C8 aromatics, separation

Kloppenburg, E., Gilles, E. D. Automatic control of the simulated moving bed process for C8 aromatics separation using asymptotically exact input/output linearization,/. Proc. Control, 1999b, 9, 41-50. 

Marteau, P., Holier, G., Zanier-Szydlowski, N., Aoufi, A., Cansell, F. Advanced control of C8 aromatics separation process with real-time multiport on-line raman spectroscopy, Process and Quality, 1994, 6, 133-140. 

P Marteau, G Hotier, N Zanier-Szydlowski, A Aoufie, F CanseU. Advanced control of C8 aromatic separation process with real-time multipoint on-line Raman spectroscopy. Process Control Qual 6 133-140, 1994. 

The Ce-Cg aromatic hydrocarbons—though present in crude oil—are generally so low in concentration that it is not technically or economically feasible to separate them. However, an aromatic-rich mixture can be obtained from catalytic reforming and cracking processes, which can be further extracted to obtain the required aromatics for petrochemical use. Liquefied petroleum gases (C3-C4) from natural gas and refinery gas streams can also be catalytically converted into a liquid hydrocarbon mixture rich in C6-C8 aromatics. 

Guo, G.-Q., Chen, H., and Ying-Cai, L. (2000) Separation of p-xylene from C8 aromatics on binder-free hydrophobic adsorbent of MFI zeolite. I. Studies on static equilibrium. Micropor. Mesopor. Mater., 39, 149-161. 

The desired C8A isomer is separated out from the full blend. Depending on the isomer, this separation may be done by fractionation, by crystallization, or by selective absorption (see Chapters 5-7). Whatever the method, the separation of that species leaves a mixture depleted in the desired isomer. C8A isomerization technology converts that mixture back to an equilibrium or near-equihbrium mixture, which is then fed back into the separation unit within the separation-isomerization loop. In this way, the full range of C8 aromatics can be converted to extinction to the desired end-product. Although EB could also be separated from the C8A mixture, this is usually too costly, so EB is typically generated by the... 

In the field of aromatic separation, the trend of research is toward isolation of pure compounds for chemical purposes. Benzene, toluene, and some of the C8 aromatics have been separated and used commercially. However, the physical properties of the C9 and Cio aromatic hydrocarbons found in reformed stocks show that other aromatics could be separated from these mixtures by distillation, crystallization, or extraction processes. It is reasonably certain that if sufficient demand develops for the pure compounds, processes for their separation will become available. Present information indicates that perhaps methylethylbenzenes and trimethylbenzenes could be isolated in relatively high purity by distillation from aromatic stocks obtained by hydroforming, but no information is available as to their industrial uses. Similarly, durene (1,2,4,5-tetramethylbenzene) possibly could be isolated from its homologs by crystallization. Furthermore, large... 

The boiling point difference between pX and mX being only 0.8°C (Table 9.2), pX must be separated from the other three isomers by other means than distillation (crystallization or adsorption). The remaining raffinate (Figure 9.1), containing mainly EB, mX and oX, is then processed into an isomerization unit where pX is reformed . The obtained C8 aromatics cut, containing the xylenes in proportion close to their thermodynamic equilibrium values, is recycled back to the xylenes column (or rerun column ) for further pX separation. A loop is then created which is called the aromatics loop . 

The typical operating conditions of xylene and EB isomerization processes are shown in Table 9.3. These conditions minimize the above side reactions. Pressure, temperature and H2/HC ratio are key parameters that define the partial pressure of C8 naphthenes intermediates for EB isomerization. Naphthene cracking and disproportionation/transalkylation are responsible for the C8 aromatics net losses that affect the overall pX yield. The C8 recycled stream from the isomerization unit to the separation unit is three times higher than the fresh feed stream (since there cannot be more than 24% of pX in the C8 aromatic cut after isomerization). This means that each percent of loss in the isomerization unit will decrease the pX yield by 3%. For example, when standard mordenite-based catalysts lead to 4% of net losses, the overall pX yield is roughly 88%. 

EB-poor C8 aromatics cuts are produced which favors further pX separation Low-value toluene and C9+ aromatics (that are generally sent to the gasoline pool because of their high octane number) are transformed into high value pX Toluene hydrodealkylation units can be easily revamped into TDP or transalkylation units... 

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). 

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... 

IFP Xylene, para Mixed xylenes Buxyl separates purified p-xylene from C8 aromatic streams 8 2000... 

Ethylbenzene Separation. Ethylbenzene [100-41-4], which is primarily used in the production of styrene, is difficult to separate from mixed C8 aromatics by fractionation. A column of about 350 trays operated at a reflux feed ratio of 20 is required. No commercial adsorptive unit to accomplish this separation has yet been installed, but the operation has been performed successfully in pilot plants (see Table 5). About 99% of the ethylbenzene in the feed was recovered at a purity of 99.7%. This operation, the UOP Ebex process, requires about 40% of the energy that is required by fractional distillation. 

This case study involves the recovery of highly valued and high demand ethylbenzene (EB) and mixed-xylenes (comprising of p-xylene (PX), m-xylene (MX) and o-xylene (OX)) from a C8-aromatics mixture (C8A). As point out above, C8A is isomers mixture, so their separation (recovery) is not simple, that why there is only one commercial process of liquid-phase adsorptive separation available for EB recovery from C8A. [8] However, this process requires high investment cost and generates huge volume of waste adsorbent that may become an environmental problem. Therefore, another green process should be considered for the EB purification. The ratio of various properties of the key components (EB and PX) were tested to examine the possibly alternatives. The result showed, by vapor pressure ratio, the solvent-based extractive distillation can be employed for their purification. [7]... 

In the early 1970s, Union Oil developed and patented a chromatographic system based on the principle of a simulated moving bed (SMB) [6-8]. A schematic of a SMB unit is shown in Figure 1.4. Streams of the mobile phase (the desorbent ) and of the feed to separate are continuously injected into the column while streams of the less retained (the raffinate ) and the more retained components (the extract ) are continuously withdrawn, all at constant flow rates. The rotary valves switch periodically the positions in the columns where these streams enter or exit. The operation of SMB imits is discussed in detail in Chapter 17. Manufacturing facilities have been built and are operated for the fractionation of various petroleiun distillates, for example, the selective separation of p-xylene, o-xylene and ethylbenzene from the C7-C8 aromatic fraction of light petroleum reformates, the separation of olefins from paraffins in feed mixtures of hydrocarbons having 10 to 14... 

In the area of C8 aromatics, o-xylene can, of course, be separated by distillation, and ethylbenzene can be fractionated out, although with... 

The octafining process is used to restore to near equilibrium concentrations C8 aromatic streams deficient in one or more of the xylene isomers. In conjunction with an appropriate separation process any of the xylene isomers can be produced by recycling to extinction the other less marketable xylene isomers and ethylbenzene. 

T e adsorption of aromatics is of great practical interest for industrial applications in separations by adsorption. For instance the various C8 aromatics can be separated by selective adsorption on Y zeolites. As an example, table 5 gives the shifts of infrared CH... 

Yan, T.Y., Separation of p-xylene and ethylbenzene from C8-aromatics using medium-pore zeolites, Ind. Eng. Chem. Res., 28(5), 572-577 (1989). 

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