Dehydration of Turbine Engine Lubricant Oil Using Cellulose Hydrogel

doi:10.21203/rs.3.rs-4212338/v1

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Dehydration of Turbine Engine Lubricant Oil Using Cellulose Hydrogel

https://doi.org/10.21203/rs.3.rs-4212338/v1

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Contamination of oils by water is a recurring problem in the industry and can damage engines and equipment. Oil dehydration systems with hydrogels have shown promise for the removal of free, soluble, and emulsified water. This work evaluates, in an unprecedented way, the dehydration of turbine lubricating oil using a cellulose hydrogel. The hydrophilic polymer was characterized through high-resolution SEM, EDS, FTIR, BET, TGA, DVS and swelling degree. The oil was evaluated regarding its composition and physicochemical properties. The performance of the hydrogel in the treatment of water-in-oil emulsion was analyzed in batch and continuous flow systems. A fixed bed apparatus was specially designed and sized according to the industry's specifications to simulate on-site application. The batch treatment was evaluated using orbital and full tumbling inversion mixing systems, both reaching removal efficiency of around 47%. Mixing by full tumbling allowed greater stability of the emulsion and control of the water concentration, but it required a longer time to enable adequate water uptake by the hydrogel. The efficiency of the hydrogel in the continuous flow system was affected by retention time and inlet water concentration. With a retention time of 12 min, it was possible to treat 1 L of oil, reducing the water concentration from 412 ppm to 197 ppm and the turbidity from Haze 6 to Haze 1. Thus, the cellulose hydrogel was efficient in dehydrating turbine lubricating oil, opening up the possibility of expanding its use to industrial facilities.

desiccant materials

hydrophilic polymers

water-oil separation

water-in-oil emulsion

water removal

Lubricants are oils used to reduce friction and wear between moving parts. They are essential for several activities, being used in turbines, reducers, circulatory systems, compressors, vacuum well pumps, hydraulic machines, oil-lubricated electric motors, and simple or anti-friction bearings (Basiron et al. 2023). These oils are usually manufactured from paraffinic petroleum derivatives and enriched with additives to enhance their characteristics (Saidi et al. 2021). To guarantee the efficacy of lubricating oil, numerous parameters and specifications must be met, and one of these critical quality control attributes is water content.

Lubricating oils can be contaminated with water during service, transport or storage (Yang et al. 2017). The presence of water alters several physicochemical properties of the oil, leading to oxidation, reduction of the pH, alterations in the stability of the additives, and formation of sludge or foam (Estevam et al. 2023). The oil can contain water in dissolved, emulsified, and free form. The dissolved water corresponds to the fraction of water that the oil can solubilize, depending on oil polarity, intermolecular forces, and on the size of the carbon chains (West et al. 2018). When water content exceeds the solubility limit, it either remains in a separate phase (creating a biphasic system) or becomes emulsified within the oil. Emulsification may occur due to the high mixing speeds typically observed in systems using lubricating oils (Mortier et al. 2011).

Water-in-oil emulsions are heterogeneous and inherently unstable systems comprising two immiscible or partially miscible liquids. The liquid in higher concentration (in this case, the oil) constitutes the continuous medium in which water droplets (liquid in lower concentration) remain dispersed (Akbari and Nour 2018). The stability of the emulsion occurs due to the presence of amphiphilic compounds, which interact with both phases of the emulsion and stabilize it (Akbari and Nour 2018). The paraffinic compounds and additives incorporated in lubricant oils can have amphiphilic properties, facilitating the occurrence of emulsion (Zhang et al. 2023). Among the additives with surfactant characteristics are those that provide detergent, dispersant, and anti-foam properties, manufactured from sulfonate, phosphonate, phenolate, and silicone compounds (Basiron et al. 2023). The instability and limited predictability of the mechanism of water dispersion of the emulsions in lubricant oils hampers the studies of water removal involving these systems (Zhang et al. 2023).

Current methods for dehydrating water-in-oil emulsions commonly incur substantial energy expenditures, require complex installations, or may contaminate the oil (Cazarolli et al. 2021; Fregolente et al. 2023). Oil dehydration using hydrogels is a new and promising technology that is associated with low energy consumption and demands simple structures that can be adapted to different configurations and sizes (Gonçalves et al. 2021b). Synthetic polymeric hydrogels reached efficiencies of around 80% in the removal of emulsified water in naphthenic insulating oil (Perez et al. 2022), marine diesel (Perez et al. 2022), diesel (Fregolente et al. 2023), and biodiesel (Gonçalves et al. 2021a). However, to consolidate the oil dehydration technology using hydrogels, it is necessary to advance in synthesizing hydrophilic polymer matrices using more sustainable and low-cost materials, apart from evaluating the process in conditions closer to the industrial reality.

Striving for a production process with enhanced environmental sustainability, this work proposes the use of cellulose hydrogels. Cellulose is a natural polymer suitable for synthesizing hydrogels due to the abundance of hydroxyl groups (Wong et al. 2021). Besides, this biopolymer is widely available and affordable (Trache et al. 2016). To assess configurations similar to those used in industrial conditions, the hydrogel's performance was evaluated in a continuous flow system employing a prototype scaled to meet industry-specific requirements.

To the best of our knowledge, this is the first work investigating the dehydration of turbine engine lubricating oil using hydrogels. The hydrophilic polymer was synthesized from cellulose, expanding the economic and sustainable aspects of the process. Comprehensive characterizations were performed on both the hydrogel and the oil. The removal of emulsified water from the lubricant oil was evaluated in batch and continuous flow systems. An additional innovation in this study lies in the application of the hydrogel within a specially designed fixed-bed apparatus, engineered specifically for oil treatment. This development allows for advancements in hydrogel-based oil dehydration technology and enables the evaluation of its performance within configurations determined by industry needs.

2.1. Hydrogel preparation

The cellulose hydrogel was synthesized according the ideal conditions devised in our previous work (Estevam et al. 2023). Briefly, a 4 wt% of microcrystalline cellulose (Synth P.A.) was dissolved in a NaOH:urea:H₂O solution (7.5:11.5:81 by weight) at 5 ± 2 ºC under constant stirring for 2 h at 750 rpm. Then, the cellulose solution was maintained at -18 ± 2 ºC for 30 min and stored at 2.5 ± 0.8°C until use. To crosslink the cellulose molecules, the solution was heated to 30°C and mixed with 10 vol% of epichlorohydrin by constant stirring at 750 rpm for 1 h. The samples were placed in Petri dishes (15 cm in diameter and about 1 cm in height) and exposed to 60 ºC for 4 h in an air-circulating oven to complete the crosslinking process. Then, the swollen hydrogels (about 13 g.g^-1) were cut into 1.0 cm diameter discs. The unreacted compounds were removed by immersing the hydrogel in distilled water for 24 h with periodic replacement of water. The samples were then dried in air-circulating oven at 60°C for 24 h.

2.2. Hydrogel characterization

Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700 equipment - Thermo Scientific) was used to identify the functional groups and chemical structures of the cellulose hydrogels and microcrystalline cellulose. The procedure was carried out with a resolution of 2 cm^-1 and absorbance ranging from 4,000 to 400 cm^-1.

The morphology of the surface and cross-section region of the oven dried hydrogel was evaluated using a high-resolution scanning electron microscope coupled with an energy dispersive X-ray detector (SEM-EDS) (Quattro S, ThermoFisher Scientific). With the EDS analysis, the atomic mass percentage of the elements composing the sample was quantified. This analysis was performed in triplicate at different points of the hydrogel particle to investigate the homogeneity of the material. Prior to SEM-EDS analysis, the sample was coated with gold using a current of 20 mA for 90 s. The equipment was operated at 10 kV voltage, with a current of 23 pA.

The surface area, volume, and diameter of the pores of the hydrogel were measured by N₂ adsorption and desorption isotherms (ASAP 2020 Plus, Micromeritrics). For this procedure, the sample was previously dried at 105 ºC for 12 h. Then, 0.2 g of hydrogel was submitted to 120 ºC and 3 µmHg for about 18 h. The surface area and porosity parameters were calculated by the Brunauer-Emmett-Teller equation (BET).

The thermal stability of the hydrogel was evaluated by thermogravimetric analysis (TGA). The procedure was carried out using TGA/DSC1 Mettler Toledo TGA equipment and Mettler Toledo MX5 microbalance. The test was conducted in a nitrogen atmosphere with a temperature range of 25 to 600°C at a heating rate of 10°C per minute. The nitrogen flow rate was maintained at 60 mL.min^-1 throughout the process. The data were analyzed based on thermogravimetry (TG) and derived thermogravimetry (DTG) curves.

The dynamic vapor sorption (DVS) technique was used to evaluate the hydrophilicity of the hydrogel. In this process, 650 mg of disk-shaped dried samples with 7–8 mm diameter were exposed to an atmosphere of 95% relative humidity at 25 ºC. The mass change was monitored for over 4 h, according to the methodology described by Perez et al. (Perez et al. 2022). The analysis was performed in duplicate using an Aqualab VSA equipment.

The swelling degree of the hydrogel was evaluated by immerging about 0.1 g of dried hydrogel in 200 mL of distilled water at 37°C in a thermostatic bath. The mass of the hydrogel was measured gravimetrically over time (from 5 min to 72 h) and the swelling degree was calculated by Eq. 1. The analysis was carried out in triplicate. The phenomenon of water diffusion into the hydrogel was evaluated by a power-law model (Eq. 2) applied to the exponential phase of the swelling kinetics (Hertle et al. 2010).

$${\text{W}}_{t}= \frac{{\text{w}}_{\text{f} }- {\text{w}}_{\text{i} }}{{\text{w}}_{\text{i} }}$$

$$\frac{{\text{W}}_{\text{t} }}{{\text{W}}_{\text{e}\text{q} }}=\text{k}.{\text{t}}^{\text{n}}$$

Where: W_t is the swelling degree (g.g^− 1) at time “t”, W_eq is the swelling degree (g.g^− 1) at equilibrium condition, w_i and w_t are respectively the masses (g) of the hydrogels at the beginning and after time “t” of swelling, k is the kinetic constant of the model, n is the diffusional exponent that indicates the transport mechanism (dimensionless),and t is the swelling time (min).

2.3. Characterization of the turbine engine lubricant oil

The turbine engine lubricant oil (LUBRAX AV47 PLUS) was purchased from a local market. The functional groups and chemical bonds of the oil were identified by FTIR spectroscopy, at a spectrum ranging from 4,000 to 550 cm^-1 and a spectral resolution of 2 cm^-1. The chemical composition of the oil was evaluated by X-ray fluorescence (XRF) technique (ASTM D6481 Standards, 2019). High-temperature gas chromatography was used to determine the distribution range of the boiling points of the oil, following the specifications of ASTM D7169 (2018). The oil was also characterized regarding color (ASTM D1500 Standards, 2017), visual appearance, density (ASTM D1298, 2017), kinematic viscosity (ASTM D445, 2022), flash point (ASTM D92, 2018) pour point (ASTM D97, 2022), acid index (ASTM D664, 2018), water content (ASTM D6304, 2021), and particle count (ISO 4406, 2021).

2.4. Preparation of water-in-oil emulsion and determination of water content

To perform the water removal test, the turbine engine lubricant oil was mixed with water at a concentration of 500 mg.kg^-1. The mixture was homogenized in an Ultra Turrax disperser (NT 138, Novatecnica) at 10,600 rpm for 10 minutes. Then, the emulsion was immediately used in the water removal tests. The water content (before and after hydrogel treatment) was quantified by Karl Fischer's coulometric titration (Metrohm 852) following the ASTM D6304 Standard (2021) and analyzed using the Tiamo^® V2.5 software.

2.5. Treatment of the water-in-oil emulsion using the cellulose hydrogel

2.5.1. Batch systems

Batch dewatering tests were carried out in triplicate at 25 ± 5 ºC, using 4 mg of hydrogel per milliliter of water-in-oil emulsion. A control sample (without hydrogel) was used to determine water losses by phenomena other than the removal by the hydrogel (such as decantation or evaporation). Given that slight temperature fluctuations and molecular collisions during mixing could potentially affect the emulsion's stability and water content within the oil (Mortier et al. 2011), two mixing patterns were studied: orbital and total tumbling inversion. Orbital mixing homogenizes the sample by horizontal circular movements, while total tumbling inversion mixing promotes vertical rotation of the sample at 360º. These systems were assessed based on their ability to mitigate water losses unrelated to hydrogel absorption, estimated by the water content in the control samples. In addition, the mixing systems were analyzed regarding their effectiveness in facilitating contact between hydrogel particles and water droplets within the emulsion, with water removal efficiency serving as the measured outcome.

Orbital mixing was performed in 100 mL glass Erlenmeyer flasks containing 25 mL of emulsion and 0.1 g of hydrogel. Stirring was maintained at 150 rpm using a Tecnal TE-4200 shaker incubator. As for the tumbling mixing experiments, performed in a Tecnal TE-165 shaker, polypropylene test tubes with a total volume of 18 mL were used. In this system a stirring speed of 10 rpm and 0.07 g of hydrogel were used, in order to maintain the proportion of hydrogel mass per volume of oil (4 mg per mL) similar in both mixing conditions. Both tests were carried out with hydrogel-oil contact time varying from 30 to 480 min and at room temperature (30 ± 5°C). The water content in the oil was measured by Karl Fischer's coulometric titration following the ASTM D6304 Standard (2021).

2.5.2. Continuous flow system

A fixed bed apparatus designed by the research group was used to evaluate the efficiency of water removal from the oil in a continuous flow system. The apparatus was conceived according to specifications established by the industry in which the technology aims to be applied. In this system, the oil is radially distributed over an annular section that holds the hydrogel (Fig. 1-A). The hydrogel reservoir section has a total volume of 11.51 cm³, in which around 2.8 g of hydrogels were added, leading to a 76.82% porosity of the bed. The structure of the fixed bed apparatus was produced in polyamide by additive manufacture (3D printing) using the Fused Deposition Modeling (FDM) technique.

The dehydration of the water-in-oil emulsion using the fixed bed apparatus was evaluated under two flow rates: 1 L.d^-1 and 2 L.d^-1, with a retention time of about 12 min and 6 min, respectively, without fluid recycling. In both cases, the process was monitored over 24 h, with periodic collection of samples at the inlet and outlet sections of the system. For better sample control assessments, a set of valves were connected to the apparatus. The process was carried out at 25 ± 5°C and the inlet reservoirs were maintained under constant magnetic stirring (150 rpm). Schott flasks of 1 or 2 L were used as inlet (water-in-oil emulsion) and outlet (treated oil) reservoirs, according to the volume of oil targeted for treatment. The reservoir flasks were maintained closed, featuring two small perforations on the lid to accommodate the tubing used for transporting the oil and a thermometer. Figure 1-B presents a schematic representation of the assembly of the continuous flow operation system. The water concentration in the oil at the inlet and outlet sections was accessed over time by Karl Fischer's coulometric titration (ASTM D6304, 2021) measurements.

The presence of emulsified water impacts the turbidity of the oil. Thus, Haze turbidity was evaluated in the water-in-oil emulsion (inlet section) and oil treated with hydrogel (outlet section), following the ASTM D4176 (2022) standard. In this process, 1 L Schott flask (10 cm diameter) containing about 900 mL of oil at 25°C was placed in front a standard bar graph provided by the method. The turbidity was measured by the number of bars observed across the oil bottle. Worth note that the required volume of oil prevents the assessment of Haze turbidity in the oil treated in batch systems.

3.1. Hydrogel characterization

The FTIR analysis shows that the cellulose hydrogel preserved the chemical composition and functional groups of the microcrystalline cellulose (Fig. 2). Both the pristine cellulose and the hydrogel present hydroxyl groups in stretching (3,372 cm^− 1) and bending (1,650 cm^− 1) vibration, and carbon-hydrogen single bonds (2,876 cm^− 1) (Lopes and Fascio 2004). The carbon-oxygen bonds (1,057 cm^− 1) can be related to alcohols, ethers, or peroxides functional groups (Abdel Ghaffar et al. 2020). The presence of the ether group is confirmed by the sequence of peaks in the 1,400 cm^− 1 region, and the alcohol group is identified by the peak at 1,650 cm^− 1 (Kundu et al. 2022).

The elemental composition of the hydrogel, identified by EDS analysis, shows that the material is mainly composed of carbon (35.13 ± 0.67%) and oxygen (49.41 ± 0.91%), confirming the FTIR analysis. Nitrogen (10.13 ± 0.49%) and sodium (2.55 ± 0.27%) were also identified in the sample. These compounds came from the solution used to dissolve the cellulose (Trache et al. 2016). The proportion of sodium hydroxide and urea used to dissolve the cellulose follows a standard methodology well established in the literature (Huber et al. 2019; Wong et al. 2021; Kundu et al. 2022; Thivya et al. 2022). However, the detection of nitrogen and sodium in the EDS analysis may suggest an excess usage of these compounds in relation to the percentage of cellulose used to prepare the hydrogel, or it could indicate a need for improvement in the washing method. Additionally, a minor fraction of chlorine (2.43 ± 0.66%) was identified, originated from epichlorohydrin molecules. The presence of nitrogen, sodium, and chlorine groups was not identified in the FTIR analysis due to their low proportion over the percentage of carbon and oxygen (around 85% of the hydrogel), which stood out in the infrared spectrum. Furthermore, the low standard deviations determined in the analysis reflect the homogeneity of the produced material produced.

High-resolution SEM of the surface (Fig. 3-A and B) and cross-section (Fig. 3-C and D) of the oven-dried hydrogels shows that the material has homogeneous structure and high roughness. Even under a magnification of 5,000 x (which is the highest possible attainable without risking damaging the sample), identifying interconnected pores remains challenging. In Fig. 3-D, small perforations are visible in the cross-section region, especially in the upper left corner. However, it was not possible to confirm whether these structures form interconnected pores or if they are surface grooves. The presence of nanopores was identified only in BET analysis, through which pores with a diameter between 19.2–16.9 Å were determined. Their occurrence can be related to the oven-drying process, which can shrink the material due to the capillary effect, forming a denser structure with a predominance of nanopores (Arthus et al. 2023b). In addition, the BET surface area was determined as 2.42 ± 0.15 m².g^-1 with a total volume of pores estimated as 0.002 cm³g^-1.

According to the TGA analysis (Fig. 4), it was observed that the cellulose hydrogel underwent two major mass loss events. The first event occurred at around 112 ºC, resulting in a 6% decrease in the weight of the sample, and is related to the dehydration of the sample. The second and most significant mass loss event, which led to a 73% reduction in sample weight, occurred in the region of 300 ºC and is associated with the breakdown of the polymer structure. These results are consistent with the typical values reported for cellulose hydrogels (decomposition between 270–330 ºC) and are attributed to the degradation of polysaccharides through the breaking of glycosidic bonds, followed by the opening of the pyranose ring (Priya et al. 2019; Satani et al. 2020; de Lima et al. 2020). It is worth noting that sample degradation occurs at temperatures considerably higher than the temperature typically employed in oil treatment procedures (between 20–70ºC) and is closer to the boiling temperature of the lubricating oil (around 330ºC).

The DVS analysis proves the hydrophilicity of the material, which retained up to 18.58 ± 0.01% of the water in a 95% atmosphere humidity at 25 ºC (Fig. 5). Within the time frame considered, the material did not reach equilibrium, indicating that the hydrogel can sorb higher moisture values. The results obtained with the cellulose hydrogel are close to those reported for polyacrylamide hydrogel (20.8%) under the same experimental conditions (Arthus et al. 2023b). These results confirm the advantages of the proposed hydrogel, which is based on a natural polymer and shows water absorption similar to that of synthetic polymeric hydrogels.

Figure 6-A presents the swelling kinetics of the cellulose hydrogel. The equilibrium swelling was obtained after 12 h, reaching a maximum degree of 19.2 ± 0.3 g.g^− 1. This result agrees with the literature, that commonly reports a swelling degree of cellulose hydrogels between 10 and 20 g.g^− 1 (Niu et al. 2020; Song et al. 2021; Wong et al. 2021). Figure 6-B presents the results of fitting the power law model to the data obtained during the exponential phase of the swelling kinetics. The model properly fit the data, with a coefficient of determination (R²) of 0.9826. The "n" parameter of the power law model was determined as 0.49. This parameter indicates the mechanism of water diffusion in the hydrogel. For the proposed hydrogel geometry, "n" values between 0.40–0.50 indicate Fickian diffusion. In this mechanism, the rate of relaxation of the polymeric chains is greater than the water diffusion rate (Arthus et al. 2023b). Similar water diffusion mechanisms were identified by Santos et al. (Santos et al. 2022) evaluating poly(sodium acrylamide-co-acrylonitrile) hydrogels and by Fregolente et al. (Fregolente et al. 2018) with polyacrylamide hydrogels.

3.2. Characterization of the turbine engine lubricant oil

The FTIR analysis confirms that the turbine engine lubricating oil is mainly composed of open-chain saturated hydrocarbons (Fig. 7). The two strong peaks identified at 2,920 cm^− 1 and 2,844 cm^− 1 represent the asymmetric axial stretching of the C-H bond with Csp₃ carbons, typical of an alkane structure (Lopes and Fascio 2004). The peak at 1,462 cm^− 1 indicates the presence of CH₂ (Nascimento and Costa 2020). Methyl groups (-CH₃) were identified by the peak at 1,374 cm^− 1 (Perez et al. 2022). The peak at 715 cm^− 1 is related to the bond between carbon and hydrogen and may indicate the oscillating vibration of long-chain alkanes (Lopes and Fascio 2004). Thus, the FTIR analysis agrees with the predicted chemical structure for paraffinic base oils (represented in Fig. 8) that generally compose more than 85% of turbine engine lubricant oils (Mortier et al. 2011).

The presence of chemical elements in low concentrations is not easily identified by FTIR since the functional groups that represent the alkanes stand out in the infrared spectrum (Nascimento and Costa 2020). The XRF technique was used to identify metallic and non-metallic elements in the oil. The analysis revealed the presence of silicon (7.1 ppm), phosphorus (28.9 ppm), sulfur (236.6 ppm), sodium (13.6 ppm), potassium (113.5 ppm ), cadmium (3.8 ppm), and strontium (11.3 ppm) elements. These compounds are probably derived from the additives used in oil processing. Silicones are generally used to impart antifoaming properties to the oil (Gazulla et al. 2013). Sulfur and phosphorus may come from additives intended to provide detergent, dispersant and anticorrosive properties (Mortier et al. 2011). These last compounds neutralize the acids that can be formed during oxidation reactions, prevent the corrosion of metallic surfaces and avert the formation of deposits from combustion or oxidation reactions (Mortier et al. 2011). Regarding the presence of sodium and potassium, they may either constitute additives or originate from contaminations during the dehydration process in salt beds (Barker, J., Cook, S., and Richards 2013).

According to the boiling point distribution (presented in Fig. 9), the lubricating oil has a boiling temperature between 330–620 ºC. The initial boiling point (near 330 ºC) is within the range (315–400 ºC) expected for mineral-based lubricating oils (Basiron et al. 2023). Upon analysis, triplicate samples exhibited low standard deviations ranging from 0.06 to 0.5 ºC, indicating high reliability of the data. It is noteworthy that the boiling points of lubricant oils can vary based on their composition, which encompass a blend of hydrocarbons and several purpose-specific additives, as discussed above.

The physicochemical characteristics of the turbine engine lubricating oil are presented in Table 1. The low acid index and the appropriate viscosity shows that the oil does not present signs of aging or oxidation (Rivera-Barrera et al. 2020). According to the pour point, the oil fluidizes in temperatures above − 18 ºC (Xie et al. 2019). The flash point refers to the lowest temperature at which the product can ignite when exposed to an open flame. In the case of the evaluated oil, this parameter exceeded the minimum requirement set by regulation. This characteristic is closely tied to the volatility and flammability of the compounds present in the oils. It serves as a crucial safety indicator by providing insights into potential fire hazard during oil transport and processing. The color and turbidity of the oil may indicate the presence of sediment, water, solids, or other contaminants (Perez et al. 2022). Despite the low values obtained for these parameters, a comprehensive investigation was conducted to ascertain the potential of contamination by water or solids. To evaluate the number of particles in the oil is important to avoid clogging and failures in the systems (Nascimento et al. 2018). Particulate concentration values are coded by ISO 4406 (2021) and correlate to the quantity and size of particles present in 1 mL of oil. For turbine engine lubricating oil, particulate analysis was determined as 16/15/11. This value indicates that 1 mL of the oil presents more than 320 particles larger than 4 µm (code 16), more than 160 particles larger than 6 µm (code 15), and more than 10 particles larger than 1 µm (code 11). Thus, the oil has a low concentration of solids, which are predominantly small. It is worth mentioning that water contamination can lead to an increase in the size and number of particles in the oil (Yang et al. 2017). In the evaluated sample, the water content (122 mg.kg^-1) is within the range stablished by the ASTM D6304 (2021).

Table 1

Physicochemical characteristics of the turbine engine lubricating oil.
Parameter	Result	Specification	Standard method
Appearance	Clear	Clear	-
Color	0.5	3.5	ASTM D1500 (2017b)
Turbidity	1	1	ASTM D4176 (2022)
Density at 20 ºC (kg.m^− 3)	0.870	-	ASTM D1298 (2017a)
Kinematic viscosity at 40 ºC (mm².s^− 1)	47.3	36.8–55.2	ASTM D445 (2022)
Acid index	0.2	2.8	ASTM D664 (2018a)
Pour point (ºC)	-18	-3	ASTM D97 (2022)
Flash point (°C)	240	226	ASTM D92 (2018b)
Water content (mg.kg^− 1)	122	250	ISO 11158 (2009) ASTM D6304 (2021)
Particle count - ISO	16/15/11	-	ISO 4406 (2021)

3.3. Dehydration of water-in-oil emulsion using cellulose hydrogel

3.3.1. Batch systems

Figure 10 illustrates the outcomes from the orbital and tumbling mixing systems regarding water concentration within the control samples (emulsion without hydrogel) and the treated samples (emulsion mixed with hydrogel) during the exposure time. The analyzes were carried out on different days and the fluctuation in the initial water concentration can potentially be attributed to variations in room temperature and air humidity during emulsion preparation.

The lowest water concentration in oil treated by the hydrogel (173 mg.kg^− 1) was obtained by using the orbital mixing system. Under this condition, the efficiency of water removal (calculated based on the difference in water concentration between the control and treatment samples) reached 47.5 ± 4.3%. The orbital stirring system exhibits a substantial volume of air within the flask, which could potentially contribute to moisture loss through evaporation (Perez et al. 2022). Nevertheless, within this system, the interaction between air and oil is less pronounced compared to the dynamics observed in tumbling mixing. Besides, orbital mixing may favor oil demulsification, leading to the coalescence of water drops (Silva et al. 2022). The coalescence and decantation phenomena can improve the hydrogel access to the water molecules and benefit water uptake (Fregolente et al. 2023). For tests employing orbital mixing, the initial water concentration in the oil was determined as 546 mg.kg^− 1. In the first two hours of treatment, water concentration in the control sample remained close to the initial value, varying between 514–540 mg.kg^− 1 (Fig. 10-A). At this time, the hydrogel treatment presented an efficiency of 48.6 ± 4.6% and the water concentration in the oil reached 277.5 ± 24.7 mg.kg^− 1. After two hours, there is an expressive reduction in the water concentration of the control samples, probably related to the demulsification. Despite this, the hydrogel treatment allows even lower water concentrations to be obtained, reaching 173.5 ± 14.2 mg.kg^− 1 of water after 8 h of treatment (Fig. 10-A). In addition, after two hours, all samples submitted to the hydrogel treatment presented a water concentration lower than 250 mg.kg^− 1, thereby adhering to the limit set by ISO 11158. Considering that all the water separated from the oil was taken by the hydrogel, the treatment removed 68.2 + 2.6% of the water initially present in the emulsion.

Evaluating the kinetics of water removal in the tumbling mixing system (Fig. 10-B), it is noted that in the first 4 h of the process, the hydrogel demonstrates a relatively modest efficiency, fluctuating between 6–18%. After 6 h, the treatment efficiency escalated to 35.5 ± 2.0%, with a final water concentration of 306.7 ± 8.8 mg.kg^− 1. The water concentration in the oil adhered to ISO 11158 only after 8 h of treatment, reaching 233.3 ± 1.4 mg.kg^− 1. In this condition, the treatment efficiency was determined as 47.8 ± 0.3%. It is also noteworthy that the water concentration in the control samples (which ranges between 432–452 mg.kg^− 1) remained similar to the initial concentration of the emulsion (determined as 482 mg.kg^− 1 for this set of experiments). Furthermore, small fluctuations in water content over time may be related to the heterogeneity of the emulsion (Perez et al. 2022). Thus, the tumbling mixing system proved capable of reducing water losses in control samples but demands longer processing times to allow the demulsification of the oil and water uptake by the hydrogel.

The obtained results indicate that both the stability of the emulsion and the efficiency of water removal by the hydrogel are influenced by the mixing system. In the orbital mixing, a higher agitation speed was used, leading to the collision of water droplets and the formation of larger clusters, thereby promoting coalescence. Additionally, there was a gradual increase in temperature over time, from 25 to 35 ºC, which facilitated the demulsification of the oil. In contrast, the tumbling agitation system demonstrated better temperature control, with variations of approximately 2 ºC (remaining around 25–27 ºC), and a lower agitation speed. These conditions allowed the emulsion to remain stable for a longer time. The presence of the hydrogel in the systems mixed by tumbling may have contributed to the demulsification of the oil by increasing the solids in the system and enhancing the collision of water molecules.

The removal of water from turbine engine lubricant oil using cellulose hydrogels is one of the novelties of this work. To the best of our knowledge, the dehydration of oils using cellulose hydrogels was evaluated only for water dispersed in biodiesel and diesel, reaching efficiencies of 62% and 82%, respectively (Estevam et al. 2023). Poly(acrylamide-co-sodium acrylate) and poly(acrylamide-co-acrylonitrile) hydrogels were used to remove water-in-oil emulsion (at 5,000 ppm of initial water concentration) in naphthenic insulating oil and marine diesel, presenting efficiencies of around 80% for both oils (Perez et al. 2022). Diesel emulsions were also properly treated using sodium polyacrylate hydrogels, reducing the water content from 630 to 100 ppm (Fregolente et al. 2023). Thus, the use of polymeric hydrogels for the treatment of water-in-oil emulsion has shown promise. The present work contributes to the advances of the studies in this field, being the first to evaluate the dehydration of turbine engine lubricant oil using a natural and biodegradable hydrogel.

3.3.2. Continuous flow systems

Cellulose hydrogels were used to remove emulsified water from the turbine engine lubricant oil within a continuous flow system, operating under two distinct conditions (depicted in Table 2). These variations affect the retention time of the oil in the bed (defining the contact time with the hydrogel) and the ratio between the mass of hydrogel and volume of oil treated. The applied variations exerted a substantial impact on the efficacy of the proposed treatment, as presented in Table 2. With a flow rate of 2 L.d^− 1, the treatment removed 42% of the water present in the 2 L of oil. However, under these conditions, the final water concentration in the outlet reservoir was around 328 ppm, not meeting the level specified by the ISO 11158 standard, which establishes a water concentration below 250 ppm. Furthermore, no reduction in oil turbidity was identified under these conditions. Meanwhile, applying a flow rate of 1 L.d^− 1, the treatment allowed the oil to comply with the ISO 11158 standard, reaching a final water concentration of 197 ppm and reducing the turbidity to Haze 1. In this condition, the turbidity and the visual appearance of the treated oil closely resembled those of the oil in natura, to which water was not added (Fig. 11). Thus, the results obtained showed that the hydrogel was effective in promoting oil demulsification and dehydration.

Table 2

Continuous flow treatment of the lubricant turbine engine oil with cellulose hydrogel using a flow rate of 1 L.d^− 1 (condition 1) and 2 L.d^− 1 (condition 2).
Parameter	Condition 1	Condition 2
Flow rate (L.d^− 1)	1	2
Volume of oil treated (L)	1	2
Retention time (min)	12	6
Hydrogel mass to oil volume ratio (g.L^− 1)	2.8	1.4
Inlet water concentration (mg.kg^− 1)	414.3 ± 5.8	565.9 ± 3.23
Inlet turbidity (Haze)	6	6
Outlet water concentration (ppm)	197.1 ± 16.1	327.8 ± 50.17
Outlet turbidity (Haze)	1	6
Water removal efficiency (%)	52.4 ± 3.2	42.0 ± 9.2

Monitoring water concentration in the inlet and outlet sections over time is of paramount importance due to the low stability of the emulsified water particles in the oil phase. Under continuous flow treatment conditions, the inlet concentration showed some variation over time (Fig. 12). This reduction is a recurring phenomenon documented in the literature (Gonçalves et al. 2021a; Santos et al. 2022; Fregolente et al. 2023) and results from the inherent thermodynamic instability of emulsions, which exhibit a propensity to segregate the continuous and dispersed phases. This behavior arises due to the reduction in interfacial energy between the aqueous and oil phases (Akbari and Nour 2018; Zhang et al. 2023). Thus, the observed shifts in inlet water concentration may be attributed to the coalescence and subsequent decantation of water droplets (Perez et al. 2022). It is important to emphasize that the tube used for oil transport was strategically positioned at the base of the inlet reservoir. This setup structure was designed to collect any water droplets that might undergo decantation.

The lowest flow rate led to a more significant drop in the inlet water concentration after 18 h of treatment (Fig. 12-A). This outcome could potentially be linked to sample homogenization dynamics. Notably, when the volume of the inlet reservoir decreased, the magnetic mixing formed an air vortex that may have benefited the exchange of moisture between the oil and the air presented in the reservoir. This effect was not observed in larger volumes of oil, probably due to the larger diameter of the flask, which allowed the use of a longer magnetic stirring bar and reduced the occurrence of air vortex. Despite the variations in the inlet water content, treatment with cellulose hydrogels consistently and effectively reduced the water concentration in the oil in all analyzed conditions.

The efficiency of water removal over time was related to the water concentration at the inlet section, as depicted in Fig. 13. In the two conditions used, it can be noted that the first outlet sample analyzed presented a slightly higher efficiency. This phenomenon arises from the time required to fill the bed with the oil emulsion. During this process, the emulsion occupies not only the hydrogel-containing annular section but also the vacant spaces within the bed, consequently leading to an extended contact time between the oil and the hydrogel.

The treatment at a flow rate of 1 L.d^− 1 showed greater variations in process efficiency. During the first 12 h the efficiency varied between 40–50%. The reduction in the inlet water concentration after 18 h led to a decrease in the water removal efficiency at this point, which reached 32%. With 24 hours of treatment, the system was able to operate at around 50% efficiency, even with lower feeding concentrations. Meanwhile, the treatment with a flow rate of 2 L.d^− 1 presented a decreasing efficiency, dropping from 60–30% at the first 4 h and then stabilized at efficiencies between 26–32% throughout the analysis period. Given that the retention time was shortened in this scenario and a slightly higher water concentration was obtained at the inlet section, lower efficiency than in condition 1 was expected. However, the variation profile of efficiency over time may indicate that the hydrogel bed presented distinct stages of hydration. In the early stages of treatment, the hydrogel matrix has a larger number of active sites available to interact with the water droplets within the oil emulsion. At the beginning of the dehydration process, water molecules probably interact preferentially with the hydrogel active sites located at the outermost layer of the matrix (Bashir et al. 2020). Subsequently, water retention extends into a multilayer pattern and/or reaches more internally situated active sites within the hydrogel (Jing et al. 2018). The interaction of water droplets with the innermost hydrophilic sites of the hydrogel generally requires a longer contact time between the hydrogel and the oil, which was probably not achieved under the operating conditions used. Thus, the amount of water that the hydrogel retains reduced to 35% under the conditions evaluated.

Silva et al. (Silva et al. 2022) explored the impact of emulsion stability by microgels and reported that gels characterized by a nanoporous structure (such as the cellulose hydrogel proposed herein) facilitate the occurrence of multi-layer water retention. In this process, the first layer of water molecules retained in the hydrogel established hydrogen bonds with the surface. These bonded water molecules then interact with water droplets, forming a multilayer water retention system. Thus, some layers of water are not directly attached to the hydrogel network but trapped within its hydrated polymer chains. These water droplets trapped in the hydrogel structure can collide with water molecules in the oil, favoring the occurrence of larger droplets within the hydrated polymer. The authors also noted that this phenomenon tends to occur more prominently in systems with low flow rates (Silva et al. 2022), analogous to the conditions employed in this study. Similarly, Fregolente et al. (Fregolente et al. 2023) evaluated the treatment of emulsion of diesel contaminated with water using a fixed bed filled with sodium acrylate hydrogel. The authors suggest that the sorption of water by the hydrogel is facilitated by the occurrence of coalescence of water droplets on the highly hygroscopic surface of the hydrogel and points to a multilayer water removal system.

The present work showed that the use of cellulose hydrogels is a promising alternative for the treatment of turbine engine lubricant oil. The treatment performed at 1 L.d^-1 allowed the demulsification and dehydration of the oil up to the levels established by ISO 11158 (2009). Under the conditions studied, a breakthrough curve was not observed, indicating that even larger volumes of oil can be efficiently treated if an appropriate contact time is guaranteed (according to our results, retention times above 12 min). The results reported herein were obtained employing a compact fixed bed apparatus that was specifically adapted for integration into industrial setup and uses a relatively low mass of hydrogel (considering most of fixed bed systems). Future investigations should encompass extended timeframes to ascertain the possibility of achieving a breakthrough curve, thereby reaching the maximum water removal capacity achievable by cellulose hydrogels. Furthermore, it is imperative to explore the potential for regenerating and reusing the hydrophilic polymer, as this aspect remains to be thoroughly examined.

Most oil dehydration technologies encounter limitations in the removal of emulsified water. Traditional methods, such as gravitational and centrifugal separation systems, are primarily designed for free and non-emulsified water, requiring the use of demulsifier for water-in-oil emulsions (Perissinotto et al. 2020). This, in turn, prolongs processing time and escalates reactant consumption. Additionally, centrifugation involves substantial energy expenditure. Techniques such as coalescing filters, vacuum distillation, and salt beds offer potential for treating water-in-oil emulsions but present some drawbacks (Arthus et al. 2023a). Coalescing filters may be less efficient with high-viscosity oils and fail to remove dissolved water effectively (Anez-Lingerfelt 2009). Vacuum distillation, although effective, entails complex and capital-intensive setups (Perissinotto et al. 2020). Salt beds carry the risk of contaminating the oil with sodium, leading to filter and injection system deposits, in addition to generating wastewater with high salt concentrations (Gonçalves et al. 2021a). In contrast, hydrogel systems emerge as versatile alternatives, exhibiting good performance across a spectrum of water types – free, emulsified, and diluted (Arthus et al. 2023a). The inert nature of cellulose hydrogel material ensures that oil remains uncontaminated, while also preventing the generation of secondary waste. Based on the characteristics outlined, it is evident that cellulose hydrogels present a highly favorable option for oil dehydration procedures.

This work showed for the first time that a cellulose hydrogel can promote the dehydration of turbine engine lubricating oil contaminated with water. The structure and chemical composition of the hydrogel preserved typical characteristics of microcrystalline cellulose. Hydrogel degradation occurs only at temperatures above 300 ºC, its surface is rough, with BET surface area of 2.42 ± 0.15 m².g^− 1. The hydrogel presented 18% of water sorption according to DVS analysis and a swelling degree of up to 19 g.g^− 1. The turbine lubricating oil used in this work is mainly composed of alkanes and elements from conventional additives. The physicochemical attributes of the employed lubricant are in full conformity with established regulatory standards, thereby confirming the suitability of the product selected for this study as an appropriate representative exemplar of lubricating oil.

Dehydration of oil by hydrogels is a complex phenomenon that involves simultaneously a series of events, such as the dispersion of water droplets, the collision between water molecules, the interaction of water with the hydrophilic network. In addition, the possible interactions of water molecules with each other and with the atmospheric air interfere with the control of operating conditions. Dehydration of oil in a batch system was effective both in orbital and tumbling mixing, with an efficiency of around 47%. Tumbling mixing led to better control of water content but required long processing times. Meanwhile, orbital mixing benefits the demulsification of the oil, contributing with water uptake by the hydrogel.

The evaluation of the process in a continuous flow system showed that the retention time and the inlet water concentration affected the efficiency of the hydrogel. With a retention time of 6 min, the treatment reduced the water concentration in 2 L of oil from 565 to 327 mg.kg^− 1 in 24 h of treatment. Despite the good performance, under these conditions, it was not possible to reach the limit of 250 mg.kg^− 1 of water in the oil. At a lower flow rate, with 12 min of retention time, it was possible to treat 1 L of oil daily, reducing water concentration from 414 to 197 mg.kg^− 1. In this condition, the demulsification and dehydration of the oil by the hydrogel allowed the reduction of the turbidity of the oil from Haze 6 to Haze 1, resulting in a treated material with visual characteristics similar to those of the oil in natura, not emulsified with water. Hence, the proposed system of oil treatment using cellulose hydrogel allowed compliance with standard regulation of water content in turbine engine lubricant oil.

Funding Sources

The authors acknowledge the financial support provided by Petrobras S.A. through the University Support Foundation (grant number 2022/00024-6); the São Paulo Research Foundation (FAPESP, grant number 2021/03472-7); the National Council for Scientific and Technological Development (CNPq, grant number 314724/2021-4); the Coordination for the Improvement of Higher Education Personnel (CAPES), finance code 001, through the Institutional Internationalization Program (CAPES- PrInt) grant number 18p52383/2023; and the Program of Academic Excellence (PROEX), grant number 33003017034p8.

Competing interests

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Bianca Ramos Estevam reports financial support was provided by Petrobras and Institutional Internationalization Program of the Coordination for the Improvement of Higher Education Personnel (CAPES- PrInt). Leonardo Vasconcelos Fregolente reports financial support was provided by State of Sao Paulo Research Foundation and by Program of Academic Excellence. Angela Maria Moraes reports financial support was provided by National Council for Scientific and Technological Development. The other authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

Bianca Ramos Estevam: conceptualization, methodology, validation, data curation, formal analysis, investigation, writing—original draft, and visualization.

Isadora Dias Perez: investigation, data curation, validation, writing—original draft, and visualization.

Karina Mayumi Tsuruta: conceptualization, methodology, validation.

Roberto Mendes Finzi Neto: conceptualization, validation.

Mechelangelo Viana Mancuzo: conceptualization, supervision, resources, funding acquisition, project administration.

Aldemir Aparecido Cavallini Junior: conceptualization, supervision, resources, funding acquisition, project administration.

Ângela Maria Moraes: conceptualization, supervision, validation, and writing—review and editing.

Leonardo Vasconcelos Fregolente: conceptualization, supervision, resources, funding acquisition, project administration, and writing—review and editing.

Data availability statements

The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

Declaration of generative ai in scientific writing

During the preparation of this work the authors used Grammarly-AI in order to improve readability and language. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

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No competing interests reported.

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Dehydration of Turbine Engine Lubricant Oil Using Cellulose Hydrogel

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Abstract

Figures

1. INTRODUCTION

2. METODOLOGY

2.1. Hydrogel preparation

2.2. Hydrogel characterization

2.3. Characterization of the turbine engine lubricant oil

2.4. Preparation of water-in-oil emulsion and determination of water content

2.5. Treatment of the water-in-oil emulsion using the cellulose hydrogel

2.5.1. Batch systems

2.5.2. Continuous flow system

3. RESULS AND DISCUSSION

3.1. Hydrogel characterization

3.2. Characterization of the turbine engine lubricant oil

3.3. Dehydration of water-in-oil emulsion using cellulose hydrogel

3.3.1. Batch systems

3.3.2. Continuous flow systems

4. CONCLUSION

Declarations

References

Additional Declarations

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