# 4.30: Untitled Page 85

## Chapter 4

Stream (1) is the feed to a distillation unit where the separation takes place according to the following specifications:

1. 98% of the benzene leaves with the distillate stream (stream #2).

2. 99% of the toluene in the feed leaves with the bottoms stream (stream #3) 3. 100% of the xylenes in the feed leaves with the bottoms stream (stream #3).

Assuming that the volumes of components are additive, and using the densities of pure components from Table I in the Appendix, compute the concentration and volumetric flow rate of the distillate (stream #2) and bottoms (stream #3) stream leaving the distillation unit.

4‐21. A standard practice in refineries is to use a holding tank in order to mix the light naphtha output of the refinery for quality control. During the first six hours of operation of the refinery, the stream feeding the holding tank at 200 kg/min had 30% by weight of n‐pentane, 40% by weight of n‐hexane, 30% by weight of n‐heptane. During the next 12 hours of operation the mass flow rate of the feed stream was 210 kg/min and the composition changed to 40% by weight of n‐pentane, 40% by weight of n‐hexane, and 20% by weight of n‐heptane.

Determine the following:

The average density of the feed streams

The concentration of the feed streams in mol/m3.

After 12 hours of operation, and assuming the tank was empty at the beginning, determine:

The volume of liquid in the tank in m3.

The concentration of the liquid in the tank, in mol/m3.

The partial density of the species in the tank.

4‐22. A distillation column is used to separate a mixture of methanol, ethanol, and isopropyl alcohol. The feed stream, with a mass flow rate of 300 kg/h, has the following composition:

Component

Species mass density

methanol

395.5 kg/m3

ethanol

197.3 kg/m3

isopropyl alcohol

196.5 kg/m3

Separation of this mixture of alcohol takes place according to the following specifications:

(a) 90% of the methanol in the feed leaves with the distillate stream

Multicomponent systems

152

(b) 5% of the ethanol in the feed leaves with the distillate stream (c) 3% of the isopropyl alcohol in the feed leaves with the distillate stream.

Assuming that the volumes of the components are additive, compute the concentration and volumetric flow rates of the distillate and bottom streams.

4‐23. A mixture of ethanol ( A) and water ( B) is separated in a distillation column.

The volumetric flow rate of the feed stream is 5 m3/h. The concentration of ethanol in the feed is

3

c  2 , 800 mol/m . The distillate leaves the column with a A

concentration of ethanol c  13 , 000

3

mol/m . The volumetric flow rate of

A

distillate is one cubic meter per hour. How much ethanol is lost through the bottoms of the column, in kilograms of ethanol per hour?

4‐24. A ternary mixture of benzene, ethylbenzene, and toluene is fed to a distillation column at a rate of

5

10 mol/h. The composition of the mixture in %

moles is: 74% benzene, 20% toluene, and 6% ethylbenzene. The distillate flows at a rate of

3

75  10 mol/h. The composition of the distillate in % moles is 97.33 %

benzene, 2% toluene, and the rest is ethylbenzene. Find the molar flow rate of the bottoms stream and the mass fractions of the three components in the distillate and bottoms stream.

4‐25. A complex mixture of aromatic compounds leaves a chemical reactor and is fed to a distillation column. The mass fractions and flow rates of distillate and bottoms streams are given in Table 4.25. Compute the molar flow rate and composition, in molar fractions, of the feed stream.

Table 4.25 Flow rate and composition of distillate and bottoms streams.

(kg/h)

ω

ω

ω

ω

Benzene

ωToluene

Benzaldehide

BenzoicAcid

MethylBenzoate

Distillate

125

0.1

0.85

0.03

0.0

0.02

Bottoms

76

0.0

0.05

0.12

0.8

0.03

4‐26. A hydrocarbon feedstock is available at a rate of

6

10 mol/h, and consists of

propane ( x  0 2

. ), n‐butane (

A

x

0 3

B

. ), n‐pentane (

0 2

C

x

. ) and n‐hexane

( x  0 . 3 ). The distillate contains all of the propane in the feed to the unit and D

80% of the pentane fed to the unit. The mole fraction of butane in the distillate is y  0 . 4 . The bottom stream contains all of the hexane fed to the unit. Calculate B

the distillate and bottoms streams flow rate and composition in terms of mole fractions.  153