Znd. Eng. Chem. Res. 1988, 27,845-847 Szekely, J.; Propster, M. “A Structural Model for Gas-Solid Reaction with a Moving Boundary-VI”. Chem. Eng. Sci. 1975, 30,
1049-1055.
Szekely, J.; Lin, C. I.; Sohn, H. Y. “A Structural Model for Gas-Solid Reactions with a Moving Boundary-V”. Chem. Eng. Sci. 1973,223, 1975-1989. Yortsos, Y. C.; Sharma, M. ”Application of Percolation Theory to Noncatalytic Gas-Solid Reactions”. AIChE Annual Meeting, San
845
Francisco, 1984. Yu, H. C.; Sotirchos, S. V. “A Generalized Pore Model for Gas-Solid Reactions with Pore Closure Behavior”. AZChE J. 1987, 33,
382-393.
Received for review July 10, 1986 Revised manuscript received August 14, 1987 Accepted October 2, 1987
Effect of the Operating Conditions on the Preparation of Stannous Octanoate from Stannous Oxide Fernando
V. DIez, Herminio Sastre, and Jose Coca*
Department of Chemical Engineering, University of Oviedo, 33071 Oviedo, Spain
Stannous octanoate (stannous 2-ethylhexanoate) has been obtained by reaction of hydrous and anhydrous stannous oxides with 2-ethylhexanoic acid. Results for the precipitation of stannous oxide from a stannous chloride solution with sodium hydroxide are reported, viz., pH, particle size distribution, and form of the particles. The effect of temperature, speed of agitation, and percentage excess of acid in the preparation of stannous odanoate was investigated. The most effective conditions for the industrial production of stannous octanoate using this process are suggested. Stannous octanoate (stannous 2-ethylhexanoate) is among the most important industrial tin catalysts. It is used as a stabilizer of rigid poly(viny1 chloride) (Custack and Smith, 1981), in the production of rigid and flexible polyurethane foams (Karpel, 19801, and as a curing agent in vulcanizing (RTV) silicones (Fuller, 1975). The main industrial manufacturing methods used to obtain stannous octanoate follow two routes depending on the starting material used: stannous chloride (Dietsch, 1973; Rapp, 1963) or stannous oxide (Goldschmidt, 1970; Ashbel et al., 1968). Both compounds can be obtained from stannic chloride which is the final product in the chlorine process for recovering tin from tin-plate scrap (Dubois and Bourgogne, 1981; Ray et al., 1974). Stannous chloride is obtained by treating stannic chloride with granulated tin. When stannous chloride is treated with sodium hydroxide, a white precipitate of hydrous stannous oxide forms: SnC1, 2NaOH SnO-xH20+ 2NaC1
+
-
If the aqueous solution is boiled, this precipitate turns black due to dehydration. Stannous octanoate can be obtained either from hydrous or anhydrous stannous oxide by reacting it with an excess of 2-ethylhexanoicacid in an inert atmosphere: SnOz + 2C,H15COOH Sn(C7H15C00)2+ H 2 0
-
The stannous octanoate dissolved in 2-ethylhexanoicacid can be separated from the solvent by vacuum distillation. In this work, the effect of the operating conditions on the preparation of stannous oxide and stannous octanoate are reported. The particle size distribution of hydrous and anhydrous stannous oxide has been determined as a function of operating conditions. The yield of stannous octanoate has been ascertained as a function of the reaction temperature, speed of agitation, and the percentage excess of 2-ethylhexanoic acid used.
Experimental Section Stannous oxide and stannous octanoate were prepared in a 700-cm3stirred tank reactor provided with a condenser, for heating under reflux or condensing a distillate,
and ports, for adding the reactants, measuring the temperature, introducing nitrogen, or creating a vacuum in the system. The reactor was heated with an electric heater, and the rate of heating was adjusted with a PID controller. The reaction temperature was measured with a Fe-constantan thermocouple immersed in the reactor through a glass well. The particle size distribution of hydrous and anhydrous stannous oxide was determined with a Malvern 2200 apparatus, which operates on the principle of the Fraunhofer diffraction. The electronic analysis of the diffraction patterns gives the particle size distribution of the sample. The particles of stannous oxide were observed under a Lentz Wetzlar microscope with magnifying powers of lOOX, 400X, and 1OOOX. Hydrous stannous oxide was prepared by adding sodium hydroxide solution to an aqueous solution of stannous chloride at room temperature and controlling the final pH. The anhydrous stannous oxide was obtained by boiling this solution for 15 min. If stannous oxide was employed to obtain stannous octanoate, it was washed 3 times with previously boiled water, and the excess water above the precipitate was removed. The stannous oxide was then reacted with an excess of 2-ethylhexanoic acid, while the reaction mixture was heated in a nitrogen atmosphere. The water present in the stannous oxide precipitate was removed by distillation and the mixture heated under reflux at the desired temperature. Once the reaction was completed, the product was filtered and the excess free acid was removed by distillation at reduced pressure. The tin content was determined by atomic absorption (Perkin-Elmer 372 spectrometer). For tin analysis in the aqueous phase during the preparation of stannous oxide a nitrous oxide-acetylene flame was used. The analysis of tin in the organic phase, during the preparation of stannous octanoate, was carried out with an air-acetylene flame. The yield of tin octanoate was approximately taken as the ratio between the concentration of tin in the organic phase (previously filtered) and the concentration of tin in the bulk of the reaction mixture (organic phase and solids), taking into account that the volume of solids is negligible
0888-5885/88/ 2627-0845$01.50/0 0 1988 American Chemical Society
846 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 100
I
1
PH
Figure 1. Tin concentration 8s a function of pH for the precipitation of stannous oxide from a stannous chloride solution.
Figure 3. Particle size distribution band in the precipitation of anhydrous stannous oxide.
Figure 2. Particle size distribution bands in the precipitation of hydrous stannous oxide. Sodium hydroxide addition time: (I) 10 min, (11) 25 min.
compared with the volume of the liquid phase. Results were checked in selected experiments by measuring the volume and determining the tin content of the water removed during stannous oxide precipitation, the different samples, and the final reaction product. The mass balance was checked within a 5% error margin. Results and Discussion Precipitation of Stannous Oxide. When a 2.5 M sodium hydroxide solution is added to a 1 M solution of stannous chloride, a step change to a pH N 12 is observed. This indicates that the stannous oxide has been precipitated, and further addition of sodium hydroxide results in the production of stannites. If the tin concentration in aqueous solution is represented as a function of pH, it can be observed that the precipitation of stannous oxide is practically quantitative between pH 6 and 9, Figure 1. Particle size is an important parameter in the kinetics of solid-fluid reactions; hence, particle size distribution of stannous oxides was studied as it influences the rate of reaction with 2-ethylhexanoic acid. Particle size distributions for hydrous stannous oxide and anhydrous stannous oxide are illustrated in Figures 2 and 3, respectively, in which the cumulative weight below a certain size is represented as a function of particle size. The different samples do not have a uniform distribution of particle size, and therefore the dashed bands are obtained representing the range of data from replicate experiments. The two bands in Figure 2 correspond to slow addition (I) and fast addition (11) of sodium hydroxide. Band I represents five of seven replicate experiments, the average maximum of the distribution being 21 pm (standard deviation, 11.7). As expected, a fast addition yields particles of larger size, the average maximum being 3.6 pm (standard deviation, 2.6). Six of seven experiments are covered by band 11.
d t (
sspm
Figure 4. Microphotographs of stannous oxide particles, 6OOx: (a) hydrous (b) anhydrous.
There is not a clear relation between the particle size distribution of anhydrous stannous oxide and the addition rate of sodium hydroxide. The band in Figure 3 covers 10 of 13 replicate experiments. The average maximum is 33.3 fim and the standard deviation 17.1. Figure 4 shows two microphotographs of hydrous and anhydrous stannous oxide particles. The particles of hydrous oxide are irregular in shape, while the anhydrous oxide crystals are more regular and approximately square in shape. Preparation of Stannous Octanoate. The preparation of stannous octanoate by reaction of 2-ethylhexanoic acid with stannous oxide, both hydrous and anhydrous, was carried out by using the following reaction conditions: (i) temperature, 100 and 200 "C; (ii) speed of agitation, 200 and 400 rpm; and (iii) excess acid, 100% and 200%. As the reaction mixture reaches a temperature of 100 "C, the water present in the stannous oxide was removed by distillation. The variation of conversion with time is shown in figures 5 and 6 for hydrous stannous oxide and in figure
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 847
5 I
I
5
2
LO
3
r min
Figure 5. Percent conversion to stannous octanoate from hydrous stannous oxide as a function of time with an excess of 2-ethyl120 OC, 200 ppm; ( 0 )140 "C, 200 rpm; hexanoic acid of 100%: (0) (0)140 "C, 400 rpm.
a
1
, , , , , , , , , 20
60
100
140
j
180
f . rnrn
Figure 6. Percent conversion to stannous octanoate from hydrous stannous oxide as a function of time with an excess of 2-ethyl120 "C, 200 rpm; ( 0 )140 "C, 200 rpm; hexanoic acid of 200%: (0) (0) 140 O C , 400 rpm.
to the next, and the separation processes preceding the analysis of the sample (sedimentation and suction) are not easily reproducible. However, some conclusions can be drawn. The rate of reaction is higher at 140 "C than at 120 "C,when 100% excess acid is used. For 200% excess acid, the rate of reaction increases and the temperature effect becomes negligible. It is not possible to infer any influence of the agitation speed with the range studied (200-400 rpm). A few results are presented (Figure 7) for anhydrous stannous oxide as the starting material. The experiments were carried out with a stirring speed of 200 rpm. The presence of excess acid was found to influence the reaction rate as for the hydrous oxide. However, no temperature effect was noticeable.
Conclusions The results obtained in this work indicate that the optimum industrial conditions for the production of stannous octanoate are (1)from hydrous stannous oxide (temperature, 120 "C; rate of agitation, 200 rpm; and excess of acid, loo%), the reaction is completed in about 50 min; and (2) from anhydrous stannous oxide (temperature, 120 OC; rate of agitation, 200 rpm; and excess of acid, 200%), the reaction is completed in about 30 min. Acknowledgment The authors thank Drs. Freshwater, Brooks, and Rice of the Department of Chemical Engineering at the University of Loughborough (U.K.) for their assistance in using their particle size distribution equipment. Registry No. SnC12, 7772-99-8; NaOH, 1310-73-2; SnO, 21651-19-4; C7HI5CO2H,149-57-5; Sn(C7H15C02)2, 301-10-0.
c
,.g
I
l 601 20
60
100 t . min
Figure 7. Percent conversion to stannous octanoate from anhydrous stannous oxide as a function of time a t 120 "C. Excess of 2-ethylloo%, ( 0 )200%. hexanoic acid: (0)
7 for anhydrous stannous oxide. The instant a t which the reaction mixture starts boiling was taken to be t = 0. The results are rather scattered because of the many factors involved in the process. The state of stannous oxide (particle size distribution, presence of the anhydrous form and partial oxidation) may change from one experiment
Literature Cited Ashbel, F. B., et al., USSR Patent 213800, 1968. Custack, P. A.; Smith, P. J. In Speciality Inorganic Chemicals; thompson, R., Ed.; Chem. SOC.Spec. Publ. No. 40; The Chemical Society: London, 1981. Dietsch, E. DDR Patent 83 990, 1973. Dubois, B.; Bourgogne, F. Mater Tech. (Paris) 1981, 69(3), 85-92. Fuller, M. J. Tin Its Uses 1975, 103, 3-7. Goldschmidt A. G. French Patent 2 007 745, 1970. Karpel, S.Tin Its Uses 1980, 125, 1-6. Rapp, W. French Patent 1319 969, 1963. Ray, H. S.; Singhal, V. C.; Dwivedi, L.; Dixit, G. J.Znst. Eng. (India), Part M M 1974, 5 4 ( 3 ) ,92-97.
Received f o r review September 18, 1986 Revised manuscript received November 13, 1987 Accepted December 13, 1987
