RAM usage is increasing in database server with time. When we restart the SQL services it take around 4 GB but after day or two it increases to 11-12 GB and even further. Application is hosted on separate server. Please suggest what could be the reason.
Instead of trying to find a way to release your RAM, you might be better served trying to determine how much of your RAM your SQL server is allowed to use.
Seeing as how you're using a 16GB server, try leaving 2-3GB for the OS (or more depending on what else is installed on there)
The following commands would set your max server memory (the amount of memory SQL server will keep a hold of) to 12GB:
sp_configure 'max server memory', 12288; GO RECONFIGURE; GO