Application of pH Meter in Acrylonitrile Plant
In recent years, with the strengthening of safety and environmental awareness, the requirements for the use of pH meters in the petrochemical industry are getting higher and higher, especially in acrylonitrile units, most of which are toxic and are produced continuously for 24 hours, so the There are very strict requirements for online maintenance, safety and environmental protection of pH meters. 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.
Instrument selection
Aiming at the online disassembly and inspection of the pH meter, the first set of acrylonitrile device in our factory uses a flow-through sheath, that is, a smaller pipeline is used to drain the water from the main pipeline to the pH meter measurement pool, and then return to the main pipeline, similar to In the auxiliary line installation method. Mettler-Toledo's InPro4800/120/Pt100 electrode and matching measuring cell are used. However, before the electrode is disassembled and inspected, it is necessary to close the front and rear valves of the measurement tank, and drain the material in the measurement tank by opening the pour valve. This causes material discharge, which affects environmental protection and clean production.
Combined with the problem of using the pH meter of the first set of acrylonitrile device, the online pluggable pH meter of Mettler-Toledo was used on the second set of acrylonitrile device, and the InTrac777M retractable sheath was used. This telescopic sheath (Steel material) is intrinsically safe equipment, which is directly installed on the main pipeline without additional materials such as valves and pipelines. The characteristics of the sheath are: METTLER TOLEDO's TRI-LOCKTM triple locking system can effectively prevent the leakage of process media.
Experience of Mettler-Toledo pH Meter
accurate measurement
METTLER TOLEDO provides thousands of electrodes for selection according to the viscosity, granularity, adsorption of the medium used, as well as the temperature and pressure of the medium, which is particularly targeted, thus improving the accuracy of measurement sex. The InPro4800 series pH electrodes currently used in our factory are especially suitable for measurement in harsh chemical environments. It can provide accurate and reliable measurements in oxidizing media, strong acids and alkalis, high pressure processes and high temperatures.
long lasting
The long service life of InPro4800 series pH electrodes is mainly manifested in: strong corrosion resistance to oxidizing media, solvents and strong acids and alkalis; long diffusion path and annular liquid junction (PTFE) effectively extend the service life of electrodes; Compensated diffusion path enables measurement at high process pressures; specially designed for high temperatures; optional flat glass membrane (built-in grounding) for measurements in media with high particle and fiber content.
little daily maintenance
At present, some pH meters in the factory use METTLER TOLEDO's new M420 transmitter, which has a rich screen display (see the figure below), and the status of the sensor is clear at a glance. For ISM smart electrodes, it can also display the electrode life and electrode operation time "Dynamic Life Indication DLI", and dynamically calculate the time that the electrode can still operate in this state according to the load of the electrode. In the actual application process, we can carry out selective maintenance according to the state of the electrodes. For electrodes with good service conditions and low loads, the maintenance cycle can be extended appropriately.
easy to use
Since the pH meter is an online analysis instrument, according to the relevant management regulations of the company's online analysis instrument, each pH meter needs to be cleaned and calibrated every month, so that the selected pH meter is required to be simple, convenient and safe to operate. And METTLER TOLEDO's online pluggable pH meter just meets the above requirements. When online disassembly and inspection is required, the electrode can be pulled out smoothly by simply pulling out the upper pull bolt of the sheath. Due to the action of the spring, the front end of the sheath automatically shrinks and closes to prevent the material from leaking out.
METTLER TOLEDO's pH transmitter also has a manually activated hold function, which keeps the output signal at a set value when the electrode is replaced or calibrated. This is very necessary for the pH meter involved in the automatic adjustment loop control, to avoid loop fluctuations caused by the operation of the pH meter.
The METTLER TOLEDO InPro4800 series pH electrode used in our factory does not need to be refilled with electrolyte, and the M420 transmitter adopts a two-wire system, which completely solves the problems of many wiring, heavy workload and easy wrong connection in the past.
Improvement of METTLER TOLEDO pH Meter
When the pH meter is used for a long time, due to the polymerization of the medium, it is difficult to pull out the electrode. A little force will damage the rubber handle at the end. The factory has improved and changed the rubber handle at the end to a white steel handle. The electrodes are easy to pull out when they are glued to the polymer.
