Desalination, 27 (1978) 175-187 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

LAS PALMAS PHASE II DESALINATION PLANT IN THE CANARY ISLANDS (SPAIN)

J. GUZMAN

Babcock & Wilcox Espaiiola, S.A., Madrid (Spain)

(Received September 22,197s)

SUMMARY

The principal features of the desalination plant: Las Palmas Phase II in the Canary Islands are described here. Two plants are of the multi-stage flash cross-flow acid-dosing type with a nominal capacity of 9000 m3 /day each. The steam supply is provided by the existing power station_ A description of the criteria adopted for the prevention of corrosion is men- tioned and a review is given of the main materials used. The economics of the cost of water from this-plant are considered.

INTRODUCTION

Grand Canary is one of the seven volcanic islands of the Canary Islands with a total population of about 550 000 inhabitants half of whom are concentrated in the capital city of Las Palmas. The total surface is 1558 km* with altitudes up to 2000 m.

The hydrological resources of the island are disappointing. Rainfall occurs with marked variation of from 200 to 1300 mm/year, but unfortu-

nately with an average of only 400mm/year. There are no permanent streams. The island has a large number of dams for the retention of sur- face waters and the amount of subterranean utilization is enormous, with more than 2000 wells with an intricate system of galleries. The average level of the wells is 150m and every year the phreatic level goes down 5 m. The overuse of these resources has been so exhaustive that the hydraulic equilibrium of the system has even been endangered.

The Las Palmas Municipal Authority has thus had to purchase water on the island market for the supply of water to Las Palmas which has always been deficient for a city that benefits from tourism, provoking a detraction of the traditional agricultural sources with the resulting rise of their price.

* Presented at the Sixth International Symposium on Fresh Water from the Sea, Las Palmas, Gran Canaria, September 17-22.1978.

176 J. GUZMAN

The solution was the construction by stages of residual water- recovery plants and the installation of a dual-purpose desalination plant of 4 x 5000 m3 /day and 2 x 10 MW saleable.

The desalination plant started producing water and electricity in 1970, and with its installation, the Las Palmas water shortage was partially solved.

UNELCO (Local Electricity Authority of the Island) designed a Group I Power Station for a power plant complex, with turbine steam extraction for a future desalination plant (with an electric capacity of 22- 33 MW, with or without the desalination plant). -This power station is located near the desalination plant which started functioning in 1973.

Faced with a critical demand for water through increased tourism in Las Palmas, the Ministry of Public Works and the local authorities of Las Palmas decided in 1973 to build another desalination plant, Las Palmas Phase II, taking steam from the Group I steam power plant, and sea water from the general distribution channel of UNELCO. It was decided that the plant would have two groups of 9000 m3 /day each located between the existing power station and desalination plant Phase I.

PROJECT DESCRIPTION

The Ministry of Public Works issued general international tender specifications in 1974, and called for bids for a complete turnkey plant of 2 x 9000 m3 /day.

The successful bidder was Babcock & Wilcox Espaiiola, S.A./Agro- man Empresa Constructora who took charge of the civil work and were selected by the Ministry of Public Works in 1975 to submit a complete detailed project of construction in four months with all specifications, layout, characteristics, materials and particulars for each item of the plant based on their proposal.

The practice of the Spanish Government is to award the most attrac- tive bidder a provisional acceptance and request him to prepare a con- struction project. The constmction project was carried out by Babcock & Wilcox Esptiola, S.A. in close contact with the Ministry of Public Works.

The most outstanding points of the technical specifications of the tender will be quoted briefly.

The desalters were specified to be of the multi-stage flash design with brine recycle and acid for the feed-water pretreatment.

The cross tube arrangement was also specified, as were the principal construction materials.

The main power supply was specified to be either from Las Palmas Phase I generators 13 kV/12.5 MW, or from external supply through two 66-13 kV transformers.

LAS PALMAS PHASE II DESALINATION PLANT 177

__ --__

178 J. GUZMAN

The control room building was specified to be an extension of the existing one, the control board of Phase II being next to that of Phase I.

The location of the sea-water intake was foreseen in the specifications (Fig. 1) of the tender, to be connected by a channel to the general sea- water distributor.

During the project of construction, the major changes that evolved were: the increase of thermal efficiency of the desalters, the doubling of the surface of the brine heaters, the changing of the gauge of the alumi- num-brass tubes from 20 to 19, the 3 mm stainless steel internal coating of the two first recovery stages and the increase of the recycle flow.

After this project was accepted, the Ministry in 1976 signed a con- tract with Babcock & Wilcox Espaiiola, S.A./Agroman Empresa Con- structor-a for a turnkey supply of the installation as was described in the project of construction. This desalination plant is to be of the greatest capacity built in Spain.

PROCESS DESCRIPTION

(a) Process and economy

The economy of the plant was fixed by the specifications of the Ministry of Public Works, since both the production and the amount and quality of steam available were predetermined_

The optimization of the cost of the desalter was made according to the number of stages, interchange area, diameter of the tubes and speed of brine.

The aid of a computer was very valuable for this purpose in order to obtain the definite design. Each desalter comprises 2 shells with a total of 26 stages (24 recovery and 2 reject) and the tubes are $” diameter and 7800 mm length. The width of the desalter is limited by the length of the tubes and the length of each stage by the diameter of the tubular bank and the demister as well as by the vapour release rate, this latter becoming especially important in the low temperature stages.

In the optimization of the cost of the desalter, an adequate dimen- sioning of the different parts also comes into play, especially the thickness of the plate for the shell and internal dividing elements against the number of stiffeners and therefore the welds. It is not possible to give general optimization criteria, as these would vary according to the price of the materials, labour, etc. Here good engineering practice can save hours of calculation in the mechanical design.

A datum which on occasions is not predetermined in the specifi- cations of the tender and which we think should always be defined, is the minimum thickness of sacrifice by corrosion of the carbon-steel evaporator plates, as in the present case, which gives rise to an important difference in the thicknesses offered by the different bidders, which logically has an

t 20

584

i

VIEW

A

t- 10

415

I -1

SECT

ION

R-8

Fig.

2.

Ass

embl

y of

900

0 to

n/da

y ev

apor

ator

un

it co

nsis

ting

of 2

6 st

ages

.

180 J. GUZMAN

effect on the price of the evaporator. Babcock & Wilcox Espaiiola, S.A. offered the bigger thickness of sacrifice by corrosion for this plant.

The dimensioning of the tube sheets deserves special mention since it has significant importance in the price of the desalter_ Here again the aid of the computer has permitted a very carefully calculated dimension_mg of these, and therefore a reduction in the cost of the desalter_ A diagram of the assembly evaporator is given in Fig. 2.

The only brine heater by line would be of two-pass and at the client’s request with a heat-transfer area double that necessary with which, on the one hand a certain temperature of the recycling, the temperature of the steam required is lower, and, on the other hand, permits a greater fouling factor of the tubes, thus maintaining good working conditions.

Each evaporator has two sets of double-stage steam ejectors with a pre-, inter- and post-condenser of barometric type, with which the necess- ary vacuum in the evaporator and degasifier is obtained, in addition to a suitable dragging of the noncondensable, mainly COz, and air the pres- ence of which causes an important reduction in the heat transfer rate of the evaporator.

The vent system is in cascade on three levels, each one discharging into each of the barometric condensers, the deaerator and the last stage discharge in the precondenser, in such a manner that an important reduc- tion in the amount of steam which enters the ejector is obtained_

The material used for the whole vent system is stainless steel AISI- 316.

An independent hogging ejector has been provided for each evapor- ator for the start-up_

Due to shortage of space, the shells have had to be arranged in two levels, the desalters being arranged longitudinally and symetrlcally with all the auxiliary equipment also symmetrically located between them, such as: the degasifier, decarbonator, pumps, dosing equipment, brine heater and ejectors (see Fig. 1).

(b) Scale control Since alkaline scales start to form rapidly as the sea water is heated

above ambient temperature, for plants like the one we are describing, a treatment that removes the constituents that form the alkaline scale is the addition of sulfuric acid to the makeup. The increase in sulfite con- centration due to the acid addition does not appreciably affect the calcium sulfite content already present. The magnesium hydroxide scale is avoided by controlling the pH brine at less than 7.8. The non-alkaline scale, cal- cium sulfite is avoided by maintaining the brine temperature below 120°C and the concentration factor below 1.7.

(c) Corrosion control The corrosion phenomena are limited by controlling pH and ensuring

LAS PALMAS PHASE II DESALINATION PLANT 181

the absence of oxygen from the makeup. Addition of sulfuric acid is made before the make-up enters the decarbonator, and in order to main- tain a constant pH in the recirculating brine between 7-7.5, caustic soda is added to the make-up after leaving the deaerator.

The sulfuric acid dosing pumps are automaticaBy controlled by make- up flow and the caustic soda dosing pumps by the recycled brine pH.

In order to remove the carbon dioxide generated after the addition of sulfuric acid, the make-up enters an atmospheric decarbonator where most of the carbon dioxide is removed.

Absence of oxygen from the make-up is ensured by a correct deaer- ator operation. To achieve this, the make-up enters the deaerator and enough vapour is added to ensure a good removal of oxygen and COz.

PLANT DESCRIPTION

The material used for the shells of the evaporators is carbon-steel. The first two stages are lined intemahy by AISI 316 plate. The water boxes are of carbon-steel covered by AISI-316-L, except the first two and the reject ones which are covered in Cu-Ni 90/10. The tubes are of alumi- num-brass except in the first two stages and the reject ones which are of Cu-Ni 90/10. The tubuiar plates are of Naval brass for those corresponding to the aluminum-brass tubes and of Cu-Ni for the rest.

The brine and product passes are of AISI-361, and ah the vent sys- tems are aIso of AISI-316.

The important process parameters and construction materials are summarized overleaf

Desalters

Manufacturer Type Production (each unit) Specific energy consumption Economy ratio Pretreatment Number of recovery stages Number of reject stages

Vessel 1 (Stages l-24): Length Height Width

Tubes in recovery stages:

Babcock & Wilcox Espaiiola MSF, Crosstube 9000 m3 /day 57.4 kcal/kg prod. 9.68 lb prod./kBtu Acid with caustic back-titration 24 2

20.35 m 2.95 m 7.79 m

Size $ inch x 20 BWG Cu-Ni 19 BWG AI. brass

182

Length Material (Stages 1,2)

(Stages 3-24)

Tubes in reject stages Size Length Material

Materials Shell Brine passes and deflectors Vent baffles

Water boxes Stages 1,2,25-26

Stages 3-28

Tube sheets P Stages 1,2,25-26 Stages 3-24

Process design Sea-water flow Recycle flow Make-up flow Blowdown flow Sea water temperature TDS in sea water TDS in recycle brine Maximum brine temperature

Brine heaters

Length Diameter Size Length Shell material Water boxes material

Tube sheet material

J. GUZMAN

7800 m 90/10 Cu-Ni Aluminum-brass

2 inch x 20 BWG Cu-Ni 7800 mm 90110 Cu-Ni

Carbon steel A-47 RCI Stainless steel 316 Stainless steel 316

Carbon steel with 90/10 Cu-Ni Iining

Carbon steel with 316-L stainless steel lining

90/10 Cu-Ni Naval Brass

1814 tons/h 2780 tons/h 765.7 tons/h 289-t tons/h 21°C 37 100 ppm 63 000 ppm 12oOc

10 506 mm 1600 mm 1 inch x 18 BWG Cu-Ni

Carbon-steel A-47 RCI Carbon-steel with 90/10 Cu-Ni

Iining Cu-Ni 90/10.

Heat and material balances for the desalting plants are shown in Fig. 3.

LAS PALMAS PHASE II DESALINATION PLANT 183

184 J. GUZMAN

MANUFACTURE AND CONSTRUCTION

The project has been a combination of shop construction and erec- tion.

The shells of the evaporator were manufactured in four sections due to transport and erection demands, each section having an average weight of 100 tons.

The four shells were transported to the site by boat in two shipments. The sections, once on the site, were erected on the foundations and joint filled, the four sections previously having been aligned in the workshop, in order to avoid problems on erection.

No unusual construction problems were experienced, with the excep- tion of some difficulties which occurred in the welding of the Naval Brass tube sheets. Although the adoption of Naval Brass for tube sheets is, at first sight, economically advantageous (cheaper price and less thickness in the plate), nevertheless, the extra costs which the welding of these entails, is worthy of consideration before deciding on the use of this material.

The welding of the Naval Brass tube sheets is carried out with bronze aluminum rod and requires a special process (MIG) the welding having to be effected in horizontal position, which makes it necessary to use very specialized labour, which is therefore scarce, and which makes it essential to have a permanent team of highly specialized welders, since the scarcity of these can provoke a bottle-neck in the manufacture of the evaporators and be the cause of delays in this. To resort to subcontracting is practically impossible because of lack of experience in the workshops which exist in this type of welding.

This is aggravated by the fact that the welding gives off extremely toxic copper gases, which demands the availability of a spacious well- ventilated zone in the workshops, which is isolated from the rest of the personnel.

The utilization of Naval Brass tube sheets can in some cases limit the possibility of finishing the fabrication of the evaporator on the site since its welding necessarily has to be carried out in the factory.

Even though we have mentioned a series of disadvantages which the use of the tubular sheets of Naval Brass presents, nevertheless, its econ- omic advantages against the possible disadvantages it presents must be duly evaluated in order to draw conclusions as to the convenience or not of their usage.

ECONOMIC CONSIDERATIONS

In the following we shall try to give an indicatory figure for the resulting cost price per m3 produced by the Las Palmas Phase II desali- nation plant.

L4S PALMAS PHASE II DESALINATKlN PLANT 185

We consider the production cost to be the sum of the exploitation expenses, cost of amortization and insurance in one year.

- The exploitation expenses are sub-divided into expenses of: steam, energy, chemical products, personnel and maintenance.

- In order to calculate the amortization costs it will be considered that the investment is self-financed without profits, estimating an amor& ization period of 20 years for 75% of the plant, and 15 for the remaining 25%. In view of the difficulty in defining the interest rate, we shall take a series of values for a period of time, which wili give us a group of pts/m3 values according to the rate of interest.

- We estimate the insurance as a total annual premium of 1% of the investment_

Therefore, we shah find that C, = (E + S + A,)/P for a certain rate of interest (i%)

Cp = [(VC~X 83.6+33.8Vb)P~+Q+M+O+kWkP~ +S+Ai]/P

for an interest of 6%:

Ai = 0.75 a 1s O-087 + 0.25 * 1.0.103

for an interest of 8%:

Ai = 0.75 -1 - O-102 + 0.25 l 1 - O-117

for an interest of 10%:

Ai = 0.75 - 1 - O-117 + O-25 - 1 . O-131

for an interest of 12%:

Ai = 0.75 - 1.0-134 + 0.25 - 1.0.147

Where :

CIJ = Cost of production (pesetas and dollars) E = Exploitation expenses (pesetas/year) Ai = Amortization for a certain interest rate S = Insurance (pesetas/year) P = Production of water (m3 /year) M = Maintenance (pesetas/year) 0 = Personnel (pesetas/yea.r) I = Initial investment (pesetas) VCZ = Metric ton of high pressure consumption steam Vb = Metric ton of low pressure consumption steam

pr = Price of kilogramme of fuel (pesetas) Q = Cost of the chemical products (pesetas) k Wk = Amount of kWh consumed Pe = Price of kWh (pesetas)

186 J. GUZMAN

We consider a utilization of 300 days a year. The annual investment is 950 million pesetas ($11650 000). The price of the electric energy con- sumed is 2.5 pesetas/kWh, which is the price at which Plant I at present sells the kWh to UNELCO. The price of steam, agreed between the Energy and Water Authorities, is 33.8 kgm of fuel per ton of low pressure con- sumption steam and 83.6 kam fuel ner ton of high pressure consumption steam, being 3.07 pesetas/kg of fuel.-

Cost per m3 of water produced Amount of water produced: 5 500 000 m3 /year

Exploitation :

Steam Chemical products Personnel Maintenance Electric energy produced

Insurance: 1% of 950 000 000 pesetas

Annual amortization : At an interest of 6%

8% 10% 12%

Water cost per m3 6% 8% 10% 12%

pesetas 72 000 000 13 500 000 23 500 000 11500 000 63 000 000

9 500 000

86 450 000 100 463 000 114 475 000 130 388 000

51.8 54.3 56.9 59.9

These figures have been placed on a graph which shows the price per m3 according to the rate of interest (See Fig. 4).

AIso there is a graph (See Fig. 5) of the water cost versus the fuel cost for different electrical costs, at a fixed interest rate of 8%.

LAS PALMAS PHASE II DESALINATION PLANT 187

S/M3 PTSd

C.875

1 Pe =2.50 pts/KVh

Pf q 3.00 pts/Kq

0.75 1” ;

0.625 50 I I I I I

6 % 8 % IO% 12% INTEREST RATE

Fig. 4. Water cost vs. interest rate of Las Palmas Phase IL

S/M3

too

0.875

0.75

0.625

0.6

PTS/h13

INTEREST RATE = 8%

50-

48 FUEL COST i pts/kg )

I I I 4.0 -xi 5.0

Fig. 5. Water cost vs. fuel price of Las Palmas Phase IL