Oil Refining and Petrochemical Wastewater Treatment

1. Background.

The rapid development of the petrochemical industry has brought huge economic effects, but also produced a large number of petrochemical industrial wastewater. At present, there are nearly 500 billion cubic meters of petrochemical industrial wastewater in the world every year, polluting 5.5 trillion cubic meters of fresh water, which is equivalent to more than 14% of the total global runoff. Therefore, for sustainable development, petrochemical industry wastewater needs to be treated before discharge.

Importantly, advanced treatment and recycling of petrochemical industrial wastewater is also an important measure for water conservation and emission reduction. The wastewater treatment of petrochemical enterprises is still one of the difficult problems at home and abroad. It is urgent to solve the technology of treatment of high salt and oil-bearing wastewater and reuse of reclaimed water.

2. Sources and Characteristics of Refinery and Petrochemical Wastewater.

The main sources of refinery and petrochemical wastewater are petrochemical industrial wastewater and refinery circulating cooling water. It has a wide variety, complex composition, high toxicity, inhibition of biodegradation, and high concentration. The specific characteristics are as follows:

(1) The water quantity is large, the water quality is complex and changes greatly.

The production scale of the petrochemical industry tends to be large-scale, because there is a large amount of sewage treated in the whole plant, and the sewage discharged every day is 10000 tons. Petrochemical enterprises need to add various solvents, additives, and additives in the production process, and then go through various reactions. Therefore, the composition of wastewater is quite complex.

(2) Serious organic pollution.

The organic matter contained in petrochemical wastewater is mainly hydrocarbons and their derivatives. The high concentration waste liquid discharged from some petrochemical units is still high after incineration or other appropriate treatment.

(3) Sewage contains heavy metals.

Because many reactions in petrochemical production are completed under the action of catalysts, dozens of catalysts are used in a large petrochemical plant, so the wastewater often contains heavy metals.

NEWater has developed ultrafiltration and reverse osmosis combined processes and equipment for wastewater treatment in two application scenarios: petrochemical industrial wastewater and refinery circulating cooling water, which can help enterprises greatly reduce the total amount of wastewater discharge and save the consumption of primary water, reduce production costs and improve economic and environmental benefits.

3. Oil Refining and Petrochemical Wastewater Treatment Application Scenarios.

3.1 Wastewater Treatment in Petrochemical Industry.

At present, the wastewater reuse technology has been popularized in iron and steel, petrochemical, and electric power industries, but the wastewater from the petrochemical industry has high COD concentration, complex residual oil composition, and large change in water quality. Affected by water quality and the production treatment process, the characteristics of membrane pollution of the same type of petrochemical wastewater at different times and places will be very different. The petrochemical wastewater reuse unit developed by NEWater uses the tertiary treated wastewater of the first purification unit as the water source, adopts the combined process of ultrafiltration and reverse osmosis, and the product water is used as the make-up water for the petrochemical industry.

Figure2 Petrochemical Industry.

Ultrafiltration is to separate pollutants through mechanical screening. Ultrafiltration membrane elements have the characteristics of selective separation. Under pressure, suspended solids, large organic particles, and oil slicks contained in wastewater are intercepted outside the ultrafiltration membrane, while small water molecules and salts from water production through the ultrafiltration membrane. A submerged ultrafiltration membrane module is adopted for ultrafiltration of the Jingyi unit, and the design recovery rate of each set is not less than 90%.

Figure3 Equipment of ultrafiltration and reverse osmosis system for petrochemical wastewater treatment.

RO membrane technology is a very mature technology for preparing ultrapure water in recent years. The main mechanism is that the same volume of dilute solution and concentrated solution are placed on both sides of the semi-permeable membrane. Due to the concentration difference, the dilute solution penetrates to the concentrated solution side. This process is called infiltration. When the permeation reaches equilibrium, the liquid level of the concentrated solution is a certain height higher than that of the dilute solution, thus forming a pressure difference, which is called osmotic pressure.

The osmotic pressure depends on the properties of the liquid, such as type, concentration, and temperature, and has nothing to do with the properties of the semi-permeable membrane. If a pressure greater than the osmotic pressure is artificially applied on one side of the concentrated solution at this time, the penetration direction of the solvent will be opposite to the original direction, that is, it will flow from the concentrated solution side to the dilute solution side. This phenomenon is called reverse osmosis.

Figure4 Process flow of refinery and petrochemical wastewater reuse.

The anti-pollution composite reverse osmosis membrane module is adopted for the reverse osmosis of the first purification unit, and the stable desalination rate is 98%. A double membrane process, namely ultrafiltration, and reverse osmosis system are adopted.

The reuse process of petrochemical wastewater is as follows: Discharged Petrochemical Wastewater up to Standard → Bactericide → Homogenization Regulation of Regulating Tank → Self-Cleaning Filter → Ultrafiltration Membrane Tank → Suction of Ultrafiltration Water Production Pump → Ultrafiltration Water Production → Ultrafiltration Water Production Tank → Scale Inhibitor/Effluent Reductant/Non-Oxidizing Bactericide→ Reverse Osmosis System → High-Pressure Pump → Output Qualified Water → Water Tank → External Supply as Water for Petrochemical Industry.

Although some petrochemical wastewater can meet the discharge standard, it will still pollute the environment. After the proper treatment, NEWater can be used as water for the petrochemical industry, which can not only reduce environmental pollution but also realize recycling and truly turn waste into treasure. The combined process of ultrafiltration and reverse osmosis can well remove various petrochemical pollutants. The wastewater treated by this process can meet the water standard of the petrochemical industry.

3.2 Refinery Circulating Cooling Water Treatment.

During the use of circulating cooling water in the refinery, through the exchange of heat and mass transfer of cooling structures, the drainage is polluted to a certain extent due to the leakage of pollutants in the air such as soil, sundries, soluble gas, and heat exchanger materials during the open operation of circulating cooling water.

Figure5 Oil refineries.

The drainage of circulating cooling water system has the following characteristics:

  • The content of suspended solids is high, the particles are fine and the turbidity is low.
  • It contains a small amount of petroleum, sulfide, and phenol. When the system leaks, the concentration of these pollutants rises sharply.
  • The cooling mode of the open circulating cooling water system is mainly evaporation and heat dissipation. With the continuous improvement of concentration multiple, the salt content and scaling ions in water also increase accordingly.
  • The concentration of nutrients in the circulating water system and other suitable conditions promote the proliferation of bacteria and algae and the increase of biological slime.

NEWater has formulated the treatment process of ultrafiltration combined reverse osmosis for the circulating cooling water of the refinery. First, introduce the process of various operation modes of ultrafiltration pretreatment:

  • Filtration: (Dosing Tank → Metering Pump) & Water Storage Tank → Disc Filter → Ultrafiltration Cartridge → Clean Water Tank.
  • Hydraulic Cleaning:
  • Backwashing (Differential Pressure Flushing): Dosing Tank → Backwashing Dosing Pump → Ultrafiltration Backwashing Pump → Ultrafiltration Filter Cartridge → Discharge, Clean Water Tank → Ultrafiltration Backwashing Pump.
  • Positive Pressure (Isobaric Flushing): Water Storage Tank → Disc Filter → Ultrafiltration Cartridge → Discharge
  • Chemical Cleaning: Chemical Cleaning Tank → Chemical Cleaning Pump → Ultrafiltration Cartridge →

Figure6 Ultrafiltration pretreatment process of refinery circulating cooling water.

Traditional pretreatment processes include lime clarification, medium filtration, and precision filtration. However, the ability of traditional filtration devices to block colloids and suspended particles is uncertain. The water quality obtained after traditional treatment is greatly affected by some specific factors.

Compared with the conventional process, the hollow fiber technology of ultrafiltration device has the advantages of standardized design, no continuous addition of chemicals, and limited labor force. At the same time, it also has the advantages of high-water production quality and stable water production. The specific effects are as follows:

  • Ultrafiltration membrane can effectively reduce the turbidity of circulating cooling water, and the removal rate of turbidity is more than 98.5%.
  • Ultrafiltration membrane has a good removal effect on the colloid, and the SDI of produced water is less than 0.8.
  • Ultrafiltration membrane has a good removal effect on CODCr, and the removal rate is 70-80%, and the removal rate of residual chlorine is more than 70%.

NEWater compares and analyzes the composition, treatment effect, and impact on reverse osmosis membrane of conventional reverse osmosis pretreatment process with hollow fiber ultrafiltration. Ultrafiltration as reverse osmosis pretreatment is better than the conventional pretreatment process and it can replace the conventional water pretreatment process.

Figure7 Reverse osmosis treatment process of refinery circulating cooling water.

The next step is the reverse osmosis treatment process of refinery circulating cooling water: Ultrafiltration Effluent → Increase Pump → Scale Inhibitor → Precision Filtration → Solenoid Valve → High-Pressure Pump → Reverse Osmosis Stage I → Reverse Osmosis Stage II → Reverse Osmosis Stage III → Concentrated Water Reflux (The other branch is Concentrated Water Discharge). After Reverse Osmosis Stage I, II, and III treatment is product water.

After reverse osmosis desalination treatment, the salt content of refinery circulating cooling water is reduced by more than, the hardness, colloid, and suspended solids are significantly reduced, and the water quality has made a qualitative leap, which can be re-input to the circulating cooling water system. The specific effects are as follows:

  • Reverse osmosis desalinates the wastewater from oil refining circulating cooling, which can generally reach a desalination rate of more than 96% to reduce a load of ion exchange resin by nearly times, thus reducing the cost of resin regeneration. In other words, it can increase the water production of resin by nearly times. In this way, the corresponding equipment can be miniaturized and the resin regeneration frequency can be reduced.
  • It cushions the change of corresponding produced water quality caused by the fluctuation of raw water quality, which is conducive to the stability of water quality in production and the continuous and safe operation of production.
  • Since reverse osmosis can effectively remove organic microorganisms such as bacteria and inorganic substances such as iron, calcium, manganese, and silicon, softeners can not be used to reduce equipment investment; And it also can prolong the service life of the terminal filter and reduce the corresponding cost.

Figure8 Equipment for ultrafiltration reverse osmosis double membrane process of refinery circulating cooling water.

Aiming at the problem of circulating cooling water in the refinery, NEWater conducted experiments with ultrafiltration effluent as reverse osmosis feed water, conducted in-depth analysis and research on the main factors affecting the operation of reverse osmosis system, such as pH, scaling ions, and recovery rate, determined the best recovery rate, optimized scale inhibitor, and developed ultrafiltration reverse osmosis double-membrane process equipment. The comprehensive effects are as follows:

  • According to the characteristics of high alkalinity, hardness and scaling ion of refinery circulating cooling wastewater, the treatment scheme of adding regulation and scale inhibitor is determined. The optimum recovery rate of the reverse osmosis system is 60%.
  • With the extension of the operation time, the operation pressure of each section is stable. The standard desalting rate of the pressure drop in the first and second stages and the pressure drop in the second and third stages is unchanged and can be kept above 99%, and the effluent quality is stable.
  • After the advanced treatment of ultrafiltration and reverse osmosis, the water quality of refinery circulating cooling sewage has changed qualitatively. The reverse osmosis effluent can meet the make-up water requirements of a low-pressure boiler. It can be used as circulating cooling water when mixed with ultrafiltration effluent, and it can be used as make-up water of a high-pressure boiler for further advanced treatment.

If you have any needs in oil refining and petrochemical wastewater treatment, please contact NEWater. The treatment scheme of ultrafiltration and reverse osmosis combined process and its equipment will be customized by our professional technical engineers to solve your problems and work with you to reuse the wastewater as new circulating cooling water and make-up water.

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