a reactor for the treatment of lignocellulosic material and related methods . the reactor includes a vessel having an upper portion and a lower portion . a pressure envelope is formed between an outer wall of the vessel and at least one lower wall of the lower portion of the vessel , and the upper portion of the vessel and the pressure envelope are operatively connected with a pressurization line , such that a pressure of the pressure envelope and a pressure of the upper portion may be equalized .