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3.1 The Factors Influencing Design and Operation
There are manny variables that influence the design and operation of liquid phase adsorption systems. This text has been prepared to highlight some of the factors that should be considered and to introduce the techniques which are available to define the effects of these variables.In general, the areas that need to be identified are:
Data on processing conditions:
- Concentration of adsorbate
- Temperature of liquid stream
- pH of liquid stream
- Flow rates and operating frequency
- Pressure drop in system
- Relative molecular mass
- Solubility of adsorbate
- Concentration relative to solubility limits
- Polarity of adsorbate
- Temperature of solution
- Study of adsorption isotherm data
- Selection of optimum activity level
- Physical requirements of adsorption system
- Cost analysis
- Consideration of thermal reactivation
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Freundlich liquid phase isotherm studies can be used to determine the adsorptive capacity of activated carbon over a range of different concentrations. Under standard conditions, the adsorptive capacity of activated carbon will increase as the concentration increases, until the maximum saturation capacity is reached. |
| Freundlich liquid phase isotherm can be used to determine the effect of solubility on the adsorptive capacity of activated carbon over a range of different concentrations. Phenol is highly soluble due to its polar nature whilst, in comparison, tetrachloroethylene has a low solubility due to being non-polar. In the isotherms illustrated, the concentration of phenol is low relative to its solubility limit and consequently, the adsorptive capacity peaks at 18% maximum. In comparison the concentration of tetrachloroethylene is relatively close to its solubility limit and, accordingly, the adsorptive capacity is exceptionally good. |  |
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The effect of linear flow rate through an activated carbon adsorption unit can be illustrated by the mass transfer zone (MTZ diagram). After a period of operation, the top portion of the bed becomes fully loaded with the adsorbed organics. This is referred to as the saturation zone. Removal of the dissolved organics takes place in the section of the bed referred to as the MTZ. The length of the MTZ can be optimised at lower flow rates. Other factors, such as concentration and particle size, will also affect the MTZ. The equilibrium zone remains unaffected until the MTZ progresses through the bed. |
| Initially, on start up of an adsorption unit, there will be a slight increase in the pH of the effluent water. This is due to trace leaching of the soluble matter from the matrix of the activated carbon. Leaching is usually controlled by efficient backwashing, which will readily remove soluble matter, following which the system will reach equilibrium with the pH of the feed water. |  |
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Adsorption efficiency can be optimised by using finer particle size products which improve the rate of diffusion to the surface of the activated carbon. This must be balanced against pressure drop limitations. |
| Efficient backwashing is an important part of the start up of an activated carbon filter. A bed expansion of 25-35% should be used to remove readily soluble matter and to stratify the particles in order to ensure that the MTZ is maintained when future backwashing is undertaken. |  |
Adsorption isotherms for a range of organic compounds are provided in the following graphs.
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