document preview

Abstract
Existing carbon dioxide (CO2) pipelines commonly contain over 90% of CO2 mixed with major impurities such as methane (CH4), hydrogen sulfide (H2S), and nitrogen (N2). Other minor impurities are carbon monoxide (CO), oxygen (O2), C2+ hydrocarbons, and H2O. CO2 is transported in the gas phase, liquid phase, or as a supercritical fluid. Phase behaviors of CO2 mixtures are based on the Vapor Liquid Equilibrium (VLE) and critical points of mixtures. The predictions of VLE are essential to the design and operation of CO2 mixture pipelines. The VLE and critical points for binary or ternary CO2 mixtures are predicted by Equations of State (EOS). This study illustrates how major impurities impact transport capacities of CO2-rich mixture pipelines. The calculations indicate that the flow capacity in the liquid phase is approximately twice as high as in the gas phase.

Introduction
Carbon dioxide (CO2) capture and storage (CCS), which involves the capture, transport, and geologic sequestration of CO2, is a promising solution for not only reducing global atmospheric emissions of CO2 but also decreasing changes in the global climate system (Houghton et al. 1996). Pipelines are the most commonly used conveyance for transporting large volumes of CO2 over distances from 10’s to thousands of kilometers. Long-distance CO2 pipelines in the United States were built to supply Enhanced Oil Recovery (EOR) in regional oil fields. More CO2 pipelines are expected to be built in support of EOR and geological carbon sequestration. The US Department of Energy (DOE) has established seven Regional Carbon Sequestration Partnerships (RCSP) which aim to inject and monitor CO2 at the demonstration scale (at least one million tons of CO2 injected). Since the first CO2 pipeline in the United States (352 km long) was built in 1970 by the Scurry Area Canyon Reef Operator (SACROC) Unit in Scurry County, Texas, many long-distance CO2 pipelines have carried large volumes of CO2 ranging from 1.1 to 19.3 MtCO2/year. Existing long-distance CO2 pipelines in North America include the Bravo Dome Pipeline (350 km), the Cortez Pipeline (808 km), the Sheep Mountain Pipeline (660 km), and the Weyburn Pipeline (328 km). However, impurities impact transport capacities in pipelines. CO2 can be transported in the gas phase, liquid phase, or as a supercritical fluid. In particular, CO2 is supercritical at or above temperature of 31.1°C and pressure of 72.9 atm. Supercritical CO2 is becoming an important subject due to its importance in carbon sequestration in which it behaves as a gas but with the density of a liquid. Typical considerations of CO2 pipeline engineering include flow rate, operating pressure, operating temperature, gas mixture composition, corrosion, and pipeline network. In addition, site-specific design considerations for CO2 pipelines are phase behavior from pressure changes, requirement for dehydration of CO2, pipeline routing topography, compressor and vale station, and risk assessment. Recently, several research programs dedicated to the engineering of CO2 pipeline transport have investigated several issues; corrosion with the use of carbon steel, water content and solubility, hydrate formation, fluid properties considering chemical reactions, experimental data exploring dynamics and physical properties of CO2 with impurities, and developing of equations of state (EOS) for CO2 with impurities. In this study we focus on Vapor Liquid Equilibrium (VLE) and critical conditions for binary CO2-rich mixtures containing common impurities (Hu et al., 2007; Li and Yan, 2009).