Pressure drop, trays calculation

The sieve tray dry pressure drop is calculated next, applying the following equations ... 

Pressure drop. Pressure drop is calculated as per Secs. 6.3.1 to 6.3.3, using Fair s pressure drop correlation. Usually, it is good practice to design for a pressure drop of 3 to 5 in of liquid (approximately 0.08 to 0.12 psi) per tray. If outside this range it is best to adjust the fractional hole area (if dry pressure drop dominates) or the weir height (if wet pressure drop dominates). 

The variation of total pressure during each time integration step is much smaller than the variations of composition and temperature. In order to simplify the procedure, the vapor pressure on each tray is considered constant along the time integration step, but it will be recomputed at the beginning of the new time step. The tray pressure drop is calculated on the base of hydraulic correlations, specific for the plate type. 

Calculate dry tray pressure drop. The pressure drop is calculated for two cases, assuming valves full open and assuming valves part open. Use... 

It is essential to realistically establish the condensing conditions of the distillation overhead vapors, and any limitations on bottoms temperature at an estimated pressure drop through the system. Preliminary calculations for the number of trays or amoimt of packing must be performed to develop a fairly reasonable system pressure drop. With this accomplished, the top and bottom column conditions can be established, and more detailed calculations performed. For trays this can be 0.1 psi/actu-al tray to be installed [149] whether atmospheric or above, and use 0.05 psi/tray equivalent for low vacuum (not low absolute pressure). 

This metliod calculates the dry tray pressure drop and allows for correcting the two-phase flow effects at various entrainment ratios. 

Compare calculated h j with value of dry tray pressure drop as given ... 

Calculate wet tray pressure drop, determine effective head from Figure 8-130. 

Determine the percent hole area in the active tray portion for pressure drop calculation. Note that hole size does not have to be set at this point (Figure 8-143.)... 

Calculate the total wet tray pressure drop, using an assumed height of clear liquid on the tray of 0.5-in. minimum to 4-in. maximum (1 to 2-in. are usual values). 

Example 8-41 Procedure for Calculating Valve Tray Pressure Drop (after Klein [201])... 

Valve hole area 1.65 sq. ft. (separate calculation) = ho Tray pressure drop and froth height... 

Fair reports that the data for mass transfer in spray, packed, and tray columns can be used for heat-transfer calculations for these columns. The pressure drop in these types of columns is usually quite low. 

Additional equations include physical property relationships to get densities and enthalpies, a vapor hydraulic equation to calculate vapor flow rates from known tray pressure drops, and a liquid hydraulic relationship to get liquid flow... 

This approximate approach is admittedly crude, but I have used it quite effectively for several distillation simulations. At each point in time the pressure Pp at the top of the column is calculated from Eq. (5.33), and new pressures on all the trays are calculated using a constant pressure drop per tray. 

Finally we can now calculate the vapor flow rate through the tray from the pressure drop through the tray (P i - P ) and the liquid height on the tray, which we can get from the weir height fi and the height of liquid over the weir... 

TOLUENE/O-XYLENE SEPARATION AT 90 MM HC RIGOROUS VAPOR-HYDRAULIC MODEL (VAPOR RATES calculated from pressure drop through TRAYS) USING LSODE IMPLICIT STIFF INTEGRATOR PROGRAM ASSUMPTIONS ... 

The pressure drop of the vapor flowing through the tray above the downcomer. (Calculating this pressure drop is discussed in Chap. 2.)... 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

As noted, the weir crest 4, is calculated on an equivalent clear-liquid basis. A more realistic approach is to recognize that in general a froth or spray flows over the outlet weir (settling can occur upstream of the weir if a large calming zone with no dispersers is used). Bennett et al. [AIChE J., 29, 434 (1983)] allowed for froth overflow in a comprehensive study of pressure drop across sieve trays their correlation for residual pressure drop h L in Eq. (14-100) is presented in detail in the previous (seventh) edition of this handbook, including a worked example. Although more difficult to use, the method of Bennett et al. was recommended when determination of pressure drop is of critical importance. 

FIG. 14-37 Aeration factor for pressure drop calculation. (a) Sieve trays. [Bolles and Fair, Encyclopedia of Chemical Processing and Design, vols. 16, 86. J. M. McKetta (ed.), Marcel Dekker, New York, 1982.] h. Valve trays. (From G. F. Klein, Chem. Eng., May 3, 1982, p. 81 reprinted courtesy of Chemical Engineering.)... 

Step 2. Next the number of gas phase transfer units NG is to be calculated. First froth height in inches HF must be calculated. If the dry tray pressure drop is 0.5 in or more, then use Eq. (3.37). 

For your calculated dry tray pressure drop DPTRAYi (inches of liquid), you may use linear interpolation of the preceding HF values calculated to find your froth height case. Vapor froth tray time may now be calculated from Eq. (3.39) ... 

Several factors are required first in order to calculate the tray pressure drop in inches of clear liquid. This liquid pressure unit is that liquid density referred to as liquid density used DI, lb/ft3. First calculate the weir length Lm ... 

The height of downcomer liquid is next calculated as an additive for the total valve tray pressure drop summation [9] wet tray downcomer AP, HDC2, inches of liquid, Eqs. (3.58), (3.59), and (3.60) ... 

The total tray pressure drop HDC2 (inches of liquid) is calculated with preceding factors installed in Eq. (3.60). 

Please note that HOLHA is the total hole area in ft2 on a single tray deck. It is used in Eq. (3.91) to calculate sieve tray jet flood and will be used to calculate sieve tray pressure drop as well. 

Using these calculated ratios, AHAA and THDIA, a hole discharge coefficient factor CFCV is calculated from a curve-fitted equation in Fig. 18-14 in the Chemical Engineer s Handbook [14], CFCV is a factor in Eq. (3.112) for calculating the sieve tray dry pressure drop. 

Before a total sieve tray pressure drop can be summed, the froth pressure in inches of clear liquid over the active area must be calculated. This froth height actually reduces the HHDS value by a factor called the aeration beta correction. This has been done by Smith, who plotted the aeration factor beta vs. FGA (see Eq. (3.120) for FGA). Equation (3.121) is a curve-fit of Smith s beta curve plot [16]. Generally a beta factor of 0.7 to 0.8 is calculated using Eq. (3.121). 

The total sieve tray pressure drop DPTRAy may now be calculated using the dry tray drop HHD from Eq. (3.114) and HHL from Eq. 

Total sieve tray pressure drop DPTRAY in inches of clear liquid is now calculated in a final equation ... 

The next step is to calculate the pressure loss to the control valve entrance. The designer has located the flow control valve as close as practical to the stabilizer feed entrance. This is a good design location for the control valve. Why The pressure drop across the control valve results in a two-phase flow, vapor and liquid flowing into the stabilizer feed tray. Thus, the downstream flow is a two-phase flow, and this problem will be finished later, in the section on two-phase flow. 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the available tray spacing. Downcomer backup can be calculated by adding the clear liquid height on the tray, the liquid backup caused by the tray pressure drop, and the liquid backup caused by the friction loss at the downcomer outlet. The downcomer backup is then divided by an aeration factor to give the aerated liquid backup. 

Pi "1 are the bounds on. The authors considered a detailed dynamic model (similar to those presented in Chapter 4) but included pressure drop model and Francis weir formula to calculate liquid flow rate from the tray. Refer to the original work for further details on modelling. 

Figure 6.22 (Continued) Tray aeration factor prediction for pressure drop calculations. (t>) A modified version of the correlation in a, suitable for valve trays. (Part b from George F. Klein. Chemical Engineering, May 3, p. 81, 1982, reprinted courtesy of Chemical Engineering)...

For valve trays, the hydraulic gradient is somewhat larger than that of sieve trays, probably not by much (13). In a similar manner to sieve trays, it is often neglected in the pressure drop calculation (7-9,71,80). In cases of a long flow path of liquid, it should be checked using the Hughmark and O Connell correlation above. 

The Bennett et al. correlation. This correlation was shown (31) to predict experimental sieve tray pressure drop data more accurately than Fair s correlation. The correlation is based on froth regime considerations and is not applicable to the spray regime. The Bennett et al. calculation of dry pressure drop is identical to Fair s, using Eqs. (6.42) and (6.43) and the Liebson et al- correlation (Fig. 6.21a). To calculate the h, term in Eq. (6.41), Bennett et al. depart from the concept of clear liquid flow corrected for aeration effects [Eq. (6.47a)]. Instead, they use Eq, (6.476) and a model of froth flow across the weir. Their residual pressure drop, hn, is a surface tension head loss term, which is important for trays with very small holes ([Pg.317]

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