JP2595155B2 - Method for producing paraformaldehyde having high formaldehyde content - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a process for producing paraformaldehyde having a high formaldehyde content.

[0002]

2. Description of the Related Art Conventionally, commercially available paraformaldehyde has a formaldehyde content of about 80% by weight (hereinafter, weight% is simply referred to as%).
In recent years, as a raw material for producing urea resin, melamine resin, polyacetal, etc., those having a higher formaldehyde content have been demanded.
%, And in recent years, 95% or more have been manufactured.

[0003] When paraformaldehyde is used as the above raw material, it is dissolved in warm water or hot water or various solvents to be used as high-concentration formalin.
Good solubility in hot water or hot water is required.

The following methods are known as methods for obtaining such paraformaldehyde having a high formaldehyde content (hereinafter abbreviated as "high-concentration paraformaldehyde").

That is, when the formaldehyde content is 37 to 5
About 0% of formalin is converted into a concentrated aqueous solution of formaldehyde having a formaldehyde content of about 80% by a method such as vacuum concentration, and the concentrated aqueous solution is cooled and solidified by a granulation tower or a cooling device, and the formaldehyde content is once reduced. After forming about 78 to 83% of granular or flaky paraformaldehyde (hereinafter abbreviated as "conventional paraformaldehyde"), the conventional paraformaldehyde is dried with hot air or far infrared ray at a temperature lower than the softening point. (Kaihei 2-167244).

[0006]

However, when the conventional paraformaldehyde is dried by far-infrared drying to produce high-concentration paraformaldehyde, the solubility in hot or hot water deteriorates with time. That is, when high-concentration paraformaldehyde is dissolved in warm water or hot water, those immediately after production and those that have passed a long time,
For example, when 30 days or more have passed, there is a large difference in the solubility in hot water, and there is a problem when using high-concentration paraformaldehyde.

When only far-infrared drying is used, the depth of penetration into the substance is small due to the properties of far-infrared rays, generally less than 1 mm, and a slightly thick or overlapping substance is uniformly dried to the inside. Difficult to do.
For this reason, the dried state is different between the surface portion and the inside of the layer, and a product having a high degree of polymerization is generated. In addition, paraformaldehyde has a high rate of absorption of far-infrared rays, local melting occurs on the surface of granular or flaky paraformaldehyde particles, the particles cling to each other and form a lump, and cannot be uniformly dried to the inside of the lump, resulting in polymerization. Variations occur in degrees.

As described above, when a phenol resin or the like is produced from high-concentration paraformaldehyde having poor solubility in hot water or hot water and having a variable degree of polymerization, the condensation reaction with phenol or the like is adversely affected. And the quality of the resulting phenolic resin is reduced.

As a criterion for judging the degree of solubility of paraformaldehyde in warm water or hot water, there is one called "warm water solubility". The solubility in hot water is indicated by the time required for paraformaldehyde to be completely dissolved by adding paraformaldehyde to ion-exchanged water adjusted to a temperature of 80 ° C. so that the formaldehyde content becomes 20% and stirring. It is correlated with the degree of polymerization of paraformaldehyde, and it can be said that a substance having a long hot water dissolution time has a high degree of polymerization. At this time, the turbidity of the paraformaldehyde solution is 50 or less in standard turbidity as a criterion for judging complete dissolution. Depending on the purpose of use of paraformaldehyde, the solubility may be examined using butanol or the like instead of hot water.

[0010] The solubility of paraformaldehyde in hot water is gradually deteriorated by standing after production or by passage of time.
In addition, it becomes worse as the formaldehyde concentration of paraformaldehyde increases. Thus, the solubility of the conventional paraformaldehyde in warm water is about 1 to 5 minutes immediately after the production obtained by cooling and solidifying, but 5 to 15 minutes after 24 hours of production and 20 to 20 on the 10th day. 40 minutes, 30
On the day, it takes about 30 to 60 minutes, and then gradually becomes longer. Furthermore, the hot water solubility of high-concentration paraformaldehyde dried by a known method is 1 to 5 minutes immediately after drying the conventional paraformaldehyde with a far-infrared heating device.
24 hours after drying, 5 to 15 minutes, 10 days, 10 to 2
Although it is about 0 minutes, it becomes considerably worse, about 40 to 100 minutes on the 30th day, and gradually becomes longer thereafter.

[0011]

Means for Solving the Problems In view of the above situation, the present inventors have conducted intensive studies on a method for economically producing high-concentration paraformaldehyde having good solubility in hot water, and as a result, the conventional paraformaldehyde was converted into a microform. It has been found that high-concentration paraformaldehyde having good solubility even after a long period of time can be obtained by wave heating and drying, and the present invention has been completed.

That is, a first aspect of the present invention is to dry paraformaldehyde obtained by cooling and solidifying an aqueous formaldehyde solution having a formaldehyde content of 78 to 83% by weight to obtain a paraformaldehyde having a formaldehyde content of 88% by weight or more. Is a method for heating and drying the paraformaldehyde obtained by cooling and solidifying in a microwave, and the second is that air at 30 to 60 ° C. is heated to 30 to 240 ° C. prior to microwave heating and drying. A first method wherein the softening point of paraformaldehyde is 60 ° C. or more or the formaldehyde content is 86% by weight or more.
A first or second method characterized by drying while passing paraformaldehyde with air at 00 ° C.,
Fourth is any one of the first to third methods characterized by drying while flowing paraformaldehyde, and fifth is performing far-infrared heat drying simultaneously with or after microwave drying. The first to first features
This is one of the fourth methods.

The present invention will be described in more detail. The microwave used in the present invention is 1 mm to 1 m, frequency 300 M
A radio wave of an extremely short wavelength of HZ to 30 GHZ, which is a kind of electromagnetic wave called an ultra-high frequency or an ultra-high frequency, which can be generally used in the industrial world is 2450 MHZ, 5800 M
HZ, 24125MHZ, but mainly 2450MHZ
Is used. Polar molecules that form paraformaldehyde by placing paraformaldehyde in this microwave electric field (molecules with electric dipole, such as water)
However, they violently vibrate or rotate to orient in the electric field. This causes collision and friction of molecular drying to generate heat, and the product temperature rises rapidly both inside and outside.

In brief, the principle of microwave heating is as follows: microwave energy → vibration / rotational motion of dipole → thermal energy → sample temperature rises sharply.

For the microwave heating drying and the far infrared heating drying performed in the present invention, a usual microwave heating device and a far infrared heating device can be used, and the type thereof is not particularly limited.

The following are examples of the microwave heating apparatus. That is, the basic configuration of the microwave heating device is a microwave oscillator (magnetron) for generating microwave energy, an irradiation unit for irradiating the object to be heated with microwaves, an isolator, a power monitor, a matching device, etc. between them. Are connected via a waveguide. The irradiation unit (cavity, oven) is a closed metal box, and includes a batch type and a conveyor type, but any type may be used.

The far-infrared ray, which may be used in combination with the microwave heating of the present invention, has a wavelength range of about 5.6 to 1000 μm in the infrared ray and is a kind of electromagnetic wave like a microwave. The wavelength region of 2 to 25 μm is mainly used. When this far-infrared ray is irradiated on paraformaldehyde, it is a far-infrared ray electromagnetic wave, which is directly absorbed by the projectile by the axial irradiation and conducts heat.

A substance has a vibration and rotation frequency peculiar to the substance due to the difference in the mass of the atoms constituting the substance and the structural arrangement, the arrangement state and the bonding force in the molecule.
When a substance is irradiated with far-infrared rays, if the frequency of the far-infrared rays matches the natural frequency of this vibration or rotation, the molecule absorbs the energy of the far-infrared rays, and the vibration or rotation becomes more intense and heat is generated. This is a well-known phenomenon called a resonance absorption phenomenon.

When the substance to be dried contains a large amount of water, microwaves are effective. However, as the drying proceeds, the water content in the substance decreases, and the effect of the microwaves decreases. Therefore, it is a preferable embodiment of the present invention to further reduce the water content by drying using the characteristics of far infrared rays.

Therefore, the microwave or far-infrared electromagnetic wave energy applied to paraformaldehyde is absorbed by paraformaldehyde and generates heat,
Although paraformaldehyde is dried, this heat generation occurs almost uniformly with respect to paraformaldehyde, so that drying of paraformaldehyde is performed uniformly with little difference between the surface layer and the inside.

In the present invention, microwaves generated by the above-described microwave generator are irradiated on conventional paraformaldehyde and dried. The conventional paraformaldehyde may be dried immediately after cooling and solidification.

However, in this case, the conventional paraformaldehyde placed in the microwave device has a softening point of 50 to 6
Since the temperature is as low as 0 ° C., it is preferable to dry at a temperature at which the temperature of the conventional paraformaldehyde does not exceed this softening point at the start of drying. Then, as the drying proceeds, the concentration of formaldehyde increases and the softening point of paraformaldehyde also increases, so that the product can be heated and dried. The drying temperature in the case of drying with microwaves can be easily adjusted by starting, stopping, or increasing or decreasing the output of microwave oscillation, but can also be adjusted by adjusting the flow rate and temperature of the air flowing through. In particular, when drying the cooled and solidified conventional paraformaldehyde, drying is easier if the conditions of the ventilation air are adjusted.

The following drying operation is also a preferred embodiment. The conventional paraformaldehyde at a temperature lower than the softening point of the conventional paraformaldehyde at 30 to 60 ° C;
Preferably placed in an apparatus maintained at 40-50 ° C,
Ventilation is performed so that the ventilation speed to paraformaldehyde of the conventional product in the device becomes 0.01 m / s or more on an air tube basis,
By aeration for 30 to 240 minutes, the softening point of conventional paraformaldehyde can be increased. If the temperature in the apparatus is lower than 30 ° C., it takes a long time to raise the softening point. If the temperature exceeds 60 ° C., the softening point of the paraform is exceeded, partial melting occurs, lumps are generated, and the dry state tends to vary. In addition, when the softening point of paraformaldehyde is increased, the softening point can be increased faster by combining microwave oscillation with the above-described method of only passing air. The ventilation conditions need to be adjusted in consideration of the initial polymerization degree and the physical properties such as the softening point of the conventional paraformaldehyde.

The softening point of the conventional paraformaldehyde can be increased simply by leaving the conventional paraformaldehyde at room temperature or at a temperature of 30 to 60 ° C. without ventilating the air. However, it takes a long time to increase the softening point. When the paraformaldehyde layer is thick, the temperature distribution in the paraformaldehyde layer is different and the softening point, that is, the degree of polymerization may vary, so that ventilation is recommended.

According to this method, the softening point of the conventional paraformaldehyde was 50 to 60 ° C. before the treatment, but increased to 70 to 80 ° C. after the treatment. However, when the conventional paraformaldehyde is aerated at a temperature of 35 ° C. or higher for 30 to 240 minutes in the above-described method, the conventional paraformaldehyde is slightly softened. It is also possible to improve the fluidity of the conventional paraformaldehyde by cooling to below.

If this cooling is not performed, the water present on the surface of the paraformaldehyde particles, that is, free water, is heated by the microwave, and a part of the paraformaldehyde is melted in the microwave heating and drying apparatus. In order to avoid inconvenience such as generation of a portion having a high degree of polymerization, it is preferable to perform aeration cooling for removing moisture. The type of the apparatus for carrying out these methods is not particularly limited, and any of known air circulation band dryers can be used.

Thereafter, the conventional paraformaldehyde is dried by using a microwave heating device or a combination of a microwave heating device and a far-infrared heating device, preferably using air heated to 30 to 90 ° C. Then, air is blown so that the ventilation speed to paraformaldehyde in the drying apparatus is 0.01 m / s or more on the basis of the empty tube, so that the product temperature during drying does not rise above the softening point of paraformaldehyde. Drying is performed while controlling the temperature. Therefore, it is preferable to provide at least two thermometers for measuring the drying temperature of paraformaldehyde during drying in the microwave heating device and the far-infrared heating device.

As soon as the formaldehyde concentration of paraformaldehyde in the microwave heating device or far-infrared heating device reaches a target concentration or more, paraformaldehyde is immediately taken out of the microwave heating device or far-infrared heating device, and cool air having a temperature lower than the outside air temperature is blown out. It is preferable to perform rapid cooling under the condition that the ventilation speed to paraformaldehyde is 0.01 m / s or more on the basis of an empty tube, and sufficiently cool the paraformaldehyde until the product temperature becomes preferably 30 ° C. or less. . Sufficient cooling after drying improves daily hot water solubility.

Since the formaldehyde in paraformaldehyde is also volatilized during drying, the air sent to the above-mentioned microwave heating device or far-infrared heating device is led out of the system and the contained formaldehyde is absorbed by water. It is recommended that they be collected by such means.

Furthermore, the softening point of paraformaldehyde changes depending on the standing time after cooling and solidification and the concentration of formaldehyde. Therefore, if a diagram showing the relationship between the concentration of formaldehyde and the softening point in consideration of the above is prepared in advance, it is convenient for controlling the temperature of paraformaldehyde during the drying. It is necessary that high-concentration paraformaldehyde has good solubility in hot water and that the solubility in hot water deteriorates with the passage of time after production.

[0031]

The present invention will be described more specifically with reference to the following examples. Example 1 Formalin having a formaldehyde concentration of 37% was vacuum-concentrated, and a concentrated aqueous formaldehyde solution having a temperature of 80 ° C. and a formaldehyde content of 80% was cooled and solidified in a granulation tower to obtain a granular conventional paraformaldehyde. Was manufactured. The solubility of this conventional paraformaldehyde in warm water is 5 minutes. 1000 g of the paraformaldehyde is put into a ventilated microwave drier having an internal volume of 4 l, and air is fed from the bottom of the microwave drier at a flow rate of 10 l / h. While starting and stopping the microwave oscillation repeatedly every 5 seconds for 10 seconds.
After drying for one minute, 705 g of high-concentration paraformaldehyde having a formaldehyde content of 92% was obtained.

The drying was performed for 30 minutes while keeping the outlet gas temperature of the microwave dryer at 40 to 45 ° C. In addition,
Since the air fed into the microwave drying apparatus contained formaldehyde gas, it was led out of the system and brought into contact with water to recover formaldehyde.

The hot water solubility of the high-concentration paraformaldehyde obtained immediately after the drying was 5 minutes and there was no difference in the hot water solubility before and after the drying, and the hot water solubility after 24 hours was measured to be 10 minutes. . Further, the solubility in warm water after being left at 30 ° C. for 30 days was 20 minutes.

Example 2 1000 g of the conventional paraformaldehyde obtained in the granulation tower of Example 1 was put into a hot air circulating drier having an inner volume of 20 liters drawn by a 32 mesh wire mesh and circulating hot air at 45 ° C. While drying for 30 minutes while the formaldehyde content of paraformaldehyde became 86%, Example 1
1 kW plate-type far-infrared heater was attached to the top of the microwave dryer, and air was introduced from the bottom of the dryer at a flow rate of 10 l / h. Drying was performed until the amount reached 88%, and then microwave oscillation was stopped, and heating by a far-infrared heater was started. During far infrared heating,
Every 5 minutes, the door of the dryer was opened, and the paraformaldehyde was dried for 10 minutes while stirring. At this time, the dryer outlet gas temperature was 40 to 43 ° C.

The high-concentration paraformaldehyde obtained had a formaldehyde content of 91.5%, a solubility in hot water immediately after drying of 5 minutes, and no difference in solubility in hot water before and after drying. The solubility was measured for 8 minutes. The solubility in hot water after 30 days is 15
Minutes.

Example 3 A batch type microwave fluidized bed in which 1000 g of the conventional paraformaldehyde produced in the same manner as in Example 1 is laid with a 32-mesh wire net and has an inner volume of 10 l and can circulate warm air at 30 to 100 ° C. The paraformaldehyde was dried in a microwave drier for 20 minutes while circulating hot air at 60 ° C. so that the air flow rate in the microwave drier was 1.5 m / s. As soon as 775 g of paraformaldehyde having a content of 91.5% were obtained, cooling was carried out for 10 minutes with 25 ° C. cold air at a ventilation rate of 5 m / s, and the temperature of the paraformaldehyde was lowered to 29%. ℃.

The hot water solubility of the obtained 91.5% paraformaldehyde immediately after production was measured, and it was 3 minutes. Further, the solubility in warm water after 30 days was 12 minutes.

COMPARATIVE EXAMPLE 1 10 of the conventional paraformaldehyde obtained in Example 1
00g immediately into a far-infrared dryer and air at 10 l /
The product was dried while being fed into the far-infrared ray drying device at a flow rate of h to obtain 750 g of high-concentration paraformaldehyde having a formaldehyde content of 92%. For drying, first, the product temperature of the conventional paraformaldehyde was dried at 50 to 55 ° C for 1 hour, and then the product temperature was increased to 70 to 80 ° C and dried for 30 minutes. The resulting high-concentration paraformaldehyde had a solubility in warm water immediately after drying of 5 minutes, a solubility in warm water after 24 hours of 10 minutes, and a solubility in warm water after 30 days of 40 minutes. Compared with Examples 1, 2, and 3, the solubility in hot water when passed over time is as follows:
It is getting much worse.

Comparative Example 2 10 of the conventional paraformaldehyde obtained in Example 1
00g is 20l with 32 mesh wire mesh
And dried for 30 minutes while circulating hot air at 45 ° C., and when the formaldehyde content of paraformaldehyde became 86%, comparison was performed using the same far-infrared ray drying apparatus as in Comparative Example 1. Drying was carried out under the same drying conditions as in Example 1 to obtain 725 g of high-concentration paraformaldehyde having a formaldehyde content of 91.8%. The product temperature of paraformaldehyde was 28 at the same cold air temperature and ventilation speed as in Example 3. Cooled down to ° C.

The hot water solubility of the obtained 91.8% high-concentration paraformaldehyde immediately after production was measured.
Minutes. The solubility in hot water after 24 hours was measured for 10 minutes, and the solubility in warm water after 30 days was 38 minutes. Although the cooling effect after drying is slightly recognized as compared with Comparative Example 1, the solubility in hot water is deteriorated with time as compared with Examples 1, 2, and 3.

[0041]

According to the present invention, high-concentration paraformaldehyde can be produced by simply drying conventional paraformaldehyde by microwave heating, which is an operationally simple method. The present invention also uses microwaves,
Since paraformaldehyde generates heat and is dried, it can be uniformly dried with little difference between the surface layer and the inside of paraformaldehyde.

As a result, compared with the high-concentration paraformaldehyde produced by the conventional method, the dispersion in the degree of polymerization is small, so that the solubility in hot water is good and the change in solubility is small even after passing through the day. The effect is that paraformaldehyde concentration can be obtained. In addition, since the heating of paraformaldehyde is performed uniformly as described above, the drying can be performed at a lower temperature than the conventional method.

In particular, the present invention has improved the long-term solubility over time, which has conventionally been a major problem in the paraformaldehyde production industry, and has a great industrial contribution. In addition, in the method of the present invention, since the heat transfer to paraformaldehyde is axial heat transfer, microwave heating directly heats only paraformaldehyde, and the heat loss is extremely small. There is also an effect that it is good. Furthermore, the method of the present invention allows immediate drying of conventional paraformaldehyde obtained by cooling and solidification.

Claims (5)

(57) [Claims] (1) a formaldehyde content of 78 to
A method for producing paraformaldehyde having a formaldehyde content of 88% by weight or more by drying paraformaldehyde obtained by cooling and solidifying an 83% by weight aqueous formaldehyde solution, wherein the paraformaldehyde is dried by microwave heating. A method for producing paraformaldehyde having a high formaldehyde content. 2. Prior to microwave heating and drying, air at 30 to 60 ° C. is passed for 30 to 240 minutes so that the softening point of paraformaldehyde is 60 ° C. or more or the formaldehyde content is 86% by weight or more. The method of claim 1, wherein: 3. The method according to claim 1, wherein drying is performed by microwave heating while passing air at 30 to 100 ° C. through paraformaldehyde. 4. The method according to claim 1, wherein microwave drying is carried out while flowing paraformaldehyde. 5. The method according to claim 1, wherein far-infrared heat drying is carried out simultaneously with or after the microwave heat drying.