Method for the reduction of iodine compounds from a process stream

Abstract

A method for the reduction in iodine content in a mixture, particularly in a mixture of carboxylic acids and/or carboxylic anhydrides with alkyl iodides and/or hydriodic acid, comprises contacting the organic medium in the vapor phase at elevated temperature with a metal salt dispersed on activated charcoal. The metal salt may be a carboxylate salt, a carbonate, an oxide, a hydroxide and any other salt, which may react with the organic medium to give a metal carboxylate salt. The metal may be a main group element, a lanthanide, an actinide, zinc and copper. This process converts alkyl iodide compounds converted to their corresponding carboxylate esters, while the iodine is bound in the purification mass as inorganic iodide.

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 m2/g of which 470 m2/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 m2/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 (<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	<0.5
F = 33 ± 8 g/h	40	2.7	F = 43 ± 4 g/h	40	<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	<0.5
F = 42 ± 3 g/h	40	5.7	(14 ml) + A.C.	60	<0.5
T = 183 ± 3° C.	80	12.4	(7 ml)	100	<0.5
5 mmole K	120	16.3	F = 41 ± 4 g/h	120	<0.5
	140	7.2	T = 209 ± 2° C.	140	<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/Al2O2 (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/SiO2 (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

Claims

1. A method for the reduction of iodine content in a process mixture, comprising contacting the organic medium in vapour phase at a temperature from 100° C. to 300° C. with a metal salt dispersed on activated charcoal.

2. A method of claim 1, wherein the metal salt is selected from a carboxylate salt, a carbonate, an oxide, a hydroxide and/or a salt being able to react with the organic medium to give a metal carboxylate salt.

3. A method of claim 1, wherein the metal salt comprises an element selected from a main group element, a lanthanide, an actinide, zinc, copper and silver.

4. A method as claimed in claim 1, wherein the metal salt comprises an element selected from the main group elements, the lanthanides, the actinides, zinc, copper, alkali metals and alkaline earth metals and combinations thereof.

5. A method as claimed in claim 1, wherein the metal salt comprises Na, K and/or Cs.

6. A method as claimed in claim 1, wherein the main component by weight in said mixture is a carboxylic acid and/or a carboxylic acid anhydride and/or a carboxylic ester and/or an alkylidene dicarboxylate.

7. A method as claimed in claim 1, wherein the main component by weight in said mixture is acetic acid.

8. A method according to anyone of the preceding claims, wherein the process mixture comprises carboxylic acids and/or carboxylic anhydrides with alkyl iodides and/or hydriodic acid.