The feed to a two-column fractionating system is 18,500 lb/h of a mixture contai

Question

The feed to a two-column fractionating system is 18,500 lb/h of a mixture containing 52% benzene (B), 28% toluene (T), and 20% xylene (X) on a mass basis. The feed is introduced into column I and results in an overhead consisting of 95% benzene, 4% toluene, and 1% xylene on a mass basis. The bottoms from column I are fed to the second column, resulting in an overhead from column II containing 5% benzene, 92% toluene, and 3% xylene on a mass basis (see diagram below). Assume that 52% of the feed appears as overhead in the first column and that 48% of the benzene fed to the second column appears as overhead, calculate the composition and flow of the bottoms stream from the second column.

Explanation / Answer

Feed to fractionating column= 18500 lb/hr

The mixture contains 52% Benzene = 18500*52/100=9620 lb/hr, 28% toluene= 18500*28/100=5180 lb/hr and xylene= 18500*20/100=3700 lb/hr

52% of the feed to the column appears as overhead, hence overhead = 18500*52/100=9620 lb/hr

Overhead from the first distillation column= 9620 lb/hr

This contains 95% Benzene, Benzene=9620*0.95=9139, Toluene= 9620*4/100=384.8 lb/hr and Xylene= 9620*1/100= 96.2 lb/hr

The feed to the first colum splits into overhead and bottom and bottom contain =18500-9620=8880 lb/hr

Benzene in bottoms= 9620-9139=481 lb/hr and toluene= 5180-384.8 =4795.2 lb/hr and xylene= 3700-96.2=3603.8 ( this total amounts to 481+4795.2+3603.8= 8880 lb/hr)

48% of benzene fed to second column appears as overhead, Benznene in the overhead= 481*0.48 lb/hr=230.88 lb/hr

This corresponds to 5% and hence overhead from second column= 230.88*100/5=4617.6 lb/hr

Overhead also contains 92% Toluene, amount of toluene= 4617.6*0.92=4248.192 lb/hr and 3% Xylene= 4617.6*3/100=138.528 lb/hr

Feed to the second distillation colum also splits into overhead as well as bottoms.

Bottoms contains Benzene= 481-230.88 = 250.12 lb/hr , toluene= 4795.2-4248.182=547.008 lb/hr xylene= 3603.8-138.528=3645.272 lb/hr ( the total at the bottom= 250.12+547.008+3645.272= 4262.4 lb/hr). Bottoms from second distillation column matches with =8880-4617.6=4262.4 lb/hr

Composition of bottoms from second distillation column : Benzene : 100*250.12/ 4262.4=5.8685%, toluene= 100*547.008/4262.4=12.83% and xylene= 100*3645.272/4262.4=81.29%

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.