Method and apparatus for quantifying the oxidation stability of a fuel

Abstract

Method for quantifying the oxidation stability of a fuel, in particular diesel fuel with admixtures of biodiesel or of biodiesel as such, wherein the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz is determined as a measure of the oxidation stability and wherein the imaginary part of the relative permittivity of the fuel is determined from the dielectric loss factor and the real part of the relative permittivity of the fuel.

Description

[0001] The invention relates to a method for quantifying the oxidation stability of a fuel. Furthermore, the invention relates to a measuring device for quantifying the oxidation stability of a fuel.

[0002] Since the enactment of the Biofuels Act (German Bundestag, 2006), a proportion of biodiesel (fatty acid methyl esters, FAME) has been blended with conventional, fossil diesel fuel. This improves the CO2 balance of motor vehicles and machinery, thus protecting the environment. According to the Federal Ministry of Food, Agriculture and Consumer Protection (BMELV), the CO2 emissions from the total consumption of gasoline and diesel fuel are to be reduced by 7 percent by 2020 through the use of biofuels, taking sustainability into account. A significantly greater reduction in CO2 emissions is targeted in the long term.

[0003] However, several studies have observed significant changes in fuel properties during oxidative aging, particularly in biofuels. These changes range from increased viscosity, presumably caused by oligomer formation, to an increase in acid number, a deterioration in lubricating properties, and even precipitate formation in blends (mixtures of biodiesel and fossil diesel fuel). Fuel aging is currently explained by a radical mechanism similar to auto-oxidation.

[0004] Radicals formed by the auto-oxidation mechanism can be neutralized by antioxidants, thereby increasing the oxidation stability of a fuel. The oxidation stability of a fuel describes its ability to withstand oxidative processes without a drastic deterioration of its chemical and physical properties and is therefore an important parameter for quality requirements of modern fuels and biodiesel (source: DIN EN 14214 for biodiesel and DIN EN 590 for B7).

[0005] In addition to the antioxidants naturally present in fuel, such as phenolic compounds or vitamin E (α-, δ-, γ-tocopherols), additional antioxidants, such as butylhydroxytoluene, are added to fuels to further increase their oxidative stability. Once the antioxidants in the fuel are depleted, the fuel begins to age. This process can lead to the formation of short-chain degradation products, as well as dimers, trimers, and, in particular, high-molecular-weight oligomers. Due to their polarity and high molecular weight, the latter can precipitate out of blends and form precipitates.

[0006] Potential damage and problems caused by aged fuel and the associated increase in fuel viscosity can occur, for example, in fuel pumps, injection systems, the engine oil circuit, and exhaust aftertreatment systems. The fuel pump and injection system are particularly sensitive to an increase in viscosity. The high injection pressures of modern diesel engines, in particular, do not tolerate viscosities outside the fuel specification. The continuous entry of fuel into the engine oil is becoming increasingly problematic due to the addition of biodiesel. While conventional fossil diesel fuel is distilled from the engine oil, biodiesel, due to its higher boiling point, remains in the oil. Here, too, biodiesel can form oligomers, leading to deposits and damage. A shortened oil change interval is the consequence.

[0007] Furthermore, aged biodiesel can also cause problems in exhaust aftertreatment. The increased viscosity of aged biodiesel leads to poorer atomization during injection, which can result in an increase in soot or unburned fuel in the exhaust aftertreatment system.

[0008] The problem of aging fuel is particularly acute in plug-in hybrid vehicles. These vehicles have both a battery and a combustion engine, which only engages when high power reserves are needed, such as during highway driving. For short trips, the battery provides sufficient power, so the combustion engine doesn't need to start. It is therefore likely that fuel storage times in plug-in hybrid vehicles will increase significantly compared to current levels. Storage times of several months to a year are realistic. Consequently, the problems associated with age-related changes in the fuel will also increase.

[0009] Long periods of fuel storage are also to be expected in emergency generators. For example, hospitals are equipped with such generators in case of a power outage. These are usually diesel-powered generators. In the event of a power outage, the emergency generator must start up within a very short time to ensure the continued medical care of patients. Therefore, such emergency situations do not tolerate any deterioration in the quality of fuel stored for extended periods.

[0010] Reliable information about the actual fuel quality and its further usability is therefore essential. Accurate, rapid, and robust quantification of the oxidation stability of biofuels or bio-based fuels is thus of enormous importance for ensuring fuel quality.

[0011] A common method for quantifying the oxidation stability of a fuel is the so-called "Rancimat method" (test method according to DIN EN 15751). In this method, the oxidation stability of the fuel is determined via the conductivity.

[0012] One disadvantage of the Rancimat method, however, is the high measurement effort associated with it. Each determination of oxidation stability requires sampling and subsequent conductivity titration, making the measurement method suitable only for stationary use in appropriately equipped laboratories.

[0013] Continuous monitoring of the oxidation stability of a fuel, for example in large-volume fuel tanks at filling stations or in refineries, and especially in plug-in hybrid vehicles, is not possible without prior sampling. However, this is necessary for comprehensive control and guarantee of fuel quality. Furthermore, the Rancimat method cannot detect non-volatile organic acids and other high-molecular-weight oxidation products. The measurements are therefore not sufficiently accurate; consequently, the reliability of the statements regarding oxidation stability is limited.

[0014] Another method for quantifying the oxidation stability of a fuel is described in DE 10 2012 020 520 A1. Here, the relative permittivity is determined as a measure of the fuel's oxidation stability by a capacitive measurement in a low-frequency range below 2 MHz.

[0015] Furthermore, methods for examining the quality of oils and lubricants are known. A measuring apparatus designed for this purpose is described, for example, in US 7,043,402 B2.

[0016] The invention is based, as a first objective, on providing a method which allows for an improved and easily implementable quantification of the oxidation stability of fuels compared to the prior art.

[0017] A second objective of the invention is to provide a measuring device by means of which a corresponding quantification of the oxidation stability can be carried out.

[0018] The first object of the invention is solved according to the invention by a method for quantifying the oxidation stability of a fuel, in particular diesel fuel with admixtures of biodiesel or of biodiesel as such, wherein the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz is determined as a measure of the oxidation stability.

[0019] The invention is based on the premise that reliable information, particularly regarding the higher molecular weight degradation products formed during aging, is necessary to quantify the oxidation stability of a fuel and to ensure fuel quality. However, conventional methods such as the Rancimat method do not allow for this, as they are incapable of detecting either non-volatile organic acids or other high molecular weight oxidation products. Therefore, a reliable quantification of the actual oxidation products formed is not possible.

[0020] Even more flexible methods for determining oxidation stability, such as quantification by determining the dielectric constant or the permittivity, capture the low molecular weight and high molecular weight oxidation products of a fuel together.

[0021] In a second step, the invention surprisingly arrives at the realization that the permittivity of a fuel can nevertheless be used to quantify the oxidation stability if the imaginary part of the relative permittivity is determined in a frequency range between 2 kHz and 500 MHz.

[0022] Surprisingly, determining the imaginary part of the relative permittivity in the specified frequency range directly enables the detection of oligomers formed during fuel aging, as will be described in more detail below. Quantitative analysis of these oligomers offers the unique opportunity to counteract fuel aging even in its initial stages, for example, by dissolving precipitates or sediments through the addition of biogenic components.

[0023] The relative permittivity of a medium is defined as the ratio of its permittivity to the permittivity of a vacuum and characterizes the field-weakening effects of the medium's dielectric polarization. In the case of a fuel containing biogenic components, the relative permittivity generally increases with increasing oxidative aging of the fuel. This increase is caused by the increase in polarity during the fuel's aging process. The formation of polar oxidation products leads to an increase in the orientational polarization within the dielectric, thus to an increasing field weakening within the medium and consequently to an increase in the relative permittivity.

[0024] For a more detailed analysis, the relative permittivity can be written as a complex-valued function depending on frequency and temperature, which allows dielectric losses to be described, especially in an alternating electric field: εr(ω,T)=εr'(ω,T)−iεr''(ω,T). Here, εr'(ω, T) denotes the real part, and εr''(ω, T) the imaginary part of the relative permittivity. The real part ε r '(ω, T) describes the dielectric polarization of induced dipole moments at atoms (displacement polarization), the alignment of permanent dipoles of polar molecules (orientation polarization), and the attachment of free charge carriers to the electrodes (interface polarization). The imaginary part ε rThe ''(ω, T) spectroscopy measures dielectric losses, such as conductivity and polarization losses in the dielectric. These losses arise, for example, from frictional losses caused by the oscillating charges aligning their dipoles or by the movement of charged ions within the dielectric. By measuring the real and imaginary parts of the relative permittivity over a broad frequency range, relaxation processes, in particular, can be detected ("dielectric relaxation spectroscopy").

[0025] In the case of oxidation products formed during aging in a fuel, oligomeric dipoles can follow the electric field without delay at low excitation frequencies and align themselves accordingly. An orientational polarization develops, which can be detected via the real part. However, due to their high molecular masses (between 300 g / mol and 3000 g / mol), alignment of these higher-molecular-weight compounds is no longer possible above a relaxation frequency. The molecules can then no longer follow the temporal evolution of the field. The orientational polarization of the oligomers, i.e., the polarization caused by their alignment in the electric field, becomes progressively less pronounced. As a consequence, in the relative permittivity of an aged fuel, the imaginary part increases at the expense of the real part with increasing frequency of the external field.By examining the imaginary part, a quantitative statement can therefore be made about the high-molecular-weight oxidation products formed and thus about the quality of fuels. The imaginary part of the relative permittivity is, in particular, a measure of the oxidation stability of the fuel.

[0026] The described method for monitoring fuel quality is relatively simple and can be implemented without extensive measurement technology. The permittivity, specifically its real and imaginary parts, can be determined directly in the fuel using capacitive measurement with a suitable sensor. In particular, on-site determination is also possible, for example, in fuel tanks at gas stations or refineries. Determining the shelf life and further usability of a fuel is possible at any desired time.

[0027] The actual aging process of a fuel begins when the additives blended into the fuel, such as antioxidants, are consumed. From this point on, the formation of undesirable oxidation products, and in particular the formation of oligomers, occurs immediately. In a particularly preferred embodiment of the invention, the degree of aging of the fuel is determined based on the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz. The method is thus suitable both for quantifying the oxidation stability and for determining the degree of aging of fuels.

[0028] Preferably, the imaginary part of the relative permittivity of the fuel is determined as a function of time. From the behavior of the imaginary part over time, the increase in the oligomers formed, and thus the oxidation stability, is then preferably determined directly. According to one variant, the imaginary part is determined at a fixed frequency. Alternatively, the imaginary part, or its respective value over time, is averaged over a plurality of frequencies.

[0029] Advantageously, an induction time until an accelerated increase is determined from the temporal evolution of the imaginary part within the specified frequency range, whereby the induction time is used as a measure of oxidation stability. If an induction time until the onset of the observed increase of the imaginary part is determined, this is a measure of the concentration of oligomers formed by aging products and thus of the oxidation stability of the fuel.

[0030] Once the induction time is reached, a fuel is no longer oxidation-stable and begins to age. From this point onward, the imaginary part of the relative permittivity increases sharply. It is particularly advantageous to determine the degree of aging of the fuel after the induction time has been reached by observing the time course of the imaginary part of the relative permittivity. Thus, the induction time can be used directly as a measure of oxidation stability, and in the subsequent period, the actual aging process of the fuel can be observed and its degree of aging determined.

[0031] Determining the degree of aging is particularly advantageous because it allows for a direct assessment of a fuel's shelf life. For example, a sensor can be used that is immersed in a fuel sample on-site at a refinery, thus revealing whether the stored fuel is still usable.

[0032] Our own investigations have shown, in particular, that by selecting excitation frequencies above 100 kHz, the oligomers formed during fuel aging can be reliably detected by measuring or determining the imaginary part. Accordingly, the imaginary part of the relative permittivity is advantageously determined at frequencies above 100 kHz using the presented method. Above this frequency, a detectable increase in the imaginary part of the relative permittivity can be observed in aged fuel.

[0033] The invention further provides that the imaginary part of the relative permittivity of the fuel is determined from the dielectric loss factor and the real part of the relative permittivity of the fuel. The dielectric loss factor describes the ratio between the real and imaginary parts of the complex permittivity and is thus given by the tangent of the angle α of the complex quantity to its imaginary part. The dielectric loss factor can be determined for an electrical quantity by a relatively simple measurement, in particular also from the angle of the phase shift between voltage and current. For the complex permittivity, the loss factor tan α is defined in particular as the quotient of the real part ε and the imaginary part α. r ' (ω, T) and the imaginary part ε r''(ω, T) of the relative permittivity and indicates, in this case, the magnitude of the dielectric losses associated with the alignment of polar molecules and / or the movement of charged ions during the aging of the fuel.

[0034] The loss factor can generally be determined computationally and / or experimentally using various methods. Direct measurement of the dielectric loss factor is preferred. An impedance analyzer is particularly suitable for this measurement. An alternative embodiment involves the use of an LCR functional measuring device. Preferably, the dielectric loss factor is measured using a measuring capacitance, in particular a suitable measuring capacitor.

[0035] In a capacitive measurement, the real part of the relative permittivity is obtained as the quotient of the open-circuit capacitance of the capacitor in air and the capacitance with a dielectric, where in this case the fuel is used as the dielectric. The capacitive determination of the real part is therefore carried out according to the equation: εr'(ω,T)=C(ω,T) / C0(ω,T)

[0036] Preferably, a sensor comprising a capacitor with an uncapacitance above 40 pF is used to determine the imaginary part of the relative permittivity of the fuel. Such a sensor enables the determination of the imaginary part of the relative permittivity using high excitation frequencies and thus the detection of the oligomers formed during aging. Preferably, the capacitor is designed as an interdigital capacitor formed by means of interlocking comb-shaped conductor tracks.

[0037] Advantageously, the capacitor used as a sensor is fully in contact with the fuel under test, or fully immersed in the fuel. This ensures that all oligomers formed during fuel aging influence the imaginary part of the relative permittivity to be determined, thus providing a reliable result regarding oxidation stability.

[0038] The imaginary part of the relative permittivity is preferably determined at temperatures above -25 °C. In particular, the determination of the imaginary part of the relative permittivity is carried out in a temperature range between 10 °C and 70 °C. This temperature range allows for the analysis of relaxation processes. Depending on the selected temperature, the sensitivity of the measurement can be shifted either in favor of the conductivity-contributing oxidation products or in favor of the oligomers. On the one hand, conductivity losses can be detected at higher temperatures due to the lower viscosity, while at lower temperatures, relaxation processes can be determined with greater accuracy. Accordingly, the imaginary part of the relative permittivity is preferably determined at temperatures below 15 °C. The conductivity losses are preferably detected at temperatures above 45 °C.

[0039] To determine the oxidation stability and degree of aging of a fuel sample in the laboratory, it must first be aged. During the determination of the imaginary part of the relative permittivity, an airflow of between 8 l / h and 12 l / h (liters per hour) is advantageously introduced into the fuel to induce aging. Preferably, the airflow is kept constant during the measurement. The airflow contains the oxygen necessary for oxidative aging, which is required for the formation of oxidation products and, in particular, oligomers. The aging state of the fuel under investigation can then be determined from the behavior of the imaginary part as a function of aging, or, if the airflow is introduced, over time.

[0040] Advantageously, when an airflow is introduced to age the fuel, the imaginary part of the relative permittivity is measured at intervals of 30 to 90 minutes over a period of 10 to 30 hours. Within this timeframe, a significant change in the imaginary part of the relative permittivity caused by the oxidative aging of the fuel can be determined and evaluated. The time interval between the measurement points accounts for the oxidation rate, which is particularly pronounced when an airflow is introduced as described above.

[0041] The airflow is preferably introduced into the fuel after a predetermined measurement temperature has been reached and is kept essentially constant throughout the measurement period. It has proven advantageous to introduce an airflow of 10 l / h uniformly into the fuel sample at several points within the measuring device. This ensures that a substantially equal amount of oxygen is available for fuel oxidation at every point within the sample.

[0042] The aforementioned procedural steps for aging a fuel by introducing air and measuring the imaginary part of the relative permittivity over the specified time periods or corresponding time intervals are used in the targeted aging of a fuel sample in the laboratory and serve as experimental proof of the measurement method. The analysis of a real fuel sample is carried out by taking samples of fuel that have aged solely through storage, use, or similar processes.

[0043] During the determination of the imaginary part of the relative permittivity, continuous temperature monitoring of the fuel is advantageous. This allows temperature influences and fluctuations to be taken into account when evaluating the determined values. In particular, the temperature is kept constant at a target temperature over the entire period, so that a correction of the determined imaginary part of the relative permittivity with respect to its temperature dependence is not necessary. If required, the fuel sample can also be cooled or heated to maintain or reach the desired target temperature. Specifically, the fuel temperature is regulated to the target temperature.

[0044] In a further advantageous embodiment, the sensor measures the conductivity of the fuel. The conductivity measurements are preferably performed in a low-frequency range, preferably below a frequency of 2 kHz. The low-frequency range is particularly well suited for determining the conductivity due to oxidation products in a sample.

[0045] This requires that the oxidation products possess a hydroxyl group, enabling proton transfer. These can be present in both short-chain oxidation products (alcohols, aldehydes, ketones, carboxylic acids) and higher-molecular-weight oxidation products (peroxides, hydroperoxides, epoxides, oligomers). Therefore, conductivity measurements in the low-frequency range detect all oxidation products simultaneously.

[0046] This method offers the advantage that, in addition to oxidation stability, the degree of aging of biofuels can also be determined. Measuring the imaginary part of the relative permittivity in a specific frequency range is sufficient to assess the fuel's quality. In particular, oligomers that lead to deposits in aged blends can be selectively visualized using dielectric relaxation spectroscopy in the high-frequency range.

[0047] The second object of the invention is achieved according to the invention by a measuring device for quantifying the oxidation stability of a fuel, in particular diesel fuel with admixtures of biodiesel or biodiesel as such, comprising a capacitive sensor and a control unit, which is configured to determine the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz using the sensor and according to the method described above. The oxidation stability of a fuel can be reliably determined using the measuring device.

[0048] The measuring device is preferably designed to determine the degree of aging of the fuel. This makes it possible to directly determine the oxidation stability and, in particular, the degree of aging of the sample when taking a sample from a storage container such as a refinery tank or a vehicle tank by using the measuring device.

[0049] Furthermore, it is advantageous to install a suitable measuring device, for example in a vehicle, to continuously monitor the oxidation stability and aging level of a fuel. Continuous measurement allows, in particular, for early intervention to counteract fuel aging, for example by adding more antioxidants.

[0050] Preferably, the sensor of the measuring device comprises a capacitor having a nominal capacitance above 40 pF. A measuring device with a capacitor having a nominal capacitance in this range is suitable for determining the oxidation stability of a fuel, particularly by detecting the oligomers formed during fuel aging, as it allows the application of high excitation frequencies.

[0051] The capacitor of the measuring device is preferably designed as an interdigital capacitor. An interdigital capacitor is a capacitive sensor with an interdigital, i.e., interlocking, comb-shaped electrode structure, which, due to its small size and high capacitance, is easily suitable for integration into tank systems. To implement such capacitors, the corresponding comb-shaped electrode structure is applied, for example, as conductive traces on a printed circuit board surface.

[0052] Advantageously, the electrode structure of the capacitor comprises a plurality of electrode fingers, each spaced between 150 µm and 250 µm. The sensitive surface area of ​​the electrode structure preferably has a size of 10 cm². 2 and 15 cm 2 on.

[0053] Advantageously, the sensor is designed and configured to measure the conductivity of the fuel. In other words, the sensor is designed in such a way that it can be used both to determine the imaginary part of the relative permittivity of a fuel and to determine or measure the conductivity of a fuel.

[0054] In particular, the use of this measuring method is intended to complement the Rancimat method. An alternative preferred embodiment provides for a handheld measuring device for the rapid and mobile monitoring of fuel quality, which is used in filling stations, diesel generators, heating oil tanks, or ship tanks where long residence times of the fuel are expected. Here, the fuel quality can then be determined quickly and on-site through regular sampling.

[0055] The sensor's preferably small size also allows for direct integration into the fuel tank system. This enables in-situ monitoring of fuel quality in plug-in hybrid vehicles, where long fuel storage times are also to be expected.

[0056] An embodiment of the invention will be explained in more detail below with reference to a drawing. The drawing shows: Fig. 1 a measuring device for quantifying the oxidation stability of a fuel in a three-dimensional representation, Fig. 2 a top view of a printed circuit board with an interdigital capacitor, Fig. 3 a section of the electrode structure of the interdigital capacitor, Fig. 4. A plot of the real and imaginary parts of the relative permittivity against the frequency for fuel samples of different aging stages at a temperature of 10 °C. Fig. 5 a gel permeation chromatogram (GPC) for fuel samples at different stages of aging, Fig. 6. A plot of the imaginary part of the relative permittivity against the frequency for a fresh fuel sample, as well as for aged fuel samples at a temperature of 10 °C. Fig. 7 a comparative plot of the imaginary part of the relative permittivity at a frequency of 13 MHz and a temperature of 10 °C, and that from the GPC according to Fig. 6 determined data against the aging time of the fuel samples, Fig. 8 a plot of the imaginary part of the relative permittivity against the frequency for fuel samples f of different aging stages at a temperature of 50 °C, as well as Fig. 9 a comparative plot of the imaginary part of the relative permittivity at a frequency of 2 kHz and a temperature of 50 °C, and the conductivity determined by the Rancimat method against the aging time of the fuel.

[0057] In Fig. Figure 1 shows an exemplary measuring device 1, designed for laboratory investigations, for quantifying the oxidation stability of fuel in a three-dimensional representation. The measuring device 1 comprises a sensor 2 with a capacitor 3 designed as an interdigital capacitor, the dimensions of which are shown in the Fig. 2 and Fig. 3 are shown in more detail. The capacitor 3 is mounted on a circuit board 5 and connected to a control device not shown.

[0058] To investigate a fuel, the capacitance of the respective fuel sample 7 is first determined. For this purpose, a predetermined quantity of the fuel sample 7 is placed in the sample chamber 9 so that the capacitor 3 is completely covered. The capacitance of the sample is then measured. Taking into account the open-circuit capacitance of the capacitor, which in this case is 50 pF, the real part of the relative permittivity is calculated. Furthermore, the loss factor of the fuel sample 7 is also measured. From the loss factor and the real part of the relative permittivity, the imaginary part of the relative permittivity is then determined.

[0059] Since the imaginary and real parts of the permittivity are temperature-dependent quantities, a constant temperature is maintained during the measurements. For this purpose, a controllable Peltier element 11 is located below the circuit board 5. A heat sink 15 is located beneath the Peltier element 11 to dissipate the heat generated by the Peltier element 11. The interdigital capacitor 3 is connected to an impedance analyzer (not shown) via connections 17. This allows, in particular, the determination of the dielectric loss factor.

[0060] Capacitor 3 features an interdigital electrode structure consisting of two comb-like electrodes 21, each with 36 conductor tracks 23. The electrode structure is mounted on a circuit board 5 made of a high-frequency circuit board material. The sensitive surface area 25 of the electrode structure measures 38 mm x 29 mm, with a conductor track height of 17.5 µm (see figure). Fig. 2) The distance a between two electrode fingers of electrodes 21 is 200 µm. This is in Fig. 3 shown.

[0061] To quantitatively investigate the oxidation stability of biodiesel, a fuel under investigation (in this case, rapeseed oil methyl ester, RME) was thermo-oxidatively aged using the Rancimat method at a temperature of 110 °C by introducing an air stream with a defined flow rate of 10 l / h. A total of 12 fuel samples were taken at one-hour intervals. A 13th fuel sample was aged for 60 hours. Subsequently, the imaginary and real parts of the relative permittivity of the aged samples were determined using measuring device 1.

[0062] At high temperatures, primarily conductivity losses of the fuel are detected due to the lower viscosity. At lower temperatures, relaxation processes are determined with high accuracy. Therefore, the measurements were carried out at 10 °C and 50 °C.

[0063] Fig. Figure 4 shows a plot (31) of the real and imaginary parts of the relative permittivity against the frequency in a range between 2 kHz and 13 MHz for a fresh fuel sample and a fuel sample aged for 12 hours at a temperature of 10 °C. The values ​​for the real part of the relative permittivity plotted on the left y-axis are taken from curves 33 (aging time 0 h) and 35 (aging time 12 hours). The values ​​for the imaginary part of the relative permittivity plotted on the right y-axis are taken from curves 37 (aging time 0 hours) and 39 (aging time 12 hours).

[0064] The fresh RME shows no significant frequency dependence in the frequency range between 2 kHz and 3 MHz; the value for the real part of the relative permittivity is ε r ' (ω, T) = 3.32 ± 0.02 (curve 33). In this frequency range, only marginal losses are measurable, as can be seen from the determined value of the imaginary part of the relative permittivity of ε. r '' (ω, T) = 0.005 ± 0.003 becomes clear (curve 37).

[0065] In the frequency range between 3 MHz and 13 MHz, relaxation processes due to the methyl esters and antioxidants contained in RME become apparent. Here, the real part of the permittivity for fresh RME decreases (ε). r ' (13 MHz) = 3.058 ± 0.001) and the imaginary part of the relative permittivity of fresh RME to (ε r '' (13 MHz) = 0.098 ± 0.001).

[0066] RME aged over a period of 12 hours shows significantly higher values ​​for the real part of the relative permittivity, which originates from polar oxidation products. In the frequency range between 2 kHz and 2 MHz, ε r ' = 4.90 ± 0.07. From a frequency of 2 MHz a clear decrease is also noticeable here; the real part is already ε at 13 MHz. r ' = 4.220 ± 0.005 (curve 35). This decrease is due to the formation of high molecular weight oxidation products.

[0067] For the actual detection of high-molecular-weight oxidation products, the imaginary part of the relative permittivity is considered due to its higher sensitivity. Caused by oligomers, the imaginary part of the relative permittivity of the fuel sample aged over a period of 12 hours decreases from ε starting at 50 kHz. r '' (50 kHz) = 0.005 ± 0.001 on ε r'' (13 MHz) = 0.295 ± 0.009. In the low-frequency range, an increase in the imaginary part of the relative permittivity of the aged fuel sample can also be observed with increasing aging time, which in turn is due to conductivity losses caused by dissociated ions. Thus, ε is r '' (2 kHz, 0 hours) = 0.004 ± 0.001 and increases to ε after a twelve-hour aging period r '' (2 kHz, 12 hours) = 0.025 ±0.009 to (curve 39).

[0068] In Fig. Figure 5 shows a gel permeation chromatography (GPC) 51 for fuel at various stages of aging. The formation of high-molecular-weight oxidation products was demonstrated using gel permeation chromatography (GPC). Fig. Figure 5 initially shows that with progressive aging, the proportion of oligomers increases from a molar mass of 425 g / mol; with increasing aging, peaks 53 and 55 grow. Simultaneously, the proportion of RME decreases in a molar mass range between 350 g / mol and 425 g / mol, partly due to its reaction to form low- and high-molecular-weight oxidation products (reduction of peak 57). The calibration of the GPC was performed using polyethylene glycol (PEG), so all mentioned molar masses are relative to PEG.

[0069] In Fig. Figure 6 shows a plot of the imaginary part of the relative permittivity against the frequency for fresh fuel (aging time 0 hours) and for aged fuels at a temperature of 10 °C. The plotted values ​​show samples with an aging time of 2 hours (curve 63), 4 hours (curve 65), 6 hours (curve 67), 8 hours (curve 69), and 10 hours (curve 71).

[0070] It can be seen that a significant increase in the imaginary part of the relative permittivity is only observed after an aging period of between 6 hours (curve 67) and 8 hours (curve 69). For samples aged for less than 6 hours, the imaginary part ε r '' (13 MHz) = 0.111 ± 0.02. After an aging period of 8 h, the imaginary part is already ε r '' (13 MHz) = 0.206 ± 0.042.

[0071] Fig. Figure 7 shows a comparative plot 81 of the imaginary part of the relative permittivity at a frequency of 13 MHz and a temperature of 10 °C (curve 83) with the values ​​obtained from the GPC according to Fig. The data obtained from curve 85 (5) were compared to the aging time of the fuel. The data extracted from the GPC were determined by integrating the area under peaks 53 and 55. The results show good comparability between the GPC investigations and the measurements of the imaginary part of the permittivity. The increase in the imaginary part at 10 °C and 13 MHz and the area fraction of the oligomer range from 425 g / mol to 3000 g / mol from the GPC investigations are evident.

[0072] The oxidation stability of the fuel sample can be quantitatively determined from the data, particularly by fitting two straight lines that represent the different slopes over time of the imaginary part of the relative permittivity and the integrated area from the GPC diagram. The "best fit" of two lines 87 and 89 is shown here in the plotted values. The intersection point 91 of lines 87 and 89 marks the induction time, i.e., the point at which the value of the imaginary part of the relative permittivity increases abruptly. Based on the behavior of curves 83 and 85 and the "best fit," an induction time of 6.7 hours was determined for both. The induction time 91 can be directly used as a measure of oxidation stability. The actual aging process of the fuel begins at this point.

[0073] In Fig. Figure 8 shows a plot (101) of the imaginary part of the relative permittivity against the frequency for fuel of various aging stages at a temperature of 50 °C. The plotted values ​​for a fresh sample (curve 103), as well as for samples with an aging time of 2 hours (curve 105), 4 hours (curve 107), 6 hours (curve 109), 8 hours (curve 111), 10 hours (curve 113) and 12 hours (curve 115) are recognizable.

[0074] The low-frequency range above 2 kHz of the imaginary part is also shown, reflecting the conductivity losses due to dissociated ions. As the temperature increases, the viscosity of the fuel decreases, thereby increasing the mobility of the molecules. The dissociated ions contributing to conductivity in aging RME can be detected more effectively at higher temperatures. Thus, the imaginary part ε r'' (2 kHz, 12 h) = 0.159 ± 0.006 at 50 °C. For comparison, the imaginary part ε was r '' (2 kHz, 12 h) = 0.025 ± 0.009 at 10 °C (plot 31 according to Fig. 4, Curve 39).

[0075] Fig. Figure 9 shows a comparative plot 121 of the imaginary part of the relative permittivity at a frequency of 2 kHz and a temperature of 50 °C (curve 123) and the conductivity determined using the Rancimat method (curve 125) against the aging time of the fuel. The Rancimat method yielded an induction time 127 of 6.3 hours, whereas the induction time 129 determined from the values ​​of the imaginary part of the relative permittivity was 7.1 hours, thus deviating slightly from the Rancimat method.

[0076] As already explained in section 81, Fig.7. The induction times 127, 129 were also determined by fitting two straight lines 131, 133, 135, 137, which show the different slopes in the time course of the imaginary part of the relative permittivity (lines 131, 133) and the conductivity (lines 135, 137).

[0077] Alternatively, assuming an initial constancy of conductivity and imaginary part of the loss factor, the first lines 131, 135 could each be fitted as a horizontal line with an ordinate value of 0 and the second lines 133, 137 could be fitted into the measurement points of the constant aging increase.

[0078] The difference between induction times 127 and 129 can be explained by the aging of the samples used to determine the imaginary part of the relative permittivity. The samples were artificially aged using the Rancimat method. During this process, volatile carboxylic acids escape from the samples and enter the conductivity measuring cell of the Rancimat. Consequently, only those acids that could not escape from the original sample were measured when determining the values ​​for the imaginary part. Reference symbol list 1 measuring device 2 Sensor 3 Capacitor 5 circuit boards 7 Fuel sample 9 sample containers 11 Peltier element 13 temperature sensors 15 coolers 17 connections 21 electrode 23 conductor tracks 25 sensitive surfaces 31. Application 33 Real part of the relative permittivity (fresh fuel sample) 35 Real part of the relative permittivity (12 h aged fuel sample) 37 Imaginary part of the relative permittivity (fresh fuel sample) 39 Imaginary part of the relative permittivity (12 h aged fuel sample) 51 Gel Permeation Chromatogram (GPC) 53 Peak Oligomers 55 Peak Oligomers 57 Peak RME 61. Order 63 Imaginary part of the relative permittivity (2 h aged fuel sample) 65 Imaginary part of the relative permittivity (4 h aged fuel sample) 67 Imaginary part of the relative permittivity (6 h aged fuel sample) 69 Imaginary part of the relative permittivity (8 h aged fuel sample) 71 Imaginary part of the relative permittivity (10 h aged fuel sample) 81 Order 83 Imaginary part of the relative permittivity 85 GPC data 87 Even 89 Even 91 Intersection point / Induction time 101 Order 103 Imaginary part of the relative permittivity (fresh fuel sample) 105 Imaginary part of the relative permittivity (2 h aged fuel sample) 107 Imaginary part of the relative permittivity (4 h aged fuel sample) 109 Imaginary part of the relative permittivity (6 h aged fuel sample) 111 Imaginary part of the relative permittivity (8 h aged fuel sample) 113 Imaginary part of the relative permittivity (10 h aged fuel sample) 115 Imaginary part of the relative permittivity (12 h aged fuel sample) 121 Order 123 Imaginary part of the relative permittivity for different aging stages 125 Rancimat conductivity values ​​for different aging stages 127 Induction time (Rancimat) 129 Induction time (imaginary part of the relative permittivity)

Claims

[1] Method for quantifying the oxidation stability of a fuel, in particular diesel fuel with admixtures of biodiesel or of biodiesel as such, wherein the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz is determined as a measure of the oxidation stability and wherein the imaginary part of the relative permittivity of the fuel is determined from the dielectric loss factor and the real part of the relative permittivity of the fuel. [2] Method according to claim 1, wherein the degree of aging of the fuel is determined on the basis of the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz. [3] Method according to claim 1 or 2, wherein the imaginary part of the relative permittivity of the fuel is determined as a function of time. [4] Method according to claim 3, wherein an induction time until an accelerated increase is determined from the time course of the imaginary part of the relative permittivity, and wherein the induction time is used as a measure of the oxidation stability. [5] Method according to claim 4, wherein the degree of aging of the fuel is determined based on the time course of the imaginary part of the relative permittivity. [6] Method according to one of the preceding claims, wherein the imaginary part of the relative permittivity of the fuel is determined in a frequency range above 100 kHz. [7] Method according to one of the preceding claims, wherein the dielectric loss factor is measured capacitively. [8] Method according to one of the preceding claims, wherein the real part of the relative permittivity of the fuel is determined by means of a capacitive measurement. [9] Method according to one of the preceding claims, wherein a capacitive sensor (2) is used to determine the imaginary part of the relative permittivity of the fuel, which in particular comprises a capacitor (3) with an unloaded capacitance above 40 pF. [10] Method according to claim 9, wherein the conductivity of the fuel is measured by means of the sensor (2). [11] Method according to claim 9 or 10, wherein the capacitor (3) is fully contacted with the fuel under investigation for determining the imaginary part of the relative permittivity. [12] Method according to one of the preceding claims, wherein the determination of the imaginary part of the relative permittivity is carried out at temperatures above -25 °C, in particular in a temperature range between 10°C and 70°C. [13] Method according to one of the preceding claims, wherein continuous temperature monitoring of the fuel is carried out during the determination of the imaginary part of the relative permittivity. [14] Measuring device (1) for quantifying the oxidation stability of a fuel, in particular diesel fuel with admixtures of biodiesel or of biodiesel as such, comprising a capacitive sensor (2) and a control device which is configured to determine the imaginary part of the relative permittivity of the fuel in a frequency range between 2 kHz and 500 MHz by means of the sensor (2) according to a method according to one of the preceding claims. [15] Measuring device (1) according to claim 14, designed to determine the degree of aging of the fuel. [16] Measuring device (1) according to claim 14 or 15, wherein the sensor (2) comprises a capacitor (3) having an idle capacitance above 40 pF. [17] Measuring device (1) according to claim 16, wherein the capacitor (3) is designed as an interdigital capacitor. [18] Measuring device (1) according to claim 16 or 17, wherein the capacitor (3) comprises an electrode structure with a plurality of electrode fingers, each having a distance a between them in a range between 150 µm and 250 µm. [19] Measuring device (1) according to claim 18, wherein the sensitive surface of the electrode structure has a size between 10 cm 2 and 15 cm 2 exhibits. [20] Measuring device (1) according to one of claims 14 to 19, wherein the sensor (2) is designed and configured to measure the conductivity of the fuel.

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