Since temperature control is so vital to sous vide cooking, the circulation serves to make sure that all of the water is moving and of an even temperature. If you just had a heating element submerged without a circulator, you would get some convection currents, but there would be hot and cold spots in your water bath. If these differences were even just a few degrees, you could end up with proteins not hitting their coagulation temperatures, connective tissues not softening, pectin not broken down, etc. All in all, it would defeat the purpose of this cooking method!
As for the differentiation between "circulators" and "water ovens", there may not be one. Frequently the actual thermal unit and pump is referred to as the "circulator" and the whole assembly with a vessel and water is referred to as the "oven", but I'd check the specs of each model to see whether it has all the parts you're looking for.
I confess I'm not familiar with the term entrecôte, but looking it up, its pretty clear this is a already-tender cut. So you don't need to tenderize it.
In fact, you probably don't want to tenderize it—at some point, it'll go from tender to mushy. Unfortunately, from frozen, at least according to the Baldwin tables, you'd need over five hours. If you additionally want it pasteurized, that'd probably be over six hours, but he doesn't have a table for that. Either of those times is risking mushy, at least on the outside.
You can reduce the heating time substantially by thawing it in the fridge. This also gives you the opportunity to sear before bagging (killing any surface bacteria, and also improving flavor). You're probably looking at around 3–3½ hours this way (maybe up to 4 for pasteurization, depending on exact thickness).
Finally, once its thawed, you could split it in half, giving you two, 25mm steaks (split before searing, of course). Then you're looking at around an hour in the water bath, or two for pasteurization.
Even with a pre-sear, you'll want to sear after cooking as normal, too.
Your final option is to up the water-bath temperature a bit, and pull before the steak reaches thermal equilibrium. Basically, the last few degrees take forever—heat transfer is proportional to the difference in temperatures—so if you're OK with a doneness-gradient, as you'd find in traditionally-cooked steaks though not as extreme, you can do this. Problem is, I don't have a table for this, so I'm not sure what the time would be at, say, 59°C. You could attempt to rig it with a probe thermometer to find out, but beware cheap probes are not waterproofed where the wire enters, and will be break if water gets there. There are i-Device apps that will calculate these times for you (PolySci Sous Vide Toolbox, Sous Vide Dash) but I haven't used either (no Android versions...) [This is called the deltaT method]
Best Answer
Surveying the top search results for sous vide prime rib, the consensus appears to be about 7 hours, with some leeway on either side.
Note that you don't want to hold prime rib for extended periods (such as overnight) in its bath, as enzymatic action will make it mushy.