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Kinetics of a non-catalytic slurry reaction: Reaction of acetylene with cuprous oxide suspended in water
Chemical Engineering Journal, 22 (1981) 15 - 24 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
15
Kinetics of a Non-catalytic Slurry Reaction: Reaction of Acetylene. with Cuprous Oxide Suspended in Water S. S.TAMHANKAR, National
S. P. GUPTE and R. V. CHAUDHARI*
Chemical Laboratory,
Poona 411 008 (India)
(Received 24 January 1980; in final form 11 August 1980)
Abstract A kinetic study of the reaction of acetylene with cuprous oxide suspended in water is reported. It has been found that the reaction starts after an induction period which is a strong function of temperature. The reaction is chemically controlled at 17 and 27 “C, while at higher temperatures gas-liquid mass transfer contributes significantly. The reaction rate constants were obtained using the Levenspiel model and the true activation energy was found to be 20.15 kcal/mol. INTRODUCTION
Reactions of gases in slurries containing suspended solids are important in many industrial processes. Though catalytic slurry reactors are more common, in a number of situations non-catalytic reactions with suspended solids are encountered (for example, SO, absorption-in a suspension of lime [l] acetylene absorption in CuCl slurries [2] ). The reaction of acetylene with cuprous oxide is important in the preparation of copper acetylide, which is known to catalyse the ethynylation reaction [ 3,4 3 . A major application of the copper acetylide catalyst is in the manufacture of 2-butyne-1,4 diol from acetylene and formaldehyde. Preparation of this catalyst from cuprous oxide may prove to be attractive as it can eliminate the complex step (reduction of Cu(I1) to Cu(1)) in the conventional method of preparation from cupric oxide. Generally, copper acetylide is prepared in liquid media, owing to the explosive hazards
*To whom correspondence
should be addressed.
of acetylide in a dry stage. Besides the application in catalyst preparation, the reaction of acetylene with cuprous oxide provides an interesting example of a non-catalytic slurry reaction, wherein a dissolved gas reacts with a suspended solid reactant. The literature on this reaction is very scanty and no studies on the kinetics have been published so far. In the present work, an experimental study on the reaction kinetics is reported. The effect of temperature, solid loading and acetylene pressure on the rate of reaction is discussed. Kinetic parameters have been obtqed and the shift in controlling regimes is discussed. Such a study is likely to be useful in the process of cuprous acetylide catalyst preparation for 2-butyne-1,4 diol manufacture.
EXPERIMENTAL
The apparatus used in this work (Fig. 1) is similar to that described by Tamhankar and Chaudhari [ 21. A magnetically stirred glass vessel, 4.5 cm in diameter and 9 cm in height, was used as the reactor. The vessel was provided with an outer jacket in which water circulated at constant temperature, within kO.05 “C by means of a thermostatic bath. The acetylene consumed was measured by a graduated gas burette. In a typical experiment, the system was first evacuated using a high vacuum pump. Then acetylene gas was introduced to atmospheric pressure by careful control of the valve. This evacuation-filling cycle was repeated 3 - 4 times to ensure that the system was thoroughly flushed with acetylene. The manometer liquid (saturated with acetylene) from the reservoir was introduced into the burette to adjust the zero time reading. Known quantities of cuprous oxide and de-
16
VACWM-=
Fig. 1. Schematic diagram of the experimental set-up. RESULTS AND DISCUSSION
Preliminary experiments gassed distilled water were then charged into the reactor and run started by switching on the stirrer. The uptake of acetylene was measured by the movement of the liquid level in a graduated burette. The pressure in the system was equalized by adjusting the water level in the manometer tube. Acetylene consumed uersus time was recorded for different operating conditions. The cuprous oxide conversion was then calculated from the stoichiometry. For low-pressure experiments, the procedure was similar to that described above, except that after charging cuprous oxide and water into the reactor the system was set at a desired pressure by controlled evacuation. The pressure in the system was indicated by a gauge. Suitable corrections for the vapour pressure of water at the temperature of the experiment were made. Cuprous oxide supplied by BDH Laboratories was used. Acetylene gas from a cylinder (supplied by Bombay Oxygen Ltd.) was purified by passing it through silica gel traps before use in order to remove traces of acetone. The purity of the acetylene, as indicated by a gas chromatograph, was greater than 99.5%.
The reaction of acetylene with cuprous oxide suspended in water is a typical example of a non-catalytic three-phase reaction. The reaction is given by
(1) c,H2@)+
Cu,W)
+
~zC,(s)
+ H,O(f)
(II)
In such reactions either the rate of absorption of the acetylene or the conversion of cuprous oxide is of interest for studying the kinetics and controlling regimes. In the present study, the volume of acetylene absorbed was observed as a function of time by the procedure described earlier. From this, by appropriate corrections for the solubility of acetylene in the solvent, the conversion of solid cuprous oxide was calculated. The range of variables investigated is given in Table 1. Some of the interesting observations made during this study are as follows. (a) After initial saturation of the liquid, an induction period is observed, which varies with cuprous oxide loading and temperature. (b) The (initial) reaction rate is of zero order with respect to the pressure of the acetylene. (c) The reaction rate reduces drastically before complete conversion of the cuprous oxide.
17 TABLE 1 Range of variables investigated Agitation speed Temperature Cuprous oxide loading, W Pressure of acetylene, Pc,.Q Average diameter of cuprous oxide particles, d,
W = 0.05
6’70 - 1500 rev/min 17 - 69.5 “C 0.05 - 0.20 g/cm3 0.056 - 0.897 atm 6.4 X 10m3 cm
a/cm5
cuprous oxide loading), a quantitative critetion was followed. In this approach the rate of absorption was compared with the maximum rate of gas to liquid mass transfer (given by kpaA*). Thus a factor (Y~can be defined as CX 1 = RJkpaA*
(1)
where RA (mol/cm3 s) is the rate of absorption of acetylene per unit volume of slurry, kpa(s-' ) is the overall gas-liquid mass transfer coefficient, and A* (mol/cm3) is the concentration of acetylene at the gas-liquid interface. The kla data reported by Tamhankar and Chaudhari [ 21 for similar equipment was used in calculating the factor 01~.The solubility of acetylene in water reported by Jadkar and Chaudhari [ 51 was used to calculate A*. If the value of (Y~is less than 1 and greater than 0.2, the mass transfer effects are important. In the present study it was observed that the values of for the data at 17 and 27 “C were less than 0.2, which suggests that gas to liquid mass transfer resistance is negligible. At higher temperatures (40 and 50 “C with cuprous oxide loading of 0.20 g/cm3) the values of e1 were found to be between 0.5 and 0.7, which means that gas to liquid mass transfer resistance is significant under these conditions.
1500
rpm Fig. 2. Effect of stirrer speed on the rate of reaction at 27 “C.
Owing to these complexities, only the initial reaction rate data were considered when evaluating the kinetic parameters. However, attempts to discuss the above observations qualitatively have been made. The influence of different parameters is discussed below in order to identify the significance of the transport and reaction resistances. Mass transfer effects The effect of stirrer speed on the initial reaction rate was studied at 27 “C with cuprous oxide loadings of 0.05 and 0.20 g/cm3. The rates were found to be independent of the speed of agitation (see Fig. 2), which suggests that gas to liquid mass transfer resistance is negligible under these conditions. In order to check the significance of gasliquid mass transfer under other operating conditions (high temperature and higher
Effect of cuprous oxide loading The effect of cuprous oxide loading on the rate of reaction is shown in Fig. 3 as a plot of l/R, us. l/W for various temperatures. It has been shown earlier [6] that when such a plot passes through the origin, the effect of gasliquid mass transfer is negligible. This was observed at 17 and 27 “C, indicating the absence of gas-liquid mass transfer resistance. On the other hand, for higher temperatures the effect of loading on the rate of absorption. is negligible, which suggests that gas-liquid mass transfer contributes significantly under these conditions. These observations do not indicate the contribution of liquid to solid mass transfer resistance, hence a criterion based on comparison of the observed rates with the maximum rate of liquid to solid mass transfer (given by k,a,A*) was followed. For this comparison a factor o2 can be defined as e2 = RA /k,a,A*
(2)
where k, (cm/s) is the liquid to solid mass transfer coefficient and up is the surface area
18
I
STIRRER
SPEED
,040
rpm.
12-
0.9-
ov 0
I
I
I
5
10
I5
Fig. 3. Plot of l/RA vs.l/w.
+,hJ/cm3r
. e 0 @
TIME
Fig_ 4.
I
I
20
300 250 200 iO0
MESH MESH MESH MESH
SIZE SIZE SIZE SIZE
, min
Effect &f particle size on converzion of cuprouz oxide.
of particles per unit volume of liquid. The value of k, (6.6 X 10m3 cm/s) reported by Tamhankar and Chaudhari [2] was used, while up was calculated from the following equation : =p = 6WlPbdp
(3)
In the present study it was observed that, for all temperatures and cuprous oxide loadings investigated, the value of a2 was less than 0.2, which suggests that the liquid-solid mass transfer rate is very much higher than the observed rate of reaction, and hence this resistance is negligible.
19
transfer effects. This is also indicated by the values of cl (0.3) and a2 (0.6) observed for the data in this region.
Fig. 5. Effect of acetylene pressure on the rate of reaction.
Effect of particle size The effect of the particle size of cuprous oxide on conversion is shown in Fig. 4 for a loading-of 0.05 g/cm3. It can be seen that below a mesh size of 200, the conversiontime behaviour is independent of the particle size. This suggests that intraparticle diffusion is unimportant in this range. For 100 mesh particles, however, the conversion is lower, indicating significant intraparticle diffusion effects. Effect of acetylene pressure The effect of the acetylene pressure on the rate of reaction was studied at 27 “C with a 0.05 g/cm3 loading in the acetylene pressure range 0.05 - 0.89 atm. Interestingly, it was found that the reaction is zero order with respect to acetylene above 0.4 atm, and shows dependence on the pressure below 0.4 atm (see Fig. 5). Below 0.4 atm, the values of the external mass transfer parameters (Ye and (~z are in the range 0.3 - 0.6, which indicates significant external mass transfer resistance. In the case of intrinsically zero-order reactions with respect to gaseous species, the rate is independent of the concentration of that species as long as the concentration is finite throughout the particle. However, in a case in which the concentration drops to zero before the centre of the particle is reached, the rates become dependent on the mass transfer parameters. The theoretical analysis of this problem has been considered by Dudukovic and Lamba [7]. In the present work, the dependence of reaction rate on partial pressure below 0.4 atm could be due to mass
Evaluation of kinetic parameters The mechanistic features of three-phase non-catalytic reactions are complicated owing to the possibility of reactions in different phases. If we consider the reaction of a gaseous reactant A with the solid reactant B which is suspended in the ‘liquid, two situations can exist: (a) reaction of dissolved gas with dissolved solid occurring in the liquid phase, and (b) reaction of dissolved gas with solid reactant occurring at the solid surface. When the solubilities of the gaseous and solid reactants are comparable or the solubility of the gas is much lower than that of the solid, situation (a) prevails. The theoretical analysis of this case has been performed earlier by Ramachandran and Sharma [8] and more recently by Tamhankar and Chaudhari [ 21. When the solid is insoluble in the liquid or the solubility of the solid reactant is much lower than that of the gas, situation (b) is likely to exist. In the present study the solubility of cuprous oxide in water (lo-’ mol/cm3 at 27 “C) is several times lower than that of acetylene in water (3.42 X 10m5 mol/cm3 at 27 “C). Therefore, we need to’consider only case (b). It is important to note that the analyses given early by Wen [9] and Levenspiel [lo] for non-catalytic fluid-solid reactions would be applicable here with the additional complexities due to gas-liquid and liquidsolid mass transfer. Based on the discussion in the earlier sections, liquid-solid mass transfer will be assumed to be negligible in this work. For a zero-order reaction with respect to the fluid phase reactant A, the conversion of solid reactant B with time for a kinetically controlled regime is given by [lo] 1 - (1 -x)49
= bk,t/p,R
(4)
where x is the fractional conversion of cuprous oxide, &, (mol/cm3) the molar density, R (cm) the average radius of the particle, kr (mol/cm2 s) the reaction rate constant, b the stoichiometric coefficient and t the time. The above equation is applicable only to spherical particles under isothermal conditions.
SOLID
LOADING
STIRRER
TIME
SPEED
0.05 g/cm= 1040
rpm.
, min
Fig. 6. Effect of temperature on conversion of cuprous oxide (at 0.06 g/cm3 loadingj.
SOLID LOADING 0.20 G/cm3 STIRRER
TIME
,
SPEED
9040
rpm.
min
Fig. ‘I. Effect of temperature on conversion of cuprous oxide (at 0.20 g/cm3 loading).
The conversion of cuprous oxide (x) vs. time plots for different temperatures and cuprous oxide loadings of 0.05 and 0.20 g/cm3 are shown in Figs. 6 and 7 respectively. A significant induction time has been observed for the reaction to occur at 17 and 27 “C. In such cases, Szekely et al. [ll] have suggested that for the purpose of kinetic modelling the induction period can be ignored, and eqn. (4) can be used as an approximation. Following this procedure the quantity 1 -
(1 - 3c)43 was plotted against time for all temperatures studied. As in the present case the additional complication of incomplete conversion of cuprous oxide is involved, only the lower conversion range (<0.25) was considered for kinetic modelling. Figures 8 and 9 are the plots of 1 - (1- x)4’ against time for cuprous oxide loadings of 0.05 and 0.20 g/cm3, respectively, at different temperatures. From eqn. (4) it can be noted that the slope of such plots would give the quantity
21 048
I
I
I
I
I
3
0.16 -
SOLID
.
LOADING
STIRRER
0.12 -
SPEED
0.05
g/cm?
,040
rpm.
.
O,lD-
. I
D D
I
2000
4000 I
Fig. 8. Plot of 1 - (1 -x)lp
LOADINQ
STIRRER
I 0
2000.
I 10000
I 12000
sac
us. time (Levenspiel model) for 0.05 g/cm3 solid loading.
SOLID
0
I 6000
6000
SPEED
0.20
g/cmS
1040
I 4000
rpm.
I 6000
t WC
Fig. 9. Plot of 1 - (1 - x)lp
us. time (Levenspiel model) for 0.20 g/cm3 solid loading.
bkr/p,R, from which the rate constant kl can be obtained. The values of kI obtained from these plots are shown in Fig. 10 as a plot of In kI us. l/T; This figure shows two regions, indicating a transition from the kinetic to the mass transfer regime. As the contribution of mass transfer would be different at the two cuprous oxide loadings, the slopes in the
second regime are also observed to be different. At lower temperatures, however, the slope of In kr us. l/T for both the cuprous oxide loadings is the same. The true activation energy obtained from the slope of the first regime was 20.15 kcal/mol. The activation energies in the second regime were found to be 8.11 and 2.59 kcal/mol for the 0.05 and
22
0.5
\ 2.6
3.0
3.2 (+I
x
103,‘K
3-4
I 3.6
-I
Fig. 10. Effect of temperature on the reaction rate constant.
0.20 g/cm3 loadings, respectively. These values are lower because of the effect of mass transfer. This is also consistent with our earlier discussion that the gas-liquid mass transfer contributes significantly at higher temperatures. The induction period observed in many experiments deserves some comment. This period is significant at low temperatures, while it disappears at higher temperatures. Szekely et al. [ 111 have made a similar observation in the case of NiO hydrogenation. According to them, the induction period occurs when some activation process is required to initiate the main reaction. It is generally agreed [12 - 151 that in the solid phase the chemical reaction proceeds by the initiation of discrete nucleus forming sites, followed by nuclei (sub-microscopic particles). Further, the chemical reaction is localized at the reactant-nucleus interface, causing the nuclei to grow in size as the reaction proceeds. Theoretical justification for this mechanism is given by Bulgakov and Boldyrev [ 161. This process of nuclei formation is slow and is a strong function of temperature, which perhaps explains the higher induction period observed at 17 “C. Alternatively, in a case where the adsorption capacity of the solid product is greater than that of the solid
reactant, initial product formation can become an important step. Here the product formation may accelerate the reaction rate. Such a mechanism in which the product of the reaction increases the rate of the main reaction is known as an autocatalytic reaction. The mechanism suggested for the induction period in this work is only a hypothesis and more experimental work has yet to be carried out. The dependence of the induction period on temperature is an interesting observation of this work. Figure 11 shows a plot of l/& us. l/T, for two values of cuprous oxide loading, which is a straight line. This plot shows no transition at a higher temperature, as observed in the case of the ln kl us.l/T plots (Fig. 10). The (numerical) value of the activation energy obtained from the slope of l/& us. l/T was 15.78 kcal/mol. This suggests that the induction period is perhaps a chemical activation process. The quantity l/tins has the dimensions of a first-order rate constant and can be taken as an approximate estimate of the rate constant for the induction process. Another interesting feature of this study is the observed incomplete conversion of the solid reactant. In none of the experiments was complete cuprous oxide conversion achieved. Such an effect has been observed earlier for
23
I
I
I
I
I
CUPROUS
OXIDE
LOADING
1
3.0
2-6
3.2 (f)
Fig. 11. Temperature
dependence
X
3.4 103,
OK -’
of the induction
period.
non-catalytic gas-solid reactions [17] and models for simple kinetic schemes have been proposed. Using the recently proposed model of Ramachandran and Smith [ 181, the maximum conversion for a particular system can be predicted by the following equation:
(x)max
=
1-
1-
Ebr/(Y 1-
e,,
- 1)
3.6
(5)
Equation (5) is applicable only for 7 > 1. For the present case the molar volume of the product (cuprous acetylide: 32.71 cm3/mol) is greater than the molar volume of the reactant (cuprous oxide: 23.83 cni3/mol) and the value of y is 1.37. Therefore, after a certain stage the pores of the solid particle are likely to get blocked, thus stopping further reaction. Using eqn. (5), (x),*~ was obtained as 46%, which comparesctiell with the observed range of maximum conversions (50 - 60%).
on the rate of reaction was investigated. It was found that the reaction is chemically controlled at 17 and 27 “C and the data in the lower conversion range were well represented by the Levenspiel [lo] model. At higher temperatures the gas to liquid inass transfer resistance was found to be significant. The reaction rate constants were obtained and the activation energy was found to be 20.15 kcal/ mol. An interesting observation of this study was that an induction period, which is a strong function of temperature, occurred before the reaction took place. It was also observed that the reaction stopped before complete conversion of the solid reactant. This was perhaps due to pore blockage by the solid product formed, which had a higher molar volume than the solid reactant.
NOMENCLATURE CONCLUSION
The reaction of acetylene with cuprous oxide suspended in water was studied. The effect of stirring speed, cuprous oxide loading, temperature and acetylene pressure
A* UP
b
concentration of acetylene at gasliquid interface, mol/cm3 surface area of particles per unit volume of liquid, cm2/cm3 stoichiometric coefficient for solid reactant
24
4 g 4 k, kla MB MG
average diameter of cuprous oxide particles, cm stoichiometric coefficient of solid product reaction rate constant, mol/cm’ s liquid to solid mass transfer coefficient, cm/s overall gas to .liquid mass transfer coefficient, s-l molecular weight of cuprous oxide, g molecular weight of cuprous acetylide, g
pressure of acetylene, atm average radius of particles, cm rate of absorption of acetylene per RA unit volume of slurry, mol/cm3 s T temperature, K t time, s induction time, s 4nd W loading of cuprous oxide, g/cm3 fractional conversion of cuprous oxide X (x )In,X maximum conversion of cuprous oxide
‘C,Hz R
Greek symbols parameter defined by eqn. (1) a1 parameter defined by eqn. (2) o2 7 h%$fG /b&MB (1 - eg) porosity of cuprous oxide particles Eb porosity of cuprous acetylide El3 density of cuprous acetylide, g/cm3 p&z density or molar density of cuprous Pb oxide particles, g/cm3 or mol/cm’
REFERENCES 1 S. Uchida, H. Moriguchi, H. Maejuma, K. Koide and S. Kageyama, Can. J. Chem. Eng., 56 (1978) 690. 2 S. S. Tamhankar and R. V. Chaudhari, Znd. Eng. Chem. Fundam., 18 (1979) 406. 3 S. S. Kale, and ,R. V. Chaudhari, Paper presented at 4th National Symposium on Catalysis, IIT, Bombay, December 1978. 4 J. W. Copenhaver and M. H. Bigelow, Acetylene and Carbon Monoxide Chemistry, Reinhold, New York, 1949. 5 P. B. Jadkar and R. V. Chaudhari, J. Chem. Eng. Data, 25 (1980) 115. 6 C. N. Satterfield, Mass Transfer in Heterogeneous Catalysis, M.I.T. Press, Cambridge, MA, 1970. 7 M. P. Dudukovic and H. S. Lamba, Chem. Eng. Sci., 33 (1978) 471. 8 P. A. Ramachandran and M. M. Sharma, Chem. Eng. Sci., 24 (1969) 1681. 9 C. Y. Wen, Znd. Eng. Chem., 60 (9) (1968). 10 0. Levenspiel, Chemical Reaction Engineering, Wiley, New Delhi, 1974, p. 367. 11 J. Szekely, C. I. Lin and H. Y. Sohn, Chem. Eng. Sci., 28 (1973) 1975. 12 M. Avrami,J. Chem. Phys., 9 (1941) 177. 13 D. A. Young, Decomposition of Solids, Pergamon Press, New York, 1966. 14 B. V. Erofeev, Reactivity of Solids, Proc. Fourth Znt. Symp., Elsevier, Amsterdam, 1961. 15 K. C. Russell, Nucleation in Solids, Phase Transformation, Am. Sot. for Metals, Metal Park, Ohio, 1970. 16 N. N. Bulgakov and V. V. Boldyrev, Kinet. KataZ., 14 (1973) 1402. 17 M. Hartman and R. W. Couglin, Znd. Eng. Chem. Process Des. Dev., 13 (1974) 248. 18 P. A. Ramachandran and J. M. Smith, AZChE J., 23 (1977) 353.
15
Kinetics of a Non-catalytic Slurry Reaction: Reaction of Acetylene. with Cuprous Oxide Suspended in Water S. S.TAMHANKAR, National
S. P. GUPTE and R. V. CHAUDHARI*
Chemical Laboratory,
Poona 411 008 (India)
(Received 24 January 1980; in final form 11 August 1980)
Abstract A kinetic study of the reaction of acetylene with cuprous oxide suspended in water is reported. It has been found that the reaction starts after an induction period which is a strong function of temperature. The reaction is chemically controlled at 17 and 27 “C, while at higher temperatures gas-liquid mass transfer contributes significantly. The reaction rate constants were obtained using the Levenspiel model and the true activation energy was found to be 20.15 kcal/mol. INTRODUCTION
Reactions of gases in slurries containing suspended solids are important in many industrial processes. Though catalytic slurry reactors are more common, in a number of situations non-catalytic reactions with suspended solids are encountered (for example, SO, absorption-in a suspension of lime [l] acetylene absorption in CuCl slurries [2] ). The reaction of acetylene with cuprous oxide is important in the preparation of copper acetylide, which is known to catalyse the ethynylation reaction [ 3,4 3 . A major application of the copper acetylide catalyst is in the manufacture of 2-butyne-1,4 diol from acetylene and formaldehyde. Preparation of this catalyst from cuprous oxide may prove to be attractive as it can eliminate the complex step (reduction of Cu(I1) to Cu(1)) in the conventional method of preparation from cupric oxide. Generally, copper acetylide is prepared in liquid media, owing to the explosive hazards
*To whom correspondence
should be addressed.
of acetylide in a dry stage. Besides the application in catalyst preparation, the reaction of acetylene with cuprous oxide provides an interesting example of a non-catalytic slurry reaction, wherein a dissolved gas reacts with a suspended solid reactant. The literature on this reaction is very scanty and no studies on the kinetics have been published so far. In the present work, an experimental study on the reaction kinetics is reported. The effect of temperature, solid loading and acetylene pressure on the rate of reaction is discussed. Kinetic parameters have been obtqed and the shift in controlling regimes is discussed. Such a study is likely to be useful in the process of cuprous acetylide catalyst preparation for 2-butyne-1,4 diol manufacture.
EXPERIMENTAL
The apparatus used in this work (Fig. 1) is similar to that described by Tamhankar and Chaudhari [ 21. A magnetically stirred glass vessel, 4.5 cm in diameter and 9 cm in height, was used as the reactor. The vessel was provided with an outer jacket in which water circulated at constant temperature, within kO.05 “C by means of a thermostatic bath. The acetylene consumed was measured by a graduated gas burette. In a typical experiment, the system was first evacuated using a high vacuum pump. Then acetylene gas was introduced to atmospheric pressure by careful control of the valve. This evacuation-filling cycle was repeated 3 - 4 times to ensure that the system was thoroughly flushed with acetylene. The manometer liquid (saturated with acetylene) from the reservoir was introduced into the burette to adjust the zero time reading. Known quantities of cuprous oxide and de-
16
VACWM-=
Fig. 1. Schematic diagram of the experimental set-up. RESULTS AND DISCUSSION
Preliminary experiments gassed distilled water were then charged into the reactor and run started by switching on the stirrer. The uptake of acetylene was measured by the movement of the liquid level in a graduated burette. The pressure in the system was equalized by adjusting the water level in the manometer tube. Acetylene consumed uersus time was recorded for different operating conditions. The cuprous oxide conversion was then calculated from the stoichiometry. For low-pressure experiments, the procedure was similar to that described above, except that after charging cuprous oxide and water into the reactor the system was set at a desired pressure by controlled evacuation. The pressure in the system was indicated by a gauge. Suitable corrections for the vapour pressure of water at the temperature of the experiment were made. Cuprous oxide supplied by BDH Laboratories was used. Acetylene gas from a cylinder (supplied by Bombay Oxygen Ltd.) was purified by passing it through silica gel traps before use in order to remove traces of acetone. The purity of the acetylene, as indicated by a gas chromatograph, was greater than 99.5%.
The reaction of acetylene with cuprous oxide suspended in water is a typical example of a non-catalytic three-phase reaction. The reaction is given by
(1) c,H2@)+
Cu,W)
+
~zC,(s)
+ H,O(f)
(II)
In such reactions either the rate of absorption of the acetylene or the conversion of cuprous oxide is of interest for studying the kinetics and controlling regimes. In the present study, the volume of acetylene absorbed was observed as a function of time by the procedure described earlier. From this, by appropriate corrections for the solubility of acetylene in the solvent, the conversion of solid cuprous oxide was calculated. The range of variables investigated is given in Table 1. Some of the interesting observations made during this study are as follows. (a) After initial saturation of the liquid, an induction period is observed, which varies with cuprous oxide loading and temperature. (b) The (initial) reaction rate is of zero order with respect to the pressure of the acetylene. (c) The reaction rate reduces drastically before complete conversion of the cuprous oxide.
17 TABLE 1 Range of variables investigated Agitation speed Temperature Cuprous oxide loading, W Pressure of acetylene, Pc,.Q Average diameter of cuprous oxide particles, d,
W = 0.05
6’70 - 1500 rev/min 17 - 69.5 “C 0.05 - 0.20 g/cm3 0.056 - 0.897 atm 6.4 X 10m3 cm
a/cm5
cuprous oxide loading), a quantitative critetion was followed. In this approach the rate of absorption was compared with the maximum rate of gas to liquid mass transfer (given by kpaA*). Thus a factor (Y~can be defined as CX 1 = RJkpaA*
(1)
where RA (mol/cm3 s) is the rate of absorption of acetylene per unit volume of slurry, kpa(s-' ) is the overall gas-liquid mass transfer coefficient, and A* (mol/cm3) is the concentration of acetylene at the gas-liquid interface. The kla data reported by Tamhankar and Chaudhari [ 21 for similar equipment was used in calculating the factor 01~.The solubility of acetylene in water reported by Jadkar and Chaudhari [ 51 was used to calculate A*. If the value of (Y~is less than 1 and greater than 0.2, the mass transfer effects are important. In the present study it was observed that the values of for the data at 17 and 27 “C were less than 0.2, which suggests that gas to liquid mass transfer resistance is negligible. At higher temperatures (40 and 50 “C with cuprous oxide loading of 0.20 g/cm3) the values of e1 were found to be between 0.5 and 0.7, which means that gas to liquid mass transfer resistance is significant under these conditions.
1500
rpm Fig. 2. Effect of stirrer speed on the rate of reaction at 27 “C.
Owing to these complexities, only the initial reaction rate data were considered when evaluating the kinetic parameters. However, attempts to discuss the above observations qualitatively have been made. The influence of different parameters is discussed below in order to identify the significance of the transport and reaction resistances. Mass transfer effects The effect of stirrer speed on the initial reaction rate was studied at 27 “C with cuprous oxide loadings of 0.05 and 0.20 g/cm3. The rates were found to be independent of the speed of agitation (see Fig. 2), which suggests that gas to liquid mass transfer resistance is negligible under these conditions. In order to check the significance of gasliquid mass transfer under other operating conditions (high temperature and higher
Effect of cuprous oxide loading The effect of cuprous oxide loading on the rate of reaction is shown in Fig. 3 as a plot of l/R, us. l/W for various temperatures. It has been shown earlier [6] that when such a plot passes through the origin, the effect of gasliquid mass transfer is negligible. This was observed at 17 and 27 “C, indicating the absence of gas-liquid mass transfer resistance. On the other hand, for higher temperatures the effect of loading on the rate of absorption. is negligible, which suggests that gas-liquid mass transfer contributes significantly under these conditions. These observations do not indicate the contribution of liquid to solid mass transfer resistance, hence a criterion based on comparison of the observed rates with the maximum rate of liquid to solid mass transfer (given by k,a,A*) was followed. For this comparison a factor o2 can be defined as e2 = RA /k,a,A*
(2)
where k, (cm/s) is the liquid to solid mass transfer coefficient and up is the surface area
18
I
STIRRER
SPEED
,040
rpm.
12-
0.9-
ov 0
I
I
I
5
10
I5
Fig. 3. Plot of l/RA vs.l/w.
+,hJ/cm3r
. e 0 @
TIME
Fig_ 4.
I
I
20
300 250 200 iO0
MESH MESH MESH MESH
SIZE SIZE SIZE SIZE
, min
Effect &f particle size on converzion of cuprouz oxide.
of particles per unit volume of liquid. The value of k, (6.6 X 10m3 cm/s) reported by Tamhankar and Chaudhari [2] was used, while up was calculated from the following equation : =p = 6WlPbdp
(3)
In the present study it was observed that, for all temperatures and cuprous oxide loadings investigated, the value of a2 was less than 0.2, which suggests that the liquid-solid mass transfer rate is very much higher than the observed rate of reaction, and hence this resistance is negligible.
19
transfer effects. This is also indicated by the values of cl (0.3) and a2 (0.6) observed for the data in this region.
Fig. 5. Effect of acetylene pressure on the rate of reaction.
Effect of particle size The effect of the particle size of cuprous oxide on conversion is shown in Fig. 4 for a loading-of 0.05 g/cm3. It can be seen that below a mesh size of 200, the conversiontime behaviour is independent of the particle size. This suggests that intraparticle diffusion is unimportant in this range. For 100 mesh particles, however, the conversion is lower, indicating significant intraparticle diffusion effects. Effect of acetylene pressure The effect of the acetylene pressure on the rate of reaction was studied at 27 “C with a 0.05 g/cm3 loading in the acetylene pressure range 0.05 - 0.89 atm. Interestingly, it was found that the reaction is zero order with respect to acetylene above 0.4 atm, and shows dependence on the pressure below 0.4 atm (see Fig. 5). Below 0.4 atm, the values of the external mass transfer parameters (Ye and (~z are in the range 0.3 - 0.6, which indicates significant external mass transfer resistance. In the case of intrinsically zero-order reactions with respect to gaseous species, the rate is independent of the concentration of that species as long as the concentration is finite throughout the particle. However, in a case in which the concentration drops to zero before the centre of the particle is reached, the rates become dependent on the mass transfer parameters. The theoretical analysis of this problem has been considered by Dudukovic and Lamba [7]. In the present work, the dependence of reaction rate on partial pressure below 0.4 atm could be due to mass
Evaluation of kinetic parameters The mechanistic features of three-phase non-catalytic reactions are complicated owing to the possibility of reactions in different phases. If we consider the reaction of a gaseous reactant A with the solid reactant B which is suspended in the ‘liquid, two situations can exist: (a) reaction of dissolved gas with dissolved solid occurring in the liquid phase, and (b) reaction of dissolved gas with solid reactant occurring at the solid surface. When the solubilities of the gaseous and solid reactants are comparable or the solubility of the gas is much lower than that of the solid, situation (a) prevails. The theoretical analysis of this case has been performed earlier by Ramachandran and Sharma [8] and more recently by Tamhankar and Chaudhari [ 21. When the solid is insoluble in the liquid or the solubility of the solid reactant is much lower than that of the gas, situation (b) is likely to exist. In the present study the solubility of cuprous oxide in water (lo-’ mol/cm3 at 27 “C) is several times lower than that of acetylene in water (3.42 X 10m5 mol/cm3 at 27 “C). Therefore, we need to’consider only case (b). It is important to note that the analyses given early by Wen [9] and Levenspiel [lo] for non-catalytic fluid-solid reactions would be applicable here with the additional complexities due to gas-liquid and liquidsolid mass transfer. Based on the discussion in the earlier sections, liquid-solid mass transfer will be assumed to be negligible in this work. For a zero-order reaction with respect to the fluid phase reactant A, the conversion of solid reactant B with time for a kinetically controlled regime is given by [lo] 1 - (1 -x)49
= bk,t/p,R
(4)
where x is the fractional conversion of cuprous oxide, &, (mol/cm3) the molar density, R (cm) the average radius of the particle, kr (mol/cm2 s) the reaction rate constant, b the stoichiometric coefficient and t the time. The above equation is applicable only to spherical particles under isothermal conditions.
SOLID
LOADING
STIRRER
TIME
SPEED
0.05 g/cm= 1040
rpm.
, min
Fig. 6. Effect of temperature on conversion of cuprous oxide (at 0.06 g/cm3 loadingj.
SOLID LOADING 0.20 G/cm3 STIRRER
TIME
,
SPEED
9040
rpm.
min
Fig. ‘I. Effect of temperature on conversion of cuprous oxide (at 0.20 g/cm3 loading).
The conversion of cuprous oxide (x) vs. time plots for different temperatures and cuprous oxide loadings of 0.05 and 0.20 g/cm3 are shown in Figs. 6 and 7 respectively. A significant induction time has been observed for the reaction to occur at 17 and 27 “C. In such cases, Szekely et al. [ll] have suggested that for the purpose of kinetic modelling the induction period can be ignored, and eqn. (4) can be used as an approximation. Following this procedure the quantity 1 -
(1 - 3c)43 was plotted against time for all temperatures studied. As in the present case the additional complication of incomplete conversion of cuprous oxide is involved, only the lower conversion range (<0.25) was considered for kinetic modelling. Figures 8 and 9 are the plots of 1 - (1- x)4’ against time for cuprous oxide loadings of 0.05 and 0.20 g/cm3, respectively, at different temperatures. From eqn. (4) it can be noted that the slope of such plots would give the quantity
21 048
I
I
I
I
I
3
0.16 -
SOLID
.
LOADING
STIRRER
0.12 -
SPEED
0.05
g/cm?
,040
rpm.
.
O,lD-
. I
D D
I
2000
4000 I
Fig. 8. Plot of 1 - (1 -x)lp
LOADINQ
STIRRER
I 0
2000.
I 10000
I 12000
sac
us. time (Levenspiel model) for 0.05 g/cm3 solid loading.
SOLID
0
I 6000
6000
SPEED
0.20
g/cmS
1040
I 4000
rpm.
I 6000
t WC
Fig. 9. Plot of 1 - (1 - x)lp
us. time (Levenspiel model) for 0.20 g/cm3 solid loading.
bkr/p,R, from which the rate constant kl can be obtained. The values of kI obtained from these plots are shown in Fig. 10 as a plot of In kI us. l/T; This figure shows two regions, indicating a transition from the kinetic to the mass transfer regime. As the contribution of mass transfer would be different at the two cuprous oxide loadings, the slopes in the
second regime are also observed to be different. At lower temperatures, however, the slope of In kr us. l/T for both the cuprous oxide loadings is the same. The true activation energy obtained from the slope of the first regime was 20.15 kcal/mol. The activation energies in the second regime were found to be 8.11 and 2.59 kcal/mol for the 0.05 and
22
0.5
\ 2.6
3.0
3.2 (+I
x
103,‘K
3-4
I 3.6
-I
Fig. 10. Effect of temperature on the reaction rate constant.
0.20 g/cm3 loadings, respectively. These values are lower because of the effect of mass transfer. This is also consistent with our earlier discussion that the gas-liquid mass transfer contributes significantly at higher temperatures. The induction period observed in many experiments deserves some comment. This period is significant at low temperatures, while it disappears at higher temperatures. Szekely et al. [ 111 have made a similar observation in the case of NiO hydrogenation. According to them, the induction period occurs when some activation process is required to initiate the main reaction. It is generally agreed [12 - 151 that in the solid phase the chemical reaction proceeds by the initiation of discrete nucleus forming sites, followed by nuclei (sub-microscopic particles). Further, the chemical reaction is localized at the reactant-nucleus interface, causing the nuclei to grow in size as the reaction proceeds. Theoretical justification for this mechanism is given by Bulgakov and Boldyrev [ 161. This process of nuclei formation is slow and is a strong function of temperature, which perhaps explains the higher induction period observed at 17 “C. Alternatively, in a case where the adsorption capacity of the solid product is greater than that of the solid
reactant, initial product formation can become an important step. Here the product formation may accelerate the reaction rate. Such a mechanism in which the product of the reaction increases the rate of the main reaction is known as an autocatalytic reaction. The mechanism suggested for the induction period in this work is only a hypothesis and more experimental work has yet to be carried out. The dependence of the induction period on temperature is an interesting observation of this work. Figure 11 shows a plot of l/& us. l/T, for two values of cuprous oxide loading, which is a straight line. This plot shows no transition at a higher temperature, as observed in the case of the ln kl us.l/T plots (Fig. 10). The (numerical) value of the activation energy obtained from the slope of l/& us. l/T was 15.78 kcal/mol. This suggests that the induction period is perhaps a chemical activation process. The quantity l/tins has the dimensions of a first-order rate constant and can be taken as an approximate estimate of the rate constant for the induction process. Another interesting feature of this study is the observed incomplete conversion of the solid reactant. In none of the experiments was complete cuprous oxide conversion achieved. Such an effect has been observed earlier for
23
I
I
I
I
I
CUPROUS
OXIDE
LOADING
1
3.0
2-6
3.2 (f)
Fig. 11. Temperature
dependence
X
3.4 103,
OK -’
of the induction
period.
non-catalytic gas-solid reactions [17] and models for simple kinetic schemes have been proposed. Using the recently proposed model of Ramachandran and Smith [ 181, the maximum conversion for a particular system can be predicted by the following equation:
(x)max
=
1-
1-
Ebr/(Y 1-
e,,
- 1)
3.6
(5)
Equation (5) is applicable only for 7 > 1. For the present case the molar volume of the product (cuprous acetylide: 32.71 cm3/mol) is greater than the molar volume of the reactant (cuprous oxide: 23.83 cni3/mol) and the value of y is 1.37. Therefore, after a certain stage the pores of the solid particle are likely to get blocked, thus stopping further reaction. Using eqn. (5), (x),*~ was obtained as 46%, which comparesctiell with the observed range of maximum conversions (50 - 60%).
on the rate of reaction was investigated. It was found that the reaction is chemically controlled at 17 and 27 “C and the data in the lower conversion range were well represented by the Levenspiel [lo] model. At higher temperatures the gas to liquid inass transfer resistance was found to be significant. The reaction rate constants were obtained and the activation energy was found to be 20.15 kcal/ mol. An interesting observation of this study was that an induction period, which is a strong function of temperature, occurred before the reaction took place. It was also observed that the reaction stopped before complete conversion of the solid reactant. This was perhaps due to pore blockage by the solid product formed, which had a higher molar volume than the solid reactant.
NOMENCLATURE CONCLUSION
The reaction of acetylene with cuprous oxide suspended in water was studied. The effect of stirring speed, cuprous oxide loading, temperature and acetylene pressure
A* UP
b
concentration of acetylene at gasliquid interface, mol/cm3 surface area of particles per unit volume of liquid, cm2/cm3 stoichiometric coefficient for solid reactant
24
4 g 4 k, kla MB MG
average diameter of cuprous oxide particles, cm stoichiometric coefficient of solid product reaction rate constant, mol/cm’ s liquid to solid mass transfer coefficient, cm/s overall gas to .liquid mass transfer coefficient, s-l molecular weight of cuprous oxide, g molecular weight of cuprous acetylide, g
pressure of acetylene, atm average radius of particles, cm rate of absorption of acetylene per RA unit volume of slurry, mol/cm3 s T temperature, K t time, s induction time, s 4nd W loading of cuprous oxide, g/cm3 fractional conversion of cuprous oxide X (x )In,X maximum conversion of cuprous oxide
‘C,Hz R
Greek symbols parameter defined by eqn. (1) a1 parameter defined by eqn. (2) o2 7 h%$fG /b&MB (1 - eg) porosity of cuprous oxide particles Eb porosity of cuprous acetylide El3 density of cuprous acetylide, g/cm3 p&z density or molar density of cuprous Pb oxide particles, g/cm3 or mol/cm’
REFERENCES 1 S. Uchida, H. Moriguchi, H. Maejuma, K. Koide and S. Kageyama, Can. J. Chem. Eng., 56 (1978) 690. 2 S. S. Tamhankar and R. V. Chaudhari, Znd. Eng. Chem. Fundam., 18 (1979) 406. 3 S. S. Kale, and ,R. V. Chaudhari, Paper presented at 4th National Symposium on Catalysis, IIT, Bombay, December 1978. 4 J. W. Copenhaver and M. H. Bigelow, Acetylene and Carbon Monoxide Chemistry, Reinhold, New York, 1949. 5 P. B. Jadkar and R. V. Chaudhari, J. Chem. Eng. Data, 25 (1980) 115. 6 C. N. Satterfield, Mass Transfer in Heterogeneous Catalysis, M.I.T. Press, Cambridge, MA, 1970. 7 M. P. Dudukovic and H. S. Lamba, Chem. Eng. Sci., 33 (1978) 471. 8 P. A. Ramachandran and M. M. Sharma, Chem. Eng. Sci., 24 (1969) 1681. 9 C. Y. Wen, Znd. Eng. Chem., 60 (9) (1968). 10 0. Levenspiel, Chemical Reaction Engineering, Wiley, New Delhi, 1974, p. 367. 11 J. Szekely, C. I. Lin and H. Y. Sohn, Chem. Eng. Sci., 28 (1973) 1975. 12 M. Avrami,J. Chem. Phys., 9 (1941) 177. 13 D. A. Young, Decomposition of Solids, Pergamon Press, New York, 1966. 14 B. V. Erofeev, Reactivity of Solids, Proc. Fourth Znt. Symp., Elsevier, Amsterdam, 1961. 15 K. C. Russell, Nucleation in Solids, Phase Transformation, Am. Sot. for Metals, Metal Park, Ohio, 1970. 16 N. N. Bulgakov and V. V. Boldyrev, Kinet. KataZ., 14 (1973) 1402. 17 M. Hartman and R. W. Couglin, Znd. Eng. Chem. Process Des. Dev., 13 (1974) 248. 18 P. A. Ramachandran and J. M. Smith, AZChE J., 23 (1977) 353.