Cámaras de sedimentación de partículas contaminantes del aire (partículas y controles)

Name: Cámaras de sedimentación de partículas contaminantes del aire (partículas y controles)
Uploaded: 2023-07-23T18:51:43.000Z
Duration: 2 h 44 min 32 s

Contamination by Particulate Matter and Control Techniques

Introduction to Particulate Matter

The lecture is presented by Engineer Martín Moros, a professor at the Universidad Nacional Experimental del Táchira, Venezuela, focusing on atmospheric contamination due to particulate matter and control techniques.

Classification of Particles

Particles are classified based on size (large, medium, small) and aggregation state (discrete or aggregated). Discrete particles exist individually while aggregated particles consist of multiple units.

Aerosols are defined as solid or liquid particles suspended in air. Their key properties include size and shape; although often simplified as spherical for calculations, they can have irregular forms.

Chemical Composition and Health Effects

Particles can be organic or inorganic, toxic or harmless (e.g., sand). Smaller particles pose greater health risks as they penetrate deeper into the respiratory system.

PM10 can reach deeper airways while PM2.5 can enter the bloodstream, affecting various organs including the brain. These smaller particles are linked to severe health issues such as heart disease and lung cancer.

Impact on Mortality Rates

The World Health Organization estimates over 3 million premature deaths annually due to airborne particulate matter. A premature death is defined as occurring before the average life expectancy.

Visibility and Environmental Effects

Atmospheric pollutants lead to reduced visibility; particle sizes close to visible light wavelengths contribute significantly to this effect.

Gases like sulfur dioxide and nitrogen oxides also affect visibility through refraction and absorption of light.

Particle Behavior in the Atmosphere

Smaller particles remain airborne longer than larger ones which sediment more easily. Rain helps cleanse the atmosphere by removing these particulates.

Role of Vegetation in Pollution Mitigation

Trees play a role in retaining atmospheric pollution; coniferous trees with larger leaves capture between 3% to 15% of airborne contaminants.

Photosynthesis Implications

Accumulated particulates on leaves may obstruct stomata, hindering photosynthesis processes in plants.

Erosion and Natural Suspension of Particles

Wind erosion naturally suspends large quantities of particulate matter into the air.

Size Classification of Particulates

PMx classification indicates particle diameter where 'x' varies (e.g., PM2.5 for fine particles). Larger particles range from 500 to 1000 micrometers depending on density.

This structured summary encapsulates key insights from Engineer Martín Moros's lecture regarding particulate matter's impact on health, environment, and its classification within atmospheric studies.

Understanding Particulate Matter: Classification and Impact

Classification of Particulate Matter

Particles up to 10 micrometers are categorized as PM10, which sediment more easily due to natural environmental processes like decantation.

PM10 particles range from 2.5 to 10 micrometers, penetrating deeper into the respiratory system beyond just the nasal passages.

The smallest category, PM2.5, consists of particles between 0 and 2.5 micrometers that can dissolve in blood and affect overall health significantly.

Larger particles (over 10 micrometers), referred to as PM1000 or PM500, can vary greatly in size (up to 1000 micrometers), but their classification is not strictly defined due to density variations.

The classification of larger particles remains ambiguous since they can float in the air at sizes ranging from 10 to potentially over 1000 micrometers.

Characteristics of Different Particle Sizes

Large particles (PM500/PM1000) include dust, ash, and aerosols; these are significant contributors to air quality issues.

Medium-sized particles (PM10) fall between sizes of 2.5 and 10 micrometers; they pose health risks by entering the respiratory tract.

Smallest particulate matter (PM2.5), ranging from below 1 micron down to nearly zero, has severe implications for visibility and health due to its ability to enter systemic circulation.

Definition and Types of Particulate Matter

Particulate matter refers to solid or liquid materials suspended in a gaseous state under normal atmospheric conditions; it includes both discrete masses and aerosols.

Sands are classified as coarse particles greater than 75 microns based on sieve size; dust ranges from solid material between approximately 1 and over 76 microns.

Smoke consists of sub-micron particles formed through condensation or chemical reactions during combustion processes; it includes residual products with varying heat capacities.

Sources of Particulate Matter

Mist refers to liquid droplets smaller than 10 micrometers; when dense enough, it becomes fog which obscures vision significantly.

Soot is a form of carbon particle produced during incomplete combustion; unlike smoke, soot retains combustible properties without ash content.

Industrial Contributions and Control Measures

Industries such as iron and steel production generate particulate matter through processes like smelting, producing controllable emissions via cyclones or electrostatic precipitators.

Oil refineries produce catalyst dust during regeneration processes that can be managed using similar filtration technologies including electrostatic precipitators.

Cement manufacturing generates particulates during drying processes that require effective management through various filtration systems.

This structured overview provides insights into the classification, characteristics, sources, and control measures related to particulate matter based on the provided transcript.

Emission Control in Industrial Processes

Overview of Emission Sources and Control Methods

Discusses the treatment of inorganic acids in precipitators or droplet separators, highlighting their high solubility in water. The example of glass manufacturing is provided, where acid droplets and alkaline oxides are generated during furnace processes.

Emphasizes the importance of emission factors for characterizing particle emissions, which aids in calculating potential particulate matter released based on specific industrial processes.

Explains various combustion processes (natural gas, distillate oil, residual oil, coal), noting that coal combustion is particularly contaminating due to its higher ash production rates.

Provides a calculation method for estimating particulate emissions from burning distillate oil in domestic furnaces, indicating that 8 pounds of particles can be produced per 1,000 pounds of fuel burned.

Highlights the role of control equipment in reducing emissions; if an 80% removal efficiency is achieved by control systems, only 20% will exit through the chimney.

Types of Industries and Their Emissions

Mentions various industries such as cement manufacturing and steel production that contribute to atmospheric particle emissions. It stresses understanding emission factors relevant to each industry type.

Stresses the significance of knowing the type and quantity of fuel used when assessing emission factors for accurate calculations during fieldwork at companies.

Trends in Particle Emission Reductions

Notes a general trend observed between 1993 and 2002 in the U.S., where suspended particle levels decreased by approximately 22%, attributed to improved control technologies like cyclones and filters.

Discusses how submicron particles behave similarly to gas molecules due to their small size, making them harder to treat as they do not easily settle out of air mixtures.

Particle Size and Control Efficiency

Describes how medium-sized particles (1 to 20 microns) follow gas movement but can sediment under certain conditions. Larger particles have higher sedimentation velocities making them easier to control.

Indicates that larger particles (>20 microns) settle more quickly from the air compared to smaller ones. This characteristic makes them less persistent as airborne contaminants.

Removal Efficiency Based on Particle Size

Illustrates a graph showing removal percentages for different particle sizes: PM10 shows high removal rates (95%-100%), while PM2.5 has significantly lower rates (0%-55%).

Concludes that controlling suspended particles focuses on capturing them at their source before selecting appropriate control devices based on several factors including particle characteristics and flow rate.

Mechanisms Behind Control Devices

Discusses considerations for choosing pollution control devices based on particle characteristics such as size distribution, flow rate, temperature, humidity content, and chemical properties like acidity or flammability.

Explains gravitational sedimentation as a fundamental mechanism behind some control devices like sedimentation tanks; heavier particles are drawn downwards by gravity effectively separating them from gases.

Sedimentation and Particle Control Mechanisms

Gravitational Sedimentation and Centrifugal Impactation

Gravitational sedimentation significantly influences larger particles, forming the basis for expansion chambers in sedimentation tanks.

Centrifugal impactation relies on circular motion where inertia causes particles to be expelled outward, collecting on the inner walls of cylinders known as cyclones.

Inertia and Particle Collision

Inertia plays a crucial role in particle behavior; heavier particles resist directional changes more than gas, leading to collisions with interceptors.

As gas moves upward through a channel with an obstructing plate, heavier particles collide with the plate due to their greater inertia, effectively removing them from the gas stream.

Direct Interception and Filtration

Smaller particles may still contact interceptors despite following laminar flow patterns; this is critical for filter operation.

Diffusion allows sub-micron particles to collide with collectors via Brownian motion, which is essential for wet collectors and fabric filters.

Electrostatic Precipitation Fundamentals

Opposite electric charges attract; by ionizing air through an electric field, charged ions adhere to particles, allowing them to be attracted to oppositely charged plates.

This principle underlies electrostatic precipitators that clean air by capturing charged particles on plates.

Types of Particle Collection Equipment

Various particle removal processes include gravitational sedimentation tanks and multiple plate sedimenters. Cyclonic separators and wet collectors are also utilized based on energy levels.

Particle collection systems can be categorized into wall collectors (sedimenters, cyclones, electrostatic precipitators) or flow division systems (surface filters, depth filters).

Introduction to Simple Particle Control Equipment

The discussion transitions into simpler equipment like sedimentation tanks that utilize gravity for solid particle removal from gas flows.

Gas enters a sedimentation chamber where its velocity decreases due to an expanded cross-sectional area. This reduction creates conditions favorable for particle settling.

This structured summary provides a comprehensive overview of key concepts related to particle control mechanisms discussed in the transcript while maintaining clarity and accessibility.

Sedimentation and Particle Control in Gas Streams

Overview of Sedimentation Processes

The process involves funnels where clean gas exits through a blue arrow, with larger particles falling into a hopper. Sedimentation chambers are effective primarily for large particle removal and are often used alongside more efficient control devices.

Expansion chambers serve as pre-treatment for air particles. They are mechanically collected, low-cost to install and maintain due to the absence of moving parts, making them particularly useful in industries requiring gas cooling before filtration.

Applications in Industry

Sedimentation chambers find applications in metal processing and mineral product industries, especially in dirty gas streams from foundries or metallurgical processes.

Different designs exist for sedimentation chambers, including circular and rectangular cross-sections. These can be adapted based on geometry and can effectively retain particles larger than 100 microns, with some efficiency for smaller particles down to 20 microns.

Particle Behavior During Sedimentation

The sedimentation velocity is influenced by particle size; larger particles (over 10 microns) settle faster compared to smaller ones (1 micron), which have negligible settling speeds.

A one-millimeter particle can sediment at speeds up to 6 meters per second, while a one-micron particle settles at an almost imperceptible speed of 0.00006 meters per second.

Forces Acting on Particles

The efficiency of sedimentation is determined by the settling velocities: large particles approach nearly 100% efficiency while small ones hover around zero due to their low settling speeds.

Analyzing discrete spherical particles reveals that gravity pulls them downward while buoyancy opposes this force. Equilibrium occurs when these forces balance out.

Dynamics of Particle Movement

When gravitational force exceeds buoyant force due to greater mass, the particle begins its descent but experiences frictional resistance from surrounding gas molecules during this movement.

Gravitational force is calculated as the volume multiplied by density times gravity; buoyant force relates similarly but considers the displaced volume of gas around the descending particle.

Understanding Frictional Forces

As a descending particle moves through air, it displaces an equivalent volume of air equal to its own volume—this displacement affects how it interacts with surrounding media during descent.

The buoyant force corresponds to the weight of displaced air during descent; thus, it’s essential for understanding how different densities affect sedimentation dynamics.

Mathematical Modeling of Forces

Frictional forces are defined mathematically using drag coefficients and other parameters related to particle characteristics and flow conditions.

By equating definitions for gravitational, buoyant, and frictional forces within mathematical models, we derive equations that describe terminal sedimentation velocity under specific conditions dominated by gravitational pull over other forces.

Understanding Particle Settling Velocity

Fundamental Equations for Settling Velocity

The equation for determining the settling velocity of particles is based on gas density and applies to any particle shape and flow regime.

The drag force experienced by a particle depends on its shape and flow regime, which is characterized by the Reynolds number (Re). A Re between 0 and 0.5 indicates laminar flow, while values above 500 indicate turbulent flow.

Reynolds Number and Flow Regimes

The Reynolds number is defined as the ratio of particle density times gas velocity times relative particle velocity to gas viscosity, influencing drag force calculations. For laminar flow, drag coefficient (Cd) can be calculated using Cd = 24/Re .

In transitional flow conditions, the drag coefficient formula adjusts to include additional terms: Cd = 24/Re + 0.15(Re^0.6) . This highlights how different regimes affect sedimentation dynamics.

Particle Shape Considerations

Particles can be regular or irregular in shape; irregular shapes complicate area and volume calculations necessary for determining settling velocities. To simplify, spherical assumptions are often made for irregular particles.

For spherical particles, the relationship between volume (V) and area (A) leads to a simplified expression where V/A = 2/3D , allowing derivation of terminal settling velocity equations based on gravitational forces and density differences.

Application of Stokes' Law

Stokes' Law provides a foundational equation for terminal velocity applicable under laminar conditions specifically for spherical particles: V_t = 4/3g(rho_p - rho_g)D^2/C_d . This law is crucial in sedimentation processes across various fluid regimes but primarily used in laminar flows due to its idealized nature.

Adjustments must be made when dealing with non-spherical particles; correction factors are applied based on particle geometry to ensure accurate diameter representation in calculations. For instance, sharp-edged particles may use a correction factor of 0.81 when calculating equivalent diameters from their geometric properties like length or width instead of true diameter measurements.

Limitations of Stokes' Law

While Stokes' Law behaves linearly under certain conditions, deviations occur at extreme sizes—both very small and very large particles—requiring corrections to maintain accuracy in predictions about sedimentation behavior within practical engineering contexts such as pollution control measures targeting fine particulate matter removal effectively.

Understanding these limitations emphasizes the importance of focusing on smaller particles during environmental assessments since they pose greater challenges regarding effective removal strategies compared to larger ones that settle more readily due to higher velocities achieved during sedimentation processes.

Understanding Particle Sedimentation and Correction Factors

Introduction to Particle Size and Corrections

The discussion begins with the need for correction in sedimentation calculations for particles smaller than 5 micrometers, emphasizing the importance of accurate measurements.

The Cunningham correction factor is introduced, which adjusts the Stokes' law velocity based on temperature (in Kelvin) and particle diameter (in micrometers), ensuring dimensionless units.

Application of Stokes' Law

By applying the Cunningham correction factor to the terminal velocity determined by Stokes' law, a more accurate sedimentation velocity can be calculated, referred to as the "Stokes-Cunningham law."

Graphical Representation of Sedimentation Velocity

Reference is made to Figure 8 in supporting materials that illustrates linear or slightly curved graphs used to determine terminal sedimentation velocities.

The axes of these graphs are differentiated: one axis represents small particles (0.1 - 30 micrometers), while another represents larger particles (10 - 3000 micrometers).

Reading Sedimentation Velocities from Graphs

Instructions are provided on how to read sedimentation velocities from the graph, indicating that small particles are read from bottom right while large particles are read from top left.

It is noted that these readings yield sedimentation velocities typically measured in centimeters per second.

Operational Parameters for Sedimentators

Effective operation requires sedimentation velocities greater than 25 feet per minute or 13 centimeters per second to efficiently draw particles downward.

For particle diameters over 50 micrometers, sedimentation chambers can be utilized if particle densities are low; however, lighter particles at this size cannot effectively settle.

Design Considerations for Sedimentation Chambers

Horizontal gas velocity must remain low (ideally below one foot per second or less than 30 centimeters per second) to prevent resuspension of settled particles.

Chamber dimensions—width, length, and height—are critical for effective design; both horizontal and vertical speeds must be optimized.

Efficiency Models in Sedimentation Chambers

Two models for determining chamber efficiency are discussed: piston flow model and complete mixing model. These models help assess gas and particle behavior within chambers.

Calculating Height and Efficiency

The relationship between chamber height and particle settling time is established; longer chambers with high terminal velocities enhance efficiency but should not be excessively tall.

An equation relating efficiency factors such as length, gravity, particle diameter squared, density versus height and gas velocity is presented.

Conclusion on Time Management in Gas Chambers

Emphasis is placed on ensuring that sedimentation time does not exceed residence time within the chamber; this balance determines optimal separation sizes.

Sedimentation Principles and Equipment Design

Theoretical Foundations of Sedimentation

The minimum particle size that can be removed in sedimentation chambers is based on theoretical principles, assuming piston flow and uniform particle distribution in laminar flow.

Simplifications assume no resuspension or reintegration of particles, focusing on the cut diameter (d50), which represents the particle size retained with 50% efficiency.

Flow Dynamics and Efficiency

In perfect mixing flow, turbulence must be present for effective mixing; however, this reduces equipment efficiency, necessitating longer sedimentation chambers to ensure particle removal.

A perfect mixing model leads to longer sedimentation chambers due to losses at entry, exit, and within the main chamber.

Pressure Loss Calculations

Total pressure drop in a sedimentation unit is calculated using gas density and velocity, factoring in losses from chamber length and hydraulic radius.

Loss factors are determined by equations related to inlet and outlet conditions; these calculations are crucial for understanding system performance.

Types of Sedimentation Chambers

Two primary types of sedimentation chambers are identified: expansion chambers and multiple plate chambers. Recommended heights range from 0.5 to 0.9 meters with design velocities between 0.3 to 2.5 m/s.

Multiple plate configurations enhance efficiency by reducing height while maintaining large flow rates; this design allows for better performance compared to traditional designs without plates.

Practical Example: Designing a Sedimenter

An example involves calculating a sedimenter designed to remove particles with a density of 1200 kg/m³ from an air stream at specific conditions (1.015 bar pressure, 350 K temperature).

Key parameters include volumetric flow rate (2 m³/s), chamber dimensions (2.5m high x 3m wide), and configuration as a mixed-type sedimenter with multiple plates.

Calculating Reynolds Number and Efficiency

The task includes determining the Reynolds number for gas flow, required chamber length for complete removal of particles sized at 60 micrometers, along with collector efficiencies for smaller sizes down to 10 micrometers.

Initial calculations focus on finding the Reynolds number using gas density derived from ideal gas laws; essential variables include pressure, molecular weight of air (29), gas constant R, and temperature in Kelvin.

This structured overview captures key insights into sedimentation processes while providing timestamps for easy reference back to specific parts of the transcript.

Fluid Dynamics and Efficiency Calculations

Understanding Flow Rate and Velocity

The flow rate is defined as velocity multiplied by the cross-sectional area. The height is 2.5 meters, width is 3 meters, leading to a gas velocity of 0.267 m/s based on a flow rate of 2.

Viscosity and Reynolds Number

The viscosity of air at 350 Kelvin is found to be 0.0748 kg/m³/h, which can be verified through various online resources or tables. This value is crucial for calculating the Reynolds number.

Substituting values into the Reynolds equation yields a result of 0.779; since this value exceeds 0.5, it indicates that the flow regime is not laminar but transitional instead.

Transition Regime Considerations

For transitional regimes, specific equations are used to calculate sedimentation rates:

Laminar: C_s = 24/Re

Turbulent: C_s = 0.44

Very turbulent: C_s = 0.1 . These constants are essential for efficiency modeling in fluid dynamics applications.

Efficiency Models in Fluid Dynamics

The complete mixing model suggests that while there may be some mixing within the chamber due to gas movement, assuming ideal conditions without such movements could lead to inefficiencies in design calculations. Thus, engineers should prefer using complete mixing efficiency models when applicable.

Terminal Velocity Calculations

Given that the exercise initially assumed laminar flow but confirmed it as transitional, terminal velocity will be calculated accordingly using transition equations before applying laminar models for efficiency assessments later on in calculations.

The formula for efficiency incorporates variables like tray numbers (n), terminal velocity (V_t), chamber length (L), width (W), and flow rate (Q). In this case:

n = 30 trays including the bottom.

Aiming for an ideal scenario with a terminal velocity greater than or equal to 13 cm/s leads to determining necessary chamber dimensions based on these parameters.

Ultimately calculated terminal velocity was found to be approximately 16.3 cm/s, meeting design criteria satisfactorily.

Further Analysis on Particle Sizes

To assess efficiencies across different particle diameters ranging from 50 micrometers down to smaller sizes like 10 micrometers:

Calculate respective Reynolds numbers and determine whether they fall under laminar or transitional regimes.

Use established equations for sedimentation rates corresponding with each regime type.

Results indicate larger particles exhibit higher removal efficiencies as expected theoretically; this relationship can be graphically represented for clarity in analysis and understanding trends over varying particle sizes.

New Problem Formulation

A new problem formulation suggests increasing volumetric flow rates up to 20 cubic meters per second while maintaining similar parameters from previous exercises—this sets up further exploration into fluid dynamics principles under altered conditions and their implications on system performance metrics moving forward.

Determining Particle Removal Efficiency in Turbulent Flow

Length Requirement for Particle Removal

The discussion begins with determining the required length of a chamber to achieve 96% particle removal efficiency for particles measuring 60 micrometers.

It also explores the efficiency rates for smaller particles (50, 40, 30, 20, and 10 micrometers) using the previously determined chamber length.

Turbulent Flow Considerations

The importance of turbulent flow is emphasized; it suggests utilizing a complete mixing efficiency model while respecting Reynolds numbers for respective scenarios.

Proposed Problems in Gravity Sedimentation

A gravity sedimentation collector problem is introduced, focusing on particles with a diameter of 100 micrometers and a density of 1.5 grams per cubic centimeter (1500 kg/m³).

The goal is to determine the maximum gas velocity achievable in a chamber that measures two meters in height and two meters in width while maintaining at least 90% collection efficiency.

Minimum Particle Diameter Calculation

Another problem involves calculating the minimum diameter of particles (density: 2 g/cm³) that can be collected with an efficiency of 85% using equipment that is twelve meters long and two meters high at a gas velocity of 0.78 m/s.

Additional Gravity Sedimentation Problem

A third scenario examines collecting particles sized at 70 micrometers with a relative density of 1.5 (1500 kg/m³), considering laminar flow conditions.

It seeks to find out the maximum gas velocity permissible in a collector measuring three meters high and five meters long without accounting for recirculation effects within the chamber.

Conclusion

The speaker expresses gratitude for attention and hopes that the class content will be beneficial to attendees.