Process for the preparation of anhydrous formaldehyde from aqueous formaldehyde

Abstract

The application provides a method for preparing anhydrous formaldehyde from formaldehyde aqueous solution. The method comprises the following steps: adding C5-C10 straight-chain aliphatic alcohol into the formaldehyde aqueous solution, reacting at 50-100 DEG C under normal pressure for 1-3 hours to generate formaldehyde hemiacetal; the molar ratio of formaldehyde to the reaction agent is 1:1.0-1.5; adding an extractant into the reaction solution, and standing to separate into upper and lower layers; the upper layer is an organic phase containing the formaldehyde hemiacetal, the reaction agent and the extractant, and the lower layer is an aqueous phase; the extractant is at least one selected from n-pentane, n-hexane, cyclohexane and toluene; the mass ratio of the reaction agent to the extractant is 1:0.4-0.6; rectifying the organic phase to obtain the formaldehyde hemiacetal; and pyrolyzing the formaldehyde hemiacetal and absorbing the formaldehyde hemiacetal with methanol and / or ethanol to obtain a small-molecule alcohol solution of anhydrous formaldehyde. The technical problem to be solved is how to significantly reduce the dehydration energy consumption, improve the formaldehyde recovery rate and simplify the operation process under the premise of ensuring the purity of the anhydrous formaldehyde.

Description

Technical Field

[0001] This invention relates to the field of organic chemical technology, and in particular to a method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution. Background Technology

[0002] Anhydrous formaldehyde is a high-value-added raw material in resin synthesis, fine chemical production, and pharmaceutical intermediate production. Industrially, its preparation using a 37-50 wt% formaldehyde aqueous solution presents two major challenges: first, formaldehyde forms strong hydrogen bonds with water molecules, creating a difficult-to-separate azeotropic system, resulting in extremely low efficiency with conventional physical separation methods; second, formaldehyde is chemically reactive and readily self-polymerizes into paraformaldehyde during high-temperature dehydration, leading to decreased yield and equipment blockage.

[0003] Several existing technical routes have significant drawbacks: vacuum distillation requires large equipment investment and consumes extremely high energy; adsorption is cumbersome to operate and consumes high energy for regeneration, making it unsuitable for large-scale production; solvent extraction has poor selectivity, and subsequent separation still requires high-energy distillation.

[0004] Literature reports a technical route that uses isooctanol to react with formaldehyde to produce formaldehyde hemiacetal, followed by separation and conversion of the formaldehyde hemiacetal. This route alters the separation target through chemical reaction, technically avoiding the difficulty of direct physical dehydration of formaldehyde aqueous solution. However, the steric hindrance caused by the branched structure of isooctanol in this process leads to a slow hemiacetalization reaction rate, low equilibrium conversion rate, and high formaldehyde residue. Furthermore, it disrupts the hydrophobicity of the organic phase, resulting in slow separation of the reaction mixture after settling and a high water content in the organic phase (generally >20wt%). The subsequent dehydration of formaldehyde hemiacetal still consumes a large amount of energy, failing to fundamentally solve the problem. Some researchers have attempted to improve hydrophobicity by increasing the carbon chain length, but this increases the boiling point of the alcohol, which in turn increases the energy consumption for subsequent recovery and pyrolysis, creating new technical contradictions. Summary of the Invention

[0005] The main objective of this invention is to provide a method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution. The technical problem to be solved is how to significantly reduce dehydration energy consumption while ensuring the purity of anhydrous formaldehyde, and at the same time improve the formaldehyde recovery rate and simplify the operation process, so as to make it more suitable for practical use.

[0006] The objective of this invention and the technical problem it solves are achieved by the following technical solution. A method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, according to this invention, includes the following steps: S1. Add a reactant to an aqueous formaldehyde solution and react at 50-100°C under normal pressure for 1.0-3.0 h to generate formaldehyde hemiacetal; the reactant is selected from at least one of C5-C10 straight-chain fatty alcohols; the molar ratio of formaldehyde to reactant is 1:1.0-1.5. After the S2 reaction is completed, an extractant is added to the reaction solution, and the mixture is allowed to stand and separate into layers. The upper layer is an organic phase containing formaldehyde hemiacetal, reactant, and extractant, and the lower layer is an aqueous phase. The extractant is selected from at least one of n-pentane, n-hexane, cyclohexane, and toluene. The amount of extractant added is 40-60% based on the mass content of the reactant being 100%. S3 performs distillation on the organic phase to remove water, obtaining formaldehyde hemiacetal with a water content ≤100ppm; S4 pyrolyzes the formaldehyde hemiacetal and absorbs it with a small molecule alcohol to obtain an anhydrous formaldehyde small molecule alcohol solution; the small molecule alcohol is methanol and / or ethanol.

[0007] The purpose of the invention and the solution to its technical problems can be further achieved by the following technical measures.

[0008] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the reactant is selected from at least one of n-pentanol, n-hexanol, n-heptanol, and n-octanol.

[0009] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the reactant is selected from at least one of n-pentanol and n-hexanol; the reaction temperature is 70~95℃, the reaction time is 1.0~2.0h; and the molar ratio of formaldehyde to reactant is 1:1.0~1.2.

[0010] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the reactant and extractant work together to ensure that the reaction solution has a settling time of ≤5 min, and the resulting organic phase has a water content of ≤10 wt% and an aqueous phase has a formaldehyde content of ≤1 wt%.

[0011] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the reactant and extractant allow the reaction solution to stand for stratification time ≤ 1 min.

[0012] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the mass concentration of the aqueous formaldehyde solution is 37-50%.

[0013] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the extractant is recovered during distillation in step S3, and the recovered extractant can be directly returned to step S2; the reactant is recovered during pyrolysis in step S4, and the recovered reactant can be directly returned to step S1.

[0014] Preferably, the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution further includes, after step S2, adding the same extractant as in step S2 to the aqueous phase for back-extraction to recover residual formaldehyde and reactant in the aqueous phase, wherein the formaldehyde content in the aqueous phase after back-extraction is ≤100mg / L.

[0015] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the back-extraction temperature is 20-30°C; and the mass ratio of the aqueous phase to the extractant added thereto is 1:0.2-0.5.

[0016] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the residual formaldehyde and reactants in the recovered aqueous phase are returned to step S1 for use as reaction raw materials.

[0017] Preferably, in the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the distillation in step S3 is carried out at a temperature below the lowest atmospheric boiling point of the reactant; or, the pyrolysis in step S4 is carried out at atmospheric pressure at a temperature above the highest atmospheric boiling point of the reactant.

[0018] By employing the above technical solution, the method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution proposed in this invention has at least the following beneficial effects: This invention proposes a method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution. It selects a specific C5-C10 straight-chain fatty alcohol and a suitable extractant, employing an innovative design of formaldehyde hemiacetalization fixation and synergistic dehydration with the extractant to convert formaldehyde into a stable hemiacetal, preventing formaldehyde self-polymerization. Simultaneously, the extractant forms a weak interaction with water, significantly reducing the difficulty of separating water and formaldehyde. Furthermore, steps S1, S3, and S4 are all performed at atmospheric pressure, eliminating the need for high-vacuum equipment. The extractant in this invention is also an azeotropic agent for distillation, allowing the organic phase to be distilled at a relatively low pressure. Rapid dehydration at low temperatures avoids the high energy consumption associated with traditional high-vacuum and high-temperature processes, significantly reducing the overall energy consumption of the dehydration process. C5-C10 straight-chain fatty alcohols, free of steric hindrance, exhibit high efficiency in the hemiacetalization reaction with formaldehyde, rapidly converting free formaldehyde into stable formaldehyde hemiacetals, thus inhibiting formaldehyde self-polymerization at the molecular level. Subsequent pyrolysis targets only the hemiacetal, avoiding the risk of free formaldehyde polymerization at high temperatures, ensuring process stability and product yield. In step S2, the reactant and extractant work synergistically to maintain an organic phase water content ≤1%. 0wt% initial dehydration is achieved, laying the foundation for subsequent deep dehydration. Step S3 further removes the water content of formaldehyde hemiacetal to ≤100ppm through distillation, followed by pyrolysis to regenerate formaldehyde and absorption with small molecule alcohols. The resulting anhydrous formaldehyde small molecule alcohol solution has high purity, meeting the purity requirements of high-end chemical applications. The entire process includes only four core steps: hemiacetalization reaction → extraction and layering → distillation and dehydration → pyrolysis and absorption. The layering time is ≤5min (preferably ≤1min), requiring no complex pretreatment or post-treatment procedures. Furthermore, the recovery... The reactants and extractants can be directly returned to steps S1 and S2 for reuse, simplifying the operation process, reducing the difficulty of process control, and making it more suitable for large-scale continuous production. The distillation and pyrolysis processes in steps S3 and S4 simultaneously recover the reactants and extractants. After step S2, the residual formaldehyde and reactants in the aqueous phase are recovered by back-extraction, and all recovered materials are returned to step S1 as raw materials. This reduces the loss of reactants and extractants as well as the waste of formaldehyde, significantly improves the material utilization rate, and reduces raw material costs and waste emissions, thus balancing economic efficiency and environmental protection.

[0019] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0020] Appendix Figure 1 This is a process flow diagram for preparing anhydrous formaldehyde from an aqueous formaldehyde solution according to the present invention. Detailed Implementation

[0021] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the following, in conjunction with the appended tables and preferred embodiments, details the specific implementation and effects of a method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution according to the present invention. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, the results of one or more embodiments can be combined in any suitable manner. These embodiments are provided to make the invention thorough and complete, and to fully express the scope of the invention to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​described in these embodiments should be interpreted as merely exemplary and not as limiting.

[0022] This invention proposes a method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, the core process flow of which is shown in the attached figure. Figure 1 As shown: First, an alcoholic reactant is added to an aqueous formaldehyde solution, and the reaction is carried out at 50-100°C and normal pressure for 1.0-3.0 hours to generate formaldehyde hemiacetal. In some specific embodiments of the present invention, the mass concentration of the aqueous formaldehyde solution is 37-50%, which can be purchased directly from the market or prepared by conventional methods such as methanol oxidative dehydrogenation. The specific preparation steps are not limited in the present invention.

[0023] After the reaction is complete, an extractant is added to the reaction solution, and the mixture is allowed to stand and separate into layers. The upper layer is an organic phase containing formaldehyde hemiacetal, reactant, and extractant, while the lower layer is an aqueous phase. This step completes the initial dehydration of formaldehyde hemiacetal. This invention achieves rapid hemiacetalization and excellent oil-water separation by optimizing the selection, dosage ratio, and process parameters of the reactant and extractant. The synergistic effect of the reactant and extractant ensures that the standing separation time of the reaction solution is ≤5 min, the resulting organic phase has a water content ≤10 wt%, and the aqueous phase has a formaldehyde content ≤1 wt%, significantly reducing the load on subsequent dehydration and separation, thereby reducing process energy consumption.

[0024] The next step involves distilling the organic phase to remove moisture, yielding formaldehyde hemiacetal with a water content ≤100 ppm. In this step, the extractant in the organic phase also functions as an azeotropic agent. Through the azeotropic distillation mechanism, the extractant and water azeotropically distill from the top of the column, while the formaldehyde hemiacetal and a small amount of reactant are collected from the bottom. This achieves deep removal of moisture from the formaldehyde hemiacetal at a relatively low temperature and with low energy consumption. The temperature and other parameters of this azeotropic distillation step can be performed using conventional methods in the art, and the present invention does not impose specific limitations on them. In some specific embodiments of the present invention, it is preferred that the distillation is carried out at a temperature below the lowest atmospheric boiling point of the reactant.

[0025] Finally, formaldehyde hemiacetal is pyrolyzed and absorbed by alcohol to decompose it into gaseous formaldehyde and liquid C5-C10 straight-chain fatty alcohol (reactant). The gaseous formaldehyde is then contacted with a small molecule alcohol, and the formaldehyde is absorbed by the small molecule alcohol to generate an anhydrous formaldehyde solution. The temperature and other parameters of pyrolysis and alcohol absorption in this step can be performed using conventional methods in the art, and the present invention does not impose specific limitations on them. In some specific embodiments of the present invention, it is preferred that the pyrolysis is carried out at atmospheric pressure at a temperature higher than the highest atmospheric pressure boiling point of the reactant.

[0026] In the above technical solution, the selection of the type of reactant is one of the core key aspects of this invention. The preferred reactant is a C5-C10 straight-chain fatty alcohol, more preferably at least one selected from n-pentanol, n-hexanol, n-heptanol, and n-octanol, and even more preferably n-pentanol and / or n-hexanol. This selection is based on the characteristics of the formaldehyde hemiacetalization reaction, the dehydration requirements of formaldehyde aqueous solution, and the synergistic compatibility with subsequent extraction layering, distillation, and pyrolysis processes. Its molecular structure and carbon chain length are highly matched to the entire process of this invention.

[0027] This invention discards branched-chain fatty alcohols, short-chain (C1~C4) fatty alcohols, and long-chain (C11 and above) fatty alcohols, selecting C5~C10 straight-chain fatty alcohols. The core reason is that short-chain fatty alcohols are infinitely miscible with water, and the hemiacetals they form with formaldehyde are easily soluble in the aqueous phase, making it impossible to effectively transfer formaldehyde from the aqueous phase to the organic phase. Moreover, these hemiacetals have poor stability and are easily hydrolyzed, making it impossible to achieve stable formaldehyde fixation. The branched structure of branched-chain fatty alcohols creates significant steric hindrance, resulting in a slow hemiacetalization reaction rate and low equilibrium conversion rate, easily inducing formaldehyde self-polymerization. At the same time, branched-chain alcohols have poor compatibility with straight-chain extractants, which reduces the efficiency of subsequent extraction and layering. Long-chain fatty alcohols all have boiling points >200℃ at normal pressure, which easily leads to premature decomposition of hemiacetals during subsequent distillation and dehydration. When pyrolyzing to regenerate formaldehyde, the temperature needs to be significantly increased, increasing process energy consumption. In addition, their high viscosity makes them prone to emulsification when mixed with extractants, destroying the layering effect.

[0028] C5-C10 straight-chain fatty alcohols have moderate carbon chain lengths, lacking the hydrophilicity of short-chain alcohols and the steric hindrance of branched alcohols, as well as the high boiling point and high viscosity issues of long-chain alcohols. They are highly compatible with the entire process of hemiacetalization, extraction layering, and distillation pyrolysis in this invention, making them the core key to achieving efficient formaldehyde fixation and low-energy dehydration. These straight-chain fatty alcohols have no steric hindrance, resulting in high efficiency in the hemiacetalization reaction with formaldehyde and low activation energy, achieving high formaldehyde conversion within 1.0-3.0 hours. Furthermore, the generated formaldehyde hemiacetal has good stability; the electron-donating effect of the straight-chain alkyl group reduces the tendency for hemiacetal hydrolysis, ensuring that formaldehyde exists in a stable form within the system and preventing the self-polymerization of free formaldehyde into paraformaldehyde. Meanwhile, this invention limits the molar ratio of formaldehyde to reactant to 1:1.0~1.5. Excessive linear fatty alcohol can drive the reaction forward, further improving the formaldehyde conversion efficiency and ensuring that free formaldehyde is almost completely converted into hemiacetal, thus eliminating the risk of formaldehyde self-polymerization from the root.

[0029] C5-C10 straight-chain fatty alcohols, as core reactants, do not merely perform the single function of hemiacetalization. Instead, they achieve a triple core effect through their molecular properties, becoming a crucial link connecting the entire process: First, they stabilize and fix formaldehyde. Under the process conditions defined in this invention, the formaldehyde conversion rate can reach over 98%, and the free formaldehyde content in the system is extremely low, completely avoiding problems such as decreased product yield and equipment blockage caused by formaldehyde self-polymerization, ensuring continuous and stable process operation. Second, they have a phase transfer function. C5-C10 straight-chain fatty alcohols are hydrophobic alcohols, and the hemiacetals they form with formaldehyde are also hydrophobic compounds, completely insoluble in the aqueous phase. This provides a foundation for subsequent layering after the addition of the extractant, enabling efficient transfer of formaldehyde from the aqueous phase to the organic phase and completing the initial separation of formaldehyde and water. Furthermore, the process exhibits synergistic compatibility; C5-C10 straight-chain fatty alcohols are infinitely miscible with the hydrophobic extractant selected in this invention, forming a homogeneous organic phase after mixing. This process optimizes the polarity of the organic phase, ensuring moderate interfacial tension between the organic and aqueous phases, preventing emulsification, and reducing the solubility of the organic phase in water. This results in excellent effects such as a standing layering time ≤5 min (preferably ≤1 min) and an organic phase water content ≤10 wt%, improving extraction layering efficiency and reducing energy consumption for subsequent deep distillation and dehydration.

[0030] Furthermore, the atmospheric boiling points of C5-C10 straight-chain fatty alcohols are in the range of 137℃ (n-pentanol) to 231℃ (n-decanol), with the commonly used n-pentanol to n-octanol having boiling points concentrated between 137℃ and 195℃. This highly matches the process temperature requirements of the distillation and pyrolysis processes of this invention: during distillation dehydration, the process temperature is lower than the lowest atmospheric boiling point of the reactants (≤137℃), so the straight-chain fatty alcohols will not volatilize, and only trace amounts of water in the system are removed. This achieves a deep dehydration effect with a formaldehyde hemiacetal water content ≤100ppm, while also avoiding... It eliminates the need for reactant loss and high vacuum conditions, and can be carried out at atmospheric pressure. During pyrolysis regeneration, the process temperature is higher than the highest atmospheric pressure boiling point of the reactant (≥231℃; if short-chain alcohols such as n-pentanol and n-hexanol are selected, the pyrolysis temperature can be adapted to 180~200℃). Formaldehyde hemiacetal can be rapidly decomposed into formaldehyde and straight-chain fatty alcohols in a reverse reaction. Formaldehyde is released in gaseous form and is rapidly absorbed by methanol / ethanol, while the straight-chain fatty alcohols are retained in liquid form, which is convenient for recycling and reuse. Moreover, the pyrolysis temperature does not need to be significantly increased, reducing process energy consumption.

[0031] Among C5-C10 straight-chain fatty alcohols, this invention further prefers n-pentanol and n-hexanol. Compared with longer-chain straight-chain fatty alcohols such as n-heptanol and n-octanol, these two have more prominent technical advantages and are more suitable for industrial production: n-pentanol and n-hexanol have lower atmospheric boiling points, and distillation dehydration and pyrolysis regeneration can be achieved at lower temperatures, further reducing energy consumption; their viscosity is significantly lower, resulting in better mass transfer efficiency during mixing reactions and extraction layering, which can further shorten reaction time and layering time, and improve the overall process efficiency; they are commonly used fatty alcohol products in industry, with ample market supply and low prices, which can effectively reduce raw material procurement costs and improve the economic efficiency of industrialization; n-pentanol and n-pentane, and n-hexanol and n-hexane are straight-chain alcohols and straight-chain alkanes with the same carbon chain, exhibiting the best compatibility, good organic phase homogeneity after mixing, and better extraction layering effect, achieving shorter layering time and lower organic phase water content.

[0032] This invention strictly limits the reactants to C5~C10 straight-chain fatty alcohols (preferably n-pentanol and n-hexanol). This is a key technology choice based on the characteristics of formaldehyde hemiacetalization reaction, the core requirement of formaldehyde aqueous solution dehydration, and the synergistic adaptability of the entire process. It directly determines the formaldehyde conversion efficiency, hemiacetal stability, extraction stratification effect, and process energy consumption level. It is one of the core keys to solving technical problems such as excessive energy consumption in formaldehyde aqueous solution dehydration and easy polymerization of formaldehyde.

[0033] In some specific embodiments of the present invention, the extractant is selected from at least one of n-pentane, n-hexane, cyclohexane, and toluene. Based on 100% of the reactant mass, the amount of extractant added is 40-60%, more preferably 50%. In this invention, the extractant does not only perform the function of extraction and layering, but also serves as an azeotropic agent in the subsequent distillation and dehydration step, achieving a dual function. This design is based on a comprehensive determination of the extractant's physicochemical properties, its compatibility with the system, and the requirements for azeotropic dehydration. It matches the requirements of the extraction and layering process while efficiently achieving water removal in the distillation stage. Through precise dosage control, the two functions of extraction and azeotropy achieve optimal synergistic effects.

[0034] The n-pentane, n-hexane, cyclohexane, and toluene selected in this invention are all non-polar / weakly polar organic solvents, which can simultaneously meet the core functional requirements of both extractants and azeotropic agents, making them the optimal material selection that balances the needs of both process stages. As an extractant, this type of solvent is insoluble in water, has a moderate density difference with water, and is completely miscible with the reactant and formaldehyde hemiacetal to form a homogeneous organic phase. It can fully encapsulate the reactant and hemiacetal, achieving efficient transfer of formaldehyde from the aqueous phase to the organic phase. Moreover, it is chemically stable and does not undergo side reactions with any component in the system within the process temperature range of this invention, only playing a role in physical dissolution and phase separation. As an azeotropic agent, this type of extractant can form a stable minimum azeotrope with water, with an azeotropic boiling point in the range of 68~84℃, which is far lower than the atmospheric pressure boiling point of water and the upper limit of the distillation process temperature. It can quickly and efficiently remove water, and does not form azeotropes with C5~C10 straight-chain fatty alcohols and formaldehyde hemiacetal, avoiding material entrainment and loss. At the same time, the azeotrope formed with water is a heterogeneous azeotrope, which can be quickly separated into an aqueous phase and a solvent phase after condensation. The azeotropic agent can be recovered and reused without additional separation methods, making the process simple and efficient.

[0035] In the above technical solutions, the compatibility priority of straight-chain alcohols with different carbon chains and extractants is determined based on "carbon chain length matching". Short-chain straight-chain alcohols (n-pentanol, n-hexanol) are compatible with n-pentane and n-hexane (alkanes with the same carbon chain) because of their high molecular structure similarity and best compatibility, with a separation time ≤1min. Long-chain straight-chain alcohols (n-heptanol, n-octanol) are compatible with cyclohexane and toluene (cyclic / aromatic extractants) because of their high molecular volume matching, with a separation time ≤10min.

[0036] In this invention, the distillation dehydration is performed at atmospheric pressure azeotropic distillation. The core principle is to utilize the characteristic of the extractant (azeotropic agent) forming a minimum azeotrope with water, breaking the difficult separation state of the formaldehyde-water system due to hydrogen bonding. This achieves efficient removal of trace amounts of water from the organic phase at a temperature below the minimum atmospheric pressure boiling point of the reactant. Before distillation, the organic phase obtained after extraction and separation has a water content ≤10wt%. The organic phase is fed into a distillation column for atmospheric pressure distillation, with the column bottom temperature controlled at 85~110℃. The extractant and water form a water-extractant minimum azeotrope, which escapes in gaseous form. After rising to the top of the distillation column, it is condensed and cooled, and then separated into an aqueous phase and an extractant phase. The lower aqueous phase is directly discharged, while the upper extractant phase can be refluxed back to the distillation column for continued use, or recovered and returned to reaction step S2 for reuse. As the distillation process continues, moisture is continuously removed from the column, ultimately yielding formaldehyde hemiacetal with a water content ≤100ppm in the bottoms. A small amount of reactant, due to its boiling point being higher than the distillation temperature, remains in the bottoms along with the formaldehyde hemiacetal and is recovered in the subsequent pyrolysis step. In this invention, the main recovery paths for the extractant and reactant are different; even if a small amount of extractant and reactant are mixed during the process, separation is unnecessary, and they can be directly recycled to the reaction step S1. This atmospheric pressure azeotropic distillation eliminates the need for high temperature and high vacuum conditions, fundamentally reducing energy consumption in the distillation process. This is one of the key technical means by which this invention solves the problem of excessive energy consumption in the dehydration of formaldehyde aqueous solutions.

[0037] This invention precisely quantifies and limits the amount of extractant added, determining the optimal range based on the phase separation requirements of extraction stratification and the material ratio requirements of azeotropic dehydration. This ensures that the extractant achieves its best effect in both process stages; excessive or insufficient addition will lead to a decrease in process efficiency. The lower limit is set at 40% to guarantee the minimum material requirements for azeotropic dehydration, ensuring sufficient extractant to fully encapsulate water in the organic phase for complete water removal. Simultaneously, it ensures the organic phase possesses sufficient hydrophobicity and volume to achieve efficient formaldehyde transfer, rapid stratification, and low water content, avoiding increased load on subsequent distillation dehydration. The upper limit is set at 60% to avoid material waste and increased energy consumption due to excessive extractant, preventing over-diluting of formaldehyde hemiacetal, which would increase throughput and energy consumption in subsequent pyrolysis stages. When the amount of extractant added is controlled at 40-60% (based on the mass of the reactant being 100%), the optimal synergistic balance between the extraction function and the azeotropic function can be achieved. This satisfies the requirements of high efficiency and low water content in extraction stratification, while ensuring the thoroughness of azeotropic dehydration, and at the same time avoiding the increase in energy consumption and cost caused by excessive material.

[0038] This invention combines the extractant and azeotropic agent into a single process design, achieving deep synergy between the two major process steps of extraction stratification and distillation dehydration. Compared to the traditional two-material system of "extractant + dedicated azeotropic agent," it has significant technical advantages: there is no need to separately purchase and store dedicated azeotropic agents, and no need to add an azeotropic agent addition step before distillation, resulting in a simpler process flow, reduced equipment investment costs, and better suitability for continuous industrial production; the extractant has already achieved preliminary dehydration of the organic phase during the extraction stage, reducing the load on distillation dehydration, and at the same time, as an azeotropic agent, it can achieve deep dehydration at medium and low temperatures and normal pressure. The energy consumption of water distillation is significantly reduced compared to traditional processes, fundamentally solving the core pain point of excessive energy consumption in formaldehyde aqueous solution dehydration. The extractant plays only a physical role in both extraction and azeotropic processes, with no chemical loss. It can be directly reused after recovery, and the loss of extractant in the entire process is almost zero, greatly improving material utilization and reducing raw material procurement and consumption costs. The use of a single extractant (azeotropic agent) avoids the problems of miscibility and azeotropic interference caused by mixing multiple solvents, ensuring the purity and stability of the process system and avoiding problems such as formaldehyde polymerization and product purity reduction caused by the introduction of impurities.

[0039] In summary, this invention selects n-pentane, n-hexane, cyclohexane, and toluene as extractants, limiting their addition to 40-60% (based on 100% of the reactant mass), and endows them with azeotropic agent functions. This is a key technical design based on material characteristics, process requirements, and industrialization requirements. This design achieves synergy between extraction and azeotropic processes, enabling a single material to perform dual core functions. It not only solves the core technical pain point of formaldehyde aqueous solution dehydration but also simplifies the process, reduces energy consumption and costs. This is an important support for the industrial feasibility and significant technical advantages of the process of this invention.

[0040] In a preferred embodiment of the present invention, the reaction temperature in step one is 70~95℃, the reaction time is 1.0~2.0h, and the molar ratio of formaldehyde to reactant is 1:1.0~1.2. Under this parameter combination, the formaldehyde conversion rate is ≥98%, the hemiacetal selectivity is ≥99%, and the process energy consumption is lower, which is more suitable for the efficiency and cost requirements of industrial production.

[0041] The preferred small molecule alcohols are methanol and / or ethanol, with methanol being more preferred. The core reasons are as follows: Both can rapidly undergo a reversible hemiacetalization reaction with gaseous formaldehyde, achieving efficient formaldehyde absorption. They only react with formaldehyde and are immiscible and non-reactive with hydrophobic C5-C10 straight-chain fatty alcohols and extractants in the system, making them easy to separate and preventing the introduction of impurities. Both have low boiling points and low viscosity, resulting in high gas-liquid mass transfer efficiency. The absorption process can be carried out under normal / low temperature and normal pressure conditions, with low energy consumption. Subsequent formaldehyde desorption only requires gentle heating, making the operation simple. Methanol can be linked with the raw materials for the preparation of formaldehyde aqueous solution (methanol oxidative dehydrogenation method), enabling the utilization of methanol as a process byproduct to realize material resource utilization. Formaldehyde-methanol / ethanol solution is a common form of formaldehyde storage, transportation, and application in industry. It can be directly sold as a product without the need for additional separation of alcohol and formaldehyde, directly meeting downstream demand. Moreover, both are bulk chemical raw materials with ample supply and low prices, requiring no special equipment, making industrialization feasible and offering significant cost advantages.

[0042] In some specific embodiments of the present invention, after step S2, the same extractant as in step S2 is added to the aqueous phase for back-extraction to recover residual formaldehyde and reactant in the aqueous phase. After back-extraction, the formaldehyde content in the aqueous phase is ≤100mg / L, and it can be discharged as normal wastewater.

[0043] The optimal back-extraction temperature is 20~30℃, which is within the normal temperature range. This ensures good compatibility and mass transfer efficiency between the extractant and residual formaldehyde and reactants in the aqueous phase, enabling rapid transfer of formaldehyde and reactants from the aqueous phase to the organic phase. It also avoids the volatilization and loss of the extractant due to high temperature, preventing formaldehyde from self-polymerizing in water. Furthermore, it does not reduce the mass transfer rate or prolong the back-extraction time due to low temperature, thus balancing efficiency and material stability. Moreover, it eliminates the need for additional heating / cooling, significantly reducing energy consumption.

[0044] The optimal mass ratio of aqueous phase to added extractant is 1:0.2~0.5. This ratio ensures sufficient extractant to fully encapsulate residual formaldehyde and reactants in the aqueous phase, achieving efficient recovery and minimizing formaldehyde loss. Simultaneously, it avoids material waste and increased load on subsequent separations due to excessive extractant, prevents excess extractant from carrying water that could affect the purity of the recovered material, controls the throughput of the back-extraction equipment, and reduces process costs. The recovered material (formaldehyde + reactant + extractant) obtained at this temperature and liquid-to-powder ratio has a purity and system ratio directly compatible with the reaction requirements of step one, allowing for reuse without additional purification. This aligns perfectly with the overall material recycling design, simplifying operation while improving the overall material utilization rate of the process.

[0045] The present invention will be further described below with reference to specific embodiments, but this should not be construed as a limitation on the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above description of the present invention still fall within the scope of protection of the present invention.

[0046] Unless otherwise specified, all materials and reagents mentioned below are commercially available products well known to those skilled in the art; unless otherwise specified, all methods described are methods known in the art. Unless otherwise defined, the technical or scientific terms used should have the ordinary meaning understood by those skilled in the art to which this invention pertains. Example 1

[0047] S1. Formaldehyde hemiacetalization reaction: Add 1170g of n-pentanol to 1000g of formaldehyde aqueous solution with a mass concentration of 37wt%, and stir the reaction at 70℃ and normal pressure for 1.5h to generate formaldehyde hemiacetal. S2. Extraction and layering: Add 585g of n-pentane to the reaction solution and let it stand for 0.8min to separate the layers. The upper layer is an organic phase containing formaldehyde hemiacetal, n-pentanol and n-pentane (mass 2307g, water content 8.5wt%), and the lower layer is an aqueous phase (mass 448g, formaldehyde content 0.8wt%). S3. Distillation, Dehydration, and Material Recovery: The organic phase is fed into a distillation column and distilled at atmospheric pressure and 95°C for 1.5 hours to remove water, yielding 1526g of formaldehyde hemiacetal with a water content of 85ppm. After condensation at the top of the column, 584g of the extractant n-pentane (purity ≥99.5%) is separated and recovered, which can be directly recycled. The unreacted n-pentanol remaining in the bottom of the column is recycled together with the formaldehyde hemiacetal in the S4 pyrolysis step. S4. Pyrolysis and Absorption: Formaldehyde hemiacetal was fed into the bottom of the pyrolysis absorption tower and pyrolyzed at atmospheric pressure and 180°C for 1.0 h. The formaldehyde gas produced was absorbed by methanol at an absorption temperature of 110°C to obtain 751 g of anhydrous formaldehyde methanol solution with a formaldehyde concentration of 49 wt% and a water content of 120 ppm. The liquid reactant obtained from pyrolysis was recovered and reused separately. S5. Back-extraction and recovery: Add 107g of n-pentane to the aqueous phase of step S2 and back-extract at 25℃ for 30min. After back-extraction, the formaldehyde content in the aqueous phase is 85mg / L. 118g of a mixture containing formaldehyde and n-pentanol is recovered and returned to step S1 as a reaction feedstock.

[0048] In this embodiment, the total formaldehyde recovery rate is 99.5%, and the energy consumption of the dehydration process is 740 kcal. Example 2

[0049] S1. Formaldehyde hemiacetalization reaction: Add 1850g of n-hexanol to 1000g of formaldehyde aqueous solution with a mass concentration of 50wt%, and stir the reaction at 95℃ and normal pressure for 1.0h to generate formaldehyde hemiacetal. S2. Extraction and layering: Add 925g of n-hexane to the reaction solution and let it stand for 0.5min to separate the layers. The upper layer is an organic phase containing formaldehyde hemiacetal, n-hexanol and n-hexane (mass 3435g, water content 5.2wt%), and the lower layer is an aqueous phase (mass 340g, formaldehyde content 0.6wt%). S3. Distillation, Dehydration, and Material Recovery: The organic phase is fed into a distillation column and distilled at atmospheric pressure and 105°C for 2.0 hours to remove water, yielding 2333g of formaldehyde hemiacetal with a water content of 72ppm. After condensation at the top of the column, 923g of hexane (purity ≥99.5%) is separated and recovered as the extractant, which can be directly recycled. The unreacted hexanol remaining in the bottom of the column is recycled along with the formaldehyde hemiacetal in the S4 pyrolysis step. S4. Pyrolysis and absorption: Formaldehyde hemiacetal was fed into the bottom of the pyrolysis absorption tower and pyrolyzed at atmospheric pressure and 200℃ for 0.8h. The generated formaldehyde gas was absorbed by ethanol at an absorption temperature of 105℃ to obtain 1288g of anhydrous formaldehyde ethanol solution with a formaldehyde concentration of 39wt% and a water content of 80ppm. S5. Back-extraction and recovery: Add 297.5g of n-hexane to the aqueous phase of step S2 and back-extract at 20℃ for 40min. After back-extraction, the formaldehyde content in the aqueous phase is 78mg / L. 306g of a mixture containing formaldehyde and n-hexanol is recovered and returned to step S1 as a reaction feedstock.

[0050] In this embodiment, the total formaldehyde recovery rate was 99.8%, and the energy consumption for the dehydration process was 630 kcal. Example 3

[0051] S1. Formaldehyde hemiacetalization reaction: Add 1320g of a mixture of n-pentanol and n-hexanol in a mass ratio of 1:1 to 1000g of a 40wt% formaldehyde aqueous solution, stir and react for 2.0h at 85℃ and normal pressure to generate formaldehyde hemiacetal. S2. Extraction and layering: Add 726g of cyclohexane to the reaction solution and let it stand for 0.6min to separate the layers. The upper layer is an organic phase containing formaldehyde hemiacetal, mixed reactants and cyclohexane (mass 2606g, water content 6.1wt%), and the lower layer is an aqueous phase (mass 440g, formaldehyde content 0.7wt%). S3. Distillation, Dehydration, and Material Recovery: The organic phase is fed into a distillation column and distilled at atmospheric pressure and 100°C for 1.8 hours to remove water, yielding 1723g of formaldehyde hemiacetal with a water content of 78ppm. After condensation at the top of the column, 724g of cyclohexane (purity ≥99.5%) of the extractant is separated and recovered, which can be directly recycled. The unreacted n-pentanol and n-hexanol remaining in the bottom of the column are recovered together with the formaldehyde hemiacetal in the S4 pyrolysis step. S4. Pyrolysis and absorption: Formaldehyde hemiacetal was fed into the bottom of the pyrolysis absorption tower and pyrolyzed at atmospheric pressure and 190°C for 1.2 hours. The formaldehyde gas produced was absorbed by a mixture of methanol and ethanol at an absorption temperature of 110°C, yielding 920g of anhydrous formaldehyde mixed alcohol solution with a formaldehyde concentration of 43wt% and a water content of 90ppm. S5. Back-extraction and recovery: Add 178.8g of cyclohexane to the aqueous phase of step S2 and back-extract at 30℃ for 35min. After back-extraction, the formaldehyde content in the aqueous phase is 92mg / L. 187g of a mixture containing formaldehyde and mixed reactants is recovered and returned to step S1 as a reaction raw material.

[0052] In this embodiment, the total formaldehyde recovery rate is 99.9%, and the energy consumption of the dehydration process is 680 kcal.

[0053] Comparative Example 1 (Traditional Vacuum Distillation Dehydration Method) Add 0.5g of polymerization inhibitor (hydroquinone) to 1000g of a 37wt% formaldehyde aqueous solution, and feed it into a vacuum distillation column. Distill at -0.09MPa and 160℃ for 3.0h to remove water and obtain 322g of anhydrous formaldehyde with a water content of 1.2wt%. The formaldehyde recovery rate is 86%, and the energy consumption of the dehydration process is 1230kcal.

[0054] Comparative Example 2 (Isooctanol Hemiacetal Method) S1. Formaldehyde hemiacetalization reaction: Add 1900g of isooctanol to 1000g of formaldehyde aqueous solution with a mass concentration of 37wt%, and stir the reaction at 70℃ and normal pressure for 4h to generate formaldehyde hemiacetal. S2, Extraction and Layering: Add 950g of toluene to the reaction solution, let stand for 30min to separate the layers. The upper organic phase weighs 3215g and contains 22wt% water, while the lower aqueous phase weighs 635g and contains 13.4wt% formaldehyde. S3 Distillation Dehydration and Material Recovery: The organic phase is fed into a distillation column and distilled at -0.08 MPa and 120 °C for 4.0 h to remove water, yielding 1597 g of a mixed solution of formaldehyde hemiacetal with a water content of 150 ppm and unreacted isooctanol; after condensation at the top of the column, 894 g of toluene, the extractant, is separated and recovered; the unreacted isooctanol remaining in the bottom of the column is recovered along with the formaldehyde hemiacetal in the S4 pyrolysis step. S4. Pyrolysis and Absorption: Formaldehyde hemiacetal was fed into the bottom of the pyrolysis absorption tower and pyrolyzed at atmospheric pressure and 110°C for 2.5 hours. The generated formaldehyde gas was absorbed by methanol at an absorption temperature of 90°C, yielding 830g of anhydrous formaldehyde methanol solution with a formaldehyde concentration of 35wt% and a water content of 180ppm.

[0055] In this comparative example, the total formaldehyde recovery rate was 78.5%, and the energy consumption for the dehydration process was 820 kcal.

[0056] As can be seen from the comparison of the above embodiments and comparative examples, the method of the present invention is significantly better than the traditional process in terms of formaldehyde recovery rate and product purity, and the dehydration energy consumption is greatly reduced. It completely solves the core problems of the prior art, such as high energy consumption for formaldehyde aqueous solution dehydration, easy polymerization of formaldehyde, and low recovery rate.

[0057] The technical features in the claims and / or specification of this invention can be combined, and the combination is not limited to the combinations obtained through reference in the claims. Technical solutions obtained by combining the technical features in the claims and / or specification are also within the scope of protection of this invention.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing anhydrous formaldehyde from an aqueous formaldehyde solution, characterized in that, It includes the following steps: S1. Add a reactant to an aqueous formaldehyde solution and react at 50-100°C under normal pressure for 1.0-3.0 h to generate formaldehyde hemiacetal; the reactant is selected from at least one of C5-C10 straight-chain fatty alcohols; the molar ratio of formaldehyde to reactant is 1:1.0-1.

5. After the S2 reaction is completed, an extractant is added to the reaction solution, and the mixture is allowed to stand and separate into layers. The upper layer is an organic phase containing formaldehyde hemiacetal, reactant, and extractant, and the lower layer is an aqueous phase. The extractant is selected from at least one of n-pentane, n-hexane, cyclohexane, and toluene. The amount of extractant added is 40-60% based on the mass content of the reactant being 100%. S3 performs distillation on the organic phase to remove water, obtaining formaldehyde hemiacetal with a water content ≤100ppm; S4 pyrolyzes the formaldehyde hemiacetal and absorbs it with a small molecule alcohol to obtain an anhydrous formaldehyde small molecule alcohol solution; the small molecule alcohol is methanol and / or ethanol.

2. The method according to claim 1, characterized in that, The reactant is selected from at least one of n-pentanol, n-hexanol, n-heptanol, and n-octanol.

3. The method according to claim 2, characterized in that, The reactant is selected from at least one of n-pentanol and n-hexanol; the reaction temperature is 70~95℃, the reaction time is 1.0~2.0h; and the molar ratio of formaldehyde to reactant is 1:1.0~1.

2.

4. The method according to claim 3, characterized in that, The reactant and extractant work together to ensure that the reaction solution has a settling time of ≤5 min, and that the resulting organic phase has a water content of ≤10 wt% and the aqueous phase has a formaldehyde content of ≤1 wt%.

5. The method according to claim 4, characterized in that, The reactant and extractant allow the reaction solution to separate into layers within a time limit of 1 minute.

6. The method according to claim 1, characterized in that, In step S3, the extractant is recovered during distillation, and the recovered extractant can be directly returned to step S2; in step S4, the reactant is recovered during pyrolysis, and the recovered reactant can be directly returned to step S1.

7. The method according to claim 1, characterized in that, After step S2, the process further includes adding the same extractant as in step S2 to the aqueous phase for back-extraction to recover residual formaldehyde and reactants in the aqueous phase. After back-extraction, the formaldehyde content in the aqueous phase is ≤100mg / L.

8. The method according to claim 7, characterized in that, The back-extraction temperature is 20~30℃; the mass ratio of the aqueous phase to the extractant added thereto is 1:0.2~0.

5.

9. The method according to claim 7, characterized in that, The residual formaldehyde and reactants in the recovered aqueous phase are returned to step S1 for use as reaction raw materials.

10. The method according to claim 5, characterized in that, The distillation described in step S3 is carried out at a temperature below the lowest atmospheric boiling point of the reactant; or, the pyrolysis described in step S4 is carried out at atmospheric pressure at a temperature above the highest atmospheric boiling point of the reactant.

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