Absorption Column Specification

The results of the design calculations are presented in this section. Detailed calculationsare included in Appendix G.3 and G.4. The final design specification based upon these calculations is summarized in Table 9.2. 

The design specification in Table 9.2 must be complimented by suitable construction tolerances. These suggested tolerances (from Refs. A10 and Al 2) are itemised below. They are based upon 

Materials of construction SS304L ( nitric acid grade )  

Design Code AS 1210 Legal construction self supporting no internal lining no insulation Class 1  

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Formulate a statement of the design criteria for the unit. What is the general design criteria What is the design (or the unit) intended to achieve For example, a reactor is usually required to achieve a particular conversion of reactants to products, a distillation column must provide a particular separation, an absorption column provides removal of a specific level of impurity, etc. 

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. 

The absorption column is required to absorb nitrous reaction gases thus producing 80% (wt.) nitric acid product (dissolved impurity-free basis). This is achieved by the countercurrent absorption of the nitrogen oxide components from the reaction gas into a water/weak-acid media. The column specification requires an operating pressure of 950 kPa and an absorption temperature in the range of 10°C to 65°C. 

The absorption column design represents a compromise between mass transfer factors and economic considerations. The final design specification is for a column of 1.8 m diameter, approximately 32 m high, and containing 59 sieve trays. 

This chapter considers the sizing, specification and selection of the pump required to deliver red nitric acid from the NOx absorption column to the bleaching column. The final product acid is prepared in... 

After consulting Ref. P3 (p.582 Figure 14.2), it is confirmed that the high suction pressure available in this application ensures that cavitation will not occur in the pump. For the calculated specific pump speed, there is a safety factor of over 500%. Cavitation should not be a problem provided that the column is only partially repressurised before turning on the pump (a usual start-up procedure on the absorption column). 

Part II contains the design of a major item of equipment (in this case study, it is a sieve-tray absorption column (Chapter 9) ), including the mechanical design, fabrication, materials specification, detailed engineering drawing, HAZOP study, control scheme and associated instrumentation. In summary, as complete and professional a design as... 

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. 

Sizing of the absorption column started from a base case that assumed complete recovery of FeEDTA2- in the bioreactor. Then, sensitivity studies provided the values of the G/L interfacial area and of the absorber volume giving maximum performance. The values of the two Damkohler numbers characterizing reactor performance were found after relaxing the assumption of complete FeEDTA2-recovery. Finally, the specification of NO concentration in the purified gases was checked, for different feed conditions. 

The limitation of membrane is that the presence of a membrane itself consists of another resistance to mass transfer, which is not encountered in absorption columns. This resistance could negatively affect the overall mass transfer and significantly lower the selectivity. This membrane resistance can be minimized by reducing the membrane thickness or by increasing its intrinsic gas permeability. Many authors [1,15,16] have specifically investigated this point. 

Both total flows Vj and Lj will be largest where Yj and Xj are largest. This is at the bottom of the column for absorption, and therefore you design the diameter at the bottom of the column. In strippers, flow rates are highest at the top of the column, so you design the diameter for the top of the column. Specific design details for absorbers and strippers are discussed by Kister et al. (2Q08) and Zenz 119971. 

The advantage of the unit operations approach is that similarities can be emphasised and that quantitative analysis developed in one area can be applied to another. Thus figure 6.3 could have been entitled a schematic diagram of a stage within an absorption column , and as a specific example, the absorption of ammonia from ammonia-laden... 

Tray Columns. The contactor, or absorber, is typically a countercurrent column containing an integral scrubber at the bottom, a central trayed or packed absorption section, and a demister at the top. The basic design correlations for absorption columns are described in Chapter 1 and only matters specific to dehydration contactors are covered in the following paragraphs. 

For amines absorption in an acid solution, or preferably adsorption onto an acid ion exchange column (acidified divinylbenzene-styrenesulfonic acid copolymer) is used. 10-50 1 of ambient air is sent over a wet 100 mmx 3 mm I.D. column the ion exchange polymer is put into a vial, made alkaline and the water solution is analysed on packed Carbowax-KOH GC-column with a thermionic selective detector (TSD), which is specific for nitrogen- and phosphorus-compounds. Trimethylamine is detected easily at 1 ppb. 

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