The United States Environmental Protection Agency (EPA) first issued emission standards for new heavy duty diesel engines targeting the 1974 model year. Since then, emission targets have been incrementally decreased. In the 2007 Heavy Duty Highway Diesel Rule, the EPA mandated respective reductions of particulate (PM2.5) and nitrogen oxide (NOx) emissions of 90% and 92%, with NOx allowances to drop an additional 3% in 2010. At the time of the mandate release, sulphur sensitive exhaust after-treatment was considered necessary to meet 2007 emission goals. As a result, the 2007 Highway Rule also requires sulphur levels in diesel fuel to drop 97% to 15 ppm.
To meet tightening emission requirements, original equipment manufacturers have relied increasingly on high pressure common rail fuel injection equipment. Combustion temperature and completeness, fundamental drivers of NOx and particulate emissions, are manipulated in the common rail through precision metering, timing, and fuel atomisation. In these systems, injection volumes are on the order of microlitres (one millionth of a litre), injections occur over millisecond intervals, and injection pressures operate in the vicinity of 30,000 psi. Injector spray holes and operating clearances are on the order of microns.
High operating pressure and micron level tolerances make injector systems vulnerable to contaminants and dependent on a minimal level of lubricity. Water is a fuel contaminant of concern for corrosion of steel components and promotion of microbiological growth. Both factors clearly interfere with durability and performance of any diesel injector. For this reason, some injector manufacturers limit fuel to a maximum dissolved water content of 200 ppm, with zero tolerance for free water. This requirement was adopted in 2005 by the European Union in diesel specification EN 590.
How water separators work
Separation of emulsified water from diesel fuel is a long standing requirement for diesel engine operation. Water removal is performed by a fuel-water separation filter mounted in the engine fuel system. The most typical filtration media found in such separators is hydrophobic barrier media, such as silicone treated cellulose. This media separates water on its water repellent surface. Water in the fuel is rejected and beads up on the upstream side of the media. As more water is rejected, beads coalesce into large drops, and drain into a collection cup. Another successful media is hydrophilic depth coalescing media, such as glass micro-fibre. This media has high affinity for water. As water in the fuel encounters the media, it associates with the glass fibres, where it is joined by more water over time, growing into massive drops. The water moves through the filter with the fuel. On the downstream side, the water falls out of the fuel flow into a collection cup due to its higher density, while the dry fuel rises out the filter exit. Effective water removal from fuel is considered fundamental to the preservation of fuel injection systems, and thus, attaining emission targets.
Introducing ULSD and biodiesel – impact on water separation
There are often unintended outcomes when a step change is made in a raw material specification. The diesel industry is replete with examples of unforeseen issues that cascade into areas seemingly unrelated to actions taken to meet a specific emission requirement. The transition to ultra low sulphur diesel (ULSD) is no exception. In order to meet mandated sulphur levels, ULSD is subjected to hydrodesulphurisation, a refining step that removes not only sulphur but also non-wax type species from the diesel fraction. The result is an ultra clean fuel. Unfortunately, it is an ultra-clean fuel that has been stripped of its native lubricity. Fuel lubricity is critical to emission compliance as it is the fuel’s lubricity that protects injector systems from catastrophic wear, ensuring precise control of combustion. Fuel lubricity is also fundamental to basic engine operation. A fuel system must hold pressure in order to inject fuel into the cylinder. Wear induced leaks can lead to engine failure due to fuel starvation. Chronic failure of ULSD to pass diesel wear resistance requirements led the industry to add fuel additives such as lubricity enhancers, rust inhibitors, and anti-wear agents to ULSD to restore required lubricity.
As lubricity deficiencies were surfacing with early ULSD prototypes, biodiesel began to take a place in the North American diesel market. Biodiesel is a blend of fatty acid methyl esters (FAME) derived from a caustic catalysed reaction between methanol and plant/animal fats. Biodiesel improved ULSD lubricity, and as a result, generated some independent impetus for its use as a blend component in diesel fuel. However, social and political attributes of biodiesel have been the primary drivers for its incursion into diesel markets. Escalating oil prices, perceived need for a domestic or “green” fuel supply, and pressure to minimise fossilised carbon
emissions have prompted state and federal governments to incentivise or simply mandate biodiesel inclusion in diesel blends. For example, Washington requires 2% biodiesel in all diesel blends, with an increase to 5% pending in-state manufacturing increases. Unfortunately, these expectations were enacted without comprehensive assessment of biodiesel impact on the machines required to burn mandated blends.
Just as hydrodesulphurisation produced unforeseen side effects in diesel fuel lubricity, additives and biodiesel create a less obvious, but equally dangerous unintended outcome: failure of existing fuel-water separators. In short, ULSD blends containing sufficient lubricity additive to pass wear requirements, and ULSD blends containing biodiesel, create conditions where commercial fuel-water separators fail to remove 40-100% of fuel-entrained water. The insidious aspect of this side effect is there is no way for an operator to know it is happening. Unlike particle filters which generate excessive pressure differentials prior to by-pass that alert the operator to end of filter life, there is nothing that communicates to the operator that the fuel-water separator is not removing water. Fuel-water separators rely on an operator or autovalve to empty a water collection chamber when it is full. If the collection chamber does not fill up, it is not an indicator of fuel-water separator failure; rather it is an indicator of dry fuel. The result is the fuel-water separator passes water continuously into the injection system without the operator’s knowledge, to the detriment of water sensitive surfaces and orifices.
The root cause of fuel-water separator failure in ULSD and ULSD-biodiesel blends is increased fuel surfactancy. Although given separate titles, rust inhibitors, lubricity enhancers, anti-wear additives, and biodiesel can all be grouped into a single molecular family: surfactants. Fuel and water are species that normally do not dissolve into one another; if forced to coexist, they are most stable as separate layers, with the fuel layer on top of the water layer. The degree to which the layers repel is measurable as the interfacial tension (IFT). If mixed, an emulsion is formed, where water transiently exists as suspended drops in the fuel. Surfactants are molecules unique in that they form strong associations with both fuel and water. When surfactants are in a fuel, they associate with water, and increase fuel-water compatibility. The increased compatibility is reflected in lower IFT between the two fluids. This unique surfactant behaviour allows more water to dissolve into the fuel.
Beyond dissolving more water into the fuel, the role of surfactants in fuel-water separator deactivation can be summed up with the three S’s: Size, Stability, and Surfaces. When a surfactant-containing fuel is mixed with water, the resulting emulsion has a smaller drop size distribution relative to a surfactant-free emulsion. This is due to the surfactant’s depression of IFT. All fuel-water separation media rely on physical interaction between water drops and the media to effect separation. Surfactants create sufficiently small water drops that many pass through the media without encountering it. Surfactants also stabilise the emulsion from separation so that drops that do impact the media are less likely to partition out of the fuel onto the media. Also, drops that impact other drops resist coalescing into the larger drops necessary for successful separation. Finally, surfactants associate with surfaces of media and water drops, and interfere with the unique surface interactions between media and water that destabilise water within the fuel and allow its separation. Collectively, the result of blending additives and biodiesel into ULSD is deactivation of the fuel-water separator and escape of water into the injection manifold.
Separation metrics – testing in obsolete fuels?
Fuel-water separating devices must prove efficacy in standardised industry tests. Water separation tests involve mixing a precise amount of water into fuel and passing the resulting emulsion through the separating device. Water content in the fuel upstream and downstream of the device is measured at regular intervals and a time-weighted average water removal efficiency for the device is calculated. Water removal testing is very much all or nothing, with most end users requiring at least 95% average water removal efficiency for any commercial device.
It is with standardised tests that another unforeseen consequence of mandated fuel change surfaces: lack of correlation with field performance. Despite high performance expectations, the end user is largely unaware of the alarming failure consistency of fuel dewatering systems in ULSD and ULSD-biodiesel blends. This is the case because the time required for a legislative body to mandate 2% biodiesel inclusion in diesel is fleeting relative to the time needed to adapt proven standardised industry tests to a new fuel. Irrespective of the procedure selected, there are currently key differences between fuel surfactancy and emulsification methods found in the field versus those specified in standardised test procedures designed to rate water separator performance. Both ISO and SAE committees are working to adapt procedures to high surfactancy blends. Until resolved, the result is a disconcerting overestimation of performance capability as measured in standardised tests relative to field performance.
To illustrate this point, consider fuel surfactancy. Fuel surfactancy drives water particle size distribution as well as water drop stability and media surface sensitivity. As such, it is a key determiner of success or failure of a water separation device. Interfacial tension (IFT) is sensitive to fuel surfactant content, where a lower value is indicative of increased surfactancy. Each of the current published standardised test procedures for water separation, SAE J1488, SAE J1839, and ISO 4020, specify only an IFT range for the fuel, allowing needed flexibility in test fuel given seasonal and regional variation. However, the specified IFT range in each of the procedures, 25-30 dynes/cm (SAE J1488, 1839) and 23-28 dynes/cm (ISO 4020) no longer represents the IFT typically found in field ULSD samples which can range from 9-25 dynes/cm and ULSD-biodiesel blends which fall in the 8-15 dynes/cm range. In their current states, specified test procedures at best specify fuels which represent the least challenging range of water separability available in North American and European diesel fuels, and totally ignore biodiesel blends.
The disconnect between surfactant levels in field fuels and interfacial tensions specified in active standards creates the bizarre situation where test laboratories must pre-treat their test fuels to reduce surfactancy – making them resemble obsolete pre-2007 fuels – in order to run the standard test as specified. This practice lowers test severity and inflates observed performance of the separator under test. Some test laboratories rely on fuel pre-treatment as an important tool to permit differentiation between separator designs which otherwise would consistently fail in current commercially available fuel. Further, the active standards do not require fuel reporting with result publication, giving the user little information regarding the fuel used to generate a reported result. Unless the product specifically claims capability in biodiesel blends, end users are wise to expect that reported water removal capability in a standardised test is based on a test run as specified, which will mean the test was run in a low surfactancy fuel which is now obsolete.
The distressingly consistent failure of current fuel dewatering systems in ULSD and ULSD-biodiesel blends has catalysed wide ranging development efforts for fuel independent water separators. Innovation in the coalescence and separation arena covers the range from complete separation systems to separation media. The systems involve multiple media types, multiple media elements, and multiple layers of media. The innovation often concerns packaging the media and flowing the emulsion in novel ways. The drawback of this approach is complexity, which translates directly to manufacturing and raw material costs. The same factors that give rise to complexity and increased cost, also limit universal applicability of the solution.
Ahlstrom Filtration has a long standing commitment to meeting diesel filtration needs and launched a media development program specifically addressing the gap in fuel-water separation capability in post 2007 fuels. The program targets in tiers a universal solution for water separation. The first target was water separation media capable of managing ULSD alone. The second tier broadened the media performance requirement to include ULSD as well as any biodiesel blend, thus providing the customer with a selection of capabilities.
The Ahlstrom media solution for water separation from ULSD involves composite media. Through the union of high surface area Ahlstrom FineFibre meltspun media with water repellent wet laid base sheets, composite barrier separation media have been developed with greater than 95% water removal capability in all but the highest surfactancy ULSD or ULSD-biodiesel blends. The FineFiber composite product line covers a range of particle removal efficiencies and dirt holding capacities, giving the end user flexibility in pairing media with specific end uses.
Ahlstrom is also preparing to commercialise an ultra-high surface area coalescing media for reliable water removal from any commercial fuel, including biodiesel blends. This new coalescing media provides a minimum of 90% water removal efficiency from 20% biodiesel blends (B20). In 7% biodiesel blends (B7), the water removal efficiency climbs above 95% under the same test conditions. Biodiesel containing emulsions exit the media in the form of clean, bright fuel with no evidence of free water. Water exits the media in the form of drops that settle out of the flow. In providing this level of water removal, the media allows compliance with EN 590 (2005). The media can be customised to specific life and durability requirements, without compromising water separation capability or compatibility with converting technologies.
Since 2007, diesel fuel has undergone a substantial increase in surfactant content that has led to deactivation of commercial fuelwater separators. As test method committees work to revise test standards to include new fuels, industry performance standards for water separation remain insensitive to the fuel-induced loss in separator performance. The filter and filtration media industries have responded with development efforts targeting new products with separation capability in high severity biodiesel blends. Together, Ahlstrom’s barrier composites and ultra-high surface area coalescing media are staged to deliver to the diesel industry water removal technologies capable of managing today’s advanced fuels.
Christine Stanfel, PhD
Ahlstrom Filtration, LLC
205 Nebo Road, Madisonville, KY 42431 USA