A method for the design of approximate models in the form of a system of ordinary differential
equations (ODE) for a class of first-order linear partial differential equations of the hyperbolic type with
applications to monovariate and multivariate population balance systems is proposed in this work. We
develop a closed moment model by utilizing a least square approximation of spatial-dependent factors
over an orthogonal polynomial basis. A bounded hollow shaped interval of convergence with respect to
the order of the approximate ODE model arises as a consequence of the structural and finite precision
computation numerical errors. The proposed modeling scheme is of interest in model-based control and
optimization of processes with distributed parameters