Packed-column flood and pressure drop

Fig. 24. Generalized method using log scales for estimating packed column flooding and pressure drop, AP, in kPa/m g = gravitational constant, 9.81 m/s t = kinematic viscosity in mm /s (= cSt) E, G have units of kg/(m s) are in kg/m and the packing factor, F, in can be found in...

FIGURE 5.0-12 Eckert method Tor estimating packed column flooding and pressure drop (Ref. 20). (Now ... 

Generalized correlation for flooding and pressure drop in packed columns. After Eckert )... 

Figure 10-27. Generalized flooding and pressure drop correlation for packed columns.

Example 13-4 This example is based on an example presented by Bolles and Fair.3 Determine the flood velocity, the column diameter, the pressure drop per foot of packing, HL, HG, and HOG for the separation of a benzene-toluene mixture in a packed column. The column conditions to be used for design purposes are as follows ... 

Air containing 5 mol% NH3 at a total flow rate of 20 kmol/h enters a packed column operating at 293 K and 1 atm where 90% of the ammonia is scrubbed by a counter-current flow of 1500 kg/h of pure liquid water. Estimate the superficial gas velocity and pressure drop at flooding, and the column inside diameter and pressure drop for operation at 70% of flooding for two packing materials ... 

In this chapter, all of the process flow rates were considered to be constrained by zero flow and the maximum flow allowable by the valve size and span of the flow measurement. The main column constraint because of flooding is associated with the vapor traffic and pressure drop across the trays or packing in the distillation column. The mass transfer rate limit for stripping light key impurity from the bottoms stream was presented. The mass transfer rate limit for absorbing heavy key impurity from the overhead vapor stream was also presented. 

Table 6-lc. List of technical data of structured, stacked packings and tube columns (TC) made of metal sheet, plastic and ceramic. G nstants Ki for determination of resistance coefficient i Fl cc. to Eq. (3-14) or form factors cpP for determination of resistance coefficient of dry packing i acc. to Eq. (3-26), gas velocity at flooding point uyFl cc. to Eq. (2-69) and pressure drop Ap/H acc. to Eqs. (4-48) and (4-54). List of form factors [x for determination of pressure drop Ap/H acc. 

It shoiild be pointed out that in most liquefaction processes, the flow of the reactants is cocurrent i q)ward. In cocurrent operation, there is no flooding limit, and greater throu put is attained compared with co mterc irrent column of similar size. Moreover, for the same values of gas and liquid flow rates, interfacial area, liqiaid mass transfer coefficient and pressure drop values in a cocurrent upward packed column are always higher than those obtained in downflow towers (l 3). Upflow operations also give a better performance due to larger liqioid holdup and better liq iid distribution throu out the catalyst bed jlkk). 

If the packing surface is discontinuous in nature, a phase inversion occurs, and gas oubbles through the liquid. The column is not unstable and can be brought back to gas-phase continuous operation by merely reducing the gas rate. Analogously to the flooding condition, the pressure drop rises rapidly as phase inversion occurs. 

For column analysis and troubleshooting it is important to have pressure drop measured with a DP cell. The differential pressure can also be used to control column traffic. A good way to do this would be to let the differential pressure control the heating medium to the reboiler. The largest application for differential pressure control is with packed columns where it is desirable to run at 80 to 100% of flood for best efficiency. 

The pressure drop through packed columns, and the flooding conditions, can be estimated from the generalized correlation of Leva (1992), shown in Fig. 13-5. The pressure gradient in millimeters of water per meter of packed height is the parameter on the curves, and interpolation is usually necessary to determine the pressure drop (note that the pressure... 

A column 0.6 m diameter and 4 m high is, packed with 25 mm ceramic Raschig rings and used in a gas absorption process carried out at 101.3 kN/m2 and 293 K. If the liquid and gas properties approximate to those of water and air respectively and their flowrates are 2.5 and 0.6 kg/m2s, what is the pressure drop across the column In making calculations, Carman s method should be used. By how much may the liquid flow rate be increased before the column floods ... 

---