Coal And Coke

Coal (hard) and coke are used in water filtration, primarily for the removal of coarse suspensions, care being taken to prevent them from scouring or washing away, because of their relative lightness and fine division. Coal is principally composed of carbon, and is inert to acids and alkalies. Its irregular shapes are advantageous at times over silica sand.

Though inert to acids, sand is affected by alkalies, and its spherical particle shape allows deeper solids penetration and quicker clogging than does coal. With the lighter weight of coal (normally 50 lb/ft3, compared with 100 lb/ft3 for sand), a greater surface area is exposed for solids entrapment.

CHARCOAL

Charcoal, whether animal or vegetable, when used as a filter medium, is required to perform the dual services of decoloring or adsorbing and filtering. The char filters used in the sugar industry are largely decoloring agents and the activated carbons used in water clarification are for deodorizing and removal of taste. There are many types of charcoal in use as filter media, ranging from ordinary wood char to specially prepared carbons.

Activated carbon in particular is very versatile as a filter media because it not only can physically separate out suspended solids, but it can adsorb materials. The adsorption process occurs at solid-solid, gas-solid, gas-liquid, liquid-liquid, or liquid-solid interfaces. Adsorption with a solid such as carbon depends on the surface area of the solid. Thus, carbon treatment of water involves the liquid-solid interface. The liquid-solid adsorption is similar to the other adsorption mechanisms. There are two methods of adsorption: physisorption and chemisorption. Both methods take place when the molecules in the liquid phase become attached to the surface of the solid as a result of the attractive forces at the solid surface (adsorbent), overcoming the kinetic energy of the liquid contaminant (adsórbate) molecules.

Physisorption occurs when, as a result of energy differences and/or electrical attractive forces (weak van der Waals forces), the adsórbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multilayered; that is, each molecular layer forms on top of the previous layer with the number of layers being proportional to the contaminant concentration. More molecular layers form with higher concentrations of contaminant in solution. When a chemical compound is produced by the reaction between the adsorbed molecule and the adsorbent, chemisorption occurs. Unlike physisorption, this process is one molecule thick and irreversible

because energy is required to form the new chemical compound at the surface of the adsorbent, and energy would be necessary to reverse the process. The reversibility of physisorption is dependent on the strength of attractive forces between adsórbate and adsorbent. If these forces are weak, desorption is readily affected. Certain organic compounds in wastewaters are resistant to biological degradation and many others are toxic or nuisances (odor, taste, color forming), even at low concentrations. Low concentrations may not be readily removed by conventional treatment methods. Activated carbon has an affinity for organics and its use for organic contaminant removal from gaseous streams and wastewaters is widespread. The effectiveness of activated carbon for the removal of organic compounds from fluids by adsorption is enhanced by its large surface area, a critical factor in the adsorption process. The surface area of activated carbon typically can range from 450 to 1,800 m2/g, with _

some carbons observed to have a surface area up to 2,500 m2/g. Some examples are given in Table 6.

The versatility of charcoal, or carbon lies in its ability to adsorb materials. Carbon has been known throughout history as an adsorbent with its usage dating back centuries before Christ. Ancient Hindus filtered their water with charcoal. In the thirteenth century, carbon materials were used in a process to purify sugar solutions. In the eighteenth century, Scheel discovered the gas adsorptive capabilities of carbon and Lowitz noted its ability to remove colors from liquids. Carbon adsorbents have been subjected to much research resulting in numerous development techniques and applications. One of these applications was begun in England in the mid-nineteenth century with the treatment of drinking waters for the removal of odors and tastes. From these beginnings, water and wastewater treatment with carbon has become widespread in municipal and industrial processes, including wineries and breweries, paper and pulp, pharmaceutical, food, petroleum and petrochemical, and other establishments of water usage. Interest in carbon use for air as well as water pollution control and traditional industrial/product applications has received increased attention since the early 1970s with the advent of more stringent environmental regulations.

Table 6. Typical Surface Areas of Activated Carbons.

Origin

Surface Area, m2/g

Bituminous coal

1,200-1,400

Bituminous coal

800-1,000

Coconut shell

1,100-1,150

Origin

Surface Area, m2/g

Pulp mill residue

550-650

Pulp mill residue

1,050-1,100

Wood

700-1,400

Of less significance than the surface area is the chemical nature of the carbon's surface. This chemical nature or polarity varies with the carbon type and can influence attractive forces between molecules. Alkaline surfaces are characteristic of carbons of vegetable origins and this type of surface polarity affects adsorption of dyes, colors, and unsaturated organic compounds. Silica gel, an adsorptive media that is not a carbon compound, has a polar surface which also exhibits an adsorptive preference for unsaturated organic com- pounds as opposed to saturated compounds. However, for the most part, activated carbon surfaces are nonpolar, making the adsorption of inorganic electrolytes difficult and the adsorption of organics easily effected.

Pores of the activated carbon exist throughout the particle in a manner illustrated in Figure 2. The pore structure of activated carbon affects the large surface-to-size ratio. The macropores do not add appreciably to the surface area of the carbon but provide a passageway to the particle interior and the micropores. The micropores are developed primarily during carbon activation and result in the large surface areas for adsorption to occur.

Figure 2. Carbon granule at work.

Macropores are those pores greater than 1,000 A; micropores range between 10-1,000 Â. Pore structure, like surface area, is a major factor in the adsorption process. Pore-size distribution determines the size distribution of molecules that can enter the carbon particle to be adsorbed. Large molecules can block off micropores, rendering useless their available surface areas. However, because of irregular shapes and constant molecule movement, the smaller molecules usually can penetrate to the smaller capillaries. Since adsorption is possible only in those pores that can be entered by molecules, the carbon adsorption process is dependent on the physical characteristics of the activated carbon and the molecular size of the adsórbate. Each application for carbon treatment must be cognizant of the characteristics of the contaminant to be removed and designed with the proper carbon type in order to attain optimum results. Basically, there are two forms of activated carbon: powdered and granular. The former are particles that are less than U.S. Sieve Series No. 50, while the latter are larger. The adsorption rate is influenced by carbon particle size, but not the adsorptive capacity which is related to the total surface area. By reducing the particle size, the surface area of a given weight is not affected. Particle size contributes mainly to a system's hydraulics, filterability, and handling characteristics.

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