Abstract: This paper suggests a late lumping method for a nonlinear state estimation algorithm for wall-cooled fixed bed reactors.The estimator in PDEs is discretized using orthogonal collocation to a set of ODEs and then integrated by the Gear method.The nonlinear algebraic estimation equations for concentration are solved by the Hybrid method.For an industrial process,in considering of the longer sampling interval and time delay of concentration measurement,the estimation of concentration is corrected by a predictive method.The performance of the state estimator is examined by both experimentation and simulation.It is observed that the convergence of the algorithm and the robustness of the estimator are satisfactory.