Design, engineering and analytical perspectives of membrane materials with smart surfaces for efficient oil/water separation
Graphical abstract
Introduction
Anthropogenic activities are the main reason for the oil spill and petroleum-based hydrocarbons release into the environment and water matrices [5,26]. International economic sanctions and environmental regulations affect the business of crude oil, so multinational oil companies are seriously concerned about the spillage of crude oil and are much interested in the mitigation of these environmental impacts [87]. The use of chemical or biological-based oil/water separation methods [15,82], may have collateral and combined effects but also shows that the spent crude oil cannot be recovered. Thus, it is equally important to look for ease and feasibility of the spent material, which can be recovered from water media [46]. The most extensively used method is chemical demulsification for the destabilization of crude oil and water emulsions [82]. Commercially available demulsifiers are mostly non-ionic surfactants, in which the main hydrophilic part is polyethylene oxide (PEO) and the hydrophobic part is polypropylene oxide (PPO) [1,7].
Smart materials based membrane technology has gained high research interests for the effective separation processes including desalination of seawater, and oil-water separation ( Xuejie et al.,2019). Membrane separation offers numerous advantages, such as (1) environmentally friendly, (2) needs fewer harsh/toxic constituents, (3) ease in processing and energy-efficient separation, (4) no phase transfer, (5) continuous separation with low maintenance charges, and (6) does not need any regeneration [81], which chemical-based methods fail to demonstrate on their own. There is a variety of fibers available with different physicochemical properties such as cotton, cotton fibers, carbon fibers, cellulose metal wire, metal oxide, carbon, and electrospinning fibers, metal wires, electrospinning fibers, carbon nanotubes and MnO2 wires that can be a better choice to fabricate membrane for oil-water separation [69,97]. Membranes with large pore size and high flow rates may be more suitable for emulsion separation [93,103]. Smart hybrid materials with wettability properties are more desirable for the efficient separation of oil/water emulsion or the separation of oil-contaminated wastewater (Fig. 1). This review provides useful information about the emulsions, chemical demulsification, and fabrication of membrane materials for effective separation of oil-contaminated wastewater, and crude oil/water emulsion. The later half of the work discusses various types of membranes and their effective exploitation for a said purpose of efficient oil/water separation.
Section snippets
Emulsion system – physiochemical and stability aspects
The emulsion is a mixture of two immiscible liquids in which one liquid in small droplets is dispersed in other and is stabilized by surfactants [40]. The internal phase is called the dispersed phase, while the external phase is referred to as the continuous phase [29,56]. Interfacial tension, nature of emulsifying agents, presence of solids and bulk properties of both oil and water influence the droplet size distribution of the resultant emulsion. A stable emulsion is one that its structure
Demulsification techniques for oil/water separation
Herein, some conventional demulsification techniques such as chemical, electrical and thermal demulsification are given with suitable examples. Generally, surface-active agents are used in chemical treatment to separate crude oil and water emulsions. A combination of electrical demulsification and microwaves/heating demulsification techniques can be used to improve the demulsification efficiency. Filtration and microfiltration are also one of the effective techniques to destabilize emulsions.
Chemical treatment for oil/water separation
The formation of stable emulsions is due to the presence of emulsifying materials (surfactants). For effective demulsification, there is a need for appropriate chemicals with high separation properties specific to the crude oil/water emulsion to deliver efficient, adaptable and cost-effective emulsion separation [16]. The process of demulsification using suitable chemical demulsifiers, as an additive, is one of the important treatments in the petroleum industry. Table 1 explains the nature and
Membranes treatment of oil/water separation
Membrane separation is the 21st-century emerging technology and recently used more frequently in different fields, such as seawater desalination, gas separation, water purification, and so on [66]. In the process of oil/water separation, the main function of the membrane is like a barrier, which allows the transport of selective substances from one side to another. Because oil and water show different interfacial effects, the important physical properties including breakthrough pressure and
Integrated treatment for oil/water separation
Most of the time, the individual treatment technique may not be suitable for the separation of oil, water emulsions with some specific characters, i.e. with emulsion with low or high specific or ˚API gravity. Thus, it is needed to combine different treatment techniques for the breakup of the immense majority of emulsion systems having different physicochemical characters. If we use only membrane treatment, there are more chances of irreversible fouling due to the presence of thick crude oil,
Ongoing challenges, conclusions, and future outlook
The very first challenge related to the emulsion in the oil industry is that emulsions cause high pressure. Furthermore, the major problem of the emulsion is its environmental damages due to the uncontrolled leakage or accidental spillage. Aiming to tackle this issue, several chemical-based treatments are usually performed, which is not only very costly but also hazardous to the environment. Thus, the proper responses to cut the environmental impact of the oil, water emulsions should be
Acknowledgments
All authors are thankful to their representative universities/institutes for the support and services used in this study.
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