BE723859A - - Google Patents

Description

  

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  "Apparatus for the continuous intimate contact of a liquid and a gas"
The present invention relates to an apparatus for the continuous intimate contacting of liquid and gas, as well as to a process for refining crude terephthalic acid to high purity using this apparatus.



   Heretofore, in order to bring an eaz into contact with a liquid, bubbling columns, tray columns, stirred vessels, packed columns or spray columns have been employed. bubbling columns are widely used for industrial applications, since they ensure good contact of a gas with a liquid phase consisting of a liquid, a solution or a suspension, but they are not

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 not suitable when it is desired to have a high reaction ratio and a high contact efficiency between the liquid phase and the gas, since almost complete mixing is formed in the liquid phase.

   When placing solid particles in suspension in a liquid, it becomes necessary to use a large amount of gas to lift the solid particles.



  Therefore, more power is required for the gas supply, as well as increased costs for the construction of the column. As a result, bubbling columns are uneconomical, since a large amount of gas is required to effect gas / liquid contact.



   Tray columns provide good gas-liquid contact efficiency and, if many trays are provided, the flow of a liquid phase can be brought to a shape similar to a buffer flow. However, when the tray column is operated under high pressure, a large pressure drop occurs between the trays and, if the column has an overflow weir, it is necessary to widen the spaces between the trays. In addition, when solid particles are suspended in the liquid, the particles are deposited on the gas blowing openings of the trays, thus obstructing the latter, so that there is a tendency to deflect a gas stream. or precipitation of solid particles, thereby making the operation unstable.

   A tray column having no overflow weir or having baffles corresponding to the trays is not limited by the pressure drop between the trays but, when solid particles are in suspension in the liquid, their residence time is different from that of liquid. Therefore, this type of column is not suitable if there is a restriction imposed on the residence time of the solid and the liquid.

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   Stirred vessels provide almost complete mixing inside when mechanical stirring is employed.



  If it is desired to have a high reaction ratio or a high contact efficiency between a liquid and a gas, it is necessary to connect several containers in series, which poses problems from the point of view of economics and maintenance.



   Packed columns and spray columns cannot ensure an extended liquid residence time and are not suitable when it is desired to maintain a high absorption efficiency or a high transformation of liquid and gas or when solid particles are suspended in a liquid.



   The present invention provides a gas / liquid contact apparatus which does not have the above drawbacks of conventional apparatuses and which makes it possible to obtain a high reaction ratio, a high contact efficiency between a liquid and a gas, this apparatus also having a high capacity, while it ensures stable operation. The apparatus of the present invention is particularly effective when the velocity of a gas stream may be relatively low or when solid particles are present in the liquid.



   According to the present invention, the objects and advantageous above are achieved by a gas / liquid contact device comprising a vertical container provided with a conical bottom, a pipe for supplying a liquid or a suspension, a opening for charging a gas to be in contact with the liquid or suspension and at least one opening for removing the liquid or suspension; the container is divided perpendicularly into at least two sections by at least one conical partition wall.

   Each of these sections is constructed in such a way that the material contained in one section can be fed successively to the bottom or part.

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 lower part of the other section by an opening made at the base of this partition wall or by a pipe passing through two sections and the downstream end of a fluid of which opens into the lower part of the other section; the first section comprises a supply line for supplying a liquid or a suspension, as well as a line for charging a desired gas in the lower part of said section, while the last section comprises at least one discharge opening for removing the gas and the suspension or the liquid being treated.

   The apparatus of the present invention will be described in more detail with reference to the accompanying drawings illustrating several parts or the whole of the apparatus of the invention without any limiting character.



   Figures 1 to 6 show various examples of the shape of the partition wall or the bottom of the apparatus of the invention, a indicating a plan view of each example and b, a vertical section. Figures 7 to 13 are vertical sections schematically illustrating different examples of the gas / liquid contact apparatus of the invention. Fig. 14 is a graph showing a comparison between the mass transfer properties of the apparatus of the present invention and those of a known gas / liquid contact apparatus. Fig. 15 is a graph showing the mixing properties of the liquid phase and the particulate phase in the apparatuses of the present invention. In the accompanying drawings, the same reference numerals denote the same parts.



   The vertical apparatus of the invention is divided at least into two sections by partition walls. There is no particular restriction on the number of these sections, but usually, preferably 3 to 50, especially 4 to 20 sections are provided. The appropriate number of sections is determined by the mixing properties of a li-

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 quide or suspension, the desired reaction ratio, the desired contact efficiency between the liquid phase and the gas, etc. The only condition to which the partition wall used in the apparatus of the invention must meet, is that it must go decreasing downwards.

   For example, it may have the different shapes shown in Figures 1 to, 6. In Figures 1 to 6, the reference numeral 1 represents a partition wall and 2, a side wall of the container of the apparatus of the device. invention. The partition wall may be in the form of a projectile, as shown in Figures 1-a and 1-b; it can have the shape of a circular cone as shown in figures 2-a and 2-b, the shape of a hexagonal cone, as shown in figures 3-a and 3-b, the shape of a cone circular concave, as shown in figures 4-a and 4-b, a section in the shape of a trapezoid as shown in figures 5-a and 5-b or the shape of a polygonal cone as shown in figures 6-a and 6-b. Likewise, the oonic surface of these types of partition walls can be replaced in part by a curved or flat surface.

   When using a conical surface, the vertical angle is generally 1200 or less, preferably 90 or less. There is no lower limit, but restrictions are naturally imposed on the structure. Generally, the lower limit is 15, preferably 30, and more preferably 45.



   The material can be transferred from one section to the other through an opening 8 made at the base of the conical partition wall 1, as shown in figure 7 or through a pipe, as shown in figures 8 and 10,
Referring to Figure 7, the interior of a container 4, closed at its top and comprising a side wall 2, as well as a conical bottom 3 tapering downward, is divided by the partition wall 1 in a section 7-a and a

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 section 7-b. A liquid or a suspension is charged in section 7-a from a supply line 5 opening into the side wall of section 7-a.

   A gas supply line 6 is provided at the lower end of a conical bottom 3 from which oxygen, air or other suitable gas is charged into section 7a of the vessel 4. Therefore , section 7a constitutes the first zone in which a liquid or a suspension and a gas are charged for the first time. The slurry or liquid charged in section 7a forms a circulating stream by the ascending effect of a gas supplied from the lower part, so that the liquid or slurry is mixed and brought into intimate contact with the gas. . The liquid or the suspension and the gas in section 7a then flow into the second section 7b through the opening 8 made at the base of the partition wall 1.



  A circulating stream of the liquid or suspension is also formed in section 7b by the upward effect of the gas and sufficient mixing of the liquid with the gas is effected.



  The gas is separated from the liquid or the slurry in section 7b and it is discharged through an exhaust port 9. On the other hand, the liquid or the slurry is withdrawn from the system through a line 10. Reference numerals 11a and 11b respectively indicate the surface area of the liquid in section 7a and section 7b.



   Therefore, with the apparatus shown in Figure 7, showing an embodiment of the invention, when successively charging a liquid or a suspension and a gas from the supply lines 5 and 6, the liquid or the suspension forms a circulating stream which is brought into intimate contact with the gas in sections 7a and 7b, to be subsequently withdrawn continuously from the system, which does not require any particular means such as mechanical agitation.

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   Figure 8 shows another embodiment of the apparatus of the present invention. In figure 8, three sections are formed inside the container 4 by two partition walls 1, that is to say the first section 7a, the second section 7b and the third section 70. The two walls separation has no opening and the material in section 7a flows into the lower part of section 7b through a pipe 12a open at both ends, The material in section 7b then flows into the lower part of section 7c via a pipe 12b.



  Furthermore, the lower part is not necessarily the lower end alone of the conical part, but it can be located anywhere in the entire conical part. The entrance to the conduits 12a and 12b respectively defines the level of the liquid surfaces 11a and 11b. A hollow guide cylinder 13, open at its two ends, is arranged in the upper part of the opening of the gas supply pipe opening into the lower end of section 7a. The guide cylinder 13 promotes the circulation of the liquid or the suspension in section 7a. Therefore, by means of the guide cylinder 13, the solid particles are suitably dispersed and suspended in the liquid, especially when processing a suspension.

   In the embodiment of Fig. 8, a liquid or a slurry charged through the supply line 5 and a gas charged through the supply line 6, are supplied in a pre-mixed state from the lower end of the tube. section 7a.



     The apparatus of Figure 9 is similar in structure to that of Figure 7, with the exception that a short downwardly extending conduit 14 is provided at the opening of the base of the partition walls 1, that a hollow guide cylinder 13 is provided above the opening of each partition wall

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 and that the container 4 is divided into seven sections by six partition walls. In this apparatus, a liquid or a supension and a gas charged respectively by the supply lines 5 and 6 pass successively through the sections 7a, 7b 7g, then they are withdrawn from the system respectively by the line 10 and the line 9.

   By providing a hollow guide cylinder 13 in each section, the circulation of the liquid or the suspension is activated. Optionally, the guide cylinder can be omitted in a desired number of sections.



   In the apparatus of figure 10, the first section 7a, the second section 7b, the third section 7c and the fourth section 7d are formed inside the container 4 by three partition walls 1, A liquid or a suspension is charged in section 7a via supply line 5 opening into the side wall of section 7a and a gas is charged in the lower part of section 7a via line 15, The liquid or suspension is withdrawn via line 10 provided in the last lower section 7d and the gas is evacuated from the system through line 9 provided in section 7d. The three dividing walls have no openings. The material in section 7a flows into the lower part of section 7b through a pipe 12a open at both ends.

   The material in section 7b flows into the lower part of section 7c through line 12b and the material in section 7c flows into the lower part of section 7d through line 12c.



  The inlet of each of the conduits 12a, 12b and 12c defines the level of the liquid surfaces 11a, 11b, 11c and 11d of sections 7a, 7b, 7c and 7d. Obviously, one can decrease or increase the number of partition walls in the apparatus of Figure 10.



   Figure 11 shows a structure in which an area

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 settling 16 is provided in the last zone 7g of the apparatus of FIG. 9. When a suspension is treated by using this type of apparatus, it is possible to recover, from the last zone, a suspension of a high concentration and a liquid having a low solid particle content separately from each other. In the last area 7g of figure 11, the conical partition wall 1 'protrudes beyond the side wall 2 of the container 4 and, along with the side wall 2', this section has a greater internal capacity than any other. what other section.



  In this section, there is provided a separation plate 17 consisting of a concentric cylinder closed at its upper part. The lower end of the partition plate 17 is spaced from the partition wall I and a settling zone 16 of a nun-shaped section is formed between the partition plate 17 and the interior of the wall. lateral 2 '.



  Therefore, in section 7g, circulating streams of the liquid phase and gas form in section 7g ', surrounded by the separation plate 17. In the settling zone 16, the liquid eet separated from the solid particles which. The solid particles are then transferred to section 7g '. As a result, the liquid becomes a suspension of high concentration in section 7g 'and, in the upper part of the settling zone, a liquid having a low solid particle content is obtained. Thus, liquid with a low solid particle content can be removed from a discharge port 18, as well as a high concentration slurry containing 'solid particles from section 7g'.

   The outer diameter of the side wall 2 'of the settling zone 16 may be the same as that of the side wall 2 of the other section, but preferably it is larger in order to make this section more efficient.



   In the apparatus shown in Figure 12, a liquid or

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 a suspension and a gas are withdrawn simultaneously through an evacuation line 20 and the liquid or the suspension is separated from the gas by a gas / liquid separator 21 provided outside the container 4. The gas is evacuated from a pipe 22 provided in the upper part of the gas / liquid separator 21 and the liquid or suspension is withdrawn from a pipe 23 provided in its lower part,
A baffle 24 may be provided above a guide cylinder 13, leaving some space between them in each of the sections. It is preferable to provide the chioane 24 because this prevents too short a passage of the liquid between the sections.



   We have found that by using the gas / liquid contacting apparatus described above according to the invention, crude terephthalic acid can be refined by a simple operation, thereby obtaining high purity terephthalic acid.



   In the conventional manufacture of a linear fiber and film-forming polyester, consisting primarily of ethylene terephthalate such as polyethylene terephthalate, an ester exchange is carried out between ethylene glycol and dialkyl terephthalate. . The reason for this characteristic is that terephthalic acid is difficult to refine, while dialkyl terephthalate can be refined quite easily by a process such as recrystallization and distillation. However, this process is not straightforward since the terephthalic acid must first be esterified with an alkyl alcohol such as methyl alcohol, and then be transesterified with ethylene glycol.

   Therefore, it is evident that it is advantageous to prepare polyethylene terephthalate by direct reaction of terephthalic acid with ethylene glycol. However, so that the

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 Polyethylene terephthalate obtained by this direct esterification can have excellent whiteness and very good transparency, while being commercially interesting, it is important that the starting terephthalic acid is very pure.



   Among the methods for preparing terephthalic acid currently adopted in commerce, there are the nitric acid oxidation process and the air oxidation process, in which p-xylene is a raw material. ; -, gine, @@@@ Henkel, wherein phthalic anhydride is a raw material. In recent years, a process has been proposed for the preparation of terephthalic acid by oxidation of a dialkylbenzene, in particular p-xylene, with molecular oxygen in a fatty acid of 2 to 4 carbon atoms. as a solvent, in the presence of a cobaltous compound at a relatively moderate temperature, that is, at a temperature not exceeding 150 C.

   This process comprises: (a) a process in which the oxidation is carried out in the presence of a methyleneoetone such as methyl ethyl ketone, as has been proposed in the specifications of United States Patent Nos. 2,853,514 and 3,036,122, (b) a process in which ozone is used as an initiator, as has been proposed in Japanese Patent Application Publication No. 4,963 / 60, (c) a process in which is carried out oxidation by addition of acetaldehyde, as proposed in the specification of US Pat. No. 2,673,217, (d) a process in which the oxidation is carried out by the addition of an ether such as isopropyl ether, as described in Japanese Patent Application Publication No. 4..465 / 64 and (e)

   a process in which the oxidation is carried out in the presence of a large amount of a cobalt salt, as proposed in the specification of US Pat. No. 3,334,135.



   Terephthalic acid formed by either of these

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 The process contains colored impurities, as well as impurities forming colored materials, and it is not suitable for the preparation of a linear polyester having excellent whiteness by direct esterification.



   The Applicant has carried out research relating to the refining of crude terephthalic acid formed by the processes (a) - (e) mentioned above using a cobalt compound catalyst and found that by adopting a refining process in which the oxidation is carried out with liquid phase molecular oxygen in an aliphatic monocarboxylic acid of 2 to 4 carbon atoms, terephthali- acid; that the system was refined to a remarkable effect when it; is contacted with molecular oxygen at a specific temperature in the slurry state, and at temperatures exceeding the upper limit or the lower limit of the specific range, refining was not completed until one insufficient degree.

   Therefore, the Applicant has proposed a process for preparing refined terephthalic acid consisting in separating crude terephthalic acid from a reaction mixture containing the crude terephthalic acid obtained by oxidation of a p-dialkylbenzene with molecular oxygen in an aliphatic monocarboxylic acid of 2 to 4 carbon atoms in the presence of a cobalt compound, then contacting with molecular oxygen, a suspension maintained at a temperature of 180 to 230 C and consisting of 6 -100 parts by weight of said crude terephthalic acid and 100 parts by weight of at least one aliphatic monocarboxylic acid chosen from the group comprising acetic acid,

   propionic acid and butyric acid or an aqueous solution of said aliphatic monocarboxylic acid containing at most 50% by weight, preferably at most 30 by weight of water.



   To carry out such a continuous refining process, it

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 The reaction apparatus to be employed must meet the following conditions (1) sufficient contact must be established between a gas and a suspension, (2) the solid particles must be sufficiently suspended and dispersed in a liquid, (3) the liquid phase should take a form close to a buffer flow, (4) the operation should be stable, etc.



   Heretofore, devices such as a bubbling column, a tray column and a stirring vessel have been employed to bring a gas into contact with a suspension of this type but, in view of the drawbacks of these devices, as mentioned at the beginning of this specification, it has been extremely difficult to meet the conditions (1), (2), (3) and (4) mentioned above.



   We have found that by applying the gas / liquid contact apparatus of the invention to the refining of crude terephthalic acid and by setting suitable conditions, the refining process is easily carried out on a commercial scale for give high purity terephthalic acid,
Accordingly, the present invention provides a process for refining crude terephthalic acid, which process comprises separating crude terephthalic acid from a reaction mixture containing crude terephthalic acid obtained by the oxidation of a p-dialkoylbenzene. with molecular oxygen in an aliphatic monocarboxylic acid of 2 to 4 carbon atoms in the presence of a cobaltic compound, then contact, with molecular oxygen,

   a suspension maintained at a temperature of 180 to 230 C and consisting of 6-100 parts by weight of said crude terephthalic acid and 100 parts by weight of at least one aliphatic monocarboxylic acid or of an aqueous solution of said containing aliphatic monocarboxylic acid, at most 50% by weight of water, this process being characterized by

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 that the suspension is introduced into one or the other section of a refining vessel divided at least into two sections in the perpendicular direction, at least by a dividing wall which decreases downwards, then by introducing a gas containing molecular oxygen into the lower part of said section at a surface speed of at least 0.1 cm / second,

   this suspension and this gas then being mixed with each other in said section, then the mixture is successively loaded into the lower part of the other sections through an opening made at the base of the partition wall or through a pipe passing through two sections and whose downstream end of a fluid stream opens into the lower part of the other section, the mixture then being withdrawn from the last section outside the container, after which the terephthalic acid.



   The process of refining crude terephthalic acid according to the present invention will be described in detail hereinafter.



   CRUDE START TEREPHTHALIC ACID
Crude terephthalic acid or a material containing terephthalic acid used in the refining process of the present invention is prepared by oxidizing a known p-dialkoylbenzene; with molecular oxygen in a fatty acid of 2 to 4 carbon atoms, in the presence of a cobaltous compound as a catalyst, for example according to methods (a) to (e) mentioned above, for example US patents n 2,853,514 and 2,673,217.



   If the reaction liquor obtained by the oxidation is subjected as it is (without separation of the terephthalic acid) to the refining treatment of the invention, the terephthalic acid j is only insufficiently refined and the objects of the invention cannot be achieved. This is why, according to the present invention, it is essential that the terephthalic acid be separated.

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 of a reaction mixture resulting from the oxidation and that the crude terephthalic acid recovered is used as a raw material.



   In this case, it is preferable to wash the separated crude terephthalic acid one to several times with a suitable solvent such as water, an aliphatic monocarboxylic acid of 2 to 4 carbon atoms or its aqueous solution before washing. using it as a raw material, since the effect of the invention is thereby improved.



  REFINING MEDIUM
As mentioned above, as the refining medium, acetic acid, propionic acid or n- or isobutyric acid can be employed, with acetic acid being particularly preferred. These media are usually used in substantially pure form, alone or as a mixture, but they can also be used in the form of an aqueous solution containing, at most, 505, in particular, at most 30% by weight of water. . When the water content is more than 50% by weight, the refining effect decreases and the objects of the invention cannot be achieved. REFINING CONDITIONS
As has already been pointed out,

   the refining effect is closely related to the ratio of the refining medium to the crude terephthalic acid used in the refining process, as well as to the temperature conditions at the time of refining. According to the invention, terephthalic acid of very high purity can be obtained by contacting, with a gas containing molecular oxygen, a suspension maintained at a temperature of 180 to 230 C, consisting of 6 to 100 parts by weight of crude terephthalic acid and 100 parts by weight of the refining medium (the monocarboxylic acid or its aqueous solution containing not more than 50%, preferably not more than 30% by weight. weight of water.

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  A. The influence on the refining effect of the ratio between crude terephthalic acid and the refining medium on the refining effect will be described below with reference to experiments in which all parts and percentages are by weight. 'Experimental Examples 1 - 14
20 parts of p-xylene, 130 parts of acetic acid and 20 parts of cobalt acetate Co (OCOCH3) 2.4H2O were charged to a stainless steel pressure reactor fitted with a gas blast inlet at its end. lower part, as well as' a stirrer. While maintaining the temperature at 120 ° C., air was introduced thereto at a pressure of 20 kg / cm 2 gauge at a flow rate (calculated as oxygen) of 0.0095 mol / p-xylene charged.minute.

   Stirring was carried out at 1200 rpm. The reaction was continued until there was virtually no more appreciable absorption of oxygen. At the end of the reaction; the reaction mixture was removed and separated into a solid and a liquid by centrifugation. The solid was washed with a small amount of glacial acetic acid and mixed with glacial acetic acid in an amount equal to three times the weight of the solid. The mixture was boiled and refluxed for 20 minutes under normal atmospheric pressure, then filtered while hot. The solid was again refluxed as described above, filtered hot and washed to obtain crude terephthalic acid which was used as the raw material of the mixture. refining.



   In each operation, crude terephthalic acid (having an optical density of 0.480 (cell: 5 cm) and containing 0.0192% was charged in a titanium pressure vessel at the ratio indicated in Table 1 below. by weight of Co calculated as Co (OCOCH3) 2.4H2O) and pure glacial acetic acid. Air was introduced under a pressure of 12 kg / cm2 and it was

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 the reaction system was stirred for two hours at a temperature of 190 ° C. Then, it was cooled to 115 ° C. and at a rate of 26 ° C. / minute. The product was removed and filtered.

   The terephthalic acid was further washed with pure glacial acetic acid, boiled with distilled water, filtered, washed again with distilled water and it was dried to obtain refined terephthalic acid. The optical density of the refined terephthalic acid thus obtained is also shown in Table 1.



   Table 1
 EMI17.1
 
<tb>
<tb> Exem- <SEP> Quantity <SEP> Quantity <SEP> Rate <SEP> of <SEP> the <SEP> frac- <SEP> Density
<tb> pies <SEP> of acid <SEP> of acid <SEP> tion <SEP> insoluble <SEP> of <SEP> optical <SEP> of
<tb> experienced <SEP> phthalic <SEP> acetic <SEP> acid <SEP> terrified- <SEP> acid
<tb> mental <SEP> raw <SEP> lacial <SEP> phthalic <SEP> loaded <SEP> terephthalic
<tb> (parts <SEP> (parts <SEP> (5;

   <SEP> in <SEP> weight) <SEP> refined
<tb> in <SEP> weight) <SEP> in <SEP> weight) <SEP>
<tb> 1 <SEP> 2 <SEP> 100 <SEP> 40 <SEP> 0.150
<tb> 2 <SEP> 3.3 <SEP> 100 <SEP> 64 <SEP> 0.121
<tb> 3 <SEP> 5.3 <SEP> 100 <SEP> 78 <SEP> 0.103
<tb> 4 <SEP> 6 <SEP>, <SEP> 6 <SEP> 100 <SEP> 82 <SEP> 0.092
<tb> 5 <SEP> 10 <SEP> 100 <SEP> 88 <SEP> 0.082
<tb> 6 <SEP> 11 <SEP> 100 <SEP> 89 <SEP> 0.086
<tb> 7 <SEP> 20 <SEP> 100 <SEP> 94 <SEP> 0.057
<tb> 8 <SEP> 25 <SEP> 100 <SEP> 95 <SEP> 0.060
<tb> 9 <SEP> 33 <SEP> 100 <SEP> 96 <SEP> 0.070
<tb> 10 <SEP> 43 <SEP> 100 <SEP> 97 <SEP> 0.078
<tb> 11 <SEP> 54 <SEP> 100 <SEP> 98 <SEP> 0.082
<tb> 12 <SEP> 67 <SEP> 100 <SEP> 98 <SEP> 0.085
<tb> 13 <SEP> 82 <SEP> 100 <SEP> 99 <SEP> 0.098
<tb> 14 <SEP> 122 <SEP> 100 <SEP> 99-100 <SEP> 0.161
<tb>
 
From Table 1,

   it can be seen that the ratio of crude terephthalic acid to the refining medium (glacial acetic acid) considerably influences the purity of the acid.

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 of refined terephthalic. Table 1 shows that the lower the optical density of refined terephthalic acid, the higher the purity of refined terephthalic acid.



  In experimental examples 1 - 3 (outside the scope of the invention), the amount of crude terephthalic acid was less than 6 parts by weight, against 100 parts by weight of glacial acetic acid and, in the experimental example 14 (outside the scope of the invention), more than 100 parts by weight of crude terephthalic acid were used against 100 parts by weight of glacial acetic acid. In either of these examples, the refined terephthalic acid had a greater optical density than the refined terephthalic acid obtained in either of the Experimental Examples 4 - 13 (falling within the scope of invention). The optical density of refined terephthalic acid in Experimental Examples 1 to 3 and 14 is more than 0.1, and this terephthalic acid cannot be used in direct esterification.

   On the other hand, the refined terephthalic acids obtained in Experimental Examples 4 - 13 have an optical density less than 0.1 and they can be used in direct esterification.



   Table 1 also indicates the percentage by weight of the insoluble fraction of crude terephthalic acid, based on glacial acetic acid as the refining medium. This given reveals that, under the conditions of the invention, the majority part (at least about 80%) of the crude terephthalic acid used as a raw material is contacted with molecular oxygen in an insoluble state, thereby yielding refined terephthalic acid of higher purity which, in fact, was very unexpected,
The optical density mentioned in this specification, was measured at 380 m with a 5 cm long cell using 25 cm3 of a solution of 1 g of a sample in

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 a 14% aqueous ammoniacal solution.



  B, According to the invention, it has been found that there is an important relationship between the temperature conditions at the time of refining and the refining effect. Catte C @@@@@@@@@@stique water illustrated in the experiments below.



  Experimental examples 15 - 25
Into a titanium pressure vessel were charged 6.6 parts of the same crude terephthalic acid as that prepared in Experimental Example 1 (having an optical density of 0.480 (cell; 5 cm) and containing 0.019% Co, calculated in Co (OCOCH3) 2.4H2O) and 100 parts of glacial acetic acid, then air was introduced at a pressure of 12 kg / cm2 gauge. The reaction system was maintained at the temperature shown in Table 2 and stirred for two hours. The same treatment process was followed as in Experimental Example 1. The optical density of refined terephthalic acid is shown in Table 2.



   Table 2
 EMI19.1
 
<tb>
<tb> Example <SEP> Temperature <SEP> Rate <SEP> of <SEP> the <SEP> fraction <SEP> Optical <SEP> density
<tb> Insoluble <SEP> <SEP> <SEP> from <SEP> acid <SEP> from <SEP> acid
<tb> mental <SEP> refining <SEP> terephthalic <SEP> crude <SEP> terephthalic
<tb> n <SEP> C <SEP> (% <SEP> in <SEP> weight) <SEP> refined
<tb> 15 <SEP> 160 <SEP> 91.0 <SEP> 0.192
<tb> 16 <SEP> 175 <SEP> 84.0 <SEP> 0.128
<tb> 17 <SEP> 180 <SEP> 83.4 <SEP> 0.096
<tb> 18 <SEP> 190 <SEP> 82.0 <SEP> 0.090
<tb> 19 <SEP> 200 <SEP> 77.4 <SEP> 0.077
<tb> 20 <SEP> 210 <SEP> 74.3 <SEP> 0.050
<tb> 21 <SEP> 220 <SEP> 60.5 <SEP> 0.070
<tb> 22 <SEP> 230 <SEP> 54.5 <SEP> 0.098
<tb> 23 <SEP> 235 <SEP> 50.0 <SEP> 0.155
<tb> 24 <SEP> 250 <SEP> 28.8 <SEP> 0.519
<tb> 25 <SEP> 261 <SEP> 9,1 <SEP> 1,

  231
<tb>
 

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From the results shown in Table 2, it was found that even the amount of crude terephthalic acid was within the range of the invention (6-100 parts by weight per 100 parts by weight of the refining medium. ), the optical density of refined terephthalic acid is greater than 0.1 when the refining temperature is below 180 C (experimental examples 15 and 16) and greater than 230 C (experimental examples 23, 24 and 25), this terephthalic acid cannot be used during direct esterification while, on the other hand, the optical density of refined terephthalic acid at a temperature of 180 to 230 C according to the invention (experimental examples 17 - 22) is less than 0.1,

   terephthalic acid can then be used during direct esterification.



   From the results of experimental examples 23, 24 and 25, it can be seen that at a refining temperature of 230 C, the optical density of refined terephthalic acid is greater than that of the starting crude terephthalic acid (0 , 1) and that when the refining is carried out at a temperature above 230 C, there is a greater formation of colored matter.



   Therefore, it is observed that by refining the crude terephthalic acid obtained by using a cobaltic compound as catalyst, it is important to work at a refining temperature of 180 to 230 C using 6 - 100 parts by weight of terephtha- acid. crude liquor against 100 parts by weight of the refining medium.
 EMI20.1
 OTHER CmmITIONS DE¯T.'Il (i $ 1] µQiQ, $ ¯
Other conditions necessary for the implementation of the invention will be described below in detail.



   The gas containing molecular oxygen, used in the refining of the crude terephthalic acid of the invention, can be practically pure oxygen or a gas containing both molecular oxygen and an inert gas such as than nitrogen and

 <Desc / Clms Page number 21>

 carbon dioxide, molecular oxygen can be contained in any proportion. Generally, air is the most economical gas containing molecular oxygen. The partial pressure of molecular oxygen in the system is not limited, but as a general rule it is preferably 1 to 50 atmospheres, more preferably 1 to 10 atmospheres.



   The total pressure of the reaction vessel for carrying out the invention is expressed by the sum of the vapor pressure of the solvent, the pressure of molecular oxygen and the partial pressure of the inert gas. According to the invention, any pressure can be employed, as long as at least a part of the refining medium is maintained in a liquid state in the reaction system. Usually it is better to work under a total pressure of 5 to 100 atmospheres.



   The refining treatment of the invention can be carried out in the presence or absence of a catalyst. As the catalyst, any known liquid phase air oxidation catalyst can be employed and, as a rule, variable-valent metals such as Co, Mn, Ni, Cr and Pb are widely employed, as well as their compounds. However, according to the present invention, the cobalt catalyst remaining in crude terephthalic acid is sufficient, since a cobalt compound is used as a catalyst in the liquid phase air oxidation of p-dialkoylbenzene.



   The time required for refining (expressed as the time during which the reaction system is maintained at the temperature specified above) is generally 5 minutes to 5 hours, especially 20 minutes to 3 hours.



  REFINING METHODS OF THE INVENTION
It has been found that by using the gas / liquid contact apparatus mentioned above in the acid preparation process

 <Desc / Clms Page number 22>

 Refined terephthalic consisting of separating crude terephthalic acid from a reaction mixture containing crude terephthalic acid obtained by oxidation of p-dialkoylbenzene with molecular oxygen in an aliphatic monocarboxylic acid of 2 to 4 carbon atoms in the presence of a cobalt compound, then bring into contact, with molecular oxygen, a suspension maintained at a temperature of 180 to 230 C,

   consisting of 6 to 100 parts by weight of said crude terephthalic acid and 100 parts by weight of at least one aliphatic monocarboxylic acid of 2 to 4 carbon atoms and containing, at most,
50% by weight of water, high purity terephthalic acid could be continuously obtained with extremely good efficiency and by a simple operation.



   According to the refining process of the invention, in the apparatus of the type shown in FIGS. 1 to 12 and in the manner described above with reference to these figures, a slurry (liquid phase) is charged. crude terephthalic acid and a gas containing molecular oxygen. Specifically, a refined terephthalic acid is obtained by introducing the suspension into one or the other section of a refining vessel divided at least into two sections in the perpendicular direction by at least one partition wall going decreasing down.

   Then, a gas containing molecular oxygen is introduced into the lower part of this section at a surface speed of at least 0.1 cm / second, this suspension and this gas are mixed with each other. in said section, the mixture is successively loaded into the lower part of the other section through an opening made at the base of the partition wall or through a pipe extending through two sections and the downstream end of which d 'a stream of a fluid opens into the lower part of the other section, then this mixture is removed from the last section to the outside

 <Desc / Clms Page number 23>

 from the container and then the terephthalic acid is separated.



   In the refining process of the invention, the velocity of the gas containing molecular oxygen must be determined such that: (1) the solid particles of the suspension are sufficiently suspended and float in the aliphatic monocarboxylic acid and (2) also such that the contact between the slurry and the gas containing molecular oxygen is sufficient to obtain a complete refining effect. To this end, according to the invention, it is necessary to adjust the surface speed of the gas in the column to.
 EMI23.1
 0.1 em3 / em2.second or more, preferably a minimum of 0.2 cm3 / cm2.second.

   By the expression "surface gas velocity" as used in this specification is meant a
 EMI23.2
 surface velocity (cm3! cm2. second i. ::.; j, mixture f. "C!" 'At1. consisting of a gas containing molecular oxygen and vapor of the medium present. Moreover, the speed The aforementioned surface area can also be expressed in cm / second, If the surface speed of the gas exceeds a certain limit, the gas passes through the suspension in the form of a column of gas and the effect of contact between the liquid and the gas is altered, thus making the operation unstable. The compressor requires more power and the quantity of the medium entrained in the gas ¯ increases. Therefore, it is necessary to choose the superficial speed at which these phenomena do not occur .



  In this regard, the superficial gas velocity in the column according to the invention is 5 cm / second or less, preferably 1 cm / sec or less.



   In the refining process of the invention, it is preferable that the number of sections formed by the partition walls in the apparatuses shown in Figures 7 - 12 is from 4 to 20.



   A preferred embodiment of the

 <Desc / Clms Page number 24>

 refining process of the invention with reference to Figure 13.



   In Figure 13, crude terephthalic acid and aliphatic monocarboxylic acid or its aqueous solution containing 50% by weight of water (liquid medium) are charged from lines 31 and 32 into a container 33, in order to prepare a suspension of departure. The vessel 33 provides a stirring effect necessary to uniformly disperse the crude solid terephthalic acid in the liquid medium. A suspension prepared in the vessel 33 is continuously transported to a preheater 35 by means of a pump 34 which is heated to 180-230 C, then this suspension is fed to a refining column 36. The preheater 35 is a heat exchanger. of the paper clip type in which the slurry flows through a titanium pipe and is heated from outside the pipe by a heat transfer medium.

   The refining column 36 has a structure similar to that shown in Figures 7 to 12 and is a titanium vessel provided with a heating jacket. It is preferable that the average residence time of the suspension in the refining column 36 is 5 minutes to 5 hours, in particular 10 minutes to 3 hours. A gas containing molecular oxygen, for example air, is sucked into a compressor 38 through a filter 37, so that the pressure prevailing inside the refining column 33 is 10-100 kg / gauge cm2, in particular 15-50 kg / gauge cm2, the superficial gas velocity being 0.1-5 cm / second, in particular 0.2-1 cm / second.

   The pressure is raised and the gas is heated by a preheating saturator 39 to 180-230 C. The gas is practically saturated with the medium and it is continuously charged to the refining column 36.



  In this refining column 36, the suspension and the gas are brought into concurrent contact with each other and they are

 <Desc / Clms Page number 25>

 separated one of the autia in the final section. From the final section, the gas is fed into a refrigerant 40 where it is cooled. Most of the medium entrained in the gas is condensed and is fed to a gas / liquid separator 41.



  The separated gas is sent to the atmosphere so that the pressure in the refining column 36 is almost constant.



  The condensed medium obtained in the gas / liquid separation device 41 is brought into a tank 42 under normal atmospheric pressure. It is continuously charged into the preheating saturator 39 by a quantitative pump 43. As it is entrained by the gas, it is returned to the refining column 36. From this refining column 36, the slurry is withdrawn for be fed into a sweep tank 44, so that the liquid level in the final section can be kept substantially constant. The purge tank 44 is a stainless steel stirring vessel provided with a condenser 45 and it is preferable to maintain the internal temperature, for example, at the boiling point of the medium used.

   While at an elevated temperature, the slurry contained in the purge tank 44 is fed to a separator 47 via the pump 46 and, in this separator, the refined terephthalic acid is separated from the filtrate. The separated filtrate is sent to a filtrate tank 48. The refined and separated terephthalic acid is dried and made into a final product.
In accordance with the present invention, the above operation allows the suspension and the molar oxygen-containing gas to be brought into contact with an efficiency greater than that of any other treatment in which one or more is used. the other known liquid / gas contact apparatus, this operation also making it possible to continuously obtain terephthalic acid of high purity.

 <Desc / Clms Page number 26>

 



   The refining process of the invention will be described in more detail by the following examples given by way of illustration only.



   Unless otherwise indicated, all parts and percentages of the examples are by weight.



  Part i
Part I shows a series of model experiments to measure mass transfer properties and mixing properties of different liquid / gas contact devices.



  Example 1.



   An apparatus for measuring the mass transfer properties of the gas / liquid continuous contact apparatus of the type shown in Fig. 9 is made of a hard and transparent vinyl chloride resin, the inside diameter of the column being 145 mm and the partition wall being conical in shape with a vertical angle of 60, while the communication between the liquid and the gas is effected by a pipe provided at the base of the conical partition wall, with an internal diameter of 3 mm and a length of 20 mm, On the lateral surface of the pipe, four slots of 1 mm wide and 10 mm long are made. The sections divided by the partition walls in the vertical direction are 7 in number and the distance between two partition walls is 275 mm.



  Each section comprises, at its base, a guide cylinder with an internal diameter of 20 mm and a length of 100 mm.



   Inside this apparatus, an aqueous solution of 0.2 N sodium sulfite containing 10-3 mole / liter of copper sulfate in contact with gaseous oxygen was raised and the change in temperature was measured. concentration of sodium sulfite.



  Next, an overall coefficient of absorption capacity (KL Ó) of oxygen gas in the liquid phase was determined. Figure 14 shows the KL Ó values, measured by varying

 <Desc / Clms Page number 27>

 the superficial gas velocity with the superficial liquid velocity maintained at 0.076 cm3 / cm2.second.



  Comparative example l
The mass transfer properties were measured using a bubbling column, as well as a multistage bubbling column. The bubbling column was made of a hard transparent vinyl chloride resin and had an inner diameter of 137mm and a length of 1096mm; The multistage bubbling column was made by inserting seven perforated plates having 79 openings with a diameter of 5 mm and an aperture ratio of 0.105 in the aforementioned bubbling column, at intervals this mm.



   In this apparatus, oxygen gas was countercurrently brought into contaot with an aqueous solution of sodium sulphite and the KLO value was measured. Figure 13 shows the KL Ó values, measured by varying the surface velocity of the gas with the surface velocity of the liquid maintained at 0.340 cm3 / cm2. second, as well as the values obtained with respect to the apparatus of the invention.



  Example 2.



   The mixing properties of the liquid phase when the gas is contacted with the liquid in a continuous gas / liquid contact apparatus shown in Example 1 were measured by the 0 response method.



   Water was flowed at a surface speed of 0.076 cm3 / cm2 / second, and an air stream was also passed with the water at a surface speed of 0.3 cm3 / cm2. second. 20 cc of concentrated sulfuric acid was immediately charged from the liquid inlet. The concentration of sulfuric acid at the outlet of the liquid was measured with time elapsing, so as to determine the response ó.



     , The response E (#) during the duration & in a device

 <Desc / Clms Page number 28>

 in which the average residence time of the liquid is Út, is expressed by the following formula Concentration of sulfuric acid retained from E (#) = leaving the apparatus during the time Ú x the apparatus
Total quantity of sulfuric acid charged in the device (# = Ú / ÚT)
The values of the mixing properties of the liquid phase are indicated by the symbol 0 in Fig. 15, in which the theoretical values of the mixing properties of a seven-stage unit are indicated. The values correspond perfectly to the response curve 6 obtained with respect to a seven-stage element.



  Example 3.



   The mixing properties of the particulate phase of a slurry were measured when the gas was contacted with the slurry in a gas% liquid continuous contact apparatus described in Example. 1, The suspension consisted of terephthalic acid and ethyl ether, and the gas was nitrogen gas. The suspension of 15 parts by weight of terephthalic acid and 85 parts of ethyl ether was allowed to flow at a surface speed of 0.076 cm 3 / cm 2 2 and, with the flow of the suspension. passing nitrogen gas at a surface speed of 0.3 cm3 / cm2. sec.

   From the intake provided for the suspension, 50 g of activated carbon were immediately charged, having the same particle size distribution as that of terephthalic acid, then the quantity of activated carbon discharged through the outlet was measured with the time elapsed and thus the response 6 of the particulate phase was measured.



   The response Ù E (#) during the duration Ú in an apparatus in which the mean residence time of the liquid is Ú T, is expressed by the following formula;

 <Desc / Clms Page number 29>

 
Activated carbon concentration Retained over time 0- x the device
 EMI29.1
 E (µE ') = ######################################
1) Total quantity of activated carbon loaded in the device (# = Ú / ÚT)
The values of the mixing properties of the particular phase are indicated by the symbol X in Fig. 15, together with the mixing properties of the liquid phase. The values correspond perfectly to the response curve obtained with respect to a seven-stage element.



  PART II
Part II indicates a series of experiments in which the purification of crude terephthalic acid was carried out using the apparatus of the invention.



  Examples
20 parts of p-xylene, 130 parts of acid were charged
 EMI29.2
 acetic and 20 parts of cobaltous acetate, Co (OCOCH3), 4I20¯% in a stainless steel pressure reactor similar to a bubbling column, this reactor having a gas inlet at its lower part, so that the average duration stay could be 8 hours. Air was charged thereto at a surface speed of 3 cm / second, at a temperature of 120 ° C. and under a pressure of 10 kg / cm 2 gauge. The reacted mixture was continuously removed and subjected to centrifugation to separate a solid from a liquid.

   The solid was mixed with acetic acid in an amount equal to three times its weight and treated for 20 minutes at 80 ° C. under atmospheric pressure. Then, a solid was separated from a liquid while hot, and then the solid was again subjected to the treatment described above. While still hot, we separated it and washed it to get crude terephthalic acid,
15 parts by weight of the crude terephthalic acid thus obtained (having a content of 4-CBA (4-carboxybenzal-

 <Desc / Clms Page number 30>

 dehyde) of 1.5% by weight and an optical density of 0.480) with
85 parts by weight of an acetic acid containing 10% water, to form a suspension which was preheated to a temperature of 220 C.

   The suspension was treated at a temperature of 220 0 and a pressure of 40 kg / cm2 gauge with compressed air in the apparatus of the present invention having the internal structure shown in Figure 8 (number of sections : 7; shape of the partition wall: conical} vertical angle of the partition wall: 60); connection opening, pipe provided with a short section 6 mm long at the base of the partition wall; ratio between the diameter of the connecting pipe and the internal diameter of the column! 0.046; distance between two sections: s 1.87 times the diameter of the column). The superficial velocity of the gas flowing through the column is shown in the following table.

   The treated suspension was continuously removed and separated into a solid and a liquid. The obtained cake was also suspended in acetic acid containing 10% water, then the suspension was heated to a temperature of 80 ° C. and then, treated for 20 minutes with stirring. Then, it was subjected to solid / liquid separation, and the obtained cake was dried. The table below indicates the optical density and the 4-CBA content of the crude terephthalic acid obtained in each example.
 EMI30.1
 
<tb>
<tb>



  Example <SEP> Speed <SEP> Density <SEP> optical <SEP> Content <SEP> in <SEP> 4-CBA <SEP> of
<tb> superficial <SEP> of <SEP> acid <SEP> acid <SEP> terephtadu <SEP> gas <SEP> (cmisec) <SEP> terephthalic <SEP> lique <SEP> refined
<tb> refined <SEP> (% <SEP> in <SEP> weight)
<tb> 4 <SEP> 0.3 <SEP> 0.032 <SEP> 0, lE
<tb> 5 <SEP> 0.6 <SEP> 0.027 <SEP> 0.19
<tb> 6 <SEP> 0.8 <SEP> 0.032 <SEP> 0.155
<tb> 7 <SEP> 3.0 <SEP> 0.026 <SEP> 0.16
<tb>
 

 <Desc / Clms Page number 31>

 Examples 8 - 11
The process of Example 5 was repeated, with the exception that the refining pressure and temperature were varied as indicated in the following table. The results are also shown in this table.
 EMI31.1
 
<tb>
<tb>



  Exem- <SEP> Tempera- <SEP> Pressure <SEP> Density <SEP> optical <SEP> Content <SEP> in <SEP> 4-CBA
<tb> ple <SEP> ture <SEP> (kg / cm2 <SEP> of <SEP> acid <SEP> of <SEP> acid <SEP> terried
<tb> (C) <SEP> manom- <SEP> terephthalic <SEP> phthalic <SEP> refined
<tb> triques) <SEP> refined <SEP> pn <SEP> in <SEP> weight)
<tb> 8 <SEP> 185 <SEP> 30 <SEP> 0.095 <SEP> 0.58
<tb> 9 <SEP> 200 <SEP> 30 <SEP> 0.070 <SEP> 0.40
<tb> 10 <SEP> 210 <SEP> 40 <SEP> 0.054 <SEP> 0.28
<tb> 11 <SEP> 225 <SEP> 40 <SEP> 0.040 <SEP> 0.17
<tb>
   Example, comparative 2
The process of Example 4 was repeated, with the exception that the superficial gas velocity was 0.08 cm / second, In one hour,

   there was a pressure difference of 20 kg / cm2 gauge between the top of a refining column and a saturator. air preheating. This resulted in a large fluctuation in the amount of the air stream and it was impossible to continue the operation.



  Examples 12 - 14
The process of Example 4 was repeated except for the changes shown in the following table. The results are also shown in this table.
 EMI31.2
 
<tb>
<tb>



  Exem- <SEP> Angle <SEP> Number <SEP> Ratio <SEP> of <SEP> Density <SEP> Content <SEP> in <SEP> 4-CBA
<tb> ple <SEP> vertical <SEP> of <SEP> sec- <SEP> distance <SEP> optical <SEP> of <SEP> from <SEP> acid <SEP>
<tb> of <SEP> the <SEP> tions <SEP> between <SEP> two <SEP> terephthalic acid <SEP>
<tb> wall <SEP> of <SEP> sections <SEP> and <SEP> terephta- <SEP> refines
<tb> separa- <SEP> the <SEP> diameter <SEP> lique <SEP> in <SEP> weight)
<tb> tion <SEP> () <SEP> inside <SEP> of <SEP> refined
<tb> the <SEP> column
<tb> 12 <SEP> 90 <SEP> 14. <SEP> 1.0 <SEP> 0.026 <SEP> 0.16
<tb> 13 <SEP> 90 <SEP> 9 <SEP> 1.54 <SEP> 0.025 <SEP> 0.18
<tb> 14 <SEP> 60 <SEP> 3 <SEP> 4.0 <SEP> 0.047 <SEP> 0.26
<tb>
 

 <Desc / Clms Page number 32>

 Example 15.



   The process of Example 5 was repeated, with the exception that the apparatus of the invention having the structure was employed; shown in figure 10 (shape of the conical partition wall; vertical angle of the partition wall: 60; communication opening, pipe having an inside diameter of 0.023 times the inside diameter of the column, one end being open to the same level as the base of the partition wall in an upstream section, the other end being open near the base of the partition wall of: this section; number of sections 7; distance ratio between two sections and internal diameter of the column: 1.86).



  Refined terephthalic acid had an optical density of 0.027 and a 4-CBA content of 0.16.



  Example 16.



   The process of Example 5 was repeated, with the exception that the apparatus of the invention having the structure shown in Figure 8 (shape of the partition wall; conical; vertical angle of the partition wall: 60; communication opening, pipe having an internal diameter of 0.023 times the internal diameter of the column, one end being open at the same level as the base of the partition wall of a section, upstream, the other end being open near the base of the partition wall of this section; number of sections; 7; distance ratio between two sections and the inside diameter of the column: 1.86).

   The obtained refined terephthalic acid had an optical density of 0.028 and a 4-CBA content of 0.18% by weight.



  Example 17.



   The crude terephthalic acid (1765 parts by weight) prepared in Example 4, as well as 100 parts by weight of acetic acid containing 1% of water were charged to a suspension preparation vessel 33, by pipes 31 and 32 respectively

 <Desc / Clms Page number 33>

 As shown in the diagram of Figure 13. Stirring was carried out in vessel 33 in order to uniformly disperse the solid particles in the acetic acid.

   The slurry prepared in vessel 33 was preheated to a temperature of 220 C by passing through a preheater 35, which was a paper clip-type heat exchanger, in which the slurry flows in a titanium pipe, to be preheated by a heat transfer medium from outside the pipe. A refining tower 36 is a titanium vessel having the same structure as that shown in Example 1, which is heated by a heating jacket so as to maintain the terriperature of the suspension in the column at practically 220 C.



  The average residence time of the suspension in the column was 35 minutes. By a compressor 38, through a filter 37, air was sucked in an amount calculated so as to establish, in the refining column, a pressure of 40 kg / cm2 gauge and a surface speed of 0.45 cm. /second.



  Then, the air pressure was increased and heated to 220 C by an air preheating saturator 39, then saturated with acetic acid, The air thus treated was continuously charged. in the refining column 36. The slurry and the gas are mounted in the refining column in contact. with each other in the same direction and they were separated in the upper room. The gas was taken to a condenser 40 and most of the acetic acid entrained in the air R was condensed. It was taken to a liquid / gas separator 41, and then the separated air was discharged into the atmosphere, so that the pressure of the refining column could be nearly constant.

   The condensed acetic acid obtained in the liquid / gas separator 41 was supplied to an acetic acid tank at atmospheric pressure 42 and it was continuously charged into the preheating air saturator 39 by a pump.

 <Desc / Clms Page number 34>

 43. In this way, it was loaded again into the refining column 36 by being entrained in air. From the refining column 36, the slurry was fed to a sweep tank 44, so that the liquid level in the upper section could be nearly constant.

   The purge tank 44, which is a stainless steel stirring vessel with a cooling jacket and condenser 45, was cooled by the cooling jacket so that the internal temperature could be 100 0. The slurry contained in the sweep tank 44 was fed into a separator 47, via a pump 46 and, in this separator, it was separated into a cake and a filtrate. The filtrate was taken to a tank 43, The cake was slurried with acetic acid containing 10% water, to obtain a slurry concentration of 15% by weight, then it was treated with stirring. for 20 minutes at 80 ° C. It was then subjected to a solid / liquid separation process, and then it was dried.



   The above process was carried out for 40 days, then the material balance of acetic acid and a high boiling point material consisting mainly of terephthalic acid was determined. The sum of the amount of acetic acid in the filtrate obtained in the liquid / solid separator 47 and the amount of acetic acid in acetic acid containing water, evaporated in the dryer and recovered, was 89, 3 parts by weight. The amount of the refined terephthalic acid removed from the dryer was 17.28 parts by weight. The amount of a high-boiling residue obtained by evaporating the filtrate to dryness under atmospheric pressure was 0.37 part by weight.



   The refined terephthalic acid obtained had an optical density of 0.027 and a 4-CBA content of 0.17 by weight,

 

Claims (1)

CLAIMS. 1.- Gas / liquid contact device comprising a vertical receptacle provided with a conical bottom, a pipe for supplying a liquid or a suspension, an opening for charging a gas to be introduced. in contact with the liquid or the suspension and at least one opening for withdrawing the liquid or the suspension, characterized in that the container is divided in the perpendicular direction at least into two sections by at least one dividing wall going in thinner , each of these sections being constructed in such a way that the material contained in one section can then be successively conducted towards the bottom or the lower part of the other section through an opening made at the base of this partition wall or by a pipe extending through two sections and whose downstream end of a fluid opens into the lower part of the other section, the first section comprising a supply pipe for a liquid or a suspension, as well as a line for charging a desired gas in the lower part of the section, the last section comprising at least one discharge opening for removing the suspension or the liquid and the gas being treated. 2.- Gas / liquid contact apparatus according to claim 1, characterized in that it is constructed so that a mixture of the liquid or the suspension with the gas is then transferred to the bottom of the second section and of subsequent sections through the opening in the partition wall or through a pipe. 3, - Gas / liquid contact apparatus according to claim 2, characterized in that a guide cylinder is fitted to the upper part of a gas supply line, to the bottom of the first section or to the 'opening made at the base of the partition wall in the second section and the following <Desc / Clms Page number 36> or on the top surface of the duct opening. 4. - Apparatus according to claim 1, characterized in that the partition wall comprises a conical surface having a vertical angle of 30 to 90, 5.- Apparatus according to claim 1, characterized in that this container is divided into four to twenty sections by dividing walls. 6. - Process for refining crude terephthalic acid, this process consisting in separating crude terephthalic acid from a reaction mixture containing crude terephthalic acid obtained by oxidation of p-dialkoylbenzene with molecular oxygen in an acid aliphatic monocarboxylic of 2 to 4 carbon atoms, in the presence of a cobaltic compound, then contacting, with molecular oxygen, a suspension maintained at a temperature of 180 to 230 0, consisting of 6-100 parts by weight of said crude terephthalic acid and 100 parts by weight of at least one aliphatic monocarboxylic acid or an aqueous solution of said aliphatic monocarboxylic acid containing, at most, 50% by weight of water, characterized in that the suspension is introduced into one or the other section of a refining realpient divided at least into two sections in the perpendicular direction by at least one partition wall which decreases downwards, is introduced a gas containing molecular oxygen in the lower part of said section at a surface speed of at least 0.1 cm / second, this suspension and this gas are mixed with each other in said section, the mixture is then loaded into the lower part of the other section through an opening made at the base of the partition wall or through a pipe extending through two sections and the downstream end of which has a stream of fluid opens into the lower part of the other section, then we <Desc / Clms Page number 37> remove the mixture from the last section, to bring it outside the container and then the terephthalic acid is separated. 7. - Method according to claim 6, characterized in that the superficial gas velocity is 0.2-2 cm3 / cm2.second. 8. A method according to claim 6, characterized in that air is used as gas containing molecular oxygen. 9. A method according to claim 6, characterized in that acetic acid is used as aliphatic monooarboxylic acid. 10. A method according to claim 6, characterized in that a slurry consisting of 100 parts by weight of the medium and 10-70 parts by weight of crude terephthalic acid is used, 11. A process according to claim 6, characterized in that the partial pressure of oxygen in the refining vessel is 1-50 atmospheres, preferably 1-10 atmospheres. 12. A method according to claim 6, characterized in that the temperature in the refining vessel is 200 to 230 0.