This invention already exists. See http://www.airproducts.com/corp/rel/98006.html
for one example.
Some text from that site:
The CRYO-QUICK Tunnel Freezer uses liquid nitrogen as a refrigerant to individually quick-freeze product as it is carried along a conveyor. Liquid nitrogen is sprayed into the discharge end of the tunnel. It then vaporizes to nitrogen gas at -320° Fahrenheit. The cold gas is then recirculated countercurrently toward the tunnel entrance by patented-design fans and flow controls. Once the ice cream sandwiches are hardened, they exit the tunnel and are sent to a packaging line and then typically to a conventional mechanical freezing system to complete the hardening process. With a small extension to its tunnel length, the Air Products freezer can also eliminate the need for the additional mechanical freezing step. The hard freeze that can be achieved using the CRYO-QUICK freezer allows the sandwiches to be wrapped immediately in a broad range of packaging materials, from plain to decorative.