What is Membrane Distillation?
Membrane Distillation (MD) is a relatively new and promising separation technology that is an alternative to more energy intensive methods such as reverse osmosis or distillation. MD is a thermally driven process where only vapor molecules transfer through a hydrophobic (water repellent), microporous membrane.
A typical membrane distillation technique features a membrane filter sandwiched between cold solution on one side and hot solution on the other side. The driving force of the MD process is the partial vapor pressure difference induced by the temperature difference across the hydrophobic membrane. The vapor then condenses on the cold side to produce the distillate.
Membrane Distillation Applications
MD membranes are suited for applications where water is the major component, including desalination, purification, and treatment of process water or for temperature-sensitive industrial products such as the following:
- Concentration of aqueous solutions in food industries
- Concentration of a range of dairy streams, such as whole milk, skim milk and whey
- Concentrating acid solutions or regenerating acids or separating acids
- Removal of volatile organics
Advantages of Membrane Distillation
Compared to conventional membrane processes, MD has the following benefits:
- Low temperature (max 88°C/190°F) compared to conventional distillation
- Low hydraulic pressure (max 100 psi) compared to pressure driven membrane separations
- Less expensive membrane compared to reverse osmosis
- Ability to use waste heat or solar energy
Filters for Membrane Distillation
Polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF), and Polyether ether ketone (PEEK) filters are used for MD because they are both hydrophobic, microporous, and exhibit the following characteristics:
- Low thermal conductivity to prevent heat loss across the membrane
- Low resistance to mass transfer to increase membrane flux
- Good thermal stability to cover a wide range of temperatures
- Optimum membrane molecular cut-off weight (MWCO) between 0.1 – 0.5μm
Membrane Distillation Lab Setup
Sterlitech’s test cells provide fast and accurate data to better evaluate membrane/membrane system performance. For MD purposes, Acrylic, Delrin, or PTFE test cells are recommended to mimic conditions of a large-scale MD system. An acrylic test cell, available in Air Gap and Direct Contact configurations, offers users a “plug and play” option to quickly get started. In addition to the test cell, the below equipment will be required in an MD lab set up:
- Low pressure pump× 2 (<100 psi)
- Low pressure tubing
- Flow meters
- Pressure gauges (pressure spikes above the liquid entry pressure could wet the membrane and let the water to permeate through)
- Feed tank and coolant/permeate tank
- Scales
- Chiller Heating jacket/heating bath or hot plate
For larger-scale MD operations, we recommend a unique skid mounted membrane distillation system equipped with pressure, flow, and temperature controls, conductivity measurements and safety features.
Contact one of our Application Engineers for more information on membrane selection and setup recommendations. For more information on this topic, get a free copy of our whitepaper on Membrane Distillation: Status and Perspective for Research and Industry.