Column cross-sectional area

The column cross section Aq and column diameter d result from a flow equation with the gas phase as the reference phase 

BL 2 Balance line for the 1 and 2 absorption stage EC Equilibrium curve 

The maximum allowable velocity of the gas phase, referred to the free absorber cross section Wg an. depends on the flooding point or upper loading limit for the selected internals, for example, as calculated by the methods presented in Chapter 2. 

The required number of theoretical stages N, for a countercurrent flow absorber can be graphically obtained by drawing the stages on to the operating diagram between the balance or operating line and the equilibrium curve (Fig. 3-7). 

From an absorbate balance over the top section of the absorption column, according Fig. 3-4, the equation of the balance line is 

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It should be noted that the fraction of column cross-sectional area available for gas dispersers (perforations, bubble caps) decreases when more than one downcomer is used. Thus, optimum design of the plate involves a balance between hquid-flow accommodation and effective use of cross section for gas flow. 

For design, the slip velocity is derated to 70-80 percent of the calculated value to give some margin of safety this sets the design value of the continuous phase velocity (V ). The column cross sectional area (and therefore diameter) is set by QJVc- With the diameter set, the other dimensions can be set using the ratios given above. 

Performance information for the purification of p-xylene indicates that nearly 100 percent of the ciystals in the feed stream are removed as produc t. This suggests that the liquid which is refluxed from the melting section is effectively refrozen oy the countercurrent stream of subcooled crystals. A high-meltingproduct of 99.0 to 99.8 weight percent p-xylene has been obtained from a 65 weight percent p-xyfene feed. The major impurity was m-xylene. Figure 22-12 illustrates the column-cross-section-area-capacity relationship for various product purities. 

To illustrate, consider the hmiting case in which the feed stream and the two liquid takeoff streams of Fig. 22-45 are each zero, thus resulting in batch operation. At steady state the rate of adsorbed carty-up will equal the rate of downward dispersion, or afV = DAdC/dh. Here a is the surface area of a bubble,/is the frequency of bubble formation. D is the dispersion (effective diffusion) coefficient based on the column cross-sectional area A, and C is the concentration at height h within the column. 

The F factor is used in the expression U = F/(pv)° to obtain the allowable superficial vapor velocity based on free column cross-sectional area (total column area minus the downcomer area). For foaming systems, the F factor should be multiplied by 0.75. 

Clear liquid velocity (ft/sec) through the downcomer is then found by multiplying DL by 0.00223. The correlation is not valid if Pl - pv is less than 301b/ft (very high pressure systems). For foaming systems, DL should be multiplied by 0.7. Frank recommends segmental downcomers of at least 5% of total column cross-sectional area, regardless of the area obtained by this correlation. 

Calculate minimum column cross-sectional area. Use the larger of... 

ATM = Minimum column cross-sectional area, tV. Further detailed design calculations may result in a change in tower diameter. 

ADM = Minimum downcomer area, fT ATM = Minimum column cross-sectional area, fr CAF = Vapor capacity factor CAFo = Flood capacity factor at zero liquid load CFS = Vapor rate, actual ftVsec DT = Tower diameter, ft DTA = Approximate tower diameter, ft FF == Flood factor or design percent of flood, fractional FPL = Tray flow path length, in. 

The gas risers must have a sufficient flow area to avoid a high gas-phase pressure drop. In addition, these gas risers must be uniformly positioned to maintain proper gas distribution. The gas risers should be equipped w ith covers to deflect the liquid raining onto this collector plate and prevent it from entering the gas risers where the high gas velocity could cause entrainment. These gas riser covers must be kept a sufficient distance below the next packed bed to allow the gas phase to come to a uniform flow rate per square foot of column cross-sectional area before entering the next bed. 

HETP = height equivalent to a theoretical plate, ft HTU = height of a transfer unit, ft L = liquid mass velocity, Ib/hr-ft m = exponent a 1.0 n = exponent 0.44 Pr = Prandtl number, dimensionless Sc = Schmidt number dimensionless U, = linear velocity of gas based on total column cross-sectional area, ft/sec... 

Calculate column cross-section area using the operational gas rate, G, and the calculated value of Gf (gas loading factor) ... 

The gas superficial velocity is defined as the ratio of gas flow rate to column cross sectional area ... 

Where u, is the mobile phase velocity at the column outlet, Fg the column volumetric flow rate, and Ag the column cross-sectional area available to the mobile phase. In a packed bed only a fraction of the column geometric cross-sectional area is available to the mobile phase, the rest is occupied by the solid (support) particles. The flow of mobile phase in a packed bed occurs predominantly through the interstitial spaces the mobile phase trapped within the porous particles is largely stagnant (37-40). 

As first trial take downcomer area as 12 per cent of total. Column cross-sectioned area... 

The column cross-sectional area and diameter for the selected pressure drop can be determined from the generalised pressure-drop correlation given in Figure 11.44. 

Wetting rates are frequently expressed in terms of mass or volume flow-rate per unit column cross-sectional area. 

Cp = cost per square foot of plate area, /ft2 A = column cross-sectional area, ft2 N = number of plates A min = minimum number of plates Cs = cost of shell, /ft3 H = distance between plates, ft Cf = cost of feed pump, ... 

Equilibrate the column with 1 M acetic acid. Apply the cold solution to the top of the column using 0.4 ml per cm2 of column cross-sectional area. The sum of the areas of any peaks eluted before the principal peak is not greater than 5.0% of the sum of the areas of all the peaks in the chromatogram. 

Procedure The chromatographic procedure may be carried out at room temperature using (a) a column (1 M x 25 mm) packed with a cross-linked dextran suitable for fractionation of globular proteins in the range of molecular weights from 5,000 to 350,000 (Sephadex G-150 is suitable), (b) mixed phosphate buffer pH 7.0 with azide as the mobile-phase with a flow rate of about 20 ml (4 ml per square centimetre) of column cross-sectional area) per hour, and (c) a detection wavelength of 280 nm. 

To use the flooding point diagram, first it is necessary to decide whether the drops produced in the extractor are circulating or oscillating. The mean diameter di,2 (see Eq. 9.1) is used for the characteristic drop size. If the flow rate ratio is known from the thermodynamic design, the superficial velocities of both phases can be determined at the flooding point. The minimum column cross-sectional area and diameter necessarily follows directly from the superficial velocity at the flooding point with Eq. 9.19. 

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