Introduction to Desalination. Fuad Nesf Alasfour

<50 100 Cu²⁺ 0.05 1.5 Fe³⁺ 0.10 1.0 NaCl 250 — pH 7–8 6.5–9

Table 1.5 Thermo‐physical properties of typical seawater at 40 000 ppm and 20 °C.

Density	1.0288 kg/m^3
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/(m^2 s)
Kinematic viscosity	10.58 × 10^−7 m^2/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 SO4^2−	2701	−2	7.68	28.1	96.062
Magnesium Mg^2+	1295	+2	3.68	53.3	24.305
Calcium Ca^2+	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 BO3^3−	27	−3	0.08	0.46	58.808
Strontium Sr^2+	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

      x_f = 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