Seawater compressibility

RPs have already been used in different structural applications, to replace conventional metal in seawater-compressed air surfacing ballast tanks in the Alvin depth vehicle. This vehicle, a first-generation deep research vehicle, also used RP in its outer hull construction to enclose the pressure tanks and aluminum frame. In the unmanned acoustical research vehicle of the Ordnance Research Laboratory called Divar, an RP cylinder with a 16 in. OD, 3/4 in. wall thickness, I2V2 in. ID with nine ribs, a 60 in. length and weight of 180 pounds went to depths of 950 m (6,500 ft.). 

Seawater Distillation. The principal thermal processes used to recover drinking water from seawater include multistage flash distillation, multi-effect distillation, and vapor compression distillation. In these processes, seawater is heated, and the relatively pure distillate is collected. Scale deposits, usually calcium carbonate, magnesium hydroxide, or calcium sulfate, lessen efficiency of these units. Dispersants such as poly(maleic acid) (39,40) inhibit scale formation, or at least modify it to form an easily removed powder, thus maintaining cleaner, more efficient heat-transfer surfaces. 

Fig. 14.20). Magnesium occurs in seawater and as the mineral dolomite, CaCOyMgCO,. Calcium also occurs as CaCO in compressed deposits of the shells of ancient marine organisms and exoskeletons of tiny one-celled organisms these deposits include limestone, calcite, and chalk (a softer variety of calcium carbonate). 

In seawater the thickness" of the double layer as given by k1 (Eq. 3.9) is a few Angstroms, equal approximately to a hydrated ion. In other words, the double layer is compressed and hydrophobic colloids, unless stabilized by specific adsorption or by polymers, should coagulate. Some of this coagulation is observed in the estuaries where river water becomes progressively enriched with electrolytes (Fig. 7.14a). That these colloids exist in seawater for reasonable time periods is caused... 

The PVT properties of aqueous solutions can be determined by direct measurements or estimated using various models for the ionic interactions that occur in electrolyte solutions. In this paper a review will be made of the methods presently being used to determine the density and compressibility of electrolyte solutions. A brief review of high-pressure equations of state used to represent the experimental PVT properties will also be made. Simple additivity methods of estimating the density of mixed electrolyte solutions like seawater and geothermal brines will be presented. The predicted PVT properties for a number of mixed electrolyte solutions are found to be in good agreement with direct measurements. 

High Pressure Sound Velocimeters. As discussed earlier, sound velocity measurements can yield precise compressibilities of solutions. Wilson (109) was the first to develop a high pressure sound velocimeter that could be used over a wide range of pressures and temperatures. He used the "sing around" system to measure the high pressure sound speeds of water (109), D2O (120), and seawater (121) to a precision of 0.2 m sec l which is equivalent to 0.012 x 10 bar- in 6s Barlow and Yazgan... 

Li (142) published a variation of the Tait-Gibson equation for seawater from both compressibility and sound speed data... 

Unfortunately, seawater is slightly compressible, so in situ density increases with increasing pressure. The rate at which pressure increases with increasing depth below the sea surface is nearly equal to 1 dbar per m. At 45° latitude, the actual rate is l.Oldbar/m from 0 to 2500m. Below 2500m, it increases to 1.02dbar/m. This rate increase is due to an increasing resistance to further compression. Because Earth is not a perfect sphere, the rate at which pressure increases with depth also varies with latitude. Thus, the depth at which 4500 dbar of pressure is attained is 4428 m at the equator and 23 m shallower at the poles (4405 m). Tabulated values of pressure as a... 

Because seawater is slightly compressible, the in-situ density of a parcel of seawater will increase as it is lowered into the deep ocean. As shown in Table 3-5, this effect is small, causing only a 4% increase if the seawater parcel is lowered from 0 to 4000 m (in the absence of any exchange of heat or salt with adjacent parcels). The hydrostatic... 

Since the degree to which seawater is compressed is largely determined by in situ pressure, the reason why some surface seawater parcels sink deeper than others is wholly due to their temperature and salinity. Oceanographers are most interested in... 

Unfortunately, the compressibility of seawater is not just a function of pressure. It is also dependent (slightly and nonlinearly) on temperature and salinity. In very deep waters, this can cause ct0 to exhibit a very slight decrease with increasing depth although increases with depth. Although this effect is minor, it can impair the... 

As noted earlier, the effect of salinity and temperature on the compressibility of seawater is slightly nonlinear. Even at a constant pressure, salinity and temperature interact in a nonlinear way to influence density. This is shown in Figure 3 5 for The curves in the diagram are lines of constant ct. As temperatures decline, the effect of increasing salinity on density increases. This is particularly pronounced at the low temperatures characteristic of the deep sea and surface polar waters. For seawater at 0°C, a rise in salinity from 35 to 36%o increases the a, density 15 times more than the effect of dropping the temperature by 1°C. 

The nonconservative behavior of seawater density and compressibility is caused partly by H bonding and partly by the electrostatic attractions exerted by the salt ions on their neighboring water molecules. The effect of these attractions can be estimated by trying to compute the density of seawater as a simple sum of the volumes of water and salt present in 1 kg of seawater (5 = 35%o and t = 4°C). As shown in Table 3.6, the actual density, as tabulated in the online appendix on the companion website (ct, = 27.81, so p = 1.02781 g/cm ), is about 1% higher than that predicted from summing the volumes of salt and water (1.0192 g/cm ). 

Injection of compressed CO2 into the deep ocean has already been tested. The goal of this approach is to emplace the CO2 into waters with low temperatures, ensuring the formation of relatively immobile gas hydrates. This strategy has the potential to sequester thousands of gigatonnes of carbon, but likely environmental impacts include (1) a change in the pH in the seawater near the emplaced gas hydrates, (2) benthic kills, (3) other ecosystem impacts, and (4) release back to the atmosphere as an eventual consequence of meridional overturning circulation. 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

Cured Epoxy Requirements. In order to comply with multiple needs identified above, the epoxy pot should have a compressive yield strength of greater than 9000 psi. In addition, for brackish and seawater applications an arbitrary specification of zero creep at 50°C at 1000 psi under seawater for 3000 hours has been established. Applications at higher temperatures would obviously demand a higher zero creep temperature. 

Salinity measurements are most often used in oceanography to determine seawater density. The conventional measure used by oceanographers for determining salinity is conductivity. This is feasible because the salt content of seawater is well defined, as is the temperature-related compressibility. As an alternative, the refractive index of water is a good descriptor of density when temperature is known or can be measured. Refractive index provides a high-precision method for determining the density of pure water. As various salts are added, the refractive index is a less exact predictor of density, although relative measurements can still be useful. 

In the 1950s Hickman developed a centrifugal vapor compression evaporator for seawater desalination (53). This device consisted of multiple spinning discs. Seawater sprayed on one side of the disc evaporated, while the centrifugal force removed the residue from the plate surface. The vapor was compressed and returned to the opposite side of the plate, where condensation provided the heat for evaporation and the desired freshwater for recovery. Overall heat transfer coefficients of 18 kW/m2-K are about three times higher than those achieved in steam turbine condensers. 

Pierrot D, Millero FJ (2000) The apparent molal volume and compressibility of seawater fit to the Pitzer equations. J Soln Chem 29 719-742... 

Table 1.10. Selected values and parameters for ion partial molal volumes and compressibilities in seawater (from Millero, 1982b and 1983).

Below the thermocline, the temperature changes only little with depth. The temperature is a non-conservative property of seawater because adiabatic compression causes a slight increase in the in situ temperature measured at depth. For instance in the Mindanao Trench in the Pacific Ocean, the temperature at 8500 and 10,000 m is 2.23 and 2.48 °C, respectively. The term potential temperature is defined to be the temperature that the water parcel would have if raised adiabatically to the ocean surface. For the examples above, the potential temperatures are 1.22 and 1.16 °C, respectively. Potential temperature of seawater is a conservative index. 

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