What Is Differential Absorption?

Absorption (English: absorption, Japanese: ) is generally used in medical science, physics, and biology, as opposed to production and excretion.

Chinese name: absorb
Pinyin: x shu

Explanation: Suck in something from the outside
Special: Pokemon grass primary skill name

Interpretation of absorbed words

1. The object absorbs certain substances from the outside into the body. The nutrients and water needed by the normal human body are absorbed into the human body through the digestive tract. 2. Acceptance. 3. The process by which the body ingests nutrients from the environment into the body. 4 The phenomenon that a substance enters from one medium phase to another. In physics, it is the process that the energy of a photon is owned by another object, usually an atomic electron, so electromagnetic energy is converted into other forms, such as thermal energy. In the process of wave conduction, the absorption of light is often called attenuation. For example, the valence electrons of an atom are switched between two different energy levels. In the process, the photons will be destroyed, and the absorbed energy will be released again in the form of radiant or thermal energy. Although in some cases (usually in optics), the medium will change its transparency due to the intensity of the passing wave and saturation absorption (or non-linear absorption), but in general, the absorption of the wave is independent of the intensity (linear absorb).

Absorption application range

In meteorology and climate, global and regional temperatures depend to a large extent on atmospheric gases (such as the greenhouse effect) and absorption and radiation from the ground.
In medicine, different tissues have different levels of absorption of X-rays (especially bones), which is the basis of X-ray imaging. A specific example is the calculation of atmospheric radio wave attenuation used in satellite connection design.
In chemistry and materials science, different materials and molecules have different degrees of absorption at different frequencies. These parameters can be used for material identification.
In optics, sunglasses, color filters, dyes, and other materials in this category are designed to absorb a certain amount of visible wavelengths.
In biology, photosynthetic organisms need to absorb wavelengths suitable for the range of chloroplast activity, so that the energy of light can be converted into sugar and chemical energy in the molecule

Absorption biology terms

Absorption of nutrients

After the substance is digested in the digestive tract, its breakdown products enter the blood and lymph through mucosal epithelial cells. Food is digested in the digestive tract and eventually decomposed into glucose, amino acids and other substances that can be absorbed by the body.

Absorption process

The process by which the body ingests nutrients from the environment into the body. Single-celled animals ingest nutrients directly from the living environment; in the digestive tract (lumen) of multicellular animals, the digestive products of various foods, water, salts and other substances enter the blood and lymph through the gastrointestinal epithelial cells, and the spine The retransportation of substances in the renal tubules of animals to the blood is considered absorption. There are many ways of absorption, but they are all for the purpose of supplying nutrients to the body and keeping the body environment constant.

Absorption: The process by which a substance absorbs other objects or energy. Gases are sucked up by liquids or solids, or liquids are sucked up by solids. During the absorption process, one substance sucks another substance into the body to melt or combine with it. For example, sulfuric acid or lime absorbs water, blood absorbs nutrients, blankets, mineral wool, soft fiberboard and expanded perlite can absorb noise. Chemical wood pulp or cotton pulp is used to make rough paper blotting paper. To dry ink, solids that absorb gas or liquid often have a porous structure. When sound waves, light waves, and electromagnetic radiation are projected onto the surface of the medium, part of it is reflected by the surface and part of it is absorbed and transformed into other forms of energy. When the energy travels in a certain direction in the medium, it is gradually absorbed by the medium with the incident depth. .

Absorption mechanism

For single-celled organisms and higher animals, the process of nutrient absorption is through the passage of substance molecules into the cell membrane, or from the cell through the cell membrane on the other side to leave the cell and enter the tissue fluid or blood. With the evolution of organisms, the special absorption mechanism for the specificity of different substances has become more important. Taking mammalian small intestine absorption as an example, the general mechanism of absorption can be summarized as simple diffusion, facilitated diffusion, and active transport.

absorb Pure diffusion (free diffusion). That is, molecules of a substance enter a region of low concentration from a region of high concentration. The cell membrane is a layer of lipid membrane between the intracellular fluid and extracellular fluid. Therefore, only lipid molecules that can dissolve in the lipid can diffuse from the high concentration side of the membrane to the low concentration side (also known as dispersion). The absorption process in the simple diffusion method does not consume energy, and the substance molecules move according to a concentration gradient or a potential gradient. Diffusion alone is not an important way for the small intestine to absorb nutrients.

Facilitate diffusion (assisted diffusion). Substance molecules are assisted by specific protein molecules (carriers) in the cell membrane through the diffusion process of the cell membrane. This facilitated diffusion, like simple diffusion, is also transmitted from the high concentration side to the low concentration side through the membrane. . This is how certain non-fat-soluble substances are absorbed. Facilitating diffusion does not require the consumption of metabolic energy.

Active transfer (active transport). A substance that needs to consume the energy of cell metabolism and can be transported through a membrane against an electrochemical gradient. Small intestine

absorb The glucose and amino acids are transported in an active manner against the concentration difference. Endocytosis. Germline occurs on the way raw food is consumed. It is through the indentation of the cell membrane to surround the food particles or extend the pseudofoot to roll the food particles into the cells. The small intestine can absorb some macromolecular substances and substance clumps, such as intact proteins and triglycerides, by endocytosis.

The absorption capacity of different parts of the digestive tract is very different, which is mainly related to the tissue structure of the various parts of the digestive tract, the length of time the food stays in the part, and the degree of food decomposition. Under normal circumstances, the oral cavity and esophagus basically have no absorption function, and the stomach can only absorb a small amount of water, inorganic salts and alcohol. The small intestine absorbs glucose, amino acids, glycerin, fatty acids, most of the water, inorganic salts and vitamins. Most of the nutrients have been absorbed in the small intestine. When the contents of the small intestine enter the large intestine, there are not many substances that can be absorbed. The large intestine mainly absorbs water, inorganic salts and some vitamins.

Absorption

Absorption of sugar Alcohol is absorbed almost exclusively in the small intestine as a monosaccharide. The absorption of glucose and galactose is very fast, and it can carry out the typical active transport mode against the concentration gradient. The absorption of some sugars, such as sorbose, xylose, and arabinose, is a simple diffusion process, while fructose is somewhere in between. The active absorption of glucose requires the presence of Na +. In the absence of Na +, glucose is absorbed in a manner that facilitates diffusion. It is generally believed that the mechanism of active glucose uptake lies in the coupling of glucose and Na + transport. Their relationship is: the carrier on the brush border membrane of small intestinal epithelial cells combines with Na +, and then with glucose to form a "Na + -glucose-carrier" complex, that is, the carrier can only be carried with Na + Combined with glucose. The "Na + -glucose-carrier" complex relies on the electrochemical gradient of Na + to migrate from the intestinal lumen surface of the membrane into the intracellular fluid. The high concentration of glucose in the cell enters the subcellular space from the bottom of the cell, and then enters the blood (Figure 2).

Absorption of protein The protein that is eaten is almost completely hydrolyzed by digestive enzymes into amino acids before it can be absorbed. Amino acid absorption is also active transport, and its mechanism is similar to glucose absorption.

Fat absorption Fat or triglyceride is decomposed into 2 molecules of free fatty acid (FFA) and 1 molecule of glycerol ethyl ester by pancreatic lipase in the intestinal lumen. These two decomposition products are fat-soluble. Before absorption, they must form water-soluble micelles with bile acids before they can pass through the water layer covering the surface of the intestinal epithelium. After such lipid micelles reach the absorption surface, FFA and monoglyceride are separated, and both can diffuse into the cell through the cell membrane. Fatty acids and monoglycerides re-synthesize triglycerides in the smooth endoplasmic reticulum of the cell. Non-polar substances such as triglycerides and cholesterol esters are used as the core, and apolipoproteins and phospholipids, which are relatively polar, are used as a shell to form chylomicrons. The size of chylomicrons is quite inconsistent, ranging from 750 to 5000 angstroms. When apolipoprotein and phospholipid synthesis is impaired, the volume of chylomicrons increases. Chylomicrons are packaged as secretory vesicles in the Golgi apparatus. The vesicles move to the side membrane of the cell and fuse with the side membrane. They are released into the paracellular space by exocytosis, pass through the basement membrane and the intrinsic membrane and finally enter the lymph Tube, so far only complete the entire process of fat absorption.

Absorption Pharmacokinetics Terms

Absorption: The process by which drugs enter the blood circulation from the site of administration is called absorption. Except for those who are directly injected into the blood vessels, they generally need to be transported through the cell membrane. Subcutaneous or intramuscular administration is absorbed through the capillary wall alone. Oral administration first needs to pass through the gastrointestinal mucosa. Weak acid drugs can be absorbed in the stomach, but most of them are absorbed in the intestine. They pass through capillaries in the gastrointestinal tract and enter the portal vein of the liver first. After inactivation and metabolism of certain drugs through the intestinal mucosa and liver, the amount of drug entering the systemic circulation decreases. This is called the first-pass effect. For example, about 99% of oral nitroglycerin can be inactivated by the first-pass effect and fail to use it. The medicine can pass through the portal vein of the liver, causing less damage and acting faster. In addition, factors affecting absorption include pH, solubility, administration site, and bioavailability (formulation), etc., which all make absorption significantly different.

Absorption chemistry term

In the chemical industry, it is often necessary to separate the components of a gas mixture in order to:

Recover or capture useful substances in the gas mixture to make products;

Remove the harmful components in the process gas to purify the gas for further processing; or remove the harmful substances in the exhaust gas exhausted by the industry to avoid polluting the atmosphere.

The actual process often has both the purpose of purification and recycling.

The separation of a gas mixture is always carried out based on some physical and chemical differences between the components in the mixture. According to the differences in different properties, different separation methods can be developed. Absorption is only one of them. It achieves the purpose of separation according to the solubility of each component of the mixture in a certain solvent.

Absorption process

As shown in Figure 3

A + B mixed gas is absorption tail gas

S solvent

A + S is called absorption liquid

A solute

B is called inert gas (chemical term, pay attention to distinguish from concepts in elementary chemistry) or inert component

Absorption industry absorption process

Now take gas debenzene as an example to explain the flow of absorption operation (Figure 4).

In the production process of coking and city gas production, coke oven gas containing a small amount of benzene and toluene low-hydrocarbon vapor (about 35 g / m3) should be separated and recovered. The absorption solvent used is a by-product of the production process of the process, that is, the refined product of coal tar is called washing oil.

The process of recovering benzene-based substances includes two parts: absorption and desorption. The benzene-containing gas enters the absorption tower from the bottom at normal temperature, the washing oil is poured from the top of the tower, and the tower is filled with wooden grids and other fillers. During the contact between gas and washing oil, the benzene vapor in the gas was dissolved in the washing oil, so that the benzene content of the gas leaving the top of the tower was reduced to a certain allowable value (<2 g / m3), and more benzene was dissolved. The solute washing oil (called rich oil) is discharged from the bottom of the absorption tower. In order to remove the benzene in the rich oil and enable the washing oil to be reused (called the regeneration of the solvent), the opposite operation to the absorption is performed in another device called a desorption tower: analysis. To this end, the rich oil can be preheated to about 170 ° C and poured from the top of the desorption tower, and superheated steam is introduced into the bottom of the tower. The benzene in the wash oil escapes at high temperature and is taken away by water vapor. The water is removed by condensation and layering, and finally a benzene liquid (crude benzene) can be obtained. After that, it can be sent to the absorption tower for reuse as an absorption solvent.

It can be seen that the following problems must be solved to achieve the separation of gas mixture by absorption operation:

Select a suitable solvent to enable selective dissolution of one (or some) separated components;

Provide proper mass transfer equipment to achieve gas-liquid two-phase contact, so that the separated components can be transferred from the gas phase to the liquid phase (absorption) or vice versa (desorption);

The regeneration of the solvent means removing the separated components dissolved therein for recycling.

In short, a complete absorption and separation process generally includes two components: absorption and desorption.

Choice of absorption solvent

Absorption operation is the process of contact mass transfer between gas and liquid phases. The success of the absorption operation is largely determined by the nature of the solvent, especially the phase equilibrium relationship between the solvent and the gas mixture. According to the knowledge of phase equilibrium in physical chemistry, the main basis for evaluating the quality of solvents should include the following points.

(1) The solvent should have greater solubility for the separated components (hereinafter referred to as the solute) in the mixed gas, or the equilibrium partial pressure of the solute should be low at a certain temperature and concentration. In this way, from the perspective of equilibrium, the amount of solvent required to process a certain amount of mixed gas is less, and the limiting residual concentration of the solute in the gas can also be reduced; as far as the process rate is concerned, the solute equilibrium partial pressure is low, the process driving force is large, and mass transfer Fast speed, small size of required equipment.

(2) The solubility of the solvent to other components in the mixed gas should be small, that is, the solvent should have high selectivity. If the selectivity of the solvent is not high, it will absorb other components in the gas mixture at the same time. Such an absorption operation can only achieve a certain degree of concentration between the components and cannot achieve a more complete separation.

(3) The solubility of the solute in the solvent should be more sensitive to changes in temperature, that is, not only the solubility at low temperature is large, the equilibrium partial pressure is small, but as the temperature increases, the solubility should decrease rapidly, and the equilibrium partial pressure should increase rapidly. In this way, the absorbed gas is easily desorbed and the solvent is easily regenerated.

(4) The vapor pressure of the solvent should be low to reduce the volatilization loss of the solvent during absorption and regeneration.

(5) The solvent should have better chemical stability to avoid deterioration during use.

(6) The solvent should have a low viscosity, and it is not easy to generate foam during the absorption process, so as to achieve good gas-liquid contact in the absorption tower and gas-liquid separation at the top of the tower. If necessary, a small amount of a defoamer can be added to the solvent.

(7) Solvents should meet economic and safety conditions such as being cheap, readily available, non-toxic, and non-combustible.

In fact, it is difficult to find an ideal solvent that can meet all these requirements. Therefore, a comprehensive evaluation of the available solvents should be made in order to make an economical and reasonable choice.

Physical absorption and chemical absorption

The absorption operation in which each component in the gas is separated due to the difference in physical solubility in the solvent is called physical absorption, and the above-mentioned gas debenzene is an example. The binding force between solute and solvent in physical absorption is weak, and desorption is more convenient.

However, in general, the solubility of a gas in a solvent is not high. With appropriate chemical reactions, the solvent's ability to absorb gases can be greatly improved. For example, the solubility of carbon dioxide in water is very low. However, if carbon dioxide is absorbed in an aqueous solution of potassium carbonate, a combination reaction of potassium carbonate, carbon dioxide and water to generate potassium bicarbonate occurs in the liquid phase, so that the aqueous solution of potassium carbonate has a higher absorption of carbon dioxide. Ability. At the same time, the high selectivity of the chemical reaction itself must confer high selectivity to the absorption operation. It can be seen that the use of chemical reactions greatly expands the scope of application of absorption operations. Such operations using chemical reactions to achieve absorption are called chemical absorption.

Chemical reactions that can be used as chemical absorption should generally satisfy the following conditions.

(1) Reversibility If the reaction is irreversible, the solvent will be difficult to regenerate and recycle. For example, when sodium carbonate is used to absorb carbon dioxide, it is difficult to regenerate because of the formation of sodium carbonate, and a large amount of sodium hydroxide is bound to be consumed. Naturally, it is another matter if the reaction product itself is the product of a process.

(2) Higher reaction rate If the chemical reaction used is slower, it should be studied to add a suitable catalyst to speed up the reaction rate.

Absorption measurement

Absorption is a quantification of how much light is absorbed by an object (not all photons are absorbed, and some are replaced by reflection or refraction). This is related to some properties of matter, which can be deduced through the Bill-Lambert law.

Accurately measure the amount of absorption at various wavelengths. With the aid of absorption spectroscopy, you can identify the characteristics of a substance, let light enter from one side of the sample, and measure the intensity of light leaving the sample in all directions. Examples of UV-Vis, IR, and X-ray absorption spectra are examples of absorption spectra in different parts of the spectrum.

Economics of Absorption Operation

The operating costs absorbed include:

Energy consumption of gas and liquid flowing through the absorption equipment;

Loss of solvent evaporation and deterioration;

The regeneration cost of the solvent is the desorption operation cost.

Of these three, the proportion of regeneration costs is the largest.

Common desorption methods include heating, decompression, and blowing. Among them, warming and blowing are especially common. The solvent is circulated between the absorption and desorption equipment, and heating and cooling, pressure relief and pressurization during this time will consume more energy. If the solubility of the solvent is poor and the concentration of the solute in the solvent leaving the absorption device is low, the required amount of solvent circulation must be large and the energy consumption during regeneration is also large. Similarly, if the solvent's ability to dissolve is not sensitive to temperature changes and the required desorption temperature is high, the energy consumption for solvent regeneration will also increase.

If the solution after absorbing the solute is the product of the process, the regeneration of the solvent is no longer required at this time, and this absorption process is naturally the most economical.

Absorptive contact method

Absorption equipment comes in many forms, but towers are the most commonly used. According to the different contact methods of the device and the liquid, the absorption equipment can be divided into two types: step contact and differential contact. Figure 5 is a schematic diagram of a typical absorption tower in these two types of equipment.

In the plate-type absorption tower shown in Fig. 5a, the gas and liquid are contacted countercurrently in steps. The gas rises from bottom to top through the small holes on the plate and comes into contact with the solvent on each plate, where the soluble components are partially dissolved. In this type of equipment, the concentration of soluble components decreases stepwise with each rise of gas in a tray; the concentration of soluble components decreases stepwise as the solvent decreases from plate to plate. However, the absorption process performed in the stepwise contact process may not change with time, and is a steady state continuous process.

In the device shown in FIG. 5b, the liquid flows down the wall in a film shape, which is a wet wall tower or a falling film tower. It is more common to fill the tower with a packing such as a ceramic ring. The liquid evenly drips from the top of the tower and flows down the surface of the packing. The gas rises through the gap between the packing and makes continuous countercurrent contact with the liquid. In this kind of equipment, the soluble components in the gas are continuously absorbed, and their concentration decreases continuously from bottom to top; the opposite is true for liquids, where the concentration of soluble components is continuously increased from top to bottom, which is Differential contact type absorption equipment.

The two types of equipment of stage and differential contact are not only used for gas absorption, but also used for other mass transfer unit operations such as liquid distillation and extraction. Two types of equipment can use completely different calculation methods.

Steady and non-stationary operation

The absorption or other mass transfer processes carried out in the two different contact methods of the mass transfer equipment can be a steady state continuous process, that is, the process parameters in the equipment do not change with time; they can also be non-stationary, that is, intermittent operation Or pulsed operation.