- Email: [email protected]
Experimental RO facility to study the heating effect of raw water on the varying main parameters
~
Vv.'vp~W
DESALINATION
I ELSEVIER
Desalination 134 (2001) 63-76 www.elsevier.com/loeate/desal
Experimental RO facility to study the heating effect of raw water on the varying main parameters M.H. Ali E1-Saie*, Yahya M.H. Ali E1-Saie, Mohamed Abd E1 Aziz Consulting Engineering Co., 19 Lo~i Baik Street, Manshiet E1 Baby, Postal Code 11757, Cairo, Egypt Tel. +20 (2) 2579533/2574940; Fax +20 (2) 4531271; e-mail: [email protected], [email protected]
Received 27 September 2000; accepted 11 October 2000
Abstract
Utilizing power plants in seawater desalination is one of the recent trends to cover the gap between the increasing water demand and the shortage in water resources in the new millenium. The fast development in the reverse osmosis (RO) desalination technique and membrane manufacturing technology renders it one of the promising desalination techniques to be coupled with power plants. Therefore, it is important to study all aspects related to thermal matching between the condenser cooling water outlet and RO system feedwater. In this concern an experimental RO facility was designed for Nuclear Power Plants Authority in Egypt (NPPA) to study the effect of the following parameters on the permeate quality, production, membrane life and aging and system's economy: feedwater temperature; feedwater pressure; recovery ratio. Three types of four-inch diameter spiral wound membranes were selected to cant out the experiments (Fluid Systems, FitrnTec and Hydranautics). Based on statistical analysis, five of each membrane type was considered as an optimum sample size. The system design included: I) Feedwater heating and temperature control facility, consisting of fresh water heater and fresh water to feedwater heat exchanger, in addition to permeate and brine to feedwater heat exchangers to recover its heat content. 2) Feedwater pressure regulating facility, consisting of hydr.a,ulie coupling and by pass throttling valve for fine tuning. Beachwells (two) were chosen for raw water feeding to mmlm17e the plant cost and the pretreatment requirements. The control supervisory functions for the plant were designed to be implemented using a distributed control and data acquisition system. Keywords: Reverse osmosis; Experimental; Research; Heating; Performance
*Corresponding author.
Presented at the International Conference on Seawater Desalination Technologies on the Threshold of the New Millennium, Kuwait, 4-7 November 2000.
0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved PII: S0011-9164(01)00116-3
64
M.H.A. El-Saie et al./ Desalination 134 (2001) 63-76
1. Introduction Utilizing a Nuclear Power Plant in seawater desalination is one of the main concerns of NPPA to secure a reliable potable water resource. As one of the main seawater desalination techniques, RO is undergoing continuous R&D studies. Therefore, NPPA decided to construct an experimental RO desalination unit at AI Dabaa, near Alexandria, Egypt, to take part in these studies and entrusted Consulting Engineering Co. (CEC) to carry out the engineering works for the project. CEC carried out the conceptual design, prepared the tender documents and issued an evaluation report, and the plant is now under contracting. The plant was designed to study the following parameters: 1. The effect of feedwater temperature on the permeate production, membrane life for different types of membranes. 2. The effect of varying the feedwater pressure and utilizing energy recovery, 3. Economics of the unit operation taking into consideration the above mentioned parameters to determine: • Operation and maintenance cost, • Unit availability and • Economical unit life time. Initially, beachwells will be used as a feedwater source. Taking into consideration the accuracy and relevancy of the results, the plant configuration (number of parallel membranes and number of trains) was selected based on statistical study. To reduce the equipment size and cost, a four-inch membrane was selected. The plant was designed to include two trains, one with feedwater heating equipment while the other without heating equipment. This will allow to simultaneously carry out the experiments with and without heating the feedwater. Spiral wound membranes from Fluid Systems, FilmTec and Hydranautics were selected for the experiments.
To enable study of the above mentioned parameters the unit was designed to be equipped with the following: 1. Feedwater heating facility: fresh water heater and three heat exchangers. 2. Instrumentation and control equipment including data acquisition system DAS and necessary software. 3. Feedwater pressure regulating device.
2. Design parameters 2.1. Plant size
Parallel operation for each individual membrane was selected to obtain its performance characteristics. A statistical analysis was carded out to determine the "sample" size for any one type of membrane tested. The basis of the analysis is as follows: • Membranes manufactured follow the normal distribution. • Standard deviation 10% of the mean value. • Sample size was v/tried from 1 to 9. • Different levels of confidence were selected (35, 50, 64, 73, 82 and 88), whereby results at each were obtained. • Relevance tolerance was set at 5% (this could be changed whereby a more strict criterion would enlarge the sample required and vice versa). • Confidence interval divided by the mean was plotted against sample size. Sample sizes selected for plotting were 1, 2, 3, 4, 5, 7, 8, 9 and 100. The minimum sample size satisfying the set tolerance was chosen. This study indicated: a sample size of 5 is optimum at a selected level of confidence of 73%. It also showed that choice of sample size is important since sensitivity will vary with sample number significantly.
M.H.A. El-Saie et al. / Desalination 134 (2001) 63-76 It is important to note that this analysis stands for the membrane as an "individual". More than one membrane in a pressure vessel, connected in series, is considered another case which needs another analysis. However, they will also require at least 5 sets if identical. Since this is not the case, more than 5 sets will be required, accordingly 5 membranes will be tested in parallel operation. As an experimental plant not intended for commercial operation, it is recommended to use 4-inch membrane. This will result in smaller capacity plant's components (feedwater, pumps, heater, etc.), which reduces the project budget, and O&M (operation and maintenance) costs. On the other hand, from the available data published by the membrane manufacturers, it can be concluded that the 4-inch membrane provides a very close equivalence to the 8-inch membrane in its key performance characteristics.
65
(800-1200psi) with different flow rates. Other performance parameters to be studied are mentioned under subsection 2.9 below.
2. 4. Method of heating feed and precautions To carry out the experiments at different temperatures, it is required to heat the feedwater. The feed seawater will be heated utilizing a fresh water to seawater heat exchanger. The fresh water will be heated by an electric water heater selected for its following advantages: • Easy installation and maintenance. No on-site fuel combustion which means clean, fume free and easy operation. • No fuel delivery and storage problems. •
At start-up, the required energy to raise the temperature of 1 m3 feedwater from 20°C to 45°C is calculated as follows:
2.2. Temperature operating range
1000 x 4.18 (45 - 20) = 104,500 kJ/m~
The maximum allowable operating temperature for the commercially available membranes as set by the manufacturers, is that for Fluid Systems, Hydranautics, FilmTec and Toyobo which is 45°C. Other membranes have lower values. Accordingly, the available membranes cannot be operated commercially beyond these limits. Hence it was selected to carry out' experiments with maximum temperatures of 45°C, since it is believed that no benefit will be gained from such experiments. Considering that the feedwater would be from beachwell the expected minimum feedwater temperature would be about 20°C. Hence the design operating raw water temperature range was selected to be 20-45°C.
At steady state and during continuous operation the permeate and brine heat content will be recovered in the feed seawater utilizing heat exchanger(s).
2.5. Recovery ratios From the performance data for the commercially available types of membranes the recovery ratio range of 7-35% will be selected in the unit design. Higher recovery ratios will not be encountered as long as the membranes are tested in parallel operation (as mentioned earlier under subsection 2.1 "Plant Size").
2.3. Pressure-flow operating range
2. 6. Beachwell intake location and requirements
The operating pressure range for the commercially available membranes is 5.5-8.27MPa
Two beachwells will be required as feedwater source for the plant.
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M.H.A. El-Saie et al. / Desalination 134 (2001) 63-76
2. 7. OuOrall type and location
The brine disposal will be through a dispersion pipe into the sea taking into consideration the prevailing current direction and the environments. 2. 8. Sanitation o f beachwell water
Hypochlorite solution will be injected into the beachweU water prior to storage in the raw water tank to prevent bacteriological contamination and development of algae slime. 2.9. Performance requirements
The plant has been designed for study of the following performance parameters of each type of membranes: • Feed temperature • Operating pressure • Feed flow • Recovery ratio • Permeate salinity • Product capacity • Membrane deterioration • Salt rejection These records are expected to show the significance of membrane deterioration over this duration.
2.10. Plant protection interlocks
The plant will be fitted with comprehensive instrumentation and automatic control equipment necessary for safe and efficient operation of the plant including the following: 1. Shutdown HP pump switches to protect parameter against over pressurization with alarm. 2. There is a time delay between the RO. HP pump unit start up and raw water feed pumps. 3. Low suction pressure switch shutdown to protect HP pump against cavitation with alarm.
4. Differential pressure switch (Ap) with pressure alarm indication to start automatically filters backwash pumps and others to detect the reliability of parameter performance operation associated with alarms. 5. Low level switches to stop chemical dosing pumps, feed pumps and product pump (if applicable) associated with alarm. 6. High level switches to stop beachwell pumps and HP pumps in case of high level raw water and product water storage tanks respectively with alarm indication. 7. Temperature switches to control feedwater heating process and protect parameter against excessive temperature exposure associated with alarm. 8. pH analysers for continuous pH measurement in the feed line in front of HP pump, and inter-locks high and low pH shutdown with alarm. 9. ORP analysers for high alarm and shutdown of the HP pump to insure that no halogenation of the membrane will occur. 10. High residual chlorine alarm in the feedwater. 11. High brine and product conductivity alarms. 12. Continuous SDI (silt density index) monitor in the feed line before the I-IP pump. 13. Continuous feedwater temperature recording with set point alarm after the HP pump. 14. Flow measurements used to set flow to and from parameter to control conversion. 15. liP pump trip signal shutdown of the feedwater pump to shutdown the unit. 16. Flow, pressure and temperature control valves are recommended to control flow, pressure and temperature in feedwater, product water and brine water. 17. Sample valves in the product and brine stream are used to monitor parameter performance. A sample valve is also needed before and after the cartridge filter to permit sampling for determining the feedwater silt density index (SDI).
M.H.A. El-Saie et al. / Desalination 134 (2001) 63-76
18. Flushing/cleaning tank provided with tank cover vent and mixer. A valve at the bottom of the tank is needed to assure that the tank can be completely drained and flushed of waste chemical solutions. A 5 or 10-micron cartridge filter should be installed in the solution feed line to avoid recirculation sediment which may dislodge from the system during the cleaning operation or originate from the cleaning chemicals.
67
provided, for each train, the experiments will be carried out on each membrane type separately. This is due to the practical difficulties encountered when operating more than one membrane type with one single high pressure pump simultaneously. The duration of the experiments will depend on the type of the membrane and its rate of deterioration.
3. Plant description 3.1. General 2.11. Data acquisition system
The control and supervisory functions required for central operation of the plant at the site will be implemented in a distributed control and data acquisition system (DC DAS) hardware. The distributed control system will be specifically designed for this operation. The operator control stations will provide access to the data acquisition functions which will include alarm, events display and logging function. The operator workstation may also give access to the control functions so the plant may be operated from keyboard. A minimum of two printers will be provided, one for alarm/event recording and the other for plant logging function with interchangeable facility, An engineering terminal complete with printer for system configuration. Each operator control station will consist of full graphic color display units (VDUS) of the smart CRT type and full alphanumeric character set keyboard with function keys.
2.12. Experiment types and durations
As mentioned above experiments will be carried out on three types of membranes. As the performance parameters are different, and one single high-pressure pump will be
Fig. 1 presents a block diagram of the plant. It was designed to be consisting of two trains extendable to three or more as required in the future. One train will operate with feedwater at normal temperature and the other will operate with heated feedwater. The latter will be provided with feedwater heating equipment. Beachwell will be used to feed the plant with the required feedwater. 3.2. Beachwells and pumps
For such low capacity RO desalination units it is recommended to use beachwells for the feedwater rather than seawater intake jetty or similar. Though the required feed capacity is small (about 25mVh), two beaehwells, were selected (one working and one standby) for the wells reliability and durability. This will have the advantages of lower costs and clean feedwater which needs minimum pretreatment requirements. The required capacity for the beachwell and filter feed pumps will be the maximum feed required, i.e. about 25 mVh. The beaehwell pumps will be operated intermittently in cases of the unit lower capacity. The filter feed pump will be provided with variable speed motor or hydraulic coupling to cater for the required capacity variation. The backwash pump capacity will be about 100 mVh.
68
M.H.A. El-Sale et at/Desalination 134 (2001) 63-76
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Materials for all pumps in contact with seawater (beaehwell, high pressure, filter feed and backwash pumps) will be duplex stainless steel or equivalent. Other pumps in contact with fresh water (cleaning, flashing, etc.) will be suitable grade of stainless steel. Materials for chemical injection pumps will be chemical resistant (PVC or equivalent).
69
temperature (45°C for the available membranes), which is calculated as follows: Maximum required feed =12.5 mVh Required heat = 12.5 x 1000 x 4.18 x (45 - 20) = 363 kW 3600
3.3. Pretreatment system Chlorination. Hypochlorite solution will be injected into the raw water storage tank to prevent bacteriological contamination. Filters. Feedwater will be filtered using dual media filters (one working and one standby). At a pre-determined pressure drop the filter will be backwashed. Second stage filtration will be through cartridge filters.
• •
•
Chemical dosing. Sulphuric acid to protect the membrane against calcium carbonate precipitation. Sodium metabisulphite for water dechlorination to protect the membrane against chlorine attack. Antiscalant to inhibit residual carbonate and non-carbonate scaling on the permeate depending on the safe operation of the acid dosing and the preset feed pH value.
3. 4. Water heating system
Train A will be provided with a heating facility to test the membranes at elevated feedwater temperatures while train B will be used to test the membranes at ambient feedwater temperature. Accordingly the maximum required heat rate for train A occurs when the raw water at its minimum temperature (about 20 ° ) and required to be heated to the maximum allowable operating
Direct heating of the raw seawater is not recommended due to scale formation problems within the heater. Therefore it is recommended to heat the feed seawater utilizing a fresh water to seawater heat exchanger. The hot fresh water will be obtained from an electrical water heater. The losses in the heat exchanger and piping system are estimated to be about 10%. Then the required water heater capacity will be about 400 kW. To reduce power consumption at steady state and during continuous operation, the heat content in the brine and the permeate will be used to preheat the feed seawater, utilizing permeate and brine to feed seawater heat exchangers. This will give the following advantages: • Reduction of power consumption in the water heater. • Reduction in permeate temperature. • Reduction in the brine temperature before disposal which will be advantageous from the environmental point of view. It is estimated that 40% of the heat energy will be lost in membranes, heat exchangers, pinging system, etc. and 60% will be recovered in the permeate and brine feedwater heat exchangers. Then the maximum required heating power during continuous operation estimated to be about 160 kW, and 240 kW will be recovered from the permeate and brine in the heat exchangers.
70
M.H.A. EI-Saie et al. / Desalination 134 (2001) 63-76
The heat exchangers will be of shell and tube construction (tubes titanium or Cu Ni 70/30, shell stainless steel 316L or equivalent). 3.5. High pressure pump with energy recovery and hydraulic coupling
As it is required to carry out experiments on different types of membranes, the high-pressure pump duty will be varying depending on the membrane type. One pump will be used for each train, coupled with a hydraulic coupling to obtain the required pressure-flow. For fine tuning, throttling valve together with a back pressure valve, will be provided. Variable speed motor can be used. To recover the brine kinetic energy, energy recovery turbine (ERT) or turbo charger will be provided. The systems provided with energy recovery turbines have the advantage of ability to operate in case the turbine is out of order, but with higher power consumption. On the other hand, systems provided with turbocharger cannot be operated without the same, and have the advantage of lower power consumption. Both systems can be accepted depending on detailed design and study.
will be provided for the system flushing in case of unit shut down.
3.8. Raw and product water tanks
The raw tank will be divided into two compartments. The first compartment will be for the raw water from the beachwell pumps and will be sized for a 3-hour unit operation. The second compartment will be for clean water coming from a branch after the multimedia filter. This clean water will be used in multimedia filter backwash. The clean water compartment size will be enough for one backwash cycle at least. The product water tank will be sized for 2day unit's operation. The raw and product water tanks will be reinforced concrete, suitably protected.
3.9. Post-treatment system
Caustic soda dosing: to adjust the product water pH value. Post chlorination: to sterilize the product water prior to storage in the storage tank.
3.6. Membranes and racking
3.10. Piping and valves
Three types of membranes in each train will be tested as mentioned above (FilmTec, Fluid Systems and Hydranantics), five numbers of each. Each type will be installed in a separate rack. The racks will be suitably designed for easy operation, maintenance and membrane replacement when necessary.
All high pressure piping in contact with seawater and brine will be stainless steel 316L. All other low pressure piping, fittings and valves will be of PVC or equivalent.
3. Z Cleaning/flusing system
Chemical cleaning system will be provided for periodic cleaning. The system will include tank and pump complete with necessary piping, valves, fitting, etc. In addition a flushing pump
3. I 1. Chemical treatment system
Dosing pumps will be a positive displacement diaphragm type with variable stroke length and stroke rate and will be of chemical resistant material. Dosing tanks will be suitably sized, and will be of chemical resistant materials (PVC or equivalent).
M.H.A. E1-Saie et al. / Desalination 134 (2001) 63-76
switch with indicator is activated when the filter reaches the preset value to start backwash process. The backwash earl be controlled from DC DAS.
3.12. P & l diagram
Figs. 2 and 3 shows P&I diagrams for train A, operating with heated feedwater and train B operating with normal feedwater temperature. 3.13. Plant layout
Plant layout is shown in Fig. 4. The layout is prepared taking into consideration accessibility for easy operation and maintenance and for future extension.
71
• The system is provided with cartridge filters (5-10~t). The purpose of these filters is to protect the HP pump and the membranes from larger particles that may be present in the water. A differential pressure switch with an indicator is provided to advise the operator to replace the cartridge. •
3.14. I&C works
The plant control system is designed to provide reliable and efficient operation through the use of a microprocessor based on distributed control and data acquisition system (DC DAS) interfaced through the main control console in the plant control room. The plant will include the following instruments: • Beachwell water is pumped directly from the wells into the raw water storage tank with chlorine solution dosing provided before the tank. •
Feedwater pumps (one duty and one standby) started if the raw water storage tank (feedwater compartment) level is low and in the case of high level it stops (and cannot be started either by automatic or manual control) same condition for backwash pump. The pumps can be started from local control panel interfaces with a distributed data acquisition system.
•
Chemical dosing system: - Chlorine solution Coagulant solution Post treatment solution The malfunction of chemical dosing system trips the liP pump leading to plant shutdown. • Automatic dual media pressure filters (one duty and one standby). A differential pressure -
-
Cartridge filters would be used to prevent particles from damaging the HP pump of fouling the permeators (one duty and one standby). • Each parameter has all instruments to indicate flow, pressure and temperature. •
high-pressure shutdown switch will be mounted in the RO permeator feed line. A
• Automatic feedwater shut-off valve. The valve automatically closes when the RO HP pump is shutdown for any reason. • Automatic reject line flush valve with a timer is automatically activated once every 24h for 30min to remove any build up sediment or precipitates the valve normally closed and energized to open. • The system will include a drawback tank to provide sufficient water for osmotic drawback through the membranes when the RO system shuts down. It also provides pure water for flushing the piping on the feed side of the RO at the same time. The control will include the following automatic safety shut down features: - High pressure pump motor over load. Chemical tank-low level. Instruments power failure. -
-
Instrument air. A compressor is always switched "on" for instrument air "except for maintenance" and will stop and start to main-
M.H.,4. El-Sale et al. / Desalination 134 (2001) 63-76
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tain a constant pressure in the instrument air header. Heat exchangers for brine and product recirculation to increase feedwater temperature and to trim the feedwater temperature to preset temperature value by using a temperature controller to open/close the 3-way valve in case of getting the feedwater temperature value. Chlorine-free detection monitor (ORP meter) with automatic unit shutdown. pH value of the filtered water is continuously monitored. HP pump shutdown in case of low feedwater suction pressure or high feedwater pressure and temperature to RO permeators. Flow control valve must be used to control the flow in the product and brine line. Float type level indicating (or differential pressure type) transmitters are installed on raw water and product water storage tanks. Its output signal received to DCDAS for indicaring continuous tank level. In addition, an independent level (low/high) switches have been installed on the tanks to activate an alarm and interlock that should staWstop the pumps. Pressure flow and temperature transmitter should be used to indicate continuous pressure, flow and temperature measurements in the control room. Flow and conductivity measurements before and after each parameter and recommended to measure differential pressure across each membrane. Meters and gauges. Meters are required to measure feedwater, brine and product flow rates. Sample valves are used in product and brine stream to monitoring a permeator performance. Motorized valves (or equivalent) are used to control flow, pressure and temperature in the plant.
75
• A flow control valve in the brine stream is used to set conversion. • A throttling valve and back pressure valve on the HP pump discharge line are used to control the feed pressure to the permeator. • For more safety a pressure relief valve is required on the HP pump discharge line used to control system pressure when pump capacity is greater than that needed to supply the permeator. • The measuring equipment mentioned in the system design whether flow, pressure, level, or temperature must have an accuracy of -~0.25 of full scale or better. • The type and make of equipment will be intemationally proven and of standard product of the manufacturer. 4. R&D merits Securing reliable potable water resources is one of the main concerns of the new millenium. Still seawater desalination is considered the most promising resource, which renders it critical to undergo more R&D, especially for RO. The main RO varying parameters are the feedwater temperature, pressure and salinity, and the product water salinity, production in addition to the recovery ratio. RO membrane manufacturers publish some data illustrating the effect of variation of these parameters on the product water productivity and salinity and the recovery ratio. However there are some uncertainties about these data, especially the effect of feedwater temperature on the product water quality and quantity and the recovery ratio. Flow and conductivity measurements before and after each permeator in addition to differential pressure across each membrane are recommended. The above renders the set up of this experimental facility and experiments to be carried out independent and reliable for RO because:
76
M.H.A. El-Sale et al. / Desalination 134 (2001) 63-76
• NPPA is a government institution and is not committed to any membrane manufacturer or brand. • The facility will be operated and the results will be concluded by highly qualified investigators from NPPA staff. Moreover the facility was designed to be provided with intensive measuring and control systems for the different parameters: temperatures, pressures, flow rates, salinities, which is not available in the commercially operated RO plants. As an example, each individual membrane was designed to be provided with flow, pressure and salinity measurement instruments. The I&C system is an advanced computerized system recording all important parameters with time for intense evaluation.
The operating bands are designed to span all commercially known membrane types. Further, the chemical pre-treatment facility is very extensive to allow for different chemicals and dosing rates. The heating facility is also designed for peak flows at worst conditions, allowing for versatile heating modes in all experimental conditions. The space available for different racking types is allowed for removing any constraints. The use of identical trains allows concurrent experiments under identical conditions varying filter parameters from one to the other. This gives direct comparison for problems and this can conclude scientific arguments with proof. As for seawater source, allowances have been made for direct from sea or beachwells. The R&D array of experiments is limitless and it should be a source of much further research.
Vv.'vp~W
DESALINATION
I ELSEVIER
Desalination 134 (2001) 63-76 www.elsevier.com/loeate/desal
Experimental RO facility to study the heating effect of raw water on the varying main parameters M.H. Ali E1-Saie*, Yahya M.H. Ali E1-Saie, Mohamed Abd E1 Aziz Consulting Engineering Co., 19 Lo~i Baik Street, Manshiet E1 Baby, Postal Code 11757, Cairo, Egypt Tel. +20 (2) 2579533/2574940; Fax +20 (2) 4531271; e-mail: [email protected], [email protected]
Received 27 September 2000; accepted 11 October 2000
Abstract
Utilizing power plants in seawater desalination is one of the recent trends to cover the gap between the increasing water demand and the shortage in water resources in the new millenium. The fast development in the reverse osmosis (RO) desalination technique and membrane manufacturing technology renders it one of the promising desalination techniques to be coupled with power plants. Therefore, it is important to study all aspects related to thermal matching between the condenser cooling water outlet and RO system feedwater. In this concern an experimental RO facility was designed for Nuclear Power Plants Authority in Egypt (NPPA) to study the effect of the following parameters on the permeate quality, production, membrane life and aging and system's economy: feedwater temperature; feedwater pressure; recovery ratio. Three types of four-inch diameter spiral wound membranes were selected to cant out the experiments (Fluid Systems, FitrnTec and Hydranautics). Based on statistical analysis, five of each membrane type was considered as an optimum sample size. The system design included: I) Feedwater heating and temperature control facility, consisting of fresh water heater and fresh water to feedwater heat exchanger, in addition to permeate and brine to feedwater heat exchangers to recover its heat content. 2) Feedwater pressure regulating facility, consisting of hydr.a,ulie coupling and by pass throttling valve for fine tuning. Beachwells (two) were chosen for raw water feeding to mmlm17e the plant cost and the pretreatment requirements. The control supervisory functions for the plant were designed to be implemented using a distributed control and data acquisition system. Keywords: Reverse osmosis; Experimental; Research; Heating; Performance
*Corresponding author.
Presented at the International Conference on Seawater Desalination Technologies on the Threshold of the New Millennium, Kuwait, 4-7 November 2000.
0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved PII: S0011-9164(01)00116-3
64
M.H.A. El-Saie et al./ Desalination 134 (2001) 63-76
1. Introduction Utilizing a Nuclear Power Plant in seawater desalination is one of the main concerns of NPPA to secure a reliable potable water resource. As one of the main seawater desalination techniques, RO is undergoing continuous R&D studies. Therefore, NPPA decided to construct an experimental RO desalination unit at AI Dabaa, near Alexandria, Egypt, to take part in these studies and entrusted Consulting Engineering Co. (CEC) to carry out the engineering works for the project. CEC carried out the conceptual design, prepared the tender documents and issued an evaluation report, and the plant is now under contracting. The plant was designed to study the following parameters: 1. The effect of feedwater temperature on the permeate production, membrane life for different types of membranes. 2. The effect of varying the feedwater pressure and utilizing energy recovery, 3. Economics of the unit operation taking into consideration the above mentioned parameters to determine: • Operation and maintenance cost, • Unit availability and • Economical unit life time. Initially, beachwells will be used as a feedwater source. Taking into consideration the accuracy and relevancy of the results, the plant configuration (number of parallel membranes and number of trains) was selected based on statistical study. To reduce the equipment size and cost, a four-inch membrane was selected. The plant was designed to include two trains, one with feedwater heating equipment while the other without heating equipment. This will allow to simultaneously carry out the experiments with and without heating the feedwater. Spiral wound membranes from Fluid Systems, FilmTec and Hydranautics were selected for the experiments.
To enable study of the above mentioned parameters the unit was designed to be equipped with the following: 1. Feedwater heating facility: fresh water heater and three heat exchangers. 2. Instrumentation and control equipment including data acquisition system DAS and necessary software. 3. Feedwater pressure regulating device.
2. Design parameters 2.1. Plant size
Parallel operation for each individual membrane was selected to obtain its performance characteristics. A statistical analysis was carded out to determine the "sample" size for any one type of membrane tested. The basis of the analysis is as follows: • Membranes manufactured follow the normal distribution. • Standard deviation 10% of the mean value. • Sample size was v/tried from 1 to 9. • Different levels of confidence were selected (35, 50, 64, 73, 82 and 88), whereby results at each were obtained. • Relevance tolerance was set at 5% (this could be changed whereby a more strict criterion would enlarge the sample required and vice versa). • Confidence interval divided by the mean was plotted against sample size. Sample sizes selected for plotting were 1, 2, 3, 4, 5, 7, 8, 9 and 100. The minimum sample size satisfying the set tolerance was chosen. This study indicated: a sample size of 5 is optimum at a selected level of confidence of 73%. It also showed that choice of sample size is important since sensitivity will vary with sample number significantly.
M.H.A. El-Saie et al. / Desalination 134 (2001) 63-76 It is important to note that this analysis stands for the membrane as an "individual". More than one membrane in a pressure vessel, connected in series, is considered another case which needs another analysis. However, they will also require at least 5 sets if identical. Since this is not the case, more than 5 sets will be required, accordingly 5 membranes will be tested in parallel operation. As an experimental plant not intended for commercial operation, it is recommended to use 4-inch membrane. This will result in smaller capacity plant's components (feedwater, pumps, heater, etc.), which reduces the project budget, and O&M (operation and maintenance) costs. On the other hand, from the available data published by the membrane manufacturers, it can be concluded that the 4-inch membrane provides a very close equivalence to the 8-inch membrane in its key performance characteristics.
65
(800-1200psi) with different flow rates. Other performance parameters to be studied are mentioned under subsection 2.9 below.
2. 4. Method of heating feed and precautions To carry out the experiments at different temperatures, it is required to heat the feedwater. The feed seawater will be heated utilizing a fresh water to seawater heat exchanger. The fresh water will be heated by an electric water heater selected for its following advantages: • Easy installation and maintenance. No on-site fuel combustion which means clean, fume free and easy operation. • No fuel delivery and storage problems. •
At start-up, the required energy to raise the temperature of 1 m3 feedwater from 20°C to 45°C is calculated as follows:
2.2. Temperature operating range
1000 x 4.18 (45 - 20) = 104,500 kJ/m~
The maximum allowable operating temperature for the commercially available membranes as set by the manufacturers, is that for Fluid Systems, Hydranautics, FilmTec and Toyobo which is 45°C. Other membranes have lower values. Accordingly, the available membranes cannot be operated commercially beyond these limits. Hence it was selected to carry out' experiments with maximum temperatures of 45°C, since it is believed that no benefit will be gained from such experiments. Considering that the feedwater would be from beachwell the expected minimum feedwater temperature would be about 20°C. Hence the design operating raw water temperature range was selected to be 20-45°C.
At steady state and during continuous operation the permeate and brine heat content will be recovered in the feed seawater utilizing heat exchanger(s).
2.5. Recovery ratios From the performance data for the commercially available types of membranes the recovery ratio range of 7-35% will be selected in the unit design. Higher recovery ratios will not be encountered as long as the membranes are tested in parallel operation (as mentioned earlier under subsection 2.1 "Plant Size").
2.3. Pressure-flow operating range
2. 6. Beachwell intake location and requirements
The operating pressure range for the commercially available membranes is 5.5-8.27MPa
Two beachwells will be required as feedwater source for the plant.
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2. 7. OuOrall type and location
The brine disposal will be through a dispersion pipe into the sea taking into consideration the prevailing current direction and the environments. 2. 8. Sanitation o f beachwell water
Hypochlorite solution will be injected into the beachweU water prior to storage in the raw water tank to prevent bacteriological contamination and development of algae slime. 2.9. Performance requirements
The plant has been designed for study of the following performance parameters of each type of membranes: • Feed temperature • Operating pressure • Feed flow • Recovery ratio • Permeate salinity • Product capacity • Membrane deterioration • Salt rejection These records are expected to show the significance of membrane deterioration over this duration.
2.10. Plant protection interlocks
The plant will be fitted with comprehensive instrumentation and automatic control equipment necessary for safe and efficient operation of the plant including the following: 1. Shutdown HP pump switches to protect parameter against over pressurization with alarm. 2. There is a time delay between the RO. HP pump unit start up and raw water feed pumps. 3. Low suction pressure switch shutdown to protect HP pump against cavitation with alarm.
4. Differential pressure switch (Ap) with pressure alarm indication to start automatically filters backwash pumps and others to detect the reliability of parameter performance operation associated with alarms. 5. Low level switches to stop chemical dosing pumps, feed pumps and product pump (if applicable) associated with alarm. 6. High level switches to stop beachwell pumps and HP pumps in case of high level raw water and product water storage tanks respectively with alarm indication. 7. Temperature switches to control feedwater heating process and protect parameter against excessive temperature exposure associated with alarm. 8. pH analysers for continuous pH measurement in the feed line in front of HP pump, and inter-locks high and low pH shutdown with alarm. 9. ORP analysers for high alarm and shutdown of the HP pump to insure that no halogenation of the membrane will occur. 10. High residual chlorine alarm in the feedwater. 11. High brine and product conductivity alarms. 12. Continuous SDI (silt density index) monitor in the feed line before the I-IP pump. 13. Continuous feedwater temperature recording with set point alarm after the HP pump. 14. Flow measurements used to set flow to and from parameter to control conversion. 15. liP pump trip signal shutdown of the feedwater pump to shutdown the unit. 16. Flow, pressure and temperature control valves are recommended to control flow, pressure and temperature in feedwater, product water and brine water. 17. Sample valves in the product and brine stream are used to monitor parameter performance. A sample valve is also needed before and after the cartridge filter to permit sampling for determining the feedwater silt density index (SDI).
M.H.A. El-Saie et al. / Desalination 134 (2001) 63-76
18. Flushing/cleaning tank provided with tank cover vent and mixer. A valve at the bottom of the tank is needed to assure that the tank can be completely drained and flushed of waste chemical solutions. A 5 or 10-micron cartridge filter should be installed in the solution feed line to avoid recirculation sediment which may dislodge from the system during the cleaning operation or originate from the cleaning chemicals.
67
provided, for each train, the experiments will be carried out on each membrane type separately. This is due to the practical difficulties encountered when operating more than one membrane type with one single high pressure pump simultaneously. The duration of the experiments will depend on the type of the membrane and its rate of deterioration.
3. Plant description 3.1. General 2.11. Data acquisition system
The control and supervisory functions required for central operation of the plant at the site will be implemented in a distributed control and data acquisition system (DC DAS) hardware. The distributed control system will be specifically designed for this operation. The operator control stations will provide access to the data acquisition functions which will include alarm, events display and logging function. The operator workstation may also give access to the control functions so the plant may be operated from keyboard. A minimum of two printers will be provided, one for alarm/event recording and the other for plant logging function with interchangeable facility, An engineering terminal complete with printer for system configuration. Each operator control station will consist of full graphic color display units (VDUS) of the smart CRT type and full alphanumeric character set keyboard with function keys.
2.12. Experiment types and durations
As mentioned above experiments will be carried out on three types of membranes. As the performance parameters are different, and one single high-pressure pump will be
Fig. 1 presents a block diagram of the plant. It was designed to be consisting of two trains extendable to three or more as required in the future. One train will operate with feedwater at normal temperature and the other will operate with heated feedwater. The latter will be provided with feedwater heating equipment. Beachwell will be used to feed the plant with the required feedwater. 3.2. Beachwells and pumps
For such low capacity RO desalination units it is recommended to use beachwells for the feedwater rather than seawater intake jetty or similar. Though the required feed capacity is small (about 25mVh), two beaehwells, were selected (one working and one standby) for the wells reliability and durability. This will have the advantages of lower costs and clean feedwater which needs minimum pretreatment requirements. The required capacity for the beachwell and filter feed pumps will be the maximum feed required, i.e. about 25 mVh. The beaehwell pumps will be operated intermittently in cases of the unit lower capacity. The filter feed pump will be provided with variable speed motor or hydraulic coupling to cater for the required capacity variation. The backwash pump capacity will be about 100 mVh.
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Materials for all pumps in contact with seawater (beaehwell, high pressure, filter feed and backwash pumps) will be duplex stainless steel or equivalent. Other pumps in contact with fresh water (cleaning, flashing, etc.) will be suitable grade of stainless steel. Materials for chemical injection pumps will be chemical resistant (PVC or equivalent).
69
temperature (45°C for the available membranes), which is calculated as follows: Maximum required feed =12.5 mVh Required heat = 12.5 x 1000 x 4.18 x (45 - 20) = 363 kW 3600
3.3. Pretreatment system Chlorination. Hypochlorite solution will be injected into the raw water storage tank to prevent bacteriological contamination. Filters. Feedwater will be filtered using dual media filters (one working and one standby). At a pre-determined pressure drop the filter will be backwashed. Second stage filtration will be through cartridge filters.
• •
•
Chemical dosing. Sulphuric acid to protect the membrane against calcium carbonate precipitation. Sodium metabisulphite for water dechlorination to protect the membrane against chlorine attack. Antiscalant to inhibit residual carbonate and non-carbonate scaling on the permeate depending on the safe operation of the acid dosing and the preset feed pH value.
3. 4. Water heating system
Train A will be provided with a heating facility to test the membranes at elevated feedwater temperatures while train B will be used to test the membranes at ambient feedwater temperature. Accordingly the maximum required heat rate for train A occurs when the raw water at its minimum temperature (about 20 ° ) and required to be heated to the maximum allowable operating
Direct heating of the raw seawater is not recommended due to scale formation problems within the heater. Therefore it is recommended to heat the feed seawater utilizing a fresh water to seawater heat exchanger. The hot fresh water will be obtained from an electrical water heater. The losses in the heat exchanger and piping system are estimated to be about 10%. Then the required water heater capacity will be about 400 kW. To reduce power consumption at steady state and during continuous operation, the heat content in the brine and the permeate will be used to preheat the feed seawater, utilizing permeate and brine to feed seawater heat exchangers. This will give the following advantages: • Reduction of power consumption in the water heater. • Reduction in permeate temperature. • Reduction in the brine temperature before disposal which will be advantageous from the environmental point of view. It is estimated that 40% of the heat energy will be lost in membranes, heat exchangers, pinging system, etc. and 60% will be recovered in the permeate and brine feedwater heat exchangers. Then the maximum required heating power during continuous operation estimated to be about 160 kW, and 240 kW will be recovered from the permeate and brine in the heat exchangers.
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M.H.A. EI-Saie et al. / Desalination 134 (2001) 63-76
The heat exchangers will be of shell and tube construction (tubes titanium or Cu Ni 70/30, shell stainless steel 316L or equivalent). 3.5. High pressure pump with energy recovery and hydraulic coupling
As it is required to carry out experiments on different types of membranes, the high-pressure pump duty will be varying depending on the membrane type. One pump will be used for each train, coupled with a hydraulic coupling to obtain the required pressure-flow. For fine tuning, throttling valve together with a back pressure valve, will be provided. Variable speed motor can be used. To recover the brine kinetic energy, energy recovery turbine (ERT) or turbo charger will be provided. The systems provided with energy recovery turbines have the advantage of ability to operate in case the turbine is out of order, but with higher power consumption. On the other hand, systems provided with turbocharger cannot be operated without the same, and have the advantage of lower power consumption. Both systems can be accepted depending on detailed design and study.
will be provided for the system flushing in case of unit shut down.
3.8. Raw and product water tanks
The raw tank will be divided into two compartments. The first compartment will be for the raw water from the beachwell pumps and will be sized for a 3-hour unit operation. The second compartment will be for clean water coming from a branch after the multimedia filter. This clean water will be used in multimedia filter backwash. The clean water compartment size will be enough for one backwash cycle at least. The product water tank will be sized for 2day unit's operation. The raw and product water tanks will be reinforced concrete, suitably protected.
3.9. Post-treatment system
Caustic soda dosing: to adjust the product water pH value. Post chlorination: to sterilize the product water prior to storage in the storage tank.
3.6. Membranes and racking
3.10. Piping and valves
Three types of membranes in each train will be tested as mentioned above (FilmTec, Fluid Systems and Hydranantics), five numbers of each. Each type will be installed in a separate rack. The racks will be suitably designed for easy operation, maintenance and membrane replacement when necessary.
All high pressure piping in contact with seawater and brine will be stainless steel 316L. All other low pressure piping, fittings and valves will be of PVC or equivalent.
3. Z Cleaning/flusing system
Chemical cleaning system will be provided for periodic cleaning. The system will include tank and pump complete with necessary piping, valves, fitting, etc. In addition a flushing pump
3. I 1. Chemical treatment system
Dosing pumps will be a positive displacement diaphragm type with variable stroke length and stroke rate and will be of chemical resistant material. Dosing tanks will be suitably sized, and will be of chemical resistant materials (PVC or equivalent).
M.H.A. E1-Saie et al. / Desalination 134 (2001) 63-76
switch with indicator is activated when the filter reaches the preset value to start backwash process. The backwash earl be controlled from DC DAS.
3.12. P & l diagram
Figs. 2 and 3 shows P&I diagrams for train A, operating with heated feedwater and train B operating with normal feedwater temperature. 3.13. Plant layout
Plant layout is shown in Fig. 4. The layout is prepared taking into consideration accessibility for easy operation and maintenance and for future extension.
71
• The system is provided with cartridge filters (5-10~t). The purpose of these filters is to protect the HP pump and the membranes from larger particles that may be present in the water. A differential pressure switch with an indicator is provided to advise the operator to replace the cartridge. •
3.14. I&C works
The plant control system is designed to provide reliable and efficient operation through the use of a microprocessor based on distributed control and data acquisition system (DC DAS) interfaced through the main control console in the plant control room. The plant will include the following instruments: • Beachwell water is pumped directly from the wells into the raw water storage tank with chlorine solution dosing provided before the tank. •
Feedwater pumps (one duty and one standby) started if the raw water storage tank (feedwater compartment) level is low and in the case of high level it stops (and cannot be started either by automatic or manual control) same condition for backwash pump. The pumps can be started from local control panel interfaces with a distributed data acquisition system.
•
Chemical dosing system: - Chlorine solution Coagulant solution Post treatment solution The malfunction of chemical dosing system trips the liP pump leading to plant shutdown. • Automatic dual media pressure filters (one duty and one standby). A differential pressure -
-
Cartridge filters would be used to prevent particles from damaging the HP pump of fouling the permeators (one duty and one standby). • Each parameter has all instruments to indicate flow, pressure and temperature. •
high-pressure shutdown switch will be mounted in the RO permeator feed line. A
• Automatic feedwater shut-off valve. The valve automatically closes when the RO HP pump is shutdown for any reason. • Automatic reject line flush valve with a timer is automatically activated once every 24h for 30min to remove any build up sediment or precipitates the valve normally closed and energized to open. • The system will include a drawback tank to provide sufficient water for osmotic drawback through the membranes when the RO system shuts down. It also provides pure water for flushing the piping on the feed side of the RO at the same time. The control will include the following automatic safety shut down features: - High pressure pump motor over load. Chemical tank-low level. Instruments power failure. -
-
Instrument air. A compressor is always switched "on" for instrument air "except for maintenance" and will stop and start to main-
M.H.,4. El-Sale et al. / Desalination 134 (2001) 63-76
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tain a constant pressure in the instrument air header. Heat exchangers for brine and product recirculation to increase feedwater temperature and to trim the feedwater temperature to preset temperature value by using a temperature controller to open/close the 3-way valve in case of getting the feedwater temperature value. Chlorine-free detection monitor (ORP meter) with automatic unit shutdown. pH value of the filtered water is continuously monitored. HP pump shutdown in case of low feedwater suction pressure or high feedwater pressure and temperature to RO permeators. Flow control valve must be used to control the flow in the product and brine line. Float type level indicating (or differential pressure type) transmitters are installed on raw water and product water storage tanks. Its output signal received to DCDAS for indicaring continuous tank level. In addition, an independent level (low/high) switches have been installed on the tanks to activate an alarm and interlock that should staWstop the pumps. Pressure flow and temperature transmitter should be used to indicate continuous pressure, flow and temperature measurements in the control room. Flow and conductivity measurements before and after each parameter and recommended to measure differential pressure across each membrane. Meters and gauges. Meters are required to measure feedwater, brine and product flow rates. Sample valves are used in product and brine stream to monitoring a permeator performance. Motorized valves (or equivalent) are used to control flow, pressure and temperature in the plant.
75
• A flow control valve in the brine stream is used to set conversion. • A throttling valve and back pressure valve on the HP pump discharge line are used to control the feed pressure to the permeator. • For more safety a pressure relief valve is required on the HP pump discharge line used to control system pressure when pump capacity is greater than that needed to supply the permeator. • The measuring equipment mentioned in the system design whether flow, pressure, level, or temperature must have an accuracy of -~0.25 of full scale or better. • The type and make of equipment will be intemationally proven and of standard product of the manufacturer. 4. R&D merits Securing reliable potable water resources is one of the main concerns of the new millenium. Still seawater desalination is considered the most promising resource, which renders it critical to undergo more R&D, especially for RO. The main RO varying parameters are the feedwater temperature, pressure and salinity, and the product water salinity, production in addition to the recovery ratio. RO membrane manufacturers publish some data illustrating the effect of variation of these parameters on the product water productivity and salinity and the recovery ratio. However there are some uncertainties about these data, especially the effect of feedwater temperature on the product water quality and quantity and the recovery ratio. Flow and conductivity measurements before and after each permeator in addition to differential pressure across each membrane are recommended. The above renders the set up of this experimental facility and experiments to be carried out independent and reliable for RO because:
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M.H.A. El-Sale et al. / Desalination 134 (2001) 63-76
• NPPA is a government institution and is not committed to any membrane manufacturer or brand. • The facility will be operated and the results will be concluded by highly qualified investigators from NPPA staff. Moreover the facility was designed to be provided with intensive measuring and control systems for the different parameters: temperatures, pressures, flow rates, salinities, which is not available in the commercially operated RO plants. As an example, each individual membrane was designed to be provided with flow, pressure and salinity measurement instruments. The I&C system is an advanced computerized system recording all important parameters with time for intense evaluation.
The operating bands are designed to span all commercially known membrane types. Further, the chemical pre-treatment facility is very extensive to allow for different chemicals and dosing rates. The heating facility is also designed for peak flows at worst conditions, allowing for versatile heating modes in all experimental conditions. The space available for different racking types is allowed for removing any constraints. The use of identical trains allows concurrent experiments under identical conditions varying filter parameters from one to the other. This gives direct comparison for problems and this can conclude scientific arguments with proof. As for seawater source, allowances have been made for direct from sea or beachwells. The R&D array of experiments is limitless and it should be a source of much further research.