Calculation of absorption plant with lattice (failed) trays - Machines and devices of chemical technologies - exchange rate

Contents

Introduction

1. General information on absorbers

2. Applications of absorption processes

3. Device and principle of absorbers operation

3.1. Device of surface absorbers

3.1.1 Surface absorbers with horizontal liquid mirror

3.1.2 Film absorbers

3.1.3 Nozzle absorbers

3.1.4 Mechanical film absorbers

3.2 Bubble absorbers arrangement

3.2.1 Absorbers with continuous bubbling layer

3.2.2 Poppet absorbers...................... 18 3.2.3 Absorbers with movable nozzle

3.2.4 Mechanical Agitated Absorbers

4. Absorption column calculation

4.1 Material Balance

4.2 Determination of gas velocity and absorber diameter

4.3. Mass transfer coefficient

4.4. Height of light liquid layer

4.5. Selection of distance between trays and determination of absorber height

4.6. Absorber trays hydraulic resistance

4.7. Selection of gas blower and water supply pump

4.8. Set support calculation

4.9. Calculation of devices for wind load

4.10. Piping Calculation

Conclusion

Literature

Application

Introduction

Absorption is the process of selective absorption of components from a gas or steam-gas mixture by a liquid absorber, in which this component is soluble [2, c.203].

If the absorptive gas does not react chemically with the absorbent, then such absorption is called physical (the non-absorptive component of the gas mixture is called inert, or inert gas). If the absorbent forms a chemical compound with the absorbent, then such a process is called chemisorption. A combination of both types of absorption is common in the art [1, c.49].

To purify gases from CO2, various processes are used, which can be divided into the following groups :

• Chemisorption processes based on the chemical interaction of CO2 with the active part of the absorbent ;

• physical absorption processes in which acid components are recovered by their solubility in organic and inorganic scavengers;

• combined processes using chemical physical absorbers at the same time;

• adsorption processes based on the extraction of gas components by solid absorbers - adsorbents (molecular sieves, activated coals, etc.)

The choice of the process of cleaning gases from acidic compounds depends on many factors, the main of which are: the composition and parameters of the raw gas, the required degree of purification and the field of use of commercial gas, the presence and parameters of energy resources, production waste, etc.

An analysis of global practices accumulated in the field of gas purification shows that the main processes for treating large gas flows are absorption processes using chemical and physical absorbers and their combination.

Adsorption processes are typically used to clean small gas streams or to fine clean gas.

The purpose of the project is to develop skills for calculating absorption processes based on the mass transfer equation.

1. General information on absorbers

Two phases are involved in absorption processes (absorption, desorption) - liquid and gas, and the transition of the substance from the gas phase to the liquid phase (during absorption) or, conversely, from the liquid phase to the gas phase (during desorption) occurs. Thus, absorption processes are one type of mass transfer processes [10].

In practice, most of the absorption is not carried out by individual gases, but by gas mixtures, the constituents of which (one or more) can be absorbed by the scavenger in noticeable amounts. These constituents are referred to as absorbent components or simply components, and non-absorbent constituents as inert gas.

Liquid phase consists of absorber and absorbed component. In many cases, the scavenger is a solution of the active component reacting chemically with the absorbent component; wherein the substance in which the active ingredient is dissolved is referred to as a solvent.

Inert gas and absorber are carriers of component in gas and liquid phases, respectively. During physical absorption, the inert gas and absorber are not consumed and are not involved in the processes of transition of the component from one phase to another. In chemosorption, the scavenger may react chemically with component [10].

When physically absorbed, the dissolution of the gas is not accompanied by a chemical reaction (or at least this reaction has no noticeable effect on the process). In this case, there is a more or less significant equilibrium pressure of the component over the solution and absorption of the latter occurs only as long as its partial pressure in the gas phase is higher than the equilibrium pressure over the solution. Complete removal of the component from the gas is possible only with countercurrent flow and supply to the absorber of a pure absorber that does not contain the component [10].

In chemosorption, the absorbed component binds in the liquid phase as a chemical compound. In an irreversible reaction, the equilibrium pressure of the component over the solution is negligible and its complete absorption is possible. With a reversible reaction over the solution, there is a noticeable component pressure, albeit less than with physical absorption.

2. Applications of absorption processes

Some of these areas are listed below:

1. Preparation of finished product by absorption of gas by liquid. Examples include SO3 absorption in sulfuric acid production; absorption of HCl to obtain hydrochloric acid; absorption of nitrogen oxides by water (nitric acid production} or alkaline solutions (nitrate production), etc. Absorption is carried out without subsequent desorption.

2. Separation of gas mixtures to isolate one or more valuable components of the mixture. In this case, the absorber used should have as much absorptive capacity as possible with respect to the component to be recovered and as little as possible with respect to other constituents of the gas mixture (selective, or selective, absorption). At the same time, absorption is usually combined with desorption in a circular process. Examples include absorption of benzene from coke gas, absorption of acetylene from natural gas cracking or pyrolysis gases, absorption of butadiene from contact gas after decomposition of ethyl alcohol, etc. [10].

3. Cleaning of gas from impurities of harmful components. Such purification is carried out primarily in order to remove impurities that are not permissible during further processing of gases (for example, purification of petroleum and coke gases from H2S, purification of the nitrogen-hydrogen mixture for the synthesis of ammonia from CO2 and CO, drying of sulfur dioxide in the production of contact sulfuric acid, etc.). In addition, the exhaust gases released into the atmosphere (e.g. cleaning flue gases from SO2; purification from C12 abgas after condensation of liquid chlorine; purification from fluorine compounds of gases released during the production of mineral fertilizers, etc.). In this case, the recovered component is usually used, so it is isolated by desorption or the solution is sent for appropriate processing. Sometimes, if the amount of the recovered component is very small and the absorber is not of value, the solution after absorption is dumped into the sewer [10].

4. Capture of valuable components from the gas mixture to prevent their loss, as well as for sanitary reasons, for example, recovery of volatile solvents (alcohols, ketones, ethers, etc.).

It should be noted that other methods are used to separate gas mixtures, clean gases and capture valuable components along with absorption: adsorption, deep cooling, etc. The choice of one or another method is determined by technical and economic considerations. Generally, absorption is preferred when very complete recovery of component [10] is not required.

3. Device and principle of absorbers operation

During absorption processes, mass exchange occurs on the contact surface of the phases. Therefore, the absorption apparatus must have a developed contact surface between the gas and the liquid. Based on the method of creating this surface, absorption devices can be divided into the following groups:

a) Surface absorbers in which the contact surface between phases is a liquid mirror (surface absorbers themselves) or the surface of a flowing liquid film (film absorbers). The same group includes nozzle absorbers, in which liquid flows over the surface of the nozzle loaded into the absorber from bodies of various shapes (rings, lump material, etc.), and mechanical film absorbers. For surface absorbers, the contact surface is determined to a certain extent by the geometric surface of the absorber elements (for example, the nozzle), although in many cases it is not equal to it [10, c.10].

b) Bubble absorbers in which the contact surface develops by flows of gas distributed in the liquid in the form of bubbles and jets. Such gas movement (bubbling) is carried out by passing it through a liquid-filled apparatus (continuous bubbling) or in column-type apparatuses with various types of trays. A similar pattern of gas-liquid interaction is also observed in nozzle absorbers with a flooded nozzle.

The same group includes bubble absorbers with mixing of liquid by mechanical stirrers. In bubbling absorbers, the contact surface is determined by the hydrodynamic mode (gas and liquid flow rates).

c) Atomizing absorbers in which the contact surface is formed by spraying the liquid in the gas mass into fine droplets. Contact surface is determined by hydrodynamic mode (liquid flow rate). This group includes absorbers in which liquid spraying is carried out by nozzles (nozzle or hollow absorbers), in the flow of gas moving at high speed (high-speed straight-flow spraying absorbers) or rotating mechanical devices (mechanical spraying absorbers) [10, p. 11].

The above classification of absorption devices is conditional, since it reflects not so much the design of the device as the nature of the contact surface. Depending on the operating conditions, the same type of set may be in different groups. For example, nozzle absorbers can operate in both film and bubble modes. In devices with bubbling trays, modes are possible when significant liquid spraying occurs and the contact surface is formed mainly by droplets.

Of the various types of devices, nozzle and bubble poppet absorbers are currently the most common. When choosing the type of absorber, it is necessary to proceed in each particular case from the physicochemical conditions of the process taking into account technical and economic factors [8].

The main dimensions of the absorber (for example, diameter and height) are determined by calculation based on the specified working conditions (productivity, the required degree of extraction of the component, etc.). Calculation requires information on process static and kinetics. Static data are found from reference tables, calculated using thermodynamic parameters or determined experimentally. The data on kinetics largely depend on the type of apparatus and its mode of operation. The most reliable results of experiments conducted under the same conditions. In some cases, such data are not available and you have to resort to calculation or experiments.

Currently, there is no well-established method for determining the mass transfer coefficient by calculation either on the basis of laboratory or model experiments. However, for some types of apparatus, it is possible to find mass transfer coefficients with sufficiently high accuracy by calculation or relatively simple experiments. [10, page 11]

3.1. Device of surface absorbers

In the group of surface absorbers, fixed surface devices are included, that is, devices in which the contact surface is determined to a certain extent by the geometric surface of the absorber elements.

These sets, in turn, can be divided into the following main types:

1) Surface absorbers with horizontal liquid mirror.

2) Film absorbers.

3) Nozzle absorbers (with fixed nozzle).

4) Mechanical film absorbers. [10, page 304]

3.1.1 Surface absorbers with horizontal liquid mirror

In these absorbers, the gas passes over the surface of the stationary or slow flowing liquid, the liquid mirror being the mass exchange surface. The size of this surface is insignificant, as a result of which surface absorbers are used only at a small scale of production. Typically, several series connected absorbers are installed with countercurrent movement of gas and liquid. To carry out gravity flow of liquid, absorbers are arranged stepwise - each device downstream of the previous one is slightly lower. [10, page 304]

Previously, surface absorbers were made in the form of horizontal cylindrical apparatuses or vessels of a special shape (turills, integral "tariffs) made of ceramics. In such devices, the liquid occupies a significant part of the total volume, so they are convenient for removing heat generated during absorption. In the simplest case, heat is removed through the walls of the apparatus by means of natural air cooling. Coils cooled by water or other coolant are installed in absorbers for more intensive heat removal. In addition, external water cooling is used by placing absorbers in boxes with running water or sprinkled with water their external walls. [10, page 305]

Surface absorbers are ineffective and are currently in limited use. They are used mainly to absorb highly soluble components from small volumes of gas while removing heat. These absorbers are particularly useful in absorbing components from highly concentrated gas mixtures.

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