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High-purity fatty acid n-octyl esters from housefly (Musca domestica) larval lipid, a potential new biolubricant source Zi-Zhe Cai, Sheng-qing Wu, Guo Sun, Yi Niu, Dong Zheng, Shi-yi Peng, Wen-zhe Yang, Yong Wang, and Depo Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01996 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017
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Energy & Fuels
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High-purity fatty acid n-octyl esters from housefly (Musca domestica L.) larval
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lipid, a potential new biolubricant source
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Zi-zhe Caia, Sheng-qing Wua, Guo Sunb, Yi Niua, Dong Zhenga, Shi-yi Penga,
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Wen-zhe Yangc, Yong Wangb,*, De-po Yanga,*
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a School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou,
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Guangdong Province, People’s Republic of China,
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b Dept. of Food Science & Engineering, Guangdong Saskatchewan Oilseed Joint Lab.,
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Jinan University, Guangzhou, Guangdong Province, People’s Republic of China,
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c Zhongshan Unicare Natural Medicine Co, Ltd, Zhongshan, Guangdong Province,
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People’s Republic of China.
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* Corresponding author.
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Abstract:
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With the traditional lipid feedstock cost constituting 70% of the total biolubricant
14
production, it is urgent to find a new lipid source biodiesel and biolubricant industry.
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With elevated acid value (61.8 mg KOH/g) of housefly larval lipid, converting free
16
fatty acid (FFA) into biolubricant could serve as a value-added way to the larvae
17
industry, other than pretreated with acid-catalyzed esterification for biodiesel
18
production. This study developed a method of producing high-purity fatty acid n-octyl
19
esters (FAOEs) from housefly (Musca domestica L.) larva. The housefly larva free
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fatty acid (HLFFA) from the larva lipids was obtained through wipe-film short path
21
distillation. FAOEs were produced by esterifying n-octanol with HLFFAs (catalyzed
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by benzenesulfonic acid under the following conditions: catalyst loading, 2 wt.%;
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molar ratio of n-octanol to FFA, 3:1; temperature, 100 °C; time, 2 h). Excess
24
was effectively removed by high vacuum (80 Pa) distillation at 90 °C. The unreacted
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FFA were neutralized using demethylated crude glycerin, which is the by-product of
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biodiesel production. A practical method of producing high purity fatty acid n-octyl
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esters (99.19 wt.%) derived from housefly larva lipid is developed and the product
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could serve as replacement of certain low viscosity mineral lubricants such as liquid
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paraffin, polyalphaolefin 6 and SN500.
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Keywords: Musca domestica L.; biolubricant; fatty acid n-octyl ester; esterification;
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crude glycerin neutralization.
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1. Introduction
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A lubricant or lube is a liquid that provides functions of reducing the friction
34
between moving surfaces, improving efficiency, reducing wear, preventing surface
35
corrosion, removing wear particles , and dissipating heat from wear surfaces 1. The
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estimated global biolubricant demand reaches 42.1 million metric tons by 2017 at an
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annual increase of 2% 2, however, the mineral-based lubricant occupies about 98.8%
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base oil market share 3. One of the critical obstacle preventing sustainable bio-based
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lubricant from broader application is the feedstock cost. As its primarily feedstock is
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virgin vegetable oil (such as palm oil
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higher than conventional mineral-based oils 6. In developing countries which are not
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self-sufficient in edible oils 7,8, such lipid source would not be suitable because of the
4
and rapeseed oil 5), the cost is four to five
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inadequate supply and high price of edible oils. Therefore, finding a low-cost and
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non-food feedstock for biolubricant production is urgently needed.
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Insect larva is a renewable source of protein and lipid. Both laboratory scale
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research (black soldier fly, Hermetia illucens L.9; flesh fly, Boettcherisca peregrine10;
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blow fly Chrysomya megacephala8; and Zophobas morio11) and pilot level production
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(blow fly, Chrysomya megacephala12) have utilized larva lipid for biodiesel
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production. With large amounts of municipal waste (nearly 1.3 billion tons worldwide)
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and livestock manure (1.9 billion tons in China) generated annually, considerable
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amount of lipid could be produced by rearing insect larva on such wastes 13. By now,
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insect lipid used as biodiesel feedstock have drawback of higher acid value. Our
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previous research has demonstrated that additional heating energy (nearly tripled) is
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required for heating acylglycerols (during esterification process of free fatty acid,
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FFA) and methanol recovery step, if acid-catalyzed pre-esterification is applied
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The acid value of lipid from reported insect species usually is higher than virginal
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vegetable oil (2-21 mg KOH/g)
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FFA into FAME via aforementioned two-step method.
14
.
9,11,15
, and it is not energy efficient converting larval
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The housefly Musca. domestica L. (Diptera: Muscidae) is a well-known
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cosmopolitan holometabolous insect with features like: can be fed with various source
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of waste materials
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reproductive rate. Compared with black solider flies (H. illucens) housefly has shorter
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larvae period (4-5 days v.s. 33 days) and longer adult longevity (over a month v.s. 15
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; contains high lipid content (around 20%)
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; faster and
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days) 17, and has similar lipid content (about 20%). Besides, after lipid extraction, the
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feed residues can be used as a fertiliser, and the defatted larvae proteins can be used
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livestock feed
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domestica L. larvae possess highly active lipase, and their fatty acid content readily
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increases during fresh larva processing in response to unfavourable conditions
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Instead of reducing the unfavourable FFA, the alternative is to converting fatty acid
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value-added product such as biolubricant. Through wipe-film distillation (small scale)
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or deodorizer (industry scale), FFA could be separated from larval lipid and served as
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low-cost feedstock for biolubricant.
18
, making it a value-added product for housefly lipid industry. M.
12
.
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Produced from esterification with another low-cost feedstock, n-octanol, which is
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the cheapest among long-chain alcohols (C8–C14) 19, fatty acid n-octyl esters (FAOEs)
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has applications (as low viscosity fluid) like modern two-stroke engines 20, cold-metal
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rolling 21, coolants for metal machinery 22 and drilling fluids for marine oil industry 23.
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Such low viscosity lubricant has advantages of low toxicity, biodegradable, miscible
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with various additives, oxidative stable and devoid of undesirable impurities
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However, there are drawbacks in reported FAOE production studies (Table 1): a)
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longer reaction time is required to achieve high conversion (above 98%) when using
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acid
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p-toluenesulfonic acid
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addition of alcohol and catalyst when using enzymatic esterification
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conversion rate is relatively low; c) unclear explanation on n-octanol removal; d)
or
supported
catalyst
(nitrogen-doped
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, and amberlyst 15H
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reduced
graphene
oxide
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.
25
,
with reaction time of 4-24 h); b)
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and
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insufficient details on removing unreacted FFA; e) comprehensive lubricity evaluation
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remains to be evaluated (e.g. water separability, non-seizure load, and weld load). In
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our previous research, 98.6% esterification rate was achieved under conditions: 3:1
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molar ratio of 2-ethyl hexanol to FFA, 2 h, 2 wt.% loading of BSA (benzenesulfonic
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acid)
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n-octanol removal requires high vacuum (near 100 Pa) other than methanol (5000 Pa),
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With larger processing capacity than Kugelrohr evaporator, n-octanol can be removed
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completely (residual n-octanol is lower than 0.5%) through wipe-film distillation,
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which is promising for massive production. Lubricant product with FFA content
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than 2% (approximately 4 mg KOH/g) is not acceptable to the ASTM and EU
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for biolubricant (< 0.6 mg KOH/g), which might cause possible damage of filtration
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system
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evaporation nor high-vacuum molecular distillation can separate FFAs and FAOEs
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because their actual evaporation pressures are close (preliminary experiment). FFA in
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the crude FAOE product can be effectively neutralized by alkali refining, which is
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30
. According to the finding of Moser et al. using Kugelrohr evaporator
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,
22
. For the unreacted FFA in the esterification product, neither low-pressure
developed in the edible oil refining industry 30.
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In the present study, high-purity FAOEs were produced using FFA from housefly
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(M. domestica) larvae lipid. The FFA fraction from housefly larval lipids (housefly
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larva free fatty acid, HLFFA) was obtained via wipe-film short path distillation and
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followed by benzenesulfonic acid (BSA)-catalyzed esterification with n-octanol.
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Excess n-octanol in the esterification product was removed through wipe-film short
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path distillation. The residual fatty acids in the product were neutralized with the
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by-product, demethylated crude glycerin (see Figure 1). The tribological properties of
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the produced housefly larval FAOEs were tested for the first time. The present study
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demonstrates a practical method to produce high-purity biolubricant (FAOEs).
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Developing an added-value product is a promising solution for biodiesel production
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because it can extend the production supply and supplement the biodiesel or edible
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refining industry, from which the by-products (FFAs) can be used as feedstock for
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FAOE production.
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2. Materials and methods
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2.1 Materials
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M. domestica larvae were obtained from Zhongshan Unicare Natural Medicine,
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Zhangshan, Guangdong. The larvae were identified by Prof. Hong Pang from the
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State Key Laboratory of Biocontrol and Institute of Entomology, Sun Yat-sen
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University. Analytical grade n-octanol, n-hexane, sodium hydroxide, benzenesulfonic
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acid were purchased from Damao Chemical Reagent Factory (Tianjin, China). The
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esterification and neutralization apparatus consisted of an oil bath with a magnetic
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stirrer, a digital temperature controller fixed in the oil bath and a 250 mL single-neck
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round-bottom flask. Kitchen waste was collected from the canteen of Zhongshan
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Unicare Natural Medicine, Zhangshan, Guangdong.
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The neutralization agent (for section 2.7), demethylated crude glycerin, was
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produced from transesterification of housefly larval acylglycerols (HLAG) with
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methanol, and characterized (supporting information). The base value of
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crude glycerin was 50.4 mg KOH/g.
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2.2 Larva inoculation
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Kitchen waste was collected 2 h before housefly larva inoculation. The kitchen
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waste was rinsed with hot water (80 °C) and cooled down. The resulting feeds for the
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larvae were spread evenly to a depth of approximately 5 cm in each plastic tank (60
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cm × 20 cm × 15 cm). The inoculum density was 8000 larvae/kg feed. After
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inoculation, the plastic tanks were maintained at a temperature of 28 °C with a
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relative humidity of 75% and a photoperiod of 12:12 (L:D).
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After 5 days, the mixture containing the feeds and larvae was blended with dry
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wheat bran to reduce the moisture and viscosity of the medium. The mixture was
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poured into a larva collector equipped with screens and two slopes. The obtained
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larvae were mostly free of feed residue and the total mass of fresh larvae was
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recorded. 2.3 Processing of fresh larvae and extraction of housefly larval oil
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For each patch, 200 g of extracted fresh larvae were introduced into a sealed glass
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box (5 cm × 20 cm × 20 cm) and placed in a 50 °C oven for 5 h. The preheated larvae
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were incapacitated with boiling water (100 °C) and then oven dried at 80 °C for 6 h.
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10 kg dried housefly larvae were crushed to a 60 mesh powder by a powder
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machine. Oil extraction was executed using a single-neck round-bottom reactor
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equipped with condenser, the addition of n-hexane is 60 kg (mass ratio of n-hexane to
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larva powder: 6:1) for 5 h at room temperature (25 °C). Subsequently, the solvent
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filtrated via vacuum filtration, then it was removed under reduced pressure using a
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rotary evaporator at 50 °C. The lipid content (%) was calculated by dividing the mass
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extract lipid with mass of larvae powder.
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2.4 Separation of HLFFAs
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HLFFA was separated by a wipe-film short path device (MD 80; Guangdong
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Hanwei Co., Ltd.) with a falling film evaporator (area: 0.1 m2) and an internal
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condenser (area: 0.05 m2). A jacketed glass vessel with a flow-regulating valve was
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used to introduce oil to the distillation equipment. The vacuum system was composed
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of a mechanical pump and a diffusion pump. The separation parameters are as follows:
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temperature, 120 °C; feeding rate, 200 mL/h; evaporator vacuum, 0.5–1.0 Pa (using
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both mechanical pump and diffusion pump); condenser temperature, 50 °C; and
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feeding temperature, 80 °C. In each patch, 1 kg of extracted larval lipid was subjected.
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The distillate (mostly HLFFAs) and the residue (mostly housefly larva acylglycerols,
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HLAGs) were collected. The HLFFAs would be used as feedstock for the subsequent
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esterification with n-octanol (biolubricant production), whereas the HLAGs would be
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transesterified with methanol (biodiesel production).
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2.5 Esterification of HLFFAs with n-octanol
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The rate of FFA esterification (ER) into n-octanol esters depends on several
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factors, such as reaction temperature, reaction time, catalyst loading, stirring rate and
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n-octanol content. Single-factor experiments were performed to evaluate the
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parameters affecting the ER. The catalyst ranged from 0 wt.% to 4 wt.% (based on the
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HLFFA mass) with an interval of 1 wt.%. The n-octanol-to-HLFFA molar ratio
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ranged from 1:1 to 9:1 with an interval of 2:1. The reaction time ranged from 1 h to 5
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h with an interval of 1 h. The reaction temperature ranged from 80 °C to 160 °C with
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an interval of 20 °C. The stirring rate was fixed (300 rpm).
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HLFFA sample (50.0 g), n-octanol and benzenesulfonic acid were introduced to a
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round-bottom flask equipped with a reflux condenser. The reaction was conducted in
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an oil bath under aforementioned reaction conditions at a vacuum level of 5000 Pa.
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Water was removed while n-octanol was refluxed throughout each reaction. After
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each experiment, the mixture was washed with hot water (80 °C) twice to remove
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benzenesulfonic acid residues and were saved for further analysis. Each experiment
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was performed in triplicate, and the means and standard deviation (means ± SD) were
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reported.
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The ER of HLFFA to FAOE was calculated using the following equation:
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ER = 1 - AVest/AV0,
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where AVest and AVo refer to the AV of the esterified product and initial AV of
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the reactant mixture (HLFFA with n-octanol), respectively.
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2.6 n-Octanol removal via wipe-film distillation
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The esterification product under optimum condition was washed with hot water
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(80 °C) twice to remove the acid catalyst and subjected for n-octanol removal
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processing. The excess n-octanol was removed using the aforementioned distillation
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device (MD 80; Guangdong Hanwei Co., Ltd.) operated without the diffusion pump
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(In this case, the device served as high vacuum distillation). As the vacuum is
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determined by the structure of the distillation apparatus, the vacuum degree was fixed
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at 80 Pa. Feeding rate was fixed at 400 mL/h, as faster feeding rate would raise
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problem of pressure stabilization due to the evaporated n-octanol. A single-factor
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experiment of evaporating temperature was conducted under conditions: vacuum
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degree, 80 Pa; condenser temperature, 20 °C; and feeding temperature, 80 °C. The
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evaporator temperatures ranged from 60 °C to 120 °C with an interval of 10 °C. The
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distillate was mostly n-octanol, and it was collected for repeated use. The residue
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enriched with FAOEs was weighed. The AV of each removal experiment was
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determined. The effectiveness of each experiment was expressed as octanol removal
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efficiency (RE) and FAOE recovery rate (RR), which were calculated via 1H-NMR
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analysis (section 3.2). Concentrations (C, wt.%) of FAOE, n-octanol in the samples
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were determined. Each experiment was performed in triplicate, and the means and
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standard deviation (means ± SD) were reported.
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The removal efficiency (RE) of n-octanol was calculated by equation:
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REn-octanol (wt.%) = (C’-Cn-octanol)/ C’
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Where C’, Cn-octanol are the n-octanol concentration (wt.%) before and after each
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208
distillation, respectively.
The recovery rate (RR) of FAOE was calculated by equation:
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RRFAOE (wt.%) = (Ccrude FAOE × mcrude FAOE - CFAOE × m FAOE)/ Ccrude FAOE × mcrude FAOE
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Where Ccrude FAOE, CFAOE, mcrude FAOE, m FAOE are FAOE concentration (wt.%) of
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crude FAOE, FAOE concentration (wt.%) of FAOE after n-octanol removal process,
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mass (g) of crude FAOE subjected to wipe film distillation, and mass (g) of FAOE
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after n-octanol removal process, respectively.
215 216
2.7 Neutralization of residual FFA with crude glycerin from biodiesel production
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After n-octanol removal, the n-octanol-removed product (under optimum
218
condition) was collected. On the basis of our preliminary experiment on crude
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glycerin neutralization (Figure S2), 5 g of the demethylated crude glycerin 5 g and 50
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g of n-octanol-removed product (under optimum condition) were introduced to a
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single-neck round-bottom flask. The mixture was heated in an oil bath at 80 °C for 20
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min with an agitation rate of 300 rpm. After the reaction, flask contents were
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separated into two layers in a separatory funnel. The upper layer was clear orange
224
(FAOEs product), which was then washed with hot water (80 °C) twice. The washed
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upper layer was dried in a vacuum oven for 2 h at 60 °C, then the mass of product is
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weight (g) and yield (wt.%) is recorded (mass of product divided by mass of crude
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FAOE used for neutralization).
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3. Analysis 11
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3.1 Acid value determination
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Acid values of sample from crude larval lipid, esterification, n-octanol removal
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process, and neutralization were determined through titration with KOH in
232
accordance with AOCS Method Ca 5a-40.
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3.2 Nuclear Magnetic Resonance (NMR) Spectroscopy.
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NMR spectra was obtained with a Bruker AM-400 spectrometer at 25 °C. NMR
235
data collection and analysis were conducted with MestReNova 11.0.3 software
236
(Mestrelab Research S.L.).
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For analysis of n-octanol removal products and the final product, deuterated
238
chloroform (NMR grade, Aladdin Industrial Corporation, Shanghai, China) and
239
3,4,5-trichloropyridine (TCP) (NMR grade, Aladdin Industrial Corporation, Shanghai,
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China) were used as solvent and internal standard, respectively. 4.83 mg internal
241
standard and 81.4 mg sample were dissolved in 500 µL solvent. The molecular weight
242
of FAOE, n-octanol, TCP are 371.5, 130.2, and 182.43 g/mol, respectively. The triplet
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at 4.07 ppm (2H, t, CH2O) refer to FAOEs
244
-CH2OH) refers to n-octanol. The singlet at 8.53 ppm (4H, s) refers to TCP (internal
245
standard). All above mentioned peaks were integrated for quantitative analysis.
32
and the triplet at 3.65 ppm (2H, t,
246
The FAOE concentration were calculated by equation:
247
CFAOE (%) =(mTCP/msample) × (IFAOE/ITCP) × (MFAOE/MTCP)× (HTCP/HFAOE)
248
The n-octanol concentration was calculated by equation:
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Cn-octanol (%) =(mTCP/msample) × (In-octanol/ITCP) × (Mn-octanol/MTCP)× (HTCP/Hn-octanol)
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where m denotes the weight in g, M the molecular weight in g/mol, H the marked
251
proton number of each compound and I the NMR integral area.
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3.3 FAOE composition via GC-MS
253
The FAOE composition of the final product was analyzed by using a Finnigan
254
GC-MS system equipped with a DB-WAX capillary column (50 m × 0.25 mm i.d.,
255
0.25 mm film thickness) (Trace-Ultra/DSQ; Thermo Electron Co., Waltham, US)
256
with helium as carrier gas at a flow rate of 1.0 mL/min and a pressure of 100 kPa. The
257
split ratio was 1:30. The oven temperature was set to 210 °C, and the holding time
258
was 21 min. Electron ionisation mode (ionisation energy of 70 eV) was used for GC–
259
MS detection. The temperature of the injector and MS transfer was 250 °C. The
260
scanning mass range selected was 30–650 m/z. The sample volume of each injection
261
was 1.0 µL. Data were managed with Xcalibur software with the NIST mass spectra
262
library database. The relative contents of each FAOE species were calculated using
263
the area normalisation method, and the results were reported as percentages of the
264
total peak area. The determination was performed in triplicate, and the means and
265
standard deviation (means ± SD) were reported.
266
3.4 Physical and chemical properties of the final product
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The final product was analyzed by Guangdong Inspection and Quarantine
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Technology Center (Guangzhou, PRC). The testing methods for viscosity, flash point, 13
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base value, AV, density, copper strip corrosion, pour point, water separability and
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sulphur content are listed in Table 3. Lubricity test, including wear scar diameter
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(WSD), last non-seizure load (PB value) and weld load (PD value), was conducted
272
using a four-ball friction and wear-testing machine according to Chinese standard
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GB/T3142-1982, which was similar with ASTM D2783. The steel ball was made of
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bearing steel GCr15 (hardness of 60 HRC), which was ultrasonically cleaned before
275
and after testing with petroleum ether and acetone. The WSD of stationary balls was
276
measured using a Nikon PFX optical microscope installed with a Nikon F-301 CCD
277
camera. The PB value was determined in accordance with ASTM D2783. The rotating
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speed was 1450 rpm, and a series of tests with 10−s duration at increasing load was
279
conducted. The PB value was determined when the corresponding WSD was not more
280
than 5% above the compensation line described in ASTMD-2783. The PD value was
281
the lowest applied load (1450 rpm, 10−s) when rotating ball welds to stationary balls.
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Free glycerin and total glycerin content was analyzed according to ASTM standard
283
D6584-13. The soap content was determined by AOCS method Cc 17-95.
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4. Results and discussion
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4.1 Processing of fresh larvae and lipid extraction.
286
Approximately 109 ± 2.8 g of fresh larvae were obtained from each kg of feeds,
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thus 23.98 ± 1.1 g dried larvae could be harvested from each kg of feeds. After
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solvent extraction, 23.2 kg larval lipid could be obtained from 100 kg dried larvae.
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The acid value of larval lipid was 61.8 mg KOH/g. (FFA content 27.9 wt.%). The
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AVs of the distillate (HLFFA) and residue (HLAG) after wipe-film distillation were
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215.97 and 0.56 mg KOH/g, respectively.
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Owing to the nature of insect lipid metabolism, the acylglcyerols were inevitably
293
hydrolysed during mass production: The larvae would response to unfavorable living
294
environment as the temperature elevated by increased population density during
295
extraction process. Arrese et al. 33 suggested that the lipase in the insect fat body is an
296
active phospholipase A1. In addition, the lipid hydrolysis process could be initiated
297
by the AKH (adipokinetic hormone) which excretes from corpora cardiac of insects.
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Similar to the active temperature of commercialised phospholipase A1, the active
299
temperature range was 30–50 °C. The lipase exhibits both triacylglycerol-lipase and
300
phospholipid-lipase abilities, indicating that the lipase hydrolysed the membrane and
301
had access to the stored acylglycerols. Among larval incapacitation methods such as
302
sun drying, freezing and refrigeration, heating at 50 °C without ventilation greatly
303
increases the lipid AV of larvae in a short period of time without adding any
304
hydrolysis agent. The elevated temperature had two possible effects on the larval lipid
305
hydrolysis: a) It stimulated the larval response towards unfavourable temperatures and
306
b) altered the membrane structure of the lipid drop in the fat body, allowing the lipase
307
to access the triacylglycerols. In addition, in the preliminary experiment at 50 °C with
308
ventilation, the AV increased to only 25.1 mg KOH/g because larvae were dehydrated.
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When processed without ventilation, the AV of the larval lipid increased to 61.8 mg
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KOH/g as less water was evaporated (as a reactant of lipid hydrolysis). As mentioned
311
in the introduction, most of the produced FAOEs involve high-purity fatty acids
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feedstock. Thus, producing such self-hydrolyzed lipid is efficient and economic
313
compared with those involving lipase for lipid hydrolysis 19. Housefly larvae fed with
314
various waste materials exhibit high adaptability and high resistance to diseases. Thus,
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the massive production of such high-AV lipids can greatly reduce the cost of
316
biolubricant production, and various lipid products like various kinds of fatty acid
317
esters can be produced with such lipid feedstock.
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4.2 Esterification of HLFFAs with n-octanol
319
As the catalyst loading increased from 0.5 wt.% to 2 wt.%, the ER (esterification
320
rate) was elevated from 78.7% to approximately 93% before it reached a plateau
321
(Figure 2a.). High conversion (over 90%) can be achieved within a short reaction time
322
(2 h vs. 6 h) and BSA requires fewer amounts of n-octanol compared with
323
p-toluenesulfonic acid
324
n-octanol in esterification and subsequence recovery step is conserved. In room
325
temperature (25°C), BSA forms white deliquescent sheet crystals or a white waxy
326
solid, with melting point and boiling point of 44 °C and 137 °C 34. Thus it is easy to
327
handle, transfer, and storage
328
catalyst melts to catalyze esterification and could be recovered for recycle usage after
329
cooling 34. Compared with other acid catalysts (e.g. heteropoly acid, sulfated zironcia,
330
zeolite, and other supported catalyst)
26
(3:1 vs. 10:1). In addition, the heating energy for both
35
. Under elevated reaction temperature (100 °C), the
25
, BSA has advantages like widely available, 16
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and no preparation is required. In addition, unlike other substituents (e.g.
332
3-amino-4-methylbenzenesulfonic acid; 4-amino-3-methylbenzenesulfonic acid),
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BSA is reported 90% biodegradable 36, and it can be recovered by high vacuum.
334
As displayed in Figure 2b, the initial molar ratio of n-octanol to FFA was 1:1, and
335
the reaction was conducted at 100 °C for 2 h with a catalyst loading of 2 wt.%. The
336
ER increased from 85.1% to 93.1% when the molar ratio of n-octanol to FFA reached
337
3:1. The ER then slightly decreased to 92.8% as the molar ratio continued to increase
338
to 9:1. The decrease was probably due to the dilution of BSA catalyst. Compared with
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enzymatic esterification
340
greatly reduced, with higher conversion rate (94% vs. 85%). With regard to energy
341
efficiency and feedstock cost, a 3:1 molar ratio of n-octanol to FFA is recommended.
19,27
, the amount of n-octanol required for esterification was
342
As illustrated in Figure 2c, molar ratio of n-octanol (3:1) to FFA is based on the
343
result of above experiment, and the ER was around 80% initially because the lower
344
temperature decreases intermolecular contact of each reactant. The ER increased to
345
approximately 92% and then reached a plateau as the temperature exceeded 100 °C.
346
In theory, high temperatures can promote the reaction equivalence towards the
347
positive direction. However, this condition could induce the evaporation of reactants,
348
especially n-octanol, and thus could reduce the probability of intermolecular contact.
349
In addition, high reaction temperatures might lead to the generation of darkly
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coloured esterification products found by reported n-octyl ester production
351
such instance, a reaction temperature of 100 °C is recommended.
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19,37
. In
352
As described in Figure 2d, the ER was 87% after 1 h of reaction. It increased to
353
approximately 93% and became stable as the reaction time was prolonged. Therefore,
354
a reaction time of 2 h is recommended, whereas reported FAOE synthesis used
355
prolonger reaction time to achieve conversion rate of above 90% (4–6 h for other
356
types of acid catalyst and 6–10 h for enzymatic esterification) 19,26,28.
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On the basis of the single-factor experiments above, the optimum reaction
358
conditions were as follows: temperature, 100 °C; catalyst loading, 2 wt.%; molar ratio
359
of n-octanol to FFA, 3:1; and time, 2 h. The ER under such conditions was 95.1%.
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4.3 n-Octanol removal via wipe-film distillation
361
As shown in Figure 3, as the temperature was elevated from 60 °C to 100 °C, the
362
RE of n-octanol increased sharply from 30.0 % to 98.8 % and then reached a plateau
363
above 99 %. This result indicates that the wipe-film distillation was effective and
364
efficient for n-octanol removal. Such results were also consistent with a previous
365
study
366
and 65 Pa. In addition, RR of FAOEs slightly decreased from 96.7 % to 91.3 % when
367
the temperature exceeded 90 °C, indicating that trace amount of FAOEs could have
368
been distilled as colour change of distillate was observed (transparent into yellow). A
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high RR of FAOEs can be achieved using high feeding rate (400 ml/h), as both
26
where excess octanol was removed through Kugelrohr evaporator at 120 °C
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distillation time and the possibility of FAOE being distilled can be decreased. The
371
decrease of RR at higher temperature also indicates that the optimum distillation
372
temperatures for FFA and FAOE are close (approximately 90 °C). Thus, it is
373
impossible to separate FFAs from FAOEs using high vacuum distillation. The AV
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slightly increased from 5 mg KOH/g to approximately 7.0 mg KOH/g and then
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decreased to 5.8 mg KOH/g when the temperature exceeded 90 °C. The increased AV
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was due to elevation of FFA concentration (n-octanol was removed). The
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subsequence AV decrease was observed because more FFA is distilled at higher
378
temperature. Considering above, the optimum evaporator temperature is 90 °C with
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n-octanol removal rate of 98.9 wt.%, FAOE recovery rate of 95.6 wt.%, and acid
380
value of 6.69 mg KOH/g.
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4.4 FAOEs neutralization by crude glycerin
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Under the optimum conditions of n-octanol removal, the AV of the crude FAOE
383
residue was 6.69 mg KOH/g. As suggested in previous studies, one drawback of
384
acid-catalyzed esterification is that the AV of the product is greater than 2.0 mg
385
KOH/g, which does not satisfy the AV requirement of the ASTM and EU standards
386
(
