Ind. Eng. C h e m . R e s . 1988,27, 735-739 f = fat phase g = glycerol m = monoglyceride t = triglyceride w = water 0 = initial condition = measured value at room conditions
-
Literature Cited Astarita, G. Mass Transfer with Chemical Reaction; Elsevier: Amsterdam, 1967. Basu, P. K. Chem. Age India 1976,27(10), 871. Burrows, K. Trans. Znst. Chem. Eng. 1953,31(10), 250. Butala, D. N. M.Tech. Dissertation, Department of Chemical Engineering, Indian Institute of Technology, Bombay, India, 1984. Desai, S. M.; Raghunathan, T. S.; Shankar, H. S. Frontiers in Chemical Reaction Engineering;Wiley Eastern: New York, 1984; Vol. 1, p 253.
735
Donders, A. J. M.; Wijffels, J. B.; Reitema, K. Proceedings of the Fourth European Symposium on Chemical Reaction Engineering, Brussels, Sept 9-11, 1968. Hilder, M. H. J. Am. Oil Chem. SOC.1968,45 703. Jeffreys, G. V.; Jenson, V. G.; Miles, P. R. Trans. Znst. Chem. Eng. 1961, 39, 389. Lascaray, L. Znd. Eng. Chem. 1949, 47, 486. Lascaray, L. J. Am. Oil Chem. SOC.1952,29, 362. Mehlenbacher, V. L. Analysis of Fats and Oils; Gerrad: Chapaign, IL, 1960. Mills, V.; McClain, H. K. Znd. Eng. Chem. 1949, 47, 1982. Mueller, H. H.; Holt, E. K. J. Am. Oil Chem. SOC.1948, 25, 305. Perry, R. H.; Chilton, C.H. Chemical Engineers Handbook, 4th ed.; McGraw-Hill: Kogakusha, Tokyo, 1973; p 3.87. Sarkar, S.; Mumford, C. J.; Phillips, C. R. Znd. Eng. Chem. Process Des. Dev. 1980, 19, 672. Sturzenegger, A.; Sturm, H. Znd. Eng. Chem. 1951, 43(2), 510.
Received for review November 19, 1985 Revised manuscript received August 14, 1987 Accepted November 30, 1987
Thermal Hydrolysis of Vegetable Oils and Fats. 2. Hydrolysis in Continuous Stirred Tank Reactor T. A. Patil, T. S. Raghunathan, and H. S. Shankar* Department of Chemical Engineering, Indian Institute of Technology, Bombay 400 076, India
The liquid-liquid thermal hydrolysis of coconut oil is studied experimentally on a laboratory-scale continuous stirred tank reador over the range 225 O C , 3000 kPa to 260 O C , 5500 kPa. Good agreement is indicated between the model prediction and data. Hydrolysis as a liquid-liquid reaction has been practiced commercially for a long time. Batch autoclaves are commonly used in small-scale operations, while continuous countercurrent columns are employed in large-scale operations. Batch operation involves high specific energy consumption and idle time. The continuous countercurrent spray towers require very high initial investment. In India both batch and continuous countercurrent operations are in use. The hydrolysis can be brought about over a batch of oil by a spray of high-pressure water. This scheme could have somewhat poor productivity in relation to continuous countercurrent operations. No work on semicontinuous hydrolysis has been reported so far. The reaction can also be conducted in a continuous stirred tank reactor. This scheme offers advantages with respect to energy integration and productivity in addition to cost advantages in terms of investment. In this scheme, a mixture of water and oil is pumped to a reactor. In this paper, we examine the behavior of this configuration theoretically by using the model developed earlier (Patil et al., 1988). The model predictions are then tested experimentally.
Experimental Apparatus A laboratory autoclave of 1-L capacity made from 316 stainless steel mounted in a compact, electrically heated furnace was used. The specifications of the autoclave are given elsewhere (Patil et al., 1988). The top flange houses a cooling coil with provision for cooling the water inlet and outlet. A 316 stainless steel rupture disc tested at 50 000 kPa and 600 "C is also provided. The flange contains a thermowell for temperature measurement and four ports. One of the ports is connected to a pressure gauge for pressure measurement. The others are connected via 0888-5885/ 8812627-0135$01.50/0
precision needle valves to sampling tubes of variable length for sampling from inside the vessel. A magnetically driven stirrer is housed at the center. Schematic representation of the arrangement is shown in Figure 1. A mixture of a predetermined quantity of coconut oil and water is prepared in a storage tank and fed to reactor via a dosing pump supplied by Jagdish Engineering Works, Bombay, capable of operating up to 20000 kPa and having adjustable stroke length with a maximum capacity of 2 L/h. The high-pressure dosing pump and reactor are connected by 316 stainless steel tube on which a check valve is mounted. This is provided in addition to a built-in nonreturn valve on the pump. The reactor outlet is connected to a cooler where cooling water circulates followed by a 316 stainless steel back-pressure regulating valve to maintain pressure inside the reactor during the experiment.
Experimental Procedure Water and oil are mixed in the desired proportion and stirred well to obtain a uniform mixture, and this was continuously pumped to the reactor by the high-pressure dosing pump. It was found that no segregation of wateroil emulsion occurred in the feed line, and also it was verified that the back flow of the reactants was completely prevented by providing the additional nonreturn valve on the feed line. Runs were conducted at chosen residence times until steady state was reached, which was determined by trial to vary between 1.5 and 2 h. Cooled product emerging out of the reactor was collected, maintaining constant pressure inside the vessel by prior setting of the backpressure regulating valve. The mixing pattern inside reactor was examined; for this purpose, a step input of a tracer solution containing freshly prepared sodium hydroxide was used. 0 1988 American Chemical Society
736 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 Table I. Hold-Up Measurement in CSTR feed, L/h temp, "C
GIL, vlv 0.83
225 240 260 260 260
1.0 1.0
1.0 0.5 1.5
water-oil
m'
0.83 0.83 0.83 0.83
total holdup, mL
fat-phase holdup, 6
Dredicted bv ea 7
920 915 900 905 902
0.54 0.55 0.55 0.55 0.55
0.547 0.547 0.547 0.547 0.547
b
sample
A - Feed storage tank
B- Hlgh pressure dosing Pump C - Non return valve D - Needle valve E - Impeller F Heat exchanger
300
G - Pressure regulating valve
260-
I
'
I
'
'
'
'
'
'
I
I
'
-
R- Reactor
220
Figure 1. Schematic diagram of CSTR.
Steady-state liquid hold-up measurements using a mixture of water and oil over the desired range of waterto-oil ratio at 225,240, and 260 OC were made. This was done by simultaneously closing both inlet and outlet valves in the running condition and noting the volume of total liquid together with its phase ratio in the reactor. In the present work, residence time of 25-500 min was used.
Theoretical Section Patil et al. (1988) proposed a kinetic model to describe the thermal hydrolysis of vegetable oils/animal fats considering a three-step reaction scheme. The rate of generation of triglyceride as per the model is given as rt = k,CaCd - k,C,C, (1)
n
-
180AV
-
140 0
100
Experimental
I
63- /
-I
20 I
L
I
I
I
I
I
I
I
I
I
I
I
A mass balance on triglyceride gives T = CmXl/(-rt) where T is the fat-phase residence time. Substituting for rt in eq 2, we get T = PX1/(-X12 + QX1 + R ) X1 = [(Q - P/T)+ [(Q - P/T)' + 4R]"2]/2
(3) The extents of reaction X2 and X3 can be given in terms of XI as per the earlier paper as x 2 = CX, - C' X, = BX2 - B' The quantities A , B, C, D,A', B', C', E , etc., are as defined in Patil et al. (1988). Aqueous-phase glycerol concentration is given by
9, = (xtJ,M&/MtG) + 9,o
(4)
Productivity of fatty acid and glycerol is given by Pa = Lfa/VR = Pff,/T P g
= G ~ ~ / V=RPa$g/T
(5) (6)
By use of these equations, performance of CSTR can now be evaluated. If the holdup of phases in the reactor is in
Results and Discussion The experimental values of holdup are shown in column 5 of Table I. The values from eq 7 are shown in column 6 of Table I. It is observed that the holdup is around 910 mL and that the average holdup can be predicted satisfactorily from eq 7. An F curve for the stirred tank is shown in Figure 2. It is seen that the curve exhibits a negative slope and intercept of unity. It is clear from the linearity, slope, and intercept of this curve that the reaction vessel used in this work is very close to an ideal back mix reactor. Coconut oil was used in all hydrolysis experiments. The residence time ( 7 ) was calculated from eq 8. Results from experiments at 225 "C, GIL = 0.9 w/w, and Qso = 0 are shown in Figures 3,4, and 5 respectively. Model prediction using parameters for coconut oil from Patil et al. (1988) is also shown. Very good agreement between the experimental data and model predicted acid value against residence time, monoglyceride levels versus residence time,
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 737
0 13
3L-A 0.010
LO
80
160
120
Residence time
200
T
240
280
min
Figure 4. Comparison between prediction and experimental levels of monoglycerides for coconut oil hydrolysis in CSTR at 225 "C, GIL = 0.9 w/w, and AVO = 3.
xE 0 0°"07
i'
o
40
~
,
,
,
,
80 120 160 Residence time
,
,
,
,
,
, , ,
40 80 120 Residence time
,
,
,
160
, 200
T min
Figure 7. Comparison between model-predicted and experimental concentrations of monoglyceride for coconut oil hydrolysis in CSTR at 260 "C, G f L = 0.9 wfw, AVO = 3, and 9, = 0.
,I
200 2'40 T min
280
Figure 5. Comparison between experimental and model-predicted aqueous glycerol concentration profiles for coconut oil hydrolysis in CSTR at 225 "C, GIL = 0.9 w/w, AVO= 3, and yfl = 0.
. - 0 0 3260
2
,
Experimental
0'0511 0
0
,
- Mode'
0 03
o o l ~ ,, ,
o.ol/
J%k%%%%+ 0
Residence time C min Figure 6. Comparison between predicted and experimental acid values for coconut oil hydrolysis in CSTR at 260 "C, GIL = 0.9 w/w, AVO= 3, and yfl = 0.
and aqueous glycerol against residence time is indicated in all the cases. The results at another condition, viz. 260 "C and G / L = 0.9 w/w, is shown in Figure 6-8. It is observed from Figure 6 that an acid value of 242 is realized only at large residence time; under identical conditions in a batch reactor, an equilibrium acid value of 242 is reached in about 45 min. Figures 7 and 8 show respectively the experimental and model-predicted behavior for monoglyceride
Residence time
T mln
Figure 8. Comparison between experimental and model-predicted concentrations of aqueous-phase glycerol for coconut oil hydrolysis in CSTR at 260 OC, G / L = 0.9 wfw, AVO = 3, and QgO = 0.
and aqueous-phase glycerol content against time. Once again, re&onable agreement is seen between the data and prediction. It may also be possible to develop other approaches to modeling the course of hydrolysis. The oil phase contains water and fatty acids besides other components. The hydrogen ion concentration in the oil phase could thus vary as fatty acid accumulates. The observed course of hydrolysis could then be viewed to be a consequence of the changes in hydrogen ion concentration in the oil phase. These possibilities were recognized during our work; however, no detailed study to discriminate between these alternatives was undertaken. Performance of batch and CSTR is compared in Figure 9 which shows acid value against batch reactor time or residence time for G I L = 0.9 w/w, at 240 "C. It is seen that, in the case of a batch reactor, 99% of the equilibrium conversion is achieved in 1.5 h. The same level of conversion is attained in over a very large residence time in CSTR; the results indicate that a rise in the acid value beyond the residence time of 2 h is small since the shape of the performance curve reaches plateau in slope. A value of residence time around this region is therefore to be preferred for CSTR practice. Hydrolysis being a reversible and sluggish reaction, one of the two products formed in the reaction has to be removed to push the reaction in the favorable direction. There seem to be two possible schemes to carry out this stagewise splitting.
738 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
Settling tank I
Settling tank II
Figure 11. Two-stage splitting using CSTRs at 240 “C and G / L = 0.7, scheme 2. Interstage separation of fatty acid. t or
T
of X = 0.96 and aqueous-phase glycerol concentration of hrs
Figure 9. Comparison between the performance of batch reactor and that of CSTR by model prediction for coconut oil hydrolysis at 240 O C , G / L = 0.9 w/w, AVO = 3, and QgO= 0.
I
Separation tank I
I
Figure 10. Two-stage splitting using CSTRs at 240 “C and G / L = 0.7, scheme 1. Interstage removal of aqueous glycerol. Table 11. Performance Comparison of Schemes 1 and 2 for Two-Stage Splitting Using CSTRs obiective function scheme 1 scheme 2 0.945 0.96 overall conversion, R aq phase glycerol content, Yg 0.188 0.16 fatty acid productivity, Pa,kg/(m3.h) 218 370.0 glycerol productivity, Pg,kg/(m3.h) 49 49.4
Scheme 1as shown in Figure 10 uses two CSTRs where lean glycerol and fresh oil enter the first reactor and the product coming out is allowed for phase separation, thereby removing aqueous glycerol. The fat phase containing fatty acid, unsplit oil, diglyceride, and monoglyceride along with fresh water is fed to the next CSTR. The lean glycerol solution produced is used in the first stage, and fatty matter of high conversion is attained. Scheme 2 as shown in Figure 11employs the interstage distillation of fatty acid. The product of the first stage is allowed for phase separation. The fat phase is led to the distillation unit to separate fatty acid. The residue mainly containing intermediates and unsplit oil is pumped to the second reactor together with fresh water. In both schemes mentioned above, hydrolysis of coconut oil at 240 “C, flow ratio G / L = 0.7 w/w, and 7 = 120 min is considered as an example. Table I1 summarizes the performance comparison of scheme 1 and 2. Scheme 1 consisting of interstage separation of aqueous glycerol uses two equal sized CSTRs, while scheme 2 involving interstage fatty acid removal requires two unequal sized vessels. Performance comparison of schemes 1 and 2 can be conveniently examined from the aqueous-phase glycerol content Qg,and productivity of fatty acid and glycerol, Pa and Pg,respectively. Table I1 summarizes these results. Scheme 1 yields an overall conversion of X = 0.945 where 8 = (2,- fa0)/xa and aqueous-phase glycerol concentration 9, = 0.188. In comparison, an overall conversion
Q, = 0.16 are obtained in scheme 2. Productivities of fatty acid (Pa)in schemes 1and 2, respectively, are 218 and 370 kg/ (m3.h) while the productivity for glycerol in both schemes is Pg= 49 kg/(m3.h). The above calculations show the usefulness of the proposed model in evaluating process alternatives. Of course, the economic viability of such alternative approaches depends on many factors.
Conclusions 1. The CSTR experimental data on thermal hydrolysis are shown to be consistent with the model of Patil et al. (1988),thus providing additional support to the proposed model. 2. Several design alternatives for carrying out liquidliquid hydrolysis exist. The continuous stirred tank reactor is one such approach, the feasibility of which is experimentally demonstrated in the present work. Nomenclature
+
A = 1/(1 m G / L ) A’ = ( x ~+. y@G/L)M,/[M (1 + mG/L)I AV = acid value, mg of K H/g of fat
gt0
B = K&/(A + K382) B’ = (A’ - K3&$mo/Cto)/(A + K362) C = K&/(1 + K362 - B ) C‘ = [B’ + (Cmo/Cto) - K382Cdo/Ctol/(l + K382 - B ) C, = concentration of species i, kmol/m3 D = 1+BC + c D’= (Ca0/Cto) - (C’+ BC’+ B’) E=1-C G = mass flow rate of aqueous phase, kg/h K , = equilibrium constant for ith reaction step, i = 1-3 k, = specific reaction rate constant in fat phase, m3/ (kmol-min) L = mass flow rate of fat phase, kg/h M , = molecular weight of species i m = equilibrium distribution coefficient for glycerol, w/w n = 1, 3, or 5 for forward reactions and 2,4, or 6 for reverse reactions Pi = productivity of species i, kg/(m3-h) P = 1/(k1Cto82[D- 81/62 + DE/(KI~,)I) Q = [ D - 2(61/62) - Cao/Cto- (C~O/C~O)(E/(K~~,))I/[D - 61/62 + DE/(Ki&Jl R = [6,/& + c a ~ / C t ~ l / [-D61/82 + DE/(K182)1 SV = saponification value, mg of KOH/g of fat T = temperature, OC X = overall conversion xi = mass fraction of species i in fat phase y , = mass fraction of species i in aqueous phase
Greek Symbols 6, = proportion of water in fat phase 82 = proportion of water in fatty acid pr = density of fat phase, kg/m3
I n d . E n g . C h e m . Res. 1988,27, 739-743 7
739
w = water 0 = initial condition = measured value
= residence time, min
-
= fat phase holdup ratio in reactor
Subscripts a n d Superscripts a = fatty acid
Literature Cited
d = diglycerides e = equilibrium state f = fat phase g = glycerol m = monoglyceride t = triglyceride
Patil, T. A.; Butala, D. N.; Raghunathan, T. S.; Shankar, H. S. Ind. Eng. Chem. Res. 1988, preceding paper in this issue.
Received for review May 8, 1986 Revised manuscript received May 19, 1987 Accepted June 18, 1987
Thermal Hydrolysis of Vegetable Oils and Fats. 3. An Analysis of Design Alternatives P. D. Namdev, T. A. P a t i l , T. S. Raghunathan, and H. S. S h a n k a r * Department of Chemical Engineering, Indian Institute of Technology, Bombay 400 076, India
An analysis of the oil hydrolysis reactor design alternatives is performed. A model reaction t == g for oil hydrolysis is used to simulate the performance of several reactor configurations. The continuous countercurrent spray column is shown t o be superior t o others in terms of productivity and conversion. A tubular plug-flow reactor module is shown t o have promising features. 1. Introduction A number of reactor configurations can be considered for oil hydrolysis. These include batch, semicontinuous, and continuous reactors. However, commercial operations employ either batch autoclaves or continuous countercurrent spray columns. Batch operations have considerable operational flexibility but involve low productivity. The countercurrent columns have higher productivity but lower operational flexibility, thus the need for evaluation of existing as well as possible design alternatives. A three-step kinetic model for the oil hydrolysis reaction is given by Patil et al. (1988a). It is possible to simulate the performance of different reactor configurations by using this model. Since the kinetic model is nonlinear, numerical procedures are required for the solutions of resulting ordinary differential equations. Namdev (1987) has simulated the performance of a continuous countercurrent spray column by using this model. In this paper, the elementary reaction t g is used to model the oil hydrolysis reaction. The single-step kinetic model is fitted to the experimental data of Sturzenneger and Sturm (1951) to estimate the parameters. Analytical solutions of the performance equations for batch, semicontinuous, and continuous hydrolyzers are given. The procedure thus provides a quick method of understanding the performance aspects of different design alternatives and would therefore be valuable for design and operating personnel. 2. The Model Reaction
Consider the model liquid-liquid reaction k
t&g k2
(1)
under the following assumptions to be equivalent to the fat/oil hydrolysis reaction described earlier by Patil et al. (1988a,b). (a) Component t is present in phase 1, where reaction 1 occurs, while product g distributes between phases 1and 2 such that the following equation applies: y g = mxg (2)
(b) Component t has no solubility in phase 2, and phases 1 and 2 are immiscible. (c) The mass and density of each phase remain constant during the reaction. (d) Reaction 1 belongs to a very slow reaction regime. The model reaction and oil hydrolysis are equivalent. Thus, t may be assumed to represent the triglycerides or oil and g to represent glycerol. The initial level of t in phase 1 (xto)is therefore to be adjusted to correspond to the glycerol content of feed oils. Also, for this model reaction, the ratio of molecular weights M J M , is obviously unity. The extent of reaction, X , can be related to the measured acid and saponification values as - AVO x = AV -SV - AVO
(3)
Also for every mole of glycerol produced, 3 mol of fatty acids is produced. Hence, the fatty acid productivity, Pa, can be determined from the g productivity by the following relationship:
Pa = fpg
(4)
where f is 3 times the ratio of molecular weights of fatty acid to glycerol. The objective functions (X)and Pg will depend upon reactor configurations and operating conditions used. We examine a number of reactor alternatives to bring out these features in the following sections. 3. Batch Reactor The balance for component't giyes dC,/dt, = rt = kzCg- k;C, _._
(5)
The overall balance of component g in both phases'gives (Lxto/MJX + (Lxg0/Mg)+ (Gygo/Mg)= ( L x g / M g )+ ( G y g / M g )
Using eq 2 for y g gives xg = ( A / D ) X
1988 American Chemical Society
+B
(6)
