How activated carbon is used

The way activated carbon is used highly depends on the form of carbon chosen. The most common way activated carbon is used is in a packed bed, where GAC is placed in a stationary vessel, and a stream of liquid or gas flows through it. This allows the GAC to adsorb contaminants from the fluid, retaining them within the GAC particles. Packed beds of GAC are widely used in the water-treatment industry to remove organic compounds from drinking water, in wastewater treatment and in liquid chemical processes. Similarly, for air and industrial-gas treatment, packed bed systems are used to remove pollutants, such as volatile organic compounds (VOCs) or H₂S. Packed bed systems allow for relatively simple operations using GAC in both liquid- and vapor-phase applications, and the packed beds themselves are often designed with ease of media installation and removal in mind. Typically, activated carbon is used as a polishing technology for relatively low concentrations in the liquid and gas phase, rather than used in concentrated streams, though there are exceptions.

PAC is too fine to be retained within a packed bed, and instead is typically added to a liquid stream in a batch process, in which the carbon particles are dispersed into the fluid and allowed to adsorb the contaminants. After the adsorption process, the PAC particles are filtered out of the fluid, typically using a mechanical filtration system. PAC can also be injected into vapor streams and removed in particulate capture devices, such as baghouses, a practice used for the removal of VOCs and dioxins, furan and mercury from flu- gases.While PAC particles have larger external surface area than GAC, there is no significant difference in specific surface area between GAC and PAC, because the majority of activated carbon’s surface area is internal. Adsorption kinetics onto activated carbon are dependent on particle size, and the small particle size of PAC results in rapid adsorption kinetics, allowing PAC to adsorb compounds at a faster rate than larger particles of carbon. However, there is no difference in the equilibrium loading of GAC and a corresponding PAC produced to an equivalent adsorption specification. Indeed, packed beds of GAC often result in better carbon usage rates than PAC. Practically, GAC used in a packed bed system often loads to a higher degree than PAC, adsorbing more contaminants per mass of carbon used.

GAC liquid adsorbers can be used with single vessels, but designs commonly include multiple adsorber vessels operating in series flow, lead-lag configuration, to allow for the greatest utilization of GAC capacity. In this configuration, when the lead vessel has reached its capacity, the lag vessel is moved into the lead position and a fresh carbon bed is put online in the lag position. Typically, at least ten minutes of empty bed contact time (EBCT) per vessel are required to account for the kinetics of adsorption and provide for an adequate bed life, depending on contaminant inlet concentrations and treatment objectives. In general, longer contact times are required as the carbon particle size increases and as the fluid viscosity increases. Decreasing the flowrate through a given vessel will reduce and sharpen the mass transfer zone (MTZ) of the bed, increasing carbon loading. Additionally, pretreatment and operational maintenance (such as backwashing) may be warranted in some applications, some of which are detailed in Table 3.

Vapor-phase adsorption applications with activated carbon differ significantly from liquid-phase applications in that the required contact time is often much less in the vapor phase, due to the kinetic advantages associated with gas adsorption. For high removal across a vapor-phase carbon bed, typically three or more seconds of contact time are required. Because adsorption is an exothermic process, and in vapor-phase applications there is not excess water present to dissipate heat, there are some guidelines for reducing thermal risk in these applications. Typically, inlet concentrations of VOCs exceeding 1,000 ppmv in air are not recommended. Additionally, some compounds, such as ketones and aldehydes, can oxidize on the carbon by atmospheric oxygen, generating additional heat, and so their inlet concentrations should be limited. Additional vapor-phase design and operating considerations are included in Table 4.

After activated carbon has reached its capacity and will no longer adsorb the target chemical species, it often must be removed from service. In packed beds using GAC, the spent GAC is removed from the adsorber vessel by vacuum or by slurry method, depending on the vessel design and utilities. Spent GAC is moved by truck and can be disposed of by landfill or incineration. A more environmentally friendly and cost-effective approach is to reactivate the spent activated carbon.

Carbon reactivation is a process where spent, contaminant-loaded GAC is returned to a facility with a suitable reactivation furnace, which operates under similar high-temperature conditions to physical carbon-activation furnaces. In the carbon reactivation furnace, contaminants are desorbed from the GAC and destroyed, and the furnace atmosphere facilitates the generation of new surface area in the GAC. A downstream abatement process ensures adequate destruction, control of acid-gas generation, and particulate-matter removal. Thus, the carbon reactivation process removes and destroys contaminants from GAC, allowing for the GAC to be re-used in the same or other processes. Reactivation is typically only practiced on granular materials with sufficient hardness and density to resist physical degradation in the aggressive furnace atmosphere.

To be eligible for carbon reactivation, the material must undergo a “Carbon Acceptance” process, where the composition of the carbon and contaminants are verified as acceptable. Carbon reactivation has proven to be effective at removing and destroying even robust compounds, such as PFAS. Ref. 1 describes a peer-reviewed study demonstrating >99.99% destruction of total PFAS compounds across a carbon reactivation furnace