Aromatics separation

Example 5.4 The data for an aromatics separation are shown in Table 5.5. Assuming the ratio of actual to minimum reflux to be 1.1, determine the best sequence using Eq. (5.8). 

For the refiner, the reduction in benzene concentration to 3% is not a major problem it is achieved by adjusting the initial point of the feed to the catalytic reformers and thereby limiting the amount of benzene precursors such as cyclohexane and Cg paraffins. Further than 3% benzene, the constraints become very severe and can even imply using specific processes alkylation of benzene to substituted aromatics, separation, etc. 

The question of whether adsorption should be done ia the gas or Hquid phase is an interesting one. Often the choice is clear. Eor example, ia the separation of nitrogen from oxygen, Hquid-phase separation is not practical because of low temperature requirements. In C q—olefin separation, a gas-phase operation is not feasible because of reactivity of feed components at high temperatures. Also, ia the case of substituted aromatics separation, such as xylene from other Cg aromatics, the inherent selectivities of iadividual components are so close to one another that a simulated moving-bed operation ia hquid phase is the only practical choice. 

Xylenes (dimethylbenzenes) are an aromatic mixture composed of three isomers (0-, m-, and p-xylene). They are normally obtained from catalytic reforming and cracking units with other Ce, C7, and Cg aromatics. Separating the aromatic mixture from the reformate is done by extraction-distillation and isomerization processes (Chapter 2). 

The simulated moving bed operational mode involves four distinct functional zones, the adsorption, purification, desorption and buffer zones. These zones are described in detail in other parts of this book. We now examine the function of each zone as it applies to p-xylene adsorption and which can be extrapolated to the other aromatics separations. 

The Parex, Toray Aromax and Axens Eluxyl processes are the three adsorptive liquid technologies for the separation and purification of p-xylene practiced on a large scale today. The MX Sorbex process is the only liquid adsorptive process for the separation and purification of m-xylene practiced on an industrial scale. We now consider a few other liquid adsorptive applications using Sorbex technology for aromatics separation that have commercial promise but have not found wide application. 

Chapter 7 gives a review of the technology and applications of zeolites in liquid adsorptive separation of petrochemical aromatic hydrocarbons. The application of zeolites to petrochemical aromatic production may be the area where zeolites have had their largest positive economic impact, accounting for the production of tens of millions of tonnes of high-value aromatic petrochemicals annually. The nonaromatic hydrocarbon liquid phase adsorption review in Chapter 8 contains both general process concepts as well as sufficient individual process details for one to understand both commercially practiced and academic non-aromatic separations. 

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... 

Figure 6 Scheme of aromatics separation via extractive distillation in the BTX process of the GTC Technology Corp. 

Reference G. Krekel, G. Birke, A. Glasmacher, et al., "Developments in Aromatics Separation," Erdol Erdgas Kohle, May 2000. 

Kloppenburg E. and Gilles E.D., Automatic control of the simulated moving bed process for Cg aromatics separation using asymptotically exact input/output-linearization. J. of Process Control 9 (2000) pp. 41-50. 

Olefins, miscellaneous hydrocarbons, water-oxygenated hydrocarbons, and aromatic separations. 

Repeatability of the Saturates—Aromatics Separation. Results obtained with the dual detectors for ten individual injections of a 20-vol % solution of a vacuum gas oil in n-heptane are shown in Table IV. The precision (2(t) for the saturates and aromatics area response from the RI detector was 6%, relative. The precision for the aromatics/ saturates response ratio was 6.9%, relative. The retention volumes were much more reproducible and had 2(7 values of the order of 1-3%, relative. 

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. 

Applications of Solvent to Aromatics Separation. There are two distinct techniques for using solvents in separation processes (2) extractive distillation and liquid-liquid extraction. Frequently these techniques or special variants can be combined in specified sequences to give a separation unobtainable by either method alone. Such has been the case for many of the solvents used or suggested in aromatics separation. The remainder of this discussion demonstrates the applicability and advantage of using dicyanobutane as a solvent for aromatics separation in petroleum refining. 

Masseting, J. J. H Mitsubishi Gas Chemical Company process far she production of high purity C aromatics somen", ACS 169th National Meeting, Aromatics Separation Session, Philadelphia, Pennsylvania (6/11 April 1975), ... 

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