Klaus Wangnick

Wangnick Consulting GmbH

Introduction

Seawater desalination is a method for the production of pure water that is being applied more and more. Desalination is performed by only a few commercially competitive processes: reverse osmosis (RO) as a membrane process, and the thermal processes multi stage flash (MSF) evaporation and multiple effect (ME) evaporation. The ME process is widely combined with thermal vapor compression (ME-TVC), mostly performed by steam ejectors. A few (smaller) installations use a mechanical compressor (ME-MVC), thus consuming only electrical energy for the drive. Smaller sizes of ME-MVC plants consist of a single stage only, whereas MSF units commonly consist of more than 15 stages, and METVC plants of about 4 evaporator stages. The following status report only covers the desalination of seawater, plants of larger sizes and thermal processes. It has to be mentioned that thermal plants are used for the treatment of water with a low salt content in exceptional cases only. This is due to the fact that the capital and operational costs of reverse osmosis plants decrease with lower salt content in raw water, whereas such costs remain more or less constant when using thermal plants.

Market Position [1]

Thermal processes still hold a very strong market position. More than 54 % (related to capacity) of the plants contracted within the past 10 years are thermal plants, of which some 76 % are MSF plants. Most of the large projects involve a thermal process.

Contrary to the average application, thermal processes are less used in countries where the coupling with power stations is not possible or difficult. On the other hand, almost 100 % of the larger plants on the Arabian peninsula use thermal processes, because here the power and water production is often combined.

Some 76 % (11,880,000 m³/d) of all thermal plants (15,700,000 m³/d) are installed on the Arabian Peninsula, with the United Arab Emirates having the largest installed or contracted capacity with 4,800,000 m³/d, followed by Saudi Arabia with 4,000,000 m3/d.

Where the raw water conditions are difficult, or very pure water and/or a heavy duty design is needed, it is not possible to do without thermal plants.

The main suppliers for thermal plants come from Europe (50 %) and Asia (40 %). American companies play no role in supplying thermal desalination plants.

Process

ME Evaporation

Evaporator stages are still conventionally arranged in series.

Feedwater is added in parallel (Figure 1), sometimes with stagewise pre-heating. Depending on the layout, the recirculation of seawater, either in all stages or in groups of stages, is also used, thus calling for additional pumps. The maximum seawater temperature in the evaporators is some 65 °C.

Because of the extremely low temperature difference between a high number of stages (the driving force), due to boiling point elevation and a maximum operating temperature that is kept as low as possible, additional losses must be kept at a minimum.Because of the energy consumption, ME evaporation plants need extremely large heat exchange surfaces.

In most cases, an ME evaporation plant is coupled with a vapor compression system. The compressor acts as a steam ejector. Motive steam is of 3 bara to some 20 bara pressure. Vapor is removed from the ME evaporator at about 0.1 bara and compressed to about 0.25 bara. The optimization of such a system is of the highest importance for the resulting price of the plant. The higher the motive steam pressure, the lower the ratio “motive steam / removed vapor”. In addition, this figure changes considerably with the difference in pressure between removed and compressed steam. Therefore, the vapor of larger plants is often not removed from the last (coldest), but from some other effect of the plant, thus resulting in different evaporator effect sizes. ME evaporation unit sizes up to some 25,000 m³/d have been delivered or are under construction.

Figure 1: Simplified flow scheme of an ME-TVC evaporation plant

Figure 2: Simplified flow scheme of an MSF evaporation plant with brine recirculation

Where the cost of electricity is very low (e.g. South Africa, Namibia) or thermal heat is not available at all, the ME evaporators can also be coupled to a mechanical vapor compressor. Larger ME-MVC plants have capacities up to 3,000 m³/d and more – using 3 effects, for example (6 units 3,000 m³/d each, Sardinia, Italy; supplied by I.D.E.). The consumption of electrical energy of such plants is some 7 kWh/m³. ME-MVC plants for capacities up to 5,000 m³/d using 6-effect evaporators are under development.

MSF Evaporation

Most of the plants employ the brine recirculation system (Figure 2), although the oncethrough system, which was used extensively in the early days of MSF evaporation, offers a number of advantages, such as a higher possible operating temperature because of the lower concentration in all stages, and therefore a lower boiling point elevation, smaller heat transfer area, lower number of pumps, venting of most of the NC gases to atmosphere, easier operation etc. The total dissolved solids (TDS) content of discharged brine is some 10 % to 15 % higher than seawater TDS, whereas brine from MSF plants of the recirculation type has a TDS content of some 50 % to 100 % higher than that of seawater.

The maximum operating temperature can be as high as 118 °C, but 112 °C is a typical limit in plants of the Arabian Peninsula. During winter operation, pre-warming of seawater is still being used, but the trend is towards avoiding this additional cycle by designing the low-temperature stages accordingly.

Feedwater is typically deaerated (the deaerator is often integrated in the evaporator) and treated with polyelectrolytes for scale and foam control, plus sodium bisulphate for scavenging oxygen (after a deaerator) and residual chlorine.

Scale in heat exchanger tubes is additionally controlled by on-line sponge ball cleaning systems (Figure 3). Antiscale additive dosing and ball cleaning systems ensure an operating period between acid cleaning of up to several years, as experienced in analyzing 28 units in Abu Dhabi.

Figure 3: Flow scheme of an on-line ball cleaning system installed in an MSF evaporation plant; cleaning system in operation in the heat rejection section and in the higher temperature stages of the heat recovery section [Taprogge]

The vacuum system for the removal of NC gases in MSF evaporation plants consists of 2 or 3 stages with ejectors and ejector condensers, plus a vent condenser as pre-condensing system upstream of the first ejector. Ejector condensers can be of the shell-and-tube or of the quench type.

Load variation is possible within a 50 - 100 % range. The load is varied by lowering the top brine temperature as well as the recirculation flow.

Type of Evaporators

In MSF plants, cross-tube evaporators are still used, and the long-tube version only in exceptional cases. Stages are increasingly arranged as a single-tier design (Figure 4), rather than in two tiers (Figure 5).

Figure 4: MSF plant 345,600 m³/d (6 units 57,600 m³/d each) at Taweelah B, Abu Dhabi, United Arab Emirates [Fisia]

Figure 5: MSF plant 98,100 m³/d (3 units 32,700 m³/d each) at Umm Al Nar East, Abu Dhabi, United Arab Emirates [Fisia]

Multiple Effect Evaporation plants – whether as a ME or ME-TVC or ME-MVC configuration - use evaporators with mostly circular shells (Figure 6) and heat exchangers with horizontal tubes, on which seawater is sprayed and in which the vapor condenses. Even the evaporator of a single-stage MVC plant is usually of this type. Vertical tube evaporators are used in exceptional cases only.

Design

ME Evaporation

Although rectangular evaporators give much more freedom of design, most of the present projects have cylindrical shells with diameters of up to 6 m and above. The shells are made of solid material.

Because of the extremely low pressure difference between the seawater and the steam side, the tube plates can be very thin. If the tubes are roller-expanded into the sheets, the plates have thicknesses of some 20 mm, whereas if the tubes are otherwise fixed, the thickness can be only 5 mm. Tube support plates are also very thin.

One manufacturer (Alfa Laval) uses heat exchange surfaces that are based on titanium plates instead of tubes, with good results.

Mist eliminators are either of the vertically and horizontally arranged vane type, exhibiting a low pressure loss, or of the horizontally arranged wire-mesh type.

Seawater is sprayed onto the tubes by means of nozzles or perforated plates.

Start-up, normal operation, load variation (between some 40 % to 100 %) and shut-down are typically performed fully automatically. Almost unmanned operation is possible.

MSF Evaporation

All evaporators are of the rectangular type. The width of the evaporator can be up to 25 m, and the length up to 100 m. The width is limited by the maximum condenser tube length that is commercially available. The length of the tubes in the large MSF evaporator of the Taweelah B project is about 19.2 m.

The length of the tubes dictates the width of the evaporator. Therefore, weir loads – even in very large evaporators – still remain at some 1,200 t/(m h) at a maximum, so that safe operation is ensured.

Interstage transfer of distillate and brine is maintained through orifices. Some of the manufacturers arrange weirs downstream of the orifices.

Mist eliminators are of the wire-mesh type in most of the evaporators.

During the past decade, the prices for MSF plants decreased considerably. This was due to a better understanding of the process in large plants, the use of finite element calculation methods for optimizing the plate thicknesses of corrosion-resistant materials, the application of modern measuring, control and automation equipment, and the acceptance of the ball cleaning system as part of the plant (this decision resulted in much smaller heat transfer area and considerably lower investment), to mention only a few reasons.

MSF plants can be started up, operated and shut down fully automatically. Almost unmanned operation is possible.

Pumps, including the brine recirculation pump, are driven by electrical motors.

Materials

ME Evaporation

Stainless steel (SS) type 316 L or duplex-type stainless steels are used for the shell, tube plates and tube support plates; only a few suppliers deliver painted or coated carbon-steel shells. Higher-grade SS and other materials are selected in exceptional cases only. The austenitic SS type 316L can be used, because the operating temperature is low and the water is deaerated immediately after entering the effect.

Exchanger tubes are made of a wide variety of materials: aluminum brass or copper nickel is often used, but titanium is increasingly being chosen. One supplier delivers very thin tubes (0.3 mm) of a high-grade SS (1.4565) (Figure 7). Another supplier is specialized in the installation of aluminum tubes. The upper tubes, on which the water is sprayed with high velocities, are made of titanium or the said SS in more or less all plants, in order to prevent erosion corrosion.

Figure 6: MED plant 48,000 m³/d (4 units) at Jamnagar, India [I.D.E.]

The shells of exchangers for heating the seawater are typically of SS 316L, and the tubes of titanium. Sacrificial anodes must be installed, if the water boxes of these exchangers are also of SS 316L.

Vane mist eliminators are of polypropylene (PP), and wire-mesh mist eliminators of SS 316L.

Because of the low operating temperatures, the piping for seawater, brine and distillate is of GRP.

The main ejector (the thermal compressor) is built of SS 316L.

Figure 7: ME evaporation plant 24,000 m³/d (2 units 12,000 m³/d each) at Rotterdam, Netherlands [VA-TECH]

MSF Evaporation

Nowadays, all parts of an evaporator in contact with either aerated or deaerated seawater are constructed of alloy material, not of painted or coated carbon steel. Carbon steel cladded with SS type 316L up the mist eliminators, and with SS 304L above said eliminators, is the first choice as the material for the shell. Duplex SS is another solution. One of the first evaporator shells of solid stainless steel was used for a project in Libya some 10 years ago (Figure 8). The first very large plant with a solid SS shell was realized in Bahrain (Hidd, 3 units 32,000 m³/d each, Fisia). Shells of copper-containing alloys are being used less and less, owing to problems with the copper which escapes from such plants into the sea, exerting an adverse effect on the flora and fauna.

All SS of types 316L or duplex (e.g. 1.4462) must be flushed with pure water during standstill of the plant, in order to prevent corrosion. In addition, austenitic SS must be painted from the outside, otherwise stress corrosion cracking may occur. This can be prevented by using duplex SS.

Exchanger tubes in the heat rejection section of the evaporator are commonly of titanium, but tubes in the heat recovery section of aluminum brass, 90/10 or 70/30 copper nickel grades or titanium. Aluminum brass is not used in the first two or three evaporator stages, because of the presence of non-condensable gases there.

Tube sheets are made of copper nickel or bronze of several types, and the water boxes of carbon steel cladded with copper nickel.

Figure 8: MSF once-through long-tube evaporation plant 10,000 m³/d (1 unit) at Sirte, Libya [VA-TECH]

Availability

The availability of the various systems depends on the quality of the purchased plant as well as the quality of the operation and maintenance staff. The commonly projected availability is 90 % for all processes, and is sometimes higher in industrialized countries. But it is a fact that these figures are not to be obtained in a number of plants. This reduced availability is a matter of stoppages on the one hand, and a decrease in output on the other. Here it can be noted that in thermal plants, especially the MSF plants, full output can be obtained even under the worst conditions, so that a reduced availability is due to stoppages only.

Energy Consumption

The typical consumption of thermal and electrical energy (Table 1), and total“harmonized” energy can be obtained from Table 2. Harmonized energy is related to the (theoretical) production of electrical energy by the steam (used for heating in a thermal plant) in a steam turbine, in order to compare thermal energy with electrical energy, and therefore processes such as RO and MSF.

It can be seen clearly that thermal plants do not need such big amounts of energy as is often claimed. In addition, thermal energy, especially if on a low energy level, is often a waste product of industrial processes and can be used for free.

Table 1: Consumption of electrical energy by desalination processes [kWh/m3]; (TURBINE) means the drive of the brine recirculation pump is a back-pressure steam turbine

Table 2: Harmonized energy consumption of desalination processes [kWh/m³]

The energy consumption of the ME-TVC process can be lower if the pressure of ejector motive steam is lower than 15 bar.

Consumption of Chemicals

The consumption of chemicals in an MSF plant is very limited. Seawater is sometimes chlorinated by taking the necessary hypochlorite solution from an on-site seawater electrolysis plant. An antiscale additive is added in an amount of some 2-4 ppm, and the addition of an antifoam additive is sometimes also necessary (0.1 ppm). Depending on whether a once-through plant or a recirculation plant is used and on the selection of materials, sodium bisulphite for scavenging oxygen may also become necessary. The total water costs from the addition of chemicals are well below 0.01 $/m³. Similar chemical consumptions have been reported for MED plants. This limited use of chemicals ensures a very low environmental impact.

Heat exchanger surfaces have to be cleaned at certain intervals. Whereas the heat exchange surfaces of MSF plants only need to be cleaned at extremely long intervals, thanks to the implementation of ball cleaning systems, the surfaces in MED plants have to be cleaned more often, on average twice a year.

Investment and Water Costs

A qualified comparison of investment costs is very difficult, because the investment depends on the scope of the project, the quality of the plant, and the conditions prevailing at the location of the installation. Table 3 shows the results of latest competition, namely the Jebel Ali L project (320,000 m³/d: 5 units of 64,000 m3/d each), the Al Jobail RO project (91,000 m³/d: 15 units of 6,060 m³/d each) and the Abu Dhabi UANW MED project (31,800 m³/d: 2 units of 15,900 m³/d each). As the time of project realization was different in each case, a price increase of 3 %/a has been included.

As can be seen from the diagram, the MSF process has the lowest investment costs whencompared to other processes under similar conditions.

Table 3: Investment costs of latest competition of very large thermal seawater desalination plants on the Arabian Peninsula [$/(m³/d)]

If water costs are compared, it must also be ensured that the conditions are the same or similar. In no way can the water costs resulting from a plant installed e.g. in the US be compared to those of a plant installed in the Middle East at the Arabian Gulf.

The main portion of the water costs (Table 4) results from the capital investment for depreciation and interest. If total capital costs are to be calculated with 7 %, the resulting water costs from capital investment are 0.23 $/m³ for the MSF and 0.28 $/m³ for MED process.

Costs for electrical energy have been calculated with 0.035 $/kWh and those for“harmonized thermal” energy with 0.025 $/kWh (because of the higher fuel consumption and higher investment for boiler and turbine, due to a larger live steam flow). A turbine has been assumed as the drive for the MSF brine recirculation pump. It should be noted that the costs of water from RO plants will increase further if a higher price has to be calculated for the electricity.

Thermal processes compared with the RO process: The chemical costs for the RO alternative are higher than for the thermal processes (which is not astonishing), and the costs for membrane replacement have to be included for the RO process only. Finally, the costs for personnel are somewhat higher for the RO process, due to the higher number of trains and a more complex plant system (pre-treatment).

This diagram shows clearly that the MSF process can be implemented very economically at locations where the conditions are difficult.

Table 4: Costs of water from very large thermal desalination plants at the Arabian Peninsula [$/m³]

A water price of < 0.7 $/m³ has been offered in BOOT plants on the Arabian peninsula, i.e. the water costs may be lower in industrialized countries.

Outlook

Thermal processes still hold a dominant share in the field of seawater desalination, especially if it is possible to couple the plant with a power station or any form of waste heat recovery, or if the local conditions are difficult and the produced water must be pure. In such cases, the investment costs and operational costs are often lower than for reverse osmosis. In addition, there is considerable room for the further development of MSF plants (materials, thinner exchanger tubes etc.) and extensive possibilities for improvement in the ME-TVC process (materials, very thin - 0.1 mm - exchanger tubes, plate heat exchangers, larger unit sizes etc.). The RO process dominates by far where the costs for thermal energy are high, raw water conditions good (low temperature, low salinity, low content of biological matter, rocky coast) and a coupling of the seawater desalination plant with a power stations not possible (e.g. in Spain).

References

[1] Wangnick, K. “2004 IDA Worldwide Desalting Plants Inventory No. 18”, June 2004, published by Wangnick Consulting.