By Dr. Evan Koslow
Summary: KX Industries describes the production process used to manufacture large volumes of carbon block filter cartridges and provides a guided tour through one of the world’s largest activated carbon filter production plants.
Activated carbon remains the primary material used in point-of-use/point-of-entry (POU/POE) water treatment devices for the control of chlorine, taste and odor. The advantage of activated carbon is its broad spectrum capacity to adsorb organic chemicals and promote catalytic/chemical reduction of chlorine disinfectants, both of which contribute to bad taste in potable water.
To meet basic chlorine, taste and odor reduction requirements, a bed of loose granular activated carbon (GAC) is often suitable. Modern consumer water filters, however, often provide additional “health claims,” including reduction of particulates such as asbestos or other submicrometer materials (for example, an NSF Class I particulate capability), heavy metals (such as mercury and lead), toxic organic chemicals (volatile organic compounds or VOCs, trihalomethanes or THMs, and specific pesticides such as Lindane and Atrazine), and select microbiological threats (Cryptosporidium and Giardia).
Breakthroughs in carbon
While, in general, activated carbon isn’t recommended by the USEPA as the sole treatment for removing mercury and lead nor is it listed as best available technology for Cryptosporidium and Giardia, recent advances in developing carbons with tighter pore composition has led to breakthroughs in performance of particular products. To achieve such a broad spectrum of health claims from a carbon filter, a microporous structure is required to control particulates—and this structure must also provide greatly enhanced chemical adsorption. This is achieved through use of powdered activated carbon, often mixed with a powdered heavy metal adsorbent, and formed into a solid monolithic “carbon block” structure using a thermoplastic binder (see Figure 1). Carbon block filter cartidges are usually in the form of a thick-walled pipe where the water is forced to pass from the outside to the inside of the porous wall. During their passage through the wall of the device, the fluid contaminants are adsorbed, intercepted or chemically reacted to provide a highly refined final fluid product.
Carbon block technology is dependent upon use of conventional activated carbons and adsorbents and the production process doesn’t enhance the adsorptive capacity of these materials. Instead, the carbon block enhances only the kinetics, or speed, of the adsorption process. In a typical consumer water filter, water has a contact time of only 3-to-12 seconds with the activated carbon. This compares to conventional beds of activated carbon used in municipal applications where contact times are usually 1,000-to-4,000 seconds—which can equate to more than an hour. The purpose of a carbon block filter is to accomplish a comprehensive purification of water in a timeframe perhaps 100-to-1,000 times faster than in a traditional activated carbon application. Remarkably, the carbon is essentially the same in both applications; it’s only the kinetics that has been dramatically enhanced using carbon block technology.
Prior to 1990, relatively modest volumes of high-priced carbon block cartridges were produced by compression molding. However, in 1988-89, a technology was developed to produce carbon block by extrusion. An extrusion process allows for the continuous production of a carbon tube, which is then cut to length rather than molding each unit individually. The technique not only allows relatively large volumes of production but provides the means to greatly reduce costs. Today, carbon block filters are often less expensive than traditional GAC filters because extrusion costs are actually less than the cost of molding a container to hold the loose carbon.
The process for manufacturing extruded carbon is now 10 years old and the scale of production has evolved enormously during this period. What follows is the first public-domain description of a facility built to implement this technology.
Figure 2 shows the layout of the KX Industries’ activated carbon profile extrusion factory and Figure 3 is a schematic of the process. This production line is rated to handle up to 20 million pounds of activated carbon and produce approximately 25 million carbon blocks per year. It’s probably the largest activated carbon filter production facility in the world. About 175 employees work at the plant, which operates around the clock. About 80 percent of the carbon block products made at this facility are sold to third party OEM customers, or original equipment manufacturers.
Incoming granular carbon arrives at the plant in 44,000 pound container loads, which are held for 72 hours to allow inspection for moisture content, surface area, pore structure and multi-element analysis of extractable trace metals. Once released for production, the carbon is sent to a duplex bulk bag unloading system and pumped via pneumatic transporters into four 15-foot-diameter-by-40-foot-tall mass-flow storage silos with a storage capacity of roughly 700,000 pounds. The 15-horsepower pneumatic silo loading system can move a truckload of activated carbon into the silos in one hour.
The granular carbon is moved from the silos by pneumatic circuits to a set of four specially designed grinders linked to both sieving and air classifying machines within a processing tower. The carbon processing tower’s function is to reduce granular carbon into precisely classified powder fractions that can then be recombined to yield specific formulations. The grinders are controlled using an in-line laser-light-scattering particle sizing system that’s fitted with direct digital feedback loops to the grinding circuit. The in-line laser is linked to a computer-controlled automatic sampling system that moves from one grinding cell to the next every few minutes to provide comprehensive monitoring and control of the multi-line process. The result is a process that can continuously monitor particle size distribution and make real-time adjustments to grinding conditions to minimize variance. Both laboratory laser diffraction and conventional sieving machines (ultrasonic and rototap) supplement the in-line laser particle size analysis. The final carbon powder sustains a less than 2 percent variance in particle size using these techniques—perhaps five times less variance than a traditional carbon grinding procedure.
Loop to loop
Carbon powder, thermoplastic binder and other ingredients are transported through an aeromechanical conveyor (see Loop 1) to a bin fitted with weigh cells. An aeromechanical conveyor consists of a cable fitted with flat disks at close intervals that travel within the pipe. The movement of the cable creates a pocket of trapped air between each pair of disks that serves to fluidize the various powders and move them without significant abrasion or wear through the various process steps. Once a fully formulated batch of powders has been compiled in the weigh bin, the contents of the bin are released to a second aeromechanical conveyor (see Loop 2) that transports the materials to one of five powder blenders. The blenders have a total capacity of 13,000 pounds and use of several blenders allows multiple formulations to be prepared simultaneously. Over 80 different formulations are currently used in a total of 350 different carbon block products. Once carefully blended, the mixed powder is released to a third aeromechanical conveyor (see Loop 3) that elevates the powders to the “racetracks.”
Each racetrack consists of a 250-foot-long aeromechanical conveyor that travels over an “extrusion bay.” Each extrusion bay consists of 20 solid state extruders. Two such extrusion bays exist, one on each side of the facility used to prepare the powdered ingredients. The function of the racetrack is to allow ingredients to be transported to any specific extruder or group of extruders. Because of their length and number of input and output valves on each racetrack, these units represent some of the longest and most complex aeromechanical conveyors installed anywhere in the world.
Raw material arriving at the extruders through the racetrack is released into hoppers located above each extruder. Each hopper is capable of holding 4,500 pounds of ingredients. This allows up to 90,000 pounds of material to be sustained as operating inventory within each extrusion bay. This is only about 48 hours of actual operating inventory, which means the preparation of ingredients must be carried out continuously to support the extrusion process.
The individual extruders are capable of producing approximately two standard filters per minute (2.5-inch-O.D.-by-10-inch-long carbon block units, where O.D. is the outer diameter) or about 5-to-6 filters per minute of the smaller end-of-tap carbon blocks now so common to the market. Continuous carbon rod is rough cut at the extruder to approximately 5-foot lengths and then transported to off-line saws that accomplish the final precision cut to the specified product length.
Rapid response flexibility
The plant is both highly productive and flexible. At critical times, production can be rapidly increased because reserve capacity is always available. For example, in one extreme case, production of one carbon block product was increased from an annual rate of two million devices to seven million in only four months. This flexibility is matched by the downstream filter finishing systems, which include 10 robotic filter assembly units. The design of these robots allows essentially all of the current 350 different filter products to be processed by any robot with a simple tool change. Hence, the traditional application of a prefiltration layer, netting and end-caps can be accomplished quickly and economically for hundreds of filter products on a single assembly machine without significant tooling investment. High-speed packaging completes the process, with both shrink-wrap and bagging machines available to encapsulate the final product. Case erectors, random-size case sealers, labeling machines and computer-controlled label printers allow custom packaging and lot coding.
Carbon block has the superficial appearance of a product that is easy to manufacture and simple in construction. However, large-scale production of such a product, with its attendant health claims and quality control issues, is actually a complex, capital intensive business. Ultra-large scale production facilities such as that of KX Industries, have made it possible to produce a high technology, high quality product at commodity prices. Such technology has made carbon block the preferred and economic choice for consumer water filters.
About the author
Dr. Evan E. Koslow is the chief executive officer of KX Industries, L.P., of Orange, Conn., which produces the MATRIKX® and PLEKX® carbon filtration product lines. He holds a bachelor’s degree in engineering, a master’s degree in forestry from Yale University and a doctorate in agronomy and agricultural engineering from Cornell University. He has written over 100 articles and papers, holds over 30 patents and is the original inventor of the solid state extrusion process for carbon block technology. Koslow can be reached at (800) 462-8745, (203) 799-9000, (203) 799-7000 (fax) or email: [email protected]
BREAKOUT: Software driven for quality control
The factory discussed in this article is a build-to-order facility, driven by both a materials resource planning (MRP) and integrated manufacturing software package. No significant inventory is retained except for customers who operate Kanban or similar inventory control practices. The ability to produce vast volumes of product upon demand is unique. It’s called the “virtual inventory system” (VIS) because product can be produced so quickly that it “appears” to emerge from inventory virtually at a moment’s notice, relatively speaking. In one case, a major customer requested a new product be prototyped, tested and delivered within eight days. Indeed, the prototypes were produced on the day of the request at a research and development (R&D) facility equipped with five prototype extruders. Testing was completed three days later using fully automated test stands operated by the R&D and quality control (QC) departments. The first 10,000 carbon blocks were produced in the two days following product approval and were received on schedule—much to the amazement of the customer!
The materials handling portion of the plant is controlled from a central control room located on a second-floor observation tower (see Figure 3). Within the same control room are computers that control the laser particle size measurement and sampling systems and monitor and control the extruders and their associated inventory of raw material. All extruders are linked to a plant-wide data highway that monitors extruder conditions and production continuously. Level sensors in the extruder feed bins transmit information on this data system to support the plant inventory control and scheduling system.
Supplementing the quality and productivity data collected continuously on each extruder, QC information that’s generated off-line is combined into a central statistical process control (SPC) database. There are nearly 20 off-line tests used to monitor the quality of the incoming raw materials and an additional 25 off-line tests to monitor the quality of the final product. This includes life testing on test stands for chlorine, VOCs, high-pH and low-pH lead reduction, particulate removal efficiency, pore structure, flexural strength and modulus of the carbon block, air and water permeability, density and other factors. This SPC system is equipped to track each lot of production at every extruder, including information on raw materials lots and their distribution within final products. This allows comprehensive tracking and reporting.