Maldistribution vapor

The above results are based on data obtained for optimized designs and under ideal test conditions. To translate our findings to the real world, one must factor in liquid and vapor maldistribution, which is far more detrimental to the efficiency of packings than trays. In addition. one also must account for poor optimization or restrictive internals, which are far more detrimental to the capacity of trays than packings. We also have cited several other factors that need to be considered when translating the findings of our analysis to real-world towers. ... 

Liquid and vapor are well distributed. Both liquid and vapor maldistribution have a major detrimental effect on packing efficiency. 

Vapor maldistribution. Most popular theoretical models (such as the AIChE and the Chan and Fair models, Sec. 7.2.1) postulate perfectly mixed vapor flow. In larga-diameter columns, vapor is more likely to rise in plug flow. Modeling work showed (143,179,180) that in the absence of stagnant zones on the tray, vapor flow pattern has generally little effect on tray efficiency. When column efficiency exceeds 30 percent (143), or when stagnant liquid zones exist (171,173,180), vapor plug flow reduces tray efficiency. 

Vapor maldistribution Packing pressure drop places a resistance in the vapor path that helps spread the vapor radially. If pressure drop is too low, vapor will tend to channel through the bed, leading to poor mass transfer. 

Vapor is easier to distribute than liqnid, but vapor maldistributior can also be troublesome. The effects of vapor maldistribution havi been investigated far less than those of liquid maldistribution. Thi following findings have been reported ... 

Vapor maldistribution may be induced by liquid maldistribution (66) when vapor flows are high. Regions of high liquid holdup impede vapor rise and channel the vapor into the lighter-loaded regions (66). Since liquid tends to accumulate near the wall, vapor will tend to channel through the center. 

With vapor and two-phase feeds, this guideline may be difficult to adhere to. Unless very large hole velocities are acceptable, this guideline calls for low pipe velocities. This, in turn, leads to impractically large pipes. A common compromise is to use a hole area of the same order as the pipe area, at the expense of the inferior distribution profile described in item 3 below (Fig. 2.5b, c). This, in turn, may lead to a tendency of vapor to flow toward one wall (Fig. 2.6a). When vapor maldistribution can be troublesome (e.g., beneath a packed bed), flow-straightening tubes (Fig. 2.66) can alleviate the problem. Tube length is usually two to three times the perforation diameter. 

Vapor is easier to distribute than liquid, but vapor maldistribution can also be troublesome. Vapor flow through packing tends to be uniform if the initial liquid and vapor distribution to the packing is uniform (217, 386). 

Where pressure drop is critical, a sparger pipe with the bottom quadrant cut out (rather than perforated) is sometimes used, but at the penalty of inferior vapor distribution. Similarly, the sparger can be entirely eliminated and substituted by a dog house baffle parallel to the direction of fluid entry. This baffle is somewhat wider than the nozzle diameter and stretches from wall to wall parallel to the direction of the incoming fluid. The author is familiar with one experience where addition of a "dog house baffle eliminated a packed-tower vapor maldistribution problem. 

The author experienced one troublesome case, which was also reported by Lieberman (237), where liquid overflow through the chimneys caused a severe loss of efficiency in the packed section above. The chimney tray had undersized downpipes that were not liquid-sealed either the undersizing or the lack of seal (or both) could have caused the overflow. Lieberman (237) suggests that the overflow led to entrainment and flooding, hence the loss in efficiency. However, subsequent pressure-drop measurements and other observations provided no supporting evidence for the existence of flooding, and the author believes that vapor maldistribution due to liquid overflow (guideline 14 above) caused the loss in efficiency. 

Midspan I-beams can be deep, sometimes 1 to 2 ft. When the bottom of the I-beam is too close to the liquid level, it may interfere with the distribution of vapor entering the packed bed (Fig. 8.6). This vapor maldistribution may lower the efficiency of the column and cause premature flooding. 

I-beam interference can be just as troublesome in the space above a chimney tray. In one case history contributed by D. W. Reay (334), this interference is believed to have led to severe vapor maldistribution in a refinery vacuum tower (Fig. 8.66). The maldistributed vapor profile was displayed as a carbon deposit on the siuTace of the bottom packing. The deposit formed an annular ring about 5 ft wide that extended about 1 in into the bed. In that case, liquid was known to overflow the chimneys for several months because of an incorrect location of level tappings. This overflow caused liquid entrainment. Some entrained droplets ultimately carbonized on the base of the bed. Had the vapor profile been uniform, entrainment (and therefore deposit laydown) would have been more uniform. It is believed that vapor from the side chimneys was blocked by the beams and preferentially ascended around the periphery. If liquid overflow (down the risers) had been uneven, the maldistribution could have been further aggravated. 

With partial condensers, vapor maldistribution among parallel condensers is less troublesome and can usually be avoided by careful exchanger design (237). 

Refineiy A new set of condensers was added in parallel to an existing set in order to increase condensation q>acity. Instead of increasing condensation ce xicity decreased. Vapor maldistribution was the cause. Total condensers are best added in series. If added in paraUel, beware of maldistribution. 

Disk and donut tr just above the feed and trays below the HCX) draw were replaced by grid Erratic temperatures and poor heat transfer resulted caused by vapor maldistribution. A V-shaped wedge baffle was installed directly at the vapor inlet, but did not help. Baffle and giM ctdced aH r 10 months operation, causing a ct >aci1ybottlmleck. 

The packed hei t throng which a nonuniform profile persists is a function of column diameter (156). In pilot-scale columns, vapor maldistribution was found to posist for abed he t of the order of 1 ft (155,157). In large-diameter columns, this maldiattibution persists to a much greater hei t (15,152,154). In a number of 15-ft-diameter absorbers (154), vapor maldistribution persisted throu a 50-ft bed the efficiency was about balf that encountered during good vapor distribution. 

Vapor maldistribution theoretically could lead to the same lack of performance, as illustrated in Figure 7-5. There is, however, a much better radial mixing of the vapor phase in the packed bed because it almost always is in the turbulent flow regime. Vapor distribution normally is not a problem as long as the pressure drop through the packed bed is at least 0.10 in. H20/ft of packed depth, and the inlet vapor nozzles are operating at Fg vapor rates not greater than 22 Ib /ft s. For columns in which the packed depth is less than the column diameter, vapor maldistribution can be a problem. 

Because a packing support plate usually is located immediately above the gas inlet in an absorber or the reboiler return in a distillation column, this plate could be used to control vapor distribution. Obviously, vapor maldistribution can reduce column efficiency in the same way as liquid maldistribution although due to the turbulence in the vapor phase, its rate of radial cross-mixing is at least three times that of the liquid phase. The potential for vapor maldistribution increases as column diameters or operating pressures increase. Fortunately, the vapor phase tends to maintain a uniform distribution once it has been established. Thus, usually only the packing support plate immediately above the vapor inlet needs to act as a vapor distributor. This support plate should be located at least one vapor-inlet diameter plus 12-in. above the center-line of the vapor inlet nozzle. 

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