Water Desalination Through MED-TVC IMPROVEMENT OF MULTI-EFFECT DESALINATION SYSTEMS: LOCATION OF VAPOR COMPRESSOR Mohammed Antar Professor Mechanical Engineering Department KFUPM antar@kfupm.edu.sa
2 Outlines 1 Introduction 2 3 4 5 6 7 8 Conclusion
NEED FOR WATER FOR WATER DESALINATION The volume of the water available in the earth is 1.4 10^9 Km^3 covers 70% of the earth surface area. 97.5 % of this water is salt water 80 % of the rest is frozen in the icecaps or combined as a soil moisture The remaining quantity which is (20% of 2.5% = 0.5 %) of the total quantity available in the earth used to support the live in our planet 11/2/2017 3
NEED FOR WATER FOR WATER DESALINATION The water quantity is almost constant The population is increasing significantly http://www.kivu.com/ 11/2/2017 4
Definition of desalination processes Desalination process is a process of separation of fresh water from saline water Desalination process based on thermal or membrane separation. Thermal Separation Include Evaporation followed be condensation (MSF, MED, HDH) Freezing followed by melting The membrane separation include Reverse osmosis (RO) https://www.emaze.com/ 11/2/2017 5
Electrical Energy Thermal Energy Mechanical Energy ED Steam Solar MVC RO MED MSF Solar Still Heating Steam HDH 11/2/2017 6
Multi effect evaporation system Developments Darwish* and Abdulrahim Feed water arrangement in multi effect desalination systems It is a formed a sequence of single effect evaporators The vapor created in the first effect is used as a source of heat in the next effect Avoid rejection of heated brine, which was the main drawback of the single effect system 11/2/2017 7
Multi effect evaporation system Developments http://www.sidem-desalination.com/en/process/med/process/ It is a formed a sequence of single effect evaporators The vapor created in the first effect is used as a source of heat in the next effect Avoid rejection of heated brine, which was the main drawback of the single effect system 11/2/2017 8
Schemes of supplying feed water to the evaporators Forward feed MED distillation system Backward feed MED distillation system Parallel feed MED distillation system Darwish* and Abdulrahim Feed water arrangement in multi effect desalination systems 11/2/2017 9
Authors Year El-Dessouky et al. 1998 Introduced mathematical model describing the MED system They concluded that the PR of the plant is nearly independent of the TBT El-Dessouky et al. 2000 Running of both systems parallel/cross flow and parallel flow systems is preferential at higher temperatures as a result of the extreme reduction in the specific heat transfer area Ali and El-Figi 2003 Studied the performance of MED-FF system, they pointed that the PR is notably dependent on the number of effects rather than the TBT Ophir and Lokiec 2005 Reported that the MED is better thermodynamically and it is known as lower energy consumption compared to MSF system Very low specific energy costs for water desalination. 11/2/2017 10
Authors Year Darwish et al. 2006 Normal MED system has the advantage of exploiting a low-temperature heat source when it works at low TBT The heat transfer areas increase considerably due to decreases of ΔT to less than 2 C The Multi Effect Boiling system consumes about half of the Multi Stage Flash system pumping power. Darwish and Abdulrahim 2008 Developed MED model and analyzed different arrangements In all arrangements, increasing the number of effects increases the gain ratio, and the used specific heat transfer area. Mistry et al. 2013 Illustrated that the advantage of Cogeneration systems is being able to produce both water and power at lower costs. 11/2/2017 11
Authors Year HAMED et al. 1996 MED-MVC and MED-TVC for 4 effects They reported that the MED-TVC system is more efficient than MED-MVC system. El- Dessouky et al. 2000 MED-MVC (P & PC). The specific power consumption for MED-MVC parallel cross is lower than MED-MVC parallel feed. Bahar et al. 2004 MED-MVC. Results showed that the brine concentration rate affects the distillate flow rate. 11/2/2017 12
Authors Year Ophir et al. 2007 MED with turbo-compressor at low temperature An auxiliary turbine and a compressor of higher efficiency than thermo-compressor results in considerable energy savings. Lara et al. 2008 MVC system operating at high temperature At high temperature, heat transfer area is small, compression work is low. They used a small compressor to reduce the capital cost. Fuad et al. 2011 The effect of stage temperature drop on MED-MVC The specific power consumption decreases as MVC brine temperature increase, and volume flow rate is decreased as MVC brine temperature increase. 11/2/2017 13
To develop mathematical model for design and operation of multi effect desalination system based on energy and mass balances To assess several layouts of MED-TVC. Improving MED-MVC performance through the use of a secondary compressor that extracts vapor from one of the effects. To study the effect of changing the position of thermal vapor compression (TVC or MVC) 11/2/2017 14
Forward Feed MED Model 11/2/2017 15
Forward Feed MED TVC Model 11/2/2017 16
Forward Feed MED TVC Model Results At lower steam temperature, PR is high, as the steam temperature increases, PR decreases due to increasing of the motive steam flow rate to get higher compression ratio. Increasing the number of effects increases PR due to better use of energy and vapors gained. 11/2/2017 17
Forward Feed MED TVC Model Results Increasing heating steam temperature increases the cooling water flow rate due to increase of the last effect thermal load as a result of increasing the compression ratio and the heat load of the first effect. In addition, increasing the number of effects reduces the specific cooling water flow rate due to reducing the thermal heat load by increasing the number of effects. 11/2/2017 18
Forward Feed MED TVC different ejector positions Model The heat load enters to the condenser 11/2/2017 19
Forward Feed MED TVC different ejector positions Model Results 11/2/2017 20
Exergatic Efficiency Introduction Forward Feed MED TVC Exergy Analysis Results 90 80 70 60 50 Exergy Analysis Break Down 40 30 TVC After effect 4 20 10 0 11/2/2017 21
Parallel Feed MED TVC Model 11/2/2017 22
PR and CR [-] Flow rate ratio [-] PR and CR[-] Flow rate ratio [-] Introduction TVC 9 0.14 10 0.14 8 7 6 Forward Feed -TVC- 8 effects system Ts=80 ºC 0.12 0.1 9 8 7 Forward Feed-TVC- 8 effects system 0.12 0.1 5 4 3 2 1 0 Compression ratio Performance ratio Motive steam flow rate ratio 0 1 2 3 4 5 6 7 8 9 Effect number 0.08 0.06 0.04 0.02 0 6 5 4 3 2 1 0 Ts=70 ºC 0.08 0.06 0.04 Compression ratio 0.02 Performance ratio 0 0 2 4 6 8 10 Effect number Effect of changing the location of TVC on the performance ratio at steam temperature of 80 Cfor a MED-FF system, N = 8 Effect of changing the location of TVC on the performance ratio at steam temperature of 70 C for a MED-FF system, N = 8 KFUPM ME Dept. 11/2/2017 23
Performance ratio[-] Introduction TVC 5.2 Steam temprature of 60 C 5 Steam temperature of 70 C Steam temperaturre of 80 4.8 4.6 4.4 4.2 4 0 500 1000 1500 2000 2500 3000 3500 4000 Motive steam pressure [Kpa] Effect of changing motive steam pressure on performance ratio KFUPM ME Dept. 11/2/2017 24
MVC Features: Compact Independent of external heating source Remote areas, operated by Diesel engine, wind turbine. http://www.sidem-desalination.com/zoom?media=zoomimg&doc=26716&id=c12238873151-img&src=kit_vwst_rwd KFUPM ME Dept. 11/2/2017 25
MVC PROPOSED Compressed Vapor The second Mechanical Compressor Feed seawater Extraction mf, Tcw The First Mechanical Compressor C2 C1 (1) (2) (3) (n) Flashing Box Distillate md Brine mb Patent Disclosed Feed mf, Tcw KFUPM ME Dept. 11/2/2017 26
MVC This addition improves the system performance through Effective heating of the sprayed seawater in the first effect Generating more vapor that may eventually increase the system productivity Reduced specific power consumption Is there a best location for secondary compressor?? Patent Disclosed KFUPM ME Dept. 11/2/2017 27
MVC Specific power consumption (kwh/m^3) 12 11 10 9 8 E.1 E.2 E.3 E.4 E.5 n = 6 without Extraction 7 0 0.2 0.4 0.6 0.8 1 Extraction Sepcific power Consumption ( Kwh/m^3) 9 8 7 E.1 E. 2 E. 3 E. 4 E.5 E.7 E.6 n = 8 without Extraction 6 0 0.2 0.4 0.6 0.8 1 Extraction Change in the consumed power for the parallel feed (MED-MVC) with Extraction for n = 6,8 effects. KFUPM ME Dept. 11/2/2017 28
MVC TVC increases PR of the system and reduces the specific cooling water flow rate. Changing the position of the ejector affects the Performance ratio and the specific cooling water flow rate However, the best performance occurs for wide range of heating steam temperature when the ejector is situated in the middle Increasing the number of effects increases the second law efficiency. 11/2/2017 29
Adding a secondary compressor improves Performance of MED- MVC-PF desalination system by about 10%. Decrease in the vapor specific volume at higher operating temperature reduction in specific power for vapor compression. Extracting formed vapor from the middle effect (n/2) results in a best for the system performance. Insignificant effect of extraction rate on the specific heat transfer area. 11/2/2017 30
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