Organic solvents are used in a very wide range of industries dealing with: flexible packaging manufacturing, adhesive tapes, linings, foils, rubber, explosives, pharmaceutical and chemical processes, degreasing, extraction plants, painting and varnishing processes, polyurethane and acrylic resins processes.
The main organic solvents used in the above industries are: ethylacetate, ethanol, ketones, isopropanol, tetrahydrofurane, hexane, toluene, methoxipropanol etc., their boiling point being between 60 and 120°C.
Hot air is normally used to evaporate the solvents and the solvent laden air contains generally between 2 and 15 grams/m3 of solvent, and depending on the season, 5 20 grams/m³ of water.
The recovery of the solvent by straight refrigeration is difficult because of the presence of water and because the low solvent concentration requires very low temperatures around- 80° C.
The recovery of the solvent by liquid absorption is also difficult because very high circulation rates of the absorbing liquid are necessary.
Adsorption on porous solids is the preferred process and the eminently suitable adsorbent is activated carbon. Contrary to silicagel, activated alumins, and typical molecular sieves, activated carbon adsorbs by preference less polar organic solvents, while adsorbing relatively little water.
On activated carbon, the water loading becomes important only if the water partial pressure is above some 30% of the saturation pressure at the given temperature.
Therefore activated carbon not only separates the organic solvents from the air, but it also allows passing most of the water (95-98%) unabsorbed. Special molecular sieves with low water adsorption are very expensive.
Activated carbons have also a high porosity, their pore surface being typically in the range of 1000- 1200 m2/gram and therefore at total saturation they can take up as much as 0,5 kg solvent /kg carbon (50%), and at the typical solvent recovery conditions of 30- 400C, 0.15- 0.25 kg/kg carbon (15-25%).
The adsorption cycle may be carried out on moving beds, fluid beds, rotating beds and fixed beds. The first three options have all the advantage, that no switching valves are necessary, and the piping is also simpler, but there are problems with erosion, carbon attrition and clogging or special and expensive carbon, and with rotating beds leaking and thermal stress.
These options allow also a more efficient heat recovery, but nevertheless, they are generally limited to special applications. The most widespread process is the one using fixed beds and switching valves.
The typical adsorbent pellet size in solvent recovery is between 3 and 5 mm.
Smaller pellets have better mass transfer properties, resulting in higher solvent loading, but their disadvantage is the higher pressure drop and higher blower power.
The height of the adsorbent bed is generally between 500 and 1000 mm if the pressure is close to atmospheric. At high operating pressure the bed height may be in excess of 2000 mm. Increased bed height has the advantage of higher solvent loading, but the disadvantage of higher pressure drop.
The duration of the adsorption step may be 1- 60 and more hours, but in most cases it is between 4 and 16 hours.
The pressure swing or vacuum desorption process may be suitable for solvent recovery from gases containing over 300 grams solvent / m3 i.e. more than 0.1 mol% solvent, but then it competes with straight refrigeration or liquid adsorption.
For low solvent content, the thermal swing or steam stripping processes are clearly better. The steam stripping was a long time more widely used, but it disadvantage is that the solvents are recovered in various mixtures containing large amounts of water.
The recovery of solvents is complicated because most current solvents produce azeotropes with water, and there is generally a serious problem with the waste water as environmental regulations become more stringent.
For these reasons, in the last 15 years, the adsorbent regeneration with hot inert gas became the increasingly favoured and more modern process.