Application example: Sealing of centrifuges in the HDPE production

Issue and challenge:

In pre­cip­i­ta­tion poly­meri­sa­tion (the slurry process), ethylene reacts in an inert gas at­mos­phere (hydrogen) con­tain­ing a solvent (hexane) in the presence of cat­alytic agents to form poly­eth­yl­ene. Under the con­di­tions used in this process, ethylene is soluble in hexane but poly­eth­yl­ene is in­sol­u­ble, and pre­cip­i­tates in the form of a fine sus­pen­sion. The polymer is sep­a­rated from the hexane in cen­trifuges. The sep­a­rated hexane is returned again to the poly­meri­sa­tion process. The cen­trifuges used for sep­a­rat­ing the two phases (poly­eth­yl­ene – hexane) requires a seal for the inlet pipe and two casing seals, each having a leakage and flushing con­nec­tion. Because the hexane is in­flam­ma­ble, an ATEX version is nec­es­sary.

Solution from EagleBurgmann Espey:

For the inlet pipe seal and the two casing seals, Espey designed a combined Espey® WD 200HDT/500 with six seal rings in an ATEX version, with barrier gas and flushing con­nec­tions as well as leakage removal and the cor­re­spond­ing con­nec­tions for in­tro­duc­ing ethylene and hexane into the cen­trifuge, designed for a tem­per­a­ture of 90 °C (194 °F), a vacuum of 0.6 bar (8.7 PSI), and a rotation speed of 1400 rpm. Nitrogen is used as barrier gas. The seals are easily fitted, thanks to the split casing. Because the shaft of the cen­trifuge does not have suitable wear pro­tec­tion, Espey designed two-piece shaft sleeves with a suitable coating to protect the shaft. The seal solution has a long service life, is main­te­nance-free, and is designed for dry running.