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Cost effective evaporators for desalination
DESALINATION ELSEVIER
Desalination
108 (1996) 357-360
Cost effective evaporators for desalination Heikki Jaakkola Chemitec
Consulting Oy/AQUAMAX Oy, Sinikalliontie 14, 02630 Espoo. Finland. Tel.: +3S8-9-5021034; Fax.: +3.58-9-529032 Received
19 August 1996; accepted 24 August 1996
Abstract
AQUAMAX Keywords:
evaporatorsare described.
Evaporators
1. Introduction
2. Heat transfer in plastic heat exchanger
Production costs of desalinated water depends on operation costs and capital investment. To reduce operation costs in evaporative desalination processes, basically energy consumption, means generally more heat transfer area and/or increased number of evaporation stages resulting higher investment costs. In order to overcome the cost circle, AQUAMAX has developed a new, patented evaporation technology, which makes possible to build low energy consumption evaporators with reasonable investment costs. The unique innovation of the technology is the evaporation heat exchanger, which is made of high quality, flexible plastic materials.
The AQUAMAX heat exchanger is made of thin plastic film, which is welded to plastic envelope elements, “plastic bags”. The elements are put together with liquid distributor plates in the upper part and the distillate collectors in the bottom parts of the elements to form a vertical falling film heat exchanger cassette, where heating steam flows inside the elements and brine on the outer surface as a thin, evenly spread film. Thermal conductivity of plastic materials is low, 0.2-0.5 W/m”C, compared to that of metallic materials, 15-45 W/m”C. When the walls of a plastic heat exchanger are very thin, less than 0.1 mm, the difference in overall heat transfer coefficients between plastic and metallic heat exchangers is only 20-30%, as shown in Fig. 1.
Presented at the Second Annual Meeting of the Euro ean Desalination Society (EDS) on Desalination an dp the Environment, Genoa, Italy, October 20-23, 1996. 001 l-9164/97/$17.00 Copyright PZZ SOOll-9164(97)00044-l
B 1997 Elsevier Science B.V. All rights reserved.
H. Jaakkola / Desalination
3.58
p 4000 (u E 2
3000
3
2000
2
1000
Metallic
Plastic
0 0
095
1
1,5
Wall thickness / mm
Fig. 1. Typical heat transfer coefficients evaporators.
3. Physical changer
strength
of plastic
in saline water
heat
ex-
AQUAMAX evaporators are operated at low temperatures, 40-60°C, with low temperature differences, 2.3-3°C. In these circumstances the pressure difference over the exchanger walls is only lo-25 cm water column and the tensile stress is 0.5-l N/mm”. Typical yield strength for plastic films is lO_ 25 N/mm2 exceeding well the requirements. Also the corrosion resistance of the plastics against seawater and most waste waters and chemicals is very good as generally known.
IO8 (1996) 357-360
film. When the water film flows downward on the surface it is heated by the steam flowing inside the heat transfer cassettes. The steam is condensed and the released heat flows through the plastic film to the water film, which is partly evaporated and the salinity of the remaining water is increased. The evaporated vapor flows in to the fan, which compress the vapor to higher pressure and temperature. The heated vapor is then used as heating steam in the heat transfer cassettes. The condensed steam, distilled water, is collected from the bottom parts of the cassettes. Leakage air is removed from the evaporator through the condenser by vacuum pump. Air is saturated with water vapor, which is condensed in the condenser and that condensate is led to the main condensate stream. The brine is pumped from the evaporator with brine pump trough preheater and led further in the sewer. The energy consumption of the compressor in a single stage MVC-evaporator can be estimated by Eq. (1) P = c *M
*AT
(1)
where = consumption of electricity, kW = coefficient depending on the efficiency of the compressor, typically 2.5-3 = rate of compressed vapor (=evaporation capacity), t/h = rise of vapor saturation temperature, “C
P
4. MVC evaporator When water vapor is compressed to higher pressure, its condensation temperature increases at the same time. In AQUAMAX VC-evaporator (mechanical Vapor reCompression) water vapor evaporated from saline water is compressed by a fan and the compressed vapor is utilized as heating steam in the same stage and no other energy is required for evaporation than little electricity to run the fan and the pumps (Fig. 2). Raw water is led in to the evaporator chamber trough preheaters. Water in the bottom part of the chamber is pumped with circulation pump in to the liquid distributor on the top of heat transfer cassettes. The liquid distributor supplies water on to the heat transfer surface as thin, evenly spread water
c
M AT
Eq. (1) shows that decreasing AT will decrease energy consumption. On the other hand, required heat transfer area is inversely proportional to AT; the smaller AT the larger heat transfer area as can be seen from Eq. (2) M
-
k *A
*AT
(2)
where k
= overall heat transfer
A
m2.‘C = heat transfer area, m2
coefficient,
W/
H. Jaakkola /Desalination
359
108 (1996) 357-360
COMPRESSOR
VACUUM
PUMP
METERING
PUMP WATER CTHIXICAL
SFA WAlER
w PREHi3’l-ERS
BRINE PUMP
Fig. 2. Operating
principle
of AQUAMAX
VC-evaporator.
To find the most cost effective evaporator means to optimize operation and investment costs through Eqs. (1) and (2). Generally, the optimization of MVC evaporators with metallic heat exchangers leads to AT of 46°C. Then energy consumption, including the pumps, is about 12-18 kWh/m3 distilled water. Due to very economical costs of plastic heat exchanger the optimum AT for AQUAMAX VC-evaporator is only 2-2.3”C corresponding specific energy consumption of 8-9 kWh/m3 distilled water. Fig. 3. shows the relative investment cost structures of AQUAMAX and conventional MVC-evaporators. The figure shows that the optimal conventional MVC-evaporator requires about 40% more energy and is about 10% more expensive than AQUAMAX VC-evaporator. On the other hand, the conventional MVCevaporator is about 30% more expensive than AQUAMAX VC-evaporator with the same specific energy consumption.
z
I,0
& 0.8 .$
0.6
d
0,4
02 0.0 AOUAiWAX
Conventional Optimum
Conventional Low energy
5
2.3
23
ATI%
PtotlkWhlt
8
hs
Heat transfer
0
Compressor
8
14
surface
? ?Vessel
g auxiliary
equipment
Fig. 3. Relative investment costs of MVC-evaporators and corresponding specific energy consumption.
5. Other applications Falling film principle is also widely used in multi effect distillation (MED) and thermal vapor recompression (TVC) processes. MED
360
H. Jaakkola /Desalination
plant can be even more price effective than if low pressure steam is MVC-process, available. According to Rautenbach [ 11, power generation penalty of 3.8 kWh/m3 can be achieved with a 11 effect evaporator utilizing 0.3 bar (abs) steam, t, = 70°C, resulting a total power consumption of 5.8 kWh/m3 including power for pumping. In these circumstances, thanks to low cost plastic heat exchanger, AQUAMAX MED plant’s investment costs would be about 30% lower than those of a conventional MED plant. 6. Conclusions Several AQUAMAX MVC-evaporators have proved their efficiency by producing distilled water with specific energy consumption of 8-9 kWNm3. The applications at
108 (1996) 357-360
the moment include desalination and waste water evaporation in the capacity range of 50-1,200 m3/d. Due the modular cassette construction of evaporation heat exchangers AQUAMAX technology can be applied economically to all plant sizes and also to low temperature MED and TVC processes. In addition to low manufacturing costs of plastic heat exchangers the AQUAMAX plants are operated in low temperatures yielding low potentiality of scale formation, corrosion and the cost of pretreatment.
Reference [l]
R. Rautenbach, J. Widua, and S. Schlfer, Reflections on desalination processes for the 21st Century, Proc. of IDA World Congress, Abu Dhabi, 1995, Vol. I, pp. 117-136.
Desalination
108 (1996) 357-360
Cost effective evaporators for desalination Heikki Jaakkola Chemitec
Consulting Oy/AQUAMAX Oy, Sinikalliontie 14, 02630 Espoo. Finland. Tel.: +3S8-9-5021034; Fax.: +3.58-9-529032 Received
19 August 1996; accepted 24 August 1996
Abstract
AQUAMAX Keywords:
evaporatorsare described.
Evaporators
1. Introduction
2. Heat transfer in plastic heat exchanger
Production costs of desalinated water depends on operation costs and capital investment. To reduce operation costs in evaporative desalination processes, basically energy consumption, means generally more heat transfer area and/or increased number of evaporation stages resulting higher investment costs. In order to overcome the cost circle, AQUAMAX has developed a new, patented evaporation technology, which makes possible to build low energy consumption evaporators with reasonable investment costs. The unique innovation of the technology is the evaporation heat exchanger, which is made of high quality, flexible plastic materials.
The AQUAMAX heat exchanger is made of thin plastic film, which is welded to plastic envelope elements, “plastic bags”. The elements are put together with liquid distributor plates in the upper part and the distillate collectors in the bottom parts of the elements to form a vertical falling film heat exchanger cassette, where heating steam flows inside the elements and brine on the outer surface as a thin, evenly spread film. Thermal conductivity of plastic materials is low, 0.2-0.5 W/m”C, compared to that of metallic materials, 15-45 W/m”C. When the walls of a plastic heat exchanger are very thin, less than 0.1 mm, the difference in overall heat transfer coefficients between plastic and metallic heat exchangers is only 20-30%, as shown in Fig. 1.
Presented at the Second Annual Meeting of the Euro ean Desalination Society (EDS) on Desalination an dp the Environment, Genoa, Italy, October 20-23, 1996. 001 l-9164/97/$17.00 Copyright PZZ SOOll-9164(97)00044-l
B 1997 Elsevier Science B.V. All rights reserved.
H. Jaakkola / Desalination
3.58
p 4000 (u E 2
3000
3
2000
2
1000
Metallic
Plastic
0 0
095
1
1,5
Wall thickness / mm
Fig. 1. Typical heat transfer coefficients evaporators.
3. Physical changer
strength
of plastic
in saline water
heat
ex-
AQUAMAX evaporators are operated at low temperatures, 40-60°C, with low temperature differences, 2.3-3°C. In these circumstances the pressure difference over the exchanger walls is only lo-25 cm water column and the tensile stress is 0.5-l N/mm”. Typical yield strength for plastic films is lO_ 25 N/mm2 exceeding well the requirements. Also the corrosion resistance of the plastics against seawater and most waste waters and chemicals is very good as generally known.
IO8 (1996) 357-360
film. When the water film flows downward on the surface it is heated by the steam flowing inside the heat transfer cassettes. The steam is condensed and the released heat flows through the plastic film to the water film, which is partly evaporated and the salinity of the remaining water is increased. The evaporated vapor flows in to the fan, which compress the vapor to higher pressure and temperature. The heated vapor is then used as heating steam in the heat transfer cassettes. The condensed steam, distilled water, is collected from the bottom parts of the cassettes. Leakage air is removed from the evaporator through the condenser by vacuum pump. Air is saturated with water vapor, which is condensed in the condenser and that condensate is led to the main condensate stream. The brine is pumped from the evaporator with brine pump trough preheater and led further in the sewer. The energy consumption of the compressor in a single stage MVC-evaporator can be estimated by Eq. (1) P = c *M
*AT
(1)
where = consumption of electricity, kW = coefficient depending on the efficiency of the compressor, typically 2.5-3 = rate of compressed vapor (=evaporation capacity), t/h = rise of vapor saturation temperature, “C
P
4. MVC evaporator When water vapor is compressed to higher pressure, its condensation temperature increases at the same time. In AQUAMAX VC-evaporator (mechanical Vapor reCompression) water vapor evaporated from saline water is compressed by a fan and the compressed vapor is utilized as heating steam in the same stage and no other energy is required for evaporation than little electricity to run the fan and the pumps (Fig. 2). Raw water is led in to the evaporator chamber trough preheaters. Water in the bottom part of the chamber is pumped with circulation pump in to the liquid distributor on the top of heat transfer cassettes. The liquid distributor supplies water on to the heat transfer surface as thin, evenly spread water
c
M AT
Eq. (1) shows that decreasing AT will decrease energy consumption. On the other hand, required heat transfer area is inversely proportional to AT; the smaller AT the larger heat transfer area as can be seen from Eq. (2) M
-
k *A
*AT
(2)
where k
= overall heat transfer
A
m2.‘C = heat transfer area, m2
coefficient,
W/
H. Jaakkola /Desalination
359
108 (1996) 357-360
COMPRESSOR
VACUUM
PUMP
METERING
PUMP WATER CTHIXICAL
SFA WAlER
w PREHi3’l-ERS
BRINE PUMP
Fig. 2. Operating
principle
of AQUAMAX
VC-evaporator.
To find the most cost effective evaporator means to optimize operation and investment costs through Eqs. (1) and (2). Generally, the optimization of MVC evaporators with metallic heat exchangers leads to AT of 46°C. Then energy consumption, including the pumps, is about 12-18 kWh/m3 distilled water. Due to very economical costs of plastic heat exchanger the optimum AT for AQUAMAX VC-evaporator is only 2-2.3”C corresponding specific energy consumption of 8-9 kWh/m3 distilled water. Fig. 3. shows the relative investment cost structures of AQUAMAX and conventional MVC-evaporators. The figure shows that the optimal conventional MVC-evaporator requires about 40% more energy and is about 10% more expensive than AQUAMAX VC-evaporator. On the other hand, the conventional MVCevaporator is about 30% more expensive than AQUAMAX VC-evaporator with the same specific energy consumption.
z
I,0
& 0.8 .$
0.6
d
0,4
02 0.0 AOUAiWAX
Conventional Optimum
Conventional Low energy
5
2.3
23
ATI%
PtotlkWhlt
8
hs
Heat transfer
0
Compressor
8
14
surface
? ?Vessel
g auxiliary
equipment
Fig. 3. Relative investment costs of MVC-evaporators and corresponding specific energy consumption.
5. Other applications Falling film principle is also widely used in multi effect distillation (MED) and thermal vapor recompression (TVC) processes. MED
360
H. Jaakkola /Desalination
plant can be even more price effective than if low pressure steam is MVC-process, available. According to Rautenbach [ 11, power generation penalty of 3.8 kWh/m3 can be achieved with a 11 effect evaporator utilizing 0.3 bar (abs) steam, t, = 70°C, resulting a total power consumption of 5.8 kWh/m3 including power for pumping. In these circumstances, thanks to low cost plastic heat exchanger, AQUAMAX MED plant’s investment costs would be about 30% lower than those of a conventional MED plant. 6. Conclusions Several AQUAMAX MVC-evaporators have proved their efficiency by producing distilled water with specific energy consumption of 8-9 kWNm3. The applications at
108 (1996) 357-360
the moment include desalination and waste water evaporation in the capacity range of 50-1,200 m3/d. Due the modular cassette construction of evaporation heat exchangers AQUAMAX technology can be applied economically to all plant sizes and also to low temperature MED and TVC processes. In addition to low manufacturing costs of plastic heat exchangers the AQUAMAX plants are operated in low temperatures yielding low potentiality of scale formation, corrosion and the cost of pretreatment.
Reference [l]
R. Rautenbach, J. Widua, and S. Schlfer, Reflections on desalination processes for the 21st Century, Proc. of IDA World Congress, Abu Dhabi, 1995, Vol. I, pp. 117-136.