Absorption column design discussion

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. 

Spray contactors ate particularly important for the absorption of imparities from large volumes of Hue gas where tow pressure drop is of key importance. They are used where materials in the liquid phase (e.g., particles of limestone) or in the gas phase (e.g., droplets of tar) may cause plugging of packing or trays. Other important applications of spray contactors (which are outside the scope of this discussion) include particulate removal and hot gas quenching. When used for absorption, spray devices are not applicable to difficult separations and geueratty are limited to about Four transfer units even with countercurrent spray column designs. The tow efficiency of spray columns is believed to be due to entrainment of droplets in the gas and beckmixing of the gas induced by the sprays. 

This chapter covers all important aspects of distillation column rating or design. It follows generally the sequence of the steps noted immediately above. The common case for discussion is conventional distillation operated in the continuous mode. Exceptions to this mode, such as extractive distillation or batch distillation, will be handled separately. Much of the material in Sections 5.7-5.10, dealing with hydraulics and mass transfer in columns, can also be used in the design and analysis of absorption columns. Perusal of Chapter 6 will show the reader how the connection may be ttuide. 

For further design and optimization of absorption units with countercurrent flow and solvent regeneration, see [3.4, 3.25-3.30]. The calculations for adiabatic operated absorbers, in which large heat effects have to be considered, are described in [3.31]. Process control of absorption columns can be found in [3.32]. The dynamic behavior of absorption tray columns is discussed in [3.33]. 

The previous discussions cover column design procedures based primarily on empirical data. Such procedures have proven adequate for plants designed essentially for complete removal of acid gases from gas streams because an overly conservative design, with a few extra trays, can only improve performance. This is not true for selective absorption, however, because too many trays can destroy selectivity while too few can cause the production of... 

In the design of an absorption tower it is necessary to take into account the characteristics of the packing elements and the flow behaviour discussed in Chapter 4, together with the considerations given in the following sections concerning the performance of columns under operating conditions. 

This chapter concerns the most important reactive separation processes reactive absorption, reactive distillation, and reactive extraction. These operations combining the separation and reaction steps inside a single column are advantageous as compared to traditional unit operations. The three considered processes are similar and at the same time very different. Therefore, their common modeling basis is discussed and their peculiarities are illustrated with a number of industrially relevant case studies. The theoretical description is supported by the results of laboratory-, pilot-, and industrial-scale experimental investigations. Both steady-state and dynamic issues are treated in addition, the design of column internals is addressed. 

Venturi scrubbers are designed on the basis of the venturi flow-metering device. The flow channel is narrowed down so that the velocity will greatly increase at the throat. Then, as in the flow-metering device the flow channel widens ouf. For ease of fabricalion venturi scrubbers are designed with a rectangular cross section. The absorption fluid is injected into the Venturi at the throat where the velocity is the greatest. For particulate removal plain water could be used. As in the plate columns discussed above, for the simultaneous removal of particulate matter and sulfur dioxide, soda ash or caustic soda slurries could be used for absorption of the gas. Venturis are frequently used in conjunction with plate towers. They also serve as stand-alone removal devices in some cases. 

Two different approaches have evolved for the simulation and design of multicomponent distillation columns. The conventional approach is through the use of an equilibrium stage model together with methods for estimating the tray efficiency. This approach is discussed in Chapter 13. An alternative approach based on direct use of matrix models of multicomponent mass transfer is developed in Chapter 14. This nonequilibrium stage model is also applicable, with only minor modification, to gas absorption and liquid-liquid extraction and to operations in trayed or packed columns. 

To determine the required size of an absorption or stripping nrtl, it is necessary to know not only the equilibrium soluhility of the solute in the solvent and the material balance atound the column bas also the rate at which solute is transferred from one phase to the other within the tower. This rale directly affects the volume of packing needed in a packed tower, the degree of dispersion requited in a spray contactor, and (somewhat less directly) the number of trays required in a nay tower. The last effect occurs as a result of the influence of mass transfer rms on tray efficiency which is discussed in a later section. Because of its direct effect ou packed tower design and the importance of this type of contactor in absoiption. this discussion of mass transfer is aimed primarily at the packed tower case. A more detailed review of mass transfer theoty is given in Chapter 2. 

Chapters 7 and 8 present models and data for mass transfer and reaction in gas-liquid and gas-liquid-solid systems. Many diagrams are used to illustrate the concentration profiles for gas absorption plus reaction and to explain the controlling steps for different cases. Published correlations for mass transfer in bubble columns and stirred tanks are reviewed, with recommendations for design or interpretation of laboratory results. The data for slurry reactors and trickle-bed reactors are also reviewed and shown to fit relatively simple models. However, scaleup can be a problem because of changes in gas velocity and uncertainty in the mass transfer coefficients. The advantages of a scaledown approach are discussed. 

While Example 5.2 may seem very different from the traditional ideas of absorption, it does give incredible insight as to what can be done in a single CS. While such a CS may not necessarily operate on its own, it may very well form part of an arrangement of CS in a complex column. This then gives a designer incredible freedom, especially when a number of these CSs have to be linked. Such considerations are discussed in Chapters 6 and 7. 

Since separations are ubiquitous in chemical plants and petroleum refineries, chemical engineers must be familiar with a variety of separation methods. We will first focus on some of the most common chemical engineering separation methods flash distillation, continuous column distillation, batch distillation, absorption, stripping, and extraction. These separations all contact two phases and can be designed and analyzed as equilibrium stage processes. Several other separation methods that can also be considered equilibrium stage processes will be briefly discussed. Chapters 17 and 18 e5q)lore two inportant separations—membrane separators and adsorption processes— that do not operate as equilibrium stage systems. 

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. 

In the absorption of sparingly soluble gases such as CO2, H2S, and Cl2> the liquid film resistance is normally controlling, very low overall absorption coefficients are observed, and relatively tall countetcurrent packed or tray columns are required. The design of such equipment is discussed in detail in Chapter 1. 

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