Log[10] // N

Ethylbenzene to styrene: EB -> Styrene + H2, first order rxn...

r = 8.314;
k0 = 4240*1000;   
eA = 
      90826;  
temp = 500 + 273;
k = k0*Exp[-eA/r/temp];
press = 
      137.8*1000;   
cEB0 = press/r/temp  
vol = 0.770;   
length = 
      3;   
diam = (4*vol/Pi/
              length)^0.5
eps = 1;   
flowRateEB0 = 
      547.9*1000/3600;   
tau = 
      vol*cEB0/flowRateEB0  
Solve[k*tau == (1 + eps)*Log[1/(1 - xEB)] - eps*xEB, 
      xEB]