Paraxylene separation

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). 

In this paper, after a general presentation of separation processes, we propose some key features to seize the world of paraxylene separation, the... 

For paraxylene separation, both kinds of selectivity can be observed. In the MFI structure, the aperture of the pores is sufficiently close to the dimensions of the molecules to make shape selectivity appear. However, the kinetic diameters of paraxylene and of ethylbenzene are identical, so that the selectivity is not effective for these two components. Moreover, the capacity of MFI zeolites is weak compared to other structures. More open structures which provide the opportunity to use equilibrium selectivity are preferred. The problem is that the selectivity is mainly due to interactions between the zeolite and the aromatic ring which are identical for all the xylenes. It will be shown in the following sections that this problem can be solved by using chosen FAU zeolites. 

Only displacement desorption is used for paraxylene separation processes, which is carried out in continuous countercurrent systems. 

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 The para-depleted liquid C8 aromatics raffinate stream from the paraxylene separation unit, along with hydrogen-rich recycle gas are pumped through feed/effluent exchangers and the charge heater (1) and into the reactor (2). Vapor then flows down through... 

Mendez, C., Myers, J., Roberts, S., Logdson, J., Vaia, A., Grossmann, I., 2005. MINLP model for synthesis of paraxylene separation processes based on crystallization technology. In Puigjaner, L. (ed.), European Symposium on Gomputer Aided Process Engineering, Vol. 15. Elsevier, Amsterdam, the Netherlands. 

Ethylbenzene is separated from mixed xylenes by fractionation using 360 trays and a high reflux ratio. Ethylbenzene is separated from the closest isomer paraxylene whose normal boiling point is only 3.90°F higher. The average relative volatility between ethylbenzene and paraxylene in the fractionation is about 1.06. The fractionator feed is entirely Cg aromatics which are prepared by the extraction of powerformate by the sulfolane process and by fractionation of the aromatic extract. 

Paraxylene is recovered from Cg aromatics by crystallization. Fortunately, the solidification point of the para isomer is unusually high, -1-55.9°F, considerably above the meta and orthoxylenes which freeze at -54.2 and -13.3°F, respectively. The separation of para from meta by distillation is impractical because the spread in their normal boiling points is only 1.4°F. 

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. 

The selectivity of X and Y zeolites for paraxylene is a complex phenomenon. The molecules to be separated are very similar, as is shown in Table 10.1. Consequently, the selectivities are low and very sensitive to various parameters. The first one is the nature of the cation, which is able to reverse the selectivity. For example, NaY is selective for metaxylene and KY selective for paraxylene. This is clearly illustrated by the values given in patent literature (6) (some of the figures are reported in Table 10.2). They clearly demonstrate that, depending on the cation, the molecule which is preferentially adsorbed is either paraxylene, metaxylene or ethylbenzene. 

Paraxylene selectivity is a complex phenomenon which can only be observed at high loading and for bulky and weakly charged cations. This phenomenon is related to entropy effects which allow paraxylene to be more efficiently packed in the zeolite micropores. This leads to paraxylene/metaxylene selectivity over three and paraxylene/ethylbenzene over two, which is enough for a separation process. It is now established that steric effects are very important to make the adsorbent selective for paraxylene. These effects are driven by the size, the number and the position of the compensation cations. The charge of the cation also plays an important role by controlling these effects. 

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... 

In the PX-Plus technology, fresh toluene is combined with recycle gas, heated and fed to a fixed-bed reactor. The para-selective catalyst produces xylene product with 90% paraxylene in the xylenes. Reactor effluent flows to a separator, where the recycle gas is recovered, and the liquid product is sent to a stripper. 

Optimize the design of a distillation column to separate 225 metric tons per hour of an equimolar mixture of benzene, toluene, ethylbenzene, paraxylene, and orthoxylene with minimum total annualized cost. The feed is a saturated liquid at 330 kPa. The recovery of toluene in the distillate should be greater than 99%, and the recovery of ethylbenzene in the bottoms should be greater than 99%. 

Fig. 9.30. Separation and permeation behaviour of a mixture of 0.31 kPa paraxylene ( ) and 0.26 kPa o-xylene (A) as a function of temperature. Single gas permeation data are also given 0.62 kPa px (A) and 0.52 kPa ox (V). The total pressure was 100 kPa, the balance being He. After Vroon et al.

The separation behaviour of a p/o xylene mixture is given in Fig. 9.30. The permeation of the paraxylene is much larger than that of the o-xylene at higher temperature, the last one has a permeation which is at the detection limit of the equipment used. The molecule has a diameter which is larger than that of the pore diameter of the MFI and so we have here an example of separation by size exclusion. The flux of p-xylene shows a weak maximum as a /(T) and consequently the separation factor does the same with a peak value of a = 100 at =400 K under the given conditions. The separation factors and the permselec-tivities are equal as expected for the size exclusion mechanism. 

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. 

Discriminant plots were obtained for the adaptive wavelet coefficients which produced the results in Table 2. Although the classifier used in the AWA was BLDA, it was decided to supply the coefficients available upon termination of the AWA to Fisher s linear discriminant analysis, so we could visualize the spatial separation between the classes. The discriminant plots are produced using the testing data only. There is a good deal of separation for the seagrass data (Fig. 5), while for the paraxylene data (Fig. 6) there is some overlap between the objects of class I and 3. Quite clearly, the butanol data (Fig. 7) post a challenge in discriminating between the two classes. 

The main applications in the field of hydrocarbon separation concern the recovery of -paraffins contained in the light cuts from C4 to Cg, or in heavier cuts such as kerosene from Cio to C15 approximately, and that of some isomers contained in the aromatic cuts (e.g., the recovery of paraxylene from Cg aromatic cuts) (Table 2). 

A chromatographic process for separation of Cg aromatic isomers has been recently developed by Asahi. The process operates in the liquid phase and is put forward as an alternative to the UOP Sorbex process (see Section 12.5). The adsorbent is an A or T zeolite but details of the ionic form have not been released. A schematic of the process, which uses three separation columns, is shown in Figure 10.6. The main column separates the mixed-feed stream into four cuts containing (a) ortho + meta + ethylbenzene (trace), (b) ortho + meta -f ethylbenzene + paraxylene (trace), (c) paraxylene + ethylenebenzene, and (d) pure paraxylene. Cut (d) is passed directly to product while cut (b) is returned to the isomerization unit. Cuts (a) and (c) are passed to further columns in order to produce essentially pure ethylbenzene and paraxylene as products. 

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