How do you calculate pressure drop across a packed bed reactor?

Ergun equation combines viscous and inertial losses: dP/dL = 150*mu*v*(1-epsilon)^2/(dp^2*epsilon^3) + 1.75*rho*v^2*(1-epsilon)/(dp*epsilon^3), where epsilon is bed void fraction and dp is particle diameter. First term dominates at low Reynolds number (laminar), second at high Re (turbulent). Smaller particles increase pressure drop significantly. For reacting systems, must account for molar expansion/contraction. High pressure drop requires balance: smaller particles improve effectiveness factor but increase dP.