Seperation Technologies Flashcards

Gas Cleaning

1
Q

What are the gases which are to be cleaned often?

A

Gases to be cleaned are often flue-gases from combustion, industrial processes, ventilation flows, etc.

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2
Q

Aspects to consider when choosing a technique

A

Operational parameters: Consider gas and particle temperature, flow rate, aggressive chemicals to ensure the installation can withstand the environment.

Separation efficiency: Define the required separation and cleaning efficiency; ask “how clean is sufficiently clean?”

By-product management: Determine how to handle separated materials and if by-products can be useful; assess any specific requirements.

Maintenance and resources: Consider the need for maintenance, skills, energy, and external resources for each technique.

Life cycle cost: Evaluate investment and operational costs over time, as abatement techniques rarely generate income but can avoid significant future costs.

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3
Q

What is PM?

A

A complex mixture of very small particles and liquid droplets. Some particles, such as dust, soot, dirt, etc., are large enough to be visible. Others are very small and need certain devices for detection.

The size of particles is directly linked to their potential for causing health problems.

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4
Q

Where can particulates in gas flows originate from?

A
  • Incomplete combustion of organic materials (e.g. soot) and inorganic materials (e.g. minerals, ashes, etc.).
  • Industrial processes e.g. cutting and polishing stone, flour handling in mills, metal dust in exhaust, etc.
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5
Q

What are the typical techniques for separation of PM from gas flows?

A
  • Settling chamber (gravity)
  • Cyclones, centrifuges (centrifugal forces)
  • Momentum separators (momentum)
  • Filters (barrier)
  • Wet scrubber (solubility)
  • Electrostatic Precipitators (electrostatic forces)

Removal of particulates is often arranged in a sequence starting with the large particles, to end with the smallest.

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6
Q

What is a settling chamber, and how does it work?

A

A settling chamber is a simple air pollution control device used to remove particulate matter from air or gas streams by allowing the particles to settle out of the flowing gas due to gravity.

  1. How It Works:

Slow Gas Flow: The gas stream containing particles is passed through a large chamber where the velocity of the gas is reduced.

Gravity Settling: As the gas slows down, the heavier particles lose momentum and settle out of the gas stream due to gravity, falling to the bottom of the chamber.

Particle Collection: The collected particles are removed from the chamber, often through a hopper or similar device at the base of the chamber.

  1. Key Characteristics:

Simple design: No moving parts, easy to construct and maintain. Not much external power needed.

Effective for large particles: Works well for larger, heavier particulates but is less effective for fine particles or very small dust (which tend to remain suspended in the gas stream).

Low cost: Economical in terms of both operation and maintenance.

  1. Key Applications:

Used in industries like cement, foundries, and power plants where large particles need to be separated from the gas stream before further filtration or discharge into the atmosphere.

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7
Q

What are cyclone seperators?

A
  • A cyclone separator is a device used to remove particulate matter from gas or air streams by using centrifugal force.
  • It is widely used in industries to capture dust, ash, and other airborne particles.
  • Has a relatively low pressure drop. A low pressure drop means that the gas flowing through the cyclone experiences minimal resistance. This is important because a high pressure drop would require more energy to maintain airflow.
  • Since the cyclone has a low pressure drop, it uses less energy to operate compared to other separation technologies. This makes it cost-effective in terms of both energy usage and maintenance.
  • Offers high seperation efficiency for particles down to 10mm.
  • Can be installed in parallel for increased capacity or in sequence for removal of gradually smaller particles.
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8
Q

How do cyclone seperators work?

A

Spinning Motion: The gas stream enters the cyclone in a spiral or circular pattern at high speed. This spinning motion forces the heavier particulate matter towards the outer walls of the cyclone.

Centrifugal Force: The particles are pushed outward due to centrifugal force, where they lose momentum and fall to the bottom of the cyclone chamber.

Particle Collection: The collected particles are removed from the bottom of the cyclone, often through a hopper, while the cleaned gas exits through the top or center of the cyclone.

Smaller diameter = Better separation of small particles: A cyclone with a smaller diameter is better at separating smaller particles from the gas stream. This is because the tighter vortex created in a smaller cyclone generates a higher centrifugal force, which helps to push even small particles toward the outer walls for collection.

Cost of larger pressure drop and energy consumption: However, a smaller diameter also causes a larger pressure drop in the system. This means the gas has to work harder to pass through the cyclone, requiring more energy to maintain the airflow. As a result, operating costs will increase.

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9
Q

What is a centrifuge?

A

A centrifuge is a device that separates substances (usually liquids and solids or different liquids) based on density by spinning them at high speed.

While centrifuges are not typically used for air purification, they are essential for liquid-solid separation or liquid-liquid separation in various industries, including pharmaceuticals, wastewater treatment, and food processing.

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10
Q

How do centrifuges work?

A

Spinning Motion: The centrifuge spins a mixture at high speeds, creating centrifugal force. This force pushes the denser materials (solids or heavier liquids) outward to the edges or bottom of the centrifuge.

Separation by Density: The less dense components (lighter liquids or particles) stay closer to the center. The difference in density between the components drives the separation process.

Collection: The separated materials (solid or denser liquid) are collected in the outer chamber, while the lighter material remains in the inner part of the centrifuge or is extracted separately.

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11
Q

What are momentum separators?

A

A momentum separator is a device used to separate particulate matter from a gas stream by taking advantage of the difference in momentum between the particles and the gas.

It works based on the principle that heavier particles have more momentum and thus do not follow the gas stream when it changes direction abruptly.

Momentum separators are typically used for removing large particles like dust or debris from the gas flow.

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12
Q

What are key characteristics of momentum seperators?

A

No moving parts: Like cyclone separators, momentum separators are simple in design, with no moving parts, making them easy to maintain.

Effective for large particles: They are efficient at removing larger, heavier particles but less effective for smaller or fine particles.

Low energy consumption: Momentum separators typically require less energy compared to more complex filtration systems because they rely on the natural properties of momentum rather than external forces like filters or electricity.

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13
Q

How does a momentum seperator work?

A

Gas Flow and Particle Momentum:

The gas stream containing particles enters the separator at a high velocity. As the gas flows through the separator, it is forced to make a sudden change in direction (for example, a sharp bend or a rapid expansion).

Separation by Momentum:

Heavier particles (which have more momentum) tend to keep moving in a straight line due to their inertia, and they cannot follow the gas stream as it changes direction.
As a result, these particles are separated from the gas stream and are collected in a chamber or hopper.

Particle Collection:

The separated particles fall to the bottom of the separator due to gravity and are collected for disposal or further processing.
The cleaned gas, now free of larger particles, exits through another outlet.

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14
Q

What are fabric and textile filters?

A
  • Include all types of bag filters where filter media is textile fabric arranged as set of plates or tubes supported by an external framework.
  • The fabric/textile filters can be produced for cut-off down to 1 micrometre particle
    size, including fly ash, sulfur, hydrochloric acid.
  • Since filters don’t continuously transport separated particles away will they build up “filer cakes” on the surface of the fabrics. These filer cakes must periodically be removed in order to avoid too large pressure drops. These are removed through mechanical shaking of the filters, or reversing the gas flow (letting the filter cake drop to the bottom).
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15
Q

What are important operational parameters for fabric filters?

A
  1. The gas flow velocity. A low flow avoids compaction of the filter cake. A compact cake leads to a higher pressure drop, which requires higher electricity consumption to drive the gas flow through the filter.
  2. Gas temperature as fabrics and textiles tend to meld or burn at high temperatures. There could be exceptions.
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16
Q

What are additives included in filter bags?

A

Lime is injected into the system to react with and neutralize sulfur and HCl, protecting the bag filters from corrosion and helping with pollutant removal.

Activated carbon is also used to capture heavy metals, such as mercury, from the gas stream.

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17
Q

What is a wet scrubber?

A

Wet scrubbers can be used for most kinds of removal of
particulates as well as many gas cleaning operations, including for removal of SOx.

They are highly effective for capturing dust, acid gases, and other pollutants, particularly in industries like power plants, chemical manufacturing, and waste incineration.

There are different types of wet scrubbers:
- Spray tower,
- Venturi scrubber,
- Packed bed scrubber.

Advantages:
- Effective for both gases and particulates: Wet scrubbers can remove both dust and gaseous pollutants in one system.
- Can neutralize corrosive gases: Scrubbing liquids can be tailored to neutralize acidic or alkaline gases.
- Flexibility: Wet scrubbers can handle a variety of gas streams with varying temperatures, moisture levels, and pollutant types.

Disadvantages:
- Creates wastewater: The liquid used in the scrubber absorbs pollutants, creating a contaminated liquid waste stream that needs to be treated.
- Energy consumption: The process can require significant energy to pump and circulate the liquid.
- Corrosion: Wet scrubbers can be prone to corrosion, especially when dealing with acidic gases, unless made of corrosion-resistant materials.

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18
Q

How do wet scrubbers work for the removal of PM?

A

Wet scrubbers work by using liquid (usually water) to capture and remove particulate matter (PM) from gas streams.

Gas and Liquid Contact:

The contaminated gas stream containing particulate matter is passed through a chamber where it comes into contact with a liquid spray or is bubbled through a liquid. The liquid droplets collide with the particles in the gas, causing the particles to be captured by the liquid.

Particle Capture:

The liquid droplets trap or encapsulate the particulate matter, washing the particles out of the gas stream. The particulates are either absorbed into the liquid or stick to the droplets.

Particle and Liquid Separation:

The liquid, now containing the particulate matter, falls to the bottom of the scrubber or is collected in a basin. The cleaned gas exits through the top of the scrubber, often passing through a mist eliminator to remove any remaining liquid droplets.

Disposal or Treatment of Collected Material:

The liquid, which now contains the captured particulate matter, is either treated and recycled or sent for disposal, depending on the application and the type of particles collected.

Wet scrubbers can also include arrangements where they are combined with a cyclone to increase the removal efficiency.

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19
Q

What are electrostatic precipitators?

A

Electrostatic preciptators (ESP), as often the final stage in the train of particulates removal. It is an air pollution control device that removes particulate matter (PM) from a gas stream by using electrical charges.

Can capture very fine material, from smaller that 1 micrometer up to 10 micrometers. ESP is
highly efficient for small particle sizes.

The ESP can work at relatively high temeperatures, up to 500 C but does not require higher
temepratures. An advantage is that ESP cause low pressure drops but has a high electricity
consumption.

You find ESP in many different applications from power plants, combustion, cement industry
and even in the small air-cleaners for home use.

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20
Q

How do ESPs work?

A

Ionization:

As the gas containing particulate matter enters the ESP, it passes through a set of high-voltage wires (called discharge electrodes).
These electrodes generate a strong electric field, which ionizes the gas, giving the particles a negative electrical charge.

Particle Attraction:

The negatively charged particles are then attracted to positively charged collecting plates (called collector electrodes) that are located further downstream in the gas flow. The charged particles stick to these plates due to the electrostatic force.

Collection of Particulate Matter:

Over time, the particles build up on the collecting plates. The plates must be periodically cleaned to remove the accumulated particulate matter. This is usually done by rapping or vibrating the plates to dislodge the particles, which fall into a collection hopper below the plates.

Clean Gas Exits:

The cleaned gas, now free of most particulate matter, flows out of the ESP and is released into the atmosphere or further processed.

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21
Q

What is carbon capture and storage (CCS)?

A

Carbon capture and storage involves capturing the CO2 before it is emitted to the atmosphere,
transporting it to a safe location and isolate it from the atmosphere.

Certain geological
formations could be examples of storage locations.

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22
Q

How does CCS work?

A

There are three methods.

Post-combustion system:

CO2 is removed after combustion of the fuel. CO2 is captured from the flue gas and stored or utilized.

Pre-combustion system:

Processes the fuel before combustion using steam and air/oxygen. Converts fuel into carbon monoxide, which reacts with steam to produce hydrogen gas and CO2. CO2 is separated and managed. This system has been used for natural gas and oil extraction and can capture 85-95% of CO2 emissions. Requires 10-40% more energy but reduces net CO2 emissions by 80-90%.

Oxy-fuel combustion:

Uses pure oxygen instead of air for combustion. Produces flue gas mainly containing water vapor and CO2. Water vapor is removed, and CO2 is compressed for storage or transportation. Still under development but promising for capturing CO2.

Other points:

CO2 can be stored in porous geological formations where it can remain as a liquid or fossilize into minerals.

Captured CO2 may also be used in industrial processes, though potential applications are limited by the large volumes of CO2 produced.

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23
Q

What happens to sulphur when burned (oxidized)?

A

Sulphur forms hydrogen sulphide (H₂S) and sulphur dioxide (SO₂), which combine with water to form acid substances like sulphuric acid.

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24
Q

Why do sulphur oxides cause high flue gas temperatures and corrosion?

A

Sulphur oxides form acidic substances when combined with water, leading to acid condensation inside chimneys, which causes rapid corrosion.

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25
Q

What are the environmental and health impacts of sulphur oxides?

A

Sulphur oxides can cause respiratory health problems and contribute to acidification in the environment, which leads to corrosion and degradation of buildings.

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26
Q

What is the sulphur content range in coal, crude oil, light and heavy fuel oil, wood, waste, natural gas, industrial raw materials?

A

Coal contains 0.2 - 5% sulphur, depending on the type (from lignite to anthracite).

Crude oil contains 1 - 4% sulphur, depending on its origin.

Light fuel oil: Less than 0.5% (refined oil).

Heavy fuel oil: Less than 5% (less refined oil).

Wood contains less than 0.1% sulphur, naturally low levels.

Waste contains less than 2% sulphur, depending on its composition.

Natural gas usually has no sulphur.

Industrial raw materials can also contain sulphur and sulphur compounds

27
Q

What are the three methods for cleaning fuels?

A

Fuels can be cleaned using physical, chemical, or biological methods, such as sulphur content reduction in oil refineries.

28
Q

What is an example of a modern fuel cleaning technique?

A

Modern diesel qualities have less than 0.005% sulphur due to cleaning in oil refineries

29
Q

What is fluidised bed combustion (FBC)?

A

FBC is an advanced combustion technique where limestone is injected as a sorbent of sulphur into the boiler, resulting in efficient combustion and reducing sulphur emissions by around 50%.

30
Q

How does the fluidised bed work in FBC?

A

Air is injected at high velocity from the bottom of a bed of sand, mixing fuel and lime, which captures about half of the sulphur during combustion.

31
Q

Where is FBC typically used?

A

FBC is used in medium to large installations.

32
Q

What is the principle of the Flue Gas Desulphurisation (FGD) process?

A

FGD uses alkaline sorbents (like lime or limestone) to neutralize sulphur dioxide (SO₂) from flue gases, forming calcium sulphite or gypsum.

33
Q

What is the chemical reaction in wet scrubbing using limestone?

A

Limestone (CaCO₃) reacts with SO₂ in the flue gas to produce calcium sulphite (CaSO₃) and CO₂.

34
Q

What is the end product in forced oxidation of FGD systems?

A

Gypsum (CaSO₄ x2H₂O) is produced, which can be used for products like wall plaster boards.

35
Q

What are the four main types of wet scrubbers?

A

The four types are spray towers, venturi scrubbers, plate towers, and packed beds

36
Q

How does a spray tower work in wet scrubbing?

A

The dirty flue gas meets a sprayed mist of lime or limestone slurry to capture the sulphur.

Advantages include simple design and operation, effectiveness for capturing larger particles and sulphur dioxide. Can handle large gas volumes.

Disadvantages include limited efficiency for small particles, creation of wastewater, requires regular maintenance.

37
Q

How does a venturi scrubber work?

A

The gas flows through a narrow throat, accelerating the gas and creating turbulence, which atomizes the slurry and increases gas-liquid contact for better sulphur removal.

High efficiency for capturing fine particles and gases. Creates high turbulence which maximises contact between gas and liquid.

Has a high pressure drop, leading to increased energy costs. Requires more power to operate due to gas flow resistance in the throat. Can generate big amounts of wastewater.

38
Q

What is the function of a plate tower in wet scrubbing?

A

A countercurrent process where flue gas moves upwards, meeting the slurry flowing downwards across horizontal plates for gas-liquid interaction.

Efficient gas-liquid contact due to countercurrent design. Suitable for particulate removal and gas absorption. Easy to maintain and clean.

Less efficient than venturi scrubbers for fine particles. Requires more space due to its vertical design and possibility of flooding if gas and liquid flows are not controlled properly.

39
Q

What is the purpose of packing material in packed bed scrubbers?

A

The packing material maximizes the contact area between the dirty gas and the scrubbing liquid, improving efficiency.

High absorption efficiency, operates with a lower pressure drop, effective for gas absorption including SO2 and NOx removal.

Packing material can become clogged, less effective for very fine particles, higher risk of corrosion.

40
Q

What is dry scrubbing?

A

A process where a sorbent slurry is sprayed into the flue gas, drying and reacting with SO₂ to form a dry powder, such as calcium sulphide and calcium sulphate.

Produces dry waste, reducing disposal costs and the need for wastewater treatment.
Lower water consumption compared to wet scrubbers.
Simpler waste handling as the byproduct is a dry powder.
Effective for medium-sized power plants (up to around 200 MW).

Generally less efficient than wet scrubbers for removing SO₂ (though over 90% efficiency is possible). Limited application for large-scale industrial plants due to size and efficiency constraints.
Requires precise control of temperature and humidity for optimal efficiency.

41
Q

What are the components of a dry scrubber?

A

Components include a mixer (for mixing dust, water, and lime), a reactor (fabric filter), and a hopper (to store and recirculate dust).

42
Q

What does NOx refer to?

A

NOx is a general term for nitrogen oxide compounds like nitrogen monoxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O).

43
Q

How does NOx affect the environment and health?

A

NOx contributes to acidification, eutrophication, ground-level ozone, and photochemical smog, leading to human health problems and damage to plants and crops.

44
Q

What are the main sources of NOx emissions?

A

NOx mainly forms during combustion processes (motor vehicles and industries) when nitrogen gas (N₂) in the air is oxidized at high temperatures (above 800°C, significant formation at 1200°C).

45
Q

What are the nitrogen content ranges in different fuels?

A

Coal: 0.5 – 3%
Crude oil: < 1%
Natural gas: 0.5 - 20%
Light fuel oil: approx. 0.2%
Heavy fuel oil: up to 5%
Wood: 0.1 – 0.5%
Waste: 1 - 5%

46
Q

How can combustion be optimized to prevent NOx formation?

A

Techniques include:
- Lowering combustion temperature.
- Reducing residence time in high-temperature zones.
- Adjusting the air/fuel ratio.
- Using stepwise air, fuel, or steam inlet (staging).

47
Q

How do Low NOx Burners (LNB) reduce NOx emissions?

A

LNBs reduce NOx emissions by up to 80% through multi-phase combustion, fuel re-burning, and low-air combustion.

48
Q

What is the purpose of flue-gas recirculation?

A

Flue-gas recirculation helps control and maintain a suitable combustion temperature, reducing NOx formation.

49
Q

How does using oxygen for combustion help prevent NOx?

A

Using pure oxygen (instead of air) reduces nitrogen availability for NOx formation, but it is expensive.

50
Q

What is SCR and how does it work?

A

SCR injects ammonia into the flue gas stream, which reacts with NOx in the presence of a noble metal catalyst at 300-400°C, converting NOx into nitrogen (N₂) and water (H₂O) with up to 90% removal efficiency.

51
Q

What are the main chemical reactions in SCR for NOx removal?

A

4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O
2 NO₂ + 4 NH₃ + O₂ → 3 N₂ + 6 H₂O

These reactions convert harmful nitrogen oxides (NO and NO₂), which contribute to air pollution, into harmless nitrogen (N₂) and water (H₂O). This process occurs in the SCR system, which uses a catalyst to facilitate the reaction at specific temperatures (300-400°C), and results in the removal of up to 90% of NOx emissions from flue gases.

52
Q

What is SNCR and how does it differ from SCR?

A

SNCR injects ammonia or urea into the combustion chamber at about 950°C without using a catalyst. It has a lower NOx removal efficiency (35-70%) but lower installation costs than SCR.

53
Q

What is the benefit of combining SCR and SNCR?

A

Combining SCR and SNCR improves NOx removal efficiency while balancing costs and installation requirements.

54
Q

What does VOC stand for and what does it include?

A

VOC stands for Volatile Organic Compounds, which include all volatile hydrocarbons and organic solvents. It is defined by the measurement method used for sampling and analysis.

55
Q

How are VOC and HC related?

A

VOCs are a subgroup of hydrocarbons (HCs), but not all hydrocarbons are volatile, so the terms are used interchangeably depending on the context.

56
Q

What are the health and environmental risks associated with VOCs?

A

VOCs can pose human health risks, such as cancer and allergies, and may contribute to flammable/explosive air mixes at high concentrations.

57
Q

What are the main sources of VOC emissions?

A

Transport system: Incomplete combustion in engines, fuel storage losses.

Energy sector: Incomplete combustion, storage leaks.

Industrial processes: Painting, metal surface treatment, printing, oil refineries.

58
Q

What are some methods to prevent VOC emissions?

A
  • Replacing organic solvents with water.
  • Using solvent-free methods like powder coating.
  • Using solvents with lower vapor pressure.
  • Proper sealing, storage, and handling to avoid leaks.
  • Recovering and recycling VOCs via condensation, absorption, or adsorption.
59
Q

What are common end-of-pipe approaches to VOC removal?

A

Thermal combustion
Catalytic combustion
Feeding VOCs into boiler combustion chambers
Absorption
Adsorption
Condensation

60
Q

How does thermal combustion remove VOCs?

A

Takes place at 700-1000°C in a furnace, removing 95-99% of organic material. Heat from combustion can be recovered.

61
Q

What is the difference between catalytic combustion for diesel and petrol engines?

A

Diesel: Converts CO and hydrocarbons to CO₂ and H₂O at 170-350°C.

Petrol: Also reduces NOx to N₂ at 600-800°C with 90% reductions for VOC, NOx, and CO.

62
Q

How does adsorption work for VOC removal?

A

VOC molecules are captured in the pores of a material like activated carbon, zeolites, or polymers. The material holds VOC molecules until it becomes saturated.

63
Q

How can adsorption materials be regenerated after saturation?

A

Regeneration can be done through hot air, steam, or reduced pressure, releasing the captured VOC molecules for combustion or reuse.

64
Q

When is condensation used for VOC removal?

A

Used when VOC concentration is high and consists of only one or a few substances. The gas flow is cooled below the condensation temperature of the VOCs.