Efficient regeneration method and system for liquefied gas sweetening inactivated alkali liquor

Abstract

The invention discloses an efficient regeneration method and system for inactivated alkali liquor in a liquefied gas extraction-oxidation mercaptan removal device, and belongs to the field of petrochemical engineering. The method comprises the following steps: carrying out reduced pressure distillation and concentration on inactivated alkali liquor, adsorbing clear liquid through activated carbon, adding calcium hydroxide for chemical precipitation, and enabling filtrate to sequentially pass through anion exchange resin and a nanofiltration membrane, so as to obtain purified alkali liquor. The method provided by the invention can effectively regenerate the inactivated reduced pressure, reduces the discharge amount of alkaline residues by more than 95%, and has remarkable environmental protection and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of petrochemicals, specifically relating to a method and system for the deep purification and regeneration of deactivated alkaline solution in a liquefied petroleum gas (LPG) desulfurization process, particularly suitable for situations where the desulfurization effect is reduced due to excessive thermal stabilizer salt causing alkaline solution deactivation. Background Technology

[0002] As an important chemical raw material and clean fuel, the sulfur content of liquefied petroleum gas (LPG) is a key quality indicator. With increasingly stringent environmental regulations and the rising requirements for raw material purity in downstream chemical plants (such as propane dehydrogenation, alkylation, and polypropylene), the total sulfur content of refined LPG is typically required to be controlled below 50 ppm, and some high-end chemical applications even require it to be below 20 ppm.

[0003] Industrially, the alkaline extraction-oxidative desulfurization process is widely used to remove sulfur-containing species such as thiols and hydrogen sulfide from liquefied petroleum gas (LPG). The core of this process lies in the recycling of the alkaline solution. However, in actual long-term operation, the alkaline solution faces a severe problem of deactivation and regeneration.

[0004] During operation, the alkali solution extracts thiols (RSH) to form sodium thiolate (RSNa) or potassium thiolate (RSK). Simultaneously, side reactions generate thermally stable salts such as sodium / potassium formate, sodium / potassium acetate, sodium / potassium carbonate, and sodium / potassium thiosulfate (collectively referred to as thermally stable salts). The continuous accumulation of these substances leads to a sharp decline in the alkali solution's ability to extract thiols, causing a rebound in sulfur content in the refined liquefied petroleum gas (LPG) to exceed standards. This further results in excessive sulfur content in downstream olefin products (such as propylene and butene), severely impacting the polymerization catalyst lifespan and product quality. Carbonates typically account for 30%-50% of the thermally stable salts and are a significant factor contributing to alkali solution deactivation.

[0005] When the activity of the alkali solution cannot meet the desulfurization requirements, enterprises can only periodically discharge the spent alkali solution and replenish it with fresh alkali solution. Spent alkali solution (alkali residue) is classified as hazardous solid waste (HW35) by the state. It has extremely high chemical oxygen demand (COD) and contains a large amount of sulfides, thiols and phenolic substances. Traditional neutralization, wet oxidation and other methods are costly to treat and are difficult to meet environmental emission standards.

[0006] Frequent replacement of alkali solution not only increases the procurement cost of fresh alkali solution but also generates huge hazardous waste disposal costs. Meanwhile, the transportation and storage of high-sulfur alkali residue also pose significant safety and environmental risks.

[0007] In summary, developing a technology capable of online deep regeneration of deactivated alkali solutions, particularly one that efficiently removes heat-stable salts and restores alkali concentration to achieve long-term recycling of alkali solutions and significantly reduce alkali residue emissions, has become a pressing industrial challenge for the LPG refining industry. However, existing regeneration technologies (such as wet air oxidation) primarily target thiolate conversion and have almost no ability to remove heat-stable salts; simple distillation concentration or membrane separation technologies suffer from high energy consumption, severe membrane fouling, and an inability to selectively remove heat-stable salts, making it difficult to meet the requirements of deep purification. Summary of the Invention

[0008] The technical problem to be solved by the present invention

[0009] The present invention aims to provide a highly efficient regeneration method and system for deactivated alkaline solution from liquefied petroleum gas (LPG) to solve the technical problems of difficult thermal salt removal, insufficient alkali concentration recovery, large amount of alkali residue discharge, and high operating costs in the prior art.

[0010] Technical solution

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] A highly efficient regeneration method for deactivated alkaline solution from liquefied petroleum gas (LPG) desulfurization includes the following steps:

[0013] Step 1: Concentration Pretreatment – ​​Vacuum Distillation

[0014] The deactivated alkaline solution in the LPG desulfurization unit is drawn out and pumped to the alkaline solution regeneration unit; it first enters the alkaline solution vacuum distillation system. The deactivated alkaline solution usually contains 4-15 wt% dissolved NaOH or KOH and 2-20 wt% heat-stable salt, of which carbonates usually account for 30%-50% of the heat-stable salt and are one of the main components of the heat-stable salt.

[0015] The operating conditions for step one, vacuum distillation, are as follows:

[0016] (1) Absolute pressure: 5-30 kPa (preferably 10-15 kPa)

[0017] (2) Operating temperature: 60-90℃ (preferably 70-80℃)

[0018] (3) Concentration factor: Concentrate the volume of the deactivated alkali solution to 1 / 3 to 1 / 2 of its original volume.

[0019] Under these conditions, water and some low-boiling-point organic matter are evaporated, and after condensation, reusable distilled condensate is obtained. The alkali concentration in the concentrated alkali solution increases from the original 10-15 wt% to 20-30 wt%. Due to the decreased solubility of heat-stable salts during the concentration process, some heat-stable salts, mainly polyvalent salts such as carbonates and thiosulfates, will precipitate out in solid form, forming a solid-liquid mixture.

[0020] Step 2: Preliminary removal of thermally stabilized salts – First filtration unit

[0021] The concentrated alkaline solution containing heat-stable salt solids obtained in step one is sent to the first filtration unit to remove the precipitated heat-stable salt solids and obtain a clear solution.

[0022] The first filtration unit is selected from candle filters, microfiltration filters, or combinations thereof, with a filtration accuracy of 1-50 micrometers (preferably 5-20 micrometers). The filtration temperature is controlled at 50-80°C to maintain the fluidity of the filtrate.

[0023] This step removes 30%-60% of the heat-stable salts from the concentrated alkaline solution, significantly reducing the load on subsequent chemical precipitation, ion exchange, and nanofiltration units. The removed heat-stable salt solids can be recovered as a byproduct for use as chemical raw materials or in salt processing.

[0024] Step 3: Removal of organic matter – Activated carbon adsorption

[0025] After cooling the clarified liquid obtained in step two to 40-50℃, it is sent to an activated carbon adsorption tower. The activated carbon selected is coal-based or nutshell-based granular activated carbon with an iodine value ≥900 mg / g and a specific surface area ≥1000 m² / g.

[0026] Activated carbon can effectively adsorb disulfide oils, thiols, and other organic pollutants in alkaline solutions. The flow rate of the alkaline solution through the activated carbon tower is controlled at 0.5-2 BV / h.

[0027] Step 4: Chemical precipitation to remove carbonate – calcium hydroxide treatment

[0028] The alkaline solution after activated carbon adsorption still contains a high concentration of dissolved carbonate (CO3²⁻). Carbonate is one of the main components of heat-stable salts, placing a significant load on ion exchange resins and nanofiltration membranes. This invention employs chemical precipitation to selectively remove carbonate ions.

[0029] Reaction principle:

[0030] Ca(OH)₂ + CO₃²⁻ 2− →CaCO3↓+2OH−Ca(OH)2+CO3 2− →CaCO3↓+2OH −

[0031] Step 4 Operating conditions:

[0032] (1) Reaction temperature: 40-70℃ (preferably 50-60℃)

[0033] (2) Reaction time: 10-60 minutes (preferably 20-30 minutes)

[0034] (3) Stirring speed: 50-200 rpm

[0035] (4) Amount of calcium hydroxide added: Calculated based on the carbonate content in the alkaline solution, it is 1.0-1.5 times the theoretical amount (preferably 1.1-1.2 times).

[0036] Calcium hydroxide can be added as a solid powder or in the form of lime milk. The calcium carbonate produced by the reaction is a white precipitate with relatively large particles (mainly distributed in the range of 5-20 micrometers), which is easily removed by filtration.

[0037] This step can remove 60%-90% of carbonate ions from the alkaline solution. Simultaneously, the OH⁻ generated during the reaction replenishes the alkalinity of the solution, slightly increasing the alkali concentration (by approximately 1-2 percentage points).

[0038] Step 5: Solid-Liquid Separation – Filtering

[0039] The alkaline solution after chemical precipitation contains calcium carbonate precipitate and a small amount of activated carbon detachment, which requires solid-liquid separation.

[0040] The alkaline solution after the reaction is passed through a polypropylene melt-blown filter cartridge with a filtration accuracy of 1-10 micrometers (preferably 5 micrometers) and an operating pressure ≤0.3 MPa. This filter cartridge has high dirt-holding capacity and low pressure drop characteristics, and can effectively retain calcium carbonate precipitate and activated carbon powder, with suspended solids in the effluent <5 mg / L.

[0041] Step Six: Residual Heat-Stable Salts – Anion Exchange Resin

[0042] The filtered clear liquid enters the anion exchange resin tower. After the carbonate ions are removed by chemical precipitation, the dissolved thermally stable salt load entering the ion exchange unit is significantly reduced, and the main residues are formate, acetate, carbonate, thiosulfate, etc.

[0043] The resin selected is a strong basic styrene-based anion exchange resin with the functional group -N⁺(CH3)3, which has good exchange capacity for residual dissolved anions in the heat-stable salt.

[0044] Step 6 operating parameters:

[0045] (1) Working exchange capacity: 0.5-1.2 mol / L resin

[0046] (2) Operating flow rate: 1-3 BV / h

[0047] (3) Temperature: ≤50℃

[0048] As a preferred embodiment, the anion exchange resin tower adopts a recycling regeneration method: the regeneration alkali solution is prepared from the distillation condensate obtained in step one, and the regenerated waste alkali solution is returned to the inlet of the vacuum distillation step through the reflux pipeline, mixed with fresh deactivated alkali solution, and then re-enters the treatment process to achieve zero discharge of regenerated waste liquid.

[0049] Step 7: Deep Desalination – Nanofiltration Membrane Separation

[0050] To remove residual dissolved thermally stable salts to the target level (<0.5 wt%), the ion-exchange effluent is further fed into a nanofiltration membrane system.

[0051] Depending on the alkaline solution system, the nanofiltration membrane selection and operating parameters are as follows:

[0052] parameter NaOH system KOH system Molecular weight cutoff / Da 150-250 200-300 Retention rate of multivalent anions ≥92% ≥93% OH⁻ retention rate ≤30% ≤25% Operating pressure / MPa 1.5-3.0 1.2-2.2 Operating temperature / °C 20-50 35-45

[0053] Step 8: Purify and reuse the alkaline solution

[0054] The final purified alkaline solution can be directly returned to the LPG desulfurization unit for recycling.

[0055] Explanation of Innovation Points—Key Inventive Points of this Invention

[0056] This invention is not a simple combination of known units, but rather constitutes a complete inventive concept through the following synergistic mechanisms and unexpected technical effects:

[0057] Invention Point 1: Parameter Coupling of Vacuum Distillation and Nanofiltration Membrane

[0058] The inventors have discovered for the first time that vacuum distillation not only concentrates and dehydrates salts, but more importantly, alters the ionic distribution of heat-stable salts. During vacuum distillation, low-molecular-weight monovalent heat-stable salts, such as formates and acetates, preferentially volatilize with steam or undergo thermal decomposition, while high-molecular-weight polyvalent heat-stable salts, such as carbonates and thiosulfates, are concentrated and enriched. Experiments show that, under the optimized distillation conditions of this invention—an absolute pressure of 10-15 kPa and a temperature of 70-80°C—the proportion of polyvalent anions in the concentrated alkaline solution can be increased from approximately 30% in the original deactivated alkaline solution to 60%-75%.

[0059] The significance of this discovery lies in the fact that nanofiltration membranes exhibit a significantly higher rejection rate for multivalent anions (≥92%) than for monovalent anions (≤30%). Therefore, the distillation step, by altering the ion speciation, synergistically enhances the desalination efficiency of the nanofiltration membrane. This parametric coupling effect of "distillation-nanofiltration" is something that those skilled in the art could not have foreseen based on conventional knowledge of the individual steps.

[0060] Invention Point Two: Internal Circulation of Anion Exchange Resin Based on Alkali Regeneration and Distillation Recovery

[0061] In traditional processes, the regeneration of ion exchange resins generates high-salt, high-alkali secondary waste liquid, becoming a new source of pollution. This invention creatively returns the regenerated waste liquid to a vacuum distillation system, utilizing the distillation process to recover the alkali solution and concentrate and precipitate the thermally stable salt. This design achieves:

[0062] (1) Zero discharge of recycled waste liquid

[0063] (2) Closed-loop circulation of alkali solution

[0064] (3) Resource utilization of thermally stable salt (can be recycled as by-product salt)

[0065] Invention Point 3: Optimization of nanofiltration membrane compatibility for KOH systems

[0066] In the KOH alkaline solution regeneration scenario, since the hydration radius of K⁺ is smaller than that of Na⁺, and the KOH solution has higher conductivity and lower osmotic pressure, this invention specifically optimizes the nanofiltration membrane model. By selecting a nanofiltration membrane with a K⁺ rejection rate ≤20% and a polyvalent anion rejection rate ≥92%, efficient retention of thermally stable salts such as CO₃²⁻ and S₂O₃²⁻ can be achieved while ensuring efficient KOH permeation. Compared to nanofiltration membranes that directly use the NaOH system, the KOH retention rate can be increased from 82% to 87%, and the thermally stable salt removal rate can be increased from 88% to 92%.

[0067] Invention Point 4: Selective Removal of Carbonate Ions via Chemical Precipitation

[0068] This invention introduces a calcium hydroxide chemical precipitation step into the alkaline regeneration process for the first time, used to selectively remove carbonate ions. This design has the following significant advantages:

[0069] 1. Selective decarbonation: Calcium hydroxide reacts with carbonate ions to form calcium carbonate precipitate, while having no effect on OH⁻, formate, acetate, etc. in the alkaline solution, thus achieving selective removal of carbonate ions.

[0070] 2. Significantly reduce ion exchange load: Carbonates typically account for 30%-50% of thermally stable salts. After pre-removal through chemical precipitation, the load on the ion exchange resin can be reduced by 40%-60%, and the resin regeneration cycle can be extended by 2-3 times.

[0071] 3. Synergistic Effect: Chemical precipitation decarbonization, vacuum distillation concentration, ion exchange, and nanofiltration membrane separation form a staged desalination system—distillation removes water and some solid salts, chemical precipitation selectively removes carbonate ions, ion exchange removes residual anions, and nanofiltration membrane purification. Each step has a clear division of labor and synergistic effects.

[0072] 4. By-product resource utilization: The generated calcium carbonate is a high-purity precipitate that can be used as an industrial raw material, such as in building materials and flue gas desulfurization agents.

[0073] 5. Alkalinity replenishment: The reaction generates OH⁻, which slightly increases the concentration of the alkali solution, reducing the need for subsequent alkali replenishment.

[0074] Beneficial effects

[0075] Compared with the prior art, the present invention has the following beneficial effects:

[0076] 1. Deep removal of thermally stable salts to ensure desulfurization accuracy: Through four-stage desalination (solid filtration + chemical precipitation + ion exchange + nanofiltration) and the coupling of distillation-nanofiltration parameters, the total removal rate of thermally stable salts can reach over 95%. When the regenerated alkali solution is used in the desulfurization process of liquefied petroleum gas (LPG), the total sulfur content of the refined LPG is stably controlled within 50 ppm, reaching 15-35 ppm.

[0077] 2. Selective decarbonation with significant effect: The removal rate of carbonate ions by calcium hydroxide chemical precipitation reaches 75%-90%, and the proportion of carbonate ions in the heat-stable salt is reduced from 30%-50% to less than 10%.

[0078] 3. Significantly reduced ion exchange load: After chemical precipitation decarbonization, the working cycle of ion exchange resin is extended by 2-3 times, the regeneration frequency is significantly reduced, and the service life of the resin is extended.

[0079] 4. Synergistic Effect of Technology – Enhanced Enrichment of Multivalent Ions: Under the optimized distillation conditions of this invention, the proportion of multivalent anions in the heat-stable salt increases from 30% to over 60%, thereby increasing the retention efficiency of the nanofiltration membrane for the heat-stable salt by 25%-40% compared to the case without optimized distillation conditions. This synergistic effect cannot be achieved by a single technology.

[0080] 5. Closed-loop operation, near-zero emissions: Resin regeneration waste liquid is returned to the distillation system, and nanofiltration concentrate is returned to the distillation system, realizing a closed-loop circulation throughout the entire process, reducing alkali residue emissions by more than 98%.

[0081] 6. By-product resource utilization: Thermally stable salt solids, such as carbonates, thiosulfates, and calcium carbonate precipitates, can be recycled as industrial raw materials, turning waste into treasure.

[0082] 7. Dual-system adaptation: This invention can optimize nanofiltration membrane selection and operating parameters for both NaOH and KOH systems. The alkali retention rate of the KOH system is increased by 5 percentage points compared to directly applying the NaOH parameters.

[0083] 8. Significantly reduced operating costs: Overall operating costs are reduced by more than 70%, and the investment payback period is approximately 1-1.5 years. Attached Figure Description

[0084] Figure 1 This is a process flow diagram of the high-efficiency regeneration system for deactivated alkaline solution in liquefied petroleum gas (LPG) provided in an embodiment of the present invention. In the diagram:

[0085] a) Deactivated alkali solution tank

[0086] b) Alkali heater

[0087] c) Vacuum distillation column

[0088] d) Buffer tank

[0089] e) First filtration unit (thermally stable salt solids filtration)

[0090] f) Activated carbon filter

[0091] g) Chemical precipitation reactor (calcium hydroxide precipitation)

[0092] h) Filter

[0093] i) Anion exchange resin tower

[0094] j) Nanofiltration membrane module

[0095] k) Solid alkali tank

[0096] l) Solid alkali dissolving tank

[0097] m) Thermally stabilized salt solid recovery tank

[0098] n) Calcium carbonate precipitation recovery tank

[0099] o) Pressure reduction system Detailed Implementation

[0100] The present invention will be further described in detail below with reference to embodiments and comparative examples, but the scope of protection of the present invention is not limited thereto.

[0101] Example 1

[0102] Taking the deactivated NaOH alkaline solution produced by the liquefied petroleum gas desulfurization unit of a certain refinery as an example, its initial composition is as follows:

[0103] NaOH concentration: 11.5 wt%

[0104] Total heat-stable salt content: 8.8 wt% (of which sodium carbonate accounts for 43% of the heat-stable salt);

[0105] COD: 8500 mg / L;

[0106] Suspended solids: 120 mg / L;

[0107] The specific steps of processing using the method of this invention are as follows:

[0108] 1. Vacuum distillation

[0109] The deactivated alkali solution was heated to 75°C and fed into a distillation column at a flow rate of 10 L / h, with the absolute pressure controlled at 12 kPa, and concentrated to 40% of its original volume. The distillation condensate (COD ≤ 200 mg / L) was collected, and the alkalinity of the concentrated alkali solution increased to 27.5 wt%. At this point, a significant solid precipitate appeared at the bottom of the column, mainly sodium carbonate and sodium thiosulfate. Analysis showed that the proportion of dissolved polyvalent anions (CO3²⁻, S2O3²⁻) in the concentrated alkali solution increased from 31% before distillation to 68% of the total dissolved thermally stable salts.

[0110] 2. Thermally stabilized body filtration

[0111] The concentrated alkaline solution containing solids (at approximately 50°C) was passed through a microporous filter with a filtration precision of 10 micrometers and an operating pressure of 0.3 MPa. The filtrate was clear and transparent, and approximately 4.2 wt% (48% of the original total heat-stable salt) of solidified thermally stable salts was removed. The removed heat-stable salt solids were dried and recovered as a byproduct.

[0112] 3. Activated carbon adsorption

[0113] After the filtrate was cooled to 45°C, it was passed through a coconut shell activated carbon column (iodine value 1050 mg / g) at a flow rate of 1 BV / h. The COD of the effluent was reduced to 2100 mg / L.

[0114] 4. Chemical precipitation to decarbonate ions

[0115] Add calcium hydroxide (in the form of lime milk, concentration 15 wt%) to the activated carbon effluent, the amount added being 1.1 times the theoretical amount. The reaction temperature is 55℃, the stirring speed is 100 rpm, and the reaction time is 25 minutes.

[0116] Testing revealed that the carbonate concentration decreased from 3.8 wt% before the reaction to 0.4 wt%, representing a removal rate of 90%. The calcium carbonate precipitate formed during the reaction had a particle size mainly ranging from 5 to 15 micrometers.

[0117] 5. Filter filtration

[0118] The alkaline solution after the reaction was passed through a 5 μm polypropylene melt-blown filter at an operating pressure of 0.25 MPa to remove calcium carbonate precipitate and activated carbon powder. The effluent suspended solids were <2 mg / L. The removed calcium carbonate precipitate was dried and recycled as an industrial raw material.

[0119] 6. Ion exchange

[0120] After filtration, the alkaline solution enters the anion exchange resin tower, operating at a flow rate of 2 BV / h and a working exchange capacity controlled at 0.8 mol / L. The dissolved thermally stable salt content in the effluent is reduced to 0.5 wt%.

[0121] 7. Nanofiltration membrane separation

[0122] The nanofiltration membrane is an alkali-resistant polyamide composite membrane (molecular weight cutoff 200 Da), with an operating pressure of 1.8 MPa and a temperature of 40℃.

[0123] Final purified alkali solution indicators:

[0124] (1) NaOH concentration: 27.2 wt% (an increase of about 0.5 wt% compared to before chemical precipitation)

[0125] (2) Content of dissolved thermally stable salt: 0.3 wt% (total removal rate 97.0%)

[0126] (3) COD: 280 mg / L

[0127] (4) Suspended solids: Not detected

[0128] Effect of regenerated alkali solution reuse:

[0129] Using regenerated alkali solution to extract liquefied petroleum gas (LPG) reduces the total sulfur content of LPG to 18-32 ppm, which is far better than the control requirement of 50 ppm.

[0130] Operational stability:

[0131] (1) Regeneration cycle of ion exchange resin: 25 h (79% longer than 14 h without chemical precipitation);

[0132] (2) Chemical cleaning cycle of nanofiltration membrane: 48 h (33% longer than 36 h without chemical precipitation);

[0133] (4) Ion exchange resin fouling rate: reduced by 75% compared to when there is no meltblown filtration;

[0134] By-product recycling:

[0135] (1) Heat-stable salt precipitation (step 2): The main components are sodium carbonate and sodium thiosulfate.

[0136] (2) Calcium carbonate precipitation (step 5): purity > 95%

[0137] Example 2

[0138] A refinery's LPG desulfurization unit uses KOH solution as the extraction alkali. The regeneration effect was investigated using the refinery's spent KOH alkali solution as the subject. Its initial composition was as follows:

[0139] KOH concentration: 12.8 wt%

[0140] Total heat-stable salt content: 3.5 wt% (of which potassium carbonate accounts for approximately 1.4 wt%, or 40% of the heat-stable salt content).

[0141] COD: 10200 mg / L

[0142] Suspended solids: 180 mg / L

[0143] The following parameters were optimized for the KOH system using the method of this invention:

[0144] 1. Vacuum distillation

[0145] The deactivated alkali solution was heated to 68°C in a heater, and then fed into a vacuum distillation column at a flow rate of 5 L / h, maintaining an absolute pressure of 8 kPa, to concentrate it to 35% of its original volume. After concentration, the KOH alkali solution concentration increased to 31.5 wt%. Due to the higher solubility of potassium salts, the precipitated solids were slightly less than those in the sodium salt system; analysis showed that the precipitated solids, specifically heat-stable salts, accounted for approximately 32% of the original total heat-stable salts. The proportion of dissolved polyvalent anions in the concentrated alkali solution increased from 29% before distillation to 65%.

[0146] 2. Thermally stable salt solid filtration

[0147] The concentrated alkaline solution containing solids (at approximately 65°C) was fed into a microfiltration machine with a filtration precision of 20 microns and an operating pressure of 0.4 MPa. The filtrate became clear, and approximately 1.1 wt% of thermally stable solid salts were removed.

[0148] 3. Activated carbon adsorption

[0149] After the filtrate was cooled to 42°C, it was passed through an apricot shell activated carbon column (iodine value 1100 mg / g) at a flow rate of 0.8 BV / h, and the COD of the effluent was reduced to 2450 mg / L.

[0150] 4. Chemical precipitation to decarbonate ions

[0151] Add calcium hydroxide (1.15 times the theoretical amount) to the activated carbon effluent, react at 50℃ (appropriately lower for KOH system), stir at 80 rpm, and react for 30 minutes.

[0152] The carbonate concentration decreased from 1.32 wt% before the reaction to 0.10 wt%, with a removal rate of 92.4%.

[0153] 5. Filter filtration

[0154] The alkaline solution after the reaction was passed through a 3 μm polypropylene melt-blown filter element at an operating pressure of 0.22 MPa, and the suspended solids in the effluent were <2 mg / L.

[0155] 6. Ion exchange

[0156] After filtration, the KOH alkaline solution enters the anion exchange resin tower at an operating flow rate of 1.5 BV / h, and the dissolved thermally stable salt content in the effluent is reduced to 0.6 wt%.

[0157] 7. Nanofiltration membrane separation

[0158] The nanofiltration membrane is an alkali-resistant polyamide composite membrane, which has a K⁺ rejection rate of ≤20% and a polyvalent anion rejection rate of ≥95%. The operating pressure is 1.6 MPa and the temperature is 40℃.

[0159] Final purified KOH alkaline solution indicators:

[0160] (1) KOH concentration: 30.8 wt%

[0161] (2) Content of dissolved thermally stable salt: 0.2 wt% (total removal rate 94.3%);

[0162] (3) KOH retention rate: 88%;

[0163] (4) COD: 310 mg / L;

[0164] (5) Suspended solids: Not detected.

[0165] Effect of regenerated KOH alkali solution reuse: When regenerated KOH alkali solution is used to extract and treat liquefied petroleum gas (LPG), the total sulfur content of the LPG after treatment is 15-28 ppm, which is far better than the control requirement of 50 ppm.

[0166] Comparative Example 1 (no chemical precipitation of decarbonate)

[0167] The difference from Example 1 is that the activated carbon adsorbs directly into the filter without undergoing calcium hydroxide chemical precipitation.

[0168] Execution result:

[0169] (1) The proportion of carbonate in the heat-stable salt load of the ion exchange resin is still as high as 40% or more.

[0170] (2) Regeneration cycle of ion exchange resin: 14 h (25 h in Example 1)

[0171] (3) Chemical cleaning cycle of nanofiltration membrane: 36 h (48 h in Example 1)

[0172] (4) The final purified alkaline solution has a heat-stable salt content of 1.0 wt% and a refined liquefied petroleum gas sulfur content of 30-42 ppm (18-32 ppm in Example 1).

[0173] Comparative Example 1 demonstrates that chemical precipitation to remove carbonate ions can significantly reduce the load on ion exchange and nanofiltration, extend the operating cycle, and improve the purification depth.

[0174] Comparative Example 2 (insufficient calcium hydroxide)

[0175] The difference from Example 1 is that the amount of calcium hydroxide added is 0.7 times the theoretical amount.

[0176] Execution result:

[0177] (1) The carbonate removal rate was only 62%, with a residual carbonate content of 0.44 wt%;

[0178] (2) Ion exchange resin regeneration cycle: 17 h (lower than 25 h in Example 1);

[0179] (3) Calcium carbonate solid deposits appear on the concentrate side of the nanofiltration membrane. It needs to be disassembled and cleaned after 90 hours of operation.

[0180] Comparative Example 2 demonstrates that the amount of calcium hydroxide added needs to be more than 1.0 times the theoretical amount to ensure that carbonate ions are fully removed and to avoid scaling in subsequent units.

[0181] Comparative Example 3 (without distillation-nanofiltration parameter coupling)

[0182] The difference from Example 1 is that vacuum distillation was performed under conventional conditions (50 kPa absolute pressure, concentrated to 80% of the original volume). Testing showed no significant increase in the proportion of polyvalent anions in the concentrated alkali solution. The subsequent nanofiltration membrane only achieved a heat-stable salt rejection rate of 82%. The final purified alkali solution contained 0.21 wt% heat-stable salts, and the refined liquefied petroleum gas (LPG) contained 55-72 ppm sulfur, which could not be stably controlled below 50 ppm.

[0183] Comparative Example 4 (Preliminary filtration with heat-free stable salt)

[0184] The difference from Example 1 is that: after vacuum distillation, the concentrated alkaline solution containing solids is directly sent to the activated carbon adsorption unit without passing through the first filtration unit.

[0185] Execution result:

[0186] (1) The activated carbon filter experiences a sudden increase in pressure differential after 15 hours due to solid salt blockage, requiring frequent backwashing (Example 1: Activated carbon service life > 60 hours).

[0187] (2) The regeneration cycle of ion exchange resin is shortened to 6 h (14 h in Example 1).

[0188] (3) The cleaning cycle of nanofiltration membrane is shortened to 12 h (48 h in Example 1).

[0189] (4) Solid salt deposits in subsequent pipelines, resulting in three pipeline blockage incidents.

[0190] Comparative Example 5 (no filtration, no removal of activated carbon powder and calcium carbonate)

[0191] The difference from Example 1 is that after chemical precipitation, the material does not pass through a polypropylene meltblown filter element and directly enters the ion exchange resin tower.

[0192] Execution result:

[0193] (1) Activated carbon powder and calcium carbonate precipitate enter the ion exchange resin tower, causing the pressure drop of the resin bed to gradually increase;

[0194] (2) After 20 hours of operation, the pressure difference between the inlet and outlet of the resin tower increased from 0.05 MPa to 0.28 MPa, requiring backwashing;

[0195] (3) The resin exchange capacity decreased by approximately 28% (due to powder covering the exchange sites).

[0196] (4) Carbon powder and calcium carbonate deposits were detected on the surface of the nanofiltration membrane, and the cleaning cycle was shortened to 4 hours;

[0197] (5) Carbon powder and calcium carbonate deposits were detected on the surface of the nanofiltration membrane, leading to short-term clogging failure.

[0198] Comparative Example 5 demonstrates that the filtration unit is specifically designed to remove activated carbon shedding and calcium carbonate precipitate, effectively protecting the ion exchange resin and nanofiltration membrane and significantly extending their service life.

[0199] Comparative Example 6 (Conventional Combinations, No Collaborative Design – Used to Prove Non-Obviousness)

[0200] Following the conventional approach in this field, vacuum distillation, activated carbon filtration, ion exchange, and nanofiltration membrane are simply connected in series. Each unit adopts industry-standard parameters (distillation pressure 50 kPa, temperature 85℃, no heat-stable salt filtration, no chemical precipitation, no melt-blown filtration, ion exchange flow rate 5 BV / h, no reflux pipeline, no parameter coupling).

[0201] Execution result:

[0202] (1) The regeneration cycle of the ion exchange resin is 2 hours;

[0203] (2) The flux of the nanofiltration membrane decreased by 50% after 8 hours of operation and it was scrapped after 15 hours;

[0204] (3) The final purification of alkaline solution has a thermally stable salt removal rate of only 72%, and the sulfur content of refined liquefied gas is 65-85 ppm, which cannot be stably met.

[0205] Comparative Example 7 demonstrates that simple, non-optimal combinations of the units cannot achieve the technical effects of the present invention. The parameter coupling and collaborative design between the steps of the present invention produce a synergistic effect of "1+1>2", which is non-obvious.

[0206] Industrial applicability

[0207] The liquefied petroleum gas desulfurization deactivation alkaline solution regeneration method and system provided by this invention effectively solves industry problems such as difficulty in removing alkaline solution with thermal stability, large amount of alkaline residue discharge, and high operating costs through the synergistic effect of multiple innovative points, including four-stage desalination (solid filtration + chemical precipitation (decarbonization) + ion exchange + nanofiltration), distillation-nanofiltration parameter coupling, internal circulation of resin regeneration waste liquid, and optimization of nanofiltration membrane adaptability for NaOH / KOH dual system.

[0208] It has the following particular advantages:

[0209] (1) The chemical precipitation method for selectively removing carbonate ions can significantly reduce the ion exchange load and extend the operating cycle, while producing high-purity calcium carbonate as a byproduct, thus realizing resource utilization.

[0210] (2) The parameter coupling of vacuum distillation and nanofiltration, by changing the distribution of thermally stable salt ions, synergistically enhances the desalination efficiency of nanofiltration membrane;

[0211] (3) The closed-loop design achieves zero discharge of regenerated waste liquid and near-zero loss of alkali liquid.

[0212] This technology can be retrofitted onto existing LPG desulfurization units, with moderate investment (expected payback period of 1-1.5 years), low operating costs (overall operating costs reduced by more than 70%), broad industrial application prospects, and significant environmental benefits, aligning with the development direction of green chemistry and circular economy.

Claims

1. A method for regenerating deactivated alkaline solution in a liquefied petroleum gas extraction-oxidative desulfurization process, characterized in that, The regeneration process of deactivated alkali solution includes the following steps: Step 1, Concentration Pretreatment - Vacuum Distillation: The deactivated alkaline solution produced by the LPG desulfurization unit is drawn out and transported to the skid-mounted regeneration unit; firstly, the alkaline solution is subjected to vacuum distillation, with the absolute pressure controlled at 5-30 kPa and the temperature at 60-90℃, concentrating it to 1 / 3 to 1 / 2 of its original volume, to obtain concentrated alkaline solution containing heat-stable salt solids and distilled condensate. Step 2, Preliminary Removal of Heat-Stable Salts – First Filtration Unit: The concentrated alkaline solution described in Step 1 is passed through the first filtration unit to remove the heat-stable salt solids precipitated during vacuum distillation, resulting in a clear solution; Step 3, Organic matter removal – activated carbon adsorption: The clear liquid from step 2 is passed through an activated carbon adsorption unit to remove organic matter; Step 4, Chemical precipitation to remove carbonate ions – calcium hydroxide treatment: Add calcium hydroxide to the alkaline solution after activated carbon adsorption in Step 3 to convert the carbonate ions in the alkaline solution into calcium carbonate precipitate. The reaction temperature is 40-70℃, the reaction time is 10-60 minutes, and the amount of calcium hydroxide added is 1.0-1.5 times the theoretical amount. Step 5, Solid-Liquid Separation – Filtering: The alkaline solution after chemical precipitation in Step 4 is passed through a filtration unit to remove calcium carbonate precipitate and activated carbon sludge. Step 6, Heat-stable salt ion exchange removal – anion exchange resin: Pass the alkaline solution filtered in step 5 into a strong base anion exchange resin tower to selectively remove dissolved heat-stable salts. Step 7, Deep Desalination – Nanofiltration Membrane Separation: The alkaline solution after ion exchange in Step 6 is passed into the nanofiltration membrane separation unit for deep desalination to obtain regenerated alkaline solution.

2. The method according to claim 1, characterized in that, The operating parameters of the vacuum distillation step are matched with the molecular weight cutoff of the nanofiltration membrane separation unit: the distillation endpoint is controlled to increase the proportion of polyvalent anions in the dissolved thermally stable salt in the concentrated alkali solution to more than 60%, so that the nanofiltration membrane can achieve a rejection rate of ≥92% for polyvalent anions and a rejection rate of ≤30% for monovalent OH⁻.

3. The method according to claim 1, characterized in that, In step four, the chemical precipitation step, the reaction temperature is 40-70℃ (preferably 50-60℃); the reaction time is 10-60 minutes (preferably 20-30 minutes); the stirring speed is 50-200 rpm; and the amount of calcium hydroxide added is calculated based on the carbonate content in the alkaline solution, which is 1.0-1.5 times the theoretical amount (preferably 1.1-1.2 times).

4. The method according to claim 1, characterized in that, The anion exchange resin tower adopts a circulating regeneration method: the regeneration alkali solution is prepared from the distillation condensate obtained in step one, and the waste alkali solution after regeneration is returned to the inlet of the vacuum distillation step through the reflux pipeline.

5. The method according to claim 1, characterized in that, When the deactivating alkaline solution is a sodium hydroxide solution, the nanofiltration membrane separation unit uses a polyamide composite membrane with a molecular weight cutoff of 150-250 Da and an operating pressure of 1.5-3.0 MPa; when the deactivating alkaline solution is a potassium hydroxide solution, the nanofiltration membrane separation unit uses a nanofiltration membrane with a K⁺ rejection rate ≤20% and a polyvalent anion rejection rate ≥92%, an operating pressure of 1.2-2.2 MPa, and an operating temperature of 35-45℃.

6. A high-efficiency regeneration system for liquefied petroleum gas desulfurization deactivation alkaline solution used in implementing the method according to any one of claims 1-5, characterized in that, The system comprises, in sequence, a vacuum distillation tower, a first filtration unit, an activated carbon filter, a chemical precipitation reactor, a cartridge filter, an anion exchange resin tower, and a nanofiltration membrane assembly, all connected by pipes. The concentrate side of the nanofiltration membrane assembly is connected to the inlet of the vacuum distillation tower via a reflux pipeline. The regeneration waste liquid outlet of the anion exchange resin tower is also connected to the inlet of the vacuum distillation tower via a reflux pipeline.

Images