evaporation - unimap portalportal.unimap.edu.my/portal/page/portal30/lecturer notes... · the...

EVAPORATIONType of evaporation equipment and

operation methods, calculation method for single effect and multiple effect

evaporators.

TYPES OF EVAPORATORSThe chief types of steam-heated tubular evaporators are:1.Long-tube vertical evaporator

a)

Upward flow (climbing-film)b)

Downward flow (falling-film)

c)

Forced circulation

2.Agitated-film evaporators

Prepared by, Dr. Nora JULLOK/UniMAP 2

Long‐tube evaporator with upward flow

Prepared by, Dr. Nora JULLOK/UniMAP

Fig. 16.1: Evaporators: (a) vertical climbing film, long-tube unit

1) A tubular exchanger with steam in the shell and liquid to be concentrated in the tubes

2) A separator or a vapor space for removing entrained liquid from the vapor.

3) When operated as a circulation unit, a return leg for the liquid from the separator to the bottom of the exchanger.

Essential parts:

3


Fig. 16.1: Evaporators: (b) forced-circulation unit with separate two-

pass horizontal heating element.

4

Falling‐film evaporatorsO

Used for heat-sensitive materials such as fruit juices and milk.

O

Liquidenters

topflows

downstream inside the

heated tube as a film leaves bottom

O

Vapor evolved from the liquid is usually carried downward with the liquid and leaves from the bottom.

O

These evaporators resemble long, vertical, tubular exchangers with a liquid-vapor separators at the bottom and a distributor fro the liquid at the top.


Example 16.1O

Condensed milk is produced by evaporation of milk in a falling-film evaporator containing stainless steel tubes 32 mm in diameter and 6 m long. Evaporation takes place at 60oC, which is the boiling point of milk at 2.7lbf

/in2.absolute, using steam at 70oC. The feed rate is 40 kg/h per tube at 60oC.

a)

Estimate the internal coefficient hi

and overall coefficient U.b)

What is the evaporation rate per tube?c)

If the raw milk has 13.5% fat plus solids, what is the concentration of the condensed milk?

d)

Calculate the average residence time in the evaporator.The properties of milk at 60oC are:


µ, cP Ρ, kg/m3 Κ, W/m.K λ, J/g

Raw milk25% solids

0.941.6

10101030

0.620.55

23572357

6

O

From the solution given, derive the following:i.

k = 0.662 W/m.K

ii.

µ

= 0.406 cPiii.

λ

= 2,331 J/g @ 70oC


Performance of tubular evaporator

O

The principal measures of the performance:i.

Capacity

= the number of kgs

of water vaporized

per hour.ii.

Economy

= the number of kgs

vaporized per kg of

steam fed to the unit.O

In a single-effect evaporator the economy is nearly always < 1

O

In multiple-effect equipment it way considerably greater.

O

Steam consumption (kg/h) = Capacity / economy.


Evaporator capacityO

The rate of heat transfer, q = product of three factors: Area of the heat-transfer surface A, the overall heat-transfer coefficient U, and the overall temperature drop, ∆T or

q = UA∆T

(16.1)


O

Flash evaporation:O

If feed is at a temperature above the boiling point in the vapor space, a portion of the feed evaporates spontaneously by adiabatic equilibrium with the vapor-space pressure and the capacity is greater than that corresponding to q.

O

Actual temperature drop across the heating surface depends on:1.

the solution being evaporated

2.

The difference in pressure between the steam chest and the vapor space above the boiling liquid

3.

Depth of liquid in the tubes, due to frictional loss in the tubes which increases the effective pressure of the liquid.


Boiling‐point elevation and Dühring’s

rule

O

The vapor pressure of aqueous solution is less than that of water at the same temperature.

O

Consequently, for a given pressure the boiling point of the solution is higher than that of pure water.

O

Boiling-point elevation (BPE): the increase of boiling point over that of water.

O

Dühring’s

rule: the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure.



Fig. 16.3: Dühring

lines, system sodium hydroxide-water (After McCabe)

12

Effect of liquid head and friction on temperature drop

O

When velocity of liquid is large, frictional loss in the tubes further increases the average pressure of the liquid.

O

Therefore, in any actual evaporator, the average boiling point of the liquid in the tubes is higher than the boiling point corresponding to the pressure in the vapor space.



Fig. 16.4: Temperature history of liquor in tubes and temperature drops in long-tube vertical evaporator

•

Relation between the temperature in an evaporator and the distance along the tube, measured from the bottom.

•

Applies to a long-tube vertical evaporator with upflow

of liquid.

•

The entering steam may be slightly superheated at Th

.

•

The superheated is quickly given up, an the steam drops to saturation temperature, Ts

.

•

Before the condensate leaves the steam space, it may be cooled slightly to temperature Tc

.

(Detail of the description can be found in the text book on page 495. )

14

Pressure changeO

When the velocity inside an evaporator tube is such that boiling starts inside the tube, the liquid in the nonboiling

section moves slowly and

pressure drop from friction is small.

O

In the boiling section, the mixture of vapor and liquid has a high velocity, and the friction loss is large.

O

Thus the pressure in the tube falls slowly in the lower part of the tubes and much more rapidly in the upper section, where the velocity is high.


Heat‐transfer coefficientsO

The heat flux and the evaporator capacity are affected by changes both in the temperature drop and in the overall heat-

transfer coefficient.

O

The overall coefficient is strongly influenced by the design ad method of operation of the evaporator.

O

The overall resistance to heat transfer between the steam and the boiling liquid = ΣThe steam-film resistance+2 scale resistance (inside and outside the tube)+ tube wall resistance +resistance from the boiling liquid.

O

The overall coefficient = 1/the overall resistance.


Heat‐transfer coefficients Steam‐film coefficients

O

The steam-film coefficient is characteristically high even when condensation is filmwise. Promoters are sometimes added to the steam to give dropwise

condensation and still higher coefficient.

O

Since the presence of noncondensable

gas seriously reduces the steam-film coefficient, provision must be made to vent noncondensables

fro the steam chest and to prevent leakage of air inward when the steam is at a pressure below atm.


Heat‐transfer coefficients Liquid‐side coefficients

O

The liquid-side coefficient depends to a large extent on the velocity of the liquor over the heated surface.

O

For falling-film evaporators the inside coefficient is about the same as that for film-type condensation on a vertical surface.

O

Most of the evaporation takes plave

at the liquid-vapor interface.

O

The film coefficient is greater than for purely laminar flow and can be estimated using Fig. 13.2.


Overall coefficient

Type Overall coefficient U

W/m2

.°C Btu/ft2

.h.°F

Long-tube vertical evaporatorsNatural circulationForced circulationAgitated-film evaporator, newtonian

liquid, viscosity1 cP1 P100P

1000-25002000-5000

20001500600

200-500400-1000

400300120


Table 16.1: Typical overall coefficients in evaporators

•

Due to the difficulty of measuring the high individual film-coefficients in an evaporator, experimental results are usually expressed in terms of overall coefficients.

19

Evaporator EconomyO

The main factor influencing the economy of an evaporator system is te

number of effects.

O

The economy also is influenced by the temperature of the feed.O

If the temperature:O

Below the boiling pointin

the first effect, the heating

load uses a part of the enthalpy of vaporization of the steam and only a fraction is left for evaporation.

O

above the boiling , the accompanying flash contributes some evaporation over and above that generated by the enthalpy of evaportaion

in the steam.

O

Quantitatively, evaporator economy is entirely a matter of enthalpy balances.


Enthalpy balances for single‐ effect evaporator


O

The latent heat of condensation of the steam is transferred through a heating surface to vaporize water from a boiling solution.

O

2 enthalpy balances are needed, 1 for the steam and one for the vapor or liquid side.

21


Fig. 16.5: Material and enthalpy balances in evaporator

Where q = rate of heat transfer from heating surface to liquid

Hv

= specific enthalpy of vaporHc

= specific enthalpy of thin liquorH = specific enthalpy of thick liquor

(16.2)

(16.3)

Combining Eq. 16.2 and 16.3 becomes

(16.4)22

Enthalpy balance with negligible heat of dilution

O

The heat-transfer rate q on liquor sides is

O

If the specific heat of the thin liquor is assumed constant over the temperature range from Tf

to T then,

O

Where cpf

= specific heat of thin liquor, λv

= latent heat of vaporization from thick liquor

O

If the boiling-point elevation of the thick liquor is negligible, λv

= λ


(16.5)

(16.6)

(16.7)

(16.8)

23

Enthalpy balance with appreciable heat of dilution; enthalpy‐

concentration diagramO

If the heat of dilution of the liquor being concentrated is too large to be neglected, an enthalpy-concentration diagram is used for the values of Hf

and H

in Eq. (16.4).O

Fig. 16.6 is an enthalpy-concentration diagram for solution of sodium hydroxide and water.



Fig. 16.6: Enthalpy-concentration diagram, system sodium hydroxide-water.

25

Single‐effect calculationsO

The use of material balances, enthalpy balances, and the capacity equation (16.1) in the design of single-effect evaporators is shown in Example 16.2.


Multiple‐effect evaporatorsO

Fig. 16.7 shows 3 long-tube natural-circulation evaporators connected to form a triple-effect system.


Fig. 16.7: Triple-effect evaporator: I, II, III, first, second and third effects; F1

, F2

, F3

feed or liquor control valves; S1

, steam valve, p1, p2

, p3

pressures; Ts

, T1

, T2

, T3

, temperatures.


(16.9)

(16.10)

(16.11)

The heating surface in the first effect will transmit per hour an amount of heat given by the equation

The heat transmitted in the second effect, however, is given by the equation

As has just been shown, q1

and q2

are nearly equal, and therefore

This same reasoning may be extended to show that, roughly

(16.12)


O

In ordinary practice the heating areas in all the effects of a multiple=effect evaporator are equal. This is to obtain economy of construction. Since q1

= q2

= q3

,

O

From this, it follows that the temperature drops in a multiple-effect evaporator are approximately inversely proportional to the heat-transfer coefficient.

(16.13)

Example 16.3O

A triple-effect evaporator is concentrating a liquid that has no appreciable elevation in boiling point. The temperature of steam to the first effect is 108oC, and the boiling point of the solution in the last effect is 52oC. The overall heat transfer coefficient, in W/m2.oC, are 2,500 in the first effect, 2,000 in the second effect. And 1,500 in the third effect. At what temperature will the liquid boil in the first and second effects?


Method of feedingO

Forward feed

O

Backward feedO

Mixed feed

O

Parallel feed


Capacity and economy of multiple‐effect evaporators


(16.14)

(16.15)

(16.16)


Fig. 16.8: Patterns of liquor flow in multiple-effect evaporators: (a) Forward feed; (b) backward feed; (c) mixed feed; (d) parallel feed (-----) Liquor stream (-----) Steam and vapor condensate streams.

33


Effect of liquid head and boiling-point elevation.

Fig. 16.9: Effect of boiling-point elevation on capacity of evaporators.

Optimum number of effectsO

The cost of each effect of an evaporator per square meter or square foot of surface is a function of its total area and decreases with area, approaching an asymptote for very large installation.

O

Thus the investment required for an N-effect evaporator is about N times that for single-effect evaporator of the same capacity.

O

The optimum number of effects must be found from an economic balance between the savings in steam obtained by multiple-effect operation and added investment required.


Multiple‐effect calculationsO

For a triple-effect evaporator, 7 equations may be written. i.

An enthalpy balance for each effectii.

A capacity equation for each effectiii.

The known total evaporationiv.

The difference between the thin-and thick liquor rateO

If the amount of heating surface in each effect is assumed to be the same, there are 7 unknowns in these equations:

1.

The rate of steam flow to the first effect2.

(2) to (4) the rate of flow from each effect5.

The boiling temperature in the first effect6.

The boiling temperature in the second effect7.

The heating surface per effect


O

Another method of calculation is as follow:1.

Assume values for the boiling temperatures in the first and second effects.

2.

From enthalpy balances find the rates of steam flow and of liquor from effect to effect.

3.

Calculate the heating surface needed in each effect from the capacity equations.

4.

If the heating areas so found are not nearly equal, estimate new values for the boiling temperatures and repeat items 2 and 3 until the heating surfaces are equal.

O

In practice these calculation are done by computer.


Example 16.4O

A triple-effect forced-circulation evaporator is to be fed with 60,000 lb/h (27,215 kg/h) of 10% caustic soda solution at a temperature 180oF (82.2oC). The concentrated liquor is to be 50% NaOH. Saturated steam at 50 lbf

/in2

(3.43mm) abs is to be used, and the condensing temperature of vapor from the third effect is 100oF (37.8oC). The feed order is II, III, I. Radiation and undercooling

of condensate may be neglected.

Estimated overall coefficients corrected for boiling-point elevationb

are given in Table 16.2. Calculate:

a)

The heating surface required in each effect, assuming equal surfaces in each

b)

The steam consumptionc)

The steam economy.


Tutorial 3: Due 22/10/15O

Problems:

O

16.1O

16.2

O

16.12


evaporation - unimap portalportal.unimap.edu.my/portal/page/portal30/lecturer notes... · the...

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