The invention provides a method and means of predicting, in near real-time, the detailed chemical composition of major streams in a refinery, including those downstream from the crude distillation column. The dynamically updated feed characterizations can be derived in near real time from an assay database of current crude diet, RTO information about upstream refinery units and various analyzer inputs. The chemical compositions of each stream are stored and updated in a plant wide database. A broad range of property calculations can be ascertained, either automatically or on demand, from the compositional information. An application permits users, such as operations, planning and engineering personnel, to access the database directly or indirectly (e.g., on line). This information is then used by each user to further optimize the operation of a given refinery unit and/or the plant as a whole.