3.0 Process Units Basic Functions -.pdf

AECH2111: Principles of chemical engineering-I Topic 2: Process Units: Basic functions Process Units: Basic Functions In this section, a brief description is given of the most frequently used unit operations in chemical engineering processes. The explanation is focused on typical operations...

AECH2111: Principles of chemical engineering-I Topic 2: Process Units: Basic functions Process Units: Basic Functions In this section, a brief description is given of the most frequently used unit operations in chemical engineering processes. The explanation is focused on typical operations involving the transfer of mass through physical or chemical routes Divider (Splitter) A splitter is used to divide the flow rate in a certain stream into two or more streams with different flow rates. In this case, the composition of streams F1, F2, and F3 is the same since no operation is taking place between inlet and exit streams. Mixer (Blender) The mixing process has the following characteristics: There are two or more entering streams, and only one exit stream resulting from the blending of the incoming streams. The streams can be in any phase, that is, gas, liquid, or solid. The mixing process flow sheet is shown in Figure below. Dryer (Direct Heating) Drying is a mass transfer process resulting in the removal of moisture by evaporation from a solid, semisolid, or liquid to produce a solid state. To achieve this operation, the dryer is supplied with a source of heat. Vapor is produced in the process. The flow sheet of the drying process is shown in Figure below. Solvent stream leaves as a pure vapor and is free of solids. Resulting dried products are in solid phase. Dried solids may not be solvent free. Feed can be solid, slurry, or solution.  Filter Filtration is a technique used either to remove impurities from a liquid or to isolate a solid from a fluid. Filtration is commonly a mechanical or a physical operation that is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. A cross section of the filter press is shown in Figure below. https://www.youtube.com/watch?v=60Big9Ut6Mc https://www.youtube.com/watch?v=5qVIPUMJaRA Distillation Column Distillation is a method of separating chemical substances based on differences in their volatilities. Distillation usually forms part of a larger chemical process. In the distillation column, more volatile components are in the distillate, while less volatile components are in the bottoms. Separation is accomplished by boiling. https://www.youtube.com/watch?v=2xzYf8IL_FE Distillation Column Dehumidification It is a device that reduces the level of humidity in air or a gas stream. A dehumidification process has the following characteristics: Feed stream contains a condensable component and a noncondensable component, and the condensate is a liquid with the condensable component only, such as water in air. Humidifier Humidifier is a device that increases the amount of moisture in indoor air or a stream of air. It operates by allowing water to evaporate from a pan or a wetted surface, or by circulating air through an air-washer compartment that contains moisture. Humidifier processes have the following characteristics: Feed gas is not saturated, liquid is evaporated in the process unit, and exit product may or may not be saturated. Flash Separator Flash separator splits a liquid feed into vapor- and liquid-phase products. Flash units have the following characteristics: The process is the same as that of a partial condenser except that the feed is a liquid, and vaporization is caused by reducing the pressure or by heating. Vapor and liquid streams are in equilibrium. Absorption Column In gas absorption, a soluble component is absorbed by contact with a liquid phase in which the component is soluble. This system is used for absorbing impurities from a gas stream of certain components such as hydrogen sulfide, carbon dioxide, and ammonia, using a suitable solvent. The purpose of the unit is to have the liquid absorb a component from the feed gas. The liquid stream flows down through the tower due to gravity, while the gas stream is pumped upward through the tower. Absorption Column Reactors A chemical reactor carries out a chemical reaction that converts molecular species in the input (whereby a species loses its identity) to different molecular species in the output. Figure below shows a typical reactor that has two reactant feed streams and a recycle stream. A reactor is often named by the chemical reaction taking place within it. A reactor is sometimes preceded by a fictitious mixer, if the combined reactor feed is specified or must be determined. Multiple exit streams are shown to remind you to watch for streams that separate because of their different phases There are various types of reactors used in industry. The most common ones are the batch reactor, plug flow reactor (PFR), packed bed reactor (PBR), continuous stirred tank reactor (CSTR), and fluidized bed reactor. Batch Reactor A typical batch reactor consists of a tank with an agitator and integral heating/cooling system. These vessels may vary in size from less than 1 litre to more than 15,000 litres. They are usually fabricated in steel, stainless steel, glass- lined steel, glass or exotic alloy. Liquids and solids are usually charged via connections in the top cover of the reactor. Vapors and gases also discharge through connections in the top. Liquids are usually discharged out of the bottom. Batch processes are suitable for small production and for processes where a range of different products or grades is to be produced in the same equipment for example, pigments, dye stuff and polymers. 3.12.1 Batch Reactor Continuous Stirred Tank (CSTR) Continuous stirred-tank reactors (CSTRs) are open systems, where material is free to enter or exit the system, that operate on a steady-state basis, where the conditions in the reactor don't change with time. Reactants are continuously introduced into the reactor, while products are continuously removed. CSTRs are very well mixed, so the contents have relatively uniform properties such as temperature, density, etc. throughout. Also, conditions in the reactor's exit stream are the same as those inside the tank. Packed Bed Reactor (PBRs) Packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing. Packed beds may also contain catalyst particles or adsorbents such as zeolite pellets, granular activated carbon, etc. The purpose of a packed bed is typically to improve contact between two phases in a chemical or similar process. Packed beds can be used in a chemical reactor, a distillation process, or a scrubber, but packed beds have also been used to store heat in chemical plants. In this case, hot gases are allowed to escape through a vessel that is packed with a refractory material until the packing is hot. Air or other cool gas is then fed back to the plant through the hot bed, thereby pre-heating the air or gas feed.. Fluidized Bed Reactor (FBRs) A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a solid granular material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization, imparts many important advantages to the FBR. As a result, the fluidized bed reactor is now used in many industrial applications. Basic diagram of a fluidized bed reactorr