Effect of Holdup

These observations are too qualitative, but it is important to note that holdup is a factor that must be taken into consideration when evaluating batch processes, especially when holdups reach 5-10% or higher relative to the liquid in the reboiler. The more rigorous mathematical solution methods incorporate liquid holdup and quantify its effects. 

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Although a number of studies were made and approximate methods developed for predicting the effect of liquid holdup in the period of the 1950s and 1960s, as summarized in the 6th edition of Peny .s Chemical Engineers Handbook, the complexity of the effect of liqmd holdup is such that it is now best to use computer-based batch-distillation algorithms to determine the effect of holdup on a case-bycase basis. 

To evaluate the effect of holdup on bubble velocity, Marrucci (M3) used a spherical cell model of radius b such that... 

Rose et al. (1950) and Rose and O Brien (1952) studied the effect of holdup for binary and ternary mixtures in a laboratory batch column. They qualitatively defined the term sharpness of separation as the sharpness in the break between successive components in the graph of instantaneous distillate composition against percentage distilled. They showed that an increase in column holdup enhanced the sharpness of separation at low reflux ratio but did not have any effect at a very high reflux ratio. 

Using binary mixtures, Luyben (1971) studied the effects of holdup, number of plates, relative volatility, etc. on the capacity (total products/hr). For an arbitrarily assumed constant reflux ratio the author observed both positive and negative effects of tray holdup on the capacity for columns with larger number of plates, while only negative effects were observed for columns with smaller number of plates. It is apparent that these observations are related to the degree of difficulty of separation. 

Mayur and Jackson (1971) simulated the effect of holdup in a three-plate column for a binary mixture, having about 13% of the initial charge distributed as plate holdup and no condenser holdup. They found that for both constant reflux and optimal reflux operation, the batch time was about 15-20% higher for the holdup case compared to the negligible holdup case. Rose (1985) drew similar conclusion about column holdup but mentioned that the adverse effects of column holdup depends entirely on the system, on the performance required (amount of product, purity), and on the amount of holdup. Logsdon (1990) found that column holdup had a small but positive effect on their column operation. 

Using binary mixtures, Mujtaba and Macchietto (1998) carried out further investigation to explain the effect of holdup qualitatively or quantitatively and to... 

When column holdup is significant, and especially when multicomponent mixtures must be considered, the analysis of the effect of holdup is extremely complex. This problem has been addressed by a number of investigators (11,14, 16, 17), however, and it has been concluded the column holdups on the order of 10 to 15 percent actually are beneficial in obtaining better separation. The interested reader should consult the references cited. 

For preliminary studies of batch rectification of multicomponent mixtures, shortcut methods that assume constant molar overflow and negligible vapor and liquid holdup are useful in some cases (see the discussion above concerning the effects of holdup). The method of Diwekar and Madhaven [Ind. Eng. Chem. Res., 30, 713 (1991)] can be used for constant reflux or constant overhead rate. The method of Sundaram and Evans [Ind. Eng. Chem. Res., 32, 511 (1993)] applies only to the case of constant reflux, but is eaty to implement. Both methods employ the Fenske-Underwood-Giluland (FUG) shortcut procedure at successive time steps. Thus, batch rectification is treated as a sequence of continuous, steady-state rectifications. 

It is important to note that in using computer-aided models for batch distillation, the various assumptions of the model can have a significant impact on the accuracy of the results e.g., see the discussion of the effects of holdup above. Uncertainties in the physical and chemical parameters in the models can be addressed most effectively by a combination of sensitivity calculations using simulation tools, along with comparison to data. The mathematical treatment of stiffness in the model equations can also be very important, and there is often a substantial advantage in using simulation tools that take special account of this stiffness. (See the 7th edition of Perry s Chemical Engineers Handbook for a more detailed discussion of this aspect). 

Table 10-6 Statement of Examples 10-2 and 10-3 effect of holdup on distillation columns at total reflux, type 1 (D = 0, B = 0, F = 0)...

Paraskos, J. A., J. A. Prayer and Y. T. Shah. Effect of Holdup, Incomplete Catalyst Wetting and Backmixing During Hydroprocessing in Trickle-Bed Reactors. Ind. Eng. Chem. Process Des. Dev. 14 (1975) 315-322. 

A similar analysis has been applied by some writers to the case where the distillation has been discontinued at a finite refiux ratio. The usual straight operating line was employed to determine the plate composition at the end of the distillation, but in view of the fact that the operating line is curved for such a case, the calculations are probably of little value. If the operating lines corresponding to Eq. (14-16) or (14-17) could be evaluated for the end of the distillation, then an analysis of the effect of holdup could be made. 

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