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Achemical plant is anindustrial processplant that manufactures (or otherwise processes)chemicals, usually on a large scale.[1] The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials.[2] Chemical plants use specialized equipment, units, and technology in the manufacturing process. Other kinds of plants, such as polymer, pharmaceutical, food, and some beverage production facilities,power plants,oil refineries or otherrefineries,natural gas processing and biochemical plants,water andwastewater treatment, andpollution control equipment use many technologies that have similarities to chemical plant technology such as fluid systems and chemical reactor systems. Some would consider anoil refinery or a pharmaceutical orpolymermanufacturer to be effectively a chemical plant.
Petrochemical plants (plants using chemicals from petroleum as a raw material or feedstock) are usually located adjacent to anoil refinery tominimizetransportation costs for the feedstocks produced by the refinery.Speciality chemical[3] andfine chemical plants are usually much smaller and not as sensitive to location. Tools have been developed for converting a base project cost from one geographic location to another.[4]
Chemical plants usechemical processes, which are detailed industrial-scale methods, to transform feedstock chemicals into products. The same chemical process can be used at more than one chemical plant, with possibly differently scaled capacities at each plant. Also, a chemical plant at a site may be constructed to utilize more than one chemical process, for instance to produce multiple products.
A chemical plant commonly has usually large vessels or sections calledunits orlines that are interconnected bypiping or other material-moving equipment which can carrystreams of material. Such material streams can includefluids (gas orliquid carried in piping) or sometimessolids or mixtures such asslurries. An overall chemical process is commonly made up of steps calledunit operations which occur in the individual units. A raw material going into a chemical process or plant as input to be converted into a product is commonly called afeedstock, or simplyfeed. In addition to feedstocks for the plant, as a whole, an input stream of material to be processed in a particular unit can similarly be considered feed for that unit. Output streams from the plant as a whole are final products and sometimes output streams from individual units may be considered intermediate products for their units. However, final products from one plant may be intermediate chemicals used as feedstock in another plant for further processing. For example, some products from an oil refinery may be used as feedstock in petrochemical plants, which may in turn produce feedstocks for pharmaceutical plants.
Either the feedstock(s), the product(s), or both may be individualcompounds or mixtures. It is often not worthwhile separating the components in these mixtures completely; specific levels of purity depend on product requirements and process economics.
Chemical processes may be run incontinuous orbatch operation.
Inbatch operation, production occurs in time-sequential steps in discrete batches. A batch of feedstock(s) is fed (orcharged) into a process or unit, then the chemical process takes place, then the product(s) and any other outputs are removed. Such batch production may be repeated over again and again with new batches of feedstock. Batch operation is commonly used in smaller scale plants such as pharmaceutical or specialty chemicals production, for purposes of improved traceability as well as flexibility.Continuous plants are usually used to manufacturecommodity orpetrochemicals while batch plants are more common inspeciality andfine chemical production as well asactive pharmaceutical ingredient (API) manufacture.
Incontinuous operation, all steps are ongoing continuously in time.[5] During usual continuous operation, the feeding and product removal are ongoing streams of moving material, which together with the process itself, all take place simultaneously and continuously. Chemical plants or units in continuous operation are usually in asteady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation. Such constant quantities include streamflow rates, heating or cooling rates,temperatures,pressures, and chemical compositions at any given point (location). Continuous operation is more efficient in many large-scale operations like petroleum refineries. It is possible for some units to operate continuously and others be in batch operation in a chemical plant; for example, seeContinuous distillation andBatch distillation. The amount of primary feedstock or product per unit of time which a plant or unit can process is referred to as thecapacity of that plant or unit. For examples: the capacity of an oil refinery may be given in terms ofbarrels ofcrude oil refined per day; alternatively chemical plant capacity may be given intons of product produced per day. In actual daily operation, a plant (or unit) will operate at a percentage of its full capacity. Engineers typically assume 90% operating time for plants which work primarily with fluids, and 80% uptime for plants which primarily work with solids.
Specificunit operations are conducted in specific kinds of units. Although some units may operate at ambient temperature or pressure, many units operate at higher or lower temperatures or pressures. Vessels in chemical plants are oftencylindrical with rounded ends, a shape which can be suited to hold either high pressure orvacuum.Chemical reactions can convert certain kinds ofcompounds into other compounds inchemical reactors. Chemical reactors may bepacked beds and may have solid heterogeneouscatalysts which stay in the reactors as fluids move through, or may simply be stirred vessels in which reactions occur. Since the surface of solid heterogeneous catalysts may sometimes become "poisoned" from deposits such ascoke, regeneration of catalysts may be necessary.Fluidized beds may also be used in some cases to ensure good mixing. There can also be units (or subunits) formixing (including dissolving),separation, heating, cooling, or some combination of these. For example, chemical reactors often have stirring for mixing and heating or cooling to maintain temperature. When designing plants on a large scale,heat produced or absorbed by chemical reactions must be considered. Some plants may have units with organism cultures for biochemical processes such asfermentation orenzyme production.
Separation processes includefiltration,settling (sedimentation),extraction or leaching,distillation,recrystallization orprecipitation (followed by filtration or settling),reverse osmosis,drying, andadsorption.Heat exchangers are often used for heating or cooling, includingboiling orcondensation, often in conjunction with other units such as distillation towers. There may also be storagetanks for storing feedstock, intermediate or final products, or waste. Storage tanks commonly have level indicators to show how full they are. There may be structures holding or supporting sometimes massive units and their associated equipment. There are often stairs, ladders, or other steps for personnel to reach points in the units for sampling, inspection, or maintenance. An area of a plant or facility with numerous storage tanks is sometimes called atank farm, especially at anoil depot.
Fluid systems for carrying liquids and gases include piping and tubing of various diameter sizes, various types ofvalves for controlling or stopping flow,pumps for moving or pressurizing liquid, andcompressors for pressurizing or moving gases. Vessels, piping, tubing, and sometimes other equipment at high or very low temperatures are commonly covered withinsulation for personnel safety and to maintain temperature inside. Fluid systems and units commonly haveinstrumentation such as temperature and pressure sensors andflow measuring devices at select locations in a plant. Onlineanalyzers for chemical or physical property analysis have become more common.Solvents can sometimes be used to dissolvereactants or materials such as solids for extraction or leaching, to provide a suitable medium for certain chemical reactions to run, or so they can otherwise be treated as fluids.
Today, the fundamental aspects ofdesigning chemical plants are done bychemical engineers. Historically, this was not always the case, and many chemical plants were constructed haphazardly before the discipline ofchemical engineering became established. Chemical engineering was first established as a profession in the United Kingdom when the first chemical engineering course was given at the University of Manchester in 1887 byGeorge E. Davis in the form of twelve lectures covering various aspects of industrial chemical practice.[6] As a consequenceGeorge E. Davis is regarded as the world's first chemical engineer. Today chemical engineering is a profession and those professional chemical engineers with experience can gain "Chartered" engineer status through theInstitution of Chemical Engineers.
In plant design, typically less than 1 percent of ideas for new designs ever become commercialized. During this solution process, typically, cost studies are used as an initial screening to eliminate unprofitable designs. If a process appears profitable, then other factors are considered, such as safety, environmental constraints, controllability, etc.[2] The general goal in plant design, is to construct or synthesize "optimum designs" in the neighborhood of the desired constraints.[7]
Many timeschemists research chemical reactions or other chemical principles in alaboratory, commonly on a small scale in a "batch-type" experiment. Chemistry information obtained is then used by chemical engineers, along with expertise of their own, to convert to a chemical process and scale up the batch size or capacity. Commonly, a small chemical plant called apilot plant is built to provide design and operating information before construction of a large plant. From data and operating experience obtained from the pilot plant, a scaled-up plant can be designed for higher or full capacity. After the fundamental aspects of a plant design are determined,mechanical orelectrical engineers may become involved with mechanical or electrical details, respectively.Structural engineers may become involved in the plant design to ensure the structures can support theweight of the units, piping, and other equipment.
The units, streams, and fluid systems of chemical plants or processes can be represented byblock flow diagrams which are very simplified diagrams, orprocess flow diagrams which are somewhat more detailed. The streams and other piping are shown as lines with arrow heads showing usual direction of material flow. In block diagrams, units are often simply shown as blocks. Process flow diagrams may use more detailed symbols and show pumps, compressors, and major valves. Likely values or ranges of material flow rates for the various streams are determined based on desired plant capacity using material balance calculations. Energy balances are also done based onheats of reaction,heat capacities, expected temperatures, and pressures at various points to calculate amounts of heating and cooling needed in various places and to size heat exchangers. Chemical plant design can be shown in fuller detail in apiping and instrumentation diagram (P&ID) which shows all piping, tubing, valves, and instrumentation, typically with special symbols. Showing a full plant is often complicated in a P&ID, so often only individual units or specific fluid systems are shown in a single P&ID.
In the plant design, the units are sized for the maximum capacity each may have to handle. Similarly, sizes for pipes, pumps, compressors, and associated equipment are chosen for the flow capacity they have to handle. Utility systems such aselectric power andwater supply should also be included in the plant design. Additional piping lines for non-routine or alternate operating procedures, such as plant or unit startups and shutdowns, may have to be included. Fluid systems design commonly includes isolation valves around various units or parts of a plant so that a section of a plant could be isolated in case of a problem such as aleak in a unit. If pneumatically or hydraulically actuated valves are used, a system of pressurizing lines to the actuators is needed. Any points where process samples may have to be taken should have sampling lines, valves, and access to them included in the detailed design. If necessary, provisions should be made for reducing high pressure or temperature of a sampling stream, such including apressure reducing valve or sample cooler.
Units and fluid systems in the plant including all vessels, piping, tubing, valves, pumps, compressors, and other equipment must be rated or designed to be able to withstand the entire range of pressures, temperatures, and other conditions which they could possibly encounter, including any appropriatesafety factors. All such units and equipment should also be checked formaterials compatibility to ensure they can withstand long-term exposure to the chemicals they will come in contact with. Any closed system in a plant which has a means of pressurizing possibly beyond the rating of its equipment, such as heating, exothermic reactions, or certain pumps or compressors, should have an appropriately sized pressurerelief valve included to prevent overpressurization for safety. Frequently all of these parameters (temperatures, pressures, flow, etc.) are exhaustively analyzed in combination through aHazop orfault tree analysis, to ensure that the plant has no known risk of serious hazard.
Within any constraints the plant is subject to, design parameters areoptimized for good economic performance while ensuring the safety and welfare of personnel and the surrounding community. For flexibility, a plant may be designed to operate in a range around some optimal design parameters in case feedstock or economic conditions change and re-optimization is desirable. In more modern times,computer simulations or other computer calculations have been used to help in chemical plant design or optimization.
Inprocess control, information gathered automatically from various sensors or other devices in the plant is used to control various equipment for running the plant, thereby controlling operation of the plant. Instruments receiving such information signals and sending out control signals to perform this function automatically are processcontrollers. Previously,pneumatic controls were sometimes used.Electrical controls are now common. A plant often has acontrol room with displays of parameters such as key temperatures, pressures, fluid flow rates and levels, operating positions of key valves, pumps, and other equipment, etc. In addition, operators in the control room can control various aspects of the plant operation, often including overriding automatic control. Process control with a computer represents more modern technology. Based on possible changing feedstock composition, changing products requirements or economics, or other changes in constraints, operating conditions may be re-optimized to maximize profit.
As in any industrial setting, there are a variety of workers working throughout a chemical plant facility, often organized into departments, sections, or other work groups. Such workers typically includeengineers,plant operators, and maintenance technicians. Other personnel at the site could include chemists, management/administration, and office workers. Types of engineers involved in operations or maintenance may include chemical process engineers, mechanical engineers for maintaining mechanical equipment, and electrical/computer engineers for electrical orcomputer equipment.
Large quantities of fluid feedstock or product may enter or leave a plant bypipeline, railroadtank car, ortanker truck. For example, petroleum commonly comes to a refinery by pipeline. Pipelines can also carry petrochemical feedstock from a refinery to a nearby petrochemical plant.Natural gas is a product which comes all the way from a natural gas processing plant to final consumers by pipeline or tubing. Large quantities of liquid feedstock are typically pumped into process units. Smaller quantities of feedstock or product may be shipped to or from a plant indrums. Use of drums about 55 gallons in capacity is common forpackaging industrial quantities of chemicals. Smaller batches of feedstock may be added from drums or other containers to process units by workers.
In addition to feeding and operating the plant, and packaging or preparing the product for shipping, plant workers are needed for taking samples for routine and troubleshooting analysis and for performing routine and non-routine maintenance.Routine maintenance can include periodic inspections and replacement of worn catalyst, analyzer reagents, various sensors, or mechanical parts. Non-routine maintenance can include investigating problems and then fixing them, such as leaks, failure to meet feed or product specifications, mechanical failures of valves, pumps, compressors, sensors, etc.
When working with chemicals,safety is a concern in order to avoid problems such aschemical accidents. In theUnited States, the law requires that employers provide workers working with chemicals with access to amaterial safety data sheet (MSDS) for every kind of chemical they work with. An MSDS for a certain chemical is prepared and provided by the supplier to whoever buys the chemical. Other laws covering chemical safety, hazardous waste, and pollution must be observed, including statutes such as theResource Conservation and Recovery Act (RCRA) and theToxic Substances Control Act (TSCA), and regulations such as theChemical Facility Anti-Terrorism Standards in the United States.Hazmat (hazardous materials) teams are trained to deal with chemical leaks or spills.Process Hazard Analysis (PHA) is used to assess potentialhazards in chemical plants. In 1998, theU. S. Chemical Safety and Hazard Investigation Board has become operational.
Chemical Plants used particularly forcommodity chemical andpetrochemical manufacture, are located in relatively few manufacturing locations around the world largely due to infrastructural needs. This is less important forspeciality orfine chemical batch plants. Not all commodity/petrochemicals are produced in any one location but groups of related materials often are, to induce industrial symbiosis as well as material, energy and utility efficiency and othereconomies of scale. These manufacturing locations often havebusiness clusters of units called chemical plants that share utilities and large scale infrastructure such as power stations, port facilities, road and rail terminals. In the United Kingdom for example there are four main locations for commodity chemical manufacture: near theRiver Mersey in Northwest England, on the Humber on the East coast of Yorkshire, in Grangemouth near the Firth of Forth in Scotland and onTeesside as part of theNortheast of England Process Industry Cluster (NEPIC).[8] Approximately 50% of the UK's petrochemicals, which are also commodity chemicals, are produced by the industry cluster companies onTeesside at the mouth of theRiver Tees on three large chemical parks atWilton,[9]Billingham andSeal Sands.
Corrosion in chemical process plants is a major issue that consumes billions of dollars yearly. Electrochemical corrosion of metals is pronounced in chemical process plants due to the presence of acid fumes and other electrolytic interactions. Recently, FRP (Fibre-reinforced plastic) is used as a material of construction. The British standard specificationBS4994 is widely used for design and construction of the vessels, tanks, etc.
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