Batch distillation operating procedure

Step 1 Start-up. In this step, the acetone and methanol mixture is placed in the bottom of the column together with some entrainer. Heat is added in the reboiler and vapor boil-up moves up the column. At the same time, entrainer water is fed continuously into the middle of the column at a flowrate (to be specified). The column is run under total reflux conditions until the acetone of the top product reaches its purity specification of 95 mol%. 

Step 2 Production of Acetone (PI). After the acetone teaches its purity specification in the top product, a continuous draw-off of this product into the acetone product tank is conducted with a constant reflux ratio pohcy (to be specified), fit this step, the continuous feeding of the entrainer remains until the end of this step when the acetone purity in the product tank can no longer meet its pmity specification. At the end of this step, the continuous feeding of the entrainer is stopped. 

Step 3 Slop-cut (SI) Collection. When the acetone purity in the product tank can no longer meet its purity specification, the product draw-off is diverted into another slop-cut tank until the time when the methanol of the top product reaches its purity specification of 92 mol%. Notice that, in this step, the draw-off of the product does not need to use the same reflux ratio as in Step 2. 

Step 5 Production of Water (P3) at the Column Bottoms. At the end of the Step 4, the water product in the column bottom typically has already reached its purity specification of 99 mol%. The column is shut-down and the bottom product is collected. In some cases another step of collecting another slop-cut (S2) from the top of the column may be required until the bottom water product reaches its purity specification. However, in the acetone-methanol case, when the purity of the product in the methanol product tank can no longer meet the purity specification, the bottom product has already reached its purity specification thus the step for collecting the slop-cut (S2) is omitted. 

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Sundaram, S., and L. B. Evans, Shortcut Procedure for Simulating Batch Distillation Operations, Ind. ScEng. Chem. Res. 32 (1993) 511-518. 

Instead of distilling off 50% of the solvent after the end of the batch, as required by the operating procedure, the operators distilled off only 15%. 

The graphical procedure is applicable to any binary batch distillation process and is not limited to operations at constant reflux ratio or constant distillate composition. 

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.. 

Of course, the above presentation of arithmetic methods is not exhaustive. Fohl [157] published numerical methods for ideal mixtures and batch as well as continuous operation at infinite and finite reflux ratios which make possible a rapid and relatively simple determination of the plate number. The contributions of Stage and Juilfs [71] should also be mentioned in which further accurate and approximate methods are summarized. The same applies to the book of Rose et al. [153]. Zuiderweg [158] reports a procedure which considers the operating hold-up (see chap. 4.10.5) and the magnitude of the transition fraction in batch distillation. 

Analysis of complex mixtures often requires separation and isolation of components, or classes of components. Examples in noninstrumental analysis include extraction, precipitation, and distillation. These procedures partition components between two phases based on differences in the components physical properties. In liquid-liquid extraction components are distributed between two immiscible liquids based on their similarity in polarity to the two liquids (i.e., like dissolves like ). In precipitation, the separation between solid and liquid phases depends on relative solubility in the liquid phase. In distillation the partition between the mixture liquid phase and its vapor (prior to recondensation of the separated vapor) is primarily governed by the relative vapor pressures of the components at different temperatures (i.e., differences in boiling points). When the relevant physical properties of the two components are very similar, their distribution between the phases at equilibrium will result in shght enrichment of each in one of the phases, rather than complete separation. To attain nearly complete separation the partition process must be repeated multiple times, and the partially separated fractions recombined and repartitioned multiple times in a carefully organized fashion. This is achieved in the laborious batch processes of countercurrent liquid—liquid extraction, fractional crystallization, and fractional distillation. The latter appears to operate continuously, as the vapors from a single equilibration chamber are drawn off and recondensed, but the equilibration in each of the chambers or plates of a fractional distillation tower represents a discrete equihbration at a characteristic temperature. 

Table 4.2 lists the equipment models (or procedures) and operations in each of the two simulators. Some of the models carry out simple material balances given specifications for the feed stream(s) and the batch (or vessel) size or batch time. Others, like the batch distillation... 

This chapter introduces how continuous distillation columns work and serves as the lead to a series of nine chapters on distillation. The basic calculation procedures for binary distillation are developed in Chapter 4. Multicomponent distillation is introduced in Chapter 5. detailed conputer calculation procedures for these systems are developed in Chapter 6. and sinplified shortcut methods are covered in Chapter 7. More complex distillation operations such as extractive and azeotropic distillation are the subject of Chapter 8. Chapter 9 switches to batch distillation, which is commonly used for smaller systems. Detailed design procedures for both staged and packed columns are discussed in Chapter 10. Finally, Chapter 11 looks at the economics of distillation and methods to save energy (and money) in distillation systems. 

The batch distillation column can also be operated with variable reflux ratio to keep x constant. The operating Eq. 19-261 is still valid. Now the slope will vary, but the intersection with the y=x line will be constant at Xp,. The McCabe-Thiele diagram for this case is shown in Figure 9-7. This diagram relates Xs to Xp). Since Xp, is kept constant, the calculation procedure is somewhat different. 

For Step 4 of the operating procedure, the value of the reflux ratio is determined by maximizing capacity factor (CAP) proposed by Luyben for regular batch distillation. The CAP is defined as ... 

The graphical procedure is applicable to any binary batch distillation process and is not limited to operations at constant reflux ratio or constant distillate composition. The result for a given number of stages is a tabular relationship between the distillate composition, the reboiler composition, and the LIV ratio ... 

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 procedure for processing a given batch charge of mixture m (operation m), can be viewed as a sequence of NTm distillation tasks to produce one or more main-cuts, possibly some intermediate off-cuts and a final bottom residue or product (Figure 7.1). For a ternary mixture this can be represented in the form of a STN shown in Figure 7.2. Each state s is characterised by a name (e.g. Dl), an amount Ss (e.g. SDi) and a composition vector xs (e.g. xD1). The molar fraction of an individual component j in state 5 is denoted by The sets of external feed states, main-cuts and off-cuts states in operation m are defined as EFm, MPm, and OPm, respectively. For example, Figure 7.2 shows operation 1 for a ternary mixture distillation with NTt=4 tasks, EFj= F0, MPt= Dl, D2, Bf] and OP/=[Rl, R2. Several feed states could occur, for example in the preparation of a mixed charge or... 

All of the previous discussion has considered distillation processes in terms of a constant feed of beer of uniform alcohol content. Such processes can be operated either as a continuous or as a batch procedure. 

The campaigns in the FCPI are typically operated on a train/stream approach where a solid key reagent is introduced and a crystalline product is obtained. The average characteristics of a production campaign are presented in Table 13.4. Unit operations such as reaction, distillation, liquid-liquid extraction and crystallization are fundamental procedures in a multi-purpose train. A conventional batch vessel is able to perform all these operations, allowing flexibility and versatility in the production. 

Remember we are using the same batch and the same column for this operation. Since in this case neither distillate composition nor reflux ratio is constant, the following procedure is used for integration of the Rayleigh equation. 

It should be noted that besides the extraction procedures described above, there are other operations like rectification (i.e. an additional distillation), fractional distillation (i.e. collecting the distillate in different batches), terpene removal (because some terpene and sesquiterpene obtained from certain plants are difficult-to-solve in ethanol and are easily oxidized and polymerized), decolourization, etc., that are needed in order to obtain good-quality extracts. 

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