I am looking to start a business up which will be producing sea salt. I am looking at ways to reduce the energy costs and would like to explore vacuum evaporation of sea water. My question is how much energy would this require if had to evaporate 75% of the water in a vacuum to obtain the salt. Assuming I am starting with one litre of water.

have you tried experimenting and measuring the energy used?

what sort of vacuum are you going to have it under? this will greatly affect the result.

With evaporation (by heat), you put in heat. That requires almost precisely as much energy as needed for the evaporation.

 

With vacuum evaporation, you will also need to put this heat of evaporation into the system. However, two differences:

1) your heat will be at a lower temperature. That is most likely a good thing for your energy costs (note: you don't need less energy, but you just need it at a lower temperature!).

2) you have a vacuum pump running constantly. This is effectively a gas pump, also known as a compressor. Types vary with the desired flow and pressure (the degree of vacuum). The important bit is that this vacuum pump will not nearly be 100% efficient. In fact, thermodynamics will show you that it's not so good.

 

You must also look at the type of energy you will purchase. Heat can be bought in the form of gas, or any other fuel. Vacuum pumps will almost certainly require electricity.

 

Just my 2 cents. I add a disclaimer that this was written in a hurry...

i think the point of vacuum evapouration in this case is that the heat can be taken from ambient rather than having to be supplied.

 

usually it is just to dry a substance that is damaged by heating.

Nope, you are much better off with evaporation. Get a big tower, spray water down it, blow wind up it. Or just let it sit in a shallow pool in the sun until it dries up.

Depending on ambient temperatures, and the desired water content of the product, a spray tower may be simple enough.

 

Vacuum evaporation, as insane_alien said, should use ambient for heating. In industry, vacuum evaporation is used in order to utilize waste heat.