Method for Reducing the Content of Saturated Monoglycerides in a Raw Biodiesel

Abstract

The invention relates to a method for reducing the content of monoglycerides (MG), in particular saturated monoglycerides (GMG), in a raw biodiesel (RB), which has a content of monoglycerides (MG) of between 0.4 and 0.7 wt % and a content of free fatty acids (FFA) of less than or equal to 0.25 wt %, comprising the following steps: A) providing the raw biodiesel (RB) having the constituents indicated above in the concentrations indicated above; B) adding (1) an alkaline aqueous solution (L) to the raw biodiesel (RB); C) mixing the alkaline aqueous solution (L), and thus glycerides in the biodiesel, preferably monoglycerides (MG), in particular saturated monoglycerides (GMG), being hydrolyzed; D) performing a first centrifugal separation (2) of a heavy phase (6) comprising the alkaline aqueous solution (L) with the hydrolyzed constituents from a light phase (7) comprising the biodiesel; E) drying (5) the light phase (7) or (9) in order to provide a processed biodiesel for use as a fuel having a content of monoglycerides of less than 0.4 wt %. The invention further relates to a use according to the invention.

Claims

1.-13. (canceled)

14. A method for reducing the content of monoglycerides (MG) in a crude biodiesel (CB) which has a content of monoglycerides (MG) of between 0.4% and 0.7% by weight and a content of free fatty acids (FFA) of less than or equal to 0.25% by weight, characterized by the following steps: A provision of the crude biodiesel (CB) with the abovementioned ingredients in the abovementioned concentrations; B addition (1) of an alkaline aqueous solution (S) in the form of sodium hydroxide solution to the crude biodiesel (CB), wherein the NaOH concentration in the sodium hydroxide solution is less than 4.03 mol/l and wherein the amount of sodium hydroxide solution metered in is between 1.0% and 3.0% by weight, based on the amount of crude biodiesel (CB) and wherein the temperature of the crude biodiesel (CB) during the addition of the aqueous alkaline solution (S) is at least 30 C.; C mixing of the alkaline aqueous solution (S) to hydrolyze glycerides in the biodiesel, preferably monoglycerides (MG); wherein the holding time after addition of the aqueous alkaline solution (S) and before the first centrifugal separation (2) is at least 15 minutes; D first centrifugal separation (2) of a heavy phase (6), comprising the alkaline aqueous solution (S) with the hydrolyzed ingredients, from a light phase (7) comprising the biodiesel by a separator at a speed of 4400 to 7200 rpm; E drying (5) of the light phase (7) and/or (9) to provide a processed biodiesel (PB) for use as fuel with a content of monoglycerides of less than 0.4% by weight, wherein the content of methanol in the biodiesel in steps A-E is less than 0.7% by weight, based on the total weight of crude biodiesel (CB) used or processed biodiesel (PB) in the respective step.

15. The method as claimed in claim 14, characterized in that, after the centrifugal separation (2) as per step D and before the drying (5) as per step E, washing (3) of the light phase (7) with water (W) and/or with a dilute acid (A) is effected, as is a second centrifugal separation (4) of a heavy phase (8) comprising the water (W) or the dilute acid (A), and any precipitated salts of cations of the alkaline aqueous solution (S) added, from a light phase (9) comprising the biodiesel.

16. The method as claimed in claim 14, characterized in that the crude biodiesel (CB) has a content of monoacylglycerides of less than 0.7% by weight.

17. The method as claimed in claim 14, characterized in that the NaOH concentration in the sodium hydroxide solution is between 0.51 mol/l and 2.77 mol/1.

18. The method as claimed in claim 14, characterized in that the temperature of the crude biodiesel (CB) during the addition of the aqueous alkaline solution (S) is at least 50 C.

19. The method of claim 14, characterized in that the holding time after the addition of the aqueous alkaline solution (S) and before the first centrifugal separation (2) is 30 to 180 minutes.

20. The method as claimed in claim 15, characterized in that the pH of the dilute acid during the washing (3) is between pH=3 and pH=5.

21. The method as claimed in claim 15, characterized in that the dilute acid is a phosphoric and/or citric acid.

22. The method as claimed in claim 14, characterized in that the content of methanol in the biodiesel in steps A-E is less than 0.2% by weight based on the total weight of crude biodiesel (CB) used or processed biodiesel (PB) in the respective step.

23. The method as claimed in claim 14, characterized in that steps A-E are effected without addition of alcohol to the crude biodiesel or to the intermediate product from the crude biodiesel during the method steps used in the method.

24. The method as claimed in claim 14, characterized in that steps A-E are effected without addition of hydrogen to the crude biodiesel or to the intermediate product from the crude biodiesel during the method steps used in the method.

25. The method as claimed in claim 14, characterized in that the monoglycerides (MG) are saturated monoglycerides (SMG).

26. The method as claimed in claim 16, characterized in that the crude biodiesel (CB) has a content of monoacylglycerides of between 0.41% to 0.69% by weight.

27. The method as claimed in claim 22, characterized in that the total alcohol content in the biodiesel in steps A-E is less than 0.2% by weight based on the total weight of crude biodiesel (CB) used or processed biodiesel (PB) in the respective step.

28. The method as claimed in claim 22, characterized in that the content of methanol in the biodiesel in steps A-E is less than 0.05% by weight, based on the total weight of crude biodiesel (CB) used or processed biodiesel (PB) in the respective step.

Description

[0052] The invention will be explained in more detail hereafter on the basis of a specific exemplary embodiment and multiple examples. In the FIGURE:

[0053] FIG. 1 shows a flowchart of a working example of the method of the invention.

[0054] The starting material for treatment in the present method is a crude biodiesel. This crude biodiesel may already have undergone a number of preceding processing steps and may already comply with the guideline specifications in respect of its composition according to the standard EN14214:2014-06 in every parameter.

[0055] However, the crude biodiesel differs from the processed biodiesel in its higher proportion of MG after treatment thereof by the method of the invention.

[0056] For instance, the crude biodiesel CB may have a proportion of MG of more than 0.40% by weight, preferably between 0.6% to 0.8% by weight, or a total content of glycerol of 0.25% by weight.

[0057] The present method can reduce the MG content in the processed biodiesel to below 0.40% by weight.

[0058] The proportion of saturated monoglycerides (SMG) in the MG can also be improved, especially proportionally to the reduction in MG.

[0059] The processed biodiesel PB has a substantially higher quality, displays less of a tendency to deposits and has better compatibility with the engine of an internal combustion machine.

[0060] According to the US standard ASTM D6751-15ce, the MG content in biodiesel should be below 0.40% by weight. Such a content can be realized by the present method with comparatively uncomplicated method execution. The MG content can preferably even be lowered to below 0.30% by weight.

[0061] The crude biodiesel used can be a biodiesel which already satisfies the standard EN14214:2014-06, that is to say satisfies an MG content of below 0.70% by weight, but which does not satisfy the MG content according to ASTM D6751-15ce 1-B, that is to say has an MG content of higher than 0.40% by weight.

[0062] FIG. 1 gives a schematic description of a configuration variant of the method of the invention, consisting of a reaction stage 1 with subsequent centrifugal separation 2 of MG, a washing stage 3 with subsequent centrifugal separation 4 of the water phase, and a drying stage 5.

[0063] Reaction stage 1 comprises initially supplying crude biodiesel CB to a mixing tank. An aqueous alkaline solution S, preferably a sodium hydroxide solution, is then added.

[0064] The concentration of sodium hydroxide in the sodium hydroxide solution metered in is advantageously less than 4.03 mol/l, particularly preferably between 0.51 mol/l and 2.77 mol/l.

[0065] The amount of sodium hydroxide solution metered in, based on the amount of crude biodiesel initially charged, is preferably between 0.5% and 3.0% by weight per kg of crude biodiesel, particularly preferably between 1.5% and 2.5% by weight per kg of crude biodiesel. The sodium hydroxide solution preferably contains water as solvent.

[0066] The aqueous alkaline solution is preferably metered into the crude biodiesel in continuous operation.

[0067] The temperature in the mixing tank is preferably 20 to 70 C., particularly preferably between 30 to 60 C. For the mixing of the alkali and the crude biodiesel, a dynamic or a static mixer may preferably be used.

[0068] During the reaction of the alkali with the various glycerides present in the crude biodiesel, a transfer occurs of the now-hydrolyzed monoacylglycerides from the crude biodiesel into the aqueous phase/aqueous alkaline solution.

[0069] The mixture of biodiesel and aqueous alkaline solution S is then conveyed into a first stirred vessel. The holding time within the stirred vessel is 15 to 180 minutes, preferably 60 to 90 minutes. A stirrer system is arranged within the stirred vessel and operates at a speed of preferably 25 to 120 rpm.

[0070] From the first stirred vessel, the mixture of biodiesel and aqueous alkaline solution can be separated within the scope of a first centrifugal separation 2 into a heavy phase 6 and a light phase 7. For this purpose, a separator for example may be used, preferably one with a vertical axis of rotation.

[0071] The heavy phase 6 in this case is the alkaline aqueous solution containing the various hydrolyzed MGs, especially SMG, and methyl esters.

[0072] The light phase 7 essentially comprises biodiesel which has a markedly lower content of MG, especially SMG and methyl esters. It has surprisingly been found here that the reduction in the content of SMG is at least proportional to the reduction in the total content of MG.

[0073] The centrifugal separation 2 is preferably effected between 4400 and 7200 rpm, depending on the plant performance. Continuous feed rates of 5 m.sup.3/h to 50 m.sup.3/h or more per centrifuge can thus be treated. The high centrifugal force (g-force) of up to 10 000G or more means that a maximum separating efficiency is achieved during continuous operation. These separators are equipped with disk stacks which offer a large equivalent clarifying area of up to approximately 400 000 m.sup.2 and hence very effective for the separation of suspensions that consist of two or more phases having different densities. They can be used for a liquid-liquid, liquid-liquid-solid or for a liquid-solid separation. In each case, the liquid phase is discharged continuously. There are different variants for the discharge of solids: discontinuous (for example in the case of solid bowl centrifuges that need to be stopped in order to manually remove the accumulated solid), semicontinuous (self-cleaning centrifuges that automatically periodically empty the solid that has accumulated in the sludge space).

[0074] Separators can be equipped with a so-called finetuner which allows optimal separation of the suspensions.

[0075] Separators having a hydrohermetic infeed pipe may be used. This specially designed infeed pipe protects the product from high shear forces by means of gentle infeed and simultaneously prevents oxygen uptake.

[0076] The separators are available with various drive types: gear drive, belt drive, direct drive and integrated direct drive. The integrated direct drive represents the latest stage of development in the process of separator design, because it operates without gears, belts, clutch and motor supports. The small number of installed components reduces not only energy losses but also maintenance costs, and increases machine availability. The space requirement of the integrated direct drive is around one third smaller than in comparable machines with gear drive or flat belt drive. Separators with integrated direct drive can in addition be operated in a very flexible manner. Within a certain range, the bowl speed is infinitely variable without a change in the transmission ratio. Changing the bowl speed offers a further option in terms of separation efficiency. The innovative design of the integrated direct drive moreover allows exchange of the motor complete with drive within only a few hours in the event of maintenance.

[0077] The light phase, i.e. the biodiesel, is then washed with water W in a washing stage 3. For the purpose of increasing the solubility of hydrolyzed constituents in the biodiesel, the temperature of the water is higher than the temperature of the biodiesel, preferably more than 30 C., particularly preferably between 40 and 70 C.

[0078] The water used is preferably demineralized water in order to avoid any additional introduction of ions.

[0079] The amount of water supplied in washing stage 3 is between 2.0% and 10.0% by weight, based on the weight of the biodiesel, preferably between 3.0% and 5.0% by weight.

[0080] The water W is mixed with the biodiesel after or during the supplying of the water. For the mixing of the water and the biodiesel, a dynamic or a static mixer may preferably be used.

[0081] The water W supplied in the washing stage 3 can alternatively take the form of dilute acid A, for which purpose an acid is preferably metered into the abovementioned hot demineralized water in order thus to prepare the dilute acid A. The acid metered in is preferably citric acid or particularly preferably phosphoric acid. The acid can be metered in in concentrated or dilute form, wherein the pH of the dilute acid A used for the washing in the washing stage 3 is preferably between pH=3 and 5, in particular between pH=3.5 and 4. Sodium salts can be precipitated by the addition of acid. They can then be separated out together with the heavy phase 8 during the subsequent centrifugal separation.

[0082] The mixture of water W, dilute acid A and biodiesel is separated in a second centrifugal separation 4 into a light phase and a heavy phase 9, 8. The centrifugal separation may preferably be effected in a separator, especially in a separator with a vertical axis of rotation.

[0083] The centrifugal separation 4 is effected with similar or identical centrifuges to those in the centrifugal separation 2.

[0084] The heavy phase 8 comprises the water/acid phase including the salts formed. During normal operation, no methyl ester is anticipated in the heavy phase.

[0085] The light phase 9 essentially comprises biodiesel. The latter has a content of MG of less than 0.40% by weight, preferably of less than 0.30% by weight, and a markedly reduced proportion of SMG.

[0086] The biodiesel 9 coming from the centrifugal separation then has to be conveyed into a dryer from residual water contents in a drying stage 5. The dryer is operated under negative pressure, especially under vacuum, in order thus to lower the drying temperature and at the same time save steam consumption. This provides the final product, a processed biodiesel PB.

[0087] A number of preferred specific experimental examples are presented hereinafter for better elucidation of the present invention.

EXAMPLE 1

COMPARATIVE EXAMPLE

[0088] Crude biodiesel made from soybean oil was analyzed according to DIN EN 14105:2011 in respect of its glyceride content. The data are as follows (table 2):

TABLE-US-00002 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.38 0.06 <0.01 <0.001 0.106

[0089] 1000 g of the abovementioned crude biodiesel were heated to 50 C. 3.0% by weight of demineralized water was mixed intensively with the biodiesel for several seconds. The mixture was mixed with a constant stirring speed of 100 rpm for 60 minutes at constant temperature. The mixture was centrifuged and separated into a light phase and a heavy phase by centrifugal separation. The light phase separated off was analyzed according to DIN EN 14105:2011 in respect of its glyceride content. The results are as follows (table 3):

TABLE-US-00003 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.38 0.06 <0.01 <0.001 0.106

[0090] As table 3 shows, there is no reduction in MG.

[0091] In addition, SMGs were analyzed both in the untreated sample and in the treated sample. The results are listed below in the following table, table 4:

TABLE-US-00004 Untreated sample Treated sample Saturated monoglycerides Saturated monoglycerides (% by weight) (% by weight) 0.05 0.05

[0092] As can be seen from table 4, there is no reduction in SMG.

EXAMPLE 2

[0093] Example 2 was repeated but this time with addition of an aqueous alkaline solution instead of water. The conditions were as follows:

[0094] 1000 g of the abovementioned crude biodiesel were heated to 50 C. 3.0% by weight of dilute sodium hydroxide solution (0.51 mol/l NaOH) was mixed intensively with the biodiesel for several seconds. The mixture was mixed with a constant stirring speed of 100 rpm for 60 minutes at constant temperature. The mixture was centrifuged and separated into a light phase and a heavy phase by centrifugal separation. The light phase separated off was analyzed according to DIN EN 14105:2011 in respect of its glyceride content.

[0095] The results were as follows (table 5):

TABLE-US-00005 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.05 0.1 <0.01 <0.001 0.027

[0096] As can be seen from table 5, the MG was reduced well below 0.30% by weight, corresponding to a reduction of 86%. The total glycerol was also reduced by the MG reduction.

[0097] In addition, SMGs were analyzed both in the untreated sample and in the treated sample. The results are listed below in the following table, table 6:

TABLE-US-00006 Untreated sample Treated sample Saturated monoglycerides Saturated monoglycerides (% by weight) (% by weight) 0.05 <0.01

[0098] As can be seen from table 6, SMG was reduced proportionally with the reduction in the MG, corresponding to a reduction of more than 80%.

[0099] In addition, sodium (Na) was analyzed using the test method DIN EN 14538 both in the untreated sample and in the treated sample. The results are listed below in the following table, table 7.

TABLE-US-00007 Untreated sample Treated sample Sodium (mg/kg) Sodium (mg/kg) <0.5 <0.5

[0100] As can be seen from table 7, after the treatment, sodium stays below the stipulated threshold value of the US standard ASTM D6751-15ce for grades 1-B and 2-B, below 5 mg/kg (Na+K), and of the European standard EN14214:2014-06, below 5 mg/kg (Na+K).

EXAMPLE 3

[0101] Crude biodiesel made from rapeseed oil was analyzed according to DIN EN 14105: 2011 in respect of the glyceride content. The analyzed data of the crude biodiesel are listed in table 8 below.

TABLE-US-00008 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.50 0.08 <0.01 0.002 0.143

[0102] 1000 g of the abovementioned crude biodiesel were heated to 60 C. 6.0% by weight of dilute sodium hydroxide solution (0.51 mol/L NaOH) was mixed intensively with the biodiesel for several seconds. The mixture was mixed with a constant stirring speed of 100 rpm for 60 minutes at constant temperature. The mixture was centrifuged and separated into a light phase and a heavy phase by centrifugal separation. The light phase, that is to say the biodiesel, was then washed with hot demineralized water at a temperature of 60 C. The amount of water, based on the amount of biodiesel, was 10% by weight. The water had been adjusted beforehand to a pH of 3 by addition of phosphoric acid. A centrifugal separation was subsequently effected and the light phase separated off was analyzed according to DIN EN 14105:2011 in respect of its glyceride content. The results are listed below (table 9):

TABLE-US-00009 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.28 0.10 0.01 0.005 0.088

[0103] As can be seen from table 9, the MG content was reduced to below 0.30% by weight, corresponding to a reduction of 44%. The total glycerol was also reduced by the MG reduction. A slight increase in diglycerides was detected.

EXAMPLE 4

[0104] Crude biodiesel made from soybean oil biodiesel was analyzed according to DIN EN 14105: 2011 in respect of the glyceride content. The analyzed data are listed in table 10 below.

TABLE-US-00010 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.60 0.16 0.07 0.006 0.19

[0105] 1000 g of the abovementioned crude biodiesel were heated to 60 C. 3.0% by weight of dilute sodium hydroxide solution (1.05 mol/l NaOH) was mixed intensively with the biodiesel for several seconds. The mixture was mixed with a constant stirring speed of 100 rpm for 60 minutes at constant temperature. The mixture was centrifuged and separated into a light phase and a heavy phase by centrifugal separation. The light phase, that is to say the biodiesel, was then washed with hot demineralized water at a temperature of 60 C. The amount of water, based on the amount of biodiesel, was 10% by weight. The water had been adjusted beforehand to a pH of 3 by addition of phosphoric acid. A centrifugal separation was subsequently effected and the light phase separated off was analyzed according to DIN EN 14105:2011 in respect of its glyceride content. The results are listed below (table 11):

TABLE-US-00011 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.27 0.19 0.07 <0.001 0.104

[0106] As can be seen from table 11, the MG was reduced to below 0.30% by weight, corresponding to a reduction of 55%. The total glycerol was also reduced by the MG reduction.

[0107] In addition, SMGs were analyzed both in the untreated sample and in the treated sample. The results are listed below in the following table, table 12:

TABLE-US-00012 Untreated sample Treated sample Saturated monoglycerides Saturated monoglycerides (% by weight) (% by weight) 0.09 0.04

[0108] As can be seen from table 12, SMG was reduced proportionally with the reduction in the MG, corresponding to a reduction of 55%.

[0109] In addition, sodium (Na) was analyzed using the test method DIN EN 14538 both in the untreated sample and in the treated sample. The results are listed below in the following table, table 13.

TABLE-US-00013 Untreated sample Treated sample Sodium (mg/kg) Sodium (mg/kg) <1 <1

[0110] As can be seen from table 12, after the treatment, sodium stays below the stipulated threshold value of the US standard ASTM D6751-15ce for grades 1-B and 2-B, below 5 mg/kg (Na+K), and of the European standard EN14214:2014-06, below 5 mg/kg (Na+K).

EXAMPLE 5

[0111] A crude biodiesel made from palm oil was analyzed in respect of the glyceride content (DIN EN 14105: 2011). The analyzed data are listed below (table 14):

TABLE-US-00014 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.57 0.22 0.29 0.003 0.208

[0112] 1000 g of the abovementioned crude biodiesel were heated to 60 C. 3.0% by weight of dilute sodium hydroxide solution (1.05 mol/l NaOH) was mixed intensively with the biodiesel for several seconds. The mixture was mixed with a constant stirring speed of 100 rpm for 60 minutes at constant temperature. The mixture was centrifuged and separated into a light phase and a heavy phase by centrifugal separation. The light phase, that is to say the biodiesel, was then washed with hot demineralized water at a temperature of 60 C. The amount of water, based on the amount of biodiesel, was 5% by weight. The water had been adjusted beforehand to a pH of 3 by addition of citric acid. A centrifugal separation was subsequently effected and the light phase separated off was analyzed according to DIN EN 14105:2011 in respect of its glyceride content. The results are presented below (table 15):

TABLE-US-00015 Mono- Free Total glycerides Diglycerides Triglycerides glycerol glycerol (% by (% by (% by (% by (% by weight) weight) weight) weight) weight) 0.30 0.25 0.30 <0.01 0.143

[0113] As can be seen from table 15, the MG content was reduced to 0.30% by weight, corresponding to a reduction of 48%. The total glycerol was also reduced by the MG reduction. A slight increase in diglyceride was detected.

[0114] The abovementioned invention is not restricted solely to the above-described examples. It can be modified insofar as the claims listed below are complied with.

[0115] The results above show that with first-generation biodiesel, which is produced from starting materials such as, for example: soybean oil, palm oil or rapeseed oil, an uncomplicated extension of the method which is usable in a supplementary manner in any biodiesel production plant with little additional apparatus complexity, leads to increased quality of the end product.

[0116] For second-generation biodiesel, which is produced from starting materials such as, for example, used cooking oil (UCO), animal fats and/or fatty acids, this method can lead to better performance in the downstream biodiesel distillation stage, in that the amount of heavy distilled residue is reduced.

[0117] The method is therefore advantageously but not exclusively applicable to first-generation biodiesel, and is also applicable to second-generation biodiesel.

[0118] US 2010 0175312A1 and WO2015183744A1 disclose a conventional neutralization of free fatty acid during biodiesel production. In the present case, the starting material, the crude biodiesel, is already a biodiesel with a proportion of free fatty acids of less than 0.25% by weight, corresponding to an acid number of 0.5 mg KOH/g. In the European standard EN14214:2014-06 and in the US standard ASTM D6751-15ce a maximum acid number of 0.5 mg KOH/g is permitted. Furthermore, it is advantageously possible not to use any alcohol in the present method of the invention and hence to dispense with complex and costly safety measures or explosion protection measures.

REFERENCE SIGNS

[0119] 1 addition of aqueous alkaline solution

[0120] 2 first centrifugal separation

[0121] 3 washing

[0122] 4 second centrifugal separation

[0123] 5 drying

[0124] 6 heavy phase

[0125] 7 light phase

[0126] 8 heavy phase

[0127] 9 light phase

[0128] CB crude biodiesel

[0129] PB processed biodiesel

[0130] S aqueous alkaline solution

[0131] W water

[0132] A acid