An apparatus, for the steam explosion treatment of biomass, having a pressurized reactor vessel to receive a biomass material and steam, discharge lines connecting the pressurized reactor vessel to the separation device, the lines sized and positioned to allow for the steam explosion of biomass material. At the outlet end of the discharge lines is a collection-expansion manifold to connect the outlet end of each of the discharge lines to a collection line, wherein the collection line provides a passage for biomass material and steam flowing from the discharge lines. The collection line passage has a substantially larger cross-sectional area than the cross-sectional area of a single discharge line. The collection line is coupled to a separation device such that the separation device receives the biomass material and steam from the collection line.