separations Article Separation and Purification of !-6 Linoleic Acid from Crude Tall Oil Md Shariful Islam, Lew P. Christopher and Md Nur Alam * Biorefining Research Institute, Lakehead University, 1294 Balmoral Street, Thunder Bay, ON P7B 5Z5, Canada; [email protected] (M.S.I.); [email protected] (L.P.C.) * Correspondence: [email protected] Received: 28 November 2019; Accepted: 22 January 2020; Published: 2 February 2020 Abstract: Crude tall oil (CTO) is the third largest by-product at kraft pulp and paper mills. Due the large presence of value-added fatty and resin acids, CTO has a huge valorization potential as a biobased, readily available, non-food, and low-cost biorefinery feedstock. The objective of this work was to present a method for the isolation of high-value linoleic acid (LA), an omega (!)-6 essential fatty acid, from CTO using a combination of pretreatment, fractionation, and purification techniques. Following the distillation of CTO to separate the tall oil fatty acids (TOFAs) from CTO, LA was isolated and purified from TOFAs by urea complexation (UC) and low-temperature crystallization (LTC) in the temperature range between 7 and 15 C. The crystallization yield of LA from CTO in − − ◦ that range was 7.8 w/w at 95.2% purity, with 3.8% w/w of !-6 γ-linolenic acid (GLA) and 1.0% w/w of !-3 α-linolenic (ALA) present as contaminants. This is the first report on the isolation of LA from CTO. The approach presented here can be applied to recover other valuable fatty acids. Furthermore, once the targeted fatty acid(s) are isolated, the rest of the TOFAs can be utilized for the production of biodiesel, biobased surfactants, or other valuable bioproducts. Keywords: crude tall oil; essential fatty acids; linoleic acid; γ-linolenic acid; α-linolenic acid; distillation; urea complexation; low-temperature crystallization 1. Introduction The pulp and paper industry has joined the global movement toward cleaner and greener products in their effort to transition to forest biorefineries. This transition is based on the circular bioeconomy concept, which implies the most efficient recycling of wastes, co-products, and residues as a strategy to preserve our bio-based resources and minimize the use of fossil fuel-derived products of environmental concern. As bio-based fuels and products are carbon-neutral, they do not result in net green-house gas (GHG) emissions due to the balance between CO2 emitted and CO2 trapped back by biomass. After lignin and hemicellulose, CTO is the third largest chemical by-product in a kraft pulp and paper mill, with a yield from highly resinous coniferous species in the range of 30–50 kg per ton pulp [1]. CTO is extracted from the partially concentrated black liquor, derived from kraft pulping of softwoods, by skimming off the kraft oil soap. CTO has a complex composition of saponified fatty acids (30–60%), resin acids (40–60%), and unsaponifiables (5–10%) [2]. However, the exact yield and composition of CTO depend on a number of factors such as pulping conditions, wood species, and their length of storage prior to pulping, geographical location, climate conditions, etc. [3]. If not used internally as a lime kiln hoq fuel, mills most commonly sell CTO at a profit of 1–1.5% of their total revenue to chemical companies for further refining to various products. For example, CTO is applied in producing metal-working fluids for metal-rolling and metal-working operations, in producing drill fluids and separating fluids for the concrete products industry as well as to obtain corrosion inhibitors [4]. The TOFAs are primarily used in the production of soaps, detergents, paints, Separations 2020, 7, 9; doi:10.3390/separations7010009 www.mdpi.com/journal/separations Separations 2019, 6, x FOR PEER REVIEW 2 of 11 producing drill fluids and separating fluids for the concrete products industry as well as to obtain corrosion inhibitors [4]. The TOFAs are primarily used in the production of soaps, detergents, paints, coatings,Separations plastic2020, 7, additives, 9 fuel additives, lubricants, and adhesives [5–10]. Alkyd resins account2 of 11 for the largest demand among all intermediates manufactured from TOFAs, whereas soaps, detergents, and coatings hold the largest market share of TOFAs in terms of volume. More recently, TOFAs have beencoatings, investigated plastic additives,for the production fuel additives, of lubricants,biofuels including and adhesives drop-in [5–10 hydrocarbon]. Alkyd resins fuels account [11] for and the largest demand among all intermediates manufactured from TOFAs, whereas soaps, detergents, biodiesel [12,13] The market for TOFAs is expected to reach USD 1 billion in 2022 [14]. In the U.S., and coatings hold the largest market share of TOFAs in terms of volume. More recently, TOFAs CTO has a floor price the equivalent of the value of natural gas as CTO can be used as a substitute to have been investigated for the production of biofuels including drop-in hydrocarbon fuels [11] and fossil fuels as a pulp mill lime kiln process fuel. The current global annual production of CTO is biodiesel [12,13] The market for TOFAs is expected to reach USD 1 billion in 2022 [14]. In the U.S., approximately 1.8–2 million tons (t), however, the 320 chemical pulp mills worldwide have the CTO has a floor price the equivalent of the value of natural gas as CTO can be used as a substitute potentialto fossil to fuels produce as a pulp2.6 million mill lime t per kiln year process [15]. About fuel. 65% The currentof the total global CTO annual production production is concentrated of CTO in isNorth approximately America. In 1.8–2 Canada, million 12 tonskraft (t), mills however, currentl they generate 320 chemical 75,000 pulp t CTO, mills whereas worldwide in the have U.S., the two thirdspotential of the to TOFA produce supplied 2.6 million to the t per chemical year [15]. indust Aboutry 65% are of derived the total from CTO CTO, production with another is concentrated 100,000 t of inCTO North expected America. to become In Canada, available 12 kraft by mills2020 [16]. currently generate 75,000 t CTO, whereas in the U.S., twoThe thirds main of theTOFAs TOFA in supplied CTO are to theLAchemical (C18:2n6) industry (Figure are 1), derived oleic acid from (C18:1), CTO, with GLA another (C18:3n6), 100,000 and t palmiticof CTO acid expected (C16:0) to become[17]. Typically, available the by 2020TOFAs [16]. consist of 75–85% unsaturated fatty acids (UFAs), mainlyThe polyunsaturated main TOFAs in fatty CTO areacids LA (PUFAs), (C18:2n6) (Figurewith LA1), comprising oleic acid (C18:1), about GLA half (C18:3n6), the total TOFAs and palmitic [13,18]. LAacid is an (C16:0) 18 carbon [17]. Typically,polyunsaturated the TOFAs fatty consist acid with of 75–85% the two unsaturated double bonds fatty at acids C9 (UFAs),and C12 mainly in a cis- configurationpolyunsaturated (all-cis-9,12-octadecadienoic fatty acids (PUFAs), with LA acid comprising) (Figure about1). LA half is denoted the total TOFAsas an omega [13,18]. ( LAω)-6 is anfatty acid18 carbonand together polyunsaturated with ALA fatty ω-3 acid acid with (18:3n3) the two (Figure double bonds1) form at C9theand two C12 essential in a cis-configuration fatty acids that mammals(all-cis-9,12-octadecadienoic (humans and animals) acid) cannot (Figure synthesize1). LA is denoted and must as anbe omegaacquired ( ! through)-6 fatty acidtheir and diet. together Another ω-6with fatty ALA acid! -3of acid health (18:3n3) importance (Figure 1is) formGLA the (18:3n6), two essential which fattyis classified acids that as mammals a conditionally (humans essential and fattyanimals) acid that cannot can synthesize become essential and must under be acquired developme throughntal their or disease diet. Another conditions!-6 fatty(Figure acid 1). of LA health as an essentialimportance fatty is acid GLA plays (18:3n6), an whichimportant is classified role in as metabolic a conditionally processes essential related fatty acidto human that can health become and nutrition,essential and under may developmental be a factor in or a disease number conditions of degenerative (Figure1 illnesses). LA as an such essential as cancer, fatty acidatherosclerosis, plays an osteoporosis,important role and in cardiovascular metabolic processes disease related [19–21]. to human Nowadays, health the and public nutrition, awareness and may of bethe a nutritional factor in a number of degenerative illnesses such as cancer, atherosclerosis, osteoporosis, and cardiovascular and health benefits of LA is growing, and the market demand is expected to rise in the future. Aside disease [19–21]. Nowadays, the public awareness of the nutritional and health benefits of LA is from its health benefits, LA is used in making oil paints and varnishes, in cosmetics, and as a growing, and the market demand is expected to rise in the future. Aside from its health benefits, LA is surfactant [22,23]. Sources of LA include fish and shellfish, vegetable oils, nuts, and seeds. However, used in making oil paints and varnishes, in cosmetics, and as a surfactant [22,23]. Sources of LA include most of these sources are food-based, seasonal, of limited availability, and fairly expensive. In fish and shellfish, vegetable oils, nuts, and seeds. However, most of these sources are food-based, addition, fish may be contaminated with methyl mercury and lead, which are heavy metals with high seasonal, of limited availability, and fairly expensive. In addition, fish may be contaminated with environmentalmethyl mercury and and human lead, whichtoxicity are [24]. heavy metals with high environmental and human toxicity [24]. Figure 1. Chemical structures of essential (LA, ALA) and conditionally essential (GLA) fatty acids.