How to Choose the Right Activated Carbon for the Job

Selecting the most cost-effective activated carbon for the job requires looking beyond per-pound price tags. N/A offers a range of products of varying grades and characteristics. However, to determine the most cost-effective purchase, customers are advised to work with their representatives to evaluate price/performance characteristics in light of specific applications.

Based on the demands of the application, some activated carbons outperform others—and the difference can be substantial. One refinery reported a 64% reduction in reactivated carbon use during a six-month period when they switched from a general purpose product to a high-performance carbon. An electronics manufacturer more than tripled the bed life of its carbon in a groundwater cleanup project by changing suppliers. Clearly, buyers do well to extend their selection process to the investigation of where and how competing carbons are made—and from what.

Quality In, Quality Out
The characteristics and resulting performance of an activated carbon is directly related to the starting material prior to activation. Proper selection of a base determines a carbon's inherent pore structure and will directly influence the properties and performance. Many buyers believe all carbons are alike, but in fact, in any individual application, there can be significant differences between products based on sub-bituminous coal, anthracite, peat, wood and coconut. To intelligently compare products, buyers need to know the source and base of any activated carbon under consideration. Proper selection of an application-specific starting material is crucial to the overall value of a product. It is the fundamental reason why some activated carbons outperform and/or prove more cost-effective than others in specific applications.

Step-by-Step
The manufacturing process itself also contributes to carbon performance. High-performance carbons are manufactured through the following process:

1) A high-grade raw material is pulverized to a powder.
2) A binder is added.
3) The product is reagglomerated into briquettes.
4) The briquettes are crushed.
5) The briquettes are sized.
6) The carbon is baked.
7) Finally, the carbon is thermally activated.

General-purpose carbons are often produced through a manufacturing process called "direct activation." It begins with an inexpensive raw material and proceeds directly to crushing, sizing, baking and activation. To save production costs, the pulverizing, binding and reagglomerating steps are eliminated. While direct activation results in a lower price-per-pound carbon, it may compromise long-term product performance in many applications. However, in other applications, the same product may provide a suitable result.

How well a carbon performs is directly related to its internal pore structure. The internal pore structure of a carbon granule can be compared to the infrastructure of roads in the United States. There are superhighways (macropores), highways (mesopores), regular roads and dirt roads (micropores). The larger pore structures (super highways and highways) provide faster access to where the organic removal occurs. The tighter pore structure (regular roads and dirt roads) are where the majority of the organic molecules are removed through adsorption. By eliminating the steps of grinding, binding and reagglomerating, general purpose carbons exhibit fewer superhighways and highways that allow organics to travel to the dirt roads, where adsorption takes place. In many demanding applications, the lack of additional carbon pore infrastructure equates to reduced performance and shorter bed life.

Differences between high-performance and general purpose products affect different applications to varying degrees. General purpose products are initially less expensive on a dollar-per-pound basis, however, they may remove less organic contaminants, causing the user to change out more often to meet demanding treatment objectives. The adsorption capacities of many general purpose carbons are significantly lower. Typically, they are less resistant to abrasion which results in higher transfer losses (backwash) and fines. The general purpose products have approximately 6.2 % fines, compared to 0.18% for high-performance carbon. In addition, general purpose carbons can have higher ash content, resulting in more leachables and lower adsorption capacities. They have approximately 14% ash, compared to 5-7% for high-performance carbon. Based on fines (lost in backwash) and ash, the general purpose products offer 6%+7% = 13% unusable product or 13% higher cost based on pounds.

Whether choosing general purpose or high performance activated products, buyers should be aware that any activated carbon not manufactured in ISO-certified facilities offers no guarantee of ingredients or other materials that may have been mixed in.

Reactivated Carbon
Reactivated carbons require the same level of thoughtful evaluation as their virgin counterparts. Three factors play a role in reactivated carbon performance: 1) the contents of the reactivated pool; 2) the reactivation process itself; and 3) the reactivation company's resources and expertise.

The contents of the reactivated carbon pools are key to realistic performance expectations. Pools that are predominantly made up of general purpose carbons produce reactivated products that may be unable to optimally perform in many applications. The adage of trying to make a silk purse from a sow's ear holds true for activated carbon.

The reactivation process itself also contributes to the end-product. To lower the cost of reactivation, some facilities increase the furnace throughput of the spent carbon. If spent carbon is not properly reactivated, adsorbed organic molecules may remain in the carbon pores. A closely-monitored, high-performance reactivation system adjusts critical operating variables such as heat, steam and oxygen to achieve the proper residence time in the thermal regeneration system. These parameters must be properly balanced and controlled to provide a high performance reactivated product.

Whether choosing high performance or general purpose products, reactivated carbon buyers should seek out manufacturers that have the necessary capabilities and resources to conduct extensive research into the proper reactivation of their products. ISO 9002 certification ensures that a facility produces a consistent product throughout its day-to-day operations with stable, reliable operating procedures and techniques. Buyers are also advised to select vendors with an established track record in reactivating carbon.

Proven Differences
Differences between high-performance and general purpose products are measurable through testing protocols that translate into real-world performance. TACTIC (Total Adsorption Characterization via Temperature Influence Correction) studies were conducted to measure the energy levels (cal/ml) of both high-performance and general purpose carbon pore structure. The high-performance carbon analyzed was an ISO-certified product randomly collected from a Calgon Carbon vapor-phase reactivated product pool. The two, general purpose samples were collected from a carbon supply to a major Southern California refinery prior to being installed in a large vapor-phase control system. The results indicate remarkable differences between general purpose and high-performance carbons.

Using the earlier example where the internal structure of a carbon granule was compared to that of the US highway infrastructure, the superhighways (macropores) have lower energy levels (0-9 cal/ml), highways (mesopores) have medium energy levels (6-18 cal/ml) and regular roads and dirt roads (micropores) have the highest energy levels (15-24 cal/ml). The tighter pore structure (regular roads and dirt roads) are where the majority of the organics are adsorbed. Both general purpose carbons sampled offered significantly less macropores, mesopores and micropores than the high-performance carbon.

Making the Right Choice
It is clear that the advice, "Caveat Emptor—Let the Buyer Beware," applies to companies selecting carbon based on price-per-pound alone. In many cases, general purpose carbons may provide satisfactory results and bottom-line benefits. In more demanding applications, however, higher quality raw materials combined with exacting manufacturing or reactivation processes result in activated carbons that, through higher performance, prove more cost-effective in the long term.