Batch distillation with reflux

BATCH DISTILLATION WITH REFLUX. Batch distillation with only a simple still does not give a good separation unless the relative volatility is very high. In many cases, a rectifying column tvith reflux is used to improve the performance of the batch still. If the column is not too large, it may be mounted on top of the still, as shown in Fig. 17.1, or it may be supported independently, with connecting pipes for the vapor and liquid streams. 

Batch distillation with reflux in a multistage column... 

Figure 8.1.34. Batch distillation with reflux in a multiplate column, (a) Schematic of the device with total condensation, liquid reflwc and distillate product, (b) McCabe-Thiele diagram for constant reflux ratio with enriching section operating lines for three times, t = 0, t = tj, t = t2.

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

Sorensen (1999) has suggested a cyclic operation policy for batch distillation with repeated filling and dumping of the reflux drum. This configuration achieves the maximum attainable separation and requires minimal control. Furthermore, such a column can be operated very safely. 

Batch distillation with continuous control of distillate composition via the regulation of reflux ratio is illustrated in the simulation example BSTILL. In this an initial total reflux condition, required to establish the initial concentration profile with the column, is represented in the simulation by a high initial value of R, which then changes to the controller equation for conditions of distillate removal. 

In a single-stage batch distillation with no reflux (differential distillation) at 100 kPa, the still is charged with 100 kmol of an equimolar binary mixture of hexane and heptane. The A -values are given by In A" = A, - B,/T, where T is in degrees Kelvin. 

An initial charge of a 100 kmol mixture of 40 mol% benzene (1) and 60 mol% toluene (2) is distilled by differential batch distillation with no reflux. The process is stopped when 70% of the benzene is distilled. Find the amount and average composition of the distillate and the amount and composition of the residue. The relative volatility of benzene to toluene is ai,2 = 2.5. 

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 ... 

Binary Batch Distillation with Constant Reflux... 

The fermentation of com produces a mixture of 3.3 mole% ethyl alcohol in water. If 20 mole% of this mixture is distilled at 1 atm by a simple batch distillation, calculate and plot the instantaneous vapor composition as a function of mole percent of batch distilled. If reflux with three theoretical stages were used, what is the maximum purity of ethyl alcohol that could be produced by batch distillation ... 

A reflux splitter is difficult to incorporate in continuous distillation MB control schemes because it couples variations of the material balance stream (i.e., the distillate) with reflux rate changes. This is especially troublesome when reflux ratio is high, and a rise in distillate rate will effect a severalfold increase in reflux rate. The author experienced one case of poor continuous distillation control which was attributed to the above shortcoming. The author recommends restricting application of reflux splitters to batch distillation, where they work well. In continuous distillation, Sutro weir dividers are far more suitable. 

Fig. 5.2-40 Batch distillation with constant reflux (A) and constant product concentration (B)...

FIGURE S.5-11 Batch distillation with constant distillate composition and varying reflux (three theoretical stages). 

Figure 6.3.12A. Batch distillation with total condensation and no reflux.

Residue curves in batch distillation The illustrated forms of the Rayleigh equations (6.3.87) and (6.3.91) allow one to relate the change in the total number of moles in the residual liquid phase with the liquid-phase composition change in simple batch distillation with no reflux and total condensation. One would like to know how the liquid-phase composition changes with time. Visualization of this composition change pathway is going to be quite illustrative. For ternary systems, such visualizations are likely to be particularly useful. The topic of residue curve maps is briefly introduced here to that end. 

FIG. 13-98 Typical variation in distillate and reboiler compositions with amount distilled in binary batch distillation at a constant-reflux ratio. 

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

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 5.22. (e) Batch distillation, reflux flow cascaded with temperature to maintain constant top composition... 

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