Liquid carbon dioxide, properties

Liquid carbon dioxide is discussed on page 261. Carbon dioxide gas is commonly used for carbonating drinks, in fire extinguishers, for gas-shielding of welding and in shell moulding in foundries. Its physical and toxicological properties are summarized in Tables 8.5, 8.6 and 5.29. 

Liquid carbon dioxide produces a colourless, dense, non-flammable vapour with a slightly pungent odour and characteristic acid taste . Physical properties are given in Table 8.5 (see also page 277). Figure 8.1 demonstrates the effect of temperature on vapour pressure. 

A supercritical fluid exhibits physical-chemical properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those in normal gaseous states. In the vicinity of the critical point the diffusion coefficient is more than 10 times that of a liquid. Carbon dioxide can be compressed readily to form a liquid. Under typical borehole conditions, carbon dioxide is a supercritical fluid. 

The separation properties in SFE are dependent on the choice of solvents, as well as on the solutes. The most popular solvent, carbon dioxide, is a rather nonpolar solvent, which dissolves mainly nonpolar solutes. Solubilities of selected compounds in liquid carbon dioxide are given in Table 10.6. The solubility and selectivity can be altered by adding small amounts of polar solvents, called entrainers (e.g., water or ethanol). 

A way around this issue may have been found with the use of supercritical fluids. These materials, such as liquid carbon dioxide, have many interesting properties from the point of view of pharmacutical processing since they combine liquid-like solvent properties with gas-like transportation properties. Small changes in the applied pressure or temperature can result in large changes of the fluid density and, correspondingly, the solvent capacity and properties of the resultant particles. 

Fig. 3. Effect of using either liquid oi supercritical carbon dioxide on the textural properties of titama aerogels calcined at the temperatures shown. ( ), dried with liquid carbon dioxide at 6 MPa and 283 K (- - -), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

Figure 12.3. Properties of liquid carbon dioxide. Adapted from Daubert, T. E. Danner, R. P. Data Compilation Tables of Properties of Pure Compounds. Design Institute for Physical Property Data, American Institute of Chemical Engineers New York, 1985.

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. 

Introduced in the 1980s and now very popular, this technique uses as the solvent liquid carbon dioxide under conditions described as supercritical. In the normal solvent extraction technique, an organic solvent is used that is liquid at room temperature. By application of pressure to the extraction equipment, solvents that are normally gases at room temperature can be compressed and liquiefied (see Organization of matter in Ch. 1). Carbon dioxide, when pressurized in this way, becomes a supercritical fluid at above 33 °C. In the supercritical state it is too hot to be a conventional liquid and too pressurized to be a conventional gas. In this state it has excellent solvent properties for organic molecules. Once the extraction is complete, the pressure can be released and carbon dioxide becomes a gas again. 

Carbon dioxide has most of the properties of a typical covalent compound. It has a low melting point (—79°C). At certain pressures and temperatures, carbon dioxide is a liquid. Liquid carbon dioxide is a weak conductor of electricity. 

Table 2.2 Properties of saturated liquid carbon dioxide.

Using mainly water non-misdble solvents several approaches are possible. In most cases, the organic solvent has to be saturated with water in order not to remove the boundary water surrounding the enzyme, which otherwise results in deactivation. In such microaqueous systems the pH of this tiny amount of water should be carefully chosen for optimal enzyme activity. The control of water activity can be achieved by addition of salts or utilization of saturated salt solutions I81, 821. The simplest way of using an enzyme in organic solvents is to suspend the insoluble enzyme in the required solvent. This technique was first reported in 1900 [83] and has been extended over the last few years to encompass many systems (mainly proteases and lipases) [75, 84, 85L Organic solvents may be replaced by supercritical liquid carbon dioxide, which exhibits similar properties to hexane[86, 146]. 

One of the most important properties of an SCF is that its physical properties can vary dramatically as a result of relatively small changes in temperature and pressure. This effect is most pronounced around the critical point, where the density of carbon dioxide is approximately 0.46 g/mL. If the pressure is doubled, the density of the fluid increases dramatically, reaching a density comparable to that of liquid carbon dioxide (Figure 4.3). 

The phase diagram for carbon dioxide (Fig. 16.58) differs significantly from that for water. The solid/liquid line has a positive slope, since solid carbon dioxide is more dense than liquid carbon dioxide. The triple point for carbon dioxide occurs at 5.1 atm and -56.6°C, and the critical point occurs at 72.8 atm and 31°C. At a pressure of 1 atm, solid carbon dioxide sublimes at -78°C, a property that leads to its common name, dry ice. No liquid phase occurs under normal atmospheric conditions, making dry ice a convenient coolant. 

Solvents are also used in food processing. The largest uses are of aliphatic hydrocarbons, such as -hexane, which are important for oil seed extraction and purifying oils good solvency and ease of separation from the extracted oils (low boiling point) are key properties. Extraction is important in many other parts of the food industry to produce, for example, flavour extracts a variety of solvents from hydrocarbon to alcohols are used in such applications, but volumes are smaller. Isopropanol is used to extract fish protein from ground fish. Methylene chloride was used to extract caffeine from coffee, but has been largely superseded by liquid carbon dioxide. For all human food applications, food contact approval is vital if human consumption is involved. 

Carbon Dioxide (CO2) properties, uses, apphcations, CO2 gas and liquid carbon dioxide, http // www.uigi.com/carbondioxide.html. 

Up till now ultrasound is rarely studied at higher pressures, because in most cases a high static pressure hampers the growth of cavities. Recently, we have shown that cavitation is possible in pressurized C02. Unhke ordinary liquids, carbon dioxide has a high vapor pressure, which counteracts the static pressure.t Cavitation is possible if the difference between the static and vapor pressure is smaller than the maximum acoustic pressure that can be applied. Dense-phase fluids (with a strong emphasis on CO2) provide possibilities for the development of sustainable polymer pro-cesses.t Additionally, ultrasound combined with high-pressure carbon dioxide allows the development of clean routes to produce polymers with specific properties, since no organic anti-solvents are required. 

Negretti Automation supplies the computer-controlled Gibitre low-temperature check apparatus designed to test the low-temperature properties (-73 °C, liquid carbon dioxide), i.e., crystallisation effects and elastic recovery, of rubbers and elastomers according to ASTM D1329 [140] by means of the TR test and brittleness point. ATS FAAR supplies an instrument for carrying out the same measurements. 

Essential oils are secondary metabolites produced in the plants as volatile aromatic principles. They are colorless and rarely colored and soluble in organic solvents. It is fotmd in all part of the plants and is stored in epidermal cells, trichomes, secretory cells, canals, and cavities. They are extracted by the use of liquid carbon dioxide or microwaves, expression and mainly steam or hydrodistillation process. Due to their biological, nutritive, and pharmaceutical properties, they are more attractive alternatives to synthetic chemical products to protect the equilibrium. 

Carbon dioxide is easily liquefiable and has a triple point (see Fig. 6.1). Some of the essential properties of the liquid carbon dioxide are listed below [6.10, 6.11] ... 

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