Distillation column holdup

The first step in CTO distillation is depitching. A relatively small distillation column is used as a pitch stripper. The vapor from the pitch stripper is fed directiy into the rosin column, where rosin and fatty acids are separated. Rosin is taken from the bottoms of the column and fatty acids as a sidestream near the top. Palmitic acid and light neutrals are removed in the rosin column as heads. The operation is designed to minimize holdup and product decomposition. Care is taken to prevent carryover of some of the heavier neutrals, such as the sterols, from the depitcher to the rosin column (24). 

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-... 

CONSTANT HOLDUP BINARY BATCH DISTILLATION COLUMN EQUILIBRIUM PLATE BEHAVIOUR... 

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

A 45-cm. spinning band column was employed by the submitter, but any distillation column with a low holdup may be used. Since the products have widely different boiling points, careful fractionation during distillation is not needed. Because the phenylsulfur trifluoride and benzenesulfinyl fluoride slowly attack glass, all equipment should be rinsed with water and acetone immediately after use to minimize etching. 

The Claisen distillation head was filled with glass wool to avoid formation of foam. The checkers found that constant heating of the distillation apparatus with a heat gun greatly facilitates the rate of distillation and minimizes the column holdup. 

The model of a multicomponent batch distillation column was derived in Sec. 3.13. For a simulation example, let us consider a ternary mixture. Three products will be produced and two slop cuts may also be produced. Constant relative volatility, equimolal overflow, constant tray holdup, and ideal trays are assumed. 

A. LIQUID HOLDUPS. The most common and most important trade-off is that of specifying holdup volumes in tanks, column bases, reflux drums, etc. From a steadystate standpoint, these volumes should be kept as small as possible because this will minimize capital investment. The more holdup that is needed in the base of a distillation column, the taller the column must be. In addition, if the material in the base of the column is heat-sensitive, it is very desirable to keep the holdup in the base as small as possible in order to reduce the time that the material is at the high base temperature. Large holdups also increase the potential pollution and safety risks if hazardous or toxic material is being handled. 

The older tall oil distillation columns used bubble cap trays. In new columns, structured packing is preferred. Because of the low pressure drop of structured packing, steam injection is no longer necessary. The low liquid holdup of this packing minimizes the reactions of the fatty and resin acids. A specific distillation sequence for vacuum columns using structured packing of Sulzer has been described (26). Depitching is carried out at a vacuum of... 

Operation of a batch distillation is an unsteady state process whose mathematical formulation is in terms of differential equations since the compositions in the still and of the holdups on individual trays change with time. This problem and methods of solution are treated at length in the literature, for instance, by Holland and Liapis (Computer Methods for Solving Dynamic Separation Problems, 1983, pp. 177-213). In the present section, a simplified analysis will be made of batch distillation of binary mixtures in columns with negligible holdup on the trays. Two principal modes of operating batch distillation columns may be employed ... 

Ellenberger J, Krishna R. Countercurrent operation of structured catalytically packed distillation columns pressure drop, holdup, and mixing. Chem Eng Sci 1999 54 1339-1345. 

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. 

An extensive literature survey indicates that the role of column holdup on the performance of batch distillation has been the subject of some controversy until recently. The following paragraphs outline briefly the investigations carried out on column holdup since 1950. Most of the investigations were restricted to conventional batch distillation columns and binary mixtures. The readers are directed to the original work to develop further understanding of the topic. 

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. 

For binary mixtures Converse and Huber (1965) found that in all cases studied, column holdup causes a decrease in the amount of maximum distillate obtained for a fixed time of operation. In another way, for a fixed amount of distillate and purity of the lighter component, higher column holdup increased the batch time. The authors concluded that the presence of significant holdup is bad anyway. 

The distillate accumulator, plate 1 (top) and the reboiler liquid composition (for benzene) profiles for case 3 are presented in Figures 3.20-3.22. Figure 3.23 presents the plate 1 liquid composition profile for case 4. Using these composition profiles Mujtaba and Macchietto (1998) made the following observations for better understanding of the role of column holdup. 

Note that Sundaram and Evans (1993a,b) used FUG method of continuous distillation directly and developed time explicit model, while Diwekar (1992) developed modified FUG method as described above and time implicit model for batch distillation. Sundaram and Evans used time as an independent variable of the model while Diwekar (1992) used reboiler composition as independent variable. Both models are based on zero column holdup and does not include plate-to-plate calculations. See the original references for further details. 

Seader and Henley (1998) considered the separation of a ternary mixture in a batch distillation column with B0 = 100 moles, xB0 = = <0.33, 0.33, 0.34> molefraction, relative volatility a= <2.0, 1.5, 1.0>, theoretical plates N = 3, reflux ratio R = 10 and vapour boilup ratio V = 110 kmol/hr. The column operation was simulated using the short-cut model of Sundaram and Evans (1993a). The results in terms of reboiler holdup (Bj), reboiler composition profile (xBI), accumulated distillate composition profile (xa), minimum number of plates (Nmin) and minimum... 

Converse and Huber (1965), Robinson (1970), Mayur and Jackson (1971), Luyben (1988) and Mujtaba (1997) used this model for simulation and optimisation of conventional batch distillation. Domenech and Enjalbert (1981) used similar model in their simulation study with the exception that they used temperature dependent phase equilibria instead of constant relative volatility. Christiansen et al. (1995) used this model (excluding column holdup) to study parametric sensitivity of ideal binary columns. 

Nad and Spiegel (1987) carried out experimentation in a conventional packed batch distillation column using a cyclohexane-heptane-toluene mixture. The column consists of 20 theoretical stages (equivalent) including the condenser and reboiler. The feed to the column was 2.93 kmol of which 1.9% was total column holdup and 1.2% was condenser holdup. The column underwent an initial total reflux operation for about 2.54 hr before any product was collected. After then the mixture was separated into 3 main-cuts with 2 off-cuts in between, leaving a final product in the reboiler. 

However, in batch distillation, the system is frequently very stiff, owing either to wide ranges in relative volatilities or large differences in tray and reboiler holdups. Therefore, if methods for non-stiff problems are applied to stiff problems (ODE models but having column holdup and/or energy balances), a very small integration step must be used to ensure that the solution remains stable (Meadow, 1963 Distefano, 1968 Boston et al., 1980 Holland and Liapis 1983, etc.). 

Rigorous and stiff batch distillation models considering mass and energy balances, column holdup and physical properties result in a coupled system of DAEs. Solution of such model equations without any reformulation was developed by Gear (1971) and Hindmarsh (1980) based on Backward Differentiation Formula (BDF). BDF methods are basically predictor-corrector methods. At each step a prediction is made of the differential variable at the next point in time. A correction procedure corrects the prediction. If the difference between the predicted and corrected states is less than the required local error, the step is accepted. Otherwise the step length is reduced and another attempt is made. The step length may also be increased if possible and the order of prediction is changed when this seems useful. 

Robinson (1970) considered an industrial 10-component batch distillation operation. The feed condition is shown in Table 5.3. The distillation column was currently producing the desired product using constant reflux ratio scheme. Table 5.4 summarises the results of the application of minimum time problem using simple model with and without column holdup. 

Referring to Simple Model (with zero column holdup) presented in section 5.5.1, a maximum distillate problem can be formulated as ... 

For single separation duty, Diwekar et al. (1989) considered the multiperiod optimisation problem and for each individual mixture selected the column size (number of plates) and the optimal amounts of each fraction by maximising a profit function, with a predefined conventional reflux policy. For multicomponent mixtures, both single and multiple product options were considered. The authors used a simple model with the assumptions of equimolal overflow, constant relative volatility and negligible column holdup, then applied an extended shortcut method commonly used for continuous distillation and based on the assumption that the batch distillation column can be considered as a continuous column with changing feed (see Type II model in Chapter 4). In other words, the bottom product of one time step forms the feed of the next time step. The pseudo-continuous distillation model thus obtained was then solved using a modified Fenske-Underwood-Gilliland method (see Type II model in Chapter 4) with no plate-to-plate calculations. The... 

The column initialisation is only required for the first inner loop optimisation problem (described in section 6.2). The liquid composition on the plates, condenser holdup tank and in the distillate accumulator (differential variables) at time t = 0 are set equal to the fresh charge composition (xB0) to the reboiler. The DAE model equations are solved at time t = 0 to provide a consistent initialisation of all the remaining variables. The final values of all these variables at the end of the distillation task in each inner loop problem are stored and used for column initialisation for the subsequent inner loop optimisation problems. At the beginning of each task, the distillate accumulator holdup is set/reset to zero. 

The batch distillation column consisted of 3 internal plates, reboiler and a total condenser. The reboiler was charged with a fresh feed of 5 kmol with Benzene molefraction 0.6. The total column holdup was 4 % of the charge. Half the holdup was in the condenser and the rest was distributed over the plates. The vapour load to the condenser was 3 kmol/hr. The required product purities were x oi = 0.90 and x B2 = 0.15. The solution of Equations 8.1-8.4 therefore gives DJ = 3.0 kmol and B2 = 2 kmol. This problem is same as case 3 shown in Table 8.1. Three reflux ratio (control) intervals were used to achieve (Dl, x Di) and one control interval to achieve (B2, x B2). 

The maximum conversion, the corresponding amount of product, optimal constant reflux ratio and heat load profiles for different batch times are shown in Figures 9.3-9.6. The maximum conversion profile achieved under total reflux operation (where no product is withdrawn) is also shown in Figure 9.3. The latter approximates the conversion which would be achieved in the absence of distillation. Note that if there is a large column holdup, the conversion under total reflux will not approximate the conversion achieved in the absence of distillation. 

Let us consider a CSTR/separator/recycle system, where the first-order reaction A —> P takes place. Figure 4.3(a) presents the conventional control of the plant. The fresh feed flow rate is kept constant at the value F0. The reactor holdup V is controlled by the effluent. The reaction takes place at a constant temperature, which is achieved by manipulating the utility streams. Dual-composition control of the distillation column ensures the purities of the recycle and product streams. 

The control structure discussed in this section is presented in Figure 4.4(a). The reactor-inlet flow rate is fixed at the value l. Reactor effluent controls the reactor holdup V, while the coolant flow rate controls the reactor temperature. Dual composition control is used for the distillation column. The reactant is fed on level control. For illustration purposes, a buffer vessel was considered. This increases the equipment cost and might be unacceptable due to safety or environmental concerns. An alternative is to feed the reactant in the condenser dram of the distillation column. This strategy achieves the regulation of reactant inventory, because any imbalance is reflected by a change of the holdup. 

Distillation columns have a large inventory in the reboiler, and typically, an inventory several times greater in the column itself. Column holdup may be reduced by using low holdup internals. Conventional trays and packings differ by a factor of about 10 in inventory per theoretical plate (Kletz, 1985a). An estimate of holdup per theoretical plate is shown (Lees, 1980). 

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