Gaz-Carbonique

Carbon dioxyde

General:
Carbon dioxide is the result of combination of two elements: a carbon atom C and two oxygen atoms O [chemical formula = CO₂].
Molar mass 44.01 g mol^-1.
It is produced by different processes : in particular during aerobic fermentation or combustion of organic compounds, and when the respiration of living beings, and plants. For the latter, photosynthesis trap much more CO₂ than respiration produces.
It is found at low levels in the atmospheric air or is absorbed by plants, which in turn produce oxygen. There is present in an amount approximately equal to 0.04% by volume for the current decade (WMO) or 400 ppmv (parts per million by volume) or 591 parts per million by mass. But it increases rapidly from about 1.43 ppmv / year by human activities consumption of fossil fuels: coal, oil, gas.

VALUES OF THE PARTIAL PRESSURE CO2:
In the atmosphere, it was in the late 1970s 2.10^-4 atm, and twenty years later, due to the evolution of human activities is average 3.4 10^-4 atm and currently 3.89 10^-4 atm (0.394 hPa). It is a greenhouse gas (GHG see here on the Hydro-Land french site) whose relative contribution to this phenomenon would be 49% (methane CH4 18%).

(Sciences et Avenir, 04/2012)

Properties.
CO₂ gas has a slightly irritating odor, is colorless and heavier than air.
In aqueous solution, it forms carbonic acid H₂CO₃ (H₂O + CO₂ <==> H₂CO₃) is too unstable to be isolated easily.

Chart Vapor pressure depending on the temperature:

(P critique=critical pressure)
------------------------------

The phase diagram of carbon dioxide (CO2, or carbonic anhydride) below shows that, unlike water, the solid curve (dry ice) and liquid (liquid CO2) has a positive slope: ice CO2 is denser than the liquefied gas.
Note: Density of the liquid phase (at -20 ° C and 19.7 bar): 1032 kg/m3 - Density of solid: 1562 kg/m3.
Only gas present in four forms: solid, liquid, gas and supercritical.

(reminder: 1 bar = 1000 hPa = 0,1 MPa)

Furthermore, the triple point (temperature = 56.6° C (216.5 K) and pressure: 5180 hPa [5.18 bar]) is greater than the atmospheric "normal" pressure of 1013.25 hPa (1.013 bar ) and therefore the liquid CO2 can not exist at this pressure (it can not melt or boil at this pressure), but it can sublimate has all lower pressures.

The Critical Point is located Pc = 7.375 MPa (73.75 bar) and Tc = 30.95 ° C [304.1 K].
The solid CO2 is called dry ice (because it does not melt, it sublimates). Note that at 1 atm, the temperature of the sublimation of dry ice is -78.2 ° C (194.95 K). The CO2 is normally carried in the liquid state into steel cylinders under pressure (- 20 ° C and 20 bar).

Gas solubility in water (101.3 kPa and 0° C): 1.7 vol / vol.
> Solubility in grams per liter, depending on the temperature (under NTP > 20°C (293.15 K, 68°F) and 1 atm (101.325 kN/m2, 101.325 kPa):

Note - equilibrium,

CO2 gas + H2O <<>> H2CO3

is governed by Henry's constant whose value is (101.3 kPa and 25°C):

K_H = ( [H2CO3] / pCO2 ) = 10^-1,46

with pCO2 = partial pressure of CO2 and [H2CO3] = activity.

- This constant varies with temperature according to the empirical expression:

log K_H = 1,46 + 0,0127 (t-25)

with t, temperature in ° Celsius.

- Note that the activity [H2CO3] represents the real molecule H2CO3 (carbonic acid) and dissolved molecular CO2 present in the form of microbubbles.

Thus for example, in a water surface at 25 ° C, we have:

[H2CO3] = K_H.pCO2 > 10^-1,46 x 3,4 10^-4 = 1,2.x10^-5 (mol/kg),

about 0.53 g of free CO2 per kg pure water, or 0.53 mg / liter.

------------------

Some features:

Inert , colorless and non-flammable.

Equivalent gas / liquid (1013 hPa and 15°C [ kg solid ] ) : 845 vol / vol

Latent heat of vaporization of liquid CO2 (1013 hPa at boiling point ) : 571.08 kJ kg-1

Vapor pressure of liquid CO2 ( 20°C): 5.85 MPa (58.5 bar)

Triple point temperature = -56.6 ° C - Pressure: 5180 hPa (5.18 bar)

Latent heat of fusion of dry ice (1013 hPa , at triple point ) : 196.104 kJ kg-1

Critical point : Temperature: 31°C ( 304 K) and critical pressure: 7.38 MPa (73.8 bar)

Density at the critical point (liquid / gas): 464 kg/m3

Density of gas at 1013 hPa ( 1 atm ) : 2.814 kg m3 -1 at -78°C and 1.87 kg m3 -1 at 15 ° C

Specific heat of gas at constant pressure (Cp ) (1013 hPa and 25°C ) : 37 J / ( mole.K )

Specific heat of gas at constant volume ( Cv) (1013 hPa and 25°C): 28 J / ( mole.K )

Viscosity of gas (1013 hPa and 0 ° C ) : 0.0001372 Poise

Average concentration of CO2 in the air (mid 2010): 0.0389 % by volume (or 0.0585 % by weight) .

The reaction mechanism to water.
The addition of CO2 gas is on the pH increase in free CO2, and thus possibly aggressive CO2 (carbonic acid H2CO3). The pH thus decrease depending on the dose of CO2 added.
TAC (Alkalinity), Calcium, Dry Residue and pHs do not vary.
Note: graphically (Hallopeau method) the representative point of water moves vertically downwards along a line (TAC [Alkalinity] = constant).

Standardization.
Standard products used for the production of drinking water: French official bulletin link - Carbon dioxide, Carbon dioxide NF EN 936.

Uses.
General.
CO2 can be used for many things is the main lights. Many datacenters (full computer servers) or records room is equipped with a system that, in case of fire, filled the room with CO2 as the effect of extinguishing the fire. It is also used in the aquarium for better plant growth. It allows to control the pH of the aqueous effluents (great alternative to control pH with sulfuric acid).
It can also be used in liquid form, such as:

refrigerant to freeze certain foods,
refrigerant in the electronics industry,
extinction in the so-called "carbon dioxide" extinguishing agent, sometimes called dry ice because the liquid CO2 solidifies immediately exit extinguisher producing a white powder propellant (and sometimes also carbonating) for drinks served on draft.

and solid or Dry Ice (also known as Carbonice). The dry ice sublimates, leaving no residue and produced very quickly a large amount of cold ( -78.5 ° C; -109.3 °F; 194.7 K), the man has quickly found him many uses:

in the form of "dry ice pellets" (size of about 3 mm in diameter) essentially for cleaning and pickling cryogenic,
in "sticks" (16 mm rod) for the storage, transport refrigerated products (pharmaceutical products, transport of frozen specimen transport etc..)
- for basic or applied research,
- for winemaking: fermentation slows and prevents oxidation
in "wafer" or "blocks" for aerial refueling (catering), caterers, funeral etc..
in "bread" to thick smoke (special effects, film, video).

Like water, beyond its critical point (see above), carbon dioxide enters a phase called "supercritical".
The gas liquid equilibrium curve is interrupted at the critical point, ensuring a supercritical phase in the continuum of physical and chemical properties without phase change. It is as dense as a liquid phase, but providing transport properties (viscosity, diffusion) near a gas.
The supercritical carbon dioxide is used as a "green" solvent, the extracts are free of traces of solvent:

decaffeination of coffee,
treatment taste of the plug into the cork,
formatting micronised active ingredients and in nanometric form,
extraction of biological or chemical compounds,
purification of chemical compounds (mobile phase in chromatography, including HPLC)
for transport and storage in geological sequestration of carbon dioxide (rare industrial applications)
when maniipulation artificial organs, and in admixture with air or oxygen for respiration boost.
also used for surgical dilation intraabdominal insufflation.

Industrial applications.
Pulp and Paper .
Carbon dioxide finely regulate the pH of the recycled mechanical or chemical after alkaline pulp bleaching. It can also be used in the neutralization of "tall oil" and to improve the operation of paper machines.

Chemical industry.
Carbon dioxide is used in chemical synthesis or for the temperature control of reactors. CO2 is also used to neutralize alkaline effluents.
It is operated under supercritical conditions or to perform operations purifications dyes polymer or vegetable or animal fibers.

Pharmaceutical industry.
Carbon dioxide is used for inerting, chemical synthesis in supercritical extraction (SFE), neutralization (pH) of wastewater or transportation of goods at low temperature (-78°C or -108°F).
Food and Drink .
CO2 is used in the food sector in these main areas:

Carbonation of soft drinks such as sodas, mineral water or beer,
In food packaging, properties inerting and bacteriostatic combine with those of nitrogen and increase the life of foods,
As cryogenic fluid in the operations of cooling or freezing or as dry ice for temperature control during the distribution of food,
Caffeine is removed from coffee by supercritical CO2.

Health.
The CO2 atmosphere produces a near physiological conditions during handling of artificial organs.
It is used in admixture with air or oxygen for respiration boost.
It is also used for surgical dilation intraabdominal insufflation.

Metal industry

Carbon dioxide is commonly used for environmental protection:

CO2 is used to remove the reddish smoke while loading the scrap and coal, to reduce the intake of nitrogen during casting from an EAF and to stir the mixture at the sole,
In the non-ferrous metallurgy, carbon dioxide is used to remove the fumes during transfer matte (production of Cu / Ni) or noble metal ingots (production of Zn / Pb) in the ladle,
Small amounts of liquid CO2 may be used in the methods of recycling water from the drains of the mines,
CO2 lasers are powered by suitable carbonic gas purity.

Laboratories and analyzes.
CO2 is used in the mobile phase extraction processes or supercritical fluid chromatography.

Electronic.
Carbon dioxide is commonly used in the wastewater treatment or as a cooling medium in the climatic test of electronic components.
It can be used to increase the conductivity of the ultrapure water or for the abrasive cleaning of parts in the form of dry ice in the cleaning process own photoresists supercritical CO2 in order to avoid the use of organic solvents.
Environment.
The injection of carbon dioxide makes pH control of the aqueous effluents. CO2 is a great alternative to control pH with sulfuric acid.
Other industries.
Carbon dioxide extinguisher.
Control and regulation of the pH of the waste water, swimming pools, etc.

MODE OF PROCUREMENT:
Carbon dioxide can be distributed:

bottled
in tanks for liquefied gas
solid ("dry" ice temperature)
Note: Dry ice sublimates, leaving no residue, and quickly produces a large quantity of cold (150 frigories/kg/h to -78.5°C)
- Reminder: 1 frigorie (fg) = -4185.106 joules and exact opposite of the thermie (th).The thermie is equivalent to 1,000 kilocalories, 4.1868 megajoules or 3968.3 BTU.

Major risks:

Produced under high pressure; inhalation
Toxicity (Am. Ind. Hygienists ACGIH Conf Of 2000 Edition Gov...): 5000 ppm
Flammability limits in air (STP normal conditions): Non flammable
Odor: None
Code UN : UN1013 (gaz); UN2187 (refrigerated liquid); UN1845 (solid)
EINECS Number : 204-696-9
U.S. Code for transport (DOT Label): NFG
Hazard Class USA (DOT Hazard class): Nonflammable gas

Link (Canadian) on the effects of CO2 for health

MATERIAL SAFETY DATA SHEET (French INRS): pdf file to download possibly (71 kb).

(use your browser)

Sources part of this document:: >>> Air Liquide