(1) heterogeneous separation

Raw materials, semi-finished products and discharged wastes in chemical production are mostly mixtures. In order to obtain high-purity products and meet the needs of environmental protection, it is often necessary to separate the mixture. The mixture can be divided into homogeneous (mixed) system and heterogeneous (mixed) system. In heterogeneous systems, one phase is in a dispersed state, which is called dispersed phase, such as water droplets in fog, dust particles in smoke and dust, solid particles in suspension and liquid phase dispersed into small droplets in emulsion; The other phase is in a continuous state, called continuous phase (or dispersion medium), such as gas phase in fog and smoke, liquid phase in suspension and liquid phase in emulsion. From the point of view of toxic and harmful substances treatment, heterogeneous separation process is the purification process, absorption process or concentration separation process of these substances. In industrial production, mechanical methods are often used to separate two phases, such as sedimentation separation, filtration separation, electrostatic separation and wet washing separation. In addition, there are acoustic dust removal, thermal dust removal and other methods.

Safety measures in filtration process:

1. If flammable, explosive and harmful gases are emitted during the pressure filtration process, a closed filter should be adopted. And use compressed air or inert gas to maintain pressure: when taking filter residue, release the pressure first.

2. In the process of fire and explosion, centrifugal filter should not be used, but rotary drum or belt vacuum filter should be used. If the centrifugal filter must be used, the installation quality of the motor should be strictly controlled, and the speed limit device should be installed. Be careful not to run at critical speed.

3. The centrifugal filter should pay attention to material selection and welding quality, and the drum, shell, cover and base should be made of ductile metal.

(2) Heating and heat transfer

The application of heat transfer in chemical production mainly includes creating and maintaining the temperature conditions needed for chemical reaction, creating and maintaining the temperature conditions needed for unit operation, heat energy synthesis and recovery, heat insulation and heat limitation.

There are three basic ways of heat transfer: heat conduction, heat convection and heat radiation. In fact, the heat transfer process often does not appear in a single heat transfer mode, but in a combination of two or three heat transfer modes. Heat exchange in chemical production is usually carried out between two fluids. The purpose of heat exchange is to heat (evaporate) or cool (condense) the process fluid.

Safety analysis of heating process;

The heating process is dangerous. The heating modes of the device are generally steam or hot water heating, heat carrier heating and electric heating.

1. When heating with steam or hot water, check the compressive strength of steam jacket and pipeline regularly, and install pressure gauge and safety valve. Materials that react with water cannot be heated with steam or hot water.

2. When oil-filled jacket is used for heating, it is necessary to isolate the heating furnace door from the reaction equipment with brick walls, or set the heating furnace outside the workshop. The oil circulation system should be strictly sealed, and hot oil leakage is not allowed.

3. In order to improve the safety and reliability of induction heating equipment, large-section conductors can be used to prevent overload; Adopt moisture-proof, anti-corrosion and high-temperature insulation to increase the thickness of insulation layer. Add insulation protective layer and other measures. The induction coil should be sealed to prevent contact with flammable materials.

4. There should be good insulation between the wire of the electric heater and the wall of the heated equipment, so as to prevent short circuit from causing electric sparks, breaking through the wall and causing flammable substances or leaked gas and steam in the equipment to burn or explode. When heating or drying flammable substances and substances that will volatilize flammable gas or steam when heated, enclosed electric heaters should be used. Electric heaters should not be placed near flammable substances. The load capacity of the conductor should meet the requirements of the heater, and the plug should be connected with the socket. In any case, industrial electric heaters should be equipped with separate circuits and appropriate fuses.

5. When the direct fire heating process is adopted, the furnace door of the heating furnace and the heating equipment should be completely isolated with brick walls, so that there is no open flame in the workshop. The residue in the heating pot should be removed frequently to avoid local overheating and cracking at the bottom of the pot. When using pulverized coal as fuel, the hopper should keep a certain stock, and it is not allowed to be emptied to avoid air entry and pulverized coal explosion; The pulverizing system shall be equipped with bursting discs. When gas or liquid is used as fuel, the furnace should be purged before ignition to eliminate the accumulated explosive mixed gas and prevent explosion during ignition. When the heating temperature approaches or exceeds the self-ignition point of the material, inert gas should be used for protection.

(3) distillation and rectification

In chemical production, it is often necessary to separate the mixture in order to purify and recycle the product or refine the raw materials. For homogeneous liquid mixtures, distillation is the most commonly used separation method. In order to achieve high purity separation of mixed liquid, rectification operation is needed.

Hazard analysis of distillation process;

In atmospheric distillation, attention should be paid to the fact that the distillation heat source of flammable liquid cannot be naked flame, but it is safer to heat it with steam or superheated steam. When distilling corrosive liquids, it is necessary to prevent corrosion of tower walls and trays. When exposed to open flames or charred furnace walls, flammable liquids or vapors will escape and burn. When distilling liquid with low self-ignition point, attention should be paid to the sealing of distillation system to prevent high-temperature leakage from spontaneous combustion in the air. For the high-temperature distillation system, cooling water should be prevented from suddenly leaking into the tower, so that the water can vaporize quickly, and the pressure in the tower suddenly rises, rushing out of materials or exploding. Before start-up, the condensed water in the tower and steam pipeline should be drained before use. In the process of atmospheric distillation, attention should also be paid to prevent pipes and valves from being blocked by substances with high freezing point, which will lead to the increase of pressure in the tower and the explosion. When heating high-boiling substances (such as phthalic anhydride) with direct fire, it is necessary to prevent resin oil with low self-ignition point from spontaneous combustion in air. At the same time, it is necessary to prevent residue evaporation, coking, scaling, local overheating and fire explosion. Oil coke and residue should be removed frequently. The cooling water or frozen brine in the condensation system cannot be interrupted, otherwise the uncondensed flammable vapor will escape to raise the temperature of the local absorption system, or it will be ignited when an open flame escapes.

Vacuum distillation (vacuum distillation) is a safe distillation method. Vacuum distillation is more suitable for high-boiling substances that can cause decomposition, explosion and polymerization during high-temperature distillation. For example, nitrotoluene is decomposed and exploded at high temperature, and styrene is easy to polymerize at high temperature. The distillation of similar substances must adopt vacuum distillation to reduce the boiling point of the fluid. So as to reduce the distillation temperature and ensure the distillation safety.

(4) gas absorption and desorption

Gas absorption can be divided into physical absorption and chemical absorption according to whether there is a significant chemical reaction between solute and solvent. According to the different absorption components, it can be divided into single-component absorption and multi-component absorption; According to whether the temperature of absorption system (mainly liquid phase) changes significantly, it can be divided into isothermal absorption and non-isothermal absorption. Pay attention to solubility, selectivity, volatility and viscosity when selecting absorbent. The absorption towers used in industrial production mainly include plate tower, packed tower, turbulent ball tower, spray tower and jet absorption tower.

Desorption, also known as desorption, is the process of removing absorbed solute from absorbent and making solute escape from liquid phase to gas phase. The pure gas solute needed for production is obtained by desorption process, and the solvent is recycled and returned to the absorption tower. The commonly used desorption methods in industry include heating desorption, decompression desorption, inert gas desorption and rectification.

(5) drying

According to the way of supplying heat to wet materials, drying can be divided into conduction drying, convection drying, radiation drying and dielectric heating drying. Drying can be divided into normal pressure drying and reduced pressure drying according to operating pressure; According to the operation mode, it can be divided into intermittent drying and continuous drying. Commonly used drying equipment includes box dryer, rotary dryer, air dryer, fluidized bed dryer and spray drier. In order to prevent fire, explosion and poisoning accidents, the following safety measures should be taken during drying:

1 When the dry material contains low spontaneous combustion point or other harmful impurities, it must be thoroughly removed before drying, and no substances liable to spontaneous combustion shall be placed in the drying room.

2. The drying room should be separated from the production workshop by a firewall, and good ventilation equipment should be installed. Electrical equipment should be explosion-proof or switch should be installed outdoors. When operating in the drying room or drying box, the combustible dry matter should be prevented from directly contacting the heat source, so as not to cause combustion.

3 to dry flammable and explosive substances, a vacuum drying oven heated by steam should be used. When vacuumizing after drying, it must be put into the air after the temperature is lowered; When drying flammable and explosive substances with large flow of hot air, the exhaust equipment and motor should be explosion-proof; When baking substances that can evaporate flammable vapor in an electric oven, the wires should be completely closed and the box should be equipped with explosion-proof doors; When combustible materials are directly heated by flue gas, an explosion-proof disc should be installed on the drum or dryer to prevent the explosion caused by the mixture of flue gas and carbon monoxide.

4. Intermittent drying, most materials are transported by manpower, and the heat source adopts natural circulation of hot air or forced circulation of blower, so the temperature is difficult to control, which is easy to cause local overheating and cause material decomposition to cause fire or explosion. Therefore, in the drying process, the temperature should be strictly controlled.

5. When using tunnel dryer and drum dryer for drying, it is mainly to prevent mechanical damage. Flue gas and hot air are often used as drying heat sources in air drying, spray drying, fluidized bed drying and drum drying.

6. Combustible gas and dust generated in the drying process are easy to reach the explosion limit when mixed with air. In airflow drying, materials collide violently due to rapid movement, and friction is easy to generate static electricity; In the process of drum drying, the scraper sometimes rubs against the drum wall to generate sparks. Therefore, the wind speed of dry air flow should be strictly controlled and the equipment should be grounded. When the drum is drying, the gap between the scraper and the drum wall should be properly adjusted, and the scraper should be fixed firmly, or the scraper should be made of non-ferrous metal materials to prevent sparks. The drum dryer heated by flue gas should be heated evenly, and the drum should not stop running halfway. When the hopper is cut off or stopped, cut off the flue gas and introduce nitrogen. A bursting disc should be installed on the drying equipment. (6) evaporation

According to the pressure of evaporation, it can be atmospheric evaporation, pressurized evaporation and reduced pressure evaporation (vacuum evaporation). According to the utilization times of heat required for evaporation, it can be divided into single-effect evaporation and multi-effect evaporation. Attention should be paid to the following issues during evaporation:

The selection of 1 evaporator should consider the properties of the delayed solution, such as viscosity, foaming, corrosiveness, heat sensitivity, and whether it is easy to scale and crystallize.

Scaling on the inner wall of the tube is inevitable in evaporation operation, especially when dealing with materials that are easy to crystallize and corrode, which will reduce heat transfer. In these evaporation operations, on the one hand, it is necessary to stop regularly for cleaning and descaling; On the other hand, the structure of the evaporator is improved, such as the heating pipe of the evaporator is processed smoothly, which is not easy to produce dirt, and even if it does, it is easy to clean, which improves the circulation speed of the solution and thus reduces the speed of dirt generation.

(7) crystallization

Crystallization is a process in which solid substances are precipitated from steam, solution or melt in a crystalline state. Crystallization is an important chemical unit operation, which is mainly used to prepare products and intermediate products to obtain high-purity pure solid substances.

Stirring devices are often used in the crystallization process. Stir the liquid to make it circulate in some way, so as to make the materials mix evenly or accelerate the operation of physical and chemical processes.

The agitator during crystallization should pay attention to the following safety issues:

1. When there is an explosive mixture of flammable liquid vapor and air in the crystallization equipment, static electricity should be prevented to avoid fire and explosion accidents.

2. Avoid oil leakage from the stuffing box of the stirring shaft, because the oil in the stuffing box will be dangerous if it leaks into the reactor. For example, during nitration, there is concentrated nitric acid in the reactor. If the lubricating oil leaks in, the oil will be oxidized and heated under the action of concentrated nitric acid, which will raise the temperature of the reaction material, which may lead to material erosion and combustion explosion. There is a similar danger when there is a strong oxidant in the reactor.

For dangerous flammable materials, don't stop stirring halfway. Because when the stirring stops, the materials can not be fully mixed, the reaction is not good, and a large number of piles are piled up; However, when stirring is resumed, a large number of unreacted materials are mixed quickly and react violently, which often causes material erosion and is in danger of combustion and explosion. If the stirring stops due to failure, the feeding should be stopped immediately and cooled quickly; When the stirring is resumed, the feeding can be continued until the temperature is stable, the reaction is normal, and the normal operation can be resumed.

4. The mixer should be maintained regularly to prevent the uneven mixing of materials caused by the broken mixer. Finally, the mixer should be flexible to prevent the sudden reaction from causing the motor to catch fire due to excessive temperature rise. The agitator should have sufficient mechanical strength to prevent accidents caused by friction with the reactor wall due to deformation.

(8) extraction

The choice of solvent is the key of extraction operation, and the nature of extractant determines the danger and characteristics of extraction process. Selectivity, physical properties (density, interfacial tension, viscosity), chemical properties (stability, thermal stability, antioxidant stability), the difficulty of extracting agent recovery, and the safety of extraction (toxicity, flammability, and explosiveness) are issues that need special consideration when selecting extractant. There are many kinds of extraction processes used in industrial production, mainly including single stage and multi-stage

The main performance of the extraction equipment is that it can provide sufficient mixing and separation conditions for the two liquid phases, so that there is a large contact area between the two liquid phases. This interface is usually formed by dispersing one liquid phase in another liquid phase, and the two-phase fluid runs in countercurrent in the extraction equipment. The extraction equipment includes packing extraction tower, sieve plate extraction tower, rotary table extraction tower, reciprocating vibrating screen plate tower and pulse extraction tower.

(9) Refrigeration

The main difference between cooling and condensation is whether the cooled substance changes phase, if it changes phase, it becomes condensation, otherwise it is cooling without changing phase. Cooling and condensing operation is very important in chemical production, which not only involves production, but also seriously affects fire safety. Reaction equipment and materials are often the cause of fire and explosion because they cannot be cooled or condensed in time. In the process of industrial production, the liquefaction of steam and gas, the low-temperature separation of some components, and the transportation and storage of some articles often require a lower material temperature than water or surrounding air. This operation is called freezing or refrigeration.

The essence of freezing operation is that the refrigerant itself takes away heat from the frozen object through the cycle process of compression-cooling-evaporation (or throttling expansion) and transfers it to high-temperature substances (water or air) to reduce the temperature of the frozen object. Generally speaking, the freezing degree is related to the freezing operation technology. Any freezing range within-100℃ is called freezing. When the temperature is-100 ~-200℃ or lower, it is called deep freezing or deep freezing for short.

Key points of risk control during cooling (freezing) and freezing are as follows:

(1) The cooling equipment and coolant should be correctly selected according to the temperature, pressure, physical and chemical properties of the cooled material and the required cooling process conditions. Do not use water as coolant to cool water-repellent materials, and take special measures when necessary.

(2) Pay strict attention to the tightness of cooling equipment to prevent materials from entering the cooling liquid or the cooling liquid from entering the materials.

(3) During the cooling operation, the cooling medium cannot be interrupted, otherwise it will cause heat accumulation, abnormal reaction, system temperature and pressure rise, and cause fire or explosion. Therefore, it is best to control the temperature of the cooling medium with an automatic regulating device.

(4) Before driving, the liquid in the condenser should be removed first; When driving, the cooling medium should be introduced first, and then the high-temperature material should be introduced; When parking, stop the material first, and then stop the cooling system.

(5) In order to ensure the safe evacuation of non-condensable combustible gas, nitrogen can be filled for protection.

(6) Materials with high freezing point are easy to get sticky or solidify after cooling. When cooling, the temperature should be controlled to prevent materials from blocking the mixer or blocking equipment and pipelines.

2. Safety measures in the freezing process

(1) The compressor, condenser, evaporator and piping system of the refrigeration system should pay attention to the pressure grade and air tightness to prevent the equipment and piping from cracking and leaking. In addition, the inspection and maintenance of pressure gauges and safety valves should be strengthened.

(2) For low-temperature parts, attention should be paid to the selection of low-temperature materials to prevent low-temperature embrittlement.

(3) When the refrigeration system has an accident or an emergency stop, attention should be paid to the emptying and treatment of frozen materials.

(4) The ammonia compressor should use non-sparking electrical equipment; The compressor should use lubricating oil that does not freeze at low temperature and does not react chemically with refrigerant, and the oil separator should be located outdoors.

(5) Pay attention to the anticorrosion of cold carrier brine system.

(10) Screening and filtering

1. Check

In industrial production, in order to meet the requirements of production technology, it is often necessary to screen solid raw materials and products to choose the particle size that meets the technical requirements. This operation process is called screening. Screening is divided into manual screening and mechanical screening. The equipment used for screening is called a sieve, and the particle size of the material is controlled through the holes of the sieve. According to the shape of the screen, it can be divided into two types: rotating type and flat type.

Key points of risk control in screening process. When screening combustible materials, measures should be taken to prevent collision and ignition and eliminate static electricity to prevent dust explosion and fire accidents caused by collision and static electricity.

filter

Filtration is the operation of making the suspension pass through the fine-pore object under the action of gravity, vacuum, pressurization and centrifugation, and intercepting the solid suspended particles for separation. According to the operation method, filtration can be divided into intermittent filtration and continuous filtration; According to the driving force, it can be divided into gravity filtration, pressure filtration, vacuum filtration and centrifugal filtration. The device used for filtration is a filter.

(1 1) Material transportation

In the process of industrial production, it is often necessary to transport various raw materials, intermediates, products, by-products and wastes from one place to another. These transportation processes are material transportation. In modern industrial enterprises, material transportation is realized by various transportation machinery and equipment. Due to the different shapes of substances (block, powder, liquid, gas, etc.). ), the conveying equipment used is also different.

Length of liquid material transportation

Liquid substances can be transported to lower places along pipelines by using their potential energy. From low to high, or from one place to another (horizontal transportation), or from low pressure to high pressure, pumps are needed to transport, and pressure heads are needed to ensure a certain flow and overcome resistance. There are many kinds of pumps, including reciprocating pumps, centrifugal pumps, rotary pumps and fluid-acting pumps.

The key points to control the transportation danger of liquid materials are as follows:

(1) The steam reciprocating pump should be used to transport flammable liquid. If a centrifugal pump is used, the impeller of the pump should be made of non-ferrous metal to prevent sparks from collision. Equipment and pipelines should be well grounded to prevent fire caused by static electricity. Siphon and gravity transportation are safer and should be preferred.

(2) For flammable liquids, compressed air cannot be used for pressure transportation, because air mixed with flammable liquid vapor can form an explosive mixture and may generate static electricity. For flammable liquids with very low flash point, inert gases such as nitrogen or carbon dioxide should be used for pressure transportation. Combustible liquid with high flash point and boiling point above 130℃ can be pumped by air if there is a good grounding device.

(3) The joint between the pump and the pipeline (hose) that temporarily transports combustible liquid must be tight and firm, so as to avoid the fire caused by the leakage of the pipeline due to pressure during transportation.

(4) When transporting flammable liquids with various pumps, the flow velocity in the pipeline should not exceed the safe speed, and the pipeline should have reliable grounding measures to prevent static electricity accumulation. At the same time, negative pressure should be avoided at the suction port to prevent air from entering the system and causing explosion or equipment collapse.

2. Transport of gaseous substances

The gas material is transported by the compressor. According to the movement mode of gas, compressors can be divided into reciprocating compressors and rotary compressors.

The key points of risk control of gas transportation are as follows:

(1) Liquid ring pump should be used to transport liquefied combustible gas, because liquid ring pump is safer. However, when pumping or pressurizing combustible gas, the air inlet should maintain a certain residual pressure to avoid negative pressure sucking air to form explosive mixture.

(2) In order to avoid the explosion of compressor cylinder, gas storage tank and conveying pipeline due to pressure rise, these components are required to have sufficient strength. In addition, pressure gauges and safety valves (or bursting discs) that have been verified to be accurate and reliable must be installed. The pressure relief function of the safety valve should guide the dangerous gas to a safe place. Extra-high pressure alarm, automatic regulating device or extra-high pressure automatic parking device can also be installed.

(3) The lubricating oil and cooling water shall not be interrupted during the operation of the compressor, and the cooling water shall not enter the cylinder to prevent water hammer.

(4) Gaskets on gas extraction and compression equipment are easy to be damaged and leaked, so attention should be paid to regular inspection and timely replacement.

(5) The compressor conveying special gas under pressure should take corresponding fire prevention measures according to the chemical properties of the gas materials under pressure. For example, the parts of acetylene compressor in contact with acetylene are not allowed to be made of copper, in case of producing acetylene copper, which may cause explosion hazard.

(6) The pipeline of combustible gas should always maintain positive pressure, and safety devices such as check valve, water seal and flame arrester should be installed according to actual needs, and the flow in the pipeline should not be too large. The pipeline should have a good grounding device to prevent fire caused by electrostatic aggregate discharge.

(7) The motor part of the pumping and compressing equipment for combustible gas and combustible vapor shall be electrical equipment meeting the requirements of explosion-proof grade, otherwise it shall be isolated by walls.

(8) When the pipeline conveying combustible gas is on fire, fire extinguishing measures shall be taken in time. For pipelines with a diameter below 150 mm, generally, the gate valve can be directly closed for flameout; When the pipeline with the pipe diameter above 150 ram is on fire, it is not allowed to directly close the gate valve to extinguish the fire, and the air pressure should be gradually reduced. Fire extinguishing measures by introducing a large amount of steam or ammonia. But the gas pressure shall not be lower than 50 ~ 100 Pa. It is forbidden to suddenly close the gate valve or water seal. To prevent backfire and explosion. When the fire tube burns red, it shall not be cooled suddenly with water.