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Separation of Water out of Highly Concentrated Electrolyte Solutions Using Multistage Vacuum Membrane Distillation

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Separation of Water out of Highly Concentrated Electrolyte Solutions Using Multistage Vacuum Membrane Distillation Separation of water out of highly concentrated electrolyte solutions using multistage vacuum membrane distillation Bin Jiang Master of Science Thesis KTH School of Industrial Engineering and Management Energy Technology EGI-2013-082MSC EKV967 Division of Heat and Power SE-100 44 STOCKHOLM Master of Science Thesis EGI-2013-082MSC EKV967 Separation of water out of highly concentrated electrolyte solutions using multistage vacuum membrane distillation Bin Jiang Approved Examiner Supervisor 26.09.2013 Andrew Martin Daniel Minilu Woldemariam Commissioner Contact person Abstract Absorption dehumidification requires regeneration system to regenerate diluted desiccant solutions, which are still highly concentrated. A novel multi-stage vacuum membrane distillation system was applied for separating water out of the highly concentrated solution. The performance of this novel membrane distillation system with high concentration solution is studied, as well as the effect of solution concentration, heating temperature and feed flow rate on concentration increase, permeate flux and specific energy consumption was studied. Feed solutions are LiCl solution (22-30 wt%) and CH3COOK solution (50-60 wt%).Other experimental parameters studied were: heating temperature, 70-80 °C, feed flow rate, 1.2-2.0 l/min. Response surface method is applied for model building, in order to provide a better understanding of the interactions between different parameters. Compared with pure water, high concentration solution has lower vapor pressure, which leads to lower permeate flux. The highest concentration the system can reach is 36.5 wt% for LiCl solution and over 70 wt% for CH3COOK solution, when the heating temperature is 80 °C. Lower concentration and higher heating temperature will result in larger increase in concentration, higher permeate flux and also lower specific energy consumption. But due to the configuration of the system, optimal flow rates can be found under different conditions. Within the testing region, the permeate flux ranges between 0.147-1.802 l/(m2h) for LiCl solution and 0.189-1.263 l/(m2h) for CH3COOK solution. With low concentration, high heating temperature and low feed flow rate, low specific energy consumptions, 0.85 kWh/l and 0.94 kWh/l for LiCl and CH3COOK solutions are observed respectively. With external heating recovery system, this value can be further reduced. -2- Table of Contents 1 Introduction .......................................................................................................................................................... 8 2 High concentrated electrolyte solution ...........................................................................................................10 2.1 Basic thermophysical properties of aqueous solutions ........................................................................10 2.1.1 Phase diagram ..........................................................................................................................10 2.1.2 Density ......................................................................................................................................12 2.1.3 Specific heat capacity ..............................................................................................................12 2.1.4 Thermal conductivity .............................................................................................................12 2.2 Working fluid ..............................................................................................................................................12 2.2.1 Lithium chloride (LiCl) ..........................................................................................................12 2.2.2 Potassium acetate (CH3COOK) ...........................................................................................14 3 Membrane distillation ........................................................................................................................................16 3.1 Introduction of membrane distillation ...................................................................................................16 3.2 Mass transfer ...............................................................................................................................................18 3.3 Heat transfer ...............................................................................................................................................19 3.4 Temperature & concentration polarization ...........................................................................................21 3.5 Effect of process conditions and membrane parameters on permeate flux ....................................21 3.5.1 Feed temperature ....................................................................................................................21 3.5.2 Feed concentration .................................................................................................................22 3.5.3 Feed flow rate ..........................................................................................................................22 3.5.4 Vacuum pressure.....................................................................................................................22 3.5.5 Cooling temperature ...............................................................................................................22 3.5.6 Membrane pore size and thickness ......................................................................................23 3.6 Experimental set-up ..................................................................................................................................23 4 Preparation and experimental design ..............................................................................................................29 4.1 Concentration measurement ....................................................................................................................29 4.1.1 Density method .......................................................................................................................29 4.1.2 Equipment information .........................................................................................................29 4.1.3 Preparation of samples ...........................................................................................................29 4.1.4 Results and modeling .............................................................................................................30 4.2 Method: Response Surface Method (RSM) ...........................................................................................31 4.3 Experimental procedure: ..........................................................................................................................33 4.3.1 General operation ...................................................................................................................33 4.3.2 High concentration tests ........................................................................................................33 -3- 4.3.3 Response surface design ........................................................................................................33 4.4 Performance indicators (responses) ........................................................................................................35 4.4.1 Permeate flux rate ...................................................................................................................35 4.4.2 Concentration increase ................................................................................................35 4.4.3 Specific energy consumption (SEC) ....................................................................................35 4.4.4 Distillate quality .......................................................................................................................36 5 Results and analysis ............................................................................................................................................37 5.1 General operation ......................................................................................................................................37 5.1.1 Influence of heating temperature .........................................................................................38 5.1.2 Influence of flow rate .............................................................................................................40 5.1.3 Influence of vacuum pressure ...............................................................................................41 5.1.4 Influence of cooling temperature .........................................................................................44 5.2 High concentration test.............................................................................................................................46 5.3 Comparison between potassium acetate solution and lithium chloride solution ............................47 5.3.1 Effect of heating temperature ...............................................................................................50 5.3.2 Effect of concentration ..........................................................................................................53 5.3.3 Effect of flow rate ..................................................................................................................56 5.4 Modeling with RSM method
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