Reverse osmosis seawater desalination for power plant

Reverse osmosis seawater desalination for power plant

Desalination, 96 (1994) 359-368 Elsevier Science B.V. Amsterdam - 359 Printed in The Netherlands Reverse osmosis seawater desalination for power pla...

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Desalination, 96 (1994) 359-368 Elsevier Science B.V. Amsterdam -

359 Printed in The Netherlands

Reverse osmosis seawater desalination for power plant Masanobu Noshita Japan Organ0 Co., Ltd., Toray Industries, Inc., lke Kanh Kobe Steel, Ltd., Chiyodiz-ku, Tokyo 100 (Japan)

Electric Power Co., Inc.,

SUMMARY

The Miyazu Energy Research Center of the Kansai Electric Power Company in Kyoto prefecture has an electric power generating facility of 750,000 kW. As it is located in a small peninsula of a remote area, fresh water cannot easily be obtained and almost all of the fresh water required is provided by seawater desalination. The seawater desalination facility consists of two trains of the reverse osmosis (RO) equipment which produce 1600 m3/d fresh water. For the RO membrane, spiral-type polyether composite membranes (PEC-1000) made by Toray were adopted; there are 300 modules (60 vessels). The pretreatment, the dual-media filter and the single-medium filter are equipped in series and ferric chloride and sodium hypochlorite are injected for the removal of suspended particles in the raw seawater and the prevention of bacterial growth in the equipment. Before feeding the seawater to the membrane modules, the residual chlorine and dissolved oxygen are removed completely by vacuum declaration and addition of sodium bisulfite. Then the seawater is pressurized to 66 kg/cm3 and fed to the membrane modules. The seawater temperature changes from 9-37°C according to the season, and it causes the fluctuation of the fresh water production. In order to avoid scale formation on the membrane surface, the maximum recovery ratio was set at 45% (1800 m3/d). Therefore, the OOll-9X4/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SsDIOOll-9164(94)00052-P

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number of the membrane modules in operation is changed by the seawater temperature to keep the fresh water production between the minimum (1600 m3/d). The permeated water is decarbonated, and sodium hydroxide is added to adjust the pH between 6.5-7.5. The concentrated brine is discharged after oxidizing sodium bisulfite by aeration. The facility was constructed in 1988 and has been operating smoothly without serious problems since then. The outline of the operational results is reported in the paper.

INTRODUCTION

This paper outlines the RO seawater desalination plant of the Miyazu Energy Research Center of the Kansai Electric Power Co., which inaugurated operation in 1988 and reports the operating results. General information on the desalination plant

The Miyazu Energy Research Center is a seaside steam power thermal electric plant constructed on the Kurita Peninsula of the Miyazu Bay. Fresh water used in the plant is all supplied by the desalination plant. The steam power plant is capable of generating electric power of 750,000 kW (375,000 kW times two units). Process flow

This desalination plant comprises seawater intake equipment, pretreatment equipment, RO processing equipment, post-treatment equipment, and waste water treatment equipment, and consists of two trains which can produce 1600 m3/d of fresh water (water temperature 16”C, recovery ratio 40%). At high water temperature the plant operates at 1800 m3/dXtwo trains (recovery ratio 45%). Fig. 1 outlines the process flow of this plant. Equipment spec#kations

1. Seawater intake equipment - Part of, seawater taken in for cooling is sent directly to the dual-media filter after shellfish are removed by the auto strainer.

Fil:ed

Media

V&et

Fi!ter

Figure 1. Proc&s Fiow.

Dual

Polishing

Safety

U

Filter

Filter

2earelater

Brine

RO

Recovery

Basin

Tank

Turbine

Decabonat or

Suckback

Product Vht er Tank

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2. Pretreatment equipment - The pretreatment equipment comprises dual-media filters, polishing filters, safety filters, and vacuum deaeration towers. The dual-media filter and polishing filter remove suspended solids in seawater, the safety filter prevents entrainment of foreign matter into high pressure pumps and RO membranes, and the vacuum deaeration tower removes oxygen dissolved in seawater. The dual-media filter is backwashed once every 3 days, and the polishing filter once every 30 days. Anthracite coal and sand are used for the dual-media filter material, while even finer sand is used for the polishing filter. 3. Reverse osmosis processing equipment - The RO processing equipment comprises RO membranes, high-pressure pumps, energy recovery turbines, and a suckback tank. For the RO membranes 300 pieces of Toray’s spiral-wound type module (SP-120) are used. This module requires removal of residual chlorine and dissolved oxygen but has high salt rejection capabilities and greatly reduces the load to the downstream demineralizing equipment. The high-pressure lump and the energy recovery turbine are both stainless steel multistage turbine pumps. Seawater pressurized to 5.7-6.6 MPa (57-66 kg/cm2) by the high-pressure pump is fed to the RO membranes, and the product water is fed to the post-treatment equipment via the suckback tank, while brine is fed to the waste water treatment equipment after energy is recovered from residual pressure by the energy recovery turbine. 4. Post-treatment equipment - The post-treatment equipment comprises decarbonation towers and product water pumps and adjusts the pH of the product water. 5. Waste water treatment equipment - This equipment primarily consists of backwashing waste water from filters and brine from membranes. In the backwashing waste water, because sludge treatment is done using synthetic waste water treatment equipment next to the desalination plant, which treats waste water in the power plant, treated water (fresh water) of the synthetic waste water treatment equipment is used for backwashing water for filters. Consequently, when the filters enter the backwashing process, seawater in the filters is removed and replaced with the treated water; then backwashing takes place. The backwashing water is fed to the sedimentation, while sludge is dewatered with a filter press at the synthetic waste water treatment equipment and disposed as industrial waste. The overflow from the sedimentation tank is fed to and treated at the synthetic waste water treatment equipment via the waste water polishing

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filter and discharged outside. Because the brine contains a high concentrated sodium bisulfite solution for membrane preservation at the start of the operation, aeration and pH adjustment are carried out to reduce COD. When COD and pH fall below the restricted effluent values, brine is discharged, as it is together with warm waste water. 6. Chemical dosing equipment - The following chemicals are used in this plant: (a) sodium hypochlorite, used to prevent propagation of marine growth in seawater; (b) ferric chloride, used as coagulant for pretreatment process and sludge treatment; (c) sulfuric acid, used to control pH of the feed seawater; (d) sodium bisulfite, used to remove dissolved oxygen and chlorine in feed seawater and sterilization of anaerobes; (e) caustic soda, used to control pH of product water and waste water. Features of the system

This equipment is the ancillary equipment for the power plant and is designed to be operated fully automatically. Automation takes place at water quality check of the seawater fed to membranes and selection of the equipment starting and stopping operations in accordance with the longterm or short-term stopping. The waste water discharged from the equipment is stringently checked for quality. The chemically treated waste water and waste water from filters are thoroughly treated before discharged outside to minimize the impact on the environment.

Operation results Operation data

The No. 1 system was inaugurated into operation in June 1988, and the No. 2 system in July 1988. They have both been satisfactorily operating for 5 years. The recovery ratio is controlled by switching the number of membranes at three stages: 300 pieces (for about 3.5 months/y), 200 pieces (for about 3.5 months/y), and 200 pieces (for about 5 months/y), in accord with the change of water temperature. Fig. 2 shows the operation data for 5 years. The product water quality is 432 &cm on average for 5 y and is 463 M/cm for the past year, with 70% being the maximum tolerances of

364

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700 @cm, greatly reducing the load to the downstream demineralization equipment. The maximum recovery ratio is 45%, the upper limit of the designed value, but there is no scale formation or pressure loss of the membranes, indicating the possibility of operating at a 45% recovery ratio in Japan. Water quality Table I shows the water quality at the start-up of operation and after 5 years. Because for 1 year after the start-up of operation the number of elements per pressure vessel was four (five after 1 year), the TDS of the product water increased only by about 10%) even after 5 years. Fig. 3 shows the changes of values A and B and salt passage ratio, and Fig. 4 shows the membrane replacement ratio. The designed membrane replacement ratio was 25% per year, but since the increase rate of product water quality is low, it became 12.6% for the third year and 25% per year thereafter; the overall replacement ratio for 5 years was 62.6%. Consequently, the salt passage ratio and value B increased greatly in the third year, but thereafter have remained stable and are about 2.5 times the initial values, respectively.

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33,540

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PH Electric conductivity 18,590

Product water

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First year for operation

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Item

Water quality for Miyazu RO plant (mg/l)

TABLE I

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Fig. 3. Performance of Miyazu RO plant.

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Value A tends to increase as Value B increases. This is assuming that the effects of the flow rate increased by loosening of membranes are greater than those of the flow rate decreased by compaction of membranes. Reverse osmosis membranes

For the RO membrane, Toray’s SP-1200 spiral-wound type polyetherbased composite membranes were used, and five elements each were packed into one pressure vessel; 60 such pressure vessels are installed in each piece of equipment. Fig. 5 shows stuck materials and pressure loss for each element. About 55% of the stuck materials adhere to the lead element (No. 1) and pressure loss also reaches about 54%. The stuck materials of the element are mostly white paste-like substances, which are assumed to be caused by propagation of microorganisms. In this element system adhesion of the paste-like substance to net results is an increase of pressure loss. Crystal-like substances are distributed as inorganic matter, primarily silica and magnesium. The membranes were only cleaned with chemicals three times for the No. 1 system and twice for the No. 2 system for the past 5 years. This may be attributed to small leakage of coagulant from the pretreatment process and the pressure loss primarily caused by microorganisms and soluble 100 "

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silica. However, it is apparent that product water quality has been improved by chemical cleaning, possibly suggesting the necessity of cleaning once per year in the future even if the increase in pressure loss is small. Maintenqnce and inspection

In this desalination plant periodical inspection including overhauling is carried out for filters, safety filters, and RO membranes once per year and pumps once every 3 years. The majority of problems is related to corrosion, which includes crevice corrosion at flanges, corrosion at steam pipes and heated’portions, and scale formed at chemical dosing pipes. The availability of the desalination plant greatly varies from 0.2~!90%, but there is no detrimental effect resulting from this varied availability, indicating that the RO type desalination equipment is capable of operating flexibly in accordance with water demands.

CONCLUSIONS

The RO desalination plant delivered as our first large-scale plant for power has been fully exhibiting the desired capabilities and continues operating satisfactorily, while providing a large amount of valuable findings from the operation for the past 5 years. Making the best use of these results to understand the membrane life and improve operating conditions, will reduce operating and maintenance costs in the future.