HEMATRA

Question 1

NH3 and N2 are diffusing in counterdiffusion through a straight glass tube 2ft long with
inside diameter of 0.08ft at 298K and 101.32kPa. Both ends of the tube are connected to
large mixed chambers at 101.32kPa. The partial pressure of NH3 in one chamber is
constant at 20kPa and 6.67kPa in the other chamber. If the diffusivity at 298K and
101.32kPa is 2.3x10-5m2

/s, calculate the diffusion of NH3 in lbmol/hr.

a. 1.57x10-7
lbmol/hr
b. 5.17x10-7
lbmol/hr

c. 7.25x10-7
lbmol/hr
d. 7.52x10-7
lbmol/hr

Answer 1

d. 7.52x10-7
lbmol/hr

Question 2

Ammonium sulfate is to be crystallized from a solution containing
48% (NH4)2SO4 by cooling it in a counter flow crystallizer from 85 to
350C. During cooling the amount of water that evaporates is 5% of
the mass of the feed solution.
Data:
Feed rate = 1000 kg/h
Solubility of N(H4)2SO4 at 350C = 75 kg /100 kg of water
Specific heat of 48% (NH4)2SO4 at solution = 2.97 kJ/kg-K
Heat of crystallization of (NH4)2SO4 = 75.2 kJ/kg

41. Calculate the rate of formation of crystals.
    a. 127.5 kg/h c. 822.5 kg/h
    b. 678.3 kg/h d. 321.7 kh/h
42. What is the cooling water rate, if it is heated from 18 to 290C?
    a. 764 kg/h c. 810 kg/h
    b. 505 kg/h d. 427 kg/h
43. Determine the required cooling surface, if the overall heat transfer is
    125 W/m2
    -K.
    a. 3.13 m2 c. 1.95 m2
    b. 2.52 m2 d. 4.02 m2

Answer 2

a. 127.5 kg/h

c. 810 kg/h

b. 2.52 m2

Question 3

Wet solid is dried in a batch dryer under constant drying conditions
from 35% to 10% in 7 hours. The critical moisture content is 20% and
the equilibrium moisture is 4%. All moisture contents are on dry
basis. How long should it take to dry the same solid from 10% to 5%
moisture under the same drying conditions.
a. 6.5 hrs c. 5.1 hrs
b. 3.3 hrs d. 4.8 hrs

Answer 3

a. 6.5 hrs

Question 4

A solution with a negligible boiling point rise is being evaporated in a
triple effect evaporator using saturated steam at 121.1°C. The
pressure in the vapor of the last effect is 25.6 kPa abs. The heat
transfer coefficients are U1 = 2840, U2 = 1988, and U3 = 1420 W/m2
·K
and the areas are equal. Estimate the boiling point in each of the
evaporators.

22. T1
    a. 381.7 K c. 286.2 K
    b. 592.3 K d. 440.9 K
23. T2
    a. 289.1 K b. 363.8 K
    b. 501.5 K d. 411.1 K
24. T3
    a. 210.5 K c. 495.2 K
    b. 338.7 K d. 526.2 K

Answer 4

a. 381.7 K
b. 363.8 K
b. 338.7 K

Question 5

A single effect evaporator is to concentrate 20000 lb/hr of a 20%
solution NaOH to 50% solids. Saturated steam is available at 20 psig.
The absolute vapor pressure in the vapor space is to be 100 mmHg.
The overall coefficient is to be estimated to be 250 BTU/hr-ft2
-
0F. The

feed temperature is 1000F. Calculate the following:

13. The amount of steam consumed
    a. 15400 lb/h c. 12800 lb/h
    b. 13500 lb/h d. 19100 lb/hr
14. The steam economy
    a. 0.72 c. 0.78
    b. 0.88 d. 0.90
15. The heating surface required.
    a. 930 ft2 c. 810 ft2
    b. 670 ft2 d. 550 ft2

Answer 5

a. 15400 lb/h
c. 0.78
a. 930 ft2

Question 6

The vapor pressure of styrene is 100 mmHg at 820C and 200 mmHg

at 1000C. A gas that consists of 10 mol% styrene and 90 mol% non-
condensable is contained in a tank at 1000C and 1000 mmHg.

Calculate:
59. The dew point of the gas
a. 770C c. 820C
b. 1000C d. 960C

60. The relative saturation
    a. 20% c. 30%
    b. 40% d. 50%
61. The molal saturation
    a. 0.111 c. 0.087
    b. 0.206 d. 0.150
62. The percentage saturation
    a. 33.3% c. 44.4%
    b. 55.5% d. 66.6%

Answer 6

c. 820C
d. 50%
a. 0.111
c. 44.4%

Question 7

A gas mixture contains 0.0083 mol of water vapor per mol of dry
methane at a temperature of 270C and a total pressure of 200 kPa.

63. Percent relative saturation of the mixture
    a. 46% c. 58%
    b. 33% d. 60%
64. Percent saturation of the mixture
    a. 65% c. 46%
    b. 77% d. 50%
65. Temperature to which the mixture must be heated at 200 kPa in order
    that the relative saturation will be 0.20.
    a. 530C c. 370C
    b. 640C d. 420C

Answer 7

a. 46%
c. 46%
d. 420C

Question 8

The air supply for a dryer has a dry-bulb temperature of 700F and a
wet bulb temperature of 600F. It is heated to 2000F by coils and
blown into the dryer. In the dryer it cools along an adiabatic cooling
line and leaves the dryer fully saturated.

68. What is the dew point of the initial air?
    a. 620F c. 400F
    b. 350F d. 540F
69. What is its humidity?
    a. 0.0066 c. 0.0087
    b. 0.0090 d. 0.0052
70. What is its percentage relative humidity?
    a. 33% c. 55%
    b. 44% d. 66%
71. How much heat is needed to heat 100 ft3 to 2000F?
    a. 235 BTU c. 168 BTU
    b. 107 BTU d. 240 BTU
72. How much water will be evaporated per 100 ft3 of entering air?
    a. 0.262 lb c. 0.185 lb
    b. 0.333 lb d. 0.497 lb
73. At what temperature does the air leave the dryer?
    a. 860F c. 720F
    b. 930F d. 610F

Answer 8

d. 540F
c. 0.0087
c. 55%
a. 235 BTU
c. 0.185 lb
b. 930F

Question 9

Air in an amount 1000 cfm at 1500F, 20% RH is passed over a
refrigerated coil and thereby brought to 600F, 90% RH with the
condensed moisture withdrawn at 550F. The air is then reheated by
means of an electric heating coil to 1500F.

88. Compute the moisture removed in lb/min.
    a. 1.44 c. 3.67
    b. 2.08 d. 4.19
89. Compute the heat removed by the refrigerated coil, expressed as
    tons of refrigerated coil, expressed as tons of refrigeration.
    a. 11 c. 17
    b. 15 d. 13
90. Calculate the wattage of the heating coil required.
    a. 38 kW c. 25 kW
    b. 16 kW d. 44 kW

Answer 9

a. 1.44
b. 15
c. 25 kW

Question 10

A hallow metal sphere is heated so that the inside wall temperature
is 3000F. The sphere has an internal diameter of 6 inches and is 2
inches thick. What is the heat loss from the sphere if the outer surface
is maintained at 2120F? The thermal conductivity of the metal is 8
BTU-ft/hr-ft2
-0F?
a. 7650 BTU/hr c. 1802 BTU/hr
b. 4156 BTU/hr d. 5529 BTU/hr

Answer 10

d. 5529 BTU/hr

Question 11

A current of 250 amperes is passing through a stainless wire with
diameter of 5.08 mm. The wire is 2.44 m long and has an electrical
resistance of 0.0843 ohms. The outer surface is held constant at
154.6 0C. The thermal conductivity of the wire is 22.5 W/m-K. The
center temperature of the wire is
a. 435 K c. 256 K
b. 380 K d. 621 K

Question 12

A steel pipeline, 2 in-sch40, contains saturated steam at 121.1 0C.
The pipeline is insulated with 1 inch of asbestos (k=0.182 SI units).
Assuming that the inside surface temperature of the pipe wall is at
121.10C and the outer surface of the insulation is at 26.70C.

17. Calculate the heat loss (in Watts) for L=30.5 m.
    a. 5400 c. 4910
    b. 4375 d. 3862
18. How much steam in kg/hr is condensed due to the heat loss?
    a. 10.9 c. 8.13
    b. 8.81 b. 9.74

Answer 12

a. 5400
b. 8.81

Question 13

In an oil cooler, 216 kg/h of hot oil enters a thin metal pipe of diameter
25 mm. An equal mass of cooling water flows through the annular
space between the pipe and a larger concentric pipe; the oil and
water moving in opposite directions. The oil enters at 420 K and is to
be cooled to 320 K. If the water enters at 290 K, what length of pipe
will be required? Take coefficients of 1.6 kW/m2 K on the oil side and
3.6 kW/m2 K on the water side and 2 kJ/kg-K for the specific heat of
the oil.

a. 1.9 m c. 3.5 m
b. 4.8 m d. 2.7 m

Answer 13

d. 2.7 m

Question 14

It is desired to warm an oil of specific heat 2.0 kJ/kg K from 300 to
325 K by passing it through a tubular heat exchanger containing
metal tubes of inner diameter 10 mm. Along the outside of the tubes
flows water, inlet temperature 372 K, and outlet temperature 361 K.
The overall heat transfer coefficient from water to oil, based on the
inside area of the tubes, may be assumed constant at 230 W/m2 K,
and 0.075 kg/s of oil is to be passed through each tube. The oil is to
make two passes through the heater and the water makes one pass
along the outside of the tubes. Calculate the length of the tubes
required.

a. 3 m c. 4 m
b. 5 m d. 6 m

Answer 14

b. 5 m