Trays, fractionating efficiency

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. 

American Institute of Chemical Engineers, Bubble Tray Design Manual, Prediction of Fractionation Efficiency, Amer. Inst. Chem. Engrs. (1958). 

AIChE, Bubble tray design manual, Prediction of fractionation efficiency, AIChE, New-York (1958). 

Bubble-cap trays may be operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve, or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation ... 

Let us assume that both the reflux rate and the overhead propane product rate are constant. This means that the total heat flow into the tower is constant. Or, the sum of the reboiler duty, plus the feed preheater duty, is constant. If the steam flow to the feed preheater is increased, then it follows that the reboiler duty will fall. How does this increase in feed preheat affect the flow of vapor through the trays and the fractionation efficiency of the trays ... 

As the flow of vapor through the absorption section trays is unaffected by feed preheat, the fractionation efficiency of the trays in the upper part of the tower will not change as feed preheat is increased. On the other hand, the reduced vapor flow through the stripping section may increase or decrease fractionation efficiency—but why ... 

Meanwhile, all the liquid flow must drain through that portion of the tray that is lower. The net result, as can be seen in Fig. 7.2, is dry packing in one portion of the tower, and overrefluxing in the adjacent portion. This is called vapor-liquid channeling, and this is the root cause of poor fractionation efficiency in any tower. 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

The way we increase the fractionation efficiency of trays is to make the trays work harder. The correct engineering way to say this is To improve the separation efficiency between a light and heavy product, the vapor flow rate through the trays is increased, and the internal reflux flowing across the trays is increased. ... 

Reducing the pumparound duty increases the tray loadings on trays 1 through 7. But in so doing, the trays operate closer to their incipient flood point. This is fine. The incipient flood point corresponds to the optimum tray performance. But if we cross over the incipient flood point, and trays 5, 6, and 7 actually start to flood, their fractionation efficiency will be adversely affected. Then, as we decrease the pumparound heat-removal duty, the mutual contamination of diesel and gas oil will increase. 

Select the method of calculation for tray efficiency. Two methods are presented the O Connell method and the two-film method. In the programs accompanying this book, you may select the O Connell method by entering either an F for fractionator or an A for absorbers. In 1946, O Connell [4] published curves on log-log plots showing both absorber and fractionator efficiencies vs. equilibrium-viscosity-density factored equations. Separate curves for absorbers and fractionators were given. Such data have been curve-fit using a modified least-squares method in conjunction with a log scale setup. The fit is found to be reasonably close to the O Connell published curves. 

Input the liquid viscosity in centipoise. The liquid here is that on the subject tray. This is an important factor in determining tray efficiency. The value should be entered accurately and derived from reliable data sources, such as a reliable computer program for tray-to-tray fractionation calculations. Values greater than the normal range may occur, causing lower tray efficiencies. 

The actual number of stages is equal to the number of equihbrimn stages divided by the fractionator efficiency(overall column efficiency). Although the tray efficiency will vary, we will use the fractionator efficiency. The fiactionator efficiency is obtained from the O Coimel correlation given in Figure 6.17. Vital et al. [46] have reviewed and tabulated fractionator and absorber efficiencies for many systems. These data may help to arrive at a reasonable fractionator efficiency. 

Trays, fractionating assembly of sieve trays, 428 bubblecap, 428,430-433 capacity, F-factor, 429 capacity, Jersey Critical, 432 capacity, Souders-Brown, 432 cartridge, 428 design data sheet, 429 dualflow, 426 efficiency, 439-456 Linde, 430 ripple, 426 sieve, 428,429 turbogrid, 426 types, 426 valve. 429.430.432 Trickle reactors, 576, 607 Tridiagonal matrix, 407 Trommels, 335... 

Trays can flood even below design loads because of fouling of the tray decks. Flooded trays lose fractionating efficiency (see Chapter 19). Check the pressure drop across the suspect trays. If the pressure drop per... 

These surface temperatures correlate with the temperature of the vapor exiting from the chimney tray, which in turn is a function of the reflux rate to a particular quadrant of the packed bed. (This assumes that the vapor flow to the bottom of the bed is well-distributed. Vapor distribution is much more easily achieved than liquid distribution. However, vapor channelling will also result in diminished fractionation efficiency.)... 

Most often, tower packing supports are dislodged, or distillation trays upset, when a tower is operated with an excessive bottom liquid level. Forcing heat into a tower when the liquid level is several feet above the bottom tray deck often results in dislodging the bottom few trays. Occasionally, trays are mis-assembled during a turnaround. The results of either of these misadventures is diminished fractionation efficiency. 

The term incipient flood is that point in a trayed tower s operation when the spray height of liquid from the tray below begins to impinge on the tray above to the extent that entrainment reduces fractionation efficiency. Incipient flood in a packed column is that point in the column s operation at which liquid hold-up increases to an extent that reduces fractionation efficiency. 

FIGURE 12-7 Methods to enhance tray capacity without sacrificing fractionation efficiency... 

To answer this fundamental question, we should realize that reducing the tower pressure will also reduce both the tower-top temperature and the tower-bottom temperature. So the change in these temperatures, by themselves, is not particularly informative. But if we look at the difference between the bottom and top temperatures, this difference is an excellent indication of fractionation efficiency. The bigger this temperature difference, the better the split. For instance, if the tower-top and tower-bottom temperatures are the same for a 25-tray tower, what is the average tray efficiency (Answer 100 percent + 25 = 4 percent.)... 

The basis for the 100 percent is that if the tower top and bottom temperatures are identical (plus or minus a few degrees), then the entire tower is functioning as if it represented a single perfect theoretical fractionation stage. The fractionation efficiency of a theoretical stage is equal to one perfect tray operating at 100-percent efficiency. 

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