I'm just guessing but your description makes me think that the pneumatic regulator is requiring a lot more current that you think.  I base this on you saying that your 7812 regulator is running so hot.

Couple of things:

1) You don't need the regulator.  The LM324 is good for a maximum supply voltage of 34 (36?) Vdc.

2) Add an emitter follower to the output of the op-amp.  I'd be tempted to use a physically-large transistor just because of dissipation issues.  NPN, TO-220 package, gain  (Hfe) of at least 100.  Collector goes to +24V, Base comes from output of op-amp, Emitter feeds both the pneumatic regulator and the right-hand end of R34.  Note that the transistor is inside the feedback loop and doesn't introduce any error.

Question: is your regulator a voltage-input device or current-input?  My company makes interface cards like this for a variety of different actuators and the ones that we work with are all current-input devices.  We drive them with a low-frequency PWM signal where the output current is proportional to the control input.  This eliminates temperature effects on the actuator's solenoid coil and the vibration that the low-frequency PWM produces reduces stiction in the actuator. 

Opamp Voltage Doubler Unstable (After Extended Operation)
Recently I configured a Sharepoint site for a client, and it's working pretty well.

Except the picture search feature.

The picture should point to:
"http://www.contoso.com/sites/conglom-o/Beeldbank/_t/MH130925-BW047-LR.jpg"

Instead, it points to:
"http://contoso.local/sites/conglom-o/Beeldbank/_t/MH130925-BW047-LR.jpg"

Everything else works but those picture icons.

Anyone else had this problem?
