Vapour-liquid-solid process

The prediction of the thermal conductivity is not as simple for the composite reinforced with SiC whiskers. In fact, as mentioned earlier, the thermal conductivity of the reinforcement phase plays an important role. However, for SiC whiskers, their chemical composition can vary drastically depending on the manufacturing process, which is not the case for Si3N4 whiskers. In fact, the influence of the manufacturing process is drastic the thermal conductivity of SiC(w) produced by the vapour-solid process is around 20 W/m K as opposed to 100-250 W/m K for whiskers produced by the vapour-liquid-solid process. Using a Si3N4 matrix with a nominal thermal conductivity of... 

In the vapour-liquid-solid process, as described in Figures 4.38a and b, a liquid alloy droplet composed of metal cntalyst component (An, Fe, etc.) and nanowire component (Si, M-V component, E-VI compound, oxide, etc.) is first formed under... 

Figure 5 Demonstration of the activity of printed particles. In printed arrays, 60-nm Au particles retain both their catalytic and optical activities, a. Silicon nanowires grown through a vapour-liquid-solid process from a printed array of Au particles (inset is tilted), b, AFM topography and the corresponding dark-field image of particles on the printing plate (top row) and an SEM image and the corresponding dark-field micrograph on a silicon substrate (bottom row the images were mirrored for convenience).

It is crucial that printed nanoparticles retain their useful properties during integration. Here, a surface-sensitive application was chosen to demonstmte that the individual gold nanocrystals are still catalytically active. Figure 5a shows silicon nanowires (SiNW) that were grown from an array of 60-nm particles using a vapour— liquid-solid (VLS) process. The particles were printed onto a... 

The term distillation is applied to vaporisation and subsequent condensation according to (i) it should also be applied to (ii) since it is really the liquid which is converted into vapour and is first formed by condensation. Strictly speaking, the term sublimation should be applied to changes according to (iii). However, in practice, a substance when heated may first melt and then boil, but on cooling it may pass directly from the vapour to the solid the process is then also called sublimation. Indeed the mode of vaporisation, whether directly from solid to vapour or through the intermediate formation of a liquid, is of secondary importance it is the direct conversion of vapour to solid which is really the outstanding feature of sublimation in the laboratory. 

Solids processes clearly differ from vapour liquid systems in two main ways. Firstly and most obviously, solid systems have a particle size or size distribution added to their specification. Secondly, solids processing systems use different... 

The non-random two-liquid segment activity coefficient model is a recent development of Chen and Song at Aspen Technology, Inc., [1], It is derived from the polymer NRTL model of Chen [26], which in turn is developed from the original NRTL model of Renon and Prausznitz [27]. The NRTL-SAC model is proposed in support of pharmaceutical and fine chemicals process and product design, for the qualitative tasks of solvent selection and the first approximation of phase equilibrium behavior in vapour liquid and liquid systems, where dissolved or solid phase pharmaceutical solutes are present. The application of NRTL-SAC is demonstrated here with a case study on the active pharmaceutical intermediate Cimetidine, and the design of a suitable crystallization process. 

The catalytic hydration of olefins can also be performed in a three-phase system solid catalyst, liquid water (with the alcohol formed dissolved in it) and gaseous olefin [258,279,280]. The olefin conversion is raised, in comparison with the vapour phase processes, by the increase in solubility of the product alcohol in the excess of water [258]. For these systems with liquid and vapour phases simultaneously present, the equilibrium composition of both phases can be estimated together with vapour-liquid equilibrium data [281]. For the three-phase systems, ion exchangers, especially, have proved to be very efficient catalysts [260,280]. With higher olefins (2-methylpropene), the reaction was also performed in a two-phase liquid system with an ion exchanger as catalyst [282]. It is evident that the kinetic characteristics differ according to the arrangement (phase conditions), i.e. whether the vapour system, liquid vapour system or two-phase liquid system is used. However, most kinetic and mechanistic studies of olefin hydration were carried out in vapour phase systems. 

For a long time, only a liquid phase process was employed industrially for the hydration of acetylene to acetaldehyde mercury salts in acidic solution were used as catalysts. Only recent reports can be found in the literature (e.g. ref. 300) on the industrial utilisation of the direct vapour phase hydration of acetylene over solid catalysts. 

The physical nature of the process stream. Is it single phase or two phase Is it liquid, solid, vapour 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 ... 

A schematic diagram of a general separation process is shown in Figure 1. The feed mixture can be vapour, liquid or solid, while the two... 

The most common industrial technique involves the creation of a second phase (vapour, liquid or solid) that is immiscible with the feed phase (Figure 2a). The creation is accomplished by energy transfer (heat and/or shaft work) to or from the process or by pressure reduction. 

Chromatography can be defined as the separation of mixtures by distribution between two or more immiscible phases. Some of these immiscible phases can be gas-liquid, gas-solid, liquid-liquid, liquid-solid, gas-liquid-solid and liquid-liquid-solid. Strictly speaking, a simple liquid-liquid extraction is in fact a chromatographic process. Similarly, distillation is a chromatographic process that involves separation of liquids by condensation of their respective vapours at different points in a column. 

A new developed process PGSS (Particles from Gas Saturated Solutions) was applied for generation of powder from polyethyleneglycols. Principle of PGSS process is described and phase equilibrium data for the binary systems PEG-CO2 for the vapour-liquid and the solid-liquid range are presented in a master diagram . The influence of the process parameters on particle size, particle size distribution, shape, bulk density and crystallinity is discussed. 

For the design of a process for formation of solid particles using supercritical fluids, data on solid - liquid and vapour - liquid phase equilibrium are essential. PGSS process is only possible for systems where enough gas is solubilized in the liquid. 

Until relatively recently, the fact that an experimental isotherm necessarily contained composite information concerning the adsorption of the two components of a binary solution was considered to be a major problem. For a rigorous interpretation it was felt necessary to process the data to obtain the so-called individual adsorption isotherm or separate adsorption isotherm of each component. However, this is not at all straightforward and requires the introduction of a number of assumptions relating to the structure of the adsorbed layer. The main problem is of course to know the composition of the adsorbed layer. One assumption often used in the case of volatile components is that introduced by Williams (1913) the solid will adsorb the same amount of each component from the vapour in equilibrium with the solution as from the solution itself. This of course implies that the adsorbed layer has the same composition at the liquid-solid and gas-solid interfaces and it requires numerous gravimetric measurements from the vapour... 

Similar changes are produced when the volume of the system is altered. Alteration of volume may take place either while transference of heat to or from the system is cut off (adiabatic change), or while such transference may occur (isothermal change). In the latter case, the temperature of the system will remain constant in the former case, since at the triple point the pressure must be constant so long as the three phases are present, increase of volume must be compensated by the evaporation of liquid. This, however, would cause the temperature to fall (since communication of heat from the outside is supposed to be cut off), and a portion of the liquid must therefore freeze. In this way the latent heat of evaporation is counterbalanced by the latent heat of fusion. As the result of increase of volume, therefore, the process occurs L S V. Diminution of volume, without transference of heat, will bring about the opposite change, S + V -> L. In the former case there is ultimately obtained the univariant system S—Y in the latter case there will be obtained either S—L or L— V, according as the vapour or solid phase disappears first. 

Figure 5.1 presents the behaviour of a pure species that can exist as solid, liquid or vapour in a pressure-temperature diagram. We may have three types of two-phase equilibrium solid/liquid, vapour/liquid and solid/vapour. There is a point where all three phases coexist, designated by the triple point. Here the phase rule gives F=C+2-P= +2-3=Q degrees of freedom. Neither pressure nor temperature can be used to modify the equilibrium. If only two phases can be found at equilibrium F=l+2-2=l, and either pressure or temperature can vary. The most important equilibrium in process engineering is vapour-liquid equilibrium, abbreviate as VLE. It may be observed that the two phases will coexist up to a point where it is difficult to make a distinction between vapour and liquid. This is the critical point, a fundamental physical property characterised by critical parameters and. Above the critical point the state... 

So far, in the discussion of industrial crystallization processes, only the crystallization of a solid phase from a supersaturated or supercooled liquid phase has been considered. However, the crystallization of a solid substance can be induced from a supersaturated vapour by the process generally known as sublimation . Strictly speaking, of course, the term sublimation refers only to the phase change solid vapour without the intervention of the liquid phase. In its industrial application, however, the term is commonly used to include the condensation (crystallization) process as well, i.e. solid vapour... 

Let us assume that in the absence of surfactant the drop forms an equilibrium contact angle above If the water contains surfactants then three transfer processes take place from the liquid onto all three interfaces surfactant adsorption at both (i) the inner liquid-solid interface and (ii) the liquid-vapor interface, and (iii) transfer from the drop onto the solid-vapor interface just in front of the drop. Adsorption processes (i) and (ii) result in a decrease of corresponding interfacial tensions, and y. The transfer of surfactant molecules onto the solid-vapour interface in front of the drop results in an increase of a local free energy, however, the total free energy of the system decreases. That is, surfactant molecule transfer ii) goes via a relatively high potential barrier and, hence, goes considerably slower than adsorption processes (i) and (ii). Hence, they are "fast" processes as compared with the third process (iii). 

An Au-catalysed chemical vapour transport and condensation (CVTC) process was used to produce ZnO nanorods and nanowires on Si02 and Si substrates [58], ZnO nanorods with a wide band gap (3.37eV) are regarded as promising candidates for the fabrication of nanoelectronic devices. In this work, EDX spectra of the tip and the body of ZnO nanorods were captured which indicates that Au-Zn alloyed droplets were present at the tips of the fabricated nanorods pointing to a nanorod growth via a vapor-liquid-solid (VLS) mechanism. 

Abstract Instantaneous distribution of mass and thermal fluxes inside and outside of an evaporating sessile droplet is considered using computer simulations. The latter distribution is calculated in a self consistent way by considering an intercoimected problem of vapour transfer in the vapour phase outside the droplet heat transfer in vapour, liquid and solid substrate and Marangoni convection inside the liquid droplet. The influence of thermal conductivity of the solid support on the evaporation process is evaluated. The deduced dependences of instantaneous fluxes can be applied for self-consistent calculations of time evolution of the evaporation processes of sessile droplets. 

Under a number of reasonable approximations the evaporation of small enough sessile droplets has been investigated in a self cOTisistent way by considering the interconnected problem of vapour transfer heat transfer in vapour, liquid and solid support and the Marangoni convection inside the liquid droplet The influence of the thermal conductivity of the solid support on the evaporation process has been analyzed. The calculated total evaporation flux has been compared with the result in the case of isothermal evaporation. It has been shown that the lower the thermal conductivity of the solid support the higher the deviations appear from the isothermal case. However, if the mean temperature of the droplet surface is used instead of the temperature of the surrounding air for the vapour concentration on the droplet surface flien the results found coincide with those known for the isothermal case. 

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