Breaking a process into separate units to comply with prevention program requirements

Do I have to do my hazard review, process hazard analysis, or other prevention activity on the whole process or can I break it into separate units?

Once you have determined that you have a covered process, you can divide the covered process any way you want to implement the prevention program. If you have multiple interconnected storage and reactor vessels in your process, you may want to treat them separately when you conduct the hazard review or process hazard analysis, if only to make the analyses easier to manage. Storage and reactor vessels may require separate maintenance programs. You should do what makes sense for your process.