Method and device for granulating polymer melts containing blowing agent
The invention relates to a process and an apparatus for pelletizing polymer melts comprising blowing agent in a pelletizing chamber through which a liquid flows, its pressure being above the ambient pressure. In a first step, the polymer melt is injected into the pelletizing chamber, in a second step the polymer melt is cut via a cutting apparatus into individual pellets and, in a third step, the pellets produced in the pelletizing process are discharged with the liquid from the pelletizing chamber and are then isolated from the liquid. This process also comprises at least one of the following steps: (a) comminution of agglomerates or of pellets which exceed a prescribed maximum size, in a comminuting machine downstream of the pelletizing chamber or in a comminuting unit downstream of the cutting apparatus, (b) depressurization of the liquid in a depressurizing machine, (c) depressurization of the liquid in a throttle apparatus, where there is, upstream of the throttle apparatus, a pressure-equalizing container, (d) isolation of the pellets from the liquid, without any prior depressurization of the liquid with the pellets which it comprises.
1. A process comprising the steps of:
pelletizing polymer melts comprising blowing agent by injecting the polymer melts into a pelletizing chamber and then cutting the injected melts into individual pellets, wherein the cutting is conducted by a cutting apparatus while the melts are submerged in a liquid of a liquid circuit flowing through the pelletizing chamber, and wherein a pressure of the liquid in the chamber is elevated above ambient pressure during pelletizing;
discharging the pellets from the pelletizing chamber with the liquid of the liquid circuit as a pellet-loaded liquid;
comminuting agglomerates or pellets within the liquid which exceed a prescribed maximum size, wherein the comminuting takes place within a comminuting machine arranged downstream of the pelletizing chamber or by a comminuting unit arranged within the pelletizing chamber downstream of the cutting apparatus, wherein all pellets and agglomerates are distributed within said liquid so that agglomerates, pellets, and liquid are fed through the comminuting unit or comminuting machine during the comminuting, wherein the comminuting reduces an overall number of agglomerates, and wherein the comminuting machine comprises a knife rotating on a cutting plate;
depressurizing the pellet-loaded liquid from the elevated pressure to a lower pressure level using a depressurizing machine, wherein the depressurizing machine comprises a throttle apparatus, a turbine, at least one pump, or a rotating sluice valve, and wherein a pressure-equalizing chamber arranged upstream of the throttle apparatus keeps in essence constant each of said elevated liquid pressure and a liquid circuit flow rate through the pelletizing chamber; and
isolating the pellets from the pellet-loaded liquid.
2. The process according to claim 1 , wherein the elevated pressure in the pelletizing chamber is in a range from 1 to 50 bar.
3. The process according to claim 1 , wherein for start-up of the process, the pelletizing chamber is treated with gas at a process pressure by way of a pressure-equalizing line to the pressure-equalizing container.
4. The process according to claim 1 , wherein said isolating takes place without any prior depressurization of the pellet-loaded liquid.
5. The process according to claim 1 , wherein said isolating takes place within a solids separator.
6. The process according to claim 5 , wherein the solids separator comprises a hydrocyclone, a filter, or a centrifuge.
7. The process according to claim 1 , wherein the depressurizing machine comprises a throttle apparatus.
8. The process according to claim 1 , wherein the depressurizing machine comprises a turbine.
9. The process according to claim 1 , wherein the depressurizing machine comprises a rotating sluice valve.
10. The process according to claim 1 , wherein the pellet-loaded liquid is guided through all parts of the depressurizing machine.
11. The process according to claim 10 , wherein the depressurizing machine comprises two pumps arranged so that downstream of a first of the two pumps which conveys in a flow direction of the liquid circuit there is a second of the two pumps which has a lower power rating than the first pump, a flow direction of the second pump being the same as or opposed to the conveying direction of the first pump.
12. The process according to claim 1 , wherein a pressure in the pressure-equalizing container is kept constant by way of a gas whereby in order to increase the container pressure, gas is introduced under pressure into the pressure-equalizing container, and in order to reduce the container pressure, gas is dissipated from the pressure-equalizing container.
13. The process according to claim 12 , wherein a liquid level in the pressure-equalizing container is adjusted via the depressurizing machine.