Fractionating Column Auxiliaries

Extractive distillation and azeotropic distillation share as a common characteristic, in that an auxiliary solvent is added to the crude aromatics fraction to achieve better separation by distillation. Extractive distillation takes place in the presence of an extractive material with high solvent power for aromatics, which has relatively low volatility compared with the compounds which are to be separated, and is constantly added at the top of the fractionation column. The purpose of the auxiliary solvent is to change the vapor pressures of the hydrocarbon components in such a way that they can be more easily separated by distillation e.g. the vapor pressure of benzene is lowered to the point when the accompanying non-aromatics can be distilled off as an overhead fraction. 

In addition to the fractionating column, there are certain auxiliaries necessary for its operation. These include the condenser, reboiler, pumps, instruments, valves, and receivers. 

Figure 5.16 Fractions of global-warming scores (% of total impact) for Auxiliary (R) and base used to recycle Auxiliary (R) at different stages of process development. Corresponding mass metrics can be found in Figure 5.10 (see column S ) The base is used as substrate (S) and the auxiliary material (Aux (R)) is used during the reaction.

Size-exclusion chromatography combined with RP-HPLC-MS was employed for the separation of pyranoanthocyanins from red wine. Wine samples (10 ml) were acidified with 3 M HC1 to pH 1 then sodium bisulphite was added at a concentration of 400 g/1. After 15 min reaction time the treated wine was loaded into a gel column (200 X 15 mm i.d.). Pigments were eluted with 95 per cent ethanol followed with 100 per cent methanol. The various fractions were acidified to pH 1, concentrated and redissolved in water. HPLC-DAD was carried out in an ODS column (150 X 4.6 mm i.d. particle size 5 /nn) at 35°C. Solvents were 0.1 per cent aqueous TFA (A) and ACN (B). The gradient started with 10 per cent B for 5 min to 15 per cent B for 15 min isocratic for 5 min to 18 per cent B for 5 min to 35 per cent B for 20 min. The flow rate was 0.5 ml/min and analytes were detected at 520 nm. MS conditions were sheath and auxiliary gas were a mixture of nitrogen and... 

Electrolyte container. 2 — Cells, J — Catholyte container, 4 — Anolyte (persulphnrlo acid) container, 5 — Lead distillation coil, 6 — Heating steam inlet, 7 — Tank tor steam condensate, S — Separator of liquid from vapour, 9 — Auxiliary distillation equipment, 10 — Main column for fractional condensation, 11 — Second column, 12 — Barometric oondenser, 13 — Vacuum pump, 14 — Acid dilution vessel. 13 — Acid purifier. 

The hydrogen peroxide and water vapour leaving the lead coil and tho auxiliary distillation column combine and are introduced into the main column for fractional condensation. This first column is scrubbed with a diluted solution of hydrogen peroxide (approx. I to 2 per cent H202) obtained in the second column. The final product which contains about 35 per cent b. w. HjOj is run off from the bottom of the column and collected in the receiving tourrils. 

The process scheme with partial flow reversal in column II is shown in Fig. 5. The solution of the mixture of salts, AX and DX, is used for displacement, the auxiliary ion having to be sorbed bener than the A ion over the range where the equivalent fraction of the D ion changes from 0 to Xq, the equivalent fraction of the D ion in the mbtture used for displacement. The ion exchanger, loaded with ions A and D, leaves column II for regeneration to CR and transfer to column I. The CX is removed from solution upon regeneration and the resultant solution of salts AX and DX is directed to column II. 

The measurements carried out with this apparatus confirmed the expectations. From the head vapor condenser, water, methanol, and ethanol formed a heavy polar phase to be discarded, while essentially all of the acetone and the acetaldehyde dimethyl acetal as well as some diacetyl ended up in the light hexane phase. From the latter, in an auxiliary distillation column, the hexane and the diacetyl were readily recovered as entrainer to replace the pure hexane, whereas acetone and acetaldehyde dimethyl acetal were separated as an impurity fraction to be discarded. 

The head stream (570 kg/h) containing 38 % by weight of furfural goes to an auxiliary column (not shown) where the furfural is recovered as the sump fraction. This sump fraction is the stream of 429 kg/h entering the decanter from the right-hand side. The head fraction of the auxiliary colunm is called raw solvent . Specified in chapter 16.6 (page 129), it contains the low boilers as well as the diacetyl and 2,3-pentanedione by-products. 

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