Efficiency, tray pressure

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. 

An alternate form of catalyst is pellets. The pellets are available in various diameters or extruded forms. The pellets can have an aluminum oxide coating with a noble metal deposited as the catalyst. The beads are placed in a tray or bed and have a depth of anywhere from 6 to 10 inches. The larger the bead (1/4 inch versus 1/8 inch) the less the pressure drop through the catalyst bed. However, the larger the bead, the less surface area is present in the same volume which translates to less destruction efficiency. Higher pressure drop translates into higher horsepower required for the oxidation system. The noble metal monoliths have a relatively low pressure drop and are typically more expensive than the pellets for the same application. 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

The main fractionator may be the limitation on the unit. Packing can reduce pressure drop, and more efficient trays can give better product separations [11]. 

When the dry tray pressure drop is significantly less than the hydraulic tray pressure drop, then the tray will start to leak or weep, and tray efficiency will be adversely affected. 

For a tray to function reasonably close to its best efficiency point, the dry tray pressure drop must be roughly equal ( 50 percent) to the hydraulic tray pressure drop ... 

A packed tower can successfully fractionate with a very small pressure drop, as compared to a tray. For a modern trayed tower, to produce one single theoretical tray worth of separation (that s like a single, 100 percent efficient tray), a pressure drop of about 6 in of liquid is needed. A bed of structured packing can do the same job, with one inch of liquid pressure drop, even when allowing for the vapor distributor. In low-pressure fractionators, especially vacuum towers used to make lubricating oils and waxes, this can be of critical importance. 

The clear liquid height, or the liquid holdup, is the height to which the aerated mass would collapse in the absence of vapor flow. The clear liquid height gives a measure of the liquid level on the tray, and is used in efficiency, flooding, pressure drop, downcomer backup, weep-... 

In addition to the critical design factors for finite-stage contactors of number of theoretical trays, maximum allowable vapor velocity, column efficiency, and pressure drop as discussed earlier, a number of other factors are of importance in the development of the design. These factors are discussed in the following sections. 

The liquid-phase composition profiles are shown in Figure 14.8, the temperature profiles in Figure 14.9. In this example, the liquid and vapor temperatures are almost equal. The pressure profile is not shown since it was more or less a straight line between the specified top tray pressure and the computed bottom pressure reported in Table 14.3. The pseudo McCabe-Thiele diagram and efficiency profiles are shown in Figures 14.10 and 14.11. In this case we see that the efficiencies of acetone and methanol are essentially equal over most of the column with the efficiency of water somewhat higher. Interestingly, the efficiencies decrease in the lower portion of the column below the acetone-methanol feed. 

Sieve, valve, or plate trays, tray efficiency 60% pressure drop 0.7 to 1.4kPa/tray orO.3 to 0.65 kPa/theoretical stage (see Section 16.11.2.1)... 

Description Urea is formed from CO (18) and NH3 (19) in the HP loop (1) in a reactor (2) fitted with Casale-Dente high-efficiency trays. The urea solution (3) from the reactor (2), which still contains unreacted NH3 and CO2, is first treated in a stripper (4), operating at the same pressure as the reactor, heated with steam and using CO (18) as stripping agents to recover most of the unreacted NH3 and CO. ... 

Swept-back weirs (Fig. 6.76) are sometimes used at high liquid loads. They extend the weir length, which in turn lowers the effective liquid load (gallons per minute per inch of weir length), without changing tray or downcomer area. Swept-back weirs reduce tray pressure drop and downcomer backup, improve liquid distribution on the tray, and improve tray efficiency by inducing liquid flow into peripheral stagnant zones. However, the above improvements are usually small. 

Smaller weirs associated with small downcomers distort the liquid flow pattern as it approaches the weir ( weir constriction effect ), which increases tray pressure drop (48, 257, 319, 371) and promotes the formation of stagnant regions near the tray periphery (243, 376). Such stagnant regions may be detrimental to column efficiency. 

Liquid enters from the downcomer at the left, flows through the zone aerated by the upflowing vapor, and departs into the downcomer at the right. A two-phase mixture exists on the tray and it may be either liquid-continuous (a bubbly froth) or vapor-continuous (a spray), or some combination of the two. The objective of the designer is to determine the mass transfer efficiency and pressure drop brought about by this contacting action resultant typical performance profiles are shown in Fig. S.7-S. 

Figure 2-69 shows the dependence of the tray efficiency and tray pressure drop A/>, on the vapor load factor F for different types of column trays. 

The separation of ethylbenzene and styrene is relatively intricate, because of the close boiling points (136.2 °C, and 145.2 °C resp.). To avoid overheating the bottoms, highly-efficient trays with low pressure drop are used. 

In both downcomer back up and choke cases, downcomer liquid inventory increases and downcomer liquid backs up until the downcomer froth level reaches the tray above H > Hs). This phenomenon is called downcomer flood. When downcomer flood occurs to any tray, the whole tower will be flooded very quickly. A tower under downcomer flood provides virtually no distillation. In contrast, under tray flood, liquid can still leave the tower and the tower could stiU operate if the control system allows it although distillation efficiency suffers. Downcomer flood can be prevented in design by providing adequate downcomer area and clearance underneath the downcomer and minimizing tray pressure drop. Reducing reflux rate in operation could be effective in avoiding downcomer flood in operation. 

At low vapor rates, tray pressure drop decreases hence, tray leakage is increased. This reduces tray fractionation efficiency. Then, to achieve the desired product split, a higher reflux ratio, which wastes reboiler energy, is needed. Figure 9-3 shows a typical relationship of tray efficiency vs load. When assembly of the tray sections is less than perfect, the turndown efficiency of the tray is further degraded. 

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