Desalinate seawater at full vacuum level

[faslan]

ok Water boils and transform in to steam in room temperature at full vacuum.

why can't we use this method to desalinate seawater. we don't need any fuel for heating water but just only energy need for

maintenance ,pump water ,make water containers full vacuum

[Ophiolite]

ok Water boils and transform in to steam in room temperature at full vacuum.

why can't we use this method to desalinate seawater. we don't need any fuel for heating water but just only energy need for

maintenance ,pump water ,make water containers full vacuum

1. The energy required to create the vacuum is substantial and thus renders the overall process expensive.

2. How do you intend to remove the salt from the vacuum chamber?

[Enthalpy]

Boiling by low pressure takes essentially the same energy as boiling by high temperature, that is, an awful lot - and in a less convenient form.

 

Reverse osmosis is hugely better than boiling, because it separates only the salts from the water, while boiling separates the water molecules from an other, which takes much more energy since water molecules and more abundent, and is useless when one produces liquid water.

 

In a somewhat smarter operation, heat (typically from sunlight) boils some salty water at a higher pressure, and the condensation of (salt-free) vapour releases heat that is used a second time to boil more salty water at a less high pressure, and so on one dozen times. But even with cheap Solar heat and this reuse scheme, it's more expensive than reverse osmosis.

http://en.wikipedia.org/wiki/Reverse_osmosis

[npts2020]

ok Water boils and transform in to steam in room temperature at full vacuum.

why can't we use this method to desalinate seawater. we don't need any fuel for heating water but just only energy need for

maintenance ,pump water ,make water containers full vacuum

Nearly every ship in the US Navy makes fresh water exactly via this method. The equipment is called a flash evaporator (or distilling unit), most having several stages where the water is boiled, sent through a series of chevrons to separate water droplets out of the steam, recondensed and boiled again. A typical evaporator will have 3-8 stages and the water is VERY pure at the end of the process. Waste steam from the engineering plant is used to heat the water and vacuum is maintained by a combination of condensing steam and air ejectors (which work similar to a jet pump). Note, not all of the water is boiled off, the remaining "brine" is pumped away. Evaporators on a ship the size of an aircraft carrier are rated at 100,000 gallons per day and ones on something like a destroyer might make about 20,000 gallons per day. The Navy has entire week and two week long schools on their operation and maintenance. The reason these types of distilling units aren't more commonplace is that they are pretty expensive and require a fair amount of maintenance.

[Enthalpy]

That's a good use of waste heat available at moderate temperature from the engines on ships.

 

Faslan wanted to boil seawater at room temperature by vacuum only, and while this would work, my bet is that it's less convenient than heating, which is in turn far less good than reverse osmosis.

 

How old is this Navy's technology: older than widespread reverse osmosis? Presently on big merchant ships, the part of the heat not transformed into work by the internal combustion engine powers a steam turbine, so the final waste heat has a low temperature, and these combined cycle engines (well over 60% efficient, wow) must coexist better with reverse osmosis.

[npts2020]

You can boil water at room temperature but it takes a near perfect vacuum (around 1/50 of an atmosphere or less) to do so. IIRC the distilling units onboard ship operate at about 120º F at about 29.8" of mercury (close to pressure required to boil water at room temperature). The efficiency is not the biggest consideration because the option is to get a small amount of work out of the steam (distilling water) or just recondensing it and not getting anything out of it. The steam is around 500-600ºF (for a nuclear powered ship) to upwards of 1,000ºF for a 1200psi conventional ship, so you are already extracting the vast majority of the energy from the steam beforehand. Also, there is a fair amount of other equipment besides the main engines that are steam powered from catapults on an aircraft carrier to jet pumps for pumping bilges and more, so there is plenty of waste steam even if the main engines were 100% efficient (they aren't even close).

The technology is substantially older than reverse osmosis 1860's vs late 1940's. The main problems with steam distillation are relatively high maintenance and high level of technical skill required to run them efficiently.

Edited July 14, 2014 by npts2020