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Evening program-3rd SemesterMaster in Nutritional science and Food

Technology

Course Title: Food Processing and Preservation

Technology (Assignment)Course No: 5015

Group--BSession: Summer-(2013-14) Batch-B01

Submitted to:Abul Kasem (Lecturer)

Submitted by:S. M. Mainul IslamRoll: 130-116-079

Department of Applied Nutrition and Food Technology (ANFT)

FACULTY OF APPLIED SCIENCE AND TECHNOLOGY

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ISLAMIC UNIVERSITY, KUSHTIA-7003 Some definitions of Evaporation

Evaporation: conversion of a liquid to a vapourTranspiration: that part of evaporation which enters the atmosphere through plantsTotal Evaporation: The combined processes of evaporation and transpiration, this has been referred to as evapotranspiration.Potential evaporation (PE): evaporation that would occur from an extensive well-watered surfaceActual evaporation (AE): evaporation that actually occurs, given water availability

Evaporation:Evaporation is the process whereby liquid water is converted to water vapor (vaporization) and removed from the evaporating surface (vapor removal). Water evaporates from a variety of surfaces, such as lakes, rivers, pavements, soils and wet vegetation.Energy is required to change the state of the molecules of water from liquid to vapor. Direct solar radiation and, to a lesser extent, the ambient temperature of the air provide this energy.Evaporators: An evaporator is a device used to turn the liquid form of a chemical into its gaseous form. The liquid is evaporated, or vaporized, into a gas.

Used of evaporators: An evaporator is used in an air-conditioning system to allow a compressed cooling chemical, such as R-22 (Freon) or R-410A, to evaporate from liquid to gas while absorbing heat in the process. It can also be used to remove water or other liquids from mixtures. The process of evaporation is widely used to concentrate foods and chemicals as well as salvage solvents. One of the most important applications of evaporation is in the food and beverage industry. Foods or beverages that need to last for a considerable amount of time or need to have certain consistency, like coffee, go through an evaporation step during processing.In the pharmaceutical industry, the evaporation process is used to eliminate excess moisture, providing an easily handled product and improving product stability. Preservation of long-term activity or stabilization of enzymes in laboratories is greatly assisted by the evaporation process.

How an evaporator worksThe solution containing the desired product is fed into the evaporator and passes across a heat source. The applied heat converts the water in the solution into vapor. The vapor is removed from the rest of the solution and is condensed while the now-concentrated solution is either fed into a second evaporator or is removed. The evaporator, as a machine, generally consists of four sections. The heating section contains the heating medium, which can vary. Steam is fed into this section. The most common medium consists of parallel tubes but others have plates or coils typically made from copper or aluminum. The concentrating and separating section removes the vapor being produced from the solution. The condenser condenses the separated vapor, then the vacuum or pump provides pressure to increase circulation.

Types of evaporators used todayThe more common types of evaporators include:

1. Long tube vertical evaporators (a. Climbing(Rising) Film evaporators, b. Falling film evaporators)2. Short tube (Calandria) vertical evaporators3. Horizontal tube evaporators4. Forced Circulation Evaporators

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5. Agitated, thin film evaporators and othersBatch type pan, Plate equivalents, Basket-type evaporators & Vapor-compression evaporators

1. Long-tube vertical evaporators : The three types of long-tube vertical evaporators are the most popular evaporators used today. More evaporation is accomplished in these units than in all other types combined. While they are natural-circulation evaporators, they are also categorized individually as rising-film, falling-film, and rising/falling-film types.Advantages:

i. This is the most economical design, since a large heat-transfer surface can be packed into a given body;

ii. These evaporators occupy little floor space. iii. Heat-transfer coefficients are high, and the units are ideal for substantial evaporation duties. iv. Highly versatile, they are used in various industries.v. They are especially suited for foaming or frothing liquors, as the foam is broken due to the

liquid/vapor mixture striking an impingement baffle.Disadvantages:

i. These vertical units require high headroom. ii. Generally, they are unsuitable for scaling or salting liquors, and iii. Are sensitive to changes in operating conditions.

Industrial applications: the once-through type is used in pulp-and-paper plants for concentrating black liquor. Other versions of this evaporator are discussed later.

It’s also two types===

a. Rising-film evaporators The rising-film evaporator is the original version of the long-tube vertical evaporator. Steam condenses on the outside surfaces of vertical tubes. The liquid inside the tubes is brought to a boil, with the vapor generated occupying the core of the tube. As the fluid moves up the tube, more vapor is formed, resulting in a higher central-core velocity that forces the remaining liquid to the tubewall. This leads to a thinner and more rapidly moving liquid film. As the film moves more rapidly, heat-transfer coefficients increase and residence times drop.Since the vapor and liquid both flow in the same direction, the thinning of the liquid film is not as pronounced as in a falling-film type of evaporator, and the possibility of tube dry out is less. This makes the rising-film evaporator particularly suited to services having mild scaling tendencies.Advantages:

i. Since feed enters at the bottom, the feed liquor is distributed evenly to all tubes. ii. Other advantages are those of the long-tube vertical unit, described before.

Disadvantages:i. Heat transfer is difficult to predict; pressure drop is higher than for falling-film types.ii. Performance is extremely sensitive to the temperature driving force. iii. Heat transfer falls off at low temperature differences (less than 25°F) or at low temperatures (about

250°F).Industrial applications: Major uses of rising-film evaporators include concentrating black liquors in pulp-and-paper mills, and concentrating nitrates, spin-bath liquors, electrolytic tinning liquors, etc.

b. Falling-film evaporators: Falling-film evaporators evolved as a means to solve the problems associated with the rising-film types. Specifically, the hydrostatic head necessary for the operation of rising-film units leads to problems with some heat-sensitive products.Another phenomenon common to falling-film evaporators is dry-patch formation, which reduces thermal performance. The dry patches may be caused by a liquid flow rate insufficient to maintain a continuous liquid film or by the evaporator's not being exactly vertical.

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Advantages:i. Falling-film evaporators offer all advantages of rising-film units, plus higher heat-transfer coefficients

satisfactory operation at low temperature driving forces (10-1 25°F), andii. Concentration of heat-sensitive and viscous chemical products.

Disadvantages: i. These are the same as for rising-film types, except that, in addition, feed distribution is a major

problem. ii. Temperature driving force is not limiting and iii. A broader range of applications is possible.

2. Short-tube (Calandria) vertical evaporators:  These are often referred to as calandria or standard evaporators, the latter because of earlier popularity with users. Units consist of short tubes, 4-6 ft long, and 2-4 in. dia., set between two horizontal tube sheets that span the evaporator-body diameter. The tube bundle contains a large circular down comer that returns concentrated liquor above the top tube sheet to below the bottom tube sheet for product withdrawal. The driving force for flow of liquid through the tubes is the difference in density between the liquid in the down comer and two-phase mixture in the tubes.Advantages:  

i. These evaporators can be used with scaling liquids, ii. since evaporation takes place inside the tubes, iii. Its accessible for cleaningiv. Fairly high heat-transfer coefficients are obtained with thin liquors v. Units are relatively inexpensive,

Disadvantages:i. A large area is required since the units are squat.ii. Heat-transfer coefficients are sensitive to the temperature difference and liquor viscosity, and, due

to large liquid holdup,iii. These evaporators cannot be used with heat-sensitive materialsiv. Turndown and flexibility are low — turndown being <2:1

Industrial applications:These short-tube vertical evaporators are suitable for noncorrosive (e.g., cane sugar), clear and no crystallizing liquors.

3. Horizontal evaporators: Horizontal tube evaporators use a tube and spray method of heat transfer. Advantages:

i. They are relatively low costii. They are very low headroom

Disadvantages: i. Not suitable for salting or scaling liquidsii. They have smaller capacity than others evaporators

Industrial Use:The major use is for making distilled water for boiler feed. Horizontal tube evaporators used in the pharmaceutical industry, pulp and paper industry etc.

4. Natural/Forced-circulation evaporators These are made in a variety of arrangements for services where the feed and/or product liquor has a tendency to salt or scale, and where the viscosities of the solutions are so high that natural circulation is not feasible. Thermal and flow characteristics of the process liquor are so poor that use of forced circulation is necessary.The type of vapor head used, ranging from a simple centrifugal separator to a crystallizing chamber, is selected on the basis of product characteristics.Advantages:  

i. Forced-circulation evaporators are the most versatile of all evaporators.

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ii. High heat-transfer coefficients can be achieved for problem liquors, and hence required surface area is kept to a minimum.

iii. The economics are especially favorable for applications that require the more expensive alloys such as stainless steels, high-nickel alloys, etc.

iv. Also since material is pumped around the unit, fouling can be controlled well. Operation is not limited by the liquid/vapor ratio, and turndown can be as low as 5% of capacity.Disadvantages:  

i. These evaporators are usually less economical than other types, due to operating and maintenance costs for the pumps.

ii. Corrosion-erosion can occur, due to high circulation velocities. Also, plugging of tubes where liquor enters can be a problem in salting services where the salt deposits detach and accumulate at the bottom.Industrial applications: In the fertilizer industry, these evaporators are used to concentrate urea, phosphoric acid ammonium nitrate, etc. Falling-film evaporators are also employed for processing food and dairy products, and for desalting Seawater.

5. Agitated thin( wiped –film) film evaporators These are essentially large-diameter jacketed tubes, in which the product is vigorously agitated and continuously removed from the tube wall by scraper blades (or wipers) mounted on a shaft inside the tube. Thus, the material to be processed is continuously spread as a thin film on the tube wall by a mechanical agitator. This permits processing of extremely viscous and heat-sensitive materials, as well as of crystallizing and fouling products.Advantages: 

i. These devices can process extremely viscous (to 100,000 or even 1 million cP), heat-sensitive or crystallizing liquids, as well as slurries.

ii. In some applications, agitated thin-film evaporators are, in fact, the only evaporators that will work. iii. Continuous scraping of the tube wall allows processing of severely scaling or fouling liquids. iv. Applications include services in which liquid loads are so small as to cause dry-patch formation in

falling-film units.Disadvantages: 

i. Agitated thin-film evaporators are the most expensive of all evaporators. ii. Also, due to the moving parts, operating and maintenance costs can be higher than for some of the

other types. iii. The heat-transfer surface area is limiting, which may require use of a high-temperature heating

medium to achieve higher capacities.

6. Batch type pan: Advantages:

i. It’s one of the simplest and oldest types of evaporators that used in the food industry. ii. The product is heated in a steam jacketed spherical vessel. And iii. Heating vessel may be open to the atmosphere or connected to a condenser and vacuum.

Disadvantages:i. The heat transfer area per unit volume in pan evaporators is small.ii. Poor heat transferiii. Reduces the thermal damage to heat sensitive products.

7. Basket-type evaporators : These are similar to calandria-type units except that the tube bundle is removable and the liquor down take occurs between the bundle and the shell, instead of in a central down comer.Advantages: 

i. The heating surface is removable,ii. Allowing easy cleaning and maintenance. iii. Also, due to the construction, differential thermal expansion is not a problem.

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Disadvantages: These are the same as for short-tube evaporators.i. A large area is required since the units are squat.ii. Heat-transfer coefficients are sensitive to the temperature difference and liquor viscosity, and, due

to large liquid holdup,iii. These evaporators cannot be used with heat-sensitive materialsiv. Turndown and flexibility are low — turndown being <2:1.

Industrial applications: These are the same as for short-tube evaporators

8. Plate-type evaporators : These evaporators are especially suited to the dairy, brewery and food-processing industries since there are no dead zones in which undesired bacterial growth could occur, and frequent and efficient cleaning can be done to meet stringent hygiene requirements. Maximum protection is provided for product flavor and quality since liquid holding-volume is low, and exposure to high temperature is short.Advantages:

i. Plate-type evaporators have low installation costs. ii. Thus, they are economical for the more costly materials (e.g., stainless steels, high-nickel alloys,

titanium, etc.). iii. Large Heat-transfer areas can be packed into a smaller volume, and heat-transfer coefficients are

usually higher than for tubular evaporators. iv. Capacity can be changed by simple addition or removal of plates.

Disadvantages:i. Maximum design conditions are only about 150 psig and 400°F, due to limitations of gasketing

materials, ii. The probability of fluid leakage is higher than for tubular types.

Industrial applications: This permits processing of extremely viscous and heat-sensitive materials, as well as of crystallizing and fouling products.

9. Vapor-compression evaporators: These evaporators are available in four configurations: rising/falling-film, falling-film, forced-circulation with suppressed boiling, and agitated thin-film. For the last type, film thinning is achieved by a combination of fluid hydrodynamics and plate geometry, rather than by a mechanical device. Advantages: 

i. These units are economical for processes handling heat-sensitive materials. ii. There is a distinct economic advantage in applications that require both multiple-effect operation and

use of more expensive alloys. iii. These evaporators can be used in remote locations where utility steam is unavailable.iv. In existing facilities undergoing expansion, vapor-compression evaporators may be the only choice,

due either to plot-plan limitations or to shortages of utility steam.Disadvantages:

i. Vapor-compression evaporators are difficult to justify where low-pressure steam is readily available. ii. These evaporators cannot be used when process liquors are fouling or show a high boiling-point

rise, iii. Maintenance is significant with mechanical compressors.

Industrial applications: vapor recompression evaporators are characterized by low temperature driving forces across heat-transfer surfaces, due to low compression ratios used for mechanical or thermal compressors

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