According to the definition of "process" in 40 CFR §68.3, any group of vessels that are interconnected is considered to be a single process. If a stationary source has two interconnected vessels and one contains 6,000 pounds of butane while the other contains 6,000 pounds of propane, is this a covered process under 40 CFR Part 68?

No. Although the two interconnected vessels are considered a single process, in order for that process to be subject to the risk management program regulations it must contain more than a threshold quantity of a regulated substance (40 CFR §68.10(a)). The threshold quantity for both butane and propane (and all other regulated flammable substances) is 10,000 pounds (40 CFR §68.130). The amounts of different regulated substances present in a single process need not be aggregated to determine whether a threshold is exceeded. If, at any time, more than 10,000 pounds of either butane or propane (when not used or sold as fuel) is present in a process, that process is covered by the regulations at 40 CFR Part 68.

Although this process as described is not subject to part 68, it is subject to CAA Section 112(r)(1), the general duty clause (see fact sheet  on General Duty Clause). If butane and propane are present in a mixture in the process, then the threshold quantity must be calculated differently. Because a mixture of propane and butane would meet the NFPA 4 flammability criteria, the entire weight of the mixture needs to be treated as the regulated substance and added up to account for the threshold quantity.