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2.1 The Factors Influencing Design and Operation
There are a wide number of variables which influence the design and operation of vapour phase adsorption systems. This text has been prepared to highlight some of the factors which should be considered and to introduce the techniques which are available to define the effects of these variables.In general, the areas which need to be identified are;
Data on processing conditions:
- Concentration of adsorbate
- Temperature of vapour stream
- Relative humidity of vapour stream
- Flow rates and operating frequency
- Process operating pressure
- Pressure drop in system
Identification of characteristics of adsorbate:
- Relative molecular mass
- Boiling point
- Concentration relative to saturation vapour pressure
- Polarity of molecule
Selection of adsorbent for optimum efficiency:
- Study of adsorption isotherm data
- Selection of optimum activity level
- Physical requirements of adsorption system
- Cost analysis
- Consideration of thermal reactivation
These points are considered in further detail in the following text, using the example of benzene.
| Vapour phase isotherm studies can be used to determine the adsorptive capacity of activated carbon over a range of different concentrations. For standard conditions of temperature, relative humidity and pressure, the adsorptive capacity of activated carbon will increase as the concentration increases, until the maximum saturation capacity is achieved. |  |
| Vapour phase isotherm studies can be used to determine the adsorptive capacity of activated carbon over a range of different temperatures. For standard conditions of relative humidity and pressure the adsorptive capacity of activated carbon will decrease as the temperature increases.
This effect is caused by an increase in the energy of the system. |
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| Penetration testing can be used to determine the effect of humidity on adsorption, by measuring the critical bed depth needed to reduce the concentration of benzene by 90% under standard conditions of temperature, pressure and flow. Taking measurements at several bed depths gives an intercept on the x-axis which is the critical bed depth. The value for a dry stream will be lower than that of a humid system. This effect is caused by moisture present in the pores of activated carbon, reducing adsorption efficiency. |
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| Penetration testing can be used to determine the effect of linear flow on adsorption by measuring the critical bed depth needed to reduce the concentration of benzene by 90% under standard conditions of temperature, pressure and flow. Taking measurements at several bed depths gives an intercept on the x-axis, which is the critical bed depth. The higher the rate of flow, the greater the critical bed depth. Reductions in the flow will give proportional reductions in the critical bed depth. This effect is caused by the rate of diffusion into the pores of the carbon. |
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| Vapour phase isotherm studies can be used to determine the adsorptive capacity of activated carbon over a range of concentrations at different pressures. For standard conditions of relative humidity and temperature, the adsorptive capacity of activated carbon will increase as the pressure increases. This effect is determined from the partial pressure of the vapour, at the working pressure. |
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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 increased pressure drop and consequently higher energy consumption. |
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