Designing high-caliber nonwoven filter mats for coalescence filtration of oil/water emulsions

Significance Statement

Oil–water separation has attracted considerable attention because of tightened environmental regulations and the need for water recycle and reuse. Compared with the separation of common oil–water mixtures, effective separation of oil-in-water emulsions is much more challenging due to the emulsion’s stability. Coalescence separation, as one of the traditional technologies, can be used to tackle the problem. However, it is often constrained by the choice of available materials that permit high efficiency of separation, especially when the oil droplet size is small and when there are surfactants present.

In principle, coalescence process allows the emulsion to pass through the material where the dispersed oil droplets are captured and enlarged inside the material, released and separated by gravitational/buoyancy force downstream outside the material.  It is an opposite process to membrane separation that relies on the relatively smaller pore size with respect to droplet size as well as surface oil repellency to separate the droplets. Good separation can still be accomplished by using a coalescing material even when its pore size is larger than the droplet size. Besides geometric properties such as pore size and material thickness, surface properties of a coalescing material are of significant importance in governing the effectiveness of coalescence separation.

Coalescing materials can be made of polymeric resins and nonwovens. Nonwoven fibrous media can be made by either wet-laid or dry-laid process using various stapled fibers. For efficient separation of small oil droplets, original nonwoven typically require surface treatment to balance their surface properties for oil droplet capture and release. However, the formulation of such treatment is not easy to come by. In this work, we developed a counter-intuitive treatment method that allowed for effective coalescence separation of four kinds of 2µm oil-in-water emulsions, representative of gasoline, diesel, vegetable and lubricant oils.

We started by fabricating a non-woven fibrous material using a mixture of glass and cellulose fibers with controlled pore size and thickness through a simple wet-laid process. The material did not have any physical integrity in water so a binder resin was added it to it to gain workable mechanical strength.  Yet before the binder was fully-dried and solidified, the material was put into a solvent bath dosed with a surfactant. Ultrasonication was then applied to the bath to help create an emulsification process of the resin, after which the material was taken out and ventilation-dried. The whole procedure allowed the material to be endowed with a roughened and ragged fiber surface that was proven to be effective in enhancing coalescence separation. Moreover, such treated materials, even when pre-saturated with the target oil for separation, one of the worst case scenarios a coalescing material could ever encounter in the field, was still effective in separation. In a sense that the material seemed to have the right balance of oil affinity and repellency under water due to the treatment that created a nano-micro structure on the surface of the fibers. The tradeoff was a reduced but still sufficient material tensile strength, which could be further enforced by using a plastic or metal screen backing. We expect such a simple surface treatment technique to be widely applied in soft coalescing material design, its effectiveness in helping separate surfactant emulsified oil-in-water emulsions is being evaluated and will be reported soon.  

About the author

Dan Hu is currently a PhD student at Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS) under the supervision of Professor Chuanfang Yang. She received her B.S. degree in chemical engineering and technology at Beijing University of Chemical Technology in 2012. Her on-going research is centered on fibrous materials, nano-enhanced surface modification and wetting behavior studies toward the goal of developing effective coalescence materials for oil-in-water emulsion separation. She has co-authored three related peer-reviewed papers published in Separation and Purification Technology, Journal of Materials Chemistry A, and RSC Advances. She has also one invention patent pending.

About the author

Dr. Chuanfang Yang, trained as a chemical engineer for all his degrees, is now a professor of Institute of Process Engineering, Chinese Academy of Sciences. Prior to this, he spent 4 years conducting research at the Department of Chemical Engineering and Materials Science of University of Minnesota, followed by 10 years industrial R&D experience in filtration and separation, during which his main responsibility was to carry out and lead applied research and be heavily engaged in product development and commercialization. He’s well published in peer-reviewed journals including AIChE J, Chemical Engineering J, J. of Membrane Science, J. of Hazardous Materials, J.  Colloid & Interface Science and so on. He is also a frequent speaker in many academic and industry oriented conferences internationally. On the application side, he has 20 granted and pending USA, EU and China patens under his name and contributed significantly to several new filtration products development.  He was the Senior Scientist winner of American Filtration & Separation Society in 2011 in recognition of his contribution to filtration technology advancement. His current research focuses on nanotech-enhanced environmental separation that covers coalescence separation, membrane separation, adsorption, functional synthetic and natural nanomaterials, surface sciences and heterogeneous photocatalysts.  

 

Advances In Engineering, Designing high-caliber nonwoven filter mats for coalescence filtration of oil/water emulsions

Journal Reference

Separation and Purification Technology, Volume 149, 2015, Pages 65-73.

Dan Hu1,2, Xiaoyu Li1, Lei Li1,2, Chuanfang Yang1

[expand title=”Show Affiliations”]
  1. National Key Laboratory of Biochemical Engineering and Key Laboratory of Green Process & Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
  2. University of Chinese Academy of Sciences, Beijing 100049, China
[/expand]

Abstract

Oily wastewater is a major problem in industries that can cause severe environmental pollution without proper treatment. In this paper, an easily fabricated, robust and durable nonwoven fibrous filter mat was prepared for coalescence filtration of four kinds of oil-in-water emulsions, namely hexadecane/water, octane/water, soybean oil/water and engine oil/water. An aromatic thermoplastic polyurethane resin (TPU) was selected to bind the fibers together for structure integrity of the mat. Special roughening treatment to the mat was conducted to deliver the required wettability for effective coalescence separation of these emulsions. The filter mat thus prepared reached a tensile strength of 2.99 MPa, 20 times stronger than that of the pristine mat. The mat surface was made both hydrophilic and superoleophilic, allowing it to have separation efficiency as high as 99.61% in a single pass flow. Moreover, even pre-saturated with oil, the mat remained effective in separation and permitted high flow, proving its high caliber for potential industrial applications.

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