Present-day technology utilises various processes which contribute towards compressed air purity. One of these processes is the application of adsorption technology using adsorption media. By adsorption media, one understands substances with a widely open pore structure and, a large internal surface. Examples of adsorption media are aluminium oxide (activated alumina), silica gel, molecular sieve and activated charcoal. This section deals with the adsorption of gases and vapours through solid adsorbents to form adsorbates. It does not deal with adsorption in the liquid phase. Adsorption makes use of the characteristics of porous solids, endowed with large surfaces, in order to separate low concentrations of vapour selectively from a mixture of gases. The adsorption process with porous adsorbents possessing extended internal surfaces is made up from three kinetic part processes :
- Transfer of matter in the boundary layer
- Diffusion of the substance to be adsorbed in the pore system
- Sorption at the internal surface of the adsorber
Physical adsorption on the surface of solid adsorption media is, to an extent, accompanied by other processes. For this reason, adsorption is regarded as a general sorption process. Within the micropores of the adsorption medium, capillary condensation of the vapours takes place at higher pressures, or the component to be separated is diffused within the solid material. Chemical adsorption, as a chemical reaction between gaseous components and the solid substance, is also possible. Physical adsorption and chemical adsorption differ in their accompanying heat phenomena. Adsorption makes possible the total separation of low concentrations of vapours from gaseous mixtures. In cases of high concentration of the matter to be adsorbed, this separation process is often uneconomical, as high concentrations of the substance to be adsorbed call for a high proportion of adsorption medium in relation to the quantity of gas, or frequent regeneration of the loaded up adsorption medium. Adsorption heat causes an increase in temperature of about 10 - 20°C in adsorbers, however, no cooling is necessary with adsorption, as temperature dependence of the sorptive quantities picked up is relatively small.
Table 6.1.1 - Physical and chemical properties of drying media| Characteristics | Aluminium Oxide | Silica Gel | Molecular Sieve | Activated Charcoal |
|---|
| Bead Size mm | 2 - 9 | 2 - 8 | 1 - 6 | 1 - 6 |
| Porosity % | 50 - 60 | 50 - 65 | 45 - 60 | 52 - 75 |
| Specific Surface m2/g | 100 - 400 | 300 - 800 | 500 - 900 | 100 -1500 |
| Pore Volume ml/g | 0,3 - 0,5 | 0,4 - 1,0 | 0,5 - 1,1 | 0,5 - 1,6 |
| Pore Size Ä | 15 - 100 | 21 - 100 | 4 - 15 | 10 -250 |
| Specific Heat kcal/kg°C | 0,21 - 0,25 | 0,22 - 0,25 | 0,19 - 0,31 | 0,19 |
| Heaped Volume kg/m3 | 600 - 900 | 450 - 800 | 600 - 900 | 200 - 500 |
| Static Activity kg/kg | 0,2 - 0,3 | 0,2 - 0,4 | 0,3 - 0,5 | 0,5 - 0,9 |
| Adsorption Temperature °C | 0 - 30 | 5 - 40 | 5 - 50 | 5 -55 |
| Regeneration Temperature °C | 170 - 320 | 140 -250 | 190 - 320 | 110 -180 |
| Ignition Temperature | non combustible | 250 - 400 |
Table 6.1.1 shows the characteristics of the most important adsorbents in technical use. The high specific surface of adsorption media is of paramount significance. Gas velocity inside adsorbers is in the range of 0.1 - 0.6 m/s. Aluminium oxide, silica gel, molecular sieve and activated charcoal differ in their fields of technical application. Their micropore diameters are in the region of 4 - 250 nm. Aluminium oxide, silica gel and molecular sieve are particularly suitable for the adsorption of polar compounds, in particular for drying air and gases. The fields of application of activated charcoal encompass purification and the removal or attenuation of odours from air and gases. In recent years, molecular sieves have found increasing application. These natural or artificial zeolites are crystalline alkali or earth-alkali aluminosilicates. SiO4 and AlO4 tetrahydrons form a cubo-octahydron if alternatively arranged as a complex structural component. These cubo-octahydron network three-dimensionally to form a multiplicity of possible zeolite structures. This causes well defined and evenly formed systems of voids (micropores) linked by canals. These can act as physical sieves towards molecules depending on the geometrical dimensions of the latter. At the same time, there are interaction effects between molecules and heteropolar internal void surfaces, with adsorptive effect.
The hollow cross-section of the type dependent highly uniform and constant microchannels lies in the range of 0.3 - 1 nm (kinetic pore diameter). They make possible the separation of mixtures in accordance with the molecular dimensions, e.g. with branched or unbranched hydrocarbons. In addition, zeolites adsorb polar substances such as water, so that they can be used for the intensive drying of gases. Apart from adsorption at atmospheric pressure, there is the possibility of pressure adsorption under pressurised conditions, as this increases the partial pressure of the adsorbate and thus the equilibrium load of the adsorbent. If adsorption under pressure is followed by desorption at low pressure, e.g. atmospheric or vacuum, a fractionated (insulating through vapourisation) desorption of the constituents takes place upon depressurisation and this can be used for separating individual components.
Within the pore system of the adsorbent particles, the conveyance of matter takes place in accordance with various mechanisms. The large pores act as access pores and the smaller ones as adsorption pores. Table 6.1.2 gives a survey of the types of diffusion prevalent in adsorptive pores.
Table 6.1.2 - Mechanisms which convey matter| Pore Size | Prevalent Conveyance Mechanism |
|---|
| nm | Description | Process |
|---|
| 2 - 10 | Activated Split Diffusion | Force field reinforcement through super imposition of the force fileds of pore walls lying opposite each other |
| 102 - 103 | Surface Diffusion | Concentration gradient along the adsorbate pore surface |
| 103-5*104 | Molecular Diffusion | Pore diameter smaller than the free wave length of the molecules |
| >5*104 | Normal- diffusion | Free gas diffusion |
Multilayer adsorption in larger pores is accompanied by capillary condensation in the micropores. The latter takes place particularly in adsorbents with a high constituent of mesopores in the pore radius range 1 - 50 mm. Apart from the conveyance of matter through diffusion, liquid is displaced by capillary action. The conveyance of matter through adsorption takes place as a transition phase of matter, i.e. the substance to be adsorbed is diffused to the solid matter surface (phase boundary) from the flowing gas phase through the boundary layer. The adsorption speed of matter transition varies within wide limits depending on the nature of the system, and may last from fractions of a second to a duration of hours up to the onset of adsorption equilibrium.
Temperature, pressure, molecular mass of the substance to the adsorbed and porosity of the adsorbent, influence the speed of matter transition. The capillary structure of the adsorption medium delays the onset of the equilibrium state. Speed of adsorption is influenced by :
- Flow conditions within the adsorber
- Matter displacement of the fluid phase to the adsorber surface
- Pore diffusion of the adsorbed substance within the adsorbent
- Speed of adsorbate formation
- Surface migration of the substance to be adsorbed in the adsorbing layer
The activity of the adsorbent and the time of adsorption determine the technical sequence of adsorption and characterise the adsorbing effort in completing the separating task. The activity of the adsorbing medium indicates the adsorbing capacity as quantity of substance adsorbed per unit of mass of the adsorbent, i.e. the activity equals the adsorbent loading. One has to distinguish between the static activity and the dynamic activity. Static activity presupposes the setting in of a complete equilibrium state between the content in the raw gas of substance to be adsorbed and the loading of the adsorbing medium with this substance and counts as a characteristic of the selective qualities of the adsorbent. Static activity diminishes with rising temperature. The number of adsorption/desorption cycles in service also influences the static activity. Equilibrium loading and adsorption speed are lowered due to ageing of the adsorbent.
Dynamic activity is represented by the adsorption behaviour towards the gases in a state of flow. The displacement of matter inside the adsorbent pores in the form of surface diffusion within the range of the surface tensions of the adsorbent delays the onset of a state of equilibrium. This results in the rising heat of adsorption through the wave of warmth migrating through the adsorbent layer, the equilibrium state evolves in the direction of diminishing adsorbent loading capacity. To this, one has to add mixed adsorption, leading to the sorption displacement of already adsorbed components through the more easily adsorbable constituents which make their appearance only later in the higher sorbent layers, thus bringing about a duplex and mutually impeding matter displacement at the phase boundary surface. For example, elements of water vapour replace already adsorbed components from the adsorbent in hydrophile (hygroscopic) means of adsorption such as silica gel. Combined with only part loading, all these phenomena lead to a diminuition of dynamic activity in comparison to static activity. It is at the entry point of the gas that adsorber layers begin to be saturated with the substance to be adsorbed, in this case moisture. It is there that a dynamic adsorption equilibrium between raw gas and adsorbent being loaded up is established. As time goes on, the equilibrium zone penetrates further into the static adsorber layer. The level of the total layer, within which adsorbent loading decreases from the equilibrium value (maximum valve) to zero load, is called adsorption or also mass transition zone. The higher the velocity of adsorption, the narrower.the adsorption zone. The adsorption zone travels ahead of the equilibrium zone through the overall adsorbent layer at a specific zone migration velocity. Finally, we arrive at break-through, the emerging gas now contains steadily increasing quantities of the substance to be adsorbed. This adsorption zone model, so far applied to the adsorption of a single component, is in principle valid also for two, or more component mixed adsorption. In such a case, the component being most weakly adsorbed, forms the basis for the design of the apparatus. In each case, the only weakly adsorbed component travels through the adsorber with the highest velocity of zone migration, for this reason it rushes ahead of the other components, initially meets up with unloaded adsorber layers, and is, therefore, adsorbed to a stronger degree than the mixed adsorption equilibrium because of the absence of a more readily adsorbed component. In the end, the succeeding and more strongly adsorbed components replace excess quantities (partial desorption) of adsorbate, thus establishing the mixed adsorption equilibrium characteristic for the particular mixture.
