the e百luentgas during the NO absorption， where呂S Figure 5 shows the absorption spectra of N02 only diluted with N

，

.Judging from comparison of ab sorption spectra depict巴din Figs. 3 through 5， there exists CIO

，

in the e任luentgas during th巴NOab -sorption. That is， the form of absorption spectra in the range of 350-400nm in wav巴lengthin Fig. 4 Fig.5

工藤市兵衛・近藤高司・佐田栄三・熊沢英博

in NO (A1) and first-order in NaCI02 (B) and its rate constant at 25 .C is estimated as L8x 1012 _(1/mol)2/sec

[1J . Reaction(II)is found to be second-order in N02 (A2) and first-order in NaCI02 (B) and the third-order reaction rate constant at 25 .C is derived to be 7.32 x 10' (1/moW/sec [2J . Reaction(皿)(hydrolysis of N02) can be also expressed by second-order i

n

:

N02 and its rate censtant at 25 .C is derived as 3.09 x 10' 1/mol sec [2J When Reactions (1)， (II)and (田)are taking palce in the liquid phase， mass balance equations for components A1， A2 and B can be written as follows: 。

6

i

0.4₁₅ E N '

.

園

、

.

，

.

_

"2 '-... A

τ

企 -A N a 電_o a マ_o a 目

。

Q v a ロ a V A S A “ F A a マ ロ 。 ロ 企 . ， 企 マ マ O l --﹁ l i i l i r i -ー -ー ト 2 A V 5 主

u s

"

と E -J Z E 0 4 2 ム O A ﹃ t a v 企 - A 回 国 マ 畠 企 凡 可 企 可 。 ロ A_マ マ 。 A 邑 92 .. ..・・.'"

(

1

)

d2C属 千 DA，石ず=klC'i，ム W， Wt%

v

.

， t:....7.5 0・ " ~己 30 40 50 time. mln ロ

.

_.-80 c 盆1.50開 附1/1 YA1f"215ppm Vt=15.3白 河 _..J_ 10 d (2) 2C

，

DA

，

U_d';;:'=kIICi

，

CB+khYdCi

，

-klCAん 60 20 16刷

。

(3) ) ) a A -A 戸 hd ( ( 、 白 目目 目 目 目 目 目 目目 目 目 目 目 目回 目 、 r ' a a g a E B B -E E E E E E E B E E -J d2

_C

_R

一 一 DBUd';;>"

=

νムCi

，

CB+ν山ICi

，

CB The boundary conditions are given by at Z = 0; CA，=CA，i， dC DA

， す =

kGA

，

(

CA，i-CA'9 dCB n

dz

V Absorption rate of NO and desorption rate of N02. Effect of NaCI02 concentration on the variation of NA1 and N'A2 with time.This fact favorably supports the speculation that there con -siderably exists a conversion of NO to N02 in the gas phase 3

.

s

imulation of the processes of the NO absorption and the accompanying N02 desorption Fig.7. Z =ZL; CA

，

=CA

，

=

0

，

CB=CBO

The mass balance equations and boundary condi -tions may be put into the following dimensionless form: at Dissolved NO reacts with ClO-2 in an alkaline solution of NaCI02 to produce N02 by 2NO+CIO-

→

，

2NO，+Cl- ( i ) Some of N02 produced is consumed by the liquid -phase reactions:

(6)

d2_YA， ，."，

Ud;t'= M1 YA，YB

d2YA. fM1sr

ハー

fM1s'r2 ¥"2

d 伶=(..~U.. jYA，YB十 円 -jYL-x-

，

rl /

，

rl / ) H ( 4N02+CIO-，十40H→4NO-，+Cl一+2H20 and

(

7

)

(8)

(

半

)

Y

i

1Y d2_YB f MI

¥

"

，

，

，

_，fM1s¥ ー d u=17lYLYB+lF7)YLYB _{x- ¥rIQII}

，

_{r1Q2 /} at x

=

0; YA，

=

1， (9) dYー

す

=BI(YA，i -YA'9) ω) The parameters included in Eqs.(6)to(10) take following quantities for廿le conditions developed here:

SF-h

=1.14×10-4 -kICBO s=

与

=4.07X10-4 KI Some evolves into the gas phase without undergoing any chemical reaction. In the following， the processes of the NO absorption and the accompanying N02 desorption are formulatde on the basis of the film model， and the magnitude of the ratio of N02 desorp -tion rate to NO absorption rate will be estimated.

The r巴actionsrelevant to NO and N02 can be described by the following relations: AI+vB→A2十

P

1 (ν1=1/2) (I) A2十 吟B→

P

2 (ν'2=1/4) (II) A2一→P

，

(III) First， the problem will be considered for the absorption of NO in an aqueous mixed solution of L5 molar NaCI02 and 0.2 molar NaOH. The gas-phase concentration of NO is put 1250 ppm (CAu=2x10-. mo!/，)lwhere the reaction rate expression is well established [1J . Under such a high concentration region， reaction ( 1 ) can be expressed by second-order ) -1 ・ 1 ・ 1 ( 2N02+ H20→HNO

，

+HN02 dYB ^ dx -V x = 1; Y A

，

= Y A

，

= 0

，

Y B = 1 at

大気汚物質除去に関する研究

ql=4旦ニ1.5 X 105 q2 = . '<;:;'0 = 3 X 106

ν1しA1i ν2vA1i

M

，

=3.12x10' r

，

=0.745 r2エ1.35

The Biot number B

，

takes 730 under the present experimental conditions and Y A2g is assumed to be

zero The above ordinary differential equations， Eqs. (6) to (8)， are nonlinear :therfore analytical solutions are not expected. Then， a set of differential equations were approximated by the time一centered implicit finite di妊巴renceequations. These implicit equations were simplified by linearinzing th邑reactionterms. A set of resultant simultaneous linear equations was solved by the method of tridiagonal equations. Nu merical results were expressed in t邑rmsof enhance ment factor φAI and ratio N'A2/NAI defined by れ 二

(

!

l

YA

，

¥

¥ dx ! X~O ) -i ( and NAー(_r_1\(互工~\

N

A， -¥ r2

o

A， ) ¥ dx ) X~O (12) as a function of reaction一diffusionmodulus ~ In the course of computation， scales of ql and q2 were reduc巴dby 103 because of rapid convergence

Thereby M

，

d巴cre呂sesby a factor 10-3， whereas s' increases by呂 factor 103. Numerical results are

shown in fig. 8. N'Az/NA1 is calculated as 0.29 when

s=4.07xlO s'=0.114 0.8 Z 4 0 6 、 、

‘

i

'

0.4 0.2

。

10 20 50 100 200 93 1. 0 I I 200 -4 5=:=4.07xlD s'=6.44xlO-2 0.8 ~← / 寸 100

、

、

く

:

メ

7 l山

r

O 730

I -

d

!

0.4 ト ー 1 20 1 1 0 10 20 50 100 300 ~

Fig.9. Theoぽre出tti児caLr閃elation0ぱfNAμ，jNA， 刊Vs/函1a白n

4

仇九A，V刊5/百~ for the NO-NaC口IO，jCa剖(OH)2₂ system 1.0 5"=0 s'=5.42xlO-2 200 100 10OO --150 7l3o0o 、lSl.'" 2 0 10 300 Mwd 叫 z

2

2

0 6 、 、 0.4 0.2 10

府

;

'0-'" Fig. 10. Theoretical relation of NA，/NA， vs/函1and OA， VS

A

for the NO-NaCIO，jMg(OH)2 system 5 300

1

M

，

Fig.8司 Theor巴ticalrelation of NA'/N A， vsjj立1and

OA， VS /百~for th巴NO-NaCI02/NaOHsystem.

1M

戸 177and B

二

，

730.

For the NaCIO，jCa(OH)2 slurry system， OH concentration is evaluated 0.046 g-ion/l:thereby k

二

，

3.20x 1012_{(l/mol)2/sec and s'二 0.0644.The s value is} assumed to be equal to that for the previous case (s二 4.07X 10-4). In this case， ql and q2 are also r巴ducedby a factor 103. Figure 9 shows computational result as a plot of N'A2/NA1 vsjj百1，where N'A2/NA1 is obtai田d as 0.24 when 1/而~=235. and B

，

=730. This prediction a bit exceeds experimental results (0.16 to 0.19).

For the NaCI02/Mg(OH，)system， the OH-con centration is negligibly small and there appears no contribution is negligibly small and there appears no contribution of reaction (II). Therefore sニO.The s' value reduces to 0.0542 (k

，

=3.80 x 1012(l/mol)2/s巴c) Numerical results呂reshown in Fig. 10. N'A2/NA

，

is calculated as 0.24 when JK，f;'=284 and B

ニ

，

730.0n the other hand， experimental values of N'A2/NA' ran -ged from 0.43 ot 0.52 for y Alf二450 ppm as shown in

Fig. 6 and from 0.28 to 0.41 forYAlf=215 ppm. Italso

suggests that there significantly occur both desorp -tion of the decomposition product CI02 into the gas phase and gas-phase oxidation of NO with CI02 to produce N02

9

4

工 藤 市 兵 衛a近 藤 高 司 @ 佐 回 栄 三 ー 熊 沢 英 博

CONCLUSION

The rates of absorption of NO and the ac -companying d色sorptionof NO

，

the NaCIOz/Ca(OH)

，

slurry system， were satisIactorily expectεd from the previous observation in the aqueous mixed solution of NaCIO

，

and NaOH with higher OH-concentration，

where旦sfor the NaCl02/Mg(OH)

，

slurry system， the

absorption rate of NO noticeably exceξded that for the former systems and the ratio of the NO

，

d己sorp tion rate to the NO且bsorption rate considerably exceeded the theoretical prediction for gas absorp tion with the consecutive reaction. The maximum d巴viationbεtween two factors has呂ttain巴d117

%

Also， chlorine dioxide w呂sdetectedIil the g旦sphasε It was deduced from th日seexpεrimental evidences that there significantly occur both desorption of the decomposition product C10

，

into the gas phas己and gas-phase oxidation of NO with C102 to produce

NO

，

Acknowledgements-Th芭 authors wish to express

their thanks to Japan S巴curitiesScholarship Foundーョ tion for its finan-cial support NOTATION Ap surface area of solid particlぉ=6w/ρdp，cm'/ B

，

c

cm3-dispersion O Biot number= kGA，fku12 concentration in liquid phase， mol/cm3 or mol/l D bi在usivityin liquid phase， cm'/s dp average diameter of solid particles， cm k" rate constant of reaction(II)， (l/mol')/sec kL mass transfer coe伍cietin liquid phase， cm/ sec M1 reaction-diffusion modl山s=k

，

CAlICBO/(ιA1)' V

，

total gas f10w rate， cm3/sec W concentration oI solid， g/cm3-dispξrsion of wt%

*

relative to that in the bulk of liguid or at solid surface p density of solid， g/ cm 3 o without chemical reaction REFERENCES

[lJ Sada E.， Kumazawa H.，Kudo 1. and Kondo T.，

Chem. Engng Sci. 1978 33 315

[2J Sada E.， Kumazawa H.， Kudo 1. and Kondo T under contribution.