At steady-state, the accumulation term is zero. Momentum can go 'in' and 'out' of the shell by both the convective and molecular mechanisms. Since vz(r) is the same at both ends of the system, the convective terms cancel out because (ρ vz vz 2πr Δr)|z = 0 = (ρ vz vz 2πr Δr)|z = L.