DOI: 10.3303/CET25117105 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 26 December 2024; Revised: 12 April 2025; Accepted: 30 April 2025 
Please cite this article as: Giustacori P., Brunazzi E., 2025, Analysis of Mass Transfer Efficiency in a Compact Wet Scrubber, Chemical Engineering 
Transactions, 117, 625-630  DOI:10.3303/CET25117105 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 117, 2025 

A publication of 

 

The Italian Association 

of Chemical Engineering 

Online at www.cetjournal.it 

Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto 

Copyright © 2025, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-17-5; ISSN 2283-9216 

Analysis of Mass Transfer Efficiency in a Compact Wet 

Scrubber 

Pietro Giustacori, Elisabetta Brunazzi* 

Dipartimento di Ingegneria Civile e Industriale, Università di Pisa, Largo Lucio Lazzarino 2, Pisa, 56122, Italy 

elisabetta.brunazzi@unipi.it 

This study investigates a compact wet scrubber, a versatile pollution control device designed to efficiently 

remove particulates and contaminant gases from industrial gas streams. The system features a nozzle 

integrated into a vertical channel as its core element. The nozzle allows the scrubbing liquid to be injected 

countercurrently to the descending gas flow. This configuration is intended to create a highly turbulent foam 

zone, that can significantly enhance mass transfer efficiency. The lack of internal components provides flexibility 

in handling high-flow gas streams with varying particulate concentrations, making it potentially suitable for a 

variety of industrial applications. Despite the growing interest in this type of compact scrubber for specific 

industrial sectors, there is limited knowledge about its design and performance. To address this gap, the present 

study introduces an experimental setup for characterizing the performance of the compact scrubber. Gas 

pressure drop measurements across the liquid jet were used as a key tool to identify the scrubber’s operating 

window. Additionally, absorption tests of volatile organic compounds with different solubilities were conducted 

to assess mass transfer performance. Key parameters, including overall mass transfer coefficient and 

absorption efficiency, were determined for various gas and liquid flow rates. The results showed remarkable 

absorption efficiencies of up to 90%, with gas-liquid contact times of less than 1 second. 

1. Introduction 

Stringent environmental regulations require refineries, cement kilns, power plants, tanneries, chemical 

manufacturers, and many other industrial facilities to remove pollutants and particulates from flue gases before 

releasing them into the atmosphere. Meeting the increasingly stringent concentration limits set by these 

regulations often results in significant energy-related operating costs, driving ongoing efforts to develop more 

efficient pollution control technologies (EEA, 2024). A wet scrubber is a versatile system that uses a liquid 

medium to remove particulates and gaseous pollutants from gas streams. Particulates are primarily captured 

by mechanisms such as inertial impaction, diffusion, and interception, while gaseous pollutants are absorbed 

into the liquid phase (Danzomo, 2012). Particulate removal is aided by high gas pressure drop and small droplet 

size, while control of the gas-liquid ratio is critical for pollutant absorption (Park, 2006). Over time, various types 

of wet scrubbers have been developed to effectively control both particulate and gaseous emissions. These 

include spray towers, packed or plate towers, ejector or jet scrubbers, cyclone or vortex scrubbers, venturi 

scrubbers, and others. 

When space constraints become critical, not all conventional solutions remain viable. This is particularly true for 

instance in the marine and offshore sectors, where space is at a premium, necessitating the development of 

compact scrubber systems (Long, 2025). In these applications, chemical absorption processes with fast reaction 

kinetics are often preferred (Yao, 2014), reducing the demand for benefits typically associated with conventional 

packed bed scrubbers (Brunazzi, 2001). For such scenarios, co-current scrubbing emerges as the preferred 

solution, often implemented with static mixers to ensure efficient operation (Couvert, 2008). However, 

alternatives to co-current scrubbing are gaining traction, particularly in specialized applications where effective 

gas-liquid contact is essential, pressure drops must remain low, and gas quenching is necessary. These 

alternatives are particularly advantageous when gas streams to be treated are loaded with particulate matter, 

which can lead to clogging in static mixers. 

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The objective of this study is to optimize the design of an alternative scrubbing system capable of efficiently 

treating gaseous pollutants and highly dust-laden gas streams. The compact scrubber investigated in this study 

performs its scrubbing operation by directing the scrubbing liquid countercurrently to the gas flow within the 

scrubbing chamber, which is designed as an open channel. The gas then reverses the liquid jet, and scrubbing 

continues in co-current mode. The liquid is sprayed using a full-cone nozzle to ensure complete coverage of the 

gas passage, preventing potential bypasses. High velocity spraying facilitates effective interaction between the 

liquid and the gas, promoting efficient mass transfer. This arrangement ensures intimate contact between the 

two phases, significantly enhancing contaminant removal efficiency. In addition, gas flow through the open 

channel design minimizes pressure drop and prevents blockage, improving overall system performance. After 

the contact section, a calming section is incorporated to separate most of the entrained liquid. Before exiting the 

calming section, the gas passes through a demister that removes any remaining fine liquid droplets. The calming 

tank often serves as a reservoir for liquid accumulation. A centrifugal pump is typically installed to recirculate 

the liquid back to the nozzle, ensuring continuous operation. 

To optimize the design of the compact scrubber, an experimental setup was designed and constructed. The 

fluid dynamics were analyzed through visual inspection of the jet during operation and by measuring the gas 

pressure drop across the liquid jet. Absorption tests with two volatile organic compounds (VOCs) were 

performed to evaluate mass transfer efficiency. These experimental data are essential for developing 

comprehensive design criteria that will enable the formulation of correlations and the development or validation 

of mechanistic models for fluid dynamics and mass transfer in the compact scrubber. 

2. Experimental 

Figure 1 (left) shows a schematic of the experimental setup. Ambient air enters the system and first passes 

through the gas-liquid contact section (C) where the scrubbing operation takes place, after the gas enters the 

separation vessel (D). The liquid is introduced into the contact section through a solid cone nozzle (N), sprayed 

upward into the gas flow, which moves in the opposite direction. After reaching a maximum distance from the 

inlet, the liquid reverses direction and returns to the tank, dragged by the opposing gas stream. The contact 

section has a diameter of 300 mm and is equipped with three observation windows to allow visual monitoring of 

the liquid jet dynamics. The airflow is driven by a centrifugal fan, with the gas flow rate measured by a pitot tube 

and regulated by adjustments to the fan motor inverter. Before exiting the separation tank, the gas stream 

passes through a chevron-type demister to remove entrained liquid droplets. The liquid is circulated in a closed 

loop by a centrifugal pump (G), the flow rate is regulated by a control valve and monitored with a flow meter. 

Figure 1 (right) presents pictures of the experimental setup, showing the separation tank with the liquid reservoir 

and the contact section with its three observation windows. 

 

 
Figure 1: Experimental test rig details (left: schematic of the test rig, right: test rig).  

 

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Air-water system was used for fluid dynamic characterization. Gas pressure drops across the liquid jet were 

measured using a differential pressure transmitter, with sensor ports placed upstream and downstream of the 

jet.  

To evaluate the mass transfer performance of the compact scrubber, absorption tests were performed using 

two VOCs with distinct water solubilities. Isopropanol (IPA), selected for its high water solubility and low toxicity 

under the tested conditions, served as the high water-soluble model compound. In contrast, Butyl acetate (BA) 

was chosen to represent a VOC with poor water solubility, due to its limited solubility in water and its low 

environmental risk at the concentrations used (Lamprecht and Burger, 2018; Marchini, 2021). VOC 

concentrations were measured upstream and downstream of the jet using two multi-gas sensors equipped with 

photon ionization detectors. The VOCs were introduced into the gas line using a diaphragm pump designed to 

provide a steady and consistent flow into the system. VOC delivery was facilitated by a dual-fluid nozzle that 

produced a fine mist, ensuring uniform dispersion within the gas stream and promoting efficient vaporization. In 

the gas stream, VOC concentrations range from a maximum of 600 ppmv at the inlet to a minimum of 50 ppmv 

downstream of the liquid jet. Gas and liquid temperatures were monitored during the experiments. 

The experimental setup was designed to provide appropriate liquid-to-gas ratios and gas surface velocities 

greater than the flooding velocity. Flooding velocity is defined as the minimum gas velocity sufficient to retain 

liquid flow in an open tube (Roy 1962). The compact scrubber can be considered as an upside-down distillation 

tray, allowing the flooding velocity to be estimated by considering the total entrainment of the tray (Green, 2008). 

Based on an estimation of the average droplet diameter, the entrainment (𝐸) can be expressed as follows: 

 

𝐸 = (
𝑢𝑠,𝐺

8 𝜌𝐺
4

[𝑔(𝜌𝐿 − 𝜌𝐺)]2.5𝜎1.5
) (1) 

 

Where 𝑢𝑠,𝐺 is the gas superficial velocity, 𝜌𝐺 and 𝜌𝐿 are the gas and liquid densities, 𝑔 is the gravitational 

acceleration and 𝜎 is the surface tension. Consequently, flooding conditions occur when the entrainment value 

is 1. Considering this condition, the value of the flooding velocity is determined (Steinmeyer, 1995). 

 

𝑈𝐹𝑙𝑜𝑜𝑑 = (
[𝑔(𝜌𝐿 − 𝜌𝐺)]2.5𝜎1.5

𝜌𝐺
4 )

1
8

(2) 

 

According to the Equation (2), assuming the liquid phase is pure water, and the gas is air, the flooding velocity 

is estimated to be about 4 m/s. With reference to the diameter of the contact section, the operating flow ranges 

of the compact scrubber were then determined. Liquid flow rates (𝐿𝑉) vary between 15 and 45 m3/h, and gas 

flow rates (𝐺𝑉) between 500 and 3500 m3/h. For a detailed description of the experimental setup, the reader is 

referred to Giustacori and Brunazzi 2025. 

3. Results and discussion 

The VOC concentration in the gas phase is measured across the liquid jet. These measurements allow for the 

quantification of the instantaneous absorption of the tracer. The liquid is assumed to be homogeneously mixed 

within the tank, based on the liquid flow rate relative to the tank volume and recirculation loop, as well as the 

location of the liquid suction point relative to the tank return. Given the selected operating conditions and the 

short duration of the experiment, the liquid phase can be approximated as a dilute solution. The behavior of the 

jet, which initially sprays upward before reversing direction to fall back into the tank, along with the intense 

turbulence, supports the use of a fully mixed model for the liquid in the contact section. In addition, since the 

gas phase is diluted, VOC transfer within this section is minimal, resulting in negligible changes in both gas and 

liquid molar flow rates and an almost imperceptible change in liquid temperature. Therefore, the absorption 

efficiency (𝐸𝑌) is determined as follows. 

𝐸𝑌 =
𝑦𝐼𝑁 − 𝑦𝑂𝑈𝑇

𝑦𝐼𝑁 − 𝑦𝑂𝑈𝑇
∗ (3) 

 

Here, 𝑦𝐼𝑁 represents the molar fraction of VOCs in the gas phase entering the liquid jet zone, while 𝑦𝑂𝑈𝑇 is the 

VOC molar fraction in the gas phase after passing through the jet. The term 𝑦𝑂𝑈𝑇
∗  refers to the equilibrium VOC 

molar fraction in the gas phase, corresponding to the liquid phase composition exiting the jet zone. This 

formulation accounts for both the actual VOC removal and the theoretical equilibrium limit, providing a measure 

of the jet's absorption efficiency relative to the maximum possible absorption. Under the assumption of dilute 

mixture, the VOC water solubility can be described by Henry's law. The Henry constant was calculated 

considering for the IPA H0 = 130 mol/(kg bar) at 25 °C and a temperature dependence d(ln(H))/d(1/T) = 7500; 

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instead for the BA H0 = 3.6 mol/(kg bar) at 25 °C and a temperature dependence d(ln(H))/d(1/T) = 6000 (NIST, 

2024). 

Figure 2 (left) shows the trend of absorption efficiency as a function of the F-factor (defined as the product 

between the gas superficial velocity and the square root of the gas density, 𝐹𝐹 = 𝑢𝑠,𝐺√𝜌𝐺) for various liquid flow 

rates. As the gas flow rate increases, corresponding to a larger F-factor, the absorption efficiency decreases. A 

high gas flow rate causes the liquid jet to compress into a region of reduced height, which results in a significant 

reduction in the contact time between the gas and liquid phases. Varying the liquid flow rate, on the other hand, 

has different effects. Increased liquid flow enhances jet turbulence, which promotes better mixing and a larger 

interfacial area for mass transfer. This, combined with increased phase contact time due to the more vigorous 

jet behavior and the extension of the jet over a larger zone, leads to improved absorption efficiencies. Efficiency 

trends are similar for both VOCs; however, it is noteworthy that the removal efficiencies of BA are consistently 

lower than those of IPA under the same liquid flow rate and F-factor conditions. 

The gas phase was selected as the reference phase for the mass transfer modeling. The molar flux of VOC 

transferred between the two phases (𝑁𝑉𝑂𝐶) was modeled as the product of the overall gas side mass transfer 

coefficient (𝐾𝑂𝑌), contact area (𝐴) and a driving force. Equating this to the molar flux expressed in terms of gas 

concentration drop across the jet, an analytical expression for the product of the mass transfer coefficient and 

the interfacial area is derived (Green, 2008). 

 

𝑁𝑉𝑂𝐶 = 𝐺(𝑦𝐼𝑁 − 𝑦𝑂𝑈𝑇) = 𝐾𝑂𝑌𝐴Δ𝑦𝐿𝑀 (4) 

 

𝐾𝑂𝑌𝐴 = 𝐺
𝑦𝐼𝑁 − 𝑦𝑂𝑈𝑇

Δ𝑦𝐿𝑀

(5) 

 

Where 𝐺 is the molar gas flow rate and Δ𝑦𝐿𝑀 the logarithmic mean driving force. Consistent with the absorption 

efficiency analysis, a completely mixed model is used to describe the liquid in the contact section, and liquid in 

the tank is considered well mixed. 

The trends of 𝐾𝑂𝑌𝐴 as a function of the F-factor for different liquid flow rates are shown in Figure 2 (right). The 

results show that 𝐾𝑂𝑌𝐴 experiences a significant increase with higher liquid flow rates, indicating the degree of 

increase in absorption surface area in the contact zone. The 𝐾𝑂𝑌𝐴 values for butyl acetate were significantly 

lower than those for isopropanol under comparable operating conditions. This result is consistent with 

expectations and is primarily attributable to the lower water solubility of BA. The lower solubility leads to a 

decrease in the overall mass transfer coefficient, which includes mass transfer resistances in both the liquid and 

gas phases. Specifically, the lower solubility of BA in water limits its diffusion and subsequent transfer across 

phase boundaries. 

 

 
Figure 2: Mass transfer performance results (left: absorption efficiency, right: mass transfer coefficient). 

 

The measurement of the gas pressure drop allowed for the definition of the scrubber operating window; results 

are shown in Figure 3 (left). Values are shown as a function of the F-factor and for different liquid flow rates. As 

expected, the pressure drops increase with the liquid flow rate. While for the same value of liquid flow rate, 

higher F-factors, corresponding to higher gas flow rates result in greater pressure drops. Interestingly, the 

proposed compact scrubber shows lower pressure drops compared to static mixer solutions commonly used in 

industrial applications (Couvert, 2008). The major contribution to pressure drop is from the liquid jet. However, 

628



given the limited size of the contact zone, the pressure drops are limited. This difference is mainly due to the 

main feature of the scrubber, since there is no impeding to the gas flow except crossing the liquid jet. 

During pressure drop tests, it was found that for high liquid flow rates and reduced gas flow rates, the system 

enters a pulsed operating mode. In this regime the liquid tends to accumulate at the top of the jet and, when 

critical condition is reached, the liquid piston is discharged. This condition happens cyclically and can cause 

mechanical problems.  

 

 
Figure 3: Pressure drops test results (left: pressure drops, right: ratio between absorption efficiency and gas 

power loss) 

 

The power dissipated by the gas in passing through the liquid jet can be expressed as the product between the 

gas volumetric flow rate and the pressure drop experienced by the gas passing through the liquid jet (Δ𝑝). 

 

𝑃𝑔𝑎𝑠 = 𝐺𝑉 ⋅ Δ𝑝 (6) 

 

As expressed in equation (6), the power dissipated in the system is directly proportional to the gas flow rate and 

the pressure drop, which in turn is affected by both the gas and liquid flow rates. Figure 3 (right) illustrates the 

relationship between scrubbing efficiency per unit of power and the F-factor. Notably, the data for both test 

systems converge on a single curve under all experimental conditions. This convergence demonstrates that the 

absorption efficiency per unit power has a consistent proportional dependence on the gas flow rate, regardless 

of the specific system configuration or test conditions. This observation suggests a fundamental principle 

governing scrubber operation, with significant implications for design optimization. By integrating this trend with 

additional data on pressure drop and operating gas flow rates, a more complete understanding of scrubber 

performance can be achieved. This insight facilitates the development of advanced design and operation 

strategies aimed at maximizing the efficiency of compact scrubbers for enhanced removal of gaseous pollutants. 

4. Conclusions 

This study investigates a compact scrubber with the goal of establishing optimized design criteria for this 

technology, focusing on pollutant removal performance. In order to define the operating ranges of the equipment 

and to improve the mass transfer efficiency, an experimental setup was designed and constructed. Fluid 

dynamic characterization was performed by measuring the gas pressure drop across the liquid jet and by visual 

inspection of the jet. The experimental results showed that the pressure drops increased with higher gas and 

liquid flow rates. Pulsation in the system was observed when the scrubber was operated at high liquid flow rates 

and low gas flow rates, which was attributed to cyclic jet behavior. Mass transfer tests were conducted using 

two different test systems to evaluate the absorption efficiencies. The experiments demonstrated high 

absorption efficiencies, with values up to 90% for highly soluble VOCs. The mass transfer within the jet was 

modeled, and the total mass transfer coefficients of the compact scrubber were determined. The results showed 

that the product of the total mass transfer coefficient and the interfacial area (𝐾𝑂𝑌𝐴) increased with liquid and 

gas flow rates. 

This study highlights critical relationships that govern the performance of compact scrubbers. As shown by the 

data in Figure 3, the absorption efficiency per unit power exhibits a consistent proportional dependence on gas 

flow rate, regardless of system configuration or test conditions. This trend underscores a fundamental principle 

of scrubber operation with significant implications for design optimization. The critical role of gas pressure drops 

629



across the liquid jet in influencing scrubbing efficiency further emphasizes the importance of optimizing the 

relationship between absorption efficiency per unit power and volumetric gas flow. These findings not only 

provide a basis for mass transfer characterization, but also lead the way to the development of advanced design 

strategies. By integrating the observed trends with further studies of mass transfer mechanisms, compact 

scrubbers can be better designed to maximize pollutant removal while minimizing energy consumption. This 

has direct implications for industrial applications, where enhanced scrubber efficiency can lead to lower 

operational costs, reduced environmental impact, and improved compliance with air quality regulations. Future 

research could focus on refining design parameters and exploring alternative liquid-gas configurations to further 

enhance performance and adaptability in diverse industrial settings. 

Nomenclature

𝐴 – interfacial area, m2 

𝐸 – entrainment, - 

𝐸𝑌 – absorption efficiency, - 

𝐹𝐹 – F-factor, Pa0.5 

𝑔 – gravity acceleration, m/s2 

𝐺 – gas molar flowrate, kmol/s 

𝐺𝑉 – gas volumetric flowrate, m3/s 

H – Henry constant, mol/(kg bar) 

𝐾𝑂𝑌 – overall mass transfer coefficient, kmol/m2s 

𝐿𝑉 – gas volumetric flowrate, m3/s 

𝑁𝑉𝑂𝐶 – molar flux, kmol/s 

𝑃𝑔𝑎𝑠 – gas power dissipation, kW 

𝑢𝑠,𝐺 – gas superficial velocity, m/s 

𝑈𝐹𝑙𝑜𝑜𝑑 – flooding velocity, m/s 

𝑦𝐼𝑁 – gas inlet molar fraction, - 

𝑦𝑂𝑈𝑇 – gas outlet molar fraction, - 

𝑦𝑂𝑈𝑇
∗  - gas outlet equilibrium molar fraction, - 

Δ𝑝 – gas pressure drops, Pa 

Δ𝑦𝐿𝑀 – logarithmic mean driving force, - 

𝜌𝐺 – gas density, kg/m3 

𝜌𝐿 – liquid density, kg/m3 

𝜎 – surface tension, N/m 

Acknowledgments 

The authors acknowledge the financial support of the Regione Toscana for the TRAFUCER project (Innovative 

process for treating gaseous effluents from ceramics production) through the fund PR FESR TOSCANA 2021-

27, AZ 1.1.4 (Bando 2) (CUP: 7717.29122023.043000343). 

References 

Brunazzi E., Paglianti A., Pintus S., 2001, A capacitance probe and a new model to identify and predict the 

capacity of columns equipped with structured packings, 40 (4), 1205-1212 

Couvert A., Sanchez C., Laplanche A., Renner C., Levasseur J.-P., 2008, Design of a new compact scrubber 

for deodorisation, Chemical Engineering and Processing: Process Intensification, 47 (9-10), 1793-1798 

Danzomo B. A., Salami M. J. E., Jabirin S., Khan M. R., Nor I. M., 2012, Performance evaluation of wet scrubber 

system for industrial air pollution control, Engineering Environmental Science, 1669–1677 

EEA, Air pollution, European Environment Agency, <https://www.eea.europa.eu/en/topics/in-depth/air-

pollution>, accessed 13.12.2024 

Giustacori P., Brunazzi E., Characterization of a Reverse Jet Scrubber for gas/liquid absorption, 2025 Chemical 

Engineering Science, 308, 121391. 

Green, D.W., Perry R.H., Perry’s Chemical Engineers’ Handbook, Eighth Edition McGraw Hill professional, 

McGraw-Hill Education, 2008 

Lamprecht H., Burger A., 2018, Desorption of Isobutyl Acetate into Air as a Low-Cost Alternative System for the 

Measurement of Liquid Phase Mass Transfer Coefficients, Chemical Engineering Transactions, 69, 7-12 

Long N.V.D., Lee D.Y., Kwag C., Lee Y.M., Lee S.W., Lewis D., Rebrov E., Hessel V., Lee M., 2025, Removal 

of marine NOx, SOx, and CO2 from flue gas: Simulation and experiment on a pilot plant, Separation and 

Purification Technology, 359 

Marchini S., Vishwakarma V., Schubert M., Brunazzi E., Hampel U., 2021, Direct Tray and Point Efficiency 

Measurements Including Weeping Effects through a Convenient Add-On for Air-Water Simulators, Industrial 

& Engineering Chemistry Research, 60 (6), 2600–2612 

NIST, NIST Chemistry WebBook, <https://webbook.nist.gov/chemistry/>, accessed 13.12.2024 

Park, K. L., 2006, Prediction for particle removal efficiency of a reverse jet scrubber. Journal of Aerosol Science, 

1826-1839 

Roy D. H., 1962, Method for contacting fluids in countercurrent, Domtar Inc, U.S. Patent 3,350,075. 

Steinmeyer D., 1995, Use power/mass to estimate drop size in gas/liquid contactors, Chemical engineering 

progress, 91, 72–80 

Yao H., Hansen M.J., Feilberg A., 2014, DMS removal in a bubble reactor by using peroxone (O3/H2O2) 

reactions, Chemical Engineering Transactions, 40 (Special Issue), 229-234 

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