Allowable vapor velocity

Figure 14-25 or Eq. (14-92) may be used for sieve plates, valve plates, or bubble-cap plates. The value of the flooding vapor velocity must be considered as approximate, and prudent designs call for approaches to flooding of 75 to 85 percent. The value of the capacity parameter (ordinate term in Fig. 14-25) may be used to calculate the maximum allowable vapor velocity through the net area of the plate ... 

N = number of theoretical plates Gb = allowable vapor velocity in heat exchangers, (lbmol)/(hft ) h = hours of operation... 

The diameter of a tower is established by the volume of vapors which must be handled and by the maximum allowable vapor velocity which can be tolerated without encountering excessive entrainment of liquid from one plate to the plate above. Entrainment can occur by splashing and/or suspension of small droplets in the vapor as a mist. It tends to defeat the purpose of fractionation even a small amount may be serious when rigid specifications on color or impurities must be met. 

Allowable vapor velocity (mesh in horizontal position)... 

V (separator) = Separator vapor velocity evaluated for the gas or vapor at flotving conditions, ft/sec V = Vapor velocity entering unit, lbs, per minute per square foot of inlet pipe cross section Va = Maximum allowable vapor velocity across inlet face of mesh calculated by relation, ft/sec Van Actual operating superficial gas velocity, ft/sec or ft/min, for tvire mesh pad Vu = Design vapor velocity (or selected design value), ft/sec... 

Turndown ratio is the ratio of the maximum allowable vapor rale at or near flooding conditions (rates) to the minimum vapor rate when weeping or liquid leakage becomes significant it may be termed the minimum allowable vapor velocity [193, 199, 200]. 

The allowable vapor velocity and the corresponding tray diameter are represented by the work of Souders and Brown, which is cited in standard textbooks, for example Treybal (1980). Its equivalent is the Jersey Critical formula,... 

The large effective heat capacity of the liquid-solid slurry absorbent enables relatively small slurry flows to absorb the carbon dioxide heat of condensation with only modest absorber temperature rise. This contrasts with other acid gas removal processes in which solvent flows to the carbon dioxide absorber are considerably larger than flows determined by vapor-liquid equilibrium constraints. Large flows are required to provide sensible heat capacity for the large absorber heat effects. Small slurry absorbent flows permit smaller tower diameters because allowable vapor velocities generally increase with reduced liquid loading (8). 

The vapor velocity in a finite-stage contactor column can be limited by the liquid handling capacity of the downcomers or by entrainment of liquid droplets in the rising gases. In most cases, however, downcomer limitations do not set the allowable vapor velocity instead, the common design basis for choosing allowable vapor velocities is a function of the amount of gas entrainment which can result in improper operation or flooding of the column. 

An alternate approach for estimating maximum allowable velocities has been presented by Fair (see reference given in footnote for preceding paragraph) which is based on data obtained with sieve-tray and other types of finite-stage columns and takes into account the effect of surface tension of the liquid in the column, the ratio of the liquid flow rate to the gas flow rate, gas and liquid densities, and dimensions and arrangement of the contactor. In this method, the basic equation for the maximum allowable vapor velocity, equiva-... 

In many cases, the vapor rate changes over the length of the tower, and the theoretical diameter based on the allowable vapor velocity varies. Occasionally, two different diameters are used for different sections of one tower. Cost considerations, however, usually make it impractical to vary the diameter, and the constant diameter should be based on the tower location where allowable velocity and throughput rates require the largest diameter. 

Example 1 Determination of distillation-column diameter on basis of allowable vapor velocity. A sieve-tray distillation tower is to be operated under the following conditions ... 

The limiting diameter occurs at the bottom of the tower therefore, the minimum diameter based on the maximum allowable vapor velocity is 3.6 ft obtained with the use of Fig. 16-6. 

In addition to the critical design factors for finite-stage contactors of number of theoretical trays, maximum allowable vapor velocity, column efficiency, and pressure drop as discussed earlier, a number of other factors are of importance in the development of the design. These factors are discussed in the following sections. 

Size the column by using the Kv from both of the plots given in Chap. 16 for maximum allowable vapor velocity, and use an 85 percent safety factor on the maximum allowable vapor velocity. For comparison purposes, give an answer for each of the two Kv estimates. 

Reestimate the required free cross-sectional area, this time to accommodate the maximum allowable vapor velocity. Since mass flow rate equals density times cross-sectional area times velocity, free column area AF = (69,000 lb/h)/[(0.02 lb/ft3)(3600 s/h)(10.74 ft/s)] = 89.23 ft2. Note that this turns out to be slightly larger than the area calculated in step 2. 

The FRI data for dualflow trays have been used to form a tentative design method (Garcia and Fair, 2002). Data for Turbogrid trays have been included in the model. A generalized chart for predicting allowable vapor velocity, similar to that for cross flow trays given in Fig. 13.32(b), is included. 

The allowable vapor velocity for bubblecap trays is about the same as for sieve trays, and Figure 13.32(b) may be used. The most complete published source of design and performance prediction of these trays is given by W. L. Bolles (in Smith, 1963). 

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