Abstract:
The present invention discloses a method for removing iodine compounds from acetic acid, in which the iodine compound is removed by using a solid adsorbent in the form of an activated carbon fiber having a large strength, a large bulk density, and a large specific surface, so that the treatment of large amounts should be possible, that the acetic acid should not be contaminated during the extraction of foreign materials from the adsorbent, and that the adsorbent can be repeatedly used by regenerating it. The method includes the steps of: preparing a filter in the usual manner by using an activated carbon fiber as the adsorbent; and making acetic acid containing an iodide pass through the activated carbon fiber filter, whereby the iodide in acetic acid is removed by being adsorbed by the activated carbon fiber filter.

Description:
The application is a 371 of PCT/KR94/00027 Mar. 30, 1994. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for removing iodine compounds, which is applicable to the process for preparing acetic acid. Particularly, the present invention relates to a method for removing iodine compound by using activated carbon fibers as the adsorbent. 
     BACKGROUND OF THE INVENTION 
     Generally, in the case where acetic acid is prepared based on the methanol carbonylation reaction, the iodine compound is used as the reaction promoting agent in the form of methyl or hydrogen iodide (CH 3  I or HI). 
     The iodide which is used as the reaction promoting agent is mostly recovered by distillation. 
     However, usually the iodine compound remains in the form of iodine ions I - , iodine molecules I 2 , or alkyl iodine (particularly methyl iodine) in small amounts of several or several scores ppm. 
     In the case where acetic acid in which such small amount of iodine compound remains is used as the raw material, if the catalyst is very sensitive to the small amount of the iodine compound, the concentration of the iodine compound used as the raw material has to be lowered to an extremely small amount (several scores ppb) so as for the iodine compound to be suitable to the process. 
     Thus it is required that the concentration of the iodine compound remaining in the acetic acid be lowered to an extremely low level. 
     As the conventional iodine or iodine compound (to be called iodides) removing methods, there is an oxidation method (U.S. Pat. No. 3,709,705), methods using chemicals as a scavenger (U.S. Pat. Nos. 4,246,195 and 4,664,753), a hydrogenation method (U.S. Pat. No. 4,792,420), a distillation method (U.S. Pat. No. 4,029,553), and a crystallizing method (Japanese Patent Laid-open No. Sho-50-126610). 
     In these methods, although there are some differences, the cost for the distillation or the cost for the chemical salts constitutes the major expenses, thereby making them uneconomical. Further, the regeneration of the chemical salts used is difficult. 
     Meanwhile, there is known a method in which the iodine compound is removed by using a solid adsorbent. This method is different from the above cited methods, and has an advantage that the expense for the distillation and the expense of the chemical substance can be saved. 
     The typical methods are: the method using ion exchange resin (U.S. Pat. No. 3,943,229), the method using zeolite with a metal supported therein (U.S. Pat. No. 4,088,737), the method using silica with a metal supported therein (U.S. Pat. No. 3,838,554), the method using a ceramic with triethylene diamine supported therein, and the method using an activated charcoal with SnI 2  supported therein (J. Nucl. Science and Technol 9(4), 197, 1972). All the above methods remove the iodine compound by using the adsorbent. 
     However, in the case where the iodine compound is removed by using a solid adsorbent, not only the treatment of large amounts is impossible, but also the metal salts supported on the adsorbent or the chemical compounds are extracted, thereby contaminating the acetic acid. 
     SUMMARY OF THE INVENTION 
     The present inventors made studies on how to solve the problem of the conventional methods, and the present invention is based on these studies. 
     It is the object of the present invention to provide a method for removing iodine compounds from acetic acid, in which the iodine compound is removed by using a solid adsorbent agent in the form of an activated carbon fiber having a high strength, a large bulk density, and a large specific surface area, so that the treatment of large amounts should be possible, and the acetic acid should not be contaminated by the foreign materials extracted from the adsorbent, and the adsorbent can be repeatedly used by regenerating it. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which: 
     FIG. 1 is a graphical illustration of the regeneration of activated carbon fiber with N 2  flushing at the temperature of 20°-400° C. The iodine removal efficiency is plotted in FIG. 1 against I 2  feeding defined as milligram of I 2  fed to the adsorbent bed per gram of adsorbent accumulated from the start of the run to the point when the measurement was made. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method for removing the iodine compounds from acetic acid by using a solid adsorbent. The solid adsorbent agent is an activated carbon fiber. 
     The present invention will be described in more detail. 
     The activated carbon fiber should have preferably a strength of 100-250 MPa, a bulk density of 0.01-0.2 g/cm 3 , and a specific surface area of 1,000-2,500 m 2/  g. 
     As the method for activating the carbon fiber, there is a method for steam activating the pitch-based carbon fibers, and a method of carbonizing the carbon fiber at a high temperature after activating it by means of phosphoric acid or ZnCl 2 . The most desirable of the aforementioned two is the steam activating method conducted at a temperature of 500°-1000° C. 
     In the present invention, the activated carbon fiber thus prepared can be used as a adsorbent after treated by ammonia, hydrogen gas, chlorine gas, or water. 
     The activated carbon fiber having the above physical properties is installed in the form of a filter, and the acetic acid containing the iodine compound is passed through the fiber filter at a certain flow rate, so that the iodine compound contained in the acetic acid is removed by being caught by the activated carbon fibers. 
     When the acetic acid containing the iodine compounds passes through the activated carbon fibers, the flow rate of the acetic acid should be preferably 1.0-50.0 ml/min per g of adsorbent. The reason is that, if the flow rate is less than 1.0 ml/min, the productivity becomes too low, and that, if the flow rate is more than 50 ml/min, the regeneration period for the adsorbent agent becomes too short, thereby making it uneconomical, and making it impossible to operate the apparatus continuously. Further, in the case of the latter, too much iodine compound will remain in the acetic acid. 
     If the regenerating period for the activated carbon fiber is considered, the more preferable flow rate of the acetic acid should be 1-10 ml/min per g of adsorbent. 
     Further, when the iodine compound is removed from the acetic acid, the temperature should be preferably lower than 70° C., and if the temperature is higher than 70° C., the removal efficiency of the activated carbon fiber decreases. 
     In the present invention, water can be used as a promoter for the removal of the iodine compound from the acetic acid. If water is used as the promoter, the iodine removal rate can be improved. 
     If water is to be used as the promoter, water is added to the acetic acid or is saturated in the activated carbon fiber. 
     Further, in order to obtain the final pure acetic acid in the practical process, a plurality of water removing steps have to be carried out. The process of the present invention should be desirably applied to the step immediately preceding the last step of removing the water content. 
     According to the present invention, the activated carbon fibers which have been used in removing the iodine compound from the acetic acid can be repeatedly used after regenerating them. The regenerating process is carried out in such a manner that the used activated carbon fiber is heated to a temperature of 200°-500° C. for 2.0-6.0 hours while flowing inert gases such as N 2 , He, or argon. 
     If the regenerating temperature is below 200° C., the regenerating efficiency is lowered, thereby making it impossible to make a complete restoration. If the regenerating temperature is over 500° C., the activated carbon fibers may be damaged, although a complete regeneration is possible. Therefore, the regenerating temperature should come within the range of 200°-500° C. 
     When the iodine compound is removed from the acetic acid according to the method of the present invention, the capability of the activated carbon fiber for adsorbing the iodine compound is 200-300 mg per g of the activated carbon fiber. 
     The application of the present invention is not limited to the above described method, but extends to the removal of the iodine compounds from an ethanol or methanol solution or other aqueous solutions. 
     Now the present invention will be described based on actual examples. 
     &lt;EXAMPLE 1&gt; 
     (Example 1 of the present invention) 
     One g of an ACF1 adsorbent having 1650 m 2  /g (pore volume: 0.8 ml/g) was used, and 200 ml of acetic acid containing 800 ppm of iodine I 2  was agitated at room temperature. The amount of the removed iodine was checked per unit of time, and it was found that an equilibrium point was reached after two hours of adsorption. At the equilibrium point, 305 mg of iodine was removed per g of activated carbon fiber (ACF). 
     The ACF1 represents the activated carbon fiber which is obtained by activating the pitch-based carbon fiber by steaming. 
     &lt;Comparative Examples 1-6&gt; 
     The process was carried out in the same manner as that of Example 1, except that, instead of the activated carbon fiber, the various adsorbents as shown in Table 1 below were used. The amount of iodine removed per g adsorbent and the time to reach equilibrium are as shown in Table 1 below. 
     
                       TABLE 1______________________________________                       I.sub.2 removed per                       Eqtime g  Adsorbent Spc srf area                       adsorbentExample No.    (1 g)       (m.sup.2 /g)                           (mg/g)  (h)______________________________________C. Example 1    Actvtd carbon                860        195     24C. Example 2    Ag/NaY.sup.1                550        235     24C. Example 3    Ag/Amb.sup.2                300        180     24C. Example 4    NaY-zeolite 580         0      --C. Example 5    SiO.sub.2   350         0      --C. Example 6    Al.sub.2 O.sub.3                200         0      --______________________________________ 1. Ag/NaY indicates that in which 2 wt % Ag is ionexchanged in NaY. 2. Ag/Amb indicates that in which 2 wt % Ag is ionexchanged in Amberlyst XN 1010 (trade name). 
    
     According to the above results, in the case where the activated carbon fiber is used as the adsorbent, the equilibrium time is shorter than the cases of the comparative examples (1-6), and the iodine removal amount is much larger. 
     &lt;EXAMPLE 2&gt; 
     (Example 2 of the present invention) 
     One g of the ACF1 adsorbent (having a specific surface area of 1650 m 2  /g) was filled into a glass tube having a diameter of 15 mm, so that 10 ml of uniform adsorbent should be formed. Then 1 cm layers of glass fibers were filled in the upper and lower portions of the adsorbent to support the adsorbent. 
     The adsorbent was maintained at the room temperature, and then, an acetic acid solution containing 800 ppm of iodine was made to pass through the adsorbent from above downward by means of a liquid pump. Under this condition, the flow rate was varied within the range of 1.0-50.0 ml/min as shown in Table 2 below. 
     Then 3 ml of the solution which has passed through adsorbent was taken as a test sample to measure the average concentration of iodine within the acetic acid. 
     The iodine concentration within the acetic acid before the filtering is indicated by C 0 , while the average concentration of the iodine within 3 ml of the effluent acetic acid is called C. The iodine removal efficiency (%) by the adsorbent was calculated based on a formula [(C 0  -C)/C 0  ]×100. 
     The point at which the iodine removal efficiency is lowered to 99.8% is called E 1 . Then the time T 1  which elapsed before E 1  is reached in accordance with the flow rate, and the cumulative iodine removal amount R 1  until the time T 1 , are as shown in Table 2 below. 
     
                       TABLE 2______________________________________      Flow rateExample No.      (ml/min)     T.sub.1 (min)                            R.sub.1 (mg/g)______________________________________Example ofprsnt invtn3           1           325      260.04            1.7        184      250.25            3.5        86       240.86           7             37.5   212.87          16           26       208.08          25           10       200.09          50            5       193.7______________________________________ 
    
     As shown in table 2 above, the smaller the flow rate of the acetic acid, the longer the regenerating period, i.e., the time, T 1 ,which is the time elapsed before reaching E 1 , as well as increasing the accumulated iodine removal amount. 
     &lt;EXAMPLE 3&gt; 
     (Example 10-14 of the present invention) 
     The processes were carried out in the same manner as that of Example 2, except that the flow rate is 1.7 ml/min, and that 1 g each of ACF2, ACF3, ACF4, ACF5 and ACF6 as the adsorbent was used as shown in Table 3 below. The time T 1  which elapsed before arriving at E 1 , and the cumulative iodine removal amount R 1  until the time T 1 , were as shown in the table below. 
     
                       TABLE 3______________________________________               Surface area      R.sub.1Example No.     Adsorbent (m.sup.2 /g)                          T.sub.1 (min)                                 (mg/g)______________________________________Prnt inventionNo.10        ACF2      1550       170.6  232.011        ACF3      1500       166.0  225.712        ACF4      1780       198.8  270.313        ACF5      1380       154.8  210.514        ACF6      1280       143.5  195.1______________________________________ 
    
     In the above, ACF2 indicates an activated carbon fiber which was obtained by activating a pitch-based series carbon fiber with phosphoric acid, and by carbonizing at a temperature. ACF3 indicates an activated carbon fiber which was obtained by activating with ZnCl 2 , and by carbonizing at a temperature. ACF 4 indicates an activated carbon fiber which was obtained by treating the ACF1 of Example 1 with ammonia at 800° C. ACF5 indicates an activated carbon fiber which was obtained by treating the ACF1 with hydrogen at 900° C. ACF6 indicates an activated carbon fiber which was obtained by treating the ACF1 with chlorine gases at 450° C. 
     &lt;Comparative Examples 7 and 8&gt; 
     The process was carried out in the same manner as that of Example 2, except that the flow rate of acetic acid was 1.7 ml/min, and that the adsorbent consisted of an activated carbon (specific surface area: 860 m 2  /g) and Ag/NaY (specific surface area: 580 m 2  /g) in which 2 wt % Ag was ion-exchanged in NaY. Here, in order to make the volume of the adsorbent layer become 10 ml, small glass beads were uniformly mixed. 
     When the flow rate was 1.7 ml/min, the time T 1  for arriving at E 1 , and the cumulative iodine removal amount R 1  were as shown in Table 4 below. 
     
                       TABLE 4______________________________________Example No.    adsorbent      T.sub.1 (min)                            R.sub.1 (mg/g)______________________________________C. Example 7    Activated carbon                   15       20.4C. Example 8    Ag/NaY           5.0     6.8______________________________________ 
    
     From the above table, it is seen that the case of using ACF2-ACF6 as the adsorbent (Examples 10-14 of the present invention) is superior in T 1  and R 1  compared with the case of using an activated carbon and Ag/NaY (Comparative Examples 7 and 8). 
     &lt;EXAMPLE 4&gt; 
     (Example 15 of the present invention) 
     The process was carried out in the same manner as that of Example 2, except that the flow rate was 3.5 ml/min, and that the iodine concentration in the acetic acid was varied within the range of 15-1500 ppm as shown in Table 5 below. The time T 1  for arriving at E 1  and the cumulative iodine removal amount R 1  were as shown in Table 5 below. 
     
                       TABLE 5______________________________________Iodine concentration in acetic acid(ppm)               T.sub.1 (min)                        R.sub.1 (mg/g)______________________________________ 15                 3053     160.3100                 511      178.9300                 172      180.6500                 106      185.51100                 65      250.31500                 49      257.3______________________________________ 
    
     As shown in Table 5 above, it is possible to remove iodine within the iodine concentration range of 15-1500 ppm. 
     &lt;EXAMPLE 5&gt; 
     (Example 16 of the present invention) 
     The process was carried out in the same manner as that of Example 2, except that the flow rate was 3.5 ml/min, and that the solvent used was methanol, ethanol, water or a mixture of 50 volume % of water and 50 volume % of acetic acid instead of pure acetic acid as shown in Table 6 below. The time T 1  for arriving at E 1  and the cumulative iodine removal amount R 1  until the time T 1  were as shown in Table 6 below. 
     
                       TABLE 6______________________________________      IodineSolvent    Concentration (ppm)                     T.sub.1 (min)                              R.sub.1 (mg/g)______________________________________Methanol   500            554      959.5Ethanol    800             79      221.2Water      300            1533     1610Water 50% +      800            275      770Acetic acid 50%______________________________________ 
    
     As shown in Table 6 above, the activated carbon fiber of the present invention can be applied not only to acetic acid but also methanol, ethanol, water and a mixture of water and acetic acid for removing iodine. 
     &lt;EXAMPLE 6&gt; 
     (Example 17 of the present invention) 
     The process was carried out in the same manner as that of Example 2, except that the activated carbon fiber ACF1 saturated with distilled water for one hour was used as the adsorbent, and that the flow rate was 3.5 ml/min. The time T 1  for arriving at E 1  and the cumulative iodine removal amount R 1  until the time T 1  were 118 minutes and 330.4 mg per g of adsorbent, respectively. 
     From the above results, it can be seen that the case of using water as the promoter for removing iodine from acetic acid was superior in T 1  and R 1  compared with the cases which is not used water as a promoter. 
     &lt;EXAMPLE 7&gt; 
     (Example 18 of the present invention) 
     The process was carried out in the same manner as that of Example 2, except that the flow rate was 3.5 ml/min, and that acetic acid containing 800 ppm of hydrogen iodide (HI) instead of iodine was used. The time T 1  for arriving at E 1  and the cumulative iodine removal amount R 1  until the time T 1  were 125 minutes and 350.0 mg per g of ACF1 respectively. 
     It is seen from the above results that the activated carbon fiber of the present invention is effective in removing not only iodine but also iodine compounds such as hydrogen iodide. 
     &lt;EXAMPLE 8&gt; 
     The activated carbon fiber (used material) which was used in Example 2 for removing iodine (used ACF1) was regenerated by heating at 20° C. for 24 hours (Comparative Example 9), at 100° C. for 20 hours (Comparative Example 10), at 150° C. for 12 hours (Comparative Example 11), at 200° C. for 6 hours (Example 19 of the present invention), at 300° C. for 4 hours (Example 20 of the present invention), and at 400° C. for 3 hours (Example 21 of the present invention), while flowing N 2  gas at a flow rate of 300 ml/min. The process was carried out in the same manner as that of Example 2 after regenerating the used ACF1, according to comparative example 9-11 and example 19-21, thereby removing iodine from the acetic acid. FIG. 1 illustrates the iodine removal efficiency of activated carbon fiber regenerated at a temperature of 20°-400° C. 
     As shown in FIG. 1, it is apparent that the Examples 19-21 of the present invention mostly restored the removal capacity of fresh ACF1. 
     According to the present invention as described above, an activated carbon fiber is used as the adsorbent, with the result that the iodine removal efficiency is markedly increased. Not only iodine, but also iodides such as hydrogen iodide, methyl iodide and the like can be removed. Further, not only from acetic acid, but also iodine can be removed from water, ethanol, methanol and a mixture of water and acetic acid. Further, the activated carbon fiber can be regenerated after use for repeated use.