Sorbents, Desiccants, and Adsorption

What is adsorption?

By definition adsorption refers to the concentration of a fluid component (gas or liquid phase) onto the surface of a solid. Activated aluminas refer to that class of aluminum oxides that are very hydrophilic in nature and possess high degrees of bulk porosity. As a result of chemical structural properties activated aluminas exhibit high adsorption capacity for water and well as other polar substances. Once saturated with the adsorbed material the alumina are easily regenerated by thermal desorption of the adsorbed species. Their traditional use lay as bulk desiccation agents in the dehydration of gases and organic solvents. Adsorbate binding energies on alumina are greater than that of silica, making activated aluminas more attractive when low to moderate water levels are specified in process effluent. Activated aluminas are low in cost, costing less than ¼ of the price of molecular sieves, and are the best adsorbent in terms of resistance of acid, performing far better than silica gels, alumino silicates, or molecular sieves. Spherical activated alumina has high surface area, adsorptive capacity and abrasion resistance. It is regenerated for reuse by purging or evacuating at elevated temperatures.

What is the adsorption capacity?

Adsorption capacity is defined as the accumulation of the solute molecules at the surface of a solid. This capacity is directly proportional to the area of the surface exposed and is dependent on the solute partial pressure and the temperature. An increase in temperature reduces the adsorption capacity of activated alumina as the adsorption of water on alumina is exothermic.

Adsorption capacity depends on the surface site reactivity and is measured by the volume of adsorbed water per unit of surface area. Water in the air actually sticks to the alumina itself in between the tiny passages as the air passes through them. The water molecules become trapped so that the air is dried out as it passes through the filter. This process is reversible, and when the alumina desiccant is heated to between 350-600 F (177-316 C) it releases all of the water stored inside it. The process of heating the activated alumina is called regenerating the desiccant.

When a gas is compressed the partial pressure of the water present increases. At a constant temperature the adsorptive capacity for water increases with increasing water partial pressure (and relative humidity).

Desiccants are manufactured to meet standards such as “Standard Methods of Testing Sorbent Performance of Absorbents” published by ASTM International, as well as other US government standards.

Water removal from ambient air is the important first step in the production of nitrogen enriched gas from nitrogen containing gas mixtures such as air. This need for water removal is essential for all gas separating processes including cryogenics, membrane permeation and adsorption techniques.

What is activated alumina?

Activated alumina is manufactured from aluminum hydroxide by dehydroxylating it in a way that produces a highly porous material. This material yields a surface area of between 120-250 square meters per gram of material. Activated aluminas do not soften or disintegrate when immersed in liquids. A high internal surface area through the presence of pores or micropores is necessary to create adsorption sites.

What are desiccants?

Desiccants and sorbents are specialized moisture absorbing minerals or chemicals. Alumina, silica gel, calcium sulfate, barium oxide, lithium chloride, perchlorates (such as lithium, barium or magnesium perchlorate) and molecular sieves such as zeolite are commonly used as desiccants. These materials are used as desiccants because they possess both high water adsorption capacities as well as favorable water adsorption isotherm shapes. (discussed below) The water adsorption capacity of these materials varies from 20 to 50 wt %.

Desiccants attract moisture from gases and liquids. The desiccant material becomes saturated as moisture is adsorbed onto its surface. The best adsorbents will therefore have the greatest surface area available for adsorption. Superior adsorbents are designed to maximize the available surface area for moisture removal. DrysphereTM by Dynamic Adsorbents, Inc. is designed to increase surface area by containing multiple channels and pores which dramatically increase the available surface area for physical and chemical interaction.

How are desiccants and sorbents different?

Desiccants and sorbents are related products, but differ. Sorbents recover liquids through absorption, adsorption and chemically react with or otherwise remove water. Absorbents retain liquid through the molecular structure, causing more than 50% swelling. Adsorbents are coated by a liquid on the surface (including pores) without swelling more than 50%. Adsorption is a phenomenon which involves the fixation of a material present in a fluid on a solid. A selective mass transfer occurs between these two phases. Since adsorption is a surface phenomenon the best adsorbents have large surface areas per unit mass and high attractive forces for the compounds they are adsorbing.

Desiccants and sorbents differ in how performance is measured. One unit of desiccant will adsorb 3 grams of water vapor at 20% relative humidity, or 6 grams of water vapor at 40% relative humidity at 77 C. To achieve very dry air, a dehumidifier desiccant can be rotated through process air and heated to remove moisture. Desiccants can be reactivated using heat as the method to remove captured moisture, thus allowing the desiccant to once again become fully activated. Superior desiccant materials such as alumina break down slowly and therefore can be used and regenerated multiple times.

How does one determine how much moisture can be removed?

The water content of a gas is defined as the weight or volume of water vapor per unit weight or volume of gas. This is expressed as parts per million weight (ppmw) or parts per million volume (ppmv). For any given temperature water content can also be expressed by its relative humidity, which is defined as the ratio of its partial pressure to its saturation pressure.

The water adsorption isotherm relates the equilibrium amount of water adsorbed onto a solid and the water content in a fluid at any constant temperature and pressure. The amount of water trapped on a solid at a given relative humidity and temperature depends on its chemical affinity for the solid and the number of available sites for interaction. The capacity of a desiccant for water is expressed as the mass of water adsorbed per mass of desiccant.

Water adsorption is a combined result of three phenomena:

  1. Chemisorption
  2. Physisorption – hydrogen bonding with some surface forces (Van der Waals’ forces) due to the formation of multiple layers by hydrogen bonding in the pores of the desiccant.
  3. Capillary Condensation – growth of multilayers of water (Kelvin’s law) where localized condensation takes places at temperatures above that of the bulk fluids dew point.

As the relative humidity begins to increase the first step in the process of adsorbing a liquid or gas onto a solid is chemisorption with a monomolecular layer forming on the surface of the adsorbent. As the relative humidity increases, the less active sites adsorb water more gradually, with the monolayer completely bound when the relative humidity reaches approximately 10%. At this point multilayers of water vapor form, reflective of the fact that physisorption is taking place. When the water vapor pressure (relative humidity) reaches 40% the pores begin to fill in by capillary condensation. When the relative humidity of the inlet gas attains 100% and the bed is in equilibrium there is no further adsorption through these three mechanisms of adsorption and complete saturation occurs.

Alumina and other materials used as desiccants have water adsorption isotherms that are concave to the pressure axis, particularly at low pressure, which helps in forming short, sharp mass transfer zones. (See below) These materials are polar, and as such they selectively adsorb polar molecules like water and alcohol, even though they adsorb all liquid and gases to some extent. For example, water as a polar compound is more strongly adsorbed than hydrocarbons.

How much desiccant is required for any given job?

The amount of desiccant required depends on several factors including the amount of water present, the capacity of the selected desiccant to take up water, and the presentation of the desiccant relative to the components containing water. Stream conditions such as pressure, concentration and molecular weight of the molecules, temperature and site competing molecules affect the efficiency of adsorption.


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