BIBLIOTHEEK TU Delft P 1276 4444
HIGH-PRESSURE HYDROGENATION OF FATTY ACID
ESTERS TO FATTY ALCOHOLS
HIGH-PRESSURE HYDROGENATION OF
FATTY ACID ESTERS TO FATTY
ALCOHOLS
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H. J. DE WIJS, HOOGLERAAR IN DE AFDELING DER MIJNBOUW-KUNDE, VOOR EEN COMMISSIE U I T DE SENAAT TE VERDEDIGEN OP WOENSDAG 18 MEI 1966
DES NAMIDDAGS TE 4 UUR DOOR
KLAUS MAX KARL MUTTZALL
Diplom-Chemiker geboren te Berlijn
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR PROF. DRS. P. J. VAN DEN BERG
A C K N O W L E D G E M E N T
T h e author wishes to express his thankfulness to the Management of the Unilever Research L a b o r a t o r y -Vlaardingen and of Unilever N.V. for their permission to carry out this investigation and to publish the results in this form. T h e valuable assistance of colleagues is gratefully acknowledged.
C O N T E N T S
page CHAPTER 1 I n t r o d u c t i o n
1.1 Use of fatty alcohols 9 1.2 P r e p a r a t i o n of fatty alcohols 9
CHAPTER 2 Survey of literature
2.1 Catalytic hydrogenation of fatty acids to fatty alcohols 13
2.2 C o p p e r c h r o m i u m oxide catalysts 15
CHAPTER 3 Experimental details
3.1 A p p a r a t u s 17 3.2 Starting materials 18
3.3 Analysis 20 3.4 Procedure 21
CHAPTER 4 Results of experiments
4.1 Solubility of hydrogen a n d w a t e r in fatty alcohol 24
4.2 H y d r o g e n a t i o n reaction 27 4.3 R e a c t i o n kinetics 32 4.4 Influence of process conditions on reaction-rate constants 37
4.5 F o r m a t i o n of hydrocarbons 44
4.6 Conclusions 48
CHAPTER 5 Mechanism of the catalytic hydrogenation
5.1 T h e r m o d y n a m i c estimation of equilibrium constant 51
5.2 Mechanism of reaction on catalyst 54
CHAPTER 6 Influence of reaction conditions on the formation of fatty alcohols
6.1 Batch reaction 57 6.2 Continuous reaction 59
Summary 63 Samenvatting 65 List of symbols used 67
CHAPTER 1
I N T R O D U C T I O N
1.1 U s e o f fatty a l c o h o l s
T h e term fatty alcohols is applied to straight-chain primary organic alcohols having the general formula
C H 3 — ( C H , ) „ — C H . O H
where n is an even number between 6 and 18. These alcohols are valuable raw materials for many branches of industry. In 1962, about 125,000 tons of fatty alcohols with more than 10 C-atoms were produced in the United States with a total value of $ 48,000,000.— [1]. More than 70% of this production was used in the detergents industry.
Fatty alcohol sulfates, which are excellent anionic detergents, are obtained by reaction of fatty alcohols with a sulfonating agent (e.g. sulfuric acid, sulfur trioxide, chlorosulfonic acid) and subsequent neutralization according to the reaction scheme:
C H , — ( C H j ) ^ — C H j O H + SO3 -> C H 3 — ( C H 2 ) „ — C H j O S O j H C H 3 ( C H 2 ) „ C H 2 0 S 0 3 H + N a O H - > C H 3 ( C H 2 ) „ C H 2 0 S 0 3 N a + H , 0
Reaction of ethylene oxide with fatty alcohols according to
C H 3 — ( C H , ) „ — C H j O H + m C H j — C H ^ -> C H » — ( C H ^ ) » — C H 3 — ( O G H j C H ^ ) ^ — O H \ /
O
yields ethoxylated alcohols, which are used as nonionic detergents.
Octanol and decanol are used for the production of plasticizers, such as phthalic esters.
Fatty alcohols with higher carbon numbers are applied by the cosmetics industry in for instance creams and ointments.
1.2 P r e p a r a t i o n o f fatty a l c o h o l s
T h e oldest method for the preparation of fatty alcohols is the alkaline saponifica-tion of sperm oil [2, 3], consisting for about 70% of esters of fatty acids and fatty alcohols, e.g. cetyl oleate,
C H 3 — ( C H j ) , , — C H . O C O — ( C H 3 ) , — C H = C H — ( C H 3 ) , — C H 3
and 3 0 % triglycerides. T h e alcohols obtained after saponification and distillation are partly unsaturated and have an even number of C-atoms between 14
and 20. Saturated fatty alcohols are obtained by hydrogenation of the double bonds.
In 1903, BouvEAULT and BLANC discovered the sodium reduction of fats or
fatty acid esters to fatty alcohols. T h e reaction can be described by the following equations:
R C O O C 2 H s + 4 N a + 2 C j H 5 0 H -> R C H 3 0 N a + 3 C 3 H 5 0 N a
R C H 3 0 N a + 3 C 2 H 5 0 N a + 4 H j O -> R C H 3 O H + 3C3H5OH + 4 N a O H
T h e fatty alcohol obtained has the same chain-length as the starting fatty acid and the carbon-carbon double bonds remain intact. Since 1930, this method has been greatly improved and used for the commercial production of fatty alcohols [4, 5]. A disadvantage of this process is the high processing costs, due to the high price of sodium.
Hydrogen, which is much cheaper than sodium, can also be used as reducing agent. In this case, however, the reduction must be carried out in autoclaves at high temperatures and high pressures in the presence of a catalyst. In most cases, carbon-carbon double bonds are hydrogenated as well, yielding saturated fatty alcohols having the same carbon chain as the starting fatty acids. Table 1 gives a survey of some continuous processes [6].
Table 1 Technical processes for the production of saturated fatty alcohols by catalytic hydrogenation
T e m p e r a t u r e Pressure
R a w material Catalyst (°C) (atm) References Fats, fatty acid esters suspended copper 330-340 230-250 7 , 8
c h r o m i u m oxide
F a t t y acid methyl esters fixed catalyst b e d ; copper 200-300 100-500 6 , 9 zinc c h r o m i u m oxide
P a l m kernel oil suspended copper 270-320 240 6 Tallow c h r o m i u m oxide
F a t t y acids * suspended copper 240-320 200-700 6, 10 F a t t y acid esters c h r o m i u m oxide
* F a t t y acids form intermediate fatty alcohol esters.
For most technical processes copper chromium oxide catalysts are used. Since these catalysts are destroyed by free acids, esters of fatty acids must be used as starting material [3]. Fatty acid methyl esters - prepared separately - are mostly used. During hydrogenation methanol is re-formed according to
R C O O C H 3 + 2 H j - * R C H j O H + C H 3 0 H
which can be used again. If triglycerides are used as starting material, the glycerol is converted into 2-propanol according to [6, 11]
CH^—OCOR, CH3
I I
CH—OCOR2 + 8 H j ^ C H O H + R i C H 2 0 H + R j C H j O H + R 3 C H 3 0 H + 2 H 3 0 I I
CHj—OCOR3 CH3
T h e processes for t h e p r o d u c t i o n of fatty alcohols d e s c r i b e d so far a r e b a s e d o n t h e use of fats as r a w m a t e r i a l . H y d r o c a r b o n s a r e also i m p o r t a n t r a w m a t e -rials for t h e p r o d u c t i o n of a l c o h o l s . A c c o r d i n g to t h e O x o - p r o c e s s , alkenes c a n b e c o n v e r t e d i n t o alcohols b y r e a c t i o n w i t h c a r b o n m o n o x i d e a n d h y d r o g e n u n d e r h i g h p r e s s u r e in t h e p r e s e n c e of a c a t a l y s t [12, 13]. A m i x t u r e of i s o m e r i c a l d e h y d e s is f o r m e d a c c o r d i n g t o : 2 R — C H ^ C H j + 2CO + 2H, -> R—CH3—CH3—CHO + R—CH—CH, CHO T h e a l d e h y d e s a r e r e d u c e d to t h e c o r r e s p o n d i n g p r i m a r y a n d s e c o n d a r y a l c o h o l s h a v i n g o n e C - a t o m m o r e t h a n t h e s t a r t i n g a l k e n e s .
W h e r e a s t h e O x o - p r o c e s s gives a m i x t u r e of i s o m e r i c alcohols t h e Alfol-process yields s y n t h e t i c s t r a i g h t - c h a i n e v e n - n u m b e r e d fatty alcohols e q u a l to those o b t a i n e d from fats as s t a r t i n g m a t e r i a l . T h i s process, w h i c h w a s d e v e l o p e d b y ZiEGLER, p r o c e e d s in t w o steps [14, 15, 1 6 ] :
a. a d d i t i o n of e t h y l e n e to t r i e t h y l a l u m i n i u m u n d e r h i g h p r e s s u r e , y i e l d i n g
trialkyl a l u m i n i u m : , (C2Hi)„+iH
(CjH4)„+iH
b . o x i d a t i o n of t h e trialkyl a l u m i n i u m a n d acidolysis t o fatty a l c o h o l s :
2A1—R, + 3 0 ,
-OR, * 2A1—OR,
R3 O R , ORi
2A1—OR2 + 3 H j S 0 4 - ^ Al3(S04)3 + 2RiOH + 2R3OH + 2R3OH \
O R ,
S i n c e 1962, this process h a s b e e n used for t h e c o m m e r c i a l p r o d u c t i o n of fatty alcohols [ 1 7 ] .
T o d a y , t h e c a t a l y t i c h y d r o g e n a t i o n of fatty a c i d esters is still t h e p r i n c i p a l process for t h e p r o d u c t i o n of fatty a l c o h o l s . H o w e v e r , little is k n o w n a b o u t t h e m e c h a n i s m a n d kinetics of this h y d r o g e n a t i o n r e a c t i o n .
T h i s thesis d e a l s w i t h a n i n v e s t i g a t i o n i n t o t h e m e c h a n i s m a n d kinetics of t h e h i g h - p r e s s u r e h y d r o g e n a t i o n of esters of fatty acids a n d fatty alcohols u s i n g
copper chromium oxide as catalyst. For most experiments, technical grade coconut and tallow fatty acids, which are mixtures of homologous fatty acids with 8 to 18 C-atoms, were used as starting material because of their great economic importance. In order to verify the results obtained, some experiments were carried out with esters of fatty acids and fatty alcohols of one definite chain-length.
T h e hydrogenation of fatty acid esters to fatty alcohols is a gas-liquid reac-tion. A solid catalyst is suspended in the reaction mixture. T h e reaction proceeds at about 300 °C and at pressures above 100 atm. T h e experiments carried out had to give information about the influence of the different reaction-conditions on the course of the hydrogenation. It was therefore necessary to use an appara-tus in which different reaction-conditions could be applied and in which the conditions chosen could be kept constant during an experiment. In addition, it was necessary to follow the course of the reaction closely, which could best be achieved by drawing and analysing samples during the reaction. It was therefore decided to use a mechanically stirred batch autoclave with a volume of 5 1 for the experiments. T h e large volume made it possible to draw several small samples during an experiment without disturbing the reaction. In order to avoid any uncontrollable influences of the catalyst used, all experiments were carried out with one thoroughly mixed batch of catalyst.
T h e purpose of this investigation was to find quantitative relations about the influence of process conditions on the yield of fatty alcohol and the reaction rate. These relations should make it possible to calculate and design reactors for continuous systems.
CHAPTER 2
S U R V E Y O F L I T E R A T U R E
2.1 Catalytic h y d r o g e n a t i o n o f fatty a c i d s t o fatty a l c o h o l s
T h e catalytic high-pressure hydrogenation of carboxylic compounds to alcohols
was almost simultaneously discovered by ADKINS, NORMANN, SCHMIDT and
S c H R A U T H i n 1 9 3 1 .
ADKINS et al. used copper chromium oxide catalyst for the liquid-phase hydrogenation of various organic compounds including methyl, ethyl and butyl esters of lauric and stearic acid [18, 19, 20]. They also hydrogenated spermaceti, the cetyl palmitate fraction from sperm oil, and obtained cetyl alcohol [21].
NORMANN used copper carbonate on silica as catalyst for the hydrogenation of several fatty acids, fatty acid methyl esters and triglycerides to their cor-responding fatty alcohols [22, 23, 24].
SCHMIDT found that the reduction of ethyl oleate can also be carried out in the gas phase at atmospheric pressure using copper chromite on silica as catalyst [25, 26].
SCHRAUTH et al. established that the high-pressure hydrogenation of fatty acid esters with copper catalysts yields fatty alcohols below 320 °C and hydro-carbons above 350 °C [27].
So far, the kinetics of the high-pressure hydrogenation have not been the subject of extensive investigations, which is probably due to experimental difficulties, which arise when working at high pressures.
NORMANN studied the hydrogenation of coconut oil at 310-315 °C and at pressures varying between 120 and 500 atm. H e observed that at higher pres-sures, higher yields of fatty alcohols are obtained. However, no further
explana-tion was given. NORMANN proposed a theory according to which the
hydrogena-tion of esters would proceed via the hemiacetal:
O H
+ H2 I + H 3 R C O O C H 3 R > R — C — O C H j R >• 2 R C H j O H
I
H
T h e hemiacetal was not isolated. In the above reaction, the rate of hydrogena-tion of the ester would most probably depend linearly on the hydrogen pressure, if there are no physical limitations.
According to this author, hydrocarbons can be formed according to the following reactions:
a. the hemiacetal can be hydrogenated to an ether and water and the ether is then hydrogenated to fatty alcohol and hydrocarbon:
OH
R—C—OCHjR + Hj ^ RCHj—O—CHjR + H^O I
H
RCHj—O—CH,R + H3 ^ RCH2OH + R C H ,
b. the ester is hydrogenated directly to fatty acid and hydrocarbon:
RCOOCH2R + Hj ^ R C O O H + RCH3
For both reactions, the rate of formation of hydrocarbons would be linearly dependent on the hydrogen pressure.
ADKINS et al. studied the catalytic hydrogenation of esters to alcohols. They observed, that the hydrogenation reaction is reversible. From a series of experi-ments with octyl caprylate they concluded that the yield of alcohol and the reaction rate depend on the hydrogen pressure [28]. However, from the results no thermodynamically significant equilibrium constant could be calculated.
ADKINS thought the hydrogenation to proceed via intermediate products and suggested two different reaction mechanisms [29]. T h e first is similar to that
proposed by NORMANN, but ADKINS assumes that the hemiacetal decomposes
into fatty alcohol and aldehyde:
H H R—C—OR+H2 ^ R—C—OR ^ R—C + R O H
I
I A
o OH o T h e aldehyde is subsequently hydrogenated to alcohol:
H
R—C + Hj ^ RCHjOH
i
For the other mechanism it is assumed that ester reacts with hydrogen directly to alcohol and aldehyde:
H
R—C—OR + H, ^ R—C + ROH
h h
Again, the aldehyde is hydrogenated to the corresponding alcohol.
For both reaction mechanisms the rate of hydrogenation of the ester would depend linearly on the hydrogen pressure, if there are no physical limitations.
hydro-genation of methyl laurate to lauryl alcohol [30]. These conditions were found to be 300 °C, 4 % copper chromium oxide catalyst, 300 atm and a reaction time of 20 minutes. T h e authors noticed, that the life time of the catalyst was long and that good agitation of the reaction mixture was necessary.
HAIDEGGER et al. determined the optimum reaction conditions for the batch-wise and continuous hydrogenation of triglycerides to fatty alcohols using a copper chromium oxide catalyst [31, 32]. T h e highest yields were obtained in batch experiments at 310-330 °C and 300 atm using 1-2% catalyst and reac-tion times of 10-60 min. Although these authors studied the influence of process conditions extensively, especially in continuous experiments, no attempt was made to determine the kinetics or the reaction mechanism of the hydroge-nation.
Fatty acids and fatty acid methyl esters can be hydrogenated to saturated fatty alcohols with copper soap as catalyst [33]. When using a mixture of copper and cadmium soaps, the carbon-carbon double bonds of the starting material are not hydrogenated and unsaturated fatty alcohols are obtained.
According to STOUTHAMER, copper hydride is the active form of copper
cadmium soaps in the hydrogenation of oleic acid to oleyl alcohol in hydro-carbon solution [34, 35]. T h e reaction rate is determined by the reaction of copper hydride with oleic acid. T h e rate of hydrogenation of oleic acid to oleyl alcohol is linearly dependent on the hydrogen pressure and copper concentra-tion and independent of the concentraconcentra-tion of oleic acid.
2.2 Copper c h r o m i u m o x i d e c a t a l y s t s
ADKINS et al. were the first to prepare and use copper chromium oxide catalysts [18, 19]. If barium is incorporated - probably present as B a C r 0 4 - the catalyst does not become red and inactive during hydrogenation [36].
Copper chromium oxide catalysts are prepared by mixing solutions of copper and barium nitrate with ammonium dichromate and ammonia. A brown precipitate of basic copper ammonium chromate, C u ( O H ) N H 4 C r 0 4 , and barium chromate is formed. T h e catalyst is obtained by thermal decomposition of this precipitate at 300-350 °C [37].
T h e structure of copper chromium oxide catalysts has been investigated
by several authors. ADKINS found that the catalyst is inactive after removal of
the copper [38]. Finely distributed metallic copper - formed by reduction of copper oxide - seems to be the active constituent [39]. T h e behaviour of the catalyst on heating in air and when used in reduction experiments was studied with the aid of X-ray diffraction [40]. T h e fresh catalyst was found to consist of C u O and CuCr204. Consequently, the following scheme was proposed for activation, inactivation and regeneration of the catalyst:
CuO + CuCr^O, Cu + CuCrjO,
I
CujCr^O, CuO + CuCrjO, (fresh catalyst) (activated catalyst) (inactive catalyst) (regenerated catalyst)According to this scheme, the activated catalyst consists of metallic copper on copper(II)chromate as carrier. T h e catalyst is inactivated by the formation of copper(I)chromate, during which process the metallic copper present is consumed. T h e inactive catalyst can be regenerated by oxidation to copper oxide and copper(II)chromate.
CHAPTER 3
E X P E R I M E N T A L D E T A I L S
3.1 A p p a r a t u s
T h e experiments described in this thesis were carried out in a batch autoclave (Fig. 1) of the firm of A. Hofer, Germany. T h e autoclave, which had a total volume of 5 1, had an inside diameter of 125 mm and an inside height of 420 mm. T h e construction material was stainless steel according to German material numbers 4571/4580, DIN-code X 10 CrNiMoTi 18 10 and X 10
Fig. 1 High-pressure autoclave
a. M a g n e t b . W a t e r inlet c. Cooler d. Sampling valve e. D i p tube f Electric heater g. T h e r m o m e t e r pocket h. Stirrer i. Air distribution ring
CrNiMoNb 18 10. T h e maximum working pressure and temperature of the apparatus were 300 atm and 350 °C. T h e autoclave could be heated electrically (6 kW) and the heating rate was ca. 5 °C min~i. An automatic temperature control device kept the temperature at the desired value with an accuracy of ± 2 °C. Cooling of the autoclave was achieved by blowing a current of cold air along the reactor wall. T h e cooling rate was about 1.2 ° C - m i n - i .
T h e autoclave was equipped with an up- and downwards moving stirrer with electromagnetic drive. T h e advantage of this type of drive is that no stuffing
box is needed. T h e stirrer consisted of a vertical rod with three horizontal perforated plates. This rod was moved up- and downwards with an amplitude of ca. 150 m m . In a glass model filled with water, it could be observed that this type of stirrer is very effective for the dispersion of gas in liquids. Directly after starting the stirrer, the liquid contains about 20 vol.% of gas bubbles having diameters between 2 and 8 mm. T h e number of strokes per min of the stirrer could be adjusted between 30 and 160.
Furthermore, the autoclave was equipped with a manometer, a safety valve, a gas inlet and gas outlet valve, the latter with reflux condenser and a dip-tube with pressure cooler and valve for taking liquid samples.
T h e sampling device used was very satisfactory. It consisted of a dip tube, a cooler (length about 60 mm - inside diameter 6 mm) with circulating water, the temperature of which was just above the melting point of the product, and a valve with a diameter of 2 mm. T h e samples were immediately cooled to about 70 °C under pressure, so that the reaction composition was frozen in. The course of the reaction could be very easily followed by means of this sampling device. However, abrasion - caused by the suspended catalyst par-ticles - was observed on the sampling valve after a certain time. Therefore, this valve was replaced after about every 50 experiments.
3.2 Starting m a t e r i a l s
Fatty acids
Esters prepared from fatty acids and fatty alcohols were used in the experi-ments described in this thesis.
Distilled coconut and tallow fatty acids were obtained from Union N.V., Baasrode. Analytical data of these fatty acids are given in Table 2.
Table 2 Characteristics and composition (in wt.%) of coconut and tallow fatty acids
Acid value Saponification value Iodine value Unsaponifiable (%) Sulfur (ppm) M e a n molecular weight Coconut 267.3 267.3 9.3 0.1 12^14 210 Tallow 208.9 210.0 49.1 0.1 10-11 267 2.5 4.5 27.0 66.0 8.5 9.0 49.0 16.5 7.5 9.5
Pelargonic acid was obtained from Unilever-Emery N.V., Gouda, and pelar-gonic alcohol from Messrs. Polak & Schwarz, Zaandam. Gaschromatographic analysis showed that the purity of both substances was higher than 9 8 % . T h e GLC-analysis was carried out using a Carlo Erba type C I D F F apparatus. T h e stationary phase was PEGA, 5 % on Diaport S 80-100 mesh. T h e temperature was 180 °C and nitrogen was used as carrier gas.
Fattj) alcohols
Fatty alcohols were prepared from fatty acids by high-pressure hydrogenation and purified by distillation. T h e distillation was carried out at a pressure of 6 mm Hg and temperatures of 120-190 and 140-200 °C for coconut and tallow fatty alcohols respectively. In both cases, about 5 % residue was obtained con-sisting mainly of unconverted ester. T h e alcohols obtained had the same chain-length distribution as the starting fatty acids.
Esters
T h e esterification was carried out in a 50 1 stainless steel vessel. A mixture of 45 mole % fatty acid and 55 mole % fatty alcohol was heated to 240 °C, while a slow nitrogen stream (approx. 100 l-h~i) was passed through the vessel. No catalyst was used. T h e mixture was cooled when the free fatty acid content of the mixture had dropped below 0.8%, which was generally the case after 4-6 h at 240 °C. T h e ester obtained was used as such for the hydrogenation experi-ments. T h e compositions of the esters used are given in Table 3.
Table 3 Composition (wt.%) of esters of fatty acids and fatty alcohols
T y p e of fatty a c i d / f a t t y alcohol Coconut Coconut Coconut Coconut Coconut Tallow Coconut Pelargonic Ester 96.1 80.0 84.2 88.5 88.3 83.5 90.2 89.0 Alcohol 3.0 19.4 15.7 Il.O 11.2 15.7 8.9 11.0 Acid 0.8 0.3 0.5 0.45 0.7 0.7 1.1 0.03 W a t e r _ 0.10 -0.15 0.07 0.03 0.03 0.08 Hydrogen
Electrolytic hydrogen was supplied in pressure (200 atm) steel cylinders by N.V. Maatschappij Oxygenium, Schiedam. According to suppliers, the hydro-gen contained < 10 ppm oxyhydro-gen, < 40 p p m nitrohydro-gen and < 30 p p m water.
Catalyst
For all experiments one batch of copper chromium oxide catalyst (ex Deutsche Gold und Silberscheideanstalt, Frankfort, Germany) was used, containing 34.7% copper, 2 9 . 3 % chromium and 7.8% barium, the balance being oxygen. T h e composition of the catalyst is therefore:
1.16CuO • CuCr204 • 0.23BaCrO4
T h e particle-size distribution of the catalyst - measured with a Coulter Counter - is given in Table 4. T h e mean particle diameter appeared to be about 4 [X.
Table 4 Particle-size distribution of catalyst
Particle diameter {\j.) Vol. %
> 15 11 10-15 6 6-10 14 4-6 25 2-4 35 < 2 9
T h e bulk density of the catalyst powder was 4.6 g • cm ^^ and the total surface
area appeared to be 19 m^ • g-i (measured by Mr. T. OSINGA - Unilever Research
Laboratory, Vlaardingen).
3.3 A n a l y s i s
T h e samples taken during a hydrogenation experiment might, after removal of the catalyst by centrifugation, contain:
fatty alcohol
fatty acid - fatty alcohol ester fatty acid
hydrocarbons
unsaturated compounds.
T h e percentage of fatty alcohol, ester, fatty acid and the degree of unsatura-tion can be determined from the hydroxyl, saponificaunsatura-tion, acid and iodine values.
If MA, ME, MZ are the mean molecular weights of fatty alcohol, ester and fatty acid respectively, the following percentages can be calculated:
% fatty alcohol = — ^ • iOHV-AV)
"" ^ 560.8 ^ '
ME
% fatty acid - fatty alcohol ester = • {SV—A V) MZ
% fatty acid = ^ - g ^ . ^ F
T h e above values were determined according to standard methods described in the literature [41].
All analyses were carried out by the Analytical Service Department of U.R.L., Vlaardingen.
Since no method for the accurate determination of small amounts of hydro-carbons in fatty alcohol was found in the literature, a suitable method was
developed by Mr. VAN GALEN of U . R . L . [42].
According to this method (accuracy about 2 % ) , a solution of the fatty alcohol sample in carbon tetrachloride is passed over a column of activated alumina. Fatty alcohol, acid and ester are adsorbed and the effluent contains all the hydrocarbons present in the sample. T h e infrared adsorption of this hydro-carbon-CCU-solution is measured between 2200 and 3050 m[x,. T h e adsorption-bands of the — CH3 and > CH2-groups of hydrocarbons have maxima at 2315 and 2355 m\i. With the specific extinctions of hydrocarbons, which were deter-mined using pure compounds, the hydrocarbon content of the CCL-solution and therefore also of the sample could be determined from the height of the adsorption maxima.
T h e determination is disturbed if the fatty alcohol sample contains more than 5 0 % ester. If the chromatographic separation is not satisfactory, the IR-spectrum will show other adsorption maxima at 2760 m[A (fatty alcohol), 2840 mjji (fatty acid) or 2905 mji, (ester).
In this thesis, concentrations are expressed in parts by weight of the reaction mixture without catalyst. T h e concentration of ester for instance is indicated as the weight of ester in the mixture divided by the total weight of ester, alcohol and hydrocarbon.
3.4 P r o c e d u r e
T h e copper chromium oxide catalyst was suspended in 1 kg molten ester and sucked into the autoclave by means of vacuum. Subsequently, the autoclave was washed twice with nitrogen and twice with hydrogen, pressurized and heated from 60 to 300 °C in about 50 min.
T h e reaction starts already during the heating-up period of the autoclave under hydrogen pressure. When reaching the reaction temperature desired,
20 to 4 0 % of the starting material is already converted into fatty alcohol, which
could not be avoided. STOUTHAMER, who studied the homogeneously catalysed
hydrogenation of oleic acid to oleyl alcohol in solution, first heated the solvent to the temperature desired and subsequently injected the material to be hydro-genated [34]. In our case, injection of material into the preheated empty autoclave would have been dangerous in connection with possible temperature stresses [43]. In fact, manufacturers of the autoclave strongly advise against this procedure [44].
When the required temperature had been reached, the hydrogen pressure was set at the value desired, after which the first sample was taken. T h e acid and iodine values of this sample were invariably lower than 0.1 and 1.0 respec-tively. This indicates that the small amount of free fatty acid present in the starting material is esterified during the heating-up period and that saturation of double bonds (iodine values of starting material ca. 4-20) occurs at the same time.
T h e experiments were carried out at temperatures between 270 and 320 °C and at hydrogen pressures ranging from 50 to 170 atm. At temperatures below 270 °C, the hydrogenation reaction proceeds very slowly but above 320 °C it proceeds so fast that temperature control of the autoclave is impossible. At pressures below 50 atm and particularly below 5 atm, side reactions occur resulting in the formation of up to 4 0 % of unidentified substances. Since the hydrogen used for the experiments was taken from steel cylinders at 200 atm, no experiments could be carried out at constant pressures above approximately
170 atm.
Temperature and hydrogen pressure were kept constant during the experi-ment (accuracy ± 2 °C, ± 2 atm).
At certain intervals samples were drawn from the autoclave.
After termination of the reaction, the autoclave was cooled by an air-stream along the outside of the reactor. T h e hydrogen was blown off" at about 100 °C, after which the autoclave was washed twice with nitrogen and the product collected. T h e reactor was washed several times with the starting material to be used for the next experiment. This washing was necessary since the reactor could not be emptied completely via the downpipe; about 350 g product was left. This amount was taken into account when calculating the catalyst con-centration.
In the experiments where the catalyst was repeatedly used, the autoclave was allowed to cool down for 5-6 h without stirring. T h e catalyst had then settled on the bottom and the supernatant clear liquid could be drawn off via the downpipe. T h e catalyst present in the samples taken was removed by centrifugation, taken up in ester and fed back into the autoclave.
determined. T o this end, a weighed amount of reaction mixture was filtered through a glass filter. T h e catalyst was washed with light petroleum, dried and weighed. Catalyst losses were in the order of 0 . 1 % per run.
Since it was found that used copper chromium oxide catalyst is highly pyrophoric when free from fatty alcohol, only small amounts of catalyst were isolated in the way described.
CHAPTER 4
R E S U L T S O F E X P E R I M E N T S
4.1 Solubility o f h y d r o g e n a n d w a t e r in fatty alcohol
T h e hydrogenation of fatty acid esters to fatty alcohols proceeds in the liquid phase, while hydrogen is dissolved in the reaction mixture. Therefore, the solubility of hydrogen in fatty alcohol - containing diff'erent amounts of ester or water - was measured at various temperatures and pressures.
T h e autoclave was filled with ca. 2 kg fatty alcohol and heated to the required temperature, which was kept constant. After setting of the pressure, the alcohol was saturated with hydrogen by vigorous stirring. T h e stirrer was stopped and the dip tube for sampling rinsed by drawing off ca. 100 g alcohol. A glass flask was then connected to the sampling device and a weighed amount of fatty alcohol was drawn off. T h e volume of hydrogen liberated was measured with a wet gas meter and corrected for NTP-conditions.
Fig. 2 shows the solubility of hydrogen at 302 °C in different fatty alcohols as a function of pressure. There is a linear relationship between solubility and
en "g 70 u 0. 5 60 I 50 •a ^ 40 "o I" 30 15 J 20 J . 10 0 100 200 300 *- Pressure (atm.)
Fig. 2 Solubility of hydrogen in fatty alcohol
a. Lauryl alcohol b . Coconut fatty alcohol c. Tallow fatty alcohol d. Stearyl alcohol
pressure indicating that Henry's law is valid. T h e slopes of the lines represent the solubility coefficients A given in Table 5. T h e solubility coefficients appear to decrease slightly at increasing molecular weight of the solvent. T h e same influence of molecular weight on the solubility of hydrogen was observed for hydrocarbons [45].
Table 5 Solubility coefficients of hydrogen at 302 °C
Solvent Mean mol. weight ( N T P c m ' g - ' - a t m 1) Lauryl alcohol *
Coconut fatty alcohol Tallow fatty alcohol Stearyl alcohol * 186 196 253 270 0.231 0.214 0.202 0.200 * ex Merck, Darmstadt, Germany.
Fig. 3 shows the solubility of hydrogen at 302 °C in coconut fatty alcohol, containing different amounts of ester. For alcohol containing different amounts of water, the solubility of hydrogen is given by:
CH = 0.217ipn-pn^o) ( N T P c m ^ - g - i )
T h e solubility coefficient is practically the same as that of dry coconut fatty
100 - ^ Pressure (atm.)
200 300
Fig. 3 Solubility of hydrogen in coconut fatty alcohol at 302 °C
a. 0% ester b. 25% ester c. 50% ester d. 86.3% ester e. 1%, water in fatty alcohol
alcohol, which shows that water has no influence on the solubility coefficient of hydrogen.
Fig. 4 shows the influence of the ester concentration in coconut fatty alcohol on the solubility coefficient at 302 °C, which can be described by:
A = A o ( l - 0 . 2 E ) ; Ao = 0.214
Fig. 5 shows the influence of temperature on the solubility coefficient of hy-drogen in coconut fatty alcohol. This influence is given by:
/ - 1 3 0 0 \
0.2 0.4 0.6 - » • Ester (parts by weight)
Fig. 4 Influence of ester content in coconut fatty alcohol on solubility coefficient of hydrogen at 302 °C
T h e solubility of water in coconut fatty alcohol was determined at temperatures between 280 and 320 °C. Coconut fatty alcohol containing a known amount of water was heated in the autoclave to the temperature desired. Via the
pres-0 2 2
-^ Reciprocal temperature ("K' xlO )
Fig. 5 Influence of t e m p e r a t u r e on solubility coefficient of hydrogen in coconut fatty alcohol
sure cooler, samples were taken and analysed for water content. From the initial amount of water and the concentration of water in fatty alcohol, the amount and partial pressure of water in the gas phase were calculated.
With Henry's law:
C H J O = AHjO/'HaO
the solubility coefficient XH^O (% water atm-i) of water was calculated. Table 6
shows the mean values of AHJO and their standard deviations obtained in three
Table 6 Solubility coefficient of water in coconut fatty alcohol
Temperature (°C) -^HzO (% water atm-»)
280 0.096 ± 0.007 300 0.099 ± 0 . 0 1 2 320 0.088 ± 0.009
The temperature has no significant influence, so that in the temperature range investigated the mean value
AHJO = 0.094 ± 0.005% water a t m - i = 1 . 1 7 N T P cm3 • g-i • a t m - i may be used as the solubility coefficient of water.
4.2 H y d r o g e n a t i o n r e a c t i o n 4.2.1 Course of reaction
In Fig. 6 the compositions of samples taken during a hydrogenation experiment with coconut fatty alcohol have been plotted against reaction time. As can be seen, the concentration of unreacted ester first decreases rapidly and much slower later on, while corresponding amounts of fatty alcohol are formed. At a certain ester concentration, the hydrogenation of ester to fatty alcohol stops. Attempts to obtain a higher degree of conversion of ester to fatty alcohol by introduction of fresh hydrogen, use of a starting material of different composi-tion or by addicomposi-tion of more fresh catalyst, were not successful. This phenomenon seemed to point to a chemical equilibrium.
Reaction time (h)
Fig. 6 Course of hydrogenation of coconut ester a n d alcohol T e m p e r a t u r e 302 ° C ; pH 165 a t m ; 3 % catalyst
In order to check whether the catalytic high-pressure reduction of esters to
fatty alcohols is indeed a reversible reaction as suggested by ADKINS, some
experiments were carried out using distilled fatty alcohol as starting material. Analysis of the samples drawn showed that the concentration of fatty alcohol had indeed decreased and that ester had been formed, which proves the revers-ibility of the reaction. T h e results of experiments carried out at hydrogen pressures of 165 and 96 atm respectively and in which ester or fatty alcohol had been used as starting material have been plotted graphically in Figs. 6 and 7. In both cases, the same equilibrium concentrations were reached.
Fig. 6 shows that during the reaction small amounts of hydrocarbons are formed, the concentration of which increases linearly with reaction time. After a reaction time of 3 h, the hydrocarbon content of the reaction mixture was 0.015 parts by weight at a reaction temperature of 285 °C; at 300 °C it was about 0.025 parts by weight. T h e formation of these hydrocarbons will be discussed in Chapter 4.5.
4.2.2 Determination of equilibrium constant for coconut fatty alcohol
In order to find an expression for the equilibrium constant, the equilibrium concentrations of ester and coconut fatty alcohol were determined at 302 °C at various pressures.
T h e general reaction equation for the reduction of esters to fatty alcohols in the liquid phase is
R — C O O C H 2 R + 2H2 ?^ 2RCH2OH (1) If A and E are the concentrations of fatty alcohol and ester in the reaction
mixture, the equilibrium constant K for this reaction would be:
A2
ETC^
According to Chapter 4.1, the concentration of hydrogen in the liquid phase can be expressed as:
C H = A ( 1 - 0 . 2 E ) / ) H (3)
Substitution of (3) in (2) gives:
A2
E ( l - a 2 ' E ) %
T h e equilibrium compositions found at various pressures and the equilibrium constants calculated according to equation (4) are given in Table 7. It appears that K is constant, the mean value being K = 4.95 • 10"* with a standard devia-tion offf = 0.06-10-4.
^ = i r ^ (2)
Table 7 Equilibrium constants of formation of coconut fatty alcohol at 302 °C Starting material Ester Ester Ester Ester Alcohol Alcohol Ester Ester Alcohol Ester Alcohol Ester PH{ 165 165 165 165 165 136 136 96 96 76 47 47 i t m ) ) \ Equilibrium E 0.062 0.060 0.060 0.067 0.065 0.095 0.090 0.150* 0.221 0.380 * concentrations A 0.910 0.890 0.880 0.915 0.930 0.900 0.880 0.840 * 0.759 0.600 * A:-10* 4.96 4.98 4.86 4.72 5.02 4.78 4.83 5.41 4.93 5.02 Extrapolated.
In two series of experiments, in which ester and fatty alcohol were used as starting material, the reaction time was not long enough to reach equilibrium. If E" and Ei are the concentrations of ester in experiments starting from ester and fatty alcohol respectively, it can be shown mathematically that a plot of EO-Ei against E" or E^ yields in good approximation a straight line. At EO-Ei = 0
1.0 r
1 2 3 4
—k' Reaction time(h)
Fig. 7 Course of hydrogenation of coconut ester and alcohol Temperature 302 °C; pH 96 atm; 3 % catalyst
O) '5 , » \ / IB \ / Q- 1 / -" 0-2" i y UJ \ / UJ 1 / 0.1 - i </ O 0.1 0.2 0.3 0.4 0.5 * . Ester ( p a r t s by w e i g i i t )
Fig. 8 Extrapolation to equilibrium concentration a. E » - E ' against E ' b . E»-Ei against E«
this line intersects the E" or E^ coordinate at the equilibrium concentration. This extrapolation is shown in Fig. 8. Fig. 7 shows the course of the concentra-tions during these experiments.
4.2.3 Influence of temperature
Equilibrium constants for the formation of coconut and tallow fatty alcohol were determined at temperatures between 270 and 318 °C. Results are given in Table 8.
Table 8 Influence of temperature on equilibrium constants for the formation of coconut and tallow fatty alcohol
Starting ester Coconut Coconut Coconut Coconut Coconut Coconut Coconut Coconut Tallow Tallow Tallow Tallow Tallow Tallow T e m p e r a t u r e (°C) 270 283 285 286 286 302 316 318 270 285 286 288 301 318 pa (atm) 138 137 137 137 137 see T a b l e 7 134 134 140 140 140 140 140 140 Equilibrium E 0.060 0.071 0.080 0.074 0.074 see T a b l e 7 0.114 0.111 0.048 0.056 0.060 0.064 0.077 0.079 concentrations A 0.930 0.913 0.900 0.910 0.908 see T a b l e 7 0.880 0.870 0.930 0.930 0.923 0.925 0.908 0.860 K-\0* 7.75 6.44 5.57 6.13 6.10 4.95 3.96 3.88 9.00 8.03 7.43 7.00 5.63 4.93
A logarithmic plot of the equilibrium constants against reciprocal temperature is shown in Fig. 9. T h e straight lines obtained can be represented by the following equations:
-.-Reciprocal temperature (10 . K' )
Fig. 9 Influence of temperature on equilibrium constant a. Tallow fatty alcohol b. Coconut fatty alcohol
/ 4 9 0 0 \
/Teoconut = 9.65 • 10"» e x p ( ^ - ^ j ( r in ° K )
^tai.ow = 1 6 . 8 - 1 0 - 8 e x p ( ^ ^ ) ( r i n ° K )
T h e heat Q of the reaction
R C O O C H 2 R + 2H2 dissolved ?^ 2 R C H 2 O H + Q
is 9.7 and 9.4 kcal per mole reduced coconut and tallow ester respectively. T h e heat effect Q^ in the overall reaction
R C O O C H 2 R + 2H2gas ^ 2 R C H 2 O H -f Ql
is then 14.9 and 14.6 kcal per mole reduced coconut and tallow ester respec-tively, when using the heat of solution of hydrogen in fatty alcohol.
4.2.4 Influence of molecular weight
Starting from gaschromatographically pure fatty alcohols with 8 to 18 carbon
10 12 14 16 —.- C-number of fatty alcofiol
atoms, the equilibrium concentrations of ester and fatty alcohol were deter-mined at 285 °C and 140 atm, using 3 % catalyst. T h e equilibrium constant of formation of pelargonic alcohol was found in another series of experiments starting from ester. T h e results obtained are summarized in Table 9. In Fig. 10 the equilibrium constants have been plotted against the C-number of fatty alcohol.
Table 9 Equilibrium constants at 285 °C
Chain-length fatty alcohol Ca
c.
C.o Cu Cuc„
C,s Coconut Tallow Mol. weight fatty alcohol 130.2 144.2 158.3 186.3 214.4 242.4 270.5 196 253 pH (atm) 128 133 135 138 139 140 140 ^ -Equilibrium E 0.081 0.078 0.069 0.068 0.060 0.058 0.057 -concentrations A 0.900 0.912 0.911 0.912 0.920 0.922 0.923 -K\0* 6.3 6.3 6.6 6.8 7.5 7.6 7.8 6.3 7.7Table 9 also shows the equilibrium constants of coconut and tallow fatty alcohol, calculated for 285 °C by means of the equations stated in Chapter 4.2.3. These equilibrium constants are somewhat lower than would be expected considering the mean molecular weights of these alcohols, which are mixtures of homologous fatty alcohols. T h e non-converted esters present in the equilib-rium mixture also consist of homologous fatty acids and fatty alcohols.
Only the total concentrations of fatty alcohol and ester could be determined analytically. T h e equilibrium constants calculated for coconut and tallow fatty alcohol are defined as
( 1 - 0 . 2 SE)2SE/)^
Apparently, the equilibrium constant of a mixture of fatty alcohols is determined by the lowest equilibrium constant of the alcohols present.
4.3 R e a c t i o n k i n e t i c s 4.3.1 Reaction-rate equation
If the catalytic liquid-phase hydrogenation of esters to fatty alcohols proceeds according to the reaction equation
the rate equation would be
^ = - ^ = kaEC'^-hA^ (5)
if there is no physical limitation.
When the hydrogen concentration C H in the liquid (Chapter 4.1) is ex-pressed by:
C H = A(1-0.2E)/)H (3) the rate equation with k], = ka^^ is
dA
— = A : E ( 1 - 0 . 2 E ) 2 / „ - / ; , A 2 (6)
It was found, however, that the reaction-rate constants of equation (6) are linearly dependent on the catalyst concentration n (Chapter 4.3.3). T h e com-plete rate equation is therefore given by
dA , d E
— = kinE{l-0.2E)^pJi-k2nA^= -— (7)
in which K-\_ = — = — Ka and K2 = ^ .
71 71 71
T h e reaction-rate constants k\ and k^ will be used throughout this thesis. When equilibrium has been reached, i.e. when dA/d< = 0, the equilibrium constant is found from equation (7)
K = '±= ^ ^ (4)
k^ E ( l - 0 . 2 E ) 2 / > ^ ^ '
4.3.2 Determination of reaction-rate constants
T h e reaction-rate constants were determined from the plot of the concentra-tions of ester and fatty alcohol against reaction time. At four different reaction times, tangents were drawn to the ester and alcohol line. T h e slopes of these tangents - representing dE/d< and dA/d/ - were determined. T h e absolute values of dE/di and dA/d< at the same reaction time were averaged. With equation (7) and the known ester and fatty alcohol concentrations, these values yield four equations, from which the average values of the unknown k\ and A2 were calculated.
T h e reaction-rate constants of some experiments determined in this way were checked by calculating the theoretical course of the ester and fatty alcohol concentrations. T h e agreement between the measured and recalculated values was good. In most cases, deviations were smaller than 0.01 parts by weight.
For the calculation of the course of the reaction from the reaction-rate constants, equation (7) had to be integrated. This can be done if the formation of hydrocarbons - being of the order of magnitude of 0.005 parts by weight/ hour - is neglected, so that the sum of ester and alcohol concentration is unity. During a reaction, the concentration of ester soon becomes much smaller than unity. T h e term (1—0.2 E)^ may therefore be simplified to (1—0.4 E). Equa-tion (7) becomes then
dA
--= ki7i{l-A){0.6+0AA)pli-k27iA2 (8)
which can be integrated between t = 0, A = AQ and t, A to give:
j ^ _ _ 1 im-0.2kijipji)e'^t+{m+0.2ki7tpl)p 2iOAkiJip^^+k27c) ' e»'-iS • • U with and m = 7iptiVk]p^ + 2Akik2 2AQ{0Aki7ipli+k27i)+0.2ki7Tpii-m /5 2Ao{OAki7ipli+k27i) + 0.2ki7Tpli+m 4.3.3 Experimental results
T h e reproducibility of the hydrogenation experiments carried out was reason-ably good. In a series of 8 experiments carried out under the same conditions, the standard deviation of the mean value of the reaction-rate constants ki was 3.9%. I n another series of 7 experiments the standard deviation was 4 . 5 % . Table 10 shows the results of experiments carried out at different pressures. T h e ki and k2 values calculated from the course of the reaction are indeed constant, their mean values and standard deviations being
ki = (1.95 ± 0.11)10-5 and k2 = (4.0 ± 0.18)10-2
Table 10 Hydrogenation experiments with coconut ester ( r = 302 °C, fresh catalyst) pH n k^-Xf^" Aa-lO^ ^#2-10* 165 165 136 136 136 96 76 47 2.3 3.0 3.0 6.0 1.0 3.0 3.0 3.0 1.9 2.0 1.5 1.9 1.8 1.8 2.6 2.1 4.0 3.8 3.4 3.7 3.8 3.9 5.2 4.1 4.8 5.3 4.4 5.1 4.7 4.6 5.0 5.1
T h e results in Table 10 show that equation (7) gives a correct description of the influence of pressure on the reaction rate. T h e mean value kilk2 = 4.8 • 10~* is in good agreement with the value of the equilibrium constant K = 4.95 • 10"* found at this temperature from the equilibrium composition of the reaction product.
Fig. 11 shows the influence of catalyst concentration on the reaction-rate
-». Catalyst Concentration 7t (wt.%)
Fig. 11 Influence of catalyst concentration on reaction-rate constants k'^ and k,, Full line: fresh catalyst; broken line: used catalyst
constants kl and kb of equation (6). Results of experiments with fresh and used catalysts are shown (Chapter 4.4.2). Both kl and kb are linearly dependent on catalyst concentration. Therefore, kl = ki7i and kb = k27t, which indicates that equation (7) gives also a correct description of the influence of catalyst con-centration on the reaction rate.
T h e reaction-rate constants might be regarded as a measure for the activity of the catalyst used. T h e size of the reaction-rate constants is influenced by some reaction conditions such as temperature, type of catalyst, amount of water etc. (see Chapter 4.4). It was found that under all conditions, the course of the reaction could always be described by rate equation (7). This is illustrated in Figs. 12 and 13 where the compositions of samples drawn during several experi-ments carried out under different conditions (pressure ranging from 47 to
165 atm, temperature from 285 to 302 °C, catalyst concentration from 2.3 to 4.4%) have been plotted against reaction time. These figures also show the theoretical course of the reactions, which was calculated from the integrated rate equation (9) using the reaction-rate constants derived from the experi-mental results.
1 2 3
-*• Reaction time (h)
Fig. 12 Comparison of the measured (dots) and calculated (curves) course of hydrogenation of coconut ester
Fresh catalyst; t e m p e r a t u r e 302 °C a. pH = 4 7 ; b . ^H = 136; c. pH = 165; n = 3.0; n = 3.0; 71 = 2 . 3 ; kl = 2 . 1 - 1 0 - ' ; /ti = 1.5-10-^; kl = 1.9-10-5; k^ = 4.1-10-2 k^ = 3.4-10-2 k^ = 4.0-10-2 1.0 r 0.5 1 1.5 - . - Reaction time (h)
Fig. 13 Comparison of the measured (dots) a n d calculated (curves) course of hydrogenation of:
Coconut ester with 4 . 4 % used catalyst a n d 5 g w a t e r ; ^H = 134; T= 286 kl 1.9-10- k, =
3.2-10-b . Coconut ester with 3 . 0 % used catalyst;
/.H = 137; r = 2 8 5 ; 4^ = 3.5-10-*; A:j = 5.8-10-2 c. Pelargonic ester with 3 . 0 % fresh catalyst;
/»H = 134; r = 2 8 5 ; k^ = A.G-W-^; A, = 6.1-10-^ d. Tallow ester with 2 . 8 % used catalyst;
-4.4 Influence o f p r o c e s s c o n d i t i o n s o n reaction-rate c o n s t a n t s 4.4.1 Influence of stirring
In the previous chapter it was stated that the reaction rate is linearly dependent on catalyst concentration. This leads to the assumption that the reaction-rate is determined by reactions on the catalyst surface. If this is true, the transport of hydrogen from the gas phase to the liquid phase, which is determined by the stirring speed, would have no influence on the reaction rate, i.e. the reac-tion-rate constants would be independent of the stirring speed. Table 11 gives the results of some experiments in which different stirring speeds were applied. Table 11 Influence of stirring speed on the hydrogenation of coconut ester
[pn = 136 a t m ; 5.2% used catalyst) T 288 285 287 285 Stirring 33 60 60 155
rate (strokes min- ') it,-10* 3.7 3.9 3.6 3.6 A,- 102 6.2 5.9 6.1 5.6
In the range investigated, the stirring speed had indeed no influence on the reaction rate. In connection with the special construction of the stirrer (Chap-ter 3.1) the energy input could not be de(Chap-termined. Experiments in which no stirring would be applied, will yield unreliable results since the catalyst would not remain in suspension. T h e settling velocity of the catalyst in fatty alcohol at 300 °C is ca. 30 cm min-i calculated for hindered settling [46]. Experiments at lower temperatures showed that hindered settling occurred at catalyst con-centrations above n = 0.4%. T h e settling velocities measured were in good agreement with the values calculated.
4.4.2 Reuse of catalyst
A used catalyst invariably yielded higher reaction-rate constants than a fresh
catalyst. Table 12 shows some results in illustration of this finding. Table 12 Influence of catalyst on reaction-rate constants; 3.0% catalyst
Starting ester Coconut Tallow Pelargonic Pn 136 140 134 T 284 287 285 yti-10* fresh 2.7 2.8 4.0 used 3.8 5.3 5.1 A,-102 catalyst fresh 4.3 3.8 6.1 used 6.4 7.3 7.9
If the same catalyst was used several times, the reaction-rate constants remained unchanged after the first increase (Table 13).
Table 13 Influence of reuse of catalyst on reaction-rate constants
(coconut ester, p^i = 136 atm, TI = 5.45%)
Catalyst Fresh Used 2nd time Used 6th time Used 8th time Used 10th time Used 13th time ki-W 2.1 3.2 4.0 3.3 3.7 3.7 A,-102 3.3 5.3 6.4 5.6 5.8 6.1
As can be seen from this table, the reused catalyst yielded the same high reaction-rate constants, even after 13 experiments. Each experiment lasted 2 hours.
T h e higher activity of the used catalyst could be explained when it was established that water has a negative influence on the reaction rate (see Chap-ter 4.4.3).
It was assumed, that water is formed by reaction of fresh catalyst with hydro-gen, which would explain the low reaction-rate constants. To test this, the hydrogen was blown off from the autoclave at 280 °C via a cooling trap ( — 73 °C) at the end of some experiments. T h e amount of water collected in the trap was measured. T h e amount of water introduced into the autoclave with the starting material (water content of ester and water formed by esterif-ication of free fatty acid) and the amount of water formed during hydrocarbon formation (assuming that per mole hydrocarbon one mole of water is formed) was calculated from a material balance. T h e difference between the amounts of water introduced and formed in the way stated above and that collected must therefore have been formed by the catalyst. T h e results obtained are given in Table 14.
T h e results in this table show that about 5.4 g water is formed from 100 g fresh catalyst. This amount corresponds with the amount of water which would be obtained on reduction of 23.8 g copper oxide. According to the analysis of the copper chromium oxide catalyst (Chapter 3.2), 100 g of this catalyst will contain 23.2 g copper oxide. From this it may be concluded that the amount of water collected was formed by the reduction of copper oxide and that a used catalyst contains metallic copper. This is in agreement with the finding that metallic copper is the active component of the copper chromium oxide catalyst (Chapter 2.3).
Table 14 Amount of water formed during hydrogenation (/>H = 136 atm, T = 285 °C) Starting ester Coconut Coconut Coconut Coconut Coconut Coconut Coconut Coconut Tallow Tallow Catalyst Fresh 2nd use 3rd use 4th use 5th use 6th use Fresh 2nd use Fresh 2nd use 71 5.5 5.4 5.3 5.2 5.1 5.0 3.0 2.95 5.5 5.4 Ai-10* 2.1 3.9 3.6 3.6 3.7 3.6 2.7 3.8 2.3 4.9 A,-102 3.3 5.9 6.1 5.6 6.2 6.0 4.3 6.4 3.1 6.6 Water introduced and formed (g) 2.00 2.55 2.6 2.7 2.9 3.35 2.0 2.3 1.40 1.50 Water collected (g) 6.2 3.2 3.05 2.6 3.3 3.8 4.55 2.4 5.05 2.0 Water formed per 100 g catalyst (g) 5.4 0.8 0.6 -0.5 0.6 6.1 0.2 4.7 0.6
T h e amount of water liberated from the used catalyst (ca. 0.6 g/100 g catal-yst) might have been formed by partial oxidation of the metallic copper. During an experiment several samples were taken. T h e catalyst was isolated from these samples by centrifugation and introduced again into the autoclave. T h e whole process, during which the catalyst was always in contact with fatty alcohol, was carried out in air, so that oxidation of the catalyst seems possible. A used catalyst, which was washed free of fatty alcohol with ether and dried, was strongly pyrophoric and burnt to a grey powder on contact with air.
4.4.3 Influence of water
T h e results of experiments with fresh and used catalyst indicated that water has a negative influence on the reaction-rate constants. In order to establish this influence quantitatively, a series of experiments was carried out in which different amounts of water had been added to the reaction mixture. T h e results of these experiments are shown in Table 15.
Table 15 Influence of water on the hydrogenation rate of coconut ester (reused catalyst, T = 285 °C, />H = 136 atm)
W a t e r (g) in autoclave * C H , O ** (%) ki-\Q' k,-W 2.9 2.9 5.25 4.75 5.15 4.4 5.3 1.32 2.12 2.35 2.75 5.65 7.75 12.0 0.048 0.076 0.084 0.098 0.203 0.280 0.432 5.1 3.9 3.7 3.8 2.2 1.9 1.2 7.2 6.5 5.9 6.1 3.7 3.2 2.0
* W a t e r from ester + water formed by esterification of free fatty acid in ester + water formed by hydrocarbon formation in half the reaction time + water from catalyst + water a d d e d . ** Calculated with AHJO = 0.094%, w a t e r / a t m .
T h e influence of water on the catalyst activity is reversible. If the water present in the reaction mixture is removed by evaporation, the catalyst shows its original high activity.
In Fig. 14 the reciprocal reaction-rate constants have been plotted against
Fig. 14 Influence of water on reaction-rate constants
the concentration of water in the reaction mixture. Straight lines are obtained, which can be described by the equations
8.5-10-5 (10) ki = l - f l 4 C H , o 13.9-10-2 : i i ) 1 + 1 4 C H , O
T h e influence of water on the catalyst activity can be explained if water is assumed to act as a catalyst poison adsorbed preferably (see Chapter 5.2). From equations (10) and (11) the following maximum reaction-rate constants
at CHJO = 0 and 285 °C are obtained:
A : i _ = 8.5-10-5 (h-i) A 2 _ = 13.9-10-2 (h-i)
T h e quotient of equations (10) and (11) represents the equilibrium constant X = - ^ = 6.1-10-4
«2
(12) T h e equilibrium constant is independent of the concentration of water as should be the case if water affects only the activity of the catalyst.
T h e amount of water formed during some experiments with a fresh catalyst has also been measured. In addition, the maximum reaction-rate constants for a fresh catalyst have been calculated by means of equations (10) and (11) (Table 16).
Table 16 Maximum rate constants for the hydrogenation of coconut ester with fresh catalyst ( T = 285 °C; /^H = 136 atm)
71 5.5 5.5 3.0 Water present (g) A, - 10^ 5.85 2.1 4.80 2.3 4.80 2.7 k^W' 3.3 3.1 4.3 ^ . n , a . - 1 0 ' 8.2 7.8 8.6 *2„,ax-10^ 12.9 10.6 13.8
For these calculations, the amount of water present after half the reaction time has been used. T h e maximum rate constants obtained are in agreement with the A;max-values for a used catalyst, derived from equations (10) and (11). It can therefore be concluded that the lower activity of a fresh catalyst is only caused by the formation of water during the reduction of copper oxide to metallic copper.
For the above calculations the amount of water adsorbed on the catalyst surface was neglected. An estimation of the amount of water adsorbed by the catalyst shows that the error made as a result of this neglect is only small. If the whole surface area of the catalyst (19 m^/g) is assumed to be covered with a monomolecular layer of water, the amount of water adsorbed would be 5.5 mg/g catalyst. T h e amount of water actually adsorbed on the catalyst surface will be much smaller than this value.
4.4.4 Influence of temperature on reaction-rate constants
Below 270 °C, the high-pressure catalytic hydrogenation of fatty acid esters is very slow but above 320 °C the reaction proceeds so fast that temperature control of the autoclave is impossible. In addition, a considerable amount of hydrocarbons is formed (approx. 0.02 parts by weight/h) at 320 °C. T h e in-fluence of temperature on the reaction-rate constants is shown in Table 17. T h e Amax-values have been calculated with equations (10) and (11), assuming that these equations are also valid for other temperatures.
In Fig. 15 the maximum reaction-rate constants have been plotted log-arithmically against reciprocal absolute temperature. There appears to be little difference between the A:i-values for the hydrogenation of coconut and tallow ester. T h e straight lines obtained are described by the equations:
Table 17 Influence of temperature on reaction-rate constants (used catalyst) Starting ester Coconut Coconut Coconut Coconut Coconut Coconut Tallow Tallow Tallow Tallow Tallow Tallow T 268 284 286 286 300 318 270 286 286 287 301 318 CH.O (%) 0.057 0.076 0.084 0.098 0.067 0.078 0.041 0.056 0.048 0.048 0.055 0.112 ^1-10'^ 2.6 3.8 3.7 3.8 5.6 8.2 3.3 4.9 5.0 5.3 7.5 8.6 A,- 102 3.3 6.4 5.9 6.1 10.9 21.4 3.7 6.6 7.2 7.3 13.1 17.0
^wio=
4.8 7.9 8.0 9.0 10.9 17.1 5.2 8.6 8.4 8.8 13.2 21.8* w i o ^
5.9 13.2 12.8 14.4 21.2 44.8 5.8 11.8 11.9 12.2 23.2 43.2 1 10 n E 1.70 1.74 1.78 1.82 1.86 ^ Reciprocal temperature (°K x 10 )Fig. 15 Influence of temperature on reaction-rate constants Full lines: coconut ester; broken lines: tallow ester
/ - 8 0 0 0 \
coconut ester ki^^^— 1.3-102 exp I———1 (13) / - 1 2 6 0 0 \ ^ W = 8.8-108 exp ^ ^ ^ ^ j (14) /_9400\ tallow ester ^Wx = l - ^ - 1 0 ' e x p l — — \ (15) /'-13900\ , , ^2„,ax = 7 . 7 - 1 0 « e x p ( - ^ ^ ) (16)
There is little difference in activation energy between the two starting materials. From the above equations, the following average activation energy of the hydrogenation reaction can be calculated:
AH\ydrogenation = —17.2 kcal - mole"!^ For the dehydrogenation reaction:
AH dehydrogenation = — 26.2 kcal • mole-1
T h e difference between the two activation energies represents the reaction heat. This value is 9.0 kcal - mole-i, which is in good agreement with the values of 9.7 and 9.4 kcal - mole-i calculated from the dependence of the equilibrium constant on temperature (Chapter 4.2.3).
4.4.5 Influence of starting material on reaction rate
T h e catalytic hydrogenation of esters to fatty alcohols has been studied using pelargonic, coconut and tallow esters as starting material. No significant dif-ferences in the reaction-rate constants have been found. This is illustrated in Table 18 stating the Ajmax-values of the three esters.
T h e pelargonic ester is a p u r e compound but the coconut and tallow esters are composed of mixtures of homologous fatty acids and fatty alcohols of different carbon chain-length. T h e reaction-rate constants are defined by the equation
dA
— = ki7iE{\-Q.2EYpli-k27iA^ (7) at
Table 18 Reaction-rate constants of diflferent starting materials (total pressure 140 atm, T = 285 °C, 3 % used catalyst)
Starting ester Pelargonic Coconut Tallöw CHZO 0.049 0.048 ^,-10* 5.0 5.1 A j - 1 0 2 8.0 7.2 ^ w - l O ^ 8.4 8.5 8.6 *2rr..x-10^ 13.5 13.9 12.1
For a mixture of esters and alcohols of different chain-length, this rate equation would be
d(i;A)
dt = 7r-/»^(l-0.2 SE)2SA;iE-7rSyt2A2 (17)
which is equal to equation (7) if the reaction-rate constants are independent of the chain-length of the starting materials and the products. T h e results of Table 18 show that this is indeed the case.
T h e fact that the length of the carbon-chain has no influence on the reaction-rate constant might be explained by the following speculation: T h e hydrogena-tion proceeds on the catalyst surface between adsorbed hydrogen and ester molecules. As will be shown later (Chapter 5.2), the reaction on the catalyst surface determines the reaction rate. Of the ester molecule, only the carboxyl group takes part in the reaction. We might now imagine that the ester molecule is attached to the catalyst surface by its carboxyl group, the carbon chains stretching away from the catalyst surface. If this is the case, the chain-length of the molecule would indeed have practically no influence on the reaction-rate. T h e picture would be quite diff'erent if diffusion of the ester molecule to the catalyst surface were rate-determining. Then the length of the carbon chain would have a strong influence on the reaction rate.
4.5 F o r m a t i o n o f h y d r o c a r b o n s
During the high-pressure catalytic reduction of fatty acid esters to fatty alcohols, small amounts of hydrocarbons are formed. Practically straight lines are ob-tained, if the hydrocarbon concentrations during an experiment are plotted against reaction time, which indicates that the rate of formation is almost constant (Fig. 16).
•*- Reaction time (h)
Fig. 16 Formation of hydrocarbons d u r i n g hydrogenation of coconut ester (/>H = 136 a t m ) Broken line: ester concentration with T = 300, JI = 1
Curves a - d represent 1 0 x H C . T e m p e r a t u r e ( ° C ) : a. 315; b . 300; c. 300; d. 288 Catalyst concentration ji: a. 3 ; b . 6; c. 1; d. 5.5
From the experimental results, the following qualitative conclusions can be drawn regarding the influence of process conditions on the rate of formation of hydrocarbons:
- Temperature: increased hydrocarbon formation at higher temperatures. - Hydrogen pressure: no influence.
- Catalyst concentration: increased hydrocarbon formation at higher catalyst concentrations.
- W a t e r : slight decrease of hydrocarbon formation at higher water concentra-tions.
Hydrocarbons could be formed from ester or fatty alcohol according to the reactions:
R—COO—CH2—CH2—R ^ R C O O H - f C H 2 = C H — R . . (18) R — C H 2 — C H 2 — O H - ^ R — C H = C H 2 + H2O . . . . (19) For both reactions, the rate of formation of 1-alkenes would be independent of hydrogen pressure. T h e double bonds of the 1-alkenes formed would be hydro-genated immediately, yielding saturated hydrocarbons. T h e fatty acid formed by reaction (18) would esterify with the fatty alcohol present according to
R C O O H + R—CH2OH ^ R C O O C H 2 — R + H2O . . . . (20) T h e dehydration of alcohols (reaction 19) is catalysed by metal oxides [47] such as aluminium chromium oxide [48] and copper chromium oxide [49]. T h e reaction on aluminium chromium oxide proceeds already at 200-300 °C. Pyrolysis of esters to fatty acids and 1-alkenes proceeds easily at higher tem-peratures and is used for the preparation of 1-alkenes [50, 51]. Some experi-ments were carried out in order to check whether the pyrolysis of fatty esters already proceeds at the temperatures used for high-pressure hydrogenation. About 200 g coconut ester was heated for several hours under reflux in glass flasks. Samples were drawn and analysed for 1-alkene content. T h e results shown in Fig. 17 indicate that at 300 °C already, considerable amounts of 1-alkenes are formed. If there is no physical limitation, the rate equation for the formation of hydrocarbons by pyrolysis of ester according to reaction (18) would be
^ ' - ' S ^ ( ^ ' )
if the hydrogenation of 1-alkenes to saturated hydrocarbons is assumed to be very fast. For the dehydration of alcohols the rate equation would be
dHC
^ = k37lA = k37t{\-E) (22) at
if it is assumed that the dehydration is catalysed by the copper chromium oxide catalyst.
T h e course of the hydrocarbon concentration during a hydrogenation experi-ment according to rate equations (21) and (22) was calculated on an analogue
-^ Reaction time(h)
Fig. 17 Formation of 1 -alkenes by pyrolysis of coconut ester T e m p e r a t u r e ( ° C ) : a. 312 ± 3 ; b . 301 ± 2 ; c. 284 ± 2
1.0
|-I
I °* \
-.- Reaction time (h)
Fig. 18 Formation of hydrocarbons by pyrolysis of ester calculated with E q . (21) Broken line: ester concentration. Curves a - d : tenfold HC-concentration
V a l u e of kl in h " ' for c u r v e : a. 0.07; b . 0.05; c. 0.03; d. 0.01
computer at the Technological University in Delft * (Chapter 6.1). The results have been plotted in Figs. 18 and 19. Fig. 18 shows that by pyrolysis of ester, the * T h e calculations were m a d e u n d e r the guidance of Mr. J . M . VALSTAR.
greater part of the hydrocarbons is formed at the beginning of the reaction. As can be seem, the curves flatten after a reaction period of ca. 2 h. For Aij = 0.05 h - i , the concentration of hydrocarbons is then about 0.03 and the rate of formation is constant at ca. 0.0018 h~^. T h e experimental results (Fig. 16) indicate about the same rate of formation but a much lower starting concentra-tion of hydrocarbons. As a result of the necessary heating-up period of the autoclave, the concentration of ester has already dropped to ca. 0.6 at i = 0; the hydrocarbon concentration of the esters used for the experiments was about 0.005. Apparently, hydrocarbons are not exclusively formed by pyrolysis of ester. Fig. 19 shows the formation of hydrocarbons by dehydration of fatty alcohol.
I.U 0.8 0 6 0.4 0.2 ' / / / / / * 1 1 1 1 1 1 ,
L^=
aX
y"^ b c — d 1 1 1 Reaction time (h)Fig. 19 Formation of hydrocarbons by dehydration of fatty alcohol calculated with Eq. (22) Broken line: alcohol concentration. Curves a - d : tenfold HC-concentration.
ValuesofAjTt in h ' ' % for c u r v e : a. 0.015; b . 0.010; c. 0.005; d. 0.0025
T h e rate of formation becomes constant after 1 h. For kzn = 0.0025 h - i % , the rate of formation is 0.0024 h - i ; the extrapolated starting concentration of hydrocarbons at < = 0 would be 0.001 less than the actual starting concentra-tion. These findings suggest that hydrocarbons are formed by dehydration of fatty alcohol according to
R — C H 2 — C H a — O H - ^ R — C H = C H 2 + H2O (19) However, the possibility that part of the hydrocarbons is formed by pyrolysis
of ester cannot be excluded. Fig. 20 shows the theoretical course of the hydro-carbon concentration if hydrohydro-carbons are simultaneously formed both by pyrolysis of ester and dehydration of fatty alcohol.
During some experiments, small amounts of nickel were added as nickel-soap to the reaction mixture. It was noted that this led to a strong increase in the
