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Effect of additives to improve the performance of biodiesel at low temperatures

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Effect of additives to improve the performance of biodiesel at low temperatures Evaluation of a novel type of chemistry to improve the old Filter Plugging Point of Fatty Acids Methyl Esters Lopes, P.M.
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Effect of additives to improve the performance of biodiesel at low temperatures Evaluation of a novel type of chemistry to improve the old Filter Plugging Point of Fatty Acids Methyl Esters Lopes, P.M. 1 ; Muller, D. 1 ; Harrison, R. 1 ; Bordado, J Arizona hemical B.V., European leochemicals Research & Development Group, Almere, The Netherlands 2 Instituto Superior Técnico, Departamento de Engenharia Química, Lisboa, Portugal Abstract: Using several experimental techniques, such as differential scanning calorimetry (DS), thermomiscroscopy and determination of cloud point (P), pour point () and cold filter plugging point (F), polyamides were tested in fatty acids methyl esters (FAME) obtained from vegetable oil triglycerides and fossil diesel fuel as a cold flow improver (FI) additives. The methyl esters were derived from rapeseed, soybean and palm oil feedstock, and the prototype polyamide additive was benchmarked against industry standard ethylene vinyl acetate (EVA) additives. The experimental results showed that the polyamide additive was very effective in the depression of the P and of rapeseed methyl esters (RME). This work also supports the state of the art knowledge regarding the fact that the effectiveness of FI additives is decreased with the blend of esters derived from vegetable oils with higher concentration of saturated fatty acids in the fatty acid profile. Given that the polyamide FI additive is formulated in a matrix of free fatty acids (FFA), it possesses unique properties since it was established that it also provides lubricity and anti-corrosion to ultra low sulphur diesel fuel. Keywords: biodiesel, cloud point, cold filter plugging point, cold flow improvers, crystallization of saturated fatty acids, diesel fuel, kinematic viscosity, low temperature properties, methyl esters, polyamides, pour point. 1. Introduction It is a well established fact that the methyl esters obtained from vegetable oil and animal fats lipids (triglycerides) can be used to power compression-ignition (diesel) engines. In fact, Rudolph Diesel, inventor of the engine with the same name, ran the prototype presented in the 1900 World Exhibition of Paris on peanut oil (Knothe, 2001). Triglycerides are too viscous to enable an effective injection in modern diesel engines, but by promoting the transesterification of these materials in the presence of methanol and a catalyst, the properties of the FAME compares very well with fossil diesel, in terms of cetane number, viscosity and enthalpy of combustion (Suppes, 2005). Biodiesel is a material that is readily degradable through biological processes if spilled on the environment and, because has very high flash points (above 150 ), this fuel is safer to handle and store compared with fossil fuels. Additionally, the combustion of FAME lower the amount of hazardous materials that are released to the atmosphere, such as particulate matter, carbon mono and dioxide, sulphur oxides compounds, etc. The only increase in the emissions is related with Nx, due to the lower combustion temperatures that are registered in the engine cylinder chambers (Mcormik, 2003). Until very recently, the utilization of FAME was also said to have either a negative or zero carbon footprint, meaning that the overall carbon released to the atmosphere during the preparation of the fields, planting, fertilization, cropping, transportation of the crops, transesterification and combustion of the final biodiesel fuel is less than the 2 emitted from the combustion of an equivalent amount of fossil diesel, since the fossilized plants deceased million of years ago do not possess a present counterpart to absorb the carbon during the plant s growth (van de Heuvel, 2005). However, more recent studies are demonstrating, to much controversy, that in fact there is an effective increase in the carbon by utilizing biodiesel as a fuel (Doornbosch, 2007). The low temperature performance is a crucial technical parameter that renders problematic the utilization of FAME in diesel engines in regions of moderate temperature. The performance issues of diesel fuel derived from the formation of crystalline structures at low temperatures are known for many decades by refineries and engine manufacturers. a) b) c) Fig. 1 - Different crystal morphologies: a) plate, b) irregular and c) need shape. In fossil diesel fuel, the main crystalline structures are formed by linear, branched or cyclic hydrocarbons. Pure paraffin crystallizes as monoclinic (chains larger than 26), triclinic (chains shorter than 26) or orthorhombic. However, the detailed discussion of the arbon Footprint and Life ycle Assessment (LA) is being the scope of this paper. (chains with odd number of carbon atoms) structures (Marie, 2005). These considerations are however far from applicable to a common fuel matrix: due to the complexity and number of different molecular species, crystals are never derived from pure but from material with a broad distribution of carbon chain lengths. A study showed that the paraffinic material present in diesel fuels, when crystallizes out of solution, is enriched with the higher molecular weight paraffin with respect to the composition of the initial mixture (Marie, E. et al, 2004). In the case of biodiesel, as the methyl esters are cooled towards its melting point, the fuel will initially become cloudy and upon further cooling, the viscosity will increase until reaching complete solidification. There are three main parameters that can be used to evaluate the performance at low temperatures of a fuel: cloud point, cold filter plugging point and pour point. loud Point The cloud point (P) is defined as the temperature at which crystals first appear in the fuel, and the ASTM D2500 is the standard method for this determination. It is well established that heavy n-alkanes and saturated methyl esters are, respectively, responsible for the P of diesel and biodiesel fuels (Dunn, R.., 2004). This transition corresponds to an equilibrium controlled by kinetics (crystal side) and thermodynamics relationships (solvent side), and although initially the crystals are submicron in size and invisible to the human eye, as temperatures drop further, the particles will reach approximately 0.5 m in size and the crystals will become visible and the fuel will appear as turbid. Pour Point As temperature drops further below the P, the crystals formed will begin to agglomerate first and form aggregates afterwards. This eventually leads to complete solidification, and the fuel, behaving now more like a solid, is no longer able to flow, i.e., no longer pourable (Dunn, R.., 1996). This point, measured accordingly to ASTM D97, is defined as the Pour Point (), and can occur with as little as 1.0 to 2.0 weight percent of precipitated material. old Filter Plugging Point As effective measures of the fuel ability to be used in real life situations, the P and are not acceptable. Upon closer look, the P can be considered as a pessimistic value, since the fuel, even though revealing some turbidity, is still able to flow freely and no issues are expected to occur to the mechanic components. n the other hand, the is an over optimistic value: it is more than certain that a fuel that has reached this point, not only will not flow through the fuel injection system, but most probably already caused severe operational issues, such as completely plugged fuel lines and engine shutdown due to fuel starvation. Therefore, due to the need of having a parameter that correlate well with real life situation, the old Filter Plugging Point (F) was created, correlating to the temperature at which a fuel will plug a 45 m filter (similar mesh opening as installed in vehicles) under standardized injection pressure conditions. There are three main methods to improve the performance of biodiesel at low temperatures: winterization blends with fossil diesel and utilization of FI additives. Winterization Winterization is a popular technique used to reduce the P of vegetable oils in the food industry and used extensively in the manufacture of salads and other dressing s formulations. ne common technique for industrial winterization of biodiesel is to carefully control the refrigeration rate of large storage tanks for a well defined period of time. Afterwards, the liquid portion is separated from the partially solidified material that has settled at the bottom. From a conceptual standpoint, this procedure could be employed in biodiesel production in order to improve the LTP. It has been demonstrated experimentally that winterization improves the low temperature operatibility of FAME. Winterization increases the iodine value, since the remaining species in the fuel are unsaturated. As a trade-off, the oxidative stability is extensively decreased. Winterization also decreases the cetane number of the winterized fuel (Dunn, 1997). Blends with Fossil Diesel Fuel The route of improving the low temperature performance of a fuel by means of diluting with a second fuel with better LTP is well known and used for many years in the petrochemical industry. Refiners have obtained fossil diesel fuel with excellent performance at low temperatures by blending it with kerosene (aviation fuel). However, this is a simple remediation solution, since by diluting the fuel with lighter streams, wax-related problems can be minimized, but not eliminated. The main economical issue related with this procedure is the cost penalty that refineries have to accept from displacing large volumes of a far more profitable fuel to another market (Dunn, R.., 1995). In the same manner as blending vegetable oil with diesel reduces the viscosity, blending biodiesel with fossil diesel causes an enhancement in the LTP of the biodiesel, due to the dilution of the fraction of saturated long-chain methyl esters in biodiesel, therefore lowering the P and (Johnson, 2004). Utilization of FI Additives It is known that the crystal morphology and/or growth rate is profoundly affected by the presence of impurities in the system. Some impurities suppress growth while some others stop growth completely in one or more directions. Some theoretical models have been established that centre the action of the impurity on certain crystallographic faces of the structure, and Arizona hemical / Instituto Superior Técnico, September therefore impurities can also be used to modify the crystal habit (Kubota, 2001). When impurities are added specifically to produce a well defined and desired morphological effect on the crystalline structure, they are then referred as additives. The concentration at which the additives will impart an effect varies, depending on the particular system. An important class of additives is so called tailor-made additives, which are designed to interact in a very specific way with selected faces of crystalline materials. These additives are designed to contain some chemical groups (moieties) that mimic the solute molecule and are therefore readily adsorbed at growth sites on the crystal surface. Impurities and additives operate by binding to growth sites and thereby modifying the crystal growth rate. The overall shape of the growing crystal is determined by the relative rates of growth of its various faces. The slower the growth rate, the larger will be the face. The individual crystals faces will also have their own growth-rate dependence on temperature and super saturation (Hatakka, 2006). Additives to improve the LTP of fossil diesel fuel have been used since the 1960 s (Brown, 1990). Despite the additives companies claiming otherwise, the additives used until recently in FAME are not novelty chemical species, but instead adjustments from the historical fossil diesel FI additives chemistries (Lopes, 2005). The most common type of wax crystal modifiers used to enhance the LTP of diesel is based on ethylene vinyl acetate (EVA) copolymer chemistry. Due to the polymerization flexibility, these compounds can vary in molecular weight and acetate ratio in order to obtain a polymer specifically to a given fuel. It is known that the performance of an EVA copolymer can be enhanced by blending with a FI additive of different chemistry. ther species, such as vinyl acetate-fumarate copolymers, styrene-esters copolymers, diester-alpha olefin copolymers, malanstyrene esters and polymethacrylates are also used for this porpoise. As mentioned, it has long been established that trace amounts of specific impurities can greatly affect crystallization. Petroleum companies have developed specific polymeric additives to both enhance wax nucleation (to produce many small crystal rather than a few big ones) and slow crystal growth. Based on macroscopic evidence, it is generally believed that impurities bond preferentially to specific faces on the growing crystals, inhibiting therefore the growth along those faces. Recently, a more complex model has been proposed, designated by kinetic growth inhibition. It was found that the solidification can occur in the form of macroscopic bands (with several hundred micrometers) parallel to the front. In this growth mode, the front periodically stops growing, allowing a new front to nucleate and spread laterally along the arrested front. These bands are controlled by the thermodynamics of the process, hence the designation of the effect. As the crystallization proceeds, solute is depleted near the front, and as the sample moves though the gradient, a region ahead of this depletion zone becomes supersaturated, a situation designated as constitutional supercooling. However, if a kinetic inhibitor additive is present, the crystals do not grow appreciably (Alkane rystalization Theory, 2007). The interaction between the additives and the crystals in the fuel may take place either in solution, at the surface or inside the crystalline structure, in the case of co-crystallization. 2. Experimental 2.1 Methods loud/pour Point Measurements. The measurements were carried out on a Herzog HP 852 apparatus. The experimental conditions are described on test methods ASTM D2500 and ASTM D97 standards, respectively, for the determination of the P and the. old Filter Plugging Point Measurements. The measurements were carried out on a Herzog HP 842 apparatus. The experimental conditions are described on test method ASTM D6371. hromatography Analysis. G-FID on fatty acid composition according to internal procedure AQM022. orrosion. The corrosion was assessed by the petrochemical industry standard NAE TM0172 test method. Differential Scanning alorimetry (DS). The measurements were carried out sing a standard Mettler apparatus. The sample was placed in a 40 µl aluminum pan (approximately 10 mg sample mass) and the temperature program ran as follows: 25 to -65 with a cooling rate of -5 K/min and -65 to 40 with two heating rates of 2 and 5 K/min, under an atmosphere of 50 ml/min N2. The calorimeter signal was recorded and stored to be used off-line. Lubricity. The lubricity parameter was measure by the High Frequency Reciprocating Rig (HFRR) method in the petrochemical laboratories of the BR refinery in Poland, according to ASTMD6079.The parameter assessed was the Wear Scar Diameter measured in millimetres. Thermomicroscopy. This type of investigation is excellent for providing the correlation between morphological changes with the thermal effects observed with DS experiments. The equipment used was a DS823 e with intracooler and a hot stage (Mettler FP82) with circulating liquid nitrogen for cooling. This equipment can operate can operate in the range from -70 to 300, controlled by a FP80 control unit. The samples were placed between glass slides for the microscope camera. The temperature ranged from 25 to -60 with a cooling rate of -5 K/min. The magnification used was of 6.5 times. Arizona hemical / Instituto Superior Técnico, September 2.2 Products The biodiesel samples were obtained from leading EU producers, such as ADM (Leer and Hamburg in Germany), INES (Verdun in France), etc. Given the nature of the work, these samples did not contain any kind of additives. The fossil diesel fuel was obtained from GALP Refinery in Sines, Portugal, and no additives were present in the fuel. The polyamide material tested was part of the current Arizona hemical s portfolio and the benchmarking EVA additives are commercial products offered by leading companies in the area of fuel additivation. 2.3 Protocol The experimental analysis can be divided in two main stages: an initial screening exercise where a great number of polyamide material was tested to establish a correlation between chemistry/performance and a second stage, where the optimal carrier solvent and treat rate of the polyamide was evaluated and benchmarked with commercially available FI additives. The performance of the Arizona hemical prototype additive was also evaluated in fossil diesel fuel. 3. Results and Discussion 3.1 Stage ne of the Polyamide Evaluation Besides RME, the fatty acid profile of soybean methyl esters (SME) and tall oil fatty acids methyl esters (TFA ME) was measured by G. Table 1 Fatty acid profile of TFA ME, RME and SME Fatty Acid TFA ME (%) RME (%) SME (%) 16:0 Palmitic leic (18:1-9 cis) Linoleic 18:2 cis 9, cis thers Total Saturated ontent A total of seven different samples were selected for the initial evaluation, the criteria used based the molecular weight, the degree of dimer hydrogenation and type of polymer termination group. These samples are referred as samples A to G. Additives A and B are polyamides polymers based on ethylene diamine (EDA), hydrogenated dimer and terminated with an amine group, the main difference being the molecular weight (additive A lower). Additives, D, E, F and G are similar in structure to polyamides utilized in the formulation of flexographic inks, the differing aspect being the molecular weight and overall formulation cost. The following figure indicates the polymer structure of ester terminated polyamides (ETPA) and tertiary amide terminated polyamide (ATPA). R 1 R 2 R 1 Fig. 2 - Polymeric structure of two types of polyamides from Arizona hemical (AZ) portfolio. The results from the initial evaluation of the P and of the polyamides are presented in the following table. Table 2 Experimental results of the behaviuor of RME dosed with diffrent concentrations of different additives. Additive Polyamide treat rate loud point RME Pour Point n.a n.a AZ (B) n.a n.a n.a AZ (A) AZ (H) AZ (I) AZ (J) 500 n.a n.a n.a AZ (K) 400 n.a n.a n.a. -36 AZ (L) n.a. -36 AZ (M) 500 n.a n.a. -33 AZ (N) ommercial Additive 1 Dimer Ester Terminated Polyamide (ETPA) N Dimer NH-H 2 H 2 NH Dimer n.a Based in these experimental results, it was confirmed that the polyamide with the best FI performance were the ATPA type, with additive B providing the best cost structure and being therefore the preferred additive. A more in-depth concentration analysis was conducted with this additive, the optimal treat rate of the being established at approximately 500 ppm. Further increase NH-H 2 H 2 NH Dimer Tertiary Amide Terminated Polyamide (ATPA) R 1 n R 2 N n R 1 Arizona hemical / Instituto Superior Técnico, September Viscosity (mpa.s) Effect of Additives to Improve the Performance at Low Temperatures of Biodiesel in the concentration of this additive causes a decrease in the response. This is completely the opposite to the response of increase in concentration of FI additives known to the art. It was also verified experimentally that the EVA copolymer additive gave a linear response in depression with the increase in concentration. It was surprising to observe that the commercial additive required nearly six times the concentration of additive B to produce similar depressing effect. The following figure indicates the response in terms of depression with increasing treat rate of RME with additive B and the commercial additive 1. An extensive test program was performed to determine the b
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