DESCRIPTION OF THE REFERENCE SEAWATER DESALINATION PLANTS The following seawater desalination processes were considered as the most interesting

for large scale seawater desalination in integrated co-production plants:

Multi-stage flash once through distillation (MSF-OT);

Multiple effect distillation (MED) and - Reverse osmosis (RO).

4.2.1. MSF-OT plants

The MSF-OT units chosen are of modular design. The modules are arranged parallel to each other and connected by U-turns of the brine flow in the condensers and the evaporators.

Each module contains several evaporation stages placed in long tube arrangement. The condenser tubes are arranged in 2 parallel bundles per module. Figures 13 and 14 show the module arrangement of a long tube MSF-OT unit with 44 stages, and the cross-sectional view of one of its modules respectively.

As construction materials for the MSF-OT units, stainless steel for evaporation shells and titanium (TiPa) for the tubes are used. The units are operated without separate deaerators and decarbonators.

37

LOCAL CONTROL AND SW1TCHGEAR ROOM BELOW V6

BRINE HEATER BELOW V1

STAGE 44 ——3.

SEAWATER IN —

19600

>6o 3000 sbAo

FIG 13 Module arrangement of a 44 stages MSF-OT long tube unit (72 000 m/d) [JO]

I 9 S O O

3 350 2 800 2 800 3 350

SECTIONS TO BE —J PREMANUFACTLIRED K

FIG 14 Cross-sectional vie\\ of a MSF-OT long tube unit [10]

TABLE Xll TECHNICAL PERFORMANCE DATA OF MSF-OT UNITS ALL OF 7 2 0 0 0 m3/ d [ l l ]

GOR

Number of stages

Maximum brine temperature Brine blow-down temperature Steam temperature in brine heater Thermal heat consumption a* Electncit) consumption Seawater flow b)

Seawater design parameters 30°C, 45

°C

a) without steam supply for vacuum units

b) including cooling water demand of multi-stage steam ejector vacuum system of barometric t>pe

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The nominal net capacity of a single MSF-OT unit is 72 000 m³/d, taking a seawater temperature of 30°C and a seawater TDS of 45000 ppm as a basis (conservative assumptions) Therefore, 4 MSF-OT units of 72000 m3/d are required to produce the reference water quantity Four different long tube MSF-OT units with different GOR, all of modular long tube designs, were considered Table Xn shows the most important technical performance data of the different MSF-OT units

4.2.2. MED plants

Four different low temperature horizontal tube multi-effect (LT-HTME) processes with a GOR of 7 5 to 135, as well as two high temperature vertical tube evaporation (HT-VTE) processes with a GOR of 17 and 21 respectively were chosen At seawater design conditions (30°C, 45 000 ppm), the nominal net capacity of each unit is 36 000 m³/d Table XIH contains the most important technical performance data of the various MED units

Dependant on the GOR, the LT-HTME plants contain 8 to 18 evaporation effects with brine flash chambers, which are located below the evaporators, and one heat rejection condenser To remove non-condensable gases, a steam jet ejector is assembled at the coolest end of the heat rejection condenser. The feed is treated with a harmless polyphosphate additive to inhibit scaling on the heat transferring outer surface of the tube bundles Low-temperature operation enables the utilization of low cost construction materials such as aluminium tubes, plastic piping and epoxy painted steel shells

The HT-VTE plants contain 28 and 23 evaporation effects respectively, in which the brine runs down as a thin film in the condenser tubes and partly evaporates In contrast to LT-HTME plants, thin-walled titanium or high-grade steel alloys are used as tube bundle material in the top effects, because of the high temperature operation The evaporation effects and the final condenser are arranged in a vacuum-tight concrete or carbon steel shell

TABLE Xffl TECHNICAL PERFORMANCE DATA OF MED UNITS ALL OF 36 000 mVd [11]

HT- HT- LT-VTE-1 VTE-2

HTME-1 2 3 4

GOR

Number of effects

Maximum brine temperature Bnne blow-down temperature Steam temperature in brine heater A$ in final condenser

Thermal heat consumption a)

Electricity consumption Seawater flow bl

°C Seawater design parameters 30°C. 45 OOP ppm

a) without steam supply for vacuum units

b) including cooling water demand of multi-stage steam ejector vacuum system of barometric type

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4.2.3. RO plant

To produce the reference water quantity, 12 parallel trains are used, each with a net capacity of 24 000 m³/d As membrane module configuration, hollow fibre membranes are chosen, which are operated in single stage mode The pumping energy in the bnne blow-down is partly recovered by Pelton turbines Figure 11 in Section 2 illustrates the simplified flow diagram of the RO plant

The design of the RO trains was performed for a temperature range of 23 to 35°C and a TDS of 45 000 ppm, assuming a five year operation time of the membrane modules (warranty reasons) The recovery ratio of the RO trains is kept constant at 35% by regulating the feed water pressure as a function of temperature (so-called "temperature/pressure guidelines") [19] In Annex I, the chemical analysis of the produced water is given The results show that the WHO drinking water standards can be fulfilled in the single stage module arrangement

In the economic assessment, an average annual seawater temperature of 27°C was assumed (conservative assumption), so that an average feedwater pressure of 72 bar is required to produce the reference water quantity Table XIV shows some relevant technical data of the RO trains, as well as the breakdown of their electricity consumption

No separate assessment of RO plants with spiral wound membrane modules was performed, since similar specific water production cost is to be expected The pre-heating of feedwater in the condenser of the power plant beyond 35°C, which could increase the membrane performance, was not considered This is currently not state-of-the-art, and

TABLE XIV TECHNICAL DATA OF THE 24 000 m3/d RO TRAINS [17,19]

Membrane configuration Net water capacity

Seawater design temperature Seawater design TDS

Feedwater pressure at seawater design conditions Recovery ratio

Number of membrane modules Electricity consumption:

Seawater pumps (Ap=l 7 bar, T|P=0 85, nM=0 96) Booster pumps (Ap=3 3 bar, T|P=0 85, T|M=0 96) High pressure pumps (Ap=71 bar, T)p=0 85, T|M=O Energy recovery (T)Pei=0 85)

Other power (0 979 kW(e) h/m3) Total specific electricity consumption

* other plant configurations may reduce the total specify

m3/d

°C ppm bar

%

MW MW 96, T]^C=0 97) MW MW MW kW(e) h/m3

.c electricity consumption bv abou

hollow fibre 24000 27 45000 72 35 795

017 033 7 3 3 -3 19

1 00 55*

t 0 5 kW(e)/m Subscripts p^pump M^motor C=hydrauhc coupling Pel=pelton turbine

40

furthermore, the proportion of membrane equipment cost in total RO plant investment cost is low (about 10 to 15%), so that appreciable benefits in specific water cost are not achieved.