Membrane distillation

In membrane distillation is a thermally -driven separation process in which the separation takes place due to a phase change. A hydrophobic membrane in this case represents a barrier for the liquid phase (e.g., salt water) of a fluid stream, the vapor phase (e.g. water vapor ) may permeiren through the pores of the membrane. The driving force for the process is a Partialdampfdruckgefälle, which is usually caused by a temperature difference.

Principle of the membrane distillation

In the conventional separation methods in which the separation of the mass flow takes place through a membrane, a static pressure difference ( for example, RO), a concentration gradient (dialysis) or an electric field (ED) is imparted to the driving force between the two interfaces. The selectivity of the corresponding membrane is caused by their pore size relative to the size of the retentive material, their diffusion coefficients or their electrical polarity. The selective property of a membrane, which is used for membrane distillation (MD ), however, is based on the retention of liquid water with simultaneous permeability to free water molecules that Water vapor. These membranes are made of a hydrophobic plastic (e.g. PTFE, PVDF or PP), and have pores with a diameter of usually 0.1 to 0.5 microns. Since water has strong dipole, while the membrane material is non-polar, it does not come to a wetting of the material through the liquid. Although the pores are much larger than the molecule, the penetration of liquid phase is prevented in the pores of the high surface tension of water, wherein, a convex meniscus is formed in the pore inside. This effect is known as capillary depression. The penetration depth is dependent inter alia on the external pressure acting on the liquid. A measure for the penetration of the liquid into the pores, the contact angle Θ = 180 - Θ ' as long as the following applies: Θ > 90 ° and Θ '> 0 ° is no wetting of the pores instead. If the external pressure is greater than the so-called wetting pressure, then Θ is = 90 ° and there is a short circuit in the pore. The driving force, which promotes the steam through the membrane to recover it as a product on the permeate side can, the water vapor partial pressure difference between the two interfaces of the membrane. This partial pressure is a result of a temperature difference between the two interfaces. As can be seen in the illustration, the membrane on one side with a warm feedwater flow and on the other side is exposed to a cooled permeate stream. The temperature difference across the membrane, which is usually in the range of 5 to 20 K, brings a corresponding partial pressure differential with it, which ensures that the resultant at the membrane interface vapor pressure gradient following permeates through the membrane pores through and condense on the colder side.

Membrane distillation process

In the art, various processes are used in membrane distillation. Essentially, there are four methods that differ primarily by the structure of the distillate channel or its operation. Commonly used are the following technologies:

  • Direct Contact MD method ( DCMD )
  • Air Gap MD method ( AGMD )
  • Vacuum MD method ( VMD)
  • Sweeping Gas MD method ( SWGMD )

Direct Contact MD

When DCMD process both sides of the membrane with liquid are applied. On the evaporator side is the hot feed water, while the permeate permeate is cooled. The condensation of the vapor permeating through the membrane takes place in the liquid phase at the membrane boundary layer directly. Since in this case only the membrane precludes an obstacle to the mass transport, relatively high area- specific permeate can be reached here. The disadvantage is the effect of, however, that even for the sensitive heat conduction of the opportunity provided by the membrane resistance is low, thereby providing a relatively high heat loss from the evaporator to the condenser is formed. This amount of heat is the distillation process is not available, which makes the process of its efficiency.

Air Gap MD

Wherein said air gap MD method, the structure of the evaporator, the passage of DCMD process while the permeate from the permeate side of the membrane and a cooled wall and is filled with air. The permeate through the membrane of water vapor has to override this air gap before it condenses on the cold wall. The advantage of this method is that the air gap has a good thermal insulation is to the condenser, whereby the heat conduction loss can be significantly reduced. The disadvantage, however, is that the air gap for the transport of materials represents an additional resistance, making the area-specific permeate back against the DCMD process. Another advantage of the AGMD opposite the DCMD is that volatile substances having a low surface tension such as alcohol or other solvent can be separated from aqueous solutions since in the AGMD no contact between the liquid permeate and the membrane is that the wetting to the would result.

Sweeping Gas MD

In sweeping gas MD, referred to in the art as air stripping, a channel structure with a free gap is used on the permeate side, which corresponds to the structure in the AGMD. This gap is flushed with a gas during the SWGMD that travels with the vapor permeated through the membrane and out of the carrying module. The condensation of the steam takes place in a condenser located outside the module MD. As with the AGMD also volatiles with low surface tension can be distilled with this procedure. The advantage of SWGMD against AGMD is that the mass transfer inhibiting transfer resistance of the air gap can be significantly reduced by the forced flow. This allows a much higher specific surface material flow than the AGMD reach. A disadvantage of the SWGMD that due to the gas content and the associated overall mass flow higher condenser capacity is needed. When using less gas mass flow, however, there is a risk that the gas at the warmer membrane to excessive heating and thus there is a reduction of the partial pressure difference, and thus the driving force. A solution to this problem is suggested by a cooled wall of the permeate is used in the same as in the SWGMD AGMD through which the purge gas is heated.

Vacuum MD

The Vacuum MD a channel structure is also used with an air gap. The permeated vapor through the membrane is removed through a vacuum from the permeate and condensed as the SWGMD method outside the module. VCMD can be as SWGMD used to remove volatile substances from an aqueous solution, or also for the production of pure water from a concentrated brine. An advantage of acts that are exhausted by the vacuum undissolved inert gases that can block the membrane pores and thus, a larger effective membrane area available. In addition is achieved by the reduction of the boiling point even at lower temperatures and lower absolute temperature differences across the membrane a similar productivity. Due to the lower be impressed temperature difference of the specific thermal energy demand is reduced. The disadvantage here is that the generation of negative pressure must be adjusted according to the temperature of the brine, requires high effort. The MD - modules must be vacuum tight and stable. The electric energy consumption is considerably higher than in the DCMD and the AGMD method. In addition, here is the problem that the pH value increases, since the feed water CO2 is removed.

Permeate Gap MD

Hereinafter the basic channel structure or the operation of a normal DCMD module as well as a module with separated DCMD permeate will be explained. Although the structure shown on the right in the picture sketched a shallow channel arrangement, but can be understood as a scheme for disk modules, hollow fiber modules or spiral-wound modules. The channel consists of the capacitor channel with inlet and outlet and the evaporator channel with inlet and outlet. Evaporator channel and condenser channel are separated by the hydrophobic, microporous membrane. The capacitor channel through which flows for cooling fresh water, the evaporator channel, for example, saline feed water. The cooling water flows at a at a temperature of for example 20 ° C in the condenser channel. The permeate through the membrane of water vapor condensed in the cooling water, its latent heat is released again, leading to a rise in temperature of the cooling water. In addition, heat is added to the cooling water sensitive due to heat conduction through the membrane. Characterized in that a mass transport through the membrane takes place therethrough in the evaporator decreases, the mass flow channel, while it increases in the condenser channel by the same amount. The preheated cooling water mass flow leaving the condenser duct having a temperature of 72 ° C and is fed to a heat exchanger, to serve for preheating the feed water. The preheated feed water is supplied to the reheating to a further heat source, and then passed at a temperature of 80 ° C into the evaporator channel of the MD modulus. Through the formation of steam to the feedwater latent heat is removed, as there will be a further slowdown in the flow direction. In addition, the feed water is withdrawn as a result of sensible heat conduction through the membrane heat. The cooled feed water leaving the evaporator channel with about 28 ° C so here is the same difference in temperature from the condenser inlet is as been impressed between the condenser outlet and evaporator inlet ist.Bei a PGMD module, the permeate is separated with a condensing surface from the condenser channel. The water flowing through the condenser cooling water channel may be directly the saline feed water in this case, since it does not come into contact with the permeate. The entering with the temperature T1 in the condenser cooling or feedwater is now to the permeate in the permeate channel to cool. The condensation of the steam takes place in the liquid permeate. The preheated feed water, having served for the cooling of the condenser, after leaving the capacitor can be fed directly to a heat source for reheating and then passed to the temperature T3 to the evaporator with the temperature T2. The permeate is removed with the temperature T5, the cooled concentrate is discharged with the temperature T4. A great advantage of PGMD process over the DCMD process is that the feed water can be fed directly to the cooling by the module and then the total mass flow enters only via an external heat exchanger to the evaporator. This reduces losses due to heat transfer and saves costly components. A further advantage is that the Permeatüberschuss need not be withdrawn from the cooling water as the permeate has been separated, and the cooling -water mass flow in the capacitor remains constant. The disadvantage is that the permeate in the permeate has a minimal flow rate and therefore the heat transfer from the membrane interface to Kondensatorwandung is very bad. This produces relatively high temperatures on the permeate side of the membrane interface (temperature polarization), leading to a reduction of the vapor pressure difference, and thus the driving force. The advantage is that the losses are reduced by heat conduction through the membrane due to the poor heat transfer. Opposite the AGMD still higher specific surface permeate stream is obtained since the stream is not additionally hampered by the diffusion resistance of an air layer.

Applications

Typical applications of membrane distillation are / can be:

  • Desalination
  • Brackish water desalination
  • Process water treatment
  • Ultrapure water
  • Ammonium removal / concentration
  • Material flow recirculation
  • Wertstoffaufkonzentration

Solar -powered membrane distillation

The membrane distillation, especially in the execution of a spiral wound module for a patent of GORE from 1985 is ideally suited for compact, completely solar-powered desalination for small to medium daily capacity ≤ 10,000 L / day. As part of the EU project started in 2003 MEMDIS were started together with project partners MD modules to construct and develop two different solar-powered systems to install and to investigate at the Fraunhofer Institute for Solar Energy Systems (ISE ). In the first system is a designated as a compact system installation for the production of about 100-120 l / day drinking water from seawater or brackish water. Objective in this system design was to create especially a simply constructed, energy-independent, maintenance-free and robust system for target markets in infrastructure vulnerable areas of the arid and semi-arid zones. The second system was described as a two- circuit system plant with a capacity of about 2.000 L / day. The collector circuit is decoupled from the desalination circle here by a salt- water-resistant heat exchanger. Based on these two systems, several prototype systems have been developed, installed and measured.

With the default configuration of the present (2011) compact system can be a distillate yield of up to 150 l / day to achieve. The thermal energy is provided by a 6.5 m² solar thermal collector field, the electric power from a 75 W PV module. The system will be further developed and marketed by the SolarSpring GmbH, a spin-off from the Fraunhofer Institute for Solar Energy Systems ISE. As part of another EU project, the MEDIRAS project, an advanced dual-circuit system was established in 2011 in Gran Canaria. The system is installed in a 20 ft shipping container and allows with a collector area of ​​225 m² and a heat storage, distillate yields up to 3,000 l / day. Other applications of up to 5000 l / day were achieved, the method is operated either 100 % solar powered or hybrid method in combination with heat.

  • Example plants

Collector field of a dual-circuit system

Solar-powered dual-circuit system with 12 MD - modules and a capacity ~ 3000 L / day ( Location: Gran Canaria)

Compact plant with a daily capacity Oryx150 ~ 150 L / day (location: Mexico)

Activities

Worldwide, and research several companies and research institutions and the process of membrane distillation. Currently active are, inter alia, the following:

  • Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany
  • SolarSpring GmbH, Freiburg, Germany
  • Keppel Seghers
  • Scarab Development AB, Sweden
  • Plataforma Solar de Almería, Spain
  • ITM - CNR. Istituto per la Tecnologia delle membrane, Italy
  • Instituto Tecnológico de Canarias, S. A., Spain
  • Università Degli Studi di Palermo, Italy
  • Deukum GmbH, Frick Hausen, Germany
  • Institute of Environmental Process Engineering, University of Bremen, Germany
  • Memsys clearwater Pte. Ltd.. , Singapore and Grafing
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