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Chemical gas absorption process

I. Analysis of chemical absorption process

chemical absorption refers to the absorption process in which the absorbent has an obvious chemical reaction with the absorbent. For chemical absorption, the mass transfer mechanism of solute from the gas phase main body to the gas-liquid interface is exactly the same as that of physical absorption, and its complexity lies in the mass transfer in the liquid phase. In the process of solute diffusion from the interface to the main body of the liquid phase, it will distribute biochemistry with the absorbent or other active groups in the liquid phase, which will not only affect the normal metrological reaction of the experimental machine. Therefore, the change of solute composition along the diffusion path is not only related to its own diffusion rate, but also related to the reverse diffusion rate of active components in the liquid phase, chemical reaction rate and the diffusion rate of reaction products

due to the chemical reaction of the solute in the liquid phase, the solute in the liquid phase presents two ways: physical dissolution and chemical combination, and the equilibrium partial pressure of the solute is only related to the solute in the physical state in the liquid phase. Therefore, the chemical reaction consumes the absorbent entering the liquid phase, significantly increasing the effective solubility of the absorbent and reducing the equilibrium partial pressure, thereby increasing the driving force of the absorption process; At the same time, some solutes are consumed due to chemical reactions during the diffusion in the liquid membrane, which reduces the process resistance and increases the absorption coefficient. Therefore, the occurrence of chemical reactions will always improve the absorption rate to varying degrees

most industrial absorption operations are chemical absorption, because:

① chemical reaction improves the selectivity of absorption

② accelerate the absorption rate, so as to reduce the volume of the equipment

③ the reaction increases the solubility of solute in liquid phase and reduces the amount of absorbent

④ the reaction reduces the equilibrium partial pressure of solute in the gas phase and can completely remove a small amount of harmful gas in the gas phase

as shown in Figure 11-9, the purification process of synthetic ammonia feed gas (containing about 230% C02) is to remove C02 in the refining process, and the obtained Co: gas is also the raw material for preparing urea, ammonium bicarbonate and dry ice. Therefore, the combined absorption and desorption process of alcohol amine method is adopted. The feed gas of synthetic ammonia enters the absorption tower from the bottom, and ethanolamine liquid is sprayed on the top of the tower. After absorbing coz, ethanolamine is discharged from the bottom of the tower. The content of C02 in the gas discharged from the top of the tower can be reduced to 0.2% - 0.5%. The ethanolamine containing solution discharged from the bottom of the absorption tower is pumped to the heater, heated (about 130 ° C) and sprayed from the top of the desorption tower. Water vapor is introduced into the bottom of the tower, and ethanol desorbs from the solution at high temperature and low pressure (about 300kPa). The gas discharged from the top of the desorption tower is cooled and condensed to obtain usable coz. The solution discharged from the bottom of the desorption tower is still used as absorbent after cooling (about 50 ° C) and pressurization (about 1800kpa). In this way, the absorbent can be recycled and the solute gas can be recovered

II. High content gas absorption

when the content of absorbent in the mixed gas entering the tower is higher than 10%, it is often called high content gas absorption in engineering. Due to the high content of adsorbate, the transfer of adsorbate from gas phase to liquid phase is large in the absorption process. Therefore, the absorption of high content gas has its own characteristics

(1) the molar flow of gas-liquid two phases changes greatly along the tower height. During the absorption process, the molar flow of mixture and absorbent at different sections in the tower are different, and they change significantly along the tower height and can no longer be regarded as constants. However, the molar flow of inert gas is basically unchanged along the tower height. If the volatility of absorbent is not considered, the molar flow of pure absorbent is also constant

(2) the absorption process has a significant thermal effect. Because more solutes are absorbed, more heat of dissolution is generated. If the liquid gas ratio in the absorption process is small or the heat dissipation effect of the absorption tower is not good, the temperature of the absorption liquid will increase significantly. At this time, the gas absorption is non isothermal absorption. However, if the heat of solute dissolution is not large, the absorbed liquid gas ratio is large, or the heat dissipation effect of the absorption tower is good, then the gas absorption can still be regarded as isothermal absorption

(3) the absorption coefficient is not constant. Due to the influence of gas velocity, the absorption coefficient decreases gradually from the bottom to the top of the tower. However, when the change of gas-liquid phase molar flow in different sections of the tower does not exceed 10, the absorption coefficient can be taken as the average value of the absorption system at the top and bottom of the tower, and can be regarded as a constant for relevant calculation

as shown in Figure 11-10, it is a two-stage packed stripper used to treat wastewater with high content of volatile phenol. After the waste water is heated to 100 ° C by the heat exchanger, it is sent to the stripping section and drenched from the top of the stripping tower. In the stripping section, it is in countercurrent contact with the steam at 105 ° C. The volatile phenol in the waste water is transferred to the gas phase and brought out of the tower by the steam to become phenol containing steam. The phenol concentration of the wastewater after stripping can be reduced to less than 400mg/L, which will be sent to the next treatment process for further treatment after being cooled by water sealed pipe and heat exchanger. The phenol containing steam is sent to the regeneration section by blower and contacted with 10% NaOH solution at 102 ° C in countercurrent. After chemical absorption, phenol sodium salt is generated to recover the phenol. The purified steam enters the stripping section for recycling. In order to increase the content of phenol sodium salt, the circulating lye often flows back to the regeneration section, and then recovers phenol after saturation

III. multicomponent absorption

in the multicomponent absorption process, due to the existence of other components, the equilibrium relationship of absorbent in gas-liquid two-phase has changed. Therefore, the calculation of multicomponent absorption is more complex than that of single component absorption. However, for the absorption of low content gas with a large amount of spray, the mutual interference between absorbing substances can be ignored, and its equilibrium relationship can still be considered to obey Henry's law. Therefore, each absorbent component can be calculated separately. The phase equilibrium constants of different absorbent components are different, and the content of each component in the gas entering and leaving the absorption equipment is also different. Therefore, each absorbent component has an equilibrium line and an operation line

key components refer to components whose absorption rate must first reach the predetermined index in the absorption operation. For example, the main purpose of the oil absorption tower in the treatment of petroleum cracking gas is to recover ethylene from cracking gas, which is the key component of this process. In production, it is generally required that the development momentum of wearable equipment of ethylene is no less than that of intelligent recovery rate (displayed only if it is used), large deformation (displayed only if it is used) and other relevant information. Is there a numerical display in the re experiment process; 98% to 99%, which must be guaranteed. Therefore, although this process belongs to multi-component absorption, it can be regarded as a single component absorption process of using oil to absorb ethylene in the mixture during calculation

in the multi-component absorption process, in order to improve the content of solute in the absorption solution, the absorption evaporation process can be used. As shown in Figure 11-11, oil is used to absorb and separate the cracked gas. The upper part of the tower is an absorption tower and the lower part is a stripper. The cracked gas enters from the middle of the tower. C4 fraction is used as the absorption liquid to absorb the cl-c3 fraction in the cracked gas. The absorption liquid evaporates methane, hydrogen and other gases through the lower tower section, so that the tower kettle can obtain C2-C3 fraction with high purity. The absorption liquid of the tower kettle enters the C2 and C3 separation towers to achieve the purpose of separation

IV. desorption process

desorption, also known as desorption, is the process of removing the absorbed solute in the absorbent and making the solute escape from the liquid phase to the gas phase. The desorption process in production has two purposes:

① obtain the required purer gas solute

② it is more economically reasonable to regenerate the solvent and return it to the absorption tower for recycling

in industrial production, the combined operation of absorption and desorption is often used. As shown in Figure 11-12, use Na2CO3 aqueous solution to purify and remove H2S in the gas. The solution from the bottom of the absorption tower is pumped to the desorption tower, and then desorbed with air. The desorbed solution (absorbent) is pumped back to the top of the absorption tower for spraying. In this process, absorption and desorption are carried out at room temperature

desorption is the process of solute transferring from liquid phase to gas phase. Therefore, The necessary condition for desorption is that the actual partial pressure (or y) of the gas phase solute must be less than the equilibrium partial pressure p* (the operation of the experimental machine is flexible and convenient. The lifting and locking of the moving beam and the clamping of the sample are all completed by button operation. Advanced hydraulic servo drive technology is adopted for loading, high-precision dynamic load sensor and high-resolution magnetostrictive displacement sensor are used to measure the force value and displacement or y*) of the sample, and the difference is the driving force of the desorption process. There are several desorption methods commonly used in industry

(1) heating desorption heating up the solution or increasing the equilibrium partial pressure of the solute in the solution to reduce the solubility of the solute, then some solute must be released from the liquid phase, which is conducive to the separation of solute and solvent. If the "thermal deoxidation" method is used to treat boiler water, it is to make dissolved oxygen escape from the water by heating

(2) decompression desorption if the solution that was originally at a higher pressure is decompressed, the partial pressure of the solute in the gas phase will be correspondingly reduced after the total pressure is reduced, and the solute will be released from the absorption solution. The degree of desorption of solute depends on the final pressure and temperature of desorption

(3) desorb in inert gas. Heat the solution and send it to the top of the desorber to make countercurrent contact with the inert gas (or water vapor) introduced into the bottom of the tower. Because the partial pressure of solute in the inert gas of the tower is p = 0, it is conducive to the desorption process

the desorption process operated in countercurrent mode is similar to countercurrent absorption. The absorption liquid is sprayed from the top of the desorption tower, and the inert gas (air, steam or other gases) flows from the bottom to the top. In the process of countercurrent contact between gas and liquid phases, the solute will be continuously transferred from liquid phase to gas phase, mixed with inert gas and sent out from the top of the tower, and the desorbed solution will be led out from the bottom of the tower, as shown in Figure 11-12. If the solute is non condensable gas or the solute condensate is insoluble in water, the solute component with high purity can be obtained by steam condensation. If the washing oil solution of benzene and toluene is desorbed by steam, benzene and toluene can be separated from the condensate. The concentrated end of the desorption tower is at the top and the dilute end is at the bottom, just opposite to absorption

(4) the solute is dissolved in the solvent by distillation, and the resulting solution can be separated from the solvent by distillation, so as to recover the solute and obtain fresh absorbent for recycling