Adsorption: Gas & Liquid Removal

Adsorption is a process that uses special solids (called adsorbents) to remove substances from either gaseous or liquid mixtures. The term “adsorption” was first coined in the late 19th century, but the process itself was not widely used until the 1940s and 50s when activated carbon was first used for municipal water treatment.

These days, it is not always adsorption that matters, but desorption. This is because the simple uptake of a component generally only changes its phase (i.e., from either gas- or liquid-phase to adsorbed- or solid-phase). In order to be economical and to protect the environment, it is usually important to desorb the component in a fluid that is richer than that from which it was originally adsorbed. This accomplishes enrichment or purification in a cyclic manner.

There are a variety of standard methods to force an adsorbent to take-up then release certain components. One, that applies only to gas-phase separations, is cycling the pressure – resulting in what is called pressure swing adsorption (PSA). Another method, which works for either gases or liquids, is temperature swing adsorption (TSA). In addition to those, some chemicals are used as regenerants. Some function as a diluent or carrier, and they elute adsorbed components from an adsorbent, because they are less strongly adsorbed. Others work by displacing adsorbed components. Since they are more strongly adsorbed than the ones being displaced, there must be a subsequent step to drive those off the adsorbent.

What can be accomplished using Adsorption?

Adsorption is effective for purifications, e.g. taking a contaminant ranging from 1 ppb to 1000 ppm out of a stream of gas or liquid. In addition, adsorption is good for bulk separations, e.g. taking 1 to 50% out of a stream of gas, or maybe 1 to 10% out of a liquid. Adsorption is also used for recovery of certain constituents (solvents from air), preventing pollution, purifying materials that will react, and so on.

The above by no means represents a comprehensive list of the ways that adsorption can be used. As with any science, adsorption is constantly evolving and research is ongoing.

How has Adsorption benefited industry?

Over the years, advancements in the field have resulted in substantial cost benefits from and expanded usage of adsorption in a number of industrial settings. One prominent example is in refineries and petrochemical plants where pressure swing adsorption (PSA) has replaced cryogenic distillation as the most economical method for separating hydrogen from various compounds used in these settings.

By replacing cryogenic distillation with PSA, refineries and petrochemical plants have been able to reduce costs by anywhere from 60% – 90%.

Another example of the cost benefits of adsorption is ARI’s recent development of the Sulfur Dioxide Recovery System (SDRS). In this instance, the SDRS uses PSA techniques pioneered by ARI to enable foundry operators to recover and reuse more than 98% of the sulfur dioxide used in forming cold box molds for metalcasting. According to ARI’s estimates, a mid-size foundry running three shifts a day can expect to save up to 40% of their annual cost of SO₂ by using the SDRS.

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