Raising pros and cons of falling film and packed column for absorption of NH3 by a NH3–H2O solution

Licianne Pimentel Santa Rosa; Karen Valverde Pontes

doi:10.1515/cppm-2024-0117

Article

Raising pros and cons of falling film and packed column for absorption of NH₃ by a NH₃–H₂O solution

Licianne Pimentel Santa Rosa and Karen Valverde Pontes

Published/Copyright: June 2, 2025

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From the journal Chemical Product and Process Modeling

Abstract

The refrigeration absorption cycle is a mature technology to obtain refrigerant effect at low temperatures, and the absorber unit plays a key role on the cycle efficiency. Falling liquid films and packed columns are usually employed in refrigeration cycle with ammonia and water. This study addresses the pros and cons of each equipment, comparing both in terms of absorption capacity, equipment cost and cooling demand by mathematical modelling and simulations. The packed absorption column is modeled in Aspen Plus, and the falling film, by an equation-oriented approach in Matlab. The results indicate that the falling film ensures a uniform coolant distribution, enhancing ammonia absorption efficiency. Consequently, for the same absorption capacity, the falling film enables nearly 80 % reduction in the required volume of the column. However, this space-saving advantage comes at the cost of a 51 % increase in capital and installation expenses. Therefore, the falling film offers a more compact unit but, when space limitation is not an issue, absorption columns might take advantage.

Keywords: absorption; ammonia and water; falling film; packed column

Corresponding author: Licianne Pimentel Santa Rosa, Department of Chemical Engineering, Federal University of Sergipe, Av. Marcelo Déda Chagas, s/n, Rosa Elze, São Cristóvão, 49107-230, Sergipe, Brazil, E-mail: liciannep@academico.ufs.br

Acknowledgments

We express our heartfelt gratitude to professor Glória Meyberg Nunes Costa (in memoriam) for her invaluable contributions to this research. Her dedication and legacy continue to inspire and guide us. This work is a testament to her enduring impact on the field.

Research ethics: Not applicable.
Informed consent: Not applicable.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Use of Large Language Models, AI and Machine Learning Tools: None declared.
Conflict of interest: The author states no conflict of interest.
Research funding: None declared.
Data availability: Not applicable.

Notation

A: Area (m²)
Ø: Packed column diameter (m)
z: Falling film absorber length (m)
Z: Column high (m)
δ: Thickness of the film (m)
m: Mass flowrates (kg.s⁻¹)
n ˙: Mass Fluxes (kg·m⁻²s⁻)
PF: Packing factor
FA: Dimensionless parameter
φ: Liquid specific density
U: Overall heat transfer coefficient though the wall (W-m⁻²K⁻¹)
q: Sensible heat flux (W-m⁻²)
H: Enthalpy (J-kg⁻¹)
h ˜: Partial Mass Enthalpy (J-kg⁻¹)
Q: Heat flux (W-m⁻²)
L: Length of the heat and mass exchange area
P: Pressure (bar)
R: Ideal gas constant (cm³-atm⁻¹mol⁻¹)
T: Temperature (K)
d: Diameter (m)
x: Molar/mass fraction at the liquid phase
y: Molar/mass fraction at the vapour phase
α: heat transfer coefficient (W-m⁻²K⁻¹)
F: Mass transfer coefficient (kg-m⁻²s⁻¹)
FZ: Mass fraction of ammonia in the absorbed/desorbed flux.
ψ: Correction factor.
C _P: Specific heat (J-kg⁻¹K⁻¹)
M: Molecular Weight (g-mol⁻¹)
S c: Schmidt number
Pr: Prandtl number
D: Diffusivity (m-s⁻²)
µ: Viscosity (Pa-s⁻¹)
λ: Thermal conductive (W-m⁻¹K⁻¹)
ρ: Density (kg-m⁻³)
R e: Reynolds Number
I _C: Desired percentage of speed in the flood condition
G f ′: The surface mass flow of the vapour in the flood condition
α: Packing parameters
β: Packing parameters
ABS: The rate of solute absorbed
k _OG a: Empiric mass transfer coefficient
C ₁: Packing parameters
C 2: Packing parameters
C ₃: Packing parameters
V: Specific velocity (ft/s)
a_w: Effective interfacial area of packing per unit volume
a: Actual area of packing per unit volume
L W *: Liquid mass flow rate per unit column cross-sectional area (kg/m²s)
V W *: Liquid mass flow rate per unit column cross-sectional area
d p: Parameter packing size (m)
K ₅: Parameter packing sizes
a: Cost estimation parameter
b: Cost estimation parameter
C: Equipment cost
S: Size/capacity of the equipment parameter
n: Equipment type
ε: Thickness of the column (m)
E: Welding efficiency
MT: Maximum tension alowded (kPa)
CA: Corrosion Factor
IF: Installation factor
LFA: Allocation factor

Subscript

hv: Hydraulic diameter of the vapour in the control volume
L: Liquid Phase
V: Vapour Phase
c: Coolant
i: component index
int: Interface
W: Absorber length (m)
w: Wall
VL: Vapor-liquid

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Received: 2024-11-29

Accepted: 2025-05-17

Published Online: 2025-06-02

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https://doi.org/10.1515/cppm-2024-0117

Keywords for this article

absorption; ammonia and water; falling film; packed column