JP3827455B2 - Propylene oxide production method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a novel method for producing propylene oxide. Specifically, a method for producing high-purity propylene oxide that can suppress by-products of acetaldehyde very effectively and, in some cases, can simplify the process of removing low boilers by rectification of the resulting propylene oxide. It is.
[0002]
[Prior art]
Conventionally, a method for producing propylene oxide (hereinafter abbreviated as PO) includes an indirect oxidation method in which propylene is converted to PO using an oxidizing agent in the presence of a catalyst, and hypochlorous acid is allowed to act on propylene in an aqueous medium. A chlorohydrin method in which propylene chlorohydrin (hereinafter abbreviated as PCH) is once obtained and dehydrochlorinated (referred to as saponification) to obtain PO is industrially carried out.
[0003]
Each of these methods has advantages and disadvantages, and which one is adopted depends on the circumstances of each company, but the present invention relates to an improvement of the chlorohydrin method.
[0004]
In the chlorohydrin method, hypochlorous acid produced by, for example, dissolving chlorine in an aqueous medium is reacted with propylene to produce an aqueous solution containing 3 to 6% by weight of PCH and hydrogen chloride produced as a by-product at the same time. Then, after removing hydrogen chloride by distillation or neutralization, or simultaneously with neutralization, saponification is performed with alkali such as slaked lime or sodium hydroxide to obtain PO.
[0005]
When the PO produced in the saponification process comes into contact with hydrogen chloride, other by-products or metal compounds, by-products such as aldehydes and glycols are promoted. Generally, steam is introduced in the saponification process, The PO generated at the same time as the conversion is instantaneously stripped. Further, the stripped PO is also purified by removing low-boiling impurities by distillation and rectifying the PO.
[0006]
[Problems to be solved by the invention]
PO is polycondensed into a polyol mainly for polyurethane raw materials, surfactants and other uses. However, polyols using PO obtained by the chlorohydrin method are often colored. As a result of pursuing the cause, it has been found that it is caused by a trace amount of acetaldehyde, which is an impurity contained in PO.
[0007]
Accordingly, as a result of investigating how much acetaldehyde can be reduced to avoid coloring, it has been found that acetaldehyde in PO needs to be kept at 15 ppm or less, preferably 10 ppm or less.
[0008]
Regarding the separation of acetaldehyde, the boiling point is 20.2 ° C., and the boiling point of PO is 34 ° C., and the boiling point is not more than 10 ° C. This requires tens of shelves and consumes a large amount of energy, which is not economical.
[0009]
It is needless to say that the most preferable is to suppress the by-production of acetaldehyde.
[0010]
Therefore, the present inventors have investigated the mechanism by which acetaldehyde is produced as a by-product, and elucidated that it is mainly a saponification process. Next, paying attention to the amount of acetaldehyde produced, and as a result of intensive studies on the factors that can reduce the amount produced, the concentration of hypochlorous acid in the aqueous reaction solution supplied from the reaction step to the saponification step is determined. It was found that the amount of acetaldehyde in PO produced in the saponification step can be controlled to a significantly low level by controlling it to a specific value or less, and the present invention has been completed.
[0011]
[Means for Solving the Problems]
That is, the present invention provides a reaction step of supplying chlorine and propylene in an aqueous medium to obtain a reaction aqueous solution containing propylene chlorohydrin, and a saponification step of dehydrochlorinating the propylene chlorohydrin with an alkali. Provided is a method for producing propylene oxide, characterized in that, in the oxide production process, the concentration of hypochlorous acid in the reaction aqueous solution supplied to the saponification step is controlled to 10 ppm or less, preferably 5 ppm or less.
[0012]
The greatest feature of the present invention is that, in the chlorohydrin method, the concentration of hypochlorous acid in the aqueous solution containing PCH supplied to the saponification step is suppressed, so that acetaldehyde in the product PO is 15 ppm or less, preferably 10 ppm. Is to: Therefore, any conventionally known means can be adopted as the various processing means and the like performed in the step of obtaining PCH, the saponification step, before, after or in the middle thereof.
[0013]
Furthermore, the present inventors have also found a method suitable for controlling the hypochlorous acid concentration in the aqueous reaction solution supplied to the saponification step. That is, the present invention detects the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification step, and based on the detected value, (1) propylene / chlorine supply ratio, (2) reaction in the reaction step. There is also provided a method for producing propylene oxide, characterized by controlling at least one of the pressure in the gas phase in the step and (3) the flow rate in the reaction zone.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improvement in a method for producing PO by a conventionally known chlorohydrin method. An example of the manufacturing process will be described with reference to FIG.
[0015]
In the case of the chlorohydrin method, the entire process generally consists of three steps: a reaction step, a saponification step, and a purification step. In the reaction step, hypochlorous acid and propylene are reacted to obtain PCH. In general, chlorine dissolves in an aqueous medium to produce hypochlorous acid. Therefore, chlorine is supplied from the bottom of the reaction tower, and propylene is supplied to the top thereof, so that hypochlorous acid obtained by dissolving chlorine in water is supplied. Chloric acid and propylene come into contact to produce PCH. At this time, if chlorine dissolved in water comes into contact with propylene as it is, addition of chlorine to propylene occurs and by-products such as dichloropropane are produced, so that chlorine must be promptly converted to hypochlorous acid. For this reason, it is possible to add a separate chlorine dissolution step and supply the obtained hypochlorous acid aqueous solution to the reaction step. However, as in the reactor 1 shown in FIG. A preferred reaction method is to form a ring reactor, supply chlorine to the sub-column which is the downward flow, and supply propylene from the lower part of the main column which is the reaction zone. In this case, the driving force for circulation of the aqueous medium in the reactor is formed by a lift action by blowing gas such as propylene, that is, an apparent specific gravity difference between the liquid in the main tower and the sub tower. Therefore, in FIG. 1, unreacted propylene and inert gases such as propane contained in propylene, and in some cases, in order to dilute propylene, an inert gas such as mixed nitrogen is extracted from the pipe 11 and, if necessary, a washing tower. 2 and is circulated to the reaction tower via the pipes 12 and 13 by the circulating gas blower 3 to control the flow rate of the liquid flow in the main tower. That is, if this method is employed, the flow rate in the reactor can be easily controlled by changing the amount of circulating gas.
[0016]
The amount ratio of chlorine and propylene newly supplied to the reactor is theoretically 1 to 1 mole. Therefore, the propylene to chlorine ratio, that is, the propylene / chlorine molar ratio can be appropriately selected within the range of 0.9 to 1.1.
[0017]
On the other hand, the PCH produced in the reaction zone of the reactor is obtained in an aqueous solution, but if the PCH concentration to be produced is set low, the scale of the reactor etc. increases, and if it is set high, by-products such as dichloropropane. Both are not economical. Therefore, the PCH concentration in the normal aqueous solution is preferably 3 to 6% by weight. The produced PCH aqueous solution is extracted from the liquid reservoir (drum) at the top of the reactor through the pipe 14 together with hydrogen chloride by-produced simultaneously with the PCH, passes through the PCH buffer tank 4 and passes through the PCH buffer tank 4 to the saponification tower 5. It is transferred to.
[0018]
The present invention is characterized in that the concentration of hypochlorous acid in the reaction solution transferred to the saponification tower is kept at 10 ppm or less.
[0019]
The saponification tower 5 is supplied with alkali such as lime milk and steam to saponify PCH at a tower temperature of 85 to 100 ° C. Since the boiling point of PO produced by saponification of PCH is 34 ° C., it immediately evaporates and flows out from the pipe 16 together with water vapor as crude PO. In the case of the process shown in FIG. 1, simultaneously with saponification of PCH, by-product hydrogen chloride is also neutralized by lime milk [Ca (OH) ₂ ] to produce calcium chloride. They are discharged from the pipe 17 together with the reaction liquid from which PO has been removed. Of course, the reaction aqueous solution is distilled between the reactor and the saponification tower to remove most of the hydrogen chloride, or the aqueous solution is contacted with limestone [CaCO ₃ ] to convert the hydrogen chloride into calcium chloride. It may be removed, and may be preferable from the viewpoint of reducing the consumption of lime milk in the saponification tower by half.
[0020]
The crude PO discharged from the pipe 16 is then removed from the pipe 18 by low-boiling tower 6 by distilling low-boiling substances, that is, organic compounds having a boiling point lower than that of PO. Although acetaldehyde can also be removed by this low boiling tower, as described above, it is necessary to use a several tens stage distillation tower, which is not economical. If the production of acetaldehyde is reduced by carrying out the present invention, the low boiling tower can be simply distilled or have a simple structure of several stages.
[0021]
The aqueous solution of PO is guided from the pipe 19 to the rectification column 7, rectified, and taken out as a product PO from the top of the column through the pipe 20. On the other hand, high boilers such as water are taken out from the bottom of the tower through a pipe 21 and re-supplied to the reactor as process water as necessary.
[0022]
As described above, in the PO production process of the chlorohydrin method, the present invention is characterized in that the concentration of hypochlorous acid in the reaction aqueous solution supplied to the saponification tower is controlled to 10 ppm or less. Thereby, the density | concentration of the acetaldehyde in PO can be reduced to 15 ppm or less. As an example, FIG. 2 shows the relationship between hypochlorous acid concentration in the reaction solution and acetaldehyde.
[0023]
The concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower can be easily known by measuring the oxidation-reduction potential (ORP) of the solution. The relationship between the oxidation-reduction potential (mV) and the hypochlorous acid concentration is illustrated in FIG.
[0024]
A means for controlling the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower is not particularly limited. The simplest means is a reducing agent such as sodium thiosulfate, sodium sulfite, ammonium sulfide, sodium sulfide, sodium polysulfide, sulfur dioxide, etc., depending on the concentration of hypochlorous acid remaining in the reaction aqueous solution. A reducing metal compound such as lithium aluminum hydroxide, sodium aluminum hydroxide, lithium hydride, or sodium hydride, and a reducing gas such as carbon monoxide are appropriately used.
[0025]
Furthermore, in the present invention, the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower is changed by changing the ratio of propylene / chlorine supplied to the reaction tower within a range of 0.9 to 1.1. Can be controlled. That is, as the amount of propylene increases with respect to chlorine, the concentration of hypochlorous acid tends to decrease. However, since unreacted propylene increases, it is not appropriate to increase propylene beyond 1.1.
[0026]
The hypochlorous acid concentration can also be controlled by adjusting the internal pressure of the reactor, particularly the gas phase pressure at the top of the reactor. For example, as shown in FIG. 4, the acetaldehyde concentration in PO can be decreased by increasing the gas phase pressure in the column from 16 kPa to 17 kPa. Of course, the higher the internal pressure of the column, the lower the concentration of acetaldehyde in PO. However, it gradually converges to a constant value, and the equipment cost increases as the pressure becomes higher. In addition, it can be understood from FIG. 4 that the effect of reducing the acetaldehyde concentration by pressure tends to become larger as the flow rate in the reactor is slower.
[0027]
Here, the flow rate in the reactor, that is, the apparent rising speed of the liquid in the reaction region (in the main tower) is generally about 0.1 to 3 m / second, preferably about 0.5 to 1.5 m / second. However, since this flow rate generally has a correlation with the amount of circulating gas, it can be controlled by the amount of circulating gas. Of course, the specific numerical value varies depending on the shape and size of the reactor, but the correlation coefficient can be easily obtained by a simple experiment. FIG. 5 collectively shows an example of the relationship between the pressure inside the tower, the amount of circulating gas (Nm ³ / H), and the acetaldehyde concentration (ppm) in PO. The reactor used in this experiment was a U-shaped main tower (inner diameter: 2000 mm) having a different diameter and a sub tower (inner diameter: 900 mm) connected at the top by a drum.
[0028]
Reducing the circulating gas volume, that is, to reduce the reaction area flow rate, the increased influence of the tower pressure circulation gas amount, increasing to 1000 Nm ³ / H exceed 900 Nm ³ / H, the effect of pressure is reduced, Moreover, it is possible to reduce the acetaldehyde in PO to 5 ppm or less, but increasing the amount of circulating gas requires a huge blower and increases the pressure in the circulation system, resulting in a large energy loss. Therefore, it is not preferable economically.
[0029]
As explained above, as a factor for controlling the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower, (1) the ratio of propylene / chlorine supplied to the reaction tower, (2) in the reaction step At least one of the pressure in the gas phase and (3) the flow velocity in the reaction zone (this can be controlled by the amount of gas circulation), preferably two or more, particularly the pressure and the amount of gas circulation are simultaneously controlled. Thus, the concentration of hypochlorous acid in the solution can be controlled to 10 ppm or less, preferably 5 ppm or less, and further 3 ppm or less. Thus, the acetaldehyde concentration in PO can be reduced to 15 ppm or less, preferably 10 ppm or less, and further 5 ppm or less. Of course, the specific change amounts of (1), (2) and (3) are respectively different depending on the form and size of the equipment or the operating conditions, but the parties can set them very easily based on the above description. be able to.
[0030]
According to another preferred embodiment for carrying out the present invention, the reaction aqueous solution supplied to the saponification tower is measured for potential by a redox potential measuring device, and manually or automatically by a signal based on the measurement. The purpose is to control the chloric acid concentration. In this case, the control amount may be input of a reducing agent, but preferably, the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification step is detected, and the reaction step is performed based on the detected value. (1) Propylene / chlorine supply ratio, (2) Pressure in the gas phase in the reaction step, and (3) Flow rate in the reaction zone are controlled at least one of them.
[0031]
【Example】
Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.
[0032]
Example 1
Experiments were performed using the flow sheet apparatus shown in FIG.
[0033]
As the reactor, a U-shaped main tower having a different diameter and a Ti tower in which the upper part of the sub tower was connected by a drum were used. The inner diameter of the main tower was 2000 mm, the inner diameter of the sub tower was 900 mm, chlorine was blown into the sub tower side, and propylene and circulating gas were added to the main tower side, respectively, and the propylene / chlorine molar ratio was 1.1. . The supply amount of process water was adjusted so that the PCH concentration in the aqueous solution was 4 wt%. The reactor was operated at a pressure of 17 kPa and a circulating gas amount of 800 Nm ³ / H (main tower side flow rate 1.2 m / s) to generate a PCH aqueous solution. The produced PCH aqueous solution was mixed with lime milk and stripped with steam in a saponification tower, and PCH was dehydrochlorinated to be extracted as crude PO from the tower top. The amount of steam input to the saponification tower was controlled so that the temperature of the saponification tower was 100 ° C. in the range of 4 to 6 tons per ton of PCH aqueous solution.
[0034]
The composition of the aqueous PCH solution obtained in this experiment was 4% PCH, 4 ppm hypochlorous acid, and the indicated value of the ORP meter provided on the pipe 15 was 690 mV. The acetaldehyde concentration in the obtained PO was 5 ppm or less.
[0035]
Example 2
In Example 1, the reactor pressure 16～17KPa, in a range of circulating gas amount 800~900Nm ³ / H (main column side flow rate 1.2~1.5m / s), the indicated value of the ORP meter PCH solution Was performed for a long period of time in the same manner as in Example 1 except that each was adjusted to 500 to 600 mV.
[0036]
The concentration of acetaldehyde in the obtained PO remained at 5 ppm or less as shown in FIG. Moreover, the hypochlorous acid concentration in the PCH aqueous solution produced | generated at the reaction process was changing in the range of 0-5 ppm.
[0037]
Comparative Example 1
In Example 1, the operation was performed for a long time in the same manner as in Example 1 except that the reactor was operated at a pressure of 15.5 to 16 kPa and a circulating gas amount of 800 Nm ³ / H (main tower side flow rate 1.2 m / s).
[0038]
The acetaldehyde concentration in the obtained PO changed in the range of 10 to 35 ppm as shown in FIG. Moreover, the hypochlorous acid density | concentration in PCH aqueous solution produced | generated at the reaction process was changing in the range of 15-30 ppm.
[0039]
【The invention's effect】
In the present invention, by controlling the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower to be 10 ppm or less, it becomes possible to reduce the acetaldehyde in the product PO to 15 ppm or less. Can avoid the coloring of polyols made from PO.
[0040]
The present invention also makes it possible to simplify the low boiling point removal step in the purification step.
[Brief description of the drawings]
FIG. 1 is a diagram showing a flow of production of propylene oxide by the chlorohydrin method of the present invention.
FIG. 2 is a graph showing the relationship between the concentration of hypochlorous acid in the aqueous chlorohydrin solution supplied to the saponification tower and the concentration of acetaldehyde in the crude propylene oxide in the present invention.
FIG. 3 is a graph showing the relationship between the ORP value of the chlorohydrin aqueous solution supplied to the saponification tower and the hypochlorous acid concentration.
FIG. 4 is a graph showing the relationship between the change in pressure in the reaction tower and the production concentration of acetaldehyde.
FIG. 5 is a graph showing the relationship between the amount of circulating gas and the production concentration of acetaldehyde.
[Explanation of symbols]
1: Reactor 5 for reacting propylene with hypochlorous acid 6: Saponification column 6: Distillation column for low-boiling substances 7: Rectification column for propylene oxide

Claims (2)

In a process for producing propylene oxide, which includes a reaction step of supplying chlorine and propylene in an aqueous medium to obtain a reaction aqueous solution containing propylene chlorohydrin, and a saponification step of dehydrochlorinating the propylene chlorohydrin with an alkali. A method for producing propylene oxide, comprising controlling a concentration of hypochlorous acid in a reaction aqueous solution supplied to a saponification step to 10 ppm or less. The concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification step is detected, and based on the detected value, (1) propylene / chlorine supply ratio in the reaction step, and (2) the gas phase portion in the reaction step. 2. The method for producing propylene oxide according to claim 1, wherein at least one of the pressure and the flow rate in the reaction zone (3) is controlled.

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