Slop cut

The eolunin is run on total reflux until the overhead distillate eomposition of the lightest component (component 1) reaches its specification purity. Then a distillate product, which is the lightest component, is withdrawn at some rate. Eventually the amount of eomponent 1 in the still pot gets very low and the Xdi purity of the distillate drops. There is a period of time when the distillate contains too little of component 1 to be used for that product and also too little of component 2 to be used for the next heavier product. Therefore a slop cut must be withdrawn until Xj)2 builds up to its specifieation. Then a seeond product is withdrawn. Thus multiple produets can be made from a single eolumn. 

Theoretical trays, equimolal overflow, and constant relative volatilities are assumed. The total amount of material charged to the column is M q (moles). This material ean be fresh feed with composition Zj or a mixture of fresh feed and the slop cuts. The composition in the still pot at the begiiming of the batch is Xgoj. The composition in the still pot at any point in time is Xgj. The instantaneous holdup in the still pot is Mg. Tray liquid holdup and reflux drum holdup are assumed constant. The vapor boilup rate is constant at V (moles per hour). The reflux drum, eolumn trays, and still pot are all initially filled with material of eomposition Xg j. 

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. 

C START SLOP CUT NO. 1 FLAGPl = 1 IF SLOP NO. 1 IS BEING REMOVED IF(XD1AV.LT.XD1SP)THEN FLAGPls . 

Three products (PI, P2, aod P3) and two slop cuts (SI and S2) are produced. The average composition of the products are 95 mole percent. The PI product is mostly the lightest component (component 1). The P2 product is mostly intermediate component (number 2) with some impurities of both the light and the heavy components. The final product P3 is what is left in the still pot and on the trays. The times to produce the various products and slop cuts are given in the results shown in Table 5.14. The total time for the batch distillation in this example is 6.4 hours. 

Note that the 70.92 motes of the first slop cut contain mostly light and intermediate component (25/75 mol %), while the 34.5 moles of second slop cut contain mostly intermediate and heavy components (52/48 mol %). Recycling these slop cuts back to the next batch cycle makes little thermodynamic sense, but that is the normal procedure in practice. 

The ternary mixture can be separated into three relatively pure fractions by the use of varying reflux to obtain high overhead concentrations, with intermediate slop cuts being taken during the transition between components. 

Estimate the required size of a batch still, with vapor rectification, to recover a dye intermediate from its coproduct and some low- and high-boiling impurities. It has been specified that 13,000 lb (5900 kg), consisting of fresh reactor product and recycled slop cuts, must be processed per batch. 

Recover (i.e., distill off) the dye intermediate while increasing the reflux ratio one or two times 6 h Distill off center (slop) cut while further increasing reflux ratio one or two times 6 h... 

If the coproduct must also be recovered at a reasonably high purity, then steps 5 through 8 should also be repeated for distilling off the slop cut (essentially a mixture of dye intermediate and coproduct) and for the coproduct cut (a mixture of coproduct and high-boiling residue). 

As can be seen, the total batch time is nearly 8 hr, the amounts of 99.9 mol% methanol and 99.98 mol% propylene glycol products are 29.88 Ibmol and 33.03 Ibmol, respectively. Note that after step 4 the methanol product accumulator contains 0.999 X 29.88 = 29.85 Ibmol methanol. The remainder, 33.33 - 29.85 = 3.48 Ibmol methanol is recovered initially in the water product accumulator during step 6. Hence, the water product accumulator contains a slop cut of water. Nearly all of the propylene glycol is recovered in the still. These results can be reproduced using the BATCHHiAC file, EXAM 12-3. bkp, on the multimedia CD-ROM. ... 

The RCMs and the equivolatility curves of this chemical system ean be seen in Figure 13.1, where the numbers in the equivolatility emwes denote the relative volatility of acetone versus methanol in the presence of water. The RCM indicates that any mixture of acetone and methanol, even premixed with water, will produce the acetone-methanol azeotrope at the top of the column. However, by continuously adding water (a heavy entrai-ner) into the column, it can be seen from the equivolatility curves that the acetone is becoming more and more volatile than the methanol in the extractive section. Acetone and methanol can then be separated in the extractive section if the number of trays in this section is sufficient. Acetone will go toward the top of the column while methanol will be carried with the water toward the column bottom. In the rectifying section, owing to the lack of methanol in this section, only the separation of acetone and water is performed. Pure acetone will preferably go to the top of the batch extractive distillation column. After the draw-off of the acetone product and a slop-cut period, where the acetone in the column is completely depleted, the methanol product can be collected at the top of the column. The heavy entrainer (water) can be collected at the column bottom. 

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. 

For Step 3 of the operating procedure, several different values of the reflux ratio have been simulated. The objective is to minimize the batch time until the end of this step because the slop-cut is considered to be waste material that is recycled in the next batch mn. Table 13.3... 

At time = 5.34 h, the purity in the PI product tank can no longer be maintained thus the draw-off is switched to the slop-cut (51) tank until time = 8.48 h. By the end of the slop-cut collection step, the methanol purity in the reflux drum has built up to the purity specification of 92 mol%. From time = 8.48 to 12.26 h, the methanol product is continuously drawn-off... 

From time = 0 to time = 0.96 h, the IPA purity in the reflux drum continuously builds up to the purity specification of 98 mol%. From time = 0.96 to 3.19 h, the IPA product is continuously drawn-off to the PI tank. At time = 3.19 h, the purity in the PI product tank can no longer be maintained, so the draw-off is switched to the slop-cut (51) tank until time = 4.68 h. At this time, the water purity in the reflux drum has built-up to the purity specification of 92 mol%. From time = 4.68 to 7.92 h, the water product is continuously drawn-off to the P2 tank. At time = 7.92 h, the purity in the P2 product tank can no longer be maintained. However, at the same time, the DMSO composition in column bottom has become very pure (99.5 mol%), so the batch mn can be stopped and the bottom product collected. 

Another factor limiting the amount of recoverable distillate is the holdup of liquid in the trays of the column. Before the next higher boiling product can be withdrawn as distillate, the mixture held on the trays must be removed as a slop cut. This material is collected in its own receiver and returned to the reboiler with a later batch of feed... 

It can be seen that multicomponent separations may be accommodated without difficulty in a batch still. A separate receiver is necessary for each product, and manual operations are required to change receivers and to readjust the set point of the temperature (or composition) controller. But with each additional product cut there is also a slop cut. Hence as the number of products increases, the percentage of the batch recovered as product diminishes. 

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