US2636843A - Cracked naphtha desulfurization - Google Patents

Description

April 28, 1953 R. c. ARNOLD ETAL 2,636,843

CRACKED NAPHTHA DESULFURIZATION Filed Oct. 9, 1950 CAT. CRACKED NAPHTHA.279%S TOPPING 25% OvERI-IEAD .I5%s

(DIsT-I BOILING BELOW 270'F.

75% BOTTOMS'.3l5%S I BOILING ABOvE 270F.

ACID TREAT E IOI5 /BBL 2.7% (2.0%) SLUDGE I H2804 9OIOO%, 98%

3* 97.3% (73.0%) TREATED OIL RERuN 94.7% (69%) OvERHEAD.O?e%s (.DIST.) BOILING BELOW 395E 5.3% (4%) BOTTOMS- 3.3%s

HYDRODESULFURIZE SULFACTIVE CAT.

700 --950 F. H28 2oOI5OO I=. s.I.

.sI5 V/HR/V 1 H2 PRESENT 92% (3.6%) LIQ. PROD- .75%s

FRACTI ONATE 32% (I.9%I OVERHEAD .2e%s

(DIsTI I I LINE 48%(I.T%I BOTTOMS I.3%s g s (FURNACE OIL) I INvENTORs;

ROBERT C. ARNOLD ARTHUR B. LIEN JOHN F. TERs BY: M A *BASED ON ORIG. CHG.

ATTORNEY:

Patented Apr. 28, 1953 ascetic I caaonnn NAPHTHA DESULFURIZATIUN Robert C. Arnold, Park Forest, 111., Arthur 1?. Lien, Highland, and John F. Deters, Valparaiso, ind, assignors to Standard Oil Company, Chicago, l[ll., a corporation of Indiana Application October 9, 1950, Serial No. 189g154 5 (Claims. 1

This invention relates to the desulfurization of cracked naphthas and it pertains more particularly to an improved combination process for refining a relatively wide boiling range cracked naphtha which is characterized by a high olefin content.

Various processes have been proposed for the desulfurization of cracked naphthas of the gasoline boiling range, but heretofore such processes have always been subject to severe disadvantages. Acid treating such naphthas results in undesirable olefin polymerization and/or such reactions as alkylation, condensation, etc., and it has heretofore resulted in relatively large losses to sludge. Solvent extraction processes result in removal of olefins and aromatics as well as sulfur compounds, create an extract disposal problem and usually are characterized by undesirably low yields and often by loss in octane number. I-Iydrodesulfurization (i. e. hydroforming, hydrofining, etc.) effects lowering of sulfur contents with only small treating losses but requires very expensive equipment as well as high operating costs; furthermore, hydrodesulfurization processes are subject to disability of effecting olefin saturation which not only consumes valuable hydrogen but lowers the octane number value of the resulting product. The object of our invention is to provide a relatively inexpensive process Which will avoid the objectionable features of prior art processes and which will at the same time result in the production of maximum yields of low sulfur gasoline without impairing octane number.

A further object is to provide an improved topping-acid treating-topping process which will minimize treating losses, maximize yields and produce a product which meets strict sulfur specifications without suffering appreciable octane number loss. An important object is to alter the chemical composition of the sulfur compounds in the acid treating step so that most of the sulfur compounds can be removed from acid treated oil in a simple distillation (topping) or rerun step and concentrated in a bottoms fraction which constitutes only about 5% of the initial cracked naphtha and which provides a unique and advantageous charging stock for hydrodesulfurization by processes such as hydroforming or hydrofining. Still another object is to convert these rerun bottoms for markedly increasing the ultimate yield of desulfurized gasoline and for producing a by-product suitable for use as furnace oil. Other objects will be apparent as the detailed description of the invention proceeds.

Our invention is particularly applicable to the desulfurization of catalytically cracked naphthas but it .is also applicable to thermally cracked naphthas, such, for example, as coke still naphtha, which is one of the most difficult to desulfurize. We first subject the high sulfur cracked naphtha charging stock to a topping distillation step and take overhead components boiling below about 250 to 300 F., the preferred cut point being at 270 F.; we have found that the overhead fraction from the topping step is particular- 1y rich in olefins but contains less than half the amount of sulfur that is contained in an equivalent amount of the higher boiling bottoms from the topping step.

Next we acid treat the bottoms from the topping step, preferably employing about to pounds per barrel of a strong acid, such as 90 to 100%, e. g. 98%, sulfuric acid. The acid treating is preferably at a controlled low temperature in the range of about to F., although advantageous results are obtainable with treating temperatures as high as to F. The sludge loss in such an operation amounts only to about 1 to 3% based on the initial cracked naphtha,

this loss being minimized by the absence of olefins and other low boiling components removed in the initial topping step. The acid treated oil may be neutralized and/or water washed, in the conventional manner, and this oil (which constitutes about "73% of the original charging stock) is rerun in a second topping step to take overhead all gasoline boiling range components and leave as bottoms only components which are higher boiling than gasoline. The overhead from this rerunning step may constitute about 69 or 70% of the original charging stock and it is characterized by a sulfur content of only about 07% so that when it is blended with the overhead from the first topping step a 94: weight per cent yield of gasoline is obtainable with a sulfur content of less than .1% and with an octane number which is substantially the same as that of the original charging stock.

The bottoms from the rerun or second topping step may amount to about 4 to 5% of the original charging stock and may contain 3 to 4% of sulfur. Such bottoms are Very different in chemical composition from extracts obtained by use of selective solvents since the nature of the sulfur compounds has been radically changed in the acid treating step. These rerun bottoms are subjected to hydrodesulfurization, for example, to hydrofining by contact with cobalt-molybdatecan-alumina catalyst at a pressure of about 1500 p. s. i. in the presence of added hydrogen. We

components.

find that the products produced are not of the same approximate boiling range as the charge to the hydrofining step (which is usually the case in hydrofining operations) but on the contrary contains about 52% of components boiling in the gasoline boiling range and containing only about 26% sulfur. By subjecting only about 4 or 5% of the original charging stock to hydrofining conditions, which can be done in very small equipment and with only a small amount of added hydrogen, we can obtain about a 2% increase in gasoline production and at the same time obtain a valuable by-product oil. Thus, by our unique combination of treating steps we are able to obtain from cracked naphthas containing .2% to 3% sulfur a finished gasoline containing less than .1% sulfur and with substantially no change in octane number. We are able to obtain yields of about 96 weight per cent (even greater on a volume basis) along with a valuable by-product oil and weare able to obtain this phenomenal result in relatively inexpensive equipment and at low cost since the volume of the hydrodesulfurization charge is only about 5% of the volume of the initial high sulfur cracked naphtha.

The invention will be more clearly understood from the following detailed description of specific examples read in conjunction with the accompanying drawing, which is a schematic flow diagram illustrating our improved sequence of steps.

Referring first to the process illustrated in the drawing, we will describe results obtained by treating a high sulfur, stabilized, catalytically cracked naphtha having an initial boiling point in the range of 150 to 175 F. and an end point of approximately 400 F., said naphtha having a sulfur content of 279% by weight. We first top this charge stock by distillation in any conventional manner to remove an overhead stream containing components boiling below 270 F.; in. this case the overhead stream constituted about 25% of the initial charge, was

'highly olefinic and contained only about sulfur. It is important that the cut point in this initial topping operation be in the range of about 250 to about 300 F. and for,most stabilized cracked naphthas, atopping at this out point gives an overhead amounting to about to 50% of the initial charge, best resultsbeing obtainable with a cut point of approximately .perature in the range of about 40 to 100 F..

preferably in the range of 40 to 60 F., followed by the conventional sludge separation, water washing and neutralization steps, there was a loss of hydrocarbon in the acid sludge corresponding to 2.7% by weight based on the charge to the acid treating step (2% by weight based on initial charge). This acid treated stream is then rerun in a second topping distillation step to remove overhead all gasoline boiling range The cut point will depend on particular gasoline specifications and is usually approximately 400 F., but in this example the cut point was at 395 F. and the overhead stream, which constituted 94.7% of the material charged to the rerun step or about 69% of the initial charge, was characterized by a sulfur content of only 076%. When this stream was blended with the overhead stream from the first topping step, a 94 weight per cent gasoline yield was obtained and the product was characterized by a sulfur content of only 094%, its clear and leaded octane numbers being substantially the same as those of the original charging stock. While the yield and quality of gasoline thus far produced are remarkably good, we have discovered that the ultimate gasoline yield can be still further enhanced by further treatment of the bottoms from the rerun steps which constitute about 5.3% of the material charged to the rerun step or 4% of the initial charge and which is characterized by a sulfur content of about 3.3%, a refractive index m of 1.5105 and a density of 0.926.

When this relatively small quantity of rerun bottoms was subjected to conventional hydrofining conditions, i. e. contacted with a cobaltmolybdate-on-alumina catalyst at a temperature in the range of 700-800 F., at a pressure in the range of 1300-1500 p. s. i., in the presence of added hydrogen with a reaction time of about 4 hours (test conditions to duplicate as closely as possible results that would be obtained in a commercial hydrofining operation), we obtained a 92 volume per cent yield of a liquid product stream which contained only 375% sulfur, had a refractive index (11 of 1.4800 and a density of 0.345. This product stream constituted in this case 3.6% by weight of the total stock charged to the first topping operation. When the liquid stream from the hydrofining step was distilled, it was found that 52% of this stream constituted components in the gasoline boiling range, had an initial of 95 F., a 50% point of 266 F. and an end point of 401 F.

This gasoline material was characterized by a sulfur content of only 26%, a refractive index (72, of 1.3990 and a density of 0.778. Ordinary charging stocks which are subjected to hydrofining conditions are not materially a1- tered in boiling range during the hydrofining step, but in this case more than half of the stantially the same as that of the initial high sulfur cracked naphtha. In addition to this remarkable gasoline yield, we obtain as bottoms from the last distillation step an oil which contains less than about 1.3% sulfur and which can thus be employed as a valuable component in furnace oils or other marketable products.

The hydrodesulfurization step when applied to the acid treating rerun bottoms gives results which are entirely and radically different from results obtainable when ordinary solvent extraction extracts are subjected to hydrodesulfurization because such extracts do not give a marked change in product boiling range. In the specific example hereinabove described, the hydrodesulfurization was effected with cobalt molybdate catalyst, but it should be understood that it may be effected by use of other known hydrofining or hydroforming catalysts such as molybdena on aaluminacoraother nth. roup metaloxides or sul- .fides on, alumina either'ralone' or ino njlm tlfl with-an oxide -orssulfide.of-znickel :or: cobalt. This class of catalysts is commonly known in. art

asflsulf active hymcgenation (or dehydrogenation) catalysts. alysts cobalt molubclate or nickel tungstate (suppontedand: in the. form of sulfides :as well as For hydrofining with such-oatoxides'), theltreatment is preferably at a tempera.- tureiinthe range :of: about 650 110 .809" unclera pressure in the range of about 260 to 2000 p. s. i., usually. 1500 to 1580 p. s. i., with a space velocity oftheiorder. of .2 to l5,,e. g. about :5 volumes of liuuidpharging.stockper' hour volume-of cat-- alyst; with a gas recyclerate ofcaboutrlmlo to 5900 7 the gas recycle rate being substantially the same in both cases and the pressure being in the range of'ZOU to 500p. s. i. It should be understood, of course, that by using increased amounts of hydrogen and increasing the. severity of the treatment, the desul'fur'ization may be to a much greater extent than set forth in the above described example.

While in the example hereinabove described we employed catalytically cracked high sulfur naphtha having a sulfur content of approximately 3%, the invention is also applicable to thermally cracked naphtha such as the coke still naphtha which is produced by thermally cracking high sulfur gas oil and/or reduced crude, at a temperature of about 900 to 950 under a pressure of about atmospheric to 109 p. s. i. with such holding time that solid coke is formed. The naphtha fraction of the overhead stream from a coking operation may have a sulfur content of about .6 to 37%. When such a coke still naphtha is topped to about 25%, the overhead stream in one run was found to contain 28% sulfur while the bottoms contained 379% sulfur. When these bottoms were treated with 98% sulfuric acid (12 pounds per barrel), a treating loss to sludge of 2.3% was obtained and the acid treated oil before rerunning had a sulfur content of 58%. The overhead from the rerunning step had a sulfur content of while the bottoms from the rerunning step had a sulfur content of 3.2%. The rerun bottoms in this case may be hydrodesulfurized as hereinabove described to augment the total gasoline recovery. In the case of coke still naphtha, the desulfurization is not nearly as complete as it is in the case of catalytically cracked naphtha and the resulting product does not meet strict sulfur specifications. While our invention may thus be advantageously employed with coke still naphtha, it will be apparent that coke still naphtha is by no means the equivalent of catalytically cracked naphtha in our process and for best results our process should be employed for the desulfurization of catalytically cracked naphtha.

In the acid treating step, we prefer to employ sulfuric acid at about 90 to 1.00% acid strength, preferably about 98%. The amount of acid should be of the order of about to 15 pounds of acid per barrel of initial bottoms charged to the :acid. treating: step. Sludger'losses are: minimized in. the. acidi treatingrbyr holding: the.;treating temperature.intherange; of; aboutzeorto 56.05" While sulfuric acid gives: excellent results. it should. be: understood that. other. strong" acids, suchas hydrogen fluoride-may: be employed under conditions to actually react witlncomponents of the charge. and to effect the'conuersion' of at leastabout3 to 5% zofithecharge:intornaterials higher. boiling than. gasoline. .Such. materials serve as. a charging stoiclrifor. our hydrodesulfurization step.

lnanother. example ofpur inventicniasapplied to. a; high sulfur catalytically' cracked. naphtha,

of which the initial cut point in the first toppin step was. 270.? 7.5%. bottoms. from the first topping step. (boilin in. the range. of 270 to 395 F.) weretreated. with. 12 pounds of 98% sulfuricacid at a temperature ofiabout. 5'5 and the acid treated oil was rerun to produce 395F. end point gasoline. In this case, we. obtained inthe rerunning step a gasoline yield of 94 weight vper....cent of the. materials charged to said acid treating anda. desulfurization of. 81% basedon said material. Basedon total cracked napht the gasoline yieldpriorto'the final hydrodesulfurization step was 95.5 weight per cent of a gasoline containing only 118%. sulfur, the sludge loss in this case being only 1.8 weight per cent and the rerun bottoms being about 2.7 weight per cent based on initial charge. By hydro'desulfurization of the rerun bottoms, it willbe seen that the total gasoline yield can be of theorder' of 97%, the gasoline containing less than .l% sulfur and with substantially the same octane number as the original catalytioally cracked naphtha. In addition to this remarkably high yield of high quality gasoline, we obtain a by-product oil which serves as a valuable component in furnace oil or the like.

From the above examples, it will be seen that we have accomplished the objects of our invention. We have increased the amount of desulfurization accomplished per unit amount of acid used, lowered gasoline losses to sludge, augmented gasoline yields by the hydrodesulfurization of rerun bottoms and obtained about 95 to 97% yields of high quality gasoline which meets strict sulfur requirements, i. e. contains less than .1% sulfur. Only a very small amount of material requires treatment by hydrodesulfurization and this material is initially all higher boiling than gasoline but is converted chiefly into gasoline boiling range hydrocarbons. Saturation of olefins is minimized in the acid treating step and is nil in the hydrodesuifurization step. The high octane number of the charging stock is not materially altered.

We claim:

1. A process for desulfurizing a high sulfur catalytically cracked naphtha which process comprises topping said cracked naphtha at a cut point in the range of 256' to 300 F. to obtain a first overhead stream wh ch is rich in olefins and low in sulfur content, acid treating the bottoms from the topping step with the equivalent of about 98% sulfuric acid employed in amounts of 10 to 15 pounds per barrel, at a temperature in the range of ll) to 100 F. to form about 1 to 3% of sludge and an acid treated oil containing com ponents higher boiling than gasoline, rerunning said acid treated oil. to separate gasoline boiling range components from higher boiling range components, hydrodesulfurizing said higher boilin range components to produce additional amounts of gasoline boiling range components and blending said additional amounts of said gasoline boiling range components with gasoline boiling range components from the rerunning step and with low boiling components from the topping step to obtain a finished gasoline of low sulfur content.

2. The method of claim 1 wherein the hydrodesulfurization step is effected by contacting the higher boiling range components from the rerunning step with a sulf-active hydrogenation catalyst at a temperature in the range of I to 950 F., under a pressure in the range of 200 to 1500 p. s. i. and in the presence of added hydrogen.

3. The method of desulfurizing a cracked naphtha boiling chiefly in the range of about 150 to 400 F. and containing at least about .2% sulfur, which method comprises topping said cracked naphtha to remove as a first overhead stream components lower boiling than about 270 F., acid treating the bottoms from the topping step with sulfuric acid under conditions for effecting substantial desulfurization and the production of components boiling above 400 F., separating sludge formed in the acid treating step, rerunning the acid treated naphtha to obtain a second overhead stream consisting essentially of gasoline boiling range components and higher boiling oil formed in the acid treating step, hydrodesulfurizing said higher boiling oil to produce additional amounts of gasoline boiling range components and blending said last named components with gasoline boiling range components contained in said first overhead stream and gasoline boiling range components in said second overhead stream to obtain a finished gasoline of low sulfur content.

4. The method of claim 3 wherein said hydrodesulfurizing is efiected with an alumina-supported cobalt molybdate catalyst at a temperature in the range of about 700 to 800 F. under a pressure in the range of about 700 to 1500 p. s. i. and a space velocity in the range of about 2 to 15.

5. The method of claim 3 wherein hydrodesulfurizing is effected with an alumina-supported molybdenum oxide catalyst at a temperature in the range of 800 to 950 F., a pressure in the range of 200 to 500 p. s. i. and a space velocity in the range of .5 to 2.

ROBERT C. ARNOLD. ARTHUR P. LIEN. JOHN F. DETERS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,881,534 Harding Oct. 11, 1932 2,070,295 Morrell Feb. 9, 1937 2,219,109 McCormick Oct. 22, 1940 2,255,394 Schulze Sept. 9, 1941 2,487,466 Nahin Nov. 8, 1949 2,574 Porter et a1 Nov. 6, 1951 FOREIGN PATENTS Number Country Date 345,738 Great Britain Apr. 2, 1931

Claims (1)

1. 3. THE METHOD OF DESULFURIZING A CRACKED NAPHTHA BOILING CHIEFLY IN THE RANGE OF ABOUT 150 TO 400*F. AND CONTAINING AT LEAST ABOUT .2% SULFUR, WHICH METHOD COMPRISES TOPPING SAID CRACKED NAPHTHA TO REMOVE AS A FIRST OVERHEAD STREAM COMPONENTS LOWER BOILING THAN ABOUT 270*F., ACID TREATING THE BOTTOMS FROM THE TOPPING STEP WITH SULFURIC ACID UNDER CONDITIONS FOR EFFECTING SUBSTANTIAL DESULFURIZATION AND THE PRODUCTION OF COMPONENTS BOILING ABOVE 400*F., SEPARATING SLUDGE FORMED IN THE ACID TREATING STEP, RERUNNING THE ACID TREATED NAPHTHA TO OBTAIN A SECOND OVERHEAD STREAM CONSISTING ESSENTIALLY OF GASOLINE BOILING RANGE COMPONENTS AND HIGHER BOILING OIL FORMED IN THE ACID TREATING STEP, HYDRODESULFURIZING SAID HIGHER BOILING OIL

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