Negative Pressure Membrane Distillation for Excellent Gypsum Scaling Resistance and Flux Enhancement

Abstract

Membrane distillation (MD) has potential to become a competitive technology for managing hypersaline brine but not until the critical challenge of mineral scaling is addressed. The state-of-the-art approach for mitigating mineral scaling in MD involves the use of superhydrophobic membranes that are difficult to fabricate and are commercially unavailable. This study explores a novel operational strategy, namely, negative pressure direct contact membrane distillation (NP-DCMD) that can minimize mineral scaling with commercially available hydrophobic membranes and at the same time enhance the water vapor flux substantially. By applying a negative gauge pressure on the feed stream, NP-DCMD achieved prolonged resistance to CaSO4 scaling and a dramatic vapor flux enhancement up to 62%. The exceptional scaling resistance is attributable to the formation of a concave liquid–gas under a negative pressure that changes the position of the water–air interface to hinder interfacial nucleation and crystal growth. The substantial flux enhancement is caused by the reduced molecular diffusion resistance within the pores and the enhanced heat transfer kinetics across the boundary layer in NP-DCMD. Achieving substantial performance improvement in both the scaling resistance and vapor flux with commercial membranes, NP-DCMD is a significant innovation with vast potential for practical adoption due to its simplicity and effectiveness.

Negative Pressure Membrane Distillation: A Novel Strategy for Wetting Mitigation

“…A commercial polyvinylidene fluoride (PVDF) membrane (GVHP00010) was procured from Millipore, USA. Based on the literature, the mean pore size, thickness, water contact angle, and LEP of the PVDF membrane are 0.22 μm, 125 μm, 130°, and 240 kPa, respectively. Sodium chloride (NaCl, AR, Sigma-Aldrich), ethanol (AR, Sigma-Aldrich), and SDS (AR, Sinopharm Chemical Reagent Co. Ltd.) were purchased and used without further purification.…”

“…According to our previous study, the negative feed pressure could reduce the vapor transfer resistance inside the membrane pores and mitigate the temperature polarization in the feed, thereby resulting in an enhanced vapor flux. 26 However, with the SDS concentration elevated to 0.2 mM, the PVDF membrane became wetted in negative pressure DCMD with a negative feed pressure of −10 kPa (Figure 2B). Unlike conventional DCMD (Figure 2A and Figure S3), in negative pressure DCMD, once the membrane was wetted, the vapor flux drastically decreased while the distillate conductivity remained relatively unchanged.…”

“…However, such a strategy has seldom been investigated because the applied hydraulic pressures in MD operations are often considered to be negligible. Very recently, we developed a novel MD operation mode named negative pressure membrane distillation (NPMD) and demonstrated that NPMD can effectively mitigate membrane scaling in MD operations . In NPMD, a negative gauge pressure (i.e., relative to atmospheric pressure) is applied on the feed stream to induce a negative transmembrane hydraulic pressure.…”

“…Very recently, we developed a novel MD operation mode named negative pressure membrane distillation (NPMD) and demonstrated that NPMD can effectively mitigate membrane scaling in MD operations. 26 In NPMD, a negative gauge pressure (i.e., relative to atmospheric pressure) is applied on the feed stream to induce a negative transmembrane hydraulic pressure. We hypothesize that NPMD can impart wetting resistance even with the use of conventional hydrophobic membranes.…”

Development of Robust Fluorinated SiO2/PVDF Composite Hollow Fiber Membrane for Bromine Resources Recovery from Brine via Membrane Distillation

“…In a similar manner, membrane distillation (MD) can also be used to extract bromine, where the permeate vaporized bromine can be collected directly without any absorbent. MD is a thermally induced desalination process driven by the vapor pressure gradient difference between the two sides (a hot feed and a cold permeate) of a microporous and hydrophobic membrane. − MD has similar advantages to membrane absorption but is more efficient in bromine extraction by imposing extra vacuum forces. , The hydrophobicity of the membrane prevents the infiltration of the aqueous solution, because only vapor can be transported through the pore channels, so MD can theoretically achieve a nearly complete rejection of nonvolatile salts. , Tang et al employed a membrane distillation process to separate bromine–water mixtures with different flat polyurethane membranes. However, normal polymeric such as polyurethane membrane tends to suffer from a low vapor flux and a short life span of the membrane process, owing to the challenging operation condition (at relatively low pH values of 2.0) that easily degrades membrane materials.…”