You CAN NOT substitute tannin for raisins.
Raisisns indeed add tannin to a wine but they do much more.
- They add sugar
- They add flavor
- they add tannin
- they add nutrients
As raisins contain nutrients your must may lack these if you ommit the raisins. As raisins contain sugar, your must has a lower Sg as intended and your wine will have a lower ABV.
Read my entire story on raisins here:
http://wijnmaker.blogspot.com/2007/11/rozijnen-raisins.html
Be aware that if you ommit raisins and substitute tannin for it you will be ommitting a lot more, so your wine will not be the wine that was intended in the recipe.
Luc