Absorption column design solutions

This section concerns the modelling of countercurrent flow, differential mass transfer applications, for both steady-state and non-steady-state design or simulation purposes. For simplicity, the treatment is restricted to the case of a single solute, transferring between two inert phases, as in the standard treatments of liquid-liquid extraction or gas absorption column design. 

In phase equilibrium, mole fraction in each of the phases is the most common way to express composition that goes along with mole balances around the system being studied. There is also the possibility to use mass fraction for defining composition. Mass balances around the system can then be made. For some situations, absorption column design, for example, the composition may be expressed as mass of solute per mass of solvent. 

The amount of a component absorbed or stripped in a column is dependent on the column design (the number of stages), the component solubility, and the gas and liquid rates. The fraction absorbed can be estimated using the absorption factor method, attributed to Kremser (1930) (see Volume 2, Chapter 12). If the concentration of solute in the solvent feed to the column is zero, or can be neglected, then for the solute component the fraction absorbed =... 

Sulphur dioxide produced by the combustion of sulphur in air is absorbed in water. Pure S02 is then recovered from the solution by steam stripping. Make a preliminary design for the absorption column. The feed will be 5000 kg/h of gas containing 8 per cent v/v S02. The gas will be cooled to 20°C. A 95 per cent recovery of the sulphur dioxide is required. 

This section discusses the design of a suitable absorption column for the production of nitric acid by the single-pressure process. A comprehensive design study determined the column mechanical details and physical characteristics, together with its anticipated performance. This chapter contains the operating criteria and product specifications, the solution method, and finally the results of the design calculations. Details of the design calculations and all associated data are included in Appendix G. 

Example 8.8 A wastewater containing 25 mg/L of phenol and having the characteristic breakthrough of the previous example is to be treated by adsorption onto an activated carbon bed. The flow rate during the breakthrough experiment is 0.11 mVs this is equivalent to a surficial velocity of 0.0088 m/s. The XIM ratio of the bed for the desired effluent of 0.06 mg/L is 0.02 kg solute per kg carbon. If the flow rate for design is also 0.11 mVs, design the absorption column. Assume the influent is introduced at the top of the bed. The packed density of the carbon bed is 721.58 kg/ml... 

Ammonia is considered a gas at normal pressure and ambient temperature, although is not supercritical (7 c=405.5 K, Pc=11.3 atm). Traditionally, the design of an absorption column for ammonia in water makes use of the Henry law, but this approach may be justified only when the solution is very diluted. Investigate the possibility to handle more concentrated NH, solutions by using symmetric and asymmetric convention. 

Density and viscosity are important for the process and plant design, especially relating to fluid dynamics calculations for the absorption column, gas-liquid mass transfer resistance and the selection of pumps and packings. The measurement systems were first validated with 30 w1% MEA solutions. As seen in Figure 5, the dynamic viscosity of MEA solutions is significantly increased by the dissolution of CO2 and the measured data agree well with the values predicted from Weiland s correlation [20]. The maximum deviation is less than 3%. 

A study of industrial apphcatious by Taylor, Kooijmau, and Woodman [IChemE. Symp. Ser. Distillation and Absorption 1992, A415-A427 (1992)] concluded that rate-based models are particularly desirable when simulating or designing (1) packed columns, (2) systems with strongly uonide liquid solutions, (3) systems with trace compo-... 

In an attempt to test the surface renewal theory of gas absorption, Danckwerts and Kennedy measured the transient rate of absorption of carbon dioxide into various solutions by means of a rotating drum which carried a film of liquid through the gas. Results so obtained were compared with those for absorption in a packed column and it was shown that exposure times of at least one second were required to give a strict comparison this was longer than could be obtained with the rotating drum. Roberts and Danckwerts therefore used a wetted-wall column to extend the times of contact up to 1.3 s. The column was carefully designed to eliminate entry and exit effects and the formation of ripples. The experimental results and conclusions are reported by Danckwerts, Kennedy, and Roberts110 who showed that they could be used, on the basis of the penetration theory model, to predict the performance of a packed column to within about 10 per cent. 

The design must consider three feed streams and two product streams. The three inlet feed streams are strong reaction gases, weak nitric acid solution and make-up water. Two outlet streams flow from the column. These are a lean reaction gas (tail-gas) stream and red product acid. Absorption of nitrous oxides increases as the temperature is reduced. This effect, together with the exothermic oxidation/ absorption reactions, requires installation of an internal cooling circuit. 

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