US2770580A

US2770580A - Alkaline treatment of petroleum vapors

Info

Publication number: US2770580A
Application number: US380642A
Authority: US
Inventors: Earl M Honeycutt; Curtiss C Wallin
Original assignee: Sun Oil Co
Current assignee: Sunoco Inc
Priority date: 1953-09-17
Filing date: 1953-09-17
Publication date: 1956-11-13
Anticipated expiration: 1973-11-13

Description

Filed Sept. 17, 1953 26 Cuuslic Soclu Aqueous Alcohol Nuphlhu A6 4 42 l L J 'l L a?) l M f 22 Exlruction Solweul I I Zone Recovery 2% 24 40 ll 4 /A 8 Aqueous Shame O|l@7' w Alcoholic Sodium l2 Nuphihenme INVENTOR. EARL M. HONEYGUTT BY CURTISS C. WALLlM Patented Nov. 13, 1956 ALKALINE TREATMENT OF PETROLEUM VAPORS Earl M. Honeycutt, Claymont, Del., and Curtiss C. Wallin, Springfield, Pa., assignors to Sun Oil Company, Philadelphia, Pin, a corporation of New Jersey Application September 17, 1953, Serial No. 380,642

8 Claims. (Cl. 196-436) This invention relates to the alkaline treatment of lubricating oil vapors for removal from the vapors of naphthenic acids contained therein.

It is known in the art to contact lubricating oil vapors containing naphthenic acids with molten, or liquid aqueous alkali metal hydroxide in order to react the naphthenic acids with the alkali to form alkali metal naphthenates in admixture with the alkali metal hydroxide.

There are certain problems involved in such alkaline treatment of petroleum vapors, which problems have not been satisfactorily solved prior to the present invention. One such problem is the tendency for organic material. and particularly the alkali metal naphthenates, in the alkaline material to decompose under the influence of the relatively high temperatures, e. g. in the neighborhood of 600 F. to 700 R, which prevail in the treating zone when, for example, a topped crude oil is vacuum distilled at commonly employed pressures, e. g. about 40 mm, Hg absolute. This decomposition is highly disadvantageous because of the tendency for the decomposition products to plug up the apparatus, and also because of the decrease in yield of valuable alkali metal naphthenates that it causes, It has been found that, in ordinary operation as practiced in the prior art, decomposition products may be formed in amount up to 25 percent of the amount of naphthenic acid removed from the oil vapors.

Another problem involved in alkaline treatment of petroleum vapors for removalof naphthenic acids is the obtaining of satisfactory contacting conditions in the treating zone. Even whenthe best apparatus available for contacting rising vapors with descending liquid is employed, it has been found that, in ordinary operation as practiced in the prior art, the consumption of alkaline material and the extent of removal of naphthenic acids from the vapors is substantially less than it should be for satisfactory results, thus indicating that the. contact between alkaline material and naphthenic acids is un satisfactory.

According to the present invention, an alkaline treatment of petroleum vapors is provided which overcomes in a novel manner the above-mentioned problems involved in such treatment. This is done by maintaining, in the contacting zone where oil vapors contact liquid alkaline material, an unusually low concentration of akali metal hydroxide and an unusually high concentration of liquid mineral oil in the liquid alkaline material. By virtue of the maintenance of these conditions, the liquid alkaline material according to the invention is a singlephase solution of alkali metal naphthenate and alkali metal hydroxide in oil, or at least an exceptionally homogeneous and stable, fine emulsion of alkali metal naphthemate and alkali metal hydroxide in oil.

To provide the advantages of the present invention,"

the relative proportions of materials in the liquid alkaline material in the contacting zone should be Within the following limits:

Parts by weight Liquid oil 60-95 Alkali metal naphthenates 3-3O Alkali metal hydroxide s 0.210

Most advantageously, the proportions are as follows:

i Parts by weight Liquid oil 86-95 Alkali metal naphthenates 4-l3 Alkali metal hydroxide 15 From the above, it is seen that the liquid alkaline material employed according to the invention is characterized by high oil content and low alkali metal hydroxide content. These features are necessary in order to avoid excessive decomposition of organic constituents, chiefly alkali naphthenates, in the liquid alkaline material. It has been found, for example, that a low oil content in the liquid alkaline material, e. g. an oil content of about 50, results in the formation of up to 45 times as much decomposition product as is obtained when the oil content is in the range 6070. The concentration of alkali metal hydroxide should also be maintained at a low level, in order to avoid formation of a caustic phase separate from the oil phase, since the formation of such separate phase results in increased decomposition of organic constituents of the liquid alkaline material.

The process of the present invention involves unobvious features in that it involves the use of an alkaline treating material containing an unusually small proportion of free alkali, wherea it would be expected that more advantageous results would be obtained with greater proportions of alkali. However, it has been found that the process of the invention gives a remarkably complete removal of naphthenic acids from the oil vapors, probably because of the relatively low viscosity and consequently better contact between oil vapors and alkaline treating agent which is obtained With a high oil content in the treating agent as compared with treating agents wherein there is a low oil content or wherein there are two liquid phases. It has been found-that the process of the present invention solves very satisfactorily the problem of obtaining good contact between alkaline material and naphthenic acids.

The contact between lubricating oil vapors and the liquid alkaline material according to one embodiment of the invention, can be brought about by continuously cir culating the alkaline material through a system including a contacting zone wherein the alkaline material flows downwardly in countercurrent contact with the rising oil vapors. The circulating alkaline material is withdrawn from a lower portion of the contacting zone and recycled to an upper portion thereof. The circulating alkalinen1aterial generally comprises a mixture of alkali metal naphthenate and alkali metal hydroxide, and in order to maintain the desired concentration of alkali metal hydroxide in the circulating material, fresh alkali metal hydroxide can be added thereto, and a drag stream removed there from. The drag stream contains valuable alkali metal naphthenates which can be recovered therefrom by suitable known methods.

Instead of continuously circulating alkaline reacting material through the system, it is possible to introduce fresh alkali metal hydroxide into the contacting zone for contact with the oil vapors, and to remove from the contacting zone alkali metal naphthenates or a mixture thereof with alkali metal hydroxide without reintroducing the material thus removed into the contacting zone. In the subsequent discussion of alkaline treatments of petroleum vapors, reference will be made to processes where a circulating alkaline material is employed, but it is to be understood that the same principles apply where the alkaline material passes only once through the treatmg zone.

The invention will be further described with reference to the attached drawing, which is a schematic representation of distillation apparatus in which the process of the present invention can be carried out.

In operation, charge oil, e. g. a topped naphthenic crude oil, is, after preheating to a suitable temperature, introduced into flash section 11 ofdistillation tower through line 12. Vapors are evolved and pass upwardly into caustic treating section 13 of tower 10. The vapors pass upwardly through apertures in trays 14, and then upwardly into fractionation section 15 of tower 10. The vapors pass upwardly through the trays 16, and a portion of the vapors is condensed on each tray 16, and various distillate fractions are removed through the lines 17 from the trays 16.

On the trays 14 of the caustic treating section 13, there is maintained a liquid alkaline material comprising liquid lubricating oil, sodium hydroxide, and sodium naphthenates. The liquid alkaline material on lowest tray 22 of treating section 13 comprises, for example, 78 parts by weight of liquid lubricating oil, 18.6 parts of sodium naphthenates, and about 1.8 parts of sodium hydroxide. This alkaline material is withdrawn through line 23, and at least a portion is recycled through line 25 and line 20, or through line 25 and line 26, to treating section 13. If introduced into treating section 13 through line 26, the alkaline material is commingled with caustic soda from line 21 on the uppermost tray 14 of treating section 13. If introduced into treating section 10 through line 20, the commingling takes place after the caustic soda introduced through line 21 has contacted oil vapors on the top tray of treating section 13 and has flowed down to the next lower tray.

The liquid oil content of the alkaline material on the trays 14 can be maintained by condensation of oil from the vapors rising through the treating section 13; or by introduction into treating section 13 of a separate stream of liquid oil which is condensed in fractionating section 15 and introduced from the lowest tray 18 of fractionating section 15 into treating section 13 through lines 19 and 20, or through lines 19 and 26; or by recycling oil removed from the alkaline material in extraction zone 40 as subsequently described; or by a combination of such means. Instead of the

exterior lines

19 and 20 or 26, a downcomer, not shown, from tray 18 to the top tray 14 of treating section 13 could be used to reflux oil from fractionating section 15 to treating section 13.

The sodium naphthenates on the trays 14 are derived from reaction of sodium hydroxide in the liquid alkaline material on the trays 14 with naphthenic acids contained in the oil vapors. The sodium hydroxide on the trays 14 is derived from preheated, fresh caustic soda introduced through line 21 onto the uppermost tray 14 of the treating section 13. The caustic introduced through line 21 can be either aqueous caustic soda or substantially anhydrous, molten caustic soda. If aqueous caustic is added, the temperature in treating section 13 is generally high enough that substantially all the water is vaporized from the caustic upon introduction into treating section 13 so that the caustic is molten as it passes downwardly therein.

In operation where the liquid alkaline material is introduced through line 20 into treating section 13, caustic soda, together with whatever sodium naphthenates are formed on the top tray and with whatever oil is condensed on the top tray, flows from the top tray to the next lower tray, where it is admixed with liquid alkaline material, which is introduced through line 20. The admixture flows downwardly from tray to tray, contacting rising oil vapors and increasing in oil and naphthenate content, until it reaches lowermost tray 22 in treating section .13, by which time the composition of the alkaline material is about 78 percent oil, 18.6 percent sodium naphthenates, and 1.8 percent free sodium hydroxide.

This alkaline material is removed from treating section 13 through line 23, and a portion is removed through line 24. The rest is passed through line 25 and re-introduced into treating section 13 through line 20 or line 26.

The alkaline material removed through line 24 can be introduced into extraction Zone 40 and contacted therein with aqueous isopropanol and petroleum naphtha introduced through lines 41 and 42 respectively. Upon layer separation, an aqueous phase containing sodium naphthenates and sodium hydroxide dissolved in aqueous isopropanol and a hydrocarbon phase containing oil dissolved in petroleum naphtha are obtained. The aqueous phase is removed through line 43 for further treatment by means not shown to recover the sodium naphthenate or naphthenic acid product. The hydrocarbon phase is removed through line 44 and introduced into solvent recovery zone 45, wherein naphtha is stripped off through line 46 and recycled to the extraction zone. All or a part of the recovered oil can be withdrawn through line 47 and re-introduced through line 20 or line 26 into treating section 13. Any recovered oil not returned to the treating section can be removed from the system through line 48.

Preferred operation according to the invention is that wherein all or a part of the recovered oil is recycled through line 47, since it is diificult or impossible to maintain the optimum oil content in the liquid alkaliine materal without such recycling. There are practical limits to the amounts of oil available by condensation in treating section 13 and by reflux from fractionating section 15, and optimum prevention of plugging can be suitably obtained only by recycling oil recovered from the extraction step.

Any suitable method can be employed for removal of oil from the liquid alkaline material in the extraction operation. For example, the method disclosed in U. S. Patent 2,600,537 to Earl M. Honeycutt for separating oil from naphthenates in a residual oil can be employed to separate oil from naphthenates in the liquid alkaline material of the present invention.

The amounts of oil in the liquid alkaline material which are provided by condensation in treating section 13 depend on the temperatures in treating section 13; generally, higher oil contents are obtained with lower temperatures at the top of treating section 13, relative to the temperature at the bottom of treating section 13. The temperatures employed depend on the nature of the charge oil and other factors, and in the light of the present specification, a person skilled in the art can determine the proper temperatures to employ to obtain or assist in obtaining the required oil content of the liquid alkaline material. The control of the temperature in treating section 13 can be effected by control of the temperature of the liquid alkaline material upon introduction into treating section 13 through line 20, and the circulating liquid alkaline material can be heated or cooled as required to effect this control.

Under most practical operating conditions, there will I be some condensation of oil in treating section 13, so

that the oil content of the liquid alkaline material therein is derived in part from such condensation. it is necessary according to the present invention to provide a substantially greater oil content in the alkaline material than that which is obtained according to ordinary operation wherein no affirmative measures are taken .to provide an oil content as required to obtain the ad- However,

treating section than can be obtained where the only method of control is by regulation of the temperature in the treating section.

In the operation illustrated in the drawing, fresh caustic soda is introduced onto the top tray of the treating section, and recycled liquid alkaline material may be introduced onto the next tray from the top in the treating section. The operation is particularly advantageousin that it provides contact on the top tray, of freshcaustic soda with oil vapors which have been freed from nearly all of their naphthenic acid content, and fresh caustic is particularly eifective in removing the small remaining amounts of naphthenic acids from the vapors. The

amounts of liquid oil on the top tray, of course, are very much below the amounts required'on the lower trays, but this is not detrimental since the amounts of sodium naphthenates formed on that top tray are relatively small, and there is no danger of excess decomposition. It is on the lower trays, where large amounts of naphthenates are formed that it is essential to maintain at least 60 percent of liquid oil in the alkaline material.

It is to be noted that it is only on the uppermost tray or trays that fresh caustic soda may be employed, and that on the lowermost tray or trays of the treating section, the oil content of the liquid alkaline material must be maintained in the previously specified, relatively high range, and the alkali metal hydroxide content must be maintained in the previously specified, relatively low range, since it is on the lowermost tray or trays that large amounts of sodium naphthenates are formed, and the stated proportions must be maintained in order that excessive decomposition of the naphthenates does not occur.

Thus, according to the present invention, fresh caustic is added to the system at a location such that the caustic does not come in contact withoil vapors having high naphthenic acid content until that soda has become diluted with oil to the extent that the liquid alkaline material contains 60-95 percent oil. Thus the fresh caustic should not be contacted with oil vapors before the latter have had their naphthenic acid content substantially reduced by contact with liquid alkaline material containing 60-95 percent oil, and the fresh caustic is preferably added either to the alkaline material after removal from the treating section and before recycle thereto, or to one of the upper trays of the treating section.

In the operation described previously in connection with the drawing, a portion of the liquid alkaline material removed from treating section 13 through line 23 is recycled to the treating section 13 through line 20 or line 26. It is to be understood, however, that all of the alkaline material can be withdrawn through line 24, so that the only introduction of sodium hydroxide to treating section 13 is through line 21. In this case, liquid oil condensed in fractionating section 15 or recycle oil from the extraction operation or both are introduced through lines or .26 into the treating section 13, and generally at a higher rate than that employed when such liquid oil isintroduced through line 20 or line ,26 together with recycled liquid alkaline material. Whether or not the alkaline material is recycled, it is important to maintain, in at least a lowermost portion of treating section 13, an alkaline treating material composition containing 60-95 percent oil, 3-30 percent sodium naphthenates, and 02-10 percent sodium hydroxide.

The following example illustrates the invention:

The charge oil, a topped n'aphthenic crude, was preheated to 735 F. and introduced into a flash zone wherein an absolute pressure of 42mm. of Hg was maintained. The flashed vapors were introduced into a treating section wherein they were countercurrently contacted with descending liquid alkaline material which had, on the lowermost tray of the treating section, the following com- The liquid alkaline material removed from the lowermost tray of the treating section was found to contain only 1.6 parts by weight of naphtha-insoluble materials, the latter being in all likelihood decomposition products resulting from decomposition of organic constituents of the liquid alkaline material. It is seen that the decomposition obtained in this example was only slight and did not adversely aifect the yield of sodium naphthenates to any substantial extent.

The liquid alkaline material removed. from the lowermost tray of the treating section was recycled to an upper portion of the treating section. Fresh 38 B. caustic soda was introduced into an upper portion of the treating section at a volume rate equivalent to 0.6 percent of the volume rate of introduction of charge oil to the flash zone. To maintain a substantially constant amount of alkaline material in the treating section, a product stream was withdrawn from the alkaline ma terial after removal from the treating section and before reintroduction thereinto. The volume rate of withdrawal of product stream was 5.6 percent of the volume rate of introduction of charge oil into the flash. zone.

The oil content of the liquid alkaline material on the lowermost tray of the treating section was maintained at 78 percent by regulation of the vapor temperature at the top of the treating section at 646 F. The liquid temperature on the lowermost tray of the treating section was 656 F. The uncondensed vapors passed from the top of the treating section into the fractionating section and upwardly therethrough. No condensate was returned from the fractionating section to the treating sec tion, and the oil content of the liquid alkaline material in the treating section was maintained entirely by condensation occurring in the treating section.

Comparison example To determine the effect of decreasing the liquid oil content of the alkaline material in the treating section, another run was made wherein the composition of the alkaline material on the lowermost tray of the treating section Was:

Parts by weight Liquid oil 45.7 Sodium naphthenates 44.2 Excess sodium hydroxide 1.4 The relatively low oil content was obtained by maintaining the temperatures at the top and bottom of the treating section at 653 F. and 641 F. respectively. The naphthainsoluble content of this alkaline material was found to be 8.7 parts by weight indicating more than five times as much decomposition of organic materials in the alkaline material in this run as in the preceding example wherein the oil content was 78 percent.

In a series. of additional runs, it was found that in five runs where the oil content was in the range 46-57, the percent of naphtha-insolubles ranged from 17 to 26, Whereas in 11 runs where the oil content was in the range 64-84, the percent of naphtha-insolubles was never greater than 10, and in a majority of runs was 5 or less.

Example 2 Reduced naphthenic crude was preheated and introduced at a rate of parts by volume per hour into a flash zone wherein a temperature of 690 F. and an absolute pressure of 65-90 mm. of Hg were maintained. The flashed vapors were introduced into a treating section wherein they were countercurrently contacted with descending liquid alkaline material which had,- on' the lowermost tray of the treating section, the following composition:

Weight percent Liquid oil 88-92 Sodium naphthenates 6-11 Excess sodium hydroxide 1-2 Liquid alkaline material was removed from the lowermost tray of the treating section at a rate of 57 parts by volume per hour. A portion of the removed liquid alkaline material was recycled to an upper portion of the treating section at a rate of 50 parts by volume per hour. The remainder of the alkaline material was introduced into an extraction operation at the rate of 7 parts by volume per hour. Liquid oil recovered from the alkaline material in the extraction operation was recycled to an upper portion of the treating section at a rate of 2 parts by volume per hour.

Fresh 50 B. caustic soda wa introduced into an upper portion of the treating section at a rate sufficient to maintain the above concentrations of sodium naphthenates and excess sodium hydroxide in the liquid alkaline material on the lowermost tray of the treating section The liquid temperature on the lowermost tray of the treating section was about 650 F., and the vapor temperature at the top of the treating section was about 630 F.

The uncondensed vapors passed from the top of the treating section into the fractionating section and upwardly therethrough. Normal reflux was maintained from the fractionating section to the treating section by means of a liquid downcomer from the lowermost tray of the fractionating section to the top tray of the treating section.

In the operation described, the oil content of the alkaline material was maintained at a high levelby three means: condensation within the treating section, reflux from the fractionating section, and recycle from the extraction operation. No excessive plugging of the apparatus occurred. In operation in a similar manner but without recycle of oil from the extraction operation, the oil content of the liquid alkaline material on the lowermost tray of the treating section was 8385 weight percent, and greater tendency for plugging of the apparatus was observed than when oil was recycled from the extraction operation.

In the operation of this example, the oil recycled from the extraction operation was injected into the portion of the liquid alkaline material which was returned to the upper portion of the treating section, the injection occurring before reintroduction of that portion into the treating section. Alternatively, the recycled oil could be injected into the treating section separately from the liquid alkaline material; e. g. the recycled oil could be injected into the treating section at a higher level than that at which the liquid alkaline material is introduced into the treating section.

In the operation of this example, the liquid alkaline material removed from the treating section was in part returned to the treating section and in part introduced into an extraction operation. Alternatively, all of the liquid alkaline material could be returned to the treating section, or all could be introduced into the extraction operation.

In the operation of this example, only part of the oil recovered in the extraction operation was recycled to the treating section. Alternatively, all or none of the oil recovered could be recycled to the treating section.

This application is a continuation-in-part of application Serial No. 315,222 filed October 17, 1952, now abandoned.

The invention claimed is:

1. In the removal of naphthenic acids from heavy petroleum lubricating oil vapors by contact with liquid alkaline material in a contacting zone at a temperature within the approximate range from 600 F. to 700 F under reduced pressure permitting vapor phase contacting atsaid temperature, the improvement which comprises: initiating contact of said vapors with said alkaline material in a portion of said contacting zone wherein said alkaline material comprises 60 to parts by weight of liquid lubricating oil, 3 to 30 parts of alkali metal naphthenates, and 0.2 to 10 parts of alkali metal hydroxide, whereby formation of carbonaceous deposits normally occurring in said contactingv zone at said temperature, is inhibited.

2. Method according to claim 1 wherein a stream of liquid lubricating oil is introduced into said contacting zone separately from said lubricating oil vapors in order to maintain said 60 to 95 parts of lubricatingoil in said liquid alkaline material.

3. Methodaccording to claim 2 wherein said lubricat ing oil vapors, after contact with said liquid mixture, are removed from said contacting zone and subsequently at least partiallycondensed, and wherein said stream of liquid lubricating oil comprises at least part of the condensate thereby obtained.

4. Method according to claim 1 wherein alkali metal hydroxide is continuously introduced into said contacting zone, and said liquid alkaline material is continuously removed therefrom, and a portion of the liquid alkaline material thus removed is continuously re-introduced into said contacting zone, thereby to maintain a substantially constant volume of said liquid alkaline material in said contacting zone and a substantially constant proportion of alkali metal hydroxide in said liquid alkaline material at a given level in said contacting Zone.

5. Method according to claim 4 wherein alkali metal hydroxide thus introduced is contacted in an upper portion of said contacting zone, in the absence of said liquid alkaline material, with lubricating oil vapors which have previously been contacted with said liquid alkaline material, and wherein the alkali metal hydroxide is then passed downwardly and admixed with said liquid alkaline material.

6. Method for removal of naphthenic acids from heavy mineral lubricating oil which comprises: introducing lubricating oil vapors-containing naphthenic acids into a lower portion of a contacting zone; passing said vapors upwardly from said lower portion into an upper portion of said contacting zone; introducing into said upper portion of said contacting zone a liquid alkaline material comprising lubricatingoil, alkali metal naphthenate, and alkali metal hydroxide; contacting said liquid alkaline material with said lubricating oil vapors at a temperature within the approximate range from 600 F. to 700 F. under reduced pressure permitting vapor phase contacting at said temperature, thereby to react naphthenic acids in said lubricating oil vapors with said alkali metal hydroxide to form alkali metal naphthenate in said liquid alkaline material; passing said liquid alkaline materal downwardly from said upper portion into said lower portion; withdrawing from said lower portion of said contacting zone said liquid alkaline material, now containing 60 to 95 parts by weight of lubricating oil, 3 to 30 parts by weight of alkali metal naphthenate, and 0.2 to 10 parts by weight of alkali'metal hydroxide; recycling a portion of the liquid alkaline material thuswithdrawn to said upper portion of said contacting zone; removing oil from another portion of the liquid alkaline material thus withdrawn; introducing the oil thus removed into said upper portion-of said contacting zone; introducing alkali metal hydroxide into said upper portion; commingling alkali metal hydroxide thus introduced into said upper portion with said liquid alkaline material in said upper portion; removing lubricating oil vapors from said upper portion of said contacting zone; introducing the vapors thus removed into' a fractionating zone; introducing liquid lubricating oil condensate from said fractionating zone into said upper portion of said contacting zone; andmaintaining the concentrations of constituents of said liquid alkaline material in said lower portion of said contacting zone within the recited ranges, thereby to inhibit formation of carbonaceous deposits normally occurring in said contacting zone at said temperature.

7. Method according to claim 1 wherein liquid alkaline material is removed from said contacting zone, and wherein lubricating oil is separated from the alkaline material thus removed, and wherein at least part of the separated lubricating oil is reintroduced into said contacting zone.

8. Method according to claim 1 wherein said liquid lubricating oil consists essentially of distillate material.

References Cited in the file of this patent UNITED STATES PATENTS Rogers July 2, 1935

Claims

1. IN THE REMOVAL OF NAPHTHENIC ACIDS FROM HEAVY PETROLEUM LUBRICATING OIL VAPORS BY CONTACT WITH LIQUID ALKALINE MATERIAL IN A CONTACTING ZONE AT A TEMPERATURE WITHIN THE APPROXIMATE RANGE FROM 600* F. TO 700* F. UNDER REDUCED PRESSURE PERMITTING VAPOR PHASE CONTACTING AT SAID TEMPERATURE, THE IMPROVEMENT WHICH COMPRISES: INITIATING CONTACT OF SAID VAPORS WITH SAID ALKALINE MATERIAL IN A PORTION OF SAID CONTACTING ZONE WHEREIN SAID ALKALINE MATERIAL COMPRISES 60 TO 95 PARTS BY WEIGHT OF LIQUID LUBRICATING OIL, 3 TO 30 PARTS ALKALI METAL NAPHTHENATES, AND 0.2 TO 10 PARTS OF ALKALI METAL HYDROXIDE, WHEREBY FORMATION OF CARBONACEOUS DEPOSITS NORMALLY OCCURRING IN SAID CONTACTIG ZONE AT SAID TEMPERATURE IS INHIBITED.