Reverse Osmosis
03 August 2009
Water, is taught by thirst.
Land -- by the Oceans passed.
What is Reverse Osmosis (RO)
Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. Membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
Osmosis describes how solvent moves between two solutions separated by a semi permeable membrane to reduce concentration differences between the solutions. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semi permeable membrane. The semi permeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.
Whew!!! I wish I wrote that.
Simply put, if you took a cup full of salt water and a cup full of fresh water (two different concentrate solutions) and poured them into a larger container they would instantly mix and become one. Now, if you were able to separate the salt water and fresh water in that container by nothing more than a standard paper towel eventually the salt water and fresh water would mix on their own by the fresh water migrating into the salt water through the paper towel. The fresh water being a more dilute solution would migrate to the salt waters more concentrate solution. That is called "Osmosis"
Now if you were to replace that standard paper towel with a semi-permeable membrane designed for watermakers and then poured a cup of salt water on one side and a cup of fresh water on the other, what would happen? Nothing! These membranes are far too dense to let the fresh water migrate into the salt water. But! If we can add pressure to the salt water side, like about 600-1000 pounds worth then we would force the salt water solution to the fresh water side. That is called "Reverse Osmosis."
Now it does get a bit more complicated. For example even though we apply 600-100 pounds of pressure and the Reverse Osmosis process begins, there are some things even under that kind of pressure that cannot pass through the membrane and there are things that can. Those things that cannot pass through the membrane of course are salt. Those things that can are of course water. There are other things that can and cannot pass through the membrane that are found in sea water but for now lets stick to fresh and salt water in a somewhat pure form.
If we continue to keep pressure on the salt solution side eventually (in theory) we would in essence squeeze out all the fresh water while concentrating more and more the salt water side until we had nothing left but salt. This is were we introduce flow. No, not Flo down at the diner.
Watermakers
02 August 2009
Water, water, every where,
And all the boards did shrink ;
Water, water, every where,
Nor any drop to drink.
The Rime of the Ancient Mariner
One of the reasons I've started this blog was to delve deeper into watermakers. I get scores of questions about watermakers. Most of the time the questions are the same. So I thought that this would be a good place to perhaps help guide people to so they can get their questions answered. I will attempt to cover all the questions I believe people who are looking into a watermaker for their boat will need answers for. One of my recurring mantras is "There is no "One size fits all" watermaker".There are questions that have to be asked and answered to make sure you end up with a watermaker that best fits you and your needs. You will have some questions and I will have even more for you that will also need to be answered. We will cover a very large area of information. If you are new to watermakers and just want some basic information to start out with, you are in the right place. If you already have a watermaker, you are in the right place. If you think the cheapest watermaker you can buy will work for you, you are in the right place. If you think you're going to need a watermaker that makes fifty gallons an hour, you are in the right place. If you are looking to get into a different or larger watermaker, you are in the right place. If your buddy or the crowd at the bar have a watermaker they tell you is the one you have to have, you are in the right place. If you swear a watermaker is the last thing you or anybody else needs on their boat, you are in the right place. We will cover all these issues and lots more. So if this topic interests you, you are in the right place. Feel free to join in with your comments, observations and certainly your experiences good and bad. We will start out with what exactly is a watermaker and how they basically work and break them down into their components. We will then move on to the different types and how they differ and why that will be important to you. As I gain more experiance with this site I will be adding plenty of pictures pertaining to watermakers. I might throw in some personal stuff as the mood strikes me as well.
First attempt
01 August 2009
I have finally decided it is time to attempt a blog. For many reasons I have put it off and have limited myself to the many good sailing boards/forums that abound on the net. I suppose it only fair to let those who I have interacted with for so long get a better idea of who I am, what I do, and why I see or believe the things the way I do. As the common preamble to all such things undertaken for the first time this will be a work in progress and will no doubt change with the tides, new information learned, or just how well I embarrass myself.
A little useless information about myself. I was born and raised in South Florida. I have been around the water all my life. I have owned several boats throughout my life. My belief has always been "The best way to have a boat is to have a friend who owns one". So much for beliefs. We currently own a 95 FP Athena 38' Catamaran named Poerava. We've had her for about four years now. She is the first Catamaran I have owned. Since the first day I brought her home my decision to go with a Cat over a Mono has been rewarded with an ever growing appreciation for Cats. At least for this particular Cat.
I first learned/taught myself to sail in a 9' plastic trimaran my parents had at their second get away home in the Florida Keys. It was bayside just north of Long Key state park and just south of Fiesta Key back in the late 60s early 70's. A time when a brown paper bag with a half dozen P&Js and a one gallon glass milk jug filled with Koolaid was enough preparation for a long day exploring or an overnighter further south or north. I look at my boats systems now, then think back to that plastic boat. I wonder if I haven't gone so far as to forget why I like to sail in the first place.