General chemical technology, part I: Manual for students of chemical specialties of higher education institutions 9786010427969

Table of contents :
Chapter 4
Chemical Reactors
1. Classification of chemical reactors
A semi-batch reactor is a variation of a batch reactor in which one reactant may be added intermittently or continuously to another reactant contained in a vessel as reaction proceeds.
Reactors operating in a steady-state and transient state mode are distinguished according to the nature of the change in process parameters over time.
Printed in the printing office of the «Kazakh university» publishing house.

Citation preview

AL-FARABI KAZAKH NATIONAL UNIVERSITY

G. K. Vassilina

GENERAL CHEMICAL TECHNOLOGY PART I

Educational manual

Almaty «Qazaq university» 2017

UDC 66.0 (075.8) LBC 35.50 я 73 V 30 Recommended for publication by the decision of the Academic Council of the Faculty of Chemistry and Chemical Technology, Editorial and Publishing Council of Al-Farabi Kazakh National University (Protocol №5 dated 11.07.2017); Educational and methodical association on groups of specialties «Natural sciences», «Engineering and technology» of Republican educational-methodical council on basis Al-Farabi Kazakh National University (Protocol №2 dated 29.06.2017) Reviewers: Doctor of chemical sciences, Associate professor D.N. Akbayeva PhD, Associate Professor K.O. Kishibayev PhD K.Kh. Khakimbolatova

V 30

Vassilina G.K. General chemical technology, part I: Manual for students of chemical specialties of higher education institutions / G.K. Vassilina. – Almaty: Qazaq university, 2017. – 130 p. ISBN 978-601-04-2796-9 The book is educational manual on the course «General chemical technology» for students of chemical specialties of higher education institutions. The general regularities of reactionary processes of chemical technology, basis of the theory and the choice of the chemical reactor are stated in the first part of educational manual. The chemical production is considered as a chemical-technological system, approaches to its synthesis and analysis are given. The problems of rational use of raw materials and energy in chemical production are outlined and also materials on energy technology facilities are included. Published in authorial release.

UDC 66.0 (075.8) LBC 35.50 я 73 ISBN 978-601-04-2796-9

© Vassilina G.K., 2017 © Al-Farabi KazNU, 2017

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PREFACE The manual is designated for students of chemical specialties of higher education institutions. This manual is compiled in accordance with the basic provisions of the curriculum of the subject called «General chemical technology»; it includes 6 chapters, which disclose general particularities of the reaction processes in chemical technology, basics of the theory and selection of a chemical reactor, consideration of chemical production as a chemical technology scheme, providing also the approaches to the synthesis and analysis of the chemical production, along with the parameters of technological process efficiencies and the criteria characterizing the extent of technological perfection of the technological systems. Much attention is paid to the issues of raw material and energy base of the chemical industry, complex and reasonable consumption of raw materials and energy; there are also included the aspects related to applying of energy-efficient installations. The questions for performance of self-control are provided at the end of each chapter; it facilitates comprehension of the theoretically studied learning material. The manual of general chemical technology is a timely contribution into improvement of the process of teaching and general engineering training of the students specialized in the chemistry and chemical technology disciplines.

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INTRODUCTION. MODERN STATE AND PROSPECTS OF DEVELOPMENT CHEMICAL TECHNOLOGY In the technical education system, the main purpose of the study of «General Chemical Technology» course is the formation of ideas about the components of the technosphere, modern production and technology used in it. The development of the technological approach as a universal algorithm for transforming activity defines the overall objectives of the «General Chemical Technology» discipline. Program of study provides the formation of ideas about the technological culture of production and labor, development of the system of technical and technological knowledge and skills, training of personal labor qualities, promotes professional self-determination, formation of a pragmatic-oriented outlook, socially reasonable value orientations. The technology is a combination of methods, processes and materials used in any field of activity, as well as the scientific description of production methods. The technology (from Greek words τέχνη – art, skill, craft and Λόγος – the collection of techniques, skills, methods and processes used in the production of goods) is a set of organizational measures, operations and techniques, aimed at production, maintenance, repair and operation of a product with a specified quality and economic cost, which are caused by the current level of knowledge science, technology and society development. Modern chemical technology, using knowledge of natural and technology sciences, studies and develops physical processes, optimal ways to implement these processes and control them in the industrial production of various substances, products, materials and items. Thus, chemical technology is the science of the most cost-effective and environmentally sound methods and processes of chemical 4

processing of raw natural materials in consumer goods and capital goods. The term «processing methods and processes» means a series of successive operations carried out with the raw materials in different machines and devices in order to receive the required product. Allocation of technology in a special branch of knowledge began in the second half of the XVIII century, when the foundations of chemical technology, as a science and academic discipline, were established. For the first time, the term «technology» was used in this sense in 1772 by a professor of the University of Gottingen J. Beckmann, who published the first comprehensive works, covering the technique of many chemical industries, and were the first textbook on chemical technology. In 1795, in Germany there was a two-volume study course «Manual of Technical Chemistry» by I.F. Gmelin. Chemical Technology at the end of the XVIII century became mandatory academic discipline in universities of the higher technical educational institutions in Europe. The main trends in the global chemical industry development are: − International program «Responsible Care» developed by REACH is adopted and implemented – it is a system of registration, evaluation of properties and permission for the production of various types of chemical products aimed at strengthening of control over the production of chemicals; − Rise in the cost of energy resources when trying to optimize costs to ensure the competitiveness of products forces major companies to transfer production of basic chemical products to countries with more liberal laws and low-cost resources. When constructing logistic schemes, there are considered not only the factors of sales markets, mining areas placement, raw materials production areas, availability of cheap energy, but also technological solutions; − Combining the chemical industry, agriculture and energy in a new cluster; − Transition to safe technologies, companies’ specialization in the high value-added products; 5

− Rapid development of technology, frequent removability.The focus in new technologies is on ensuring the quality of products, reducing the consumption of raw materials, reducing the number of stages of chemical processes; − Focus on small lines producing small-tonnage chemical products. The effectiveness of specialized lines is supported by extrahigh prices for unique products and the acceleration of technological cycle reduces the time during which companies ensure return on investment; − The growth of production concentration through the creation of transnational companies. Formation of consolidated chemical concerns, the effectiveness of functioning and competitiveness of which are caused by a synergistic effect from the integration of the oil and gas processing; − Change of the largest companies’ strategies. The traditional strategy based on the effective management of assets and costs minimization is gradually replaced by a strategy based on effective management of know-how and skills on the effective use of its internal capacity possessed by the company. The main competitive advantage of the world's leading companies such as DuPont, BASF, are innovations in technologies and products; − The growth of the importance of the petrochemical industry. In the developed countries, the growth rate of petrochemical products production is by 1.5-2 times higher than the growth rate of gross domestic product. This is due to the rapid development of scientific and technical progress in the industry, which allows to create new materials with predetermined properties, introduce resource-saving technologies, increase the effectiveness.As a result, the role of petrochemical industry grows in the formation of progressive structure of production and consumption, providing economic and energy security of states, solving the problems of improving the environmental situation and social problems; − Increase in the proportion of gas feedstock (methane, propane, butane). In the United States, Chile and several other countries, the «gas chemical wing» occupies an important place in the petrochemical industry. Technology based on the processing of natural gas and gas condensate, allows to reach high produceability and pro6

duction efficiency. At this there is ensured compliance with modern environmental safety standards, minimized wastes; − Role of the chemical energy increases significantly. Its objectives is the development of highly efficient methods of energy storage in the power-consuming substances such as hydrogen and methane, which are easily transported and can store energy indefinitely; − Hydrogen resources are not limited and renewable. Using hydrogen technology, the environmental and commodity issues are completely eliminated. In case of limited oil resources, a great importance has coal and other low-calorie fuel as a raw material for chemical and petrochemical products. Currently, the work on creation and development of economical processes and methods of complex processing of coal, other non-oil types of fossil fuels in ennobled solid, liquid and gaseous types of fuel and chemical raw materials. One of the main problems is an extensive use of renewable raw materials and energy sources, especially biomass. About 90% of the biomass of the biosphere is terrestrial plant's biomass. The total reserves of biomass on Earth is estimated at 18 bln. tons with energy content of 27 bln. GJ that is equivalent to 64 bln. tons of oil. One of the most important tasks of modern technology is the development of processes excluding harmful emissions into the atmosphere and water. The main direction of solving environmental problems is the integrated use of raw materials and accelerated introduction of low-waste production processes. The most important areas of basic and applied research in the field of chemical technology are as follows: − chemical safety and environmental protection; − highly efficient chemical-technological processes, including catalytic, membrane, steel ones,electrochemical, as well as the processes associated with the use of high energy and physical methods of acceleration of chemical reactions; − new processes of in-depth and complex chemical processing of mineral raw materials, oil, gas and solid fossil fuels; − chemical power production and creation of new chemical power sources and energy conversion systems;

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− fine organic, inorganic, and element-organic synthesis, to create new substances and materials; − new construction and functional organic or inorganic (polymeric, composite, ceramic and metal), elastomers, artificial and synthetic fibers, materials, as well as ways to protect them from corrosion and wear; − new methods of instrumental chemical analysis, chemical monitoring and diagnosis of chemical processes; − chemicalinformatics. The main feature of the new technological ideology is the scientific systematic approach, considering physicochemical, social aspects of the organization of production in conjunction.

8

CHAPTER

1

GENERAL INFORMATION 1. Technological and economic criteria of efficiency of the chemical-technological process The effectiveness of any industrial process implementation is judged primarily by economic indicators, such as reduced costs, production costs, etc. Naturally, the final assessment of the chemicaltechnological process effectiveness is derived from these criteria. However, they characterize the whole process, its final result, without detailed consideration of the process features. To assess the effectiveness of the individual process steps in addition to general economic indicators, there are used the performance criteria that reflect the chemical and physicochemical nature of the phenomena occurring in the individual devices of the technological scheme. As such indicators, the initial reactant conversion degree and product yield are used. They characterize the completeness of the opportunities of a particular chemical reaction from different sides. Thus, technical excellence and efficiency of chemical and technological processes are characterized by technical and economic indicators. The basic indicators will be considered below. Performance is the indicator which characterizes the efficiency of machines, apparatus, facilities, workshops and plants operation in general. Performance is the amount of the produced product or recycled raw material per unit of time: P=

G

τ

9

,

(1.1)

where Р – performance; G – amount of the produced product or recycled raw material; τ – time. Performance expressed in tons per day (t/day), of kilogram per hour (kg/h), in tons per year (t/year), of cubic meters per day (m3/day). The maximum possible performance of the device (under optimal conditions) is called capacity. To compare operation of devices of different tools and sizes with the same processes, the term «intensity» is used. Intensity is a performance referred to any value, characterizing the device size, its volume, cross-sectional area: I=

P G = V V ⋅τ

(1.2)

I=

P G , = S S ⋅τ

(1.3)

where I – intensity; V – volume of device; S – cross-sectional area of device. The intensity is measured by the number of tons or kg of product obtained during the day or the hour unit with unit volume – t/(day·m3), kg/ h m3 or the number of tons of product obtained during the day with cross-sectional area of device – t/(day·m2). Consumption index is the ratio of raw materials, fuel, energy expended for the chemical and technological process to the produced desired product. In general, it can be written as follows: η=

Q , B

(1.4)

where Q – amount of expended raw materials, fuel, energy; B – amount of produced product. Consumption indexes show the amount of raw materials, fuel and energy expended per unit of production. Consumption indexes expressed in tonnes per tonne t·t-1, of cubic meters per tonne m3·t-1, of kilowatt-hours per tonne kVt·h·t-1 etc. 10

Practical consumption index is always higher than the theoretical ηprac> ηtheor. In production conditions in order to reduce consumption indexes, the aim is to achieve the highest values of the raw material conversion degree, product yield, selectivity of the process. To characterize the process, you need to know to what extent raw materials are used, how deep its chemical transformation. It can be found, if you compare the amount of reacted substance with its initial amount. For example, a simple irreversible reaction takes place. А → В. Assume that NAo – starting amount of a substance A (weight, volume, number of moles) and NA – amount of substance A after process. It is necessary to establish to what extent chemical conversion of agent A has passed. For this, we shall find the amount of reacted substance A, it will be equal to NAo-NA=ΔNA, and refer it to the original amount of substance A. The obtained value gives the proportion of chemically reacted substance A. It is denoted by XA and is called the degree of transformation:

XA =

N Aο − N A ∆N A . = N Aο N Aο

(1.5)

The conversion degree is the ratio of reacted reactant amount to its initial amount. The reactant conversion degree shows to what extent feedstock or share of initial reactant is used in the chemical and technological process, and it is expressed in fractions or percentage, i.e. change range of the conversion degree is determined by: 0≤χ≤1 or 0 %≤χ≤100 %. For different processes, the conversion degree is called degree of oxidation, degree of conversion, degree of polymerization, degree of absorption, etc. According to the equation (1.5) N A = N Aο (1 − X A ). 11

(1.6)

If the reaction proceeds without volume change, then

XA =

c Aο − c A c =1− A , c Aο c Aο

(1.7)

where сАо и сА – the concentration of the initial reagent A at the beginning and end of the process. From equation (1.7) we find that c A = c Aο (1 − X A ).

(1.8)

When the volume of the reaction mixture changes, functional dependence c=f(XA) includes relative change of the system volume expressed by the equation:

εA =

V XA=1 − V XA=0 V XA=1 = − 1, V XA=0 V XA=0

(1.9)

where εА – relative change of the system volume; 𝑉𝑉𝑋𝑋 𝐴𝐴 =0 and 𝑉𝑉𝑋𝑋 𝐴𝐴 =1 – volume of the reaction mixture at XA=0 and XA=1 respectively. In the special case – at linear change of the volume of the reaction mixture in time – volume dependent on the conversion degree can be written (1.10) V = V0 (1 + ε A X A ). Considering equations (1.6) and (1.10) the current concentration of the initial reactant can be expressed as

cA =

N A N Aο (1 − X A ) = V Vο (1 + ε A X A )

(1.11)

but

N Aο = c Aο Vο 12

(1.12)

there for

1− X A 1+ ε A X A

(1.13)

c Aο − c A . c Aο + c Aε A

(1.14)

c A = c Aο or

XA =

The product yield is a ratio of the received desired product to its maximum possible amount, which would be obtained under the given conditions of performing the chemical reaction. The product yield is expressed as a share of some maximum possible value and varies within 0≤QR≤1 or 0 %≤QR≤100 %. For irreversible reactions A → R, the maximum possible quantity of product R will be obtained, if the whole A reactant reacts. QR =

nR n R ,max

,

(1.15)

where QR – amount of the desired product; nR – amount of the product R at the end of the process; nR,max – maximum possible amount of the product R. However, in this case nR,max=nAo, and nR=nAo-nA, therefore for irreversible processes QR =

nR nR ,max

=

n Aο − n A = χ A. n Aο

(1.16)

For reversible reactions, the important concept is the equilibrium conversion degree; for reaction A ↔ R the maximum possible amount of the product R is determined by the equation. X A∗ =

n Aο − n ∗A , n Aο

(1.17)

where X А∗ – equilibrium degree of conversion; n ∗A – amount of initial reagent A in a state of equilibrium. 13

∗

For reversible reactions n R ,max = n R , therefore QR =

nR , n R∗

(1.18)

where n R∗ – amount of product R at equilibrium. ∗

∗

However, п R = n Aο − n A and n R = n Aο X A , therefore it follows from equation (1.18) that for the reversible reaction.

QR =

nR n Aο − n A X A = = ∗. nR∗ n Aο X A∗ XA

(1.19)

Thus, for reversible reactions, the product yield is equal to a proportion of actually reached degree of conversion for such conditions of reaction. For complex reactions when the same starting substance can be used in several chemical reactions and different reaction products can be produced, it is insufficient to assess the process only by the conversion degree.The conversion degree may be high, i.e. most of the raw materials enter into a chemical reaction, but this transformation will not always lead to the formation of the desired (target) products. In addition to the desired products, waste products (co-products) may be formed. The more target products and less co-products are formed, the more efficient process flows. To characterize such complex processes the concept of selectivity is used. Selectivity is the ratio of the starting substance consumed on the target reaction to the total amount of starting substance consumed on all the reactions (target and side reactions). For example, if a parallel reaction proceeds in the process

and the target product is R, then the selectivity is expressed as: 14

ϕR =

nR , n R + nS

(1.20)

where φR – selectivity; nR and nS – total products R и S. Since for this parallel reaction nR-nS=nAo-nA, then

ϕR =

nR . n Aο − n A

(1.21)

Production cost is the monetary value of the costs of the given company for output and sale of a production unit. Company costs directly relating to the output of products are composed of the following articles: 1) raw materials, semi-finished products and basic materials directly involved in the chemical reactions; 2) fuel and energy for technological purposes; 3) wages of the main production workers; 4) depreciation, i.e. deductions for compensation of capital consumption: buildings, structures, equipment, etc.; 5) shop expenses consisting of the costs for the maintenance and current repairs of basic production assets (including salary of support and maintenance workers), as well as the costs for the maintenance of administrative and managerial staff, occupational health and safety; 6) a tax for pollution of the environment; 7) plant costs. Usually, the cost of by-products produced from the same raw material is reduced from the basic product prime cost. Calculation of prime cost is made in a separately designated form in which all cost components are reflected. Quality of products. In most cases, the quality of chemical products is determined by their purity or content of the base material. Superior grade products are considered to be the materials containing the maximum amount of the base material and minimum of impurities. The quality of the chemical product, i.e. its contents and properties must meet the requirements set out in national or international standards, where the key indicators characterizing the pro15

duct are specified. Requirements for novel products on which no standards has been established yet, shall be determined by technical specifications.

2. Material and heat balances To analyze the process, compare different production methods and schemes, choose reactors and other devices in the design of a new production process, the process design is performed. For this, this material and heat balances are prepared. The material balance is based on the law of conservation of mass. Regarding the chemical-technological process, it means that mass of substances received for process step (input) is equal to mass of substances obtained in this operation (output): ∑ min = ∑ mout ,

(1.22)

where ∑min – total mass of the starting substance; ∑mout – total mass of the final products of the process. Any chemical production can be considered as an aggregate of material flows, involved feed components, intermediates and byproducts, the desired product and production waste. To prepare the material balance it is useful to use schemes that reflect the movement and transformation of all material participants of the process. As for the reaction: AB+CD→AD+BD+BC scheme of material flow may be represented as follows (figure 1):

Fugure 1. Scheme of material flow of technological process

16

The material balance is calculated by equation of a chemical reaction taking into account parallel and side reactions. Raw materials often have a complex composition, include a main component and impurities. Income items are the masses of useful material components and impurities in raw materials; expenditure items are the masses of the target product, co-products, waste production and losses. The material balance is accounted per unit of mass of the designed product or a separate apparatus or unit of time (hour, year). Usually for periodic processes, the balance is accounted for one operation, for continuous processes – per unit time. In addition to the balance for all substances, the individual components balance is used. The results of the material balance calculation are provide in table 1. Table 1 The material balance Input Substance Raw material 1 A B Raw material 2 C D Total

Output Amount kg %

Ѵ

100

Substance Desired product BC Byproduct DA Waste BD Total

Amount kg

%

Ѵ

100

According to the conditions of the mass conservation, figures marked Ѵ should match. However, due to rounding of the obtained results, these data may be slightly different. Based on the material balance the consumption ratios are calculated, dimensions of devices are defined, the optimal values of technological mode parameters are set. The heat balance is based on the law of conservation of energy. Regarding the chemical-technological systems, this law is formulated as follows: the amount of heat received for process step (input) is equal to output of heat in this operation: ∑ Q in = ∑ Q out ,

(1.23)

where ∑Qin – total input of the heat; ∑Qout – total output of the heat. 17

Income and expenditure items in heat balance are the heat effect of reaction ∆𝐻𝐻, the heat of phase transitions Q1, the heat introduced by the raw materials and removed by products Q2, the heat supplied from outside and removing it with the reaction products, and through the vessel walls Q3, a heat loss in this technological operation Q4: ∆𝐻𝐻 + 𝑄𝑄1 + 𝑄𝑄2 + 𝑄𝑄3 + 𝑄𝑄4 = ∆𝐻𝐻1 + 𝑄𝑄11 + 𝑄𝑄21 + 𝑄𝑄31 + 𝑄𝑄41 ,

(1.24)

where: index (1) refers to output items The heat introduced by the raw materials and removed by products (heat content of substances) is calculated by the formula: 𝑄𝑄2 = 𝑚𝑚 ∙ 𝑐𝑐 ∙ 𝑡𝑡,

(1.25)

where m – mass of substance, c – heat capacity, t – temperature. Heat effect of chemical reaction Qp or reaction enthalpy change is calculated according to the Hess's law, where Qp = – ΔHp: ∆𝐻𝐻 = � ∆𝐻𝐻𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝

− � ∆𝐻𝐻𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖

𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

,

(1.26)

The values of the enthalpy of reaction products and starting substances are taken from tables. The heat of phase transitions Q1is calculated according to the formula: (1.27) 𝑄𝑄1 = 𝑚𝑚 ∙ 𝑞𝑞, where m – mass of substance, q – the specific heat of the corresponding phase transition (evaporation, condensation, dissolution, crystallization, etc.). Heat supply and extraction is calculated by the formula: 𝑄𝑄3 = 𝑚𝑚 ∙ 𝑐𝑐 ∙ �𝑡𝑡𝑖𝑖𝑖𝑖 − 𝑡𝑡𝑓𝑓 �,

(1.28)

where: m – mass of the heat carrier, c – heat capacity of the heat carrier, tin and tf – the initial and final temperature of the heat carrier. 18

A heat loss to the environment through the vessel wall is calculated by the formula of heat transfer through the wall: 𝑄𝑄3 = 𝐾𝐾 ∙ 𝐹𝐹 ∙ (𝑡𝑡𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 − 𝑡𝑡𝑎𝑎𝑎𝑎𝑎𝑎 ) ∙ 𝜏𝜏,

(1.29)

where К – heat transfer coefficient, F – heat exchange surface, 𝑡𝑡𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 and 𝑡𝑡𝑎𝑎𝑎𝑎𝑎𝑎 – temperature of device wall and the ambient temperature, τ – time. Table of heat balance looks similar to the table of material balance. Self-Control Questions 1. What kind of technological criteria of efficiency of chemical and technological process do you know? Give their definitions. 2. What is called productivity, power, intensity? 3. Please, describe the relationship between: a) productivity and conversion of reagent; b) productivity and yield of a target product? 4. What are limits of conversion, yield and selectivity measurements? 5. What does it mean: "reagents are taken in the stoichiometric ratio"? 6. What is the difference between the real and the equilibrium degrees of reagent's conversion? 7. Derive the equation of the relationship between conversions of the two reagents entering into the reaction aA+bB→rR+sS if it is known that nA0 mole of reagent A and nB0 mole of reagent B is taken to carry out the reaction. 8. Derive the equation of the relationship between the yield of the target product P, the degree of conversion of reagent A and the total selectivity of φ when carrying out two irreversible sequential reactions A→R (primary reaction) R→S (side reaction). 9. Derive the equation of the relationship between the yield of the target product, the degree of conversion of the reagent and the total selectivity for parallel reversible reactions a1A+b1B→rR (primary reaction) a2A+b2B→rR+sS (side reaction). 10. Calculate the total selectivity during the sequential reaction A→R+M (primary reaction) R→S+N (side reaction). 11. What is the material balance of chemical production? Decipher its structure by the example of one of the productions. 12. What is determined on the basis of the heat balance of the equipment?

19

CHAPTER

2

BASIC REGULARITIES IN CHEMICAL TECHNOLOGY 1. Classification of chemical reactions There are a number of features upon which chemical reactions can be classified. By mechanism of reacting, there are simple (singlestage) and complex (multi-stage) ones. In the course of a simple reaction: А→R or A+B→R one stoichiometric equation is enough to describe it. Complex reactions refer to reactions comprised of more than one simple reactions interrelated between each other: for example parallel reactions: 𝐴𝐴 → 𝑅𝑅

and serial reactions:

R or

A

𝐴𝐴 → 𝑆𝑆

S

A→R→N. To describe a complex reaction, several stoichiometric equations are necessary. In terms of reversibility, chemical reactions are divided into reversible and irreversible. In fact, all reactions are reversible, since any reaction can go at noticeable speed both forward and backward under appropriate circumstances. However, under these 20

circumstances, many chemical reactions go in one of directions at near-zero speed, an therefore such reactions are considered to be irreversible by convention. In terms of kinetics, chemical reactions are classified by molecularity of reaction or by reaction order. Molecularity of reaction is characterized by the number of molecules, in simultaneous interaction of which elementary act of chemical interaction occurs. According to this feature, chemical reactions can be divided into mono-, bi- and trimolecular ones. You may talk about higher molecularity, but in fact, even simultaneous collision of three molecules is already improbable. When chemical equation indicates that there is a large number of molecules taking part in any reaction, the sum of stoichiometric coefficients a and b in the equation 𝑎𝑎𝑎𝑎 + 𝑏𝑏𝑏𝑏 → 𝑟𝑟𝑟𝑟 + 𝑠𝑠𝑠𝑠 ± 𝑄𝑄

(2.1)

is more than three. The process proceeds substantially harder and runs in two or more stages. In each of these reactions, interaction is carried out in collision of two or rarely three molecules. The order of reaction n is the sum of exponents of concentrations in equation expressing the dependence of reaction rate on concentration of reacting agents. For example, as for the above reaction considered as a model, on-rate is expressed by the following equation: 𝑏𝑏 (2.2) 𝑟𝑟 = 𝑘𝑘𝐶𝐶𝐴𝐴𝑎𝑎 𝐶𝐶𝐵𝐵.

Based on the definition, the order of this reaction is n=a+b. In terms of order, reactions are divided into first, second and third order. There are reactions the order of which is expressed by fraction and zero (zero-order reaction). The order of catalytic reactions is almost always lower than their molecularity. In chemical engineering, classification by phase feature is commonly used, according to which there are homogeneous reactions, where reacting agents contained in a single phase, and heterogeneous reactions, where reacting agents contained in separate phases. Reactors, where they are executed, are divided on the same account. 21

Depending on application or non-application of special agents – catalysts to change reaction rate, there are catalytic and uncatalytic reactions, and thus chemical technology processes. The vast majority of chemical reactions, on which industrial chemical technology processes are based includes catalytic reactions. Chemical reactions on the sign of the thermal effect are divided into exothermic (–ΔΗ) and endothermic (+ ΔΗ). Chemical reactions on reversibility are divided into reversible and irreversible. Irreversible processes proceed in one direction only. Reversible reactions differ from the irreversible ones by the fact that reaction products turn into initial products again.

2. Thermodynamic calculations of chemical-technological processes When designing technological processes, thermodynamic calculations of chemical reactions are vital. They allow giving findings on possibility of this chemical transformation in principle, preselecting arrangements for the process, and determining equilibrium composition of the product that is necessary for composition of energy balances. Thermodynamic parameters fall under extensive and intensive ones. Amounts proportional to the mass of thermodynamic system are extensive amounts, these are volume, internal energy, entropy, enthalpy. Extensive parameters tend to be additive. Intensive parameters do not depend on the mass of thermodynamic system, and they serve as the thermodynamic state parameters only. These are temperature, pressure, as well as extensive parameters per unit of mass, volume or amount of substance. Change of intensive parameters in order to speed up the chemical technology process is known as intensification. Reaction orientation. Chemical process is physically feasible, if any reaction proceeds with reduction of chemical potential, which represents isobaric potential or Gibbs energy, i.e. possibility of its progress is defined by equality. ∆𝐺𝐺𝑃𝑃,𝑇𝑇 < 0, 22

(2.3)

where ∆𝐺𝐺𝑃𝑃,𝑇𝑇 – the change in Gibbs energy when the initial substances are converted into products. The following data and ratios are used for ∆𝐺𝐺𝑃𝑃,𝑇𝑇 calculation. Thermodynamics reference literature contains the values of standard Gibbs energies of material formation (∆𝐺𝐺298 )form at the temperature of 298 K. For the reaction 0 0 ∆G298 = � vi (∆G298 )form i . i

(2.4)

Dependence of Gibbs energy on temperature: ∆𝐺𝐺𝑇𝑇0 = ∆𝐻𝐻𝑇𝑇0 − 𝑇𝑇∆𝑆𝑆,

(2.5)

where ∆𝐻𝐻𝑇𝑇0 , ∆𝑆𝑆 – changes in enthalpy and entropy, which are calculated by formulas similar to (2.4). Dependence of Gibbs energy on the composition of the reaction mixture (for gas mixtures): 𝑣𝑣

∆𝐺𝐺𝑃𝑃,𝑇𝑇 = ∆𝐺𝐺𝑇𝑇0 + 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅П𝑝𝑝𝑖𝑖 𝑖𝑖 ,

(2.6)

where П – Sign of multiplication, pi – partial pressure of components. Inequality (2.3) allows determining the possibility of using the process to obtain the desired product – this is the beginning of development of new production method. Another use of inequality (2.3) is in finding the conditions that prevent the occurrence of adverse reactions. Thermal effect of reaction. Another thermodynamic quantity important for production research is thermal effect Qp, or reaction enthalpy change. They are opposite in sign: Qp=-ΔН. Enthalpy change is calculated through the enthalpies of formation of materials involved in the reaction. Depending on the sign of ΔH or Qp: ΔН0 – exothermic reaction; ΔН