How to manage the safety risks of diazotization enterprises

Diazotization reaction refers to the reaction of primary amine and nitrite acid at low temperature to generate diazo salt, diazotization process is an exothermic process, most of the raw materials used in the production process are flammable, toxic, corrosive, once the risk of leakage is greater, in 2009 was included in the "first batch of key supervision of hazardous chemical process catalog"
.

Diazo salt refers to salts containing diazo groups (-N + ≡N), in addition to some diazo salts (such as fluoroborate diazo salt, pyrazole diazo salt, acid black nitrate oxygen, tripterene diazo salt, etc.
) is relatively stable, the vast majority of diazo salts are extremely unstable, strong activity, heating or friction, impact and other effects of decomposition speed is extremely fast, a short time to release a large amount of gas and heat, decomposition heat energy is generally 125-450kJ/mol, for a typical chemical explosion
.
At present, there are more than 200 diazotization enterprises in China, involving medicine, pesticides, dyes, polymer materials and other industries
.
In recent years, there have been many production accidents in China due to overheating of diazotization reaction materials, spontaneous combustion of diazo salt filter residue, reaction tail gas pipeline fire, etc.
, among which the accidents caused by overheating of reaction materials are particularly prominent, on November 27, 2007, the operator of the dispersed blue 79# filter cake diazotization process in the 5 workshop of a company in Jiangsu did not close the steam valve in place, causing the diazotization reactor to be continued to heat up during the insulation process, and the diazo salt in the kettle decomposed violently, and a chemical explosion occurred, resulting in the death of 8 rescue personnel
。

Therefore, how to control the reaction temperature of the diazotization process, control the risk of diazo salt decomposition, and avoid accidents is particularly important
.
Combined with the spirit of recent relevant documents, it is recommended to focus on the following measures to prevent and control major risks and eliminate major hidden dangers
.

01Improve the intrinsic safety level of diazotization process

The first is to ensure the maturity and reliability
of process technology.
The diazotization process must have a reliable technical source and technology transfer contract, especially the newly developed and domestically used process for the first time, which must be gradually scaled up to industrial production on the basis of small-scale test, pilot test and industrial test, and be demonstrated
by the safety and reliability of the relevant departments of the provincial people's government.
The second is to carry out the whole process of response safety risk assessment
.
Carry out the whole process reaction safety risk assessment of the production plant, and the risk assessment temperature should be consistent with the reaction temperature of the diazotization process; The thermal stability of related raw materials, intermediate products, products and by-products, kettle bottom residues, filter residues, etc.
should also be tested
.
Third, it must be formally designed
.
Diazotization enterprises should be designed by qualified designers to control source risks, especially process devices involving high temperature and high pressure reaction conditions, and raise the design threshold to ensure the intrinsic safety level of
diazotization processes.

02Strict diazotization process and upstream and downstream process parameter monitoring and interlocking control

The first is to implement process safety control requirements
.
In accordance with the key supervision of hazardous process safety control requirements, combined with the results of process thermal risk assessment and HAZOP analysis, the process parameters such as temperature, liquid level, pressure, pH value, stirring rate and temperature of the post-treatment unit in the diazotization reactor should be monitored
.
In view of the risk of diazo salts in the downstream process of diazotization process, temperature monitoring of post-treatment units such as dilution, distillation, extraction, drying and storage should be configured, and interlocking control
should be formed with stirring, cooling system and discharge system.
The second is to improve the automation level of
solid material feeding.
Pipe chain, winch cage and other feeding methods are adopted to avoid the contact between operators and flammable, explosive, toxic and harmful materials to the greatest extent, reduce the number of on-site operators, and reduce the risk
of exposure.
The third is to strengthen the interlocking control of
process parameters.
The diazotization reactor should be equipped with automatic control valves for feed and refrigerant flow, the reaction temperature should be controlled interlocking with the feed volume and refrigerant flow, and an inert gas protection interlock device
should be set up for diazotization reactions involving Class A and B organic solvents.
Equipment involving diazo drying should be equipped with interlocking devices
for temperature measurement, heating heat source switch, and inert gas protection.
If the diazotization reactor involves multiple reaction processes, a sequence control procedure is established according to the reaction time, temperature, pressure and other parameters, and the inspection, judgment and control
are carried out in a prespecified order.
The fourth is to realize the automatic stratification
of diazo salts.
After the reaction is completed and static stratification, the interlock control of the discharge valve and the monitoring parameters is used to achieve the effect
of automatic stratification of the material by means of light detection or testing of the conductivity of the medium.
Fifth, improve the emergency discharge system
.
Set high and low alarms for the temperature of the diazotization reactor and stirring current, and the high and low alarms are interlocked with the raw material feed and refrigerant, and the process equipment has the function of
emergency stopping.
For the device with high concentration sulfuric acid as the solvent and nitrosyl sulfuric acid as the diazotizing reagent, an emergency discharge system should be provided, the emergency discharge tank should be set on the periphery of the workshop, and cold sulfuric acid should be placed in the tank in advance for cooling treatment
.
Equipment involving diazo salt storage should be added to pressure relief or emergency discharge facilities
.
The sixth is to carry out process exhaust gas analysis
.
If the exhaust gas system is merged between the equipment, a safety risk analysis shall be carried out, and it is strictly forbidden to mix the exhaust gas that may block the
pipeline, generate a new source of danger or form an explosive gas.

03Pay attention to the decomposition of diazo salts and residues

First, the pipeline transporting diazo salts should be equipped with cold tracing, emergency pressure relief and purging measures to ensure the stability
of the material remaining in the pipeline.
Second, the diazotized materials and wastes to be stored should be tested for self-accelerated decomposition temperature SADT, and control measures
to prevent overheating should be taken.
Third, the hazardous waste warehouse where diazotized filter residue is stored must be equipped with safety facilities such as forced ventilation, infrared thermal imaging monitoring alarm and video surveillance, and the storage conditions and cycle of the filter residue should be clarified through risk assessment, and control measures
to prevent spontaneous combustion of the filter residue should be taken.