(164b) Adsorption: The Forgotten Unit Operation

Benzene is a much more common chemical than generally supposed. Thus, the opportunity to contaminate a second chemical inventory with trace quantities of benzene is not insubstantial. Such contamination occurs in tank farms and transfer lines. It can also occur in poorly flushed batch and semi-batch reactors. Also, many chemical processes use benzene-containing ligands to activate a catalytic metal. Unfortunately, such ligands decompose into a variety of by-products, one of which is benzene. Such decomposition contaminates the process as well as the desired product with benzene

Removing benzene by distillation generally produces a large benzene-rich by-product stream and a product stream containing low parts per million benzene. Thus, removing parts per million benzene from a product stream is expensive.

Removing benzene via adsorption produces a product stream at or below one part per million benzene without producing a large benzene-rich by-product stream. Adsorbed benzene is recovered by heating the adsorbent, volatizing the benzene-rich hydrocarbon, then condensing the resulting vapor. The benzene-rich hydrocarbon can be sold for fuel or consumed as fuel at the site.

Unfortunately, due to its molecular structure, benzene does not adsorb onto activated alumina or activated carbon. Benzene must, therefore, be adsorbed onto a size exclusion adsorbent, such as molecular sieve.

This presentation discusses using 10X and 13X molecular sieve to adsorb benzene from 1-hexene. We present the benzene adsorption isotherms for each molecular sieve and discuss their implications.