Seawater desalination by reverse osmosis (case study)

Transcription

1 DE!HLlNATON Desalination 153 (2002) Seawater desalination by reverse osmosis (case study) M. Abou Rayana*,. Khaledb Mansoura University, Mansoura, Egypt Tel. +20 (3) ; Fax ; Desalination Sector Sinai Development Organization South Sinai, Egypt Received 9 Apri 2002; accepted 16 April 2002 Abstract This paper presents a case study of the operation and maintenance of 2000 m /d desalination plant erected in1995. The results have been obtained over 6 years of operation. The plant consists of four units with a capacity 500 m3/d each. The results obtained are used to evaluate and develop the optimum plant operating pressure and temperature. The daily feed salinity, temperature and pressure were recorded, and finally the tota cost for product of potabe water was calculated. Recommendations were issued regarding the optimum operating conditions and the most pertinent operating problems. A technoeconomical analysis is undertaken in order to evaluate the cost of the water produced and technical reliability of the technology. The objective is to present field results of the reverse osmoses plant operation in order to evaluate the reliability of this technology in comparison with other technologies. Keywords: Seawater desalination; Reverse osmosis; Economic evaluation 1. ntroduction The present study gives an outline description of a reverse osmosis (RO) technology. The considered plant has been constructed on the site of Dahab city in south of Sinai, Egypt. The required electrical power for the plants is supplied to the site from 8 generators, 500 KVA each. *Corresponding author. The saline water is supplied from 8 beachwells on the beach of the Aquaba gulf. The salinity of feed seawater is in the order of 44,000 ppm. The station consists of 4 units; each unit plant has maximum capacity of 500 m3/d. The plant started production in November The objectives of this study are: The analysis of the performance in order to evaluate the determining operating factors. Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, May 4-6, /02/$- See front matter Elsevier Science B.V. Ah rights reserved P: SO01 l-9164(02)

2 246 M Abou Rayan,. Khaled / Desalination 153 (2002) The evaluation of the reverse osmosis efftciency under the actual operating conditions. The determination of the optimum operating parameters. The evaluation ofthe overall system reliability for long-term automatic operation. The evaluation of the operation since 1995 to 2002 in order to explore the necessary maintenance work and the reliability of the technology. The establishment of the best maintenance procedure for the membrane system. The plant is still working under continuous operation without any significant operational problems. 2. General plant overview The plant consists of 4 units. Each 500 m3/dunit consists of 1) ntake 2) Multi media filters 3) Cartridge filters 4) Chemical system 5) RO unit 2.1. ntake The intake is a beachwell system. The RO unit requires 75 m3/h flowrate of raw seawater for one unit producing 500 m3/d. Based on the permeability of the soil, one beachwell can deliver a discharge of 75 m3/h. Safety margins and standby requirements for only 50% capacity are considered: For one 500 m3/d unit 2 wells are required. For four 500 m3/d units 8 wells are required. The beachwells are constructed according to the standard. The wells is equipped with submersible pumps. The following specification are applied: mechanical digging with a O-inch diameter and depth of about 15 m. Applying bentonite and taking various samples, each 1 m length to determine the quality of penetrating subsoil, the thickness and quality of water layer in order to assure the best design to the well casing. There are 1 O-inch diameter PVC pipes The well flushing and cleaning is done via high pressure pumps. The pump is a multistage submersible motor pumping set suitable for water application, particularly resistant to erosion and saline water, with the following characteristics: The nominal discharge at due point 75 m3/h The suction from the well improves water quality, particularly regarding fine surrounding materials. The sand is acting as a natural filter Seawater chlorination system The seawater chlorination system is a complete automatically as well as manually actuated sodium hypochlorine solution dosing plant. The proposed plant comprises the following: a) PE solution tanks: Chemically resistant PE transparent cylindrical tank of 500 bit capacity. The tank is equipped with a seawater level gauge, drain valve and top cover. The post-chlorination is particularly important in order to disinfect the water introduced to the potable water network. b) Electrically activated agitator with stainless steel shaft and mixing blades mounted vertically. A small electric motor of fraction of horse power was used. The pump capacity is 27 /h Discharge The outfall system includes one buried PVC pipe from the plant up to the seashore. The remaining part from the pipe (offshore pipes) is high-density PE laid down on the seabed with special covering. At the end of the outfall pipe a distributor header is fit to discharge the reject over a wide area Multi mediajilters Each filter unit consists of 2 filters made from GRP -the first filter for sands and the other for activated carbon. The two filters are in series. The filter system is completely automatic with backwash facilities. The backwash water is collected in the brine outfall line.

3 M. Abou Rayan,. Khaled / Desalination 153 (2002) Cartridge filters The plant is provided with one cartridge filter which ensures that particles larger than 5 micron, carried over from the dual madia filters, will not enter the membranes. This filter is constructed from SS for total corrosion resistance Chemical treatment system Chemical cleaning system Periodic cleaning of RO plant membranes is necessary. t gives an indication of the type of fouling that may take place. The system has been designed so that one bank of membrane from each module can be cleaned. Circulation ofthe cleaning fluids is handled by one chemical-cleaning pump Permeate/flushing tank Each plant is provided with one drawback. The drawback is the process of natural osmosis which occurs when the plant is shutdown. Under normal circumstances when a shutdown occurs, the permeate pumps will start and flush out the seawater from the membranes Post-treatment Post-treatment of the product water consists of chlorinating to allow a chlorine residual and ph adjustment within the range of Economic analysis n order evaluate precisely the RO process, an economic analysis is important. The major factors, which influence the unit cost, are fuel cost, chemical products cost, operating and maintenance cost in addition to capital cost Cost of maintenance and running Fuel consumption. Power generating unit of 500 KVA is used. The rate of fuel consumption is 93 LE/h. The price of 1 of solar (diesel) is 0.40 LE. Table 1 Effect of feed pressure on productivity and concentrate Pressure, Product water Concentrate water bar m /h TDS m3/h TDS Table 2 Effect of temperature on productivity Temperature, C Productivity, % Table 3 Energy power requirements Motor kw Consumed kwh/m3 eff 90% power product Sea pump Filter pump Additive pumps High pressure RO pump Energy recovery turbine Total The price of fuel/m3 of water = 93x0.4x = LE/mof product water Oil consumption (5% of fuel consumption) = x 0.05 = 0.09 LE/m3

4 248 M. A bou Ruyan, 1. Khaled / Desalination 53 (2002) Total fuel and oil consumption = = 1.88 LE/m Chemical requirement per m3 of product surprisingly chemical treatment cost is low in percentage. This is may be due to the fact that the plant is new. g/m3 cost, LE CaOCl (60%) HCl (30%) FeC, CaOCl (past) NaCO, Others Total 0.94 Maintenance and repair per m3 of product LE Cartridge filter 0.1 Pump motors 0.12 Electrical controls, etc nstruments, etc Miscellaneous 0.05 Total 0.40 Labor and administration Approx LE/m3 product water from 1, 2, 3 and 4. Cost of 1 m product = Cost of 1 m3 produced = 3.97 LE Capital cost 4,500,OOO LE/unit Cost of 1 m3 product = o~~~~~~oo = LE/m3 Cost of 1 m3product30m wells including civil work cost = 500,000 20x365~500 = LE/m3 Final cost of 1 m3 product: from 4,5 and 6 = = 6.57 LE/m3 4. Discussion 3. Maintenance and repairs During this period of operation of 6 years the following items were effected. These items require intervention and in most cases complete change. a) Seawater pump ( beachwell pumps): Due to the erosive nature of saline water the pumps were eroded shortly after starting the operation. During these 6 years of operation 50% of pumps were out of service and removed. The total number of working pumps was 8. Four of them have been replaced with new pumps. And the other four have been completely revamped. The increment in total cost due to the change of pumps is 0.12 LE/m3 of water produced. b) Multi media j?lters: The activated carbon has been changed for all filters. c) CartridgeJilters: The core of the cartridge filter has been changed regularly each 3 months. t has been included in the cost breakdown d) Membrane: The membrane is the heart of the plant and the lifetime was considered 5 years - 20%/year depreciation which is reasonable. The plant lifetime is considered 10 years. The total cost of product will be increased by the cost of the membrane which is approximately 20% of the total cost. The resultant increase in the cost is 0.25 LE/m3 of water produced. Finally the cost of water produced is = 6.94 LE. This cost is relatively high for RO units. But the reason is the high capital cost which is equivalent to 9000 LE/m3 installed. The reason is the high cost of civil work and the beachwell. Now for new plants this figure has been considerably decreased LE/m3 installed. t appears from the breakdown cost that the The discussion will emphasize the two aspects major factor in cost is capital cost and fuel cost, - the technical and economics of the technical

5 M. Abou Rayan,. Khaled / Desalination 53 (2002) Transit tank Beachwells Chlorine feed system,t, Pllmn 1. 1, t Membrane To storage tank R.W. Tank, Csulant DS Flushing pump Fig. 1. Desalination unit RO with 500 m3/d capacity in Dahab water station. aspects. Fig. 1 shows the schematic ofthe system. The system is conceived in such a manner as to conserve energy. The energy rejected from the discharged water is recovered in a recovery turbine. Figs. 2-5 show the influence of different parameters on operation - increasing water temperature increases productivity. The relation is linear between pressure and productivity. Thus in hot and arid zone, where water temperature is high, the RO productivity can be increased which results in low total cost. s 125 LOO -,, 200 $ i 225 The RO system is reliable for operation and does not require any skilled operator and is simple to install. The only inconvenience is its high operating and maintenance cost. n this case the high operation cost comes from two items - capital cost contribution is 34% and fuel cost 26%. Maintenance and repairs present only 0.5% and chemical treatment 11%. The high capital cost is due to the well system. The energy consumption is reasonable. Desalination is a high-energy consumption process. The energy amounts to 3560% of the total production cost. Due to the rapid 140 El ; SD s 2s.- -_-_- / \ \ / 75% r&y 500 ppm l&cl 25 c Passage 75% recoverv 0 SO Transmembrane pressure, psi Fig. 2. Effect of feed pressure on productivity and salt passage. The quantity of water production is increased with pressure increment. The quantity of salt produced is decreased with increasing pressure Temperature, C Fig. 3. Effect of temperature on productivity.

6 250 M. Abou Rayan,. Khaled / Desalination 53 (2002) Middle East 43,000 TDC 120 W MSF Caribbean TDC r oocl l % Feed salinity, ppm NaCl Fig. 4. Effect of feed salinity on productivity wt WJ) (10) (100) d,& Plant capacity (1000 GPD) Fig. 6. Capital cost of reverse osmosis vs. multistage flash distillation.(from [2]). 6 P L z. Z El SO%,, The desalination process is generally energy consuming. The thermal processes consume more energy than RO. The advantages of (RO) over the more energy extensive thermal processes will increase as world energy costs rise Feed salinity, ppm NaCl Fig. 5. Effect of feed salinity on salt passage. progress in seawater reverse osmosis technology, the cost was decreased. The present cost now is as low as $US 1. Thus thermal desalination can be economically considered only if the low heat energy cost is available. RO plants have a considerable scope for future development in terms of membrane performance, and cost reduction for stand-alone and for small-scale production. RO is likely to be the main process in this region. Fig. 6 shows the approximate cost of RO and MSF. t appears that RO is competitive compared to MSF. 5. Conclusions The following conclusions are obtained based on a 6-year operating period of the RO desalination plant: 9 The economic analysis shows that RO is competitive to thermal processes. The reverse osmosis system is sensible to change in feedwater temperature, and the product quality is sensitive to the working pressure. After more than 6 years of operation the plant is now runnjng continuously with minimal operator interference. The only plant components, which gave trouble, were the seawater pumps (wells pumps), pipe lines, power cables and one chemical dosing pump.

7 M. Abou Rayan,. Khaled/ Desalination 1.53 (2002) Reference [l] 0. Benchikh and M. Abou Rayan, Technologie de Sessalement de Eau, UNESCO Publication, Paris, [2] A. Naim, The prospects of solar desalination in the Gaza Strip, First nternational Water Technology Conference, Alexandria, Egypt, February [3] R. Morris, Solar Desalination Options for Small Communities, ODA, London, [4] A. El-Nashar, Recent development in membrane technology, Cairo nternational Conference on Energy, 1994, Egypt. The manuals of the following companies have been used: ndustrial and Engineering Enterprises Co. (1.E.E) Weir Company Culligan Company V.(S.) Frenhel E.M.S. Company T. Gourgi E.M.S. Company