A method for removing organic acids from water of a fischer-tropsch synthesis

Abstract

The application provides a method for removing organic acid in Fischer-Tropsch synthesis water, and the Fischer-Tropsch synthesis water containing the organic acid is treated by using an electrodialysis device, and the treatment conditions include: controlling the membrane voltage to be 1V-10V, and / or controlling the current density to be 1mA / cm 2 ~30mA / cm 2 The treatment method can effectively reduce the energy consumption of the wastewater treatment unit flow, and can produce great economic benefits.

Description

Technical Field

[0001] This invention relates to the field of Fischer-Tropsch synthesis water treatment technology, and more specifically to a method for removing organic acids from Fischer-Tropsch synthesis water. Background Technology

[0002] In current coal indirect liquefaction projects, Fischer-Tropsch syngas water is produced during the Fischer-Tropsch reaction of syngas to generate synthetic oil, with a water-to-oil ratio of approximately 1:1.2. This Fischer-Tropsch syngas water contains about 1% organic acids and about 5% other oxygen-containing organic compounds, including alcohols, acids, aldehydes, and ketones, which accumulate to a considerable amount. Water treatment units are typically installed to purify and recover these oxygen-containing organic compounds. To prevent the organic acids in the Fischer-Tropsch water from directly entering the distillation column and causing corrosion, existing treatment processes use alkali neutralization at the front end to remove acid, adding inorganic alkali to maintain a near-neutral pH for subsequent distillation. However, neutralization is costly, and the organic acid salts produced during neutralization require further hazardous waste disposal at the end.

[0003] Patent CN103435211A discloses a method for removing organic acids and purifying and recovering other oxygenated organic compounds from Fischer-Tropsch synthesis water. The patent employs an alkali-addition method for neutralizing and deacidifying organic acids, with the resulting carboxylates then removed via resin or membrane methods. Other oxygenated organic compounds are separated and recovered using distillation. The organic acid neutralization and removal method for Fischer-Tropsch synthesis water described in patent CN103435211A requires the addition of a large amount of inorganic alkali to the Fischer-Tropsch synthesis water for an acid-base neutralization reaction, resulting in Fischer-Tropsch synthesis water containing oxygenated organic compounds and carboxylates. The carboxylates generated during neutralization need to be removed in the water recovery process using ion exchange or membrane methods. Furthermore, the generated carboxylates are hazardous waste, requiring additional disposal costs. The neutralization reagent consumption and hazardous waste treatment costs associated with this process are extremely high.

[0004] Patent CN10239886A discloses a method for separating and recovering organic matter from water in high-temperature Fischer-Tropsch synthesis reactions. The patent effectively and systematically separates and recovers acetaldehyde, acetone, butanone, ethanol, and n-propanol from the water in these reactions, achieving a purity of over 95% and a yield of over 90% for each product. The initial process involves using an acid distillation column to separate organic acids and alcohols (C4 and above) from the Fischer-Tropsch synthesis water, followed by further purification and recovery. However, due to the high acidity of the Fischer-Tropsch synthesis water and the large volume of water directly entering the acid distillation column, the column's capacity and material requirements are very high, necessitating significant equipment investment for this method.

[0005] Patent CN105819604A discloses a water treatment system and method for separating organic acids from Fischer-Tropsch synthesis wastewater. The organic acids are separated from other oxygen-containing organic compounds via electrodialysis, thereby recovering the oxygen-containing organic compounds, increasing the added value of the Fischer-Tropsch synthesis process, and effectively solving the corrosion problem of organic acids on downstream equipment. While patent CN105819604A discloses the electrodialysis method for separating organic acids from Fischer-Tropsch synthesis wastewater, it only discloses the removal rate of organic acids by the electrodialysis method; the specific operational procedures are unclear.

[0006] In existing technologies, there are few reports on the application of electrodialysis to ion migration in weak electrolytes. The main reason is that weak electrolytes have low dissociation degrees, resulting in low ionic strength in the solution containing the migrating ions, poor ion loading current capacity, and low per-membrane throughput. Therefore, this technology has low efficiency in removing organic acids from Fischer-Tropsch water. Mechanistically, the organic acids in Fischer-Tropsch water are formic acid, acetic acid, propionic acid, butyric acid, and valeric acid, with acetic acid having the highest content, approximately 75%. Taking acetic acid as an example, analyzing the ionization of organic acids, the dissociation constant Ka of acetic acid is 1.8 × 10^6. -5 When the concentration of organic acid in Fischer-Tropsch water is 0.84 wt%, if we approximate it to the concentration of acetic acid, then a 0.84% ​​acetic acid solution has a concentration of approximately 0.14 mol / L. Therefore, the concentration of acetic acid in its molecular state can be calculated to be 0.0016 mol / L, meaning that ionized acetic acid accounts for 1.14% of the total acetic acid. Thus, the electro-driven ion migration of organic acid in Fischer-Tropsch water results in a very low concentration of ionized ions. Consequently, the ionic strength of organic acid in Fischer-Tropsch water and the electro-membrane treatment capacity of the equipment need to be improved.

[0007] Electrodialysis utilizes the selective permeability of ion-exchange membranes to cations and anions. Under the influence of a direct current electric field, cations and anions migrate in a directional manner, thereby achieving the separation, purification, and concentration of electrolyte solutions. Therefore, ion-exchange membranes and direct current are two essential conditions for the electrodialysis process.

[0008] Currently, the most common and basic application of electrodialysis technology is for the desalination or concentration of aqueous solutions. However, there are few reports on its application to the ion migration of weak electrolytes. This is mainly because weak electrolytes have low dissociation degrees, resulting in low ionic strength in the solution containing the migrating ions, poor ion loading current capacity, and low membrane throughput. Consequently, this technology is inefficient in removing organic acids from Fischer-Tropsch water. Therefore, this invention primarily addresses the problem of low ionization degrees and enhanced ionization of organic acids, which are weak electrolytes, in the removal of organic acids from Fischer-Tropsch synthesis water using electro-driven membranes. Summary of the Invention

[0009] In view of the problems existing in the prior art, the object of the present invention is to provide a method for removing organic acids from Fischer-Tropsch synthesis water.

[0010] The inventors of this application discovered in their research that, in the application of electrodialysis technology, to avoid concentration polarization and reduce current utilization efficiency, it is generally necessary to first explore the limiting current density of the wastewater to be treated. Concentration polarization is of great significance to the operation of electrodialysis because it theoretically requires electrodialysis to operate at low current densities, thus greatly limiting the unit processing capacity of electrodialysis equipment. Concentration polarization is a special phenomenon occurring at the membrane-solution interface, where the balance between ion input and output is disrupted. Essentially, when the current density increases to a certain value, the amount of ions input at the interface is less than the amount output, and the ions are depleted at this interface. The moment when the ion concentration at the membrane-solution interface is 0 is the limiting current density. Therefore, in the limiting current density curves of common inorganic salts, the resistance suddenly increases near the limiting current density point. When the limiting current density is exceeded, a very large potential drop occurs through the interface, forcing water molecules to dissociate. The OH- produced by water dissociation... - and H + It acts as a current carrier, passing through the ion exchange membrane, and its resistance decreases accordingly after water dissociation. This is because the current is carried by the OH groups. - and H + As a carrier, it is ineffectively consumed, reducing current efficiency and causing polyvalent ion precipitation and scaling. Therefore, in the prior art, when using the electrodialysis process, the load is often avoided to exceed the limit current density, that is, the electrodialysis device is operated below the limit current density.

[0011] Wherein, current density refers to the current intensity per unit effective area of ​​the electrodialysis ion exchange membrane, expressed in mA / cm². 2 It indicates that current efficiency is the percentage of ions actually removed by an electrodialysis device relative to the theoretically removed ions.

[0012] Contrary to existing technologies, this invention discovers that when using an electrodialysis unit to treat Fischer-Tropsch synthesis water, operating the unit above its limiting current density can actually reduce the power consumption per ton of water treated per unit time for the same membrane area. Based on this, this invention is proposed.

[0013] The technical solution of the present invention includes:

[0014] A method for removing organic acids from Fischer-Tropsch synthesis water, characterized by using an electrodialysis device to treat the Fischer-Tropsch synthesis water containing organic acids, wherein the treatment conditions include: controlling the membrane voltage to be 1V to 10V, and / or controlling the current density to be 1mA / cm². 2 ~30mA / cm 2 .

[0015] In some preferred embodiments of the present invention, the processing conditions include: controlling the membrane voltage to be 2.5V to 6V, and / or controlling the current density to be 8mA / cm². 2 ~30mA / cm 2 .

[0016] In some specific embodiments of the present invention, the control membrane voltage is 1V, 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V and any value between them.

[0017] In some specific embodiments of the present invention, the current density is controlled to be 1 mA / cm². 2 2mA / cm 2 3mA / cm 2 4mA / cm 2 5mA / cm 2 6mA / cm 2 7mA / cm 2 8mA / cm 2 9mA / cm 2 10mA / cm 2 11mA / cm 2 12mA / cm 2 13mA / cm 2 14mA / cm 2 15mA / cm 2 16mA / cm 2 17mA / cm 2 18mA / cm 2 19mA / cm 2 20mA / cm 2 21mA / cm 2 22mA / cm 2 23mA / cm 2 24mA / cm 2 25mA / cm 2 26mA / cm 2 27mA / cm 2 28mA / cm 2 29mA / cm 2 30mA / cm 2 And any values ​​in between.

[0018] In some preferred embodiments of the present invention, the processing conditions further include controlling the circulation flow rate of the Fischer-Tropsch synthesis water to be 10-100 L / h, preferably 20-80 L / h.

[0019] According to the present invention, the treated Fischer-Tropsch synthesis water flows through the membrane stack in a multi-cycle operation, with some feed liquid being replenished and some feed liquid being discharged. The specific process involves concentration control according to different water quality requirements.

[0020] In some preferred embodiments of the present invention, the processing conditions further include: the operating temperature of the electrodialysis device is 5°C to 30°C.

[0021] According to the present invention, an operating temperature of 5°C to 30°C can effectively improve the service life of the membrane stack.

[0022] In some preferred embodiments of the present invention, the treatment conditions further include: the endpoint of the treatment is that the content of organic acids in the Fischer-Tropsch synthesis water is <0.1 wt%.

[0023] In some preferred embodiments of the present invention, the organic acid is selected from at least one of formic acid, acetic acid, propionic acid, butyric acid, and valeric acid.

[0024] According to the present invention, the method provided by the present invention can not only treat Fischer-Tropsch synthesis water containing the above-mentioned organic acids, but is also suitable for separating or purifying weakly acidic electrolytes.

[0025] In some preferred embodiments of the present invention, the organic acid has a mass concentration of 0.5 wt% to 5 wt% in the Fischer-Tropsch synthesis water.

[0026] In some specific embodiments of the present invention, the organic acid in the Fischer-Tropsch synthesis water has a mass concentration of 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%, 4.5 wt%, 5.0 wt%, and any value between them.

[0027] In some preferred embodiments of the present invention, the Fischer-Tropsch synthesis water containing organic acids is filtered before entering the electrodialysis device. Preferably, the filtration process includes passing the Fischer-Tropsch synthesis water containing organic acids through a 20-mesh to 100-mesh filter.

[0028] In some preferred embodiments of the present invention, the Fischer-Tropsch synthesis water containing organic acids is introduced into the dilute chamber of the electrodialysis device, and a receiving liquid is introduced into the concentrate chamber. Preferably, the receiving liquid is deionized water.

[0029] According to the present invention, a trace amount of electrolyte can be added to the concentration chamber to reduce the power consumption of the entire system, wherein the electrolyte is preferably an inorganic salt electrolyte.

[0030] In some preferred embodiments of the present invention, the membrane stack in the electrodialysis device is a homogeneous ion exchange membrane; and / or the electrodes of the electrodialysis device are titanium-coated ruthenium electrodes.

[0031] The beneficial effects of this invention are at least in the following aspects:

[0032] (1) From a technical perspective, electrodialysis technology removes organic acids from Fischer-Tropsch water in one step. Compared with the neutralization method, it does not consume high amounts of chemicals and does not generate hazardous waste of organic acid salts. At the same time, it realizes the recovery of organic acid by-products.

[0033] (2) From the perspective of equipment, conventional electrodialysis devices can be used, with a traditional two-compartment structure, which is simple to operate and control.

[0034] (3) From a process perspective, the limiting current density test curve of Fischer-Tropsch synthesis water was provided, and its special characteristics were found. Combined with the different variation law of high current values ​​compared with inorganic salts, and by increasing the operating current density above the inflection point, combined with current efficiency and power consumption, it serves as the basis for deciding the current density operating range of electrodialysis. Attached Figure Description

[0035] Figure 1 These are UI curves for limiting current density tests of different systems.

[0036] Figure 2 This is a UI curve of the limiting current density test under different circulation flow rates in conventional electrodialysis.

[0037] Figure 3 This is a graph showing the trend of current efficiency in ion-medium electrodialysis as a function of current density.

[0038] Figure 4 This is a graph showing the trend of power consumption in ion-conducting electrodialysis as a function of current density.

[0039] Figure 5 This is a graph showing the trend of electricity consumption per ton of water as a function of current density under the same membrane area in ion-conducting electrodialysis. Detailed Implementation

[0040] The present invention will be described in detail below through embodiments, but the scope of protection of the present invention is not limited to the following description.

[0041] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0042] like Figure 1As shown, limiting current density tests were conducted on Fischer-Tropsch synthesis water in an organic acid-weak electrolyte system. Electrodialysis UI curve tests revealed its unique characteristics: specifically, as current density increases, the rate of increase in the overall voltage of the Fischer-Tropsch water system decreases after a certain point, exhibiting a clear inflection point. This differs from the behavior of inorganic salts with high current values. Taking sodium chloride as an example, as current density increases, the rate of increase in the overall voltage of the sodium chloride system increases after a certain point, also exhibiting a clear inflection point. Increasing the operating current density above the inflection point, combined with current efficiency, served as the basis for determining the operating current density range for electrodialysis. In the limiting current density curves of common inorganic salts, the resistance suddenly increases near the limiting current density point. When the limiting current density is exceeded, a very large potential drop occurs at the interface, forcing water molecules to dissociate. The OH- and H+ ions produced by water dissociation act as current carriers, and the resistance value decreases accordingly after water dissociation.

[0043] in, Figure 1 The results were obtained by measuring the limiting current density of Fischer-Tropsch synthesis water using a homogeneous ion exchange membrane prepared by a certain company at 25°C and a circulation flow rate of 50 L / h.

[0044] In short, according to Figure 1 It can be observed that the voltage increase caused by strong electrolytes exceeding the limiting current density is not significant; instead, an inflection point appears where the voltage increase decreases. This inflection point is approximately at 5 mA / cm². 2 This is a new discovery that organic acids, which are weak electrolytes, differ from strong electrolytes.

[0045] Figure 2 As can be seen, the limiting current density (UI) curves under different circulation flow rates also exhibit similar patterns. Similarly, taking ion-conducting medium-filled electrodialysis as an example, UI curve measurements revealed the same phenomenon, with the current density inflection point around 7 mA / cm². This inflection point is larger than that of traditional electrodialysis resin because the addition of an ion-conducting medium reduces the overall resistance of the system, allowing the membrane stack to provide a larger current at the same voltage. This structure offers energy efficiency advantages in practical applications.

[0046] Taking ion-conducting medium-filled electrodialysis as an example, Figure 3 Figure 1 shows the trend of current efficiency as a function of current density, and Figure 4 shows the trend of power consumption as a function of current density. A range of 3-18 mA / cm² was selected. 2 Experiments were conducted using 12 sets of current densities. This range was selected based on the rectifier's range and the power consumption level from the perspective of electrodialysis applications, and a comprehensive analysis of the specific deacidification efficiency of electrodialysis was performed. The results are as follows: Figure 4 As shown. Single-variable adjustment of current density across the inflection point from 3 mA / cm² 2 Increased to 18 mA / cm 2Twelve constant current experiments were conducted. The results showed that the current efficiency remained around 90% across the entire current density range. Furthermore, the rate of increase in power consumption gradually slowed down across the entire tested current density range, which is consistent with... Figure 4 trend.

[0047] It is evident that a certain increase in current density accelerates the ionization of organic acids in Fischer-Tropsch water within the system, and this change is beneficial, promoting the electromigration of organic acid ions. Simultaneously, this increase in current density, within a certain range, does not lead to significant concentration polarization at the interface, meaning it does not cause a significant decrease in current efficiency. From an energy consumption perspective, a certain increase in current density does increase system energy consumption, consistent with Ohm's law. However, this invention finds that the operating trend of the electrodialysis device above the limiting current density gradually slows down, which is equivalent to a certain increase in current density. This translates to a decrease in energy consumption per ton of Fischer-Tropsch water per unit time for the same membrane area, consistent with... Figure 5 The trend is clear. Increasing current density directly improves the unit membrane throughput, significantly reducing the investment cost of ion exchange membranes and related systems. Specific applications should be considered in conjunction with power consumption levels, as these directly determine the operating cost of electrodialysis for removing organic acids.

[0048] In the following embodiments, the water quality of the treated Fischer-Tropsch synthesis water is shown in Table 1 below.

[0049] Table 1

[0050] project Content / mg / L Formic acid 121 Acetic acid 6589 propionic acid 902 butyric acid 188 Valeric acid 119

[0051] Example 1

[0052] An electrodialysis unit using a dilute chamber as the ion-conducting medium was used to treat Fischer-Tropsch synthesis water with the composition shown in Table 1. The electrodialysis unit was operated under conditions not exceeding the limiting current density, as shown in Table 2. The treatment results are also shown in Table 2.

[0053] Example 2

[0054] An electrodialysis unit using a dilute chamber as the ion-conducting medium was used to treat Fischer-Tropsch synthesis water with the composition shown in Table 1. The electrodialysis unit was operated at a current density exceeding its limiting limit, and the specific operating conditions are shown in Table 2. The treatment results are also shown in Table 2.

[0055] Example 3

[0056] Fischer-Tropsch synthesis water with the components listed in Table 1 was treated using conventional electrodialysis. The electrodialysis unit was operated under conditions not exceeding the limiting current density, as shown in Table 2. The treatment results are also shown in Table 2.

[0057] Example 4

[0058] Fischer-Tropsch synthesis water with the composition shown in Table 1 was treated using conventional electrodialysis. The electrodialysis unit was operated at a current density exceeding the limiting current density, and the specific operating conditions are shown in Table 2. The treatment results are also shown in Table 2.

[0059] Comparative Example 1

[0060] A 0.8 wt% sodium chloride aqueous solution was treated using conventional electrodialysis. The electrodialysis unit was operated under conditions not exceeding the limiting current density, as shown in Table 2. The treatment results are also shown in Table 2.

[0061] Comparative Example 2

[0062] A 0.8 wt% sodium chloride aqueous solution was treated using conventional electrodialysis. The electrodialysis unit was operated at a current density exceeding its limiting limit; specific operating conditions are shown in Table 2, and the treatment results are also shown in Table 2.

[0063] Comparative Example 3

[0064] An 8 wt% sodium chloride aqueous solution was treated using conventional electrodialysis. The electrodialysis unit was operated under conditions not exceeding the limiting current density, as shown in Table 2. The treatment results are also shown in Table 2.

[0065] Table 2

[0066]

[0067]

[0068] Note: In the table above, "ED" refers to electrodialysis.

[0069] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A method for removing organic acids from Fischer-Tropsch synthesis water, characterized in that, Electrodialysis is used to treat Fischer-Tropsch synthesis water containing organic acids. The treatment conditions include: controlling the membrane voltage to be 2.5V~6V, or the treatment conditions include: controlling the membrane voltage to be 2.5V~6V, and controlling the current density above the limiting current density and at a current density of 8mA / cm². 2 ~30mA / cm 2 .

2. The method according to claim 1, characterized in that, The processing conditions also include controlling the circulation flow rate of the Fischer-Tropsch synthesis water to be 10~100L / h.

3. The method according to claim 2, characterized in that, The circulation flow rate of the Fischer-Tropsch synthesis water is controlled to be 20~80 L / h.

4. The method according to any one of claims 1-3, characterized in that, The conditions for the treatment also include: the operating temperature of the electrodialysis device is 5℃~30℃.

5. The method according to any one of claims 1-3, characterized in that, The conditions for the treatment also include: the endpoint of the treatment is that the content of organic acids in the Fischer-Tropsch synthesis water is <0.1 wt%.

6. The method according to any one of claims 1-3, characterized in that, The organic acid is selected from at least one of formic acid, acetic acid, propionic acid, butyric acid, and valeric acid.

7. The method according to any one of claims 1-3, characterized in that, In the Fischer-Tropsch synthesis water, the mass concentration of the organic acid is 0.5 wt% to 5 wt%.

8. The method according to any one of claims 1-3, characterized in that, The Fischer-Tropsch synthesis water containing organic acids is filtered before entering the electrodialysis device.

9. The method according to claim 8, characterized in that, The filtration process includes passing the Fischer-Tropsch synthesis water containing organic acids through a 20-100 mesh filter.

10. The method according to any one of claims 1-3, characterized in that, The Fischer-Tropsch synthesis water containing organic acids is introduced into the dilute chamber of the electrodialysis device, and the receiving liquid is introduced into the concentrate chamber.

11. The method according to claim 10, characterized in that, The receiving liquid is deionized water.

12. The method according to any one of claims 1-3, characterized in that, The membrane stack in the electrodialysis device is a homogeneous ion exchange membrane; and / or the anode of the electrodialysis device is a titanium-coated ruthenium electrode, and the cathode is a graphite electrode.

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