My first inclination would be to use logic to switch between one reboiler and the other rather than a splitter. But this is something I haven’t really seen, so I have to ask… Are the reboilers more or less identical, or are they different and selected based on what is being separated? If the reboilers are not essentially identical substitutions, then you are probably going to have a different process gain and your temperature loop tuning will need to be changed depending on which reboiler is in service.
Is the process bottoms diverted from one to the other depending on which reboiler is in service? Are you using logic to manage flow on the process side? Most of the examples of dual reboilers I have observed used both reboilers simultaneously. In one case, the column was built with two identical reboilers and we used a simple splitter which sent equal outputs to both reboiler steam flow loops. In other cases, the reboilers are heated with streams of differing value, perhaps a utility like steam or hot oil in one and waste heat from another part of the process in the other. Generally in that case, it was desirable to minimize the use of the utility, which simply supplemented the recovered process heat.
If you use a splitter or a selector, then the bkcal from the downstream flow loops both come back to the block and it generates the required bkcal for the temperature loop PID block. Alternatively, you can intercept the bkcals in a calc block and output the one you want back to the PID block. I’ve done both ways in the past. If you decide to use Predict Pro, there will be some issues with switching the MPC PRO block between local and MPC as regards the mode of the downstream loop, so it needs to be planned out, but it isn’t that difficult. I’ve done that before, as well.
Regards,
Lou Heavner