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Solvent refining refers to a process of removing impurities and non-ideal components contained in a raw material (or semi-finished product) by extraction. It is also a common process for waste oil regeneration.

Solvent

The term broadly refers to an excess of the component contained in a homogeneous mixture. In a narrow sense, a liquid that does not undergo any change in chemical composition and that dissolves other substances (generally referred to as solids), or a liquid that chemically reacts with a solid and dissolves the solid. The homogeneous mixture system formed by dissolution is called a solution. The excess component in the solution is called a solvent; the lesser component is called a solute. The solvent is also referred to as a solvent, that is, a medium containing a dissolved solute. However, the solvent referred to in the industry generally means a single compound or a mixture of two or more kinds which is capable of dissolving fats and oils, waxes, resins (most of which are insoluble in water) to form a homogeneous solution. Such solvents other than water are referred to as nonaqueous solvents or organic solvents, and water, liquid ammonia, liquid metals, inorganic gases, and the like are referred to as inorganic solvents.

Solvent solvency judgment

The solvent’s ability to dissolve, in short, refers to the ability to dissolve a substance, that is, the ability of a solute to be dispersed and dissolved. It is generally measured in terms of solubility in aqueous solution, which is only suitable for dissolving low molecular crystalline compounds. For solutions of organic solvents, especially high molecular substances, the solubility is often expressed in the rate of formation of a certain concentration of solution and the viscosity of a certain concentration of solution, which cannot be clearly expressed by solubility. Therefore, solvent solvency should include the following aspects:

(1) The ability to disperse a substance into small particles; (2) The rate at which the substance is dissolved; (3) The ability to dissolve the substance to a certain concentration; (4) The ability to dissolve most substances; (5) The ability to mix with a diluent to form a mixed solvent. Industrial methods for determining the solvent dissolving ability include a dilution ratio method, a constant viscosity method, a viscosity phase diagram method, a shell rosin-butanol (dissolution) test, and an aniline point test.

Solvent classification

1. Classified by boiling point

(1) Low boiling point solvent (boiling point below 100 ° C) These solvents are characterized by fast evaporation, easy drying, low viscosity, and mostly aromatic odor. Typical of such solvents are active solvents or diluents. For example: methyl ether, diethyl ether, propyl ether, methanol, ethanol, propanol, isopropanol, ethyl acetate, acetone, and the like. (2) Medium boiling point solvent (boiling point is 100-150 ° C) These solvents are used for nitro spray paint and have good leveling performance. For example: butanol, isobutanol, pentanol, cyclohexanone, amyl propionate, methyl stupid and the like. (3) High-boiling solvent (boiling point in the range of 150-200 ° C) These solvents are characterized by slow evaporation rate and strong dissolving power. When used as a coating solvent, the coating film has good fluidity and can prevent precipitation and whitening of the coating film. For example: benzyl alcohol, decyl alcohol, cyclohexanone, ethyl lactate, ethyl benzoate and the like. (4) Plasticizers and softeners (boiling point at 300 ° C) These solvents are characterized by a good bond strength and toughness of the formed film. For example, camphor for nitrocellulose, dimethyl phthalate for ethyl cellulose, dioctyl phthalate for polyvinyl chloride, and the like.

2. Sort by evaporation speed

(1) Rapid evaporation of solvent The evaporation rate is more than three times that of butyl acetate, such as acetone, ethyl acetate, benzene, and the like.

(2) Medium-speed evaporation solvent The evaporation rate is 1.5 times or more of butyl acetate, such as ethanol, toluene, sec-butyl acetate, and the like. (3) Slow evaporation of solvent The evaporation rate is faster than industrial pentanol and slower than sec-butyl acetate, such as butyl acetate, pentanol, ethylene glycol-ether. (4) Extra slow evaporation solvent The evaporation rate is slower than industrial pentanol, such as ethyl lactate, diacetone alcohol and the like.

3. Classification according to the polarity of the solvent

(1) Polar solvent means a solvent containing a polar group such as a hydroxyl group or a carbonyl group. Such solvents have strong polarities and large dielectric constants, such as ethanol, and polar solvents can dissolve phenolic resins and alkyd resins. (2) Non-polar solvent refers to a type of solvent having a low dielectric constant, such as petroleum hydrocarbons, benzene, and the like. The non-polar solvent dissolves oil-soluble phenolic resin, coumarone resin, etc., and is mainly used for the manufacture of varnish.

4. Classification by chemical composition

(1) Organic solvent (omitted); (2) Inorganic solvent (omitted).

Drying of solvent

Some solvents always contain impurities for various reasons. These impurities can be used directly if they have no effect on the purpose of use of the solvent. However, impurities must be removed during chemical experiments and some special chemical reactions. Although it is difficult to remove all the impurities, at least the impurities should be reduced to a limit that does not hinder the purpose of use. The operation of removing impurities is called refining of the solvent, so that the purification of the solvent is almost always carried out by dehydration, and then the other impurities are removed.

The incorporation of water in the solvent is often carried out as a by-product in the manufacture, treatment or side-reaction of the solvent, and secondly during the preservation process, moisture is also absorbed. The presence of water not only has a negative impact on many chemical reactions, but also on a series of chemical experiments, such as recrystallization, extraction, and washing. Therefore, the dehydration and drying of the solvent are important in chemical experiments and are often performed. Although moisture is sometimes added when removing other impurities in the solvent, dehydration and drying are carried out at the end. The solvent that is sufficiently dried after refining must also be added with a suitable desiccant during storage to prevent the solvent from absorbing moisture. There are several methods for dehydrating a solvent.

(1) Dehydration of desiccant

This is the most commonly used method for dehydrating and drying liquid solvents at normal temperatures. The desiccant can be a solid, a liquid or a gas, and is classified into an acidic substance, a basic substance, a neutral substance, and a metal or a metal hydride. The properties of the desiccant are different. The characteristics of the desiccant and the nature of the substance to be dried should be fully considered in order to effectively achieve the purpose of drying.

When selecting a desiccant, first ensure that the dried material does not react with the desiccant; when the desiccant acts as a catalyst, it should not decompose and polymerize with the solvent, and no adduct is formed between the desiccant and the solvent. In addition, the drying speed, the drying effect and the amount of water absorbed by the desiccant are also taken into consideration. In the specific use, it is preferable to use an acidic desiccant for drying the acidic substance, an alkaline desiccant for drying the alkaline substance, and a neutral desiccant for drying the neutral substance. If there is a large amount of water in the solvent, avoid using a desiccant that is in contact with water (such as metal sodium) or a violent drier. You can use a mildening agent such as calcium chloride for drying and dehydration, so that the water is reduced and then used. The metal sodium is dry. After adding the desiccant, it should be stirred and left for one night. The temperature can be considered in relation to the drying speed depending on the nature of the desiccant. The amount of desiccant should be slightly excessive. In the case of a large amount of water, the desiccant is partially or completely dissolved by the absorption of water to form a liquid or a mud, and is separated into two layers. At this time, separation should be carried out and a new desiccant added. The separation of the solvent and the desiccant is generally carried out by decantation, and the residue is filtered, but the filtration time is too long or the surrounding humidity is excessively absorbed again to allow the moisture to mix in. Therefore, a special filtration device that is isolated from the atmosphere may be used. When some desiccant is dangerous to operate, it can be carried out in a safety box. The safety box contains a desiccant that allows the tank to dry sufficiently (I know it is anhydrous phosphorus pentoxide) or blow dry air or nitrogen. When a desiccant such as molecular sieve or activated alumina is used, it should be added to the glass tube, and the solvent flows from the top to the bottom to perform dehydration, and the contact with the outside is not good. Most solvents can be used in this dehydration process, and the desiccant can be recycled.

Commonly used desiccants are:

1. metal, metal hydride

Al, Ca, Mg: commonly used for drying alcohol solvents
Na, K: Suitable for drying solvents such as hydrocarbons, ethers, cyclohexylamine, liquid ammonia, and the like. Note that there is a risk of explosion when used in halogenated hydrocarbons and must never be used. It can also not be used to dry methanol, esters, acids, ketones, aldehydes and certain amines. The alcohol contains a trace amount of water and can be directly distilled by adding a small amount of sodium metal.
CaH: 1g of calcium hydride is quantitatively reacted with 0.85g of water, so it has better drying effect than alkali metal and phosphorus pentoxide. It is suitable for drying of hydrocarbons, halogenated hydrocarbons, alcohols, amines, ethers, etc., especially cyclic ethers such as tetrahydrofuran, dimethyl hydrazine, hexamethylphosphoramide and the like. Polar aprotic solvents commonly used in organic reactions are also dried by this method.
LiAlH4: Drying of solvents such as ethers.

2. neutral desiccant

CaSO4, NaSO4, MgSO4: suitable for drying solvents such as hydrocarbons, halogenated hydrocarbons, ethers, esters, nitromethane, amides, nitriles, and the like.
CuSO4: Anhydrous copper sulfate is white, and turns into a blue color when it contains 5 molecules of crystal water. It is commonly used to detect trace amounts of water in a solvent. CuSO4 is suitable for the dehydration of alcohols, ethers, esters and lower fatty acids. Methanol and CuSO4 can form adducts, so it is not suitable for use.
CaC2: Suitable for alcohol drying. Note that when calcium carbide with poor purity is used, malodorous gases such as hydrogen sulfide and phosphine may occur.
CaCl2: Suitable for dry hydrocarbons, halogenated hydrocarbons, ether nitro compounds, cyclohexylamine, nitriles, carbon disulfide, and the like. CaCl2 can form adducts with primary alcohols, glycerol, phenols, certain types of amines, esters, etc., and is therefore not suitable.
Activated Alumina: Suitable for drying of hydrocarbons, amines, esters, and formamides.
Molecular sieve: Molecular sieve has obvious hygroscopic capacity when the partial pressure of water vapor is low and the temperature is high. Compared with other desiccants, the hygroscopic capacity is very large. The table below compares the hygroscopicity of various desiccants (refers to the number of milligrams of residual water in 1 liter of air dried at a sufficient temperature by a sufficient amount of desiccant at normal temperature). Molecular sieves are second only to phosphorus pentoxide in various desiccants. Since almost all kinds of solvents can be dehydrated by molecular sieves, they are widely used in the laboratory and industry.

3. alkaline desiccant KOH,

NaOH: Suitable for basic substances such as dry amines and cyclic ethers such as tetrahydrofuran. Acids, phenols, aldehydes, ketones, alcohols, esters, amides, etc. are not suitable.
K2CO3: It is suitable for drying solvents such as alkaline substances, halogenated hydrocarbons, alcohols, ketones, esters, nitriles and cellosolves. Not applicable to acidic substances.
BaO, CaO: suitable for drying alcohols, alkaline substances, nitriles, amides. Not suitable for ketones, acids and esters.

4. acidic desiccant

H2SO4: Suitable for drying saturated hydrocarbons, halogenated hydrocarbons, nitric acid, bromine, etc. Alcohols, phenols, ketones, unsaturated hydrocarbons, etc. are not suitable.
P2O5: Suitable for drying of hydrocarbons, halogenated hydrocarbons, esters, acetic acid, nitriles, carbon disulfide, liquid sulfur dioxide. Ethers, ketones, alcohols, amines, etc. are not suitable.

(2) Fractional dehydration,

a solvent having a large difference in boiling point from the boiling point of water can be dehydrated by a distillation column (rectification column) having a high fractionation efficiency, which is a commonly used dehydration method.

(3) Azeotropic distillation dehydration,

a solvent which forms an azeotrope with water cannot be subjected to fractional dehydration. If a solvent containing a very small amount of water is removed by azeotropic distillation, although a small amount of solvent is lost, most of the water can be removed. Most solvents generally form an azeotrope with water.

(4) Evaporation,

Distillation Drying The solvent to be dried is difficult to volatilize and cannot form an azeotrope with water, and the water can be preferentially removed by heating or distillation under reduced pressure. For example, solvents such as ethylene glycol, ethylene glycol-butyl ether, diethylene glycol-diethyl ether, polyethylene glycol, polypropylene glycol, and glycerin are suitable.

(5) Drying with a dry gas.

When drying a solvent that is difficult to volatilize, it is usually slowly refluxed, and a sufficiently dry air or nitrogen gas is blown in, and the gas is carried away from the solvent in the drying tube at the end of the condenser. release. This method is suitable for drying with solvents such as ethylene glycol and glycerin.

(6) Others Under special circumstances,

acetic acid dehydration may be carried out by adding acetic anhydride in an equivalent amount to acetic acid or directly adding acetic anhydride. The dehydration of formic acid can be melted by heating with boric acid at a high temperature, and the anhydrous boric acid obtained after cooling and pulverization is dehydrated and dried. There is also a method of cooling and drying. For example, hydrocarbons are cooled with a refrigerant, in which moisture is formed into ice for dehydration purposes.