Desorbents zeolite/desorbent combination

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

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. 

Zeolite/Desorbent Combination A light desorbent, that is a desorbent that boils lighter than the mixed xylene feed, is used in the MX Sorbex process. This means that the energy demand of the distillation columns per unit feed for the MX Sorbex... 

When selecting candidate zeolitic materials for effecting an adsorptive separation the researcher faces an enormous number of possible combinations of materials and desorbents. In the absence of any established algorithm for selecting materials the researcher is forced to rely on analogy and published experience to make choices for experimentation. 

Contrary to catalytic applications, zeolite adsorbents are mostly applied in a fixed-bed operation. A number of columns packed with zeolite adsorbent(s) are interconnected with an automatic valve system to facilitate a continuous flow of the industrial stream being processed. Each bed, however, goes through a stepwise cyclic operation, and during each cycle the adsorbed molecules in the zeolite bed are desorbed by raising the bed temperature, lowering the bed pressure, displacing the adsorbate with another adsorbate, or combination. 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

The oxygen-enriched product gas (-90% O2) is produced at the feed air pressure (Pa) by this process. Table 7 describes the performance of this process using LiX zeolite and specific pressure levels [Pa=1.43, Pi=0.61, Pd=0.34 atmospheres] of operation. The combined waste gases (steps c, d, and e) contain 88.7% N2 along with the desorbed CO2, H2O and some O2. A two column embodiment of the process is described in Figure (8b). A surge tank is needed to smooth out product flow and composition. The operation of the vacuum pump is intermittent for this case. 

This technique consists of saturating a zeolite with adsorbate and then heating the zeolite in vacuuo. The molecular composition of the desorbed products as a function of temperamre is then determined using mass and infra-red spectroscopic methods. This when combined with Thermogravimetric Analysis... 

Adsorptive distillation is an integrated operation in, which adsorption is combined with distillation to separate the close boiling components or constant boiling liquid mixtures. It is a three-phase mass transfer operation in which the adsorbent, usually in the form of (fine) fluidized powder, is introduced into the eolumn along with an inert carrier gas. The adsorbent selectively adsorbs one of the eomponents and flows into the desorption column in, which the adsorbed eomponent is desorbed. Thus adsorptive distillation is successful in separation and in avoiding the formation of azeotropes. Most commonly used adsorbents in the industry are silica gel, activated carbon, zeolite and alumina. Though adsorptive distillation has been reported long back, its industrial and commercial applications are very limited. However, potential application fields for adsorptive distillation inelude separation of toluene/methyl eyclohexane, naphtha reformates, p-xylene/ m-xylene, etc. 

The thin films can equilibrate and desorb vapors within a few seconds to minutes, often at room temperature, while the bulk materials need substantial heating (ca. 200-300°C) to remove the absorbed vapors (the last ppm desorption steps of water from polar zeolite films at r.t. can take 30-50 min). Thus the sensor response occurs at a satisfactory time scale. The kinetics of vapor desorption from the zeolite layers are strongly dependent on the adsorbate/zeolite combination, thus providing an additional capability for molecular recognition. 

---