How To Design A Large Msf Desalination Plant

Transcription

1 OVERVIEW OF DESIGN FEATURES AND PERFORMANCE CHARACTERISTICS OF SALINE WATER CONVERSION CORPORATION (SWCC) MSF PLANTS 1 Osman A. Hamed, Mohammad AK. Al-Sofi,Ghulam M. Mustafa, Khalid Bamardouf and Hamad Al Washmi Saline Water Conversion Corporation P.O.Bo x 32, Al-Jubail -311, Saudi Arabia Tel: , Fax: rdc@swcc.gov.sa ABSTRACT Saline Water Conversion Corporation (SWCC) is internationally recognized as the main producer of desalinated water. SWCC is currently operating 13 large MSF desalination plants with a total installed capacity of 2. MIGD. MSF plants are coupled with either back-pressure or extraction condensing turbines for the simultaneous production of power and water. SWCC MSF distillers are characterized by a wide range of design features and performance characteristics. Distiller production capacity ranges from as low as 2. MIGD to as high as MIGD in vary between. and. kg/232 kj. guidance for future planning and selection of improved MSF design. Overview of sented in this paper. Most important operating parameters e.g. temperatures and mass flow rates, and fouling factors of MSF plants are reported. The paper also reviews other design features such as surface area, tube dimensions, materials of construction and energy requirements. Performance characteristics of MSF plants, which have been in operation for more than twenty years are highlighted. SWCC successful attempts to improve the performance of its MSF distillers and reduce operating cost especially with regard to scale control are reviewed. 1 Proceedings of the th Gulf Water Conference Doha State of Qatar, 24-2 March 2001.

2 Key Words : SWCC, Desalination, MSF, Design Features, Performance Characteristics. INTRODUCTION The Saline Water Conversion Corporation (SWCC) of Saudi Arabia is currently producing more than 1 percent of the total world production of desalinated water [1]. The majority of SWCC desalination plants are employing the multistage flash process producing more than 3 percent of its total water production. All large MSF plants are coupled with power generation facilities for simultaneous production of power and water. The first two seawater condensers built in Saudi Arabia were installed in Jeddah during - first large scale MSF plant in Jeddah. Historical developments of SWCC desalination plants were recently reported [2]. SWCC MSF plants are located along the Arabian Gulf and the Red Sea Coasts. In the East, along the Arabian Gulf, MSF plants are located in Al-Khafji, Al-Jubail and Al-Khobar. Along the Red Sea the large MSF plants are located in Jeddah, Yanbu, Al-Shugayg and Shoibah. SWCC MSF distillers are characterized by a wide range of design and operating characteristics [3-].The objective of this paper is to give a comprehensive overview of the salient design and operating features of SWCC MSF desalination plants and to provide an insight into the basic relationship between the different designs parameters which can then be used together with the vast amount of operational experience as a guide for future planning of an improved MSF design and operation. DESIGN FEATURES All SWCC large MSF plants operate within the context of dual purpose facilities for the simultaneous production of power and water. Such co-generation arrangement uses either back-pressure or extraction condensing turbine. Some MSF/Power plants are also combined with RO plants (Jeddah, Yanbu & Al-Jubail) taking advantages of common seawater intake and outfall, common pretreatment facilities and blending of permeate in 2

3 appropriate proportions from MSF and RO plants if the need arises as well as reducing power to water ratio. Salient features of SWCC MSF plants are shown in Table-1. All the MSF distillers are operating with brine recirculation modes and the majority is with cross flow configuration except Jeddah II & IV, which are having long tube configurations. Number of stages varies from 1 in Alkhobar and Jeddah III up to 34 in the unique design configuration of Jeddah II. Distillate production ranges from as low as 2. MIGD in Al-Khafji and Jeddah II up to MIGD in the mostly recently built plants. The design performance ratio ranges between 2.3 to 4. kg/00 kj. Table-1 also shows the blowdown and recycle concentration ratio mass flow rate of cooling water and brine recycle with respect to the distillate production and water velocities in evaporator tubes as well as the design fouling factors of the brine heater and heat recovery section. The tube dimensions of the different sections of the distiller and flash chamber specifications are shown in Table 2. The flashing brine flow rate per unit width of the flash chamber (shell load) is an important design parameter that affects the cost of shell as well as stage-wise nonequilibrium losses in the stage []. Table 3 shows the design shell load of SWCC MSF plants. It ranges between 4,00 kg/(hr m) for plants with low rated capacity up to,30 kg/(hr m) in Shoaiba II plant. Using regression analysis the following linear relationship was obtained interrelating the shell load (SL) in kg/(hr m) of all SWCC MSF distillers with rated capacity of each distiller (D) in MIGD: SL = D The difference between the design shell load values and those predicted from the empirical relationship is +14. and 1. percent. Another important design variable is the vapor release velocity at brine surface. It has a strong influence on the liquid carryover in the flash chamber as the higher the release rate the higher will be the amount of brine entertainment in the vapor []. The vapor release in a distiller varies from stage to stage as shown in figure-1 for selected three distillers representing Al-Jubail II, Al-Khobar II and Shoaiba I plants. 3

4 For the two latter plants the vapor release velocity varies from 1 m/s in the first stage up to. or m/s in the last stage which is operating with low temperature and having high specific vapor volume. Al-Jubail Phase II distiller is generating low vapor release velocity compared to the two other plants and this is attributed to the low flash plan areas. MATERIALS OF CONSTRUCTION The most commonly used materials of construction in SWCC MSF plants are carbon steel, stainless steel, copper-nickel alloy and titanium []. The shell of brine heaters of all plants is made of carbon steel and the tubes are either 0/30 or 0/ Cu-Ni except Al Jubail Phase I which is having titanium tubes.the material of construction of flash chambers is carbon steel with and without cladding. In some plants such as Al Jubail, Al-Khafji II and Jeddah III, the first high temperature stages are cladded with stainless steel. Module 1 of Jeddah II and the first two modules of Jeddah IV are also cladded with stainless steel. Al Khobar II flash chambers are completely cladded with 0/ Cu-Ni and Al Shugayg I is also totally claded with stainless steel. The materials of construction of the heat rejection tubes of all plants are made of titanium except Jeddah plants, which are having 0/ Cu-Ni tubes. OPERATING FEATURES The operational performance of selected MSF distillers is shown in Figure-2 based on data collected during a period of 30 days. This figure shows the variation of performance ratio with time in days. Although these plants are in service for more than 1 years, the performance ratio is consistently equal to or in most cases much higher than the design value. This could be attributed primarily to the generously available surplus heat transfer surface area and equally important to the strict operating and maintenance procedures followed by SWCC. In particular, the combined use of chemical additive and on-load tube cleaning contributed greatly in reducing the fouling factor which resulted in maintaining high overall heat transfer coefficient and reduced steam consumption and hence maintained high performance ratio. 4

5 SCALE CONTROL AND ON-LOAD SPONGE BALL CLEANING Scale formation on heat transfer surfaces, which is one of the basic problems in the desalination of sea water, can be effectively controlled or minimized by the addition of chemical additives and use of on-load ball cleaning. A number of optimization tests have been carried out by SWCC thus leading to successful operation at low antiscalant dose rates [1]. Recommended dose rates to SWCC in 11 were 12. and 4. for TBT of 1 and 0 o C respectively [12] and are currently reduced to only 2.0 and 1.0 ppm for the respective temperatures. This significant reduction in dose rate is attributed to several factors such as improvement in chemical formulation, adoption of on-line sponge ball cleaning and plant operator awareness to reduce chemical dosing while maintaining effective plant performance. Although the formation of scale is combated and controlled by threshold treatment with the use of antiscalant, its complete prevention is impracticable. Sludge or distorted scale is also formed as a result of threshold treatment, which gets deposited on tube metallic surfaces, and induce resistance to heat transfer. The combined use of chemical additives and on-line tube cleaning has been proved to be the most cost effective means to combat scale formation and to avoid acid cleaning [121]. All SWCC MSF plants are employing on-load sponge ball cleaning. The chemical treatment, ball to tube ratio and frequency of cleaning in different MSF plants are shown in Table 4. The ball to tube ratio for plants using chemical additive treatment varies from as low as 0.22 in Al-Shugayg up to about 0.4 in Al Jubail Phase I and Al-Khobar plants with average frequency of three ball cleaning operations per day for all plants. The ball to tube ratio in SWCC MSF plants, thus in most cases, lie within the reported accepted range [1&1]. Larger number of ball to tube ratio may cause problems by several balls passing one tube simultaneously and getting stuck while smaller ratio will not be capable to reach all tubes. The wide variation of ball to tube ratio reveals that ball cleaning operation is not yet well established. This can be attributed to its dependence on many interacting operating and design factors such as brine chemistry, type of inhibitor and control regime, ball type and MSF design parameters such as

6 temperatures, number of stages and tube length, flow pattern and arrangement of ball injection points. CONCLUSIONS 1. The common design features of SWCC MSF plants are that: they are all using brine recirculation mode with on-stream ball cleaning. They are also having crosstube configurations except Jeddah II and IV which are having long tube configurations. 2. There is a wide diversity in the dimensions of the heat transfer tubes and flash chambers. The flashing brine flow rate per unit width ranges between 4,00 and,30 kg/hr m. 3. After more than 1 years of service, SWCC MSF distillers are currently operating with performance ratios equal to even higher than the corresponding design values. This attributed to SWCC strict maintenance and operating procedures. 4. Antiscalants dose rates are reduced to as low as 2.0 and 1.0 PPM for the low and high temperature operations, respectively.. All SWCC MSF plants are employing on-load ball tube cleaning with a ball to tube ratio in the range of 0.22 to 0.4. REFERENCES 1. 1 IDA World wide desalting plants inventory, Report 1, June 1. Prepared and Published by Wangnick consulting GmBH. 2. Saline Water Conversion Corporation. Desalination of sea water in Saudi Arabia (1). 3. Mohammed, A., Tariq, B., Al-Mobayed, A. and Anwar, E., (1), Flexibility and economic operation of Al Jubail phase II Desal and Power Plant. Proceeding of International Desalinaion Association, San Diego, Vol. IV, Nada, N. and Mughram, O., (14), Design features of Shoaiba phase II 00 MW-0 MIGD Power and Desalination Plant. Proceedings of the Second Gulf Water Conference. Bahrain, - Nov.,

7 . Al-Sofi, M.A., Al-Hussain, M.A.and Saleh, G.A., (1), Additive Scale Optimization and Operation Modes, Desalination,, Al-Mudaiheem, A.M.A., Al-Sofi, M.A.,.Al-Omaran, A.A., Al-Jardan, A.M., (11), IDA World Congress, Washington D.C, Darwish, M.A., El-Rafaee M.M. and AbdeJawad, M., (1), Development in the multistage flash desalting system, Desalination, 0, Al-Nashar, A.M., Kamaluddin, B.A., McGregor, I. D., Khan, S. and El-Hares, H., (1), Overview of the design features, performance and economics of the MSF plants operated by Abu Dhabi water and Electricity Department, Proceeding of IDA world Congress on Desalination and Water Sciences, Abu Dhabi, UAE, November 1-24, Malik, A.U. and Mayan Kutty, P.C., (12), Corrosion and material selection in desalination plants, proceeding of the seminar on operation and maintenance of desalination plant, Saline Water Conversion Corporation, April 2-2, Hamed, O.A., Al-Sofi, M.AK., Imam, M., Ba Mardouf, K., Al-Mobayed, A.S.and Ehsan, A., (1), DSB(M) Antiscalant Testing at low dose rate, Research & Development Center, Report No. TSR-30/02 Part I. 11. Hamed, O.A., Al-Sofi, M.AK., Imam, M., Ba Mardouf, K., Al-Mobayed, A.S.and Ehsan, A., (1), DSB(M) Antiscalant Testing at low dose rate in Al-Jubail Plant, Research & Development Center, Report No. TSR- 30/02 Part II. 12. Al-Sofi, M.AK., Al-Husain, M.A. and Al-Zahrani, S., (1), Additive Scale Control Optimization and Operation Modes, Desalination,, Al-Mudaiheem, A.M.A. and Szostack, R.M., (1), Evaluation of Chemical Additives for Scale Control in MSF Plants, Desalination, 13, SWCC, Saudi Arabia. 14. Nada, N., (1), Evaluation of Various Additives at Al-Jubail Phase-I during Reliability Trials, Topic in Desalination No. 13, SWCC, Saudi Arabia. 1. Al-Zahrani, S., Al-Ajlan, A.M. and Al-Jardan, A.M., (13), Using Different Type of Antiscalants at the Al-Jubail Power and Desalination Plant in Saudi Arabia. Proceedings of IDA and WRPC World Conference on Desalination and Water Treatment, Yokohama, Japan, Vol. 1, pp Al-Sofi, M.AK., Al-Husain, M.A., Al-Omaran, A.A. and Farran, K.M., (14), A Full Decade of Operating Experience on Al-Khobar-II Multistage Flash (MSF) Evaporators (12-12), Desalination,, Al-Sofi, M.AK., Khalaf, S. and Al-Omaran, A.A., (1), Practical Experience in Scale Control, Desalination, 3,

8 1. Hamed, O.A. Al-Sofi, Mohammad AK., Mustafa, G.M. and Dalvi, A.G.I., (1), The performance of different antiscalants in multistage flash distillers, Desalination, 123, load spong ball cleaning system. Encyclopedia of Desalination and Water Resource. Chapter II load tube cleaning systems and debris filters for avoidance of microfouling in MSF desalination systems, Desalination, 3, Al-Sofi, M.A., (1), Fouling Phenomena in Multistage Flash (MSF) distillers. Desalination, 12,.1-.

9 Table 1. Salient Design features of SWCC MSF Plants S. No. SWCC-MSF plants Production, MIGD (max) No. of Stages Top Brine Temp. Design PR Concentration Design Fouling Factor Brine Velocity, m/sec Ratio m 2 K/W x -3 No. of Distiller s MIGD/ Distiller Total 0 C (max) Kg./00 kj (BR) (BB) BH H. Rec. M sw/m d M R/M d BH H Rec. 1 2 Al-Jubail Phase I Phase II C2 C3 C4 C Jeddah Phase II Phase III Phase IV Al-Khobar Phase II Phase III Yanbu Phase I Phase II Shugayg Phase I Shouaiba Phase I Phase II Khafji Phase II

10 Table 2. Tube dimensions and flash chambers specification S. SWCC-MSF Tube Dimension No= PLANTS BH H Rec. H Rej. number length (m) ID (mm) number length(m) OD number length (m) (mm) OD (mm) Design Flash Chambers Specifications Width (m) Stage Length (m) Height (m) 1 2 Al-Jubail Phase I Phase II C2 C * * 3. C C Jeddah Phase II per module** Phase III * 4.2 Phase IV per module** 4. Al-Khobar Phase II Phase III Yanbu Phase I Phase II Shugayg Phase I Shuaiba Phase I Phase II Khafji Phase II * Total length of evaporator. ** Long tube configuration.

11 Table 3. Shell Load of SWCC MSF plants S. No. Plant Production (MIGD) (Shell Load) design kg/hr m (Shell Load) equation kg/hr.m % Difference 1 Al-Jubail I Al-Jubail II * Jeddah III Al-Khobar II Al-Khobar III Yanbu I Yanbu II Shugayg I Shoaiba I Shoaiba II Al-Khafji II * Note Jeddah II & Jeddah IV are long tube arrangement so they are not mentioned in the table. Table 4. On-load sponge ball cleaning S. No. Plant Ball/ Tube Ratio Chemical Treatment BH HRC Frequency of Ball Cleaning Operation No. of cycles per operation 1 Al-Jubail Ph. I Antiscalant Oper. / Day Cycles / Oper. 2 Al-Jubail Ph. II C2 & C3 Antiscalant Oper. / Day Cycles / Oper. C4 Antiscalant Oper. / Day Cycles / Oper. C Antiscalant Oper. / Day Cycles / Oper. 3 Jeddah Ph II Acid One/week 3 cycle/oper 4 Jeddah Ph III Antiscalant Oper. / Day 4 Cycles / Oper. Jeddah Ph IV Antiscalant Oper./ Week Cycles / Oper. Al-Khobar II Antiscalant Oper. / Day Cycles / Oper. Yanbu I Antiscalant Oper. / Day 12 Cycles / Oper. Acid One Oper./ Week 12 Cycles / Oper. Al-Shugayg Antiscalant Oper/ Day Cycles / Oper. (1 for high TBT) Al-Shoaiba I Antiscalant Oper/ Day 3 Cycles / Oper. Al-Khafji Antiscalant One Oper/ Day Cycles / Oper. 11

12 Vapor Release Velocity Al-Jubail II Al Shoaiba I Al-Khobar II Stage Numbers. Figure-1: Variation of vapor release velocity with stage number

13 TBT= 11 o C Jeddah II PR Kg/232 KJ 11.. (PR) d=.2 PR Kg/232 KJ.... TBT= o C Jeddah III (PR) d=.11 PR Kg/232 KJ... TBT= 1 o C Jeddah IV (PR) d=.04 PR Kg/ 232 KJ.... TBT= 0 & o C (PR) d=.0 Al-Jubail II (PR) d =. PR Kg/ 232 KJ TBT= 0 o C Al-Khobar II (PR) d=. PR Kg/ 232 KJ TBT= o C Al-Khafji (PR) d=.20 Days Figure-2: Operational performance of selected SWCC MSF distillers 13