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Rivera, Alyssa A. 5ChE-C Equipment Design

Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

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Page 1: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Rivera, Alyssa A.

5ChE-C

Equipment Design

Page 2: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

1. Introduction

Evaporation is basically a separation step which uses heat transfer to separate products

presenting differences at boiling point. In order to concentrate a non-volatile solute, such as

organic compounds, inorganic salts, acids or bases from a solvent, evaporation process is used

where in solvent is removed as vapor from a solution, slurry or suspension of a solid in a liquid.

Evaporation requires the use of a heating medium, usually steam which is in indirect contact

through a steam chest. The most common solvent in most of the evaporation systems is water.

Evaporation differs from the other mass transfer operations such as distillation and drying. In

distillation, the components of a solution are separated depending upon their distribution between

vapor and liquid phases based on the difference of relative volatility of the substances. Removal

of moisture from a substance in presence of a hot gas stream to carry away the moisture leaving a

solid residue as the product is generally called drying. Evaporation is normally stopped before the

solute starts to precipitate in the operation of an evaporator.

1.1 Evaporation

The major requirement in the field of evaporation technology is to maintain the quality of the

liquid during evaporation and to avoid damage to the product. This may require the liquid to be

exposed to the lowest possible boiling temperature for the shortest period of time.

Evaporation process comprises three main steps:

1. Pre-heating of a solution prior to evaporation

2. Removal of water (solvent) as vapor by steam heating

3. Condensing the vapor removed

Page 3: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

1.2 Types of evaporation

Because of numerous requirements and limitations for feed, products and steam, it have

resulted in a wide variation of evaporator designs today. In almost all evaporators the heating

medium is steam, which heats a product on the other side of a heat transfer surface. The following

list contains the descriptions of the most common types of evaporators: Falling Film Evaporators,

Rising Film Evaporators, Forced Circulation Evaporators and Plate Evaporators.

Typical evaporator applications: Product concentration Dryer feed pre-concentration

Volume reduction Water / solvent recovery Crystallization

Types of Evaporators

a. Falling Film Evaporators

In a falling film evaporator, the liquid is fed at the top of the tubes in a vertical tube bundle.

The liquid is allowed to flow down through the inner wall of the tubes as a film. As the liquid

travels down the tubes the solvent vaporizes and the concentration gradually increases. Vapor

and liquid are usually separated at the bottom of the tubes and the thick liquor is taken out.

Evaporator liquid is recirculated through the tubes by a pump below the vapor-liquid separator.

b. Rising or Climbing Film Evaporators

The LTV evaporator is frequently called a rising or climbing film evaporator. The liquid

starts boiling at the lower part of the tube and the liquid and vapor flow upward through the

tube. If the heat transfer rate is significantly higher, the ascending flows generated due to higher

specific volume of the vapor-liquid mixture, causes liquid and vapor to flow upwards in

parallel flow. The liquid flows as a thin film along the tube wall. This cocurrent upward

movement against gravity has the advantageous effect of creating a high degree of turbulence

in the liquid. This is useful during evaporation of highly viscous and fouling solutions.

Page 4: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

c. Forced circulation Evaporator

This system is used when the product has a strong tendency to foul the heating surfaces;

therefore, it is recirculated at a rather high rate through the tubes. Forced circulation evaporator

is commonly used for concentration of caustic and brine solutions and also in evaporation of

corrosive solution.

d. Gasketed Plate Evaporator

The gasketed-plate evaporator is also called the plate evaporator because the design is

similar to that of a plate heat exchanger. The heat transfer coefficient is greatly enhanced due

to high turbulent flow through narrow passages. This evaporator is suitable for high viscous,

fouling, foaming and heat sensitive solutions. This type of evaporators is mainly used for

concentration of food products, pharmaceuticals, emulsions, glue, etc

Other types of evaporators:

e. Stirrer Evaporator

This type of evaporation is rarely used today. It applies mostly for highly viscous feed and

for very specific products.

f. Circulation Evaporator

The operations are similar to the rising film principle, but the liquid recovered in the

separator is sent back to the bottom of the evaporator creating a closed loop.

g. Fluidized bed Evaporator

This system operates under the same principle as forced circulation evaporators and is used

when the feed solution contains particles.

1.3 Multiple Effect Evaporator

Multiple Effect Evaporation remains one of the popular methods used for the concentration

of aqueous solutions. Water is removed from a solution by boiling the liquor in an evaporator and

Page 5: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

withdrawing the vapor. If the solution contains dissolved solids, the resulting strong liquor may

become saturated so that crystals are deposited.

Evaporation is carried out by supplying heat to the solution to vaporize the solvent. The

heat is supplied basically to provide the latent heat of vaporization and by adopting methods for

recovery of heat from the vapor, it has been possible to achieve great economy in heat utilization.

The normal heating medium is generally low pressure steam (1 to 1.5 kg/cm2g).

An industrial evaporator systems generally comprises:

1. A heat exchanger to supply sensible heat and latent heat of evaporation to the feed.

Saturated steam is usually used as the heating medium.

2. A separator in which the vapour is separated from the concentrated liquid phase.

3. A condenser to effect condensation of the vapour and its removal from the system.

There are two main types of ways of improving steam economy in evaporators. One is to

use a multiple effect evaporator, the other is to use mechanical vapor recompression.

The chief factor influencing the economy of an evaporator system is the number of effects. By

increasing the number of effects we can increase the economy of an evaporator system. The first

effect of a multiple effect evaporator is the effect to which the raw steam is fed, vapors obtained

from first effect act as a heating medium for another effect.

1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator

a. Forward feed

The typical feeding method of multi-effect evaporators is forward. Both feed and steam are

introduced in the first effect and the feed passed from effect to effect parallel to the vapor from the

earlier effect. Concentration increases from the first effect to the last. Forward feeding operation

is helpful when the concentrated product may degenerate if exposed to high temperature. The

product is withdrawn from the last effect.

Page 6: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

b. Backward feed

In backward feed configuration, the feed enters at the last effect (coldest effect) and is

pumped through the successive effects. The product is withdrawn from the first effect (hottest)

where the steam is introduced. This method of feeding requires a pump between each pair of effects

to transfer liquid from lower pressure effects to higher pressure effects. It is advantageous when

cold feed entering needs to be heated to a lower temperature than in forward feed operation.

Backward feed is commonly used when products are viscous and exposure to higher temperature

increases the rate of heat transfer due to reduction in viscosity of the liquid.

c. Mixed feed

In the mixed feed operation, the dilute feed liquid enters at an intermediate effect and flows

in the next higher effect till it reaches the last effect of the series. In this section, liquid flows in

the forward feed mode. Partly concentrated liquor is then pumped back to the effect before the one

to which the fresh feed was introduced for further concentration. Mixed feed arrangement

eliminates some of the pumps needed in backward configuration as flow occurs due to pressure

difference whenever applicable.

d. Parallel feed

The fresh feed is introduced to each effect and in this configuration the product is

withdrawn of from the same effect in parallel feed operation. In parallel feeding, there is no transfer

of liquid from one effect to another effect. It is used primarily when the feed is saturated and the

product is solid containing slurry. This is most common in crystallizing evaporators.

Page 7: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Method of feeding of evaporator: a: forward feed; b: backward feed; c: mixed feed; d:

parallel feed.

2. Feed Stock Analysis

Sugar (C12H22O11) is an organic compound, colourless, sweet-tasting crystals that

dissolve in water. In addition to providing a sweet taste and flavour, sugar performs a variety

of functions in food products. Sugar is used as a preservative, to prevent large ice crystals from

forming in frozen sweet mixtures, and to support fermentation in products containing yeast.

Making sugar an important and versatile food ingredient. Sugars are found in the tissues of

most plants, but are present in sufficient concentrations for efficient extraction only in

sugarcane and sugar beet. Sugar beets accounted for 20% of the world's sugar production.

Sugar beets, after washed and sliced go through a large tank called a diffuser where raw

sugar juice is extracted. Sugar juice is now purified resulting to a solution called thin juice.

Page 8: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

This thin juice having a 10-14% solids is evaporated to 60% to be fed to a crystallizer to

prudence sugar crystals.

2.1 Operating Conditions, Assumptions and Constraint

For the evaporation for thin juice of 14% to 60%, multiple effect evaporator will be used

specifically three effects. Normally, 5 multiple evaporators are used for evaporation of thin

juice but the assumption will be, the feed that will be treated in an evaporator is less than the

feed treated in a 5 multiple effect. In this manner, triple effect evaporator will be designed.

Entering steam temperature is at its saturation with an operating pressure of 233.54kPa at 1st

effect evaporator. The 3rd evaporator will be at 550mmHg vacuum pressure. Entering feed is

at 20C. Boiling point rise will be neglected for it is a organic solution. Sucrose will only have

a significant boiling point rise if it is above 60Brix or 60% solute. For the heat transfer

coeffiecient, assume values from 2500-1600 (W/m2K) for the falling film evaporator ranges

from these values for heat transfer coefficient.

3. Rationale for equipment Selection

Multiply effect evaporator will be used for the process, specifically, triple effect

evaporator. Because the feed is at 20C and the vaporization of water in the solution will make

it viscous, backward feeding will be chosen. There are different types of evaporator today, but

the falling film tubular evaporator will be appropriate for the evaporation of thin juice. The

typical application for falling film evaporators are the concentration of dairy products (such as

whey, milk protein, skim milk, cream and hydrolyzed milk), sugar solutions, urea, phosphoric

acid, concentrated juices and black liquor.

4. Equipment Specification

Some of the problems associated with the equipment can be traced back to improper or

poor specification of the equipment. The following details are designed to assist in this process

and minimize the consequences of poor selection.

4.1 Material of Construction

Page 9: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

The two parameters which control the selection of the material of construction are

corrosion and ease of cleaning. Stainless steel will be used as the material of construction

because it does not readily corrode, rust or stain with this kind of feed solution as ordinary steel

does.

4.2 Shell Specifications

a. Material: stainless steel: type (304)

b. Maximum allowable stress of stainless steel = 14884.5psi

c. Length: 7.916 meters

d. Diameter: 1.979 meters

e. Thickness: 6.9236 mm

f. Evaporator heads(thickness): hemispherical(4.958mm) & flat head(71.174mm)

4.3 Pipe Inside the Evaporator Specifications (Assume: Based on Heuristics)

a. Material: 3 1/3 Schedule 40 stainless steel

b. Length: 4meters

c. Inside Pipe Diameter: 3.5 in

d. Outside Pipe Diameter: 3.334 in

Page 10: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

e. Number of tubes: 40 tubes

5. Theoretical Calculations and Material Balances

*Assumptions: no heat loss in the environment, no solute evaporated with the solvent, negligible

boiling point rise (BPR), same contact area for the 3 effects of the evaporators.

Step 1: Over-all Material Balance: Feed = Effluent + Liquid

10000 lb/hr = E + L

Solute Balance: FxF = LxL

10000(0.14) = (0.60)L

Solving simultaneously: E = 7.667X103 lb/hr

L= 2.333X103 lb/hr

Step 2: Assume pressure for 3rd effect. (Setting it to 550mmHg Vacuum). Entering

steam is at its saturation temperature.

Assume or setting over-all heat transfer coefficient (W/m2 K): U1=2500, U2= 2000 and

U3=1600

Po=233.54KPa To= 398.15K

T1T

1U1

U2

U1

U3

15.106

Page 11: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

∑∆T= 57.591K

Step 3: Temperature and enthalpy profile

(Hs-hc)= Lo

To=398.15K L0=2198.165 kJ/kg

T1=383.044K L1=2240.316 kJ/kg

T2=364.162K L2=2289.5605 kJ/kg

T3=340.559K L3= 2300.8843kJ/kg

Step 4: Energy Balance in 3 Evaporators

1st Effect : VoLo=(F-V3-V2)Cp(T1-T2)+V1L1

2nd Effect : V1L1=(F-V3)Cp(T2-T3)+V2L2

3rd Effect: V2L2=FCp(T3-Tf)+V3L3

Step 5: Solve for stream rates Vo,V1,V2, V3

1st eq: Vo(2198.165)=(10000-V3-V2)*(3.8029)*(109.894-91.012)+ V1*(2240.316)

2nd eq: V1(2240.316)=V2(2289.5605) + (10000-V3)(3.8029)(91.012-67.409)

3rd eq; V2*(2289.5605)=(10000)*(3.8029)*(67.409-20)+(V3)*(2300.8843)

4th eq: 7.667X103=V1+V2+V3

Vo= 3.31283x103 V1= 3.07708x103 V2=2.6932x103 V3=1.89638x103

T2U1

U2T1 18.882

T3U1

U3T1 23.603

Page 12: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Step 6: Check if the three areas are equal

1st Effect : VOLO=U1 A1 ∆T1

2nd Effect : V1L1=U2 A2 ∆T2

3rd Effect : V2L2=U3 A3 ∆T3

A1= 53.564 m2 Amean= 49.875m2

A2= 50.706 m2

A3= 45.356 m2

* Area must be less than 3 % difference with the Amean

%Diff: 𝐴𝑛−𝐴𝑚𝑒𝑎𝑛

𝐴𝑚𝑒𝑎𝑛X100

%Diff1: 7.396%

%Diff2= 1.666%

%Diff3= 9.061%

If error > 3%

∆Tn= (An* ∆Tn)/Amean

∆T1= 16.425

∆T2= 19.436

∆T3=21.731

Repeat Step 3: temperature and enthalpy profile

(Hs-hc)= Lo

To=398.15K L0=2198.165 kJ/kg

T1=381.725K L1=2243.8754 kJ/kg

T2=362.289K L2=2294.2492 kJ/kg

T3=340.558K L3= 2300.8421kJ/kg

Page 13: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Step 4: Energy Balance in 3 Evaporators

1st Effect : VoLo=(F-V3-V2)Cp(T1-T2)+V1L1

2nd Effect : V1L1=(F-V3)Cp(T2-T3)+V2L2

3rd Effect: V2L2=FCp(T3-Tf)+V3L3

Step 5: Solve for stream rates Vo,V1,V2 , V3

1st eq: Vo(2198.165)=(10000-V3-V2)*(3.8029)*(108.575-89.14)+ V1*(2243.8754)

2nd eq: V1(2243.8754)=V2(2294.2) + (10000-V3)(3.8029)(89.14-67.409)

3rd eq; V2*(2294.2492)=(10000)*(3.8029)*(67.409-20)+(V3)*(2300.8421)

4th eq: 7.667X103=V1+V2+V3

Vo= 3.3033x103 V1= 3.05841x103 V2=2.69978x103 V3=1.90847X103

Step 6: Check if the two areas are equal

1st Effect : VOLO=U1 A1 ∆T1

2nd Effect : V1L1=U2 A2 ∆T2

3rd Effect : V2L2=U3 A3 ∆T3

A1= 49.121 m2 Amean= 49.8216 m2

A2= 49.042 m2

A3= 49.484 m2

*Area should be less than 3 % difference with the Amean

%Diff: 𝐴𝑛−𝐴𝑚𝑒𝑎𝑛

𝐴𝑚𝑒𝑎𝑛X100

Page 14: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

1st Effect

U1=2500

(W/m2 K)

2nd Effect

U1=2500

(W/m2 K)

3rd Effect

U1=2500

(W/m2 K)

%Diff1: 0.192%

%Diff2= 0.354%

%Diff3= 0.546%

6. Heuristics

Treated as pressure vessel, operating pressure inside is up to 233.54KPa. Design pressure for

these evaporators are equal to operating pressure plus 25psig or 10%. Design temperature is

equal to Operating temperature plus 30Oc or 50F. Shell and head thickness is obtain with design

pressure and design temperature. Gas/Liquid separators are vertical in alignment. Used diameter:

length ratio of 1:4. Maximum allowable stress is dependent on the material of construction and

the design temperature. Internal tubes for the evaporator can be range to 19–63mm (0.75–24.8

in.) in diameter and 3.66–9.14m (12–30 ft) long.

7. Diagram

Diagram of Falling Film (Backward Feeding)

Legends:

Steam Flow

Feed Flow

Pumps

xf:0.14

F:10000

lb/hr

xl:0.60

Po= 233.54 kPa

550 mmHg vacuum

Page 15: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Steam inlet pipe

Front view of Triple effect Falling Film Evaporator

Top View

Bottom view Side views

Entering feed

to the 3rd

evaporator

(showing

internals)

Page 16: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Triple effect Falling Film evaporator with support

8. Conclusion and Recommendation

In designing an evaporator equipment, an engineer must consider a lot of condition. One must

know first what he/she wants to process and produce. Characteristics of feed and product are

important factor in designing an evaporator; their properties before and after they’re fed to the

equipment is strictly observe by the engineer. Because there are a lot of available type of

evaporator, characteristics of feed must be known to be efficient in yielding the product. Also,

knowing its characteristics, material for construction for the equipment will be easily chosen.

Operating temperature and pressure must be specified to allow the designer to make an equipment

Page 17: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

having a maximum allowable working temperature and pressure. Having the design parameters,

thickness and allowable stress can be computed.

9. References

http://nptel.ac.in/courses/103107096/module4/lecture2/lecture2.pdf

http://www.spxflow.com/cn/assets/pdf/Evaporator_Handbook_10003_01_08_2008_US.pdf

https://www.bma-worldwide.com/products/sugar-and-sweeteners/tubular-fallingfilm-

evaporator.html

http://multiple-effect-evaporation.webs.com/

Page 18: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Apendix (Calculations)

Page 19: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is
Page 20: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is
Page 21: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

A= ᴨ*D*L

Diameter and length has a ratio of 1:4

L=4D

49.231= ᴨ*D*(4D)

D=1.9793m

L= 4D= 7.9173m

Page 22: Rivera, Alyssa A. 5ChE-C Equipment Design · 1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator a. Forward feed The typical feeding method of multi-effect evaporators is

Maximum Allowable Working Pressure:

233.54kPa*(14.7/101.325)= 33.88145 psia

33.88145+ 25= 58.88psia

Maximum Allowable Temperature:

125C+ 30C= 155C= 311F

Interpolate for Stainless Steel (304):

500 − 311

500 − 300=

12.9 − 𝑥

12.9 − 15

X=14.8845 ksi= 14884.4 psi (Maximum allowable stress)

Thickness of the shell (Cylinder)

P<0.385(SE)

0.385(14884.5)(1)=5730.53

t=𝑃𝑟𝑖

𝑆𝐸−0.6𝑃+ 𝐶𝑐 =

58.88∗989.5

(14884.5)−(0.6)∗(58.88)+ 3 = 6.9236𝑚𝑚

Thickness of heads:

(Hemispherical)

t=𝑃𝑟𝑖

2(𝑆𝐸)−0.2(𝑃)+ 𝐶𝑐 =

58.88∗(989.5)

2∗(14884.5)(1)−(0,2)(58.88)+ 3 = 4.957951

(Flat Head)

t=2𝑟𝑖 ∗ √0.3𝑃

𝑆+ 𝐶𝑐 = 2(989.5) ∗ (√

0.3∗58.88

14884.5) + 3 = 71.1747