I have a code, but it keeps saying "Supplied objective function must return a scalar value.'

function Moles = Conv_func(H2, CO2, CO, CH4, H2O, T0, Pt, e11, e12, e13, e14)

% Call Sh_func to obtain entropy values

Sh_H2 = Sh_func('H2', T0);

Sh_CO2 = Sh_func('CO2', T0);

Sh_CO = Sh_func('CO', T0);

Sh_CH4 = Sh_func('CH4', T0);

Sh_H2O = Sh_func('H2O', T0);

% Compute standard entropies of reaction

Sr1 = (Sh_CH4(1) + Sh_CO2(1)) - (2*Sh_CO(1) + 2*Sh_H2(1)); % 1st reaction

Sr2 = (Sh_CO2(1) + Sh_H2(1)) - (Sh_CO(1) + Sh_H2O(1)); % 2nd reaction

Sr3 = Sh_CH4(1) - (2*Sh_H2(1)); % 3rd reaction

Sr4 = Sh_CO2(1) - (2*Sh_CO(1)); % 4th reaction

% Compute standard enthalpies of reaction

Hr1 = (Sh_CH4(2) + Sh_CO2(2)) - (2*Sh_CO(2) + 2*Sh_H2(2)); % 1st reaction

Hr2 = (Sh_CO2(2) + Sh_H2(2)) - (Sh_CO(2) + Sh_H2O(2)); % 2nd reaction

Hr3 = Sh_CH4(2) - (2*Sh_H2(2)); % 3rd reaction

Hr4 = Sh_CO2(2) - (2*Sh_CO(2)); % 4th reaction

% Compute equilibrium constants

k1 = exp(-Hr1*1000/(8.314*T0) + Sr1/8.314); % 1st reaction

k2 = exp(-Hr2*1000/(8.314*T0) + Sr2/8.314); % 2nd reaction

k3 = exp(-Hr3*1000/(8.314*T0) + Sr3/8.314); % 3rd reaction

k4 = exp(-Hr4*1000/(8.314*T0) + Sr4/8.314); % 4th reaction

% Solve for conversion using fminunc optimizer

e0 = [e11; e12; e13; e14]; % Initial condition for solver

r = fminunc(@(e) Sol_func(e, H2, CO2, CO, CH4, H2O, k1, k2, k3, k4, Pt), e0);

% Compute moles of species

Moles(1) = H2 + 2*r(1) - r(2) + 2*r(3);

Moles(2) = CO2 - r(1) - r(2) - r(4);

Moles(3) = CO + 2*r(1) + r(2) + 2*r(4);

Moles(4) = CH4 - r(1)- r(3);

Moles(5) = H2O - r(2);

end

my sol_func is :

function z = Sol_func(e, H2, CO2, CO, CH4, H2O, k1, k2, k3, k4, Pt)

e1 = e(1);

e2 = e(2);

e3 = e(3);

e4 = e(4);

ntot = H2 + CO2 + CH4 + CO + H2O; %Total number of moles

z = zeros(4, 1);

z(1) = (CH4 - e1 - e3) * (CO2 - e1 - e2 - e3) * k1 - (ntot + 2*e1+ e3+ e4)^-2 * (CO + 2*e1 + e2 + 2*e4)^2 * (H2 + 2*e1 + 2*e3)^2 * (Pt)^(-2);

z(2) = (H2 + 2*e1 + 2*e3) * (CO2 - e1 - e2 - e3) * k2 - (CO + 2*e1 + e2 + 2*e4) * (H2O + e2) * (Pt)^(-2);

z(3) = (CH4 - e1 - e3) * k3 - (ntot + 2*e1+ e3+ e4)^-1 * (H2 + 2*e1 + 2*e3)^2 * (Pt)^(-2);

z(4) = (CO2 - e1 - e2 - e3) * k4 - (ntot + 2*e1+ e3+ e4)^-1 * (CO + 2*e1 + e2 + 2*e4)^2 * (Pt)^(-2);

end

Answer 1

[Torsten]

I have a code, but it keeps saying "Supplied objective function must return a scalar value.'

Yes, that's true. Your function "Sol_func" must return a scalar value instead of a 4x1 vector. "fminunc" will then try to minimize this scalar value. Maybe you want to have the z(i) equal to 0 ; then you should return sum(z.^2).

[nima]

I got this error

''fminunc stopped because it cannot decrease the objective function along the current search direction.''

[Torsten]

What shall I say ? Bad luck :-)

If you want a constructive answer, we need code that we can execute and analyze.