Optimization of hybridized seawater desalination process

DESALINATION ELSEVIER

Desalination 131 (2000) 147-156

Optimization of hybridized seawater desalination process Mohammad AK. A1-Sofi*, Ata M. Hassan, Osman A Hamed, Abdul Ghani I. Dalvi, Mohammad N.M. Kither, Ghulam M. Mustafa, Khalid Bamardouf Saline Water Conversion Corporation, Research & Development Center, P.O. Box 8328, Al-dubai131951, Saudi Arabia Tel. +966 (3) 361-3713; Fax +966 (3) 362-1615; e-mail: swcc [email protected] Received 12 July 2000; accepted 21 July 2000

Abstract

Saline Water Conversion Corporation (SWCC), Research and Development Center (RDC) came up in recent years with a breakthrough in feed trealment to all desalination processes. Selective ions, biomass and suspended solids rejection characteristic of nanofiltration (NF) membranes were utilized to retard the ingress of hardness forming ions and other foulants into desalination downstream processing steps, e.g. SWRO and MSF. NF pretreatment using brackish water hardness removal membranes were applied to seawater pilot, demonstration and eventually commercial desalination plants. A good number of such membranes were tested in order to select the best. The screening scheme revealed couple of interesting facts related to characteristics of various brands and grades of NF membranes. In an earlier stage of this multi phase project these membranes were categorized as groups A, B and C. These groups in the same order were found to give fi'om high hardness and salinity rejection with low flow and product recovery down to low rejection with high flow and recovery. Based on the above findings, it was decided that the most suitable membranes would be those of group B which could give good flow and recovery with reasonable rejections. Membranes of group B were tested at various plant sizes, production schemes and configurations. This paper will concentrate on 4, 8 and 9 inch membrane elements used in NF or NF/SWRO combination to provide make-up to pilot plant MSF of 20 kl/d distiller. Such hybridized production configurations were optimized by testing overall performance. Few prime parameters were closely analyzed to come up with the best combination of NF membrane brand and their operating parameters along with MSF and/or SWRO-MSF hybrid conditions. In the latter case SWRO reject was used as make-up to the MSF pilot plant distiller. Moreover, the testing program locked into fut~er hybridization, whereby MSF heated reject water was used as feed to NF pretreatment section. This was done to establish the effect of feed temperature rise benefits on NF and/or NFSWRO energy input, yield(s) and product(s) flow and quality. The MSF operation was monitored and its operating results were analyzed. Main components of such analyses were gain output ratio (GOR), fouling factor (FF), top brine temperature (TBT), chemical consumption and product to make-up yield ratio and its quality. Keywords: Nanofiltration; Hybridization; Optimization; MSF; SWRO; Permeate recovery; Foulant rejection; Production cost reduction

*Corresponding author. Presented at the Conference on Membranes in Drinking and Industrial Water Production, Paris, France, 3-6 October 2000 InternationalWater Association,EuropeanDesalinationSociety,AmericanWaterWorks Association,Japan Water Works Association 0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All fights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 0 ) 0 0 1 1 0 - 7

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M.AK. Al-Sofi et al. / Desalination 131 (2000) 147-156

integrated production mode, this name is still valid, since the scheme calls for utilization of heat reject from thermal processes, e.g. MSF, to heat membrane process feed water. In 1998, 1999 and 2000 the MSF heat reject water was used during the winter as feed water to the NF to improve the process yield by as much as three per cent for every degree centigrade temperature rise. In trihybrid scheme the improved production was seen not only in the NF process but in SWRO process as well, when the whole set up was run in an integrated production mode.

I. Introduction

During 1998 few runs were conducted to check the scheme of nanofiltration (NF) and multistage flash (MSF) in a dihybrid set up. Moreover, trihybrid set up of NF-SWRO-MSF was also tested [1,2]. Furthermore, the winter season of 1998 to 2000 were utilized to check an added scheme of hybridization. Such scheme was discussed during the eighties [3,4], even ahead of NF pretreatment of seawater introduction by SWCC from 1996 onwards [5,6]. The said scheme was referred to then (in 1989)as

SW

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4 HRC Stages

D/A

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MU

Fig. 1. Schematic flow diagram ofNF, SWRO and MSF pilot plants.

HP Pump

Reject to MSF

SWRO Unit

RR

149

M.AK. Al-Sofi et al. / Desalination 131 (2000) 147-156 a. P e r m e a t e 10~

Flow Rates •

4 O

2

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NF-E

~: NF-F

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Recovery

10o 80 A

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Conductivity

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1000

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45

50 Feed

55

60

65

70

75

Pressure(bar)

Fig. 2. SWRO permeate, a, flow; b, recovery; c, conductivity from the NF-SWRO process and the conventional SWRO vs. applied pressure (feed to SWRO is permeate from NF membranes or seawater).

2. Results and discussion Pilot plant testings carried out at the Saline Water Conversion Corporation (SWCC) research and Development Center (RDC) have demonstrated nanofiltration process adaptability to either one or more seawater desalination process in hybridized fashion. This adaptability was checked in detail in both di- and trihybridized (as well as integrated production of di- and trihybrid) mode of SWRO and MSF. These hybridized modes can be referred to as NF-SWRO, NF-MSF, NF-SWRObnn~ reject-MSF or MSF heat

reject then NF and SWRO brine reject as makeup to MSF recovery section. These production schemes are shown in Fig. 1. In Fig. 1, main flow streams are identified for the purpose o f illustration. In Figs. 2--4 production data of N F - S W R O are shown for a number of NF membrane brands. From these figures one can depict four main characteristics. These are flow and yield of NF-SWRO, rejection percentages of NF and SWRO, product quality and operating pressure. One can also depict that there is an inverse relationship between flow and yield on one hand

M.AK. AI-Sofi et al. / Desalination 131 (2000) 1 4 ~ 1 5 6

150

3.,?.00

3000 2500 1610

2000

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Fig. 4. Chemical composition and physical properties of seawater, NF filtrate, and NF and SWRO rejects at different NF feed pressure.

with per cent salt rejection of various NF membranes on the other hand. These facts were utilized to come up with an optimized integrated hybrid of MSFht.reject-NFSWRObn ~eject-MSFrecsee • As a result of these tests MSF heat reject water feeding was found to be very attractive for either the NF operation as an MSF pretreatment for provision of MSF makeup of reduced salinity and more importantly depleted hardness ionic content as shown in Fig. 4. Fig. 4 also compares the make-up qualities of this mode of operation in addition to the ability of MSF operation using SWRO reject while SWRO is fed with NF permeate. Moreover, Fig. 4 compares the above with normal seawater quality as traditionally used, i.e.

M.AK. Al-Sofi et al. / Desalination 131 (2000) 147-156

Table 1 Grouping of NF membrane based on their permeate composition and flow characteristics NF permeate: Groups composition and flow A a.

Permeate condition SO4=, ppm M-alkalinity, ppm Ca++, ppm Mg++, ppm Total hardness TDS, ppm x 103

b. Flow, l/min

B

C

41-107 0-52 15-46 28-56 19-92 44-112 32-180 32-126 185-946 241-930 1 3 - 2 4 25-32

33-112 55-71 163-300 222-425 1,320-3,500 33-36

1.3-5.8

21.6--45

17-25

without NF pretreatment as make-up for the MSF process. In this optimization review of "Integrated Hybridized Seawater Desalination Process" a number o f NF membrane brands were tested to identify the most attractive ones. As a result NF membrane brands were categorized into three groups as shown in Table 1. These groups are identified as A, B and C group. Based on the said results and categorization it has been recommended to use membranes of group C with partial acidification of make-up for NF-MSF dihybrid set up. Such membranes could be used also for the integrated dihybridized mode of NF feed heating by MSF heat reject seawater. This option of combined group C merits with feed heating is found very attractive as it yields very high nanofiltration permeate (NFP) flow rates. Partial acidification of seawater ahead of NF is even of greater benefits than partial acidification of NFP make-up to MSF. This practice will reduce NF reject side scaling potential hence the possibilities of further increasing NFP flow, i.e. higher NFP recovery. This will be achieved on top of the primary objectives of reducing MSF scaling potential particularly in high temperature stages of recovery section and even more so inside the tubes of the brine heater,

151

On the other hand, group B membranes are found somewhat more attractive for trihybridized mode of operation of NF-SWRObnne ~j-MSF also for the integrated trihybrid set up. Moreover, group B membranes are also recommended in dihybrid NF-MSF. This option is found to be more attractive for MSF plants, which are not equipped with decarbonators, and especially in the absence of efficient external deaerators. The pertinence of deploying Group B membrane is mainly due to the doubtful ability of stripping the remaining carbon dioxide outside MSF distillers. Carbon dioxide removal would not be required with group B since no acid (or extremely low) dosing is used for such membranes due to their higher alkalinity rejection in comparison to group C membranes. In this regard, it is to be noted that use of group B membranes which entails reduction in yield is found attractive as a trade off between NFP higher quality against reduced recovery ratio of yield. It is also worth drawing attentions to the fact that integration of lrihybrid mode by feed heating is also available as an option for winter operation. The Saline Water Conversion Corporation is currently executing a project to augment and upgrade a half a million gallon per day SWRO train by incorporating NF into the process. It was established that due to reduced osmotic pressure of NFP compared to normal Red Sea water (as feed to this plant) the production of the one train will increase from 92 to 135 kl/h. In addition, the improved quality of SWRO feed also made potable water production by a single stage SWRO possible. Thus, the installed second stage train of brackish water reverse osmosis (BWRO) became redundant. Based on the redundancy of BWRO train the pumps of this part of the train which could deliver up to 40 bar pressure are going to be utilized as NF feed pumps. Fig. 5 shows the original set up of this train, while Fig. 6 shows the flow pattern of this train upon the introduction of nanofiltration.

3/LAK. Al-Sofi et al. /Desalination 131 (2000) 147-156

152

Clear Water Pum

SW Intake Intake Snmp To Train 200

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Gravity Filtered Filter Water Sump To Train 100 I~ Stage RO 30% Conversion 1~ Stage 414 Elements•69 PressureVessels 2"~Stage rip P u m p ~ ~ 1 0 8 mS/h HP Pump ( ~ 6 0 - 6 4 bar ~ " 360 m3]h

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ProductWater Pump

Fig. 5. Umm Lujj SWRO Plant flow diagram.

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~Outfall

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Fig. 6. Umm Lujj SWRO Plant flow diagram with NF in pretreatment system.

Product Water Pump

~

153

M.AK. Al-Sofi et aL / Desalination 131 (2000) 147-156 NF

SWRO

pretreatment step using ultrafiltration (UF). This proposed addition is shown graphically in Fig. 7, while the calculated results of various set ups are shown in Fig. 8. This figure shows annual operating costs, production and permeate unit cost o f three set ups which are: (1) existing train, (2) NF introduction and (3) UF introduction and NF expansion. Upon completion and commissioning o f NF introduction, i.e., step 1 in SWCC-RDC plans, step 2 will be activated using the resuits of step 1. Future planning also calls for recommissioning of an existing MSF distiller in order to be incorporated later into the scheme at this site. The eventual scheme would be trihybridization of NF-SWRObnnerej-MSF. Integration for feed heating is of remote application at this site due to narrow seasonal variation in the temperature of the Red Sea at this site.

252 rn3/h 6,048 m:~ld 66,578 g/h 1,597,887 gld

240 mS~ UF

266 m3/h

Fig. 7. Predicted production ofUmm Lujj SWRO Plant.

Due to the non-availability of additional seawater feed the current upgrade will lead to partial utilization of the said SWRO train. It is, therefore, envisioned to augment the existing seawater feed pretreatment at this site. Further SWCC-RDC scheme is calling for the introduction o f membrane filtration also as a first

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Fig. 8. Possible variances in production and cost of potable water before and after NF and UF+NF introduction to Umm Lujj Plant.

M.AK. Al-Sofi et al. / Desalination 131 (2000) 147-156

154

Seawater MSF (with acid & antifoam) NF-MSF (No further treatment) NF-ROR-MSF (No further treatment)

N

TBT

Caxl0

SO4x10

M-AIk

~.

~.

Cond.xl03

pHx0.1

% Recovery

Fig. 9. Brine recycle physical/chemical properties.

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T o p brine temperature, °C

Fig. 10. Predicted production capacity and performance improvement.

160

~

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M.AK. AI-Sofi et al. / Desalination 131 (2000) 147-156 50000

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Fig. 11. Predictedproduction capacity of water and power.

Looking back to pilot plant results, Fig. 9 shows MSF stream quality for three modes of operation at a TBT of 120°C. Based on these results SWCC-RDC finds that di- and trihybridization could very much help in expanding MSF top brine temperatures (TBT) into an unheard of upper limits exceeding 120 or even beyond 155°C. Thus, the expanded flash range will lead to an appreciable increase in water production and will also improve performance. Fig. 10 is a result of a simulation for TBT rise to as high as 160°C. Fig. 11 is also a result of simulation for dual production cycle showing the possibility of increased electrical power generation productivity. It is to be stressed here that this simulation is based on utilization of one existing boiler turbine generation (BTG) coupled to a pair of MSF distillers. This simulation also looked into expanding MSF cascade staging as shown in Fig. 10. It also looked into replacing the existing pair o f steam turbo-generator by a larger machine. More importantly, this simulation is based on changing the point of steam pass out from the turbine as a source of thermal energy input to the MSF brine heaters. Yet, the most

outstanding aspect of this simulation study is that it will maintain the same boiler at its present maximum rated capacity (MCR). Thus, maintaining true heat input to the dual cycle unchanged. This will eventually lead to higher efficiency dual production plants to yield not only more water but also more power.

3. Conclusion Based on the above discussion and results it can be concluded that the introduction o f nanofiltration as a feed treatment can be viewed as a break through in seawater desalination. The merits of NF-SWRO combination are clearly manifested in Fig. 8. Overall benefits of hybridization on dual-purpose plant of BTG-NF-MSF are shown in Figs. 10 and 11.

4. Recommendations 1. It is recommended to upgrade existing SWRO plants by NF incorporation as a second step seawater feed treatment.

156

~LAK. Al-Sofi et al. / Desalination 131 (2000) 147-456

2. Seawater ultrafiltration is also recommended as a quick approach for improving feed water quality to N F - S W R O , N F - M S F or N F SWRObrme r~j-MSF.

References

3. Implementation of trihybrid schemes as the one mentioned (in recommendation 2) is recommended with an expanded top brine temperature and the length o f cascade staging o f MSF distillers.

[2]

[1]

[3] 4. The incorporation o f NF to create either di or trihybrids o f N F - M S F and N F - S W R O or NF-SWRObnne rej-MSF are recommended in order to expand water and power production at reduced capital and operating costs. 5. The option of feed water heating for membrane process(s) in winter season deserves serious consideration in hybridized desalination processes.

[4] [5]

[6]

M.AK. AI-Sofi, A.M. Hassan, A.O. Hamed, G.M. Mustafa, A.G.I. Dalvi and M.N.M. Kither, Desalination, 125 (1999) 213. A.M. Hassan, M.AK. AI-Sofi, A.S. A1-Amoudi, A.T.M. Jamaluddin, A.M. Farooque, A. Rowaili, A.G.I. Dalvi, N.M.N. Kither, G.M. Mustafa and A. A1-Tisan, A., Proc., WSTA Conference, Bahrain, 2 (1999) 577. M.AK. Al-Sofi and A.R. Khan, Hybrid SWRO and MSF, Proc., 12th Conference of NWESA, Orlando, FL, USA, 1984. M.AK. AI-Sofi, Desalination, 76 (1989) 89. A.M. Hassan, M.AK. AI-Sofi, A.S. AI-Amoudi, A.T.M. Jamaluddin, A.M. Farooque, A. Rowaili, A.G.I. Dalvi, M.N.M. Kither, G.M. Mustafa and A. Al-Yisan, Desalination, 118, (1998) 35. M.AK. AI-Sofi, A. Hassan, G.M. Mustafa, A.G.I. Dalvi and M.N.M. Kither, Desalination, 118 (1998) 123.