I have a simulation in Aspen Plus for turning sulfur into sulfuric acid. In my RSTOIC reactor that turns SO2 into SO3, I am getting this error whenever I try to do an economic evaluation: "Sizing error: vessel capacity cannot be calculated. Please check stream flows" and "ERROR> 'AT - 1' QUESTION MARK FOUND IN NUMERIC DATA; ERROR> 'AT - 1' EITHER CAPACITY OR DIAMETER+HEIGHT MUST BE SPECIFIED". Has anyone had a problem like this before? What should I do to help solve this problem?

Thank you.