Experimental study of the separate existence of phases in two-phase water-supercritical carbon dioxide systems :: SPE RESST

Experimental study of the separate existence of phases in two-phase water-supercritical carbon dioxide systems

Supercritical fluid technologies are currently in the process of rapid development and expansion. This is due to the fact that the technologies of chemical production using toxic and explosive reagents do not withstand competition with more efficient and harmless supercritical fluid technologies. Supercritical fluid, as an extractant, has a number of characteristics. In comparison with a conventional solvent, it is characterized by 1-2 orders of magnitude lower viscosity, 2-3 orders of magnitude greater diffusion coefficient and lower density. This significantly (several times) reduces the extraction time. The dissolving power of the supercritical fluid depends strongly on temperature and pressure, which allows for selective extraction. Special applications of supercritical fluid technologies were obtained in the processing of spent nuclear fuel, uranium production waste and selective extraction of actinides. Despite the high degree of experimental study of this subject, the reasons for a significant increase in extraction efficiency in two-phase systems consisting of SC-CO₂ and a small (but not less than 0.3% by volume) amount of distilled water remain unclear.

Experimental equipment

Experiments to study the formation of water droplets in SC-CO₂ in situ were carried out at the SKFE-1 facility. An optical high-pressure cell with sapphire windows was located at the outlet from the reactor with a volume of 48.13 cm³. The construction of the cell is shown in Figure 1.

Figure 1 - High-pressure optical cell construction

The dimensions of the water droplets in SC-CO₂ were analyzed by diffraction patterns obtained by passing red or violet laser radiation through a high-pressure optical cell with sapphire windows. The existence of diffraction patterns in the scattering of laser radiation by water droplets in SC-CO₂ is due to the fact that the refractive indices of water and SC-CO₂ are different (as is known, diffraction gratings are formed even in a transparent liquid or gas in which the refractive index is changed by excited running or standing ultrasonic waves).

Experimental technique

A study of the diffraction of laser radiation on water droplets was carried out in accordance with the scheme shown in Fig. 2. The cell was mounted in the holder perpendicular to the outer surface of the sapphire window to the laser beam at a distance from the exit window of the laser of not less than 0.5 m. The screen was removed from the cell by a distance L = 3,3 m. Two mutually perpendicular rulers with divisions of 0.5 cm, intended for determining the dimensions of the elements of the diffraction pattern, were arranged along the perimeter of the screen.

Схема образования дифракционных картин при рассеянии лазерного излучения на каплях воды

Figure 2 - Diagram of the formation of diffraction patterns in the scattering of laser radiation on water droplets: 1 - screen; 2 - supercritical fluid, SC-CO₂; 3 - metal casing of the cell; 4 - a drop of water; 6 - source of laser radiation

As a source of laser radiation, a laser pointer based on a red laser diode was used.

Water solubility in SC-CO₂

The weight method showed that in the investigated pressure range (from 7 to 17 MPa) at a fixed temperature of 40 °C, the mole fraction of water dissolved in SC-CO₂ is practically constant.

Investigation of stationary diffraction patterns of scattering of laser radiation by a two-dimensional lattice with square cells

To estimate the size of water droplets, stationary diffraction patterns from a circular diffraction grating with square cells for red and violet laser radiation were obtained and analyzed (see Fig. 3).

$Diffraction pattern$		$Diffraction pattern$		$Diffraction pattern$
a)		b)		c)

Figure 3 - Diffraction pattern from a lattice with square cells: a) qualitative; b) for a red laser; c) for a purple laser. To estimate the dimensions of the elements of the diffraction pattern in these figures and further in the lower left corner, a white square with a side length of 10.0 mm

Investigation of diffraction patterns of scattering of laser radiation by water droplets in SC-CO₂

The filter paper impregnated with 0.5 ml of distilled water was placed in the reactor of the unit. In accordance with the carbon dioxide quality certificate, the initial water content under normal conditions (pressure 101.3 kPa, temperature 20 °C) was less than 0.076 g/m³. Estimates show that at pressures up to 16.0 MPa, the water content of carbon dioxide was significantly lower than the amount deposited on the filter paper.

The fluid in the cell with the water dissolved in it was illuminated by a red or purple laser. The laser radiation passing through the cell was directed to the screen (see Fig. 2). The time-varying (dynamic) diffraction pattern obtained at a certain pressure was recorded on the video camera for 5 minutes. The clip thus created was divided into separate frames, representing static diffraction patterns, which were then analyzed to obtain information on the size of the water droplets in SC-CO₂ (see Figures 4-6).

$Diffraction pattern$		$Diffraction pattern$
a) 0.1 MPa. The trace on the screen from the red laser in the absence of supercritical fluid in the high-pressure cell		b) 8.0 MPa. On the screen there is an intense chaotic motion of light dots of ∼3x2 mm (purple laser ∼2x1 mm)
$Diffraction pattern$		$Diffraction pattern$
c) 10.0 MPa. On the screen there is a chaotic movement of light points ∼3x2mm in size (purple laser ∼2x1mm)		d) 12.0 MPa. On the screen there is a chaotic motion of light points
$Diffraction pattern$		$Diffraction pattern$
e) 14.0 MPa. On the screen there is a slow movement of light points		f) 16.0 MPa. On the screen, there is practically no movement

Figure 4 - Photographs of diffraction patterns at different times in a holding interval of 5 min for the optical path length in a high-pressure cell 21.0 mm

$Diffraction pattern$		$Diffraction pattern$
a) 8.0 MPa. On the screen there is a chaotic movement of light points ∼3x2mm in size (purple laser ∼2x1mm)		(b) 10.0 MPa. On the screen, the motion of bright spots is absent

Figure 5 - Photographs of diffraction patterns at different times in a holding interval of 5 min for the length of the optical path in a high-pressure cell 6.0 mm

$Diffraction pattern$		$Diffraction pattern$
a) 12.0 MPa. On the screen when carbon dioxide is released, there is no movement of light spots		b) 12.0 MPa. On the screen when the pressure is released, there is a slight uniform movement of light spots

Figure 6 - Photographs of diffraction patterns at different times in a holding interval of 5 min for the length of the optical path in a high-pressure cell 0.1 mm

The image of the diffraction pattern on the screen is usually a slow moving light spot for a red laser, and a light circular spot with flickering dots on it for a purple laser. The presence of one spot in the latter case is explained by the fact that the wavelength of the violet laser is smaller than for the red one, and therefore the bright spots on the screen from the violet laser merge into one spot. Therefore, experiments in obtaining diffraction patterns used the light of a red laser.

The investigation of diffraction patterns of the scattering of laser radiation by a two-dimensional lattice with square cells showed the correspondence of the experimental results to theoretical concepts.

Analysis of diffraction patterns of scattering of laser radiation by water droplets in SC-CO₂, as well as optical microscopy at a pressure of 10.0 MPa and a temperature of 40 °C, determine the average diameter of water droplets of 55±3 μm.