- Email: [email protected]
Low-temperature distillation processes in single- and dual-purpose plants
DESALINATION Desalination 136 (2001) 189-197
ELSEVIER
www.elsevier.com/locate/desal
Low-temperature distillation processes in single- and dual-purpose plants Gustavo Kronenberg*, Fredi Lokiec IDE Technologies Ltd., PO Box 591, Raanana 43104, Israel Tel. +972 (9) 747-9777; Fax +972 (9) 747-9715; email: [email protected]
Received3 August 2000; accepted 17 August 2000 Abstract
During the last decade, intensive research and development have been invested to improve further the advantages of the low-temperature process by increasing the unit's capacities and decreasing the energy consumption. This paper presents the advantages ofthe low-temperaturedistillationprocess, describespractical commercial application for steam driven multi-effect distillation plants in dual-purpose (electricity and water production) applications and the latest developments for single-purpose mechanical vapor compression plants. Keywords: Low-temperature distillation; Multi-effect distillation; Mechanical vapor compression; Cogeneration
1. Introduction
The low-temperature distillation process has been presented in various papers [1,2] where the general technological advances have been reemphasized, demonstrating the improved economics of related desalination processes. These advances can be summarized as follows: • Development of a unique design of a falling film horizontal tube evaporator/condenser, utilizing only latent heat transfer, avoiding sensible heat pick-up. *Correspondingauthor.
• Low temperature operation (max brine temp. of 70°C). • Use of an economical and durable construction material such as aluminum alloy for heat transfer tubes, plastic process piping and epoxy-painted steel shells. • Possibility of using low-cost/low-grade heat available through cogeneration schemes to minimize the energy cost component. • Minimal requirements for intake and pretreatment systems. The practical experience with commercial plants using the above-mentioned advances has
Presented at the 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 Ecole Nationale d'lngenieurs de Tunis, September 11-13, 2000, Jerba, Tunisia.
0011-9164/01/$-- See front matter © 2001 Elsevier Science B.V. All rights reserved
PII: SOOl1-9164(01)00181-3
190
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
been analyzed and discussed in other venues, showing their remarkable stability, flexibility and reliability of the low temperature process in comparison with others.
2. Low-temperature advantages
distillation
(LTD)
Low-temperature distillation is the basis for a series of features forming the core of the plant's highly economical operation [3.] • Low corrosion rates: The reduced corrosiveness of seawater at low operating temperature and vacuum conditions permits safe and economic use of corrosion-proof plastic materials and coatings both for piping and for vessel linings, as well as the use of aluminum for heat transfer tubing and vessel internals. Low maintenance and extended plant life (exceeding 25 years) result from the combination of the low corrosion rates and the use of a mild anti-sealant. • Flexibility: LTD plants have short start-up periods with little time loss for heating up. Plants have excellent load following capabilities allowing for production to closely match both water demand and energy supply. • Thermodynamic efficiency: The use of generous heat transfer surfaces results in a reduction of heat fluxes and temperature differentials and therefore in an increase of thermal efficieneies. As a result, 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 2-2.5°C. • Minimal scaling rates: The operating temperatures (Fig. 1) are well below the saturation limits of problematic sealants found in sea and most ground waters. Scale is reduced to an insignificant level, enabling plants to operate for long periods - -
5 years in some cases - - between cleanings. Low-cost polyelectrolyte feed pretreatment is adequate. Descaling is a simple procedure, consisting of mild acid recirculation, using the plant's own recirculation pumps. • High-purity distillate: An additional advantage is the high purity of the product water (usually less than 20 ppm and as low as 25 ppm for special applications). This allows the water to be used directly for industrial processes such as in refineries, power stations, breweries etc., where boiler water quality is required or in municipal installations to reduce the production costs further by blending the high-purity distillate with local brackish or poor-quality water and satisfy the potable water standards. • Reliability: Experienced engineering, rugged construction and proven equipment combined with extremely low corrosion and scaling rates result in minimal maintenance and lead to annual plant availability in excess of 95%. • Low energy costs: The low-temperature operation in dual-purpose application enables the use of 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 lowpressure 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 system [4].
3. Cogeneration: use of low-temperature heat sources 3.1. Diesel waste heat utilization
Several LTMED plants have been in operation since 1985 using 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 water pumps of the plant. Such
G. Kronenberg, F Lokiec / Desalination 136 (2001) 189-197
191
Fig. 1. Brine concentration/temperaturecurve for LT-MEDprocess. Operation range of low-temperaturedistillation. plants operate on the Marshall Islands and in Nauru in the Pacific, and on the islands of Bonaire in the Caribbean Sea. In these diesel-cogeneration installations, the MED draws the motive energy for desalination from the waste heat recovered from the exhaust gases and the jacket water cooling system of a diesel generator power station. (Fig. 2). 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%.
In the US Virgin Islands, 14 MED plants with thermocompression have been in operation since the early 1980s. Those LT-MED units have been performing at better than nominal rating ever since their installation. In the Reliance refinery (Fig. 3) in India, four MED plants are in operation, each one with a nominal production of 12,000 m3/d. The units have proved their reliability and fiexiblity in operation and they are actually producing 10% above nominal capacity. 3.2. 2. Back-pressure coupling
3.2. Steam turbine cogeneration 3.2.1. Extraction steam coupling
The LTMED 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.
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 in the range of 55 to 70°C steam 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 LTMED process to use standard condensing turbines (Fig 4) makes it a perfect match for large, dualpurpose plants. This capability also allows the addition of a desalination plant at a later stage to
Fig. 3. Reliance refinery (India); 4 x IVIED12,000 mVd.
an existing power station, since no change in the turbine design is required. Even a heat source that has a temperature as low as 60°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 (Fig. 5) or nuclear power system.
G. Kronenberg, F Lokiec / Desalination 136 (2001) 189-197
193
Fig. 4. Direct coupling of MED with back-pressure steam.
Fig. 5. MED operating with condenser hot water.
3.2.3. Combination o f 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.
194
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
4 kWh/t compared to 6-8 kWh/t for MVC. The advantages of MVC are the following: • Purity of product. Distillate of 5 ppm and less compared to 300-500ppm permeate for SWRO. • Does not require sophisticated pretreatment of seawater and is not sensitive to contamination of seawater by oil and/or other organic matter compared to SWRO. • More durable and requires less maintenance. There is no need for membrane replacement.
Fig. 6. LTMED 10,000 m3/d Curacao.
+
3.2 MW at the KAE,
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. 6). The success of this plant led to the purchase of a second, identical unit, which was commissioned in June 1990. This plant includes a small turbine 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 10,000t/d of product water. This results in a net power consumption for desalination below 5 kWh/t.
4. Low-temperature distillation mechanical vapor compression
with
Mechanical vapor compression (MVC) distillation is inherently the most thermodynamically efficient process of single-purpose thermal desalination plants. In recent years MVC and seawater reverse osmosis (SWRO) processes became the forerunners of all single-purpose desalination processes competing for the lowest seawater desalination cost. Of these two processes, modem SWRO is superior in having a lower specific energy consumption of about
The thermodynamic efficiency of the MVC process is derived from the application of the "heat pump" principle. The latent heat required by the system (evaporation and condensation steps) is continuously recycled by a large volumetric flow compressor, thus eliminating the need for cooling water for heat rejection as in MSF or MED process. The heat generated by the compressor work is rejected in the outgoing streams of product and brine that are discharged to the sea at a higher temperature than the seawater feed. However, in order to maintain a higher operating temperature, the feed is preheated by means of two plate heat exchangers by exchanging heat with the outgoing streams. The simultaneous transfer of latent heat on both sides of the heat transfer surface of a filmtype 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. Fig. 7 describes the process flows for a three effect MVC unit. Besides the thermodynamic advantage of the MVC process over the other single-purpose thermal processes such as MSF and thermal compression MED, it has the additional advantage that it does not have to be located close to a source of steam and requires no cooling water for heat rejection. There are fewer ecological problems and the operation is comparatively simple and easy (See Fig. 7).
195
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
t
m
r M w
m
Fig. 7. Multi-effectmechanicalvapor compression.
5. Features o f IDE MVC units
The main component featuring in IDE's MVC is the unique self-developed flexible blade compressor, capable of handling a large volumetric flow of water vapor at a relatively low cost. This compressor is coupled to a falling-film horizontal-tube evaporator that provides highly efficient heat transfer capacity. Consequently, this combination allows the plant to operate at a relatively low temperature, below 70°C, which in turn permits the selection of inexpensive materials of construction such as aluminum alloy for heat transfer tubes, polypropylene piping and epoxy-coated, carbon-steel vessels. Low-temperature operation also reduces scale problems and consequently requires a simpler feed treatment system. The inexpensive, yet durable, construction material allows for the generous use of heat
transfer surface tubes, thus reducing the thermal driving force that causes reduction in the electrical consumption of the compressor. Since 1969 IDE has installed and operated over 200 vapor compression units world wide, thus acquiring considerable expertise in this field. During the last decade, intensive research and development have been invested to improve the MVC process further by increasing the unit capacities and reducing energy. Growing demands for larger MVC units by the world market on one hand, and the requirements for decreasing specific capital investment cost and electrical consumption imposed by the pressing competition on the other hand, have led IDE to undertake further development of larger capacity, high efficiency MVC units. Larger MVC units are inherently superior compared to multiples of smaller units
196
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
yielding equivalent total capacity due to economies of scale. The operating cost, including energy consumption, could also be lowered by installing more specific heat transfer area (m2/t/d). The growing capacity of the MVC units is limited by the maximum compressor' s volumetric flow and the head developed by the compressor, the operating temperature and the heat transfer capacity of the evaporator, which in turn determine the required head. Increasing the heat transfer capacity could be achieved by increasing the number of effects, which also increases the output for a given volumetric flow. Increasing the size of the effect would also contribute to increased heat transfer capacity, but this increase is limited by available transportation means. Above a capacity of 900t/d, at least two effects are required.
units were tested and their performance at design conditions conformed to the expected design expectation, as follows: 3000ffd • Capacity: • Specific energy: Compressor: 6.9 kWh/t Total 8.1 kWh/t (including product pumping) Product salinity:3 ppm This performance was made possible by the development of a new compressor with higher volumetric capacity (about 75 m3/s) and increased head (more than 5000kg/kg), obtained by increasing the diameter of the suction channel, rotor and impeller diameters, and improving flow conditions in the rotating flow channel. A similar approach has been utilized in two additional units supplied recently to a Refinery in Turkemenistan (Caspian Sea).
6. MArC - - 3000 t/d units
Until recently the maximum capacity ofIDE' s unit did not exceed 2000 t/d. In 1998 six larger units of approximately 3000t/d (Fig. 8) comprising three effects of 4.8 diameter each coupled to a larger capacity compressor were designed and installed for Sarlux in Sardinia, Italy. These
Fig. 8. Sarlux 6 x MVC 3000 m3/d.
7. Design consideration in increasing MVC capacity
The main factor in increasing MVC capacity is by developing compressors with higher volumetric flow and head. The higher head enables the utilization of more effects in the unit,
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
which yields more product for the same volumetric flow. Increasing the number of effects would also provide more specific heat transfer area and reduce the average mean effective boiling point elevation in the plant. In addition, the capacity of each individual effect can be further increased by using a more efficient heat transfer surface such as smaller diameter or grooved tubes, optimal wetting and larger diameter vessel. This would consequently decrease the specific energy consumed by the compressor. However, lowering specific energy has its limits. These would be set by the increase in the heat exchanger required to preheat the feed due to the decrease of the thermal driving force from 2.2°C to 1.2°C, corresponding to specific energy of 8-8.5 kWh/t, and 6 kWh/t, respectively. In some cases this problem could be partially solved by the introduction of waste heat into the system should it become available.
8. E x p e r i e n c e a n d c o n c l u s i o n s
I. The low-temperature distillation plants based on IDE's recent technological developments offer attractively low operating costs which compete with those of similar and alternative technologies. An additional advantage can be derived form the high purity of the produced water, allowing the water to be used directly for industrial process (boiler feed water) or to be blended with locally available brackish water.
197
2. The experience accumulated with the commercial plants during three decades and with more than 300 worldwide 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 long-term operation under remarkably stable conditions. Scale formation and corrosion are minimal or absent, and these factors lead to exceptionally high plant availabilities of 94% to 96%. 3. New developments in low-temperature distillation show the capability to enlarge the single unit capacity as follows: •
F o r M E D units:
Up to 20,000 m3/d with proven experience Up to 40,000 m3/d under development •
F o r M V C units:
Up to 3,000 m3/d with proven experience Up to 10,000 m3/d under development
References
[1] R. Matz and Z. Zimerman,Desalination,52 (1982) 201. [2] A. Ophir, A. Gendel and G. Kronenberg,Internat. DesalinationWat. Reuse Q., 4(1) (1994) 28. [3] IDE,Aquaport brochure, Fresh water from the sea, 1990. [4] A.N. Rogers, C.D. Siebenthal, R.F. Battey and L. Awerbuch,DesalinationTechnology- - Reporton the State of the Art, BechtelGroup, San Francisco, 1983.
ELSEVIER
www.elsevier.com/locate/desal
Low-temperature distillation processes in single- and dual-purpose plants Gustavo Kronenberg*, Fredi Lokiec IDE Technologies Ltd., PO Box 591, Raanana 43104, Israel Tel. +972 (9) 747-9777; Fax +972 (9) 747-9715; email: [email protected]
Received3 August 2000; accepted 17 August 2000 Abstract
During the last decade, intensive research and development have been invested to improve further the advantages of the low-temperature process by increasing the unit's capacities and decreasing the energy consumption. This paper presents the advantages ofthe low-temperaturedistillationprocess, describespractical commercial application for steam driven multi-effect distillation plants in dual-purpose (electricity and water production) applications and the latest developments for single-purpose mechanical vapor compression plants. Keywords: Low-temperature distillation; Multi-effect distillation; Mechanical vapor compression; Cogeneration
1. Introduction
The low-temperature distillation process has been presented in various papers [1,2] where the general technological advances have been reemphasized, demonstrating the improved economics of related desalination processes. These advances can be summarized as follows: • Development of a unique design of a falling film horizontal tube evaporator/condenser, utilizing only latent heat transfer, avoiding sensible heat pick-up. *Correspondingauthor.
• Low temperature operation (max brine temp. of 70°C). • Use of an economical and durable construction material such as aluminum alloy for heat transfer tubes, plastic process piping and epoxy-painted steel shells. • Possibility of using low-cost/low-grade heat available through cogeneration schemes to minimize the energy cost component. • Minimal requirements for intake and pretreatment systems. The practical experience with commercial plants using the above-mentioned advances has
Presented at the 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 Ecole Nationale d'lngenieurs de Tunis, September 11-13, 2000, Jerba, Tunisia.
0011-9164/01/$-- See front matter © 2001 Elsevier Science B.V. All rights reserved
PII: SOOl1-9164(01)00181-3
190
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
been analyzed and discussed in other venues, showing their remarkable stability, flexibility and reliability of the low temperature process in comparison with others.
2. Low-temperature advantages
distillation
(LTD)
Low-temperature distillation is the basis for a series of features forming the core of the plant's highly economical operation [3.] • Low corrosion rates: The reduced corrosiveness of seawater at low operating temperature and vacuum conditions permits safe and economic use of corrosion-proof plastic materials and coatings both for piping and for vessel linings, as well as the use of aluminum for heat transfer tubing and vessel internals. Low maintenance and extended plant life (exceeding 25 years) result from the combination of the low corrosion rates and the use of a mild anti-sealant. • Flexibility: LTD plants have short start-up periods with little time loss for heating up. Plants have excellent load following capabilities allowing for production to closely match both water demand and energy supply. • Thermodynamic efficiency: The use of generous heat transfer surfaces results in a reduction of heat fluxes and temperature differentials and therefore in an increase of thermal efficieneies. As a result, 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 2-2.5°C. • Minimal scaling rates: The operating temperatures (Fig. 1) are well below the saturation limits of problematic sealants found in sea and most ground waters. Scale is reduced to an insignificant level, enabling plants to operate for long periods - -
5 years in some cases - - between cleanings. Low-cost polyelectrolyte feed pretreatment is adequate. Descaling is a simple procedure, consisting of mild acid recirculation, using the plant's own recirculation pumps. • High-purity distillate: An additional advantage is the high purity of the product water (usually less than 20 ppm and as low as 25 ppm for special applications). This allows the water to be used directly for industrial processes such as in refineries, power stations, breweries etc., where boiler water quality is required or in municipal installations to reduce the production costs further by blending the high-purity distillate with local brackish or poor-quality water and satisfy the potable water standards. • Reliability: Experienced engineering, rugged construction and proven equipment combined with extremely low corrosion and scaling rates result in minimal maintenance and lead to annual plant availability in excess of 95%. • Low energy costs: The low-temperature operation in dual-purpose application enables the use of 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 lowpressure 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 system [4].
3. Cogeneration: use of low-temperature heat sources 3.1. Diesel waste heat utilization
Several LTMED plants have been in operation since 1985 using 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 water pumps of the plant. Such
G. Kronenberg, F Lokiec / Desalination 136 (2001) 189-197
191
Fig. 1. Brine concentration/temperaturecurve for LT-MEDprocess. Operation range of low-temperaturedistillation. plants operate on the Marshall Islands and in Nauru in the Pacific, and on the islands of Bonaire in the Caribbean Sea. In these diesel-cogeneration installations, the MED draws the motive energy for desalination from the waste heat recovered from the exhaust gases and the jacket water cooling system of a diesel generator power station. (Fig. 2). 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%.
In the US Virgin Islands, 14 MED plants with thermocompression have been in operation since the early 1980s. Those LT-MED units have been performing at better than nominal rating ever since their installation. In the Reliance refinery (Fig. 3) in India, four MED plants are in operation, each one with a nominal production of 12,000 m3/d. The units have proved their reliability and fiexiblity in operation and they are actually producing 10% above nominal capacity. 3.2. 2. Back-pressure coupling
3.2. Steam turbine cogeneration 3.2.1. Extraction steam coupling
The LTMED 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.
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 in the range of 55 to 70°C steam 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 LTMED process to use standard condensing turbines (Fig 4) makes it a perfect match for large, dualpurpose plants. This capability also allows the addition of a desalination plant at a later stage to
Fig. 3. Reliance refinery (India); 4 x IVIED12,000 mVd.
an existing power station, since no change in the turbine design is required. Even a heat source that has a temperature as low as 60°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 (Fig. 5) or nuclear power system.
G. Kronenberg, F Lokiec / Desalination 136 (2001) 189-197
193
Fig. 4. Direct coupling of MED with back-pressure steam.
Fig. 5. MED operating with condenser hot water.
3.2.3. Combination o f 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.
194
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
4 kWh/t compared to 6-8 kWh/t for MVC. The advantages of MVC are the following: • Purity of product. Distillate of 5 ppm and less compared to 300-500ppm permeate for SWRO. • Does not require sophisticated pretreatment of seawater and is not sensitive to contamination of seawater by oil and/or other organic matter compared to SWRO. • More durable and requires less maintenance. There is no need for membrane replacement.
Fig. 6. LTMED 10,000 m3/d Curacao.
+
3.2 MW at the KAE,
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. 6). The success of this plant led to the purchase of a second, identical unit, which was commissioned in June 1990. This plant includes a small turbine 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 10,000t/d of product water. This results in a net power consumption for desalination below 5 kWh/t.
4. Low-temperature distillation mechanical vapor compression
with
Mechanical vapor compression (MVC) distillation is inherently the most thermodynamically efficient process of single-purpose thermal desalination plants. In recent years MVC and seawater reverse osmosis (SWRO) processes became the forerunners of all single-purpose desalination processes competing for the lowest seawater desalination cost. Of these two processes, modem SWRO is superior in having a lower specific energy consumption of about
The thermodynamic efficiency of the MVC process is derived from the application of the "heat pump" principle. The latent heat required by the system (evaporation and condensation steps) is continuously recycled by a large volumetric flow compressor, thus eliminating the need for cooling water for heat rejection as in MSF or MED process. The heat generated by the compressor work is rejected in the outgoing streams of product and brine that are discharged to the sea at a higher temperature than the seawater feed. However, in order to maintain a higher operating temperature, the feed is preheated by means of two plate heat exchangers by exchanging heat with the outgoing streams. The simultaneous transfer of latent heat on both sides of the heat transfer surface of a filmtype 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. Fig. 7 describes the process flows for a three effect MVC unit. Besides the thermodynamic advantage of the MVC process over the other single-purpose thermal processes such as MSF and thermal compression MED, it has the additional advantage that it does not have to be located close to a source of steam and requires no cooling water for heat rejection. There are fewer ecological problems and the operation is comparatively simple and easy (See Fig. 7).
195
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
t
m
r M w
m
Fig. 7. Multi-effectmechanicalvapor compression.
5. Features o f IDE MVC units
The main component featuring in IDE's MVC is the unique self-developed flexible blade compressor, capable of handling a large volumetric flow of water vapor at a relatively low cost. This compressor is coupled to a falling-film horizontal-tube evaporator that provides highly efficient heat transfer capacity. Consequently, this combination allows the plant to operate at a relatively low temperature, below 70°C, which in turn permits the selection of inexpensive materials of construction such as aluminum alloy for heat transfer tubes, polypropylene piping and epoxy-coated, carbon-steel vessels. Low-temperature operation also reduces scale problems and consequently requires a simpler feed treatment system. The inexpensive, yet durable, construction material allows for the generous use of heat
transfer surface tubes, thus reducing the thermal driving force that causes reduction in the electrical consumption of the compressor. Since 1969 IDE has installed and operated over 200 vapor compression units world wide, thus acquiring considerable expertise in this field. During the last decade, intensive research and development have been invested to improve the MVC process further by increasing the unit capacities and reducing energy. Growing demands for larger MVC units by the world market on one hand, and the requirements for decreasing specific capital investment cost and electrical consumption imposed by the pressing competition on the other hand, have led IDE to undertake further development of larger capacity, high efficiency MVC units. Larger MVC units are inherently superior compared to multiples of smaller units
196
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
yielding equivalent total capacity due to economies of scale. The operating cost, including energy consumption, could also be lowered by installing more specific heat transfer area (m2/t/d). The growing capacity of the MVC units is limited by the maximum compressor' s volumetric flow and the head developed by the compressor, the operating temperature and the heat transfer capacity of the evaporator, which in turn determine the required head. Increasing the heat transfer capacity could be achieved by increasing the number of effects, which also increases the output for a given volumetric flow. Increasing the size of the effect would also contribute to increased heat transfer capacity, but this increase is limited by available transportation means. Above a capacity of 900t/d, at least two effects are required.
units were tested and their performance at design conditions conformed to the expected design expectation, as follows: 3000ffd • Capacity: • Specific energy: Compressor: 6.9 kWh/t Total 8.1 kWh/t (including product pumping) Product salinity:3 ppm This performance was made possible by the development of a new compressor with higher volumetric capacity (about 75 m3/s) and increased head (more than 5000kg/kg), obtained by increasing the diameter of the suction channel, rotor and impeller diameters, and improving flow conditions in the rotating flow channel. A similar approach has been utilized in two additional units supplied recently to a Refinery in Turkemenistan (Caspian Sea).
6. MArC - - 3000 t/d units
Until recently the maximum capacity ofIDE' s unit did not exceed 2000 t/d. In 1998 six larger units of approximately 3000t/d (Fig. 8) comprising three effects of 4.8 diameter each coupled to a larger capacity compressor were designed and installed for Sarlux in Sardinia, Italy. These
Fig. 8. Sarlux 6 x MVC 3000 m3/d.
7. Design consideration in increasing MVC capacity
The main factor in increasing MVC capacity is by developing compressors with higher volumetric flow and head. The higher head enables the utilization of more effects in the unit,
G. Kronenberg, F. Lokiec / Desalination 136 (2001) 189-197
which yields more product for the same volumetric flow. Increasing the number of effects would also provide more specific heat transfer area and reduce the average mean effective boiling point elevation in the plant. In addition, the capacity of each individual effect can be further increased by using a more efficient heat transfer surface such as smaller diameter or grooved tubes, optimal wetting and larger diameter vessel. This would consequently decrease the specific energy consumed by the compressor. However, lowering specific energy has its limits. These would be set by the increase in the heat exchanger required to preheat the feed due to the decrease of the thermal driving force from 2.2°C to 1.2°C, corresponding to specific energy of 8-8.5 kWh/t, and 6 kWh/t, respectively. In some cases this problem could be partially solved by the introduction of waste heat into the system should it become available.
8. E x p e r i e n c e a n d c o n c l u s i o n s
I. The low-temperature distillation plants based on IDE's recent technological developments offer attractively low operating costs which compete with those of similar and alternative technologies. An additional advantage can be derived form the high purity of the produced water, allowing the water to be used directly for industrial process (boiler feed water) or to be blended with locally available brackish water.
197
2. The experience accumulated with the commercial plants during three decades and with more than 300 worldwide 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 long-term operation under remarkably stable conditions. Scale formation and corrosion are minimal or absent, and these factors lead to exceptionally high plant availabilities of 94% to 96%. 3. New developments in low-temperature distillation show the capability to enlarge the single unit capacity as follows: •
F o r M E D units:
Up to 20,000 m3/d with proven experience Up to 40,000 m3/d under development •
F o r M V C units:
Up to 3,000 m3/d with proven experience Up to 10,000 m3/d under development
References
[1] R. Matz and Z. Zimerman,Desalination,52 (1982) 201. [2] A. Ophir, A. Gendel and G. Kronenberg,Internat. DesalinationWat. Reuse Q., 4(1) (1994) 28. [3] IDE,Aquaport brochure, Fresh water from the sea, 1990. [4] A.N. Rogers, C.D. Siebenthal, R.F. Battey and L. Awerbuch,DesalinationTechnology- - Reporton the State of the Art, BechtelGroup, San Francisco, 1983.