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Equipment and processes of chemical industry 25.01.2020 14:09
Chemical industry takes an important place in national economy, produces and supplies a wide range of consumer goods and raw materials for processing by other industries.
Chemical industry, while achieving its objectives in relation to products manufacturing, applies a wide range of technologies used for processing of feed to finished products. In this case feed can be solid, liquid or gaseous, and chemical reactions can be of various types. Therefore chemical industry is a science-intensive and complex industry and often it is a dangerous branch of national economy.
Chemical reaction that makes it possible to produce finished product from raw materials is not often as significant in chemical industry as preparation of feed, intermediate products at different process stages before performing required chemical reaction. The preparation of feed and intermediate products influences the production rate and selectivity of chemical process. For example, before performing chemical reaction it can be required to preheat feed or solve it in water, separate undesired admixtures, etc.
Process hydrodynamics understanding is very important. During design engineering of chemical vessels and pipelines it is required to perform calculation of flow hydrodynamic characteristics. For example, it is required to assess whether a flow is laminar of turbulent. Bernoulli’s equation and Navier-Stokes equations are widely applied for calculations in fluid dynamics.
Central place at all chemical industrial plants is taken by equipment for liquid medium transportation i.e. pumps. There are pumps of the following types: piston pumps, rotary-displacement pumps, centrifugal pumps, peripheral pumps, liquid jet pumps, etc. Pump selection is performed on the basis of original requirements to a flow such as flow rate, description of the pumped medium, temperature, viscosity, density, etc. At the same time performance curve defining dependence between pump head and flow rate is used for pump selection. At plants where gases and equipment operating under pressure are used, compressors are widely applied. Compressors perform gases compression and generate required pressure. There are compressors of the following types: piston compressors, diaphragm compressors, bellows compressors, etc. Fans can be used for simple gas transportation and creation of low vacuum or low excess pressure. Fans are used at almost every chemical plant in general forced ventilation (plenum and exhaust ventilation) and emergency ventilation for achieving required working area air parameters and prevention of hazardous substances emission to atmosphere in case of depressurization of equipment and process pipelines.
At chemical plants it is often required to separate liquid heterogeneous systems. In order to perform that, the following three processes are applied: settling and decantation, filtration, centrifuging. Settling and decantation are performed in special vessels (settling tanks) where higher phase drops down because of gravity and therefore separation of heterogeneous medium is achieved. Filtration is performed in batch filters and continuous filters. Batch filters have simpler arrangement but they require significant operation cost. The filtering element in batch filters is a partition that can be unlinked, grainy, fiber or fixed rigid. Continuous filters mainly are suitable for manufacturing with high production capacity. Continuous vacuum drum filters with external filtering surface are widely applied. Furthermore, continuous disc vacuum filters and continuous vacuum belt filters are used. Centrifuging process is performed in centrifuges that can be small (laboratory centrifuges) or big (industrial centrifuges).
Regarding to separation of gaseous heterogeneous systems, the following methods are applied: mechanical gas purification, wet gas cleaning, gas filtration and electrical gas purification. Mechanical gas purification is performed in settling chambers and centrifugal dust cleaners (cyclones). Wet gas cleaning is performed with the use of liquid medium. Foam vessels are widely used for that. Gas filtration is performed in gas filters where filtering partition is used for separation of gaseous heterogeneous systems. Electric filters take a special place in process of fine gas purification from mechanical impurities. Fine mechanical particles are separated from main gas stream by electric current.
Mixing is widely applied for ensuring better contact between substances interacting with each other during chemical reaction. Liquid medium separation is carried out in vessels including mechanical agitators. Agitators can be impeller mixers, propeller mixers, turbine mixers, etc. Agitator type selection depends on required rotation speed, mixing intensity, viscosity of medium being mixed, etc. More complex equipment is used for mixing solid loose and pasty medium. Screw mixers and mixing drums are applied for that.
Special vessels named heat exchangers are used for ensuring heating and cooling of process medium at chemical plants. Heat exchangers can be double-pipe, tubular, spiral, plate heat exchangers, etc. In the heat exchangers heat transfer from more heated medium to less heated medium takes place. At the same time in heat exchangers contact between the medium being heated and heating medium is eliminated. Furthermore, heating and cooling is often performed directly in a reactor. In this case reactors are equipped with jackets or coils. In some rare cases heat transfer takes place as a result of direct contact between the medium being heated and heating medium, for example in case of application of live steam i.e. steam fed directly to the medium being heated. Process medium heating can be performed in special heating furnaces, in combustion chambers of various process vessels.
Among heat exchangers there are vaporizers that have other but almost similar structure. Vaporizers are used for concentration of liquid media. For example, during vaporizing we can produce 20% solution from 30% one.
Mass transfer at chemical plants takes place during absorption, fractionation, extraction, crystallization and drying processes. Absorption that is gas trapping by liquid medium is performed in surface and wetted wall absorbers. In order to increase surface area for contact between gaseous and liquid medium, packed absorbers of column type are used. Fractionation is widely applied in chemical technology ensuring purification and separation of components. Fractionation is performed in fractionation columns of packed or tray type. Calculation of fractionation columns is a rather difficult issue based on calculation of theoretical trays that is performed for both tray columns and packed columns. Extraction is performed in special extraction units (extractors) and makes it possible to recover (extract) one or several components from a mixture. Crystallization is applied if it is required to recover a solid substance from a solvent. Crystallization is highly important if it is required to produce purified and highly purified substances. For producing such substances repeated crystallization is performed i.e. a substance is crystallized from a solvent, then it is dissolved once again and once again crystallized, etc. In this case substance purity is significantly increased on each crystallization stage. Drying is applied for removing excess moisture from substances to be produced. Drying in fluidized-bed vessels (dryers) is widely used. In the vessels hot air (or other gas) is fed with head sufficient for lifting solid (that is in a powder form) to be dried. Therefore fluidized bed is created. Fluidized bed is called by the name because its properties are similar to fluid properties though it is formed by solid powder. Fluidized-bed drying is the most effective method for reducing moisture of powder products.
For many chemical processes cooling is required. Cooling is ensured by heat transfer to air in air coolers or to water in water coolers (the latter coolers are heat exchangers), cooling towers. Furthermore cooling can be performed with the use of special units (refrigerating machines). Operating cycle of refrigerating machine (chiller) is based on Carnot cycle that is an ideal cycle. Chillers have a complex arrangement and consist of the following: compressor (a main element of refrigerant loop), vaporizer, thermostatic expansion valve, condenser and automation system ensuring coordinated operation of all elements. In addition, in some cases it is required to use two-loop refrigerating machines where simultaneously two refrigerant loops are available. As a rule fluorochlorohydrocarbons (FCHC) are used as refrigerants. Main characteristic of a refrigerating machine is cooling capacity i.e. cooling power achieved from a concrete refrigerating machine.
Furthermore in chemical industry mechanical grinding processes are widely used. They are applied mainly for finished products that shall meet defined requirements regarding particle size. In addition, mechanical grinding may be a preliminary stage before chemical reaction. In this case it is required for increasing surface area for contact between two substances. Coarse grinding (preliminary grinding) is performed in jaw crushers, cone crushers and disc crushers. Secondary and fine crushing is performed with the use of roll-hammer crushers, crushing mills and impact centrifugal mills. Fine grinding is performed in ball and rod mills, ring mills, vibrating mills and colloid mills. Grating is performed in special vessels (screeners) that can be mobile or stationary. For milled product dosing feeders are used. There are feeders of the following types: drum feeders, star feeders, roll feeders, disc feeders, swinging feeders (tray feeders), plunger feeders, plate feeders, vibrating feeders.