Adsorption Processes 8
8.3 Activated carbon
Activated carbon is manufactured from a wide variety of carbonaceous material such as coal, bituminous coal (lignite), bone, wood and coconut shell. The basic carbonaceous material is subjected to a manufacturing process which creates a huge internal area within the carbon particles.
For example one gram of ground coal has an internal surface area of only 10 m2; during the process of activation this internal surface area is increased up to 1000 m2. Hence, the process of activation transforms the carbon particle into a useful adsorbent. The most common process of activation consists of thermal treatment which is carried out in two stages.
First, the material is carbonised at about 700◦C in the absence of oxygen to reduce the volatile content and convert the raw material to a char.
This is followed by further heating up to 1000◦C in a regulated oxygen and steam environment. The steam penetrates the carbon particle and selectively oxidises the internal carbon atoms to create the large internal surface area.
Within the furnace the main gaseous reactions can be represented as follows:
H2O(g)+ C(s) ⇒ CO(g) + H2(g) endothermic CO2(g)+ C(s) ⇒ 2CO(g) endothermic then
CO(g)+ H2O(g)⇒ CO2(g)+ H2(g)
When using oxygen the reaction is exothermic and it is more difficult to control the attack on the carbon structure.
3/2 O2(g)+ C(s) ⇒ CO2(g)+ CO(g)
There are two principle forms of activated carbon used in water treatment:
granular activated carbon (GAC) and powdered activated carbon (PAC).
Although both types can be produced from the same raw material their typical average particle diameters vary from greater than 1 mm for the GAC to less than 0.1 mm for PAC. This variation in size can affect the rate of adsorption but not the amount adsorbed, since the greater proportion of the particle’s surface area lies within its internal structure (pores) and not on its external surface.
Carbons are generally categorised by pore size according to the classi-fication below:
macropore>50 nm mesopore 2–50 nm micropore 1–2 nm minimicropore<1 nm
Of the carbon sources it is known that coconut shell-based GAC has the highest surface area, generally over 1000 m2 g−1. Bituminous-based GAC has a relatively large surface area, approximately 900 m2g−1, whilst lignite-based GAC has a surface area of approximately 650 m2g−1. Lignite GAC is more suited to larger molecules whilst coconut shell-based GAC generally has a larger surface area than coal-based GAC, and a very large percentage of micropores. A comparison of carbon sources and activated carbon structures is shown in Figure 8.1.
The following main characteristics should be considered in respect of GAC specifications.
Total surface area is normally determined by measuring the amount of nitrogen adsorbed to produce an adsorption isotherm. This data is then analysed using the Brunauer Emmett Teller isotherm equation which assumes monolayer adsorption of nitrogen molecules only. The resulting surface area is given as the BET surface area, normally in units of m2g−1 of media.
Apparent density is normally expressed as the ‘apparent bulk density’.
In this instance only the density of the carbon in air is being measured.
Typical values range from 350 to 500 kg m−3.
True bulk density includes contributions from the carbon, the weight of water within the pore structure and the water attached to the carbon.
Typical values range from 900 to 1300 kg m−3.
Fig. 8.1 Comparison of activated carbon structures (with kind permission from PICA Carbons).
Hardness is measured by placing a screened and weighed sample in a
‘hardness pan’ with a number of steel balls. This is then subjected to a combined rotating and tapping action for 30 min. The degree of particle size degradation is measured as the ‘ball–pan hardness’ and is calculated as follows:
H= 100A B where
H= ball–pan hardness number
A= weight of sample retained on hardness sieve B= weight of sample used in test.
Adsorption capacity is usually estimated by the adsorption of iodine or methylene blue. The iodine number is defined as the amount of iodine (mg) adsorbed by 1.0 g of carbon at a residual concentration of 0.02 N. The test results give an indication of the surface area available within the microp-ores. Methylene blue adsorption is predominantly within the mesopores and the methylene blue number is defined as the amount of methylene blue (mg) adsorbed by 1.0 g of carbon at a residual concentration of 1.0 mg l−1. When the effluent quality from a carbon contactor reaches minimum water quality standards, the spent carbon is removed for regeneration.
Small treatment works usually find regeneration of their spent carbon at an off-site commercial reactivation facility to be the most convenient and eco-nomical method, whilst treatment works that contain at least 500 tonnes of carbon find on-site regeneration to be cost effective. Carbon regenera-tion is accomplished primarily by thermal means. Organic matter within the pores of the carbon is oxidised and thus removed from the carbon surface. The two most widely used regeneration methods are rotary kiln and multiple hearth furnaces. Approximately 5–10% of the carbon is de-stroyed in the regeneration process or lost during transport and must be replaced with virgin carbon. The capacity of the regenerated carbon is slightly less than that of virgin carbon. Repeated regeneration degrades the carbon particles until equilibrium is eventually reached providing pre-dictable long-term system performance. The main elements of the regen-eration process are:
1. Pre acid washing 2. Reactivation
3. Quenching and post treatment 4. Flue gas treatment.
An optional pre acid washing process is available which has the ability to wash the spent GAC with hydrochloric acid (pH 2) for several hours or until set process criteria are reached. The benefits of this are the removal of damaging inorganic compounds such as calcium which can catalyse the thermal degradation of the GAC structure and lead to reduced physical strength and shorter operational life. Additional benefits are also that the conditioning of the regenerated GAC on return to service is much faster due to the removal of inorganic species which can cause difficulties (Mn, Al etc). From the spent hopper the GAC is transported to the top of the furnace (using an eductor system) via a dewatering screw which removes up to 85% of the water. A typical rotary hearth furnace consists of six hearths (refractory lined) and is capable of processing up to 40 m3day−1. A revolving central shaft with attached rabble arms, sweeps the carbon from the inlet port on the middle of the top hearth to the outside, where it drops onto the hearth beneath. It is then rabbled to the inside and falls to the next hearth and so on. The constant turnover action of the rabbles produces an intimate contact between the carbon granules and the furnace gases.
Factors that affect the quality of the regenerated product are 1. Oxidising gas
2. Furnace temperature
3. Residence time in the furnace
4. Concentration and type of substance adsorbed
5. Type and concentration of inorganic impurities adsorbed 6. Type of activated carbon (i.e. wood, coal based etc).
Regeneration of the carbon can have the following deleterious effects on the integrity of the structure:
r Small pores (<2 nm) may be lost, whilst larger pores are created r The particle diameter may be reduced
r Hardness of the particle decreases.
The regeneration plant is governed by the Environmental Protection Act 1990 in the UK which requires that emissions from the plant meet certain standards. As a result it has been necessary for the installation of a number of process units to reduce the contaminants in the flue gas emissions from the furnace stack. Flue gas emission treatment firstly consists of an after-burner which provides for a retention time of 2 s at 8500◦C in a minimum 6% oxygen environment, which is necessary for the oxidation of harm-ful organic products (e.g. dioxins). This is followed by passage through
a heat exchanger to reduce the temperature of the gas. Particulates are then removed by passing through a venturi scrubber. A tray scrubber is then used to remove acidic gases. Finally, the temperature is increased to 1000◦C and discharged to atmosphere via a chimney. It is important to increase the temperature to avoid a visible plume. Continuous monitoring and independent emission testing ensure compliance with the regulations.
The excess heat removed by the heat exchanger can in part be reused.
8.4 Applications
Activated carbon is widely used in the water industry to remove organic and inorganic compounds. In particular it is used to remove (i) natural organic matter and colour, (ii) pesticides, (iii) taste and odour (iv) algal toxins. There are some general rules that can be used to predict if GAC is likely to remove a molecule:
r Larger molecules adsorb better than smaller molecules.
r Non-polar molecules adsorb better than polar molecules.
r Slightly soluble molecules adsorb better than highly soluble molecules.
r pH may have an influence on the extent of adsorption as it can control both polarity and solubility of a molecule.
Figure 8.2 compares the Freundlich isotherm constant, K for a range of problem water contaminants. The constant K is related to the capacity of the activated carbon for a specific molecule where the larger the value of K the larger the capacity will be and the molecule will be more easily adsorbed. From the data shown in Figure 8.2 it can be seen that acti-vated carbon has a significantly larger capacity for the pesticide atrazine than it does for the taste and odour causing molecules, geosmin and 2-methylisoborneol (MIB). More information on the use of activated car-bon to adsorb organic molecules is given in Chapter 10.
8.5 Adsorbers
Granular carbon is produced in the form of granules, typically around 1–2 mm diameter, which are used in deep bed contactors or ‘columns’. As water passes down the column the carbon becomes exhausted from the top downwards. If we consider the ‘exhaustion front’, that is the interface between the carbon above and the fresh carbon below we see a concen-tration profile like that shown in Figure 8.3. It is an over-simplification
Fig. 8.2 Comparison of the Freundlich isotherm constant (K ) for a range of contaminants (Awwa, 2000; Hall & Sheppard, 2005).
of the real situation, but we can assume that above the dotted line the carbon is 100% exhausted whilst below it the carbon is 100% fresh. At the interface there is a band of carbon through which the state changes and, at the bottom as the exhaustion front reaches the bottom of the bed, the sorbate will finally break through the bed.
The volume of carbon required in a treatment plant is usually deter-mined by the empty bed contact time (EBCT) and is calculated as follows:
V = Qt where
V = GAC volume (m3) Q= Flow rate (m3h−1)
t= empty bed contact time in hours.
However, it is wise to check that the capacity is sufficient to give a reasonable run time, especially if the carbon is to be taken off site for regeneration:
T = 1000VC Qc
100% EXHAUSTED
0% Exhausted Direction of f low
Concentration
Depth of Exhaustion Front
Fig. 8.3 The exhaustion front.
where
= Carbon density (typically 0.5 te m−3) c= Influent concentration of adsorbate mg l−1
C= Carbon adsorption capacity for adsorbate mg g−1.
In waterworks applications the carbon volume is usually large and con-sideration must be given to carbon handling. Most road tankers will take a carbon load of 20 m3so most GAC contactors are designed for a carbon volume of some multiple of 20 m3. Examples of a pressure adsorber are shown in Figures 8.4 and 8.5.