US2556438A - Mercaptan extraction system - Google Patents

Description

I une 12, 1951 H. A. PARKER :TAL

MERCAPTAN ExTRAcTIoN SYSTEM Filed Aug. 7, 1948 Patented june 12, 195i UNITED STATES orties MERCAPTAN EXTRACTION SYSTEM Indiana Application August 7, 1948, Serial No. 43,072

2 Claims.

This invention relates to an improved system for extracting mercaptans from distillate fuels and it pertains more particularly to an improved closed water cycle in such a system.

While the extraction of mercaptans from gasoline by caustic methanol solutions is well known, it has been found that systems designed therefor are not suitable for extraction `of mercaptans from distillate fuel hydrocarbons such as heater oils, furnace oils, and other mercaptan-containing hydrocarbons which boil chiey above the gasoline boiling range. An object of our invention is to provide an improved system for removing mercaptans from such distillate fuels.

An important object of our invention is to provide a system of maximum efficiency which can be built and operated at minimum expense and which provides maximum simplicity, flexibility and ease of control. A particular object is to minimize clogging o-r fouling of heat exchangers, strippers and other parts of the system with solid alkali metal carbonates. A further object is to minimize reagent losses and product contamination. Another object is to provide improved methods and means for separating and recovering component parts of the extracting andA washing liquids. Another object is to provide closed cycles for extracting and washing liquids for minimizing introduction of extraneous impurities and avoiding loss of valuable reagents. Other objects will be apparent as the detailed description of our invention proceeds.

In practicing the invention the sour (mercaptan-containing) distillate fuel oil is countercurrently extracted with an alkali metal hydroxide aqueous solution containing alkali metal cresylates which are retained in solution by means of methanol. Residual amounts of alkali metal hydroxide solution and methanol are removed from extracted oil by means of internally cycled water. This water which contains methanol and alkali metal hydroxide solution from the washing step is then combined with the extract solution which now contains a major portion of the mercaptans in the form of mercaptides. The wash water may be mixed with the extract solution before it enters the stripping tower and/or the two streams may be separately introduced into the stripping tower for admixture therein but the water stream is preferably introduced at a high level in the tower because this favors the desired hydrolysis for conversion of mercaptides `is wet by liquid but no liquid level is maintained 2 in the base thereof and the reheater at the base of the stripper is preferably operated at such temperature as to give a withdrawn caustic solution of desired concentration which is approximately 50 weight percent (about 47 weight percent in case of caustic soda and about 54 weight percent in case of caustic potash) The overhead from the stripping tower is introduced at an intermediate point in the methanol distillation tower from which methanol is taken overhead and water containing some methanol and mercaptans are withdrawn as bottoms. Here again the tower is operated with no liquid level at its base and the bottoms are withdrawn to a settler for separating mercaptans from water. The presence of about 1 to 5 percent of methanol in the water assists in preventing emulsion formation when separating mercaptans. The amounts of methanol allowed to remain in the water phase removed from the methanol fractionator should not be sufficient to appreciably increase the true solubility of the mercaptans therein. The separated water is recycled to the product washing step. The external ratio of wash water to extracted oil may be in the range of about 1:50 to 1:5 but for effectivewashing the total water to oil ratio (including internal recycle) is preferably in the range of about 1:10 to 2:1. The water thus serves the multiple functions of removing valuable reagents from extracted product, assisting hydrolysis of mercaptides, stripping mercaptans from alkali metal solutions and separating mercaptans from methanol. The use of the closed cycle is desirable because it prevents introduction of extraneous impurities such as carbon dioxide, the presence of which would lead to carbonate formation and consequent plugging of the stripping towers, heat exchangers, settlers, etc.

At least a part of the bottoms from the stripping tower are likewise separated in a settler so that any excess cresylate may be withdrawn from the system. The recovered methanol is preferably mixed with recycled cresylates before being mixed with recycled alkali metal hydroxide because the cresylates are insoluble in the alkali metal hydroxide in the absence of methanol and transferring and mixing the solutionsis facilitated thereby.

The invention will be more clearly understood from the following detailed description thereof read in conjunction with the accompanying drawing which forms a part of this specification and which is a schematic flow diagram of a 20,000 barrel per day commercial unit.

in the particular' example herein described, the charge is a heater oil with a boiling range of about 325 F. to 650 F. (about 90% being above the gasoline boiling range) and a copper number of about to 90. The copper number is the milligrams of mercaptan sulfur per 100 nil. of oil determined by titration with a standardized copper salt solution. The charge is preferably given a prewash with a 3 to 5% sodium hydroxide solution for removing H and any other component soluble in said prewash solution. The prcwashed charge is introduced by line I@ through mixer II to first settler i2. Solvent solution from a subsequent extracting stage is introduced by line I3 to give an external solvent-to-oil volume ratio of about 1:5 to 2:5. Recycled solvent may be introduced by line it to give a total solvent-to-oil ratio up to about l:l. vigorous and intimate mixing should be obtained in mixer H. Although an orifice mixer is preferred, a stirred mixer or any other vtype of mixer may be used.

Settler I2 is designed to provide an oil holding time of about l5 to 45 minutes or more, e. g. about minutes. The solvent layer is withdrawn from the base of settler I2 by line I5 and pump I5. Some of the withdrawn solvent may be recycled by line I4, in which case the rest oi" the solvent (corresponding to the amount introduced through line i3) withdrawn through line i? and heat exchanger Ila to stripping tower I8.

The temperature of the extraction should be below about 100 F. and is preferably about 9G F. At appreciably lower temperatures, e. g. below 80 F., components of the solvent tend to solidify and at appreciably higher temperatures the extraction is less efficient. Sufiicient pressure is employed to provide good mixing when using orice mixers and it is convenient to charge the oil at sufficient pressure to drive it through the entire system of mixers and settlers. The pressure oi the initial extraction step` may be of the order of about 1GO to 200 p. s. i. g. and will de pend chiefly on the number of extraction stages employed with the consequent pressure drop through the system.

Extracted oil from the top of settler I2 is introduced by lines id and 25 to mixer 2l and thence to second settler 22 together with fresh solvent introduced by line 23. internally recycled solvent may be introduced through line 24. The solventtooil ratios and operating conditions in this stage are substantially the same as in the irst stage and it should be understood that any number of stages may thus be employed, 2 to 4 stages being usually sucient. Alternatively the extraction may be accomplished in a countercurrent extraction tower but the stage extraction as hereinabove desc ibed offers outstanding advantages in ease of control and eincient utilization of solvent. In the last (and in this case the second) extraction stage, solvent is removed from the settler by line 25 and pump 26. Here again part of the removed solvent may be returned by line 2Q, an amount being withdrawn through line I3 corresponding to the amount of fresh solvent being introduced through line 23.

The fresh solvent preferably consists of about 1GO volumes of 15 molar aqueous caustic potash solution (about 54 weight percent KOH), about 20 to 40 or approximately 30 volumes of methanol and 2 to 20 or approximately 5 volumes of potase slum cresylate. While NaOH may be employed instead of KOH, the latter is preferred because concentrated solutions of KOH are more fluid, and because it results in a more effective solution for extracting inercaptans. The potassium cresylates are also more fluid and require less methanol to render them miscible in the KOH.

The extracted oil from the nnal settler is introduced by lines 2 and 2S to mixer 29 and product wash settler 3d together -with externally recycled wash water introduced through line 3l and internally recycled wash water introduced through line 32. The external water-tooil volume ratio may be the range of 1:50 to 1:5, e. g. may be about 1:10. The total Waterto-oil volume ratio (including internally recycled water) introduced into mixer 29 may be in the range of about 1:10 to 2:1, e. g. about 1:4. The temperature and holding time in the washing step may be substantially the same as in the extraction step but the pressure is ordinarily somewhat lower due to the pressure drop through the system.

Washed product is withdrawn from the top of settler 30 through line 33. Wash water is withdrawn from separator 3d by lines 3d and pump 35, the major part of this stream being usually recycled by line 32 and an amount corresponding to that introduced through line 3I being withdrawn through line 36 and introduced either to line Il or through one of the lines 3'! and heat exchanger 31a directly to an upper or intermediate point in stripping tower IB. The streams introduced into the stripper are preferably preheated by heat exchange with hot solutions in the system as will be hereinafter described.

The introduction of the wash water at an upper part of the stripping tower is particularly advantageous because it favors hydrolysis of mercaptides in the extract into mercaptans and markedly facilitates removal of mercaptans from the top of the stripping tower. The base of the stripping tower is heated by withdrawing total liquid through trapout 38 and passing this liquid through heater 39 and line l5 into the bottom of the tower below the trapout. To avoid corrosion in 5S, nickel alloys, e. g. Monel, are desirable. With the tower operating at a pressure of about l5 p, s. i. g., the heater should maintain a tower bottom temperature of about 320 F. in the case of KOH or about 290 F. in the case of NaOH so that a caustic solution of the desired concentration can be removed from the tower bottom. If desired to operate at higher temperatures, however, such as 350 to 400 F., the more concentrated caustic resulting can be diluted with water admitted by valved line ia to the desired concentration. Instead of employing a stripping tower we may preheat the extract and flash the heated mixture with superheated steam generated from-the recycled water, sufficient heat having been introduced to produce flash bottoms of desired concentration of caustic for recycle to the extraction step.

Above trapout 38, tower I8 is preferably packed with carbon Raschig rings or is provided with other known means for obtaining efficient countercurrent contact between descending liquid and ascending Vapor. The stripping tower is unique however in that no liquid level is maintained in the bottom thereof. When methanol is removed from the solution the potassium cresylate becomes insoluble in the potassium hydroxide solution. If attempts are made to maintain a liquid level in the tower bottom and to draw off separately a potassium cresylate layer from a lower potassium hydroxide layer difficulties are encountered because the descending potassium hydroxide interferes with proper settling and separate removal of potassium cresylate. We have avoided these difliculties icy-withdrawing liquids from the stripping tower through valved conduit 4| at such a rate that there will be no liquid level in the tower itself and the withdrawn liquids are then introduced into settler i2 wherein the upper potassium cresylate layer iiows over baffle 43 so that it can be removed by line 44 separate from the concentrated caustic solution which is withdrawn by line 45. IThe separate removal facilitates the accurate uniform metering of these two components in recycled fresh solvent.

The hot caustic solution substantially free of mercaptans, Withdrawn through line 45 and pump 46 is cooled by heat exchanger d1 and introduced into line 23. Similarly the hot cresylate solution withdrawn through line Il and pump t8 is cooled in heat exchanger 49 and introduced into line 23. At least a part of the cooling in both cases is preferably effected. by heat exchange with rich solvent solution in line il and/or wash water' solution in lines 36 and/or f 31. To avoid undue complication ofthe flow diagram such heat exchange has not been shown. Such heat exchange is not suflicient to bring the respective streams to desired temperature so that the caustic and cresylate solutions are further cooled either before or after mixture with the methanol in order to bring the total lean solvent solution to a temperature of about 96 F. After the heat exchange, rich solution in line li is preferably preheated to at least about 200 F'. and wash water introduced to line 3i is preferably preheated to a still higher temperature in order to insure the introduction of the required amount of heat and stripping steam into tower i3 to vaporize and expel from the caustic substantially all the mercaptans dissolved therein. Thus the heat exchangers Ha and 3M are intended to include not only heat exchange with hot caustic and cresylate but also to include any further heating that may be required.

When cresylic acid is present in the charging stock it is frequently necessary to withdraw excess cresylate from the system either continuously or from time to time and in our system this can be readily accomplished by separate withdrawal line 5G. Caustic potash solution can be added to the system by line 55a to make up for the loss in this way. It will be noted that the recycled potassium cresylate meets with the methanol in line 23 before this solution picks up potassium hydroxide from line 45, the potassium hydroxide and cresylate being mutually insoluble in the absence of the methanol. It is possible to employ settler 42 only for separating and removing excess cresylate and introduce the main stream from line 4| directly to line 23; however, such an operation offers difiiculties in pumping and control and is usually not desirable.

The overhead from stripping tower I8, which consists essentially of methanol, mercaptans and steam, is introduced by line 5l at an intermediate point in methanol fractionating tower 52. This tower, like tower I8, is provided with a total trapout 53 at lowermost plate 5s and the total liquid is passed through heater 55 and returned to the base of the tower through line 55. Here again the removal of methanol overhead renders the remaining water and mercaptans mutually insoluble so that no liquid level is maintained at the base of the tower but, on the convby volume.

vdensation and revaporization.

trary, the liquid is introduced by conduit 5l into mercaptan settler 58 from which mercaptans are removed through line 59 while water is removed through line 3|. Y It is not necessary to employ sufficient fractionation in 52 to remove all the methanol from the water inasmuch as recycling the latter prevents loss of methanol from the system.

Methanol substantially free of water is removed from the top of tower 52 and passed through condenser 5G to receiver 6l from which condensate is withdrawn through line E2 and pump 53 the major portion of the withdrawn condensate being returned by line 6d for reflux in tower 52 while the net recovered methanol is recycled by line 23. rIhe methanol thus recovered may contain as much as l to 3% water It is preferred to hold the amount of water to a minimum inasmuch as mercaptans are found to increase with the water content. The methanol will be contaminated with a small amount of niercaptans, usually about 1%, but such mercaptans are of the lowest molecular weight and substantially all retained by the caustic cresylate solution in the final caustic extraction stage.

As heretofore indicated, the cresylates may be obtained by reaction of the caustic in the extraction solvent with cresols naturally occurring in the oil treated. It should be understood that the cresols canrbe Obtained from any other source and that while cresols of the charging stock boiling range are preferred, particularly in the case of heater oils, other phenolic compounds may be used. The term cresylate as used herein is intended to include alkali metal phenylate, cresylate, alkyl cresylates and equivalent compounds.

From the above description it will be seen that we have accomplished the objects of our invention. Substantial savings in required heat are obtained `because the overhead from tower I8 is at the proper temperature for introduction at the mid-point of fractionator 52, without con- This iractionator can be of commercial design and requires no special nickel or alloy packing because the caustic is substantially all removed in the stripping tower. Neither wash water nor recycled solvent components are contaminated by the introduction of extraneous materials although it may of course be necessary to introduce small amounts of make-up methanol and caustic to supply inevitable losses. The recycle water stream is substantially free of cresols, inasmuch as any traces of cresols carried over by entrainment or otherwise with the methanol-mercaptan vapors from stripper I8, remain dissolved in the inercaptan layer which separates from the water in 58.

While we have specifically described a preferred embodiment of our invention it should be understood that Various modifications and additional or alternative steps will be apparent from the above description to those skilled in the art.

`We claim:

l. The process for removing high boiling mercaptans from a hydrocarbon distillate boiling in the range of about 325 F. to about 650 F. which process comprises extracting said hydrocarbon distillate with a solvent mixture consisting essentially of an aqueous potassium hydroxide methanol cresylate solution in which the aqueous potassium hydroxide is an approximately l5 molar potassium hydroxide solution, methanol is present in an amount in the range of 20 to 40 volumes 'per 100 volumes of aqueous potassium hydroxide and potassium cresylate is present in amounts of about 2 to 2-0 volumes per 100 Volumes of aqueous potassium hydroxide, washing both methanol and potassium hydroxide from extracted hydrocarbon distillate with a recycled Water stream from the source hereinafter defined, introducing solvent from the extraction step to the upper part of a stripping zone, simultaneously introducing Water from the washing step into the stripping zone, removing in vapor form from the top of said stripping Zone substantially all mercaptans and methanol and most of the water to leave as stripper bottoms potassium cresylate and concentrated aqueous potassium hydroxide containing less than 50 per cent by Weight of water, separating the overhead stream'from the stripping zone into substantially anhydrous liquid methanol, liquid meroaptans and a recycle liquid Water stream which contains no carbon dioxide or potassium hydroxide and which is substantially free from methanol and mercaptans, recycling said Water stream for use in the Washing step in the absence of Water from extraneous sources, separating potassium cresylates from concentrated potassium hydroxide solution leaving the base of the stripping zone, and combining a part of said separated potassium cresylate with the substantially anhydrous methanol and with the separated potassium hydroxide solution for recycle to the extraction step as a solvent mixture having a component composition as hereinabove dened.

2. lThe process for removing high boiling merv captans from a hydrocarbon distillate boiling in the range of about 325 F. to about 650 F. which process comprises extracting said hydrocarbon distillate with a solvent mixture consisting essentially of an aqueous potassium hydroxide methanol cresyiate solution in which the aqueous potassium hydroxide is an approximately 15 molar potassium hydroxide solution, methanol is present in an amount in the range of 20 to 40 volumes per 100 volumes of aqueous potassium hydroxide and potassium cresylate is present in amounts of about 2 to 20 volumes per 100 volumes of aqueous potassium hydroxide, Washing methanol from extracted hydrocarbon distillate with a water stream, introducing liquid Water from the Washing step into solvent from the extraction step at a temperature suiiiciently high to facilitate hydrolysis of mercaptides to mercaptans, stripping the combined mercaptan-containing mixture under conditions to remove in vapor form from the top of the stripping Zone substantially all mercaptans and methanol and most of the Water to leave as stripper bottoms potassium cresylate and concentrated aqueous potassium hydroxide containing less than per cent by weight of water, introducing at least a part of the overhead stream from the stripping zone into a vaporliquid fractionating zone at an intermediate level thereof, removing substantially anhydrous methanol vapors from the top of the fractionating zone and condensing said vapors to obtain liquid anhydrous methanol, removing liquid Water and mercaptans from the base of the fractionatng zone and separating said liquid water from said liquid mercaptans, separating potassium cresylate from concentrated potassium hydroxide solution leaving the base of the stripping zone and combining a part of said separated potassium cresylate With the liquid anhydrous methanol and with the separated potassium hydroxide solution for recycle to the extraction step as a solvent mixture having a component composition as hereinabove defined.

HOWARD A. PARKER. GEORGE W. RITTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,084,575 Day June 22, 1937 2,269,467 McCullough Jan. 13, 1942 2,317,053 Henderson Apr. 20, 1943

Claims (1)

1. THE PROCESS FOR REMOVING HIGH BOILING MERCAPTANS FROM A HYDROCARBON DISTILLATE BOILING IN THE RANGE OF ABOUT 325* F. TO ABOUT 650* F. WHICH PROCESS COMPRISES EXTRACTING SAID HYDROCARBON DISTILLATE WITH A SOLVENT MIXTURE CONSISTING ESSENTIALLY OF AN AQUEOUS POTASSIUM HYDROXIDE METHANOL CRESYLATE SOLUTION IN WHICH THE AQUEOUS POTASSIUM HYDROXIDE IS AN APPROXIMATELY 15 MOLAR POTASSIUM HYDROXIDE SOLUTION, METHANOL IS PRESENT IN AN AMOUNT IN THE RANGE OF 20 TO 40 VOLUMES PER 100 VOLUMES OF AQUEOUS POTASSIUM HYDROXIDE AND POTASSIUM CRESYLATE IS PRESENT IN AMOUNTS OF ABOUT 2 TO 20 VOLUMES PER 100 VOLUMES OF AQUEOUS POTASSIUM HYDROXIDE, WASHING BOTH METHANOL AND POTASSIUM HYDROXIDE FROM EXTRACTED HYDROCARBON DISTILLATE WITH A RECYCLED WATER STREAM FROM THE SOURCE HEREINAFTER DEFINED, INTRODUCING SOLVENT FROM THE EXTRACTION STEP TO THE UPPER PART OF A STRIPPING ZONE, SIMULTANEOUSLY INTROUCING WATER FROM THE WASHING STEP INTO THE STRIPPING ZONE, REMOVING IN VAPOR FORM FROM THE TOP OF SAID STRIPPING ZONE SUBSTANTIALLY ALL MERCAPTANS AND METHANOL AND MOST OF THE WATER TO LEAVE AS STRIPPER BOTTOMS POTASSIUM CRESYLATE AND CONCENTRATED

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