Patent Publication Number: US-2002010363-A1

Title: Method for the reduction of iodine compounds from a process stream

Description:
[0001] The present invention relates to a process for the removal of iodine from iodine-containing compounds, e.g. alkyl iodides and the like, which are contained in a mixture with other compounds, more specifically with carboxylic acids and/or carboxylic anhydrides. In particular, the present invention is suited for the purification of acetic acid and/or acetic anhydride prepared by the rhodium or iridium-catalysed, methyl iodide promoted carbonylation of methanol and/or dimethyl ether and/or methyl acetate.  
       [0002] Within industrial acetic acid production, it is well known that the final product in order to meet the demands of the purchasers in many cases must be extremely low (less than 6 parts per billion weight (ppb)) in iodide. Approximately 50% of the global acetic acid production are used for vinyl acetate manufacture in a catalytic process. The catalyst for this process is severely damaged by iodide even in very low concentrations. Iodide in acetic acid stems from the use of methyl iodide and/or hydriodic acid as co-catalyst in the carbonylation of methanol and/or dimethyl ether and/or methyl acetate. Most of the iodine is present in the form of methyl iodide in the crude acetic acid, while smaller amounts are present as hydriodic acid and higher alkyl iodides. Methyl iodide is easily separated by distillation and recycled to the reactor zone. Most of the hydriodic acid may also be recovered. The residual iodides in the product, mainly higher (C2-C8) alkyl iodides are more difficult to remove. Hexyl iodide is often mentioned since this compound is particularly troublesome to remove by distillation due to its boiling point being very close to that of acetic acid. The concentration of higher alkyl iodides is typically of the order of 100-1000 ppb (0.1-1 ppm).  
       [0003] Several methods for the purification of crude carboxylic acids and/or acetic acid and/or derivatives thereof have been described. Most of these methods, however, concern purification in the liquid phase. Thus, several patents teach the use of silver, palladium, mercury and/or rhodium-exchanged resins (U.S. Pat. Nos. 4,975,155, 5,220,058, 5,227,524, 5,300,685 and 5,801,279) for reducing the level of iodine in carboxylate streams in the liquid phase. Purification by the addition of oxidising agents is taught e.g. by U.S. Pat. No. 5,387,713 describing the use of hydrogen peroxide in the liquid phase and by U.S. Pat. No. 5,155,265, which describes the use of ozone as purifying agent in the liquid phase. More related to the present invention is U.S. Pat. No. 4,246,195, which teaches the use of caesium, potassium and sodium acetate for purification of carbonylation products with respect to iodine contaminants. Said patent, however, deals solely with operations in the liquid phase.  
       [0004] It is an object of the present invention to provide a method for reducing the iodine level in a mixture, particularly in a mixture of acetic acid with alkyl iodides and/or hydriodic acid in the vapour phase.  
       [0005] In accordance with the invention, it has surprisingly been discovered that when acetic acid contaminated with alkyl iodides in a concentration from 0.1 to 3000 ppm has been contacted in the vapour phase at elevated temperatures and normal pressure with metal acetate salts dispersed on a special support material, the alkyl iodides have been converted to the corresponding alkyl acetates, while the iodide has become bound as inorganic (non-volatile) iodide in the absorbent. Said special support material is activated charcoal. With other supports such as alumina and silica, there is almost no conversion.  
       [0006] Thus, it was found that when a solution of 258 ppb hexyl iodide in acetic acid was evaporated through a bed of potassium acetate on activated charcoal (bulk volume 7 ml) at a temperature of 189±6° C. and at a flow rate of 33±8 g/h, the concentration of hexyl iodide in the re-condensed acetic acid solution had become reduced to below 6 ppb hexyl iodide (below 4 ppb iodine). The hexyl iodide concentration was measured after 20, 40, 60, 120 and 140 minutes on stream and was in all cases found to be below 6-ppb hexyl iodide.  
       [0007] Moreover, by the present invention alkyl iodides are not simply retained on the absorbent due to some chromatographic effect. The following examples will show that alkyl iodides are converted to their corresponding acetate esters that were identified by gas chromatographic analysis. To demonstrate the scope of the present invention, 7 different absorbents were prepared (Examples 1-7). Since the present invention may have a specific application within acetic acid manufacture and purification technology, the performance of some of the absorbents towards an acetic acid solution with a content of hexyl iodide of 100-300 ppb was tested (Examples 8-14). Also, a number of comparative examples with a more concentrated solution of alkyl iodides in acetic acid were conducted (Examples 15-32). These comparative examples were not performed on solutions of hexyl iodide in acetic acid, but rather on mixtures of methyl iodide, butyl iodide and octyl iodide in acetic acid. 
     
    
    
     EXAMPLE 1  
     [0008] Preparation of a potassium on activated charcoal absorbent (KOAc/A.C.).  
     [0009] Potassium acetate (1.96 g, 0.02 mole) was dissolved in de-ionised water (4 ml) and diluted to 7.5 ml with de-ionised water to give a clear, colourless solution. Activated charcoal (10.00 g) in the form of 2.5 mm granules (Merck, QBET surface area 1190 m 2 /g of which 470 m 2 /g is microporous surface area) was contacted with the aqueous solution and shaken thoroughly in a closed container until the material appeared dry or almost dry. The material was then dried in an oven at 100° C. for 16 hours, after which the weight was recorded to be 13.10 g and the bulk volume to be 28 ml. This material was then divided into four portions of equal weight, thus containing 0.005 mole K each and these portions were stored separately. This procedure was followed in every preparation of absorbent materials.  
     EXAMPLE 2  
     [0010] An absorbent was prepared as described in Example 1 except that the potassium acetate was replaced with caesium acetate (3.84 g, 0.02 mole).  
     EXAMPLE 3  
     [0011] An absorbent was prepared as described in Example 1 except that the potassium acetate was replaced with lithium acetate dihydrate (2.04 g, 0.02 mole).  
     EXAMPLE 4  
     [0012] An absorbent was prepared as described in Example 1 except that the potassium acetate was replaced with zinc acetate dihydrate (4.39 g, 0.02 mole).  
     EXAMPLE 5  
     [0013] An absorbent was prepared as described in Example 1 except that the potassium acetate was replaced with magnesium acetate tetrahydrate (4.29 g, 0.02 mole).  
     EXAMPLE 6  
     [0014] An absorbent was prepared as described in Example 1 except that the 10.00 g activated carbon was replaced with 10.00 g calcined alumina (QBET surface area 270 m 2 /g).  
     EXAMPLE 7  
     [0015] An absorbent was prepared as described in Example 1 except that the 10.00 g activated carbon was replaced with 10.00 g silica (Merck, 100 mesh).  
     EXAMPLE 8  
     [0016] A glass reactor of inner diameter 1.0 cm was charged with one portion of the absorbent (bulk volume 7 ml, 0.005 mole potassium acetate) prepared as described in Example 1. The glass reactor was placed in a tube furnace. The lower end of the glass reactor was connected to a well isolated 250 ml round bottomed flask resting in a heating mantle, while the top end was fitted to a Claisen condenser and a receiver flask. The round-bottomed flask was equipped with a thermometer. Through the top of the glass reactor was introduced a thermoelement, which was fixed in the centre of the metal acetate bed during the distillation. The tube furnace was heated to 250° C. and the heating mantle was turned on. A solution of n-hexyl iodide (HxI) in acetic acid was introduced into the hot round-bottomed flask by means of a peristaltic pump. The temperature in the acetate bed was measured to be 189° C.±6° C. throughout the experiment. The flow rate was maintained at 33±8 g/h. At regular intervals, the condensate was withdrawn from the receiver flask, the weight of it was recorded in order to measure the flow rate, and the contents of HxI in the sample (thoroughly homogenised) was analysed with GC-MS by a standardised method. The concentration of HxI in the untreated solution was measured to be 258 ppb (258 microgram hexyliodide per kilogram solution). After 20 minutes the purified solution was collected and analysed. The concentration of HxI amounted to 5.4 ppb. The experiment was continued for 140 minutes during which period a sample was withdrawn regularly. The concentration of HxI was measured in each sample with the results displayed in Table 1. This example shows that at a temperature of 189° C. and a flow rate of 33 g/h, a volume of 7 ml of the absorbent prepared as described in Example 1 reduces the hexyl iodide content in an acetic acid stream from above 258 ppb to below 6 ppb. Furthermore, this example shows that the concentration of hexyl iodide in the treated solution is maintained during 140 minutes continuous time on stream.  
     EXAMPLES 9-12  
     [0017] The procedure described in Example 8 was repeated except for changes of the absorbent and/or changes in temperature and/or changes in flow rate and/or changes in the hexyl iodide concentration in the untreated acetic acid solution as evident from Table 1. The temperature was varied by varying the set point of the tube furnace and/or by varying the flow rate. For each example the contents of hexyl iodide in the treated samples are also displayed in Table 1. Example 9 demonstrates that also the absorbent prepared as described in Example 2 is efficient in reducing the hexyl iodide concentration at the conditions specified in Table 1. Example 10 shows that the absorbent prepared in Example 1 does not perform as well at a lower temperature and higher flow rate compared to Example 8. However, the hexyl iodide concentration is still strongly reduced compared to the hexyl iodide concentration in the untreated solution. Example 11 clearly demonstrates that at a substantially lower temperature (155° C. compared to 189° C. in Example 8), the absorbent prepared as described in Example 1 does not perform as well as it does at the higher temperature with respect to decreasing the hexyl iodide concentration. Example 12 demonstrates that with a bed volume of 14 ml (two portions of the absorbent prepared as described in Example 1) instead of a bed volume of 7 ml as used in Example 8, the reduction in hexyl iodide concentration is even larger.  
     EXAMPLE 13  
     [0018] The procedure described in Example 8 was repeated except that activated charcoal alone was used as the absorbent instead of potassium acetate on activated charcoal. The temperature, the flow rate, the amount of HxI in the untreated solution are recorded in Table 1. For each example, the contents of hexyl iodide in the treated samples are also displayed in Table 1. This example demonstrates that activated charcoal alone is apparently as efficient in decreasing the amount of hexyl iodide in a stream of acetic acid as is activated charcoal impregnated with metal acetate salts. As later examples will show, however, activated charcoal alone will only convert a minor fraction of alkyl iodides to alkyl acetates and the effect of decreasing the level of hexyl iodide in a stream of acetic acid as observed in the present example is primarily a chromtographic effect.  
     EXAMPLE 14  
     [0019] The procedure described in Example 8 was repeated except that on top of the bed of potassium acetate on activated charcoal (14) was placed another bed (7 ml) of unimpregnated activated charcoal. The temperature, the flow rate, the amount of HxI in the untreated solution are recorded in Table 1. For each example, the contents of hexyl iodide in the treated samples are also displayed in Table 1. This example demonstrates that the amount of hexyl iodide may be reduced to a level below the detection limit of our method (&lt;0.5 ppb).  
     EXAMPLES 15-32  
     [0020] The following examples are all carried out as described in Example 8 with the following deviations: (i) a mixture of methyl iodide, butyl iodide and octyl iodide was used in acetic acid instead of the hexyl iodide/acetic acid mixture (ii) the total concentration of iodide in the untreated solution was in most cases 0.037-0.040 M (37-40 mM) and in some cases 0.0037 M (3.7 mM) (iii) the identity and the amount of the absorbent, the temperature, the flow rate and the specific concentrations of methyl iodide, butyl iodide and octyl iodide were varied as indicated in Table 2. Included in Table 2 for each experiment is the amount of each alkyl iodide and each alkyl acetate measured at between three and five times. For each point in time is calculated how much of the capacity is used of the absorbent bed (Cap (%) in Table 2) calculated as:  
       Cap  (%)=( n ( MeOAc )+ n ( BuOAc )+ n ( OctOAc ))/ n ( M )*100%  
     [0021] where  
     [0022] n(MeOAc) is the total amount of mole methyl acetate formed at the time;  
     [0023] n(BuOAc) is the total amount of mole methyl acetate formed at the time;  
     [0024] n(OctOAc) is the total amount of mole methyl acetate formed at the time;  
     [0025] n(M) is the amount of moles metal in the absorbent as indicated in Table 2.  
     [0026] Similarly, for each point in time is calculated the average conversion of the three alkyl iodides to acetates (Con (%) in Table 2) as:  
       Con  (%)=(([ MeOAc]/[MeI]tot )+([ BuOAc]/[BuI]tot )+([ OctOAc]/[OctI]tot ))/3*100%,  
     [0027] where  
     [0028] [MeOAc] is the measured concentration of methyl acetate in the purified mixture;  
     [0029] [BuOAc] is the measured concentration of butyl acetate in the purified mixture;  
     [0030] [OctOAc] is the measured concentration of octyl acetate in the purified mixture;  
     [0031] [MeI]tot is the concentration of methyl iodide in the unpurified mixture;  
     [0032] [BuI]tot is the concentration of methyl iodide in the unpurified mixture;  
     [0033] [OctI]tot is the concentration of methyl iodide in the unpurified mixture.  
                                       TABLE 1                                   time   ppb       time   ppb           (min)   H × I       (min)   H × I                                                            Ex 8 231199A   —   258   Ex 12 091299C   —   130       KOAc/A.C. (7 ml)   20   5.4   KOAc/A.C. (14 ml)   20   &lt;0.5       F = 33 ± 8 g/h   40   2.7   F = 43 ± 4 g/h   40   &lt;0.5       T = 189 ± 6° C.   60   4.0   T = 191 ± 4° C.   80   0.9       5 mmole K   120   3.2   10 mmole K   120   1.7           140   5.9       140   2.2       Ex 9 241199A   —   258   Ex 13 091299A   —   215       CsOAc/A.C. (7 ml)   21   3.7   A.C. (7 ml)   62   0.6       F = 43 ± 4 g/h   86   5.1   F = 42 ± 6 g/h   80   1.3       T = 200 ± 5° C.   121   3.1   T = 179 ± 2° C.   100   0.9       5 mmole Cs   140   3.6   0 mmole metal   120   1.9           167   4.2       140   2.7       Ex 10 251199A   —   258   Ex 14 101299A   —   109       KOAc/A.C. (7 ml)   20   3.9   KOAc/A.C.   41   &lt;0.5       F = 42 ± 3 g/h   40   5.7   (14 ml) + A.C.   60   &lt;0.5       T = 183 ± 3° C.   80   12.4   (7 ml)   100   &lt;0.5       5 mmole K   120   16.3   F = 41 ± 4 g/h   120   &lt;0.5           140   7.2   T = 209 ± 2° C.   140   &lt;0.5       Ex 11 261199A   —   258   10 mmole K       KOAc/A.C. (7 ml)   41   100       F = 38 ± 8 g/h   60   116       T = 155 ± 3° C.   80   93       5 mmole K   120   125           140   114                  
 
     [0034]                                                       TABLE 2                                   time   ppm   ppm   ppm   ppm   ppm   ppm   Cap   Con           (min)   MeI   MeA   BuI   BuA   OctI   OCtA   (%)   (%)                                                                            Ex 15 151099A   —   1750   —   2250   —   3000   —   —   —       CsOAc/A.C. (14 ml)   43   0   509   794   931   0   1565   6   65       F = 29 ± 8 g/h   86   0   573   462   1172   0   1810   11   76       T = 179 ± 3° C.   113   0   572   438   1161   0   1680   15   74       10 mmole Cs   123   0   566   444   1158   0   1640   16   73       [I] = 37.0 mM       Ex 16 151099B   —   1739   —   2236   —   2981   —   —   —       LiOAc/A.C. (14 ml)   41   826   289   1476   454   0   221   1   25       F = 35 ± 10 g/h   81   1187   102   1948   102   480   922   3   21       T = 162 ± 5° C.   102   1182   65   1962   65   820   965   4   17       10 mmole Li   130   1273   85   1987   85   722   1037   6   22       [I] = 36.8 mM   150   1238   101   1920   101   559   1203   7   25       Ex 17 181099A   —   1739   —   2253   —   2939   —   —   —       Zn (OAc) 2 /A.C. (14 ml)   40   940   322   1123   688   0   238   3   32       F = 44 ± 3 g/h   81   1071   294   1126   719   0   751   7   40       T = 170 ± 3° C.   100   1086   292   1188   673   0   1022   10   43       10 mmole Zn   122   1157   258   1244   636   0   1251   12   44       [I] = 36.7 mM   141   1091   236   1303   605   0   1368   15   44       Ex 18 191099A   —   1775   —   2300   —   3000   —   —   —       Mg (OAc) 2 /A.C. (14 ml)   40   1100   308   1492   486   0   164   3   25       F = 30 ± 14 g/h   61   968   403   1169   695   0   266   4   35       T = 170 ± 5° C.   80   946   528   1125   751   0   396   5   42       10 mmole Mg   125   1166   276   1672   413   0   695   7   30       [I] = 37.5 mM   145   1321   307   1852   297   91   1003   9   33       Ex 19 191099B   —   1775   —   2300   —   3000   —   —   —       KOAc/A.C. (14 ml)   40   486   560   1019   727   0   953   4   52       F = 41 ± 7 g/h   130   619   496   1167   720   0   1637   15   60       T = 179 ± 9° C.   151   595   510   1103   695   0   1928   18   64       10 mmole K       [I] = 37.5 mM       Ex 20 201099A   —   1775   —   2300   —   3000   —   —   —       A.C. (7 ml)   42   1324   188   1514   380   0   0   —   15       F = 34 ± 3 g/h   80   1574   199   1866   258   0   0   —   13       T = 162 ± 2° C.   102   1691   207   1938   134   0   0   —   11       0 mmole metal   121   1775   125   2059   99   0   34   —   7       [I] = 37.5 mM   140   1579   190   2029   81   0   61   —   10       Exp. 21 221099A   —   1775   —   2300   —   3000   —   —   —       A.C. (7 ml)   43   1008   286   1042   710   0   0   —   27       F = 31 ± 2 g/h   60   1317   245   1286   597   0   0   —   23       T = 172 ± 3° C.   81   1372   264   1536   474   0   0   —   20       0 mmole metal   119   1568   311   1753   335   0   0   —   19       [I] = 37.5 mM   143   1627   350   1848   279   0   0   —   19       Ex 22 28109A   —   1791   —   2348   —   3024   —   —   —       KOAc/A.C. (7 ml)   80   1205   243   1943   146   622   1346   12   33       F = 44 ± 4 g/h   140   1261   190   1996   126   709   1336   22   30       T = 150 ± 1° C.   179   1378   179   2026   108   739   1217   29   28       5 mmole K   220   1386   144   2069   101   814   1229   35   26       [I] = 39.0 mM   255   1411   123   2091   92   833   1225   40   25       Ex 23 281099D   —   1753   —   2351   —   3068   —   —   —       KOAc/A.C. (7 ml)   42   1019   293   1801   302   516   1274   9   37       F = 45 ± 3 g/h   80   1035   262   1836   302   492   1409   17   38       T = 166 ± 2° C.   100   1011   275   1750   338   431   1468   21   40       5 mmole K   120   1029   285   1756   354   407   1524   26   41       [I] = 37.9 mM   140   1092   277   1797   338   424   1492   31   40       Ex 24 291099A   —   1753   —   2351   —   3068   —   —   —       KOAc/A.C. (7 ml)   40   437   673   736   929   0   1119   12   62       F = 40 ± 2 g/h   80   443   761   658   984   0   1364   26   70       T = 194 ± 2° C.   101   489   743   703   956   0   1343   33   69       5 mmole K   124   536   716   759   927   0   1307   41   67       [I] = 37.9 mM   138   546   693   804   899   0   1257   46   64       Ex 25 041199A   —   2051   —   2350   —   3047   —   —   —       CsOAc/A.C. (7 ml)   40   608   701   1109   776   139   1614   11   64       F = 35 ± 3 g/h   80   814   607   1298   679   177   1904   23   63       T = 162 ± 1° C.   100   796   602   1238   716   139   1896   29   64       5 mmole Cs   120   832   594   1244   730   115   1800   34   62       [I] = 39.9 mM   140   854   577   1262   709   117   1758   40   61       Ex 26 041199C   —   2051   —   2350   —   3047   —   —   —       Zn (OAc) 2 /A.C. (7 ml)   60   1313   249   1584   503   230   709   8   30       F = 41 ± 2 g/h   80   1505   147   1846   320   376   1215   11   30       T = 155 ± 2° C.   100   1598   120   1935   189   450   1490   14   31       5 mmole Zn   120   1624   93   1933   262   474   1672   18   34       [I] = 39.9 mM   140   1502   0   1923   233   518   1573   21   29       Ex 27 111199A   —   2051   —   2350   —   3047   —   —   —       KOAc/Al 2 O 2  (4 ml)   40   1813   0   2308   0   2961   155   1   2       F = 34 ± 3 g/h   80   1774   0   2264   0   3041   65   1   1       T = 157 ± 2° C.   100   1856   0   2302   0   3058   40   1   1       5 mmole K   126   1765   0   2239   0   3091   32   1   0       [I] = 39.9 mM   145   1781   0   2273   0   3082   31   1   0       Ex 28 111199C   —   178   —   230   —   298   —   —   —       KOAc/A.C. (7 ml)   60   117   44   205   34   99   165   2   49       F = 45 ± 2 g/h   80   121   43   204   0   98   172   2   42       T = 152 ± 2° C.   100   115   26   202   37   105   177   2   45       5 mmole K   120   121   32   201   0   103   177   3   39       [I] = 3.7 mM   140   131   41   211   0   104   161   3   40       Ex 29 121199A   —   178   —   230   —   298   —   —   —       KOAc/A.C. (7 ml)   40   61   77   113   107   0   153   1   76       F = 39 ± 3 g/h   81   68   83   121   106   15   215   3   88       T = 174 ± 2° C.   101   67   52   125   74   20   223   4   70       5 mmole K   120   73   77   129   73   20   217   4   78       [I] = 3.7 mM   140   66   73   119   61   20   213   5   73       Ex 30 171199A   —   178   —   230   —   298   —   —   —       KOAc/A.C. (7 ml)   40   0   70   42   212   35   117   1   92       F = 47 ± 3 g/h   80   0   75   83   155   22   209   3   95       T = 191 ± 2° C.   100   0   74   55   148   14   226   4   96       4 mmole K   120   40   73   88   110   25   231   5   87       [I] = 3.7 mM   140   46   73   108   126   34   240   6   93       Ex 31 181199A   —   2086   —   2404   —   2940   —   —   —       KOAc/SiO 2  (4 ml)   40   1578   110   2253   0   2430   242   2   7       F = 41 ± 2 g/h   80   1699   95   2337   0   2501   214   4   6       T = 161 ± 2° C.   100   1713   89   2342   0   2491   202   4   6       5 mmole K   120   1661   79   2300   0   2544   197   5   6       [I] = 40.0 mM   140   1766   77   2354   0   2478   183   5   5       Ex 32 181199B   —   2086   —   2404   —   2940   —   —   —       CsOAc/A.C. (7 ml)   60   341   1072   555   1295   0   1722   32   88       F = 45 ± 1 g/h   100   429   933   623   864   0   1543   53   72       T = 174 ± 2° C.   120   534   885   720   1096   38   1360   62   73       5 mmole Cs   140   656   787   821   1001   48   1276   70   66       [I] = 40.0 mM