Investigation of Membrane Distillation Crystallization for Desalination Processes

Abstract

Brine disposal remains one of the most significant environmental challenges associated with desalination, driving the need for sustainable technologies such zero liquid discharge (ZLD) systems. Among emerging ZLD techniques, membrane distillation crystallization (MDCr) offers a promising approach by integrating water recovery with salt crystallization. In MDCr, a hydrophobic membrane separates water vapor from concentrated brine via a tem-perature-driven vapor pressure gradient, while the remaining solution reaches supersatura-tion, enabling salt crystallization.
This study investigated key operational parameters in MDCr, particularly the effect of bot-tom-section mixing and feed temperature. Experiments were conducted using a polytetraflu-oroethylene (PTFE) flat sheet membrane and polyvinylidene fluoride (PVDF) hollow fiber membranes in a custom-built reactor divided into a warm distillation zone and a cooler crys-tallization zone. The influence of thermal gradients, membrane type, and mixing intensity on water flux and crystal formation was evaluated.
The tests with the flat sheet membrane demonstrated a permeate flux of 2.63 LMH and showed evidence of crystallization, though the limited surface area of the membrane result-ed in lower feed concentration and slower crystallization.
The system was then tested using the hollow fiber membrane. The results showed that in-creasing the temperature gradient from around 20 °C to 30 °C significantly improved water flux, increasing it from 1.02 LMH to a stable range of 2.25–2.35 LMH (125%). This was due to the stronger driving force for vapor transport at higher temperature differences. However, after about 120 hours of continuous operation, scaling started to appear on the membrane surface. At a feed conductivity of 160 mS/cm, the flux dropped to 0.99 LMH, and visible needle-shaped salt crystals formed on the membrane and inner walls of the reactor
In the mixing test, the membrane flux remained stable at an average of 2.1–2.2 LMH, with the temperature in the top section maintained in the beginning at approximately 52°C. How-ever, an issue arose after about one day of operation when a leak was detected in membrane I, leading to a sharp increase in feed conductivity, indicating membrane damage. After-wards, two replacement modules were tested, and membrane III was used to continue the test, but it exhibited a reduced flux (down to 1.7 LMH) and a quicker rise in permeate con-ductivity, indicating increased membrane wetting. Despite these issues, the introduction of mixing prevented the salt from settling at the bottom of the reactor, ensuring a more uni-form distribution of the solutes and keeping the concentration gradient between the top and bottom sections of the reactor. Turbidity measurements confirmed initial crystal suspension, followed by settling and growth.
Tests without mixing produced larger, more elongated crystals with prominent needle-like structures, which were primarily composed of calcium (Ca) and sulfur (S), being sodium (Na) being more abundant in square-like crystals. In contrast, crystals formed during the mixing tests were smaller and more diverse in shape.
Overall, these findings highlight the potential of MDCr as a sustainable desalination tech-nology, though challenges such as membrane scaling and wetting require further attention. Future work should focus on optimizing membrane materials and operational conditions to enhance long-term stability and scalability for industrial applications.

Popular Abstract

This thesis explores a new method for cleaning salty water and extracting salts, called Membrane Distillation Crystallization (MDCr). This method not only purifies water but also recovers valuable salts, making it a sustainable solution to two global problems: water shortage and pollution from leftover saltwater (brine).
The process works by using a special membrane that allows only water vapor to pass through, separating it from the salty water. By heating the water, the vapor moves through the membrane, which only allows gas to pass through, and is cooled on the other side to form fresh, clean water. As the salty water becomes more concentrated, salt starts to form crystals. The research looks at how different factors, such as temperature, mixing, and the type of membrane, affect how well this process works in cleaning water and forming salt crystals.
The experiments tested different types of membranes, including flat sheets and hollow fi-bers, to see how they affected the rate at which water passed through and how well the salts formed crystals. The study found that increasing the temperature helped the process work faster, but scaling (salt buildup on the membrane) still created problems. Mixing the water helped to prevent the salt from settling at the bottom, but it did not improve the quality of the crystals or the speed of the water recovery as expected.
The research shows that controlling the temperature, choosing the right membrane, and us-ing mixing are important factors to make this method more efficient. While scaling and oth-er issues remain, using seed crystals to start the crystal formation and improving the materi-als used in the membranes might help to fix these problems in the future.
Overall, this thesis demonstrates that MDCr could be a valuable tool for desalinating water and reducing waste, making it a more environmentally friendly solution. However, more work is needed to improve the technology and make it suitable for large-scale use.

• author: Hossein Panah, Mehrafarin
• supervisor: Frank Lipnizki; Maria Salud Camilleri Rumbau
• organization: Chemical Engineering (M.Sc.Eng.)
• course: METM01 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Membrane Engineering, Chemical engineering, Membrane, Distillation, Crystallization
• language: English
• id: 9205910
• date added to LUP: 2025-08-18 10:48:07
• date last changed: 2025-08-18 10:48:07