MEMBRANE DISTILLATION (MD) is a thermally driven separational program in which separation is enabled due to phase change. A hydrophobic membrane displays a barrier for the liquid phase , allowing the vapour phase (e.g. water vapour) to pass through the membrane's pores. The driving force of the process is given by a partial vapour pressure difference commonly triggered by a temperature difference.
* 1 Principle of membrane distillation
* 2 Applications * 3 Solar-powered membrane distillation * 4 Challenges * 5 References * 6 Literature
PRINCIPLE OF MEMBRANE DISTILLATION
Capillary depression of water on a hydophobic membrane Schematic image of the membrane distillation process Temperature and pressure profile through the membrane considering temperature polarisation
State of the art processes that separate mass flows by a membrane, mostly use a static pressure difference as the driving force between the two bounding surfaces (e.g. RO), a difference in concentration (dialysis ) or an electric field (ED). Selectivity of a membrane is produced by either its pore size in relation to the size of the substance to be retained, its diffusion coefficient or electrical polarity . However, the selectivity of membranes used for membrane distillation (MD) is based on avoiding passage of liquid water while allowing permeability for free water molecules and thus, for water vapour. 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. As water has strong dipole characteristics, whilst the membrane fabric is non-polar, the membrane material is not wetted by the liquid. Even though the pores are considerably larger than the molecules, the liquid phase does not enter the pores because of the high water surface tension . A convex meniscus develops into the pore. This effect is named capillary action. Amongst other factors, the depth of impression can depend on the external pressure load on the liquid. A dimension for the infiltration of the pores by the liquid is the contact angle Θ=180 – Θ'. As long as Θ > 90° and accordingly Θ' > 0° no wetting of the pores will take place. If the external pressure rises above the so-called wetting pressure, then Θ = 90°resulting in a bypass of the pore. The driving force which delivers the vapour through the membrane, in order to collect it on the permeate side as product water, is the partial water vapour pressure difference between the two bounding surfaces. This partial pressure difference is the result of a temperature difference between the two bounding surfaces. As can be seen in the image, the membrane is charged with a hot feed flow on one side and a cooled permeate flow on the other side. The temperature difference through the membrane, usually between 5 and 20 K, conveys a partial pressure difference which ensures that the vapour developing at the membrane surface follows the pressure drop, permeating through the pores and condensing on the cooler side.
MEMBRANE DISTILLATION TECHNIQUES
Schematic DCMD arrangement Schematic AGMD arrangement
Many different membrane distillation techniques exist. The basic four techniques mainly differ by the arrangement of their distillate channel or the manner in which this channel is operated. The following technologies are most common:
* Direct Contact MD (DCMD) * Air Gap MD (AGMD) * Vacuum MD (VMD) * Sweeping Gas MD (SWGMD) * Vacuum multi-effect membrane distillation (V-MEMD) * Permeate Gap MD (PGMD)
In DCMD, both sides of the membrane are charged with liquid- hot feed water on the evaporator side and cooled permeate on the permeate side. The condensation of the vapour passing through the membrane happens directly inside the liquid phase at the membrane boundary surface. Since the membrane is the only barrier blocking the mass transport, relatively high surface related permeate flows can be achieved with DCMD. 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 layer. This lost heat is not available to the distillation process whereby its efficiency is lowered .
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. A further advantage towards DCMD is the fact, 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. AGMD is especially advantageous compared to alternatives at higher salinity. Variations on it can include hydrophobic condensing surfaces for improved flux and energy efficiency.
Schematic SWGMD arrangement Schematic VCMD arrangement
Sweeping-gas MD, also known as air stripping, uses a channel
configuration with an empty gap on the permeate side. This
configuration is the same as in AGMD.
Vacuum MD contains an air gap channel configuration. Once it has passed through the membrane, the vapour is sucked out of the permeate channel and condenses outside the module as with SWGMD. VCMD and DWGMD can be applied for the separation of volatile substances from a watery solution or for the generation of pure water from concentrated salt water. One advantage of this method is that undissolved inert gasses blocking the membrane pores are sucked out by the vacuum, leaving a larger effective membrane surface active. Furthermore, a reduction of the boiling point results in a comparable amount of product at lower overall temperatures and lower temperature differences through the membrane. A lower required temperature difference leaves a lower total- and specific thermal energy demand. 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 electrical energy demand is a lot higher as with DCMD and AGMD. An additional problem is the increase of the pH value due to the removal of CO2 from the feed water. For vacuum membrane distillation to be efficient, it is often run in multistage configurations.
Schematic image of a DCMD channel configuration Schematic image of a PGMD channel configuration
In the following, the principle channel configuration and operating method of a standard DCMD module as well as a DCMD module with separate permeate gap shall be explained. 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.
The complete channel configuration consists of a condenser channel with inlet and outlet and an evaporator channel with inlet and outlet. These two channels are separated by the hydrophobic, micro porous membrane. For cooling, the condenser channel is flooded with fresh water and the evaporator e.g. with salty feed water. The coolant enters the condenser channel at a temperature of 20 °C. 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. Sensible heat conduction also heats the cooling water through the surface of the membrane. Due to the mass transport through the membrane the mass flow in the evaporator decreases whilst the condenser channel increases by the same amount. The mass flow of pre-heated coolant leaves the condenser channel at a temperature of about 72 °C and enters a heat exchanger, thus pre-heating the feed water. 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. The evaporation process extracts latent heat from the feed flow, which cools down the feed increasingly in flow direction. Additional heat reduction occurs due to sensible heat passing through the membrane. The cooled feed water leaves the evaporator channel at approximately 28 °C. Total temperature differences between condenser inlet and evaporator outlet and condenser inlet and evaporator outlet are about equal. In a PGMD module, the permeate channel is separated from the condenser channel by a condensation surface. This enables the direct use of a salt water feed as coolant, since it does not come into contact with the permeate. Considering this, the cooling-or feed water entering the condenser channel at a temperature T1 can now also be used to cool the permeate. Condensation