If I were you, I wouldn't start from here!
You have a classic X-Y problem going on.
The objective is to get rid of excess heat.
The intention is to use 12V or 24V cooling fans - so far so good.
However your reasoning then all goes pear shaped!
I assume the input voltage is >24V, probably >35V or you'd be using an off-the-shelf 12V regulator. If the input voltage is 45V, then to run a 12V fan you need to drop 33V. If the fans draw 600mA, that's approx. an extra 20W of heat to get rid of! Not Clever. 
The way out of the hole you have dug is to use a switching regulator. Tell us the abs. max. input voltage and whether you are able to do a PCB layout for a high frequency part or need an 'old skool' low frequency part that can be prototyped successfully on perfboard and we can come up with some suggestions.
Also, what's the input supply from? - your current circuit shows a -15V supply for the OPAMP, which if its beefy enough would be fairly close to what you need to run 12V fans. Add a string of 3x 1N4001 diodes to drop 2V . . .
Agreed .I understand that I could solve the problem I have much simpler. I could simply buy a few AC fans. Or use a separate 12V source . But I have what I have and didn't want to purchase any more if I can solve issues with what I have on hand . I realize that some approaches are unorthodox but with limited resources I do what I can. Heck I don't even have any tl431's even in the scrap pile.
My schematics aren't perfect and neither are my calculations because I'm trying to learn.
This circuit is simply to cool the heat sink of a high voltage high amperage power linear supply and I'm tapping the positive and negative source from that.And it's -15V because I'm running a 7915 regulator off of one half of the rectifier.This runs the other opamps for the power supply circuit.I could go with a larger heat sink but again I have what I have.
I try my hardest to wrap my head around the math and I preform experiments to verify.I, not getting frustrated with something I enjoy doing. I enjoy it because it re leaves my chronic anxiety and depression. I realize that this whole project has become a monster and I expect it to to get even more complex when I start adding digital control.
The supply I'm building is based on the infamous 0-30v TL 081.
In the diagram many of the values are incorrect to give the expected output but I did that intentionally until I can reconfigure the bread board to handle higher amperage. The tracks can only handle a maximum of maybe 10 amps before things start to melt.At 12 amps things start going south really badly . So I have to fashion some rails for the high current . Also the electronic load was only capable of 150W But I have 800W at least with 40 volts and 20 amps .So I modified the Electronic load to handle more.
Anyway at at 10 amps the heat sink for the power transistors is hitting around 70 degrees C .Anyway with the filter caps I have the voltage will probable drop off at full load. 40000uF is probably not near enough to what is needed.
As you see I'm trying to solve problems as they arise with knowledge that I'm really pushing the envelope with limited resources.
After some thought I was wondering if tapping off half the rectifier with a 7812 regulator would work to power a few fans.hmm