Thread Content
Hazard analysis of chemical unit operations: Chemical unit operations refer to the processing methods based on physical processes in various chemical production activities, including heating, cooling, pressurization, vacuum operation, freezing, material transfer, melting, drying, evaporation, and distillation. 1 Heating Heating is a necessary means to facilitate chemical reactions as well as operations such as evaporation and distillation of materials. Common heating methods include direct fire heating (flue gas heating), steam or hot water heating, carrier heating, and electric heating. (1) Excessively high temperatures will accelerate the rate of chemical reactions; in the case of exothermic reactions, the amount of heat released increases. If heat dissipation is not sufficient, the temperature gets out of control, leading to material overflow, and it may even cause fires and explosions. (2) An excessive heating rate not only easily causes the reaction to exceed safe temperature limits but also damages the equipment; for example, rapid heating can damage lined equipment as well as various types of heating furnaces and reaction reactors. (3) When the heating temperature approaches or exceeds the material’s auto-ignition point, inert gas protection should be used ； If the heating temperature is close to the decomposition temperature of the material, this production process is considered hazardous, and it is necessary to find ways to improve the process conditions, such as by using negative pressure or pressurized operations. 2 Cooling In chemical production, when cooling materials to a temperature above ambient temperature, air or circulating water can be used as cooling media ； When the cooling temperature is above 15°C, groundwater can be used ； The cooling temperature can range from 0 to 15°C, and frozen saline can be used. Cooling can also be achieved by extracting heat from the material that needs to be cooled through the evaporation of a medium with a lower boiling point; common such media include freon and ammonia. At this point, the temperature to which the material is cooled can reach around -15°C. (1) During cooling operations, the cooling medium must not be interrupted, otherwise heat accumulation will occur, leading to a sudden increase in system temperature and pressure and potentially causing an explosion. When driving, the cooling medium should be circulated first ； When stopping the machine, stop the material feed first, and then the cooling system. (2) Some materials with a high freezing point tend to become viscous or solidify when cooled; therefore, it is necessary to control the temperature during cooling to prevent the material from sticking to the mixer or blocking the equipment and pipes. 3 Pressurization operation Any operation in which the pressure exceeds atmospheric pressure is considered a pressurization operation. The equipment used for pressurization must meet the requirements for pressure vessels, and the pressurization system must not leak; otherwise, the material will be ejected at high speed under pressure, generating static electricity, which can easily lead to fires and explosions. In industrial production processes, liquefying steam and gases, separating certain components at low temperatures, as well as transporting and storing certain materials, often require reducing the temperature of these materials to levels lower than that of water or the surrounding air. This process is known as freezing or refrigeration. Generally speaking, the degree of freezing is related to the freezing techniques used; any freezing process at temperatures below –100°C is considered freezing ； Temperatures of –100 to –200°C or lower are referred to as deep freezing, or simply cryogenic conditions. (1) Some refrigerants are flammable and toxic. As with ammonia, leaks of the refrigerant should be prevented. (2) For the compressor, condenser, evaporator, and piping of a refrigeration system, attention should be paid to the pressure rating and airtightness to prevent leaks. 4 Material Transport In industrial production processes, it is often necessary to transfer various raw materials, intermediates, products, as well as by-products and waste from one stage to the next, from one workshop to another, or to storage and transportation locations. These processes of transfer are what are known as material transport. (1) In addition to malfunctions of its own, the biggest problems with air-conveying systems are system blockages and dust explosions caused by static electricity. The powder pneumatic conveying system should maintain good airtightness. The piping material should be a conductive material with good grounding; if insulated piping is used, grounding measures must be taken outside the pipes. The conveying speed should not exceed the flow rate permitted for that material; powder should not accumulate inside the pipe, and the pipe walls must be cleaned regularly. (2) When transporting flammable liquids using various types of pumps, too high a flow rate can lead to the accumulation of static electricity; therefore, the flow rate within the pipes should not exceed the safe limit. (3) When transporting explosive or flammable materials, inert gases such as nitrogen or carbon dioxide should be used in place of air to prevent combustion or explosion. (4) Pipelines transporting flammable gas materials should be kept under positive pressure at all times to prevent air from entering, and safety devices such as check valves, water seals, and flame arrestors should be installed as required. 5 Melting In chemical production, it is often necessary to melt certain solid materials (such as sodium hydroxide, potassium hydroxide, naphthalene, sulfonic acids, etc.) before carrying out chemical reactions. Alkali chips or alkali solutions splashing onto the skin or eyes during the alkaline melting process can cause burns. If impurities such as inorganic salts are present in the alkali fusion products and sulfonates, they should be removed as much as possible; otherwise, these inorganic salts, due to their inability to melt, can cause localized overheating and burning, leading to the eruption of the molten material and posing a risk of burns. The melting process generally takes place at temperatures between 150 and 350°C, and continuous stirring is necessary to prevent localized overheating. 6 Drying Drying is a unit operation that uses heat to remove moisture (or solvent) from solid materials. Dry heat sources include hot air, superheated steam, flue gas, and open flames. During the drying process, the temperature must be strictly controlled to prevent local overheating, which could lead to the decomposition and explosion of the material. Flammable and explosive gases or dusts released during the process should not come into contact with open flames or hot surfaces to prevent combustion and explosion. Anti-static measures should be taken in air flow drying, and the gap between the scraper and the drum wall should be adjusted appropriately in drum drying to prevent sparks. 7 Evaporation: Evaporation is a physical process in which the solvent contained in a solution is continuously vaporized through heating, in order to increase the concentration of the solute in the solution or to cause the solute to precipitate out. Evaporation can be classified into atmospheric, pressurized, and vacuum evaporation depending on its operating pressure. All evaporated solutions possess certain characteristics. If crystallization, precipitation, and fouling occur during the concentration process due to the solute, these phenomena can reduce heat transfer efficiency and cause localized overheating, leading to the decomposition, combustion, and explosion of the material; therefore, it is necessary to control the evaporation temperature. To prevent the decomposition of heat-sensitive substances, vacuum evaporation can be used to lower the evaporation temperature, or high-efficiency evaporators can be employed to increase the evaporation area and reduce the residence time. 8 Distillation Distillation is a process in which a liquid mixture is separated into its pure components by taking advantage of the differences in their volatilities. Distillation operations can be divided into batch distillation and continuous distillation ； Based on the pressure applied, distillation is divided into atmospheric pressure distillation, reduced-pressure distillation, and pressurized (high-pressure) distillation. In terms of safety technology, the appropriate distillation methods and equipment should be selected for different materials. When dealing with materials that are difficult to vaporize (with a boiling point above 150°C at atmospheric pressure), vacuum distillation should be used; this allows the distillation temperature to be reduced, thereby preventing the materials from decomposing, deteriorating, or polymerizing at high temperatures. Atmospheric distillation is used when dealing with moderately volatile materials (with a boiling point of around 100°C). For materials with a boiling point below 30°C, pressure distillation should be used. This post was last edited by lxzzl on 2009-2-11 09:00]