I would treat it as if you were getting pregnant while they are away.  Have an open dialogue, one would hope your 19 year old doesn't think they're being replaced.  

Are you close with your daughter? If she is pretty mature, I'm sure she would understand your raising another child!

Have you tried to sit down with her yet?

My best advice would be to sit down with your 19 year old. Having a frank, honest discussion with your child. Actively listen to what he or she says. You want them to feel like their opinion matters to you, even if you're not going to change your decision. Keep in mind that this would be a huge transition for your family, which can be scary and cause your 19 year old to lash out. Just be patient and try to be as understanding as possible. 

As with any child, it is important to include them on the discussion and baby prep. Your 19 year old may really enjoy being part of this new chapter and having a new sibling. Communication is key and you may be fortunate to have someone to include her in shopping and caregiving. Here's to happiness and expanding your family.

I think at 19 your daughter is old enough to have a heart to heart talk with. Maybe explain to her that the best part of your life has been raising her into adulthood and she now begins her journey to her own life and family. Your journey as a mother does not feel complete yet and you are wanting to share your love and life with another child in need. I bet she will accept it well! Good luck

My oldest brother is 22 years older than me. My second oldest brother is 18 years older than me and my third oldest brother is 13 years older than me. My sister is 6.5 years older than me. We are all full biological siblings. History repeated itself and when I got pregnant with my fourth child I had a 19 year old daughter, a 17 year old  son and 13 year old son. When I got pregnant with our fifth child, my daughter was 20 (turning 21 the next month) and sons were 19, 15, and 12 months. I remember meeting a woman in Target who had the exact same dynamic. It is much more common today than when I was younger, but the big ranges were there because bigger families were the norm back then.  I would sit down and let her know from your heart why you want to expand your family through adoption. That is where I would start if I were you.