First, solvent recovery
(1) Granular activated carbon
1. Overview of solvent recovery
In recent years, with the development of various industries, the types of solvents used have also increased. A variety of solvents including acetone, such as aromatic solvents such as toluene and xylene, alcohols, esters, and methyl ethyl ketone (MEK), isobutyl methyl ketone (MIBK) and Other solvents, in synthetic fibers and synthetic It is widely used in the fields of resin industry, printing industry, paper processing industry and so on. At the same time, various industrial sectors also use a variety of other solvents. Such as fluorocarbons and chlorocarbon organic compounds, used for degreasing and washing of metals and base washing in precision machine manufacturing; cyclohexenone for the manufacture of magnetic recording media; dichloromethane for the production of photographic film Wait. Preventing these solvents from evaporating and scattering during use, recycling and harmless treatment are indispensable in terms of saving resources, improving labor hygiene conditions, and protecting the global environment.
In the hydrocarbon emission regulations, the concentration of the organic solvent in the exhaust gas is generally below 50 to 100 ppm by volume. Recently, from the perspective of protecting the global scale, we are studying the issue of re-recognizing harmful substances and strengthening their regulations.
The organic solvent gas recovery device or the device for detoxification treatment is designed and manufactured according to the corresponding regulations. Among the various benchmarks specified, there is an initial operating environment benchmark. From the standpoint of labor safety, it stipulates the concentration of organic solvents and harmful substances in the air that must be observed in factories and business sites that handle organic solvents and hazardous substances.
2. Activated carbon for solvent recovery
Most of the organic solvents contained in the exhaust gas are volatilized into the air during the processes of coating, steaming, and drying. In addition to the incombustible solvent, the concentration at the time of discharge is usually several thousand ppm (volume) or less due to the limitation of the explosion limit concentration.
Since activated carbon is a relatively non-polar substance, it has a strong affinity for organic matter; even if water is present, the adsorption value can be reduced little; and it is cheaper and the like, it is used in the adsorption of organic solvents. a lot. In particular, the shaped granular activated carbon commonly used in the recovery of solvents is a product of coal, petroleum, wood, coconut shell and the like. Further, a product having a particle size of 2 to 5 mm prepared by carbonizing, crushing, molding, firing (carbonizing) and steaming the raw material is also used. Further, a crushed activated carbon having a particle size of 1 mm or less activated by steaming, crushing, and sieving is also used. The powdered activated carbon (particle size <200 mesh) is specially used for water treatment and liquid phase adsorption. The fibrous activated carbon is produced by carbonizing and activating a regenerated cellulose, a polypropylene eye, a phenol novolak resin, and a pitch fiber.
The adsorption performance of activated carbon is determined by the pore size and surface area. It can be considered that the size of the pores determines the selectivity to the adsorbate, and the size of the specific surface area determines the adsorption capacity. Activated carbon is characterized by a large specific surface area and a specific pore volume, and a large amount of adsorption per unit weight. The specific surface area is usually obtained by determining the specific molecular surface by the cross-sectional area of ​​the adsorbed molecules by determining the adsorption amount of the monolayer on the adsorption isotherm according to the BET (Brunauer Emmett Teller) equation.
In addition, for the purpose of supplying the fluidized bed and the moving layer type adsorption device, a spherical activated carbon having a diameter of 0.5 to 1 mm with good abrasion resistance was produced using petroleum pitch as a raw material. With this activated carbon, the adsorption and desorption operations of the recovery device can be continuously performed.
(1) Adsorption characteristics
Under the condition of a certain temperature, the curve indicating the relationship between the gas concentration or pressure of the adsorbed component (adsorbed matter) (strictly speaking, the pressure in the single component system of the adsorbent) and the adsorption amount per unit mass of the adsorbent is adsorption isotherm. line. Whether it is comparing the adsorbent or in actual use, the mosquito line such as adsorption has important value. In the case where the determination of activated carbon is selected, the equilibrium adsorption characteristics of the activated carbon to the solvent to be recovered are of course an important factor. In addition, activated carbon has various properties such as particle size, ignition point, strong heat residue, pH value, pore size and specific surface area, and can effectively exert its adsorption capacity under the conditions of gas concentration, humidity, temperature and pressure actually used. It is also very important. Regarding the adsorption performance, the most important is the size of the pore size and its publication, and it is necessary to select an activated carbon having a pore size suitable for the target substance. However, the solvent to be actually recovered is rarely a single component, usually a mixed solvent, and contains various organic substances ranging from a low point component to a high boiling component. Therefore, it is possible to use activated carbon having a certain proportion of micropores, intermediate pores and macropores, or to fill several adsorption activated carbons in the adsorption tank.
(2) Catalytic
When the activated carbon is heated in the desorption operation of the solvent component to which it is adsorbed, chemical reactions such as oxidation, decomposition, and polymerization generally have a catalytic action. The oxidation and decomposition products of such solvents sometimes cause a decrease in the quality of the recovered solvent. That is, a solvent which is relatively easy to react, such as a ketone, an ester, or an alcohol, is likely to undergo a chemical change during the recovery process. Moreover, when the solvent is an organic halide, a small amount of acid and halogen generated upon decomposition may also corrode the material of the adsorption tank.
(3) Deterioration of activated carbon
The reason why the adsorption performance of the activated carbon is lowered during use is considered to be as follows: accumulation of a trace amount of a polymer substance or a polymerizable substance contained in the adsorbed gas in the activated carbon; and impurities mixed from the desorbed water vapor; As can be seen in the recovery of carbon disulfide, the accumulation of sulfur which is oxidatively decomposed into a solid, and the accumulation of a polymer substance produced by the adsorbed solvent in activated carbon. In particular, when the target adsorbed material is chemically reacted by oxidation, decomposition, polymerization or the like to form those high-boiling substances which cannot be removed in the usual desorption operation, these substances are also generated inside the activated carbon, sometimes causing the adsorption device. The performance has dropped dramatically.
(4) Heating of the activated carbon layer
The most serious accidents in solvent recovery are activated carbon ignition. It can be considered that most of the fire accidents are caused by the heat of adsorption of the activated carbon to the solvent or the reaction heat generated by the oxidation reaction of the solvent in the activated carbon layer, which is caused by the abnormal temperature rise and natural ignition. Activated carbon is a porous structure with poor thermal conductivity and is liable to cause local heat storage. In normal operation, the heat generated by the adsorption is in equilibrium with the exotherm of the adsorbed gas. However, when the adsorbed solvent is oxidized and decomposed, the equilibrium is destroyed, and the oxidation and decomposition reactions are further accelerated, eventually leading to an abnormal rise in temperature. In particular, when a ketone solvent such as acetone, methyl ethyl ketone or cyclohexanone is recovered, there are many accidents of ignition.
In order to grasp the situation at the time of the fire, the temperature was maintained at 85 ° C in the activated carbon filling layer, and the air having a cyclohexanone concentration of 2000 ppm by volume was introduced, and the temperature change in the activated carbon layer and the concentration of carbon monoxide in the exhaust gas were measured over time.
(5) Desorption performance of adsorption solvent
To completely desorb the adsorbed solvent will increase the consumption of water vapor and increase the recovery cost. When the amount of water vapor is controlled, the amount of residual solvent increases, and the effective adsorption amount of activated carbon decreases. When there is a drying step after desorption, the solvent is dissipated, and the recovery rate is lowered and the concentration of the solvent in the exhaust gas is increased. Generally, when desorbing a high boiling point solvent, a large amount of desorbed water vapor is required. Not only does the cost of desorption increase, but also the cost of the subsequent refining process due to the mutual solubility of the solvent and water. The heat recovery from desorption of water vapor has been carried out before. In the case of activated carbon, a variety of activated carbons which are particularly suitable for high-boiling solvent pore size distribution and particle size desorption performance have also been developed. It is suitable for the adsorption isotherm of activated carbon for general solvent adsorption such as cyclohexanone and activated carbon suitable for adsorption of high-boiling solvent at 25 ° C and 100 ° C.
(6) Adsorption performance in wet state
When the activated carbon contains moisture or the gas flowing therethrough is wet, the adsorption rate of the activated carbon to the organic solvent generally decreases. However, by using an activated carbon which can maintain a considerable adsorption capacity in a wet state, it can be used for recovery even under conditions which are not suitable for recycling, and it is also possible to save water vapor. In particular, in the case where the solvent which is likely to generate heat due to oxidation or decomposition is recovered, by humidifying the adsorbed gas, it is possible to suppress an increase in the temperature of the activated carbon layer, suppress the occurrence of the reaction, and prevent ignition. In the case where a ketone solvent is recovered, this becomes an important condition for selecting activated carbon. The breakthrough curve of dried activated carbon and moist activated carbon adsorbing acetone. Penetration time is also one of the indicators of activated carbon adsorption capacity. This shows the effect of moisture on the shape of the adsorption zone.
3. Some problems in solvent recovery
(1) Energy saving
The method of desorbing with water vapor is one of the most commonly used methods for recovering a solvent using activated carbon. In large-scale solvent recovery equipment, the amount of water vapor is large. In particular, in the case of recovering high boiling high solvent and ketone solvent, a large amount of water vapor is used for safety reasons.
Therefore, a process of recovering heat of condensation from the desorbed water vapor can be considered. It is a method used in the desorption operation after boosting with desorption vapor. With this method, most of the latent heat of vaporization of the required water vapor can be recovered, and after deducting the energy required for the steam to be boosted, a lot of energy can be recovered.
(2) Quality of recovered solvent
Some impurities are often mixed in the solvent recovered from the activated carbon. There are two types of impurities to be mixed, one is the solvent to be recovered, partially oxidized, polymerized or hydrolyzed on the activated carbon, and then mixed into the recovery liquid; the other is added during the production process using the solvent. The substance or the product of its decomposition and polymerization enters the recovered solvent together with the solvent vapor.
An ester solvent such as ethyl acetate or propyl acetate is hydrolyzed on activated carbon to form an organic acid and an alcohol, and there is a problem that alcohol is mixed in the recovered solvent. A ketone solvent such as MEK (methyl ethyl ketone) or MIBK (methyl isobutyl ketone), in addition to an organic acid, forms an oxide such as a diketone; and a polymerization product of cyclohexanone is a cyclohexylidene group. Cyclohexanone or the like is a cause of coloring and odor of the recovered solvent.
A solvent-containing gas discharged from a production section using a solvent often contains a trace amount of a high-boiling substance. These materials are likely to cause a decrease in the quality of the recovered solvent and also cause deterioration of the activated carbon. Therefore, it is necessary to study countermeasures in advance.
As an example in which the additive substance in the resin enters the adsorption tank together with the solvent gas, there are additives such as dimethylacetamide and dimethylformamide. When they are subjected to steam desorption on activated carbon, they are hydrolyzed to form dimethylamine. Dimethylamine is not adsorbed on activated carbon and is discharged into the atmosphere. It is a substance with a very low concentration of odor, which often causes odor problems. When the substance is present alone, it can be treated by adding activated carbon to the chemical; however, when it is present in the gas together with other solvents, the adsorption capacity of the activated carbon to the component is greatly lowered.
The recovered solvent is purified by water, alkali and other chemicals. However, since there are many types of decomposition products and the solvent is repeatedly recycled, there is a problem of accumulation of trace impurities. On the other hand, the COD and BOD values ​​of the treatment device are increased, and some solvents generate substances such as phenol during the treatment. Therefore, the purification of the solvent should be considered together with the drainage treatment.
(3) Dehydration and purification of recovered solvent
The solvent is recovered by a water-soluble solvent such as ethanol, acetone or THF (tetrahydrofuran), a water-insoluble solvent such as cyclohexanone or toluene, or a partially water-soluble solvent such as acetic acid or a partial solvent such as MEK. Even in the case of desorption by an inert gas, the activated carbon adsorbs moisture mixed in the gas during adsorption, and is desorbed together with the solvent. Therefore, when the amount of solvent to be adsorbed is small, the proportion of moisture is large. In order to allow the recovered solvent to be reused, the solvent is dehydrated according to the conditions of use. Although it is affected by the kind of the solvent, in general, when the allowable water content in the solvent is 100 ppm by volume or more, it can be dehydrated by distillation; if it is less than this value, desiccant is further used.
1 distillation dehydration
A solvent which differs greatly from the boiling point of water and does not form an azeotropic mixture with water can be dehydrated by fractional distillation.
Although an azeotropic mixture is formed with water, the azeotrope can be separated into a solvent of a solvent phase (large concentration of solvent) and a water phase (large concentration of water), which can be nearly azeotropic from the top of the distillation column. After the mixture was composed, it was separated into two groups by a decanter. The solvent phase liquid is returned to the column for rectification together with the feed liquid fed to the rectification column, and a dehydration solvent is obtained from the bottom of the column. Dichloromethane is an example of this.
For forming an azeotrope with water, and the azeotrope is a homogeneous liquid phase (such as methyl ethyl ketone, tetrahydrofuran, dioxane, acetonitrile, etc.), or although it does not form an azeotrope with water, However, a type of solvent having a relative volatility close to 1.0 (for example, acetone having a low water content) is subjected to dehydration distillation after the addition of the third component, and the third component added is practically insoluble in water with water. However, it can azeotrope with water, has a higher boiling point than the solvent, and does not azeotrope with the solvent to form a uniform phase.
2 The desiccant used for dehydration and dehydration with a desiccant must not react chemically with the solvent to be dehydrated. Further, the desiccant used in the industry is capable of being regenerated (dried) and used repeatedly. Synthetic zeolites, activated alumina and silica gel are generally used as drying agents. These desiccants are typically filled in two columns and subjected to water absorption and dehydration operations in turn.
Synthetic zeolites can be used in the dehydration of various organic solvents. For example, it can dehydrate aqueous ethanol to a moisture content of about 10 ppm. The adsorption operation of the solvent dehydration is carried out in the vicinity of normal temperature; and the regeneration and drying operation of the dehydrating agent is carried out by heating at 200 to 300 °C. There are reports of dehydration of various solvents using synthetic zeolites.
The silica gel and alumina gel adsorbent are regenerated using a heating gas of 150 to 350 ° C depending on the application. There are also reports on the dehydration of various solvents using silica and alumina gel-based adsorbents.
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