(37b) Turbo Expanders – High Enthalpy Wheel Design

Turboexpander Systems are widely used in ethylene plants to optimize cryogenic recovery and optimize the recompression of residue gas.  The light hydrogen rich residue gas, in particular if produced by ethane or E/P cracker, challenges the design of the Turboexpander System as the machines would need to handle very high enthalpy drop per stage.

Turboexpanders for residue / tail gas are designed mainly with two types of loading devices: generator load and compressor load.

This paper will focus mainly on Turboexpander systems with recompressor load. A cascade of units are typically used to meet the required efficiency and to providing a reliable solution mechanical solution.

Wheel design and rotordynamics are very important in the optimization process, particularly when active magnetic bearings are used. Active magnetic bearings are characterized by very low losses thus loading more the recompressor than traditional oil bearing units where mechanical losses can brake significantly the expander wheel. The recompressor loading is often the main constraint for residue gas expander aerodynamic optimization.

 A new patented wheel design is presented; this design allows the compressor to run at very high tip speed with minimal centrifugal stresses. This solution eventually can reduce the number of stages or vice versa to improve significantly the efficiency of the expander. Technology limits for the new solution as well as for traditional recompressor wheel design are compared with other different loading devices.