Process for producing hydrogen cyanide

A process of producing hydrogen cyanide comprising:

• a) providing at least one hydrocarbon, at least one nitrogen containing gas, and at least one oxygen containing gas;
• b) reacting the at least one hydrocarbon, at least one nitrogen containing gas, and at last one oxygen containing gas in an apparatus to form hydrogen cyanide, and
• c) supplying heat by a simultaneous combustion reaction with the at least one oxygen containing gas in the apparatus;
• wherein the apparatus comprises:
• an inlet piping section with a first cross-sectional dimension;
• a downstream process section with a second cross-sectional dimension;
• and
• an inlet transition section connecting the inlet piping section and downstream process section;
• wherein the inlet transition section comprises internal insulation comprising refractory ceramic fiber and wherein the internal insulation forms a conical interior surface within said inlet transition section.