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Mechanical vapour compression desalination plants — A case study
DESALINATION
ELSEVIER
Desalination 101 (1995) 1-10
Mechanical vapour compression desalination plants-A case study Jos6 M. Veza Departamento de ]ngenierfa de Procesos, Universidad de Las Palmas de Gran Canaria, 7rafira Baja, 350! 7 Las Palmas de Gran Canaria, Spain Rece:ved 22 September 1993; accepted 2 December 1993
Abstract Over 120 desalination plants are spread over the Canary Islands due to a scarcity of conventional water resources. These units are varied in technology and size, and accordingly a fairly good knowledge of their technology as well as experience on their operation has been developed. As a case study, The Las Palmas Port Authority desalination plant consists of two low-temperature vapour compression units, with a production capacity of 500 m3/d each. These units were conmaissioned in 1987 and 1989 and have been producing water with high levels of availability, namely average plant factors of 87.3 % and 90.2 %, respectively. Product water is consistently below 20 #S/cm conductivity. Energy consumption ranges from 10.4-11.2 kWh/m3, and the standard energy efficiency for the equipment is around 58.9 % tbr the ~ompressors. This report summarizes the design parameters for the system as well as its operational features.
Keywords: Vapour compression, Distillation, Operating results
1. introduction The Canary Islands, particularly Gran Canaria, Lanzarote and Fuerteventura, have suffered a long-time water scarcity as shown by the low rates of consumption. For the City of Las Palmas, the supplies have increased over the last years, reaching 188 I/person/d available in 1991.
Amongst other actions taken to alleviate the situation, 122 sea and brackish water desalination systems have been built with a global production capacity of ever 212,000 m3/d in 1990 [1-3]. These plants include most technologies in the market: multistage flash, vapour compression, reverse osmosis and electrodialysis.
001 I-9164195/$09.50 © 1993 Elsevier Science B.V. All rights reserved SSD! 001 I-9164(95 )00002-X
2
J.M. Veza /Desalination 101 (1995) 1-10
Between 1988 and 1990 the increase in production capacity rose to 32% due to the commissioning of a new plant (Las Palmas III, 36,000 m3/d capacity) as well as some plants scheduled on the Desalination Plants Scheme for the Canary Islands. The Scheme is the outcome of an agreement between the Ministry '~f Public Works and the Regional Government. The first stage began in 1988 and includes an overall new capacity of 40,000 m3/d in ten plants for a 5-year period. An extension was added in 1992 to cover an add;tional 55,000 m3/d capaci~ up to 1996. Among these plants is the one built for the Las Palmas Port Authority, whose design and operation will be described in this report.
were the only technologies taken into account for the required production capacity. RO technology was discarded because of the feed seawater in the Port containing oils which would have made necessary a full line of pretreatment, complicated to operate. In addition, the higher TDS in product was not adequate in case a ship would require water for boiler make-up. Other VC manufacturers offered more sophisticated designs, with slightly lower energy consumption, but they did not have a long enough record of experience. Water cost was considered, of course, and it was reasonable for uhe VC process compared to RO proposals. In any event costs were not the determining factor. Above all, reliability was sought in order to secure water supply to ships at all times.
2. Project description Las Palmas Port supports heavy shipping traffic in Spain, numbering 12,442 units in 1991 and 43.38 million tons in gross weight. The total amount of water supplied through the Port Authority was 773,013 m 3. Out of this quantity, 311,159 m 3 were delivered to shipping. In the past water was provided by the Munic.ipal Water Company and through purchases fi-om private suppliers~ although the Port Authority started in 1980 to survey the feasibility of having their own water supply, mainly because of the low availability of resources then. In 1987 it was decided to acquire a desalination unit with a production capacity of 500 m 3/d with vapour compression technology, which was later extended to a second unit commissioned in 1989 to reach an overall capacity of 1000 m3/d. Both units were supplied by IDE Technologies and are located adjacent to the workshop and warehouse areas. The tender and process selection was ~ssisted by Centro de Estudios Hidrogr~tficos, Madrid (Center for Hydrology Studies). Vapour compression (VC) and reverse osmosis (RO)
3. Description of the vapour compression process Desalination by vapour compression is described elsewhere [4-6], and it consists basica!Wy of a seawater evaporator which uses as a heating medium the vapour produced in the chamber itself after compression and heating by mechanical means. Our reference plant consists of two units of 500 m3/d each, with a common water intake, apart from the auxiliary facilities. Feed water is pumped from a 5 m deep, 2 m diameter well drilled inside the site. There are two stainless steel pumps, each rated 15 HP and capable of conveying 50 m 3/h. The pretreatment applied to feed water includes passing it through a 2 m long, 0.2 m diameter cartridge filled with aluminium pellets to prevent tube corrosion in the evaporator. An antiscalant is dosed to pt'event precipitation of salts. Some time after commissioning, two 0.4 m diameter hydrocyclones were added in order to reduce suspended solids in water, thus extending the time between cleanings of the hea~ exchangers.
J.M. Veza / Desalination 101 (1995) 1-10
3
Evaporator
I I Product Pzoduct ~'
Sea water
-,O Brine
Heat exchangers (product and .brine)
Brine
Fig. 1. Flow diagram for the vapour compression process. Feed water is then taken through two plate heat exchangers in parallel, each one of them heated by the product water and brine, respectively, which are discharged from the evaporator (Fig. 1). Seawater is then mixed with recycled brine from the evaporator and pumped to the evaporator through spray nozzles which spread the water over the horizontal tube bundle. Enough water is provided in order to get a thin continuous film over the tubes, therefore increasing the heat transfer rate. The evaporator-condenser consists of a horizontal tube bundle made from an aluminium alloy and falling film in a steel vessel where water is sprayed through nozzles. The tubes are 5.13 m long with a total surface area of 2598 m 2 while the vessel has a 4 m diameter. Operating temperature is around 59"C. The vapour compressor produces a vacuum when suetioning in such a way that pressure is lower than that of equilibrium at feed tempera-
ture; therefore, part of the seawater evapot'ates. After passing a separation mesh, vapour is absorbed by the rotating compressor where it is slightly compressed and heated and then discharged inside the tubes where it condenses. The vapour c gmpressor is a radial centrifuge type capable of processing 21 t/h vapour, an aluminium rotor and titanium blades designed and developed by Zarehin-IDE. Non-condensable gases are continually extracted from the system by means of a vacuum pump connected to an auxiliary condenser while also producing the initial start-up vacuum. The auxiliary condenser is a shell and tube type of a titanium alloy. Part of the brine is recirculated and mixed with make-up feed water to be pumped through the spray nozzles over the tubes. Product water is treated with soda ash to adjust pH, ~hus neutralizing its slight acidity. Heat transfer in horizontal tubes and thin
4
J.M. Veza/Desalitmtion 101 (1995) 1-10
film allows high transfer coefficients, even in operation at moderate temperatures. At operation tetaperatures around 60°C, scaling is reduced and heat losses through the vessel are minimized with minimum requirements for thermal isolation.
• • • •
• •
Some other advantages are: modular type: capable of extending capacity simplicity of pretreatment and operation good product quality flexible operation: load adjustment through temperature variation,,; Some significant drawback may be: unit capacity limited to some 1200 m 3/d energy consumption rather high compared to other processes, although this is decreasing due to technological improvements
The most significant features of the equipment are listed in Table 1.
Both distillation units were supplied with a 28,000 m3/month production warranty, which is equivalent to 162,500 m 3/y at 325 d/y. The assured product water quality is lower than 100 ppm TDS, with power consumption below 12.5 kWh/m 3 product water, with a breakdown as follows: • Seawater feed pumping -- 0.3 • • •
Production Product pumping Total
- - 1 1.5 - - 0.2 - - 12
Certain conditions were agreed upon with the contractor: energy consumption in excess of 12.5 kWh/m 3 (on a quarterly basis) is charged to the contractor; non-produced water is also t:harged; and product water over 100 ppm TDS is not paid for.
4. Operational features Table 1 Main design parameters Technology Number of units Unit capacity, m3/d Feed water flow rate, m3/h Feed water salinity, % TDS Feed temperature, *C Feed pressure, bar Pretreatment, ant;.sealaot~ ppm Product water flow rate, m3/h Product salinity, ppm TDS Post-treatment, sodium carbonate, ppm Brine discharge flow rate, m3/h Brine salinity, % TDS Evaporation temperature, *C Operating pressure, mm Hg Recovery ratio, % Power required, kW Total installed powera, kW Specific energy consumption", kWh/m 3 aFeed intake pumping not included. bAmbient temperature vapour compression.
4.1. Actual operating parameters ATVCb 2
500 50 3.8 21 2 5 20.83 < 30 3 29 6.5 59 150 45 239.6 292 11.5
The system usually operates close to the c~e~ign conditions. Table 2 shows the operating conditions for unit #2 as of May7, 1991. Table 2 Operating conditions Feed water flow rate, m3/h Raw feed temperature, °C Feed temperature, product heat exchanger outlet, °C Feed temperature, brine heat exchanger outlet, °C Recycle brine temperature, °C Recycle pump pressure, bar Product pump pressure, bar Brine pump pressure, bar Evaporator pressure, bac Compressor discharge pressure, bar Product flow rate, m3/h Brine flow rate, m3/h Product quality, ~tS/cm
50 20 55 53 59 0.7 3.8
3 0.19
0.21 21 29 6.5
J.M. Veza I Desalination 101 (1995) 1-10
It may be worthwhile to note the low operating temperature in the evaporator (below 60°C), as well as the low compression ratio at the vapour compressor (1.10).
5
average plant factor of 87.3% when the plant factor is defined as follows: plant factor = actual monthly production
(1)
days x daily design production month
4. 2. Evolution along time
Operating parameters for both units are alike and fairly stationary along time. No remarkable deviations have been recorded [7]. As an example, Fig. 2 shows the temperatures of feed water after heat exchangers and recycling brine. No substantial variations appear. Recycle temperature, for example, is always kept ber,veen 56-58°C except for one day. The figures presented were recorded daily during the period from April 7 to August 28, 1991. Evaporation pressure does not vary noticeably either, particularly when considering the narrow range of operation (scarcely 0.2 bar between high and low pressme).
Av~e
davy ~oducUon. m~day ,, |
II IIIt
".
°
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0
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t~
II
l |
~I •
#e I
tl q~ ||
~."
tt tf
|
t
~
i
t~ tl
4. 3. Production results I 11 1 i i i i i i]~ i | ii
Unit #1 has been producing water at a monthly average of 479.9 m3/d (between January 1988 and December 1992). This means an
61
i i i i lllrll
i i i i~ i $ i ~ I i I i
i 1 11 [[(
| i i I-i'i
I I ~Jll
| i i i I i i i
Fig. 3. Water production.
U~t# 1
62
i i|
Mon~, Jan 1988 to Din= 1992
1.2
plant fac~, %
Tempmlture, "C 1.1
60
It t
59 55
.
.
.
o.
.
, .... j,..,,.,]
57 56
Ill
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49 411
111111111 i W l l l l l i ~ | l l l
it i i ii~rl ill
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ii iii
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Procluc~exd~nger ~ n e ~
0.3O.2-
---- Unlt# 1
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! ii i ii i i! i iiiiiill
28.1901)
Fig. 2 Operation,e..perat,a..,s.
t
iig~ll
i i ¢1111 i ! 1 1 i i ! ! i i ! 1 1
0
vwwv
vt
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iwv~lwwir~mwwvll
ewll
I d o l 1 ~ Jan 1ge8 Io Dec 19g~
Fig. 4. Plant facter.
| | iw
i l l l l l v ~ l l w l ,
l i l l l l
lrrl-
t
6
..... ,., J . M . Veza / D e s a l i n a t i o n 101 (1995) 1 - 1 0
As for unit #2, it was commissioned in April 1989, and as of December 1992 i~ produced a monthly average of 476.3 m3/d, with an average plant factor of 90.3%. Monthly average production and plant factors are shown in Figs. 3 and 4, respecti,rely. The data indicate an average plant availability of 26.6 and 27.5 d/month for each unit in terms of utilizati.,-~ time. A~ the goal of the Port Authority is to achieve~:_ .much production as possible -- thus the units are actually operating continuously-one can in a first approach impute downtime to unforeseen reasons, either maintenance requirements or energy supply failures. The only exceptions took place in August and September :992 when the units were turned off due to an excess of water stored in product tanks.
4. 4. Energy consumption Energy consumption is mainly due to the vapour compressor motor and the elec~.rical feed water intake pumps. Other minor consumptions are due to vacuum pumps, product and brine pumps, as well as the heating resistance at the start-up boiler and other ancillaries. Expected energy consumptions are shown in Table 3 for each 500 m 3/d unit. A distinction is made between actual shaft power required and the installed power (or nominal, as in motor plates), the difference being mechanical and electrical efficiencies. The expected specific energy consumption is therefore 11.5 kWh/m 3 of product water when feed water pumping power is not included, and up to a maximum of 12.5 kWh/m ~ when included. The actual performance recorded at the units between 1988 and 1992 is as follows: Unit #1 had an average energy consumption of 10.36 kWh/m 3 of product water in the compressor proper and 10.48 kWh/m 3 of global consumption when including the feed pump, start-up boiler and auxiliary pumps. As for
Table 3 Energy consumption Motor
Required shaft Installedpower in in power, kW motors, kW 197.4 265
Vapour compressor Recycle pump 8.9 Brine pump 3.3 Product pump 3.1 Vacuum pump 3.4 Oil pump 0.15 Dosing pump 0.15 ,- (;~retreatment) Dosing pump 0.15 (post-treatment) Total" 217
12.4 4.5 4.5 4.5 0.25 0.25 0.25 292
aNot including power for feedwater pumping. unit #2, recorded average consumptions are from 11.17 kWh/m 3 in the compressor to 11.24 kWh/m 3 when feed pumping and auxiliaries are included. It should be noted that unit #2 has consistently shown an energy consumption which is 7% higher than t.'nit #I, despite the tact that both units are reportedly of the same design. No expl,.natory reason has been provided by the user or by the manufacturer for this deviation in energy pet !brmance. 14 Energyoonsumption, kWh/m3 13.5 t
Nominal
131 12.5 -
in
B
Compressor
1211.511 10.5 10 9.5
9
I.hllt# 1 Unit # 2 Unit# 1, averageJan 88- Dec 92;,Unit# 2, averageApt 8.9. Dec92
Fig. 5. Energy consumption.
J.M. Veza / Desalination 101 (1995) 1-10
Fig. 5 shows energy consumption for units #1 and #2 within their respective operation periods, differentiating between compressor and total consumption. Nominal or expected total consumption is also plotted to provide a reference.
4.5. Product water quality Product water quality is checked weekly through analysis for both units and the final mixture supplied to end-users. Usually conductivity is kept below 20 pS/cm, as shown in Fig. 6, which shows eonductivities for various samples taken between March and November 1991.
7
Table 4 Sources and uses ot water supplies (m 3, 199!) Sources:
Self-production (desalination) Purchased flora third parties Uses: Supplies to ships Other supplies" Supplies not accounted for b
357,249 404,224 311,159 260,707 189,607
aHarbour services, housing. bGardening, losses. ing, harbour services), and finally some uses not accounted for (gardening, cleaning, etc.). Table 4 lists the distributed amounts for 1991.
4. 6. Water supply
4. Z Maintenance requirements
The aforementioned desalination units are part of the Port Authority water supply scheme, for which we provide an overview. Desalination provides around 47% of total supply, and the balance is bought from thirdparty suppliers. Water is then distributed to ships as well as to other end-users (e.g, hous-
The evaporator #1 has been opened twice from 1987 to date, while #2 has been opened only once. The heat transfer tubes have been found in good condition with no indication of corrosion whatsoever. The vapour compressor is a unique feature in these VC units. Both compressors have performed satisfactorily with only two blade breakages in unit #1. When the control system fails to keep the brine level down in the vessel, the blade tips in their lower position may smash against the bottom brine, thus breaking. As for the plate heat exchangers, the effect of laying on the hydroeyeiones has been beneficial, reducing the cleaning requirements for exchangers. Thus, before the hydroeyelones were installed, it was necessary to open and clean the exchangers every other month, whereas with the new pretreatment cleaning is performed only once a year.
2-
cor~.o~aty..S/cm
20-
~
Unit#1
/
Unit#2
1816
14 121086-
42
m
III
Sompkmtak~ belwNn I ~ . ~
Fig. 6. Product conductivity.
1991
5. Operating conditions analysis It is intended here to work out a comparison between predicted and actual performance or to
8
J.M. Veza/Desalination 101 (~995) 1-10
determine the operation efficiencies for some pieces of equipment in the plant.
5.1. Heat exchanger performance As indicated above, there are two plate heat exchangers in each unit acting as preheaters for the feed water whose flow is split into two streams in parallel. Effectiveness for these exchangers may be expressed as t:--
q
qmax where q is the heat load through the plates and qmax is the maximum available heat load, if eventually the cold fluid were heated up to the hot fluid inlet temperature (i.e., if Too= Thi). For the product water heat exchanger, the cold water raises its temperature from 20°C to 55"C while the hot water cools from 59°C down to 250C. Thus, effectiveness t turns out to be 87.1%. Such a figure /br effectiveness is a fairly good value due to the close approach temperatures. On the other hand, an analysis of the energy quality for a piece of equipment or system must include the available energy (exergy) efficiency since it is this available energy which is really capable of doing work, and it is also that energy which is lose in processes due to degradation of different forms of energy. From the point of view of useful energy, one has to evaluate the relation 18-101: exergy efficiency -- exergy output exergy input The appropriate exergy balance must be first established, including the loss of exergy (irreversibilities, or lost work):
E EXin = ~ EXout÷ lost work
When the exergy flows are evaluated with the standard water flow rates and temperatures and a reference state of liquid at 2O°C, the summation of inlet and outlet exergies is 58.99 and 53.11 kW, respectively, and therefore the lost work amounts to 5.88 kW. Exergy efficiency, or the Second Law of Efficiency, ,s then 90.0%, which is also high when compared to other processes such as steam boilers (49%) or diesel engines (36%) [I I]. This means that the preheaters used are highly effective since temperature differences are low, which in turn reduces irreversibilities and provides an adequate performance regarding useful energy. So far heat exchangers have been considered by themselves and not in accounting for their role in the plant. Within the framework of the plant as a system, good performance of the exchangers must be viewed as an indication of low energy consumption. Since approach temperatures are close, heat losses are reduced, and no additional energy is to be supplied to the system. This result requires large heat transfer surfaces for the exchangers.
5.2. Vapour compressor efficiency Regarding vapour compression, this will first be analyzed on the basis of an isentropic operation with pv n=cOnstant, to be compared later to the actual operating conditions. For an isentropic process in the vapour compressor, with n=~,, the work input W would be
W--
tl
n--"-IPl vl
ill ] P2
n-l _ 1
whel'e the inlet and outlet vapour pressures are 0.19 and 0.21 bar, respectively; specific volume 8.17 m3/kg; and 3,= 1.32, which gives an energy consumption in the compressor such as
J.M. Veza / Desalination 101 (1995) 1-10
W = 15.718 kJ/kg product water = 3.46 kWh/m 3 product This figure has to be compared to the actual energy consumption at the compressor. The average consumptions measured in the plant are 10.25 and 10.98 kWh/m 3 for units #1 and #2, respectively, as previously stated. If we assume an average 10.61 kWh/m 3, from this point of view the compressor isentropic efficiency would be 58.9%.
6. Cost analysis A brief cost analysis has been attempted in order to estimate roughly the costs of water production. Only the most relevant cost items are included here, not including chemicals or overheads. The only aim is to provide a rough figure which might be useful to readers, without a detailed account, which would go beyond the scope of this report. The analysis is based on the following assumptions, as stated in Table 5. According to the above-mentioned assumptions, total cost of production due to the main items would be as shown in Table 6, reaching a total cost of 334.7 PTA/m 3. If only the operation and maintenance expenses are accounted for, the cost would be 213.3 PTA/m 3. Note that energy expenses take over half the O&M cost (51.5 %), while labour and maintenance account for 23.6% and 24.8%, respectively. This indicates that the cost of water is expensive when compared to the price one can buy water on the water market, which is around 100 PTAIm 3. Nevertheless, at the time this cost was considered worthwhile since the priority was to offer a good service to ships and other customers. Finally, it is to be remembered that product water is mixed with other waters, the total distributed flow being 1900m3/d with some 400ppm TDS. The
9
Table 5 Cost assumptions Capital costs (both units, including civil works), MPTAa Capital recovery factor (10% interest, 15 y) Operation (six shifts × 1 operator), MPTAJy Maintenance contract, PTA/m3 Energy cost, FTA/kWh
330 0.~314 18 53 10.27
aMPTA=million pesetas. US$= 104 PTA (1991). Table 6 Cost summary (PTA/m 3) Amortization Labour Energy Maintenance
121.4 50.4 109.9 53.0
Total
334.7
selling price to ships was set by the Spanish Directorate General of Ports at 285 PTA/m 3 ( ~,989) prices.
7. Conclusions The desalination plant at the Las Palmas Port Authority was conceived as a means to provide the required water for the harbour services at a time when there was a need for a supply of its own. Throughout the years, both vapour compression units have shown good performance, as a reliable process, providing water with high plant factors. Water produced is of a remarkable good quality, which even allows it to be mixed with other supplies to reach the required amount to be distributed for consumption. Energy consumption at the plant, although kept below specifications, is somewhat high as compared
IP,
J.M. Veza / DesalimTtion 101 (1995) 1-10
with membrane desalination processes. The newer VC ur~.~,, aowever, are being produced with lower energy requirements. Production costs are also rather expensive as compared to local prices, mainly due to operating expenses.
8. Acknowledgment I gratefully acknowledge the assistance and cooperation for the preparation of this report, which I received from the management and staff of the Las Palmas Port Authority, as well as from Centre de Estudios Hidrogr~ficos who assisted with the plant selection.
References [1] A. G6mez Gotor, J.M. Veza and J. P6rez Castillo, Estado actual de la tecnoiogfa de desalaci6n en Canarias: I. Plantas de agua salobre; II. Plantas
de agua de mar. Symposium on ~e~liration p~sr~t operations, EDA-CANAGUA, Las Palmas, 1990. 12] J.M. Veza and A. G6mez Gotor, S~tu,~ of desalination in Canary Islands. Energy related aspects. Seminar on new technologies for the use of renewable energies in water desalination. Commission of the European Communities, Athens, I991. [3] J.M. Veza, A. G6mez Gotor and J. P6rez Castilio, Desalination, 85 (1992) 147. [4] M.A. Darwish, Desalination, 69 (1988) 275. [5] D. Hoffman, Low temperature evaporation plants, Chemical Engineering Progress, October 1981. [6] The Aquaport vapor comp~'~sioti di:~;.iilation at ambient temperatures, IDE Bulletin, 1990. [7] Daily and monthly plant operation reports. [81 J.E. Ahern, The Exergy Method of Energy Systems Analysis, Wiley, New York. 1980. [9] M.J. Moran, Availability Analysis: A Guide to Efficient Energy Use, Prentice Hall, Englewood, NJ, 1982. 11Ol T.J. Kotas, The Exergy Method of Thermal Plant Analysis, Butterworths, London, 1985. 1111 H.W. Hevert and S.C. Hevert, Prec., Energy, 5 (1980) 865.
ELSEVIER
Desalination 101 (1995) 1-10
Mechanical vapour compression desalination plants-A case study Jos6 M. Veza Departamento de ]ngenierfa de Procesos, Universidad de Las Palmas de Gran Canaria, 7rafira Baja, 350! 7 Las Palmas de Gran Canaria, Spain Rece:ved 22 September 1993; accepted 2 December 1993
Abstract Over 120 desalination plants are spread over the Canary Islands due to a scarcity of conventional water resources. These units are varied in technology and size, and accordingly a fairly good knowledge of their technology as well as experience on their operation has been developed. As a case study, The Las Palmas Port Authority desalination plant consists of two low-temperature vapour compression units, with a production capacity of 500 m3/d each. These units were conmaissioned in 1987 and 1989 and have been producing water with high levels of availability, namely average plant factors of 87.3 % and 90.2 %, respectively. Product water is consistently below 20 #S/cm conductivity. Energy consumption ranges from 10.4-11.2 kWh/m3, and the standard energy efficiency for the equipment is around 58.9 % tbr the ~ompressors. This report summarizes the design parameters for the system as well as its operational features.
Keywords: Vapour compression, Distillation, Operating results
1. introduction The Canary Islands, particularly Gran Canaria, Lanzarote and Fuerteventura, have suffered a long-time water scarcity as shown by the low rates of consumption. For the City of Las Palmas, the supplies have increased over the last years, reaching 188 I/person/d available in 1991.
Amongst other actions taken to alleviate the situation, 122 sea and brackish water desalination systems have been built with a global production capacity of ever 212,000 m3/d in 1990 [1-3]. These plants include most technologies in the market: multistage flash, vapour compression, reverse osmosis and electrodialysis.
001 I-9164195/$09.50 © 1993 Elsevier Science B.V. All rights reserved SSD! 001 I-9164(95 )00002-X
2
J.M. Veza /Desalination 101 (1995) 1-10
Between 1988 and 1990 the increase in production capacity rose to 32% due to the commissioning of a new plant (Las Palmas III, 36,000 m3/d capacity) as well as some plants scheduled on the Desalination Plants Scheme for the Canary Islands. The Scheme is the outcome of an agreement between the Ministry '~f Public Works and the Regional Government. The first stage began in 1988 and includes an overall new capacity of 40,000 m3/d in ten plants for a 5-year period. An extension was added in 1992 to cover an add;tional 55,000 m3/d capaci~ up to 1996. Among these plants is the one built for the Las Palmas Port Authority, whose design and operation will be described in this report.
were the only technologies taken into account for the required production capacity. RO technology was discarded because of the feed seawater in the Port containing oils which would have made necessary a full line of pretreatment, complicated to operate. In addition, the higher TDS in product was not adequate in case a ship would require water for boiler make-up. Other VC manufacturers offered more sophisticated designs, with slightly lower energy consumption, but they did not have a long enough record of experience. Water cost was considered, of course, and it was reasonable for uhe VC process compared to RO proposals. In any event costs were not the determining factor. Above all, reliability was sought in order to secure water supply to ships at all times.
2. Project description Las Palmas Port supports heavy shipping traffic in Spain, numbering 12,442 units in 1991 and 43.38 million tons in gross weight. The total amount of water supplied through the Port Authority was 773,013 m 3. Out of this quantity, 311,159 m 3 were delivered to shipping. In the past water was provided by the Munic.ipal Water Company and through purchases fi-om private suppliers~ although the Port Authority started in 1980 to survey the feasibility of having their own water supply, mainly because of the low availability of resources then. In 1987 it was decided to acquire a desalination unit with a production capacity of 500 m 3/d with vapour compression technology, which was later extended to a second unit commissioned in 1989 to reach an overall capacity of 1000 m3/d. Both units were supplied by IDE Technologies and are located adjacent to the workshop and warehouse areas. The tender and process selection was ~ssisted by Centro de Estudios Hidrogr~tficos, Madrid (Center for Hydrology Studies). Vapour compression (VC) and reverse osmosis (RO)
3. Description of the vapour compression process Desalination by vapour compression is described elsewhere [4-6], and it consists basica!Wy of a seawater evaporator which uses as a heating medium the vapour produced in the chamber itself after compression and heating by mechanical means. Our reference plant consists of two units of 500 m3/d each, with a common water intake, apart from the auxiliary facilities. Feed water is pumped from a 5 m deep, 2 m diameter well drilled inside the site. There are two stainless steel pumps, each rated 15 HP and capable of conveying 50 m 3/h. The pretreatment applied to feed water includes passing it through a 2 m long, 0.2 m diameter cartridge filled with aluminium pellets to prevent tube corrosion in the evaporator. An antiscalant is dosed to pt'event precipitation of salts. Some time after commissioning, two 0.4 m diameter hydrocyclones were added in order to reduce suspended solids in water, thus extending the time between cleanings of the hea~ exchangers.
J.M. Veza / Desalination 101 (1995) 1-10
3
Evaporator
I I Product Pzoduct ~'
Sea water
-,O Brine
Heat exchangers (product and .brine)
Brine
Fig. 1. Flow diagram for the vapour compression process. Feed water is then taken through two plate heat exchangers in parallel, each one of them heated by the product water and brine, respectively, which are discharged from the evaporator (Fig. 1). Seawater is then mixed with recycled brine from the evaporator and pumped to the evaporator through spray nozzles which spread the water over the horizontal tube bundle. Enough water is provided in order to get a thin continuous film over the tubes, therefore increasing the heat transfer rate. The evaporator-condenser consists of a horizontal tube bundle made from an aluminium alloy and falling film in a steel vessel where water is sprayed through nozzles. The tubes are 5.13 m long with a total surface area of 2598 m 2 while the vessel has a 4 m diameter. Operating temperature is around 59"C. The vapour compressor produces a vacuum when suetioning in such a way that pressure is lower than that of equilibrium at feed tempera-
ture; therefore, part of the seawater evapot'ates. After passing a separation mesh, vapour is absorbed by the rotating compressor where it is slightly compressed and heated and then discharged inside the tubes where it condenses. The vapour c gmpressor is a radial centrifuge type capable of processing 21 t/h vapour, an aluminium rotor and titanium blades designed and developed by Zarehin-IDE. Non-condensable gases are continually extracted from the system by means of a vacuum pump connected to an auxiliary condenser while also producing the initial start-up vacuum. The auxiliary condenser is a shell and tube type of a titanium alloy. Part of the brine is recirculated and mixed with make-up feed water to be pumped through the spray nozzles over the tubes. Product water is treated with soda ash to adjust pH, ~hus neutralizing its slight acidity. Heat transfer in horizontal tubes and thin
4
J.M. Veza/Desalitmtion 101 (1995) 1-10
film allows high transfer coefficients, even in operation at moderate temperatures. At operation tetaperatures around 60°C, scaling is reduced and heat losses through the vessel are minimized with minimum requirements for thermal isolation.
• • • •
• •
Some other advantages are: modular type: capable of extending capacity simplicity of pretreatment and operation good product quality flexible operation: load adjustment through temperature variation,,; Some significant drawback may be: unit capacity limited to some 1200 m 3/d energy consumption rather high compared to other processes, although this is decreasing due to technological improvements
The most significant features of the equipment are listed in Table 1.
Both distillation units were supplied with a 28,000 m3/month production warranty, which is equivalent to 162,500 m 3/y at 325 d/y. The assured product water quality is lower than 100 ppm TDS, with power consumption below 12.5 kWh/m 3 product water, with a breakdown as follows: • Seawater feed pumping -- 0.3 • • •
Production Product pumping Total
- - 1 1.5 - - 0.2 - - 12
Certain conditions were agreed upon with the contractor: energy consumption in excess of 12.5 kWh/m 3 (on a quarterly basis) is charged to the contractor; non-produced water is also t:harged; and product water over 100 ppm TDS is not paid for.
4. Operational features Table 1 Main design parameters Technology Number of units Unit capacity, m3/d Feed water flow rate, m3/h Feed water salinity, % TDS Feed temperature, *C Feed pressure, bar Pretreatment, ant;.sealaot~ ppm Product water flow rate, m3/h Product salinity, ppm TDS Post-treatment, sodium carbonate, ppm Brine discharge flow rate, m3/h Brine salinity, % TDS Evaporation temperature, *C Operating pressure, mm Hg Recovery ratio, % Power required, kW Total installed powera, kW Specific energy consumption", kWh/m 3 aFeed intake pumping not included. bAmbient temperature vapour compression.
4.1. Actual operating parameters ATVCb 2
500 50 3.8 21 2 5 20.83 < 30 3 29 6.5 59 150 45 239.6 292 11.5
The system usually operates close to the c~e~ign conditions. Table 2 shows the operating conditions for unit #2 as of May7, 1991. Table 2 Operating conditions Feed water flow rate, m3/h Raw feed temperature, °C Feed temperature, product heat exchanger outlet, °C Feed temperature, brine heat exchanger outlet, °C Recycle brine temperature, °C Recycle pump pressure, bar Product pump pressure, bar Brine pump pressure, bar Evaporator pressure, bac Compressor discharge pressure, bar Product flow rate, m3/h Brine flow rate, m3/h Product quality, ~tS/cm
50 20 55 53 59 0.7 3.8
3 0.19
0.21 21 29 6.5
J.M. Veza I Desalination 101 (1995) 1-10
It may be worthwhile to note the low operating temperature in the evaporator (below 60°C), as well as the low compression ratio at the vapour compressor (1.10).
5
average plant factor of 87.3% when the plant factor is defined as follows: plant factor = actual monthly production
(1)
days x daily design production month
4. 2. Evolution along time
Operating parameters for both units are alike and fairly stationary along time. No remarkable deviations have been recorded [7]. As an example, Fig. 2 shows the temperatures of feed water after heat exchangers and recycling brine. No substantial variations appear. Recycle temperature, for example, is always kept ber,veen 56-58°C except for one day. The figures presented were recorded daily during the period from April 7 to August 28, 1991. Evaporation pressure does not vary noticeably either, particularly when considering the narrow range of operation (scarcely 0.2 bar between high and low pressme).
Av~e
davy ~oducUon. m~day ,, |
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°
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t~
II
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~."
tt tf
|
t
~
i
t~ tl
4. 3. Production results I 11 1 i i i i i i]~ i | ii
Unit #1 has been producing water at a monthly average of 479.9 m3/d (between January 1988 and December 1992). This means an
61
i i i i lllrll
i i i i~ i $ i ~ I i I i
i 1 11 [[(
| i i I-i'i
I I ~Jll
| i i i I i i i
Fig. 3. Water production.
U~t# 1
62
i i|
Mon~, Jan 1988 to Din= 1992
1.2
plant fac~, %
Tempmlture, "C 1.1
60
It t
59 55
.
.
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o.
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57 56
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49 411
111111111 i W l l l l l i ~ | l l l
it i i ii~rl ill
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0.3O.2-
---- Unlt# 1
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! ii i ii i i! i iiiiiill
28.1901)
Fig. 2 Operation,e..perat,a..,s.
t
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i i ¢1111 i ! 1 1 i i ! ! i i ! 1 1
0
vwwv
vt
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iwv~lwwir~mwwvll
ewll
I d o l 1 ~ Jan 1ge8 Io Dec 19g~
Fig. 4. Plant facter.
| | iw
i l l l l l v ~ l l w l ,
l i l l l l
lrrl-
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6
..... ,., J . M . Veza / D e s a l i n a t i o n 101 (1995) 1 - 1 0
As for unit #2, it was commissioned in April 1989, and as of December 1992 i~ produced a monthly average of 476.3 m3/d, with an average plant factor of 90.3%. Monthly average production and plant factors are shown in Figs. 3 and 4, respecti,rely. The data indicate an average plant availability of 26.6 and 27.5 d/month for each unit in terms of utilizati.,-~ time. A~ the goal of the Port Authority is to achieve~:_ .much production as possible -- thus the units are actually operating continuously-one can in a first approach impute downtime to unforeseen reasons, either maintenance requirements or energy supply failures. The only exceptions took place in August and September :992 when the units were turned off due to an excess of water stored in product tanks.
4. 4. Energy consumption Energy consumption is mainly due to the vapour compressor motor and the elec~.rical feed water intake pumps. Other minor consumptions are due to vacuum pumps, product and brine pumps, as well as the heating resistance at the start-up boiler and other ancillaries. Expected energy consumptions are shown in Table 3 for each 500 m 3/d unit. A distinction is made between actual shaft power required and the installed power (or nominal, as in motor plates), the difference being mechanical and electrical efficiencies. The expected specific energy consumption is therefore 11.5 kWh/m 3 of product water when feed water pumping power is not included, and up to a maximum of 12.5 kWh/m ~ when included. The actual performance recorded at the units between 1988 and 1992 is as follows: Unit #1 had an average energy consumption of 10.36 kWh/m 3 of product water in the compressor proper and 10.48 kWh/m 3 of global consumption when including the feed pump, start-up boiler and auxiliary pumps. As for
Table 3 Energy consumption Motor
Required shaft Installedpower in in power, kW motors, kW 197.4 265
Vapour compressor Recycle pump 8.9 Brine pump 3.3 Product pump 3.1 Vacuum pump 3.4 Oil pump 0.15 Dosing pump 0.15 ,- (;~retreatment) Dosing pump 0.15 (post-treatment) Total" 217
12.4 4.5 4.5 4.5 0.25 0.25 0.25 292
aNot including power for feedwater pumping. unit #2, recorded average consumptions are from 11.17 kWh/m 3 in the compressor to 11.24 kWh/m 3 when feed pumping and auxiliaries are included. It should be noted that unit #2 has consistently shown an energy consumption which is 7% higher than t.'nit #I, despite the tact that both units are reportedly of the same design. No expl,.natory reason has been provided by the user or by the manufacturer for this deviation in energy pet !brmance. 14 Energyoonsumption, kWh/m3 13.5 t
Nominal
131 12.5 -
in
B
Compressor
1211.511 10.5 10 9.5
9
I.hllt# 1 Unit # 2 Unit# 1, averageJan 88- Dec 92;,Unit# 2, averageApt 8.9. Dec92
Fig. 5. Energy consumption.
J.M. Veza / Desalination 101 (1995) 1-10
Fig. 5 shows energy consumption for units #1 and #2 within their respective operation periods, differentiating between compressor and total consumption. Nominal or expected total consumption is also plotted to provide a reference.
4.5. Product water quality Product water quality is checked weekly through analysis for both units and the final mixture supplied to end-users. Usually conductivity is kept below 20 pS/cm, as shown in Fig. 6, which shows eonductivities for various samples taken between March and November 1991.
7
Table 4 Sources and uses ot water supplies (m 3, 199!) Sources:
Self-production (desalination) Purchased flora third parties Uses: Supplies to ships Other supplies" Supplies not accounted for b
357,249 404,224 311,159 260,707 189,607
aHarbour services, housing. bGardening, losses. ing, harbour services), and finally some uses not accounted for (gardening, cleaning, etc.). Table 4 lists the distributed amounts for 1991.
4. 6. Water supply
4. Z Maintenance requirements
The aforementioned desalination units are part of the Port Authority water supply scheme, for which we provide an overview. Desalination provides around 47% of total supply, and the balance is bought from thirdparty suppliers. Water is then distributed to ships as well as to other end-users (e.g, hous-
The evaporator #1 has been opened twice from 1987 to date, while #2 has been opened only once. The heat transfer tubes have been found in good condition with no indication of corrosion whatsoever. The vapour compressor is a unique feature in these VC units. Both compressors have performed satisfactorily with only two blade breakages in unit #1. When the control system fails to keep the brine level down in the vessel, the blade tips in their lower position may smash against the bottom brine, thus breaking. As for the plate heat exchangers, the effect of laying on the hydroeyeiones has been beneficial, reducing the cleaning requirements for exchangers. Thus, before the hydroeyelones were installed, it was necessary to open and clean the exchangers every other month, whereas with the new pretreatment cleaning is performed only once a year.
2-
cor~.o~aty..S/cm
20-
~
Unit#1
/
Unit#2
1816
14 121086-
42
m
III
Sompkmtak~ belwNn I ~ . ~
Fig. 6. Product conductivity.
1991
5. Operating conditions analysis It is intended here to work out a comparison between predicted and actual performance or to
8
J.M. Veza/Desalination 101 (~995) 1-10
determine the operation efficiencies for some pieces of equipment in the plant.
5.1. Heat exchanger performance As indicated above, there are two plate heat exchangers in each unit acting as preheaters for the feed water whose flow is split into two streams in parallel. Effectiveness for these exchangers may be expressed as t:--
q
qmax where q is the heat load through the plates and qmax is the maximum available heat load, if eventually the cold fluid were heated up to the hot fluid inlet temperature (i.e., if Too= Thi). For the product water heat exchanger, the cold water raises its temperature from 20°C to 55"C while the hot water cools from 59°C down to 250C. Thus, effectiveness t turns out to be 87.1%. Such a figure /br effectiveness is a fairly good value due to the close approach temperatures. On the other hand, an analysis of the energy quality for a piece of equipment or system must include the available energy (exergy) efficiency since it is this available energy which is really capable of doing work, and it is also that energy which is lose in processes due to degradation of different forms of energy. From the point of view of useful energy, one has to evaluate the relation 18-101: exergy efficiency -- exergy output exergy input The appropriate exergy balance must be first established, including the loss of exergy (irreversibilities, or lost work):
E EXin = ~ EXout÷ lost work
When the exergy flows are evaluated with the standard water flow rates and temperatures and a reference state of liquid at 2O°C, the summation of inlet and outlet exergies is 58.99 and 53.11 kW, respectively, and therefore the lost work amounts to 5.88 kW. Exergy efficiency, or the Second Law of Efficiency, ,s then 90.0%, which is also high when compared to other processes such as steam boilers (49%) or diesel engines (36%) [I I]. This means that the preheaters used are highly effective since temperature differences are low, which in turn reduces irreversibilities and provides an adequate performance regarding useful energy. So far heat exchangers have been considered by themselves and not in accounting for their role in the plant. Within the framework of the plant as a system, good performance of the exchangers must be viewed as an indication of low energy consumption. Since approach temperatures are close, heat losses are reduced, and no additional energy is to be supplied to the system. This result requires large heat transfer surfaces for the exchangers.
5.2. Vapour compressor efficiency Regarding vapour compression, this will first be analyzed on the basis of an isentropic operation with pv n=cOnstant, to be compared later to the actual operating conditions. For an isentropic process in the vapour compressor, with n=~,, the work input W would be
W--
tl
n--"-IPl vl
ill ] P2
n-l _ 1
whel'e the inlet and outlet vapour pressures are 0.19 and 0.21 bar, respectively; specific volume 8.17 m3/kg; and 3,= 1.32, which gives an energy consumption in the compressor such as
J.M. Veza / Desalination 101 (1995) 1-10
W = 15.718 kJ/kg product water = 3.46 kWh/m 3 product This figure has to be compared to the actual energy consumption at the compressor. The average consumptions measured in the plant are 10.25 and 10.98 kWh/m 3 for units #1 and #2, respectively, as previously stated. If we assume an average 10.61 kWh/m 3, from this point of view the compressor isentropic efficiency would be 58.9%.
6. Cost analysis A brief cost analysis has been attempted in order to estimate roughly the costs of water production. Only the most relevant cost items are included here, not including chemicals or overheads. The only aim is to provide a rough figure which might be useful to readers, without a detailed account, which would go beyond the scope of this report. The analysis is based on the following assumptions, as stated in Table 5. According to the above-mentioned assumptions, total cost of production due to the main items would be as shown in Table 6, reaching a total cost of 334.7 PTA/m 3. If only the operation and maintenance expenses are accounted for, the cost would be 213.3 PTA/m 3. Note that energy expenses take over half the O&M cost (51.5 %), while labour and maintenance account for 23.6% and 24.8%, respectively. This indicates that the cost of water is expensive when compared to the price one can buy water on the water market, which is around 100 PTAIm 3. Nevertheless, at the time this cost was considered worthwhile since the priority was to offer a good service to ships and other customers. Finally, it is to be remembered that product water is mixed with other waters, the total distributed flow being 1900m3/d with some 400ppm TDS. The
9
Table 5 Cost assumptions Capital costs (both units, including civil works), MPTAa Capital recovery factor (10% interest, 15 y) Operation (six shifts × 1 operator), MPTAJy Maintenance contract, PTA/m3 Energy cost, FTA/kWh
330 0.~314 18 53 10.27
aMPTA=million pesetas. US$= 104 PTA (1991). Table 6 Cost summary (PTA/m 3) Amortization Labour Energy Maintenance
121.4 50.4 109.9 53.0
Total
334.7
selling price to ships was set by the Spanish Directorate General of Ports at 285 PTA/m 3 ( ~,989) prices.
7. Conclusions The desalination plant at the Las Palmas Port Authority was conceived as a means to provide the required water for the harbour services at a time when there was a need for a supply of its own. Throughout the years, both vapour compression units have shown good performance, as a reliable process, providing water with high plant factors. Water produced is of a remarkable good quality, which even allows it to be mixed with other supplies to reach the required amount to be distributed for consumption. Energy consumption at the plant, although kept below specifications, is somewhat high as compared
IP,
J.M. Veza / DesalimTtion 101 (1995) 1-10
with membrane desalination processes. The newer VC ur~.~,, aowever, are being produced with lower energy requirements. Production costs are also rather expensive as compared to local prices, mainly due to operating expenses.
8. Acknowledgment I gratefully acknowledge the assistance and cooperation for the preparation of this report, which I received from the management and staff of the Las Palmas Port Authority, as well as from Centre de Estudios Hidrogr~ficos who assisted with the plant selection.
References [1] A. G6mez Gotor, J.M. Veza and J. P6rez Castillo, Estado actual de la tecnoiogfa de desalaci6n en Canarias: I. Plantas de agua salobre; II. Plantas
de agua de mar. Symposium on ~e~liration p~sr~t operations, EDA-CANAGUA, Las Palmas, 1990. 12] J.M. Veza and A. G6mez Gotor, S~tu,~ of desalination in Canary Islands. Energy related aspects. Seminar on new technologies for the use of renewable energies in water desalination. Commission of the European Communities, Athens, I991. [3] J.M. Veza, A. G6mez Gotor and J. P6rez Castilio, Desalination, 85 (1992) 147. [4] M.A. Darwish, Desalination, 69 (1988) 275. [5] D. Hoffman, Low temperature evaporation plants, Chemical Engineering Progress, October 1981. [6] The Aquaport vapor comp~'~sioti di:~;.iilation at ambient temperatures, IDE Bulletin, 1990. [7] Daily and monthly plant operation reports. [81 J.E. Ahern, The Exergy Method of Energy Systems Analysis, Wiley, New York. 1980. [9] M.J. Moran, Availability Analysis: A Guide to Efficient Energy Use, Prentice Hall, Englewood, NJ, 1982. 11Ol T.J. Kotas, The Exergy Method of Thermal Plant Analysis, Butterworths, London, 1985. 1111 H.W. Hevert and S.C. Hevert, Prec., Energy, 5 (1980) 865.