ABSTRACT
This work focused on the chemical synthesis and characterization of palm
kernel oil (PKO) for bio-lubricant production using transesterification
of palm kernel methyl ester (PKME) with trimethylolpropane (TMP) and
epoxidation-esterification methods. The PKO was extracted using solvent
extraction method. The physicochemical characteristics of the PKO and
produced bio-lubricant samples were determined using standard methods.
Fourier Transform Infrared (FTIR) spectrometry and Gas Chromatographic
analyses, were respectively, used to determine the predominant
functional groups and fatty acids of PKO and the produced bio-lubricant
samples. At 55 °C, 150 min and 0.5 mm particle size, kernel oil yield
was 49.82 % (by weight). The viscosities at 40 °C, 100 °C, viscosity
index, pour and flash points of the bio-lubricants produced by
transesterification of TMP (PKBLT) and
epoxidation-esterification (PKBLE) methods, were
[42.53 cSt, 10.65 cSt, 139, - 11 °C, 235 °C] and [44.69 cSt, 11.42
cSt, 132, - 12 °C, 240 °C], respectively. Time, mole ratio and
temperature effects were the main factors that significantly influenced
the transesterification and epoxidation processes. The obtained
physicochemical properties of PKBLE and
PKBLT samples showed conformity with ISO VG 32 standard,
hence, their possible application as bio-lubricant basestock.
Keywords: Bio-lubricant; palm kernel; Transesterification;
Trimethylolpropane; Epoxidation-esterification.
Introduction
Globally, the crude oil reserves have significantly drooped, due to its
high consumption, as well as the slow natural mineral deposit formation.
There is therefore possible impeding energy crisis if not quickly
addressed [1]. In other to avert this, scientist and researchers
have therefore sought for alternative energy sources using bio-based oil
materials. As such, alternative oil sources for chemical process
industries, which are renewable, biodegradable, as well as eco-friendly,
are currently being researched. Mostly, vegetable oils obtained from
seeds and nuts are often used as a suitable substitute to mineral oil
obtained from petroleum [2]. There is therefore need for the
development/modification of the vegetable oils into industrial products
like biodiesel [3], biolubricants [4], transformer fluid
[2], as well as other vital fuels that could be used as replacement
for conventional mineral base fuels/fluids.
In Nigeria, due to the availability of huge crude oil reserves, as well
as the production, exploration, and refining of petroleum into products
like, petrol, kerosene, diesel, our interest is on the production of
petrochemical products like bio-lubricants. This is because of the
environmental friendliness of the bio-lubricants, when compared to its
petroleum base counterpart [4].
In the recent past, researchers have successfully synthesized
bio-lubricants from different vegetable oil sources for use as
environmentally friendly lubricant. Some of these include but not
limited to Jatropha curcas oil [4], palm kernel oil [1],
palm oil [5], Fluted Pumpkin Seed oil [6], and castor bean
biodiesel [7]. It is on this premises that this project seeks to
synthesize bio-lubricant from palm kernel oil using different process
routes, in other to evaluate the method with better lubricant
properties.
Palm kernel oil (PKO) is edible oil that is obtained from the kernel of
palm fruit, otherwise scientifically known as Elaeis guinueensis[8-9]. It is yellowish in color and the tree plant (palm tree)
originates from tropical Africa and some part of Asia [8]. Palm
kernels are essential by-products obtained during the palm oil milling
and processing. It is about 45 to 48 % (by weight) of the palm nut of
the Elaeis guinueensis fruit. The oil yield of palm kernel is
about 47 to 59 % by weight [9-10]. The predominant fatty acid in
palm kernel oil is lauric acid, with about 48 % of the fatty acid
composition of the oil. It also contains muriatic acid and oleic acid at
16 % and 15 % compositions, respectively [9,11]. Due to the
relatively high oil yield of PKO, a number of studies be carried out on
its utilization for the production of biodiesel [12], as well as in
bio-lubricant production [1].
It is as a result of the foregoing, that the authors extend this present
study to the use of PKO (because of its high oil yield, availability, as
well as bio-degradability), in bio-lubricant production, using a
modified process routes, for the purpose of comparison, since limited or
no studies by authors, have been channeled towards this direction.
Previously, a good number of studies have been reported on bio-lubricant
production using vegetable oils, for potential use as alternative to
mineral base lubricant, using the two stage transesterification and
epoxidation-esterification methods. In case of two stage
transesterification using methanol (using alkali catalyst) and
subsequently trimethylolpropane, have been extensively reported in the
literature. For instance, Heikal et al. [13], Menkiti et al.
[4], Yunus et al. [14], da Silva et al. [7] and Shote et al.
[15], successfully used this approach for the production of
bio-lubricants using palm oil, jatropha curcas oil, palm-based methyl
ester, castor bean biodiesel, and palm kernel oil, respectively. In this
method, the main drawback of straight alkali catalyzed
transesterification stage, without initial esterification pretreatment
stage, is its low alkyl ester yield, as well as product separation
difficulty, due to formation of soap [2]. As such, in this work,
successive two-step conversion process: an acid-catalyzed esterification
(aimed at lowering the FFA content), followed by the alkali-catalyzed
transesterification (aimed at improving the alky ester yield and
purity), prior to the final stage of transesterification with
trimethylolpropane, was chosen. The essence is to eliminate these
short-coming and ensure higher high grade bio-lubricant with high purity
level and yield [4].
For the epoxidation-esterification procedure, the method is often
considered because of its advantage of better thermo-oxidative
stability, as well as the associated low temperature properties of the
final product [2]. Epoxidation reaction is the reaction of the
double bonds and peracetic acid to produce epoxy ring [2,16]. In
this work, it is worth mentioning that instead of using the PKO, the
obtained alkyl ester form the alkali catalyzed transesterification, was
reacted with the peracetic acid to get the desired epoxy methyl ester.
This decision is based on the improved thermo-oxidative stability and
lower pour point of the final desired product [2,16]. Within the
authors’ knowledge disposal, there is no existing published work that
compares bio-lubricants produced from PKO using these two approaches.
Though, several works have reported the production of bio-lubricant from
PKO using transesterification with trimethylolpropane [1,14,17].
Therefore, this study focuses on bio-lubricant production from PKO,
using alkali-catalyzed transesterification, followed by
transesterification with trimethylolpropane and
transesterification-epoxidation-esterification methods. The qualities of
the produced bio-lubricants were evaluated on the basis of the two
methods used, and on the referenced standards. Furthermore, the
physicochemical properties of the PKO and the PKO bio-lubricants samples
produced were evaluated using standard procedures. Also, the prevalent
functional groups, and the fatty acid compositions of the PKO and the
PKO bio-lubricant samples, were determined using Fourier Transform
Infrared (FTIR), and Gas Chromatography, respectively.
- Materials and methods
- Materials
Palm kernels were obtained from Aguneze, Ahiazu-Mbaise, in Imo State.
Trimethylolpropane was purchased from Sigma Aldrich, Germany. Methanol,
sulphuric acid (H2SO4), orto-phosphoric
(H3PO4) and analytical grade n-hexane
were purchased from Conraws, Presidental Road, Enugu. The reagents had
purity levels of above 99 %, and they were used without further
purification.
Extraction of PKO
Oil extraction from the milled palm kernel oil sample was carried out
according to Association of Official Analytical Chemists (AOAC) 963.15
method [18], using soxhlet extractor unit for the five different
average particle sizes (0.5, 1.0, 1.5, 2.0, and 2.5 mm). Soxhlet
extractor was chosen in order to enhance the solubility of the palm
kernel sample in the chosen solvents used. Milled kernels of a
particular average particle size (15 g) were packed in a thimble of the
soxhlet extractor and the extractor was filled with 150 ml of n-hexane.
Oil extraction was performed at temperatures of 35, 40, 45, 50, and 55
°C using n-hexane. For each temperature, extraction was performed for
five average particle size (0.5, 1.0, 1.5, 2.0 and 2.5 mm). At a
specific temperature and particle size, extraction was carried out for
30, 60, 90, 120, and 150 min. The oil yield obtained at the end of every
extraction time for every extraction condition was calculated and
recorded. The extraction temperature was measured using an electronic
thermometer (Hanna HI-9063), while the time was measured using a stop
watch. The oil yield was calculated (see equation 3.1) using AOAC method
no. 920.85 at the end of each extraction cycle (30, 60, 90, 120, and 150
min). After each extraction batch/cycle, the solvent was removed at 60
°C using rotary evaporator (model N- 1000S-W, EYELA, Tokyo, Japan). The
solute to solvent ratio used for the entire extraction was 1:5 (15 g:
150 ml). Similarly, the extraction times were 30, 60, 90, 120 and 150
min. The entire extraction process carried out under each set of
conditions was performed three times and the average values reported,
while the total extraction yield was obtained using AOAC 920.85 standard
method.
The oil yield of sample was calculated using equation 3.1.
( 1)
The extracted oil sample was palm kernel oil (PKO).
Physicochemical properties of PKO and PKO bio-lubricant
samples
The oil yield (AOAC 920.85) was determined according to AOAC approved
techniques [18]. On the other hand, viscosity index (ASTM D2270),
viscosity (ASTM D445) and specific gravity (ASTM D1217 – 15), were
determined using ASTM standard methods. Also, the pour and flash points
were determined using ASTM D97 and ASTM D93 standard methods,
respectively. Each physicochemical property was measured three times,
and the average values of the properties were determined and noted.
Fatty acid composition of PKO and PKO bio-lubricants samples
Fatty acid profile was evaluated in line with the AOAC 996.06 (1990). In
this procedure, a gas chromatograph (GC) (Shimadzu GC–14B, Model 910),
was used to determine quantitatively, the prevalent fatty acids in the
PKO and PKBL samples. According to the equipment, a HP 88 capillary
column (0.25 mm i.d. × 100 m, film thickness 0.25 µm – Shimadzu
Corporation, Tokyo, Japan), was used to equip the GC’s flame ionization
detector and integrator. This was achieved by using 250 °C temperatures
for both the injector and detector. However, the oven temperature was
retained at 190 °C for a period of 15 min. Thereafter, this temperature
was then increased intermittently, up to 230 °C, at the rate of 5 °C per
min. Afterwards it was maintained at this temperature for the same time
interval as the initial step. The carrier gas used was nitrogen, and was
maintained at a pressure of 500 kPa. Finally, the prevalent fatty acids
were identified and compared with standard compounds; while the quantity
of each fatty acid was calculated from the percentage area of the
individual fatty acid [19]. The analysis was carried out three
times.
- Transesterification experiment
- Synthesis of Palm kernel methyl ester
25 ml of each of the palm kernel oil (Triglyceride) sample was poured
into 250 ml conical flasks and heated to 60 °C using a water bath.
Solution batches of potassium methoxide were prepared by dissolving 5.1
g (30 wt. %) of KOH pellets in an agitated 250 ml beaker containing 150
ml anhydrous methanol. The potassium methoxide solution was then
transferred into warm 25 ml palm kernel oil (triglyceride) sample at a
methanol to oil ratio of 6:1. The solution was then stirred strongly,
using magnetic stirrer at 500 revolutions per minute (RPM) for 120 min.
In other to ensure proper settling, the mixture was left undisturbed in
a separating funnel for 24 hrs. At ending of the settling process, the
upper layer (methyl ester sample) was poured into a beaker and
afterwards properly washed with distilled water. This was aimed at
removing unreacted methanol, catalyst, glycerin, soap and other
impurities. The demoisturization of the fatty acid methyl ester (FAME)
sample was then carried out by heating slowly to constant temperature of
100 °C. Finally, the lower layer that consists of glycerol and soap was
collected via the bottom of the funnel.
The percentage methyl ester yield of the palm kernel oil sample was
calculated using the relationship in equation 2:
(2)
The product of the transesterification of the extracted palm kernel oil
is the palm kernel methyl ester (PKME), or generally referred to as
fatty acid methyl ester (FAME); see equation 3.
Triglyceride Methanol Glycerol Methyl Esters
(Extracted oil) (FAME) (3)
On the other hand, in the present method, FAME of PKO was also obtained
using this procedure. Here, a mixture comprising of 300 g of palm kernel
oil (Triglyceride), 100 g of methanol, as well as 1% wt/wt
orthophosphoric acid catalyst were decanted into continuously stirred
reactor. This reactor was that was equipped with a water-cooled reflux
condenser to ensure complete reaction. Thereafter, the mixture was then
heated to 65 °C and maintained at this temperature for 1hr 30min. After
the reaction, the mixture was dosed with 0.2 molar solution of sodium
trioxocarbonate IV. This was to ensure complete neutralization of the
acid, and eventually stop the reaction. The mixture was then poured into
a separating funnel and subsequently allowed to stand for 24 hrs. This
was to ensure complete separation of methyl esters and glycerol phases.
Glycerol phase at the bottom was emptied into a clean container and
allowed to stand. The PKME (FAME) was then heated to 65 °C, so as to
ensure the removal of the residual methanol. Finally, the remaining
catalyst in the PKME was removed by successively rinsing with hot
distilled water at 80 °C. Thereafter, the remaining water in the PKME
was eliminated by oven-heating at 100 °C [4].
Synthesis of Bio-lubricant from PKME using trimethylolpropane
(TMP)
The synthesis of biolubricant adapted was as described by Surapoj et al.
[20] with slight modifications. The obtained palm kernel methyl
ester (PKME) or generally Fatty acid methyl ester (FAME) was synthesized
with trimethylolpropane (TMP) to obtain trimethylolpropane ester or palm
kernel biolubricat (PKBLT) as shown in equation 4.
(4)
Here, trimethylolpropane (TMP) was initially heated using a
transesterification experimental set-up. This set-up comprises of 50 mL
three necked round-bottom flask that are fitted to a water-cooled reflux
condenser, a thermometer, Kipp’s apparatus and a stirrer operated at
1000 rpm. At 1000 rpm, and under the flow of CO2, the
TMP in the flask was heated to 110 °C, and maintained at this
temperature for 15 min, before allowing cooling. In other to ensure the
evolution of moisture from the TMP, the temperature was maintained at
110 °C. Thereafter, a Ca(OH)2 catalyzed batch
transesterification reactions between PKME (FAME) and already cooled TMP
were conducted at PKME–TMP ratios: 3:1, 4:1, 5:1, 6:1 and 7:1, using
the same experimental set-up. At 80, 100, 120, 140 and 160 °C, each of
the stated PKME–TMP ratios was subjected to transesterification. At
intervals of 1, 2, 3, 4, and 5 h, samples from the respective individual
runs of the experiment (at a particular mole ratio and temperature) were
monitored, collected and analyzed. After every reaction, the mixture was
allowed to cool to room temperature, prior to carrying out filtration
process, so as to separate the residual solid catalyst from the liquid
mixture, which is the palm kernel biolubricat (PKBLT).
The filtered palm kernel bio-based stock was analyzed using the GC to
determine the product composition. Furthermore, prior to the
characterization of the biobased TMP ester, unreacted methyl ester was
not expunged. This was aimed at the improvement of the wear resistance
of the biobased TMP ester, as well as to prevent conjugation reaction
that occurs at elevated temperatures (180 – 200 °C), which involves
poly unsaturated fatty acid (PUFA) [14,21].
Synthesis of Biolubricant from PKME using
epoxidation-esterification reaction
Fifty (50) g of the palm kernel methyl ester sample was placed in
three-necked flasks. 7 g of acetic acid was placed in the round bottom
flask, followed by the addition of 0.02 g (1.5 wt. %) of
H2SO4 to the flasks. The mixtures were
stirred continuously, followed by the addition of 15 g (1.5 moles) of
hydrogen peroxide (15 wt. %). The mixture was added to the flask
containing 7 g acetic acid to produce peracetic acid (PAA). The methyl
ester sample in the three-necked flask was then heated to 70 ºC,
followed by the addition of PAA mixtures. These were allowed to react at
70 °C for 7 hours with continuous stirring at 1,200 rpm. Samples were
taken out every 1 hour from each of the reaction set up for FTIR
analysis to determine the effect of reaction time on the yields (i.e
changes in functional groups as the reaction proceeds) at 70 °C. At the
end of the reaction, 3 ml of the epoxy methyl ester sample was then
removed two times using diethyl ether (2 x 20 ml) in separating funnels.
The organic phases (oily layer) of samples were purified (washed) three
times with 5% saturated sodium bicarbonate NaHCO3 (3 x
15 ml) to neutralize the unreacted acid present. Thereafter, organic
phase of sample was also washed three times with saturated solutions of
sodium chloride NaCl (3 x 15 ml) to obtain epoxy methyl ester of the
sample. The sample was dried over anhydrous magnesium sulphate. The
solvent (diethyl ether) was then removed using a rotary evaporator. The
epoxy oxygen (oxirane) content and iodine value (IV) of the sample was
measured according to procedure described by Arumugam et al. [22]
and Arumugam and Sriram [23].
The obtained epoxy methyl ester sample was epoxy methyl ester (oxirane)
of palm kernel oil (EMPKOo). Fig. 1 shows the reaction
scheme for the epoxidation reaction of PKME to obtain
EMPKOo.
Fig. 1: Reaction Scheme for Epoxidation Reaction of PKME to
obtain EMPKOo [24].