Paraxylene, adsorption

Cain, J.J. (2001) Adsorption process for paraxylene purifacation using Cs SSZ-25 adsorbent with benzene desorbent. U.S. Patent 5,281,405. 

Figure 15.19. The Toray simulated continuous adsorption process, (a) Showing the main valving for a seven-chamber adsorption system [Otani et al., U.S. Pat. 3,761,533, (25 Sep. 1973)]. (b) Flowsketch for recovery of paraxylene by continuous adsorption [Otard et al., Chem. Economy Eng. Rev. 3(6), 56-59 (1971)].

It should, however, be emphasized that new catalysts with zeolites other than MOR or MFI which give higher paraxylene yields were recently developed for the isomerization of the C8 aromatic cut. Moreover, adsorption on FAU zeolites is now the main technique used for paraxylene separation (Chapter 10). 

Crystallization and adsorption are both widely used to perform the separation distillation is not used (except for orthoxylene separation) because of too small differences between the boiling points (Table 10.1). Despite the still high importance of crystallization, adsorption becomes the most widely used technique because of its high efficiency. The adsorbents which are used for selective adsorption of paraxylene are X or Y zeolites exchanged with adequate cations. Liquid phase Simulated Counter Current adsorption, which is the most efficient process, is generally used (1). In addition to the complexity of this process, the choice of an adsorbent selective for paraxylene is the critical point. 

More recently, it was shown (10, 11) for a series of FAU samples that the zero loading adsorption enthalpies were very similar for paraxylene and metaxylene (Table 10.3). This indicates that at low loading, very few differences can be expected for the adsorption of paraxylene and metaxylene. 

Table 10.3 Adsorption heats of paraxylene and metaxylene on X and Y zeolites (10, II)...

Simulated counter current adsorption for paraxylene separation... 

X and Y zeolites exchanged with potassium and barium cations and associated with an adequate solvent are efficient for paraxylene separation by adsorption. The design of the adsorbent as well as of the process are both critical to reach economical production of paraxylene. The complete system, adsorbent plus process with the chosen configuration - stand alone or hybrid (adsorption + crystallization) - has to be optimized. 

Description Aromatics are produced from naphtha in the Aromizing section (1), and separated by conventional distillation. The xylene fraction is sent to the Eluxyl unit (2), which produces 99.9% paraxylene via simulated countercurrent adsorption. The PX-depleted raffinate is isomerized back to equilibrium in the isomerization section (3) with either EB dealkylation-type (XyMax) processes or EB isomerization-type (Oparis) catalysts. High-purity benzene and toluene are separated from non-aromatic compounds with extractive distillation (Morphylane ) processes (4). Toluene and C9 to Cn aromatics are converted to more valued benzene and mixed xylenes in the TransPlus process (5), leading to incremental paraxylene production. 

The xylene splitter overhead is sent directly to a Parex unit (11), where 99.9 wt% pure paraxylene is recovered by adsorptive separation... 

Design variations are used to recover paraxylene efficiently from feedstocks (-22% PX) in a multi-stage system, competitive with adsorption-based systems. 

The sensitivity of lattice modes to structural changes is illustrated by the recent study of Mueller and Connor [25] on the effects of cyclohexane adsorption on the structure of MFI zeolites. The adsorption of molecules such as paraxylene and benzene into MFI zeolites causes a structural transition from monoclinic to orthorhombic symmetry, which has been characterized by X-ray powder diffraction and 29 si NMR spectroscopy [26]. Cyclohexane has a slightly larger kinetic diameter than benzene or paraxylene (0.60 nm compared with 0.585nm), but does not cause the same structural transition. Cyclohexane adsorption does however affect the zeolite lattice mode vibrational frequencies. Figure 7 shows spectra taken from reference 25 before and after (upper spectrum) adsorption of cyclohexane in a low aluminium MFI zeolite. 

Zn species have also a significant effet on the distribution of aromatics. Whether it be from propene or from propane, the production of the less bulky compounds (benzene, toluene, paraxylene) is more favored )n ZnHZSMS than on HZSMS. This can be related to diffusion limitations created by the Zn species and shown by the adsorption of nitrogen and of m-xylene (Table 1). 

The xylene splitter overhead is sent directly to a Parex unit (11), where 99.9 wt% pure paraxylene is recovered by adsorptive separation at very high recovery. The raffinate from the Parex unit is almost entirely depleted of paraxylene and is sent to an Isomar unit (12). In the Isomar unit, additional paraxylene is produced by re-establishing an equilibrium distribution of xylene isomers. The effluent from the Isomar unit is sent to a deheptanizer column (13). The bottoms from the deheptanizer are recycled back to the xylene splitter column. 

Feed concentration of PX is used efficiently Technology is flexible to process a range of feed concentrations (75 wt%-95 wt% PX) using a single refrigeration system Design variations are used to recover paraxylene efficiently from feedstocks (-22% PX) in a multi-stage system, competitive with adsorption-based systems. 

The diffusion of gaseous benzene and paraxylene during their adsorption in a fixed bed of HZSM-5 zeolite crystallite has been studied by Xe NMR of adsorbed xenon used as a probe. The equations of diffusion in the macropores and micropores have been analytically solved, giving the hydrocarbon concentration profiles against time in both types of pores and allowing the simulation of the Xe NMR spectra. The comparison of simulated and experimental spectra leads to the value of the intracrystallite diffusion coefficients which are in good agreement with the literature. 

Here, we present the application of these techniques to the diffusion of hydrocarbons (benzene, n-hexane, paraxylene), pure and mixed, in a fixed bed of HZSM-5 zeolite during their adsorption at room temperature. 

We also investigated the paraxylene/HZSM-S syston with the same technique. Due to the lower intracrystalline diffusion rate the "non uniform" model was not able to simulate the NMR spectra correctly. A simpler "core shrinking" model was used [9]. The crystallites are divided into two zones a core free of diffusing molecules and a shell with a uniform hydrocarbon concentration. During the adsorption there is a diffiision front in the crystallites, the shell region increases at the expense of the core. Then, the NMR spectra is simply the sum of two lines whose intensity inversely varies widi time (Fig. 3b). 

The paraxylene process (Figure 11-6) takes mixed xylenes from reformers or steam crackers to produce high-purity paraxylene. Feedstock is pumped to a feed rerun column that removes C9 (and heavier) materials out the bottom and mixed xylenes out the top. The overhead product is sent to a set of adsorption columns where paraxylene is removed and purified to 99.9%. A series of distillation columns is used to separate or recycle the rest of the products. 

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