A vapor-liquid contactor includes a plurality of vertically stacked trays within a containment column, the central downcomer provides a liquid supply and removal area at the same location on each tray, so that the horizontal liquid flow pattern is the same on each tray. To maintain the security of the sensitive data present in a processor, the scan controller denies access to the scan chain until data is cleared from scan-observable portions of the processor, then clears the scan chain again prior to exiting test mode and resuming normal operation. A first flow state characterizing the first flow is then retrieved.