Vapor film liquid column

An interesting class of exact self-similar solutions (H2) can be deduced for the case where the newly formed phase density is a function of temperature only. The method involves a transformation to Lagrangian coordinates, based upon the principle of conservation of mass within the new phase. A similarity variable akin to that employed by Zener (Z2) is then introduced which immobilizes the moving boundary in the transformed space. A particular case which has been studied in detail is that of a column of liquid, initially at the saturation temperature T , in contact with a flat, horizontal plate whose temperature is suddenly increased to a large value, Tw T . Suppose that the density of nucleation sites is so great that individual bubbles coalesce immediately upon formation into a continuous vapor film of uniform thickness, which increases with time. Eventually the liquid-vapor interface becomes severely distorted, in part due to Taylor instability but the vapor film growth, before such effects become important, can be treated as a one-dimensional problem. This problem is closely related to reactor safety problems associated with fast power transients. The assumptions made are ... 

The rate-based models suggested up to now do not take liquid back-mixing into consideration. The only exception is the nonequilibrium-cell model for multicomponent reactive distillation in tray columns presented in Ref. 169. In this work a single distillation tray is treated by a series of cells along the vapor and liquid flow paths, whereas each cell is described by the two-film model (see Section 2.3). Using different numbers of cells in both flow paths allows one to describe various flow patterns. However, a consistent experimental determination of necessary model parameters (e.g., cell film thickness) appears difficult, whereas the complex iterative character of the calculation procedure in the dynamic case limits the applicability of the nonequilibrium cell model. 

The thin film evaporator operates based on action of three forces applied by an electric field to polar molecules of a liquid. The first force pumps the liquid between the two electrodes that are generating the electric field. A second force generated due to the difference between the dielectric constant of the vapor and the liquid phase forces the liquid layer to the surface. A third force pushes down the liquid column raised between the two electrodes. A balance between the first and the third forces, gravity, and viscous forces at the fluid-solid interface determines the height of the liquid between the electrodes. 

A liquid film and a vapor film are assumed to exist at the interface between the two phases, at which point the mole fractions in the liquid and vapor are X and Y. These compositions are, in general, different from the bulk liquid and vapor compositions and are assumed to correspond to equilibrium compositions. Thus, the point lies on the equilibrium curve (Figure 15.3a) at the same column height as point... 

In the remainder of this chapter, the two-film theory with equations of the form of Eq. (13-2) is applied to packed columns and to plate efficiencies. The essence of the two-film theory is the additivity of the vapor and liquid film resistances which was first proposed by Lewis and Whitman.26,37... 

Most of the vapor from the top of the column is condensed and subcooled in a condenser. This subcooled liquid then flows into the base of the reflux drum. There is heat transfer between the hot vapor and the cooler vapor-liquid interface (at temperature T) and between the vapor-liquid interface and the cooler-subcooled reflux in the tank (at temperature Tr). The vapor film coefficient is 10 Btu/hr °C ft, and the liquid film coefficient is 30 Btu/hr °C The heat transfer area on the surface of the liquid is 72 ft". The heat of vaporization of isobutane is 120 Btu/lbm. 

In packed columns, liquid reflux flows as a falling film, or as a streamlet, from top to bottom counterflow to the upflowing vapor. Both liquid and vapor phases are in continual contact (Fig. 2-5 8 b and c). Mass and heat transfer occur at the inside and outside surfaces of the randomly packed filling material or the arranged packing elements in reflux film. The exchange area is the surface area. In the case of spraypack fabrics, the reflux liquid is sprayed. The contact area is the total surface area of the liquid droplets. 

In rotary columns, the reflux is sprayed by means of a rotor with a funnel shaped distributor or is distributed by means of a rotor as a film on a heated tube wall. The phase contact upflowing vapor-downflowing liquid is stepwise in the first case and continuous in the latter (Fig. 2-58 d). 

This equation is based on the two-film theory, which assumes that the vapor film is in equilibrium with the liquid film at the interface. The operating line slope for a distillation column is L /0. The ratio of the equilibrium curve slope to the operating line slope is called the lambda factor. 

D. Rectification in vertical wetted wall column with turbulent vapor flow, Johnstone and Pigford correlation =0.0.328(Wi) Wi P>vP 3000 < NL < 40,000, 0.5 < Ns. < 3 N=, v,.gi = gas velocity relative to R. liquid film = — in film -1 2 " [E] Use logarithmic mean driving force at two ends of column. Based on four systems with gas-side resistance only, = logarithmic mean partial pressure of nondiffusing species B in binary mixture. p = total pressure Modified form is used for structured packings (See Table 5-28-H). 

Henry s Law constant for the equilibrium is KH = (Vc/Vg)((t/to) — 1). Vc and Vg are the volumes of condensed and vapor phases in the column (i.e. for gas-liquid chromatography, Vc is the volume of the liquid film on the supported packing or open tubular wall, and Vg the volume of void space, respectively). If the column is in the linear range (small loading) the resolution is,... 

Figure 1 Flowsheet of the RCH/RP hydroformylation process 38 1 Continuous flow stirred tank reactor,424 2 Phase separator, 3 Stripping column, 4 Distillation column, 5 Heat exchanger, 6 Falling film evaporator, 7 Liquid vapor separator.

Solvent mixtures that contain heat-sensitive compounds, are viscous, or have high boihng points can be separated using wiped-film evaporators (WFE) [57, 58]. A WFE operates by receiving a liquid feed into a column that contains several wiper blades. The walls of the WFE are heated at a constant temperature in order to vaporize the solvent film. As the solvent vapors migrate to the center of the WFE, they come into contact with a coohng unit that condenses the vapors, allowing them to flow down the condenser to the outlet receiver. The thin solvent film and reduced system pressure (in the case of vacuum operation) allows the solvents to be separated slowly at lower temperatures [57-59]. 

Fig. 4. Flow diagram of the Ruhrchemie/Rhone-Poulenc process (137) 1, continuous-flow, stirred tank reactor 2, phase separator 3, stripping column 4, distillation column 5, heat exchanger 6, falling film evaporator 7, liquid-vapor separator.

Its formation is accompanied by the generation of a spray, resulting from the vibrations at the liquid surface and cavitation at the liquid-gas interface. The quantity of spray is a function of the intensity. Ultrasonic atomization is accomplished using an appropriate transducer made of PZT located at the bottom of the liquid container. A 500-1000 kHz transducer is generally adequate. The atomized spray which goes up in a column fixed to the liquid container is deposited onto a suitable solid substrate and then heat treated to obtain the film of the material concerned. The flow rate of the spray is controlled by the flow rate of air or any other gas. The liquid is heated to some extent, but its vaporization should be avoided. 

Example 6.3 Diffusion of species from a gas mixture to a falling liquid film In a wetted wall column, the falling liquid film consists of a mixture of acetone (1) and benzene (2). This falling film is in contact with a downward flowing gas mixture of acetone, benzene, and helium (3). The pressure, temperature, and thickness of the film are P = 1 atm, T = 28.1°C, and 8 = 0.044 cm. The gas entering at the top of the column contains 10 mol% acetone and no benzene, and the composition of the vapor flow at the interface is... 

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