How To Design A Better Carbon-Filtration Solution

Being forced to add carbon filtration in order to comply with drinking water and wastewater regulations or other extreme industrial filtration requirements is challenging enough. Worrying about performance limitations that the treatment process might impose only compounds the problem. Here is a look at how synthetic carbon technology makes it easier to design a better filtration medium tailored to the most challenging process demands.

While there are multiple types of carbon filtration options, not all of them offer the same range of performance capabilities. That is why evaluating media by specific design and performance principles for large packed beds or inline cartridges is so critical — particularly with liquid or gas streams facing extremely demanding requirements.

Finding The Best Raw Material For Worst-Case Scenarios

There’s nothing wrong with using commodity granular activated carbon (GAC) for run-of-the-mill applications. But when those off-the-shelf solutions fail to achieve the required results in more extreme environments, exploring the opportunities and advantages of synthetic carbon filtration media tailored to the specific demands of the application can make the difference between success and failure.

The first step is to understand the dynamics at work. The second is to apply the proper attributes to the task at hand. Comparing the unique core capabilities of the Carboxen® family of synthetic carbon media is a good place to start (Figure 1).

• Synthetic Carbon Consistency. While naturally sourced carbons offer a range of performance characteristics, they also bring inherent limitations. For example, the naturally variable composition of mined coal makes it hard to produce a consistent GAC for highly reproducible results in extremely challenging filtration applications. Synthetic carbons are more pure and consistent than carbon from plant-based or animal-based natural sources, so there is less concern over unavoidable metallic impurities in mined bituminous, sub-bituminous, or lignite coals or inconsistencies in wood or coconut shell pore structures.

• Structural Efficiency. The consistent spherical shape of Carboxen synthetic carbon offers several structural advantages relative to the irregular shapes of natural carbons.

• Packed Bed Performance. Spherical or bead-shaped carbons pack better than granular shapes to deliver higher removal capacity in the same space, so they deliver better efficiency.

• Lower Backpressure In The System. Spherical carbons with narrow particle size distributions also experience lower pressure drops in packed columns than amorphous natural carbons do.

• Non-Friable Nature. With proven ability to withstand up to 16,000 psi in industrial applications, Carboxen spherical synthetic carbon has no risk of fracturing during normal transportation or use — unlike granular carbon that can fracture into smaller particles and fine powders. Fines are problematic because they can increase backpressures or contaminate the product. Non-friable characteristics also provide better performance for onsite regeneration of used media, without risk of particle deterioration. That means significant savings over the landfill costs of single-use GAC materials.

• Uncompromised Durability. Because synthetic carbon spheres are chemically bonded together, as opposed to pelletized charcoals, there are no fragments or fines to contaminate the final filtered output and no buildup within the bed to restrict throughput or increase backpressure during the process.

Photo courtesy of MilliporeSigma

Figure 1. Durable synthetic carbon media in spherical shapes (right) offer several advantages over granular carbon formats in water treatment applications. The spherical shapes nestle well to provide good surface exposure without building problematic backpressure. Also, being non-friable, they stand up to handling, processing, and regeneration more successfully than traditional GAC.

Engineering Better Carbon Filtration Solutions

Even though synthetic carbons offer certain advantages over natural-source carbons, not all synthetic carbons are created equal. Because key aspects of Carboxen synthetic carbon media are controllable — by particle size, pore size/structure, and surface pH — their performance can be tailored closely to specific adsorbate application requirements. This is critical because those properties of the carbon structure have an important impact on the different types of molecules to be adsorbed and separated from a stream.

• Controllable Pore Structures. Unlike natural GAC, which is limited to a microporous structure, synthetic carbons can incorporate a variety of pore sizes to fit the size of targeted molecules. They can even include tapered pores that offer significant advantages for mass transfer of the target molecules into the pore structure.

Forming synthetic structures from chemically consistent polymer materials provides the ability to control the mix of pore structure, size, and distribution for specific needs. Carbonizing those structures then yields a consistent composition that can be modified to produce a filtration medium highly targeted to the desired performance characteristics of the final application. Manufacturing control over its attributes means more consistent performance, without unpredictable structural variability or impurities from natural materials and without residues from chemically induced modifications.

• Controllable Particle Size. Being able to control particle sizes offers several advantages in treatment performance. Reducing particle size helps to achieve longer breakthrough times, while smaller particle size to column diameter ratios produce sharper breakthrough curves. A sharper curve from initial breakthrough to saturation makes it easier to maximize media replacement cycle efficiency while minimizing the risk of unacceptable contaminant throughput.

• Surface Properties. Experience shows that surface properties of carbon structures have an impact on the adsorption of molecules targeted for removal. For example, a basic carbon helps with retention of acidic analytes, contaminants, or pollutants targeted for removal, while an acidic carbon helps with the retention of basic molecules (Figures 2 and 3).

• Surface Chemistry. The ability of Carboxen synthetic carbon to control surface pH ranging from 2.5 to 10.5 provides more freedom to meet specific application needs. Because its pH is achieved through physical activation, not chemical treatments, there are no concerns about chemical leaching or off-gassing that could affect the process or complicate shipping requirements. No other natural or synthetic activated carbon offers that advantage.

• Hydrophobic/Hydrophilic Properties. Laboratory research shows that instilling synthetic carbon with hydrophobic or hydrophilic tendencies also provides advantages for improving separation capacity with compatible materials or environments. For example, hydrophobic carbons work better in high-humidity environments, without loss of retention capacity.

Manufacturing Better Filtration Options

While GAC formats offer some choice in terms of carbon source performance — e.g., coconut shell, wood, anthracite/bituminous coal, etc. — synthetic carbons can literally be designed and manufactured to satisfy specific desired performance criteria. And unlike synthetic carbons available in limited formats, the eight primary Carboxen products cover a wide range of conditions and are easily modified to impart the most desirable performance characteristics. To enable preliminary testing, prepared sampler kits are available for comparison in specific adsorbate applications.

Graphic courtesy of MilliporeSigma

Figure 2. Compared to a high-pH GAC (pink) that shows near-instantaneous, significant breakthrough of benzoic acid, a 10.5-pH version of synthetic activated carbon (blue) demonstrates a longer time to initial breakthrough and saturation.

Graphic courtesy of MilliporeSigma

Figure 3. Compared to an outsourced synthetic activated carbon (gold), a pH-adjusted acidic Carboxen 1032 synthetic activated carbon (blue) displays a longer period until initial breakthrough of a basic organic amine (benzylamine), plus a longer time before maximum saturation. Note also how the sharp rise in the saturation curve demonstrates superior performance almost right up to the time for media replacement.