Engineering a New Generation of Depth Filter Technology
By David J. Paulson
Summary: A new technology allows users to be more specific when making a filter selection for their particular applications. Along with more efficient filters, the trend will increasingly become more cost-effective products.
Industrial demand for melt-blown filters has experienced tremendous growth over the past 20 years. At the same time, advances in filtration material and construction techniques have presented manufacturers with the challenge—and the opportunity—to bring to market more precise, longer-lasting filters without sacrificing efficiency and strength.
While purchase price often plays a key role in filter selection, the trend is to focus on overall operating cost of filtration. Customers today consider all facets of filtration performance when making purchase decisions, which include filter life, flow rates, energy costs as well as price.
Striking a balance
Whether a process uses one or 1,000 filters, customers invariably want ones that last longer, but there’s more to filtration performance than simply achieving long life. The ideal filter needs to strike a balance between filtering too much and too little. For example, a filter will have a shorter life if its structure is excessively tight and designed to intercept particles smaller than those necessary for the given application. Such filters perform the desired separation, but will have a higher frequency of change-outs and higher operating costs. On the other hand, filters need to deliver a certain acceptable life at the level of removal efficiency required to meet the basic filtration objectives applications demand. A new melt-blown filter manufacturing technology effectively achieves this balance. This technology produces filters designed to exceed the dirt-holding capacity of traditional melt-blown filters, maintain removal performance throughout its life cycle, and can be tailored to meet specific application requirements.
Optimize filter performance
The manufacturing process produces depth filters with a significantly increased void volume and lower pressure drop. Void volume is the volume occupied by the spaces between the fibers of the filter media. This technology employs smaller diameter polypropylene fibers than used in conventional melt-blown filters and a second technique is designed to create a unique structure of fibers in a bi-directional arrangement. The transverse fibers in the matrix allow for an extraordinary degree of filter layer “lofting,” which increases the available space within the filtration matrix to collect contaminants.
The filter technology uses a three-dimensional fiber matrix, which also provides a more rigid strength from the core to the outer layer without the presence of a support core. This transverse matrix significantly increases the filter’s ability to hold dirt even under large surges in flow and pressure. The transverse fibers add increased matrix stability that prohibit unloading of trapped dirt even under variable process flows and the increasing pressure drop as the cartridge loads up over its life.
Live and in 3-D
Adding another fiber into an already complex media is a fundamentally different orientation compared to the old technology. First, it’s key to understand that while the traditional spray pattern involves multiple fiber spray in a relatively perpendicular fashion, the fibers are collected on a cylindrical structure, which is both spinning circumferentially and moving perpendicular to the spray. This approach creates vast improvements in cartridge filters in widespread use today. By adding this newly oriented fiber, however, the traditional matrix is locked more completely into its structure, which allows use of finer fibers and, therefore, creates greater void volume—two characteristics beneficial to filter performance.
The unique 3-D fiber web helps deliver benefits to users in four key areas—fiber shedding, dirt-holding capacity, filter life and pressure drop. Some conventional melt-blown filter media lack fiber-bonding integrity, leading to an undesirable frayed exterior, media shedding and jeopardizing general cleanliness of the filter. By using a transverse fiber melt-bonding process, the filter’s exterior and core surface fibers are fully bonded, which reduces fiber shedding and provides a cleaner appearance. Gradation of density from the outer shell to the inner core is preserved through the original melt-blown technology concept of progressively finer layers—where each zone acts as a pre-filter to the next (see Combing through the Layers). This design spreads the work of dirt removal over the entire depth of the media.
In addition, more continuous fibers that run from the inside to the outside of the filter increase filter voids and maintain strength. The result is an efficient filter with up to 100 percent greater dirt-holding capacity and 100 percent longer filter life compared to conventional depth filters at equivalent competitive efficiencies, which means fewer change-outs and less downtime. For example, change-outs at one municipal RO treatment plant decreased as a result of the longer filter life. While the previous filters had to be changed every three to four months, the filters were only at half their full-load capacity after running for five months.
Lower pressure drop is another key benefit of this technology. By using finer fibers to increase void volume throughout the filter, less pressure is required to achieve the desired flow. These new filters typically have initial pressure drop values as low as half those of competitive filters. This means it takes twice as long for the transverse filter to reach its terminal pressure differential, allowing users to experience greater filter life and reduced pumping requirements and energy consumption.
Perhaps one of the most important advantages of this technology is its inherent manufacturing flexibility, which increases the ability to design depth filter characteristics based on specific application concerns. The unique media matrix allows tighter control of porosity gradation, pore sizes and pore uniformity. This allows users to purchase filters engineered to their specific application requirements without having to pay for qualities that aren’t important in the process. For example, a filter intended for deep well protection is designed with much higher pore size gradation—rate of change of pore sizes—than a filter intended for protecting membrane systems where the target particles in silt density index (SDI) reduction are more consistent in size.
Reduction of suspended matter in feed water to RO and nanofiltration systems is a standard prerequisite to ensure cost-efficient water treatment by protecting expensive crossflow membrane elements from excessive fouling. Effective pre-RO treatment is measured as a reduced SDI, less wear and tear on elements, fewer cleaning—or clean-in-place (CIP)—cycles, longer membrane element life and lower maintenance costs.
Hoffman Engineering, a manufacturer of industrial electronic enclosures in Minneapolis, has experienced these benefits firsthand. Hoffman uses filters specifically manufactured for its RO pretreatment system, which provides a pure rinse for metal products prior to painting. Before installing the new generation filters, Hoffman used a 5-micron (µm) conventional melt-blown filter that wasn’t providing the desired level of filtration.
Although it wanted a higher level of foulant (SDI) removal, Hoffman had experienced difficulties with increased pressure drop at the front end of the RO system and didn’t want this filter change to magnify the problem. To meet its requirements, the company switched to a 1-µm filter and eliminated these fears. The SDI value was reduced and, as an added benefit, operating pressure went from 60 pounds per square inch (psi) to 40 psi.
“Deciding to move to a one-micron filter meant the potential for increased pressure drops and, in turn, reduced process efficiency,” said Mike Stone, maintenance manager of Hoffman Engineering. “These new filters allow us to have both a high level of pre-filtration and reduced operating pressure. It’s a significant improvement over previous filters we’ve used.”
Oil & gas
A common filtration application in oil and gas production is deep well injection of produced water for enhanced secondary recovery and wastewater disposal. Here, depth filters protect the oil-bearing strata from contaminants in the re-injection water that degrade well performance by plugging the local strata. Cartridge filtration is the preferred method in deep well filtration to minimize downtime and protect long-term viability of these valuable wells. Typically, filters used in this application have low dirt-holding capacities and require frequent filter change-outs, which consume valuable well up-time and reduce a well’s profits. New technology cartridges are engineered specifically to maximize life in the produced water application while enhancing particulate removal and decreasing pressure differential.
The described depth filter technology takes the “one size fits all” proposition out of filter selection. While cost will always be a primary factor, filters tailored to specific applications are emerging as key to the purchase decision. As technology continues to advance, other possibilities include filters with unique flow patterns and separation characteristics. Whether it’s customization or the ability to design special separation characteristics, all of these improvements contribute to better filter performance and lower overall cost filtration for the user.
About the author
David J. Paulson is director of corporate technical services at GE Osmonics, of Minnetonka, Minn. This article is based on the company’s new, patent-pending Z.Plex™ filter technology, which includes the proprietary ROsave.Zs™ and WellPro™ product lines. He can be reached at (952) 988-6113 (fax) or email: firstname.lastname@example.org