Adsorption zone

The developer is generally a solvent in which the components of the mixture are not too soluble and is usually a solvent of low molecular weight. The adsorbent is selected so that the solvent is adsorbed somewhat but not too strongly if the solvent is adsorbed to some extent, it helps to ensure that the components of the mixture to be adsorbed will not be too firmly bound. Usually an adsorbate adheres to any one adsorbent more firmly in a less polar solvent, consequently when, as frequently occurs, a single dense adsorption zone is obtained with light petroleum and develops only slowly when washed with this solvent, the development may be accelerated by passing to a more polar solvent. Numerous adsorbat are broken up by methyl alcohol, ethyl alcohol or acetone. It is not generally necessary to employ the pure alcohol the addition from 0 5 to 2 per cent, to the solvent actually used suffices in most cases. 

Fig. 4. Adsorption zone and breakthrough curve for fixed bed of granular or shaped activated carbon.

Fig. 7. Movement of adsorption zone through fixed bed adsorber. Reprinted from [32], copyright 1986 Gulf Publishing Company, with permission...

The adsorption of hydrocarbons by activated carbon is characterized by the development of adsorption isotherms, adsorption mass and energy balances, and dynamic adsorption zone flow through a fixed bed. 

Figure 7.15 shows the adsorption of ion exchangers, downflow pattern. The bed has an adsorption zone with respect to time and is saturated with solute. 

Bed behind adsorption zone saturated with adsorbate... 

Equation 17.75 is important as it illustrates, for the equilibrium case, a principle that applies also to the non-equilibrium cases more commonly encountered. The principle concerns the way in which the shape of the adsorption wave changes as it moves along the bed. If an isotherm is concave to the fluid concentration axis it is termed favourable, and points of high concentration in the adsorption wave move more rapidly than points of low concentration. Since it is physically impossible for points of high concentration to overtake points of low concentration, the effect is for the adsorption zone to become narrower as it moves along the bed. It is, therefore, termed self-sharpening. 

An isotherm which is convex to the fluid concentration axis is termed unfavourable. This leads to an adsorption zone which gradually increases in length as it moves through the bed. For the case of a linear isotherm, the zone goes through the bed unchanged. Figure 17.19 illustrates the development of the zone for these three conditions. 

In order to design moving-bed equipment, the velocity of the adsorption zone relative to the solid has to be calculated. This gives the velocity at which the solids must move in... 

In Figure 17.28, C/Ce is plotted as a function of z /za and the unsaturated fraction i of the adsorption zone may then be found. Since the zone in a moving bed is essentially the... 

Figure 17.28. Dimensionless breakthrough curve showing fractional unsaturation of the adsorption zone...

An adsorption unit is to be designed to dry air using silica gel. A moving-bed design is considered in which silica gel moves down a cylindrical column in plug flow while air flows up the column. Air enters the unit at the rate of 0.129 kg of dry air/m2s and with a humidity of 0.00267 kg water/kg dry air. It leaves essentially bone dry. There is equilibrium between air and gel at the entrance to and the exit from the adsorption zone. Experiments were carried out to find the relative resistances of the external gas film and pellet diffusion. Referred to a driving force expressed as mass ratios then ... 

The integral may be evaluated graphically from a plot of l/(yr — y ) against yr over the concentration range of the adsorption zone. 

The time ta for the adsorption zone to move its own length za is given by ... 

Hence the minimum length of bed to contain the adsorption zone is 0.185 m. In practice, a somewhat greater length would be used to allow for variations in the length ot the zone that might result from fluctuations in operating conditions. The data are summarised as follows ... 

Ce Concentration of adsorbate at the emergence of the adsorption zone kmol/m3 NL-3... 

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 faujasite zeolite in the UOP Parex process has some finite affinity for aU the aromatic species in the mixed xylene feed, indicated by the fact that selectivities between the components are typically less than five. Because the adsorbent has the tendency to adsorb all aromatic species in the feed to some extent, the fundamental variable dictating the adsorption zone operation is the ratio of zeolitic selective pore volume circulated past the feedpoint by the stepping action of the rotary valve per the volume of aromatics conveyed to the adsorption chambers. Typically this ratio is set to obtain a certain target recovery of p-xylene. 

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. 

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