Gas mixture

Gas mixtures are gases made up of at least two different chemical elements or chemical compounds .

The best known and most common gas mixture is the air in the earth's atmosphere . Gas mixtures are always homogeneous mixtures of substances .

Mixed properties

The physical properties of gas mixtures can be calculated approximately from the physical properties of the individual components through interpolation and mixing rules. In the following denotes the molar mass, the mole fraction and the mass fraction of the -th species (component) of the gas mixture. ${\displaystyle M_{i}}$ ${\displaystyle x_{i}}$ ${\displaystyle w_{i}}$ ${\displaystyle i}$

The following relationships apply to ideal mixtures:

Mean molar mass :
${\displaystyle M=\sum _{i}x_{i}M_{i}=\left(\sum _{i}{\frac {w_{i}}{M_{i}}}\right)^{-1}}$

Mass fraction :
${\displaystyle w_{i}={\frac {x_{i}M_{i}}{M}}}$

Density :(see Amagat's law )
${\displaystyle \rho (T)=\left(\sum _{i}{\frac {w_{i}}{\rho _{i}(T)}}\right)^{-1}}$

Specific enthalpy :
${\displaystyle h(T)=\sum _{i}w_{i}\cdot h_{i}(T)}$

Specific heat :
${\displaystyle c(T)=\sum _{i}w_{i}\cdot c_{i}(T)}$

Specific entropy :
${\displaystyle s(T)=\sum _{i}w_{i}\cdot s_{i}(T)+R\cdot \sum _{i}{\frac {w_{i}}{M_{i}}}\cdot \ln \left({\frac {M_{i}}{Mw_{i}}}\right)}$

Diffusion coefficient :
${\displaystyle D_{im}={\frac {1-x_{i}}{\sum _{j}{\frac {x_{j}}{D_{ij}}}}}}$

${\displaystyle D_{ij}}$is the diffusion coefficient of the species in the component ${\displaystyle i}$ ${\displaystyle j}$

${\displaystyle D_{im}}$the diffusion coefficient of the species in the mixture. ${\displaystyle i}$

Viscosity and thermal conductivity : ${\displaystyle \eta }$ ${\displaystyle \lambda }$
${\displaystyle \eta (T)=\sum _{i}{\frac {x_{i}\eta _{i}(T)}{\sum _{j}x_{j}\Phi _{ij}(T)}}}$

${\displaystyle \lambda (T)=\sum _{i}{\frac {x_{i}\lambda _{i}(T)}{\sum _{j}x_{j}\Phi _{ij}(T)}}}$ (Mixing formula according to Wassiljewa)

According to Mason and Saxena, the correction factors result from the viscosity coefficients and the molar masses of the components: ${\displaystyle \Phi _{ij}(T)}$ ${\displaystyle \eta _{i}(T)}$ ${\displaystyle M_{i}}$

${\displaystyle \Phi _{ij}(T)={\frac {1}{2{\sqrt {2}}}}\left(1+{\frac {M_{i}}{M_{j}}}\right)^{-{\frac {1}{2}}}\cdot \left[1+\left({\frac {\eta _{i}(T)}{\eta _{j}(T)}}\right)^{\frac {1}{2}}\cdot \left({\frac {M_{j}}{M_{i}}}\right)^{\frac {1}{4}}\right]^{2}}$

Adiabatic exponent
The adiabatic exponent of an ideal gas mixture results from the adiabatic exponent of the individual components: ${\displaystyle \kappa }$ ${\displaystyle \kappa _{i}}$

${\displaystyle \kappa ={\frac {\sum _{i}{\frac {x_{i}\cdot \kappa _{i}}{\kappa _{i}-1}}}{\sum _{i}{\frac {x_{i}}{\kappa _{i}-1}}}}}$

Gas mixers and gas mixing systems

Technical gas mixtures are generated with the help of gas mixers and gas mixing systems (other names are also e.g. gas mixers, blenders, gas mixing stations or gas mixing systems) from individual gases or from already mixed gases (gas mixtures), e.g. B. as

• Shielding gas for welding technology (e.g. Ar / CO ₂ )
• Synthetic air for the medicine or chemical industry (mixtures of air with oxygen for enriched air or of air or oxygen with nitrogen for lean air )
• Protective gas for the food industry (e.g. N ₂ / CO ₂ )
• Reaction gas for thermal deformation technology (e.g. Ar / O ₂ / H ₂ )
• Forming gas for steel and rolling mills (e.g. N ₂ / H ₂ ) or protective gas moistened with water vapor ( N ₂ / H ₂ / H ₂ O )
• Forming gas for glass production (e.g. N ₂ / H ₂ )
• Admixture of CO ₂ in biotechnology for gassing fermenters z. B. to regulate the pH of the medium
• Test gas for high analysis accuracy due to high purity and low manufacturing tolerance

These and many other gas mixtures are used in many industries, e.g. B.

• Plant and mechanical engineering
• Automotive industry
• chemistry
• Ferrous / non-ferrous metals
• power supply
• Electroplating
• Glass industry
• Semiconductor technology
• Industrial furnace construction
• Synthetic resins
• Copper processing
• food industry
• Medical technology
• Pharmaceutical industry
• Welding and laser technology
• steel
• Shipyards

Gas mixers are mostly used for mass-produced devices with capacities from a few liters per minute to approx. 500 Nm³ / h. Gas mixing systems, on the other hand, usually refer to devices with higher performance levels, which can be up to 10,000 Nm³ / h and are individually planned and manufactured.

Gas mixers are usually used when one or more gas mixtures are required in larger quantities or mixtures with variable components. In such cases, the use of a local gas mixer or a gas mixing system is more economical than the external supply of gases in gas cylinders or gas bundles. For example, the economical use of gas mixers to generate a typical welding gas mixture (8% CO ₂ in 92% argon ) begins with around five welding points, depending on the gas purchase price and duty cycle. With a normal material flow, these locations would have a monthly gas requirement of 750 liters (corresponds to about 40 bottles of 18L), so that savings can be made when the unmixed gases and mixtures are obtained on site with the help of a gas mixer. The same applies to variable gas points such as hospitals, industrial plants, measuring systems and similar applications with variable scattering.

Gas mixers can be used as static, i.e. H. with one or more control valves or as dynamic systems, i.e. with automatic control valves. The gas mixtures are often checked by a gas analyzer and stored in buffer containers and normalized for pressure before they are sent to the point of consumption.

Types of mixed media

Gas mixers can be designed in four different versions. The difference lies in the mixing technique used and is determined by the application.

• Gas mixer with mechanical mixing valve

• Gas mixer with electronic mixing valve

• Gas mixer with pneumatic mass flow controller

• Gas mixer with mass flow controller

Gas mixer with mechanical mixing valve

Gas mixers with mechanical mixing valves have been in use since the early Middle Ages . With this method, gas mixtures can be reliably generated from all gases . The mechanical mixing valve has gas inlets and a gas outlet for the mixed gas. By turning the valve , the flow rates of the individual gases are regulated in an interplay of orifices and pistons, thus generating the required gas mixture. Gas mixers with mechanical mixing valves are suitable for continuous withdrawal or, with a gas container, also for discontinuous withdrawal.

Gas mixer with electric mixing valve

As with the mechanical mixing process, proportional or individual mixing valves for the inlet gases are moved here. A piston in connection with different orifices regulates the flow rate of the gases and thus produces the desired mixture. In contrast to gas mixers with mechanical mixing valves, the electric mixing valves are not operated by hand using a rotary knob, but by means of electric motors. These electric motors are usually operated via an electronic control.

Gas mixer with pneumatic flow regulator

The gases are mixed pneumatically using a sintered body . Depending on the mixing ratio, the individual gases are offered a surface area of ​​the sintered body of different sizes for flow.

Gas mixer with flow regulator

In contrast to the mechanical gas mixer, the volume flows of each individual gas involved are regulated here. A mass flow controller (MFC) is used for each gas. The volume flow of the gases is recorded in the respective mass flow controller by means of thermal conductivity and then regulated.

The volume flows of the individual gases are only then combined into a mixture. This enables a control that compensates the mass flow of the gases against disturbances such as pressure fluctuations or temperature influences.

Examples

• Breathing gas
• Forming gas
• Protective gas
• Protective gas for food
• Lean air

Classification in the scheme of chemical substances

Schematic classification of the substances

material                                                                                                                               mixture                       Pure substance                                                                                               homogeneous mixture                         connection   element                                                                                           Gas mixture Mixture of several gases   Alloy Mixture with metallic properties , contains at least one metal   Solution Solid ,  liquid , gas dissolved  in a liquid             molecular   Ionic                                                       heterogeneous mixture                                                                                                                             Foam Gaseous bubbles in a liquid   Rigid foam Gaseous bubbles in a solid   Aerosol   Suspension Solid particles in a liquid   Emulsion Mixture of several immiscible liquids   Batch mixture of several not mixable solid                                                                             Smoke Solid particles in a gas   Fog Liquid particles in a gas

literature

1. ^ VDI Society for Process Technology and Chemical Engineering (GVC): VDI-Wärmeatlas; 6th edition, VDI-Verlag, Düsseldorf 1991
2. ↑ Mason, EA, et al. SC Saxena: Phys. Fluids 1 (1958), 361
3. ↑ Industries / References | LT gas technology. In: LT Gas Technology. Retrieved April 7, 2016 . 
4. ↑ Series gas mixer. Retrieved February 17, 2020 . 
5. ↑ Gas mixer. Retrieved September 20, 2019 . 
6. ↑ Bingenheimer, Klaus: The air heaters of the Middle Ages: on the typology and development of a technological-historical phenomenon . Ed .: Verlag Dr. Kovac. Kovač, Hamburg 1998, ISBN 3-86064-788-1 .

Web links

Wiktionary: gas mixture  - explanations of meanings, word origins, synonyms, translations

• Calculation of the thermal conductivity of gas mixtures with improved interaction coefficients, In: Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications , NASA Reference Publication 1311, 1994, p. 22