Mohammed Abdulla Sunday, December 19, 2010 Desalination Mohammed Abdulla Sunday, December 19, 2010

Outlines Salinity Desalination Processes Desalination Barriers. Abu Dhabi Solar Desalination Plant. Useful Resources. First of all I will define the main term in desalination which is salinity.

Desalination Plants Rejection 1. Salinity Salinity Concentration of salt in water measured in grams per liter or parts per thousand. Fresh Water Brackish Water Saline Water Brine Less than 0.5 g/l Desalinated Water 0.5-30 g/l Underground Water 30-50 g/l Sea Water More than 50 g/l Desalination Plants Rejection Based on the salinity, water is divided into four parts.

2. Desalination Processes Mainly, we have two inputs, and two outputs. Now, lets sea what the separation process could be?

2. Desalination Processes Separation Processes Thermal Multi Stage Flash Multiple Effect Evaporation Freezing Humidification-Dehumidification Membrane Reverse Osmosis Electro-Dialysis

Specific Heat Capacity 2.1 Thermal Desalination Specific Heat Capacity Heat required to raise a unit mass by a unit temperature at constant volume or pressure (J/kg.°C). Sensible Heat Latent Heat Heat added or removed and causes change in temperature. Heat added or removed and causes change in phase. I would like to review some thermodynamics terminologies as they are essential in thermal desalination. Water Vapor Water Steam Gas phase of water. Water vapor at high pressure and temperature.

Water Saturation Pressure 2.1 Thermal Desalination Water Saturation Pressure Pressure at which water changes its phase at certain temperature. In thermal desalination, we always need vapor. In order to keep the water vapor, its pressure should always be below the saturation pressure. This will become clear when we come to the thermal processes.

2.1.1 Multi Stage Flash Most widely, 60% in the world. It consists mainly of several stage depending on the capacity of the plant, condensers, and heater. Moving forward in the consecutive stages, the temperature decreases.

2.1.1 Multi Stage Flash Demister is like a mesh that separates brine droplets from fresh water vapor. Part of the feed water is for cooling and is rejected after condenser because this is a single stage that is not able to handle all the feedwater. The most challenging part in thermal desalination is the accurate control of temperature, pressure, brine level, vibration, insulation, This is very simple. You can do something like this in your project prviding heating water from solar panels. (El-Dessouky & Ettouney, 2002)

2.1.2 Multiple Effect Evaporation

2.1.2 Multiple Effect Evaporation (El-Dessouky & Ettouney, 2002)

2.2 Membrane Desalination (Reverse Osmosis) Osmosis (Natural Osmosis) Water movement through a membrane from low salinity side to the high salinity side.

2.2 Membrane Desalination (Reverse Osmosis)

2.2 Membrane Desalination (Reverse Osmosis)

2.2 Membrane Desalination (Reverse Osmosis) Tampa Bay RO Desalination Plant, Florida, US, 2007

3. Desalination Barriers Energy Consumption Cost Scaling Environment Impact Main desalination barriers are energy consumption, cost, scaling, and environmental impact.

3.1 Energy Consumption Multi Stage Flash & Multiple Effect Evaporation (Thermal) Reverse Osmosis (Membrane) 10 – 200 kWh/m³ 1 – 10 kWh/m³ Steam Turbines Gas Turbines Steam and Gas Turbines Power (MW) to Water (MIGD) Ratio 5 : 1 10 : 1 15 : 1 Water desalination plants usually exist in conjunction with power generation plants. One reason is to reduce energy consumption significantly specially for thermal desalination. Also, when building new city they first build desalination and power plants together. If they want water more than power, they will get steam turbines. If they want

3.1 Energy Consumption This is a desalination plant that consists of both thermal and membrane desalination.

3.2 Cost (Semiat, 2000)

3.3 Scaling What is scaling? The accumulation of salt particles on the desalination plant components. How to minimize scaling? Addition of chemicals, like Polyphosphate. Accumulation over condensers, evaporators. Act as thermal insulation and decreases efficiency.

3.4 Environmental Impact Energy consumption and carbon footprint. Rejection of brine back into sea. Brine is hot and has high salinity. It could destroy marine life. Should be diluted and cooled down before rejection.

4. Abu Dhabi Solar Desalination Plant Built in 1984. It consists mainly of a solar collector field, heat accumulator tanks, and evaporator. Evacuated tube flat plate collectors. They want this plant to run for 24 hours and to minimize the temperature flactuation, so they added heat accumulator tanks. Most solar desalination plants use vertical MEE because (El-Nashar, 2008)

5. Useful Resources Fundamentals of Salt Water Desalination, by El-Dessouky & Ettouney Thermal analysis, mass balance, energy balance, efficiencies, ratios. Desalination Research Progress, by Delgado & Moreno Renewable energies and desalination. Multiple Effect Distillation of Seawater Using Solar Energy, by El-Nashar Solar desalination plant in Abu-Dhabi.