Summary of experience in chemical engineering design

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Throughout the entire chemical engineering design process, we must be familiar with its principles and spirit in order to apply them. Flexibly apply the details in the design process to ensure compliance with standards without compromising production. Achieve maximum utilization of resources. Now let’s gain a further understanding of the requirements for chemical engineering design and related topics. Since safety is of primary importance in chemical production, we must analyze the safety aspects involved in the production process. 　　Be familiar with the properties of the raw materials and products used in the design process, so that we can match them accordingly. Chemical industry standards classify the degree of hazard of substances into five levels. This classification is determined primarily by taking into account the physicochemical properties, quantities of raw materials, intermediates, and finished products used, produced, and stored during the manufacturing process, as well as their fire and explosion hazards, along with the characteristics of the production process itself. Only by considering the different hazard levels can we determine the storage and transportation methods for raw materials, the material of equipment, fire separation distances, and explosion protection ratings. He played a decisive role in our choice of equipment, instruments, operating methods, and fire-fighting gear. 　 Among the five categories of A, B, C, D, and E, hazardous materials classified as Category A are defined as follows: 1. Liquids with a flash point of less than 28°C; 2. Liquids with an explosion limit of less than 10%; 3. Substances that can decompose on their own at room temperature or oxidize in air, thereby causing rapid spontaneous combustion or explosion; 4. Substances that, when exposed to water or water vapor in air at room temperature, produce flammable gases that can lead to combustion or explosion; 5. Strong oxidizers that are highly prone to combustion or explosion when exposed to acids, heat, impact, friction, catalysts, or flammable inorganic substances such as organic compounds or sulfur; 6. Substances that can cause combustion or explosion upon impact, friction, or contact with oxidizers or organic compounds; 7. Substances used in closed systems where the operating temperature is equal to or greater than the substance’s own auto-ignition point. Under these conditions, as long as one of them is met, the material is classified as a Category A explosive hazard; substances such as benzene, toluene, and propylene all fall into this category. For Class A hazardous materials, the relevant regulations are as follows: 1. Fire separation distance for above-ground Class A fixed-roof vertical storage tanks: when the volume of a single tank is greater than 1,000 cubic meters, the distance between tanks should be 0.6D; when the volume is 1,000 cubic meters or less, the distance between tanks should be 0.75D. However, it is also stipulated that for above-ground fixed storage tanks holding Class A and B liquids with a single-tank capacity of not more than 1,000 cubic meters, the fire separation distance may be no less than 0.6D when a fixed cooling fire protection system is used. It is also stipulated that when equipped with submersible foam fire extinguishing systems, fixed cooling water systems, and foam fire extinguishing systems for extinguishing liquid fires within fire dikes, the spacing between storage tanks can be reduced appropriately; however, the distance between above-ground storage tanks should not be less than 0.4D. 　 2. The fire separation distance between above-ground Class A fixed-roof vertical storage tanks and process units must be greater than 25 meters ； The fire separation distance from Class A warehouse areas is between 25 and 35 meters. 　 3. The fire separation distance between Class A process units and power distribution rooms and pump rooms should not be less than 20 meters, with a minimum of 15 meters ； The fire protection distance from open flames and emission sources should not be less than 30 meters ； The fire separation distance from sewage treatment plants should not be less than 30 meters. Fire separation distance from roads outside the plant: 15 meters ； A fire separation distance of 10 meters from the main roads within the factory ； 5 meters of fire separation from secondary roads within the plant ； The fire separation distance from civil buildings shall be no less than 25 meters ； The fire separation distance from important public buildings shall be no less than 50 meters. 　 4. Class A explosion-proof workshops with explosion hazards should be located separately, and should be of open or semi-open design. They should employ a frame or truss structure supported by reinforced concrete columns or steel columns, with the steel columns being equipped with fire-resistant protective layers ； There should be no fewer than two safety exits in the factory building. 　 5. The distance between the control room and the production equipment shall be no less than 15 meters, and it should be located away from electrical explosion-proof areas. If it is necessary to be in an explosion-proof area, no doors or windows should be installed on the walls facing the hazardous area, and explosion-proof positive pressure measures must be taken to maintain a positive pressure of 5–10 mm of water column inside, thereby preventing hazardous gases from entering the control room. In addition, other factors must also be taken into consideration: A: It should be located upwind of equipment that emits toxic gases. B: Care should be taken to stay away from equipment that is exposed to high temperatures and pressures, as well as from equipment that contains flammable or toxic substances. C: At least one side of the control room should not face the production facilities. 6. Regarding the classification of explosion-proof ratings in explosive gas environments: An example is dⅡBT4. “d” denotes an explosion-proof type; this parameter indicates the selection of electrical equipment for use in explosive gas environments, that is, it defines the safety levels required for different electrical devices in such environments. For the selection of rotary motors, they are classified into flameproof type, positive pressure type, increased safety type, and spark-free type ； The selection of lighting fixtures is divided into explosion-proof type and increased safety type ； The selection of signals and alarm devices is divided into intrinsically safe, flameproof, and enhanced safety types. 　 IIB represents the classification of the explosiveness of gas or vapor mixtures, and is divided into three categories: IIA, IIB, and IIC. Among them, IIA has the lowest level of explosiveness; substances belonging to category IIB include acetylene, ethylene, cyclopropane, 1,2-epoxycyclopropane, and coke oven gas ； It is distinguished by the highest or lowest ignition current of ⅡC. 　　T4 represents 135＜T≤200℃; it is defined as a temperature range within which a certain medium can be ignited. It is mainly divided into T1 – 450℃ < T, T2 – 300 < T ≤ 450℃, T3 – 200 < T ≤ 300℃, T4 – 135 < T ≤ 200℃, T5 – 100 < T ≤ 135, and T6 – 85 < T ≤ 100℃. 7. Terminology explanations: 7.1 Flash point: Also known as the ignition point. One of the indicators for the properties of flammable liquids. It is the temperature at which the mixture of vapor and air on the surface of a liquid produces a flash of blue flame when it comes into contact with fire. When measured using standard instruments, there are two types: open-cup and closed-cup. Generally, the former is used to measure liquids with a high flash point, while the latter is used to measure liquids with a low flash point. The temperature is a bit lower than the ignition point. The flash point and ignition point of flammable liquids indicate the likelihood of an explosion or fire, and are of great significance for the safety of their transportation, storage, and use. 　　C. Design of plant area utility tunnels and roads: When the utility tunnels cross above roads, their clear height should be: 4.5 meters or more for secondary roads; 6 meters or more for primary roads; and 7 meters or more for railways. When there are supports in the pipe gallery, the calculation should be based on the height at the bottom of the supports. The clear height of the maintenance passage under the pipe gallery shall be not less than 3.1 m. The clear height when the pipeline passes above platforms and walkways shall be not less than 2.1 meters. When the combined units are arranged side by side, a common maintenance road is provided, with a width of 5.2–6 m, and secondary roads have a width of 4.5 m. 　　D. There are specific requirements regarding the installation and layout of pumps; they must meet the needs of safe operation and maintenance. In addition, attention must also be paid to the spacing between the operating surfaces. The clear distance in the operation passage between pumps should be no less than 1 m, the distance in the operation passage in front of the pumps should be no less than 1.25 m, and the maintenance passage for the pumps should be no less than 3.5 m to allow cranes and trucks to access them; the width of the maintenance passage for small pumps can be reduced to 2.5 m. 　　When arranging pumps of different sizes, there are generally three methods: 1. Align the centerlines of the pump outlets; the advantage is that it simplifies operational coordination. 　 2. The pump foundation surface should be leveled to facilitate the installation of sewage pipes or ditches, as well as to ease foundation construction. 　 3. Align the foundation surface of the power end. The advantages are that cable wiring is easy and cost-effective; the pump switch and ammeter are located on the same line, and the motor is easy to operate. Of course, if the sizes of the pumps differ too much, it will result in an excessively long suction pipe. Another thing to note is that the distance between the valve handle and any protruding parts of the pump or columns should be at least 750 mm, while the distance between electric motors should be 1.5–1.8 meters. When installing the pump, the foundation surface is generally 200–600 mm higher than the floor level, and the installation height must be consistent. 　 Of course, when considering the installation height, the pump’s net positive suction head factor should be taken into account. 　　When installing a filter in front of the pump inlet, its base height should be chosen to facilitate the cleaning and removal of the filter. In addition, the drainage funnel and buried pipes also need to be considered. 　 If the pump is used to draw liquid from a tank, a bottom valve must be installed at the bottom of the suction pipe, and there must be a column of liquid at the suction height when the pump is started. 　 The pump outlet isolation valve should preferably be a valve with low resistance, such as a gate valve; globe valves should be avoided as much as possible, in order to reduce pressure drops and prevent damage to the pump. 　 The pump outlet pressure gauge should be installed between the pump outlet and the first shut-off valve. 　 The bearings of pumps generally require cooling water for cooling; cooling water pipes should be equipped with flow detectors or funnels to monitor the water flow and prevent interruptions in flow. In winter, measures must be taken to prevent freezing.