Stepped trays

Stepped (cascade) trays (Fig. 6.10) are sometimes used in large-diameter columns. They are used when there is a concern that an excessive hydraulic gradient will induce vapor to preferentially channel through the outlet portion of the tray. Since the hydraulic gradient is 

Each step of the tray should be designed with the same hydraulic gradient, the same liquid head over the weir, and the same hole area (86, 88). Intermediate weirs should rise above the calculated liquid height immediately downstream (48, 86, 257, 371, 409) if submerged, they would not operate as true weirs. 

Guidelines for setting the number of passes and minimizing operating problems with multipass trays are listed below  

There is an incentive to use as few passes as possible (88, 307). Increasing the number of passes decreases efficiency, increases the sensitivity to maldistribution, and increases costs. 

To avoid excessively short liquid paths, several designers recommend that two-pass trays should only be installed when column diameter exceeds 4 to 6 ft (38, 48, 73, 138, 172, 211, 257, 319, 371, 409) three-pass trays should only be installed when column diameter exceeds 7 to 9 feet (38, 138, 211) and four-pass trays should only be installed when column diameter exceeds 10 to 12 ft (38, 138, 211). 

---

The next day comes and the hung-over chemist wakens to see a dark red solution stirring away. In some cases where the chemist had made an enormous batch of this stuff, there may be seen a small mass of crystalline precipitate at the bottom of the flask. This is no big deal and will go away in the next step. If the chemist had made this in a flat-bottomed flask (which she really should have for convenience) then the ice tray is removed, the flask returned to the stir plate, a distillation setup attached, and the acetone is vacuum distilled from the flask. After all the acetone has come over the chemist can proceed in two different ways. One way is to just keep on distilling the solution until all of the formic acid has been removed. The chemist knows that just about all the formic has been removed when there is about 300mL of thick black liquid remaining in the reaction flask and hardly any clear formic acid is dripping over into the collection flask. If one were to swirl the reaction flask, the liquid will appear syrupy and kind of coat the sides of the flask. This is more evident when the flask cools. A quick sniff of the flask may indicate that some formic is still in there, but it should be too minimal to be of any concern. 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

One-Step Cameras and Processors. The eadiest one-step cameras used roU film and completed processing inside a dark chamber within the camera (9). The first instant color film, Polacolor, was provided in roU film format to fit these cameras. Flat-pack film cameras (Fig. 2), introduced in 1963, permitted the film to be drawn between processing rollers and out of the camera before processing was completed (10). Film holders for instant 10 X 13 cm (4x5 in.) film packets contain retractable rollers that permit the film to be loaded without mpturing the pod (11). For 20 x 25 cm (8 x 10 in.) films, the processing rollers are part of a tabletop processor. The exposed film, contained in a protective black envelope, and a positive sheet with pod attached are inserted into separate slots of a tray that leads into the processor. The film passes through the rollers into a covered compartment within which processing is completed. 

The application of a 50 percent Murphree vapor-phase efficiency on a y-x magram is illustrated in Fig. 13-40. A pseudo-equilibrium cui ve is drawn halfway (on a vertical line) between the operating hnes and the true-equilibrium cui ve. The true-equilibrium cui ve is used for the first stage (the partial reboiler is assumed to be an equilibrium stage), but for 1 other stages the vapor leaving each stage is assumed to approach the equilibrium value only 50 percent of me way Consequently, the steps in Fig. 13-40 represent actual trays. 

Derivatives or rates of change of tray and condenser-reflux drum hquid holdup with respecl to time are sufficiently small compared with total flow rates that these derivatives can be approximated by incremental changes over the previous time step. Derivatives of liquid enthalpy with respect to time eveiywhere can oe approximated in the same way. The derivative of the liquid holdup in the reboiler can likewise be approximated in the same way except when reflux ratios are low. 

Berth operator to take reasonably practicable steps to control flammable or toxic gas escapes (e.g. hose support, flange couplings liquid- or gas-tight, drip trays). 

The effect of the partial condenser is indicated in Figure 8-13 and is otherwise represented by the relations for the rectifying and stripping sections as just given. The key point to note is that the product is a vapor that is in equilibrium with the reflux to the column top tray, and hence the partial condenser is actually serving as an external tray for the system and should be considered as the top tray when using the equations for total reflux conditions. This requires just a little care in step-wise ctdculation of the column performance. 

As the reflux ratio is decreased from infinity for the total reflux condition, more theoretical steps or trays are required to complete a given separation, until the limiting condition of Figure 8-23 is reached where the operating line touches the equilibrium line and the number of steps to go from the rectifying to stripping sections becomes infinite. 

Point F on the figure represents conditions in the kettle or still with Xj, yj, or Xq, yo- Line DF represents slope of the operating line at minimum reflux. The step-wise development from point D cannot cross the intersection, F, where the slope intersects the equilibrium line, and leads to an infinite condition, as point F is approached. Thus, an infinite number of theoretical trays/stages is required, and... 

The batch distillation of a binary is somewhat simplified, as L/V values can be assumed, and since there is only enrichment of the overhead involved, only one operating line is used per operating condition. Theoretical trays can be stepped off and xib values read to correspond. The plots involved are the same as previously described. 

Continue step-wise calculations until the ratio of light to heavy key on a tray equals (or nearly so) that ratio in the liquid portion of the feed. This is then considered the feed tray. 

For partial condenser replace Dho by DHd in Step 3. A dew point on compositions of yp (vapor) give to or total pressure. Also get liquid composition x (liquid reflux in equilibrium with product vapor yo. Overhead vapor is sum of compositions of yp and xp. A dew point on this vapor (overhead from tray one top)) gives top tray temperature, tj. 

From values of Sj calculated (=Sg), read Ej, values from Figure 8-58 at the number of theoretical trays assumed in Step 2. Note that the Sg corresponds to the number of trays selected, hence will give a value for performance of the system under these particular conditions. 

Read Eaj values for fraction absorbed from Figure 8-58 using the A values of Step 9 and the fixed or assumed theoretical trays. 

Cascading the tray by using weirs as dams to divide the tray in steps, each step or section of the tray having no significant gradient from its inlet to outlet. This is usually only considered for trays 10 ft in dia. and larger, as it adds considerably to the cost of each tray. 

Calculate liquid entrainment, W e, as pounds of liquid per square foot of net tray area, equals (Step... 

For stepped caps (tray level, risers of different heights) or cascade tray (tray consisting of two or more levels) care must be taken to analyze all the conditions associated with level changes on the tray. Reference 5 discusses this in some detail. 

From Figure 8-144 or by calculation determine the plate area required for the holes on the pitch selected. Several selections may be tried to be used with the tray layout. These should be checked to agree with the assumed per cent hole area of Step 3. 

Example 11.8 With highly reactive absorbents, the mass transfer resistance in the gas phase can be controlling. Determine the number of trays needed to reduce the CO2 concentration in a methane stream from 5% to 100 ppm (by volume), assuming the liquid mass transfer and reaction steps are fast. A 0.9-m diameter column is to be operated at 8 atm and 50°C with a gas feed rate of 0.2m /s. The trays are bubble caps operated with a 0.1-m liquid level. Literature correlations suggest = 0.002 m/s and A, = 20m per square meter of tray area. 

---