Reliable nonlinear parameter estimation in VLE modeling

The reliable solution of nonlinear parameter estimation problems is an impo rtant computational problem in the modeling of vapor-liquid equilibrium (VL E). Conventional solution methods may not be reliable since they do not gua rantee convergence to the global optimum sought in the parameter estimation problem. We demonstrate here a technique that is based on interval analysi s, which can solve the nonlinear parameter estimation problem with complete reliability, and provides a mathematical and computational guarantee that the global optimum is found. As an example, we consider the estimation of p arameters in the Wilson equation, using VLE data sets from a variety of bin ary systems. Results indicate that several sets of parameter values publish ed in the DECHEMA VLE Data Collection correspond to local optima only, with new globally optimal parameter values found by using the interval approach . When applied to VLE modeling, the globally optimal parameters can provide significant improvements in predictive capability. For example, in one cas e, when the previously published locally optimal parameters are used, the W ilson equation does not predict experimentally observed homogeneous azeotro pes, but, when the globally optimal parameters are used, the azeotropes are predicted. (C) 2000 Elsevier Science B.V. All rights reserved.