‘But nothing comes out on the graph.’
The result of the int function is a constant. At best, you will get a straight line.
I am not certain what you want. You can get it to plot as a function of the upper limit of integration by creating a function from it:
syms x a
kw = 91371;
K = 1.29;
fun = FaO*((1+K*CaO*(1-x))/(kw*CaO*(1-x)));
W(a) = int(fun, x, 0, a);