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Desorption of isoamylene from loaded acid liquors
Chemical Engineering
Science, 1975, Vol. 30, pp. 729-733.
Pergamon Press.
Printed in Great Britain
DESORPTION OF ISOAMYLENE FROM LOADED ACID LIQUORS D. S. SANKHOLKAR and M. M. SHARMA Department of Chemical Technology, University of Bombay, Matunga Road, Bombay 400019,India (Received 8 August 1974;accepted 16 January 1975) Abstract-The desorption of 2-methyl-2-butene (isoamylene) from loaded sulphuric acid solutions, having an acid strength of about 60 per cent (w/w) and loading upto 0.45g mole of isoamyleneper g moleof H,SO,, was found to be precededby a fast reaction,whichoccurred in the film adjacent to the interface. The specificrates of desorption of isoamylene into inert hydrocarbons-n-heptane and toluene and an inert gas-nitrogen, were found to be proportional to the isoamylene concentration j, expressed as g mole of isoamylene per g mole of H,SO,, and agreed among themselves at the same value of j. The technique of desorption preceded by a fast reaction was employed for the measurement of effective interfacial area in liquid-liquid and gas-liquid agitated contactors. The absorption of isobutylene into fresh and loaded solutions of sulphuric acid was also used for the measurement of effective interfacial area in gas-liquid agitated contactors for comparative purposes. The values of effective interfacial area for the gas-liquid system obtained by the desorption technique were found to be comparable with those obtained from the absorption of isobutylene in fresh and loaded solutions of sulphuric acid, under otherwise comparable conditions.
INTRODUCTION The &-fraction from naphtha crackers contains about 14
per cent tert-isoamylenes (namely, 2-methyl-I-butene and 2-methyl-2-butene). If, however, cyclopentadiene and isoprene are recovered, the remaining &-fraction would contain about 20 per cent tert-isoamylenes. The tetiisoamylenes are industrially important as precursor for isoprene and for a variety of alkylated derivatives. The telt-isoamylenes can be separated from the less reactive n-pentenes and inert paraffins of the &fraction by selective extraction into 6&70 per cent (w/w) aqueous sulphuric acid solutions in the temperature range of 1&3O”C[ 1,2]. The teti-isoamylenes can be recovered from the loaded sulphuric acid solutions, usually referred to as ‘fat’ acids, by contacting them with a relatively high boiling hydrocarbon solvent, at 30-38°C. Thus, it appears that the reaction of isoamylenes with sulphuric acid is reversible. In fact, this reversibility is the basis of the process developed by Sinclair Research Laboratories, U.S.A.[l]. They have reported the results obtained on a typical run on a continuous integrated pilot plant consisting of a single acid extraction stage and a single acid regeneration stage. A product containing 99 per cent isoamylenes is claimed to have been obtained by this method from a &-fraction containing 17 per cent isoamylenes. The solvent used for the extraction of isoamylenes from the fat acid was n-heptane. It has also been claimed that a co-current pipeline contactor can be advantageously employed for carrying out the extraction of isoamylenes [3]. In a previous paper, we have reported the kinetics of absorption of 2-methyl-2-butene (a representative of tert-isoamylenes) into aqueous solutions of sulphuric acid in the range of acid concentration employed in practice [4]. The absorption was found to conform to fast pseudo-first order mechanism, in the range of acid concentration from 61.5 per cent w/w (9.4 g mole/l) to 75
per cent w/w (12.85 gmole/l). The pseudo-first order reaction rate constant at 30°C was found to vary from about 4.2 x lo’ to 2.2 x lo* se? over the above concentration range. Zakharova et al.[5] have studied the dependence of density, viscosity and surface tension of the fat acid on its isoamylene concentration. They have also reported the equilibrium data for distribution of isoamylenes between 65 per cent (w/w) sulphuric acid and n-amylenes at 2, 10 and 20°C. There is, however, no information in the literature concerning the kinetics of extraction of isoamylenes from the fat acids into an inert hydrocarbon. This work was, therefore, undertaken to study the kinetics of desorption of 2-methyl-2-butene (- representative of tert-isoamylenes and hereafter called simply “isoamylene”) from fat acids into inert hydrocarbons-nheptane and toluene and an inert gas-nitrogen. The concentration of aqueous sulphuric acid solution chosen was 60 per cent (w/w), because isoamylene is not polymerized by the acid of this concentration, even after a substantially long period[6] and hence is unlikely to interfere with the process of desorption of isoamylene from the fat acids. Gehlawat and Sharma[7] have shown that the absorption of isobutylene in 60 and 65 per cent (w/w) aqueous sulphuric acid solutions conforms to fast pseudo-first order mechanism and it is expected that the absorption of isobutylene in aqueous sulphuric acid solutions loaded with isobutylene (also referred to as “fat” acids) would show a similar behaviour. The above systems can, therefore, be conveniently used for the measurement of effective interfacial area in gas-liquid contactors. It was thought that the desorption of isoamylene from fat acid liquors into an inert hydrocarbon or an inert gas may prove to be a unique system for the measurement of effective interfacial area in liquid-liquid or gas-liquid contactors, respectively.
D. S. SANKHOLKAR and M. M. SHARMA
730 MA-
Isoamylene (Zmethyl-2-butene) was prepared by the dehydration of tert-amyl alcohol (Zmethyl-Zbutanol) with aqueous sulphuric acid (approx. 48 per cent w/w) [8]. n-Heptane and toluene were laboratory chemical grade materials. Sulphuric acid was of commercial grade. Aqueous solutions of sulphuric acid were prepared with deionised water. A Cd-fraction from a local naphtha cracker, after the extraction of butadiene, containing 58 per cent isobutylene, was also used for the measurement of effective interfacial area for comparative purposes. ANALYSIS
rate of extraction of isoamylene from the fat acid into the hydrocarbon phase was followed by determining the concentration of isoamylene in the hydrocarbon phase and/or from the increase in the density of the fat acid at the end of the run, whenever this increase was substantial. The rate of desorption of isoamylene into nitrogen was followed by determining the concentration of isoamylene in the outlet gas stream, if this were conveniently measurable by G.L.C. analysis (e.g. in the case of mechanically agitated gas-liquid contactors) or from the increase in the density of a batch of fat acid at the end of the run of a suitable period (e.g. in the case of the stirred cell). The concentration of isoamylene in the fat acid was calibrated against the density of the fat acid, by preparing fat acids of known isoamylene concentration and determining their densities at 30°C. Both the liquid and the gaseous samples containing isoamylene were analysed chromatographically on an F and M Model 720 Dual Column Programmed Temperature Gas Chromatograph. The components of the organic phase were analysed by a 6 mm id. 3 m long copper column packed with 10per cent Carbowax 20 M on Chromosorb W. The components of the gaseous sample were analysed by a 6 mm i.d. 11m long copper column packed with AgNOJDiethylene glycol (DEG) on chromosorb W.‘The rate of absorption of isobutylene (from Cd-fraction) into fresh and fat aqueous sulphuric acid solutions was calculated from the concentration of isobutylene in the inlet and outlet gas streams, determined chromatographically on the AgNOJDEG column, with a probable error of +I per cent (of the olefin concentration). The
EXPERIMENTAL. Stirred cells
The fat acids of various known isoamylene concentration were contacted with a known amount of n-heptane or toluene in a 5.5 cm id. glass stirred cell. The design of the stirred cell was similar to that used by Gehlawat and Sharma[7]. In all the cases, the fat acid phase was below the hydrocarbon phase. Both the phases were stirred, so as to renew the interface continuously without disturbing it significantly. The design of the stirrer was similar to that used by Sankholkar and Sharma[9]. In all the experiments, the concentration of isoamylene in the organic phase was not allowed to build up above 3 per cent (w/w), in order to keep the concentration of isoamylene at a relatively low level. In some experiments, the speed of stirring was varied from 20 to SSrev/min to study the
effect of the speed of stirring on the specific rate of extraction. In most cases, the stirred cell was run for a period of 3-10 hr. The desorption (into an inert gas) experiments were carried out in a 9*5cm i.d. glass stirred cell. A known amount of fat acid of a known isoamylene concentration was taken in the stirred cell. It was continuously stirred by a cruciform type of glass stirrer, with four flat blades, which just dipped into the fat acid. Nitrogen was continuously passed through the stirred cell, at a flow rate sufficient to keep the concentration of isoamylene in the outgoing gas stream negligibly small (less than about 0.3 per cent v/v). The bulk gas phase above the fat acid was also stirred. In some experiments, the speed of stirring was varied from 40 to 1OOrevlmin to study the effect of the speed of stirring on the specific rate of desorption. In most cases, the stirred cell was run for a period of about 6 hr. Some experiments were also carried out in the 9.5 cm i.d. glass stirred cell, to obtain the values of specific rate of absorption of isobutylene into fat sulphuric acid solutions containing 60 per cent (w/w) sulphuric acid. The uptake method described by Sankholkar and Sharma[4] was used. The values of the specific rate of absorption of isobutylene into fresh 60 and 65 per cent (w/w) aqueous sulphuric acid solutions were also measured and were found to be in close agreement with those obtained by Gehlawat and Sharma 171.
Agitated co&actors
A 9.5cm i.d. glass vessel was used for liquid-liquid extraction experiments which was provided with four vertical baffles, each one-tenth the diameter of the vessel, mounted against the wall at right angles to it and spaced at 90” intervals around the tank. A four-bladed straight paddle impeller of glass with a diameter of 4.8cm was used. All the experiments were carried out with the fat acid as the dispersed phase with a dispersed phase hold up of about 30 per cent. Here, a known amount of fat acid of a known isoamylene concentration was charged to the previously cleaned contactor. A known volume of n-heptane or toluene was then added carefully without significantly disturbing the interface between the two liquids. In most cases, the batch experiments were carried out for a period of 3-15 min. The speed of agitation was varied from 650 to 1150rev/min. The phase separation after the cessation of agitation, was found to occur in less than about 15 sec. The desorption (into nitrogen) experiments were carried out in the 9.5 cm i.d. glass vessel described above. A known amount of fat acid of a known isoamylene concentration was charged to the previously cleaned contactor. Nitrogen was bubbled through the fat acid at a constant known volumetric flow rate, below the agitator. The speed of agitation was varied from 850 to 1800rev/min. In this contactor, some runs were also made on the absorption of isobutylene from &-fraction into 60 per cent (w/w), 65 per cent (w/w) and 60 per cent (w/w) fat sulphuric acid solutions.
731
Desorptionof isoamylenefromloadedacidliquors Determination of physical properties
The densities, viscosities and surface tensions of the fat acids loaded with isobutylene and isoamylene were measured by specific gravity bottle, Ostwald viscometer and stalagmometer, respectively. The interfacial tensions between the relevant liquid pairs were measured by Du Notiy interfacial tensiometer. RESULT.9 ANDDISCUSION Stirred cells
The specific rates of desorption of isoamylene from the fat acid of a particular isoamylene concentration, into the hydrocarbon phase and nitrogen, were found to be independent of the speed of stirring in the range of 20-55 rev/min and 4&100 rev/min, respectively. The specific rates of desorption were also found to be independent of the volume of the fat acid over a two-fold range. The specific rates of desorption were found to be proportional to the isoamylene concentration in the fat acid, j, expressed as g mole of isoamylene per g mole of HzS04 and agreed within 10 per cent among themselves, at the same value of j (Fig. 1). (The relatively higher rates of desorption into nitrogen may be due to almost negligible concentration of isoamylene in the outgoing
gaseous stream as compared to a finite concentration of isoamylene in the hydrocarbon phase.) Thus, the desorption of isoamylene from fat sulphuric acid solutions is preceded by a fast reaction. The specific rate of desorption under these conditions is a unique function of the physico-chemical properties of the system and is independent of the hydrodynamics of the system. However, the value of the reaction rate constant could not be calculated due to the lack of equilibrium data. The specific rates of absorption of isobutylene into 60 per cent (w/w) aqueous sulphuric acid solutions loaded with isobutylene were also found to be independent of the speed of stirring and of the volume of the fat acid. The relevant data are reported in Table 1. Agitated contactors The values of the effective interfacial area in the
mechanically agitated contactors were obtained by the chemical method, the theoretical aspects of which have been discussed by Nanda and Sharma[lOl and Sharma and Danckwerts[ 111and will not be repeated here. The values of effective interfacial area a obtained from the various extraction experiments, with an average dispersed phase hold up of about 30 per cent are shown in Fig. 2. The relevant physical properties of the various phases involved are given in Table 2. It can be seen from Table 2 that, for a given continuous phase, the value of interfacial area increased with an increase in the loading of the fat acid (that is-increase in j). This is probably due Table I. Specific rates of absorption of isobutylene into 60 per cent w/w aqueous sulphuric acid solutions loaded with isobutylene, in the stirred cell (temperature = 30°C)
Concentration
of isoamylene in the fat acid,
j, g mole isoamylene/g
mole H,SO,
Fig. 1. Effect of isoamylene loading on the specific rate of desorption of isoamylene from fat acids into n-heptane, toluene and nitrogen, in the stirred cell (temperature = 30°C);0 n-heptane, @toluene,Onitrogen.
0
7.4.
0.11
7.94
0.20
8.30
0.28
8.57
* taken from the work of Cehlawat
7.6”
and Sharma
Table 2. Effect of physical properties of liquids on effective interfacial area in the agitated liquid-liquid contactor [temperature = 30°C;speed of agitation = 850rev/min; fractional dispersed phase hold up = 0.30 (average)] Density,
ayrrtem Continuous phase
Dispersed phase
No.
I
fat acid; j = 0.067
2
fat
3
fat acid; j = 0.136
4
fat acid; j =
acid;j
=
0.067
0.136
”
bepram
tduene
1.
lleptane
tOl”.?lle
Dispersed phase
g/cm3 ContinwuB phase
cP
Interfacial
ContinuouB IAhase
g:‘.41,
Viscosity. Dispersed ohase
Interfacial y;t;
=53
1.452
0.676
5.25
0.37
25
33
1.452
0.854
5.25
0.52
15.5
46
1.419
0.676
5.85
0.37
17.5
44
1.419
0.854
5.85
0.52
10.3
JO
[7]
D. S. SANKHOW and M. M. SW
732
“E $16
I8
E 614 . : 12 B B IO ‘3 0 58 E ._ 6 .-f 24 z
0 600
700 Speed
of
I
I
800
900
ogitation,
E, lOGO
1100
1200
800
rev/min
Dispersed fat acid; j fat acid; j fat acid; j fat acid; j
phase =0467 =0467 =0.136 =0*136
Continuous phase n-heptane toluene n-heptane toluene
to the increase in the viscosity of the fat acid and decrease in the interfacial tension with an increase in j. The values of interfacial area for the fat acid of a particular j, were found to be higher when toluene was used as the continuous phase because of the lower interfacial tension between toluene and the fat acid and higher viscosity of toleune as compared to that for n-heptane. The observations made in the present work regarding the effect of physical properties on the effective interfacial area in agitated liquid-liquid contactors are, thus, in general agreement with those made by Fernandes and Sharma[l2]. The values of effective interfacial area obtained from the desorption (into an inert gas) experiments are plotted against the speed of agitation in Fig. 3. The values of effective interfacial area obtained from the absorption of isobutylene (from Cd-fraction) into fresh and loaded 60
1200
1400
of agitation,
1600
1800
2000
rev/min
Fig. 3. Effect of speed of agitation on the effective interfacial area in agitated gas-liquid contactor (temperature = 30°C).Absorption of isobutylene in fresh sulphuric acid solutions; 0 60 per cent (w/w), A 65 per cent (w/w). Absorption of isobutylene in 60 per cent (w/w) sulphuric acid solutions loaded with isobutylene, 8 j = 0.15, I j = 0.28. Desorption of isoamylene from 60 per cent (w/w) sulphuric acid solutions loaded with isoamylene; @ j = 0.19, 0 j = 0.30.
Fig. 2. Effect of speed of agitation on the effective interfacial area in the agitated liquid-liquid contactor (temperature = 30°C): 0 0 A A
1000 Speed
per cent (w/w) and fresh 65 per cent (w/w) sulphuric acid solutions are also plotted for comparison. These values are unlikely to have error of more than 15per cent in most of the runs. The superficial velocity of the gas phase was 1cm/set in all the experiments and the gas phase was assumed to be back-mixed. It can be seen from Fig. 3 that the interfacial area varies linearly with the speed of agitation in the range covered in this work. The effect of the physical properties of the liquid on the effective interfacial area can be seen from Table 3. Thus, the higher values of a obtained with the fat acids as compared to 60 and 65 per cent (w/w) fresh acids are due to the higher viscosity and lower surface tension of the fat acids. These observations are in agreement with those made by Mehta and Sharma[13]. Further, it can be seen that the values of effective interfacial area obtained by the desorption technique are quite comparable with those obtained from
Table 3. ERect of physical properties of liquids on effective interfacial area in the agitated gas-liquid contactor [temperature = 30°C;speed of agitation = 1500revlmin; superficialgas velocity = 1cm/set (average)] NO.
Density,
Liquid
*/cm3
Viscosity, CP
Surface tenem, dyne/cm
Effective interfacial area. 5. cm2lo3
60 % (w/w) sulpburic
acid
1.49
4.9
78.0
9.0
65 % (w/w) sulphuric
acid
1.55
6.5
83.4
9.5
60 % (w/w) sulphuric acid loaded with isobutylene ; j = 0.15
1.42
5.8
41.0
11.8
60 % (w/w) sulphuric acid loaded with isobutylene i j = 0.28
1.371
7.3
36.5
12.5
60 ?A (w/w) sulphuric acid loaded wi”, imamylene; j = 0.19
1.396
6.4
34.5
12.7
60 + (w/w) aulphuric acid loaded with i~oamylens; j = 0.30
1.353
7.9
31.0
14.3
733
Desorptionof isoamylene fromloadedacidliquors the absorption of isobutylene in fresh and loaded solutions of sulphuric acid, under otherwise comparable conditions. CONCLUSIONS The desorption of isoamylene from fat acids having an acid strength of 60 per cent (w/w) and loading up to 0.45 g mole of isoamylene per g mole of H2S04, is preceded by a fast reaction which occurs in the fdm adjacent to the interface. The scaling up of desorption column can be done on the basis of the knowledge of the specific rate of desorption coupled with that of effective interfacial area in the type of equipment envisaged. The technique of desorption with reaction can be employed for the measurement of effective interfacial area in liquid-liquid and gas-liquid systems. Acknowledgement--One of us (DSS) wishes to thank the University Grants Commission for an award of a scholarship which enabled this work to be carried out. NOTATION
a i R
effective interfacial area, cm2/cm3 concentration of isobutylene or isoamylene in the fat acid, g mole olefinlg mole H2S04 specific rate of absorption or desorption, g mole/cm* set
REFERENCES [l] Foster R. L., WunderlichD. K., Patinkin S. H. and Sanford R. A., Petrol. Refiner l%O 39 229.
[2] LiakumovichA. G., ZakharovaN. V. and SmaginaL. R., Khimiya Tekhnol. Topl. Masel 1%16 14; Chem. Abstr. 1%2 56 8996. [3] Edwards W. R. and Wesselhoft R. D., U.S. Patent 3,217,055 July 16,1%2 (to Esso Research and Engg. Co.); Chem. Abstr. 196664 507.
[4] SankholkarD. S. aad SharmaM. M., Chem. Engng Sci. 1973 28 49. [5] Zakharova N. V., Liakumovich A. G., Parfenenkova L. R. and Vasil’evaA. G., Khimiya Tekhnol. Topl. Masel 19649 18; Chem. Abstr. 196461 14498.
[6] NorrisJ. F. andJoubert J. M., J. Am. Chem.Sot. 192749873. [A GehlawatJ. K. and Sharma M. M., Chem. Engng Sci. 196823 1173.
[8] Vogel A. I., A TextbookofPractical Organic Chemistry, 2nd Edn. p. 242. Longmans Green, London 1951. [9] Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1973 28 2089. [lo] Nanda A. K. and Sharma M. M., Chem. Engng Sci. 1966 21 ?#I? I”,.
[ll] Sharma M. M. and Danckwerts P. V., &it. Chem. Engng 19701s 522. [12] Femandes J. B. and Sharma M. M., Chem. Engng Sci. 196722
1267. 119 MehtaV. D. and SharraaM. M., Chem. Engng Sci. 197126 461.
Science, 1975, Vol. 30, pp. 729-733.
Pergamon Press.
Printed in Great Britain
DESORPTION OF ISOAMYLENE FROM LOADED ACID LIQUORS D. S. SANKHOLKAR and M. M. SHARMA Department of Chemical Technology, University of Bombay, Matunga Road, Bombay 400019,India (Received 8 August 1974;accepted 16 January 1975) Abstract-The desorption of 2-methyl-2-butene (isoamylene) from loaded sulphuric acid solutions, having an acid strength of about 60 per cent (w/w) and loading upto 0.45g mole of isoamyleneper g moleof H,SO,, was found to be precededby a fast reaction,whichoccurred in the film adjacent to the interface. The specificrates of desorption of isoamylene into inert hydrocarbons-n-heptane and toluene and an inert gas-nitrogen, were found to be proportional to the isoamylene concentration j, expressed as g mole of isoamylene per g mole of H,SO,, and agreed among themselves at the same value of j. The technique of desorption preceded by a fast reaction was employed for the measurement of effective interfacial area in liquid-liquid and gas-liquid agitated contactors. The absorption of isobutylene into fresh and loaded solutions of sulphuric acid was also used for the measurement of effective interfacial area in gas-liquid agitated contactors for comparative purposes. The values of effective interfacial area for the gas-liquid system obtained by the desorption technique were found to be comparable with those obtained from the absorption of isobutylene in fresh and loaded solutions of sulphuric acid, under otherwise comparable conditions.
INTRODUCTION The &-fraction from naphtha crackers contains about 14
per cent tert-isoamylenes (namely, 2-methyl-I-butene and 2-methyl-2-butene). If, however, cyclopentadiene and isoprene are recovered, the remaining &-fraction would contain about 20 per cent tert-isoamylenes. The tetiisoamylenes are industrially important as precursor for isoprene and for a variety of alkylated derivatives. The telt-isoamylenes can be separated from the less reactive n-pentenes and inert paraffins of the &fraction by selective extraction into 6&70 per cent (w/w) aqueous sulphuric acid solutions in the temperature range of 1&3O”C[ 1,2]. The teti-isoamylenes can be recovered from the loaded sulphuric acid solutions, usually referred to as ‘fat’ acids, by contacting them with a relatively high boiling hydrocarbon solvent, at 30-38°C. Thus, it appears that the reaction of isoamylenes with sulphuric acid is reversible. In fact, this reversibility is the basis of the process developed by Sinclair Research Laboratories, U.S.A.[l]. They have reported the results obtained on a typical run on a continuous integrated pilot plant consisting of a single acid extraction stage and a single acid regeneration stage. A product containing 99 per cent isoamylenes is claimed to have been obtained by this method from a &-fraction containing 17 per cent isoamylenes. The solvent used for the extraction of isoamylenes from the fat acid was n-heptane. It has also been claimed that a co-current pipeline contactor can be advantageously employed for carrying out the extraction of isoamylenes [3]. In a previous paper, we have reported the kinetics of absorption of 2-methyl-2-butene (a representative of tert-isoamylenes) into aqueous solutions of sulphuric acid in the range of acid concentration employed in practice [4]. The absorption was found to conform to fast pseudo-first order mechanism, in the range of acid concentration from 61.5 per cent w/w (9.4 g mole/l) to 75
per cent w/w (12.85 gmole/l). The pseudo-first order reaction rate constant at 30°C was found to vary from about 4.2 x lo’ to 2.2 x lo* se? over the above concentration range. Zakharova et al.[5] have studied the dependence of density, viscosity and surface tension of the fat acid on its isoamylene concentration. They have also reported the equilibrium data for distribution of isoamylenes between 65 per cent (w/w) sulphuric acid and n-amylenes at 2, 10 and 20°C. There is, however, no information in the literature concerning the kinetics of extraction of isoamylenes from the fat acids into an inert hydrocarbon. This work was, therefore, undertaken to study the kinetics of desorption of 2-methyl-2-butene (- representative of tert-isoamylenes and hereafter called simply “isoamylene”) from fat acids into inert hydrocarbons-nheptane and toluene and an inert gas-nitrogen. The concentration of aqueous sulphuric acid solution chosen was 60 per cent (w/w), because isoamylene is not polymerized by the acid of this concentration, even after a substantially long period[6] and hence is unlikely to interfere with the process of desorption of isoamylene from the fat acids. Gehlawat and Sharma[7] have shown that the absorption of isobutylene in 60 and 65 per cent (w/w) aqueous sulphuric acid solutions conforms to fast pseudo-first order mechanism and it is expected that the absorption of isobutylene in aqueous sulphuric acid solutions loaded with isobutylene (also referred to as “fat” acids) would show a similar behaviour. The above systems can, therefore, be conveniently used for the measurement of effective interfacial area in gas-liquid contactors. It was thought that the desorption of isoamylene from fat acid liquors into an inert hydrocarbon or an inert gas may prove to be a unique system for the measurement of effective interfacial area in liquid-liquid or gas-liquid contactors, respectively.
D. S. SANKHOLKAR and M. M. SHARMA
730 MA-
Isoamylene (Zmethyl-2-butene) was prepared by the dehydration of tert-amyl alcohol (Zmethyl-Zbutanol) with aqueous sulphuric acid (approx. 48 per cent w/w) [8]. n-Heptane and toluene were laboratory chemical grade materials. Sulphuric acid was of commercial grade. Aqueous solutions of sulphuric acid were prepared with deionised water. A Cd-fraction from a local naphtha cracker, after the extraction of butadiene, containing 58 per cent isobutylene, was also used for the measurement of effective interfacial area for comparative purposes. ANALYSIS
rate of extraction of isoamylene from the fat acid into the hydrocarbon phase was followed by determining the concentration of isoamylene in the hydrocarbon phase and/or from the increase in the density of the fat acid at the end of the run, whenever this increase was substantial. The rate of desorption of isoamylene into nitrogen was followed by determining the concentration of isoamylene in the outlet gas stream, if this were conveniently measurable by G.L.C. analysis (e.g. in the case of mechanically agitated gas-liquid contactors) or from the increase in the density of a batch of fat acid at the end of the run of a suitable period (e.g. in the case of the stirred cell). The concentration of isoamylene in the fat acid was calibrated against the density of the fat acid, by preparing fat acids of known isoamylene concentration and determining their densities at 30°C. Both the liquid and the gaseous samples containing isoamylene were analysed chromatographically on an F and M Model 720 Dual Column Programmed Temperature Gas Chromatograph. The components of the organic phase were analysed by a 6 mm id. 3 m long copper column packed with 10per cent Carbowax 20 M on Chromosorb W. The components of the gaseous sample were analysed by a 6 mm i.d. 11m long copper column packed with AgNOJDiethylene glycol (DEG) on chromosorb W.‘The rate of absorption of isobutylene (from Cd-fraction) into fresh and fat aqueous sulphuric acid solutions was calculated from the concentration of isobutylene in the inlet and outlet gas streams, determined chromatographically on the AgNOJDEG column, with a probable error of +I per cent (of the olefin concentration). The
EXPERIMENTAL. Stirred cells
The fat acids of various known isoamylene concentration were contacted with a known amount of n-heptane or toluene in a 5.5 cm id. glass stirred cell. The design of the stirred cell was similar to that used by Gehlawat and Sharma[7]. In all the cases, the fat acid phase was below the hydrocarbon phase. Both the phases were stirred, so as to renew the interface continuously without disturbing it significantly. The design of the stirrer was similar to that used by Sankholkar and Sharma[9]. In all the experiments, the concentration of isoamylene in the organic phase was not allowed to build up above 3 per cent (w/w), in order to keep the concentration of isoamylene at a relatively low level. In some experiments, the speed of stirring was varied from 20 to SSrev/min to study the
effect of the speed of stirring on the specific rate of extraction. In most cases, the stirred cell was run for a period of 3-10 hr. The desorption (into an inert gas) experiments were carried out in a 9*5cm i.d. glass stirred cell. A known amount of fat acid of a known isoamylene concentration was taken in the stirred cell. It was continuously stirred by a cruciform type of glass stirrer, with four flat blades, which just dipped into the fat acid. Nitrogen was continuously passed through the stirred cell, at a flow rate sufficient to keep the concentration of isoamylene in the outgoing gas stream negligibly small (less than about 0.3 per cent v/v). The bulk gas phase above the fat acid was also stirred. In some experiments, the speed of stirring was varied from 40 to 1OOrevlmin to study the effect of the speed of stirring on the specific rate of desorption. In most cases, the stirred cell was run for a period of about 6 hr. Some experiments were also carried out in the 9.5 cm i.d. glass stirred cell, to obtain the values of specific rate of absorption of isobutylene into fat sulphuric acid solutions containing 60 per cent (w/w) sulphuric acid. The uptake method described by Sankholkar and Sharma[4] was used. The values of the specific rate of absorption of isobutylene into fresh 60 and 65 per cent (w/w) aqueous sulphuric acid solutions were also measured and were found to be in close agreement with those obtained by Gehlawat and Sharma 171.
Agitated co&actors
A 9.5cm i.d. glass vessel was used for liquid-liquid extraction experiments which was provided with four vertical baffles, each one-tenth the diameter of the vessel, mounted against the wall at right angles to it and spaced at 90” intervals around the tank. A four-bladed straight paddle impeller of glass with a diameter of 4.8cm was used. All the experiments were carried out with the fat acid as the dispersed phase with a dispersed phase hold up of about 30 per cent. Here, a known amount of fat acid of a known isoamylene concentration was charged to the previously cleaned contactor. A known volume of n-heptane or toluene was then added carefully without significantly disturbing the interface between the two liquids. In most cases, the batch experiments were carried out for a period of 3-15 min. The speed of agitation was varied from 650 to 1150rev/min. The phase separation after the cessation of agitation, was found to occur in less than about 15 sec. The desorption (into nitrogen) experiments were carried out in the 9.5 cm i.d. glass vessel described above. A known amount of fat acid of a known isoamylene concentration was charged to the previously cleaned contactor. Nitrogen was bubbled through the fat acid at a constant known volumetric flow rate, below the agitator. The speed of agitation was varied from 850 to 1800rev/min. In this contactor, some runs were also made on the absorption of isobutylene from &-fraction into 60 per cent (w/w), 65 per cent (w/w) and 60 per cent (w/w) fat sulphuric acid solutions.
731
Desorptionof isoamylenefromloadedacidliquors Determination of physical properties
The densities, viscosities and surface tensions of the fat acids loaded with isobutylene and isoamylene were measured by specific gravity bottle, Ostwald viscometer and stalagmometer, respectively. The interfacial tensions between the relevant liquid pairs were measured by Du Notiy interfacial tensiometer. RESULT.9 ANDDISCUSION Stirred cells
The specific rates of desorption of isoamylene from the fat acid of a particular isoamylene concentration, into the hydrocarbon phase and nitrogen, were found to be independent of the speed of stirring in the range of 20-55 rev/min and 4&100 rev/min, respectively. The specific rates of desorption were also found to be independent of the volume of the fat acid over a two-fold range. The specific rates of desorption were found to be proportional to the isoamylene concentration in the fat acid, j, expressed as g mole of isoamylene per g mole of HzS04 and agreed within 10 per cent among themselves, at the same value of j (Fig. 1). (The relatively higher rates of desorption into nitrogen may be due to almost negligible concentration of isoamylene in the outgoing
gaseous stream as compared to a finite concentration of isoamylene in the hydrocarbon phase.) Thus, the desorption of isoamylene from fat sulphuric acid solutions is preceded by a fast reaction. The specific rate of desorption under these conditions is a unique function of the physico-chemical properties of the system and is independent of the hydrodynamics of the system. However, the value of the reaction rate constant could not be calculated due to the lack of equilibrium data. The specific rates of absorption of isobutylene into 60 per cent (w/w) aqueous sulphuric acid solutions loaded with isobutylene were also found to be independent of the speed of stirring and of the volume of the fat acid. The relevant data are reported in Table 1. Agitated contactors The values of the effective interfacial area in the
mechanically agitated contactors were obtained by the chemical method, the theoretical aspects of which have been discussed by Nanda and Sharma[lOl and Sharma and Danckwerts[ 111and will not be repeated here. The values of effective interfacial area a obtained from the various extraction experiments, with an average dispersed phase hold up of about 30 per cent are shown in Fig. 2. The relevant physical properties of the various phases involved are given in Table 2. It can be seen from Table 2 that, for a given continuous phase, the value of interfacial area increased with an increase in the loading of the fat acid (that is-increase in j). This is probably due Table I. Specific rates of absorption of isobutylene into 60 per cent w/w aqueous sulphuric acid solutions loaded with isobutylene, in the stirred cell (temperature = 30°C)
Concentration
of isoamylene in the fat acid,
j, g mole isoamylene/g
mole H,SO,
Fig. 1. Effect of isoamylene loading on the specific rate of desorption of isoamylene from fat acids into n-heptane, toluene and nitrogen, in the stirred cell (temperature = 30°C);0 n-heptane, @toluene,Onitrogen.
0
7.4.
0.11
7.94
0.20
8.30
0.28
8.57
* taken from the work of Cehlawat
7.6”
and Sharma
Table 2. Effect of physical properties of liquids on effective interfacial area in the agitated liquid-liquid contactor [temperature = 30°C;speed of agitation = 850rev/min; fractional dispersed phase hold up = 0.30 (average)] Density,
ayrrtem Continuous phase
Dispersed phase
No.
I
fat acid; j = 0.067
2
fat
3
fat acid; j = 0.136
4
fat acid; j =
acid;j
=
0.067
0.136
”
bepram
tduene
1.
lleptane
tOl”.?lle
Dispersed phase
g/cm3 ContinwuB phase
cP
Interfacial
ContinuouB IAhase
g:‘.41,
Viscosity. Dispersed ohase
Interfacial y;t;
=53
1.452
0.676
5.25
0.37
25
33
1.452
0.854
5.25
0.52
15.5
46
1.419
0.676
5.85
0.37
17.5
44
1.419
0.854
5.85
0.52
10.3
JO
[7]
D. S. SANKHOW and M. M. SW
732
“E $16
I8
E 614 . : 12 B B IO ‘3 0 58 E ._ 6 .-f 24 z
0 600
700 Speed
of
I
I
800
900
ogitation,
E, lOGO
1100
1200
800
rev/min
Dispersed fat acid; j fat acid; j fat acid; j fat acid; j
phase =0467 =0467 =0.136 =0*136
Continuous phase n-heptane toluene n-heptane toluene
to the increase in the viscosity of the fat acid and decrease in the interfacial tension with an increase in j. The values of interfacial area for the fat acid of a particular j, were found to be higher when toluene was used as the continuous phase because of the lower interfacial tension between toluene and the fat acid and higher viscosity of toleune as compared to that for n-heptane. The observations made in the present work regarding the effect of physical properties on the effective interfacial area in agitated liquid-liquid contactors are, thus, in general agreement with those made by Fernandes and Sharma[l2]. The values of effective interfacial area obtained from the desorption (into an inert gas) experiments are plotted against the speed of agitation in Fig. 3. The values of effective interfacial area obtained from the absorption of isobutylene (from Cd-fraction) into fresh and loaded 60
1200
1400
of agitation,
1600
1800
2000
rev/min
Fig. 3. Effect of speed of agitation on the effective interfacial area in agitated gas-liquid contactor (temperature = 30°C).Absorption of isobutylene in fresh sulphuric acid solutions; 0 60 per cent (w/w), A 65 per cent (w/w). Absorption of isobutylene in 60 per cent (w/w) sulphuric acid solutions loaded with isobutylene, 8 j = 0.15, I j = 0.28. Desorption of isoamylene from 60 per cent (w/w) sulphuric acid solutions loaded with isoamylene; @ j = 0.19, 0 j = 0.30.
Fig. 2. Effect of speed of agitation on the effective interfacial area in the agitated liquid-liquid contactor (temperature = 30°C): 0 0 A A
1000 Speed
per cent (w/w) and fresh 65 per cent (w/w) sulphuric acid solutions are also plotted for comparison. These values are unlikely to have error of more than 15per cent in most of the runs. The superficial velocity of the gas phase was 1cm/set in all the experiments and the gas phase was assumed to be back-mixed. It can be seen from Fig. 3 that the interfacial area varies linearly with the speed of agitation in the range covered in this work. The effect of the physical properties of the liquid on the effective interfacial area can be seen from Table 3. Thus, the higher values of a obtained with the fat acids as compared to 60 and 65 per cent (w/w) fresh acids are due to the higher viscosity and lower surface tension of the fat acids. These observations are in agreement with those made by Mehta and Sharma[13]. Further, it can be seen that the values of effective interfacial area obtained by the desorption technique are quite comparable with those obtained from
Table 3. ERect of physical properties of liquids on effective interfacial area in the agitated gas-liquid contactor [temperature = 30°C;speed of agitation = 1500revlmin; superficialgas velocity = 1cm/set (average)] NO.
Density,
Liquid
*/cm3
Viscosity, CP
Surface tenem, dyne/cm
Effective interfacial area. 5. cm2lo3
60 % (w/w) sulpburic
acid
1.49
4.9
78.0
9.0
65 % (w/w) sulphuric
acid
1.55
6.5
83.4
9.5
60 % (w/w) sulphuric acid loaded with isobutylene ; j = 0.15
1.42
5.8
41.0
11.8
60 % (w/w) sulphuric acid loaded with isobutylene i j = 0.28
1.371
7.3
36.5
12.5
60 ?A (w/w) sulphuric acid loaded wi”, imamylene; j = 0.19
1.396
6.4
34.5
12.7
60 + (w/w) aulphuric acid loaded with i~oamylens; j = 0.30
1.353
7.9
31.0
14.3
733
Desorptionof isoamylene fromloadedacidliquors the absorption of isobutylene in fresh and loaded solutions of sulphuric acid, under otherwise comparable conditions. CONCLUSIONS The desorption of isoamylene from fat acids having an acid strength of 60 per cent (w/w) and loading up to 0.45 g mole of isoamylene per g mole of H2S04, is preceded by a fast reaction which occurs in the fdm adjacent to the interface. The scaling up of desorption column can be done on the basis of the knowledge of the specific rate of desorption coupled with that of effective interfacial area in the type of equipment envisaged. The technique of desorption with reaction can be employed for the measurement of effective interfacial area in liquid-liquid and gas-liquid systems. Acknowledgement--One of us (DSS) wishes to thank the University Grants Commission for an award of a scholarship which enabled this work to be carried out. NOTATION
a i R
effective interfacial area, cm2/cm3 concentration of isobutylene or isoamylene in the fat acid, g mole olefinlg mole H2S04 specific rate of absorption or desorption, g mole/cm* set
REFERENCES [l] Foster R. L., WunderlichD. K., Patinkin S. H. and Sanford R. A., Petrol. Refiner l%O 39 229.
[2] LiakumovichA. G., ZakharovaN. V. and SmaginaL. R., Khimiya Tekhnol. Topl. Masel 1%16 14; Chem. Abstr. 1%2 56 8996. [3] Edwards W. R. and Wesselhoft R. D., U.S. Patent 3,217,055 July 16,1%2 (to Esso Research and Engg. Co.); Chem. Abstr. 196664 507.
[4] SankholkarD. S. aad SharmaM. M., Chem. Engng Sci. 1973 28 49. [5] Zakharova N. V., Liakumovich A. G., Parfenenkova L. R. and Vasil’evaA. G., Khimiya Tekhnol. Topl. Masel 19649 18; Chem. Abstr. 196461 14498.
[6] NorrisJ. F. andJoubert J. M., J. Am. Chem.Sot. 192749873. [A GehlawatJ. K. and Sharma M. M., Chem. Engng Sci. 196823 1173.
[8] Vogel A. I., A TextbookofPractical Organic Chemistry, 2nd Edn. p. 242. Longmans Green, London 1951. [9] Sankholkar D. S. and Sharma M. M., Chem. Engng Sci. 1973 28 2089. [lo] Nanda A. K. and Sharma M. M., Chem. Engng Sci. 1966 21 ?#I? I”,.
[ll] Sharma M. M. and Danckwerts P. V., &it. Chem. Engng 19701s 522. [12] Femandes J. B. and Sharma M. M., Chem. Engng Sci. 196722
1267. 119 MehtaV. D. and SharraaM. M., Chem. Engng Sci. 197126 461.