Activated Carbon: Fundamentals and New Applications

1 Cover Story Activated Carbon: Fundamentals and New Applications Activated carbon sorbents are important tools in water purification and air-pollution control. This article provides information on the Fundamentals of this diverse sorbent and on new Applications for which it is being employed G. Cabot Ken Koehlert lobal sustain- Cabot Corp. ability trends are creating in- creased demand IN BRIEF for purification of air and Activated CARBON water, as well as more en- BASICS vironmentally friendly pro- cess alternatives. Activated RAW MATERIALS carbon (AC) sorbents play ACTIVATION PROCESSES important roles in applica- tions throughout the chemi- POST-PROCESSING. cal process industries (CPI). ADSORPTION These range from traditional 1,000 nm 100 nm Fundamentals Applications , such as mu- FIGURE 1. These images from a helium-ion microscope show the pore structure of FACTORS AFFECTING nicipal water purification lignite-based Activated carbon PERFORMANCE and fluegas treatment, to cutting-edge Applications , such as ad- ner.

2 The degree of order varies based on APPLICATION TYPES. sorbed natural gas storage and double- the starting raw material and thermal his- BIOGAS PURIFICATION layer capacitors. tory. Graphitic platelets in steam- Activated SEDIMENT REMEDIATION In this article we review the basics of ac- coal are somewhat ordered, while more tivated carbon, as well as the link between amorphous aromatic structures are found in Activated carbon properties and adsorption chemically Activated wood. performance. In addition, the article provides Randomized bonding creates a highly po- an overview of two emerging Applications for rous structure with numerous cracks, crev- Activated carbon sorbents biogas purifi- ices and voids between the carbon layers. cation and sediment remediation. Activated carbon's molecular size porosity and the resulting enormous internal surface Activated carbon basics area make this material extremely effective for Activated carbon is a highly porous, high- adsorbing a wide range of impurities from liq- surface-area adsorptive material with a uids and gases.

3 To provide some perspective largely amorphous structure. It is composed for the internal surface area of Activated car- primarily of aromatic configurations of car- bon, the annual Activated carbon production bon atoms joined by random cross-linkages. of the author's employer has a surface area Activated carbon differs from another form nearly equal to the total land area on Earth of carbon graphite in that Activated (148 million km ). Figure 1 shows two micro- carbon has sheets or groups of atoms that graphs of the internal structure of steam-ac- are stacked unevenly in a disorganized man- tivated lignite. 32 CHEMICAL ENGINEERING JULY 2017. Cabot Activated carbon sorbents are tailored reactions in the raw material at tempera- for specific Applications mainly based on tures between 100 and 400 C. pore size and pore volume requirements. Charring. Higher-molecular-weight or- Porosity and other parameters are con- ganic materials are converted to a car- trolled by the following: 1) raw material bonaceous char residue at temperatures selection; 2) activation process condi- of 400 600 C.

4 At this point, incipient tions; and 3) post-processing steps. porosity and increased internal surface Depending on the application, Activated area begin to form. A B. carbon may be in the form of powder Activation. Activation is generally con- (PAC), granule (GAC) or extrudate (EAC). ducted in a steam atmosphere at tem- All three forms are available in a range of peratures between 700 and 1,050 C, particle sizes. depending on the pore structure of the carbon being produced. The desired re- Raw materials action is shown in Equation (1). Almost any carbon-containing mate- rial can be used to produce Activated C + H2O CO + H2 (1) C D. carbon. In practice, economics and FIGURE 2. This series of diagrams il- target product properties are the deter- This reaction gasifies portions of the lustrates pore development during steam activation mining factors in the selection of raw solid carbon to create pore volume. materials. The base raw material has a Typically, about half of the carbona- significant impact on the final product ceous char material entering the activa- properties, including pore size distribu- tion step will be reacted away to create tion and volume, hardness and purity.

5 The desired internal pore structure. The Most commercial Activated carbons diagrams in Figure 2 depict the develop- are manufactured from the following ment of porosity during steam activation. raw materials: Chemical activation is used to pro- Coal (anthracite, bituminous, sub- duce carbons with pore structures and bituminous, lignite) compositions that are somewhat differ- Coconut shell ent from those in steam- Activated car- Wood bons. For example, chemically Activated Some types of Activated carbon are wood has higher mesoporosity and produced from less conventional raw higher oxygen content than a steam- materials, such as peat, olive stones, Activated carbon. The chemical activa- fruit pits, petroleum coke, pitch, syn- tion process consists of mixing raw bio- thetic polymers, scrap tires and waste mass with a strong dehydrating agent cellulose materials. and heating to about 400 to 700 C. The Raw materials may undergo pre-pro- dehydrating agent (typically phosphoric cessing steps to control size, form and acid or zinc chloride) extracts the mois- other properties.

6 They may be crushed, ture from the raw material and fixes the milled, briquetted or mixed with binders volatile component of the biomass while and extruded prior to activation. the activation occurs. The degree of activation is determined by the ratio of Activation processes raw material to dehydrating agent and Activated carbons are manufactured via by the heating time and temperature. one of two processes: steam activation After activation, the product is extracted at high temperature or chemical activa- to yield a highly porous Activated carbon tion using a strong dehydrating agent. product and the the dehydrating agent Steam activation is the most com- is recovered. monly used method for Activated car- Chemical activation with potassium bon production. It is performed in rotary hydroxide is of great recent interest be- kilns, shaft kilns, multi-hearth furnaces cause it can produce highly micropo- or fluidized beds, and proceeds through rous carbons with specific surface areas the following steps: at or beyond the theoretical value for Drying.

7 Activated carbon raw materi- graphene (about 2,600 m2/g). als in commercial use contain residual moisture that must be removed before Post-processing activation can take place. Following activation, Activated carbon Devolatilization. Volatile organic com- sorbents may undergo a series of post- pounds (VOCs) are formed by cracking processing steps, including the following: CHEMICAL ENGINEERING JULY 2017 33. Cabot Bituminous range of Applications . Silver is commonly Lignite used to disinfect water. A more special- Wood ized example is impregnation with metal Peat salts and amines for military gasmasks. Pore volume, mL/g On a larger scale, impregnation is used to promote the oxidation of mercury in coal-fired utility fluegas Applications . Products for enhanced sulfur removal are impregnated with potassium iodide or sodium hydroxide. Another example of impregnation by end-users is the de- 0 position of precious metals on Activated < 2 nm 2-50 nm > 50 nm Pore diameter carbon to form heterogeneous catalysts.

8 Re-activation. Some granular and ex- FIGURE 3. The distribution of pore vol- Washing. Activated carbon may be truded products can be re- Activated umes differs depending on the starting acid-washed to reduce ash content and and recycled after use to minimize cost material for the Activated carbon remove soluble impurities, such as iron. and environmental impact. In this pro- Varying levels of increased purity are re- cess, spent carbon loaded with organic quired for some Applications , including contaminants is thermally treated in a double-layer capacitors, pharmaceuti- process similar to steam activation. Re- cals and food and beverages. activation desorbs and destroys volatile Sizing. Granular products may be contaminants and restores much of the sieved to specific particle size ranges. original pore volume and adsorptive ca- Common mesh-size specifications pacity. Typical product loss during re- range from 20 x 40 to 4 x 8 mesh (ap- activation is 5 15%, requiring makeup proximately mm x mm to 2 mm with virgin material.)

9 X 5 mm). Extrudates are produced with diameters ranging from to 5 mm and Adsorption Fundamentals length-to-diameter ratios between about Activated carbon sorbents remove low 1:1 to 4:1. Granule and extrudate sizes concentrations of chemicals (adsorbates). are specified to balance mass transfer from a fluid (liquid or gas) by adsorption, and pressure drop. Powder products the process of accumulation of materials are often ground in roller, hammer or onto a solid surface. Adsorption occurs jet mills to achieve a target particle-size within the Activated carbon pore struc- distribution (PSD), often specified by the ture by two distinct mechanisms: physi- terms d5, d50 and d90, which are mea- cal and chemical adsorption. sures of particle diameters. D50 (median Physical adsorption. Molecules are particle size) can range from 5 to about attached to the carbon surface by van 20 micrometers ( m). Smaller particles der Waals attractive forces. These in- improve mass transport, but can lead termolecular forces are very weak and to filterability challenges.

10 Tight control of diminish with increasing distance be- PSD can optimize the balance. tween the carbon surface and the ad- Shaping. Powdered Activated car- sorbate molecule. Because the weak bons may be formulated with binders attractive forces are greatly dependent and shaped into cylinders, tubes, hon- on distance, physical adsorption occurs eycombs and other extruded forms to primarily within pores that have a radius optimize surface-to-volume ratio and only a few times greater than the molec- pressure drop for various Applications . ular diameter of the adsorbed molecule. Granules and extrudates may also be Pores that are smaller than the size of formed into filter blocks and plates using the impurity molecule are inaccessible polymeric binders. and do not participate in the adsorp- Impregnation. Some contaminants, tion process. Pores that are significantly such as formaldehyde, elemental mer- larger than the adsorbate molecule are cury and hydrogen sulfide are not ad- not as effective at adsorption because sorbed on Activated carbon.