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When a gas or vapour comes into contact with a solid surface, part of it adheres to the solid. The fraction of the gas or vapour that adheres to the solid enters it or remains attached to its surface, a phenomenon that is called adsorption. Adsorption is an exothermic process in which there is a fall in system entropy.
In simplified terms, adsorption is the attraction and adhesion of gas and/or liquid molecules to the surface of a solid and its internal cavities. That attraction is due to the unequal forces that exist on the surface of all solids. Such unequal forces are the result of unsatisfied valences on the surface of the solid and give rise to what is referred to as solid surface tension. The degree to which adsorption takes place depends on the specific nature of the adsorbent and that of the adsorbed material alike.
Given that adsorption is a surface phenomenon, good adsorbents have large surface areas per unit of mass and substantial forces of attraction. The degree to which adsorption takes place is directly proportional to the surface area exposed and to the concentration of the material adsorbed. All kinds of molecules can be adsorbed to a certain extent, although those with a higher polarity are more liable to be strongly adsorbed than those with a lower polarity.
Adsorption can occur in both static and dynamic conditions. Static adsorption capacity, also known as equilibrium capacity, is the maximum capacity it is possible to obtain for a given level of relative humidity. The packaging of pharmaceutical products with adsorbent materials to keep humidity levels low is an example of the application of static drying. If a fluid to be dried is in motion but confined within a closed circuit, this is also deemed to constitute a case of static conditions. However, most applications of adsorption take place in dynamic conditions, an example being the drying of the water vapour contained in compressed air for subsequent use in production processes.
As the fluid to be treated passes through the adsorbent, the efficiency of the latter decreases progressively until it becomes saturated. Energy is released in the form of heat during the drying process. The regeneration process makes it possible to re-establish the adsorptive properties of the bed. There are various methods of regeneration, the most commonly used being heat-driven regeneration, regeneration by means of pressure reduction and regeneration by means of purging with a fluid with a low level of humidity.
      
Aluminas have been used in the catalysis and adsorption industries for several decades. Their adsorptive characteristics are essentially determined by their structure. There are five types of thermodynamically stable aluminas and various metastable phases. Most types of alumina are obtained by means of the Bayer process.
The process currently used to obtain alumina is, in essence, that invented by Karl Bayer in 1887. It involves the use of sodium hydroxide (caustic soda) to selectively dissolve the Al²O³ of bauxite. Doing so produces a complex solution of sodium aluminates, from which Al(OH)³ is subsequently precipitated. The Al(OH)³ then undergoes calcination, resulting in the production of aluminium oxide.
Generally speaking, crystalline aluminas, which originate from oxides and hydroxides, are not considered to be useful for adsorption, as they have very little porosity and contact surface area. It is metastable phases of aluminas (transition aluminas) which are used as adsorbent materials. Aluminas of the kind in question are obtained through the thermal decomposition of hydroxides, as the loss of hydroxyl groups gives rise to crystalline structures lacking in oxygen and aluminium atoms. Depending on the metastable phase being obtained, the Al+³ y O-² ions may be tetrahedrally or octahedrally arranged in close-packed cubic or hexagonal systems.
By way of an example, gamma alumina, which is the best known type of alumina, has a defect in its spinel structure, in which the oxygens are in a close-packed cubic structure. The oxygen atom sublattice is highly ordered, but there is a significant degree of disorder in the aluminium atom sublattice. That distortion of the lattice structure entails a greater concentration of surface acid sites.
Depending on the method of synthesis used, the transition aluminas obtained may initially be amorphous or have a low level of sensitivity to the diffraction of X-rays. Many of the commercial products available at present are actually amorphous, as is the case of gamma alumina. By definition, amorphous transition aluminas have a greater concentration of lattice structure defects than partially crystalline transition aluminas do. During the transition from an amorphous structure to a crystalline structure, a significant rearrangement takes place in the anionic and cationic crystalline substructures. Defects are generated in the lattice structure during the rearrangement process, giving rise to active adsorption sites.
Beta aluminas are another group of aluminas with adsorptive properties. They consist of alkali-substituted aluminates and their derived compounds. The best known is sodium beta aluminate (Na²O-Al²O³-2CO²-2H²O). A derived compound and sodium beta aluminate are the main components of the so-called alkaline alumina process for desulphurising calcination flue gases. Another group of aluminas is that of alpha aluminas, although they are not usually considered to be adsorbent due to their virtually total dehydroxylation, reduced contact surface area and almost non-existent porosity. Nonetheless, they have a greater degree of Lewis acidity per unit of contact surface area than transition aluminas do, and that acidity means that they can have greater adsorptive properties. Alpha aluminas are highly effective adsorbents of chlorides, water vapour and inorganic acids, for example.
The chemical nature of active adsorption sites of aluminas is not fully known at present, as it is not possible to attribute the entire adsorption process to anionic vacancies. Activated aluminas can be considered to have Lewis and Brönsted acid and basic sites of varying strength and concentration. The acidity is due to unsaturated Al³+ ions, protonated hydroxyls and some acidic hydroxyls. The basicity is the result of anionic vacancies of O²- and basic hydroxyls.
Lastly, five different forms of surface hydroxyl groups have been identified in activated aluminas. The groups in question have differing degrees of acidic and basic characteristics in adsorption, depending on their synthesis, geometric orientation and concentration. Increasingly greater importance has been attributed to the study and modification of the surface chemistry of aluminas over recent years, with a view to enhancing their selective adsorption of certain compounds. The most common method of modifying transition aluminas consists of controlling the thermal treatment that takes place during the process by virtue of which they are obtained. Such control makes it possible to modify the concentration of hydroxyls and of Lewis and Brönsted acid and basic sites.
Aluminas are used for the drying of air, olefins, natural gas, synthetic gas, gas recycled from catalytic reforming processes, propylene, butane, butadiene, petrol, chlorinated hydrocarbons, fluorocarbons and aromatic solvents, as well as for the purification of gases and liquids.
      
Molecular sieves are crystalline zeolites whose general formula is M²/nO ˇ Al²O³ ˇ xSiO² ˇ zH²O, where M is an n+ valence cation. They were discovered 200 years ago by the mineralogist Cronstedt. It was he who coined the term zeolite, upon noting that such material apparently boiled and underwent evaporation upon being heated. The term zeolite comes from the Greek words "zeo", meaning boil, and "lithos", which means stone.
Although of a similar composition, zeolites are completely different from amorphous aluminium silicates. The fundamental component of the structure of the crystals of a molecular sieve is a tetrahedron with four oxygen anions surrounding a smaller silicon or aluminium cation. Sodium, potassium or calcium ions serve to compensate the deficiency in terms of positive electrical charge in alumina tetrahedrons. Each of the four oxygen anions is, in turn, shared with another silica or alumina tetrahedron, extending the lattice structure in three dimensions. The resulting lattice structure has alveoli with relatively large cavities. Each of those cavities is connected to other, adjacent cavities by means of channels or pores.
Of the different types of molecular sieves, the most commonly used are those of the 4A and the 13X varieties, which have the following formulae:
4A : Na¹² [ (AlO²)¹² (SiO²)¹² ] ˇ 27H²O
13X: Na86 [ (AlO²)86 (SiO²)106 ] ˇ 27H²O
In both cases, the sodium ion can be exchanged for other ions, making it possible to form different types of molecular sieves for a range of applications. The hydration water is extracted by means of heating before the material is used.
Type-A molecular sieves have a lattice structure formed by truncated octahedrons linked in a cubic array. That array produces a central truncated cube octahedron with an internal cavity with a diameter of 11 Angstroms. Each central cavity, referred to as the α cage, is entered through six circular openings, formed by an almost regular ring of eight oxygen atoms with a free diameter of 4.2 Angstroms. The cavities are thus arranged in a continuous three-dimensional pattern, forming a system of channels with a maximum diameter of 11 Angstroms and a minimum diameter of 4.2 Angstroms. In turn, the truncated octahedrons enclose a second set of cavities with an internal diameter of less than 6.6 Angstroms, referred to as α cages, which are connected to the larger cavities by means of a distorted ring of six oxygen atoms with a diameter of 2.2 Angstroms.
In the process by virtue of which type-A molecular sieves are obtained, sodium silicate, alumina trihydrate and sodium hydroxide are dispensed into mixing vessels and stirred until the mixture is homogeneous. The resulting gel is pumped to a crystallisation vessel, where it is kept under controlled conditions. When the crystallisation process is complete, the compound obtained is filtered and washed. In cases in which the sodium of the lattice structure of the compound obtained is to be replaced with calcium or another cation, it should be mixed with a solution of the corresponding metal salt and heated, and the newly obtained compound must subsequently be filtered and washed. The compound is mixed with clay binding agents and extruded in order to produce the end commercial form. After doing so, the final step is to heat the material obtained so as to extract its water and activate the molecular sieve.
Unlike other adsorbent materials, molecular sieves have an extraordinary capacity for the adsorption of water and polar molecules, provided that there is a low concentration thereof in the fluid to be treated.
In addition, the uniform size of the pores of molecular sieves, a characteristic attributable to their crystalline structure, means that they can be used as screen filters for particles or molecules of a specific size.
Molecular sieves are used for the drying of air, CO, CO², H²S, SO², NH³, N², O², methane, methanol, ethanol, ethylene, acetylene, propylene, ethylene oxide and n-propanol.
      
Silica gel is a porous, granular, amorphous form of silicon. It is obtained by taking advantage of the difference between the surface tension of a colloidal solution (a chemical reaction between sulphuric acid and sodium silicate) and that of a precipitant for the formation of gels. The process by virtue of which silica gel is obtained produces spherical beads. Regardless of their internal structure, that spherical form ensures the regular, uniform distribution of the adsorbent material, a reduced level of abrasion and the smooth passage of the fluid to be treated, without the formation of channels or cavities in the bed.
Silica gel is chemically stable, non-toxic, inert and non-corrosive. Its chemical composition, in terms of weight, is 97% SiO² and 3% Al²O³. Its composition renders it compatible with most industrial chemical products, with the exceptions of strong bases and hydrofluoric acid.
While amorphous, the internal structure of silica gel is formed by a vast network of interconnected micropores. Standard silica gels have an internal surface area of approximately 800 m²/g.
Silica gels are used for the drying of gases and liquids, with the exceptions of strong bases and hydrofluoric acid.
      
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