Maldistribution, packing, vapor

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. 

In packed columns, liquid overflowing the chimneys may preferentially descend into some risers (often the peripheral), with vapor ascending through the others (often the central risers). This may cause severe maldistribution of vapor to the bed above. 

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. 

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. 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

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). 

When a sparger pipe enters an intermediate feed, vapor jets must not impinge on redistributor liquid surface or other packed-column internals. Liquid surface agitation in redistributors, or mechanical damage to internals, can cause maldistribution, which can be detrimental to column efficiency. 

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. 

Following replacement of trays with structured packing, the product failed to meet specs and tamdown was poor. Glycol rate had to be doubled to achieve dehydration. The most likely cause was gas maldistribution induced by an inlet vapor velocity head of 56 in of water. The column achieved design performance after new structured packing as well as new vapor and liquid distributors were installed. 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

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. 

---