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Day 2 > Heat and Mass Transfer > Flashcards

Heat and Mass Transfer Flashcards

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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
Why is saturated steam used during evaporation?
Saturated Steam has higher heat transfer coefficient (U) compared to Superheated steam
26
Capacity of Evaporator
Amount of Vapor evaporated/removed per unit time
27
Economy of an evaporator
E = Mv/Ms
28
When does flash evaporation occurs?
When Tfeed > Tvapor
29
Enthalpy balance for evaporation
Mf•Hf + Ms•Hs = Mv•Hv + M•H
30
Heat balance for evaporation without BPE
Qevap = -Qsteam Qevap = Mf•Cpf•(T - Tf) + Mv•Hv Qsteam = Ms•Hs T=Tv=Tref since BPE=0
31
Crystal size in continuous crystallizer depends on a. rate of heat transfer b. degree of turbulence c. degree of supersaturation d. all
All of these
32
True or false. Heat waves travel in straight line and can be reflected by a mirror.
TRUE
33
Opaque surface towards radiation
Gases and Liquids
34
Black liquor from paper manufacturing is concentrated using a. Single effect b. Single effect with crystallizer c. Multiple effect d. Multiple effect with crystallizer
Multiple effect
35
Stanton number is proportional to
St = heat transfer coefficient/heat capacity = h/rho*v*Cp St proportional to f
36
TRUE OR FALSE. The fouling factor is dimensionless
FALSE Fouling factor accounts the additional resistance to heat flow caused by scaling. It decreases ∆Tlm
37
Steam ejector a. Superheats steam b. Removes condensate from steam pipelines d. Creates vacuum
Creates vacuum
38
For shell and tube, with increasing area, the purchased cost per unit area a. increases b. decreases c. stays constant d. passes through maxima
Passes through maxima
39
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
Relieve stress caused by thermal expansion
40
Leidenfrost point
Boiling of a liquid in a hot surface Phenomenon where liquid produces an insulating vapor that prevents the liquid from boiling rapidly
41
Which if the following is not used as medium for high temperature heating? a. Dowtherm b. Mercury c. Liquid Metal d. Fused Salts
Mercury It has the lowest heat capacity out of the four
42
With increase in temperature, the Col-burn jH factor a. increases b. decreases c. stays the same
increases see chapter 5 table 5-17
43
Liquid entrainment in evaporators is caused by a. high vacuum b. increased rate of evaporation c. foaming
Foaming
44
Purpose of baffles in a shell and tube heat exchanger
Increase the shell side heat transfer coefficient
45
Economy of multiple effect evaporator is not influenced by a. BPE b. Tf c. Q d. F/P
BPE
46
Equivalent diameter for pressure drop is ____ that of heat exchanger a. lower than b. higher than c. equal
lower
47
Lewis Number
Ratio of thermal diffusivity to mass diffusivity Use to characterize fluid flows where mass and heat transfers are simultaneously happening
48
Relationship of fanning friction factor, stanton number, and prandtl number
Chilton-Colburn analogy St = (f/2)(Pr)⅔
49
Dittus-Boelter equation is used to characterize ___ flow a. laminar b. turbulent c. transition
Turbulent Re>10000
50
Reynold's analogy and Prandtl's analogy are equal if a. Pr = 1 b. Pr = 1.5 c. Pr = 0.5 d. Pr = 2
Pr= 1
51
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
Constant flow rate Assumptions: Steady flow No phase change Constant Cps Negligible ∆KE and ∆PE U is constant
52
Heat flux increases with temperature drop beyond the Leiden frost point for a boiling liquid because a. convection becomes important b. conduction c. radiation
Radiation
53
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
film boiling
54
Solid angle subtended by finite surface at the radiating element
Angle of vision
55
Planck's distribution law is applicable for? a. black body b. white c. colored d. all bodies
colored
56
Relation of product and seed masses
Wp=Ws(Dp/Ds)³ C=Wp-Ws Dp=Ds+∆D
57
A dimensionless number that characterizes transient heat conduction
Fourier number
58
Baffles are added to heat exchangers to increase a. fouling factor b. heat transfer area c. heat transfer coefficient
Coefficient
59
Prandtl number for water ranges from a. 0.5 to 1 b. 5 to 10 c. 50 to 100 d. 500 to 1000
5 to 10
60
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
1/t / inversely proportional
61
During sensible heating, wet bulb temperature is _____ a. increased b. decreased c. not affected
increased
62
Difference between Tdb and Twb is called a. Dry bulb depression b. Wet bulb depression c. Dew point depression
Wet bulb depression
63
As relative humidity decreases, the dew point temperature will be _____ wet bulb temperature
lower than
64
Mass transfer rate between two fluids does not depend on
chemical properties of the two phases
65
Fenske equation determines the
minimum number of theoretical plates
66
Extractor in a system having very low density difference
Centrifugal extractor
67
Fenske equation holds good when
relative volatility is constant
68
When temperature and humidity of air is low, we use ____ draft cooling tower a. induced b. draft c. natural
natural
69
True or False. The selectivity of solvent extraction is unity at plait point.
True
70
What column is used for liquid dispersion in a continuous gas phase?
Packed
71
Number of stages is the ratio of
Ideal/Actual
72
Controlling factor in a drum drier a. Mass transfer b. Heat transfer c. Both
Heat transfer
73
Drying operation under vacuum
reduces drying temperature
74
Total reflux
Minimum number of stages
75
Minimum reflux
Number of stages is infinite
76
Humidity can be determined by measuring the ____ of the fiber
Electrical resistance
77
Mass, thermal, and momentum diffusivity if Pr and Sc are equal to
1
78
True or False. HETP increases with increasing absorption factor
TRUE
79
Dew point changes directly with a. temperature b. pressure
Pressure Decreasing pressuring will also decrease dew point
80
Operating line in absorption is always ______ the equilibrium curve a. Above b. Below c. On same level
Absorption - Above Stripping - Below
81
Which parameter can be controlled in a co-current absorber?
Flow rate of liquid
82
Absorption accompanied with rxn is exemplified by tye absorption of
SO2 in alkaline solution
83
Thiele-Geddes equation
Stages in multicomponent distillation
84
Increase in pressure, _____ in relative volatility
decrease
85
Relationship of K and D in film theory
K is directly proportional to D
86
Prandt number for Colburn equation
0.6 to 120
87
In forward feeding, pressure drop
will be least at the outlet of the last effect
88
Ratio of capacities of single and tripe effect if same heat transfer area and terminal conditions
1
89
K = Ko + aT unit of a
W/m K²
90
Film condensation is promoted on a
clean and smooth surface
91
Dietus Boelter equation is used for molten metal due to its low
Prandlt Number
92
Remove entrained liquid
Catchall
93
Multiple effect evaporators are used in
processing fats and paper
94
Best arrangement
Vertical HE with steam at the shell side
95
Dropwise condensation
contaminated cooling
96
higher BWG, higher
cross-sectional area
97
Film boiling occurs at
Very high pressure
98
Fins are
extended heat transfer surface
99
Exchanger for heavy heat loads
Series and parallel sets of shell and tube

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