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Strong Acid System - Mist Eliminators
July 28, 2003
Introduction
Within an acid plant there is the necessity to remove acid
mist and droplets from the gas stream exiting the drying and absorption towers. The
primary reasons for trying to capture, collect and remove the mist and droplets are:
- To prevent damage to downstream equipment
- To avoid undesirable atmospheric emissions
- To recover valuable acid from the gas stream
Acid mist and droplets are formed in one of three ways;
Mechanical, Condensation or Chemical Reaction.
Mechanically formed droplets usually range in size from 10 to
100 microns. They generally form when acid is re-entrained due to localized high
velocity gas tearing droplets away from a liquid film or from the splashing or spray
generated from a liquid distribution device.
Mist or fume are much smaller in size (3 microns or less) and
are generated from the sudden or shock cooling of hot gas containing sulphur trioxide.
Chemical reaction between sulphur trioxide and water will also produce a mist or
fume. This generally occurs at the inlet of the absorber towers.
Fibre filters have proven to be an effective device for
capture, collection and removal of mist and droplets from gases. In sulphuric acid
applications, fibre bed mist eliminators are used to remove acid mist and entrained
droplets from the process gas exiting the drying and absorbing towers. Proper and
effective removal of the acid mist and droplets will extend the life of the downstream
equipment and prevent undesirable atmospheric emissions.
Acid Mist
The size and quantity of mist generated will depend on the
type of plant, gas source, operating parameters, acid strength, type of distributor,
etc. In a sulphur burning plant the quality of sulphur affect the amount of mist
generated. 'Dark' sulphur will produce considerably more mist than 'bright' sulphur
due to the amount of hydrocarbon/water present in the sulphur.
The following table summarizes the typical mist loads and
particle size for various types of plant and duties.
|
Mist Load |
Particle
Size |
Comments |
| Drying Tower |
|
|
|
| Sulphur Burning |
500 mg/m³ |
+ 3 mm, relatively large |
|
| Spent Acid, Metallurgical |
175 to 3,530
mg/m³ |
0.6 to 10 mm, 1.0 mm mean |
|
|
|
|
|
| Intermediate Absorber |
|
|
|
| No Oleum, Bright Sulphur |
500 to 1,766
mg/m³ |
1 to 2 mm, fine to moderate |
|
| No Oleum, Dark Sulphur |
3,000 mg/m³ |
fine to moderate |
|
| Metallurgical |
500 mg/m³ |
fine to moderate |
|
| Oleum |
2,000 mg/m³ |
fine to moderate |
stronger oleum produces smaller
particles |
|
|
|
|
| Final Absorber |
|
|
|
| No Oleum |
500 mg/m³ |
moderate |
|
| Oleum |
2,000 mg/m³ |
1 mm |
stronger oleum produces smaller
particles |
| Spent Acid |
3,000 mg/m³ |
2 mm average |
|
|
|
|
|
| Absorber (Single
Absorption) |
|
|
|
| No Oleum |
500 to 700 mg/m³ |
1 to 2 mm, moderate |
|
| Oleum |
2,000 mg/m³ |
0.6 to 1 mm |
stronger oleum produces smaller
particles |
|
|
|
|
| Crossflow Stripper |
500 mg/m³ |
high granulometry |
|
|
|
|
|
| Product Stripper |
500 mg/m³ |
high granulometry |
|
|
|
|
|
| Acid Concentrator |
10,000 mg/m³ |
|
|
|
|
|
|
| Wet Process |
35,300 to 100,000
mg/m³ |
2 mm average |
|
Collection
Mechanisms
Mist or droplets are collected in four different ways:
- Inertial Impaction
- Direct Interception
- Brownian Movement/Diffusion
- Induced Electrostatic Forces
The first three collection mechanisms will occur to varying
degrees in all fibre bed mist eliminators.
Inertial Impaction
Large droplets (3 microns or larger) are collected when their
momentum prevents them from following the gas streamlines around a fibre. The
momentum of the droplet causes it to leave the streamline and strike the fibre and become
collected. Since momentum is the product of mass and velocity, it follows that large
droplets will be collected more efficiently than small droplets travelling at the same
velocity.
Interception
Interception of a droplet occurs when the size of the
particle allows it to follow the gas streamline around an object in its path. As the
particle follows the gas streamline around the object it may come sufficiently close to
the object such that it will touch the object and become collected. Interception as
a collection mechanism is less important than inertial impaction.
Brownian Diffusion
Extremely small acid particles or mist are so small that they
do not follow the gas streamlines but exhibit a random path as they collide with gas
molecules. These submicron particles will be collected when they collide or touch an
object.
Types of Mist
Eliminators
There are generally two types of mist eliminators: Impaction
and Brownian Diffusion Types so-called because of the primary collection mechanism
employed in their design. Impaction type mist eliminators employ impaction and to a
lesser extend interception methods to capture, collect and remove acid mist. As
such, impaction devices are effective for the the larger particles and are less efficient
for the smaller submicron particles. To collect the submicron particles, a Brownian
diffusion device must be used.
Mesh
Pads
Mesh pads operate primarily on inertia impaction and are
efficient in removing particles 5 microns or larger. Mesh pads are size for
relatively high gas velocities which relies on the fact that the size of the particle that
can follow the gas streamline decreases as gas velocity increases. However, at
higher gas velocities, the possibility of droplet re-entrainment occurs. A t lower gas velocities, the momentum of the particle
decreases which decreases the collection efficiency. The effective operating of a
typical mesh pad is approximately 30 to 110% of design gas flow.
Mesh pads are usually made of woven metal wire that is
crimped and formed into a flat pad and fitted into the tower. The mesh is held
together by a grid place above and below the pad.
To enhance the collection efficiency of a mesh pad, glass or
PTFE fibres can be co-knitted with the wire to form a composite pad. The smaller
diameter glass or PTFE fibres increase the number or targets in the pad without the need
to make the pad denser which increases pressure drop.
Impaction Candles
Impaction candles utilize inertial impaction as the primary
means of particle collection but they offer improved collection of particles in the 1 to 3
micron range that mesh pads are only capable of removing to a small degree.
Impaction candles are made of glass fibres hand packed in
between two metal cages or machine wound onto the inner cage with an outer cage added on
top. The candles are installed in either the hanging or standing position.
Normal bed velocities are in the range of 1.27 to 1.63 m/s
(250 to 320 ft/min). Since inertial impaction is the primary collection mechanism,
the turndown capability is limited to 75% of the design value.
Brownian
Diffusuion Candles
All three collection mechanisms are employed in Brownian
diffusion candles but Brownian diffusion is the mechanism which allows these elements to
achieve the high collection efficiencies.
Impaction candles are made of glass fibres hand packed in
between two metal cages or machine wound onto the inner cage with an outer cage added on
top. The candles are installed in either the hanging or standing position.
Design bed velocities are very low and range from 0.025 to
0.2 m/s (5 to 40 ft/min) depending on the pressure drop and collection efficiency desired.
In contrast to the other type of mist eliminators, the collection efficiency of
Brownian diffusion candles increases as the gas velocity decreases. At lower
velocities the residence time of the mist particle increases and the fibre bed so the
chance that it will be captured increases.
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