The influence of feed space velocity and pressure on the cold flow properties of diesel fuel

1 Introduction

Diesel fuel is widely utilized in passenger vehicles, buses, tractor-trailers, farm machinery, locomotive engines, boats, power generators, and other high-power equipment. The primary performance indicators of diesel fuel serve to assess its quality and suitability for use in diesel engines. These indicators evaluate fuel efficiency, safety, emissions, and overall performance.

Among these indicators, cold flow properties (CFPs) are particularly critical, as they determine the fuel’s ability to remain fluid and avoid solidification under low ambient temperatures. Favorable CFPs ensure proper fuel system operation and prevent clogging caused by wax crystal formation.

In the modern oil and gas industry, a range of refining processes is employed to improve or modify the properties of crude oil fractions. These processes include isomerization, dearomatization, deasphalting, and dewaxing. Dewaxing is a key process designed to remove normal paraffins from kerosene-gas oil and other middle distillate fractions to produce winter-grade diesel fuels. This process reduces the pour point (PP) of the fuel, improving its resistance to cold climates and minimizing damage to fuel systems due to ice formation. Normal paraffins, which have high pour points, are removed to achieve this improvement [1]. Additionally, dewaxing enhances the operational characteristics of diesel fractions, including their PP, cloud point, cold filter plugging point (CFPP), cetane number, and cetane index. It is important to note that, in addition to the low-temperature properties, other characteristics of diesel fuel play a significant role. For example, higher heating values enhance engine power and fuel economy. Viscosity, density, cetane number, and heating value influence combustion efficiency, while flash point and vapor pressure are critical for fuel safety [2,3,4,5].

For both fuels and lubricating oils, several additional parameters are also crucial. For fuels, thermal stability is of particular importance, as higher thermal stability reduces coke formation during combustion, which in turn protects metal components. For lubricating oils, key parameters include volatility and viscosity. Dewaxing reduces oil volatility at high temperatures during engine operation, thereby minimizing vapor formation. Simultaneously, it allows for viscosity control, ensuring that it remains within an optimal range. Low viscosity may fail to provide adequate lubrication, leading to increased wear, while excessive viscosity can result in higher energy losses during engine operation.

Currently, two main dewaxing methods are employed: solvent dewaxing (typically with urea) and catalytic dewaxing.

Solvent dewaxing, which has been in use since the mid-20th century, often relies on aqueous urea solutions [6]. This method exploits the ability of urea (NH₂CONH₂) to form crystalline complexes with normal paraffins. These complexes, known as clathrates, consist of urea molecules forming a hexagonal spiral structure held together by hydrogen bonds. The spiral forms channels of fixed diameter (0.5–0.6 nm), accommodating normal paraffin molecules but excluding branched paraffins, aromatic hydrocarbons, and cycloalkanes due to their larger effective diameters. The clathrate precipitates at room temperature and is subsequently heated to 80°C to release the paraffins. The recovered urea is then recycled. Despite its historical significance, solvent dewaxing has largely been replaced by catalytic methods in modern refining processes [7].

Catalytic dewaxing, a form of hydrocracking, is widely employed to improve the CFPs of middle distillates and lubricants by selectively cracking normal paraffins. The process involves the use of catalysts and specific reaction conditions, including temperatures of 200–480°C and pressures of 0.7–7.5 MPa, depending on the catalyst and feedstock. When conducted in the presence of hydrogen, this method is referred to as hydrodewaxing.

An alternative approach, such as Chevron’s isodewaxing process, relies on isomerization rather than hydrocracking. Isomerization converts normal paraffins into isoparaffins, which have lower melting points. This improves the PP and viscosity of middle distillates and oils, the cloud point of diesel fuel, and the freezing point of jet fuel [8].

Until the late 20th century, catalytic dewaxing primarily focused on hydrocracking reactions to lower the PP of feedstocks. However, hydrocracking of n-paraffins to light hydrocarbons often results in significant yield losses of the target product. In contrast, isodewaxing enhances CFPs through hydroisomerization reactions of n-paraffins, where the resulting isoparaffins are retained in the target product. Hydroisomerization involves converting higher n-alkanes in hydrocarbon fractions into branched hydrocarbons [9,10,11,12,13]. This process aligns with green chemistry principles by enabling the efficient conversion of diesel fraction components into value-added products while minimizing by-products.

Optimizing diesel fuel production is a complex challenge influenced by feedstock composition, process conditions, catalyst performance, and other factors [14]. The authors developed a mathematical model for diesel fuel hydroisomerization, demonstrating that higher temperatures favor the endothermic dehydrogenation of n-paraffins, a key intermediate step in hydroisomerization. Conversely, increased pressure enhances the conversion of high-molecular-weight n-paraffins via hydrocracking and promotes the transformation of low-molecular-weight n-paraffins into isoparaffins due to higher hydrogen partial pressure.

Bifunctional catalysts are widely used in n-alkane hydroisomerization [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. Previously, the authors [29,30] developed bifunctional catalysts based on mesoporous aluminosilicate (MAS) and bentonite (from the Tagan field, Kazakhstan) as supports, with nickel, molybdenum, and a Ni–Mo bimetallic system as promoters. These catalysts were tested in the hydroisodewaxing of n-hexadecane (a model compound) and diesel fractions. For the model compound, maximum iso-paraffin yields of 44–46 wt% and selectivities of 91–95% were achieved using Ni/MAS-bentonite and Mo/MAS-bentonite catalysts at 320°C, atmospheric pressure, and a feed space velocity of 0.8–0.9 ml/min. Testing these catalysts in diesel fraction hydroisodewaxing showed that the 5% Ni–1% Mo/MAS-H-bentonite catalyst provided a maximum diesel fraction yield of 97.4% at 320°C, 2 MPa pressure, and a feed space velocity of 1 h⁻¹. The main distinction of this study from the latest research [31,32,33,34] in the field lies in the fact that most studies focus on microporous zeolites (such as ZSM-5) or SAPO as the basis. Zeolitic materials with pore sizes in the microporous range experience diffusion limitations. In contrast, mesopores facilitate faster mass transfer and act as “molecular highways.” Transport limitations negatively affect the activity and selectivity of adsorbents and catalysts and also reduce their service life. One promising approach to addressing this issue is either reducing the diffusion path or enhancing diffusion efficiency within the pores, which can be effectively achieved through the use of mesoporous materials.

Additionally, in most cases, the use of promoting additives includes not only transition metals but also rare-earth or noble metals, significantly increasing catalyst costs. To reduce the final cost of catalysts, this study utilized a combination of MAS and bentonite as the acidic component of the catalyst instead of solely using MAS. Bentonite serves as a binder (between MAS and metals) and also influences the acidity of the catalyst, as confirmed by analyses such as temperature-programmed desorption of ammonia (TPD-NH₃), diffuse reflection infrared Fourier transform (DRIFT) spectroscopy, and others. Consequently, the final cost of the catalyst is significantly lower, as the MAS-to-H-bentonite ratio of 35:65 allows for a more economical use of the costly MAS while maintaining competitive production costs compared to existing counterparts.

It is noteworthy that Kazakhstan has substantial reserves of bentonite, making it possible to rationally utilize domestic resources. The choice of nickel, molybdenum, and the bimetallic Ni–Mo system as promoting additives was driven by the fact that nickel can compete with platinum- and palladium-based catalysts in terms of hydrogenation activity and imparts dehydrogenation properties to the catalyst. At the same time, molybdenum demonstrates high resistance to poisoning by sulfur- and nitrogen-containing compounds. The combination of nickel and molybdenum endows their mixture with the ability to facilitate both homolytic and heterolytic reactions while exhibiting resistance to the poisoning effects of sulfur- and nitrogen-containing compounds present in petroleum feedstock [35].

This study focuses on the influence of feed space velocity and pressure on diesel fuel performance characteristics. Experiments were conducted at 320°C, as identified as optimal in prior research, while varying feed space velocity and pressure. The relationships between process parameters and the CFPs of the diesel fraction were evaluated, providing further insights into optimizing the hydroisodewaxing process.

2 Methods

The synthesis methods of bifunctional catalysts, along with techniques for analyzing their physicochemical properties, were thoroughly detailed in previously published materials [29,30]. These include low-temperature N₂ adsorption/desorption, X-ray diffraction (XRD), TPD-NH₃, temperature-programmed reduction of hydrogen (TPR-H₂), DRIFT spectroscopy, and Fourier transform infrared spectroscopy. Additionally, methods for assessing the composition and physicochemical characteristics of raw materials and process products were covered, such as chromatography–mass spectrometry, pour point determination of diesel fractions, CFPP analysis, fractional composition analysis, and sulfur content determination. The testing of catalysts in a flow-through unit and the evaluation of experimental errors were also discussed [29].

The process of hydroisodewaxing of diesel fraction was carried out on a flow-type unit (Figure 1) with a fixed catalyst bed at a temperature of 320°C; feed rates were 1–3 h⁻¹ and pressure 0.5–2.0 MPa. The volume ratio H₂:feedstock = 100 was controlled using a flow meter. In this study and reactor, the process takes place in the presence of hydrogen and in the absence of oxygen. Oxygen from the air is used during catalyst regeneration. After 5–6 h of catalyst operation, oxidative regeneration is carried out using air at a temperature of 500°C for 5 h. To determine the yield of the diesel fraction, the products obtained as a result of hydroisodewaxing were stabilized in order to remove low-boiling products.

Figure 1

Schematic diagram of the laboratory catalytic unit.

3 Results and discussion

CFPs of diesel fuel directly depend on its chemical composition, in particular, on the content of various hydrocarbons: paraffins, iso-alkanes, naphthenes, and aromatic compounds. In this article, the authors consider two main low-temperature characteristics – the CFPP and the PP of diesel fuel. The choice was made based on the fact that the maximum filterability temperature is of great importance for drivers, since even if the fuel remains fluid, paraffin crystals may already appear, and it becomes more viscous and clogs filters. As the temperature drops, crystallization worsens and goes into the second stage of freezing. It is this intermediate stage that is characterized as CFPP – an indicator that says at what lowest temperature the car will start on frosty days. It shows the temperature limit of diesel fuel passing through the filter. The second important characteristic, the PP, is of great practical importance when transporting and pumping fuel from storage sites. The PP indicates a complete loss of mobility. In addition, the PP is used as an evaluation parameter when calculating the cost of pumping and technical and economic indicators of pipeline networks. The content of n-paraffins is crucial for both low-temperature characteristics. The higher the paraffin content in diesel fuel, the higher the temperatures at which it solidifies and will start on cold days of the year.

The content of paraffinic hydrocarbons in diesel fuel can be controlled through the conditions of the dewaxing process, particularly the hydroisomerization reaction. Hydroisomerization reactions aim to convert linear hydrocarbons into branched isomers. The reaction conditions, including temperature, pressure, catalyst composition, and catalyst volume, determine the yield, composition, and performance characteristics of the final product. The influence of temperature and catalyst composition on diesel fuel performance was explored in previous studies by the authors [29,30], which identified 320°C as the optimal temperature for the process. This article examines the impact of feed space velocity (0.5–2.0 h⁻¹) and pressure (1–3 MPa) on the CFPP and PP, which depend on the group hydrocarbon composition of the reaction products. Optimizing hydroisomerization conditions is crucial for producing diesel fuel with improved CFPs. The best catalysts in the authors’ series were selected for the process: among the nickel-based catalysts, 5% Ni/MAS-H-bentonite, among the molybdenum-based catalysts, 1% Mo/MAS-H-bentonite, and the bimetallic catalyst 5% Ni–1% Mo/MAS-H-bentonite [30]. The results were compared with the initial parameters and the group hydrocarbon composition of the diesel fraction before the hydroisodewaxing process [30].

The graph (Figure 2) illustrates the relationship between feed space velocity (h⁻¹) and three key parameters: PP, CFPP, and the yield of the diesel fraction (%). The hydroisomerization process over a Ni/MAS-H-bentonite catalyst demonstrates a significant improvement in the operational properties of the diesel fraction compared to the initial feedstock (Table 1).

Figure 2

Dependence of diesel fuel’s CFPP, PP, and the yield of the diesel fraction after hydroisodewaxing process on Ni/MAS-H-bentonite on the feed space velocity.

However, as the feed space velocity increases, the PP and CFPP values deteriorate. At a feed space velocity of 2 h⁻¹, the PP remains unchanged. Conversely, a critical parameter in the process is the yield of the diesel fraction, which consistently increases with higher feed space velocity.

The optimal balance between improved CFPs and diesel fraction yield on the nickel-based catalyst is achieved at a feed space velocity of 1 h⁻¹. Under these conditions, the PP is −27°C, the CFPP is −31°C, and the diesel fraction yield reaches 95.4%.

The graph (Figure 3) illustrates the relationship between pressure (MPa) and three key parameters: PP, CFPP, and the yield of the diesel fraction (%). In this case, an inverse correlation is observed: as pressure increases, the yield of the diesel fraction decreases, whereas the operational properties are improved across all pressure values, achieving optimal levels at 2 MPa.

Figure 3

Dependence of diesel fuel’s CFPP, PP, and the yield of the diesel fraction after hydroisodewaxing process on Ni/MAS-H-bentonite on the pressure.

Thus, at the optimal pressure for conducting the hydroisomerization process over the Ni/MAS-H-bentonite catalyst, the following parameters are achieved: PP = −30°C, CFPP = −31°C, and the diesel fraction yield = 95.4%.

The identified trends regarding the effects of feed space velocity and pressure on CFPs and the yield of the diesel fraction during the hydroisomerization of diesel fractions over a nickel-based catalyst are also observed when using a molybdenum-containing catalyst (Figures 4 and 5).

Figure 4

Dependence of diesel fuel’s CFPP, PP, and the yield of the diesel fraction after hydroisodewaxing process on Mo/MAS-H-bentonite on the feed space velocity.

Figure 5

Dependence of diesel fuel’s CFPP, PP, and the yield of the diesel fraction after hydroisodewaxing process on Mo/MAS-H-bentonite on the pressure.

The optimal process conditions at which a balance between improved PP and CFPP values and the diesel fraction yield is achieved are as follows: feed space velocity = 1 h⁻¹ and pressure = 2 MPa. Under these conditions, the process achieves a PP of −38°C, a CFPP of −38°C, and a diesel fraction yield of 85.2%.

Similar to monometallic catalysts, the optimal conditions for the hydroisomerization process over a bimetallic Ni-Mo/MAS-H-bentonite catalyst are a feed space velocity of 1 h⁻¹ and a pressure of 2 MPa (Figures 6 and 7). Under these conditions, the process achieves a PP of −36°C, a CFPP of −33°C, and a diesel fraction yield of 97.4%.

Figure 6

Dependence of diesel fuel’s CFPP, PP, and the yield of the diesel fraction after hydroisodewaxing process on Ni-Mo/MAS-H-bentonite on the feed space velocity.

Figure 7

Dependence of diesel fuel’s CFPP, PP, and the yield of the diesel fraction after hydroisodewaxing process on Ni-Mo/MAS-H-bentonite on the pressure.

Comparing the performance characteristics of the hydroisodewaxing process of diesel fraction (Table 2) using the synthesized bifunctional catalysts (Mo/MAS-H-bentonite, Ni/MAS-H-bentonite, and Ni–Mo/MAS-H-bentonite), it can be concluded that the molybdenum-based catalyst provides the best low-temperature properties, exhibiting the lowest CFPP and PP values. However, the diesel fraction yield does not exceed 88.6%. In contrast, the Ni–Mo/MAS-H-bentonite catalyst offers an optimal balance between low-temperature performance and diesel fraction yield, outperforming the monometallic catalysts in this regard.

The hydroisodewaxing process on all the studied bifunctional catalysts proceeds in two main directions: hydroisomerization and hydrocracking. As shown in Tables 3 and 4, increasing the feed space velocity from 0.5 to 2.0 h⁻¹ and the process pressure from 1 to 3 MPa results in a decrease in the concentration of higher long-chain paraffin hydrocarbons. This can be attributed to the increased rate of the hydrocracking reaction, leading to a higher number of paraffins undergoing cracking. Additionally, an increase in pressure leads to a slight rise in the concentration of naphthenes in the reaction products, which is likely due to the hydrogenation of monoaromatic hydrocarbons.

However, when the process pressure is raised to 3.0 MPa, the performance characteristics of the resulting products are negatively affected, possibly due to the increased concentration of hydrogen molecules. It is understood that n-paraffins undergo the first stage of hydroisomerization on the metal sites of the catalyst, where they are dehydrogenated to olefins. These olefins are then isomerized at the acid sites of the catalyst after being protonated to form carbocations.

Considering these findings, it can be concluded that a decrease in pressure accelerates the hydroisomerization process by enhancing the dehydrogenation of n-paraffins to olefins. Therefore, the hydrodewaxing process of the diesel fraction using the synthesized bifunctional catalysts is recommended to be conducted at a pressure of 2 MPa for optimal performance.

The proposed reaction mechanism for the dewaxing process of diesel fuels was developed based on experimental and literature data [36,37]. Since the feedstock is not a model compound but a mixture of hydrocarbons, including each individual component in the reaction scheme is impractical. Therefore, the mechanism was proposed using n-heptadecane as a representative example, as its content predominates over other n-paraffins in the diesel fraction according to hydrocarbon composition analysis of the feedstock.

The analysis of the hydrocarbon group composition in the feedstock and its products indicates that the key and competing reactions in the hydroisodewaxing process of the diesel fraction are hydroisomerization and hydrocracking. Other possible reactions, such as dearomatization and cyclization, have a negligible contribution and were therefore not included in the proposed reaction mechanism scheme for the dewaxing process on the developed catalysts (Figure 7). In this study, the reaction mechanism and hydrocarbon composition of the fuel before and after the process were hypothesized based on literature. Therefore, the primary focus of this study is on low-temperature properties and product yields.

According to Figure 8, depending on the catalyst, selective hydrocracking and hydroisomerization of n-paraffins occur. In the first stage (1), n-paraffin and hydrogen, in the presence in which the experiments were conducted, are adsorbed on the catalyst surface.

Figure 8

Proposed reaction mechanism scheme for the dewaxing process on bifunctional catalysts based on MAS.

At the metallic centers (Ni, Mo, Ni–Mo), in the second stage (2), a dehydrogenation reaction takes place, resulting in the formation of an alkene. The third stage (3) is characterized by the diffusion of the hydrocarbon molecule to the acid sites and the formation of alkylcarbenium ions through protonation. Next, a rearrangement of carbocations occurs, leading to the formation of mono-, di-, and trimethylparaffins (4, 4′, and 4″). At this stage, the subsequent mechanism can proceed in two directions: hydrocracking (5, 5′, and 5″) and hydroisomerization (6), depending on the balance of the acid and metallic centers.

In the case of hydrocracking, alkylcarbenium ions undergo primary and secondary β-scission. Since the carbon number in the starting material is C > 12 and incomplete cracking occurs, the primary cracking products do not desorb rapidly, causing the largest fragments to undergo secondary β-scission.

In the hydroisomerization stage (6), the deprotonation of poly-substituted alkylcarbenium ions occurs, followed by further diffusion back to the metallic centers. At the metallic centers, the molecule undergoes hydrogenation (7). The mechanism concludes with the desorption of the reaction products and dehydrogenated hydrogen from the catalyst surface (8).

Thus, the proposed mechanism reflects the two main reaction pathways of the dewaxing process of n-paraffins present in the investigated diesel fraction. Based on the data on group hydrocarbon composition, it can be inferred that hydroisomerization processes predominate over hydrocracking. Furthermore, if hydrocracking were to dominate, a significant proportion of lighter hydrocarbons would be formed, leading to a considerable loss in diesel fraction yield. In contrast, hydroisomerization processes enhance the operational properties of diesel fuel, improving its low-temperature performance without excessive loss of high-value products.

Increasing the feed space velocity from 0.5 to 2 h⁻¹ at the same temperature and pressure resulted in a decrease in the depth of reactions such as isomerization and hydrocracking. This is probably due to the fact that higher feed space velocities are equivalent to lower feed contact times with the catalyst. As a result, an increase in the n-paraffin content and a decrease in the iso-paraffin content are observed, since the latter are formed as a result of n-paraffin isomerization, which requires a longer contact time. The optimal rate of the studied process is 1 h⁻¹.

4 Conclusion

The changes observed in the hydrocarbon composition of the diesel fraction conversion products are influenced by a combination of factors related to the process conditions and catalyst characteristics. These factors affect the extent of isomerization, hydrocracking, and hydrogenation reactions under varying conditions. As feed space velocity increases from 0.5 to 2.0 h⁻¹, the significance of cracking reactions diminishes, leading to higher yields of iso-paraffins (up to 27.5%, compared to the initial 16.08%) and a reduction in hydrocracking products. This is due to the fact that more intensive treatment is required at lower feed rates to achieve deeper cracking. Additionally, increasing the pressure (from 1 to 3 MPa) during hydroisodewaxing of the diesel fraction enhances the concentration of hydrogen on the catalyst surface, thereby intensifying hydrogenation and hydrocracking reactions. The bimetallic catalyst (Ni–Mo/MAS-H-bentonite) proved to be the most effective in the studied process, ensuring balanced productivity under various reaction conditions – particularly at a pressure of 2 MPa and a feed space velocity of 1 h⁻¹ – achieving a diesel fraction yield of 97.4%, a CFPP of −33°C, and a PP of −36°C.

The findings of this study provide significant environmental and practical benefits by offering an economically viable and environmentally friendly alternative to traditional zeolite-based catalysts. The use of locally sourced bentonite from Kazakhstan reduces reliance on imported materials, minimizes the carbon footprint from transportation, and aligns with green chemistry principles by using less expensive and more abundant nickel and molybdenum as promoters for the catalysts. The optimized hydroisodewaxing process improves CFPs of diesel fuels, reduces wax formation, and minimizes yield losses, enhancing fuel efficiency and lowering maintenance costs. This approach supports sustainable diesel fuel production by balancing performance with ecological and economic considerations.