Courtesy, Chemical Construction Corporation
BATTERY OF SIX CONVERTERS Three are in a plant using a cold purification system and consequently the converters are preqeded by heat exohtngers where the sulfur dioxide is heated sufficiently for satisfactory conversion. The other three converters are part of a sulfur-burnin plant (located In the same building) and consequently receive hot Babe8 direct from the waste heat %oiler.
1
I
VANADIUM CATALYSTS Performance and Poisoning by Arsenic in Sulfuric Acid Manufacture JOHN C. OLSEN AND HERMAN MAISNER Polytechnic Institute, Brooklyn,N. Y.
HE origin of the contact process F t e s back to 1831 when finbly divided platinum was patented by Phillips (14) for the catalytic manufacture of sulfuric acid. Development of the contact process, because of the gradual diminution of the conversion capacity of the platinum while in service, was hindered seriously u n t i l K n i e t s c h (9) in 1901, s e v e n t y y e a r s after the discovery of the process, explained it to be the result of traces of mmnic carried in the burner gases. Careful purification of burner gases to remove all traces of harmful i m p u r i t i e s , such as arsenic, selenium, and antimony, was adopted as a preventive measure. This and other improvements placed the contact process, using platinum, on a competitive basis with the lead chamber process. How-
T
Vanadium contact masses are subject to poisoning by arsenic, and distinction is made here between arsenic incorporated in the contact mass and arsenic introduced into the gas stream by sublimation of arsenious oxide. The results obtained tend t o indicate that all types of vanadium contact masses succumb t o the effects of arsenic in the gas stream, provided the test is not discontinued prematurely. The conversion efficiency of a poisoned vanadium contact mass rises with increased temperature, but the conversion returns to the original value upon restoration of former temperature conditions. The resistivity of a contact mass towards poisoning by arsenic is not increased by the addition of barium, which serves as promoter for the vanadium. 254
ever, up to date no real remedy had been found to immunize the platinum catalyst against the effects of arsenic.
,
Early Developments The difficulties a r i s i n g i n connection with the use of platinum catalysts in the manufacture of sulfuric acid and the excessively high cost of platinum led to the i n v e s t i g a t i o n of various substances in an effort to r e p l a c e t h i s metal by a cheaper catalyst which would be capable of giving satisfactory conversion of sulfur dioxide to the trioxide. Among o t h e r s , vanadium pentoxide on an asbestos carrier was patented for the first time by de Haen in 1901 (5) for the catalytic manufacture of sulfuric acid, claiming 84 per cent conversion a t 645' C. Such low conversion capacity
1
, ,
1
MARCH, 1937
INDUSTRIAL AND ENGINEERING 'CHEMISTRY
was unsuitable for commercial purposes, for it was far below the mark set by the more efficient platinum catalyst and by the lead chamber process. Further attempts to replace platinum by vanadium pentoxide were made in Europe, and in 1913 another patent was issued to a Belgian firm (3). It is also claimed that during the World War vanadium contact masses were used successfully in Germany in the manufacture of sulfuric acid. There was no significant progress in the development of vanadium contact masses for the manufacture of sulfuric acid until 1926, when the Monsanto Chemical Works (10) patented a commercially satisfactory vanadium catalyst in the form of an artificial zeolite, containing vanadium combination so that it was not exchangeable for bases, and giving 97 to 98 per cent conversion of sulfur dioxide t o sulfur trioxide. This catalyst was claimed (IS) to be capable of large overload and not poisoned by arsenic and hydrochloric acid. A whole series of vanadium contact masses followed, making use of nonsiliceous base-exchange bodies in which the vanadium was present in the nonexchangeable nucleus, and systematically claiming immunity from catalytic poisons such as arsenic, selenium, antimony, hydrochloric acid, and hydrofluoric acid, all of which are so destructively harmful to platinum catalysts. I n fact, a patent applied for in 1928 (17) asserted that the vanadium contact masses described therein may be used with gases containing gaseous poisons for platinum.
255
facturing sulfuric acid from a mixture of sulfur dioxide and air, were immune to poisoning by arsenic. There is no indication in the chemical literature that any previous work has been done in the United States on the poisoning of vpnadium catalysts by arsenic. However, some work of this nhture has been carried out in Russian universities. There are many kinds of vanadium contact masses, with different carriers and promoters, which could be used for the purpose of this study, but it was desirable to make use of a commercial type of contact mass that has found actual application in industry. For the purpose of comparison, a somewhat different type of catalyst (barium-promoted) was also investigated.
Apparatus The set-up used in this investigation is shown in Figure 1. It consists of two purification systems, one for air and one for sulfur dioxide; two flowmeters, one for air and one for sulfur dioxide; a mixing bottle with a three-way glass cock; a converter (Figure 2) which contains the contact mass; two absorption bottles, one of them under vacuum to remove the sulfur trioxide fumes; an electric furnace with two rheostats and an extra resistance coil; a pyrometer with millivoltmeter; a 650" C. thermometer; and a stop watch. The air is furnished from a steel cylinder under about 4 atmospheres (60 pounds per square inch) pressure; its flow is regulated by a Hoke-Phoenix pressure-reducing valve. The sulfur dioxide is furnished from a cylinder of the liquid and its flow is controlled by means of a needle valve.
Method of Introducing Arsenic There are practically no experimental data in the American chemical literature relating to the effects of arsenic upon vanadium contact masses in the manufacture of sulfuric acid, although it has been stated (6)that some prominent sulfuric acid company in the United States subjected a vanadium contact mass to the action of excessive amounts of catalytic poisons, such as compounds of arsenic, selenium, antimony, hydrochloric acid, and hydrofluoric acid ; the total weight of the poison being about two thirds of the total weight of the contact mass, and no diminution in activity was shown. Unfortunately, experimental results were not given and the mode of introducing arsenic into the system in this case is not known. This point is mentioned because of the important role which it evidently plays in the poisoning of vanadium catalysts by means of arsenic. Neumann (12) reported that the addition of 10 per cent arsenic to vanadium pentoxide or t o copper vanadate contact masses produced no poisoning effects. It will, however, be shown later that even smaller amounts of arsenic, when introduced into the gas stream and not added t o the catalyst in the course of its preparation, produced distinct poisoning effects upon contact masses containing vanadium pentoxide. It is therefore necessary to make a distinction between arsenic added to the contact mass and arsenic introduced into the gas stream.
Purpose of Research The major objective of this research was to determine whether or not vanadium contact masses, as used for the purpose of manu-
FIGURE1. FLOW DIAGRAM OF APPARATUS 1. Air supply 2. Glass wool 3. 4, 10. Concentrated sulfuric acid 5. Air accumulator 6, 11. Calcium chloride 7 12. Pressure regulators 8: 13. Flowpleters 9. Sulfur dioxide supply 14. Gas mixing bottle 15. Three-way stopcock
16. 17. 18. 19.
Converter Thermocouple Millivoltmeter
Biirnane ._
20, 21.
Absorption bottles 22 23. Rheostats Resistance coil 25, 26. Electric switches Needle valve 27 28. Hoke pressure reducer
24:
INDUSTRIAL AND ENGINEERING CHEMISTRY
256
PURIFICATION SYSTENS.The object of the two purification chains was to remove from the air and the sulfur dioxide any impurities, such as oil, dust particles, and moisture. Glass wool, concentrated sulfuric acid, and calcium chloride served as scrubbing mediums. The larger accumulator bottle, 5 (Figure l), was used to minimize the sensitivity of the system to sudden pressure changes. Pressure regulators 7 and 12 served as safety valves in case of sudden, excessively large flow of gas or air; they practically eliminated fluctuation of the menisci of the flowmeters. FLOWMETERB. The continuous operation and the small volumes 'to be measured necessitated the use of flowmeters. They were calibrated by determining the time required for a known volume of air or gas to pass through them. The volume of air was measured by displacement with water, conditions being varied so as to cover the range of rates of flow to be used. The accuracy for the air flowmeter was about 2.5 cc. per minute, amounting t o about 2.5 per cent for the range of volumes used most. The accuracy for the flow of sulfur dioxide was about 0.2 cc. per minute, or about 3.2 per cent in the range of rates of flow used. ELECTRIC FURNACE. The temperature within the heating space of an electric furnace varies usually from point to point. It was therefore desirable t o know these characteristics in the furnace employed. Both ends of the heating chamber were plbgged, in order to minimize the circulation of air through it. After sufficient time had been allowed for the temperature to become constant, temperature readings were taken, the procedure being repeated for different positions within the heating chamber of the furnace. The results in Table I show fairly uniform temperature rise from both ends towards the middle section. WITHIN TABLE I. VARIATION OF TEMPERATURN
Distance from Right End of Furnace
Cm.
In.
Temp. of Furnace
c.
THE
Distance from Right End of Furnace Cm.
In.
peratures used most, the thermometer readings were about 4O C. higher than the corresponding pyrometer readings. CONVERTER.The construction of the converter is shown in Figure 2. It was made of heat-resistant glass, and the gas passages were 1 cm. inside diameter. The rear end of the converter was provided with a ground-glass cover to permit introduction and removal of the contact mass and the capsule containing the arsenious oxide.
A. THERMOMETER
1. CONTACT MASS
E. T H E W C O U P L E
F. GROUND-EMS COVER 0 . PVRNAOE
C.
CAFSJLe
-
D, GLASS W O O L
H. CONVERTER
FIGURE 2. CONVERTER IN POSITION IN FURNACN
The construction of the converter was such as to permit the gases to complete a double pass through the furnace, giving them a chance to become preheated before entering upon the catalyst. The furnace with the converter was tilted slightly forward, so as to permit drainage of any conFURNACE densate which might appear a t the discharge end of the converter. All connections were made with rubber tubing, Temp. of Furnace except a t the outlet from the converter, where paraffined c. cork was used because sulfur trioxide is very corrosive if not perfectly dry. The converter proved satisfactory and convenient to operate; the heat exchange feature was probably helpful in obtaining good temperature control.
It is thus evident that the maximum heating effect occurs All temperature measurements were made with the bulb of the thermometer and the weld of the thermocouple a t a point which was 17.8 cm. (7 inches) from the right end of the furnace, or close to t h e maximum heating position of the furnace. Recorded temperatures will therefore be the maximum temperhire of reaction, but not necessarily the final equilibdurn conditions of the products leaving the furnace, and will serve the purpose of recording analogous operating conditions within the system with the various catalysts to be investigated. MILLIVOLTMETER AND THERMOMETER. The thermometer and thermocouple, both mounted on a disk of asbestos board, with the weld of the couple and the bulb of the thermometer about even, were inserted into the furnace. Both ends of the heating space were closed in order to minimize circulation of air. Thermometer and millivoltmeter readings were taken at different temperatures, after sufficient time had been allowed for conditions within the furnace to become constant. The millivoltmeter was mounted conveniently on the wall and served for continuous observation of temperature conditions. All recorded 'temperatures were obtained with the thermometer because it gave closer readings, the pyrometer being used for control purposes. Within the range of temat about the mid-point of the furnace.
VOL. 29, NO. 3
Method of Operation An acidified solution of potassium permanganate was used for the determination of sulfur dioxide. The original gas mixture, high in sulfur dioxide content, was analyzed by bubbling it through a 0.1 N solution; the residual gases issuing from the converter, low in sulfur dioxide content, were analyzed by bubbling them through a 0.01 N solution. The sulfur trioxide absor tion bottles were all of the same shape and size. They coulf therefore be used interchangeably because the level of a certain volume of solution stood at the same height in any one of them, resulting in the same amount of pressure upon the system. The permanganate in these absorption bottles was always diluted with 185 cc. of distilled water and acidified with 15 cc. of concentrated sulfuric acid. To make a run, an absorption bottle Containing 225 CC. of solution was plared under converter outlet (Figure 1) and the flow of air and sulfiir dioxide thrnugh the converter was adjusted to the desired rates of each. Another absorption bottle containing the same volume of solution was then connerted to the three-way stopcock, 15, where the mixed gases could be analyzed before going t o the converter. The stream of gas could then be diverted from one ahsorption bottle to the other merely by turning the three-way glass cock. This arrangement made it possible to adjust the rate of flow and then to divert the flow of gas to the desired absorption bottle, placed either before the converter or at its discharge end. When the furnace was up to reaction temperature, an absorption bottle, containing standard potassium permangsnate, was connected to the outlet end of the converter, while the properly adjusted gas mixture was allowed to flow into an ahsorption bottle connected to the three-way glass cook. The gas stream
MARCH, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
was then switched over to the converter, and the time required to decolorize the permanganate recorded with a stop watch. The original sulfur dioxide content of the mixture could be determined before or after a run through the converter.
Calculation of Conversion To calculate the percentage of sulfur dioxide converted to sulfur trioxide it was necessary to know the following: A , the time required to decolorize 25 cc. of 0.01 N potassium permanganate solution with the original gas mixture, before going through the converter. B , the time required to decolorize 35 cc. of 0.01 N potassium permanganate solution with the residual gases, after going through the converter.
It was found preferable to use 0.1 N solution for the analysis of the original gases, high in sulfur dioxide, and to convert the results to the equivalent of 0.01 N . On this basis, the percentage of sulfur dioxide in the residual gases, unconverted, is: (AIB) x 100
The percentage conversion-that is, the percentage of sulfur dioxide converted into sulfur trioxide-is : (1 - A / B ) x 100 I n some of the tables it will be found that for the same values of B two different tests may not give the same conversions, although the sulfur dioxide concentrations were the same. This came about as a result of slight variations in the value of A , too slight to affect the sulfur dioxide concentration in the first decimal place. For example, if in two tests the respective values of A were 4.43 and 4.48 minutes, the per cent sulfur dioxide would remain unaffected in the first decimal place, as reported in the tables. But, the conversions would then be 98.53 and 98.51 per cent, respectively, even though the value of B is reported to be 30 minutes for each of the two tests. NORMAL LOADING.The normal loading constitutes the passage of 135 liters of 7 per cent sulfur dioxide gas per hour
257
per 200 cc. of contact mass-that is, 225 cc. of mixed gases per minute per 20 cc. of contact mass 0 r . a proportionate volume of mixed gases for a smaller or larger volume of catalyst used. It was adopted merely as a inatter of convenience for the purpose of comparing the conversion performance of the catalysts. Although not absolutely accurate, it gives a fairly good idea of the relative efficiency of the different contact masses. The procedure has been used in a lawsuit involving patent infringement, to compare the conversion efficiency of vanadium contact masses (General Chemical Company os. Selden Company, KO. 2358 in equity, District Court of the United States for the Western District of Pennsylvania).
Preparation of Catalyst A vanadium contact mass was used with an inert carrier containing potassium hydroxide which, it is claimed, protects the catalyst against the effects of higher temperatures (18). It was prepared by treating 316 grams of kieselguhr with an aqueous solution of 50 grams of ammonium metavanadate and 56 grams of potassium hydroxide. After thorough mixing, the mass was evaporated to a consistency which permitted molding into granules. The mass was activated a t 480’ C. with a mixture of air and sulfur dioxide for about 2 hours and then for a while with air only. The activated contact mass contained 3.7 per cent vanadium, equivalent to 6.6 per cent vanadium pentoxide. The finished catalyst contained all of the vanadium originally added, since no fltration was resorted to in the process of preparation.
Conversion Performance of Catalyst 1 , First Charge For the purpose of comparison, it was necessary to establish first the conversion performance of the contact mass, before the effects of arsenic upon it could be judged. The first charge of catalyst 1 placed in the converter weighed 4.353 grams and had a volume of 9.4 cc. Normal
-
A
TO 503-GAS COOLERS
405-410’C.
73L CONVERTER A
-
PARTLY CONVEkTED GAS
II
-
-
50s-CAS
Reproduced by courtesy of Reinhold Publishing Corporation from A . C . 8.Monograph BS on “Sulfuric Acid Manufacture,” by A M . Fazrlrs DIAQRAM OF
GAS FLOW IN
A TWO-STAQE CONVERTER AND HEATEXCHANQE SYSTEM OB THE VERTERS ARE ADAPTEDFOR USE WITH VANADIUM MASSES
BADISCHE TYPE;THI CON-
INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 29, NO. 3 loading conditions would call for the passage of 106 cc. of the fact that conversion begins to take place a t fairly low mixed gases per minute. The rate of flow actually used was temperatures, below 400" C. It rises extremely rapidly with 113 cc. of mixed gases per minute, about 106 per cent of nortemperature, reaching 98 per cent conversion at 450" C., mal loading. 99 per cent a t 460" C., and slightly higher conversion with The results obtained (Table I1 and Figure 3) show that the further temperature rise. Up to about 550" C. the catalyst catalyst possessed a high conversion capacity, even though continues to give 99 per cent conversion or better. Beyond operating a t 106 per cent normal loading. this temperature, conversion decreases slowly, falling tto 98 per cent at 585" C.-that is, a 1 per cent drop in conversion for a 35" C. temperature rise. However, comparison cannot be made between the temperatures observed in this investigation and operating temperatures of commercial installations. The data of Table I show that there is a wide variation in temperature within the small furnace used. The temperature wae also read from a thermometer and thermocouple which were not located in the catalytic mass where temperature changes would take place from the heat of reaction of the chemical reaction. The temperatures, therefore, cannot be directly TEYZCRATUPE IN DE&. C. compared with temperatures given 'by F. C. Zeisberg (4). 0 5 6 Y li! 15 18 U PCRCEWC As205 INZRORICDD IN90 W E SYSTEM. The primary purpose of this research was not to determine temperature-conversion curves but to study the effect of FIGURE3. CONVERSION DATAFOR CATALYST 1, FIRST arsenic upon the performance of the catalyst. CHARGE 258
A . Temperature 118. conversion B . Per cent arsenious oxide vs. conversion
Method of Introducing Arsenic into Gas Stream
TABLE11. TEMPERATVRE-CONVERSION DATAFOR CATALYST 1, FIRSTCHARGE" Time of Run Conversion Temp. Min % c. 495 57.00 99.16 514 503 54.00 99.09 514 514 62.00 99.18 520 530 63.00 99.24 480 530 60.00 99.20 " 6.2 per cent SO2 in the gas mixture.
Temp. O
c.
Time of Run Conversion Min. % 58.00 99.15 54.00 99.09 60.00 99.20 59.00 99.18
EFFECT OF INCREASE IN GASFLOW.To see how the contact mass would behave under a heavier overload, 4.353 grams of catalyst (9.4-cc. volume) were subjected to 177 cc. of 7.0 per cent sulfur dioxide gas per minute. This was equivalent to 167 per cent of normal loading. The following figures show that the catalyst lost only about 0.2 per cent of its conversion capacity from what it was at 106 per cent loading, as a result of this much heavier overload. This contact mass evidently possessed a very high capacity for operating under heavy overload conditions : Temp. O
c.
480 480
602 in
Gas Mixt.
% 7.0 7.0
Time of Run Min. 25.00 26.00
As has already been shown, addition of arsenic to the contact mass would not be equivalent to its introduction into the gas stream. Besides, such a procedure would not simulate plant operating conditions. It was therefore necessary to devise a scheme whereby sublimed arsenious oxide could be injected into the gases going to the converter. All attempts to introduce the sublimed arsenic into the gas stream a t any distance away from the converter and t o
99
P 97
E b
0s ll
E
;93 I O
Conversion
% 98.95 98.99
Conversion Performance of Catalyst 1 , Second Charge Owing to the fact that different portions of the same contact mass might behave somewhat differently, another charge of catalyst 1 was tested. The first charge was removed from the converter and set aside for further use later. The second charge, weighing 4.313 grams and having a volume of 9.4 cc., was loaded in the converter and tested under conditions similar to those used with the first charge. The load was again maintained at 106 per cent of normal, but this charge was investigated much more thoroughly in order t o establish the optimum temperature range for the catalyst under the operating conditions adopted, and to serve as a guide in comparing results, without the aid of existing equilibrium curves for use of platinum catalysts. The results (Table I11 and Figure 4) show good confirmation of the conversions obtained with the first charge. The exceptionally high quality of the contact mass is shown by
01 ~~
430
6 0
410
490
510
5jC
550
570
S'EMPZRAWRE IN DEO. 0.
FIGURE4. TEMPERATURE-CONVERSION DATAFOR CATALYST 1, SECOND CHARQE TABLE 111. TEMPERATURE-CONVERSION DATAFOR CATALYST 1, SECOND CHARGE 902 in Gas Temp. Mixt. O C. % 407 6.4 407 6.4 429 6.4 429 6.4 431 6.4 433 6.4 433 6.4 6.4 442 442 6.4 450 6.4 450 6.4 6.4 461 6.4 461 6.4 472 6.4 472 6.4 473
Time of Run
Min. 1.30 1.33 1.67 1.72 4.97 5.55 5.58 12.50 12.67 23.00 23.00 39.00 41.00 48.00 48.00 51.00
Conversion
% 66.00
66.75 73.55 74.35 91.10 92.03 92.08 96.47 96.51 98.08 9s.08 98.87 98.92 99.os 99.08 99.13
802 in Gas
Temp. Mixt. C. % 6.4 6.4 6.4 6.4 6.4 6.0 6.2 6.2 6.2 6.2 6.4 6.4 6.4 6.4 6.4 6.4 6.4
Time of Run Min. 45.00 50.00 53.00 53.00 53.00 62.00 58.00 53.00 63.00 60.00 32.00 42.00 40.00 28.00 26.00 24.00 23.00
Coyersion
% 99.02 99.12 99.16 99.16 99.16 99.18 99.17 99.09 99.24 99.20 98.62 98.95 98.90 98.42 98.30 98.16 98.08
MARCH, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
maintain it successfully in the gases until they reached the catalyst failed. It was therefore thought best to try to make use of that part of the converter which served to preheat the gases. The construction of the converter (Figure 2) was such as to permit access from the back of the furnace to the catalyst and to the preheater. As porcelain boats were too long to pass the bend in the converter, capsules were made of heatresistant glass tubing and proved to be very satisfactory. The glass capsule, containing a weighed amount of arsenious oxide, was placed in the preheater tube of the converter with its open end in the direction of the gas stream. Any subliming arsenious oxide would then be carried along by the gases which swept over the capsule. At the rear end of the converter, the gases were well churned, reversing their direction of flow just before entering upon the catalyst. This arrangement kept the sublimed arsenious oxide hot all the way into the catalyst chamber and well mixed wiih the gas mixture. A number of tests were made, varying the position of the glass capsule containing the arsenic. It was found that zones could be located where no sublimation of arsenic took place. By moving the capsule progressively into the furnace, the rate of sublimation of arsenious oxide could be varied a t will. This method proved entirely satisfactory, and the following procedme was adopted : A capsule containing a weighed amount of arsenious oxide was placed in the preheater tube through the ground-glass seal. A light plug of glass wool in the preheater tube served to locate the desired position of the capsule with arsenic for the proper rate of sublimation. The glass cup was then replaced upon the converter opening, and a determination started. After completion of each test, lasting from 3 t o 5 hours, exclusive of heating and cooling the system, the capsule was removed from the converter and reweighed. The loss in weight represented the amount of arsenious oxide sublimed and passed over the contact mass with the gases; there was always a small amount of arsenious oxide left in the capsule after each test. Without disturbing the contact mass in the converter, the process was continued day after day until conversion had been lowered sufficiently t o show that a downward trend was in
259
The amount of arsenious oxide introduced into the system and the conversions obtained are given in Table IV and plotted in Figure 5. The charge of contact mass weighed 4.313 grams, had a volume of 9.4 cc., and was operated under a load of 113.2 cc. of 6.4 per cent sulfur dioxide gas per minute, or 106 per cent of normal loading. Table IV and Figure 5 show that the total amount of arsenious oxide introduced in the system was 0.5425 gram, equivalent to 12.61 per cent of the weight of the contact mass, or nearly twice the amount of vanadium pentoxide present. The net result was a 4.12 per cent lowering in conversion-that is, from 99.21 to 95.09 per cent a t 480’ C. TABLEIV. EFFECTOF AMOUNTOF ARSENICON CONVIRSION WITH CATALYST 1, SECOND CHARQE Amount of As203 Introduoed Gram % 0.0150 0.35
Total As208 Introduced Gram % 0.0150 0.35
0.0090
0.21
0.0240
0.56
0.0135
0.31
0.0375
0.87
0 0100
0.23
0.0475
1.10
0 0090
0.21
0.0565
1.32
0 0110
0.25
0.0675
1.57
0.0150
0.35
0.0825
1.92
0.0480
1.12
0.1305
3.04
0.0660
1.53
0.1965
4.57
0.0640
1.49
0.2605
6.05
0.0460
1.07
0.3065
7.13
0.0830
1.93
0.3895
9.05
0 0770
1.79
0.4665
10.88
0.0760
1.77
0.5425
12.61
progress.
Effect of Arsenic upon Catalyst 1 , Second Charge The second charge of catalyst 1 was then subjected to the treatment with arsenic, after its conversion performance had been investigated thoroughly and a satisfactory method found for introducing arsenic into the gas stream. In the absence of information as to the quantitative effect of arsenic upon the conversion capacity of vanadium contact masses, it was deemed advisable to proceed cautiously with the introduction of arsenic into the system, in order to avoid the possibility of bringing about suddenly abrupt changes in conversion. Tests were therefore begun with very small amounts of arsenious oxide a t very low rates of sublimation. Figure 4 shows that there was very little change in the conversion capacity of this catalyst over a considerable temperature range. The temperature a t which all arsenic poisoning tests were conducted-namely, 480 O C.-lies within this range. Since temperature fluctuations within a few degrees of either side of 480’ C. would produce practically no change in conversion, danger of obtaining results which might have been influenced by slight temperature variations was therefore eliminated. The rate of flow and the strength of gas were maintained the same as in the conversion performance investigation. The results are therefore comparable, and any change in conversion capacity of the catalyst must be due to the effect produced by the presence of arsenic in the gas stream.
Duration of Test Conversion Hours % 99.21 4.5 99.19 3.0 99.19 99.19 99.19 4.0 99.19 99.19 5.0 98.99 99.19 3.0 99.19 3.0 99.19 99.19 99.19 3.0 99 * 19 99.19 3.0 99.19 99.19 3.0 99.08 3.0 99.02 98.74 98.74 5.0 98.68 98.58 98.36 98.36 98.36 98.16 5.0 97.90 97.79 97.64 98.16 5.0 97.90 97.05 90.32 96.32 90.32 5.0 96.00 96.60 96.32 95.98 95.58 95.09 95 09
Over a considerable length of time, up to the introduction of a total of 0.1965 gram of arsenious oxide (equivalent to 4.57 per cent of the weight of the contact mass) no ilL effects were observed. In fact, up to this point, over a period of 9 days, the results tend to indicate that vanadium catalysts were immune to poisoning by arsenic, as claimed so frequently in the chemical literature. However, the effects of arsenic poisoning’ became apparent when the amount of arsenious oxide added reached 6.05 per cent of the weight of the contact mass. Further addition of arsenic continued to depress the conversion capacity of the catalyst slowly but steadily. When the conversion had been reduced to 95 per cent, it was decided to discontinue the 1 The amount of arsenious oxide introduced into the contact mass wan so large compared with the vanadium pentoxide present that objection may be raised t o designating the effects of the arsenic as “poisoning,” which is usually underbtood to designate the introduction of small amounts of foreign material. The results reported here constitute only a part of a research pioject undertaken t o ascertain (1) how much of a catalyst poison such as arsenious trioxide must be introduced to affect the properties of the catalyst and (2) what is the mechanism of the action of the ”poison” upon the catalyst. If the answer t o the second question can be found, it may be possible t o suggest in a future publication a more appropriate term than “catalyst poison,” particularly when large quantities are involved rather than relatively small amounts.
Figure 6. ARSENICLEAVING THE CONVERTER. To keep track of any arsenic which might be carried over the contact mass and out of the converter, qualitative tests were made with hydrogen sulfide on the sulfur trioxide absorber contents, because it would naturally be retained there. Any arsenic depositing in the short length of tube outside the furnace would be noticeable, especially if an appreciable amount of it had accumulated. No such accumulation of arsenic had been observed anywhere in the system. The hydrogen sulfide tests on the absorber contents revealed no trace of arsenic until about 9 per cent of arsenious oxide had been introduced into the gas stream. Only slight turbidities formed a t any time, indicating that an inconsiderable amount of the total arsenic introduced into the system could have left it by way of the exit gases. EFFECT OF TEMPERATURE. The influence of temperature upon the conversion capacity of a poisoned vanadium catalyst is shown in the following table; the figures were obtained by raising the temperature of the system after about 7 per cent of arsenious oxide had been introduced into the gas stream and the Conversion had fallen to 98.36 per cent a t 480" C.: Temp., 480 630 546 630 480
O
C.
the poisoned and unpoisoned catalyst was 0.76 per cent at 480" C. and only half as large a t 530" C. Upon returning to the original temperature (480" C.), the conversion difference increased again to nearly what it was a t the beginning. It seems that the conversion of the poisoned catalyst remains always somewhat lower than that of the normal, unpoisoned material a t any particular higher temperature, although it has been claimed by Neumann (la)that the conversion capacity of silver vanadate, poisoned by addition of arsenic, is practically restored to its normal value when the temperature is raised above 500" C. The positive effect produced by higher temperatures is not permanent, as evidenced by the return to lower conversions when the temperature is lowered to the starting point, 480" C. The sulfur trioxide absorber liquors gave no test for arsenic with hydrogen sulfide, indicating that no arsenic had been volatiliaed from the poisoned contact mass as a result of its subjection to higher temperatures.
Sampling and Analysis of Poisoned Contact Mass (Second Charge) From the appearance of the contact mass a t the time of its removal from the converter it was evident that the distribution of arsenic was not uniform over the entire length of the charge. It was therefore thought best to determine the relative accumulation of arsenic along the path traversed by the arsenic-laden gases, rather than to determine the average content of arsenic in the mass. Besides, the total amount of arsenic introduced into the system was known, and only a small amount of it could have passed out of the
r Per Cent Conversion Unpoisoned oatalyst Poisoned aatalyst
99.12 99.26 98.90 99.20 99.12
98.36 98.87 98.48 98.74 98.48
Evidently, a rise in temperature produces a beneficial effect upon the conversion capacity of a poisoned vanadium contact mass. The original difference in conversion between 260
MARCH, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
converter with the exit gases; hence little would be gained by knowing exactly how much arsenic was retained by the catalyst, although such a figure might have been of some interest. It was therefore decided to divide the second charge of catalyst P into three sections, a t the time of its removal from the converter. The portion of the contact mass where the gases first enter the converter represented the first section, the middle portion represented the second section, and the portion where the gases leave the converter represented the third section. Only those portions of the contact mass which were weighed for analysis were ground finely, because it was desired to preserve some of the original, poisoned specimen for future reference. This procedure necessitated the picking out a t random of several pieces for analysis and considering them as representative of the particular section of the contact mass. Obviously, this may not give an average representative sample, but the approximation will be close enough for the purpose. Arsenic oxide is fairly soluble in water, but the trioxide must be dissolved in alkali, thus making it necessary to leach the poisoned contact mass with caustic to ensure complete removal of arsenic, in case both forms should be present. The caustic treatment would also bring vanadium into solution, because vanadium tetroxide is soluble in acids and alkalies, and vanadium pentoxide is slightly soluble in water and soluble in concentrated acids and alkalies, calling for a separation of arsenic from vanadium. A method for the separation of arsenic from antimony (7), applicable to the separation of arsenic from vanadium, was adopted. It is based on the principle that only the arsenic sulfide is precipitated by hydrogen sulfide from a strongly acidified solution containing arsenic, antimony, tin, and vanadium. I n this precipitation, especially from a hot solution the arsenic is reduced to the trivalent state because the pentasulfide is stable only under ice cold conditions and decomposes into the trisulfide and sulfur when dissolved in hot solution (15).
261
arsenious sulfide precipitate. The percentage of arsenic in the second charge of catalyst 1 was as follows: Seotion
Per Cent AsaOs 16.9 12.0 1.5
Location in Converter Where gases enter Middle Where gases leave
1
2 3
Practically all of the arsenic seemed to be present in the pentavalent condition, because direct titration of the dissolved material indicated only a small amount of either vanadium tetroxide or arsenious oxide.
99 0
E
R 98 5
T
c
97
I V
B
e
R 96
0
1. 95
460
416
492 0
508
5'24
mwmAIvm IY KO. e. e 4 6
PERCENT
AS+O,
540
556
8 INTRODUCED inro THE s x s w
LO
572
12
FIGURE 6. DEVIATION OF CONVERSION CAPACITY OF POISONED CONTACT MASS FROM NORMAL CATALYST (SECOND CHARQE OF CATALYST 1) A . Temperature ns. conversion B. Per cent arsenious oxide 08. conversion
The fact that accumulation of arsenic upon the contact mass decreased along the path traversed by the gas stream indicates that adsorption of it by the catalyst was very rapid, that most of it was removed from the gas stream as soon as it reached the catalyst, and that conversion was maintained by a continuously decreasing amount of the contact mass.
Effect of Arsenic on Catalyst 1, First Charge After the behavior of the contact mass under the influence of arsenic had been established, it was desirable to check these results. To do this, it was necessary again to know beforehand the exact conversion performance of the particular charge to be treated. For this purpose, the ftrst charge of catalyst 1, previously tested and put aside, was now ready for checking purposes. The charge was placed in the converter and subjected to the effects of arsenic in the gas stream. It seemed of interest to see whether the rate of introducing arsenic into the system would exert any particular effect upon the behavior of the 1.6 3.2 4.8 6.4 8.0 9.6 11.2 ?ERCEWT As& INTROIUCED IUTO THE IrSTBM contact mass. Consequently, larger amounts of arsenious FIGURE 5 . PER CENT ARSENIOUSOXIDEas. CONVERSION oxide were sublimed in the converter in shorter periods of AT 480' C. WITH CATALYST 1 (SECOND CHARGE) time than in the previous investigation. 0.010 gram of arsenious oxide sublimed per hour The results are given in Table V and Figure 3. The weight of the contact mass was 4.313 grams (9.4-cc. volume), and The arsenious sulfide is dissolved in ammonium hydroxide, it was operated a t 113.2 cc. of 6.4 per cent sulfur dioxide gas acidsed with sulfuric acid, and reduced to sulfur trioxide per minute-that is, a t 106 per cent of normal loading. fumes. The clear solution is then cooled, made ammoniacal, Introduction of 5.2 per cent arsenious oxide resulted in and reacidified slightly; an excess of sodium bicarbonate is practically no lowered conversion performance of the contact added, and the mixture titrated with standard iodine solution. mass, and 12.73 per cent arsenious oxide reduced the conThe method was tried on synthetic samples of vanadium version 1.54 per cent, from 99.08 to 97.54 per cent. and arsenic and found sufficiently accurate for the purpose; Beyond this point, and up to the introduction of 15.45 92 to 94 per cent of the arsenic was recovered. Known mixper cent arsenious oxide, the conversion capacity of the tures of vanadium and arsenic were also run simultaneously catalyst rose 1.03 per cent. This anomaly might be due to a with the actual determinations. The hydrogen sulfide treatrearrangement of the individual granules of the loosely packed ment was repeated until the hot filtrate yielded no more contact mass in the converter, occasioned by vibration or
262
INDUSTRIAL AND ENGINEERING CHEMISTRY
some other disturbance of the converter and its contents and resulting in exposure of new surfaces and less affected by arsenic surfaces of the catalyst. It has been shown (1) that a vanadium contact mass, with its conversion reduced to 89.4 per cent from an original value of 96.5 per cent by poisoning with arsenic, gave again 96.3 per cent conversion when removed from the converter and replaced immediately. Evidently it may be assumed that the side of the individual granules of the contact mass which faces directly the arsenicladen gas stream is affected most by deposition of arsenic upon it. The other side, facing downstream, suffers much less from the effects of the poison, and the superimposed or touching faces are probably affected least, when the rate of introduction of arsenic into the system is high.
VOL. 29, NO. 3
of the catalyst and mainly upon purely adsorptive centersthat is, on areas which are inactive catalytically for sulfur trioxide formation.
Sampling and Analysis of Poisoned Contact Mass (First Charge) The first charge of catalyst 1 was also divided into three sections for analysis a t the time of its removal from the converter. The analytic procedure was the same as that used for the second charge, and the percentage-of arsenic present was found to be as follows: Section
1 2
3 .
Location in Converter Where gases enter Middle Where gases leave
Per Cent AsaOo 21.3 18.5 1.4
TABLEV. EFFECTOF AMOUNTOF ARSENICON CONVERSION The distribution of arsenic along the path traversed by WITH CATALYST 1, FIRSTCHARQE the gases is in the same direction as was found with the second Amount of AsaOs Total AsnOs Duration charge, except that the first and second sections of this charge Introduced Introduced of Test Conversion ntained a hi e of arsenic. Dram 5% Gam % Hours % The larger enic introduced into the system 0.2260 5.20 0.2260 5 20 5.0 99.08 99.02 and the much higher rate of sublimation had no effect upon 99.02 the general tendency of the catalyst to remove the arsenic 0.3270 7.53 0.5530 12.73 5.0 98.57 98.10 from the gases as soon as they came in contact. This is 98.16 shown by the heavy deposit of arsenic upon the first and 97.90 97 _ .54 _second sections and the relatively light accumulation towards 97.54 98.57 0.1180 2.72 0.6710 15.45 4.0 the end of the path traversed by the gases. 98.57 ARSENICLEAVINGCONVERTER.As in the case of the 98.67 98.57 second charge, hydrogen sulfide tests were made continuously 97.54 0.2000 4.61 0.8710 20.06 4.0 97.40 on the contents of the sulfur trioxide absorbers. Up to the 97.24 introduction of about 5 per cent arsenious oxide into the gas 97.24 97.24 stream, no qualitative test for arsenic was obtained. Afterwards, traces of arsenic could be detected in the absorber contents, but the amounts remained small a t all times. Introduction of further amounts of arsenic into the gas Although only a small amount of the total arsenic introstream caused the conversion capacity of the contact mass duced into the system could have escaped from the converter, to fall to 97.24 per cent when the total amount of arsenious it was undoubtedly greater than in the case of the second oxide reached the value of 20.06 per cent, equivalent to three charge. This might easily be accounted for by the greater times the amount of vanadium pentoxide contained in the total amount of arsenic introduced and the higher rate of contact mass. sublimation employed.
Rate of Introducing Arsenic into Gas Stream The second charge of catalyst 1was treated with about 0.010 gram of arsenious oxide per hour, and the first charge with about 0.049 gram per hour. The difference in results is shown by the fact that in the slow treatment 9.05 per cent of arsenic produced a reduction in conversion to 97.54 per cent, and 12.61 per cent of arsenic reduced the conversion to 95.09 per cent, whereas in the rapid method of sublimation it required 12.73 per cent of arsenic to bring the conversion to 97.54 per cent and 20.06 per cent of arsenic to bring the conversion to 97.24 per cent. It is possible that these results were influenced to some extent by the sudden rise in conversion which is thought to have been caused by exposure of less affected surfaces, but the indications are that the higher rate of sublimation of arsenic requires a larger amount of arsenious oxide to produce the same amount of reduction in conversion as the lower rate of sublimation. That a sudden rush of arsenic upon the catalyst is less detrimental than a slow, gradual treatment with the poison, is perhaps due to the fact that in the process of slow sublimation the gradual entrance of arsenic upon the contact mass permits of more selective adsorption. The arsenic molecule. are lodged more regularly in the lattice of the catalytic surface and therefore exert a more pronounced poisoning effect upon the action centers. On the other hand, the introduction of larger amounts of arsenic at one time may result in more irregular, random collection of its molecules upon the surface
As has been shown, catalyst 1 possesses a very high efficiency for converting sulfur dioxide to sulfur trioxide, but, although its resistivity to poisoning by arsenic is remarkably high, especially as compared with platinum contact masses, it cannot be pronounced immune to poisoning. Therefore, the statement so often encountered in the literature which stresses the immunity of vanadium contact masses to poisoning by arsenic must be accepted with reservation and treated as relative only.
Periodic Ring Formation When the contact mass was removed from the converter, after treatment with arsenious oxide, a certain pattern was observed upon the individual particles. It extended all the way through the material, as was shown when some of the pieces were broken in two. Figure 7 shows that the pattern resembles the periodic structure of the well-known Liesegang concentric rings. Judging from the appearance of the photomicrograph and the fact that no such structure could be found on any of the particles of the catalyst which were not treated with arsenic, this structure must have been formed by the arsenic. Whether the white rings in the photomicrograph were composed of arsenic alone could not be ascertained analytically, because there was not sufficient material available for this purpose. The phenomenon was most pronounced in that section of the contact mass where the gases leave the converter and where the accumulation of arsenic was lightest. It is possible
MARCH. 1937
1NDUSTRlAL AND ESGlNEERlNG CHEMISTRY
that the pattern existed throughout the contact mass, but was obliterated by the heavy accumulation of the arsenic. The photomicrograph shows clearly the porous structure of the contact mass which evidently plays an important role in its high conversion efficiency. If the arsenic has a natural tendency to group itself as sliown in the photomicrograph, rather than t o disperse evenly throughout t h e mass, then the pores would remain open much longer and the resistance towards poisoning by arsenic would he materially increased.
263
Although the load under which this catalyst operated was 32 per cent above normal, the results show fairly satisfactory
conversion capacity. The performance does not, however, come up to that of catalyst 1which gave 99.18 per cent conversion at 480' C. when operating at 106 per cent of normal loading, and 98.99 per cent conversion when operating a t 167 per cent. of normal loading. Besides, catalyst 1 cont.ained only 15.6 per cent vanadium pentoxide; catalyst 2 contained 8.6 per cent vanadium pentoxide. P R Y s I C A L STRENGTH.
The physical s t r e n g t h of catalyst 2 was not satisfacPreparation of tory for commercial appliCatalyst 2 cation. Its crushing Barium and calcium arc strength was unsuitable for both capable of acting as the purposeof shipping from promoters for v a n a d i u m the manufacturer to the contact masses in the oxidaplant, and it could not withtion of sulfur dioxide to sulstand the additional hanfur trioxide. Preference is d l i n g occasioned by the given to barium because it loading of the contact mass precipitates vanadium into the converters without much more c o m p l e t e l y alargeamount of crumbling. than rloes calcium, thus re Some binding m a t e r i a l ducing t h e a m o u n t of might possibly help to overvanadium lost in the filcome this defect of the contrate. tact mass, or the amount of FIGURE 7. PATTERN OF FIRST CHARGE OF CATALI-~T 1 Catalyst 2 was a bariurnsodium silicate added should Dromoted vanadium contact perhaps be increased. mass, prepared in the following manner (16): To 2.33 grams EFmm OF ARSENIC.Although this catalyst was unsuitable of potassium hydroxide, dissolved in 60 cc. of water, 4.78 for industrial use without further modification, it was interestgrams of ammonium metavanadate were added, and the ing to see what influence the promoter substance, barium, mixture was boiled gently to drive offthe liberated ammonia. would exert upon the resistivity of the contact mass towards This solution and 30 cc. of 40' BC.. commercial water glass poisoning by arsenic in the gas stream. were added simultaneonsly, very slowly, with vigorous The same charge which had been used in bhe conversion stirring, to a solution of 41.25 grams of barium chloridc jierforniaoee test was subjected to treatment with arsenic. (BaC12.211,0)in 915 cc. of water. After settliiig for 2 hours, The t,emperature was held const,ant a t 480' C., and the rate the precipitated mass was filtered off, dried, formed into of flow gas was 113.2 cc. of 6.4 per cent sulfur dioxide per granules, and activated by heating in air for 2 lionrs at 200minute, or 132 per cent of normal loading. The results 300" c. (Figure 8) are as follows: The discarded filtrate contained a considerable amount of .Amount ul Introduced Duintioii oi 'TsTt Cuweraioi~ barium chloride and 11 little vanadium. After correcting for Gram % Min. % the vanadium lost in the filtrate, the finished contact mass contained 4.8 per oent vanadium, tlie equiwlent of 8.6 per cent vanadium pentoxide.
Performance of Catalyst 2 COXVERSION.A charge of 4.920 grams of contact mass, with a volume of 7.6 cc., was placed in the converter. For normal loading conditions this would require 86 cc. of mixed gases per minute, but i t actually operated at 113.2 cc. of 6.4 per cent sulfur dioxide gases per minute, equivalent to 132 per cent of normal loading. The results arc given in Table VI and Figure 8. TABLE VI. tern^.
c.
470 480 490 498
* 6.4
A total of 0.2950 gram of arsenious oxide, equivalent to G.O per cent of the weight of the contact mass, or 70 per cent
TEMPER~TURE-C~~.~ERSION DATA^ FOR CATALYST 2
Time of Hun Convernion x*n. 28.00 27.50 29.50 28 25
7% 98.34 98.32 98.40 08.33
per oent SOn in the gss mixture.
-
Temp. Time ai Run Conversion c. Min. % 503 27.00 98.25 50s 25.50 88.16 508 28.60 88.22 520 24.50 98.11 580 23.50 98.03
A . Temperature 1 8 . convereion B . Per cent aiseniou oxide sa. conversion
264
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the vanadium pentoxide in it was introduced into the system in 3 hours, subliming about 0.1 gram of arsenious ,oxide per hour. The first test was made after the system had been in operation about half an hour; the other tests were made a t 20-minute intervals during the remainder of the 3 hours. Conversion dropped rapidly, most of the reduction occurring within the first hour of the test. The last three determinations were made within the last hour of the run and show that conditions had become constant. The resistance of this catalyst towards the effects of arsenic in the gas stream was evidently low, as compared with catalyst 1; hence no beneficial effect accrued from the presence of barium in the contact mass. The contents of the sulfur trioxide absorbers showed the presence of arsenic when tested with hydrogen sulfide, indicating that some of it had passed by the catalyst and out of the converter with the gases. APPEARANCE OF CONTACT MASS. The contact mass before activation was light pink, turning slightly yellowish with heating, and reddish yellow when treated with sulfur dioxide. The poisoned contact mass was bluish gray with orange and green specks. This would seem to indicate that, besides vanadium pentoxide which is orange or red, there was also present in the poisoned catalyst, vanadium tetroxide which is bluish, and vanadium trioxide which is greenish in color. According to Neumann ( I I ) , vanadium tetroxide appears as an intermediary compound in the mechanism of sulfur trioxide formation. ANALYSISOF POISONED CATALYST.The poisoned contact mass was analyzed according to the method used with catalyst 1. It was found to contain 4.5 per cent of arsenic when calculated as arsenious oxide. As in the case of catalyst 1, only a portion of the contact mass was used for analysis, reserving the rest for possible future reference. The results are therefore not necessarily representative of the entire charge of contact mass, but they show retention of considerable amounts of arsenic by the catalyst.
Catalyst 3 I n the preparation of catalyst 2, the excess barium was discarded with the filtrate and the precipitated mass was not washed, because it had been shown (8) that washing produces an inferior product. For example, a uranium-promoted vanadium contact mass gave 91 per cent conversion unwashed and only 24 per cent conversion when the mass was subjected to washing. A lead-promoted vanadium catalyst gave 99.2 per cent conversion when it was simply filtered off and not washed; the same mass, washed, gave only 65 per cent conversion. The reason for this variation in conversion efficiency as a result of washing is not clearly understood. However, since lead metavanadate is only slightly soluble in water (g), it might be inferred that it is not a question of loss of vanadium but rather of loss of water-soluble salts which are held in the contact mass by adsorption and act as promoters. This led to the preparation of catalyst 3 in the same manner as catalyst 2, but the mass was neither filtered nor washed. The original mixture was evaporated directly to a consistency which permitted forming of the mass into grains of desired siae. Drying was then continued in air, raising the temperature gradually to 200' C. and holding the catalyst there for 2 hours for the purpose of activation. The additional amount of barium in this catalyst resulted in a somewhat lower percentage of vanadium, as compared with catalyst 2. The finished product contained 3.72 per cent vanadium, equivalent to 6.7 per cent vanadium pentoxide.
VOL. 29, NO. 3
PHYSICAL STRENQTH.There was no noticeable improvement in physical strength over that of catalyst 2. The contact mass was just as friable and lacked the necessary cohesive and crushing strength which would make for industrial durability. CONVERSION PERFORMANCE. The charge of contact mass loaded into the converter for testing its conversion capacity weighed 3.930 grams and had a volume of 7.2 cc. Normal loading conditions would call for 81 cc. of mixed gases per minute, but it actually operated a t 113.2 cc. of 6.4 per cent sulfur dioxide gas per minute, or the equivalent of 140 per cent of normal. The results are given in Table VI1 and plotted in Figure 9. TABLD VII. TDMPERATURE-CONVDRSION DATAFOR CATALYST 3" Temp. O
Time of Run Conversion
c.
475 490 497 603 503 a 6.4 per cent SO1 in the
R
I
I
S
I
A.
I
I
I
O
98.16 98.16 98.26 98.03 97.78 gas mixture.
I
I
I
I
Time of Run Conversion Min. % 19.25 97.75 19.50 97,7a 21.25 97.96 20.60 97.89
Temp.
%
Min. 24.42 24.00 25.50 22.00 19.50
c.
508 514 514 533
I
I
I
I
I
I
h
I
I
N
I
I
I
I
I
Temperature vs. aonversion v8. conversion
B . Per cent arsenious oxide
The conversions are somewhat lower than those for catalyst 2, but, although the rate of gas flow was the same as that with catalyst 2, the volume of contact mass of the present charge was smaller than that of catalyst 2; that is, catalyst 3 operated under a load of 140 per cent normal, and catalyst 2 under a load of 132 per cent normal. This difference might account for the somewhat lower conversions obtained with catalyst 3 and place both contact masses on a nearly competitive basis. It may therefore be assumed that the conversion capacity of the contact mass has not been changed materially by the presence of an excessive amount of barium. VOLATILIZATION OF VANADIUMFROM CONTACTMASS. With catalyst 2, a very light film of reddish brown material accumulated on the walls of the converter tube just outside the furnace; but, with catalyst 3 in the converter, it became fairly heavy. Evidently, the excess salts (in the form of chlorides) present in this contact mass caused the formation of volatile vanadium compounds which deposited in the cooler portion of the converter tube. EFFECT OF ARSENIC. To test the effect of arsenic upon catalyst 3, the same charge, whose temperature-conversion performance was determined, was subjected to treatment with sublimed arsenious oxide in the gas stream. The gas velocities and sulfur dioxide concentration were maintained the same as above-namely, 113.2 cc. of 6.4 per cent sulfur dioxide per minute, equivalent to 140 per cent of normal loading. The temperature was maintained constant throughout a t 480' C. The results are recorded in the table which follows and plotted in Figure 9.
MARCH, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
Amount of AszOs Introduced
&am 0.2710
%
Duration of Teat
Conversion
Min.
%
6.9
194 to 132 per cent of normal. The following are the results obtained with this much lower load: Temp. O
c.
SO1 in Gas Mixt.
Time of Run
Conversion
%
Min. 13.45 13.50
95.48 95.50
530 530
Over a period of 3 hours, 0.2710 gram of arsenious oxide, equivalent to 6.9 per cent of the weight of the charge and about equal t o the amount of vanadium pentoxide present, was introduced into the system, subliming about 0.09 gram of arsenious oxide per hour. The results obtained with this catalyst do not differ much from those obtained with catalyst 2. There is practically no difference in resistivity towards poisoning by arsenic between the two catalysts. The lower final conversion of poisoned catalyst 3 was due to the somewhat larger amount of arsenic injected into the system. ANALYSIS OF THE POISONED CATALYST. The method used t o analyze poisoned catalyst 1 was also used to determine the arsenic content of this contact mass. It was found to contain 5.6 per cent arsenic, calculated as arsenious oxide. As in the case of catalyst 2, this figure does not necessarily represent the average arsenic content of the entire charge of poisoned contact mass, because only a portion of it was used for analysis, saving the remainder for future reference.
265
7.1 7.1
%
This increased conversion performance tends to show that there was nothing wrong with the quality of the catalyst and that the overload was too heavy for proper functioning of the contact mass. TABLE VIII. TEMPERATURE-CONVERSION DATAFOR CATALYST 4, FIRSTCHARGED Temp. O
Temp. Time of Run Conversion
Time of Run Conversion
c.
c.
%
Min.
487 4.17 487 4.20 498 4.43 4.38 498 4.83 508 4.67 508 a 6.2 per cent SO2 in the
88.90 89.00 89.55 89.43 90.68 90.36 gas mixture.
%
Min. 4.67 4.59 4.47 4.42
514 514 530 530
90.36 90.18 89.92 89.82
P 91 E
R E
Catalyst 4 I n an attempt t o improve the physical strength of the barium-promoted catalyst, kieselguhr was added to serve as an inert skeleton and carrier base. The method of preparation followed was the same as that for catalyst 3, except for the addition of 12.1 per cent kieselguhr. Filtration was not resorted to; the original mixture was evaporated sufficiently t o permit formation into granules and activated in air by heating a t 200' C. for a period of about 2 hours. On the basis of activated contact mass, it contained 3.2 per cent vanadium, equivalent to 5.7 per cent vanadium pentoxide. PHYSICAL STRENQTH.The addition of kieselguhr to catalyst 4 resulted in a very much improved physical structure. The texture of the contact mass was smoother, the grain and pore space finer, and the crushing strength satisfactory for industrial application in the manufacture of sulfuric acid. Physically, the catalyst improved materially with use; that is, subjection to working temperature conditions gave the contact mass better cohesive strength and reduced the tendency towards crumbling to a minimum.
Conversion Performance of Catalyst 4 FIRSTCHARGE.The greatly improved appearance of the contact mass led to the belief that its performance might also be of a high order. To test this, two ways of procedure suggested themselves: that is, the rate of gas flow could be increased and the volume of the catalyst maintained about the same as in previous tests, or the rate of gas flow could be maintained as on previous occasions and the volume of contact mass reduced. The latter procedure was deemed preferable. A much smaller charge, weighing 2.791 grams and having a volume of 5.2 cc., was placed in the converter. This charge required, according to normal loading, 58.6 cc. of mixed gases, but it actually operated with 106 cc. of air and 7.0 cc. of sulfur dioxide per minute, the equivalent of 194 per cent of normal loading. The conversion performance is given in Table VI11 and plotted in Figure 10. These conversions are quite unsatisfactory, and it was thought that the excessively high overload might be responsible for the poor performance of the contact mass. Accordingly, the rate of gas flow was reduced to 72 cc. of air and 5.5 cc. of sulfur dioxide per minute, decreasing the load from
N T 9O C 0
N
EI 9
E9
I
l
l
l
l
l
l
l
l
l
l
l
l
l
o
n.
"'
501
TEnPEmTum
rn
509 Dpo.
c.
5 17
525
5 33
FIGURE 10. TEMPERATURE-CONVERSION CURVE FOR CATALYST 4, FIRST CHARQE
If the rates of gas flow were reduced still further, commensurate with the volume of contact mass used, to nearly normal loading, or the volume of catalyst increased to the point where the rates of gas flow used would be nearly those required for normal loading, the percentage conversion obtained would probably also rise considerably. A larger volume of contact mass, operating within the preferred range of rates of gas flow, was therefore tried. SECONDCHARGE.As a result of the greatly improved conversion obtained by reduction of the loading imposed upon the catalyst, a larger charge was investigated a t a still further reduced rate of flow of gases. The new charge weighed 5.240 grams, had a volume of 9.1 cc., and called for 102 cc. of mixed gases per minute for normal loading. It actually operated at 113.2 cc. of 6.4 per cent sulfur dioxide gas, or 111 per cent of normal loading. The results are given in Table IX and plotted in Figure 11. These results are of about the same magnitude as those obtained with catalyst 2. However, the present charge contained only 0.299 gram of vanadium pentoxide, whereas the charge of catalyst 2 contained 0.423 gram of vanadium pentoxide. Although direct comparison is made somewhat difficult by the fact that catalyst 2 operated under 132 per cent of normal loading, and the second charge of catalyst 4 operated under 111 per cent of normal loading, judging by the large difference in vanadium pentoxide content of the two charges, it seems that catalyst 4 should be considered the more efficient, EFFECT OF ARSENIC.The second charge of catalyst 4 was subjected to treatment with arsenious oxide in the gas stream. The temperature was maintained constant at 480" C. and the rate of flow of gas was 113.2 cc. of 6.4 per cent sulfur dioxide per minute.
INDUSTRIAL AND ENGINEERlNG CHEMISTRY
266
R
I
I
I
1 .
I
0
\.
FIGURE 11. CONVERSION DATAFOR CATALYST 4, SECOND
CHARGE
A. B.
Temperature 218. conversion Per cent arsenious oxide IS. conversion
TABLE IX. TEMPERATURE-COXVERSION DATAFOR CATALYST 4, Temp.
c.
SECOND CHARGE5 Time of Run Conversion Temp. Time of Run Conversion
490 490 503 503 508 508 a 6.4 per
Min.
%
29.00 30.00 29.00 30.00 27.00 27.00
98.47 98.62 98.46 98.50 98.36 98.36
c.
Min.
%
26.00 26.00 2Fj.00 24.00
98.30 98.30 98.14 98.21
cent SO1 in gas mixture.
A total of 0.2955 gram of arsenious oxide, representing 5.7 per cent of the weight of the contact mass in the converter, or about equal to the amount of vanadium pentoxide contained therein, was introduced into the system over an interval of 4 hours; the rate of sublimation was about 0.075 gram of arsenious oxide per hour. The results are given in Figure 11 and the following table: Amount of As203 Introduced Gram % 0.2955
Duration of Test
Min.
Conversion
% 96. 08 93.68 93.68 92.63 91.16 89.00 89.00
These results tend to show that neither the excess barium nor the added kieselguhr had improved the resistivity of the catalyst towards the effects of arsenic in the gas stream. Catalyst 3 was about the most resistant towards poisoning by arsenic, but it was also the least satisfactory of the barium-promoted contact masses with regard to stability under the temperature conditions. APPEARANCE OF CONTACT MASS. The original contact mass before activation was grayish with a slight shade of pink. The activated mass was light yellowish, and the poisoned catalyst was speckled with light greenish blue, indicating the presence of vanadium in a lower form than the pentoxide. ANALYSISOF THE POISONED CATALYST. The poisoned contact mass was analyzed according to the method used in connection with the other catalyst. It was found to contain 5.2 per cent arsenic, calculated to arsenious oxide. As in the case of the other two barium-promoted catalysts, the analysis was made on a portion of the charge used. It may therefore not be strictly representative of the entire charge, but it does show high retention of arsenic by the catalyst.
Summary Catalyst 1 was a vanadium CONTACT MASSESPREPARED. contact mass, without addition of any promoter, consisting of an excessive amount of potassium which protected it
VOL. 29, NO. 3
against deterioration in conversion capacity a t higher temperatures, an inert siliceous carrier, and 6.6 per cent of vanadium pentoxide in the finished product. Catalyst 2 was a vanadium contact mass promoted with barium, the excess of which was discarded with the filtrate. Commercial 40" BB. water glass was incorporated at the time of its preparation, and the finished product contained 8.6 per cent vanadium pentoxide. Catalyst 3 was a vanadium contact mass promoted with an excessive amount of barium. It was prepared the same way as catalyst 2 but was not filtered and contained 6.7 per cent vanadium pentoxide. Catalyst 4 was a vanadium contact mass promoted with an excessive amount of barium and containing an inert siliceous carrier substance. It was prepared the same way as catalyst 3, without filtration, and contained 5.7 per cent vanadium pentoxide. CONVERSION PERFORMANCE. Contact mass 1 was found to be capable of handling satisfactorily much higher rates of flow of gases than any of the three barium-promoted contact masses, 2, 3, 4. This was shown by its operation at 480" C. under a load of 167 per cent normal (188 cc. of 7.0 per cent sulfur dioxide gas per minute per 10 cc. of catalyst), with only 0.2 per cent reduction in conversion from that a t 106 per cent normal loading at the same temperature. The first charge of this catalyst gave 99.18 per cent maximum conversion and the second, 99.16 per cent maximum conversion, when operating at 106 per cent of normal loading (120 cc. of 6.2 per cent or 6.4 per cent sulfur dioxide gas per minute per 10 cc. of contact mass). Catalyst 2 gave 98.32 per cent a t 480" C. when operating a t 132 per cent of normal loading (149 cc. of 6.4 per cent sulfur dioxide per minute per 10 cc. of catalyst). Catalyst 3 gave 98.16 per cent a t 480" C. when operating at 140 per cent of normal loading (157 cc. of 6.4 per cent sulfur dioxide per minute per 10 cc. of catalyst). Catalyst 4 gave 88.90 per cent conversion at 487" C. and 194 per cent of normal loading (218 cc. of 6.2 per cent sulfur dioxide per minute per 10 cc. of catalyst); 95.50 per cent conversion at 530" C. and 132 per cent of normal loading (149 cc. of 7.1 per cent sulfur dioxide per minute per 10 cc. of contact mass); and 98.47 per cent conversion a t 490" C. when operating at 111 per cent of normal loading (124 cc. of 6.4 per cent sulfur dioxide per minute per 10 cc. of contact mass). PHYSICAL STRUCTURE. Catalyst 1 possessed the most satisfactory physical structure of all the catalysts prepared. The physical strength of contact mass 2 was such as to make it least suitable for commercial use. Contact mass 3 was affected most adversely of all the catalysts prepared by the working temperature conditions. The physical structure of contact mass 4 was the most satisfactory of the barium-promoted catalysts, but it did not measure up to that of catalyst 1. EFFECT OF ARSENIC IN GASSTREAM.The second charge of catalyst 1 was treated with 12.61 per cent of arsenious oxide a t 480" C. which reduced its conversion from 99.16 to 95.05 per cent. The first charge of catalyst 1 was treated with 20.06 per cent of arsenious oxide at 480" C. which reduced its conversion from 99.18 to 97.24 per cent. Catalyst 2 was treated with 6.0 per cent of arsenious oxide a t 480" C. which caused its conversion capacity to fall from 98.32 to 90.17 per cent. Catalyst 3 was treated with 6.9 per cent of arsenious oxide, and the conversion fell from 98.16 to 89.00 per cent, a t 480" C. Catalyst 4 was treated with 5.7 per cent arsenious oxide at 480" C. which lowered its conversion from 98.47 to 89.00 per cent.
INDUSTRIAL AND ENGINEERING CHEMISTRY
MARCH, 1937
Literature Cited
(10) Monsanto Chemical Works (by A. 0. Jaeger and J. A. Bertsch),
(1) Adadurov, I. E.,and Guminskaya, M. A., J. Applied Chem. (U.S.S. R.),5, NO.6-7, 722-35 (1932). (2) Comey, A. M., and Hahn, Dorothy, “Dictionary of Chemical Solubilities,” 2nd ed., 1921. (3) Conidelon Soci6t6 Anonyme, British Patent 5174 (May 29,1913). (4) Fairlie, “Sulfuric Acid Manufacture,” p. 43. (5) Haen, C. J. E. de, U. S. Patent 687,834(Dec. 3, 1901). (6) Jaeger, A. O.,IND. ENQ.CREM.,21,627-32 (1929). (7) Keffer, R.,“Methods in Non-Ferrous Metallurgical Analysis,” 1928. (8) Kharmandaryan, M. O . , and Brodovitch, K. I., Ukrain. Khem. Z ~ U T8, . , NO.1, 49-57 (1933). (9) Knietsch, Ber., 34, 4069-115 (1901).
STALING
use
267
British Patent 266,007 (May 14, 1928). (11) Neumann, B.. 2. Elektrochem., 35, 42-51 (1929). (12) Neumann, B., and Juettner, H., Ibid., 36,87-96 (1930). (13) Nickell, Chem. & Met., 35, 153 (1928). (14) Phillips, Peregrine, Jr., British Patent 6096 (Sept. 14, 1831). (15) Scott, W. W.,“Standard Methods of Chemical Analysis,” New York, D. Van Nostrand GO., 1925. (16) Scott and Layfield, IND. ENQ.CHEW,23,617-20 (1931). (17) Selden Go., British Patent 314,858 (June 19, 1930). (18) Slama, Franz, and Wolf, Hans, U. S. Patent 1,371,004(March 8, 1921; reissued Sept. 21, 1934). RECEIVED June 9, 1936. Submitted in partial fulfillment of the requirementa for the degree of doctor of philosophy in the Faaulty of Chemical Engineering, Polytechnic Institute of Brooklyn.
RANCIDITY IN
ROASTED COFFEE Oxygen Absorption
by the Fat Fraction LUCIUS W. ELDER, JR. General Foods Corporation, Battle Creek, Mich.
T
HE development of stale flavor in coffee as a result of its exposure to air has been the subject of much study (2, 3, 17-22, 28). Staling has frequently been attributed to the autoxidative’ development of rancidity in the fat fraction of coffee. This is perhaps a natural assumption in view of the important role played by rancidity in the field of vegetable oil technology. However, some investigators have questioned the participation of fat oxidation in the staling of roasted coffee (17, 19, 21). Part of the confusion which has appeared in this connection is due to a loose definition of the term “rancid” and to the failure to distinguish between the properties of an unrefined fat extract and those of its component fatty acid glycerides. In the following presentation the term “rancid” is used to designate the odor and flavor characteristic of the products resulting from the autoxidation1 of the glycerides of unsaturated fatty acids. This is the sense in which the term has been most commonly used (96,27). The experimental data reported here show that the development of stale flavor in coffee is not identified with the development of rancidity in the fat fraction.
Analytical Methods Available Fatty oil technologists have developed a variety of tests to appraise the degree of oxidative deterioration which may have occurred in processing or storage. None is infallible and none is significant without prior experimental correlation with organoleptic tests. Of the more common tests the Kreis I The term ”autoxidation” is now generally accepted as a n abbreviation of the more cumbersome but more oorreot term “autocatalytic oxidation” [of. Milas, N. A., Chem. Rea., 10, 296 (1932)l.
Analytical methods commonly used for evaluating the tendency of fats t o become rancid are discussed in relation to their applicability to the fat fraction of roasted coffee. Many such tests are based on the detection of products of fatty decomposition. In the presence of the highly reactive aroma constituents of coffee fat, these tests cannot be interpreted in the conventional manner without misleading conclusions. The method involving measurement of the oxygen absorption induction period is reasonably free from such objections. On the basis of oxygen absorption measurements it is shown that the fat extract from coffee stored in air for periods up t o 13 weeks has an induction period identical with that of the fat extract from vacuum-packed coffee in spite of the fact that differences in cup quality in favor of the vacuum-packed coffee were apparent after 2 weeks. I t is concluded that the staling of coffee proceeds by a reaction or group of reactions which are not identified with the development of true rancidity in the fat fraction.
test (8, 16)) peroxide value (11, 16, 29), Schibsted fataldehyde value (24), von Fellenberg test (6), and methylene blue fading test (12, 23) which involve color comparisons or colorimetric titration are not directly applicable to the fat extract from roasted coffee on account of its deep pigmentation. The ordinary fat constants such as free acidity, iodine value, etc., are not usually employed because significant changes do not occur until some time after changes detectable by organoleptic tests are well advanced (26).
