Evaporation

Question 1

Evaporation

Answer 1

Process of producing a concentrated solution of a VOLATILE solvent and a NON-VOLATILE solute.

Question 2

MULTIPLE EVAPORATOR Pres & Temp:
*Forward Feed
*Backward Feed

Answer 2

*BOTH Forward and Backward Feed
-Decreasing P = Decreasing T

Question 3

Evaporation Heat Source

Answer 3

Steam
* Provide latent heat of vaporization

Question 4

Evaporator Capacity

Answer 4

*Vapor evaporated

*amount of water vaporized in a certain period of time.

Question 5

Steam Economy

Answer 5

*Vapor evaporated / Steam used

*Amount of vapor evaporated per amount of steam used.

Question 6

Factor affecting Evaporation

Answer 6

1. Feed Condition
   *temperature, pressure, solubility, xf
   *foaming, temperature sensitivity
   *Vessel and Condenser Pressure
   *Enthalpy of Solution
   *Thick Liquor Temperature (T = Tvap + BPE)

Question 7

EVAPORATOR TYPES:
Forward-Feed

Answer 7

*Fresh feed in first (hottest) effect
*Used for heat sensitive feed

Question 8

EVAPORATOR TYPES:
Backward-Feed

Answer 8

*Fresh feed in last (coldest) effect
*Used for Viscous feed
*Higher capacity then forward
*Pump required

Question 9

Relationship of Capacity and Economy

Answer 9

Higher capacity = Higher economy

Question 10

BPE

Answer 10

*Boiling Point Elevation
*Function of concentration and species

Question 11

*Duhring Rule
*Duhring Lines

Answer 11

*Duhring Rule
-Boiling point of solution a function of water boiling point and concentration

*Duhring Lines
-Graph used in Water vs NaOH

Question 12

Factors affecting Evaporation

Answer 12

1. Feed/Product Parameters
   *Temperature
   -higher T = Lower area
   *Species
   *Concentration
2. Steam Temp/ Pressure
3. Evaporator Operating Pressure
   *higher dT = lower area
   *lower P = lower boiling point
4. Scaling / Fouling

Question 13

For evaporation of viscous solution in a multiple effect evaporator, the preferred feeding scheme is ___

Answer 13

backward

Question 14

In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient ____

Answer 14

decreases

Question 15

What is the steam economy in case of a single effect evaporator system ?

Answer 15

< 1

Question 16

The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
_____

Answer 16

arrest the entrained liquid

Question 17

In a multiple effect evaporator, the effect of boiling point elevation is to ___

Answer 17

reduce the capacity

Question 18

Vacuum is generally maintained in the vapor space of an evaporator mainly to
_____

Answer 18

get economical temperature difference by using moderate pressure steam.

Question 19

Double pipe heat exchangers are used
_____

Answer 19

when heat transfer area required is very low, i.e (100-200 ft2)

Question 20

Economy of an evaporator is influenced by the ____

Answer 20

*feed temperature
*number of effect

Question 21

Viscous & heat sensitive liquids are concentrated in __________ evaporators.

Answer 21

agitated film

Question 22

In a backward feed multiple effect evaporator _____

Answer 22

feed flows from low pressure to high pressure.

Question 23

When to use Single Effect Evaporator?

Answer 23

*HIGH Construction Cost
*LOW Energy Cost

Question 24

Driving force of Evaporation is___

Answer 24

Temperature difference of solution boiling pt. and Steam temperature

Question 25

Capacity and Economy:
Single vs Multiple Effect Evaporator

Answer 25

*EQUAL CAPACITY
-For same surface area and terminal conditions

*MORE ECONOMY: Multiple Effect

Question 26

Common Charts used in Evaporation calculations

Answer 26

*Duhring Lines
*Enthalpy Concentration

Question 27

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.

Answer 27

feed flows from low pressure to high pressure.

Question 28

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

Answer 28

boiling point (at the same pressure)

Question 29

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.

Answer 29

large scale evaporation of liquor is needed.

Question 30

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).

Answer 30

both (b) and (c).

Question 31

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.

Answer 31

most concentrated liquid is at highest temperature.

Question 32

Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.

super

sensible

latent

both(b) & (c)

Answer 32

latent

Question 33

In evaporators, lowering the feed temperature ___

increases the heating area required.

reduces the economy.

both (a) and (b).

decreases the heating area required.

Answer 33

both (a) and (b).

Question 34

Colligative properties

Answer 34

Properties dependent on amount of solute in solvent