AIChE / EPA MACT
Advanced
Mass Transfer & Separation Technology
Packed Gas Absorption Tower & Flooding Hydraulics
Simulate countercurrent solvent scrubbing, calculate packing height Z via HTU-NTU, and detect Sherwood flooding limits.
Models continuous gas scrubbing in a random/structured packed bed absorption column (e.g. acid gas CO2/H2S removal with chemical solvent). Solves Whitman two-film interphase mass transfer, computes packing depth Z = HTU x NTU, and monitors hydraulic flooding limits.
Governing Physical Law & Equations
N_A = K_y(y - y^*), \quad Z = \text{HTU}_{OG} \cdot \text{NTU}_{OG}, \quad \text{NTU}_{OG} = \int_{y_2}^{y_1} \frac{dy}{y - y^*}
Interphase mass transfer flux and packed column height based on overall gas-phase transfer units.
Law: Whitman Two-Film Theory & Sherwood-Eckert Flooding Hydrodynamics | Standard Reference: AIChE Equipment Testing Procedures / EPA 40 CFR Part 63 (MACT Scrubbers) / Perry’s Chemical Engineers Handbook
Adjustable System Parameters
| Parameter |
Nominal Value |
Dynamic Range |
Physical Role |
| Inlet Gas Flow Rate (G_m) |
18 kmol/h |
5 to 40 kmol/h |
Molar flow rate of gas feed stream |
| Solvent Liquid/Gas Ratio (L/G) |
2.8 |
1 to 6 |
Molar ratio of descending liquid solvent to ascending gas |
| Inlet Solute Conc y_in (y_{in}) |
8 % |
1 to 15 % |
Pollutant mole percentage in raw entering gas |
| Henry's Law Slope H (H) |
1.2 |
0.5 to 3 |
Equilibrium vapor-liquid distribution partition coefficient |
Analytical Proof & Derivation
Whitman’s two-film theory balances steady interfacial solute flux: N_A = k_y (y - y_i) = k_x (x_i - x) = K_{OG} (y - y^*). In a differential packed slice dz with specific surface area a_v, the solute transferred is G_m dy = K_{OG} a_v (y - y^*) S dz. Integrating over inlet y₁ and outlet y₂ yields total bed height Z = [G_m / (K_{OG} a_v S)] ∫ dy / (y - y^*) = HTU_{OG} × NTU_{OG}. Flooding velocity is calculated via the Sherwood-Eckert correlation: (u_G² a_p / g ε³) (ρ_G / ρ_L) μ_L^{0.2} = f[(L/G)√(ρ_G / ρ_L)].
Verification Benchmark
EPA MACT Acid Gas Absorber benchmark: Gas rate G = 18 kmol/h, inlet pollutant y_in = 0.08 (8%), target outlet y_out = 0.004 (95% scrubbing). Solvent ratio L/G = 2.8, Henry constant H = 1.2. Log-mean driving force gives NTU_{OG} = 4.25. For 25mm Pall rings (HTU_{OG} = 0.65 m), required packed depth Z = 2.76 m. Hydrodynamic loading is at 62% of Sherwood flooding limit (stable non-flooding regime).
Field Engineering Insights
Industrial scrubbers operate at 60–75% of flood velocity. Exceeding 85% causes liquid hold-up to surge dramatically, blocking vapor channels and triggering massive liquid entrainment, differential pressure surges (ΔP > 2 mbar/m), and severe breakdown of emission compliance.