Porous PTFE Membranes

porous ptfe for membrane distillation

ECLIPSE Membranes® for Membrane Distillation

Markel : Porous PTFE Membranes : Contacting and Separation : ECLIPSE Membranes® for Membrane Distillation

Why Membrane Distillation

Distillation as a means of separating two liquids or purifying one liquid has been in existence since at least 200 AD. There is ample evidence of distillation in many forms in both Greek and Egyptian cultures. In its conventional form, distillation is a thermal process for separating multiple liquids or liquids from solids via differences in boiling point. Why now explore membrane distillation? There are many advantages to osmotic and membrane distillation over conventional thermal distillation processes.

 

• Separation of thermally sensitive materials – many membrane distillation processes (osmotic distillation) rely on concentration differences, or differences created via partial pressure to effect a separation, rather than a pure thermal driving force, making them ideal for isolation and recovery of thermally sensitive materials.

 

• A theoretical 100% rejection1 of ions, macromolecules, colloids, cells, and other non-volatiles from the process stream.

 

• Low operating temperatures and operating pressures

 

• Reduced energy requirements

 

• Mobility – in this age of decentralization of the chemical process industry, the ability to set up smaller, localized, processing facilities is increasingly important.   The compact and modular nature of membrane distillation units make them particularly appealing when there are advantages of bringing the process to the raw material rather than the other way around.

 

• Scalability – Having developed a separation process on a small or pilot scale system, scaling up membrane operations are relatively risk free and straightforward as it is based simply on available surface area of membrane.

 

1. Evans, Lindsey R., and James E. Miller, “Sweeping Gas Membrane Desalination using Commercial Hydrophobic Hollow Fiber Membranes”, Sandia national Laboratories, January 2002

Characteristics of all Methods of Membrane Distillation

Characteristics common to all the methods of membrane distillation outlined below include1:

 

• The membrane is not wetted by the process liquids, that is, no liquid passes through the membrane wall. Moreover, any penetration by a liquid greatly inhibits the transport of the distillate from one side of the membrane to the other.

 

• The membrane is porous (as opposed to semi-permeable membranes which are non-porous)

 

• No capillary condensation takes place within the pores of the membrane

 

• The driving force for mass transfer is differences in partial pressure of the species of interest between the two sides of the membrane.

 

• One side of the membrane is in direct contact with the liquid

 

It is in this fifth category that many forms of membrane distillation differ from one another.  In all cases the feed stream is in direct contact with the membrane, but in various modes of membrane distillation, the distillate side differs markedly.

 

1. Evans, Lindsey R., and James E. Miller, “Sweeping Gas Membrane Desalination using Commercial Hydrophobic Hollow Fiber Membranes”, Sandia national Laboratories, January 2002

Methods of Membrane Distillation

As outlined by Evans and Miller in their report on sweeping gas membrane distillation1, there are several variants on membrane distillation:

 

Direct Contact Membrane Distillation (DCMD)

Direct Contact Membrane Distillation (DCMD) employs the porous hydrophobic membrane to separate two liquid streams that are both in contact with the membrane. The transport of the species of interest is driven by temperature differences between the two liquid streams and moves across the membrane as a vapor. The mass transfer occurs from the hot liquid feed to the colder permeate stream through evaporation at the pore entrance, transport through the pore as a vapor, and condensation into the cold liquid permeate stream on the outside of the membrane. Along with the distillate, some of the heat from the feed stream is transported across the membrane through conduction as well as losses due through the latent heat of vaporization.

 

Direct contact membrane distillation is useful for purifying a liquid feed stream (removal of pollutants, desalination, etc.) or where one desires to concentrate the feed stream as in the case of concentrating foods, beverages, or other products.

 

Air Gap Membrane Distillation (AGMD)

Air Gap Membrane Distillation (AGMD) solves some of the shortcomings of direct contact membrane distillation by providing an air space between the outside membrane wall and the condensing surface. This eliminates the loss of heat through conduction through the membrane wall.

 

Sweeping Gas Membrane Distillation (SGMD)

Sweeping Gas Membrane Distillation (SGMD) employs an inert sweeping gas moving concurrently or counter-currently on the outside of the membrane to maintain a low vapor pressure for the diffusing species. The distilled species must then be condensed out or otherwise removed from the sweep gas to recover it.

 

Vacuum Membrane Distillation (VMD)

Vacuum Membrane Distillation (VMD) is similar to the sweeping gas membrane distillation but instead of an inert gas, a vacuum is applied to the distillate side of the membrane. Again, the species being removed must then be condensed out or trapped from the discharge of the vacuum system.

 

Osmotic Distillation (OD)

Osmotic distillation is a separation process in which a liquid mixture containing a volatile component is contacted with a second liquid phase with an affinity for that volatile component. The two liquid phases are contacted through a porous, hydrophobic membrane.

 

Like membrane distillation, osmotic distillation employs the differences in vapor pressures of the contacting liquid phases to drive the mass transfer. Like membrane distillation, water moves across the (OD) membrane by evaporating, diffusing through the pores, and condensing on the other side of the pores. Also like membrane distillation, osmotic distillation requires a tough, chemical resistant, extremely hydrophobic membrane.

 

The difference between the two is that osmotic distillation achieves this difference not by requiring the feed stream to be heated, but by exposing the feed stream to a stripping stream in which the vapor pressure is low either due to its having a high concentration of inorganic compounds or through the use of low vapor pressure organic solvents. This allows actual separation of water from other components in the feed stream without the other components "following" the water as it moves across the membrane, but more importantly allows this separation to occur without the use of heat.

 

While osmotic distillation is not useful in desalination processes or other water purification processes, it is an extremely valuable tool in concentrating foods, beverages, flavors, fragrances, or pharmaceutical products that are sensitive to temperature.

 

Bessarbov and Twardowski authored an excellent summary of one of the new opportunities for osmotic membrane distillation for the chlor-alkali industry (“New Opportunities for Osmotic Membrane Distillation” Membrane Technology, July 2006). In it they offer that fluoropolymers are the only economical family of polymers that offer the required properties including thermal stability, chemical resistance, and hydrophobicity. They go on to point out the shortcomings of PVDF (low chemical resistance in the presence of NaOH) and the cost of some of the fully fluorinated amorphous polymers such as Hyflon AD. The authors acknowledge that e-PTFE is an optimal choices in the fluoropolymer family for this significant commercial opportunity.

 

Regardless of the technique, and it is evident that there are many, porous PTFE hollow fiber membranes are ideal candidates for these developing systems. Whether the application is concentration of fragrances, recovery of flavors, extraction of vitamins, desalination of water, or other chemical operations, the chemical resistance, hydrophobicity, and chemical inertness of PTFE is the best choice for designing these systems.

 

The Markel potting technology, wherein the potting system is every bit as robust as the PTFE fiber itself is a key enabling technology for all forms of membrane distillation. Without the ability to mount, seal, and hold the fibers and tolerate the extreme and often times harsh chemical environments found in certain membrane distillation systems, even the PTFE hollow fiber membranes would be of little use.

 

Contact Markel to discuss the details of your separation application and see if the combination of the Markel fiber and potting technology can give you a viable and economic solution to your problem.

 

1. Evans, Lindsey R., and James E. Miller, “Sweeping Gas Membrane Desalination using Commercial Hydrophobic Hollow Fiber Membranes”, Sandia national Laboratories, January 2002