What Is an Absorption Tower?

An absorption tower is a device that performs an absorption operation. There are three types of gas-liquid contact forms. The first type is a plate column, a bubble absorption tower, and a stirred bubble absorption tower in which gas is dispersed in the liquid phase in the form of bubbles; the second type is an ejector, a Venturi tube, and a spray, in which liquid is dispersed in the gas phase in the form of droplets Tower; The third type is the packed absorption tower and falling film absorption tower where the liquid comes into contact with the gas phase in a film-like motion. Gas-liquid two-phase flow in the tower can be countercurrent or co-current. Generally, countercurrent operation is adopted. The absorbent is added from the top of the tower to flow from the top to the bottom, and contacts the gas flowing from the bottom to the top. The absorbent liquid is discharged from the bottom of the tower, and the purified gas is discharged from the top of the tower.

The industrial absorption tower should have the following basic requirements:

Absorption tower

(1) Determine the design plan according to the design tasks and process requirements;

(2) According to the requirements of the system and separation, choose a suitable filler;

(3) Determine process dimensions such as tower diameter and packing layer height (considering spray density);

(4) Calculate tower height and pressure drop of packing layer; ^[2]

Discrimination of absorption tower failure mode

The pressure vessel failure process information refers to the physical and chemical process information inferred from the internal inducing factors, including various types of rules, mechanisms, models, etc. It uses a tree structure method, uses scientific search countermeasures, and adopts certain interpretation procedures to further reason the failure process. ^[3]

Failure mechanism analysis of pressure vessel in absorption tower

1) Failure mechanism of ductile fracture

The significant macroscopic plastic deformation that occurs before a member breaks is called ductile fracture, and it is one of the ways of failure and failure of metal materials. For tough materials. When the compressive load of the material is greater than the strength limit of the material itself, it is easy to cause tough fracture. The failure characteristics of ductile fracture are: obvious macroscopic plastic deformation occurs around the fracture of the material; and the tensile fracture is cup-shaped, the fracture direction is perpendicular to the principal stress, the cone direction is parallel to the maximum shear stress, but sometimes the entire macro fracture direction It is parallel to the maximum shear stress and will produce a shear fracture of 45 °; the color of the fracture is gray and the surface is fibrous.

2) Failure mechanism of brittle fracture

Brittle fracture failure refers to the failure failure mode caused by obvious plastic deformation during the steady propagation of cracks in a component. Brittle fracture is a very dangerous failure mode in actual engineering structures. The failure characteristics of brittle fracture are: no obvious plastic deformation is found before fracture, but in actual analysis cases, it is found that the fracture is usually perpendicular to the normal stress and the fracture surface is flush; however, the edges often lack shear lips, Fractured cut lips are smaller. The fracture color of the brittle fracture of the component appears bright or dark. The macro relief of the bright fracture of the brittle fracture sometimes produces emissive lines during the rapid expansion of the crack. When the brittle fracture is rotated, a reflective facet usually appears; for the brittle fracture, the macro state is formed. Rough and unshaped surfaces, and occasionally the appearance of fractures will appear grains.

3) Failure mechanism of fatigue fracture

Under the long-term action of alternating load and strain, the damage of metal materials or parts due to continuous accumulation causes fatigue fracture failure. The specific process of fatigue cracks is the occurrence of fatigue cracks, the damage propagation into a subcritical state of fatigue cracks, and finally the expansion of fatigue cracks into instability. Its failure characteristics are: fracture problems caused by a certain number of week cycles under the action of alternating load; low stress brittle fracture sudden characteristics will occur during fatigue fracture; fatigue failure failure process is reflected in local regional; Under the action of alternating load, there is no macro plastic deformation characteristic around the failure fracture of the metal member, and the fracture has obvious shell pattern.

4) Creep fracture failure mechanism

Constant temperature and constant stress act on metal materials for a long time to form slow plastic deformation, that is, metal creep. Its failure characteristics are: the working conditions are constant temperature, constant force and long-term action, slow permanent deformation speed; the macro fracture is marked by oxidation color or black; the micro fracture is generally fractured along the crystal, without the characteristics of fatigue streaks.

5) Uniform corrosion failure mechanism

Corrosion occurs uniformly on the entire surface of the metal. Its failure characteristics are: the chemical composition of the corroded metal components is uniform and the surface of the microstructure is uniform, and the uniform corrosion environment covers the metal surface without restriction; uniform corrosion can be understood as Local electrolytic corrosion occurs on the metal surface; the metal has a dark and smooth surface morphology under uniform corrosion, or the surface is rough due to the corrosion of large pieces of metal.

6) Pitting corrosion failure mechanism

Pitting corrosion failure refers to failure caused by chemical action of free materials in metal materials and the environment. Its failure characteristics are: corrosion appears in the local area of the component, there are obvious sharp small holes, the small holes further expand into deep holes or even penetrate; the water film condenses on the metal surface in a humid environment or the atmosphere, making the metal surface often point Corrosion; pitting pits were visually enlarged to find that the edges were smooth, and the bottom of the pit was dark gray because the bottom of the pit was covered by corrosion products. The pits were observed vertically through the abrasive pads and found that most of the pits were circular or polygonal.

7) Stress corrosion failure mechanism

Under the combined action of static load tension and corrosive environment, local corrosion cracking formed by metal materials is called stress corrosion failure. Its failure characteristics are: sensitive corrosive media and stress working environment; macro fractures appear in two areas: corrosion fracture zone and transient fault zone. The stress corrosion fracture zone is dark gray, and the fracture structure is rough, and at the same time it is covered by corrosion products. Most of the fresh fractures appearing in the transient fracture zone are fibrous and accompanied by radiation ridges. The stress corrosion crack has a dendritic shape and is bifurcated. Cracks are also the result of cumulative effects of corrosion products. ^[3]

Judgment and preventive measures of failure of absorption tower

1. Judging the cause of failure

The cause of failure should be analyzed from the aspects of material selection, structural design, environment, and operation.

(1) Material selection: whether the selection of materials is reasonable, whether the material's chemical composition, metallurgical process quality, and surface state are normal. In particular, various factors such as material strength, stiffness, and toughness need to be understood. If the material cannot be selected correctly, the application temperature is significantly higher than the creep temperature of the material; or the material is degraded, and it is easy to cause carburization of the material when it is used in a high temperature environment for a long time.

(2) Structural design: whether the geometry, cross-section size, fillet radius, surface smoothness, etc. of the component are scientific; the defects generated in the structure form a large residual stress.

(3) Environmental factors: the environment in which the metal is located has a higher or lower concentration of corrosives; an increase in temperature will also increase the corrosion rate of the metal; the compatibility of metal materials and environmental media is poor, and due to unsuitable humidity The oxide film on the surface of the material is directly damaged; the surface of the material appears uneven.

(4) Operation and operation factors: operating irregularly or failure of safety accessories resulting in sudden temperature changes or operating temperatures lower than the toughness transition temperature of the material; causing chemical abnormal reactions inside the pressure vessel; under the action of alternating or alternating loads, causing stress concentration areas Fatigue cracks gradually expand towards unstable fractures; under the combined action of alternating loads and corrosive media, corrosion fatigue fractures eventually occur.

2. Failure prevention measures

(1) Scientific material selection, the use of non-low temperature steel is strictly prohibited at low temperature, and the material with high corrosion resistance is selected.

(2) Improve the structural design. During the design of the structure, try to use smooth transitions to reduce the stress concentration of the components. During the operation of the equipment, avoid frequent alternating loads as much as possible, avoid excessive temperature operation and local overheating, and prevent the occurrence of Water stagnation can effectively eliminate residual stress through heat treatment.

(3) Strengthen the periodic inspection of material performance, strictly inspect the corrosive agent concentration in the environment where metal components or equipment are located, and reduce the effective temperature of environmental media.

(4) Improve the medium environment, reduce oxidized cations, add appropriate elements to the material, improve the corrosion resistance of the material, and adopt surface protection methods.

Calculate the stress value on the inner wall of the absorption tower through the corresponding formula. From the calculated results, we can know that the circumferential stress value of the absorption tower is the largest. However, due to the excessive concentration of stresses near the weld and the nozzle, and the combined effect of welding residual and thermal stresses, the inner wall of the absorption tower eventually formed a large stress, which provided a very important condition for stress corrosion. Through in-depth analysis of the failure and destruction of the absorption tower, it was found that the failure of the absorption tower is due to the combined action of concentrated stress and corrosion-sensitive media, which resulted in dendritic cracks, corrosion products appeared at the micro fractures, and there were Corrosion pits. Therefore, it can be judged that the absorption tower belongs to stress corrosion failure. The stress concentration in the local area of the absorption tower is relatively high, such as residual stress and thermal stress, and the alkali embrittlement of the material caused by KOH. These factors exacerbate the stress corrosion failure of the absorption tower. Select materials scientifically, improve conditions, and avoid corrosive environments as much as possible; in the process of structural design, in order to avoid excessive stress concentration, install ring-shaped angle steel support frames in the stress concentration area of the absorption tower to reduce residual stress. Because of the gradually expanding fatigue crack problem in pressure vessel damage accidents, it accounts for about 40%. Therefore, it is of great significance to systematically study the characteristics, causes and measures of fatigue failure. ^[3]