advanced med process for most economical sea water desalination

Advanced MED process for most economical sea waterdesalination

A. Ophir*, F. Lokiec

IDE Technologies Ltd.POB 5016, Hamatechet Street, Hasharon Industrial Park, kadima 60920, IsraelTel. þ972 9 892 9740/892 9777; Fax þ972 9 892 9715; email: [email protected]

Received 8 February 2005; accepted 21 February 2005

Abstract

The low temperature horizontal tube multi-effect desalination (MED) process is thermodynamically the

most efficient of all thermal distillation processes.

A comprehensive multi-disciplinary development and design approach resulted in prevention of corrosion

and scale formation on the plant’s heat transfer surfaces. It also allows the successful and most economical use

of aluminum alloys for heat transfer tubes, as well as carbon steel epoxy coated shells for the evaporator body.

The ability to use economically low grade heat, such as waste heat, exhaust steam from power station

turbines as the primary heat source for MED, yields very low specific energy costs for sea water desalination.

Recent developments of very economical low temperature deep pool nuclear heat reactors, when acting as the

primary energy source for large MED plants, yield very low specific desalination energy costs

The combination of economical specific MED plant costs with low energy cost, together with the inherent

durability of low temperature MED avoiding the necessity of comprehensive sea water pretreatment (such as

with RO plants) make the MED process one of the best candidates for safe and durable large capacity

economical desalination options.

This paper describes the design principles and various energy considerations that result in this uniquely

economical MED process and plant. It also provides an overview of various cases of waste heat utilization, and

cogeneration MED plants operating for many years.

Keywords: Sea water desalination, Multi-effects, Thermal processes, Low grade heat, System approach,

Prevention of scale and corrosion, Nuclear heat reactors

Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005.

European Desalination Society.

0011-9164/05/$– See front matter � 2005 Elsevier B.V. All rights reserved

*Corresponding author.

Desalination 182 (2005) 187–198

doi:10.1016/j.desal.2005.02.026

1. Introduction

Low Temperature Multi Effect Distillation(LT-MED) is the most efficient thermal desa-lination processes currently in use. It incorpo-rates technological advances which haveresulted in reliable, durable and economicaldesalination plants producing high purityproduct water.

These advances are as follows:1. Development of a unique design of a fall-ing film horizontal tube evaporator/conden-ser with high heat transfer coefficient,utilizing only latent-heat transfer, avoidingsensible heat pick-up.2. Superior thermodynamic efficiency andvery low pressure drops at high volumetricvapor flows, as prevailing in low temperatureoperation. This enabled the optimization ofthe process for operation at a maximum brinetemperature of 70�C.3. The low temperature operation aided by acomprehensive multi-disciplinary develop-ment and design approach has made possiblethe utilization of economical and durablematerials of construction such as aluminumalloy for heat transfer tubes, plastic processpiping and epoxy-painted carbon steel shellswhich show a better resistance when matchedwith aluminum alloy or titanium.4. The economy of using aluminum tubes forheat transfer as compared with copper alloytubes, which are essential for higher temperatureplants (used by other distillationmanufacturers),enables the increase of the heat transfer area perton of water produced in the desalination plantfor the same investment costs.5. The significant increase in heat transferarea, in addition to the thermodynamicsuperiority of MED over the MSF process,results in a very low temperature drop pereffect (1.5–2.5�C), enabling the incorporationof a large number of effects (10–16) even witha maximum brine temperature as low as

70�C, consequently resulting in very higheconomy ratios (product to steam).6. Possibility of using low-cost/low-grade heatavailable through cogeneration schemes tominimize the energy cost component.7. Minimal requirements for intake and pre-treatment systems.

The practical experience with commercialplants using the above mentioned advanceshas shown the remarkable stability, flexibilityand reliability of the low temperature pro-cess in comparison with others. Continuousresearch and development improved furtherthe advantages of the low temperature pro-cess by increasing the unit’s capacities;decreasing the energy consumption; and low-ering levels of scaling and corrosion, bothbeing significantly reduced due the low tem-perature operation.

2. Brief description of the MED process

MED plants utilize horizontal tube, fall-ing-film evaporative condensers in a serialarrangement, to produce through repetitivesteps of evaporation and condensation, eachat a lower temperature and pressure, a multi-ple quantity of distillate from a given quan-tity of low grade input steam.

Any number of evaporative condensers(effects) may be incorporated in the plants’heat recovery sections, depending on the tem-perature and costs of the available low gradeheat and the optimal trade-off point betweeninvestment and steam economy. Technicallythe number of effects is limited only by thetemperature difference between the steam andseawater inlet temperatures (defining the hotand cold ends of the unit) and the minimumtemperature differential allowed on eacheffect.

The low temperature differential allowanceon each evaporator (effect) in the train allowsa large number of effects to be utilized while

188 A. Ophir, F. Lokiec / Desalination 182 (2005) 187–198

maintaining the maximum brine temperaturebelow 70�C, thus significantly increasing thegain operation ratio (or economy ratio).MED units are powered by heat availablefrom very low pressure steam (0.2–0.4 ata)or hot water sources above 55�C. Wherehigher pressure steam is available (over2.0 ata), the plant can be supplied as a ther-mal vapor compression (TVC) unit.

MEDunits are available with capacities of upto 40,000 m3/d in a single unit with larger plantsbeing realized by multiple unit installations.

2.1. MED cogeneration principles

The ability of low temperature distillationplants to make effective use of low cost, lowgrade heat, or, where available, even zero costwaste heat, reduces to a minimum the motiveenergy requirements of these installations.

Low grade heat is available throughcogeneration schemes with diesel generator,steam turbine, nuclear power reactors andgas turbine power plants. Waste heat is also

obtained through waste heat recovery fromindustrial cooling waters and exhaust gases,from solid waste incinerators, solar pondsand geothermal waters.

3. LT-MED and TVC economics

Low temperature distillation is the basisfor a series of features, forming the core ofthe plants’ highly economical capital andoperational costs:1. The simultaneous transfer of latent heat onboth sides of the heat transfer surface of afilm type horizontal tube evaporator occursat a constant temperature so no loss of theeffective thermal driving force due to sensibleheating of liquid takes place.2. The design of the evaporator is character-ized by good sealing between its main com-ponents to prevent leakage of the brine to theproduct (there is no pressurized brine flow,and brine pressures are always lower thansteam or product pressures), a low thermalload which in turn will cause low vapor

Fig. 1. Typical evaporator effect

assembly.

A. Ophir, F. Lokiec / Desalination 182 (2005) 187–198 189

velocity and hence decrease carry-over, andcompact utilization of heat transfer area andcarry-over separator. The design also affordsexcellent scavenging of Non-Condensable-Gases (NCG), preventing corrosion andavoiding air pockets and the blanketing ofcondensation heat transfer surface with diffu-sion barriers.3. The core of the evaporator is a bundle ofhorizontal aluminum tubes sealed by tubesheets at the front and rear ends by meansof rubber grommets (Fig. 1). The rubbergrommets also provide electrical insulation,preventing galvanic corrosion. The heattransfer load is evenly distributed between

all tubes, making bundle performanceapproach single tube coefficients.4. The utilization of inexpensive aluminumtubes permits a large heat transfer area, thusreducing thermal loads as well as vapor velo-cities hence contributing to higher distillatepurity. This allows a lower carry-over and alower energy requirement. A special designprevents galvanic and pitting corrosion byinsulation between different metals, ensuringfilm flow free of stagnation spots, and elimi-nating heavy metal ions by utilizing ion traps.5. The use of generous heat transfer surfacescauses a reduction of heat fluxes and tem-perature differentials, thus an increase of

Fig. 2. Brine concentration/temperature curve for LT-MED process. Operation range of low-temperature

distillation.


thermal efficiencies. Consequently the eva-porative condensers operate with overall tem-perature differentials, including thermaldriving forces, boiling point elevations andnon-condensable gases and fouling factors,as low as 1.5–2.5�C.6. The operating temperatures are well belowthe saturation limits of problematic scalantsfound in sea and ground waters (Fig. 2). Scaleis reduced to an insignificant level, enablingplants to operate for long periods—5 years insome cases—between chemical cleanings.Low cost polyelectrolyte feed pre-treatmentis convenient. Descaling simply consists of

mild acid recirculation, using the plant’sown recirculation pumps.7. The reduced corrosiveness of seawater, atthe low operating temperature and vacuumconditions (deaerated feed water), allowssafe and economic use of corrosion proofmaterials and coatings both for piping andfor vessels lining, as well as the use of alumi-num for heat transfer tubing and vessel inter-nals. Lower capital and maintenance costs,and extended plant life (exceeding twenty-five years) result from the combination ofthe low corrosion rates and the use of amild anti-scalant.

Fig. 3. LT-MED coupled to a diesel generator.


8. LT-MED’s pre-treatment is simpler than thatrequired for other thermal (andmembrane) pro-cesses. Rough seawater filters with screens of 3mm open pores are sufficient for a safe and longterm operation, since no clogging of the spraynozzles (1/20 opening) is experienced. Equallyimportant is the fact that, at the low operatingtemperatures, a low cost and harmless polyelec-trolyte additive is used for feed pre-treatment,rather than sulfuric acid dosing which is oftenrequired with high temperature plants.9. Flexibility is achieved since MED’s plantshave short start-up periods with little timeloss for heating up. The plants have excellentload following capabilities, allowing for pro-duction to closely match both water demandand energy supply.10. The high purity of the distillate (usuallyless than 20 ppm, and for special applications

as low as 2–5 ppm), allows the product waterto be used directly to industrial processeswhere boiler feed water quality is required,or in municipal schemes, to reduce furtherthe production costs by blending the highpurity distillate with local brackish or poorquality water and improve and satisfy thepotable water standards.11. As the energy share in costs breakdown ofthermal desalination is high, the quest for lessexpensive energy sources is a task of primaryimportance. The low temperature operationenables the Low Temperature MED Distilla-tion units to utilize low grade, low cost sourcesof heat, which would otherwise be lost throughbeing released into the environment in the formof stack gases, cooling water streams or lowpressure exhaust steam. The motive energycost component for the desalination process is

Fig. 4. Reliance Refinery (India) – 4xMED 12,000 m3/day.


reduced to a minimum and consequently thewater production costs are lower than anyother seawater desalination thermal system.12. High reliability is achieved thanks toexperienced engineering, rugged construction,few moving parts and proven equipmentcombined with extremely low corrosion andscaling rates, resulting in simple operation,minimal maintenance and leading to annualplant availability in excess of 95%.

4. Experience

A few examples of commercial LT-MEDplants are presented below, emphasizing the

plants’ highly economical capital and opera-tional costs.

4.1. Diesel waste heat utilization

Several LT-MED plants have been inoperation utilizing the waste heat from dieselgenerator power stations as the sole heatsource. The only prime energy consumptionis 2.0 kWh/t used for the plant ancillarywater pumps. In these types of installations,the MED draws the motive energy for desa-lination from the waste heat recovered fromthe exhaust gases and the jacket water, lubeoil and air cooling system of a diesel

Fig. 5. Direct coupling of MED with back-pressure steam.


generator power station (Fig. 3). This vir-tually free energy brings the operating costsof the desalination unit down to a minimumand the thermal efficiency of the diesel powerstation up from approximately 40% to over80%.

4.2. Steam turbine cogeneration

4.2.1. Extraction steam: The LT-MEDprocess is extremely efficient as a replacementfor aging MSF plants where extraction steamin the range of 1.5–2.5 barg (originally selectedfor MSF) is available. In these plants theexisting extraction steam is used to activate a

thermocompressor, thus increasing the econ-omy ratio of the desalination plant. Thermo-compressors (ejectors) are relativelyinexpensive and durable (no moving parts),but they have a relatively low adiabatic effi-ciency compared to mechanical turbines andcompressors.

In the US Virgin Islands, 15 MED plantswith thermocompression have been in opera-tion since the early 1980s. The recent unitsare of a new, compact design, with up tothree (3) effects packed into one evaporatorvessel, thus reducing their capital costs andspace requirements. Those LT-MED units

Fig. 6. MED operating with condenser hot water.


have been performing at better than nominalrating ever since their installation.

In the island of Las Palmas 2 MED plantsoperating with ejectors utilizing motive steampressure as low as 1 bara. The plants consistsor 14 effects each, producing 20000 t/d with arecovery ratio of 11 (Fig. 8).

At the Reliance Refinery (Fig. 4) in India,four MED plants are in operation since 1998,each one with a nominal production of12,000 m3/d. The units have proved theirreliability and flexibility in operation andthey are continuously producing 10% abovenominal capacity. A fifth MED unit of14,400 m3/d capacity is scheduled to be deliv-ered by February 2005.

4.2.2. Back-pressure coupling:: For verylarge, dual-purpose applications ranging from

50 to 500 MWE and 20,000–200,000 t/d ofwater, respectively, the capability of operatingwith exhaust steam of 55–60�C means thatstandard condensing turbines can be usedinstead of specially designed back-pressure tur-bines required for higher temperature distilla-tion plants.

For obvious reasons, high reliability andavailability are desirable features for power uti-lity companies. Thus the ability of the LT-MED process to use standard condensing tur-bines (Fig. 5) makes it a perfect match for large,dual purpose plants. This capability also allowsthe addition of a desalination plant at a laterstage to an existing power station, since nochange in the turbine design is required.

Even a heat source that has a temperature aslow as 55�C (for a seawater sink of up to 30�C)

Fig. 7. LT-MED 10,000 m3/day þ 3.2 MW.


can be economically utilized. Such low heatsources could be available from almost anyconventional (Fig. 6) or nuclear power system.

4.2.3. Combination of extraction steam withan auxiliary turbine:: In this scheme theextraction steam (i.e. at 1.5 bar or above) isfirst used to activate an auxiliary turbine,thus using the energy to produce electricity

to the grid and then discharge it at the neededpressure 0.3 bar into the tubes of the firsteffect of the MED plant.

This principle was adopted in a 10,000 t/dplant for the Kompania di Awa e Electrisidad(KAE) of Curacao installed in 1988 (Fig. 7).The success of this plant led to the purchaseof a second, identical unit, which was

Table 1

A desalinated water calculation of a large capacity 100,000 m3/day production coupled to a back pressure turbine

of 0.35 ata operating at a base load mode

MED

Plant Configuration 5 � 20,000Daily Production m3/day 100,000Availability % 95%Annual Production m3/yr 34,675,000Interest rate % 6%Contractual Period years 20Capital Cost: Desalination Equipment MUSD 85Capital cost: Erection and Balance of Plant ’ 21.25Total Capital Investment ’ 106.25Amortization USD/m3 0.27

Operating Costs (excluding steam consumption)

Electricity Cost USD/kwh 0.05Electrical Consumption kwh/m3 1.2Electricity Cost USD/m3 0.060Chemicals USD/m3 0.050Spare Parts (1) USD/m3 0.031Labor (2) USD/m3 0.015Operating Costs (excluding steam consumption) USD/m3 0.156

Desalted Water Cost (excluding steam consumption) USD/m3 0.42

Calculation of the steam cost (@ 70 �C, 0.35 ata):The thermal energy (steam) cost chargeable to the

desalination is composed of the additional fuel costin an enlarged boiler and the incremental capitalcost of enlarging the boiler, required for thecompensation for generation loss.

Electrical Generation Loss: kWh/ton 4.5

Assuming for fuel cost USD/kWh 0.02

Boiler amortization USD/kWh 0.005

Total steam cost = 4.5x(.02 þ .005) USD/m3 0.1125

Total water cost USD/m3 0.54

Notes:(1) 1% of Capital Cost; (2) 13 workers, @$40,000/year each.Note that the total water cost, while assuming a turbine operating at a base load mode, reaches the low mark of0.54 USD/m3 which can compete with RO processes.


commissioned in June 1990. This plantincludes an auxiliary low pressure steamturbo generator where 48 t/h of 1.5 bargextraction steam (from the main turbine)expands to 0.35 bara, yielding 3.2 MWelectricity, and then enters the MED to pro-duce 10,000 t/d of product water. This resultsin net power consumption for desalinationbelow 5 kWh/t.

5. MED desalinated water cost calculation

The following table summarizes a desali-nated water calculation of a large capacity100,000 m3/day production coupled to a

back pressure turbine of 0.35 ata operatingat a base load mode (>Table 1). The genera-tion loss chargeable to the desalination, dueto operating at such a back pressure,is com-pensated by increasing the boiler size andincreasing the amount of fuel to producemore steam.

6. Closing remarks

This paper described the main processadvantages of the LT-MED technology.These process advantages have a significantimpact on the economics of the installationby reducing both capital and operation costs,

Fig. 8. Las Palmas MED 20,000 m3/day.


increasing the availability and extending thelife expectancy of the plant.

As shown above, the low-temperature MEDprocess offers attractive low costs, which cancompete with alternative technologies.. Thehigh purity of the produced water also allowsthe water to be used directly for industrial pro-cesses (boiler feed water), or to be blended withlocally available brackish water.

The experience accumulated with commer-cial plants in more than three decades ofactivity and of more than 350 world-wideinstallations, shows that such plants havesuperior technological characteristics in com-parison with other systems for seawater appli-cations. These characteristics, resulting fromthe low temperature design, provide simpleand long term operation under remarkable

stable conditions. Scale formation and corro-sion are minimal or absent and these factorslead to exceptional high plant availabilities.

6.1. Future development

Recent nuclear reactor design trend inChina is to manufacture plants to generatesteam for heating alone at a temperaturerange between 70 to 120�C. This equipment,while dedicated only to large desalination,can be cheap and when coupled withdesalination plants, its cost could be as lowas 1/4 of the total investment in project, andthe fuel cost could be lower by one half thanfossil fuel. This heat source could beadvantageous for MED plants, yieldinglower desalination costs.


advanced med process for most economical sea water desalination

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