Single-component Adsorption Isotherms Research Articles

Fabricating a Robust Ultramicroporous Metal-Organic Framework for Purifying Natural Gas and Coal Mine Methane.

Purifying methane (CH4) from natural gas and coal mine methane (CMM) is of great significance but challenging in the chemical industry. Herein, a robust ultramicroporous metal-organic framework (MOF) is reported, which can be synthesized on a gram scale by stirring under room temperature. Single-component adsorption isotherms of gases (CH4, ethane (C2H6), propane (C3H8), nitrogen (N2)) and breakthrough experiments indicate that the MOF can separate CH4 efficiently from CH4/C2H6/C3H8 ternary mixture, with super high purity-CH4 production of 154.7 cm3g-1. Additionally, the MOF shows higher CH4 capacity than N2, resulting in excellent separation performance for the CH4/N2 mixture. Notably, the binding sites of gases can be precisely determined by single-crystal X-ray data, further confirmed by molecular simulation. It is found that there are multiple hydrogen bonds and C─H···π interactions between the gases and the framework. This work offers an excellent candidate material for CH4 purification with both high capacity and separation efficiency.

Facile shaping of flexible MIL-53(Al) for effective separation of propylene over propane

A facile granulation approach using polyvinyl butyral (PVB) as a polymer binder was developed to control the textural properties of the flexible metal–organic framework MIL-53(Al). The large pore (LP) structure of MIL-53(Al) contracted after granulation due to a strong interaction between MOF and polymer binder. This contraction enhances propylene's selectivity over propane at saturation pressure (region II), as confirmed by single-component adsorption isotherms. The adsorption kinetics showed faster adsorption of C3H6 compared to C3H8 over time in the case of the granules, while no difference was observed for the powder sample. Breakthrough experiments successfully separated C3H6 over C3H8 using MIL-53(Al)-B10% granules, whereas the pristine MIL-53(Al) powder did not separate the C3H8-C3H6 gas mixture. The breakthrough finding reaffirms that the observed gate-opening effect in MIL-53(Al) within pressure region I is ineffective for separating a mixed gas system containing propane and propylene. The constrained pores of the shaped granules could facilitate strong π-π stacking interactions with C3H6 and decrease the diffusion rate for C3H8 molecules. These findings indicate that the simple granulation approach can modulate the textural properties of flexible MIL-53(Al) and improve its selectivity for C3H6/C3H8 separation resulting from increased MOF-propylene affinity and kinetic factors.

Reversed CO2/C2H2 separation with ultrahigh carbon dioxide adsorption capacity in Zn-based pillared metal-organic frameworks

The removal of CO2 impurities from CO2/C2H2 gas mixtures to obtain high-purity acetylene used for manufacturing of various commodity chemicals, is very important for the chemical purifying of acetylene. However, CO2 and C2H2 exhibit analogous physical properties as well as molecular sizes causing their separation to become an industrially challenging process, and developing energy-efficient CO2-selective adsorbents is a class of materials of vast value. In this work, a pillared-layer ultramicroporous metal-organic framework (MOF) constructed from zinc (Ⅱ) and triazolium planar layers with oxalic acid, which has been termed as CALF-20, can preferentially absorb CO2 from C2H2. Unique pore environment and electrostatic effects enable this material foremost capture CO2. CALF-20 shows higher affinities towards CO2 than C2H2 based on heats of adsorption computed from single-component adsorption isotherms. Grand Canonical Monte Carlo simulations confirm the affinity interactions and multiple host-guest interactions between CO2 and CALF-20. Sorption experiments revealed that CALF-20 has excellent trapping volume with extremely high CO2 uptake capacity (140.2 cm3 cm−3).

Separation of high-purity propylene through propane-selective CAU-3 isomorphs

The development of a suitable C3H8-selective adsorbent has been of interest to achieve high energy efficiency. Herein, we report an energy-efficient C3H8-selective CAU-3 BDC and CAU-3-NDC adsorbent. The separation performance was evaluated using a single-component adsorption isotherm, and preferential C3H8 uptake over C3H6 was observed, revealing their C3H8-selective features. To demonstrate the separation ability, a breakthrough experiment was performed on CAU-3-BDC with a 1/15 (v/v) C3H8/C3H6 gas mixture, and the high-purity C3H6 (>99.95%) can be produced during multi-cycle operation. Further, the binding affinity of the C3H8 molecule was determined using computational calculations.

Ideal Adsorbed Solution Theory (IAST) of Carbon Dioxide and Methane Adsorption Using Magnesium Gallate Metal-Organic Framework (Mg-gallate)

Ideal Adsorbed Solution Theory (IAST) is a predictive model that does not require any mixture data. In gas purification and separation processes, IAST is used to predict multicomponent adsorption equilibrium and selectivity based solely on experimental single-component adsorption isotherms. In this work, the mixed gas adsorption isotherms were predicted using IAST calculations with the Python package (pyIAST). The experimental CO2 and CH4 single-component adsorption isotherms of Mg-gallate were first fitted to isotherm models in which the experimental data best fit the Langmuir model. The presence of CH4 in the gas mixture contributed to a lower predicted amount of adsorbed CO2 due to the competitive adsorption among the different components. Nevertheless, CO2 adsorption was more favorable and resulted in a higher predicted adsorbed amount than CH4. Mg-gallate showed a stronger affinity for CO2 molecules and hence contributed to a higher CO2 adsorption capacity even with the coexistence of a CO2/CH4 mixture. Very high IAST selectivity values for CO2/CH4 were obtained which increased as the gas phase mole fraction of CO2 approached unity. Therefore, IAST calculations suggest that Mg-gallate can act as a potential adsorbent for the separation of CO2/CH4 mixed gas.

Engineering Metal-Organic Frameworks for Selective Separation of Hexane Isomers Using 3-Dimensional Linkers.

Metal-organic frameworks (MOFs) are highly tunable materials with potential for use as porous media in non-thermal adsorption or membrane-based separations. However, many separations target molecules with sub-angstrom differences in size, requiring precise control over the pore size. Herein, we demonstrate that this precise control can be achieved by installing a three-dimensional linker in an MOF with one-dimensional channels. Specifically, we synthesized single crystals and bulk powder of NU-2002, an isostructural framework to MIL-53 with bicyclo[1.1.1]pentane-1,3-dicarboxylic acid as the organic linker component. Using variable-temperature X-ray diffraction studies, we show that increasing linker dimensionality limits structural breathing relative to MIL-53. Furthermore, single-component adsorption isotherms demonstrate the efficacy of this material for separating hexane isomers based on the different sizes and shapes of these isomers.

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Highly Selective Separation of C2H2/CO2 and C2H2/C2H4 in an N-Rich Cage-Based Microporous Metal-Organic Framework

The separation of acetylene (C2H2) from carbon dioxide (CO2) and the purification of ethylene (C2H4) from C2H2 are quite essential processes for the chemical industry. However, these processes are challenging due to their similar physical properties, including molecule sizes and boiling points. Herein, we report an N-rich cage-based microporous metal-organic framework (MOF), [Cd5(Tz)9](NO3) (termed as Cd-TZ, TZ stands for tetrazole), and its highly efficient separation of C2H2/CO2 and C2H2/C2H4. Single-component gas adsorption isotherms reveal that Cd-TZ exhibits high C2H2 adsorption capacity (3.10 mmol g-1 at 298 K and 1 bar). The N-rich cages in Cd-TZ can trap C2H2 with a higher isosteric heat of adsorption (40.8 kJ mol-1) than CO2 and C2H4 owing to the robust host-guest interactions between the noncoordinated N atoms and C2H2, which has been verified by molecular modeling studies. Cd-TZ shows a high IAST selectivity for C2H2/CO2 (8.3) and C2H2/C2H4 (13.3). The breakthrough simulations confirm the potential for separating C2H2/CO2 and the purification of C2H4 from C2H2.

Constructing C2H2 anchoring traps within MOF interpenetration nets as C2H2/CO2 and C2H2/C2H4 bifunctional separator

Efficient separation of C2H2/CO2 and C2H2/C2H4 is necessary and challenging for industrial processes. The traditional method of pore sieving has great limitations for separating gases with similar characteristics. Herein, we report a new case (UPC-80) with the help of C2H2 anchoring traps constructed by 4-fold interpenetration nets for gas separation. The single-component adsorption isotherms indicate that UPC-80 adsorbs more C2H2 than C2H4 and CO2, resulting in high adsorption selectivities of 6.34 and 4.78 for C2H2/CO2 and C2H2/C2H4 at 298 K, respectively. Laboratory-scale fixed-bed breakthrough experiments verify the efficient separation ability of UPC-80 toward the C2H2/CO2 and C2H2/C2H4 gas mixtures, remaining separation stability with negligible capacity loss. Theoretical simulations confirm that the good separation performance can be attributed to the anchoring of C2H2 by two Tb2(COO)6 clusters in adjacent interpenetration nets, as well as the strong electrostatic interaction. UPC-80 makes the first exploration to obtain specific C2H2 binding sites by interpenetrated networks, becoming an excellent candidate as porous adsorbent for separating C2H2/CO2 and C2H2/C2H4.

On the Role of Flexibility for Adsorptive Separation.

Chemical separations, mostly based on heat-driven techniques such as distillation, account for a large portion of the world's energy consumption. In principle, differential adsorption is a more energy-efficient separation method, but conventional adsorbent materials are still not effective for many industry-relevant mixtures. Porous coordination polymers (PCPs), or metal-organic frameworks (MOFs), are attractive for their well-defined, designable, modifiable, and flexible structures connecting to various potential applications. While the importance of the structural flexibility of MOFs in adsorption-based functions has been demonstrated, the understanding of this special feature is still in its infancy and mostly stays at the periodic structural transformation at the equilibrium state and the special shapes of single-component adsorption isotherms. There are many confusions about the categorization and roles of various types of flexibility. This Account discusses the role of flexibility of MOFs for adsorptive separation, mainly from the thermodynamic and kinetic points of view.As the classic type of framework flexibility, guest-driven structural transformations and the corresponding adsorption isotherms can be thermodynamically described by the energies of the host-guest system. The highly guest-specific pore-opening action showing contrasting single-component adsorption isotherms is regarded as a strategy for achieving molecular sieving without the need for aperture size control, but its effect and role for mixture separation are still controversial. Quantitative mixture adsorption/separation experiments showed that the common periodic (cooperative) pore-opening action leads to coadsorption of molecules smaller than the opened aperture, while the aperiodic (noncooperative) one can achieve inversed molecular sieving under a thermodynamic mechanism.The energy barrier and structure in the nonequilibrium state are also important for flexibility and adsorption/separation. With suitable energy barriers between metastable structures, new types of framework flexibility such as aperture gating can be realized. While kinetically controlled gating flexibility is usually ignored because of the difficulty of characterization or considered as disadvantageous for separation because of the variable aperture size, it plays a critical role in most kinetic separation systems, including adsorbents conventionally regarded as rigid. With the concept of gating flexibility, the meanings of aperture and guest sizes for judging molecular sieving need to be reconsidered. Gating flexibility depends on not only the host itself but also the guest, the host-guest interaction, and the external environment such as temperature, which can be rationally tuned to achieve special adsorption/separation behaviors such as inversed temperature dependence, molecular sieving, and even inversed thermodynamic selectivity. The comprehensive understanding of the thermodynamic and kinetic bases of flexibility will give a new horizon for next-generation separation materials beyond MOFs and adsorbents.

Temperature Effects in Flexible Adsorption Processes for Amorphous Microporous Polymers.

A collection of atomistic molecular simulations is reported that illustrate the impact of adsorption temperature on species uptake and adsorbate-induced structural rearrangement for amorphous polymers of intrinsic microporosity. Temperature-sensitive structural rearrangement is evaluated by contrasting two methods: standard grand canonical Monte Carlo simulations using a rigid framework approximation and a combined Monte Carlo/molecular dynamics approach that fully incorporates framework flexibility. We report single-component gas phase adsorption isotherms for CH4, C2H4, C2H6, C3H6, C3H8, and CO2 across a temperature range of 250-400 K for models of an archetypal polymer of intrinsic microporosity, PIM-1. A quadratic model is presented that captures two main mechanisms of temperature-dependent adsorption-induced deformation of PIM-1 up to a relative swelling of 1.15: thermal expansion and an increased propensity to swell as a function of species uptake. Two case studies are reported that highlight the critical role of operating temperature in industrial storage and separation applications. The first study focuses on methane storage and delivery applications using a pressure-temperature swing adsorption application (PTSA). We demonstrate that larger working capacities are accompanied by increased volumetric strain between adsorption-desorption steps. The second case study considers PIM-1 as an adsorbent to separate an exemplar ternary syngas mixture at operating temperatures ranging 300-550 K. A temperature threshold of ∼400 K is identified, beyond which adsorption-induced PIM-1 swelling is negligible and the solubility selectivity-loading curve transitions to exhibiting a nearly linear relationship.

Modeling the effect of humidity and temperature on VOC removal efficiency in a multistage fluidized bed adsorber

The competitive adsorption of volatile organic compounds (VOCs) and water vapor onto beaded activated carbon (BAC) in a fluidized bed adsorber is developed in the form of a two-phase model using the Manes method. The Manes method uses as input single component adsorption isotherms described by the Langmuir model for adsorption of VOCs and the Qi-Hay-Rood (QHR) model for adsorption of water vapor. The effect of temperature is accounted for using the linear van’t Hoff relationship for Langmuir affinity coefficient. The effects of relative humidity (RH) and temperature in the simulations are validated using experimental data, with very good agreement observed (e.g. R2 = 0.98 comparing overall removal efficiencies). Humidity begins to have an effect on the adsorption of 1,2,4-trimethylbenzene (TMB) on BAC starting at 70% RH in the form of a reduction in overall removal efficiency due to fewer available adsorption sites. The overall removal efficiency decreases from 93.0% at 70% RH to 85.4% at 90% RH, and then plateaus with further increase in RH up to 100%. In dry air, temperature variation has a small effect on removal efficiency, showing a reduction in modeled overall removal efficiency from 93.0% to 90.2% when the adsorption temperature increases from 22 to 50 ˚C. On the other hand, at high RH values, temperature has a larger and positive effect on removal efficiency due to RH change. Increasing the temperature of a stream of humid air (RH = 95%) from 22 to 27 °C increases the TMB removal efficiency by 6.9% from 85.3% to 92.2%. Taking into account the effect of humidity and temperature, the model can be used to help optimize fluidized bed adsorber operations in industrial applications.

Stable titanium metal-organic framework with strong binding affinity for ethane removal

Abstract Direct separation of high purity ethylene (C2H4) from an ethane (C2H6)/ethylene mixture is a critical and challenging task owing to the very similar molecular size and physical properties of the two components. While some studies have attempted this separation, there is a lack of excellent porous materials with strong binding affinity for C2H6-selective adsorption via an energy-efficient adsorptive separation process. Herein, we report a titanium metal-organic framework with strong binding affinity and excellent stability for the highly efficient removal of C2H6 from C2H6/C2H4 mixtures. Single component adsorption isotherms demonstrated a larger amount of adsorbed ethane (1.16 mmol·g−1 under 1 kPa) and high C2H6/C2H4 selectivity (2.7) for equimolar C2H6/C2H4 mixtures, especially in the low-pressure range, which is further confirmed by the results of grand canonical Monte Carlo simulations for C2H6 adsorption in this framework. The experimental breakthrough curves showed that C2H4 with a high purity was collected directly from both 1:9 and 1:15 C2H6/C2H4 (volume ratio) mixtures at 298 K and 100 kPa. Moreover, the unchanged adsorption and separation performance after cycling experiments confirmed the promising applicability of this material in future.

Efficient separation of C4 olefins using tantalum pentafluor oxide anion-pillared hybrid microporous material

With the increasing demand for synthetic rubber, the purification of 1,3-butadiene (C4H6) is of great industrial significance. Herein, the successful removal of n-butene (n-C4H8) and iso-butene (iso-C4H8) from 1,3-butadiene (C4H6) was realized by synthesizing a novel TaOF52− anion-pillared ultramicroporous material TaOFFIVE-3-Ni (also referred to as ZU-96, TaOFFIVE = TaOF52−, 3 = pyrazine). Single-component adsorption isotherms show that TaOFFIVE-3-Ni can achieve the exclusion of n-C4H8 and iso-C4H8 in the low pressure region (0–30 kPa), and uptake C4H6 with a high capacity of 92.78 cm3·cm−3 (298 K and 100 kPa). The uptake ratio of C4H6/iso-C4H8 on TaOFFIVE-3-Ni was 20.83 (298 K and 100 kPa), which was the highest among the state-of-the-art adsorbents reported so far. With the rotation of anion and pyrazine ring, the pore size changes continuously, which makes smaller-size C4H6 enter the channel while larger-size n-C4H8 and iso-C4H8 are completely blocked. The excellent breakthrough performance of TaOFFIVE-3-Ni shows great potential in industrial separation of C4 olefins. The specific adsorption binding sites within ZU-96 was further revealed through the modeling calculation.

Coadsorption Equilibria on Molecular Sieves 3A and Densities of Liquid Mixtures Containing Formaldehyde, Methanol, and Water at 295.15 and 313.15 K

Liquid solutions of formaldehyde, methanol, and water are reactive mixtures in which many oligomeric species are formed. The coadsorption equilibrium from such mixtures on molecular sieves 3A is studied in form of isotherms at 295.15 and 313.15 K. The densities of the liquid phase are measured and reported. Single-component adsorption isotherms of water from 2-propanol on molecular sieves 3A at both temperatures are also presented. A model, which simultaneously considers the physical adsorption equilibrium and the chemical equilibrium in the bulk phase, is regressed to the experimental adsorption data of the three binary subsystems. This model predicts the adsorption from ternary mixtures with good accuracy and is used to estimate separation factors and adsorption enthalpies. Thus, it provides a solid basis for the design of adsorptive processes for drying mixtures containing formaldehyde.

An Ultramicroporous Metal-Organic Framework with Record High Selectivity for Inverse CO2/C2H2 Separation

Abstract Removal of CO2 to purify CO2/C2H2 mixtures through porous material is a more energy saving method compared with the traditional cryogenic distillation. Those CO2-selective adsorption porous materials are ideal for directly producing high-purity C2H2, especially allowing sieving separation with infinite selectivity but without sacrificing any uptake capacity. Here, we report an ultramicroporous metal–organic framework (MOF) [Cu(hfipbb)(H2hfipbb0.5)] (1), [H2hfipbb is 4,4′-(hexafluoroisopropylidene) bis(benzoic acid)] has been studied for inverse separation of CO2 from C2H2 under ambient conditions. This Cu-based MOF material was comprehensively demonstrated as an efficient CO2-selective adsorbent used for C2H2 purification by single-component adsorption isotherms, showing promise for industrial acetylene purification.

Modeling a biogas upgrading PSA unit with a sustainable activated carbon derived from pine sawdust. Sensitivity analysis on the adsorption of CO2 and CH4 mixtures

Pressure Swing Adsorption (PSA) is one of the implemented technologies for removing carbon dioxide in biogas streams. Different adsorbents, mostly zeolite-based, and process configurations have been patented and commercially demonstrated. In this study, we have developed a numerical model to successfully describe the dynamic performance of biomass-derived activated carbon in biogas purification. It is the first step in designing a biomass-based carbon capture unit within the bioenergy and circular economy context.Microporous activated carbon pellets prepared from pine sawdust by physical activation with CO2 was the adsorbent material choice. The model was built with the fittings of single-component adsorption isotherms of CO2 and CH4 at different temperatures to the Langmuir-Freundlich model and the Ideal Adsorbed Solution Theory (IAST) to account for multicomponent adsorption. The kinetics of mass transfer in the solid phase was described by the Linear Driving Force model (LDF).The dynamic simulations were performed with the aid of the commercial software Aspen Adsorption and experimental data previously obtained in the laboratory used for the model validation [1]. The model was applied to address the separation performance of a biogas upgrading biomass-based PSA process by running a parametric study to determine the influence of key performance parameters. The sensitivity analysis concluded that a single stage 4-step PSA can produce methane with a purity above 95% and a recovery of around 60% in a configuration with P/F ratios (quotients of molar flows of CH4 in the purge and the feed streams) between 0.67 and 1 for an adsorption pressure of 3 bar.

Bi-functionalized ionic liquid-grafted mesoporous alumina: Synthesis, characterization and CO2/N2 selectivity

In this work, short-chain bi-functionalized ionic liquid (IL) was grafted onto mesoporous alumina (MA) for ideal CO2/N2 selectivity. First, bi-functionalized IL 1-(2-carboxymethyl)-3-methoxyethyl-imidazolium bromide ([CmMOEim][Br]) with ether and carboxyl group was successfully synthesized and the initial decomposition temperature is 165 °C. Then low-cost hydroxyl-MA (MA-OH) was obtained from ordered MA (OMA) with the assist of 30 wt% H2O2 solution to prepare [CmMOEim][Br]-grafted MA with different molar ratios of [CmMOEim][Br] and MA-OH via facile chemical reaction without the introduction of alkoxysilanes. MA-OH material presents slit-like structure and more hydroxyl groups compared to OMA sample before hydroxylation. Crystal structure and morphology of grafted material remains unchanged after grafting process. [CmMOEim][Br] is successfully grafted onto MA-OH via the chemical link of carboxyl group on IL and hydroxyl group on MA-OH. Finally, CO2 and N2 capture capacities of different materials were separately evaluated at 0.1 MPa and 25 °C. With the increase of IL dosage, CO2 capture capacity of the grafting material first enhances and then decreases while N2 capture capacity gradually falls owing to the high CO2 affinity of [CmMOEim][Br]. Wherein, [CmMOEim][Br]-grafted MA-0.15 exhibits the optimum CO2 capture capacity of 45.3 cm3/g at 25 °C and ambient pressure. The ideal CO2/N2 selectivity of [CmMOEim][Br]-grafted MA can reach up to 30 based on single-component adsorption isotherms, which is higher than that of [CmMOEim][Br] and MA-OH. Furthermore, CO2 capture capacity, morphology and structure of [CmMOEim][Br]-grafted MA maintain well even after ten cycles.

Overloading behavior of fenoprofen and naproxen as two model compounds on a non-porous silicon pillar array column

In this study, the adsorption behavior of naproxen and fenoprofen as two model compounds on a non-porous pillar array column (NPAC) was investigated under reverse phase liquid chromatography conditions. Band profiles of both analytes were recorded in overloaded concentrations using 30% methanol/water (v/v) as the mobile phase. Breakthrough experiments under the same chromatographic condition were carried out to measure the adsorption isotherms. Single-component adsorption isotherm data were acquired by frontal analysis for each analyte. The isotherms were found to be concave upward and downward for naproxen and fenoprofen, respectively. To find the best agreement between the experimental data points and the adsorption isotherm models, the obtained isotherms were modeled using several isotherm models. The Langmuir-Freundlich and anti-Langmuir models provided the best fitting for fenoprofen and naproxen, respectively. The solute and stationary phase properties determine the appropriate model. Adsorbate–adsorbate interaction is important in the case of naproxen, while the adsorbate- adsorbent (stationary phase) plays the main role in retention of fenoprofen on the NPAC. The validity of the selected isotherm models were checked by comparing calculated and experimental band profiles and plate heights. An excellent agreement was observed for the whole concentration range of both analytes, which confirmed the accuracy of the selected models.

Process intensification in binary adsorption of VOC-water vapor on zeolite in a countercurrent fluidized bed adsorber

In this study, fast ideal adsorbed solution theory was coupled with a two-phase bubbling bed approach to describe the adsorption of volatile organic compounds (VOCs) on zeolite in the presence of water vapor in a multistage fluidized bed adsorber. The binary adsorption of VOC-water vapor was predicted using their single component adsorption isotherms. The model was verified by experimental data obtained at a wide range of operating conditions before being used for studying the process intensification. Validation tests revealed that the model could accurately predict the experimental removal efficiencies (R2 = 0.94), as well as VOC concentration profiles inside the bed (R2 = 0.98). The intensification simulations showed that increasing the adsorbent feed rate is effective when there is a need for more adsorption sites (e.g. at high inlet concentrations), and is quite ineffective when the adsorption process is limited by low solid-gas contact time (e.g. high air flow rates). Increasing the adsorbent feed rate can also diminish the interference of water vapor in adsorption of VOC (even at RH as high as 75%). Reducing air flow rate at constant VOC load is always effective specially when there are enough adsorption sites available (e.g. high adsorbent feed rate, and low VOC loads and RHs). Similarly, increasing the number of stages can effectively improve the fluidized bed performance at high adsorbent feed rates and low VOC inlet concentrations and RHs. Using 3 adsorbers of 2 stages instead of 1 adsorber of 6 stages can improve the removal efficiency up to 34.5% in the range of operating conditions simulated. While having the same high weir height along the bed yields better performance than having the same low weir height, an optimized arrangement of weir heights in a descending order from the top to the bottom of the bed would maximize the removal efficiency.