Adsorbent materials are usually very porous materials with high surface areas. They include activated carbon, alumina (aluminium oxide), silica gel, synthetic resins, or hydrous aluminosilicates known as a ‘zeolites’. Different gases will have different tendencies to ‘adsorb’ (stick) to a given material. For this reason, adsorbent materials can be used for gas separation. This has many useful applications, such as the separation of nitrogen and oxygen, the removal of carbon dioxide, the removal of hydrogen sulphide (‘sweetening’), or the drying of gases.
An adsorbent material will become less and less effective over time. There are a number of reasons why this might occur. For example, it might become saturated with adsorbed molecules and then those molecules need to be ‘desorbed’ so that the material can be re-used. This process is called the ‘regeneration’ of the material. Since gases are more readily adsorbed at low temperatures and high pressures, regeneration usually involves increasing the temperature, reducing the pressure, or both.
Adsorbent materials are used, regenerated, re-used and so on, in a cyclical process until, eventually, they have degraded to such an extent that they must be discarded and replaced.
In many cases these processes must be free of oil (Oil Free)
‘Heterogeneous catalysts’ are solid catalysts used in gas processing. They are usually elemental metals, such as copper-zinc, iron, molybdenum-cobalt, nickel, palladium, platinum, rhodium or silver, or they are a metallic, inorganic compound, typically an oxide or chloride, such as iron oxide, magnesium chloride, titanium chloride or vanadium oxide. The greater the surface area of the catalyst, the more effective it is. For this reason, these catalysts are usually in the form of a fine powder. The catalyst powder is usually dispersed on a ‘supporting’ material to maximise the surface area and to prevent the catalyst particles from clumping or ‘sintering’ (binding to each other). Since adsorption of the reacting gas molecules onto the catalyst is an essential step in the process, the adsorbent materials mentioned above are ideally suited as supports for catalysts.
The activity of a catalyst bed will decline over time. One of the ways in which this can occur is by the formation of unwanted deposits, such as carbon, or by ‘poisoning’ which is when contaminants bind to the catalyst, interfering with its chemical properties. In some cases it is possible to regenerate the catalyst (also known as ‘rejuvenation’) with a regeneration gas, using the methods already described above.
Molecular sieves are solids to which only molecules of a particular size or polarity will ‘adsorb’ (stick to the surface). They are typically made from zeolites, which have channels that run through the crystalline lattice. These ‘pores’ are in the range of 3 to 10 Ångström, with 4 to 8 Ångström being most common in the gas phase. Foreign molecules can enter these pores. The pore size determines the size of the molecules that can enter.
The material can be regenerated by slowly heating it up whilst a blower is used to pass a ‘regeneration gas’ over it. This is typically a carrier gas – one that will not react with the material – containing controlled amounts of oxygen, hydrogen or other gas which will oxidise or reduce the chemicals that need to be desorbed. The regeneration gas must itself be free from contaminants and the gases that need to be desorbed. Because the carrier gas used is typically nitrogen, and because regeneration always proceeds the main process, this method is often known as ‘nitrogen start-up’.
Mechanical vacuum boosters or dry vacuum pumps (name depends on the manufacturer) add a second or third pumping stage to your vacuum operation, boosting the pumping speed and assisting to achieve a deeper ultimate vacuum level faster. In hybrid vacuum systems, Boosters can be added to your existing roughing pump including liquid ring, piston, mechanical vane, dry screw, or dry multistage roots pumps to boost performance.
Adsorbent materials can be regenerated by lowering the gas pressure. This is the case in the ‘pressure swing adsorption’ (PSA) and ‘vacuum swing adsorption’ (VSA) processes.
Roots Systems’ machines acting as dry vacuum pumps are ideally suited to these applications.
