Membrane distillation

[1] These membranes are made of hydrophobic synthetic material (e.g. PTFE, PVDF or PP) and offer pores with a standard diameter between 0.1 and 0.5 μm (3.9×10−6 and 1.97×10−5 in).

[8] A disadvantage is the high sensible heat loss, as the insulating properties of the single membrane layer are low.

However, a high heat loss between evaporator and condenser is also the result of the single membrane layer.

[10] In air-gap MD, the evaporator channel resembles that in DCMD, whereas the permeate gap lies between the membrane and a cooled walling and is filled with air.

The vapour passing through the membrane must additionally overcome this air gap before condensing on the cooler surface.

The advantage of this method is the high thermal insulation towards the condenser channel, thus minimizing heat conduction losses.

However, the disadvantage is that the air gap represents an additional barrier for mass transport, reducing the surface- related permeate output compared to DCMD.

[13] A further advantage over DCMD is that volatile substances with a low surface tension such as alcohol or other solvents can be separated from diluted solutions, due to the fact that there is no contact between the liquid permeate and the membrane with AGMD.

[17] Sweeping-gas MD, also known as air stripping, uses a channel configuration with an empty gap on the permeate side.

[18] The advantage of SWGMD over AGMD is the significant reduction of the barrier to the mass transport through forced flow.

One solution of this problem for SWGMD and for AGMD is the use of a cooled walling for the permeate channel, and maintaining temperature by flushing it with gas.

However, the generation of a vacuum, which must be adjusted to the salt water temperature, requires complex technical equipment and is therefore a disadvantage to this method.

The design in the adjacent image depicts a flat channel configuration, but can also be understood as a schema for flat-, hollow fibre - or spiral wound modules.

After passing through the membrane, the vapour condenses in the cooling water, releasing its latent heat and leading to a temperature increase in the coolant.

This feed water is then delivered to a further heat source and finally enters the evaporator channel of the MD module at a temperature of 80 °C (176 °F).

Therefore, the permeate does not have to be extracted later in the process and the coolant's mass flow in the condenser channel remains constant.

[22][23] Compared to AGMD, in PGMD or CGMD, a higher surface related permeate output is achieved, as the mass flow is not additionally inhibited by the diffusion resistance of an air layer.

[7] The typical vacuum multi-effect membrane distillation (e.g. the memsys brand[clarification needed] V-MEMD) module consists of a steam raiser, evaporation–condensation stages, and a condenser.

Stage 1: Steam from the evaporator condenses on a PP foil at pressure level P1 and corresponding temperature T1.

Although it is called a ‘dead-end’ frame, it does contain a small channel to remove the non-condensable gases and to apply the vacuum.

The heat of condensation is transported through the foil and is immediately converted into evaporation energy, generating new vapour in the seawater feed channel.

Memsys has developed a highly automated production line for the modules and could be easily extended.

[clarification needed] As the memsys process works at modest low temperatures (less than 90 °C or 194 °F) and moderate negative pressure, all module components are made of polypropylene (PP).

Within the MEMDIS project, which kicked off in 2003, the Fraunhofer Institute for Solar Energy Systems ISE began developing MD modules as well as installing and analysing two different solar powered operating systems, together with other project partners.

The standard configuration of today's (2011) compact system is able to produce a distillate output of up to 150 litres per day (40 US gal/d).

Within the MEDIRAS project, a further EU-project, an enhanced two-loop system was installed on the Island of Gran Canaria.

Further applications with up to 5,000 litres per day (1,300 US gal/d) have also been implemented, either 100% solar powered or as hybrid projects in combination with waste heat.

[29] The single biggest challenge for membrane distillation to be cost effective is the energy efficiency.