Carbon black feedstocks

The market for tar-based road binders has declined considerably for a variety of reasons. Less cmde tar is available and the profits from the sales of electrode pitch and wood-preservation creosote or creosote as carbon-black feedstock are higher than those from road tar. In most industrial countries, road constmction in more recent years has been concentrated on high speed motorways. Concrete, petroleum bitumen, or lake asphalt are used in the constmction of these motorways. In the United Kingdom, for example, the use of tar products in road making and maintenance had fallen from 330,000 t in 1960 to 100,000 t in 1975 and is less than 100 t in 1994, mainly based on low temperature pitch which is not suitable for electrode or briquetting binders, but which is perfectly satisfactory as the basis for road binders. 

Aromaticity is the most important property of a carbon black feedstock. It is generally measured by the Bureau of Mines Correlation Index (BMCI) and is an indication of the carbon-to-hydrogen ratio. The sulfur content is limited to reduce corrosion, loss of yield, and sulfur in the product. It may be limited in certain locations for environmental reasons. The boiling range must be low enough so that it will be completely volatilized under furnace time—temperature conditions. Alkane insolubles or asphaltenes must be kept below critical levels in order to maintain product quaUty. Excessive asphaltene content results in a loss of reinforcement and poor treadwear in tire appHcations. 

The pricing of carbon black feedstocks depends on their alternate market as residual fuel oil, especially that of high sulfur No. 6 fuel oil. The actual price is deterrnined by the supply/demand relationships for these two markets. Feedstock cost contributes about 60% of the total manufacturing cost. The market price of carbon black is strongly dependent on the feedstock cost as shown in Figure 8. 

The heaviest bottoms product from the main column is commonly called slurry or decant oil. (In this book, these terms are used interchangeably.) The decant oil is often used as a cutter stock with vacuum bottoms to make No. 6 fuel oil. High-quality decant oil (low sulfur, low metals, low ash) can be used for carbon black feedstocks. 

DO is the heaviest product from a cat cracker. DO is also called slurry oil, clarified oil, bottoms, and FCC residue. Depending on the refinery location and market availability, DO is typically blended into No. 6 fuel, sold as a carbon black feedstock (CBFS), or even recycled to extinction. 

In the case of carbochemical oils, the BMCI may not reflect the true aromaticity of the product. For this reason, the carbon/hydrogen ratio is more favored for carbochemical products. However, as this measurement is also superior to BMCI, even for petrochemical products, the carbon/hydrogen ratio or the carbon content are becoming the preferred criteria for all carbon black feedstocks. 

The preheated feedstock is atomized radially to the smallest possible droplets in a tubular reactor, the so-called carbon black furnace . There is a zone of high turbulence immediately in front of the injector in which the atomized feedstock is intimately mixed with hot gases from the combustion of the fuel (preferably natural gas, but also coking gas or liquid gas) in an excess of preheated air. The air excess is so adjusted that up to ca. 50% by weight of the carbon black-feedstock is consumed. The temperature obtained in the pyrolysis zone is 1200 to ca. 1800°C. Carbon black formation is weakly endothermic. 

In the furnace process, which today dominates carbon black production, oils rich in aromatics from naphtha or gas oil pyrolysis, cat-cracker residues (decant oils) together with mixtures of aromatics from coal tar, are used as feedstock. Table 13.5 summarizes the characteristic data for decant oil, pyrolysis oil from naphtha cracking and a carbon black feedstock derived from coal tar. 

The Bureau of Mines Correlation Index (BMCI) defined below, is a particularly important criterion of the quality of the carbon black feedstock ... 

Decant oil Pyrolysis oil Coal-tar carbon black feedstock... 

The pre-heated carbon black oil is sprayed into the reactor, where it is cracked in a high temperature zone (1200 to 1800 °C). The reaction temperature is maintained by burning an additional energy carrier, such as natural gas, with an excess of atmospheric oxygen the unused oxygen leads to partial combustion of the carbon black feedstock. 

Furnace carbon black is produced from the incomplete combustion of what is called carbon black oil feedstock, which consists of heavy aromatic residue oils. In the United States this oil is commonly the bottoms from catalytic cracker units. They are commonly referred to as cat cracker bottoms and contain relatively low hydrogen content (and conversely high carbon content). In Europe and other locations, the carbon black oil used is commonly a byproduct of high-temperature steam cracking of such products as naphtha, gas condensate, and gas oil to produce ethylene, propylene, and other olefins. Here, no catalysts are used in the cracking process. These types of carbon black oils are mainly unsaturated hydrocarbons. A third source of carbon black feedstock is coal tar, which is commonly used in China to manufacture carbon black. 

Approximately 50—55% of the product from a coal-tar refinery is pitch and another 30% is creosote. The remaining 15—20% is the chemical oil, about half of which is naphthalene. Creosote is used as a feedstock for production of carbon black and as a wood preservative. Because of modifications to modem coking processes, tar acids such as phenol and cresyUc acids are contained in coal tar in lower quantity than in the past. To achieve economies of scale, these tar acids are removed from cmde coal tar with a caustic wash and sent to a central processing plant where materials from a number of refiners are combined for recovery. 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

The aimual worldwide production of carbon blacks, which iaclude a large variety of carbonaceous products, was estimated to be around six million metric tons ia 1994. More than 90% of this pigment is consumed by the mbber iadustries, ia particular, by the tire iadustry as a reinforcing agent. The rest (- 500, 000 t) is used for coloring plastics, printing inks, and paints. Particle size of carbon blacks varies from 5 to 500 p.m and can be controlled by the process conditions and feedstock (see Carbon, carbon black). 

The production process or the feedstock is sometimes reflected ia the name of the product such as lamp black, acetylene black, bone black, furnace black, or thermal black. The reason for the variety of processes used to produce carbon blacks is that there exists a unique link between the manufactuting process and the performance features of carbon black. 

Until the end of World War II, coal tar was the main source of these aromatic chemicals. However, the enormously increased demands by the rapidly expanding plastics and synthetic-fiber industries have greatly outstripped the potential supply from coal carbonization. This situation was exacerbated by the cessation of the manufacture in Europe of town gas from coal in the eady 1970s, a process carried out preponderantly in the continuous vertical retorts (CVRs), which has led to production from petroleum. Over 90% of the world production of aromatic chemicals in the 1990s is derived from the petrochemical industry, whereas coal tar is chiefly a source of anticorrosion coatings, wood preservatives, feedstocks for carbon-black manufacture, and binders for road surfacings and electrodes. 

Timber-preservation creosotes are mainly blends of wash oil, strained anthracene oil, and heavy oil having minor amounts of oils boiling in the 200—250°C range. Coal-tar creosote is also a feedstock for carbon black manufacture (see Carbon, carbon black). Almost any blend of tar oils is suitable for this purpose, but the heavier oils are preferred. Other smaller markets for creosote were for fluxing coal tar, pitch, and bitumen in the manufacture of road binders and for the production of horticultural winter wash oils and disinfectant emulsions. 

Carbon Blacks. Carbon blacks are occasionally used as components in mixes to make various types of carbon products. Carbon blacks are generally prepared by deposition from the vapor phase using petroleum distillate or gaseous hydrocarbon feedstocks (see Carbon, carbon black). 

In addition to combined hydrogen and oxygen, carbon blacks may contain as much as 1.2% combined sulfur resulting from the sulfur content of the aromatic feedstock that contains thiophenes, mercaptans, and sulfides. The combined sulfur appears to be inert and does not contribute to sulfur cross-linking during the vulcanization of mbber compounds. 

The quality and yield of carbon black depends on the quaUty of the feedstock, reactor design, and input variables. The stmcture is controlled by the addition of alkaU metals to the reaction or mixing 2ones. Usual practice is to use aqueous solutions of alkaU metal salts such as potassium chloride or potassium hydroxide sprayed into the combustion chamber or added to the make oil in the oil injector. Alkaline-earth compounds such as calcium acetate that increase the specific surface area are introduced in a similar manner. 

Carbon black is produced by the partial combustion or the thermal decomposition of natural gas or petroleum distillates and residues. Petroleum products rich in aromatics such as tars produced from catalytic and thermal cracking units are more suitable feedstocks due to their high carbon/hydrogen ratios. These feeds produce blacks with a... 

Carbon blacks are manufactured from hydrocarbon feedstocks by partial combustion or thermal decomposition in the gas phase at high temperatures. World production is today dominated by a continuous furnace black process, which involves the treatment of viscous residual oil hydrocarbons that contain a high proportion of aromatics with a restricted amount of air at temperatures of 1400-1600 °C. 

Furnace Black One of the three principal processes used for making carbon black the others being the Thermal Black and the Channel Black processes. In the Furnace Black process, aromatic fuel oils and residues are injected into a high velocity stream of combustion gases from the complete burning of an auxiliary fuel with an excess of air. Some of the feedstock is burned, but most of it is cracked to yield carbon black and hydrogen. The products are quenched with water. 

Thermal Black One of the processes used to make carbon black. The feedstock is usually natural gas. The gas is pyrolyzed in one of a pair of refractory reactors which has been preheated by burning part of the feed and hydrogen from the process. When the temperature has fallen, the functions of the reactors are interchanged. 

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