US3023084A

US3023084A - Caustic regeneration process

Info

Publication number: US3023084A
Application number: US786854A
Authority: US
Inventors: Jr Arthur M Thomas
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1959-01-14
Filing date: 1959-01-14
Publication date: 1962-02-27
Anticipated expiration: 1979-02-27

Description

Feb. 27, 1962 A. M. THOMAS, JR

CAUSTIC REGENERATION PROCESS Filed Jan. 14, 1959 3 uzoN zQEexo w o. J; 6 EEEEE 305:. mm T 2 1. :75 @2308 Em; 4 Q :2

Arthur M. Thomas, Jr. Inventor By 'R 6-+3.?1M M% Agenf United States Patent 3,023,084 CAUSTlC REGENERATEUN PROtIESS Arthur M. Thomas, .lr., Railway, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Jan. 14, 1959, Ser. No. 7863M 7 (Ilaims. (Cl. 23-484) The present invention is concerned with an improved method of regenerating alkaline solution which has been employed to wash a hydrocarbon oil containing mercaptans and phenolic constituents. More particularly, it deals with a combination process whereby both extracted mercaptans and phenolates may be removed from the spent alkaline solution, thus liberating alkali for further oil extraction.

The use of an alkaline wash solution to treat hydrocarbon oils is Well known in the art. Hydrocarbon fractions such as LPG, naphthas, and heating oils and light and heavy catalytic naphthas, generally boiling in the range of 0 to 600 F., contain mercaptans and other sulfur-containing compounds. Mercaptans are particularly undesirable due to their noxious odor, and sulfur-containing materials are in general deleterious since they adversely afiect stability as well as combustion properties. Sulfur removal is also necessary in cases where material is to be fed to some catalytic reforming or polymerization process where small amounts of sulfur are catalyst poisons. Sulfur in naphtha lowers lead response; in heating oils'it burns to form corrosive gases. By subjecting such oils to extraction with an alkaline solution, e.g. sodium or potassium hydroxide, mercaptans and sulfur-containing compounds are removed into the alkaline phase. Since the feed hydrocarbon oils invariably also contain phenolic constituents characterized by para tertiary amyl phenol, the alkaline extract will also contain phenolates. Gradually, the concentration of extracted materials increases and it is periodically necessary to replace the alkaline solution with fresher material. Due to the large volumes of alkali involved, it is economically necessary that the spent alkaline solution containing mercaptides, phenolates, sulfides, etc. be regenerated.

Numerous means have been advanced in the art for regenerating such alkaline solutions. Since the primary purpose of the alkaline Wash is the removal of mercaptans and sulfides, the prior art processes are principally concerned with removing these materials.

In one such well known process, it has been suggested to subject the spent alkaline solution to steam stripping at pressures of 50 p.s.i.g. and less, and temperatures of 100 to 300 F. It has been shown that these mildstripping conditions are effective in removing mercaptides from the spent solution.

Competitive with the above process is the oxidative regeneration process. This process accomplishes the same purpose as the mild stripping in that it removes mercaptides. The mercaptides are oxidized to disulfides which are readily separated from the alkaline solution.

Both conventional steam stripping step and conventional oxidative regeneration have little or no effect on phenolate removal.

Though the above processes have individually met with some success, they have encountered difiiculties. In spite of regeneration by these methods, ultimately concentrations of carboxylic acids and phenolates as well as other materials prevent indefinite regeneration and use of regenerated solution. These compounds tie up the free sodium ions leaving none to react with and extract the phenolates, mercaptans, sulfides and acid oils from the hydrocarbon stream. The resulting alkaline solution disposal problem is'quite' severe since due to its high phenolic content it is detrimental to aquatic life and thus normally can not be simply discharged into streams, lakes, or the like. The increasing numbers of municipal restrictions regarding waste disposal have aggravated this matter. Relatively costly means of reducing phenol content in spent solution, such as by a biological oxidation process, have been reverted to in attempts to reduce pollution upon discharging spent alkaline Wash solutions. Another means suggested for reducing this problem is the use of carbon dioxide so as to spring phenolates out of solution by converting them to phenols. However, the latter process has severe limitations since the treatment of spent caustic with carbon dioxide as proposed in the prior art also Will convert free caustic to carbonates thus. depleting free alkali. The resultant buildup of carbonates necessitates ultimate discard of the caustic.

Thus there exists a need in the art for a process whereby the above difiiculties may be substantially eliminated. The present invention serves to satisfy this need.

In accordance With the present invention, spent alkali solution (containing mercaptans, phenolates, sulfides, etc.) is first subjected to a severe stripping treatment at pressures above about 500 p.s.i.g. and a temperature of at least 400 F. Normally, stripping pressures will be in the range of 500 to 2500 p.s.i.g. and temperatures of 400 to 700 F. are employed. It has been found that these severe stripping conditions result in removal of phenolates due to their hydrolysis to phenols, the phenols being removed from the solution by the stripping action of the steam. Although this severe treat serves primarily to remove phenolates, some sulfides as Well as mercaptans will also be removed.

The treated solution having a substantially decreased concentration of phenolates, e.g. less than 50 wt. percent of the concentration in the initial spent alkaline solution, is then subjected to a fairly conventional oxidative regeneration step such as treatment with air at moderate pressure and temperature.

The present invention offers numerous advantages. It has been found that the prior severe stripping treat not only serves to partially regenerate the caustic, but by having removed phenolates and some sulfides prior to oxidation, the oxidation step itself is enhanced. The mercaptan oxidation rate is substantially greater at reduced concentration of phenolates, as Well as being greater at reduced concentration of sulfides. Further, the amount of alkali metal thiosulfate normally formed from sulfides in the oxidation step is reduced thereby reducing the loss of alkali metal held in this manner.

In addition to improving the oxidation step, the phenolate concentration in the recycled alkaline wash is reduced as is the concentration of phenolics in the solution ultimately to be sent to disposal. Further, the phenols removed from the severe stripping treatment may be recovered and used for the manufacture of chemicals among them phenolic type resins, plastics and plasticizers. The stripping step does not spend already present tree alkali but treats only those alkali metal ions which are combined with the contaminating organic molecules.

It should be clearly understood that the present process is distinguished from simply employing a severe heat soaking of spent alkaline solution. The present process requires the use of a stripping gas, such as steam. If no stripping gas is supplied, only a small amount of the phenolates, e.g. less than about 20%, are removed. In the absence of stripping gas, phenolates are simply held in the Vapor space above the solution, thus building up to an equilibrium with the phenolates in the liquid phase and only a small degree of phenolate removal is obtained. Secondly, the present invention contemplates a two step regeneration in which mercaptideremoval is sacrificed in the first step in favor of phenolate removal, the oxidative regeneration step thereafter efiiciently serving to reduce mercaptide concentration to desired levels.

Similarly, conventional mild stripping conditions do not serve to remove phenolates, but rather are an alternative toprior art oxidation procedures.

By way of clarifying nomenclature, the term alkaline solution denotes the basic solutions (as opposed to acidic) formed from the hydroxides or salts of the alkali metals, particularly those of sodium and potassium. The term oxidative regeneration includes the various prior art procedures wherein mercaptides are reacted with oxygen to form 'disulfides. The oxygen may be pure oxygen, air, ozone, etc.

The various aspects and modifications of the present invention will be made more clearly apparent by reference to the following description, examples and accompanying drawing. 1

The drawing is a simplified illustration of the present system consisting principally of stripper and oxidation zone 24. By way of supplying a specific embodiment, it is desired to regenerate the spent caustic solution resulting from caustic washing of a heavy catalytic naphtha fraction with an initially 50 B. sodium hydroxide.

The spent caustic fed to unit 10 through line 11 contains 4 wt. percent phenols and based on milligram per 100 cc. of caustic it has a mercaptan number of 91 and a sulfide number of 16. The phenols are in the form of sodium phenolates. Substantially all the contaminants were initially derived from the treated hydrocarbon oil.

Stripping zone 10 operates at a pressure of 600 p.s.i.g. and a temperature of 500 F. A stripping gas such as steam is introduced into the lower portion of zone 10 via line 12, and serves to strip phenols from the spent caustic solution, phenols being removed overhead through outlet 18 along with the stripping gas. Additionally, some sulfides and mercaptides will also be removed by the stripping gas. About pounds of steam are employed per pound of spent alkaline solution treated in unit 10, stripping being etiectecl for a period of about 4 hours. The contact time is capable of Wide variation, it normally being adjusted to strip out at least 50 wt. percent of the phenols which are initially in the spent solution in the form of phenolates.

The bulk of the caustic is withdrawn through line 13. its composition is tabulated below:

The stripped alkaline solution is then circulated to oxidation zone 24 by means of pump 14 and lines 15 and 17. If desired, a portion may be recycled to unit 19 through conduit 16.

Oxidation zone 24 may be any one of a number of conventional oxidative regeneration processes such as air blowing at moderate or elevated pressures at moderate or elevated temperatures. In the present embodiment, zone 24 operates at about 120 F. and 40 p.s.i.g. Air or other oxygen-containing gas is supplied through line 25 and serves to convert mercaptides to disulfides. In addition sulfides are converted. to thiosulfates. Gases are vented through line 22, preferably into the same outlet system employed for venting the stripping zone gases, as will be later described.

Oxidized caustic solution is passed to separation drum 27 by conduit 26. Disullides may normally be readily separated by gravity settling, the lighter disulfide layer being removed through outlet 28. Regenerated caustic is Withdrawn through line 29, and may be recycled for further hydrocarbon oil extraction.

The regenerated caustic in line 29 now has a mercaptan No. of less than 5.0 and a sulfide No. of less than 5.0. The oxidation step has substantially no eitect on the concentration of phenols.

Returning to the stripping gas-phenols eifiuent of zone 10, phenols may readily be separated by cooling the stream in unit 19. Phenols and sulfides are withdrawn through outlet 20 for further processing or are simply collected. The separated stripping gas may be vented through

lines

21 and 23, or if desired may be recycled to the stripping zone, by means not shown.

In many cases, it Will be desirable to heat exchange the stripping gas sent to unit 11 with the hot etfiuent solution withdrawn through line 13 in order to conserve heat.

Various modifications may be made to the system described. For example, only a part of the total spent alkaline solution might be subjected to the present two-step rocess while the remaining solution is simply regenerated by conventional means, the two fractions thereafter being combined for hydrocarbon extraction. As the spent alkaline solution composition changes due to refinery operational changes or as pollution regulations become more severe, the fraction of spent solution treated in the present manner may be varied. Further, although steam is preferred for use as the stripping gas, other gases such as nitrogen, flue gases, etc., may be utilized.

Various experiments will now be detailed in order to illustrate the operation of the present invention and the advantageous results to be realized thereby.

Example 1 Tabulated below are the results of subjecting various spent caustic solutions which had previously been used forhydrocarbon extractions to the present high pressure, high temperature stripping treatment.

TABLE 2 HIGE PRESSURE STRIPPING OF SPENT ALKALINE SOLUTION Operating conditions:

Temperature, F. 500 179 496 553 60" 610 608 Pressure, p.s.i.g.. 600 600 600 1,000 1, 500 1, 500 1, 500 Steaming rate, Min... 0. 99 0.98 0.99 1.02 1.0 1.00 0. 51 Water inlet temperature, F 200 201 200 201 200 200 201 Phenoiate concentration,

weight percent' Initial".-. 20. 4 9. 0 5. 3 9. 0 9. 0 20. 4 20. 4 After 1 non 18. 9 7. 7 4. 7 7. 7 7. 5 17.7 After 3 hours. 14.0 6.6 3. 4 5. 8 5. 4 11.0 11. 0 After 5 hours 10.0 4. 6 1. 5 3. 6 3. 4 7. 7 7. 0 Attertlhours 8.5 4.0 1.6 3.3 2.9 3.7 6.1 Percent removed.. 58.4 63.3 69.8 67.0 71.0 81.8 70.0

Mercaptan number,

rugs/100 cc;

n' 1 560 388 5 388 388 560 560 After 6 hours... 264 127 61 144 150 280 280 Percent remove 52. 9 07.3 41. 9 62.9 61. 3 50.0 50.0

Sulfide number, rugs/100 Initial 140 70 35 70 70 140 M0 After 6 hours 58 6 54 60 106 Percent removed...- 39. 3 17. 2 82.8 22.9 14. 3' 24. 3 28.1 Caustic concentration,

weight percent:

Initial 3. 8 7. 6 7. 6 14. 4 14. 1 After 6 hours. 9. 2 15. 2 14.1 23.8 18.2 Percent incre 142.1 100.0 85. 5 65.3 26. 4 Steam consumption, 1b./1b. of spent NaOH solution 41. 3 33. 5 73. 3 53. 6 61.1 42. 5 53. 7

The various experimental runs depicted in Table 2 illustrate that the present high pressure-high temperature stripping treatment substantially reduces the concentration of phenolates. Further, it reduces the concentration of sulfides in the alkaline solution and at least partially regenerates the solution.

It is also to be noted that under the conditions of the stripping'step a substantial portion of the mercaptides will remain in the alkaline solution.

Example 2 To illustrate the difierence between the present stripping step and steam stripping under conventional mild conditions, spent caustic was subjected to steam stripping under the conditions of the prior art.

Table 3, particularly when compared to Table 2, shows that conventional stripping does not remove appreciable quantities of phenols, but rather conditions are such as to favor sulfide removal. Thus, the present high temperature stripping is readily distinguished from the practices of the prior art.

Example 3 The data presented below show the benefits of employing the present stripping operation prior to a conventional oxidative regeneration treatment.

The results in the following table are the mercaptide oxidation rates at a mercaptide level of 1000'mgs./ 100 cc. of solution for two levels of phenolate concentration. The data are from the air oxidation of an initially 25 B. caustic.

TABLE 4 Temperature, Oxidation Phenolates, vol. percent F. rate, rugs/100 ccJhr.

It is thus seen that the oxidation rate in the absence of phenolates is nearly six times greater than when the spent caustic contains 36 vol. percent phenol (present in the form of phenolates). This is in spite of the fact that the temperature is 23 F. lower in the case of low phenolate concentration.

The above results show the advantage of reducing the phenolate content of the spent alkaline solution prior to subjecting it to oxidative regeneration, as is done by the present combination process.

Similarly, Table 5 illustrates that for approximately the same mercaptan level, reducing the amount of sulfide compoundspresent in the solution to be subjected to air oxidation will enhance the rate of oxidation. The data was obtained from the air oxidation of an originally 18 B. spent caustic at 100 F. and 50 p.s.i.g. in a stirred vessel. No phenolates were present.

TABLE 5 Sulfide Oxidation Mercaptlde number, mgs./l00 cc. number rate, mgs./100

mgs./100 cc. cc. r.

The above data show that the mercaptide oxidation rate is appreciably greater at low concentration of sulfides. Thus, there is a further improvement in the oxidation step because of the previous stripping which had reduced sulfide concentration. (See Table 2.)

summarily, it is seen that the present combination process offers substantial advantages over both conventional steam stripping and direct oxidation of spent alkaline solutions. That which is sought to be protected is set forth in the following appended claims.

What is claimed is:

1. An improved process for regenerating a spent alkaline solution which has previously been employed to wash a hydrocarbon oil containing mercaptans and phenolic materials which comprises, in combination; subjecting said spent alkaline solution to stripping with a gas at a pressure of at least 500 p.s.i.g., and a temperature of at least 400 F. so as to remove a substantial portion of the phenolates from said spent alkaline solution; thereafter removing mercaptans from said thus treated alkaline solution by means of an oxidative regeneration step wherein said mercaptans are converted to disulfides by reaction with oxygen.

2. The improved process of claim 1 wherein said spent alkaline solution is subjected to stripping at a temperature in the range of 400 to 700 F. anda pressure in the range of 500 to 2500 p.s.i.g., and steam is employed as the stripping gas.

3. The improved process of claim 1 wherein said spent alkaline solution is a spent solution of sodium hydroxide, and wherein at least 50 weight percent of the phenolates in said spent sodium hydroxide solution are removed by said high pressure stripping prior to passing said solution to said oxidative regeneration step.

4. The improved process of claim 1 wherein an oxygencontaining gas is employed in said oxidative regeneration step to convert mercaptans to disulfides.

5. The improved process of claim 1 wherein said spent alkaline solution is a spent caustic solution.

6. In the oxidative regeneration process for regenerating spent alkaline solution which has been employed to treat a hydrocarbon oil containing mercaptans and thiophenols, the improvement which comprises, gas stripping said alkaline solution at a pressure in the range of 500 to 2500 p.s.i.g., and a temperature in the range of 400 to 700 P. so as to remove at least 50 wt. percent of the phenolates present in said spent alkaline solution prior to subjecting said alkaline solution to oxidative regeneration wherein mercaptans are converted to disulfides by reaction with oxygen.

7. The improvement of claim 5 wherein steam is employed for gas stripping said alkaline solution and said alkaline solution is spent caustic.

References Cited in the file of this patent UNITED STATES PATENTS Happel et al Aug. 5, 1952 Petty Dec. 2, 1958 OTHER REFERENCES

Claims

1. AN IMPROVED PROCESS FOR REGENERATING A SPENT ALKALINE SOLUTION WHICH HAS PREVIOUSLY BEEN EMPLOYED TO WASH A HYDROCARBON OIL CONTAINING MERCAPTANS AND PHENOLIC MATERIALS WHICH COMPRISES, IN COMBINATION; SUBJECTING SAID SPENT ALKALINE SOLUTION TO STRIPPING WITH A GAS AT A PRESSURE OF AT LEAST 500 P.S.I.G., AND A TEMPERATURE OF AT LEAST 400*F. SO AS TO REMOVE A SUBSTANTIAL PORTION OF THE PHENOLATES FROM SAID SPENT ALKALINE SOLUTION; THEREAFTER REMOVING MERCAPTANS FROM SAID THUS TREATED ALKALINE SOLUTION BY MEANS OF AN OXIDATIVE REGENERATION STEP WHEREIN SAID MERCAPTANS ARE CONVERTED TO DISULFIDES BY REACTION WITH OXYGEN.