A process for producing acetic acid by: a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, an ionic iodide, and methyl iodide in a carbonylation reactor, a flash distillation step for continuously feeding a flasher with a reaction mixture from the reactor and evaporating a volatile component at least containing product acetic acid, methyl acetate, and methyl iodide by flash distillation to separate the volatile component and a liquid catalyst mixture at least containing the metal catalyst and the ionic iodide, and an acetic acid collection step for separating a stream containing acetic acid from the volatile component to collect acetic acid; wherein, in the flash distillation step, the flash distillation is conducted under the condition that the concentration of methyl acetate in the liquid catalyst mixture is not less than 0.6% by weight.
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0. 14. A process for producing acetic acid, which comprises
a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, an ionic iodide, and methyl iodide in a carbonylation reactor,
a flash distillation step for continuously feeding a flasher with a reaction mixture from the reactor and evaporating a volatile component at least containing product acetic acid, methyl acetate, and methyl iodide by flash evaporation to separate the volatile component and a liquid catalyst mixture at least containing the metal catalyst and the ionic iodide, a recycling step for recycling the liquid catalyst mixture to the reactor, and an acetic acid collection step comprising distilling and separating the volatile component into a lower boiling point fraction as a first overhead, a side stream containing acetic acid, and a higher boiling point fraction and distilling and separating the side stream into a lower boiling point component as a second overhead, and an acetic acid stream as a side or bottom stream for separating a stream containing acetic acid from the volatile component to collect acetic acid,
wherein the metal in the metal catalyst comprises a rhodium catalyst, and
in the flash distillation step, the flash evaporation is conducted at a temperature of about 100 to 260° C. under an absolute pressure of about 0.03 to 1 MPa while maintaining the concentration of the ionic iodide of 1 to 35% by weight, the concentration of methyl iodide of 0.01 to 2% by weight, the concentration of acetic acid of 45 to 90% by weight, the concentration of methyl acetate of about 0.6 to 3% by weight and the concentration of water of about 0.8 to 8% by weight in the liquid catalyst mixture,
wherein the concentration of hydrogen iodide in the liquid catalyst mixture is not more than 1% by weight.
0. 33. A method for inhibiting production of hydrogen iodide or increased concentration of hydrogen iodide in a flasher in a production process of acetic acid, the production process, which comprises
a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, an ionic iodide, and methyl iodide in a carbonylation reactor,
a flash evaporation step for continuously feeding a flasher with a reaction mixture from the reactor and evaporating a volatile component at least containing product acetic acid, methyl acetate, and methyl iodide by flash evaporation to separate the volatile component and a liquid catalyst mixture at least containing the metal catalyst and the ionic iodide,
a recycling step for recycling the liquid catalyst mixture to the reactor, and
an acetic acid collection step comprising distilling and separating the volatile component into a lower boiling point fraction as a first overhead, a side stream containing acetic acid, and a higher boiling point fraction and distilling and separating the side stream into a lower boiling point component as a second overhead, and an acetic acid stream as a side or bottom stream for separating a stream containing acetic acid from the volatile component to collect acetic acid,
wherein the metal in the metal catalyst comprises a rhodium catalyst, and
in the flash evaporation step, the flash evaporation is conducted at a temperature of about 100 to 260° C. under an absolute pressure of about 0.03 to 1 MPa while maintaining the concentration of the ionic iodide of 1 to 35% by weight, the concentration of methyl iodide of 0.01 to 2% by weight, the concentration of acetic acid of 45 to 90% by weight, the concentration of methyl acetate of about 0.6 to 3% by weight and the concentration of water of about 0.8 to 8% by weight in the liquid catalytic mixture,
wherein the concentration of hydrogen iodide in the liquid catalyst mixture is not more than 1% by weight.
0. 1. A process for producing acetic acid, which comprises
a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, an ionic iodide, and methyl iodide in a carbonylation reactor,
a flash evaporation step for continuously feeding a flasher with a reaction mixture from the reactor and evaporating a volatile component at least containing product acetic acid, methyl acetate, and methyl iodide by flash evaporation to separate the volatile component and a liquid catalyst mixture at least containing the metal catalyst and the ionic iodide,
a recycling step for recycling the liquid crystal mixture to the reactor, and
an acetic acid collection step comprising distilling and separating the volatile component into a lower boiling point fraction as a first overhead, a side stream containing acetic acid, and a higher boiling point fraction and distilling and separating the side stream into a lower boiling point component as a second overhead, and an acetic acid stream as a side or bottom stream for separating a stream containing acetic acid from the volatile component to collect acetic acid,
wherein the metal catalyst comprises a rhodium catalyst, and
in the flash evaporation step, the flash evaporation is conducted at a temperature of about 100 to 260° C. under an absolute pressure of about 0.03 to 1 MPa while maintaining the concentration of methyl acetate of about 0.6 to 15% by weight and the concentration of water of about 0.8 to 8% by weight in the liquid catalyst mixture.
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0. 12. A method for inhibiting production of hydrogen iodide or increased concentration of hydrogen iodide in a flasher in a production process of acetic acid, the production process comprising
a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, an ionic iodide, and methyl iodide in a carbonylation reactor,
a flash evaporation step for continuously feeding a flasher with a reaction mixture from the reactor and evaporating a volatile component at least containing product acetic acid, methyl acetate, and methyl iodide by flash evaporation to separate the voltile component and a liquid catalyst mixture at least containing the metal catalyst and the ionic iodide,
a recycling step for recycling the liquid catalyst mixture to the reactor, and
an acetic acid collection step comprising distilling and separating the volatile component into a lower boiling point fraction as a first overhead, a side stream containing acetic acid, and a higher boiling point fraction and distilling and separating the side stream into a lower boiling point component as a second overhead, and an acetic acid stream as a side or bottom stream for separating a stream containing acetic acid from the volatile component to collect acetic acid,
wherein the metal catalyst comprises a rhodium catalyst, and
in the flash evaporation step, the flash evaporation is conducted at a temperature of about 100 to 260° C. under an absolute pressure of about 0.03 to 1 MPa while maintaining the concentration of methyl acetate of about 0.6 to 15% by weight and the concentration of water of about 0.8 to 8% by weight in the liquid catalyst mixture.
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0. 34. A process or method according to
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The concentration of methyl acetate in the liquid catalyst mixture may be selected from the range of not less than 0.6% by weight (e.g., 0.6 to 20% by weight), and may for example be not less than 0.7% by weight (e.g., about 0.7 to 15% by weight), preferably not less than 0.8% by weight (e.g., about 0.8 to 10% by weight), more preferably about 0.9 to 5% by weight, and usually about 0.7 to 5% by weight (e.g., about 0.7 to 3% by weight, preferably about 0.8 to 2% by weight, and more preferably about 0.9 to 1.5% by weight). Moreover, the concentration of methyl acetate in the liquid catalyst mixture may be as high as not less than 1% by weight (e.g., about 1.2 to 10% by weight), preferably not less than 1.3% by weight (e.g., about 1.4 to 8% by weight), more preferably not less than 1.5% by weight (e.g., about 1.7 to 7% by weight), and particularly not less than 2% by weight (e.g., about 2.2 to 5% by weight).
Incidentally, when the concentration of methyl acetate in the flasher is excessively high, the production or increased concentration of hydrogen iodide can be inhibited, while the succeeding steps are hindered by lowered separation of the organic phase and the aqueous phase, or others. Therefore, there are some cases where the process cannot be performed stably.
The concentration of water in the liquid catalyst mixture may for example be selected from the range of not more than 15% by weight (e.g., 0.1 to 12% by weight), and may for example be not more than 10% by weight (e.g., about 0.5 to 10% by weight), preferably not more than 8% by weight (e.g., about 0.8 to 8% by weight), more preferably not more than 4% by weight (e.g., about 0.8 to 4% by weight), and particularly not more than 2% by weight (e.g., about 0.8 to 2% by weight).
Moreover, the concentration of acetic acid in the liquid catalyst mixture may for example be not less than 30% by weight (e.g., about 35 to 95% by weight), preferably not less than 40% by weight (e.g., about 45 to 90% by weight), and more preferably not less than 50% by weight (e.g., about 50 to 85% by weight) and may usually be about 60 to 90% by weight.
Further, the concentration of methyl iodide in the liquid catalyst mixture may be selected from the range of not more than 10% by weight (e.g., 0.001 to 8% by weight), and may for example be not more than 7% by weight (e.g., about 0.005 to 6% by weight), preferably not more than 5% by weight (e.g., about 0.01 to 4% by weight), more preferably not more than 3% by weight (e.g., about 0.05 to 2.5% by weight), particularly not more than 2% by weight (e.g., about 0.1 to 1.8% by weight) and may usually be about 0.1 to 3% by weight (e.g., about 0.3 to 2.5% by weight and preferably about 0.5 to 2% by weight).
Furthermore, the concentration of the ionic iodide in the liquid catalyst mixture may for example be not more than 60% by weight (e.g., about 1 to 55% by weight), preferably not more than 50% by weight (e.g., about 2 to 45% by weight), more preferably not more than 40% by weight (e.g., about 3 to 37% by weight), and particularly not more than 36% by weight (e.g., about 5 to 35% by weight) and may usually be about 5 to 25% by weight (e.g., about 8 to 20% by weight). Multiple factors are also involved in the reason why the increase in concentration of hydrogen iodide is prevented by adjusting the concentration of the ionic iodide, and one of the factors includes consumption of hydrogen iodide by the following equilibrium reaction.
MI+CH3COOH⇔CH3COOM+HI
[In the formula, M represents a residue of an ionic iodide (or cationic group, e.g., an alkali metal such as lithium)]
Incidentally, in terms of the inhibition of the corrosion, it is preferable that the amount of the component (such as methyl iodide or lithium iodide) producing hydrogen iodide in an equilibrium reaction be small.
Incidentally, the concentration of the metal catalyst in the liquid catalyst mixture may for example be not less than 100 ppm (e.g., about 150 to 10000 ppm), preferably not less than 200 ppm (e.g., about 250 to 5000 ppm), and more preferably not less than 300 ppm (e.g., about 350 to 3000 ppm) on the basis of weight.
Moreover, the concentration of methanol in the liquid catalyst mixture may for example be not more than 1% by weight (e.g., about 0 to 0.8% by weight), preferably not more than 0.5% by weight (e.g., about 0 to 0.3% by weight), and more preferably not more than 0.3% by weight (e.g., about 0 to 0.2% by weight). As described later, as the concentration of methanol is higher, the concentration of methyl acetate in the liquid catalyst mixture is easily and efficiently increased.
The adjustment of the concentrations of the constituents in the liquid catalyst mixture (increase or decrease in concentration) is not particularly limited to a specific one, and the concentrations may be adjusted by the flash distillation condition, the quantity of the process solution to be recycled from the succeeding reaction (step(s)), and others. If necessary, in order to adjust the concentration of each component, a component for increasing or decreasing the concentration of each component [for example, an ester (e.g., an acetate ester), an alcohol, and an ether] may be added to the reaction mixture and/or the flash evaporator. Such a component may be a component (a basic component) reactive to hydrogen iodide.
For example, the concentration of methyl acetate in the liquid catalyst mixture can efficiently be increased by increasing the concentration of methanol in the reaction mixture (or liquid catalyst mixture). That is, as represented by the following formula, methanol is allowed to react with acetic acid to produce methyl acetate (equilibrium reaction). Thus the production reaction of methyl acetate easily occurs as the concentration of methanol increases. As a result, the concentration of methyl acetate in the liquid catalyst mixture can be increased.
CH3OH+CH3COOH⇔CH3COOCH3+H2O
In the range that the production efficiency of acetic acid is ensured sufficiently, the concentration of methanol can be increased by increasing the concentration of methanol to be fed in the reaction or by decreasing the reaction rate to inhibit consumption of methanol. The reaction rate can be adjusted by suitably selecting the reaction temperature, the concentration of the catalyst (e.g., the concentration of methyl iodide and the concentration of the metal catalyst), the concentration of carbon monoxide (or carbon monoxide partial pressure), and others. The concentration of methanol may be adjusted by adding methanol directly, as described later.
Incidentally, as the concentration of methyl acetate or methanol in the reaction solution is higher, the amount of production of acetic acid can be increased, and the concentration of hydrogen iodide in the flasher may easily be reduced. However, there are some cases where the reaction is unstable and it is difficult to control the temperature or the pressure. Accordingly, it is preferable that the concentration of methyl acetate or methanol as a methyl acetate source be lowered in the reaction solution and that the concentration of methyl acetate in the flasher (or the liquid catalyst mixture) be adjusted to the above-mentioned concentration.
Moreover, the concentration of methyl acetate in the liquid catalyst mixture may be adjusted by adding methyl acetate and/or a component for producing methyl acetate (e.g., methanol and dimethyl ether). Incidentally, as described above, methanol is allowed to react with acetic acid to produce methyl acetate; and dimethyl ether is allowed to react with hydrogen iodide or others to give methanol, which is allowed to react with acetic acid to produce methyl acetate. If necessary, a component for increasing or decreasing the concentration of each component may be added or mixed in the form of a mixture containing a solvent.
When the increasing or decreasing component is added to the reaction mixture, the position (or timing) of addition is not particularly limited to a specific one as far as the increasing or decreasing component is added before the reaction mixture is fed to the flash evaporator. The increasing or decreasing component may be fed to the reactor. In terms of process efficiency, the increasing or decreasing component may be fed to the reaction mixture after the reaction mixture is discharged from the reactor and before the reaction mixture is fed to the flash evaporator (for example, as shown in the FIGURE, the increasing or decreasing component may be fed to a line for feeding the flash evaporator with the reaction mixture discharged from the reactor).
Moreover, when the increasing or decreasing component is added to the flash evaporator (or the increasing or decreasing component is mixed to the reaction mixture in the flash evaporator), the position (height level) of addition is not particularly limited to a specific one. The increasing or decreasing component may be added to either the liquid phase portion or the gaseous phase portion in the flash evaporator, or both. The increasing or decreasing component may be added to the process solution to be recycled from the succeeding step(s) to the flash evaporator.
The volatile component (acetic acid stream) separated in the flasher contains product acetic acid, in addition, methyl iodide, an ester of the product acetic acid with methanol (e.g., methyl acetate), water, a very small amount of by-product(s) (e.g., acetaldehyde and propionic acid) and others. The volatile component may be distilled in the first distillation column and the second distillation column to produce purified acetic acid.
According to the present invention, as described above, the production or increased concentration of hydrogen iodide in the flasher can be inhibited. Thus the concentration of hydrogen iodide in the volatile component may for example be regulated to not more than 1% by weight (e.g., about 0 or detection limit to 0.8% by weight), preferably not more than 0.6% by weight (e.g., about 0.001 to 0.5% by weight), more preferably not more than 0.3% by weight (e.g., about 0.01 to 0.2% by weight), and particularly not more than 0.1% by weight (e.g., about 0.02 to 0.09% by weight). Moreover, the concentration of hydrogen iodide in the liquid catalyst mixture may for example be regulated to not more than 1% by weight (e.g., about 0 or detection limit to 0.8% by weight), preferably not more than 0.6% by weight (e.g., about 0.001 to 0.5% by weight), more preferably not more than 0.3% by weight (e.g., about 0.01 to 0.2% by weight), and particularly not more than 0.1% by weight (e.g., about 0.02 to 0.09% by weight).
The concentration of hydrogen iodide may be measured directly or measured (or calculated) indirectly. For example, the concentration of the iodide ion derived from the iodide salt [for example, an iodide derived from the co-catalyst such as LiI, and a metal iodide (e.g., an iodide of a corroded metal (such as Fe, Ni, Cr, Mo, or Zn) produced in the process of the acetic acid production)] may be subtracted from the total concentration of iodide ions (I−) to determine (or calculate) the concentration of hydrogen iodide.
Part of the separated volatile component (acetic acid stream) may be introduced into a condenser or a heat exchanger for cooling or heat-removal, as the embodiment illustrated in the FIGURE. Since the reaction heat transferred from the reaction solution to the flash vapor can partly be cooled by the heat removal, the heat removal efficiency can be improved, and acetic acid with a high purity can be produced without installing an external circulation cooling unit in the reactor. Moreover, the cooled volatile component may be recycled to the reaction system, as the embodiment illustrated in the FIGURE. On the other hand, the gaseous component in the cooled volatile component may be introduced into the scrubber system.
(Acetic Acid Collection Step)
In the acetic acid collection step (distillation step), acetic acid is collected by separating a stream containing acetic acid from the volatile component. The separation method is not particularly limited to a specific one. Usually, the separated volatile component is fed to the distillation column (splitter column), and separated into a lower boiling point fraction (overhead) containing a lower boiling point component (e.g., methyl iodide, acetic acid, methyl acetate, and by-product acetaldehyde) and a stream containing acetic acid (acetic acid stream) by distillation. The acetic acid collection step is not necessarily the embodiment shown in the FIGURE, and may be a step in which a treatment for removing the lower boiling point component and a treatment for removing water are carried out in a single distillation column (for example, a step utilizing a distillation column described in Japanese Patent No. 3616400 publication) or a step in which a treatment for removing the lower boiling point component and a treatment for removing water in a first distillation column is followed by a further purification step in a second distillation column. Considering the purification efficiency and others, a preferably usable step includes a distillation step in which the treatment for removing the lower boiling point component is mainly carried out in the first distillation column and the treatment for removing water is mainly carried out in the second distillation column.
(First Distillation Column)
Part of the acetic acid stream (lower boiling point fraction) fed from the flasher is introduced into the heat exchanger, and the remaining (residual) acetic acid stream is fed to the first distillation column. In the first distillation column, a lower boiling point fraction (or first lower boiling point fraction or first overhead) containing at least part of an lower boiling point component (e.g., methyl iodide, methyl acetate, and acetaldehyde) and a higher boiling point fraction (or bottom fraction) containing at least part of a higher boiling point component (e.g., propionic acid and water) are separated from the acetic acid stream, and a stream containing at least acetic acid is withdrawn. In the embodiment of
As described above, the acetic acid stream fed to the first distillation column is not limited to an acetic acid stream obtained by removing the rhodium catalyst component from the reaction mixture of the reaction system. The acetic acid stream may contain at least acetic acid, the lower boiling point component, the higher boiling point component, and others; or simply may be a mixture of these components.
As the first distillation column, there may be used, for example, a conventional distillation column, e.g., a distillation column such as a plate column or a packed column. The material of (or forming) the first distillation column may include the same material as that of the flasher. According to the present invention, the production or increased concentration of hydrogen iodide in the flash distillation step can be inhibited. Thus as the first distillation column, there may be used a distillation column made of the same material, which is relatively inexpensive material (e.g., an alloy), as that of the flash evaporator.
The distillation temperature and pressure in the first distillation column may suitably be selected depending on the condition such as the species of the distillation column, or the main subject (target) for removal selected from the lower boiling point component and the higher boiling point component. For example, for the plate column, the inner pressure of the column (usually, the pressure of the column top) may be about 0.01 to 1 MPa, preferably about 0.01 to 0.7 MPa, and more preferably about 0.05 to 0.5 MPa in terms of gauge pressure.
Moreover, in the first distillation column, the inner temperature of the column (usually, the temperature of the column top) may be adjusted by adjusting the inner pressure of the column, and may be, for example, about 20 to 180° C., preferably about 50 to 150° C., and more preferably about 100 to 140° C.
Moreover, for the plate column, the theoretical number of plates is not particularly limited to a specific one, and, depending on the species of the component to be separated, is about 5 to 50, preferably about 7 to 35, and more preferably about 8 to 30. Further, in order to separate acetaldehyde highly (or with a high precision) in the first distillation column, the theoretical number of plates may be about 10 to 80, preferably about 12 to 60, and more preferably about 15 to 40.
In the first distillation column, the reflux ratio may be selected from, for example, about 0.5 to 3,000, and preferably about 0.8 to 2,000 depending on the above-mentioned theoretical number of plates, or may be reduced by increasing the theoretical number of plates. Incidentally, in the first distillation column, the distillation may be carried out without reflux.
Since the lower boiling point fraction separated from the first distillation column contains a useful component (e.g., methyl iodide and methyl acetate), the lower boiling point fraction may directly be recycled to the reaction system (or reactor) and/or the first distillation column, or may be liquefied by heat-removing part of the reaction heat in the reaction system (e.g., the reactor) using a condenser, a heat exchanger, or other means and then recycled to the reactor and/or the first distillation column. For example, the lower boiling point fraction withdrawn from the first distillation column is not necessary recycled to the first distillation column after condensation by the condenser as the embodiment of
The higher boiling point fraction (bottom fraction or first higher boiling point fraction) separated in the first distillation column contains water, acetic acid, an entrained rhodium catalyst, lithium iodide, in addition, acetic acid remaining without being evaporated, the lower boiling point impurities, and others. Thus, if necessary, the higher boiling point fraction may be recycled to the reaction system (reactor) and/or the flasher. Incidentally, prior to recycling, propionic acid, which deteriorates the quality of acetic acid as a final product, may be removed off.
(Second Distillation Column)
In the second distillation column, hydrogen iodide, a lower boiling point component, and a higher boiling point component, each of which remains without being separated, in the first distillation column are removed with further high precision. As the second distillation column, there may be used a conventional distillation column, for example, a plate column, a packed column, and other columns. The material of (or forming) the second distillation column may include the same material as that of the first distillation column. Moreover, the inner temperature of the column, the inner pressure of the column, the theoretical number of plates, and the reflux ratio in the second distillation column may be selected depending on the species of the distillation column, for example, may be selected from the same (similar) range with the range of the above first distillation column.
Since the lower boiling point fraction (second lower boiling point fraction or second overhead) separated from the second distillation column contains a useful component such as methyl iodide or methyl acetate, the lower boiling point fraction may directly be recycled to the reaction system (e.g., the reactor) and/or the second distillation column. In order to remove part of the reaction heat, as the same manner as the lower boiling point fraction withdrawn from the first distillation column, the lower boiling point fraction may be liquefied by a condenser, a heat exchanger, or other means and then recycled. Moreover, since the lower boiling point fraction sometimes contains acetaldehyde, the lower boiling point fraction may for example be recycled after removing acetaldehyde with the after-mentioned aldehyde-separating column, if necessary. Incidentally, the offgas component may be introduced into the scrubber system.
Further, the higher boiling point fraction (second higher boiling point fraction) may be discharged from the bottom or lower part of the column. Since the higher boiling point fraction separated from the second distillation column contains propionic acid, and others, the higher boiling point fraction may directly be discarded (or removed off). Moreover, since the higher boiling point fraction further sometimes contains acetic acid, if necessary, the higher boiling point fraction from which propionic acid is removed and/or recovered may be recycled to the reaction system (e.g., the reactor).
In the second distillation column, the purified acetic acid stream is withdrawn by side cut in the embodiment of
(Iodide Removal Step)
The purified acetic acid recovered is usually introduced into a column for product acetic acid and obtained as product acetic acid. Prior or posterior to introduction into the column for product acetic acid, the purified acetic acid may further be subjected to an iodide-removing step to remove an iodide (e.g., a C1-15alkyl iodide such as hexyl iodide or decyl iodide).
In the iodide removal step (or iodide-removing step), the acetic acid stream may be contacted with a remover (removing agent or material) having an iodide-removability or iodide-adsorbability (e.g., a zeolite, an activated carbon, and an ion exchange resin). In order to efficiently remove the iodide from the acetic acid stream which is continuously obtained (in a continuous system), an ion exchange resin having iodide-removability or iodide-adsorbability, particularly an iodide-removing column provided with the ion exchange resin therein is advantageously used.
The ion exchange resin to be used is usually an ion exchange resin (usually a cation exchange resin) in which at least part of the active site (e.g., usually an acidic group such as a sulfone group, a carboxyl group, a phenolic hydroxyl group, or a phosphone group) is substituted or exchanged with a metal. The metal may include, for example, at least one member selected from the group consisting of silver (Ag), mercury (Hg), and cupper (Cu). The cation exchange resin as a base (substrate) may be any one of a strong acidic cation exchange resin and a weak (mild) acidic cation exchange resin, and the preferred one includes a strong acidic cation exchange resin, for example, a macroreticular ion exchange resin, and the like.
In the ion exchange resin, the proportion of the active site exchanged to the metal (or substituted with the metal) may be, for example, about 10 to 80% by mol, preferably about 25 to 75% by mol, and more preferably about 30 to 70% by mol.
At least contacting of the acetic acid stream from the second distillation column with the ion exchange resin (preferably passing of the acetic acid stream through the ion exchange resin) achieves removal of the iodide. While contacting with (or passing through) the ion exchange resin, if necessary, the temperature of the acetic acid stream may be increased (or elevated) stepwise. The stepwise temperature elevation ensures to inhibit outflow or effusion of the metal from the ion exchange resin, as well as to remove the iodide efficiently.
Examples of the iodide-removing column may include a packed column packing inside thereof at least the ion exchange resin which is exchanged with a metal, a column provided with a bed of an ion exchange resin (e.g., a bed comprising a particulate resin) (a guard bed) and the like. The iodide-removing column may be provided with the metal-exchanged ion exchange resin, and in addition, another ion exchange resin (e.g., a cation exchange resin, an anion exchange resin, and a nonion exchange resin) inside thereof. Even when the metal is effused from the metal-exchanged ion exchange resin, arrangement of the cation exchange resin at the downstream side of the metal-exchanged ion exchange resin (e.g., arrangement of the cation exchange resin by packing, or arrangement of the cation exchange resin as a resin bed) allows the effused metal to be captured with the cation exchange resin and be removed from the carboxylic acid stream.
The temperature of the iodide-removing column may be, for example, about 18 to 100° C., preferably about 30 to 70° C., and more preferably about 40 to 60° C.
The rate of the acetic acid stream to be passed through is not limited to a specific one, and may be, for example, in an iodide-removing column utilizing a guard bed, e.g., about 3 to 15 BV/h (bed volume per hour), preferably about 5 to 12 BV/h, and more preferably about 6 to 10 BV/h.
In the iodide-removing step, the acetic acid stream may be at least contacted with the metal-exchanged ion exchange resin. For example, the iodide-removing column may comprise a column provided with the metal-exchanged ion exchange resin and a column provided with another ion exchange resin. For example, the iodide-removing column may comprise an anion exchange resin column, and a metal-exchanged ion exchange resin column on the downstream side of the anion exchange resin column, or may comprise a metal-exchanged ion exchange resin column, and a cation exchange resin column on the downstream side of the metal-exchanged ion exchange resin column. The details of the former example can be referred by WO02/062740, and others.
(Acetaldehyde Separation Step)
When the fraction containing acetaldehyde generated by the reaction is recycled and circulated to the reaction system, the amount of by-product(s) such as propionic acid, an unsaturated aldehyde, or an alkyl iodide increases. Thus, it is preferred to remove acetaldehyde in the solution to be recycled. In particular, removal of acetaldehyde is preferred, because it is unnecessary to separate and remove propionic acid, which makes acetic acid sub-standard, in the second distillation column. The method for separating acetaldehyde may comprise feeding a recycle solution (a solution to be recycled) to the acetaldehyde-separating column to separate a lower boiling point fraction containing acetaldehyde and a higher boiling point fraction containing methyl iodide, methyl acetate, water, and others, and then separating acetaldehyde from the top or upper part of the aldehyde-separating column, with the offgas component (e.g., carbon monoxide and hydrogen). Further, the offgas component may be previously removed off with a condenser or a cooling unit, prior to the separation of acetaldehyde. Furthermore, since the higher boiling point fraction obtained by removing acetaldehyde as the lower boiling point fraction contains methyl iodide, water, methyl acetate, acetic acid, and the like, the higher boiling point fraction may be recycled to the reaction system.
As the aldehyde-separating column, for example, there may be used a conventional distillation column, e.g., a plate column, a packed column, a flash evaporator, and others.
The temperature (the temperature of the column top) and the pressure (the pressure of the column top)) in the acetaldehyde-separating column may be selected depending on the species of the distillation column and others, and is not particularly limited to a specific one as far as at least acetaldehyde is separable as a lower boiling point fraction from the recycle solution [for example, the lower boiling point fraction(s) obtained in the first and/or second distillation column(s)] by utilizing difference between acetaldehyde and other components (particularly methyl iodide) in boiling point. For example, for the plate column, the pressure may be about 0.01 to 1 MPa, preferably about 0.01 to 0.7 MPa, and more preferably about 0.05 to 0.5 MPa as a gauge pressure. The inner temperature of the column is, for example, about 10 to 150° C., preferably about 20 to 130° C., and more preferably about 40 to 120° C. The theoretical number of plates may be, for example, about 5 to 150, preferably about 8 to 120, and more preferably about 10 to 100.
In the acetaldehyde-separating column, the reflux ratio may be selected from about 1 to 1000, preferably about 10 to 800, and preferably about 50 to 600 (e.g., about 70 to 400) depending on the above-mentioned theoretical number of plates.
The following examples are intended to describe this invention in further detail and should by no means be interpreted as defining the scope of the invention.
When the production process of acetic acid described in
Test pieces of various materials were added to the liquid catalyst mixture of the flasher 2, and the continuous production process of acetic acid was performed for 100 hours in a state in which the test pieces were left in the flasher. After the continuous production process for 100 hours was completed, each test piece was examined for a corrosion test.
The corrosion test was evaluated on the basis of the following criteria in Comparative Examples 1 to 2 and Examples 1 to 3 and evaluated on the observed corrosion amount in Comparative Examples 3 to 5 and Examples 4 to 8.
“A”: Test piece is not corroded at all.
“B”: Test piece is hardly corroded.
“C”: Test piece is slightly corroded.
“D”: Test piece is significantly corroded.
The composition of the liquid catalyst mixture and the results of the corrosion test are shown in Tables 1 and 2. In Tables 1 and 2, “wt %” means % by weight, “Ac” represents acetic acid, “MA” represents methyl acetate, “MeOH” represents methanol, “MeI” represents methyl iodide, “Zr” represents zirconium, “HB2” represents a nickel-based alloy (HASTELLOY B2 manufactured by Oda Koki Co., Ltd.), “HC” represents a nickel-based alloy (HASTELLOY C manufactured by Oda Koki Co., Ltd.), and the unit “mm/Y” means the corrosion rate of the test piece per year (the decreased thickness (mm) of the test piece per year). The concentration of the iodide ion derived from the iodide salt was subtracted from the total concentration of iodide ions (I−) to calculate the concentration of hydrogen iodide (HI).
TABLE 1
Liquid catalyst mixture
Ac
MA
MeOH
HI
MeI
H2O
LiI
Corrosion test
wt %
wt %
wt %
wt %
wt %
wt %
wt %
Zr
HB2
HC
Comparative
76.5
0.1
less than
0.3
1
7
15
A
B
D
Example 1
0.1
Comparative
75.4
0.1
less than
0.4
2
7
15
A
B
D
Example 2
0.1
Example 1
75.7
0.9
less than
less than
1
7
15
A
A
C
0.1
0.1
Example 2
74.8
0.9
less than
less than
2
7
15
A
A
C
0.1
0.1
Example 3
74.7
1.9
less than
less than
1
7
15
A
A
B
0.1
0.1
TABLE 2
Liquid catalyst mixture
Corrosion test
Ac
MA
MeOH
HI
MeI
H2O
LiI
Zr
HB2
HC
wt %
wt %
wt %
wt %
wt %
wt %
wt %
mm/Y
mm/Y
mm/Y
Comparative
80.9
0.1
less than
0.1
0.7
3.1
15.0
less than
0.1
0.52
Example 3
0.1
0.03
Comparative
76.5
0.1
less than
0.2
5.0
3.0
15.0
less than
0.12
0.62
Example 4
0.1
0.03
Comparative
70.5
0.1
less than
0.2
5.0
9.0
15.0
less than
0.12
0.62
Example 5
0.1
0.03
Example 4
80.3
0.9
less than
less than
0.9
3.0
14.7
less than
0.04
0.16
0.1
0.1
0.03
Example 5
79.3
0.9
less than
less than
1.6
3.0
15.0
less than
0.05
0.20
0.1
0.1
0.03
Example 6
79.3
1.9
less than
less than
1.4
2.9
14.3
less than
less than
0.04
0.1
0.1
0.03
0.03
Example 7
82.1
0.7
less than
less than
1.2
1.0
14.8
less than
0.05
0.15
0.1
0.1
0.03
Example 8
80.8
1.8
less than
less than
0.9
1.5
14.8
less than
less than
less than
0.1
0.1
0.03
0.03
0.03
As apparent from the tables, the production and increased concentration of hydrogen iodide (HI) and the corrosion of the test pieces were prevented by adjusting the composition of the liquid catalyst mixture in the flasher to specific components and specific proportions.
The production process of the present invention is extremely useful as a process for producing acetic acid while efficiently inhibiting the production or increased concentration of hydrogen iodide in the flash evaporator for distilling the reaction mixture obtained from the reactor.
Ueno, Takashi, Shimizu, Masahiko, Miura, Hiroyuki, Nakajima, Hidehiko, Saito, Ryuji
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