Common chemical desiccants can be divided into three categories:
One type is an acidic desiccant, which is rich in H2SO4, phosphorus pentoxide P2O5 (non-dryable NH3), silica gel desiccant, etc.
The second type is an alkaline desiccant, which has a solid caustic soda, a lime desiccant and soda lime (a mixture of sodium hydroxide and calcium oxide);
The third type is a neutral desiccant such as anhydrous calcium chloride desiccant, anhydrous magnesium sulfate and the like.
1. Concentrated sulfuric acid H2SO4: It has strong water absorption and strong oxidizing property. It is commonly used to remove water in gas which does not react with concentrated sulfuric acid H2SO4, that is, acid gas and neutral gas can be dried: for example, as N2, Cl2, H2 , desiccant of O2, CO, CO2, SO2, HCl, CH4 and other gases. However, concentrated sulfuric acid H2SO4 cannot be used to dry basic gases such as NH3 and strong reducing gases such as H2S. (Recommended reaction equation: H2SO4 (concentrated) + H2S = S + SO2 + 2H2O)
2. Anhydrous calcium chloride desiccant CaCl2: There is a complex reaction of the complex, which mainly absorbs water and reacts with ammonia gas, and is widely used because of its low cost and strong drying ability. It has a fast drying speed and can be regenerated with a dehydration temperature of 473K. H2, O2, HCl, CO2, CO, N2, SO2, etc. can be dried. Can not be used to dry ammonia, alcohol, amines, acyl, ketones, aldehydes, esters, etc. [CaCl2+nH2O=CaCl2·nH2O (n can be 8), CaCl2+8NH3=CaCl2·8NH3 (calcium octaammonium chloride, which is a compounding process)]
3. Anhydrous magnesium sulfate MgSO4: It has a strong drying ability and generates MgSO4.7H2O after water absorption. It has a fast water absorption effect, high efficiency and low cost, and is a good desiccant. Often used to dry organic reagents. (Note that magnesium sulfate MgSO4 has a certain coordination ability (Mg2+) for alcohols and amines, so the drying range is limited when the organic matter is dried.)
4, anhydrous sodium sulfate NaSO4: with anhydrous calcium chloride desiccant CaCl2. The drying temperature must be controlled within 30 ° C, and the drying property is far worse than that of the anhydrous magnesium sulfate desiccant, and it is not used much.
5, solid sodium hydroxide NaOH and soda lime: fast water absorption, high efficiency, low price, is an excellent desiccant, but can not be used to dry acidic substances. Commonly used to dry hydrogen H2, oxygen O2, nitrogen N2, methane CH4 and other neutral gases; can dry neutral such as phosphorus pentoxide P2O5; additionally soda lime can dry ammonia NH3; can not dry acid gas: such as hydrogen chloride HCl Gas (because a complex is formed), CO2, SO2, and the like. The same drying agent, barium hydroxide Ba(OH) 2, potassium hydroxide KOH has the same drying performance.
6, calcium sulfate CaSO4: white monoclinic crystal or crystalline powder, odorless, hygroscopic. Very low solubility in hot water, very slowly soluble in glycerin, almost insoluble in ethanol and most organic solvents, soluble in acid, sodium thiosulfate and ammonium salt solution, relative density of 2.32, irritating. It usually contains two crystal waters, CaSO4·2H2O, which exist in the form of gypsum ore in nature. It can dry H2, O2, CO2, CO, N2, Cl2, HCl, H2S, NH3, CH4, etc. (Not recommended) Its water absorption is much weaker than anhydrous magnesium sulfate MgSO4, which is less used as a desiccant and is not recommended.
7. Anhydrous sodium sulfate NaSO4: with anhydrous calcium chloride CaCl2. The temperature must be controlled within 30 ° C, and the drying property is worse than that of anhydrous magnesium sulfate, and it is not used much.
8, activated alumina (Al2O3): the surface is loose and porous, can absorb water. It has a large water absorption, fast drying speed and can be regenerated (400-500K baking).
9. Silica gel desiccant: There are 3 kinds of common (transparent, blue, orange), which are commonly used to keep the instrument and balance dry. After the blue water absorbs red, the orange becomes green after moisture absorption. The transparent silica gel desiccant can be used for drying foods and medicines, and the failed silica gel desiccant can be used after drying and regeneration. Dry amine, NH3, O2, N2, etc.
10, molecular sieve desiccant, the current water absorption capacity is stronger than the silica gel desiccant, and does not contain heavy metals, non-toxic, desiccant in the infrared instrument is the molecular sieve desiccant. The specific scope of application depends on the specific specifications. When using it, it must be optimistic about its molecular diameter.
11. Montmorillonite desiccant (Al2O3·4SiO2·nH2O): It is made of pure natural montmorillonite as raw material, dry and activated, without any additives and solubles. It is a non-corrosive, non-toxic and pollution-free green. Environmentally friendly products, excellent compatibility; can be treated as general waste. Product color: purple, gray, purple. The moisture absorption performance is stable under various temperature conditions, even if it is directly in contact with metal.
