Ench3mt Main Nov 2015 Section a Questions

SCHOOL OF ENGINEERING EXAMINATIONS: NOVEMBER 2015 COURSE AND CODE: MASS TRANSFER

ENCH3MT

DURATION: 3 HOURS TOTAL MARKS: 100

INTERNAL EXAMINER(S):

DR K OSMAN DR C NARASIGADU

INTERNAL MODERATOR(S):

EXTERNAL MODERATOR:

INSTRUCTIONS: 1.

ALL questions should be attempted.

2.

Two Answer Books are provided. Label them Books 1 and 2 respectively. Answer Section A in Book 1 and Section B in Book 2.

3

Any programmable or non-programmable calculator may be used provided it has been cleared of any information that would subvert the purpose of the examination.

4.

Calculations must be shown in sufficient detail to illustrate your understanding of the procedure.

5.

ENSURE THAT PAGE (5) IS DETACHED AND HANDED IN WITH BOOK 2

6.

This examination question paper, together with all associated diagrams, MUST BE HANDED IN TOGETHER WITH YOUR SCRIPT.

UNIVERSITY OF KWAZULU-NATAL, MAIN EXAMINATIONS: NOVEMBER 2015 COURSE AND CODE: MASS TRANSFER PAGE: 1 of 10

ENCH3MT

SECTION A [ANSWER IN BOOK 1] QUESTION 1 Thymol, a main constituent if thyme, is an important ingredient in men’s fragrances, disinfectants and oral hygiene applications. It can be produced industrially in an alkylation process using m-cresol. The product from the alkylation process contains 75 mol% thymol and 25 mol% unreacted m-cresol. M-Cresol is the more volatile product The mixture is to be distilled using a packed column to recover m-cresol at 95% purity in the distillate (by mole). The bottoms may contain 5% m-cresol (by mole). The distillation column is fitted with a total condenser and a partial reboiler. A reflux ratio of 1 and a boilup ratio of 1.7 are proposed. To simplify the problem, the relative volatility of m-cresol to thymol may be taken to be 6. Find Algebraically: a)

The equation of the rectifying section operating line, the feed stage operating line, and the stripping section operating line. (6)

b)

The number of theoretical equilibrium stages (NT) and optimum feed stage location. (9)

c)

The packed height using the HETP method and the information below:

Rectifying section Strippint section

HG/ft HL/ft 1.12 0.5 0.85 0.53

(15) Data and supporting material is provided on pages 6 – 10. TOTAL

/30/

UNIVERSITY OF KWAZULU-NATAL, MAIN EXAMINATIONS: NOVEMBER 2015 COURSE AND CODE: MASS TRANSFER PAGE: 2 of 10

ENCH3MT

QUESTION 2 A gaseous reactor effluent consisting of 2 mol% ethylene oxide in an inert gas is scrubbed with water at 30oC atm. The total gas feed rate is 1500 lbmol/hr and the water rate entering the scrubber is 2300 lbmol/hr. The column has a diameter of 2.4 ft and is to contain 1.5 inch metal Pall rings. Under the operating conditions for the scrubber, the K value of ethylene oxide is 0.77 and the estimated values of kya and kxa are 225 lbmol/hr.ft3-∆y and 180 lbmol/hr.ft3.∆x respectively. Calculate: a) Kya

(2)

b) HOG

(3)

c) NOG

(5)

d) lT

(2)

TOTAL

/12/

QUESTION 3 a) Explain the term “channelling” in packed columns and how to avoid it from occurring (4) b) Explain packed column operation in the “Pre-loading range” and why it is advisable to operate in this range. (4)

TOTAL

/8/

UNIVERSITY OF KWAZULU-NATAL, MAIN EXAMINATIONS: NOVEMBER 2015 COURSE AND CODE: MASS TRANSFER PAGE: 3 of 10

ENCH3MT

SUPPLEMENTARY MATERIAL AND DATA SHEET

McCabe-Thiele Analysis of Binary Distillation:

y

Equilibrium line:

Reflux Ratio: R 

αx 1  (α  1) x

L ; D

Operating Lines:

x

y α  (α  1) y

Boil-up Ratio: VB 

xD  R  top: y   x R 1  R 1

V B

V  1 x  x  B bottom: y   B VB  VB 

q-line:

q

H F,sat.vap.  H F,feed T ; H F,sat.vap.  H F,sat.liq

for partially vapourised feed (0