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which separates water from thearomatics. Stream S-10 is the water rich phase and stream S-11 isthe aromatic rich phase. Because methanol is soluble in water,it leaves the decanter as part of the stream, S-10, which is pumpedand sent to the distillation columnDST-100 to separatemethanolfrom water. Methanol, being the lighter compound, is separatedas a distillate (stream S-14). The aromatic rich stream S-11contains unreacted toluene, benzene, and xylenes. Benzene, beingthe lightest, is separated in the distillation column DST-101 asa distillate. The bottom stream from DST-101, S-17, is sent tothe distillation column DST-102 for separation of toluenefrom xylenes. Toluene has a lower boiling point, 110.6 °C, andis separated as a distillate (stream S-18). It is mixed with themethanol recycle stream S-14 in a mixer, MIX-101. The mixedstream is heated to the reactor temperature and mixed with thefresh feeds of toluene and methanol in the MIX-100 mixer.5. P-XYLENE SEPARATIONTypically, theminimump-xylene purity required is 99.5%,whereas99.8%is considered ultrapure.4The bottomstreamfromDST-102is a mixed xylene stream with 97.5% p-xylene. It is a challenge toseparate p-xylene from its isomers due to the very close boingpoints as shown in Table 1. The technologies mainly usedindustrially for xylene separation are adsorption and crystalliza-tion.4But here these two techniques pose a serious problem as thep-xylene streamis already 97.5%pure. In adsorption, the simulatedmoving bed (SMB) technology is used,22but p-xylene is the onethat is being adsorbed due to its smaller size (kinetic diameter:6.7Åfor p-xylene, 7.3 Å for o-xylene, and 7.4 Å for m-xylene23).This means that a large adsorption bed would be required forseparation, which is not economical. The same dilemma exists in the case of crystallization as crystallizing out 97.5% of a stream isnot feasible. The other option for xylene separation is bydistillation. o-Xylene can be separated from p-xylene by extensivedistillation as the relative volatility is around
1.17. m-Xylene andp-xylene, however, can be hardly separated by ordinary distillationdue to a relative volatility of around 1.02. Saito el al.24reportedthe separation of m-xylene from p-xylene by reactive distillation.m-Xylene reacts preferentially with di-tert-butyl-benzene (DTBB)and tert-butyl-benzene (TBB) to form tert-butyl m-xylene(TBMX) and benzene (B). For modeling the reactive distillationcolumn in the Aspen Plus, the reactions can be considered to reachequilibrium25as in Venkataraman 27, based on the experimentalwork of Saito et al.26Equilibrium data is given by Saito et al.24Thereactions are shown in eqs 8 and 9,24,25and the correspondingequilibrium constants are given in eqs 10 and 11, respectively.24+‐↔ + DTBB m X TBMX TBB (8)+‐↔ + TBB m X TBMX B (9)==‐Kxxxx0.6 9R TBMX TBBDTBB m X (10)==‐Kxxxx0.16 10R TBMX BTBB m X (11)The mixed xylene stream S-19 is fed to reactive distillationcolumn DST-103. The standard inside-out algorithm by Bostonand Britt23is used for convergence. The column operates at1 bar, which results in a suitable temperature in the condenser,137 °C, so that cooling water can be used as a coolant. Theequilibrium reactions shown in eqs 8 and 9 are added to thedistillation block. The solvent streamcontaining DTBB and TBBis added at the 10th stage (from the top) of the distillationcolumn. Separation of o-xylene requires a large number of trays,a high reflux ratio, and a high reboiler duty. To get optimumresults, a sensitivity analysis is made. The mole fraction ofp-xylene in the distillate and the mole fraction of o-xylene in thebottom are set using the design specifications tool within theRadFrac environment as 0.99 and 0.16, respectively. Thesecomposition values result in 99.7% p-xylene purity in the productstream S-25. The reflux ratio and distillate rate are varied to getthe set values of compositions in the column top and bottom. Foreach set of the total number of equilibrium stages (includingcondenser and reboiler) and feed stage location, the RadFracblock converges only if the design spec tool also converges. Then,the NQ Curves tool, available in RadFrac block,21is used to findthe optimum location of the feed stage that gives the minimumreboiler duty. The results of this analysis are shown in Table 6for a range 40−60 for the total number of equilibrium stages.
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