Distillation columns weeping

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

Fig. 2-4. Examples of typical scan profiles obtained for various problems met on distillation columns, respectively from up and left to bottom and right normal column, collapsed tray, flooding, entrainment, weeping and foaming (IAEA, 2002).

Determine the tray layout and pressure drops for the distillation column in Examples 10-1 and 10-2. Determine if entrainment or weeping is a problem. Determine if the downcomers will work properly. Do these calculations only at the top of the column. 

In addition to one or more product compositions, other process variables need to be controlled. Consider the separation of a binary mixture and the conventional tray-distillation column shown in Fig. 13.2. Assume that the chief control objective is to control both product compositions, Xjy and jc. However, the liquid levels in the reflux drum, and the column base (or sump), Hb, must be kept between upper and lower limits (Guideline 2). The column pressure, P, must also be controlled in order to avoid weeping or flooding in the column and to control the vapor inventory in the column. Thus, this column has a total of five CVs. 

In addition to entrainment flooding, we need to be concerned with the turndown ratio, the maximum vapor flow divided by the minimum vapor flow. This ratio is important because it determines the flexibility of our distillation column. In an ideal world, we would want to operate our separation at full capacity all the time. In a real world, we will not always have a high, steady demand for our product, or we may have a fluctuating feedstock. In this real world, we may want to run at less than full capacity without the possibility of problems like weeping. 

Perforated plates such as sieve trays used in absorption, distillation or extraction columns. The holes can be covered by caps or valves to avoid weeping in the range of low superficial gas or vapor velocities. The two phases are moving in a crossflow on a tray. 

Maximum column capacity is increased since manipulated variables— reflux or distillate, boilup or bottom product—are not changed as much as if unaided feedback controllers were used. Similarly, the column is less likely to weep or dump. 

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