Recovery and Purification of Crude Glycerol from Vegetable Oil Transesterification: A Review

Separation and Purification Reviews (2014)

Recovery and Purification of Crude Glycerol from Vegetable Oil TransesterificationWan Nor Roslam Wan Isahak1,2, Zatil Amali Che Ramli2, Manal Ismail1, Jamaliah Mohd Jahim1, Mohd Ambar Yarmo2,*1Department of Chemical Engineering and Process, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia.

2School of Chemical Sciences and Food Technology, Faculty of Science and Technology,

Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia.

Submitted August 19, 2011; revised September 24, 2013; accepted September 30, 2013Short running title: Glycerol recovery from vegetable oil

ABSTRACT: This article reviews the purification techniques involved in producing high-purity glycerol in the biodiesel industry. Utilization of glycerol by-products (containing less than 50 wt% of glycerol in water, salts, unreacted alcohol and catalyst) in biodiesel production affords greener and less costly processes. Research has focused on several purification steps that are capable of producing high-purity glycerol. Various new techniques for purifying glycerol with better quality and lower cost and simple technologies are required to fulfill the increasing worldwide glycerol demand. Neutralisation, ultrafiltration, the use of ion exchange resins, and vacuum distillation are methods that have been utilised in single or multiple stages. Recent studies have demonstrated that the combination of more than one technique produces high-purity glycerol (>99.2%). Purifications cost can be as low as US$0.15 per kg. For many applications, such as hydrogen production, methanol production, pharmaceuticals and food additives, high-purity glycerol is needed. The glycerol purification methods are reviewed.Key words: Crude glycerol, Purification, Ion exchange resins, Separation technique, Ultrafiltration, Vacuum distillation.

Address correspondence to Prof. Dr. Mohd Ambar Yarmo, School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Malaysia. E-mail: ambar@ukm.myINTRODUCTIONGlycerol, or glycerine, or 1,2,3-propanetriol, can be produced from the transesterification or hydrolysis of natural fats, vegetable oils or petrochemicals (1). In Malaysian biodiesel processes, palm oil is the primary raw material from which glycerol is produced as a transesterification byproduct. In these processes, palm oil is treated with methanol and a basic homogeneous catalyst. Alternatively, acidic, basic or enzymatic heterogeneous catalysts are used because of their ease of separation from the products.

Crude glycerol production from biodiesel conversion is increasing yearly. From 2008 to 2011, total worldwide crude glycerol output increased from 2.06 to 2.88 million tons (2, 3). The global demand for glycerol was 2 millions in 2011 and is expected to reach 3 millions tons by 2018, growing at a Compounded Annual Growth Rate (CAGR) of 6.3% from 2012 to 2018 (4). The Malaysian palm-based oleochemical industry is growing rapidly and produces products such as fatty acid methyl esters (FAMEs), fatty alcohols and crude glycerol (5, 6). The abundant crude glycerol generated by this industry affords a great opportunity for scientists to explore new glycerol applications. High-purity glycerol finds wide use as an ingredient or processing aid in healthcare products, fuel additives, lubricants, personal care products, cosmetics and food (7, 8).

However, the glycerol produced as a byproduct of transesterification from biodiesel processes is not pure enough for direct use in high-tech applications. To overcome this problem, numerous treatments are required to remove impurities. Moreover, the manufacturing and pharmaceutical industries have increasingly demanded high-quality, pure glycerol because of its superior physical properties, low contamination and odorlessness (9).

Therefore, an effective, efficient glycerol purification process is needed to minimize production costs, minimize industrial waste and maximize the utility of biodiesel industrial processes. Because of the enormous demand for the production of glycerol from biodiesel waste, we have thoroughly reviewed vegetable oil transesterification and hydrolysis as a glycerol synthetic route. Various purification methods for producing high-purity glycerol are herein discussed, and some glycerol conversion processes are summarized.

CHEMICAL COMPOSITION OF CRUDE GLYCEROL

The factors that influence the quality of crude glycerol derived from biodiesel production processes include catalyst type and quantity, recovery methods, unreacted methanol and other impurities. For example, a crude glycerol extracted from sunflower oil biodiesel had a composition (w/w) of 30% glycerol, 50% methanol, 13% soap, 2% moisture, 2-3% salts (primarily sodium and potassium) and 2-3% other impurities (10). In contrast, Hansen et al. (11) reported glycerol contents of 38 to 96% in a set of 11 crude glycerol samples collected from 7 different Australian biodiesel producers. Some of those samples contained more than 14% methanol and 29% ash. Because most biodiesel production uses low-grade methanol and homogeneous alkaline catalysts (sodium methoxide or potassium hydroxide), the quality of the afforded glycerol is poor. Saman et al. (2008) identified several contaminants in crude glycerol methanol, soaps, catalysts, salts, non-glycerol organic matter and excessive water (12).

Even when identical feedstocks were employed, the crude glycerol produced from alkali- and lipase-catalysed transesterifications was reported to differ in purity (13). For biodiesel production that utilized homogeneous alkaline catalysts, the crude glycerol produced contained 5 to 7% salts (14), making conventional purification techniques more costly. Heterogeneous processes using enzymes and solid metal-oxide catalysts have been promoted as alternatives that afford higher-quality crude glycerol. However, with heterogeneous catalysts, impurities present in natural raw feedstock tend to accumulate in the glycerol phase. Therefore, purification remains a requirement for meeting current standards. Moreover, each contaminant requires a different method of removal.

PRODUCTION OF GLYCEROL FROM VEGETABLE OILThe two primary processes for biodiesel production are hydrolysis and transesterification. Hydrolysis refers to a chemical reaction in which water molecules are split into hydrogen and hydroxide anions, whereas a biodiesel transesterification refers to a reaction that occurs between a triglyceride or fat and an alcohol to form alkyl esters (biodiesel fuel) and glycerol (Figure 1). The theoretical stoichiometric ratio of alcohol to lipids for these transesterifications is 3:1. In reality, a 6:1 ratio is necessary to achieve practical yields. The alcohol molecules displace the triglyceride (triacylglycerol) molecules in forming an ester. This process is also known as alcoholysis because cleavage of an alcohol is involved. Most biodiesel producers utilize homogeneous alkaline catalysts such as sodium hydroxide or potassium hydroxide (15). These catalysts also saponify the starting materials into foams. Consequently, yields decrease, and major problems in catalyst recovery, product separation and product purification are encountered (16-18).

Transesterification requires an alcohol. Methanol and ethanol are the most frequently used alcohols for biodiesel transesterification reactions; propanol and butanol are also widely employed. For environmentally friendly processes, ethanol is chosen because it can be derived from agricultural products or other renewable resources. Alternatively, methanol is chosen for its lower cost, high polarity and short alkyl chain (19).

Hydrolysis Processes

Vegetable oil hydrolysis is achieved using an acid or base catalyst and produces glycerol and free fatty acids or soaps. Base-catalyzed ester hydrolysis is commonly called saponification. Both processes are shown in Figure 2. The performances of various vegetable oil hydrolyses are summarized in Table 1. In previous studies, Hammond and Inmok (20) reported that lipase split triglycerides into free fatty acids and glycerol. Their hydrolyses were performed with 17 to 44% moisture, and water was applied by various suitable techniques, e.g., soaking and spraying.

Hydrolysis without a catalyst at 270-350 C, 20 MPa and a water/oil feed ratio of 50/50 (v/v) afforded approximately 90% biodiesel and 10% glycerol (25,26). The water concentration was sufficient for both hydrolysis and triglyceride cracking (24, 27, 28). Commonly, vegetable oil hydrolysis involves the use of rotating hydrothermal reactors operated at high temperatures and pressures. Several companies in Malaysia namely, Cognis Oleochemical Industries, FPG Oleochemicals Sdn Bhd and Pacific Oleochemicals Sdn Bhd are using this catalyst-free hydrolytic technology in their biodiesel production processes.

Transesterification Reactions

In transesterification, basic, acidic or enzymatic catalysts are employed (29, 30). Major differences exist between homogeneous and heterogeneous catalysts in terms of activity, product separation and production cost (31). In Malaysia, transesterification reactions are widely applied in biodiesel production by Malaysian Palm Oil Board, Golden Hope Plantation Sdn Bhd and Emery Oleochemicals. The other international companies such as P & G Chemicals (USA) and BASF Chemical (Germany) also produced glycerol in huge volume. Unfortunately, the glycerol produced by their processes is mediocre. The poor glycerol quality provided by these companies is attributed to difficult separations and the high costs associated with the development of purification techniques.

Homogeneous catalytic systems

The most active catalysts, alkaline metal alkoxides such as sodium methoxide (CH3ONa), are commonly used in methanolyses because of their high conversions (>98%), short reaction times (approximately 30 minutes) and low molar concentrations (0.5 mole %). However, anhydrous requirements have rendered those catalysts inappropriate for typical industrial processes (18). Moreover, the separation of the homogeneous catalyst from the glycerol mixture has been cost-prohibitive (32). Transesterification reactions are also performed using acid catalysts. In many cases, the reactions with acid catalysts have been reported to be slower than the reactions with base catalysts. However, acid catalysts exhibit high activity at high temperatures and high oil-to-alcohol ratios. Among the catalysts reported in the literature, trifluoroacetic acid was been observed to perform the best, affording 98.4% conversion in 5 hours with an alcohol-to-oil molar ratio of 20:1 and at a reaction temperature of 120 C (33).

Catalytic sodium hydroxide was observed to produce side reactions and form sodium soaps easily. This sodium soap formation was also observed when catalytic sodium methylate was employed in the presence of trace water amounts. These sodium soaps were soluble in the glycerol phase. The soaps required neutralisation to fatty acids and decantation (34). Furthermore, even when a water-free alcohol/oil mixture was used, some water was introduced into the reactor system by the deprotonation of the alcohol by hydroxide. The presence of water enabled hydrolysis and resulted in soap formation. This undesirable saponification reaction reduced fatty acid methyl ester yields and considerably hindered glycerol recovery due to emulsion formation (18). The performances of various homogeneous catalysts are shown in Table 2.

Heterogeneous catalytic systems

The heterogeneous catalysts of vegetable oil transesterification can be categorized as either acidic or basic. Alkali catalysts are commonly used in transesterification and exhibit higher activities than acidic catalysts. Furthermore, basic catalysts have afforded particularly high conversions when supported on alumina, metal or zeolites. A comparatively high reaction temperature is required to achieve only a slow reaction rate in acid-catalyzed transesterifications.

Previous studies have reported that vegetable oil transesterification using heterogeneous acid catalysts is not a practical process because it requires high temperatures, lengthy reaction times and large catalyst charges. In addition, synthesizing the catalysts was reported to be complicated and uneconomical. Catalyst leaching also presented a risk of product contamination. These drawbacks led to higher separation costs and created additional problems. However, the solid acid catalysts could be regenerated and reused.

Supported basic heterogeneous catalysts, such as potassium hydroxide on alumina (KOH/Al2O3), have exhibited high activities and basicity (11). One optimized KOH/Al2O3 reaction afforded 90.54% diesel and 9.46% glycerol. Arzamendi et al. (42) reported conversions up to 99% when sodium hydroxide on alumina (KOH/Al2O3) was employed. The high conversions resulted from the catalysts high number of active sites and the catalysts basicity. Sparingly soluble catalysts such as calcium oxide, sodium methoxide and barium hydroxide have exhibited high activities for rapeseed oil transesterification (43). During vegetable oil transesterification using calcium oxide, calcium glyceroxide was produced through the reaction of calcium oxide with glycerol. This byproduct created more active sites and thus enhanced the reaction rate (44). The activities of heterogeneously catalyzed reactions are listed in Table 3.

The supercritical methanol system

Transesterification reactions using basic or acidic catalysts are relatively time-consuming and require complex separations, resulting in high production costs and energy consumption. To overcome these problems, supercritical methanol (SCM) has been proposed for catalyst-free vegetable oil transesterifications (57, 58, 59). Methanol is supercritical above 10 MPa and 290C. Whereas vegetable oil transesterifications with regular methanol are biphasic reactions, the lower dielectric constant of supercritical methanol results in a one-phase reaction solution. The single phase allows for a short reaction time (60). Compared with catalytic processes carried out at atmospheric pressure, the non-catalytic SCM process involves a considerably simpler purification step, a lower reaction time and lower energy. In addition, the SCM method is more environmentally friendly.

In investigating product separation problems, Hawash et al. (61) reported that a non-catalytic transesterification reaction using supercritical methanol afforded a 100% ester yield within four minutes. However, a reaction temperature of 320C (593 K) and a reaction pressure of 8.4 MPa were necessary. Moreover, a high molar ratio of methanol to oil was utilized (61, 62). Although high, the cost of the SCM process could be offset because this reaction produces high-purity methyl esters (99.6%) and glycerol (96.5%) (63). Glycerol production using the SCM technique is summarized in Table 4.

Immobilised enzyme catalytic system

The transesterification reaction is also performed using enzyme catalysts. Lipase is the most efficient and active enzyme for the reaction. To immobilize the enzyme, the carbodiimide activation method is the most effective. The penicillium expansum lipase (PEL) systems discussed by Yang et al. (66) and Xu & Ma (67) are summarized in Table 5.

Nanoparticle catalytic systems

Over the past three years, nanoparticle heterogeneous catalysts have been used in vegetable oil transesterifications to easily separate the catalyst from the glycerol phase. The activity and performance was reported to increase for nano-sized catalytic particles because of their higher surface area and availability of active sites. Boz et al. (68) demonstrated KF-loaded nano--Al2O3 as a versatile catalyst for transesterification. The catalytic performance of various nanoparticle catalysts is summarized in Table 6. Ionic liquid (ILs) catalytic systems

In other works, imidazolium-based ionic liquids and multiphase acidic or basic conditions have been used to produce glycerol from vegetable oil transesterification. High yields of biodiesel (> 98%) were afforded from soybean oil transesterification when the ionic liquid 1-n-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ([BMIM] NTf2), alcohols and K2CO3 or sulfuric acid were used (72). The lack of a solid catalyst resulted in a clean process. However, this catalytic process was not practical because of the high IL cost and difficulty in handling.

Interestingly, Vidya and Chadha (73) reported that hydrophobic ILs such as [BMIM] PF6 and [BMIM] NTf2 were better media for vegetable oil transesterifications than the hydrophilic [BMIM] BF4. They also indicated that the IL anions strongly affected the catalytic performance of Pseudomonas cepacia lipase (73). Comparing two hydrophobic ILs, [BMIM] NTf2 performed better than [BMIM] PF6. The higher viscosity of [BMIM] PF6 limited mass transfer of the substrates and products to and from the enzyme active sites and thus led to lower catalytic activity (76). Isahak et al. (77) reported that the use of ionic liquids, namely choline chloride, produced higher-quality biodiesels and glycerol. The activities of various ionic liquid catalysts are summarized in Table 7.

PRODUCTS SEPARATION AND CATALYST RECOVERY STAGE

Homogeneous catalysts are the most active catalysts for the vegetable oil transesterification reaction. However, the higher residual catalyst amounts associated with these catalysts compared with those encountered in heterogeneous processes lead to higher separation costs. Furthermore, unlike heterogeneous catalysts, homogeneous catalysts cannot be recycled for reuse because they remain in the product. One technique for removing excess homogeneous catalyst is a titrative method by which the acid or base catalyst is converted into its salt (38). Because of their ease of separation and ability to be regenerated, heterogeneous catalysts are good, clean and cost-effective alternatives for producing FAMEs and glycerol from vegetable oils. The solid catalyst can be removed by filtration, resulting in a less complex recovery of biodiesel and glycerol (78, 79).

Recovering glycerol from FAME phases was studied by Saleh et al. (80). The researchers found that an ultrafiltration technique successfully separated the small amount of glycerol contained in the FAME phase. Temperature significantly increased water solubility in various commercial biodiesels (81). Consequently, using higher temperatures increased glycerol solubility in FAMEs and made the subsequent separations more challenging. Wang et al. (82) reported that using ceramic membrane separation at 60 C reduced the glycerol dissolved in FAME; however, this result was achieved only after removing the methanol.

For glycerol produced by vegetable oil transesterification with an ionic liquid, the catalyst can be separated from the product mixture by a crystallization and freezing technique based on the boiling-point differences between glycerol and the ionic liquid (72). The glycerol producers using super critical methanol (SCM) and hydrolysis were not apprehensive of any excess catalyst. However, unreacted triglyceride remained in the product mixtures. To remove the unreacted material, a solvent extraction method that involves overnight separation into layers based on weight and polarity can be used. Otherwise, a centrifugation technique must be employed to separate the products.

Homogeneous Catalyst Recovery Processes

The homogeneous catalysts remain in the product after transesterification; therefore, the homogeneous catalysts are not reusable. However, transition metal homogeneous catalysts can be recovered by using a zeolite membrane that has a crystalline structure with pores smaller than those of transition metal catalysts (82). Alternatively, homogeneous catalysts can be neutralized into salts, and the salts can then be removed by filtration. Catalyst removal after acid washing is discussed in section 6.0.

Heterogeneous Catalyst Recovery Processes

Few studies of heterogeneous catalyst regeneration exist. Solid-phase heterogeneous vegetable oil transesterification catalysts can generally be recycled a few times without adverse effects. They can be removed easily from the products by filtration or centrifugation (42, 68, 56, 83, 84). Commonly, the heterogeneous catalysts are then washed with organic solvent and drying overnight (85-87). Following re-calcination under N2 after an extensive methanol wash, the catalysts are ready for additional reactions (88, 89). Some researchers report that heterogeneous catalysts can be reused without any treatment and without any significant loss in activity (90).Supercritical Methanol Recovery Process

Catalyst-free supercritical methanol transesterification is performed to increase the reaction rate and thus shorten the reaction time. However, the excess of methanol required leads to some difficulty in separation. The problem can be overcome by an evaporation and layer separation technique (63).

Immobilised Enzyme Catalyst Recovery Process

Immobilized enzymes are efficient catalysts for vegetable oil transesterification. However, the production costs are high and require catalyst reuse. Otherwise, difficulties in handling are the primary problems associated with immobilized enzymes. For reuse, the enzyme is isolated using centrifugation. The recovered lipase is washed with organic solvents and is then ready for another reaction (68, 91).

Nanoparticle Catalyst Recovery Processes

Nanoparticle catalysts are difficult to remove using conventional filtration. Polymeric membranes can recover nanoparticle catalysts. The filtration efficiency depends on the membrane size. The nanoparticle catalysts used for vegetable oil transesterification processes resist separation and recovery. However, centrifugation has been demonstrated to separate these catalysts from products at high recovery levels (70).

Ionic Liquid Catalyst Recovery Process

Ionic liquids are efficient and versatile catalysts because of their physicochemical properties. Ionic liquids are salts that consist of easily separated anions and cations. Because of melting-point differences between the ionic liquids and remainder of the reaction chemicals, freezing techniques are common methods for separating ionic liquids from the products and unreacted starting material (72,73).

CRUDE GLYCEROL RECOVERY PROCESS

In this section, glycerol recovery from hydrolysis, saponification and transesterification reactions is reviewed. Various practical methods and techniques have been used for glycerol recovery and enrichment.

Hydrolysis

Hydrolysis is divided into two processes: acid-catalyzed hydrolysis and base-catalyzed hydrolysis (saponification). The reaction produces two layers of product that can be separated by using a separating funnel or by decantation. Homogenous catalysts can be recovered by neutralization to salts and centrifugation (22); heterogeneous catalysts can be removed by filtration. Crude glycerol is obtained from the lower phase by removing water through vacuum distillation (23). An advanced glycerol recovery technique was developed by modification of an ionic liquid-glycerol mixture to form deep eutectic solvents (DES). The synthesized DES was used to extract the glycerol from the biodiesel (92).

Transesterification reactionTheoretically, the glycerol of vegetable oil transesterification constitutes approximately 10% of the products. Typically, however, the recovered glycerol constitutes only 9 to 9.6% of the products (Table 8). In an effective biodiesel production process, only small amounts of the unreacted starting materials remain in the glycerol phase. Glycerol is also an important byproduct in soap production. When fats and oils are saponified by caustic soda in the soap production process, glycerol is dissolved in the soap lye and in the crude soap as an impurity.

Glycerol from fats and oil in soap manufacturing usually comprise approximately 10% of the total products. Recovering usable materials is vital to the profitability of any soap production process. Unfortunately, many small- and medium-scale soap producers discard the lye. The specific recovery technique employed is critical to recovering glycerol from spent soap lye. The liquor that remains after soap manufacture must be allowed to settle for 20 minutes after stirring. The clear phase is decanted and discarded, and the remaining phase is heat treated at 60 C in a conical flask (100).

The basic and acidic catalysts employed for glycerol production by transesterification are recovered via chemical treatment. Either sulfuric acid (H2SO4) or sodium hydroxide (NaOH) are used to neutralize the catalysts to salts. For example, H2SO4 neutralizes NaOH in glycerol samples to sodium sulfate (Na2SO4). Fortunately, Na2SO4 has low solubility in the aqueous glycerol solution, which is saturated with sodium chloride (NaCl). Indeed, NaCl remains primarily in the glycerol layer (101, 102). Hence, the Na2SO4 salts can be removed by decantation and filtration.

The highest glycerol yields are obtained by the bleaching recovery technique (100). Bleaching (alkaline system) both purifies the glycerol and further saponifies the free triglycerides (103). The amount of recovered glycerol depends on the recovery point and on the purification stage during which the technique is utilized (104). The variation in glycerol recovery amounts obtained across the soap industry is due to the different soap types of lye and methods of treatment employed. For example, during the recovery stages, glycerol can be lost through washing, graining and desalting. Moreover, if a temperature of 60 C is exceeded during treatment, side reactions may occur. Glycerol decomposes to acrolein at higher temperatures (>140 C).

The crude glycerol derived from vegetable oil can be recovered from the biodiesel phase by centrifugation. Centrifugation is followed by hydrochloric acid treatment to convert any contaminant soaps to free acids or salts (105,106). Methanol and water contaminants are removed by distillation (107). Afterward, the glycerol layer is neutralized with caustic soda, producing 80% (w/w) crude glycerol. In the next section, some glycerol purification processes are presented. In addition, a recovery technique that employs fixed silica gel beds to adsorb glycerol from methanol-free biodiesel streams is discussed (108).

OVERVIEW OF GLYCEROL PURIFICATION

For many years, glycerol has been purified to make it more useful for various manufacturing activities. Crude glycerol is obtained as a byproduct from three different processes: soap manufacture, fatty acid production and fatty ester production (109). High-purity glycerol is used commercially in pharmaceuticals, food processing, lubrication and cosmetics. For use as animal food, several glycerol purification steps are required to remove impurities (110).

The purity of crude glycerol obtained from vegetable oil transesterification depends on three parameters: the type of catalyst used, the amount of excess alcohol and the conversion achieved (111). The purity ranges of crude glycerol produced by transesterification using homogeneous catalysts, heterogeneous catalysts and supercritical methanol (SCM) are 55%-70%, 75%-85% and 96.5%, respectively (112).

Currently, much attention is being focussed on employing green catalytic transesterification processes to convert bio-renewable vegetable oils to commodity chemicals and clean fuels. These reactions are performed at lower temperatures and atmospheric pressure using homogeneous or heterogeneous catalysts and excess methanol. However, the excessive unreacted methanol presents a problem. Methanol is dangerous and can adversely affect human health and the environment. To overcome this problem, the excess methanol is recovered by processes such as evaporation and recycled to the reactor for additional transesterification cycles.

Crude glycerol residue contains 20% glycerol, 6.6% fatty acids (as soap) and 64.4% salt. Thus, 91.1% of crude glycerol residue consists of components that are potentially useful for other applications (113). According to van Gerpen et al. (114), crude glycerol obtained by transesterification is composed of 50% or less glycerol. The remaining contents are primarily water, salts, unreacted alcohol and catalyst. To produce high-quality glycerol, these contaminants must be removed.

Purification Techniques for Glycerol Synthesized with Inorganic Catalysts

Recently, crude glycerol separation and purification activities have expanded considerably, and academic institutions have explored more innovative methods, theories and process designs in these respects.

Salt separation

For crude glycerol derived with an alkaline catalyst, treatment begins by neutralization using certain acids. This technique efficiently removes alkaline matter, including excess catalyst and the abundant soaps formed during transesterification processes employing homogeneous catalysts. The neutralization separates the reaction mixture into three phases using a strong- or medium-strength mineral. The three phases consist of the catalyst in the bottom phase, the neutralized glycerol and methanol in the middle phase and the free fatty acids (FFAs) in the top phase (115).

Acids are used to neutralize excess alkaline catalysts, whereas bases are used to neutralize acidic catalysts. Sometimes, hydrochloric or sulfuric acid is employed in a re-neutralization step and produces sodium chloride or potassium sulfate, respectively (97). However, using phosphoric acid is more environmentally friendly. Phosphoric acid neutralizations produce a phosphate salt that is widely used as a fertilizer. Sulfuric and hydrochloric acids produce environmentally harmful substances during neutralization. The amount and concentration of acids used in neutralization exert major effects on the separation time and the removal of free fatty acids and salts (116).

Usually, the crude glycerol is reacted with greater than one mole of 85 wt.% sulfuric acid. Afterward, sodium borohydride or sodium hydroxide solution is added to neutralize the excess acid and to remove colored impurities. Hajek and Skopal (93) demonstrated that sequential neutralizations or saponifications could yield 84% purity glycerol. Furthermore, Kongjao et al. (117) asserted that acidifying the crude glycerol with mineral acids (such as sulfuric acid) converted soap impurities into insoluble fatty acids according to reaction (1).

(1)

Crystallization or precipitation

In another separation technique, catalyst salts in solution after acidic treatment are removed by precipitation as hydroxyapatite (HAP). The co-addition of lime (Ca(OH)2) and phosphoric acid to the pre-treated glycerol results in calcium apatite (Ca5(PO4)3(OH)) formation. This chemical reaction removes solubilized catalyst from glycerol samples (98). The reaction and precipitation is driven by calcium-ion and hydroxide-ion attraction. Separation of the calcium apatite by gravity or centrifugation removes nearly all of the excess catalyst.

Methanol removal and recycling

Excess un-reacted methanol is a major contaminant in crude glycerol. High methanol levels are toxic, particularly in animal feeds and pharmaceuticals. Methanol is inherently toxic but not directly poisonous. Alcohol dehydrogenase enzyme in the liver converts methanol to formic acid and formaldehyde, which causes blindness by the destruction of the optic nerve (118). The excess methanol must be removed to achieve the level deemed safe by the United States Food and Drug Administration (FDA). Brockmann et al. (119) reported excess methanol removal using a flash evaporation. This technique, based on the boiling point of alcohols, removed nearly 100% of the methanol. In summary, a methanol removal step is needed to meet the general usage requirements set by international standards (ASTM and EN in Table 9).

Removal of solid contaminants

Heterogeneous catalysts are better suited for glycerol production than homogeneous catalysts. Heterogeneous catalysts afford a considerably cleaner crude glycerol, and heterogeneous catalysts can be easily removed by simple filtration. The disadvantages of heterogeneous catalysts include their high cost and difficult syntheses. Homogeneous catalysts are better focused. However, neutralizing homogeneous catalysts produce more salt.

Furthermore, years ago, the Wurster & Sanger single-effect glycerine evaporator was developed to overcome the salt removal problem (122). The first of the three apparatuses had a large chamber that functioned to collect salts. After neutralization, the entire mixture was dropped into a tank with a false bottom comprising a filter bed of wire screen and filter cloth. The crude glycerol was pumped away from below the false bottom. The salt was washed with lye and then with water. The wash liquors were pumped back into the evaporator feed tank. Depending on the crude glycerol content, this procedure decreased the salt content to 0.5 to 2.0 wt.% (122). This method for removing salt was used only in single-effect evaporations. The second method, which is still extensively used in small and moderately sized plants, has the evaporator bottoms connected to salt filters, salt boxes or salt extractors. For a double-effect evaporator, three salt extractors are typically used. The setup allows for both evaporators to drop salt while one extractor is emptied. Salt is allowed to accumulate in the evaporator during the time required to steam, dry and empty its extractor. Furthermore, this second apparatus allows for the salt to be removed from the evaporators continuously and dried. The third apparatus utilizes salt drums and centrifuges for complete salt removal.

Recently, Buenemann et al. (1991) reported an advanced technology for removing solids from crude glycerol. This technique employs microfiltration or ultrafiltration using ceramic-supported zirconia or alumina filters (123). The ceramic material has a high mechanical resistance and tolerates a wide range of temperatures and pH values. Theoretically, the micro-sized catalysts and salts are easily isolated using these ceramic-supported zirconia or alumina filter membranes (124). This process has produced high-quality glycerol without any significant loss in yield.

Gomes et al. reported that ceramic membranes made of tubular-type -Al2O3/TiO2 are able to purify glycerol to high purity levels (125). The microfiltration process proposed by Gomes et al. consisted of two stages. First, a 3.5-kg mixture was prepared with a mass composition of 80% biodiesel, 10% alcohol and 10% glycerol. In the second stage, the membrane that yielded the best permeate flux and free glycerol retention was identified. This microfiltration membrane has also been used to filter other micro-sized materials from glycerol (126). Large glycerol streams can be purified continuously, effectively and economically even with frequent provenance changes. This technique has produced analytical-grade glycerol with purity above 99.2% w/w.

Saleh et al. (80) reported an ultrafiltration (UF) technique for separating crude glycerol from the fatty acid methyl ester (FAME) phase. This pressure-driven technique was performed using 1-100-nm membranes. Specifically, this UF technique removed high-molecular-weight substances, colloidal materials, organic and inorganic molecules. The technique was employed in several other applications, including virus prevention and bacteria and waste water recycling (127). This application can effectively recover and separate crude glycerol from the FAME phase. In the Saleh et al. study, adding a small amount of water (approximately 0.06 mass %) improved separation and efficiency (80). Conversely, a nanofiltration technique was used to remove 1- to 100-nm particles in a high-viscosity separation (128).

Removing ions and colored contaminants by adsorption

During the reaction, some catalysts dissolve into the reaction medium as free ions. To remove these free ions, ion exchange resins have been used. Both column and batch methods have been investigated (129). Synthetic ion exchange resins have been produced commercially since 1960s. Strong acid cation exchange resins and strong base anion exchange resins, which fully ionize over the entire pH range, are supported on three-dimensional polystyrene cross linked with an agent such as divinylbenzene. To convert the cross-liked polystyrene to a hydrogel with ion exchange capability, ionic functional groups are attached to the polymeric network by a variety of chemical means. For example, sulfonating a styrene-divinylbenzene copolymer permanently attaches sulfite (-SO3) groups, affording a negatively charged matrix and exchangeable, mobile and positive hydrogen ions (130). The specific linkages and three-dimensional structures play important roles in adsorbing contaminants.

Two separate ion exchange resins can be used successively to exchange cations for hydrogen ions (H+) and anions for hydroxyl ions (OH-). The hydrogen and hydroxyl ions subsequently combine to form pure water, as shown in Figure 4. If maintaining neutrality is desired, Na+, Ca2+, K+ or Mg2+ resins can be utilized instead of H+ resins. Likewise, Cl-, HCO3-, SO42- or NO3- resins can replace strongly basic OH- resins. The maximum ion exchange capacity of strong acid cation or strong base anion exchangers is stoichiometric i.e., the capacity is based on the equivalents of mobile charge within the particular resins. Thus, one mole of H+ corresponds to one equivalent. One mole of Ca2+ is two equivalents. Anion and cation exchange resins used together ensure that the ion exchange resins capture both free anions and cations from the crude glycerol sample, maximizing performance. This process has produced glycerol of purities higher than 99% w/w.

In another case, acidic ion exchange resin beads were used to separate fatty acid salts and inorganic salts from glycerol (131). This purification was effective when high-quality resins consisting of 4 to 65 wt.% crosslinker were used. Uniformity coefficients no greater than 1.15 were necessary to ensure that the glycerol passed through the bed at a minimum flow rate of 0.3 bead volumes per hour. The gel-type resins in this crosslinker range were more suitable for the separation of soluble substances than resins with less than 10% crosslinker. High crosslinking affected separation efficiency. Rezkallah (131) also claimed that the salts and colored impurities eluted from the column earlier than the glycerol. The afforded glycerol exhibited a considerably lower ion and colored impurity content.

Amberlite-252, a strong acid cationic exchange resin, has also been employed. Carmona et al. (132) reported that the macroporous Amberlite could be used for sodium ion removal from glycerol/water solutions containing high salt concentrations. This resin was capable of yielding technical-grade glycerol from many different processes. Purification using Amberlit-252 was particularly efficient because of its ability to be regenerated more than five times without any significant loss of exchange capacity (129).

Distillation as a single purification step

Traditionally, crude glycerol was purified using a simple distillation unit. The raw glycerol contained ash, non-glycerol organic matter, water and soap (108). The distillation was reported as a successful method for purifying crude glycerol that is similar in composition to the source of commercial glycerol used today. An informative comparison of crude, purified and commercial-grade glycerol is shown in Table 10. The corresponding analyses were performed based on standard methods: glycerol content ISO 2879-1975; ash content ISO 2098-1972; and matter organic non-glycerol (MONG) ISO 2464-1973.

The ash in crude glycerol was primarily sodium catalyst salts (96). During glycerol recovery, trace amounts of short- and medium-chain fatty acids were retained in the crude glycerol (106, 135). At the distillation temperatures, the free sodium hydroxide reacted with the fatty acids, forming short- and medium-chain soaps. Higher pH levels, due to the presence of more sodium hydroxide, resulted in greater soap formation. Sodium hydroxide also catalyzed glycerol polymerization to polyglycerol (136).

Many works have explored the distillation of the glycerol phase as a method for removing methanol (137). This technique operates based on boiling points. Two or more materials with different boiling points can be separated using vacuum distillation, and this concept was applied to glycerol purification. Before distillation, the glycerol was acidified (134). The success of vacuum distillations depends on temperature (T) and pressure (P). The crude glycerol was successfully distilled at 120-126 C and 4.0 x 10-1 to 4.0 x 10-2 mbar, producing glycerol 96.6% pure. The optimum pH for the distillation was less than five, which obviated foaming.

This technique is sensitive and must be monitored to avoid undesirable reactions. Three possible reactions can reduce the glycerol yield during distillation: polymerization, dehydration and oxidation. The polymerization of glycerol to polyglycerol occurs readily at high pH values, excess NaOH concentrations and high temperatures (>200 C) (136, 138). Glycerol dehydrates to acrolein (bp. 52C) at low pH (99, 139), that accumulates in the cold trap during distillation. In addition, glycerol can oxidize to glycerose, glyceraldehyde and dihydroxyacetone (140).

Recent Industrial Purification Processes

Recently, environmental issues have propelled the refinery industry to develop new technologies for glycerol purification. Many techniques are combined into a single step or a limited number of steps to enhance recovery and purification, e.g., soap splitting followed by salt and methanol removal. Some separation techniques have employed vacuums because of glycerol heat sensitivity (glycerol decomposes at 180 C) (119).

As established methods, these following technologies may be used to further enhance the purity of glycerol after the soap splitting step: fractional distillation, ion exchange, adsorption, precipitation, extraction, crystallization and dialysis. The most common purification pathway, in sequential order, is soap splitting, combined methanol/water removal, fractional distillation, ion exchange (zeolite or resins) and adsorption (active carbon powder) (120, 131). Well-known companies manufacture glycerol purification equipments. For example, companies such as Desmetballestra and Buss-SMS Canzler market ion exchange equipments. Other chemical companies, such as Rohm & Haas and Lanxess, supply ion exchange granulates, whereas Norit Company supplies powder and granulated activated carbon as glycerol bleaching and decolorizing agents (119). Their activated carbon, with its large surface area and high porosity, adsorbs pigments and organic matter easily for large glycerol samples. Ion exchange applications may be performed by either a column or batch technique (129). In addition, higher capacity ion exchanges that will make high-purity glycerol production more facile are being developed.

Many glycerol refining plants exist today. Biodiesel-based glycerol is manufactured in various grades by treatment using single or multiple steps neutralization, heating, condensing, refluxing and distillation (31, 141). Purification of glycerol from biodiesel processing using the ion exchange resin Ambersep BD50 (142), is an effective and innovative process. The biodiesel industry generates a tremendous amount of crude glycerol that must be purified at minimum cost (14). For most applications in the food and pharmaceutical sectors, crude glycerol needs be purified to pharmaceutical grade (99.7%). This high-quality grade can be realized by employing a combination of techniques- e.g., heating, evaporation, splitting, decantation, adsorption and vacuum distillation (143). This combinatory process produces glycerol with a purity greater than 99.5% from typical crude glycerol. A flow diagram of typical glycerol purification is shown in Figure 5.

The Tennessee based EET Corporation patented a high efficiency electrodialysis process: the HEED technology to produce high-purity glycerol. This electrodialysis processing equipment is an economical solution for glycerol purification in the biodiesel and soap industries. By using EETs technology, crude glycerol from biodiesel and saponification processes can be refined to achieve the USP-grade quality requirement of 99.7% purity (144). Alternatively, lower-cost and intermediate-purity grades can be produced for direct use or chemical conversion into other compounds such as propylene glycol and ethylene glycol. In addition, EETs membrane-based technology avoids important problems associated with stand-alone evaporation and distillation, such as foaming, cross-contamination, corrosion, limited recovery and high costs in energy, maintenance and operation.

The robustness of EETs technology allows it to be applied to neutralized glycerol either before or after methanol removal and over a range of feed compositions. Other HEED applications include purification of refined glycerol that has been distilled or evaporated but nevertheless contains residual salts or organic substances. EET's glycerol purification process begins with a pre-treatment to remove solids and fouling organics and to partially remove color-causing organics.

The HEED (also known as HEEPM) system configuration combines customized automated controls and control logic to provide the optimal desalting of a particular pre-treated crude glycerol. This established technique produces colorless glycerol with low salt content. The process is considered a good, efficient technique for producing high-purity glycerol. However, its complicated technology contributes to high production costs and therefore makes the systems uneconomical. Efficient technologies need to be developed for producing high-quality glycerol at lower cost.

Disposing of crude glycerol is both costly and wasteful. An applied technology for crude glycerol purification was introduced by the California based SRS Engineering Company (Temecula, CA 92590) (145). Incorporated into SRSs high-purity glycerol purification system (the SRXG-Series distillation column) is an ideal combination of processing steps. The SRXG-Series system produces high-purity glycerol without any significant loss in yield. In summary, these technologies illustrate that investing in the development of purification technologies can eliminate disposal costs and provide a new venue for profit in the form of purified glycerol. The SRS system was able to purify crude glycerol to technical-grade glycerol (over 97%) (145). Furthermore, the purification of crude glycerol with high methanol and water content was successfully performed by Rototherm mechanically agitated thin-film processors. These thin-film processors can be operated continuously and in combination with distillation. They can also be used with products containing sensitive solids (146).

Cost Estimates for Glycerol Purification Processes

Pharmaceutical (99.5% pure) glycerol can be bought for US$ 1000 per ton (2013 value). It is difficult to obtain data on cost estimation for glycerol purification. Posada et al. (2011) estimated the cost of glycerol purification achieved up to 98 wt % by combination of neutralization, centrifugation, evaporation and column distillation (147). During the purification process, methanol at 99 wt % was recovered and thus for the economic assessment, two scenarios were analyzed. In the first scenario, the obtained methanol is considered as a process waste. In the second scenario the methanol is considered as a co-product which could be recycled to the transesterification process and an economic value is given to this stream. The lowest cost for glycerol purification was obtained under the second scenario conditions producing an estimated US$ 0.149 per glycerol kg. In another work, the enrichment process of crude glycerol was performed via chemical extraction and physical adsorption processes (148). For the whole enrichment process, based on equal quantity of crude glycerol, it was noticed that the adsorption process was the less expensive process with US$ 0.46 per liter of crude glycerol producing a slightly colored 87% pure glycerol, while a combined process of chemical extraction with n-propanol and adsorption was skyrocketing to US$ 17.1 per liter of crude glycerol for an almost clear 99% glycerol and even US$ 41.2 per liter of crude glycerol to obtain a perfectly clear 99.1% pure glycerol through n-butanol extraction (148).Glycerol Conversion to Other Chemicals

Recently, numerous papers have been published on the direct utilization of glycerol. For example, glycerol can be converted into value-added products by pyrolysis, steam gasication or catalytic treatment. Glycerol can be catalytically converted into many other liquid products, including acetaldehyde, acrolein, formaldehyde and hydroxyacetone. Buhler et al. (149) reported the production of methanol, acetaldehyde, acrolein, allyl alcohol, acetone, ethanol, carbon dioxide, carbon monoxide and hydrogen from glycerol under supercritical conditions.

Additionally, Kunkes et al. (150) reported the conversion of glycerol to syngas (H2, CO, CO2) using Re on Pt/C as a catalyst. The syngas was subsequently used to produce a series of alcohols (151, 152). Thiruchitrambalam (153) reported that glycerol can be completely converted into H2-rich syngas through pyrolysis at 800 C in a xed-bed reactor. Cortright et al. (154) reported H2 production from the aqueous phase carbohydrate reforming of glycerol over Pt/Al2O3 catalysts. In this reaction, hydrogen (H2) was afforded in 64.8 mol% yield. Buhler et al. (149) produced allyl alcohol, acetaldehyde, acrolein, methanol, CO, CO2 and H2 by treating glycerol under supercritical conditions. However, a low glycerol conversion (0.431 wt.%) was reported in this work.

Chaudhari and Bakhshi (155) converted glycerol to hydrogen by steam gasification. The steam gasication was performed at steam flow rates of 2.5, 5.0 and 10 g/h at 600 and 700C. The glycerol ow rate was 4 g/h. An approximate 80% conversion was achieved when a steam ow rate of 10 g/h at 700C was used. Chaudhari and Bakhshi (155) illustrated that the steam gasication of glycerol does not produce liquid product at 600 and 700 C in a xed-bed reactor. In contrast, Stein and Antal (156) demonstrated that steam gasication of glycerol afforded acrolein and acetaldehyde liquid products at 600675 C in a laminar flow reactor.

High-purity glycerol can be reacted with oleic acid to form monoacylglycerols and diacylglycerols, which are widely used as biolubricant additives (157-161). One acylglycerol synthesis reported was the esterification of glycerol with lauric acid to form glycerol laurate (162), another interesting biolubricant.CONCLUSIONSIn this review, we showed that crude glycerol can be easily recovered from biodiesel by centrifugation or gravitational settling. However, the challenge is purification of this crude glycerol to food-grade glycerol. For most applications, glycerol needs to be free of impurities particularly catalysts, salts and soap to avoid the formation of unwanted byproducts during manufacturing. Currently, the glycerol purification process is expensive and is plagued with handling and separation problems. Many methods have been employed to purify glycerol, including neutralization, splitting, heating, ultrafiltration, ion exchange chromatography and vacuum distillation. The combination of more than one of these techniques can successfully yield pharmaceutical-grade glycerol, and recovered and purified glycerol has been converted into many valuable products, e.g., methanol, hydrogen, 1,3-propanediol, glycerol tert-butyl ether (GTBE).

Acknowledgements

The authors thank Universiti Kebangsaan Malaysia (UKM) for funding this project under research grant number UKM-GUP-BTK-08-14-306/Dana Lonjakan, LRGS/BU/2011/USM-UKM/PG/02, DPP-2013-056 and DIP-2012-022.

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Table 1: Performance of various vegetable oil hydrolytic processesType of catalyst/reactionMolar Ratio (Oil: water)Temperature (C)Time (h)Conversion (%)References

Lipase-catalysed hydrolysis

Lipase-catalysed hydrolysis

Base-catalysed hydrolysis

Acid-catalysed hydrolysis

Non-catalysed hydrolysis -

-

-

1:20

-40

27

100

190

270-3502

5

3

8

15 min95

88

98

99.4

100(20)

(21)

(22)

(23)

(24)

Table 2: Homogeneous catalysts in glycerol productionType of catalyst/reactionMolar RatioTemperature (C)Time (h)Conversion (%)References

Homogeneous basic

NaOH

KOH

NaOCH3 6:1

6:1

6:145

60

600.25

1

298

100

97.1(35)

(36)

(37)

Homogeneous acidic

AlCl3 and ZnCl2

H2SO4H2SO4H2SO4Trifluoroacetic acid 24:1

50:1

20:1

245:1

20:1110

80

95

70

12018

4

20

4

598

97

>90

99

98.4(38)

(39)

(40)

(41)

(33)

Table 3: Various heterogeneous catalytic systems employed in glycerol productionType of catalyst/reactionMolar RatioTemperature (C)Time (h)Conversion (%)References

Heterogeneous basic

Ca (NO3) 2/Al2O3

CaO/Al2O3

KOH/Al2O3

Mg-Al hydrotalcite

CaO

CaO/ZnO

Sulfated zirconia

Sr-Mg

Alum (KAl (SO4)212H2O)65:1

12:1

15:1

65:1

12:1

30:1

20:1

9:1

18:160

65

98(58)

(61)

(64)

(65)

(63)

Table 5: Performance of transesterification reactions using immobilised enzymes as catalystsType of catalyst/reactionMolar RatioTemperature (C)Time (h)Conversion (%)References

Immobilised lipase NOVO435

Immobilised lipase on magnetic nano-

Particles2.2:1

1:1

43

45

36

25

100

94

(91)

(67)

Table 6: Performance of transesterification reactions using nanoparticle catalystsType of catalyst/reactionMolar RatioTemperature (C)Time (h)Conversion (%)References

KF-loaded nano--Al2O3CaO nano-powder

Nano-MgO

Nanocrystalline CaO15:1

15:1

36:1

27:1

65

65

240

Room temperature8

2.5

16 min

-97.7

94

99.28

99(68)

(69)

(70)

(71)

Table 7: Performance of transesterification reactions using ionic liquids as catalystsType of catalyst/reactionMolar Ratio

(MeOH:Oil)Temperature (C)Time (h)Conversion (%)References

[Bmim]NTf2[Bmim]PF6[C3mim]Cl

N-Methyl-2-pyrrolidone hydrogen sulphate7.5:1

-

1:1

2:170

50

80

801.5

24

3

3>98

98

96

95(72)

(73)

(74)

(75)

Table 8: Typical percentages of transesterification products Products of transesterificationPercentage range (%)References

Biodiesel90-91(11,93,94)

Glycerol 9.0-9.6(93,94,95,96,97)

Unreacted products (methanol, MG, DG, TG)0.4-1.0(11,93,98,99)

Note: MG: Monoglyceride, DG: Diglyceride, TG: Triglyceride

Table 9: Standard glycerol characterisation methodsPhysical propertiesUnited States Pharmacopeia (USP)ASTMEuropean Standard Method (EN)References

Glycerol contentUSP 26(97)

DensityD5002-9414214(120)

ViscosityD445-9614214(110)

Ash valueD0482-03(97)

AcidityD1093-98(97)

Moisture content D4377-00E01(97)

Heat of combustionD0240-92(121)

Table 10: Characterisation of crude, purified and commercial-grade glycerolParameterCrude glycerolPurified glycerolCommercial glycerolReferences

Glycerol content (%)60-8099.1-99.899.2-99.98(97)

Moisture (%)1.5-6.50.11-0.800.14-0.29(133)

Ash (%)1.5-2.50.054