PROCESS SIMULATION OF VACUUM DESALINATION SYSTEMS FOR PRODUCING POTABLE DRINKING WATER FROM SALINE SEA WATER

Transcription

1 International Journal of Mecanical Engineering an Tecnology (IJMET) Volume 8, Issue 7, July 2017, pp , Article ID: IJMET_08_07_132 Available online at ttp:// ISSN Print: an ISSN Online: IAEME Publication Scopus Inexe PROCESS SIMULATION OF VACUUM DESALINATION SYSTEMS FOR PRODUCING POTABLE DRINKING WATER FROM SALINE SEA WATER S. Taela Vacuum Tecnology Laboratory, IIT Karagpur, Karagpur, West Bengal, Inia. Raja Sekar Donapati Lovely Professional University, Pagwara, Punjab, Inia. V. V. Rao Vacuum Tecnology Laboratory, IIT Karagpur, Karagpur, West Bengal, Inia. ABSTRACT Pure water is te basic requirement for mankin s survival an is an essential requirement for rinking, agricultural an inustrial purposes. Due to te rapi increment of worl s population accompanie by te rise in living stanars, existing natural sources of pure water are unable to meet te over growing eman for potable water. To cope up wit te situation, new tecniques of esalination for te generation of potable water, wic assures minimum energy requirement an economic viability are of greater interest now. In tis line of researc, an analysis on Vacuum Desalination system, in wic te evaporation of warm sea water taken from upper surface of te sea an successive conensation of te same using col sea water taken from te eep layer of te sea for prouction of rinking water was presente. By employing vacuum esalination tecnique, vacuum is create insie a flas camber uner wic sea water can be rapily evaporate at muc lower temperatures, followe by conensation in a sell an tube conenser. Te scope of te present work iscusses about te evelopment of a numerical moel to simulate te operation of a esalination plant an to investigate te effect of operational parameters on te performance of te system. Also te esign of plant, wic comprise of te esign of vacuum flas camber, istillate conenser, vacuum pumping system an water pumping system, was inclue. For te analysis of te system, te governing equations are obtaine from mass, momentum an energy balances an te resulting equations are solve using a simulation. Base on te results obtaine from simulation, te effect of key process variables suc as vacuum camber pressure an inlet temperature of te warm seawater on te system s operational performance in terms of fres water prouce (yiel) as been evaluate. ttp:// eitor@iaeme.com

2 S. Taela, Raja Sekar Donapati an V. V. Rao Key wors: Desalination, Vacuum, Yiel, Saline Sea water, Drinking water. Cite tis Article: S. Taela, Raja Sekar Donapati an V. V. Rao Process Simulation of Vacuum Desalination Systems for Proucing Potable Drinking Water from Saline Sea Water. International Journal of Mecanical Engineering an Tecnology, 8(7), 2017, pp ttp:// 1. INTRODUCTION Water is te source of life for any society an is a precious resource tat is require for all te living organisms on eart. Te oceans, wic are sprea over tree-fourts of te eart's surface, contain uge quantities of water but its ig salt content is making it unfit for rinking, inustrial an various purposes. Drinking water resources are uner eavy pressure ue to population growt rates, rising living stanars, inustrialization an irrigation in agriculture. Hence te evelopment of an economic an reliable tecnique for seawater esalination as been a callenge pursue over te years. As water scarcity an contamination problems are more acute in rural an coastal areas, implementation of esalination tecnology will elp for proviing safe rinking water. Currently esalination of saline water appears to be te solution for escalating rinking water sortage as oceans are te only vast source of water. Basically tere are two variants of esalination namely, Distillation (involving cange of state) an Reverse osmosis (using membrane filtration). Vacuum esalination (involves cange of state) is a process in wic te seawater is vaporise at lower temperature wen subjecte to vacuum an te resulting vapor is conense for proucing fres water [1]. Te principle tat te boiling point of water rops wit ecreasing absolute pressure forms te basis for vacuum esalination. Low an Tay [1] first reporte te vacuum esalination process for prouction of sea water by utilizing te waste eat from a steam turbine. Kawla Abul Mosen Al-Sayji [2] as one moeling, simulation, an optimization of large scale commercial esalination plants. M. AI-Sammiri [3] as stuie te Evaporation rate as a function of water salinity. Te results a sown a ecrease in te evaporation rate wit an increase of water salinity because of te reuction in te vapor pressure at te water surface. Anil K. Rajvansi [4] as reporte a sceme to esalinate te sea water using solar energy to prouce about 5.25 x 10 m 3 /yr of fres water wit km 2 of collector area for Tar Desert of Inia. A. Mani et. al [5] as presente an analysis on a jet pump assiste vacuum esalination system using power plant waste eat. Te simulation results a sown an increase in yiel of te system wit ecrease in conenser temperature an ecrease of camber pressure. A. E. Mutunayagam et. al [6] as propose te concept of flas vaporization of warm sea water an its subsequent conensation using te col water taken from eep layers of te sea, for a low energy esalination system. An experimental stuy on a typical esalination system, wic utilizes ocean termal energy, was given by Sentil Kumar et. al [7]. Kali S. E. Al-Malay et. al [8], investigate te corrosion beavior for wie range of materials in seawater environment. Te meto of assessment of corrosion was mae by te use of electrocemical monitoring in te form of anoic potentio-ynamic polarization scans. Te work of M. Elimelec an William A. Pillip [9] gave insigts into te future of sea water esalination by empasizing on te potential role of avance materials an innovative tecnologies in improving te performance of esalination plants. A brief review [10] on te solar assiste sea water esalination was also available in te literature. Currently in Inia, NIOT is actively working on tis tecnology an as successfully emonstrate te vacuum esalination of sea water wit two plants, a 100 m 3 /ay esalination plant at kavaratti an a 1000 m 3 /ay barge mounte esalination plant 40 km off Cennai coast. Te current ttp:// eitor@iaeme.com

3 Process Simulation of Vacuum Desalination Systems for Proucing Potable Drinking Water from Saline Sea Water work is performe to se some ligt on te esign etails an operational performance of a esalination plant couple wit vacuum system, wic can prouce 1, 00,000 litres (100 m 3 /ay) of rinking water per ay by utilizing te naturally available sea water termal graient. For te entire process, using a simulation program, te impact of ifferent operating conitions on te performance of te esalination system in terms of fres water prouce was stuie. 2. PRINCIPLE OF OPERATION Fig. I sows a scematic layout of te propose esalination plant. Te system comprises of a flas camber, a sell an tube conenser, a water pumping system (bot warm seawater pump an col seawater pump), a vacuum system, associate piping an instrumentation. Te stream of warm seawater from te surface layer of te ocean, wic is usually in te temperature range of C is pumpe using a warm water pump to te vacuum flas camber. Due to te provision of vacuum insie te camber, te incoming seawater gets subjecte to a pressure rop below its saturation pressure. Te water may also be preeate to C by running it troug a suitably esigne solar termal eat-excanger. In tis conition all te eat cannot be containe in te seawater an tereby te surplus eat is transforme into latent eat of vaporization. In tis way a conition of saturation equilibrium is maintaine insie te camber at a temperature some egrees lower wit respect to incoming seawater. Tis creates a continuous flasing of seawater an tereby small portion evaporates using te latent eat of evaporation from incoming warm seawater. Te vacuum system (operating at an absolute pressure in te range of 25 mbar) maintains te saturation pressure in orer to facilitate te evaporation of vapour in te flas camber an its successive conensation in te conenser sell. Furter, uring te process of evaporation in flas camber, te non-conensable gases an air tat leaks troug te flange joints of components an pipes, wic are release ue to te continuous flasing of sea water are constantly purge to atmospere by vacuum pump connecte to te system. Te flas camber is maintaine at barometric eigt (10.13 m) for gravity iscarge of resiual liqui in te flas camber into te sea after flasing. Te vapor generate uring te flasing passes troug te emister, were seawater roplets carrie away wit te vapor are separate. Te resulting vapor is in-turn riven into a conenser, were it gets conense wen passe over a bunle of tubes, by losing its latent eat of conensation to te stream of col seawater (at a temperature of 13 0 C, wic is less tan te saturation temperature of water at 25 mbar of flas camber) circulating troug te tubes, wic was taken from eep layers of te sea. Finally, te istillate (fres water) prouce at te conenser, is collecte in te sump. Tis water of rinking quality is ten pumpe to atmosperic pressure troug a suitably esigne centrifugal pump wose outlet is connecte to a fres water storage tank. ttp:// eitor@iaeme.com

4 S. Taela, Raja Sekar Donapati an V. V. Rao Figure 1 Layout of Vacuum esalination plant 3. DESCRIPTION AND DESIGN OF THE SYSTEM 3.1. Vacuum Flas Camber Vacuum flas camber is te place were actual flasing an subsequent evaporation of warm seawater occurs. A istillate conenser is connecte on te top sie of te camber in suc a way tat etacment of flow oesn t occur along te vapor pat. Te flas camber an istillate conenser are in turn connecte to te vacuum system, wic elps in maintaining a conition of saturation equilibrium lower wit respect to fee seawater. Te camber is well insulate wit Poly-Uretane Foam (PUF) insulation to prevent te eat loss from te camber to te ambient. Te steam generate after flasing carries away some roplets of brine. Tis unevaporate brine is separate at te emister wic is place before te conenser. Te camber tickness is evaluate base on te internal pressure, imensions of te camber an te yiel strengt of material cosen (etails given in Table I). In view of low temperatures an vacuum associate wit te process, stainless steel (SS 316L) is cosen for te construction of flas camber ue to its ig corrosion resistance, ig yiel strengt, welability an comparable operation cost. Table 1 Design etails of te flas camber (SS 316L) S. No. Name Dimension 1 Camber lengt 2.5 m 2 Camber iameter 1.2 m 3 Camber wall Tickness 6 mm 3.2. Process Conenser A sell an tube configuration as been cosen (etails given in Table II) for te purpose of istillate conenser. Te resulting vapour from te flas camber is facilitate to flow raially over te peripery of te tube bunle in te conenser camber, losing latent eat of conensation to te col seawater (at a temperature of 13 0 C) circulating troug te tubes of ttp:// eitor@iaeme.com

5 Process Simulation of Vacuum Desalination Systems for Proucing Potable Drinking Water from Saline Sea Water te conenser. In te esign of conenser, te effects of te non-conensable gases tat are carrie away along wit te vapor are consiere, as tey can alter te eat transfer caracteristics of te conenser. Te tubes of te conenser are mae out of SS 316L as tey ave to witstan te corrosion in te saline meium. On te oter an te sell sie of te conenser is constructe from 90Cu-10Ni alloy for anling te pure water vapor. Table 2 Design etails of te Process Conenser S. No. Name Dimension 1 Inner iameter of te tube 16 mm 2 Outer iameter of te tube 20 mm 3 Tube wall tickness 2 mm 4 No. of tubes (for 2 passes) Lengt of eac tube 3.48 m 6 Total area m Vacuum System Te vacuum system comprises of roots blowers an water ring pump. Te most critical aspect of te plant is te maintenance of vacuum in te flas camber an conenser. Te vacuum system installe soul be able enoug to maintain te esigne saturation pressure for facilitating te evaporation of vapour in te flas camber an its subsequent conensation in te conenser. Also because of te uge amounts of incoming fee seawater compare to te istillate prouce, a significant amount of issolve air (approximately 1%) is release in te flas camber uring flasing of seawater. Tis air as to be vente to te atmospere continuously to maintain te saturation conition in te flas camber. Oterwise ue to te ig amount of air release, te pressure in te camber increases resulting in ig saturation temperature corresponing to te new pressure evelope. As a consequence, te amount of seawater evaporate ecreases causing a rop in te yiel prouce. Hence uring te selection of vacuum systems for esalination process, along wit te power requirement, tecnical an economical aspects, te effect of te non-conensable gases wic are liberate from te water at lower pressures an air leakages from flange joints of various components were taken into consieration. In te present system, in orer to minimize te total power consumption of te plant, a water ring pump of 223 ACFM volumetric capacity couple wit a booster stage of Roots Blower (rating) is cosen to meet te process requirement of vacuum of Torr an to anle air loa (non-conensable gases) of 15 kg/r. Te vacuum system is connecte at sell sie of te conenser for facilitating maximum utilization of vapour generate troug flasing of seawater, wic woul oterwise escape into atmospere if connecte irectly to flas camber wit minimum amount of vapour entering te conenser Water Pumping System A esalination plant basically requires pumps to pump uge quantity of warm seawater from te surface of sea to vacuum flas camber; pumps to pump col seawater from m. water ept to te conenser; an pumps for pumping istillate (rinking water) to a storage vessel. In a typical esalination plant, te components tat require power are tat of te pumps for sea water (warm sea water an col sea water) an vacuum system. Hence for minimizing te power consumption an to meet te requirements, pumps wic eliver ig ttp:// eitor@iaeme.com

6 S. Taela, Raja Sekar Donapati an V. V. Rao iscarge (uge amount of incoming fee water) an low ea must be selecte. Also uring te selection of pumps, it must be ensure tat te losses in te pipelines, bens an fittings are taken into consieration. Base on te quantity of water to be pumpe, Centrifugal pumps connecte in series (elivering ig ea) are propose for te current system. Owing to its ig egraation resistance at low temperatures, PVC material is cosen as te pipe material for pumping warm an col sea water an for te pumping of istillate 316L stainless steel is cosen. Sipon arrangement as been aopte for te piping of warm water flow troug te flas camber an for te flow troug te conenser so tat ea losses are minimal. 4. PROCESS SIMULATION A simulation program, to establis te impact of variation of ifferent process parameters an operating conitions on te performance of te esalination system as been performe. Te flow cart (Fig. II) enumerates te typical proceure followe for te analysis of te esalination system. Te main objective of tis analysis is to know te effect of variation of input parameters on te yiel of te plant. Yiel of te plant is estimate by varying one parameter an keeping all te oter parameters constant. In te process of calculation, we are proviing te inlet temperatures of te fee sea water (bot warm sea water an col sea water), flow rates at various positions of te plant an pressure in te vacuum flas camber wic are te key parameters influencing te yiel of te plant. Te parameters use in te simulation of te plant are summarize in Table III. Table 3 Process parameters use in te simulation of plant S. No. Parameter Range 1 Warm water inlet temperature ( 0 C) 28,30,32 2 Col water inlet temperature ( 0 C) 13 3 Non conensable loa (kg/r) 15 4 Operating pressure in te Flas camber an conenser (Torr) 25 Te governing equations are obtaine from momentum an energy balances across various components an te resulting equations ave been use for simulation of te system. At first, te saturation temperature corresponing to te pressure in te flas camber is calculate. Ten by knowing te entalpies of seawater at upstream temperature, ownstream temperature an entalpy of saturate water vapour at te operating pressure of te flas camber (property correlations obtaine from [11]), we can compute te amount of seawater evaporate from te mass balance an eat balance one across te flas camber wic is given by equation (4). Mass balance across te flas camber gives: m& = m& + m& (1) fsw wvap rsw From te Heat balance across te flas camber m& m& m& (2) L v L fsw u = w vap + rsw m& m& - ( & & ) L v L wvap = fsw u - m fsw m wvap (3) ttp:// eitor@iaeme.com

7 Process Simulation of Vacuum Desalination Systems for Proucing Potable Drinking Water from Saline Sea Water L L ( u - ) ( - ) & = m& (4) m wvap fsw V L Ten from te eat balance across te conenser, te yiel of te plant is evaluate from te equation (7) given below. From te Heat balance across te conenser m& m& m& m& m& V L L L V wvap + isw = isw osw + osw + nc nc m& = m& = m& were isw osw ( ) V V L L ( ) + m& ( - ) L V ( - ) m& m & m & m & & & V L L L V wvap + isw = osw + + m wvap - m nc Y iel = m & = m& - w vap nc isw osw n c (5) (6) (7) Figure 2 Flow cart for te analysis of esalination system ttp:// eitor@iaeme.com

8 S. Taela, Raja Sekar Donapati an V. V. Rao 5. RESULTS AND DISCUSSIONS In te present work to analyze te operational performance of te plant, a numerical moel was evelope an te simulation as been carrie out. By keeping all oter process variables fixe, te effect of key process parameters suc as inlet temperature of te warm fee seawater an vacuum camber pressure over te yiel of te plant are presente. Fig. III sows te variation in te yiel of esalination plant wit respect to te warm seawater inlet temperature at various flow rates of fee seawater an constant operational pressure of flas camber. Figure 3 Effect of warm fee seawater inlet temperature on te yiel at various mass flow rates of warm sea water From te plot, it is observe tat at a particular flow rate of te fee sea water, yiel of te plant is increasing wit an increase in fee seawater temperature ranging from 26 0 C to 40 0 C. Tis increase in yiel is mainly because of te eat input provie to te system wic results in a significant increase in evaporation rate ue to te temperature ifference between te fee seawater temperature an saturation temperature corresponing to te pressure in te flas camber. Also at a constant operational pressure of flas camber i.e. 25 Torr an at various flow rates of fee sea water, it is seen tat te yiel obtaine is iger in case of operation wit iger flow rate ( kg/r) of te warm sea water. Tis increase is a result of te aitional amount of vapour tat is flasing insie te camber at ig flow rates, wic inturn conenses to fres water in given unit time. Also te increase in yiel wit respect to fee seawater temperature is limite by conensing capacity of te col seawater. Fig. IV presents te variation in te yiel of esalination plant wit respect to te warm seawater inlet temperature at constant flow rates of fee seawater an varying operational pressure of te flas camber. ttp:// eitor@iaeme.com

9 Process Simulation of Vacuum Desalination Systems for Proucing Potable Drinking Water from Saline Sea Water Figure 4 Effect of warm fee seawater inlet temperature on te yiel at various operational pressures of te flas camber Wit te flow rate of fee seawater being maintaine constant, it is observe tat te yiel obtaine from te plant is iger wen te flas camber is operate at lower pressure (23 Torr). Tis increase in yiel is attribute to te lower saturation temperature corresponing to te lower pressure insie te flas camber wic evelops aitional temperature ifference wit respect to incoming seawater, tereby resulting an increase vapour. Furter, at a fixe operating pressure of flas camber, a steep rise in te yiel of te plant was seen wit an increase in inlet temperature of fee seawater. Fig. V iscerns te effect of flas camber pressure ranging from Torr over te yiel of te plant at various flow rates, keeping te inlet temperature of te warm seawater constant. Figure 5 Effect of Flas camber pressure on te yiel at various flow rates of inlet sea water A ecrease in yiel of te system as been observe wit increasing flas camber pressure from Torr. Tis ecrease in yiel is an outcome of increase boiling point of te warm sea water corresponing to te iger pressure wic inturn results in a ecrease in te evaporation rate. Fig. VI reveals te variation in te yiel at ifferent inlet temperatures of warm fee sea water by keeping te inlet flow rates constant. From te figure it can be inferre tat te increase temperature graient between te flas camber an conenser is responsible for te improve yiel at iger inlet temperatures of fee water. ttp:// eitor@iaeme.com

10 S. Taela, Raja Sekar Donapati an V. V. Rao Figure 6 Effect of Flas camber pressure on te yiel at various inlet sea water temperatures 5. CONCLUSIONS Te feasibility of an innovative, tecnically viable, low energy intensive an corrosion resistant esalination system riven by utilizing naturally available sea water termal graient was stuie. A numerical moel using ifferent process variables as been evelope an te resulting equations were solve using a simulation program an te results were reporte to illustrate te operational performance of te esalination plant in wic a flas camber an a conenser are couple to a vacuum system. By working at fixe operating conitions, te significant effects of inlet temperature of fee seawater an of flas camber pressure on te yiel of te system were analyze an te following main conclusions were rawn. Base on te results of simulation, it was unerstoo tat ig flow rates an ig inlet temperature of fee sea water (temperature ranging between 28 0 C C) an low operational pressure of flas camber allows for obtaining more fres water (yiel) in typical Inian Ocean conitions. Furter muc lesser flow rate an low inlet temperature of fee sea water an ig pressures of flas camber pose a treat of poor performance of te system. In future, a etaile exergy analysis nees to be carrie out to optimize te plant performance as well as its energy requirements. P Flas Camber pressure (Torr) Tisw Inlet Fee sea water temperature ( 0 C) Tosw Col sea water temperature ( 0 C) Symbols Tu Saturation temperature of upstream seawater ( 0 C) T Saturation temperature of ownstream seawater ( 0 C) L u L v Entalpy of upstream seawater at upstream pressure an corresponing saturation temperature (J/kg) Entalpy of ownstream seawater at ownstream pressure an corresponing saturation temperature (J/kg) Entalpy of te flase vapor at ownstream pressure an corresponing saturation temperature (J/kg) L Entalpy of inlet warm seawater (J/kg) isw L Entalpy of outlet col seawater (J/kg) osw v Entalpy of non-conensable gases (J/kg) nc m fsw Mass flow rate of te fee seawater to te flas camber (kg/s) ttp:// eitor@iaeme.com

11 Process Simulation of Vacuum Desalination Systems for Proucing Potable Drinking Water from Saline Sea Water m wvap Mass flow rate of water vapor (kg/s) m REFERENCES Mass flow rate of istillate (kg/s) [1] J. H. Tay, S. C. Low, an S. Jeyaseelan, Vacuum esalination for water purification using waste eat, Desalination, vol. 106 (1), pp , [2] K. A. Al-Sayji, Moeling, Simulation an Optimization of Large-Scale Commercial Desalination Plants, Tesis, Virginia Polytecnic Institute an State Unversity, [3] M. Al-Sammiri, Evaporation rate as a function of water salinity, Desalination, vol. 150 (2), pp , [4] A. K. Rajvansi, A sceme for large scale esalination of sea water by solar energy, Solar Energy, vol. 24 (6), pp , [5] R. Sentil Kumar, A. Mani, an S. Kumaraswamy, Analysis of a jet-pump-assiste vacuum esalination system using power plant waste eat, Desalination, vol. 179 (1), pp , [6] A. E. Mutunayagam, K. Ramamurti, an J. R. Paen, Low temperature flas vaporization for esalination, Desalination, vol. 180 (1), pp , [7] R. Sentil Kumar, A. Mani, an S. Kumaraswamy, Experimental stuies on esalination system for ocean termal energy utilisation, Desalination, vol. 207 (1), pp. 1 8, [8] Kali S. E. Al-Malay, T. Hogkiess, Comparative stuies of te seawater corrosion beaviour of a range of materials, Desalination, vol. 158 (1), pp , [9] Menacem Elimelec an William A. Pillip, Te Future of Seawater Desalination: Energy, Tecnology, an te Environment, Science, vol. 333, pp , [10] Cennan Li, Yogi Goswami an Elias Stefanakos, Solar assiste sea water esalination: A review, Renewable an Sustainable Energy Reviews, vol. 19, pp , [11] A. Husain, A. Wolai, A. AI-Raif, A. Kesou, R. Borsani, H. Sultan, an P. B. Despaney, Moelling an simulation of a multistage flas (MSF) esalination plant, Desalination, vol. 97 (1), pp , [12] : Hazem M. Sale, Eab M. Mina, Raouf N. Abelmessi Effect of Some Design an Operation Parameters on te Performance of A Water Desalination Unit Using Humiification Deumiification. International Journal of Mecanical Engineering an Tecnology, 8(5), 2017, pp [13] Anantan D Tampi, Ajit C Menon, Ceric Beneict, Amal Sreenivas, Fabrication An Analysis Of Portable Desalination System. International Journal of Avance Researc in Engineering an Tecnology (IJARET), 5(5), 2014, pp ttp:// eitor@iaeme.com