PROCESS OPTIMIZATION AND MODELLING OF LUBRICANT BASE STOCK SYNTHESIS FROM CRUDE PALM OIL
C. Nkem and M.S. Nwakaudu
Chemical Engineering Department,
Federal University of Technology, Owerri, Nigeria.
In this study, crude palm oil was used as a precursor for the synthesis of biolubricant base stock by a two-step transesterification process. Palm oil methyl ester produced from the first step was subsequently reacted with trimethylolpropane using calcium hydroxide as the catalyst to produce palm based TMP ester, a biolubricant. Optimization of lubricant base stock synthesis from palm oil methyl ester was carried out using response surface methodology, central composite design (CCD). The optimum synthesis conditions developed by the CCD model for prediction of TE yield were reaction temperature of 159.9oC, mole ratio of 4.99, catalyst loading of 1.16 and reaction time of 211.63 minutes under vacuum pressure for 84.681% biolubricant yield. The statistical analyses of the data lead to development of the second order quadratic polynomial regression model which establishes the relationship between triester (TE) yield and the process variables with temperature and mole ratio the most significant variable. The coefficient of regression R2 was 0.9477 which validates the fitness of the model equation implying that 94.77% of the variability in the response can be explained by the model. Palm based TMP ester was found to have the following tribological properties: kinematic viscosity of 40.44 and 10.03 cSt at 40oC and 100oC respectively, viscosity index of 198, flash point of 187oC and pour point of -6oC. The properties of palm based TMP ester met the requirement for ISO VG32 and VG46 viscosity grades, and were found comparable to other plant based biolubricants such as jatropha, sesame and canola biolubricant base stock and indicates good potential as base stock in biodegradable lubricant formulation.
Keywords: Biolubricant, Crude palm oil, Response surface methodology Transesterification, Trimethylolpropane.
Petroleum based lubricants have been attributed with lot of negative impact on our environments owing majorly to its inherent properties of non-biodegrability, toxicity and non-renewability. This is evident from pollution, threats to occupational, human and aquatic safety caused by accidental discharge, total loss applications, refinery processes, etc. Also, the combustion of mineral oils as a lubricant has been proven to emit traces of metals, such as calcium, phosphorous, zinc, magnesium, and iron nanoparticles (Miller et al., 2007). Moreover, Tung and McMillan, (2004) analyzed current and future prospects of mineral oils as lubricants in automobile engines and anticipated a declined future prospects. These have rekindled researchers interest on the search for a better alternative to petroleum based lubricants, one that will be easily renewable, non-toxic, biodegradable, ecofriendly and at the same time posses good lubricity property. Over the years, researchers has explored the prospects of biodegradable synthetic products, a renewable source as an altenative to petroleum based lubricants for industrial and transportation application just like bio-diesels (Ebtisam et al., 2016), Zubir & Chin, 2010 also attested to the prospects of vegetable oils as an alternative fuels. Bio- lubricants possess lower volatility, higher flash/ fire points, less vapor emissions and oil mist, and constant viscosity that make them offer better safety (Mohammed et al., 2015)
Vegetable oils are promising alternative to petroleum based base oil because of their good lubricity, non-toxic and biodegradable nature, low volatility and are also renewable. However, vegetable oils have some drawbacks which limit their application as lubricant base oil. Such properties include low thermal oxidative stability and high melting point (Fox & Stachowiala, 2007). Vegetable oil is made up of natural triglyceride (TAGs) consisting of glycerol back-bone and three esterified long chain fatty acids, the ? hydrogen in glycerol is not suitable because of its instability and tendency to undergo elimination reaction which causes molecule degradation (Jieyu, 2012). These drawbacks can be reduced by converting natural fatty acyl esters into synthetic esters using a more resistant polyol to replace the glycerol backbone (Campanella et al., 2010; Arbain & Salimon, 2009).
The chemical modification involves transesterification reaction whereby trimethylolpropane (TMP), a polyol is used to displace the glycerol backbone to yield synthetic ester (Fox & Stachowiak, 2007). This compound is also used to produce triesters (TE) compounds that can replace triacylgycerol in lubricants (Arbain & Salimon, 2010; Schneider, 2006) and produce vegetable oil based lubricants with improved properties. The major constituent of vegetable oil is triacylgycerol which is made up of carbon, hydrogen and oxygen that determines their characteristics. The general principle of synthesis of biolubricant from vegetable oil involves a two-step transesterification process, firstly, the base-transesterification reaction of triglyceride and alcohol to produce fatty acid acyl ester (FAAE) and subsequent reaction of FAAE and TMP to yield TMP ester. Previous studies reported over 85% yield of Fatty acid methyl ester (FAME) at 60oC reaction temperature, 1% sodium hydroxide catalyst, methanol-to-oil molar ratio of 6.0 and 1 hour reaction time (Hoda, 2010; Rashid et al., 2009; Meher, et al., 2006). For the second step, the use of sodium methoxide to catalyze the reaction has been reported by many researchers. (Siti et al., 2007; Musa et al., 2015; Nuhu, 2015).
Both edible and non-edible vegetable oils have been investigated for the synthesis of biolubricant. Menkiti et al., 2016 reported 75.0% yield of melon-based biolubricant successfully produced at the optimum mole ratio, time and temperature of 4:1, 5 hours, and 1500C respectively while jatropha curcas has also been reported as a feedstock for biolubricant production (Bilal et al., 2013; Muhammad et al., 2011). The process variables that affects the conversion efficiency of the reaction has been reported to include temperature, catalyst loading, reaction time, molar ratio and pressure (Menkiti et al., 2015, Ebtisam et al., 2016; Bilal et al., 2013). Siti et al., (2007) used a mini pilot batch reactor for the process and obtained the following optimal conditions; temperature: 120°C, pressure: 20 mbar, molar ratio: 3.8: 1 (POME to TMP), 2 hours reaction time, catalyst: 0.9 w/w% and speed of agitation: 180 rpm