Actually, there's at least one, possibly two, kalk reactors that can be operated in a sealed and pressurized manner, but neither one are "nielsen reactors" that incorporate a stirrer.
The first one is the Tunze kalk reactor/reservior that's designed to work with their Universal Osmolator, it's designated the "Calcium Dispenser 5074".
The second one I can think of is the Two Little Fishies Kalk Reactor 300.
Another possibility might be one of the kalk reactors that use a sealed chamber and a pump to stir the contents rather than a stirrer. The Precision Marine one found
here would likely work, though you might need to call Precision Marine to verify that the reaction chamber is completely sealed.