Expander Applications

One application for hot gas expanders is for use in refineries to recover the energy in the tail gas of a catalytic cracking unit. In the recent past, other uses included certain process applications that used air for an oxygen source. If the residual pressure after the process reaction was sufficiently high, an expander was used to recover the remaining energy. A variation might have included auxiliary firing if the residual oxygen content was high enough to support the combustion. Unfortunately, the economics were diminished because of environmental concerns with stack emissions and the advent of direct oxygen use. A potential for the hot gas expander is in binary bottoming cycles, to convert low level heat to usable shaft work.

While many of the applications use the expander to drive a generator, a compressor is a good alternative candidate for the load. Expanders are generally custom-sized and can, therefore, be readily matched to the centrifugal or axial compressor. It also will match the screw compressor of the dry type, at least in the larger frames. Basic sizing to the compressĀ«): should follow the same guidelines as for steam turbines.

The expander is somewhat difficult to govern, and is sometimes used with a helper steam turbine for use in starting and speed control.

A variation of the application of expanders to a compressor train is to include an induction motor-generator. This arrangement does not provide precise speed control, but works well in other aspects. The motor-generator acts as a starter to bring the compressor train to speed and permits the process to start. As the expander begins to recover energy, it first takes load from the compressor and when excess torque becomes available, the induction machine acts as a generator. Because an induction generator is a varless machine, it must be tied to the grid to produce power. This system is inherently speed regulated: the faster the generator turns the more load it takes from the expander. Regulation is within a few percent, general! y in a range acceptable to a centrifugal or axial compressor.

References

\. National Electrical Code 1984, National Fire Protection Association, Quincy, MA, 1983.

2. API RP 500, Recommended Practice for Classification of Locations for Electrical Installations in Petroleum Facilities, First Edition, Washington, D.C . i American Petroleum Institute, 1991.

3. API RP 540, Electrical Installations in Petroleum Processing Plants'. Third Edition, Washington, DC: American Petroleum Institute, 1991,

4. Jeumont-Schneider, R. Champrade, High Power Adjustable Speed Drive with Synchronous Motor, Power Conversion International, Sept./Qci 1979, pp. 83-89.

5. Scholey, D., "How Adjustable Frequency Drivers Affect Induction Motor Operation," Plant Engineering, July 26, 1984, pp. 44-47.

6. Lazor, D. A. and Bryson, E. J., "Efficiency Improvements for Steam Turbine Drivers," Hydrocarbon Processing, January 1984, pp. 61-63,

7. Evans, Frank L,, Jr., Equipment Design Handbook for Refineries and Chemical Plants, Vol. 1, Houston, TX: Gulf Publishing Company, 1979.

8. Scholey, D., "Induction Motors for Variable Frequency Power Supplies." IEEE Transactions, July 1982, pp. 368-372.

9. LeMone, C. P., Large Adjustable Speed Drives and Their Application to a High Speed Centrifugal Compressor, Proceedings of the 10th Annual f ur-bomachinery Symposium, Texas A&M University, College Station, TX, 1981, pp. 81-85.

10. API Standard 611, General-Purpose Steam Turbines for Refinery Services, Third Edition, Washington, DC: American Petroleum Institute, 1988, Reaffirmed 1991.

11. API Standard 612, Special-Purpose Steam Turbines for Petroleum, Chemical, and Gas Industry Services, Fourth Edition, Washington, DC: Ameri can Petroleum Institute, 1995.

12. API Standard 616, Combustion Gas Turbines for Refinery Service, Third Edition, Washington, DC: American Petroleum Institute, 1992.

13. Boyce, Meherwan P., Gas Turbine Engineering Handbook, Houston, TX: Gulf Publishing Company, 1982.

14. Siemens, High-Voltage Three-Phase Motors, Catalogue M-2 Supplement, 1975, Siemens West Germany.

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