tx69299 ch1

© 2003 by CRC Press LLC

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Introduction to Unit Operations:

Fundamental Concepts

1.1 Process

Process is the set of activities or industrial operations that modify the prop-erties of raw materials with the purpose of obtaining products to satisfy theneeds of a society. Such modifications of natural raw materials are directedto obtain products with greater acceptance in the market, or with betterpossibilities of storage and transport.

The primary needs of every human being, individually or as a society,have not varied excessively throughout history; food, clothing, and housingwere needed for survival by prehistoric man as well as by modern man. Thesatisfaction of these necessities is carried out by employing, transforming,and consuming resources available in natural surroundings.

In the early stages of mankind’s social development, natural products wereused directly or with only small physical modifications. This simple produc-tive scheme changed as society developed, so that, at the present time, rawmaterials are not used directly to satisfy necessities, but rather are subjectedto physical and chemical transformations that convert them into productswith different properties.

This way, not only do raw materials satisfy the necessities of consumers,but also those products derived from the manipulation of such raw materials.

1.2 Food Process Engineering

By analogy with other engineering branches, different definitions of foodprocess engineering can be given. Thus, according to one definition, “foodprocess engineering includes the part of human activity in which the knowl-edge of physical, natural, and economic sciences is applied to agriculturalproducts as related to their composition, energetic content, or physical state.”

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Unit Operations in Food Engineering

Food process engineering can also be defined as “the science of conceiving,calculating, designing, building, and running the facilities where the trans-formation processes of agricultural products, at the industrial level and aseconomically as possible, are carried out.”

Thus, an engineer in the food industry should know the basic principlesof process engineering and be able to develop new production techniquesfor agricultural products. He should also be capable of designing the equip-ment to be used in a given process. The main objective of food processengineering is to study the principles and laws governing the physical,chemical, or biochemical stages of different processes, and the apparatus orequipment by which such stages are industrially carried out. The studiesshould be focused on the transformation processes of agricultural raw mate-rials into final products, or on conservation of materials and products.

1.3 Transformation and Commercialization of Agricultural Products

For efficient commercialization, agricultural products should be easy to han-dle and to place in the market. As a general rule, products obtained directlyfrom the harvest cannot be commercialized as they are, but must undergocertain transformations. Products that can be directly used should be ade-quately packaged according to requirements of the market. These productsare generally used as food and should be conveniently prepared for use.

One problem during handling of agricultural products is their transportfrom the fields to the consumer. Since many agricultural products have ashort shelf life, treatment and preservation methods that allow their lateruse should be developed. As mentioned earlier, many of these productscannot be directly used as food but can serve as raw material to obtain otherproducts. Developed countries tend to elaborate such products in the harvestzone, avoiding perishable products that deteriorate during transport fromthe production zone to the processing plant.

1.4 Flow Charts and Description of Some Food Processes

Food processes are usually schematized by means of flow charts. These arediagrams of all processes that indicate different manufacturing steps, as wellas the flow of materials and energy in the process.

There are different types of flow charts; the most common use “blocks” or“rectangles.” In these charts each stage of the process is represented by ablock or rectangle connected by arrows to indicate the way in which thematerials flow. The stage represented is written within the rectangle.



Introduction to Unit Operations: Fundamental Concepts

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Other types of flow charts are “equipment” and “instrumentation.”Figures 1.1, 1.2, and 1.3 show some flow charts of food processes.

1.5 Steady and Unsteady States

A system is said to be under steady state when all the physical variablesremain constant and invariable along time, at any point of the system; how-ever, they may be different from one point to another. On the other hand,when the characteristic intensive variables of the operation vary through thesystem at a given moment and the variables corresponding to each system’spoint vary along time, the state is called unsteady. The physical variables toconsider may be mechanical or thermodynamic. Among the former are vol-ume, velocity, etc., while the thermodynamic variables are viscosity, concen-tration, temperature, pressure, etc.

1.6 Discontinuous, Continuous, and Semicontinuous Operations

The operations carried out in the industrial processes may be performed inthree different ways. In a discontinuous operation the raw material is loaded

FIGURE 1.1

Extraction of olive oil.

Bagasse oil

CENTRIFUGATION

Oil frompress

Virgin oil

Exhaustedbagasse

DRYING

EXTRACTION

Bagasse

Olives

WASHING

PRESSING



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in the equipment; after performing the required transformation, the obtainedproducts are unloaded. These operations, also called “batch” or “intermit-tent,” are carried out in steps:

1. Loading of equipment with raw materials2. Preparation of conditions for transformation3. Required transformation4. Unloading products5. Cleaning equipment

The batch operation takes place under an unsteady state, since its intensiveproperties vary along time. An example of this batch process is the crushingof oily seeds to obtain oil.

FIGURE 1.2

Production of fruit concentrated juices.

Fruit

Juice 12 ºBrix

Pulp

Water andaromas

Juice 15 ºBrix

Water

Juice 70 ºBrix

CRUSHING

PRESSING

PRE-CONCENTRATION

ENZYMATICTREATMENT

CLARIFICATION

EVAPORATION

COOLING

STORAGE




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In continuous operations the loading, transformation, and unloadingstages are performed simultaneously. Equipment cleaning is carried outevery given time, depending on the nature of the process and the materialsused. To carry out the cleaning, production must be stopped. Continuousoperations take place under steady state, in such a way that the characteristicintensive variables of the operation may vary at each point of the systembut do not vary along time. It is difficult to reach an absolute steady state,since there may be some unavoidable fluctuations. An example of a contin-uous operation is the rectification of an alcohol–water mixture.

In some cases it is difficult to have a continuous operation; this type ofoperation is called semicontinuous. A semicontinuous operation may occurby loading some materials in the equipment that will remain there for agiven time in a discontinuous way, while other materials enter or exit con-tinuously. Sometimes it is necessary to unload those accumulated materials.For example, in the extraction of oil by solvents, flour is loaded and thesolvent is fed in a continuous way; after some time, the flour runs out of oiland must be replaced.

FIGURE 1.3

Elaboration of soluble coffee.

Roastedcoffee

Coffee exhaust(diluted solution)

Coffee extract(concentrated solution)

Soluble coffee

Hotwater

Solidwaste

Watervapor

Water

EXTRACTION

GRINDING

EVAPORATION

DRYING



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These different ways of operation present advantages and disadvantages.Advantages of continuous operation include:

1. Loading and unloading stages are eliminated.2. It allows automation of the operation, thus reducing the work force.3. Composition of products is more uniform.4. There is better use of thermal energy.

Disadvantages of continuous operation are:

1. Raw materials should have a uniform composition to avoid oper-ation fluctuations.

2. Is usually expensive to start the operation, so stops should beavoided.

3. Fluctuations in product demand require availability of consider-able quantities of raw materials and products in stock.

4. Due to automation of operation, equipment is more expensive anddelicate.

Continuous operation is performed under an unsteady state during startsand stops but, once adequately running, may be considered to be workingunder steady state. This is not completely true, however, since there couldbe fluctuations due to variations in the composition of the raw materials anddue to modifications of external agents.

When selecting a form of operation, the advantages and disadvantages ofeach type should be considered. However, when low productions arerequired, it is recommended to work under discontinuous conditions. Whenhigh productions are required, it is more profitable to operate in a continuousway.

1.7 Unit Operations: Classification

When analyzing the flow charts of different processes described in othersections, it can be observed that some of the stages are found in all of them.Each of these stages is called basic or unit operation, in common with manyindustrial processes. The individual operations have common techniquesand are based on the same scientific principles, simplifying the study of theseoperations and the treatment of these processes.

There are different types of unit operations depending on the nature ofthe transformation performed; thus, physical, chemical, and biochemicalstages can be distinguished:




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• Physical stages: grinding, sieving, mixture, fluidization, sedimen-tation, flotation, filtration, rectification, absorption, extraction,adsorption, heat exchange, evaporation, drying, etc.

• Chemical stages: refining, chemical peeling• Biochemical stages: fermentation, sterilization, pasteurization,

enzymatic peeling

Hence, the group of physical, chemical, and biochemical stages that takeplace in the transformation processes of agricultural products constitute theso-called unit operations of the food industry, the purpose of which is theseparation of two or more substances present in a mixture, or the exchangeof a property due to a gradient. Separation is achieved by means of a sepa-rating agent that is different, depending on the transferred property.

Unit operations can be classified into different groups depending on thetransferred property, since the possible changes that a body may undergoare defined by variations in either its mass, energy, or velocity. Thus, unitoperations are classified under mass transfer, heat transfer, or momentumtransfer.

Besides the unit operations considered in each mentioned group, thereexist those of simultaneous heat and mass transfer, as well as other opera-tions that cannot be classified in any of these groups and are called comple-mentary unit operations.

All the unit operations grouped in these sections are found in physicalprocesses; however, certain operations that include chemical reactions canbe included.

1.7.1 Momentum Transfer Unit Operations

These operations study the processes in which two phases at different veloc-ities are in contact. The operations included in this section are generallydivided into three groups:

Internal circulation of fluids: study of the movement of fluids throughthe interior of the tubing; also includes the study of equipmentused to impel the fluids (pumps, compressors, blowers, and fans)and the mechanisms used to measure the properties of fluids(diaphragms, venturi meters, rotameters, etc.).

External circulation of fluids: the fluid circulates through the externalpart of a solid. This circulation includes the flow of fluids throughporous fixed beds, fluidized beds (fluidization), and pneumatictransport.

Solids movement within fluids: the base for separation of solids with-in a fluid. This type of separation includes: sedimentation, filtration,and ultrafiltration, among others.



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1.7.2 Mass Transfer Unit Operations

These operations are controlled by the diffusion of a component within amixture. Some of the operations included in this group are:

Distillation: separation of one or more components by taking advan-tage of vapor pressure differences.

Absorption: a component of a gas mixture is absorbed by a liquidaccording to the solubility of the gas in the liquid. Absorption mayoccur with or without chemical reaction. The opposite process iscalled desorption.

Extraction: based on the dissolution of a mixture (liquid or solid) ina selective solvent, which can be liquid–liquid or solid–liquid. Thelatter is also called washing, lixiviation, etc.

Adsorption: also called sorption, adsorption involves the eliminationof one or more components of a fluid (liquid or gas) by retentionon the surface of a solid.

Ionic exchange: substitution of one or more ions of a solution withanother exchange agent.

1.7.3 Heat Transfer Unit Operations

These operations are controlled by temperature gradients. They depend onthe mechanism by which heat is transferred:

Conduction: in continuous material media, heat flows in the directionof temperature decrease and there is no macroscopic movement ofmass.

Convection: the enthalpy flow associated with a moving fluid is calledconvective flow of heat. Convection can be natural or forced.

Radiation: energy transmission by electromagnetic waves. No mate-rial media are needed for its transmission.

Thermal treatments (sterilization and pasteurization), evaporation, heatexchangers, ovens, solar plates, etc. are studied based on these heat transfermechanisms.

1.7.4 Simultaneous Mass–Heat Transfer Unit Operations

In these operations a concentration and a temperature gradient exist at thesame time:

Humidification and dehumidification: include the objectives of hu-midification and dehumidification of a gas and cooling of a liquid.

Crystallization: formation of solid glassy particles within a homoge-neous liquid phase.




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Dehydration: elimination of a liquid contained within a solid. Theapplication of heat changes the liquid, contained in a solid, into avapor phase. In freeze-drying, the liquid in solid phase is removedby sublimation, i.e., by changing it into a vapor phase.

1.7.5 Complementary Unit Operations

One series of operations is not included in this classification because theseare not based on any of the transport phenomena cited previously. Theseoperations include grinding, milling, sieving, mixing of solids and pastes, etc.

1.8 Mathematical Setup of the Problems

The problems set up in the study of unit operations are very diverse,although in all of them the conservation laws (mass, energy, momentum,and stochiometric) of chemical reactions apply. Applying these laws to agiven problem is done to perform a balance of the “property” studied insuch a problem. In a general way, the expression of the mass, energy, andmomentum balances related to the unit time can be expressed as:

This is, that which enters into the system of the considered property isequal to that which leaves what is accumulated. In a schematic way:

In cases where a chemical reaction exists, when carrying out a balance fora component, an additional generation term may appear. In these cases thebalance expression will be:

When solving a given problem, a certain number of unknown quantitiesor variables (

V

) are present, and a set of relationships or equations (

R

) isobtained from the balances. According to values of

V

and

R

, the followingcases can arise:

• If

V

<

R

, the problem is established incorrectly, or one equation isrepeated.

• If

V

=

R,

the problem has only one solution.• If

V

>

R

, different solutions can be obtained; the best solution isfound by optimizing the process.

Property entering the system Property exiting the system

Property that accumulates

( ) = ( )+ ( )

E S A= +

E G S A+ = +



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There are

design variables. The different types of problems presented depend on thetype of equation obtained when performing the corresponding balances.Thus,

• Algebraic equations have an easy mathematical solution obtainedby analytical methods.

• Differential equations are usually obtained for unsteady continu-ous processes. The solution of the mathematical model establishedwith the balances can be carried out through analytical or approx-imate methods. In some cases, differential equations may have ananalytical solution; however, when it is not possible to analyticallysolve the mathematical model, it is necessary to appeal to approx-imate methods of numerical integration (digital calculus) orgraphic (analogic calculus).

• Equations in finite differences are solved by means of analogiccomputers which give the result in a graphic form. In some casesthe exact solution can be obtained by numerical methods.

F V R= −
