Novel process for producing biodiesel with reduced monoacylglycerol content

Abstract

A process for the post-treatment of crude biodiesel containing residual monoacylglycerol (MAG) and comprising contacting the crude biodiesel with a concentrated alkali solution, performing dual countercurrent washing, after each washing, performing phase separation and drying. The process is economical in terms of chemicals and wash water.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Patent Application No. 63 / 343,551, filed on May 19, 2022, entitled “NOVEL PROCESS FOR PRODUCING BIODIESEL WITH REDUCED MONOACYLGLYCEROL CONTENT,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to the production of lower alkyl esters of fatty acids having a reduced monoacylglycerol content for use as fuel in compression ignition engines. In particular, the novel process is economical and is characterized by the use of a minimum number of expensive equipment, chemicals and water compared to the prior art. Background Art

[0004] Biodiesel is the fatty acid ester of lower alkyl alcohol, such as but not limited to fatty acid methyl ester (FAME).FAME is the most common biodiesel produced by transesterification reaction, in which triglyceride such as plant or animal oil and fat or its blend is reacted with methanol in the presence of catalyst, preferably alkaline catalyst and even more preferably sodium methylate (NaOCH ).This transesterification reaction is almost complete, because one of the reaction products, glycerol (glycerol) (normally glycerol (glycerin) or glycerol) is insoluble in FAME to a great extent, and therefore can be easily separated from the reaction mixture, for example, by decantation, which causes the reaction equilibrium to move to FAME.Certainly, transesterification reaction moves to FAME and is also subject to the promotion of other parameters such as methanol stoichiometric excess and catalyst concentration, and the catalyst is actually also reacted and at least partially destroyed in the reaction.However, the material produced by this destruction still shows basicity and some catalytic activity. In a recent review published in Wiley Online Library and freely accessible (https: / / onlinelibrary.wiley.com / doi / full / 10.1002 / ejlt.202000188, at the time of writing this specification), AJ Dijkstra pointed out that the structure of the actual catalytic intermediates is still the subject of speculation, with arguments supporting enolate and / or organic glycerate anion species. However, it is clear that sodium methoxide is only a precursor to the actual catalyst, and since the actual catalyst is highly polar, it is much more preferentially dissolved in the heavy glycerol phase, and therefore a large amount of catalytic activity is lost when the light FAME phase is separated from the heavy glycerol phase. Therefore, it may be more appropriate to point out that the residual catalytic activity of any form of catalyst is at least partially lost due to physical separation and lack of contact with the reactants. Such considerations are consistent with the observation that transesterification shifts to FAME (and glycerol) if a larger amount of catalyst is used. It is also clear that despite the apparent simplicity of the chemical reaction that produces biodiesel, the exact mechanism and kinetics are still not fully understood.

[0005] The size of the current industrial plants for producing FAME by base-catalyzed transesterification (usually with sodium methoxide) is designed for a given excess of methanol. Typically, they are designed for a stoichiometric excess of methanol of 100%, and the consumption of sodium methoxide catalyst is about 4 kg (about 0.4%) per ton of converted feedstock. Those plants are also designed to carry out transesterification in three steps, each of which ends by removing one of the reaction products (glycerol) by phase separation in order to shift the reaction toward FAME. Under those conditions, about 98% to 99% of the glycerides are converted to produce crude FAME, which typically contains monoacylglycerol (MAG) typically in the range of about 0.4% to about 0.8% in addition to other non-FAME components, and diacylglycerol (DAG) and triacylglycerol (TAG) both typically in the range of about 0.1% to about 0.4% (unless otherwise stated, all percentages are expressed as weight / weight percentages). Prior art biodiesel facilities produce biodiesel with a MAG content that is typically close to but still exceeds 0.4%. Indeed, it is desirable to minimize the residual partial glycerides (MAG and DAG) of biodiesel in order to maximize the conversion yield of FAME, regardless of the purity specifications.

[0006] Crude biodiesel must be washed in order to remove various impurities such as residual catalyst, soaps, glycerol, excess reactants. Standard washing involves contacting the crude biodiesel with an acidic aqueous solution whose acidity serves the purpose of neutralizing the residual alkalinity originating from the catalyst and / or its degradation products. However, this acid washing procedure does not reduce the concentration of residual MAGs.

[0007] As a result, most current biodiesel plants currently produce biodiesel with residual MAG concentrations exceeding the new limits set for winter-grade biodiesel in the United States (ASTM 6515), which is a maximum of 0.4% for winter-grade biodiesel. This new, stricter limit has been implemented to improve the long-term stability and filtration characteristics of biodiesels such as FAME, which is by far the most prevalent biodiesel. It is expected that this stricter specification may also be implemented in Europe and other parts of the world.

[0008] In Asia, future directives will require higher blending ratios of biodiesel in petroleum diesel (20% and higher, i.e. B20). In order to ensure trouble-free use of blends containing such high concentrations of biodiesel, it is expected that the MAG content of biodiesel should reach even lower values, such as 0.2% or lower, even for use in mild and hot climates. Therefore, it is expected that in the future, at least in large world markets such as Malaysia and Indonesia, the tolerable content of MAG in biodiesel will be set at a maximum of 0.2%.

[0009] Several technical solutions are known for reducing the residual MAG concentration in biodiesel. However, each of them has serious disadvantages.

[0010] The residual MAG concentration in the crude biodiesel can be reduced by further shifting the transesterification equilibrium, for example, in the case of FAME, by using a greater excess of methanol or by using more catalyst. However, the preferred catalyst sodium methoxide is expensive, and in order to keep the production of biodiesel competitive, it is preferred to minimize its amount per unit of converted raw material. Using a higher degree of methanol stoichiometric excess will require adjusting the size of the container and / or reducing the output of the device. When the excess methanol is recovered and recycled, this methanol is actually distilled to obtain dry methanol. Therefore, it is not desirable to use a large amount of excess methanol. Therefore, both methods are not cost-effective and / or will require a large number of changes to the existing biodiesel device. Even for new devices to be established, there is a strong motivation to make the device as compact as possible and minimize catalyst consumption because this catalyst is expensive. In addition, this method is limited in efficiency because it cannot reduce the MAG content of biodiesel to very low levels (such as 0.2% or lower) because chemical equilibrium must be observed and a given concentration of reactants and reaction products must be imposed at equilibrium. Shifting the equilibrium to the reaction product to obtain such a low concentration of MAG will impose an unrealistic excess of reactants, particularly methanol. Therefore, industrially, a post-treatment process that can reduce the residual MAG present in crude biodiesel is preferred. In addition, the post-treatment should be as economical as possible.

[0011] One possible post-treatment is to distill the crude biodiesel. This method satisfactorily purifies the crude biodiesel, but is very expensive and causes a large reduction in yield in the form of residual bitumen. In addition, distillation also removes natural antioxidants, which is undesirable because natural antioxidants impart stability to biodiesel, which is an important feature. Therefore, alternative, more cost-effective post-treatment methods have been proposed.

[0012] WO 2016 / 098025 discloses a method for purifying crude biodiesel obtained by reacting triacylglycerol with an alcohol in the presence of a catalyst, the method comprising a series of water washing operations in order to reduce the total amount of contaminants and in particular sterol glucosides and MAG. The invention is characterized in that water is added to the reaction mixture before the glycerol phase is separated from the reaction mixture in order to achieve an initial washing of the entire reaction mixture. This initial washing comprises mixing the reaction mixture with the added water for a sufficient time and separating the aqueous phase (containing impurities) from the biodiesel (e.g. by decantation). After this initial washing of the entire reaction mixture, two consecutive additional washing steps are required in the absence of alkalinity to sufficiently reduce the residual MAG concentration of the biodiesel.

[0013] WO 2018210573 A1 discloses a method for reducing the content of monoglycerides (MG), in particular saturated monoglycerides (GMG), in crude biodiesel (RB), wherein the crude biodiesel has a monoglyceride (MG) content of 0.4% to 0.7% by weight and a free fatty acid (FFA) content of less than or equal to 0.25% by weight, the method being characterized by the following steps:

[0014] A) providing crude biodiesel (RB) having the above components in the above concentrations,

[0015] B) adding (1) an alkaline aqueous solution (L) in the form of a sodium hydroxide solution to the crude biodiesel (RB), wherein the concentration of NaOH in the sodium hydroxide solution is less than 4.03 mol / l and the amount of the sodium hydroxide solution added is from 1.0 wt. % to 3.0 wt. %, based on the amount of the crude biodiesel (RB), and wherein during the addition of the alkaline aqueous solution (L), the temperature of the crude biodiesel (RB) is at least 30° C.,

[0016] C) mixing the alkaline aqueous solution (L) with the hydrolyzed glycerides, preferably monoglycerides (MG), in particular saturated monoglycerides (GMG) in the biodiesel, wherein the residence time after the addition of the alkaline aqueous solution (L) and before the first centrifugation (2) is at least 15 minutes,

[0017] D) first centrifugally separating (2) a heavy phase (6) containing an alkaline aqueous solution (L) and hydrolyzed components from a light phase (7) containing biodiesel by a separator at a speed of 4400 to 7200 rpm,

[0018] E) drying (5) the light phase (7) and / or (9) to provide a processed biodiesel (AB) for use as fuel, the biodiesel having a monoglyceride content of less than 0.4 wt.-%, based on the total weight of the crude biodiesel (RB) used or the processed biodiesel (AB) in the corresponding step, the methanol content in the biodiesel in steps AE being less than 0.7 wt.-%.

[0019] Therefore, a washing method capable of reducing the MAG content of crude biodiesel can be obtained. However, these methods involve a large amount of water in order to achieve three consecutive washing steps or a large amount of chemicals in the form of dilute alkaline washing solutions and use several expensive centrifuges. In addition, in the method disclosed in WO 2018210573 A1, based on the gross weight of crude biodiesel, the methanol (MeOH) content of crude biodiesel must be less than 0.7% by weight, preferably less than 0.2% by weight, and particularly preferably less than 0.05% by weight. This mandatory low content of methanol in crude biodiesel can cause a reverse reaction, reducing the yield due to reverse transesterification, especially when the crude biodiesel still contains an active catalyst and / or is in contact with a strong base that may act as a catalyst. Therefore, this requirement is particularly potentially destructive, and in order to prevent the reverse transesterification of the crude biodiesel after post-treatment when the crude biodiesel after post-treatment still contains an active catalyst, it is advantageous to design a new and creative post-treatment method that is still effective even if the residual methanol content in the crude biodiesel subjected to post-treatment is significantly higher.

[0020] Purpose of the Invention

[0021] The object of the present invention is to overcome the various drawbacks and disadvantages of the prior art methods for post-treatment of crude biodiesel with the aim of reducing its residual MAG content. In particular, there is a specific need for a method for post-treatment of crude biodiesel capable of reducing the residual MAG content of the crude biodiesel to a concentration below 0.4% and even preferably to a concentration below 0.2%, and requiring the use of less water and less chemicals than the known methods. The innovative method should not cause a substantial saponification of the biodiesel. Optionally, the innovative method should cause at least a portion of the residual MAG to be converted into biodiesel, thereby increasing the conversion rate of the feedstock.

[0022] Advantages of the invention

[0023] An advantage of the present invention is that a method for post-treatment of crude biodiesel is disclosed, which method is able to reduce the MAG residual content of the crude biodiesel to a concentration below 0.4% and even preferably to a concentration below 0.2%.

[0024] A further advantage of the present invention is that a method for post-treating crude biodiesel is disclosed which enables the use of chemicals to be reduced compared to known methods, in particular the use of bases such as sodium hydroxide and acids such as citric acid or phosphorous acid, while still maintaining a substantial reduction in the residual concentration of MAG from the post-treated crude biodiesel.

[0025] Another advantage of the present invention is that a method for post-treatment of crude biodiesel is disclosed which is able to reduce water usage by implementing two countercurrent washing steps using only one volume of fresh acidic water (for the second washing step) which is recycled and used in the first washing step.

[0026] A further advantage of the present invention is that a method for post-processing crude biodiesel is disclosed which can be implemented in existing biodiesel plants without any major modifications and without requiring large investments.

[0027] Another advantage of the present invention is that a method for post-processing crude biodiesel is disclosed, which method is capable of preventing saponification of crude biodiesel during said post-processing.

[0028] A further advantage of the present invention is that a method for post-treatment of crude biodiesel is disclosed, which method is capable of optionally converting a portion of the residual MAG contained in the crude biodiesel into biodiesel.

[0029] A further advantage of the present invention is that a method for post-treatment of crude biodiesel is disclosed, which method is able to improve the long-term stability and filtration characteristics of crude biodiesel. In fact, reducing the MAG residual content of biodiesel is a prerequisite for its long-term stability and filtration characteristics, especially for fuels used at low temperatures and / or produced by high blending ratios (such as 20% or more biodiesel blended into petrodiesel, i.e. B20).

[0030] A further advantage of the process for post-treatment of crude biodiesel according to the invention is that it is effective even if the crude biodiesel contains a large amount (ie above 1%) of monohydric lower alkyl alcohols such as, for example, MeOH, in order to exclude any reverse transesterification.

[0031] These and other advantages will become apparent from the description of methods and embodiments according to the invention. Summary of the invention

[0032] The content of the present disclosure is presented below to provide a basic understanding of some aspects. The present disclosure is neither intended to identify key or important elements, nor is it intended to limit any limitation of embodiments or claims. The present disclosure can provide a simplified overview of some aspects that can be described in more detail in other parts of the present disclosure. In addition, any of the described aspects can be separated or combined with other described aspects, without being limited to the same effect as it has been clearly described individually and in each possible combination.

[0033] It has surprisingly been found that the above objects can be achieved with a process for the post-treatment of crude biodiesel, which contains residual MAG and which comprises the following steps:

[0034] (a) providing crude biodiesel,

[0035] (b) contacting the crude biodiesel with a concentrated alkali solution and stirring the resulting mixture to obtain a biodiesel treated with concentrated alkali,

[0036] (c) contacting the concentrated alkali-treated biodiesel of step (b) with recycled water and stirring the resulting mixture to obtain a first stirred mixture,

[0037] (d) phase separating the first stirred mixture produced from step (c) to obtain a pre-washed biodiesel and wastewater phase,

[0038] (e) contacting the pre-washed biodiesel of step (d) with fresh acidic water and stirring the resulting mixture to obtain a second stirred mixture,

[0039] (f) phase separating the second stirred mixture of step (e) to obtain an aqueous phase and fully washed biodiesel, wherein the aqueous phase is recycled as the recycle water in step (c),

[0040] (g) drying the fully washed biodiesel of step (f) to obtain purified biodiesel,

[0041] Wherein the concentrated alkali solution used in step b) is at least 5 mol / L and its volume is a maximum of 1% of the volume of the crude biodiesel, and wherein the residual MAG concentration of the purified biodiesel does not exceed 0.4%, even preferably not more than 0.2%.

[0042] In another embodiment, the crude biodiesel from step (a) is FAME and is produced by the transesterification of fatty material with methanol, catalyzed by a basic catalyst such as sodium methoxide.

[0043] In another embodiment, the crude biodiesel from step (a) is FAME and is produced from the transesterification of fatty material with methanol, catalyzed by a basic catalyst, and still contains 1% to 4% of residual methanol.

[0044] In another embodiment, the crude biodiesel from step (a) is FAME and is produced by transesterification of fatty material with methanol, the transesterification being catalyzed by a basic catalyst and the transesterification being further achieved in several steps, each step ending with a phase separation of a light crude FAME phase and a heavy glycerol phase.

[0045] In another embodiment, the crude biodiesel from step (a) is FAME and is produced by transesterification of fatty material with methanol, the transesterification being catalyzed by a basic catalyst and the transesterification being further achieved in several steps, each step ending with a phase separation of a light crude FAME phase and a heavy glycerol phase, and the final light crude FAME does not contain more than 0.5% of residual glycerol but contains more than 0.4% of MAG.

[0046] In another embodiment, the mixture of step (b) resulting from contacting the crude biodiesel with the concentrated alkaline solution is agitated with a high shear mixing device at a vigorous to severe agitation intensity for a period of time ranging from 0.1 minutes to 5 minutes.

[0047] In another embodiment, the mixture of step (b) resulting from contacting the crude biodiesel with the concentrated alkaline solution is stirred in a cavitation reactor, the stirring being maintained for a period of time ranging from 0.01 minutes to 0.1 minutes.

[0048] In another embodiment, the first and / or second stirred mixture of step (c) and / or step (e) is stirred at a light to moderate intensity for a period of time ranging from 1 minute to 60 minutes.

[0049] In another embodiment, the reduction of MAG provided by the above disclosed method is achieved by converting the MAG to soap by saponification with alkali, and / or converting the MAG to biodiesel by transesterification with monohydric lower alkyl alcohols.

[0050] In another embodiment, the above disclosed process is continuous and the concentrated alkaline solution of step b) is contacted with the crude biodiesel in-line using a metering pump.

[0051] In another embodiment, the purified biodiesel produced by the above disclosed method has a MAG content of less than 0.4%.

[0052] In another embodiment, the purified biodiesel produced by the above disclosed method has a MAG content of less than 0.2%.

[0053] In another embodiment, the purified biodiesel produced by the method disclosed above enters a blend containing petroleum diesel and 10% or more of the purified biodiesel, such as a blend referred to as B10, B20, B30, B40, B50, B60, B70, B80 or B90 and containing 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% purified biodiesel, respectively.

[0054] The following description and the accompanying drawings disclose various illustrative aspects. Some improvements and novel aspects can be explicitly identified, while other aspects can be apparent from the description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The drawings illustrate various systems, devices, apparatuses, and methods, wherein like reference numerals refer to like parts throughout, and wherein:

[0056] Figure 1 A method for post-processing crude biodiesel according to an embodiment of the present invention is schematically represented.

[0057] The present invention may be implemented in several forms without departing from the spirit or essential features of the present invention. The scope of the present invention is defined by the appended claims rather than by the specific description thereafter. Therefore, all embodiments falling within the equivalent meaning and scope of the claims are intended to be included in the claims. DETAILED DESCRIPTION

[0058] Reference will now be made to exemplary embodiments, examples of which are shown in the accompanying drawings. It should be understood that other embodiments may be utilized and that structural and functional changes may be made. In addition, the features of the various embodiments may be combined or changed. Therefore, the following description is presented only in an illustrative manner and should not limit in any way the various substitutions and modifications that may be made to the illustrated embodiments. In the present disclosure, many specific details provide a thorough understanding of the present disclosure. It should be understood that various aspects of the present disclosure may be practiced with other embodiments, etc. that do not necessarily include all aspects described herein.

[0059] As used herein, the words "example" and "exemplary" mean an instance or illustration. The words "example" or "exemplary" do not indicate a key or preferred aspect or implementation. Unless the context indicates otherwise, the word "or" is intended to be inclusive rather than exclusive. As an example, the phrase "A employs B or C" includes any inclusive permutation (e.g., A employs B; A employs C; or A employs both B and C). As another matter, the articles "a" and "an" are generally intended to mean "one or more" unless the context indicates otherwise.

[0060] kind".

[0061] Step (a) of the method according to the invention comprises providing crude biodiesel. Typically, crude biodiesel is produced by an ester exchange reaction in which a fat mixture comprising fatty acid glycerides and monohydric lower alkyl alcohols is reacted in the presence of an alkaline ester exchange catalyst to produce a mixture comprising fatty acid lower alkyl esters and glycerol as main components.

[0062] In the context of the present invention, such monohydric lower alkyl alcohol is defined as a C1-C4 alcohol selected from methanol, ethanol, propanol, isopropanol and butanol. Preferably, the monohydric lower alkyl alcohol is methanol, and therefore the fatty acid lower alkyl esters are referred to as FAME, which is by far the most common biodiesel currently produced. In fact, when the term "alcohol" is used, it will refer to the monohydric lower alkyl alcohol.

[0063] In the present specification, the term "processed biodiesel" will denote partially post-treated crude biodiesel at any stage of the process according to the invention, and the term biodiesel includes FAME.

[0064] However, the present invention is not limited to FAME or biodiesel produced by transesterification catalyzed by alkaline catalysts. Crude biodiesel produced by transesterification catalyzed by acid catalysts or enzyme catalysts can be an effective starting material for the process according to the present invention because even if those transesterification reactions are favorably shifted, they are still not completely completed and therefore residual MAG remains in the reaction product.

[0065] However, in most cases, the crude biodiesel to be worked up by the present invention will have been produced by the transesterification of fatty material with methanol catalyzed by sodium methoxide, the transesterification being achieved in several steps, each step ending in a phase separation, producing a light crude FAME phase and a heavy glycerol phase.

[0066] Preferably, the crude biodiesel in step (a) of the process of the present invention contains not more than 1% glycerol, even more preferably not more than 0.5%.

[0067] Preferably, the crude biodiesel in step (a) still contains 1% to 4% of monohydric lower alkyl alcohols, such as methanol, for example. This advantageously limits the reverse transesterification reaction during the subsequent steps of the method according to the invention, and when the crude biodiesel is contacted with a concentrated alkaline solution, the presence of a large amount of monohydric lower alkyl alcohols such as MeOH may lead to the conversion of MAG contained in the crude biodiesel into FAME. It is believed that the concentrated alkaline solution can also act as an esterification catalyst, especially under strong mixing, and in particular when the MAG concentration of the crude biodiesel is above about 0.4%. If the MAG concentration of the crude biodiesel is already close to or below 0.4%, the addition of the concentrated alkaline solution mainly leads to the saponification of those MAGs. However, it was observed that when MeOH was present in the crude biodiesel, no substantial saponification of FAME was observed. Thus, surprisingly, it has been observed that the presence of very small amounts of MeOH in crude biodiesel (such as 2% for example) allows the selective saponification of MAG, which is a minor component in crude biodiesel (less than 1%), while FAME, which is the most abundant component in crude biodiesel, is not saponified, even though both components contain ester functions. In fact, this surprising observation could be related to the presence of two hydroxyl functions in the MAG molecule that could act as co-catalysts during saponification and / or esterification. However, to our knowledge, this effect has never been demonstrated in the presence of residual MAG in crude biodiesel, so this explanation remains purely speculative.

[0068] Therefore, the preferred crude biodiesel of step (a) is FAME and is produced by a transesterification reaction catalyzed by an alkaline catalyst such as sodium methoxide (also known as sodium methanolate or sodium methylate). It can be added as is or as a solution in a lower alkyl alcohol. It is commercially available in the form of a 30% methanol solution. In an industrial plant, it is expected that about 4 Kg of sodium methoxide per ton of fat mixture will be sufficient. In fact, catalysts are expensive and there has been concern in the art to limit their use as much as possible. This includes high-quality starting materials (with low water, pollutant and FFA content) and achieving transesterification reactions in multiple stages each separated by phase separation, wherein fresh methanol and catalyst are added for each of these steps.

[0069] In practice, the transesterification is preferably carried out in stages separated from one another by addition of a fresh solution of catalyst and methanol followed by removal of the glycerol, generally by decantation. Thus, if several stages are involved, the FAME obtained after separation of the heavy glycerol phase released by the transesterification during the last stage constitutes the preferred starting crude biodiesel provided in step (a) of the process according to the invention.

[0070] Optionally, the heavy glycerol phase obtained after the last stage is used as a pretreatment of the fat mixture raw material selected for the production of FAME. Alternatively, one or more of the heavy glycerol phases obtained after each transesterification stage are used as a pretreatment of the fat mixture raw material. In fact, this or those heavy glycerol phases still contain active catalysts and some methanol. Therefore, this recycling can be used to neutralize the FFA that may be present in the raw material and has already triggered the transesterification reaction. Therefore, this recycling saves catalysts and therefore reduces the chemicals required for neutralizing residual catalysts.

[0071] Alternatively, the provided crude biodiesel is produced by an ester exchange reaction catalyzed by an acid catalyst such as sulfuric acid, sulfonic acid or a solid acid catalyst (such as aluminum silicate and / or zeolite). Alternatively, the provided crude biodiesel is produced by an ester exchange reaction catalyzed by one or more enzymes that generally belong to the lipase family. However, after the one or more reactions steps themselves, at least one final sedimentation and phase separation of the reaction mixture will occur to remove most of glycerol from the final reaction mixture, so as to provide useful crude biodiesel. Due to the fact that conversion cannot be complete with acid and / or with an enzyme catalyst, the resulting crude biodiesel also contains a large amount of MAG, and therefore will benefit from the method according to the present invention.

[0072] Crude biodiesel generally contains some active catalyst species, a portion of the excess alcohol (e.g. methanol in the case of FAME), some glycerol which may remain soluble and a large portion of minor components present in the raw material, such as tocopherols. It also typically contains significant amounts of MAG, typically about 0.4% to about 0.8%. For given production parameters, variations in the MAG content of crude biodiesel may be due to variations in the quality of the feedstock, but are primarily related to the degree of completeness of the transesterification: in general, the more complete this transesterification is, the lower the residual MAG concentration will be.

[0073] Optionally, the crude biodiesel provided in the step (a) of the method of the present invention is FAME, and the residual methanol contained in the crude biodiesel can be partially evaporated before step (a). During methanol evaporation, care should be taken not to reverse the transesterification reaction because at this stage, the active catalyst material is still present in the crude biodiesel. Therefore, this optional step is usually achieved under a moderate vacuum to remove about 50% of the methanol content. The resulting flashed crude biodiesel contains several percent, typically 1% to 6%, preferably about 1.5% to 4% methanol. However, the method according to the present invention is still advantageous in the absence of this optional methanol evaporation step to remove a part of the methanol from the crude biodiesel. In fact, another advantage of this partial methanol removal is to provide a dry methanol stream that can be recycled upstream. In fact, in most cases, this is the reason for this methanol evaporation step. If no partial flash is performed at this stage, the residual methanol will be contaminated by water, and therefore it will require more energy to recover it in dry form.

[0074] Optionally, the crude biodiesel provided in step (a) of the method of the present invention is FAME, and may have been subjected to one or more standard purification procedures, and may therefore meet old quality specifications, and may therefore still contain up to 0.7% of MAG. This biodiesel can also be considered as an effective starting material for the method of the present invention, and is considered as crude biodiesel in the context of the present invention. However, this biodiesel has a reduced methanol content (maximum 0.2%) and no residual catalyst. For this type of starting material, it is advantageous to add some methanol before proceeding to step (b) of the method according to the present invention. In fact, when a concentrated alkali solution is contacted with this crude biodiesel, the addition of methanol will prevent the saponification of FAME and may be conducive to the conversion of MAG to FAME. Alternatively, the concentrated alkali solution also contains methanol. In fact, even if there are no catalyst residues in this biodiesel, the reverse reaction is still possible due to the addition of a concentrated solution of a strong base to the crude biodiesel.

[0075] Most surprisingly, it was observed that the concentration of residual MAG present in the crude biodiesel was reduced more effectively when the same crude biodiesel was contacted with a small amount of concentrated alkali solution than when the crude biodiesel was contacted with a large amount of dilute alkali solution, even though the absolute amount of alkali was the same in both cases. Thus, it was unexpectedly observed that, despite the global use of the same amount of alkali per ton of crude biodiesel, the concentrated alkali solution was more effective than the dilute alkali solution in reducing the concentration of residual MAG in the crude biodiesel. This unexpected observation results in an effective reduction of the MAG contained in the crude biodiesel with significantly less chemicals, in particular with less alkali. Since less alkali is used, less acid is subsequently required to neutralize the residual alkali present in the processed crude biodiesel. In addition, the use of the alkali solution in concentrated form enables two washing steps to be carried out in countercurrent with only one volume of fresh acidic water (for the second washing), which is recycled for the first washing, and therefore the method according to the invention requires considerably less washing water per ton of processed crude biodiesel compared to known methods. Details of this unexpected observation are disclosed in the Examples, and in particular Examples 2a, 2b and 2c.

[0076] Therefore, in the method according to the present invention, step (b) includes contacting the crude biodiesel with a small amount of concentrated alkali solution, and vigorously stirring the resulting mixture to obtain the crude biodiesel treated with alkali. Preferably, the alkali solution is an aqueous solution. Alternatively, the alkali can be dissolved or dispersed in a lower alkyl alcohol such as methanol or ethanol. However, even in this case, some water must be present to support the dissociation of the alkali and produce hydroxyl anions. Preferably, the concentrated alkali solution used in step b) is at least 5 mol / L, and its volume does not exceed 1%, preferably 0.5% of the volume of the treated crude biodiesel. Even more preferably, the volume of the concentrated alkali solution used in step b) is in the range of about 0.4% to about 0.1% of the volume of the treated crude biodiesel. Preferably, in a continuous industrial device, a metering pump is used to add this small amount of concentrated alkali solution online to the crude biodiesel. Preferably, the stirring of the resulting mixture is achieved in a high shear mixer or with a cavitation reactor. Both devices are known in the art and are often used in the field of biodiesel and edible oil refining. These conditions are sufficient to reduce the concentration of MAG present in the crude biodiesel by about 50% or more while using reduced amounts of alkali, compared to known methods of contacting the crude biodiesel with a dilute alkali solution.

[0077] In fact, a small amount of contacting concentrated alkali solution may favor the formation of small droplets when vigorously stirred with crude biodiesel, since for such a small fraction of the dispersed phase, coalescence of droplets (during intensive mixing) is almost non-existent. Therefore, even though it is counter-intuitive, when a small amount of concentrated alkali solution is dispersed in the crude biodiesel, a greater degree of contact between the crude biodiesel and the concentrated alkali solution can be obtained, and surprisingly, a greater degree of reduction in MAG is caused.

[0078] In fact, the process according to the invention is able to reduce the concentration of MAG from typical values ​​ranging from about 0.4% to about 0.8% to values ​​ranging from about 0.4% to about 0.2% or even lower with very small amounts of alkali, such as, for example, about 1000 ppm NaOH per ton of crude biodiesel treated. In contrast, if a more dilute alkali solution is used, 1500 ppm NaOH per ton of crude biodiesel is required to similarly reduce the MAG of the crude biodiesel. The details of this observation are disclosed in the Examples and in particular Examples 3a and 3b. Of course, the amount of alkali to be used will depend on the exact residual MAG concentration of the crude biodiesel and the target value of residual MAG in the purified biodiesel.

[0079] The present invention focuses on the reduction of MAGs, but may have additional benefits, especially the reduction of sterol glucosides.

[0080] Even though this observation is completely unexpected and is not fully understood at present, it is believed that contacting the crude biodiesel with a concentrated alkali solution will enhance the saponification of MAG and may cause some conversion of MAG to FAME, since a large amount of methanol is still present in the crude biodiesel and under those conditions, the concentrated alkali may act as a transesterification catalyst. Therefore, this mechanism may also have a moderate effect on the yield of the transesterification reaction of the processed biodiesel. It is believed that if a more dilute alkali solution is contacted with the crude biodiesel, this conversion to FAME is unlikely. However, it is believed that the conversion of MAG to FAME only occurs when the residual MAG concentration in the crude biodiesel exceeds about 0.4%. In fact, when the concentration of MAG in the crude biodiesel is already moderate (about 0.45%), it has been shown that the reduction of MAG caused by the method according to the present invention corresponds to a similar consumption of alkali (in moles). Therefore, it can be inferred that all the removed MAG has been saponified and no FAME saponification has occurred. When the concentration of MAG in the crude biodiesel is more significant (about 0.85%), it has been shown that the reduction of MAG caused by the process according to the invention is much more important when the crude biodiesel contains 2% methanol, compared to the presence of very low amounts of methanol (0.05%). It can be inferred that when methanol is present in significant amounts, in addition to the MAG saponification reaction caused by the concentrated alkaline solution, transesterification (with methanol) occurs, thereby converting at least part of the residual MAG into FAME. It is believed that transesterification with methanol is only possible when the crude biodiesel contains more significant amounts of residual MAG, due to the need to comply with the transesterification chemical equilibrium constant. Unfortunately, there is a lack of theoretical data to confirm this hypothesis. The unexpected observations that led to this hypothesis are shown in Examples 2 and 4.

[0081] The stirring during step b) is preferably intense. Intense stirring will facilitate the fine dispersion of the droplets of the concentrated alkali solution in the crude biodiesel, which in turn will facilitate strong contact and enhance chemical reactions and / or diffusion and exchange between the phases. This is achieved by intense mixing, which is characterized by "powerful" to "violent" in the mixing scale (or stirring scale) discussed below. The mixing scale is used to quantify the stirring intensity in a given process. Characterizing stirring is complicated because its calculation depends on viscosity, interfacial tension, density, shear force, and various models are proposed in addition. In industry, more practical and empirical methods have been proposed to characterize stirring, particularly for mixing devices. This scale is proposed by a mixing device manufacturer (Dispersetech, US). The mixing scale is a 1 to 10 stirring scale, which is based on the impeller pumping capacity divided by the tank cross-sectional area, thereby obtaining an overall fluid velocity. This speed (in feet per minute (ft / min.)) is divided by 6 to obtain an arbitrary stirring scale. This simple 1 to 10 scale has been used as a valuable tool for describing mixing intensity. In the mixing scale, values ​​of 1 to 3 correspond to light mixing, values ​​of 4 to 6 correspond to moderate mixing, values ​​of 7 to 8 correspond to vigorous mixing, and values ​​of 9 to 10 correspond to intense mixing. Visually, vigorous mixing produces some splashing, boiling events at the surface of the liquid, and occasional eddies. Intense mixing produces persistent splashing, large wave formation, and unstable eddies.

[0082] It is understood that agitation includes mixing (such as provided by a mixing device) and also includes cavitation (such as provided by a hydrodynamic cavitation reactor). In this specification, light to moderate mixing / agitation is collectively referred to as moderate mixing / agitation, and vigorous to violent mixing is collectively referred to as intensive mixing / agitation.

[0083] In fact, a convenient procedure for starting the process according to the invention is to set the mixing intensity of step (b) to about 6, depending on the mixing scale, evaluate its effect on the MAG reduction, and finally adjust the mixing intensity as required. Alternatively, if the mixing device used in step (b) provides a very narrow range of mixing scales and / or its nominal mixing capacity is limited to the "powerful" range, the concentration of the base can be increased in order to obtain the desired MAG reduction. Such mixing tanks and devices capable of providing the appropriate mixing intensity required for the process according to the invention are widely used in various industries and are available as standard equipment from several manufacturers.

[0084] Alternatively, the stirring of the mixture obtained in step (b) of the method according to the invention can be achieved in a cavitation reactor, preferably a hydrodynamic cavitation reactor. A hydrodynamic cavitation reactor is capable of intensely mixing fluids without the use of an impeller. Instead, mixing is produced at local acceleration / deceleration of the fluid, thereby causing extremely high and low local pressures, creating cavities in the fluid, which, once collapsed, produce a high energy release, leading to intense mixing (and other effects). The advantages of cavitation reactors are their small size, short residence time and reliability due to the absence of moving parts. On the other hand, they require a large initial investment and generally require high-capacity pumps to feed the fluid to the cavitation reactor at an appropriate speed and pressure. However, cavitation reactors generally allow chemical reactions to be completed with fewer reactants, such as in the degumming and / or neutralization of edible oils, particularly for the degumming and / or neutralization of soybean oil, when the cavitation reactor is firmly built. It can be expected that the chemicals required to reduce the MAG in crude biodiesel are also reduced (reduced by up to 50%). Among the hydrodynamic cavitation reactors, a relatively new category is called "low-pressure cavitation reactors", and is able to produce sufficient cavitation at low hydraulic velocities and pressures, and therefore does not require a high-capacity feed pump. In addition, they are generally cheaper. Therefore, those "low-pressure cavitation reactors" are particularly preferred because they are more economical. Cavitation reactors (including high-pressure hydrodynamic cavitation reactors and low-pressure hydrodynamic cavitation reactors) are supplied, for example, by Cavitation Technology Inc (US).

[0085] In addition, the method according to the invention enables the recycling of fresh acidic water used for the second (and final) washing of the pre-washed biodiesel. The acidity of this fresh acidic water serves the purpose of deactivating any alkalinity still present in the pre-washed biodiesel. In step (e) of the method according to the invention, the second and final washing steps of the pre-washed biodiesel are typically carried out with fresh acidic water containing about 0.15% to about 0.5% citric acid. The acid must be slightly in excess compared to the total alkalinity of the processed biodiesel, in particular the alkalinity contained in the pre-washed biodiesel, in order to ensure complete deactivation of residual catalytic activity, in particular alkaline anions. Therefore, in step (f) after the second and final washing, the aqueous phase recovered by phase separation will still be slightly acidic, but closer to neutral, because part of the acid will be neutralized. This recovered aqueous phase is called "recycled water". Therefore, in the method according to the invention, this recycled water is used again and is contacted with the biodiesel treated with concentrated alkali produced by step (b). Therefore, since concentrated alkali is added to the crude biodiesel in a separate step, all the added alkali will be available for reaction with the MAG before this still slightly acidic recirculated water is added. The process according to the invention thus significantly reduces the required chemicals, in particular the alkali, and also significantly reduces the water required for washing the crude biodiesel, since the same water is used twice in countercurrent.

[0086] Therefore, in the step (c) of the method according to the invention, recirculating water is contacted with the biodiesel through concentrated alkali treatment obtained from step (b). The contact is preferably realized in a large-capacity mixing tank, allowing a residence time of about 1 minute to about 60 minutes, preferably about 10 minutes to about 30 minutes. Typically, the volume of recirculating water accounts for about 3% to about 6% of the biodiesel through concentrated alkali treatment obtained from step (b). Step (c) is preferably realized under medium stirring corresponding to "slight" to "moderate" stirring in the mixing scale discussed above. Medium stirring will be conducive to the coalescence of the very small droplets produced during step (b). In fact, these droplets are so small that it may be difficult and inefficient to phase separate such a mixture potentially. On the contrary, it has been observed that a long maturation of this first mixture (which results from contacting the concentrated alkali treated biodiesel from step (b) with the recirculation water obtained from step (f)) under moderate agitation produces a coarser mixture which is easily separated by conventional phase separation techniques such as decanters, settlers, centrifuges or other common liquid / liquid phase separation devices. Furthermore, the addition of the still slightly acidic recirculation water already contributes to the neutralization of the residual alkalinity present in the concentrated alkali treated biodiesel obtained from step (b). The residual slight acidity of the recirculation water comes from the fact that the recirculation water originates from the final washing and neutralization (step (e) of the process according to the invention) and that, in step (e), the fresh acidic water is contacted with the pre-washed biodiesel in order to deactivate any alkalinity present in the pre-washed biodiesel. Since the fresh acidic water intentionally contains a stoichiometric excess of acid to ensure complete neutralization of the alkalinity during the final washing step, the remaining recirculation water is still slightly acidic. In addition, the long contact time between the recycled water and the concentrated alkali treated biodiesel from step b) is conducive to the diffusion of any polar components present in the processed biodiesel to the aqueous phase. The moderate mixing that occurs in step (c) is conducive to obtaining a non-emulsified mixture that is easy to phase separate. This phase separation is realized in step (d) of the method according to the present invention, and can be carried out with various methods and devices known in the art such as decanters, settlers and centrifuges. The phase separation operated in step (d) produces wastewater, which is discarded and pre-washed biodiesel, which will be further processed.

[0087] In step (e) of the process according to the invention, the pre-washed biodiesel resulting from the phase separation performed in step (d) is subjected to final and complete deactivation of catalytic intermediates and / or any residual alkalinity. This is achieved by contacting fresh acidified water with the pre-washed biodiesel and maintaining sufficient stirring time of about 1 minute to about 60 minutes, preferably about 10 minutes to about 30 minutes. Agitation is preferably moderate, corresponding to mild to moderate in the mixing scale discussed previously, to again allow maturation and diffusion between the phases. Emulsification should be avoided so as not to jeopardize the downstream phase separation.

[0088] It has been found that the type of acid is not critical, provided that it is strong enough to deactivate the catalytic intermediates present in the pre-washed biodiesel. It has been found that citric acid is effective, but other acids, such as but not limited to phosphoric acid, may also be used in the method according to the invention. The amount of acid used is preferably stoichiometrically excessive for the residual alkalinity still present in the pre-washed biodiesel obtained by step (d). The molar concentration of the base can be determined by titration. The use of a large amount of excess acid constitutes an unnecessary expense that should be avoided. Generally, a stoichiometric excess of about 5% to about 50% is satisfactory. Alternatively, the acid can first be added to the pre-washed biodiesel in the form of a concentrated solution, which is strongly mixed in a short time, such as during 1 minute. After this initial strong mixing, the mixture is transferred to a conventional tank containing fresh water, and the resulting new, more dilute mixture is stirred at a medium intensity during a longer period of time, typically during about 1 minute to about 60 minutes, preferably about 10 minutes to about 30 minutes. In an intermittent process, a standard agitator will be fully sufficient to disperse the acidified aqueous solution into the processed biodiesel. In continuous systems, simple mixing devices such as static mixers have been found to be satisfactory, provided that the acidified aqueous solution is metered accurately enough into the processed biodiesel stream. In the case of fluctuations, the amount of acid must be increased so that its minimum value still exceeds the stoichiometric requirement.

[0089] The temperature during step (e) is not critical. Therefore, step (e) is preferably carried out at the same temperature as steps (a) to (d) of the process according to the invention. However, since step (g) (drying the processed biodiesel) can be carried out at a higher temperature, this means a heating stage, the pre-washed biodiesel of step (d) can be heated to about 90° C. before adding fresh acidic water. In this case, the temperature of the fresh acidic water will also be about 90° C.

[0090] The final phase separation of the process according to the invention is achieved in step (f) and can be carried out with various methods and devices known in the art such as decanters, settlers and centrifuges. Thus, this phase separation produces recycle water that is still slightly acidic and fully washed and neutralized biodiesel. The recycle water is recycled upstream of the process according to the invention (in step (c)), and the fully washed and neutralized biodiesel only needs to be dried in step (g) to meet international quality standards.

[0091] In step (g) of the process according to the invention, the water content and possible methanol content of the fully washed and neutralized biodiesel of step (f) are reduced to at least 0.05% and 0.2%, respectively, by evaporation under reduced pressure. The purified biodiesel produced by step (g) complies with the latest quality standards, in particular with ASTM D6751-15ce requiring a MAG of less than 0.4%. This drying step is systematically implemented in a biodiesel production facility, so that the skilled person does not need additional details in order to fully complete step (g).

[0092] Figure 1 The method according to the invention is schematically represented. However, Figure 1 This should not be interpreted as a limitation of the method according to the invention. Figure 1 This is only one of the possible embodiments of an apparatus for post-treatment of crude biodiesel, in particular for reducing the MAG contained in said crude biodiesel, using the method according to the invention. The invention is defined by the claims and is limited only by them.

[0093] like Figure 1As shown in , a crude biodiesel (1) is continuously provided and can be derived from various transesterification technologies. This crude biodiesel typically contains MAG in a concentration range of about 0.4% to about 0.8%. This crude biodiesel is contacted with a concentrated alkali solution stored in a storage tank (2). Typically, the concentrated alkali solution is introduced into the crude biodiesel online (3) and accurately dosed by a metering pump (4). The crude biodiesel contacted with a small amount of alkali is introduced into a high shear mixer (5) and mixed strongly. The size of the high shear mixer is designed to allow a residence time of about one minute. Alternatively, the high shear mixer can be replaced by a cavitation reactor, particularly a low-pressure cavitation reactor, wherein a pump is provided upstream of the low-pressure cavitation reactor to transfer sufficient hydraulic velocity to a mixture made of crude biodiesel and concentrated alkali solution. In the latter case, the range of the retention time will be less than 1 second to several seconds. After this intense stirring, the biodiesel treated with concentrated alkali is guided (6) to a large storage tank (7) and contacted with recirculating water (8). Typically, the size of the design tank (7) is to allow a residence time of about 1 minute to about 60 minutes, preferably under moderate stirring. During this aging period, the polar components contained in the biodiesel treated with concentrated alkali will migrate in the aqueous phase, and at the same time, the concentrated alkali solution that will undergo fine dispersion will coalesce into a coarser dispersion that is easier to separate. Then, the mixture is guided (9) to a phase separation device (10), where the mixture is separated into wastewater containing most of the impurities present in the crude biodiesel and pre-washed biodiesel. The wastewater (11) is guided to a suitable treatment unit for the wastewater, such as a biodigester (not shown). The processing of this wastewater is highly dependent on local regulations and the environment. The pre-washed biodiesel (12) is guided to a second storage tank (13) and contacted with fresh acidic water (14). The mixture made of pre-washed biodiesel and fresh acidic water is moderately stirred during about 1 minute to about 60 minutes. In this step, all remaining alkaline components are neutralized and removed, and the alkaline components are derived from the catalyst and / or the concentrated alkali solution still present in the pre-washed biodiesel. Alternatively, the acid can be introduced online in the form of concentrated acid solution, and the resulting mixture is stirred vigorously, for example, in a high shear mixer, with a residence time of about one minute. Then, pure water can be added to this strongly stirred mixture. In fact, the addition mode of the acid is not critical, because MAG reduction occurs when the concentrated alkali solution contacts the raw biodiesel. Then, this mixture stirred in (13) is guided (15) to the second phase separation device (16), wherein the mixture is separated into the fully washed biodiesel (17) and the recirculation water (8) actually recycled to carry out the first washing step. Then, the fully washed biodiesel is fully purified, and will be further dried (18) at low pressure and high temperature by known methods.After this drying step to remove low molecular weight contaminants (19) such as water and any remaining lower alkyl alcohols (usually methanol) and other remaining volatile components that may be present, the final purified biodiesel (20) that meets commercial specifications is stored. Various phase separation devices, such as decanters or centrifugal separators, can be used, and the phase separation devices are known in the art. Generally, phase separation is not a problem, provided that the soap concentration is minimized to less than 0.05%. This is also known in the art and is achieved by reducing the amount of free fatty acids present in the raw material, particularly when the transesterification is catalyzed by sodium methoxide and / or sodium hydroxide. In. Figure 1 In the drawings, steps (a), (b), (c), (d), (e), (f) and (g) of the method according to the present invention have been presented to illustrate the positioning of these different steps.

[0094] Example

[0095] Crude biodiesel synthesis

[0096] Different batches of crude biodiesel have been synthesized using laboratory equipment and containers, but with a procedure that simulates a typical industrial process for producing crude FAME. The raw material is refined, bleached and deodorized (RBD) palm oil, the alcohol is methanol, and the catalyst is a 30% solution of sodium methoxide in methanol. Table 1 reports the characteristics of the RBD palm oil raw material supplied by a major international palm oil processor. The characteristics of this RBD palm oil are unique to good quality raw materials, which will be used in high-quality industrial biodiesel production facilities. The key parameters are low FFA (less than 0.05%), low total insoluble impurities (less than 100ppm), low moisture content (less than 100ppm) and a P content of less than 10-15ppm is required.

[0097] Table 1. Raw material specifications (commercial RBD palm oil)

[0098]

[0099]

[0100] The transesterification reaction is realized in three different steps, each of which is separated into a light FAME phase and a heavy glycerol phase by phase separation. In this batch laboratory setting, it is not practical to recycle one or more of the heavy glycerol phases separated after each step to pretreat the raw material (in order to recover the catalytic activity that preferably migrates in the heavy glycerol phase). Therefore, as expected, more catalysts are needed to achieve satisfactory raw material conversion. In fact, 0.6% of the catalyst is used, compared with 0.4% in industrial facilities where such recycling (usually in a continuous process) is achieved. First, the raw material (RBD palm oil, as described in Table 1) is preheated at 68°C and transferred to a multi-neck batch reactor, which is maintained at a temperature of 60°C under stirring at 200rpm. Transesterification is carried out in three steps by adding part of methanol and catalyst to the raw material. The exact conditions are described in Table 2. After the set reaction time, decantation is carried out at 60°C, and the light FAME phase is recovered and used as a new raw material for the next step of transesterification. The heavy glycerol phase is discarded.

[0101] Table 2: Synthesis conditions of crude biodiesel samples used in Examples 1a, 1b, 2a, 2b, 2c, 3a and 3b

[0102]

[0103] The third decantation thus produces the final crude biodiesel, which is further flashed under reduced pressure to remove about 50% of its methanol content (a portion of the excess methanol remains in the light phase during phase separation). This methanol flash is carried out at 65° C., 250 mbara for 10 minutes. This methanol removal has the advantage of economically recovering a dry methanol stream, which, in an industrial setting, is recycled upstream. Several batches of crude biodiesel have been prepared, and their characteristics are presented in Table 3. In fact, it can be seen that the characteristics of the different batches are completely within the specifications expected for industrial crude biodiesel. For different batches with MAG concentrations in the range of 0.45% to 0.49%, the characteristics are very similar. In addition, it can be seen that the removal of methanol causes a slight reverse reaction, because the partial glyceride content of the flashed crude biodiesel batch is (moderately) higher than the partial glyceride content of the crude biodiesel batch that has not been flashed.

[0104] Table 3: Composition of crude biodiesel samples used in Examples 1a, 1b, 2a, 2b, 2c, 3a and 3b

[0105]

[0106]

[0107] Examples 1a and 1b: Double washing with water and acidified water.

[0108] In Example 1a, the crude biodiesel (batch 1, after partial methanol flashing) was washed successively, first with distilled water and secondly with an acidified aqueous solution containing 0.5% citric acid (which corresponds to 300 ppm of acid in the biodiesel). In fact, the acid was first added to the crude biodiesel in the form of a concentrated solution containing 30% citric acid, and the resulting mixture was intensively mixed in a high shear mixer for 1 minute. After this initial intensive mixing, fresh distilled water was added and the mixture was transferred to a maturation vessel, in which the mixture was moderately stirred at 200 rpm at 60° C. for 15 minutes. Both washes were carried out with 6% added water. The washing steps consisted of mixing the crude biodiesel with the washing solution at 60° C. for 15 minutes under stirring at 200 rpm. After each wash, the mixture was centrifuged at 2,000 rpm for 15 minutes. These two washes reduced the MAG concentration from 0.46% to 0.43% (after the first wash) and 0.42% (after the second wash), respectively. This demonstrates that the standard washing procedure does not significantly reduce the MAG concentration of the biodiesel, even though it certainly effectively neutralizes and / or eliminates any residual catalyst species present in the crude biodiesel.

[0109] In Example 1b, the same crude biodiesel (batch 1 after partial methanol flashing) was also washed sequentially, but the first wash was achieved with recycled water obtained by phase separation after the second wash of Example 1a. The second wash was achieved with acidified water containing 0.15% citric acid (this corresponds to 100 ppm of acid in the biodiesel), which was added as a concentrated acid solution containing 30% citric acid and initially mixed intensively for 1 minute using a high shear mixer. Again, both washes were carried out with 6% countercurrent use of water. These two consecutive washes reduced the MAG concentration from 0.46% to 0.43% and from 0.43% to the same value, respectively, indicating that the second wash had no effect on MAG reduction. This experiment once again confirms that the standard washing procedure does not significantly reduce the MAG concentration of biodiesel and that the use of recycled water has no benefit for MAG reduction in crude biodiesel.

[0110] In Examples 1a and 1b, the washed biodiesel samples were further dried and their specifications were within specification except for the MAG content. All results are summarized in Table 4. It must be noted that this washing procedure had no or very limited effect on the partial glycerides of the biodiesel. This indicates that no reverse reaction occurred and no further transesterification took place.

[0111] Table 4: Dual standard washing of crude biodiesel. Effect of countercurrent recirculation of the second wash water.

[0112]

[0113]

[0114] (a) expressed as w / w % of crude biodiesel; (b) expressed as w / w ppm of crude biodiesel

[0115] Examples 2a, 2b and 2c: Initial caustic wash and countercurrent wash were performed with 1000 ppm NaOH.

[0116] In Examples 2a, 2b and 2c, a countercurrent double wash with fresh or recycled washing water solution was studied, the first wash being achieved in the presence of an alkali. However, it has been unexpectedly observed that the efficiency of washing (at least for MAG reduction) is significantly improved if the alkali is contacted with the raw biodiesel in the form of a concentrated alkali solution, which is strongly mixed into the biodiesel in a short time (1 minute) in a high shear mixer (HSM). On the contrary, if the alkali is contacted with the raw biodiesel in the form of a dilute alkali solution, the MAG reduction is less obvious, even if the same amount of alkali (1000ppm) is used per unit of raw biodiesel. This unexpected observation can be obtained by comparing the results obtained in Example 2a with Example 2b and Example 2b with Example 2c. These results are compiled in Table 5. In Examples 2a and 2c, the crude biodiesel was first contacted with a concentrated alkali solution (resulting in a 66.7% and 68.1% reduction in MAG), and in Example 2b, the crude biodiesel was contacted with a dilute alkali solution (resulting in a 44.4% reduction in MAG). However, the total amount of alkali was the same in each example. A discussion of the full details and results of these experiments is disclosed below.

[0117] In Example 2a, crude biodiesel (batch 2 after partial methanol flashing) was sequentially contacted with concentrated alkali solutions and washed according to the following scheme: 0.33% of a 7.5Mol / L NaOH aqueous solution was added to the biodiesel and the resulting mixture was directly high shear mixed for 1 minute. This mixture was then contacted with 6% (weight / weight of crude biodiesel) distilled water and stirred at 200 rpm for 15 minutes at 60°C. After this moderate mixing, the mixture was separated by centrifugation at 2000 rpm for 15 minutes to produce wastewater and pre-washed biodiesel. The pre-washed biodiesel obtained was then washed a second time by contacting the obtained pre-washed biodiesel with fresh acidic water containing citric acid, which was added as a concentrated acid solution containing 30% citric acid and initially mixed vigorously for 1 minute using a high shear mixer (the exact final concentration is reported in Table 5). After this intense mixing, 6% distilled water was added directly and the resulting mixture was matured by moderate stirring at 200 rpm and 60°C for 15 minutes. After aging, phase separation is performed by centrifugation at 2000 rpm for 15 minutes, and a fully washed biodiesel and an aqueous phase that is still acidic are produced. Finally, the fully purified biodiesel is exposed to low pressure and high temperature using a Rotavapor (125° C., 50 mbara, 30 minutes) and dried. The dried and purified biodiesel obtained is analyzed and the results are included in Table 5 (FAME phase). The MAG of crude biodiesel has been reduced from 0.45% to 0.15%, which represents a significant reduction of 67%.

[0118] In Example 2b, the washing procedure is almost identical to that used in Example 2a, except that instead of adding the concentrated NaOH solution directly to the crude biodiesel, it is mixed with the recycled water resulting from the phase separation of the acid wash achieved in Example 2a (since all examples are based on batch experiments achieved with laboratory equipment). Another difference compared to Example 2a is that the second wash is achieved with much less citric acid: 100 ppm instead of 300 ppm. Surprisingly, even though the same total amount of caustic is used per unit of crude biodiesel, the MAG reduction is significantly lower compared to Example 2a. Indeed, the final MAG concentration is 0.25%, which corresponds to a reduction of only 44%.

[0119] In Example 2c, the concentration of chemicals was the same as used in Example 2b, but caustic was again added to the crude biodiesel (Batch 3, after flashing a portion of the methanol for evaporation) as a concentrated NaOH solution (0.33% 7.5 Mol / L NaOH in water), and the resulting mixture was directly high shear mixed for 1 minute. Again, the MAG reduction was significant (68%) and similar to the reduction observed in Example 2a (67%). Example 2c demonstrates that the addition of caustic solution in concentrated form is beneficial to the reduction of MAG and allows for the recycling of water produced by the phase separation of the second acid wash. This use of a small amount of concentrated caustic solution (0.33% in Examples 2a and 2c) in direct contact with the crude biodiesel results in a significant reduction in the chemicals and water required to wash the crude biodiesel compared to known methods.

[0120] It is also evident that the MAG reduction is caused by the alkaline wash and not the acid wash. Even though very small changes in MAG content can be observed after the second (acid) wash, it is believed that these small changes are due to normal experimental variations and not to a true effect of the acid wash.

[0121] In Examples 2a, 2b and 2c, the wastewater resulting from the phase separation achieved after the first washing has a strongly alkaline pH, which confirms that the crude biodiesel contains a significant alkalinity, which indicates that the added NaOH is not completely consumed and does not react strongly with the FAME. In fact, from the pH values ​​of the various aqueous phases it can be calculated that the MAG reduction corresponds to a similar base consumption (in moles). This observation suggests that the MAG is probably saponified to soap and glycerol and that no or at least very limited FAME saponification occurs, since no significantly more base is consumed.

[0122] In fact, when crude biodiesel containing methanol and MAG is contacted with alkali (NaOH), two possible reactions of the MAG are saponification and / or transesterification.

[0123] Saponification in MAG In this case, NaOH is a reactant and is consumed, and the reacted NaOH will no longer contribute to the pH of the water resulting from the washing of the crude biodiesel treated with the concentrated alkali solution.

[0124] Transesterification in MAG In this case, the NaOH acts as a catalyst and is not consumed, so all the added NaOH will contribute to the pH of the water resulting from the washing of the crude biodiesel treated with the concentrated alkaline solution.

[0125] Example 1a shows that there is some alkalinity left in the crude biodiesel. In fact, if this crude biodiesel is washed with 6% distilled water, the pH of the decanted water obtained is 12.1, which corresponds to 0.01 Mol / L of dissociated OH- in the wastewater. Therefore, we can assume the same alkalinity level in the crude biodiesel batch used in Example 2a. In fact, this alkalinity is relatively modest, since only 6% of water concentrates most of the alkalinity of the crude biodiesel.

[0126] The pH of the wastewater obtained in Example 2a was observed to be 13.4, which corresponds to a concentration of dissociated OH- of 0.25 Mol / L.

[0127] Since we can assume that 0.01 Mol / L of dissociated OH- comes from the crude biodiesel, we can assume that 0.25-0.01=0.24 Mol / L comes from the added NaOH that is ultimately present in the wastewater.

[0128] In Example 2a, 1000 ppm NaOH has been added to the crude biodiesel and any unconsumed (unreacted) NaOH will be almost completely transferred to the aqueous phase during washing. It is easy to calculate that if the NaOH is not consumed, it will result in a concentration of 0.42 Mol / L NaOH in the wastewater (which will correspond to a pH of 13.6).

[0129] Furthermore, in Example 2a, the concentration of MAG has been reduced by 0.30% (from 0.45% to 0.15%). If we assume that this amount is converted into soap and glycerol, a corresponding amount of NaOH will be consumed (mol / mol). It can be calculated that the resulting transport of remaining NaOH in the wastewater will be 0.27 Mol / L.

[0130] Since the measured value of the residual alkalinity of the wastewater (0.24 Mol / L) is very close to the theoretical value (which is 0.27 Mol / L based on complete saponification of the removed MAG), it is likely that the main mechanism leading to MAG removal in crude biodiesel is saponification, and at least when the MAG content in crude biodiesel is about 0.4% or less, transesterification with methanol is not very frequent. It is also likely that biodiesel saponification is very limited, because a large amount of biodiesel saponification will consume an equally large amount of NaOH.

[0131] Therefore, it can indeed be concluded from Example 2a that at least when the content of treated crude biodiesel is about 0.4%, those MAGs are mostly saponified and converted into soaps when the crude biodiesel is contacted with a concentrated alkaline solution.

[0132] Table 5: Double countercurrent washing. Effect of the concentration of the alkaline solution in contact with the crude biodiesel.

[0133]

[0134]

[0135]

[0136] (a) expressed as w / w % of crude biodiesel; (b) expressed as w / w ppm of crude biodiesel

[0137] Examples 3a and 3b (Dilute caustic wash with 1500 ppm NaOH)

[0138] In Examples 3a and 3b, the crude biodiesel (batch 4 after partial MeOH flashing) was washed with a dilute aqueous caustic solution. However, more caustic was used because 1500 ppm (expressed as ppm in biodiesel) of caustic was diluted in 6% distilled water (Example 3a) or 6% recycled water (Example 3b) resulting from the phase separation of the acid wash achieved in Example 2a. This dilute caustic solution was then contacted with the crude biodiesel using the same procedure used in the other examples. The second wash was achieved with acidified distilled water (300 ppm citric acid in Example 3a or 100 ppm citric acid in Example 3b). Both washing procedures resulted in a 67% reduction in MAG. These examples demonstrate that using a dilute form of alkali requires a greater total amount of alkali (per unit of crude biodiesel) to obtain the same MAG reduction when the crude biodiesel is contacted with a concentrated alkali solution corresponding to 1000 ppm NaOH. These examples also demonstrate that recycling of the wash water (i.e., recycle water) resulting from the phase separation of the acid wash (second wash) does not reduce the intermediate or final reduction of MAG. In fact, in Examples 3a and 3b, the same reduction in MAG was observed after both the first and second washes. In other words, the residual slight acidity contained in the recycle water has no effect on the MAG reduction, because only an insignificant amount of alkalinity is neutralized by this slight acidity, and therefore most of the added alkalinity remains available for reaction with MAG. Calculations show that less than 1% of the added alkalinity is neutralized by the residual acidity of the recycle water. The results related to Examples 3a and 3b are compiled in Table 6.

[0139] Table 6: Double countercurrent washing. Effect of recirculation of wash water (ie recirculation water).

[0140]

[0141]

[0142] (a) expressed as w / w % of crude biodiesel; (b) expressed as w / w ppm of crude biodiesel

[0143] Examples 4a and 4b (Caustic Washing of Methanol-Depleted Crude Biodiesel with Dilute Alkaline Solution)

[0144] Examples 4a and 4b show the effect of the significant lack of methanol in crude biodiesel on the reduction of MAG and its saponification. In order to be able to show these mechanisms more accurately and more convincingly, crude biodiesel containing a higher concentration of MAG is required. Therefore, two crude biodiesel batches were synthesized in two stages instead of three stages, resulting in significantly higher MAG contamination levels (1.71% and 1.52% respectively). This crude biodiesel has been fully washed with acidified water (so as to deactivate any residual catalytic activity retained in the biodiesel and exclude any reverse reaction caused by the initial presence of residual catalyst in the crude biodiesel, and then fully vacuum dried to reduce water and MeOH to very low levels (500ppm). Since biodiesel contains a high content of MAG, a more concentrated alkaline solution is used (2500ppm and 3370ppm respectively).

[0145] Example 4a shows that contacting and intensively mixing a concentrated alkali solution with a methanol-depleted biodiesel allows reducing its MAG concentration from 1.71% to 0.33% (after water washing). Therefore, it can be concluded that the presence of MeOH in the crude biodiesel is not mandatory for achieving a significant reduction in MAG, as long as a sufficient amount of alkali is contacted with the crude biodiesel. In addition, the analysis of the aqueous phase resulting from the water washing revealed the presence of glycerol (0.19%), which proves and confirms that MAG is saponified when in contact with a concentrated alkali solution. In addition, this aqueous phase also contains MeOH, which indicates that a very small proportion of FAME is also saponified (corresponding to 23 ppm of methanol). However, the actual methanol content in a well-washed biodiesel is likely to be underestimated, since part of the methanol may be lost by evaporation during washing and related operations. In fact, methanol is very volatile. In addition, it cannot be ruled out that some MAG reacts with the biodiesel to produce DAG and methanol.

[0146] Example 4b confirms the results obtained with Example 4a: in fact, the use of a more concentrated alkali solution leads to an even greater reduction in MAG and more glycerol present in the aqueous phase resulting from the water wash. Due to the inadequacy of the analytical equipment, the methanol content was not obtained. The results obtained with Examples 4a and 4b are compiled in Table 7.

[0147] It must be noted that in Examples 4a and 4b, a single wash with distilled water was achieved after treatment with concentrated alkali solution in order to enable optimal analysis of the resulting wastewater. This approach is acceptable since it was previously determined that the second acid wash had no effect on MAG reduction.

[0148] Table 7: Caustic washing of methanol-depleted crude biodiesel with dilute alkaline solutions

[0149]

[0150]

[0151] (a) expressed as w / w% of crude biodiesel; (b) expressed as w / w ppm of crude biodiesel; (c) ND: not detectable; (d) expressed as % or ppm in FAME phase; (f) NA: not obtained

[0152] Examples 5a and 5b (Specific effect of methanol concentration in crude biodiesel on MAG reduction)

[0153] The purpose of Examples 5a and 5b is to show the specific effect of the presence of methanol in the crude biodiesel and whether the process according to the invention can reduce the residual MAG content of biodiesel to very low values ​​even if the starting crude biodiesel contains the maximum MAG content found in commercial biodiesel.

[0154] Therefore, two batches of crude biodiesel have been synthesized with MAG contents reaching 0.8% to 0.9%, in order to examine whether the process according to the invention is still able to reduce the concentration of MAG remaining in the post-treated biodiesel to values ​​below 0.4% and even preferably below 0.2% with limited amounts of alkali, and, moreover, to determine the specific influence of the presence of methanol in the crude biodiesel on the reduction of MAG. Again, in Examples 5a and 5b, the crude biodiesel samples have been thoroughly washed with acidified water in order to deactivate any residual catalytic activity remaining in the biodiesel, and then thoroughly vacuum dried to reduce water and MeOH to very low contents (500 ppm). However, for Example 5b, a controlled amount of MeOH (20,000 ppm) has been added to this thoroughly washed and dried biodiesel.

[0155] It has been observed that the reduction of MAG caused by contacting those crude biodiesel samples with concentrated alkali solutions (2,000 ppm) is significantly more important when methanol is added to the crude biodiesel (i.e., in the case of Example 5b). It is possible that the presence of MeOH in the biodiesel causes the transesterification of MAG into FAME, and that this mechanism is in addition to their saponification. The combination of these two mechanisms leads to an excellent reduction of MAG when the crude biodiesel contains a large amount of methanol. In fact, it is possible that the transesterification caused by the presence of a large amount of methanol causes a reduction in DAG and TAG in the post-treated biodiesel, while for the crude biodiesel depleted in methanol, the DAG and TAG concentrations increase, which can confirm the occurrence of the reaction of MAG with biodiesel to produce DAG and methanol. Therefore, the presence of methanol in the crude biodiesel is advantageous because it enhances the removal of residual MAGs (and possible DAGs and TAGs) in the post-treated biodiesel, and in addition because the resulting soaps and glycerol are then removed from the FAME phase during subsequent washings, which can cause a portion of those MAGs to be converted to FAMEs, which corresponds to an increase in yield compared to the reduction of MAGs that relies solely on saponification. In addition, the results obtained by Example 5b are also consistent with the absence of FAME saponification during treatment with concentrated alkaline solutions. In fact, the residual concentration of MeOH in the post-treated biodiesel (4,900 ppm) and the residual concentration of MeOH in the aqueous phase (12,239 ppm) are consistent with the consumption of some MeOH during the esterification of MAGs, DAGs and TAGs. On the contrary, considering that the amount of MeOH is likely to be underestimated, for Example 5a, when the starting crude biodiesel contains only a very limited amount of methanol, the presence of MeOH in the aqueous phase (503 ppm) is also consistent with some FAME saponification.

[0156] Table 8 Specific effects of methanol concentration in crude biodiesel on MAG reduction

[0157]

[0158]

[0159] (a) expressed as w / w% of crude biodiesel; (b) expressed as w / w ppm of crude biodiesel; (c) expressed as % or ppm in FAME phase

[0160] Therefore, the present invention provides a particularly economical method in which the same water is contacted twice with the processed biodiesel in countercurrent. It has been found that 3% to 6% of water is sufficient for effective washing. In addition, unexpectedly, compared with dilute alkaline solutions, when contacted with crude biodiesel, the efficiency of concentrated alkaline solutions increases, reducing MAG to less than 0.4%, even less than 0.2%, not only further reducing the alkali required during this post-treatment itself, but also reducing the acid required for subsequent neutralization of any alkalinity still present in the pre-washed biodiesel. Therefore, the method according to the present invention is particularly economical in terms of chemical substances and water consumption while achieving a significant reduction in MAG. In addition, the method of the present invention can convert some MAG present in crude biodiesel into biodiesel (such as FAME). The method of the present invention does not cause significant saponification of biodiesel. Therefore, the method is able to deliver stable biodiesel with improved filtering characteristics, which can be safely blended with petrochemical diesel in high proportions. Finally, the implementation of the method of the present invention in existing production facilities is simple and does not require large equipment and a large amount of investment.

[0161] The above description includes embodiments of this specification. Of course, it is not possible to describe every conceivable combination of components or methods for the purpose of describing this specification, but those of ordinary skill in the art will recognize that many other combinations and arrangements of this specification are possible. Each of the above components or methods can be combined or added together in any arrangement. Therefore, the present invention is intended to cover all these substitutions, modifications and changes that fall within the spirit and scope of the appended claims. In addition, with respect to the use of the term "comprising" in a specific embodiment or in the claims, this term is intended to be inclusive in a manner similar to the term "comprising" being interpreted as "comprising" when used as a transitional word in the claims.

Claims

1. A method for post-processing crude biodiesel, wherein the crude biodiesel contains residual monoacylglycerol (MAG) and the method comprises the following steps: (a) providing crude biodiesel, (b) contacting the crude biodiesel with a concentrated alkali solution and stirring the resulting mixture to obtain a biodiesel treated with concentrated alkali, (c) contacting the concentrated alkali-treated biodiesel of step (b) with recycled water and stirring the resulting mixture to obtain a first stirred mixture, (d) phase separating the first stirred mixture produced from step (c) to obtain a pre-washed biodiesel and wastewater phase, (e) contacting the pre-washed biodiesel of step (d) with fresh acidic water and stirring the resulting mixture to obtain a second stirred mixture, (f) phase separating the second stirred mixture of step (e) to obtain an aqueous phase and fully washed biodiesel, wherein the aqueous phase is recycled as the recycle water in step (c), (g) drying the fully washed biodiesel of step (f) to obtain purified biodiesel, Wherein the concentrated alkali solution used in step b) is at least 5 mol / L and its volume is a maximum of 1% of the volume of the crude biodiesel, and wherein the residual MAG concentration of the purified biodiesel does not exceed 0.4%, even preferably not more than 0.2%.

2. A process according to claim 1, wherein the crude biodiesel from step (a) is fatty acid methyl esters (FAME) and is produced by transesterification of fatty material with methanol, the transesterification being catalyzed by a basic catalyst such as sodium methoxide.

3. The process according to claim 1, wherein the crude biodiesel from step (a) is FAME and is produced by transesterification of fatty material with methanol, the transesterification being catalyzed by a basic catalyst and still containing 1% to 4% of residual methanol.

4. The process according to claim 1, wherein the crude biodiesel from step (a) is FAME and is produced by transesterification of fatty material with methanol, the transesterification being catalyzed by a basic catalyst and the transesterification being further performed in several steps, each step ending with a phase separation of a light crude FAME phase and a heavy glycerol phase.

5. The process according to claim 1, wherein the crude biodiesel from step (a) is FAME and is produced by transesterification of a fatty material with methanol, the transesterification being catalyzed by a basic catalyst and the transesterification being further achieved in several steps, each step ending with a phase separation of a light crude FAME phase and a heavy glycerol phase, and the final light crude FAME does not contain more than 0.5% of residual glycerol but contains more than 0.4% of MAG.

6. The process of claim 1, wherein the mixture of step (b) resulting from contacting the crude biodiesel with a concentrated alkali solution is stirred with a high shear mixing device at a vigorous to severe stirring intensity for a period of time ranging from 0.1 minutes to 5 minutes.

7. The process of claim 1, wherein the mixture of step (b) produced by contacting the crude biodiesel with a concentrated alkali solution is stirred in a cavitation reactor, and the stirring is maintained for a period of time ranging from 0.01 minutes to 0.1 minutes.

8. The method of claim 1, wherein the first and / or second stirred mixture of step (c) and / or step (e) is stirred at a light to moderate intensity for a period of time ranging from 1 minute to 60 minutes.

9. The method of claim 1, wherein the reduction of MAG is achieved by converting the MAG into soap by saponification with alkali, and / or converting the MAG into FAME by transesterification with MeOH.

10. The method of claim 1, wherein the method is continuous and the concentrated alkaline solution of step b) is contacted with the crude biodiesel in-line using a metering pump.

11. A purified biodiesel produced by the method of claim 1, the purified biodiesel having a MAG content of less than 0.4%.

12. A purified biodiesel produced by the method of claim 1, the purified biodiesel having a MAG content of less than 0.2%.

13. A blend containing petroleum diesel and 10% or more purified biodiesel produced by the method according to claim 1, such as a blend referred to as B10, B20, B30, B40, B50, B60, B70, B80 or B90 and containing 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% purified biodiesel, respectively.

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