1. Fachtagung TestRig - Juni 2022 73 Accelerated ageing test bench - BigOxy Chandra Kanth Kosuru Tec4Fuels GmbH, Herzogenrath, Germany Hichame Ait El Mallali Tec4Fuels GmbH, Herzogenrath, Germany Simon Eiden Tec4Fuels GmbH, Herzogenrath, Germany Summary Accelerated ageing methods are very important to forecast the behaviour of the fuel over extended period of time. Tec- 4Fuels has developed an accelerated ageing test bench named “BigOxy” to study the fluid quality over longer periods in a short amount of time. With increase in research and development of several alternative fuels such as OME (Oxy-methylene ether), Alcohol blends with diesel, MTG (methanol to gasoline), etc., it is necessary to identify the behaviour of the fuels in long term. In BigOxy, we can use different types of fuels such as Diesel, Paraffinic fuels, Biodiesels (FAME) or Gasoline as well as other fluids like lubricants or cooling media. Additionally, the system is able to investigate various material compatibilities by immersion of materials or system components in the fluid during the test run. This paper will explain the test setup BigOxy and the principle of its working in various applications with a variety of fluids. 1. Introduction Accelerated ageing methods are used to identify the longterm behaviour (ageing behaviour) of fuels in a short period of time. Storing the fuels for longer periods to observe their behaviour seems impractical in all the cases. These methodologies help to identify the stability of the fuel over time and allows to take the necessary action to address the issue. Alongside the stability check, the fuel-material interaction can also be testing in a quick time. Fuel degradation or fuel ageing has been a problematic phenomenon during long term storage or when subjected to higher temperatures [1]. This phenomenon is observed in various applications such as domestic heating systems, automotive, generators, etc [2-4]. Degradation of fuels can lead to formation of heavy molecular compounds, normally known as deposits or sediments [1, 4]. These sediments can lead to injector blocking, irregular spray formations, fuel lines blocking, deteriorating the fuels properties leading to material incompatibilities, etc. To avoid this phenomenon, the fuels are mixed with different types of fuel stabilizers, which will help in increasing the fuels resistance towards ageing. There has been a significant research in progress towards the implementation of Novel-fuels as diesel or gasoline fuel blends or replacements. These Novel fuels include GTL (Gas to liquid), HVO (hydrotreated vegetable oil, blends of alcohols, and E-fuels such as OME (Oxy-methylene Ether), etc. With the introduction of these novel fuels, the importance of testing their stability and compatibility is high before market introduction. The accelerated ageing methods help in identifying their behaviour and compatibility over longer periods of time. 1.1 Auto-oxidation The phenomenon of fuel ageing during long term storage or when exposed to higher temperatures is due to auto-oxidation reaction [1]. When the fuels are exposed to atmospheric oxygen, the hydrocarbon molecules in the fuel react with this oxygen and start the auto-oxidation reaction. The ability of the fuel to resist this reaction is called fuel stability. The auto-oxidation reaction is described as follows: Figure 1: Mechanism of auto-oxidation [1, 5] 74 1. Fachtagung TestRig - Juni 2022 Accelerated ageing test bench - BigOxy The auto-oxidation occurs when the hydrocarbons from the fuel are susceptible to react with oxygen in the atmosphere leading to the formation of alkyl radicals [1]. This is called the initiation step. This step is the slowest step of the reaction and once initiated the next steps follow rapidly. This initiation step is the determining factor for the fluid stability. Further, this radical would react the available oxygen and lead to a sequence of chain growth reactions (propagation step) forming hydroperoxides [1, 5]. Finally, these molecules further polymerise into long chain products or higher molecular products also called the sediments or deposits in the Termination step [1]. The accelerating ageing methods concentrate on the initiation step, by providing enough activation energy to kick start the initiation step by inducing higher temperatures [4]. Further steps are self-sustaining and fast occurring reactions [1, 6]. 1.2 Accelerated ageing methods There have been several types of accelerated ageing methods using the principle of PetroOxy [4, 5], Rancimat [6] and titrations with Carbonyl [3]. Alternatively, Rancimat method is also considered as an effective method to perform accelerated ageing method. Flitsch et al. [6] has use the Rancimat method to quantify the ageing products in Bio-diesel. But the volumes of the samples obtained after the test are in smaller quantities. Black et al. [3] use the method of titrating the sample with carbonyl to obtain an indirect parameter to obtain the age of the Pyrolysis oil. This method also includes the subjection of fuel sample to as high as 80 °C. The current paper focusses on the methodology derived from the principle of PetroOxy. Relative to the above mentioned methodologies, the principle of BigOxy provides more flexibility and opportunity to investigate bigger sample volumes [4, 5]. The major influential parameter in this method is the temperature. Also, this method has the advantage of avoiding any influence of external agents such as carbonyl. But also gives the flexibility to use ageing catalysts, if required [4, 5]. 2. BigOxy BigOxy is an accelerated ageing testbench, which uses the principle of PetroOxy (DIN EN 16091) but on a larger scale [5]. BigOxy consists of a larger reactor volume of 1.5 litre compared to 20 ml in PetroOxy. The basic principle of subjecting the fuel volume to higher temperatures to induce accelerated ageing remains the same. This method can age up to 1 litre of fluids at different temperatures and pressures. This section explains the test setup of the BigOxy and its principle in detail. 2.1 Test Bench The Piping and instrumentation diagram (P&ID) of the BigOxy testbench is shown in Figure 2. The test bench consists of a big thermal oil bath, where the reactors are placed to heat up to required temperatures. The bath can accommodate maximum of 8 reactors for each experiment. The bath is heated up using a heat-pump which regulates the temperature setpoint according to the requirement. Two stirrers are used to keep the temperature of the bath homogeneous across all reactor positions. Figure 2: P&ID of the BigOxy test bench The reactors are made of stainless steel, with a total volume of 1.5 litre. Figure 3 shows the image of the BigOxy reactor. The reactor is fitted with a steel tube on its side, where all the electrical components are mounted. The top of the reactor consists of PT100 thermocouple which measures the fluid temperature throughout the test. The side mount consists of a pressure transducer and solenoid valve. The pressure transducer is used to measure the gas phase pressure inside the reactor, while the solenoid valve is used to regulate the gas flow in and out of the reactor. Figure 3: BigOxy Reactor The setpoint for the heat pump is determined proportional to the required fluid temperature. Each reactor is independent to each other in case of pressure setting and the 1. Fachtagung TestRig - Juni 2022 75 Accelerated ageing test bench - BigOxy type of fluid that can be used; but heated to only one temperature set point in each instance due to the common oil bath. The reactors can accommodate fluid volumes from 500 ml to 1 litre. The gas phase can be pressurised around 1.2 - 5 bar (depending on requirement), considering the expansion of gases at higher temperatures. The oil bath is heated up to as high as 180 °C which corresponds to a fluid temperature of 150 °C. The temperature selection of the fluids is completely flexible depending on the requirement and the type of fluid that is used in the reactor for testing. For each experiment, the reactors are cleaned using solvents to completely remove the remains of the old sample and a new sample is filled. Then the reactor closed tightly and filled with gases to the required pressure. The reactors are subjected to leakage test for at least 10 hours and then place in the heated oil bath. The duration of this experiment is flexible, depending on requirement and the fluid type. The typical durations are 24 h, 48 h, 72 h and 128 h. The temperature and the pressure profiles are logged throughout the experiment and used for further analysis. 2.2 Data Analysis The pressure and temperature are continuously monitored over the entire experiment time. Then the two curves are plotted as a function of time, as shown in the Figure 4. Figure 4: Pressure and Temperature curves of a BigOxy experiment The light blue line indicates the pressure profile of the gas phase during the experiment, while the Dark blue line indicates the temperature profile of the fluid inside the reactor. The arrow indicates the respective axis that the curves belong to. The pressure increases as the gas expands due to the heat up of the reactor. The pressure stays constant as long as the fluid shows resistance to react with the atmospheric gas (reactor gas phase). One the reaction starts the fluid consumes the gas phase reactant molecules and lead to a pressure drop. Once the fluid consumes the required amount of reactant molecules, the pressure stabilises and keeps further stable until the end of the experiment. In case of liquid fuels, air is usually used as the gas phase and the reactant molecule is oxygen. The fuel consumes oxygen to start the auto-oxidation reaction. The stable phase until the start of the pressure drop is considered aa the induction time, which indicate the stability of fuel over time. The second stable phase after the pressure drop is considered as the termination stage where the effect is only due to heat (temperature). The induction time is used to compare the stability of different fuel samples. The auto-oxidation reaction which is considered to take longer periods is being accelerated with the influence of temperature of and pressure, which makes the current method an accelerated ageing method. 2.3 Fluid Analysis The samples that are subjected to accelerated ageing must be analysed to observe their change in physio chemical properties. Due to the process of ageing, the fluids tend to change their chemical composition and lead to the formation of new products [4]. For example, in liquid fuels, the fuels tend to lose their oxidation stability, and form more acids and water molecules [1]. In this case, the fuel samples must be analysed before and after the test method to compare the extent of change in their properties [1, 4, 5]. The analytics performed are Oxidation stability analysis (DIN EN 16091 and DIN EN 15751), Acid content (DIN EN 14104), Water content (DIN EN ISO 12937), etc. For other fluids such as cooling fluids the density, viscosity and vapour pressure could also be some critical properties to be analysed and compared with the fresh samples. This would indicate the expected change in properties upon ageing over extended periods of time. This comparison also provides the opportunity to test the effect of various property improvers, stabilizers, etc., also known as additives. The differentiation between different additives and the respective fluids should give the opportunity to provide an optimum final fluid sample for respective applications [5]. Figure 5: Chemical analysis of a middle distillate fuel Figure 5 shows an example on how the chemical properties could change due to ageing of a typical middle distillate fuel (Diesel). The X-axis indicate the initial pressure 76 1. Fachtagung TestRig - Juni 2022 Accelerated ageing test bench - BigOxy applied in the gas phase of the reactor with same fuel volume of 500 ml in all the reactors. The PetroOxy is a measure of oxidation stability. This shows a reduction due to ageing, which is expected upon auto-oxidation reaction of the fuel [5]. The water content tends to rise with ageing, which is a by-product of ageing, as seen in the auto-oxidation process [1, 5] (Chapter 1.1). 3. Compatibility and Reproducibility To identify the compatibility of the testbench with the respective fluids, the fluid and the component interaction must be checked. The components that are in contact with fluids are mostly made of corrosion resistant stainless steel. The heat pump used for the heat up of thermal oil is an oil-specific heat pump. The thermal oil must be exchanged every 6 months, due to its deterioration of heat transfer properties. The reactors are made of stainless steel to avoid any fluid and metal interactions. The electrical components such as pressure transducer, thermocouple and solenoid valve are also resistant to the fluids or any gas phase interactions. This built up also makes the testbench highly reproducible while all the reactors are made of identical components and materials. Each test will have specific temperature as the oil bath is common for all reactors. To have a reproducible result in between experiments, the fluids are always sampled using a standard measuring cylinder. The gas phase is controlled using pressure regulator, which allows us to fill the gas phase to the required pressure in all the cases. The pressure transducers have an accuracy range of 0.5 % of the nominal value and the thermocouple sensor is at maximum deviation of 1 °C. The thermal oil bath is also heated to a specific setpoint depending on the fluid temperature requirement and the accuracy does remain at a range of ± 1 °C. 4. Applications The accelerated ageing method has been a part of different industrial and research projects. The methodology was used to differentiate between the fossil and bio-fuel stability and also differentiating the stability behaviour between the fossil fuels and biofuels and their blends [4]. Not only biofuels but also the system was found to be compatible with the testing of e-fuels of both diesel and gasoline like properties [5]. The test method was also used for the analysis of the fuel additive behaviour and the compatibility of the specific additive with different fuel types. Not only fuel but also different cooling fluids are tested to analyse their long-term behaviour at higher temperature applications. These fluids are tested at different parameters depending on the requirement of the customer. 5. Conclusion and Outlook The testbench is currently focussed on the adaptation to marine fuels. The testing is also focussed on the standardising the methodology for Gasoline fuels. The test bench is also being considered for the testing of fluids with the introduction of catalysts or catalytic materials dipped in the fluids. This could identify the material compatibility of the fluids in the practical applications. Also using different metals in the fluid while ageing can help in identifying the long-term behaviour of the specific metals when subjected to specific fluids at elevated temperatures and pressures. Correspondingly, several property improvers and stabilisers can be proposed upon requirement. The test bench is also further upgraded towards electric heating replacing the thermal oil heating. This will allow the testbench to be operated completely automatic and more variety of testing parameter and flexibility in the testing characteristics can be introduced. References [1] Owczuk M, Kołodziejczyk K. Liquid Fuel Ageing Processes in Long-term Storage Conditions. In: Biernat K (Ed.). Storage Stability of Fuels. InTech (2015). [2] Meshkatodd MR. Aging Study and Lifetime Estimation of Transformer Mineral Oil. American J. of Engineering and Applied Sciences 1(4), 384-388 (2008). [3] Black S, Ferrell JR. Accelerated aging of fast pyrolysis bio-oil: a new method based on carbonyl titration. RSC Adv. 10(17), 10046-10054 (2020). [4] Koch W, Eiden S, Feldhoff S, Diarra D. Entwicklung Einer Neuen Prüfmethode Zur Bewertung Der Stabilität Von Heizölen Mit Biogenen Anteilen: Winfried Koch, Simon Eiden, Sebastian Feldhoff, David Diarra. Deutsche Wissenschaftliche Gesellschaft für Erdöl, Hamburg (2017). [5] Chandra Kanth Kosuru, Natalia Eurich, Simon Eiden (Ed.). Development of a Test method for Accelerated ageing of Gasoline fuels (2019). [6] Flitsch S, Neu PM, Schober S, Kienzl N, Ullmann J, Mittelbach M. Quantitation of Aging Products Formed in Biodiesel during the Rancimat Accelerated Oxidation Test. Energy Fuels 28(9), 5849- 5856 (2014).