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Extraction of pinenes into aqueous sulphuric acid: A convenient system for the measurement of effective interfacial area in liquid-liquid contactors
843
Shorter Communications CONCLUSIONS 1. When compared under equivalent conditions of bed height and excess gas velocity (U - U,,,,),at 200and 300°Cbubbles were 75-85% of the sizes observed at room temperature. 2. Independent observations of the onset of slugging confirm this trend in that higher excess gas velocities were achieved before the bed started to slug. 3. The minimum bubbling velocity appears to be less sensitive to temperature than U,,,, values reported in the literature. Bradford University Bradford, Yorkshire England
D. GELDART D. S. KAPOOR
Chemical Engineering Science. 1976, Vol. 31, pp. 843-845.
Pergamon Press.
NOTATION Rem, particle Reynolds number at miniium fluidization
U,, minimum fluidization velocity, cm/s u rnb minimum bubbling velocity, cm/s U superficial gas velocity, cm/s REFERENCES [l] Singh B., Rigby G. R. and Callcott T. G., Trans. Instn. Gem. Engrs. 197351 93. 121Mii T., Yoshida K. and Kunii D., J. Chem. Engng Japan 19736 100. [3] Yoshida K., Fugii S. and Kunii D., Proc. of Int. Conf. on Fluidization. Engineering Foundation, New York, Asilomar 1975.
Printed in Great Britain
Extraction of pinenes into aqueous sulphuric acid: a convenient system for the measurement of effective interfacial area in liquid-liquid contactors (Receioed 15 Nooember 1975; accepted 29 Januaryl976)
A knowledge of the effective interfacial area, a, for the design of liquid-liquid contactors, where liquid extraction is accompanied by chemical reaction, is essential. The chemical method for the measurement of a in liquid-liquid contactors was first suggested bv Nanda and Sharmalll. Thev showed that it is uossible to measure a in contactors-by extracting sparingly soluble esters of formic acid, such as butyl formate, amyl formate, etc. into aqueous solutions of caustic soda. This method has been subsequently adopted by many workers to obtain values of effective interfacial area in various types of liquid-liquid contactors [2-lo]. Esters of acids other than formic acid have also been employed but these materials require great care in handling as they are somewhat hazardous. Sankholkar and Sharma[ll] have recently reported that the extraction of a tertiary olefin, such as diisobutylene, into aqueous sulphuric acid can be conveniently used for the measurement of a in liquid-liquid agitated contactors. Sankholkar and Sharma[lZ, 131have also found that extraction of isoamylene into aqueous solutions of sulphuric acid (61.5-75%w/w), and desorption of the same from the loaded acid solutions into inert hydrocarbons such as n-heptane and toluene, which are industrially important, can also be conveniently adopted for the measurement of a. It was thought desirable to extend this method to cover some other industrially important reactions involving the use of olefins similar to diisobutylene. The hydration of pinenes (fractions of turpentine distillation) by sulphuric acid of relatively low concentrations is an important industrial process in the manufacture of a-terpineol, which is a perfumery product. Both a - and p -pinene are tertiary olefins and are expected to have reaction rate constants much higher than that of diisobutylene for the same concentration of sulphuric acid. In view of this, the extraction of pinenes into sulphuric acid of low concentrations appeared to be an attractive method for obtaining: values of a in agitated liquid-liquid contactors. Another attractive feature of this system is that, as in the case of diisobutylene, the specific rate of extraction of pinene into sulphuric acid is expected to be very low and hence would enable the measurement of a in an agitated contactor by operating it batch-wise without causing appreciable changes in the properties of the contacting liquids and the holdup of the dispersed phase. The rate of hydration of /3-pinene has been reported to be dependent upon the agitation of the reacting liquids [ 141,indicating that the reaction is essentially mass transfer controlled. Also the solubility of P-pinene in water is extremely low, thus fulfilling the conditions for the validity of chemical method of measuring a. The theoretical aspects of the chemical method of measuring a have been discussed by Nanda and Sharma[l] and Sharma and DanckwertsllS] and will not be repeated here.
For the sake of convenience in analysis, /3-pinene was diluted with monochlorobenzene which has extremely low solubility in sulphuric acid and which also remained unaffected by the acid in the range of concentration used. Since the solubility of pinene in the aqueous phase is extremely low, the resistance to mass transfer would be expected to lie in the aqueous phase, and so the choice of the diluent is purely a matter of convenience. The products of /3-pinene hydration are strongly dependent upon the concentration of sulphuric acid used and the temperature of the reaction. At room temperature and at low concentration of acid the main product is terpin hydrate (a desired product) and at concentrations above 50% w/w acid dimerisation and polymerisation of p-pinene occurs. The maximum concentration of acid which does not lead to appreciable dimerisation and polymerisation at 20°Chas been reported to be 45% w/w acid[l6]. An initial concentration of 42%w/w acid was therefore chosen for measuring the specific rate of P-pinene extraction. MATERIALS /3-Pinene and monochlorobenzene were obtained from firms of repute and were tested for their purity on the gas-liquidchromatograph. Monochlorobenzene was found to be chemically pure while P-pinene was found to contain 7% a-pinene. Since the analysis in the actual extraction experiments was based upon the disappearance of /3-pinene, there was no need to distil the fi-pinene sample and it was used as such. The concentrated sulphuric acid used was of commercial grade. Analysis
The extent of reaction can be conveniently followed by noting the fall of P-pinene concentration in the organic phase which can be accurately analysed by gas-liquid-chromatography. A 10ft long 6 mm i.d. copper column packed with 10%carbowax 20 M on Chromosorb W was used for this purpose. The column temperature was maintained at 90°Cand hydrogen was used as the carrier gas. EXPERIMRNTAL Stirred cell
In order to measure values of a with the P-pinene-aqueous sulphuric acid system, it is necessary to determine the specific rate of extraction of /3-pinene from the organic phase. Solutions of p-pinene in monochlorobenzene were contacted with 42% aqueous sulphuric acid for known length of time in stirred cells. Each batch consisted of 25 ml of organic phase and 200ml of sulphuric acid phase. All the experiments were carried out in 9.2 cm i.d. glass stirred cells which had effective interfacial area of 65 cm2. The design of the stirred cell was similar to that used by Gehlawat and Sharma[l7]. In all the cases the aqueous phase was
844
Shorter Communications
below the organic phase. Both the phases were stirred, without disturbing the interface considerably, so as to renew the interface continuously. Experiments were carried out for 12-70hr as the rate of extraction of @pinene into sulphuric acid was expected to be very low. In order to keep the duration of the experiments to a minimum, a very small quantity of the organic phase (25 ml) was taken. The problem of stirring the top organic layer was solved by using two thin nichrome wires (1 mm dia.), fixed to the glass stirrer, as agitator blades. To avoid any corrosion of these wires by sulphuric acid, care was taken to ensure that they did not come in contact with the aqueous sulphuric acid phase. The lower aqueous phase was stirred with the conventional cruciform type of stirrer. The speed of stirring was varied from 26 to 50rpm to study the effect of stirring on the rate of extraction of p -pinene. Mechanically agitated contactor
Experiments were carried out in a 16.5cm i.d. mechanically agitated glass contactor. It was provided with four 1.5cm wide vertical high density polyethylene baffles perpendicular to the wall of the contactor, and at 90”to each other. A six bladed 6.5 cm dia. high density polyethylene turbine impeller was used for agitation. High density polyethylene was found to be a suitable material of construction as it remained unaffected by both the sulphuric acid and pinene-chlorobenzene mixture. The speed of agitation was varied from 675 to 1450rpm. In all experiments the organic phase was dispersed in the aqueous sulphuric acid phase since such a mode of operation was expected to yield higher values of a compared to the other mode of operation, namely, aqueous phase dispersed in the organic phase[ll]. Preliminary experiments showed that the phase separation occurred within 60 set after the cessation of agitation. Each batch of the experiment consisted of 500ml of organic phase containing 0.0975 mole fraction of P-pinene and 21oOml of 42% w/w sulphuric acid. The organic phase hold-up in the batch was thus 19.2%.Experiments were conducted for periods ranging from IO min at the highest speed of agitation to 45 min for the lowest speed of agitation. The organic phase was analysed for its pinene content at the end of each run. It was observed in the preliminary experiments that the temperature of the liquid tended to go up appreciably in the course of the reaction. External cooling was, therefore, employed to keep the temperature within 30? l”C, by placing the contactor in a thermostatic bath.
Speed
rev/m!n
of stmng,
5.0 2C
P
7
;4.0 E
-
P e sx3o .; e
-
ii ; 2.0
-
2 e
0
z u
1.0
-
/I
=:
/I 1’ I
I
0.1
02
01 0
Mole
fraction
of
I 0.3 pmene
Fig. 1. Effect of stirring and concentration of pinene on the specific rate of extraction in stirred cells (temp. = 30”(J),stirring speed = 38rpm, initial mole fraction of pinene = 0.0975. I
I
I
I
I
RESULTS ANDDISCUSSION Stirred cell
The values of the specific rate of extraction of P-pinene into sulphuric acid are plotted in Fig. I against the speed of stirring and the mole fraction of P-pinene in the organic phase. It is observed that the specific rate of extraction is independent of the speed of stirring in the range of speed covered, that is from 26 to 50 rpm and remained constant at an average value of 1.395X lo-” mole/cm* set at 30°Cfor an initial mole fraction of p-pinene of 0.0975in the organic phase. Experiments conducted at 38rpm with higher concentrations of /3-pinene gave the values of specific rate of extraction to be 2.815x lo-” and 4.416x 10ml’mole/cm’ set at the mole fractions of 0.209 and 0.336 of /3-pinene, respectively. The values of specific rate of extraction when plotted against the mole fraction of P-pinene gave a straight line passing through the origin (Fig. I) indicating that the extraction of fl-pinene into aqueous sulphuric acid is accompanied by a fast pseudo first order reaction with respect to P-pinene.
120 500
I 700
I
I
I
900
1100
1300
Speed of ogltatian, rev/min Fig. 2. Effect of speed of agitation on effective interfacial area in the agitated contactor (temp. = 30°C).
dilutents and different concentrations of sulphuric acid so as to cover a wide range of physical properties such as density, viscosity and interfacial surface tension. It is also proposed to use this system for the measurement of effective interfacial area in other contactors, such as rotating disc contactor, etc.
Agitated contactor
The values of a obtained in the mechanically agitated contactor for an average organic phase fractional hold-up of 0.188 are plotted against the speed of agitation in Fig. 2. The effective interfacial area varies linearly with the speed of agitation. This observation is in agreement with that of Fernandes and Sharma[Zl and Sankholkar and Sharma[ll]. In the present investigation monochlorobenzene was the only diluent used. It is well known that the physical properties of the contacting phases can have marked effect upon the values of effective interfacial area in any contactor[ll]. It is, therefore, proposed to carry out further work using a variety of organic
CONCLUSION The extraction of /3-pinene from its solutions in an organic solvent, such as monochlorobenzene, into aqueous sulphuric acid solutions is a convenient method for the measurement of a in a variety of agitated liquid-liquid contactors. This system is particularly useful for batch operation of contactors, thus simplifying the measurement of a. This system is also of industrial importance and the scale-up of plants manufacturing terpin hydrate will require a knowledge of specific rate of extraction and effective interfacial area.
845
Shorter Communications Acknowledgement-One of us (S.S.L.) wishes to thank the University Grants Commission for an award of a scholarship which enabled this work to be carried out.
[6] de Santiago M. and Trambouze P., Chem. Engng Sci. 197126 1803. [7] de Santiago M. and Binder M. S., Chem. Engng Sci. 197126 175. 181Onda K., Takeuchi H. and Takahashi M., Kagaku Kogaku 197135 221. [91 Shah A. K. and Sharma M. M., Can. J. Chem. Engng 197149 5%. UOI Fernandes J. B., A.I.Ch.E. Symp. Ser. No. 120197268 124. r111 Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1973 L-1
Department of Chemical Technology University of Bombay Matunga Road Bombay-400 019 India
S. S. LADDHA M. M. SHARMA
28 2089.
WI Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1973
REFERENCES
2n 49.
[I] Nanda A. K. and Sharma M. M., Chem. Engng Sci. 196621 707. 121Femandes J. B. and Sharma M. M., Gem. Engng Sci. I%7 22 1267. [3] Femandes J. B. and Sharma M. M., Chem. Engng Sci. 196823 9. [4] Sharma M. M. and Nanda A. K., Trans. Instn Chem. Engrs. 1%8 44 T44. [5] Puranik S. A. and Sharma M. M., Chem. Engng Sci. 197025 257.
Chemical Engineering Science, 1976, Vol. 31. pp. 845447.
Pcrgamon Press.
[131Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1975 30 729.
[I41 Simonsen J. L., The Terpenes Vol. 2, p. 139. Cambridge University Press, London 1932. [151Sharma M. M. and Danckwerts P. V., Brit. Chem. Engng 1970 15 522. [161Austerweil G., Bull. Sot. Chim. (iv), 192639 690; C.A., 1927 21 1806. [I71 Gehlawat J. K. and Sharma M. M., Chem. Engng Sci. 196823 1173.
Printed in Great Britain
The correlation of axial dispersion data for beds of small particles (Received 10October 1975; acceptedinrevisedfor
12January 1976)
Axial dispersion in packed beds has drawn considerable attention[l-91. Still a conclusive theory has not been formulated which explains heat and mass transfer and dispersion, especially for the case of beds packed with small particles (dia. < 0.5 mm). In this study a tentative interpretation is given of the fact that in general in the case of beds packed with fine particles reported P&let numbers are lower than in the case of beds packed with large particles. Axial dispersion is usually formulated based on the onedimensional dispersed plug flow model which reads for a bed packed with porous particles with infinite fast exchange between interparticle and intraparticle space:
For a bed packed with non-porous particles this equation reduces to: 5
t 2t
Figure 1surveys the measurements of axial dispersion published so far together with some of our own measurements. The curve related to large particles (dia. > 2 mm) has been found by many authors and can be interpreted as follows: (a) At very low Reynolds numbers axial dispersion is mainly caused by molecular diffusion; the dispersion coefficient is given by: D=lDAB. 7
0.01
2
5
0.1
2
(4)
(b) At high Reynolds numbers the axial dispersion coefficient can be derived by matching the dispersion model to a model in which the packed bed is represented by a series of perfectly stirred tanks, thelengthof atankbeingequaltotheheightofaparticlelayer. If the number of layers is large enough so that end effects are
5
IO
2
5
IO
Rep%
Fig. 1. Summary of literature data of axial dispersion of gases in beds packed with fine particles.
4
(3)
This leads to the following relation: Pe, = r Re,Sc.
0.001~
1. Edwards and Richardson 2. Suzuki and Smith 3. Suzuki and Smith 4. Edwards and Richardson 5. Edwards and Richardson 6. Edwards and Richardson 0 column 1 0 column 2 present A column 3 investigations v column 4
>500 wrn 160225 pm 114+lO~m 158290 firn 97k30p.m 146+70pm 117+3 pm 101+22 Km 195226 pm 21627 pm
Shorter Communications CONCLUSIONS 1. When compared under equivalent conditions of bed height and excess gas velocity (U - U,,,,),at 200and 300°Cbubbles were 75-85% of the sizes observed at room temperature. 2. Independent observations of the onset of slugging confirm this trend in that higher excess gas velocities were achieved before the bed started to slug. 3. The minimum bubbling velocity appears to be less sensitive to temperature than U,,,, values reported in the literature. Bradford University Bradford, Yorkshire England
D. GELDART D. S. KAPOOR
Chemical Engineering Science. 1976, Vol. 31, pp. 843-845.
Pergamon Press.
NOTATION Rem, particle Reynolds number at miniium fluidization
U,, minimum fluidization velocity, cm/s u rnb minimum bubbling velocity, cm/s U superficial gas velocity, cm/s REFERENCES [l] Singh B., Rigby G. R. and Callcott T. G., Trans. Instn. Gem. Engrs. 197351 93. 121Mii T., Yoshida K. and Kunii D., J. Chem. Engng Japan 19736 100. [3] Yoshida K., Fugii S. and Kunii D., Proc. of Int. Conf. on Fluidization. Engineering Foundation, New York, Asilomar 1975.
Printed in Great Britain
Extraction of pinenes into aqueous sulphuric acid: a convenient system for the measurement of effective interfacial area in liquid-liquid contactors (Receioed 15 Nooember 1975; accepted 29 Januaryl976)
A knowledge of the effective interfacial area, a, for the design of liquid-liquid contactors, where liquid extraction is accompanied by chemical reaction, is essential. The chemical method for the measurement of a in liquid-liquid contactors was first suggested bv Nanda and Sharmalll. Thev showed that it is uossible to measure a in contactors-by extracting sparingly soluble esters of formic acid, such as butyl formate, amyl formate, etc. into aqueous solutions of caustic soda. This method has been subsequently adopted by many workers to obtain values of effective interfacial area in various types of liquid-liquid contactors [2-lo]. Esters of acids other than formic acid have also been employed but these materials require great care in handling as they are somewhat hazardous. Sankholkar and Sharma[ll] have recently reported that the extraction of a tertiary olefin, such as diisobutylene, into aqueous sulphuric acid can be conveniently used for the measurement of a in liquid-liquid agitated contactors. Sankholkar and Sharma[lZ, 131have also found that extraction of isoamylene into aqueous solutions of sulphuric acid (61.5-75%w/w), and desorption of the same from the loaded acid solutions into inert hydrocarbons such as n-heptane and toluene, which are industrially important, can also be conveniently adopted for the measurement of a. It was thought desirable to extend this method to cover some other industrially important reactions involving the use of olefins similar to diisobutylene. The hydration of pinenes (fractions of turpentine distillation) by sulphuric acid of relatively low concentrations is an important industrial process in the manufacture of a-terpineol, which is a perfumery product. Both a - and p -pinene are tertiary olefins and are expected to have reaction rate constants much higher than that of diisobutylene for the same concentration of sulphuric acid. In view of this, the extraction of pinenes into sulphuric acid of low concentrations appeared to be an attractive method for obtaining: values of a in agitated liquid-liquid contactors. Another attractive feature of this system is that, as in the case of diisobutylene, the specific rate of extraction of pinene into sulphuric acid is expected to be very low and hence would enable the measurement of a in an agitated contactor by operating it batch-wise without causing appreciable changes in the properties of the contacting liquids and the holdup of the dispersed phase. The rate of hydration of /3-pinene has been reported to be dependent upon the agitation of the reacting liquids [ 141,indicating that the reaction is essentially mass transfer controlled. Also the solubility of P-pinene in water is extremely low, thus fulfilling the conditions for the validity of chemical method of measuring a. The theoretical aspects of the chemical method of measuring a have been discussed by Nanda and Sharma[l] and Sharma and DanckwertsllS] and will not be repeated here.
For the sake of convenience in analysis, /3-pinene was diluted with monochlorobenzene which has extremely low solubility in sulphuric acid and which also remained unaffected by the acid in the range of concentration used. Since the solubility of pinene in the aqueous phase is extremely low, the resistance to mass transfer would be expected to lie in the aqueous phase, and so the choice of the diluent is purely a matter of convenience. The products of /3-pinene hydration are strongly dependent upon the concentration of sulphuric acid used and the temperature of the reaction. At room temperature and at low concentration of acid the main product is terpin hydrate (a desired product) and at concentrations above 50% w/w acid dimerisation and polymerisation of p-pinene occurs. The maximum concentration of acid which does not lead to appreciable dimerisation and polymerisation at 20°Chas been reported to be 45% w/w acid[l6]. An initial concentration of 42%w/w acid was therefore chosen for measuring the specific rate of P-pinene extraction. MATERIALS /3-Pinene and monochlorobenzene were obtained from firms of repute and were tested for their purity on the gas-liquidchromatograph. Monochlorobenzene was found to be chemically pure while P-pinene was found to contain 7% a-pinene. Since the analysis in the actual extraction experiments was based upon the disappearance of /3-pinene, there was no need to distil the fi-pinene sample and it was used as such. The concentrated sulphuric acid used was of commercial grade. Analysis
The extent of reaction can be conveniently followed by noting the fall of P-pinene concentration in the organic phase which can be accurately analysed by gas-liquid-chromatography. A 10ft long 6 mm i.d. copper column packed with 10%carbowax 20 M on Chromosorb W was used for this purpose. The column temperature was maintained at 90°Cand hydrogen was used as the carrier gas. EXPERIMRNTAL Stirred cell
In order to measure values of a with the P-pinene-aqueous sulphuric acid system, it is necessary to determine the specific rate of extraction of /3-pinene from the organic phase. Solutions of p-pinene in monochlorobenzene were contacted with 42% aqueous sulphuric acid for known length of time in stirred cells. Each batch consisted of 25 ml of organic phase and 200ml of sulphuric acid phase. All the experiments were carried out in 9.2 cm i.d. glass stirred cells which had effective interfacial area of 65 cm2. The design of the stirred cell was similar to that used by Gehlawat and Sharma[l7]. In all the cases the aqueous phase was
844
Shorter Communications
below the organic phase. Both the phases were stirred, without disturbing the interface considerably, so as to renew the interface continuously. Experiments were carried out for 12-70hr as the rate of extraction of @pinene into sulphuric acid was expected to be very low. In order to keep the duration of the experiments to a minimum, a very small quantity of the organic phase (25 ml) was taken. The problem of stirring the top organic layer was solved by using two thin nichrome wires (1 mm dia.), fixed to the glass stirrer, as agitator blades. To avoid any corrosion of these wires by sulphuric acid, care was taken to ensure that they did not come in contact with the aqueous sulphuric acid phase. The lower aqueous phase was stirred with the conventional cruciform type of stirrer. The speed of stirring was varied from 26 to 50rpm to study the effect of stirring on the rate of extraction of p -pinene. Mechanically agitated contactor
Experiments were carried out in a 16.5cm i.d. mechanically agitated glass contactor. It was provided with four 1.5cm wide vertical high density polyethylene baffles perpendicular to the wall of the contactor, and at 90”to each other. A six bladed 6.5 cm dia. high density polyethylene turbine impeller was used for agitation. High density polyethylene was found to be a suitable material of construction as it remained unaffected by both the sulphuric acid and pinene-chlorobenzene mixture. The speed of agitation was varied from 675 to 1450rpm. In all experiments the organic phase was dispersed in the aqueous sulphuric acid phase since such a mode of operation was expected to yield higher values of a compared to the other mode of operation, namely, aqueous phase dispersed in the organic phase[ll]. Preliminary experiments showed that the phase separation occurred within 60 set after the cessation of agitation. Each batch of the experiment consisted of 500ml of organic phase containing 0.0975 mole fraction of P-pinene and 21oOml of 42% w/w sulphuric acid. The organic phase hold-up in the batch was thus 19.2%.Experiments were conducted for periods ranging from IO min at the highest speed of agitation to 45 min for the lowest speed of agitation. The organic phase was analysed for its pinene content at the end of each run. It was observed in the preliminary experiments that the temperature of the liquid tended to go up appreciably in the course of the reaction. External cooling was, therefore, employed to keep the temperature within 30? l”C, by placing the contactor in a thermostatic bath.
Speed
rev/m!n
of stmng,
5.0 2C
P
7
;4.0 E
-
P e sx3o .; e
-
ii ; 2.0
-
2 e
0
z u
1.0
-
/I
=:
/I 1’ I
I
0.1
02
01 0
Mole
fraction
of
I 0.3 pmene
Fig. 1. Effect of stirring and concentration of pinene on the specific rate of extraction in stirred cells (temp. = 30”(J),stirring speed = 38rpm, initial mole fraction of pinene = 0.0975. I
I
I
I
I
RESULTS ANDDISCUSSION Stirred cell
The values of the specific rate of extraction of P-pinene into sulphuric acid are plotted in Fig. I against the speed of stirring and the mole fraction of P-pinene in the organic phase. It is observed that the specific rate of extraction is independent of the speed of stirring in the range of speed covered, that is from 26 to 50 rpm and remained constant at an average value of 1.395X lo-” mole/cm* set at 30°Cfor an initial mole fraction of p-pinene of 0.0975in the organic phase. Experiments conducted at 38rpm with higher concentrations of /3-pinene gave the values of specific rate of extraction to be 2.815x lo-” and 4.416x 10ml’mole/cm’ set at the mole fractions of 0.209 and 0.336 of /3-pinene, respectively. The values of specific rate of extraction when plotted against the mole fraction of P-pinene gave a straight line passing through the origin (Fig. I) indicating that the extraction of fl-pinene into aqueous sulphuric acid is accompanied by a fast pseudo first order reaction with respect to P-pinene.
120 500
I 700
I
I
I
900
1100
1300
Speed of ogltatian, rev/min Fig. 2. Effect of speed of agitation on effective interfacial area in the agitated contactor (temp. = 30°C).
dilutents and different concentrations of sulphuric acid so as to cover a wide range of physical properties such as density, viscosity and interfacial surface tension. It is also proposed to use this system for the measurement of effective interfacial area in other contactors, such as rotating disc contactor, etc.
Agitated contactor
The values of a obtained in the mechanically agitated contactor for an average organic phase fractional hold-up of 0.188 are plotted against the speed of agitation in Fig. 2. The effective interfacial area varies linearly with the speed of agitation. This observation is in agreement with that of Fernandes and Sharma[Zl and Sankholkar and Sharma[ll]. In the present investigation monochlorobenzene was the only diluent used. It is well known that the physical properties of the contacting phases can have marked effect upon the values of effective interfacial area in any contactor[ll]. It is, therefore, proposed to carry out further work using a variety of organic
CONCLUSION The extraction of /3-pinene from its solutions in an organic solvent, such as monochlorobenzene, into aqueous sulphuric acid solutions is a convenient method for the measurement of a in a variety of agitated liquid-liquid contactors. This system is particularly useful for batch operation of contactors, thus simplifying the measurement of a. This system is also of industrial importance and the scale-up of plants manufacturing terpin hydrate will require a knowledge of specific rate of extraction and effective interfacial area.
845
Shorter Communications Acknowledgement-One of us (S.S.L.) wishes to thank the University Grants Commission for an award of a scholarship which enabled this work to be carried out.
[6] de Santiago M. and Trambouze P., Chem. Engng Sci. 197126 1803. [7] de Santiago M. and Binder M. S., Chem. Engng Sci. 197126 175. 181Onda K., Takeuchi H. and Takahashi M., Kagaku Kogaku 197135 221. [91 Shah A. K. and Sharma M. M., Can. J. Chem. Engng 197149 5%. UOI Fernandes J. B., A.I.Ch.E. Symp. Ser. No. 120197268 124. r111 Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1973 L-1
Department of Chemical Technology University of Bombay Matunga Road Bombay-400 019 India
S. S. LADDHA M. M. SHARMA
28 2089.
WI Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1973
REFERENCES
2n 49.
[I] Nanda A. K. and Sharma M. M., Chem. Engng Sci. 196621 707. 121Femandes J. B. and Sharma M. M., Gem. Engng Sci. I%7 22 1267. [3] Femandes J. B. and Sharma M. M., Chem. Engng Sci. 196823 9. [4] Sharma M. M. and Nanda A. K., Trans. Instn Chem. Engrs. 1%8 44 T44. [5] Puranik S. A. and Sharma M. M., Chem. Engng Sci. 197025 257.
Chemical Engineering Science, 1976, Vol. 31. pp. 845447.
Pcrgamon Press.
[131Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1975 30 729.
[I41 Simonsen J. L., The Terpenes Vol. 2, p. 139. Cambridge University Press, London 1932. [151Sharma M. M. and Danckwerts P. V., Brit. Chem. Engng 1970 15 522. [161Austerweil G., Bull. Sot. Chim. (iv), 192639 690; C.A., 1927 21 1806. [I71 Gehlawat J. K. and Sharma M. M., Chem. Engng Sci. 196823 1173.
Printed in Great Britain
The correlation of axial dispersion data for beds of small particles (Received 10October 1975; acceptedinrevisedfor
12January 1976)
Axial dispersion in packed beds has drawn considerable attention[l-91. Still a conclusive theory has not been formulated which explains heat and mass transfer and dispersion, especially for the case of beds packed with small particles (dia. < 0.5 mm). In this study a tentative interpretation is given of the fact that in general in the case of beds packed with fine particles reported P&let numbers are lower than in the case of beds packed with large particles. Axial dispersion is usually formulated based on the onedimensional dispersed plug flow model which reads for a bed packed with porous particles with infinite fast exchange between interparticle and intraparticle space:
For a bed packed with non-porous particles this equation reduces to: 5
t 2t
Figure 1surveys the measurements of axial dispersion published so far together with some of our own measurements. The curve related to large particles (dia. > 2 mm) has been found by many authors and can be interpreted as follows: (a) At very low Reynolds numbers axial dispersion is mainly caused by molecular diffusion; the dispersion coefficient is given by: D=lDAB. 7
0.01
2
5
0.1
2
(4)
(b) At high Reynolds numbers the axial dispersion coefficient can be derived by matching the dispersion model to a model in which the packed bed is represented by a series of perfectly stirred tanks, thelengthof atankbeingequaltotheheightofaparticlelayer. If the number of layers is large enough so that end effects are
5
IO
2
5
IO
Rep%
Fig. 1. Summary of literature data of axial dispersion of gases in beds packed with fine particles.
4
(3)
This leads to the following relation: Pe, = r Re,Sc.
0.001~
1. Edwards and Richardson 2. Suzuki and Smith 3. Suzuki and Smith 4. Edwards and Richardson 5. Edwards and Richardson 6. Edwards and Richardson 0 column 1 0 column 2 present A column 3 investigations v column 4
>500 wrn 160225 pm 114+lO~m 158290 firn 97k30p.m 146+70pm 117+3 pm 101+22 Km 195226 pm 21627 pm