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Incentives to increase efficiency and reduce CO2 emissions have led to the development of many plant process designs incorporating turbo-expanders. Turbo-expanders provide the most efficient solution when it is required to reduce the pressure of a fluid stream. By expanding high pressure fluid, energy in the high-pressure fluid entering the turbo-expander can be efficiently used for electrical power generation. The energy recovered from the expansion is supplementary, thereby improving the thermal efficiency of the plant without leaving any noticeable trace of greenhouse gas emissions. This article focuses on both mechanical and electrical considerations for the implementation of turbo-expander generators. In an example of LNG liquefaction plant, the article discusses the plant thermal efficiency, turbo-expander performance curve, generator sizing, and the choice between the induction and the synchronous generator. The impact on the plant real and reactive power balance and the system stability is shown in an example. Furthermore, this article evaluates the options for the turbo-expander generator tie-in points within the electrical system. [ABSTRACT FROM AUTHOR]

Centrifugal compressors are major components in LNG liquefaction plants. Design considerations for optimal selection and proper sizing of compressor drivers include large starting torque requirements to enable compressor start from settle-out conditions and to prevent flaring the gas. Start-up capabilities of the motor driver can significantly impact the reliability and operability of the compressor train. The article reviews the requirements of API 617 and API 546, and addresses a number of important design and sizing criteria used to properly match the large centrifugal compressor and its synchronous motor driver. In the effort to optimize plant design and maintain the performance requirements, the article discusses optimization options, such as direct on line starting method to explore the motor rating limits, and the use of synchronous motors for power factor correction to eliminate or reduce the need for reactive power compensation by capacitor banks. This article presents a novel approach to show constant reactive power lines on traditional V curves. It also complements capability curves of synchronous motors with lines of constant efficiency. [ABSTRACT FROM AUTHOR]