Discussion on the application of pH meter in acrylonitrile plant
In recent years, with the strengthening of safety and environmental protection awareness, the petrochemical industry has higher and higher requirements for the use of pH meters, especially in acrylonitrile plants, most of which are toxic and are produced continuously for 24 hours, so pH meters There are very strict requirements for online maintenance, safety and environmental protection. The online pluggable pH meter produced by Mettler-Toledo is used for the pH value detection of the acrylonitrile unit, which has the characteristics of real-time, safety and cleanliness.
The acrylonitrile plant of Jilin Petrochemical Company covers an area of 54.64 hectares and has fixed assets of 3.37 billion yuan. It is currently the largest production base of acrylonitrile and methyl methacrylate in the country. Existing 106,000 t/a acrylonitrile (2 sets), 212,000 t/a acrylonitrile, 50,000 t/a methyl methacrylate, 100,000 t/a methyl methacrylate, 300,000 t/a sulfuric acid , 60,000 t/a acetone cyanohydrin, 120,000 t/a acetone cyanohydrin, 6,400 t/a acetonitrile (2 sets), 8,000 t/a ammonium sulfate, a total of 11 sets of production equipment. The main products are acrylonitrile, acetone cyanohydrin, methyl methacrylate, acetonitrile, sulfuric acid, ammonium sulfate, and the products are sold to South China, North China, East China and other regions.
After many years of practice, the acrylonitrile plant currently uses 44 pH meters, including 4 Beijing Defei T28, 2 E+H CM224, 8 Emerson OXMT-P-HT-11, Mei 22 sets of Teller-Toledo 2100e and 8 sets of Mettler-Toledo M420. The online pluggable pH meter produced by Mettler-Toledo is better: transmitter M420, electrode InPro4800i/SG/225, sheathed InTrac777M/100/4404/D00/Ka/A00, featuring real-time, safe and clean production.
Difficulties in using a pH meter
At present, the acrylonitrile plant of Jilin Petrochemical Company applies the technology of BP Company of the United States to produce acrylonitrile by propylene and ammoxidation, and at the same time produces acetonitrile and hydrocyanic acid as by-products. In the process operation, it is required to detect the pH value of the water phase of the upper and lower parts of the quenching tower and the water phase of the large circulation and recovery tower stratifier.
The pH value of the upper section of the quench tower is required to be controlled at 3.0~6.0, the lower section is required to be controlled at 6.5~7.0, the large circulation system requires a pH value of 6.5~7.5, and the pH value of the water phase of the recovery tower stratifier is required to be controlled at 6.0~7.2. Within the range, it can not only effectively remove the remaining ammonia in the reaction, but also remove ACL and ACO in the reaction gas, so that the cyanohydrin produced with HCN is relatively stable (the cyanohydrin has a high boiling point and is easy to discharge from the tower kettle). In the production, the pH value of the large cycle is adjusted by adding Na2CO3. If the control is not good, the yield of acrylonitrile will be reduced and the output will be affected; while the total cyanide content in the feed liquid of the recovery tower kettle, four-effect evaporation and light organic matter stripping tower will increase. If it is too high, it will reduce the quality of environmental comprehensive sewage discharge.
According to the above requirements, the use of pH meters has the following difficulties:
It must be able to realize online dismantling and inspection. Since the acrylonitrile unit is in continuous production for 24 hours, it is required that once the pH meter fails, it must be able to be dismantled and maintained online.
Safety must be ensured during disassembly and inspection. Since the material of the acrylonitrile plant is poisonous, and the pipelines in the production of the plant are all under pressure, it is necessary to ensure that the material does not splash out during the online dismantling and inspection process.
In view of the increasing requirements for safety, environmental protection and clean production in recent years, it is required to minimize or eliminate material discharge during maintenance.
