Chemical industrial equipment
Swiss production-engineering company ENCE GmbH was founded in 1999, has large staff of specialists (over 200 persons) involved in chemical equipment production and can deliver various chemical equipment to industrial enterprises of CIS countries, carry out service works, including contract supervision, commissioning, warranty and post-warranty service using its own resources as well as with the involvement of wide network of its representation offices in CIS countries.
We offer equipment for the following industries:
|Mining and processing industry
Oil and gas producing industry
|Oil-and-gas and chemical industry
Primary industrial equipment of chemical production and its selection
Chemical industry involves a great amount of all sorts of industrial equipment which can be divided into the following classes:
- transport facilities
- Apparatus is an engineering installation which has working volume and equipped with technological process power and instrumentation control and monitoring facilities.
- Machine is an engineering installation in which technological process behavior is accompanied with introduction of mechanical energy in the working volume by means of equipment actuators.
- Working volume (reaction space) is a place of technological process behavior.
- The second type of reactors has maximum high production capacity and is less sophisticated in design, including technological process control and monitoring facilities, but allows to get a very limited number of types of final products.
Moreover, depending on its purpose all chemical equipment is divided into:
- Universal equipment – this equipment is used at enterprises as it is, without being retrofitted. It is called general-purpose equipment or, otherwise, all-factory equipment.
- aggressive media pumps (for more details see pumps)
- mixers, dryers, centrifuges and separators (for more details see centrifuges)
- compressors (for more details see compressors)
- fans (for more details see fans and gas compression)
- filters, dust catching and gas treating systems (for more details see filters and gas treatment)
- delivery facilities.
- Special-purpose equipment is equipment which is used in one technological oft different changes. It includes:
- heat exchange assemblies (for more details see heat-exchange equipment);
- rectification (spacer liquid mixtures) columns;
- absorption apparatuses.
- Dedicated equipment is equipment which is used only for performance of one production process. It includes:
Moreover process equipment is also divided into:
- Basic equipment is machines, installations and apparatuses in which different technological operations and processes take place (physical-chemical, chemical, and so on), resulting in production of any final product (or products).
Basic industrial equipment for chemical production includes the following apparatuses:
- reaction-type – contact devices, reactors, shaft converters, converters (ammonia production) and other devices;
- machines and apparatuses for performance of physical and chemical operations and processes – heat-exchanging and evaporating apparatuses, saturation towers, extractors, absorbers, roll mills, dryers, presses, calenders and so on.
- Auxiliary equipment is various reservoirs, tanks and storages.
Auxiliary equipment is designed for performance of additional production processes. So auxiliary equipment provides storage and delivery of the following substances:
- Liquidized gases;
- Loose materials.
Thus auxiliary equipment includes tanks capable to store and transport various types of substances and materials:
- Gas holders;
It should be noted that any product (or products), as a rule, are obtained not on a single, but on a number of equipment which represent one integrated technological process. In addition, the change of chemical composition or physical form takes place in each piece of this equipment and, correspondingly, all machines and apparatuses which make up one piece of equipment will operate under its individual operating conditions. For normal technological process behavior each of these machines should maximally correspond to the product being processed under such parameters, as size, form and intrinsic property.
It follows that the type of machine or apparatuses is selected according to the aggregate state, chemical properties, temperature, thermal effect and pressure of substances being a part in the technological process.
Equipment classification by process category
Equipment of each manufacturing enterprise can be divided into dedicated and general industrial equipment. Dedicated equipment is typical for specific production and cannot be used in other technological processes. General industry equipment has universal nature and it is installed at all enterprises regardless of sector affiliation.
Processing equipment provides behavior of various technological processes in manufacturing industries. By type of process that takes place thanks to process equipment operation the latter are subdivided into classes:
- equipment implementing mechanical processes;
- equipment implementing hydromechanical processes;
- equipment implementing thermal processes;
- equipment implementing mass-exchanging processes;
- equipment implementing chemical processes.
Each individual class of process equipment is subdivided into groups in accordance with functional purpose. Groups that include certain technological apparatuses are commonly subdivided into types.
Classes of process equipment are subdivided into groups of apparatuses in accordance with their functional purpose. The term “classification” implies distribution of equipment by groups in accordance with purpose, nature and organization of technological processes, performance (power), control methods (manual, automatic, combined). Chemical production processing equipment is dedicated equipment and intended for implementation of successive chain of technological processes starting from preparation of incoming fresh raw materials to final product fabrication:
- Mechanical equipment used for preparation of fresh raw materials for processing - crushing-grinding (crushers, mills), screening (sifting machines, separators, sieves, sorters), as well as feeders, dispensers.
- Hydromechanical equipment for purification of fresh raw materials against impurities, using different densities of its components. Hydromechanical processes: centrifuges, separators, sedimentation reservoirs, cyclones, hydrocyclones, filters, scrubber;
- Equipment for air and electromechanical cleaning – cyclones, scrubbers, direct-action filters, electrical, catalytic, etc;
- Thermal processes: heat pipes and furnaces, plasmatrons, regenerative and open-type heat exchangers, crystallization and evaporation apparatuses; Thermal (heat) equipment – dryers, regenerative heat exchangers, plasmatrons, recuperators, regenerators.
- Mass exchange processes: adsorbers, dryers, apparatuses for diffusion and baromembrane processes, desalinization and dissolution equipment, ion-exchange apparatuses, rectifiers, extractors, dissolvers, neutralizers, membrane equipment, and others.
- Chemical processes: furnaces for implementation of chemical processes and chemical reactors.
The above-listed groups of equipment make it possible to decontaminate fresh raw materials, bring it to homogeneous state by mechanical, physical and chemical properties for further treatment. Final product is obtained in the next group of process equipment under generalized term “chemical reactor”. Depending on the nature of processes that take place in them, the reactors can be of cold and hot type.
Chemical reactions which proceed under natural conditions are carried out in cold reactors. If high temperatures or catalysts are required in order to obtain a product, then furnaces, autoclaves and special vessels are used as reactors. In its turn, groups of equipment are distributed into types (by mode of operation, type of drive, etc), and those in its turn – into standard sizes (by overall dimensions, power, efficiency).
For accompanying technological processes auxiliary equipment is intended – it provides collection and storage of by-products, delivery and accumulation of fresh raw materials and final product (reservoirs, gas holders, bunkers, etc). According to the method of influence on raw materials chemical reactors are divided into apparatuses and machines. Apparatus is called a facility inside of which operation zone (chemical reactions space) locates, where technological process proceeds without involvement of mechanical devices. If normal process flow requires additional mechanisms (agitators, rippers and so on), apparatus becomes a machine.
Description and classification of machines
Depending on the technological process structure and final product range, chemical reactors are subdivided into apparatuses and machines of continuous or periodic action.
Apparatuses and machines of periodic action make it possible to manufacture different types of products close in composition (for example, polymers) using the same equipment, Change-over of such reactors requires minimal consumption of time and means. Manufacturing flexibility is a major advantage of intermittent action equipment.
According to principle of technological process structure the machines are commonly divided into equipment of continuous and intermittent action.
- In intermittent action machines different technological process stages proceed within one working volume, but with different time intervals. The key advantage of such equipment is degree of flexibility, i.e., ability to quickly change-over from one type of product to another without degrading its quality.
- In continuous action machines different technological process stages proceed within different working volumes, but at one and the same time interval. The key advantage of such equipment is relatively low metal and power intensity, simplicity of design and high performance.
According to automation level the machines are commonly divided into the following types:
- Ordinary machines (operates under operator’s control) with simple (manual) control which entirely depends on operator’s experience, knowledge and skill.
- Semi-automatic machines (they perform basic operations thanks to installed program, while operator’s functions include loading/unloading, control and adjustment); semi-automats are the equipment with several programmed functions or operating modes, for the selection of which the operator is responsible.
- Automatic machines (all operations after loading and activation are carried out independently according to the installed program); automats provide full cycle of product manufacture (starting from loading of raw materials to issue of final product), and if they go beyond programmed range of parameters the technological process is interrupted on its own.
- Adapting machines (perform logical operations taking into account various conditions). More advanced systems of equipment control can take into account changing conditions of technological process behavior and independently correct operation in accordance with the current situation.
General trends in design of chemical production equipment
Design and engineering of industrial equipment
Design engineering is called a process of creation of certain technical project. Structural design is one of the directions of design engineering (detailed engineering/ project engineering). The outcome of the engineering process is a design project developed by the design engineer. Design project defines:
- elements of article;
- design of article;
- principle of article operation;
- data required for article manufacture, application, maintenance and repair.
In the development of modern chemical industrial equipment the following concepts are used:
– use of analogous (very similar) in design machines and apparatuses in different branches of chemical industry. Such concept allows to make not only some individual parts, units and assembly units, but also a number of machines and apparatuses uniform.
Unification of chemical equipment notably facilitates not only work at the stage of machines and apparatuses design, but also makes their manufacture and maintenance much easier. On top of that, introduction of unification considerably enhances effectiveness of their intended use. For example, the unification of heat exchange apparatuses at chemical plants will allow to significantly reduce the cost of their repair, maintenance and replacement and also shorten time of repair and emergency downtime. Besides, unification helps to improve methods of necessary repair works, reduces wastes of materials and parts, reduces the quantity of personnel engaged in maintenance and repair of such machines and apparatuses.
– sharp rise of intensity and effectiveness of manufacturing equipment due to some increase of scale of this or that chemical production. As a rule, this is achieved through conversion of the technological process by using higher temperatures and pressure, increase of velocity, use of more active catalysts, positive change of hydraulic modes in machines and apparatuses, and so on. For example, today many Russian chemical enterprises have already mastered synthetic process of such products, as ammonia, methanol, butyl alcohol and carbamide, which are carried out in pressurized apparatuses.
It should be noted that at the present time intensification of local chemical processes is also assisted by introduction of up-to-date technologies, the basis of which was formed by plasma, membrane, pulse, electron-beam and other chemical and physical fine processes. For example, application of plasma technology makes it possible to carry out complex chemical reactions, which require high temperature, pressure and long time within minimal short intervals.
Membrane technology is often used in industrial gas separation systems (for enrichment of air with oxygen and nitrogen), and also in production of products, such as caustic soda (sodium hydrate) and chlorine. Also new and renovating mercury processes are equipped with membrane bipolar electrolyzers. Membrane technology is often used in industrial systems.
Today pulse technology is quite often used in industrial shredders and other units for fine shredding of industrial materials, as well as bag filters and equipment for vacuum systems.
Ultrasound effect found application in acoustic filters used to clean fluids from mechanical suspensions.
- reliability enhancement
– mechanical reliability, uninterrupted and prolonged operation of chemical equipment are determined by such properties of equipment, as durability, rigidity, air-tightness, stability and long service life. Reliability of chemical production installations is very closely tied up with temperatures, pressure and aggressiveness of process media, shortly, with specific conditions of production equipment operation.
Today it is very important to enhance reliability of modern industrial equipment, installations and apparatuses, as they quite often operate at very high, or on the contrary, very low temperatures, pressures, high velocities and can process various media, including toxic, explosive and flammable.
Durability and reliability are given first priority in the design of such chemical equipment as high-pressure apparatuses. In their development special attention is paid to such requirement, as absence of necessity to open, inspect and repair vessels and internals of high-pressure apparatuses during the operational process. In particular, such actions are inadmissible for aggregates having large unit power, as their even short downtime leads to huge losses of yield.
By no means unimportant condition of reliable operation of chemical operation is also air-tightness. Ar-tightness of machines and apparatuses that handle toxic, explosive and flammable media is very important, as any leakage of processed fluid or gas to the environment may lead to most deplorable consequences – explosion, ignition, fire or personnel poisoning may occur.
The use of air-tight equipment helps to avoid such situations. Today reliable and air-tight apparatuses have a widespread application in various branches of domestic industry.
– As technological processes are continuously mechanized and automated, changing labor conditions, it is very important that a man felt comfortable while working with new equipment. For this reason in design of chemical aggregates it is very important to take into account properties and abilities of people who will maintain this industrial equipment. Principal elements of ergonomics are esthetic, hygienic and physiological requirements for chemical equipment design. Quality of machines, devices and apparatuses cannot be judged only by their effectiveness; the designed aggregates should not create unfavorable labor conditions at the production site. Equipment control and maintenance should be maximum simple and easy. A man should not perform too many operations on chemical equipment, should not make too much efforts, fling about and work in inconvenient working posture. Production equipment by its exterior, painting and proportions should trigger only pleasant emotions.
- equipment enlargement
– due to the fact that today heavy tonnage production more and more demands the increase of chemical equipment unit power, necessity to create enlarged, in other words, large-sized machines, combined devices and integrated apparatuses appears. The use of enlarged chemical equipment at production sites makes it possible to increase its performance manyfold, and noticeably reduce all capital costs and operating charges due to reduction of number of machines and apparatuses, instrumentation, shortening of production service lines, reduction of sites and working personnel.
In creation of large-sized chemical equipment designers must take into account the circumstance that that it should be transported. Performance enhancement of machines and apparatuses, as said above, inevitably leads to their enlargement, so the equipment which is earlier was quite suitable for transportation can become absolutely unadapted for transportation in final assembled form. In particular, it concerns its transportation by railway due to limited dimensions of rolling stock.
Therefore, as early as at the equipment design stage designers should be concerned with all individual parts (assemblies and units) of aggregates being transportable and having minimal dimensions in their top and side sections.
Main stages of design documentation preparation
Main stages of design documentation preparation include the following five:
- technical assignment;
- draft proposal;
- concept design;
- engineering design;
- detail documentation.
Project designer defines key requirements for the future article at the technical assignment stage. The technical assignment stage is original design. At this stage designer solves problems defined in the technical assignment. Concept design presupposes development of documentation that determines:
- key article parameters;
- principle of article operation;
- view drawings, internal diagrams, etc.
In certain cases concept design is coordinated with customer and supervising bodies.
Engineering design stipulates development of detailed and final engineering solutions that give an insight into an article. Detail documentation required for manufacture of all article elements and its assembly is fully based on this data.
Sequence of design stages
Design of equipment and objects of higher hierarchy starts with technical assignment, where the customer sets forth equipment (object) purpose, key technical characteristics and specific conditions. Content and drawing up of the technical assignment should correspond to the established requirements.
In response to the technical assignment designer prepares his draft proposal, where on the basis of analysis of information contained in the technical assignment he shows feasibility of customer’s requirements, possible engineering and technological techniques for solution of tasks assigned, and other tentative results. Documents covering draft proposal are designated by letter “P” and agreed upon with customer.
Next stage is concept design containing design and technological solutions, principal diagrams of equipment and its crucial assemblies, scale models and dummies. Documentation is labeled by letter “E”, approved by the customer and is coordinated with state control and supervision authorities.
Upon approval of concept design main activity starts - engineering design, and by its results final solutions are determined and prepared in such areas as:
- General structure of equipment and its individual assemblies with preparation of drawings;
- Arrangement of equipment main systems – kinematics, drive, hydraulics, pneumatics, electrical diagram;
- Calculations of equipment reliability and its assemblies;
- Assessment of technological effectiveness – machine tools, machining attachments, tools and consumables required for equipment manufacture;
- Performance characteristics – maintainability, operation control and equipment protection, serviceability, resistance to external influence, etc;
- Calculations and evaluation of ergonomic and esthetic parameters, ecological considerations;
- Determination of levels of standardization and unification of equipment and its assemblies;
- Compilation of checklist of materials and purchased constituent articles specifying their qualitative and operating characteristics;
- Full description of work site arrangement and layout of workspace for operation, maintenance and repair of equipment, including labor protection and safety requirements;
- Determination of patent clearance, calculation of equipment technical-and-economic efficiency, and performance of other actions set by the technical assignment.
Documentation drawn up by results of engineering design is labeled with letter “T”, approved and agreed upon in the same manner, as concept design, and after this it becomes the basis for detail documentation preparation.
Working documents (drawings, process flow charts, specifications) provide manufacture of equipment prototype models, and after their testing technical documentation is corrected to the extent of drawbacks found out. In this case documents are labeled with letter “O” with the test serial number.
Positive results of prototype models tests allow to proceed to the stage of issue of the equipment adjustment series which is enclosed with technical documentation with letter “A”. After an article is finally ready for full-fledged serial production, and after amendments are introduced by results of adjustment series work, design documentation receives letter “B” for mass- and letter “B” for single-part and small-scale manufacturing.
Techniques of industrial equipment design
In the course of new technological design development the ordered search of optimal version is often carried out. This method includes several stages:
- definition of design parameters
- determination of parameters to be defined in the course of design;
- determination of target significance;
- identification of mutual dependence of variables;
- forecast of independent variables;
- determination of limit values of variables;
- assignment of numerical value to each solution factor and variable calculation;
- selection of design optimal version.
The essence of linear dimensions change method consists in changing equipment performance in proportion to the change of all equipment basic dimensions. This method is widely used in design of cyclones, augers, roller units, etc.
Basic aggregate technique consists in basic construction being constant while only certain elements of the design are changing. Such method is used for solution of technological tasks on drum-type aggregates and column hardware.
Conversion technique consists in the use of basic model for carrying out new technological process.
Partitioning method consists in equipment separation into equal sections and creation of the new type of equipment through build-up of unified sections.
Equipment design approaches
Proceeding to design of equipment for chemical enterprises specialists firstly assess possibility of use of existing samples of machines and apparatuses of similar purpose as a reference point. Current versions of kinematic diagrams, drives, tools and degree of freedom to make own decisions (solution factors) are under consideration. Simple item-by-item examination of possibilities can give too many versions, so one should use principles of ordered search based on such actions:
- Solution factors are determined – degree of freedom of choice for each of the parameters of equipment under design;
- Independent variables (parameters) are determined, those which designers cannot change basically or at all;
- Interrelations between freedom of choice and independent variable are set;
- Each technical parameter is gradated by the degree of significance;
- Limitations for all parameters are revealed;
- Expert estimate is given to values of independent variables for concrete equipment operation conditions;
- Obtained information is summarized in order to establish optimal ratios between all factors and parameters ensuring achievement of the set target.
At the present time engineering computer programs are used in order to implement the ordered search principle. These technologies help to establish promising areas of further work in design of industrial equipment.
One of the fastest ways of achievement of the result is the design according to the method of linear dimensions change. With such an approach apparatus or machine of similar purpose is selected, parameters of working zone, mechanisms or tools are determined with a view to obtain equipment of required performance, after that the design taken as basic is proportionally recalculated. In such a way one may quickly prepare design documentation for mixers, granulators, cyclones, auger mechanisms, etc.
Good results are also obtained by means of techniques using basic aggregates, and conversion in design of drum-type machines and apparatuses, when minor changes are introduced to final design, or auxiliary devices (mechanisms) are added. In such a way one can obtain equipment for processing of new kinds of raw materials or change its performance in the required direction.
Partitioning method of design is based on the development of modules of which equipment of required power and performance is made up. Such approach is applied, for example, in design of chemical equipment for production of acids, column-type apparatuses and machines, filtering systems, etc.
Regulatory system for equipment design
Design of chemical and any other equipment is made according to single normative-technical base which sets requirements for development procedure, content and documentation execution.
Unified systems of standards for documentation preparation
Creation of effective equipment is impossible without qualitatively prepared documentation, for the development of which unified systems of standards are used for each stage of design works:
- Design documentation - ESKD;
- Technological documentation – ESTD;
- Protection of materials and articles against corrosion and aging – ESZKS;
- Tolerances and sittings – ESDP;
- Labor safety – SSBT, and others.
Moreover, in design of chemical equipment for operation under specific conditions, for example, under high and overpressure, high or extreme temperatures, requirements of “Rules for Design and Safe Operation of Pressure Vessels”, and other regulatory documents of State Committee for Industrial and Mining Safety Supervision should be taken into account. In some branches own normative documents, which set additional requirements for technological equipment and take into account branch or individual features of production, are in force.
The term “design” is more widely used notion, including design engineering, informational and technological preparation, development of all kinds of technical documentation, including its correction by the results of testing equipment prototype models. Essential feature of apparatuses and machines for chemical industry is that they are part of higher hierarchy objects (process lines, productions and enterprises) with extensive infrastructure (engineering service lines, environmental protection systems, and so on). Due to this reason creation of such equipment turns into process called design of mega complexes.
Design (engineering) documentation determines constituent parts of equipment, principles of its operation, materials used for manufacture, operation rules, maintenance and repair. Technological (functional) documentation determines rules of equipment parts manufacture (machine tools, tools, consumables, processing modes, etc), protection against corrosion, layout and erection of machines, apparatuses and accompanying infrastructure, etc.
Documentation on labor safety provides for work site arrangement and equipping (tools, personal protection equipment, materials, accessories), rules (instructions) of handling with equipment, measures of personal and production safety, personnel qualification level, and so on. Complete set of of documentation makes up a project which is implemented in the following order.
Initial data required for the industrial equipment design
Chemical equipment is defined by two key factors: nature of the proceeding process, and sufficient affinity of its structural forms, materials used, machine-building technology applied.
In accordance with these key factors all range of chemical equipment can be divided into fifteen main groups, and then each group is subdivided into types and unit sizes.
Typical technological process is called a process used for manufacture of group of articles having common design and technological features. It is developed for manufacture of an article (in certain production environment), which for this group of articles will be their typical representative. The article, manufacture of which requires maximal number of operations (basic and auxiliary), which are characteristic for manufacture of all articles of this group, are usually attributed Tto such typical representatives.
Grouping of articles by initial technological and design features is mandatory condition for type design practice of technological processes, which promote implementation of most progressive and innovative forms of production.
Based on achievements of science and engineering in the field of innovative technologies, application of typical technological processes can shorten time required for industrial mastering brand new articles. It also leads to rational use of material and labor resources, makes it possible to set up databanks including initial information, later used in design.
Initial data for calculation and selection of industrial equipment used for filtration processes
- Data on characteristics of filtered suspension:
- for liquid phase: boiling and crystallization temperature, viscosity and specific weight, explosiveness and toxicity;
- for solid phase: chemical composition, granulometric composition, its specific weight, nature of particles making it up (amorphous or crystalline), deposition rate during settlement;
- ratio of phases (content of solid phase in suspension);
- additional data that define suspension yield, conditions of its transport across pipes.
- Sediment characteristics:
- tendency to cracking during vacuum-filtration, easiness of coming off fabric, is it smearing, sticky, dense, loose;
- tendency to shredding in augers when transported across pipelines;
- its bulk weight, content of liquid phase (depending on the selected filtration method);
- its composition both before and after flush.
- Data on filtration mode:
- is it necessary to perform water flush or treatment using other fluids;
- temperature of rinsing fluid, its composition, recommendations for its application, consumption per one kg of sediment;
- total filtration time, duration of one cycle of filtration determined on pilot installation, filtration temperature;
- considerations used in selection of modern filtering aggregate produced by industry;
- thickness of sediment layer left on the filter;
- recommended and used fabric for filtration, mesh or porous material of another nature;
- recommended material of hardware used for filtration and how corrosion influence of medium affects it;
- means used for mesh (filtering fabric) regeneration);
- recommended regeneration mode, time within which it should be performed;
- regeneration solutions, recommendations for their use.
Initial data used in design of industrial equipment for processes of absorption, distillation and rectification
- Recommendations for application of separation method used in the process.
- Composition of initial mixture involved in the process and possible range of its change, as well as composition of still bottoms and distillate.
- Type of mixture (heterogeneous or homogeneous), characteristics of solution with indication of deviations from ideal mixture:
- availability and characteristic of azeotropic mixtures (boiling temperature and make up);
- influence of medium pH, high temperatures, influence of impurities contained in the fresh raw materials on stability of individual components and mixture as a whole, possibility of forming solid-state deposits, formation of explosive products;
- data (for ideal mixtures) on components vapour pressure or vapor-liquid equilibrium, information on influence of pressure on value of coefficient describing relative volatility of key mixture;
- viscosity and density of components being in liquid state, heat capacity and latent heat in evaporation of components, and viscosity of mixture being in vapor phase.
- Typical recommendations which determine selection of the type of structures for contact devices (headers and trays).
- Tray performance under operation conditions or height of header which is equivalent in the theory of one tray, composition of still bottoms, distillate and feed, reflux-to-product ratios, and total number of theoretically perfect trays.
- Specific requirements related to design of rectification aggregates in case of solid salts precipitation should also be taken into account.
- Data used in calculation of heat-and-mass balance for processes of absorption, distillation and rectification.
Initial data required in design of drying process
- Humidity of product delivered to drying at the process onset.
- Humidity of dried up product at the process end.
- Consistence of processed product before and after drying (briquette, powder, paste).
- Chemical and physical properties or processed product before and after drying: ability for balling, arching, caking, sticking, angle of repose, electrolyzing and abrasive properties, powder granulometric composition(size of particles, real and bulk density), chemical composition, product tendency to self-ignition or decomposition, temperatures of decomposition, softening, fusion.
- Recommendations for selection of dryer type and data on its specific performance by final product.
- Recommendations for optimal drying mode, selection of temperature, calculation of holding time, media characteristics, required pressure
- Composition of environmental pollutants and recommendations for purification of air discharged during drying.
- Data used for calculation of heat-and-mass balance of the drying process.
Initial data used in calculation and selection of industrial equipment for evaporation units
- Concentration of evaporated solution at the process onset.
- Its final concentration.
- Solution density at operation temperatures and concentrations.
- Solution viscosity at operation temperatures and concentrations.
- Temperature of solution freeze and boiling.
- Tendency of evaporated solution to foaming, gumming, decomposition, etc. Recommendations which relate to prevention of these phenomena.
- Conditions under which evaporated solution can be transported; duration of apparatus continuous operation in-between flush, cleaning with minimal calculated sediment precipitation on surfaces of heat-exchangers
- Can evaporation be made before crystal haze; what is solid fraction/liquid ration in this case; pulp properties (crystals precipitation rate, transportability).
- Recommendations regarding method of particulate emission from evaporated solution.
- Recommendations regarding selection of materials for pipelines and hardware proper.
- Recommendations regarding selection of the evaporation system type.
- Recommendations regarding disposal or decontamination of juice vapor.
- Information on components stability for the case of their multiple recirculation in the course of production cycle (recirculation of mother liquid).
- Initial data used in calculation of heat-and-mass balance for the evaporation unit.
Initial data used in design of crystallization processes
- Initial characteristics of solution delivered for crystallization:
- Chemical and physical properties of solution, including solvent and dissolved substances: dependence of mixture solubility level on temperature, dependence of solution density on various content of substances dissolved in it, dependence of solution heat capacity on its concentration and temperature;
- Concentration of substance dissolved in solution at the processes start.
- Temperature at which crystallization process proceeds. Recommendations regarding chilling rate.
- For new substances crystallization heat.
- Characteristics of suspension obtained in the course of crystallization:
- granulometric compositions (size of crystals);
- their hardness;
- ability to grind during pumping or mxing.
- Time required for crystallization and sufficient growth of crystals in order to obtain properly filtered suspension.
- Data required for calculation of heat-and-mass balance of crystallization process.
Initial data required for design of technological assemblies providing dozing, mixing and grinding
- Chemical and physical properties of components fed to apparatus, and also mixtures thereof: angle of repose, hardness, caking ability, real and bulk density, heat capacity, softening and melting temperature, flowing property, viscosity (for components of fluids), sorption capacity or porosity (if necessary).
- Accuracy of components permissible dosage. Deviations allowed for the ratio set for mixture. Mean relative and absolute errors of mixture components distribution.
- Physical state inherent in mixture after passing through mixer, the need for grinding and chilling.
- Recommendations for mixture chilling or heating in the mixing technological process, its optimal mode.
- Vapors and gases emitted during mixing. Chemical processes that might proceed during mixing.
- The need for grinding components during mixing process, particle size resulting after passing through mixer.
- Recommendations for this process by types of dispenser and mixer.
- Information on components ability for grinding (fragility, hardness according to Mohs scale, adhesion, grinding ability, softening ability).
- Recommendations regarding the selection of types of grinders for pre-grinding and fine grinding stages. Information on recommended grinders for grinding fineness and performance of specific material processing.
- Recommended characteristics of products transport:
- pneumatic transport speeds recommended for each of the materials;
- optimal ratio of air and solid substance;
- is preliminary products loosening needed before being supplied to pneumatic pipeline;
- type of feeder;
- possibility of forming explosive concentrations and static electricity during pneumatic transport
- What type of containers is recommended for final products packing.
Initial data used in design of thermal decontamination units for still bottoms and industrial waste at chemical production
- Form of still bottoms or industrial waste delivered for decontamination:
- suspension, solution;
- alkaline, acidic, neutral;
- their tendency to polymerization;
- possibility of their conveyance across pipelines and recommendations for pumps selection;
- specific weight, heat capacity, heat conductivity, viscosity;
- composition of initial products.
- Physical and chemical properties of solid precipitate: temperatures of boiling, melting, hardening, sublimation, decomposition, water solubility.
- Marketable appearance (in case of further use), GOST, OST, TU.
- Probable composition of products formed during thermal decontamination of still bottoms or solution.
- Corrosion properties of still bottoms or solution, and recommended selection of materials and pumping facilities.
- Toxicity of parent compounds and newly formed, still bottoms, solutions and products resulting from thermal decontamination.
Data used in design of electrolytic processes:
- Characteristic of electrolyte delivered for electrolysis.
- Chemical and physical properties of the solution (heat capacity, electric conductivity, solubility, density, etc.
- Conductivity, solubility, density, etc.
- Characteristic of liquid and gaseous products obtained during electrolysis (heat capacity, solubility, density, explosive concentrations, toxic properties).
- Recommendations for electrolyzer design based on comprehensive testing ( voltage, current output, current intensity, current density, optimal temperature mode, diaphragms, construction materials for cathodes and anodes, electrolyzer installation drawings).
- Operation manuals for production, erection and maintenance of electrolyzer, including instructions for process start-up and shut-down.
- Main provisions for electric safety and general safety engineering.
- Diagrams of dependence of electrolytic process parameters on current density (current efficiency, voltage, consumption of materials, etc), in order to determine optimal operation mode in specific economic conditions.
- Calculated specific consumption indices.
- Instructions pertaining to repair of electrolyzers, and recommendations for design of repair shops (anode shop, replacement of diaphragms, rods’ filling, impregnation, etc).
- List of arrangement for control of current leaks, required in sampling, pipeline laying, etc.
- Recommendations regarding the selection of piping materials and fittings.
- Recommendations regarding the selection of instrumentation, remote control, adjustment, both local and switchboard-mounted, etc.
Selection of materials and protective coatings for chemical equipment production
Materials for chemical equipment manufacture should be selected with regard to specifics of their operation and bearing in mind, that physical and chemical properties of materials may change under influence of process and aggressive media, high or low temperatures and chemical and technological processes proceeding at the production site. Therefore, in selection of material for this or that equipment one should be guided by standards established in our country.
In order to select proper material for the equipment, firstly, one has to find out in what operating environment it will be installed. In other words, what temperature, pressure, processed media and its concentration will be. In selection of constructional material for chemical equipment, it is very important to take into account physical (thermal expansion, heat expansion) and mechanical (hardness, cracking resistance, breaking strength, and so on) properties of material, as well as its chemical durability.
It should be noted that mechanical properties of materials of which equipment is made, may change, and rather noticeably at low and, on the contrary, high temperatures. One more necessary prerequisite in material selection is material welding ability, as in chemical machine building the major method of fixed connections making is namely welding, and on rare occasions – soldering.
However one of the major requirements for material used in chemical equipment manufacture is its corrosion resistance. As a rule, not outer surface of chemical apparatuses, but inner one is subjected to corrosion. Namely corrosion resistance determines how long industrial equipment will serve.
Protection of chemical production equipment
The economy of our country suffers very acutely from loses inflicted by corrosion to national economy. It has destructive effect on metal, metal articles, building structures and production equipment. It should be noted that corrosion destructions often become the cause of downtimes and emergency situations at the enterprises, and damage by them sometimes exceeds direct metal loss.
Foreign and Russian practice showed that today quite significant part of existing acid-proof steels can be extremely well replaced by ordinary carbon steel having protective coating. Alongside with it one may save a lot of financial means and scarce metals.
Some chemical installations and service lines (pipelines) require thermal insulation due to high-temperature reactions or reactions using high-temperature heat carriers proceeding in them. Thermal insulation is also required for equipment where reactions are performed using cooling agents with negative temperatures.
As a rule, today chemical equipment is protected by several methods:
- Lining arrangement – protective inside lining of chemical apparatuses is made;
- Coating deposition – it can be rubber, enamel or polymers;
- Equipment painting;
- Insulation provision (for example, by glass fiber).
In selection of materials, protective coating design, specific conditions of equipment operation, its purpose and installation site have to be taken into account. If the hardware with non-metallic inner coating (like, enamel, lacquer, rubber, and so on) is planned to be mounted outdoors, its coating should have mechanical resistance to low and variable temperatures. If the equipment cannot be provided with coating resistant enough to low ambient temperatures, it is installed in heated premises.
Selection of protective coating and fettling for chemical equipment insulation
Today it is very important to make maximum adequate choice of coating material of particular chemical equipment, thus making it possible to do without costly and scarce construction materials used for its creation. Besides, there are numerous chemical processes where corrosive media even prohibits the use of machines and apparatuses made of special steels.
Service life of protective industrial equipment depends not only on resistance of protective material to aggressive media, but also on quality of fettling works. Properly made fettling is a guarantee of reliability and uninterrupted hardware operation.
Today key materials used for protection of chemical equipment are acid proof and moler bricks, andesite stones, diabasic slabs, acid proof cements, diatomaceous-silica asbestos earth, sheet asbestos and polyisobutylene.
Protective coatings made of polymer materials
Among all presently existing anti-corrosion protective coating for chemical equipment, coatings made of polymer materials are considered the most effective. Particularly efficient are fluoroplastics of most varied grades that are noted for their high durability, chemical-, thermal- and wear-resistance in combination with excellent anti-adhesion (sticking-resistance) properties.
For example, presently in sulfuric acid production plate-type heat exchangers with carbon steel elements came into use. They are protected by polymer anti-corrosion coating having increased heat conductivity. Modular plate-type heat exchanging apparatus fabricated of carbon steel and having anticorrosion protective coating was specially designed for this production. Moreover, heat-transmitting plates have such dimensions and configuration that make it possible to generate very intensive turbulent flow for process media. All this, with identical energy consumption required to overcome hydraulic resistance, makes it possible to enhance heating capacity in this chemical equipment.
Before starting deposition of anticorrosion coating and fettling, durability, sealing and quality of the equipment surface pre-treatment is thoroughly inspected. Protective coating is applied only if treated surface is dry, has uniform grey-dim tint and is free of traces of rust, scale or any soiling, oil in particular.
Enameled protective coatings are more reliable in terms of service and most wide-spread. Chemical equipment enameled coatings are very resistant to action of acids, both organic and non-organic, as well as their salts and alkaline solutions. On the other hand enamel coatings are not distinguished for their resistance to hydrofluosilicic and hydrofluoric acids and their salts.
Chemical equipment painting
For protection of equipment and service lines against influence of external corrosion they are painted by perchlorovinyl enamel, oil paint, or coatings using aluminum, zinc (by metallization) and other materials are applied, with regard to features of atmosphere, environment and operation conditions
Insulation of chemical production equipment
Equipment and process service lines are insulated if heated surfaces of industrial equipment have temperature over 45 degrees and pipelines over 60 degrees Celsius.
Principal objectives of insulation are:
- provision of chemical equipment with specified temperature mode;
- exclusion of heat or cold emission to environment;
- creation of normal sanitary and hygienic conditions for maintenance personnel.
For chemical equipment insulation mineral-cotton, vulcanite, powellite, diatomaceous materials and glass fiber items are used.
Special requirements are set to all heat insulating coatings of chemical equipment without distinction. Today such wide-spread heat-insulating materials as glass wool, slag wool and asbestos cement unfortunately cannot properly provide quality insulation due to their rather low mechanical strength and high moisture absorption ability. Besides, their service life in aggressive media and in the presence of atmospheric moisture is very short, as they are quickly destructed.
Due to all this principally new designs of heat-insulating coatings were developed, optimally suitable for use in the chemical production environment. In such designs mainly foam plastic acts as heat-insulating material; these are polystyrene foam and foamed polyurethane distinguished for their durability, low heat conductivity, small volume weight, and practically do not absorb moisture.
Selection of materials and protective coatings for chemical hardware installed in outdoor areas
During selection of material for manufacture of chemical equipment to be installed and maintained in outdoor areas, all hardware operating conditions have to be taken into account, without forgetting about its possible overcooling under negative ambient temperatures. Sudden drops of temperature should be also considered. It should also be borne in mind that upon overcooling metal tends to sudden destruction. So, for the environment like this doped or carbon steel is preferable.
Equipment installed in the outdoor areas must be protected against corrosion, and the products processed in it must be protected against freezing in winter season, and must be protected against overheating under sunshine in summer time. Due to this such machines and apparatuses need high-quality heat-insulating coatings.
The chemical equipment insulation layer, being in the open air, should be protected from the top with reliable coating resistant to atmospheric precipitations. Such protective coating can be asbestos-cement sheets or plastering, as well as metal housings made up of corrugated aluminum sheets, or thin-layer galvanized steel (it is usually used for protection of insulation material of cold surfaces).
Specifics of construction material selection for chemical equipment
Selection of materials used for chemical production equipment is stipulated by specific operation conditions having direct influence on properties of metals, polymers, mechanical rubber goods, lubricants, etc. Among key influencing factors are:
- Pressure – from deep vacuum to excessive values;
- Temperature mode – from liquid nitrogen temperature to iron ore melting;
- All types of corrosion – chemical, thermal and ordinary;
- High mechanical loads in aggressive environment.
Under such influence metals radically change plasticity, impact resistance, dynamic and static loads, their ageing and fatigue develop in different ways. Sealants and materials for inner surface fettling should resist extreme temperatures, acidic and alkaline medium.
Technical literature offers sufficient number of reference tables, which reflect dependence of yield point variation (σт) under static loads or elastic behavior (modulus E and Poisson's constant μ) on operation temperature. At low temperature ferrous metals become brittle, and at high temperature become permeable for diffusion of active compounds.
Disturbance of mechanical properties of ordinary metals induces development of additional cooling systems for compensation of mechanical loads, inherent stresses, hydromechanical shocks and other actions. The norms of Russian Federal Service for Ecological, Technical and Atomic Supervision provide for very limited application of carbon steel and cast iron in the extreme operation environment. That is why special grades of doped steel, non-ferrous metals (copper, aluminum) and their alloys are among basic construction materials for chemical equipment.
Plastic characteristics of these materials do not change a lot under influence of low temperatures, besides the fact that they are much more stable to all types of corrosion, including chemical processes using electrolytes. Summing it up, one can note that construction material selected for chemical industrial equipment should comply with the following requirements:
- Sufficient corrosion resistance in aggressive medium with concentration of chemically active compounds, temperature range and pressure level set by the technological process;
- Sufficient mechanical stability under these same conditions with regard to additional loads from own weight, wind, installation method, support area;
- Possibility of obtaining durable welded joints stable to fracture and mechanical loads under extreme conditions;
- Material availability in a sense of cost and prevalence;
- Safe and simple utilization after operating life termination.
Alongside with metals, similar approach is used in selection of other construction materials by parameters of heat resistance, stability to impact loads and vibration. Correct choice of materials is a guarantee of durability and reliability of chemical equipment.
Key requirements specified for industrial equipment
Industrial equipment should occupy small in size production floor space. i.e., be whenever possible compact and function with proper production capacity. As for material of its parts, they should be of high quality, reliable and durable, and also be capable to operate in aggressive chemical environment.
Machine (apparatus) itself should also be reliable, safe and user friendly to maximum. Also whenever possible, time a machine (apparatus) spends for manufacture of one unit of produce, should also be reduced to minimum, and to maximum consumption of converted power resources(electric power, water, steam, heat and compressed air) should be reduced.
To steel welded vessels and apparatuses their own technical requirements are set by industry standards. All vessels and apparatuses used in production should be equipped with hatches providing access inside for cleaning or repair purposes.
The enterprise allows for use of heat-exchanging apparatuses, vessels with jackets and apparatuses with removable bottoms or covers without manholes.
Requirements for chemical equipment design
An important aspect of process equipment requirements are the requirements set for quality of machines and their operation conditions.
Depending on the stage of process equipment manufacture the following parameters are differentiated:
General design requirements for any technological equipment include a group of indices to be taken into account in the development of design and detail documentation for all branches of industry, including chemical production:
- Purpose – it includes parameters of power, performance, power supply capacity, operating speed of actuating mechanisms, etc;
- Reliability ‑ capability of equipment for prolonged continuous operation, maintainability of an apparatus or machine as a whole, and its individual parts;
- Ergonomics ‑ parameters of man-machine system (the issues of sanitary and hygiene, biomechanics of movements, influence on human physiology and psychic in production environment);
- Esthetics – under otherwise equal conditions, namely design becomes decisive argument of equipment selection; pleasant outside appearance of equipment also favorably tells on quality of personnel work;
- Manufacturability – provides optimization of expenses for equipment manufacture both in quantity and cost (materials, tools, labor costs);
- Transportability – provides equipment mobility in conveyance, costs reduction of loading/unloading operations, possibility of using standard transport vehicles, etc;
- Standardization and unification – indicate ratio of original and standard structural elements used in equipment design;
- Legal (patent) clearance – someone else’s developments cannot be claimed as one’s own, still own original ideas should be protected by patents;
- Ecology ‑ indices of negative impact of equipment in the environment are getting more and more crucial, value of emissions should be included with some margin for potential norms tightening;
- Safety ‑ an integral index encompassing elements of all groups of parameters that influence safety of maintenance personnel, as well as effectiveness of automatics and protection systems in determining hazardous situations, actuation speed, etc.
Meeting equipment design requirements should not be blind – one has also to take into account key tendencies of industry development, prospects of changes in the field of labor protection and environment, just look ahead for the project not to turn obsolete before being embodied in metal:
- Effectiveness – optimal power parameters, intellectual systems of control, automatics and protection, complete set of auxiliary equipment.
- Design and technological perfection – application of computer engineering programs making it possible to maximally simplify equipment design and documentation, reduce material intensity, find replacement for expensive materials.
- Operation characteristics – simple and convenient erection, maintenance and operation of the equipment, high maintainability, ergonomic and ecologic indices, absence of vibration, etc.
- Economic effectiveness – equipment should be able to produce more products at low costs, have lower operation costs (power, lubricants and consumables, maintenance and repair).
- Observance of requirements of current normative and technical documentation, without which the project is impossible to be put into production
All above-given requirements are interdependent. It is difficult, or even impossible to provide perfect parameters for one group of requirements without prejudicing others. High power parameters are directly proportional to the level of harmful emissions, which implies inevitability of creating high-performance purification systems. But on the other hand, enhancement of equipment reliability will automatically lead to the rise of safe maintenance.
Key requirements for process equipment design are effectiveness, reliability, designability, transportability and operational simplicity.
Effectiveness of equipment operation is its performance parameter under continuous technological process conditions.
Reliability index integrates such parameters as durability, strength and stability.
Designability presupposes simplicity of design, small weight and overall dimensions.
Equipment transportability implies possibility to transfer equipment in blocks or as a complete set.
Operational simplicity consists in convenience of erection, repair and control over industrial equipment operation.
Complex of contradicting requirements set for modern process equipment brings up an issue of optimal process solution selection.
In other words, good equipment design is the result of reasonable trade-off between observance of design requirements, search for maximum number of points in common in contradictory situations, and orientation for best specimen of existing machinery. Only in such a way one can achieve optimal result not just in separate areas, but in broad terms, when a designed machine or apparatus is integrated in the existing chemical equipment.
All parameters out of the enumerated groups of requirements are predictable at the stage of technical assignment issue, production-type – during development of design and technical documentation and prototype testing, and operation – after equipment starts operation at a specific enterprise.
In design of chemical equipment it is particularly important to take into account exclusive requirements set for materials stability to all types of corrosion (chemical, thermal and environmental). If equipment is supposed to be installed outdoors, these requirements are getting even more strict. There are also special requirements for industrial equipment depending on environmental conditions at operating site, which have to be taken into account in design and indicated in articles labeling:
- Moderate climate – “U”;
- Moderate and cold climate – “UHL”;
- Tropical climate – “Т”;
- General climatic modification for dry land – “О”;
- General climatic modification – “В”.
Numeric notations near letter designation indicate machinery version for open-air operation (1), inside light buildings and under tents (2), indoors with climatic conditions adjustment (3), inside climate-controlled premises (4), inside high-humidity premises (5). Hence temperature variations, influence of atmospheric pressure, abrasive actions and other parameters affecting selection of materials, lubricants, sealants, etc. are taken into account.
Description and requirements for construction materials
For manufacture of industrial equipment the chemical and petrochemical industries use materials that offer high degree of resistance to aggressive media, mechanical durability, low tendency to ageing, etc. The reasons for such strict requirements lie in the fact, that chemical equipment is functioning in the wide range of pressures.
Key requirements for equipment construction materials used in chemical and petrochemical industries can be defined in the following way:
- corrosion resistance in the context of technological process;
- high index of mechanical resistance;
- good material welding ability on condition that in the process of welding the above-listed characteristics are retained;
- low cost of material and its availability;
- simplicity of utilization.
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.