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Membrane material for brackish water application for utilities - an evaluation
Desalination, 91 (1993) 155-162 Elsevier Science Publishers B.V.,Amsterdam - Printed in TlreNetherlands
155
Membrane material for brackish water application for utilities - an evaluation L. Raghuraman Tata Consulting Engineers, 73/l, St. Mark’s Road, Bangalote 560 001, Karnataka State (India)
(Received September 23,1992)
SUMMARY
Although both cellulose acetate (CA) and polyamide/thin film composite (TFC) membranes are suitable for brackish water application, the performance characteristics of these membrane materials vary significantly with respect to annual membrane replacement, auxiliary power consumption and chemical requirements of the downstream water treatment plant. A techno-economic evaluation based on expected membrane performance considering these factors, favours the use of polyamide/TFC membranes instead of CA membranes for utilities requiring water for steam generation and also where a demineralisation (DM) plant is provided downstream of the reverse osmosis (RO) plant. While assessing equivalent annual costs, only the major factors that affect techno-economic comparison such as power consumption, acid consumption, membrane replacement, membrane performance projections and downstream (DM) plant chemical consumption and resin replacement cost have been considered. All other annual costs towards maintenance of the RO plant and DM plant, consumption of other chemicals for the RO plant and operating personnel costs are considered to remain the same both for the CA option and polyamide/TFC membrane option.
001 l-9 164193/$06.00 @ 1993Elsevier Science Publishers B.V.All rights reserved.
156
Even though the polyamide!TPC option is marginally higher in initial capital investment cost, the overall k&no-economic comparison favours the use of polyamide/TPC membranes instead of CA membranes. This is mainly due to low power consumption of RO plant high pressure pumps, low acid consumption in RO Plant and better permeate water quality, thus reducing the size and chemical consumption of downstream DM plant. INTRODUCTION
A typical application of the use of brackish water for a 3 x210 MW coastal power station required the supply of raw water from borewells. Due to proximity to the sea, there was an ingress of salt water into the borewells, resulting in progressive deterioration of water quality. The design analysis of the borewell water is given in Table I below. The borewell water analysis indicates a high total dissolved solids (TDS) content of about 2500 ppm. Hence it was decided to install RO plant to treat the borewell water as to make it fit for further treatment in the DM plant before introduction into the steam generation system as make-up. A part of the treated water also meets the potable water demand. The major design parameters specified for the RO plant are furnished in Table II below. TABLE I Design water analysis of borewell water for RO plant Constituents
As such/as ion
As CaCOs
Turbidity, NTU P-Alkalinity, ppm M-Alkalinity, ppm Sulphate, ppm Chloride, ppm Total anions, ppm
Nil Nil 612 221 883 1722
Nil 502 236 1245 1983
Calcium, ppm Magnesium, ppm Sodium, ppm Total cations, ppm
281 88 423 792
701 362 920 1983
Reactive silica as SiOa, ppm PHDissolved iron, ppm
35 7.5 Nil
Nil
157 TABLE II Major design parameters of RO plant Description
Value specified
RO plant capacity, m3/d No. of RO streams
4225 2
No. of sub-streams, in each stream, %
3~33.3% or 4x25%
Membrane material
Cellulose acetate or polyamid&TFC
Minimal salt rejection, %
96
Overall recovery, %
75
Design temperature, “C
30
TDS in permeate water, ppm
Not more than 250
Out of 4225 m3/d of permeate water, about 2850 m3/d will be utilised for the steam generation system make-up water treatment plant (DM plant) including the waste water quantity of the DM plant. The balance is used for other plant services such as plant potable water system and make up water for air conditioning systems. PLANT DETAILS The scheme of the RO plant is presented in Fig. 1. Global tenders were invited for this project. The specification called for both CA option and polyamide/TFC option. Both spiral wound and hollow fine fibre configurations were specified. In order to ensure optimum selection of membrane material, suitable evaluation factors were specified for differential loading based on guaranteed values of relevant factors such as rate of membrane failure per annum, auxiliary power consumption and consumption of chemicals. Pre-bid discussions were held with the prospective bidders to furnish clarifications on the specification documents before bidding. Borewell water received in a raw water storage tank is pumped to the RO plant by three (2 working + 1 standby) raw water pumps. Since the analysis of raw water does not indicate the presence of suspended solids, coagulant and coagulant aid dosing systems and pressure sand filtration units are not provided at present. However, provision has been made in the system for installation of these systems at a later date. Chlorine is dosed at the raw water pump suction and two contact tanks are provided with
158
.
3NlSO3
dlWS
L
I
I
P <
4 . L
159
adequate retention time for the chlorine to react. Automatic acid dosing pumps with pH recorder and controller for controlling pH of the water and automatic sodium sulphite dosing pumps for dechlorination with residual chlorine analyser, recorder and controller are provided. Sodium hexa-meta phosphate dosing pumps to avoid sulphate scale precipitation on membrane surface and a static mixer arc also provided. Three (2 working + 1 standby) polypropylene disposable type 10 micron cartridge filters are provided as a back-up to protect membranes from fouling in the pretreatment section. The pre-treated raw water is then pumped to the RO modules with the help of three (2 working + 1 standby) RO plant high pressure pumps. The RO plant is divided into two 50% capacity streams to provide flexibility of operation. Each stream is further divided into 3~33.3% capacity or 4 x 25% capacity sub-streams to increase the availability of the system. The permeate water is led to two degasser towers to strip the carbon dioxide in the permeate water to a level of 5 ppm as COz. The degassed permeate water is collected in two permeate water storage tanks for further distribution to the DM plant and plant potable water system. Part of the permeate water is led to a flush water tank to facilitate flushing of the RO modules in case of plant shut down or total power failure. For this purpose, one electric motor driven and a standby diesel engine-driven flush water pump are provided in the system. A recirculation line is also provided to facilitate changing of water in the flush water tank and to test the flush water pump operation regularly. The major technical parameters as furnished by the bidders, which affect the techno-economic comparison of the CA option and polyamide/TFC option, are tabulated in Table III. ANALYSIS
A techno-economic comparison between the CA option and polyamide/ TFC option was carried out based on guaranteed performance as furnished by the bidders. From Table III, the following main observations can be made: l The pumping head and hence pumping cost in the case of CA membrane option is higher than the polyamide/TFC membrane option. l
In the pre-treatment section the acid requirement of the CA membrane option is more than the polyamide/TFC membrane option.
160 TABLE III Major technical parameters as furnished by bidders Bidder B
Bidder A CA Guaranteed RO plant high pressure pump motor power consumption, kW 30% HCl consumption in RO plant, kg
Polyamide!lFC
CA
Bidder C PolyamideJl”PC
CA
PolyamideJI’FC
175
71.4
147
108
143.5
86.2
5759
5526
6580
5250
3817
3445
180
180
144
200
216
180
33.3
5
15
12
12
12
60
9
22
24
26
22
Permeate water quality, ppm as ions Ca 2.8 5.4 Mg 0.9 1.7 Na 40.1 37.6 HCo3 2.3 11.4 so4 0.7 3.2 Cl 67.4 63.4
7.7 2.4 76.0 7.6 2.9 132.4
9.7 3.2 24.7 11.9 7.2 52.3
3.3 1.5 45.8 9.2 0.8 74.8
5.1 5.1 25.5 5.8 2.3 58.0
h4embranes(spiral wound) Total no. of membranes used Guaranteed membrane failure in %ly Guaranteed membrane failure/y
Silica, in ppm as SiO2
6.3
1.3
6.0
1.1
5.7
3.6
TDS
114.2
122.7
229.0
109.0
135.4
101.8
RO plant high pressure pumphead, in MWC
335.0
150.0
270.3
204.0
280.0
180.0
161 The
quality of permeate water in terms of TDS and silica is better in the case of the polyamide!I’FC membrane option in comparison with the CA option except in the case of bidder A. This will result in lower ionic load on the downstream DM plant with a consequent reduction in the capital as well as the annual operating cost of the downstream DM plant. In the case of bidder A, the quality of permeate water in both the CA and polyamide options is almost the same but at the expense of higher annual membrane replacement in the case of the CA option. The results of the techno-economic comparison study are furnished in Table IV. Only the difference in annual acid consumption for pre-treatment (as 30% HCl) based on 330 days of operation is considered. This recurring annual expenditure is converted into equivalent cost (capitalized cost) as at the time of bid analysis by multiplying the differential annual cost by a present worth factor computed on the basis of an interest rate of 15% and a plant life of 30 years. The annual membrane replacement cost, which is computed based on the guaranteed membrane failure rate/annum as indicated by the bidder and the quoted unit rate of the membrane, has been capitalized. The annual 0
TABLE lV Techno-economic comparison between cellulose acetate and polyamidm about 2500 ppm TDS Bidder A CA 1.
membranes for brackish water of
Bidder B PolyamideJl’FC 32.32
Bidder C
CA
PolyamideflFC
CA
Polyamidem
Base
25.37
Base
34.63
Base
2.
Difference in capital cost *Capitalized annual consumption cost of 30% HCI
5.23
Btie
29.85
Base
3.
‘C’Ciaalii
annual replace-
82.68
14.59
84.82
104.17
100.24
95.49
4.
*Capitalized power consumption cost
95.20
38.84
79.97
58.75
78.06
46.89
5.
Estimated difference in capitalized annual chemical consumption cost (including capital cost) in the DM plant
Base
3.50
140.00
Base
41.77
Base
6.
Total evaluated cost (1 to 5 above)
183.11
89.25
334.64
188.29
228.42
177.01
7.
Differential
(+) 93.86
Base
(+)146.35
8.35
Base
ment cost
l
Base
(+) 51.41
Annual costs have been capitalized based on an interest rate of 15% and plant life of 30 years.
Base
162
pumping cost has been capitalized on similar lines. The difference between the annual chemical consumption costs and the investment cost of the DM side plant have been converted into equivalent cost (capitalized cost). From the analysis in Table IV, it is found that RO plants using polyamide/ TFC membranes involve a marginally higher investment cost than RO plants using CA membranes (ranging from 5.5 to 10% of CA option cost). However, on an overall basis, polyamide/TFC membranes are more economical than CA membrane materia.l, considering capitalized recurring operating costs. The main factors favouring the choice of polyamide/TFC membranes in the techno-economic analysis are the lower power consumption, lower membrane failure rate, reduced chemical consumption in the RO plant, and reduced capital and chemical consumption cost in the downstream DM plant. CONCLUSION
In a power plant where water is required for steam generation system make-up, the use of polyamic/TFC membranes in the RO plant upstream of the DM plant for brackish water application would be beneficial as it will result in considerable saving in RO plant power consumption, and chemical consumption costs, in addition to substantial saving in the downstream DM plant chemical consumption costs. ACKNOWLEDGEMENT The author thanks the management of Tata Consulting Engineers for the support given in preparing this article.
Desalination, 91 (1993) 163475 Elsevier Science Publishers B.V., Amsterdam- Printed in The Netherlands
163
Production of sulphuric acid and caustic soda fi=omsodium sulphate by electromembrane processes. Comparison between electroelectrodialysis and electrodialysis on bipolar membrane Didier Raucq, Gerald Pourcelly and Claude Gavach* Laboratoryof Physical Chemistryof PolyphasicSystem, CNRS URA330,BP5051,34033 MontpellierCedex (France) Tel: 33 (67) 613409, Fan: 33 (67) 042820 (Received December 29,1992; in revised form February8,1993) SUMMARY
In order to generate HzS04 and NaOH from Na#Od solutions, the electro-electrodialysis @ED) and the electrodialysis (ED) on bipolar membrane (BPM) are compared. For both these processes, the electroosmosis associated to the transport of the sodium ions through the cation exchange membrane (CEM) limits the soda concentration. Due to the co-ion leakages through the BPM which increase with the concentrations of the acid and the base on both sides of this membrane, the current efficiencies of both the acid and the base production are higher for the EED than for the ED on BPM. However, the investment cost for EED is higher than for ED due to the need for a couple of metallic electrodes per each elementary cell.
*The author to whom correspondence should be sent OOll-9164/93 /$06.00 01993
Elsevier Science Publishers B.V. All rights reserved.
155
Membrane material for brackish water application for utilities - an evaluation L. Raghuraman Tata Consulting Engineers, 73/l, St. Mark’s Road, Bangalote 560 001, Karnataka State (India)
(Received September 23,1992)
SUMMARY
Although both cellulose acetate (CA) and polyamide/thin film composite (TFC) membranes are suitable for brackish water application, the performance characteristics of these membrane materials vary significantly with respect to annual membrane replacement, auxiliary power consumption and chemical requirements of the downstream water treatment plant. A techno-economic evaluation based on expected membrane performance considering these factors, favours the use of polyamide/TFC membranes instead of CA membranes for utilities requiring water for steam generation and also where a demineralisation (DM) plant is provided downstream of the reverse osmosis (RO) plant. While assessing equivalent annual costs, only the major factors that affect techno-economic comparison such as power consumption, acid consumption, membrane replacement, membrane performance projections and downstream (DM) plant chemical consumption and resin replacement cost have been considered. All other annual costs towards maintenance of the RO plant and DM plant, consumption of other chemicals for the RO plant and operating personnel costs are considered to remain the same both for the CA option and polyamide/TFC membrane option.
001 l-9 164193/$06.00 @ 1993Elsevier Science Publishers B.V.All rights reserved.
156
Even though the polyamide!TPC option is marginally higher in initial capital investment cost, the overall k&no-economic comparison favours the use of polyamide/TPC membranes instead of CA membranes. This is mainly due to low power consumption of RO plant high pressure pumps, low acid consumption in RO Plant and better permeate water quality, thus reducing the size and chemical consumption of downstream DM plant. INTRODUCTION
A typical application of the use of brackish water for a 3 x210 MW coastal power station required the supply of raw water from borewells. Due to proximity to the sea, there was an ingress of salt water into the borewells, resulting in progressive deterioration of water quality. The design analysis of the borewell water is given in Table I below. The borewell water analysis indicates a high total dissolved solids (TDS) content of about 2500 ppm. Hence it was decided to install RO plant to treat the borewell water as to make it fit for further treatment in the DM plant before introduction into the steam generation system as make-up. A part of the treated water also meets the potable water demand. The major design parameters specified for the RO plant are furnished in Table II below. TABLE I Design water analysis of borewell water for RO plant Constituents
As such/as ion
As CaCOs
Turbidity, NTU P-Alkalinity, ppm M-Alkalinity, ppm Sulphate, ppm Chloride, ppm Total anions, ppm
Nil Nil 612 221 883 1722
Nil 502 236 1245 1983
Calcium, ppm Magnesium, ppm Sodium, ppm Total cations, ppm
281 88 423 792
701 362 920 1983
Reactive silica as SiOa, ppm PHDissolved iron, ppm
35 7.5 Nil
Nil
157 TABLE II Major design parameters of RO plant Description
Value specified
RO plant capacity, m3/d No. of RO streams
4225 2
No. of sub-streams, in each stream, %
3~33.3% or 4x25%
Membrane material
Cellulose acetate or polyamid&TFC
Minimal salt rejection, %
96
Overall recovery, %
75
Design temperature, “C
30
TDS in permeate water, ppm
Not more than 250
Out of 4225 m3/d of permeate water, about 2850 m3/d will be utilised for the steam generation system make-up water treatment plant (DM plant) including the waste water quantity of the DM plant. The balance is used for other plant services such as plant potable water system and make up water for air conditioning systems. PLANT DETAILS The scheme of the RO plant is presented in Fig. 1. Global tenders were invited for this project. The specification called for both CA option and polyamide/TFC option. Both spiral wound and hollow fine fibre configurations were specified. In order to ensure optimum selection of membrane material, suitable evaluation factors were specified for differential loading based on guaranteed values of relevant factors such as rate of membrane failure per annum, auxiliary power consumption and consumption of chemicals. Pre-bid discussions were held with the prospective bidders to furnish clarifications on the specification documents before bidding. Borewell water received in a raw water storage tank is pumped to the RO plant by three (2 working + 1 standby) raw water pumps. Since the analysis of raw water does not indicate the presence of suspended solids, coagulant and coagulant aid dosing systems and pressure sand filtration units are not provided at present. However, provision has been made in the system for installation of these systems at a later date. Chlorine is dosed at the raw water pump suction and two contact tanks are provided with
158
.
3NlSO3
dlWS
L
I
I
P <
4 . L
159
adequate retention time for the chlorine to react. Automatic acid dosing pumps with pH recorder and controller for controlling pH of the water and automatic sodium sulphite dosing pumps for dechlorination with residual chlorine analyser, recorder and controller are provided. Sodium hexa-meta phosphate dosing pumps to avoid sulphate scale precipitation on membrane surface and a static mixer arc also provided. Three (2 working + 1 standby) polypropylene disposable type 10 micron cartridge filters are provided as a back-up to protect membranes from fouling in the pretreatment section. The pre-treated raw water is then pumped to the RO modules with the help of three (2 working + 1 standby) RO plant high pressure pumps. The RO plant is divided into two 50% capacity streams to provide flexibility of operation. Each stream is further divided into 3~33.3% capacity or 4 x 25% capacity sub-streams to increase the availability of the system. The permeate water is led to two degasser towers to strip the carbon dioxide in the permeate water to a level of 5 ppm as COz. The degassed permeate water is collected in two permeate water storage tanks for further distribution to the DM plant and plant potable water system. Part of the permeate water is led to a flush water tank to facilitate flushing of the RO modules in case of plant shut down or total power failure. For this purpose, one electric motor driven and a standby diesel engine-driven flush water pump are provided in the system. A recirculation line is also provided to facilitate changing of water in the flush water tank and to test the flush water pump operation regularly. The major technical parameters as furnished by the bidders, which affect the techno-economic comparison of the CA option and polyamide/TFC option, are tabulated in Table III. ANALYSIS
A techno-economic comparison between the CA option and polyamide/ TFC option was carried out based on guaranteed performance as furnished by the bidders. From Table III, the following main observations can be made: l The pumping head and hence pumping cost in the case of CA membrane option is higher than the polyamide/TFC membrane option. l
In the pre-treatment section the acid requirement of the CA membrane option is more than the polyamide/TFC membrane option.
160 TABLE III Major technical parameters as furnished by bidders Bidder B
Bidder A CA Guaranteed RO plant high pressure pump motor power consumption, kW 30% HCl consumption in RO plant, kg
Polyamide!lFC
CA
Bidder C PolyamideJl”PC
CA
PolyamideJI’FC
175
71.4
147
108
143.5
86.2
5759
5526
6580
5250
3817
3445
180
180
144
200
216
180
33.3
5
15
12
12
12
60
9
22
24
26
22
Permeate water quality, ppm as ions Ca 2.8 5.4 Mg 0.9 1.7 Na 40.1 37.6 HCo3 2.3 11.4 so4 0.7 3.2 Cl 67.4 63.4
7.7 2.4 76.0 7.6 2.9 132.4
9.7 3.2 24.7 11.9 7.2 52.3
3.3 1.5 45.8 9.2 0.8 74.8
5.1 5.1 25.5 5.8 2.3 58.0
h4embranes(spiral wound) Total no. of membranes used Guaranteed membrane failure in %ly Guaranteed membrane failure/y
Silica, in ppm as SiO2
6.3
1.3
6.0
1.1
5.7
3.6
TDS
114.2
122.7
229.0
109.0
135.4
101.8
RO plant high pressure pumphead, in MWC
335.0
150.0
270.3
204.0
280.0
180.0
161 The
quality of permeate water in terms of TDS and silica is better in the case of the polyamide!I’FC membrane option in comparison with the CA option except in the case of bidder A. This will result in lower ionic load on the downstream DM plant with a consequent reduction in the capital as well as the annual operating cost of the downstream DM plant. In the case of bidder A, the quality of permeate water in both the CA and polyamide options is almost the same but at the expense of higher annual membrane replacement in the case of the CA option. The results of the techno-economic comparison study are furnished in Table IV. Only the difference in annual acid consumption for pre-treatment (as 30% HCl) based on 330 days of operation is considered. This recurring annual expenditure is converted into equivalent cost (capitalized cost) as at the time of bid analysis by multiplying the differential annual cost by a present worth factor computed on the basis of an interest rate of 15% and a plant life of 30 years. The annual membrane replacement cost, which is computed based on the guaranteed membrane failure rate/annum as indicated by the bidder and the quoted unit rate of the membrane, has been capitalized. The annual 0
TABLE lV Techno-economic comparison between cellulose acetate and polyamidm about 2500 ppm TDS Bidder A CA 1.
membranes for brackish water of
Bidder B PolyamideJl’FC 32.32
Bidder C
CA
PolyamideflFC
CA
Polyamidem
Base
25.37
Base
34.63
Base
2.
Difference in capital cost *Capitalized annual consumption cost of 30% HCI
5.23
Btie
29.85
Base
3.
‘C’Ciaalii
annual replace-
82.68
14.59
84.82
104.17
100.24
95.49
4.
*Capitalized power consumption cost
95.20
38.84
79.97
58.75
78.06
46.89
5.
Estimated difference in capitalized annual chemical consumption cost (including capital cost) in the DM plant
Base
3.50
140.00
Base
41.77
Base
6.
Total evaluated cost (1 to 5 above)
183.11
89.25
334.64
188.29
228.42
177.01
7.
Differential
(+) 93.86
Base
(+)146.35
8.35
Base
ment cost
l
Base
(+) 51.41
Annual costs have been capitalized based on an interest rate of 15% and plant life of 30 years.
Base
162
pumping cost has been capitalized on similar lines. The difference between the annual chemical consumption costs and the investment cost of the DM side plant have been converted into equivalent cost (capitalized cost). From the analysis in Table IV, it is found that RO plants using polyamide/ TFC membranes involve a marginally higher investment cost than RO plants using CA membranes (ranging from 5.5 to 10% of CA option cost). However, on an overall basis, polyamide/TFC membranes are more economical than CA membrane materia.l, considering capitalized recurring operating costs. The main factors favouring the choice of polyamide/TFC membranes in the techno-economic analysis are the lower power consumption, lower membrane failure rate, reduced chemical consumption in the RO plant, and reduced capital and chemical consumption cost in the downstream DM plant. CONCLUSION
In a power plant where water is required for steam generation system make-up, the use of polyamic/TFC membranes in the RO plant upstream of the DM plant for brackish water application would be beneficial as it will result in considerable saving in RO plant power consumption, and chemical consumption costs, in addition to substantial saving in the downstream DM plant chemical consumption costs. ACKNOWLEDGEMENT The author thanks the management of Tata Consulting Engineers for the support given in preparing this article.
Desalination, 91 (1993) 163475 Elsevier Science Publishers B.V., Amsterdam- Printed in The Netherlands
163
Production of sulphuric acid and caustic soda fi=omsodium sulphate by electromembrane processes. Comparison between electroelectrodialysis and electrodialysis on bipolar membrane Didier Raucq, Gerald Pourcelly and Claude Gavach* Laboratoryof Physical Chemistryof PolyphasicSystem, CNRS URA330,BP5051,34033 MontpellierCedex (France) Tel: 33 (67) 613409, Fan: 33 (67) 042820 (Received December 29,1992; in revised form February8,1993) SUMMARY
In order to generate HzS04 and NaOH from Na#Od solutions, the electro-electrodialysis @ED) and the electrodialysis (ED) on bipolar membrane (BPM) are compared. For both these processes, the electroosmosis associated to the transport of the sodium ions through the cation exchange membrane (CEM) limits the soda concentration. Due to the co-ion leakages through the BPM which increase with the concentrations of the acid and the base on both sides of this membrane, the current efficiencies of both the acid and the base production are higher for the EED than for the ED on BPM. However, the investment cost for EED is higher than for ED due to the need for a couple of metallic electrodes per each elementary cell.
*The author to whom correspondence should be sent OOll-9164/93 /$06.00 01993
Elsevier Science Publishers B.V. All rights reserved.