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Abstract

Recovery of natural gas liquids (NGL) components in gas not only may be required for hydrocarbon dew point control in a natural gas stream (to avoid the unsafe formation of a liquid phase during transport), but also yields a source of revenue, as NGLs normally have significantly greater value as separate marketable products than as part of the natural gas stream. One of the key parameters in the recovery of ethane and heavier products from natural gas is the effect of the extraction on the heat content of the residue gas. In this work a simple-to-use generalized correlation is developed for the ethane recovery limit to attain a minimum higher heating value (HHV) for various feed gas compositions. As can be seen from this correlation results, the quantity of inerts in the feed gas has an impact on the ethane recovery level which can be targeted in a plant design. A simple correlation is also developed here to estimate the propane and butanes recovery percent as a function of ethane recovery percent. The tool developed in this study can be of immense practical value for petroleum and gas engineers to have a quick check of the heat content of the residue gas after recovery of ethane and heavier components from natural gas at various conditions without opting for any experimental trials. In particular, engineers would find the approach to be user-friendly with transparent calculations involving no complex expressions.

Introduction

Most natural gas is processed to remove the heavier hydrocarbon liquids from the natural gas stream (Baek et al, 2011). These heavier hydrocarbon liquids, commonly referred to as natural gas liquids (NGLs), include ethane, propane, butanes, and natural gasoline (condensate). Lighter NGL fractions, such as ethane, propane, and butanes, can be sold as fuel or feedstock to refineries and petrochemical plants, while the heavier portion can be used as gasoline-blending stock (Serag-Eldin, 1989; Oliveti et al, 2012). Regardless of the economic incentive, however, gas usually must be processed to meet the specification for safe delivery and combustion ( Bahadori and Vuthaluru 2009 and 2010). The gas composition has a major impact on the economics of NGL recovery and the process selection. In general, gas with a greater quantity of liquefiable hydrocarbons produces a greater quantity of products and hence greater revenues for the gas processing facility (Jensen and Skogestad; 2009). The other major consideration in the evaluation of NGL recovery options is the specification of the residue sales gas. Sales specifications are usually concerned with a minimum Higher Heating Value (HHV) of the gas, but in some instances the maximum HHV can also be a consideration.