INTRODUCTION
LNG (liquefied natural gas) receiving terminals have been designed and built at many locations around the world for storing and vaporizing LNG cargos for delivery to existing pipeline infrastructures. The composition and heating value of the vaporized LNG sent to pipeline is determined by the properties of the LNG produced at the source. Since most pipeline contracts specify a range of acceptable heating values for gas delivered into a particular market, the variability of the inlet cargo properties can be a problem. In many instances, the heating value of LNG shipped to market is higher than acceptable in a specific pipeline grid. Some options available to reduce the heating value include: blending with other gas streams; injecting an inert gas (typically nitrogen) into the vaporized stream; or vaporizing the rich LNG stream and sending it to a conventional NGL / LPG (natural gas liquids / liquefied petroleum gas) recovery plant for processing prior to flowing to the pipeline grid.
In most cases, there is not an abundance of natural gas available for blending to adequately lower the heating value of a rich LNG stream. Nitrogen injection is usually very expensive and generally provides no other economic benefit besides lowering the heating value of the sales stream. Also, there are usually specifications covering total inert content which may not allow enough nitrogen
injection to adequately lower the heating value. Vaporization of the gas stream and then processing in a conventional gas plant does provide the economic advantage of selling the higher value heavier components separately (i.e., typical gas plant economics), but generally at a high operating cost. When LNG is vaporized, heated, and then processed in a conventional gas plant, a significant amount of compression is required to re-refrigerate the gas (via expansion) in order to recover the hydrocarbon components and then re-inject the residue gas at pipeline pressure.
A better method for controlling the delivery heating value is to recover NGL or LPG by integrating the recovery step into the vaporization step, eliminating the need for re-compression and taking advantage of the refrigeration available in the LNG. This provides the lowest capital and operating cost alternative for controlling heating value, while also providing a significant additional revenue stream.
In most LNG liquefaction plants, heavy hydrocarbon removal (generally C5+) is considered a feed conditioning step for the liquefaction process, so the resultant LNG will contain most of the hydrocarbon components lighter than pentane. For more than 30 years, Ortloff has been developing technology for recovering liquids from natural gas that offers higher recovery, better efficiency, greater simplicity, and better reliability than other available processes. This natural gas liquids recovery
technology can be extended to allow efficient liquids recovery in LNG terminals through integration of the recovery step with the re-vaporization step.
The Ortloff LNG Fractionation Processes (LFP) eliminate the need for recompression in the liquids recovery step, reducing the overall power required by as much as 90% or more compared to processing in a conventional gas plant. An additional benefit of LFP is that process performance is not sensitive to changes in inlet LNG composition, providing the terminal operator with maximum flexibility in processing LNG cargos from essentially anywhere in the world. This paper presents the results of case studies conducted by Ortloff to determine the recovery and efficiency performance of
the LFP liquids recovery technology when applied to a typical LNG feedstock.
Krunal Yuvaraj Bhosale
Chemical Engineer
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