My Projects

Topic: Supercritical Extraction
Title: Flash Precision Cleaning


The animation illustrates the cleaning of cleavages, lacunas, and other cavities by a supercritical fluid.
During a pressure increase, the cleaning agent gets compressed …
… and thereby immersed normally to the contaminated surface.
In return, during a subsequent pressure release the expansion removes the contaminants normally from that surface.
Even cavities with complex forms get cleaned precisely, traceless and quickly by this procedure

What one doesn’t see:
Supercritical fluids SCF are gases under such high pressure that their density equals that of fluids, however, at such high temperatures that their reactivity remains that of gases. In consequence, the usual phase difference between gaseous and liquid state has vanished: According to solubility SCF provides the exchange rates of gases, e.g. diffusivity and viscosity, as well as the mass action of liquids, e.g. turnover and drag.

What’s behind it:
Cleaning is a solution process where a dissolving agent is mixed with the contaminants to dissolve. In a first step the dissolver infiltrates the contaminants in order to mobilize them, i.e. the flow of the dissolver entrains the contaminants. At the end of this process even the least traces of contaminants get dissolved and removed with the entraining dissolver. However, liquid dissolvers are incompressible, which causes problems with continuity and wettability: Narrow cleavages or other dead-end cavities prohibit any formation of a continuous flow for entrainment – and an adhering liquid film leaves always traces when drying. Vapor condensation cleaning is but an option to envisage for precision cleaning, yet rather time-consuming due to the very low density of vapor as an entrainer. The application of SCF, however, offers a way to apply a compressible fluid with high density: By compression the dissolver infiltrates the contaminants and by subsequent flash decompression the mixture is detached from the surface.

What you can obtain from it:
Turbine blades are stuffed with wax to protect the fine inner cooling fins during manufacturing. However, the wax has to be removed traceless for quality control. A vapor condensation cleaning lasts for hours and requires low-boiling hydrocarbons, which are either problematic in regard to operational safety, e.g. explosive and unhealthy, or in regard to the environment, e.g. haloalkanes like CFC. Therefore, an autoclave is anyway required. Carbon dioxide becomes already supercritical SCCO2 at a temperature of 31°C and a pressure of 74 bars. In a closed circuit process of an autoclave vessel, it provides good solubility for organic contaminants. Please note that CO2 is just applied in such a process – not produced – so that occasional emissions are just and only neutral to the atmospheric content. But the improved reaction kinetics speed up the cleaning process from several hours to just a few minutes.

My related publications:
Process and Apparatus for Cleaning Articles with a Compressed Cleaning Fluid
patent US 20020074022 (2002)