Isopropyl mercaptan is an important pharmaceutical intermediate and chemical material. And thiourea and triglycol arethe main materials for the synthesis of isopropyl mercaptan. Therefore the dissolution of thiourea in triglycol solution is very important for the production of isopropyl mercaptan. The aims of this study are to examine the dissolution kinetics of thiourea in triglycol solution, and to present an alternative process for producing isopropyl mercaptan. In order to investigate the dissolution kinetics of thiourea in triglycol solution, the concentrations of solution and reaction temperature were selected as experimental parameters. It was determined that the dissolution rate of thiourea increased with the increase in solution concentration and temperature. An empirical equation was used in fitting the data. Statistical analysis indicated small errors and the results should be reliable.
INTRODUCTION
Isopropyl mercaptan is an important pharmaceutical intermediate and chemical material. In recent years, the market demand is increasing steadily, and has aroused attention from both home and abroad. Thiourea and triglycol are the main materials for the production of isopropyl mercaptan;
1
,
2
the solubilities of thiourea in triglycol increase with rising temperature. But so far, no study has been found in the literature concerning the dissolution kinetics of thiourea in triglycol solution. However in the study of the preparation methods of isopropyl mercaptan, it is necessary to investigate the dissolution rate at different temperatures. Hence the dissolution kinetics of thiourea in triglycol solution is investigated in this paper.
EXPERIMENTAL
 Materials
Thiourea and triglycol were of AR grade and were obtained from Shanghai Chemical Reagent Co. with purities of 0.995 in mass fraction. Deionized water was used.
 Apparatus and Procedure
The dissolution experiments were carried out in a 50 mL cylindrical glass reactor equipped with a mechanical stirrer, a reaction temperature control unit (a constanttemperature bath), and a condenser for avoiding loss of solution by evaporation. The apparatus was shown in
. 1
. The experimental procedure was as follows: the triglycol solution at a definite concentration (the mass fractions of triglycol were 0.72, 0.82, 0.91 and 1.00 respectively) was placed in the glass reactor. The reactor jacket was heated to the desired temperature and the stirring speed was set at a rate of 600 ~ 700 r/min. A given amount of solid sample was added into the solution. The dissolution process was carried out for various reaction time. During the experimental process, a sample was taken out from the reactor at intervals and the concentration of thiouea in triglycol solution was determined throngh solidliquid separation using the cooling method.
Solubility experimental apparatus. (1) laser generator; (2) magnetic stirrer; (3) condenser; (4) microthermometer; (5) dissolution vessel; (6) port of circulating water for temperature controlling; (7) photoelectric converter; (8) digital display.
Each experiment was replicated twice and an arithmetic average of the sample was used. These experiments could be repeated with a maximum deviation of approximately 1.5%.
 Test of apparatus
In order to ensure proper operation of the apparatus, the solubility of NaCl in water was measured and compared with the values reported in the literature.
3
The experimental measurements agreed with the reported values with a mean relative deviation of 0.20%. The measured values were listed in
1
.
Solubility of NaCl in water
Solubility of NaCl in water
RESULTS AND DISCUSSION
 Effect of temperatures on the dissolution of thiourea
The solubility curves of thiourea in triglycol solution at intervals and at different temperatures, and different triglycol mass fractions were presented in
2

5
, respectively.
2

5
showed that the solubility of thiourea increased as the temperature rised.
Data for the dissolution of thiourea in triglycol + water mixtures (w= 0.72)
Data for the dissolution of thiourea in triglycol + water mixtures (w = 0.72)
Data for the dissolution of thiourea in triglycol + water mixtures (w= 0.82)
Data for the dissolution of thiourea in triglycol + water mixtures (w = 0.82)
Data for the dissolution of thiourea in triglycol + water mixtures (w= 0.91)
Data for the dissolution of thiourea in triglycol + water mixtures (w = 0.91)
Data for the dissolution of thiourea in triglycol + water mixtures (w= 1.00)
Data for the dissolution of thiourea in triglycol + water mixtures (w = 1.00)
 Effect of triglycol mass fractions on the dissolution of thiourea
To investigate the effect of the triglycol mass fractions on dissolution rate, the experiments were carried out in the mass fractions of 0.72, 0.82, 0.91 and 1.00 respectively. The results plotted in
. 2

5
showed that the dissolution rate decreased with the increase in triglycol mass fraction.
Ct curve for the dissolution of thiourea in triglycolwater mixtures (w = 0.72). Symbols: ▼: 298.15 K, ▲: 308.25 K, ●: 317.25 K, ■: 332.35 K.
 Kinetic analysis
The kinetic equation which describes the dissolution rate of solid could be expressed in the Stumm equation.
4
,
5
,
6
Where
C
_{s}
is the saturation concentration of thiouea; C represents the concentration at t time; K is the rate constant; n is reaction order.
The calculated concentrations of thiourea in triglycol solution at intervals in eq 1 were given in
2

5
. The
Ct
curves in eq 1 were shown in
. 2

5
. The values of parameters
K
,
n
, the correlation coefficient (R
^{2}
) and the rootmeansquare deviation (σ) were listed in
6
.
Ct curve for the dissolution of thiourea in triglycolwater mixtures (w = 0.82). Symbols: ▼: 297.65 K, ▲: 307.55 K, ●: 318.25 K, ■: 332.75 K.
Ct curve for the dissolution of thiourea in triglycolwater mixtures (w = 0.91). Symbols: ▼: 298.45 K, ▲: 308.65 K, ●: 318.75 K, ■: 333.65 K.
Where
N
is the number of experimental points; cci represents the concentration calculated from eq 1;
c
_{i}
represents the experimental concentration value.
The relative deviations between the experimental value and calculated value were also listed in
2

5
. The relative deviations (RD) were calculated by using the following formula.
Ct curve for the dissolution of thiourea in triglycolwater mixtures (w = 1.00). Symbols: ▼: 298.65 K, ▲: 308.75 K, ●: 317.65 K, ■: 332.95 K.
From
2

5
，it could be seen that the relative deviations in eq 1 among all of these values did not exceed 2.74%; from
6
, it could also be learned that the correlation coefficient (R
^{2}
) was greater than 0.99 and the rootmeansquare deviation did not exceed 6.19%, which indicated that the Stumm equation was fit to correlate the dissolution kinetics data of thiourea in triglycol + water mixtures and the dissolution process of thiourea in triglycol + water mixtures was a diffusion process. From
. 2

5
, it was shown that for the mixed solvent of the same mass fraction of triglycol, the higher the temperature, the greater the solubility of thiourea, the faster the rate of proliferation, and the less the dissolution time.
Calculation results ofK,nduring the dissolution process of thiourea in triglycol + water mixtures
Calculation results of K, n during the dissolution process of thiourea in triglycol + water mixtures
 Activation energies
The temperature dependence of the chemical reactions can be given in the Arrhenius equation:
Calculation results of activation energy according to the Arrhenius equation
Calculation results of activation energy according to the Arrhenius equation
Where
K
is rate constant;
K
_{0}
is a preexponential factor;
E
_{a}
represents activation energy;
R
is gas constant;
T
represents temperature.
According to this equation, the slope of the curve between ln K versus 1/T should give a straight line whose slope equals to －E
_{a}
/
R
.
. 6
showed the Arrhenius plots for dissolution of thiourea in different triglycol solutions. The activation energies derived from these curves were found as 12.31 kJ/mol, 17.77 kJ/mol, 38.95 kJ/mol, 59.20 kJ/mol for the triglycol mass fractions of 0.72, 0.82, 0.91 and 1.00 respectively. This indicated that the triglycol mass fractions significantly affected the dissolution of thiourea. The activation energy of thiourea in triglycol solution increased with the increasing triglycol mass fraction. The results of activation energy were listed in
7
.
Arrhenius plot of the dissolution process. Symbols: ■: w = 0.72, ●: w = 0.82, ▲: w = 0.91, ▼: w = 1.00.
CONCLUSION
The solubilities and dissolution rate of thiourea in triglycol solution were measured at intervals, at different temperatures and in different triglycol mass fractions. The higher the temperature, the better the dissolution of thiourea; the dissolution rate increased with that of the temperature.
The Stumm equation was used to correlate the dissolution kinetics data and the calculated value in the models was in good agreement with the experimental data. The result of fitting indicated the dissolution process of thiourea in triglycol + water mixtures was a diffusion process.
The experimental dissolution kinetics and correlation equation in this work can be used as essential data and model for the synthesis of isopropyl mercaptan.
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