Treatment method of spent caustic by membrane system, acidic and oxidizing gases

Abstract

The spent caustic is the wastewater resulting from removing acidic gases with a pH higher than 14 and containing hazardous environmental pollutants and a very unpleasant smell. There are many serious process and material selection challenges for treating this wastewater. The existing method for its treatment is to use wet air oxidation (WAO), which is conducted under high temperatures and pressure and is faced with many operational challenges for implementation in a refinery or petrochemical plant environment. In this invention, the membrane system is used to inject carbon dioxide for treatment. This system operates under ambient temperature and pressure and does not experience acid injection problems in a semi-closed system. This product is equipped with modular gasification units. Also, to complete the spent caustic treatment cycle, an advanced oxidation process (AOP) system can be added to the membrane system.

Description

DescriptionTitle of Invention : TREATMENT METHOD OF SPENT CAUSTIC BY MEMBRANE SYSTEM, ACIDIC AND OXIDIZING GASESTechnical Field

[0001] The technical field of this invention relates to the treatment of spent caustic by membrane system and acidic gas, which is carried out under ambient temperature and pressure without any toxic gas emission.Background Art

[0002] A patent with publication number US2017 / 0001893 entitled Process and Device for Treating Spent Caustic Solution has been published in the United States Patent Office. In this project, the ozonated spent caustic (incomplete treatment) is exposed to carbon dioxide (to the extent that bubbles are created in the system) until the treatment is complete and pH = 7-11 , if the pH reaches the range of 10 to 11 , the process is repeated, and if it is lower than this value, it is desirable for this treatment and there is no need to repeat the process. To supply carbon dioxide enormous methods including carbon dioxide tankers, etc., have been used but the best source for supplying carbon dioxide gas is off-gas and hydrogen gas, which off-gas consists of oxygen gas, ozone, hydrogen sulfide, and carbon dioxide.

[0003] A patent with publication number US2017 / 0001893 entitled process and device for treating used caustic solution has been published in the United States Patent Office. In this patent, various methods are applied to contact spent caustic with ozone. In the first method, spent caustic enters the first reactor and is exposed to the ozone, and then the obtained solution enters the second reactor and is exposed to the carbon dioxide. This process is repeated in a circular and closed circuit until the final treatment. In the second method, these two gases enter the first reactor simultaneously and treatment is continued. (It is recommended to mix these two gases before entering the reactor). In the third method, a mixed gas is used, and the best approach is to use a combination of nitrogen, oxygen, ozone, and carbon dioxide.

[0004] In the stage of carbon dioxide injection, a specified amount of carbon dioxide can be added into the solution in one step, which in turn makes treatment faster, but in the alternative method, this injection can be performed several times until the treatment stage which is highly exothermic and occurs due to the contact of carbon dioxide and solution, is controlled and the system temperature rising is prevented.Technical Problem

[0005] The spent caustic is the wastewater resulting from removing acidic gases with a pH higher than 14 and containing hazardous environmental pollutants and a very unpleasant smell. This wastewater is hugely produced in oil and gas refineries as well as petrochemical plants and is considered a very hazardous environmental pollutant.

[0006] There are many serious process and material selection challenges for treating this wastewater. The existing method for its treatment is to use wet air oxidation (WAO), which is conducted under high temperatures and pressure and is faced with many operational challenges for implementation in a refinery or petrochemical plant environment. Another treatment method is the use of acids, especially sulfuric acid, which in turn involves many issues in materials selection, higher TDS, toxic gas generation, foaming, environmental issues, etc.

[0007] In this invention, the membrane system is used to inject carbon dioxide for treatment. This system operates under ambient temperature and pressure and does not experience acid injection problems in a semi-closed system. This product is equipped with modular gasification units. Also, to complete the spent caustic treatment cycle, an advanced oxidation process (AOP) system can be added to the membrane system.Solution to Problem

[0008] The present invention discloses spent caustic treatment in two direct and indirect methods. Also, a complementary method to reduce the Chemical Oxygen Demand (COD) of treated caustic is presented.

[0009] First method: direct treatment method using liquid-gas contactor

[0010] The proposed system for the treatment of spent caustic is a closed circulatory system. To treat the spent caustic, first, this solution enters the suitable tank with the separation capacity, then it is transferred into the membrane reactor through a pump or without a pump. On the other side, in this system, another pipeline enters another entrance of the membrane system, which is responsible for transferring acidic gas (preferably carbon dioxide gas) from the tank / source of this gas to the membrane system.

[0011] Additionally, this device is equipped with the liquid-gas contactor system, preferably micro and nano-bubble-producing systems, and more preferably the membrane module (or the membrane reactor), which is the main part that distinguishes this device from others. In this part, which is preferably a membrane system, the spent caustic solution phase is in contact with the acidic gas phase, and in these systems, the reaction can start or, preferably, the reaction can be completed.

[0012] Given the very high contact area in liquid-gas contactor systems, preferably the membrane module system, the amount of acidic gas, preferably carbon dioxide, in the liquid phase is increased and increases the composition of the percentage of gas in the form of microbubbles or nanobubbles or preferably increases dissolution without creating bubbles (dissolution on a molecular scale) in the liquid phase and finally prevents the loss of this gas. At the same time, because of this increase in the amount of gas in a small amount of solution compared to other treatment systems, a low temperature is applied to the solution and preferably the temperature is same as the ambient temperature, and the system can perform the treatment reaction quickly without the need for heating.

[0013] Once the reaction occurs on the outer surface of the liquid-gas contactor, the treated material can enter the flash drum system and be divided into two gas and liquid phases. The gas phase containing acidic gas, preferably excess and undissolved carbon dioxide gas, and free hydrogen sulfide gas and etc., can exit from the upper outlet of the flash drum and enter the scrubber, in this part there can be a small amount of fresh caustic or absorbent material that absorbs theresidual gas such as carbon dioxide and it prevents its loss and transfers hydrogen sulfide gas, etc. to the safe area.

[0014] The outlet of the liquid phase from the liquid-gas contactor system, preferably the lower outlet of the flash drum area, is used to discharge the treated spent caustic solution. In the first circulation the pH value reaches the range of 12-14, and then the circulation of this liquid is repeated until the pH value reaches a range of less than 9. Until the pH reaches the sedimentation range, the spent caustic return line in the tank has a UF membrane that prevents suspended particles from entering the tank. After reaching the sedimentation range (pH less than 9), the return line to the tank is changed and transferred to the line with liquid phase separation systems from solid (sediment separation systems), preferably cyclone system and sludge collection system, and then liquid circulation is again continued until it reaches neutral pH (pH=7).

[0015] The treated fluid after reaching the sedimentation range in each round of fluid circulation in the system, can enter the first separation system, preferably a cyclone system, and the solid phase (carbonate, bicarbonate, etc.) consisting of the caustic solution is separated and the caustic solution is returned to the tank for the treatment cycle. The wet solid phase can be removed from the first separation system, preferably the cyclone, and transferred to the secondary liquid-solid separation system, preferably the sludge collection system, and the solid sludge and supersaturated gases can be separated from the separated solution and the resulting spent caustic solution can enter the temporary storage tank first and then enters the tank again after passing through the UF membrane. The gases released in the secondary liquid-solid separation system, preferably the sludge collection system, can also enter the scrubber system and the process of absorbing acidic gas, preferably carbon dioxide and the rest of the residual gases such as hydrogen sulfide can be performed again.

[0016] This product after reaching pH=7 and stabilization in the sludge collection system, leaves the system as a completely treated solution. In this system, as well as treatment, dissolved COD is also significantly reduced, which facilitates complete treatment in the subsystems.

[0017] Second method: indirect treatment method using liquid-gas contactor

[0018] In the second method, the device is configured in such a way that first, water enters the liquid-gas contactor system from the water tank, preferably the contactor system that does this by creating micro and nanobubbles, and more preferably the liquid-membrane contactor system and on the other hand acidic gas, preferably carbon dioxide gas enters the liquid-gas contactor system, preferably the contactor system that does this by creating micro and nanobubbles, and more preferably the liquid-gas membrane contactor system and acidic water exits from the outlet of the module.

[0019] The acidic water removed in the previous step enters the next tank, and the spent caustic also enters this tank with a flow corresponding to the water flow, in this tank the acidic water and the spent caustic are mixed and the treatment process is started.

[0020] The volume of the water tank, the spent caustic tank, the volume of the treatment tank, and the volume of the membrane should be in coordination with each other.

[0021] After the treatment stage, the produced gases from the gas outlet of the treatment tank, enter the scrubber, where they are mixed and absorbed with the absorbent material, preferably the spent untreated caustic, and the remaining gas enters the safe space from the scrubber outlet. The liquid of the separation tank enters the solid-liquid separation system, preferably the sludge collection system, and the solid part is removed as sludge, the liquid part returns to the spent caustic tank until it reaches a neutral state (pH=7). After reaching the neutral state (pH=7), if the COD of the system reaches the desired value, the liquid enters the outlet line, and if the COD of the system is not favorable, first the treated solution enters the AOP unit and after reaching the desired COD, the treated solution enters the outlet line.

[0022] In both methods, the Clean-in-place (CIP) system is embedded next to the membrane contactor system, which is used to wash the system when the system is at rest.

[0023] Complementary method: reduction of COD from a solution using an oxidizing gas by membrane contactor and oxidizing substances and rays.

[0024] In the treatment system, which can be a supplementary system for the treatment of spent caustic, after the completion of the treatment stage and the completion of the treatment of the entire substance in the tank, the semi-treated spent caustic liquid can enter the full treatment cycle and through the pump, it can enter into the gas-liquid contactor system.

[0025] At the inlet or outlet or inside the liquid-gas contactor system, preferably the membrane contactor system, the treated spent caustic can be mixed with a small amount of peroxide material (such as hydrogen peroxide H2O2) or exposed to oxidizing radiation, preferably this mixing process can be performed in the inlet.

[0026] Strong oxidizing gases, preferably oxygen gas and more preferably ozone gas from the supply system of these substances enter the gas inlet of the gas-liquid contactor system, preferably the gas-liquid membrane contactor system, and are treated in the gas-liquid contactor system and are treated in spent caustic solution and the peroxide substance that can enter and come into contact with the liquid part of the gas-liquid contactor system and start the treatment process.

[0027] When the strong oxidizing gases, preferably oxygen gas and more preferably ozone gas are mixed with the treated spent caustic solution in the gas-liquid contactor system and treatment is started, the treated material can enter the flash drum system and is split into two the gas and liquid phases, excess and undissolved gases, released gases, etc. exit from the upper outlet of the flash drum and can enter the scrubber, in this part, a small amount of fresh caustic or suitable absorbent for excess and released gases is added to absorb the remaining gases and prevent the loss of the gases, and the insoluble gases can be transported to a safe area.

[0028] The outlet of the liquid-gas contactor system, preferably the lower outlet of the flash drum area, can be utilized to discharge the treated spent caustic solution, in the first circulation, the COD value reaches about half of the COD value of the untreated spent caustic, and then the circulation of this liquid can be repeated until the value of COD reaches a range lower than the desired level for entering normal industrial wastewater (about COD = 2,000 ppm or lower). Then the liquid exiting the liquid-gas contactor system, preferably the exiting liquid from the flash drum can enter the line with solid-liquid separation systems, preferably cycloneand sludge collection systems (or without it), and the liquid circulation can continue unit COD reach the desired level.

[0029] If necessary, in each round of fluid circulation in the system, the treated fluid enters the primary solid-liquid separation system, preferably the cyclone system, and if there is a solid phase (carbonate, bicarbonate, etc.), it is separated from the caustic solution and the caustic solution can be returned to the tank after passing through the UF filter for the treatment cycle. The wet solid phase is removed from the primary solid-liquid separation area, preferably the cyclone system, and can be transferred to the secondary solid-liquid separation system, preferably the sludge collection system, to separate the solid sludge and supersaturated gases from the solution, then the resulting caustic solution enters the temporary storage tank first and can enter the tank if needed after passing through the UF membrane, the gases released in the secondary solid-liquid separation system can also enter the scrubber system and the process of excess gas absorption and disposal of insoluble gases should be performed again.

[0030] After reaching the desired COD and stabilizing it while passing through the secondary solid-liquid separation system, this product leaves the system as a treated solution.

[0031] In case, after the treatment stage, the dissolved COD is enough to enter the wastewater into the normal industrial wastewater, then only the treatment system can be used and the supplementary treatment system can be removed from the system.Advantageous Effects of Invention

[0032] • Substitution of low-volume membrane system with large treatment plants

[0033] • Reduction of the operation temperature from 150-300 °C to the ambient temperature

[0034] • Reduction of the operating pressure from 7-35 bar to the ambient pressure

[0035] • Remarkable reduction of Total Dissolved Solids (TDS) in the treatment stage

[0036] • Reduction of COD in the treatment stage

[0037] • Modular system with the potential to increase / decrease the capacity

[0038] • Gasification without making bubbles and with the lowest waste

[0039] • Low CO2 consumption

[0040] • High stability under neutral pH

[0041] • The manufactured membrane is asymmetric or composite and assists in controlling and solving operational problems including leakage and bubbling.Brief Description of Drawings

[0042] Figure 1 . Proposed schematic of spent caustic treatment system through reaction with acidic gas (direct method).

[0043] Figure 2. Proposed schematic of spent caustic treatment system through reaction with acidic gas (indirect method).

[0044] Figure 3. Schematic of the modified system for the complementary treatment of spent caustic.

[0045] 01 : Tank

[0046] 02: Pomp

[0047] 03: Filter

[0048] 04: Drum

[0049] 05: Scrubber

[0050] 06: Gas

[0051] 07: CIP Tank

[0052] 08: FT

[0053] 09: PG

[0054] 10: LM

[0055] 11 : Cyclone

[0056] 12: Dewatering

[0057] 13: Sludge Caket

[0058] 14: Neutralized Spent Caustic

[0059] 15: Safe Location

[0060] 16: Spent Caustic

[0061] 17: AOP

[0062] 18: Water

[0063] 19: 03

[0064] 20: Treated Spent Caustic

[0065] 21 : Membrane Contactor or Bubble Reactors

[0066] 22: 100 cc / min Syringe PumpIndustrial Applicability

[0067] This method can be applied in oil and gas refineries as well as petrochemical plants to treat the wastewater resulting from the removal of acidic gases, which contains very hazardous environmental pollutants.

Claims

Claims

1. Spent caustic membrane treatment method with a circulation system under the ambient temperature comprising: a. Tank; b. Membrane contactor; c. Acidic gas; d. Flash drum system; e. Continuous stirred-tank reactor; f. Sludge dewatering system; in which the spent caustic enters from the tank to the membrane contactor and the acidic gas enters the reactor from another inlet and the spent caustic solution phase comes into contact with the acidic gas phase in a way that the gas and liquid phases come into contact with each other at a very high level and the reaction between the two phases takes place in the reactor, then the treated materials enter the flash drum system, which allows adjusting the pressure of the system at values higher than the atmosphere, and finally, in the spent caustic flash drum, it is divided into two gas and liquid phases and then the gas phase is transferred out of the system and the liquid phase is transferred to the sludge dewatering system to remove possible deposits from the reaction.

2. The spent caustic membrane treatment method with a circulation system according to claim 1 , so that first water enters the membrane contactor system from the tank, and on the other hand, acidic gas enters the membrane contactor system and acidic water exits the module outlet which in turn enters the continuous stirred-tank reactor, and also the spent caustic enters this stirred reactor from the corresponding tank, in this tank, acidic water and spent caustic are mixed and reacted together and the treatment process begins.

3. The spent caustic membrane treatment method according to claim 2, so that after the treatment step, the gases generated from the outlet of thetreatment stirred-tank reactor enter the scrubber and are mixed and absorbed with the fresh caustic absorbent material and the remaining gas from the scrubber outlet enters the safe area and the liquid of the reactor with separation mixer enters the sludge dewatering system and the solid part is removed as sludge, the liquid part until it reaches a neutral state (pH=7) enters the return path to the spent caustic tank.

4. The spent caustic membrane complementary treatment method to reduce COD according to claim 1 , where at the inlet of the membrane contactor system, spent caustic treated is mixed with hydrogen peroxide H2O2 and ozone gas enters the gas inlet of the membrane contactor system and comes into contact with spent caustic treated solution and the peroxide substance, then it enters the flash drum system and is divided into two gas and liquid phases, the excess, undissolved gases, and the released gases exit from the upper outlet of the flash drum.

5. In the similar method, neutralization of acidic solutions using alkaline gases or gases which make the solution alkaline after dissolving in water can be performed by the techniques according to the claim 1 -3.

6. The membrane contactor in claim 1 , can be flat, spiral wound or other types and more preferably hollow fiber membrane contactor.

Images