Membrane distillation (MD) is generally a nonthermal technology which has gathered momentum, having come to the fore about 50 years ago. Back in 1963, researchers began to explore different membrane systems for commercial processing of all sorts of liquid streams.
In all cases of MD a hydrophobic membrane forms the barrier between a liquid phase and a liquid or gas phase. The vapour phase of interest but usually water passes through the membrane. The driving force of the whole process is the partial vapour pressure difference which is usually promoted by employing a temperature difference between the feed and the collecting stream.
The phenomenon can be described as a three phase sequence (Jiao et al., 2004):
(1) formation of a vapour gap at the warm solution–membrane interface;
(2) transport of the vapour phase through the microporous system;
(3) its condensation at the cold side membrane–solution interface.
Distillation performance as measured by permeate flux also depends on membrane porosity and its thickness. Flux is proportional to porosity and inversely proportional to membrane thickness and tortuosity. The recirculation rate dictates the level of boundary layer resistance. A high recirculation rate reduces the boundary later resistance thus maximising the heat transfer coefficients. That leads to improved permeate flux and better removal of water vapour.
*Its worth noting that MD is a thermally driven process because the molecules are transported through the hydrophobic membranes.
There are two common types of membrane configurations in use:
1) Hollow fiber membrane mainly prepared from polypropylene (PP), polyvinylidenefluoride (PVDF) and PVDF – polytetrafluoroethylene (PTFE), various composite materials
2) Flat sheet membrane mainly prepared from PP, PTFE, and PVDF.
Composite materials have been prepared from porous SiOC ceramic membranes (PSCM) which are based on polydimethylsiloxane) for example. Membranes are prepared using a phase inversion method. The techniques produces a hollow fibre with either a symmetric or unsymmetric structure which can be modified to accommodate various operational aspects, the type of feed material and even fouling materials.
In another example, a supported flat sheet polyvinylidene fluoride – polytetrafluoroethylene (PVDF-PTFE) nanocomposite membranes has been tested too (Li et al., 2019).
Membrane Geometries For MD.
At the moment hollow fibre membranes are preferred to flat membrane or plate systems because of their compact configuration. The hollow-fibre system has a lower mass transfer resistance with high packing density.
Some very good reviews about the process have been written. Take that of Curcio and Drioli, (2005); Alkhudhiri et al., (2012). Theoretical modelling of MD has been handled by Khayet (2011).
- It will handle any feedstock stream and produce a highly pure distillate/permeate.
- It only needs relatively low-grade heat for warming up the feedstock.
- The process is relatively simple and does not rely on the addition of any chemicals.
- It does not rely on pressure-driven processing unlike reverse osmosis for example.
- The recovery ratios are high.
One challenge is membrane wetting where the feedstock permeates the membrane and alters its permeability and contaminates the permeate. Various fouling materials which deposit on the membrane surface can promote this issue. The other main issue is its cost-effectiveness especially where energy consumption is concerned. Thermal distillation technologies are still highly competitive in terms of energy consumption and this aspect is also improving all the time so that membrane distillation cannot catch up with the development of alternative technologies.
Fouling which plays a major part in all membrane processes has been the subject of a major review by Tijing et al., (2015).
A number of types exist:
- air gap membrane distillation (AGMD)
- sweeping gas membrane distillation (SGMD)
- vacuum multi-effect membrane distillation (V-MEMD)
- vacuum membrane distillation (VMD)
- direct contact MD (DCMD)
- permeate gap MD (PGMD)
Direct contact Membrane Distillation (DCMD)
A type of membrane distillation (MD) where both the liquid feed and permeate are keep contact with the membrane. There is a temperature difference between the two solutions that produces a trans-membrane vapor pressure difference that drives the flux across the membrane. The feed solution is usually hotter than the permeate side to maintain the vapour pressure difference.
A variant of DCMD is osmotic membrane distillation. Here the permeate/distillate side has a brine solution to offer an extra driving force to the movement of water vapour across the membrane.
It is the most common of all the MD processes and found in the fruit juice industry.
Air Gap Membrane Distillation (AGMD)
Here a hot feed solution passes over a membrane which has a stagnant air gap on the other side. The water vapour condenses on a cold surface on the other side of the air gap and is drawn away. The benefit of this design is the reduced heat lost by conduction. However, additional resistance to mass transfer is created, which is a significant disadvantage.
The configuration is ideal for desalination and removing volatiles from juices and other aqueous feedstreams. The main disadvantage of this configuration is that a small volume of permeate diffuses in a large sweep gas volume, requiring a large condenser.
In some cases AGMD and SGMD are combined is a process known as thermostatic sweeping gas membrane distillation (TSGMD). The inert gas in this case is passed through the gap between the membrane and the condensation surface. Some of the vapour is condensed over the condensation surface as in AGMD and the remainder is condensed outside the membrane cell by an external condenser as in SGMD (Khayet, 2011; Garcia-Payo et al., 2002).
Sweeping Gas Membrane Distillation (SGMD)
Here the warm feed solution passes over a membrane which has a gas sweeping across the other side of the surface. This draws away water vapour and other small molecules where they are condensed.
The vacuum MD process operates in a similar way to both the sweeping-gas MD and air-gap MD methods. A vacuum is applied on the other (permeate) side of the membrane. This literally sucks out vapour from the permeate channel once it has crossed the membrane and is then allowed to condense away from the membrane system. The method is ideal for producing very pure water from extremely concentrated brines which can be found inland for example. It will also remove volatiles from water streams which can be recovered if needs be.
A major benefit of the technology is the reduction in fouling that occurs when undissolved inert gases block the pores of the membrane. These are removed with other gases and water is condensed elsewhere. It also means a larger and more effective membrane surface is available for processing.
As with fruit juice processing which contains oily constituents, it pays to reduce the hydrophobicity of the membranes using hydrophilic polymers. These applications are discussed later but on a more general point, membranes such as polyvinyl chloride (PVC) have been modified for VMD using ethyl acrylate monomer using free radical graft coploymerization (Tooma et al., 2015).
in terms of capital cost, a vacuum generating device is needed and it must be adjusted as the salt water temperature alters during processing. Energy usage is higher for VMD than for DCMD and AGMD. There is also an increase in the pH value because dissolved carbon dioxide is extracted from the feed water. Most VMD systems are operated in multistage configurations.
Desalination Of water
Membrane distillation is more effective than other traditional desalination for treating saline solutions and brine, and especially ones with a high saline content. In this process water is purified from seawater by using a suitable hydrophobic membrane which is permeable to water vapour bit not liquid water. In this process a hot saline feed solution flows over the membrane. The increased water vapour pressure from the higher temperature then drives vapour through the pores. The pore sizes are between 0.2 and 0.4 microns. The vapour is collected on the other side and recondenses.
Air-gap MD appears to be one of the most effective methods and there are numerous examples in the literature of the technology being successfully applied.
A VMD example used a flat sheet polyvinylidene fluoride – polytetrafluoroethylene (PVDF – PTFE) nanocomposite membrane. This modified membrane showed better flux (46% higher) than a conventional membrane when desalinating a brine solution (Li et al., 2019).
Membranes for distillation perform in equivalence to reverse osmosis systems. Salt rejection of 99.9 per cent is feasible with NaCl solutions at a maximum concentration of 16%/wt. with MD. Similarly sugar solutions which mimic food syrups and juices are popular models.
Fruit Juice Concentration With Membrane Distillation
In fruit juice processing, thermal evaporation has held sway as the first and predominant method of choice. Unlike membrane processes this technology is well understood and many processors have still retained the equipment because of its versatility.
However, consumers want better quality foods and ingredients and thermal technologies modify these, very often to detrimental levels. Membrane distillation along with a host of other foods offers the processor some useful alternatives to other processes but it is still after so many years yet to supersede traditional thermal technologies.
Unlike pressure-drive membrane processes like microfiltration, ultrafiltration and RO, it is possible to achieve Brix values (concentration levels) in juices with MD which are equivalent to most thermal concentration technologies. A typical commercial brix value is between 60 and 65 °Brix. However, to achieve the necessary water vapour pressure gradient requires heating the feedstream which reduces the quality of the juice. It also means a significant loss in volatiles in a few cases as they cross the membrane. VMD for example will comfortably concentrate a fruit juice to 50 Brix.
Pretreatment of the juice improves the performance of MD applied afterwards. Ultrafiltration (UF) is very effective for the initial treatment of fruit juice to remove solids such as pulp and pectins produced by milling. The MD permeate flux is claimed to remain constant after UF treatment until the concentration of the fruit juice exceeds a doubling in its concentration. After this the permeate flux exponentially declines (Drioli et al., 1992).
Enrico Drioli’s research team at the University of Calabria in Italy have often studied membrane processes. A typical model fruit juice is apple. They produced a highly concentrated apple juice (of 64 Brix) with a hollow-fibre system (Lagana et al., 2000).
In 2006, researchers at the University of Nottingham in the UK studied DCMD (Gunko et al., 2006) and found that increasing the temperature of the feedstream meant a lower temperature was needed on the permeate side to improve vapour transmission.
Orange juice was investigated at the Universidade Nova de Lisboa in Caparica, Portugal by comparing two similar processes MD with osmotic evaporation (OE) (Alves & Coelhoso, 2005). The MD system was a hollow fibre membrane. In this system, OE was better than MD because water flux across the membrane was higher for the same energy input whilst aroma compounds such as citral and ethyl butyrate were retained.
Osmotic distillation has been successfully applied to concentrating a range of fruit juices too. Pomegranate juice was concentrated from 10 Brix to 52 Brix (Rehman et al., 2019).
A mandarin orange fruit juice was concentrated using osmotic membrane distillation with a view to retaining antioxidant properties. Here the juice was initially clarified by ultrafiltration to a bric of 9 before MD was applied to generate a final brix of 64. Other factors important to fruit juice appeared unaffected by the processing such as the vitamin C content, total phenolics and various measures of antioxidant potential as measured by FRAP, ABTS and DPPH (Kumar et al., 2019). Perhaps most importantly, the sensory quality of the mandarin concentrate on reconstitution was better than that produced by thermal evaporation.
Must from a wine process has been concentrated using VMD (Bandini & Sarti, 2002).
A sweeping gas (SWMD) process was compared with a VMD system for recovering volatile fruit juice aromas in a lab-scale system. In this comparison blackcurrant juice was used (Bagger-Jorgensen et al., 2011). They compared various operating parameters. The flux of SGMD increased with an increase in temperature, feed flow rate or sweeping gas flow rate. Increased temperature and feed flow rate also increased the concentration factors (Cpermeate/Cfeed) of the aroma compounds. The aroma recovery was between 73 and 84 vol.% with the most volatile compounds.
Compared to VMD, the aroma recovery with SGMD was less affected by the feed flow rate but temperature was more influential. Higher fluxes were achieved during concentration by VMD and this reduced the operation time. This in turn reduced the degradation of anthocyanins and polyphenolic compounds in the juice which is an important attribute.
One of the challenges of concentrating any fruit juice if it contains volatile oils like citrus juices is to handle membrane wetting. The high hydrophobic nature of the membrane means it will readily adsorb volatiles such as limonene and citral. To overcome this, membranes are coated with hydrophilic polymers such as polyvinyl alchol (PVA) (Mansouri & Fane, 1999), alginates (Xu et al., 2004) or chitosan (Chanachai et al., 2010). These types of coated membrane work better because they simply do not get as wet and a more stable permeate flux is possible. Coated membranes used to concentrate the oil solution (limonene 2%, v/v) for 5 h were not wetted out during flux measurement and no visual damage was observed indicating the stability on the base membrane.
Dealcoholization Of Wine Using Membrane Distillation
Membrane distillation has been used on a few occasions but one of the techniques to remove alcohol from wine is osmotic distillation. In this technology the wine is passed through a hydrophobic hollow-fibre membrane whilst degassed water is passed along the other side. Because of the difference in vapour pressure between the two sides, some of the alcohol in the wine literally evaporates into the water on the other side of the membrane. The process is possible at room temperature without having to use high pressures save for for a pressure that gently pumps wine through the hollow-fibre membrane. The common alternative is reverse osmosis and that relies on both high pressure. The other is vacuum distillation which needs high temperature for optimum performance. The conditions in both technologies have damaged wine quality.
The level of alcohol removal is between 1 and 2 per cent which is significant even in a sensory sense so the technology has not widely caught on because of the poor flavour impact.
Milk Concentration Using Membrane Distillation
An air gap MD process has been tested for simulating milk concentration at Wageningen University in The Netherlands (Moejes et al., 2019). A combination of reverse osmosis (RO) with air gap membrane distillation (AGMD) was examined. The air gap process was highly energy intense compared to RO.
In all cases low permeate flux coupled to membrane fouling by milk proteins and fat hampered the commercial value of the process. Fouling, as with many fluid food streams increases energy costs of pumping. The optimal system for AGMD was a one stage process that operated at a high concentration with relatively low flux rate. The researchers identified the possibilities of increasing both hot and cold side temperatures to the maximum value, and to develop spacers that allowed lower linear flow velocities in the system implying lower recirculation rates. Waste heat from processing was also available to improve MD performance by heating the incoming feedstream.
Manufacturers Of Membrane Distillation Units
Condorchem Envitech (Spain)
Deltapore (CT Beuningen. The Netherlands) – produce a range of membrane process products.
Maintech Systems (Germany)
Markel (Plymouth Meeting, Pennsylvania. USA)
Memsys (Schwabmunchen, Germany).
SolarSpring GmbH (Freiburg, Germany).
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