Holdup, liquid batch distillation

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. 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

Consider the binary batch distillation column, represented in Fig. 3.58, and based on that of Luyben (1973, 1990). The still contains Mb moles with liquid mole fraction composition xg. The liquid holdup on each plate n of the column is M with liquid composition x and a corresponding vapour phase composition y,. The liquid flow from plate to plate varies along the column with consequent variations in M . Overhead vapours are condensed in a total condenser and the condensate collected in a reflux drum with a liquid holdup volume Mg and liquid composition xq. From here part of the condensate is returned to the top plate of the column as reflux at the rate Lq and composition xq. Product is removed from the reflux drum at a composition xd and rate D which is controlled by a simple proportional controller acting on the reflux drum level and is proportional to Md-... 

To increase product recovery in batch distillations, as a result to the lower liquid holdup in a packed column. 

Batch distillation. Because of the smaller liquid holdup of packing, a higher percentage of the liquid can be recovered as top product. 

In a steady state continuous distillation with the assumption of a well mixed liquid and vapour on the plates, the holdup has no effect on the analysis (modelling of such columns does not usually include column holdup) since any quantity of liquid holdup in the system has no effect on the mass flows in the system (Rose, 1985). Batch distillation however is inherently an unsteady state process and the liquid holdup in the system become sinks (accumulators) of material which affect the rate of change of flows and hence the whole dynamic response of the system. 

A liquid binary mixture with Bo = 10 kmol and xbo = <0.6, 0.4> molefraction is subject to conventional batch distillation shown in Figure 4.3. The relative volatility of the mixture over the operating temperature range is assumed constant with a value of (a=) 2. The total number of plates is, N = 20. The vapour boilup rate is, V = 5.0 kmol/hr and the reflux ratio is, r = 0.75. The condenser and total plate holdups are 0.2 and 0.2 kmol respectively. 

The basic theory of batch distillation is given in Hart (1997), Perry et al. (1997), Richardson et al. (2002) and Walas (1990). In the simple theoretical analysis of batch distillation columns, the liquid holdup in the column is usually ignored. This holdup can have a significant effect on the separating efficiency and should be taken into account when designing batch distillation columns. The practical design of batch distillation columns is covered by Hengstebeck (1976), Ellerbe (1997), and Hart (1997). 

With negligible liquid holdup, negligible pressure drop, constant vapor and liquid flow rates from tray to tray, time-invariant vapor flow rate, and ideal thermodynamics, the batch distillation problem for product k may be stated as follows (Farhat et al., 1990) ... 

A three-stage batch distillation column is charged with a 100 kmol mixture containing 60% mole component 1 and 40% mole component 2. The column pressure is maintained at 100 kPa, and the distillate rate is 20 kmol/h. It is desired to produce two distillation cuts. The first cut will be produced by continuously adjusting the reflux ratio to maintain the distillate composition at 90% mole component 1. Production of the second cut starts when the L/V ratio is 0.80. The L/V ratio will be fixed at this value until the second cut cumulative composition is 75% mole component 1. Determine the amount of each cut. Assume negligible tray holdups and use vapor-liquid equilibrium data from Problem 6.1. 

E3. In inverted batch distillation the charge of feed is placed in the accumulator at the top of the column fFigure 9-8). Liquid is fed to the top of the column. At the bottom of the column bottoms are continuously withdrawn and part of the stream is sent to a total reboiler, vaporized and sent back up the column. During the course of the batch distillation the less volatile component is slowly removed from the liquid in the accumulator and the mole fraction more volatile component increases. Assuming that holdup in the total reboiler, total condenser and the trays is small compared to the holdup in the accumulator, the Rayleigh equation for inverted batch distillation is. 

Equilibration time is of interest in continuous system startups as welt as in batch distillation. Cieariy, the amount of liquid holdup in the condenser, cohimn, and stillpot is a major factor in the time requirement. For a stage n (see Fig. 5. 1), the basic material balance equation is... 

The batch distillation of binary mixtures will be considered fpr the cases of (1) no rectification, (2) rectification without liquid holdup in the column, and (3) rectification with holdup. 

When a batch distillation is carried out by the variable reflux ratio method to give a constant value of xd, and the distillation is continued until the reflux ratio is essentially total reflux, the amount of holdup in the column at the end of the distillation can be easily calculated by using the y x line as the operating line. Such a procedure gives the composition of the liquid on each plate, and a correction can be applied for effect of, the holdup on the percentage yield of a given fraction. 

The state of the art for the batch distillation of multicomponent mixtures is even less satisfactory than for binary mixtures, and except for total reflux, no accurate and practical method is available even without liquid holdup in the column. This difficulty arises from the fact that, starting with a given liquid composition in the still, it is possible to calculate the equilibrium vapor leaving the still, but the composition of the overflow to the still from the first plate cannot be calculated without knowing the composition of the distillate leaving the system. In a binary system, it is possible to choose the composi-... 

As a guide to the characteristics of multicomponent batch distillation, the case of (1) total reflux with no liquid holdup in the column and (2) finite reflux ratio with no liquid holdup by an approximate method, will be considered. 

The design methods for batch distillation allowing for liquid holdup in the column are very unsatisfactory, and it is a field that should be actively studied in view of the importance of the operation. Improvements in the design calculations for multicomponent mixtures with no liquid holdup in the column are also needed. 

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