WO2007065679A2 - Method for removing impurities from fatty acid methyl esters based on natural fats and oils - Google Patents

Abstract

The invention relates to a method for removing impurities from fatty acid methyl esters based on natural fats and oils, especially for separating glycerine and traces of alkali metal compounds. As a result of the disadvantages of prior art, the aim of the invention is to produce a method by which means impurities such as glycerine and traces of alkali metal compounds are approximately completely removed from fatty acid methyl esters based on natural fats and oils, with relatively simple means. To this end: a) the FAME raw product is treated with a neutralisation means that is insoluble in the raw product in order to remove the transesterification catalyst, b) methanol is then removed by distillation, and c) the methanol-free FAME raw product is brought into contact at least once with phosphorylated and carbamated starch phosphate particles, the glycerine contained in the FAME raw product being absorbed by the starch phosphate particles. The cited starch phosphate particles can absorb up to ten times the quantity of their own weight in glycerine from biodiesel and therefore achieve a maximum residual content of glycerine of 0.02 %.

Description

 Process for removing impurities from fatty acid methyl esters based on natural fats and oils

The invention relates to a method for removing impurities from fatty acid methyl esters based on natural fats and oils, in particular for the separation of glycerol and traces of alkali metal compounds.

 Fatty acid methyl ester (FAME = Fatty Acid Methyl Ester) based on natural fats and oils is also referred to as biodiesel, which has become increasingly important as a diesel fuel.

 In the alkaline-catalyzed transesterification of natural oils and fats to fatty acid methyl esters, a crude product is obtained which contains excess methanol, a small part of the glycerol released and small amounts of catalyst, such as NaOH or Na methylate or KOH or K methylate.

 This composition inevitably arises during the transesterification process, since it is necessary to work with excess methanol in order to achieve the most complete transesterification, because the glycerol released separates only incompletely and is therefore not completely removed from the equilibrium reaction.

 Glycerin must be removed by a final cleaning and the remaining of its fatty acid esters (mono-, di- and triglycerides) must be avoided by as complete a conversion as possible, because otherwise deposits of carbonization products can occur during the combustion process in the diesel engine.

 In addition, the existing alkali metal compounds (catalyst residues) and methanol must be freed from the crude product. Alkali metal compounds lead to ash deposits during the combustion process of the diesel fuel and methanol lowers the flash point of the biodiesel.

 It is known from practice to remove methanol, glycerol and catalyst residues from the crude product by water washing, a certain proportion of the methanol used either being lost or having to be recovered by distillation. Furthermore, problems often arise in the phase separation from the wash water due to emulsifying soaps. Soaps are created by free fatty acids or traces of water in the raw material under the influence of alkaline catalysts. It is difficult to prevent their education.

Furthermore, for reasons of corrosion protection, the biodiesel must be dewatered after washing, for example by treatment under vacuum and at temperatures above 100 ^° C.

To reduce the glycerol content in biodiesel, it is known (DE 197 29 203 A1) to bring biodiesel into contact with acidic ion exchangers.

The examples in this document show that 3 to 4 liters of biodiesel with an initial glycerol content of 300 mg / kg can be cleaned with 10 ml of ion exchanger (residual glycerol content 100 mg / kg). This is followed by regeneration with 200 ml of methanol and washing out the methanol residues (called "conditioning") with 20 ml of biodiesel (= 2 bed volumes mina) required to enable the ion exchanger to be reused. The resulting mixtures (a) methanol with biodiesel and glycerol and (b) biodiesel with methanol lead to increased expenditure due to the required separation by distillation into the mixture components for further use. From the example, it can be concluded that relatively large amounts of ion exchanger are required, since the glycerol uptake / amount of ion exchanger is around 10%.

 The invention has for its object to provide a method with which it is possible to almost completely remove impurities such as glycerol and traces of alkali metal compounds from fatty acid methyl esters based on natural fats and oils with relatively little effort.

 According to the invention the object is achieved by the features specified in claim 1. Advantageous refinements and developments of the method are the subject of claims 2 to 7. Claims 8 to 12 relate to advantageous uses of the phosphorylated and carbamidated starch phosphates.

 The following procedure is proposed for removing impurities from fatty acid methyl esters (FAME) based on natural fats and oils:

In a first process step a), the crude product obtained from the transesterification process is treated with a neutralizing agent which is insoluble in the crude product before being brought into contact with the absorbent. This is necessary because, before the methanol is distilled off, the transesterification catalyst residues NaOH and Na methylate or KOH and K methylate must be rendered ineffective. Otherwise, as is well known, there is a risk that the glycerol residues will slightly reverse the transesterification when the methanol disappears when distilling off.

 For the elimination or removal of the hydroxides and methylates, all acids and salts which are insoluble in the raw FAME product and have sufficient neutralizing capacity are suitable, but must not release fatty acids from any soaps formed. Sodium bicarbonate, which is safe to handle, is preferably used. The resulting soda is so sparingly soluble in the ester that it is practically freed from alkali.

 At the same time, the soda is so much weaker basic that it cannot catalyze the transesterification. Acid anhydrides are also suitable, but they only bind the hydroxides. Carbon dioxide is particularly well suited, since an excess of carbon dioxide remaining in the biodiesel when it comes into contact with water can only give a very weak acid and is not corrosive. Methylate only reacts with carbon dioxide when the exact amount of water required to hydrolyze the methylate is added. The resulting salts can be separated very well and completely by settling alone.

It is advisable to check for existing catalyst residues by stirring a small sample with water and then checking its pH, which may not exceed 8.5. In a second process stage b), the crude product treated with neutralizing agent is freed from methanol by distillation, preferably by means of an evaporator, such as a thin-film evaporator. The resulting methanol is anhydrous and can be immediately returned to the Transesterification process can be traced back, which is economically advantageous. To avoid high temperatures, distillation at reduced pressure and correspondingly low condenser temperatures is recommended.

 After the methanol has been distilled off, most of the glycerol previously contained separates out, since the methanol is probably missing as a "solubilizer". A small amount of glycerin is in finely divided form and causes turbidity.

 Another part of glycerin in the raw product is really dissolved and is in the concentration range of about 500 to 1000 mg / kg. In accordance with the final process step c), the crude product containing glycerol is brought into contact with a phosphorylated and carbamidated starch phosphate, both genuinely dissolved and dispersed glycerol and traces of alkali metals being absorbed by the latter. The anhydroglucose units of the starch phosphate are substituted by phosphate ester groups and carbamide groups, preferably with an average degree of substitution DS of the carbamide groups and the phosphate ester groups of 0.1 to 1. This known absorbent (EP 1 157 044 B1) is surprisingly very suitable, larger amounts of glycerol to record. So far, it was only known that this, as a so-called "superabsorber", has a particularly high swelling capacity for water. Other known superabsorbers, such as those based on acrylate or specially thermally treated starch products which swell very well in water, do not absorb glycerol. The property of binding traces of alkali metals represents an advantageous side effect when cleaning raw biodiesel product.

 The starch phosphates are preferably used as a granular product. They have a density that is significantly higher than that of glycerin. Due to the difference in density between glycerin and biodiesel, separation takes place by sedimentation even when a high level of saturation with glycerin has already been reached and the density of the swollen particles approaches that of glycerin. The starch phosphates used according to the invention can be used as unpurified raw products which still contain unreacted, excess urea and ammonium phosphates / polyphosphates as impurities. This does not increase the phosphorus and nitrogen content of the biodiesel, as control studies have shown.

 The absorption is preferably carried out in the filter bed or by means of batch sorption. The required quantities of starch phosphate are 0.1 g to 1.0 g per liter of FAME raw product. To clean this crude product, all of the starch phosphates specified in the aforementioned publication (EP 1 157 044 B1) can be used. Starch phosphates with a high water retention capacity are particularly suitable.

The glycerol dispersed in the biodiesel can, of course, also be separated off by simple settling or by means of a centrifuge, as a result of which the consumption of absorbent can be reduced. This phase separation poses hardly any problems in comparison with the water washing known per se, since on the one hand the difference in density is more than twice as large and on the other hand the interfacial activity of possible impurities, in particular of Soaps (alkali salts of fatty acids) do not play a role here. In contrast to soap solutions in water, soap solutions in glycerin do not emulsify oils and fats.

 As an additional side effect, a reduction in the alkali content in biodiesel is achieved during the separation process, since the starch phosphates are present as ammonium salts and have an ion-exchanging effect. The sodium or potassium salts of the fatty acids ("soaps"), which are dissolved to a very small extent in the biodiesel, are then converted into the corresponding ammonium compounds, which do not lead to ash residues when the biodiesel is burned in the engine.

 As usual with sorptive processes, the residual content of the substance to be separated depends on the loading of the sorbent. With increasing saturation of the starch phosphate particles with glycerin, the sorption equilibrium naturally approaches the solubility limit of the glycerol in biodiesel. This behavior depends on the synthesis conditions of the carbamidated and phosphorylated starch, in particular the starch used, but also the degree of phosphorylation and carbamidation and the absorption capacity for glycerol achieved in this way.

 By varying the amounts of the phosphorylating agents and the urea and / or the reaction times, starch phosphates with different degrees of substitution, both of the carbamide groups and of the phosphate ester groups, preferably in the range from 0.1 to 1, can be prepared. As starting strengths e.g. Pea, wheat, corn or potato starches as well as so-called starch-soluble as well as cold water-soluble starches can be used.

 Phosphoric acid of any technical quality can be used as the phosphorylating agent, the commercially available 85% strength being particularly suitable. Instead of phosphoric acid, ammonium phosphates, potassium phosphates or sodium phosphates and mixtures of these phosphates can also be used. The reaction takes place under reduced pressure of less than 13.3 kPa. The carbamidation and phosphorylation reaction is preferably carried out under vacuum at temperatures from 90 to 140 ° C.

At reaction temperatures above 120 ⁰ C starch phosphates are obtained which give particles with a high water retention. These have a high absorption capacity for glycerin. The starch phosphates according to the invention can thus also be used as glycerol stores.

 Experiments using starch phosphates according to the invention with a water retention capacity of approx. 2500% and larger showed that the sorbent particles are able to absorb approx. 10 times their own weight of glycerol from biodiesel and thereby a maximum residual content of 0.02% to reach. The swelling capacity in pure glycerin is approximately the same as that in water, i.e. 1 g of such starch phosphates takes up about 25 g of glycerol. This high swelling capacity in glycerin can be used to remove dispersed glycerin from the biodiesel without time-consuming phase separation or centrifuges.

In practice, it is expedient to carry out the separation process in stirred cascades or columns, since this procedure entails a high loading of the starch phosphate particles Glycerin is achieved. When using column beds, the crude product to be cleaned should be passed through the columns in the so-called "upflow process" in order to avoid densification by swelling due to glycerol absorption and to achieve the greatest possible loading of the starch phosphate particles with glycerol.

 It should be noted here that the flow rate is not too high, as swirling the particle bed has a negative effect on the cleaning effect.

 Since the starch phosphate particles are able to absorb several times their own weight of glycerin, a very good cleaning effect is achieved.

 Another advantage is the easy disposal of the starch phosphate particles loaded with glycerin. Since these are made from biodegradable material, there are several possible uses. The sorbent particles can be disposed of by composting or, if necessary, also used for biogas production. In both cases, phosphorus and nitrogen are formed in non-toxic, plant-available form during the degradation process.

example 1

 Neutralization (process stage a))

 1 1 crude product of a technical rapeseed oil methyl ester plant (after separation of the lower, glycerol-rich phase from the alkaline transesterification mixture, without water washing) was stirred for 3 hours with 2 g of sodium bicarbonate at room temperature and decanted from the sediment after sitting for 15 minutes. The cloudy product was completely clear after a few minutes of exposure (in contrast, a comparison sample with untreated crude product was still cloudy even after standing for several days and gave a pH of 11.5 in the test described below).

 A 1 ml sample of the crude product treated with bicarbonate was shaken with 1 ml of distilled water in a test tube and then a pH of 8.0 was determined in the aqueous phase.

 Distilling off the methanol (process stage b))

The crude product treated with bicarbonate was heated to 100 ^° C. in a laboratory rotary evaporator. When no more methanol distilled off, condenser and receiver was cooled to -5 ⁰ C and gradually, a vacuum of 4 kPa is applied and held for temperature and pressure for 15 minutes. 55 ml of pure, anhydrous methanol were obtained as the distillate. A few milliliters of a viscous, brown liquid had settled on the bottom of the distillation residue and was separated off. After this liquid had been separated off, 940 ml of biodiesel were obtained, which were slightly cloudy. According to the determination methods DIN EN 14105 (for glycerin) and E DIN EN 14538 (for the alkali content), the following values were determined for the contaminated biodiesel (FAME raw product):

Glycerin content: 0.11%

Alkali content (Na + K): 4.9 mg / kg. Treatment of biodiesel (process stage c))

 A phosphorylated and carbamidated starch phosphate based on potato starch with the following properties was used to treat biodiesel:

 Phosphorus content: 7%

Nitrogen content. 2.5%

DS (carbamide groups): 0.41

 DS (phosphate ester groups): 0.52

Water retention capacity (WRHV): 2530%

 The raw crude starch phosphate product was broken up into small particles (approx. 2 to 5 mm large granules or grains).

 1 g of this granulate was placed in a beaker into which the amount of 940 ml of contaminated biodiesel (FAME crude product) obtained after process step b) was placed. The mixture was stirred at room temperature. After just a few minutes of stirring, the appearance of the biodiesel changed from cloudy to clear. The starch phosphate particles settled very quickly after the stirring had ended (stirring time 30 min). No changes in the starch phosphate particles were visible externally. The supernatant, purified biodiesel was decanted. The density, glycerol content and alkali content were then determined.

 Density: 0.88 kg / l

 Glycerol content: 0.01% (according to DIN EN 14105)

Alkali content (Na + K): 1.0 mg / kg (according to E DIN EN 14538)

Example 2

 In an analogous manner as in Example 1 (process steps a) and b)), crude product treated with bicarbonate was prepared and methanol was distilled off.

 After the methanol had been distilled off, a completely clear liquid was formed by resting the distillation residue for three days. 940 ml of biodiesel were obtained. According to the determination methods DIN EN 14105 (for glycerin) and E DIN EN 14538 (for the alkali content), the following values were determined for the contaminated biodiesel (raw FAME product):

Glycerin content: 0.05%

Alkali content (Na + K): 1.7 mg / kg.

Treatment of biodiesel (process stage c))

 A phosphorylated and carbamidated starch phosphate based on wheat flour with the following properties was used to treat biodiesel:

Phosphorus content: 9.8%

Nitrogen content. 0.84%

DS (carbamide groups): 0.15

DS (phosphate ester groups): 0.79

Water retention capacity (WRHV): 2300% The raw crude starch phosphate product was broken up into small particles (approx. 2 to 5 mm granules). 1 g of this granulate was placed in a beaker into which the amount of 940 ml of contaminated biodiesel (FAME crude product) obtained after process step b) was placed. The mixture was stirred at room temperature. The starch phosphate particles settled very quickly after the stirring had ended (stirring time 30 min). No changes in the starch phosphate particles were visible externally. The supernatant, purified biodiesel was decanted. The density, glycerol content and alkali content were then determined.

Density: 0.88 kg / l

 Glycerol content: 0.01% (according to DIN EN 14105)

Alkali content (Na + K): 1.0 mg / kg (according to E DIN EN 14538).

Example 3

 In a manner analogous to that in Example 2 (process steps a) and b)), a larger amount of crude product was treated with bicarbonate, methanol was distilled off and clarified by letting it sit for several days. 30 l of contaminated, clear biodiesel were obtained.

According to the determination methods DIN EN 14105 (for glycerin) and E DIN EN 14538 (for the alkali content), the following values were determined for the contaminated biodiesel (FAME raw product):

Glycerin content: 0.05%

Alkali content (Na + K): 1.7 mg / kg.

Treatment of biodiesel (process stage c))

 A phosphorylated and carbamide-based starch phosphate based on wheat flour with the following properties was used to treat biodiesel:

Phosphorus content: 7.4%

Nitrogen content. 3.9%

DS (carbamide groups): 0.71

DS (phosphate ester groups): 0.61

Water retention capacity (WRHV): 3450%

 The raw crude starch phosphate product was broken up into small particles (approx. 2 to 5 mm granules).

 30 tests were carried out to determine the possible loading until the residual glycerol content of 0.02% permitted by DIN EN 14214 was reached as an equilibrium value of the sorption, with 1 g of granules being repeated successively (30 times) with 1 l of contaminated biodiesel for 3 hours was stirred. After the granules had been left to settle, the liquid was decanted as completely as possible and the glycerol and alkali content were determined by the abovementioned methods.

The following results were achieved:

Experiments 1 to 28 resulted in biodiesel with a glycerol content of at most 0.01% and an alkali content reduced to 1 mg / kg. Sorption tests 29 and 30 gave one Glycerol content increased to 0.02%, with unchanged alkali content of 1 mg / kg.

 The loading of the granulate was calculated from the decrease in the glycerol content of the first 28 samples, from which the maximum glycerol content of

0.02% was reached:

 28 [I] x 0.88 [kg / l] x (0.05-0.01) [g / 100g] x 10 [100g / kg] = 9.856 g

 It has thus been demonstrated that 1 g of the starch phosphate used is capable of approximately 10 g

Take up glycerin in compliance with the prescribed residual content.

Claims

Claims 1. A process for removing impurities from fatty acid methyl esters (FAME) based on natural fats and oils, which are obtained by transesterifying the starting oil with methanol in the presence of a catalyst, and the FAME crude product is brought into contact with an absorbent, characterized by the following process steps in the order given:  a) the FAME crude product is treated to remove the transesterification catalyst with a neutralizing agent which is insoluble in the crude product,  b) then methanol is distilled off and  c) the methanol-free FAME raw product is then brought into one or more contact with phosphorylated and carbamide-based starch phosphate particles, glycerol contained in the FAME raw product being absorbed by the starch phosphate particles. 2. The method according to claim 1, characterized in that an acid, an acid anhydride, a salt or carbon dioxide is used as neutralizing agent. 3. The method according to any one of claims 1 or 2, characterized in that sodium bicarbonate is used as salt. 4. The method according to any one of claims 1 to 3, characterized in that when carbon dioxide is used, water is added in the amount required for the hydrolysis of the methylate. 5. The method according to any one of claims 1 to 4, characterized in that the amount of starch phosphate used is 0.1 to 1 g per liter of methanol-free FAME crude product. 6. The method according to any one of claims 1 to 5, characterized in that the contacting of the methanol-free FAME raw product with the starch phosphate takes place in a stirred cascade or column arrangement. 7. The method according to any one of claims 1 to 6, characterized in that products are used as starch phosphates whose anhydroglucose units have an average degree of substitution DS of the carbamide groups and the phosphate ester groups of 0.1 to 1 each. 8. Use of phosphorylated and carbamidated starch phosphates, the anhydroglucose units of which are substituted by phosphate ester groups and carbamide groups, as a granular absorbent for the removal of glycerol from fatty acid methyl esters (FAME) based on natural fats and oils. 9. Use of the starch phosphates according to claim 8 in amounts of 0.1 to 1 g per liter of FAME raw product. 10. Use of starch phosphates according to one of claims 8 or 9, whose anhydroglucose units have an average degree of substitution DS of the carbamide groups and the phosphate ester groups of 0.1 to 1 each. 11. Use of the starch phosphates according to one of claims 8 to 10 for the purification of biodiesel. 12. Use of phosphorylated and carbamidated starch phosphates, the anhydroglucose units of which are substituted by phosphate ester groups and carbamide groups, as superabsorbents for glycerol.