Development of an adsorption process to selectively remove

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Development of an adsorption process to selectively remove oligomers in aging engine oil resulting from the use of biodiesel or its blends Jerome Kpan, Anja Singer, and Jürgen Krahl Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02146 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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Development of an adsorption process to selectively remove oligomers in aging engine oil resulting from the use of biodiesel or its blends

Jerome Kpan1,2, Anja Singer1, Jürgen Krahl1,3,4 1

Coburg University of Applied Sciences and Arts, Friedrich-Streib-Straße 2, 96450 Coburg, Germany

2

Dept. of Chemistry, University of Cape Coast, Cape Coast. Ghana

3

Ostwestfalen-Lippe University of Applied Sciences, Langenbruch 23, 32657 Lemgo, Germany

4

www.fuels-jrg.de

Corresponding author: [email protected]

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ABSTRACT: When biodiesel finds its way into engine oil, it causes dilution of the oil, increasing viscosity, and causing the lubricating oil to oxidize leading to premature formation of sludge and deposits in the crankcase resulting in short period of oil drain interval. This study was carried out to use adsorption as a means of mitigating this negative impact of the use of biodiesel on crankcase oil by selectively removing oligomers formed in the oil as a result of degradation of biodiesel. It was also to determine a suitable adsorbent for the use of this purpose. Neat base oil mixed with Rapeseed oil methyl ester (RME) biodiesel (20 % RME and 80 % neat base oil by volume) were thermo oxidatively aged at 170 °C and for 80 h. Also unaged Longlife engine oil was used in this study to observe the impact of the adsorbents on additives. The adsorbents used in this study were 20 % Dimethyldichlorosilane (DMDCS) chemically deactivated silica gel, 20 % DMDCS deactivated alumina and 25 % water deactivated silica gel. 20 g of the selected adsorbents were used to create an adsorption bed in a chromatographic column for the separation process. The total acid number and changes in molecular masses before and after separation were determined. The adsorption data of oligomers onto adsorbents were attained and described by Langmuir and Freundlich adsorption isotherms. Thermodynamic parameter, ΔGo was calculated, which indicated that the adsorption was spontaneous in nature. The adsorption also followed both surface adsorption and intra-particle diffusion mechanisms. Total acid number of the aged base oil and RME mixture without separation, 10.46 mg KOH/g was reduced to 1.03, 1.49, and 7.37 mg KOH/g by 20 % DMDCSalumina, 25 % water deactivated silica and 20 % DMDCS-silica respectively. The 20 % DMDCS-alumina adsorbed 57 % of oligomers while 25 % water deactivated silica adsorbed 48 % and 20 % DMDCS-silica had 37 %. The results of this study have shown that adsorption is a potential useful tool for oligomers removal from aging engine oil.

Keywords: biodiesel, engine oil, dilution, oligomers, adsorption.

Introduction With increasing energy demand and declining conventional fossil fuel reserves, biodiesel is seen as an alternative fuel. The renewed interest in alternative fuels has given a boost to cleaner and more environmentally friendly fuels. Increase in greenhouse gases (GHG) is the driving force for switching from fossil fuels to alternative fuels. Numerous regulations have been adopted by the European Commission. Member States have committed themselves to reducing greenhouse gas emissions (GHG) by 20 %, increasing the share of renewables in the EU's energy mix to 20 %, and achieving the 20 % energy efficiency target by 2020 [1]. By 2050, the EU should cut greenhouse gas emissions to 80 % below 1990 levels. Milestones to achieve this are 40 % emissions cuts by 2030, 60 % by 2040 and 80 % by 2050. According to the Commission, by 2050 transport emissions, excluding international maritime transport must be cut by 54-67 % [1, 2]. Also after the oil crises of the 1970’s, the search for alternative fuels was intensified. Due to advances in technology the petroleum reserves are admittedly declining more slowly than first assumed, however, the resources are still finite [3]. Socio-political schemes have great interest in the addition of biogenic components to the fossil fuel to increase the share of renewable energies and reduce the dependence on the mineral oil as well as reduce the CO 2 emissions. Currently up to 7 % of commercial diesel fuel consists of rapeseed oil methyl ester in Germany [3]. The most common biofuel is biodiesel which can be used in diesel engines. Biodiesel fuel is chemically fatty acid methyl ester (FAME) and is produced from various feedstocks such as vegetable oil, animal fat or used cooking oil. In Europe, mainly rapeseed oil is used as the raw material [4]. Biodiesel fuel is a mixture of molecules of different molecular weight with ester functionality and often with one or two double bonds in the alkyl group associated with the fatty acid. Such double bonds are chemically active groups that make the biodiesel fuel chemically and kinetically unstable [5]. Biodiesel is less volatile than conventional diesel fuel and tends to concentrate in the sump of an engine over time. Hence the level of unburnt fuel in the engine oil can build up. The viscosity of the lubricant is reduced as fuel dilution increases [6]. As such in the internal combustion engine, there is always a certain amount of unburnt fuel. The fuel results from complete or incomplete combustion and passes through the piston rings and cylinder and finds its way into the lubricating oil [7]. Modern diesel engines are equipped with diesel particulate filters (DPF). In these engines, fuel is post-injected into the cylinder to increase the exhaust gas temperature and help burn the carbon-based deposits. Post-injection for after-treatment system regeneration leads to unburnt fuel on the cylinder walls which finally finds its way into the crankcase. In the crankcase, the biodiesel is less likely to evaporate out of the engine oil due to its higher boiling range of about 340 °C to 375 °C [8, 9]. This therefore, leads to higher levels of oil dilution [10]. Fuel dilution increases viscosity and oil oxidation causing biodiesel fuel itself and the lubricating oil to oxidize, leading to premature formation of sludge and deposits in the crankcase resulting in short period of oil drain interval [6, 7, 11, 12, 13, 14, 15]. From the above literature, studies have been conducted on combustion characteristics, emissions, engine performance, fuel consumption, and the effects on crankcase oil properties when using biodiesel. The use of additives to control the stability of biodiesel has also been documented [11]. This study has been carried out to use adsorption as a means of mitigating the negative impact of the use of biodiesel on the crankcase oil by selectively removing oligomers formed in the oil as a result of the degradation of biodiesel. The separation of the oligomers was carried out through fixed bed columns using 20 g of 20 % Dimethyldichlorosilane (DMDCS) deactivated alumina, silica and 25 % water deactivated silica gel respectively. Gel permeation chromatography was employed to measure the molecular weight of the aged products before and after the separation with the deactivated adsorbents. These deactivated adsorbents have been extensively used in this project in view of getting the parameters necessary to establish a tailor made adsorbent for the purpose of selectivity. This process was undertaken with the aim of

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optimizing the physical and chemical parameters such as the polarity in order to observe the adsorbents’ impact on additives. In addition, the equilibrium adsorption data were fitted to various adsorption models to obtain the constants related to the adsorption phenomena. Equilibrium and kinetic analysis were conducted to determine the factors controlling the rate of adsorption, the optimization of various parameters in the additives recovery and to find out the possibility of using this material as a low-cost adsorbent for the removal of oligomers in aging crankcase oil during usage in the engine. In this study, Longlife engine oil was also used in order to determine the impact of the adsorbents on additives in the oil. Using inductively coupled plasma, selected elemental concentration in the engine oil was measured before and after separation with the adsorbents. Materials and Methods Chemicals From Sigma Aldrich Chemie GmbH, Taufkirchen, Germany, dichlorodimethylsilane (DMDCS)-97 %, silica gel-60 and Aluminum oxide were ordered. Apart from the adsorbents, dichlorodimethylsilane was used without any modification. Tetrahdrofuran-99.9 %, toluene-99.8 %, and isopropanol-99.9 %, all UV/IR grade, methanol-99.9 % p.a. and KOH in isopropanol-0.1 mol/l-0.1 N volumetric standard solution, nitric acid-supra quality and hydrogen peroxide-ultra quality were ordered from Carl Roth GmbH+Co. KG, Karlsruhe, Germany. Oil mixtures Base Oil was obtained from a project partner, Volkswagen AG. Rapeseed oil methyl ester was ordered from ADM. Hamburg, Germany. The RME supplied was within the specification for biodiesel as sated in DIN EN 14214. For the aging experiments, a mixture of pure neat base oil with rapeseed oil methyl ester (RME) was used. The mixture had 80 % base oil and 20 % RME all by percent volume and was aged for 80 h at 170 °C to create oligomers for subsequent separation with adsorbents. Physical parameters of these oils are listed in Table 1

Table 1: The physical properties of base oil and RME Sample

Base oil

RME

Kinematic viscosity at 40 °C

30.97 mm²/s

4.52 mm²/s

Kinematic viscosity at 100 °C

5.91 mm²/s

1.77

Density at 15 °C

0.87

kg/m3

0.88 kg/m3

The mixing ratios of the mixtures were selected based on engine oil samples that were examined as part of a fleet project with biogenic fuel at Volkswagen AG [16]. In diesel fuel, Fang and McCormick [5] defined the biodiesel admixture of 20 % up to 30 % as worst-case. They have proved that mixtures containing 20 % to 30 % biodiesel in diesel fuel have the highest tendency for deposit formation. Aging equipment The construction of the aging apparatus is based on the oil stability index measurements (OSI) according to the standard DIN EN 14214 for FAME [17]. However, unlike the prescribed temperature of 110 °C, air flow of 10 L /h and a mass of 7.50 g of sample in the OSI, a temperature of 170 °C, air flow of 300 mL/min and a volume of 200 mL of sample were used in the aging. The aging apparatus consists of a glass vessel with a two way cork which allows air into it from one side and leave through the other. An air flow at rate of 300 mL/min generated by a pump passes through the oil-fuel mixture. This air simulate the combustion occurring in the reaction gas flowing through the engine oil and hence contribute to real aging. The volatile decomposition products formed during the aging process passes through a silicone tube into collection vessels. In the work of Singer et al. (2014), an aging process which simplified some known aging procedures like the defined temperature program was developed. It proved that the same results could be achieved with a constant temperature of 170 °C as used in this study. Therefore, an engine oil change interval of 30,000 km can be simulated within 40 h [18]. Adsorbents The adsorbent was chemically deactivated using Dimethyldichlorosilane (DMDCS). This is because the active sites of the adsorbent does not possess the same energy of interaction with any given solute. Also considering the fact that the oligomers are of higher polarity than the additives in the oil, the polarity of the adsorbent has to be reduced to enable it selectively adsorb only the oligomers. The silicone layer, Si-OH group was therefore, masked in order to reduce the polarity of the silica gel. For the deactivation, 20 % of DMDCS by volume was prepared in a bottle by taking 100 mL of DMDCS and diluting it to 500 mL with toluene. The deactivation solution was poured gradually with continuous stirring into the silica gel. The 50 g silica gel was soaked in this deactivation solution for about 30 min. After rinsing it twice with pure toluene, it was again soaked in methanol for about 30 min and rinsed with methanol and then dried under nitrogen atmosphere. A mass of 50 g of aluminum oxide was also deactivated using the same 20 % DMDCS. The process of deactivation of the silica gel was repeated with the aluminum oxide. The deactivation of silica gel was also carried out using water. So preparing 50 g of 25 % (mass percent) water deactivated silica gel, 25 g of water was added drop by drop with agitation on a laboratory shaker to 75 g silica gel. Upon adding all


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the water, the mechanical agitation continued till the silica gel was free flowing and no lumps were present. The deactivated silica was allowed to equilibrate for about 10 h before it was used in the adsorption process. Separation of Oligomers The column bed was loaded with the deactivated adsorbent and fed with 30 mL of the aged mixture of neat base oil and RME. Aged sample was poured gradually unto the adsorbent bed in order not to spread the adsorbent causing unevenness on the bed. It was essentially a frontal chromatography process and the flow through the adsorbent bed was under gravity and at selected temperatures. The time taken for the mixture to flow through the adsorbent was recorded with the aid of a stop watch. The influence of the different parameters such as adsorbent dose, temperature, contact time etc. was evaluated by varying the parameter under investigation while keeping all other parameters constant. The equilibrium solid phase concentration qe (mg/g) was calculated using the equation: qe = (Ci - Ce) × (v/m ) (1) where qe (mg/g) is the amount of the oligomers adsorbed by the adsorbent; C i and Ce (mg/g) the initial and equilibrium liquid phase concentration of the oligomers; V (L), the initial volume of the aged oil; m (g) is the weight of the adsorbent. [19, 20, 21] The adsorption of oligomers onto the adsorbents was studied by varying the amount of the adsorbent. It is observed that the percentage of adsorption increased linearly as the adsorbent dose increased. At 20 g/L dose, there was more than 60 % removal of oligomers and therefore, this amount was thus used for all other experiments. To evaluate the impact of the adsorbent on the additives in the engine oil, 30 mL of longlife engine oil was fed onto the adsorbent bed just as the treatment meted out to the aged sample. The flow through the bed was done under gravity. An amount of 0.2 g of the filtrate was acid digested using SPD-Discover microwave from CEM. To the sample, 2 mL of hydrogen peroxide and 6 mL of concentrated nitric acid were added and digested. The microwave digestion was carried out at the pressure of 26 bar and temperature of 200 °C. The microwave was set to first ramp to a temperature of 165 °C in 3 mins with a holding time of 5 mins. A further ramp to 175 °C in 5 mins and held for 10 mins. Finally, it came to 200 °C in a ramp time of 10 mins and a holding time of 15 mins. The digested sample was transferred into a 50 mL sample vial. Deionized water was used to top it up to the 50 mL mark. The concentration of selected ions before and after the separation with the adsorbent was determined using the ICP-OES. Analytical methods Fourier Transform Infrared Spectroscopy (FTIR) For Fourier transform infrared (FTIR) spectroscopy analysis, a Nicolet 6700 FTIR from thermo scientific company was used. The thermo scientific omnic software was used for the evaluation of the measurements. This instrument is equipped with an ATR-crystal for ATR spectroscopy. The crystal needs just a drop of the sample which is sufficient for analysis. A DTGS detector is in use in the machine. It has an XT-KBr beam splitter. The background and sample were each scanned 16 times. Gas chromatograph with mass spectrometry (GC-MS) For the GC-MS measurements, an Agilent GC7890A gas chromatograph coupled with Agilent 5973 quadrupole mass spectrometer was used. A phenomenex zebron ZB-5 column with a length of 30 m, inner diameter of 0,25 mm and film thickness of 0,25 µm. Helium was used as carrier gas with flow rate of 13.3 mL/min. A temperature ramp from 110 °C up to 280 °C was used. 20 µL of sample was mixed with 1 mL Cyclohexane and used for the analysis. Size Exclusion-Chromatography (SEC) Size exclusion chromatograms (SEC) were measured using Agilent Technologies 1260 Infinity quaternary LC Systems with a column length of 30 cm, pore size 5 microns, porosity of 100 Å, refractive index detector and using tetrahydrofuran (THF) as solvent. Three columns were used in series with a flow rate of 0.5 mL/min at 45 °C. Calibration was done with polyethylene glycol standards (Agilent PEG calibration kit Part No.: PL2070-0100) with different molecular masses between 106 and 4040 Da. Using PEG for calibration, the molar mass of methyl oleate was determined to be about 335 Da but the real molar mass of methyl oleate is 296.45 Da. Since the molecular size of the calibration standards and the analytes were not identical, it is difficult to make quantitative statements. The SEC measurements were done with 10 mg of the sample mixed with 1000 µL THF. Potentiometric determination of the Total Acid Number (TAN) Total acid number was measured using a fully automated measurement system developed by Metrohm (Titrando 888). The potentiometric determination was carried out according to ASTM D664. 0.1 N volumetric standard solution of KOH in isopropanol was the titrant while the solvent was made of 500 mL toluene, 495 mL isopropanol and 5 mL CO 2 free H2O. 50 mL of the solvent was added to 0.2 g of the sample and titrated against the standard KOH in isopropanol for the


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acid value. The acid number is the amount of potassium hydroxide in mg that is required to neutralize acids contained in one g of fuel sample. Results and discussion Aging of neat base oil with RME During the aging of the mixture of neat base oil and RME, the formation of oligomers could be detected. The aging products from the reaction vessel were analyzed using Fourier transform infrared spectroscopy (FTIR), gas chromatograph with mass spectrometer (GC-MS), size-exclusion-chromatography (SEC) and Metrohm potentiometric titrator. Figure 1 shows the IR-Spectra of the mixture during the aging process of the base oil mixed with RME. The IR indicates functional groups and has been used in this work to study the oxidative changes in FAME blends. The differential spectrum of the non-aged base oil and RME mixture and that of the aged mixture highlights the oxidative changes. There are clear increases in signals for hydroxy groups (ca. 3000-3600 cm−1), as evidenced by the increase signal in this region of the spectrum (insert). The OH band in this region could be attributed to organic compounds such as water, alcohol, hydroperoxide, and carboxylic acids containing the OH functional group. These signals that point to oxidation products are however absent in the unaged mixture. The increase in these signals translates into higher acid content generated. This is also reflected in the increase in the signal in the finger print region which is most attributable to increase in acid number. This is because fatty acid methyl esters are known to react with oxygen to produce allylic radicals which may isomerize to more stable isomers. The higher signal of the OH bands is collaborated with the total acid number determined in this study. The region of carbonyl groups or mostly oxidative products (ca. 1600-1800 cm−1) also registered enhanced absorption in the spectrum. The influence of biofuel, RME on the aging process is exhibited here. The base oil with RME before aging showed an ester vibration but as the ageing proceeded, the aged samples showed wider carbonyl bonds than an ester vibration. This is characteristic of aging as other carbonyl products such as ketones, aldehydes, and others are formed after chain scission. A decrease of the CH, CH2 and CH3 vibrations were detected within the range of 2800 cm-1 to 3000 cm-1.

Figure 1: FTIR-spectrum of aging of neat base oil for 80 h In the GC-MS analysis shown in Figure 2, neat base oil component showed up in a retention time range of 25 and 45 min. The neat base oil component peaks however, degraded during the aging for 80 h. This declining and degradation of peaks effect correlates the FTIR analysis shown in Figure 1 where a decrease of the CH, CH2 and CH3 vibrations could be detected at wave numbers between 2800 cm-1 and 3000 cm-1. The analysis of the MS-spectra showed a decomposition of the long hydrocarbon chains of the neat base oil. Few shorter carbonyl substances were formed. However, it does not correspond with the big decrease in the abundance of hydrocarbons in the neat base oil range. The possible explanation for this effect is the inability of oligomers to evaporate due to their size and therefore, could not be detected in GC-MS. Between a retention time of 16 and 26 min typically shows up the signals of RME. The presence of RME in the unaged mixture is very prominent and conspicuous as shown in Figure 2. The degradation of the RME signals after the aging for 80 h at 170 °C is visibly detected within this range. At 80 h, the RME is virtually degraded and this confirms the fact that it is oxidatively unstable and more prone to oxidation due to its higher degree of unsaturation. Allylic hydrogen


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atoms in RME are susceptible to radical attack which lead to the formation of peroxides. When the rate of peroxide degradation exceeds the rate of peroxide formation, an exponential increase in the production of acids and other degradation products occur leading to an accelerated aging of the mixture of base oil and RME. The buildup of oxidative products such acids, aldehydes, ketones etc is exemplified in the FTIR-analysis shown in Figure 1. Here, the carbonyl vibration bands resulting from the aging process could be detected at wave number of 1700 cm-1 for RME. It is important to show evidence of the formation of bigger molecules with the SEC because these molecules cannot be detected by use of GC-MS and FTIR.

Figure 2 GC chromatogram of neat base oil mixed with RME and aged for 80 h

As already seen in the FTIR spectrum, the analysis with SEC presents the molar masses of the neat base oil mixture with RME aged and separated with the adsorbents. SEC was employed to gain information on the degree of oligomerization of the mixture and also the impact of the adsorption in removing the oligomers formed. In SEC is an indication of the buildup of bigger molecules during the aging process. In Figure 3, the chromatograms of the aged base oil mixed with RME before and after separation with the adsorbents are shown. There were no changes in the molar mass distribution for the base oil-RME mixture without any ageing. These results collaborated those of the IR and GC/MS measurements of the unaged mixture. As the ageing proceeded, a noticeable increase in the oxidized products sets in at about 400 g/mol. This could be attributed to the FAME in the mixture. Signs of dimers appeared at about 600 g/mol. With increased aging time, the maximum of the dimers moved to higher masses resulting from further uptake of oxygen into the dimers and higher oligomers. The molar mass of the formed oligomers was approximately 1600 g/mol but after the separation with the adsorbents, it reduced to about 1200 g/mol. These figures are just approximation since the molecular size of the calibration standards and the analytes were not identical in their chemical structure. The area under the SEC curve reflects the amount of oligomers formed. This area under the curve is therefore, dependant on the successful removal of higher built molecular mass substances by the different adsorbents. By calculation, the area under the curve generated by the mixture of neat base oil mixed with RME and aged for 80 h at 170 °C without any separation stood at 190.18 [g/mol]2. This area has been successfully reduced to 80.99, 97.12 and 120.05 [g/mol] 2 by 20 % DMDCS-Alumina, 25 % D-Silica and 20 % DMDCS-silica respectively, Figure 4. Imperatively 20 % DMDCS deactivated alumina adsorbed about 57 % of the oligomers formed in the aged sample followed by 25 % D-silica with 48 % and then 20 %DMDCS-silica had 36.88 % adsorption. The level of removal can be seen in the difference in the curves


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as shown in the insert of Figure 3. This is an indication that adsorption has great potential in removing the bigger molecules formed during aging process.

Figure 3: SEC of aged base oil mixed with RME before and after separation with adsorbents

Figure 4 Area generated after separation with adsorbents Effect of adsorbent dosage Equation 1 was applied to arrive at the amount of oligomers removed. The results are recorded in Table 2 and graphically represented in Figure 5 with respect to the amount of oligomers removed by the adsorbent at different dosages of 5.00 g, 10.00 g, 15.00 g, and 20.00 g per 30.00 mL of aged oil. In Figure 5, it is observed that the adsorption of the oligomers increases as the amount of the adsorbent also increases. This is due to the increase in surface area as the amount of the adsorbent increases and therefore the availability of more adsorption sites. Of the three deactivated adsorbents used, the deactivated alumina showed a better adsorption of the oligomers with 23.58 mg/g followed by 25 % D-Silica gel, 19.8 mg/g and 20 % DCDMS-Silica gel having 8.95 mg/g. This is in consideration of the fact that the choice of any stationary phase is usually governed by the polarity of the feed components of the mixture.


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Table 2 Amount of oligomers removed Sample

Volume of Solvent [mL]

Mass of adsorbent [g]

TAN, Ce [mg KOH/g]

X=Ci-Ce [mg KOH/g]

V/m [mL/g]

qe [mg/g]

B20 aged 80 h @ 170°C

50.00

25 % D-Silica gel

50.00

5.00

10.46

10.46

50.00

523

5.55

4.91

10.00

49.10

25 % D-Silica gel

50.00

10.00

4.12

6.34

5.00

31.70

25 % D-Silica gel

50.00

15.00

2.80

7.66

3.33

25.53

25 %D-Silica gel

50.00

20.00

2.54

7.92

2.50

19.80

20 % DCDMS-Alumina

50.00

5.00

4.09

5.99

10.00

59.90

20 % DCDMS-Alumina

50.00

10.00

2.50

7.75

5.00

38.75

20 % DCDMS-Alumina

50.00

15.00

0.37

8.56

3.33

28.53

20 % DCDMS-Alumina

50.00

20.00

0.21

9.43

2.50

23.58

20 % DCDMS-Silica gel

50.00

5.00

8.20

2.26

10.00

22.60


50.00

10.00

7.75

2.71

5.00

13.55


50.00

15.00

7.36

3.10

3.30

10.33


50.00

20.00

6.88

3.58

2.50

8.95

Figure 5 Removal of oligomers by the adsorbents at different dosages


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Adsorption isotherms The Langmuir and Freundlich isotherm models were used to fit the experimental data which are represented in Figure 6 and Figure 7 and the values of the parameters are summarized in Table 3, Table 4 and Table 5. The expression of the Langmuir model is based on assumptions that there is a fixed number of vacant or adsorption sites which are homogeneously distributed on the surface of the adsorbent; all the vacant sites are of equal size and shape on the surface of adsorbent. However, the active sites of the adsorbent have same affinity for adsorption of a mono molecular layer and there is no interaction between adsorbed molecules. The Langmuir model contains two useful parameters, qmax and b which reflect the characteristics of the adsorption system. Langmuir constant b = 1/K which is related to the energy of adsorption through the Arrhenius equation. The higher the b, the smaller the K, the higher is the affinity of the adsorbent for the adsorbate. qmax is interpreted as the total number of binding sites that are available for adsorption. From Table 4, 20 % DMDCS deactivated alumina has the highest b value and therefore has the highest affinity for the oligomers. It is therefore, not surprising that it registered the highest amount of the oligomers adsorbed from the aged oil. It is as a result of the fact that alumina is a basic adsorbent and therefore, has greater affinity for acidic components. The Freundlich isotherm on the other hand applies to adsorption on a heterogeneous surface. There is however, room for interaction between the adsorbed molecules but is not necessarily limited to the formation of a monolayer. The assumption of this model includes that as the adsorbate concentration increases; the concentration of the adsorbate on the adsorbent surface also increases.


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Table 3: Values used for the adsorption isotherms Sample

Mass sample [g]

25%D-Silica gel

of

titre value [mL]

Volume of solvent [mL]

mass of adsorbent [g]

TAN,Ce [mg KOH/g]

X=Ci-Ce [mg KOH/g]

X/m [mg]

log Ce

V/M [m/g]

qe [mg/g]

Ce/qe

1/Ce

1/qe

log qe

2.00

1.98

50.00

5.00

5.55

4.91

0.98

0.78

10.00

49.10

0.11

0.18

0.02

1.65

25%D-Silica gel

2.01

1.48

50.00

10.00

4.12

6.34

0.63

0.62

5.00

31.70

0.13

0.24

0.03

1.50

25%D-Silica gel

2.01

1.00

50.00

15.00

2.80

7.66

0.51

0.52

3.33

25.53

0.11

0.38

0.04

1.38

25%D-Silica gel

2.00

0.91

50.00

20.00

2.54

7.92

0.40

0.39

2.50

19.80

0.13

0.39

0.05

1.30

20%DCDMSAlumina

2.01

1.47

50.00

5.00

4.09

5.99

1.27

0.65

10.00

59.90

0.07

0.24

0.02

1.78

20%DCDMSAlumina

2.02

0.90

50.00

10.00

2.50

7.75

0.80

0.43

5.00

38.75

0.06

0.40

0.03

1.59

20%DCDMSAlumina

2.02

0.13

50.00

15.00

0.37

8.56

0.67

0.28

3.33

28.53

0.01

2.70

0.03

1.46

20%DCDMSAlumina

2.02

0.07

50.00

20.00

0.21

9.43

0.51

0.01

2.50

23.58

0.01

4.76

0.04

1.37

20%DCDMS-Silica gel

2.00

2.93

50.00

5.00

8.20

2.26

0.45

0.91

10.00

22.60

0.36

0.12

0.04

1.35

20%DCDMS-Silica gel

2.02

2.78

50.00

10.00

7.75

2.71

0.27

0.89

5.00

13.55

0.57

0.13

0.07

1.13

20%DCDMS-Silica gel

2.02

2.65

50.00

15.00

7.36

3.10

0.21

0.87

3.33

10.33

0.71

0.13

0.10

1.01

20%DCDMS-Silica gel

2.01

2.46

50.00

20.00

6.88

3.58

0.18

0.84

2.50

8.95

0.77

0.14

0.12

0.95


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Energy & Fuels

Correspondingly, the sorption energy therefore exponentially decreases on completion of the sorption centers of the adsorbent. The correlation coefficients were calculated using the Langmuir and Freundlich isotherms. The results as clearly depicted by Figure 6 and Figure 7 fits well with the Freundlich model. A good and clearly fitted Freundlich model is an indication of a good fit for a physical adsorption as well as a heterogeneous distribution of active sites on the adsorbent surface. The correlation coefficients for Freundlich isotherms were 0.9844, 0.9433 and 0.9094 while that of Langmuir were 0.9399, 0.8627 and 0.9126 respectively. The Freundlich’s constant ‘n’ signifies a measure of the deviation of the adsorption from linearity. When the value of n is equal to unity then the adsorption is linear. On the other hand, when the value of n is less than unity, it implies that the adsorption process is unfavorable while when the value of n is greater than unity, it means the adsorption is favorable [19, 20, 21].

Figure 6 Langmuir isotherm for adsorption of oligomers


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7 Freundlich isotherm for adsorption of oligomers


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Page 13 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 4: Langmuir isotherm constants Adsorbent

q max

b

R2

20 % DMDCS-Silica gel

2.04

-0.12

0.9126

20 % DMDCS-Alumina

52.91

4.50

0.8627

-909.09

-8.85*10-3

0.9399

25 % D-Silica gel

Table 5: Freundlich isotherm constants Adsorbent

KF

n

R2


3.54*10-4

0.19

0.9094

20 % DMDCS-Alumina

21.30

1.56

0.9433

25 % D-Silica gel

8.29

1.07

0.9844

In this present study, the values of n at equilibrium for 20 % DMDCS-Alumina and 25 % D-Silica gel, 1.56 and 1.07 respectively are greater than unity implying a favorable adsorption process for oligomers. The correlation co-efficient (R2), 0.9094, 0.9433 and 0.9844 respectively indicating the adsorption of oligomers on adsorbents fitted adequately to the Freundlich model. This is an indication of a multilayer adsorption process. Adsorption kinetics The kinetics of oligomers adsorption by adsorbents was considered at the different concentration levels before and after the adsorption process. For evaluating the adsorption kinetics, pseudo first order, pseudo second order, and intra-particle diffusion models were used to fit the experimental data by using linear regression analysis method. The calculated value of the amount of oligomers adsorbed (Qcal), the experimental determined amount of oligomers adsorbed (Qexpt) and their corresponding regression coefficient values (R2) are presented in Table 6, Table 7 and Table 8. From the kinetic data, the calculated qcal value from pseudo first order model is less than that of the experimental value qexpt (Table 7). But the calculated qcal value from pseudo second order model is nearly the same as that of the experimental value qexpt. Knowing that when the calculated value is equal or nearly the same as that of the experimental value, it is an indication of the best model for the adsorption process. In this present study, the plot of t/qt against time for the pseudo second order reaction for the adsorption of oligomers on the deactivated adsorbents is shown in Figure 8.

Table 6: Pseudo first and second order kinetic models Adsorbent

Time [min]

Equilibrum adsorbate, qe [mg/g]

Adsorbate at time t, qt [mg/g]

In (qe-qt)

t/qt


0.00

1.30

1.48

-

0.00


3.00

1.30

1.40

-

2.14


6.00

1.30

1.25

-2.96

4.79


9.00

1.30

1.24

-2.69

7.28


12.00

1.30

0.86

-0.81

13.95

20 % DCDMS-Alumina

0.00

2.38

2.13

-1.41

0.00

20 % DCDMS-Alumina

3.00

2.38

2.33

-3.04

1.29

20 % DCDMS-Alumina

6.00

2.38

2.32

-2.96

2.58

20 % DCDMS-Alumina

9.00

2.38

2.35

-3.72

3.83

20 % DCDMS-Alumina

12.00

2.38

2.49

-

4.82

25 % D-Silica gel

0.00

2.49

1.90

-0.53

0.00

25 % D-Silica gel

3.00

2.49

2.15

-1.08

1.40

25 % D-Silica gel

6.00

2.49

2.31

-1.71

2.60

25 % D-Silica gel

9.00

2.49

2.40

-2.39

3.76

25 % D-Silica gel

12.00

2.49

2.15

-1.08

5.59


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

The plots of pseudo second order reaction passes through the origin as seen in Figure 8. To describe the success of the kinetics of the adsorption, the plots must pass through the origin of the graphs. It is therefore, right to conclude that pseudo second order reaction is the best model by which oligomer adsorption follows.

Figure 8 Pseudo second order kinetic model Also considering the correlation coefficients for pseudo second order reaction kinetics in Table 7 (0.9232, 0.9976 and 0.9931) are nearly unity. The pseudo second order reaction recorded a standard deviation of 0.32. These results give an indication that the adsorption of oligomers is better represented by pseudo second order reaction model. The correlation coefficient (R2), averagely 0.9713 is higher for pseudo second order and thus shows that the adsorption of oligomers by the deactivated adsorbent follows pseudo second order kinetic model.

Table 7: The parameters of pseudo second order kinetic model Adsorbent

Q expt

Q cal

K1

R2


2.256

2.441

0.764

0.9232

20 % DMDCS-Alumina

2.333

2.238

0.420

0.9976

25 % D-Silica gel

2.231

2.506

0.399

0.9931

The same experimental data were used for intra-particle diffusion model and intra-particle diffusion constant [Ki], intercept and correlation co-efficient (R2) are thus calculated (Table 8 and Figure 9). The intra-particle diffusion in the adsorption of oligomers onto the deactivated adsorbents at various temperatures was plotted as qt against t1/2 and used in the determination of diffusion rate parameters. The intra-particle diffusion plot has time as a limiting factor and this might be due to mass transfer effect. The initial portion of the plot reflects the boundary layer effect and the other portion represents the intra-particle diffusion effect. The correlation coefficients for intra-particle diffusion (R2), 0.9044, 0.8832, 0.8385 (Table 9) were lower than that of pseudo-second order kinetics. Also, the plots do not pass through the origin. This is an indication that there is some level of boundary layer control. 20 % DMDCS deactivated alumina is thus best suited here for the adsorption of the oligomers.

Table 8: Intra-particle diffusion model Sample

mass of sample[g]

titre [mL]

B20 aged 80 h at 150°C

2.01

2.46

20%DMDCS-Silica gel

2.03

1.31

value

Time[min]

t0.5

qt [mg/g]

45.00

6.71

1.30


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Energy & Fuels

20%DMDCS-Silica gel

1.56

1.15

12.00

3.46

0.86

20%DMDCS-Silica gel

1.87

1.26

9.00

3.00

1.24

20%DMDCS-Silica gel

1.56

1.04

6.00

2.45

1.25

20%DMDCS-Silica gel

1.80

1.08

3.00

1.73

1.40

20%DMDCS-Silica gel

1.67

0.95

0.00

0.00

1.48

20%DMDCS-Alumina

2.01

0.34

45.00

6.71

2.38

20%DMDCS-Alumina

2.02

0.39

12.00

3.46

2.49

20%DMDCS-Alumina

2.02

0.36

9.00

3.00

2.35

20%DMDCS-Alumina

2.06

0.24

6.00

2.45

2.32

20%DMDCS-Alumina

2.00

0.38

3.00

1.73

2.33

20%DMDCS-Alumina

2.04

0.56

0.00

0.00

2.13

25%D-Silica gel

2.01

0.40

50.00

7,07

2.49

25%D-Silica gel

0.50

0.14

12.00

3.46

2.15

25%D-Silica gel

2.00

0.14

9.00

3.00

2.40

25%D-Silica gel

2.05

0.24

6.00

2.45

2.31

25%D-Silica gel

2.02

0.29

3.00

1.73

2.15

25%D-Silica gel

1.69

0.64

0.00

0.00

1.90


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

From the above, the intercept value indicates that the graph is not passing through the origin. Therefore, some other process could control the adsorption. The values of the correlation co-efficient (R2), 0.9044, 0.8832, and 0.8385 been less than that of pseudo second order model, 0.9232, 0.9976, and 0.9931 confirms it.

Figure 9 Intra-particle diffusion model

Table 9: The parameters of Intra-particle diffusion model Adsorbent

C

Ki

R2


1.68

0.10

0.9044

20 % DMDCS-Alumina

2.19

0.05

0.8832

25 % D-Silica gel

2.00

0.08

0.8385

Thermodynamic parameter [∆G°] The thermodynamic parameter, the changes in standard free energy change [ΔG0] for the adsorption process of oligomers by the adsorbent was calculated based on the adsorptions at different temperatures. The values of these calculated parameters are shown in Table 10. Gibbs free energy represents the energy that is free to do useful work for a spontaneous reaction process. Essentially when a process results in a negative change in the amount of Gibbs free energy, it is said to occur spontaneously. Therefore, the negative sign simply means that energy is released when a chemical reaction occurs while a positive sign means you have to provide energy to make the chemical reaction happen [19, 20, 21]. The adsorption data indicates that ΔG° values for all the adsorbents except 20 % DMDCS-Silica gel were negative at all temperatures. The magnitude of ΔG° suggests that the adsorption is a physical adsorption process. From Table 10, the 20 % DMDCS deactivated alumina recorded the lowest energy at both temperatures. The 20 % DMDCS-Silica gel has a higher energy barrier to overcome and therefore, its adsorption would take a longer time to occur.


Page 16 of 20

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Energy & Fuels

Table 10: Thermodynamic parameters Sample

Mass of sample [g]

titre value [mL]

TAN, Ce [mg KOH/g]

Ci [mg KOH/g]

X=Ci-Ce [mg KOH/g]

X/m

log Ce

log (X/m)

Ce/(X/m)

1/T

In Ce

Temp [K]

∆G° [J/mol]

X/Ce

In Kd

25 %D-Silica gel

2.00

0.88

2.47

10.46

7.99

0.40

0.39

-0.40

6.18

0.0034

0.90

298.00

-2908.60

3.23

1.17

25 %D-Silica gel

2.01

0.53

1.49

10.46

8.97

0.45

0.17

-0.35

3.32

0.0025

0.40

393.00

-5865.34

6.02

1.80

20 %DMDCSSilica gel

2.01

2.46

6.88

10.46

3.58

0.18

0.84

-0.75

38.44

0.0034

1.93

298.00

1618.49

0.52

-0.65

20 %DMDCSSilica gel

2.01

2.65

7.37

10.46

3.09

0.15

0.87

-0.81

47.70

0.0025

1.10

393.00

2840.18

0.42

-0.87

20 %DMDCSAlumina

2.00

0.77

2.17

10.46

8.29

0.41

0.34

-0.38

5.24

0.0034

0.77

298.00

-3320.75

3.82

1.34

20 %DMDCSAlumina

2.02

0.37

1.03

10.46

9.43

0.47

0.01

-0.33

2.18

0.0025

0.03

393.00

-7235.13

9.16

2.21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

Total Acid Number (TAN) Acid number is a measure of the amount of acid in the fuel. These acids emanate from two sources: during the production of the biodiesel; and as a byproduct from oxidation degradation [21]. The degradation products of biodiesel include acidic products. These acidic volatile decomposition products whose presence causes changes in the level of the total acid number are formed during the aging. This also gives an indication of the effectiveness of the adsorbent in excluding some of the oligomers. As shown in Table 10 and Figure 10, there is decrease in the total acid number of the samples which were adsorbed. This confirms that the adsorption process did remove some oligomers from the aged base oil-RME mixture. The deactivated alumina removed the most acidic products having recorded a total acid number of 1.03 mg KOH/ g which is the least acid number. 25 % distilled water deactivated silica gel also followed with 1.49 mg KOH / g as total acid number. The order shown here collaborates the trend exhibited with respect to the energy barriers and the spontaneity of the adsorption reactions.

Figure 10 Total acid number determination before and after separation with adsorbents 4.8 Impact on additives The impact of the adsorption process on the additives in motor oil was detected using the measure of concentrations of some selected elements before and after the adsorption process. Calcium, Phosphorus and Zinc were selected and their concentrations were determined using ICP-OES. The results as indicated in Figure 11 shows that there is essentially no net significant impact on the additives


18

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

by the adsorbents. The concentration of the elements before and after the separation registered marginal differences. The results of the other adsorbents, 25 % D-silica gel and 20 % DMDCS-silica gel did not differ much from this.

Figure 11 Concentration of selected elements before and after separation with 20 % DMDCS-Alumina Conclusion This study investigated the use of adsorption to remove oligomers formed in aging mixture of base oil and RME as a function of adsorbent dose, initial concentration, and temperature. It can be stated that the adsorption equilibrium correlated quite well with Freundlich isotherm. The Freundlich isotherm recorded a correlation coefficient of 0.9844 and 0.9433 for 25 % D-silica gel and 20 % DMDCS-Alumina respectively while that of Langmuir were 0.9399 and 0.8627 respectively too. The Freundlich’s constants for deviation from linearity of adsorption were 1.56 and 1.07 for 20 % DMDCS-Alumina and 25 % D-Silica gel respectively indicating that the adsorption of oligomers was very favorable. The pseudo first order reaction recorded a standard deviation of 2.64 while that of pseudo second order reaction reported 0.32 indicating a better fit for the description of the adsorption process. The changes in standard free energy change [ΔG0] for the adsorption process of oligomers at 25 °C and 120 °C by 20 % DMDCSAlumina, 25 % D-silica gel and 20 % DMDCS-silica gel were -3.32 KJ/mol, -7.24 KJ/mol; -2.91 KJ/mol, -5.87 KJ/mol; and 1.62 KJ/mol, 2.84 KJ/mol respectively. These adsorption kinetics of oligomers onto the adsorbent, the thermodynamic results suggest that adsorption of oligomers on the adsorbent is spontaneous and physical in nature. The total acidity of 10.46 mg KOH/ g of the mixture of base oil and RME aged for 80 h after the adsorption was reduced to 1.03 mg KOH/ g, 1.49 mg KOH/ g, and 7.37 mg KOH/ g by 20 % DMDCS-Alumina, 25 % D-silica gel and 20 % DMDCS-silica gel respectively. This adsorption process has shown reduction of the acidity to between 90 and 30 % according to the adsorbent used. 20 % DMDCS-Alumina adsorbed about 57 % of the oligomers while that of 25 %D-silica was 48 % with 20 %DMDCS-silica adsorbing about 36.88 % of the oligomers. It can therefore, be concluded that adsorption has considerable potential for the removal of oligomers. CONFLICT OF INTEREST: None


19

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 20

References [1] https://ec.europa.eu/clima/policies/strategies/2050_en (assessed 14/2/17) [2] EU Regulation CO2 emissions from new passenger cars as of 2020 cep Policy Brief No. 2012-46 of 12 November 2012 http://www.cep.eu/en/analyses-of-eu-policy/climate-protection/low-carbon-economy (assessed 13/2/17) [3] BioKraftQuG (2006). Gesetz zur Einführung einer Biokraftstoffquote durch Änderung des Bundes [4] https://www.mbusa.com/vcm/MB/DigitalAssets/pdfmb/serviceandparts/biodiesel_Brochure5.pdf) (assessed 9/2/17) [5] Fang, H., McCormick, R. (2006); Spectroscopic Study of Biodiesel Degradation Pathways; SAE paper 2006-01-3300 [6] Dick Beercheck, Green Fuels give engine oils the blues, L’n’G | Europe – Middle East – Africa May/June 2008 [7] Levent Yüksek, Hakan Kaleli, Orkun Özener, Berk Özoğuz. The Effect and Comparison of Biodiesel-Diesel Fuel on Crankcase Oil, Diesel Engine Performance and Emissions, FME Transactions (2009) 37, 91-97 [8] Xin He, Aaron M. Williams, Earl D. Christensen, Jonathan L. Burton, and Robert L. McCormick National Renewable Energy Laboratory October 5, 2011 [9] Goodrum, J.W. Volatility and boiling points of biodiesel from vegetable oils and tallow, Biomass and Bioenergy 22 (2002) 205 – 211 [10] Shanta, Sultana Mahbuba, "Investigations of the Tribological Effects of Engine Oil Dilution by Vegetable and Animal Fat Feedstock Biodiesel on Selected Surfaces" (2011). Electronic Theses & Dissertations. 776. http://digitalcommons.georgiasouthern.edu/etd/776 [11] Kiatkong Suwannakij, Teerapong Baitiang, Manida Tongroon,*, Sathaporn Chunhakitiyanon and Nuwong Chollacoop Biodiesel Contamination in Engine Lube Oil, The Second TSME International Conference on Mechanical Engineering, 19-21 October, 2011, Krabi [12] Matthew J. Thornton, Teresa L. Alleman, Jon Luecke, and Robert L. McCormick, Impacts of Biodiesel Fuel Blends Oil Dilution on Light-Duty Diesel Engine Operation, Presented at the 2009 SAE International Powertrains, Fuels, and Lubricants Meeting, Florence, Italy, June 15−17, 2009 [13] Ihwan Haryono, M Taufiq Suryantoro, Effects of Using Biodiesel on Engine Generator Components, International Journal of Engineering and Technology (IJET) – Volume 4 No. 5, May, 2014 [14] (https://www.deere.com/en_US/industry/engines_and_drivetrain/learn_more/biodiesel/what_every_biodeisel_user_need s_to_know/every_biodiesel_user.page) [15] http://www.lubricants.total.com/news/fuel-dilution-of-engine-oil-causes-and-effects.html ( 9/02/2017) [16] Schütte, T (2006) Bewertung eines neuartigen Dieselkraftstoffs hinsichtlich Emissionen und Auswirkungen auf Motorkomponenten im Flottenversuch; Diplomarbeit, Volkswagen AG Wolfsburg [17] https://ec.europa.eu/energy/intelligent/projects/sites/iee-projects/files/projects/documents/biofuel_marketplace_biofuel_standards_for_transport_in_the_eu.pdf (9/02/2017) [18] Singer, A., Ruck, W., Krahl, J.: Influence of Different Biogenic Fuels on Base Oil Aging, SAE-Technical Paper 2014-01-2788, 7 S., 2014. [19] Salwa M. Al-Rashed and Amani A. Al-Gaid Kinetic and thermodynamic studies on the adsorption behavior of Rhodamine B dye on Duolite C-20 resin, Journal of Saudi Chemical Society, Volume 16, Issue 2, April 2012, (209–215) [20] Santhi M., Kumar P.E. Adsorption of Rhodamine B from an Aqueous Solution: Kinetic, Equilibrium and Thermodynamic Studies. International Journal of Innovative Research in Science, Engineering and Technology. Vol. 4(2) February 2015 [21] Biodiesel guidelines, March 2009, Worldwide Fuel Charter Committee


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Development of an adsorption process to selectively remove

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