An unusual type of low-pressure VLE behavior is that of double azeotropy, in which the dew and bubble curves are S-shaped, thus yielding at different compositions both a minimum-pressure and a maximum-pressure azeotrope. Assuming that Eq. (13.57) applies, determine under what circumstances double azeotropy is likely to occur.

Eq. (13.57)

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P = x171Pj + x2Y2P sat sat (13.57)