Evaporation Flashcards
Evaporation
Process of producing a concentrated solution of a VOLATILE solvent and a NON-VOLATILE solute.
MULTIPLE EVAPORATOR Pres & Temp:
*Forward Feed
*Backward Feed
*BOTH Forward and Backward Feed
-Decreasing P = Decreasing T
Evaporation Heat Source
Steam
* Provide latent heat of vaporization
Evaporator Capacity
*Vapor evaporated
*amount of water vaporized in a certain period of time.
Steam Economy
*Vapor evaporated / Steam used
*Amount of vapor evaporated per amount of steam used.
Factor affecting Evaporation
- Feed Condition
*temperature, pressure, solubility, xf
*foaming, temperature sensitivity
*Vessel and Condenser Pressure
*Enthalpy of Solution
*Thick Liquor Temperature (T = Tvap + BPE)
EVAPORATOR TYPES:
Forward-Feed
*Fresh feed in first (hottest) effect
*Used for heat sensitive feed
EVAPORATOR TYPES:
Backward-Feed
*Fresh feed in last (coldest) effect
*Used for Viscous feed
*Higher capacity then forward
*Pump required
Relationship of Capacity and Economy
Higher capacity = Higher economy
BPE
*Boiling Point Elevation
*Function of concentration and species
*Duhring Rule
*Duhring Lines
*Duhring Rule
-Boiling point of solution a function of water boiling point and concentration
*Duhring Lines
-Graph used in Water vs NaOH
Factors affecting Evaporation
- Feed/Product Parameters
*Temperature
-higher T = Lower area
*Species
*Concentration - Steam Temp/ Pressure
- Evaporator Operating Pressure
*higher dT = lower area
*lower P = lower boiling point - Scaling / Fouling
For evaporation of viscous solution in a multiple effect evaporator, the preferred feeding scheme is ___
backward
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient ____
decreases
What is the steam economy in case of a single effect evaporator system ?
< 1
The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
_____
arrest the entrained liquid
In a multiple effect evaporator, the effect of boiling point elevation is to ___
reduce the capacity
Vacuum is generally maintained in the vapor space of an evaporator mainly to
_____
get economical temperature difference by using moderate pressure steam.
Double pipe heat exchangers are used
_____
when heat transfer area required is very low, i.e (100-200 ft2)
Economy of an evaporator is influenced by the ____
*feed temperature
*number of effect
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
agitated film
In a backward feed multiple effect evaporator _____
feed flows from low pressure to high pressure.
When to use Single Effect Evaporator?
*HIGH Construction Cost
*LOW Energy Cost
Driving force of Evaporation is___
Temperature difference of solution boiling pt. and Steam temperature
Capacity and Economy:
Single vs Multiple Effect Evaporator
*EQUAL CAPACITY
-For same surface area and terminal conditions
*MORE ECONOMY: Multiple Effect
Common Charts used in Evaporation calculations
*Duhring Lines
*Enthalpy Concentration
In a backward feed multiple effect evaporator ___
feed is introduced in the first effect.
feed flows from low pressure to high pressure.
no pumps are required between successive effects.
none of these.
feed flows from low pressure to high pressure.
Boiling point of a solution according to Duhring’s rule is a linear function of the __________ of water.
boiling point (at the same pressure)
viscosity
density
thermal conductivity
boiling point (at the same pressure)
Multiple effect evaporation is generally recommended, when the ___
large scale evaporation of liquor is needed.
corrosive liquids are to be concentrated.
fuel is cheaply available.
evaporation on small scale is to be done.
large scale evaporation of liquor is needed.
Boiling point elevation of a solution of NaOH ___
increases rapidly with temperature rise.
is almost independent of temperature.
is almost independent of pressure.
both (b) and (c).
both (b) and (c).
The advantage of backward feed multiple effect evaporators over forward feed units is that ___
heat sensitive material can be handled.
there is no additional cost of pumping.
most concentrated liquid is at highest temperature.
equal heat transfer co-efficients exist in various effects.
most concentrated liquid is at highest temperature.
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
super
sensible
latent
both(b) & (c)
latent
In evaporators, lowering the feed temperature ___
increases the heating area required.
reduces the economy.
both (a) and (b).
decreases the heating area required.
both (a) and (b).
Colligative properties
Properties dependent on amount of solute in solvent
