The accuracy of linear approximate models increases upon time irrespectively of the value of the Thiele modulus, while for the nonlinear ones the accuracy increases upon time for sufficiently small values of the Thiele modulus. For higher values of the Thiele modulus there appears an unacceptable steady-state error. To reduce this drawback in the present work the approximate model for the single reaction with a non-linear kinetic equation and with a modified Thiele modulus was derived and tested.