Technology

Process Summary

ET Black™ is a modified furnace black process. It has been developed across several years of operating experience in commercial plants where particular attention being given to: i) product quality and consistency, ii) collection efficiency on a very fine particles size (nearly 100% efficiency), iii) pelletizing and drying operations, iv) operational costs reduction also by means of steam recovery and electricity generation.

The production of carbon black in the reactor is accomplished by the thermal decomposition of hydrocarbon oil at high temperature across specific reaction times (the order of micro-seconds) for each grade. The reactor is the heart of the process, where the conversion oil feedstock is ultimately transformed in carbon black of defined characteristics.

Feedstock storage and pumping

The feedstock is unloaded into fixed roof storage tanks where it is kept at 60-90°C by steam-heating or by heating through the excess hot feedstock return from the reactor section.

The feedstock is viscous in nature, therefore steam jacketing and special pumps are required for ease of handling and pumping. The feedstock is supplied to the reactors through pumps, suitable for handling high viscosity fluids. In order to make the fluid movable, steam heated suction heaters are provided. The feedstock is therefore filtered, through fine mesh strainers/filters to remove extraneous materials, and preheated to 200-250°C for better atomization into the reactor.

Reaction

The reaction chamber, lined with high temperature chrome-alumina refractory, is operated at 0.5 barg, at approximately 1900°C in case of hard black and 1400°C for soft blacks. In the HB reactor, the reaction temperature is raised to higher level through the combustion of auxiliary fuel. The heat developed during the combustion provides the energy required for the endothermic thermal cracking reaction. The reaction products moving at very high velocities are cooled to about 800-1000°C by water sprays, located at predetermined points inside the quenching section of the reactor.

The reactor gases are an aerosol of fluffy carbon particles suspended in a mixture of hot combustible gases and steam. They are first conveyed to an air-preheater, where the reaction air is heated up to 600-800°C, then passed through a reactor boiler for producing 20 barg saturated steam and a conversion oil pre-heater. Before heading to the main bag filter for separation they pass through the quench tower where they are cooled down to 280-300°C for safeguarding the integrity the main bag filter bags. Nevertheless the quench tower temperature should be sufficiently high to avoid condensation that might cause corrosion further downstream.

Carbon black separation and homogenization

The carbon black is separated from the gaseous stream (a mixture of CO, CO₂, CH₄, C₂H₂, N₂, H₂, air and water vapours) in the main bag filter, where an appropriate number of compartments contain bags typically made of graphite/silicone-coated fiberglass. 280-300°C smoke from reactor section enters the main bag filter where the bags capture the carbon particles. The gas then exits the filter from the gas header. Filter bags cleaning is performed by either reverse flow or pulse-jet techniques, depending upon customers’ preference.
The carbon black powder is collected in the hopper, discharged and forwarded to micro pulverizers where carbonaceous particles are reduced to 325 mesh max.
Most of the off gases (tail gas) collected in the off gas header is routed to the tail gas boiler for full combustion and generation of medium pressure steam. The remaining fraction of the tail gas is diverted to the drying section and fired in the dryer combustion furnace for providing the necessary drying heat.

Pelletizing

The collected carbon black material is sent to a pelletizer where it gets mixed with water, molasses solution and kerosene to form strong wet pellets.
The pelletizer, a piece of equipment of special design, has a rotating shaft with sharp edge pins in a double helix configuration. The close gap between the pins and the inner smooth surface of the pelletizer accompanied by the conveying and rotating action converts the mixture of carbon black particles and water into wet and strong spherical pellets.

Drying

The wet pellets are fed into the rotary dryer through the dryer drum screw feeder in order to preserve their integrity. Therein, slow tumbling and agitating action dry the pellets. The dryer shell is enclosed in a refractory lined box all along its length and the heat is supplied by burning a small part of the off-gases from the main bag filter. Dry pellets with moisture less than 1 % exit at the other end of the dryer before being transferred to storage in silos.

Purge gas filtration

At the very end of the dryer drum the flue gases, together with vaporized water and some powdered carbon black material are removed by the purge gas blower. The hot gases are sent to a pulse-jet bag filter called purge gas filter whose purpose is to remove and collect the carbon black particles, on one side, and let out clean gas to the atmosphere, on the other side. Carbon black collected in the conical hopper is fed to the surge tank through a rotary valve, so no carbon black loss occurs throughout the operation of the dryer.

Handling, storage and packing

Dried pellets coming from the dryer are fed into a bucket elevator that lifts them up to the storage silos top and, through screw conveyor, are distributed therein according to the relevant grade before being cleared by a magnetic drum separator which removes any metallic particles. Off-spec product is routed to an off-spec silo for further treatment.
Carbon black is typically shipped in bulk by tank trucks, tank train cars, or in big-bags. Packing area and warehouse are kept de-dusted under slightly vacuum in order to provide the workers with a safe and clean environment.

Energy Recovery

The balance tail gas not utilized in the dryer combustion furnace, is burned in a tail gas boiler for producing steam. About 2% of the steam is utilized in the ET Black™ process, while the remaining part can be either exported to local users or supplied to a power block for the generation of electricity. Before being released to the atmosphere the flue gases from tail gas boiler and from the dryer are sent to a flue gas treatment system for the removal of NO_x/SO_x.

Reference

Eurotecnica, CARBON BLACK PROCESS.