Lu An Jian Fa [2011] No. 140, Guidelines on Safety Control for Distillation Systems

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Guidelines on Safety Control for Distillation Systems, Lu An Jian Fa [2011] No. 140: These guidelines were formulated in accordance with the \"Guiding Opinions on Further Strengthening Work Safety in the Handling of Hazardous Chemicals\" issued by the Office of the State Council’s Work Safety Committee (An Wei Ban 2008] No. 26), the \"Opinions on Further Strengthening Work Safety in the Handling of Hazardous Chemicals\" issued by the provincial government’s office (Lu Zheng Ban Fa 2008] No. 68), as well as relevant laws and standards. The purpose of these guidelines is to effectively prevent accidents that may occur during distillation processes involving flammable, explosive, and toxic substances; to guide chemical and pharmaceutical enterprises across the province in upgrading their safety control systems for distillation operations and revising relevant safety procedures; to enhance the intrinsic safety level of such operations; and to provide a reference for design firms when developing corresponding safety control systems. 1. Overview 1.1 Process Introduction Distillation is a type of chemical unit operation that utilizes the differences in volatility among the components in a liquid mixture, through partial vaporization or partial condensation, to achieve the relative separation of these components. Distillation is widely used in industries such as chemicals and pharmaceuticals, including atmospheric and vacuum distillation in petroleum refining, the purification of basic organic chemical products, the refinement of fine chemical products, solvent recovery in chemical manufacturing processes, and air separation. 1.2 Classification 1.2.1 Based on components, it can be divided into two-component and multi-component distillation. 1.2.2 Based on the operating pressure, distillation can be divided into atmospheric distillation and vacuum distillation. Vacuum distillation is used for mixtures of substances that have high boiling points under normal conditions, are difficult to vaporize, or are thermally sensitive and may decompose, explode, or polymerize when distilled at high temperatures. By applying vacuum, the boiling point of these substances can be reduced, thereby lowering the distillation temperature and enhancing safety; for example, nitrotoluene decomposes and explodes at high temperatures, while styrene tends to polymerize at high temperatures ； It can also be used for the distillation of highly toxic materials to prevent leaks into the air), as well as for pressure distillation (for substances that are gases at atmospheric pressure). 1.2.3 Based on production continuity, it can be divided into continuous distillation and batch distillation. 1.2.4 Based on the operation method, it can be divided into distillation without reflux and distillation with reflux ； Among them, distillation without reflux is further divided into simple distillation and equilibrium distillation, while distillation with reflux is called rectification ； Distillation can be further divided into ordinary distillation, azeotropic distillation, extractive distillation, steam distillation, etc. 1.3 Composition of the unit: A complete distillation system generally consists of components such as a distillation tower, reboiler, condenser, reflux tank, material pump, and product storage tank; it may also be a combination of two or several of these components. A simple distillation system typically consists of a distillation vessel along with its heating device, a condensation cooler, and a receiver tank. The heating devices for batch distillation include external jacket types and internal coil types; for some materials, direct fire heating or electric heating outside the tank is also used. 2. Hazard analysis: The materials used in distillation are, for the most part, flammable, explosive, toxic, or corrosive ； During the distillation process, changes in pressure within the system (equipment) also occur. Therefore, the main hazards of distillation systems include: fire, explosion, poisoning, asphyxiation, burns, etc. 2.1 Accidents that may be caused by hazardous materials 2.1.1 Explosive hazards: The accumulation of certain impurities in a specific location within the tower can lead to explosions; examples include poly-nitro compounds in nitro compounds, hydrocarbons in liquid oxygen, and catalysts that can cause the polymerization of ethylene oxide. 2.1.2 During the distillation process, a gas-liquid coexistence state persists within the system; if flammable or explosive materials leak out or come into contact with air, an explosive gas mixture can be formed. Especially when distilling materials with a low auto-ignition point at high temperatures, if such hot materials leak out and come into contact with air, they can ignite spontaneously, leading to fire accidents. 2.1.3 The residues at the bottom of the distillation vessel, especially those from batch distillation processes, are complex mixtures with high boiling points, high viscosity, and a tendency to decompose or undergo polymerization at high temperatures; such residues are highly prone to thermal decomposition, self-polymerization, or spontaneous ignition due to heat accumulation. When the residues contain heat-sensitive or flammable and explosive substances, the risk of fire and explosion increases further. 2.1.4 When distilling flammable liquids, especially those that are poor conductors of electricity, the material flows at high speeds through the pipes, and the liquid in the distillation vessel undergoes intense stirring, friction, and splashing – all of which can generate static electricity, which tends to accumulate. There is a risk of fire resulting from static discharge. 2.1.5 In distillation equipment operating at high temperatures, if cold water or other low-boiling-point substances enter, it can cause instant vaporization in large quantities, leading to a sudden increase in pressure inside the equipment and resulting in container explosion accidents. 2.1.6 For substances with a high distillation freezing point, the outlet pipes of the equipment can become coated with condensate and blocked, which leads to an increase in pressure inside the equipment and may result in the container exploding. 2.1.7 When distilling toxic or corrosive materials, equipment leaks can easily lead to poisoning or chemical burns. 2.1.8 During the distillation process, if materials at high or low temperatures are present, inadequate protection can lead to burns or frostbite. 2.2 Accidents that may occur due to violation of operating procedures 2.2.1 Continuous distillation: The operation of continuous distillation is generally complex, involving many auxiliary devices. Any deviation in a certain control parameter or in a particular operational step during the distillation process can disrupt the balance of the entire distillation system, leading to accidents. If the distillation temperature is too high, there is a risk of overpressure explosions, liquid overflow, material ejection, thermal decomposition, and spontaneous ignition ； If the temperature is too low, there is a risk of tower flooding. If the feeding rate exceeds the limit, in the case of column distillation, it can increase the amount of vaporization, causing uncondensed vapor to enter the liquid receiver and leading to overpressure and explosion in the receiver. When the backflow increases, it not only lowers the operating temperature within the system but also easily leads to tower flooding, resulting in loss of control over the operation. 2.2.2 For flammable and explosive materials, intermittent distillation can lead to incomplete displacement due to periodic feeding and discharging, which allows oxygen to mix in and trigger accidents. If the flow rate of the heating medium is too high, excessive vaporization will occur, leading to overpressure in the equipment. If the liquid level in the distillation kettle is too low, it can cause the kettle to dry out and lead to accidents. An overload due to excessive feeding amount can cause a boiling fire. If heating begins as soon as the distillate discharge valve is closed, it can easily cause overpressure in the system. 2.2.3 In vacuum distillation, an increase in pressure due to various reasons can raise the boiling point of the material, potentially leading to overheating and accidents. Illegal operations during production can cause a large amount of air to be drawn into the vacuum tower, potentially leading to fires or even explosions in the vacuum tower. 2.2.4 Equilibrium distillation: If the feed temperature is too high, resulting in excessive liquid superheat, rapid flashing occurs in the flash tower, leading to a pressure overload accident. 2.3 Accidents caused by defects in equipment and facilities 2.3.1 Distillation is mostly carried out at high temperatures, which can lead to metal fatigue in equipment and pipelines; improper material selection may result in high-temperature creep failure. 2.3.2 Poor sealing of high-temperature and high-pressure equipment and flanges can lead to leaks of hazardous materials. 2.3.3 Distillation operations involving corrosive materials can cause corrosion and perforation of equipment and pipelines, thinning of the wall thickness, and an accelerated rate of coking, which in turn leads to a loss of structural strength and may result in leaks that could trigger fires. 2.3.4 An open flame or sparks are generated at the bottom of the distillation vessel heated directly by fire, which can cause fires or explosions when in contact with flammable materials. 2.3.5 During depressurization operations, poor sealing of equipment and facilities allows air or other oxidizing agents to enter, resulting in the formation of explosive mixtures. Furthermore, sudden interruptions in utilities such as water supply, power supply and distribution, steam supply, cooling, and ventilation can alter the process conditions of distillation operations, potentially leading to overpressure and leakage accidents. 3. Process parameters under close monitoring 3.1 Distillation tower (vessel): Tower and vessel temperature, liquid level; key plate temperatures, component composition, feed flow rate and temperature, top temperature, pressure (vacuum level), and reflux volume. 3.2 Reboiler: temperature, pressure (vacuum level), flow rate, temperature, and pressure of the heating medium. 3.3 Condenser: temperature, cooling medium flow rate, temperature, pressure. 3.4 Reflux tank: liquid level, pressure (vacuum). 4. Key safety control requirements: During the distillation process, it is essential to strictly control operational parameters such as temperature, pressure, liquid level, feed rate, and reflux rate. Attention must also be paid to the interactions and interdependencies among these parameters. Automatic control systems should be used as much as possible to minimize human error. 4.1 Common requirements 4.1.1 In areas of the tower where hazardous materials are concentrated (such as poly-nitro compounds in nitro compounds, hydrocarbons in liquid oxygen, catalysts that can cause the polymerization of ethylene oxide, etc.), component analysis should be carried out. This can be achieved by using component analysis instruments to monitor the content of these components either in real time or on a regular basis, or by conducting manual sampling for analysis at regular intervals. Regular discharge measures should also be implemented. 4.1.2 For reboilers heated by steam or other high-temperature gases, flow rate display devices and control valves shall be installed on the steam or high-temperature gas pipelines, so as to adjust the amount of steam (gas) according to the temperature of the reactor ； When steam flows through the shell side of a vertical reboiler, a control valve can also be installed on the condensate pipeline. A reboiler using liquid heating is employed, with a control valve installed on the pipeline for the heating liquid; the flow rate of this heating liquid is adjusted based on the temperature of the heating liquid and the temperature inside the reactor, and a filter is placed before the control valve. 4.1.3 The cooling water pipes of the condenser are equipped with centralized flow display and alarm functions; a low-low flow alarm for the cooling water triggers an interlock to stop the heating medium. 4.1.4 A centralized display for the pressure (or vacuum level) at the top of the tower should be provided. Distillation equipment with a maximum operating pressure at the top exceeding 0.03 MPa shall be equipped with explosion-proof pressure relief systems such as safety valves or burst discs. The safety valve is installed at the top of the equipment or on the vapor distillation pipeline at the top. 4.1.5 When using a heating furnace for heating, a temperature display for the distillation heating medium should be installed, automatic control of fuel or air flow should be provided, and a flame-out protection device for the heating furnace should be in place. 4.2 Personality Requirements 4.2.1 Atmospheric distillation towers and pressurized distillation towers shall be equipped with high bottom pressure alarms, as well as high-high interlock systems to shut off the heating of the material. 4.2.2 The pressure automatic regulation is installed on the material-side vent pipeline of the pressurized distillation system. 4.2.3 The vacuum distillation tower is equipped with a low-vacuum alarm at the bottom of the tower. In cases where a decrease in vacuum level leads to an increase in temperature, posing an explosion risk to the material, an interlock system is installed to cut off heating of the material when the vacuum level at the bottom of the tower drops too low. 4.2.4 For continuous distillation, a centralized display of the column feed flow rate and automatic control valves are provided. 4.2.5 Intermittent distillation should be equipped with a discharge valve for the distilled product, as a safety measure to ensure that the pressure inside the tower remains within normal limits once this valve is closed. 4.2.6 Intermittent distillation should be equipped with high and low liquid level alarms for the distillation vessel, as well as a low-low liquid level interlock to shut off the heating medium supply system. 4.2.7 The equilibrium distillation system features centralized display and alarm for feed temperature. 4.3 Other safety design requirements 4.3.1 Distillation units that are prone to explosion should be equipped with a safety interlock shutdown system or other systems with such functionality, in order to ensure the safety of operators and the proper operation of the equipment. 4.3.2 Measures should be in place to prevent the pipes from being clogged by substances with high freezing points, which could lead to an increase in pressure inside the tower and thereby cause an explosion. Such measures include pipe heating, the use of dual pressure gauges, and the installation of rupture discs in series before the safety valves. 4.3.3 For the distillation of flammable and combustible liquids, open flames or electric heating devices should not be used as heat sources; instead, steam, superheated water, or other heat carriers should be employed for heating. 4.3.4 When distilling materials that can react chemically with water, it is not advisable to use water or steam as heating media or refrigerants, to avoid leaks that could lead to uncontrolled reactions and subsequent fires or explosions. 4.3.5 The distillation units should be arranged in an open or exposed area, as close as possible to the edge of the production area. No other equipment or rooms should be placed above the distillation units. Distillation vessels with a high risk of explosion should be separated from other areas by explosion-proof walls. 4.3.6 Attention should be paid to the compatibility between the tray and packing materials, the bottom pump, and the heat exchange equipment, as well as the material being processed; this includes factors such as stress corrosion caused by certain material combinations, and whether the medium reacts with the materials of the equipment. 4.3.7 The vacuum pump used for the vacuum distillation of flammable materials shall be equipped with a check valve. 4.3.8 For the level indication of occupational exposure toxins that pose extreme hazards (Level I) or high-level hazards (Level II), as well as of materials with high temperatures and strong corrosivity, glass tube level gauges shall not be used. Glass tube level gauges should generally not be used in containers holding flammable, explosive, and moderately toxic hazardous materials. 4.3.9 According to the requirements of the process, when additives different from the operating medium are injected into the overhead distillate pipeline, a check valve and a shut-off valve should be installed on the connection fitting. 4.3.10 The condensate pipeline must have a slope, sloping toward the return tank. 4.3.11 Distillation units that require a safety interlock shutdown system should be equipped with a backup power source or emergency power supply to ensure they can start functioning properly in the event of a power outage of the main power supply. 4.3.12 In distillation units, many devices operate at temperatures equal to or above their auto-ignition points, and there are also devices that use open-flame heaters; the fire separation distances between these devices and other equipment in terms of layout must comply with the provisions of the \"Code for Fire Protection Design of Petrochemical Enterprises\" GB50160-2008. 4.4 Basic requirements for the safety monitoring of major equipment Sequence Number Process parameters and monitoring requirements Remarks I. Distillation tower 1. Centralized display of the bottom of the tower temperature, with high-limit alarms; interlock mechanism to cut off heating when the temperature reaches a high level. 2. On-site liquid level measurement at the bottom of the tower; automatic control valves can be used as needed to regulate the liquid level there as well as the flow rate of the liquid pump. 3. Centralized display of the top temperature of the tower, with high-limit alarms; automatic control based on the top temperature or a sensitive temperature point, as well as the reflux flow rate. 4. Centralized display of the temperature at an appropriate position in the middle of the tower, with high-limit alarms. 5. Centralized display of the pressure at an appropriate position in the middle of the tower, with high-limit alarms. 6. Centralized display of the pressure difference between the top and bottom of the tower, with high-limit alarms. 7. Centralized display of the feed flow rate, with automatic control. 8. Centralized display of the feed temperature. 9. Display and automatic control of pump pressure and flow rate. 1. On-site, control panel-based pump shutdown, automatic pump shutdown system, temperature display and automatic control of the feed heat exchanger. 2. Reboiler (distillation vessel): 1) Centralized display of pressure in the heating steam pipeline; 2) Centralized display of the temperature of the vapor rise pipe; 3) Installation of a safety valve upstream of the return pipe when the liquid is being heated; 4) Centralized temperature display on the hot liquid circulation pipeline during liquid heating; 5) Direct installation of on-site level gauges and automatically controlled level gauges on the batch reboiler, with the flow rate of the vessel liquid pump adjusted based on the liquid level; 6) Temperature displays installed on the vessel liquid circulation pipeline and the outlet pipeline of the batch reboiler; 7) Dual temperature displays in cases where the distilled residue poses an explosion risk. III. Condenser: 1. A centralized temperature display is provided for the 1-liter gas pipe. 2. A centralized temperature display is also provided for the condensate pipe. 3. The condenser is equipped with control valves for regulating the amount of cooling water; the flow rate of cooling water is controlled based on the temperature at the material outlet. 4. An alarm is triggered when the cooling water pressure is low, and an interlock mechanism stops the heating medium in case of extremely low pressure. 5. Pressure and temperature displays are provided for the cooling return pipe. 6. A safety valve is installed upstream of the shut-off valve on the cooling water outlet pipe.

IV. Reflux Tank: 1. On-site level gauges are available. 2. Automated level gauges are used for centralized monitoring, display, and control; the level of liquid in the reflux tank is used to control the amount of reflux or distillate. 3. The reflux pipe is equipped with flow rate displays and control valves. 4. The distillate pipe has flow rate displays and control valves as well. 5. A liquid seal is installed on the reflux pipe. 6. Flow rate displays and control valves are provided on pipes for gas distillate. 7. A centralized pressure display is available on pipes for gas distillate.

Note: Due to the variety of distillation processes and the wide differences in the properties of materials, it is not possible to cover all scenarios. Moreover, not all distillation processes require all of the above control measures; specific safety control measures must be determined based on the particular distillation setup.