JP2000095772A - Production of propylene oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain propylene oxide useful for producing polyols with no discoloration through effectively suppressing byproduct acetaldehyde formation by controlling at a specified level or lower before saponification the concentration of the hypochlorous acid in a propylene chlorohydrin-contg. reaction aqueous solution produced from chlorine, etc., through reaction in an aqueous medium. SOLUTION: This propylene oxide is obtained by the following steps: an aqueous medium is fed with (A) chlorine and (B) propylene to produce (C) a propylene chlorohydrin-contg. reaction aqueous solution (reaction process), the component C is then dehydrochlorinated with an alkali to perform a saponification (saponification process); wherein the hypochlorous acid concentration of the reaction aqueous solution to be fed to the saponification process is controlled at <=10 ppm; it is preferable that the above control operation is performed as follows: the hypochlorous acid concentration of the above reaction aqueous solution to be fed to the saponification process is detected, based on the result, one or more parameters among the feed ratio B/A in the reaction process, the vapor phase pressure in the reaction process and the flow rate within the reaction zone are controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a novel method for producing propylene oxide. Specifically, it is possible to extremely effectively suppress the by-product of acetaldehyde,
In some cases, the method is a method for producing high-purity propylene oxide which can simplify the step of removing low-boiling substances by rectifying the obtained propylene oxide.

[0002]

2. Description of the Related Art Conventionally, propylene oxide (hereinafter referred to as PO
Are abbreviated as propylene) in the presence of a catalyst in the presence of a catalyst using an indirect oxidation method to form PO using an oxidizing agent, and by causing hypochlorous acid to act on propylene in an aqueous medium to once produce propylene chlorohydrin ( The chlorohydrin method for obtaining PO, which is abbreviated as PCH, and then dehydrochlorinating (hereinafter referred to as saponification) to obtain PO, is industrially performed.

[0003] Each of these methods has advantages and disadvantages, and which one to employ depends on the circumstances of each individual company. However, the present invention relates to an improvement on the chlorhydrin method.

In the chlorhydrin method, hypochlorous acid generated by dissolving chlorine in an aqueous medium is reacted with propylene to produce 3 to 6% by weight of PCH and simultaneously produce hydrogen chloride by-produced. This method involves obtaining an aqueous solution containing the compound, and then once removing hydrogen chloride by distillation or neutralization, or simultaneously with the neutralization, saponifying with an alkali such as slaked lime or sodium hydroxide to obtain PO.

When the PO produced in the saponification step comes into contact with hydrogen chloride, other by-products, or metal compounds, by-products such as aldehydes and glycols are promoted, so that steam is generally introduced in the saponification step. Then, the PO generated at the same time as the saponification is instantaneously stripped. Further, the stripped PO is also purified by a process such as removal of low boiling impurities by distillation and rectification of PO.

[0006]

PO is mainly polycondensed into a polyol for use as a raw material for polyurethane, a surfactant, and other uses. However, a polyol using PO obtained by the chlorhydrin method is often colored. As a result of pursuing the cause, P
It turned out that it was caused by a trace amount of acetaldehyde, which is an impurity contained in O.

[0007] Then, as a result of investigating how much acetaldehyde can be reduced to prevent coloring, it was found that the acetaldehyde in PO needs to be kept at 15 ppm or less, preferably 10 ppm or less.

Regarding the separation of acetaldehyde, its boiling point is 20.2 ° C., the boiling point of PO is not more than 10 ° C. between 34 ° C., and it is difficult to completely separate both by rectification. However, it requires several tens of trays in the distillation column and consumes a large amount of energy, which is not economical.

It is needless to say that the most preferable is to suppress the by-product of acetaldehyde.

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, as a result of intensive studies on factors that can reduce the amount of acetaldehyde, the concentration of hypochlorous acid in the reaction aqueous solution supplied from the reaction step to the saponification step was determined. By controlling the amount of acetaldehyde in the PO produced in the saponification step to a level not higher than the specific value, it has been found that the amount of acetaldehyde can be controlled to an extremely low level, and the present invention has been completed.

[0011]

That is, the present invention provides a reaction step in which chlorine and propylene are supplied to an aqueous medium to obtain a reaction aqueous solution containing propylene chlorohydrin, and the propylene chlorohydrin is removed with an alkali. In a process for producing propylene oxide having a saponification step of hydrogen chloride, the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification step is controlled to 10 ppm or less, preferably 5 ppm or less. And a method for producing the same.

The most important feature of the present invention is that in the chlorhydrin method, the concentration of hypochlorous acid in the aqueous solution containing PCH supplied to the saponification step is suppressed to reduce the acetaldehyde in the product PO to 15 ppm or less. It is preferably at most 10 ppm. Therefore, the step of obtaining PCH, the saponification step, or various treatment means performed before, after, or in the middle thereof can be adopted without any limitation to conventionally known means.

Further, the present inventors have found a method suitable for controlling the concentration of hypochlorous acid 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, the propylene / chlorine supply ratio in the reaction step, Also provided is a method for producing propylene oxide, wherein at least one of the pressure and the flow rate in the reaction zone is controlled.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to an improvement in a conventionally known method for producing PO by the chlorohydrin method. An example of the manufacturing process will be described with reference to FIG.

In the case of the chlorohydrin method, the whole process generally comprises three steps: a reaction step, a saponification step, and a purification step. In the reaction step, hypochlorous acid is reacted with propylene to obtain PCH. Generally, chlorine is dissolved in an aqueous medium to produce hypochlorous acid, so that chlorine is supplied from the bottom of the reaction tower, and propylene is supplied to the top of the reaction tower, so that hypochlorous acid obtained by dissolving chlorine in water is obtained. Chloric acid and propylene contact to form PCH. At this time, if chlorine dissolved in water comes into contact with propylene as it is, chlorine is added to propylene and by-products such as dichloropropane are generated, so that chlorine must be promptly converted to hypochlorous acid. For this reason, a separate chlorine dissolving step can be added, and the resulting aqueous solution of hypochlorous acid can be supplied to the reaction step. However, as in the reactor 1 shown in FIG. A preferred reaction system is a method in which a cyclic reactor is connected, and chlorine is supplied to the sub-column, which is a downward flow, and propylene is supplied from the lower part of the main column, which is a reaction zone. In this case, a 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 liquid in the sub-column. Therefore, in FIG. 1, unreacted propylene and inert gases such as propane contained in propylene,
In some cases, for dilution of propylene, mixed inert gas such as nitrogen is withdrawn from pipe 11, washed with washing tower 2 if necessary, and pipe 1 is circulated with circulating gas blower 3.
It circulates to the reaction tower via 2 and 13, and the flow rate of the liquid flow in the main tower can be controlled. That is, if this method is adopted, the flow rate in the reactor can be easily controlled by changing the amount of circulating gas.

The ratio of chlorine to propylene newly supplied to the reactor is theoretically 1: 1 molar amount. Accordingly, 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.

On the other hand, PC generated in the reaction zone of the reactor
H is obtained in an aqueous solution, but when the PCH concentration to be generated is set low, the scale of the apparatus such as a reactor increases, and when the PCH concentration is set high, by-products such as dichloropropane increase, and both are economical. I can't say. Therefore, it is considered that the PCH concentration in the aqueous solution is preferably 3 to 6% by weight. The generated aqueous PCH solution, together with hydrogen chloride by-produced at the same time as the PCH, is withdrawn from a liquid reservoir (drum) at the upper part of the reactor through a pipe 14, passed through a PCH buffer tank 4, and then passed through a pipe 15 to a saponification tower 5. Is transferred to

The present invention is characterized in that the concentration of hypochlorous acid in the reaction solution transferred to the saponification tower is maintained at 10 ppm or less.

An alkali such as lime milk and steam are supplied to the saponification tower 5, and the temperature in the tower is 85 to 100 ° C.
Saponify H. PO produced by saponification of PCH
Since its boiling point is 34 ° C., it evaporates immediately and flows out together with water vapor from the pipe 16 as crude PO. In the case of the process shown in FIG. 1, at the same time as saponification of PCH, by-product hydrogen chloride is also neutralized by milk of lime [Ca (OH) ₂ ] to produce calcium chloride. These are discharged from the pipe 17 together with the reaction solution from which the 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 brought into contact with limestone [CaCO ₃ ] to convert the hydrogen chloride into calcium chloride. It is also possible to remove it, and it may be preferable from the viewpoint that the consumption of lime milk in the saponification tower can be halved.

The crude PO discharged from the pipe 16 is then distilled in the low-boiling tower 6 to remove low-boiling substances, that is, organic compounds having a boiling point lower than that of the PO from the pipe 18. Acetaldehyde can be removed by this low-boiling column, but as described above, it is necessary to use several tens of distillation columns, which is not economical. If the production of acetaldehyde is reduced by the practice of the present invention, the low-boiling column can have a simple distillation or a simple configuration of about several stages.

The aqueous solution of PO is supplied from the pipe 19 to the rectification tower 7.
To the product PO
Is taken out as On the other hand, high-boiling substances such as water are taken out from the bottom of the tower by a pipe 21 and are re-supplied to the reactor as process water if necessary.

In the chlorohydrin PO production process as described above, 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 concentration of acetaldehyde in PO can be reduced to 15 ppm or less. As an example, FIG. 2 shows the relationship between the concentration of hypochlorous acid in the reaction solution and acetaldehyde.

The concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower depends on the oxidation-reduction potential (OR
It can be easily known from the measurement of P). FIG. 3 illustrates the relationship between the oxidation-reduction potential (mV) and the concentration of hypochlorous acid.

Means for controlling the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower is not particularly limited. As the simplest means, a reducing agent depending on the concentration of hypochlorous acid remaining in the reaction aqueous solution, for example, sodium thiosulfate, sodium sulfite, ammonium sulfide, sodium sulfide, sodium polysulfide, sulfur compounds such as sulfur dioxide A reducing metal compound such as lithium aluminum hydroxide, sodium aluminum hydroxide, lithium hydride, and sodium hydride, and a reducing gas such as carbon monoxide are appropriately used.

Further, 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 the range of 0.9 to 1.1. By doing so, it can be controlled. That is, as the amount of propylene with respect to chlorine increases, the concentration of hypochlorous acid tends to decrease. However, increasing the propylene beyond 1.1 is not appropriate because unreacted propylene increases.

The hypochlorous acid concentration can also be controlled by adjusting the internal pressure of the reactor, in particular, the pressure of the gas phase in the upper part of the reactor. For example, as shown in FIG. 4, the concentration of acetaldehyde in PO can be reduced by increasing the gas 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 the PO. However, the higher the pressure, the higher the equipment cost. Further, it can be understood from FIG. 4 that the effect of reducing the acetaldehyde concentration by the pressure tends to increase as the flow rate in the reactor decreases.

Here, the flow rate in the reactor, that is, the apparent rise rate of the liquid in the reaction zone (in the main tower) is generally 0.1 to 0.1.
The flow rate is generally about 3 m / sec, preferably about 0.5 to 1.5 m / sec. However, since this flow rate generally has a correlation with the circulating gas amount, it can be controlled by the circulating gas amount. Of course,
Specific values vary depending on the shape and size of the reactor,
The correlation coefficient can be easily obtained by a simple experiment. FIG.
Table 1 shows an example of the relationship between the pressure in the tower, the amount of circulating gas (Nm ³ / H), and the concentration of acetaldehyde in PO (ppm). The reactor used in this experiment was a U-shaped main tower (with an inner diameter of 2000 mm) with a different diameter, and a sub tower (with an inner diameter of 900 m).
m) connected by a drum at the top.

[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 influence of the pressure Although it is possible to reduce the amount of acetaldehyde in PO to 5 ppm or less, increasing the amount of circulating gas requires a huge blower and increasing the pressure in the circulating system. It is not economically favorable because the loss increases.

As described above, as a factor for controlling the hypochlorous acid concentration in the aqueous reaction solution supplied to the saponification tower, the ratio of propylene / chlorine supplied to the reaction tower, Pressure and
By controlling at least one, preferably two or more of the flow rates in the reaction zone (which can be controlled by the gas circulation amount), particularly the pressure and the gas circulation amount at the same time, the hypochlorous acid in the solution is controlled. The concentration can be controlled to 10 ppm or less, preferably 5 ppm or less, and more preferably 3 ppm or less. Thus, the concentration of acetaldehyde in PO can be reduced to 15 ppm or less, preferably 10 ppm or less, and further to 5 ppm or less. Of course, the above-mentioned and the specific amount of change are each depending on the form and size of the equipment or the operating conditions,
The setting can be made very easily by the parties based on the above description.

In another preferred embodiment for carrying out the present invention, the potential of the reaction aqueous solution supplied to the saponification tower is measured by an oxidation-reduction potential measuring instrument, and a signal based on the measurement is used for manual or automatic operation. Another object is to control the concentration of hypochlorous acid. In this case, the control amount may be charging of a reducing agent, but preferably, the concentration of hypochlorous acid in the reaction aqueous solution supplied to the saponification step is detected, and the reaction step is performed based on the detected value. Of the propylene / chlorine supply ratio, the pressure of the gas phase in the reaction step, and the flow rate in the reaction zone.

[0031]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

Example 1 An experiment was conducted using the flow sheet apparatus shown in FIG.

The reactor comprises a U-shaped main tower of different diameter,
The thing made of Ti which connected the upper part of the side column with the drum was used.
The inner diameter of the main tower was 2000 mm, and the inner diameter of the sub tower was 900 mm. Chlorine was added to the sub tower and propylene and circulating gas were added to the main tower, and the propylene / chlorine molar ratio was 1.1. . PCH concentration in aqueous solution is 4wt
% Of the process water was adjusted. 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 an aqueous PCH solution. The produced PCH aqueous solution was mixed with lime milk, stripped by steam in a saponification tower, and subjected to a dehydrochlorination reaction of PCH, and extracted as crude PO from the top of the tower. The amount of steam input to the saponification tower is 4 to
The temperature was controlled so that the temperature of the saponification tower was 100 ° C. within a range of 6 tons.

The composition of the aqueous PCH solution obtained in this experiment was as follows: PCH 4%, hypochlorous acid 4 ppm, pipe 15
The indicated value of the ORP meter provided at 690 mV was 690 mV. The acetaldehyde concentration in the obtained PO was 5 pp.
m or less.

Example 2 In Example 1, the reactor pressure was 16 to 17 kPa and the amount of circulating gas was 800 to 900 Nm ³ / H (main tower side flow rate 1.
The operation was performed for a long period of time in the same manner as in Example 1, except that the indicated value of the ORP meter for the aqueous PCH solution was adjusted to 500 to 600 mV in the range of 2 to 1.5 m / s).

The acetaldehyde concentration in the obtained PO fluctuated below 5 ppm as shown in FIG. Further, the concentration of hypochlorous acid in the aqueous PCH solution generated in the reaction step was in the range of 0 to 5 ppm.

Comparative Example 1 In Example 1, the reactor pressure was increased to 15.5 to 16 kP.
a, the amount of circulating gas was 800 Nm ³ / H (main tower side flow rate 1.2
m / s), except that the operation was performed for a long period of time as in Example 1.

The acetaldehyde concentration in the obtained PO was in the range of 10 to 35 ppm as shown in FIG. Further, the concentration of hypochlorous acid in the aqueous PCH solution generated in the reaction step was in the range of 15 to 30 ppm.

[0039]

According to the present invention, by controlling the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification tower to 10 ppm or less, it is possible to reduce the acetaldehyde in the product PO to 15 ppm or less. This makes it possible to avoid coloring of the polyol using PO as a raw material.

The present invention also makes it possible to simplify the low boiling point removing step in the purification step.

[Brief description of the drawings]

FIG. 1 is a view showing a flow of propylene oxide production by a chlorohydrin method of the present invention.

FIG. 2 is a graph showing the relationship between the concentration of hypochlorous acid in an aqueous chlorohydrin solution supplied to a saponification tower and the concentration of acetaldehyde in crude propylene oxide in the present invention.

FIG. 3 is a diagram showing a relationship between an ORP value of an aqueous chlorhydrin solution supplied to a saponification tower and a concentration of hypochlorous acid.

FIG. 4 is a diagram showing a relationship between a change in pressure inside a reaction tower and a concentration of acetaldehyde formed.

FIG. 5 is a diagram illustrating a relationship between a circulating gas amount and an acetaldehyde generation concentration.

[Explanation of symbols]

 1: Reactor for reacting propylene with hypochlorous acid 5: Saponification tower 6: Distillation tower for low-boiling substances 7: Rectification tower for propylene oxide

Claims (2)

[Claims] 1. A propylene oxide comprising: a reaction step of supplying chlorine and propylene into an aqueous medium to obtain a reaction aqueous solution containing propylene chlorohydrin; and a saponification step of dehydrochlorinating the propylene chlorohydrin with an alkali. Wherein the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification step is controlled to 10 ppm or less. 2. Detecting the concentration of hypochlorous acid in the aqueous reaction solution supplied to the saponification step, and based on the detected value, the propylene / chlorine supply ratio in the reaction step and the pressure of the gas phase in the reaction step. The method for producing propylene oxide according to claim 1, wherein at least one of the flow rates in the reaction zone is controlled.