Study of the N-Paraffins Addition Effect on the Effectiveness of Depressant Additives for the Production of Low-Freezing Diesel Fuels

Abstract

The use of depressant additives is the most common method for producing diesel fuels with improved low-temperature properties. However, the depressants effectiveness largely depends on the composition of the diesel fuel, in particular on the content of n-paraffinic hydrocarbons, which to the greatest extent determine the low-temperature properties of the fuel. The work revealed the regularities of the n-paraffins addition influence on the depressant additives effectiveness. It was found that the addition of n-paraffins to blends of diesel fuels with depressants in low concentrations (0.05–0.50% wt.) enhances the depressants effectiveness in relation to the cold filter plugging point: maximum at 6–16 °C depending on the diesel fuel sample. It is shown that the effect is observed for DF of various compositions, various depressants, and also n-paraffins of various compositions. It was established that the positive effect of adding n-paraffins increases with the heavier added n-paraffins. Recommendations have been developed for obtaining diesel fuels with improved low-temperature properties and enhancing the depressant additives effectiveness: for a sample of straight-run diesel fuel F₁, it is recommended to use a blend of fuel, depressant A₁ and 0.50% wt. n-paraffins, separated from heavy gasoil; for a sample of straight-run diesel fuel F₂, it is recommended to use a blend of fuel, depressant A₂ and 0.50% wt. n-paraffins, separated from heavy gasoil or a blend of fuel, depressant A₁ and 0.05% wt. n-paraffins, separated from highly paraffinic oil fraction. The revealed patterns and the developed recommendations will allow increasing the production of low-freezing DF brands, and also offer a resource-efficient option for using heavy gas oil fractions. The results obtained in the work contribute to expanding the understanding of the mechanism of interaction between diesel fuel hydrocarbons and the active ingredients of depressant additives.

1. Introduction

The global automotive diesel engine market by region includes North America, Asia-Pacific, Europe, South America, and the Middle East and Africa [1]. The Asia-Pacific region shows clear dominance, accounting for the largest market share (45%), due to its rapid industrialization and infrastructure development, which require reliable energy sources and the use of heavy equipment [2]. According to a global report, the global automotive diesel engine market is expected to grow in average of approximately 4% between 2023 and 2028, with Asia-Pacific being the fastest growing region, followed by North America and Europe [3]. Active economic and scientific development of the Arctic and northern territories has led to a sharp increase in the demand of diesel fuel with improved low-temperature properties [4].

Low temperature properties determine the ability of fuel to flow smoothly into the engine from the fuel tank. At low temperatures, the mobility of the fuel decreases, which leads to deterioration in pumpability through pipelines and filters due to the crystallization of n-paraffin hydrocarbons contained in its composition. Low-temperature properties include three indicators: cloud point, cold filter plugging point and pour point.

The cloud point (CP) characterizes a change in the phase composition of the fuel, namely the formation of not only the liquid phase of the fuel, but also the solid one. Cloudiness manifests itself as a change in the appearance of the fuel from clear to cloudy. As the temperature decreases, the crystallization of n-paraffins that are present in the fuel begins, but at this stage, the crystals are still small in size, which still allows the fuel to be pumped through the filters, forming only a thin film of n-paraffins on them.

With a further decrease in temperature, the amount of solid phase increases, which is explained by the growth and association of n-paraffin crystals, and also the formation of framework structures. The fluidity of the fuel decreases, which prevents it from further passing through the fuel filters. The temperature at which fuel stops passing through the filters is called the cold filter plugging point (CFPP).

With an even greater decrease in temperature, the fuel finally loses its mobility, the frame structures become more and more branched, the length of n-paraffin chains increases, and the fuel solidifies. The temperature at which this phenomenon occurs is called the pour point (PP).

Diesel fuel has different requirements in different regions of the world. According to the standard [5], in conformity the requirements for CFPP, diesel fuels for temperate climates are divided into 6 grades. Each of the varieties has its own temperature, so for grade A CFPP should not exceed 5 °C, for grade B 0 °C, for grade C −5 °C, for grade D −10 °C, for grade E −15 °C, for grade A −20 °C. According to the CFPP requirements, diesel fuels for cold and arctic climates are divided into 5 classes. So, for class 0, the CP should be no more than −10 °C, the CFPP should not be more than −20 °C; for class 1: −16 °C and −26 °C; for class 2: −22 °C and −32 °C; for class 3: −28 °C and −38 °C, for class 4: −34 °C and −44 °C, respectively. On the territory of the Russian Federation, the requirements for the CFPP of diesel fuels are determined according to [6]. Thus, the CFPP for a summer grade of diesel fuel should not exceed −5 °C, for an inter-season grade −15 °C, for a winter grade −25 °C; for the Arctic −45 °C.

To production, diesel fuel with improved low-temperature properties, processes that improve the PP and CFPP are introduced into its production technology. Such processes include urea and zeolite dewaxing processes, lowering the boiling point of the diesel fraction, blending the diesel fraction with the kerosene fraction, removing n-paraffins from diesel fuel by extractive crystallization, catalytic isomerization, catalytic dewaxing and the using of depressant additives [7,8]. The method of lightening the fractional composition is ineffective in terms of re-sources, since a decrease in the end of boiling leads to a decrease in the volume of diesel fuel production [9,10]. The use of catalytic, zeolite and urea dewaxing leads to the selective extraction of paraffinic hydrocarbons, making it possible to obtain fuel with improved low-temperature properties; however, the effectiveness of the process is highly dependent on the impurities contained in the purified fraction. One of the most economically profitable and promising ways to obtain low-freezing fuels is the involvement of depressants [11]. The principle of operation of the depressant additive is based on reducing the size of n-paraffin crystals in the composition of diesel fuel, changing the morphology of the n-paraffin crystal and providing more favorable crystal forms. The adding of copolymers of a PP depressant changes the lamellar shape of the n-paraffin crystal to a more granular one, forming a microcrystalline structure. The lighter structure of n-paraffin clogs fuel filters at a lower rate, which leads to an improvement in the CFPP, the main criteria for assessing the low-temperature properties of the fuel.

Substances used as depressants to improve PP and CFPP can be divided into polymeric (copolymers of ethylene with polar monomers, polymers based on esters of acrylic and methacrylic acids, polyolefins, copolymers of maleic anhydride with alpha-olefins, alkylphenol-formaldehyde resins) and organic non-polymeric compounds (alkylnaphthalenes, esters of polyhydric alcohols, esters of dibasic acids, nitrogen-containing compounds, nanohybrid additives and petroleum refining residues) [12,13].

The most common additives are mixtures based on copolymers of vinyl acetate and ethylene. Thus, the authors of [14] conducted research on the addition of copolymers of vinyl acetate with ethylene and methacrylate with acrylamide. Additives based on ethylene vinyl acetate copolymer showed the best reduction in PP and CFPP of diesel fuel. The authors of [15] assessed the ability to improve the low-temperature properties of diesel fuels by adding additives from various manufacturers. It has been found that the most effective depressant additives are also copolymers of ethylene and vinyl acetate. The authors of [16] conducted research on the feasibility of adding various substances such as ethanol, toluene, n-heptane and xylene as additives to improve PP and CFPP. It was found that the addition of these substances to diesel fuel did not lead to a significant decrease in the CP of the fuel, but the addition of toluene had the best effect. Some researchers synthesize additives themselves and then introduce them into diesel fuel [17,18,19,20]. For example, the authors of [21] synthesized depressant additives based on heterocyclic compounds of acrylic acid, such as benzoxazolone, benzoxazolethione, benzthiazolone, benzthiazolethione. The resulting polymers were used as depressant additives, the introduction of which in small quantities into diesel fuels led to a significant decrease in the PP and improved fluidity at low temperatures. Some authors are studying the effectiveness of simultaneous introduction of depressants and dispersants into diesel fuel. Thus, the authors of [22,23] found that this experiment improves the low-temperature properties of diesel fuel, including sedimentation stability, and also leads to a synergistic action of the components of the complex depressant additive. In the work of [24] studied the results of the influence of sixteen different commercial depressants on PP and CFPP on commercial and straight-run diesel fuel. The authors found that the reduction in CFPP depends not only on the selected additive and its concentration, but also on the composition of the fuel. The authors of work [25], together with depressant and depressant-dispersant additives, introduced light delayed coking gasoil, heavy diesel fractions from vacuum columns of primary oil refining plants, and heavy diesel fractions from an atmospheric column into diesel fuel and vacuum distillates of various fractional compositions. According to the test results, it was noted that this method significantly reduces the PP of the diesel fuel and gives high, positive depression indicators. In [26], the authors studied the composition of diesel fuel before adding various depressant additives to it. It has been established that the same additive works differently in fuels of different compositions. The authors of [27] note the influence of the fractional composition of diesel fuel on the effectiveness depressant additive. And in work [28], the structural-group composition of three winter diesel fuels was studied and its influence on the low-temperature properties of the fuels was considered. The influence of the component composition of diesel fuels on the effectiveness of anti-wear and depressant-dispersant additives is noted and it is shown that a high content of saturated hydrocarbons, primarily medium-molecular n-alkanes and arenes with a higher proportion of substitution, leads to a deterioration in low-temperature properties. The works [29] presents the results of the depressant additives effectiveness on the diesel fuel low-temperature properties study, using the summer fuel sample. The authors found that the depressant additives effectiveness depends on the total content of n-paraffin hydrocarbons in diesel fuel. In the works of authors [30,31,32,33] a hypothesis was put forward about the existence of an optimal concentration of n-paraffin hydrocarbons in diesel fuel, within which the greatest effect of the depressant additive is achieved.

As a literature review has shown, there are many studies devoted to the synthesis of new depressant additives or comparison of their effectiveness. In recent years, more and more researchers are paying attention to the composition of diesel fuel as a factor determining the effectiveness of depressor additives. An important practical consequence of these studies is the possibility of increasing the efficiency of depressant additives by modifying the composition of diesel fuel. The addition of substances that enhance the action of depressant additives can make it possible to produce fuels with improved low-temperature properties without significant costs for technological conversions. In addition, these studies make a significant contribution to understanding the mechanism of interaction between substances included in depressant additives and compounds that make up diesel fuels.

Therefore, it is extremely important to identify patterns and develop ways to in-crease the depressants effectiveness by adjusting the composition of diesel fuel, and the purpose of this work is to study the effect of adding n-paraffins on the depressant additives effectiveness.

2. Materials and Methods

The objects of study in the work are:

• 2 samples of straight-run diesel fuel (DF): F₁, F₂, obtained from atmospheric distillation units of oil from various fields in Western Siberia;
• 2 samples of n-paraffins: P₁—n-paraffins separated from heavy gasoil, obtained as a residue of atmospheric distillation of oil at an oil refinery and P₂—n-paraffins separated from highly paraffinic oil fraction;
• 2 samples of commercial depressants: A₁, A₂. According to information from the manufacturer, the type of active component of the additives is the copolymers of vinyl acetate and ethylene.

The following techniques were used to determine the properties and composition of the samples:

• CP was determined according to the method presented in [34];
• CFPP was determined according to the method presented in [35];
• PP was determined according to the method presented in [36];

The method for determining CP and PP is as follows: a double-walled test tube filled with the test sample is immersed in a liquid low-temperature thermostat and gradually cooled. Every 1–2 °C, the test tube is removed from the thermostat and compared with a previously prepared test fuel sample at room temperature. If there is visible turbidity of the fuel relative to the standard, CP is recorded. With a further decrease in temperature, the fuel solidifies; PP is fixed when, the test tube tilting at 45° and holding it in this position for 1 min, the meniscus of the sample does not shift.

The method for determining CFPP is as follows: in a low-temperature liquid thermostat, using a CFPP determination unit on a cold filter, the sample under study was gradually cooled. At intervals of 1 °C, the test sample was automatically pumped through a standardized wire mesh filter into a glass pipette under a controlled vacuum. If the test sample did not have time to fill the pipette within 60 s, then the experiment was stopped and CFPP was recorded, because fuel has lost the ability to pass through the filter.

• The content of n-paraffins was determined by gas chromatography (chromatograph “Chromatek-Kristall 2000” with a quartz capillary column 30 m × 0.25 mm, stationary phase—SE-54, carrier gas—helium).

The essence of the method is the chromatographic separation of liquid hydrocarbon blends on a capillary column with a non-polar stationary phase, followed by registration of hydrocarbons with a flame ionization detector and automated processing of the obtained information using software.

A sample of the liquid hydrocarbon blend is introduced into a gas chromatograph equipped with a quartz capillary column containing a stationary phase. Under the influence of a carrier gas, the sample passes through a column in which its components are separated. The components are detected by a flame ionization detector as they elute from the column. The detector signal is processed by an electronic data acquisition system or an integrating computer. Each resulting peak is identified by retention index or visual comparison with standard chromatograms.

3. Experimental

3.1. N-Paraffins Separation

N-paraffins were separated according to the method presented in [37].

The essence of this method is the removal of polycycloaromatic hydrocarbons (PCA) and heavy aromatic hydrocarbons from petroleum products using extraction and adsorption methods and further freezing of n-paraffins at a temperature of −20 °C. The technique contains two main parts: the separated of n-paraffins and their freezing.

N-paraffins separation. For this stage, the following was previously carried out: calcination of the silica gel to remove moisture from it; removal of aromatic hydrocarbons from the original sample by extraction using petroleum ether and subsequent filtration of the mixture obtained as a result of extraction. The separation process was carried out using an adsorption column filled with prepared silica gel and a washing solution. When the resulting filtrate passes through the column, the remaining heavy aromatic hydrocarbons and n-paraffins are adsorbed on silica gel; the latter were removed from the column using a washing solution. At the end of the separation, the washing solution was removed from the resulting filtrate using simple distillation.

N-paraffins freezing. For freezing, the following was previously carried out: preparing a cooling bath (installing funnels with Schott filters in the bath, filling the bath with chilled alcohol, placing Bunsen flasks under the bath and connecting a pump to them) and preparing a washing solution for purifying n-paraffins from oils. Next, the distillation residue along with the washing solution was added in portions to the funnel with the filter, and as a result of cooling, n-paraffins are released from the liquid in the funnel, and the washing solution containing oils flows from the filter into the Bunsen flask under the action of a pump. Then the cooled alcohol was removed from the bath and the bath was gradually heated with water to melt the white mass of n-paraffins obtained on the filter. In this case, n-paraffins flow from the filter into the flask. The remaining n-paraffins on the filter were washed off with toluene. Finally, the resulting mass of n-paraffins was dried in a fume hood to remove residual solvent.

A diagram of the main stages of n-paraffins separation from petroleum products is shown in Figure 1.

3.2. Blends Preparation

In the course of the study, blends of straight-run DF samples and depressant additives (FA), as well as blends of straight-run DF samples, n-paraffins and depressant additives (FPA) were prepared.

N-paraffins were added to the blends at concentrations of 0.50/0.25/0.10 and 0.05% wt.

The depressant additives were used in the concentrations recommended by the manufacturers: A₁ additive at a concentration of 0.26 mL per 100 mL of DF; additive A₂ at a concentration of 0.30 mL per 100 mL of DF.

To prepare the blend, 100 mL of a diesel fuel sample was taken using a graduated cylinder; the flask with the test sample was closed using a stopper with a thermometer so that the thermometer was immersed in the diesel fuel and, at the same time, did not touch the bottom and walls of the flask.

Next the flask was placed in a liquid thermostat and withstanded to a temperature of 25 °C for 30 min, stirring occasionally. Upon reaching the specified temperature, a depressant additive was introduced into the diesel fuel sample.

The blend was stirred for 1 min, placed in a thermostat and then thermostatted for 10 min. The resulting blend was left for 1 day in a cold, dark place.

4. Results and Discussion

4.1. Results of Determining the Low-Temperature Properties of the Studied Samples

Properties and composition of the studied straight-run DF samples are presented in Appendix A.

The results of determining the low-temperature properties of the studied samples of straight-run DF, as well as blends of straight-run DF with depressant additives, are presented in

Table 1. Low-temperature properties of straight-run DF with depressant additives blends: ↓ is decrease.

Blend	CP	CFPP	PP	Δ CP	Δ CFPP	Δ PP
	°C
F1	−4	−5	−16	–
F2	0	0	−8	–
F1A1	−5	−25	−42	1↓	20↓	26↓
F1A2	−4	−10	−26	0	5↓	10↓
F2A1	−2	−16	−37	2↓	16↓	29↓
F2A2	0	−2	−37	0	2↓	29↓

: Δ are presented in comparison with pure samples of straight-run DF without the involvement of depressants.

According to requirements [5,6] by CFPP, a sample of straight-run DF F₁ corresponds to summer brand of DF. A sample of straight-run DF F₂ does not meet the requirements [5,6] for low-temperature properties. As shown by the results presented in Table 1, the use of the investigated DF samples in cold climatic conditions is impossible without the use of depressants. The unsatisfactory low-temperature properties of samples F₁ and F₂ can be explained by the fact that the samples are straight-run diesel fuel obtained by direct distillation of oil and do not contain any additives [8,10].

From the results presented in Table 1 it can be seen:

• the addition of both depressants for both samples of straight-run DF resulted in an improvement in CFPP and PP;
• the addition of the A₁ depressant is on average more effective than the addition of the A₂ depressant;
• for a sample of straight-run DF F₁, the addition of a depressant A₁ allows to obtain fuel of winter brand, in accordance with the requirements [5,6]; the addition of A₂ depressant does not change the DF brand (summer brand according to the requirements [5,6]);
• for a sample of straight-run DF F₂, the addition of a depressant A₁ allows to obtain fuel inter-season brand, in accordance with the requirements [5,6]; the addition of A₂ depressant does not allow obtaining fuel that meets the requirements [5,6].

Next, the low-temperature properties of blends of straight-run DF samples with n-paraffins P₁ and depressants were determined (

Table 2. Low-temperature properties of straight-run DF samples with n-paraffins P₁ and depressants blends.

Content of N-Paraffins, % wt.	Blend	CP	CFPP	PP
		°C
0.50	F1A1P1	−1	−31	−39
	F1A2P1	1	−1	−21
	F2A1P1	2	−20	−36
	F2A2P1	3	−6	−31
	F2A1P2	3	−19	−37
0.25	F1A1P1	−1	−28	−39
	F1A2P1	0	−5	−21
	F2A1P1	2	−19	−37
	F2A2P1	2	−12	−31
	F2A1P2	2	−21	−38
0.10	F1A1P1	−1	−26	−39
	F1A2P1	−1	−8	−22
	F2A1P1	1	−17	−37
	F2A2P1	0	−14	−35
	F2A1P2	−1	−21	−42
0.05	F1A1P1	−3	−23	−39
	F1A2P1	−1	−11	−26
	F2A1P1	−1	−12	−37
	F2A2P1	0	−18	−36
	F2A1P2	−1	−23	−42

).

From the results presented in Table 2 it’s clear that:

• adding n-paraffins P₁ at a concentration of 0.50% wt. to the F₁A₂ blend does not allow obtaining fuel that meets the requirements [5,6] for all brands;
• adding n-paraffins P₁ in concentrations of 0.05–0.25% wt. to the F₁A₂ blend, at concentrations of 0.10–0.50% wt. to a blend of F₂A₂, as well as at a concentration of 0.05% wt. to the F₂A₁ blend allows to obtain fuel that meets the requirements for low-temperature properties [5,6] for summer brand of DF;
• adding n-paraffins P1 at a concentration of 0.05% wt. to blends F₁A₁ and F₂A₂, as well as in concentrations of 0.10–0.50% wt. to the F₂A₁ blend allows to obtain fuel that meets the requirements for low-temperature properties [5,6] for winter-season brand;
• adding n-paraffins P₂ in concentrations of 0.05–0.50% wt. to the F₂A₁ blend allows to obtain fuel that meets the requirements for low-temperature properties [5,6] for inter-season brand;
• adding n-paraffins P₁ in concentrations of 0.10–0.50% wt. to the F₁A₁ blend allows to obtain fuel that meets the requirements for low-temperature properties [5,6] for winter brand.

The obtained results are consistent with the results of studies about changes in the composition of diesel fuel [28,29,33].

4.2. Results of Determining the Composition of Separated N-Paraffins

Results of determining the composition of separated n-paraffins P₁ and P₂ presented in

Table 3. Hydrocarbon composition of P₁ and P₂ n-paraffins.

Number of Carbon Atoms	Content, % wt.
	P1	P2
C17–C22	4.247	0.059
C23	8.668	0.018
C24	16.050	0.042
C25	19.269	0.168
C26	19.533	0.916
C27	14.048	4.417
C28	9.408	13.466
C29	6.329	25.314
C30	2.448	23.539
C31	0.000	15.258
C32	0.000	11.658
C33–C34	0.000	5.143

.

As can be seen from the results presented in Table 3, as part of n-paraffins separated from vacuum gasoil (P₁) are dominated hydrocarbons with the number of carbon atoms from 24 to 27, while in the composition of n-paraffins separated from the highly paraffinic oil fraction (P₂)—hydrocarbons with the number of carbon atoms from 28 up to 31. Thus, it can be concluded that P₂ n-paraffins are heavier than P₁ n-paraffins.

4.3. Analysis of the Effect of Adding N-Paraffins on the Depressant Additives Effectiveness

Effects of n-paraffins P₁ and P₂ addition, on the low-temperature properties of straight-run DF F₁ and F₂ with depressants A₁ and A₂ blends, are presented in

Table 4. Influence of adding n-paraffins to DF blends on the depressants effectiveness: ↓ is decrease, ↑ is increase.

Content of N-Paraffins, % wt.	Blend	CP	CFPP	PP
		°C
0.50	F1A1P1	4↑	6↓	3↑
	F1A2P1	4↑	4↓	1↑
	F2A1P1	5↑	9↑	5↑
	F2A2P1	3↑	4↓	6↑
	F2A1P2	5↑	3↓	0
0.25	F1A1P1	4↑	3↓	3↑
	F1A2P1	4↑	3↓	0
	F2A1P1	4↑	5↑	5↑
	F2A2P1	2↑	10↓	6↑
	F2A1P2	4↑	5↓	1↓
0.10	F1A1P1	4↑	1↓	3↑
	F1A2P1	3↑	1↓	0
	F2A1P1	3↑	2↑	4↑
	F2A2P1	0	12↓	2↑
	F2A1P2	1↑	5↓	5↓
0.05	F1A1P1	2↑	2↑	3↑
	F1A2P1	1↑	4↑	0
	F2A1P1	3↑	1↓	0
	F2A2P1	0	16↓	1↑
	F2A1P2	1↑	7↓	5↓

: Δ presented in comparison with blends F₁A₁ and F₂A₁ without the addition of n-paraffins.

As we can see from the data presented in Table 4 adding n-paraffins P₁ to blends F₁A₁ and F₂A₁ leads to:

• deterioration of the depressant effectiveness in relation to CP (by 2–4 °C for a blend F₁A₁ and 1–4 °C for the blend F₂A₁), no change or deterioration in the depressant effectiveness in relation to PP (by 3 °C for the blend F₁A₁ and at 0–1 °C for the blend F₂A₁);
• at concentrations of 0.10–0.50% wt.—improving the depressant effectiveness in relation to CFPP (by 1–6 °C for the blend F₁A₁ and 1–4 °C for the blend F₂A₁); at a concentration of 0.05% wt.—deterioration of the depressant effectiveness in relation to CFPP (by 2 °C for the blend F₁A₁ and 4 °C for the blend F₂A₁).

In addition, it can be seen that with a decrease in the concentration of added n-paraffins weaken both the negative effect on the depressant effectiveness in relation to CP and PP, and the positive effect on the depressant effectiveness in relation to CFPP.

Also from the data provided in Table 4, it can be seen that adding n-paraffins P₁ to blends F₁A₂ and F₂A₂ leads to:

• no change or deterioration in the depressant effectiveness in relation to CP (by 3–5 °C for the blend F₁A₂ and at 0–3 °C for the blend F₂A₂), no change or deterioration in the depressant effectiveness in relation to PP (by 0–5 °C for a blend F₁A₂ and 1–6 °C for the blend F₂A₂);
• for blend F₁A₂: at concentrations of 0.10–0.50% wt.—deterioration of the depressant effectiveness in relation to CFPP (by 2–9 °C); at a concentration of 0.05% wt.—improving the depressant effectiveness in relation to CFPP (by 1 °C);
• for blend F₂A₂: in all concentrations—to improve the depressant effectiveness in relation to CFPP (by 3–7 °C).

In addition, it can be seen that with a decrease in the concentration of added n-paraffins:

• for all blends, the negative effect on the depressant effectiveness in relation to CP and PP is weakened;
• for blends with an additive A₁ the positive effect on the depressant effectiveness in relation to CFPP is weakened;
• for blends with an additive A₂ the positive effect on the depressant effectiveness in relation to CFPP is enhanced.

Also, from the data presented in Table 4, it can be seen that the addition of P₂ n-paraffins to the F₂A₁ blend leads to:

• deterioration of the depressant effectiveness in relation to CP (at 1–4 °C);
• no change or improvement in the depressant effectiveness in relation to PP (by 0–5 °C);
• improving the depressant effectiveness in relation to CFPP (by 3–7 °C).

In addition, it can be seen that with a decrease in the concentration of added n-paraffins weaken the negative effect on the depressant effectiveness with respect to CP, and the positive effect on the depressant effectiveness with respect to CFPP and PP is enhanced.

The revealed regularities are explained in the mechanism of depressants action: the introduction of additional heavy n-paraffins creates initial crystallization centers, triggers the action of a depressant, which prevents the growth and coarsening of these crystals, allowing fuel at more negative temperatures to pass through a standard filter element (improving the CFPP). A decrease in the positive effect with respect to CFPP with a decrease in the concentration of added n-paraffins is due to the fact that at low concentrations of added high-melting hydrocarbons, the number of initial crystallization centers is insufficient, the effect described earlier is leveled.

Regularities of a negative nature in relation to the effectiveness of the additive action on CP and PP are associated with the fact that the added heavy n-paraffins themselves are characterized by unsatisfactory low-temperature properties and, upon adding, determine (worsen) the CP and PP of blends. In addition, depressants are aimed at limiting the growth and aggregation of large crystals of n-paraffins, however, they do not prevent their appearance, as a result of which, with a decrease in temperature, the precipitated crystals of n-paraffins solidify and limit the fluidity of DF (deterioration of PP). With a decrease in the concentration of added high-melting hydrocarbons, the described effects with respect to CP and PP, similarly, are leveled.

Difference in the regularities of the influence of the n-paraffins addition on the depressant additives A₁ and A₂ effectiveness can be explained as follows: the additive A₂ is weaker than the additive A₁ (Table 1) and in case of adding n-paraffins in high concentrations, its capabilities are not enough to simultaneously stop the growth of all crystallization centers, the effect of the additive is stronger at low concentrations of added high-melting hydrocarbons.

The difference in the regularities of the influence of the n-paraffins P₁ and P₂ addition on the depressant additives effectiveness can be explained as follows: n-paraffins P₂ are heavier than n-paraffins P₁ (Table 3), with the introduction of n-paraffins P₂ the formation of initial crystallization centers will begin at less negative temperatures, the depressant will begin to act earlier, thereby providing a more finely dispersed fuel structure with smooth cooling, which will lead to an improvement not only in the CFPP, but PP of fuel. Strengthening the effect with decreasing concentration of n-paraffins is explained by an increase in the number of additive molecules interacting with each initial crystallization center with a slight reduction in their number.

5. Conclusions

• It was found that the addition of n-paraffins to blends of DF with depressants in low concentrations enhances the depressants effectiveness in relation to CFPP. It is shown that the effect is observed for DF of various compositions, various depressants, and also n-paraffins of various compositions.
• It is shown that the optimal concentration of added n-paraffins depends on the depressant additive effectiveness—for less effective (weak) additives, a lower concentration of n-paraffins is required.
• It was revealed that the positive effect of n-paraffins on CFPP increases with the heavier added n-paraffins.
• Recommendations developed to obtain DF with improved low-temperature properties and enhance the depressant additives effectiveness by adding low concentrations of n-paraffins: for a sample of straight-run DF F₁, it is recommended to use a blend of fuel, depressant A₁ and 0.50% wt. n-paraffins P₁; for a sample of straight-run DF F₂, it is recommended to use a blend of fuel, depressant A₂ and 0.50% wt. n-paraffins P₁ or a blend of fuel, depressant A₁ and 0.05% wt. n-paraffins P₂.

The indicated recommendations make it possible to obtain DF of inter-season with a large margin of quality in terms of CFPP. The revealed patterns and the recommendations developed will allow increasing the production of low-freezing DF brands, and also offer a resource-efficient option for using heavy gas oil fractions.

The work describes for the first time a method for increasing the effectiveness of depressant additives by modifying the composition of diesel fuel (adding a small amount of n-paraffins). The results obtained in the work contribute to expanding the understanding of the mechanism of interaction between diesel fuel hydrocarbons and the active ingredients of depressant additives.