Vacuum Technologies for Polyreaction
Product qualities which require higher levels of devolatilisation make deeper vacuum levels essential.
There are many systems for creating vacuum on the market. Mostly, these are multistage systems using screw compressor / roots blower technologies.
The difficulty when applying these technologies when processing PET melt is to protect the vacuum machinery from the products which are extracted from the polymer by the extruder, which are not restricted to water vapour or ethylene glycol. In the devolatilisation process of the MRS extrusion systems, in addition to water vapour and ethylene glycol, significant quantities of monomers, dimers, waxes and other sticky substances are extracted, which would block the tight clearances of conventional vacuum pumps within a short period of time. Therefore, it is important to concentrate on separating out these substances between the vacuum dome on the extruder and the vacuum pump itself.
During recent years, Gneuss has developed several different vacuum systems in cooperation with international experts and renowned manufacturers of vacuum equipment, which are exactly tailored and optimised to the relevant application.
During the chemical reaction inside the polycondensation system, “JUMP”, gases are released which are for the most part glycol vapour. Most of the sticky substances which are normally released during devolatilisation have already been removed along with residual water during the upstream extrusion process where the MRS extruder carries out the first devolatilising stage with the help of a simple and rugged water ring vacuum pump.
With the “JUMP” vacuum system, the hot gas which is extracted from the reactor is passed through spray jets of cold ethylene glycol. Since the extracted gas consists for the most part of glycol vapour, this is also selected for the cooling medium. Glycol is pumped in circulation and any possible foreign particles are removed in bag filters and cooled in heat exchangers, before the glycol is returned to the condenser.
Polyreaction Extruder MRS
The main component of the gases extracted from the PET melt in the devolatilisation section of the MRS extruder is water vapour, in many cases up to 1 % of the throughput rate or more. For a typical extruder with a throughput rate of 1.000 kg/h., this means 10 litres/h of water, or 10m³/h of water vapour which need to be extracted. Under vacuum, this represents a theoretical suction rate of 10.000 m³/h, which makes a large system necessary. The vacuum system of the MRS uses an ingeneous cooling trap technology. This is cooled with an extremely cold circulation medium so that the extracted water vapour goes into the solid phase. The ice which accumulates on the heat transfer surfaces (together with all the other products which are to be removed) can either mechanically scraped away during the process or flushed out with hot water.
The Gneuss vacuum technologies for polyreaction drastically reduce the required volumes for the vacuum pumps. Depending on the application and requirements, different systems offer the optimum solution for operation and protection of the vacuum pumps themselves.