Characteristics of the Desorbent

In liquid phase adsorption, some particular components of the feed steam are selectively adsorbed or extracted by a solid zeoUtic adsorbent. At the same time, other components of the feed stream are rejected by the adsorbent. At equilibrium, the liquid composition within the zeolite pores differs from that of the liquid surrounding the zeolite. In the process, a second liquid component, the desorbent, is also introduced into the system. The function of the desorbent is to desorb and recover the extracted feed components from the adsorbent In order for the desorbent to perform well in the process, a suitable interactive force between the desorbent and the extracted components to the adsorbent is required. If the selectivity is too high, it requires high desorbent volume to desorb the extracted components from the adsorbent. If the selectivity is too low, the desorbent tends to compete with extracted components for capacity of adsorbent. 

A basic guideline for choosing a desorbent is to match the chemical properties of the extracted components and the desorbent, along with appropriate selection of boiling point differences to allow recovery from the feed components after the 

Adsorbent Desorbent PXnet retention volume (ml) PX/EB selectivity PX/MX selectivity PX/OX selectivity PX stage time (s) 

Desorbent features are illustrated in Table 6.8 using Cg-aromatic adsorbates with BaX and KY adsorbents. The results in Table 6.8 further emphasize the desorbent characteristic requirement mentioned above. For instance, phenyldecane is a suitable desorbent for PX separation using BaX adsorbent. However, phenyldecane is too weak to desorb PX from KY adsorbents. In contrast, diphenylmethane offers good separation of PX with KY adsorbent but not with BaX. With BaX adsorbent, PX is separated from other Cg-aromatics using 1,4-diisopropylbenzene but not with other isomers of diisopropylbenzene, such as 1,3-diisopropylbenzene. 

---