sess_expire_on_close not working

I have a problem..in $config['sess_expire_on_close'] = TRUE; when i close the browser..the session still save in the database. and i can still see the data in user_data column.

config.php

  $config['sess_cookie_name']     = 'ci_session';
  $config['sess_expiration']        = 2400;
   $config['sess_expire_on_close']  = TRUE;
  $config['sess_encrypt_cookie']    = FALSE;
  $config['sess_use_database']  = TRUE;
  $config['sess_table_name']    = 'ci_sessions';
  $config['sess_match_ip']  = FALSE;
  $config['sess_match_useragent']   = TRUE;
 $config['sess_time_to_update'] = 300;

Need help.. the purpose is to show all online users.

I'm using CI version 2.1.3