Objective
To build a pilot plant to test the process and later to build an industrial unit to produce polyamide parts with substantial energy saving.
The new process is almost continuous where polymerisation and moulding follow each other avoiding the cooling and re-melting steps.
Problems encountered: the automatic process control failed causing melted polyamide to pass from the vacuum stage into the mixing condenser and receiver where it then solidified. To prevent this happening again a series of additional safty features have been installed.
It was found that some of the stuffing boxes at pumps and gagues were not fit to stand the strain of continuous operation. They will be replaced by teflon rings or graphite stuffing boxes.
In spite of carefull insulation of the plant a heat loss occurred of about 500 kWh per ton of polyamide 6. During the winter this serves to warm the buildings housing the production facilities, but in the summer this heat must all be removed. Nevertheless, the energy saving, as compared to the conventional process, has been very substantial. Between 850 and 1700 KWH/ton being required to produce saleable polyamide injection moulding grades.
The equipment comprises two autoclaves with mixing equipment, one vacuum pump and a special extruder, with a capacity of up to 20 t/d (4,000 t/y).
Caprolactam, the polyamide raw material, supplied in a molten form at 80 deg C is poured into a mixing autoclave with other necessary additives (incl. Hexamethylene diamine) under a nitrogen atmosphere. It is then heated to 230-270 deg C under a pressure of max. 20 bar. After 6-8 hours the polymerisate is transferred under vacuum in an intermediary storage vessel where further polymerisation takes place at some 520 deg C eliminating volatiles. The mass is then pumped into an extruder where temperature and vacuum can be adjusted to produce the required polymerisation grade. Additives such as plasticisers, heat stabilisers, colours, fillers, etc are added. The polymer is then extruded, cooled in water and chopped. The vapours from the intermediate containers and degassing steps are condensed, recovered and fed back into the reactor.
For a2,000-4,000 t/y production the energy saving can reach 15,000-30,000 kWh per day or 3-6 million kWh/yr.
The project started in November 1984 with the engineering work completed in January 1985. The pilot plant was built between February 1985 and August 1985. Tests followed until November 1985 when construction of the industrial unit started. Completion is expected by December 1987. At variance with the original project description, a fourth storage tank for molten caprolactam, heated by means of thermal oil and of 29,000 litres capacity, was installed. This makes it possible to keep complete tankcar loads of approximately 24 tons apart from each other in two storage systems. This ensures that caprolactam from different suppliers and possibly of different quality may be stored separately.
Filters were installed upstream of the lactam storage tanks and downstream of the lactam melting vessel to prevent impurities from the tankcar from entering the storage tanks and to ensure that the contents of the melting vessel may be pumped into the autoclaves free from contaminates. Opposite the pilot plant a second autoclave of 7.4 m3 capacity has been installed next to the existing 5 m3 autoclave. Both autoclaves are equipped with stirrers and heated with thermal oil. The level is radiometrically monitored. The process control is carried out automatically by the Baelz computer.
The polyamide melt from the two autoclaves is pumped into the vacuum stage by means of a spinning pump. The pressure is monitored and should an over-pressure of more than 15 bar occur then the pump automatically shuts off In addition the piping is protected by a 25 bar bursting disc. In the vacuum stage, the melt flows down, as a thin film, along the walls of the tube-type vessel and collects in the conical bottom part where its level is radiometrically monitored and controlled. The vacuum system generates a vacuum in the order of 3 to 14 hPa. This causes 8 to 9% of caprolactam monomer to distil off.
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
6114 GROSS-UMSTADT
Germany