In the "Chemical Process Computations" by Raghu Raman, Elsevier Applied Science Publishers LTD, ISBN 0-85334-341-1,I found Peng Robinson equation in the form:
P=R*T/(V-b)- a(T)/(V*(V+b)+b*(V-b)),
where
b=0,0778*R*Tc/Pc,
a(T)=0,45724*(R^2)*(Tc^2)/Pc*alfa(T),
alfa^0,5=1+m*(1-Tr^0,5),
m=0,37464+1,54226*FI-0,26992*FI^2;
Tc=critical T
Tr= reduced T
Pc =critical P
and acentric factor FI if not known, could be estimated as
FI=(3/7)*Tbr/(1-Tbr)*LOG(Pc/101,3)-1
where Tbr is the reduced normal boiling point.
I hope this will help you.
m777182