Improvement in Low-Temperature Properties of Fatty Acid Methyl Esters

Abstract

The European Union requirements related to the quality parameters for fatty acid methyl esters (FAMEs) are gathered in the standard EN 14214:2012 + A2:2019 that also includes reference to low-temperature properties. This paper presents studies on the obtaining of modified FAMEs, featuring improved low-temperature properties. Investigated fatty acid methyl esters (FAMEs) were subjected to a solvent dewaxing process with a methyl ethyl ketone and the mixture of methyl ethyl ketone—Toluene. It was found that the application of a process carried out under conditions similar to solvent dewaxing, used as a standard procedure for oils dewaxing and slack waxes of petroleum origin deoiling usually used in refinery industrial installations, allows us to achieve the intended goal. The modification of three different types of FAME by the dewaxing process with MEK-TOL and MEK solvents allows for the improvement of low-temperature properties of the obtained modified FAME, consistent in obtaining reduced cloud points, flow points and CFPP. The tests of fatty acid profiles show a clear increase in the content of glycerides of saturated acids in the separated sludge, as compared to the charges from the dewaxing process, which confirms that the selectivity of the dewaxing process is maintained for the atypical raw material, which are fatty acid methyl esters.

1. Introduction

The care of the natural environment, the depletion of petroleum resources, and the increase in the costs of its mining and processing have drawn the attention to the use of biofuels as alternative energy sources. The substitution of conventional fuels for biofuels, or introducing them as components of petroleum fuels, reduces the emissions of CO, SO₂, and NOx, stops adverse climate changes, increases the fuel security through reduction of the dependence on the petroleum supplies, and causes the activation of agriculture and the creation of new jobs [1].

One of the widely used biofuels, substituting for conventional diesel fuels and heating oils, is the biodiesel—Bioesters—Fatty acid methyl esters (FAMEs). They are a mixture of fatty acid methyl esters, produced from vegetable or animal oil, which feature very good miscibility with conventional diesel fuel at any ratio [1].

The addition of the FAME biocomponent to diesel fuel improves its lubricity, increases the cetane number, reduces the emission of harmful exhaust components, but deteriorates its oxidation stability, low-temperature properties, and also reduces the energy value of the fuel [1].

The quality of the final product, FAME-containing biofuel or diesel fuel, depends on the quality of components used. Both the conventional diesel fuel and FAME must have an appropriate quality and satisfy requirements specified in the product standards [1,2,3]. It is widely known that FAMEs feature a number of adverse properties, which include, e.g., low oxidation stability [4]. The problems of FAME application in diesel fuel are also related to its low-temperature properties [5]. Relatively high values of the temperature of cold filter plugging point and the cloud and solidification points limit biofuels’ applications, in particular at negative ambient temperatures. The introduction of FAMEs as an independent fuel or as a component of diesel fuels caused manufacturers and designers of diesel engines to impose strict requirements related to the quality of produced biofuels.

1.1. General Description of Biofuels

Conventional diesel fuels, FAMEs, and their compositions should feature physical, chemical, and practical properties that are invariable over time, because since the moment of their production or composition they are exposed to the operation of factors, which cause the activation and acceleration of ageing processes and deteriorating fuel properties [1].

It was shown in paper [6] and patent [7] that basic problems related to FAME use in engine fuels comprise issues related to the low-temperature properties, foaming, low oxidation stability, deposits and sealing-wax forming on injectors, lower resistance to water operation, both from the point of view of hydrolytic stability and propensity to form permanent emulsion with water, higher propensity to biodegradation processes, and increased fuel corrosivity.

Diesel fuels produced now from petroleum are deeply refined blends of aliphatic-cycloalkane hydrocarbons with more and more limited amounts of aromatic hydrocarbons and sulphur compounds. Distillation temperatures of a typical diesel fuel range from approx. 170 to 360 °C. The number of carbon atoms in a molecule of hydrocarbons, comprised by the diesel fuel, is 12 to 30. These fuels feature good stability properties.

Biofuels, which are independent fuels or compositions of petroleum-origin fractions and fatty acid methyl esters, have characteristics of both components. It was found that biofuels, which are a blend of petroleum fractions, even highly-refined, and fatty acid esters show greater propensity to degradation than the biocomponent itself. Already a share of 10 vol.% of esters in a blend with conventional fuel deteriorates the stability properties of the fuel, and the amounts of deposits determined in tests grow significantly [1]. The degradation of FAMEs, as manifested by a low oxidation stability, is related to the existence in esters of unsaturated structures, in particular allyl ones, showing the propensity to create, as a result of reaction with oxygen present in the air, radicals and peroxy structures, which are precursors of FAME oligomerisation [8].

The FAME quality issues are also related to compounds other than esters, existing in trace amounts [1]. Vegetable oils are the raw material for FAME production. They are acylglycerols contain small amounts of enzymes, proteins, steroids, phospholipids, sterol glucosides, and other compounds, which affect physicochemical and practical properties of FAMEs. Additional issues are created by products of destruction and oligomerisation, related to FAME ageing processes [4,8].

The phenomenon of an adverse impact of monoacylglycerols and sterol glucosides present in FAMEs on operating properties of the fuel have been described in a number of papers [9,10,11]. It is not considered by standardized requirements now.

Paper [11] describes results of studies on the impact of fatty acid methyl esters of various origin on low-temperature practical properties of biofuels. For the studied biofuel samples containing distilled FAME, based on the assessment of the filter blocking trend (FBT), it was found that such biofuels feature a definitely smaller value of the index of fuel filters blocking trend. Instead, in paper [12] it was found that the removal of sterol glucosides during FAME distillation does not result in lowering the CFFP, as compared with the initial FAME.

1.2. Low-Temperature Properties of Fatty Acid Methyl Esters, Fame

FAMEs crystallise at low temperatures. Crystals, forming as plates or needles, with sizes from a few to a few hundred micrometres, as a result of coagulation form three-dimensional crystal structures, containing occluded particles of liquid fuel. This process results in the loss of fluidity and an increase in the fuel solidification point, which limits the use of FAMEs during the winter period [11].

FAMEs feature higher cloud and solidification points than hydrocarbon diesel fuel. Because of that, the use of esters in biofuels is normally limited to blends, usually containing up to 20 vol.% of biodiesel (B20) [13,14].

The most recent version of the European standard EN 14214:2012 + A2:2019 related to requirements and test methods for fatty acid methyl esters (FAME) raises the issue of low-temperature properties of diesel fuel blends connected with the quality of FAME used as the component and states that there is a negative impact of monoacylglycerols and sterols glucosides, which exist in FAMEs, on the low-temperature properties of the fuel. However, as there are no standardised methods for those compounds’ determination, reference to requirements defining the CFPP and the cloud point is a temporary solution.

Literature sources [15,16,17] report that the CFPP typical of FAMEs usually ranges between 263 K (−10 °C) and 298 K (+25 °C), and that it is higher than that characteristic of petroleum-origin fuels (from 246 K (−25 °C) to 258 K (−15 °C)).

At the cloud point (CP), crystals of ‘solid wax’ originate, which are minimum 0.5 μm in diameter, causing the fuel to become opaque.

Certain authors [17,18,19] consider the cloud point of biofuels as the most important parameter, which influences the biofuel quality at low temperatures, defining the temperature at which problems with the engine operation start occurring caused by the precipitation of solids from the fuel.

Scientific papers [20,21] describe various methods for improvement in low-temperature FAME properties, which consist in the modification of their composition, e.g., through freezing long-chain esters or removing esters with a saturated hydrocarbon chain in extraction processes with hexane. The second group consists of chemical modifications such as the alkoxylation of the ester hydrocarbon chain, catalytic isomerisation, partial ozonolysis of esters, or the use of fatty acid esters and alcohols with longer carbon chains (C2-C4). Another method to improve low-temperature properties of fatty acid methyl esters is their replacement with alcohol esters with a higher number of carbon atoms (mainly C2-C4), or the use of additives, which modify their fluidity— depressants.

Additionally, many patent solutions are devoted to the issue of improvement in low-temperature FAME properties.

Patents [22,23,24] present methods directed towards obtaining blends of esters containing more than 50% of medium-chain fatty acids (12–18 carbon atoms), which feature low melting points. The use of appropriately genetically modified plants was suggested, which would allow raw material (vegetable oil) to be obtained for FAME production with a required content of such fatty acids.

Patent US5 389113 [25] describes a solution based on mixtures of fatty acid esters that have good low-temperature properties. The fuel composition consists essentially of: 58 to 95% m/m ester or a mixture of 15% esters selected from the group consisting of fatty acid esters of 12–22 carbon atoms and aliphatic alcohols of one to four carbon atoms, said esters having an iodine value ranging from 50 to 150.

The patent EP 716139 [26] shows a biofuel produced by the reaction of vegetable oils and sec-butyl alcohol, which is characterised by improved low-temperature properties.

Patents [5,27] describe various groups of depressants, including polymethacrylates, methacrylic acid and polyether copolymers, or long-chain alcohols (PAMA) chosen depending on the used vegetable material and the method for fatty acid alkyl esters production.

The paper describes studies on obtaining modified FAMEs featuring improved low-temperature properties due to the application of a process carried out under conditions similar to solvent dewaxing, used as a standard procedure for oils dewaxing and slack waxes of petroleum origin deoiling.

2. Materials and Methods

Fatty acid methyl esters (FAMEs) produced in industrial technological FAME plants were used in the studies as a raw material.

Table 1. Properties of studied fatty acid methyl esters (FAME).

No	Property	Unit	FAME I	FAME II	FAME III	Test Method
1	Fatty acid methyl esters (FAME) content	% (m/m)	97.5	97.1	96.4	PN-EN 14103:2012
2	Density at 15 °C	kg/m3	883.0	882.8	882.7	PN-EN ISO 12185:2002
3	Kinematic viscosity at 40 °C	mm2/s	4.537	4.480	4.379	PN-EN ISO 3104:2021-03
5	Sulphur content	mg/kg	<5	3.4	3.8	PN-EN ISO 20884:2012
6	Sulphate ash	% (m/m)	0.01	0.01	0.01	PN-ISO 3987:2005
9	Corroding action Cu/50 °C/3 h	degree of corrosion	1	1	1	PN-EN ISO 2160:2004
10	Oxidation stability at 110 °C	h	9.7	10.9	9.4	PN-EN 14112:2004
11	Acid number	mg KOH/g	0.26	0.21	0.24	PN-EN 14104:2004
12	Iodine number	g of iodine/100 g	109	108	101	PN-EN 14111:2004
13	Linolenic acid methyl ester content	% (m/m)	8.3	8.6	8.2	PN-EN 14103:2012
15	Monoacylglycerols content	% (m/m)	0.59	0.57	0.78	PN-EN 14105:2012
16	Diacylglycerols content	% (m/m)	<0.10	<0.10	<0.10	PN-EN 14105:2012
17	Triacylglycerols content	% (m/m)	<0.10	<0.10	<0.10	PN-EN 14105:2012
23	Cloud point	°C	−4	−5	+1	PN-ISO 3016:2005
24	Pour point	°C	−12	−12	0	PN-ISO 3016:2005
25	Cold filter plugging point (CFPP)	°C	−15.0	−12.4	−1.0	PN-ISO 3016:2005
26	Sterol glucosides content	ppm	32.0	−	14.8	INIG-PIB method

presents the selected physicochemical properties of those samples.

Experiment Description

The crystallisation of fractions precipitating from FAME was carried out according to an own method, developed for the needs to model the process of solvent dewaxing in the INIG-PIB, consisting of gradual cooling of the mixture of dewaxed material (in this case FAME) contained in the crystalliser in an appropriate solvent, and then filtering the cooled mixture—Figure 1.

The crystalliser, containing 300 g raw FAME, is situated in a cooling bath equipped with a cooling cycle programmer, which allows us to set the final temperature of crystallisations and appropriate cooling rates in consecutive stages of the process.

The crystallisations are carried out by the method of dilution, by adding consecutive portions of a cold solvent to the cooled mixture of material and solvent.

During the crystallisation, the crystalliser content is continuously mixed by means of a mixer with an anchor tip, with the cooling rate adapted to the increasing viscosity of the mixture.

Once the final crystallisation temperature is reached, the precipitated solids are filtered, which contain an occluded solvent, with the use of a vacuum nutsche filter. The filtered solid substances are washed with a few portions of a cold solvent; its remains are then removed in the process of stripping with nitrogen. A biofuel with improved low-temperature properties was obtained as a result of solvent removal from the filtrate in a vacuum evaporator.

Table 2. Technological parameters and the mass balance of carried out tests, and properties of modified biofuels, when the FAME I was used as a raw material.

Test No.	IA	IB	IC	ID
Solvent type	MEK-TOL	MEK-TOL	MEK-TOL	MEK
Symbol of obtained biofuel sample	FAME IA	FAME IB	FAME IC	FAME ID
Process technological parameters
Solvent, mass ratio	40:60	40:60	40:60	-
Crystallisation/filtration temperature, °C	−20	−29	−29	−25
Total solvent to material ratio, (m/m)	4:1	2:1	0.85:1	2:1
Mass balance of processes, averaged results
Yield of modified biofuel, % (m/m)	96.6	88.0	85.0	93.5
Yield of precipitated solids, % (m/m)	−	8	8	3.5
Losses, % (m/m)	3.4	4	7	2
Filtration time	49	40	120	45
Vacuum conditions, bar	0.12	0.12	0.12	0.12
Properties of modified biofuel
Cloud point, °C	−4	−8	−4	−15
Pour point, °C	−12	−14	−12	−17
Cold filter plugging point (CFPP), °C	−15	−15	−15	−18
Kinematic viscosity at 40 °C, mm2/s	4.637	4.850	4.823	4.914
Sterol glucosides content, mg/kg	31	25	28	29

,

Table 3. Technological parameters and the mass balance of carried out tests, and properties of modified biofuels, when the FAME II was used as a raw material.

No. of Dewaxing	IIA	IIB	IIC	IID	IIE	IIF	IIG
Solvent Type	MEK-TOL	MEK-TOL	MEK-TOL	MEK	MEK	MEK	MEK
Viscosity modifier	−	−	Viscoplex 9—327	−	−	Viscoplex 9—327	−
Symbol of modified biofuel sample	FAME IIA	FAME IIB	FAME IIC	FAME IID	FAME IIE	FAME IIF	FAME IIG
Technological parameters of dewaxing processes
Solvent, mass ratio	60:40	50:50	40:60	−	−	−	−
Crystallisation/filtration temperature, °C	−28	−28	−28	−21	−28	−28	−28
Total solvent to material ratio, (m/m)	3.2:1	2.9:1	5.5:1	2.0:1	2.9:1	2.9:1	6.0:1
Washing at filtration temperature, (m/m)	0.6:1	0.2:1	0.6:1	0.4:1	0.2:1	0.2:1	0.4:1
Mass balance of dewaxing processes, averaged results
Yield of modified biofuel, % (m/m)	96.6	90.0	87.0	92.8	90.0	94.6	91.6
Yield of precipitated solids, % (m/m)	3.4	8.0	9.0	4.0	5.0	3.5	5.0
Losses, % (m/m)	3.0	2.0	4.0	3.2	5.0	1.9	3.4
Filtration time, seconds	3 min 19 s	40 s	2 min 20 s	1 min 59 s	2 min 24 s	1 min 52 s	1 min 38 s
Vacuum conditions, bar	0.12	0.12	0.12	0.12	0.12	0.12	0.12
Properties of modified biofuel
Cloud point, °C	−9	−8	−14	−7	−14	−12	−12
Pour point, °C	−15	−14	−15	−9	−15	−14	−16
Cold filter plugging point (CFPP), °C	−15.1	−15.0	−17.0	−10.2	−15.4	−15	−16.4
Kinematic viscosity at 40 °C, mm2/s	4.714	4.841	4.910	4.735	4.832	4.729	4.768

and

Table 4. Technological parameters and the mass balance of carried out tests, and properties of modified biofuels, when the FAME III was used as a raw material.

No. of Dewaxing	IIIA	IIIB	IIIC	IIID	IIIE
Solvent type	MEK-TOL	MEK-TOL	MEK-TOL	MEK	MEK
Viscosity modifier	−	−	Viscoplex 9—327	−	−
Symbol of dewaxed material sample	FAME IIIA	FAME IIIB	FAME IIIC	FAME IIID	FAME IIIE
Technological parameters of dewaxing processes
Solvent, mass ratio	40:60	70:30	50:50	−	−
Crystallisation/filtration temperature, °C	−28	−21	−21	−21	−21
Total solvent to material ratio, (m/m)	5.5:1	5.5:1	3.5:1	5.5:1	3.8:1
Washing at filtration temperature, (m/m)	0.5:1	0.5:1	0.5:1	0.5:1	0.2:1
Mass balance of dewaxing processes, averaged results
Yield of dewaxed material, % (m/m)	96.0	80.6	85.4	89.4	90.1
Yield of slack wax, % (m/m)	1.7	13.7	9.8	4.7	4.5
Losses, % (m/m)	2.3	5.7	4.8	5.9	5.4
Filtration time, seconds	46 s	62 s	85 s	62 s	63 s
Vacuum conditions, bar	0.12	0.12	0.12	0.12	0.12
Properties of modified biofuel
Cloud point, °C	+1	−9	−10	−7	−8
Pour point, °C	0	−9	−11	−8	−9
Cold filter plugging point (CFPP), °C	−1.4	−11.6	−11.8	−9.4	−10.6
Kinematic viscosity at 40 °C, mm2/s	4.463	4.527	4.568	4.618	4.598

present the description of technological parameters of carried out tests.

3. Results

This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.

3.1. Improvement in FAME I Properties

Technological parameters of performed experiments using the FAME I and methyl ethyl ketone (MEK) and the mixture of methyl ethyl ketone—Toluene (MEK-TOL) solvents, as well as the mass balance of the obtained modified FAME biofuel and precipitated solids, are presented below together with their physicochemical properties (Table 2).

Tests IA-ID resulted in obtaining 85% up to more than 96% of modified biofuel. For tests IA and IC, no improvement in low-temperature properties was obtained. The best results were obtained in test ID, with the use of MEK as a solvent. FAME ID biofuel had a lower cloud point by 11 °C as compared with raw FAME I, the pour point went down by 5 °C, while the cold filter plugging point (CFPP) lowered by 3 °C. Figure 2 presents a graphical illustration of the impact of the process carried out under conditions of solvent dewaxing on low-temperature properties of FAME I.

The performed tests show low selectivity of sterol glucosides removal from FAME in the studied process. This is probably caused by good solubility of sterol glucosides in polar solvents.

3.2. Improvement in FAME II Properties

Technological parameters of performed experiments using the FAME II sample and MEK-TOL (three tests) and MEK (four tests) solvents, as well as the mass balance of the obtained modified FAME and of precipitated solids, are presented below together with their physicochemical properties (Table 3).

In the tests carried out to improve low-temperature properties with the use of MEK-TOL and MEK solvents, the yield of modified biofuels ranged from 87.0% (m/m) to 96.6% (m/m), while the yield of precipitated solids was within a range of 3.4% (m/m) to 9.0% (m/m). High variability of filtration times was observed, within a range of 40 s to more than 3 min. A crystallisation modifier was used in tests IIC and IIF.

As a result of carried out experiments it was found that in most cases the modified biofuel obtained from FAME II features improved low-temperature properties. It was observed that the cloud point went down by 2 to 9 °C, the pour point lowered by 2 to 4 °C, and the CFPP decreased by 2.2 to 4.6 °C. Only in the case of the test with the modified biofuel IID was the CFPP found to increase by 2.2 °C. In addition, a small increase in the viscosity at 40 °C was observed for modified biofuels.

Figure 3 presents a graphical illustration of the impact of the process carried out under conditions of solvent dewaxing on low-temperature properties of FAME II.

In the studied variation range of technological parameters, i.e. the solvent to charge ratio, filtration temperature, and solvent type, a similar effectiveness of low-temperature properties of FAME II was obtained. When carrying out a detailed analysis it is possible to state that the use of the MEK solvent is the most effective, and the effectiveness of the cloud, solidification, and CFPP point lowering increases with a growing solvent to charge ratio.

3.3. Improvement in FAME III Properties

Technological parameters of performed experiments using the FAME III sample and MEK-TOL (three tests) and MEK (two tests) solvents, as well as the mass balance of the obtained modified FAME and of precipitated solids, are presented below together with their physicochemical properties (Table 4).

In the tests carried out to improve low-temperature properties with the use of MEK-TOL and MEK solvents, the yield of modified biofuels ranged from 80.6% (m/m) to 96.0% (m/m), while the yield of precipitated solids was within a range of 1.7% (m/m) to 13.7% (m/m). Various filtration times were observed, ranging from 46 s to more than 1 min. A crystallisation modifier was used in test IIIC.

As a result of carried out experiments it was found that in most cases the modified biofuel obtained from FAME III features improved low-temperature properties. It was observed that the cloud point went down by 0 to 10 °C, the pour point lowered by 0 to 9 °C, and the CFPP decreased by 1.0 to 10.6 °C. In addition, a small increase in the viscosity at 40 °C was observed for modified biofuels.

Figure 4 presents a graphical illustration of the impact of the process carried out under conditions of solvent dewaxing on low-temperature properties.

In the studied variation range of technological parameters, i.e. the solvent to charge ratio, filtration temperature, and solvent type, a similar effectiveness of low-temperature properties of FAME III improvement was obtained, except for test IIIA. When carrying out a detailed analysis it is possible to state that the use of the MEK solvent is the most effective, and the effectiveness of the cloud, solidification, and CFPP point lowering increases with a growing solvent to charge ratio, similarly as in the previously described cases.

3.4. Profiles of Fatty Acids

The results of studies on fatty acid profiles (

Table 5. Results of studies on fatty acid profiles in raw FAME (II and III), modified biofuels, and precipitated solids obtained in tests IIA and IIIB.

No.	Fatty Acids	Unit	Raw FAME		Precipitated Solids		Modified FAME
			II	III	IIAs	IIIBs	IIA	IIIB
1	<C16	% (m/m)	−	−	1.4	1.2	0.1	0.1
2	C14:0	% (m/m)	−	0.2	−	−	0.2	0.1
3	C16:0	% (m/m)	4.5	15.5	5.2	54.4	4.3	13.6
4	C16:1	% (m/m)	0.3	0.2	−	0.2	0.3	0.2
5	C16 unident.	% (m/m)	0.4	0.2	0.6	0.3	0.3	0.2
6	C18:0	% (m/m)	1.7	3.4	10.9	20.7	1.2	3.0
7	C18:1	% (m/m)	63.2	32.5	9.5	6.1	63.4	34.2
8	C18:2	% (m/m)	18.5	40.7	0.1	1.1	18.7	41.5
9	C18 unident.	% (m/m)	0.2	0.1	0.4	−	0.1	0.2
10	C18:3	% (m/m)	7.8	5.6	0.2	0.1	8.3	5.8
11	C20:0	% (m/m)	0.6	0.4	20.8	9.9	0.1	0.2
12	C20:1	% (m/m)	1.3	0.4	0.3	0.2	1.5	0.4
13	C20 unident.	% (m/m)	0.2	0.1	1.2	1.3	0.2	0.1
14	C22:0	% (m/m)	0.3	0.4	26.1	3.4	0.1	0.2
15	C22:1	% (m/m)	0.4	0.1	0.6	0.2	0.7	0.0
16	C22 unident.	% (m/m)	0.1	0.1	4.3	0.3	0.1	0.1
17	C24:0	% (m/m)	0.2	0.1	12.4	0.6	0.0	0.1
18	C24:1	% (m/m)	0.3	−	1.2	−	0.3	−
19	C24 +	% (m/m)	−	−	4.8	−	0.1	−

) were carried out according to the ISO 12966-1:2014 standard [28]. They were determined in raw FAME, modified biofuels, and precipitated solids obtained in tests IIA and IIIB, as presented below.

Figure 5 presents the impact of dewaxing processes on a change of fatty acids content versus the degree of their unsaturation in raw materials (FAME II and III), and products obtained in tests IIA and IIIB (modified FAME IIA and IIIB, and precipitated solids IIAs and IIBs).

The analysis of fatty acid profiles allows us to state that the studied process causes the removal of solids from the raw FAME, which are mainly saturated triglycerides, which are the cause for high values of PP, CP, and CFPP. As a result of the studied process, the low-temperature properties substantially improve at small changes of the biofuel composition, reduction by approx. 1.5–3% (m/m) of the content of saturated fatty acids, and an increase in the total content of unsaturated acids by 1.4 to 2.8% (m/m) in modified FAME.

Based on the above balance, it is possible to presume that compounds, in which two or three ester bonds are present between glycerine hydroxyl groups and saturated fatty acids, are the main component of removed triglycerides. Instead, the structures containing ester bonds between glycerine hydroxyl groups and one saturated fatty acid radical and two unsaturated acid radicals transfer to the modified biofuel and do not have an adverse impact on its low-temperature properties.

4. Discussion

Biodiesel fuel, usually fatty acid methyl esters, is an alternative to or a component of conventional diesel fuel. The requirements and test methods, which must be satisfied by FAME, used as a fuel for diesel engines or a component of fuel for car diesel engines, are specified by the standard PN-EN 14214:2012 + A2:2019. It is widely known that, apart from unquestionable advantages, FAMEs feature a number of adverse properties, which include, e.g., low oxidation stability or adverse low-temperature properties. Low oxidation stability is related to the existence of unsaturated structures, in particular allyl ones, showing the propensity to create, as a result of the reaction with oxygen present in the air, radicals and peroxy structures, which are precursors of oligomerization and FAME destruction [4,8]. Instead, unfavourable values of the cold filter plugging point, cloud point, and solidification point, which reduce the range of FAME application at negative temperatures, are related to the presence of saturated fatty acid triglycerides, and some pollutants, such as mono- and diglycerides. The cloud point (CP) of fuels is frequently considered the most important parameter affecting the biofuel quality at low temperatures. Crystalline structures of ‘solid wax’ originate at the cloud point, which are at least 0.5 μm in diameter, causing the fuel to become opaque and problems with the engine operation start appearing due to the accumulation of deposits in the fuel system.

As a result of the carried out studies, it was found that the process of removing substances precipitating from FAME at low temperatures, performed under conditions similar to solvent dewaxing, which is used as a standard procedure for oils dewaxing and slack waxes of petroleum origin deoiling, allows for an effective improvement in low-temperature properties of biofuel as compared with the initial values. If the process was carried out with the use of MEK-TOL and MEK as solvents, the cloud point was reduced even to −15 °C, the pour point to −16 °C, and the cold filter plugging point to −16 °C. The best effects are observed for raw FAME with CP, PP, and CFPP of approx. 0 °C.

Profiles of FAME fatty acids used as the raw material for the process and obtained products show a clear increase in the content of saturated acid glycerides in precipitated solids, which confirms the maintaining of process selectivity carried out under conditions similar to solvent dewaxing for an atypical raw material, which are fatty acid methyl esters.

5. Patents

PL236708 Method of producing a modified FAME biofuel for automotive compression ignition engines and the modified FAME biofuel for automotive compression ignition engines, INiG-PIB, 2021-02-08.

PL236705 Method of producing a modified FAME biofuel for automotive compression ignition engines and the modified FAME biofuel for automotive compression ignition engines, INiG-PIB, 2021-02-08.