DESALINATION
There are two basic methods for desalination of water for which titanium is best suited. A discussion of these two methods-- multi-stage flash evaporation and evaporation with vapor recompression--is given below.

MULTI-STAGE FLASH (MSF)
The MSF units have been in use for over thirty years in both shipboard and landbased plants. The shipboard plants were primarily U.S. Navy ships requiring drinking water and boiler feedwater. The land-based plants were largely to supply resort hotels on Caribbean Islands with drinking water. Due to their success, the use of MSF plants has spread to industrial complexes in that area. The first titanium-tubed plant went into service on the Island of St. Croix in 1968, beginning the change from traditionally used copper alloys.

One of the inherent advantages of MSF plants is that their heat source can come from extraction steam from a steam power plant. A typical plant is shown schematically in Figure 13.^(14)

In this design, there are three major sections:

1. Heat Rejection Section
2. Heat Recovery Section
3. Brine Heater

The incoming raw seawater (see Figure 13) passes through the tubes in the top of the heat rejection section and is heated by the steam flashed from the bottom section. A major portion of the raw seawater is then discharged from the system. The remaining portion is treated either by polyphosphates or acid to prevent carbonate precipitation and is also degassed. This treated water then passes through the tubes of the heat recovery section where it is heated again by the flashing steam which is condensed and collected on trays. After passing through the highest state of the heat recovery section, the water goes to the brine heater where it reaches its highest temperature using steam, usually extracted from a steam turbine power plant located adjacent to the MSF plant. The air ejector, which creates the necessary below atmospheric pressure to create low-pressure steam, is also usually supplied with extraction steam. In the first stage, the pressure and temperature of both steam and water are at their highest. Both the temperature and the pressure drop from the brine heater to the last stage of the heat rejection section. This causes flash boiling of the incoming brine in each stage. A part of the brine in the last stage is discharged and the remaining portion is mixed with incoming seawater and used as circulating brine to continue the cycle.

In recent years, countries in the Middle East such as Saudi Arabia, Kuwait, Bahrain and the United Arab Emirates have developed requirements for great quantities of fresh water to be used in their new industrial complexes. They have opted to convert sea water to fresh water by MSF. Because of the success of titanium tubing in this service in the Caribbean, it was selected for many of these Middle Eastern plants.

In two locations alone, Al-Jobail and Al-Khobar, in Saudi Arabia, the two desalination plants^(15) with combined capacity of almost 350 million gallons of water per day (MWD) used 6,854,440 lbs. of titanium tubing. Based on 1 1/4" OD x .020" gage tubing, this represents 45,363,600 linear feet of titanium tubing. Desalination service is an excellent environment for thin wall tubing since the corrosion rate on both the steam side and the water side is essentially nil. Tube wall thickness as low as .016" have been used successfully resulting in a heat transfer rate higher than a .049" ga 90/10 copper-nickel tube. Beside the improved heat transfer, the thin wall tubing is an economical choice.

In a properly designed MSF plant, vibration is not a serious condition because steam velocities are extremely low compared to a power plant surface condenser.

Titanium tubes are recommended for all three sections of a MSF plant. The heat rejection sections handles raw water containing sand, silt, and debris similar to the conditions in a power plant surface condenser. The heat recovery section operates at higher temperatures and the water does contain some oxygen and carbon dioxide which are harmful to copper alloys over long periods, of time, but which are not harmful to titanium. In the brine heater, both the steam and the water are at their highest temperature level. In addition, there are often deposits of calcium carbonate on the inside surfaces of the tubes. It is suggested, therefore, that if the brine temperature is over 200 degrees F, commercially pure titanium tubes, TIMETAL 50A, not be used in the brine heater. Instead TIMETAL Code-12 tubing becomes the preferred choice. Another alloy with palladium, TIMETAL 50A Pd, could also be used but at a much higher cost.

EVAPORATION WITH VAPOR RECOMPRESSION
In a vapor compression unit (see Figure 14)^(16), treated water is pumped through a heat exchanger 1 which raises its temperature to near the boiling point. It then passes through a deaerator for CO2, NO2 and O2 removal. 2 The feed then goes to the brine sump at the bottom of the unit 3 where it combines with a brine slurry. the slurry is pumped to the top of the unit and distributed to the top of the vertical titanium tubes which are covered with distribution caps. The slurry then forms a falling film on the inside of the tubes. The vapor condenses on the outside of the tubes due to the temperature differential between the vapor and the brine slurry film. A sensing device controls the concentration in the sump and the supersaturated brine slurry waste is discharged for final disposal. The product water gives up its heat to the incoming waste stream in the heat exchanger. The fresh water is then pumped for distribution.

Although this type of unit can be and has been used in desalination, it is most commonly used as a brine concentrator taking power plant cooling tower blow down waste, concentrating it, and removing fresh water. The concentrated waste from the unit is piped to drying ponds where the remaining moisture is evaporated and the solid waste piped away. Figure 15^(17) shows the relative corrosion rates of the various materials considered for this application and aptly illustrates why titanium was chosen over previously used copper and aluminum alloys.