Continuous fibers vapor phase processes

Even more perfect single wall (or shell) carbon nanotubes were reported by a scaled-up version [81] of the laser oven method described above [12]. This process yields a nearly endless, highly tangled raw nanotube material. It is purified in nitric acid and the resulting nanotube ropes, non-woven mats, or bucky paper can be cut into individual intermediate length (100-300 nm) macromolecules for further evaluation [81]. This method, which yields potentially endless nanotubes, will also be discussed in Chapter 3, which deals with continuous or potentially continuous fibers from the vapor phase. 

The technology of growing short carbon fibers from the vapor phase dates back to 1889 when a patent [46] issued claiming their growth by using iron catalyzed pyrolysis of methane. About a hundred years later a continuous process has become commercially available [47-48] potentially opening the door to commercial development [51]. 

This chapter deals with four continuous processes capable of yielding continuous, potentially continuous, and discontinuous fibers from the vapor phase. One is a commercial process the others are experimental processes. 

Mass transfer in metal catalyzed and in laser assisted CVD processes is driven by highly localized temperature gradients. The relatively small area of either a hot molten metal particle or of a hot laser focus affords whiskers [4] or continuous fibers, respectively [2] [18-19]. The transfer of an equal mass from the vapor to the solid phase in a conventional chemical vapor deposition results in a thin coating over the relatively large area of a hot surface, i.e., that of a flat complex shaped composites part. 

Chemical vapor infiltration (CVI) is a process whereby reactive chemical species are generated in the vapor phase and allowed to react with a solid substrate thus modifying its chemistry. A successful use of this process requires (1) an absolutely continuous and stoichiometric conversion of the initial to the final chemistry in the solid state, and (2) a continuous and eventually complete conversion of the morphology, density, surface tension, mechanical as well all other properties. Discontinuous or incomplete results cause a steep drop in strength, the premier measure of uniformity. By combining the complexities of chemical vapor deposition (this chapter) with those of fiber formation from a precursor fiber (Chapters 8 to 12), the process is therefore intrinsically more difficult to control than any other. 

When fabricating ceramic composites, the reaction between the reinforcement and matrix must be minimized. Consequently, a suitable low processing temperature should be selected to minimize degradation of the reinforcing phases. Use of continuous fibers can modify the mechanical properties if suitable fiber orientations are chosen. A well-designed hber architecture is hrst prepared, and the matrix material introduced into the voids of the structure. Infiltration into the hber architecture is often performed using chemical vapor inhltration or... 

In the Chemische Werke Witten process, which was further developed by Dynamit Nobel and Hercules, p-xylene, air and the catalyst are fed continuously into the oxidation reactor, to which recirculated p-methylbenzoic acid methyl ester is also added. Oxidation is effected at a temperature of 140 to 170 °C and a pressure of 4 to 7 bar. The heat of reaction is removed by the vaporization of water and excess p-xylene. The further reaction with methanol is carried out at 200 to 250 °C under slightly raised pressure (20 bar) in the esterification reactor, to keep the reaction mixture in the liquid-phase. The esterification products flow to the crude ester column, where p-methylbenzoic add methyl ester is separated from the crude dimethyl terephthalate. p-Methylbenzoic acid methyl ester is recycled to the oxidation reactor, where oxidation of the second methyl group occurs. The crude dimethyl terephthalate is purified to fiber grade quality by distillation and crystallization from methanol, and subsequent redistillation in a column with around 30 trays. The yield of dimethyl terephthalate (m.p. 141 °C) is generally about 87 mol%. 

Until 1987, the only route to short carbon fibers was a metal catalyzed chemical vapor deposition. Since then, a novel process has become available [19] that facilitates the growth of discontinuous carbon fibers from mesopitch by a continuous liquid phase centrifuge process. Pitch may be considered to consist of a complex mixture of polycyclic aromatic hydrocarbons. It is a semisolid at room temperature but, depending on the composition, it melts above 100°C. Pitch has two phases, a high melting anisotropic, and a low melting isotropic, phase. The anisotropic phase, called mesopitch, is preferred for this process. 

A liquid phase, as opposed to a vapor or solid phase, includes dispersions, solutions and melts. Several processes, which yield continuous inorganic fibers directly from the melt, have been discussed in Chapter 4. Only one generic process, dry spinning, is known to yield one specific amorphous oxide fiber directly from a liquid phase other than that of a melt. All other processes which start with a liquid phase (see Chapters 8-12) yield first a solid, non-functional precursor or green fiber, and then a functional, nano- or polycrystailine ceramic fiber. Such refractory ceramic fibers are therefore directly derived from a solid phase, a precursor or a green fiber, and only indirectly from a liquid phase. 

Membrane contactors provide a continuous process for contacting two different phases in which one of the phases must be a fluid. Whether using a flat-sheet, hollow-fiber, or spiral-wound type, the membrane acts as a separator for two interfaces as it has two sides compared to conventional separation processes, which involve only one interface in a two-phase system. Therefore, it allows the formation of an immobilized phase interface between the two phases participating in the separation process [9]. Generally, there are five different classes of contacting operations gas-liquid, liquid-liquid, supercritical fluid-liquid, liquid-solid, and contactors as reactors [10]. The most commonly used operation in industry are gas-liquid also known as vapor-liquid, liquid-liquid, and supercritical fluid-liquid. Each class of system has its own modes of operation but in this study, emphasis will be focused on the gas-Uquid contacting systans. Table 9.1 describes the membrane contactor in summary. 

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