Liquid flow, unevenness

Liquid nonuniformity effect. Liquid flows unevenly through the packing and tends to concentrate at the wall (Sec. 9.2.4). 

Maldistribution tends to be a greater problem at low liquid flow rates than at high liquid flow rates [Zuiderweg, Hoek, and Lahm, 1. ChemE Symp. Ser. 104, A217 (1987)]. The tendency to pinch and to spread unevenly is generally higher at the lower liquid flow rates. 

Liquid profile unevenness is more severe at low liquid flow rates (140,142,147). 

Optimum performance of the monolith reactor requires uniform and stable distribution of gas and liquid over the cross section of the monolith. Because the monolith consists of many small channels, it may be difficult to obtain a good distribution of the gas and liquid flows within the monolith. This is very important for the monolith reactor, since an uneven inlet distribution would be propagated throughout the reactor. On the other hand, if the inlet distribution is appropriate, no nonuniformity will occur along the reactor. 

As a residual solid or viscous liquid is concentrated to dryness, stirring becomes difficult, especially in an immobile vessel. Due to the small surface area-to-volume ratio of large equipment, heat transfer and distillation take longer than similar operations in the laboratory. Decomposition may occur with extended heating, especially if there is uneven heat distribution. Complete removal of residual solids and viscous liquids is difficult from vessels with access limited through manways. Residual solvents may be removed ( stripped ) from liquids using specialized stills, e.g., wiped film evaporators, in which the mobile liquid flows over a heated zone. Only rarely are extracts concentrated to dryness. 

The perforated-pipe distributor has a relatively low turndown ratio, roughly 2 1 to 2.5 1 (111, 305). Excessive liquid flow rates may generate fine mist, while deficient liquid flow rates may generate uneven irrigation (Fig. 2.5d). The turndown can be enhanced by using a dual liquid distributor (Fig. 3.65 Sec. 3.8). 

Disadvantages of fixed beds include lower effectiveness factors because of the larger particle size and lower coefficients for gas-liquid and liquid-solid mass transfer. Fixed beds with two fluid phases are also difficult to scale up or scale down because of incomplete wetting and changes in gas and liquid flow distribution. Finally, there is a risk of temperature excursions with exothermic reactions in packed beds, since radial heat transfer is poor. Large reactors are often operated adiabatically, but hot spots may occur because of uneven flow distribution. 

In principle, a colrrrrm tray can be operated even with very small liquid loads because the necessary height of the two-phase layer (froth) on the tray is provided by the exit weir. At extremely low liquid loads, however, the liquid will flow in an uneven pattern across the tray resulting in some degree of maldistribution of liquid. Accordingly, it is recommended to ensure a ttunimum liquid flow rate over the exit weir larger than Fl/ w 2 m /(m h). In small diameter columns, however, the liquid load can be considerably lower. 

Marangoni effects can be encountered in both single- and multicomponent liquid systems. In a pure liquid, surface tension gradients result from differences in temperature (or evaporation rate) from one point to another in the system. It is generally found that an increase in temperature lowers ctlv so that where hot spots occur, liquid flows away to cooler regions of the liquid (Fig. 6.13 ). The result of such a phenomenon can be the formation of dimples in a surface that dries or solidifies under uneven temperature conditions. 

As previously explained, problems arise from an uneven distribution of the liquid around the catalyst pellets in TBRs In fact only a part of the external particle surface is effectively wetted by the flowing liquid the remaining zones are partly in contact with semistagnant liquid pockets and partly covered by an almost motionless thin film The chosen reaction is not significantly exothermic and the catalyst pores can therefore be assumed completely filled by liquid No particular problem exists if the controlling reactant is fed with the liquid because the most active zones of the catalyst are very likely those characteri- ed by a flowing liquid and the conversion rate increases as the liquid flow... 

Uneven liquid flow across the tray deck is particularly detrimental to good vapor-liquid mixing. For example, if half of the tray deck has stagnant liquid, then the vapor bubbling through the stagnant liquid cannot alter its composition. 

Uneven liquid flow is promoted by the outlet weir being out of level. Liquid will tend to flow across that portion of the tray with a lower than average weir height. The portion of the tray upstream of the high part of the outlet weir will contain stagnant liquid. However, if the crest height (i.e., the height of... 

Each tray in a tower is inherently a vapor-liquid redistributor. The holes in the tray decks redistribute the vapor flowing up the tower. The outlet weir, or more exactly the crest height of the liquid overflowing the weir, forces the liquid to flow evenly across the tray. Even if the weir height is uneven on a tray, and liquid flow is distorted on that particular tray, the liquid will be properly redistributed on the tray below. 

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