10&11 Flashcards
may start as dilute solutions similar to water, but become more unique as they are concentrations
concentration
Liquid properties which affect evaporators
- concentration
- foaming
- temperature sensitivity
- dissolves solids or scale
some materials generate foam during vapourisation which can be entrained in vapour
foaming
fine chemicals, pharmaceuticals and food can be degraded at moderate temperatures. Vacuum operation is desirable
temperature sensitivity
some solutions can precipitate out dissolves solids, which forms deposits that affect the overall heat transfer coefficient
dissolves solids or scale
Liquid characteristics and properties
- specific hear
- heat of concentration
- freeze point
- gas liberation on boiling
- toxicity
- explosive hazards
- sterile operation
modes of operation
- 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
Single evaporator is used the vapour produced is condensed and discarded.
This is simple but also wasteful
Single effect evaporator
- 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
Multiple effect evaporator
Heat balance equations
Q = mc delta T
Q = UA delta T
Q = πΞ»
Q = Heat transferred
U = Overall coefficient of heat transfer
A = Area of which the heat transfer occurs
βT = Temperature difference between the two streams
effect which can affect the value of delta T used in π = ππ΄βπ
Boiling point rise
empirical rule used to obtain BPR when it cannot be calculated from the physical data
DΓΌhringβs Rule
boiling point of a given solution is a linear function of the boiling point of pure water
DΓΌhringβs rule
These straight lines for different solution concentrations are known as DΓΌhring
Lines.
U
Overall coefficient of heat transfer
Heat transfer coefficient
1/π = 1/βπ + 1/βπ + 1/βπ€+ 1/βπ π + 1/βπ π
Where steam film coefficients are
outside of the tube
Where liquid side coefficients are
inside the tube
- characteristically high, even when condensation occurs
- typical values 5-10 kW/m^2 K
steam film coefficients
- 1-2 kW/m^2 K for natural circulation
- forced circulation is about double this
liquid side coefficients
- normally a fixed value
- build up of scale on the inside and outside of the tube wall introduced additional resistances
wall coefficients
Evaporator economy
(quantity of vapour produced/ quantity of steam consumed) * 100%
Heat balances for single effect
- balance for the steam side
- balance for the liquid/vapour side
Enthalpy balance on the steam side
ππ = πΞ»π
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
Enthalpy balance on liquor side
π = ππ + ππ£
π = π. πππ πβ² β ππ + π·Ξ»π£
Q = Rate of heat transfer from the heating
surface to the liquid
Tf = Temperature of Feed
Tβ = 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
General Principles of Multiple Effect Evaporators
π1 = π2 = π3
π1π΄1βπ1= π2π΄2βπ2= π3π΄3βπ3
π = πππ£ βπ1 + βπ2 + βπ3 π΄
Simple mechanically with two pumps. Steam and feed enter the 1st evaporator
Forward feed
Backward feed is most efficient arrangement, but requires a lot of pumps
Backward feed
A compromise between the forward and backward feed systems
Mixed feed
No transfer of liquid from one effect to another
Parallel feed
removal of water from the solution by boiling the liquor and withdrawing the vapour
evaporation
Backward feed system shows
- reduction in steam consumption
- improved economy
- reduction in condenser load
- small reduction in heat transfer area
