Multicomponent vapor, condensation

For multicomponent vapor-liquid equilibria, the equation of equilibrium for every condensable component i is... 

FIGURE 1 Typical temperature profiles for several process heat exchanger applications (a) product cooler (b) feed heater with condensing stream (c) multicomponent feed heater with vaporization and superheating (d) pure-component product condenser (e) multicomponent product condenser (f) typical feed-effluent heat exchanger. 

Particle formation events from gaseous precursors are observed frequently almost everywhere in the troposphere, both in polluted cities and remote clean areas [4]. It is likely that different nucleation mechanisms are at work in different conditions, but no formation mechanism has been identified so far. It is, however, clear that particles are formed by nucleation of a multicomponent vapor mixture. Water vapor is the most abundant condensable gas in the atmosphere, but it can not form particles on its own homogeneous nucleation requires such a high supersaturation, that heterogeneous nucleation on omnipresent pre-existing particles always starts first and consumes the vapor. However, vapor that is un-... 

Condensation of only part of a multicomponent vapor mixture, all components of which are theoretically condensable. This situation will occur where the dew point of some of the lighter components is above the coolant temperature. The uncondensed component may be soluble in the condensed liquid, such as in the condensation of some hydrocarbon mixtures containing light gaseous components. 

In this section we review the experimental studies that have been carried out with a view to testing models of multicomponent condensation. There is a great shortage of experimental data on mass transfer in multicomponent vapor (plus inert gas)-liquid systems. Most published works deal with absorption (or condensation or evaporation) of a single species in the presence of a nontransferring component. Thus, this review is necessarily brief. 

Bandrowski, J. and Kubaczka, A., On the Condensation of Multicomponent Vapors in the Presence of Inert Gases, Int. J. Heat Mass Transfer, 24, 147-153 (1981). 

Webb, D. R., Heat and Mass Transfer in Condensation of Multicomponent Vapors, Proceedings of the Seventh International Heat Transfer Conference, Munich, Germany, 5, 167-174 (1982). 

Chapter 12 presents models of mass transfer on distillation trays. This material is used to develop procedures for the estimation of point and tray efficiencies in multicomponent distillation in Chapter 13. Chapter 14 uses the material of Chapter 12 in quite a different way in an alternative approach to the simulation and design of distillation and absorption columns that has been termed the nonequilibrium stage model. This model is applicable to liquid-liquid extraction with very little modification. Chapter 15 considers the design of mixed vapor condensers. 

SCV - Single-component vapor MCV - Multicomponent vapor SC - Subcooled condensate Ap - Pressure drop C - Coolant. Acceptability G - good F - fair P - poor X - not acceptable or not recommended. 

In some services (e.g., refinery fractionators), vapor approaching the chimney tray is hotter than the chimney tray liquid. Heat will be transferred from the vapor to the liquid. If the vapor is condensable, some will condense on the bottom face of the chimney tray. The net result is analogous to leakage. The author is familiar with situations where refractory was installed on the bottom face of the chimney tray. In all these cases, steps were also taken to minimize leakage, making it difficult to independently assess the effectiveness of the refractory. For multicomponent, partially condensable vapor condensing on an uninsulated bottom face of a chimney tray (e.g., in a refinery fractionator), a typical heat transfer coefficient is 15 Btu/(h ft °F) (237). 

Total reflux exists in a distillation column, whether a binary or multicomponent system, when all the overhead vapor from the top tray or stage is condensed and returned to the top tray. Usually a column is brought to equilibrium at total reflux for test or for a temporary plant condition which requires discontinuing feed. Rather than shut down, drain and then re-establish operating conditions later, it is usually more convenient and requires less... 

To be useful, this type of simulator must calculate the thermodynamic properties of multicomponent mixtures in both liquid and vapor phases while predicting bubble and dew points or partial vaporizations or condensations. Using this basic information, the simulator must then make calculations for other processes, such as gas cooling by expansion, gas compression, multiple flashes condensations, and separations by absorption... 

A similar model has been applied to the modeling of porous media with condensation in the pores. Capillary condensation in the pores of the catalyst in hydroprocessing reactors operated close to the dew point leads to a decrease of conversion at the particle center owing to the loss of surface area available for vapor-phase reaction, and to the liquid-filled pores that contribute less to the flux of reactants (Wood et al., 2002b). Significant changes in catalyst performance thus occur when reactions are accompanied by capillary condensation. A pore-network model incorporates reaction-diffusion processes and the pore filling by capillary condensation. The multicomponent bulk and Knudsen diffusion of vapors in each pore is represented by the Maxwell-Stefan model. 

Moreover, the convection of the vapors in the pores was considered and the condensation in the pores was described by the multicomponent Kelvin equation (Shapiro and Stenby, 1977). 

In some operations, where the top product is required as a vapor, only sufficient liquid is condensed to provide the reflux flow to the column, and the condenser is referred to as a partial condenser. When the liquid is totally condensed, the liquid returned to the column will have the same composition as the top product. In a partial condenser the reflux will be in equilibrium with the vapor leaving the condenser. Virtually pure top and bottom products can be obtained in a single column from a binary feed, but where the feed contains more than two components, only a single pure product can be produced, either from the top or bottom of the column. Several columns will be needed to separate a multicomponent feed into its constituent parts. 

Integral condensation in which the liquid remains in equilibrium with the uncondensed vapor. The condensation curve can be determined using procedures similar to those for multicomponent flash distillation given in Chapter f f. This will be a relatively simple calculation for a binary mixture, but complex and tedious for mixtures of more than two components. 

The coefficients cited for condensation in the presence of noncondensable gases or for multicomponent mixtures are only for very rough estimation purposes because of the presence of mass transfer resistances in the vapor (and to some extent, in the liquid) phase. Also, for these cases, the vapor-phases temperature is not constant, and the coefficient given is to be used with the mean temperature differences estimated using vapor-phase inlet and exit temperatures, together with the coolant temperatures. 

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