Hybrid Membrane Process Research Articles

Membrane-based technologies for per- and poly-fluoroalkyl substances (PFASs) removal from water: Removal mechanisms, applications, challenges and perspectives.

Water purification from per- and poly-fluoroalkyl substances (PFASs), as a group of persistent and mobile fluoro-organic contaminants, is receiving increasing attention worldwide due to the ubiquitous presence of these highly toxic compounds. To reduce the risk of exposure of human life to PFASs and their dispersion in the environment, various techniques, primarily based on membrane technologies, have been rapidly developed. Here we critically review and analyze the current state-of-the-art of membrane-based techniques for PFASs removal, including direct membrane filtrations, adsorption-based membranes, and hybrid membrane processes. Membranes performance, treatment efficiencies, characteristic parameters and mechanisms for PFASs removal are discussed in detail. We highlight and discuss advantages and limitations, as well as challenges and prospects of individual membrane-based PFASs treatments, pointing towards the practical and sustainable application of these technologies.

A hybrid ultrafiltration/nanofiltration/pervaporation membrane process for intensifying the refining of crude canola oil and solvent recovery

Conventional edible oil processing methods with all their drawbacks have not changed over the past decades. Therefore, the main novelty of this study is to develop a three-stage hybrid membrane process to remove phospholipids, pigments, and solvent from the crude canola oil miscella by combining ultrafiltration, nanofiltration, and pervaporation processes. For the first and second stages, ultrafiltration and nanofiltration membranes were prepared by phase inversion method using polyethersulfone as polymer, and triethylene glycol and polyethylene glycol as additives. Pervaporation for the first time was conducted for solvent recovery, using commercial polydimethylsiloxane composite membranes with different active layer thicknesses. Almost full rejection of phospholipids (>99.9%) was obtained by addition of both additives, and approximately remained constant by changing the additive concentration. A 91.49% reduction in the color of crude oil with a flux of 1.17 L/m2.h was obtained at the end of the bleaching stage. The solvent recovery by the pervaporation process with the polydimethylsiloxane composite membranes showed the 100% oil rejection with acceptable flux. The proposed hybrid ultrafiltration/nanofiltration/pervaporation membrane process showed an excellent performance in terms of permeation flux, phospholipids rejection, and color reduction for refining crude canola oil and solvent recovery processes and meets the criteria of process intensification.

On optimisation of N2 and CO2-selective hybrid membrane process systems for post-combustion CO2 capture from coal-fired power plants

A superstructure-based optimisation model for the CO2 capture process using an N2-selective and CO2-selective membranes is presented. The model automatically selects either of the membranes for each membrane stage to have a hybrid membrane CO2 capture process. The superstructure embeds numerous process routes and allows the collection of product streams and residue streams from either the retentate side or permeate side of the multi-membrane stages. This results in novel process flowsheets for post-combustion CO2 capture using membrane technology. In the study, the optimisation of carbon capture by an all N2-selective membrane process is also carried out. At 90% recovery and 95% product purity, the multi-stage optimisation of the N2–CO2 hybrid membrane process reduces the total membrane area by 46% and the cost of capture by 14% compared to the optimised CO2-selective membrane process.

Modeling and fouling control in a hybrid membrane process using CuO-catalytic membrane coupled to ozone

Membrane filtration methods are prone to significant fouling, which lowers the efficiency of the operation. Hybrid techniques combining ozone and membrane filtration are promising for removing dissolved contaminants and reducing membrane fouling. However, a detailed knowledge of fouling processes is necessary to incorporate these technologies in large-scale water and wastewater treatment plants. Although fouling models for conventional membrane filtration systems are well established in the literature, modeling data for hybrid systems (combining membrane/advanced oxidative processes) with antifouling features are scarce. Therefore, a ceramic α-Al2O3/ZrO2 commercial membrane with a molecular weight cut off of 10 kDa was functionalized applying layer-by-layer deposition of CuO for application in a hybrid system (membrane/catalytic ozonation) to treat contaminated surface water. The findings show that CuO-membranes can mitigate membrane fouling during hybrid processes, enhancing total filtration efficiency in the treatment of surface water. A fitting model that incorporated Hermia’s law as well as surface and bulk reactions that contribute to fouling removal is proposed and was successfully applied. The model allowed parameters to be estimated for the coated and uncoated membranes in the presence or absence of ozone. Membrane reusability was also investigated.

Reuse of textile wastewater for dyeing cotton knitted fabric with hybrid treatment: Coagulation/sand filtration/UF/NF-RO

The aim of this study was to investigate the usefulness of a membrane hybrid process for the treatment of real textile wastewater (TWW) and its potential reuse in the dyeing of cotton knitted fabric (DCF) process. To determine a suitable pretreatment, sand filtration, coagulation, and UF hollow fiber (UF–HF) were compared on a laboratory scale in terms of turbidity, color, and total organic carbon (TOC). Here, UF-HF provided the best removal results of 93.6%, 99.0%, and 29.0%, respectively. The second stage involves the study of UF flat sheet membranes (5, 10, 20, and 50 kDa). The 5 kDa membrane provided the best permeate quality according to the chemical oxygen demand (COD), turbidity, TOC, conductivity, and color by 54.5%, 83.9%, 94.2%, and 45.7–83.3%, respectively. The final step was treatment with nanofiltration (NF) and reverse osmosis (RO) and these effluents were reused for dyeing. Finally, the effluents from UF-HF/5 kDa UF/RO (Scenario 1) and UF-HF/5 kDa UF/NF (Scenario 2) were analyzed for turbidity, COD, TOC, biological oxygen demand, conductivity, hardness, anions and cations, and color. Both scenarios provided high removal results of 76.3–83.5%, 94.6–97.7%, 88.5–99%, 95.4–98.0%, 59.2–99.0%, 88.7–98.7%, 60.7–99.1%, and 80.0–100%, respectively. They also satisfied the DCF tests compared to the standard DCF samples. The innovative aspect of this research is as follows: 1) the complete analysis of hybrid membrane separation processes for the purpose of reuse of treated textile wastewater and 2) the proposal of a new criterion for reuse for DCF.

Insights into colloidal membrane fouling mechanisms for nanofiltration of surface water using single and hybrid membrane processes

Colloidal membrane fouling mechanisms were modeled for experimental data provided for nanofiltration of surface water by single and hybrid membrane systems. The hybrid system is composed of coagulation/activated carbon adsorption and bare polyethersulfone or mixed‐matrix membranes. The comprehensive modeling of these membrane systems was performed by three explicit equations to evaluate the effects of combined cake‐filtration and pore‐blocking on the fouling processes. These equations were derived from Darcy's law applied under constant operating pressure. The results showed that by using the nonlinear regression method, the combined cake‐filtration/pore‐blocking models demonstrated more accurate fits to the experimental permeate volume data compared to individual blocking models. The accuracy achieved by such nonlinear fits is expressed in terms of a mean squared error of approximately 10−10 (m3/m2)2 and R2 > 0.999. Furthermore, the impact of a pretreatment stage and the type of membrane used to prevent or reduce the fouling are also investigated. Experimental results reveal that the colloidal fouling depends on the type of membrane and the characteristics of raw water and is not only related to the cake filtration.

Sustainable recovery of high-saline papermaking wastewater: Optimized separation for salts and organics via membrane-hybrid process

In this work, a novel zero liquid discharge technology combining of electrodialysis (ED), electrochemical catalytic oxidation (ECO), reserve osmosis (RO) and bipolar membrane electrodialysis (BMED) was developed for sustainable recovery of high-saline papermaking wastewater. The effect of membrane type, volume ratio, membrane fouling and voltage were investigated in ED process; subsequently parameter such as electrolyte conductivity was studied in ECO process. The FKB/FAB stack shows good separation performance for the salts and organics, allowing a relatively high COD rejection of 97.80%. Meanwhile, membrane fouling was investigated through a comparison between virgin and fouled membranes. To achieve a high-saline solution with low organics, a second-stage ED process was applied. The concentration factor was increased to 1.799 with a COD rejection of 96.54% at voltage of 12 V. Simultaneously, the ECO process exhibited superior COD removal of 94.99% at the electrolyte conductivity of 4 mS/cm. Through the application of BMED at Vf:Vb:Va (volume ratio of feed, base and acid solution) of 4:1:1, the final acid and base content could approach a high value of 12.30 wt% and 10.60 wt%, respectively. Therefore, this hybrid ED-ECO-RO-BMED combined process demonstrates a strongly technical applicability for sustainable resource recovery from high-saline papermaking wastewater.

Nanofiltration for separation and purification of saccharides from biomass

Saccharide production is critical to the development of biotechnology in the field of food and biofuel. The extraction of saccharide from biomass-based hydrolysate mixtures has become a trend due to low cost and abundant biomass reserves. Compared to conventional methods of fractionation and recovery of saccharides, nanofiltration (NF) has received considerable attention in recent decades because of its high selectivity and low energy consumption and environmental impact. In this review the advantages and challenges of NF based technology in the separation of saccharides are critically evaluated. Hybrid membrane processes, i.e., combining NF with ultrafiltration, can complement each other to provide an efficient approach for removal of unwanted solutes to obtain higher purity saccharides. However, use of NF membrane separation technology is limited due to irreversible membrane fouling that results in high capital and operating costs. Future development of NF membrane technology should therefore focus on improving material stability, antifouling ability and saccharide targeting selectivity, as well as on engineering aspects such as process optimisation and membrane module design.

Inline dosing of powdered activated carbon and coagulant prior to ultrafiltration at pilot-scale – Effects on trace organic chemical removal and operational stability

Hybrid membrane processes such as inline dosing of powdered activated carbon (PAC) prior to ultrafiltration membranes (UF) have already shown promising potential for the abatement of trace organic chemicals (TOrCs). However, questions regarding the optimization of the operational stability by the employment of coagulation and its interferences with inline dosed PAC, have not yet been comprehensively investigated. Within the scope of this pilot-scale study, inline dosing of different sized PAC types at different dosages was combined with or without the addition of polyaluminium chloride (PACl) coagulant prior to UF. As expected, when PAC was not employed, negligible TOrC removal was observed, whereas all the operational modes with the application of PAC inline dosing showed significant TOrC removal. Coagulation with PACl clearly reduced the build-up of transmembrane pressure, especially owing to maintaining hydraulic backwash efficiency. The operational mode of precoating the UF with PACl combined with continuous inline dosing of PAC exhibited particularly good TOrC removal results along with optimized membrane fouling mitigation. In contrast, the simultaneous and continuous dosing of PAC and PACl is not recommended, in particular owing to detrimental effects of the coagulant on TOrC removal efficiency by PAC.

Clarification of Citrus fruit (Mosambi) juice by hybrid (Pretreatment and Membrane) process

Clarify citrus fruit mosambi (sweet lime) juice demand were increased in non-seasoning and pandemic situation due to high content of vitamin C and for enhance immunity. Membrane separation is the unique process to clarify the raw juice and store it for prolonged duration at appropriate environments. Low permeates flux and high fouling rate with operating time is the major drawback in membrane separation process. Enhance permeates flux and fouling was improved by pretreatment of raw juice before membrane separation process. Common pretreatment namely, centrifugation and fining agent (single or combination of two) were applied previously for clarification of juice. Detailed studies for pretreatment of sweet lime juice were done individually. On the basis of these pretreatment method estimated the value of optimum rpm for centrifugation machine and dose concentration in fining agent pretreatment. At optimum conditions in different pretreatments process, membrane operation was coupled to achieve clarified juice from pretreatment juice. Dead end filtration cell (batch cell) with material of stainless steel at capacity of 1000 ml was used for clarification of juice. Polymeric (polyamide) membrane with average pore size 2.5 µm at operating pressure 69 kPa was used to conduct the experiment. Physicochemical properties of raw, pretreated and clarified juice were compared each other’s.

Application of Hybrid Membrane Processes Coupling Separation and Biological or Chemical Reaction in Advanced Wastewater Treatment.

The rapid urbanization and water shortage impose an urgent need in improving sustainable water management without compromising the socioeconomic development all around the world. In this context, reclaimed wastewater has been recognized as a sustainable water management strategy since it represents an alternative water resource for non-potable or (indirect) potable use. The conventional wastewater remediation approaches for the removal of different emerging contaminants (pharmaceuticals, dyes, metal ions, etc.) are unable to remove/destroy them completely. Hybrid membrane processes (HMPs) are a powerful solution for removing emerging pollutants from wastewater. On this aspect, the present paper focused on HMPs obtained by the synergic coupling of biological and/or chemical reaction driven processes with membrane processes, giving a critical overview and particular emphasis on some case studies reported in the pertinent literature. By using these processes, a satisfactory quality of treated water can be achieved, permitting its sustainable reuse in the hydrologic cycle while minimizing environmental and economic impact.

Utilization of Bidirectional Cation Transport in a Thin Film Composite Membrane: Selective Removal and Reclamation of Ammonium from Synthetic Digested Sludge Centrate via an Osmosis-Distillation Hybrid Membrane Process.

Selective removal and resource recovery of ammonium nitrogen (NH4+-N) from high-strength ammonium waste streams is of practical importance for biological wastewater treatment and environmental protection. In this study, we demonstrate the simultaneous removal and reclamation of ammonium from synthetic digested sludge centrate via a novel osmosis-distillation hybrid membrane (ODHM) process. Using NaHCO3 as the draw solute, ammonium diffuses from the synthetic centrate to the draw solution by utilizing the bidirectional cation transport nature of the thin film composite (TFC) membrane. Then, NH4+ is converted to gaseous NH3 at 60 °C and recovered by a sweeping gas membrane distillation (SGMD) process. Herein, the bidirectional transport of monovalent cations in the osmotic process, selectivity of TFC membranes for different cations, and recovery of the draw solution following the extraction of ammonia through the SGMD process were systematically investigated. The removal of NH4+-N from the synthetic centrate achieved 21.34% during a 6-h continuous operation of the ODHM system, with ammonium fluxes through the TFC and SGMD membranes at 1.39 and 0.57 mol m-2 h-1, respectively. A secondary interfacial polymerization was proposed to further enhance ammonium transport through the TFC membrane. Results reported here highlight the potential of the ODHM process for the selective removal and reclamation of ammonium from ammonium-rich waste streams.

Hybrid neutralization and membrane process for fluoride removal from an industrial effluent

This study aims to investigate at a laboratory scale fluorides removal from an industrial wastewater having excessive F- concentration through a hybrid process combining neutralization and membrane separation. For the membrane separation operation, both Reverse Osmosis (RO) and Nanofiltration (NF) were investigated and confronted. The optimized neutralization step with hydrated lime allowed reaching fluoride removal rates of 99.1 ± 0.4 %. To simulate continuous process, consecutive batch treatments with full recirculation of membrane process brines were conducted. Despite the relatively high super saturations with respect to CaF2, no membrane cloaking was observed. The RO polishing treatment allowed decreasing the permeate fluoride concentration to 0.9 &#xb1 0.3 mg/L with a fluoride rejection rate of 93&#xb1 2% at the optimal transmembrane pressure of around 100 psi. When NF membrane was used to treat neutralization filtrate, the permeate fluoride concentration dropped to 1.1&#xb1 0.4 mg/L with a fluoride rejection rate of 88&#xb1 5% at the optimal pressure of around 80 psi. Thus, with respect to RO, NF allowed roughly 20% decrease of the driving pressure at the expense of only 5% drop of rejection rate. Both NF and RO permeates at optimal operating transmembrane pressures respect environmental regulations for reject streams discharge into the environment.

Removal of contaminants of emerging concern from real wastewater by an innovative hybrid membrane process – UltraSound, Adsorption, and Membrane ultrafiltration (USAMe®)

The low-level presence of emerging contaminants (ECs) in the environment has raised a great concern due to their persistence, chronic toxicological, and endocrine disrupting effects on terrestrial and aquatic organisms. Wastewater treatment plants (WWTPs) have become hotspots for the spread of these contaminants to the environment as conventional processes are not efficient in removing them. Thus, the integration of advanced treatment methods within the chain of WWTPs is very essential. In this study, the innovative hybrid process USAMe® which integrates ultrasound irradiation (US), adsorption (A) and membrane filtration (Me) was investigated for the removal of ECs from secondary effluents. Diclofenac, carbamazepine, and amoxicillin were selected due to their large consumption and frequent presence in the aquatic environment. All three ECs were spiked into real secondary wastewater effluent at two concentrations of 10 ppm and 100 ppb. Membrane ultrafiltration and its combination with US (USMe) or adsorption (AMe) were also studied as control tests. The hybrid combination of all the three methods in the USAMe® processes elevated the EC removals to above 99% as compared to only around 90% in the AMe process. All effluents of the hybrid USAMe® processes gave “No Effect” to D. magna, with immobilization of ≤20%. Therefore, results showed that the USAMe® process was efficient in not only removing ECs, but also in generating safe and less toxic treated effluents; thereby displaying its potential as an advanced method for wastewater treatment.

Conventional water treatment improvement through enhanced conventional and hybrid membrane processes to remove Ag, CuO and TiO2 nanoparticles mixture in surface waters

This study proposes, for the first time, to improve the most used process in water treatment, conventional coagulation (coagulation/flocculation/sedimentation, C/F/S), to maximally remove a mixture of engineered nanoparticles (ENPs) from surface waters. Thus, conventional coagulation enhanced with powdered activated carbon (C/F/S + PAC) and hybrid membrane treatment, integrating conventional C/F/S with ultrafiltration (C/F/S → UF), were optimised to maximise the removal of a mixture of ENPs (TiO2, CuO and Ag, 1:1:1) from surface waters. Results demonstrated that both optimized C/F/S + PAC and C/F/S → UF improved the removal of the metal-based ENPs in mixture, compared to conventional C/F/S, from hydrophilic natural waters with low and medium turbidity. In C/F/S + PAC treatment, Ag, Cu and Ti removal efficiencies were higher than 99%, and only ca. 3.0–2.9 µg Ag/L and 2.0–2.5 µg Cu/L were found in waters, while Ti concentrations were below the detection limit, representing a significant improvement to C/F/S performance, where in same situations the residual concentrations reached the 16 µg/L. In this treatment, PAC acted as an adsorbent and also enhanced the settleability of the aggregates (ENPs-natural organic matter (NOM)) formed during C/F. In the C/F/S → UF, at 90% of water recovery rate, Ti and Cu removal efficiencies were approximately 100%, but ca. 5.0–7.0 µg/L of Ag were found in the permeate water. After C/F/S, the Ti/Cu-NOM aggregates were removed by UF membranes, due to the size exclusion mechanism but, because of some Ag dissolution, the ionic Ag passed through the membrane pores. Overall, the processes optimisation to the ENPs removal did not affect their ability to remove other target compounds (turbidity, dissolved organic carbon, specific UV absorbance and aluminium).

Conceptual process design and simulation of membrane systems for integrated natural gas dehydration and sweetening

Subsea natural gas processing attracts increased interest due to the smaller environmental footprint. Natural gas (NG) dehydration and sweetening are the main processing steps to avoid pipeline plugging and corrosion caused by the presence of water and CO2. Triethylene glycol (TEG) and amine absorption are the commercial technologies for these applications. However, membrane technology is considered as promising solutions for alternative subsea gas processing technologies, which provides unmanned operations without the requirements for rapidly periodical maintenance. In this work, a hybrid membrane process was designed for integrated dehydration and sweetening of a saturated natural gas containing 10 mol% CO2, and the process operating parameters such as inter-stage feed and permeate pressures are investigated. The simulation results indicated that the optimal permeate pressure in the 2nd -stage unit is 4 bar, and the optimal 3rd-stage feed and permeate pressures are15bar and 2 bar, respectively. The minimum specific cost of <2.71 × 10−3 $/m3 sweet natural gas was estimated to achieve the separation requirement of <2.5 mol% CO2 in purified NG together with captured high purity CO2 (>95 mol%) for enhanced gas recovery. However, due to the relatively low water selectivity of the dehydration membranes at high pressure of 60 bar used in the simulation, the hydrocarbon loss is still quite higher. Thus, advanced membranes with high H2O/CH4 selectivity at high pressure should be pursued to promote the application of the designed membrane system for subsea natural gas dehydration and sweetening.

Hybrid membrane process for post-combustion CO2 capture from coal-fired power plant

Post-combustion CO2 capture is a promising way to reduce CO2 emissions at the coal-fired power plants. Even though several studies have been conducted regarding a wide variety of CO2 capture techniques, gas separation membrane process has shown potential as an energy-efficient, low-cost CO2 capture option. However, most of the previous researches focused only on CO2-selective membrane, and there's limited literature about the application of N2-selective membrane.In this study, both CO2-selective and N2-selective membranes are considered and relevant issues regarding the CO2-selective membrane, such as pressure ratio, attainability, and multistage configuration, are studied and extended to the N2-selective membrane. Polaris® membrane is served as a typical CO2-selective membrane; on the other hand, membrane properties of N2-selective membrane with selectivity up to 200 are considered. Hybrid membrane process applying both CO2- and N2-selective membranes for capturing at least 90% CO2 from a typical 550 MW subcritical coal-fired power plant is designed, simulated and analyzed using Aspen Plus® simulation software. The optimization and economic analysis are also applied with the cost factor defined in this work, which is a comprehensive index to evaluate the applicability of the membrane, including permeance, membrane lifetime and membrane price. The results reveal the relationship between the optimal design variables and membrane properties. Moreover, the selectivity-limited region is identified, which leads to two different dominant strategies for the cost reduction in a different region, either by the increase in selectivity or the reduction of the cost factor.

Zero liquid discharge hybrid membrane process for separation and recovery of ions with equivalent and similar molecular weights

Separation of similar ions through nanofiltration (NF) is advantageous for many different applications such as, resource recovery and the reduction of waste discharge. The production of pigments generates a solution comprising of acetate (Ac−) chloride (Cl−) ions, in which the Ac− can be profit as recovered chemical. To recover these ions in this work we propose the use of a hybrid zero liquid discharge (ZLD) system. The ions rejections in this work reached 81.8% and −7.6%, for Ac− and Cl− ions respectively. This selectivity was achieved using NF with a separation ratio of 5.9. The recovery of these chemicals is highly advantageous as it can not only significantly reduce the waste discharge for the pigment industry, but also offset the cost of water treatment. When using mechanical vapor recompression (MVR), a cost is incurred (−0.04 $·t−1) and without the use of MVR a profit is made (2.73 $·t−1). The proposed strategy in this work may shed light on the heavily polluting industry towards environmentally friendly and economical wastewater treatments.

Application of volume retarded osmosis – Low pressure membrane hybrid process for recovery of heavy metals in acid mine drainage

Recovery of heavy metals in acid mine drainage (AMD) such as Mn, Fe, Cu, Zn, As, Cd and Pb was evaluated using volume retarded osmosis and low-pressure membrane (VRO-LPM) process. In VRO-LPM process, the draw solution (DS) is regenerated by the naturally generated pressure, giving its economic value. Ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na) and Poly (sodium-4-styrenesulfonate, PSS-Na) were used and compared to determine more suitable DS in heavy metal recovery from the AMD. Forward osmosis (FO) and nanofiltration (NF) membrane were employed in VRO-LPM process, due to the low EDTA-4Na rejection (about 50%) in ultrafiltration (UF) process. For the FO part in the VRO-LPM process, PSS-Na had flux values of 0.12, 0.11 and 0.05 L m−2 h−1 and at osmotic pressure of 8.9, 12 and 13 bar, respectively. Unlike the flux values, the RSF of PSS remained at 0.01 mmol h−1 at all osmotic pressures. For EDTA-4Na, the flux values were 0.10, 0.06 and 0.04 L m−2 h−1, which are relatively higher than those of PSS-Na; and the RSF values were 0.1, 1.2, 2.2 mmol h−1, which are higher compared to those of PSS-Na. Unlike PSS-Na, RSF for EDTA-4Na increased as the concentration increases. In the NF part of the VRO-LPM process, PSS-Na had higher water flux and rejection than EDTA-4Na, and the flux and rejection both decreased with concentration for both PSS-Na and EDTA-4Na. The overall rejection in VRO-LPM process was over 95% for all heavy metal ions. Therefore, VRO-LPM process has proven its ability to be used in AMD treatment for heavy metal removal.

Amine–membrane hybrid process economics for natural gas sweetening

In this study, natural gas sweetening using the amine–membrane hybrid process was investigated by economic evaluation. The Amine–membrane hybrid system consists of a membrane section applied to reduce the bulk flow acid gases followed by the amine section to enhance gas quality. Membrane feed pretreatment operation is also considered to approach the real membrane unit. The hybrid system is economically optimized by considering both single and two-stage membrane sections applying different methane and membrane prices, feed flow rates, membrane type and presence of other impurities (N2). Economic evaluations show that the two-stage cellulose acetate (CA) membrane section, in the hybrid system, is unsuitable and a single-stage membrane hybrid system for feed with high acid gas content (about 20 mol%) is competitive with the amine sweetening unit due to the decrease in Total Operating Cost (TOC) with a small increase in Total Capital Investment (TCI). The product price increases rapidly by scale-down in the unit capacity below 100 MMSCFD feed rate. Economic assessment indicates that application of higher selective membranes than CA reduces the TOC but at the same time enhances the TCI. It seems, therefore, that the use of a semi selective membrane such as CA is beneficial.