We present a new modeling approach for dividing-wall columns (DWCs) that is amenable to equation-oriented flowsheet simulation and optimization. The material, equilibrium, summation, and heat (MESH) equations describing a DWC are highly coupled and nonlinear, making DWC-based process flowsheets challenging to simulate. Design optimization poses further challenges, typically requiring integer variables to select the number of column stages. To address these difficulties, we represent DWCs as networks of pseudo-transient (differential-algebraic) subunit models. We show that these networks have the same steady-state solution as the original (algebraic) MESH equations, but present significant numerical benefits. We then embed these models in a previously developed pseudo-transient flowsheet modeling and optimization framework. We further reformulate the models to require only continuous decision variables when selecting the optimal number of stages during design optimization. To illustrate these concepts, we discuss the DWC-based intensification of the dimethyl ether process. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 704-716, 2016