A) OPERACIONES CORRIENTES
3.1 ESTADOS SEGÚN SU CLASIFICACIÓN ECONÓMICA
3.1.1 Estado de ingresos
Objective:
After reading this unit, you should able to Know what physical chemistry is.Know what adsorption process is.
Know the physical chemistry keywords.
READING 1
Physical chemistry is the study of the physical basis of chemical systems and processes. It is concerned with the measurement, description, and prediction of the characteristics of chemical systems and their interactions with each other with respect to the transfer of mass and energy. There are some important areas of its study.
1. Chemical thermodynamics deals with the transfer of energy in chemical changes and seeks to characterize the equilibrium state of chemical systems.
2. Chemical kinetics deals with the rate and mechanism of chemical changes.
3. Structure of matter is a broad area of experimental and theoretical description of the properties of matter at the atomic and molecular level.
4. Quantum theory explains the nature of chemical bonding while the spectra of atoms and molecules are explained by quantum mechanics.
5. The discipline that allows us to ring our knowledge of molecular structure to bear on the problems of equilibrium and kinetics is found in the study of statistical mechanics.
TASK 1: MATCHING
Match a word or phrase in A with its definition in B
A B
1) Chemical changes a) condition in which no change occurs
2) Equilibrium b) strong attractive force that holds together atoms in molecules 3) Structure of matter c) arrangement and interaction of the particles of a substance
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4) Quantum mechanics d) modern theory of matter5) Chemical bonding e) process in which one set of reactants is transformed in to a new set of products
TASK 2: MAIN TOPICS
Complete the passages with the appropriate phrases describing the major branches of physical chemistry
1. ……… which revolutionized physics in the early part of this century, is required for an understanding of chemistry. The spectra of atoms and molecules are explained by quantum mechanics, and their theoretical treatment yields quantities of importance in various areas of physical chemistry. The nature of chemical bonding is explained by quantum theory.
2. ……… has been developed to provide an interpretation of the properties of matter in terms of the properties of molecules, atoms, ions, and electrons. Both thermodynamic properties and kinetic properties of matter may be calculated using statistical mechanics, provided that certain information about molecules is known from spectroscopic or other measurements.
3. ……… involves the time factor and is concerned with molecules and mechanisms. Many reactions in organic chemistry, inorganic chemistry, and many industrial processes, the products are not in the state of equilibrium, and the yields are controlled more by the relative rates of reaction than by thermodynamics. Chemical kinetics is based on almost all of physical chemistry. Kinetic theory is based upon certain assumptions about molecules. The results of kinetic theory based upon classical physics have been very useful but come into direct contradiction with certain experimental results such as the dependence of heat capacity upon temperature. However, classical kinetic theory is very helpful in understanding the results of both thermodynamics sand chemical kinetics. It is necessary to obtain a completely satisfactory theory.
4. ……… can be determined from X-ray diffraction, electron diffraction, and molecular spectra.
Information on molecular structure is of importance for understanding chemical reactions and for calculating thermodynamic and kinetic behavior. Certain types of chemical behavior can be predicted when the molecular structure is known.
5.
……… is one of the most powerful tools of physical chemistry. It provides exact relations between energy and properties of systems without any information about molecules or mechanisms of20
processes. Thermodynamics applies to systems at equilibrium and is concerned only with initial and final states. It has nothing to do with time. Thermodynamics provides an answer to the question,
―How far will this particular reaction go before equilibrium is reaches?‖
READING 2
To be technically effective in a commercial separation process, whether this be a bulk separation or a purification, an adsorbent material must have a high internal volume which is accessible to the components being removed from the fluid. Such a highly porous solid may be carbonaceous or inorganic in nature, synthetic or naturally occurring, and in certain circumstances may have true molecular sieving properties. The adsorbent must also have good mechanical properties such as strength and resistance to attrition and it must have good kinetic properties, that is, it must be capable of transferring adsorbing molecules rapidly to the adsorption sites. In most applications the adsorbent must be regenerated after use and therefore it is desirable that regeneration can be carried out efficiently and without damage to mechanical and adsorptive properties. The raw materials and methods for producing adsorbents must ultimately be inexpensive for adsorption to compete successfully on economic grounds with alternative separation processes.
The high internal surface area of an adsorbent creates the high capacity needed for a successful separation or purification process. Adsorbents can be made with internal surface areas which range from about 100 m2/g to over 3000 m2/g. For practical applications, however, the range is normally restricted to about 300 – 1200 m2/g. For most adsorbents the internal surface area is created from pores of various size. Many adsorbent materials, such as carbons, silica gels and aluminas, are amorphous and contain complex networks of interconnected micropores, mesopores and macropores. In contrast, in zeolitic adsorbents the pores or channels have precise dimensions although a macroporous structure is created when pellets are manufactured from the zeolite crystals by the addition of a binder. Fluid molecules which are to be adsorbed on the internal surface must first pass through the fluid film which is external to the adsorbent particle, thence through the macroporous structure into the micropores where the bulk of the molecules are adsorbed.
Adsorbent particles have a finite capacity for fluid phase molecules and therefore extended contact with a feedstock will ultimately lead to the creation of a thermodynamic equilibrium between the solid and fluid phases. At this equilibrium condition the rates of adsorption and desorption are equal and the net loading
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on the solid cannot increase further. It now becomes necessary either to regenerate the adsorbent or to dispose of it. For those applications in which it is economically favorable to regenerate the adsorbent it is necessary to devise processes in which the regeneration method can be incorporated.
TASK 3: COMPREHENSION QUESTIONS Answer the questions below
1. What are the required properties of an adsorbent material to be effective in a separate process?
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2. Why is it necessary that adsorbents be regenerated?
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3. What is the internal surface area range of adsorbents in practical applications?
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4. In what case are the rates of adsorption and desorption equal?
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5. How many types of pores are there in silica gels?
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TASK 4: GAP FILLING
Complete the sentences with appropriate words or phrases
The (1) ………. properties of activated carbons are essentially due to their surface area, universal (2)
………. effect, highly microporous structure, and a high degree of surface reactivity. The availability of favorable pore size makes the internal surface accessible and enhances the adsorption rate. The most widely used activated carbons have a specific (3) ………. of 800 to 1500 m2/g. This surface area is contained predominantly within micropores that have effective diameters smaller than 2 nm. In fact, a particle of active carbon consists of a network of pores that have been classified into micropores (diameters < 2 nm), mesoporous (diameter between 2 and 50 nm) and macropores (diam. > 50 nm). The (4) ………. do not contribute significantly toward surface areas but act as conduits for the passage of the adsorbate into the interior (5) ………. and the (6) ………. surface where most of the adsorption takes place. Although the adsorption capacity of active carbons is determined by their physical or porous structure, it is strongly influenced by the chemical structure of their surface. In graphites, for example, which have a highly ordered crystalline structure, the adsorption capacity is determined by the dispersion component of London forces. In the case of active carbons, however, the (7) ………. ordering of the aromatic sheets causes a variation in the arrangement of electron clouds in the carbon skeleton, which
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results in the creation of unpaired electrons and incompletely saturated valencies that would undoubtedly influence the adsorption behavior. In addition (8), ………. are generally associated with oxygen and hydrogen, which are present in the form of carbonoxygen and carbon-hydrogen surface groups. These surface groups are bonded at the edges of the aromatic sheets. Because these edges constitute the main adsorption surface, these surface groups profoundly influence the adsorption behavior of active carbons.
Besides, the active carbon surface has (9) ………. in the form of edges, dislocations and discontinuities that determine the chemical reactions and the catalytic properties of active carbon.
READING 3
Adsorption decolorization of sugar solutions is the final step in the purification process. The sugar juices are first cleared of the impurities by processes such as clarification (liming), saturation, affination, centrifugation, and filtration. These treatments help the activated carbon to retain its adsorption capacity.
The activated carbon treatment is the last stage of the purification process before the sugar juices are boiled to produce white mother liquor from which white sugar can be obtained. In the manufacture of beet sugar, activated carbon is used for decolorizing thin juice, thick juice, and liquors.
Sugar solutions contain different types of coloring matter. They can be classified into (i) caramels (i.e., nitrogen-free coloring substances formed by the partial thermal decomposition of sugars containing phenolic and quinoid groups, (ii) melanoidines (i.e., nitrogen containing coloring substances formed by reactions of reducing sugars with amino compounds, and (iii) iron containing polyphenolic complexes.
These coloring substances are present both as dissociated and nondissociated compounds, but frequently the anion types are prevalent. The substances with molecular masses between 8000 and 15000 occurring in colloidal form produce the most intense color in sugar solutions.
The decolorizing capacity of an active carbon depends upon the physical structure that involves pore size and the chemical structure that includes the acidity or alkalinity of the carbon surface as also on the nature of the coloring matter present in the sugar solution. Within the range of pH values met with in the sugar manufacturing process, lowering of the pH value usually improves decolorization. A pH of 4.5 is optimum for decolorization, but it cannot be used because inversion of sugar may take place. It is advisable to reduce the alkalinity of sugar juices by sulfitation before treating with active carbons. Sulfitation improves decolorization by active carbons. The decolorizing activity of an active carbon is measured in terms of the molasses number.
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The treatment of sugar solutions with active carbons only slightly increases the purity of the solution, usually 0.1 percent at most, but gives the solution better optical appearance. It also markedly enhances the processing properties. For example, by removing surface active agents and colloidal substances, the surface tension of the solution is enhanced and its viscosity is decreased. These changes result in higher rates of sucrose crystallization and improve the separation of syrup from crystals during centrifugation.
TASK 5: TRUE OR FALSE
Decide whether the following statements are true (T) or false (F).
1. Sugar solutions contain various kinds of dissociated colorants.
2. The physical structure of activated carbon determines its decolorizing capacity.
3. The sugar juices are cleared of the impurities before activated carbon treatment so that the activated carbon retains its adsorption capacity.
4. The decolorization of sugar solutions works best at pH of 4.5.
5. The purity of sugar solutions is greatly improved after the activated carbon treatment.
TASK 6: MATCHING
Match a word or phrase in A with its definition in B
1) Filtration a) the process by which crystals are obtained from a saturated solution of a substance, by allowing the solvent to evaporate slowly
2) Centrifugation b) changing from an adsorbed state on a surface to a gaseous or liquid state
3) Desorption c) the process of removing undesirable chemicals, materials, and biological contaminants from contaminated mixture
4) Purification d) the process whereby fluids pass through a filter or a filtering medium
5) Crystallization e) the process of separating substances of different densities by the use of a centrifuge
READING 4
Decolorization of sugar solutions using powdered active carbons can be carried out by two methods:
contact batch method and the continuous layer filtration method. There is a third method, which is a
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combination of both the methods. The contact batch method is a batch process in that a given amount (5 to 10 kg.m–3 of syrup) of the activated carbon is added to the sugar syrup placed in a container. The resulting suspension is kept at a temperature of 80 to 90oC for about 20 min, which is sufficient time to attain adsorption equilibrium. The suspension is then pumped into filter presses to remove the active carbon.
During filtration the thickness of the filter cake (carbon bed) increases to 25 to 35 mm, and this results in an increase in the hydrodynamic resistance from 2 x 105 to 4 x 105 Pa. Therefore, the pressure during filtration has to be increased from 2 atm to 4 atm. About 600 kg of carbon is collected in 1m 3 of filter volume. When the filter press is full, the active carbon is sweetened off by washing with 6 to 9 times its weight of water. The active carbon is then removed and the filter press assembled again.
Continuous layer filtration involves filtering the sugar syrup through a layer of activated carbon. Several types of filters are used, such as pressure leaf filters with metal frames on which a filter cloth that may be cotton, polyamide, or wire mesh is fixed; rotary leaf filters or bed filters in which the filtering medium is a ceramic or sintered plate, wire mesh, or finely perforated metal plate. The latter filters are usually coated with a layer of filter aid that may be a diatomaceous earth. A suspension of active carbon in water or liquor is passed through the filter until a uniform layer of active carbon bed 10 to 15 mm thick builds up.
The filter is then ready for filtration of the liquor that must flow to the filter at a uniform rate to avoid breaking the layer. It is worth mentioning here that each of the two methods described above requires different activated carbons with different properties. For example, in the contact batch method, the active carbons used should have good filtering properties, because the flow rates here are about 10 times higher than in the continuous layer filtration method, although the decolorizing ability of the carbon may be lower. This method is very flexible, and dosage of the carbon can be easily varied depending upon the color of the syrup or juice. The continuous layer filtration, on the other hand, requires an active carbon with high decolorizing properties. In this case, the filtering properties of the active carbons are not important. Another important difference between the two methods is the consumption of the active carbon that amounts to 0.3 to 1.0% of refined sugar in the contact batch method and about 10 times smaller (i.e., 0.05 to 0.1%) in the continuous layer filtration method. However, the quality of the active carbon in the latter method has to be higher. The choice between the two methods depends upon the size of the plant and the economic considerations.
The combined method utilizes the advantages of the above two methods. The filter is precoated with a layer of fresh active carbon, and a dose of active carbon can be added to the sugar solution. The decolorized solution is refiltered through a bed of diatomaceous earth, which serves to remove active carbon that might have passed through a damaged filter, cloth, or channel in the filter cake. Without refiltration, the final product may have a gray color.
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Compare the contact batch method and the continuous layer filtration method in terms of principle, the quality and the consumption of activated carbon.
TASK 9: How to find good keywords
Find the keywords in reading paragraphs 3 and 4 TASK 10: Summary
In about 5 sentences, summarize the main idea in paragraphs 3 and 5
TASK 11: Glossary
Search your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
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No Terminology Definition Vietnamese
1 thermodynamics
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20 micropores21 mesopores 22 macropores 23 pellets 24 particle 25 adsorption 26 desorption 27 activated
carbons 28 porous 29 effective
diameters 30 valencies 31 saturation 32 affination 33 centrifugation 34 acidity