Process intensification in the fields to separate, recycle and reuse waste through membrane technology

Abstract

Recycling and reusing wastewater from diverse industries by adopting the simple dynamics of process intensification (PI) have emerged as a promising development route for the chemical process industry due to their potential to offer innovative and sustainable alternatives. This review summarizes the routes for recycling wastewater via various processes and separation techniques, which can be implemented at different scales, such as the phenomenon scale and task scale. Recent trends in process intensification have highlighted the importance of the widespread adoption of membrane-based processes owing to their low cost, compactness, energy efficiency, modularity and sustainable operation. Various intensifying approaches such as membrane-based, reactive, and hybrid separation and the intensification of various types of membrane systems, including liquid, vapor and gas separation steps such as pervaporation and vapor permeation while covering a wide range of operations and other processes for wastewater treatment are presented in this review. According to the literature, the advantages of PI for industrial application include cost reduction, increased safety, reduced emissions and environmental footprint, and improved resource efficiency using energy and water resources more efficiently. Overall, herein, we provide a comprehensive overview of recycling and reusing steps using the process intensification route from an engineering perspective, focusing on sustainable membrane-based techniques using hybrid and integrated technology.

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Swapna Rekha Panda

Dr. Swapna Rekha Panda, an academic professional, graduated from IGIT with a B.E. and earned her MTech 2006 and Ph.D. from IIT Kharagpur, 2015. She has held key roles at institutions such as CUTM, Odisha; NITH, Himachal Pradesh; LPU, Punjab; and UPL University of Sustainable Technology, Ankleshwar, Gujarat, India. Dr. Panda, a luminary in chemical engineering, is the recipient of the “International Schlumberger Award” for two consecutive years 2016-17 and 2017-18, a recognition that saw participation from 78 countries and was sponsored by the Netherlands Government. Her international experience includes serving as an academic visitor as well as a postdoctoral visiting scholar at NUS, Singapore, (QS world ranking 10) and UTM, Malaysia. Specializing in membrane separation technology, her expertise encompasses membrane synthesis (flat & hollow fibres), anti-fouling properties, desalination, ionic separation, functionalization, and solvent separability of various industrial effluents. Noteworthy projects include contributions to the BRNS and Tata Steel projects during her doctoral research and consultancy project at UPL Ltd afterwards. Dr. Panda's prolific research is evident in her 40 publications in esteemed journals and conferences (National/international/UGC), authored one book chapter (Taylor & Francis), filed four patents (2 of them granted and 2 published). She is a member of several prestigious professional organizations such as IIChE, ISC, AIChE and IEI. Currently Dr. Panda supervises four PhD scholars, further cementing her reputation as a leading researcher in multidisciplinary fields.

Sudeep Asthana

Professor Sudeep Asthana, with 34 years of expertise, is renowned in wastewater treatment. He began his journey with a B.Tech. from HBTI, Kanpur, pursued an M.Tech. from IIT BHU Varanasi in 1992, and earned a Ph.D. from GGSIP University, New Delhi, in 2019. With 6 years in industry at GHCL and Maya Agro Products Ltd., he transitioned to teaching for 28 years at esteemed institutions such as HBTI Kanpur, Dr. K.N. Modi IET in Modi Nagar, HCST in Mathura. Currently, he shares his wealth of professional knowledge as an academician at LPU in Punjab. Mentoring 100+ B.Tech. students and supervising 2 Ph.D. candidates, he has authored 25 journal publications, and holds 2 patents as his research credential. His research spans various areas, including ion exchange resin, adsorption studies, zero liquid discharge (ZLD) systems, biological treatment processes, nanotechnology applications, wastewater purification, and resource recovery.

Krunal J. Suthar

Dr. Krunal J. Suthar is Assistant Professor at Shroff S. R. Rotary Institute of Chemical Technology, UPL University of Sustainable Technology, Ankleshwar. His research interests include process intensification, thermodynamics, modeling and simulation, properties prediction, and green solvents. Dr. Suthar has contributed to chemical engineering education and explored the potential of green solvents, focusing on their properties and applications. He earned his Ph.D. in Chemical Engineering from Nirma University, Ahmedabad. He engages in teaching, mentoring, and supervising research projects on wastewater treatment and green solvents, contributing to the academic community's growth and development.

Arvind S. Madalgi

Arvind S. Madalgi holds a B.E. in Chemical Engineering from VTU, and currently is pursuing an integrated Ph.D. in Chemical Engineering at UPL University, focusing on the treatment of industrial effluent via separation, dehydration, set-up design and its scale-up using advanced membrane technology. Alongside his academic pursuits, he serves as Assistant Vice President/Head of Process Engineering at one of India's largest chlor-alkali companies. With over 19 years of cross-functional experience, Arvind is a resourceful technologist with expertise spanning various technical domains, including technical services, manufacturing plant operations, pilot plant management, and technology transfer/assimilation. He specializes in process scale-up, pilot plant validation, and technology transfer projects within the pharmaceutical, agro-chemical, chlor-alkali, and flavor and fragrance industries.

Koshal Kishor

Dr. Koshal Kishor, certified in Chemical Engineering with a PhD from IIT Kanpur, has over 8 years of research and teaching experience at IIT Kanpur, HBTU Kanpur, UNIST South Korea, SNPIT&RC Surat, and IAR Gandhinagar. At UNIST's Energy and Chemical Engineering Department, he specializes in cell types and electrode design, optimization, stack engineering, and electrochemical characterizations for seawater batteries. His expertise includes OER and CER electrocatalysts, MEA, desalination seawater batteries, electrolysers, and fuel cells. Dr. Kishor has led projects funded by ISRO, DST, and EWP Korea, He has two patents & 14 publications in reputed journals and currently manages a fifty Lakhs SERB & IAR research grant.

Ahmad Fauzi Ismail

Professor Ahmad Fauzi Ismail, Vice Chancellor, and Founding Director of UTM's Advanced Membrane Technology Research Centre AMTEC, Malaysia, boasts an impressive academic journey, including a Ph.D. from the University of Strathclyde as a Commonwealth Academic Staff Scholarship recipient. With over 1000 journal papers, 50 book chapters, 6 authored/co-authored books, and 20 patents, his expertise is unmatched. Supervising over 70 PhD, 50 MSc students, and 10 PDFs, he has made significant contributions to academia. He has been serving as Vice President of Japan Society for the Promotion of Science (JSPS), a Qatar University Press Advisory Board member since 2018 and an International Evaluation Panel member of University of Hradec Kralove (UHK), Czech Republic, since 2020, where his influence extends globally. Prof. Fauzi's research on membrane technology addresses crucial issues such as water desalination, fuel cells, haemodialysis, wastewater treatment, and gas separation, securing substantial research grants exceeding RM 60 million.

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Water impact

Process intensification and its application for wastewater separation and purification using membrane filtration processes provide better energy savings and promising economic benefits of recycling, reuse of water to reduce cost, lower disposal expenses, and generation of potential revenue streams from recovered resources. It also has good social and community benefit in terms of health and safety. By adopting PI technology, the society will benefits in terms of better scalability, adaptability, and greener route options.

1 Introduction

The modern world encounters a complex challenge of meeting the growing demand for process upgradation, industrial globalization, new process innovation, long-term sustainability, scientific growth, and collaboration with academia for future growth on an industrial scale. It is indeed a great challenge in the process industry sector to satisfy the increasing demand for raw materials, energy, and products while adhering to the constraints of sustainable development. However, possible solutions can be found by the rational integration and implementation of new industrial, economic, environmental, and social strategies. Currently, the chemical industry is particularly engaged in finding sustainable solutions that improve production and product purity while minimizing costs, energy requirements, and the environmental footprint. In this context, the future challenges of chemical engineering involve several key aspects, including focusing on achieving higher productivity and selectivity through process intensification and multi-scale control of processes, designing innovative equipment to reduce energy consumption and improve the overall process efficiency, adopting new chemical engineering methodologies to fit end-use properties required by the customer, ensuring that products meet specific market needs, and with technological advancements, multi-scale applications of computational chemical engineering are realized, from the molecular scale to complex production scales to optimize processes and minimize their size and cost. Accordingly, process intensification (PI) provides a viable solution to all these challenges. The focus on process intensification (PI) has seen exponential growth in the past two decades and continues to be a major field of research. Process intensification strategically aims to significantly reduce equipment size, capital costs, energy consumption, and waste generation while achieving production goals; minimizing environmental impact; and increasing safety, remote control, and automation. In this regard, membrane technology is a promising approach that can contribute to achieving these strategic goals. Membrane operations have several intrinsic characteristics that make them attractive for industrial processes such as they are relatively simple to operate and maintain, offer high selectivity and permeability for specific components, can be easily integrated into existing systems, offer flexibility in scaling-up, require less energy compared to conventional separation techniques, and stable under diverse operating conditions and environmentally friendly.

There are several areas of study of PI in the chemical industry, where the main area of intensification is mass transfer, heat transfer, gas and liquid dispersion, mixing processes and separation processes.^1–8 To date, PI is known to be a method that makes industrial processes more efficient, faster, and more eco-friendly, thereby increasing the affordability of the process industry. The development of new apparatus, gadgets, and industrial devices are examples of the implementation of PI that leads to a significant improvement in performance such as speed and quality, making it a promising strategy for industrial development, as discussed in the literature.^9–11 In 2009, Van Gerven and Stankiewicz analysed the fundamentals of process improvement. Currently, in terms of approaches and principles, PI is generally an applied term used as the main development path and significant advancement zone for a synthetic procedure mainly for the chemical process industry and one of the most important prospective areas of research in chemical engineering.¹ As discussed by Van Gerven and Stankiewicz, PI reduces the energy consumption and working costs, making it particularly essential in the chemical industry, where it involves modifying a process to achieve a similar performance in a smaller size compared to conventional bigger-sized units. Ramshaw first characterized that PI achieves a dramatic reduction in volume in the chemical industry.² Ramshaw et al.² reported that PI also aims to improve the safety and efficiency of chemical processes by reducing the size of equipment and increase operational control, for example, the use of novel types of reactors, such as the oscillatory baffle reactor (OBR), can enable the conversion of batch processes to continuous processing, which can result in significant reductions in reactor size up to 100 fold, while providing greater flexibility and control.² The size and operability of equipment can be intensified through PI technology, as reported by Jensen K. F., on the topic, “Should micro reaction engineering should be small or big”? Jensen, K. E. shed light on the efficiency of the process, and one of the examples of equipment is to develop micro reactors from macro reactor types.⁶ Extremely high rates of heat transfer are feasible in micro-scale reactors, considering the isothermal states of extremely exothermic conditions. This becomes especially significant in dynamic investigations for carrying out kinetic studies. The extremely low reaction-volume/surface area ratio of small-scale reactors make them attractive for processes involving toxic/poisonous/explosive/dangerous reactants as an intensified process route compared to the conventional techniques.^4,12 Zheyu Jiang and colleagues explored the concept of intensified multicomponent distillation using a ‘heat mass Integration’ strategy (HMI).¹³ HMI reduced the number of condensers and reboilers used for multicomponent multiple distillation towers, thereby minimizing the heat duty for both exchangers. Additionally, replacing conventional distillation with divided wall columns (DWC) resulted in a 30% reduction in capital costs an actual example of process intensification.¹³ Hui Ding and colleagues successfully enhanced the classical reactive distillation by incorporating microwaves. Their method, microwave-assisted reactive distillation (MRD), showed significant improvements in terms reaction time and product purity for the esterification of acetic acid and ethanol catalyzed by sulfuric acid.¹⁴ They found that a higher reboiler duty facilitated rapid conversion and increased the product purity, which is likely due to the heightened ethanol vaporization in the liquid phase. Conversely, an increase in microwave power led to a decrease ethanol conversion and product purity. These findings were supported by Aspen Plus simulations, confirming the efficacy of microwave-assisted reactive distillation in process intensification.¹⁴ Messaouda Gabli and team developed a methodology to intensify the process of removing Ni(II) from water by the combination of electrodialysis and ion-exchange processes as an intensified method.¹⁵ Among the various processes, few models were detailed with examples formulated such as (i) competent membrane technologies for a worldwide clean water supply, (ii) inexpensive small-scale processing technologies for manufacturing applications in variable atmospheres, (iii) fuel cells and PI using a multisource multiproduct strategy and (iv) control at the molecular level of chemical conversions. Membrane technology is extensively applied in various industries, particularly for water and wastewater treatment, where it effectively eliminates contaminants such as suspended solids, bacteria, viruses, and dissolved salts.^16,17 Besides, it plays a crucial role in the purification of pharmaceuticals and biotechnology products, such as proteins and vaccines. It also finds utility in cell harvesting and separation within bioreactors. In the chemical industry, membrane technology is instrumental in separation and purification processes, while it is also integral for gas separation applications such as separating hydrogen from natural gas. Process intensification (PI) techniques are applied in operations such as distillation, extraction, leaching, and membrane separation, as well as biofuel manufacturing, catalysis, oil recovery, and pharmaceutical production processes. Also, the relevance of PI has been examined in the design and operation of various reactors, including batch, continuous, and microreactors. In chemical-based industries, PI strategies such as heat mass integration, catalytic cyclic distillation, and microwave-assisted reactive distillation have been tested for applications such as the esterification process. Furthermore, enzymatic reactive distillation has been explored for bio-reactive processes. Notably, this review discusses the adoption of hybrid methods and integrated steps in basic chemical processes and membrane-based separations in subsequent sections. This review explores PI in the energy sector, including petroleum and bio-gas separation and biodiesel production, where equipment such as ultrasonic cavitation reactors and hydrodynamic cavitation reactors are used. Additionally, the use of PI and hybrid separation in textile effluent treatment, wastewater removal, desalination through membrane operations for zero liquid discharge, and the recycling of chemical compounds from refineries to enhance product outcomes are discussed briefly.

2 Integration of PI technology in the energy sector with membranes

The integration of different membrane operations has promising applications in the petrochemical industry, such as in the ethylene steam-cracking process. Currently, the annual ethylene production is more than 110 million tons, of which more than 97% is obtained by steam cracking. As reported by Bernardo et al., conventional separation reaction steps can be replaced by a membrane reactor for ethane oxidative dehydrogenation; a single- and multi-stage membrane gas separation system for H₂ recovery; gas liquid membrane contactor (GLMC) for the removal/recovery of hydrocarbons from water and for acid gas removal; MF for coke removal from the scrubbing water in the decoking phase; and a unit for oxygen-enriched air (OEA), which can be used, for example, to improve the combustion efficiency in flares and during decoking.⁸ An exergetic analysis showed that the replacement of traditional operations with membrane units results in lower exergetic loss with respect to the traditional cycle.⁸

Membrane operations have become a cornerstone of innovative separation, conversion, and upgrading processes, playing a crucial role in the implementation of “green process engineering” principles across various sectors. Recycling and reuse of waste using PI strategies are one of the promising technologies to reduce the energy cost for separation processes, where chemical process industries consume significant energy for only separation. PI has been applied in diverse industries ranging from wastewater treatment to the petroleum sector and chemical processing industries, which were found to be the top energy-consuming manufacturing sectors in 2010. PI can be applied in these sectors to reduce the usage of energy and save the related significant expenses. The top ten energy consuming sectors include petroleum, chemical, paper, iron and steel, balance of manufacturing, metal-based durables, food, aluminium, glass and other manufacturers. The schematic in Fig. 1(a)¹⁸ shows the end-use consumption sectors and feed stocks on the real axis and energy consumed in 1000 TBTUs (trillion British thermal units) on the imaginary axis. The top ten sectors are listed in the figure, where most industries consume petroleum and natural gas products.^9,18,19 The energy consumption and the band widths are compared, where the opportunity and possibility of energy consumption were analysed. The possibility and the impracticalness describe the current typical (CT) energy consumption in 2022 by chemical manufacturing companies. This figure contains the CT-energy consumption, existing and practicing best available technologies, research and development, and thermodynamic conditions.^18,19 Separation processes consume a lot of energy and vary from industry to industry. It has been found that the energy consumption for separation is greater in the petroleum and chemical industries compared to other industries. A comparison of the total industrial consumption of energy is compared with the energy consumption for separation in different sectors in Fig. 1(b).²⁰ The implementation of energy conservation strategies for separation processes, which include PI methodologies and membrane-based separations, results in lower energy consumption and increases the product quality.^18,19 Process intensification technologies, including reactive distillation, dividing wall column distillation (DWC) and reverse flow reactors (RFR), have been implemented on the commercial scale in the petrochemical industry more than 100 times each. A comprehensive overview of process intensification, in the context of the chemical process industry and types of methods and equipment is presented in Table 1.^21–23 These technologies have been analysed and discussed by Stankiewicz, A. I. and Moulijn by year with four drivers for innovation in the chemical process industry in terms of feedstock cost reduction, capital expenditure reduction, energy reduction and safety risk reduction, including four hurdles for innovation such as risk of failure, scale-up knowledge uncertainty, equipment unreliability and higher safety, health, environmental risks compared to other conventional technologies (not using process intensification).^22,23 The article by Stankiewicz, A. I. and Moulijn concluded that process intensification technologies will probably be rapidly implemented on the commercial scale when at least one of the above-mentioned drivers exists and when all above-mentioned hurdles are addressed. The petroleum and petrochemical industries have invested a lot in separation processes. One of the majorly used separation techniques in the petrochemical industry is membrane-based gas separation (GS). In the development stage of this process, TPX (poly(4methyl1pentene) and ethyl cellulose membranes with the selectivity for O₂/N₂ of up to 4 were used. CO₂ removal from natural gas is the major step because of its corrosive nature and scale formation, in addition to a decrease in the calorific value. Therefore, CO₂ removal is the major step. The treatment shows positive results for both CO₂ and H₂S removal using polymeric membranes even at higher pressures. In the case of higher quality, multiple membrane units must be used. One of the majorly used materials for the removal of CO₂ and H₂S, which is known as the sweetening process, is cellulose acetate (CA). However, the use of cellulose acetate has negative effects such as plasticization. Thus, to reduce the effect of plasticization, polyimides are used, which are chemically, mechanically and thermally more stable compared to CA. Polyimides possess higher selectivity and permeability for CO₂ compared to CA. Therefore, polyimides with crosslinking gives better results in the removal of CO₂ from the natural gas stream.²⁴ PI has been used in the detoxification of ethanol produced from biomass, which included a membrane pervaporation bioreactor and vacuum membrane distillation bioreactors. Also, the implementation of PI in ethylene and ethanol production has already been reported. To intensify the process for ethanol production, microchannel reactors and membrane reactors were incorporated in the water–gas-shift reaction, as reported by Jansen et al. in 2010 and 2011.^19,24,25 Another application of the gas separation technique is in the production of nitrogen in the petroleum industry, which is separated from oxygen, the second major component of air, by using dense inorganic membranes that selectively permit oxygen to pass through at temperatures higher than 700 °C. These membranes are also used for hydrogen recovery in the oil industry, which is one of the major aspects and assumed to be three-times more valuable if recovered and used as a fuel.²⁶Table 2 shows the applications of gas separation techniques in the petrochemical industry.²⁷ Composite membranes are being developed and commercialized for these applications, offering improved resistance to aromatics and other contaminants, good separation performance, and competitive capital and operating costs. Phenolphthalein-based polyether-ether-ketone PEEK, a compound with a lactone group on its backbone, endows materials with good mechanical and thermal properties. This compound is amorphous and non-polar and is soluble in chlorohydrocarbons, amides, and ethers. Therefore, phase separation techniques are performed employing polymeric membranes prepared using this compound. Alternatively, plasticization-resistant perfluorinated membranes are used given that they are resistant to plasticization, and therefore no swelling of the membrane occurs. One of the most widely used membranes in the petrochemical industry is per-fluoropolymer membranes.^27,28 Presently, the reuse of waste for PI is still an emerging field and still under research, and some studies have progressed to the pilot plant stage. Table 3 (ref. 29 and 30) summarizes the present status of the strategic implementation of PI in biodiesel production from vegetable oils and animal fats, which are scarcely commercially employed, and some studies are still at the laboratory and pilot plant scales. One of the applications of PI in biodiesel production is the use of membrane reactors, which have the residence time of 1–3 h, and the expenditure for operation is lower, and as reported by Kiss and Suszwalak, they are easy to control, but membrane reactors are still at the pilot plant scale. Biogas is used as an alternative to natural gas, which is composed of methane and carbon dioxide, in addition to traces of ammonia and hydrogen sulphide.²⁹ A major technique used for carbon dioxide separation is chemical and physical absorption. The removal of carbon dioxide is the major step given that carbon dioxide is corrosive in nature and reduces the heating value of the gas stream. The conventional separation method is expensive with reduced efficiencies. In this case, membrane-assisted gas separation is an intensified alternative method to conventional processes such as adsorption and absorption, which offers reduced capital costs, ease of control, and high efficiency, unlike conventional processes. Polyimides show positive results in the separation of methane and carbon dioxide, together with thermal and chemical stability. However, they are also affected by plasticization because of the presence of humidified gases, and thus in their absence, the membrane does not exhibit any of the above-mentioned problems. As reported in the work by Brunetti et al., the use of Matrimid 5218 offers high selectivity and can be used in one-stage and two-stage separation systems, while Hyflon offers high permeability and only used in one-stage separation systems. Both polymers can yield higher that 95% of methane and 2% of carbon dioxide in a single-stage system. To intensify the process, they used a double-stage system, where first Hyflon was used followed by Matrimid 5218 to get better results, demonstrating that multistage systems can be adopted.³¹ Vialkova and Malyshkina reported the process intensification of petroleum product extraction from the aqueous solutions using natural absorbents.³² Finally, their effort resulted in the intensification of the esterification reaction for propyl butyrate production using pervaporation, a process that significantly reduced the energy consumption and enabled operation beyond vapor–liquid equilibria. Consistent with this process intensification with integrated membrane technology, we advocate the re-design of traditional biogas plants to integrate or replace conventional operations with advanced membrane units. Francesco Zito et al. and team used a multistep membrane process for the upgrading of a mixture containing 60% of methane and 40% of carbon dioxide. They worked on 3 case studies, which covered a wide CO₂ permeability range (from 5 to 180 Barrer) and CO₂/CH₄ selectivity (from 30 to 200),³³ as schematically shown in Fig. 2(a). Francesco Zito et al. aimed to determine the extent to which increasing the number of steps positively impacts the separation performance and reduces the total membrane area and recycled flow rate needed to achieve a methane purity of 98%, i.e., membrane-integrated process intensification.³³ Another example is biogas derived from bio-digester gas streams, which is rich in valuable products such as biomethane, volatile organic compounds (VOCs), and volatile fatty acids. Their recovery offers significant advantages for environmental protection, energy efficiency, and waste valorization. In this case, advanced membrane units can efficiently separate and recover these components, establishing environmentally sustainable and compact separation processes. A schematic diagram showing the use of various types of membranes for a series of operations is presented in Fig. 2(b). Adele Brunetti and Giuseppe Barbieri proposed a process comprised of three membrane operations to intensify the process and enhance the overall efficiency and environmental sustainability of biogas upgrading, including a membrane condenser and gas separation unit, MF/UF unit for VOC and VFA, and a pervaporation unit as discussed in Fig. 2(b).³⁴ The introduction of a membrane condenser reduces the need for conventional pre-treatment units prior to gas separation. Meanwhile, the use of advanced membranes with higher tolerance towards H₂O, H₂S, and other contaminants will enable a reduction in the load on these pre-treatment units. The microfiltration/ultrafiltration (MF/UF) unit can concentrate the condensate, recovering water for reuse within the plant and a concentrated stream of VFA and/or VOCs. The recovery of VFA and VOCs present in the biogas stream increases the diversification of the process, given that these compounds are valuable chemicals used in various industries as building blocks of various organic compounds including alcohols, aldehydes, ketones, esters, and olefins to several other biotechnological applications from manufacturing to biotechnology. This shift towards small-scale, energy-efficient separation processes aligns with the principles of the circular economy and holds great potential for various industries.³⁴ Similar examples of process intensification for tertiary effluent are described in section 4.^35,36

Fig. 1 (a) Top 10 energy-consuming manufacturing sectors and the energy consumed; online open-source available data from https://www.eia.gov/consumption/manufacturing reproduced from ref. 18. (b) In-plant and separation energy use for energy-intensive industries; online open-source available data from Department of Energy, BCS, Columbia reproduced ref. 20.

Table 1 Process intensification technologies in the petrochemical industry, their innovation drivers and hurdles in various aspects²¹

Technologies	Hurdles faced during initial introduction
	Feed stock cost reduction	Capital cost reduction	Energy reduction	Commercial implementation	Failure risk	Scale-up knowledge	Equipment reliability	Safety
Reactive distillation		20–8%	20–8%	>150	Low	Low	Low	No
DWC distillation		10–30%	10–30%	>100	Low	Low	Low	No
Reverse flow reactor		>20%	Low	>100	Low	Low	Low	No
Microchannels reactor	Yes	Yes: small scale		Only in fine chemical sector	Low	Low	Low	No
Centrifugal field absorbers	Case dependent	Case dependent		A few	Low	Low	Medium	No
External field PI					High	High	High	Low

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Table 2 Application of gas separation techniques in the petrochemical industry²⁷

Petrochemical	Monomer recovery in polyolefin production (resin degassing vents/reactor purge). Monomer recovery in polyvinyl chloride production
	Ethylene recovery in ethylene oxide production
	Syngas H/CO ratio adjustment for oxo-alcohol production, GTL, etc.
Refining	H2 recovery from hydrodesulphurization/hydrocracker large streams
	H2 recovery from catalytic cracker off-gas, H2 recovery from refinery fuel
	H2 recovery from fluidized catalytically cracked (FCC) overhead gas
	H2 recovery from pressure swing adsorption (PSA) tail gas
	Catalytic reformer hydrogen upgrading, LPG recovery
Natural gas	CO2 and N2 removal, fuel gas conditioning
	NGL concentration, treatment and recovery
	Natural gas treatment to meet pipeline specifications
	CO2 for enhanced oil recovery and sales gas from CO2-rich stream
	Production of sales gas from CO2 fractured well digester off gas treating
	Landfill gas upgrading

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Table 3 PI technologies to produce biodiesel from vegetable oils and animal fats^29,37

Equipment	Residence time	Operating and capital cost	Temperature control	Status
Static mixer	∼30 min	Low	Good	Lab-scale
Micro-channel reactor	28 s to several minutes	Low	Good	Lab-scale
Oscillatory flow reactor	30 min	Low	Good	Pilot-plant
Cavitational reactor	Microseconds to several seconds	Low	Good	Commercial scale
Spinning tube in the tube reactor	<1 min	Low	Good	Commercial scale
Microwave reactor	Several minutes	Low	Good	Lab-scale
Membrane reactor	1–3 h	Low	Easy	Pilot-plant
Reactive distillation	Several minutes	Low	Easy	Pilot-plant
Centrifugal contactor	∼1 min	Low	Easy	Commercial scale

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Fig. 2 (a) Multistep membrane process for biogas upgradation and improved purity.³³ Reproduced with permission from ref. 33, published by Elsevier, 2022. (b) Process schemes for biogas upgrading to biomethane: (A) integrated membrane process and (B) traditional process. Adapted from Frontiers Media SA publication, Lausanne, Switzerland.³⁴

3 PI application in textile and other process industries integrated with membranes (IWM)

Generally, textile industrial effluents are characterized by high chemical oxygen demand (COD) and the presence of non-biodegradable components such as dyes, pigments, some types of newly introduced sizing polymers or chemicals and some types of heavy metals, pH, and suspended solids.³⁵ A 90% textile wastewater mixture contains several flowing flagellates with single and double flagella, single ciliates and numerous anoxic bacteria. Single cysts of roundworms, small protozoans, slowly flowing ciliates and anoxic bacteria were also found in the microscopic view of 30% textile wastewater mixture. Accordingly, effluents from the textile industry have a complex composition. Firstly, the presence of non-biodegradable components leads to the development of numerous oxidation processes for the partial or complete destruction of dyes and pigments, which further cannot be recovered after processing.³⁸ Secondly, the dyeing process in the textile industry is characterized by high water usage such as for multiple washing and rinsing cycles, hence utilizing a large amount of water, where generally underground water is used as process water.³⁹ After usage, this water must be discharged. Thus, it is mandatory to develop technology for the recovery and reuse of water and other chemicals from textile wastewater.

Integrated membrane technology, which is developed by adopting PI strategies, is used in the textile industry for effluent treatment.²¹ Given that the textile industry intensively uses processing chemicals, membrane separation using microfiltration (MF) and ultrafiltration (UF) allows the significant volume reduction, separation and recycling of useful chemicals, dyes such as synthetic sizing agents and some types of intermediate chemicals, as reported in,^40,41 using conventional techniques. The use of ultrafiltration partly removes colour but is not effective in terms of recycling of chemicals and dyestuff. Alternatively, nanofiltration (NF) and reverse osmosis (RO) enable size reduction, recycling of water and chemicals and maximum dye removal together with the used salt content in the effluent.^42–47 One of the most used dyes in dye houses is azo dyes, and thus the biodegradation of azo dyes was the main objective of a study. These dyes possess one or more azo bonds (–N=N–) and cannot be degraded under aerobic conditions in wastewater treatment plants. However, under anaerobic conditions, many bacteria possess enzymes that can break the azo-bonds and release aromatic amines such as azoreductases. Many aromatic amines are carcinogenic; therefore, they are further degraded under aerobic conditions.⁴⁶ These observations confirm that correct operation intensification such as the usage of a bioreactor and activated sludge adaptation technique is required.^35,45 Usually, textile effluents require more than a single treatment technology to achieve the above-mentioned two approaches. Generally, there are two approaches for PI in separation techniques considering the management of textile wastewater. The first approach of PI sheds its light on the fulfilment of environmental legislations. The second approach of PI is maximum recycling and reuse with maximum savings to approach minimum possible discharge, and consequently minimum cost directed to end-of the pipe-treatment. The Fenton reagent is a powerful oxidizing agent, resulting in both rapid decolourization and COD reduction.^44,45 PI is performed for biological treatment, which includes the treatment of aerobic textile effluents that are moderately biodegradable and about 40–50% colour removal has been reported⁴⁵ due to the biodegradation and adsorption of dyes on flocculated sludge. Employing this biological treatment, the COD removal was found to be as high as 70%.⁴⁵ PI for separation using the combination of physical–chemical and biological processes enabled more than 85% COD removal.^44,45 Adsorption using activated carbon as an adsorbent involves the use of a polishing step for the removal of colour and COD. Powdered activated carbon treatment (PACT), involving the addition of powdered activated carbon to the activated sludge process, is generally mentioned as PACT treatment and adopted to achieve high BOD, COD and colour removal efficiencies depending on the dosage. Several integrated processes for textile wastewater treatment have been explored based on the ability and property of the processes, and thus accordingly arranged as reported. TEXPERT, a textile expert program, has been developed on an Excel spreadsheet by Harmsen J.²¹ This program enables the selection of appropriate modules according to technical criteria based on the wastewater characteristics, capacities, removal efficiencies and limiting quality criteria. The input data of the TEXPERT program generally include the capacity, sizing material and wastewater characteristics of the proposed scenarios covering typical cases from the textile industry. The results of the TEXPERT program shed some light on the ability to deal with different situations in the textile sector. This program also revealed the importance of integrating membrane recycling systems within the scope of the treatment matrix to achieve permissible discharge limits. The TEXPERT program also enables the users at the planning level and financial institutions to decide on the capital needed at the sectorial level.⁴⁶ Generally, the recommended hybrid process in the textile industry for effluent treatment, which includes several steps such as aeration, primary and final clarification, ultrafiltration, nanofiltration, and reverse osmosis, is depicted step-by-step in Fig. 3.⁴⁶ Activated sludge and chemical coagulation have been widely used to treat textile wastewater. However, the cost of the treatment increases because a higher dosage of ozone and coagulant is required.³⁶ Thus, an integrated coagulation–precipitation/ozonation process is mainly used to increase both the colour and COD removal in India. In addition, using integrated membrane systems, the total expenses incurred for water treatment and recovery using an RO/NF membrane were reported by Ismail and Lau to be about INR 134/m of effluent.³⁶ Membrane contactors is a technique mainly used to remove oxygen and carbon, which affects the performance and material life of the plant and removal of pollutants that are non-volatile such as boron and argon. An integrated membrane coagulated reactor was used for the biological treatment of BOD and SS removal. Li et al. reported³⁹ an improved integrated membrane coagulated reactor mainly consisting of two units, as follows: 1) a coagulation unit, in which polyaluminium chloride is used as a coagulant and 2) a membrane separation unit, in which for maintaining the pH, NaOH or HCl is added. The percentage colour removal was calculated by comparing the absorbance values of the treated sample using a spectrophotometer. The dyes were only transformed from wastewater into floss, which could be separated more easily, whereas the molecular structures of the dyes remained unchanged. An industrial sedimentation tank in its PI version was applied, where sedimentation occurs as pre-treatment of the effluent, as reported by ElDefrawy and H. F. Shaalan,⁴⁶ together with an integrated membrane coagulated reactor with different molecular weight materials.³⁹ Thin film composites (TFC) are the process intensified version of NF membranes. If NF is used in the textile industry, the only solution to meet the maximum requirements is mainly for the process with an objective of high permeability of solvent, which is generally water, and the rejection of salts in thin film composite NF membranes (TFC-NF). Higher salt rejections were observed by optimizing TFC-NF membranes compared to asymmetric nanofiltration membranes. Membranes such as NF 45 and DK 1073 were observed to exhibit a good performance in terms of dye retention. Another PI method reported by Xu et al. implemented electrocatalytic oxidation in combination with NF.⁴⁵ The application of a DC current across the NF membrane is a major technique using electrophoresis and electro-osmosis together. Electrophoresis reduces the polymer concentration and fouling, which increases the flux by pulling the particles out of the membrane. Also, it possesses high potential to reduce dye in wastewater. The integration of NF and electrocatalytic oxidation enhanced the permeate flux up to 70%, as reported by Xu et al., and hence the process was intensified compared to conventionally adopted methods.⁴⁵

Fig. 3 Recommended hybrid process in textile industry for effluent treatment; reproduced with permission from ref. 46, published by Elsevier, 2007.

3.1 PI technology in other process industry wastewater purification IWM

Process intensification (PI) is a term that is currently widely used in wastewater treatment, especially in desalination plants, and in various process industries to intensify membrane technology systems, which is aimed at making processes economically feasible and sustainable for future growth strategies, rather than simply increasing production.^5,6 All four approaches and principles of PI, which were discussed in the previous sections, can be satisfied by membrane technology, which offers a wide range of advantages such as less energy usage compared to the traditional separation techniques, lower cost and higher separation efficiency. Membrane operations have desirable intrinsic characteristics such as operational simplicity, high efficiency, high permeability and selectivity for the transport of specific components, environmental compatibility, less energy requirements, stability under operating conditions, easy control, capability of being scaled up, and flexibility, offering interesting options for the advanced rationalization of industrial processes. Accordingly, it is the most feasible option for the implementation of PI strategy commercially by decreasing the equipment size, raw material and energy utilization, and waste generation. Process intensification using membrane-based separation was adopted to treat tertiary effluent (TDS ∼1150 mg L⁻¹) from the petrochemical industry using a pilot-scale electrodialysis reversal (EDR) process followed by reverse osmosis (RO).³⁵ Two ED modules, each comprised of 75 cell pairs with an area of 28.8 m², were employed either in series or parallel configuration. The EDR stage achieved TDS removal rates of approximately 90%, while the EDR–RO hybrid system demonstrated overall removal efficiencies exceeding 90% for various physico-chemical parameters, with a water recovery rate of 41% (75% in EDR and 50% in RO). Hybrid systems, where electrodialysis (ED) treats the brine retentate from RO, have the potential to enhance the water recovery.³⁶ Further, PI in membrane technology offers a wide industrial utilization range of newer integrated techniques, which has been found to be very effective in terms of efficiency and separation.^41,42 Metallic membranes are expensive compared to polymeric or ceramic membranes. Gallucci, et al. and Fernandez et al. in 2013 and 2015, respectively, reported the strategic implementation of PI in metal-based membranes because of their advantages of high strength and porous structure, which are currently used for H₂ production. Palladium-based membranes are generally used for hydrogen (H₂) separation given that they are the most selective.^48,49 However, finding a good support for the membrane is the prime objective to enhance performance of the processes, which can be intensified drastically. Generally, ceramic supports are used, but these supports lack mechanical strength compared to porous metal supports. The simultaneous use of electrode plating Pd–Ag membranes on Al₂O₃ porous ceramic membrane enhanced the permittivity and selectivity for H₂ compared to the membranes with ceramic-based supports, as reported by Medrano, J. A. et al. and Fernandez, E. et al. in 2015.^50,51 The emulsion liquid membrane (ELM) an example of PI that works even at low concentration, which can be utilized to evacuate and recuperate the overwhelming heavy metals, as reported by Fouad and Bart in 2008.⁵² Generally, it is utilized in the pharma sector, which deals with a wide range of dangerous toxins that contain phenol and chloro- and nitro-phenol. The main advantage of ELM is its high mass transfer surface to area ratio, and also high surface area to volume ratio. In a dispersion-free and hollow shape, a fibre contractor can be used for the ELM operation. It can provide an emulsion liquid membrane with a better option for stripping and high shearing in a stirring contractor. Stability is not important in the extraction of Hg, Cu, and Ni, which resulted in 99% and 96% extraction of Ni and Hg, respectively, according to the study by Fouad and Bart. The limitation was the poor resistance of the emulsion globules to liquid shear, implying film breakage and spillage. The zinc extraction process is one of the examples of ELM that depends on process intensification, which utilizes liquid ion exchangers, where an increase in the liquid in membrane layer increases the extraction of zinc.⁵² Membrane-based reactive separation systems provide a combination of diverse functions, leading to significant improvements in the process performance.^29,30 These improvements include better yields/selectivity, better energy management, more compact design, and prolonged catalyst lifetime, as reported in detail by Sirkar et al. and shown in Fig. 4.⁵³ As discussed earlier in the introduction, processes intensification can be applied either to equipment or the method. As discussed by Drioli et al., process intensification can be applied either to equipment or the method using membranes.⁵⁴ For example, FO is an intensified method of RO. FO is not the same as pressure-driven RO processes, which is a natural process where the permeate is an osmotic pressure gradient, driving the fluid across the membrane. The lower energy consumption in this process and the fouling tendency are the major advantages of FO over the RO process. However, the recovery of water-soluble particles is a major challenge in the FO process, which are recovered by other hybrid processes such as integrated MD and NF. This process is mainly used for the removal of water from an aqueous product, which is easier than other processes, as reported by Sirkar et al.⁵⁵ Similarly, pressure retarded osmosis (PRO) is an intensified separation technique compared to the RO technique. In PRO, the major advantage is the lower hydraulic pressure compared to than that of the osmotic pressure gradient across the membrane. Subsequently, the net water flow is from low salinity to high salinity, which is also in the same direction in FO. The enlarged volume of pressurized fluid can be used to perform suitable work such as driving a hydro turbine for power generation. The world's first osmotic power plant using river water and seawater with a capacity of 4 kW was installed by Statkraft, an European company specializing in renewable energy, in November 2009 in Tofte, Norway. In catalytic membrane reactors, catalytic nanoparticles are coated on the membrane layers, which are used in steam deformation and dehydrogenation. In membrane bioreactors, membranes with biocatalysts are useful for immobilizing whole cells or separation and purification of bioactive molecules, where concentration of the product occurs in single integrated unit operation.

Fig. 4 Membrane functions in chemical reactor systems; reproduced with permission from ref. 53, published by ACS Publications, 1999.

The techniques used in PI requires the minimal use of toxicants. Given that PI is an upcoming and rising technique, it can be used for addressing the major problems encountered in industry. The integration of membranes holds significant potential in achieving the objectives of process intensification (PI), as presented in Fig. 5(a). An example of this is seen in seawater desalination, which is a prime illustration of the accomplishments possible through membrane integration, as reported by Drioli and Curcio.⁵⁶ However, it also highlights the ongoing challenges faced by integrated membrane systems.

Fig. 5 (a) Scheme of an integrated desalination system; reproduced with permission from ref. 56, published by Wiley Analytical Science, 2007. (b) Flow chart of ammonia treatment process; reproduced with permission from ref. 57, published by Elsevier 2020. (c) Schematic of resin water EDI configuration and process streams; reproduced with permission from ref. 58, published by ACS Publications, 2015.

Another example of the application of process intensification is the removal of ammonia from the effluent gas stream, given that ammonia is toxic to the environment, which should be treated before its release into the atmosphere. Generally, ammonia is treated with sulphuric acid in two packed columns. Alternatively, PI membrane contactor technology emerged, where the interfacial area is approximately 40-times greater per unit volume. In this process, both fluids come into non-dispersive contact, during which ammonia moves to the acid through a hydrophobic porous membrane using air stripping. This process is known as transmembrane chemisorption. The product formed is ammonium sulphate, which is removed in crystalline form by integrating chemically reactive membrane crystallization with transmembrane chemisorption, resulting in the final process in one step, as reported by Davey et al. and shown in Fig. 5(b).⁵⁷ Reverse electrodialysis is a hybrid system in desalination plants, where the left-over concentrated brine from the desalination process is used for power generation. The power generated using this process is sufficient to run the desalination process. This technique is used either before or after reverse osmosis, resulting in integration of reverse electrodialysis with reverse osmosis. This can also be done by integrating membrane distillation with reverse electrodialysis, which produces collaborative effects. Using the retentate of reverse osmosis in membrane distillation, which is attached to reverse electrodialysis, enhances the recovery factor. The electrodeionization process is a combination of two process, i.e., electrodialysis and ion exchange chromatography. As shown in Fig. 5(c), Ashu et al. reported the process of electro-deionization, which was denoted as EDI.⁵⁸ In the case of conductive ion exchange chromatography, two columns are used, where one is under operation, whereas the other is in regeneration mode.

A newly developed method is the degradation of effluents by photocatalysis, which is mainly used for the oxidation of detrimental compounds in effluents, as discussed by Iglesias et al.⁵⁹ ZnO, CdS, and TiO₂ are the major semiconductor materials used as photocatalysts. Considering its stability, low cost and efficacy, TiO₂ is preferred for the degradation of organic pollutants but its operation is limited to the UV region. Also, TiO₂ catalysts are generally used in suspended powder form, making their separation difficult. Accordingly, processes such as coagulation, sedimentation, and filtration can be used to overcome this drawback. Given that membranes offer several advantages in terms of energy and operational control, that can be the best option for obtaining recycled products for reuse. Photocatalysis with membrane filtration mostly employs micro-, nano-, and ultra-filtration based on particle size. Considering cost, polymeric membranes are mostly chosen but they have the drawback of fouling, requiring backwash. This hybrid system can be operated in two modes of external and submerged membranes, where the external mode enables continuous operation. Air is circulated to maintain a high dissolved O₂ content and catalyst molecules are suspended in the stream. Photocatalytic membrane reactors are membrane filters that offer both catalytic activity and filtration operations simultaneously, which achieve the four goals of PI. During the manufacturing of the membrane, a catalyst is coated on it or the membrane is composed of a catalyst, where the most used membranes are inorganic and polymeric membranes. The photocatalyst can be coated in many ways such as “dip-coating”, “electro-spraying of TiO₂”, and “magnetron spattering”. In effluent treatment, photocatalyst-coated membranes are used for dye treatment and removal of PE, glycols, phenolic compounds and other pollutants, as reported by Iglesias et al. in 2016.⁵⁹ This is one of the promising examples in the implementation of the process intensification route, as reported by Kim et al. and Senthilkumar et al.^60,61 The same process is intensified by using a porous support for the catalysts that are used in the process, as studied by Kishor et al.⁶² Various design and operational methods are adopted to increase the mass transfer performance of membrane modules, such as geometry perturbation and modifying the flow conditions, to overcome the problems encountered with conventional membranes in terms of mixing, flooding, loading, foaming, etc. Singh et al.⁶³ reported the increased performance of carbonation in the beverage industry with respect to taste, odour, and colour. They studied three different hollow fibre membrane modules, viz., straight tube, helical coil, and coiled flow inverter (CFI) membrane modules, as shown in Fig. 6.⁶³ A higher permeate flux was reported for the CFI membrane module compared to the helical coil module with similar design parameters with a lower pressure drop. The process intensification of the straight tube membrane module and CFI membrane module resulted in merit values of 2.5 for oxygenation and 2.7 for carbonation. The outcome of their research after intensifying the process will be potentially helpful to design new membrane units for improved mass transfer efficiency.

Fig. 6 Schematic diagram. (a) Straight tube module; (b) helical coil module; (c) CFI module; (d) helical module–CFI module design view. Reproduced with permission from ref. 63, published by MDPI publications, 2016.

One of the best-developed membrane technologies is pervaporation, exhibiting great potential for the intensification of various industrial processes, such as breaking azeotropes and removing volatile organic compounds (VOCs) from liquids.⁶⁴ Polyether-block-amide (PEBA) is suitable for pervaporating low-volatility aromatics^65,66 and vanillin.⁶⁷ Böddeker and co-workers reported that PEBA-dense membranes were suitable for use in pervaporation reactors to produce vanillin as a product. Using the hybrid mode of operation, i.e., reaction with pervaporation, the enrichment factor of vanillin was enhanced. This process-intensified step resulted in thick PEBA membranes, which decreased the resistance for vanillin permeation, as presented in Fig. 7.⁶⁴ In contrast, the resistance to water permeation increased, which resulted in a 10-fold increase in vanillin flux and competitive yield compared with the conventional method of separation.^64,68–70 Gregorius Rionugroho Harvianto et al. reported process intensification by studying a hybrid process combining thermally coupled reactive distillation (RD) with membrane-based pervaporation.⁷⁰ They used this process for enhancing the production of n-butyl acetate from n-butanol and methyl acetate. They found that this hybrid technique improved the energy efficiency of the RD process by preventing remixing effect and nullifying the azeotropic nature of methanol (product) and methyl acetate in the recycle stream. This method reduced the reboiler duty by 63% and annual cost by 43% compared to conventional RD. This method proved to be advantageous in cases of low feed rate, high methanol concentration in the liquid split stream, low methanol concentration in rectifying stages, and high conversion.⁷⁰ Process intensification of the esterification reaction to produce propyl butyrate by pervaporation was reported by Chandrakant R. Khudsange and Kailas L. Wasewar.⁷¹ Traditionally, esters are formed by the reaction between acids and alcohol in the presence of catalysts such as HCl and HI. They are produced in various reactors such as reactive distillation column and microreactor, leading to high energy consumption, which depend on the vapor–liquid equilibrium of the system and azeotropic mixture. Accordingly, a hybrid method was adopted to intensify the process stream by combining pervaporation with esterification. The intensified process led to a decrease the energy consumption, enabled the operation beyond vapor–liquid equilibrium, handled azeotrope, and made the process environmentally friendly. The catalyst p-toluene-sulfonic acid was used for the reaction between butyric acid and n-propanol. A composite membrane made of polyvinyl alcohol (PVA)-polyethersulphone (PES) was used for pervaporation.⁷¹ The authors reported that temperature and molar ratio are the key tuning parameters, which are directly proportional for the conversion of the acid. The conversion increased rapidly from 72% to 93% due to the removal of water with an increase in catalyst concentration from 1 to 2.5 wt% at 353 K. PVA and PES as hydrophilic polymers supported the pervaporation mechanism for efficient water removal from the reaction mixture. Mohd Hizami Mohd Yusoff et al. intensified the membrane distillation (MD) process by combining it with a wet scrubber (WS).⁷² The MD + WS system simultaneously recovered water and heat through the MD permeate using polytetrafluoroethylene (PTFE) as the membrane polymer. The scrubber achieved complete water and heat recovery. At a higher flue gas temperature (FGT), the authors reported an increase in mass and heat flux in MD due to the increase in flue gas flow rates with total energy. Due to the small membrane area, the conductive heat transfer was low. Also, a low flow rate resulted in excessive heat loss across the MD tubing. Enrico Drioli et al. studied the role of membrane engineering in achieving the objectives of process intensification in the field of desalination.⁷³ The membrane had features such as high selectivity and permeability for the transport of specific components, small environmental footprint, high safety, low cost, and ease of integration with other processes. The disadvantage of conventional desalination is the pressure-driven process, which hinders its wide application. In their study, the authors reported the use of a membrane crystallizer (MCr) in an integrated approach with RO for seawater desalination. Initially, the seawater was treated by NF and RO, followed by MCr treatment (temperature gradient as the driving force) of RO concentrate for NaCl crystal formation, which completed the cycle. According to their results, 50% to 90% recovery of fresh water was obtained in combination with salt recovery. Unlike RO, the process step used 40% of solar or wind energy and the remaining 60% of the desalination was occupied by RO, resulting in a better performance and boosting the economy of the process. The goal of obtaining zero liquid discharge was achieved by integrating it with a conventional energy source such as a diesel generator because solar and wind energy do not allow continuous operation without battery back-up at the back end. The design parameters such as productivity, variation in pressure, temperature, feed composition, flexibility, and modularity were explored to realize an enhanced performance.⁷³ Membrane-based hybrid separation, MD, membrane absorption/stripping, membrane chromatography, membrane extraction, and membrane crystallization are hybrid separation processes that integrate two or more different separation methods in a single operation.^74–78 Post-treatment membrane distillations use a hydrophobic membrane for the separation of aqueous solutions, which is brought to contact on the retentive side, preventing the penetration of aqueous solution. The membrane allows volatile content to pass through due to the temperature difference, which is the driving force. MD is one of the units of a membrane contactor. The hot aqueous solution is kept on one side and separated by a microporous hydrophobic membrane, through which liquid cannot pass, but the volatile species evaporate and pass through the membrane. All the non-volatile species such as salts and colloid particles and other large particles are not allowed to cross the membrane. This results in the maximum retention of macromolecules. In general, the feed temperatures vary in the range of 30–50 °C. The major problem in using the membrane technology is the concentration polarization, which is not seen in this process. This is the main advantage of membrane distillation. The PI version of MD is osmotic distillation (OD). Two aqueous solutions of different concentrations are kept in contact by placing a microporous hydrophobic membrane between them. This causes a vapour pressure difference, which activates the mass transport through the membrane. Gas separation, purification, controlled release of drugs or bioactive species to specific targets, biomedical applications, etc., are the major processes employing membranes for separation, and their application is found in biomedical operations such as dialysis and electrochemical membrane sensors. Recently, direct contact membrane distillation (DCMD) with conductive heat transfer reported by Hickenbottom and Cath showed nearly 100% inorganic salt rejection with no additional energy consumption.⁷⁶ This technique developed by Hickenbottom and Cath employed a high TDS content as the feed. Also, it utilized temperature and flow reversals on an alternating basis to control the variations in water flux and salt concentration using thin film composite membranes of Teflon (TS22) active stratum and polypropylene (PP22) as a support layer. As reported by Hickenbottom and Cath, the membrane having 175 μm of overall thickness, effective surface area of 89 cm², pore size of 0.22 μm and porosity of 70% exhibited 100% salt removal capacity, as shown in Fig. 8.^76,77 The hybrid process developed in the MD unit resulted in a higher water flux and salt concentration at a lower temperature difference of 20 °C at the interface of the TS22 membrane, as reported by Stankiewicz and Moulijn in 2000.²² To achieve the ambitious objective of “zero liquid discharge”, different membrane operations can be coupled in integrated systems and using novel separation processes including PRO, MD, RO and reverse electrodialysis (RED).^37,79–90Fig. 9(a) shows the qualitative analysis of various generations of desalination technology based on different process intensification parameters.⁷⁹ Seawater brine contains a few essential and strategic elements including sodium, magnesium, barium, and lithium, and thus can serve as a form of open sky mine for the recovery of these components. Recently, a dramatic increases in the consumption of some of these materials has been observed. For example, the use of strontium has increased in the oil and gas industry as a weighing material in mud. Similarly, the traditional sources of lithium may not be sufficient to fulfil its requirements in the electronic industry (especially considering hybrid vehicles). The amount of Na and Mg in brine from current desalination capacities is more than that obtained through conventional mining, as reported by Quist-Jensen et al. and Drioli et al.^91,92 Thus, process intensification in recovery of minerals from brine using membrane distillation/crystallization can be a future outlook for the mining industry. This situation can further be improvised for the upcoming planned desalination on the completion of contracted and planned projects by adopting process-intensified desalination techniques such as crystallization (MCr) treatment for seawater desalination in an integrated approach with RO across the globe discussed by Drioli et al., Profio et al., and Quist-Jensen et al.^93–95 Pressure retarded osmosis (PRO) is one of the most interesting membrane processes to deal with clean energy from a salinity gradient. In PRO, a semipermeable membrane is applied to separate high and low salinity solutions.^96–102 The osmotic pressure extracts the fresh water from the dilute to the concentrated solution. A pilot-scale PRO-hybrid research project, as presented in Fig. 9(b),¹⁰³ under the name “Global MVP (membrane distillation, valuable resource recovery, pressure retarded osmosis) Project” in Korea was covered in the report “Global MVP 2013–2018”. The objective of this project was to evaluate the feasibility of the RO–MD–PRO hybrid process in terms of reducing the discharged water concentration and the energy consumption. Fig. 9(c) presents various process intensification matrices for the comparison of different overall desalination processes.¹⁰⁴ The data reported by Klaysom et al.⁹⁷ showed different processes for desalination such as MSF (multi-stage flash), which is widely used for thermal desalination process, MD and RO against various key operating parameters MI (mass intensity), productivity/weight ratio (PW), and productivity/size ratio (PS). RO shows the maximum value of productivity/weight ratio (PW) due to the high membrane permeability and elimination of heavy metallic parts, which are essential components of MSF plants. This aspect is particularly important for off-shore or remote installations.^105–108 Overall, the comparison indicated the intensification of membrane operations (MD in the current example) that can be optimum candidates to overcome the drawbacks (limited MI) of conventional RO processes.^109,110

Fig. 7 Schematic of (a) experimental set-up for membrane pervaporation and (b) schematic of the integrated photocatalysis–pervaporation continuous process; reproduced with permission from ref. 64, published by MDPI publications, 2014.

Fig. 8 (a) Schematic process flow diagram of a typical membrane distillation unit.⁷⁶ (b) Variation in total solid content (TDS) in pre-filtered brackish water samples at the feed side for different types of membrane materials (a) TS22 membrane (b) PP22 membrane at a temperature difference of 20 °C; reproduced with permission from ref. 76, published by Elsevier, 2014. (c) Schematic diagram of the hybrid unit that co mbines wet scrubber and MD cells; reproduced with permission from ref. 77, published by Elsevier, 2011.

Fig. 9 (a) Parameters of the process intensification strategy; reproduced with permission from ref. 79, published by ACS Publications, 2021. (b) Schematic diagram of hybrid RO–MD–PRO in Korea implemented under Global MVP Project; open resources available online.¹⁰³ (c) Process intensification metrics for MD, multistage flash (MSF), and RO. MI, mass intensity; PW, productivity/weight ratio; PS, productivity/size ratio. Reproduced with permission from ref. 104, published by RSC, 2015.

In their study, Gregorius Rionugroho Harvianto et al. explored a novel hybrid process that combines thermally coupled reactive distillation with membrane-based pervaporation.¹¹¹ Their research aimed to improve the production of n-butyl acetate from n-butanol and methyl acetate. This hybrid technique significantly enhanced the energy efficiency of the reactive distillation process by preventing the remixing effect and nullifying the azeotropic nature of methanol (a product) and methyl acetate in the recycle stream. By implementing this method, the researchers observed a remarkable reduction in reboiler duty by 63% and annual cost by 43% compared to conventional reactive distillation methods. This hybrid technique particularly excelled in situations involving low feed rates, high methanol concentrations in the liquid split stream, low methanol concentrations in the rectifying stages, and high conversion rates in reactive distillation. Overall, this hybrid process presents a promising advancement in the field of chemical engineering, offering improved efficiency and reduced costs to produce n-butyl acetate from n-butanol and methyl acetate.¹¹¹ Yusoff et al. explored the enhancement of membrane distillation (MD) through a combined approach with a wet scrubber (WS).¹¹² This innovative MD + WS system effectively recovered both water and heat from the MD permeate, which was achieved through the complete water and heat recovery process of the scrubber. Using polytetrafluoroethylene (PTFE) as a membrane, they conducted experiments using artificial flue gas comprised solely of water vapor and air. Two types of membrane condensers, membrane condenser (MC) and transport membrane condenser (TMC), were utilized. The MC utilized the hydrophobic nature of the membrane to recover water from the flue gas by allowing water vapor to pass through and condense in the permeate side. Conversely, the TMC employed saturated flue gas in the lumen side of the membrane and coolant water in the shell side. The researchers observed an increase in both mass and heat flux in MD at higher flue gas temperatures, which was attributed to the higher flue gas flow rates carrying more total energy. Due to the smaller membrane area, the conductive heat transfer was minimal, while excessive heat loss occurred across the tubing of MD at low flow rates.¹¹² Enrico Drioli et al. conducted research on membrane engineering and its role in achieving process intensification, focusing on desalination.¹¹³ They highlighted several features of membranes that make them attractive for this purpose, including high selectivity and permeability, small environmental footprint, safety, low cost, and ease of integration with other processes. The challenge with conventional desalination methods is the pressure-driven process, which limits their scalability. In a different study, Drioli et al. proposed an innovative approach that involved using a membrane crystallizer (MCr) in combination with reverse osmosis (RO) for seawater desalination. Initially, seawater was treated using nanofiltration and RO, followed by MCr treatment of the RO concentrate to produce NaCl. This integrated process resulted in a significant increase in freshwater recovery from 50% to 90%, while simultaneously recovering salt. Unlike RO, MCr operates on a temperature gradient rather than osmotic pressure, making it less susceptible to osmotic phenomena. Most desalination processes (60%) involved RO, which was combined with renewable energy sources such as solar or wind energy to improve the performance and reduce the economic burden. Ultimately, this approach enabled the “zero liquid discharge” goal to be achieved.^113,114

A membrane-based approach to intensify the process is adopted by substituting cation exchange membranes (CEMs) with NF membranes, as illustrated in Fig. 10(a) through ED. To safeguard against acid and metal seepage, the utilization of proton-blocking anion exchange membranes (AEMs) and monovalent selective cation exchange membranes (MVCs) is indispensable for executing viable recovery procedures through ED.¹¹⁵ Various methods were employed to fabricate MVCs, achieving a perm selectivity of 34.4.^116,117 Custom-made NF membranes were employed for acid recovery from a synthetic solution containing Zn²⁺ (the dilate feed was comprised of 0.5 mol L⁻¹ H₂SO₄ and 0.23 mol L⁻¹ ZnSO₄, while the concentrate contained 0.05 mol L⁻¹ H₂SO₄). The modified ED system showed a superior performance compared to the stack equipped with MVCs, elevating the perm selectivity from 15 to 354. An example of a novel approach for process intensification is the combination of membrane distillation (MD) and reverse electrodialysis (RED) for the dual recovery of water and energy from urine in off-grid scenarios.¹¹⁸

Fig. 10 (a) Modified ED equipped with an NF membrane in place of CEM. Monovalent cations A⁺ (e.g., H⁺) can move across the NF membrane, whereas divalent cations B²⁺ (e.g., Zn²⁺) are retained. Reproduced with permission from ref. 115, published by Elsevier, 2016. (b) Scheme of the FO–ED integrated process for wastewater (or brackish water) reclamation. Reproduced (adapted) from ref. 119, published by the American Chemical Society, 2013. (c) Scheme of the IX–RO–ED system for concentrating LiCl from industrial lithium-containing wastewater. Reproduced from ref. 120, published by the American Chemical Society, 2019. (d) Scheme of ED desalination of polymer flooding produced water. Reproduced from ref. 121, published by Elsevier, 2018.

Using real urine feed characterized by the conductivity of 12.65 mS cm⁻¹, ammonia nitrogen content of 207 mg L⁻¹, and chemical oxygen demand of 6.33 g L⁻¹, MD produced a retentate with double the conductivity (24.1 mS cm⁻¹) and a permeate with conductivity measuring 0.21 mS cm⁻¹. Subsequently, these output streams were utilized as feeds for an RED unit, facilitating partial remixing together with energy recovery. The maximum power density (P_d,max) achieved (∼0.2 W m⁻²) was comparable to that attained with NaCl solutions (0.32 W m⁻²). The experimental data revealed that increasing the temperature from 22 °C to 50 °C could enhance P_d,max by 70%, suggesting the potential utilization of waste heat. RED tests incorporating recirculation yielded the extraction of approximately 47% of the Gibbs free energy. Further optimization of the system could enhance the energy efficiency, while maintaining the desired quality of the final dilate.

Separation intensification using various integrated schemes using membranes is a combined electrodialysis (ED) with forward osmosis (FO) and a novel idea adopted for the efficient removal of salt from the draw stream.¹¹⁹ As an illustration, in one scheme, FO was utilized to extract water from secondary effluent with a salt concentration of 0.05 M. Subsequently, the extracted water was supplied to a draw solution containing NaCl, and then directed to the ED stage (refer to Fig. 10(b)). In the FO process, ions, organic, and inorganic substances are retained in the FO retentate, while the draw stream, enriched in water through osmosis (diluted from 0.5 M to 0.2 M), proceeds to the ED stage. This integrated process resulted in the production of high-quality water with a conductivity in the range of 0.81 to 0.88 mS cm⁻¹, as well as the generation of draw solution for the FO process.

Membranes were applied for the intensification of lithium (Li) separation from industrial wastewater (see Fig. 10(c)) using a combination of ion exchange (IEX)–RO–ED method.¹²⁰ Initially, the effluent (with 1268.9 mg L⁻¹ Li⁺ and conductivity of 17.87 mS cm⁻¹) originating from a lithium-ion battery manufacturing facility underwent softening to prevent scaling. Subsequently, it was concentrated via reverse osmosis (RO), followed by two-stage electrodialysis (ED), yielding fresh water (RO permeate) with enhanced recovery (ED dilate recycle) and a concentrated solution suitable for lithium carbonate (Li₂CO₃) precipitation through the addition of sodium carbonate (Na₂CO₃). Under the optimized conditions, the RO retentate (with a conductivity of 60 mS cm⁻¹) was divided into dilate and concentrate ED feed solutions in a 3 : 1 volume ratio. This achieved water recoveries of 67.51%, 78.73%, and 69.44% in the RO step, first ED stage, and second ED stage, respectively (with specific energy consumption (E_spec) in the ED stages of approximately 30 and 50 kW h m⁻³). Ultimately, a final lithium chloride (LiCl) concentration of around 87 g L⁻¹ was attained, with an average efficiency (η) of 67.52%, total E_spec of 0.772 kW h kg⁻¹, and overall cost of 0.47 $ per kg (at a process capacity of 282 kg per year).¹²⁰

For betterment of the intensification in the separation process using single membranes, a small modification of the ED–desalination system was performed for the enhanced recovery of oil from wastewater. Polymer flooding is technique for the enhanced recovery of oil from wastewater. This method requires a TDS value in the range of 500–1000 ppm to prevent excessive viscosity reduction of the solution. ED–desalination can be applied (Fig. 10(d))¹²¹ and has been demonstrated by pilot-/large-scale plants.^122,123 However, serious fouling issues occurred and their mechanisms were investigated together with characterisation of the fouled membranes,^124,125 proposing the need for chemical cleaning strategies.^123,126 A small modification during the separation step was performed to reduce the fouling and enhance the oil recovery. A synthetic solution containing TDS levels of 5000 mg L⁻¹ (representative of brackish water) and 32 000 mg L⁻¹ (seawater) and 1.0 g L⁻¹ of partially hydrolyzed polyacrylamide (HPAM) was effectively treated through ED desalination. The concentrate feed contained 5 g L⁻¹ NaCl and a small polymer replenishment (∼25% was retained in the stack) was required. The removal efficiency ranged from 85% to 99%, with the specific energy consumption (E_spec) ranging from 0.5 to 6 kW h m⁻³.

4 Concluding remark

Despite the advantages provided by PI routes, technical gaps have become hurdles in their industrial implementation. Although rigorous research on technical advancements in the process of PI is ongoing, there are many technical and non-technical barriers observed in the running condition, where PI is still lacking lab-scale and pilot plant studies.¹⁰ The overall implementation of PI often requires significant investment in new equipment and technology, which can be a barrier for some companies. Broadly, regarding process complexity, PI may require complex and intricate designs, which can be difficult to manage and operate especially for smaller-scale companies. In terms of scalability, some PI technologies may not be scalable for larger industrial applications, making it challenging to apply them in commercial settings. In terms of technical challenges, other factors for the implementation of PI may require overcoming several technical challenges such as material selection, thermal management, and process control.^127–129

4.1 Challenges and solutions proposed for different sectors discussed IWM

4.1.1 PI technology in energy sector IWM. Integrating membrane-based separation technologies in the energy sector, particularly in the petroleum and biogas industries, can offer several benefits including increased energy efficiency, reduced emissions, and lower operating costs. However, these applications also present a unique set of challenges, such as handling multiple complex feed streams containing a variety of components, including impurities such as sulphur compounds and other volatile organic compounds (VOCs). High operating pressure and temperature and corrosive environment are the primary issues, which can damage and degrade membrane materials and shorten their service life. Thus, to address the above-mentioned challenges, membranes should have the ability to withstand high operating pressures, high temperature and corrosive environment. This can be achieved as follows:

Using advanced membrane materials and module designs capable of withstanding high pressure and temperature such as fabricating inorganic membranes and designing hollow fiber membrane modules, together with flat sheet membranes.

Scaling-up and integration with lab-scale membrane separations have been found to be more promising compared to the challenges faced by industrial processes.

Other points such as working on process optimization using energy recovery devices such as pressure exchangers and turbine generators can significantly improve the energy efficiency to a satisfactory level.

Other solution-based strategies include focusing on the long-term benefits, including energy savings, regulatory compliance, and product quality improvements. In this case, financial models should be considered such as developing individual start-up models and building public-private partnerships through small NGOs to spread out costs or seek funding opportunities for the development of energy-efficient technologies integrated with membranes.

The process proposed for biogas treatment can also be applied to other waste gaseous streams, such as that from biomass gasification and chemical and petrochemical plants, given that the challenges of the process and the advantages of the novel approach are similar.

Safety concerns are the main issue, and thus adopting new technologies and equipment can introduce new safety risks and require additional safety measures, where operators can be trained accordingly. Also, PI may require obtaining regulatory approval, which can be time-consuming and costly.

4.1.2 PI technology in textile industry IWM. The intensification of textile processes using membranes can address some challenges such as fouling, scaling, high operating cost, high energy consumption, and limited lifespan. In addition, the other challenges including satisfying the regulatory compliance of the pollution control board for the proper permissible discharge of pollutants, process complexity, material compatibility, and membrane flux decline, and these issues cannot be ignored. Thus, to address these issues and for the successful implementation of process intensification using membranes in the textile industry, firstly, it is essential to address these challenges through careful process design, material selection, and ongoing monitoring and maintenance. We can start by working on the following points:

To address the low permeate quality issue, advanced membrane types can be employed, choosing membranes with smaller pore sizes or select membranes specifically designed to reject certain types of pollutants. A multi-stage filtration system integrated with different types of membranes (e.g., microfiltration, ultrafiltration, and nanofiltration) can be implemented to remove a wider range of pollutants once through operation. Other post treatment technologies, such as activated carbon adsorption, advanced oxidation processes, and microbial separation can further polish the permeate and remove residual pollutants.

Secondly, the energy consumption should be optimised by employing energy recovery systems, such as pressure exchangers and turbine generators, to capture and reuse energy from high-pressure brine streams. The system operation can be further optimized by adjusting the flow rate, feed concentration, and operating pressure to minimize the energy consumption, which is an option that cannot be ignored. Alternatively, membranes with lower fouling tendencies and higher permeability can be employed to reduce the energy required for filtration.

Thirdly, considering the key point of the huge discharge of liquid waste into water bodies, to meet environment regulatory compliance, the installation of advanced monitoring systems for real-time monitoring and track pollutant concentrations to adjust treatment processes accordingly is necessary.

To address these challenges, national and international collaboration and working closely with regulatory agencies to understand specific discharge requirements and develop treatment strategies that meet or exceed those requirements are options. By addressing these challenges and implementing the suggested solutions, the textile industry can effectively integrate membrane separation technology in its wastewater treatment processes, leading to an improved environmental performance and regulatory compliance.

4.1.2.1 PI technology in other process industry wastewater purification IWM. PI technology in wastewater separation using membranes is continually facing similar challenges as mentioned above in the textile industry and energy sectors. Thus, the following solutions are proposed to deal with these issues:

Integrating a set of innovative technologies can pave the way for the development of breakthrough membrane-based integrated plants for biomethane purification, waste recovery of valuable compounds (e.g., VFA), and water vapor, thus reducing the need for pre-treatments. This goal can be achieved by combining and integrating advanced membrane operations that can overcome the current limitations of conventional technologies, as well as by developing novel multifunctional materials with optimized separation properties capable of withstanding aggressive environments.

More focus should be given to improving the communication among areas such as advanced membrane materials, module design, economics, membrane morphology, supramolecular engineering, mass production, and process control. This also involves analysing the benefits, establishing clear processing protocols, comparison of indexes and metrics, and accurate modelling and optimization.

In desalination by membrane operation processes, process intensification has achieved the ambitious goal of “zero liquid discharge”. However, some disadvantages have been noted, such as increased complexity and the need for more complex control systems, as well as the necessity of enhancing plant safety.

Encouraging progress is already being made, with innovative membrane-integrated processes being designed for water desalination, agro-food and petrochemical industries, biotechnological processes, and other industrial fields. This progress makes the goal of sustainable industrial growth more attainable.

5 Conclusion

PI strategies are particularly useful in remote areas and process extension works where traditional processes may not be feasible. However, the large-scale industrial implementation of PI requires addressing the challenges associated with integrating multiple processes into one process. In this case, further research, development, and demonstration are required to overcome the challenges and uncertainties associated with its implementation in upcoming research.

PI offers significant advantages for process industries; however, it is important to consider its limitations and challenges that must be overcome to realize its full potential. Regarding membranes, no solution has been found to remove fouling from membrane technology. Membranes are also dependent on their chemical and biocompatibility with the corresponding feed. The membrane operation is dependent on multiple parameters, such as flux, pH of the fluid stream, hydrophobicity, hydrophilicity, and pore size of the membrane, where many of these parameters acts as hurdles for the industrial implementation of membranes.^130–133 Membrane operation is limited by the erosion and damage of the membrane if it encounters lager particles in the process stream, which may also clog the membrane surface, resulting in the requirement of a pre-filtration process.⁴¹ Accordingly, to overcome these hurdles, hybrid membrane separation can intensify the process to a larger extent, maintaining the inherent properties of membranes, while enabling process intensification using hybrid separation techniques. RD&D programs should focus on improving the hardware, operating conditions, and methodologies, as well as phase transition, integration of processes and equipment, and process design and flexibility for better usage. Moreover, these programs should also address the challenges and uncertainties associated with PI, including selectivity in separation processes, yield, throughput, durability, and risk management.^134,135

Author contributions

Swapna Rekha Panda (S. R. Panda): writing – conceptualization, methodology original draft preparation and editing. Prof. Datuk Ts. Dr. Ahmad Fauzi Ismail (Ahmad F. Ismail): supervision, writing – review & editing. Sudeep Asthana: visualization, investigation. Krunal Suthar: visualization and data presentation. Arvind S. Madalgi: visualisation. Amit Kumar: commentary or revision. Haresh Dave: formatting and visualization. Rakesh K. Sinha: commentary or revision. Koshal Kishor: funding and editing.

Conflicts of interest

There are no conflicts of interest to declare under “Conflicts of interest” heading. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author Swapna Rekha Panda would like to express her gratitude to the UPL University of Sustainable Technology and Prof. Ismail, University of Technology, UTM, Malaysia for providing required information, resources, and support. Koshal Kishor also acknowledges the support from the Science and Engineering Research Board, Department of Science and Technology, and Government of India for supporting this work via Grant no. SERB/F/9286/2022-2023. Further Dr. Panda acknowledges Dr. Anirban Roy, BITs Goa for his suggestion, insight, and vision to initiate writing a review paper on the said topic.

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