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Recent achievements of low temperature multiple effect desalination in the western areas of Abu Dhabi. UAE
111
Desalination, 93 (1993) 11 l-l 18 Elsevier Science Publishers B.V.,
Amsterdam
Recent achievements of low temperature multiple effect desalination in the western areas of Abu Dhabi, UAE T. Michels Sidem, 54 rue de clichy, 75009 Paris (France). Tel. 33-l-49 Fax. 33-l-49 95 76 95
95 76 76;
SUMMARY
The multiple effect (ME) process allows optimised desalination at an attractive cost while conserving the proven reliability of seawater distillation. ME has already superseded the more classic MSF in the range of unit productions up to about 10,000 TPD and tends to increase its leading position further to larger unit capacities.
INTRODUCTION
In the second half of 1989 in view of improving rapidly and in a reliable way the water supply situation of the remote western areas of the Emirate of Abu Dhabi, UAE, the Water and Electricity Department issued an international tender for tbe supply and erection of the desalination plants to be located in three townships with the following capacities: Mirfa Jebel Dhanna Sila
2x1 mIgpd 1x1 mIgpd 1x1 mIgpd
The consultant was Ewbank-Preece of the UK. Sea water distillation was specified. The tender requested quotations for either MSF (multistage flash) or for ME (multiple effect) evaporators. OfIll-9164/93L$O6.00
0 1993 Elsevier Science Publishers B.V.
All rights reserved.
112
Whilst the merits of MSF are well known, the results of the tender showed once again the outstanding features of ME with thermal vapour compression, called ejectocompression with the following features: Low temperature operation lowering corrosion and scaling (top brine temperature is below 60°C) Low energy consumptions both thermal and electrical Good economics including erection, civil works and the seawater intake (35% cheaper than the MSF plants) (3) Short delivery time Easy operation and maintenance Proven reliability in Gulf conditions At the end of 1989 the contract was awarded to SIDEM on the basis of its ejectocompression system. At each site there were to be installed: a boiler plant with one diesel oil fired boiler per evaporator, a sea water intake, a desalination plant, and a distillate treatment plant. After a period of only two years of construction including all tests - in particular the 30-&y reliability run - the three plants were successfully put into commercial operation.
Tables I and II and Fig. 1 give the main data of the distillers, their heat and mass balance diagram as well as the sea water characteristics. While ejectocompression is able to achieve GGRs (gain output ratio, i.e. produced distillate to heating steam ratio) up to 17 [1,2] without excessive investment cost, a GGR of only 8 was specified because of the prevailing rather low fuel costs and with regard to the output capacity of the plants. This choice corresponds with the fact that for MSF, which was alternatively admitted in the tender, the investment costs rise sharply with higher GGRs thus further decreasing its competitiveness. Each evaporator has only four effects. This is sufficient to reach the required GGR of 8, thanks to the heat pump effect provided by the ejectocompression as explained hereafter. After being heated up in the last effect condenser, the incoming sea water is sprayed on the main tube bundles of each effect, where it partially evaporates in film outside the tubes thanks to the heat released by the condensing vapour inside the tubes which constitutes the distillate production of each effect. Vapour generated by this evaporation goes to the next
113
adjacent effect where it condenses inside the tubes forming again the
distillate production of that effect. TABLE I Emirates of Abu Dhabi - western remote areas. Four 1 mIgpd sea water desalination units Main technical performance (one unit) values Distillate Daily production: t/d t/h Temperature (after cooler), “C Maximum salinity (IDS), ppm
4548 189.5 38 10
Sea water Design temperature, “C Flow, t/h Required pressure, bar g
33116 1300 3.5
Steam to evaporator (saturated) Pressure, bar abs Flow, t/h Condensate temperature, “C Condensate salinity (IDS), ppm
24 22 62.7 5
Steam to vacuum ejector (saturated) Pressure, bar abs Flow (approx.), t/h
24 0.7
Electricity Consumption per ton of distillate produced (without sea water pump), kWh/ton
1.0
Antiscale additives Dosing rate, ppm Daily consumption, kg
3 42
Modules’ dimensions and weights (four modules for each evaporator) Length, m Diameter, m Weight, t
12 4.8 60
114 TABLE II Sea water characteristics of four 1 mIgpd desalination units. Emirate of Abu Dhabi, UAE - western remote areas ppm Calcium Magnesium Sodium and potassium Chloride Sulphate Bicarbonate Total dissolved solids
480 1750 15180 26390 3840 158 47800
PH
8.3
The process is repeated in each of the four effects. Distillate and brine from each effect are cascaded to the last effect where they are extracted by two horizontal centrifugal type pumps, one for distillate and one for brine. In the last effect only part of the vapour produced is condensed in its condenser while the other part is drawn by the ejectocompressor - the heat pump - and sent together with the motive steam as heating vapour to the first effect. The motive steam comes directly from the associated boiler without the need to reduce pressure; water tube boilers were selected because of the required boiler size and pressure. Several other important aspects should be highlighted: Particular attention was paid to the top brine temperature occurring in the first effect. It was kept below 60°C; this is of paramount importance to minimize evaporator scaling. Due to the absence of a brine recycle pump, the electric consumption of the evaporator is only about 1 kWh per ton of distillate produced. The condensate is drawn off from the first evaporator effect at a temperature of 62.7”C. The condensate flow is slightly higher than the total steam flow including the vacuum ejector, thus compensating directly for all losses of the steam/water cycle. Prior to its return to the boiler the condensate is heated to 105°C in the deaerator. Chemicals used for the boiler feed water treatment are compatible with the use of the produced distillate as drinking water.
(tncludlng
8tean
to
ventfng
8y8tem)
. BRINE BLOHDOWN O- 362.5 T/H T- 46.4 lC s- 71.5 C/KG
0
t ISTIUATE O- 169.5 TM T- 47.0 ‘C
T/H
UP 572.0 T/H 44.0 .C 47.6 wuc
REJECT a- 7ze.e b
OTs-
Fig. 1. Emirate of Abu Dhabi, UAE, western remote areas. Four 1 m&d desalination units. Heat and mass balance diagram; nominal conditions.
ZYNDENS~ITE RETURN O22.9 T/H T- 62.7 lC
STERM ‘P24.6 ATA 0- 22.7 T/H I T- 221 lC
K VI
116 CONSTRUCTION
Due concern was given to do as much as possible fabrication in the shop in order to limit the erection works at site while keeping the overall dimensions and weights of each evaporator block within reasonable limits in order to facilitate transportation. A cylindrical concept was chosen, as illustrated on Fig. 2, leading to four separate blocks consisting each of one effect and having about the following characteristics: Diameter 4.8 m Length 12 m Weight 60 tons Each block is delivered to the site completely tubed and pretested in the workshop. Erection work at site is therefore limited to the installationof the distillate condenser which comes as a separate block, the vapour connections between each block, the installation of the thermocompressor in between the first and fourth effect and of all required auxiliaries (pumps, valves and piping).
Fig. 2. Jebel Dhanna, Abu Dhabi. Multi-effect thermo-vapour compression -
1 x4T 4550.
117
Materials selected for the evaporator have a proven reliability and compatibility with brine and sea water. They are given for reference in Table III. TABLE III
Four 1 mIgpd seawater desalination units - evaporator materials (Emirate of Abu I Dhabi, UAE, western remote areas)
Evaporator vessel . . . .
Shell in contact with sea water Shell in contact with vapour External reinforcement Steam and distillate boxes
Heat tubes bundles . Tubes (3 top rows)
Stainless steel 316L Stainless steel 316L Carbon steel stainless steel
. Tubes (all others) . Tube-plates . Support plates
Titanium Aluminium brass 7612212 Stainless steel 316L Stainless steel 316L
Demisters
Stainless steel 316
Spray nozzles
Stainless steel 3 161
Distillate condenser . Shell
.Tubes . Tube-plates . Support plates . Water boxes
Stainless steel 316L Aluminium brass 7612212 Stainless steel 316L Stainless steel 316L Stainless steel 316L
OPERATION
Each unit underwent its commissioning tests, a 30&y reliability run and finally the performance test. The performance test results showed 10% higher production and 10% better GOR towards the guarantee figures. Distillate salinity was less than the guarantee of 10 ppm. These good results allowed a prompt provisional acceptance. Now after more than six months of service, operation and maintenance are fully satisfactory.
118 CONCLUSIONS
The described plants illustrate the advantages of a process which allows by its flexibility a good cost optimisation in most of the encountered situations of seawater desalination: 1. For single purpose plants, where the steam is generated by a dedicated boiler either: l l
when fuel costs are low it allows for very low investment costs, or in case of higher fuel price, GORs up to 17 can be reached resulting in a reduced fuel consumption of only 4 kg per ton of distillate produced, while the plant costs stay within reasonable limits.
2. A combination with a gas turbine or diesel engine heat recovery boiler is very attractive since the ejectocompression takes full advantage of the available steam pressure and a wide range of water productions can be achieved for a given steam quantity. 3. Lastly pass-out steam of a steam turbine can be used efficiently by a simple ME unit down to a steam outlet temperature of only 70°C.
REFERENCES 1
2
F. Murat and T. Michels, The high performance ejecto-compression process at low temperature applied to sea water desalination (3 X2500 TPD in St. Marteen-Caribbean Sea), Sidem, 1989. N. Greco, A. Durante and F. Murat, Application of the ME process at low temperature to a large seawater de&nation plant (Trapani, Sicily, 6X9000 TPD), Washington Desalination Conference, 1991.
Desalination, 93 (1993) 11 l-l 18 Elsevier Science Publishers B.V.,
Amsterdam
Recent achievements of low temperature multiple effect desalination in the western areas of Abu Dhabi, UAE T. Michels Sidem, 54 rue de clichy, 75009 Paris (France). Tel. 33-l-49 Fax. 33-l-49 95 76 95
95 76 76;
SUMMARY
The multiple effect (ME) process allows optimised desalination at an attractive cost while conserving the proven reliability of seawater distillation. ME has already superseded the more classic MSF in the range of unit productions up to about 10,000 TPD and tends to increase its leading position further to larger unit capacities.
INTRODUCTION
In the second half of 1989 in view of improving rapidly and in a reliable way the water supply situation of the remote western areas of the Emirate of Abu Dhabi, UAE, the Water and Electricity Department issued an international tender for tbe supply and erection of the desalination plants to be located in three townships with the following capacities: Mirfa Jebel Dhanna Sila
2x1 mIgpd 1x1 mIgpd 1x1 mIgpd
The consultant was Ewbank-Preece of the UK. Sea water distillation was specified. The tender requested quotations for either MSF (multistage flash) or for ME (multiple effect) evaporators. OfIll-9164/93L$O6.00
0 1993 Elsevier Science Publishers B.V.
All rights reserved.
112
Whilst the merits of MSF are well known, the results of the tender showed once again the outstanding features of ME with thermal vapour compression, called ejectocompression with the following features: Low temperature operation lowering corrosion and scaling (top brine temperature is below 60°C) Low energy consumptions both thermal and electrical Good economics including erection, civil works and the seawater intake (35% cheaper than the MSF plants) (3) Short delivery time Easy operation and maintenance Proven reliability in Gulf conditions At the end of 1989 the contract was awarded to SIDEM on the basis of its ejectocompression system. At each site there were to be installed: a boiler plant with one diesel oil fired boiler per evaporator, a sea water intake, a desalination plant, and a distillate treatment plant. After a period of only two years of construction including all tests - in particular the 30-&y reliability run - the three plants were successfully put into commercial operation.
Tables I and II and Fig. 1 give the main data of the distillers, their heat and mass balance diagram as well as the sea water characteristics. While ejectocompression is able to achieve GGRs (gain output ratio, i.e. produced distillate to heating steam ratio) up to 17 [1,2] without excessive investment cost, a GGR of only 8 was specified because of the prevailing rather low fuel costs and with regard to the output capacity of the plants. This choice corresponds with the fact that for MSF, which was alternatively admitted in the tender, the investment costs rise sharply with higher GGRs thus further decreasing its competitiveness. Each evaporator has only four effects. This is sufficient to reach the required GGR of 8, thanks to the heat pump effect provided by the ejectocompression as explained hereafter. After being heated up in the last effect condenser, the incoming sea water is sprayed on the main tube bundles of each effect, where it partially evaporates in film outside the tubes thanks to the heat released by the condensing vapour inside the tubes which constitutes the distillate production of each effect. Vapour generated by this evaporation goes to the next
113
adjacent effect where it condenses inside the tubes forming again the
distillate production of that effect. TABLE I Emirates of Abu Dhabi - western remote areas. Four 1 mIgpd sea water desalination units Main technical performance (one unit) values Distillate Daily production: t/d t/h Temperature (after cooler), “C Maximum salinity (IDS), ppm
4548 189.5 38 10
Sea water Design temperature, “C Flow, t/h Required pressure, bar g
33116 1300 3.5
Steam to evaporator (saturated) Pressure, bar abs Flow, t/h Condensate temperature, “C Condensate salinity (IDS), ppm
24 22 62.7 5
Steam to vacuum ejector (saturated) Pressure, bar abs Flow (approx.), t/h
24 0.7
Electricity Consumption per ton of distillate produced (without sea water pump), kWh/ton
1.0
Antiscale additives Dosing rate, ppm Daily consumption, kg
3 42
Modules’ dimensions and weights (four modules for each evaporator) Length, m Diameter, m Weight, t
12 4.8 60
114 TABLE II Sea water characteristics of four 1 mIgpd desalination units. Emirate of Abu Dhabi, UAE - western remote areas ppm Calcium Magnesium Sodium and potassium Chloride Sulphate Bicarbonate Total dissolved solids
480 1750 15180 26390 3840 158 47800
PH
8.3
The process is repeated in each of the four effects. Distillate and brine from each effect are cascaded to the last effect where they are extracted by two horizontal centrifugal type pumps, one for distillate and one for brine. In the last effect only part of the vapour produced is condensed in its condenser while the other part is drawn by the ejectocompressor - the heat pump - and sent together with the motive steam as heating vapour to the first effect. The motive steam comes directly from the associated boiler without the need to reduce pressure; water tube boilers were selected because of the required boiler size and pressure. Several other important aspects should be highlighted: Particular attention was paid to the top brine temperature occurring in the first effect. It was kept below 60°C; this is of paramount importance to minimize evaporator scaling. Due to the absence of a brine recycle pump, the electric consumption of the evaporator is only about 1 kWh per ton of distillate produced. The condensate is drawn off from the first evaporator effect at a temperature of 62.7”C. The condensate flow is slightly higher than the total steam flow including the vacuum ejector, thus compensating directly for all losses of the steam/water cycle. Prior to its return to the boiler the condensate is heated to 105°C in the deaerator. Chemicals used for the boiler feed water treatment are compatible with the use of the produced distillate as drinking water.
(tncludlng
8tean
to
ventfng
8y8tem)
. BRINE BLOHDOWN O- 362.5 T/H T- 46.4 lC s- 71.5 C/KG
0
t ISTIUATE O- 169.5 TM T- 47.0 ‘C
T/H
UP 572.0 T/H 44.0 .C 47.6 wuc
REJECT a- 7ze.e b
OTs-
Fig. 1. Emirate of Abu Dhabi, UAE, western remote areas. Four 1 m&d desalination units. Heat and mass balance diagram; nominal conditions.
ZYNDENS~ITE RETURN O22.9 T/H T- 62.7 lC
STERM ‘P24.6 ATA 0- 22.7 T/H I T- 221 lC
K VI
116 CONSTRUCTION
Due concern was given to do as much as possible fabrication in the shop in order to limit the erection works at site while keeping the overall dimensions and weights of each evaporator block within reasonable limits in order to facilitate transportation. A cylindrical concept was chosen, as illustrated on Fig. 2, leading to four separate blocks consisting each of one effect and having about the following characteristics: Diameter 4.8 m Length 12 m Weight 60 tons Each block is delivered to the site completely tubed and pretested in the workshop. Erection work at site is therefore limited to the installationof the distillate condenser which comes as a separate block, the vapour connections between each block, the installation of the thermocompressor in between the first and fourth effect and of all required auxiliaries (pumps, valves and piping).
Fig. 2. Jebel Dhanna, Abu Dhabi. Multi-effect thermo-vapour compression -
1 x4T 4550.
117
Materials selected for the evaporator have a proven reliability and compatibility with brine and sea water. They are given for reference in Table III. TABLE III
Four 1 mIgpd seawater desalination units - evaporator materials (Emirate of Abu I Dhabi, UAE, western remote areas)
Evaporator vessel . . . .
Shell in contact with sea water Shell in contact with vapour External reinforcement Steam and distillate boxes
Heat tubes bundles . Tubes (3 top rows)
Stainless steel 316L Stainless steel 316L Carbon steel stainless steel
. Tubes (all others) . Tube-plates . Support plates
Titanium Aluminium brass 7612212 Stainless steel 316L Stainless steel 316L
Demisters
Stainless steel 316
Spray nozzles
Stainless steel 3 161
Distillate condenser . Shell
.Tubes . Tube-plates . Support plates . Water boxes
Stainless steel 316L Aluminium brass 7612212 Stainless steel 316L Stainless steel 316L Stainless steel 316L
OPERATION
Each unit underwent its commissioning tests, a 30&y reliability run and finally the performance test. The performance test results showed 10% higher production and 10% better GOR towards the guarantee figures. Distillate salinity was less than the guarantee of 10 ppm. These good results allowed a prompt provisional acceptance. Now after more than six months of service, operation and maintenance are fully satisfactory.
118 CONCLUSIONS
The described plants illustrate the advantages of a process which allows by its flexibility a good cost optimisation in most of the encountered situations of seawater desalination: 1. For single purpose plants, where the steam is generated by a dedicated boiler either: l l
when fuel costs are low it allows for very low investment costs, or in case of higher fuel price, GORs up to 17 can be reached resulting in a reduced fuel consumption of only 4 kg per ton of distillate produced, while the plant costs stay within reasonable limits.
2. A combination with a gas turbine or diesel engine heat recovery boiler is very attractive since the ejectocompression takes full advantage of the available steam pressure and a wide range of water productions can be achieved for a given steam quantity. 3. Lastly pass-out steam of a steam turbine can be used efficiently by a simple ME unit down to a steam outlet temperature of only 70°C.
REFERENCES 1
2
F. Murat and T. Michels, The high performance ejecto-compression process at low temperature applied to sea water desalination (3 X2500 TPD in St. Marteen-Caribbean Sea), Sidem, 1989. N. Greco, A. Durante and F. Murat, Application of the ME process at low temperature to a large seawater de&nation plant (Trapani, Sicily, 6X9000 TPD), Washington Desalination Conference, 1991.