A reactor for producing phosgene, the reactor comprising: tube located in a shell and a space located between the tube and the shell; a cooling medium located in the space and a catalyst located in the tube or cooling medium located in the tube and a catalyst located in the space; a feed inlet; and a product mixture outlet; wherein the tube comprises one or more of: a mini-tube and a second tube section; a first concentric tube concentrically located in the shell; a twisted tube; an internal scaffold; and an external scaffold.