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Separation Processes > 10&11 > Flashcards

10&11 Flashcards

1
Q

may start as dilute solutions similar to water, but become more unique as they are concentrations

A

concentration

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2
Q

Liquid properties which affect evaporators

A
  • concentration
  • foaming
  • temperature sensitivity
  • dissolves solids or scale
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3
Q

some materials generate foam during vapourisation which can be entrained in vapour

A

foaming

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4
Q

fine chemicals, pharmaceuticals and food can be degraded at moderate temperatures. Vacuum operation is desirable

A

temperature sensitivity

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5
Q

some solutions can precipitate out dissolves solids, which forms deposits that affect the overall heat transfer coefficient

A

dissolves solids or scale

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6
Q

Liquid characteristics and properties

A
  • specific hear
  • heat of concentration
  • freeze point
  • gas liberation on boiling
  • toxicity
  • explosive hazards
  • sterile operation
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7
Q

modes of operation

A
  • most evaporators utilise the condensation of steam to supply the heat required
  • nearly always the material to be evaporated is inside the tubes
  • steam is introduced at low pressures
  • often the lqiuid to be evaporated is held under a moderate vacuum up to 0.05 atm to reduce the voiling temperature
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8
Q

Single evaporator is used the vapour produced is condensed and discarded.
This is simple but also wasteful

A

Single effect evaporator

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9
Q
  • Vapour produced in the first effect is passed to the steam chest of the next effect in line
  • this effectively reuses the heat of the original steam supplied to the 1st effect
  • in the case of a double effect evaporator the evaporation of liquid per mass of steam is approximately doubled
  • it is typical to use a series of effects to maximise the amount of liquid that can be evaporated per mass of steam used
A

Multiple effect evaporator

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10
Q

Heat balance equations

A

Q = mc delta T
Q = UA delta T
Q = π‘šΞ»

Q = Heat transferred
U = Overall coefficient of heat transfer
A = Area of which the heat transfer occurs
βˆ†T = Temperature difference between the two streams

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11
Q

effect which can affect the value of delta T used in 𝑄 = π‘ˆπ΄βˆ†π‘‡

A

Boiling point rise

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12
Q

empirical rule used to obtain BPR when it cannot be calculated from the physical data

A

DΓΌhring’s Rule

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13
Q

boiling point of a given solution is a linear function of the boiling point of pure water

A

DΓΌhring’s rule

These straight lines for different solution concentrations are known as DΓΌhring
Lines.

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14
Q

U

A

Overall coefficient of heat transfer

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15
Q

Heat transfer coefficient

A

1/π‘ˆ = 1/β„Žπ‘œ + 1/β„Žπ‘– + 1/β„Žπ‘€+ 1/β„Žπ‘ π‘œ + 1/β„Žπ‘ π‘–

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16
Q

Where steam film coefficients are

A

outside of the tube

17
Q

Where liquid side coefficients are

A

inside the tube

18
Q
  • characteristically high, even when condensation occurs
  • typical values 5-10 kW/m^2 K
A

steam film coefficients

19
Q
  • 1-2 kW/m^2 K for natural circulation
  • forced circulation is about double this
A

liquid side coefficients

20
Q
  • normally a fixed value
  • build up of scale on the inside and outside of the tube wall introduced additional resistances
A

wall coefficients

21
Q

Evaporator economy

A

(quantity of vapour produced/ quantity of steam consumed) * 100%

22
Q

Heat balances for single effect

A
  1. balance for the steam side
  2. balance for the liquid/vapour side
23
Q

Enthalpy balance on the steam side

A

𝑄𝑠 = 𝑆λ𝑠

Qs = Rate of heat transfer through
the surface from steam
S= Mass flow of Steam
Ξ»s = Latent heat of condensation
Ts = Temperature of steam

24
Q

Enthalpy balance on liquor side

A

𝑄 = 𝑄𝑓 + 𝑄𝑣
𝑄 = π‘Š. 𝑐𝑝𝑓 𝑇′ βˆ’ 𝑇𝑓 + 𝐷λ𝑣

Q = Rate of heat transfer from the heating
surface to the liquid
Tf = Temperature of Feed
T’ = Boiling temperature of liquor in evaporator
T = Boiling temperature of water vapour
W = Mass flow of Liquid Feed
D = Mass flow of Vapour Produced
cpf = Specific heat capacity of feed
Ξ»v = Latent heat of vapourisation
Ts = Temperature of steam

25
Q

General Principles of Multiple Effect Evaporators

A

𝑄1 = 𝑄2 = 𝑄3
π‘ˆ1𝐴1βˆ†π‘‡1= π‘ˆ2𝐴2βˆ†π‘‡2= π‘ˆ3𝐴3βˆ†π‘‡3
𝑄 = π‘ˆπ‘Žπ‘£ βˆ†π‘‡1 + βˆ†π‘‡2 + βˆ†π‘‡3 𝐴

26
Q

Simple mechanically with two pumps. Steam and feed enter the 1st evaporator

A

Forward feed

27
Q

Backward feed is most efficient arrangement, but requires a lot of pumps

A

Backward feed

28
Q

A compromise between the forward and backward feed systems

A

Mixed feed

29
Q

No transfer of liquid from one effect to another

A

Parallel feed

30
Q
A
31
Q

removal of water from the solution by boiling the liquor and withdrawing the vapour

A

evaporation

32
Q

Backward feed system shows

A
  • reduction in steam consumption
  • improved economy
  • reduction in condenser load
  • small reduction in heat transfer area

Separation Processes (6 decks)

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