Batch distillation overhead

Batch distillation (see Fig. 3) typically is used for small amounts of solvent wastes that are concentrated and consist of very volatile components that are easily separated from the nonvolatile fraction. Batch distillation is amenable to small quantities of spent solvents which allows these wastes to be recovered onsite. With batch distillation, the waste is placed in the unit and volatile components are vaporized by applying heat through a steam jacket or boiler. The vapor stream is collected overhead, cooled, and condensed. As the waste s more volatile, high vapor pressure components are driven off, the boiling point temperature of the remaining material increases. Less volatile components begin to vaporize and once their concentration in the overhead vapors becomes excessive, the batch process is terrninated. Alternatively, the process can be terrninated when the boiling point temperature reaches a certain level. The residual materials that are not vaporized are called still bottoms. 

The batch distillation of a binary is somewhat simplified, as L/V values can be assumed, and since there is only enrichment of the overhead involved, only one operating line is used per operating condition. Theoretical trays can be stepped off and xib values read to correspond. The plots involved are the same as previously described. 

Figure 8-39. Batch distillation with trays constant overhead product.

Mols of distillate or overhead product, lb mols/hr or batch distillation, mols Mols component, i, in distillate Vaporization efficiency of steam distillation Overall column efficiency Overall tray efficiency Eqg = Murphree point efficiency, fraction Murphree plate/tray efficiency, = E ... 

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-... 

Figure 14.9 Reflux ratio can be varied in batch distillation to maintain overhead product purity. (Reproduced from Smith R and...

Whilst most batch distillation operations involve the purification of a single-product overhead, it can also... 

Figure 13.10. Batch distillation McCabe-Thiele constructions and control modes, (a) Construction for constant overhead composition with continuously adjusted reflux rate, (b) Construction at constant reflux at a series of overhead compositions with an objective of specified average overhead composition, (c) Instrumentation for constant vaporization rate and constant overhead composition. For constant reflux rate, the temperature or composition controller is replaced by a flow controller.

Hence the reflux ratio, the amount of distillate, and the bottoms composition can be related to the fractional distillation time. This is done in Example 13.4, which studies batch distillations at constant overhead composition and also finds the suitable constant reflux ratio that enables meeting required overhead and residue specifications. Although the variable reflux operation is slightly more difficult to control, this example shows that it is substantially more efficient thermally—the average reflux ratio is much lower—than the other type of operation. 

Batch distillation (Fig. 16) is often preferable to continuous distillation when small quantities of feed material are processed. A liquid feed is charged to a still pot and heated until vaporization occurs. Vapor leaves the top of the column, and after condensation, part is removed as product and the rest returned to the column as reflux. As distillation proceeds, the contents of the still pot and the overhead product become richer in less volatile components. When operated at a fixed reflux ratio, an overhead product cut is collected until the product composition becomes unaccceptable. As an alternative, the reflux ratio can be gradually increased to hold the product composition constant as the cut is taken. For a fixed rate of heat addition to the still pot, the latter option results in a steadily declining product flow rate. After the first cut, subsequent... 

However, in batch distillation, as the overhead composition varies during the operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main products are usually determined by market or downstream process requirements but the amounts... 

In batch distillation, as the overhead composition varies during operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main-cuts are usually determined by the market or downstream process requirements but the amounts recovered must be selected based on the economic trade off between longer distillation times (hence productivity), reflux ratio levels (hence energy costs), product values, etc. Increasing the recovery of a particular species in a particular cut may have strong effects on the recovery of other species in subsequent cuts or, in fact, on the ability to achieve at all the required purity specifications in subsequent cuts. The profitable operation of such processes therefore requires consideration of the whole (multiperiod) operation. 

Comtant reflux, varying overhead composition. The reflux is set at a predetermined value at which it is maintained for the entire run. Since the pot liquid composition is changing, the instantaneous composition of the distillate also changes. The progress of the distillate and pot compositions in a particular binary separation is illustrated in Fig. 13-120. The variation of the distillate composition for a multicomponent batch distillation is shown in Fig. 13-121 (these distillate product cuts have relatively low purity). The shapes of the curves are functions of volatility, reflux ratio, and number of theoretical plates. The distillation is continued until the average distillate... 

Batch Distillation With Fractionation li-ays—Constant Overhead Product Composition, Multicomponent and Binary... 

The foregoing eqontions apply to simple takeover batch distillation, with no reflux. If a column is installed abuve the stillpot and provision is made to return a portion of the condensed overhead vapor to the column as reflux, it is possible to obtain a much better separation. As an extreme case, near-total reflux is used to get the best possible separation, without regard to the length of time required for die revalorization of the reflux. Uader such a condition, the Fenske relationship [Eq. (5.3-28)] may be ased to relate the siillpot vapor composition to that of the vapor passing over to the condenser ... 

An alternative method of running a batch distillation is to fix the reflux ratio and let the overhead product purity vary with time, stopping the distillation when the amount of product or the average concentration in the total product reaches... 

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