Degree of division

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The degree of division , also known as granularity , is a term used in chemical and mechanical process engineering . It describes the ratio of the surface to the volume of a substance .

Geometric characterization

For a given volume, a sphere has the smallest possible surface and thus the smallest degree of division. A powder made of the same material with the same mass, on the other hand, has a very large surface and thus a very high degree of division.

Math background

The volume increases to the third power of the edge length of a cube or the radius of a sphere, but the surface only increases to the second power.

A simple idea of ​​this can be obtained by mentally dividing a cube with three cuts (halving in width, length and height) into 8 small cubes with half the edge length; Then, in addition to the initial six square outer surfaces, there are six more equally large cut surfaces; so the surface has doubled.

Biological background

Metabolic processes between different systems run faster if the corresponding biochemical reactions are favored by increasing the surface area. An example is the biconcave shape of erythrocytes , which facilitates the transport of oxygen.

Example calculation for a molecular breakdown

The following extreme example can be used to mentally demonstrate the degree of division down to the molecular level:

One mole of water ( molar mass = 18 g · mol −1 ) has a volume of 18 cm³ ("a badly filled shot glass"). An ice cube made from it has an edge length of 2.62 cm with six side surfaces each approx. 6.87 cm²; the total surface of the ice cube is 41.2 cm².

According to definition (see Avogadro's number ), one mole of water contains approx. 6.022 · 10 23 water molecules. The third root of this is 8.44 · 10 7 . This is how many water molecules (around 84,445,000) form one edge of the cube. If you cut this cube into so many (over 84 million) “slices” in all three spatial directions, you will win “each water molecule individually”. More than 84 million · 6 = 507 million new areas (each 6.87 cm²) were created with a total of 3,480,000,000 cm² or 348,000 m² - that is a total surface of approx. 50 soccer fields!

Measurement

The degree of division is used to classify solids of different grain sizes.

Material with different degrees of division is, for example, separated (" classified ") by sieving , as is mixtures of substances with different degrees of division.

The sieve analysis is used for the quantitative measurement of the grain size distribution ( grain size analysis , grain size classification ) , the bulk goods according to DIN 66165 with sieve mesh sizes in the gradations 1 - 0.71 - 0.5 - 0.355 - 0.25 - 0.18 - 0.125 - 0.09 - 0.063 - <0.063 mm mesh size classified. The distances correspond to the decadic logarithm; This ensures that the distances on a logarithmic grain size scale are the same.

The sieve analysis leads to slightly different results for different bulk solids, because the shape of the particles plays a role: Long, thin particles also pass through small meshes despite their relative size. For this reason, all such measurements and grain size specifications are never exactly comparable values.

Further characteristic values ​​for the degree of division of a bulk material are the numerical ratios “bulk weight to material density” and “vibrating weight to bulk weight”; from them, exact values ​​for the sum of the (air) gaps remaining in the bulk material can be read off.

Applications

Depending on the degree of division of the disperse (distributed) phase of a multiphase substance, a distinction is made between coarsely disperse substances ( emulsion , suspension ), colloidal substances (glue-like solutions ) and molecularly disperse substances (real solution).

With the degree of division, the surface of the substance increases and the probability of an encounter with the reaction partner increases. Substances with a high degree of fragmentation (large reactive surface) react faster (and therefore more violently) than substances with a low degree of fragmentation.

The activation energy for a reaction is independent of the size of the reacting particles, but it can be reached locally in the case of small particles without being immediately dissipated by conduction, as is the case with a large body. This is especially true for molecularly distributed substances, where more often than with larger particles some molecules are in an energetic state above the required activation energy due to the Maxwell-Boltzmann distribution and can initiate a reaction.

Examples

  • For example, wood chips are easier to light than a thick log, iron powder easier than an iron nail. The degree of division of the fuel therefore plays an important role in assessing the combustibility (see diesel engine ).
  • Another example of the dependence of the reaction rate on the degree of fragmentation is the enzymatic decomposition of food.
  • Because of its low degree of division, rock candy takes much longer to dissolve in water than the same amount of fine-grained sugar (or lump sugar, which despite its compact shape has a high degree of division due to the many cavities).
  • The grading curve of the aggregate , which is used as an aggregate or filler for concrete and mortar , has a major influence on the material properties of the set material and its processability.

See also

Other meanings

  • The term degree of division is also used synonymously for the feathering of leaves, see also leaf shape .
  • The term degree of division is introduced as a measure of the quantification of landscape division. The fragmentation of the landscape influences the dispersal behavior of animals and their habitat, e.g. B. through settlements, roads, canals etc.
  • The term degree of comminution also comes from chemical and mechanical process engineering, but has a completely different meaning. It is a measure of the effectiveness of a crushing process and is calculated from the diameter of the particles before and after the crushing process.

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