Examples of applications of membrane distillation.

Since I began my doctoral studies in 1988, I have been continuously working on the research and application of membrane distillation. But it’s difficult. First of all, it is quite difficult to produce qualified microporous hydrophobic membranes. Polypropylene PP and polyvinylidene fluoride PVDF membrane materials lack sufficient hydrophobicity, resulting in a very short service life. Polytetrafluoroethylene PTFE has good hydrophobicity, oxidation resistance, acid and alkali resistance, as well as high-temperature resistance, but it is difficult to process. As for hollow fibers, there are indeed some manufacturers that can produce PP and PVDF hollow fibers, but almost no manufacturers can produce PTFE hollow fiber membranes suitable for membrane distillation. It was not until the beginning of this year that we began mass-producing polytetrafluoroethylene hollow-fiber microporous hydrophobic membranes suitable for membrane distillation. Currently, the inner diameter can be reduced to below 0.45 mm, making it the thinnest polytetrafluoroethylene hollow fiber microporous hydrophobic membrane in the world. Hollow fiber membranes with large inner diameters are not suitable for industrial membrane distillation, as their economic efficiency is not comparable to that of conventional multi-effect evaporation and multi-stage flash evaporation. Secondly, it is difficult to produce qualified membrane modules. Micro-porous, water-repellent flat membrane versions of the materials mentioned above can be produced, but it is difficult to create them as large-scale components.

The third consideration is cost, mainly operating costs. As we all know, both multi-effect evaporation and multi-stage flash evaporation, which are widely used today, have a water production ratio as an important characteristic parameter. The higher the water production ratio, the greater the thermal efficiency and the better the economic viability. For example, the largest desalination plant in our country at present is the Beijiang Power Plant Desalination Plant located in Tianjin, which produces 200,000 tons of fresh water per day. It uses 15-effect evaporators, with a water production ratio of 14 (in practice, hot steam at 170 degrees Celsius, emitted by the thermal power plant, is used as the heat source). What is the water production ratio of traditional membrane distillation? I tell you: it’s generally 0.3 to 1.0. Therefore, conventional membrane distillation simply cannot compete with traditional multi-effect evaporation and multi-stage flash evaporation, neither in terms of scale nor in energy savings.

Over the past decade or so, people have continuously made improvements in energy savings through membrane distillation, but progress has been limited. Generally speaking, in the published articles, the water production ratio obtained through experiments has not exceeded 3, which is equivalent only to the energy-saving effect of a 4-effect evaporator. I began considering the energy-saving aspects of membrane distillation in 2002, and introduced the concept of multi-effect membrane distillation in 2006. Multi-effect membrane distillation modules manufactured using the world’s highest-performance microporous hydrophobic membranes – PP membrane fibers produced by the German company Membrana (with a porosity of ≥70% and inner/outer diameters of 0.33/0.66 mm) – can achieve a water production ratio of 5–16 when using steam at 100–120 degrees Celsius as the heat source. This is indeed a remarkable advancement, but German membrane fibers are too expensive – usually 150 euros per square meter – making them unaffordable. Additionally, their long-term service life is poor, generally not exceeding 6 months.

Small-scale projects are now being carried out using PTFE multi-effect membrane distillation modules; of course, the production cost of these membrane modules remains high.

So it is mainly used to concentrate those liquid streams that cannot be concentrated by multi-effect evaporation, multi-stage flash evaporation, reverse osmosis, or MVR, such as dilute salt acids.

I don’t know why it keeps saying that the content I submitted contains inappropriate information.

“How can “concentration of sodium carbonate solution” be considered bad information?

“*Is “concentration of sodium aoate solution” bad information? By the way. Found a **.

Or take on small-scale projects – those that traditional equipment manufacturers consider unworthy of attention, such as the deep concentration of sodium carbonate and sodium *ao*ate solutions. (Multi-effect membrane distillation can be used for ultra-small projects with a daily processing capacity of one ton; how can multi-effect evaporators of this scale be developed?) Based on the data collected from our construction site, when an 8% sodium *aoate aqueous solution is concentrated to 30%, the average water production ratio is around 6, which is equivalent to the energy-saving effect of an 8-effect evaporator. Of course, there is still considerable room for improvement in this water generation ratio. We expect it to be greater than 10. I hope to exchange more ideas with my chemical industry colleagues who are interested in membrane distillation.

I currently have an MVR unit here that is used for concentrating sulfuric acid AN. However, the concentration of sulfuric acid AN is quite low, and feeding it directly into the MVR requires a significant amount of energy. So I plan to first use membrane distillation to process it before feeding it into the MVR, in order to save energy. Additionally, I am quite interested in the applications you mentioned regarding dilute salt acids. I have a large amount of dilute salt acid at around 3%, and at present it can only be disposed of by concentration; I hope we can work together on this.