Report

4

SHMT Project Report

Group Members

Hifza InamAbubakar KhalidMuhammad AqibSyed Hammad Andrabi

Due Date12/29/2014

Project DescriptionNew membrane distillation configurations were investigated. The performance of various membranes was analyzed, vacuum assistance was provided to see if DCMD could be used to economically desalinate water while keeping the feed water temperature around 40 C.

IntroductionMembrane Distillation (MD)It is a separation process involving mass as well as heat transfer through a porous hydrophobic membrane. Generally has four basic configurations are used. Air Gap MD(AGMD) Vacuum MD (VMD) Direct Contact MD (DCMD) Sweeping Gas MD (SWGMD) *MD=Membrane Distillation.In this process transfer of mass occurs due to evaporation of a volatile solute (such as in Benzene toluene mixture benzene is MVC) or a volatile solvent (water), if the solute is relatively less-volatile. The process of transfer of mass is driven by the difference of vapor pressure across the microporous membrane.Direct Contact Membrane Distillation (DCMD) Here we are discussing vacuum assisted direct contact membrane distillation. In this case high temperature feed is in contact with membranes one side and there is cold water in contact on the other side. Temperature difference of liquids and solute concentration induce vapor pressure gradient which serves as the driving force for the transfer of mass across the membrane. Effect of temperature difference is more pronounced on mass transfer rate. Mass transfer in DMCD involves following steps: Molecular flux from feed water to the membrane boundary. Convective and molecular transfer of vapors through pores (of membrane). Vapors are condensed on the product side.

Inefficiencies in Membrane DistillationEnergetic inefficiencies in membrane distillation are due to following reasons: Temperature polarization across the membrane. Resistance to mass transfer due to trapped air in the pores. Heat loss by conduction through the membrane.Temperature Polarization In MD heat is transferred mainly through two channels. Transfer of latent heat of evaporation with vapors across the microporous membrane being one mode of heat transfer. The other is transfer of heat due to conduction through the membrane and boundary of the system, this conductive heat transport is a source of inefficiency as this energy cannot be reused for evaporation and is simply dissipated to the environment as wasted.Thermal boundary layer is developed on membrane surface due to above mentioned phenomenon. Heat transfer through this thermal boundary layer inflicts a limit on the transfer quantity i.e. (mass). Temperature polarization coefficient (TPC) is calculated as, energy required for transport of vapors divided by the energy supplied (total). The TPC can be increased by better design of module used (configuration of equipment, material of construction etc) or operating parameters. The value of TPC should be closer to unity for better performance.

TPC =All values are temperatures i.e.Tp= Bulk permeate Tmp = Interfacial permeate Tmf = Interfacial feed Tf =Bulk feed Resistance to Mass TransferAir molecules trapped in membrane pores offer resistance to vapors moving across the membrane. Several studies showed that degasification of working fluid and vacuum control of partial pressure of air in the pores increases the (mass) flux of vapors. Moreover properties of membrane like (below) dictate the resistance to mass transfer through membranes in MD. Porosity Pore size Tortuosity Membrane thickness Therefore, selecting a membrane posing relatively less resistance to transfer of mass is important.Conductive Loss of HeatHeat loss through conduction is difficult to control as a thicker membrane provides better insulation while a thinner membrane offers relatively less resistance to transport of mass across the membrane. This loss can be reduced by using more porous membrane or membrane of higher porosity, because air is an excellent insulator however, membranes with 80% or more are available and nanotube technology when matures can allow for higher porosity. Design Considerations for MD ModulesWe need a minimum pressure, in order to maintain a flow in a channel, at the entrance of the membrane boundary. This pressure drop can be expressed as:

Here f = friction factor d = Diameter of the pipe = Density of the fluid u = Velocity of fluidThen there is another factor referred to as LEPW. It is the (minimum) pressure required for water to penetrate the pores after overcoming the hydrophobic forces of membrane.

Here B = geometric factor depends on pore structure L =liquid surface tension rmax = largest pore size = contact angle of liquid-solid.

Methods and MaterialsMicroporous MembranesWe analyzed performance of four membranes for this study. Out of four, three membranes have thin active layer of polytetrafluoroethylene (PTFE) and a sub support layer of polypropylene (PP). The other membrane is made of polypropylene (PP) and is an isotropic and symmetric membrane. Properties of membranes used are as follows:Test Unit DCMDWe analyzed the performance of the DCMD processes by varying parameters like impurity concentrations (solution chemistry) and conditions, in an isolated (closed loop) small scale test unit. Various configurations evaluated are shown below:Solution chemistries like salts presence is observed by monitoring conductivities of permeate and feed.Results and DiscussionThe experiment was first carried out with a traditional DCMD (Direct Contact Membrane Distillation) three times to establish an accurate baseline to serve as the control. The control experiment was then compared with the new configurations, each time only one of the three variables (velocity of various streams, temperature gradient within microporous hydrophobic membrane, and the overall positive pressure of the system) were altered and resulting changes in flux were noted. Stream Velocity Vs FluxFigure 1 (shown on the left) highlights the performance of 4 membranes as a function of the feed and product flow channel velocities. Experiments were conducted at 40C of feed temperature and 20C being the permeate temperature. The concentration of the feed was kept constant for all experiments at 0.6g/l NaCl. The rate of salt rejection exceeded 99.9% in all cases.

Feed Temperature Vs FluxFigure 2 (shown on the left) shows the relation between the feed temperature and the flux (of permeate) for the three membranes (TS45, TS22, PP22). The permeate velocity being 1.75 m/s and temperature being 20C. Salt rejection exceeded 99.8% in all cases. The fluxes of membranes TS22 and TS45 came out as exponential functions of temperature, which is reasonable considering the Antoine equation, where vapor pressure varies exponentially with the temperature.The membrane PP22 shows a linear variation with temperature; this is probably due to increased effect of the thickness of the membrane over the thermodynamic effects.System absolute Pressure Vs FluxFigure 3 (shown on the left) represents the effect of system absolute pressure on the permeate flux for the composite membrane TS45 with feed at 40C and permeate temperature of 20C respectively. Stream velocity and concentration of NaCl being, 1.05 m/s and 0.6 g/l respectively.As the graph shows, positive pressure has virtually no effect on the permeate flux.

Performance of DCMD Setup (traditional)Even in the traditional DCMD setup, the efficiency and the permeate flux rates were significantly better as compared to those found in previous studies using similar sized pores and operating under similar conditions. The fluxes obtained (highest) in past studies for the TS22 (22 m pore size) at 20C and 40C were 13.5 and 41.1 kg/(m2hr)respectively while with the current study the fluxes obtained were 24.7 and 81.5 kg/(m2hr). This represents a flux increase of almost 100%. Similar results were obtained when testing the TS45 (45 m pore size). The improve in performance can be attributed to a number of reasons High agitation in the boundary layer (thermal) improving transfer of heat via conduction and therefore maintaining a higher thermal polarization within the membrane Use of insulating plastic (specifically acrylic) as a construction material to avoid heat losses to the surroundings Use of thin active layer composite membranes, providing lower resistance to flux of mass.The new DCMD/Vacuum Configuration

Figure 3 compares the results of Flow velocity on flux of the membrane TS22 in a traditional DCMD and a DCMD/Vacuum configuration. The pressure on the permeate side was reduced to below atmospheric (from 108 kPa to 94 kPa). The stream flow rate increased to 2.1 from 0.7m/s. The temperature of feed 40C and 20C was the permeate temperature. Thus flux was increased up to reasonable value of 15% with the vacuum configuration.

Figure 5 shows the implications of pressure reduction (on the side of permeate) on the flux. The permeate flux is seen to increase linearly with the decrease in pressure and increases 84% for the lowest pressure (55 kPa).

Figure 6 studies the effect of varying concentration of the salt NaCl on the flux. The concentration being increased from moderate 0.6 g/l to relatively higher 73 g/l. While the temperature of feed maintained at 40C and 20C was the permeate temperature. The feed and permeate velocity were maintained at previous value i.e. 1.4 m/s and the pressure of permeate was kept around 68 kPa. Study showed a flux decrease of 9% over the concentration range specified. In all cases the salt rejection rate was above 99.85% with large chunk over 99.9%. This is one of the most important advantages of the DCMD/Vacuum system, As it is barely effected by salt concentration it can be easily and very effectively used in large scale desalination projects to obtain fresh water from seawater. DCMD with vacuum-vacuum configurationThe main aim of the this arrangement was to distinguish between implications of convective transfer of mass because of total pressure gradient and the results of decreased flux in the membranes pores because of incomplete removal of air film in the pores. The new configuration having both sides under vacuum was worked with concurrent mode so that at all points, similar pressures are produced.The figures show the results of the experiments conducted with membranes While the temperature of feed maintained at 40C and 20C was the permeate temperature. From the figure it is deduced that the water fluxes were greater in counter current arrangement as opposed to concurrent mode. And the vacuum-vacuum configuration adds to the decrease in flux.The thickness of the membrane is the most important to the flux of water vapor through the partially permeable membrane. Hence the properties of the membrane such as its thickness are a barrier to the effects of improving the operating conditions and therefore mass transfer increases very slightly.

Economic benefitsIt is difficult to judge overall performance of the enhanced DCMD process; there are some advantages of the enhanced DCMD process over the RO process that are worth consideringFor an accurate economic analysis, pilot scale tests can be conducted by using the large module operating in the enhanced DCMD process. When the manufactures performance data for sea water desalination using RO (reverse osmosis) process is analyzed, the flux ranges between 18/(m2 h) and 341/ (m2 h). The enhanced DCMD process has an advantage as it provides flux greater than 351 m2/h at 40C feed temperature considering that most RO processes have a flux lower than the stated range. More over the graph relating flux and feed temperature shows that for an increase in 1C of feed temperature, the flux increases considerably by more than 21/ (m2 h). Among major benefits, of this process used to desalinate water, one is that the performance of the system is minutely altered by the concentration of salt in the feed. In the process, if the concentration of salt is increased it only negligibly decreases the vapor pressure of water and leads to minimal decrease in the mass transfer driving force. Comparatively in the RO process, increasing the salt concentration greatly reduces the mass transfer driving force as the membrane allows the salt to pass through it. Since the feed is heated to a specific temperature, hence using the DCMD plant with any power plant or waste heat ejection plant may be useful and would reduce the cost for heating the water. Other ways of energy can also be reduce to make the process cost effective such as the renewable solar energy and geothermal energy. The use of these renewable sources may require a high capital investment for setting up the equipment but in future would result in lower operating costs.An RO process uses only one pump as opposed to 2 pumps required in DCMD process, the pumps used in DCMD process operate at low pressure. These pumps have lower capital and operating costs than those operating at high pressures. A vacuum permeate pump may be used if enhanced DCMD process is employed but the costs will still be less because of low pressure gradient.The percentage of recovering permeate in a single RO element is about 10-15%. The small scale tests of the enhanced process show that recoveries are no more than 1%. With the scaling up of the unit, it is expected that recovery level will also rise.In processes involving membranes, membrane cleaning is considered to be important. Making membranes which are chemical agents resistant is vital. Membranes made of polymers which can tolerate oxidizing agents such as chlorine etc. reduce the need for replacing membranes regularly and makes them more robust.ConclusionThis investigation was for an improved approach for construction and operation of the direct contact membrane distillation (DCMD) system used for the purpose of desalination. It was shown that an appropriate choice and configuration can reduce obstructions in permeability as well as polarization of temperature in the distillation (membrane) of salt solutions. Fluxes of more than double than the original DCMD mode of operation can be achieved at lower temperatures. The concentration of salt in the solution of feed does not affect the elimination of salts which is always high. With improving the research in nanotechnology, newer materials and surfaces with unvarying pores of nano size can be produced which would boost membrane absorbency and enhance the membrane distillation process.

THE END