I feel your pain! My now 4 yr old was pretty good at throwing fits too. What we started, and have continued with our 2 1/2 yr old is to make the time in their room long enough for them to truly calm down. Remember that the "time out" isn't a punishment in and of itself -- it is what it is, time to cool off and reconsider their actions. Granted, a 21-month old can't do a lot of reconsidering, but time to truly get calm will make a difference.
You're doing the right thing by taking her to her room. When you take her in there, tell her that it looks like she needs some time to calm down (or whatever you would normally say: "Oh, looks like it's room time for Sally")
Then you find a way to keep her in her room (a baby gate, put her in her crib, set the playpen up in her room and leave it there for this purpose...for a kid who can break out of all that, I've heard that a door knob cover on the INside of the door works).
Tell her that when she hears the timer go beep she can come back and join you with whatever you were doing. The catch is that the timer doesn't start until she's quiet. At her age you'd do a minute and a half or two minutes. Those little Pampered Chef kitchen timers are great for this. The magnet/clip back usually sticks to something near the kid's room so they are sure to hear it beeps.
I was having the same experience you described with our 2 1/2 yr old when he was around 21-22 months. He'd throw a fit and I'd send him to his room. He'd go to his room and get quiet, then come back and start throwing his fit again the minute he saw me.
After a while I realized we weren't giving him enough time to calm down, so started doing time out this way again and it really helped.