Low temperature, multi-effect distillation for cogeneration yielding the most efficient sea water desalination system

Desalination, 84 (1991) 85-101 Elsevier Science Publishers B .V., Amsterdam

LOW TEMPERATURE, MULTI-EFFECT DISTILLATION FOR COGENERATION YIELDING THE MOST EFFICIENT SEA WATER DESALINATION SYSTEM

A . OPHIR, IDE Technologies Ltd ., P .O . Box

591,

Raanana, 43104, Israel

ABSTRACT

IDE Technologies Ltd . (IDE) has commissioned in March 1988 and in July 1990, two 10,000 tons per day low temperature multi-effect distillation (LT-MED) plants each linked to a low pressure 3 .1 MW turbine (as a replacement to an aging MSF plant), demonstrating a total specific energy consumption of below 5 .0 kWh per ton of desalinated water .

During the last four years IDE has also commissioned three smaller LT-MED plants, 1200 to 2200 t/d, utilizing exhaust waste heat of diesel generator power stations with a total net specific power consumption of only 2 .5 kWh per ton of desalinated water . This paper describes the process principles of the LT-MED plants that made these achievements possible . Also, a description of the Curacao, 2x10,000 t/d and the Ebeye, 1,100 t/d MED plants will be given .

1 . INTRODUCTION

The LT-MED Distillation process supplied by IDE has been described in various previous papers (Ref . 1), nevertheless it is essential to emphasize and clarify the technological principles that have resulted in achieving such maximum economies in total power consumption as 5 kWh/ton whilst using prime energy and only 2 .5 kWh/ton whilst utilizing waste heat . 85



86 These principles are as follows : 1 . Development of a unique design of a falling film horizontal tube MED effect (evaporator/condenser) with high heat transfer coefficient), superior thermodynamic efficiency (compared to MSF) and very low pressure drops at high volumetric vapor flows, as are encountered at low temperature (70°C to 30°C) operation . This development enabled the optimal utilization of the process for operation at maximum brine temperatures of 70°C for sea water temperatures of up to 32°C . 2 . The low temperature operation has made possible the utilization of economical and durable construction materials for the LT-MED process such as : Aluminium Alloy for heat transfer tubes . • Polypropylene plastic piping for most liquid conduits . • Epoxy painted steel for evaporator shells . 3 . The economical use of aluminium tubes for heat transfer as compared with copper alloy tubes which are essential at higher temperature plants (used by other distillation manufacturers) enables more than double the heat transfer area per ton of produced water in the desalination plant for the same investment costs . 4 . The significant increase in heat transfer area in addition to the thermodynamic superiority of MED over the MSF process results in a very low temperature drop per effect (1 .5 - 3°C) enabling the incorporation of a large number of effects (10 to 16) even with a low maximum temperature such as 70°C, consequently resulting in very high economy ratios (product to steam) . The above mentioned technological principles result in : a . minimal power consumption (as will be shown in the following examples) b . minimizing problems of scaling and corrosion, both being significantly reduced due to low temperature operation . Consequently the LT-MED commercial plants that have emerged are characterized by very low water cost and very durable long life, trouble free operation .

87 2 . CURACAO 2x10,000ton/dayPLANT 2 .1 GeneralDescription Curacao's Kompania Di Awa I Elektrisidat Di Korsou (KAE), the contracted to replace its water and power company of Curacao, older and less efficient MSF plants with two 10,000 m3 /day low temperature, horizontal tube, falling film, LT-MED plants . The LT-MED plants operate at a top brine temperature of 70°C and require steam at only 0 .34 bar a . However, motive steam is available, as extraction from the power station's turbines, at 2 .55 bar a, 155°C, as required by the higher temperature MSF plant . This excess pressure potential is used to generate 3 .1 MW of power in an auxiliary low pressure steam turbo-generator, thereby improving overall economy and lowering desalted water cost to a level unequalled by any other system . The paper describes the power station turbine, auxiliary low pressure turbo generator and LT-MED plant combined scheme, its operational features and economics . 2 .1 .1 SiteHistory Desalination started in Curacao in 1928, when the first submerged tube MED 60 m 3 /day plant was installed . This was followed by eleven 240 m 3 / day .plants of the same type, installed between the years 1929 to 1948 . In 1959 the first dual purpose plant was installed consisting of two 2000 m 3 /day submerged tube MED units coupled to two 3 .78 MW turbo generators . These five MED plants required steam at 30 psig (3 bar a) . This was followed by seven MSF plants, two 6000 m 3 /day units in 1963, two 4000 m 3 /day units in 1970/71, two-8500 m3/day units in 1977/78 and one 10,000 M3 /day unit in 1984 . Of these, only the first two MSF units were installed together with two new turbo-generators of 7 .6 MW each . The remaining extensions to the power plant two gas-turbines of 14 .9 and 20 .8 MW in 1973 and 1979 respectively and two 25 MW turbo-generators in 1981 and 1982 were installed not in conjunction with any particular desalination plants . The average consumption is about 40,000 m3 /day . The anticipated rate of growth is approximately 1% to 2% per year . 2 .1 .2 Recent Installations For their current expansion KAE selected two (2) 12 effect LT-MED, 10,000 m 3 /day units . As mentioned before, each plant includes a supplementary turbine through which the required 48

88 tons/hr of the available 2 .55 bar a steam will be expanded to 0 .34 bar a to generate 3 .1 MW before passing out to the evaporator . For additional flexibility, each LT-MED plant is supplied with a thermo-compressor and the required connections so that the plant can be operated directly with the 2 .55 bar a extraction steam using a vapour recycle system (MEVC) . In this mode of operation, the quantity of motive steam required is reduced from 48 tons/hr to 33 .25 tons/hr, achieving an economy ratio (ER) of 12 .5 . The control system of the plant has been so designed that it is possible to easily and swiftly transfer from one mode of operation to the other . The transfer from the turbine mode of operation to that with vapour recycle can be carried out in minutes - the time required for the automatic closing and opening of the appropriate steam inlet and discharge valves . The limiting factor, when transferring from the vapour recycle to the turbine mode is the start-up of the turbine .

2 .2 Process Description The LT-MED evaporation is similar to the other MED plants supplied by our company and has been described in various previous papers (Ref .1) . One new feature used in the design of this plant to suit the dual mode of operation is that the design of the first six hot effects is different to that of the second six cold effects . To accommodate the additional amount of vapour that has to be initially handled, when operating is the vapour recycle mode, more and longer tubes, but with a slightly smaller diameter are used in the first six hot effects . In addition, as specified by the owners, standby pumps and NCG removal systems have been installed . Figures 1 and 2 illustrate the flow diagram and layout of the plant . The supplementary turbine selected is a multi-stage type condensing turbine housed in a single casing . The first effect of the evaporator acts as the condenser for the turbine . Due to possible presence of hydrozine, used in the boiler feed treatment in the motive steam, all of the condensate from the first effect is returned as boiler feed even during the vapour recycle mode of operation when the amount of condensate exceeds the amount of motive steam .

89 Though the turbine control system allows it to be operated for that to independent or parallel operation it has been agreed the maximum benefit from the scheme, the turbine will be obtain operated as a base load supply at its rated output . However the desalination plant operates efficiently to well below fifty the turbine percent of its rated capacity and is able to follow loading automatically so as not to provide any limitation to the turbine operation . 2 .3 PerformanceEvaluation In assessing the performance of the different modes of operation, MED with power generation or MEVC (which evaluation would be identical for an MSF plant with an equivelent steam economy), one faces the difficulty of finding a common footing on which to base the comparison . For this assessment, the performance curves supplied by the turbine manufacturer reproduced in figures 3 and 4 will be used . These give steam and fuel requirements as a function of power output with extraction steam quantity as an additional parameter . The comparison of the two processes will be made by comparing the specific power consumption for both cases based on the same fuel consumption of the turbine . To determine at what power output to make the comparison, we will assume that there is no loss of power generating capacity as is generally the case, and express the loss of power generated due to the extraction of the steam as the equivalent power requirement in kWh/m 3 . From Fig . 3 we find that when generating 25 MW with no extraction steam, the power loss is approximately 4 .3 MW when 48 tons/hr of steam is extracted . From this 4 .3 MW must'be deducted the 3 .1 MW generated by the supplementary turbine . Hence the net loss is 1 .2 MW . Attributing this loss to the 10,000 m 3 /day water produced, we have as an equivalent power consumption of 2 .9 kWh/m 3 . To this figure must be added the pumping energy for the desalination plant . The total equivalent energy consumption is then 5 .0 kWh/m 3 . Actually the total consumption of the desalination plant is lower because 0 .5 kWh/m 3 of this energy should be attributed to the pumping energy that would otherwise be required for condensation of the 48 tons/hr extraction steam in the turbine condenser .





7 a

SEA WATER

_ MCG R-rMCVAL SYSTEM

TRI NE a7lCE TAMK

PRODUCT PUMP

PRODUCT SURGE TANK

BRU.E PUMP

I

CGGI.AMT PUMP

Figure 1 .

I

7

-

11

6

MAIN EACTCR

BRINE FLASH CHAMBERS

1ST . FEED 'A'

GENERATCRI

4

INT. PEE) 'S'

5

-

)I

3

Principle flow diagram of med 10.000 M 3/day. KAE

PRODUCT PLASH CHAMBERS

1 1 1 1 1 1 1 1 1

PEED PUw

MO7TIVE STEAM

TURBINE

CZHOENSATE

COHDi'SAtC SURGE TANK

P~U ..P





I

(D

~~

!a

VORKING

AIR HIGH PURITY DIST . 1'

INT . FEED B PUMPS

T

CONDENSATE OUT a A'

l-- -

LOV PRESSURE STEAM a 16'

HIGH PURITY MUMS

CONDENSATE PUMPS

INT .FEED C PUMPS


FEED TREATMENT STATION

~"

NIB

/

FEED PUMPS

-=-

~II

II

11111115

(D

-

-giv-

.I

DESALINATION UNIT

SEA VATER a2A'

BRINE 6 COOLANT OUT a 38 -

no

CONDENSER

1

/ COOLANT PUMPS

BRINE PUMPS

PRODUCT PUMPS

NIGH PRESSURE STEAM a 4'

Figure 2. Plant layout of med . 10.000 M 3/day. KAE . View A

INSTRUMENT AIR

ACID CLEAN STATION TT'9

,All

I

INT .FEED A PUMPS

PRODUCT PIPE a 12'



92

Fig . 2 .

PLANT LAYOUT OF LIED 10 .000 M3/DAY . KAE . YIEV B

.100 1111

2 .5

NO N

2

501111

0O

25 till 01111

17 d

N M

Y

C 0 . 0

C 00

v

5

10

15

20

25 (Mw)

Output at generator terminal Fig . 3 . PERFORMANCE DIAGRAM OF STEAM TURBINE . ELECTRICAL OUTPUT AT ALTERNATOR TERMINALS AS A FUNCTION OF STEAM CONSUMPTION 111T11 PASS-OUT QUANTITY AS A PARAMETER .

93

Fig . 4 .

PERFORMANCE CURVES

3 . EBEYE1,100ton/dayPLANT 3 .1 GeneralDescription The Ebeye (Marshall Islands) sea water desalination project produces 300,000 US GPD (1100 m3 /day) of pure distilled water utili .ing as its prime energy input the waste heat discharge from an adjacent disesel generator station, operating at an average The only other energy requirement is electric load of 3 .2 MW . power for process, heat recovery and sea water supply pumping at about 9 .5 kWh/1000 gal . (2 .5 kWh/ m3 ) . The desalination plant consists of one 12 effect low temperature top brine multi-effect distillation unit, operating at a temperature of 158°F (70°C) with simple polyphosphate feed pretreatment . The diesel generator station consists of two 2 .4 MW diesels, with provision for the addition of a third as demand increases . Heat is recovered from the diesel's exhaust gases, jacket cooling water, lube oil and compressed air after coolers .

94 3 .1 .1 SiteHistory One of the many coral islands forming part of the Republic of the Marshall Islands is Kwajalein Atoll,- portions of which are leased to the United States Defense Department . The resulting need to relocate Marshallese from these leased islands let to dramatic population increases on Ebeye Island which now has the highest population density in the Pacific . Whereas similar Islands support a population of only several hundreds, Ebeye is home to approximately eight thousand five hundred people . This rapid population growth led to serious problems in the supply of power and water . Power was undependable with frequent outages, some lasting several days, which caused damage to commercial and private electrical equipment .'Water was in short supply and frequently contaminated . The principle source was from individual roof cathments which fed private cisterns linked to the Island's distribution system . Contamination of one cistern could thus affect the entire system . As a supplement, water was barged from a neighbouring Island where a military installation was located . In 1982 the Kwajalein Atoll Development Authority (aka KADA) was formed to oversee the development of the non-military Islands in the Kwajalein Atoll . To this end International Bridge Corporation (IBC), then engaged in construction activities and consulting work for the authority, was commissioned to evaluate various methods of increasing water supply and ensuring its year round availability . However, projections as to reservoir capacity based on catchment area were rendered somewhat irrelevant when the Pacific Basin was struck by drought in late 1984 and early 1985 . The resultant water shortage convinced both the members of the Authority and IBC officials that a source other than catchment was necessary if the random acts of nature were not to inflict great suffering on the population of Ebeye . IBC then examined sea water desalination in its many aspects . Single purpose systems such as Reverse Osmosis, Vapour Compression and Multi-stage Flash were evaluated but problems with small reverse osmosis plants in other areas of the Marshall Islands and the cost of water from such single purpose units caused attention to be focused on co-generation possibilites and the utilization of waste heat from the proposed new diesel power plant . The potential cost savings of such configurations led KADA to authorize a detailed evaluation of a waste heat driven

95 desalination facility to be constructed jointly with the new Power Plant . Given the decision to explore desalination driven by waster heat, the capabilities of many systems and manufacturers were reviewed . Based on the heat available from the engines in meeting the Ebeye electrical load, and the water requirements the design proposed by IDE Technologies Ltd . (IDE) was chosen as best suited to the needs .

3 .1 .2 Plant Description The turnkey project consisted of a plant for the cogeneration of electrical power and potable water including wells for sea water supply, all civil works including the building housing the diesel generators and all auxiliary items, including the waste heat recovery equipment (waste heat boilers, heat exchangers) and tie-ins to the water and power distribution networks . The plant is installed on the sea shore and adjacent to the existing power station and rain water catchment area . Power Plant The new Power Plant consists of two model DSR-46 diesel engines manufactured by the Enterprise Engine division of Trans-America Delaval each coupled to a Brush Electrical Machines generator . The engines are rated to produce 3656 Hp at 450 rpm using either diesel or heavy (1700 redwood) fuel and the generators are rated at 3254 Kva at 0 .8 power factor, 2603 Kw, 13 .8 Kv . The generators are brushless with rotating exciters and the excititation and voltage regulation gear are also supplied by Brush . A foundation and extension to the main switch gear have been installed to accept a third unit when demand requires . The piping system was designed to resist corrosion . The salt water system utilizes fiberglass and P .V .C . products .

Heat Recovery Equipment Engine waste heat is recovered from the economiser, the jacket water cooling system, the lube oil cooling system and the aftercooler . This is done by means of titanium plate type heat exchangers . These provide extreme corrosion resistance and allow very close approach temperatures for efficient recovery of the relatively low grade waste heat .

96 gases from the engines are ducted to two "Vapor The exhaust Phase" waste heat boilers manufactured by Engineering Control, Inc . The boilers can each produce 4,700 lb/hr or 50 p .s .i . steam with the diesel operating at 100% capacity . They are fitted with the thermal efficiency of the system . economisers to increase Figure 5 illustrates the heat flow diagram for the diesel engine system . DesalinationEquipment The desalination plant is a low temperature, horizontal tube, falling film multi effect distillation (MED) unit supplied by IDE Technologies Ltd . (IDE) . The unit consists of a series of 7 pre-assembled modular elements, six housing two heat recovery evaporator-condenser effects, and the seventh, a heat rejection condenser and a flash chamber . These modular elements, assembled in-line, are mounted on concrete foundations . Installed adjacent to the unit are all the process pumps, piping, valves, steam jet ejectors and field located instruments . The desalination unit is operated from individual control panels located in the Cogeneration Plant's central control room . 3 .2 ProcessDescription Sea water is supplied to the plant from two 1800 gpm salt wells drilled to a depth of 75 feet adjacent to the plant .

water

The sea water is fed into the heat rejection condenser where it at absorbs the heat rejected from the process and is deaerated the same time . The outgoing stream, which has now been heated by about 10° C is split into two streams . One of while a small and is effects

the streams is returned to the sea as coolant discharge the other becomes the feed to the unit . It is treated with to inhibit scaling quantity of polysphosphate additive fed into the lower temperature group of heat recovery . The flow diagram is illustrated in Fig . 6 .

SpecialFeatures the Ebeye While the plant is a standard IDE LT-MED unit, installation is unique in its utilization of low grade waste heat for motive energy . Whereas desalination plants have been driven







PRODUCT

BRINE

COOLING OUT

UNIT - MEVC

DESALINATION

I

Fig . 6

vx BOILER

FEVC-DIESEL FLOP-DIAGRAI

MEVC-DIESEL SYSTEM

STEAM JET THERMOCOMPRESSOR

COOLING VATER

RECYCLE VAPOUR

STEAM AT 8 ATA

J

I

EXHAUST GASES

COOLING VATER HEAT EX.

A "

AIR COOLER

DIESEL ENGINE

LUB . OIL COOLER

Fig . 5

or

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MEVC-DIESEL HEAT FLOW DIAGRAM

COOLING WATER

W BmCATING OIL

COOLING WATER

CYLINDER HEADS JACKET

COOLING WATER FOR CHARGING AIR COOLER

'A

FUEL OIL

CHARGING AIR

99 by steam from waste heat boilers in the past, in this case, all the possible waste heat streams from the diesel engines are utilized from the low temperature turbocharger aftercoolers to the high temperature exhaust gases . While this capability required an additional investment in the heat recovery equipment this was easily compensated by the essentially free energy for the water plant in the future . Another special feature of the installation, which is inherent in the basic design of the MED unit, is its capability to "follow" the power station load without any instability in the process . While the plant has been designed to produce 300,000 GPD when the diesels are producing 2 .4 MW each, initially only one engine runs at a load varying between 60% and 80% of its rated capacity (1 .44 to 1 .92 MW) and the plant performs satisfactorily at this partial load . With only one diesel operating at 60% load the plant is in fact producing at only 30% of its rated capacity, an indication of how flexible the load following capacity is . The general procedure for the estimation of water capacity for different diesel engines is attached in Appendix A .

3 .3 Performance Evaluation Schedule The time span of the project was set at two years . Equipment purchase orders were placed in mid 1984 . The desalination plant was installed during May and June of 1986 but as the power station was not yet complete the plant could not be commissioned then . Final assembly and acceptance testing took place in January and early February, 1987 .

Performance Test Results The 72 hour acceptance test run was held between the 5th and 8th of February . The following table and figure shows the warranted figures, the actual test results and the variance .

loo TABLE The values are for a diesel generator output of 1 .8 mw .

GUARANTEED VALUE ACTUAL VALUE

ITEM PRODUCT (GPD) WATER QUALITY (ppm TDS)

140,000

166,900

50

25

VARIANCE 19 .2% gain

100% gain

During the test period power station load varied from 1360 kw to 1890 kw and hourly production from 18 cu .m/hr to 31 cu .m/hr (equivalent to 115,000 gpd and 187,500 gpd respectively) . The production rate over the whole range of power loads exceeded the guaranteed figures in each case .

REFERENCE

- A 1 . D . Hoffman, Low Temperature Distillatiuon Plants Comparison with Sea Water Reverse Osmosis, Proceeds N W S I A Washington D .C ., May 31/June 4, 1981

APPENDIX1

USEOFTHEDIAGRAM The quantity and temperature of the exhaust gas leaving the diesel generator are usally known . If the exhaust gar rate of the engine Is not known, an approximation can be made from the engine type and power, from the diagram on the right . The equivalent generated steam rate Is the amount of heat recovered from the exhaust gases and cooling water with an outlet waste heat boiler of 2000C ., as the standard arrangement . (See special arrangement on request) .



10 1 The total water production rate can be obtained from the diagram below for several equipment combinations . Small capacity plants require low initial investment, and large capacity plants will obtain lower operating costs . Example : Given : a 2 stroke 15 .5 MW engine exhaust-gas temperature

with

320°C outlet

The exhaust-gas flow weight

1)10 .000 Kg/hr

The equivalent generated steam

9 .000 Kg/hr

The MEVC-Diesel water production with a D204 - 7 effects

1,1300 m 3 /day

ENGINE OUTPUT XI0 3 Kw

30 2 STROKE ENGIN=

25

too

4 STRO E ENGINI

2C0

2,0

EXIIAIIST-GAS FLOW WEIGHT X10 3 (Kq/hr) EQUIVALENT STEAM (Ton/hr)

EQUIVALENT STEAM (ton/hr)

MEVC-D204

40 01910-ima Pro WAN

\1 = 10

- 1.50°C 400°C __ 3500C

MEVC-0202 w

=

_--300°G EXHAUST-GAS DIESEL TEMPERATURE

W

500 1100

010

3000

WATER PRODUCTION RATE

4000

(CUM/Day)

5000

50

100

150

200

EXHAUST-GAS FLOW WEIGHT X10 3

250