Tray efficiency problems

The net result of this unpleasant scenario is loss of both vapor-liquid contacting and tray efficiency. Note how the mechanical problems (i.e., levelness) of tray 1 ruins the tray efficiency of both trays 1 and 2. 

Figure 14-42 shows that errors in relative volatility are a problem only at low relative volatilities for a > 1.5 to 2.0, VLE errors have negligible direct impact on tray efficiency. 

The above problem is not unique to the Chan and Fair correlation. In fact, the author feels that this is the most reliable published theoretical efficiency correlation currently available. The current correlation inherited these high efficiency predictions from the AlChE model, and the problem extends to all other theoretical tray efficiency correlations the author has experience with. When the column diameter exceeds 4 ft, one can almost count on a theoretical correlation to predict between 80 and 100 percent efficiency, regardless of the service. In the real world, most columns run closer to 60 percent efficiency. Which of the limitations listed above, and to what extent, generates the problem is unknown. The author would not trust any theoretical tray efficiency correlation for obtaining design efficiencies unless proven that it has actually overcome the above overestimating problem. 

Although real-life processes may not actually reach equilibrium, the assumption of an equilibrium stage is essential for a rigorous solution of a problem and for providing a sound basis for column design and performance evaluation. The application of equilibrium stage calculation results to actual performance information which is usually accomplished by utilizing tray efficiencies, a concept that is discussed in Chapter 14. 

For the separation problem described in Examples 12.1 and 12.4, estimate the number of trays required if the condenser can handle a maximum vapor rate of 150 kmol/h and the overall tray efficiency is 65%. 

If operating data from a laboratory, pilot-plant, or plant column are available, it can be used to determine what physical property method fits the column data. There could be a problem in using column data because the tray efficiency is also not known, and the VLB parameters cannot be decoupled from the efficiency. 

The common problem with measuring reflux rate is that reflux meters are typieaUy set at startup and then never adjusted again. Therefore, the reflux flow rate is typically not reliable. The reflux ratio is checked and monitored as an important operating parameter, but the absolute value of the reflux rate is rarely monitored. However, to have a correct heat balance, the reflux flow meter must be checked and calibrated to achieve at least 5% closure of heat balance ([total heat input total heat output]/total heat input). Only with this accuracy of heat balance, tray efficiency can be accurately determined (Summers, 2009). 

Deterministic optimization has been the common approach for batch distillation operation in previous studies. Since uncertainties exist, the results obtained by deterministic approaches may cause a high risk of constraint violations. In this work, we propose to use a stochastic optimization approach under chance constraints to address this problem. A new scheme for computing the probabilities and their gradients applicable to large scale nonlinear dynamic processes has been developed and applied to a semibatch reactive distillation process. The kinetic parameters and the tray efficiency are considered to be uncertain. The product purity specifications are to be ensured with chance constraints. The comparison of the stochastic results with the deterministic results is presented to indicate the robustness of the stochastic optimization. 

The problem is exacerbated by the composition controllers on the column. The drop in tray efficiency will result in a reduction in product purity. The controllers will respond by... 

It should be noted that, at 50% scale-down, it is possible that weeping or low tray efficiency may be observed (see Section 18.3.2 and Figure 18.191. This must be considered before recommending such a scale-down. Furthermore, reduction of the cooling water flowrate in the condenser, combined with increased cooling water tenperatures, could cause fouling problems, as pointed out in Example 19.5. Care must taken to understand the consequences of process modifications. 

The general principles of design of multicomponent fractionators are the same in many respects as those for binary systems, but the dearth of adequate vapor-liquid equilibrium data imposes severe restrictions on their application. These are especially needed for liquids which are not ideal, and the danger of attempting new designs without adequate equilibrium data or pilot-plant study for such solutions cannot be overemphasized. Inadequate methods of dealing with tray efficiencies for multicomponents represent another serious problem still to be solved. 

The problem we have been discussing—loss of tray efficiency due to low vapor velocity—is commonly called turndown. It is the opposite of flooding, which is indicated by loss of tray efficiency at high vapor velocity. To discriminate between flooding and weeping trays, we... 

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