Issue 40/2012 - Main discharge path of impurities in caprolactam production

Caprolactam is an important organic chemical raw material, which is a monomer for the production of nylon-6 fiber and nylon-6 engineering plastics, which are widely used in textile, automotive, electronics, machinery and other fields
.
However, the quality problem of caprolactam is a worldwide problem, and its production process produces more than 30 kinds of impurities, although the content is small, but the presence of these impurities seriously affects the quality and use
of caprolactam downstream products.
In addition to raw materials, oximization, liquid phase rearrangement and caprolactam refining all have a great influence
on product quality.

The caprolactam production process mainly includes five reaction steps, namely benzene hydrogenation to cyclohexene; cyclohexene hydrated to cyclohexanol; dehydrogenation of cyclohexanol to cyclohexanone; cyclohexanone is amxime to cyclohexanone oxime; The cyclohexanone oxime liquid phase rearranges to produce caprolactam
.
The discharge of impurities generated by each step of the reaction or carried in the feed is as follows:
1.
The discharge
of impurities in the benzene hydrogenation process The water-soluble impurities produced by the benzene hydrogenation process are mainly discharged
after rectification in the dehydration tower reflux tank and benzene stratification.
Here, the reflux tank of the dewatering tower plays the role of
separating the oil phase from the water phase and removing water-soluble impurities.
The complete removal of water lays the foundation
for the next step of extractive rectification separation of benzene, cyclohexene and cyclohexane mixtures.
The organic impurities produced by the benzene hydrogenation process are mainly distilled into the organic solvent N,N-dimethylacetamide, and are discharged from the system by the tower after rectification operation in the solvent recovery column
.

2
.
The water-soluble impurities generated in the hydration
process are mainly sent back to the cyclohexene washing separator in the form of gas phase from the top of the column after the rectification operation of the cyclohexanol separation column.
The organic impurities produced by the hydration process are mainly removed from the heavy oil by the tower after the rectification operation of the cyclohexene column, and the light oil
is removed by the top of the column after the rectification operation of the cyclohexanol refining column.

3.
The discharge
of impurities in the cyclohexanol dehydrogenation process The water-soluble impurities produced by the process are mainly separated from the organic phase through the drying tower, and then discharged from the top of the tower through the wastewater stripping tower; The organic impurities produced are mainly removed from the top of the column by rectification operation in the light column, and then removed from the bottom of the column after rectification operation by the alcohol column
.

4.
The discharge
of impurities in the amxime process The water-soluble impurities produced in the process are extracted by the water extraction tower and removed from the bottom of the tower, and then sent to the wastewater stripper tower, and enter the wastewater main pipe from the bottom of the wastewater stripping tower, which is also the only water-soluble impurity discharge outlet
in the oxime process.
The organic impurities produced by the amxime process are mainly discharged
by the tower after rectification operation through the toluene deketone column.

5.
After the discharge
of impurities in the Beckmann rearrangement process is rearranged and extracted, the process is the process with the most impurities in the intermediate, the main impurities are cyclohexanone, 2-methylcyclopentanone, 3-allyl cyclohexanone, N-methylcaprolactam, 2-cyclohexenylcyclohexanone, valtanelactam, 3-methylcaprolactam, 6-methylcaprolactam and some unknown impurities
。 After the ion exchange process, the caprolactam aqueous solution and the hydrogenated caprolactam aqueous solution are equipped with cyclohexanone, 2-methylcyclopentanone, valeric am, 6-methylcaprolactam left over from the previous process; The presence of pentanolactam, 2-cyclohexenylcyclohexanone, 3-methylcaprolactam in the unqualified caprolactam; The superior caprolactam only exists in valfalam
.
These impurities are very similar to the structure, physical and chemical properties of caprolactam, which are the most easily introduced impurities into the product and are difficult to effectively remove
in the post-process.
It can be seen that the impurities in caprolactam are mainly produced
during the Beckmann rearrangement rearrangement reaction.
Therefore, controlling the reaction conditions of Beckmann rearrangement is key
to reducing impurities in caprolactam.
Other non-polar and weakly polar impurities are mainly removed
by rectification, extraction, reverse extraction and other means in the previous process.

The water-soluble impurities generated by the Beckmann rearrangement process are mainly separated from the organic phase after extraction by caprolactam extraction tower, and then the benzene dissolved in it is recovered by the condensate stripper and discharged into the wastewater main.
The discharge route of inorganic ions is to remove a small amount of ammonium ions and sulfate ions through ion exchange resin; Organic impurities are mainly discharged from polyolketones and organic impurities
dissolved in them through the bottom of the column after the rectification operation of the benzene distillation column.

6.
Discharge
of gas impurities (1) benzene hydrogenation reaction tail gas benzene hydrogenation reaction tail gas contains cyclohexane and unreacted hydrogen, through the hydrogenation tail gas absorption tower to absorb the cyclohexane, from the top of the absorption tower discharged non-condensable gas still contains a small amount of cyclohexane, through the activated carbon adsorber further adsorption recovery, the remaining gas
is mainly hydrogen, but also contains trace amounts of cyclohexane and nitrogen
。
In order to ensure that benzene can be completely converted, the hydrogen content of benzene hydrogenation reaction needs to be more than 30%, the pressure of the reactor is maintained at about 1MPa, and the gas must be vented
every interval.
After the tail gas of benzene hydrogenation reaction is absorbed and adsorbed, the hydrogen content reaches 99%, and it is transported to the aniline plant as a reaction gas
through the pipeline network.
Therefore, the treatment method adopted is feasible and reliable, and there is no secondary pollution problem
.

(2) The main component of the tail gas
of the oxidative dehydrogenation reaction of the oxidation reaction is cyclohexane, and the exhaust gas is absorbed by the exhaust gas absorption system and discharged from the top of the absorption tower to the torch main pipe
.
The exhaust gas absorption system of cyclohexanone device is composed of
exhaust gas absorption tower, absorption tower circulating pump, absorption tower circulating cooler, absorption tower feed cooler, etc.
The exhaust gas and cyclohexane recovery system from the scrubber containing cyclohexane inert gas enters the bottom of the exhaust gas absorption tower, and the alcohol and ketone liquid flowing from the top of the tower are in contact, most of the cyclohexane is absorbed by alcohol and ketone liquid, the alcohol and ketone liquid are sent to the top of the exhaust gas absorption tower by the absorption tower feeding cooler through the absorption tower feed cooler, and the temperature at the top of the tower is controlled at ≤10 °C
.
The alcohol and ketone liquid dissolved in cyclohexane at the bottom of the exhaust gas absorption tower are partially circulated by the circulation pump of the absorption tower through the circulation cooler of the absorption tower, and part of the mixing tank
of the refining system is removed.
The absorbed exhaust gas is discharged through the pressure control valve and sent to the torch, and the pressure at the top of the exhaust gas absorption tower is controlled at about
1250kPa (absolute pressure).
The use of exhaust gas absorption device reduces the discharge of pollutants, saves raw materials, brings certain economic benefits to manufacturers, and is technically feasible
.
The main components of the above treated exhaust gas are alkanes and other organic substances, which are sent to the aniline device flare system for combustion treatment, and are mainly CO2 and H2O after combustion, which can be directly discharged
at high altitude.

The torch is a safe and effective way to treat flammable exhaust gases, and the combustion products of organic substances such as alkanes are CO2 and H2O
.
From the actual operation data, the incineration removal rate of alcohols, ketones, alkanes and other combustible gases can reach 100%, the operation effect is reliable, and the equipment is simple and the operating cost is low
.

(3) Waste alkali incinerator flue gas waste lye is hazardous waste, hazardous waste number is HW35, according to the information provided by the construction and feasibility study, the main equipment incinerator of the proposed incineration system is built according to the standard of hazardous waste incinerator, using negative pressure design, using heavy oil as fuel to increase the combustion temperature, incineration temperature ≥ 1100 °C, flue gas
residence time ≥ 2s, combustion efficiency ≥ 99.
9%, incineration removal rate ≥ 99.
99%, The thermal ignition reduction rate of incineration residue is 4%.


Conclusion High-quality
caprolactam can be used in the production of high-speed spun chips and civil yarn, while low-quality caprolactam can only be used to produce industrial yarn
.
Therefore, timely discharge of impurities in each production process of caprolactam helps to control the quality of caprolactam's products, produce more qualified products and even high-grade products, and create more economic value
.