An apparatus and process for isomerizing a hydrocarbon stream rich in a C4 hydrocarbon and/or at least one of a C5 and C6 hydrocarbon which includes a first drier and a second drier; and a reaction zone communicating with at least the first drier. The first drier operates at a first condition to dry the reactant and the second drier operates at a second condition during regeneration. The used regenerant remaining in the second drier after regeneration can (1) pass through a vent-to-flare assembly in a batch-wise manner; (2) pass through a downflow-depressure-to-low-pressure-device assembly in a batch-wise manner; (3) pass through a cross-over piping purge assembly to minimize upsets in the reaction and fractionation zones when the second drier is placed back in operation; or any combination of (1) (2) and/or (3) to minimize upsets in the reaction and fractionation zones when the second drier is placed back in operation.