One of the most employed SMB based technologies for p-xylene separation is UOP’s Parex. The studied aromatic complex uses this
technology consisting of 24 adsorbent beds with length and diam-
Minceva et al. [8]: 23.6 wt% p-xylene; 49.7 wt% m-xylene;
12.7 wt% o-xylene; 14 wt% ethylbenzene.
This work is foreseen as a modification of the current aromatic complex; that is the main reason to maintain the physical dimen- sions of the equipment. In case that the resulting flow rates are below or above the downstream units, a second train with the same characteristics could be installed to guarantee optimal oper- ation of said units. It is strongly recommended, whenever possible, to use similar units to the original ones while installing second trains in revamp and/or expansion projects in order to keep oper- ation simplicity.
3.1. Adsorption and reaction data
The normal operating conditions for the Parex process is around
180 C and 9 bar [13]. Pressure shall be high enough to maintain the operation in liquid phase and to avoid failure of associated equipment (e.g. pump cavitation) due to pressure drop in lines and columns; in other words, the influence on adsorption and reac- tion data is neglected. Temperature, on the other hand, will defi- nitely influence the reaction and adsorption data. According to Minceva et al. [9], increase in temperature leads to lower adsorp- tion capacity and faster isomerization, which means that a com- promise shall exist somewhere above normal Parex operation.
Bergeot [14] carried out adsorption and reaction experiments of xylenes at 200 C in liquid phase. The adsorbent used was low sil- ica X zeolite exchanged with barium (BaLSX); the author claimed that the adsorbent presented better selectivity compared to that of BaX, specifically on ethylbenzene. The adsorption equilibrium is described with the generalized Langmuir isotherm:
bi Ci
eter of 1.14 and 4.12 m respectively, p-diethylbenzene as desor- bent, particle diameter of 0.062 cm, and a time switch of
qi ¼ qsat 1
þ j bj Cj
ð1Þ
1.15 min [3]. The SMBR unit will keep the geometric characteristics of the Parex unit, i.e. 24 adsorbent beds, with the possibility to modify the location of inlets and outlets in order to use the appro- priate number of columns in each zone since column configuration plays an important role when dealing with different product con- centrations [11]. Moreover, p-diethylbenzene cannot be used since it isomerizes into o-diethylbenzene and m-diethylbenzene over acid catalysts; toluene, which has been used in the industry, is used as desorbent [10].
Generally, the feed to the Parex unit contains naphthenic frac- tion which are involved in the ethylbenzene isomerization in the Isomar unit. These nonaromatic compounds increase the utility consumption of the unit; however, they do not affect the xylene adsorption [12]. The feed used, as a first attempt, is that used by
The catalyst used in the isomerization tests was HZSM-5, which is industrially used in xylene isomerization in gas phase. The reac- tion scheme followed is presented in Fig 2. According to Cappel- lazzo et al. [15], the triangular scheme adds to the mechanism the direct conversion between o- and p-xylene, which actually does not occur, to account for the influence of intracrystalline mass-transfer resistance. Following the triangular scheme, the reaction rates for each species are given by Eqs. (2–4):
RPX ¼ k5 COX þ k3 CMX k6 CPX k4 CPX ð2Þ RMX ¼ k1 COX þ k4 CPX k2 CMX k3 CMX ð3Þ ROX ¼ k2 CMX þ k6 CPX k5 COX k1 COX ð4Þ