Characteristics of Zeolitic Adsorptive Process

The two adsorbent chambers contain the zeolitic adsorbent, the liquid xylenes and p-diethylbenzene desorbent. Proper loading of the adsorbent into the large diameter vessels in industrial production plants is of critical importance to maximize adsorbent mass in the fixed vessel volume and not generate low and high density areas within the adsorbent bed. Density inconsistencies could adversely affect liquid flow distribution and thereby have a detrimental effect on the performance of the process. Adsorbent loading methods are a matter of proprietary know how of the technology licensors. However, Seko has published a paper on the practical matters involved in an actual problem case [20]. 

The UOP rotary valve has been used in hundreds of Sorbex units across a variety of applications. The purpose of the rotary valve is to simply move the inlet and outlet ports of the net streams (feed, desorbent, raffinate, extract) around the 24 beds in stepwise fashion, creating a semi-continuous countercurrent flow of adsorbent relative to the entry and exit points of the net streams to and from the adsorbent chambers [25]. The rotary valve consists of a rotor plate pressed against a stator plate inside a pressure vessel that indexes the net desorbent, feed, extract and raffinate streams around the adsorbent chambers [26]. An alternative method of moving the inlet and outlet streams around the adsorbent chambers is used in other technologies where multiple automatic on-off valves at each distributor grid inlet are employed [5]. 

Zeolite/Desorbent Combination The desorbent used in the UOP Parex unit is p-diethylbenzene (PDEB) [28]. It has been found to have approximately the same affinity for the faujasite zeoHte as does p-xylene, balancing the amount of desorbent required for p-xylene desorption while not excluding the p-xylene from adsorbing in the adsorption zone. 

The adsorbent typically consists of the selectively adsorbing zeoHte and a binder. The binding material is required to make parHcles of the zeoHtes of a large enough size to allow for practical use in a commercial appHcation. The particle formation method and resulting characteristics of the binder/zeoHte combinaHon is of criHcal importance, especially as it relates to mass transfer through the particle and the characteristics of the zeoHte [30]. The practical aspects of forming and 

The simulated moving bed operational mode involves four distinct functional zones, the adsorption, purification, desorption and buffer zones. These zones are described in detail in other parts of this book. We now examine the function of each zone as it applies to p-xylene adsorption and which can be extrapolated to the other aromatics separations. 

---