Trays, sieve examples

Aspen Plus has an easy-to-use tray sizing capability. Click the item Tray Sizing under the Cl block, and then click New and OK for the identification number. A window will open on which the column sections to be sized, and the type of tray can be entered. Figure 3.53a shows the parameter values used in the example. The stages run from Stage 2 (the top tray) to Stage 31 (the bottom tray). Sieve trays are specified. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

For weeping sieve trays, see Figures 8-131 and 8-132, and example in later paragraph. 

Example 8-37 Sieve Tray Splitter Design for Entrainment Flooding Using Fair s Method (used by permission [183])... 

Example 8-38 Sieve Tray Design (Perforated) with Downcomer... 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

When vapor flows through a tray deck, the vapor velocity increases as the vapor flows through the small openings provided by the valve caps, or sieve holes. The energy to increase the vapor velocity comes from the pressure of the flowing vapor. A common example of this is the pressure drop we measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the vapor as it flows through the orifice plate comes from the pressure drop of the vapor itself. 

The conditions of Example 13.15 will be used. This is the case of a standard sieve tray with 24 in. spacing and to operate at 80% of flooding. The entrainment correlation is Figure 18.4 for which the value of the abscissa was found to be... 

Example 9 Flooding, of a Distillation Trau An available sieve tray... 

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. 

Example 11 Pressure Drop, Sieve Tray For the conditions of Example 9, estimate the pressure drop for flow across one tray. The thickness of the tray metal is 2 mm. The superficial F-factor is 2.08 m/s kg/mph... 

Solution Table 14-12 presents measurements by Billet (loc. cit.) for ethyl-benzene-styrene under similar pressure with sieve and valve trays. The column diameter and tray spacing in Billets tests were close to those in Example 9. Since both have single-pass trays, the flow path lengths are similar. The fractional hole area (14 percent in Example 9) is close to that in Table 14-12 (12.3 percent for the tested sieve trays, 14 to 15 percent for standard valve trays). So the values in Table 14-12 should be directly applicable, that is, 70 to 85 percent. So a conservative estimate would be 70 percent. The actual efficiency should be about 5 to 10 percent higher. 

Cross-sectional view of finite-stage contactor tower in operation showing an example of a sieve tray, a valve tray, and a bubble-cap tray. 

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 ... 

Examples 3 and 4 presented in the following illustrate methods for estimating pressure drop with bubble-cap contactors and with sieve-tray contactors. The examples also give information as to typical design conditions for the two types of contactors. 

Calculate the crystal size distribution function n. The crystal size distribution for the ith sieve tray is n, = 1012M3AVTj/(L3ALj), where AIT, is the weight fraction retained on the ith screen, /., is the average screen size of material retained on the ith screen (see Example 10.7, step 2), and A/., is the difference in particle sizes on the ith screen (see Example 10.7, step 3). For instance, for the Tyler mesh 100 screen, nio = 1012(0.119)(0.040)/(1613 x 28) = 40.7 crystals per cubic centimeter per micron. Table 10.5 shows the results for each sieve screen. 

Example 4.9 Entropy production in separation process Distillation Distillation columns generally operate far from their thermodynamically optimum conditions. In absorption, desorption, membrane separation, and rectification, the major irreversibility is due to mass transfer. The analysis of a sieve tray distillation column reveals that the irreversibility on the tray is mostly due to bubble-liquid interaction on the tray, and mass transfer is the largest contributor to the irreversibility. 

Liquid-liquid systems are encountered in many practical applications involving physical separations of which extraction processes performed in both sieve-tray and packed columns are well-known examples. In principle, all three methods discussed in Section III,B,2 can be used to model liquid-liquid two-phase flow problems. The added complexity in this case is the possible deformation of the interface and the occurrence of flow inside the droplet. 

Siemens-Martin furnace-regenerator, 590 Sieve tray extractors, 483 capacity, 484,487 diameters, 483, 487 efliciency. 483.487 pulsed, 478,483,487 sizing example, 486 Sieve trays, 428 assembly in a tower, 428 comparison with other types, example, 431... 

As expected, the Type 2 packing has a higher capacity than the sieve tray with 24-inch spacing (Example 13.17). The sieve tray has an expected overall efficiency in the range of 80-90%, or an equivalent HETP of 24/0.9 = 26.7 in. to 24/0.8 = 30 in. This range of HETP = 0.68-0.76 m does not approach the efficiency of structured packing. 

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