The present invention pertains to a system for processing semiconductor wafers. The processing may involve the removal of material from the wafers or deposition of material on the wafers. Various aspects of the invention include specialized pressurization, process vessel, recirculation, chemical addition, depressurization, and recapture-recycle subsystems. A solvent delivery mechanism can convert a liquid-state sub-critical solution to a supercritical processing solution and introduce it into a process vessel that contains a batch of wafers. The wafers may be rotated within the supercritical processing solution. The supercritical processing solution is preferably recirculated through the process vessel by a recirculation system. When chemical additives are added to a supercritical solvent, the momentum of the chemical additives are preferably matched to the momentum of the supercritical solvent. Additives may be added at a higher initial flow rate, then ramped down a lower flow rate, e.g., a steady-state flow rate.