For large ethylene plants, parking in just a few hours will cause economic losses of millions or even tens of millions of dollars, which will affect the efficiency of the enterprise. Therefore, it is necessary to improve the reliability of the compressor and keep the long-term operation of the compressor. The key to smooth operation of the ethylene plant. From the equipment itself, the coke in the compressor impeller, diaphragm and runner seriously affects the long-term operation of the compressor. In particular, the problem of polymerized coking in the cracker compressor system becomes a difficult point affecting the production of the device and the long-term smooth operation. It is one of the important reasons for the full-line parking of the device.

1 Unit overview

Lanzhou Petrochemical 240,000 tons of ethylene / year old plant cracking gas compressor is a two-cylinder four-stage compressor produced by Italy New Billion Company, low pressure cylinder model is 2M CL806, high pressure cylinder model is 2BCL508, turbine is back pressure extraction EHNK40/ 45, transferred from Dushanzi Petrochemical in Xinjiang during the renovation of 240,000 tons in 2003. On June 12, 2003, the plant was successfully driven once. By 2006, the unit had undergone three disintegration overhauls, and the unit operating cycle was basically one year, which seriously restricted the long-term operation of the ethylene plant. In the overhaul of the plant in 2006, the workshop carried out technical breakthroughs through summing up the experience, which greatly improved the operation of the unit. There has been no unplanned parking incident since 2007, and the ethylene plant has been upgraded to three years. Step forward. Design parameters of the compressor.

2 Problems in the operation of the unit

During the overhaul of the unit in 2004, the unit was found to be severely cokeed, and the compressor after the third stage of the compressor was also severely cokeed. From 2003 to June 2005, the compressor has been using the washing oil to cool the compressor section. The temperature is not controlled, and it can reach more than 100 at the highest, which increases the coking between the segments. After June 2005, the workshop carried out the initial technical transformation of the cooling system between compressor sections, from oil injection to water injection; in 2006, the second technical transformation was carried out on water injection, because the pump used in the water injection reform in 2004 was diaphragm type measurement. The compressor has a high back pressure of about 4. 0 M Pa. This pump often fails, and the frequency of inspection is high, which seriously affects the normal flow of the compressor. Due to the factors of the water injection pump, the temperature between the sections cannot be caused. Effective control.

Therefore, in 2006, the water injection system was reformed again, and the original diaphragm metering pump was changed to a high-speed centrifugal pump. At this point, the temperature between the compressor sections was controlled. On January 28, 2006, the USP of the electrical workshop was faulty, causing the compressor to stop. After several times of driving, the compressor could not be operated at all times. According to the condition of the driving, it was initially judged that the high-pressure cylinder section was sealed with helium gas, so the compression was performed. The high-pressure cylinder of the machine was repaired. It was found that the gas seal of the third and fourth sections of the unit was severely corroded, the gas seal had been destroyed, and the sealing function could not be achieved. At the same time, the four-stage gas seal also had great corrosion.

3 solutions

The main cause of polymerization coking in the compressor system is unsaturated olefins, peroxides, etc. contained in the petroleum cracking gas. In particular, butadiene and styrene in the cracking gas will be polymerized at a certain temperature. These polymers are bonded to the impeller, baffle and runner of the compressor. Over time, these polymers will accumulate more and eventually reduce the flow rate of the compressor and increase the vibration, directly affecting the compressor. Aerodynamic performance, mechanical properties and service life.

Most of these polymerization reactions are free radical reactions, and the primary factor causing them to undergo polymerization is temperature, because these free radicals are all produced by diolefins after being heated, and peroxides also generate free radicals and polymerization of diolefins when heated. . The key to preventing or reducing these polymerizations is to reduce the formation of free radicals and reduce the rate of polymerization.

It can be seen that as the temperature increases, the rate of polymerization increases, and when the temperature exceeds 85, the rate of polymerization increases rapidly. Therefore, the most effective way to prevent and reduce the polymerization coking of the compressor system is to lower the outlet temperature of the compressor and control the outlet temperature to not exceed 85, so that the polymerization reaction occurs less.

3. 1 Compressor uses interstage water injection to reduce the temperature between the sections

The water injection between the stages is sprayed on the impeller and the diffuser inlet through the water injection nozzle. After the water is atomized by the water injection nozzle, the water enters the compressor casing, and is mixed with the cracking gas to be gasified, and absorbs a large amount of heat to reduce the temperature of the cracking gas. It is closer to isothermal compression, reducing compressor power consumption, reducing polymer coking, increasing compressor aerodynamics, reducing power loss, and reducing polymer coking. It also controls the compressor section temperature to 85. Hereinafter, the occurrence of the polymerization reaction is effectively reduced. Therefore, the compressor was refilled and rebuilt. Two metering pumps were used for the initial transformation. The pump manufacturer was PUL SAFEEDER, which was opened in the United States. The pump model is 7660- 4-AE, 4 cylinders per pump; the drive motor model is A-EF-160LE, power 15 kW.

Metering pump 4 cylinder design parameter flow rate: 1 398 L / h, pressure 8. 3 10 5 Pa flow: 2 253 L / h, pressure 17. 6? 10 5 Pa flow rate: 928. 9 L/h, pressure 41. 4 10 5 Pa flow rate: 1 940 L/h, pressure 6. 2 10 5 Pa metering pump flow signal is connected to the workshop DCS system, each cylinder stroke can be in the middle Control room control. Actually use 3 cylinders of the metering pump, there is 1 cylinder in operation (flow 1 940 L / h, pressure 6. 2 10 5 Pa) stroke is zero; there is 1 cylinder (flow 2 253 L / h, pressure 17. 6 10 5 Pa) Water supply to the second and third sections. Flow chart of water injection transformation.

The problems with water injection are as follows.

1) Insufficient water injection: Since the four-stage operating pressure of the cracking gas compressor is 3. 6 M Pa, the metering pump has a higher back pressure, and the diaphragm material is tetrafluoroethylene. The diaphragm is broken due to too high pressure, and the turbine is at the same time. The worm and connecting rod often break or damage, and the pump failure rate is high, which greatly affects the effect of water injection and cooling. The water injection is not very stable, because the water injection volume often changes, and the water injection nozzle has a small aperture, with the accumulation of coke powder. After a long time, the water injection hole is often blocked, and the water injection nozzle can only be backflushed online, which is dangerous, the temperature between the sections is not controlled, and the overheating operation is often performed, which makes the coking more serious. Therefore, the workshop has carried out a secondary transformation of the water injection system, and the original water injection pump was changed to a high-speed centrifugal pump. The pump flow rate is 6 m 3 and the head is 420 m. The water injection point is the original water injection point, which is shown after the process transformation.

After the second transformation, the continuity and stability of water injection between the compressor sections are greatly improved. The temperature between the compressor sections can be strictly controlled within 85 90, and the cylinder coking is slowed down, effectively ensuring long-term operation of the compressor. .

2) Gas seal corrosion: Since the compressor adopts two-cylinder four-stage compression, the cracking gas is alkali-washed at the three-stage outlet of the compressor and the four-stage inlet and alkali washing tower to eliminate acidic substances in the cracked gas, such as H 2 S. , CO, CO 2, etc. Therefore, after water injection, a small amount of alkali liquid (mainly containing NaOH) must be brought to the fourth section of the compressor, and the gas seals of the third and fourth sections of the compressor and the gas seals of the four stages are all wrought aluminum LD5, alkali water injection two. The secondary reformation flow will react with aluminum, so the four-stage gas seal will be corroded by alkali.

In order not to affect the operating cycle of the compressor, it must be air-sealed. The selection of the main material is very important. Firstly, the material needs to be resistant to alkali corrosion. Secondly, the hardness should not be too high. It should be equivalent to LD5, which can prevent the compressor rotor from being damaged by gas seal under abnormal mining. Taking into account the above situation, the data found that the nickel alloy has high alkali resistance and corrosion rate comparison. Since the gas seal teeth need to be directly driven out on the sealing body, the material selection requires a certain mechanical processing property, so the strength is higher. Nickel alloy (CZ100), its material composition, physical properties. The strength of the nickel-based alloy can fully meet the requirements of the structural design size, and the hardness is not high, which is lower than the hardness of the compressor shaft (the hardness of the shaft surface is about 268 HB). When the shaft is rubbed against the improper installation, it will not The shaft is scratched and will itself wear out in a short period of time. This material has poor adhesion and does not cause tearing of the sealing teeth when it is rubbed against the shaft. The worn powder does not stick to the surface of the shaft, ensuring the life of the seal. Material Corrosion Rate The working environment is 14% NaOH temperature 88 corrosion rate / mm a - 1 titanium alloy. Nickel alloy 0. 000 5 M onel (Monel) 0. 001 3 carbon steel 0. 208 3 cast iron 0. 208 3 Inconel (Inconel) 0. 000 76 stainless steel 0. 001 alloy steel 0. 1 3 . 2 coking of the heat exchanger between compressor sections

From the previous operation of the compressor, the coke between the compressor sections also seriously affects the operating cycle of the compressor, mainly in the three and four sections of the compressor, and the three-stage aftercooler (EA-205) The coking is most significant. Since the coking affects the flow rate of the cracking gas, the outlet pressure of the compressor is also increased, which increases the load of the compressor, and the heat exchange effect is also reduced. The temperature between the sections is increased, the coking is intensified, and the cracking is carried out. The gas is not sufficiently cooled, and then the temperature becomes higher after entering the next stage of compression, forming a vicious circle. Therefore, the coke problem of the three-stage aftercooler of the compressor is also the bottleneck affecting the long cycle of the compressor, and the EA-205 heat exchanger is only a single unit, which cannot complete the online cleaning. It is necessary to ensure the heat exchange of EA-205 for the long cycle of the compressor. The effect and the flowability of the tube bundle, so a heat exchanger is added as a backup heat exchanger, and the heat exchanger is cut out to ensure the cooling effect between the sections of the compressor and the flowability of the cracking gas flow, so as to clean up the long-cycle operation of the unit. bottleneck.

4 Conclusion

Water injection between the compressor sections can not only effectively reduce the coking degree of the compressor, but also reduce the power consumption of the compressor. After the water injection reform, the gas seal between the third and fourth sections of the compressor and the four-stage gas seal can prevent water injection. After the corrosion of the gas seal, the long-term operation of the compressor is guaranteed. Increasing the spare heat exchanger, cleaning the tube bundle and the heat exchanger shell on-line, can effectively reduce the temperature between the sections and ensure the flow of the heat exchanger, and is also one of the effective ways to improve the long-term operation of the compressor. After the above modification of the compressor, the cracking gas compressor of the device basically realized long-term operation, and took a solid step toward the goal of three years, and at the same time played a vital role in energy saving and consumption reduction of the device.

In terms of economic benefits, not only the maintenance cost is saved, but also the economic benefits of the enterprise are improved by avoiding unnecessary parking maintenance.

However, since the hardness of the new gas seal material is greater than that of the forged aluminum LD5, the friction of the gas seal to the compressor rotor under abnormal working conditions needs further study. If the four stages of the compressor can increase the spare heat exchanger, the compressor operates reliably.

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