Waste gas treatment clearly is a must in the printing industry.

Methods to clean waste gases are grouped whether they remove dust and aerosols or volatile substances.

Which technique is employed, mainly depends on the physical and chemical nature and the amount of the contaminant, but also on the gas flow to be treated, etc.

The following priciples are of special importance to the printing industry:

Dust removal techniques are of interest only to finishing plants; removal of gaseous contaminants by absorption into liquids and by techniques which rely on condensing the vapours are not regularly employed.

Application of waste gas cleaning technologies

  Gases Dust Recuperation of
  inorganic organic aerosols substance energy
Cyclones - - x x -
Wet dust scrubblers - - x - -
Surface filters - - x x -
Electrical precipitators - - x x -
Condensation (x) x x x -
Absorption x x - x -
Chemisorption x x - x -
Adsorption x x - x -
Oxidation
      thermal
- x - - x
      catalytic - x - - x
Bioscrubbers - x - - -
Biofilters - x - - -


Dust Removal Devices

The most important groups of dust removal technologies are


Cyclones

operate by the inertia of the dust particles. Air is forced to flow in a helical manner, the diameter of the circles becoming smaller and smaller. As the centrifugal force is given by

F = m · v² / r,

it increases more and more. The dust particles happen to strike the cyclone's wall some time, are slowed down, fall to the bottom of the hopper, and are removed.

Dust removal by a cyclone
Dust removal by a cyclone         Multi-cyclone




(a) polluted gas
(b) hopper
(c) dust bunker
(d) dust particles
(e) cleaned gas




Multi-cyclone

 (Source: Fonds der Chemischen Industrie, Germany; imageseries "Luft")


Wet dust scrubbers

Below some basic types of scrubbers are shown.

Spray tower

Spray tower

              Self-induced spray scrubber
Self-induced
spray scrubber


Rotational scrubber
Rotational scrubber
Venturi scrubber
Venturi scrubber

Their common feature are more or less elaborate systems to wet the dust; if this doesn't happen, the effect of the device is negligible.


Dust removal by Venturi scrubbers

Low pressure
Venturi

illustration of a low-level venturi
 part of a low-level venturi        
High-level venturi 
illustration of a high-level venturi
High pressure
Venturi

(Source: Fonds der Chemischen Industrie, Germany; imageseries "Luft")


Surface filters

are virtually rather simple, but very effective pieces of equipment for dust removal. Modern filter media are made of glass, polymer, or ceramic fibres. The actual filtering medium, however, is the dust layer deposited on that fibre material. The filters are cleaned by reversing the air flow for a short time by a pulse of compressed air.

Dust removal by surface filters
Scheme of a tubefilter   (a) cleaned gas
(b) polluted gas
(c) compressed air
(d) cleaning jet
(e) filter
(f) removed dust



Scheme of
a tubefilter

Dust removal by surface filters

 (Source: Fonds der Chemischen Industrie, Germany; imageseries "Luft")


Electrical precipitators

are mainly used for very big flows of air, e.g. in the chimneys of power stations. Dust particles are sprayed with electrons emanating from a high-voltage corona discharge (60 ... 80 kV) and removed by electrostatic attraction.

Sections of an electrical precipitator (at right angles to each other);

Dust removal by electrical precipitators
Dust removal by electrical precipitators 
illustration of the principle
(a) polluted gas
(b) wash jets
(c) high-voltage power
(d) emission electrode
(e) earthing
(f) cleaned gas
(g) to the dust bunker
 Dust removal by electrical precipitators

 (Source: Fonds der Chemischen Industrie, Germany; imageseries "Luft")


Vapour removing devices

consentration of the saturated vapourCondensation

is not a very effctive means of removing vapours from air. It is suitable as a pre-cleaning step at most. The vapour pressure diagrams beside show that at ambient temperatures concentrations in the several hundred grams per cubic meter region survive. Temperatures below 0 °C are not advisable, as then water vapour freezes out.










Absorption

is the removal of a gaseous contaminant from air by contacting it with a liquid. If in this process a chemical reaction takes place, the expression is "chemisorption". Again, the main problem is to bring the reactants into contact as intense as possible. The standard design is a column absorber packed with some material to provide a large surface.

Recycling of solvents by absorption
Recycling of solvents by absorption  
left side:
(a) feed gas
(b) clean gas
(c) solvent
(d) loaded solvent
(e) disperser
(f) packing

right side:
(a) clean gas
(b) feed gas
(c) solvent
(d) substance recovered from feed gas
                                rectification
          absorber           column
  
illustration of the principle

 (Source: Fonds der Chemischen Industrie, Germany; imageseries "Luft")


Adsorbers

illustration of a tandem adsorber arrangementOrganic gases are retained by some solid material with a very large, fissured surface.

The most popular adsorbant ist activated charcoal. Charcoal adsorbers work all the better, the less polar the gas is.

Hence, adsorption is the main procedure to treat waste gas from gravure presses, that is, to remove toluene.

Remaining pollutant concentrations are in the range of 100 ... 150 mg/m³.

Adsorbed substances can be recovered, e.g. by desorbing them with steam.

Adsobers are usually built in a tandem arrangement, so that always one portion is ready for service.

As the adsorption process is exothermic, charcoal adsorbers may catch fire.


Thermal waste gas treatment ("after-burning")

illustrationOrganic gases are oxidized by combustion.

The waste gas must be preheated, up to 500 °C.

As a rule, the waste gas does not contain enough solvent to maintain a flame, so that additional fuel is needed. Natural gas is the best choice.

This results in wasting thermal energy, unless it is used in-house or sold (burning temperature about 800 °C). A thermal after-burning unit may well replace the factory's central heating.

Remaining solvent concentrations are below 20 mg/m³; in turn, the flame produces new pollutants, mainly and CO. The most important green-house gas, , is also formed.

Thermal after-burning is usually employed with web offset presses to burn the mineral oil vaporized in the dryer.


Catalytic conversion

Organic gases are oxidized employing a catalytic converter.

The converter only works, after a certain minimum temperature has been reached. Hence, initial heating is necessary, until the catalytic combustion produces enough heat to propagate the process.

If the solvent concentration in the waste gas is too low, continuous heating may be unevitable.

Remaining solvent concentrations can be well below 10 mg/m³.

The catalyst is mechanically rugged, but chemically rather delicate: it can easily be "poisoned" and thus put out of action.

Basically, catalytic converters can be employed with every printing process. It is dangerous, however, to use it with web offset presses, as silicon oils are quite potent catalyst poisons.

Example

diagramm

  solvent concentration
in waste gas (mg/m³)
solvetn concentration
in clean gas (mg/m³)
dimethyl formamide 54 3
methyl ethyl ketone 727 1
methyl isobutyl kentone 688 1
toluene 94 2
isobutyl acetate 35 n.d.


Bioactive treatment

Bioactive devices rely on bacteria, which live on the chemicals contained in the waste gas.

They are divided into bio-active scrubbers and filters. Both have been employed in the graphic arts industry, but with problems.

Bioactive scrubbers are comparable to sewage treatment plants, while biofilters make use of solid materials, e.g. bark from spruce, which carry bacteria.

Bioactive devices work best, when solvent concentrations are always the same. Short-term peaks may not be treated properly, shortages may need the bacteria to be "fed" in order to prevent them from dying.

Solvents are treated all the better, the better they are soluble in water. Hence, bioactive devices are best suited for flexo presses.

Remaining pollutant concentrations are better than 100 mg/m³.


Bioactive scrubber
Bacteria suspended in water

Bioactive scrubber

Example: varnishing of soft drink cans

  waste gas clean gas
gas flow (m³) 85.000 60.000
temperature (°C) 160 35
solvents, water-soluble (kg/h) 35 0.35
aromaties (kg/h) 4 1.6
COD of effluent (mg/l)   < 500
solvents in effiuent (mg/l)   <20


Bioactive filter
Bacteria fixed on solid carrier

Bioactive filter

Example: flexo printing on packaging films

  waste gas clean gas
gas flow (m³) 20.000 20.000
temperature (°C) 25 25
solvents, water-soluble (mg/m³) 650 ... 1300 40 ... 100


Cost of waste gas treatment units

The following figures were quoted at the 1994 "Woche der Druckindustrie" ("the printing industry's week") for installations designed for 5,000 m³/h. Bigger units are more expensive, but not proportionally.

Purchase cost

adsorption 200,000 DM
thermal 250,000 DM
catalytic 450,000 DM
bioactive 200,000 DM

Operating cost

adsorption 80,000 DM / y
thermal 100,000 DM / y
catalytic 70,000 DM / y
bioactive 60,000 DM / y




           Copyright © 1997     gomail Feedback     Updated on: 20. January 1998