A binary mixture of butane and butadiene is to be separated in an extractive distillation using furfural. Furfural lowers the activity of butadiene more than it does for butane. Consider the distillation sequence shown in the figure. Match the component labels in Group I with the chemical species in Group II.
Group I
Group II
P. Component X
1. Butane
Q. Component Y
2. Butadiene
R. Component Z
3. Furfural
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First fix Z using boiling points (furfural, about 161 C, must always be the least volatile bottoms product); then use the fact that furfural suppresses butadiene's activity more than butane's to decide which C4 species leaves overhead of the extractive column.
Fresh feed enters Column 1, the extractive distillation column: solvent-rich liquid enters near the top and hydrocarbon feed enters lower down, so descending solvent preferentially retains one C4 species. Column 1 gives a nearly pure overhead product X. The bottoms of Column 1 (retained C4 plus furfural) feed Column 2, an ordinary solvent-recovery column, producing overhead product Y and bottoms product Z, recycled as lean solvent.
Step 2: Fix Z using boiling points.
Furfural boils at about 161C, while butane (about -0.5C) and butadiene (about -4.4C) are light C4 gases. Column 2 has no extractive effect, so the huge boiling-point gap means the C4 species leaves overhead as Y, and furfural, far less volatile, always leaves as bottoms Z. R maps to furfural (species 3), R-3.
Step 3: Fix X and Y using solvent-selectivity.
Furfural lowers the activity of butadiene the most, so butadiene is preferentially held back in the liquid phase with furfural, while butane's volatility is comparatively unaffected. In Column 1, butane is the species not retained, leaving as overhead product X, while butadiene travels down with the solvent-rich liquid and is recovered as Y from Column 2.
Step 4: Assign matches.
X = Butane, P-1. Y = Butadiene, Q-2. Z = Furfural, R-3.
