Absorption column height

The height of an absorption column depends on the feed conditions, the product purity specifications, the solvent used and the extent of separation through the absorption equilibrium relationship, but also on the rate of separation. If the rate of mass transfer of the gaseous component from the gas phase into the liquid phase is slow, then the column needs to be longer to ensure that the required amount is removed. The rate of mass transfer depends on the mass-transfer coefficient, normally denoted kG or k. The value of the mass-transfer coefficient depends on the components in the gas feed and on the solvent used and is often determined experimentally. The type of packing used in the column will also have an impact on the column height as for distillation. 

The diameter of the column is determined mainly by the vapour flow rate and correlations for this can be found in textbooks. If using commercial programmes, suitable correlations for different trays/pack-ings will normally be incorporated. 

This chapter has mainly been based on textbooks generally used in undergraduate courses on mass transfer and separation processes.1 6 Most of these textbooks contain sections on both basic calculations as well as some design considerations. Coulson et al is devoted entirely to design. Perry,7 in addition to basic theory, also includes thermodynamic data for many common mixtures which may be used for preliminary 

Stichlmair and Fair11 and Rose8 are textbooks devoted entirely to design and operation of distillation columns, Ruthven12 considers absorption and Astarita et alP absorption with chemical reaction. Ho14 is a handbook for membrane separations and Guiochon15 considers chromatographic separations. 

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To calculate the exact temperature and concentration profile over the active absorption column height, the appropriately adapted equation system in Chapter 1.9.3 is used.)... 

Adsorption The design of gas-adsorption equipment is in many ways analogous to the design of gas-absorption equipment, with a solid adsorbent replacing the liqiiid solvent (see Secs. 16 and 19). Similarity is evident in the material- and energy-balance equations as well as in the methods employed to determine the column height. The final choice, as one would expect, rests with the overall process economics. 

Once packing heights are determined in other sections from HETP (distillation) or Koa (absorption), the height allowances for the internals (from Figure 1) can be added to determine the overall column height. Column diameter is determined in sections on capacity and pressure drop for the selected packing (random dumped or structured). 

Massimilla et al. (M5) measured the rate of absorption of carbon dioxide in water from a mixture of carbon dioxide and nitrogen. Used as solid phase were silica sand particles of average equivalent diameter 0.22 mm, or glass ballotini of average equivalent diameter 0.50 and 0.80 mm. Columns of 30-and 90-mm i.d. were used, and the column height was varied from 100 to 1200 mm. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Absorption column diameter 1 m, vessel overall height 15 m, packed height 12 m, packing 25 mm ceramic intalox saddles, vessel carbon steel, operating pressure 5 bar. 

Absorption column packed column, diameter 0.5 m, height 6.0 m, packing height 4.5 m, packing 25 mm ceramic saddles, design pressure 2 bar, material carbon steel. 

Mass transfer in packed columns is a continuous, differential, process, so the transfer unit method should be used to determine the column height, as used in absorption see Section 11.14.2. However, it often convenient to treat them as staged processes and use the HETS for the packing employed. For random packings the HETS will, typically, range from 0.5 to 1.5 m, depending on the type and size of packing used. 

Next, to determine packed column height use Table 8-12 for distillation HETP values, leaning towards the high side of the range for studies. For use of Kqa values, see Section 9—Absorption. Bed height per packed bed runs up to 20-30 ft for metal or ceramic packings, but plastic packing is usually limited to 24 ft. 

Thus for an absorption column that is 57.31 m high, we will achieve slightly less than the desired maximal output Xe = 1.930556. If we increase the height to 78 to, we achieve our objective, however ... 

The same issues apply to the height and diameter of an absorption column as for a distillation column the more difficult the separation, the taller the column and the more liquid and vapour flowing inside the column, the wider the column diameter. 

Height Equivalent to a Theoretical Plate. Provided both the equilibrium and operating lines are straight, HETP values may be estimated by combining the HG and HL values predicted by the above correlations and by translating the resulting HQG into HETP by combining equations 47, 51, and 56 with equation 85, which is discussed under bubble tray absorption columns ... 

Imagine that the height of an absorption column varies directly as the natural logarithm of the ratio of the exiting and entering contaminant concentration ... 

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