A large number of industrial processes generate foul air / gas streams which contain toxic or corrosive materials and therefore, require to be removed from the gas stream before the air / gas stream is released to the atmosphere outside / environment.
A good number of solutions / designs are available of these fume / vapour extraction systems to stop dispersion of the contaminants in the environment. One of the most cost effective methods for cleaning of contaminated air / gas stream is by using a wet scrubber. This is a device in which the dirty gas is brought in contact with a liquid that will dissolve the contaminant and thus the gas stream is cleaned by way of absorption of contaminant into the liquid / solvent.
- There are many designs of these proprietary scrubbers, which have its own gas and liquid / solvent contacting mechanism. Most of these scrubbers can be classified as :
- 1. Packed tower scrubbers, in which the liquid / solvent flows down and gas flows up through a bed packings / raschig rings.
- 2. Perforated plate-scrubbers in which contaminated air / gas is bubbled through pools of liquid / solvent retained on these plates.
Variations within these designs are numerous.
While packed tower scrubbers are cheaper than plate type scrubbers, these use large amount of liquids. Since the liquid get contaminated as the gas is cleaned, higher rate of use of liquid is an important consideration in designing of a total pollution control system.
Comparison / Advantages :
There are a number of advantages in making use of VENTECH designed Acid Fume Scrubber of perforated plate configuration in comparison to conventional packed tower (raschig rings). Packed tower scrubbers require a lot of maintenance due to corrosive conditions and use of rotaryequipments like pumps, etc. and also generate a large volume of dilute waste (effluent) water. Also the packed tower scrubbers tend to plug up due to entrapment of solid particulate matters which may come in the air stream.
PACKED SCRUBBER CONFIGURATIONS
PLATE SCRUBBER
- The distinct advantages of plate scrubbers are as under :
- 1. USE VERY LITTLE WATER FLOW RATE SO GENERATE LESS EFFLUENT WATER
- 2. NO MOVING PARTS, SPRAY NOZZLES OR CIRCULATING PUMPS TO MAINTAIN AND THUS MAINTENANCE COST IS VIRTUALLY ZERO
- 3. WILL NOT EASILY PLUG UP WITH AIR BORNE SOLIDS
- 4. STAND UP TO THE CORROSIVE EFFECTS OF ACIDS ( POLLUTANTS ) AS PROVIDED IN ALL PVC / FRP MATERIAL OF CONSTRUCTION
- 5. COST OF EFFLUENT TREATMENT PLANT IS MINIMISED AS QUANTITY OF EFFLUENT WATER GENERATED IS SMALL
Principle:
In Plate Scrubbers, each plate represents a reservoir which holds water through which the contaminated air is bubbled, thereby scrubbing out the contaminant. The water displaced by the steady feed of water on the top plate overflows through a down comer, and so on, until the final effluent water is withdrawn from the bottom of the scrubber. Since the plates are always full of liquid on it, there is no need to maintain a high flow rate of water to keep the tower flooded. The reservoir of water on the platesenables the tower to handle sudden surge of acid gas, which sometimes occur when the work is added to the process tanks.
Each plate contains hundreds of holes, and each hole acts as a miniature venturi scrubber so that the whole plate is a mass of bubbling water providing highly efficient contact of foul air and water. As the water overflows from upper to the lower of the plate in the tower it becomes stronger and stronger in recovered acid, and as the air passes through each plate upwardly it becomes cleaner and cleaner.
Application :
Fume Scrubbers have found a big application field in the steel pickling industry. Even before the present day pressures for environmental clean-up they were needed to prevent damage to roof structures, buildings, EOT and other vehicles around the plant, and to protect adjacent properties. Now-a-days with environmental protection act in application and other regulatory agencies placing stringent limits on discharges to atmosphere, the requirements of fume scrubbers are greater than ever.
In the past, the large water flow was no disadvantage as slightly acidic water pouring from the bottom of the scrubber could be discharged to a nearby sewer / drain. However, as of now, restrictions on quality of effluent water discharges have made this type of operations both uneconomical and illegal.
With the once through water flow, the contaminated air stream is in counter-current flow with the scrubbing water. At the bottom of the scrubber the dirtiest air is being scrubbed by the most contaminated water, whereas, at the top of the scrubber the final air discharging to atmosphere is being scrubbed by pure water. However, when we recirculate water in a scrubber system this / counter – current flow, and the air at the discharge of the scrubber comes in contact with contaminated water as like the dirty inlet air. If no fresh water is added up in re-circulatory system, the contaminant level in the water system builds up to such a level where the scrubber is completely ineffective, so some water is bleededoff and a moderate flow of fresh make up water is added.
The Plate Scrubbers of VENTECH design are primarily for use in the field of air pollution control in the steel and metal processing industries. The suitability of these scrubbers is generally for following application parameters :
- • Gas scrubbed :Contaminated ( acid fume laden ) air
- • Contaminant : Low Concentration
- • Scrubbing liquid : Water or other solvent ( aqueous solutions )
- • Pressure : Near atmosphere
- •Temperature : Below 90 deg. C
- • Nature of contaminant : Corrosive
The typical applications of the scrubbers are as below :
- i. Fume Extraction Scrubbing on batch, rod or coil pickling tanks having acid baths of Sulphuric, Nitric, or Hydro-chloric acids.
- ii. Fume extraction scrubbing on continuous strip pickling tanks having acid baths of Sulphuric, Nitric, or Hydrochloric acids.
- iii. Fume extraction scrubbing in Glass Etching industry using Hydro-flouric acid.
- iv. Fume Extraction scrubbing in Electro plating industry using Chromic acid, and other acid baths.
- v. Fume extraction scrubbing in process industry involving acid fumes / vapours, and water soluble particulate matters / powders.