Abstract: Method and apparatus for contacting particulate solid matter with a fluid in a pressure vessel through which the fluid continuously moves at high, supercritical, pressure while the particulate solid matter is periodically moved via isolatable pressurizable vessels which are periodically pressurized and de-pressurized to and from the supercritical pressure to facilitate periodic movement of the particulate solids between the pressure vessel and the isolatable pressurizable vessels. A supply of the fluid is maintained at an intermediate pressure at which the fluid exists in both the liquid and gaseous states. The pressurizable vessels are raised to supercritical pressure by first using intermediate pressure fluid in the gaseous state and then in the liquid state, followed by introducing fluid at the supercritical pressure. The vessels are de-pressurized by first venting and then pumping to the intermediate pressure fluid supply.