A recent report issued by Grand View Research Inc. indicates the global market for specialty fuel additives is expected to reach $8.5 billion by 2020. Surging global demand for ultra-low sulfur diesel (ULSD) is predicted to position diesel ahead of gasoline as the leading application market for specialty fuel additives by 2020. GVR estimates the diesel market for specialty fuel additives will grow at a compound annual growth rate (CAGR) of 6.5 percent from 2014 to 2020. More specifically, cold flow improvers (CFIs) are predicted to be the fastest growing product segment with an estimated CAGR of 7.9 percent from 2014 to 2020. Clearly there is money to be made and room for improvement in the biodiesel additive market. Researchers around the world are exploring new ways to improve cold flow additives and diluents, and better understand interactions of additives with aging fuel blends’ petroleum and biogenic components.

In Malaysia, researchers published a paper this year titled, “Influence of Chemical Blends on Palm Oil Methyl Esters’ Cold Flow Properties and Fuel Characteristics.” The authors—Universiti Malaysia Pahang’s Obed M. Ali, Rizalman Mamat and Abdul Adam Abdullah, University of Southern Queensland’s Talal Yusaf, and Universiti Teknologi MARA’s Nik R. Abdullah—set out to evaluate the improvement of properties of palm biodiesel from adding ethanol, butanol and diethyl ether, and how these additions affect energy content of the fuel, engine power and fuel consumption.

The researchers note that addition of ethanol, butanol and diethyl ether can cause a regular low temperature operability improvement of palm oil biodiesel with the increase in additive proportion.Increasing additive content resulted in a significant improvement in pour point (PP) with a maximum decrease of 5 degrees Celsius in PP at 5 percent diethyl ether compared to palm B100. Additionally, a statistically significant PP variation between the different chemical additives was observed as the mean palm B100 PP temperature with diethyl ether being around 1 C less than that with ethanol and 2 C less than that with butanol at 5 percent blending ratio. A linear reduction in palm oil biodiesel kinematic viscosity and density was indicated with an increase in the chemical additive blending ratios. The lower viscosity was for blends of biodiesel-diethyl ether blend mixtures with 16.5 percent reductions at 5 percent blending ratio compared to palm B100, whereas biodiesel-butanol blends mixtures were progressively more viscous. The research shows that palm B100 with diethyl ether blend exhibited optimum properties with slightly superior cold flow performance, kinematic viscosity, heating value, acid value and engine performance in comparison to ethanol and butanol, suggesting that diethyl ether may be the most prudent choice among the selected additive-biodiesel blends.

In the U.S., USDA Agricultural Research Service scientist Robert Dunn is well-known for his work over the years to understand and improve cold flow characteristics of biodiesel. This year, he and colleagues from USDA-ARS Helen Ngo and Michael Haas conducted a study titled, “Improving the Cold Flow Properties of Biodiesel by Skeletal Isomerization of Fatty Acid Chains.” 

“I wanted to know if the basic chemical structure of biodiesel could be altered by organic synthesis to improve their cold flow properties,” Dunn tells Biodiesel Magazine.  Co-author Ngo came up with a way to alter the normal structure of oleic acid and produce mixtures of iso-oleic and iso-stearic acids that could be esterified and then mixed with ordinary biodiesel. He says the objective was to alter the straight hydrocarbon chain part of the fatty acid methyl ester (FAME) structure to introduce a small degree of branching. “This would allow the synthesized ‘BC (branched chain)-FAME’ to have very good cold flow properties with hopefully very little increase in fluid viscosity. Knowing that the synthesis process might prove to be expensive, it was my idea to try them as additives/diluents in mixtures with ordinary biodiesel.”

BC-FAME isomers were tested in B100 from canola, soybean and palm oils. CFIs made for biodiesel blends generally work only on the petroleum diesel portion of the blend. “CFI additives are most effective in blends with lower ratios of biodiesel present,” Dunn says. “According to the literature, most CFI additives are composed of comb-shaped copolymers that have a hydrocarbon backbone with small functional groups periodically attached along the chain. The hydrocarbon chain resembles a saturated alkane while the functional groups act like teeth to hinder crystalline growth. Some additives are designed to coat crystal nuclei as the form to prevent them from growing and agglomerating to form larger crystals. Since the crystal/additive complexes have generally a higher density, they eventually begin to settle. Thus, the second type of additive is known as wax anti-settling additives.”

Dunn says in order for a CFI additive to work, crystal nuclei must have begun to form. Afterward, the additives co-crystallize inside the crystal allowing them to activate and disrupt the packing inside the lattice. “This causes the crystal shape and growth rate to change, keeping them small and needle-shaped, and allowing them to penetrate the filters and other obstructions in various fueling systems,” he says. “For an additive to be effective for biodiesel, it is my belief that a similar strategy would be effective. That is, the main structure of the additive would resemble a normal, saturated C18-C20 hydrocarbon chain and have small functional groups attached to the chain that disrupt the packing inside the crystal. An easy way to test this hypothesis was to synthesize the two BC-FAME isomers and test them in mixtures with ordinary biodiesel.”

The best results were seen with higher concentrations of BC-FAME. “The additives were more effective when present at 20 to 39 percent by mass and probably should be classified as diluents,” Dunn says. He defined “additive” levels as being up to 2 percent by mass, and diluents levels has being higher. The results were not greatly different when compared to weighted average cloud point (CP) and PP values taken between the unmixed ordinary biodiesel and BC-FAME. “That is,” he explains, “the results appear to be linear at all mixture levels between 0.5 and 50 percent by mass. Effects became noticeable only at the higher BC-FAME concentrations.”

All the biodiesels tested responded in the same manner with no particular feedstock demonstrating a favorable response. “The study also found that the iso-stearic BC-FAME were as effective as the iso-oleic BC-FAME, at least in terms of reducing the CP and PP of soybean and palm oil FAME.” He says the canola oil FAME demonstrated a split response. The iso-stearate BC-FAME was more effective in reducing CP of canola oil FAME while the iso-oleate BC-FAME was more effective in reducing PP of canola oil FAME. “My explanation for the latter has to do with similarities between the hydrocarbon chain structures where both are C17 or C18 and monounsaturated,” Dunn says. “I do not have an explanation for the former, and can only say that it may have to do with compatibilities between the higher-melting components in the canola oil FAME—that is, the 6 percent saturated FAME—and the iso-stearate BC-FAME. If the higher melting components begin to crystallize and precipitate first, then they may be complexing with the BC-FAME when it is present in the mixture.”

The CP and PP of ordinary biodiesel decreased mainly as a result of dilution with the BC-FAME. “Statistical analysis indicated that 5 to 10 percent by mass BC-FAME was necessary to effectively decrease CP and PP,” Dunn says. These decreases, however, were relatively small at less than 2 C. “The more practical comparison, made by decreasing CP or PP by at least 5 C, indicated 20 to 39 percent by mass of the BC-FAME was necessary,” he says.

Though Dunn admits using BC-FAME to improve biodiesel cold flow is not economical, he says the research may demonstrate that biodiesel composed of at least partially branched-chain esters are more attractive with respect to improved cold flow properties. “The procedure may be applied in two forms,” he says. “Synthesis from oleic acid followed by mixing with ordinary biodiesel, or partial synthesis performed directly on the ordinary diesel. In terms of developing CFI additives for biodiesel, it shows that the main structure should resemble a FAME molecule, not necessarily an alkane such as what has been done in the past. Additives or diluents specific to improving the cold flow properties of biodiesel need to incorporate the FAME structure, not just a medium chain alkane, as a basis for development. These structures may be more sensitive to co-crystallizing with crystal nuclei as they form in biodiesel.”

Finally, Dunn notes that many of the previous attempts to modify the fatty acid tailgroup structure have resulted in molecules with enhanced cold flow properties at the expense of significantly increased fluid viscosities. “The BC-FAME tested in our study had very good cold flow properties in unmixed form and relatively low viscosities—5.6 and 6.0 centistokes at 40 C,” he says.

Hamburg, Germany-based DGMK is managing a project in cooperation with AGQM for the Oel-Waerme-Institut GmbH investigating how aging, additized heating oil blended with biodiesel interacts with freshly added fuel. The project summary states, “It is necessary to clarify if and which interaction may occur between existing aging products and components of the fresh fuel delivery; additives and their mechanisms as well as their effects on storage, compatibility with materials and combustion will be focused on in particular.” The objective of the 30-month long, ongoing project is to determine whether fuel aging can be depicted by a standardized test method. “Aging products that form in fuel-FAME blends are analyzed to be able to identify those aging products responsible for potentially negative interaction,” the project summary states. “Furthermore, probable interaction between aging products and additives shall be illustrated. For that purpose, the refueling circumstances are imitated by aging a fuel admixed with performance additives both in a laboratory and on a pump test stand and then mixing it with fresh fuel also admixed with additives.”

R&D at the Oil Additives Team of Germany-based Evonik Corp. is currently focused on market-driven application testing, says Brian Hess, Evonik account manager. “For example, we’re looking at different combinations of ULSD No. 2 with biodiesel and different ratios of biodiesel and fossil diesel.” Through intensive innovation efforts over the past few years, Evonik has built a full range of CFIs for various feedstock-based biodiesels. “In addition, we laid the groundwork for introducing additives that improve the low temperature performance of biodiesel blends and fossil diesel,” Hess adds. Evonik’s latest biodiesel R&D has led to the development of new multifunctional CFIs. For example, Evonik’s Viscoplex 10-608 is able to treat both fossil fuel and biodiesel blends. Viscoplex 10-780 provides both cold flow improvement and oxidation stability. 

“Our objective is to make our products more robust and more versatile for use across a variety of feedstocks,” Hess tells Biodiesel Magazine. “We have a full range of CFIs adapted to different feedstocks and each of Evonik’s biodiesel products have been evaluated with the complete battery of relevant tests: CP, PP; cold filter plugging point (CFPP): and the cold soak filtration test (CSFT).Compared to blending with less saturated, more expensive B100, the use of cold flow additives can often be a cost-effective solution to achieve desired low-temperature properties.”
 
Author: Ron Kotrba
Editor, Biodiesel Magazine
218-745-8347
rkotrba@bbiinternational.com