Introduction: (Initial Observation)
Water, as a resource is the most basic and fundamental component to life. Yet, only 1% of the Earth’s water is fresh water. This small amount is becoming more inadequate, particularly in certain geographic areas of the world and in high-density populated areas, due to pollution and shortages of rainfall. Desalinating seawater has long been considered as a solution to the world’s current and future water problems. However, desalination has been costly and un-affordable in places where it is most needed. Desalinated plants around the world today produce more than several billion gallons of water per day. However, the demand for water continues to exceed the available supply.
To understand the problem it is good to know that less that 1% of fresh water is consumed by human and another 99% has industrial uses. Production of steel, aluminum, gasoline, synthetic fiber and many other products relies on accessibility to large amounts of water.
There are more than 7,500 desalting plants in operation worldwide, producing several billion gallons of water per day. 57% are in the Middle East and 12% in the Americas, mostly located in the Caribbean and Florida. However, as drought conditions continue and concerns over water availability increase, desalination projects are being proposed at numerous locations.
In this project you will study one or more of the problems we face in desalination.
Find out about desalination methods, their costs and their problems. Read books, magazines or ask professionals who might know in order to learn about material, equipment and energy sources used in desalination. Keep track of where you got your information from. Following are samples of information that you may find:
Desalination is a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination, including reverse osmosis (RO), distillation, electro-dialysis, and vacuum freezing. Two of these technologies, RO and distillation, are being considered by municipalities, water districts, and private companies for development of seawater desalination. These methods are described below.
Reverse Osmosis (RO)
Membrane processes or Reverse Osmosis are usually used with brackish inland water, the salt content of which, though undesirable, is considerably below that of seawater. In such a process, fizz water (carbonated water) is pumped at high pressure through permeable membranes. Fresh water passes through while the concentrated mineral salts remain behind. The fizz water is pretreated to remove particles that would clog the membranes. The quality of the water produced depends on the pressure, the concentration of salts in the fizz water, and the salt permeation constant of the membranes. Product water quality can be improved by adding a second pass of membranes, whereby product water from the first pass is fed to the second pass.
Distillation remains the most widely used desalination process. In the distillation process, water needs to be boiled and evaporate and then condensed. This requires a lot of energy and makes the process expensive. In order to save energy during the distillation process, two different methods are being used. The first method is known as multiple-effect and the second method is called flash.
Multiple-effect evaporator consists of a series of evaporators in which salt water is heated and vaporized in long, vertical tubes. The hot vapor is used to heat salt water entering the next evaporator; in doing so, the vapor is cooled and condensed into fresh water. Because the multiple-effect evaporator reuses heat, it requires less fuel to treat incoming water than a single evaporator.
In flash evaporation, heated seawater is sprayed into a tank kept under reduced pressure. At this reduced pressure, the water vaporizes at a lower temperature, so that flash evaporators require less heat and thus less fuel. Multistage-flash distillation systems consist of a series of flash chambers operating at decreasing pressures. Such systems are more efficient and have greater capacity than single-stage units, and so are employed in very large desalination plants.
Solar distillation – In regions where salt water and intense sunlight are both abundant, a simple distillation apparatus can be used. The heat of the Sun partially vaporizes salt water under a transparent cover; on the underside of the cover, the vapor condenses and flows into a collecting trough. The principal difficulty in this process is concentrating the energy of the sunlight within a small area.
Since distillation is the most commonly used process, we focus our research on the distillation process.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
What are the problems with desalination using distillation process? What are the solutions for such problems?
One major problem with desalination is the energy cost. That includes the cost of fuel and electricity used for evaporation of salt water. Specific questions that can be the subject of study in this project are:
- How does the choice of energy affect the cost of desalination of salt water?
- You may experiment desalination by evaporation using a gas fuel, a liquid fuel, and electricity. Compare the cost of energy per liter (or per gallon) of the produced fresh water.
- How does the concentration of salt in water affect the cost of desalination? You may experiment desalination of water with different concentrations of salt. Compare the cost of energy per liter or per gallon of the produced fresh water.
- Does weather temperatures affect the cost of desalination?
- You may experiment desalination of water at different weather conditions and compare the cost of energy per liter or per gallon of the produced fresh water.
What you see above are just samples of questions that may be studied in relation to desalination. You may come up with questions that focus on design or material used in desalination system. In either case you will need to repeat desalination experiment to see how certain factors affect the distillation rate or cost.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Following is an example of defining variables for our first question:
The independent variable (also known as manipulated variable) is the type of energy. Possible values are gas fuel, electricity, oil, solar energy. (Note that the cost of solar energy is the cost of lease for the land where you install your equipment.)
The dependent variable (also known as responding variable) is the cost of energy used to produce one liter (or one gallon) of fresh water.
Controlled variables are the weather temperature and moisture. (You should do all your experiments at the same time or in identical weather conditions.)
Constants are the evaporation method, apparatus, type of salt water, initial temperature of salt water.
Following is an example of defining variables for our second question:
Independent variable is the concentration of salt. Possible values are 5% salt, 10% salt, 15% salt.
Dependent variable is the cost of energy used to produce one liter (or one gallon) of fresh water.
Controlled variables are the weather temperature and moisture. (You should do all your experiments at the same time or in identical weather conditions)
Constants are the evaporation method, apparatus, type of energy, initial temperature of salt water.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
This is a sample hypothesis for the first question.
The cost of energy depends on the location. In some areas, certain types of energy may be cheaper due to higher availability. In New Jersey shoreline gas may be the best source of energy due to high availability. I estimate the production cost to be close to $1 per gallon. My hypothesis is based on my previous information about refineries in New Jersey and the price of bottled water.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: How does the choice of energy affect the cost of desalination of salt water?
Introduction: You may experiment desalination by evaporation using a gas fuel, a liquid fuel, and electricity. Compare the cost of energy per liter (or per gallon) of the produced fresh water.
Prepare a distillation setup that can be used with different sources of energy. Your distillation apparatus that can be built at home may consist of :
- A Boiling Chamber — which can be a Pressure Cooker, Tea Kettle or Other Boiling Container.
- A Plastic Bucket in which to install the Stainless Steel Condensing Coil.
- Extra High Capacity Spiral Condensing Coil. (Stainless steel Spiral condensing coils are hard to find. You may use any glass, copper or plastic coil instead)
- A plastic or rubber tube to connect the condensing coil to the boiling chamber.
As a heat source for your boiling chamber you may use a portable gas stove, an electric heater, a solar furnace, an oil burning stove, or a charcoal burning stove. (Only select those who you can access and use safely.)
When your setup is ready, record the time, temperature and humidity and start your experiment with the first fuel or heat source.
Stop when your production of fresh water is equal to 1/2 of the salt water that you started with.
Measure or estimate the amount and the cost of each fuel. Record your results in a table like this:
|Energy Source||Amount of fuel/ energy||Cost of used energy||Amount of water produced||Cost of fuel per Litter|
Calculate the cost of fuel per litter of water and write it in the last column.
You can use the same idea to develop your own experiment procedures if you choose to study any other questions.
Distillation is probably the most common technique for purifying liquids. In simple distillation, a liquid is boiled and the vapors work through the apparatus until they reach the condenser where they are cooled and re-liquified.
The process is relatively simple:
a) the dirty water or salt water is heated
b) to the boiling point and thus vaporizes
c) (becomes steam), while other substances remain in solid state, in boiler. Steam is then directed into a cooler
d) where it cools down and returns to liquid water
e) and the end result is a water, purified of additional substances found in it before distillation.
For chemical experiments and small amounts of liquids, we are not usually concerned about the rate of production or the amount of energy used for the distillation process. In industrial production however, specially for water distillation, cost is a major concern. We should design the equipment for minimum energy consumption and maximum production of distilled water.
Another simple distillation apparatus that can be built at home consists of :
- A Boiling Chamber — which can be a Pressure Cooker, Tea Kettle or Other Boiling Container.
- A Plastic Bucket in which to install the Stainless Steel Condensing Coil.
- Extra High Capacity Spiral Condensing Coil.
This setup is much more productive and efficient than laboratory glassware used in the first example. But the boiling chamber will still waste some heat energy.
Small but quality constructed water distillers are well insulated and save more energy. The following picture shows one of these distillers.
The hot side of this distiller is insulated to save energy.
1-Hot chamber, 2-Heat element, 3-Condenser coil, 4-Fan, 5-distilled water
A simple water distiller can be made of a stainless steel box that from inside is divided to 2 parts.
The left part that is a hot chamber has a heat element and insulated double walls. The right side that is cold chamber has no insulation, instead it has radiators or heat sync for better heat exchange. Water will be boiled in the left side and vapors travel to the right side, where they condense and become distilled water. Such simple box distillers are used to produce distilled water for laboratories and some hospitals.
To construct a simple distillation device with household items, you may use two disposable aluminum containers. One container will hold the salt water and sit on an electric hotplate. The other one will be about a foot away and two containers will be connected to each other using a PVC pipe.
Containers usually come with a lid, but if they don’t, cut a piece of cardboard and cover it with aluminum foil to be used as a lid. Make a hole on both lids to fit the PVC pipe, connect the pipe and seal it with hot-melt glue (applied by a glue gun). One extra small hole on each lid covered with a rubber, plastic or wooden plug can be helpful because you will be able to use these holes to add salt water or drain distilled water. The hole on the cold side can remain open to ensure smooth flow of steam from the hot side to the cold side. It is also necessary for the pipe to have a slope toward the cold container.
In a commercial desalination plant, different methods are used to accelerate evaporation and save energy while producing distilled water in a continuous process. Some of these methods are as follows:
- As the salt water evaporates, the salt concentration increases, so the system continuously discharges some concentrated salt water that usually goes back to the sea. This salt water is hot. So before it gets out of the facility, it goes trough heat exchangers and transfers its heat to incoming salt water. So when the new salt water enters the system, it is already hot and needs a little more heat to evaporate.
- Hot steam also goes through condensers that are also heat exchangers. Hot steam also will give its heat to incoming salt water.
- A turbine constantly sucks the air and steam from hot tank and force it in to a condenser tank. This will reduce the pressure in the hot tank, so it will boil at lower temperatures. It also accelerates the condensation process.
In the above picture, green pipes represent salt water and blue pipes indicate distilled water. The red areas indicates heat or heat exchange. compressor at the top is actually a turbine.
Materials and Equipment:
List of material may vary depending on the material that you can find for your experiments. This is a sample list of material.
- Glass distillation set (2 Flasks, one condenser and connecting glass elbows)
- Electric heater
- Rubber tubes (used to send cold water to the condenser)
- Glass Thermometer
- Pressure Cooker
- Plastic Bottle
- Clear PVC tube
Results of Experiment (Observation):
After each water purification or distillation, you may want to see the result. So you may test the filtered or distilled water with more advanced test equipment that can detect minor amounts of contaminants. However in this experiments we can evaluate a filtered or distilled water by it’s color, smell or taste. Experiment of distilling salt water produced a clear water with no taste an no odor.
In more advanced level, students may want to calculate the amount of heat energy used to produce certain amount of distilled water.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
List of References