Can I use butane torch to loosen this A/C hose fitting?
 

Just use two wrenches (27 & 32mm?), it will loosen right up.

I wouldn't use heat.

Try a 50/50 mix of ATF and acetone as your penetrant.
Another tip is to slightly tighten the union to break the corrosion seal, before you try to loosen the union.
Hammer strikes to each of the six flats of the coupling nut may also help loosen the stubborn fitting.

For really tough fittings, I extend the length of the wrench(es) to increase the mechanical advantage.

HEAT and Refrigerants ARE NOT A GOOD IDEA
 

As you've been told.

Premier "Fittings Loosener" is "Kroil" from Kano Labs in Tennessee!

What you're experiencing is the Bane of Commercial/Industrial Electricians!

Zomeone Prior to their "Ministrations" has tightened Aluminum Fittings
WITHOUT USING "NO LOX".
(Almost as if the Bauxite Extraction has the ability to Weld to Itself AT WILL!)

Aluminum + Some Stainless in Marine Environs has an even worse result...
[If that's even Possible]

Good Advice in earlier replys!

I recently rebuilt my AC and had a fight with that one and the fitting about that size coming off the evaporator. In both cases I used two wrenches with a piece of pipe over them so I had an extra 3 feet of leverage. Oiled the threads with PB blaster or whatever I had around and went to town, they broke loose wit quite the bang.

The combination of ATF and Acetone ( or lacquer thinner ) is well regarded as a homemade penetrant.
The Kroil is well regarded as a commercially available penetrant.

The problem is that just putting those onto a joint provides little incentive for it to go INTO that joint... as the rule for a liquid is that is follows the path of least resistance.

IF you have TIME....on an important joint.... consider building a little ' cup ' with RTV or something... which will in itself require a day curing time...

so that you can make a puddle of the penatrent which includes part of the joint in question... and leave overnight.. or longer...THAT provides hydraulic pressure due to air pressure... not much .. but so much more than just putting it onto the outside of a joint that it may be worth it...

Are you not going to use the color coded , for specific refrigerant , thickened refrigerant oil to connect the AC hoses ? Nylog ....anything which keeps the threads from being in physical contact will help.....and the Nylog is recommended for AC systems..
https://www.google.com/#q=nylog+thread+sealant

If the threads and seals have nylog on them they can't hardly do any corrosion dance.
I would use Castrol Premium cold flow AC oil..because that is what my Guru in San Antonio .. and others... have suggested over the years...
There is nothing I trust Diesel Giant's word on .. have discussed that in the past many times.. and he sells Duracool..
Whatever Nylog maker says is correct color currently is correct one to use...
You are not wanting ' luck '... you are wanting the latest Chemical engineers in the AC business opinions..... which Diesel Giant is not even close...

Lots of discussion if you google it..
bottom line
it is a Blend... so you can not just top it off when it starts getting low.. due to smaller molecules leaking out and changing the ratio...
Duracool is a combo of R134a and something..... and many advertised it wrongly as a ' direct drop in replacement ' for R12....

Use only legal refrigerants which you can get serviced by legit garages ( they do not want to contaminate their equipment ) unless you are sure you will be doing all the AC work yourself and never want to sell the car...

I read the entire thread, and have come to the realization that these HC refrigerants may not be as bad as people are making them out to be. In this particular case, it was the stop leak that caused most if not all the problems with the A/C system in the guy's TDI. PLEASE READ THE ENTIRE THREAD!