vpasolve gives wrong solution

Hi, I'm Alessio and I'm a chemical engineer. I have basic knowlwdge of matlab and I have some problems with a system of equations that I can't manage to solve. Could you please help me ?

Tc=200;
Pc=45;
R=0.0821;
Z=0.30;

syms Vc a b c;
EoS = (R*Tc)/(Vc-b)-(a-c*sqrt(Tc))/(Vc*(Vc+b)+b*(Vc-b)) - Pc;
eq1 = (R*Tc)/(Vc-b)-(a-c*sqrt(Tc))/(Vc*(Vc+b)+b*(Vc-b)) - Pc == 0;
eq2 = diff (EoS, Vc) == 0;
eq3 = diff (EoS,Vc, 2) == 0;
eq4=(Pc*Vc)/(R*Tc)-Z==0;
sol = vpasolve([eq1,eq2,eq3,eq4],[a,b,c,Vc]);
Vc = sol.Vc

Vc = 

0.10946666666666666666666666666667

b = sol.b

b = 

0.26427591129444147134215819234349

a = sol.a

a = 

0

c = sol.c

c = 

0

[Srijith Kasaragod]

Could you please share what issues you are facing? In case you are facing any errors, please share the error message.

Answer 1

[Neelanshu]

Hi Alessio,

As Srijith mentioned kindly share the error message and the issue faced by you. Since the code runs without any error, I assume that you are referring to the solutions of a,b,c,Vc not having the desired values as the problem.

This is likely to be the case because the system of equations has more than one solution. If no initial guess is mentioned using the "init_param" argument, the "vpasolve" function returns the first solution it finds.

You may refer to the following documentation to learn more about "vpasolve" :

https://www.mathworks.com/help/symbolic/sym.vpasolve.html

Hope this helps.