Membrane distillation is a thermal process in which only vapour molecules can pass through a hydrophobic membrane. The driving force for the separation is the vapour pressure and not the total pressure, as with reverse osmosis. Increasing the temperature of the feed increases the vapour pressure and therefore the vapour pressure gradient, which is the driving force.
Membrane distillation equipment consists of several stages, or effects, leading to improved separation and greater energy efficiency.
As with the case of evaporation, the membrane distillation process is not limited by equilibrium; thus, the desired water recovery factors and reject concentration can be achieved. Unlike reverse osmosis, there is no equilibrium imposing a limit upon the separation.
Membrane distillation has a number of advantages that make it very competitive compared to other techniques. The main ones are:
- In general, the technology requires no feed pre-treatment to extend the life of the membrane, as this is not in contact with the solution but clean steam.
- The efficiency of the system and good quality of the water produced are practically independent of the feed contaminant concentration.
- 100% of non-volatile solutes are rejected.
- Corrosive and acid effluents can be treated, which is difficult for conventional distillation due to the materials required.
- Investment costs are low because plastic materials are used.
- Operating costs are low, because of the high process efficiency and because pre-treatment of the feed is not usually necessary.
- It has a wide range of possible applications, due to its modularity and the possibility of treating many different types of solutions.
Membrane distillation has been successfully used in the following applications:
Desalination
- Sea water can be converted to drinking or process water.
- Brine can be concentrated to increase the capacity of existing desalination plants.
- Brackish water from contaminated aquifers can be desalinated.
- As a step towards the implementation of a zero discharge system, when water recovery is high.
- Using solar energy.
Extraction from petroleum
- Reduction in the volume of water produced during petroleum extraction.
- Water treatment in conventional and unconventional extraction systems (e.g. fracking).
- Wastewater treatment in refineries.
Mining and industrial processes
- Concentration of molasses.
- Obtaining recoverable substances from wastewater.
- Organic effluent concentration.
- Halogenated alkaline effluent treatment.
- Concentration of acid drainage from mining for near zero waste.
- Concentration of volatile organic compounds (VOC).
Food industry
- Wastewater treatment.
- Concentration and separation of wastewater from olive oil production.
- Separation of recoverable substances and concentration of whey.
- Water distillation for hydroponic farming.
- Fruit juice concentration and milk processing.
Condorchem Envitech offers its customers the design and implementation of a membrane distillation separation process. Specifically, there are two complementary treatment options, designed for different treatment volumes:
memDist 4-6.4
- Compact unit consisting of a steam generator, 4 effects, a condenser and a total membrane area of 6.4m2.
- Excellent for treating small volumes or for testing in different applications.
- Produces 50 L/h of distillate.
memDist 6-52
- Standard unit consisting of a steam generator, 6 effects, a condenser and a total membrane area of 52m2.
- Can operate in parallel with other units to treat high flow rates.
- Produces 160-310 L/h of distillate.
