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Table 1.5 Thermo‐physical properties of typical seawater at 40 000 ppm and 20 °C.
| Density | 1.0288 kg/m3 |
| Specific heat capacity | 3.973 kJ/(kg °C) |
| Boiling point elevation, at 20 °C | 0.344 K |
| Boiling point elevation, at 90 °C | 0.565 K |
| Thermal conductivity | 0.601 W/(m K) |
| Dynamic viscosity | 1.089 × 10−3 kg/(m2 s) |
| Kinematic viscosity | 10.58 × 10−7 m2/s |
| Latent heat of vaporization | 2355.4 kJ/kg |
1.6.2 Seawater
Table 1.5 shows thermo‐physical properties of standard seawater at 40 000 ppm and 25 °C.
Note that the comprehensive thermo‐physical properties of seawater as a function of temperature and salinity are presented in Appendix A.
The major six elements that comprise about 99% of seawater are chlorine (Cl−), sodium (Na+), sulfate (SO42−), magnesium (Mg+2), calcium (Ca+2), and potassium (K+), and standard seawater composition at 35 000 ppm are presented in Table 1.6.
Table 1.6 Standard seawater composition at salinity equal to ≈35 000 ppm.
| Chemical ion | Concentration (ppm) | Valence | Total salt content (%) | mmol/kg | Molecular weight |
| Chlorine Cl− | 19 345 | −1 | 55.03 | 546 | 35.453 |
| Sodium Na+ | 10 752 | +1 | 30.59 | 468 | 22.990 |
| Sulfate SO42− | 2701 | −2 | 7.68 | 28.1 | 96.062 |
| Magnesium Mg2+ | 1295 | +2 | 3.68 | 53.3 | 24.305 |
| Calcium Ca2+ | 416 | +2 | 1.18 | 10.4 | 40.078 |
| Potassium K+ | 390 | +1 | 1.11 | 9.97 | 39.098 |
| Bicarbonate HCO3− | 145 | −1 | 0.41 | 2.34 | 61.016 |
| Bromide Br− | 66 | −1 | 0.19 | 0.83 | 79.904 |
| Borate BO33− | 27 | −3 | 0.08 | 0.46 | 58.808 |
| Strontium Sr2+ | 13 | +2 | 0.04 | 0.091 | 87.620 |
| Fluoride F− | 1 | −1 | 0.003 | 0.068 | 18.998 |
| Σxi = 35 151 ppm | |||||
Note that Cl makes up to 55% of salt in seawater and NaCl makes up to 86% of salt in seawater.
Example 1.5 Heat Exchanger with Feed Seawater
An adiabatic heat exchanger provides thermal load to feed seawater, such that feed seawater can be supplied to MEE desalination system at 70 °C.
Steam stream: 1 kg/s, 120 °C
State 1: saturated vapor
State 2: saturated liquid
Feed seawater stream:
State 3: 20 °C
State 4: 70 °C
xf = 40 000 ppm
Find:
1 Feed seawater flow rate.
2 Assume feed is fresh water, find feed flow rate.
3 If steam (thermal load) is generated by conventional boiler using natural gas as a fuel, find fuel flow rate and emitted amount of CO2.
4 Provide emission table for three types of fuels to generate steam (thermal load).
Solution
1 Energy balance (first law): of thermal load (≈11 time of steam flow rate).Alternative