Particulate Emission Control by Mechanical Separation & Wet Gas Scrubbing | Environmental Engineering - Civil Engineering (CE) PDF Download

Particulate Emission Control By Mechanical Separation

The basic mechanisms by which particulate matter is removed from a gas stream are:

• Gravitational settling
• Centrifugal impaction
• Inertial impaction
• Direct interception
• Diffusion
• Electrostatic attraction

Equipment that remove particles make use of one or more of these mechanisms. Broad categories of mechanical particulate control devices are:

• Gravitational settling chambers
• Cyclone separators
• Fabric filters (bag-house)
• Electrostatic precipitators

Gravitational Settling Chambers

Description: Gravitational settling chambers are simple large-volume enclosures that reduce the horizontal velocity of a gas stream so that particles settle under gravity to collection surfaces. They are typically used for removing coarse, abrasive particles (commonly >50 µm) where large floor area is available. Typical design gas velocities through settling chambers range from 0.5 to 2.5 m/s.

Advantages: Low pressure loss; simple design and maintenance.

Disadvantages: Large footprint required; low collection efficiency for small particles; effective mainly for large particles.

Design using Stokes' law

When particle Reynolds number is small and Stokes' law applies, the terminal settling velocity of a spherical particle is given by:

v_t = ( (ρ_p - ρ_f) · g · d_p² ) / (18 · μ)

Where:

• v_t = terminal settling velocity (m/s)
• ρ_p = particle density (kg/m³)
• ρ_f = fluid (gas) density (kg/m³)
• g = acceleration due to gravity (9.81 m/s²)
• d_p = particle diameter (m)
• μ = dynamic viscosity of the gas (Pa·s)

If the horizontal gas velocity in the chamber is u, a particle will be collected (reach the floor) if its settling velocity v_t is at least equal to u · (h / L) depending on chamber geometry; commonly the simple criterion used is v_t ≥ u for full-width chambers where the residence time is L/u. Solving for particle diameter gives:

d_p = √( (18 · μ · v_t) / ( (ρ_p - ρ_f) · g ) )

Using this relation one can compute the minimum particle diameter that will be removed with near 100% theoretical efficiency for a given chamber geometry and gas conditions.

  (2.2.1)  

(2.2.2)  
   Where, g=gravitational constant, m/s²; ρ_p=density of particle, kg/m³; ρ_a=density air, kg/m³; d_p=diameter of particle, m; µ_a=viscosity of air, kg/m s; H=height of settling chamber, m; v_h=horizontal flow-through velocity, m/s; and L=length of settling chamber, m. 

(2.2.3)  
 All particles larger than d_p will also be removed with 100% efficiency, while the efficiency for smaller particles is the ratio of their settling velocities to the settling velocity of the d_p particle. 

Cyclone Separators

Description: A cyclone separator is a centrifugal device consisting of a cylindrical body, a conical base and a tangential inlet. Dust-laden gas enters tangentially and sets up a swirling flow; centrifugal forces drive particles toward the wall where they slide down into a hopper, while cleaned gas spirals upward and exits at the centre.

         (2.2.4) 
 Where, F_c=centrifugal force, N; M_p=particulate mass, Kg; V_i equals particle velocity and R equals radius of the cyclone, m/s. From this equation, it can be seen that the centrifugal force on the particles, and thus the collection efficiency of the cyclone collector can be increased by decreasing R. Large-diameter cyclone have good collection efficiencies for particles 40 to 50 µm in diameter.  

Factors affecting performance: Collection efficiency depends on centrifugal force, which increases with particle mass, gas velocity (swirl intensity) and decreasing cyclone diameter. Smaller particles require higher centrifugal acceleration (higher peripheral velocity or smaller diameter) for good capture.

Advantages:

• Relatively inexpensive; simple to design and maintain.
• Requires less floor area than settling chambers.
• Low to moderate pressure drop compared with some other devices.

Disadvantages:

• Require headroom (height) for the conical section and separation.
• Collection efficiency drops for small particles (submicron range).
• Sensitive to variations in dust loading and flow rate.

Fabric Filters (Bag-house)

Description: In a fabric filter, the particulate-laden gas passes through a woven or felted fabric. Particles are retained by a combination of direct interception, inertial impaction, diffusion and electrostatic attraction. As a dust cake forms on the fabric, sieving becomes an important mechanism and submicron particles can be collected effectively.

Filter bags are usually tubular or envelope-shaped and may be between 1.8 and 9 m long in industrial installations. The bag-house supports and arranges the bags so deposited dust can be removed into a hopper.

Fabric and fibre characteristics: Fabrics may be:

• Woven - fibres arranged in a regular repeating pattern; gas flows through interstices and particles are bridged by impaction and interception.
• Felted (needled) - randomly oriented fibres compressed into a mat; provides a more tortuous path and higher initial capture of fine particles.

The choice of fibre depends on operating temperature, abrasion and chemical exposure. Common fibres include cotton (low-cost, low-temperature service), glass fibre (high-temperature service - typically used with protective coatings or lubricants to reduce abrasion) and synthetic fibres (e.g., PTFE, aramids) for chemically aggressive or high-temperature service.

Cleaning methods: Fabric filters are cleaned to restore permeability. Common methods are:

• Mechanical shaking (periodic)
• Reverse-air cleaning (periodic)
• Pulse-jet cleaning using short bursts of compressed air (intermittent or continuous)

Advantages: Very high collection efficiency, capable of removing particles down to 0.1 µm in substantial quantities; suitable for dry handling and final polishing of gas streams.

Disadvantages: Hot gases often must be cooled; gas must be kept dry to avoid condensation and clogging; fabrics are subject to chemical attack and abrasion; periodic maintenance is required to remove the dust cake.

Electrostatic Precipitator (ESP)

Description: An electrostatic precipitator charges particles electrically and collects them on oppositely charged collector plates under the influence of an electric field. ESPs are widely used in power plants, cement works, paper mills and refineries.

Operating principle: The system comprises discharge electrodes (often negatively charged wires) and grounded collecting surfaces (plates). At sufficiently high DC voltage (often of the order of 50 kV) a corona discharge occurs near the electrode. The corona produces ions which charge particles in the passing gas. Once charged, particles drift toward the collector plates and are deposited. Deposited dust is removed periodically by mechanical rapping or vibration; the dislodged dust falls into hoppers for disposal.

Advantages:

• Low routine maintenance when corrosive or adhesive substances are absent.
• Few moving parts.
• Can operate at elevated temperatures (commonly up to 300-450 °C depending on design and materials).

Disadvantages:

• Higher initial capital cost than some alternatives.
• Sensitivity to variable dust loading and sudden changes in gas flow.
• Requires high voltage with associated safety considerations.
• Collection efficiency can decline without proper maintenance (e.g., due to rapping damage, coating or re-entrainment).

Particulate Emission Control By Wet Gas Scrubbing

Description: Wet scrubbers remove particulate matter by transferring particles into liquid droplets or by contacting particles with a wetted surface. The key aim is to provide close contact between scrubbing liquid (commonly water) and the particulate. Collection mechanisms are the same as for filters - inertial impaction, interception and diffusion - but the interaction is between particles and liquid droplets or wet surfaces.

Particle-size dependence: For many scrubbers, impaction and interception dominate for particles larger than about 3 µm; diffusion becomes important for particles smaller than about 0.3 µm. Between these ranges there is typically a most-difficult-to-collect size (a minimum in efficiency).

Major types of wet scrubbers are:

• Plate scrubber
• Packed-bed scrubber
• Spray scrubber
• Venturi scrubber
• Cyclone scrubber
• Impingement-entrainment scrubber
• Fluidized-bed scrubber

Plate Scrubber

Description: A plate scrubber (sieve-tray scrubber) is a vertical tower fitted with perforated horizontal plates. Gas flows upward through the perforations while water flows downward across the plates (countercurrent). The perforations create droplets and wet surfaces that capture particles.

Collection efficiency improves with smaller perforation diameter because smaller holes produce finer droplets and greater contact area. For example, sieve plates with 3.2 mm diameter holes are often cited in literature as providing appreciable collection of fine particles; finer holes or additional trays increase removal of smaller particles.

Packed-bed Scrubber

Description: A packed-bed scrubber operates like a packed-column gas absorber: gas flows through a column packed with structured or random packing while liquid flows countercurrent or cocurrent over the packing. The packing provides large wetted surface area for gas-liquid contact and particle capture.

Performance: Collection efficiency increases as packing size is reduced (larger surface area and better mixing). Practical packed columns packed with elements about 2.5 cm in size can attain cut diameters on the order of 1.5 µm (50% collection diameter) under good operating conditions.

Spray Scrubber

Description: In spray scrubbers, liquid is atomised into droplets by nozzles and the gas stream passes through the droplet cloud. Particles are collected by capture within droplets via impaction, interception and diffusion.

Key parameters: Droplet size, gas velocity, liquid-to-gas ratio and droplet trajectories determine performance. For droplets falling at terminal velocity, the optimum droplet diameter for fine-particle collection is commonly in the range 100-500 µm. Typical liquid-to-gas ratios are in the range 0.001 to 0.01 m³ liquid per m³ gas. Spray scrubbers in simple gravitational arrangements can achieve cut diameters on the order of a few micrometres (e.g., about 2.0 µm) for coarse applications.

Venturi Scrubber

Description: A venturi scrubber uses the gas stream velocity to atomise the scrubbing liquid. The throat of the venturi is a region of very high gas velocity; high relative velocities between droplets and particles increase inertial impaction and capture.

Operating range: Typical gas velocities in venturi throat regions are in the range 60-120 m/s. Venturi scrubbers give high collection efficiency for small particles at the cost of higher pressure drop and energy consumption compared to simple spray or packed-bed scrubbers.

Cyclone Scrubber

Description: A cyclone scrubber combines centrifugal separation with wetting: liquid droplets are introduced into the swirling flow to capture particles by impaction and by creating a wetted wall. Sprays can be directed outward from a central manifold or inward from the cyclone wall, depending on design.

Notes: Cyclone scrubbers are suitable where moderate collection with lower liquid usage is desired; they are often used as pre-cleaners ahead of other devices.

Impingement-Entrainment Scrubber

Description: Gas is forced to impinge on a liquid surface so that particles contact the liquid film. Some liquid atomises and becomes entrained in the gas; gas exits are designed to minimise loss of entrained droplets while maximising particle capture by the liquid surface.

Fluidized-bed Scrubber

Description: A fluidized-bed scrubber contains a zone of fluidized packing where gas and liquid mix intensively. Gas passes upward through the packing while liquid is sprayed from the bottom or flows down; the fluidized packing promotes intimate gas-liquid contact and good particle removal for a given liquid flow.

Selection considerations for particulate control: When selecting a control device, engineers balance required collection efficiency, particle size distribution, gas temperature, corrosiveness/abrasiveness of dust, available footprint and headroom, pressure-drop limits, liquid handling (for wet systems), maintenance capability, and capital cost. Often a combination of devices (e.g., cyclone pre-cleaner followed by fabric filter or ESP) gives the best overall performance and economy.

Final summary: Mechanical devices (settling chambers, cyclones, fabric filters, ESPs) are preferred for dry systems and large particles or when low water use is required. Wet scrubbers are flexible for handling sticky or hot gases and for simultaneous gas-phase pollutant removal, but they require liquid handling and wastewater treatment. The correct choice depends on particle sizes, gas properties, plant constraints and regulatory requirements.