Direct separate of m-xylene

Given the problems raised by the presence of m-xyleoe in the separation of a high yield of p xylene at the required degree of purity, one possibility consists in isolating m-xylene selectively. In this way, in treating the aromatic cut, it is no longer necessary in principle to resort to crystallization or adsorption to recover the p-xylene, and each of the isomers can then be obtained in the pure state by distillation. 

Among the different alternatives proposed, only the technique devdoped by Mitsubishi Gas Chemical (formerly Japan Gas Chemica ) has been commercially implemented. 

This process achieves the extraction of m-xylene by means of HF—BFj. It exploits the complexing properties of boron trifluoride on. aromatic compounds, and the particular ability of m-xylene to fonn the most stable complex. Subsequently, this isomer, added to a mixture of Cg aromatics which does not conCrin it, displaces the other components from their respective complexes, and the composition of the medium depends on the values of the equilibrium constants, or m-xylene, 2 for o-xylene, 0.14 

By adjusting the amount of boron trifluoride, introduced in gaseous form in hydrofluoric add in contact with the organic phase, it is therefore possible to extract the m-xylene from an aromatic Cg cut after agitation. The HF—BFj solvent is also an isomerization catalyst whose activity is related to the operating conditions, so that the combination of extraction and isomerization (see Section 4.4.Z2) offers a number of economic advantages. 

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