Nearly all electrocoat (e-coat) ultrafiltration systems being installed today use spiral-wound membranes, since these membranes provide the lowest capital and operating costs compared to other types. However, most conventional spiral-wound membranes - including tailed, brine seal, and flanged spiral configurations - have a significant drawback. In each case, the membrane element is separate from the housing. As a result, the membrane must be fitted into the housing during installation and then removed from the housing after use.

Recently, a new design has been developed that incorporates a potted membrane element in a polyvinyl chloride (PVC) shell. This new "potted spiral" design eliminates the extra steps typically associated with spiral membranes, thereby helping e-coaters reduce costs and increase productivity.

Spiral-Wound Technology

A spiral-wound module is shown in Figure 1. The feed solution (i.e., e-coat paint) enters the module under pressure, typically at 50 psi, and components of the paint having a larger molecular weight are rejected by the ultrafiltration membrane. Water, dissolved salts, solvent, solubilizer and low-molecular-weight resin pass through the membrane as ultrafiltrate or permeate. As the paint travels through the membrane, it becomes increasingly concentrated as permeate is withdrawn. The exiting paint, which is known as the concentrate, typically exits at 20 psi. The permeate generally leaves through a central permeate tube at atmospheric pressure.

The spiral-wound module consists of flat membrane sheets glued together and wrapped around a central permeate collection tube in a spiral fashion. The membrane sheets are separated by spacers to allow the feed and permeate to flow inside the spiral element. The feed spacers can be used at different thicknesses and geometries to promote turbulence and to ensure unrestricted paint flow. The spiral-wound structure is wrapped on the outside with a hard layer - typically tape, netting material or a fiberglass/epoxy composite - to prevent loosening or unwinding from occurring during operation. Traditionally, the spiral-wound element is inserted into a cylindrical pressure vessel or housing made of PVC, fiberglass or stainless steel.

Ultrafiltration spiral elements are available in different membrane materials, such as polyvinylidene fluoride, polysulfone, polyacrylonitrile and cellulose acetate. The most commonly used material for e-coat paint ultrafiltration is polyvinylidene fluoride, due to its chemical resistance and ability to withstand a wide pH range.

Production-sized elements for e-coat paint are typically available in 4-, 6-, and 8-in. diameters. Lengths can range from 33 to 40 in. Feed channel spacers vary in size and design, depending on the membrane manufacturer and product, but spacers used for e-coat typically range in thickness from 0.030 to 0.043 in.

Spirals used for e-coat paint are generally mounted vertically to avoid paint settling on the membrane if the paint flow is stopped due to a power outage or system failure. However, some e-coat spirals in France have been installed horizontally.

Conventional Designs

Spiral-wound elements for e-coat paint ultrafiltration are available in different construction designs and configurations. Each design has advantages and disadvantages.

Tailed Spiral. The tailed spiral-wound module (also known as a netted module) was among the first spiral ultrafilter elements to be applied to e-coat paint. The outer wrap of this element is made of plastic netting material, and comes with extra outer wrap net called tail. During installation, part of this tail can be sliced off to ensure a snug fit inside the module housing. This feature is especially useful with PVC housings, which have a non-controlled inside diameter.

However, even with a good fit, a significant amount of paint can bypass the feed channels. As a result, an 8-in. diameter tailed spiral module requires a paint feed rate of 120 gpm or more. If the tailed spiral does not fit properly inside the housing, the module's feed spacer can shift inside the spiral element, causing non-uniform paint flow. The result is inefficient use of total membrane area, along with increased fouling in certain channels.

Brine Seal Spiral. A conventional brine seal spiral is composed of a fiberglass/epoxy outer wrap and an end seal (see Figure 2). The seal (often called a brine or U-cup seal) is a rubber gasket that presses against the inside of the housing and reduces - but does not eliminate - paint bypass around the spiral element. In e-coat paint ultrafiltration, the seal is usually placed at the top or exit end of the spiral rather than at the bottom or feed end for ease of installation.

The required paint feed rate for 8-in. diameter brine seal elements can range from 60 to 100 gpm, depending on the element diameter, feed spacer size and desired crossflow pressure drop. Larger diameter elements (e.g., 7.9 in.) are used for thinner-wall, 8-in. stainless steel housings, whereas smaller diameter elements (e.g., 7.2 to 7.4 in.) are used for thicker-wall, 8-in. PVC housings. Greater paint feed flows are required for larger diameter elements, thicker spacers and larger crossflow pressure drops.

Potential problems with this design include difficult or improper fitting of the membrane element with the brine seal in the housing; seals that flip during installation; and a large bypass of paint feed. These problems can occur as a result of a non-controlled inside diameter for PVC housings. In addition, stagnant or low-flow areas between the outside of the fiberglass wrap and the inside of the housing can cause paint to settle, which could be a source of dirt and cause difficulty during membrane removal (i.e., the membrane becomes stuck inside the housing). A variable productivity due to channeling or preferential flow around the membrane during processing is not uncommon with brine seal spiral elements.

Flanged Spiral. The flanged spiral is patented and uses an integral rubber flange at each end of the module (see Figure 3). Each flange acts as a gasket at the housing end cap to eliminate bypass and direct all of the paint (or cleaning solution) through the module. The flanged spiral is more efficient at using paint to generate permeate than either the tailed or brine seal spiral. The paint feed rate is low (typically 50 gpm per 8-in. spiral) enabling smaller pumps, valves, fittings and piping to be used compared to other spiral configurations. A hard fiberglass/epoxy outer wrap is used, similar to the conventional brine seal spiral. Use of the fiberglass/epoxy outer wrap enables the rubber flanges to be integral with the element. Anti-telescoping devices and positive seals at both ends of the flanged modules enable cleaning by reverse flow.

Potted Spiral Design

Each of the conventional spiral configurations (tailed, brine seal and flanged) has a spiral element that is separate from its housing. During installation, the membrane must be fitted inside the housing.

Potted designs have been available for years for tubular, hollow fiber, and 4-in. spiral membranes, but could not be obtained for large diameter spirals. Recently, a new "potted spiral" design has been developed for 8- and 10-in. diameter spirals that eliminates membrane fitting problems during installation and reduces the labor involved when removing spirals (see Figure 4). With this new design, the spiral-wound membrane is secured by epoxy to the inside of a PVC shell. The membrane and shell form one integral unit; no field fitting of the membrane in the shell is required. The potted spiral is easily connected to its stainless steel end caps by common couplings.

The potted spiral design offers the following benefits over other spiral element configurations:

• Equivalent or higher permeate output. The zero-bypass design ensures that 100% of the paint flow is used to generate permeate. The potted spiral design allows more membrane area to be placed into the shell.
  
  
• Equivalent or higher flux recovery after cleaning. The zero-bypass design enables higher turbulence during cleaning.
  
  
• Easier membrane installation and replacement. The membrane is integrally attached to the PVC shell and does not need to be fitted into the housing during installation or removed from the housing during replacement. Stainless steel end caps are easy to connect and disconnect.
  
  
• Reduced work strain and safety hazards. Installation or removal of the membranes is done at floor level; no ladder is required.
  
  
• Lower system fabrication costs. The cost of permanently installed stainless steel or PVC housings is eliminated.

The potted spiral membranes operate at a pressure profile similar to conventional spirals (e.g., 50 psi inlet and 20 psi outlet). The designed paint flow rate at a 30 psi crossflow pressure drop for an 8-in. potted spiral is 65 gpm, while the flow rate for a 10-in. potted spiral is 105 gpm.

The performance of an 8-in. potted spiral versus a conventional 8-in. brine seal spiral at an automotive assembly plant is shown in Figure 5. Each spiral has a 0.030-in. feed spacer and operates on unleaded gray cathodic epoxy paint. The permeate rate of the potted spiral exceeds that of the conventional brine seal spiral.

The performance of an 8-in. potted spiral versus an 8-in. tailed spiral at an appliance manufacturer is shown in Figure 6. Each spiral has a 0.030-in. feed spacer and operates on unleaded white cathodic acrylic paint. The permeate rate of the potted spiral exceeds that of the conventional tailed spiral.

Case Studies
Auto Parts Manufacturer

In January 2004, an automotive parts supplier replaced its tubular cabinet containing 296 tubular membranes with two spiral racks, each containing six 8-in. diameter potted spirals, to treat its unleaded black cathodic epoxy paint. The company's reasons for switching from a tubular to spiral configuration included a newer technology, better and more permeate output relative to floor space, and easier maintenance (i.e., easier membrane replacement).

Prior to the conversion, the tubular system produced 10 to 20 gpm of permeate. Cleaning was required about twice a year, and complete retubing was needed about every three years. After the conversion to 8-in. potted spirals, the plant has produced 36 gpm of permeate and cleans once a year. Flux recovery after cleaning has been nearly 100%. The 8-in. potted spirals are currently more than two years old.

Recreational Vehicle Manufacturer

In May 2004, a recreational vehicle manufacturer replaced its 4-in. spiral rack containing thirteen 4-in. potted spirals with an 8-in. spiral rack containing two 8-in. potted spirals (see Figure 7, p. 52).

The driver for this change was an increase in permeate rate that was required when the plant converted from leaded to unleaded black cathodic epoxy paint. The user purchased the new equipment to reduce long-term membrane replacement costs and to permit cleaning of individual elements during normal production hours, which would reduce overtime labor expenses.

The original 4-in. potted spiral system with thirteen 4-inch diameter spirals was designed to produce 3.5 to 5 gpm of permeate. However, the permeate rate was allowed to drop to 1 gpm. Additionally, the membranes were never cleaned since the entire rack would have to be brought offline, which was not practical given a heavy production schedule. The new system with two 8-in. diameter potted spirals has generated an average total permeate rate of 4 gpm. The 8-in. diameter potted spirals have been cleaned twice since May 2004. The original set was replaced after a year of operation, but the replacement cost was 80% less compared to replacing the original thirteen 4-inch diameter spirals.

Auto Assembly Plants

During 2004-2005, two auto assembly plants completely replaced their 8-in. drop-in, brine seal type spirals with 8-in. potted spirals. In both cases, there was a substantial increase in permeate rate. One of the plants needed an increase in ultrafilter permeate rate to ensure adequate feed flow to the nanofilter downstream of the ultrafilter. The other plant has been able to take one of its three spiral racks offline.

In January 2006, another auto assembly plant replaced its tubular cabinet with two spiral racks, each containing four 10-in. potted spirals (see photo, page xx). Prior to the conversion, the permeate rate for the 264-tube cabinet ranged from 18 to 25 gpm. Cleaning occurred every six months for the first two years after a complete retube, and then every three to four months for tubes older than two years. Since the entire ultrafilter had to be cleaned all at once, cleaning had to occur during non-production time, requiring overtime labor. The tubular membrane life was about four to five years.

Since the changeover to the potted spirals, the auto manufacturer has achieved 60 gpm from six 10-in. diameter potted spirals. Each spiral rack is designed to have individual module cleaning, which allows cleaning of an individual element while the others remain in production. Since the cleaning can occur during production, overtime labor costs for cleaning can be eliminated. Each rack is operated with three spirals online and one spiral offline. Every week, one spiral is taken offline and flushed with deionized water while another is placed online. To date, the membranes have required just one cleaning.

Lawn and Garden Equipment Manufacturer

In October 2005, a lawn and garden equipment manufacturer replaced its two tubular units, each containing 162 tubes, with two spiral racks, each containing four 10-in. potted spirals. The company cited easier maintenance (i.e., membrane replacement) with the spiral system versus the tubular system and an increased permeate rate as the reasons for the change. The paint type is an anodic acrylic, and there are two paint tanks. Colors, which include orange, red, green, blue and black, are changed in each tank every few days.

Prior to the conversion to potted spirals, the permeate rate from each tubular ultrafilter unit after eight years of operation was 8 gpm. The units were designed to produce 20 gpm each. After the conversion, the permeate rate per spiral rack, with four 10-in. potted spirals operating, has ranged from 35 to 40 gpm. Each rack is set up for individual module cleaning. To date, no cleanings have been required.

Numerous Advantages

The potted spiral configuration offers significant advantages over conventional spiral configurations, including equivalent or increased permeate output, equivalent or increased flux recovery after cleaning, easier/safer installation and replacement of spirals, and lower-cost systems. Potted spirals have been used to successfully retrofit other spiral configurations on existing systems, and new systems with 8- or 10-in. diameter potted spirals have successfully replaced tubular and conventional spiral wound ultrafiltration systems.

Mark Rizzone, Ph.D., is sales manager of specialty applications, Henia Yacubowicz is applications engineering manager, and Kevin Donahue is business manager of specialty applications for Koch Membrane Systems, Inc., Wilmington, MA. For more information about potted spiral ultrafiltration membranes, call 888.677.5624 or 978.694.7000, e-mail info@kochmembrane.com, or visit www.kochmembrane.com.