Desiccant is also called adsorbent, which is used to prevent moisture and mildew from drying. The desiccants used in daily life to remove water vapor by adsorption method include silica gel, alumina gel, molecular sieve, activated carbon, bone char, or activated clay. Common adsorbents used to remove water vapor by chemical absorption include calcium chloride, quicklime or phosphorus pentoxide, which have a high chemical affinity with water vapor. At present, desiccants are widely used in food, agricultural products, furniture, integrated circuits, precision electronics, leather products, instrumentation, textiles, books and documents, cultural relics, calligraphy and painting, containers, auto parts and other products to prevent moisture from being stored in warehouses, Containers, cartons, wooden boxes and wooden boxes are damaged by moisture and condensation.
The desiccant can be divided into:
- Acidic desiccant: concentrated sulfuric acid, phosphorus pentoxide, silica gel.
- Neutral desiccant: anhydrous calcium chloride, anhydrous magnesium sulfate, calcium sulfate, sodium sulfate, alumina.
- Alkaline desiccant: soda lime (main ingredients CaO and NaOH), quicklime, solid sodium hydroxide, potassium hydroxide, calcium oxide, barium oxide.
According to its redox properties, it can be divided into:
- Oxidizing desiccant: concentrated sulfuric acid, etc.
- Reducing desiccant: iron powder, etc.
Desiccants are divided into the following categories: silica gel desiccants that are purified and processed from natural minerals into granules or beads; the appearance is a small gray ball, which is most suitable for absorbing moisture in an environment below 50℃; artificially synthesized and effective Molecular sieve desiccant with strong adsorption of water molecules; mineral desiccant composed of several natural minerals with an appearance of off-white pellets; composed of plant fiber, with good moisture absorption effect, fiber desiccant with a wider range of applications It is made of calcium chloride and some other natural plant materials through special craftsmanship. It is safe, convenient and practical without taking up space.
Characteristic and application
- Concentrated sulfuric acid: It has strong water absorption and can dry neutral gases such as N2, O2, H2, CH4, CO, and non-reducing acid gases such as CO2, SO2, NO2, Cl2, and HCl. Because of its acidity, it cannot be used for drying Alkaline gases such as NH3 cannot dry reducing gases such as H2S, HBr, HI, and C2H4 (olefins) because of their strong oxidizing properties.
- Phosphorus pentoxide: white powder, said to be "the strongest dehydrating agent in the laboratory", can be used to dry neutral gases such as O2, H2, N2, CO, CH4 and acid gases such as CO2, SO2, NO2, Cl2, HCl .
- Silica gel: It is chemically stable, insoluble in water and any solvent, and has a very fine pore network structure inside, which has a strong adsorption effect on water. It can be used as a desiccant for bagged food and is also commonly used to keep the instrument dry. It can be dried O2, N2, NH3, etc.
- Soda lime and solid sodium hydroxide: it absorbs water quickly and is an excellent desiccant. It is commonly used to dry neutral gases such as N2, O2, H2, CH4, CO, and alkaline gases such as NH3, but not Cl2, HCl, H2S, SO2 , CO2, NO2 and other acid gases.
- Quicklime: white solid, alkaline oxide, can be used to dry neutral and alkaline gases
- Anhydrous calcium chloride: white porous solid, strong drying ability, generally used to fill the dryer, can dry a variety of drugs and most gases such as N2, O2, H2, CH4, CO, CO2, SO2, NO2, Cl2, HCl , Can not be used to dry ammonia, alcohol, amine, acyl, ketone, aldehyde, ester, etc.
- Anhydrous Magnesium Sulfate: It has rapid water absorption and strong drying ability. It is often used to dry organic reagents.
- Calcium sulfate: can dry H2, O2, CO2, CO, N2, Cl, HCl, H2S, NH3, CH4, etc.
- Activated alumina: large water absorption and fast drying speed.
- Anhydrous copper sulfate: It has a certain degree of drying and less water absorption. It is generally used to test the presence of water. It turns blue after absorbing water and is not used as a desiccant.
The table shows the comparison of the drying performance of several commonly used desiccants
|Desiccant||Drying capacity||Regeneration temperature|
|Residual moisture in the air at 20℃ / mg·L-1||Drying speed||Water absorption|
|Activated alumina oxide||0.003||中||
Desiccant is a kind of dewatering agent that absorbs moisture from the atmosphere. Its drying principle is to physically adsorb water molecules in its own structure or chemically absorb water molecules and change its chemical structure to become another substance.
Specifies the size of desiccant particles. The desiccant particles include spherical particles and columnar particles. The size of the spherical particles is the size of the particle size, and the size of the cultured particles is the size of the column length and the cross-sectional diameter. The size of common desiccants is generally on the order of several millimeters.
Particle strength refers to the compressive strength and abrasion loss of the desiccant particles. The desiccant has high particle strength and low abrasion loss during use, and it is not easy to break during use, and the use efficiency of the desiccant is also higher.
Moisture absorption rate
The moisture absorption rate of a desiccant refers to the ratio of the mass of moisture absorbed by the desiccant to the mass of the desiccant under specific conditions.
Moisture absorption rate
Moisture absorption rate refers to how fast the desiccant absorbs moisture. That is, under certain humidity conditions, the time required for the desiccant to reach the hygroscopic saturated state, the shorter the time, the faster the moisture absorption rate of the desiccant. Desiccant does not have very high requirements for moisture absorption rate.
Renewability refers to the characteristic that the desiccant can continue to be used after it has absorbed moisture. That is, the desiccant evaporates the water by heating and other methods after absorbing water, so that the desiccant can be used again.
Whether there is poison, pollution, etc.
The scope and location of the desiccant are specified. The desiccant must not be toxic, corrosive, polluting, radioactive, etc. to the target material to be dried and the surrounding environment. The food industry has relatively high requirements for desiccants.
Desiccant is often used in moisture-proof packaging, such as cameras and photosensitive materials, precision equipment/electrical appliances, food, medicine, shoes, clothes, leather, weapons, telecommunication equipment, etc.; it can also be used for air dehumidification, such as home closets, cabinets, and floors , Musical instruments, etc.; can be used in air drying, such as warehouses, pharmaceutical factories, precision machinery, electronic equipment manufacturing plants, compressed air, instrument air drying. In addition, active mineral desiccants are widely used in products that cannot be sealed in oil or gas phase, such as optical instruments, electronic products, medical health care, food packaging, and dry air sealing of military and civilian products.
The choice of desiccant in daily life
First of all, when using a desiccant, the water absorption capacity and drying performance of the desiccant must also be considered. Water absorption capacity refers to the amount of water absorbed per unit weight of desiccant; drying efficiency refers to the degree of liquid drying to reach equilibrium. For inorganic salt desiccants that form hydrates, the vapor pressure of crystal water after water absorption is used to express. In addition, the drying speed and price should be considered when choosing a desiccant. In addition, the drying of liquid organic compounds usually involves direct contact with the desiccant, so the desiccant used must not chemically react or catalyze the substance, and must not dissolve in the liquid. For example, alkaline desiccants cannot be used for acidic substances, and acidic desiccants cannot be used for alkaline substances. Some desiccants can form complexes with certain dried substances. For example, calcium chloride is easy to interact with alcohols. Amines form complexes and cannot be used to dry these liquids. Strong alkaline desiccants such as calcium oxide and sodium hydroxide can catalyze the condensation and auto-oxidation of certain aldehydes or ketones, as well as the hydrolysis of lipids or amides. Potassium hydroxide (sodium) can also be significantly dissolved in lower alcohols. Secondly, the basic principle of selecting desiccant is that the selected desiccant cannot react with the gas to be dried, and no new substances can be introduced during the drying process. Generally speaking, acidic desiccant can not dry alkaline gas, it can dry acid gas and neutral gas; alkaline desiccant can not dry acid gas, it can dry alkaline gas and neutral gas; neutral desiccant can dry various gases .
Use desiccant to store tea in daily life
The use of desiccant can extend the storage time of tea to about one year. The type of desiccant used can be determined according to the type of tea and the convenience of the material. For storing green tea, use lumpy undeliquescent lime; for storing black tea and scented tea, use dry charcoal; if possible, use discoloring silica gel. When preserving tea leaves with quicklime, you can first wrap the loose tea with thin kraft paper (a pack of several to half a catty), tie it firmly, and arrange the layers in a dry, odorless, intact jar or a rustless, odorless small iron cylinder Place a bag or several bags of unweathered quicklime between the altar and the tube, and then put a few packets of tea leaves on it, and then block the opening of the altar or tube with kraft paper or cotton pad, then close the lid and store in a dry place. Generally, the lime is replaced once every 1-2 months. As long as the lime is replaced on time, the tea will not absorb moisture and deteriorate.
Use desiccant to preserve medicines in daily life.
In order to keep the medicine dry or to dry the produced gas, a desiccant must be used. There are three types of commonly used desiccants: one is acid desiccant, including concentrated sulfuric acid, phosphorus pentoxide, silica gel, etc.; the second is alkaline desiccant, including solid caustic soda, lime and soda lime, etc.; the third is medium Desiccant, such as anhydrous calcium chloride, anhydrous magnesium sulfate, etc. The properties and uses of commonly used desiccants are as follows: Concentrated H2SO4 has strong water absorption and is commonly used to remove moisture in gases that do not react with H2SO4. It is often used as a desiccant for H2, O2, CO, SO2 and other gases; anhydrous calcium chloride It has strong drying ability and is widely used. It has fast drying speed and can be regenerated. The dehydration temperature is 473K. It is generally used to fill dryers and drying towers, dry medicines and various gases; anhydrous magnesium sulfate has strong drying ability and is formed after absorbing water MgSO4.7H2O is commonly used to dry organic reagents; solid sodium hydroxide and soda lime are excellent desiccants with fast water absorption and high efficiency. They are commonly used to dry hydrogen, oxygen and other gases; color-changing silica gel is commonly used to keep instruments and balances dry.
Application of desiccant in daily food packaging
The desiccant is put in the food packaging to reduce the humidity in the food bag to prevent the food from spoiling. Commonly used desiccants in food are calcium oxide (CalciumOxide) and silica gel (SilicaGel). Calcium oxide is what we often call quicklime. It is a white or off-white lump and has a strong function of absorbing moisture in the air. The desiccant is put into food packaging to reduce the humidity in the food bag and prevent the food from spoiling. It is most commonly used in the packaging of children's food. Desiccants in food are generally non-toxic, tasteless, non-contact corrosive, and environmentally free; while laboratory desiccants such as concentrated sulfuric acid, soda lime, etc. are too dangerous, but they also have advantages, such as extremely fast water absorption. High efficiency, low price, etc.
Active mineral desiccant is refined from pure natural raw material minerals and active hygroscopic agent. It is environmentally friendly, non-toxic, tasteless, and harmless to humans. The product has good adsorption performance at room temperature and general humidity, with adsorption activity, static dehumidification and odor removal. And the price is low, the adsorption speed is fast, and the preparation cost is low. Compared with other types of desiccants currently on the market, it not only has fast adsorption speed, high adsorption capacity, and is non-toxic, odorless, non-contact corrosive, and non-environmental pollution. It is especially used in food packaging and is harmless to humans.
Montmorillonite desiccant uses pure natural raw material bentonite, which is environmentally friendly, non-toxic, tasteless, and harmless to humans. It has good adsorption performance at room temperature and general humidity, with adsorption activity, static dehumidification and odor removal. Not only the adsorption speed is fast, the adsorption capacity is high, and it is non-toxic, odorless, non-contact corrosive, and non-environmental pollution. It is especially used in food packaging and is harmless to the human body. It is widely used in dry air sealing of food packaging and military products and civilian products that cannot be sealed by oil seals and gas phase.
Main reference materials
- 刘启飞. 凹凸棒石集装箱新型复合干燥剂的制备[D].安徽理工大学,2016.