What is this fuel system part called?

Recently I removed a part from my friend's car and put a fuel filter in its place. I have come to find out that it was not the fuel filter and cannot find any information on what it is so I can replace it. It was a small saucer shaped cylinder that connected to the fuel line just before the fuel rail. It was located in the top of the engine bay by the firewall, connected to the fuel rail by a 6 inch hose. I originally thought that it was the fuel pressure regulator, but now am not sure. It had no vacuum lines or electrical/signal wires attached. It only had an inlet and outlet for the fuel. Do you have any idea what this part is?