Plate tower design

The design of a plate tower for gas-absorption or gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical plates needed to achieve a specified composition change (see Sec. 13). Distillation differs from gas absorption in that it involves the separation of components based on the distribution of the various substances between a gas phase and a hquid phase when all the components are present in Doth phases. In distillation, the new phase is generated From the original feed mixture by vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

In gas absorption, the new phase consists of an inert nonvolatile solvent (absorption) or an inert nonsoluble gas (stripping), and normally no reflux is involved. The following paragraphs discuss some of the considerations peculiar to gas-absorption calculations for plate towers and some of the approximate design methods that can be employed when simplifying assumptions are vahd. 

FIG. 14-79 Cost of trays in plate towers. Price includes tray deck, bubble caps, risers, downcomers, and structural-steel parts. The stainless steel designated is type 410 Peters and Timmerhaus, Plant Design and Economics for Cbemical Engineers, 4th ed., McGraw-Hill, New York, 1.9.91). 

Spray towers, and plate and packed columns are used, as well as a variety of proprietary designs. Spray towers have a low pressure drop but are not suitable for removing very fine particles, below 10 /j.m. The collecting efficiency can be improved by the use of plates or packing but at the expense of a higher pressure drop. 

Control device vendors estimate that removal efficiencies range from 95 to 99%. For S02 control, removal efficiencies vary from 80 to greater than 99%, depending upon the type of reagent used and the plate tower design. Most current applications have an S02 removal efficiency greater than 90%. 

When absorption is used for VOC control, packed towers are usually more cost effective than impingement-plate towers. However, in certain cases, the impingement-plate design is preferred over packed-tower columns either when internal cooling is desired or where low liquid flow rates would inadequately wet the packing. 

A variety of designs have been developed to maximize the relative motion between particles and spray droplets or the contact between liquid and particles. These include jet impingement devices, packed or sieved plate towers, preformed and gas atomized spray towers, and venturi sprayers. 

Because of liquid-dispersion difficulties in packed towers, the design of plate towers is considerably more reliable and requires less safety factor when the ratio of liquid mass velocity to gas mass velocity is low. 

Plate towers can be designed to handle wide ranges of liquid rates without flooding. 

The total weight of a dry plate tower is usually less than that of a packed tower designed for the same duty. However, if liquid holdup during operation is taken into account, both types of towers have about the same weight. 

Design information for plate towers is generally more readily available and more reliable than that for packed towers. 

Random-packed towers are seldom designed with diameters larger than 4 ft, and diameters of commercial plate towers are seldom less than 2 ft. 

The pressure drop through packed towers may be less than the pressure drop through plate towers designed for the same duty. This advantage, plus the fact that the packing serves to lessen the possibility of tower-wall collapse, makes packed towers particularly desirable for vacuum operation. 

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