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A new technique to measure the deactivation rate of edible oil catalysts due to sulphur poisoning
117
Catalysis Today, I1 .( 1991) 117-128 Elsevier Science Publishers B.V., Amsterdam
A NEW TECHNIQUE TO MEASURE THE DEACTIVATION RATE OF EDIBLE OIL CATALYSTS DUE TO SULPHUR POISONING W.T. Koetsiei,
C.M. Lok! and R. Rtitjesb
’ Unilever Research Vlaardingen, Olivier van Noortln 120, Nl-3133 AT Vlaardingen, and b Unichema International, Steintor 9 , D-4240 Emmerich Abstract A new experimental set up has been applied to accurately monitor the Hz absorption rate in edible oil hydrogenation by using a Peteric Pressflow Controller. This technique was used to determine the deleterious effect on the rate of hydrogenation of a S-component, namely dibutyl sulphide, added to soybean oil in amounts of 0.5, 1, 2 and 4 times the amount required to poison the Ni surface with one S per two Ni atoms. This new technique proved to be a valuable method which made it possible to follow the iodine value as a function of time and to calculate the reduction in the hydrogenation rate accurately. The turnover numbers calculated for the initial rates and based on two different, estimated, values for the effective Hz diffusion coefficient were about a factor 5 to 8 times lower due to the presence of sulphur.
l.INTRODUCTION The poisoning of an edible oil catalyst during hydrogenation of a fish oil can be caused by a number of contaminants, such as sulphur and phosphorus components. For example, the hydrogenation rate of a fish oil containing 12 wppm S has been illustrated in fig 1 for two levels of Ni. 100,
1
_0
20
00
HYDROQ&ATION i&EIN MIN
loo
Figure 1 : The change in the refractive index as a function of the hyrogenation time for a hydrogenation experiment at 180°C of a soybean oil and a fish oil. 092~5861/91/$04.20
0 199 1 Elsevier Science Publishers B.V. All rights reserved.
118 This rate is about equal to the rate observed for a soybean oil hydrogenation if at least 4 times more Ni is used. This lower rate causes that a considerable higher Ni consumption has to be used, if the same hydrogenation time has to be achieved as required for the hydrogenation of a soybean oil. Moreover, in case the poisoning is caused by S, then it has also an effect on the selectivity. Sulphided nickel is more selective in the hydrogenation of diolefins versus mono-olefms. In addition, it enhances the cis/trans isomerisation reaction. The poisoning effect caused by Cl, S, P and N has been studied in fatty acid hydrogenation by Klimmek’. Chlorine was the most effective poison. An amount of 10 wppm on 100 wppm Ni was already dramatic. The Iodine Value dropped only 10 points instead of 50. The same occurred with 25 wppm S and about 80 wppm P. The deleterious effect of nitrogen was much less. The data presented in his paper indicate that this poisoning reaction is a first order process with a time constant of 30 min. Edible oils contain sulphur compounds of a different nature. Cristensen* has identified some of the sulphur compounds in fish oil: - 20 wt% were volatile components consisting of sulphides and lower aliphatic thiol esters; - 55 wt% consisted of polar substances with a molecular weight of less than 700; and, - 25 wt% was unidentified.’ The poisoning of a Ni surface by S has been studied by many authorr?~4J~6~7~**9. In general, it can be concluded the extent of poisoning to depend on the severity of the conditions, the type and the concentration of this S component. Usually mild conditions cause a S coverage of the free Ni surface of one S atom per two Ni atoms. It has been the objective of this study to determine the effect of only sulphur on the hydrogenation rate of a clean oil, e.g. soybean oil in order to demonstrate the value of this new technique, For this purpose we used a simple S-containing component, viz. dibuthyl sulphide.
2.EXPERIMENTAL In the past it has been a common practice to determine the rate of an edible oil hydrogenation by taking liquid samples out of the batch reactor at constant time intervals of 15 min to 30 min and analysing the iodine value or, alternatively, the refractive index. For example, the data given in fig 1 have been obtained by using this method. We are now using a new method which does not require taking liquid samples, but which is based on the continuous analysis of the total amount of hydrogen absorbed during this hydrogenation process. This continuous monitoring of the total amount of hydrogen makes it possible to calculate the iodine value at considerable smaller time intervals. Moreover, it is also possible to accurately calculate the hydrogenation rate at these same time intervals. The experiments were performed in a 0.4 litre Medimex autoclave and using 0.2 litre of oil. This autoclave was continuously well stirred and supplied with hydrogen to keep a constant pressure in the system. This hydrogen supply rate could be continuously monitored using a Press Flow Controller sold by Peteric Ltd and this whole unit was operated through a computer system. This technique enabled us to follow the change in the iodiie value in time and, hence, it is a method to make it possible to calculate with good accuracy the hydrogenation rate (being the derivative) as a function of the iodine value. Moreover, it is less time consuming, because it can be fully automated.
119 3.INTERPRiiTATION
OF
DATA
Inorckrto understandatw~h~concentrationthehydrogenation oftheoilandthe desulphurisation of the sulphur component occurs it is mquired to calcuhtte the hydiogen concentration at the particleinterl%ceand to c&ulate the Thiele modulus using an estimated value for the effective axon uxfficient of I!&.
3.1 Hydrogen concentration at the IiquWparticie interface In the activity teat H, is continuously transferred from the gas phase via the bulk liquid phase to the catalyst particles where it reacts with the unsaturated bonds on the nickel is determined by the temperature and the surface. The maximum Hz concentration, C-, pressure in the reactor. The higher the temperature, the better dissolves Ii2 in oils. At the temperanne of lsO”C and a pressure of 2 bar which we used in our experiments the maximum solubility is 7.2 moWm3. The rate of this absorption process is determined by the overall driving force, C,,,, and a series of resistances. These resistances are of a chemical and physical nature. The effect of these various resistances on the hydrogen concentration distribution as present in between the gas/liquid interface and the centre of the catalyst particle has been depicted qualitatively in the picture given below. Near the gas/liquid interface is a small zone, usually not more than 10 to 100 microns thick, where the concentration drops from its maximumvalue to the level which is present in the bulk of the liquid phase. The mass transport through this zone is the result of diffusion as well as convection due to the presence of eddies in the liquid near the interface. gas phase
liquid phase
catalyst particle
<___~~~~~~~~~~~_~~~~~~>
hydrogen concentration distribution in bulk liquid phase
<--------
d
-_---_-_-_>
particle 'diameter; hydrogen concentration at which reaction occurs
Figure 2 : HYDROGEN CONCENTRATION DISTRIRUTION DEPICTED QUALITATIVELY The equation which describes the mass transfer rate of H, from the gas/liquid interface to the bulk of the liquid is given by:
JX2 = k6Lso( G~,m9x - Gmulk 1 vail
mole Hz/s
(11
120 In the bulk of the liquid phase this rate of transport is the result of convection only and it is so good that the concentration is the same everywhere throughout the whole reactor volume. The H, concentration drops again near the surface of the catalyst particles. The thickness of this layer is also not more than 10 to 100 micron. And in this case the mass transfer rate from the bulk of the liquid to the surface of the catalyst particles is given by:
JH.2= L& ( G,w - ‘&,i 1 Vain
mole Hz/s
(2)
In the two above given equations the parameters ku..Land kr_s are the partial mass transfer coefficients for the gas/liquid and liquid/solid interface, respectively. The parameters S, and S, are the interfacial areas in mz per unit liquid volume for the gas and solid phase, respectively. The two fractions l/k&& and l/kr_& are the partial resistances for the mass transfer of Hz at these two interfaces. The parameter S, is of particular interest to us, because we want to study the effects occurring at the liquid/solid interface. This parameter can be correlated to the mean particle diameter dj.* (the so-called Sauter diameter) by using the following three equations:
sp= ‘b ~a8
mz cat/m’ oil
and 4 = 6&z in which
m* cat/m’ cat
(4)
E, = v,,N,
By elimination of the unknown hydrogen bulk concentration from equations 1 and 2 an expression is obtained from which the inter&e concentration Crui can be calculated:
in which Sh is k,d,,/D,, 4.i is defined by: C d(i)‘/C d(i) and D, is the liquid phase diffusion coefficient of H,. It may be assumed that for particles in the range of 1 to 10 micron this Sherwood number is a constant, According to Brian and Halesi it will have a value of 3 to 4. Once the hydrogen molecules have arrived at the outer surface of the catalyst particles the transport into the particles is caused by diffusion only.
3.2. Reaction within the particle; in the absence of sulphur The other factor, which affects the overall rate of our absorption process., are the chemical reactions occurring at the Ni surface. In fact, there are several reactions, such as hydrogenation of double bonds and isomerisation of double bonds to other positions within the chain and to the trans isomers. For our analysis we consider only the hydrogenation reactions. It is the hydrogenation of unsaturated -C=C- bonds, which are present in fatty acid chains of different length and as mono- and poly-olefinic bonds. Each individual double bond will be hydrogenated at an intrinsic rate which is characteristic for that particular component in which that double bond is present. Di-olefinic bonds hydrogenate much faster than mono-olefinic bonds, but there is certainly also an effect on the position of these bonds within the molecule.
121
A rigorous analysis requires to take all these et%& into account. Such an snalysis is too time consuming,because it demandsa detailed componentanalysis. It is a custom to ose a Imped kinetic model, namely to measure only tha iodinevale and the rate it dropsin time. At most, bonds are being saturated is the two different rates at which poly- verses ~lefi~c accounted for. We will limit our analysis by using just ona simple equation, namely: %-
(mole Hr/m3cat/s}
4sN1(1GZ,itf
et
in which Sxi is the Ni surke area in m2/gram Ni, q gmm Ni/m3cat and Q is the affectivity factor which is a fun&n of the Thiele mod&s # as de&red by Cka&” :
Equatiou (‘7)is bssed on the assumptionthat the hydrogeuationreaction is 1“ order in Hz (the concentrationof which is about 2 mole/n?) and zero order in oil molecules (the concentration of which is about 500 mole/m3). The hydrogen Sow into the sphere is derived in Qankl* as: mole Hr/m3cat/s This equation can be used to calculatethe Thiele modulus4 by using an estimatedvalue of the effective diffusion coefficient, for example we have assumedthat Doffis in between0.3 D, and 0.1 D,. For example, Colen et al” were able to fit their &ta obtained at 1OOCby using an ef%ctive diflksion coefficient of 0.2 D,. If, also in the above equations 7 and 6 the hy~ogen ~~en~tion C& is eIimiM~d, then the following general equation is obtained: 1
GlZ.-=:
A
+
B*-
(IO) Eclr
Jn2~oi,
This is the classical equation which has been used by many authors to Euler parameters A and B which both have the cation of seconds:
1 A=-
and k+L%
B
=
$
+
Bz =
4.1 + 6 Sh D,
1 k %i
the two
(11) 4 1
The ex~rimen~l ~~~i~tion of A and B can be done accurately, if Jm is ~~~i~d for various vahtes of C~ and ~/J~~~ is plotted on the Y-axes vs l& on the X-axes. Once the value of B, has been determined, it is possibIe to calculate the l&&c constant k, and the turnover number, which is defined as the number of molecules of y converted per surface nickel atom and per unit of time.
3.3 Reaction wIthIn the particle;in the presence of sulphur In the presence of a sulky component the Ni surface will be covered with a layer of sulphur atoms, namely one atom per two nickel atoms. The rate at which this nickel surface will be suiphided is determinedby the type of sulphur component, the concentration of this component, the hydrogen concentration, the temperature, and the rate at which the sulphur component can be transferred to the catalyst particles.
122
The hydrogenation rate of the oil molecules can now be described by the following equation: mole H,/m’cat/s
(12)
usually the rate on a sulphided nickel surface is much lower than on a clean surface, therefore, n is expected to be about one. The rate at which the Ni surface is sulphided requires a monitoring of the change in the S concentration in the oil. This cannot be done accurately yet.
4.RESULTS In our program we have completed a limited number of experiments by using only three different catalyst samples with a pore size, calculated as two times the pore volume divided by the BET surface, ranging between 2.5 nm and 10 nm, namely: Catalyst X; a wide pore (10 run) Ni-catalyst with a Sauter mean diameter of 3.76 microns; CataZysf E a medium pore (5 run) Ni-catalyst with a Sauter mean diameter of 6.65 microns, and Cutdyst 2; a narrow pore (2.5 nm) Ni-catalyst with a Sauter mean diameter of 3.96 microns. In addition to our experiments without using sulphur, soybean oils containing 4 different sulphur levels, namely 2.5 wppm, 5 wppm, 10 wppm and 20 wppm have been used. All the relevant data have been summarised in table 1 and the fig 3 through 9. Based on the Hz chemisorption analysis of these three catalysts and the observation made by other scientists that per two nickel atoms only one S-atom adsorbs onto the nickel surface, we calculated that, respectively, for catalyst X 5.2 wppm S, for catalyst Y 6.1 wppm S and for catalyst Z 5.6 wppm S, could be adsorbed. All experiments have been performed in the same reactor at a temperature of 180°C and at a pressure of 2 bar. From a first set of two experiments we determined the value of the time constant A, defined in equation 11. The value of the time constant Bi has been calculated from the mean diameter d,,, and the Sh = 3.5. In these experiments the amount of Hz added to keep a constant pressure in the system was measured every 4 minutes. Therefore, this new technique enabled us to calculate the iodine value, IV, accurately as a function of time. The results of our experiments using this technique will be summarised in the paragraphs 4.2 through 4.4 which have been given below. 4.1 Rate at which S is transferred from the oil to the catalyst In two experiments oil samples to analyze the S-content were taken immediately upon addition of the sulphur component to the oil in the reactor and at intervals of 25 min. In figure 3 the result has been plotted of these S-in-oil analyses. This result reveals that the sulphur has been adsorbed onto the catalyst very quickly. Whether it is a physical adsorption on the Ni surface or a chemical reaction with this surface cannot be concluded from this observation. Only, it can be estimated that the time constant for this process is in the order of 5 min. And after at most 25 min the equilibrium level had almost been obtained. The amount of sulphur adsorbed onto the catalyst is in agreement with the amount expected assuming one S per two surface Ni.
123 WPPM S IN OIL
‘~fg-yzq
catalyotY 2 t "0
20
76
HYDROQENATION
TIME IN MN
26
100
Figure 3 : The concentration of the sulphur in the oil phase as a function of the hydrogenation time. 4.2 Hydrogenation rate in the absence of S In the absence of sulphur the hydrogenation rate appeared to be about equal for the two catalyst samples X and Y which, in fact, is quit unexpected for two reasons: one is the larger particle size and the other is the smaller pore size of catalyst Y. It can be calculated that the fhm at zero time of Hz in mole/m* cat/s for catalyst Y is 2.6 times as observed for catalyst X. For catalyst Z the initial hydrogenation rate is 20% less, but the rate at IV 90 is already 50% lower then observed for X and Y. 4.3 Hydrogenation rate using a strIpbided catalyst Using a different sulphiding procedure we have made a sulphided medium pore catalyst containing one S per two surface Ni atoms. We have measured the hydrogenation rate (data added to fig 4 and 5) under the same conditions as used in all the other experiments for the following two cases: * one in the absence of any S component in the oil. This hydrogenation rate appeared to be a constant rate of 0.09 IV/min or 0.054 mole Hz/m3 oil/s, and * one in the presence of 5 wppm S in the oil. This hydrogenation rate appeared to be a constant rate of 0.065 IV/min or 0.04 mole Hz/m3 oil/s. 4.4 Effect of S on the hydrogenation rate In all cases the addition of the S-component to the soybean oil caused a reduction in the hydrogenation rate. The effect on the hydrogenation rate as a function of the IV depends on the type of catalyst and on the amount of S added. For example, for the lowest S-level the medium pore catalyst is the least sensitive for the sulphur poisoning. The time required to drop the N from 132 to 80 has increased only from 65 min to 115 min, while for the wide pore and small pore catalysts it has increased to 240 min and 280 min, respectively. However, for the highest S level this effect reverses and the wide pore catalyst is the one which is now the least affected. We don’t understand yet as to why at the lower S-levels catalyst Y is so much less affected by this sulphur poisoning. One is certainly the extent of poisoning of the nickel surface, namely 41% for Y and 48% and 45% for X and Z, respectively. The hydrogenation rate as a function of the iodine value appeared not to be a constant as has been observed for the sulphided catalysts, but it decreases in time as can be seen from the
124
THE IODINE VALUE AS A FUNCTION OF THE HYDROGNATION TIME OF A SOYBEAN OIL HYDROGENATION AT 180°C OF THE THREE DIFFERENT NICKEL CATALYSTS
0 datasulphided
Figure 4 : Data for catalyst X
HYDROQENATION
cat
TIME In MN
1 t
1
0
20
40
Figure 5 : Data of catalyst Y
20
lO0
120
HYDROGENATION
70 0
Figure 6 : Data of catalyst Z
20
a 20
time ’ 40
( 20
,
,
,
l40
I20
180 200
TIME In MN
,
,
(
20loO120l4om~200220240
HYDROGENATION
220 240
TME
in MN
,
,
125
THE CALCULATED HYDROGENATION RATE AS A FUNCTION OF THE IODINE VALUE , 2 HYDRDQENATION
CATALm
RATE in DELTA IV I MM
X
0.0
100 &I
t
0’ 00
Figure 7 : Data of catalyst X
loo
110
lw
uo
110
190
l40
lQ0
lQ0
uo
110
IODINE WLUE
-IQ0
90
RATE In DELTA IV / MIN
100
lm
IODINE MLUE
Qo
Figure 9 : Data of catalyst Z
f
I
w
HYDRDQENATION
Figure 8 : Data of catalyst Y
Ni
Qo
lQ0
m IODINE ULUE
126 figures 7, 8 and 9. The hydrogenation rate levels off to a value of 0.04 mole Hz/m3/s, a value which has been observed for all three catalyst samples and it is also the same value which has been measured for the sulphided catalyst in presence of an additional amount of S. The rate at which the the hydrogenation rate decreases is not in line with what one would expect based on the rate the S-component is being absorbed by the nickel surface as has been depicted in figure 3. After 25 minutes the S adsorption seems to be completed, but the hydrogenation rate still continues to decrease and reaches its lowest level only after about 100 min. Why the hydrogenation rate has not become a constant value after 25 min despite the fact the Scontamination has reached a level of one S per two Ni is not yet understood. rABLE 1 : SUMMARY OF DATA CAT X
CAT Y
CAT Z
CATALYST DATA mean diameter d3,s in microns
3.76
6.65
3.96
mean diameter 4.r in microns
2.56
4.93
1.93
SNiin m* Ni I gram Ni q * 1W6in g Ni/m3 catalyst
100 0.8
130 1.15
115 1.51
ABSORPTION RATE DATA ; 0 WPPM S; RATE AT ZERO TIME J&V,
* lo-* at t=O
C!m,iin moles/m3 Thiele modulus 4
if D, = 0.3 D,
0.54
0.78
0.83
1.56
1.3
2.59
1.16
3.4
1.18
turnover number * 10r
4.3
6.5
1.8
Thiele modulus 4
2.35
8.2
2.4
if D, = 0.1 D,
turnover number * l@
5.9
12
ABSORPTION RATE DATA ; 10 WPPM S; RATE AT ZERO TIME
2.5
127 4.5 Effect of S on turnover number for rate at zero time For each hydrogenation experiment it is possible to calculate the concentration at the particle/liquid inter&e at any time by using the equations given in the paragraph 3. The result at t=O of these calculations for the experiments using 0 wppm and 10 wppm S have been given in table 1. The lower the hydrogenation rate, the higher is the interface concentration. For the set of experiments in which 0 and 10 wppm S has been used this interface concentration varies between 1.3 and 2.59 mole/m’ and 4.2 and 5.2 mole/m3, respectively. The turnover numbers and the Thiele moduli have been calculated for two estimated values of the effective diffusion coefficient, namely 0.3 and 0.i times the liquid phase diffision coefficient. In the absence of S the turnover number for the medium pore catalyst is 1.5 to 2 times larger than for the wide pore catalyst. Moreover, if we take into account the effective diffusion coefficient for the medium pore catalyst to be smaller than for the wide pore catalyst, the difference in the turnover numbers for these two catalysts is still higher than reflected by the numbers calculated on the basis of equal effective diffusion coefficient. Apparently, the nickel in the medium pore catalyst is for the hydrogenation of poly-olefinic bonds in fat molecules in terms of activity a much more effective catalyst than the nickel in the wide pore catalyst. Loki3 has determined the selectivity of the wide pore catalyst to be better than for the medium pore catalyst. An observation which supports the fact that the Thiele modulus of the medium pore catalyst is indeed larger than for the wide pore catalyst. An experimental determination of the effective diffusion coefficient should be done to get a better understanding of these observations. The calculated Thiele moduli are just in the range where the hydrogenation rate is affected by the diffusional limitation of hydrogen. In particular, this is the case for catalyst Y. The turnover numbers at t=O for the case of 10 wppm S in the soybean oil are 5 to 8 times smaller. It reflects the inhibiting effect of the S-contamination. Again the’tumover numbers for the medium pore catalyst are 1.5 times larger than the turnover number for the wide pore catalyst. The effect of the difference in the two values of the effective diffusion coefficient exists only for the medium pore catalyst. Apparently, there is hardly any effect of diffusion limitation of hydrogen molecules for the hydrogenation rates of these sulphided catalysts.
5.CONCLUSIONS The new technique to monitor continuously the hydrogen absorption rate proved to be a valuable technique. By using this technique it has been possible to calculate the hydrogenation rate over a wide range of iodine values from 132 to 80 for the hydrogenation of a soybean oil. As a result we were able to follow the inhibiting effect due to small amounts of a sulphur component added to the soybean oil. The medium pore catalysts shows unexpectedly the highest turnover numbers. An observation which is not understood yet. An experimental determination of the effective diffusion coefficient will be very helpful to clarify this observation.
6.REFERENCES 1. 2.
H. Klimmek; JAOCS &I(2), (1984) 200 B.W. Cristensen et al; JAOCS 58(12) (1981) 1053
128 3. 4. 5.
6. 7. 8. 9.
10. 11. 12. 13.
C.J. Duyverman et al; Proceedings IIIth Int. Congmss on Cat., Amsterdam 1964, vol II, 1416 K.H. Boume et al; ibid page 1400 M. Perdereau et al; Surf. Sci. 2Q (1970) 80 Jens R. Rostrup-Nielsen et al; J. Catal. a (1979) 395 J. Gudar; Cat. Rev. a(2) (1980) 171 P.K. Agrawal et al; in proceedings l* Catalyst Deactivation Symposium; Elsevier Scientific Publishing Company (1980) 179 C.H. Bartholomew et al; ibid page 375 P.L.T. Hales et al ; A.1.Ch.E.J. U (1%9) 727 J. Crank, The Mathematics of Diffusion; At the Clarendon Press; Oxford 1957 G.C.M. Colen et al; Proceedings Symposium on Catalyst Performance Testing; Appl. Cat. $j, 1988, 339 C.M. Lok; presentation at the AGCS meeting of May 1991 in Chicago
Catalysis Today, I1 .( 1991) 117-128 Elsevier Science Publishers B.V., Amsterdam
A NEW TECHNIQUE TO MEASURE THE DEACTIVATION RATE OF EDIBLE OIL CATALYSTS DUE TO SULPHUR POISONING W.T. Koetsiei,
C.M. Lok! and R. Rtitjesb
’ Unilever Research Vlaardingen, Olivier van Noortln 120, Nl-3133 AT Vlaardingen, and b Unichema International, Steintor 9 , D-4240 Emmerich Abstract A new experimental set up has been applied to accurately monitor the Hz absorption rate in edible oil hydrogenation by using a Peteric Pressflow Controller. This technique was used to determine the deleterious effect on the rate of hydrogenation of a S-component, namely dibutyl sulphide, added to soybean oil in amounts of 0.5, 1, 2 and 4 times the amount required to poison the Ni surface with one S per two Ni atoms. This new technique proved to be a valuable method which made it possible to follow the iodine value as a function of time and to calculate the reduction in the hydrogenation rate accurately. The turnover numbers calculated for the initial rates and based on two different, estimated, values for the effective Hz diffusion coefficient were about a factor 5 to 8 times lower due to the presence of sulphur.
l.INTRODUCTION The poisoning of an edible oil catalyst during hydrogenation of a fish oil can be caused by a number of contaminants, such as sulphur and phosphorus components. For example, the hydrogenation rate of a fish oil containing 12 wppm S has been illustrated in fig 1 for two levels of Ni. 100,
1
_0
20
00
HYDROQ&ATION i&EIN MIN
loo
Figure 1 : The change in the refractive index as a function of the hyrogenation time for a hydrogenation experiment at 180°C of a soybean oil and a fish oil. 092~5861/91/$04.20
0 199 1 Elsevier Science Publishers B.V. All rights reserved.
118 This rate is about equal to the rate observed for a soybean oil hydrogenation if at least 4 times more Ni is used. This lower rate causes that a considerable higher Ni consumption has to be used, if the same hydrogenation time has to be achieved as required for the hydrogenation of a soybean oil. Moreover, in case the poisoning is caused by S, then it has also an effect on the selectivity. Sulphided nickel is more selective in the hydrogenation of diolefins versus mono-olefms. In addition, it enhances the cis/trans isomerisation reaction. The poisoning effect caused by Cl, S, P and N has been studied in fatty acid hydrogenation by Klimmek’. Chlorine was the most effective poison. An amount of 10 wppm on 100 wppm Ni was already dramatic. The Iodine Value dropped only 10 points instead of 50. The same occurred with 25 wppm S and about 80 wppm P. The deleterious effect of nitrogen was much less. The data presented in his paper indicate that this poisoning reaction is a first order process with a time constant of 30 min. Edible oils contain sulphur compounds of a different nature. Cristensen* has identified some of the sulphur compounds in fish oil: - 20 wt% were volatile components consisting of sulphides and lower aliphatic thiol esters; - 55 wt% consisted of polar substances with a molecular weight of less than 700; and, - 25 wt% was unidentified.’ The poisoning of a Ni surface by S has been studied by many authorr?~4J~6~7~**9. In general, it can be concluded the extent of poisoning to depend on the severity of the conditions, the type and the concentration of this S component. Usually mild conditions cause a S coverage of the free Ni surface of one S atom per two Ni atoms. It has been the objective of this study to determine the effect of only sulphur on the hydrogenation rate of a clean oil, e.g. soybean oil in order to demonstrate the value of this new technique, For this purpose we used a simple S-containing component, viz. dibuthyl sulphide.
2.EXPERIMENTAL In the past it has been a common practice to determine the rate of an edible oil hydrogenation by taking liquid samples out of the batch reactor at constant time intervals of 15 min to 30 min and analysing the iodine value or, alternatively, the refractive index. For example, the data given in fig 1 have been obtained by using this method. We are now using a new method which does not require taking liquid samples, but which is based on the continuous analysis of the total amount of hydrogen absorbed during this hydrogenation process. This continuous monitoring of the total amount of hydrogen makes it possible to calculate the iodine value at considerable smaller time intervals. Moreover, it is also possible to accurately calculate the hydrogenation rate at these same time intervals. The experiments were performed in a 0.4 litre Medimex autoclave and using 0.2 litre of oil. This autoclave was continuously well stirred and supplied with hydrogen to keep a constant pressure in the system. This hydrogen supply rate could be continuously monitored using a Press Flow Controller sold by Peteric Ltd and this whole unit was operated through a computer system. This technique enabled us to follow the change in the iodiie value in time and, hence, it is a method to make it possible to calculate with good accuracy the hydrogenation rate (being the derivative) as a function of the iodine value. Moreover, it is less time consuming, because it can be fully automated.
119 3.INTERPRiiTATION
OF
DATA
Inorckrto understandatw~h~concentrationthehydrogenation oftheoilandthe desulphurisation of the sulphur component occurs it is mquired to calcuhtte the hydiogen concentration at the particleinterl%ceand to c&ulate the Thiele modulus using an estimated value for the effective axon uxfficient of I!&.
3.1 Hydrogen concentration at the IiquWparticie interface In the activity teat H, is continuously transferred from the gas phase via the bulk liquid phase to the catalyst particles where it reacts with the unsaturated bonds on the nickel is determined by the temperature and the surface. The maximum Hz concentration, C-, pressure in the reactor. The higher the temperature, the better dissolves Ii2 in oils. At the temperanne of lsO”C and a pressure of 2 bar which we used in our experiments the maximum solubility is 7.2 moWm3. The rate of this absorption process is determined by the overall driving force, C,,,, and a series of resistances. These resistances are of a chemical and physical nature. The effect of these various resistances on the hydrogen concentration distribution as present in between the gas/liquid interface and the centre of the catalyst particle has been depicted qualitatively in the picture given below. Near the gas/liquid interface is a small zone, usually not more than 10 to 100 microns thick, where the concentration drops from its maximumvalue to the level which is present in the bulk of the liquid phase. The mass transport through this zone is the result of diffusion as well as convection due to the presence of eddies in the liquid near the interface. gas phase
liquid phase
catalyst particle
<___~~~~~~~~~~~_~~~~~~>
hydrogen concentration distribution in bulk liquid phase
<--------
d
-_---_-_-_>
particle 'diameter; hydrogen concentration at which reaction occurs
Figure 2 : HYDROGEN CONCENTRATION DISTRIRUTION DEPICTED QUALITATIVELY The equation which describes the mass transfer rate of H, from the gas/liquid interface to the bulk of the liquid is given by:
JX2 = k6Lso( G~,m9x - Gmulk 1 vail
mole Hz/s
(11
120 In the bulk of the liquid phase this rate of transport is the result of convection only and it is so good that the concentration is the same everywhere throughout the whole reactor volume. The H, concentration drops again near the surface of the catalyst particles. The thickness of this layer is also not more than 10 to 100 micron. And in this case the mass transfer rate from the bulk of the liquid to the surface of the catalyst particles is given by:
JH.2= L& ( G,w - ‘&,i 1 Vain
mole Hz/s
(2)
In the two above given equations the parameters ku..Land kr_s are the partial mass transfer coefficients for the gas/liquid and liquid/solid interface, respectively. The parameters S, and S, are the interfacial areas in mz per unit liquid volume for the gas and solid phase, respectively. The two fractions l/k&& and l/kr_& are the partial resistances for the mass transfer of Hz at these two interfaces. The parameter S, is of particular interest to us, because we want to study the effects occurring at the liquid/solid interface. This parameter can be correlated to the mean particle diameter dj.* (the so-called Sauter diameter) by using the following three equations:
sp= ‘b ~a8
mz cat/m’ oil
and 4 = 6&z in which
m* cat/m’ cat
(4)
E, = v,,N,
By elimination of the unknown hydrogen bulk concentration from equations 1 and 2 an expression is obtained from which the inter&e concentration Crui can be calculated:
in which Sh is k,d,,/D,, 4.i is defined by: C d(i)‘/C d(i) and D, is the liquid phase diffusion coefficient of H,. It may be assumed that for particles in the range of 1 to 10 micron this Sherwood number is a constant, According to Brian and Halesi it will have a value of 3 to 4. Once the hydrogen molecules have arrived at the outer surface of the catalyst particles the transport into the particles is caused by diffusion only.
3.2. Reaction within the particle; in the absence of sulphur The other factor, which affects the overall rate of our absorption process., are the chemical reactions occurring at the Ni surface. In fact, there are several reactions, such as hydrogenation of double bonds and isomerisation of double bonds to other positions within the chain and to the trans isomers. For our analysis we consider only the hydrogenation reactions. It is the hydrogenation of unsaturated -C=C- bonds, which are present in fatty acid chains of different length and as mono- and poly-olefinic bonds. Each individual double bond will be hydrogenated at an intrinsic rate which is characteristic for that particular component in which that double bond is present. Di-olefinic bonds hydrogenate much faster than mono-olefinic bonds, but there is certainly also an effect on the position of these bonds within the molecule.
121
A rigorous analysis requires to take all these et%& into account. Such an snalysis is too time consuming,because it demandsa detailed componentanalysis. It is a custom to ose a Imped kinetic model, namely to measure only tha iodinevale and the rate it dropsin time. At most, bonds are being saturated is the two different rates at which poly- verses ~lefi~c accounted for. We will limit our analysis by using just ona simple equation, namely: %-
(mole Hr/m3cat/s}
4sN1(1GZ,itf
et
in which Sxi is the Ni surke area in m2/gram Ni, q gmm Ni/m3cat and Q is the affectivity factor which is a fun&n of the Thiele mod&s # as de&red by Cka&” :
Equatiou (‘7)is bssed on the assumptionthat the hydrogeuationreaction is 1“ order in Hz (the concentrationof which is about 2 mole/n?) and zero order in oil molecules (the concentration of which is about 500 mole/m3). The hydrogen Sow into the sphere is derived in Qankl* as: mole Hr/m3cat/s This equation can be used to calculatethe Thiele modulus4 by using an estimatedvalue of the effective diffusion coefficient, for example we have assumedthat Doffis in between0.3 D, and 0.1 D,. For example, Colen et al” were able to fit their &ta obtained at 1OOCby using an ef%ctive diflksion coefficient of 0.2 D,. If, also in the above equations 7 and 6 the hy~ogen ~~en~tion C& is eIimiM~d, then the following general equation is obtained: 1
GlZ.-=:
A
+
B*-
(IO) Eclr
Jn2~oi,
This is the classical equation which has been used by many authors to Euler parameters A and B which both have the cation of seconds:
1 A=-
and k+L%
B
=
$
+
Bz =
4.1 + 6 Sh D,
1 k %i
the two
(11) 4 1
The ex~rimen~l ~~~i~tion of A and B can be done accurately, if Jm is ~~~i~d for various vahtes of C~ and ~/J~~~ is plotted on the Y-axes vs l& on the X-axes. Once the value of B, has been determined, it is possibIe to calculate the l&&c constant k, and the turnover number, which is defined as the number of molecules of y converted per surface nickel atom and per unit of time.
3.3 Reaction wIthIn the particle;in the presence of sulphur In the presence of a sulky component the Ni surface will be covered with a layer of sulphur atoms, namely one atom per two nickel atoms. The rate at which this nickel surface will be suiphided is determinedby the type of sulphur component, the concentration of this component, the hydrogen concentration, the temperature, and the rate at which the sulphur component can be transferred to the catalyst particles.
122
The hydrogenation rate of the oil molecules can now be described by the following equation: mole H,/m’cat/s
(12)
usually the rate on a sulphided nickel surface is much lower than on a clean surface, therefore, n is expected to be about one. The rate at which the Ni surface is sulphided requires a monitoring of the change in the S concentration in the oil. This cannot be done accurately yet.
4.RESULTS In our program we have completed a limited number of experiments by using only three different catalyst samples with a pore size, calculated as two times the pore volume divided by the BET surface, ranging between 2.5 nm and 10 nm, namely: Catalyst X; a wide pore (10 run) Ni-catalyst with a Sauter mean diameter of 3.76 microns; CataZysf E a medium pore (5 run) Ni-catalyst with a Sauter mean diameter of 6.65 microns, and Cutdyst 2; a narrow pore (2.5 nm) Ni-catalyst with a Sauter mean diameter of 3.96 microns. In addition to our experiments without using sulphur, soybean oils containing 4 different sulphur levels, namely 2.5 wppm, 5 wppm, 10 wppm and 20 wppm have been used. All the relevant data have been summarised in table 1 and the fig 3 through 9. Based on the Hz chemisorption analysis of these three catalysts and the observation made by other scientists that per two nickel atoms only one S-atom adsorbs onto the nickel surface, we calculated that, respectively, for catalyst X 5.2 wppm S, for catalyst Y 6.1 wppm S and for catalyst Z 5.6 wppm S, could be adsorbed. All experiments have been performed in the same reactor at a temperature of 180°C and at a pressure of 2 bar. From a first set of two experiments we determined the value of the time constant A, defined in equation 11. The value of the time constant Bi has been calculated from the mean diameter d,,, and the Sh = 3.5. In these experiments the amount of Hz added to keep a constant pressure in the system was measured every 4 minutes. Therefore, this new technique enabled us to calculate the iodine value, IV, accurately as a function of time. The results of our experiments using this technique will be summarised in the paragraphs 4.2 through 4.4 which have been given below. 4.1 Rate at which S is transferred from the oil to the catalyst In two experiments oil samples to analyze the S-content were taken immediately upon addition of the sulphur component to the oil in the reactor and at intervals of 25 min. In figure 3 the result has been plotted of these S-in-oil analyses. This result reveals that the sulphur has been adsorbed onto the catalyst very quickly. Whether it is a physical adsorption on the Ni surface or a chemical reaction with this surface cannot be concluded from this observation. Only, it can be estimated that the time constant for this process is in the order of 5 min. And after at most 25 min the equilibrium level had almost been obtained. The amount of sulphur adsorbed onto the catalyst is in agreement with the amount expected assuming one S per two surface Ni.
123 WPPM S IN OIL
‘~fg-yzq
catalyotY 2 t "0
20
76
HYDROQENATION
TIME IN MN
26
100
Figure 3 : The concentration of the sulphur in the oil phase as a function of the hydrogenation time. 4.2 Hydrogenation rate in the absence of S In the absence of sulphur the hydrogenation rate appeared to be about equal for the two catalyst samples X and Y which, in fact, is quit unexpected for two reasons: one is the larger particle size and the other is the smaller pore size of catalyst Y. It can be calculated that the fhm at zero time of Hz in mole/m* cat/s for catalyst Y is 2.6 times as observed for catalyst X. For catalyst Z the initial hydrogenation rate is 20% less, but the rate at IV 90 is already 50% lower then observed for X and Y. 4.3 Hydrogenation rate using a strIpbided catalyst Using a different sulphiding procedure we have made a sulphided medium pore catalyst containing one S per two surface Ni atoms. We have measured the hydrogenation rate (data added to fig 4 and 5) under the same conditions as used in all the other experiments for the following two cases: * one in the absence of any S component in the oil. This hydrogenation rate appeared to be a constant rate of 0.09 IV/min or 0.054 mole Hz/m3 oil/s, and * one in the presence of 5 wppm S in the oil. This hydrogenation rate appeared to be a constant rate of 0.065 IV/min or 0.04 mole Hz/m3 oil/s. 4.4 Effect of S on the hydrogenation rate In all cases the addition of the S-component to the soybean oil caused a reduction in the hydrogenation rate. The effect on the hydrogenation rate as a function of the IV depends on the type of catalyst and on the amount of S added. For example, for the lowest S-level the medium pore catalyst is the least sensitive for the sulphur poisoning. The time required to drop the N from 132 to 80 has increased only from 65 min to 115 min, while for the wide pore and small pore catalysts it has increased to 240 min and 280 min, respectively. However, for the highest S level this effect reverses and the wide pore catalyst is the one which is now the least affected. We don’t understand yet as to why at the lower S-levels catalyst Y is so much less affected by this sulphur poisoning. One is certainly the extent of poisoning of the nickel surface, namely 41% for Y and 48% and 45% for X and Z, respectively. The hydrogenation rate as a function of the iodine value appeared not to be a constant as has been observed for the sulphided catalysts, but it decreases in time as can be seen from the
124
THE IODINE VALUE AS A FUNCTION OF THE HYDROGNATION TIME OF A SOYBEAN OIL HYDROGENATION AT 180°C OF THE THREE DIFFERENT NICKEL CATALYSTS
0 datasulphided
Figure 4 : Data for catalyst X
HYDROQENATION
cat
TIME In MN
1 t
1
0
20
40
Figure 5 : Data of catalyst Y
20
lO0
120
HYDROGENATION
70 0
Figure 6 : Data of catalyst Z
20
a 20
time ’ 40
( 20
,
,
,
l40
I20
180 200
TIME In MN
,
,
(
20loO120l4om~200220240
HYDROGENATION
220 240
TME
in MN
,
,
125
THE CALCULATED HYDROGENATION RATE AS A FUNCTION OF THE IODINE VALUE , 2 HYDRDQENATION
CATALm
RATE in DELTA IV I MM
X
0.0
100 &I
t
0’ 00
Figure 7 : Data of catalyst X
loo
110
lw
uo
110
190
l40
lQ0
lQ0
uo
110
IODINE WLUE
-IQ0
90
RATE In DELTA IV / MIN
100
lm
IODINE MLUE
Qo
Figure 9 : Data of catalyst Z
f
I
w
HYDRDQENATION
Figure 8 : Data of catalyst Y
Ni
Qo
lQ0
m IODINE ULUE
126 figures 7, 8 and 9. The hydrogenation rate levels off to a value of 0.04 mole Hz/m3/s, a value which has been observed for all three catalyst samples and it is also the same value which has been measured for the sulphided catalyst in presence of an additional amount of S. The rate at which the the hydrogenation rate decreases is not in line with what one would expect based on the rate the S-component is being absorbed by the nickel surface as has been depicted in figure 3. After 25 minutes the S adsorption seems to be completed, but the hydrogenation rate still continues to decrease and reaches its lowest level only after about 100 min. Why the hydrogenation rate has not become a constant value after 25 min despite the fact the Scontamination has reached a level of one S per two Ni is not yet understood. rABLE 1 : SUMMARY OF DATA CAT X
CAT Y
CAT Z
CATALYST DATA mean diameter d3,s in microns
3.76
6.65
3.96
mean diameter 4.r in microns
2.56
4.93
1.93
SNiin m* Ni I gram Ni q * 1W6in g Ni/m3 catalyst
100 0.8
130 1.15
115 1.51
ABSORPTION RATE DATA ; 0 WPPM S; RATE AT ZERO TIME J&V,
* lo-* at t=O
C!m,iin moles/m3 Thiele modulus 4
if D, = 0.3 D,
0.54
0.78
0.83
1.56
1.3
2.59
1.16
3.4
1.18
turnover number * 10r
4.3
6.5
1.8
Thiele modulus 4
2.35
8.2
2.4
if D, = 0.1 D,
turnover number * l@
5.9
12
ABSORPTION RATE DATA ; 10 WPPM S; RATE AT ZERO TIME
2.5
127 4.5 Effect of S on turnover number for rate at zero time For each hydrogenation experiment it is possible to calculate the concentration at the particle/liquid inter&e at any time by using the equations given in the paragraph 3. The result at t=O of these calculations for the experiments using 0 wppm and 10 wppm S have been given in table 1. The lower the hydrogenation rate, the higher is the interface concentration. For the set of experiments in which 0 and 10 wppm S has been used this interface concentration varies between 1.3 and 2.59 mole/m’ and 4.2 and 5.2 mole/m3, respectively. The turnover numbers and the Thiele moduli have been calculated for two estimated values of the effective diffusion coefficient, namely 0.3 and 0.i times the liquid phase diffision coefficient. In the absence of S the turnover number for the medium pore catalyst is 1.5 to 2 times larger than for the wide pore catalyst. Moreover, if we take into account the effective diffusion coefficient for the medium pore catalyst to be smaller than for the wide pore catalyst, the difference in the turnover numbers for these two catalysts is still higher than reflected by the numbers calculated on the basis of equal effective diffusion coefficient. Apparently, the nickel in the medium pore catalyst is for the hydrogenation of poly-olefinic bonds in fat molecules in terms of activity a much more effective catalyst than the nickel in the wide pore catalyst. Loki3 has determined the selectivity of the wide pore catalyst to be better than for the medium pore catalyst. An observation which supports the fact that the Thiele modulus of the medium pore catalyst is indeed larger than for the wide pore catalyst. An experimental determination of the effective diffusion coefficient should be done to get a better understanding of these observations. The calculated Thiele moduli are just in the range where the hydrogenation rate is affected by the diffusional limitation of hydrogen. In particular, this is the case for catalyst Y. The turnover numbers at t=O for the case of 10 wppm S in the soybean oil are 5 to 8 times smaller. It reflects the inhibiting effect of the S-contamination. Again the’tumover numbers for the medium pore catalyst are 1.5 times larger than the turnover number for the wide pore catalyst. The effect of the difference in the two values of the effective diffusion coefficient exists only for the medium pore catalyst. Apparently, there is hardly any effect of diffusion limitation of hydrogen molecules for the hydrogenation rates of these sulphided catalysts.
5.CONCLUSIONS The new technique to monitor continuously the hydrogen absorption rate proved to be a valuable technique. By using this technique it has been possible to calculate the hydrogenation rate over a wide range of iodine values from 132 to 80 for the hydrogenation of a soybean oil. As a result we were able to follow the inhibiting effect due to small amounts of a sulphur component added to the soybean oil. The medium pore catalysts shows unexpectedly the highest turnover numbers. An observation which is not understood yet. An experimental determination of the effective diffusion coefficient will be very helpful to clarify this observation.
6.REFERENCES 1. 2.
H. Klimmek; JAOCS &I(2), (1984) 200 B.W. Cristensen et al; JAOCS 58(12) (1981) 1053
128 3. 4. 5.
6. 7. 8. 9.
10. 11. 12. 13.
C.J. Duyverman et al; Proceedings IIIth Int. Congmss on Cat., Amsterdam 1964, vol II, 1416 K.H. Boume et al; ibid page 1400 M. Perdereau et al; Surf. Sci. 2Q (1970) 80 Jens R. Rostrup-Nielsen et al; J. Catal. a (1979) 395 J. Gudar; Cat. Rev. a(2) (1980) 171 P.K. Agrawal et al; in proceedings l* Catalyst Deactivation Symposium; Elsevier Scientific Publishing Company (1980) 179 C.H. Bartholomew et al; ibid page 375 P.L.T. Hales et al ; A.1.Ch.E.J. U (1%9) 727 J. Crank, The Mathematics of Diffusion; At the Clarendon Press; Oxford 1957 G.C.M. Colen et al; Proceedings Symposium on Catalyst Performance Testing; Appl. Cat. $j, 1988, 339 C.M. Lok; presentation at the AGCS meeting of May 1991 in Chicago