Purification zone

At a given amount of selective pore volume circulation, p-xylene is adsorbed by an adsorbent that is highly selective for p-xylene over the other mixed xylene components. A more selective adsorbent effectively gives the adsorbent more capacity to carry p-xylene out of the adsorption zone into the purification zone, fncreasing the volumetric flow of p-xylene in the fresh feed by concentrating the p-xylene in the feed increases the required selective pore volume circulation per aromatic fresh feed by a ratio that is somewhat less than one-to-one. Because the required rise in adsorbent circulation is less than the increase in the volumetric rate of p-xylene in the feed, the overall Parex unit capacity is effectively increased for feeds that are more concentrated in p-xylene. Finally, the desorbent that enters the adsorption zone from the purification zone has a significant impact on the amount of p-xylene that can be adsorbed for a given selective pore volume... 

Since the Sorbex process is a liquid-phase fixed-bed process, the selection of particle size is an important consideration for pressure drop and process hydraulics. The exact particle size is optimized for each particular Molex process to balance the liquid phase diffusion rates and adsorbent bed frictional pressure drop. The Sorbex process consists of a finite number of interconnected adsorbent beds. These beds are allocated between the following four Sorbex zones zone 1 is identified as the adsorption zone, zone 2 is identified as the purification zone, zone 3 is identified as the desorption and zone 4 is identified as the buffer zone. The total number of beds and their allocation between the different Sorbex zones is dependent on the desired performance of the particular Molex process. Molex process performance is defined by two parameters extract normal paraffin purity and degree of normal paraffin recovery from the corresponding feedstock. Details about the zone and the bed allocations for each Molex process are covered in subsequent discussions about each process. 

The actual purification is somewhat less than that predicted by Equation (10.15) and by Figure 10.11 because mixing is not perfect in the liquid. A thin boundary layer of impurity forms in the liquid just ahead of the interface. See Figure 10.12. The formation of a boundary layer causes the concentration of the impurity in the liquid at the liquid-solid interface to be greater than for perfect mixing. Therefore, there is less purification. Zone refining is most efficient in relatively pure materials, where the boundary layer builds up. 

The second factor is a lack of normally developed shelf along 70% of the sea coast and also the small size of the self-purification zone because in the Black Sea it is limited by the upper oxygen layer which is 120 to 150 m thick. 

NMA was obtained from Eastman Kodak Co., fractionally crystallized once, and then continuously zone-refined 10-20 times. The zone refiner had three purification zones the product from the first zone was used,... 

The idea of a column was followed by Schildknecht (see, e.g., Maas and Schildknecht 1963) in many papers. The column was fed in the center and used a spiral conveyer to transport the crystals upward countercurrently to melt through the purification zone. 

The fourth (final) stage of the process is the purification zone the liquid Al alloy produced floats and flows through an overflow weir to a tank, where dissolved C and entrained AI4C3 particles can be removed by proprietary technology to recover pure Al. 

A low-biuret product can be produced by recycling the biuret-containing effluents from the purification zone back to the converter. See U.S. Patent 3,232,984. The stripping NH3 actually is fed to the reactor as excess, which inhibits the formation of biuret. Snamprogetti also claim lower maintenance costs, and many new plants based on this process were installed in the early 1980s. 

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