Near-Critical CO 2 Flow for Carbon Capture and Storage

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1 7EN-174 Topic: Environmental aspects of combustion 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 213 Near-Critical CO 2 Flow for Carbon Capture and Storage Farzan Kazemifar 1, 2 1, 2, 3 Dimitrios C. Kyritsis 1 Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 2 International Institute for Carbon Neutral Energy Research (WPI-I 2 CNER), Kyushu University, Fukuoka, Japan 3 Khalifa University of Science Technology and Research, Abu-Dhabi, UAE Handling CO2 flow rates for the purposes of capture and sequestration necessitates high pressure and low temperature (in order to maximize density and minimize volumetric flow rate) which brings the flow close to the critical point of CO2 at approximately 74 bar and 31 C. We are presenting a first experimental investigation of several properties of two near-critical CO2 flows, namely pipe flow and Joule-Thompson throttling. Shadowgraph technique was employed to visualize the pipe flow structure in an optically accessible test section. CO2 was compressed in a 1-liter hydraulic accumulator using highpressure nitrogen. Downstream of the cylinder was the test section and a needle valve that controlled the mass flow rate. The results indicated a strong sensitivity of the pressure drop in the pipe on inlet conditions near the critical point. Due to the fact that the isothermal compressibility is very large near the critical point it is very difficult to control the density in the vicinity of the critical point. 1. Introduction Carbon capture and storage (CCS) is one of the proposed solutions for reducing greenhouse gas emissions. With this technique, CO 2 is separated from flue gases at single large sources, such as fossil fuel thermal power plants, compressed and injected into geological formations for storage [1, 2]. The critical point of CO 2 is at 73.8 bar and 31. C; conditions in the vicinity of which will be encountered in transition from high temperature and atmospheric pressure of CO 2 streams to high pressure and low temperature of transport and storage. The thermophysical and transport properties of fluids undergo abrupt changes near the critical point. Kinematic viscosity, thermal diffusivity, and mass diffusivity reach a minimum, while isobaric heat capacity and thermal conductivity reach a maximum. Also, in the supercritical region, surface tension and latent heat of vaporization go to zero as the interface between saturated liquid and saturated vapor disappears [3, 4]. These drastic changes in properties make near-critical flows unique, since even small fluctuation in pressure and temperature can have significant effects on fluid properties and consequently on flow behavior. Properties of near-critical fluids such as density and dynamic viscosity are strong functions of both pressure and temperature. This dependence can possibly provide an additional degree of freedom for controlling fluid properties [4] in order to tune them to properties desired for specific processes. The extreme sensitivity of properties on temperature highlights the importance of the Joule-Thomson coefficient. Due to the Joule-Thomson effect CO 2 temperature can drop sharply as a result of expansion inside geological formations after injection. Should the temperature drop below freezing temperature, there will be possibility of CO 2-hydrate formation and residual water freezing. Such effects can alter the porous structure of the rocks and create thermal stresses [5]. Last, but not least, a phenomenon that poses a challenge to the visualization of near-critical flows is critical opalescence [6, 7]. Our purpose in this paper is to provide seminal results for the challenging and intriguing field of near-critical CO 2 flow. An experimental framework has been set up for systematic study of CO 2 flow in flow configurations relevant to CCS. In this work near critical CO 2 pipe flow has been investigated. Detailed measurements of pressure losses in pipes were performed together with measurements of the Joule-Thomson coefficient. Shadowgraph technique was used for flow visualization in order to rationalize the measurements of the thermodynamic quantities

2 2. Experimental Setup The experimental setup is shown schematically in Figure 1. High pressure nitrogen from a cylinder was used to compress CO 2 in a 1-liter Parker piston accumulator. As a result of the compression, CO 2 temperature raised from near room temperature to near the critical temperature of CO 2. Electric tape heater or ice-filled cold packs were used to further heat or cool down the fluid. Three different test sections were used in this setup; a stainless steel pipe, 2 ft long with ¼ in OD and.84 in ID, a stainless steel.14 in diameter orifice (O Keefe Controls Co.), and an optically accessible high-pressure chamber. The pipe was insulated using.5 in thick rubber foam. The metal casing of the optically accessible chamber was insulated using the same rubber foam, however, heat transfer was possible through the glass windows. The chamber, was a Jerguson liquid level gage. It incorporated two tempered borosilicate glass windows located on opposite sides of the chamber and a rectangular flow cross-section, with the inlet and outlet at the top and the bottom, respectively. The chamber was approximately 5 in long with a visible range of 3 ¾ in, rated at 2 psi ( 138 bar) for operation at 38 C. Pressure and temperature at the inlet and outlet of the test section were measured using pressure transducers and T-type thermocouples respectively. The pressure transducers were Setra model 29 pressure sensors that measured the gauge pressure up to 3 psi (26.8 bar) with ±.25% full-scale accuracy (±.5 bar). Pressure drop in the pipe was measured using a Rosemount 351C differential pressure transducer with -1 in-h 2O (-25 kpa) range and.15% accuracy. Ungrounded T-type thermocouples in.62 in diameter sheath were used for temperature measurements. Cold junction compensation (CJC) was accomplished using the DAQ Assistant virtual instrument for T-type thermocouples. For the Joule-Thomson experiment, pressure drop across the orifice was obtained by subtracting individual pressure transducers readings. Mass flow rate was measured using a Micro Motion CFM1 Coriolis mass flow meter with.1% accuracy. Flow rate was controlled using a needle valve located downstream of the test section. Data acquisition and monitoring of the measurement devices were performed using National Instruments LabVIEW software. The voltage signal from diagnostic instruments including the thermocouples, mass flow meter, and pressure transducers were captured by a National Instruments BNC-211 shielded connector block, connected to a National Instruments PCI-636E card installed in a PC. 3. Results and Discussion 3.1. Pipe Flow Experiment N2 Hydraulic Piston Accumulator Coriolis Mass Flow Meter Cylinder Thermocouple Pressure Transducer Figure 1 Schematic diagram of experimental setup Optically Accessible Test Figures 2-7 report pressure drop per pipe unit length vs. mass flow rate for several cases of upstream/downstream pressures and temperatures. Figures 2 and 3 show the effect of pressure and Figures 4-7show the effect of temperature on pressure drop in the pipe. As shown in Figure 2 the pressure drop is not affected by decreasing inlet pressure from 8 bar to 78 bar at constant temperature of 35.7 C. However, further pressure decrease to 76 bar results in ~22% decrease in ΔP for ~3.7 g/s mass flow rate and also same ΔP (~59 kpa.m -1 ) for 14% greater mass flow rate (5.6 g/s vs. 4.9 g/s). Moreover, Figure 3 indicates a significant spread in data points and there is no clear relation between ΔP and P in in the pressuretemperature range presented here. CO2 2

3 The triads represent :, T in. If only one number: T in , 37.5, , 58.5, , 3.6, , 59.4, Inlet Conditions 35.7±.5 C 8 bar 78 bar 76 bar Figure 2 Pressure Drop per Unit Pipe Length vs. Mass Flow Rate at Tin = 35.7±.5 C The triads represent :, T in. If only one number: T in , 63.9, , 52.3, , 3., , 4.9, bar 74 bar Figure 3 Pressure Drop per Unit Pipe Length vs. Mass Flow Rate at Tin = 34.2±.5 C Inlet Conditions 34.2±.5 C In contrast to inlet pressure, the effect of inlet temperature on ΔP is more pronounced. As shown in Figure 4, at 8 bar, temperature decrease from 37.5 C to 29.5 C results in 42% decrease in ΔP for 6.4 g/s mass flow rate, while at ~4.4 g/s, same temperature decrease does not yield any significant change in pressure drop. This suggests that depending on the mass flow rate, there are distinct pipe flow regimes that can affect pressure drop substantially. According to Figure 5, at 78 bar there is no substantial change in pressure drop as inlet temperature varies in the 32.7 C-37 C range. However, for T in=29.2 C, the results show a 47% decrease in pressure drop at 6.1 g/s mass flow rate. As shown in Figure 6, at 76 bar, ΔP increases by 56% for the same mass flow rate (4.2 g/s) as inlet temperature increases from ºC to 34.5ºC. Figure 7 indicates that at 74 bar, ΔP is not sensitive to changes in inlet temperature in the 32.7 C-35.4 C range for mass flow rates less than 4.5 g/s. Nevertheless if inlet temperature is further reduced to 27.5 C, a ~5% reduction in ΔP occurs for m =3. g/s. Moreover, for m =5.7 g/s a temperature decrease from 35.1 C to C results in 25% decrease in ΔP, and further reduction to 28.8 C yields ~5% decrease (compared to ΔP at C) bar 37.5±.4 C 8 bar 35.7±.3 C 8 bar 29.5±.1 C 8 bar 23.6 C 6.4, bar 37.±.2 C 78 bar 36.±.3 C 78 bar 35.3±.4 C 78 bar 32.7±.4 C 78 bar 29.2 C 6.1, , , , , Figure 4 Pressure Drop per Unit Pipe Length vs. Mass Flow Rate at 8 bar Figure 5 Pressure Drop per Unit Pipe Length vs. Mass Flow Rate at 78 bar 3

4 Pressure [bar] bar 36.±.4 C 76 bar 35.±.4 C 76 bar 34.±.5 C 76 bar 29.1 C 4.2, bar 35.1±.3 C 74 bar 34.1±.3 C 74 bar 33.±.3 C 74 bar C 5.7, , , , , , Figure 6 Pressure Drop per Unit Pipe Length vs. Mass Flow Rate at 76 bar 2 3, Mass Flow Rate [g/s] Figure 7 Pressure Drop per Unit Pipe Length vs. Mass Flow Rate at 74 bar 3.3. Shadowgraph Visualization Figure 9 shows shadowgraphs of the flow structure at six different stations during the flow across the optically accessible test section. The prevailing pressure and temperature conditions have been positioned on the P-T phase diagram along with the saturation curve in Figure 8. The shadowgraphs depict the transition of CO 2 from a state at supercritical pressure and subcritical temperature to a subcritical saturated liquid/vapor. At point #1, CO 2 is at supercritical pressure and subcritical temperature. As fluid expands and flows through the test section, initially pressure and temperature go in opposite directions, then temperature increases at almost constant pressure until fluid reaches the saturation curve. After this, pressure and temperature decrease simultaneously. As fluid reaches the saturation curve, initially fluid is in saturated liquid phase (#3) which is followed by emergence of the interface between the vapor and liquid phase (#4). As the flow progresses the interfaces advances from top to bottom towards test section exit (#4-5). Presumably, fluid above the interface is saturated vapor while fluid below is saturated liquid. At the end (#6) it seems that CO 2 is in saturated vapor condition due to the resemblance of shadowgraph structure to the structure visible above the interface (#4-5) Inlet Images Saturation Temperature [ C] Figure 8 P-T diagram for inlet conditions in shadowgraph experiment

5 (1) (2) (3) 3.2. Joule-Thomson Experiment (4) (5) (6) Figure 9 Shadowgraphs of near-critical CO2 flow, Flow direction from top to bottom Joule-Thomson coefficient data are presented in Figure 1 and compared with reference data available in NIST Chemistry WebBook [8]. The relative error of these measurements with respect to NIST values, demonstrates a clearly bimodal behavior. For certain inlet conditions, the error is less than ~25% while for different conditions it reaches 2%. There are two possible controlling factors: the inlet enthalpy, h in, and the quality (vapor mass fraction) at the exit, x out. As indicated in Figure 11, the spike in relative error occurs at approximately 335 kj/kg, which is very closely the enthalpy of CO 2 at its critical point (h crit = kj/kg). For data points with h in > h crit, the error magnitude is less than 25% for almost all cases, while for data points with h in < h crit the error is significantly larger. One contributing factor is that, for the conditions studied, the Joule-Thomson coefficient reported in literature is smaller for cases with h in < h crit compared to those with h in > h crit (.2-.5 C/bar vs C/bar). Nevertheless, as signified in Figure 12, the absolute error is still higher for the cases with h in < h crit. 5

6 Joule-Thomson Coefficient, µ JT [ C/bar] Figure 13 shows the relative error in Joule-Thomson coefficient measurements vs. the quality at the exit (x out). The error demonstrates a bimodal behavior similar to what was described above. The spike in this case occurs at approximately x out =.6. As shown in this graph, all data points with x out >.6 have inlet enthalpy greater than h crit, which suggests that the two controlling factors (h in and x out) are not independent from one another. Absolute Error in µ JT [ C/bar] 1 25%.8 2%.6.4 Experiment Relative Error in µ JT 15% 1% 5%.2 NIST % h in [kj/kg] Figure 1 Joule-Thomson coefficient measurements compared with NIST published data [5] -5% h in [kj/kg] Figure 11 Relative error in Joule-Thomson coefficient measurements vs. inlet enthalpy of CO2.6 25% h_in > h_crit.4 2% h_in < h_crit.2 Relative Error in µ JT 15% 1% 5% -.2 % h in [kj/kg] Figure 12 Absolute error in Joule-Thomson coefficient measurements vs. inlet enthalpy of CO2-5% X out Figure 13 Relative error in Joule-Thomson coefficient measurements vs. vapor mass fraction at the exit The significance of h crit and x out can be explained by visualizing in terms of the P-h diagram shown in Figure 14. In this diagram, inlet and outlet data points are presented along with the saturation curve, h = h crit constant enthalpy line, and x =.6 line. Joule-Thomson throttling process is essentially an isenthalpic process which lies on a vertical line in the P- h diagram, thus for plotting the outlet data points h out is assumed to be equal to h in. In our experiments, for the inlet conditions and mass flow rates studied, the fluid at orifice exit fell in the two phase region of the P-h diagram (below the saturation bell-shaped curve) for almost all cases. Also the difference between the h=h crit line and x =.6 line is smaller than 15 kj/kg for all pressures in the 15 bar to 73.8 bar range. Thus, we suppose the increased error has to do with reduced vapor content of the fluid at the orifice exit, however the mechanism by which this occurs is currently unclear. 6

7 Pressure [bar] 4. Conclusions A system has been set up to study CO 2 flow in configurations relevant to CCS. Pipe flow measurements indicate the sensitivity of ΔP to inlet pressure and temperature; especially the latter. However, there seems to be certain conditions (pressure, temperature, and mass flow rate) for which ΔP is almost independent of T in and P in, which suggests that there are distinct pipe flow regimes based on flow rate and inlet conditions. Acknowledgements The authors acknowledge the support of the International Institute for Carbon Neutral Energy Research (WPI-I 2 CNER), sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan. References Saturation Inlet Outlet Critical Enthalpy x = Enthalpy [kj / kg] Figure 14 P-h diagram for inlet and outlet data points in the Joule-Thomson Experiment 1. Ciferno, J. P., Fout, T. E., Jones, A. P., & Murphy, J. T. (29). Capturing carbon from existing coal-fired power plants. Chemical Engineering Progress, 15(4), Haszeldine, R. S. (29). Carbon capture and storage: how green can black be?. Science, 325(5948), Reid, R. C., Prausnitz, J. M., & Poling, B. E. (1987). The properties of gases and liquids. 4. Clifford, T. (1999). Fundamentals of supercritical fluids. New York: Oxford University Press 5. Oldenburg, C. M. (27). Joule-Thomson cooling due to CO 2 injection into natural gas reservoirs. Energy Conversion and Management, 48(6), Einstein, A. "Theory of the Opalescence of Homogeneous and of Mixed Liquids in the Neighborhood of the Critical Region." Ann. Physik 33 (191): White, John A., and Bruce S. Maccabee. "Temperature dependence of critical opalescence in carbon dioxide." Physical Review Letters (1971): Lemmon, E. W., M.O. McLinden & D.G. Friend, "Thermophysical Properties of Fluid Systems" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, Gaithersburg MD, 2899, (retrieved March 8, 213). 7