FIGURE t?i?.i SEPARATION P1AHT RH RANGEHENT
cftse study ho. 12.
In order to counter the reduced recovery, the process was modified by adding- a new residue vapor-i-ziitg ("squeezing") column. The modification is shown as the dashed lines in Figure 12.12.1. Bottoms from the product column entered this new column, which stripped additional product from the residue under more severe conditions.
I Water ar.d ether volatile impurities could be tolerated in the overheads of this column, because the overheads stream was recycled back to the vacuum column.
The residue squeezing column was a small, simple column and required little investment. In addition to solving the problem, this modification also led to a higher overall plant product recovery than ever before, because it enabled additional stripping of product previously lost in the product column bottoms.
(Contributed by F. Wetherill, Santa Fe Braun Inc., Alhamfcra, Calif)
INSTALLATION A product column in a specialty chemical plant producing a heavy, water-soluble alcohol. The process is similar to that described in Case Study No. 12. The column separates product from high boiling residues. The column is shown as the heavy lines in Figure 12.13.1.
PROBLEM Although water was removed from the column feed, and water-forming reactions were supressed by lowering the base temperature in a manner similar to that which was described in Case Study No. 12, a very small quantity of water (about 0.1%) was still present in the product. It was economical to remove even th? amount of water from the product.
INVESTIGATION This amount of water was very small, and could have originated either in the column feed or from water-forming condensation reactions at the column base. Tackling this problem at the source would have been difficult.
It was realized that the product was very hygroscopic. Therefore, it was suspected that after the product is condensed and subcooled in the overhead condenser, it re-absorbs water from the inerts stream.
SOLUTION It appeared beneficial to withdraw the product upstream of the point where it was being subcooled. A suitable point was the top tray of the column. The column was modified to withdraw product from this tray, as shown by the dashed lines in Figure 12.13.1. This eliminated the water problem.
POST MORTEM The relative volatility for product-water separation was large (the atmospheric boiling point of the product was greater than 400°F). Any liquid water present in the reflux stream, therefore, easily vaporized on the top tray.
It may appear that withdrawing water from the top tray, instead of from the reflux drum, would have enriched the product with the heavier impurity, because the condenser stage was no longer available for the product-residues separation. This reasoning, however, is incorrect, because even before the modification, the condenser behaved as a total condenser from the product-residue separation viewpoint (product was withdrawn as liquid), and had therefore contributed little to the product-residue separation.
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