A method for preparing an adsorbent for tail gas produced by MDI

Abstract

The invention discloses a method for preparing an adsorbent for tail gas produced by MDI, wherein the adsorbent is prepared by ultrasonic mixing and vacuum drying of a mesoporous graphene phase carbon nitride (g-C3N4) carrier and a 4-aminopyridine reagent in an N,N-dimethylformamide solution after thermal exfoliation. The adsorbent for tail gas treatment of an MDI vacuum unit of the invention has the following advantages: the adsorbent has a large adsorption capacity, high adsorption selectivity, mild adsorption conditions, simplifies the tail gas treatment process, and improves the operating stability of the vacuum unit.

Description

Technical Field

[0001] The invention relates to the field of distillation tower tail gas treatment, and in particular to a method for preparing an adsorbent for MDI production tail gas. Background Art

[0002] Diphenylmethane diisocyanate (MDI) is a polyurethane raw material with excellent performance and wide application. At present, the main production process of MDI is phosgenation. First, aniline and formaldehyde are condensed to form polyamine (DAM), then DAM reacts with phosgene to form crude MDI, and finally, crude MDI is distilled to prepare pure MDI and polymerized MDI.

[0003] Since MDI is a heat-sensitive material, the crude MDI distillation needs to be carried out under high vacuum conditions, and the top of the distillation tower is directly connected to the vacuum unit. During the crude MDI distillation process, a small amount of light component impurities such as PI (phenyl isocyanate) and MDI will enter the vacuum unit in the form of gas phase, and PI and MDI are prone to liquefaction and solidification in the vacuum pump, causing the unit to block and lock, affecting the long-term stable operation of the MDI vacuum unit.

[0004] To avoid the above effects, vacuum units are generally designed with condensers when leaving the factory. However, the design of the condensers in the prior art cannot completely condense the MDI and PI in the exhaust gas, and the residual MDI and PI in the exhaust gas will cause the unit to frequently become blocked and locked. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing an efficient adsorbent for treating tail gas from MDI production. The adsorbent has the advantages of large adsorption capacity and high selectivity, simplifies the treatment process of tail gas from a vacuum unit, and realizes stable operation of the vacuum unit.

[0006] In order to achieve the above invention object, the present invention adopts the following technical solutions:

[0007] A method for preparing an adsorbent for treating tail gas from MDI production comprises the following steps:

[0008] 1) preparing mesoporous graphene phase carbon nitride (g-C3N4) as a carrier;

[0009] 2) The carrier prepared in step 1) is mixed with oxalic acid and placed in a tubular furnace in a nitrogen atmosphere, heated to 400-500°C at a heating rate of 10-20°C / min, kept at a constant temperature for 3-7h, and cooled to obtain thermally exfoliated g-C3N4.

[0010] 3) The thermally stripped g-C3N4 carrier prepared in step 2) and the 4-aminopyridine reagent are added to DMF and ultrasonicated for a period of time, and then vacuum dried to obtain an adsorbent.

[0011] In a specific embodiment, the precursor for preparing the mesoporous graphene phase carbon nitride (g-C3N4) in step 1) is not particularly limited, as long as the carrier required by the present invention can be prepared. Specifically, it can be prepared by a method recognized by the industry, for example, the preparation method in "Chemical Synthesis and Application of Graphite Phase Carbon Nitride" in Acta Physico-Chimica Sinica (DOI: 10.3866 / PKU.WHXB201306173), wherein the carbon source and nitrogen source for preparing the mesoporous graphene phase carbon nitride (g-C3N4) include melamine, cyanamide, urea, etc., but are not limited thereto. Preferably, the thermal polymerization temperature for preparing the mesoporous graphene phase carbon nitride (g-C3N4) is 500-600°C.

[0012] In a specific embodiment, the mass ratio of the carrier to oxalic acid in step 2) is 1:0.2-1, preferably, the mass ratio of the carrier to oxalic acid is 1:0.3-0.6, for example including but not limited to 1:0.2, 1:0.4, 1:0.6, 1:0.8, 1:1;

[0013] The constant temperature of step 2) is 400-500° C., including but not limited to 400° C., 420° C., 440° C., 460° C., 480° C., and 500° C. The constant temperature time is 3-7 hours, including but not limited to 3 hours, 4 hours, 5 hours, 6 hours, and 7 hours.

[0014] In a specific embodiment, the mass ratio of the g-C3N4 carrier to 4-aminopyridine after thermal stripping in step 3) is 1:0.1-1, for example, but not limited to 1:0.1, 1:0.3, 1:0.5, 1:0.8, 1:1;

[0015] Wherein, in step 3), the mixing conditions of the thermal stripping carrier and the 4-aminopyridine reagent in DMF are: the ultrasonic temperature is 40-70°C, for example, but not limited to 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, and the ultrasonic time is 0.5-3h, for example, but not limited to 0.5h, 1h, 1.5h, 2h, 2.5h, 3h; the preferred ultrasonic temperature is 50-60°C, and the preferred ultrasonic time is 1.5-3h;

[0016] After the ultrasonic treatment is completed, the mixed solution is subjected to a period of vacuum drying treatment to remove the DMF solvent in the mixed solution. Specifically, the vacuum degree is 50-80Kpa, the drying temperature is 80-110°C, and the drying time is 8-16h; preferably, the vacuum degree is 70-75Kpa, the drying temperature is 90-100°C, and the drying time is 10-12h;

[0017] The solid powder obtained after vacuum drying is ground to obtain a g-C3N4 and 4-aminopyridine composite adsorbent.

[0018] In another aspect of the present invention, there is provided an adsorbent prepared according to the above method for treating tail gas from MDI production.

[0019] Another aspect of the present invention provides the use of the above-mentioned adsorbent in the treatment of tail gas from MDI production.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The adsorbent for exhaust gas treatment of MDI vacuum units of the present invention uses mesoporous graphene phase carbon nitride (g-C3N4) as a carrier, which is mixed with oxalic acid. After high-temperature thermal stripping, the pore structure of the g-C3N4 carrier is richer, the specific surface area is significantly improved, and the adsorption capacity is also significantly increased.

[0022] (2) The adsorbent of the present invention is used for treating the tail gas of the MDI vacuum unit. The combination of g-C3N4 after thermal stripping and 4-aminopyridine greatly increases the amino groups on the surface of the adsorbent, thereby effectively improving the adsorption amount of organic matter such as MDI, PI, MCB in the tail gas of the MDI vacuum unit by the adsorbent, and at the same time also improving the adsorption selectivity of the adsorbent for PI in the tail gas. DETAILED DESCRIPTION

[0023] The technical scheme of the present invention and its effects are further described below by specific examples. The following examples are only used to illustrate the content of the present invention and are not used to limit the protection scope of the present invention. Simple changes made to the present invention by applying the concept of the present invention are all within the scope of the protection claimed by the present invention.

[0024] Unless otherwise specified, the raw materials used in the following examples or comparative examples can be obtained from commercial channels.

[0025] Preparation example: Preparation of adsorbent carrier mesoporous g-C3N4

[0026] The use of thermal polymerization to induce the precursor to undergo polycondensation to form g-C3N4 is a relatively simple preparation method. The operation is as follows: first, a certain mass of melamine precursor is placed in a ceramic crucible, and then placed in a tubular furnace in a nitrogen atmosphere for calcination at a temperature of 550°C for 5 hours. After natural cooling, the product is taken out and ground to obtain mesoporous g-C3N4.

[0027] Example 1: Preparation of adsorbent #1

[0028] Take 20 g of the carrier obtained in the preparation example and mix it with 6 g of oxalic acid, heat it to 500 ° C in a tube furnace at a heating rate of 10 ° C / min, keep the temperature constant for 3 hours, and obtain the exfoliated g-C3N4 after cooling; take 20 g of the g-C3N4 carrier after oxalic acid stripping and 6 g of 4-aminopyridine, put them into 50 g of DMF for ultrasonic mixing, the ultrasonic temperature is 60 ° C, and the ultrasonic time is 1.5 hours; the mixed solution is placed in a vacuum drying oven for drying, the vacuum degree is 70 Kpa, the drying temperature is 90 ° C, and the drying time is 10 hours, and 1# adsorbent is obtained.

[0029] Example 2: Preparation of adsorbent #2

[0030] Take 20 g of the carrier obtained in the preparation example and mix it with 8 g of oxalic acid, heat it to 500 ° C in a tube furnace at a heating rate of 15 ° C / min, keep the temperature for 5 hours, and obtain the stripped g-C3N4 after cooling; take 20 g of the g-C3N4 carrier stripped by oxalic acid and 10 g of 4-aminopyridine, put it into 50 g of DMF for ultrasonic mixing, the ultrasonic temperature is 60 ° C, and the ultrasonic time is 2 hours; the mixed solution is placed in a vacuum drying oven for drying, the vacuum degree is 70 Kpa, the drying temperature is 90 ° C, and the drying time is 10 hours, and 2# adsorbent is obtained.

[0031] Example 3: Preparation of 3# adsorbent

[0032] Take 20 g of the carrier obtained in the preparation example and mix it with 10 g of oxalic acid, heat it to 420 ° C in a tube furnace at a heating rate of 15 ° C / min, keep the temperature constant for 7 hours, and obtain the stripped g-C3N4 after cooling; take 20 g of the g-C3N4 carrier stripped by oxalic acid and 12 g of 4-aminopyridine, put them into 50 g of DMF for ultrasonic mixing, the ultrasonic temperature is 50 ° C, and the ultrasonic time is 2.5 hours; the mixed solution is placed in a vacuum drying oven for drying, the vacuum degree is 70 Kpa, the drying temperature is 90 ° C, and the drying time is 10 hours, to obtain 3# adsorbent.

[0033] Example 4: Preparation of 4# adsorbent

[0034] Take 20 g of the carrier obtained in the preparation example and mix it with 10 g of oxalic acid, heat it to 450 ° C in a tube furnace at a heating rate of 15 ° C / min, keep the temperature constant for 5 hours, and obtain the exfoliated g-C3N4 after cooling; take 20 g of the g-C3N4 carrier after oxalic acid stripping and 15 g of 4-aminopyridine, put them into 50 g of DMF for ultrasonic mixing, the ultrasonic temperature is 55 ° C, and the ultrasonic time is 3 hours; the mixed solution is placed in a vacuum drying oven for drying, the vacuum degree is 70 Kpa, the drying temperature is 90 ° C, and the drying time is 10 hours, and 4# adsorbent is obtained.

[0035] Example 5: Preparation of 5# adsorbent

[0036] Take 20 g of the carrier obtained in the preparation example and mix it with 12 g of oxalic acid, heat it to 500 ° C in a tube furnace at a heating rate of 15 ° C / min, keep the temperature for 5 hours, and obtain the stripped g-C3N4 after cooling; take 20 g of the g-C3N4 carrier stripped by oxalic acid and 18 g of 4-aminopyridine, put them into 50 g of DMF for ultrasonic mixing, the ultrasonic temperature is 60 ° C, and the ultrasonic time is 3 hours; the mixed solution is placed in a vacuum drying oven for drying, the vacuum degree is 70 Kpa, the drying temperature is 90 ° C, and the drying time is 10 hours, and 5# adsorbent is obtained.

[0037] Comparative Example 1: Preparation of 6# adsorbent

[0038] Take 20 g of the g-C3N4 carrier obtained in the preparation example and heat it to 500°C in a tube furnace at a heating rate of 15°C / min and keep the temperature constant for 5 hours. After cooling, obtain thermally stripped g-C3N4 and obtain adsorbent #6.

[0039] Comparative Example 2: Preparation of 7# adsorbent

[0040] Take 20 g of the carrier obtained in the preparation example and mix it with 8 g of oxalic acid, heat it to 450°C in a tube furnace at a heating rate of 15°C / min, keep the temperature constant for 5 hours, and obtain the exfoliated g-C3N4 after cooling, and obtain adsorbent #7.

[0041] Comparative Example 3: Preparation of 8# adsorbent

[0042] Take 20g of the g-C3N4 carrier obtained in the preparation example and 10g of 4-aminopyridine, put them into 50g of DMF for ultrasonic mixing, the ultrasonic temperature is 60°C, and the ultrasonic time is 1.5h; the mixture is placed in a vacuum drying oven for drying, the vacuum degree is 70Kpa, the drying temperature is 90°C, and the drying time is 10h, to obtain adsorbent #8.

[0043] The tail gas is obtained through laboratory configuration, which is close to the composition of MDI industrial tail gas. The concentrations of MCB, PI, MDI, etc. in the tail gas are determined by gas chromatography; the test method is: take 10g of adsorbent in the adsorption column, and the MDI tail gas is heated for 20h. -1 The air velocity is passed and the mass concentration of the tail gas before and after adsorption is analyzed.

[0044] The MDI tail gas treatment results of different adsorbents are shown in Table 1.

[0045] Table 1 MDI tail gas treatment results with different adsorbents

[0046] Non-condensable gas% MDI% PI% MCB% Initial exhaust gas composition 25.00 1.50 5.00 68.50 1# adsorbent after adsorption 42.01 0.08 0.44 57.47 2# adsorbent after adsorption 41.02 0.06 0.51 58.41 3# adsorbent after adsorption 41.79 0.05 0.48 57.68 4# adsorbent after adsorption 41.95 0.07 0.42 57.56 5# adsorbent after adsorption 41.57 0.03 0.20 58.20 6# adsorbent after adsorption 28.84 1.15 3.11 66.90 7# adsorbent after adsorption 33.88 1.08 2.71 62.33 8# adsorbent after adsorption 36.42 0.22 2.18 61.18

[0047] It can be seen from adsorbent #1-5 that the MDI content in the MDI tail gas treated with the g-C3N4 and 4-aminopyridine composite adsorbent is less than 0.1%, and the PI content is less than 1%, and the adsorption effect is excellent.

[0048] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as limiting the present invention. Those skilled in the art will appreciate that, under the guidance of this specification, some modifications or adjustments may be made to the present invention. These modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A method for preparing an adsorbent, characterized in that: The following steps are involved: 1) Mesoporous graphene-phase carbon nitride is used as a carrier, mixed with oxalic acid, heated for a period of time, and then cooled to obtain thermally exfoliated g-C3N4; 2) The thermally stripped g-C3N4 is mixed with 4-aminopyridine and dried to obtain an adsorbent.

2. The preparation method according to claim 1, characterized in that: In step 1), the mass ratio of mesoporous graphene phase carbon nitride to oxalic acid is 1:0.2-1.

3. The preparation method according to claim 1, characterized in that: In step 1), the mass ratio of mesoporous graphene phase carbon nitride to oxalic acid is 1:0.3-0.

6.

4. The preparation method according to claim 1 or 2, characterized in that: In step 1), after mixing, the mixture is heated to 400-500° C. at a heating rate of 10-20° C. / min and kept at a constant temperature for 3-7 hours.

5. The preparation method according to claim 1, characterized in that: In step 2), the mass ratio of thermally stripped g-C3N4 to 4-aminopyridine is 1:0.1-1.

6. The preparation method according to claim 1, characterized in that: In step 2), thermally exfoliated g-C3N4 is mixed with 4-aminopyridine in DMF under the following conditions: ultrasonic temperature is 40-70°C and ultrasonic time is 0.5-3h.

7. The preparation method according to claim 1, characterized in that: Step 2) The drying is performed by vacuum drying, the vacuum degree is 50-80Kpa, the drying temperature is 80-110°C, and the drying time is 8-16h.

8. An adsorbent prepared according to the preparation method according to any one of claims 1 to 7.

9. A method for treating tail gas from MDI production, characterized in that: The adsorbent according to claim 8 is used.