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Day 2 > Heat and Mass Transfer > Flashcards

Heat and Mass Transfer Flashcards

1
Q

Two-phase mixture of mother liquor and crystal of all sizes

A

Magma

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2
Q

A growing crystal that maintains geometric similarity during growth

A

Invariant

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3
Q

Equilibrium in crystallization is reached when…

A

Solution is saturated

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4
Q

Number of new particles formed per unit time per unit volume of magma

A

Rate of nucleation

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5
Q

Ways of supersaturating the solution

A

1.Evaporation
2. Cooling
3. Vacuum cooling - adiabatic evaporation
4. Chemical reaction
5. Seeding - addition of third component to start crystallizatoon

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6
Q

Concentration difference between the supersaturated solution in which crystal is growing and that of a solution in equilibrium with the crystal

A

Supersaturation
∆y=y-ys

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7
Q

First step of crystal formation

A

Nucleation

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8
Q

Occurs in a bulk of fluid without solid-liquid interface

A

Homogeneous or Primary Nucleation

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9
Q

Occurs in the presence of other surfaces other than those of the crystals

A

Secondary or Heterogeneous Nucleation

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10
Q

Stages of Crystal Formation

A
  1. Cluster - loose aggregation
  2. Embryo - start of lattice arrangement
  3. Nucleus - unstable equipibrium
  4. Crystal
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11
Q

How to get solubility from mole fraction

A

x = S / (100+S)

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12
Q

Assumption in crystallization

A

Mother Liquor is always saturated thus its solute fraction can be obtained from the solubility at the given temperature

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13
Q

Flow preferred for crystallization

A

Countercurrent

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14
Q

Solute fraction for hydrate and non-hydrate

A

Hydrate: Xc= MWdry/MWtotal
NonHydrate: Xc = 1

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15
Q

Heat balance for crystallization

A
  • Q crystals = Q water

Q crystals = F Cpf (Tl - Tf) + VHv + CHc
Q water = m Cp (t2 - t1)

Cp of water = 4.184 J/g-K
Hv = Hsv - Hsl
Hc is always negative (Hl&raquo_space;>Hs)

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16
Q

Design Equation for crystallization

A

Q = UA ∆Tlm

Use ∆Tlm since temperature of cooling medium varies

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17
Q

Supersaturating mechanism used by Swenson-Walker Crystallizer

A

Cooling; V=0

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18
Q

Forced circulation evaporator with internal heater used for production of large and uniform crystals

A

Oslo Crystallizer

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19
Q

Assumption for Vacuum Crystallizer

A

Adiabatic; Q=0

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20
Q

Other term for evaporation

A

Water distillation

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21
Q

How does concentration of solution in evaporation affects overall heat transfer coefficient?

A

High Concentration
High Density
High Viscosity
High Boiling Point
Low Heat Transfer Coefficient

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22
Q

Design equation for evaporation

A

Q = U A ∆T

∆T = Tsteam - Tsolution

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23
Q

Why is vacuum pressure used in evaporator?

A

Low Operating Pressure
Low Boiling Point
Solvent will evaporate quicker
∆T will be larger

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24
Q

Solution properties that affect evaporation

A
  1. Concentration (needs to be low)
  2. Temperature sensitivity (food, pharma, biologicals can degrade fast at high T)
  3. Foaming/Frothing (caustic solutions, fatty acids can foam that causes heavy entrainment losses)
  4. Scale deposition (decreases U)
  5. Materials of construction
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25
Q

Why is saturated steam used during evaporation?

A

Saturated Steam has higher heat transfer coefficient (U) compared to Superheated steam

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26
Q

Capacity of Evaporator

A

Amount of Vapor evaporated/removed per unit time

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27
Q

Economy of an evaporator

A

E = Mv/Ms

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28
Q

When does flash evaporation occurs?

A

When Tfeed > Tvapor

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29
Q

Enthalpy balance for evaporation

A

Mf•Hf + Ms•Hs = Mv•Hv + M•H

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30
Q

Heat balance for evaporation without BPE

A

Qevap = -Qsteam

Qevap = Mf•Cpf•(T - Tf) + Mv•Hv
Qsteam = Ms•Hs

T=Tv=Tref since BPE=0

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31
Q

Crystal size in continuous crystallizer depends on

a. rate of heat transfer
b. degree of turbulence
c. degree of supersaturation
d. all

A

All of these

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32
Q

True or false. Heat waves travel in straight line and can be reflected by a mirror.

A

TRUE

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33
Q

Opaque surface towards radiation

A

Gases and Liquids

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34
Q

Black liquor from paper manufacturing is concentrated using

a. Single effect
b. Single effect with crystallizer
c. Multiple effect
d. Multiple effect with crystallizer

A

Multiple effect

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35
Q

Stanton number is proportional to

A

St = heat transfer coefficient/heat capacity = h/rhovCp

St proportional to f

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36
Q

TRUE OR FALSE. The fouling factor is dimensionless

A

FALSE

Fouling factor accounts the additional resistance to heat flow caused by scaling. It decreases ∆Tlm

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37
Q

Steam ejector

a. Superheats steam
b. Removes condensate from steam pipelines
d. Creates vacuum

A

Creates vacuum

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38
Q

For shell and tube, with increasing area, the purchased cost per unit area

a. increases
b. decreases
c. stays constant
d. passes through maxima

A

Passes through maxima

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39
Q

Floating head in an exchanger is used to

a. facilitate cleaning of exchanger
b. increase heat transfer area
c. relieve stress caused by expansion
d. increase log mean temperature

A

Relieve stress caused by thermal expansion

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40
Q

Leidenfrost point

A

Boiling of a liquid in a hot surface

Phenomenon where liquid produces an insulating vapor that prevents the liquid from boiling rapidly

41
Q

Which if the following is not used as medium for high temperature heating?

a. Dowtherm
b. Mercury
c. Liquid Metal
d. Fused Salts

A

Mercury

It has the lowest heat capacity out of the four

42
Q

With increase in temperature, the Col-burn jH factor

a. increases
b. decreases
c. stays the same

A

increases

see chapter 5 table 5-17

43
Q

Liquid entrainment in evaporators is caused by

a. high vacuum
b. increased rate of evaporation
c. foaming

44
Q

Purpose of baffles in a shell and tube heat exchanger

A

Increase the shell side heat transfer coefficient

45
Q

Economy of multiple effect evaporator is not influenced by

a. BPE
b. Tf
c. Q
d. F/P

46
Q

Equivalent diameter for pressure drop is ____ that of heat exchanger

a. lower than
b. higher than
c. equal

47
Q

Lewis Number

A

Ratio of thermal diffusivity to mass diffusivity

Use to characterize fluid flows where mass and heat transfers are simultaneously happening

48
Q

Relationship of fanning friction factor, stanton number, and prandtl number

A

Chilton-Colburn analogy

St = (f/2)(Pr)⅔

49
Q

Dittus-Boelter equation is used to characterize ___ flow

a. laminar
b. turbulent
c. transition

A

Turbulent

Re>10000

50
Q

Reynold’s analogy and Prandtl’s analogy are equal if

a. Pr = 1
b. Pr = 1.5
c. Pr = 0.5
d. Pr = 2

51
Q

Assumption in computing LMTD which gives the largest deviation in practice

a. Constant U
b. Constant flow rate
c. Constant Cp
d. No phase change

A

Constant flow rate

Assumptions:
Steady flow
No phase change
Constant Cps
Negligible ∆KE and ∆PE
U is constant

52
Q

Heat flux increases with temperature drop beyond the Leiden frost point for a boiling liquid because

a. convection becomes important
b. conduction
c. radiation

53
Q

When vaporization takes place through a blanketing film of gas, the phenomenon is termed as

a. pool boiling
b. nucleate boiling
c. transition boiling
d. film boiling

A

film boiling

54
Q

Solid angle subtended by finite surface at the radiating element

A

Angle of vision

55
Q

Planck’s distribution law is applicable for?

a. black body
b. white
c. colored
d. all bodies

56
Q

Relation of product and seed masses

A

Wp=Ws(Dp/Ds)³

C=Wp-Ws

Dp=Ds+∆D

57
Q

A dimensionless number that characterizes transient heat conduction

A

Fourier number

58
Q

Baffles are added to heat exchangers to increase

a. fouling factor
b. heat transfer area
c. heat transfer coefficient

A

Coefficient

59
Q

Prandtl number for water ranges from

a. 0.5 to 1
b. 5 to 10
c. 50 to 100
d. 500 to 1000

60
Q

Local surface conductance for laminar film condensation on vertical surface is

a. directly proportional to thickness
b. directly proportional to square of
c. inversely proportional to
d. inversely proportional to square root of

A

1/t / inversely proportional

61
Q

During sensible heating, wet bulb temperature is _____

a. increased
b. decreased
c. not affected

62
Q

Difference between Tdb and Twb is called

a. Dry bulb depression
b. Wet bulb depression
c. Dew point depression

A

Wet bulb depression

63
Q

As relative humidity decreases, the dew point temperature will be _____ wet bulb temperature

A

lower than

64
Q

Mass transfer rate between two fluids does not depend on

A

chemical properties of the two phases

65
Q

Fenske equation determines the

A

minimum number of theoretical plates

66
Q

Extractor in a system having very low density difference

A

Centrifugal extractor

67
Q

Fenske equation holds good when

A

relative volatility is constant

68
Q

When temperature and humidity of air is low, we use ____ draft cooling tower

a. induced
b. draft
c. natural

69
Q

True or False. The selectivity of solvent extraction is unity at plait point.

70
Q

What column is used for liquid dispersion in a continuous gas phase?

71
Q

Number of stages is the ratio of

A

Ideal/Actual

72
Q

Controlling factor in a drum drier

a. Mass transfer
b. Heat transfer
c. Both

A

Heat transfer

73
Q

Drying operation under vacuum

A

reduces drying temperature

74
Q

Total reflux

A

Minimum number of stages

75
Q

Minimum reflux

A

Number of stages is infinite

76
Q

Humidity can be determined by measuring the ____ of the fiber

A

Electrical resistance

77
Q

Mass, thermal, and momentum diffusivity if Pr and Sc are equal to

78
Q

True or False. HETP increases with increasing absorption factor

79
Q

Dew point changes directly with

a. temperature
b. pressure

A

Pressure

Decreasing pressuring will also decrease dew point

80
Q

Operating line in absorption is always ______ the equilibrium curve

a. Above
b. Below
c. On same level

A

Absorption - Above
Stripping - Below

81
Q

Which parameter can be controlled in a co-current absorber?

A

Flow rate of liquid

82
Q

Absorption accompanied with rxn is exemplified by tye absorption of

A

SO2 in alkaline solution

83
Q

Thiele-Geddes equation

A

Stages in multicomponent distillation

84
Q

Increase in pressure, _____ in relative volatility

85
Q

Relationship of K and D in film theory

A

K is directly proportional to D

86
Q

Prandt number for Colburn equation

A

0.6 to 120

87
Q

In forward feeding, pressure drop

A

will be least at the outlet of the last effect

88
Q

Ratio of capacities of single and tripe effect if same heat transfer area and terminal conditions

89
Q

K = Ko + aT unit of a

90
Q

Film condensation is promoted on a

A

clean and smooth surface

91
Q

Dietus Boelter equation is used for molten metal due to its low

A

Prandlt Number

92
Q

Remove entrained liquid

93
Q

Multiple effect evaporators are used in

A

processing fats and paper

94
Q

Best arrangement

A

Vertical HE with steam at the shell side

95
Q

Dropwise condensation

A

contaminated cooling

96
Q

higher BWG, higher

A

cross-sectional area

97
Q

Film boiling occurs at

A

Very high pressure

98
Q

Fins are

A

extended heat transfer surface

99
Q

Exchanger for heavy heat loads

A

Series and parallel sets of shell and tube

Day 2 (6 decks)

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