Petrochemical Processing Parex process

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Since the early 1970s p-xylene has grown to become a large volume petrochemical. It is used primarily for the production of polyester fibers, films and resins, such as PET (polyethylene terephthalate) [7]. Demand for p-xylene has increased tenfold since 1970 to about 26xl0 t/year. Almost all of this additional production has been by the UOP Parex process as shown in Figure 7.1. A baseline production ofp-xylene is maintained by crystallization based sites that existed before the SMB adsorptive separation technology was established [8]. 

The aromatics complex converts approximately 75% of the feed naphtha to petrochemical aromatics with the vast majority of the remainder being exported as raffinate and some hydrogen. With a modern aromatics complex flowscheme, a little over half of the mixed xylenes are produced in the Tatoray unit while the rest are produced in the CCR Platforming unit directly from the naphtha reforming. Having reviewed the framework of an aromatics complex we are now in a better position to understand the context of the continuous countercurrent liquid adsorptive Parex process which produces the primary aromatics complex product, p-xylene. 

The SMB technology was developed by UOP and its major field of application is in the area of binary separations. For example, SMB has been used in the chemical industry for several separations known as SORBEX processes [1-3], which include, among others, the PAREX process for p-xylene separation from a Cs aromatic fraction [4], the OLEX process for the separation of olefins from paraffins, the SAREX process to separate fructose from glucose [4] and the MOLEX process [5]. Simulated moving bed is being used particularly for separation of enantiomers from racemic mixtures or from the products of enantioselective synthesis [6,7]. It has been used for the production of fine chemicals, and petrochemical intermediates, such as Cg-hydrocarbons [8], food chemistry such as fatty acids [2], or certain sugars from carbohydrate mixtures [8] and protein desalination [9]. 

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