WO2016011723A1

WO2016011723A1 - Paraformaldehyde production method

Info

Publication number: WO2016011723A1
Application number: PCT/CN2014/089886
Authority: WO
Inventors: 叶子茂; 向家勇; 张鸿伟; 胡敏; 杨科岐; 郭兵
Original assignee: 江苏凯茂石化科技有限公司
Priority date: 2014-07-21
Filing date: 2014-12-03
Publication date: 2016-01-28
Also published as: CN104130360A

Abstract

Disclosed is a paraformaldehyde production method. A formaldehyde solution is used as the raw material, and is evaporated and concentrated to produce fine, soluble, and fluid paraformaldehyde after spray granulation in a spray granulation tower. The paraformaldehyde is transformed into a fluidized bed for drying, thereby obtaining dry paraformaldehyde particles; after concentration, formaldehyde is delivered to the spray granulation tower under a constant pressure and blown out of the top of the tower; at the same time, nitrogen gas and formaldehyde are delivered into the top portion of the spray granulation tower to conduct contact heat exchange in parallel flows. In the present invention, formaldehyde is delivered under a constant pressure and by means of a specially made nozzle, such that the formaldehyde solution can be rapidly dispersed and conduct contact heat exchange with nitrogen gas in parallel flows, thereby ensuring the degree of polymerization and degree of consistency in particle diameters for the paraformaldehyde product. Additionally, paraformaldehyde particles in a cyclonic separator is recovered and delivered to the granulation tower, thereby further enhancing the size consistency of the paraformaldehyde products.

Description

Process for producing paraformaldehyde

Technical field

The invention belongs to the technical field of chemical production, and particularly relates to a production process method of paraformaldehyde.

Background technique

Paraformaldehyde is a product obtained by dehydration, concentration and drying of aqueous formaldehyde solution. It is a mixture of formaldehyde molecule-polyoxymethylene glycol HO(CH2O)nH and is a chemically important linear polymer. At present, the production method of domestic paraformaldehyde is mainly produced by a conventional vacuum dehydration method. The degree of polymerization of paraformaldehyde obtained by the conventional production method is uncontrollable, resulting in uneven particle size of the paraformaldehyde particles, which seriously affects the product quality of paraformaldehyde.

The aqueous formaldehyde solution can be polymerized during long-term storage or concentration operations to form a polymer of paraformaldehyde-white powdery linear structure. The reaction equation is as follows:

The equilibrium reaction generated is greatly affected by the H+ concentration, and the presence of trace polar substances such as acid, alkali and water accelerates the polymerization. The temperature also has an effect. When the temperature is low, the reaction moves toward the direction in which the polymer is formed, and when the temperature rises, it moves in the opposite direction. When the temperature is high, it will even completely depolymerize to form monomers, especially in the presence of acid, it is easier to depolymerize into gaseous formaldehyde monomer.

The key to preparing paraformaldehyde is to control the degree of polymerization of the product, prevent the polymerization of formaldehyde, shorten the drying and dehydration time, and improve the product yield. The formaldehyde solution is pumped into the prilling tower, and is sprayed and cooled by a spray head to form a uniform particle size of paraformaldehyde having a polymerization degree of about n=11. However, the prior art nozzle does not have a premixing dispersion effect, so that formaldehyde in the formaldehyde solution has partially polymerized, thereby seriously affecting the control of the subsequent polymerization degree, resulting in a high degree of polymerization of paraformaldehyde and a decrease in product quality.

Summary of the invention

OBJECTS OF THE INVENTION In view of the above problems and drawbacks, the present invention provides a process for producing paraformaldehyde, which has a uniform particle size of 20 to 60 mesh and a polymerization degree of about 11.

Technical Solution: In order to achieve the above object, the present invention adopts the following technical scheme: a process for producing paraformaldehyde, which uses formaldehyde solution as a raw material, is concentrated by evaporation, and is spray-granulated in a spray granulation tower to obtain fine particles and water-soluble. Sexual and fluid paraformaldehyde, then transferred to a fluidized bed for drying Paraformaldehyde particles; wherein, after being concentrated, the formaldehyde is sent to the top of the spray granulation tower at a constant pressure, and the upper part of the spray granulation tower is fed with nitrogen gas. After passing through the gas distributor, the nitrogen gas is passed downstream with the formaldehyde. Contact heat exchange, the temperature in the spray granulation tower is 5-30 ° C; the gas phase after heat exchange in the spray granulation tower is sent to a cyclone separator to collect and collect paraformaldehyde dust, and is again sent with nitrogen The top or bottom of the prilling tower is randomly collided with paraformaldehyde.

Further improved, the paraformaldehyde obtained by the spray granulation tower is dried in a fluidized bed and is divided into three stages of drying. The first stage drying temperature is 45-80 ° C to obtain 90-91% of paraformaldehyde; the second stage drying temperature is More than 95% of paraformaldehyde is obtained at 50 to 100 ° C; the third stage drying temperature is 10 to 40 ° C, and paraformaldehyde is cooled and dried.

Further, the formaldehyde solution is concentrated as a raw material through a two-stage evaporation concentration device, and after being evaporated and separated by a vaporizer, the concentration of the formaldehyde solution is 70-85% (mass%), and the concentration of the formaldehyde solution is evaporated and separated by the two-stage evaporator. It is 85 to 95% by mass.

Further, the temperature in the one-stage evaporation concentration device is 60 to 70 ° C, and the degree of vacuum is -30 to -60 KPa.

Further, the temperature in the two-stage evaporation concentration device is 80 to 100 ° C, and the degree of vacuum is -30 to -70 KPa.

Further, the oxygen content in the spray granulation tower is controlled to be 10% or less. (Specific use of 0 ~ 10 mbar negative pressure, thus preventing explosions to reduce leakage, improve the working environment and safe production)

Further, the gas phase at the top of the one-stage evaporation concentrating device and the two-stage evaporation concentrating device is sent to the absorption tower through the pipeline to absorb the diluted formaldehyde.

Further, before entering the evaporator, urotropine, triethylamine or dimethylamine having a mass content of less than 1% is added to the formaldehyde solution. (Thereby increasing the water solubility of the paraformaldehyde product, the dissolution time is reduced by half compared to before the additive is added).

Advantageous Effects: Compared with the prior art, the present invention can deliver formaldehyde at a high speed by combining formaldehyde with a special nozzle, and can exchange heat with a downstream nitrogen gas to ensure the polymerization degree of the paraformaldehyde product. The particle size uniformity is simultaneously sent to the prilling tower by recovering the paraformaldehyde particles of the cyclone separator, thereby further improving the uniform paraformaldehyde product.

DRAWINGS

1 is a schematic structural view of a formaldehyde solution evaporation concentration device according to the present invention;

Figure 2 is a schematic view showing the structure of the spray granulation and drying device of the present invention;

Figure 3 is a schematic structural view of the exhaust gas absorbing device of the present invention;

Figure 4 is a schematic view showing the structure of the spray granulation tower of the present invention;

Figure 5 is a schematic view showing the structure of a gas distribution plate in the spray granulation tower of the present invention;

Figure 6 is a schematic view showing the structure of the dispersing nozzle of the present invention;

Figure 7 is a partial enlarged view of the area A in Figure 6;

Figure 8 is a schematic structural view of the squeegee separator of the present invention;

Figure 9 is a schematic cross-sectional view of the squeegee separator of the present invention.

Among them, the evaporator 1, the primary separator 2, the secondary separator 3, the buffer tank 4, the spray granulation tower 5, the fluidized bed dryer 6, the cyclone separator 7, the primary absorption tower 8, and the first absorption tower 9. Second absorption tower 10, third absorption tower 11, heat exchanger 12, transfer pump 13, constant pressure pump 14, central shaft 31, connecting rod 32, peripheral dry 33, fixed ring 34, elastic scraper 35, block Strip 36; buffer tank regulator pump anti-blocking diagram; tower body 51, airflow distributor 52, inner ring duct 521, outer ring duct 522, nitrogen inlet 523, air passage 524, nozzle 53, nozzle 531, pipe joint 532, the nozzle body 533, the nozzle hole 534, the spiral flow path vane 535, the outer end surface 536, the stepped curved surface 537, the fluidized bed bottom plate 61, and the bulge 62.

detailed description

The invention will be further clarified with reference to the drawings and specific embodiments, which are to be construed as illustrative only and not to limit the scope of the invention. Modifications of equivalent forms are intended to fall within the scope defined by the appended claims.

As shown in Fig. 1, a paraformaldehyde production process device and a process method mainly include three parts: a formaldehyde solution evaporation concentration device, a spray granulation and drying device, and an exhaust gas absorption device.

Part I: Evaporation and concentration of formaldehyde solution

Among them, the formaldehyde solution evaporation and concentration device mainly comprises an evaporator, a primary separator, a secondary separator and a buffer tank which are connected in sequence. In order to improve the efficiency of evaporation and concentration, in this embodiment, about 40% of the dilute formaldehyde solution is sent to the evaporator through the raw material pump, and the vapor in the evaporator jacket is evaporated and then enters the primary separator, at 60-70. Gas-liquid separation is carried out under the conditions of °C and pressure -30~-60KPa, and the gas phase is taken out from the top of the primary separator (as a preferred solution, the gas phase is sent to the absorption tower for recovery), and after concentration, 70-85% of the formaldehyde solution is obtained. The bottom of the primary separator flows out and is sent to a secondary separator via a formaldehyde transfer pump for further concentration. The secondary separator is a scraper type, which includes an air inlet, a cylindrical cylinder, and a cage drum And an elastic scraper, the cage drum is disposed at the center of the cylindrical cylinder, one side of the elastic scraper is disposed on the circumference of the cage drum, and the other side is disposed on the inner wall of the cylinder; The airflow inlet is disposed tangentially along the cylinder and is disposed in the same direction as the cage drum. When the formaldehyde solution enters the secondary separator (scraper separator), the scraper rotates and agitates the formaldehyde solution. Under the left and right side of the scraper rotating side of the scraper, the formaldehyde solution is more likely to be centrifugally upward and upward, thereby improving the cylinder body. The height of the liquid film on the wall accelerates the evaporation and concentration. The formaldehyde solution is fed into the secondary separator (scraper separator). The inlet is placed in the tangential direction of the separator, so that the formaldehyde solution can enter the tangential direction along the inner wall. The evaporation effect is improved; at the same time, due to the presence of the baffle, a moderate disturbance to the liquid flow and a small region solution reflux can be prevented, thereby preventing the prepolymerization of formaldehyde and facilitating the particle uniformity of the subsequent spray granulation paraformaldehyde. Another consideration of the strip scraper in this embodiment is that if ordinary rotating blades are used, on the one hand, there is no liquid flow disturbance, and formaldehyde is easily polymerized when evaporating and concentrating in a film forming state, and on the other hand, the film forming area can be increased, thereby improving Evaporation concentration rate. At the same time, due to the presence of the baffle, the liquid flow can be moderately disturbed and the small area solution is refluxed, which can prevent the pre-polymerization of formaldehyde and facilitate the particle uniformity of the subsequent spray granulation of paraformaldehyde.

Preferably, a first absorption tower may be disposed in the evaporation concentration device, and the formaldehyde-containing gas from the top of the primary separator and the secondary separator is sent to the bottom of the first absorption tower through a pipeline for formaldehyde absorption to obtain a diluted formaldehyde. Therefore, the formaldehyde is recovered to reduce the cost, the emission is reduced, and the environment is polluted, and the absorbed exhaust gas is sent to the exhaust gas absorption device for treatment. According to a further improvement, the first absorption tower is divided into three absorption sections from bottom to top, and the outlet of the bottom of each absorption section is connected to the top inlet of the corresponding absorption section through a formaldehyde circulation pump to form a circulation.

Preferably, the pipe for connecting the formaldehyde evaporation and concentration device to the inlet of the spray granulation dryer is provided with a branch pipe for circulating pure water, and the branch pipe is provided with an anti-blocking valve. Therefore, when the process device needs to be shut down for maintenance, the formaldehyde delivery valve is closed, the anti-blocking valve is opened, and the pure water is sent to clean the pipeline of the entire process device to prevent the polymerization of formaldehyde or paraformaldehyde and block the pipeline.

As a preferred solution, the secondary separator (two-stage evaporation concentration) is divided into three absorption sections from bottom to top, and the outlet at the bottom of each absorption section is connected to the top inlet of the corresponding absorption section through a formaldehyde circulation pump to form a circulation, thereby greatly accelerating the formaldehyde. Concentrate by evaporation.

A buffer tank with agitation is further disposed between the secondary separator and the spray granulation tower, and the outlet of the secondary separator is inserted into the bottom of the buffer tank through the pipeline and is below the surface of the formaldehyde liquid, leaving a space above the formaldehyde liquid surface. And the top of the buffer tank is provided with a nitrogen-passing pipe, and the bottom of the buffer tank is connected to the inlet of the spray granulation tower through a pipe. After the second stage concentration, the concentration of the formaldehyde solution can reach 85-95%, and enter the buffer tank. Since a certain pressure of nitrogen is introduced above the formaldehyde liquid surface, the formaldehyde solution fed into the granulation tower can be ensured to be a constant pressure, thereby passing When the spray head forms a spray, it can be sufficiently dispersed to obtain a paraformaldehyde particle having a uniform particle size.

Through repeated debugging and verification, it was found that in order to obtain a paraformaldehyde target product having a polymerization degree of about 12 and a particle diameter of 10 to 100 mesh (preferably 20 to 60 mesh), one of the keys is to ensure feeding into the spray prilling tower. The concentrated formaldehyde solution must be fed at a constant pressure, and is sprayed at a high speed through a nozzle to form a highly dispersed formaldehyde mist and is cooled by contact with a downstream nitrogen gas. The invention provides two kinds of voltage stabilization embodiments: (1) a buffer tank with stirring is arranged between the second stage separator and the spray granulation tower, and the outlet of the second stage separator is directly inserted into the bottom of the buffer tank through the pipeline and is in the form of formaldehyde solution. Below the liquid level, there is space above the liquid level. At this time, the upper part of the tank body is provided with a nitrogen gas inlet, and the pressure of the nitrogen gas is controlled by the valve to be about 5 MPa, thereby ensuring the constant pressure of formaldehyde fed into the granulation tower. (2) The second method is to separately set the pulsation damper on the pipelines on both sides of the pump. The volume of the pulsation damper is more than 1% of the delivery amount of the formaldehyde solution per unit hour, so that the pressure amplitude can be kept below 3%. .

Part II: Spray granulation and drying

The spray granulation tower is cylindrical, and the lower bottom is a conical head, which mainly comprises a tower body, a product outlet provided at the bottom of the tower body, a formaldehyde solution inlet provided at the top of the tower body, and a nitrogen inlet provided at the upper part of the tower body. The nitrogen inlet is connected with a gas flow distributor; a nozzle is arranged inside the tower body, the nozzle is mainly composed of a nozzle and a pipe joint, wherein the nozzle comprises a nozzle body and a nozzle hole provided at a front end of the body, the nozzle body is a right-angled shape, and the tail end is The pipe joint is screwed, the center of the nozzle body is a tubular passage, and the spiral flow path blade is embedded in the passage; at the same time, the nozzle hole includes a conical section and a straight hole section of the small hole, so that the formaldehyde solution flowing through forms a high-speed rotation dispersion. The shape is sprayed out to form a highly dispersed mist, and the spiral flow path vanes can be easily disassembled by means of embedding. At the same time, we further improve the nozzle hole, the outer end surface of the nozzle hole is designed as a concave stepped arc surface, the height of the step is 1mm, and the step is rounded, so that the formaldehyde solution is sprayed from the nozzle hole at a high speed along the curved end surface. Forming a sloping downward sloping shape, which is advantageous for dispersion and forms a strong convective heat exchange with the nitrogen entering the upper part of the granulation tower. As a preferred solution, the curved end surface is designed to be stepped to effectively improve the atomization effect, thereby performing heat exchange cooling with nitrogen. In addition, the nozzle body is formed by a right angled corner, so that the formaldehyde solution entering from the pipe joint forms a mixed flow at the corner to avoid the flat flow of the formaldehyde solution when entering, because when the length of the straight pipe reaches a certain level, the liquid flowing through will form a relative flow. At rest, the relative flow is reduced, thereby increasing the risk of polymerization of formaldehyde.

The spray granulation tower must also include a regulated frequency-adjusting pump. The combination of the constant-pressure pump and the special-structured spray head can effectively disperse the spray liquid so that the specific surface area of the formaldehyde polymer is greatly improved, and finally obtained by freeze-drying. The degree of polymerization of paraformaldehyde is about 11, and the particle size distribution is uniform. The operating conditions in the spray granulation tower are 5-30 ° C, under vacuum conditions, nitrogen is fed from the nitrogen inlet in the upper part of the tower body, and the low-temperature nitrogen gas is fed into the mist-formaldehyde solution through the nitrogen gas flow distribution plate. The heat exchange is cooled, and the formaldehyde mist sprayed with the nozzle is in a downstream manner, thereby avoiding the formation of disturbance and end current in the countercurrent manner, resulting in excessive local polymerization of paraformaldehyde. As a modification, the airflow distributor includes an inner ring air duct and an outer ring air duct, the outer ring air duct is provided with a nitrogen inlet, and a diameter of a side of the outer ring air duct away from the nitrogen gas inlet is relatively small; The inner side of the inner ring duct and the outer side of the outer ring duct are connected by a plurality of air passages.

The upper part of the spray granulation tower is provided with a nitrogen outlet. After the heat exchange with the formaldehyde spray, the nitrogen gas is still mixed with a small amount of paraformaldehyde product, and is taken out from the nitrogen outlet, and then sent to two cyclone separators for gas-liquid separation. A clean nitrogen tail gas is used for internal circulation. In order to control the oxygen content of the recycle gas below the explosion value, it is necessary to maintain a small amount of purge gas which is discharged after passing through the gas scrubber. The solid paraformaldehyde separated from the cyclone is sent from the bottom to the dry fluidized bed for further recovery and drying. Preferably, the bottom of the cyclone separator is further provided with a screw conveyor which is respectively connected to the lower middle portion and the top portion of the prilling tower through a pipe and a fan. Therefore, the drier paraformaldehyde dust collected by the cyclone is transported by nitrogen to the top or the middle of the prilling tower, thereby forming a larger amount of paraformaldehyde particles in the prilling tower. The random collision accelerates the mixing and agglomerates into a uniform particle size of paraformaldehyde, which accelerates the drying and dehydration.

After the freeze polymerization of the spray granulation tower, a paraformaldehyde having a polymerization degree of n=11 is obtained, and since the surface is moist and has a large amount of water, it is also required to be dried by a vibrating dry fluidized bed. The dry fluidized bed is a vibration three-stage structure, comprising a first-stage drying section, a second-stage drying section and a cooling section, and each floor of the section is uniformly provided with a plurality of pores for passing nitrogen gas, and the pores are opened in the convex bulge on. Wherein, the vent opening on the bulge at the periphery of the bottom plate of the primary drying section faces the central portion, and the vent opening on the central bulge faces the secondary drying section; the opening on the bottom drum of the secondary drying section and the third-stage drying section faces downward In the first-stage drying section, a gas blower is arranged at the bottom of the three sections, and nitrogen gas is blown through the paraformaldehyde solid to be dried by the fan to accelerate drying and recover heat. And the bottom plate is oscillating vibration as a whole, specifically through a swing arm disposed under the bottom plate, and is driven by a motor. In the process control, the temperature of the first-stage drying section is controlled at 45-80 °C. The temperature in this section is low to prevent excessive drying of the surface of the paraformaldehyde particles; the temperature of the second-stage drying section is controlled at 50~ At 100 ° C, this section is a strengthening section. On the one hand, the paraformaldehyde is further dried, and at the same time, due to the concentrated heating and drying of the section, the nitrogen flowing through can recover more heat; the third-stage drying section is a cooling section, and the temperature is this section. Controlled at 10 to 40 °C.

In addition, in order to recover the paraformaldehyde solids in the blown nitrogen, we send the blown gas to the third cyclone for cyclone separation, and the paraformaldehyde collected by the cyclone is recovered and sent back to the dry fluidized bed to continue drying. The gas separated from the top of the cyclone is used for internal circulation.

Part III: Exhaust gas absorption device

The exhaust gas absorbing device in the present invention comprises a first absorption tower, a second absorption tower and a third absorption tower which are sequentially connected, and an outlet of each absorption tower is provided with a heat exchanger for recycling and absorbing heat, thereby reducing steam Consumption.

Claims

The invention relates to a process for producing paraformaldehyde, which is characterized in that: formaldehyde solution is used as a raw material, and after evaporation and concentration, spray granulation is carried out in a spray granulation tower to obtain fine granules, water-soluble and fluid paraformaldehyde, and then transferred. Drying into a fluidized bed to obtain dried paraformaldehyde particles; wherein, after being concentrated, the formaldehyde is sent to the top of the spray granulation tower at a constant pressure, and the upper part of the spray granulation tower is fed with nitrogen gas, and the nitrogen gas passes through the gas distributor. Then, the contact heat exchange with formaldehyde is carried out in a downstream manner, and the temperature in the spray granulation tower is 5 to 30 ° C; the gas phase after the heat exchange in the spray granulation tower is sent to a cyclone separator to separate and collect the paraformaldehyde dust. And again with nitrogen to the top of the spray granulation tower or the bottom of the tower is randomly collided with paraformaldehyde.
The method for producing paraformaldehyde according to claim 1, wherein the paraformaldehyde obtained by the spray granulation tower is dried in a fluidized bed and is divided into three sections, and the first drying temperature is 45 to 80 ° C to obtain 90. ～91% of paraformaldehyde; the second stage drying temperature is 50-100 ° C to obtain more than 95% of paraformaldehyde; the third stage drying temperature is 10 ~ 40 ° C, paraformaldehyde is cooled and dried.
The method for producing paraformaldehyde according to claim 2, wherein the formaldehyde solution is concentrated as a raw material through a two-stage evaporation concentration device, and after being evaporated and separated by a vaporizer, the concentration of the formaldehyde solution is 70-85% (mass%). The concentration of the formaldehyde solution after evaporation and separation by the two-stage evaporator is 85 to 95% by mass.
The method for producing paraformaldehyde according to claim 3, wherein the temperature in the one-stage evaporation concentration device is 60 to 70 ° C, and the degree of vacuum is -30 to -60 KPa.
The method for producing paraformaldehyde according to claim 3, wherein the temperature in the two-stage evaporation concentration device is 80 to 100 ° C, and the degree of vacuum is -30 to -70 KPa.
The method for producing paraformaldehyde according to claim 3, characterized in that the oxygen content in the spray granulation tower is controlled to be less than 10%.
The process for producing paraformaldehyde according to claim 3, characterized in that the gas phase at the top of the one-stage evaporation concentrating device and the two-stage evaporation concentrating device is sent to the absorption tower through a pipe to absorb the diluted formaldehyde.
The method for producing paraformaldehyde according to claim 3, characterized in that before entering the evaporator, urotropine, triethylamine or dimethylamine having a mass content of less than 1% is added to the formaldehyde solution.