Carbon dioxide triple point crystallization

The process features of carbon dioxide triple-point crystallization and slurry absorption of carbon dioxide have been demonstrated with the first generation bench-scale apparatus. Current efforts are focused on the design and construction of an improved version of the carbon dioxide triple-point crystallizer in cooperation with the U S Department of Energy. Future efforts are planned to design and construct absorption units to study multi-stage slurry absorption of carbon dioxide, and the more conventional gas-liquid absorption of sulfuruous compounds with liquid carbon dioxide. 

The CNG process removes sulfurous compounds, trace contaminants, and carbon dioxide from medium to high pressure gas streams containing substantial amounts of carbon dioxide. Process features include 1) absorption of sulfurous compounds and trace contaminants with pure liquid carbon dioxide, 2) regeneration of pure carbon dioxide with simultaneous concentration of hydrogen sulfide and trace contaminants by triple-point crystallization, and 3) absorption of carbon dioxide with a slurry of organic liquid containing solid carbon dioxide. These process features utilize unique properties of carbon dioxide, and enable small driving forces for heat and mass transfer, small absorbent flows, and relatively small process equipment. 

The CNG acid gas removal process is distinguished from existing AGR processes by three features. The first feature is the use of pure liquid carbon dioxide as absorbent for sulfurous compounds the second feature is the use of triple-point crystallization to separate pure carbon dioxide from sulfurous compounds the third feature is the use of a liquid-solid slurry to absorb carbon dioxide below the triple point temperature of carbon dioxide. Pure liquid carbon dioxide is a uniquely effective absorbent for sulfurous compounds and trace contaminants triple-point crystallization economically produces pure carbon dioxide and concentrated hydrogen sulfide for bulk carbon dioxide absorption the slurry absorbent diminishes absorbent flow and limits the carbon dioxide absorber temperature rise to an acceptable low value. The sequence of gas treatment is shown in Figure 1, an overview of the CNG acid gas removal process. 

The triple-point crystallization of carbon dioxide is illustrated in Figure 7, which shows a schematic carbon dioxide phase diagram expanded about the triple-point and a closed-cycle triple-point crystallizer operating with pure carbon dioxide. The operation of this closed-cycle unit is identical to that of a unit in the stripping section of a continous crystallizer cascade, except that in the cascade vapor would pass to the unit above, and liquid would pass to the unit below. 

Concentration changes observed between mother liquor in the flash zone and liquid product in the melt zone of an experimental triple-point crystallizer have been dramatic. A qualitative concentration profile typical of those observed in the experimental unit is shown in Figure 8. The mother liquor concentration is relatively uniform above the packed bed, but a sharp drop in contaminant concentration occurs within the top several inches of the loosely packed crystal bed. Concentration changes of the order 500 to 5000 have been observed for representative sulfurous compounds and trace contaminants, including hydrogen sulfide, carbonyl sulfide, methyl mercaptan, ethane, and ethylene. Concentration profiles calculated for the packed bed of solid carbon dioxide using a conventional packed bed axial dispersion model agree very well with the observed experimental profiles. 

CNG [Consolidated Natural Gas] A process for removing acid gases from natural gas and syngas, using supercritical carbon dioxide. Under development since 1973 by the Consolidated Natural Gas Research Company with assistance from the U.S. Department of Energy and Helipump Corporation. Liquid carbon dioxide is first used to extract the sulfur compounds. Crystallization at the triple point separates these sulfur compounds from the... 

The adiabatic flash pressure Pf, maintained slightly below the triple-point pressure, causes liquid to spontaneously vaporize and solidify. The ratio of solid to vapor is determined by the heats of fusion and vaporization for carbon dioxide about 1.7 moles of solid are formed for each mole vaporized. The solid, more dense than the liquid, falls through a liquid head and forms a loosely packed crystal bed at the bottom. The liquid head is about 10-12 feet, and increases the hydrostatic pressure on the solid to melter pressure Pm. The crystal bed depth is about two... 

For most substances, including water (see Fig. 10.23c), atmospheric pressure occurs somewhere between the triple-point pressure and the critical pressure, so in our ordinary experience, all three phases—gas, liquid, and solid—are observed. For a few substances, the triple-point pressure lies above P = 1 atm, and under atmospheric conditions, there is a direct transition called sublimation from solid to gas, without an intermediate liquid state. Carbon dioxide is such a substance (see Fig. 10.23b) its triple-point pressure is 5.117 atm (the triple-point temperature is —56.57°C). Solid CO2 (dry ice) sublimes directly to gaseous CO2 at atmospheric pressure. In this respect, it differs from ordinary ice, which melts before it evaporates and sublimes only at pressures below its triple-point pressure, 0.0060 atm. This fact is used in freeze-drying, a process in which foods are frozen and then put in a vacuum chamber at a pressure of less than 0.0060 atm. The ice crystals that formed on freezing then sublime, leaving a dried food that can be reconstituted by adding water. 

Fig. 1. Typical phase diagram (of carbon dioxide) in coordinates T (temperature) and p (pressure). D is the triple point DAi, DA2 and DA3 are the curves of melting, boiling and sublimation, respectively S, L and G are the areas of existence of solid (crystal), hquid and gaseous states of matter.

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