I am recieving an error in creating the solution however it still prints a result.

clc
clear all
close all

%Homework 7 Question 3

Tin=298; %K
Cp=80; %J/(mol K)
CAin=2000; %mol/m^3
k0=10e-7; %m3/(mol min)
Ea_R=5000; %K
rho=1000; %mol/m^3

CA=linspace(2,2000,61);
xA=(CAin-CA)/CAin;

T_sol=zeros(1,length(CA));
k_sol=zeros(1,length(CA));
tau_sol=zeros(1,length(CA));

DHR=-50000; %J/mol exothermic (gets hot)

for i=1:length(CA)
    syms T k tau
    eqn1=k==k0*exp((-Ea_R)*((1/T)-(1/Tin)));
    eqn2=2*k*(CA(i)^2)==(CAin-CA(i))/tau;
    eqn3=k*tau*(CA(i))^2*DHR==rho*Cp*(Tin-T);
    solution=solve([eqn1,eqn2,eqn3],[T,k,tau]);
    T_sol(i)=double(solution.T);
    k_sol(i)=double(solution.k);
    tau_sol(i)=double(solution.tau);
end

figure(1)
plot(log10(tau_sol),xA,'-bo');
xlabel('log_{10}(\tau, min)')
ylabel('x_A')

Answer 1

[Star Strider]

In the last iteration of the loop, the results are all empty. One way to avoid the error is to test for at least one field to be empty:

if ~any(structfun(@isempty, solution))

T_sol(i)=double(solution.T);

k_sol(i)=double(solution.k);

tau_sol(i)=double(solution.tau);

end

Try this —

clc
clear all
close all

%Homework 7 Question 3

Tin=298; %K
Cp=80; %J/(mol K)
CAin=2000; %mol/m^3
k0=10e-7; %m3/(mol min)
Ea_R=5000; %K
rho=1000; %mol/m^3

CA=linspace(2,2000,61);
xA=(CAin-CA)/CAin;

T_sol=zeros(1,length(CA));
k_sol=zeros(1,length(CA));
tau_sol=zeros(1,length(CA));

DHR=-50000; %J/mol exothermic (gets hot)

for i=1:length(CA)
    syms T k tau
    eqn1=k==k0*exp((-Ea_R)*((1/T)-(1/Tin)));
    eqn2=2*k*(CA(i)^2)==(CAin-CA(i))/tau;
    eqn3=k*tau*(CA(i))^2*DHR==rho*Cp*(Tin-T);
    solution=solve([eqn1,eqn2,eqn3],[T,k,tau]);
    if ~any(structfun(@isempty, solution))
        T_sol(i)=double(solution.T);
        k_sol(i)=double(solution.k);
        tau_sol(i)=double(solution.tau);
    end
end

figure(1)
plot(log10(tau_sol),xA,'-bo');
xlabel('log_{10}(\tau, min)')
ylabel('x_A')

EDIT — (21 Apr 2024 at 20:14)

Changed if condition to use:

structfun(@isempty, solution)

Code otherwise unchanged.

.

[Lily]

Worked perfectly thanks!

[Star Strider]

As always, my pleasure!