How to Select Adsorption Equipment
Adsorption equipment is used to bind or adhere molecules or particles to a surface in a process stream. This equipment is used for industrial applications such as odor control, the recovery of volatile solvents such as benzene, ethanol, and trichloroethylene, and the drying of process gas streams. They also are incorporated into environmental systems for reclamation or remediation of contaminated water or air streams.
What is Adsorption?
Adsorption involves the adherence of molecules or particles to an adsorbing media. It can take place through either physical or chemical means. Physical adsorption involves a weak bonding of molecules (typically gases) to the adsorbing solid. Physical adsorption can be reversed by the application of heat or pressure, otherwise known as regeneration.
Chemical adsorption, or chemisorption, involves actual chemical bonding by reaction of the adsorbate with the adsorbing solid. The oxidation of SO2 to SO3 by activated carbon is an example of chemisorption. Except in specialized applications, this process cannot be reversed and the adsorbed material cannot be recovered.
Types of Adsorption Equipment
Adsorption systems can be referred to as packed beds since the adsorbing material is packed together to filter fluid streams. The two different design implementations are fixed bed adsorbers and fluidized-bed adsorbers.
- Fixed-bed adsorbers utilize a stationary adsorbent to filter streams. These systems are used in applications ranging from large industrial operations to remove harmful VOCs (volatile organic carbons) to small consumer uses as filters and gas masks. Important properties including pressure drop and life expectancy of the adsorbent are very predictable in these types of systems.
- Fluidized-bed adsorbers utilize a more complex moving (fluidized) adsorbent to filter streams. This is achieved by inducing a stream velocity high enough to suspend the adsorbent particles. Fluidization allows for continuous regeneration and a uniform temperature gradient, giving these systems the advantage of running continuously. However, energy costs to maintain suspension velocity and deal with large pressure drops are considerably more than in fixed-bed systems. Also, the behavior of fluidized-beds is much harder to predict since much is still unknown. Fluidized-beds require a larger chamber and are usually only implemented in high-volume industrial applications.
Common media used in adsorption equipment include activated carbon, activated alumina, silica gel, and molecular sieves.
- Activated alumina is an adsorbent made of aluminum oxide (Al2O3). It is used as a desiccant for drying gases and air and as a fluoride filter for drinking water. It has specific use as a silica gel replacement in certain environments due to its thermal shock resistance and physical constancy when immersed in water. It also commonly used as an alternative to activated carbon for VOC removal.
- Activated carbon is roasted organic material (coconut shell, bone, wood) that forms porous granules. It is a versatile and inexpensive adsorbent that comes in many sizes and has a range of applications from gas, water, and metal purification to air filtration. It is the most commonly used industrial adsorbent.
- Molecular sieves or zeolites are naturally occurring adsorbents with uniform pore size that can be tuned to be highly selective. They are used as dehumidifiers and air purifiers due to their high retention and adsorption capacities even at high temperatures. Zeolites are often combined with activated carbon for combined effectiveness.
- Silica gel or silicon dioxide is a common desiccant used in food preservation, humidity control, and various medical devices. It has a higher water adsorption capacity than clay silicates (40% of its weight in moisture), it is very inert, and it can be regenerated through heating.
When selecting adsorption equipment, the primary considerations are selectivity, capacity, and regeneration.
- Selectivity is the amount of specificity an adsorbent has in the materials that it can capture. A very unselective adsorbent will readily capture many different particles, ideal for streams with many pollutants. Highly selective adsorbents will only capture certain types of particles, making them suited for filtering out specific contaminants from a product stream.
- Capacity is the amount of contaminant adsorption equipment can capture before the adsorbent is saturated and requires renewal. This is a crucial factor in fixed-bed systems where the material must eventually be replaced or regenerated.
- Regeneration is the ability of the adsorbent in the system to be reused. This is important for systems with large-volume or high-cost materials where iterative replacements would be expensive.
Another factor that affects system design is the equipment’s pressure drop gradient or required operating power.
In addition to manufacturing adsorption equipment, many suppliers offer a number of related services to help maintain purchased equipment. These include media reactivation and dual vessel system assembly. In media reactivation, the supplier collects saturated adsorbent and regenerates it for future use. This can help cut down on both cost and waste generated by adsorption equipment. Suppliers that engage in dual vessel system assembly make sure that the adsorption equipment is both the proper fit for the user, and help to incorporate the equipment into an existing process line or remediation system.
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