Natural gas containing saturated water during natural gas pipeline transportation is prone to generate hydrates in the pipeline, which can block pipeline valves and affect normal production. Generally, ethylene glycol is injected to prevent the formation of hydrates. To reduce the investment cost of ethylene glycol, it is necessary to regenerate and recover it.
However, there are many problems in the ethylene glycol regeneration process, such as poor separation efficiency of alcohol and hydrocarbon, equipment corrosion and scaling, and ethylene glycol foaming, which greatly reduce the economy and reliability of the alcohol injection process. By determining the correct regeneration operation method, adding chemicals reasonably, and optimizing regeneration equipment, various drawbacks of ethylene glycol regeneration process can be solved.
1 Existing problems
1.1 Poor separation effect of alcohol and hydrocarbon. The mixture of alcohol, hydrocarbon, and water enters the three-phase separator for separation. If the temperature inside the separator is too low or the mixture stays in the separator for too short, it may cause poor separation effect of alcohol and hydrocarbon. At this point, some of the condensate enters the ethylene glycol regeneration tower, causing problems such as foaming, flushing, and device fouling.
1.2 Scaling and blockage of the regeneration device: Generally, the mineralization degree of the produced water in the gas field is high. If a mechanical filtration device is not installed at the inlet of the regeneration system, the corrosive impurities brought by circulation and the salt precipitated from the water will gradually accumulate, causing scaling of the pipeline, heat exchanger, reboiler and other equipment, and also causing blockage of the underwater ethylene glycol injection pipeline.
1.3 Corrosion of regeneration equipment: Natural gas contains acidic gases such as H2S and CO2. When ethylene glycol is injected into the natural gas system, it will absorb some of the acidic gases, causing the acidity of ethylene glycol to continuously increase. When it enters the regeneration system, carbon steel corrosion will accelerate rapidly, causing significant damage to the equipment. At the same time, the ethylene glycol system contains dissolved oxygen, which can accelerate the corrosion process of metal equipment.
1.4 Poor heat exchange effect of lean and rich solution During the regeneration process, the ethylene glycol rich solution flowing out from the bottom of the reboiler needs to exchange heat with the lean solution. Generally, a sleeve type heat exchanger is used for heat exchange, but due to the small diameter and heat exchange area of the sleeve type heat exchanger, the heat exchange effect of the lean and rich solution is poor. After heat exchange, the temperature of the ethylene glycol lean solution is high. To ensure the service life of the lean solution pump, a cooler must be installed in front of the lean solution pump, which is not conducive to simplifying the process; At the same time, the rich liquid cannot be fully heated after heat exchange, which increases the load of the regeneration tower and exacerbates heat energy loss.
After investigation, it was found that there are several reasons for the large loss of 1.5 ethylene glycol:
Excessive injection of ethylene glycol. This will lead to an increase in the amount of ethylene glycol carried in the gas phase, exacerbating the evaporation loss of ethylene glycol.
The bottom temperature of the regeneration tower is too high. If the temperature at the bottom of the tower is too high, it will cause the ethylene glycol rich liquid in the reboiler to regasify and exit the tower together with the evaporated water vapor, causing waste. At the same time, it will also increase heat loss.
Due to the lack of experience of on-site operators, they were unable to accurately control the position of the liquid level gauge in the three-phase separator, resulting in the mixing of ethylene glycol into the condensate system.
2 Solutions
2.1 Standardized Operation and Additive Addition 2.1.1 Improvement of Alcohol and Hydrocarbon Separation Effect Due to the high viscosity and easy emulsification of ethylene glycol rich liquid at lower temperatures, which is not conducive to alcohol and hydrocarbon separation, low temperature in the three-phase separator can lead to poor alcohol and hydrocarbon separation effect. Therefore, a heat exchanger is set up in front of the separator to heat the alcohol and hydrocarbon mixture, and a heat tracing pipe is added around the liquid package of the three-phase separator, Stabilizing the temperature inside the liquid hydrocarbon separator at around 50 ℃ can effectively improve the separation efficiency of alcohol and hydrocarbon.
2.1.2 Anti scaling of the device To effectively suppress scaling, the following points should be achieved: a mechanical filtration device should be installed between the rich liquid buffer tank and the lean and rich liquid heat exchanger; Regularly disassemble and clean dirt after installation of heat exchange equipment; To prevent scaling of the packed tower, loose packing should be used; Less dead zones and low flow rate zones should be taken in the heat exchanger; Ensure uniform flow velocity distribution in the heat exchanger to avoid large velocity gradients and ensure uniform temperature distribution; Increase the flow rate appropriately and use anti scaling agents in conjunction.
2.1.3 Acid gas and oxygen corrosion control: Monitor the pH value of ethylene glycol to ensure that it is between 8.15 and 7.13. If the pH value drops to 7, use borax as a neutralizing agent for treatment; All chemical storage tanks, ethylene glycol storage tanks, etc. are sealed with nitrogen to avoid air pollution. Add a certain amount of activated carbon deoxidizer to the regeneration system for control.
2.1.4 Reduce ethylene glycol loss
(1) Using the Hammersmith semi empirical formula to determine the required inhibitor * low rich liquid concentration for the gas field, and then derive a reasonable injection amount of ethylene glycol.
(2) Moderately reduce the temperature of the reboiler. Generally, stabilizing the temperature of the reboiler at around 106 ℃ can meet the requirements for ethylene glycol concentration and reduce the amount of ethylene glycol carried loss.
(3) Accurately control the position of the liquid level control gauge to prevent condensate/MEG from flowing together.
2.2 Optimizing the device
(1) The room temperature pipeline connected to the regeneration tower is lined with polypropylene, and the valve is a ball valve lined with tetrafluoroethylene. Perform three coating processes on the inner and outer walls of the tower and the tube bundles inside the high-temperature heat exchange equipment, with a coating thickness of 350-500 Lm.
(2) Select an efficient three-phase separator and install a mist collector at the outlet to improve the separation effect of alcohol and hydrocarbon.
(3) Replacing plate heat exchangers with tube bundle heat exchangers not only facilitates cleaning and descaling, but also greatly improves heat exchange efficiency.
(4) Install an efficient vertical heat exchanger at the top of the regeneration tower.
Not only can it improve the heat exchange efficiency of the rich and poor solution, but it can also reduce the loss of ethylene glycol carrying. At the same time, it is reasonable to choose the number of regeneration trays. Generally, the number of regeneration trays is 5, and the reflux ratio is 0.2. At this time, there is a good regeneration effect.
3 Conclusion
In the process of ethylene glycol regeneration, due to errors by on-site operators, outdated process equipment, and a lack of coordination in adding chemical agents, serious corrosion and scaling of the regeneration equipment have been caused. The quality of ethylene glycol regeneration is substandard, the regeneration yield is low, and the heat energy loss is large. By investigating existing mature regeneration process plans and making reasonable improvements to the gas field ethylene glycol regeneration process on-site, a practical and feasible regeneration optimization plan has been proposed, greatly improving the economy and reliability of the regeneration process.