The simulation procedure used in the code, CRYDIS-2, is greatly improved. The previous procedure used the Newton-Raphson method choosing a set of temperatures and liquid flow rates for the independent variables. Considering the property that the convergence characteristics of the liquid flow rates are much less sensitive to the type of the iterative method than those of the temperatures, the iterative loop is divided into two loops — the inner loop of the quasi-Newton method for temperature corrections and the outer loop of the successive iteration for flow rate corrections. The corrections of the deviation coefficients are also made in the outer loop, together with the flow rate corrections, when the nonideality of the hydrogen isotope solution is incorporated in the model. Since the order of the Jacobian matrix is halved, and the numerical evaluation of the Jacobian matrix and its inversion are needed only once, both the computer storage requirements and computation time are remarkably reduced. Thus, a new computer code, CRYDIS-N, which uses an efficient simulation procedure, is developed. Also, a simple but powerful method for estimating the initial set of temperatures is proposed, and it assures rapid achievement of convergence. The simulation procedure is a verison particularly developed for simulating hydrogen isotope distillation columns.