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A Direct Contact Membrane Distillation (DCMD) rig was built to test to the performance of the membranes in direct contact membrane distillation. Membrane Distillation can be operated in four modes [59], but DCMD exhibit high flux as the air-gap is limited to the membrane thickness thus making materials comparison more accurate and precise. Furthermore as both sides get contacted with water, it is also the best method to compare materials properties such as limit entry pressures, wetting or impact of surface pore size. It is the most restrictive setup as the membranes need to be hydrophobic on both sides and present a very low thermal conductivity, which is less critical in the case of air-gap or vacuum MD. This section will first briefly describe the design of the module and of the setup before detailing the working conditions in which the membranes were tested.
In DCMD of saline waters, the membrane is used as a separation barrier between a hot brine feed and a cold permeate of fresh water (Figure 2-12). This schematic figure describes water vapour transport across a porous membrane during DCMD. Thus, the two liquid streams are only connected by the air-gap present in the membrane.
Gradient of temperature
Gradient of temperature
Vapour transport
Feed Membrane + vapour gap Permeate
Gradient of temperature
Gradient of temperature
Vapour transport
Feed Membrane + vapour gap Permeate
Figure 2-12 Principle of DCMD
Tests were performed in a module in counter current flow mode, with deionised water, on the cold side, and synthetic seawater on the hot feed side
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(35 g/L NaCl solutions). The cell was designed to offer turbulent flow, thus preventing the formation of stagnant layers at the membrane surface. An angle of 45º was formed between the inlet and outlets with the membrane cavity, thus forcing the water to come in contact with the membrane. The module was made in poly(tetra fluoro ethylene) (PTFE) to benefit from the good insulation properties of the material, limiting the thermal losses and heat exchanges which were not related to the process during the test. Vuiton ® o- rings were used to seal the membrane and the module, and Tygon ® tubing were used to transfer water from the reservoirs to the DCMD module. The chamber where the membrane was sealed was a flat cylinder, which diameter and height were respectively 25 mm and 2 mm. The membrane test area was a disc of 5 cm2. A small module was used to reduce the effect of module size related to temperature drops. Therefore, flux or permeance is less affected by temperature profiles as temperature was almost constant on each side of the membrane. A schematic diagram of the membrane module is given in Figure 2-16. Periodic cleaning with deionised water and propan-2-ol of the module, pump heads, glassware and tubing between tests was conducted to remove possible salts deposited in the equipment.
The experimental design involved the use of a double head peristaltic pump (Pump: Cole Palmer Masterflex, model 7521-25; Head: Easyload II, Model 77200-60), to pump the hot and cold streams through 50 cm long heat exchangers. The hot and cold heat exchangers were respectively connected to a CS Lauda C6 heater and to a Thermo Scientific Neslab RTE-7 cooler. The heater (CS Lauda C6) and cooler (Thermo Scientific Neslab RTE-7) enabled a wide range of operating temperatures tuneable between 2ºC and 100ºC.
Solution conductivity, temperature at both inlets and outlets of the module and water volume on the cold side were recorded over time to assess performance of the membrane and to ensure that salt rejection remained high. Typically, the flow rate for each stream, setup on the pump, was maintained between 200 and 300 mL/min corresponding to velocities in the module between 0.006 and 0.01 m/s, while the inlet temperatures of the water streams were kept constant. The cold side temperature was generally kept
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constant over each series of tests while the hot side was varied to achieve differences in the partial vapor pressure of water across the membrane.
The volume change on the cold side was monitored with a graduated column directly connected to the cold water reservoir. Pictures were recorded with a webcam every 10s. The precision of the reading linked to the water level fluctuation and to the reading was of ~ 0.5 mL. This allowed in situ, real time measurement of water transport across the membrane. Temperature and conductivity on the cold side were data logged. A schematic diagram of the DCMD test rig is shown in Figure 2-13 and pictures of the rig are shown in Figure 2-14.
The driving force in MD is the difference in water vapor pressure between the two sides of the membrane air-gap. The absolute variations of pressure (∆P) were derived from Antoine’s law and calculated as a function of the temperature measured at the inlets of the module on both side of the membrane (Figure 2-16). Antoine’s law is given in Equation 7. Depending upon the feed and permeate temperature variations from one up to a few degrees were found between the inlets and outlets. The module outlet temperatures were found to vary by 1 - 3 % compared with the inlet temperatures. C T B A P + − = °) log( (2-7)
Where P is the vapor pressure, T the absolute temperature in K and A, B, C are constants that need to be determined through experiments. Tables are available in the Handbook of Chemistry and Physics (63rd edition, pages D- 196 to D-198) for water vapor and were used to fit the curve given in Figure 2-15.
132 Figure 2-13 Schematic diagram of the test rig
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Figure 2-15 Graph of vapor pressure as a function of temperature (Antoine's law)
The results of the tests were considered if no leak had occurred during the test and if the membrane integrity was intact after at least 2 h of continuous testing. The level of water, as well as the temperatures, flow rates and conductivity were also carefully recorded so that the results were as reproducible as possible. The results will be reported as flux and permeabilities, which was found to be the most suitable way to compare membranes of different thickness and geometrical properties.
134 Inlet cold Inlet hot outlet hot outlet cold Safety O-rings Sealing o-rings Teflon module Hot part Cavity Membrane Inlet cold Inlet hot outlet hot outlet cold Safety O-rings Sealing o-rings Teflon module Hot part Cavity Membrane
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