50
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
Hydrogenation of Glucose, Fructose, and Their Mixtures Jaime Wisniak' and Rozita Simon Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel
Glucose, fructose, and their mixtures (inverted sugar) have been hydrogenated in the range 85 to 130 "C,50 to 1000 psig, and catalyst loading 0.5 to 9% wt with Raney nickel, ruthenium, and rhodium catalyst. In every case the reaction follows a pseudo-first-order course. Ruthenium is the most effective catalyst, glucose is not hydrogenated in the presence of rhodium, and fructose is not hydrogenated with Raney nickel. Hydrogenation of glucose with Raney nickel and fructose with ruthenium can be explained by a mechanism involving the surface reaction between unadsorbed sugar and atomic chemisorbed hydrogen.
Monosaccharides and dissacharides have long been used and developed as natural sweetening agents, with glucose, sucrose, and starch hydrolysates being the most commonly utilized in the diet. For health and dietary reasons an increasing number of people have found it necessary to use nonnutritive sweeteners instead of these sugars. Inglett (1971,1976) lists a large number of sweeteners that exhibit some resemblance to sucrose-type sweetness and indicates that only a few can be considered suitable for human consumption. Almost all the building blocks of foodstuffs-carbohydrates, proteins, or lipids-are among the substances generally recognized as safe. Noncaloric sweeteners have been known since the discovery of saccharin in 1879, and did capture a significant market in the field of canned foods, low-calorie soft drinks, gums, and candies. Cyclamates were banned in the U S . in 1969 by the FDA after indications that they may be cancer-promoting agents. Presently, the largest selling artificial sweetener is saccharin and it has been removed from the GRAS list. Sucrose is the most widely used natural sweetener. I t is a dissaccharide which by hydrolysis forms one mole of glucose (dextrose) and one mole of fructose (levulose). This inversion can be brought about by heating the sucrose solution with diluted acids, by enzymatic action, or with ion-exchange resins. The two invert sugars are called reducing sugar because they reduce copper in a Fehling's solution. Glucose and fructose are also present in 10-1270 in the molasses produced in the manufacture of cane sugar. Hydrogenation of glucose yields sorbitol, an important chemical commodity. Hydrogenation of fructose will yield, on the other hand, a mixture of sorbitol and mannitol. Mannitol is about half as sweet as glucose; it is absorbed so slowly and excreted so completely by the human body that it is essentially nonglucogenic. Its major disadvantage is that ingestion of more than 1G20 g/day causes laxation. Sorbitol is calorigenic but it does not appreciably raise the sugar level in the blood; it also causes laxation if more than 50 g/day is ingested. A general survey on sorbitol and mannitol (Wright, 1974) indicates that glucose is normally hydrogenated in aqueous solution a t temperatures of 120-160 "C and hydrogen pressure of 70 to 140 atm, with a supported nickel or Raney nickel catalyst. At 120 "C the rate of reaction is first order with respect to glucose concentration and first order with respect to catalyst concentration. The rate order on hydrogen pressure is about 0.7. The energy of activation is given as 17.6 kcal/mol. Brahme e t al. (1964) studied the hydrogenation of glucose to sorbitol in an aqueous medium, using Raney nickel as a catalyst. The range of variables was: tem0019-7890/79/1218-0050$01 .OO/O
perature, 100 to 150 "C; pressure, 200 to 700 psig; catalyst, 24 to 54 wt/vol. Initial rates of reaction were found to be first order in the catalyst concentration and glucose concentration and half-order with respect to hydrogen pressure. The apparent activation energy was reported to be 5.2 kcal/mol. Substantial increase in the rate of reaction was achieved when the aqueous medium was replaced by an alcoholic solution. More recently, Brahme and Doraiswamy (1976) have reported on the hydrogenation of glucose on Raney nickel in a stirred reactor. Operating conditions were: pressure, 4.4 to 21.4 atm; temperature, 77 to 145.5 "C; and catalyst loading, 10.0 to 38.8 g/L. The bulk of the runs was performed using a 25% glucose solution, but some runs were made at initial concentrations of 0.60 to 0.99 mol of glucose/L, to test the effect of this variable on some of the kinetic parameters. Kinetic modeling of the reaction was undertaken a t temperatures below 100 "C since above this level the reaction was diffusion controlled. I t was found that the results could be explained by a model involving the surface reaction between molecularly adsorbed hydrogen and glucose in the liquid phase, with the adsorption of sorbitol as the controlling step. Activation energies for 10 g/L catalyst loading were 10.5 kcal/mol between 77 and 100 "C, and 1.4 kcal/mol between 100 and 145 "C. The activation energy for hydrogen adsorption was 9.20 kcal/mol. Solubility of hydrogen in a 25% glucose solution was also reported a t 27, 65, and 100 "C in the pressure range 8 to 55 atm. Extensive kinetic data are available on the hydrogenation of the pentose xylose to xylitol (Wisniak e t al., 1974a,b) using Raney nickel, ruthenium, rhodium, and palladium under a wide range of variables: xylose concentration, 1 to 3.5 M; catalyst concentration, 1 to 18% wt; pressures, 100 to 800 psig; temperature, 80 to 140 "C; and agitation rates, 300 to 1200 rpm. Rates of agitation were sufficiently high to assure that the reaction was controlled by the chemical resistance. For Raney nickel it was found that the reaction followed a pseudo-first-order course with a surface-reaction controlling step between atomic adsorbed hydrogen and adsorbed xylose a t 100 "C and unadsorbed xylose a t higher temperatures. In the experimental range of conditions the reaction constant varied between 3 and 22 h-*, and a t a catalyst loading of 2% and 200 psig, the activation energy was 6.5 kcal/mol. With platinum metal catalyst the reaction was also of pseudo-first-order and was controlled by the surface reaction between atomic adsorbed hydrogen and unadsorbed xylose. With ruthenium catalyst the activation energy was 12 kcal/mol and the heat of adsorption for hydrogen 13.8 kcal/mol. It was concluded that the activity of the catalyst 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
51
Table I. Range of Operating Variables concn, %' pressure. Dsig ruthenium rhodium Raney nickel
1-7 3-9 1-4
50-1000 200-700 200-800
t e m p . "C
85-130 100-130 85-130
' Catalyst as is, weight of sugar decreased in the order Ru > Ni > R h > Pd. A patent by Boyers (1959) describes the hydrogenation of several carbohydrates with different catalysts based on ruthenium. It is stated that the preferred catalysts are the ones containing elemental ruthenium supported on alumina or carbon and in concentration between 0.1 and 10% by weight of the active metal. The rate is said to increase up to hydrogen pressures of 100 to 150 atm only. Drastic conditions may lead to cleavage of the carbon chain. With hexoses degradation leads mainly to the formation of glycerol and ethylene glycols (Van Ling and Blugter, 1969). The composition of mixtures of glucose-sorbitol can be determined by gas-liquid chromatography (Weiss, 1972) or by direct titration of the aldehyde group with hydroxylamine hydrochloride in methanol (Patchornik, 1956). Mixtures of sorbitol-mannitol can be easily analyzed by GLC if the polyalcohols are first converted to their cyclic-n-butylboronate ester (Rabinowitz, 1974).
Experimental Equipment and Procedures Hydrogenation runs were made in a dead-end "/,-gal batch autoclave Model AF, 150, manufactured by Autoclave Engineers for a working pressure of 5000 psig at 650 O F . Description of the experimental setup as well as of the operating and analytical procedures have been given elsewhere (Wisniak et al., 1974a). The analysis of mixtures containing sorbitol and mannitol was done by GLC. Before injection into the column the samples were treated with n-butylboronic acid according to the procedure described by Rabinowitz (1974). Calibration graphs were prepared using methyl nonadecanoate as the internal standard. The analysis is believed to be accurate to within f l % . The GLC analysis was performed in a Packard-Becker Model 417 apparatus provided with a hydrogen flame ionization detector and an electronic integrator. The glass column was 183 cm long, 4 mm i.d., filled with 3% SP-2300 (cyanoalkyl phenylsilicone) on silanized diatomaceous earth. Operating conditions were: column, 209 "C; detector, 260 "C; injection port, 245 "C; nitrogen, 50 mL/min; hydrogen, 30 mL/min; air, 300 mL/min. Glucose monohydrate was purchased from Sigma and fructose from Merck. Raney nickel (W2) was obtained from Doduco Chemie, Elsenz, Germany. Platinum metal catalysts were purchased from Engelhard Industries, Newark, N.J., as 5% active metal on active carbon and had a bulk density of 28-31 lb/ft3 and a surface area of 1100 m2/g. These catalysts were used as received without prior treatment. In all the following figures percentage catalyst refers to bulk catalyst per unit weight solids. Results and Discussion A total of 150 hydrogenation runs were made at 980 rpm in the variable range described in Table I. At this agitation rate, the reaction occurs in the mixing range that is not affected by the impeller speed (Wisniak et al., 1974a). The course of the reaction was followed by plotting the logarithm of sugar concentration C (glucose, fructose, or their mixtures) vs. the time of the reaction, t. In every case, the plot obtained was a straight line so that the overall rate
'
O'-
1OO'C 600 Psig Run NO
1
1 2 D 3 ' h R U
005-
60 + 3'1. R o n r y NI
L 15
002 0
30
15
75
60
90
105
T I M E , MIN
Figure 1. Typical hydrogenation runs.
could be represented by the following pseudo-first-order reaction r = -dC/dt = kC (1) Figure 1 shows a typical experiment with glucose. The first-order constant k could thus be used to characterize a particular set of variables. This kinetic behavior is similar to the one observed in the hydrogenation of sugars (Brahme and Doraiswamy, 1976; Wright, 1974; Wisniak et al., 1974a,b). Table I1 reports the value of k for a wide range of operating variables. For glucose we have
and G = G 0e-kit For fructose we have
(4)
kz
F-S
k3
F+M
(5) Assuming first-order kinetics for each branch, we get
E' = FOe-(kz+k3)t dM/dt = k3F = k3F0e-(k2+k3)t
(6) (7)
M-
k2 s-s()=k , + k , F0 ''
-
The selectivity of reaction 5 can be defined as k3 M - M 0 g=-=-
(11) k2 S - So A typical run for fructose hydrogenation appears in Figure 2. If we assume that in a mixture of glucose and fructose, each one reacts according to a first-order reaction we get G + F = GOe-klt + F,,e-(k~fkdt (12)
The difference ( M - MO)is the same as given by eq 9. It is probable that for a given set of conditions the actual
52
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979 Run hi0 103
130 "C 400 5 I G
. 0 J
I z
P
k
l x I-
z u w
z V 0
*-/ 3
c
'0
EO
33
50
4:
53
73
TIVE, V'h 5
10
25
20
15
Figure 4. Raney nickel inhibition by mannitol.
REACTlON T I M E , MIN
Figure 2. Hydrogenation of fructose.
100°C 0 Ruthenium, 2 0 0 p s 1 g
+
koney n chel 4 O O p 5 ~ g
0001
C
.
2
3
4
CfiTALYST
5
6
7
8
9
22
24
26
TEMPERATURE^
28
30
32
~o~/T,"K-'
PERCENT
Figure 3. Effect of catalyst loading-glucose
hydrogenation.
values of h l , h2, and h3 will be different from those calculated from eq 2 and 5 . Mass Transfer Effects. Previous hydrogenation runs made at various agitation regimes have already shown that the rate was independent of the impeller speed above 900 rpm (Wisniak e t al., 1974a). These results, together with those obtained at different catalyst loadings, indicated that in the range of operating variables the system was probably controlled by the chemical reaction. Catalyst. For glucose Figure 3 indicates that there is an almost linear variation of h , with catalyst concentration and that ruthenium is a more effective catalyst than Raney nickel. Two-hour long runs with rhodium in the range 3 to 9% catalyst, 200 t o 700 psig, and 100 to 130 "C failed to produce any detectable hydrogenation of the aldehyde group. The rate constant h = h2 + h3 for fructose hydrogenation varies linearly with ruthenium concentration in the range 0.5 to 3% wt. Two-hour long runs with Raney nickel in the range 2 to 3% catalyst, 200 to 800 psig, and 100 to 130 "C failed to hydrogenate fructose a t all. The failure of Raney nickel to hydrogenate cannot be due t o its incapability of reducing the ketone group, since there is ample evidence that this catalyst can hydrogenate ketones under mild conditions (Wisniak et al., 1976). A better possibility is that the reaction products inhibit the catalyst by being more strongly adsorbed than fructose. This hypothesis was tested by adding mannitol in a concentration of 0.1 M to a glucose solution. The results shown in Figure 4 indicate that the rate of hydrogenation of this mixture was about one-third that of glucose alone. The experimental results reported in Table I11 indicate that the selectivity in the hydrogenation of fructose is
independent of catalyst concentration. The differences in the ratio h3/h2are well within the error range. Although each constant increases with an increase in catalyst concentration, the relative increase is the same so that the same ratio is retained. For a given set of conditions the rate of reaction of glucose-fructose mixtures is slower than that of fructose alone. T o obtain similar rates, it is necessary to increase the ruthenium concentration from 1 to 3% (Table 11). Glucose is probably adsorbed more strongly than fructose and thus inhibits the catalyst. The linear variation of the rate with catalyst loading, and its independence of the rate of agitation, permits us to assume that the reaction is controlled by the chemical steps. Temperature. In the experimental range of this work all reaction rate constants were found to follow Arrhenius' law. For glucose hydrogenation at 600 psig the activation energy was 4500 cal/mol with 3% Raney nickel and 16 800 cal/mol with 3% ruthenium (Figure 5 ) . The value for Raney nickel is close to that of 5200 cal/mol reported by Brahme et al. (1974), and that with ruthenium is of the same order (13840 cal/mol) as that for xylose hydrogenation (Wisniak et al., 1974b). For fructose hydrogenation a t 700 psig and 1% the energy of activation was 7800 cal/mol. This value is well within the range 4500-7500 cal/mol reported by Sokolskii et al. (1974) for several aliphatic ketones. The small value of the activation energy with Raney nickel would normally suggest the presence of diffusion effects, but this is not consistent with the results of agitation rate and catalyst loading (Figure 3). Table IV reports the variation of selectivity u and percent mannitol for hydrogenation of fructose at different
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979 53
Table 11. Reaction Rate Constant, Eq 1 catalyst,
pressure, psig
%
ruthenium
200 200 300 400 600 200 300 600 400 100 200 50 800 200 800 700 500 600 900 1000
1 1 1 1 1 3 3 3 3 3 3 3 3 3
3 3 3 3 3 3 3 3 3 3 3 3 5
ruthenium
1000 1000 800 600 550 400 200 200 400 800 200 200 400 600 800 200 600 800 700 400 800 500 100 300 400
1 1 1 1 1 1 1 1 1 1 1 1 1 1
temp, "C 100 100 100 100 100 100 100 100 100 100 100 100 100 130 100 100 100 100 100 100 130 85 85 130 85 130 85 130 85 130 100
100 100 100 100 130 130 130 130 130 130 130 100 130 85
k . 1O3/ 2.30 min-'
catalyst,
pressure, psig
%
A. Hydrogenation of Glucose, G 2.24 ruthenium 2.24 3.36 rhodium 3.36 3.48 9.2 10.2 Raney nickel 14.5 11.8 5.1 8.3 3.25 13 15.8 14 14.2 13.7 13.4 12.6 12.3 36.5 4.7 5.25 37.5 5.15 29 2.58 15.8 44 48 9.6 B. Hydrogenation of Fructose, F ruthenium 11.8 26.0 31.1 40.5 14.2 41.0 Raney nickel 89.0 70.0 35.0 85.0 38.5 8.2 21.0 10.8
7 9 3 7 9 9 3
temp, "C
100
2QO 200 200 200 200 700
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 2 4
100 200 400 600 800 100 100 200 240 400 400 600 8013 100 200 4pO 660 600 800 800 400 400 400
1 0.5 2 3 1 1 2 2 3 3 3 3 3
400 200 200 200 200 7 00 200 200 400 800 600 600 600
3
100 100 100 100 130 85 85 85 85 85 100 100 109 100 10'0 100 100 100 13d 130 130 130 i 30 130 130
ratio G/F
1:l 1:l 1:l 1:l 1:l 1:l 1:l
%
1 1 1 3 3 3 3
100 100 100 100 100 100 100
0 0 0 7.5 19.7 25.0 28.6
Table 111. Effect of Catalyst Loading on the Ratio k,/k, T = 100 "C P = 200 psig % Ru
kJk,= u
0.5 1.15
1 1.07
2 1.12
3 1.10
pressures and temperatures. The values listed indicate that the selectivity, and thus the amount of mannitol, increases with an increase in temperature. Pressure. The behavior of the reaction constant with hydrogenation pressure for the different sugars appear in
3 3 3 3 3 3
1:l 1:l 1:l 1:l 0.5: 1 2:l
0 0
100 100
85 100 100 100 100 85 130 100 130 130 100 100 100
11.8 39.0 27.5 45.0 11.8 22 0 0 0 0 16 16 4.3
C. Hydrogenation of Glucose-Fructose Mixtures. (Ruthenium Catalyst) -~ presk lo'/ cataprestemp, 2.30 sure, ratio temp, lyst, sure, Dsie "C min-l G/F % Dsie "C
200 200 800 200 400 800 600
13.7 16 0 0 1.68 2.6 3.6 4.01 4.01 1.86 2.05 2.67 2.80 3.04 3.68 4.15 4.33 2.97 3.86 6.38 7.8 8.05 1j.3 11.7 1.5 2.68 4.2
1q0
~~
catalyst,
k.103/ 2.30 min-l
~
130 130 130 130 100 100
200 600 400 800 400 400
~~~
k.1O3/ 2.30 min-' 31.0 34.6 31.0 29.0 25.4 8.5
Table IV. Selectivity of Fructose Hydrogenation-Temperature Effect temp, "C
200
85 k J k ,
%M
100 k ~ k
%M 130 k j h ,
%M
-
300
-
pressure, psig 400 500 600
0.98 49 , 1.07 1-04 52 51.5 1.3 1.35 1.17 1.24 57 57.5 54 55.5
-
-
-
700
800
1 50
-
-
1.08 1.08 52 - 52 1.28 1.3 1.25 56.8 57 56
54
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
-
7 -
0
-
x
j-
1
3L-0
I
I
l
HYDROGEN
W
i
30C 4CC
'00 23C
5Ci
l
533
7CC
am
PRESSURE, P S I A
2-
F LL Q
F i g u r e 9. Effect of hydrogen pressure. Glucoso/fructose = 1:l.
1 -
O
I
0
L
-
100
I
200
,
1
500 603
300 4 0 i
703 330 N
HYDROGEN P R E S S U R E , P 5 I A
>
F i g u r e 6. Effect of hydrogen pressure. Glucose-Raney nickel.
5
-33
1
16
c
I 0
L
0
5
L 1
,"
----.-Ai 1
5
2
2
5
3
0
2
MOLAR R A T I O GLUCOSE/FRUCTOSE
F i g u r e 10. Effect of glucose-fructose ratio. Table V . Hydrogenation of 1:l Mixture of Glucose a n d Fructose pressure, psig
temp, "C 600
400
200
0
HYDROGEN
800
PRESSURE, P
1000
1200
F i g u r e 7. Effect of hydrogen pressure. Glucose-ruthenium.
q
L_---L--.-IA 0
100
200
302
HYDROGEN
4C3
500
100
SIA
---
630 7 3 3
3CC
PRESSURE, P S I A
F i g u r e 8. Effect of hydrogen pressure. Fructose-ruthenium.
Figures 6-9. It can be seen that there are striking differences according to the pair sugar-catalyst. For glucose with Raney nickel at 85 and 100 "C the increase in rate is limited, while a t 130 " C there is a sudden increase in the rate a t pressures above 600 psig (Figure 6); a similar behavior is observed with fructose-ruthenium (Figure 8). T h e rapid increase at higher pressures has also been observed in the hydrogenation of xylose with Raney nickel a t 125 "C (Wisniak e t al., 1974a). Glucose and glucosefructose mixtures in the molar ratio 1:l exhibit a normal behavior (Figures 7 and 9).
130
h,/h,
%M k3/k2 %M
200
400
600
800
1.45 29.4 1.5 30
1.8 32.1 1.8 32.1
2.2 34.5 2 33.3
1.45 29.5 1.75 31.8
Table IV shows that the selectivity in fructose hydrogenation is not affected by changes in pressure. This is to be expected because the ratio h3h, should be essentially a function of temperature alone. Calculation of the selectivity in the hydrogenation of mixtures of glucose and fructose was simplified by the fact that fructose would totally disappear before the mixture was completely hydrogenated. Thus could be calculated from the initial and final compositions if it was assumed that it depended only on the temperature. Pertinent results are given in Table V. The amount of mannitol produced increased with an increase in the operating pressure and reached a maximum value a t about 600 psig. Ratio Glucose/Fructose. Most of the runs with mixtures of glucose and fructose were done a t the constant molar ratio 1:1,that is, the ratio present in inverted sugar. This mixture hydrogenated a t a slower rate than pure glucose (Table 11). The rate decreased for ratios larger than 1:l and increased for ratios smaller than 1:l (Figure 10). The ratio also affected the selectivity of the reaction. Addition of glucose to the mixture increased the amount of mannitol produced (increase in selectivity). From the results obtained on the hydrogenation of fructose it should have been expected that mannitol would be present to the tone of 25-26 % , but Table V shows that the content grows to 29-34%. The common product sorbitol is probably the cause of this behavior (Le Chatelier's principle). Mechanism of the Reaction. The values of the reaction constants that appear in Figures 1-9 suggest the following conclusions.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
1. In the pressure range studied the gas phase is essentially hydrogen so that its mass transfer resistances may be neglected. 2. Mass transfer resistances in the liquid phase can be assumed negligible based on the fact that the rate of reaction is independent of the agitation regime and varies linearly with catalyst loading. The controlling step can be then assumed to be a chemical one and the experimental results can be further interpreted by microscopic kinetic models, such as those developed by Langmuir-Hinshelwood and detailed by Hougen and Watson (1947). In furthering this method use will be made of the experimental fact that hydrogen solubility in aqueous sugar solutions follows Henry's law up to very high pressures (Brahme et al., 1976; Wisniak et al., 1974~).Hydrogen pressure in the gas phase can then be assumed to be a direct and proportional measurement of hydrogen concentration a t the catalyst surface. In addition, it will be assumed that hydrogen is preferentially adsorbed in an atomic form (Smith, 1948; Lapujoulade, 1972; Csuros et al., 1961). T h e possible mechanisms can then involve only adsorption, desorption, and/or surface reaction steps as the controlling stage. Further examination of the experimental data permits their screening as follows. 3. The adsorption of hydrogen cannot control the process as this would require that the rate vary proportionally to hydrogen pressure and inversely proportional to the reactant concentration. This requirement is not supported by the data illustrated in Figures 1, 6-9. 4. The adsorption of reacting sugar does not control the rate as this would require a decrease in rate with an increase in hydrogen pressure. Figures 6-9 contradict this condition. 5 . The adsorption of the product cannot be rate controlling as this would require the rate to increase to a constant value when the hydrogen pressure and/or reacting sugar concentration are increased. 6. The rate-controlling step is not the collision of one unadsorbed species on the other adsorbed. This would require that the rate be first order in the hydrogen pressure, which is not the case. The Langmuir-Hinshelwood treatment limits then the possible mechanisms to those involving control by the surface reaction. For glucose and Raney nickel it can be assumed that the controlling step is the surface reaction between atomic chemisorbed hydrogen and unadsorbed glucose. The rate equation is
c
c32
335
304
l/
339
55
3
6
Figure 11. Test of eq 16 for glucose-Raney nickel.
c5
L-
-
-1035
-0002
32 -
-3001
100005
302
-
~00002
TEMPERATURS', 1 0 3 / ~o
~
l
Figure 12. Arrhenius equations for KH and k ' X , (eq 16).
2
Z L
s
1
33
dL2
753
L4
3'
3
D
c
i
1
1G
Figure 13. Test of eq 16 for fructose-ruthenium
Combination of eq 1 and 14 and neglecting the adsorption of the alcohol gives
kl=
k'XtKHP (1 +
.t(,p!'
Equation 15 can be linearized as follows
Equation 16 is the equation of a straight line and can be tested with the experimental data as shown in Figure 11. It is seen that there is a good fit for the three temperature levels (at 130 "C the fit is appropriate up to 600
psig). The intercept and the slope of the straight lines permit evaluation of the different kinetic constants at the proper temperature (Figure 11). The calculated constants were fitted with an Arrhenius type equation and yielded 15 500 cal/mol as the activation energy for hydrogen adsorption and 4400 cal/mol for the surface reaction (Figure 12). No mechanism is suggested for hydrogenation at 130 "C and pressures above 600 psig; the reaction is too fast (full conversion within 10 min) and it is probably not controlled by chemical steps. The same mechanisms were tested successfully for hydrogenation of fructose with ruthenium. The respective results are given in Figure 13. For the hydrogenation of glucose with ruthenium catalyst the kinetic data indicated that the rate goes through a maximum value with an increase in hydrogen pressure. According to the Langmuir-Hinshelwood model, this sort
56
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
-
KH 3% R u
+
E=-15000 c a l j m o l
05
02 40;
X
/
r
x
0001
01
1
36
005
-
24 20
ioooo2
002
f+ '
0
00005
1
'
4
1
'
"
~
'
16
12
8
'
"
20
'
24
~
'
28
'
*
'
36
32
TEMPERATURf', lo3/ T'K-'
Figure 16. Arrhenius equation for K H and k'X,(eq 16).
Figure 14. Test of eq 19 for glucose-ruthenium. .
0
Table VI. Simulation of Invert Sugar Hydrogenation (100 "C, 800 psig, 3%wt Ruthenium) reactmannitol ion sugar concn concna sorbitol concnb time, min exptl theor exptl theor exptl theor
2 3% R U
+
14
30%
-
0 5 10 15 20 25
12 -
aS-S,.
I
1.43 0.86 0.76 0.62 0.35 0.21
1.43 0.94 0.64 0.45 0.33 0.24
0 0.15 0.23 0.28 0.28 0.28
0 0.15 0.23 0.27 0.29 0.30
0 0.52 0.63 0.74 0.83 1.0
0 0.32 0.54 0.69 0.80 0.88
bM-M,.
Table VII. Simulation of Invert Sugar Hydrogenation. Effect of Pressure and Temperature pressure, psig 0
02
04
06
08
'2
200
14
400
600
800
temp, "C 1 1 F
100 130 100 130 100 130 100 130
Figure 15. Test of eq 16 for glucose-ruthenium.
of behavior is possible only for the surface reaction between atomic chemisorbed hydrogen and adsorbed glucose.
Assuming that glucose and sorbitol are weakly adsorbed we get
k =
k'X,2KHK$ (1 + V z T 3 1 3
(18)
or
For this mechanism to be correct a plot of (P/k)'l3 against (P)'12should give a straight line. The pertinent results appear in Figure 14 and show that this is not true. Each isotherm may be considered to be composed of two straight lines that intercept at a pressure of about 50ct600 psig, which corresponds approximately to the maximum value in the curve k / P . A possible explanation of this behavior is that two mechanisms are present and that each controls the rate in a different pressure range (before and
k , ~ 1 0 2 , m i n - '0.64 3.6 3.5 4.1 4.8 6.1 k,.102,min-' 0.78 9.8 9.5 7.1 4.6 7.2
5.6 5.2 5.8 7.7
after the maximum value of k ) . If it is assumed that for pressures below 500-600 psig the controlling mechanism is the one involving the surface reaction between atomic chemisorbed hydrogen and unadsorbed glucose, then eq 14-16 are valid and can be tested successfully as shown in Figures 15 and 16. In this case, the activation energies are 20 000 cal/mol for hydrogen adsorption and 15 000 cal/mol for the surface reaction. Simulation of Hydrogenation in Glucose-Fructose Mixtures. The hypothesis that in the hydrogenation of glucose-fructose mixtures (1:l)each sugar reacts according to a first-order reaction was tested by writing an optimization computer simulation program based on eq 9, 12, and 13 and finding the appropriate values of k , and k2. Table VI gives some typical results for a run a t 100 O C , 800 psig, and 3% ruthenium. The values of kl and k 2 for different pressures and temperatures appear in Table VII. Examination of these results shows that the rate constant for glucose ( k , ) a t 100 "C increases with an increase in pressure while at 130 "C it goes through a maximum value a t about 600 psig, which is in accordance with the experimental evidence for glucose alone. A similar behavior is observed for the hydrogenation of fructose. A comparison of the values of k l and kz that appear in Tables
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979 57
Table VIII. Simulation of Invert Sugar Hydrogenation. Influence of the Glucose/Fructose Ratio ( G / F ) GIF ~
0.5:l k , . 1 0 ' 2 , min-' k 2 . 1 0 - 2 min-' ,
5.4 3
1:l 3.3 3.5
2: 1 2.4 3.65
X,= total concentration of active sites Subscripts 0 = initial 1, G = glucose 2, S = sorbitol 3, M = mannitol H = hydrogen
I1 and VI1 indicates that mixtures of glucose-fructose
Literature Cited
hydrogenate a t a slower rate than each reagent alone. Significant information was obtained from simulating the influence of the ratio glucose/fructose (Table VIII). I t can be seen that the more fructose present in the solution the larger the value of k l (hydrogenation of glucose); as more mannitol is present glucose tends to produce more sorbitol (Le Chatelier's principle). On the other hand, the value of k 2 decreases with an increase in the ratio glucose/fructose.
Boyers, G. G..U.S. Patent 2 868 847 (1959). Brahme, P. H., Doraiswamy, L. K., Ind. Eng. Chem. Process Des. Dev., 15, 130 (1976). Brahme, P. H., Pai, M. U., Narsimhan, G., Brit. Chem. Eng., 9(10), 685 (1964). Csuros, A., Petro, J., Holly, S., Acta Chim. (Budapest), 29, 351 (1961). Hougen, 0. A,, Watson, K. M., "Chemical Process Principles", pp 947-949, Wiley, New York, N.Y., 1947. Inglett, G. E., J . Toxicol. Environ. Health, 2, 207 (1976). Inglett, G. E., "Recent Sweetener Research", 2nd ed,Botanicals, P.O. Box 3034, Peoria, Ill., 1971. Lapujoulade, J.. "Hydrogen in Metals", International Congress, Paris, 1972. Patchornik, A., Doctoral Thesis, The Hebrew University of Jerusalem, Jerusalem, 1956. Rabinowitz, M. P., J . Pharm. Sei., 63, 1601 (1974). Smith, D. P., "Hydrogen in Metals", The University of Chicago Press, Chicago, Ill.. 1948. Sokolskii, D. V., Sokoiskaya, A. M., Lyashenko, A . I., Byabinina, S. A,, Dokl. Akad. Nauk SSR, 219(2), 400 (1974). Van Ling, G., Blugter, J. C., J . Appl. Chem., 19, 43 (1969). Weiss, A. H., Tambawala, J., J . Chromatogr. Sci., I O , 120 (1972). Wisniak, J., Hershkowitz, M., Liebowitz, R., Stein, S., Ind. fng. Chem. Prod. Res. Dev., 13, 75 (1974a). Wisniak, J., Hershkowitz, M., Stein, S., Ind. Eng. Chem. Prod. Res. Dev., 13. 232 (1974b). Wisniak, J., Hershkovitz, M., Leibowitz R., Stein, S.. J . Chem. f n g . Data. 19, 247 ( 1 9 7 4 ~ ) . Wisniak, J., Hershkovitz, M., Roffe, D., Smilovitz, S.,Ind. Eng. Chem. Prod. Res. Dev., 13, 163 (1976). Wright, L. W., CHfMTECH, 4, 45 (1974).
Nomenclature (I = concentration of sugar F = concentration of fructose G = concentration of glucose k = reaction rate constant, min-' k ' = surface reaction constant, min KG = adsorption constant of glucose KH = adsorption constant of atomic hydrogen K s = adsorption constant of sorbitol M = concentration of mannitol 1' = pressure r = rate of reaction S = concentration of sorbitol t = time, min
Received for recieu' March 1 3 , 1978 Accepted October 9, 1978
Polyelectrolyte Flocculation of Oil-Water Emulsions Nicholas D. Sylvester,* John J. Byeseda, and Baheru Yadeta Chemical Engineering Department, Resources Engineering Division, University of Tulsa, Tulsa, Oklahoma 74 104
The relative effectiveness of several polyelectrolytes (one nonionic, eight cationic, and three anionic) as flocculants for oil-water emulsions prepared from a petroleum sulfonate stabilized paraffinic oil in 1% NaCl solution was evaluated. I t was found that the best of the polyelectrolytes was capable of reducing turbidity by approximately 20 to 30% at its optimum dosage.
Introduction Impurity substances in water can vary in particle size from a few nanometers to several hundred micrometers (Le., to cm). Since these micro-substances are too small for effective gravitational separation, chemical alteration of their surface properties has been advantageously used to promote their aggregation and eventual separation from turbid water. Both the adsorption of the chemicals (polymers) to the particle surface and the bridging mechanism resulting in particle-polymer-particle formation are felt to contribute to the processes of flocculation promoting turbidity removal. Some of the most common flocculating agents have been the sulfates and/or chlorides of aluminum and iron, and to a lesser extent, bentonites and activated silica. More recently, however, long chain, high molecular weight polyelectrolytes, which 00 19-7890/79/1218-0057$01 .OO/O
possess ionizable sites along their chain length, have rapidly increased in application as efficient flocculants. I t is important to note that the complete separation of most liquid-liquid dispersions is a two-step process. The initial droplets first form multiple droplets (aggregation and flocculation). Then these droplets combine into a single larger droplet (coalescence). However, interfacial material may inhibit the flocculation process by presenting an electrostatic or mechanical barrier to droplets approaching within the range of van der Waals forces. T h e coalescence step is also affected by the interfacial material. Both flocculation and coalescence are dynamic processes and, since they generally occur in series, the slower process will be rate-determining for the overall phase separation. Polymeric flocculants are generally used in small dosages in liquid form. T h e efficiency of a particular polymer
0 1979 American Chemical Society
