function [sys,x0] = colds(t,x,u,flag)%%  Simulink interface to coldmod.m     %% Inputs:    t    - time in [min].%            X    - State, the first 41 states are compositions of light%                   component A with reboiler/bottom stage as X(1) and %                   condenser as X(41). State X(42)is holdup in reboiler/%                   bottom stage and X(82) is hold-up in condenser. %            U(1) - reflux L,%            U(2) - boilup V,%            U(3) - top or distillate product flow D,%            U(4) - bottom product flow B,%            U(5) - feed rate F,%            U(6) - feed composition, zF.%            U(7) - liquid feed fraction, qF.%% Outputs:   sys and x0 as described in the SIMULINK manual.%            when flag is 0 sys contains sizes and x0 contains %            initial condition. %            when flag is 1, sys contains the derivatives,%            and when flag is 3 sys contains outputs; %            y(1)    - top composition,%            y(2)    - bottom composition,%            y(3)    - condenser holdup,%            y(4)    - reboiler holdup,%            y(5:115) - tray composition, y(5) is reboiler y(115) is top.NT = 111;if abs(flag) == 1  % Return state derivatives.  sys = coldmod(t,x,u);elseif abs(flag) == 3  % Return system outputs.  sys(1,1) = x(NT);         % Top composition.  sys(2,1) = x(1);          % Bottom composition.  sys(3,1) = x(2*NT);       % Holdup in condenser.  sys(4,1) = x(NT+1);       % Holdup in reboiler.  sys(5:NT+4,1)= x(1:NT);   % Stage Compositions.%  sys(5:NT+4,1)= x(NT+1:2*NT);   % Stage holdups.elseif flag == 0  % Ininitalize the system%  load cold_init . change 02 Aug 2005cold_init  x0 = Xinit;  NT=111;  sys = [2*NT, 0, NT+4, 7, 0, 0];else  sys = [];  end