A nitration reactor for use in nitrating aromatic organic compounds, comprising: (a) a first vertically-oriented reactor section; (b) a second vertically-oriented reactor section; (c) a connecting section between the first reactor section and the second reactor section for the flow of nitration reactants from the first reactor section to the second reactor section; (d) one or more inlets (20, 22) for introduction of the nitration reactants into the reactor; (e) an outlet for the removal of nitration reaction products from the reactor; (f) a vertically-downward flowpath for the nitration reactants in at least one of the first reactor section, the connecting section and the second reactor section; and (g) a cross-sectional area in the section of the reactor having the vertically-downward flowpath selected such that a stability parameter Φ is in the interval of 0 < Φ ≤ 1, alternatively 0 < Φ ≤ 0.75, alternatively 0 < Φ ≤ 0.5, where φ = β a ⋅ Ri + b ⋅ Eo + c a = − 1.1836 × 10 − 1 b = 2.2873 × 10 − 5 c = 1.1904 × 10 − 1 Ri = Richardson Number Ri = gD ρ c − ρ d ρ c U 2 β = volumetric fraction of a dispersed, organic phase, β = Q d Q d + Q c Eo = Eötvös Number Eo = ρ c − ρ d gD 2 σ U = bulk fluid velocity U = Q d + Q c A D = downflow section hydraulic diameter D = 4 A P A = downflow section cross-sectional area, P = downflow section cross-sectional perimeter, g = gravitational acceleration constant, ρ c = density of continuous phase, ρ d = density of dispersed phase, Q c = volumetric flow of continuous phase, Q d = volumetric flow of dispersed phase, and σ = interfacial tension. (a) a first vertically-oriented reactor section; (b) a second vertically-oriented reactor section; (c) a connecting section between the first reactor section and the second reactor section for the flow of nitration reactants from the first reactor section to the second reactor section; (d) one or more inlets (20, 22) for introduction of the nitration reactants into the reactor; (e) an outlet for the removal of nitration reaction products from the reactor; (f) a vertically-downward flowpath for the nitration reactants in at least one of the first reactor section, the connecting section and the second reactor section; and (g) a cross-sectional area in the section of the reactor having the vertically-downward flowpath selected such that a stability parameter Φ is in the interval of 0 < Φ ≤ 1, alternatively 0 < Φ ≤ 0.75, alternatively 0 < Φ ≤ 0.5, where φ = β a ⋅ Ri + b ⋅ Eo + c a = − 1.1836 × 10 − 1 b = 2.2873 × 10 − 5 c = 1.1904 × 10 − 1 Ri = Richardson Number Ri = gD ρ c − ρ d ρ c U 2 β = volumetric fraction of a dispersed, organic phase, β = Q d Q d + Q c Eo = Eötvös Number Eo = ρ c − ρ d gD 2 σ U = bulk fluid velocity U = Q d + Q c A D = downflow section hydraulic diameter D = 4 A P A = downflow section cross-sectional area, P = downflow section cross-sectional perimeter, g = gravitational acceleration constant, ρ c = density of continuous phase, ρ d = density of dispersed phase, Q c = volumetric flow of continuous phase, Q d = volumetric flow of dispersed phase, and σ = interfacial tension. β = volumetric fraction of a dispersed, organic phase, β = Q d Q d + Q c Eo = Eötvös Number Eo = ρ c − ρ d gD 2 σ U = bulk fluid velocity U = Q d + Q c A D = downflow section hydraulic diameter D = 4 A P A = downflow section cross-sectional area, P = downflow section cross-sectional perimeter, g = gravitational acceleration constant, ρ c = density of continuous phase, ρ d = density of dispersed phase, Q c = volumetric flow of continuous phase, Q d = volumetric flow of dispersed phase, and σ = interfacial tension.