*Corresponding author. Tel.: #86-591-7893524.E-mail address: clin@fzu.edu.cn (C. Lin).

Chemical Engineering Science 56 (2001) 667}671

Modeling of hydrazodicarbonamide oxidation by chlorinein a gas}liquid}solid system

Cheng Lin*, Ji-Yu ZhangInstitute of Chemical Engineering and Technology, Fuzhou University, Fuzhou 350002, People's Republic of China

Abstract

The oxidation reaction of solid hydrazodicarbonamide with chlorine in aqueous solution is experimentally studied in a 1 l agitatedglass vessel reactor with a four-blade paddle stirrer. Based on the "lm theory, a model is "rst developed for this complicated system bytaking account of the gas}liquid and solid}liquid mass transfer, solid dissolution, and instantaneous reaction, especially the reductionof solid}liquid mass transfer interfacial area arising from the solid reactant dissolution. The model can reasonably and well describethe experimental data. According to the values of gas}liquid and solid}liquid mass transfer coe$cients estimated with the proposedmodel, it is found that the gas}liquid mass transfer is a rate-controlling step in the majority of the reaction course, but the solid}liquidmass transfer becomes a rate-controlling step when the reaction is close to the "nal stage of the process. ( 2001 Elsevier Science Ltd.All rights reserved.

Keywords: Azodicarbonamide; Mass transfer; Solid dissolution; Film theory; Gas}liquid}solid reaction; Rate-controlling step

1. Introduction

Azodicarbonamide, one of the important blowingagents, is being used in plastics and the rubbers, etc.Although azodicarbonamide has been commerciallymanufactured by the oxidation reaction of solid hy-drazodicarbonamide with chlorine in aqueous solutionsince late 1960s, some problems still exist with regard tomodeling of elementary steps of the process, namelythe absorption of chlorine into the liquid phase,hydrazodicarbonamide dissolution, reaction of the dis-solved chlorine with the dissolved hydrazodicarbonam-ide, and azodicarbonamide crystallization. No publishedwork deals with the reaction kinetics of this complicatedreaction system, which is very important for the optimumdesign and operation of this reaction system and is alsoacademically interesting.

In the past decades, many theoretical and experimentalstudies were done on gas}liquid}solid three phase reac-tion systems (Ramachandran & Sharma, 1969; Kojima,Hakuta, Kudoh, Ichinoseki & Midorikawa, 1989; Lan-cia, Musmarra, Pepe & Volpicelli, 1994; Lancia, Mus-marra & Pepe, 1997), but these studies mainly focused on

the gas absorption process. Little attention was paid tothe e!ect of the dissolution of solid reactant on theoverall process rate, and dealt with the conversion ofsolid reactant, which, in practice, much attention has tobe drawn to such gas}liquid}solid multiphase systems asthe oxidation reaction of hydrazodicarbonamide withchlorine.

Based on the "lm theory, we had previously developeda theoretical model to describe the enhancement factor ofgas absorption or solid dissolution by considering thee!ect of solid dissolution (Lin, & Zhang, 1998, 2000). Inthis study, the model is extended to express the relation-ship of conversion of solid hydrazodicarbonamide toreaction time, and is employed to evaluate the role ofeach step of process kinetics in the reaction system.

2. Theoretical considerations

The overall oxidation reaction of solid hydrazodicar-bonamide with chlorine in aqueous solution may bewritten as

NH2CONHHNCON

2(s)#Cl

2(g)H2O,C!5

&&&"

NH2CONNCONH

2(s)#2HCl.

0009-2509/01/$ - see front matter ( 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 2 7 4 - 8

As mentioned earlier, the oxidation of hydrazodicar-bonamide by pure chlorine is a complex multiphasereaction process. Three important steps, viz., chlorineabsorption into liquid phase, hydrazodicarbonamide dis-solution into liquid phase and reaction of the dissolvedchlorine with hydrazodicarbonamide are taken into ac-count in this study. Since chlorine and solid hy-drazodicarbonamide are sparingly soluble in water andthe mass transfer is often regarded as a rate-controllingstep for the overall reaction process in such reactionsystems (Albal, Shan & Schumpe, 1983; Gholap, Kolhe,Chaudhari, Emig & Hofmann, 1987); it is considered thatthe reaction between the dissolved chlorine and hy-drazodicarbonamide is relatively fast and is assumed tobe an instantaneous reaction. When the reaction is oper-ated continuously with respect to chlorine and batchwisewith respect to the solid reactant in the slurry, the pro-cesses can be expressed in the manner analogous to thehandling of the gas}liquid instantaneous reaction basedon the "lm theory in our previous study (Lin and Zhang,1998, 2000). The three steps can be written as

Case 1 (reaction takes place in the liquid "lm near thegas}liquid interface):

(1) Transport of chlorine (A) into the liquid phase onaccount of the enhancement of instantaneous reaction

N@A"k

LAaC

AiA1#D

LBC

BLD

LAC

AiB. (1)

(2) Transport of dissolved hydrazodicarbonamide (B)into the liquid phase

N@B"k

SA

P(C

BS!C

BL)

"kSnpd2

p(C

BS!C

BL). (2)

(3) Dissolution rate of solid reactant hydrazodicar-bonamide

!

dnB

< dt"N@

B. (3)

Case 2 (reaction takes place in the liquid "lm aroundsolid particles):

(1) Transport of chlorine (A) into the liquid phase

N@A"k

LAa(C

Ai!C

AL). (4)

(2) Transport of dissolved hydrazodicarbonamide (B)into the liquid phase on account of enhancement of theinstantaneous reaction

N@B"k

Sntpd2

pC

BSA1#D

LAC

ALD

LBC

BSB. (5)

(3) Dissolution rate of solid reactant hydrazodicar-bonamide

!

dnB

< dt"N@

B. (6)

The relationship between the conversion of solid hy-drazodicarbonamide and the ratio of the mean diameterof unreacted solid particles to the initial mean diameterof solid hydrazodicarbonamide is given as follows:

X"

=0!=

=0

"1!(dp/d

p0)3. (7)

Assuming that the series mass transfer steps are at thequasi-stationary state and that the mass transfer coe$-cients, k

LAa and k

Sare constant in the whole reaction

course, combining Eqs. (1)}(3) or Eqs. (4)}(6) with Eq. (7)gives the same following expression for the relationshipof hydrazodicarbonamide conversion to reaction timeaccording to the reaction stoichiometry

t"aX#b[1!(1!X)1@3], (8)

where

a"=

0/<

kLA

aMBC

BS(D

LB/D

LA#C

Ai/C

BS), (9)

b"oBdp0

(DLB

/DLA

)

2ksM

BC

BS(D

LB/D

LA#C

Ai/C

BS). (10)

and a, b represent the resistance of gas}liquid masstransfer and of solid}liquid mass transfer, respectively.

It is seen from the Eq. (8), that if the gas}liquid masstransfer process is rate-controlling, then the solid conver-sion depends linearly on time. Conversely, a nonlineardependence is obtained when solid dissolution is rate-controlling.

3. Experimental

Experiments are performed in a 1 l agitated glass vesselreactor with a four-blade paddle stirrer. The reactor iskept in a thermostatic bath to maintain reaction temper-ature within $1 di!erence. Pure chlorine, taken froma lique"ed chlorine cylinder, is used as a gas reactant. It#ows through a rotameter and sparges into the reactorthrough a gas distributor at the bottom of reactor. Thevolumetric #ow rates of pure chlorine are varied from1.938]10~6 to 4.772]10~6 m3/s. Solid reactant is hy-drazodicarbonamide with di!erent mean diameters(6.04]10~2, 4.50]10~2 and 2.20]10~2 cm) of narrowsize distribution by sieving. Slurry phase is prepared bymixing distilled water with solid hydrazodicarbonamidein di!erent solid concentrations of 90.9, 145.5, 181.8 and272.7 kg/m3, respectively.

In a typical run, a 550 cm3 of slurry containinga known amount of solid hydrazodicarbonamide anda catalyst is charged into the reactor. When the desiredtemperature is reached, the slurry is stirred at a speci"edstirrer speed, which is measured by a phototachometer,then pure chlorine is introduced into the reactor to startthe run. Several slurry samples, each of only 1 cm3 are

668 C. Lin, J.-Y. Zhang / Chemical Engineering Science 56 (2001) 667}671

Fig. 1. Reaction time curves vs. degree of conversion at di!erent Cl2

#ux.

Fig. 2. Reaction time curves vs. degree of conversion at di!erent stir-ring speeds.

Fig. 3. Reaction time curves vs. degree of conversion at di!erent solidloadings.

pipetted out at di!erent reaction time. The samples are"rst handled by adding 30% (w/w) sodium hydroxidesolution until the product azodicarbonamide is dissolvedover. Then, the remainder, unreacted solid particles, isanalyzed for their mean diameter by using a Particle SizeAnalyzer (Coulter LS230, USA) to calculate the conver-sion of solid hydrazodicarbonamide at di!erent samplingtimes according to Eq. (7).

4. Experimental results and discussions

Detailed experiments are conducted under di!erentchlorine #ow rate, stirring speed, solid particles diameter,solid loading, and reaction temperature, to determine therate-controlling step in this reaction system.

4.1. Ewect of chlorine yow rate

One of the major parameters that a!ect the processrate is the chlorine #ow rate. The e!ect of di!erentchlorine #ow rates on the reaction time is shown inFig. 1 under certain other operation conditions. It can beseen that the reaction time, needed for the same conver-sion, is reduced with an increase of chlorine #ow rate.The increase of chlorine #ow rate will cause the increaseof the gas}liquid interfacial area so as to enhance thegas}liquid mass transfer rate. Consequently, the reactiontime is reduced.

4.2. Ewect of stirring speed

The e!ect of a stirring speed on reaction time is studiedin the range of 6.67}15.0 Hz. It is observed that thestirring speed strongly a!ects the reaction time and thereis over a two-fold increase in the reaction time when thestirring speed is changed from 15 to 6.67 Hz as given inFig. 2. This behavior is expected since the increase ofstirring speed will enhance slurry turbulence and bubblesbreakage, which results in an increase of mass transfer

rate. This implies that the mass transfer process may playan important role in the reaction process.

4.3. Ewect of solid loading

Fig. 3 exhibits the e!ect of solid loading on reactiontime. The solid loading is varied from 90.9 to 272.7 kg/m3

while a constant slurry volume is maintained to ensurea constant mean residence time of pure chlorine. It can beclearly seen that the lower the solid loading, the less thereaction time required.

4.4. Ewect of solid particle diameter

Particle sizes can a!ect the solid}liquid mass transferrate and the complete dissolution time of solid particles.The smaller the particles size, the shorter the dissolutiontime, hence the shorter the reaction time. Fig. 4 showsthis phenomenon of the solid hydrazodicarbonamide sizee!ect on the reaction time for particles of three di!erentsizes.

4.5. Ewect of temperature

Although the experimental results of temperature ef-fects on the reaction time are not provided in this paper,

C. Lin, J.-Y. Zhang / Chemical Engineering Science 56 (2001) 667}671 669

Fig. 4. Reaction time curves vs. degree of conversion at di!erent meandiameters of solid particles.

Table 1Parameter values used in the model

Parameter Value Source

DLA

1.48!1.80]10~9 m2/s Hikita et al. (1975)D

LB1]10~9 m2/s Assumed

CAi

3.40!6.25]10~2 kmol/m3 Hikita et al. (1975)C

BS1.35!1.69]10~3 kmol/m3 Determined

kLA

a 3.21!9.60]10~3 s~1 EstimatedkS

0.667!3.11]10~4m/s Estimated

it is found that the reaction time is only slightly a!ectedwithin the experimental temperature range from 308 to326 K. This con"rms that the overall reaction rate iscontrolled not by reaction but by mass transfer.

4.6. Analysis of the experimental results

As demonstrated in the Figs. 1}4, the conversion ofhydrazodicarbonamide is almost linear with reactiontime in most of the reaction course under various operat-ing conditions, except the "nal stage of the reaction. Thissuggests that the reaction is a zero order or more likelyinstantaneous one, therefore, we attempt to usethe model proposed by this paper to interpret theexperimental data.

In order to compare the experimental results with thetheoretical predictions, it is necessary to know the valueof the parameters used in the model. The values of di!us-ivity and solubility of chlorine in water are taken fromthe literature (Hikita, Asai, Ishikawa & Saito, 1975). Nodata are available for the di!usivity and solubility ofhydrazodicarbonamide in the liquid phase. Generally,the value of the di!usivity of dissolved solid is in theorder of 10~9 m2/s (Sada, Kumazawa & Butt, 1977),therefore, the di!usivity of hydrazodicarbonamide in theliquid phase is assumed as 1]10~9 m2/s, and the solubil-ity of hydrazodicarbonamide in the liquid phase is mea-sured in this study. The values are listed in Table 1. Itshould be pointed out that, although the hydrochloricacid produced in reaction may a!ect the solubility ofhydrazodicarbonamide, we still neglect this e!ect in thiswork since the solubility of hydrazodicarbonamide inHCl aqueous solution is only slightly higher than in purewater according to our experiment.

The volumetric gas}liquid mass transfer coe$cientkLA

a and the solid}liquid mass transfer coe$cient kS

areestimated by the non-linear least-squares method. Theresults are also given in Table 1. It is seen that the valuesestimated in the present study fall into the normal magni-tude range of the value of mass transfer coe$cients. As

demonstrated in Figs. 1}4, the predictions of the modelare in good agreement with the experimental data. Thiscon"rms that the model proposed in this paper is accept-able and is reliable to describe this complicated reactionsystem. According to the estimated values of themass-transfer coe$cients, one can "nd that the followingcondition is satis"ed in most of our experiments

kLA

aCAi(k

SA

pC

BS. (11)

This indicates that the gas}liquid mass transfer playsa dominating role in the reaction process. Therefore, theconversion of solid reactants depend almost linearly onthe reaction time as shown in Figs. 1}4. However, itshould be pointed out that as the reaction proceeds, theinterfacial area of solid}liquid mass transfer, A

pin

Eq. (11), decreases gradually with the dissolution of solidparticles. Especially, when the reaction process is close tothe end of reaction, the interfacial area decreases dra-matically. As a consequence, the rate-controlling stepwould shift from gas}liquid mass transfer to solid}liquidmass transfer. This can elucidate, as Eq. (8) indicates, whythe experimental curves of the hydrazodicarbonamideconversion versus reaction time exhibit the character-istics of non-linearity in the "nal stage of the reaction.

5. Conclusions

(1) Based on the "lm theory, a model is "rst developedfor the oxidation reaction of hydrazodicarbonamide withpure chlorine in aqueous solution by taking account ofthe gas}liquid and solid}liquid mass transfer,solid reactant dissolution, and instantaneous reaction ofthe dissolved chlorine with the dissolved hydrazodicar-bonamide. A good agreement between experimental re-sults and model predictions indicates that the model candescribe, reasonably well, this complicated process.

(2) The e!ects of chlorine #ow rate, stirring speed, solidparticles diameter, solid loading, and reaction temper-ature on the overall reaction rate are experimentallyinvestigated to assess the rate-controlling step of thereaction process. The results show that the gas}liquidmass transfer is a rate-controlling step in most of thereaction course, according to the value of gas}liquid andsolid}liquid mass transfer rates estimated with theproposed model. It is, therefore, necessary to intensify the

670 C. Lin, J.-Y. Zhang / Chemical Engineering Science 56 (2001) 667}671

gas}liquid mass transfer process in order to speedup theoverall reaction rate.

Notationa gas}liquid interfacial area, m2/m3

Ap

solid}liquid interfacial area, m2/m3

CAi

chlorine equilibrium concentration in water,kmol/m3

CAL

chlorine concentration in bulk liquid, kmol/m3

CBL

hydrazodicarbonamide concentration in bulkliquid, kmol/m3

CBS

hydrazodicarbonamide equilibrium concentra-tion in water, kmol/m3

dp

diameter of solid particle, mdp0

initial mean diameter of solid particles, mD

LAdi!usivity of chlorine in water, m2/s

DLB

di!usivity of hydrazodicarbonamide in water,m2/s

kLA

mass transfer coe$cient of chlorine in liquidphase, m/s

kS

mass transfer coe$cient of hydrazodicarbonam-ide in liquid phase, m/s

MB

molecular weight of hydrazodicarbonamide,kg/mol

n number of solid particles per unit reaction vol-ume, 1/m3

nB

molecular number of hydrazodicarbonamide,kmol

t reaction time, s< reaction volume, m3

= weight of unreacted hydrazodicarbonamide, kg=

0initial weight of hydrazodicarbonamide, kg

X conversion

Greek lettersa de"ned in Eq. (6), sb de"ned in Eq. (7), soB

hydrazodicarbonamide density, kg/m3

Acknowledgements

The "nancial support of the Education Committee ofFujian Province, China, is gratefully acknowledged.

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C. Lin, J.-Y. Zhang / Chemical Engineering Science 56 (2001) 667}671 671