An apparatus for processing material at elevated pressure, the apparatus comprising: (a) two or more radial restraint structures defining an interior region configured to receive a processing chamber, the radial restraint structures being configured to resist an outward radial force from the interior region; (b) upper and lower crown members being disposed axially on either end of the interior region and configured to resist an outward axial force from the interior region; (c) a first axial restraint structure coupling the upper crown member and the lower crown member to provide axial restraint of the upper crown member and the lower crown; and (d) a second axial restraint structure compressing the two or more radial restraint structures to provide an axial restraint of the two or more radial restraint structures.