TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The support plates are separated by a distance of three-quarters of the diameter of the vessel and 1.5 times the diameter. The biomass enters the vessel through the upper inlet, flows through the flow passage and is discharged through the bottom discharge. The support plates are separated by a vertical distance of one diameter between an apex (24) of one of the support plates and a trailing edge (22) of a vertically adjacent support plate. The support plates is planar, has a straight trailing edge and a generally hyperbolic upper edge, and the upper edge abuts the sidewall. The support plates forms an angle of 30 degrees or 10-45 degrees between an outer surface of the plate and an interior surface of the sidewall. The angular offset is an offset of 90 degrees . The flow column defined by trailing edges of: each of the pairs of plate has a geometric center coaxial with a vertical axis (13) of the vessel; or at least two pairs of plates which are sequential along a vertical direction has a geometric center offset from a vertical axis of the vessel. The adjacent pairs of support plates are offset at 90 degrees and horizontal component of the compaction of the biomass due to each pair of plates shifts by 90 degrees as the biomass moves between one pair of the support plates to between the next pair of support plates. The upper level of the biomass is higher than apexes of the pair of support plates at the uppermost elevation in the vessel. The rate of liquid added to the vessel is sufficient to saturate the biomass Preferred Method: The gradual compaction and rapid release by each of the pairs of support plates suppresses the formation of arches in the biomass. The liquid level is maintained at or below trailing edges of the lowermost pair of support plates and the bottom outlet. The method further comprises adding steam to an upper region of the vessel; and stirring or agitating the biomass at the bottom of the vessel.