US3436340A

US3436340A - Denitrification process with recycle of extracted nitrogen compounds

Info

Publication number: US3436340A
Application number: US598985A
Authority: US
Inventors: Robert L Jacobson
Original assignee: Chevron Research and Technology Co
Current assignee: Chevron USA Inc
Priority date: 1966-12-05
Filing date: 1966-12-05
Publication date: 1969-04-01
Anticipated expiration: 1986-04-01

Description

P" 1969 R. L. JACOBSON 3,

DENITRIFICATION PROCESS WITH RECYCLE OF EXTRACTED NITROGEN COMPOUNDS Filed Dec. 5. 1966 /s 2 S 4 5 l4 9 Lu /9 HYDROGEN a0 REACTION ZONE 2.?

EXTRACTION ZONE 2 DJ J o (D 24 2 LIQUID 29 PRODUCT 23 Fl 34 /9 E FEED 5 HYDROGEN as 36 //a /0 INVENTOR F G 2 ROBERT L. JA coaso/v United States Patent US. Cl. 208-254 4 Claims ABSTRACT OF THE DISCLOSURE A process for denitrification which permits higher yields at milder conditions and comprises hydrodenitrification of a portion of the feed oil under specified conditions, solvent extraction of the remaining nitrogen compounds, and recycle of the extracted nitrogen compounds to the hydrodenitrification step.

This mvention relates to processes for the selective hydrogenative removal of nitrogen compounds from hydrocarbon streams. In a particular embodiment, the invention is concerned with a catalytic and extractive process for the substantially complete removal of nitrogenous contaminants from hydrocarbon oils to be used as feedstocks in subsequent catalytic conversion processes.

Hydrocarbon oils contain varying amounts of nitrogen compounds, ranging from several parts per million up to 1-3%, depending on the boiling range of the oil and the nature of the crude petroleum or other hydrocarbonaceous deposit from which it was derived. The organic nitrogen compounds are believed to be primarily basic ring compounds, such as pyridines and quinolines, with some nonbasic ring compounds, such as indoles, carbazoles, and pyrroles. It is not uncommon to report both total nitrogen and basic nitrogen in analyzing hydrocarbon oils. As used herein, the term nitrogen content refers to total nitrogen, determined by Kjeldahl analysis, expressed in parts per million of nitrogen.

This invention provides a process useful for removing nitrogen from hydrocarbon oils any time a product oil low in nitrogen content is desired. The process is of particular value when the nitrogen content of an oil is to be reduced to a low residual level, e.g., 1 to ppm. or less, so that the oil may be treated in a subsequent catalytic conversion process, such as reforming, hydrocracking, isomerizing, or other similar conversion processes. For example, one process which advantageously uses the denitrified product from the process of this invention is a low temperature hydrocracking process which uses sulfided cobalt or nickel on a silica-alumina cracking catalyst support under controlled reaction conditions to effect a substantial conversion of the denitrified feed to liquid products boiling below the initial boiling point of the denitrified feed and containing a high ratio of isoto normal-paraifins, with negligible coke production and only a very small product of light gaseous hydrocarbons. In such a process it is desirable to have a maximum nitrogen content in the feed of not over 10 p.p.m. and additional operational advantages may be obtained if the maximum is less than 1 p.p.m.

Several factors have a multiplying effect to make the removal of nitrogen compounds from hydrocarbon oils by catalytic hydrogenation progressively more difiicult as feeds of increasingly higher boiling point are considered. Thus, the rate of hydrogenation of nitrogen compounds is slower with feeds of higher boiling point and With feeds of higher nitrogen content; and higher boiling feeds ordinarily contain higher concentrations of nitrogen compounds. Hence, if a given low nitrogen content is desired in the product, more nitrogen compounds must be hydrogenated. Yet another factor is that the reaction by which nitrogen compounds are converted to ammonia follows pseudo first order reaction kinetics. Consequently, to convert, for example, of the nitrogen content of an oil requires twice the catalyst volume or contacting time as to convert 50% of the nitrogen.

The above factors all dictate the use of relatively more severe conditions when treating higher boiling oils for the removal of nitrogen compounds by hydrogenation. Because there are practical limitations on the extent to which the contacting time can be raised, the reaction rate must be increased, for example, by using a higher temperature. Because higher temperatures tend to increase the rate of coking and catalyst deactivation, a higher hydrogen partial pressure must be used to counteract this effect. Unfortunately, the use of higher temperatures also tends to increase the production of light hydrocarbons by hydrocracking, thereby diluting the hydrogen and reducing the hydrogen partial pressure. It would therefore be desirable to provide a method whereby hydrocarbon oils, and in particular higher boiling oils, could be denitrified under milder conditions of pressure, temperature, and other reaction variables to minimize these detrimental effects of treating under severe conditions.

The process of this invention provides such a desired method, which comprises increasing the concentration of organic nitrogen compounds in contact with hydrogenation catalyst in at least a portion of a hydrotreating zone during hydrotreating by adding to a hydrocarbon oil at a point prior to the end of the oils passage through the hydrotreating zone additional organic nitrogen compounds in an amount of at least 1 weight percent of the orgnic nitrogen compounds present in the oil at the point at which the addition is made. The organic nitrogen compounds required for the addition are supplied within the process itself by an extraction and recycle section.

The process achieves the desired result of permitting nitrogen removal under milder conditions, by increasing the concentration of organic nitrogen compounds and thus allowing removal by hydrofining of large amounts of these compounds more readily. This results in reduction of the nitrogen content of the oil to a level from which subsequent solvent extraction can more easily remove substantially all the remaining nitrogen.

Broadly, the process of this invention involves hydrodenitrifying an oil feed while controlling the rate of nitrogen removal by increasing the concentration of organic nitrogen compounds either in the oil feed itself or after the oil feed has been partially denitrified. The organic nitrogen compounds used to increase the concentration are supplied by solvent extracting from the oil substantially all the organic nitrogen compounds that remain after hydrodenitrification, separating these compounds from the solvent, and recycling the separated organic nitrogen compounds to hydrodenitrification.

More particularly, the invention involves three related steps. First, an augmented oil, hereinafter defined, is contacted with hydrogen in a catalytic hydrodenitrification reaction zone wherein a major portion of the organic nitrogen compounds present are converted to ammonia, which is separated from the hydrotreated oil. This oil is then solvent extracted to remove substantially all the remaining organic nitrogen compounds and yield a substantially nitrogen-free hydrocarbon oil. The extracted organic nitrogen compounds are recycled back to be combined into the augmented oil.

Nitrogen remaining in the oil after hydrodenitrification must be at least 1 weight percent of the nitrogen that is present in the oil at the point at which the additional nitrogen is to be added. This requirement has both a technological and an economic basis. First, if less than 1 percent of the nitrogen is left in the oil as organic nitrogen compounds after hydrodenitrification, this amount will not cause a significant increase in the concentration of organic nitrogen compounds in the oil upon recycling and addition to the oil, and consequently there will be no significant decrease in the severity of conditions required for the desired nitrogen removal. Secondly, it is found not economically attractive to operate the hydrodenitrification reactor for excessively high nitrogen removal since that would require that the solvent extractor be operated considerably below its optimum for nitrogen removal. By considering both of these criteria it has been determined that the process operates at an optimum when approximately 75 to 85% of the organic nitrogen compounds are converted in the hydrodenitrification reactor. Substantially all the remaining organic nitrogen compounds are then removed by the solvent extraction and recycled to hydrodenitrification, increasing the nitrogen concentration by at least 1%, but preferably by about 15-25%.

For clarity in the following detailed discussion of the invention, the following definitions of terms will apply throughout. Feedstock refers to any organic nitrogencontaining hydrocarbon oil which it is desired to denitrify by this process, and applies to such oil until the point at which either (1) the oil, unaltered by any addition of organic nitrogen compounds, enters the hydrotreating zone, or (2) additional organic nitrogen compounds are added prior to hydrotreating. Partially denitrified feedstock refers to oil which has passed through a portion of the hydrotreating zone but which contains no added organic nitrogen compounds. Augmented oil refers to either of the above oils after additional organic nitrogen compounds have been added. Any of these hydrocarbon materials may contain added hydrogen and other light gases.

The process of this invention may be more fully understood by reference to two specific embodiments thereof, illustrated schematically in FIGURES I and II of the attached drawing. FIGURE I shows one possible arrange ment of equipment for that embodiment of the process wherein the nitrogen content is to be increased over the entire hydrodenitrification reaction zone FIGURE II shows an embodiment of the process where the equipment arrangement shown in FIGURE I is modified to provide for increasing the nitrogen content in only a portion of the reaction zone. This latter embodiment may be more desirable when the reactor contains several separate beds or zones of hydrodenitrification catalyst and the nitrogen content is sought to be increased in less than the total number of zones.

Referring now to FIGURE I, the feedstock enters mixing zone 1 through line 2. In mixing zone 1 the feedstock is thoroughly mixed with the organic nitrogen compounds being recycled through line 3. The resulting augmented oil is then passed through line 4 into mixing zone 5 in which it is mixed with a stream containing a major proportion of hydrogen and minor amounts of ammonia, H 5, and light hydrocarbons, principally methane. The hydrogen and augmented oil, after being heated to the desired reaction temperature, are passed through line 6 into reaction zone 7 in which they are maintained at a temperature, pressure, and LHSV sufficient to convert the desired quantity of organic nitrogen compounds to ammonia and other light volatile nitrogen compounds. The effluent of reaction zone 7 is discharged through line 8 into hydrogen separation zone 9 in which most of the unconverted hydrogen and a small portion of the ammonia, H 8, and light hydrocarbon gases are separated from the treated oil. If desired, water may be injected into line 8 through line 10 to aid in removal of the ammonia. The ammonia, rich water is removed from hydrogen separation zone 9 through lines 11 and 12. It may also be desirable to inject a small amount of caustic into line 8 through line 10 to neutralize any acid which may have formed in reaction zone 7. This water and caustic injection is accomplished by conventional methods. The

gases are recycled back through line 13 to mixing zone 14 where they are mixed with fresh hydrogen which is added through line 15. It may be desirable to permit a portion of this stream to be vented through

lines

16 and 17 or 16 and 18 to prevent buildup of the nonhydrogen gases in the recycle hydrogen stream. The resulting hydrogen-rich gas is then passed into mixing zone 5 through line 19 and is recycled back into reaction zone 7.

The hydrotreated oil, now substantially hydrogen-free, is passed from hydrogen separation zone 9 through line 11 into stripping zone 20 wherein substantially all the remaining H 8, ammonia, hydrogen, and volatile hydrocarbons (C are removed through line 18 and may be separated and recovered by conventional gas separation and recovery equipment not shown. The gas-free oil then passes through line 21 into countercurrent single or multistage extraction zone 22 wherein it is contacted with a polar solvent which enters the extraction zone through line 23. Contact time, temperature, and pressure are all controlled such that the conditions in the extraction zone are sufificient for the solvent to extract from the oil essentially all the remaining unconverted organic nitrogen compounds. The organic nitrogen-free oil is removed through line 24. The organic nitrogen-rich solvent is removed through line 25 and passed into solvent separation zone 26 wherein substantially all the organic nitrogen compounds are removed from the solvent by conventional means such as flashing or distillation. The solvent, containing little residual organic nitrogen, is removed from solvent separation Zone 26 through line 27 and passed into mixing zone 28 wherein it is mixed with fresh makeup solvent which enters through line 29 in a quantity sufiicient to replace any solvent which is lost 'by removal with the oil through line 24. The combined solvent streams are recycled to extraction zone 22 through line 23. The extracted organic nitrogen compounds are recycled through line 3 to mixing zone 1.

If a significant amount of solvent is absorbed in the oil leaving extraction zone 22 through line 24, it may be desirable, depending on the type of subsequent processing planned for the oil, to have line 24 lead directly to a conventional solvent recovery unit from which recovered solvent could be recycled to mixing zone 28 or added to fresh solvent in line 29.

If the solvent leaving solvent separation zone 26 through line 27 contains excessive amounts of oil, organic nitrogen compounds, and/or other impurities, a portion of the solvent may be drawn off through line 30 for purification by conventional means not shown. Oil so recovered could be added to the oil leaving solvent extraction zone 22 through line 24.

FIGURE 11 depicts schematically a modified hydrodenitrification reaction zone section of the equipment arrangement shown in FIGURE I. In this embodiment of the process the reaction zone 7 contains a plurality of catalyst beds or zones designated 31, 32 33. Conversion of some portion of the organic nitrogen compounds to ammonia occurs in each of these beds, while little conversion occurs in the spaces between the beds. The feedstock enters mixing zone 5 through line 34. In mixing zone 5 it is mixed with the hydrogen-rich stream recycled through line 13, mixing zone 14, and line 19 as described in the discussion of FIGUR-E I. The hydrogen and feedstock, after being heated to the desired reaction temperature, are passed into reaction zone 7 through line 6. A portion of the organic nitrogen compounds are reacted in the first section of the catalyst, bed 31, before the oil is augmented by additional organic nitrogen compounds.

After passing through the first portion of the catalyst many of the easily removable organic nitrogen compounds have been converted to ammonia, and the ease of nitrogen removal from the partially denitrified feedstock has decreased substantially because of the deceased concentration of the organic nitrogen compounds in the oil. As noted above, previous denitrification processes sought to overcome this problem of increasing difliculty of nitrogen removal by increasing the catalyst temperature or otherwise increasing the severity of the operating conditions. In the process of this invention, however, the problem is solved without increasing the severity of operating conditions by increasing the concentration of organic nitrogen compounds at this intermediate point by adding additional organic nitrogen compounds to reaction zone 7 through line 35 to create an augmented oil.

Alternatively, the oil may be reacted in several catalyst beds (e.g., beds 31 and 32) before mixing with the added organic nitrogen compounds to produce the augmented oil. In this case the added organic nitrogen compounds are added through line 36.

Lines

35 and 36 both come from solvent separation Zone 26, as shown in FIGURE II, in the same manner as that described for line 3 in FIGURE 1. As another alternative, concentrations of organic nitrogen compounds can be increased at more than one point by passing portions of the total amount of organic nitrogen compounds from solvent separation zone 26 through a plurality of lines such as

lines

3, 35 and 36 to reaction zone 7.

Lines

11, 15 and 16 in FIGURE II are identical to those described under the same numbers in the description of FIGURE I, and serve to connect the modified reaction zone reactor with the remaining portion of the equipment, which remaining portion is identical to that shown in FIGURE I.

In a typical operation of the embodiment of the process shown in FIGURE I, a feedstock containing 1700 p.p.m. of nitrogen would be denitrified to 300 p.p.m. in the hydrotreating zone, with 295 p.p.m. nitrogen being extracted and recycled to mixing zone 1 to make an augmented oil with 1995 p.p.m. of nitrogen to be hydro treated, producing a final product with only 5 p.p.m. of nitrogen. In a typical operation of the embodiment of the process shown in FIGURE II, a feedstock containing 3000 p.p.m. of nitrogen could be denitrified to 1000 p.p.m. in catalyst bed 31 of reaction zone 7, augmented by addition of 200 p.p.m. of additional nitrogen added through line 34, and further denitrified to 210 p.p.m. nitrogen in

beds

32 and 33. Extraction of 200 p.p.m. of nitrogen for recycle would leave a product with only p.p.m. of nitrogen.

The process of this invention may be used to denitrify any normally liquid hydrocarbon oil containing up to about 1-3% (10,00030,000 p.p.m.) nitrogen. In common practice, the process of this invention might profitably be used to denitrify a variety of oils and oil fractions which are principally composed of moderate to heavy components, i.e., those oils and oil fractions which contain a major proportion of components boiling above 200 F. The process of this invention is most profitably employed to denitrify high-boiling hydrocarbon oils. By high-boiling hydrocarbon oils is meant those hydrocarbon oils and oil fractions which remain substantially entirely in the liquid phase at reaction conditions. Specifically, the high-boiling oils boil at least 90% above 600 F. and at least 10% above 750 F. It is particularly suitable as a process for the removal of nitrogen com pounds from oils boiling more than 50% above 750 F. Examples of such oils are reduced crude, deasphalted residuum, heavy straight-run gas oils, lube oils, waxes, heavy cracked cycle oils, coker gas oils, shale oil distillates, etc., and raflinates or other components of such oils.

Conditions of temperature, pressure, hydrogen throughput, and space velocity in the reactor are correlated to provide the desired degree of nitrogen removal. Higher temperatures, pressures, and/or hydrogen throughputs are required when treating the higher boiling feedstocks and those containing the more refractory nitrogen compounds. A particular advantage of the process is that it permits the hydrodenitrification of refractory stocks at comparatively mild conditions of temperature and pressure and/ or the use of high space velocities. The term refractory is used herein with reference to the relative diificulty with which nitrogen is removed from the respective feedstocks. In general, the complex nitrogen compounds found in high boiling hydrocarbon fractions and cracked cycle oils and more resistant to hydrogenation than the lower boiling compounds.

Temperature has a large influence on the rate of conversion of the nitrogen compounds and is adjusted -upwards to maintain the hydrodenitrification rate as the catalyst ages or is deactivated through protracted use. The temperature should be in the range 500-850 F., preferably 600-800 F. The rate of hydrodenitrification is fairly low at temperatures below 550 F. At temperatures much above 800 F. substantial cracking of the hydrocarbon oil and coke formation normally occurs, and the production of light gases increases markedly.

Elevated pressures advantageously influence the rate of hydrodenitrification as well as extending the catalyst activity and life. Pressures as low as 200 p.s.i.g. may be employed when treating light naphthas, whereas pressures up to 4000 p.s.i.g. may be advisable for the substantially complete hydrodenitrification of highly refractory and high boiling stocks.

Hydrogen throughput rate is maintained above about 500 s.c.f./bbl. of hydrocarbon oil, and is preferably in the range WOO-20,000 s.c.f./bbl. More generally, at least sufiicient hydrogen is provided to supply that consumed in the conversion of the nitrogen compounds and to compensate for incidental hydrogenation of unsaturates and oxygen, sulfur, and halogen compounds, while maintaining a significant hydrogen partial pressure. The use of more than 20,000 s.c.f. of hydrogen/bbl. does not generally produce sufficient improvement in conversion rate to justify the increased cost.

The catalysts used in the hydrotreating reaction zone in this process may be any of the sulfactive hydrogenationdenitrification catalysts. The preferred catalysts have as their main active ingredient one or more oxides or sulfides of the transition metals, such as cobalt, molybdenum, nickel, and tungsten. These various materials may be used in a variety of combinations with or without such stabilizers and promoters as the oxides and carbonates of K, Ag, Be, Mg, Ca, Sr, Ba, Ce, Bi, Cr, Th, Si, Al and Zr. These various catalysts may be employed per se, but preferably in combination with various conventional supporting materials. Examples of the latter are charcoal, fullers earth, kieselguhr, silica gel, alumina, bauxite, and magnesia. While any of the noted classes of conventional sulfactive hydrogenation catalysts may be employed, it has been found that a molybdenum oxide catalyst promoted by a minor amount of cobalt oxide and supported upon an activated alumina or a tungsten sulfide on activated alumina are particularly preferred catalysts for this hydrofining operation. Sulfided nickel and molybdenum on predominately alumina carriers, i.e., containing up to about 40% silica, have also proved to be particularly effective catalysts in this process. The catalyst may be in the form of fragments or formed pieces such as pellets, extrudates, and cast pieces of any suitable form or shape.

The solvent used in the extraction zone 22 is to be a solvent which possesses two basic characteristics. First, it should be selective for removal of organic nitrogen compounds, so as not to reduce the oil yield in any significant amount. Second, it should be relatively easily separated from the extracted organic nitrogen compounds. The latter property is of lesser importance than the former, however, and is satisfied if the solvent-nitrogen compound separation is sufiiciently complete such that the organic nitrogen compound content of the solvent returning to extraction zone 22 through

lines

23 and 27 is low enough that the extraction rate of the organic nitrogen compounds from the oil by the recycled solvent is not significantly diminished. Further, one may permit a certain small amount of some solvents to be recycled with the organic nitrogen compounds to the hydrotreating zone through lines 3, 4 and 6. The amount of any solvent thus carried into the hydrotreating zone should be no more than the minimum amount needed to serve as a liquid carrier fluid for the organic nitrogen compounds and permit them to flow through line 3 back to mixing zone 1. Such a procedure is not recommended, however, for it is much preferred to have no solvent in the organic nitrogen recycle stream. Further, it is rarely necessary to use a liquid carrier fluid for the organic nitrogen compounds since they are themselves normally liquid at the operating temperatures of this process. Typical solvents which may be used in this process include dimethyl sulfoxide, acetic anhydride, sulfuric acid, and HF.

The following example illustrates that the process of this invention permits use of a considerably smaller hydrodenitrification reactor and less catalyst to process the same volume of feed, as compared to a prior art process.

EXAMPLE I Nitrogen content, p.p m of Feed 1, 000 1,000 1,200 210 Extractor efiluent 10 Amount of nitrogen recycled, p.p.m 100 LHSV of oil through catalyst bed 1. 2.18

It is apparent that, since the process of this invention permits the LHSV to be doubled while processing the same volume of feedstock, the size of the hydrotreating reactor and amount of catalyst required can be reduced to about half of that required in conventional hydrodenitrification processes. This is obviously a significant advantage.

The following example illustrates that if the reactor and catalyst now used in a conventional process were adapted for use in the process of this invention, milder reaction conditions could be used in the reactor, with a resultant decrease in maintenance and replacement costs of reactor and catalyst. In this example the feed in two parallel runs contains 1000 p.p.m. of nitrogen. Reaction pressure, hydrogen consumption, and LHSV are the same for both runs. Run C illustrates conventional denitrification processes, while Run D illustrates the improvement of the process of this invention. In Run D organic nitrogen compounds are added upstream of the inlet of the reactor.

EXAMPLE II Nitrogen content, p.p.m of- Oil at reactor inle 1, 000 1, 200

Reactor efliuent-.. 210

Extractor etlluent 10 Amount of nitrogen recycled, p.p.m 200 Reaction temperature, F 780 740-750 E F G 90 Anhydrous Number of stages 4 4 2 Solvent dosage per stage, weight percent; of oil"... 2 2 20 Temperature, F- 75 75 75 Nitrogen content of p Feed to extractor 148 243 Extractor product l. 5 0. 3 4

A particularly advantageous solvent for use in this process is anhydrous hydrogen fluoride. Its high degree of selectivity for extraction of organic nitrogen compounds results in higher yields of oil products in the solvent extraction zone efiluent, and less necessity to draw off solvent for separation of extracted oil from the solvent after separation of the organic nitrogen compounds from the solvent. Use of HF also permits use of fewer extraction stages than required by other solvents, or, conversely, less acid dosage in each stage if the same number of stages is used. Further, HF has less tendency to react with nonnitrogenous components of the oil than some other solvents, such as sulfuric acid.

The above-described flow system and operating conditions are given for illustrative purposes only. It is apparent that many widely ditferent embodiments of this invention may be made without departing from the scope and spirit thereof and therefore it is not intended to be limited except as indicated in the appended claims.

I claim:

1. A process for the production of a low-nitrogen content hydrocarbon oil from a heavy nitrogen-contaminated hydrocarbon oil, 90 weight percent of whose components boil above 600 F., under conditions in which coking and production of light hydrocarbons is minimized, thereby increasing the yield of heavier oil, which comprises:

(A) passing said heavy oil into a hydrotreating zone wherein it is contacted with hydrogen in the presence of a sulfactive hydrogenation catalyst;

(B) adding to said heavy oil at a point prior to the end of its passage through said hydrotreating zone organic nitrogen compounds in the amount of at least 1 weight percent of the organic nitrogen compounds present in the oil at the point at which said organic nitrogen compounds are added;

(C) maintaining in said hydrotreating zone a temperature in the range of 550-800" F., a hydrogen throughput rate in the range of SOD-20,000 s.c.f./bbl., and a pressure in the range of 200-4,000 p.s.i.g.;

(D) converting a portion of the total amount of organic nitrogen compounds present to ammonia in said hydrotreating zone;

(B) passing the treated oil from said hydrotreating zone to an extraction zone and extracting substantially all of the unconverted organic nitrogen compounds with a solvent;

(F) separating a major portion of said solvent from the extracted organic nitrogen compounds;

(G) recycling essentially all of the extracted organic nitrogen compounds to said hydrotreating zone as said organic nitrogen compounds which are added to step (B); and

(H) recovering from said extraction zone a heavy oil product having an organic nitrogen content no greater than the order of 10* p.p.m. nitrogen.

2. The process of claim 1 wherein said solvent specified in step (D) of said claim 1 is a solvent which is selective for polar organic nitrogen compounds.

3. The process of claim 2 wherein said solvent comprises hydrogen fluoride.

4. The process of claim 1 wherein in said hydrodenitrification zone the nitrogen is removed from 75-85 percent of the nitrogen-containing compounds and said additional organic nitrogen compounds contain nitrogen in an amount equal to 15-25 percent of the total organic nitrogen compounds present at the point at which said additional organic nitrogen compounds are added.

References Cited 5 UNITED STATES PATENTS 3,052,742 9/1962 Mills 208-254 3,201,345 8/1965 Hamilton et 211. 3,309,309 3/1967 Hess 208-236 10 DELBERT E. GANTZ, Primary Examiner.

G. I. CRASANAKIS, Assistant Examiner.