US2431920A - Catalytic treatment of sulfurbearing hydrocarbon distillates - Google Patents

Description

Dec. 2, 1947. R. M. COLE 2,431,920

CATA-LY'I'IC TREATMENT OF SULFUR-BEARING HYDROCARBON DISTILLATES Filed D00. 21, 1944 2 Sheets-Sheet 1 Removal DcsuKurizcd Produd Desulfur'lzcd Produci Pmdud Recycle Fig. 11

lnvenl'or: Rcberi M. Cole Dec. 2, 1947. R. M. COLE 2.431.

CATALYTIC TREATMENT OF SULFUR-BEARING HYDROCAHBON DISTILLATES Filed Dec. 21, 1944 2 Sheets-Sheet 2 15 Ramova\ Rgad'or- Fd Soiurizar 3Q Tra+mnt Dasulfurizzd E Product Racgcle. Product Fig.1.

H 3 Rzmoval Frzzd Dodor Truamr Dzsufiurizad Produd Rzcgclz V Produd' Fiqfl Low 5 Fradion W Reader lnvznhar RobaH M. Cola Patented Dec. 2, 1947 caranmo murmur SULFUR- BEARING HYDROCARBON- DISTILLATES RobertMCole,LongBeach,Calif., amignorto Shell Development Oompany, San Francisco, Calif., a corporation of Delaware Application December 21, 1944, Serial No. 569,234

This invention relates to a new and improved process for the treatment of cracked gasolines, thermally reformed gasolines and similar olefinic hydrocarbon distillates obtained from high sulfur petroleums and containing more than 0.10% sulfur. The purpose and result of the treatment are (1) to effect a substantial reduction of the sulfur content, (2) to saturate and render innocuous certain highly unsaturated gum-forming constituents of such distillates and to saturate at least a part of the olefins if these are present, and, (3) to produce aromatic hydrocarbons in said materials by the catalytic dehydrogenation of naphthenic and cyclic olefin components thereof. A further purpose and result of the process may be stated to be a substantially complete desulfurization of the material in a substantially continuous treatment with excellent catalyst life under conditions affording an improvement in the anti-knock properties of the material.

Numerous processes have been suggested for the catalytic dehydrogenation, usually under hydrogen pressure. of various gasoline fractions. Such treatments, usually referredto as catalytic reforming or hydroforming, are adapted for and applied to straight run gasolines and similar materials of low octane number and usually negligible sulfur and olefin content and are effective primarily through dehydrogenation of naphthenic hydrocarbons to aromatic hydrocarbons. It is recognized that in these processes some desulfurlzation is also usually obtained. If these processes are used to treat cracked gasolines a very much smaller improvement in the antiknock properties is obtained and the catalyst life is usually very short. See, for example, U. S. Patent 2,143,078.

Also, numerous processes have been suggested for the catalytic hydrogenation of various hydrocarbon materials including gasoline at low temperatures to remove sulfur compounds. These treatments are very efilcient if applied to straight run gasolines, but straight run gasolines generally do not contain appreciable amounts of sulfur and do not require such treatment. When these latter processes are applied to cracked gasolines and similar unsaturated materials of relatively high octane number, on the other hand, they are effective for removing sulfur, but generally cause a large depreciation in the anti-knock properties of the material due to the hydrogenation of oleflns and aromatic constituents. This is morefully described in U. S. Patent 2,336,736.

At present there is no really good method for treating such materials as catalytically cracked aviation base stocks which contain considerable amounts of olefins and aromatic hydrocarbons and have relatively high octane numbers without decreasing the anti-knock properties. The catalytic treatment of such materials by the above- 2 mentioned reforming and hydroforming processes has been accomplished on a small scale but only at the expense of prohibitively short catalyst life.

This problem is particularly important since it is well known (1) that even traces or sulfur compounds in such materials have large disproportionate detrimental effects on the lead susceptibility and gum-forming tendency and (2) the commercial catalytic cracking processes using clay type catalysts do not reduce or destroy sulfur compounds very effectively and consequently yield gasolines of rather high sulfur content, for ex-v ample, 0.12% to 0.35% sulfur.

In view of the above-described state of affairs the refiners have had to resort to acid refining with its high losses or have had to resort to one of the following three alternatives: (1) Effect very limited and incomplete desulfurization by low temperature hydrogenation (to avoid prohibitive loss of anti-knock properties). (2) elect and crack only low sulfur crudes. (3) Attempt to desulfurize the crudes prior to cracking. None of these alternatives offer a real solution to the problem.

The process of the present invention involves passing the vapors of the sulfur-bearing cracked gasoline or similar distillate in the presence of hydrogen in contact with a preformed metal sulfide hydrogenation-dehydrogenation catalyst. It is desirable before describing the process of the invention infurther detail to point out a few basic facts which underlie such treatments. If a cracked gasoline or similar material containing various saturated and unsaturated hydrocarbons, aromatic hydrocarbons and sulfur compounds is treated under hydrogen pressure with a hydrogenation-dehydrogenation catalyst at a. low temperature, for example; in the order of 500-600 F. simple hydrogenation results. No dehydrogenation takes place and if the treatment is carried anywhere near to completion the anti-knock properties of the material are severely depreciated. There may be a slight selectivity in the hydrogenation action, particularly if a very weak or spent catalyst is used and the treatment is limited. Thus, in general, during the initial stages of the treatment when the concentrations are not too small, highly unsaturated materials such as dioleflns and acetylenic compounds tend to hydrogenate most readily, mercaptans and disulfldes a little less readily, tertiary oleflns still less readily, thiophenic sulfur compounds still less readily, normal oleflns still less readily, and

-' aromatic hydrocarbons still less readily. However, these dlflerences are relatively small. Furthermore, the relative amounts of sulfur compounds is usually small and if the treatment is carried out to effect a substantial hydrogenation of the sulfur compounds it is found that the ole- 3 v fins are substantially completely hydrogenated and the aromatic hydrocarbons are partly hydroequilibriums of the systems, naphthenesz aromatics, and cyclic olefinsz aromatics, are displaced toward the right so that at temperatures above about 850 F. dehydrogenation of naphthenes and cyclic olefins takes place simultaneously with hydrogenation of open chain olefins and sulfur compounds. This simultaneous dehydrogenation tends to counteract the loss of anti-knock properties due to the hydrogenation of olefins and in many cases may result in an overall improvement in the anti-knock properties. As explained above, this type of treatment is conventionally applied to straight run distillates and similar materials. However, if the treatment of cracked ga'solines and similar materials under such conditions is attempted it is found that the catalyst loses activity at a rapid rate. This type of process can therefore be carried out in an intermittent manner with an oxide catalyst which is regenerated every few hours, but generally cannot be carried out continuously with a sulfide catalyst.

It has now been found that the damaging effect of cracked gasolines and similar materials upon the catalytic activity of metal sulfide dehydrogenation catalysts, hitherto ascribed to the highly unsaturated materials such as diolefins, acetylenic compounds and certain polymerizable olefins, is

largely due to the sulfur compounds in such feeds.

It has been found (1) that the concentration of sulfur in the feed is a controlling factor, and, (2) that some sulfur compounds, and particularly mercaptan sulfur compounds, are more damaging than other sulfur compounds when present above certain minimum concentrations. Thus, it is found that whereas at lower temperatures any amount of sulfur may be tolerated in the feed, at temperatures above about 850 F. the maximum permissible sulfur concentration is close to 0.10%. Also, it is found that considerable improvement may be obtained if mercaptan sulfur is first removed or converted to a less detrimental sulfur compound.

' The process of the present invention in its broader aspect comprises treating cracked gasoline or similar material containing more than 0.10% sulfur in the vapor phase in the presence of between about 1 and 30 volumes of hydrogen at a pressure of at least 20 atmospheres with certain preformed metal sulfide hydrogenation-dehydrogenation catalysts while maintaining the temperature between about 850 F. and 910 F, and while maintaining the sulfur concentration in the hydrocarbon feed below 0.10%. The sulfur concentration in the feed may be maintained below 0.10% by any method, but is preferably maintained below 0.10% by the methods hereinafter described.

According to one embodiment of the invention the sulfur concentration in the feed is reduced to below 0.10% by treatment with a sulf-active hydrogenation catalyst at a temperature below 850 via line I. cracked gasoline fraction such for instance .as a

F. and preferably between about 500 F. and 810 F. This treatment is carried out in the vapor phase in the presence of between about 1 and 30 volumes of hydrogen at a pressure above about 10 atmospheres. v is generally very high, for example, between about 4 and 30 and is adjusted to reduce the sulfur concentration to 0.10% or below, for example 0.08%. No attempt is made to effect a complete desulfurization in this step and, in fact, the treatment is made as mild as possible consistent with the necessary sulfur removal. This embodiment of the invention is illustrated by the flow diagram given in Figure I of the accompanying drawing. Referring to Figure I the hydrocarbon feed enters This feed may be, for example, a

debutanized 300 F. endpoint aviation base stock fraction obtained by the catalytic cracking of a high sulfur petroleum oil with a clay-type cracking catalyst. Such stocks contain considerable amounts of olefins and aromatic hydrocarbons. An appreciable amount of the olefinic hydrocarbons are usually cyclic olefins such as cyclohexene. The sulfur content'of such distillates is, for example, in the order of 0.18 to 0.52% and this sulfur is largely in the form of cyclic sulfur compounds such as thiophene and its homologs. The

' feed is mixed with recycled hydrogen from line 2.

The amount of hydrogen recycled is, for example, between 1 and 30 volumes per volume of hydrocarbon feed. The mixture of hydrogen and hydrocarbon after preheating in preheater 3 passes through a catalytic reactor 4 containing the sulfactive hydrogenation catalyst. The temperaure in reactor 4 is maintained below 850 F. and preferably between about 500 F. and 750? F. The pressure is maintained above about 10 atmcspheres, for example, 500-1000 p. s. i. The liquid hourly space velocity is usually between about 4 and 30 and is adjusted to redu"e the sulfur to 0.10% or below, for example, 0.08%. Under such conditions the olefins in the feed are substantially unaffected or only a part of them are hydrogenated. The catalyst used in reactor 4 may be any one of the various suit-active catalysts generally used by the catalytic hydrogenation of sulfur-bearing feeds. Particular catalysts of this type are those comprising metals of the fifth, sixth and seventh groups of the periodic system and the iron group and zinc and copper as such and in particular in the form of their sulfides and/or oxides. These materials may be applied in various mixtures and/or if desired in con- Junction with various extender or carrier materials such as alumina, magnesia, silica, etc. Particularly suitable catalysts are, for example. cobalt thiomolybdate-alumina, tungsten-nickel sulfide, tungsten-iron sulfide, nickel sulfide-alumina, molybdenum oxide-zinc oxide-magnesia, moybdenum oxide-chromium oxide-alumina, nickelcopper-alumina.

The mixture of hydrogen and partially bydrogenated product is cooled and passed to a separator 5 or other equivalent apparatus for the separation of the hydrogen recycle gas. This gas contains hydrogen sulfide formed by the reduction of the sulfur compounds and is preferably cycled through a hydrogen sulfide removal system 6 of any of the conventional designs. Part of the hydrogen from which most of the hydrogen sulfide has been removed is recycled via lines I and 2. Fresh hydrogen may be supplied to the system via line l3.

The product from the described preliminary The liquid hourl space velocity treatment now contains 0.10% sulfur or less and a large part of the olefins. This material is then subjected to a second treatment with a metal sulfide catalyst under hydrogen pressure at a temperature in the range of 850-970 1''. Thus, the material is fed via line I, pump and heater I0 to reactor ll. Hydrogen is supplied via line II. The catalyst in reactor ll preferably comprises preformed sulfide of a metal of the iron group. Particularly suitable catalysts of this type .are cobalt thiomolybdate-alumina-silica, nickel 'thiomolybdate-alumina, tungsten sulfide, nickel sulfide and tungsten sulfide-nickel sulfide. The ratio or hydrogen to hydrocarbon may vary, for example, between 1:1 and 30:1. The pressure is 20 atmospheres or more but insufllcient to give destructive hydrogenation. Typical pressures are, for example, between about 500 and 1000 p. s. i. The liquid hourly space velocity is usually somewhat lower than in the first step and is usually in the order of 5 to 20.

The treatment in the second step is carried out substantially to completion; that is, the olefinic content is reduced to a small quantity. If desired, however, the treatment may be carried out at very high; space velocities to reduce the sulfur content to say only 0.005% while still leaving an appreciable amount of the oleflns unaffected. Under either of these conditions desulfurization can be obtained with an overall increase in antiknock properties and while operating in a continuous manner at a relatively high space velocity. Thus, instead of operating at a space velocity of 0.2 as heretofore necessary, a liquid hourly space velocity in the order of may be utilized over a period of several thousand hours of continuous operation,

An alternative mode of operation which has the advantage of requiring only one reactor system is illustrated in the flow diagram in Figure II. In this system the concentration of sulfur in the feed is maintained below 0.10% by recycling 8. part of the product. Thus, referring to Figure II the feed, for example, a thermally cracked gasoline reformed over bauxite and containing about 0.14% sulfur as thiophenic sulfur and 0.02% sulfur as mercaptan and disulfide sulfur enters via line 20. An amount of product containing, for example, 0.005% sulfur is recycled to the feed via lines 2| and 22 to lower the total sulfur content of the combined feed to 0.10% or less; for

example 85 volumes of recycle to 100 volumes of fresh feed. Recycle hydrogen in an amount between about 1 and 30 volumes per volume of combined feed is introduced via line 23. The mixture after preheating to the desired temperature is passed to reactor 24 wherein it is contacted with the hydrogenation-dehydrogenation catalyst. The temperature in reactor 24 is between 850 F. and 910 F. The pressure is at least 20 atmospheres, for example, 500-1000 p. s. i. The liquid hourly space velocity with respect to the mixed hydrocarbon feed is between about 4 and 30 but with respect to fresh feed is therefore considerably lower. The catalyst in reactor 24 may be any one of the metal sulfide hydrogenation-dehydrogenation catalysts mentioned above for the high temperature treatment. Thus, sulfides of metals of the iron group in combination with sulfldes of the metals of Group VI are particularly suitable for this application. Particularly suitable catalysts are, for example, tungsten sulfidenickel sulfide and cobalt thiomolybdate-aluminasilica. This method has the disadvantage that it requires the recycling of a large amount of product, particularly if the desulfurization is carried only to a permissible maximum such as, for example, 0.03% and this necessitates separating and recycling very large amounts of hydrogen. However, only one reactor is requiredand the method is quite advantageous in certain cases. for exampie, where the sulfur content of the charging stock is just above 0.10% and the'desulfurlzation is carried out substantially to completion.

In some cases the sulfur content of the feed maybe maintained below 0.10% very economically by utilizing the diluting effect of a low sulfur feed. Thus, for example, in some cases the sulfur concentration may be reduced by diluting the feed with a sulfur-free olefin polymer or a relatively sulfur-free straight run gasoline fractlon. In the case of the above described two step method, polymers, if added, are introduced via line I 3 prior to the first treating step whereas straight rim gasoline, if added, is introduced via line It prior to the second treating step. In the case of the above-described single step method using product recycle, the use of such diluents may decrease appreciably the amount of recycling necessary. This is particularly the case if the polymer or straight run gasoline used boils outside of the range of the high sulfur feed treated. In this case it may be separated and fresh polymer or straight run feed added via line 25 to replace part or. all of the product recycle of line 22.

In some cases where the sulfur content is, for

example, in the order of 0.11 to 0.15% it may be more desirable to operate in the following manner. The feed is first separated into a high and a low sulfur fraction containing more and less than 0.10% sulfur by extractive distillation (see, for example, U. S: Patent 2,341,812) or by other known methods. The high sulfur fraction is treated by the two step method as described above; the low sulfur fraction is recombined after the first treating step and the mixture is treated in the second treating step. Thus, referring to a Figure I the feed is first separated into the high and low sulfur fractions by means not-shown. The high sulfur fraction containing more than 0.10% sulfur is charged via line I and the low sulfur fraction containing less than 0.10% sulfur is introduced via line It.

This modification of the process of the invention is further illustrated in the fiow diagram of Figure V of the drawing. Thus, referring to Figure V, the feed stock containing more than 0.1% sulfur enters column 5| of an extractive distillation system via line and is contacted therein under extractive distillation conditions with a polar solvent entering via line 52. A low sulfur fraction containing less than 0.1% sulfur is removed overhead via line 53. The bottom product of column 5| passes via line 54 to olumn 55 wherein the polar solvent is separated from the high sulfur fraction which later passes overhead via line 56. The high sulfur fraction is given a preliminary treatment under low temperature conditions as described above and then blended with the lowsulfur fraction and the'blend treated under high temperature conditions as described above.

As explained above, it has also been found that certain sulfur compounds, and in particular mercaptan sulfur compounds, are more detrimental than other sulfur compounds in adversely effecting the activity of the catalyst when operating at temperatures above 850 F. Such sulfur com- P unds may be easily removed or converted into 7 disulfides by a variety of known methods. Thus, these materials are usually decomposed quite readily by materials of high surface such as active alumina, active bauxite, active clays, etc. The clay type cracking catalysts (usually consisting of treated clays or synthetic blends of silica and alumina) invariably have large available surfaces and decompose mercaptan sulfur compounds quite readily. The feed stocks of the type in question generally do not contain appreciable amounts of mercaptans. In such cases where they do, however (for instance thermally cracked or thermally reformed stocks), a further improvement may be obtained by first removing the mercaptans or converting them to other less harmful sulfur compounds. One method for accomplishing this is to subject the feed to a socalled solutizer treatment (such as described in U. S. Patent 2,228,295) which selectively extracts mercaptans. Another method is to pass the distillate over bauxite, over active clay, or over a metal oxide catalyst such as Luxmass, iron oxide, molybdenum oxide or the like which tends to decompose mercaptans andbecome converted to the corresponding metal sulfide. The metal sulfide so formed may then be used in the described hydrogenation step or steps or it may be regenerated to form the oxide. Still another method is to pass the distillate in the presence of a small regulated amount of an oxidant such as air or steam over clay or the like to oxidize the mercaptans to the disulfldes. In some cases the oxidation may be carried out in the wet way by.

means of a doctor treatment.

These modifications are further illustrated in 85 the fiow diagrams of Figures III and IV respectively, of the drawing. Thus, referring to Figure III, the feed containing mercaptans and more than 0.1% sulfur entering via line is first subjected to a solutizer treatment to selectively extract and remove mercaptans. The feed then passes via line 3| to a treating system wherein it is treated as described in connection with Figure I.

Referring to Figure IV, the feed containing mercaptans and more than 0.1% sulfur entering via line is first subjected to a doctor treatment to oxidize the mercaptans. The feed then passes via line H to a treating system wherein it is treated as described in connection with Figure I.

A catalyst frequently recommended for catalytic desulfurization is molybdenum sulfide. The superior life of catalysts of the type herein specifled (containing a sulfide of a metal of the iron group) as compared tomolybdenum sulfide is illustrated by the following results: A good and typical molybdenum sulfide catalyst was used for the hydrogenation-desulfurization of a third cut cracked gasoline having the following inspection conditions. The catalyst activity decline rate (expressed in per cent increase in sulfur retention per 1000 bbls. of feed treated per bbl. of catalyst) in the case of the molybdenum sulfide catalyst was 13.0; that for the nickel sulfidetungsten sulfide catalyst was less than 0.2.

The above results also illustrate the fact that when operating at temperatures below about 850 F. with catalysts of the type specified the operation may be carried out continuously for extended periods without appreciable loss of efficiency even when the feed stock contains large amounts of sulfur compounds.

A nickel sulfide-tungsten sulfide catalyst was used for the hydrogenation-desulfurization of a catalytically cracked gasoline having the following inspection data:

Boiling range F 118-324 Gravity A P. I 55.0 Bromine number 58.0 Sulfur per cent 0.17 Aromatics per cent by volume 24.5 Maleic anhydride value 24 The treating conditions were as follows:

Temperature F 850-890 Pressure p. s. i 720 Liquid hourly space velocity 6 Hydrogen rate cubic feet per bbl 6000 The life of the catalyst is expressed in terms of the number of hours of continuous processing before the removal of the sulfur drops to 90% of completion or before the saturation of olefins drops to 85% of completion (1. e., the processing hours corresponding to 10% sulfur retention or 15% olefin retention). Under the above conditions these figures are about 310 hours and 310 hours respectively.-

n The same catalyst was used under the same conditions to treat a catalytically cracked gasoline having the following inspection data:

. Boiling range F 110-330 Gravity A. P. I 52.0 Bromine number 45 Sulfur per cent 0.14 Aromatics per cent by volume 32.5

Maleic anhydride value 15 ness of the catalyst.

data:

Boiling range, F 250-370 Gravity, A. P. I 44.8 R. 1., N 1.4466 Per cent S--- 1.04 Bromine number 50.2 Aromatics, per cent by volume 16 The processing conditions were as follows:

Temperature F 700 Pressure p. s. i 700 M01 ratio of recycle gas to feed 7:1 Liquid hourly space velocity 10 A catalytically cracked gasoline similar to those described above was also treated with the same catalyst under the same conditions. However, this catalytic gasoline was blended with such an amount of hydrogenated product that the sulfur content was reduced to 0.09%. The mixed feed had the following inspection data:

Boiling range F 118-324 Gravity 56.0 Bromine number 27.0 Sulfur per cent- 0.09 Aromatics per cent by volume 29.5 Maleic anhydride value 10.5

The life of the catalyst, as expressed above was more than 1250 hours.

This result illustrates the fact that when operating with catalysts of the type specified and at temperatures of 850 F. or above, a great improvement in the catalyst life which allows the process to be converted from an essentially intermittent process to an essentially continuous process may be obtained by the simple expedient of decreasing the sulfur content of the feed stock to the process from above 0.10% to below 0.10% sulfur.

The preliminary reduction of the sulfur content to below 0.10% may be effected by dilution with desulfurized product. Also, as illustrated by other results given, such a preliminary sulfur reduc tion may be carried out with the same or a different catalyst without loss of efliciency, providing that the treatment is carried out at temperatures below 850 Rand preferably below about 810 F.

The advantage of being able to operate continuously instead of having to stop to regenerate the catalyst after each short period of use is, of course, obvious. This advantage is even more important in this particular case than might be expected because the regeneration of metal sulfide catalysts is much more time consuming and complicated than the regeneration of oxide catalysts and furthermore the sulfide catalysts tend to lose mechanical strength upon regeneration and are consequently able to withstand relatively few regeneration treatments, as compared to oxide catalysts. The advantages of being able to operate at temperatures of 850 F. or above are also important.- Thus, operation at these temperatures affords a lower hydrogen consumption, (due to the fact that additional aromatics and hydrogen are formed by the dehydrogenation of naphthenic and cyclic olefin components in the field and/or to the fact that somewhat less olefins are hydrogenated). Also operation at these temperatures generally affords products of better anti-knock characteristics (probably due to avoidance of hydrogenation of aromatics and/or to production of aromatics).

One further point regarding the described twostep process should be mentioned. As pointed out, when operating attemperatures of 850 F.-

or above, aromatic hydrocarbons in the feed are not hydrogenated as they are prone to do when operating at lower temperatures, but additional aromatic hydrocarbons may be produced in the process by dehydrogenation of naphthenes and cyclic. olefins (and possibly also by dehydrocyclization). These dehydrogenation reactions are highly endothermic. The hydrogenation of sulfur compounds and oleflns, on the other hand, is exothermic. By limiting the treatment in the first step to bring the sulfur concentration down only to just below 0.10% the -major amount of the olefins remains unchanged. Most of these oleflns are then hydrogenated in the second (high temperature) step and serve to supply the heat for the endothermic dehydrogenation reaco tions.

This allows an adiabatic reactor to'be used.

I claim as my invention:

1. A process for the improvement of cracked gasolines and similar hydrocarbon distillates containing sulfur compounds equivalent to more than 0.10% sulfur which comprises selectively extracting mercaptans from said material, continuously passing the demercaptanized material in the vapor phase in admixture with from about 1 to about 30 volumes of hydrogen through a reaction zone containing a hydrogenation-dehydrogenation sulfide of a metal of the iron group at a temperature between about 850 F. and 910 F. and at a pressure of at least 20 atmospheres insufficient to cause destructive hydrogenation, to increase the amount of aromatic hydrocarbons, decrease the amount of olefins and effect a substantial desulfurization, and maintaining the sulfur concentration in the hydrocarbon feed to said reaction zone below 0.10% by recycling a portion of the thus-treated product.

2. A process for the improvement of cracked gasolines and similar hydrocarbon distillates containing sulfur compounds equivalent to more than 0.10% sulfur which comprises selectively extracting mercaptans from said material, continuously passing .the demercaptanzed material in the vapor phase in admixture with from about 1 to about 30 volumes of hydrogen through a re- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,025,255 Taylor et a1. Dec. 24, 1935 2,167,602 ,Schulze I July 25, 1939 2,183,591- 'Schulze II Dec. 19, 1939 2,232,909 Gohr Feb. 25, 1941 2,273,298 Szayna Feb. 17, 1942 2,273,299 Szayna Feb. 17, 1942 2,315,530 Loyd Apr. 6, 1943 2,316,092 Loyd Apr. 6, 1943 2,325,034 Byrns July 27, 1943 2,370,707 Archibald Mar. 6, 1945 FOREIGN PATENTS Number Country Date 418,926 Great Britain Nov. 2, 1934

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