Absorption columns plate efficiency

A. Assuming (a) temperature and total pressure throughout the column to be constant, (b) no change in molar flowrates due to gas absorption, (c) plate efficiencies to be 100 per cent, (d) the equilibrium relation to be given by yn = mx + b, (e) the holdup of liquid on each plate to be constant and equal to H, and (f) the holdup of gas between plates to be negligible, show that the variations of the liquid compositions on each plate are given by ... 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Entrainment Entrainment in a plate column is that liquid which is carried with the vapor from a plate to the plate above. It is detrimental in that the effective plate efficiency is lowered because hquid from a plate of lower volatility is carried to a plate of higher volatility, thereby diluting distillation or absorption effects. Entrainment is also detrimental when nonvolatile impurities are carried upward to contaminate the overhead product from the column. 

The actual stage can be a mixing vessel, as in a mixer-settler used for solvent extraction applications, or a plate of a distillation or gas absorption column. In order to allow for non-ideal conditions in which the compositions of the two exit streams do not achieve full equilibrium, an actual number of stages can be related to the number of theoretical stages, via the use of a stage-efficiency factor. 

To translate ideal plates into actual plates, the plate efficiency must be known. The following discussion applies to both absorption and fractionating columns. 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

The variety of equipment used for extraction is much greater than for distillation, absorption, and stripping. Efficient contacting and separating of two liquid phases is considerably more difficult than contacting and separating a vapor and a liquid. In addition to plate and packed (random, structured and membrane) columns, many specialized pieces of equipment have been developed. Some of these are... 

The mass transfer between gas and liquid phases on a column tray consisting of two phases is usually expressed by the enrichment ratio, plate efficiency E (see Chapter 1.1, stage efficiency factor). Since the main resistance to the mass transfer occurs in the gas phase during rectification and absorption, E is mainly defined as the efficiency with respect to gas. Eg or Eg . 

Atkins and Franklin (Ref. 1) found an over-all column efficiency of 18 per cent for a natural gasoline absorber using gas oil as the absorbing liquid. Walter (Ref. 30) obtained Murphree vapor plate efficiencies from 80 to 95 per cent in a 2-in. laboratory column for air humidification. Data taken in the same unit on the absorption of propylene and isobutylene in gas oil, heavy naphtha, and mixtures of gas and lube oil, gave plate efficiencies on the vapor basis of 5 to 36 per cent. 

We have seen in Section 8.1.3 that multistage distillation is generally carried out in a vertical multiplate column in which each plate has crossflow. Vapor bubbles rise vertically through a liquid layer flowing horizontally in cross-flow over the plate (Section 8.1.3.5) the same two-phase flow scheme is employed in a plate for gas-liquid absorption stripping. We were introduced to the notion of stage efficiency, plate efficiency, tray efficiency, etc., in Section 8.I.3.4. We will introduce here the models used to... 

Figure 2-80 shows a typical absorber tray diagram for the absorption of CO2 in MEA solution. Ibis figure is based on actual plant data from a 16 bubble-cap tray absorber treating atmospheric pressure flue gas for CO2 recovery. Because of the low values of solution loading involved, the equilibrium line is almost coincident with the x-axis and is not shown. A pseudo-equilibrium line (dashed) has been drawn to represent actual gas and liquid compositions from each tray. The plate efficiencies in this column vaiy from about 14% in the bottom of the column to slightly over 16% at the top. 

Figure 2-80. Graphical analysis of plate-efficiency data for CO2 absorption with 14.5% aqueous monoethanolamine in an atmospheric pressure bubble-cap column. Data of Kohl (1956)...

If the plate efficiency b 50 per cent, the column will behave as if it consisted of eight theoretical plates. The absorption factor, to recover 99 per cent of a component, when eight plates are used, is about 1.5 (Fig. 22-11). The equilibrium constant for isopentane at 90°F and 60 lb is about 0.37 (Fig. 15-4). 

For sieve or valve plates, h = h , outlet weir height. For bubble-cap plates, h = height of static seal. Tbe original references present vaH-dations against laboratoiy and small-commercial-column data. Modifications of tbe efficiency equation for absorption-stripping are also included. 

Solute equilibrium between the mobile and stationary phases is never achieved in the chromatographic column except possibly (as Giddings points out) at the maximum of a peak (1). As stated before, to circumvent this non equilibrium condition and allow a simple mathematical treatment of the chromatographic process, Martin and Synge (2) borrowed the plate concept from distillation theory and considered the column consisted of a series of theoretical plates in which equilibrium could be assumed to occur. In fact each plate represented a dwell time for the solute to achieve equilibrium at that point in the column and the process of distribution could be considered as incremental. It has been shown that employing this concept an equation for the elution curve can be easily obtained and, from that basic equation, others can be developed that describe the various properties of a chromatogram. Such equations will permit the calculation of efficiency, the calculation of the number of theoretical plates required to achieve a specific separation and among many applications, elucidate the function of the heat of absorption detector. 

The HAZOP study was instrumental in determining the need for an adequate alarm system on each of the specified controllers. If liquid levels within the column are not well controlled, then either flooding (too much liquid) or plate by-passing bythegas (too little liquid) will occur. Both situations lead to a substantial reduction in absorption efficiency with large increases in emission levels. The other important control parameter was shown to be the temperature. If the temperature in the cooling-coil section rises, then there is an appreciable reduction in absorption. Control of temperature is important in the upper sections of the column because it is here that the greatest effect on emission levels occurs. 

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