Imho you want a power controller for that job,
A pid is going to get you to a target temp and maintain it so perhaps determine the temperature of a rolling boil and set that as the target temp? though it will change as you boil off the vapor..
a power controller will let you dial up and down the power to the element, there are a few options, the cheapest i think is a suitably rated for your element ebay china board
http://www.ebay.co.uk/itm/3000W-High-Po ... 2a4c1ea589but i opted to use a resistance controlled ssr kit i got off ebay for just over a tenner, it included a sticky back dial, knob and potentiometer circa 500 ohm, and the ssr itself,
unlike the pid ssr its not driven by a dc current but is goverend by the resistance between its control terminals, so the potentiometer bridges the control terminals and the load is goverend between the load carrying terminals.. simple

I added a volt/amp meter and heatsink again from ebay and stuck it all in a box..
the voltage reads 244 as i took the power feed to the board from the input side not output

, amps are measured via passing the ouput live from the ssr thru a 'polo mint' attached to the meter.
and there are more expensive options out there too...