Additional information on industrial equipment and chemical engineering processes
1.1 Classification of chemical engineering processes
All chemical engineering processes are divided into several main groups depending on kinetics of process behavior:
- Hydromechanical processes – these are technological processes which proceed on the basis of momentum transfer principles in gaseous and liquid systems, and seldom – in solid-phase systems. Their basis is hydromechanical influence on products, and driving force is drop of pressures. The rate of these processes flow is determined not only by laws of hydromechanics, but mechanics as well, since mechanical processes also join this group;
- Thermal processes – these are technological processes the behavior of which is related to most varied by their nature forms of heat transfer in region with non-uniform temperature field. Their basis is variation of thermal state of mutually interacting media, and driving force is difference of temperatures of these media. The rate of these process reaction is determined by laws of heat conduction and heat transfer.
- Mass-exchange processes– these are chemical-technological processes the reaction of which is related to transfer of one or several substances from one phase to another by means of phases interface. Their basis is mass-exchange between interacting phases, and driving force – difference of concentrations of distributed substance (substances). The rate of such processes flow is determined by laws of mass transfer.
- Chemical processes – these are the processes that represent one or several chemical reactions accompanied by phenomena of transfer of energy and such quantities, as heat, momentum and mass which influence each other and flow of reactions. Their basis is profound change of structure, properties and make up of substances involved in the reaction, and driving force is difference of chemical potentials. The rate of these processes reaction is determined by laws of chemical kinetics.
By their structure main chemical engineering processes are classified into intermittent, continuous and combined (mixed) processes.
Intermittent processes proceed in single equipment, where a portion of initial reacting substances are introduced before reaction start. All stages: mixing of these substances, their chemical interrelations and production of final products, making up one cycle, proceed sequentially one after another and are periodically repeated within certain period of time. In-between cycles when raw material is loaded and final product is unloaded, the apparatus is idling. The main feature of intermittent processes is that all stages take place in a single apparatus in sequence order. Industrial equipment where intermittent processes take place can be a closed-, or open-loop system. For example, autoclave is a closed-loop system, since it is tightly closed during the cycle. But batch rectification column due to continuous distillate drain in the course of operation is an open-loop system.
Continuous processes – proceed without auxiliary stages in flow apparatuses. It means that loading of fresh raw material into equipment and final product unloading take place without equipment downtime, in other words, continuously. All process stages are carried out simultaneously, but either in different zones of a single apparatus, or in different apparatuses. And besides, all parameters of this process, like temperature, pressure and so on remain invariable in time.
Combined processes – these can be both continuous processes when some stages are performed intermittently, and intermittent processes when one or several stages proceed continuously.
The use of continuous processes thanks to their numerous merits considerably enhances hardware performance, quality and homogeneousness of products, reduces the need for maintenance personnel, provides more extensive mechanization and facilitates production automatic control, and on top of that improves labor conditions.
Intermittent processes, though displaced by continuous ones, still retain their significance. Thanks to their advantages (versatility, use of cheap means of reagents dispensing, small amount of equipment) intermittent processes today find application at small production sites with sufficiently versatile product range. They make it possible to achieve greater flexibility of equipment operation at relatively low capital costs.
Depending on behavior of process parameters (temperature, concentration, rate, pressure and so on), distinguished are stationary (steady-state) and nonstationary (non-steady) processes.
In the first (stationary) processes any parameter can vary from one point to another inside chemical equipment, still it retains its value in time.
As to nonstationary processes, values of parameters (unlike that of stationary) vary both in time and space. They include all intermittent processes, as well as semi-continuous.
Nonstationary processes in continuously running apparatuses are considered all transient processes that result from variation of operation parameters. Analysis of nonstationary processes is much more complicated as compared to the analysis of stationary processes due to the fact, that all their parameters are time-dependent.
Depending on the number of phases (by quantity) involved in the process, distinguished are homogeneous (proceeding within one phase) and heterogeneous (proceeding at the interface of two phases) processes.
Depending on the number of components (their quantity) distinguished are processes with single-component and multicomponent streams. Theoretical foundation of hydromechanical, thermal and mass-exchange processes is made up of such principal laws of nature, as laws of transfer, retention of substance and thermodynamic equilibrium.
General principles applied in calculations of chemical engineering processes and equipment
The aim of the calculation is the assessment of costs (power and material costs) required for processing, creation of optimal conditions for process flow, and also calculations of dimensions (basic) of apparatuses to be used.
The following sequence is observed in calculation:
- by laws of thermodynamics and hydrodynamics direction of process flow and its equilibrium conditions are determined;
- proceeding from equilibrium data obtained, starting and finite process points are selected;
- heat-and-mass balances are drawn up proceeding from conservation laws;
- process driving force is determined by quantities which define equilibrium and operation parameters;
- coefficient of process flow rate is calculated on the basis of kinetics laws;
- basic equipment size is determined by data obtained. As basic size the following areas can be accounted for:
- cross-sectional area;
- heating surfaces;
- phase contact surface.
Many processes used in chemical technology are multi-stage, in other words, they come through a number of stages (steps) and develop in several ways. And only one out of possible stages is, as a rule, process-limiting, So naturally namely limiting stage should be influenced.
Determination of the limiting stage depends on the ratio of rates at all stages, their sequence or different ways of flow. If the process can proceed in parallel using two or more different methods, the limiting is the method having maximum process intensity (rate of flow). In the event the process proceeds strictly sequentially, the limiting is the slowest step that takes the longest time. The following definition can be given: the limiting stage is the stage determining total rate of multi-stage process flow, defined by ratio of rates, mutual place and sequence of stages.
Nevertheless, there are processes during which neither the highest nor the lowest rate may be determined as processes-limiting. Such thing happens when non-limiting stage at first sight substantially influences the flow of stage, which on all grounds should be considered as limiting.
Process intensification is called increase of rate at which substance transfer takes place. From the point of view of single kinetic law described by formula
↑q = (dA) / (Sdτ) = (↑Δ) / (R↓) = ↑K↑Δ
the rate of substance transfer within process is proportional to its driving force and is in an inverse proportion to resistance, in which case:
- Driving force depends on the degree of deviation from equilibrium in the current process status and hydrodynamic conditions of its flow.
- To a considerable degree resistance depends on substance transfer mechanism.
- In multi-stage process the rate is determined by its limiting stage.
Through boundary layer the substance is often transferred by the slowest of the transfer mechanisms – molecular, and namely because of that this stage is, as a rule, limiting. In other words, design and technological methods that reduce thickness of boundary layer lead to increase of this stage rate, thus accelerating substance transfer process as a whole.
Most diverse hardware of chemical engineering is used in accordance with conditions of treatment processes and properties (chemical and physical) of materials being processed.
Crucial factors that determine the type of chemical equipment used include:
- chemical properties of substances involved in the process;
- aggregative state of these substances;
- temperature at which the process proceeds;
- operation pressure;
- heat exchange intensity;
- thermal effect.
In the course of development of chemical branches of industry chemical processes are partitioned into subranges, which encompass rather narrow lists of related processes. They are characteristics of specific branches only. However it cannot go unnoticed that there are, such apparatuses and processes are available that are common for many industries. Diverse process technologies may be combined into certain groups, and their general principles considered as fundamental for processes of each such group.
Basic physical laws
Chemical and technological processes are, by definition, related to chemical and physical phenomena that take place in reality. However, in the majority of cases, these processes can be defined by a small number of physical laws. For example, physical law of conservation of energy of mass serves as a basis for energy-and-mass balances. The laws that characterize equilibrium conditions are of quite significant importance for understanding of numerous processes, as well as laws that describe variations taking place in systems not being in equilibrium condition.
Material balance equation In accordance with physical law of mass conservation, amount of substances delivered for processing (∑Gнач) equals to the amount of substances obtained after termination of processing process (∑Gкон). Then material balance equation can be given in the following form:
∑Gнач = ∑Gкон
Material balance for intermittent processes is calculated per one operation, whereas for continuous processes – per one unit of time, for example, per one hour.
Material balance can be drawn up for one apparatus, its particular sections and even for a group of apparatuses. At the same time the material balance can be made for all substances subject to processing, or only one of the components making up the mixture.
As an example we consider filtration of suspension resulting in recovery of filtrate and precipitant. We might say that in this case material being processed comprises only two components: fluid and solid substance. Then material balance equation is set up either assuming total amount of suspension involved in the process, or only for solid fraction, or solely for liquid component. Only the latter two equations from three material balance equations can be considered independent. The equation describing material balance involving total amount of suspension can be derived by term-by-term addition of material balance equations for liquid and solid substance.
Energy balance equation. In accordance with energy conservation law quantity of energy introduced in the process should equal to its quantity obtained as a result of process execution.
Energy can be introduced in the process and extract it along with substances involved in it, or independently, apart from these substances. The energy introduced and extracted along with substances is an aggregate of intrinsic energy of these substances, including their kinetic and potential energies.
Energy introduced to and extracted from the process independently from introduction and extraction of substances involved in it, can be represented by:
- heat fed to an apparatus by way of its heating across walls, or by electric current;
- mechanical work consumed in compressor or pump;
- heat irradiated to environment.
Most general form for energy balance expression in reference to chemical and technological processes is generalized equation of Bernoulli:
(ρυ²) / 2 + ρgh + p = const
In this formula
ρ — fluid density,
p — pressure in point of space where center of mass of fluid element under consideration is located,
h — height at which fluid element under consideration is located,
υ — flow rate,
g — gravity acceleration.
Description of equilibrium conditions. Any process will flow till that moment when the state of its complete equilibrium finally settles. So fluids flow from vessels with higher fluid level to vessels with their lower level until levels of fluids in all vessels equalize. Heat is transferred from more heated body to less heated body until temperature of both bodies becomes equal. Salt will dissolve in water until solution becomes saturated. Many such examples can be singled out. Namely because of this equilibrium conditions demonstrate the so-called statics of any processes and define the limits up to which they can flow.
Equilibrium conditions can be expressed by different laws: for example, the second law of thermodynamics and laws defining ratios that occur in different phases of system between concentrations of components.
Equation of process reaction rate. When any system is in nonequilibrium condition, a process that tends to bring this system to equilibrium state is sure to occur. Usually, process rate in this case is the higher, the more the system is deviated from equilibrium state. Thus, deviation from equilibrium state in any system expresses the driving force of the process that takes place in it. Due to this, at larger driving force the rate at which process takes place will also be higher. Upon approaching the equilibrium state both the driving force and process flow rate will retard and reach zero after equilibrium is obtained. The closer the system is to the equilibrium state the lower process rate is, and it will continue to drop upon further approach to equilibrium. Theoretically, in order to achieve absolutely equilibrium state infinitely long time is required. Still in practice comparatively quickly such state of the system can be achieved, that it will be so close to equilibration, that can be considered as equilibrium.
In practical calculations one should know rather accurately the process rate which it has at different stages, in other words, have data on the so-called process kinetics. It should be borne in mind, that in many cases process rate is proportional to its driving force. This most simple dependence can be observed in processes of filtration, where heat is transferred by way of convection and heat conductivity, in mass transfer processes. In all these cases the equation expressing process rate will have the following form:
(N / Fτ) = KΔ
N — amount of heat or substance being transferred in time across surface F;
К — coefficient of proportionality (process rate);
Δ — process driving force.
For thermal processes F denotes the heat exchanging surface, i.e., the surface across which heat is transferred to the system; in mass transfer processes F denotes the surface where phases adjoin.
The left-hand side of this equation describes process rate.
In its turn, coefficient of proportionality K is usually found by experiment, as making its calculations in a number of cases presents substantial difficulties.
Material and energy calculations
The terms taking part in each production chemical process are:
- materials subjected to processing;
- energy required for their processing;
- industrial chemical equipment implementing processes
Materials used in processes, both final products and semi-processed articles, are practically never absolutely pure, and represent mixtures of various components, i.e., several different individual substances.
Composition of such mixtures is usually expressed in parts, weight parts or percent. In process design composition of material mixtures is more convenient to be expressed in molecular percents or molecular parts (i.e., parts of mole).
The notion of material balance
In order to determine consumption of initial materials, calculate final product yield, evaluate dimensions and performance of apparatus to be used, preliminary calculations must be made relying on the law of material conservation, and stoichiometric ratios expressed by chemical equations.
In accordance with the law of conservation of matter, G1 (weight of material delivered for processing) should equal to G2 (weight of materials resulting from processing):
G1 = G2
Nevertheless, in practice under real process conditions some loss of materials takes place. So weight of products resulting from the process, is always somewhat less than weight of initial materials supplied for processing. Thus the primary formula should be substituted for the following:
G1=G2 + Gn (I)
where G denotes loss of material weight measured in kilograms-forces (kgf).
Namely equation (I) – is the material balance equation applied in equal measure both to a process as a whole, and to its certain operation or stage.
Material balance can be made for all materials involved in the process collectively, or component-by-component, for each of them, or any single one.
For example, for process where humid material is dried up, one-component balance can be drawn up proceeding from weight of dry matter in dried up material, and also by weight of moisture retained in it.
When drawing up material balance of chemical processes, equations that express reactions proceeding in these processes must be used.
Initial data used for drawing up material balance can be summarized in the table showing materials input and consumption, and for greater clearness a diagram can be made showing flows of materials in definite scale.
Material balance plays a major role in the correct run of technological processes. It allows to draw correctly process flow chart at the stage of new processing lines design and select dimensions of apparatuses used in it. In the course of manufacturing process material balance permits to identify nonproductive material losses, calculate quantity and composition of impurities and by-products, making it possible to single out optimization procedures.
Material balance enables to form an informed judgment about degree of technological process sophistication and general state of chemical production. The more comprehensive the balance is, the more thoroughly this technological process has been studied. The less by-products and losses the process has, the greater is the confidence that the process is running properly.
If a particular process is poorly known, its material balance is impossible to draw up. Big losses in the material balance identified when studying the process show that this technology needs improvement.
Final product yield
The final product yield, or just yield, is the ratio of product quantity obtained during process run and quantity of initial product delivered for processing, and expressed in percentage.
For different chemical production processes running of which can be expressed quantitatively by stoichiometric equations, yield is expressed in percentage of practically obtained quantity of product to its theoretically possible quantity, strictly corresponding to its stoichiometric equation for this reaction.
In practice yield is always less than 100% due to losses. The process is all the more perfect, the closer its yield is to 100%, as less initial products are consumed and, correspondingly, lower the cost of the produced product is.
One may do it in another way, if exact equation of some chemical processes run cannot be set up. In such case yield can be expressed quantitatively using ratio of final product to initial products supplied for processing, and then yield will always be less than 100%. One can also relate weight of final product to the weight of some single initial product, and then yield can prove to be more than 100%.
Process production rate
Production rate is a key characteristic of industrial equipment. It is expressed in quantity of materials brought in the process for processing in unit of time, i.e., second, minute, hour and even day. For measurement of production rate one can also use quantity of materials obtained during processing, and also calculated for unit of time. To express quantity of processed materials the following units of measurement are used:
- weight units – kilograms, tons;
- volumetric units – liters, cubic meters;
- numerable units – pieces, units.
For example, production rate of various mill and crushers is usually expressed in tf/hour or kgf/hour, for fluid pumps m3/s (/min, /hour), l/min can be used, and for plastic pressing production rate of presses can be expressed in pc/hour or pc/day, etc.
Under identical conditions production rate of different types of chemical equipment depends both on dimensions of these installations, and rates of processes that proceed in them. For larger dimensions of machines and apparatuses, higher rate of process flows is characteristic and consequently, higher production rate.
Intensity of technological processes flow
Process intensity is chemical equipment production rate referred to a particular basic unit defining this device. For example, for evaporation apparatuses intensity can be characterized by quantity of water evaporated within an hour from one square meter of heating surface.
Enhancement of any process intensity leads to reduction of required equipment rated for production volume, or reduction of overall dimensions chemical equipment being used. The costs of capital construction, equipment maintenance and repair drop, whereby labor efficiency as basic index of production economical efficiency increases.
Production intensification, being synonymous to intensity enhancement of technological processes used, is one of the most crucial conditions, making it possible to raise labor productivity and transfer chemical industry to higher than achieved engineering level. Intensification of production processes is tending towards manufacture of maximally large quantity of final products by using one and the same hardware, one and the same equipment, within one and the same period of time and by force of one and the same maintenance personnel.
We denote quantity of heat, in kcal, in the following way:
Q1 — heat introduced in process along with materials;
Q2 — heat introduced in process from outside;
Q3 — heat developed in the course of the process;
Q4 — heat withdrawn from the process along with materials;
Q5 — heat irretrievably lost in the environment.
Then heat balance equation can be written in the following way:
Q1 + Q2 + Q3 = Q4 + Q5 (II)
Efficiency coefficient (EC) and power
Industrial equipment is characterized not only by production rate, but power as well. It represents work consumed or obtained in unit of time. As a rule, power is expressed in kW (kilowatts) or h.p. (horse power) units of measurement. It should be noted that power consumed on the equipment drive shaft differs from engine power that actuates it. For any engine power due to loss of energy in transmission mechanisms should always be higher than power required for machine or apparatus shaft.
Thus, one may claim that useful power or useful work is always lower than actually consumed power or work. The ratio between useful power (N) and actually consumed power (Ne), taking into account all inevitable losses, is called EC – efficiency coefficient of chemical equipment:
η = N/Ne (III)
EC value is practically always smaller than one. For this purpose, the more perfectly a particular machine or apparatus runs, the closer their EC is to one.
Automation of technological processes and production sites
Pumps. Pump units. Pump systems