Ind. Eng. C h e m . Res. 1988,27, 727-735
727
KINETICS AND CATALYSIS Thermal Hydrolysis of Vegetable Oils and Fats. 1. Reaction Kinetics T. A. Patil, D. N. Butala, T. S. Raghunathan, and H. S. Shankar* Department of Chemical Engineering, I n d i a n Institute of Technology, Bombay 400 076, India
A model is proposed to describe the liquid-liquid thermal hydrolysis of vegetable oils and animal fats. Extensive data on hydrolysis equilibrium and rate have been obtained. Results of the present and previous studies have been compared with model predictions. Uniformly excellent agreement is indicated in all the cases. The direct saponification of oil/fat in small-scale soap manufacturing units poses many problems with regard to recovery of byproduct glycerol. The hydrolysis of oils prior to saponification provides possibilities of overcoming this problem. Fatty acids and glycerol are valuable chemical intermediates with a variety of end uses. Yet quantitative information on the kinetics, thermodynamics, and engineering aspects of this reaction is limited. The hydrolysis reactions can be conducted thermally as a liquid-liquid reaction or as a gas-liquid reaction using superheated steam. It can also be conducted using lipolytic enzymes at ambient conditions. All three modes of hydrolysis are being investigated, but we present here our results on liquid-liquid thermal hydrolysis. Commercially the reaction is carried out under the conditions of 100-260 "C and 100-7000 kPa using 0.4-1.5 w/w initial water to oil charge with or without catalysts. Feed stock flexibility, high extent of hydrolysis, minimal color degradation, and high concentration of glycerol in the aqueous phase are among the principal process requirements. Thus, several alternative such as the batch, the semicontinuous, and the continuous countercurrent processes are in use. In the batch Twitchell process, the acid-washed fat is contacted with water at 100 "C and atmospheric pressure using 25-50% w/w water and 0.75-1.25% by its own weight Twitchell reagent in covered wooden vats (Mueller and Holt, 1948). Oxidative discoloration and high steam consumption constitute the major drawbacks of this process. No direct information on the rate and equilibrium characteristics of this process is reported. In the batch autoclave process, the reaction is conducted in the presence of liquid water and wet steam under moderate conditions of 1000-3000 kPa and 180-230 "C; agitation is provided by steam or a mechanical stirrer. The degree of hydrolysis of about 90% is generally obtained which may be increased to 96% at the cost of glycerol dilution by employing a higher water-to-oil ratio (Lascaray, 1949). In the continuous countercurrent process, the preheated fat is fed at the bottom of a hydrolyzing column under pressure while water is sprayed from the top of the reactor. The descending stream of water encounters the rising fat counter currently at around 260 "C and 4700 kPa to accomplish the necessary contact needed for the reaction. No catalyst is generally used (Burrows, 1953). Hydrolysis of oils is clearly a case of chemical reaction, accompanied by mass transfer. In this heterogeneous 0888-5885/88/ 2627-0727$01.50f 0
system, water and glycerol can distribute between the aqueous and the oil phases. Thus, both reaction and phase equilibria are featured in this reaction. The dependence of the equilibrium acid value on the initial water-oil ratio is available (Lascaray, 1949,1952). These results suggest that the degree of hydrolysis at equilibrium depends on the initial water-to-oil ratio alone and is independent of temperature. Product glycerol is found to have an adverse effect on the final degree of hydrolysis, and the empirical equation X,= 1.0 - 0 . 8 ~ ~(0.8 ~ IX,I1.0) (1) is given by Lascaray (1952) to quantify this effect. Here
X,is the overall conversion and y g eis the glycerol mass fraction in the aqueous phase at equilibrium. Equation 1 shows clearly that a possible strategy for design would be to extract glycerol from the fat phase to shift the position of equilibrium in the favorable direction, thus the importance of the continuous countercurrent scheme. Mills and McClain (1949) studied the equilibria in the hydrolysis of beef tallow and coconut oil at 235 and 250 "C in the range of 90-100% completeness using a novel stirred batch reactor. A t equilibrium, the oil phase was found to contain small quantities of diglycerides, monoglycerides, and glycerol, providing evidence for the following stepwise reaction: triglyceride
+ water
11. diglyceride
+ water
I.
e
diglyceride
+ fatty acid
monoglyceride + fatty acid
111. monoglyceride + water
6
glycerol + fatty acid (2)
The glycerol distribution coefficient m ( = y g / x ) for beef tallow and coconut oil at 250 "C was found to \e 12 and 6, respectively. The reaction is said to occur in the oil phase and that water is unimportant in establishing the equilibria because it is present in large molecular proportions. However, no data on the concentration of water in the oil phase are reported. Sturzenegger and Sturm (1951) have reported batch hydrolysis data for some oils. These studies indicate an autocatalytic behavior with a clear induction period. The overall rate of reaction is also seen to be a strong function of temperature. The reaction is found not to go to completion and that equilibrium acid value is a function of 0 1988 American Chemical Society
728 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
water to oil ratio alone. The acid value-time data reveal oscillations of 2-3 units around the equilibrium region. Despite the above features, a pseudefirst-order irreversible kinetic interpretation with rate constant
(3) based on half-life is given. An activation energy E I R of 7000-8000 K is indicated. Technically, a number of reactor designs are possible. However, very limited studies are reported on the behavior of hydrolysis reactors. Jeffreys et al. (1961) assumed overall stoichiometry and pseudo-first-order irreversible kinetics to model a countercurrent spray tower as a case of simultaneous reaction in the oil phase and glycerol transfer to the aqueous phase. Reaction conditions are not specified, but results showed abnormal HTU values. An approach to describe the performance of a spray column with axial dispersion model is presented by Donders et al. (1968) using overall reaction stoichiometry. A value of the second-order rate constant for forward reaction, k, = 0.009 m3/(kmol.min) and K, = 2, is used. A simple equation is suggested to calculate the dispersion coefficient from superficial velocity of the dispersed phase. A mass-transfer coefficient of kL = 0.001 m/min and interfacial area of 87 m2/m3is suggested. No experimental data or evidence is reported. Basu (1976) presented data on the splitting of rice bran oil in a countercurrent column and compared the same with conversion predicted from irreversible pseudo-firstorder kinetics; considerable disagreementwas found. Desai et al. (1984) modeled the course of batch hydrolysis using the following assumptions: (1)reaction takes place in the oil phase as per eq 2 with the first reaction controlling; (2) diglycerides and monoglycerides are stationary at their equilibrium values; (3) the concentration of water in the oil phase is linearly related to fatty acid concentration in the oil phase; (4) the phase equilibria for glycerol hold during hydrolysis; (5) elementary kinetic representation applies so that the equilibrium constant for the reactions can be given as
In this paper, a model containing four equilibrium parameters and one rate parameter is developed to describe the course of the liquid-liquid thermal hydrolysis. The model is tested against present and previous data, and uniformly good agreement is revealed in all cases. Mathematical Model The model for batch hydrolysis developed here uses the following assumptions: (a) The reaction takes place in the oil phase as per the scheme shown in eq 2. Isomers of mono- and diglycerides which are possible are not explicitly accounted. (b) The mass and density of the phases remain unchanged during the reaction so that where i refers to component. (c) All reactions are elementary. Equilibrium constants for all the steps are the same and are temperature independent. (d) The reaction belongs to the very slow reaction regime, and phase equilibria for glycerol exist at all times so that Yg
(7)
= mxg
where g refers to glycerol. (e) Water transport to the oil phase is rapid, and its concentration can be given as (f)The first step of eq 2 is rate controlling, and the other two steps are relatively rapid. (g) Glycerides and fatty acids are insoluble in the aqueous phase. Let X1, X2,and X, be the extents of reactions with respect to triglyceride in the system; since there is no change in volume due to reaction, we have by stoichiometry
Mass balance for triglycerides gives dCt/dt = k2CdCa - klCtCw
(13)
The other two reactions being relatively rapid, we get (4) with the overall equilibrium constant given as K, = K1K2K3= [C,3C,/C,CW3],
(5)
The model applicability is tested against acid value-time data of Sturzenegger and Sturm (1951) on beef tallow. Excellent agreement is indicated. Agreement with respect to other oils was poor. Summing up, the available information on this reaction is inadequate. The equivalence between acid and glycerol formation, the concentration of water in the fat phase, and the role of mass transfer relative to reaction are not well established. No useful model exists to predict the reactor performance under different design and operating conditions, hence the motivation for the present work. The objective of this study is therefore to develop a suitable model to cover some of the above inadequacies.
Cm - -k3Cw _
-=-
c,
k5Cw
Cd
cm
k6Ca
k4Ca
(14)
Since the solubility of water in fatty acids is large relative to glyceride, eq 14 can be written approximately as The above approximation leads to a simple solution of the problem. Overall glycerol balance gives
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 729 Table I. Specifications of Autoclave Used in the Present
Work capacity outer diameter of the vessel inner diameter of the vessel depth max pressure max temp stirrer type stirring speed propeller diameter propeller width
1L 0.111 m 0.08 m 0.23 m 50 000 kPa 600 O C propeller 150-1440 rpm 0.05 m 0.017 m
Since mass and density are assumed constant when using eq 6, we get C, = Ca(AX3 + A ' ) (18) From eq 16 we get
X3 = BX2 - B' (19) x2 = cx1- C' (20) Substituting for C,, Cd,C,, and C, in terms of X 1 in eq 13 we get -P dX,/dt = XI2 - Q X 1 - R (21) The solution to eq 21 is given by aP(1 - eNt)
x1=
a
- fieNt
where a and fi are the roots of the right-hand side of eq 21.
Experimental Apparatus A laboratory autoclave of 1-L capacity made from stainless steel 316 mounted in a compact, electrically heated furnace was used. The specifications of the autoclave are summarized in Table I. The top flange is provided with a cooling coil having provision for cooling water inlet and outlet. A stainless steel 316 rupture disc tested at 500 atm and 600 "C is provided on the top flange. The flange also contained 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 precision needle valves to sampling tubes of different lengths for sampling from different points in the autoclave. A magnetically driven stirrer is housed at the center. The stirrer speed was measured with a tachometer. A precalibrated chromel-alumel thermocouple connected to a digital multimeter indicated the liquid temperature inside the reactor to within f0.5 "C. Temperature was controlled by using controllers which regulate the power input to the electrical heaters. Analytical Section Volumetric methods were used for determination of fatty acid, saponifiable matter, monoglyceride, and glycerol. Water from the oil sample was removed by percolating through anhydrous sodium sulfate prior to analysis. All analytical procedures were adopted from Mehlenbacher (1960). Free fatty acid content of the sample on a dry basis is determined via the acid value defined as the milligrams of potassium hydroxide necessary to neutralize the free acids in 1 g of sample. The saponifiable matter in the sample is determined via determination of the saponification value defined as the milligrams of potassium hydroxide required to saponify 1g of dry sample. In present work, 1-butanol is used as the solvent and an accuracy of *0.5 unit is estimated.
Three methods were tried for the determination of glycerol content in aqueous phase viz. periodic acid oxidation method, sodium metaperiodate oxidation method, and specific gravity measurement (Perry and Chilton, 1973). Standard samples of glycerol solution of 0.05-35% w/w were prepaxed to evaluate the efficacy of the methods. Excellent agreement was found between these over the range investigated. Therefore, the specific gravity method which was simple and rapid was used in the present work with frequent cross-checks against sodium metaperiodate titrations. The error of the glycerol measurement by this method was estimated as *0.026%. The fat-phase glycerol analysis was carried out by shakiig a known amount of distilled water with a relatively large fat-phase sample to extract the glycerol in water so as to facilitate accurate measurement of aqueous glycerol by the specific gravity method. The periodic acid oxidation method was also used to confirm the measurements. Levels of monoglyceride were analyzed by the periodic acid oxidation method. a-Monoglyceride could be measured by this method. Total monoglyceride was calculated as per the procedure given by Mehlenbacher (1960). A measurement error of Zm f 0.0008 is indicated here. Only clean oil free from mono- and diglyceride was used in the experiments. In general, acid value (AV), saponification value (SV), aqueous glycerol fig), and monoglyceride (Zm)were measured. A balance for fatty acid is used to obtain the overall conversion in terms of measured quantities as shown below: ( X i + Xz + X 3 ) / 3 = AV/SV - AVo/SVo (23) The quantities X 2 and X 3 could be expressed in terms of X 1 by using eq 19 and 20. The resulting equation in X I could be expressed in terms of elapsed time by using eq 22. Thus, we could relate the measured AV and SV values to time, and this relation involves the unknown parameters k,, m, K , al, and These parameters are estimated via regression analysis of present and previous data. As per the model, AV and SV data alone are necessary for the purpose of parameter estimation. In this work, 9, and Zm are also measured to provide additional checks on the methods employed. The experimental data are obtained by analyzing at room conditions the samples from the reactor. The oil phase in the reactor contains fatty acids, glycerides, glycerol, and water. During analysis at room conditions, water and glycerol are removed. Thus, measured mass fractions are larger than at the reactor conditions. Hence, a correction is required to relate measured quantities to reactor conditions. The magnitude of this correction increases with temperature and is about 3% at 260 "C. This relationship can be found from a fatty matter balance as 2, = q/(l - x, - X J (24) If model parameters are known, then this equation could be used to relate composition of the oil phase in the reactor to the measured quantities at room conditions. The mass fractions 2, and 5, are found as x', = (AV)M,/561000 (25) 2, = (SV - AV)Mt/168300 (26) Since Lx, + G y , = G9, we have by using eq 7 mG/L yg=
+ mG/L
730 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 Table 11. Experimental Conditions of Present Work temD 180-280 "C water-to-oil ratio, G / L 0.0067-2.0 W / W 120-800 rPm glycerol in initial water charge, g, 0.0.171 coconut oil 920 kg/m3 density of oil initial monoglyceride in oil 0.0 0.0 initial diglyceride in oil 253 inidial oil saponification value, SVo 2.8-50 initial oil-acid value, AVO
-
0,A-Sturzonoggcr (1.01 (19511 -
260- -
-
220-
Model oil Tollow GIL- 0 7
-
where m is at reaction temperature. The value of x g can then be calculated from eq 7. Procedure The time required to reach equilibrium at different temperatures was determined by trial. In equilibrium experiments, known amounts of reactants were introduced in the reactor and heated under stirring until equilibrium was reached. The reaction mixture was quickly cooled to room temperature by circulating chilled water through it. Samples of oil and aqueous phases were then taken for analysis. In the kinetic experiments, reactants were heated and stirred at constantly controlled temperature. Samples were drawn intermittently for analysis. Aqueous-phaseglycerol content is estimated from the end sample remaining in the reactor after attaining equilibrium. In another set of runs, reaction was halted after the desired time intervals; the acid value of the fat phase and glycerol content in the aqueous phase were measured. The rate experiments in this work were confined to the temperature range 180-200 "C only since sufficient rate data at higher temperatures were available in the literature. Experimental conditions have been so chosen as to bring out the effect of reactant ratio, feed composition, stirring speed on the equilibria, and rate. The experimental conditions used in the present work are summarized in Table 11. Model Features and Parameter Estimates The proposed model contains four equilibrium parameters and one rate parameter. Hence, extensive data at equilibrium are required for its estimation. If these parameters were known, then one measurement, say acid value, is necessary to estimate the composition of phases. Thus, all the available acid value-time data in the literature could be used for the purpose of parameter estimation. Butala (1984) estimated parameters of this model by regression analysis of the acid value-time data of Sturzenegger and Sturm (1951). Arrhenius-type temperature Table 111. Regression Estimates of peanut oil a t 225 "C 240 "C 260 O C 2.22 2.22 K 2.22 m 28 17 11.5 0.053 0.075 k," 0.040 0.55 0.60 62 0.45 0.30 0.36 0.13 6,
Time (mln)
Figure 1. Comparison of fit between model prediction and measured acid value for tallow fat.
dependences were assumed for the parameters. The regression estimates of parameters for the different oils are summarized in Table 111,while the Arrhenius temperature coefficients are summarized in Table IV. The value of m for beef tallow as shown in Table I1 is in the neighborhood of the experimental value of Mills and McClain (1949). Peanut oil shows a relatively high value of m at all temperatures, the reasons for which could be the higher level of unsaturation in the oil. The value of m for coconut oil decreases from 63 at 190 "C to 4.5 at 280 "C, which indicates a exothermic heat of solution of 43.4 kJ/mol. The distribution coefficients for other oils also show a similar trend. The magnitude of the equilibrium constant K is estimated to be 2.22 which is assumed temperature independent in the model. The concentration of water in the oil phase depends on the concentration of triglyceride and fatty acid. The contribution due to triglyceride alone can be estimated by using a1 values and due to fatty acid alone from a2 values. The contribution due to triglycerides during coconut oil hydrolysis is estimated to vary between 0.0005 and 0.004 w/w and for peanut oil to vary between 0.002 and 0.01 w/w over the temperature range 180-280 "C.The contribution to water in the fat phase from fatty acid is seen to vary between 0.025 and 0.06 w/w for coconut oil over the temperature range 180-280 O C . The parameter & increases with temperature, indicating an endothermic heat of solution of 16.7 kJ/mol. The agreement between model fitting and experimental data of Sturzenegger and Sturm (1951) is displayed in
Parameters of Model at Different Temperatures (Butala, 1984) tallow at coconut oil a t 280 OC 225 "C 240 "C 260 "C 280°C 190°C 225 O C 240 "C 260 OC 2.22 2.22 2.22 2.22 2.22 2.22 2.22 2.22 2.22 4.5 10 9 7 47 27 20 20 14 0.093 0.036 0.055 0.092 0.133 0.016 0.040 0.046 0.064 0.70 0.45 0.55 0.65 0.75 0.326 0.450 0.50 0.55 0.42 0.045 0.055 0.10 0.15 0.022 0.054 0.060 0.066
in m3/(kmol.min).
Table IV. Arrhenius Temperature Coefficients of Model Parameters (Butala, 1984)" oil 62 m k,,kmol/(m3.min) coconut exp[4.17 - 2500/T] exp[-9.6 + 6470/T] exp[7.l - 5750/T] peanut exp[3.83 - 2310/T] exp[-8.0 + 5680/T] exp[5.83 - 4505/T] beef tallow exp[4.25 - 2500/T] exp[-10.25 + 6565/T] exp[10.34 - 6825/T] T in degrees kelvin.
K 2.22 2.22
2.22
280 "C 2.22 8 0.10 0.70 0.084
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 731 0,A-Sturzeruggor el. a1 (1951)
260
/003 220
20
-
Model o i l - Tallow G/L- 0 7
40
80 o 120
300r-----7 2 60
160
200 i
Time (min)
Figure 2. Comparison of fit between model prediction and measured acid value for tallow fat.
Model Peanut G / L - 0.5
oil
220
l i m e (min)
Figure 5. Comparison of fit between model prediction and measured acid value for coconut oil.
t
i
-
o,2LkIY 0
10
20
30
40
50
Time (min)
Figure 6. Variation of predicted oil-phase mass fraction of diglyceride, monoglyceride, and glycerol during hydrolysis of tallow fat at 280 "C, G f L = 0.7, and AVO= 5.5. Time (min)
Figure 3. Comparison of fit between model prediction and measured acid value for peanut oil.
Model oil -Peanut
220
GIL- 0.5
AV
Time (min)
Figure 4. Comparison of fit between model prediction and measured acid value for peanut oil.
Figure 1which shows the variation in acid value against time at 280 and 240 " C at GIL = 0.7 for tallow. Initially the reaction is relatively slow due to the limitation in water availability in the oil phase. This is the induction period which can be seen to disappear at higher temperatures. The time course of reaction reveals a clear maxima, and the rate then falls markedly due to depletion of glycerides and consequent accumulation of glycerol in the fat phase as equilibrium is approached. Thus, the model shows most of the distinct features of the reaction. Figure 2 shows the fit for tallow at 260 and 225 "C at G / L = 0.7; the fit for peanut oil is shown in Figures 3 and 4 and that for cocount
oil in Figure 5. In all these figures, the equilibrium acid value can be seen to be clearly insensitive to temperature. The simulated variation of the concentration of the intermediates is shown in Figure 6. It is observed that diglyceride, monoglyceride, and glycerol exist in the fat phase but their levels are indeed small. For example, for MIOW at 280 "C and G / L = 0.7, Xd = 0.07, X , = 0.04, and x g = 0.02 are indicated at equilibrium.
Model Prediction and Present Data Special experiments have been conducted to bring out the strengths and weaknesses of the present model. Table V lists the comparison between equilibrium data for coconut oil hydrolysis and predictions using parameters of Table 111. We note that the parameters of Table I11 were based on literature data, and thus the comparison provides an independent check on the model proposed. In these equilibrium experiments, the effects of temperature, water-to-oil ratio, and initial composition of charge have been investigated. In all these experiments, uniformly excellent agreement is seen between model prediction and data. Table VI summarizes the experimental level of x , at equilibrium for coconut oil hydrolysis. The independent predictions from Table I11 model parameters are also shown. The agreement is satisfactory. The level of monoglyceride is seen to lie in the range 0.01-0.04 w f w. This low level of intermediates can be taken as a justification for the hypothesis that reactions I1 and I11 are rapid relative to reaction I. A comparison between the measured course and predicted course of hydrolysis at 190 "C is shown in Figure 7 . The comparison between model prediction and data to show the effect of initial water-to-oil ratio on the course
732 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
3001---!
-i
O.Ol4C
2 60
AV
s'
E O.O"t
-
- Model 0.004
0 - Experimentol oil -Coconut
o,oo2\
G/L-O.9 1 - 19oc
E
0
4
8
12
16
0
20
u 2
Figure 7. Comparison between model prediction and experimental data for coconut oil hydrolysis with feed at AVO= 3 and y@ = 0.
4
6
8
1
0
Time ( h r )
Time ( h r :
Figure 10. Comparison between model prediction and experimental values of levels of monoglycerides at 190 OC, G / L = 0.9, AVO= 3, and
ygo = 0.
300
AV
i
- Model 0 Experimentol
oil -Coconut G/L T
Time ( h r )
Figure 8. Effect of initial water-to-oil ratio on the time course of coconut oil hydrolysis at 190 OC, AVO= 3, and jlgo = 0.
wo
i
1
'
6
- 0.9 - 19oc
Q
1
iloo
lime ( h r )
3 00
Figure 11. Comparison between model prediction and experimental values of aqueous-phase glycerol concentration from termination runs with feed at AVO= 3 and jl@ = 0.0.
260
dictions using the parameters of Table 111.
220
Verification of Assumption and Parameter Estimates It is shown so far that there is good agreement between prediction and data. It is now necessary to check the goodness of parameter estimates and the applicability of the assumptions. Carefully selected experiments were therefore necessary. Hydrolysis experiments were conducted at 190 and 260 "C using suitable amounts of water so that water, oil, and other species formed in the reaction remained essentially in a single phase during the entire course of hydrolysis. The proportion of water to be added to neutral oil at different temperatures to form a homogeneous phase was judged by extrapolating the results of Hilder (1968). Acid value, glycerol, monoglyceride, and saponification value were measured. The value of kl was estimated from the initial slope of the acid value-time curve, and the equilibrium constanta K1,K,, and K3 were calculated using x,, x g , and x g data at equilibrium. The purpose of conducting the above homogeneous hydrolysis experiments was to have an independent check on the magnitude of k,,K1,K 2 ,and K3 and also test the assumption of the equality Kl = K2 = K3. Table VI1 shows the value of the rate constant k , obtained thus at 190 OC to be 0.015 m3/(kmol-min),which is close to the regression estimate of 0.016 m3/(kmol.min) shown in Table 111. Equilibrium constants K1,K 2 ,and K 3 so determined are
180 AV
I40
Time ( h r s )
Figure 9. Effect of initial composition of charge on the time course of coconut oil hydrolysis at 190 "C.
of hydrolysis is displayed in Figure 8. The comparison with respect to initial composition of charge is shown in Figure 9 and that for the intermediate x , is shown in Figure 10. Uniformally excellent agreement is seen in all these comparisons. In the hydrolysis literature, no data on Qg versus time are reported. Runs were therefore carried out so that the reaction could be frozen rapidly after desired time intervals by rapid chilling of the reaction mixture. These experiments were tedious but were nevertheless done to assess the goodness of the model proposed. Figure 11 which displays 9,-t data reveals very good agreement with pre-
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 733 Table V. Comparison between Model Prediction and Equilibrium Data for Coconut Oil Hydrolysis
8 9 10 11 12 13 14 15 20 25 26 27 28 29 30 31 33 34 35 36 37 38 39 41 42 43 45 46 47 48 49 50
260 260 260 260 260 260 260 260 260 240 240 240 240 240 240 240 220 220 220 220 220 220 220 200 190 190 190 190 190 190 190 180
'Homogeneous hydrolysis.
2.0 1.8 1.5 1.3 1.0 0.8 0.6 0.4 0.250 1.8 1.5 0.96 0.8 0.6 0.4 0.25 2.0 1.5 1.0 0.8 0.6 0.4 0.25 0.9 2.0 0.9 0.825 0.764 0.4 0.0107 0.0067 1.0
0.0650 0.0711 0.0843 0.0962 0.1220 0.1433 0.1770 0.2460 0.329 0.0720 0.0850 0.1294 0.1485 0.1840 0.2500 0.3110 0.0670 0.0864 0.1240 0.1480 0.1882 0.2526 0.3400 0.1345 0.0650 0.1330 0.2300 0.3230 0.2730
NA NA 0.1220
260 260 260 200 190 190 190 180
257.8 254.5 253.4 251.5 248.0 234.7 221.4 212.0 184.5 256.0 252.8 249.2 239.4 226.4 210.5 170.6 256.5 255.0 246.1 237.4 229.0 209.9 183.8 241.8 249.8 238.3 232.6 216.8 219.1
257.0 255.5 253.2 251.5 248.0 235.4 222.4 211.0 185.5 256.0 253.0 249.3 238.0 226.6 210.9 170.3 256.8 255.4 246.7 238.1 229.8 210.9 184.9 240.9 250.9 241.0 230.2 222.0 214.8 22.0 43.0 250.3
NA NA 249.2
NA = not available.
Table VI. Monoglyceride Levels for Hydrolysis of Coconut Oil at Equilibrium run T,"C GIL (xm)cxptl (xm),,red 8 12 15 41 42 43 47 59
0.0674 0.0750 0.0850 0.1030 0.1287 0.1534 0.1990 0.2610 0.3330 0.0750 0.0887 0.1363 0.1547 0.1960 0.2610 0.2820 0.0670 0.0890 0.1310 0.1560 0.1976 0.2580 0.3158 0.1290 0.0656 0.1430 0.2120 0.3330 0.2620 0.0023° 0.0009' 0.1220
0.010 0.017 0.035 0.013 0.004 0.012 0.017 0.010
2.0 1.0 0.4 0.9 2.0 0.9 0.4 1.0
0.024 0.042 0.078 0.014 0.005 0.011 0.022 0.008
also seen to be nearly equal and in the vicinity of the regression value shown in Table IV. Table VI11 summarises the comparison between model prediction and present data for xg, yg, and m over the temperature range 190-260 "C. Here equilibrium samples were drawn from both fat and aqueous phases. The results show good agreement between experimental m and regression values of Table 111.
In the present work, attempts were made to measure the solubility of water in coconut fatty acids alone at high temperatures. In this two-component system, fatty acids were found to dissolve upto x, = 0.2 w/w at 240 "C, confirming the results of Mills and McClain (1949). However, the batch hydrolysis model predicts only xw = 0.03-0.04 w/w in the fat phase at 240 "C. The reason for this difference in solubility is not clear at this stage. The above results also compare favorably with a value of x , = 0.01-0.017 w/w at about the same temperature from countercurrent column experiments of Jeffreys et al. (1961). The system has six components (C = 6): two liquid and one vapor phase (P= 3) and three independent reactions (R = 3). Thus, the phase rule gives (C - R ) 2 - P = 2 as the number of degrees of freedom. The net heat of reaction is small so that temperature as one degree of freedom has negligible effect. Hence, the observable degree of freedom is one. This feature explains why the liquid-
+
Table VII. Estimate of Rate and Equilibrium Constants from Homogeneous Hydrolysis of Coconut Oil (k,in m3/(kmol min)) T , "C GIL ki K1 K.7 KQ K. 190 280
0.0067 0.015
0.015
NA
2.8 2.48
2.0 2.22
2.52 2.05
14.8 11.3
Table VIII. Comparison between Model Prediction and Experimental Values of Glycerol Distribution Coefficient at Equilibrium for Coconut Oil Hydrolysis at Different Temperatures run
T , "C
(YJprd
(YJexptl
(xg)prd
(Xg)exptl
55 54 53 52 51
260 240 220 200 190
0.1310 0.1344 0.1356 0.1346 0.1329
NA
0.0094 0.0067 0.0045 0.0033 0.0028
NA
ONA = not available.
0.1297 0.1317 0.1330 0.1338
0.0063 0.0043 0.0028 0.0021
(m)pred
(mlexptl
14 20 30 40 47
NA 20.6 30.6 47.5 63.0
734
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
'O0I 1 63
0
2
4 6 Time ( h r )
8
IO
Figure 12. Effect of stirring speed on the time course of coconut oil hydrolysis at 190 O C , G / L = 0.9, AVO= 3, and QgO = 0.
liquid equilibria in this reaction depend on water-to-oil ratio alone. The effect of external mass-transfer resistance on the overall rate of reaction can be understood from runs 56, 57, and 58 in which stirrer speed was varied. The experimental results are shown as acid value-time in Figure 12. The plot based on the model which ignores external mass-transfer resistance is also shown. The close agreement indicates the role of external mass transfer to be small a t 190 O C as per model assumption. The conventional means of discerning the controlling mechanism compares the kinetic parameter and masstransfer parameter. In the present study, KLa = 3.0 min-' >> d,k,C,, = 0.09 min-l suggests that the overall process belongs to a kinetically governed situation. In these calculations, KL = 6 X lo3 m/min and a = 500 m2/m3from Sarkar et al. (1980), and kl = 0.1 m3/(kmol.min) at 280 "C, d, = 0.35, and Cto = 1.337 kmol/m3 are used. A criterion, diffusion time tD,much smaller than reaction time, t R , is given by Astarita (1967) for the applicability of the very slow reaction hypothesis. Values of tR = 1.17 min and tD= 0.0167 min are estimated using available data for D A and KL, indicating the overall process to be a case of very slow reaction. The activation energy for the reaction velocity constant k l for all the oils is generally in excess of 40 kJ/mol as shown in Table IV which suggests the overall process to be kinetically controlled. The concentration of water and glycerol in the oil phase is small. During the course of reaction, water is transferred to the fat phase and glycerol produced in the fat phase are extracted into aqueous phase. Due to these two transfer processes in opposite directions, changes in density and mass of both phases and estimated to be less than 2 % ,thus the justification for the constancy of mass and density of the two liquid phases during reaction. In the present work and in the work of Sturzenegger and Sturm (1951),oscillations of 2-3 units in acid value have been observed around the equilibrium region. These oscillations were generally 1-1.5% of the measured value. This effect was found to be more pronounced above 240 "C and G / L above 0.5. Such behavior is also seen in industrial batch autoclaves. This feature though small in magnitude is not explained by the present model. This aspect is under investigation. Summing up, we observe that the model describes the course of hydrolysis very well. The assumptions used are seen to be reasonable. In nonlinear regression, parametric insensitivities are likely. In the present work, these problems have been overcome by checking the regression estimates with experimental values.
Conclusions A model which contains four equilibrium parameters and one rate parameter is developed to describe the thermal hydrolysis of vegetable oils. The model is shown to exhibit most of the kinetic and thermodynamic features and also to give an excellent description of the present and previous hydrolysis data. The liquid-liquid thermal hydrolysis of oils could thus be viewed as a three-step reaction taking place in the oil phase with negligible change in the density of phases. The triglyceride hydrolysis is the controlling step, and the overall reaction belongs to very slow reaction regime. Mass transfer of glycerol and water across the phases is fast relative to reaction. The concentration of water in the oil phase is only 2-4% w/w even though the oil phase may contain predominantly fatty acids and in which water solubilities can be expected to be high. The reasons for the marked reduction in water "solubility" in the oil phase are not clear at this stage. Nomenclature A = l / ( m G / L + 1) A' = (xpo + y p o G / L o ) M t / x t f l g (+ l mG/L) AV = acid value, mg of KOH/g of fat B = K382/(A + K362) B' = (-4'- K ~ & C , O / C ~ O ) + / ( AK362) C = K282/(1 + K262 - B ) C' = [B' + (Cmo/Cto)- Kz&Cdo/Cto]/(1+ K282 - B ) C, = concentration of species i, kmol/m3 D=l+C+BC D'= (Cao/Cm)- (C'+ BC'+ B ' ) DA = diffusion coefficient of the species in the liquid system, mz/s E=l-C G = mass of aqueous phase, kg k , = equilibrium constant for the ith reaction step, i = 1-3 K , = overall reaction equilibrium constant KL = overall mass-transfer coefficient, m/min k , = specific reaction rate constant in the fat phase, m3/ (kmol-min) L = mass of fat phase, kg A4,= molecular weight of species i m = equilibrium distributioncoefficient for glycerol on weight basis N = (a- p ) / P n = 1, 3, or 5 for forward reactions and 2 , 4 , or 6 for reverse reactions P = l/[k18,C,(D - 61/82 + D E / ( k & ) ) ] Q = (D - 2(61/62) - c,o/Cto - C,c,E/(Ct&z&))/(D - 6 1 / 6 2 + DEI (K262)) R = (6,/6z + Ca,/C,)/(D - 61/62 + D E / ( K z ~ z ) ) SV = saponification value, mg of KOH/g of fat T = temperature, " C t = time, min tR = 1/(4klCto)= reaction time, min tD = 4D/aKL2= diffusion time, min x , = mass fraction of species i in the fat phase y , = mass fraction of species i in the aqueous phase Greek Symbols = [ Q + ( Q 2 + 4R)lI2]/2, as defined in eq 22 /3 = [ Q - ( Q 2 + 4R)'I2]/2, as defined in eq 22 6, = as defined in eq 8 d, = fat phase holdup per unit reactor volume 6, = as defined in eq 8 pf = density of the fat phase, kg/m3 (Y
Subcripts and Superscripts a = fatty acid d = diglyceride e = equilibrium state
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
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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.
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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
