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Abstract
Supercritical pipelines are most commonly used in CO2 transportation. Liquid models are always utilized for design and hydraulic checking of those pipelines. However, supercritical CO2 may show significant characteristics of gaseous under certain operating condition. This phenomenon may increase pressure drop so that pressure values will be different from calculation results from liquid models. According to comparing the compression factor and density curve of liquid and supercritical CO2, the fact that when operation parameters of supercritical pipelines occupy in certain pressure and temperature regions, compressibility of CO2 cannot be neglectd is concluded. On the basis of density variation tendency, the method and its steps which are used to delimit compressible region of supercritical CO2 are proposed. Compared with this compressible region, operating parameters of supercritical pipelines can be checked. The application example calculates the pressure drop both by liquid model and gaseous model which considers copressibility. The results verified that delimited method of compressible region is accurate and reasonable.

Introduction
Transport is that integral stage of CO2 capture, storage and EOR that links sources and storage sites. CO2 is commercially transported by tanks, ships and pipelines. Small-scale offshore transport prefers tanks and the relatively small needs of onshore transport limit the development of ship transportation. Therefore, for large-scale and long-distance transport of CO2, pipelines are the most economical way. Because of the relatively low critical parameters (critical temperature Tci=304.25K, critical pressure Pci=7.29MPa), CO2 pipelines can be divided into 3 categories: vapor pipelines, liquid pipelines and supercritical pipelines. It is estimated that there are about 3100km of pipelines worldwide with a capacity of 50Mt per year of CO2 for EOR and most of them are supercritical pipelines (Working Group III of the Intergovernmental Panel on Climate Change, 2005). Liquid models and calculation formulas are always utilized for design and hydraulic checking of those pipelines. However, supercritical CO2 may show significant characteristics of gaseous under certain operating conditions. This phenomenon may increase pressure drop so that pressure values will be different from calculation results from liquid models. According to comparing the compression factor
and density curve of liquid and supercritical CO2, the compressibility of supercritical CO2 are concluded. On this basis, the method and its steps which are used to delimit compressible region of supercritical CO2 are proposed. The method is finally verified by the application example.

Compressibility of supercritical CO2
Phase envelop of CO2
CO2 supercritical pipelines always require high-purity CO2 (volume content is higher than 95%). Figure 1 shows the P-T phase envelope of high-purity CO2 (Seevam, P., Race, J. M., Downie, P. H., 2007). This diagram contains two distinct lines: bubble point line and dew point line. Their intersection is the critical point under this content of CO2. Based on these two lines, the diagram can be divided into four regions: liquid region (A), supercritical region (B), gaseous region (C) and two phase region (D). For energy saving and safety consideration, operating parameters of CO2 pipelines should avoid occupying in region D. Therefore, the transport of CO2 could theoretically achieve by three-phase state: gaseous, liquid and supercritical.