Purification of water using vertical multiple effect distillation unit

Transcription

1 634 Journal of Scientific & Industrial Research J SCI IND RES VOL 70 AUGUST 2011 Vol. 70, August 2011, pp Purification of water using vertical multiple effect distillation unit G R Desale 1 *, Padma Vasudevan 2, J K Pothal 1, K S Zala 1, B A Bhatti 1, S N Singh 2 and P K Sen 2 1 Central Salt and Marine Chemicals Research Institute (CSMCRI), CSIR, Bhavnagar , India 2 Indian Institute of Technology Delhi, New Delhi , India Received 04 May 2011; revised 28 May 2011; accepted 01 June 2011 This study presents use of a vertical multiple effect distillation (VMED) technique for defluoridation and dearsenification of contaminated water and desalination of sea water. Distillation of tap water was also undertaken for comparison. VMED unit has been found able to remove all types of salt from water to produce good quality distilled water. Keywords: Dearsenification, Defluoridation, Desalination, Distilled water, Vertical multiple effect distillation (VMED), Water purification Introduction Drinking water supplied must be free not only from harmful fluoride, arsenic and salt, but also free from bacteriological and other chemical contaminants. Generally, processes such as coagulation/filtration, chemical treatment, reverse osmosis (RO), electro dialysis (EDI), phase change (thermal desalination), filtration, ion exchange resins, etc. are used for water purification for different applications 1-5. All these processes can be grouped into two categories: Category I (where phase change occurs, and water is removed from unsafe levels of fluoride, arsenic & salt in water) such as evaporation, freezing, purification through crystallization etc.; and Category II (where phase change does not occur for purification of water) such as filtration, RO, EDI, or chemical reaction etc. Selection of appropriate process depends on quality of feed water, required quality of treated water and its applications 6-8. RO is widely accepted for desalination of potable, brackish and sea water 9, but it can not effectively remove fluoride and arsenic ions from water 10. Thus for defluoridation and dearsenification, processes such as coagulation/filtration, lime softening, resin column, activated alumina, nanofiltration, etc. are used 11. In case of activated alumina, chemical handling requirements may make this process too complex and dangerous for many small systems; also this invites disposal problem *Author for correspondence Tel: (+91) ; Fax: (+91) grdesale@csmcri.org of highly fluoride and arsenic concentrated waste stream and the same problem is also associated with ion exchange resin technique. Processes under Category I consume higher energy due to phase change and increases operating cost. While in Category II, operating cost is comparatively less 12. However, for defluoridation, arsenic removal and desalination of water, Category II technologies have limitations, and thus the techniques under category-i are more acceptable. In this study, vertical multiple effect distillation (VMED) unit, designed & fabricated by Sen et al 13-15, is used for production of distilled water from tap water, removal of unsafe levels of fluoride and arsenic, and desalination of sea water. Experimental Section Experimental Set-up of vertical multiple effect distillation (VMED) VMED unit is used for distillation of tap water, and defluoridation, dearsenification and desalination of sea water. Hardness from sea water is pre treated by nanofiltration. Resultant water is fed to VMED unit 13, which consists of steam generator (boiler), pressure regulating valves (PRV), vertical tube evaporators (VTE), mist eliminators (MEs), falling-film condenser, distillate and brine withdrawal system, pump, economiser, measuring instruments, etc. (Fig. 1). Fire tube boiler (capacity, 100 l) is used for steam generation and single reciprocating pump is connected to boiler as well as economiser, which, with one inlet and one outlet along with valves, is fixed just above boiler and gets heated by

2 DESALE et al: PURIFICATION OF WATER USING VERTICAL MULTIPLE EFFECT DISTILLATION UNIT 635 Fig. 1 Schematic diagram of multiple effect distillation unit 13 A-A Fig. 2 Schematic diagram of vertical tube evaporator 13 fire tubes. Inlet pipe is connected to pump and outlet pipe is used to distribute cooling water to all 6 VTEs. Inside VTE, aluminium pipes (7) are fitted to pass steam for cooling. VTEs (Fig. 2) have 2 inlets (1 each for steam and feed water) and 4 outlets (1 each for fresh steam, distilled water, excess steam and cooling water). MEs (6) are fitted vertically to the side of VTEs. Experimental Procedure Procedure for distillation of tap water, defluoridation, dearsenification and desalination of sea water are almost similar for VMED unit except for removal of hardness through nanofiltration in case of desalination of sea water. Separate sets of experiments were carried out for distillation of tap water, defluoridation, dearsenification and desalination of sea water. In two separate pots containing tap water, controlled amounts of fluoride and arsenic were mixed to make potted waters respectively 9.4 ppm and 3 ppm contaminated. In another set of experiments, tap water, contaminated water and sea water were stored in a tank and pumped to boiler as well as economiser using a reciprocating pump. Wood was

3 636 J SCI IND RES VOL 70 AUGUST 2011 Fig. 3 Photographic view of experimental set up used as fuel for heating the water inside boiler and economiser. Steam produced in boiler flows to first VTE, where it passes through aluminium tubes. Economiser supplies heated water to fall on the outer surface of aluminium tubes for cooling steam inside the tubes. Preheated water from economiser absorbs heat during cooling steam tubes inside VTE and produces distilled water inside the tube and water on the outer surface of tube gets converted into steam. This steam is again passed through ME and further fed to next stage VTE, where it cools and distilled water is produced and collected through outlet. Steam produced outside aluminium tubes is passed to next VTE through ME and same steps are repeated up to the last stage. Excess water used for cooling in VTE is drained through one common pipe to tank and then pumped to economiser. Final product (distilled water) from outlet of each VTE is collected individually. Results and Discussion VMED unit (Fig. 3) was operated for 8 h with tap water, water containing fluoride, arsenic and salt individually. To ensure repeatability, experiments with different types of input water were conducted twice. Flow rate from each stage was measured and collected water was tested for quality. Desalination of Tap Water Tap water [total dissolved solids (TDS), 530 ppm] was used for passing through VMED unit to get distilled water. Flow rate of product (desalinated) water (53 l h -1 ) was measured (Fig. 4). A maximum of 16.2 l water was collected from first stage. Product water (TDS, 7.93 ppm) from all 6 stages was collected in a single vessel (Fig. 5). Generally, water (TDS d 10 ppm) Total dissolved solids, ppm Distilled water. flow rate, l/h Fig. 4 Flow rate of distilled water during experiment with tap water Fig. 5 TDS measured during experiment with tap water is described as distilled water. It is observed (Fig. 5) that all first 5 stages produce distilled water and sixth stage (last one) gives higher TDS (16.1 ppm) product water. However, mixing of sixth stage product water with all other 5 stages collectively gives distilled water. Skilled operator is required to run unit at its maximum efficiency to optimize productivity and quality of distilled (product) water. Defluoridation of Water Water (9.4 ppm fluoride conc.) was used as input in VMED unit for purification. Distilled water samples from each stage were collected and fluoride content was measured (Fig. 6). As per recommendations of World Health Organisation (WHO) 16, water (fluoride conc., < 1.5 ppm) is permissible for drinking. It is observed (Fig. 6) that product water collected from all stages (51.6 l h -1 ) is distilled water (fluoride conc., ppm) and free from fluoride contamination. Dearsenification of Water Experimental procedure for defluoridation is followed for dearsenification. Initially, water (arsenic, 3 ppm) is

4 DESALE et al: PURIFICATION OF WATER USING VERTICAL MULTIPLE EFFECT DISTILLATION UNIT 637 Fig. 6 Fluoride concentration (ppm) and product water flow rate from different stages Flow rate, LPH Fig. 7 Concentration of arsenic (in ppb) and product water flow rate from different stages Distilled water. flow rate, l/h Total dissolved solids, ppm Arscnic conc., ppb Fluoride conc., ppm Flow rate, LPH Fig. 8 Flow rate of distilled water during experiment with sea water passed through VMED unit for distillation. As per recommendations of WHO 16, water (arsenic conc., < 50 ppb) is permissible for drinking. It is observed Fig. 9 TDS measured during experiment with sea water (Fig. 7) that product water collected from all stages (56.9 l h -1 ) is distilled water (arsenic conc., 1.64 ppb) and free from arsenic contamination. Stage No. 3 had arsenic concentration of 10 ppb and 0 ppb in all other stages.

5 638 J SCI IND RES VOL 70 AUGUST 2011 Desalination of Sea Water Sea water (TDS, ppm) was passed through nano-filtration unit to get pre-treated sea water (TDS ppm), which was then used for desalination in VMED unit. It is observed (Fig. 8) that product water flow rate from the first two stages was maximum compared to other stages; product water collected from all 6 stages is 35 l h -1.Product water collected from each individual stage shows higher TDS (Fig. 9) than maximum limit for distilled water (10 ppm). Product water from all 6 stages was collected in a single vessel and its TDS was ppm. Thus, VMED unit is unable to produce distilled water of desired quality from sea water in single pass. However, unit reduces TDS by 1000 times (from to ppm), and TDS of product water is below permissible levels for potable water. With addition of appropriate quantity of minerals, product water could be made potable. Also, unit requires some modifications in order to flush out periodically the common salt deposit in powder form. This is required for prolonged operation of unit for sea-water desalination. Conclusions VMED unit is efficient to produce distilled water (7.93 ppm) from tap water (530 ppm). It can be effectively used for defluoridation and dearsenification, as it reduces fluoride concentration from 9.4 ppm to ppm and arsenic concentration from 3 ppm to 1.64 ppb. For sea water, VMED unit shows good performance to get product water of ppm from pre-treated sea water of 18,600 ppm during first run of experiment. Thus TDS of sea water is reduced by 1000 times. VMED unit is efficient for the production of distilled water from tap water. Same unit can be used for defluoridation, dearsenification and desalination of sea water. However, water produced cannot be directly used for drinking purpose as it does not contain sufficient amount of useful minerals, which may to be added in controlled amounts to make drinking water. Performance and cost effectiveness of VMED unit for desalination of sea water can be improved by suitable modifications. Acknowledgments Authors thank Dr P K Ghosh, Director, CSMCRI Bhavnagar, for encouraging and support to carry out this work; Dr K M Popat for analysis of fluoride and arsenic concentration; and Mr M R Gandhi for kind support for installation of boiler. Authors also thank Mr Sitaramji, Mr Sehgal Mr S N Patel, Mr B P Rathod, Mr S D Gohil and Mr R S Patil for installation of plant at CSMCRI Bhavnagar. Financial support under RC-UK DST, India (EP/G021937/1) project is also acknowledged. 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