Batch distillation simple

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

The vapor is thea withdrawa from the stiH as distillate. The changing Hquid composition is most coavenieafly described by foUowiag the trajectory (or residue curve) of the overall composition of all the coexistiag Hquid phases. An exteasive amouat of valuable experimental data for the water—acetoae—chloroform mixture, including biaary and ternary LLE, VLE, and VLLE data, and both simple distillation and batch distillation residue curves are available (93,101). Experimentally determined simple distillation residue curves have also been reported for the heterogeneous system water—formic acid—1,2-dichloroethane (102). 

Batch distillation, which is the process of separating a specific quantity (the charge) of a liquid mixture into products, is used extensively in the laboratory and in small production units that may have to serve for many mixtures. When there are N components in the feed, one batch column will suffice where N — 1 simple continuous-distillatiou columns would be required. 

Differential Distillation Simple Batch, No Trays or Packing Binary Mixtures, No Reflux... 

Simple Batch Distillation Constant a, with Trays or Packing, Constant BoUup, and with Reflux [129] Using x-y Diagram... 

Differential Distillation—Simple Batch, Without Trays, Multicomponent Mixture... 

Batch Distillation, 45 Differential Distillation, 46 Simple Batch Distillation, 47 Fixed Number Theoretical Trays,... 

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-... 

The basic theory of batch distillation is given in Volume 2, Chapter 11 and in several other texts Hart (1997), Perry et al. (1997) and Walas (1990). In the simple theoretical analysis of batch distillation columns the liquid hold-up in the column is usually ignored. This hold-up 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). 

Early refiners utilized simple batch distillation to prepare kerosenes and lubricating oils. As the demand for these materials expanded and new crude oils were found, certain desirable and undesirable characteristics became apparent. Crude oils were selected from which products possessing desirable characteristics could be distilled—for example, oxidation stability, low smoke tendency, low carbon-forming tendency, small viscosity change with change in temperature (high viscosity index), light color, and attractive appearance were more likely to be found in petroleum of the paraffinic or Pennsylvania type. 

Figure 4.3. Batch Distillation using Simple Model...

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. 

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... 

For single separation duty, Bernot et al. (1991) presented a method to estimate batch sizes, operating times, utility loads, costs, etc. for multicomponent batch distillation. The approach is similar to that of Diwekar et al. (1989) in the sense that a simple short cut technique is used to avoid integration of a full column model. Their simple column model assumes negligible holdup and equimolal overflow. The authors design and, for a predefined reflux or reboil ratio, minimise the total annual cost to produce a number of product fractions of specified purity from a multicomponent mixture. 

In Greaves et al. (2001) and Greaves (2003), instead of using a rigorous model (as in the methodology described above), an actual pilot plant batch distillation column is used. The differences in predictions between the actual plant and the simple model (Type III and also in Mujtaba, 1997) are defined as the dynamic process-model mismatches. The mismatches are modelled using neural network techniques as described in earlier sections and are incorporated in the simple model to develop the hybrid model that represents the predictions of the actual column. 

Greaves et al. (2001) and Greaves (2003) presented two simple algorithms which is computationally less expensive to obtain the minimum batch time for a given separation task. These algorithms are the results of the application of some of the unique properties of batch distillation process to the general optimisation framework discussed earlier. The algorithm-1 is based on experiment and is presented in section 3.4 of Chapter 3. 

Simple to detailed mathematical models for conventional and unconventional batch distillation processes... 

Since residue curves do not cross simple batch distillation boundaries, the distillate and bottoms compositions must be in the same distillation region with the mass balance line intersecting a residue curve in two places. Mass balance lines for mixing and for other separations not involving vapor-liquid equilibria, such as extraction and decantation, are of course not limited by distillation boundaries. 

Manipulation of the operating conditions such as reflux ratio or pressure during a batch distillation can be useful. In addition, the feed to the batch distillation may vary during the process. A common application is to replace one solvent with another in the presence of a heavy nonvolatile product, as may be encountered in pharmaceutical production. One option for switching solvents is to use simple distillation repeatedly Initially a portion of the first solvent is removed by boiling. Then the second solvent is added, and a simple distillation removes more of the first solvent along with some of the second. Repetition of the latter step can be used to reduce the concentration of the first solvent to very small levels. 

Gentilcore [Chem. Eng. Progr, 98(1), 56 (Jan. 2002)] describes an alternative strategy of constant-level batch distillation where the replacement solvent is added at a rate to keep the volume of liquid in the pot constant. For simple distillation without rectification the analog of Eq. (13-126) is... 

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