text: Toluene disproportionation (TDP) is the conversion of toluene to benzene and xylene.
  Given that demand for para-xylene (p-xylene) substantially exceeds demand for other xylene isomers, a refinement of the TDP process called Selective TDP (STDP) may be used. In this process, the xylene stream exiting the TDP unit is approximately 90% p-xylene. In some systems, even the benzene-to-xylenes ratio is modified to favor xylenes.