Chemical engineering largely involves the design and maintenance of chemical processes for large-scale manufacture. Chemical engineers in this branch are usually employed under the title of process engineer.
A specific example would be the synthesis of aspirin by the reaction of salicylic acid (R1) with acetic anhydride (R2) in solvent water (S) and in the presence of catalyst phosphoric acid (C). Aspirin is the product P, and acetic acid (W) is also formed.
In the laboratory 5 grams of R1 (a solid) are added to 120 ml of water in a flask. 5 ml of R2 (a liquid) are added plus 0.5 ml of phosphoric acid solution, and the flask is heated in a water bath. The contents are agitated by swirling the flask or with a laboratory stirrer and heated under reflux for about an hour.
The material is allowed to cool down and crystals of aspirin are formed, which may be filtered off, and perhaps recrystallized. A good yield would be 5 to 6 grams. The remaining solution is poured down the sink.
Now consider an industrial process in which we replace grams with tons.
Firstly suitable storage (say for two weeks of production) must be provided for the raw materials. In this case R1 is a solid and would be put in a storage silo; R2 is a corrosive liquid, combustible and sensitive to water, so would need a closed tank of resistant material. A means of transport to the reactor must be provided, such as a screw conveyor for the solid R1 and a pump and pipes for liquid R2. Chemical engineers would calculate the sizes and power requirements and specify suitable materials. Similar arrangements must be made for the solvent S and the catalyst C. In this case, water is the solvent, but ordinary tap water would not be good enough, so there will be a separate process to clean the water.
The reactor is now to contain 120 tons of water and the other ingredients so cannot be swirled. An agitator must be designed and its power consumption calculated to give the necessary mixing. Heating and cooling are considered free in the laboratory, but not in industry. The chemical engineers must first calculate the amount of heat to be added and removed, and then design suitable methods to do this, perhaps by passing steam through an outer jacket of the vessel to heat. They will probably decide to pump the reacted mixture to another vessel with a cooler, then to a filter. The solid will then go to further equipment to dissolve, crystallize and filter again, giving perhaps 5.5 tons of aspirin, which will be dried and placed in suitable storage, which must also be designed. (The drying process uses significant amounts of energy.)
However, there is about 125 tons of waste which cannot be just poured down the drain. It will contain some unreacted R1 and about 3 tons of W, which must be recovered and recycled. (In this case, W can be converted to R2 in another reactor.) The catalyst may be recovered, or made harmless by a chemical reaction before disposal. Thus there will be another set of equipment to save the cost of wasting chemicals and to protect the environment. Solvents other than water are generally recycled by distillation, but water is also re-used and recycled as far as economically feasible.
This process may run as a batch process, where reactors are filled and emptied discharging their contents through the plant upon completion. Alternatively the process may be run as a continuous process, where the reaction precursors are fed through the plant at a fixed rate. The choice of continuous or batch is dependent upon a combination of reaction kinetics, the desired amount of product and the economics of the plant's operation and would be decided during the design phase of the plant.
The individual processes used by chemical engineers (eg. distillation or filtration) are called unit operations and consist of chemical reactions, mass-, heat- and momentum- transfer operations. Unit operations are grouped together in various configurations for the purpose of chemical synthesis and/or chemical separation. Some processes are a combination of intertwined transport and separation unit operations, (e.g. reactive distillation).
Three primary physical laws underlying chemical engineering design are conservation of mass, conservation of momentum and conservation of energy. The movement of mass and energy around a chemical process are evaluated using mass balances and energy balances, laws that apply to discrete parts of equipment, unit operations, or an entire plant. In doing so, chemical engineers must also use principles of thermodynamics, reaction kinetics and transport phenomena. The task of performing these balances is now aided by process simulators, which are complex software models (see List of Chemical Process Simulators) that can solve mass and energy balances and usually have built-in modules to simulate a variety of common unit operations.
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