Thin-film Composite Hollow Fiber Membranes Research Articles

High-Degree Concentration Organic Solvent Forward Osmosis for Pharmaceutical Pre-Concentration.

Over half of the pharmaceutical industry's capital investments are related to the purification of active pharmaceutical ingredients (APIs). Thus, a cost-effective purification process with a highly concentrated solution is urgently required. In addition, the purification process should be nonthermal because most APIs and their intermediates are temperature-sensitive. This study investigated a high-degree concentration organic solvent forward osmosis (OSFO) membrane process. A polyketone-based thin-film composite hollow fiber membrane with a polyamide selective layer on the bore surface was used as the OSFO membrane to achieve a high tolerance for organic solvents and an effective concentration. MeOH, sucrose octaacetate (SoA), and 2M polyethylene glycol 400 (PEG-400)/MeOH solution were used as the solvent, model API, and a draw solution (DS), respectively. OSFO was performed at room temperature (23 ± 3 °C). Consequently, the 11 wt% SoA/MeOH solution was concentrated to 52 wt% without any SoA leakage into the DS. To our knowledge, there are no studies in which up to a 5 wt% concentration by OSFO has been demonstrated. However, the final feed solution contained 17 wt% PEG-400. This study demonstrates the promising potential of OSFO for pharmaceutical pre-concentration and the technical problems that need to be solved for social implementation.

Tailoring the pore size of intermolecular cross-linked PIMs-based thin-film composite hollow fiber membranes using different length cross-linkers for organic solvent nanofiltration

Tailoring the pore size of intermolecular cross-linked PIMs-based thin-film composite hollow fiber membranes using different length cross-linkers for organic solvent nanofiltration

Incorporation of an Intermediate Polyelectrolyte Layer for Improved Interfacial Polymerization on PAI Hollow Fiber Membranes.

In a single-step spinning process, we create a thin-walled, robust hollow fiber support made of Torlon® polyamide-imide featuring an intermediate polyethyleneimine (PEI) lumen layer to facilitate the integration and covalent attachment of a dense selective layer. Subsequently, interfacial polymerization of m-phenylenediamine and trimesoyl chloride forms a dense selective polyamide (PA) layer on the inside of the hollow fiber. The resulting thin-film composite hollow fiber membranes show high NaCl rejections of around 96% with a pure water permeability of 1.2 LMH/bar. The high success rate of fabricating the thin-film composite hollow fiber membrane proves our hypothesis of a supporting effect of the intermediate PEI layer on separation layer formation. This work marks a step towards the development of a robust method for the large-scale manufacturing of thin-film composite hollow fiber membranes for reverse osmosis and nanofiltration.

Inner-selective polyethersulfone-polydimethylsiloxane (PES-PDMS) thin film composite hollow fiber membrane for CO2/N2 separation at high pressures

Carbon dioxide (CO2) is the dominant greenhouse gas (GHG) and thus CO2 capture from flue gas of power plants, the main anthropogenic source of emission, is a key approach to mitigating the climate change. Thin-film composite (TFC) membranes have received extensive research attention recently as a promising post-combustion CO2 capture (CO2/N2) technology owing to its energy-efficiency, design versatility and upscaling potential. However, studies with the most industrially relevant membrane configuration, the hollow fibers, are rather limited for this application. Herein, we developed an inner-selective polyethersulfone-polydimethylsiloxane (PES-PDMS) TFC hollow fiber membrane by inner coating PDMS in the lumen of PES hollow fibers and then pressurizing the hollow fibers for CO2/N2 separation. The optimal PES-PDMS TFC membrane can achieve a CO2 permeance of 2150 GPU and a CO2/N2 selectivity of 20 when the operating pressure was between 20–22 bar, which holds good potential for post-combustion CO2 capture. The high separation performance arises from a combination of the stretched PES and PDMS layers. The micro-deformation of both layers under high pressures at the lumen side could possibly reduce the selective layer thickness. It may also induce polymer chain orientation, increase the fractional free volume, and potentially create free volume with smaller sizes, thus enhancing the CO₂ permeability and CO₂/N₂ selectivity from the PDMS and PES layers.

Effect of Molecular Weight and Chemical Structure of Terminal Groups on the Properties of Porous Hollow Fiber Polysulfone Membranes

For the first time, polysulfones (PSFs) were synthesized with chlorine and hydroxyl terminal groups and studied for the task of producing porous hollow fiber membranes. The synthesis was carried out in dimethylacetamide (DMAc) at various excesses of 2,2-bis(4-hydroxyphenyl)propane (Bisphenol A) and 4,4'-dichlorodiphenylsulfone, as well as at an equimolar ratio of monomers in various aprotic solvents. The synthesized polymers were studied by nuclear magnetic resonance (NMR), differential scanning calorimetry, gel permeation chromatography (GPC), and the coagulation values of 2 wt.% PSF polymer solutions in N-methyl-2-pyrollidone were determined. According to GPC data, PSFs were obtained in a wide range of molecular weights Mw from 22 to 128 kg/mol. NMR analysis confirmed the presence of terminal groups of a certain type in accordance with the use of the corresponding monomer excess in the synthesis process. Based on the obtained results on the dynamic viscosity of dope solutions, promising samples of the synthesized PSF were selected to produce porous hollow fiber membranes. The selected polymers had predominantly -OH terminal groups and their molecular weight was in the range of 55-79 kg/mol. It was found that porous hollow fiber membrane from PSF with Mw 65 kg/mol (synthesized in DMAc with an excess of Bisphenol A 1%) has a high helium permeability of 45 m3/m2∙h∙bar and selectivity α (He/N2) = 2.3. This membrane is a good candidate to be used as a porous support for thin-film composite hollow fiber membrane fabrication.

Intermolecular cross-linked polymer of intrinsic microporosity-1 (PIM-1)-based thin-film composite hollow fiber membrane for organic solvent nanofiltration

Organic solvent nanofiltration (OSN) is a membrane-based organic solution separation process that has emerged as a viable alternative to energy- and solvent-intensive separation processes. Polymers of intrinsic microporosity (PIMs) are the targets of recent active research on OSN membrane materials because of their unique physicochemical properties, such as solution processability, chemical tunability, robust porosity, and high thermal stability. However, applying PIMs-based membranes in practical OSN applications is still challenging because of their limited organic solvent stability and inevitable decrease in performance. Meanwhile, despite the potential of thin-film composite hollow fiber membranes for OSN, only a few studies have been reported to date. In this study, we fabricated a PIMs-based thin-film composite hollow fiber membrane by dip-coating and improved its organic solvent stability by intermolecular cross-linking. The membrane exhibited pure ethanol permeance of 10.9 LMH/bar and a molecular weight cut-off of 952 g/mol in ethanol. The membrane displayed stable dye rejection performance (>95%) even after immersion in ethanol, acetone, and toluene for seven days. This study demonstrates that intermolecular cross-linking between PIMs polymer chains can improve the organic solvent stability of PIMs-based membranes and provides a desirable strategy to fabricate feasible organic solvent stable PIMs-based thin-film composite hollow fiber membranes for OSN applications.

Industrial scale thin-film composite membrane modules for salinity-gradient energy harvesting through pressure retarded osmosis

The current work highlights development of novel thin-film composite hollow fiber membranes for pressure retarded osmosis applications to harvest salinity gradient energy. The membranes were developed with a specific target of harnessing the salinity gradient energy between wastewater reverse osmosis retentate and seawater reverse osmosis brine via osmotic mixing. The hollow fiber membranes were prepared by coating a polyethersulfone substrate with a thin-film composite polyamide layer via interfacial polymerization, which were assembled into modules of different diameters for lab scale and pilot scale evaluation. As the module size increased from 1-in. to 2-in., and 4-in., the water permeability, tested against a 1000 mg/L sodium chloride solution at 15 bar, decreased from 2.6 L m−2 h−1 bar−1 to 2.0 L m−2 h−1 bar−1, and 1.2 L m−2 h−1 bar−1, respectively. The power density, measured in the lab-scale unit using 1 M sodium chloride draw solution, and DI water feed solution, also decreased from 9.1 W/m2 to 5.3 W/m2 with increasing module size. Pilot scale evaluation of the 4-and 8-in. modules on a 24 m3/day unit resulted in lower power densities of 2.5 W/m2 and 1.5 W/m2 which translated to 0.024 kWh/m3 and 0.05 kWh/m3 of salinity-gradient energy harvested, respectively.

Hollow fiber composite membranes of poly(paraterphenyl-3-bromo-1,1,1-trifluoroacetone) and PVA/glycine for ethanol dehydration

Thin film composite hollow fiber membranes (TFC-HFMs) usually have permeate flux of less than 2 kg m−2 h−1 for pervaporation (PV) dehydration of ethanol, as well as complicated and pragmatic preparation process, limiting their application in the PV. In this study, the TFC-HFMs with excellent PV performance are fabricated on newly-developed poly(paraterphenyl-3-bromo-1,1,1-trifluoroacetone) (PTB) hollow fiber supports. The PTB hollow fiber supports exhibit low mass transfer resistance as a result of high pure water permeance of 1.6 × 103 L m−2 h−1 bar−1, high porosity of 63.8% and mean pore size of 2.53 μm. Furthermore, self-assembly sandwich structure dense-selective layers are formed on the lumen side of PTB hollow fiber supports via heterostructured evolution that derives from a mixture of polyvinyl alcohol (PVA) and glycine (Gly). The resulting TFC-HFMs display the excellent separation performance. Especially, the TFC4.1-HFMs show a high permeate flux of 3.49 kg m−2 h−1, 91.1 wt% water in the permeate and excellent stability for dehydrating 85 wt% ethanol aqueous solution at 50 °C. The newly developed TFC-HFMs should have a wide application in the PV dehydration of ethanol due to simple preparation progress and efficient separation.

Metal-assisted multiple-crosslinked thin film composite hollow fiber membrane for highly efficient bioethanol purification

Polyamide (PA)-based thin film composite (TFC) membranes have been extensively applied in various membrane-based separations. However, research works for bioethanol purification are relatively limited due to insufficient hydrophilicity and chain packing density of the PA selective layer by conventional interfacial polymerization. In this work, a new TFC hollow fiber (HF) membrane is fabricated by metal-assisted multiple crosslinking strategy, that is, metal-coordinated crosslinking of the pristine PA layer and coordination complexation between metal ions and phytic acid (PhA) for advanced water/ethanol separation. By tuning coordination abilities of metal ions with PA and PhA and the cycle number of PhA/metal ion assembly, PA-based TFC HF membrane is perfected. Additionally, a joint experiment/simulation characterization is deployed to reveal the coordination abilities of various metal ions with the pristine PA and grafted PhA. Ultimately, the synergistic effect between PA layer and PhA-metal ion complexes (i.e., 1 + 1 > 2 effect) is embodied in the separation performance of water/ethanol separation. The resulting TFC-Fe3+-PhA membrane with the strongest PhA-metal ion coordination ability exhibits the best water/ethanol separation performance, i.e., the separation factor of 1703 (34 times higher than pristine PA TFC HF membrane) and total flux of 1705 g/m2 h. This metal-assisted multiple crosslinking strategy is therefore believed to shed important lights on the development of next-generation high-performance TFC membranes for bioethanol purification.

Polyether-block-amide thin-film composite hollow fiber membranes for the recovery of butanol from ABE process by pervaporation

• PEBA thin-film composite hollow fiber were fabricated by coating. • Pervaporation tests were carried out with membranes of different active layer thicknesses. • Good performance of membranes for recovery of butanol from ABE mixtures. • Membrane support has a relevant contribution to mass transfer resistance. This work reports the continuation of previous efforts to recover butanol from the ABE (acetone-butanol-ethanol) fermentation process by pervaporation (PV). A key aspect to improve the efficiency of the technology is the membrane used to perform the selective butanol separation; hence, this study focuses on the implementation of hollow fiber (HF) membrane configuration for the ABE separation by PV as opposed to flat sheet membrane configuration. The HF membrane preparation was done by dip coating, a frequently used process for the production of HF membranes, which involves the deposition of a thin film of a coating solution. Different thicknesses of the active layer were obtained by modifying the polymer content in the coating solution, allowing later to evaluate the influence of the thickness on the separation performance. This study includes a description of the procedure to prepare selective membranes, its characterization and an analysis of the influence of operating conditions on membrane separation performance. SEM and water contact angle were used to characterize the produced membranes. The mass transport phenomena in the pervaporation process were characterized using a resistances-in-series model. The results allow to adopt a criterion to select the most suitable thickness for the membrane active layer, which allows to achieve an adequate separation performance, and reveal the importance in the choice of the membrane support material. Finally, a comparative analysis of the self-made hollow fiber membranes performance in terms of flux, separation factor and PSI with respect to those found in the literature is presented.

Fabrication of thin-film composite hollow fiber membranes in modules for concentrating pharmaceuticals and separating sulphate from high salinity brine in the chlor-alkali process

Nanofiltration (NF) has been widely used for sulphate removal in chlor-alkali industries and concentrating ingredients in pharmaceutical industries, but the fabrication and applications of thin-film composite (TFC) NF hollow fiber membranes and modules are still lagged behind those NF membranes in other configurations. In this work, the pressure-assisted interfacial polymerization (IP) is employed to fabricate TFC NF hollow fiber membranes in 1-inch modules through tailoring the reaction conditions. By firstly circulating a piperazine (PIP) solution in the lumen side under a positive pressure and then circulating a trimesoyl chloride (TMC) solution at atmosphere pressure, a strong, uniform and defect-free polyamide selective layer is formed on the inner surface of hollow fiber membranes. The optimal TFC NF hollow fiber membranes in 1-inch modules exhibit a pure water permeability of 8.93 L m−2 h−1 bar−1 and a molecular weight cut-off (MWCO) of 175 Da. They also show a rejection of 98.3% to MgSO4 and a high rejection of around 98% to cephalexin (molecular weight = 437 g mol−1). The optimal membranes exhibit a consistent Na2SO4 rejection of ∼85% and nearly no NaCl rejection over a 30-day performance test using a high salinity feed solution containing 200 g/L NaCl and 10 g/L Na2SO4. The impressive performance suggests that the newly developed TFC NF hollow fiber membrane has great potential to be used in the chlor-alkali and pharmaceutical industries.

Modified Graphene Oxide-Incorporated Thin-Film Composite Hollow Fiber Membranes through Interface Polymerization on Hydrophilic Substrate for CO2 Separation

Thin-film composite mixed matrix membranes (CMMMs) were fabricated using interfacial polymerization to achieve high permeance and selectivity for CO2 separation. This study revealed the role of substrate properties on performance, which are not typically considered important. In order to enhance the affinity between the substrate and the coating solution during interfacial polymerization and increase the selectivity of CO2, a mixture of polyethylene glycol (PEG) and dopamine (DOPA) was subjected to a spinning process. Then, the surface of the substrate was subjected to interfacial polymerization using polyethyleneimine (PEI), trimesoyl chloride (TMC), and sodium dodecyl sulfate (SDS). The effect of adding SDS as a surfactant on the structure and gas permeation properties of the fabricated membranes was examined. Thin-film composite hollow fiber membranes containing modified graphene oxide (mGO) were fabricated, and their characteristics were analyzed. The membranes exhibited very promising separation performance, with CO2 permeance of 73 GPU and CO2/N2 selectivity of 60. From the design of a membrane substrate for separating CO2, the CMMMs hollow fiber membrane was optimized using the active layer and mGO nanoparticles through interfacial polymerization.

Ultra-strong polymeric hollow fiber membranes for saline dewatering and desalination

Osmotically assisted reverse osmosis (OARO) has become an emerging membrane technology to tackle the limitations of a reverse osmosis (RO) process for water desalination. A strong membrane that can withstand a high hydraulic pressure is crucial for the OARO process. Here, we develop ultra-strong polymeric thin film composite (TFC) hollow fiber membranes with exceptionally high hydraulic burst pressures of up to 110 bar, while maintaining high pure water permeance of around 3 litre/(m2 h bar) and a NaCl rejection of about 98%. The ultra-strong TFC hollow fiber membranes are achieved mainly by tuning the concentration of the host polymer in spinning dopes and engineering the fiber dimension and morphology. The optimal TFC membranes display promising water permeance under the OR and OARO operation modes. This work may shed new light on the fabrication of ultra-strong TFC hollow fiber membranes for water treatments and desalination.

Thin-film composite hollow-fiber nanofiltration membranes prepared from benzonitrile containing disulfonated poly(arylene ether sulfone) random copolymers coated onto polyphenylene oxide support membranes

A new method for the preparation of chemically robust thin-film composite hollow-fiber (TFC-HF) nanofiltration membranes through a dip-coating process is reported. A negatively charged disulfonated poly(arylene ether sulfone) random copolymer (SPN-20) containing highly polar benzonitrile groups was utilized as a separation layer in a TFC-HF membrane. Only four polar aprotic solvents, N-methyl-2-pyrrolidone (NMP), dimethyl acetamide (DMAc), dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO), proved to be good solvents for SPN-20 possibly due to its rigid chemical backbone and strong intermolecular interactions. In order to avoid dissolution or irreversible swelling of the polymeric supports in the dip-coating process, a polyphenylene oxide (PPO) hollow-fiber membrane was introduced as a novel substrate for TFC-HF fabrications. Solubility investigation using a framework of Hansen solubility parameters (HSPs) revealed that the PPO membrane is a suitable substrate for coating the SPN-20 copolymer due to its tolerance of polar aprotic solvents. By using DMSO as a coating solvent for the SPN-20 copolymer, TFC-HF membranes were successfully prepared with neither dissolution nor any damage to PPO supports. The dip-coating and drying processes were analyzed in detail to optimize the thickness and separation performance of TFC-HF membranes. The resultant membranes showed excellent rejection of Na2SO4 (>98%) and an almost 100% rejection of negatively charged dyes along with a reasonable level of pure water flux (1.0–10.3 [L∙m−2∙h−1∙bar−1]).

High loading and high-selectivity H2 purification using SBC@ZIF based thin film composite hollow fiber membranes

Membrane-based separation systems have been widely explored for hydrogen production and purification in the development of next-generation fuels. In this work, we propose a thin film composite hollow fiber membrane (TFCHFM) for H2 purification, incorporating poly(styrene-co-butadiene) copolymers (SBC, 4% butadiene, for flexibility and ease of processing) and ZIF-8 nanoparticles. The TFC was supported on a PEI-based hollow fiber substrate. SBC containing a flexible butadiene segment showed enhanced compatibility through π-π interactions. The morphology of ZIF-8 was altered from a cubic shape to a smaller spherical shape by altering the molar ratio of NH4+/Zn2+. The gas separation mechanism of the ZIF-8 based TFC membrane was predicted by the Maxwell model. In the binary gas permeation test (5 vol% H2), the H2 permeance and H2/CH4 selectivity of TFCHFMs were 6.85 GPU and 152.20, respectively, with 50 wt% ZIF-8 loading. We believe that the high loading weight (50 wt% ZIF-8) and H2/CH4 selectivity presented here are unprecedented for ZIF-8 based TFC membranes reported in the literature.

Optimization of interfacial polymerization to fabricate thin-film composite hollow fiber membranes in modules for brackish water reverse osmosis

The development of commercial-scale thin-film composite (TFC) hollow fiber membranes and modules for reverse osmosis (RO) has been lagging behind spiral wound TFC flat-sheet membranes and modules for many years. The delay arises from the challenges of synthesizing a defect-free polyamide selective layer on hollow fibers in modules. In order to solve this problem, a novel interfacial polymerization method is presented in this work to fabricate 1-inch TFC hollow fiber membrane modules for RO applications. By circulating the m-phenylenediamine (MPD) solution on the lumen side under a positive pressure and circulating the trimesoyl chloride (TMC) solution at the atmosphere pressure, a strong, uniform and defect-free polyamide selective layer is formed on the inner surface of hollow fiber membranes in modules. Under pressure, not only MPD monomers are evenly distributed on the inner surface of each hollow fiber substrate in modules, but also sufficient MPD monomers can retain in the substrate pores for further reaction with TMC monomers. The best RO performance obtained by this newly developed TFC hollow fiber membrane module has a water permeability of 4.49 L m−2 h−1 bar−1 and a NaCl rejection of ~99%, using a feed solution of 1000 ppm NaCl under a transmembrane pressure of 20 bar. The novel interfacial polymerization method may provide useful insights to fabricate TFC hollow fiber membrane modules for desalination of brackish water.

Optimization of TFC-PES hollow fiber membranes for reverse osmosis (RO) and osmotically assisted reverse osmosis (OARO) applications

The osmotically assisted reverse osmosis (OARO) has been recently proposed to increase the water recovery of the reverse osmosis (RO) process, in which RO has reached its limitations. Strong membranes are essential for both OARO and RO processes. Here, we have developed high mechanical strength thin-film composite (TFC) hollow fiber membranes for both RO and OARO processes. The newly developed TFC hollow fiber membranes consist of a polyamide layer synthesized via interfacial polymerization on the inner surface of PES hollow fiber substrates that have been optimized by controlling the bore and dope fluid flow rates, fiber dimension and morphology. The TFC-PES hollow fiber membranes have a pure water permeability (PWP) of around 2.5–3 L/(m2 h bar) (LMH/bar) and a NaCl rejection of around 97.5–98% for brackish water desalination at 20 bar. The water permeability drops from 2.15 to 0.06 LMH/bar when the NaCl concentration increases from 0.035 mol/L (2000 ppm) to 1.2 mol/L at 30 bar for OARO due to the concentrative and dilutive concentration polarization (ECP and ICP) that decreases the effective driving force. The structural parameter (S) and burst pressure (PB) of the newly developed membranes increase from 550 to 800 μm and from 47 to 104 bar, respectively, with an increase in the dope to bore flow rate ratio (D/B ratio) because of the thicker substrate wall and reduction in porosity. The optimal TFC-PES hollow fiber membrane for OARO has a burst pressure of 95 bar, structural parameter of 795 μm and water permeability of 0.09 LMH/bar when using 1.2 mol/L NaCl for OARO. To the best of our knowledge, this inner-selective TFC-PES hollow fiber membrane has the highest burst pressure to date, it has impressive high mechanical strength and good RO and OARO performance.

Three-phase hybrid facilitated transport hollow fiber membranes for enhanced CO2 separation

The configuration of thin film composite (TFC) in the form of hollow fiber is desired for gas separation membranes to achieve better gas permeation and higher packing density. In this work, we developed and tested TFC hollow fiber membranes with a defect-free, ultrathin (200 nm) hybrid facilitated transport selective layer consisting of three phases, i.e., a host polymeric matrix with fixed-site carriers, a 2D inorganic filler, and, a CO2-philic mobile carrier. The effect of lateral size of graphene oxide (GO)-based fillers on CO2 permeation were studied in detail, and the modified size-optimized porous GO (pGO) fillers were found to enhance CO2 permeation at a very low loading of 0.2 wt%. The optimized hybrid materials were then combined with selected mobile carriers, which interact with CO2 reversibly to form carbonate/carbene-CO2 adduct to further enhance the CO2 permeation performance. The resulting hybrid facilitated transport membranes with mobile carriers showcase a CO2 permeance of up to 825 GPU with a CO2/N2 separation factor of 31 and a CO2/CH4 of 20. These membranes also exhibit increased resistance to carrier saturation phenomena typical of facilitated transport membranes, showing potential for CO2 separation applications also at elevated pressures.

PIM-1/PAN Thin-Film Composite Hollow Fiber Membrane as Structured Packings for Isopropanol (IPA)/Water Distillation

In this study, novel polymers of intrinsic microporosity (PIMs)-based hollow fiber structured packings were prepared by a facile pour-coating method and successfully applied as structured packings ...

Thin-film composite hollow fiber membranes incorporated with graphene oxide in polyethersulfone support layers for enhanced osmotic power density

This study focused on the development of pressure retarded osmosis (PRO) thin film composite (TFC) membranes for enhanced osmotic power using hollow fiber polyethersulfone (PES) support structure modified by incorporating hydrophilic graphene oxide (GO) nanosheets. The GO loadings in the hollow fiber substrates were varied to improve water flux performances without compromising the mechanical strength. GO embedded (≤0.2 wt%) PES hollow fiber supports revealed noticeable improvements in pure water permeability, improved structural morphologies, as well as the hydrophilicity within the support layer, without deteriorating the mechanical properties. The GO (0.2 wt%)-incorporated TFC-PRO membrane appeared to have an initial PRO flux (without any applied pressure) of 43.74 L m−2 h−1, lower specific reverse salt flux of 0.04 g L−1 and structural parameter (S) of 522 μm, significantly better than the control membrane. The maximum power density of 14.6 W m−2 was achieved at an operating pressure of 16.5 bar under the condition of DI water and 1 M NaCl as feed and draw solutions, respectively. The results obtained in this study indicate that modification of PRO hollow fiber support layer by incorporating nanoparticles such as GO nanosheet can be a useful tool to improve the PRO performance.