General [3]

Desiccant is also called adsorbent, which is used in moisture and mould proof, and plays a drying role. In daily life, there are silica gel, alumina gel, molecular sieve, activated carbon, activated carbon, activated clay and so on. The common adsorbents used to remove water gas by chemical absorption method include calcium chloride, quicklime or phosphorus pentoxy, which have a large chemical affinity with water gas. At present, desiccant is widely used in food, agricultural products, furniture, integrated circuit, precision electronics, leather products, instrument and instruments, textiles, books and documents, cultural relics, calligraphy and painting, containers, automobile accessories and other products, which can effectively avoid the goods from being damaged by moisture and condensed water in warehouses, containers, cartons, wooden boxes and wooden boxes.

Type [1]

Desiccant can be divided into three parts according to its acid base property:

1. Acid desiccant: concentrated sulfuric acid, phosphorus pentoxy, silica gel.

2. Neutral desiccant: anhydrous calcium chloride, anhydrous magnesium sulfate, calcium sulfate, sodium sulfate, alumina.

3. Alkaline desiccant: alkali lime (main components Cao and NaOH), quicklime, solid sodium hydroxide, potassium hydroxide, calcium oxide, barium oxide.

According to its redox, it can be divided into:

1. Oxidizing desiccant: concentrated sulfuric acid, etc.

2. Reducing desiccant: iron powder, etc.

Classification [3]

Desiccant can be divided into the following categories: silica gel desiccant which is granulated or bead shaped by purification of natural minerals; the appearance is gray ball, which is most suitable for The desiccant of moisture absorbing clay under 50 ℃; the molecular sieve desiccant which is synthesized artificially and has strong adsorption to water molecules; the mineral desiccant composed of several natural minerals and with gray white ball appearance; it is composed of plant fiber, which has good moisture absorption effect. The fiber desiccant with a wide range of applications shall be made of calcium chloride and other natural plant materials through special process It is refined and made, safe, convenient and practical, and does not occupy space of calcium chloride desiccant.

Nature and application [1] [2]

1. concentrated sulfuric acid: it has strong water absorption, which can dry neutral gases such as N2, O2, H2, CH4, Co, and non reductive acid gases such as CO2, SO2, NO2, Cl2 and HCl. Because of its acidity, it cannot be used to dry alkaline gases such as NH3, and H2S, HBr, hi, C2H4 (olefin) and other reducing gases due to its strong oxidation.

2. phosphorus pentoxy: white powder, with 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, etc.

3. silica gel: it is stable in chemical properties, insoluble in water and any solvent, and has a very fine mesh structure inside, which has strong adsorption effect on water. It can be used as desiccant for bagged food, and also used to keep the instrument dry, and can dry O2, N2, NH3, etc.

4. alkali lime and solid sodium hydroxide: it is a good desiccant with fast water absorption, which is often used to dry neutral gases such as N2, O2, H2, CH4, CO and other alkaline gases such as NH3. Acid gases such as Cl2, HCl, H2S, SO2, CO2, NO2, etc. cannot be dried.

5. quicklime: white solid, alkaline oxide, which can be used to dry neutral and alkaline gases.

6. anhydrous calcium chloride: white porous solid with strong drying capacity. It is generally used to fill the dryer. It can dry a variety of drugs and most gases such as N2, O2, H2, CH4, Co, CO2, SO2, NO2, Cl2, HCl, etc. and cannot be used to dry ammonia, alcohol, amine, acyl, ketone, aldehyde, ester, etc.

7. anhydrous magnesium sulfate: it has rapid water absorption and strong drying ability, and is often used to dry organic reagents.

8. calcium sulfate: can dry H2, O2, CO2, Co, N2, Cl, HCl, H2S, NH3, CH4, etc.

9. active alumina: large water absorption and quick drying speed.

10. anhydrous copper sulfate: it has certain dryness and small water absorption. It is generally used to check the existence of water. After absorbing water, it turns blue, and it is not used as desiccant.

Comparison of drying performance of several common desiccants

Table 1 Comparison of drying performance of several commonly used desiccants

Drying mechanism [3]

Desiccant is a kind of water remover which absorbs moisture from the atmosphere. Its drying principle is to absorb water molecules in their own structure or to absorb water molecules by chemical means and change their chemical structure, and then turn them into another substance.

Measurement [4]

1. particle size

The size of the desiccant particles is specified. The size of the desiccant particles is spherical and columnar, the size of the spherical particles is the size of the particles, and the size of the culture particles is the size of the column length and the diameter of the cross section. The size of the commonly used desiccant is usually several millimeter order of magnitude.

2. particle strength

Particle strength refers to the compressive strength and wear loss of the desiccant. The drying agent has high particle strength and low wear consumption in the process of use, and it is not easy to break when used, and the efficiency of desiccant is also high.

3. moisture absorption

The moisture absorption rate of desiccant refers to the ratio of moisture quality adsorbed by desiccant to that of desiccant under specific conditions.

4. moisture absorption rate

Moisture absorption rate refers to the speed and speed of desiccant moisture absorption. That is, under certain humidity conditions, the time required for the desiccant to reach the saturated state of moisture absorption is shorter, indicating the faster the moisture absorption rate of the desiccant. The requirements of desiccant for moisture absorption rate are not very high.

5. regeneration

Regeneration refers to the characteristics that desiccant can continue to use after being treated after moisture absorption. That is, after absorbing water, the desiccant evaporates water by heating, so that the desiccant w can be used again.

6. whether there is poison or pollution, etc

The scope and place of drying agent are specified. The desiccant can not be toxic, corrosive, polluted and radioactive to the dried object and surrounding environment. The food industry has higher requirements for desiccant.

Application [3]

Desiccant is often used in humidity proof packaging, such as cameras and sensitive materials, precision instruments / appliances, food, medicine, shoes, clothing, leather, weapons, telecommunication equipment, etc.; it can also be used for air dehumidification, such as the closet, cabinet, floor, instrument, etc. in the home; it can be used in air drying, such as warehouse, pharmaceutical factory, precision machinery, electronic equipment manufacturer, compressed air drying The instrument air is dry. In addition, active mineral desiccant is widely used in products that cannot be sealed by oil seal and gas phase, such as optical instruments, electronic products, medical care, food packaging and dry air storage of military and civil products.

1. selection of desiccant in daily life.

First, when using desiccant, the water absorption capacity and drying performance of desiccant should be considered. Water absorption capacity refers to the water absorbed by unit weight desiccant; drying efficiency refers to the degree of liquid drying to reach equilibrium. For inorganic salt desiccant forming hydrate, the vapor pressure of crystallized water after water absorption is used to express it. In addition, drying speed and price should be considered for the selection of desiccant. In addition, drying of liquid organic compounds usually contacts the desiccant directly, so the desiccant used must not react with the substance or catalyze it and cannot dissolve in the liquid. For example, acid substances cannot be used with alkaline desiccant, and alkaline substances cannot use acid desiccant. Some desiccants can form complexes with some dried substances. For example, calcium chloride is easy to be used with alcohols. Amines form complexes and therefore cannot be used to dry these liquids. Strong alkaline desiccants such as calcium oxide and sodium hydroxide can catalyze condensation and automatic oxidation of some aldehydes or ketones, and hydrolysis of lipids or amides. Potassium hydroxide (sodium) can also be significantly dissolved in lower alcohols. Secondly, the basic principle of choosing desiccant is that the selected desiccant can not react with the dried gas and new substances cannot be introduced in the drying process. Generally speaking, acid desiccant can not dry alkaline gas, can dry acid gas and neutral gas; alkaline desiccant can not dry acid gas, can dry alkaline gas and neutral gas; neutral desiccant can dry various gases.

2. 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 can be selected according to the type of tea and the convenience of materials. Green tea can be stored with block lime without deliquescence; black tea and scented tea can be stored with dry charcoal; if possible, color changing silica gel can also be used. When storing tea with quicklime, bulk tea can be wrapped in thin kraft paper (in two to half Jin), bound tightly, and arranged in layers around dry and tasteless jars or small iron tubes without rust and tastelessness. A bag or bags of fresh lime without weathering are placed in the middle of the altar and cylinder. Then, a few bags of tea can be placed on it, and then the altar or barrel mouth can be blocked with kraft paper and cotton pad, and then sealed tightly Cover, store in a dry place. Generally, lime is changed once in 1-2 months. As long as lime is replaced on time, tea will not be moisture absorbing and deteriorated.

3. use desiccant to keep medicine in daily life.

In order to keep the medicine dry or dry the gas, desiccant must be used. There are three kinds of desiccants commonly used: one is acid desiccant, such as concentrated sulfuric acid, phosphorus pentoxide, silica gel, etc.; the second is alkaline desiccant, such as solid caustic soda, lime and alkali lime, etc.; the third is neutral desiccant, such as anhydrous calcium chloride, anhydrous magnesium sulfate, etc. The performance and application of common desiccant are as follows: concentrated H2SO4 has strong water absorption, which is often used to remove water from gases not reacting with H2SO4, and is often used as desiccant for gases such as H2, O2, Co, SO2, etc.; anhydrous calcium chloride has strong drying ability and is widely used, with fast drying speed and regeneration, and dehydration temperature of 473k. It is generally used to fill the dryer and drying tower, drying drugs and a variety of kinds of chemicals Gas; anhydrous magnesium sulfate has strong drying capacity, and mgso4.7h2o is generated after water absorption, which is often used to dry organic reagents; solid sodium hydroxide and alkali lime are excellent desiccants with high water absorption and high efficiency, which are often used to dry gases such as hydrogen and oxygen; discoloration silica gel is often used to keep the apparatus and balance dry.

4. application of desiccant in daily food packaging

The purpose of putting desiccant into food packaging is to reduce the humidity in the food bag and prevent the food from spoilage. The desiccants commonly used in food are calcium oxide and silica gel. Calcium oxide, which is called quicklime, is a white or gray white block, which has a strong function of absorbing water in the air. Desiccant is put into food packaging in order to reduce the humidity in food bags and prevent food spoilage. It is most commonly used in children's food packaging. The desiccants in food are generally non-toxic, tasteless, non-contact, corrosive and non polluting; while the desiccants in laboratory, such as concentrated sulfuric acid, alkali lime and so on, are too dangerous, but they also have advantages, such as fast water absorption, high efficiency and low price.

The active mineral desiccant is refined from pure natural raw materials and active hygroscopic agent, which is green and environmentally friendly, non-toxic and tasteless, and has no harm to human body. The product has good adsorption performance at room temperature and general humidity, and has the functions of adsorption activity, static dehumidification and odor removal. The results show that the cost of preparation is low, the price is low, the adsorption speed is fast and the preparation cost is low. Compared with other types of desiccants sold in the market, it has not only high adsorption speed, high adsorption capacity, non-toxic, tasteless, non-contact corrosion, no environmental pollution, especially used in food packaging, harmless to human body.

Montmorillonite desiccant is made of pure natural raw material bentonite, which is green and environmentally friendly, non-toxic and tasteless, and has no harm to human body. It has good adsorption performance at room temperature and general humidity, and has the functions of adsorption activity, static dehumidification and odor removal. It is not only fast adsorption speed, high adsorption capacity, non-toxic, tasteless, non-contact corrosion, no environmental pollution, especially used in food packaging, harmless to human body. It is widely used in food packaging that cannot be sealed by oil seal and gas phase, and dry air seal of military and civil products.