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ODE System not Functioning Correctly

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Can Cakiroglu
Can Cakiroglu on 26 May 2019
Closed: MATLAB Answer Bot on 20 Aug 2021
Hello Matlab Community,
I'm a chemical engineering student, and I have to solve an ODE system. However, while one of my values change, the other values seem to stay constant, although they are deffinetly functions that are not constant. Could there be any problems in the calculation of Matlab? And if yes, how could it be solved?
The function codes are as following:
Function File
function func = fchem(W, Variables)
X = Variables(1);
T = Variables(2);
P = Variables(3);
Fao = 1487.351; % kmol/h
Tr = 273; % K
X0 = 0;
P0 = 150; %bar
T0 = 673; %Kelvin
a = (3 * 1.5) / ((3 * 1.5) + 1);
b1 = (0.5 + (0.5 / 1.5)) / 1;
b2 = (-0.5+(1.5*1.5))/1;
R = 8.314 / 1000; % kJ/mol*K
K30 = 0.0307;
k20 = 2.12 * (10 ^ 13); % should be 2.12e13 I suspect
E2 = 72100.82; % kJ/mol
E3 = -80989.98; % kJ/mol
omega = 1.564;
alpha = 0.640;
Bo = 0.254626; %atm/m
rho_c = 1600; %kg/m^3
Phi = 0.5;
D = 0.05; %Diameter of tube, m
Ac = pi * (D^2)/4; %m^2
alpha_pressure = (2*Bo)/Ac*rho_c*(1-Phi)*P0;
c1= 2.691122;
c2= 5.519265;
c3= 1.848863;
c4= 2001.6;
c5= 2.6899;
k2o = k20*exp(-E2/(R*T));
z = 0.5*X-1;
gamma = 1.5;
a_NH3 = P*z;
a_N2 = P*(a/3*gamma)*(1-b2*z);
a_H2 = P*a*(1-b1*z);
Ka = (1/T^c1)*10^(-c2*10^(-5)*T + c3 * 10^(-7)* (T^2) + (c4/T) + c5);
K3 = K30*exp(-E3/(R*T));
ra = (k2o*(a_N2*(Ka^2)-(a_NH3^2)/(a_H2^3)))/(1+K3*a_NH3/(a_H2^omega))^(2*alpha);
Cp_NH3 = 6.70 + 0.00630*T;
Cp_H2 = 6.62 + 0.00081*T;
Cp_N2 = 6.50 + 0.00100*T;
delta_H_Tr = -91630;
delta_Cp = 2*Cp_NH3-3*Cp_H2-Cp_N2;
delta_H = delta_H_Tr + (-12.96*(T-Tr)+0.00917*((T-Tr)^2)/2);
dXdW = -ra/Fao;
dTdW = (ra*delta_H)/(Fao*((Cp_N2+3*Cp_H2+2*Cp_NH3)+delta_Cp*X));
dPdW = ((-alpha_pressure/2)/P)*(T/T0)*(1-0.5*X);
func = [dXdW; dTdW; dPdW];
end
And this is the calling script
clc
Wspan = [0 500]; % Range for Catalyst Weight
y0 = [0.; 673.; 150]; % Initial values X,T,and P respectively
[W y]=ode45(@fchem,Wspan,y0);
z=size(y);
plot(W,y);

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