Carbon dioxide
Carbon dioxide is a carbon oxide with a chemical formula of CO₂ and a chemical formula weight of 44.0095. It is a colorless and odorless gas at room temperature and pressure, and its aqueous solution has a slightly sour taste. It is also a common greenhouse gas and a component of air (accounting for 0.03%-0.04% of the total volume of the atmosphere).
Carbon dioxide can generally be produced by high-temperature calcination of limestone or by the reaction of limestone and dilute hydrochloric acid. It is mainly used to refrigerate perishable foods (solid), as a refrigerant (liquid), to make carbonated soft drinks (gas), and as a solvent for homogeneous reactions (supercritical state). Regarding its toxicity, studies have shown that low concentrations of carbon dioxide are not toxic, but high concentrations of carbon dioxide can cause animal poisoning.
Basic information
| Liquid density | 0.9295kg/L (0℃, 101.3485kPa) |
| Triple point | -56.6℃ (517.97kPa) |
| Preparation method | Prepared by reaction of limestone and dilute hydrochloric acid, etc. |
| Molecular diameter | 0.35～0.51nm |
| Gas density | 1.997g/L (0℃, 101.325kPa) |
| Applications | Refrigeration of perishable foods, refrigerant, and manufacture of carbonated soft drinks |
The natural process of producing carbon dioxide
l Carbon cycle
The carbon cycle refers to the phenomenon that carbon is exchanged in the biosphere, hydrosphere, lithosphere and atmosphere on the earth, and circulates continuously with the movement of the earth. The carbon cycle in the biosphere is mainly manifested in the absorption of carbon dioxide from the atmosphere by green plants, which is converted into glucose and released oxygen through photosynthesis with the participation of water, and then the organisms use glucose to synthesize other organic compounds. Organic compounds are transmitted through the food chain and become part of other organisms such as animals and bacteria.
The process of carbon flowing between the atmosphere, soil, rock formations and plants and animals. Algae and green plants convert carbon dioxide in the atmosphere into carbohydrates through photosynthesis, and then return to the atmosphere through animal respiration, corpse decomposition and combustion. Therefore, the carbon cycle always maintains a balance.
The two largest carbon reservoirs on the earth are the lithosphere and fossil fuels, which contain about 99.9% of the total carbon on the earth.
Basic process
The basic process of the carbon cycle in nature is as follows: carbon dioxide (CO2) in the atmosphere is absorbed by plants on land and in the ocean, and then returns to the atmosphere in the form of carbon dioxide through biological or geological processes and human activities.
Cycle between organisms and atmosphere
Green plants obtain carbon dioxide from the air, which is converted into glucose through photosynthesis, and then synthesized into carbon compounds in plants. Through the food chain, it becomes carbon compounds in animals.
The respiration of plants and animals converts part of the carbon ingested into carbon dioxide and releases it into the atmosphere, while the other part constitutes the organism or is stored in the organism.
After the death of animals and plants, the carbon in the remains is also converted into carbon dioxide through the decomposition of microorganisms and finally discharged into the atmosphere.
It takes about 20 years for the carbon dioxide in the atmosphere to circulate once.
l Exchange between the atmosphere and the ocean
Carbon dioxide can enter the seawater from the atmosphere and enter the atmosphere from the seawater. This exchange occurs at the interface between air and water and is enhanced by wind and waves. The amount of carbon dioxide flowing in these two directions is roughly equal. As the amount of carbon dioxide in the atmosphere increases or decreases, the amount of carbon dioxide absorbed by the ocean also increases or decreases.
Human activities
When humans burn fossil fuels to obtain energy, they produce large amounts of carbon dioxide. From 1949 to 1969, the amount of carbon dioxide generated due to the burning of fossil fuels and other industrial activities was estimated to have increased by 4.8% per year. The result is an increase in the concentration of carbon dioxide in the atmosphere. This disrupts the original balance of nature and may lead to climate anomalies. A small part of the carbon dioxide generated by the burning of fossil fuels and discharged into the atmosphere can be dissolved by seawater, but the increase in dissolved carbon dioxide in seawater will cause changes in the acid-base balance and carbonate solubility balance in seawater.
Forest products
The amount of carbon sequestered by forest products in forest ecosystems is a highly variable factor. Generally, forest products can be divided into short-term products and long-term products according to their service life. Fuel wood and pulp wood are short-term products, while plywood and construction wood are long-term products. The length of the service life of forest products also largely determines the carbon sink function of forest ecosystems. Forest products with a long service life can delay carbon release and alleviate the increase in global atmospheric carbon concentration. Generally speaking, the service life of durable forest products can reach 100 to 200 years. During such a long period of time, a virtuous cycle of carbon can be fully achieved through reforestation. Therefore, durable and long-life forest products should be processed as much as possible.
Global Carbon Pool
Carbon is one of the main elements in living matter and an important component of organic matter.
In summary, there are four major carbon pools on the earth, namely the atmospheric carbon pool, the marine carbon pool, the terrestrial ecosystem carbon pool and the lithosphere carbon pool.
The carbon element is constantly circulating between the major carbon pools such as the atmosphere, land and ocean. Carbon in the atmosphere mainly exists in the form of gases such as carbon dioxide and methane, and in water it is mainly carbonate ions. In the lithosphere, it is the main component of carbonate rocks and sediments, and in the terrestrial ecosystem, it exists in the form of various organic or inorganic substances in vegetation and soil.
Geochemical cycle of carbon
The geochemical cycle of carbon controls the migration of carbon between surface or near-surface sediments and the atmosphere, biosphere and ocean, and is the most important control of atmospheric carbon dioxide and oceanic carbon dioxide.
Biological cycle of carbon
In the biological cycle of carbon, carbon dioxide in the atmosphere is absorbed by plants and converted into organic matter through photosynthesis, and then converted from organic matter into carbon dioxide through biological respiration and bacterial decomposition and enters the atmosphere. The biological cycle of carbon includes the migration of carbon between animals, plants and the environment.
Laboratory-scale CO2 production
Industrial production of CO2
In the 3rd century, Zhang Hua (232-300) of the Western Jin Dynasty in my country recorded in his book “Records of Natural History” that a gas produced during the burning of white stone (CaCO₃) to make lime (CaO) was produced. This gas is lime kiln gas. In addition to CO₂, lime kiln gas also contains CO, SO₂, water vapor, air, dust, etc. After purification, a higher purity carbon dioxide gas can be obtained.
Laboratory and small-scale preparation
Acid and carbonate reaction method
Steps: simple device construction (solid-liquid reactor), gas collection (upward air exhaust method).
Notes: Sulfuric acid is prohibited (CaSO₄ is generated to cover the reactants), hydrochloric acid is preferred (low cost and high efficiency).
Heating decomposition method
Key: The gas needs to be dried (use CaCl₂ or P₂O₅ to remove water).
Measurement and Monitoring of Carbon Dioxide Production
Sensors and Instruments
Infrared Sensors: Infrared sensors work on the principle that carbon dioxide molecules absorb infrared radiation of a specific wavelength. When carbon dioxide is present in the air or in a gas sample, it absorbs infrared light at a wavelength of around 4.26 µm. The sensor measures the amount of infrared radiation absorbed and calculates the concentration of carbon dioxide in the sample based on the Beer-Lambert law. Infrared sensors are widely used in environmental monitoring stations to measure carbon dioxide levels in the environment, in industrial settings to monitor carbon dioxide emissions from processes, and in some laboratory experiments to measure carbon dioxide produced during reactions.
Gas Chromatography: Gas chromatography (GC) is a powerful analytical technique for separating and analyzing components in gas mixtures. In carbon dioxide analysis, a gas sample containing carbon dioxide and other gases is injected into the GC system and the sample is carried by a carrier gas (such as helium) through a chromatographic column filled with a stationary phase. Different gases in the sample interact differently with the stationary phase and therefore elute from the column at different times. Carbon dioxide can be detected by a detector (such as a thermal conductivity detector or a flame ionization detector) and its concentration can be determined by comparing the peak area of the carbon dioxide peak to a known standard. Gas chromatography is commonly used in research laboratories and industrial quality control to accurately measure the purity and concentration of carbon dioxide in gas samples.
Innovative technologies and resource utilization
Oil and gas production increase technology
Using CO2 to increase oil and gas production and efficiency can not only achieve effective storage of CO2, but also relieve water supply pressure, reduce water lock damage and reduce environmental pollution. CO2 geological utilization technologies related to oil and gas production and efficiency increase mainly include CO2 flooding, CO2 drilling, CO2 fracturing and CO2 gas recovery.
CO2 flooding
CO2 can be said to be an efficient and pollution-free oil displacement agent, which is close to some light hydrocarbons in oil and gas reservoirs in terms of molecular thermodynamic characteristics, such as C2H6 and C3H8. In the process of CO2 flooding, on the one hand, CO2 can be fully dissolved in crude oil, which can play the role of solubilization and expansion, viscosity reduction and oil displacement, extraction of intermediate hydrocarbons and heavy hydrocarbons carrying oil; on the other hand, since CO2 is fully dissolved in crude oil and water, it will cause carbonation of crude oil and water, which can improve the oil-water mobility ratio, increase the permeability of the reservoir near the injection well, and inhibit the expansion, dispersion and migration of clay minerals such as 2:1 illite and montmorillonite mixed layers. Its oil recovery effect is also affected by reservoir characteristics. Reservoir physical properties and heterogeneity determine the distribution of fluid in the reservoir during CO2 oil recovery, which ultimately affects the oil recovery rate.
CO2 drilling
In the process of drilling and completion, using supercritical CO2 as a drilling medium is conducive to improving rock breaking efficiency and oil and gas recovery. The supercritical rock breaking mechanism is similar to water jet rock breaking, but the supercritical CO2 rock breaking threshold pressure is lower. In addition, it has high diffusivity and strong mass transfer properties, which can penetrate deep into the micro-cracks of the formation and efficiently transfer jet energy, which greatly increases the degree of rock destruction and achieves rapid drilling. In addition, supercritical CO2 has a higher density than traditional gas drilling fluids such as nitrogen, natural gas and air, and can provide sufficient torque for downhole drilling tools and drive downhole drilling tools more efficiently.
CO2 fracturing
Liquid/supercritical CO2, as a new type of fracturing fluid, has the characteristics of low fracturing pressure, complex fracturing network, and rough fracturing section. Compared with hydraulic fracturing, it can increase the scope of transformation, and the flowback after fracturing is rapid, without residue, and low damage to the reservoir. It has high technical feasibility and input-output ratio, and is considered to be a fracturing technology with great application prospects. CO2 fracturing technology mainly includes CO2 pre-energized fracturing technology, CO2 foam fracturing technology, CO2 quasi-dry fracturing technology and CO2 pure dry fracturing technology.
CO2 gas production mainly uses gravity differentiation and competitive adsorption replacement mechanism to realize the exploitation of natural gas, shale gas and coalbed methane.
| Exploitation Type | Advantages |
| Natural Gas | Replenish formation energy, efficiently displace natural gas. For gas reservoirs with edge/bottom water, inhibit and slow down water invasion, prolonging the water-free production period. |
| Shale Gas | Replenish formation energy, enhance reservoir permeability, and efficiently displace shale gas through replacement. |
| Coalbed Methane | Dissolve organic matter in coal seams to improve coal seam permeability. |
Exploitation of groundwater
Injecting CO2 into deep saline or brine layers to exploit high-salinity water can not only achieve the acquisition of high-value-added liquid mineral resources or water resources, but also release reservoir pressure and seal CO2 in saline layers on a large scale by reasonably controlling the location of pumping wells and the amount of water produced. For areas with scarce water resources, it can alleviate the pressure on water resource sustainability caused by the high water consumption of the energy industry to a certain extent. However, the economic benefits of CO2 exploitation of groundwater are closely related to the cost of CO2. Formulating a reasonable carbon pricing policy can help reduce carbon emissions.
Safety precautions in carbon dioxide production
Pressure vessel operation
Operate strictly in accordance with the process technical parameters and the technical parameters of the pressure vessel design. Overheating and overpressure are strictly prohibited.
Welding, hammering and other operations on pressure vessels are strictly prohibited.
Ventilation and protection
Carbon dioxide gas is an asphyxiating gas and should be in a good ventilation state during operation.
After carbon dioxide poisoning, the poisoned person should be quickly removed from the poisonous area for oxygen inhalation, and high-pressure oxygen treatment should be used if necessary.
Rescuers should wear oxygen breathing masks or isolation gas masks.
l Prevent leakage
Liquid carbon dioxide is easy to frostbite the human body. Low-temperature liquid carbon dioxide equipment and pipeline connections must not leak.
All ammonia systems are strictly prohibited from having leakage points. If leakage occurs, wear a protective flat gas mask, close the valve under supervision, cut off the gas source and deal with it in time.
l Equipment maintenance and repair
When the equipment is not unloaded, any repair work and welding are absolutely prohibited.
Any container or pipeline with explosive or burning gas that needs to be repaired and ignited must be approved by the relevant departments before replacement and blowing. The on-site analysis must be qualified and safety precautions must be taken.
l Temperature control
Try to avoid excessive temperature of the cylinder. The cylinder should be stored in a cool, dry place away from heat sources and should not exceed 31°C to prevent the liquefied carbon dioxide liquid from expanding in volume with the increase in temperature to form high-pressure gas, which will exert greater pressure on the cylinder and cause the risk of explosion.
FAQ
How to prepare CO₂ in the laboratory?
Common method: marble (CaCO₃) reacts with dilute hydrochloric acid (HCl).
Steps:
Solid-liquid room temperature device (Kipp generator or simple conical flask + separatory funnel)
Collect by upward exhaust air method (because CO₂ density > air)
Check fullness: place burning wood sticks at the mouth of the bottle, and it is full when the flame goes out
What are the core production equipment?
Oxygen generator: CO₂ is produced as a byproduct of air separation (suitable for large-scale use)
Gas recovery device: adsorption/membrane separation technology captures CO₂ in industrial waste gas
CO₂ generator: miniaturized equipment that produces gas instantly through chemical reactions (such as acid + carbonate), suitable for mine fire fighting and laboratories.
Purification and compression system: remove impurities such as H₂O and O₂, compress and liquefy for storage
What are the risks of CO₂ production?
Asphyxiation: Concentration > 7% can cause death within 5 minutes (normal air contains 0.04%).
Leakage hazard: heavier than air, easy to accumulate in low-lying areas (cellars and tank areas are at the highest risk)
l How to ensure production safety?
Detection equipment: infrared sensor (such as Riken GX-3R Pro, accuracy ±20ppm).
Ventilation design: forced ventilation in enclosed spaces to avoid CO₂ deposition.
Emergency measures: install fixed alarms (such as SD-1), linkage exhaust system
Source：Carbon dioxide
