Strong Acid System - Mist Eliminators
July 28, 2003

Introduction Acid Mist Collection Mechanisms     Inertial Impaction     Interception     Brownian Diffusion Types of Mist Eliminators     Mesh Pads     Impaction Candles     Brownian Diffusuion Candles

Associated Links Mesh Pads         Koch-Otto York         Begg Cousland         Monsanto Enviro-Chem Systems         Kimre         ACS Industires, Inc. Impaction Candles Brownian Diffusion Candles

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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:

 

1. Inertial Impaction
2. Direct Interception
3. Brownian Movement/Diffusion
4. 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.  At 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.