i need to add 15 albums in my itunes library to my iphone. if i synch just those albums with the rest of my edited phone library disappear?

It depends on how the music on your phone got there.

• If you Synced the music currently on your phone from the same iTunes Library that you intend to use now, then there is no reason why the previous music should be removed, unless your Sync options tell iTunes to remove it
• If the music currently on the phone was purchased from the iTunes Store, directly on the phone, then you need to ensure that you're signed into your iTunes Store account in your iTunes Library, so that your store purchases can be added to your Library. Unless that music is in your Library, it will probably get removed
• If you have music on your phone form other sources, such as Syncing with another iTunes Library, any attempt to synchronise with this second Library will result in iTunes asking if you wish to erase the current content. in other words, if you say "yes", you will lose the existing music