From Shanghai to Seoul and back

Hi

I'm from Denmark and I'm currently staying in Shanghai with a tourist single entry visa (expires 28th of november). I would love to visit Seoul for a week however, but since i have a lot of stuff here in China (i rented an apartment for 2 month) I won't risk not being able to travel back to China.
How do i make sure that I can get a new visa for when I'm leaving Korea to go back to China after aprox. 1 week in Korea?
Is there any chance that i can do it here from Shanghai or do I have to be in Seoul to do it?
Looking forward to hearing from you at your earliest convenience.

Regards

Jan Jakob Holm-Weber