Extrusion machine: How to optimize VOCs emission in the plastic industry?

Modified plastic extrusion machine

Industrialization has driven human society forward, and while we were reveling in the dividends it brought, the London smog incident of 1952 brought us back to the reality.
As time going, consumers, workers, and politicians are becoming more and more
conscious of industrial waste gases.

The introduction of new policies has also prompted the modified plastics industry to transition to environmental protection.
But how can we efficiently optimize VOCs emission in the modified plastic industry?
Now follow my step to look at the industry’s mainstream VOCs treatment methods, as well as biological process.

PP/PE/PVC plastic extrusion line：Analysis of VOCs emission

VOCs are mainly generated in the melt process and cooling process of the extruder in the co-blending modification process, and the main pollution generating site is near the die head of the extruder. The composition of waste gas generated in the modification process is different for different base materials.
Polyethylene (PE) production exhaust is dominated by particulate matter (smoke), hydrocarbons and heavy hydrocarbons (oil);
Polypropylene (PP) production exhaust is dominated by non-methane total hydrocarbons, heavy hydrocarbons, propylene, formaldehyde, acrylic acid, acetaldehyde and acetone;
Polyvinyl chloride (PVC) production exhaust is dominated by methylene chloride;
The concentrations of various pollutants in the exhaust gases of polystyrene (HIPS) and ABS production are very low (<1mg/m3 ).
Temperature also has a large impact on the type and total amount of exhaust gases produced during the production process. One study mentioned that for polypropylene homopolymer processing, with production temperatures of 205°C and 319°C, respectively, the latter produced eight times more total non-methane hydrocarbons than at 205°C, with 7.4 times more heavy hydrocarbons and 66.2 times more propylene than the former.

Common waste gas treatment processes

1.Use resin with low monomer residue
As far as possible, choose a resin with a relatively low content of residual monomers. For example, the content of VC monomer in medical PVC resin is very low, and the VC content can reach 1ppm (mg/kg) (the normal value is 5); for PP used in automobile interior panels, be sure to choose a resin with low content of small molecule volatiles.
2.Use of additives with higher molecular weight
As far as possible, use additives with high molecular weight, preferably polymer additives, to ensure that they do not volatilize at processing temperature as well as external migration.
(1) If the coupling agent is changed to a compatibilizer, specifically such as a grafted polymer or a grafted polymer, but the maleic anhydride graft part has a certain odor.
(2) Lubricant with poly pentaerythritol stearate (PETS), etc.; dispersant selection of molecular weight greater than 5000 polyethylene wax (PE-WAX).
(3) Flame retardant as far as possible to use polymer type, specifically such as brominated polystyrene, brominated epoxy resin, etc.
3. Adding adsorbent
It refers to the use of adsorbents to capture or adsorb the residual organic small molecules in the resin matrix to achieve the requirement of VOC content control. Adsorption method is divided into two categories: chemical reaction adsorption and physical adsorption. Common physical adsorption materials are porous materials, such as: zeolite, activated carbon, molecular sieve, diatomite, etc.
Chemical reaction adsorption is a method of eliminating VOCs from materials by adding additives that react with VOCs to produce a compound with a larger relative molecular mass that will not volatilize under normal conditions of use (e.g., below 100 °C) or by producing a more volatile gas.
4. Control of processing conditions
(1) Low-temperature production: Under the premise of ensuring the full plasticization of the products, the processing temperature is lower, thus reducing the generation of small molecule decomposition substances.
(2) Multi-stage evacuation: increase the number of evacuation, which can reach 3 or even 4 stages, to remove the small molecule volatiles generated in time.

5.Reduce the use of organic solvent-based materials
(1) Water-based coatings: water as a dispersion medium instead of organic volatile solvents, the current technology still needs to contain about 20% ether and alcohol solvents, can only be considered low VOC coatings, not VOC-free coatings.
(2) Solid powder coating: without any organic solvent, zero VOC emission.
(3) light-curing coatings: film-forming materials rely on light-curing reaction molding, can reduce the use of organic solvents, but often need to contain about 30% of organic solvents.
(4) Water-based adhesives and water-based release agents: currently available are organic solvent low content products.
6.High temperature dry steaming
After the plastic matrix passes through the twin-screw extruder and pelletizer, it is transported to the VOC treatment system, where it is heated at 80-120°C to make the VOC substances (benzene, toluene, ethylbenzene and other hydrocarbon substances) adsorbed by the plastic particles dry-steamed as clean as possible. The time of dry-steaming depends on the temperature, which is high and less, usually about 3 hours. At present, the large modification enterprises basically use this method.

Biological waste gas treatment process

The biological approach is feasible for two reasons:
① The concentration of the exhaust gas is very low, and the concentration of the selected odor is greatly reduced after the biological method removes the pollutants with high biodegradability from the exhaust gas, so that the emission standard can be met;
②Small amounts of short-chain hydrocarbons and methylene chloride will be adsorbed by materials such as bamboo charcoal in the filler, and may be partially or completely degraded by microorganisms slowly.

Exhaust gas collection → spray tower → electrostatic degreasing → fan → biological filtration / activated carbon → meet the standard emissions

After the organic waste gas is collected by the collection system, it enters the scrubber tower, and the temperature of the waste gas is reduced by circulating water spraying, while most of the particles and high boiling point pollutants (such as heavy hydrocarbons) in the waste gas are removed. After the waste gas comes out of the scrubber tower, it enters the electrostatic oil removal equipment, which further removes the particles and heavy hydrocarbons (oil) in the waste gas and provides conditions for the subsequent process.
The exhaust gas is then sent to the biological unit, where it enters the biofilter after a pre-wash tank, further cooling and humidification, and passes through a wetted, porous and active microbial filled filter layer, where the microorganisms in the layer adsorb, absorb and degrade the pollutants in the organic exhaust gas, decomposing them into carbon dioxide, water and other inorganic substances.
(1) Cooling tower
The main purpose of the spraying process is to reduce the temperature of the waste gas, on the one hand, so that the temperature of the gas meets the inlet requirements of biological treatment; on the other hand, as the temperature decreases, most of the heavy hydrocarbons, acrylic acid, acetone, methylene chloride and other substances in the gas phase are converted into liquid form, thus reducing the pollution load.



(2)Electrostatic oil removal device
The discharge gas from the spray tower needs to have as much moisture removed from the exhaust gas as possible before it enters the de-oiling unit. The oil removal process mainly removes residual oil droplets in the exhaust gas, mainly using electrostatic oil removal. The working principle is that the exhaust gas is introduced into a high voltage electric field, which causes the oil droplets in the exhaust gas to be charged. The charged oil droplets move toward the dust collection pole under the electric field force and are deposited, thus removing the oil droplets from the exhaust gas. Electrostatic oil removal has the advantages of high purification efficiency, small resistance loss, and large volume of gas treatment range.

(3) Biofiltration device
Biological filtration is a safe, reliable and environmentally friendly treatment method, the principle of which is to collect the waste gas from various pollution sources and send it to the biofilter centrally. After the waste gas is humidified and de-dusted, it passes through the wet, porous and active microbial filler layer, firstly, the filler and biofilm adsorb the pollutants, then the specialized microorganisms attached inside and outside the filler absorb and degrade the pollutants, decomposing the pollutants into CO2 , H2 O, SO , NO and other non-toxic and harmless simple inorganic substances, SO42- , NO3- and other non-toxic and harmless simple inorganic substances.
Low VOCs are not only a protection for workers’ health, but also one of the criteria for a good factory.
“Green water and green mountain is the silver mountain of gold”, with the increasing awareness of environmental protection, I believe the modified plastics industry will continue to transition to green.