Ind. Eng. Chem. Res. 1998, 37, 1189-1195
1189
Kinetics of TiCl4 Hydrolysis in a Moist Atmosphere M. Rigo, P. Canu,* L. Angelin, and G. Della Valle† Istituto di Impianti Chimici, Universita´ di Padova, Via Marzolo 9, 35131 Padova, Italy
The gas-phase hydrolysis of TiCl4 has been studied with several laboratory reactors. Unexpectedly, the reaction is clearly split in two steps, an almost instantaneous partial hydrolysis which produces 2 mol of HCl/mol of TiCl4 together with a salt, identified as Ti(OH)2Cl2. The latter forms a very fine dust which easily evolves into aerosols. Ti(OH)2Cl2 in a moist environment eventually hydrolyses to HCl. The kinetics of both steps have been studied. While the first step is very fast, the salt hydrolysis is much slower, its kinetics being controlled by diffusion of the gaseous reactant through the shell of the solid product covering the aerosol particles. 1. Introduction TiCl4 is a chemical product widely used in the chemical industry, mainly as a catalyst in polyolefin synthesis. Accidental releases of TiCl4 are known to be harmful due to its hydrolysis by the atmospheric humidity that produces HCl which is a toxic gas. Nonetheless, we have not been able to find in the open literature any quantitative study of the reaction kinetics at ambient conditions. High-temperature hydrolysis to Ti(OH)4 has been studied by Akhtar et al. (1994) in connection with TiCl4 oxidation to TiO2, as it occurs during the vaporphase synthesis of titania powder. The dominance of hydrolysis at low temperatures has been proved, and the formation of volatile Ti(OH)2Cl2 as an intermediate has been suggested by Gruy and Pijolat (1992) to be a rate-limiting step. The formation of a number of hydrolysis byproducts at equilibrium has been discussed in Shinkarev et al. (1973) as a function of temperature, pressure, and excess H2O. Shegrov (1964, 1966) already observed an incomplete hydrolysis of TiCl4 at intermediate oxychlorides and hydroxy chlorides, preferentially the first as temperature increases. He noted that hydroxy chlorides tend to react further with water. Without quantitative information about the kinetics, one would be tempted to model atmospheric hydrolysis of TiCl4 as instantaneous and quantitative, yielding 4 mol of HCl/mol of TiCl4. Through an experimental investigation, we show that such an assumption is much less conservative than it might appear. The complete hydrolysis assumption does not explain the observed phenomena of a bright white dense cloud that develops upon dispersion of TiCl4 vapors in the atmosphere. The white aerosol can be made up of a number of titanium products, including Ti(OH)4 and TiO2. Nonetheless, experience demonstrates that the aerosol keeps releasing HCl, which cannot be the case for the latter products. Such a cloud has been found to be more dangerous than the HCl initially developed. Because of its relative stability, it can diffuse over larger distances, be inhaled easily, and slowly release HCl in a moist environment, such as in the lungs. * Corresponding author. E-mail:
[email protected]. Tel.: (+39) 49 8275463. Fax: (+39) 49 8275461. † Present address: Montell Italia S.p.A., P.le Privato G. Donegani 12, 44100 Ferrara, Italy.
Figure 1. Schematic of reactor 1stubular with θ ) 30 s. PR ) pressure reducer, F ) flowmeter, S ) saturation bottle, LT ) liquid trap, HS ) hydraulic seal, DP ) differential U-manometer, R ) reactor, FT ) filter, and A ) absorber.
2. Experimental Reactors Three laboratory reactors have been built to study the reaction kinetics because of the different time scales which have been unexpectedly identified in the first reactor. 2.1. Reactor 1. Since the hydrolysis was expected to be quite fast, even in the gas phase, a tubular reactor was initially conceived to allow for a steady-state analysis. Intermediate steps of the fast reaction would have been isolated by varying the contact time in the apparatus. Reactor 1 is sketched in Figure 1. It is made up of a coil of polyethylene tube 12 mm i.d. and 60 m long and allows for a residence time of 30 s, at a gas flow rate of 800 L/h. The gas velocity inside the tube guarantees a high-enough degree of turbulence to assume homogeneous mixing. A posteriori analysis would suggest checking for significant axial dispersion, but that would not affect the qualitative results provided by this apparatus. A synthetic mixture of TiCl4 and H2O in a nitrogen carrier has been obtained by two independent saturation bottles (S1 and S2, respectively) put into a thermostatic bath. A nitrogen dilution line has been provided to avoid condensation into the reactor and to allow more freedom to independently set the inlet composition, at the same overall feed rate. Mixing takes place at the
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Figure 2. Apparatus to collect solid salts for the analysis. PR ) pressure reducer, F ) flowmeter, S ) saturation bottle, R ) reactor, and FT ) filter.
reactor entrance, and the mixing length is assumed negligible with respect to the overall reactor length. Liquid traps (LTs in the figure) are required to prevent gas-liquid reactions. Immediately after the reactor exit, an absolute filter has been set up to capture any solid produced as an intermediate. The filter is constructed with glass wool in a short piece of tube, and it proved very effective without significant pressure loss. Eventually, gaseous products pass through a sequence of absorbers containing an aqueous solution of NaOH. HCl has been measured by titration of samples from the absorbing solution, drawn at 5 min, 10 min, and every 10 min thereafter up to 50 min. Some of these samples have also been analyzed to detect the presence of titanium, either as a suspended solid (Ti(OH)4 does not solubilize in an alkaline solution) or dissolved, by means of atomic absorption (Perkin-Elmer 500). Solids have been analyzed through elemental analysis. A specific apparatus (Figure 2) has been set up to allow for a significant amount of solid to be collected. The solid is deposited on the filter, dried in a nitrogen stream, collected, weighed, and dissolved in a solution of 10% H2SO4. Samples of the solution have been analyzed through atomic absorption to determine the titanium composition and by a potentiometer for the Clevaluation. Concerns about HCl adsorption onto the inner surface of the PE reactors arose later on, though the influence on the observed behavior was small, as evaluated through a direct HCl adsorption test. The analysis of the experimental data accounts quantitatively for such a contribution. All the runs were performed at ambient temperature (20 °C). 2.2. Reactor 2. The results obtained from the first reactor clearly called for a shortening of the residence time to identify the kinetics of a first transformation. A second reactor was built on the structure of the first one, by simply replacing the PE tube with a linear glass tube, 12 mm i.d. and 1.5 m long, other things being equal to reactor 1 as shown in Figure 1. Such a reactor allows us to work with a residence time less than 1 s. Here, the first step of the hydrolysis has been studied. HCl production has been monitored by titration of the NaOH absorbing solution, as in the previous case. In an attempt to closely track the HCl production, pH measurements in a water-filled absorber have been used also. The test demonstrated that the first absorber can
Figure 3. Schematic of reactor 3sinternally recirculated batch type reactor. PR ) pressure reducer, F ) flowmeter, S ) saturation bottle, LT ) liquid trap, HS ) hydraulic seal, DP ) differential U-manometer, MS ) microsyringe, R ) reactor, FT ) filter, and A ) absorber.
collect all the HCl produced, since none of it was found in the subsequent one, which eventually were removed. Unfortunately, the resulting pH measurements were too noisy. 2.3. Reactor 3. Since experiments have so far produced an amount of HCl much lower than the stoichiometric amount, as will be discussed later, a different reactor was built to investigate large residence times. An internally recirculated, batch type reactor was set up as shown in Figure 3. Mixing in the vessel is provided by a turbine inside a straightener to increase local velocity throughout the whole volume (8 L). The reactor was later replaced with a spherical glass reactor (10 L) without appreciable differences in the results. An H2O saturated nitrogen stream is allowed to flow through the stirred reactor up to uniform conditions (30 min). Then the reactor is sealed and TiCl4 injected through a microsyringe so as to achieve a TiCl4/H2O mole ratio of 1/10 to 1/20. TiCl4 suddenly hydrolyzes to the white salt, which remains suspended as an aerosol, with a minimal adhesion to the reactor wall. After the given reaction time, anhydrous N2 is used to flush the internal atmosphere through the standard absorbing solution, to determine both HCl produced and TiCl4 left unreacted. Several reaction hours can be easily investigated by this reactor. 3. Results and Discussion The HCl concentration in the absorption solution at the exit of reactor 1 is shown in Figure 4 as a function of time, at different TiCl4 feed flow rates. The values clearly increase with time since HCl accumulates in the solution. Deviation from linearity reflects the existence of two mechanisms of HCl production. The most important is the gas-phase reaction in the tubular reactor at a residence time of 30 s, much shorter than the sampling interval. It accounts for the steady linear increase of HCl concentration in the absorbing solution, which reflects a constant HCl flux out of the reactor, with respect to the sampling time scale. Using a different inlet concentration of TiCl4 changes the slope of the curve; indeed, the data are better
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1191
Figure 4. HCl concentration in the absorption solution at the exit of reactor 1 compared to the theoretical value for a 1/2 TiCl4/ HCl ratio. Total mass of TiCl4 consumed ) 4.99 g; overall flowrate 800 L/h. Cl- absorption by the PE tube ) 3.1 × 10-4 mol.
Figure 5. Yield of HCl with respect to TiCl4 in reactor 1.
compared through the time derivative, which provides the flux of HCl out of the reactor, averaged on the sampling interval. The molar flow rate of HCl leaving the reactor can be compared to the inlet flow rate of TiCl4, giving the instantaneous yield of the hydrolysis to HCl, as shown in Figure 5. Besides some experimental error, significantly amplified by the numerical differentiation, all the data collected for any inlet condition show the same behavior. A yield of 2 is always observed for t f 0, clearly indicating that the gas-phase reaction in the tube hydrolyzes half of the TiCl4 fed, while a small, progressively slower secondary mechanism of HCl production takes place. The question remains as to whether the rest of the TiCl4 flows through the reactor unconverted or is transformed into other products. The first hypothesis would require titanium to be present in the absorbing solution, since it would be eventually hydrolyzed. Actually, the solution was always clear, without any suspended solid in it, and contained no detectable amount of dissolved titanium. The gas-phase hydrolysis in the reactor clearly converted all the fed TiCl4, producing something else which could not reach the solution. Indeed, it forms a solid salt that is captured by the filter, clearly observable on it after inspection as a fine white
Figure 6. Yield of HCl with respect to TiCl4 in reactor 2.
powder. Submicron size particles have been identified by optical microscopy, much smaller than the average diameter of a fiber (5-10 µm). Such an observation is consistent with the experimental results of Rubio et al. (1997) where gas-phase hydrolysis of a titanium alkoxide at ambient temperature has proven to be effective in producing uniform spherical particles with a narrow size distribution, around 0.7 µm. The excess of water vapor among the reactants causes a significant amount of water to be present after the reactor, which flows through the filter on the solid therein. A gas-solid hydrolysis takes place, producing additional HCl which gets absorbed together with the main reaction product. Such a salt hydrolysis is much slower and is responsible for the deviation from linearity in the curve shown in Figure 4 and from the constant yield of 2 shown in Figure 5. A qualitative mechanism can be shaped from these observations:
TiCl4 + H2O f 2HCl + salts salts + H2O f 2HCl + Ti(OH)4
(fast) (slow)
This agrees with the previous observation by Shegrov (1964) of oxychloride and hydroxy chloride intermediates in a first partial hydrolysis step, but the exact chemical nature of the salts remains to be determined. Such a preliminary conclusion, though purely qualitative, clearly splits the study toward shorter and larger reaction time scales. Reactor 2 was set up to verify both the reaction rate of the first step and other possible intermediates. Though contact time was as low as 1 s, no significant difference was observed with the results of reactor 1, as shown in Figure 6 for the yield. It can be concluded that the partial hydrolysis is so fast that it reaches completion in less (maybe much less) than 1 s, and no additional evolution takes place for up to 30 s. Unfortunately, such a conclusion is still qualitative, but trying to investigate a shorter time interval is not easy, because the approximations about mixing length, axial dispersion and plug flow conditions would become poor. For any practical applications, we conclude that the first hydrolysis can be assumed to be instantaneous. Most interesting is the long-term release of HCl by the solid resulting from the first hydrolysis. Such a
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Figure 7. Concentration of HCl in the batch reactor as a function of time. Initial concentration of TiCl4 ) 30, 40, 50, 60, 80, and 100 µL, corresponding to experimental curves in increasing order.
mechanism is particularly dangerous in view of an accidental spill of TiCl4. Since the intermediate solid forms an aerosol, it can be transported over fairly large distances and be easily inhaled, retaining its toxic potential, since it can hydrolyze to HCl with humidity for a relatively long time, for example, hours. To identify the chemical nature of the salt, the third reactor, suitable for long-term kinetic studies, was set up. The salt was collected through the apparatus described earlier in Figure 2 and characterized through Ti/Cl atomic ratio and molecular weight. The Ti/Cl atomic ratio was found to be 1/2 and the molecular weight was found to be 154 g/mol, as an average result of 11 measurements. The titanium salt, among the possible ones, which matches those data is Ti(OH)2Cl2, in agreement with previous hypotheses of Gruy and Pijolat (1992). The kinetics of Ti(OH)2Cl2 hydrolysis was then studied in the batch reactor described above, suitable for observing long-term transformations. The reaction proceeds between a suspended solid and the water vapor, according to the following stoichiometry:
2H2O + Ti(OH)2Cl2 f 2HCl + Ti(OH)4
(R2)
(2A + B f 2C + D)
4. Kinetic Model The experimental data of the second step (R2) have been analyzed assuming that the reaction takes place between gaseous water vapor and the solid salt in the form of spherical nonporous particles of constant initial size. Accordingly, the number of moles of Ti(OH)2Cl2 (called B in the following) in the reactor at any time is given by
NB ) VcNpF˜ B ) 4/3πrc3NpF˜ B
(R1)
The reaction progress was measured through HCl concentration in the batch atmosphere. Figure 7 shows the increase of HCl concentration according to different initial amounts of TiCl4. The salt hydrolysis appears to proceed quite slowly. More insight is given by the conversion versus time plot, shown in Figure 8. First, it can be concluded that only half of the solid conversion occurs in 6 h. Moreover, the reaction rate is continuously decreasing, so that several hours would be needed to completely convert all the available salt. It can also be observed that an increase of initial TiCl4 concentration results in a lower specific consumption rate.
(1)
where only the particle core radius changes with time. The number of particles in the reactor, Np, depends linearly upon the initial amount of TiCl4, to conform with the assumption that the initial particle size is constant. The equation above states that
NB N0B
)
() rc
r0p
3
) η3
providing a connection between the particle size and the conversion:
X)1-
where the initial salt concentration depends on the available TiCl4, according to the first quantitative step:
TiCl4 + 2H2O f 2HCl + Ti(OH)2Cl2
Figure 8. Conversion Ti(OH)2Cl2 in the batch reactor as a function of time. Initial concentration of TiCl4 ) 30, 40, 50, 60, 80, and 100 µL, corresponding to experimental curves in decreasing order. Solid lines have no meaning besides helping to identify each set of data. Note that 80 and 100 µL curves somehow overlap each other.
NB N0B
) 1 - η3
(2)
A description of the kinetics of a single particle is enough to obtain the overall conversion, through the number of particles in the system. Several mechanisms have been investigated trying to describe the set of data obtained with 30 µL of TiCl4. Note that in this case the water vapor left by the first hydrolysis is still almost 12 times the stoichiometry, allowing for the approximation of constant composition throughout the second reaction. 4.1. Chemical Control Assumption. The reaction transforms a solid into a solid product, Ti(OH)4, which is likely to remain in the outer shell of the particle, according to the well-known shrinking unreacted core model (Szekely et al., 1976). In the instance of very fast diffusion through the external product, the kinetic is controlled by the reaction at the interface between the
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1193
process, possibly nonchemical. In this case reversible kinetic would mean m R ) k+CnA - k-Cm C = k′+ - k-CC
where the HCl concentration, CC, can be related to the number of moles of solid in the system:
CC ) C0C + 2λ ) 2C0B + 2C0B(1 - η3) Substituting into the eq 3 one obtains
dη 1 ) - 0 (k′+ - k-[2C0B(2 - η3)]m) dt r F˜ p B
Figure 9. Comparison between experimental conversion-time data and model predictions: (- - -) unreacted shrinking core under chemical control, irreversible reaction; (-‚-) reversible reaction; (s) diffusion control with irreversible reaction.
two solids, the core surface. The core size changes as
drc R )dt F˜ B
(3)
Let us assume first that the reaction rate follows an irreversible power-law expression:
R ) kCnA = k′ where the large amount of water vapor (called A) allows for the last simplification. Equation 3 can be divided by r0p, allowing us to express it in terms of η, directly connected with the relative abundance of solid into the reactor through eq 2:
dη k′ ) - 0 ) -k′′ dt r F˜ p B
to be integrated with the initial condition
η(0) ) 1 This obvious result can be cast into the more useful form:
X(t) ) 1 - (1 - k′′t)3 The constant k′′, which contains the actual kinetic constant k, can be evaluated by fitting the experimental data. The initial size of the solid particles and the molar density do not need to be explicitly known for further application, as long as the solid remains the same and the nucleation always yields the same kind of solid particles, with constant diameter. The dashed line in Figure 9 shows the best agreement obtained, which is quite unsatisfactory. Indeed, the decrease of reaction rate with conversion shown by the data would call for a reversible rate expression, under the assumption of chemical control, or some other sort of rate-limiting
C0B is the salt concentration at the beginning of the second hydrolysis, as given by the TiCl4 hydrolysis. It can be evaluated by the initial amount of TiCl4 injected. The three parameters k′ + , k-, and m can be estimated by fitting the resulting X(t) to the experimental data. Figure 9 shows the better agreement found with this model (m ) 1). The reversibility correctly accounts for the asymptotic behavior clearly shown by the experimental data, which was missed by the irreversible reaction model, whose reaction rate is almost zero-order when the water vapor is largely abundant. Nonetheless, the chemical control by a reversible reaction does not explain the extremely high reaction rate observed at the beginning. 4.2. Diffusion Control Assumption. The model of shrinking unreacted core particle with diffusion control of the gaseous species through the outer shell can be very difficult to solve, since a radial concentration profile in the shell evolves in time, resulting in a twodimensional differential problem. With the quasisteady-state approximation for the core shrinkage with respect to diffusion, the model splits into two subproblems: determination of the diffusion rate through the shell at a given core radius and the connection of such diffusion rate with the core size variation. The molar flux of reactant A through the shell surrounding the particle core is given by
JA ) -4πr2D
dCA dCC ) -JC ) 4πr2D ) const dr dr
where the diffusion coefficient has been assumed to be the same for both species, according to the equimolar counterdiffusion scheme. The radial profile can be obtained by integration with the boundary conditions:
CA(r0p) ) C0A CA(rc) ) 0 where the last condition, at the core-shell interface, is based on irreversible kinetic assumption, at a rate much higher than diffusion. The C0i is the value of concentration in the gas phase in which particles are dispersed. Accordingly, the rate of diffusion results:
JA ) 4πDC0A
[ ] 1 1 rc r0
p
-1
1194 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998
Thus, the rate of particle shrinkage under diffusion control is given by
-4πr2c F˜ B
[ ]
drc JA 1 1 ) ) 2πDC0A - 0 dt 2 rc r p
-1
dη 1 ) -DCA(t) dt η - η2
The equation can be transformed in terms of η by simply dividing by (r0p)2:
dη 1 ) -DC0A dt η - η2
(4)
The differential equation can be integrated analytically but involves the solution of a cubic equation. Moreover, the differential equation has a singularity at the origin, which makes the numerical integration difficult to start precisely. The infinite rate of consumption at the beginning is a consequence of the assumption of diffusion control. Indeed, there is no diffusion layer at time zero. The equation can be formally integrated to yield
6DC0At ) 1 - 3η2 + 2η3
(5)
Such a solution is still implicit in the actual unknown η(t), but easier to apply for the estimation of D, simply treating the experimental data with the conversion as an independent variable and the time as the dependent one. The parameter D contains the diffusion coefficient D together with several unknown constants that do not need to be known, as explained above:
D)-
vapor concentration. Including the effect of variable water vapor concentration, CA(t), eq 4 is slightly modified to
D 2(r0p)2F˜ B
Equation 5 fits the experimental data quite well, as shown by the solid line in Figure 9. Note that the hypothesis of diffusion control implies that the actual rate of salt consumption is limited by the diffusion through the solid product covering the particles. Such a resistance is close to zero in the beginning, allowing for a very high initial rate, as the experimental data show. It is worth recalling that this feature was not correctly accounted for by the pure chemical control, as also shown in Figure 9. At the same time, the diffusion control also explains the asymptotic behavior, since a larger shell of solid product tends to stop the reaction. The results summarized in Figure 9 clearly indicate that the kinetic is diffusion-controlled. By each experimental run being independently fitted, corresponding to a given initial amount of TiCl4, the agreement remains very satisfactory, but the value of the parameter D varies among the six sets of data, indicating a systematic dependence upon N0B. Values constantly decrease up to a factor of 1.9 as the initial amount of salt increases 3.3 times. A possible explanation is provided by removing the hypothesis of excess water vapor in the system, allowing for a variable concentration during the reaction. Indeed, the vapor consumption can significantly reduce the excess, even more as the initial amount of TiCl4 increases. The same initial concentration of water vapor is above 11 times the stoichiometric for the smaller initial concentration of TiCl4, but can be only 2.7 times for the larger. Note from Figure 8 that a larger initial amount of TiCl4 reduces the salt conversion attainable at a given time, which can be a consequence of a relatively lower water
where 3 0 0 CA(t) ) (Cin A - 2CB) - 2CB(1 - η )
The first contribution in the rhs is due to the hydrolysis of TiCl4, while the latter accounts for the salt hydrolysis. Note that Cin A is the concentration of water in the reactor before the injection of TiCl4 (6.7 × 10-4 mol/L). It corresponds to 70% of the nitrogen atmosphere saturation, at the saturator temperature assumed to be 20 °C. An analytical solution can still be obtained for the implicit formulation t(η), though it is somewhat more complex. The fitting results are barely distinguishable from Figure 9, but now the coefficient D is more stable. The average value is
D ) 4.51 × 10-2 [m3/(mol s)] with a standard deviation computed on the six-parameter estimates equal to 7.17 × 10-3. Several models have been tested against the experimental data of Figure 8, including reversible kinetics at the surface under diffusion control and variable reaction order with respect to the water vapor. Though any mechanism based on diffusion control is able to very satisfactorily describe each single set of results, none of them give really constant parameters, where they are to be expected for physical meaning. Surprisingly, a semiempirical model that fits all the six sets of data with an almost unique parameter can be formulated as
D1 1 dη ) - 0 2/3 dt (CB) η - η2 which is consistent with the assumption of diffusion control, zero-order kinetics, and constant particle number. The latter hypothesis implies that the initial volume of the particles is proportional to the initial concentration of salt, according to eq 1. Though the last assumption is questionable, the models provide a unique description of all the experimental data, with an average parameter
D1 ) 3.25 × 10-8 [mol2/3/(m2 s)] which includes the actual diffusion coefficient times unknown physical constants, like F˜ B. Figure 10 shows the results obtained with the last model with the single parameter D1 above. The parameter standard deviation is 2.58 × 10-9, clearly better than before. Nonetheless, the variance of the parameter estimate based on a single set of data is lower, indicating that the models still miss something. The aspect which has been neglected is the sticking of the salt to the reactor internal surfaces as well as coalescence and mechanical erosion of the particles. All these contributions are undoubtedly minor but can account for the residual lack of fit of all the data with a single model. Some experimental error is also evident from Figure 8, though it is quite small
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1195 λ ) extent of the second hydrolysis (R2) [mol/L] NB ) moles of Ti(OH)2Cl2 in the reactor [mol] Np ) number of solid particles in the reactor V ) volume [m3] r ) radius [m] F˜ B ) molar density of solid Ti(OH)2Cl2 [mol/m3] η ) rc/r0p R ) reaction rate per unit area [mol/(m2 s)] k ) reaction rate constant per unit area [mol/(m2 s)] X ) conversion C ) concentration [mol/m3] D ) diffusion coefficient of gas through solid Ti(OH)4 [m2/ s] D ) adaptive coefficient for D estimation Subscripts and Superscripts Figure 10. Comparison between experimental conversion-time data and model prediction: unreacted shrinking core under diffusion control, irreversible zero-order reaction assumption.
when considering that each experimental point needs a specific run, since HCl concentration is determined by flushing the reactor. We believe that the results reported above, both in terms of D or D1, are quite useful, since the fate of atmospheric dispersion of TiCl4 can be easily predicted with these kinetics. Indeed, a little information is required by these models, namely atmospheric humidity and initial amount of TiCl4 released.
A ) referring to water vapor B ) referring to Ti(OH)2Cl2 C ) referring to HCl 0 ) at the beginning of the second hydrolysis in ) initial p ) of the particle c ) of the particle core + ) of the forward reaction - ) of the reverse reaction
Literature Cited Conclusions The gas-phase hydrolysis of TiCl4 has been shown to be clearly split in two steps, a partial hydrolysis which produces 2 mol of HCl/mol of TiCl4 together with a salt, identified as Ti(OH)2Cl2. The latter forms a very fine dust which easily evolves into aerosols. Ti(OH)2Cl2 in a moist environment eventually hydrolyzes to HCl. The first step appears to be almost instantaneous, since the same results have been obtained both at 30- and 1-s residence time. For any practical purposes connected with environmental hazard, this means instantaneous and quantitative conversion of the whole TiCl4 to produce half of the HCl attainable. The remaining HCl is produced during the second hydrolysis step, involving a solid reactant. The kinetics of the second step are much slower and have been successfully explained by the shrinking unreacted core model with diffusion through the external solid shell controlling the overall rate. The effective diffusion coefficient has been estimated, allowing for a quantitative prediction of longterm hydrolysis of the airborne aerosol. Nomenclature θ ) reactor residence time [s] Ji ) molar flux of species i [mol/(m2 s)]
Akhtar, M. K.; Vemury, S.; Pratsinis, S. E. Competition between TiCl4 Hydrolysis and Oxidation and its Effect on Product TiO2 Powder. AIChE J. 1994, 40, 1183. Gruy, F.; Pijolat, M. Kinetic of Anatase TiO2 Surface Area Reduction in a Mixture of HCl, H2O, and O2: II Quantitative Modelling. J. Am. Ceram. Soc. 1992, 75, 663. Rubio, J.; Oteo, J. L.; Villegas, M.; Duran, P. Characterization and Sintering Behavior of Submicrometre TiO2 Spherical Particles Obtained by Gas-Phase Hydrolysis of Titanium Tetrabutoxide. J. Mater. Sci. 1997, 32, 643. Shegrov, L. N. Some Properties of Products of the Partial Hydrolysis of Titanium Tetrachloride. Tr. Vses. Nauchno-Issled. Inst. Khim. Reakt. Osobo Chist. Khim. Veshchestv 1964, 26, 160. Shegrov, L. N. Gasometric Determination of Hydrogen in Hydrolysis Products of Titanium Tetrachloride. Tr. Ural. Politekh. Inst. 1966, 152, 62. Shinkarev, A. N.; Shtrambrand, Yu. M.; Shaulov, Yu. Kh.; Andreeva, N. I.; Ryabenko, E. A. Equilibrium Composition of Products of the Vapor-Phase Hydrolysis of Titanium Tetrachloride. Zh. Fiz. Khim. 1973, 47, 11, 2945. Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas-Solid Reactions; Academic Press: New York, 1976.
Received for review September 5, 1997 Revised manuscript received January 15, 1998 Accepted January 15, 1998 IE970625E
