04-l.petkov

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L. Petkov, T. Todorov , I. Dardanova, K. Boshnakov

133

Journal of the University of Chemical Technology and Metallurgy, 41, 2, 2006, 133-136

MATHEMATICAL MODELLING OF THE PROCESS OF ELECTROCHEMICAL

PRODUCTION OF NaClO FROM DILUTED CHLORIDE SOLUTIONSL. Petkov, T. Todorov , I. Dardanova, K. Boshnakov

University of Chemical Technology and Metallurgy8 Kl. Ohridski, 1756 Sofia, Bulgaria

E-mail: [email protected]

ABSTRACT

The sodium hypochlorite synthesis using diluted chloride solutions represents an actual problem considering the number of applications of this compound - as an antiseptic, for the waste waters treatment, etc . The goal of this work is the elaboration of a mathematical model describing the relationship between the quantity of the obtained hypochlorite and the electrical energy consumption from one part and the current value and the concentration of the initial chloride solution, from the other. The experiments were carried out in cell ( 200 ml ) with cathodes from iron and anodes fromruthenium and titanium oxides . The initial concentration of the chloride solutions varied in the range 0.05 0.45 g/l. The chosen experimental model was the full factorial one with two levels of the factors. According to this model, the mass of the obtained sodium hypochlorite is maximal for the maximal sodium chloride concentration and the maximal current value. The correlation coefficient is 99 %, which confirms the validity of the model. Concerning the energy consumptionE, the minimal values correspond respectively to the highest sodium chloride concentration and current values. The correlation coefficient in this case is however equal to 75 % and some supplementary studies for the modelin this respect

precision are needed.Keywords: electrosynthesis, sodium hypochlorite, mathematical model, correlation coefficient.

Received 18 November 2005 Accepted 05 January 2006

INTRODUCTION

The process of electrochemical production of NaClO from diluted chloride solutions is importantregarding the purification of natural and industrial wa-ters, the extraction of highly toxic compounds from waste

waters (e. g. cyanides), the fabrication of pharmaceuti-

cals, etc. That is why, the investigations related to theeffect of the different experimental conditions: currentdensity, composition of the solutions, rate of the flowacross the electrolysis cell on the process of produc-tion, represents an interest [1-9].

As it is known a characteristic for the electroly-sis of low concentrated solutions is the low current ef-ficiency. In order to improve this parameter of the pro-cess, electrodes and after all anodes, containing noblemetals as platinum, ruthenium etc. are used [10-14].The disadvantage of those electrodes consists in their

high price. Consequently, more of the investigations are

dedicated to the possibility of their substitution. An-odes from graphite, MnO 2, PbO 2, magnetite, etc. have

been proposed. However, because of their insufficienteffectiveness, the possibilities of using anodes covered

with an active spinel layer are studied. As known, theminerals from the class of the complex oxides with ageneral formula AB 2O4, where A is a bivalent ion Zn,

Mn, Co, Ni, etc and B is a triavalent ion Al, Fe, Cr, Mn,Ti belong to them. Useful to the solution of the prob-lem with the substitution of the noble metals could bethe inert electrodes investigated regarding some galvanicsystems chrome, alloys, etc. [21-24].

The investigation of the mentioned process is alsoimportant because of the fact that the productiveness of the electrolysis is 10 fold lower than that of the purelychemical synthesis. In this aspect, studies related tomodeling and optimization of this electrochemical pro-cess are actual. As known, due to the possibilities of the

computer data treatment, the methods of mathematical

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modeling and optimization play an important role forthe development of technological regimes and construc-tion of electrochemical reactors [25-28].

The purpose of this work is the development of models correlating some factors of the process of electro-chemical synthesis of NaClO from diluted solution of NaCland especially the mass of the product and electrical en-ergy consumption (output parameters) from one part andthe current value and concentration of the initial chloridesolution (input parameters), from the other part. Theseinvestigations represent a continuation of our previous stud-ies [29], resulting in the development of models for theprocess of electrolysis of concentrated chloride solutions.

EXPERIMENTAL

The experiments were carried out in an electroly-sis cell (PVC) with a volume of 200 ml. The cathodes(steel 1X18H9T) and the anodes (ruthenium and tita-nium oxides) were disposed in a package, vertically (4cathodes and 3 anodes) with a distance between them0.8 cm. NaCl p. a. was used for solution preparation.The experiments were performed maintaining a con-

stant current value (galvanostatically), using a regulatedpower supply (TES-9). The working solution flow acrossthe electrolysis cell was insured due to the gravitation.The rate was keeping constant through a system of stop-cocks. The hypochlorite concentration was determinedusing a iodometrical method modified for small quan-

tities of active chlorine. The experimental device is sche-

matically presented in Fig. 1.

RESULTS AND DISCUSSION

Investigations on the influence of the current densityThe relationship between the current value and the

mass of the active chlorine, the current efficiency and thespecific energy consumption is presented in Fig. 2. All of the experiments were carried out in the following condi-tions: solution volume of 1 dm 3 and initial NaCl concen-tration 0.05 g/l. The current values correspond to cur-rent density from the order of 1x10 -3 1x10 -2 A. dm -2.

As shown in the figure, the increase of the current value leads to an increase of the mass (the concentration)of the active chlorine, as well as of the current efficiency;this relationship has another character concerning thespecific energy consumption a decrease in energy con-sumption is observed. The values of the current efficiencyare relatively low and those of the specific energy con-sumption are high and the characteristic for the elec-trolysis of low concentrated solutions.Investigations on the influence of the NaCl concen-tration

The obtained experimental data are presented inFig. 3, a, b, c.

As shown in the figures, the increase of the initialNaCl concentration leads to an increase of the quantityof the active chlorine. For example, for a concentrationof the NaCl equal to 0.05 g dm -3, the mass of the activechlorine is 7.04 mg and for a NaCl concentration of 0.6g dm -3, it is 52.4 mg. By analogy of the influence of thecurrent density, here an increase of the current efficiencyand a decrease in the energy consumption are also ob-served. For the cited concentrations, the current effi-

ciency is 13.2 % and 26 % respectively and the energyconsumption 40.5 wth/g and 15.9 wth/g.Planning of the experiments

Based on the performed and describe above ex-periments, a conclusion could be made that the rela-tionship between the input parameters (NaCl concen-tration and current value I) and the output parameters(mass of the active chlorine and energy consumption)has an exponential character, i.e. it is not linear.

The elaborated plan of experiments is presentedin Table 1. A full factorial experiment with two levels

of the factors was chosen.

Fig. 1. Schem of the device for H 2O decontamination (disinfection)1. reservoir with initial NaCl solution; 2. Electrolysis cell with apacket of electrodes; 3. reservoir for disinfected water; 4. regulatedpower supply; 5. Ampermeter; 6. Voltmeter; 7. Anodes; 8.

Cathodes.

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Mathematical modelsThe obtained mathematical models according to

the plan of the experiments can be expressed with thefollowing equations: m = 1,6 4,02 C NaCl - 0,0475 I +0,0732 C NaCl I+1,86C 2

NaCl + 0,000407 I 2

E = 630 598 C NaCl -13,5 I + 1,22C NaCl I +1011 C 2

NaCl + 0,0797 I 2

with : m the mass of the sodium hypochlorite,g ; C the concentration, g/l ; I the current, mA ; E

the specific energy consumption, Wth /g ;It is evident, considering the equation of the first

model, that maximal quantities of active chlorine can be obtained for the corresponding maximal concentra-tions of the initial NaCl and for current values in thestudied ranges of these parameters. The analysis of themodel shows that the influence of the chloride concen-tration is stronger that this of the current through theelectrolysis cell. This fact can be explained taking intoconsideration that with the increase of both the chlo-ride concentration and the current, the first parameteracts in higher degree on the current efficiency of theprocess of active chlorine synthesis.

In accordance to the equation of the second model,for maximal values of the input parameters, a minimumis observed regarding the specific energy consumption.In this case the influence of the NaCl concentration ismore strongly too. The concentration increase provokesa decrease in the energy consumption of the process, be-cause of the decrease of the solution resistance. The cur-rent efficiency for the product increases, too.

The current influence is a result of the action of

opposite factors. From one side, the current efficiency

Fig. 2. Study of the influence of the current value I (1a) on the

process of active chlorine production for a constant NaClconcentration (C = 0.05 g/L)

Fig. 3. Study of the influence of the NaCl concentration. a - 0,1g/l; b - 0,3 g/l; c - 0,6 g/l.

increases, but from the other side the voltage of the

electrolysis cell increases and the energy consumption,too. It is evident (and the described experiments con-firm this fact), that the decrease of the energy consump-tion due to the increased current efficiency gets ahead of its increase because of the higher sell voltage.

The verification made for the adequacy of the mod-els shows that the correlation coefficients are 99% (for thefirst model) and 75% (for the second model) respectively.

These values of the correlation coefficients con-firm that the first model is significant.

Additional experiment are needed for the second

model, in order to reach a higer degree of correlation.

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CONCLUSIONS

The investigations on the process of electrochemi-cal production of NaClO using diluted solutions theimportant influence of the NaCl concentration and thecurrent value on the product concentration and the

electrical energy consumption. The obtained mathemati-cal models confirm that maximal NaClO masses andminimal energy consumption are present for maximal

values of the input parameters (concentration and cur-rent) in the studied ranges. The models equation are of second degree, i.e. they are not linear.

Acknowledgment

The presented investigation constitutes a part of the scientific project Purification of waste waters by elec-trolysis , financed by the National Fond for Scientific In-vestigations of the Bulgarian Ministry of Education. The authors tank for the financial support.

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Table 1. Plan of the experiment.

C NaCl ,g/l I ,mA m , mg W , W.h./g CE , %

0,05 60 0,284 96,1 7,2

0,05 80 0,550 85,13 8,94

0,05 100 1,012 62,46 13,16

0,25 60 0,355 71 6,43

0,25 80 0,923 32,6 14,7

0,25 100 2,04 20 25,87

0,45 60 0,799 28,8 14,8

0,45 80 1,668 18,08 24,84

0,45 100 2,698 13,59 33,59

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04-l.petkov