CHAPTER
FIVE EVAPORATION
5.1
Climbing Film Evaporator
Cold hearted orb that rules the night, And steals the colours colo urs from our sight. Red is gray and yellow yello w white, But we decide which is right, rig ht, And which is an illusion. illusio n.
MOODY BLUES
Chapter 5: EVAPORATION
5.1 CLIMBING FILM EVAPORATOR
Keywords: Evaporation, thermal efficiency, upward-flow (climbing-film).
5.1.1 Object
The object of this experiment is to investigate the effect of the variations in the feed rate on the concentration of the product, the effect of the operating steam pressure on the rate of evaporation achieved, and to measure the thermal efficiency of the climbing film evaporator.
5.1.2 Theory
The vaporization of a liquid for the purpose of concentrating a solution consisting of a nonvolatile solute and volatile solvent is a common unit operation in chemical processing and is performed in many ways. Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution or a thick liquor. Evaporation differs from drying in that the residue is a liquid - sometimes a highly viscous one - rather than a solid; it differs from distillation in that the vapor is usually a single component, and even when the vapor is a mixture, no attempt is made in the evaporation step to separate the vapor into fractions; it differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals. The conditions under which evaporation is carried out in practice vary widely. The liquid to be evaporated may be less viscous than water, or it may be so viscous that it will hardly flow. It may deposit scale on the heating surface; it may precipitate in crystals; it may tend to foam; it may have a very high boiling point elevation; or it may be damaged by the application of too high temperatures. This wide variety of problems has led to considerable variation in the types of mechanical construction used. Evaporator types may be classified as follows: 1.
Short-tube evaporators
2.
Long-tube vertical evaporators - Forced-circulation - Upward-flow (climbing-film) - Downward-flow (falling-film)
3.
Agitated-film evaporators
Material and Heat Balances:
Figure 5.1.1 - Energy balance for single-effect evaporator.
Figure 5.1.1 is a highly simplified diagram of an evaporator, in which the heating surface is represented by a simple coil. Let F be the kilogram of the feed to the evaporator per hour, whose solute content is x f (x is the weight fraction). Let the enthalpy of the feed per kilogram be h f . There is taken out of the evaporator L kg of thick liquor, whose composition in weight fraction of solute is xL and whose enthalpy is h L in joule per kilogram. There is also V kg of vapor having a solute concentration of y and an enthalpy of H, J/kg. In most evaporators, the vapor is pure water, and therefore y is zero. The material balance equations for this case become:
Total Material Balance:
Solute Balance:
F = L+ V
Fx F
=
Lx L
+ Vy
(5.1.1)
(5.1.2)
In order to furnish the heat necessary for evaporation, S kg of steam is supplied to the heating surface with an enthalpy of Hs (J/kg) and there is taken out S kg of condensate with an enthalpy of hc (J/kg). One simplifying assumption usually made is that in an evaporator there is very little
Chapter 5: EVAPORATION
cooling of the condensate. This leads the assumption that the condensate will leave at the condensing temperature of the steam.
The heat balance equation is: Heat in feed + Heat in steam = Heat in thick liquor + Heat in vapor + Heat in condensate + Heat lost by radiation
(5.1.3)
Neglecting losses by radiation and using the relevant symbols we get the following equation:
Fh F
+ SH S =
VH + Lh L
+ Sh C
(5.1.4)
which represents the heat balance of the evaporator.
Since the available heat transfer area is fixed then for a given steam pressure and feed rate, the overall heat transfer coefficient is steady and the rate of evaporation is constant. Variation in the feed rate will change the sensible heat loading, the overall heat transfer coefficient and the percentage evaporation rate of which affect the product concentration.
Q = ( MCp∆T) + λm
(
Q = U heatýng A h ∆Tlmheatýng
A
where
=
Ae
) (U +
evap A e ∆Tlmevap
+ Ah
Q
:
total heat load, J
M
:
mass flow rate of feed, kg/s
m
:
evaporation rate, kg/s
cp
:
specific heat of feed, J/kg•K
∆T
:
temperature increase, K
∆Tlm:
log mean temperature difference, K
U
:
heat transfer coefficient, J/m •s•K
A
:
total fixed heat transfer area, m
λ
:
latent heat of evaporation, J/kg
2
2
(5.1.5)
)
(5.1.6)
(5.1.7)
For an increase in the feed rate then the sensible heat load is larger. Since there is little change in the overall heat transfer coefficient for heating and the
∆T
will be constant then A h must be
larger.
The heat transfer area A e must be correspondingly smaller and the evaporation rate will be less. Since the volume of the feed is higher, the percentage evaporation will be lower so that the product is less concentrated.
The rate of heat transferred Q is dependent upon the overall heat transfer coefficient U, the heat transfer surface area A, and the log mean temperature difference
∆Tlm,
that is:
Q = UA∆Tlm
(5.1.8)
For a given system, A will be constant, U is virtually constant so that the rate of heat transfer depends entirely upon the log mean temperature difference. Since varying the steam pressure varies the operating steam temperature, the rate of heat transfer and hence the rate of evaporation is changed.
Heat taken up by the feed is the sensible heat and latent heat.
Q = (mass) × (specific heat) × (temperature rise) + (evaporation) × (latent heat)
(5.1.9)
This experiment assumes as a basis that 1 kg of dry saturated steam condensing will evaporate 1 kg of boiling water. In practice heat is lost to the atmosphere and to the side streams purged from the system. The latent heat of the steam is used to preheat the feed into the evaporator and allowance must be made in the calculation for this. Water carried over by the steam is collected in the purge receiver and effectively lost from all dryness factor value (DF) is required for the steam supplied. This may in practice vary from 0.85 to 1.00.
steam used to preheat feed =
Evaporative efficiency =
mass of water in the feed×Sp heat of feed× temperatur e rise of water Latent heat of steam mass of water evaporated×100
(mass
)
of steam condensed − mass of preheat steam × DF
-
(5.1.10)
(5.1.11)
Chapter 5: EVAPORATION
5.1.3 Apparatus
The apparatus used in this experiment is shown in Figure 5.1.2.
Figure 5.1.2 - QVF climbing film evaporator.
1.) Feed inlet and drain valve
10.) Bottom receiving vessel
2.) Condensate outlet
11.) Vacuum and vent cock
3.) Stream inlet
12.) Top receiving vessel
4.) Calandria
13.) Vacuum cock
5.) Calandria vent
14.) Water inlet
6.) Cyclone
15.) Condenser
7.) Condensate receiver
16.) Water outlet
8.) Three-way cock
17.) Thermometer pocket
9.) Condensate drain
5.1.4 Experimental Procedure
Attention: During the experiment the floor gets wet, so wear suitable clothes. 1.
Turn the steam generator on.
2.
Prepare 30 dm of a 10 % w/w solution of glycerol in water.
3.
Close all drain cocks on the plant.
4.
Turn on the cooling water to the condenser, (No.15).
5.
Adjust steam pressure to a specified value.
6.
Check that the steam trap is functioning correctly.
7.
Open the feed inlet cock, (No.1), and control the feed rate by the rotameter.
8.
Allow the liquid level in the calandria to reach the steam inlet neck before finally adjusting
3
the feed rate. The liquid will begin to boil and expanding bubbles will rise rapidly in the tube giving climbing film operation. 9.
Adjust the feed rate to a value and allow a minimum of 5 minutes of operation under these conditions so that the unit can settle down, i.e., come to steady state.
10. Once the unit is operating smoothly close the valve between the condensate receivers and note the liquid level in the graduated concentrate receiver. For a minimum of 15 minutes operation ensure that the steam pressure, feed rate, and vacuum are constant. 11. At the end of the test period close the valve between the twin condensate receivers and note the level in the graduated concentrate receiver. 12. Measure the volume of the condensate collected, calculate the volume of the feed to the unit and the volume of the concentrate produced. 13. Repeat the experiment with two other feed rates. 14. Repeat the experiment keeping the flow rate constant and varying the pressure using two other different pressures. 15. In one of these five data calculate (find) the amount of steam used to evaporate the feed by placing the steam pipe into a specified amount of cold water. 16. Shut the unit down following the procedure described below:
IMPORTANT: •
Close the feedcock.
•
Close the steam control valve and all other steam valves on the path between the steam generator and control table.
•
Close the steam generator.
Chapter 5: EVAPORATION
•
Stop the pump.
•
Close the condensate cooling water valve.
•
Open each of the drain cocks in turn and drain contents to a suitable receiver.
•
Leave all the drain cocks open.
5.1.5 Report Objectives
1.
State the assumptions used in your calculations.
2.
Why is glycerol used?
3.
What are the properties of a climbing film evaporator?
4.
Calculate the percent concentration of the concentrate.
5.
Calculate the heat transferred in producing the product concentrate and condensate.
6.
Calculate the percent of feed evaporated.
7.
Calculate the total heat required to heat the concentrate water and heat required to evaporate the condensate. Compare this value with the heat available from the steam, which was condensed and collected from the steam trap.
8.
State the factors that affect the evaporative efficiency.
9.
What is the enthalpy difference between 1 kg of steam having a DF of 0.95 and 1 kg of steam having a DF of 0.85?
10. Comment on the effects of vacuum operation on evaporative efficiency. 11. Calculate the evaporative efficiency.
5.1.6 References
1.
Badger, L. W., and J. T. Banchero, Introduction to Chemical Engineering, McGraw-Hill, New York, 1955.
2.
Coulson, J. M., and J. F. Richardson, Chemical Engineering Unit Operations, Pergamon Press, 1962.
3.
Perry, R. H., and D. Green, Perry’s Chemical Engineers’ Handbook , 6th edition, McGrawHill, 1988.
