US2984617A - Denitrogenizing reformer feed - Google Patents

Description

May 16, 1961 v. DE cHELLxs ETAL 2,984,617

DENITROGENIZING REFORMER FEED Filed June 13, 1957 s sheets-sheet 2 (Mm) 70 RfFmM/.FR

f/zFm/v #mear/95,47 0.4514141 N [A4/w) T0 REFORME/e lNVENTORS Engine/1d El: 5 Zz'l wil HGENT May 16, 1961 1. v. DE cHELLls ET AL 2,984,617

DENITROGENIZING REFORMER FEED Filed June 13, 1957 5 Sheets-Sheet I5 'mm/.of ya m. SIPUN- J0 mz. THE/maur wwf/fo INVENTORS fiala 1./ de ke/lis @mauri/ aldi Patented May 1.6, 1961 fine DENITROGENIZING REFORMER FEED Italo V. de Chellis, Woodbury, and Raymond R. Halik, Pitman, NJ., assignors to Socony Mobil @il Company, Inc., a corporation of New York Filed June 13, 1957, Ser. No. 665,381

2 Claims. (Cl. 208-88) The present invention relates to the removal of nitrogen from the feed to a reformer and, more particularly, to the removal of nitrogen from the feed to a reformer employing a platinum-type catalyst to provide a reformer feed containing not more than about two parts per million of nitrogen (2 p.p.m. N).

Many reforming units are provided With facilities to remove sulfur because (l) sulfur poisons the catalyst; (2) the sulfur pollutes the air; (3) the sulfur causes rapid corrosion of all but alloy steels or clad steels. However, sulfur can be removed from naphtha by catalytic hydrodesulfurization to a satisfactory extent Without reducing the nitrogen content of the naphtha to an innocuous level.

Thus, for example, a naphtha can be hydrodesulfurized to a satisfactory level at 660 to 730 F. under a pressure of about 400 to about 500 p.s.i.g. over a cobalt oxidemolybdenum oxide on alumina catalyst at a liquid hourly space velocity (LHSV) of about 6. On the other hand to reduce the nitrogen content of a coker gasoline from 80 p.p.m. to 1.6 p.p.m. requires a temperature of 750 F., a reactor pressure of 1000 p.s.i.g. and a liquid hourly space velocity of 3. In other Words, employing the same catalyst the conditions necessary to reduce the nitrogen content to a satisfactory level (2 p.p.m.) for reforming over a platinum-type catalyst are a reactor pressure of more than twice that required for satisfactory hydrodesulfurization, and a space velocity about one-half that required for satisfactory hydrodesulfurization.

It has now been discovered that the load on the hydrodesulfurization unit can be reduced when removing nitrogen from paphtha or the hydrodesulfurization unit can be eliminated when reducing the sulfur and nitrogen content of a reformer feed naphtha by subjecting the feed naphtha to one or more treatments other than hydrogenation prior to reforming.

A unit capable of treating 8000 b./d. of naphtha to reduce the nitrogen content of the naphtha from 80 to 1.6 p.p.m. at 3 LHSV, 750 F. and 1000 p.s.i.g. costs about $1,500,000. On the other hand, a unit to produce a treated naphtha containing 1.6 p.p.m. nitrogen at 6 LHSV, 700 F. and 500 p.s.i.g. costs about $1,100,000.

The equipment to pretreat the naphtha in accordance With the principlees of the present invention prior to hydrodenitrogenizing costs about $25,000 to about $70,000. Consequently, there is a saving of about $330,000 to about $375,000 in capital costs when denitrogenizing reformer feed in accordance with the principles of the present invention even though the hydrotreating unit is not eliminated.

An additional advantage accrues from the use of the present invention to denitrogenize naphtha feed. Recently, considerable diiiiculty has been encountered in the heat exchangers used to preheat the feed to either the reforming unit or the hydrodesulfurization unit due to fouling of the tubes. By pretreating the naphtha feed to remove at least a part of the nitrogen before hydrodenitrogenizng the naphtha the problem of heat exchanger fouling is reduced if not eliminated.

lt has been found that when hydrodesulfurizing naphthas prior to reforming, ammonium salts are formed Which crystallize in the piping when the temperature of the hydrodesulfurizing reactor efuent is reduced to about 450 F. thereby increasing the pressure drop to prohibitive magnitude.

A number of combinations of what is hereinafter designated chemical treatment with mild hydrogenation produces a reformer feed naphtha containing not more than 2 p.p.m. of nitrogen and of satisfactorily reduced sulfur content. The block diagrams 1 through B in Figure 1 illustrate in a highly schematic manner the treatment of a naphtha feed containing about p.p.m. nitrogen to produce a reformer feed containing not more than 2 p.p.m. and in some instances less than 1 p.p.m. of nitro gen.

In the drawing Figure 1 the legend, Acid Treat, means treatment of the feed naphtha with 1 percent to 98 percent sulfuric acid at the rate of about 1.0 to about 15.0 pounds of makeup plus recycle acid per barrel of naphtha. In the drawing Figure 1, the legend, Hydropretreat (Mild) designates treatment over 'a hydrodesulfurization catalyst at about 600 to about 700 F. at a reactor pressure of about 250 to about 500 p.s.i.\g. and a liquid hourly space velocity (Ll-ISV) of about 4 to about l0 employing about 300 to about 1500 standard cubic feed (scf.) of hydrogen per barrel of naphtha feed and consuming about 25 to about 400 s.c.f. hydrogen per barrel of naphtha feed.

The legend, Hydropretreat (Severe) designates treatment over a hydrodesulfurization catalyst at about 650 to about 800 F. at a reactor pressure of about 1000- 2000 p.s.i.g. and a liquid hourly space velocity (LHSV) of about 1-6 employing about 500 to about 3000 s.c.f. of hydrogen per barrel of naphtha and consuming about 50-400 s.c.f. of hydrogen.

The legend, Percolation used in Figure 1 designates contact of the naphtha feed with activated attapulgus clay or similar absorbent at a rate of about 100 to about 5000 barrels of naphtha per ton of clay at a temperature of about 32 to about 150 F.

The legend, Acid Denitrogenation also designated strong sulfuric acid-caustic designates very brief contact with strong sulfuric acid, 98 percent or stronger, followed by neutralization with a mild caustic solution. The acid strength should be in the order of 80-98 percent Wt. White or spent alkylation acid may be used. Treat should be 1/2 to l0 pounds acid per barrel naphtha.

The legend, Solutizer Salt designates contact of the naphtha with an aqueous treating agent being the solutizer salt phase of a mixture of alkali metal hydroxide, alkali metal salt of at least one solutizer and Water in which the proportions of the three components are such that the mixture forms a solutizer salt layer comprising free alkali metal hydroxide, alkali metal salt of at least one solutizer and water substantially immiscible with an alkali metal hydroxide layer comprising alkali metal hydroxide and Water at a temperature within the range about 60 to about 150 F. Presently, it is preferred to use the alkyl phenols boiling Within the range to 650 F. as the solutizer.

The sequence of operations designated 1 in Figure l results in some loss of yield when there is more than about 10-30 percent unsaturated aliphatic hydrocarbons in the feed. On the other hand, there is less fouling of the heat exchangers and less formation of ammonia in the hydrotreater than in sequence 2, Figure 1.

The sequence of operations designated 2 in Figure l results in no loss of yield but more ammonia is formed in the pretreater than in 1 of Figure l.

The sequence of operations designated 3 in Figure 1 is the preferred combination when exceptionally low concentrations of the order of less than 1.0 p.p.m. of nitrogen in .the feed to the reformer are required.

Sequence 4 in Figure l provides an operation which results in less foulingrof the pretreater heat exchangers than 2, 3, 5, 7. Absorbents which can be used in the percolation stepare silica gel, finely divided silica-alumina cracking catalyst, clay, bauxite, and the like.

When the naphtha is rst mildly yhydrogenated and then contacted with silica gel, finely divided silica-alumina cracking catalyst, and the like, the life of the absorbent is longer but more ammonia is formed in the pretreater than in sequence 4 with the resultant greater accumulation of crystallized ammonium salts in the piping downstream of the pretreater.

The sequence designated -6 gives results similar to those provided by sequence l. However, the loss when unsaturated aliphatic hydrocarbons are present in the feed is less than in sequence 1 while the unit is smaller and more compact. In contrast, when the naphtha is first hydrogenated and then contacted by acid denitrogenation as indicated in sequence 7 there is no loss in yield but the formation of ammonia in the pretreater is greater than in sequence 6.

For the production of reformer feed having relatively low nitrogen content the three step sequence 8 provides satisfactory results. When somewhat higher nitrogen concentrations can be tolerated the third step, i.e., acid wash or acid-alkali wash or percolation can be omitted. It is to be noted that the rst step, i.e., contact with the solutizer salt reduces exchanger fouling in the pretreater unit and also reduces the nitrogen concentration about thirty percent.

The preferred treatment depends upon the charge stock, nitrogen level, etc. For example, thermally cracked charge stocks having a relatively high nitrogen content of the order of 25 to 1000 p.p.m. preferably are treated in accordance with sequence 6. On the other hand, when straight run charge stocks having a relatively low nitrogen content of 4 to 50 p.p.m. are treated it is presently preferred to use sequence 3 or 4. However, when naphthas, such as shale oil naphtha having a nitrogen content in excess of 1000 p.p.m. are treated, it is preferred to employ a three-stage treatment comprising the sequence of acid-treating followed by hydropretreating and then by a further acid treat.

In general, the present method of treating naphtha to be reformed over a particle-form solid reforming catalyst, the activity and/or selectivity of which is adversely affected by nitrogen in the feed naphtha in excess of about 2 p.p.m., to reduce the load on the hydrodesulfurizing reactor, i.e., to provide a feed the nitrogen content of which can be reduced to not more than about 2 p.p.m. by hydrogenation under mild hydrodesulfurization conditions comprises subjecting a naphtha feed containing nitrogen in excess of 2 p.p.m. to contact with a reagent selected from the group consisting of 1 to 98 percent sulfuric acid, aqueous solutizer salt solution, strong sulfuric acid-weak caustic and particle-form solid absorbent material and contacting the naphtha feed containing nitrogen in excess of 2 p.p.m. with a hydrodesulfurizing catalyst under mild conditions.

The hereinbefore described combination of treatments whereby the load on a hydrodesulfurizing reactor is reduced have been described in application to a cracked naphtha, particularly a thermally cracked naphtha, such as coker naphtha. However, it is often advantageous to reform a mixture of cracked naphtha and straight run naphtha. When a blend of straight run naphtha and cracked naphtha, for example, thermally cracked naphtha volume percent and straight run naphtha 80 volume percent, is to be reformed it is advantageous, by reason of reduced requirements for facilities, to treat only the thermally cracked naphtha while hydrotreating the mixture.

Referring .now to'Figure 2, the combination of operations designated 9 has the advantage of reduced heat exchanger fouling and reduced load on the percolator in addition to a reduced load on the hydrotreater. The combination l0 gives similar results to those obtained with sequence 9 but with ya somewhat `greater loss in yield of treater thermally cracked gasoline. Sequences 1l and 12 result in reduced heat exchanger fouling and substantially no loss in yield while also reducing the load on the hydrotreater.

Elimination of the hydrotre'ater When the naphtha to be reformed or the naphtha blend to be reformed contains less than about 0.005 weight percent sulfur, hydrotreating of the naphtha or blend prior to reforming can be Vreplaced with chemical treating or absorption. There are several advantages which accrue from the elimination of the hydrotreater, among which are a great reduction in capital and operating cost, much simpler processing techniques involving no high pressures or high temperatures and elimination of the requirement for hydrogen thus releasing hydrogen for other refinery processing such as hydrotinishing lube oils, hydrocracking heavy stocks, etc. Three combinations of operations for treating low sulfur naphtha containing not more than about 2O p.p.m. nitrogen are illustrated in Figure 3. Thus, thermally cracked or straight run gasoline is treated with sulfuric acid or by Acid Denitrogenation or contacted with particle-form absorbent such as clay until the nitrogen content of the sot-reated naphtha is not greater than about 2 p.p.m. This pretreatment has the inherent advantage of no substantial loss of yield and capital and operation costs considerably less than those for hydrotreating the same stock to the same concentration of nitrogen compounds.

The sequence of operations numbered 14 produces a treated reformer feed stock having `a nitrogen concentration not in excess of 2 p.p.m. without hydrotreating. While the sequence of treatments for the thermally cracked portion of the blend provides for percolation followed by treatment by acid denitrogenation it is to be understood that the treatment by acid denitrogenation can be replaced by acid-treating. However, acid-treating increases the loss of yield in the treatment of the thermally cracked portion of the blend.

When the proportion of cracked, e.g., thermally cracked gasoline to straight run gasoline which is to be pretreated to provide -a reformer feed stock is low, i.e.,. less than 15:85 the sequential treatments designated 15 produces a satisfactory reformer feed.

The relative efficiency of the various methods of treating thermal naphtha to reduce the concentration of nitrogen-containing materials in the thermal naphtha is set forth in the following table.

l. A method of reforming a naphtha having an initial concentration of nitrogen compounds which is deleterious to a nitrogen-sensitive reforming catalyst which comprises contacting a naphtha containing in excess of 2 p.p.m. nitrogen with at least one liquid phase treating agent selected from the group consisting of sulfuric acid and aqueous solutizer salt, separating. the treated naphtha from said treating agent, contacting the treated naphthat with a hydrodesulfurizing catalyst under mild conditions to produce a naphtha containing not more than 2 ppm. nitrogen, and then contacting the treated naphtha with a nitrogen-sensitive reforming catalyst under reforming conditions.

2. A method of reforming a blend of straight run naphtha and a cracked naphtha having an initial concentration of nitrogen compounds which is deleterious to a nitrogen-sensitive reforming catalyst which comprises contacting at least the cracked naphtha containing in excess of 2 p.p.m. nitrogen with at least one liquid phase treating agent selected from the group consisting of sulfuric acid and aqueous solutizer salt, separating the treated naphtha from the treating agent, mixing the straight run and cracked components of the blend, con- I5 2800427 tacting said blend with a hydrodesulfurizing catalyst under mild conditions, and then contacting the hydrodesulfurized blend with a nitrogen-sensitive reforming catalyst under reforming conditions.

References Cited in the tile of this patent UNITED STATES PATENTS 1,691,266 Caldwell Nov. 13, 1928 2,728,715 Rampino Dec. 27, 1955 2,744,053 Kay et a1. May 1, 1956 2,758,064 Haensel Aug. 7, 1956 2,766,179 Penske et al Oct. 9, 1956 2,773,008 Hengstebeck Dec. 4, 1956 Junk July 23, 1957

Claims (1)

1. A METHOD OF REFORMING A NAPHTHA HAVING AN INITIAL CONCENTRATION OF NITROGEN COMPOUNDS WHICH IS DELETERIOUS TO A NITROGEN-SENSITIVE REFORMING CATALYST WHICH COMPRISES CONTACTING A NAPHTHA CONTAINING IN EXCESS OF 2 P.P.M. NITROGEN WITH AT LEAST ONE LIQUID PHASE TREATING AGENT SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID AND AQUEOUS SOLUTIZER SALT, SEPARATING THE TREATED NAPH-

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