Oil furnace carbon blacks are generally produced in a furnace-type reactor by pyrolyzing an hydrocarbon feedstock with hot combustion gases to produce combustion products containing particulate carbon black. A variety of methods for producing carbon blacks by the oil furnace process are generally known and are described in U.S. patents, such as U.S. Pat. Nos. 3,922,335; 3,401,020; and 2,785,964. An oil furnace process for producing carbon blacks is also described in the commonly assigned U.S. patent application Ser. No. 07/846,644, filed Mar. 5, 1992, the disclosure of which is hereby incorporated by reference.
In one type of a furnace carbon black reactor, such as shown in U.S. Pat. No. 3,401,020 to Kester et al., or U.S. Pat. No. 2,785,964 to Pollock, hereinafter "Kester" and "Pollock" respectively, a fuel, preferably hydrocarbonaceous, and an oxidant, preferably air, are injected into a first zone and react to form hot combustion gases. A hydrocarbon feedstock in either gaseous, vapor or liquid form is also injected into the first zone whereupon pyrolysis of the hydrocarbon feedstock commences. In this instance, pyrolysis refers to the thermal decomposition of a hydrocarbon. The resulting combustion gas mixture, in which pyrolysis is occurring, then passes into a reaction zone where completion of the carbon black forming reaction occurs.
In another type of a furnace carbon black reactor a liquid or gaseous fuel is reacted with an oxidant, preferably air, in the first zone to form hot combustion gases. These hot combustion gases pass from the first zone, downstream through the reactor, into a reaction zone and beyond. To produce carbon blacks, a hydrocarbonaceous feedstock is injected at one or more points into the path of the hot combustion gas stream. The hydrocarbonaceous feedstock may be liquid, gas or vapor, and may be the same or different than the fuel utilized to form the combustion gas stream. Generally the hydrocarbonaceous feedstock is a hydrocarbon oil or natural gas, however other hydrocarboneous feedstocks such as acetylene are known in the art. The first (or combustion) zone and the reaction zone may be divided by a choke or zone of restricted diameter which is smaller in cross section than the combustion zone or the reaction zone. The feedstock may be injected into the path of the hot combustion gases upstream of, downstream of, and/or in the restricted diameter zone. Furnace carbon black reactors of this type are generally described in U.S. Reissue Pat. No. 28,974 and U.S. Pat. No. 3,922,335.
In both types of processes and reactors described above, and in other generally known reactors and processes, the hot combustion gases are at a temperature sufficient to effect pyrolysis of the hydrocarbonaceous feedstock injected into the combustion gas stream. In one type of reactor, such as disclosed in Kester, feedstock is injected, at one or more points, into the same zone where combustion gases are being formed. In other type reactors or processes the injection of the feedstock occurs, at one or more points, after the combustion gas stream has been formed. In either type of reactor, since the hot combustion gas stream is continually flowing downstream through the reactor, pyrolysis continually occurs as the mixture of feedstock and combustion gases passes through the reaction zone. The mixture of feedstock and combustion gases in which pyrolysis is occurring is hereinafter referred to, throughout the application, as "the effluent". The residence time of the effluent in the reaction zone of the reactor is sufficient, and under conditions suitable, to allow the formation of carbon blacks. "Residence time" refers to the amount of time which has elapsed since the initial contact between the hot combustion gases and the feedstock. After carbon blacks having the desired properties are formed, the temperature of the effluent is lowered to stop pyrolysis. This lowering of the temperature of the effluent to stop pyrolysis may be accomplished by any known manner, such as by injecting a quenching fluid, through a quench, into the effluent. As generally known to those of ordinary skill in the art, pyrolysis is stopped when the desired carbon black products have been produced in the reactor. One way of determining when pyrolysis should be stopped is by sampling the effluent and measuring its toluene extract level. Toluene extract level is measured by ASTM D1618-83 "Carbon Black Extractables--Toluene Discoloration". The quench is generally located at the point where the toluene extract level of the effluent reaches an acceptable level for the desired carbon black product being produced in the reactor. After pyrolysis is stopped, the effluent generally passes through a bag filter system to separate and collect the carbon blacks.
Carbon blacks may be utilized as pigments, fillers, reinforcing agents, and for a variety of other applications. They are widely utilized as fillers and reinforcing pigments in the compounding and preparation of rubber compositions and plastic compositions. Carbon blacks are generally characterized on the basis of properties including, but not limited to, surface areas, surface chemistry, aggregate sizes, particle sizes and crystallite dimensions. The properties of carbon blacks are analytically determined by tests known to the art, including cetyl-trimethylammonium bromide adsorption (CTAB) and dibutyl phthalate adsorption (DBP).
In addition to a variety of other uses, such as tire, hoses, belts, and plastics, carbon blacks are generally utilized in compositions intended for use as semi-conductive shielding compounds for electric power cables. Electric power cables generally consist of electrically conductive wires surrounded by a dielectric insulating material which prevents escape of electricity to the environment. Semi-conductive shields are used to promote longer life in underground power cables. Two semiconductive layers are typically used: 1) the "conductor shield" which surrounds the conductor wires and is in intimate contact with the inside surface of the insulation layer, and 2) the "insulation shield" which surrounds the outer surface of the insulation layer. These shields act to increase cable life by reducing voltage stresses across the insulation layer by providing a smooth interface at both the inner and outer surface of the insulation. Requirements for both shielding layers include a specified degree of conductivity, appropriate mechanical properties, including tensile strength and elongation to break, smooth surface, as well as a high degree of chemical purity. The semi-conductive shields are composed of a mixture of a polymer and some type of conductive filler. Carbon black has been found to be conductive filler material that best meets all the requirements for this application.
A common cause of underground power cable failure is formation of "water trees" in the insulation layer of the cable. One possible cause of water trees is believed to be due to the presence of water and water soluble ions, such as sulfur and metallic cations, within the cable. Under the normal voltage gradients which exist in the cable, these ionic materials tend to migrate though the insulation layer. When a continuous pathway of water soluble, conductive ions is formed through the insulation layer, the cable fails. Analysis of cables has shown that the concentration of ionic impurities such as sulfur, calcium and the like are higher in water trees and in the insulation near the conductor shield than in the surrounding insulation material. One possible source of these ionic impurities is the carbon black filler in the conductor shield compound.
Carbon blacks with low sulfur and cation impurity levels are advantageous for use in long-life underground power cable shield compounds. The unique class of carbon blacks produced from pure acetylene, referred to as "acetylene blacks" are considered to be the industry standard for having the lowest sulfur and cation impurity levels. The calcium carbonate process and the acetylene gas process are generally used to manufacture acetylene black.
However, the commercial availability of acetylene blacks is limited and acetylene blacks are only commonly available within a narrow range of analytical and performance properties. Furthermore, the cost of acetylene blacks is generally higher than the costs of carbon blacks produced by the oil furnace process.
Thus, the majority of commercially available carbon blacks are produced using the oil furnace process due to its efficient use of raw materials and flexibility for producing carbon blacks with a wide range of analytical and performance properties.
From the foregoing discussion, it is apparent that it would be advantageous to produce carbon blacks that have ash and sulfur levels similar to, equivalent to, or lower than, acetylene blacks, using the oil furnace process. The furnace carbon blacks of the present invention achieve this advantage and other advantages that will become apparent from the following discussion and examples.