Calculation of dust and gas treatment plant

Contents

CONTENTS

ASSIGNMENT FOR THE COURSE PROJECT

INTRODUCTION

SUBSTANTIATION OF SELECTION OF SCHEMATIC DIAGRAM OF DUST AND GAS TREATMENT PLANT

APPARATUS CALCULATIONS

Method of precipitation chamber calculation

Cyclone Calculation and Cyclone Selection Procedure

Cyclone check calculation

Venturi Scrubber Calculation

Cooling chamber calculation

Calculation of packing absorber

CONCLUSIONS

LIST OF LITERATURE USED

Applications:

APPENDIX 1. Cyclone, heat exchanger, settling chamber, venturi scrubber, absorber. General views

APPENDIX 2. Absorption column. General View Drawing

APPENDIX 3. Scrubber Venturi. General View Drawing

Job for Course Project

The foundry includes a wagon for smelting cast iron. The capacity of the cupola is 6.5 t/h. Specific release of pollutants (q) is as follows: dust - 20 kg, nitrogen oxide - 19.5 kg per ton of smelted iron. The required amount of ventilation air per 1 ton of smelted iron is - 980 m3/h. Exhaust gas temperature is Tg = 225 0С. Design ambient temperature Tv = 18 0С.

Medium dispersion and friction composition of dust

The emission source is located from the settlement area at a distance of X = 30 m. It is necessary to design a dust and gas collecting plant that ensures cleaning efficiency within the established sanitary and hygienic standards and determine the MPA by dust and nitric oxide for the height of the pipe H = 15 + ΔH (m), where ΔH is identical to the value of the variant number. The concentration of emissions at the boundary of the settlement zone should not exceed the MPC.

Physical parameters of dust and exhaust gases :

Gas density (αg = 1.25 kg/m3)

Dust density (αpyl = 3050 kg/m3)

Average dust particle size and dispersion (dm = 20 μm), (αn = 0.468)

Gas pressure Pg = 120 kPa

Dynamic coefficient of viscosity of ventilating air μ = 25.2·106 n*s/sq.m

Introduction

Dust and gas treatment plants are used in various areas of production to clean the exhaust gas stream from harmful substances. This is necessary if the exhaust gas stream exceeds (after dispersion in air) the MPC limit values.

Foundries are widespread. When smelting cast iron, the following enters the ventilation system: SOx, COx, NOx, H2S, CH4, dust and compounds of arsenic, phosphorus, antimony, lead, mercury vapors, hydrogen cyanide and resinous substances .

Dust is one of the multi-tonnage emissions into the atmosphere. Technological processes in many sectors of the national economy are associated with the processing, transportation and storage of loose, finely divided materials. This leads to intense pollution of the airspace by dust. Dust refers to any particulate matter contained in gases emitted into the atmosphere. For dust collection, appropriate dust collecting equipment is used, the principle of operation of which is based on the use of inertial, gravitational, diffusion and electric forces.

To collect dust, it is necessary to install a dust and gas treatment plant in several stages for collecting dust of various dispersity, based on the fundamental features of the process of separating solid particles from the gas phase, these are:

- dry dust collecting equipment, which includes cyclones, dust-settling chambers, vortex cyclones, louvers and rotary dust collectors, electrofilters, filters;

- equipment for dust collection by wet method, which includes Venturi scrubber, nozzle scrubbers, foam devices.

To clean the gas from harmful chemical impurities, it is necessary to install an absorber, adsorber or chemosorber. In this case, we install a nozzle-type absorber with an absorbent Ca (OH) 2 - calcium milk (suspension).

Dust and gas treatment plant includes several stages:

The gas-dust treatment plant shall be 4-stage and consist of series-connected devices:

dust-settling chamber - the first stage;

cyclone - second stage;

Venturi scrubber - third stage;

absorber - fourth stage.

On assignment it is required to clean air from dust and gas NO.

The schematic diagram of the dust and gas treatment plant is presented

Substantiation of the selection of the schematic diagram of the dust and gas treatment plant

Volume of dust and air mixture ejected per second.

Calculations of devices

2.1. Method of precipitation chamber calculation

These chambers are the simplest devices for collecting dust. They are made in the form of hollow chambers of round or rectangular section with hopper at the bottom for dust collection. Particles in chambers are deposited under the influence of gravitational forces. The advantage of such devices is the ease of manufacture, low hydraulic resistance and availability of the materials used, which allows them to be manufactured in non-specialized enterprises. Disadvantages include low dust collection efficiency (40-50%), especially when collecting fine dust (< 20 μm). Therefore, they are often used as the first cleaning stage. To increase efficiency of operation vertical partitions are installed, which serve to change direction of gas flow movement. In such devices, along with the action of gravity, inertial forces also act, under the influence of which solid particles collide with obstacles and fall out of the flow. The gas velocity in the precipitation chambers should not exceed 1-1.5 m/s, otherwise the settled particles can be picked up by flow and carried away from the apparatus.

2.1.1. According to the dispersed and fractional composition of the dust, it is assumed that the diameter (d) of the particles to be deposited in the chamber is 50 and more μm.

2.2. Cyclone Calculation and Cyclone Selection Procedure

Cyclones belong to dry dust collectors - these are devices in which the separation of particles of impurities from the air flow occurs mechanically due to the forces of gravity, inertia, Coriolis. Structurally, dry dust collectors are divided into cyclones, rotary, vortex, radial, louver dust collectors.

Cyclones are most common for gas purification in industry. This is due to the relative simplicity of their structures, low hydraulic resistance, small overall dimensions and relatively high efficiency. Dust is generated in cyclones under action of centrifugal forces resulting from rotation of gas flow in apparatus body.

Dusty gas enters the cyclone through a tangentially arranged branch pipe, as a result of which it acquires a rotational movement. Having completed 2-3 turns in the annular gap between the body and the central pipe, the gas rotatably drops down, and in the cone part of the apparatus, due to a decrease in diameter, the flow rate increases. Under the action of centrifugal force, dust particles are thrown to the walls of the cyclone, so that the bulk of the dust particles is concentrated in a stream of gas moving in the immediate vicinity of the walls of the apparatus. This flow is directed to the lower part of the cone, dust particles at the same time enter the dust collector, and gas, having made a steep turn, is discharged through the central pipe from the apparatus. Thus, complex aerodynamic processes occur in the cyclone, the perfection of which depends on the efficiency of these devices.

Different types of cyclones are used in different industries depending on production conditions and cleaning requirements. Cyclones of the NIIOgas design are most widely used in industry. A distinctive feature of cyclones of the design of NIIL gas type TsN11, TsN-15, TsN15u, TsN-24 is the presence of an elongated cylindrical part of the cyclone and the inclination of the inlet branch pipe to the cylindrical part of the cyclone. Cyclones such as CN15 and CN24 have a higher throughput capacity and have a general industrial purpose, are widely used for gas purification in various industries due to the optimal ratio between dust collection efficiency and hydraulic resistance. The choice is made on cylindrical cyclone CP15.

2.4. Calculation of venturi scrubber

The Venturi scrubber is a turbulent dust collector - a unit composed of series-connected: a turbulent washer (Venturi pipe) and an inertial dust and spray collector or a wet direct-flow cyclone.

High-speed gas washers (Venturi scrubbers) - are a Venturi pipe, a gas stream moving in it at a high speed crushes its irrigating liquid into droplets on which dust is deposited and sludge is formed, removed from the apparatus.

The principle of scrubber operation is as follows. The gas to be cleaned enters the intake cylinder and is routed to the venturi. Nozzle is arranged at pipe inlet. Water is supplied to the nozzle by a centrifugal pump at a pressure of 300500 kPa, which sprays and captures dust particles, thereby cleaning the gas, after which it is drained into the sump. In the sump, dust particles are deposited on the bottom and periodically removed.

Water is supplied to centrifugal pump through water gate of sump. The purified gas is sent further through the cyclone drip trap in the process cycle. Dusty gas is introduced into the venturi confuser where the gas flow rate increases as the cross section decreases and reaches 150 m/s. At some distance from the neck, 80... 1000 kPa, which in a turbulent gas flow is sprayed onto the smallest droplets and intensively mixed with dust particles, wetting and enlarging them. In the diffuser, the flow rate drops, while further coagulation of the dust and its release in the impact-inertial dust collector or in the wet ramjet cyclone takes place.

The capacity of Venturi scrubbers varies widely and reaches 250,000 m3/h.

Wet cleaning devices operate on the principle of depositing dust particles either on the surface of liquid droplets or on the surface of a liquid film. The deposition of dust particles on the liquid occurs under the influence of inertia and Brownian motion. Inertia forces act on dust particles and water droplets as they approach. These forces depend on the mass of the droplets and particles, as well as on the speed of their movement. In addition, turbulent diffusion, interaction of electrically charged particles, condensation, evaporation processes, etc. affect the deposition process. An important factor affecting the efficiency of wet dust collectors is the wettability of particles with a liquid (the better wettability, the more efficient the cleaning process).

Venturi scrubber refers to wet dust collectors. They are widespread, as they are characterized by high efficiency of cleaning from fine dust (0.31.0 μm), as well as the ability to clean hot and explosive gases from dust. However, they also have a number of disadvantages, which limits the scope of their application: the formation of sludge in the process of cleaning, which requires special systems for its processing; removal of moisture into the atmosphere and formation of deposits in exhaust gas vents during cooling of gases to dew point; necessity of creation of cooling systems of water supply to dust collector.

The calculation of the scubber is determined by the geometric parameters of the Venturi pipe. Venturi scubber diagram is given in Fig. 2.3.1.

2.5. Calculation of gas cooler

Depending on the heat transfer method, two main groups of heat exchangers are distinguished:

1) surface heat exchangers in which heat transfer between heat exchanging media occurs through the heat exchange surface separating them;

2) mixing heat exchangers, in which heat is transferred from one medium to another at their direct contact.

Regenerative heat exchangers are much less commonly used in the chemical industry, in which the heating of liquid media occurs due to their contact with previously heated solids (nozzle) filling the apparatus periodically heated by another heat carrier.

Shell-and-tube heat exchangers. These heat exchangers are among the most commonly used surface heat exchangers. In the shell-and-tube heat exchanger, one of the heat exchanging media moves inside the tubes and the other in the tube-and-tube space.

The media is typically directed countercurrent to each other. At the same time heated medium is directed from bottom to top, and medium giving off heat in opposite direction. This direction of movement of each medium coincides with the direction in which the medium tends to move under the influence of a change in its density during heating or cooling.

Besides, at specified directions of media movement more uniform distribution of speeds and identical heat exchange conditions are achieved over cross-section area of apparatus. Otherwise, for example, when a cooler (heated) medium is supplied from above the heat exchanger, the warmer part of the liquid as lighter can accumulate in the upper part of the apparatus, forming "stagnant zones."

Pipes are fixed in grids most often by unfolding, at that particularly strong connection is achieved by arrangement in pipe grids of holes with circular grooves, which are filled with metal of pipe during its unfolding.

In addition, welding of pipes is used if the material of the pipe cannot be drawn and it is permissible to rigidly connect the pipes to the pipe grid, as well as soldering used for connecting mainly copper and brass pipes. Occasionally, connection of pipes with the grid is used by means of glands, which allow free longitudinal movement of pipes and the possibility of their quick replacement. This connection makes it possible to significantly reduce the temperature deformation of pipes, but is complex, expensive and not reliable enough.

2.6. Calculation of packing absorber

Absorption methods of gas sanitary cleaning are based on the ability of liquids to dissolve gases. Two phases - liquid and gas - are involved in the absorption process. During absorption, the substance is transferred from the gas phase to the liquid phase, and during desorption, on the contrary, from the liquid phase to the gas phase. Thus, absorption is the process of separating gas from a liquid. A substance that is contained in the gas phase and is absorbed into the liquid phase is referred to as an absorption component or an absortive. A substance that is contained in the gas phase and does not pass into the liquid phase upon absorption is called a gas carrier, or an inert gas. The substance in which the absorbed components dissolve is referred to as a solvent, absorber or absorbent. The devices in which the absorption process is carried out are called absorbers.

Distinguish between physical and chemical absorption (chemosorption).

Upon physical absorption, the absorbed component is physically dissolved in a solvent which is not accompanied by a chemical reaction.

In chemosorption, the absorbed component reacts chemically with the scavenger to form new chemical compounds in the liquid phase.

Column machines filled with a nozzle are widely used in industry as absorbers. The contact of gas with liquid in such apparatuses occurs mainly on the wetted surface of the nozzle along which the reflux liquid flows.

The nozzle absorber is made in the form of a cylinder, in the lower part of which a support grid is installed. A nozzle is laid on the grill in bulk or in regular rows. Irrigation liquid is supplied to the nozzle from above by means of special irrigation devices. In the lower part of the device there are connectors for gas supply and liquid discharge. Nozzles for liquid supply and gas discharge are located in the upper part of the apparatus. The liquid supplied from above tends to flow to the periphery. At the same time, the nozzle in the center of the apparatus remains wetless, forming the so-called dry cone. For uniform fluid flow along the nozzle, it is laid in regular rows along the entire height of the apparatus, reducing the size of the nozzle from bottom to top. For the same purpose, the nozzle is divided into sections, between which redistribution devices for liquid are installed.

The hydraulic mode and efficiency of the absorber depend on the correct choice of the type of nozzle and its laying. The largest use was made of a cylindrical annular nozzle - the Rashig ring, which is thin-walled bodies, the outer diameter of which is usually equal to the height. The nozzle is most often made of ceramics or porcelain, less often - from metals, carbon graphite and plastic masses.

Nozzle irrigation has a decisive effect on the efficiency of the nozzle absorbers. There are a large number of nozzle irrigation devices.

Sprinkling of nozzles can be carried out using compact and crushed jets. Compact jets provide non-spraying sprinklers. They are used at low liquid consumption and when large splash is unacceptable.

Tubular liquid distributors are the simplest in design. They are usually used in columns of small diameter. In this case, we choose a tubular distributor with a double-ring manifold.

Distribution plates ensure uniform distribution of liquid at the nozzle end. In this case, a smooth perforated distribution plate is used.

Conclusions

During the course design, during the calculation of the dust and gas treatment plant, the necessary maximum permissible concentrations for dust and gas emissions in the settling zone were provided.

Efficiency of each cleaning stage:

1. Sedimentation chamber: ¼ 1 = 0.95

2. Cyclone: ¼ 2 = 0.92

3. Venturi scrubber: ¼ 3 = 0.99

4. Absorber: ¼ 4 = 0.95

List of used literature

Main processes and devices of chemical technology: Design Manual/G.S. Borisov et al. Ed. Yu.I. Dytnersky, 5th ed., stereotypical. M.: Alliance Publishing House LLC, 2010-496 p.

Sinheev S.A. Methodological guidelines. - Syzran: SF SamGTU, 2010.

Sinheev S.A. Course of lectures on the discipline "Environmental Protection Technique," SF SamGTU

Kuznetsov I.E. and others. Gas Sanitary Cleaning Equipment: Handbook. - K.: Technique, 1989. – 304 pages.

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