Sieve-plate columns pressure drops

Example 18.6. A sieve-plate column operating at atmospheric pressure is to produce nearly pure methanol from an aqueous feed containing 40 mole percent methanol. The distillate product rate is 5800 kg/h. (a) For a reflux ratio of 3.5 and a plate spacing of 18 in., calculate the allowable vapor velocity and the column diameter. b) Calculate the pressure drop per plate if each sieve tray is in, thick with j-in, holes on a -in. triangular spacing and a weir height of 2 in. (c) What is the froth height in the downcomer ... 

The fact that the properties of the substance undergoing distillation have an appreciable influence on the pressure drop is apparent from the curves shown in Fig. 108 [229] and from Table 33 [200]. The data listed in the latter table were determined in an Oldershaw sieve-plate column (28 mm diameter) with 30 actual plates. 

Umholtz and van Winkle [230] also determined the pressure drop as a function of load for sieve-plate columns, which had a diameter of 25 mm (1 inch), a plate spacing of 50 mm (2 inches), a free area of 16.2% and contained a varying number of holes of different diameters. 

Pressure drop in an Olderahaw sieve-plate column during the distillation of substances having various densities and surface tensions [200]... 

From a constnictional aspect plate column may be subdivided into buiUe-plale, bubble-cap and sieve-plate columns. AH of these are employed mostly at atmospheric pressure, as their relatively high resistance to vapour flow gives rise to an appreciable pressure drop. 

Uinholtz and van Winkle [44] carried out investigations on a sieve-plate column of 25 ram (1 inch) diameter and 50 mm plate spacing. The free cross-section was 16.2%. The highest efficiency was obtained when the diameter of the holes in the plates was 1.9 mm. Tests with an o-xylene-p-xylene mixture showed that the efficiency dropped from 90 to 65% on reduction of the pressure from 750 to 50 mm Hg. When the load was increased the efficiency first rose, but from 200—260 g/cm h onwards it remained nearly constant. In a later investigation [45] the influence of the plate thickness and of the properties of the ini.xture (surface tension, density of vapour and liquid) were examined. 

Commercial preparations of these supports are available in narrow mesh-range fractions to obtain particles of uniform size the material should be sieved to the desired particle size range and repeatedly water floated to remove fine particles which contribute to excessive pressure drop in the final column. To a good approximation the height equivalent of a theoretical plate is proportional to the average particle diameter so that theoretically the smallest possible particles should be preferred in terms of column efficiency. Decreasing particle size will, however, rapidly increase the gas pressure necessary to achieve flow through the column and in practice the best choice is 80/100 mesh for a... 

Pressure drop. The pressure drop over the plates can be an important design consideration, particularly for vacuum columns. The plate pressure drop will depend on the detailed design of the plate but, in general, sieve plates give the lowest pressure drop, followed by valves, with bubble-caps giving the highest. 

Fig. 13. (a) Observed and (b) computed formation of a droplet at an orifice in a pulsed sieve-plate extraction column. (Reprinted from Chemical Engineering Science, Volume 50, Ohta M., et al. Numerical analysis of a single drop formation process under pressure pulse condition, pp. 2923-2931, copyright 1995, with permission from Elsevier Science.)... 

The upper limit of the velocity in a sieve-tray column is determined by the flooding point or by the velocity at which entrainment becomes excessive. Flooding occurs when the liquid in the downcomer backs up to the next plate, and this is determined mainly by the pressure drop across the plate and the plate spacing. Near the flooding point, most of the pressure drop comes from the term in Eq. 

A sieve-tray column with 15 plates is used to prepare 99 percent methanol from a feed containing 40 percent methanol and 60 percent water (mole percent). The plates have 8 percent open area, in. holes, and 2-in. weirs with segmental downcomers, (a) If the column is operated at atmospheric pressure, estimate the flooding limit based on conditions at the top of the column. What is the F factor and the pressure drop per plate at this limit (Z>) For the flow rate calculated in part (a) determine the F factor and the pressure drop per plate near the bottom of the column. Which section of the column will flood first as the vapor rate is increased ... 

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