A pretreatment method for waste lye discharged from an ethylene device caustic washing tower

Abstract

The application provides an ethylene device caustic washing tower waste lye discharge pretreatment process, which comprises the following steps: waste lye, hydrogenated gasoline and demulsifier are simultaneously introduced into a stirring tank for sufficient mixing, and then the mixture is introduced into an oil-water separation tank for separation. Compared with a traditional waste lye pretreatment process route, the process can quickly realize the demulsification of waste lye and yellow oil and then realize extraction separation, and the oil removal rate of the ultraviolet method can reach more than 90%.

Description

Technical Field

[0001] This invention belongs to the field of industrial waste alkali treatment technology, and relates to a method for pretreatment of waste alkali liquid discharged from the alkali washing tower of an ethylene plant. Background Technology

[0002] In the process of producing ethylene through steam cracking, in addition to products such as hydrogen, CO, methane, ethane, ethylene, propane, propylene, butene, and butadiene, some olefins react with dilution steam at high temperatures to generate large amounts of aldehydes and ketones. Under alkaline conditions, these olefins and aldehydes / ketones polymerize to form a buttery substance that is easily emulsified and difficult to extract. At high temperatures, this substance readily polymerizes to form blockages. Simply allowing the waste alkaline solution to settle and separate cannot solve the problem of high oil content. This significantly impacts the operating cycle and processing capacity of the downstream WAO wet oxidation equipment. Furthermore, the diverse types of organic matter and their toxicity pose a significant environmental impact.

[0003] The survey revealed that due to the severe emulsification of some grease in the waste alkaline solution, which cannot be effectively separated, some equipment using the WAO wet treatment process in the industry has experienced serious blockage problems. In addition, neutralization and other processes generate H2S gas, requiring the park to be equipped with corresponding treatment facilities and to solve the problem of SO2 generation. Meanwhile, precipitation regeneration and precipitation oxidation technologies are currently in the pilot stage and have low reliability. Their ability to treat oily substances in waste alkaline solution needs further verification.

[0004] CN201010205763.1 discloses a high-temperature wet oxidation method for treating waste alkaline solution. Under conditions of 220–260°C and 6.0–20 mPa, this method effectively treats S… 2- The removal rate reaches 100%, and the COD removal rate is about 80%. However, its processing capacity is limited under conditions with high organic content. At the same time, the initial investment in the equipment is large and the operating cost is high, which limits its application in actual projects.

[0005] Therefore, for the conventional WAO oxidation method to treat waste alkaline liquor from ethylene plants, adding pretreatment oil removal measures is of great significance for reducing the oil content of the waste alkaline liquor entering the WAO wet oxidation system, improving the stability of equipment operation, reducing operating costs, and reducing the COD of the waste alkaline liquor discharged into the wastewater treatment system. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a pretreatment process technology for waste alkali liquid at the front end of a waste alkali liquid WAO wet oxidation treatment system. This technology can achieve a high degree of separation between waste alkali liquid and butter, low equipment investment, high operational stability, and complete oxidation and separation of pollutants. The treated waste alkali liquid has good biochemical properties, making it an environmentally friendly waste alkali liquid treatment method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A pretreatment process for alkali wastewater discharged from an ethylene plant includes:

[0009] 1) Let the waste alkaline solution A discharged from the ethylene alkaline washing tower stand and separate into layers for 8-10 hours;

[0010] 2) Skim off the floating oil on the upper layer, and then mix the waste alkaline solution B in the lower layer with the demulsifier and hydrogenated gasoline in the extraction stirring tank to complete the demulsification and extraction process; then, separate the oil and water in the extracted waste alkaline solution to complete the pretreatment.

[0011] In this invention, the extracted waste oil can be incinerated to produce steam as a byproduct.

[0012] The ethylene unit refers to a unit that produces ethylene, propylene, and low-carbon olefins such as butadiene through light hydrocarbon steam cracking. The alkali washing system is used to remove H2S and CO2 from the cracked gas to prevent corrosion of downstream equipment. The waste alkali solution A contains approximately 0.5-1.0 wt% NaOH, 4.0-7.0 wt% Na2CO3, and 4.0-6.0 wt% NaOH, Na2S, and Na2CO3, respectively. Furthermore, the waste alkali solution is emulsified, with a significant amount of grease on the surface.

[0013] The COD content in the waste alkaline solution B is approximately 15,000–30,000 mg / L, and the mixing weight ratio with the hydrogenated gasoline is (10–25):1.

[0014] In some specific embodiments of the present invention, hydrogenated gasoline refers to gasoline produced from C6-C8 components byproducts of ethylene plants through a hydrorefining process, which mainly includes saturated hydrocarbons such as benzene and toluene, further reducing the operating costs of waste alkali treatment.

[0015] In the waste alkali pretreatment process of the present invention, the amount of demulsifier added is 500 mg / L to 1500 mg / L, based on the treatment volume of waste alkali B. In some preferred embodiments, the demulsifier has the following structure:

[0016]

[0017] Where n is 13, 14, or 15.

[0018] Because aldehydes, ketones, and olefins in pyrolysis gas readily polymerize under alkaline conditions to form easily emulsified condensates that are difficult to separate by conventional extraction, the cationic demulsifier selected in this invention is positively charged and can quickly disperse at the oil-water interface, combine with easily emulsified substances, and aggregate and separate. At the same time, the formed substances can be uniformly dispersed in the oil phase, creating better extraction conditions and reducing the operational difficulty of industrial applications.

[0019] In some specific embodiments, a demulsifier feed line is added to the feed inlet of the extraction mixing tank to add the demulsifier to the waste alkali solution, so that the waste alkali solution is fully mixed in the mixing extraction tank and the demulsification extraction process is completed.

[0020] In one specific embodiment of the waste alkali pretreatment process of the present invention, the temperature of the extraction stirring tank is 40-45°C; the stirring speed is 200-300 rpm / min; and the residence time in the stirring tank is 45-60 min.

[0021] Preferably, the oil-water separation of the extracted waste alkaline solution is carried out in an oil-water separator, with a separation time of 4-6 hours.

[0022] A process for treating waste alkaline solution discharged from an ethylene plant includes:

[0023] Based on the aforementioned pretreatment process, the waste alkaline solution after oil-water separation is fed into the downstream stripping and WAO wet oxidation system.

[0024] By adopting the above technical solution, the present invention has the following technical effects:

[0025] This invention employs a waste alkali solution demulsification and extraction pretreatment process. This process removes a large amount of grease from the waste alkali solution while preventing the formation of scale from easily polymerizable substances in the grease, ensuring the stable and efficient operation of the downstream WAO oxidation and acid-base neutralization systems. Furthermore, the extracted waste oil is incinerated to generate steam to supplement the industrial park's steam supply, achieving comprehensive resource utilization.

[0026] This invention utilizes a pretreatment process of demulsification, stirring, and extraction of waste alkali solution to achieve an oil removal rate of over 90%. Following this, a conventional WAO wet oxidation process reduces the COD in the treated waste alkali solution to below 1000 mg / L. The oxidized waste alkali solution has a high pH value. Finally, 20% sulfuric acid is used to neutralize the pH to approximately 8.0 before discharge into a biochemical treatment system. This method requires less equipment investment, ensures complete oxidation and separation of pollutants, and produces a waste alkali solution with good biochemical properties. Attached Figure Description

[0027] Figure 1 A flowchart of a waste alkali pretreatment process used in an embodiment of the present invention. Detailed Implementation

[0028] The waste alkali solution used in the following examples of the present invention is a sample from the outlet of the waste alkali storage tank at the front end of the WAO wet oxidation system of an ethylene plant.

[0029] The main raw materials for the synthesis of the demulsifier used in the following embodiments of the present invention include: 3-aminopyridine, bis(trichloromethyl) carbonate (BTC), and bromoalkanes. The specific preparation process is as follows: A magnetic rotor, thermometer, and condenser are installed in a 500ml three-necked flask. A certain amount of BTC powder and dichloromethane are added and stirred until completely dissolved. The mixture is heated to 40-50°C using an oil bath. After stabilization, 3-aminopyridine is added to the flask, and the reaction is allowed to proceed for 1 hour. The temperature is then further increased to 110-130°C, and a certain amount of bromotetradecane is added. The reaction continues for 5 hours. The tail gas is absorbed using 10% alkali solution. Finally, distillation is performed to obtain a substance with the following structural formula:

[0030]

[0031] Where n is 14.

[0032] The embodiments employ, as follows Figure 1 The process flow shown;

[0033] In the examples and comparative examples, the conditions for stripping the waste alkaline solution at the outlet of the oil-water separator and the WAO oxidation were: 1) reaction temperature of 136°C and atmospheric pressure; 2) reaction temperature of 155°C and pressure of 1.2 MPa.

[0034] Example 1

[0035] After the waste alkali liquid from the outlet of the ethylene plant's waste alkali storage tank was left to stand for 8 hours, the upper layer of floating oil was skimmed off, and the lower layer of waste alkali liquid was tested and found to have an oil content of 8930 mg / L and a COD content of 21500 mg / L using ultraviolet light.

[0036] At 40℃, the waste alkaline solution after settling and stratification, along with demulsifier and hydrogenated gasoline, are fed into a demulsification extraction stirred tank (where the flow rate of waste alkaline solution is 400 ml / h, the flow rate of hydrogenated gasoline is 25 ml / h, the extraction mass ratio of the two is approximately 20:1, and the amount of demulsifier added is 1000 mg / L). The stirring speed is 300 rpm / min, the residence time is 45 min, and after thorough mixing, the mixture is fed into an oil-water separator (the boundary of the oil-water separator is set at 60%) for oil-water separation. The residence time in the oil-water separator is 6 h, and the separated waste alkaline solution is fed into the downstream WAO wet oxidation system.

[0037] After continuous treatment for 5 hours using the above demulsification extraction process, the oil content in the waste alkaline solution at the outlet of the oil-water separator was reduced to 384 mg / L, with an oil removal rate of 95.7%. The COD of the outlet waste alkaline solution was 957 mg / L after stripping and WAO oxidation.

[0038] Example 2

[0039] After the ethylene waste alkaline solution under another working condition was allowed to stand and separate into layers according to Example 1, the UV oil content of the lower layer of waste alkaline solution was tested to be 15380 mg / L, indicating a relatively severe degree of emulsification, and the COD content was tested to be 29500 mg / L.

[0040] At 45℃, the waste alkaline solution after settling and stratification, along with demulsifier and hydrogenated gasoline, is fed into a demulsification extraction stirred tank (where the flow rate of waste alkaline solution is 800 ml / h, the flow rate of hydrogenated gasoline is 50 ml / h, the extraction mass ratio of the two is approximately 20:1, and the amount of demulsifier added is 1000 mg / L). The stirring speed is 285 rpm / min, the residence time is 55 min, and after thorough mixing, it is fed into an oil-water separator (the boundary of the oil-water separator is set at 60%) for oil-water separation. The residence time in the oil-water separator is 6 h, and the separated waste alkaline solution is fed into the downstream WAO wet oxidation system.

[0041] After continuous treatment for 5 hours using the above demulsification extraction process, the oil content of the waste alkaline solution at the outlet of the oil-water separator was measured to be reduced to 457 mg / L, with an oil removal rate of 97.1%. The COD of the waste alkaline solution after stripping and WAO oxidation was measured to be 1050 mg / L.

[0042] Example 3

[0043] After the ethylene waste alkali liquid under another operating condition was allowed to stand and separate into layers according to Example 1, the UV oil content of the lower layer of waste alkali liquid was tested to be 3785 mg / L, indicating a low degree of emulsification, and the COD content was tested to be 18500 mg / L.

[0044] At 40℃, the waste alkaline solution after settling and stratification is introduced together with demulsifier and hydrogenated gasoline into a demulsification extraction stirred tank (where the flow rate of waste alkaline solution is 800 ml / h, the flow rate of hydrogenated gasoline is 100 ml / h, the extraction mass ratio of the two is about 10:1, and the amount of demulsifier added is 1000 mg / L). The stirring speed is 250 rpm / min, the residence time is 45 min, and after thorough mixing, it is introduced into an oil-water separator (the boundary of the oil-water separator is set to 60%) for oil-water separation. The residence time in the oil-water separator is 5 h, and the separated waste alkaline solution is introduced into the downstream WAO wet oxidation system.

[0045] After continuous treatment for 5 hours using the above demulsification extraction process, the oil content of the waste alkaline solution at the outlet of the oil-water separator was measured to be reduced to 237 mg / L, with an oil removal rate of 93.7%. The COD of the waste alkaline solution after stripping and WAO oxidation was measured to be 850 mg / L.

[0046] Comparative Example 1

[0047] The difference between this comparative example and Example 1 is that no demulsifier was added to the extraction mixing tank.

[0048] After continuous treatment for 5 hours according to the process in Example 1, the oil content in the aqueous phase after oil-water separation was measured to be reduced to 3856 mg / L, and the oil removal rate was about 57%.

[0049] Comparative Example 2

[0050] The difference between this comparative example and Example 1 is that the demulsifier is replaced with Hebei Juye FP-23 type demulsifier, which is mainly polyether type, and the amount of demulsifier added is increased from 1000mg / L to 5000mg / L.

[0051] After continuous treatment for 5 hours according to the process in Example 1, the oil content in the effluent alkali after oil-water separation was measured to be reduced to 954 mg / L, and the oil removal rate was about 89.3%. The oil removal efficiency decreased, and the operating cost of the process increased.

[0052] Comparative Example 3

[0053] The difference between this comparative example and Example 1 is that the demulsifier was replaced with Shandong Huayou GN-6016 demulsifier, which is mainly polyethylene glycol in structure, and the amount of demulsifier added was increased from 1000 mg / L to 1500 mg / L.

[0054] After continuous treatment for 5 hours according to the process in Example 1, the oil content in the effluent alkali after oil-water separation was measured to be reduced to 734 mg / L, and the oil removal rate was about 91.8%, which showed good oil removal effect. However, a lot of flocculent matter was generated in the upper oil phase, which was distributed in clumps in the upper oil phase. This could easily cause clogging problems during industrial application, increasing the difficulty of operation.

[0055] Comparative Example 4

[0056] The difference between this comparative example and Example 1 is that the ligand used in the demulsifier synthesis was changed from 14-bromotetradecane to 10-bromodecadecane.

[0057] After continuous treatment for 5 hours according to the process in Example 1, the oil content in the effluent alkali after oil-water separation was measured to be reduced to 1834 mg / L, with an oil removal rate of about 79.5%, indicating a mediocre oil removal effect. The lower layer of alkali liquid was still relatively turbid, indicating a poor demulsification effect.

Claims

1. A pretreatment process for alkali wastewater discharged from an ethylene plant, comprising: 1) Allow the waste alkaline solution A discharged from the ethylene alkaline washing tower to stand and separate into layers; 2) The lower layer of waste alkaline solution B is thoroughly mixed with the demulsifier and hydrogenated gasoline to complete the demulsification extraction; wherein, the demulsifier has the following structure: n is selected from 13, 14, and 15; 3) The extracted waste alkaline solution is subjected to oil-water separation to complete the pretreatment.

2. The pretreatment process according to claim 1, characterized in that, The COD content in waste alkaline solution B is 15000-30000 mg / L.

3. The pretreatment process according to claim 1, characterized in that, The amount of demulsifier added is 500 mg / L to 1500 mg / L, based on waste alkali solution B.

4. The pretreatment process according to claim 1, characterized in that, The weight ratio of waste alkaline solution B to hydrogenated gasoline is (10~25):

1.

5. The pretreatment process according to any one of claims 1-4, characterized in that, Step 2) is carried out in an extraction stirred tank at a temperature of 40-45°C, a stirring speed of 200-300 rpm / min, and a residence time of 45-60 min.

6. The pretreatment process according to any one of claims 1-4, characterized in that, The oil-water separation is carried out in an oil-water separator, with a separation time of 4-6 hours.

7. A process for treating waste alkaline solution discharged from an ethylene plant, comprising: According to any one of claims 1-6, the pretreatment process involves separating the oil and water and then introducing the waste alkaline solution into the downstream stripping and WAO wet oxidation system.

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