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 solutioncp is calculated at then kg) of thermal load.
2 If feed is fresh water
3 The of natural gas fuel.For CO2 emission
4 Environmental impact of burning fossil fuels.
| Fuel | Calorific value (MJ/kg) | CO2 (kg/kgfuel) | CO2/energy (kg/MJ) | SO2 (kg/kgfuel) |
| Coal | 26 | 2.361 | 0.091 | 0.018 |
| Fuel oil | 42 | 3.153 | 0.075 | 0.040 |
| Natural gas | 55 | 2.750 | 0.050 | 0 |
Extra activity:
Student can perform the following:
1 Perform parametric study to investigate the effect of on . Plot and explain.
2 Perform parametric study to investigate the effect of xf on . Plot and explain.
3 Find the values of the following thermo‐physical parameters at and (xf= 40 000 ppm); boiling point elevation (BPE), μ, v, k, hfg, cp, u, s, ρ. Explain the reason behind the differences.
4 Find the values of the following thermo‐physical parameters at under two salinities; xf = 20 000 and 40 000 ppm for; BPE, μ, v, k, hfg, cp, u, s, ρ. Explain the reason behind the differences.
5 Calculate entropy generation and irreversibility per mass of feedwater flow rate.
Example 1.6 Performance of Feed Pump
Feedwater is pumped from 100 kPa, 30 °C, 1.5 kg/s to 3 MPa, if pump efficiency is 75%:
Find:
1 Actual work input
2 Reversible work
3 Exergy destruction (irreversibility)
4 Second law efficiency
Solution
1 Working fluid water:State 1then h2 = 131.4 kJ/kg and s2 = 0.4423 kJ/(kg K).
2 Wrev = Ψ2 − Ψ1
3 Exergy destruction (irreversibility):
4 Exergetic efficiency:
Extra activity:
Student can perform the following:
1 Sketch T–v, P–v, and T–s diagram for actual and isentropic processes.
2 Resolve example using seawater (x = 40 000 ppm) as feed. Refer to Appendix A.
3 Compare results of feed seawater against water. Explain and comment.
Example 1.7 Exergy Analysis
