Desal Methods- Case Study for Kuwait

8/6/2019 Desal Methods- Case Study for Kuwait

1/10

ELSEVIER Desalination 152 (2002) 83-92

Energy consumption in equivalent work by different desalting

DESALINATiON

www.elsevier.com/locate/desal

methods: case study for KuwaitM.A. Danish*, F. Al Asfour, N. Al-Najem

Mechanical Engineering Department, Kuwait University, KuwaitTel. + 965 (48) 11188; Fax + 965 (48) 47131; email: [email protected]

Received 28 March 2002; accepted 13 April 2002

AbstractKuwait needs to add more desalting units to its present installed capacity to satisfy the growing need of potablewater. The energy consumed by different desalting system is one of the main parameters affecting the choice of newdesalting system. Every desalting system consumed either thermal, or mechanical energy or both. This paper presents

a method to compare these energies on one scale that the equivalent work consumed. Based on this analysis, theenergy consumed by the multi stage flash MSF system, the only system presently used in Kuwait to desalt seawater,was calculated and found equal to 25 kWh/m. This is much higher than the energy consumed by the other tworeliable systems, namely reverse osmosis (RO) and multi-effect boiling (MEB) extensively used now in many of theArabian Gulf Countries, AGC. The average energy consumed by the RO system is 5 kWh/m, and by the MEB is inthe range of 12 kWh/m3 when steam is extracted from steam turbines at low availability.Keywords: Multi-stage flash distillation system; Reverse osmosis; Energy consumed per m3of desalted water; Multi-effect boiling

1. Intr oductionKuwait data on desalted water in the year 2000[1J show an annual production of 82,454.3 millionimperial gallons (MIG), or 375.17x106m3, and

maximum and average daily consumption of 278.5and 225.0 MIG per day (MIGD) respectively. Theaverage annual consumption increase in the last5 years is 7.6%. The present installed capacity ofdesalting units is 286.8 MIGD. This means that

*Corresponding author. the capacity needs to be doubled in 9 years, or atPresented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries:Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean.Sponsored by the European Desalination Society and Alexandria University Desalination Studies and TechnologyCenter, Sharm El Sheikh, Egypt, May 4-6, 2002.001 -91 64/02/!$- See front matter 0 2002 Elsevier Science B.V. All rights reservedPII:SOOll-9164(02)01051-2


2/10

84 M.A. Danvish et al. /Desalination 152 (2002) 83-92

least 250 MIGD capacity units are to be addedbefore 2 100 [2]. The only method used to desaltseawater in Kuwait is multi stage flash (MSF)system. Besides MSF, other systems can be addedto satisfy future needs. One of the main parametersaffecting the choice of any desalting system is itsconsumed energy. The consumed energy can bethermal, or mechanical energy, or both. A rationalbasis is needed to compare the value of energyused by each system. Steam (at different tempera-tures and pressures) is used to supply most thermallyoperated desalting systems. A method to evaluatethe applied energy on a single base is to expressit in terms of available energy, i.e. maximumtheoretical work that can be obtained from thatenergy, or the energy itself if it is mechanicalwork. However, specialists in the field prefer toexpress the applied energy in terms of the specificfuel energy consumed kJ/kg desalted water orequivalent mechanical work in kWh/m3. Thesemethods are used here to compare the consumedenergy by different desalting systems and to helpdecision makers in the choice of systems to be

added. The energy consumed by a desalting systemused in Kuwait is presented first.

2. Equivalent work or fuel energy consumedby a desalting system in Kuwait

Steam of moderately low pressure (2-3 bar)provides thermal energy to MSF desalting units.The supplied steam is either extracted from steamturbines or directly from a boiler after throttlingto the required pressure. All Kuwaiti MSF units,(except 3 units in Shuwaikh plant) are combinedwith steam turbines to get extracted steam. Powerplants producing both electric power and desaltedwater are called cogeneration power desalting plants(CPDP). The real value of steam supplied to theMSF units lies in its ability to produce work.

The flow sheet of steam turbine connected to2 MSF desalting units is shown in Fig. 1 as anexample. The nominal capacity of the turbine is300 MW electric power, W, and 196 MW thermal,Q, supplied to 2 MSF units. The steam generatorconsumes 696.23 5 MW fuel energy, Q, when the

m= mass low rate, kg/sFig. 1. Cogeneration power desalting plant used in Kuwait, W,= 225 MW, &= 196 MW. t = temperature,Cp = pressure, ar


3/10

M.A. Danvi sh et al. /Desalinati on 152 (2002) 83-92 85

turbine produces W, = 225 MW electric power andQ,= 196 MW thermal energy to the 2 MSF unitsby steam extracted from a cross pipe between thelow and intermediate pressure turbine cylinders.The 196 MW (Q,) heat produces 14.4 MIGD(758.3 kg/s) of desalted water, or 258.5 kJikg ofdesalted water when the MSF units operate at 110Ctop brine temperature.

The 77.23 kg/s steam (Msd) supplied to theMSF units would produce 43.295 MW work if itis expanded in the LP cylinder to the condenserinstead of being fed to the MSF units. This work,We (= 43.295 MW), is called equivalent work tothermal energy Qd (=196 MW), or the equivalentwork loss due to steam extraction. This gives heCPDP efficiency, T,~ defined by turbine workoutput W,, plus We divided by Q:,,;;,V, + We)l?,=(225 + 43.295) 1696.235

This rating method for the CPDP is better thanthe utilization factor term defined byUF = (W,+ Q41 Qe(,=225 + 196) 1696.235 = 0.6The UF overestimates the performance of theCPDP by adding the low availability heat Q, tothe work W,.

Fuel charged to desalting process should be43.295 IO.385 = 112.45 MW. More energy issupplied by high-pressure steam to operate thesteam ejectors of the MSF units used to reject non-condensable gases from the units (about 16 kJ/kgof desalted water or 17.8 kJ/kg fuel energy basedon 0.9 boiler efficiency, n,>;and by pumping energy(about 16.2 kJ/kg desalted water or 4.5 kWh/m3).So, fuel energy charged to produce 14.4 MGD(758.3 kg/s) is 112.45 MW (due to extractedsteam) + 13.5 MW (due to steam ejectors) +3 1.9 MW (due to 12.28 MW pumping work)= 157.85 MW. This gives:Specific fuel consumption = 157.85 x 1000/758.3= 208.2 kJ/kgMechanical work charged to desalting = 157.85x 0.385 = 60.77 MW, or

Specific mechanical work = 60.77 x 10001758.3= 80.14 kJikg = 22.26 kWh/m3

When the MSF units are driven by steamthrottled directly from boiler, called boiler drivencase (BD), more fuel energy (or equivalent work)is consumed, as compared to the extracted steamcase. Fuel charged to produce 14.4 MIGD for BDcase is:Fuel energy to produce 196 MW heat to the MSFunits =196 / qb = 2 17.78 MW, if (the boiler effici-ency) = 0.9.

This is almost twice the 112.45 MW for ex-tracted steam case. The total fuel energy wouldbe 263.18 MW when fuel energy for pumping andsteam ejector is added. This gives:Specific fuel consumption = 374 kJ/kg (comparedto 208.2 kJ/kg in extracted steam case), andSpecific equivalent mechanical work = 144 kJ/kgor 40 kWh/m3, (compared to 22.26 kWh/m3 forextracted steam).

So, direct supply of steam from boiler shouldalways be avoided, and it is used only when nooperating turbine is available for steam extraction.Table 1 gives year 2000 desalted water outputby all MSF units operating in 4 CPDP plants(Azzour South, Doha West, Doha East, andShuwaiba), and Shuwaikh plant (a plant producingdesalted water only) in Kuwait. The table givesthe hours and desalted water production when theMSF units were supplied with steam directly fromthe boiler, or boiler driven BD, beside total pro-duction, Tin m3 and million imperial gallons, MIG.

The table shows that out of 375.167~ 1O6m3total produced desalted water, 73.367~ 1O6m3 or19.6% were produced by using throttled steamdirectly from boiler, and the balance (301.8x 1O6m3or 80.4%) by extracted steam. Then, the averageconsumed energy is 240.7 MJ/m3 fuel energy or25.74 kWh/m3 equivalent work. This is certainlyhigh figure when compared with 5-7.5 kWhlm3consumed by reverse osmosis seawater desaltingsystem, the main competitor to the MSF system.


4/10

86 MA. Danvish et al. /Desalination 152 (2002) 83-92

Table 1Kuwait MSF units desalted water production in the year 2000

Unit number T,MTG BD, MIG T, 1000 m3 BD, 1000 m3 BD, no. hoursAzzour South plant, 8 ST x 300 MW and 12 MSF x 7.2 MGDDID2 4783.0 1220.4 21762.7 5552.8 4088.0D3LD4 4047.4 1129.1 18415.7 5137.6 3763.8D5/D6 4275.6 1193.5 19454.0 5430.4 3978.2D7/D8 5169.0 704.4 23519.0 3205.0 2348.0D9110 4108.0 0.0 18691.4 0.0 0.0DllD12 4665.8 66.8 21229.4 303.9 222.5

27048.8 4314.2 123072.0 19629.8Doha West Plant, 8 ST x 300 MW, and 16 MSF x 7.2 MGDDl/D2 2864.8 395.5 13034.8 1799.5 1318.3D3lD4 2285.3 53.3 10398.1 242.5 147.2D5fD6 4276.0 574.2 19455.8 2612.6 1913.9D7iD8 4363.7 42.6 19854.8 193.8 142.1D9/10 3946.0 610.9 17954.3 2779.6 2036.4Dl l/D12 4790.0 415.3 21794.5 1889.6 1384.3D13/D14 4720.9 377.0 21480.1 1715.4 1256.7D15/D16 4102.7 765.1 18667.3 3481.2 2550.3

3 1349.4 3233.9 142639.8 14714.2Doha East Plant, 7 ST x 150 MW, and 7 MSF x 6 MGDDl 1673.7 737.7 7615.3 3356.5 2842.0D2 1818.5 503.5 8274.2 2290.9 1983.7D3 1614.1 123.3 7344.2 561.0 467.2D4 1893.5 279.9 8615.4 1273.5 1074.9D5 1758.7 202.3 8002.1 920.5 781.6D6 2022.0 711.7 9200.1 3238.2 2784.7D7 1954.8 792.3 8894.3 3605.0 3045.2

12735.3 3350.7 57945.6 15245.7Shuaiba Plant, 6 ST x 134 MW, and 6 MSF x 5 MGDDl 1345.7D2 1004.4D3 1166.2D4 1393.7D5 1273.4D6 1496.8

7680.2

479.1 6122.9 2179.9 2487.7171.2 4570.0 779.0 932.0129.1 5306.2 587.4 692.0430.8 6341.3 1960.1 2305.2126.2 5794.0 574.2 674.1244.3 6810.4 1111.6 1265.4

1580.7 34944.9 7192.2


5/10

MA . Danv ish et al. /Desalinat ion 152 (2002) 83-92 87

Table 1, continuedUnit number T,MIG BD, MIG T, 1000 m3 BD, 1000 m3 BD, no. hoursShuweaikh Plant, 3 MSF x 6 MGDDl 1091.9 1091.9 4968.1 4968.1 4317.5D2 1653.8 1653.8 7524.8 7524.8 6509.3D3 894.9 894.9 4071.8 4071.8 3526.8

3640.6 3640.6 16564.7 16564.7Overall total 82454.3 16120.1 375167.1 73346.6Total/d 225.9 44.2 1027.9 200.9Ratio % 100.0 19.6 100.0 19.6r, MIG: total production in million imperial gallons; BD, MIG: production in MIG when units are supplied directly fromboiler; r,lOOO m3: total production in thousands of cubic meters; BD production in IO3m3 when units are supplied directlyfrom boiler.

3. Possible addit ion of other desalting systems Umm Al Nar B, 3 x7.5 in Mirfa B, 4x 10 in Jebel Alito Kuwait K2, and 2x 10 in Jebel Ali Kl [3].

Desalted water is the main source of potablewater in Kuwait. So, it is vital to the chosendesalting system to be designed for long life withhigh degree of reliability. Forced outage of largeplants, even for short periods, creates seriousproblems due to the limited storage capacity ofdesalted water. The MSF system is preferredbecause of its high availability. The availability hereis defined by the annual service hours (hours of unitproduction) plus reserve shutdown hours (hoursof unit shutdown due to no production needs)divided by the number of hours per year, 8760.The availability of the MSF plants worldwide isquite high. For example, it reaches 86% in theyear 2000 for Doha West MSF units, althoughthe units were built more than 18 years ago. It is amature system with proven material selection andno moving parts except pumps of known reliability.

Recently other systems of proven reliability havebeen used in Gulf countries, such as multi effectboiling (MEB) of conventional and thermal vaporcompression (TVC) types, and reverse osmosisdesalting systems.

4. Reverse osmosis desalting system and itsenergy consumption

Although the MSF system is known for its highenergy consumption compared to other systems,more MSF units are still ordered and contractedfor in the Middle East especially for large capacityunits. Examples of some MSF units ordered orunder construction in the UAE within the last 3years are (given in numbers x capacity in MIGDeach): 4 x (10-12.5) in Taweela A2, 5x12.5 in

RO seawater desalting system is the maincompetitor of the MSF system. The RO systembecame more attractive by continuous improve-ments in membrane materials, raising of both feedpressure and temperature limits [4], productionof potable water from high salinity water in theGulf area in single stage, and using two stage brineto raise conversion ratio [5]. Another conceptapplied recently is to use two RO passes, the firstone is of high permeate flow with less saltrejection to produce permeate of 700-l 100 ppm,which is entirely used as feed to low energybrackish water membranes. The brackish watermembrane can produce salinity of 50 ppm [6].The product pressure of the first stage can beraised to 9 to 15 bar to go through a second productstage without pumping. The main advantages of


6/10

88 M.A. Dam&h et al. / Desalination 152 (2002) 83-92

the RO system, over the MSF system are:1. It consumes less energy, [mechanical energydelivered by motor(s)].2. No need to combine to power plant or tointerfere with its operation. In fact, it can beoperated only during non-peak power demandperiod.3. It has simple start/stop operation.4. It is delivered in modules, no need to shutoff the whole plant for emergency or routinemaintenance.The energy consumed by the RO system andits comparison with that of the MSF system are

presented here through an example of Jeddah-1RO plant phase II [7]. The plant consists of 10 trainsto produce 12.5 MIGD. Each train gives a productrate of 5680 m3/d (65.74 kg/s). It has a conversionratio CR (product/feed) = 0.35 with 43,300 mg/lfeed salinity. The power consumption is calculatedhere for one train of the plant.Since CR is 0.35, then the feed flow rate is65.74/0.35 = 187.83 kg/s. The plant actual feedpressure is close to 60 bar (maximum allowablepressure is 70 bar). Assuming the net efficiencyof the feed pump including its driving motor E,, is0.76,E,, = E,, x Emwhere Ep and Emare the pump and the motor effici-ency respectively. Then,Feed pump power consumption = @(in m3/s) x P(in kPa) / En= (187.83 / 1000) x (6000) / (0.76)= 1482.87 kW

Fig. 2. Two brine stage RO system.

By considering 20% more energy is consumedby other pumps (e.g. seawater supply, seawaterboost, and chemical dosing pumps), then theconsumed power = 1.2 x 1633.3 = 1779.44 kWTo calculate the energy that can be recoveredfrom the brine by a turbine, the brine flow rate= 187.83 - 65.74 = 122.09 kg/s.Brine pressure = feed pressure - pressure loss inthe feed-brine side = 56 barRecovered energy = brine flow rate (in m3/s) x P(in kPa) x EIwhere E, is the turbine efficiency. Et is assumedequal to 0.67 when the turbine is reversed centri-fugal pump; and 0.84-0.88 when it is an impulse(Pelton) wheel turbine.The recovered energy = (122.09/1000) x 5600 x0.84 = 574.3 I kWNet energy consumption = 1779.44 - 574.3 1 =1205.13 kWSpecific workdone = 1205.13/65.74 = 18.33 kJ/kg= 5.09 kWh/m3

It may be noticed here that in Al Fujaira plant,using higher feed salinity than Jeddah plant, andreversed centrifugal pump as aturbine, the measuredpower consumption is 6.54 kWh/m3 [8], but theguaranteed power consumption is 7.5 kWh/m3.This plant is in operation since 199 1.Major advances of spiral wound membranesby raising the feed pressure to 80-100 bar, andthe concentration of the rejected brine to 87 mg/l,open the door for two stage brine RO with highrecovery ratio [4]. This decreases the energyconsumption (operating cost) and capital invest-ment. This point is illustrated by the next example.Consider again one train of Jeddah 11plant as afirst brine stage with feed of 187.83 kg/s, con-version ratio, CR, (product/feed) = 0.35 with feedsalinity of 43,300 mg/l, feed pressure is around60 bar, product of 5680 m3/d (65.74 kg/s and theenergy pumping power consumption of 1779.44 kWincluding parasitic power consumption. The flow


7/10

M.A. Darwish et al. /Desalination 152 (2002) 83-92 89

rate of rejection from this stage is 122.09 kg/s with Caribbean (Curacao) came down to 3.15 kWh/m3salinityof4.43/(1 -0.35)=6.815g/l,andpressure (when energy for pretreatment was not taken intoof 59 bar. account [5].When this brine is introduced to brine secondstage by using a booster pump that raises its pressureto 72 bar, and conversion rate of 15%, the productof this stage is 18.3 14 kg/s, and the rejection flowrate is 103.78 kg/s with salinity of 8.02 g/l. Theenergy consumed by the booster pump that raisesthe feed to this stage from 59 bar to 73 bar (i.e.1300 kPa pressure difference) can be calculatedas before:

5. Conventional and thermal vapor compressionmulti effect boiling desalt ing system

Booster pump power consumption = (122.09 /1000) x 1300) / 0.76 = 208.84 kWThe energy of the second stage brine, say at71 bar, can be recovered by turbine as mentionedearly to give:Turbine energy recovered = (103.78/l 000) x(7100) x 0.84 = 618.94 kWThe net energy consumption = 1779.44 + 208.84 -618.94 = 1369.34 kWThe total product of the 2 stages = 65.74 + 18.3 14= 84.054 kg/sSpecific power consumption = 1369.34 / 84.054= 16.29 kJ/kg = 4.52 kWh/m3

Conventional multi effect boiling (MEB)system was the predominantly used method todesalt seawater before 1960 when the MSF systememerged and practically terminated its use forunits of high capacity. In that period, submergedtubes evaporators were used with no feed pre-heaters. These evaporators have limited movementof boiling water covering the heating tubes. Thiscauses low heat transfer coefficient and highscaling rate. Forward feed system was used wherefeed water and heating vapor to the effects flowin the same direction to insure low salt concen-tration at high temperature effects. This gives lowperformance ratio since good fraction of suppliedsteam is used to raise the temperature of the firsteffect feed to its boiling point. These problemswere solved in recent years by using falling filmevaporators, and regenerative feed heaters to raisethe first effect feed temperature close to boilingpoint.

This is 10% less than the case when a singlestage was used. The conversion rate is 84.054 /187.83 = 44.75%. It is noticed here the efficienciesused for pumps and turbine are conservativevalues. Pump efficiency reported by Martinho [9]is in the range of 80-86 %, and turbine is in therange 84-88%, usually Pelton wheel turbines. Theabove calculation is conservative and optimizationof the system can lead to higher conversion ratioand lower energy consumption. The specific fuelconsumed is 4.52 x 3.610.38 = 42.87 kJ/kg.

Recently, the MEB system became dominantwith unit capacity up to 5 MIGD, which waspreviously the domain of MSF technology in thelast 4 decades [9]. These units can work either asconventional or thermal vapor compression(TVC) mode.

These calculations indicate clearly that the ROis much more efficient energy wise than the MSFsystem where its least equivalent energy consump-tion in Kuwait is 22.5 kWh/m3. The reportedenergy consumption for 5700 m3/d SWRO in the

Examples of some MEB units ordered or underconstruction in the UAE within the last 3 yearsare (given in numbers x capacity in MIGD each):14x3.7 in Taweela Al, 2x3.5 in Umm AI Nar 9and 10, and 2x5 in Layyah. The striking featureof these MEB is its low top brine temperature, inthe range of 60C.When combined with turbines to extract steamof low pressure at saturation temperature close to70C the units operate as conventional MEB.When the extracted steam is available at a 3-bar


8/10

90 MA. Danvish et al. /Desalination 152 (2002) 83-92

pressure, they can operate as a TVC system. Whenoperated directly by a boiler, and not combinedto turbine, they wok as a TVC system with steampressure of 20-30 bar.In the early 1990, Sidem Company, Francebuilt 4 TVC units of one MIGD each in UnitedArab Emirates with gain ratio of 8 [7]. Anotherplant of 12000 m3/d was reported by Temstet andLaborie in 1995 [lo] with a gain ratio of 13.4 whenoperated as a TVC plant with 2.5 bar motivesteam. The company claims that the unit canoperate as conventional multi effect boiling MEBdesalting unit with steam supplied at 0.32 bar, andhas a gain ratio equal to 9.8. This plant is takenhere as a reference to be considered for combinationwith Azzour West plant with layout shown inFig. 1.6. Energy consumed by conventional MEBsystem

The conventional multi effect system is verywell known. The reference plant (see Figs. 3a,b)is used here to show how the energy consumedis calculated by the same method used for the MSFsystem. The plant capacity is 2.64 MIGD, andwhen combined with Azzour-South plant, shownin Fig. 1, the steam can be extracted at the samecondition of the lowest feed heater. The thermalenergy con-sumed by the system according to thedesign is 5 1 t/h (14.1667 kg/s) steam of 2498 kJikgenthalpy and 0.3 bar and leaving condensate of366.8 kJ/kg enthalpy. The distillate output is500 t/h (138.89 kg/s). This gives specific thermalenergy of 2 17.4 kJ/kg distillate. When combinedto Azzour-South plant, the extracted steam is at0.416 bar, 2850 kJ/kg enthalpy and the enthalpyof the leaving condensate is 270 kJ/kg. The specificthermal energy of 2 17.4 kJ/kg gives a gain ratioof 10.6, and extracted steam flow rate of 13.1 kg/s.The pumping energy is almost the same as a TVCsystem, and is equal to 7.2 kJ/kg. The equivalentmechanical energy is calculated by the enthalpydifference of steam extracted to the unit (at thecondition to the lowest pressure feed heater of

2580 kJ/kg enthalpy), and at the condenser inlet(2346.5 kJikg). So, the work loss due to the extrac-tion of 1 kg of steam extracted to the conventionalMEB is 233.5 (= 2580 - 2346.5)kJ/kg steam. Sincethe gain ratio is 10.6, then the specific equivalentmechanical work is 22.03 kJ/kg. By adding7.2 kJ/kg pumping energy, the specific total equi-valent work is 29.23 kJ/kg distillate (8.14 kWh/m3).This is comparable the very efficient reverse osmosissystem that consumes on the average 6.5 kWh/m3(23.4 kJ/kg product).7. Energy consumed by a TVC desalting plant

The flow sheet of the plant when operated ina TVC mode is given in Fig. 3. The plant has12 effects with maximum and minimum brinetemperatures of 63 and 37.5C respectively.Saturation temperatures, ST, of vapor generatedin the first effect and last effects are 62.2 and36.5C respectively. This gives ST differencebetween heating and generated vapor DT equalto 2.3-2.4C in each effect. By taking an averageboiling point elevation E equal to lC, then theheat transfer temperature difference in each effectisAT= DT-E = 1.3C

This is a really low value, showing that largeand costly heat transfer area is used.The equivalent work consumed by this systemcan be calculated as before. There are two caseshere. The first is the conditions of steam supplygiven by the manufacturer, and the second is thecondition applied when combined with Azzour-South plant. The calculations with the later casewere done before [2], and are reported here.When the unit is combined with Azzour plant,it receives a motive steam at P, = 3.5 bar (higherthan Ps used by manufacturer). The last effectpressure is Pe= 6.45 kPa (ST = 37.5Q and thesteam ejector delivery pressure is P,= 24.5 kPa(ST = 64.5C). So the compression ratio is PJPr= 3.8, and the expansion ratio P/P,= 54.3. Thisgives y/Dr (motive steam to compressed vapor


9/10

M.A. Danvish et al. / Desalination lS2 (2002) 83-92 91

P=6.0 BAR GT=230.0 OCF=2.5 T/H

MAKE UPF=1644.0 T/HT=35.0 OCS=36.0 G/KG

HEATINGSTEAMP=O.31 BARF=51 .O T/HH=2496.0 KJlKG

F= 137.0 T/H F=137.0 T/H F=l37.0 T/H F=l37.0 T/HT=62.0 C T=56.2 C T=35.0 OC T=35.0 C REJECTF=3106.0 T/H

SEA WATERF=4750.0 T/HT=29.5 CS=36.0 G/KG

CONDENSATERETURNF=51 T/HT=64C

HIGA PUIRTYF=O.O T/H TO 20 T/H BRAIN BLOWDOWNF=1144.0 T/H

T=36.5 CS=51.73 G/KG

OUTPUTF=500.0 T/H(if high purity 0 T/H)Tc=36.0 OC

Fig. 3a. Multi effect boiling MEB operating with thermal vapor compression mode.

STEAMP=l.5 BAR GT=155.5CF=37.3 T/H

STEAMP=6.0 BAR GT=230.0 CF =2.5 T/H

F=l37.0 T/HT=34.5 =C

MAKE UPF=l644.0 T/H

_ T=34.5 CS=36.0 G/KGF=137.0 T/-lT= 34.5 C REJECT

SEA WATERF=4100.0 T/HT=29.5 OCS=36.0 G/KG

RETURNF=46.5 TMT=64.5 C

HIGH PUIRTYF=O.O TM TO 20 T/H BRAIN BLOW DOWN OUTPUTF=1144.0T/H F=490.6 T/H

T=37.5 C (if hiah ~uritv 0 T/H)S=51.7 GIKG ic=36.6 OC-

Fig. 3b. Multi effect boiling MEB operating with conventional mode.


10/10

92 M.A. Danvish et al. /Desalination 152 (2002) 83-92

ratio) equal to 2.4. The motive steam is extractedbetween the IP and LP cylinders. The suppliedsteam MS required to produce the same capacity(138.89 kg/s distillate) is 32.82 t/h (9.117 kg/s),and the gain ratio would be 14.9. Then the thermalenergy consumption is Q,= 9.117 (2944.2 - 270)= 24,38 1 kW, and specific thermal energy is178.8 kJ/kg.

This motive steam would give 560.5 kJ/kgwork if it was expanded in the LP turbine to thecondenser. Then the specific mechanical equivalentwork (for thermal energy) is 560.5/14.9 =37.6 kJ/kg. By adding 7.2 kJ/kg pumping energy,the specific total equivalent work is 44.8 kJ/kg(or 12.44 kWh/m3).

8. ConclusionsMore desalting units are urgently needed for

Kuwait to satisfy the growing consumption ofpotable water. The only used system to desaltseawater is MSF desalting system. The energyconsumed by the MSF system in Kuwait, based onoperation records, reached more than 25 kWh/m3.It was noticed that in the year 2000, 19.6% ofdesalted water was produced by steam supplieddirectly from boilers with consumed equivalentwork of 40 kWh/m3, while for the balance 83.4%production, the steam was extracted from turbineswith consumed work equivalent of 22.26 kWh/m3.

The RO and MEB with conventional or TVCoperations are much more efficient, energy-wisethan the presently used MSF system. The newdevelopments in RO membranes and brine stagingcan bring the consumed work to 5 kWh/m3. Thesteam supplied to the MEB has lower availability,and its pumping work is half that consumed bythe MSF system. Moreover, the capital cost anddelivery time of both RO and MEB are less thanthose of the MSF system. The delivery time may

be crucial to Kuwait where the present installeddesalting capacity is almost equal the con-sumption and the demand continues to increase.The RO and MEB systems should be consideredseriously by the Ministry of Electricity and Waterin their decision to install new units.

References111P I

[31

[41

PI

PI

[71

PI

[91

PO1

Statistical Year Book on electric energy, Ministry ofElectricity and Water, State of Kuwait, 200 1.M.A. Darwish, On electric power and desalted waterproduction in Kuwait, Desalination, 138 (200 1) 183-190.C. Sommariva and V.S.N. Syambabu, Increase inwater production in UAE, Desalination, 138 (2001)173-179.M. Kurihara, H. Yamamura,T. Nakanishi and S. Jinno,Operation ad reliability of high recovery seawaterdesalination technologies by brine conversion twostages RO desalination system, Desalination, 138(2001) 191-199.A. Grundisch and B.P. Schneider, Optimizing energyconsumption in SWRO systems with brineconcentrators, Desalination, 138 (2001) 223-229.J.A. Redondo and IV. Lanzarote, A new concept fortwo pass SWRO at low O&M cost using the newFILMTEC SW30-380, Desalination, 138 (2001) 23 l-236.A. Al-Badawi, Operation and analysis of Jeddah I,phase II plant. Proc. IDA World Congress on Desali-nation and Water Science, Abu Dhabi, 3 (1995) 41-53.M. Shierach, Seawater plant in Al Fujirah, UAE. Proc.IDA World Congress on Desalination and WaterScience, Abu Dhabi, 3 (1995) 99-107.A. Martinho, The high pressure pump train on reverseosmosis plants. Experience and current trends.Desalination, 138 (2001) 219-222.C. Temstet and J. Laborie, Dual purpose desalinationplant - high efficiency multi effect evaporatoroperating with turbine for power production. Proc.IDA World Congress on Desalination and WaterScience, Abu Dhabi, 3 (1995) 297-308.

Documents

Desal Methods- Case Study for Kuwait