Patent Application
20100175312
The present disclosure is generally directed to biodiesel production technology. More specifically, this disclosure is directed to a method for producing a biodiesel material from waste fat feedstock, such as waste fat feedstock from crude vegetable oils, animal fats and/or waste oils.
ASTM 6751-02, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels. This specification covers biodiesel (B100) Grades S15 and S500 for use as a blend component with middle distillate fuels. This specification further covers the required properties of diesel fuels at the time and place of delivery.
The disclosures of the foregoing publication is hereby incorporated by reference herein in its entirety. The appropriate components and process aspects of the foregoing publication may also be selected for the present compositions and processes in embodiments thereof.
Biodiesel is a renewable, alternative fuel for diesel engines that is gaining increased attention and popularity both in the United States and throughout the world. In addition to being a renewable fuel, other advantages of biodiesel include that it is biodegradable and non-toxic. Biodiesel fuel can also be used directly in most diesel engines without requiring extensive modifications to the already existing engine design.
Biodiesel is generally derived from vegetable oils and/or animal fats. In a process for manufacturing biodiesel, feedstock is defined as the basic starting ingredient that embodies the vegetable oil or animal fat base. Examples of various feedstocks used in biodiesel production include: crude soy oil, refined soy oil, yellow grease, edible tallow, trap grease, inedible tallow, raw trap grease, and the like. Many development programs are underway for feedstocks including jatopha, algae oil, field pennycrest, as well as bacterial sources of lipids, all of which will present unique processing challenges.
Depending on the type of feedstock selected, different pretreatment steps may be required. The pretreatment depends on the free fatty acid content and/or water content of the feedstock.
For instance, it is possible to remove excess water present in the feedstock by way of various pretreatment processes. Many low cost feedstocks are currently available for biodiesel production. Unfortunately, many of these feedstocks contain large amounts of free fatty acids or in some cases excess water or other contaminants that hinder production of biodiesel.
The amount of free fatty acids varies with the type of feedstock. For example, yellow grease feedstock may contain approximately 1-15% free fatty acid by weight, whereas trap grease may contain approximately 40-100% free fatty acid by weight. Generally, when the free fatty acid content is less than 1% by weight, such an amount can be disregarded.
As conventionally known in the art, when free fatty acid levels are above 1% by weight, one pretreatment possibility is to add extra alkali catalyst. This allows a portion of the alkali catalyst to be devoted to neutralizing the free fatty acids by forming soap, while leaving enough catalyst left to act as the reaction catalyst. This approach to neutralizing free fatty acids is sometimes effective when the free fatty acid levels are around 5% to 6% by weight of total feedstock. However losses as high as 10-20% of the feedstock results when this technique is used.
Moreover, when presented with feedstocks containing 5% to 30% fatty acid by weight of the feedstock or greater, it is important in a pretreatment process to remove a portion of the excess free fatty acid content, otherwise the process yield will be too low.
There are at least four pretreatment methods to apply to feedstocks containing high amounts of free fatty acid: (1) glycerolysis; (2) enzymatic methods; (3) acid catalysis; and (4) acid catalysis reaction of fatty acid with methanol followed by alkali catalysis.
With respect to glycerolysis as a pretreatment method for feedstocks having high concentrations of free fatty acids present, this technique involves adding glycerol to the feedstock and heating it to a high temperature (approximately 200° C.), usually in the presence of a catalyst such as zinc chloride. The glycerol reacts with the free fatty acids to form mono- and diglycerides. This approach produces a low free fatty acid feedstock that can then be processed using traditional alkali-catalyzed methods.
With respect to enzymatic methods of pretreatment for feedstocks having high concentrations of free fatty acids, these methods require use of expensive enzymes and, as a result, can be challenging and difficult to employ on any large commercial scale.
With respect to acid catalysis as a pretreatment method for feedstocks having high concentrations of free fatty acids present, this approach employs a strong acid, such as hydrochloric acid, to catalyze the esterification of the free fatty acids and the transesterification of the glycerides. Because no alkali metals are present, the reaction does not produce soap in a competing reaction, but still usually leaves a high free fatty acid content that has to be dealt with.
The acid esterification reaction of the free fatty acids to alcohol esters proceeds relatively quickly, moving substantially to completion in one hour at 60° C. when a large excess of alcohol is used. However, the transesterification reaction proceeds rather slowly, taking up to several days to complete. Although applying heat can accelerate the reaction at an added production expense, reaction times can still be at least 30-45 minutes.
A further issue with respect to acid catalysis is that the water produced by the following reaction stays in the reaction mixture and ultimately stops the reaction, usually well before the reaction reaches completion:
Free Fatty Acid+Methanol→Methyl Ester+Water
The water produced by the above acid catalysis reaction also participates in side reactions by hydrating triglycerides and methyl esters, resulting in products that have high free fatty acid contents that fail to meet the ASTM 6751-02 specifications for biodiesel fuel blend stock.
With respect to acid catalysis followed by alkali catalysis as a pretreatment method for feedstocks having high concentrations of free fatty acids present, this technique solves the above reaction rate problem of acid catalysis by using each technique to accomplish the process for which it is most appropriately suited. More specifically, since acid catalysis is a relatively fast reaction to convert free fatty acids into methyl esters, this reaction is used as a pretreatment step for the high free fatty acid feedstocks. Then, when the free fatty acid concentration in the feedstock has been adequately reduced (e.g. to 0.5% or less), an alkali catalyst is added to convert the triglycerides into methyl esters. However, this method still results in high losses of starting materials and processing problems.
Although acid catalysis followed by alkali catalysis can convert high free fatty acid feedstocks quickly and effectively into feedstocks with workable amounts of free fatty acids, water formation is still a problem during the pretreatment phase. One approach to correct this problem may be to simply add enough excess methanol during the pretreatment such that the water produced is diluted to a level where it does not limit or stop the reaction. A disadvantage to this approach is that more energy would be required to recover and purify the excess methanol, and, thus, at least an additional production step and added operating costs are included. Furthermore, the water side reactions will result in undesirable side products which present considerable process challenges.
One alternate approach to resolve the issue of water formation during an acid catalysis pretreatment would be to allow the acid-catalyzed esterification to proceed until it is stopped by the water formation from the acid catalysis process. Afterwards, the excess alcohol and water can be boiled off in a distillation process. If the free fatty acid content is still too high in the feedstock, then additional methanol can be added, and if necessary, an acid catalyst can be added to continue the reaction.
However, although this process can optionally be cycled for as many times as necessary to achieve a desired result, unfortunately an obvious disadvantage to using this approach is the significant amount of energy required to run this process. As a result, employing this technique would significantly increase overall production costs and present challenging feasibility considerations to be made.
Vegetable oils and animal fats are generally mixtures of triglycerides from various fatty acids. Triglycerides are esters of glycerol, a three-carbon alcohol having a hydroxyl group on each of its three carbon atoms with long-chain acids that are commonly referred to as fatty acids. Glycerol, also referred to as glycerin or glycerine, forms the backbone of fatty acids in fats. Chemical and physical properties of fats and oils and the esters derived from the fats and oils vary with respect to the fatty acid profile. Below, Table 1 lists some of the most common fatty acids, listed by their common names, corresponding IUPAC nomenclature, chemical formulas, and their corresponding methyl esters, which are formed by a reaction between the fatty acid and methanol.
As mentioned above, esters are formed by reacting an acid with an alcohol, that creates water as a byproduct. In a variation, the formation of esters can occur from the reaction of esters and alcohols. For example, an ester can react with another alcohol group, and a new alcohol is derived from the original ester and a new ester is derived from the original alcohol. Thus, an ethyl ester can react with methanol to form methyl ester and ethanol. This process is called transesterification, and an exemplary reaction mechanism is shown below.
In a conventional process, biodiesel is obtained by transesterifying triglycerides with methanol or other suitable alcohols in the presence of a catalyst. Methanol is a preferred alcohol for producing biodiesel because it is the least expensive and most readily available alcohol. However, other alcohols may be used as desired.
For the purposes of this disclosure, methanol and its related reaction products (i.e. methyl ether) are frequently described or mentioned as the alcohol or alkyl ester product in a biodiesel production process. However any suitable alcohol other than methanol can be appropriately used.
In order for the transesterification reaction to occur in a reasonable amount of time, a catalyst is generally added, often in small amounts, to accelerate the reaction. The catalyst is added to the mixture of feedstock and alcohol. Conventionally, the catalyst is not limited to a base and may be acidic in nature.
Transesterification is an equilibrium reaction. In this particular equilibrium reaction, it is desired for the reaction to proceed from left to right, or toward the products (or biodiesel). However, transesterifications do not easily proceed to completion.
Generally, to force an equilibrium reaction in the direction of the products (which is almost always desired) one or more parameters of the reaction may have to be modified. Modifications include, for example, varying the molar ratio of the reactants, modifying the reaction temperature, modifying the reaction pressure, and/or use of a catalyst.
Based on these available options, the equilibrium of a reaction is typically encouraged or promoted by adding excess amounts of one of the reactants or by removing one or more of the products. With particular respect to the transesterification reaction, in order for this reaction to proceed to completion, up to 6 moles of alcohol can be required to be added for every mole of triglyceride present.
In addition, it is not uncommon for conventional processes to require an excess of 60% to 100% of methanol or suitable alcohol to be added, simply to ensure that the transesterification reaction proceeds to completion.
The transesterification reaction may also be referred to as a two-phase reaction, meaning that two phases are present during the reaction and, as a result, not all of the reaction materials can be readily mixed with each other. Using, for example, feedstock such as animal fat that is added to, for example, methanol, at the start of the reaction the methanol and the feedstock do not readily mix. The alcohol and the triglycerides exist in separate phases resulting in very slow conversion rates and incomplete reactions because the two reactants are not in contact with each other. A further complication is at the completion of the reaction, the following two layers or phases are present: (1) a first phase comprising mainly glycerol; and (2) a second phase comprising methyl esters. As the first glycerol phase holds more of the alcohol and catalyst, the reaction completion is further complicated by this removal of catalyst and alcohol.
In general, how readily one compound dissolves in another depends on the structural features of the compounds, such as, for example, the existence of —OH groups. Compounds containing —OH groups, such as alcohols, and those without —OH groups, such as triglycerides, often do not readily mix with each other.
Therefore, in conventional methods of preparing biodiesels, the reactor mixing rate is often aggressively set at a significantly high and intense rate in order to blend the phases of the two-phase reaction mixture. Thus, the majority of biodiesel production processes known in the art use intense mixing and high mixing rates, especially at the beginning of the reaction, in order to incorporate the less soluble alcohol into the feedstock triglycerides.
It should be noted that if this intense mixing is maintained throughout the entire reaction, the glycerol may become dispersed in very fine, tiny-sized droplets throughout the resulting product mixture. When this happens, the glycerol dispersion requires anywhere between one to several hours of added production time to allow the droplets to come together to create a distinct glycerol phase. For at least these reasons, a need exists for a method to produce biodiesel and promote the reaction that does not require use of such intense and agitated mixing.
The ASTM D-6751 standard for biodiesel allows for 0.24% total glycerol in the final product, which includes both free glycerol and bound glycerol
Glycerol generally is insoluble in biodiesel. In fact, almost all of the glycerol in biodiesel is capable of being easily removed via a centrifuge or settling. Some glycerol may remain either as suspended droplets or in a very small amount that is dissolved in the biodiesel, and this is referred to as free glycerol. High levels of free glycerol in the final product can lead to injector deposits, clogged fueling systems, and otherwise results in undesirable buildup of free glycerol in the bottom of storage and fueling systems.
The total glycerol method is the method used to determine the level of glycerol in the resulting biodiesel fuel. Low levels of total glycerin ensure that a high conversion of the oil or fat feedstock into its mono-alkyl esters has occurred. Further, high levels of mono-, di- and triglycerides in the final product may cause undesirable injector deposits and in addition their high melting points may adversely affect cold-weather operation of vehicles by causing the fuel to gel and/or plug the filter.
In order to calculate the total glycerol present in a product, one must consider that one molecule of triglyceride contains one molecule of glycerol. Using triolein as an example, one mole of triolein would weigh 885.46 g/mol and the mole of glycerol would weigh 92.10 g/mol. Thus, triolein can be considered to consist of: (92.10 g/mol)/(885.46 g/mol)=0.104, or 10.4% glycerol. This glycerol is characterized as bound glycerol because it is chemically bound to the triolein molecule. Bound glycerol can also be bound to monoglycerides and diglycerides, as partial reaction products of the transesterification reaction. Bound glycerol is then added to any fully reacted glycerol, or free glycerol, to calculate the final amount of total glycerol.
Therefore, to provide an example, if the original feedstock contains 10.4% glycerol, and the final biodiesel can only contain a total glycerol level of 0.24% as the standard level, then this particular transesterification reaction must be: [10.4−0.24]/[10.4]×100=97.7%, or 97.7% complete.
One of the primary challenges with respect to transesterification and the conventional process for obtaining biodiesels is that in addition to triglycerides, mono- and diglycerides also are formed as intermediates during the transesterification reaction. Because hardly any chemical reaction proceeds to full completion, intermediates such as these can contaminate the final product and render the final product unusable, or at least below the standards for biodiesel as set forth in ASTM D6751.
Various competing reactions also exist during the preparation of biodiesels under conventional methods known in the art.
More specifically, it is common for feedstocks to contain small amounts of water, in addition to free fatty acids.
With respect to free fatty acids present in feedstock, if an oil or animal fat containing a free fatty acid, such as palmitic acid, is used to produce biodiesel, the alkali catalyst, such as NaOH or KOH, will react with this acid to form a soap. Compounds such as soap, in which the hydrogen of an acid has been replaced with a metal ion, can also be referred to as salts.
The reason that such compounds are formed is because compounds such as NaOH or KOH dissociate respectively into Na+, K+ and OH− ions, in which the protons and electrons are not evenly distributed, thus leading to charged particles. It is also important to remember that fatty acids are in fact acids, albeit weak acids, but nevertheless these acids can also form salts, in which the H+ is replaced by a cation such as Na+ or K+.
The formation of compounds such as soap is undesirable because it results in binding the alkali catalyst into a form that does not contribute to accelerating the overall rate of reaction. Additionally, excessive soap in the product can inhibit later processing of the biodiesel, including but not limited to glycerol separation and water washing. Furthermore, any catalyst that has been converted into soap is no longer available to accelerate the reaction. Therefore, a need exists for a biodiesel production process that significantly eliminates or entirely removes the formation of soap as an undesirable byproduct.
As described above, in addition to free fatty acids, excess water present in the feedstock can also be problematic with respect to biodiesel production. When water is present, particularly at high temperatures, it can hydrolyze the triglycerides into diglycerides and form a free fatty acid by hydrolysis reaction. As described above, in the presence of an alkali catalyst, the free fatty acid may react with the catalyst to form soap in a competing reaction. In addition, when water is present, it usually manifests itself through excessive soap production. Aside from that, water that carries forward into products presents an additional problem because it will hydrolyze methyl or alkyl esters into free fatty acids and methanol. Since the methanol is more volatile than the water, the methanol is then removed in a drying process and the resulting biodiesel product will have a high free fatty acid or acid value as a result.
The soaps of saturated fatty acids tend to solidify at room temperatures, therefore a reaction mixture having excessive soap may gel and form into a semi-solid mass that is difficult to recover to its original state.
Furthermore, there are at least three byproducts or side streams in the conventional biodiesel production process that must be considered and dealt with when designing any biodiesel production process. These include: (1) excess alcohol (e.g., methanol) that is recycled; (2) glycerol reaction product; and (3) wastewater stream.
With respect to excess alcohol, alcohol recycling is necessary based on the fact that in conventional processes, excess methanol is required to promote the transesterification equilibrium reaction to completion. Recovery of unspent methanol saves in raw material production costs and further prevents methanol emissions into the atmosphere. Prevention or reduction of methanol emissions is essential because methanol is both highly toxic and highly flammable. Methanol is classified by the Environmental Protection Agency as a Volatile Organic Compound (VOC) and therefore its emissions are regulated.
It is possible to recover methanol using distillation processes, either conventional distillation or vacuum distillation, or employing a partial recovery using a single stage flash. In an alternative to distillation, a falling-film evaporator may be used. Any of these production steps for dealing with excess alcohol recycle, such as adding distillation columns, comes with high costs associated with the addition, implementation and ongoing maintenance. It is important to note that if the methanol or alcohol is contaminated with water, then expensive multiple stage distillations will be necessary.
In a traditional transesterification reaction, the recovered glycerol and glycerol from rendered feedstocks often contains catalyst residue, residual alcohol, esters, phosphatides, sulfur compounds, aldehydes, ketones, proteins and/or insolubles such as dirt, minerals and/or fibers. As a result, various chemical refining, physical refining or glycerol purification methods such as, for example, vacuum distillation with steam injection followed by an activated carbon bleach or an ion exchange purification, are required in many traditional biodiesel production processes. Each of these treatments adds at least one or more additional production steps requiring further expenditures of time, money and resources.
The problems outlined in the paragraph immediately above is even more true with respect to recovered methyl esters or biodiesel in traditional transesterification reactions and, thus, presents an even greater challenge. Similar to the recovered glycerol as discussed above, various chemical refining, physical refining, and/or purification methods also must be incorporated to properly deal with the methyl ester reaction products. Such contaminants present in the methyl ester or biodiesel reaction products prevent these products from satisfying ASTM 6751-02 and other standards, and thus must be dealt with through additional production steps.
With respect to wastewater streams, ester washing produces approximately 1/10th of a gallon to approximately one gallon of water per gallon of ester, per wash. In addition, it is best if all process water must be softened in order to remove calcium and magnesium salts and then treated to remove copper and iron ions. In addition, glycerol ion exchange purification, for instance, can result in large quantities of low salt water being produced and therefore requires additional attention to its disposal.
Furthermore, wastewaters from biodiesel production processes ideally should meet local city, county and state waste treatment disposal requirements. If, for example, methanol is not fully recovered during the process in the production plant and is present in the wastewater stream, the methanol must be removed from the waste water stream typically in order to meet local, county and state disposal requirements.
Conventionally known biodiesel production processes generate waste water streams that contain soaps and other fatty compounds that increase the Biological Oxygen Demand (BOD) and Fat, Oil and Greases (FOG) of the discharge water, which can cause the plant to exceed the local water discharge permits and, thus, significantly increase operational costs.
It is further critical that, regardless of what type(s) of refining methods are selected for processing the methyl or alkyl ester reaction product, it meets the requirements set forth in ASTM D-6751-2. Examples of refining process steps related to the recovery of esters from the reaction mixture include: (1) ester/glycerol separation; (2) ester washing; (3) ester drying; and (4) additization, to name a few.
Ester/glycerol separation is commonly the first step of product recovery in most biodiesel production methods.
This separation process is based on the premises that: (1) fatty acid alcohol esters and glycerol are barely mutually soluble, as mentioned in the discussion of the two-phase reaction above; and (2) there is a significant difference in density between the ester and glycerol phases. Furthermore, the presence of methanol, or any other suitable alcohol, in one or both of the phases affects the solubility of ester in glycerol, and that of glycerol in ester.
Ester washing is typically used to neutralize any residual catalyst left over from the transesterification reaction, to remove residual free glycerol and methanol (or other suitable alcohol), and further to remove any soap that is formed during the esterification reaction. If any alcohol is present during the washing process, a distillation step is required to recover the alcohol. This may limit the choice of alcohols as ethanol and water form eutectic mixtures that are very expensive to separate.
Water used during an ester washing process also provides a vehicle for the addition of acid to neutralize the remaining catalyst and the water also provides a means to remove unwanted product salts.
In conventional processes, ester drying is necessary to meet required limits on the amount of water present in the final biodiesel product.
Regarding the final biodiesel product, additization is the addition of materials having the specific function to modify one or more of the fuel properties. For instance, petroleum-based diesel fuels are commonly treated with a variety of additives to improve oxidative stability, lubricity, corrosive resistance, detergency, conductivity, and other properties. Examples include but are not limited to: cloud point/pour point additives, stability enhancing agents, and antioxidants.
In particular, cloud point defines the temperature at which a cloud or haze of crystals appears in the biodiesel fuel under prescribed test conditions. These prescribed test conditions generally relate to the temperature at which crystals begin to precipitate from the fuel while in use. Biodiesel fuel generally has a higher cloud point than petroleum-based diesel fuel.
Additionally, oxidation products found in biodiesel or during the production process may be in the form of various acids or polymers. If oxidation products are present in high enough concentrations, they can lead to undesirable fuel system deposits, filter clogging, and various fuel system malfunctions. Additives designed to control the formation of oxidation products and improve oxidative stability can be useful in significantly improving the oxidation stability performance of biodiesel products.
Other fuel additives include biocides or biosides that can be added to a final biodiesel product to destroy or inhibit the growth of fungi and bacteria which can grow at fuel-water interfaces.
In summary, methanol, and correspondingly the use of any alcohol other than methanol in any conventional biodiesel production process as known in the art, greatly affects most if not all product recovery operations. The methanol used must be fully recycled in order to achieve the best economical and environmental conditions during operation.
Thus, embodiments of the present disclosure address the above needs with a method of producing biodiesel from feedstock that: (1) eliminates undesirable soap byproducts; (2) eliminates wastewater streams; (3) is designed with an efficient recycle stream; (4) yields superior quality biodiesel fuel; (5) is capable of promoting the transesterification reaction to completion without requiring intense mixing, excess alcohol, or the like; (6) minimizes product contamination and production costs; and (7) can be used with any available feedstock.
The present disclosure addresses these and other needs, by providing a process for converting waste fat feedstock into biodiesel fuel, the process having a reaction system that: (1) is at least capable of completely reducing fatty acids; (2) reducing or eliminating recycle and/or eliminating the need for distillation of methanol or any suitable alcohol used therein; (3) reuses catalyst from the glycerol recycle process; (4) is capable of being performed in a single reactor; (5) comprises a transesterification reaction that does not require the use of excess methanol or intense reaction mixing in order for the reaction to proceed as desired; and (5) reduces the overall energy requirements with respect to methanol recovery.
Embodiments also provide a process for obtaining biodiesel from waste fat further having a faster transesterification reaction rate than conventional methods that is capable of yielding pure biodiesel product with little to no contamination, and a biodiesel product quality that is equal to or better than the requirements and specifications as set forth in ASTM D6571.
Embodiments also include a transesterification reaction that is not required to proceed to completion in order for the reaction system of the present disclosure to function as intended.
Embodiments provide a process for obtaining biodiesel from waste fat where fatty acids are easier to recover from glycerol, that reduces losses and speeds processing. In embodiments, because of the reuse of catalyst found in the glycerol, and the use of low cost caustic in the pretreatment steps, operational costs may be greatly reduced.
In embodiments, the process allows for recovery of fatty acids from the glycerol stream, further reducing loss and providing a high purity glycerol byproduct that has a greater value than contaminated, less pure glycerol, as obtained by traditional methods.
More particularly, in embodiments, there is provided a method for converting waste fat feedstock into biodiesel, the method comprising:
providing a feedstock;
pretreating the feedstock;
adding glycerol and a caustic base to the pretreated feedstock and heating the resulting mixture to perform a glycerolysis reaction;
removing the water vapor in a vacuum;
combining the reaction products from the glycerolysis reaction with a slight excess of alcohol and catalyst, and performing a transesterification reaction, resulting in an alkyl ester reaction product;
separating glycerol from the alkyl ester reaction product in a glycerol separation processing step;
recovering alcohol from the alkyl ester reaction product in an alcohol recovery flash resulting in a stream of alkyl ester free from glycerol;
flashing the stream in an alkyl ester flash process to yield pure methyl ester biodiesel;
where the transesterification catalyst is recycled with glycerin, and where bottoms and partial glycerides that contain catalyst, from the methyl ester flash processes, are recycled back to the transesterification reaction.
Embodiments provide methods for producing biodiesel with little or no contaminants from waste fat feedstock in a lean transesterification reaction that does not require excess alcohol, such as methanol, for completion.
With respect to the selection of a feedstock for use in a biodiesel production process, generally speaking, the closer a feedstock is to containing pure triglycerides, the higher its quality and its cost, and the easier it will be to convert the feedstock into biodiesel.
On the other hand, the lower the cost of the feedstock, the lower its quality, and the more difficult and expensive it will be to convert the lower quality feedstock into biodiesel.
Overall, biodiesel feedstocks may contain a variety of contaminants, such as water, free fatty acids, particles (e.g. minerals, waste foods, proteins and other materials not soluble in lipids), and phospholipids, sterols and other soluble fat soluble impurities as discussed above. As a result, there is potential for each of these contaminants to affect the overall quality of the biodiesel product, and to affect the production costs in terms of at least time, money and feasibility.
Each of these factors should be taken into consideration when selecting a suitable feedstock.
In embodiments, any lipid source can be processed and used as a feedstock.
In embodiments, examples of suitable feedstock that can be incorporated include, but are not limited to: crude soy oil, refined soy oil, yellow grease, refined and crude vegetable oils, trap grease, animal tallow (including edible and inedible), raw trap grease, brown grease, corn oil, olive oil, cottonseed oil, butter, lard, poultry fat, fish oils, linseed oil, raw trap grease, restaurant waste grease, animal fat, coconut oil, recycled greases, soy, rapeseed, jatropha, mahua, mustard, flax, sunflower, palm oil, hemp, field pennycress, pongamia pinnata, algae, and other microbial sources such as bacterial sources of lipids, waste streams like acid oil from standard refining processes, and the like.
In embodiments, a suitable pretreatment step includes the addition of a bleaching clay to feedstock, with a subsequent drying process followed by a filtering process. As a suitable clay, silica or traditional bentonite clays can be used.
In this pretreatment step, the clay is added to feedstock at a temperature T in the range of 150° F.≦T≦250° F., such as 175° F.≦T≦200° F. or 190° F.≦T≦220° F. Typically, the dose level of the clay is in the range of 0.3 to 0.5 wt. % of total mixture, at a maximum of less than 1.5 wt. % of total mixture. The mixture is then filtered using a standard filtering process. Impurities are physically absorbed into the clay, and peroxides are broken down during this pretreatment step.
Optionally, filter aids may be added to enhance filtering properties. Furthermore, in cases where it is desired, traces of citric acid or phosphoric acid can be added before the clay is added, to aid in physically absorbing impurities into the clay. For example, in embodiments, 1500 ppm of phosphoric acid may be added to the feedstock prior to the clay addition step in order to hydrate phospholipids and non-hydratable gums, and split traces of soaps or salts that are present.
In embodiments, this pretreatment step can be performed under a vacuum, the treat time lasting approximately 3 to 5 minutes.
Many of the impurities that are removed during this pretreatment step are ones that would otherwise have interfered with glycerolysis. In cases where glycerolysis is conducted without first completing this preliminary pretreatment step, the glycerolysis would likely result in an undesirable outcome. For example, phospholipids and trace food particles present in the feedstock would form problematic emulsions that would require additional processing and/or time to remedy.
In alternate embodiments and where it is desired, other pretreatment steps that may be added include dewatering, degumming, and bleaching. Dewatering can include passing the feedstock through a decanter and centrifuge in order to reduce water content to acceptable levels.
After the pretreatment step, a glycerolysis step is then performed. In this glycerolysis step, crude glycerol from the transesterification reaction is added back into the feedstock, with some caustic added. In embodiments, a suitable amount of glycerol is between 1 wt. % to 10 wt. % of total reaction mixture.
“Caustic” is meant to include any strong base such as NaOH or KOH, or other strong bases, and the like.
The feedstock, glycerol, and caustic are mixed and heated to a temperature T2 within the range of 250° F.≦T2≦420°, such as 320° F., under a vacuum. The caustic acts as a catalyst to cause the fatty acids to react with the glycerol to form mono- and di-glycerides. The caustic also causes rearrangements by transesterification of the triglycerides into mono- and di-glycerides, increasing the quantity of these compound formed.
The level of caustic depends on the level of soap formed from the transesterification reaction in the glycerol. For instance, caustic levels as much as 0.01%, such as 0.05% or 0.15% by weight of total reaction mixture can be used where the glycerolysis step is performed at higher temperatures and higher vacuums. On the other hand, where no soap is present with the glycerin, then caustic levels as high as 0.5% by weight of total reaction mixture may be required. In embodiments, the caustic level is never desired to be sufficient to neutralize the amount of fatty acid present in the feedstock.
In embodiments, the intent of adding caustic in the glycerolysis process is to add a sufficient amount of caustic in order to drive the glycerolysis reaction to proceed, but not so much as to neutralize free fatty acids present in the feedstock. Therefore, in embodiments, the goal of adding caustic is not to exceed the level of free fatty acids present in the feedstock.
In cases where it is desired, catalysts other than caustic base can be used. For example, in embodiments, glycerolysis can be conducted in the presence of an acid catalyst instead of a caustic base catalyst. In embodiments where an acid catalyst is selected, neutralization would then be required in the transesterification reaction. In other embodiments, salts such as zinc chloride, zinc oxides, aluminum oxides, magnesium oxides, sodium bisulfates, and the like, can be used.
Advantages of the glycerolysis step according to the present disclosure include the following. To begin with, this step reduces the fatty acids present in the feedstock. The formation of mono- and di-glycerides reduces otherwise necessary mixing requirements in the subsequent transesterification reaction. Furthermore, this step allows for a simple and easy re-use of catalyst from the subsequent transesterification reaction because the soaps produced in the transesterification reaction will carry forward in the glycerol phase.
In embodiments, the same catalyst is used in both the glycerolysis and transesterification reaction and, therefore, a lesser amount of overall catalyst is required and production costs are reduced, compared to conventional methods. More specifically, the use of the caustic base catalyst in glycerolysis directly contributes to the reduction of catalyst use in the subsequent transesterification reaction.
In embodiments, with respect to the selection of catalysts to be used in the production of biodiesel, because of the possibility of reactions occurring that lead to formation of free fatty acids and mono- and diglycerides, the direct use of sodium or potassium alkylate (R—ONa or R—OK, where the alkylate moiety must correspond to the R-moiety of the alcohol) as catalysts are examples of suitable catalysts. Catalysts of this type are soluble in alcohol, easy to add to the reaction system, and do not introduce undesirable side reactions which may occur with water containing catalyst.
In embodiments, other examples of suitable catalysts include base catalysts such as sodium methoxide or potassium methoxide, and can also include sodium hydroxide or potassium hydroxide.
As discussed above with respect to glycerolysis, in cases where it is desired, an acid catalyst can be used in the transesterification step as well. Acid catalysts are most commonly used for the esterification of free fatty acids, but they are generally considered too slow for industrial processing. Suitable acid catalysts include, but are not limited to sulfuric acid, hydrochloric acid, and phosphoric acid.
In embodiments, the amount of catalyst added in the transesterification reaction is within the range of 0.1 wt. % to 0.35 wt. % of total reaction mixture.
With respect to the selection of an alcohol to be used, in embodiments, examples of suitable alcohols include methanol, ethanol, isopropanol, butanol, or any short chain alcohol, in accordance with ASTM 6751-02 specifications, or the like.
A key factor to consider with respect to alcohol selection is water content. Water interferes with transesterification reactions and, thus, can result in poor product yields and/or increased levels of free fatty acids, soap, and triglycerides present in the biodiesel. In addition, as long as the final biodiesel product meets the standards set forth in ASTM D6751, it is not of absolute significance which alcohol is used in the process.
In embodiments, a high free fatty acid feedstock and a small amount of catalyst are fed into a first reactor to perform a fatty acid reduction reaction. In this reactor, the fatty acid feedstock is combined with glycerol to produce glycerides and water. The water vapor created by the reaction is removed from the first reactor by a vacuum.
In embodiments, the first reactor for glycerolysis fatty acid reduction typically operates at a temperature of above 250° F., such as at a temperature T within the range of 250° F.≦T≦420°, such as 275° F.≦T≦400°, or 300° F.≦T≦350°, and at a pressure between 5 mm Hg to 15 mm Hg total pressure. In particular, the reactor temperature must be set high enough to flash the water off, but not too high so as to flash off the glycerol. Thus, the temperature and pressure conditions of the glycerolysis reactor must be sufficient to remove the water, without removing the added glycerol.
The following equations generally describe the free fatty acid reduction reaction:
Fatty Acid+Glycerin→Monoglyceride+H2O
Fatty Acids+Monoglyceride→Diglycerides+H2O
Fatty Acids+Diglyceride→Triglycerides+H2O
A side reaction that occurs during the fatty acid reduction shown immediately above in the glycerolysis reaction is described by the following reaction in the presence of a catalyst:
Triglycerides+Glycerin→A mixture of mono-, di-, and triglycerides
In cases where it is desired, an optional cooling step can be incorporated after the first reaction step is completed. However, in suitable embodiments, the cooling step can be eliminated because transesterification with added methanol, or a suitable alcohol, can proceed quicker and with less added catalyst at a high temperature and pressure.
Once glycerolysis is complete and free fatty acids are reduced to produce mono- and di-glycerides, the mixture of mono-, di- and tri-glycerides along with the soap (catalyst) is mixed with a slight excess of methanol and additional catalyst to complete the transesterification reaction. Since the same catalyst is used in both the glycerolysis and transesterification reaction, no water is added to kill or wash out the catalyst after the glycerolysis reaction.
With respect to the amount of excess methanol required in the transesterification reaction, classic chemistry requires 2.1 moles of methanol, or suitable alcohol, per mole of fatty acid. Because triglycerides have 3 moles of fatty acid per mole of glyceride, 6.3 moles of methanol are required for each mole of triglyceride. In embodiments according to the present disclosure, 1.1 to 1.5 moles of excess alcohol are required for each mole of fatty acid.
In embodiments, the mono-, di- and tri-glyceride mixture obtained after glycerolysis combined with methanol form a single phase. This reduces the mixing rate required in transesterification. In production, very low levels of mixing is required for the reaction to proceed, such as one horsepower or less.
Unlike prior methods conventionally known in the art, the transesterification reaction according to the present disclosure proceeds instantaneously and with a mixing rate of less than one horsepower.
In embodiments, the transesterification reaction proceeds at a temperature T3 within the range of 130° F.≦T3≦200° F., such as 160° F.≦T3≦190° F.
The following equation generally describes the transesterification reaction according to embodiments for triglyceride:
Mono- and di-glycerides may also react accordingly in the above reaction in place of triglyceride. In the above reaction, R1, R2, and R3 are chains of carbon and hydrogen atoms. It is noted that in the above reaction, any suitable alcohol other than methanol can be employed as desired.
In embodiments, the transesterification reaction goes to 70-90% completion, or a minimum completion rate of 30%.
In embodiments, because the reacted product and bottoms are recycled, the reaction does not have to go to 100% completion. This reduces excess methanol requirements and further reduces the energy needed to recover methanol in subsequent recovery process steps.
In embodiments, this transesterification reaction can proceed in a second reactor, or in the same first reactor where the glycerolysis fatty acid reduction reaction took place.
For example, in embodiments, separate continuous stirred tank reactors can be used for each reaction. In alternate embodiments, both reactions can proceed in a single batch reactor.
In the case of a single reactor system, an external bleaching step is first performed. Once complete, glycerolysis is run in the single reactor. Next, with the batch still inside the reactor, methanol and catalyst are added to the reactor to run the transesterification reaction. Glycerol is drained off. Then, with respect to the methyl ester reaction product, the methanol is driven off by applying straight heat and a vacuum. Finally, the reactor is heated to a suitable temperature to distill the methyl esters to a separate tank.
In other embodiments, multiple batch reactors can be employed.
Once the glycerolysis and transesterification steps are completed, the reaction products must be processed and bottoms subsequently recycled.
In embodiments, the first processing step after the transesterification reaction is a bulk separation of glycerol from methyl or alkyl esters, which does not require a distillation step.
In embodiments according to the present disclosure, the glycerol separation processing step is capable of completing in a shorter amount of time than compared to conventional methods because of the lower levels of methanol, or other suitable alcohol, present in the glycerol. These lower levels of methanol result in an increased difference in specific gravity between the glycerol and methyl or alkyl ester product. In embodiments, the bulk separation of glycerol can proceed by gravity or centrifuge.
In embodiments, the separation of glycerol processing step removes most of the glycerol present in the transesterification reaction product, leaving behind approximately 0.1 wt. % to 1.0 wt. % free glycerol in the methyl or alkyl esters, based on total reacted mixture.
After the bulk separation of glycerol, a methanol recovery flash step is performed. The key to the methanol flash is to not have water present. This is accomplished by not killing the methoxide or other catalyst with water.
In embodiments according to the present disclosure, the methanol recovery step is carried out as a flash at a relatively high temperature and low pressure. In embodiments, the temperature T4 falls within a range of 200° F.≦T4≦300° F. at a pressure of 2 to 25 mm Hg absolute pressure. Because the catalyst is not killed with water or acid addition after the transesterification reaction, some reversion of the methyl or alkyl ester occurs during the methanol recovery flash process (the same as with the glycerolysis reaction). By ‘reversion’ it is meant that some of the methyl esters react with the free glycerol thereby producing monoglycerides and releasing methanol in the process.
Advantages of not killing the catalyst by adding, for example, water or acid, include but are not limited to: (1) catalyst recycle with the bottoms from the methyl ester flash step; (2) the lack of water in the excess methanol makes methanol recovery simpler and less expensive; and (3) the lack of water makes it easier to recover glycerol with greater purity, thereby having an improved market value.
In embodiments, if the methanol recovery flash step is conducted at a high enough temperature, it also succeeds in flashing off all or substantially all of the free glycerol from the methyl ester reaction product. This eliminates the need for a subsequent water washing step, and eliminates waste water discharge.
In embodiments where it is desired, traces of free glycerol can be removed by water washing the methyl ester or biodiesel reaction product. In alternate embodiments, ion exchange columns with various resins or bleaching clays can be employed to remove traces of free glycerol from the methyl ester or biodiesel reaction product.
In embodiments, the resulting stream of glycerol free methyl esters from the methanol recovery flash step is then distilled in a methyl ester flash process step, resulting in a pure methyl ester biodiesel product.
This methyl ester flash process step is conducted at a temperature T5 ranging from 170° C.≦T5≦250° C., under a total pressure of 0.5 mm Hg to 15 mm Hg (Note: a higher vacuum requires a lower temperature).
The pure methyl ester biodiesel final product of the embodiments contains no monoglycerides or other bound glycerol, soap or metals, free fatty acids, sterols, or other various contamination that would prevent it from meeting ASTM specifications.
In embodiments, depending on how aggressive the methyl ester flash process step is carried out, this will determine the level of bound glycerides in the final product. In suitable embodiments, the final biodiesel product contains no bound glycerides and zero free fatty acids and is water white.
In embodiments where it is desired, more of the glycerin-free methyl esters from the methanol recovery step can be flashed consecutively until the final product reaches the ASTM 6751-02 specification for bound glycerin or acid value.
In alternate embodiments, less aggressive methyl ester flashing conditions can be employed and non-distilled material can be blended into highly purified biodiesel that results in a lesser quality product, that still satisfy ASTM specification requirements.
Biodiesel product obtained by the process according to the present disclosure has a higher market value than biodiesel products produced using conventional methods, for at least the reason that the product obtained in embodiments according to the present disclosure blends better with existing diesel fuels and, thus, is less problematic.
In short, the total process of a pretreatment, glycerolysis, transesterification reaction, glycerol separation step, methanol recovery flash, and methyl ester flash provides an overall process for producing biodiesel where any fatty acid containing material can be converted into high quality biodiesel fuel that meets ASTM 6751-02, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels.
Advantages of embodiments according to the present disclosure include a reduction of catalyst from the glycerol recycle stream. This reduces the overall cost of the process and reaction losses.
Further advantages according to the present disclosure are that the wastewater discharge streams of processes conventionally known in the art is eliminated by the process of the present disclosure. This also reduces material loss and processing costs associated with wastewater streams. In embodiments, there is no waste stream from the process except for the used bleaching clay in the pretreatment process.
Further advantages of embodiments according to the present disclosure include a faster transesterification reaction that does not require use of aggressive or intense mixing to incorporate the different phases, as conventionally used in the art.
Further advantages of embodiments include a biodiesel product that is pure biodiesel with almost no contamination or zero contamination present.
Additional advantages of embodiments of the present disclosure include the fact that fatty acids are easier to recover from glycerol as it contains no water and therefore feedstock loss is reduced. Glycerol processing is also easier to accomplish, resulting in an increased value byproduct stream.
Further advantages of embodiments include that because the bottoms in the reaction system are recycled, the transesterification reaction does not need to proceed to completion. This feature reduces excess methanol requirements conventionally required in biodiesel processes known in the art in addition to manufacturing and operational costs. Because catalyst is not killed with water or acid addition after the transesterification reaction and water washing is done only as an optional step in embodiments to remove traces of free glycerol, distillation of methanol is not required, unlike conventional processes known in the art.
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. For instance, alternate embodiments may comprise, for example, distillation columns as desired. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.
The embodiments will be illustrated further in the following Examples
The bleaching process begins with 1000 milliliters of feedstock (e.g. waste vegetable oil). The feedstock is placed into a 2 liter Erlenmeyer vacuum flask to which 0.5 wt. % (5 grams) of bleaching clay is added as well as 2 milliliters of 50% citric acid. This mixture is heated under vacuum to a target temperature of 220° F. and maintained until all signs of boiling and foaming have subsided.
The product of bleaching is then filtered under vacuum through a Buchner funnel and filter paper primed with diatomaceous earth to remove the bleaching clay as well as any other sediment.
The product of filtering is then placed into a clean 2 liter Erlenmeyer vacuum flask to which 60 milliliters of glycerine is added in addition to 15 grams of 50% caustic solution. The mixture is heated and agitated under strong vacuum to a temperature of 400° F. and maintained until all signs of water production have ceased and the product has stopped foaming and boiling. The product is then allowed to cool to approximately 170° F. before proceeding to the next step.
The cooled resultant of glycerolysis (170° F.) is placed into a 2 liter flat bottom flask with a Lieberg condenser attached. To this mixture is added approximately 150 milliliters of Methanol and 15 milliliters of sodium methoxide (30%). If the mixture has further cooled it must be heated to approximately 150° F. and allowed to reflux for several minutes while being agitated strongly.
The warm mixture is placed into a 2 liter seperatory funnel and allowed to sit undisturbed so that the glycerine will separate from the methyl esters and fall to the bottom of the funnel. When separation is complete, signified by two distinct phases, usually within 10 minutes, the glycerine is carefully drained off and retained for future refining. The methyl esters are then collected for methanol removal.
The methyl esters are then placed into a 2 liter flat bottom round flask with a distillation set up. (i.e. distillation adapter, condenser, vacuum adapter, receiver flask). The methyl esters are heated to approximately 220° F. without vacuum and the methanol is collected in the receiver flask. When the majority of methanol has been removed as indicated by a slowing of the methanol drip, the receiver flask is emptied and the apparatus put under vacuum for a very short time until any remaining methanol has been collected.
At this point the methyl esters may be directly distilled (skip to step 10) or washed and dried (steps 7-9) and then distilled (step 10).
The de-mentholated methyl esters are placed into a 2 liter seperatory funnel to which 150 milliliters of city water is added. The mixture is agitated gently so as to create intimate mixing without the formation of emulsion. The water is then allowed to settle out and is drained off.
The water washed methyl esters remains in the seperatory funnel to which 150 milliliters of pH 2 water is added and vigorously agitated. The mixture is allowed to settle out and the water drained.
The methyl esters are then returned to a 2 liter Erlenmeyer vacuum flask, placed under vacuum, and heated to 220° until the product clarifies and all sign of boiling and foaming have stopped (approximately 1 hour).
The washed and dried methyl esters are placed into a 5 liter vacuum distillation apparatus. The methyl esters are placed under strong vacuum (500-600 militorr) and slowly heated by both a top and bottom heating mantle to approximately 150° C. at which time free glycerine will begin to vaporize and pass through the receiver flask. When the methyl esters reach approximately 160° C. they will begin to vaporize and collect in the receiver flask. After approximately 250 milliliters have distilled the receiver flask is emptied and the distillation is allowed to continue as the temperature approaches 180° C. The collected methyl esters are now ready for analysis and treatment with antioxidant.
| Number | Date | Country | |
|---|---|---|---|
| 61193964 | Jan 2009 | US |
