Vent-and-reseal Vessel Technology
Milestone’s unique Vent-and-reseal vessels are the foundation of our microwave technology. This patented (US Patent 5,270,010) technology eliminates vessel failure in the case of an out of control exothermic reaction. The illustration shows Vent-and-reseal in action: the vessel cap is held in place by a dome-shaped spring (1). In the case of overpressure due to an out of control exothermic reaction, the spring is flattened, allowing the cap to lift up slightly (2) releasing excess pressure. Immediately the excess pressure is released, the spring reseals the vessel (3), and the process continues. Sample is not lost – no clean up needed. The microwave program continues to completion and no re-run is required. While there are other methods of venting, only Milestone has Vent-and-reseal technology. The major advantage of this technology is that it has been used for nearly all of Milestone's vessels and can therefore be used across all chemical applications, such as extraction and synthesis.
Other Methods of Venting
There are two other approaches to venting, though both are inferior to Milestone’s Vent-and-reseal.
This method employs a simple burst disk in the cap which is designed to fail in an overpressure situation, instantly releasing all pressure in the vessel. When this happens, instantaneous boiling occurs, the sample contents are lost and the run has to be manually stopped. Significant clean up of the cavity is required, and corrosion of the cavity and internal components will occur. The illustration compares the burst disk approach with Vent-and-reseal: the red line shows the complete loss of pressure when a burst disk fails, and the whole run is stopped. The blue line shows what happens with Milestone’s Vent-and-reseal technology: excess pressure is gently released at just over 30 bar, and the microwave run continues to completion, with no loss of sample.
The final method of venting is “self-regulating” (a more correct term would be “vent only”, compared to Vent-and-Reseal). This is used mainly for low pressure extractions (max. pressure 10-15 bar), and simply relies on the seal around the cap deforming to release overpressure. Since vessel cap design is simple, assembly and disassembly is fast, but there are several disadvantages. The pressure at which the vessel will vent cannot be accurately predicted (unlike Vent-and-Reseal) so there is loss control over the method. In fact venting pressure will vary with reaction temperature and even the solvent used. If the vessel does vent, the sample will boil and be lost, requiring clean up in the cavity and eventually corrosion of the cavity. And since the vessel does not reseal, sample loss will continue to dryness. More recently, the self-regulating, or vent-only approach has been applied also to high pressure rotors. All of the above limitations still apply, though at higher pressure, the consequences of venting are more severe.
Video: See Vent-and-reseal Technology in Action!
To see Vent-and-reseal in action, watch this 30 sec. video taken by a camera inside an ETHOS EX cavity during a run. For clarity, only a single vessel has been placed inside the cavity. A large sample amount was digested in order to generate an overpressure and to force the vessel to vent. The vessel vents and reseals twice during the video: at 8 sec. and 12 sec. Yellow nitric acid vapors can be clearly seen as the pressure is released. These are removed via the exhaust duct and the run continues to completion – no loss of sample and no clean up required.