Review of MED Fundamentals and Costing

Abstract- This paper presents the structure and features of the Low Temperature Multi Effect Distillation (LT-MED), which is one of the most efficient thermal desalination processes currently in use. It incorporates technological advances which have resulted in reliable, durable and economical desalination plants producing high purity product water.

A. Ophir and F. Lokiec

IDE Technologies Ltd.

I. Introduction

Low Temperature Multi Effect Distillation (LT-MED) is one of the most efficient thermal desalination processes currently in use. It incorporates technological advances which have resulted in reliable, durable and economical desalination plants producing high purity product water.

These advances are as follows:

1. Development of a unique design of a falling film horizontal tube evaporator/condenser with high heat transfer coefficient, utilizing only latent-heat transfer, avoiding sensible heat pick-up.
2. Superior thermodynamic efficiency (compared to MSF) and very low pressure drops at high volumetric vapor flows, as prevailing in low temperature operation. This enabled the optimization of the process for operation at a maximum brine temperature of 70 °C.
3. The low temperature operation has made possible the utilization of economical and durable materials of construction such as aluminum alloy for heat transfer tubes, plastic process piping and epoxy-painted carbon steel shells which show a better resistance when matched with aluminum alloy or titanium.
4. The economy of using aluminum tubes for heat transfer as compared with copper alloy tubes, which are essential for higher temperature plants (used by other distillation manufacturers), enables the increase of the heat transfer area per ton of water produced in the desalination plant for the same investment costs.
5. 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 – 2.5 °C), enabling the incorporation of a large number of effects (10 to 16) even with a maximum brine temperature as low as 70 °C, consequently resulting in very high economy ratios (product to steam).
6. Possibility of using low-cost/low-grade heat available through cogeneration schemes to minimize the energy cost component.
7. Minimal requirements for intake and pre-treatment systems.

The practical experience with commercial plants using the above mentioned advances has shown the remarkable stability, flexibility and reliability of the low temperature process in comparison with others. Continuous research and development improved further the advantages of the low temperature process by increasing the unit’s capacities; decreasing the energy consumption; and lowering levels of scaling and corrosion, both being significantly reduced due the low temperature operation.

II. Brief Description of the MED Process

MED plants utilize horizontal tube, falling-film evaporative condensers in a serial arrangement, to produce through repetitive steps of evaporation and condensation, each at a lower temperature and pressure, a multiple quantity of distillate from a given quantity of low grade input steam.

Any number of evaporative condensers (effects) may be incorporated in the plants’ heat recovery sections, depending on the temperature and costs of the available low grade heat and the optimal trade-off point between investment and steam economy. Technically the number of effects is limited only by the temperature difference between the steam and seawater inlet temperatures (defining the hot and cold ends of the unit) and the minimum temperature differential allowed on each effect.

The low temperature differential allowance on each evaporator (effect) in the train allows a large number of effects to be utilized while maintaining the maximum brine temperature below 70°C, thus significantly increasing the gain operation ratio (or economy ratio). MED units are powered by heat available from very low pressure steam (0.2 to 0.4 ata) or hot water sources above 55°C. Where higher pressure steam is available (over 2.0 ata), the plant can be supplied as a thermal vapor compression (TVC) unit.

MED units are available with capacities of up to 40,000 m³/d in a single unit with larger plants being realized by multiple unit installations.

MED cogeneration principles

The ability of low temperature distillation plants to make effective use of low cost, low grade heat, or, where available, even zero cost waste heat, reduces to a minimum the motive energy requirements of these installations,

Low grade heat is available through cogeneration schemes with diesel generator, steam turbine, nuclear power reactors and gas turbine power plants. Waste heat is also obtained through waste heat recovery from industrial cooling waters and exhaust gases, from solid waste incinerators, solar ponds and geothermal waters.

III. LT-MED and TVC Economics

Low temperature distillation is the basis for a series of features, forming the core of the plants’ highly economical capital and operational costs:

1. The simultaneous transfer of latent heat on both sides of the heat transfer surface of a film type horizontal tube evaporator occurs at a constant temperature so no loss of the effective thermal driving force due to sensible heating of liquid takes place.
2. The design of the evaporator is characterized by good sealing between its main components to prevent leakage of the brine to the product (there is no pressurized brine flow, and brine pressures are always lower than steam or product pressures), a low thermal load which in turn will cause low vapor velocity and hence decrease carry-over, and compact utilization of heat transfer area and carry-over separator. The design also affords excellent scavenging of Non-Condensable-Gases (NCG), preventing corrosion and avoiding air pockets and the blanketing of condensation heat transfer surface with diffusion barriers.
3. The core of the evaporator is a bundle of horizontal aluminum tubes sealed by tube sheets at the front and rear ends by means of rubber grommets. The rubber grommets also provide electrical insulation, preventing galvanic corrosion. The heat transfer load is evenly distributed between all tubes, making bundle performance approach single tube coefficients.
4. The utilization of inexpensive aluminum tubes permits a large heat transfer area, thus reducing thermal loads as well as vapor velocities hence contributing to higher distillate purity. This allows a lower carry-over and a lower energy requirement. A special design prevents galvanic and pitting corrosion by insulation between different metals, ensuring film flow free of stagnation spots, and eliminating heavy metal ions by utilizing ion traps.
5. The use of generous heat transfer surfaces causes a reduction of heat fluxes and temperature differentials, thus an increase of thermal efficiencies. Consequently the evaporative condensers operate with overall temperature differentials, including thermal driving forces, boiling point elevations and non-condensable gases and fouling factors, as low as 1.5 – 2.5°C.
6. The operating temperatures are well below the saturation limits of problematic scalants found in sea and ground waters. Scale is reduced to an insignificant level, enabling plants to operate for long periods – 5 years in some cases – between chemical cleanings. Low cost polyelectrolyte feed pre-treatment is convenient. Descaling simply consists of mild acid recirculation, using the plant’s own recirculation pumps.
7. The reduced corrosiveness of seawater, at the low operating temperature and vacuum conditions (deaerated feed water), allows safe and economic use of corrosion proof materials and coatings both for piping and for vessels lining, as well as the use of aluminum for heat transfer tubing and vessel internals. Lower capital and maintenance costs, and extended plant life (exceeding twenty-five years) result from the combination of the low corrosion rates and the use of a mild anti-scalant.
8. LT-MED’s pre-treatment is simpler than that required for other thermal (and membrane) processes. Rough seawater filters with screens of 3 mm open pores are sufficient for a safe and long term operation, since no clogging of the spray nozzles (1/2” opening) is experienced. Equally important is the fact that, at the low operating temperatures, a low cost and harmless polyelectrolyte additive is used for feed pre-treatment, rather than sulfuric acid dosing which is often required with high temperature plants. 
9. Flexibility is achieved since MED’s plants have short start-up periods with little time loss for heating up. The plants have excellent load following capabilities, allowing for production to closely match both water demand and energy supply.
10. The high purity of the distillate (usually less than 20 ppm, and for special applications as low as 2 – 5 ppm), allows the product water to be used directly to industrial processes where boiler feed water quality is required, or in municipal schemes, to reduce further the production costs by blending the high purity distillate with local brackish or poor quality water and improve and satisfy the potable water standards.
11. As the energy share in costs breakdown of thermal desalination is high, the quest for less expensive energy sources is a task of primary importance. The low temperature operation enables the Low Temperature MED Distillation units to utilize low grade, low cost sources of heat, which would otherwise be lost through being released into the environment in the form of stack gases, cooling water streams or low pressure exhaust steam.  The motive energy cost component for the desalination process is reduced to a minimum and consequently the water production costs are lower than any other seawater desalination thermal system.
12. High reliability is achieved thanks to experienced engineering, rugged construction, few moving parts and proven equipment combined with extremely low corrosion and scaling rates, resulting in simple operation, minimal maintenance and leading to annual plant availability in excess of 95%.

IV. Experience

A few examples of commercial LT-MED plants are presented below, emphasizing the plants’ highly economical capital and operational costs.

1. Diesel Waste Heat Utilization

Several LT-MED plants have been in operation utilizing the waste heat from diesel generator power stations as the sole heat source. The only prime energy consumption is 2.0 kWh_t used for the plant ancillary water pumps. In these types of installations, the MED draws the motive energy for desalination from the waste heat recovered from the exhaust gases and the jacket water, lube oil and air cooling system of a diesel generator power station. This virtually free energy brings the operating costs of the desalination unit down to a minimum and the thermal efficiency of the diesel power station up from approximately 40% to over 80%.  

1. Steam Turbine Cogeneration

2.1 Extraction Steam

The LT-MED process is extremely efficient as a replacement for aging MSF plants where extraction steam in the range of 1.5 to 2.5 barg (originally selected for MSF) is available. In these plants the existing extraction steam is used to activate a thermocompressor, thus increasing the economy ratio of the desalination plant. Thermocompressors (ejectors) are relatively inexpensive and durable (no moving parts), but they have a relatively low adiabatic efficiency compared to mechanical turbines and compressors.

In the US Virgin Islands, 15 MED plants with thermocompression have been in operation since the early 1980s. The recent units are of a new, compact design, with up to three (3) effects packed into one evaporator vessel, thus reducing their capital costs and space requirements. Those LT—MED units have been performing at better than nominal rating ever since their installation.

At the Reliance Refinery in India, four MED plants are in operation since 1998, each one with a nominal production of 12,000 m³/day. The units have proved their reliability and flexibility in operation and they are continuously producing 10% above nominal capacity. A fifth MED unit of 14,400 m³/day capacity was  delivered in 2005, and four additional MED units of 24,000 m³/day capacity each were recently contracted.

2.2 Back-Pressure Coupling

For very large, dual-purpose applications ranging from 50 to 500 MWE and 20,000 to 200,000 t/d of water, respectively, the capability of operating with exhaust steam of 55 to 60°C means that standard condensing turbines can be used instead of specially designed back-pressure turbines required for higher temperature distillation plants.

For obvious reasons, high reliability and availability are desirable features for power utility companies. Thus the ability of the LT-MED process to use standard condensing turbines makes it a perfect match for large, dual purpose plants. This capability also allows the addition of a desalination plant at a later stage to an existing power station, since no change in the turbine design is required.

Even a heat source that has a temperature as low as 55°C (for a seawater sink of up to 30°C) can be economically utilized. Such low heat sources could be available from almost any conventional or nuclear power system.

2.3 Combination of Extraction Steam with an Auxiliary Turbine

In this scheme the extraction steam (i.e. at 1.5 barg or above) is first used to activate an auxiliary turbine, thus using the energy to produce electricity to the grid and then discharge it at the needed pressure 0.3 bara into the tubes of the first effect of the MED plant.

This principle was adopted in a 10,000 t/d plant for the Kompania di Awa e Electrisidad (KAE) of Curacao installed in 1988 (Fig. 7). The success of this plant led to the purchase of a second, identical unit, which was commissioned in June 1990. This plant includes an auxiliary low pressure steam turbo generator where 48 t/h of 1.5 barg extraction steam (from the main turbine) expands to 0.35 bara, yielding 3.2 MW electricity, and then enters the MED to produce 10,000 t/d of product water. This results in net power consumption for desalination below 5 kWh/t.

V. Comparative Capex and Opex Calculation

There are real savings due to water production and considerable power generating gains when employing the LT-MED technology instead of alternative thermal process. These savings and gains are in addition to all the qualitative advantages pinpointed at the previous sections. The following table summarizes a typical comparison of two thermal technologies envisaging a 100,000 m3/day production. 

• VI. Closing Remarks

This paper described the main process advantages of the LT-MED technology. These process advantages have a significant impact on the economics of the installation by reducing both capital and operation costs, increasing the availability and extending the life expectancy of the plant.

As shown above, the low-temperature MED process offers attractive low capex and opex costs, which can compete with similar or alternative technologies and at the same time generate significant savings in both water and power production. The high purity of the produced water also allows the water to be used directly for industrial processes (boiler feed water), or to be blended with locally available brackish water.

The experience accumulated with commercial plants in more than three decades of activity and of more than 350 world-wide installations, shows that such plants have superior technological characteristics in comparison with other systems for seawater applications. These characteristics, resulting from the low temperature design, provide simple and long term operation under remarkable stable conditions. Scale formation and corrosion are minimal or absent and these factors lead to exceptional high plant availabilities.

All this reflects in the economics of these plants, which are shown to have lower production costs and overall economics than other seawater desalination processes.