A multi-stage process for the production of an ISO8217 compliant Product Heavy Marine Fuel Oil from ISO 8217 compliant Feedstock Heavy Marine Fuel Oil involving a core process under reactive conditions in a Reaction System composed of one or more reaction vessels, wherein one or more of the reaction vessels contains one or more catalysts in the form of a structured catalyst bed and is operated under reactive distillation conditions. The Product Heavy Marine Fuel Oil has a sulfur level has a maximum sulfur content (ISO 14596 or ISO 8754) between the range of 0.05 mass % to 1.0 mass. A process plant for conducting the process for conducting the process is disclosed.