Effects of Column Holdup

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. 

The effects of column holdup can be easily correlated in terms of q and of the minimum batch time required to achieve a given separation task. 

Effects of Column Holdup When the holdup of liquid on the trays and in the condenser and reflux accumulator is not negligible compared with the holdup in the pot, the distillate composition at constant reflux ratio changes with time at a different rate than when the column holdup is negligible because of two separate effects. 

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. 

Several final points remain to be considered, Fir, t, all of the discussions above ignored the effect of liquid holdup in the column and condenser, but because the combined volumes of these two are quite small as compared to the volumes in the still and receiver (particularly because so many batch stills include packed sections rather than trays), this assumption does not seem unwarranted, particularly for binary mixtures. 

EXAMPLES To demonstrate the effect of the holdup specifications on the steady state solution of a batch distillation column at total reflux (a column operating at total reflux of type 2 D — 0, B = 0, F = 0), Examples 10-2 and 10-3 are presented in Table 10-6. The temperature profiles given in Table 10-7 were found by solving Examples 10-2 and 10-3 by use of the calcula-tional procedure described above.. 

Krishna, R., De Swart, J.W.A., EUenberger, J., Martina, G.B., andMaretto, C. (1997), Gas holdup in slurry bubble columns Effect of column diameter and slurry concentrations, AIChE Journal, 43(2) 311-316. 

Vatai and Tekic [44] 50, 100, 150, and 200 CMC solutions 0.8-1 0.001-0.0668 Effect of column diameter on holdup and mass transfer coefficient. 

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. 

Consider a simple dynamic system, the reactor/ column plant described in Table G.l, and assume that the column dynamics are fast compared to the reactor dynamics. Table G.3 indicates that the holdups in these two units are Hr = 2,400 lb-moles and Hr + 20 Hs + Hj) = 930 lb-moles. Because each unit has the same flow rate F, the mean residence times for the two units are in the ratio of 2,400/930, or approximately 2.5. The effect of chemical reaction normally is to make the reactor time constant somewhat smaller than its mean residence time (see Eq. 4-89) however, the portion of column holdup located directly in the recycle loop, that is, the reflux drum plus the stripping stages, is only about one-half the total column holdup. Thus, the actual ratio of the basic time constants for the two units is... 

The previous section showed no surprises regarding the effect of tray holdup. In this section we look at changing the number of reactive trays. Intuition would lead us to think that the more trays the better. This is certainly the case in conventional distillation. However, as we will see, this is not the case with a steady-state reactive distillation column for this type of reaction (two reactants, two products). 

It has been reported that for diameters less than 7.62 cm, the gas holdup depends on the column diameter, whereas it is independent of it for diameters greater than 10.2 cm (Hughmark, 1967 Saxena, 1991). The same has been found in studies of the Fisher-Tropsch synthesis in slurry bubble columns, where it has been reported that the effect of the column diameter is negligible when foam is not present in the system (Fox and Degen, 1990). 

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