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Effect of different operation pressures for various membranes on the performance of RO plants
DESALINATION Desalination 155 (2003)287-291
ELSEVIER
www.elsevier.com/locate/desal
Effect of different operation pressures for various membranes on the performance of RO plants • b* Abdul Sattar Kahdim a, Alaa' AbdulrazaqJasslm , Saleh Ismail c
aNational Committee for Technology Transfer, Baghdad, Iraq bDepartment of Chemical Engineering, University of Basrah, Box 1458, Basrah, Iraq email: noor. b@uruklink net cUniversity of Basrah, Basrah, Iraq Received 19 August 2002; accepted 15 October 2002
Abstract
The aim was to study the effect of different operation pressures on the performance of reverse osmosis (RO) plants for various types of membranes. The study was conducted in a pilot plant at the University of Basrah, College of Engineering, which has a capacity of 9 m3/h. The plant is comprised of two parallel vessels containing five elements for each vessel, 8" in diameler and 40" in length. The first vessel has Saehane membranes, type RE8040BE-400 t~ manufactured in South Korea. Koch membranes, type 8822-XR-365 tt: made in the US, were used in the second vessel. The pilot plant uses brackish water from the Tigris River with TDS <600 ppm. The new type of RO membranes (Saehane) were used for the first trial for production of desalted water from brackish water less than 600 ppm and the results were compared with performance of the Koch membranes. It was found that over 180 days of continuous operation, the amount of permeate for Saehane membranes is larger than Koch membranes by about 26%. It was also observed that the quality of permeate water stream for Koch membranes is less than for the Saehane membranes by about 11%. Keywords: Reverse osmosis; Seawater desalination; Performance; Effect of pressure
1. I n t r o d u c t i o n
Reverse osmosis (RO) is a widely known technology for production of potable water from brackish or seawater. There are two common types o f RO membranes: spiral wound (SW) and hollow fiber (HF). Currently SW membranes are *Corresponding author.
used in most RO plants. Their success is a result of the development of a variety of polymer materials used for production of membranes. The efficiency and surface lifetime o f a RO system depends on effective pretreatment o f the feed water. The pretreatment includes any process which can minimize fouling, scaling and membrane degradation to optimize product water,
0011-9164/03/$- See front matter © 2003 Elsevier Science B.V. All rights reserved PII: S 0 0 1 ! - 9 1 6 4 ( 0 3 ) 0 0 3 0 6 - 0
288
A.S. Kahdim et al. / Desalination 155 (2003) 287-291
salt rejection, product recovery and operating cost. A proper pretreatment scheme for the feed water will depend on feed water source and feed water composition. There are three types of feed water. Well water has a low silt density index (SDI), typically <2, and low bacteria count, thus requiring a simple pretreatment scheme. Surface water, on the other hand, is characterized by a high SDI and can have a high bacteria count. For sea water the only scalant encountered is calcium carbonate, and preventing its precipitation is accomplished by pH adjustment to 6-6.5. The techniques that are used for fresh water pretreatment with respect to colloidal fouling are also used for seawater. The performance of an RO system is influenced by the feed water composition, feed pressure, temperature and recovery. Many workers have studied the effects of operation variables on the performance of membranes. Redondo [1] studied the methods that must be used to improve the performance of Filmtec membranes. Sallangos et al. [2] studied the performance of TFC-2832SS-540 Koch membranes and compared the experiment results with normalized data using NORM Pro-Software. Abbas and AI-Bastaki [3] studied the performance decline in brackish water Filmtec SW RO membranes for 500 days of continuous operation. Hawlader et al. [4] examined the performance of various types of membranes, SW and FH, vs. different operation variables. In the present work the effect of operation pressure on the performance of two types of membranes was studied. The Saehane membrane, type RE8040BE (Korea) was used in the first vessel, and the 8822-XR-365 Koch system membrane (US) was used in the second vessel. The influence of operation pressure on the values of permeates and rejected amounts, salt passage and salt rejection were examined for both types of membranes.
2. Pilot plant description The desalination system is capable of producing about 9 m3/h of distilled water as shown in Fig. 1. The plant consists of a feed water tank with a capacity of 4 m 3. A centrifuge booster pump draws water from the feed water tank, pumps it through the pretreatment cartridge filters and, subsequently, delivers it to a highpressure pump. The high-pressure pump delivers the pretreated water to the RO membranes. The feed pumps could increase the water pressure to values ranging between 1000--4000 kPa. The high-pressure feed stream enters the membrane module where fresh water passes through the membranes, while most of the dissolved salts are rejected by membrane and concentrates in the reject brine stream. The shape of the membrane and the direction of flow stream are shown in Fig. 2. Part of the product water is collected in a flush tank for washing the membranes after the system is shut down. Flow rate and concentrations of the product and reject water are measured by the flow meter and conductivity measurements, respectively. The reject water stream is returned to the source of brackish water. The feed water is chemically and mechanically treated to minimize deposition of fine particles, mud, and scaling compounds, including calcium and magnesium, fouling materials and biological matter. The feed water to the membrane is treated by different additives such as acid, scale inhibitor as antiscaling, or sodium metabisulfite (SMBS), which is used for removal of chlorine and as a biostatic. Raw water is passed through a pretreatment sequence before entering the production plant. The pretreatment process consists of sedimentation, chlorination, acidification, sand filtering, dechlorination using SMBS, and activated carbon filtering. Then, an antiscalant is added to the feed water.
289
A.S. Kahdim et al. /Desalination 155 (2003) 287-291
[__.4._H2SO4 . _431 ,so4
A
,
Raw w a t e r ~,IP'~
L
!
Feed pump 12m3/h, 13bar
arbonXCandle filter/film
Gravel Organo filter phosphate
Flocculatio~ and loading sedimentation pump
1
NaHSO4
Wastewater
Intermediate tank ;7 m 3
To consumer
Membrmles
Surface Permeate C02 tank pump trickling 46m3/h 8m3/h,8bar device
High pressure pump 12m3/h,32bar
Fig. 1. Flow diagramof RO pilot plant.
Feed
Reject
1
v
' r
Permeate Element No.
1
2
3
4
5
Fig. 2. Directionof flow streamsthroughmembranes.
Having completed the pretreatment processes of the raw water, the treated feed water enters the RO water desalination system. Feed water enters the system through the shut-off valve and flows through the cartridge filter to the high-pressure pump. From the high-pressure pump, the feed water flows to the feed inlet connection of the vessel.
The RO plant contains two vessels having a capacity of 9 m3/h, with a provision of adding a third vessel in order to increase the rate of production of the RO plant. The first vessel contains RE8040BE Saehane membranes and the second vessel contains 8822XR-365 Koch membranes. Each pressure vessel system consists of pressure vessel with five elements, which are
290
A.S. Kahdim et al. /Desalination 155 (2003) 287-291
Table 1 Properties of various membranes Properties
Rejection, % Recovery, % Effective area, ft2 Length, inch Diameter, inch Max. operation pressure, kPa Permeate flow, m3/d
Membrane type Koch
Saehane
99.5 15 365 40 8 4140 34.1
99.5 15 400 40 8 4160 41.6
Table 2 Brackish water analysis Species
Concentration
Ca, mg/l Mg, mg/l SO4, mg/l M-alkalinity as CaCO3, mg/I Total hardness as CaCO3, mg/l SiO2, mg/l Turbidity, NTU pH TDS Temperature, °C
92 53 510 217 145 1.7 0.078 7.3 580 35
connected in series. The vessel materials are constructed from fiber glass reinforced plastic 210" in length and 8" in diameter. The water produced from the RO membranes was collected in a product tank with capacity of 46 m 3. Instrumentation and measurements form an essential part in RO desalination plants where the variables are measured by probes or gauge instrumentation including thermometers, flow meters and pressure gauges; pH and conductivity measurements were monitored on-line by sensors while the other properties such as flow rates and pressures for permeate and reject streams were
measured by rotameters and pressure gauges, respectively, that are connected on line. Table 1 shows the specific properties to the membranes. The Wafa AL-Kaeed River with TDS less than 600 ppm was used as raw water to the pilot plant. Table 2 shows typical brackish water analysis after pretreatment and as feed to the membranes.
3. Results and discussion The influence o f different operation pressures on the performance of various types o f membranes was examined. The results were measured after 150 days o f operations where the surface lifetime and operation and the applied conditions were same for both type of membranes. In the present work the factors affecting the performance of the membranes such as feed temperature, feed water concentration and percent recovery were kept constant while the applied pressure represented the factor that has an effect on the performance o f membranes. The pressure of the feed solution was measured by a Bourdon pressure gauge (040 bar) connected at the outlet o f the pump. The pressure could be changed by means of a gate valve connected to the pipe. Three rotameters were installed in the lines of the feed, reject and permeate to measure different stream flow rates. The values of conductivity for the permeate and reject water were measured using conductivity sensors connected on line. Fig. 3 depicts the variation of productivity with applied pressure for both types of membranes. As can be seen, the relationship between the applied pressure and productivity was linear. This relationship is consistent with that derived by Lonsdale [5]. It can also be seen that the productivity of the Saehane membrane was greater than the Koch membrane. This is due to the difference in the effective area for both types of membranes.
A.S. Kahdim et al. / Desalination 155 (2003) 287-291
e
5
Saehane membranes
1 o 5
7
9
11
13
15
Applied pressure (bar) Fig. 3. Effect of applied pressure on productivity for both types of membranes.
1.8 ...........................................................................................................................................................................................
1.5
1.2 0.9 0.6
--e-- Saehane membranes --B- Koch membranes 0 t........................................................................................... 6 8 10 12 14
16
291
Fig. 4 shows the effect of applied pressure on the salt passage for both types o f membranes. The salt passage through the different membranes decreases with increasing feed water pressure, and Koch membranes give very good water quality as compared to Saehane membranes. Fig. 5 depicts the relationship between applied pressure and salt rejection, which through the membranes increases with increasing feed water pressure. For the Saehane membranes, the increase in concentration for the reject stream compared with Koch membranes leads to increased osmotic pressure. The value o f osmotic pressure affects the productivity or percent recovery as a result o f reducing applied pressure on the membrane elements, but for this case it appears that the reduction of permeate for Saehane membranes due to salt concentration is smaller than the influence of an increase in the effective area compared with Koch membranes for the same operating conditions.
Applied pressure (bar) Fig. 4. Effect of applied pressure on salt passage for both
types of membranes.
[1] J.A.Redondo, Desalination, 126 (1999) 249-259. [2] O. Sallangos and P. Moss, Desalination, 139 (2001) 125-129. [3] A. Abbas and N. AI-Bastaki, Desalination, 136 (2001) 281-286. [4] M.N. Hawlader, J.C. Ho and C.K. Teng, Desalination, 132 (2000) 275-280. [5] H.K.Lonsdale,R.E. Lacey and R.E. Loeb, Industrial Processingwith Membranes,Wiley,New York, 1972.
99.4 --e-- Saehane membranes 99.2
~
-m-- Koch m e m b r a ~ j l . , ~
99 98,8
98,6 n•
98.4
98,2
6
8
10
References
12
14
16
Applied pressure (bar)
Fig. 5. Effect of applied pressure on salt rejection for both types of membranes.
ELSEVIER
www.elsevier.com/locate/desal
Effect of different operation pressures for various membranes on the performance of RO plants • b* Abdul Sattar Kahdim a, Alaa' AbdulrazaqJasslm , Saleh Ismail c
aNational Committee for Technology Transfer, Baghdad, Iraq bDepartment of Chemical Engineering, University of Basrah, Box 1458, Basrah, Iraq email: noor. b@uruklink net cUniversity of Basrah, Basrah, Iraq Received 19 August 2002; accepted 15 October 2002
Abstract
The aim was to study the effect of different operation pressures on the performance of reverse osmosis (RO) plants for various types of membranes. The study was conducted in a pilot plant at the University of Basrah, College of Engineering, which has a capacity of 9 m3/h. The plant is comprised of two parallel vessels containing five elements for each vessel, 8" in diameler and 40" in length. The first vessel has Saehane membranes, type RE8040BE-400 t~ manufactured in South Korea. Koch membranes, type 8822-XR-365 tt: made in the US, were used in the second vessel. The pilot plant uses brackish water from the Tigris River with TDS <600 ppm. The new type of RO membranes (Saehane) were used for the first trial for production of desalted water from brackish water less than 600 ppm and the results were compared with performance of the Koch membranes. It was found that over 180 days of continuous operation, the amount of permeate for Saehane membranes is larger than Koch membranes by about 26%. It was also observed that the quality of permeate water stream for Koch membranes is less than for the Saehane membranes by about 11%. Keywords: Reverse osmosis; Seawater desalination; Performance; Effect of pressure
1. I n t r o d u c t i o n
Reverse osmosis (RO) is a widely known technology for production of potable water from brackish or seawater. There are two common types o f RO membranes: spiral wound (SW) and hollow fiber (HF). Currently SW membranes are *Corresponding author.
used in most RO plants. Their success is a result of the development of a variety of polymer materials used for production of membranes. The efficiency and surface lifetime o f a RO system depends on effective pretreatment o f the feed water. The pretreatment includes any process which can minimize fouling, scaling and membrane degradation to optimize product water,
0011-9164/03/$- See front matter © 2003 Elsevier Science B.V. All rights reserved PII: S 0 0 1 ! - 9 1 6 4 ( 0 3 ) 0 0 3 0 6 - 0
288
A.S. Kahdim et al. / Desalination 155 (2003) 287-291
salt rejection, product recovery and operating cost. A proper pretreatment scheme for the feed water will depend on feed water source and feed water composition. There are three types of feed water. Well water has a low silt density index (SDI), typically <2, and low bacteria count, thus requiring a simple pretreatment scheme. Surface water, on the other hand, is characterized by a high SDI and can have a high bacteria count. For sea water the only scalant encountered is calcium carbonate, and preventing its precipitation is accomplished by pH adjustment to 6-6.5. The techniques that are used for fresh water pretreatment with respect to colloidal fouling are also used for seawater. The performance of an RO system is influenced by the feed water composition, feed pressure, temperature and recovery. Many workers have studied the effects of operation variables on the performance of membranes. Redondo [1] studied the methods that must be used to improve the performance of Filmtec membranes. Sallangos et al. [2] studied the performance of TFC-2832SS-540 Koch membranes and compared the experiment results with normalized data using NORM Pro-Software. Abbas and AI-Bastaki [3] studied the performance decline in brackish water Filmtec SW RO membranes for 500 days of continuous operation. Hawlader et al. [4] examined the performance of various types of membranes, SW and FH, vs. different operation variables. In the present work the effect of operation pressure on the performance of two types of membranes was studied. The Saehane membrane, type RE8040BE (Korea) was used in the first vessel, and the 8822-XR-365 Koch system membrane (US) was used in the second vessel. The influence of operation pressure on the values of permeates and rejected amounts, salt passage and salt rejection were examined for both types of membranes.
2. Pilot plant description The desalination system is capable of producing about 9 m3/h of distilled water as shown in Fig. 1. The plant consists of a feed water tank with a capacity of 4 m 3. A centrifuge booster pump draws water from the feed water tank, pumps it through the pretreatment cartridge filters and, subsequently, delivers it to a highpressure pump. The high-pressure pump delivers the pretreated water to the RO membranes. The feed pumps could increase the water pressure to values ranging between 1000--4000 kPa. The high-pressure feed stream enters the membrane module where fresh water passes through the membranes, while most of the dissolved salts are rejected by membrane and concentrates in the reject brine stream. The shape of the membrane and the direction of flow stream are shown in Fig. 2. Part of the product water is collected in a flush tank for washing the membranes after the system is shut down. Flow rate and concentrations of the product and reject water are measured by the flow meter and conductivity measurements, respectively. The reject water stream is returned to the source of brackish water. The feed water is chemically and mechanically treated to minimize deposition of fine particles, mud, and scaling compounds, including calcium and magnesium, fouling materials and biological matter. The feed water to the membrane is treated by different additives such as acid, scale inhibitor as antiscaling, or sodium metabisulfite (SMBS), which is used for removal of chlorine and as a biostatic. Raw water is passed through a pretreatment sequence before entering the production plant. The pretreatment process consists of sedimentation, chlorination, acidification, sand filtering, dechlorination using SMBS, and activated carbon filtering. Then, an antiscalant is added to the feed water.
289
A.S. Kahdim et al. /Desalination 155 (2003) 287-291
[__.4._H2SO4 . _431 ,so4
A
,
Raw w a t e r ~,IP'~
L
!
Feed pump 12m3/h, 13bar
arbonXCandle filter/film
Gravel Organo filter phosphate
Flocculatio~ and loading sedimentation pump
1
NaHSO4
Wastewater
Intermediate tank ;7 m 3
To consumer
Membrmles
Surface Permeate C02 tank pump trickling 46m3/h 8m3/h,8bar device
High pressure pump 12m3/h,32bar
Fig. 1. Flow diagramof RO pilot plant.
Feed
Reject
1
v
' r
Permeate Element No.
1
2
3
4
5
Fig. 2. Directionof flow streamsthroughmembranes.
Having completed the pretreatment processes of the raw water, the treated feed water enters the RO water desalination system. Feed water enters the system through the shut-off valve and flows through the cartridge filter to the high-pressure pump. From the high-pressure pump, the feed water flows to the feed inlet connection of the vessel.
The RO plant contains two vessels having a capacity of 9 m3/h, with a provision of adding a third vessel in order to increase the rate of production of the RO plant. The first vessel contains RE8040BE Saehane membranes and the second vessel contains 8822XR-365 Koch membranes. Each pressure vessel system consists of pressure vessel with five elements, which are
290
A.S. Kahdim et al. /Desalination 155 (2003) 287-291
Table 1 Properties of various membranes Properties
Rejection, % Recovery, % Effective area, ft2 Length, inch Diameter, inch Max. operation pressure, kPa Permeate flow, m3/d
Membrane type Koch
Saehane
99.5 15 365 40 8 4140 34.1
99.5 15 400 40 8 4160 41.6
Table 2 Brackish water analysis Species
Concentration
Ca, mg/l Mg, mg/l SO4, mg/l M-alkalinity as CaCO3, mg/I Total hardness as CaCO3, mg/l SiO2, mg/l Turbidity, NTU pH TDS Temperature, °C
92 53 510 217 145 1.7 0.078 7.3 580 35
connected in series. The vessel materials are constructed from fiber glass reinforced plastic 210" in length and 8" in diameter. The water produced from the RO membranes was collected in a product tank with capacity of 46 m 3. Instrumentation and measurements form an essential part in RO desalination plants where the variables are measured by probes or gauge instrumentation including thermometers, flow meters and pressure gauges; pH and conductivity measurements were monitored on-line by sensors while the other properties such as flow rates and pressures for permeate and reject streams were
measured by rotameters and pressure gauges, respectively, that are connected on line. Table 1 shows the specific properties to the membranes. The Wafa AL-Kaeed River with TDS less than 600 ppm was used as raw water to the pilot plant. Table 2 shows typical brackish water analysis after pretreatment and as feed to the membranes.
3. Results and discussion The influence o f different operation pressures on the performance of various types o f membranes was examined. The results were measured after 150 days o f operations where the surface lifetime and operation and the applied conditions were same for both type of membranes. In the present work the factors affecting the performance of the membranes such as feed temperature, feed water concentration and percent recovery were kept constant while the applied pressure represented the factor that has an effect on the performance o f membranes. The pressure of the feed solution was measured by a Bourdon pressure gauge (040 bar) connected at the outlet o f the pump. The pressure could be changed by means of a gate valve connected to the pipe. Three rotameters were installed in the lines of the feed, reject and permeate to measure different stream flow rates. The values of conductivity for the permeate and reject water were measured using conductivity sensors connected on line. Fig. 3 depicts the variation of productivity with applied pressure for both types of membranes. As can be seen, the relationship between the applied pressure and productivity was linear. This relationship is consistent with that derived by Lonsdale [5]. It can also be seen that the productivity of the Saehane membrane was greater than the Koch membrane. This is due to the difference in the effective area for both types of membranes.
A.S. Kahdim et al. / Desalination 155 (2003) 287-291
e
5
Saehane membranes
1 o 5
7
9
11
13
15
Applied pressure (bar) Fig. 3. Effect of applied pressure on productivity for both types of membranes.
1.8 ...........................................................................................................................................................................................
1.5
1.2 0.9 0.6
--e-- Saehane membranes --B- Koch membranes 0 t........................................................................................... 6 8 10 12 14
16
291
Fig. 4 shows the effect of applied pressure on the salt passage for both types o f membranes. The salt passage through the different membranes decreases with increasing feed water pressure, and Koch membranes give very good water quality as compared to Saehane membranes. Fig. 5 depicts the relationship between applied pressure and salt rejection, which through the membranes increases with increasing feed water pressure. For the Saehane membranes, the increase in concentration for the reject stream compared with Koch membranes leads to increased osmotic pressure. The value o f osmotic pressure affects the productivity or percent recovery as a result o f reducing applied pressure on the membrane elements, but for this case it appears that the reduction of permeate for Saehane membranes due to salt concentration is smaller than the influence of an increase in the effective area compared with Koch membranes for the same operating conditions.
Applied pressure (bar) Fig. 4. Effect of applied pressure on salt passage for both
types of membranes.
[1] J.A.Redondo, Desalination, 126 (1999) 249-259. [2] O. Sallangos and P. Moss, Desalination, 139 (2001) 125-129. [3] A. Abbas and N. AI-Bastaki, Desalination, 136 (2001) 281-286. [4] M.N. Hawlader, J.C. Ho and C.K. Teng, Desalination, 132 (2000) 275-280. [5] H.K.Lonsdale,R.E. Lacey and R.E. Loeb, Industrial Processingwith Membranes,Wiley,New York, 1972.
99.4 --e-- Saehane membranes 99.2
~
-m-- Koch m e m b r a ~ j l . , ~
99 98,8
98,6 n•
98.4
98,2
6
8
10
References
12
14
16
Applied pressure (bar)
Fig. 5. Effect of applied pressure on salt rejection for both types of membranes.