Vapor-liquid-solid reaction process

Silicon carbide whiskers, usually produced from silica and carbon, are widely used for ceramic matrix composites [38]. Tbe vapor-liquid-solid (VLS) process is accepted as the mechanism for SiC whisker production (Fig. 15.1.4). This process requires formation of a liquid phase of a transition metal at the reaction sites. Elemental sources of Si and C are usually supplied in vapor phase form as SiO and CH4, respectively [39]. The vapors are deposited on the surface of the liquid metal and dissolved into the liquid metal droplets. Since the whiskers are precipitated from the dissolved components, metal droplets are often observed at the tip of the whiskers [39]. 

For example, when we consider the design of specialty chemical, polymer, biological, electronic materials, etc. processes, the separation units are usually described by transport-limited models, rather than the thermodynamically limited models encountered in petrochemical processes (flash drums, plate distillations, plate absorbers, extractions, etc.). Thus, from a design perspective, we need to estimate vapor-liquid-solid equilibria, as well as transport coefficients. Similarly, we need to estimate reaction kinetic models for all kinds of reactors, for example, chemical, polymer, biological, and electronic materials reactors, as well as crystallization kinetics, based on the molecular structures of the components present. Furthermore, it will be necessary to estimate constitutive equations for the complex materials we will encounter in new processes. 

Solid-eatalyzed reactions can occur in either the liquid or gas phase. Gas-phase reactions are not very common in the production of fine chemieals, beeause eom-plex molecules with limited volatility and thermal stability are usually involved, which makes operation at the high temperatures required for their vaporization impossible. Gas-liquid reactions with a solid catalyst probably encompass the largest number of applications in fine-chemical and pharmaceutical processes [1]. Two other classes of solid-eatalyzed reaction taking place in the liquid phase are liquid-solid reactions and liquid-liquid-solid reactions, but these are much less eommon. We shall, therefore, foeus on gas-liquid-solid reaetions, in which the solid is a heterogeneous catalyst. 

Versailles Agreement on Materials and Standards vapor phase formation and condensation process valence electron concentration vapor-liquid-solid process vapor-solid reaction wurtzitic boron nitride... 

Vessel mptures can also occur when a higher-temperature liquid or solid is combined with a cooler low boiling liquid, transferring sufficient heat from the hotter material to the colder material such that the colder material rapidly vaporizes. No chemical reactions are involved instead, the explosion occurs because the cooler liquid expands as it is converted to vapor, creating high pressures. These are called physical explosions. A common example is a steam explosion, which occurs when liquid water is accidentally introduced into a process vessel operating at an elevated temperature. If the hotter material is above the superheat limit temperature of the evaporating liquid, initial confinement by a vessel is not required to create an explosion pressure wave. 

The physical nature of the process stream. Is it single-phase or two-phase Is it liquid, solid, vapor or slurry What is its temperature and pressure at the sampling point, and how far can these be allowed to change during sampling What is its viscosity at the appropriate sample measurement temperature The chemical nature of the process stream. Is it at equilibrium (a final product) or is it to be measured mid-reaction Is sample transport possible, or must the sample be measured in situ Is it corrosive, and what material and metallurgical constraints exist ... 

According to the relationship between the lattice volume and Tc as described, cubic CssCgo would be an ultimate candidate for a higher Tc superconductor, but the conventional vapor-solid reaction affords only the thermodynamically stable CsCso and CS4C60 phases. In 1995, noncubic CssCgo was obtained by a solution process in liquid ammonia, and the superconductivity was observed below 40 K under an applied hydrostatic pressure of 1.4 GPa [311]. 

Reaction vessels are usually of two types one in which the contents are agitated or stirred in some way and the other in which the reactor and contents are stationary. The first is used with materials such as solids or liquids that need to be brought into intimate contact with the catalyst and the hydrogen. The second type is used where the substance may have sufficient vapor pressure at the temperature of operation so that a gas-phase as well as a liquid-phase reaction is possible. It is also most frequently used in continuous operation where larger quantities of material need to be processed than can be done conveniently with batch methods. 

In most applications, the reaction occurs between a dissolved gas and a liquid-phase reactant in the presence of a solid catalyst. In some cases, the liquid is an inert medium and the reaction takes place between the dissolved gases at the solid surface. These reactors have many diverse applications in catalytic processes and are used extensively in the chemical industry. Trickle-bed reactors have been developed by the petroleum industry for hydrodesulfurization, hydrocracking, and hydrotreating of various petroleum fractions of relatively high boiling point. Under reaction conditions, the hydrocarbon feed is frequently a vapor-liquid mixture that reacts at liquid hourly space velocities (LHSV in volume of fresh feed, as liquid/volume of bed, hr) in the... 

In addition to handling the conventional vapor/liquid process operations, the ASPEN library of process models includes solids handling and separation units, a set of generalized reactors, improved flash and distillation unit models and process models from the FLOWTRAN simulator. The user can also include his or her own model or key elements of a model, such as the reaction kinetics, in FORTRAN code. 

In this book we are concerned only with mass transport, or diffusion, in solids. Self-diffusion refers to atoms diffusing among others of the same type (e.g., in pure metals). Interdiffusion is the diffusion of two dissimilar substances (a diffusion couple) into one another. Impurity diffusion refers to the transport of dilute solute atoms in a host solvent. In solids, diffusion is several orders of magnitude slower than in liquids or gases. Nonetheless, diffusional processes are important to study because they are basic to our understanding of how solid-liquid, solid-vapor, and solid-solid reactions proceed, as well as [solid-solid] phase transformations in single-phase materials. 

Styrene. Styrene is the largest benzene derivative with annual consumption about 11.5 billion lb in the United States. It is produced mainly by catalytic dehydrogenation of high-purity ethylbenzene (EB) in the vapor phase. The manufacture process for EB is based on ethylene alkylation with excess benzene. This can be done in a homogeneous system with aluminum chloride catalyst or a heterogeneous solid acid catalyst in either gas or liquid-phase reaction. In the past decade, the liquid-phase alkylation with zeolite catalyst has won acceptance. Those processes have advantages of easier product separation, reducing waste stream, and less corrosion. In addition, it produces less xylene due to lower... 

HPAs, however, is their solubility in polar solvents or reactants, such as water or ethanol, which severely limits their application as recyclable solid acid catalysts in the liquid phase. Nonetheless, they exhibit high thermal stability and have been applied in a variety of vapor phase processes for the production of petrochemicals, e.g. olefin hydration and reaction of acetic acid with ethylene [100, 101]. In order to overcome the problem of solubility in polar media, HPAs have been immobilized by occlusion in a silica matrix using the sol-gel technique [101]. For example, silica-occluded H3PW1204o was used as an insoluble solid acid catalyst in several liquid phase reactions such as ester hydrolysis, esterification, hydration and Friedel-Crafts alkylations [101]. HPAs have also been widely applied as catalysts in organic synthesis [102]. 

However, this simple chemical equation conceals a more complicated sequence of events in which the reactants undergo various transformations before the product is formed. These may be summarized in a Born-Haber thermodynamic cycle (Figure 3.1). The first stage of the reaction process is the conversion of M and E into gaseous state atoms, requiring an enthalpy of atomization of A// j(M), and, if E is a solid or liquid, the enthalpy of vaporization, of E or... 

In a homogeneously catalyzed reaction the determination of the kinetic factors for the process is usually straightforward. In a solution, reactants and the soluble catalysts are uniformly distributed throughout the reaction medium and the reaction rate can be expressed as a function of the concentrations of these substances. A heterogeneously catalyzed process is more complex because the catalyst is not uniformly distributed throughout the reaction medium. Consider a two phase system, either vapor/solid or liquid/solid, with the solid phase the catalyst. In such a system several steps are needed to complete the catalytic cycle ... 

Fixed-bed catalytic reactors are widely applied to reaction systems in which the reactants are present in a single vapor phase. The scale-up and performance of commercial reactors can be predicted from experiments in small-scale reactors. On the other hand, the mixed-phase trickle bed reactor is considerably more complex to analyze and scale up. The performance of trickle bed reactors is influenced by many factors associated with mixed-phase (gas-liquid-solid) processing. Some of... 

In Fig. IB, the VLS process is illustrated. At reaction temperatures exceeding the eutectic. Si vapor dissolves in the Au seed to form a liquid alloy droplet. As more gaseous Si dissolves in the alloy, the droplet becomes supersaturated, and Si crystallizes on the droplet surface. The shape of the droplet interface and the surface tension difference between the liquid alloy and the solid semiconductor promote crystallization in one direction. In many cases, the liquid alloy droplet displaces off the surface and rides on the top of the vertically growing whisker—crystallization continues to occur at the liquid-solid interface as the metal seed is continuously fed with Si from the gas phase. 

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