1 Evaporation

Evaporation

McCabe Ch. 16, Seader Ch.17

1 Dr. Hatem Alsyouri

IMPORTANT NOTES

Review the use of steam tables

(always bring them,, to the class)

Review the concepts of enthalpy, latent and sensible

heats, and heat transfer coefficient

Pay attention to units

Familiarize your self with INTERPOLATION

Always bring the steam tables to class (in different units).

2

3

Saturated Steam Table

Superheated or Sub-cooled Steam

T1 SH TSat T2 SC

superheated saturated Sub-cooled

4

Latent heat

sensible heat sensible heat

Evaporation

Evaporation is a unit operations used to increase

concentrations of process solutions.

Examples: beverages, solvents, (in)organic salts

This is accomplished by evaporation of the

solvent in an evaporators.

Evaporator design involves determination of:

flow rates of products (vapor and thick

solution)

Amount and conditions of heating steam

area of heat transfer needed

Type of evaporator

Evaporation normally accompanies other

operations like crystallization.

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(dilute solution)

Vapor

Concentrated

solution

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7

Continuous-flow, steady state Evaporator

Model

Assumptions:

1. One volatile component in feed

2. Steam is saturated (latent heat

vaporizes the solution)

3. Boiling creates mixing

• Tv= Te

• Tp = Te

4. Tv = Tp = Te corresponding to

evaporator pressure P

5. Overall driving force for heat

transfer is T = Ts Tp

6. No heat loss

8

Mass and Energy Balance

vpf mmm

vvppff mwmwmw

Total and Solute mass balance

Energy balance on Solution

vvppff HmHmHmQ

vap

s HmQ Latent heat for

steam ()

)( ps TTAUQ

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Enthalpy of feed/product solutions

1) From chart

2) Heat capacity values are provided.

A reference temperature (datum) is needed. You can assume 0C, the reference

temp of steam table, or any suitable reference.

3) Assume water

Use steam tables

vvppsff HmHmQHm

vvrefpppsreffff HmTTCpmQTTCpm )()(

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Enthalpy of (NaOH-Water)

Solution

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Boiling Point Elevation (BPE) or

Boiling Point Rise (BPR)

Solutions have higher boiling points than pure water.

The increase of boiling point over the pure water is

called Boiling Point Elevation (BPE).

BPE is high for concentrated solutions.

BPE is calculated from empirical (experimental)

relations like Duhring rule (e.g., charts are available

for NaOH aqueous solution)

Large liquid head also causes BPE.

Neglecting the impact of BPE can yield wrong

design of evaporator.

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Calculation of

Boiling Point

Elevation (BPE)

Example:

35 wt% NaOH solution at 6 psia

From Steam Table at 6 psia

Boiling point of pure water

(Tw) = 170 F

Duhring chart

Tw and 35%

Boiling point of solution

(Tsol) = 210 F

BPE = 210 170 = 40 F

207 F

170 F

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Exercise (1)

S&H 17-39

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Heat Input Utilization

onvaporizatifeedsteam QQQ

vvfvffss mTTCpmm )(

steam

vapor

m

mEconomy

vvrefpppsreffff HmTTCpmQTTCpm )()(

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. . . (1)

. . . (2)

. . . (3)

Equation (3) is equivalent to the general energy balance equation

on the model evaporator:

. . . (4)

A solution of organic colloids in water is to be concentrated from 8 to 45% solids in a

single-effect evaporator. Steam is available at 1.03 atm gauge (120.5C). A pressure of

102 mmHg absolute is to be maintained in the vapor space. The feed rate to the

evaporator is 20,000 kg/h. Overall heat transfer coefficient (U) is 2800 W/m2.C. The

solution has a negligible BPE and negligible heat of dilution.

Calculate:

(1) the steam consumption (ms= 17782 kg/h)

(2) the economy (0.925)

(3) heating surface area required (56.4 m2)

Conditions: The temperature of the feed is: a) 51.7 C, b) 21.1 C, and c) 93.3 C.

Properties:

Specific heat of the feed solution is 3.77 J/g .C

Latent heat of vaporization of the solution can ne taken as that of water.

Exercise (2) McCabe Problem 16.1

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Answers: slight variations from the ideal answers are ok (b) 21.1 C: ms= 18,831 kg/h Economy= 0.873 A= 59.7 m2

(a) 51.7 C: ms= 17,782 kg/h Economy= 0.925 A= 56.4 m2

(c) 93.3 C: ms= 16,356 kg/h Economy= 1.005 A= 51.9 m2

1. Factors leading to boiling point elevation and impact of

ignoring BPE in calculations

2. The physical meaning of Economy in evaporation and

ways to improve economy

3. Main resistances to heat transfer in evaporation and steps

to calculate overall heat transfer coefficient

4. Heat of dilution effect

Brain Storming Points

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Heat of Dilution

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A single effect evaporator is used to concentrate 7 kg/s of a solution from 10 to 50%

solids. Steam is available at 205 kN/m2 and evaporation takes place at 13.5 kN/m2. The

overall heat transfer coefficient of heat transfer is 3 kW/m2.K. The feed enters to

evaporator at 294 K and the condensate leaves the heating space at 352.7 K. The

specific heats of the 10% and 50% solutions are 3.76 and 3.14 kJ/kg. K respectively.

Estimate:

(a) Heating surface area required

(b) Amount of steam used

Exercise (3) Coulson & Rischardson 14.1

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Classification of Evaporators

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• One pass mainly used in multiple effect evaporators

• Evaporation capacity is limited

• Useful for heat-sensitive materials by operation under vacuum

• Examples : falling film, agitated evaporators

Once through evaporators

• Multiple passes by circulation

• A pool of liquid is maintained. Feed mixes with the pool, evaporation in tubes, remaining liquid returns to pool.

• Not good for heat sensitive materials

• Wide range of concentrations

• Natural and forced circulation

• Examples: climbing film

Circulation evaporators

Types of Evaporators

Product viscosity Heat sensitivity Scale formation and deposition 22

Low viscosity

(a) Horizontal tube evaporator

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• Horizontal tubes

• Solution in shell, steam in tube.

• No agitation

• Agitation occurs due to bubbling of vapor

• Suitable for low-viscosity solutions that do not deposit scales on heating

surfaces

(b) Short vertical tube evaporators

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• Short vertical tubes

• Solution in tubes, and steam in shell

• Boiling solution in tubes provides agitation and higher U

• Not suitable for viscous liquids

(e) Falling film evaporator

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• Widely used

• Feed at top flow as film down the tube

(Downward flow ) as a film

• Feed distribution in tubes as film by perforated

plates (a problem)

• Heat sensitive solutions (food /juice) by

reducing the residence time (once-through

• Vacuum operations Vapor/liq

separator

(e) Climbing film evaporator

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• Upward flow

• Long vertical-tube exchanger with steam

in shell and solution in tube

• Space or separator for vapor

• Good for solutions that tend to foam

which break at the impenging baffle in the

separator

separator

(d) Forced circulation evaporators

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• A pump is used to force solution flow

• Upward flow

• Very viscous solutions

• Heat transfer coefficient is increased

by forced circulation.

• Liquid velocity is 2-5.5 m/s

compared to 1.5 m/s for natural flow

• Tubes under static pressure, liquid

becomes superheated in the vapor

space-> flashes

(c) Long Vertical tube evaporator

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• Separate chamber of product disengagement to vapor

and liquid

• Longer tubes so feed entering velocity can be higher

thus higher U

Evaporator Selection

Coulson & Richardson Vol. 2 29

Example on Calculating U McCabe Solved Example 16.1

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Condensed milk is produced by evapration of milk in a falling film evaporator

containing stainless steel tubes 32 mm in diameter and 6 m long. Evaporation takes

place at 60C, which is the boiling point of milk at 2.7 Ibf/in2 absolute, using steam at

70C. The feed rate is 40 kg/h per tube at 60 C.

(a) Estimate the internal coefficient hi and the over all coefficient U

(a) What is the evaporation rate per tube?

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

milk?

(c) Calculate the average residence time in the evaporator. The properties of milk at 60

C are:

(cP) (kg/m3) k (W/m.K) (J/g)

Raw milk 0.94 1010 0.62 2357

25% solids 1.6 1030 0.55 2357

1. During evaporation, steam generates vapor. Single effect evaporator

can be wasteful of energy if the vapor’s heat content is not used.

2. The latent heat can be recovered and re-used by employing a

multiple effect evaporators (MEE).

3. Types of MEE: a) forward, b) backward, and c) parallel feed.

4. How much will 1 kg steam evaporate from solution? (depends on the

feed’s temp)

5. What is the driving force for evaporation? (Tsteam-Te)

6. Why pressure should successively reduce across evaporators?

7. What is the economy for a 3-effect evaporator if 1 kg steam is used

and approximately 1 kg vapor is produced from each evaporator?

Multiple Effect Evaporators

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Uses:

• Feed is hot

• Product is heat

sensitive

Uses:

• Higher capacity (mv)

• Feed is cold

• Lower economy than

forward if feed is cold

• Product is viscous

P and T

Forward feed

Backward feed

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Parallel feed

Mixed feed

• Permits final evaporation

to be done at the highest

temperature

Cost savings by multiple effect

Coulson & Richardson Vol. 2

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Overall Heat Transfer Coefficient (U)

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Multiple Effect Evaporator Example Geankoplis p.544

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A triple-effect forward-feed evaporator is being used to evaporate a sugar

solution containing 10 wt% solids to a concentrated solution of 50 wt%. The

boiling point rise of the solution (independent of pressure) can be estimated

from BPR C = 1.78x + 6.22x2 (BPR F = 3.2x + 11.2x2) where x is the weight

fraction of sugar in solution. Saturated steam at 205.5 kPa (29.8 psi) [121.1C

(250 F) saturation temperature] is used. The pressure in the vapor space of the

3rd effect is 13.4 kPa (1.94 psi). The feed rate is 22,680 kg/h (50,000 Ibm/h) at

26.7C (80F). The heat capacity of the liquid solutions is cp = 4.19 2.35x

kJ/kg.K (1.0 0.56x btu/Ibm.F). The heat of solution is considered to be

negligible. The coefficients of heat transfer have been estimated as U1 = 3123,

U2= 1987, U3 = 1136 W/m2.K or 550, 350, and 200 btu/h.ft2.F. If each effect

has the same surface area, calculate the area, the steam rate used, and the steam

economy.