US2305538A - Execution of catalytic conversions in the presence of ferrous metals - Google Patents

Description

Patented Dec. 15, 1942 2,305,538 EXECUTION or oA'rALrrrc CONVERSIONS IN THE PRESENCE OF George E. Liedholm, Cole, Long Beach FERROUS METALS New York, N. Y., Robert M. and Irvin I. Shultz, Los

Angeles, Calif., assignors to Shell Development Company, San Francisco, Calif., a corporation of Delaware No Drawing. Application May 3, 1941,

- Serial No. 391,748

. 19 Claims. (01. 260683) Q The present invention relates to the catalytic conversion of organic materials in the vapor phase at elevated temperatures in the presence of ferrous metals. A particular aspect of the invention relates to the treatment of hydrocarbon vapors at elevated temperatures, i. e. from about 500 F. to 1300 F., with dehydrogenation catalysts such, in particular, as those compris ing an oxide of chromium, molybdenum, tungsten, vanadium, or manganese which are normally substantially free of iron and which are subject to poisoning by iron.

As is well known, one of the most practical and common methods for executing catalytic conversions of organic materials in the vapor phase is to provide a porous bed of catalyst in a suitable reaction tube, converter or catalyst case, and to pass the vapors of the carbonaceous reactant therethrough under appropriate conditions of temperature, pressure, etc. In most cases when treating carbonaceous materials at temperatures in the order of 500 F..to 1300 F. the catalyst gradually loses its the deposition thereon of carbon and tarry materials. This deactivation of the catalyst is temporary and may be indefinitely counteracted by periodically burning off the deposited materials in situ.

Many of the most effective catalysts for these various conversions are susceptible to poisoning by iron. I'hese various catalysts, when used for the vapor phase conversion of organic materials at these elevated temperatures in the'presence of iron, ferrous alloys, besides undergoing the above-mentioned temporary deactivation due to accumulation of carbon and ,tarry deposits, undergo a permanent deactivation due to an accumulation of iron or iron compounds formed by the oxidation or corrosion of the in contact therewith. Even traces of iron derived from various parts of the plant,

serious poisoning of these catalysts. general rule, it is found that the activity of these catalysts is decreased. approximately linearly from 80% of their initial activity with an iron content of 0.7% to 40% when the iron content is increased to 1.3%. This deactivation of the catalyst, unlike that due to the deposition of tarry materials, is permanent and cannot be counteracted by any of the known regeneration processes. Since these various conversions are often executed under pressure and since, furthermore, they generally require the introduction activity due to v or withdrawal of considerable quantities of heat from the reaction zone, usually through the confining walls, it is usually impractical in these processes to employ apparatus constructed of or lined with non-ferrous materials such as silicon, ceramic materials, and the like, and the use of iron, steel or ferrous alloy equipment is practi- 'of chromium.

cally unavoidable. i

The industry has searched diligently for some practical method whereby the transfer of iron to these catalysts could be avoided, while at the same time utilizing apparatus fabricated from ferrous metals. It has, for example, been proposed to employ apparatus plated with chromium, aluminum, copper, etc. Such apparatus sometimes works well for a short time. It is found, however, that these various linings invariably have minute pin holes or become scratched by the catalyst, and after a shortperiod of operation the under-metal is attacked through these imperfections with the result that the lining becomes pitted or peels, and iron is transferred to the catalyst. The contamination of these catalysts by iron has also been minimized to a considerable extent by the use of certain alloy steels such, in particular, as the more or less corrosion-resistant steels containing chromium, molybdenum, vanadium, titanium, etc. It is found, for example, that whereas iron and plain carbon steels are practically useless, the tendency to contaminate the catalyst with iron is considerably-decreased by the incorporation However, even chrome steel containing 27% chromium, which is the highest chrome steel that can be fabricated, has not proven entirely satisfactory for the purpose. The.

reaction vessels fabricated from this material.

are both expensive and short-lived. After a relatively short period of use, they suddenly begin to contaminate the catalyst with iron and must be discarded. It has also been proposed to employ ferrous metals pretreated with hydrogen constructed of the more common, less expensive-- perature of 11409 steels and ferrous alloys such as the common KAZ, KA2S, KA2M0T, KAZCb, KAZMo, and KA2ST steels, while at the same time avoiding all substantial contamination of the catalyst by iron. According to the method of our invention,

this is effected by maintaining certain specific concentrations of sulfur in the reaction zone.

It is generally known that when treating organic vapors at these elevated temperatures with these catalysts in the presence of ferrous metals in the absence of any substantial amount of sulfur, corrosion of the ferrous metal and iron poisoning of the catalyst takes place. It is also generally known that the corrosion and iron poisoning are usually increased by the presence of sulfur in the feed. We have found by careful investigation, however, that the types and characters of the corrosions occurring with sulfur-free feeds and with sulfur-bearing feeds are difierent and that there is 'a region corresponding approximately to the transition of one type of corrosion to the other where corrosion of the ferrous metal and. iron transfer to the catalyst are practically negligible. This region where corrosion and iron transfer practically do not take place corresponds to a low, specific and narrow range of sulfur concentrations. If the concentration of sulfur is increased beyond this range corrosion is greatly increased and. large quantities of iron are transferred to the catalyst, and on the other hand, if the concentration of sulfur is reduced beyond this range, corrosion of the other type and iron transfer are again increased.

The existence of this region of minimum iron transfer to the catalyst may be easily seen from These .ex-

the following illustrative examples. amples, for the sake of comparison, all relates to the dehydrogen of propane vapors at a tem- F'. at a space velocity of 35 per minute and process period of 40 minutes. The catalyst in each case was an 8-14 mesh activated alumina impregnated with chromium oxide (11% Cr). The fresh catalyst contained 0.025% iron. The reaction in each experiment was executed in a new sand-blasted KAIZS steel reaction tube.

Example I Propane vapors substantially free of sulfur were treated under the above-described conditions. During only about 18 (and 38) process cycles the catalyst became contaminated with about 0.26% (and 0.35%) iron. In other words, in check runs during 19 and 38 process periods iron was transferred from the M25 steel reaction tube to the catalyst to the extent of about 0.24% (and 0.35%) by weight of the catalyst. It is thus seen that in the absence of sulfur, contamination of the catalyst by Example If rial was treated under the conditions described iron is considerable.

above for 200 process cycles. During this time the conversion to propylene declined from 33.5% to 29.5%. The catalyst at the end of the 200 process cycles was not noticeably changed in iron content (initial iron content 0.025%). It is thus seen that in the presence of about 0.006% sulfur in the form of carbonyl sulfide, iron poisoning of the catalyst is almost completely eliminated and excellent conversions are obtained.

Example IV A similar set of experiments was made in which dimethylsulfide was added to the propane to provide the desired sulfur content. The following iron contents of the catalysts after 200 process adding various amounts of carbon disulfide, thiophene, sulfur trioxide, sulfur dioxide, ethyl mercaptan and elemental sulfur. In each case, it was found that within a limited range of sulfur concentrations the contamination of the catalyst by iron was substantially avoided whereas at both lower and high concentrations considerable iron was transferred to the catalyst. The most efiective concentration. of sulfur varied somewhat with the individual sulfur compound but was, in general, between about 0.0002% and 0.05%. Preferred limits of sulfur concentrations for individual types of sulfur compounds were found to be:

S =0.0006-0.015 COS =0.0006-0.05 Thiophene =0.0006-0.05 R-SR =0.005-0.038 R-S-H =0.0005-0.003 S03 =0.002-0.025 S02 =0.002-0.015 cs =0.002-0.02

One preferred group of sulfur compounds comprises volatile oxygenated sulfur compounds, i. e. those having at least one oxygen atom attached directly to the sulfur. Particular sulfur compounds of this class are COS nd S03. The use of appropriate quantities of sulfur compounds of this class to prevent contamination of these catalysts with iron is described and claimed in our copending application, Serial No. 356,036, filed September 9, 1940, now patent Ser. No. 2,269,028 of which the present application is a continuation-in-part. According to the process of the present invention, the transfer of iron to the catalyst is prevented by the use of specific quantities of reducible sulfur, i. e. elemental sulfur or a vaporizable sulfur compound other than hydrogen sulfide. Hydrogen sulfide, we have found, is relatively ineffective and is not equivalent to sulfur and reducible sulfur compounds for the present purpose. Hydrogen sulfide, even when employed at its optimum concentration, is not capable of reducing iron transfer to the catalyst below about 0.11% (under the above examples). Furthermore, even this poor inhibiting effect is only possible with an extremely narrow range of optimum concentrations. Thus, in order to retain the iron transfer in the the conditions of simplified.

Thus, if the concentration of hydrogen sulfide is allowed to go outside of the narrow range of 0.002% to 0.005%, iron transfer to the catalyst is concentration of hydrogen sulfide from the opti-.

mum of 0.003% to 0.0015% (a change of only 0.0015%), iron transfer is increased from 0.11% to about 0.9%. In practical application where it is difllcult to maintain the concentration within flde usually does not decrease iron poisoning of the catalyst but increases it.

Examples of sulfur compounds used according compounds is claimed in the above-mentioned cpending application. Of the available reducible non-oxygenated sulfur compounds, preferred agents are elementa sulfur and thiophenic com- P unds. I

Elemental sulfur, we have found, has the following. advantages and is a particularly desir able agent: I

1. Elemental sulfur-is very efficient and when used in optimum amounts substantially completely obviates iron poisoning.

2. The effect-concentration curve for elemental sulfur in the optimum concentration range is very flat, affording a relatively wide range of nearly optimum concentrations. Thus, when employing elemental sulfur a concentrationwithin the desired optimum range may be easily maintained in large commercial scale operation.

3. Elemental sulfur, it is found, is quite stable under the reaction conditions and has substantially no tendency to react with and sulfurize the product.

4. When elemental sulfur is employed the problem of removing sulfur from Thus, by simply with a caustic stantially completely removed.

Thiophene and its homologues, i. e. benzenoid compounds containing the characteristic grouping, =CH-SH=CH, are also preferred nonoxygenated sulfur compounds. These compounds are exceedingly effective and'can be used in a fairly wide range of concentrations. pounds are also very stable conditions.

The least desirable of the applicable reducible sulfur compounds are the mercaptans. Although mercaptans are fairly effective when used in optimum concentrations, the range of applicable concentrations is quite narrow and more difficult to maintain in practical application. Concentra- ;ions outside of the narrow applicable range neatly increase-iron These comunder the reaction transfer and are quite detrimental. This is believed to be due to the fact that mercaptans are partly converted to the undesirable hydrogen sulfide under the reaction conditions.

Other metalloid compounds such as the corresponding reducible selenium and tellurium comconsidered ferrous metal and causes the transfer of iron to the catalyst. The process is also most advantageous for such of these processes as are endothermic since in these processes the m tal-confining of naphthenes to the corresponding aromatic hydrocarbons, the catalytic cyclization of hexane and its higher homologues, the catalytic reforming and/or hydroforming of gasoline stocks, and the like.

The process of the invention is applicable with all solid regenerative catalysts which are affected by contamination with iron and are employed at temperatures above about 500 F. with organic As examples of such catalysts there may be mentioned by way of illustration the many tificial aluminum silicates, and the like.

gel, bauxite, ar-

denum oxide, and/or tungsten oxide and alumina. These catalysts are exceptionally suited for the dehydrogenation of hydrocarbons, the reforming of gasolines and the cyclization of aliphatic hydrocarbons, but suffer considerably from iron contamination when used in ferrous metal equipment. The described catalysts are not generally detrimentally afiected in their activities by sulfur. Obviously, such catalysts as nickel catalysts, platinum catalysts and the like which are poisoned by sulfur are not applicable in the present process.

The contamination of the catalyst by iron in the execution of these various reactions may be inhibited, according to the present process, in the presence of any of the steels and common ferrous and mild steel are not usually employed in these processes due to their lesser ability to withstand the more or less severe Particularly suitable materials which KAZS, KA2ST, KA2Cb, In the past these otherwise excellent steels have not been found entirely suitable due to their tendency to cause iron contamination of the catalyst after a short period of use and the industry has been forced to go to the more expensive and difficultly workable high-chrome steel such as 27-Cr, etc. These latter materials, although definitely superior to most low-chrome steels, are nevertheless far from satisfactory and cause iron contamination after a relatively short time. By utilizing the process of the present invention, any of these steels may be used indefinitely without any appreciable tendency to cause from. contamination of the catalyst. Particu larly excellent results are also obtained using nitrogenor aluminum-stabilized high-chrome steels. For example, a particularly excellent steel has the following composition:

Per cent C 0.14 Mn 0.40 S 0.012 P 0.021 Si 0.38 Cr 26.78 Ni Trace N2 0.17

Many of the organic materials serving as reactants in the processes in which the present invention may-be employed normally contain small to appreciable amounts of sulfur compounds. It will be. appreciated, in view of the above, that in order to realize the advantages of the present method the sulfur contents of these materials must be adjusted prior to treatment to. within the narrow limits of sulfur concentrations specified above. In such cases where the sulfur content is normally too low, it is merely necessary to add the required amounts of sulfur to bring the concentration within the specified limits. In such cases where the sulfur content is too high, as is often the case when treating petroleum fractions, the excess sulfur should be removed. This may be done by any of the conventional desulfurization treatments. Also, besides adjusting the concentration of sulfur to within the desired limitsit is desirable to take into consideration the type of sulfur compound present. In most cases it is found that sulfur-containing feed stocks for such processes contain the sulfur in the form of hydrogen s lfide and/or mercaptans. As pointed out above, mercaptans are the least 'pors entering aeoasse desirable of the applicable sulfur compounds and hydrogen sulfide is usually quite detrimental. According to the preferred embodiment of the invention, these sulfur compounds are substantially removed and the correct amount of a preferred type of sulfur, such as carbonyl sulfide, elemental sulfur or a thiophenic sulfur compound, is added.

Although the sulfur or sulfur compound may be added directly to the reaction zone or by any other convenient method, the most convenient and practical method for introducing the desired sulfur or sulfur compound info the reaction zone is in admixture with the reactant vapors. Thus, for instance, the correct amount of a desirable form of sulfur may be dissolved in the feed and the whole vaporized and passed into the reaction zone. or vapors of the sulfur compound may be mixed with the preheated reactant vathe reaction zone. In certain cases a convenient means for dosing the desired sulfur into the feed is to vaporize the feed and pass the vapors through a bath or solution of the sulfur compound whereupon sulfur is taken up in proportion to its vapor pressure.

The following examples illustrate the excellent resins that may be obtained, according to the above-described process:

Example V A.- propane fraction was desulfurized and then sufficient elemental sulfur was added to bring the sulfur concentration to 0.002%. The material was then dehydrogenated under the conditions uct with a 4% caustic solution the added sulfur was completely removed in a very simple manner.

Even a 0.5% solution of caustic is quite effective in removing the added sulfur.

Example VI A straight run propane fraction was desulfurized and then sufilcient elemental sulfur was added to portions to being the concentration up to 0.0045% and 0.006%. These portions were then dehydrogenated in new KAZS steel tubes under the same conditions as in the preceding examples. During 200 process cycles the iron transfer to the catalyst was in both cases not more than 0.015%.

Example VII A straight run propane fraction was desulfurized (sulfur content, not more than 0.0000%). The sulfur-free material was then passed in the vapor phase through a bath of molten sulfur maintained at 330 F. and then through a filter to remove any entrained sulfur particles. The resultant vapors having a sulfur content of 0.007'l% were then dehydrogenated under the same conditions as in the preceding examples in a 2'7-chrome steel tube (containing 0.14% Na). At the end of 600 process cycles the conversion was still 26.3% and the catalyst was found to contain only 0.04% iron. No hydrogen sulfide was formed in the product.

Example VIII A straight run propane fraction containing appreciable quantities of hydrogen sulfide was pretreated to remove substantially all of the sulfur still 28.7%.

Thiophene was then added to bring the sulfur concentration to 0.002%. This material was then dehydrogenated in a new KAZ ST steel tube under the conditions described in the foregoing examples. After 200 process cycles the conversion was The catalyst at the end of 200 cycles contained only 0.05% iron. I

Example IX Example X Ethyl mercaptan was added to a propane fraction to make the concentration of sulfur 0.000'%. The propane was then dehydrogenated under the same conditions in a new KA2S steel tube. After 200 process cycles the iron content of the catalyst was 0.07% and the conversion was 30.3%.

Example Xi A commercial propane fraction containing mercaptans was adjusted to a sulfur concentration of 0.007%, of which 0.0005% was mercaptan sulfur and the remainder was carbonyl sulfide. This material was dehydrogenated under the conditions described above. During about 1000 process cycles not more than 0.022% iron was transferred to the catalyst and the conversion gradually declined from 32% to only 26%. if the 0.0005% mercaptan sulfur is replaced by carbonyl sulfide sulfur, no increase in the iron contamination takes place and the conversions are increased about 7% of the above values.

Etrample XII A propane fraction was adjusted to contain 0.002% sulfur as carbonyl sulfide and was then dehydrogenated under the conditions described above. During about 1000 process cycles not more than 0.02% iron was transferred to the catalyst and the conversions gradually declined from 33%-34% to only 27.5%.

We claim as our invention:

1. In a process for the conversion-of carbonaceous materials in the vapor phase at an elevated temperature at least equal to 500 F. wherein the reactant vapors are contacted in the presence of a ferrous metal with a sulf-active catalyst susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of vapors of elemental sulfur.

2. In a process for the conversion of carbonaceous material in the vapor phase at an elevated temperature at least equal to 500" F. wherein the reactant vapors are contacted in the presence of a ferrous metal with a sulf-active catalyst susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of in susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the reaction zone from 0.0006% to 0.038% sulfur in the form of vapors of a dialkyl sulfide.

4. In a process for the conversion of carbona-ceous materials in the vapor phase at an elevated temperature at least equal to 500 F. wherein the reactant vapors are contacted in the presence of a ferrous metal with a supported chromium oxide catalyst and the catalyst is periodically regenerated by oxidation of ceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of vapors of ,a reducible non-oxygenated sulfur compound;

5. In a process for th conversion of carbonaceous materials in the vapor phase at an elevated temperature at least equal to 500 F. wherein the reactant vapors are contacted in the presence of a ferrous metal with a sulf-active dehydrogenation catalyst susceptible to iron pois cning and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of th catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of vapors of a reducible non-oxygenated sulfur compound.

6. In a process for the conversion of carbonaceous materials in the vapor phase at an elevated temperature at least equal to 500 F. in a chrome steel reactor with the aid of a sulf-active catalyst susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of vapors of a reducible non-oxygenated sulfur compound. 1

7. In a process for the catalytic treatment of a sulfur-bearing petroleum fraction at an elevated temperature at least equal to 500 F. wherein the hydrocarbon vapors are contacted in the presence of a ferrous metal with a sulf-active catalyst susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of preventing contamination of the catalyst by iron which comprises desulfurizing the hydrocarbon feed and then subjecting the desulfuriz'ed feed to a catalytic treatment in the vapor phase in admixture with from 0.0006% to 0.015% sulfur in the form of vapors of an added reducible nonoxygenated sulfur compound.

8. In a process for the catalytic treatment of a sulfur-bearing petroleum fraction at an elevated temperature at least equal to 500 F. wherein the hydrocarbon vapors are contacted in the presence of a'ferrous metal with a, sulf-activ catafeed to the.

carbonais periodically regenerated by lyst susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of preventing contamination of the catalyst by iron which comprises desulfurizing the hydrocarbon feed and then subjectin the desulfurized feed to a catalytic treatment in the vapor phase in admixture with from 0.0006% to 0.015% of an added reducible sulfur compound.

9. In a process for the dehydrogenation of propane in the vapor phase at an elevated temperature at least equal to 750 F. wherein propane vapo-rs are contacted in the presence of a ferrous metal with a.sulf-active dehydrogenation catalyst susceptible to iron poisoning and the catalyst oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the propane feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of vapors of a reducible sulfur compound.

10. In a process for the dehydrogenation of butane in the vapor phase at an elevated temperature at least equal to 750 F. wherein butane vapors are contacted in the presence of a ferrous metal with a sulf-active dehydrogenation cata lyst susceptible to iron poisoning and the catalyst is periodically regenerated by oxidation 01 carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the butane feed. to the reaction zone from 0.0006% to 0.015% sulfur in' the form of vapors of a reducible sulfur compound. Y

11. In a process for the conversion of carbonaceous materials in the vapor phase at an elevated temperature at least equal to 500 F. wherein the reactant vapors are contacted with a supported chromium oxide catalyst in the presence of a ferrous metal and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from a 0.0006% to 0.015% sulfur in the form of vapors of a reducible sulfur compound.

, 12. In a process for the conversion of carbonaceous materials in the vapor phase at an elevated temperature at least equal to 500 F. wherein the reactant vapors are contacted in the presence of .a ferrous metal with a sulf-active dehydrogenation catalyst susceptible to iron poisoning and the of carbonaceous deposits therefrom, the step of inhibiting the contamination of th catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of'vapors of a reducible sulfur compound.

14. In a process for the catalytic reforming of a petroleum distillate wherein distillate vapors are contacted in the presence of a ferrous metal accuses with a sulf-aotive catalyst susceptible to ccntam ination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0006% to 0.015% sulfur in the form of vapors of a reducible sulfur compound.

15. In a process for the catalytic dehydrogenation of carbonaceous materials in the vapor phase at an elevated temperature at least equal to 500 F. wherein the reactant vapors are contacted in the presence of a ferrous metal with a sulf-active catalyst susceptible to contamination by iron and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, e step of inhibiting the contamination of the cat lyst by iron which comprises maintaining in the r actant vapors in the reaction zone during the con ersion from 0.0002% to 0.05% of reducible sulfur.

16. In a process for the dehydrogenation of butane in the vapor phase at an elevated temperature at least equal to 750 F. wherein butane vapors are contacted in the presence of a ferrous metal with a chromium oxide dehydrogenation catalyst and the catalyst is periodically regenerated by oxidation of carbonaceous deposits'therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0002% to 0.05% sulfur in .the form of vapors of elemental sulfur.

17. In a process for the dehydrogenation of butane in the vapor phase at an elevated temperature at least equal to 7 50 F. wherein butane vapors are contacted in a chrom steel reactor with a chromium oxide dehydrogenation catalyst and the catalyst is periodically regenerated in situ by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by ironwhich comprises substantially freeing the butane to be treated of any naturally occurring sulfur compounds and maintaining in the feed to the reaction zone from 0.0006% to 0.05% of added sulfur in the form of vapors of elemental sulfur.

18. In a process for the dehydrogenation of butane in the vapor phase at an elevated temperature at least equal to 750 F. wherein butane vapors are contacted in the presence of a ferrous metal with a chromium oxide dehydrogenation catalyst and the catalyst is periodically regenerated by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises maintaining in the feed to the reaction zone from 0.0002% to 0.05% sulfur in the form of vapors of thiophene.

19. In a process for the dehydrogenation of butane in the vapor phase at an elevated temperature at least equal to 750 F. wherein butane vapors are contacted in a chrome steel reactor with a chromium oxide dehydrogenation catalyst and the catalyst is periodically regenerated in situ by oxidation of carbonaceous deposits therefrom, the step of inhibiting the contamination of the catalyst by iron which comprises substantially freeing the butane to be treated of any naturally occurring sulfur compounds and maintaining in the feed to the reaction zone from 0.0006% to 0.05% of added sulfur in the form of vapors of thiophene.

- GEORGE E. LIEDHOLM.

ROBERT M. COLE.

' IRVING I; SHULTZ.