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Zeolite membranes for xylenes separation

Zeolite membrane separation in p-xylene production exhibits low energy demand, low capital cost and compact unit size

Saudi Aramco
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Article Summary
The world’s major source of xylenes is reformate, which is produced via catalytic reforming in refineries. Reformate is split into two steps to separate benzene and toluene, while the remainder is further fractionated to separate C9 and heavier components, and the C8 components are sent to the 
p-xylene extraction facility. The feed stream components are mainly ethylbenzene, p-xylene, m-xylene, and o-xylene. Their boiling and melting temperatures are shown in Table 1.

As expected, the boiling temperatures of these isomers are very close. Separation of ethylbenzene or o-xylene from the rest can be achieved through distillation. This requires a superfractionating column involving large-scale energy consumption and a lot of distillation trays, which is clearly not an efficient method. However, it is almost impossible to separate p-xylene from m-xylene by means of distillation as the difference in boiling temperatures is less than 1°C. The similarity in boiling temperatures is due to the small variations in intermolecular forces, comparing one component with another; lower molecular forces decrease boiling point temperatures. On the other hand, the high symmetry of p-xylene allows for more molecular rotation of freedom, thus reducing significantly the driving force needed for melting (that is, it has a small entropy of fusion).

There are two dominant technologies for extracting p-xylene from the mixture sent to the p-xylene extraction facility: crystallisation and adsorption. The former exploits the difference in each component’s melting point, while the later uses molecular sieves in a simulated moving bed process.  Since crystallisation is the older technology, it will be discussed only briefly.

This process’s main principle is utilising the difference in melting points of the components in the feed stream to separate p-xylene. Typically, the mixture is refrigerated to solidify p-xylene so it can easily be separated. The slurry containing p-xylene crystals is fed to filtration and purification units where the slurry is washed to purify the crystals. The remaining, p-xylene deprived stream is fed to an isomerisation unit and recycled to increase p-xylene production. Handling of crystals in mechanical and rotating equipment is essential, which makes the equipment prone to failure. Thus, multiple trains are usually installed.2

In order to identify the advantages and disadvantages of a new technology (for instance membranes), it is essential to fully understand currently deployed technology. Therefore, emphasis will be placed on molecular sieve adsorption using a simulated moving bed.

In molecular sieve adsorption, the most important parameter is the size and the shape of the molecule. The size is characterised by the kinetic diameter, which is the diameter of the smallest circle generated by the rotating molecule. When two components with different kinetic diameters are to be separated, the adsorbent is chosen so as to have an intermediate micropore diameter. The result is a filtering effect that retains the larger molecules and allows penetration of the smaller ones.

Selectivity through geometry can be very high provided that it is possible to find the right size of micropore.

Adsorbent (molecular sieves)
The adsorbent has a zeolite structure with cavities of about 0.8 nm to preferentially trap p-xylene. Zeolite’s very fine particles (about 
1 mm in diameter) are referred to as micrograins. They are very efficient for adsorption, but are too small to be used in vessels, because their small size would generate a very high pressure drop. Zeolite micrograin powder is thus mixed with an inert or non-selective binder (macrograins) and reshaped into larger extrudates or beads. Their diameters range between 0.5 and 1 mm and have high internal porosity. The porosity of the binder enables the molecules to penetrate the grain and to have access to the zeolite micrograins, which corresponds to the tunnels formed by the cavities.

Counter-current adsorption (simulated moving bed)
The principle of counter-current adsorption can be summarised in three steps:
1. Adsorption: the component(s) with greater affinity for the solid adsorb more readily than the others, but selectivity is not 100%.
2. Elution: the majority of the non-adsorbed molecules are washed away by the desorbent, which is mixed with the feed in the liquid phase. Simultaneously, a small amount of the molecules of the adsorbed component are also washed away.
3. Desorption: this is the regeneration step, in which the surface of the adsorbent is able to again adsorb molecules of the component to be separated. This desorption is also achieved by the desorbent that washes away the free molecules of the adsorbed component. Then the equilibrium between the adsorbed and free molecules comes into play, allowing the adsorbed molecules to desorb progressively and enrich the liquid phase, leaving the solid with a clean surface.

The process described here follows the basic principles of chromatography. When the feed is introduced, the strongly adsorbed molecules are attracted more strongly to the solid adsorbent’s surface. The continuously injected desorbent attracts the adsorbed molecules, redistributing them continuously until no more adsorbed material remains at the feed location. Because of the stronger attraction of the adsorbent for separated molecules, two distinct concentration profiles appear and move along the bed. At any given section along the bed, an equilibrium is established between the desorbent, highly adsorbed and less adsorbed molecules in the liquid (moving) phase and the same components adsorbed on the surface of the adsorbent (stationary phase). From the same outlet, the less adsorbed molecules will exit prior to the highly adsorbed molecules, hence achieving separation. The operation is analogous to that of batch distillation by steam 
stripping; in liquid phase chromatography, the chromatograph outlet composition changes with time similarly to the overhead composition in batch distillation. However, in a counter current chromatography, the adsorbent is not stationary, but moving in the opposite direction of the desorbent, leading to different outlets for the separated components. The process is illustrated in Figure 1.

Although this process may work without problems on a laboratory scale, on an industrial scale several issues arise. Movement or circulation of the molecular sieves causes attrition, which lowers their performance. In addition, the movement will result in higher bed porosity, which is detrimental to molecular sieve applications.

A simulated moving bed is a way to overcome these limitations. The adsorbent beds are fixed while the counter-current flow is simulated by changing the locations of the inlet and outlet streams. As the liquid flow is in one direction, and the inlets and outlets are moving in the same direction, the solid seems to move in the opposite direction, thus achieving counter-current chromatography (see Figure 1). In a typical industrial application, 24 adsorbent beds are required to achieve the desired separation with high yield and purity. For each bed, there are 4-5 inlet and outlet streams with a dedicated valve on each stream. With the addition of fine-tuning valves, the number exceeds 120 valves (Axens), or significantly fewer valves but with a large rotary valve (UOP).

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