传质与分离过程考试试卷(英文)华工

诚信应考,考试作弊将带来严重后果！ 华南理工大学期末考试 《mass transfer and separation process 》试卷 A 注意事项：1. 考前请将密封线内填写清楚； 2. 所有答案请直接答在试卷上(或答题纸上)； 3．考试形式：开（闭）卷；

1. (40 points) Fill in the blanks or choose the correct answer (1) The diffusivities of gases are proportional to ( 1.5 ) power of absolute temperature and are ( inversely proportional to ) pressure of the system. (2) The relation between the flux and the driving force of mass transfer for the equimolar diffusion is ( )(21c c D N --=δ ) (3) Rising the pressure in an absorption process of gas, the Henry ’s constant m will ( decrease ), and the rate of absorption will ( increase ) (4)For a separation process in which relative volatility α =1, the compositions of component A would be ( the same ) in both phases, separation is ( not possible ) when this occurs. (5) At the azeotropic point,both the vapor and the liquid in equilibrium have ( the same ) composition. It is ( not possible ) to separate the azeotropic mixtures by using ordinary distillation. (6) The definition of ideal stage is: (the vapor stream and liquid stream leaving the stage are in equilibrium ) (7) The separations of binary mixtures by a distillation operation are based on differences in vapor- and liquid-phase compositions arising from the ( partial vaporization ) of a liquid mixture or the ( partial condensation ) of a vapor mixture. (8)The difference between the actual plate and the ideal plate is corrected by ( plate efficiency ) (9) In general, the liquid rate for the absorber should be ( D ) times the minimum

rate, too high a value requires ( A ) and the ( E ) of recovering, a small value results in ( F)

A) a large-diameter tower B) 1 C) 2.5 D) 1.1-2.0 E) cost F) a high tower

(10)“G as phase is controlling” means that the major resistance through both the gas and liquid phases is in the ( D), and the solute A in a gas phase is ( A )

A) very insoluble in the liquid B) very soluble in the liquid C) can’t make sure D) gas phase E) liquid phase F) the gas and liquid phases

(11) Flash distillation is used most for separating components that boil at ( very large ) different temperatures

(12) A single total condenser is used, the concentrations of the vapor from the top plate, of the reflux to the top plate, and of the overhead product are ( the same), replacing it by a partial condenser, then, the concentration of overhead is ( larger than) that of reflux.

(13) The stream relationships for the various thermal conditions of feed

?Cold feed ( C,D,G )

?Feed at bubble point ( B, E)

?Feed partly vapor (C,D,I )

?Feed at dew point ( F, A )

?Feed is a superheated vapor (C,D,H)

A) L=L′B)V=V′ C) L′=L+ q F D) V=V′+(1-q)F

E) L′=L+F F)V=V′+F G) q>1 H)q<0 I) 0

(14) When the rates of feed and of both the overhead and bottom products are ( 0), called total reflux and the slopes of both operating lines are ( 1 ). (15)Optimum reflux ratio is the point of ( most economical ), and ( not) much larger than the minimum reflux ratio.

(16)The approach of azeotropic distillation, choosing an entrainer to cause ( an new azeotropic mixture )

(17) A significant entrainment will ( reduce ) the plate efficiency.

(18) In the constant-rate drying period, the rate of evaporation is ( A ) of

the solid, and is ( B ) of the properties and flow type of air.

A) independent B) dependent C) uncertain

(19) The relationship among dry-bulb temperature t, wet-bulb temperature t w, dew-point t d of unsaturated moist air is( t> t w > t d)

(20) The equilibrium moisture content ( A ) with

the type of material for any given percent relative

humidity of air.

A) varies greatly B) is the same C ) varies

slightly

（21）Please read off the values of bound and unbound

moisture content from curve 2 on Fig, if the wool

sample contained 34 kg H 2O/100 kg dry solid,

( 27 ),( 7 )

Every one of the following problems is 15points

2. A mixture of 50 mole percent benzene and 50 percent toluene is subjected to flash distillation at a separator pressure of 1 atm. The incoming liquid is heated to a temperature that will cause 50 percent of the feed to flash. What are the compositions of the vapor and liquid leaving the flash chamber on assuming that the relative volatility of benzene-toluene system is 2.45 ?

solution:

y x 5.05.05.0+＝

x

x y 45.1145.2+=

x

x x 45.1145.21++＝ x x 245.112+＝

39.09.2565

.12245.1245

.1442=?±-=??+±-=x y=0.61

3. A distillation column is used for the separation of a binary mixture with relative volatility equal to 2 and desired to produce an overhead product containing mol 90% low boiler. The vapor flow rate to a given plate in the rectifying section is 150kmol/h with 60%mole fraction, and liquid flow rate to this plate is 100kmol/h. Murphree efficiency defined by vapor concentration is 0.5. Find:

(1) Operating line for the rectifying section, (2) What are the compositions of vapor and liquid phases leaving the plate?

(Assuming that the molar flow rates of vapor and liquid are constant in the column)

solution: (1)

321501001

==+=R R

V L

R=2 39.03211+=+++=

x R x x R R

y D (2) 6.039

.032

111＝+=+++=+n D

n n x R x x R R

y so the composition of liquid leaving the plate

45.02

9.036.0=-?=n x Murphree efficiency 6.06.0*1*1

--=--=++n n n n n n y y y y y y η=0.5

62.045.19.045

.0145

.0212*==+?=+=x x y n 5.06

.062.06

.0=--=n y η y n =0.61

4. A gas stream containing 3.0 percent A is passed through a packed column to remove 99 percent of the A by absorption in water. The absorber will operate at 1 atm, and the gas and liquid rates are to be 20 mol/h·m 2 and 100 mol/h·m 2, respectively. Mass-transfer coefficients and equilibrium data are given below:

Y ? = 3.1X

k Y a = 135 mol/h·m 3 ·unit mol fraction

k X a = 540 mol/h·m 3 ·unit mol fraction.

Find:

(a ) What is the minimum ratio of liquid and gas

(b ) N Oy , H Oy , and Z , assuming isothermal operation and neglecting changes in gas and liquid flow rates.

(c) What percent of the total resistance is in the gas phase?

Solution:

a) 031.031003

1=-=Y

()0031.099.0112=-=Y Y

121)(LX Y Y V =-

V L X Y Y =-121)

(

069.399.01.3)(1211

21m in =?==-=-=??? ??ηm Y Y Y m m

Y Y Y V L b)

()617

.9654.3632.262.38ln 632.262.099.01162.01ln 100201.3111ln 11Noy 21=?==??????+--?-=??????+??? ??--=L mV Y Y L mV L

mV

The relative gas-film resistance is ：

5. A dryer is used to dry material from a moisture content of 5% to 0.5% (wet basis). The capacity of dryer is 1.5 kg dry material/s. Hot air enters at 127℃ and a humidity of 0.007kg (water)/kg (dry air) and leaves at 82℃. The temperature of the material at the inlet and outlet is 21℃ and 66℃, respectively. The specific heat of dry material is

1.8kJ/(kg ?℃). If the heat loss of the dyer can be ignored, then calculate:

a) the amount of water removed from moist material;

b)the consumption of the wet air;

c) the humidity of the air leaving the dryer.

Solution:

a) The moisture content of the material (dry basis) is:

m Z m H a K a k m a k a

K Oy y x y y 53.261.9263.0263.0056.7620056.7601315.05401.3135111=?=====+=+=percent 56or 56.0056

.761

1351=

121250.0526950.50.0050399.5

() 1.5(0.05260.0503)0.0714/C X X W G X X kg s =

===∴=-=-=水

∴ s w a t e

r kg X X L W c /)(0714.0)00503.00526.0(5.1)(21=-=-= From the material balance, it can be obtained that: 0714.0)007.0(2=-H G

From the enthalpy balance, it can be obtained that:

)()(12

21I I L I I G c '-'=- where,

1112()(1.8 4.1870.0526)2142.49

(1.8 4.1870.00503)66120.2S W I C C X Q I '=+=+??='=+??=

1111(1.01 1.88)2490(1.01 1.880.007)12724900.007147.4

I H t H =++=+??+?= 222(1.01 1.88)82249082.82644I H H 2=+?H ?+=+ （2） ∴ 7.116)43.422.120(5.1)4.147(2=-=-I G

From the above, it can be obtained that: H 2=0.0178, G=6.61 kg wet air/s. s kg H G G /66.6)007.01(61.6)1(1=+=+='