US2183968A - Process of treating oil - Google Patents

Description

Dec. 19, 1939. T. F. MccoRMlcK Er AL PROCESS OPI TREATING OIL 3 lSheets-Sheet 2 Original Filed Dec. 27, 1935 27, 1955 3 Sheets-Sheet 3 q//wa/g T. F. MccoRMlcK Er AL PRocEss 0F TREATING OIL Original Filed Deo.

Dec. 19, 1939.

Patenre'ane. 19,1939

UNITED STATES PATENT OFFICE n 2,183,968 PROCESS OF TREATING OIL Thomas 'F. McCormick, (Jakland, and4 Arthur Lazar, Berkeley', Calif., assignors, by mesne assignments, to Tide Water Associated Oil Company, San Francisco, Calif., a corporation of Delaware Application December 27, 1935, seriarNo. 5639i; Y

Renewed April 13, 1939 13 claims. (c1: 26o-co9) This invention relates to improvements in desulphurizing petroleum or its distillates and hasfor a particular object the removal of mercaptans irom cracked distillate, especially cracked gaso- Another object of the invention is the reduction in, or elimination of, the usual sulphuric acid treatment` used ordinarily in sulphur removal from cracked gasolines wherebythe loss of valuable unsaturated hydrocarbons' desired in the nished product is substantially prevented.

' Another object is to increase the octane number of cracked gasoline by removal of mercaptans.

Another object is the recovery of mercaptans in purified form and the recovery of the treat- Vmg, agent so that a simplified processassuresa minimum treating cost far below that of heretofore known processes.

Another object is the recovery of mercaptan in relatively puried form.

Another object is the recovery of lower molecular weight fatty acids from a cracked distillate. Another object is the effective removal of HzS coupled with stabilization of the distillate by removal of wild gases such as methane etc. 'W

Another object isg-to recover a commercial butane substantially 'free from B2S, from cracked products.

Further objects will become apparent as the invention hereinafter is'more fully disclosed. A feature of the-.invention comprises the re- Y moval of certain compounds contained in cracked gasoline which have a deteterious effect on the selective removal of mercaptans whereby. the

mercaptans are left in a condition easilysusceptible of -removal by themselves. 'a

This will be better `understood by stating that when mixtures of organic compounds, such as straight run gasoline, cracked gasoline an'dother hydrocarbon mixtures, are treated with an alkaline reagent, such as caustic soda solution, so` dium carbonate solution and the like, a reaction t a speciiicv L% of the cracked gasoline and a heavy fracchemicals will be particularly'described.

the said light and heavy fractions;

The primary step includes the substantially complete removal'of HzS, preferably by the use of steps involving stabilization and fractionation of the distillate under'` treatment, because it has been determined that the major portion of the A5 mercaptans present in gasolines, or cracked gasolines, is contained in a fractional part thereof,

such as the rslt 30% `to 40% ofthe distillation range and, for instance, between 170 and 250 F. endboiling point and therefore a. combined treat- 10 ment i's very eifective for B2S removal and stabilization for eliminating the use of chemicals. Such fraction likewise Vcontains a relatively high percentageof unsaturated hydrocarbons comprising olenes and aromatics which are esl5 pecially desirable in the finished gasoline pri-` ,marily ona'ccount of the anti-knock value of the same and also by reason of their saving from loss due in ordinary sulphuric acid treatment to sulphonation and polymerization.

In accordance with this invention, therefore, a gasoline or total cracked gasoline is split into two orv more fractions, according 'tovo1atility, to yield a light fraction comprising vabout 20% to tion comprising from about to 50%. This may be termed a split treat, and the said light fraction may be treated for the removal and recove'ry of mercaptans so as to require a relatively light sweetening treatment thereafter.

The invention comprises alternative methods 'of treatment with respect to the HzS removal, in one of which the useof chemicals is eliminated and in the other of which a reduced chemical treatment may be used, but as 'to certain. novel steps of the invention, it is immaterial how the B2S is previouslyremovd although a preferredform of operation without the use of While the treatment ofcracked gas'olines is also particularly described, it must be understood that other forms, or cuts, of petroleum distillates, coal tar distillates, shale oil distillates, or other hydrocarbons or mixtures of the same, are included in the scoperof the invention.

In the appended drawings, Fig. 1 is a descriptive ow' sheet showing the treatment of both Figure 2 is a diagrammatic flow sheet specically illustrating the treatment of the Said light. fraction for removal and recovery of mercaptans, and regeneration of the treating agent.

Fig. 3 is a diagrammatic flow sheet illustrating apreferred method of removing B2S from a distillate from any well known cracking and fractionating system 5 through a stabilizer column 6 wherein aseparation of so-called wild vapors occurs, such as HzS gas, methane, ethane, propane,etc., said vapors and gases being eliminated through valve 1.

The liquid distillate, comprising a total cracked gasoline, thenpasses through pipe 8 into a still, or fractionator 9, wherein a desired cut is made preferably lto yield a light fraction of about 20% 'to 50% of the distillate and a heavy fraction of heat to the distillate, in which case a heater I3 may be used.- Pipe I4 is used for the supply of stabilized and purified distillate when the method The light fraction flowing upwardly through tower I5 bubbles through a body of caustic solution maintained at a level A-A, contact being venhanced by said Raschig rings, etc., which contact is usually suiiicient for the purpose of the4 process. However, additional contact efficiency values may be obtained by circulating caustic by means of pump 23 from the bottom of tower I5 through line 22, and dispersingthe same over the top of the tower packing I6 by means of a spray I8 which causes the caustic to flow downward by gravity countercurrent to the gasoline stream. When the free caustic value of the treating agent is reduced to a low figure, the spent solution is discharged through line 24 and represents a basic stock for the recovery of alkali soaps or free organic acids as by-products. Partially spent caustic from a second contact tower 20 is pumped by pump I 9`through lines 2Iand `22 to tcwer I5 and fresh caustic is charged through valve 28, line 21, and spray 2G to tower 20. If desired tomaintain a certain temperature of treating in tower I5, above atmospheric temperatures, a heater'ZIa may be cut into line 22.

'I'he once treated light fraction from tower I5 ows through line 25 to the second contact tower 20 (of similar construction to tower I5 and hav-` ing a reagent level B-B) wherein it is contacted in similar fashion with fresher caustic solution supplied as described through valve 28, line 21, and spray 26. Continuous Acirculation Vof caustic solution from the bottom of the tower 20 through spray 26 by means of pump I9 will increase the contact and eiciency of treatment and may be used. 'I'he treating agent introduced through valve 28 may comprise a caustic s oda solution of about 5 to 20 B. gravity. v

The above mentioned step countercurrent method is preferred but it is tol be understood that a true countercurrent method may be employed. In such cases fresh caustic of from 3 to 20 B. gravity is charged continuously through valve 28, line 21, and spray 26 to tower 20 and iiows downward by gravity countercurrent'to the gasoline stream, contact taking place in the Raschig rings. In such processing no caustic level is heldin tower 20, caustic being pumped by pump I9, with up to 100% of the gasoline charge to the treating system, through lines 2|, 22 and spray I8 to tower I5 wherein contact takes place in the Raschig rings by gravity counterflow. A caustic level E-E is maintained below the gasoline inlet to thistower I5. The treating agent is charged through valve 28 in amounts sufficient to accomplish substantial removal of organic acids and is withdrawn continuously through line 24. As in the preferred system the treatment may be performed hot in which heaters I3 and 2Ia are employed but in general practice the treatment is performed at ordinary atmospheric temperatures.

The treatment in towers I5 and 20 is used to take-advantage of the selective removal of organicacids because it has been found that these compounds along with the mercaptans go into solution in fresh strong caustic soda solution but, as the soda solution becomes increasingly spent by absorbing, or reacting, with such compounds, the stronger acids replace the mercaptans, which are thus liberated to again'go into solution in the gasoline.

While it is true that some mercaptans are removed in towers I and 20, the major portion of the mercaptans thus remain in solution in the light fraction and the amount of such removal is the result of an equilibrium condition in which the removal depends on the relative solubilities of mercaptans in the gasoline and caustic solution respectively. This distributional factor will be -aected by the strength of the caustic solution in free alkali, thetemperature, the quantity of the treating agent, and the concentration and type of mercaptan present in the gasoline, so that in providing initially a certain strength of caustic soda solution through valve 28 these facts must be borne in mind to effect minimum removal of mercaptans in towers I5 and 20 and a maximum recovery of stronger organic acids which represent a valuable by-product and whichsteps is eiective in preventing or minimizing the quantity of such compounds which otherwise would be removed with the mercaptans in a later step of the process.

The spent caustic solution withdrawn through line 24 contains the stronger organic acids in form of sodium salts. In order to recover these acids the solution may be acidied, whereby. the

organic acids are set free and subsequently can be separated into chemical individuals by fractional distillatio. Another method of purifica-- separated salts. "It is also possible to .accom plish .separation by esterifying the organic acid mixture and fractionating the resulting esters. It is found that the organic acids thus obtained uable in treatment of the light fraction derived from V the above described split-treat because itis found that not only is the major portion of the mercaptans from the total unsplit lcracked gasoline concentrated in the said light fraction comprising say up to aboutA40% of the gasoline. but the low boiling mercaptans within the boiling range of the light fraction are the ones most soluble in caustic soda solutions.

' HenceLby fractioni'ng out the low boiling constituents of a gasoline, or cracked gasoline, up to about 40% thereof, and treating' this fraction .separately with caustic solutions, a maximum lremoval of mercaptans' is assured together with.,

maximum desulphurization.

And, not only are the above factors important,"but the removal of mercaptans by themselves relieves the load ordinarily thrown on the doctor treatment for sweetening wherein the mercaptans are altered to disulphides, and also the caustic solution is capable of regeneration for.

reuse for several cycles.

In addition to the'above described treatment for selective'removal of rolatively strong organic acids and HzS whereby a large percentage of the mercaptans remain dissolved in the light fraction, it is found that temperature has a marked eifect on the above equilibrium condition and that as the temperature of the mercaptan containing light fraction mixed with ,caustic'soda' solution apprqachesthe freezing point of the said solution', the solubility of the mercaptans inv caustic increases in proportion.

In consequence, the treated light fraction issuing from tower 20 is passed through line 29 to a third similarly constructed contact tower ,30,- beingcooled bycooler 3l to a point preferably just above the freezing point of a caustic solution introduced into tower 30 by pump 32 from the bottom of a fourth similarly constructed' contact tower 38 through lines 33, 34, and lspray35.

The. caustic solution then passes in true countercurrent iiowA to the light fraction through tower 30, the spent caustic containing mercaptans being withdrawn to tank 36, while the light fraction relieved of part of the mercaptans flows through line 31 to a fourth similarly constructed contact tower 38, being cooled, as before, on its way by cooler 39 to a point preferably just above the freezing-point lof fresh strong 'caustic solution supplied through line 40, cooler 1 4I, and spray 42.

In tower v38, contact of the once treated cooled light fraction is completed with fresh caustic to nally remove all mercaptans in the caustic in the form of sodium" mercaptides as 'faras Ais possible under the optimum conditions of temperature and strength of soda solution. `I

In order to assure the most eilicient contact, pump 32 provides a rate of circulation from tower 38 back to tower 30 at a rate up to about 50%, orhigher, of the volume of the light fraction under treatment, which is finally withdrawn through line 43 to undergo the usual doctor and pharmaceutical lI 3 and 2la.

treatment for sweetening and is then anished light fraction ready for blending to yield a finished motor fuel. Some of the advantages of thus treating the-light fraction maybe summarized as follows:

1) The unsaturates present in the lightfraction which are highly desired for their antiknock value in nished gasolines are not at-rv tacked and lostv as in sulphuric acid treatment.

(2)v A re-run distillation with its accompanying losses may, in many cases, be dispensed with, due to the elimination of polymerization which takes place in ordinary acid treatment.

(3) The loss in octane number is not only minimized when' compared with acid treatment followed by distillation but the octane value may be actually increased.

(4) The desulphurization by mercaptan, removal is a maximum with the selective treatment 'of a light fraction.

(5) The maximum value may be obtained from the caustic ,soda solution by regeneration and re-use. i v The temperature to which the reacting mixture is reduced in towers 30 and 38 may b e just abovev thefreezing point of the caustic solution used up to 60 F., and the amountA of caustic sod'a solution used ranges from about 5 to by volume of the light fraction treated and is of a strength from about 3 to 20 B. Preferably, the .caustic strengthused is between 5 an 12 B. and about '10% by volume and the tre ting temperature intowers 30`and 38 about In cases where a more selective removal of strong organic acids is desired, a hot caustic treat is used in tower I5 bymeans of heaters the caustic solution increases as the freezing point of that'solution is approached so that as the temperature rises the quantity of mercaptans extracted wlththe organic acids is reduced.

This treatment is performed in the yliquid phase and applies to both the light and heavy frac' tions. Temperatures up to,300 F. and pressures up to 300k pounds per square inch, absolute, may

be employed and caustic strengths from 5to 30 B. utilized. In general however, it is preferred to treat at normal atmospheric temperatures which range from 30 to 110 F. and use pressures up to 100 pounds per square inch, absolute, or

higher, which pressures' may be-applied by the use of suitable valves inthe system.

The solubility of mercaptans inv Referring now to Figure 1, it will be seen that the heavy fraction from still 9, which contains a small. amount of mercaptan sulphur, for example, of the order of 0.08%, may be given a caustic soda wash lto remove relatively strong organic acids and mercaptans according to the steps already disclosed in treating the light fraction` for the same purposes.

Although the heavy mercaptans are much less soluble in caustic soda; than the lower molecular weight compounds, a partial removal is eected, thus reducing the mercaptan` content of said heavy fraction. when high acid ,rates are not required, for example on pressure distillates produced from selected cracking stocks which yield cracked gasolines ofv low sulfur content. The spent oaustics derived frcm this mercaptan removal are solu- Such processing is benecial v tions of the sodium mercaptldes of higher boiling members of the mercaptan group and represent a further separation of selected mercaptans f or by-product recovery.

gasoline, or motor fuel.

At4 times, it may be desirable to give the light fraction passing from tower 38 through lin'e 43, 'a slight acid treatment to stabilize the gasoline for color and to remove4 gum forming compounds, or to further desulphurize by removing other kinds or sulphur compounds.

This may be accomplished by contacting the light fraction, after removal of mercaptans ras described, with the acid sludge derived from the treatment of the-heavy fraction, or with fresh acid of a suitable degree of concentration at a suitable rate.

After thus acid-treating, the light fraction may be re-run anddoctor treated to be then blended with the treated heavy Afraction-to form 'a total nished gasoline.

,l It is apparent that, after separate acid treatf ments of the light and heavy fractions, the fractions may be blended and then re-run and doctor treated to obtain the finished motor fuel.

In the acid treatments of the light and heavy fractions as described above, it is preferred to use as the most eiiicient and economical-method, the processes described in `a co-pendingapplication S; N. 50,206 led Nov. 16, 1935, and in the co-pending application of Edwards and Stark, S. N. 532,000, filed April 22, 1931, which show thetrue countercurrent contacting desired, but it should beunderstood that the herein described invention is not limited to any particular type of contacting with acid, or caustic, or to the degree of concentration of either, or to any particular kind of hydrocarbons to be treated, as the disclosure comprehends the treatment of any kind of hydrocarbon containing Vmercaptans in accordance with the described steps, preferred forms only being illustrated herein. Caustic soda solution may be replaced by caustic potash, as is well known. Y

The eiliciency of the process as a whole,A while not dependent thereon, is materially increased by the regeneration and reuse of the spent caustic solution from tower 30, and the regeneration may -be accomplished by either batch or continuous operation.` While batch regeneration is eective, continuous regeneration'is to be preferredin order to vconform with the continuous operation of the preceding steps set forth.

.In the batch regeneration, by reference to Fig. 2, it will be seen that the spent caustic containing sodium mercaptides is drawn from tank 36v through line 44 to tank 45 containing a closed steam coil 45 and an open line I1 for the introduction of air, steam, or an inert gas for stripping.

A solution level X-X gives head room for the removal. of the low boiling mercaptans in vapor form through pipe 48, the same beingderived byV "the splitting up of the sodium mercaptides under -the innuence of heat supplied throughcoil 46 to brlngthe solution to or near, the boiling point.

arates by gravity to be withdrawn through pipe 63 controlled by an automatic liquid .level regu- Alternative methods of applying open steam, air, etc. may be used. If steam'alone be passed through the mixture, the mercaptan sulphur may be reduced from say grams perV liter `to as low as 2 grams per liter. If the velocity of thelsteam 5 through the mixture is great'enough to cause foaming, additional steam may be passed through spray line 49 to break the foam.

Alternatively and preferably, continuous regeneration is eected by passing the spent causl0 tic solution from tank 36 through line 4t and pump 50 to the top of a contact tower 5| containing a plurality of bubble plates 52, the caustic solution flowing down countercurrent tothe heating and stripping steam or gas supplied 15 through line 53.

' In such acontact tower 5I, when insulatedand being 4f in diameter and 32' high, using 1.8r pounds ofv steam for 5 pounds of caustic solu- 4 tion charged, it is found that With'seven plates 20 the mercaptan sulphur content will be reduce from 40 grams per liter to 6.00 grams per liter.` Using a higher ratio of steam to caustic the mercaptan content may be still further reduced, or the number of plates may be increased to pro? vide an additional reduction in mercaptan sulphur. -Y

The mercaptans .are taken as overhead through line 54 to an insulated'slop pot 55, wherein aseparation of -vapors from any caustic in the form 30 of foam takes placeLjand themercaptan vapors then pass through line 51 to a contact tower 58 containing suitable contact material 59 in which the vapors lare scrubbed with water applied through pipe 60 to condense the mercaptans at a predetermined temperature and pressure.

Vaporslare tapped olf through line'BI and are composed largely of the more volatile mercaptans as methyl and ethyl mercaptan. These mercaptans may be recovered by suitable absorption agents or may be liquied by'meansof `a refrigeration or compression in the well known manner. Mercapt'anscondensed by the water pass to a settling tank 62 wherein the water seplator 64. The desired liquidmercaptans aredrawn off through pipe B5. Y Any caustic solution passed over from tower 5| and trapped out in the slop pot may be returned by pump 56 for restripping in tower 5|.

The regenerated caustic solution flows continu! ously from tower 5I into tank 56 whence it may be withdrawn through line 51 to be used as charging caustic through line 40, In similar fashion, the regenerated caustic solution from the batch treatment (who used)j may be passed throughV line 68 to line 40.

, The life of the regenerated4 caustic solution is not necessarily indefinite as there may be a gradual accumulation of impurities in the same which are not responsive to regenerative separation, but it may be used eiectively in towers 38 and 3l for at least three cycles which gives great economy in caustic consumption. 65 Referring to Figi. 3, a preferred method of removing Has from a raw'cracked distillate, while stabilizing the same during fractionation to'obtain the before described light and heavyfractionsis shown. 70 -In`-these steps, which are preferably used in place of the stabilizer column 6 and still9 ofv Fig. 2the flow sheet and datav represent actual,

refinery operations duringthe production c. iinished cracked gasoline. from a stabilized ravis teristics:

Gravity 58. 2 Sulphur per cent... 0. 15V I Octane No. (C. F. R. Motor method) 10 Initial B. point' 90 5% Y 112 10. 124 20 160 30 191 I 40 227\ 257 283 80 343 90 366 95 394 End point 399 Recovery I 95. 0 Residue 1. 0 Loss 4.0

The raw cracked distillate from the cracking stills isforced by pump 10th'rough line 1|, into a flash tower 12, absorbing heat during its passage from partial condenser 13, heat exchangers 14 and heater 15 so that the distillate is discharged into ash tower 12 at about 362 F., or at a temperature suicient to release HzS, wild gases and certain desired low boiling fractions.

Flash tower 12 operates successfully and elli-- ciently in the present invention with nine plates, but any desired number can be used, and with a suitable reux liquid, which may conveniently be supplied from another part of the lsystem by pump 16 and line 11, to separate up to say 25% of low boiling constituents together with the greater portion of the HzS. A

Operating flash tower, 12 with entering 'dstil' late at 362 F. and under a pressure of 265 pounds per square inch, gage,l the desired light fraction is taken off as overhead with the HzS which latter will then be found to be reduced from 0.30

. gram per liter of the raw distillate down to 0.09

gram` per liter in the bottoms which are withdrawn through line 18 and passed into a pri-l mary tower 19.' Said bottoms are injected into primary tower 19 at a temperature of about 310 F. for rectification by removal of the remaining HzS and light ends oi the boiling range of the desired light fraction.

To this end, 'primary tower 19 may be a 30 plate tower, tted with a reboiler .wherein the bottoms are heated to about 375. F., operated under a pressure of about 40 pounds per square inch, gage, and supplied with suitable reflux liq.- uid through line 8| which may conveniently be supplied from an accumulator 82 wherein is collected part pf the overhead from primary-tower 19 which passes thereto by way -of line 83 and partial condensers 13 and 81|.

In primary tower 19, operating'under the above described conditions, the HzS will be reduced from 0.09 gram per liter ofthe entering stock to a trace'in the rectied heavy fraction passing out through line 85 to the'treating system described, while the overhead (comprising constituents of the desired light fraction) passes from accumulator 82 through line 86 and condenser 81' tobe collected in accumulator 88 in liquid form, incondensible gases being taken off through line 89 for recompres'sion, absorption, or other uses.

The condensed overhead is picked up by pump 16 and passed through line 90 and heat exchanger 9| at a temperature of about 160 F. into :a 34 plate secondary tower 92, being joined therein and in its passage by the vapor overhead brought from flash `tower 12 through line 93.

Secondary tower 92 is provided for the purpose of stabilizing the desired light fraction by the elimination of undesired wild gases such as methane, ethane, etc'., during which operation all HzS is separated. To this end, tower 92 is provided with a reboiler 94 adapted to heat the bottoms to a temperature of about 300 F. thereby giving la. temperature gradient through the'column and yield an overhead having a temperature of about' 120 F. at the outlet pipe 95 while operating under a. pressure of about 245 pounds per square inch,

gage. l

The overhead from secondary tower 9'2 vis taken through line 95 and condenser 96 to provide condensate in accumulator 91- which isv forced lby pump 98 through line 99 as refiux in the upper part of the towen. -Excess condensate may be taken through line. |00 Ito be,used as blending propane, -or for other purposes, While the incondensible wild gases and HzS are withdrawn through line |0|. V

Depending on the usedesired for the lig-ht fraction as to boiling range, etc., it may be withdrawn as is for the described caustic treatment through line |02, or may be passed at a temperature of about 300 F. through line |03 into a 24 plate butane tower |04 wherein a cut of commercial butane istaken as overhead, the bottoms comprising the-desired light fraction within the gasoline boiling range being withdrawn through lines |05 and |02 ,for the describedcaustic treatment.'

When a cut comprising susbtantiallybutanes and hydrocarbons of similarv boiling range is desired with, for example, an absolute vapor pressure of about 25 to.75 pounds per square inch at 70 F.,tower |04 mayibe provided with av reboiler |06 giving a bottom temperature of about 220 F. and a temperature at outlet line |01 of' about F.\while operating under a pressure of about 80 pounds per` square inch, gage.

The overhead passing through line |01, reflux condensers |08 and head condenser |09, will flow as a liquid through line ||0, part being used as reux through line and the remainder flowing through line ||2 and coolers ||3` to a treating system for the recovery of methyl mercaptan in substantially pure for'm. The incondensible gases are withdrawn through line I0.

It is found that by thus running a raw cracked l distillate, asdescribed, when a butane cut is taken mercaptans are present up to about 0.35%-v by weight mercaptan sulphur and that the mercaptan sulphur is derived from the specific mercaptan, methyl mercaptan CHSSH; which is removed by caustic soda solutions as follows:

In consequence, the butane cut from tower l|011 through line .|I2 maybe treated with caustic.

soda solution according to the steps outlined with respect to the operation of towers 30 and 38 in Fig. 2.

Depending en the use desired for the nght fraction as to boiling range, etc.,` it` may be with-l ldrawn as is for the described caustic treatment' through linejl2, or may be passed at a temperature of about 300gl F. through line |03 into a 24 plate butane tower |04 wherein a cut of caustic treatment.

When a cut comprising substantially butanes and hydrocarbons of similar boiling range is desired with, for example, an absolute vapor pressure of about 25 to 75 pounds per square inch at 70 F., tower |04 may be provided with a reboiler |06 giving a bottom'temperature of about 220 F. and a temperature at outlet line |01 of about l0 120 F. while ope'. ating under a pressure of about 80 pounds per square inch, gage.

The overhead passing through line |01, reflux condensers |08 and headl condenser |09, will ow as a liquid through line ||0, part being used as reux through line and the remainder owing through line ||2 and coolers I3 to a treating system for the recovery of methyl mercaptan in susbtantially pure form. The incondensible gases are withdrawn through. line H4.

It is found that by thus running a raw cracked distillate, as described, when a butane cut is taken mercaptans are present up. to about 0.35% by weight mercaptan sulphur and that the mercap- 'tan sulphur is derived from the specic mercaptan, methyl mercaptan CHJSH, which is removed by caustic soda solutions according to the following equation:

contacting Adue to the structure of the towers, 4 the reaction being conducted at about 30 to 40 F.

IThe spent caustic may then pass, as described in connection with Fig' 2 from tower 3o to the stance, if itis desired to operate these towers at described continuous regeneration plant when pure methyl mercaptan may be recovered from line 65.

More simply, the methyl mercaptan may be removed from the spent caustic by boiling the latter and condensingthe distillated mercaptan, or by passing steam,air, or other inert stripping gas `through the hot caustic solution and condensing the distilled mercaptan.

The preferred system, above outlined, for obtaining desired light and heavy fractions for separate treatments in the removal of impurities has many advantages. A 55 It providesV for the complete removal of HzS Vwithout recourse to the chemical treatment usually necessary.

It gives improved fractionation for the desired i. and other impurities it is possible to accomplish' ,split^wherein gaps of over 20 F. between the. oo 1A. S. T. M. Engler end point of the light fraction and the Engler initial boiling. point of thefheavy fraction can be obtained,v which is important when a concentration of mercaptans has 65 been determinedfin a given percentage of the raw-*distillate and thus limits the low boiling sulph compounds' to the light fraction. K It gles by improved fractionation a light fraction ,of a desired boiling range which is largely o uncontaminated with higher' boiling and more refractory types of sulphur compounds or. with compounds which may impair color stability.

It gives a light fractionl free from HzS of such character that the rst treatment thereof with 75 warm orhot caustic solution permitsthe selecdrawn through lines |05 and |02 for the described v vtive removal of organic, or fatty, acids uncontaminated with HzS reaction products.

'It minimizes the use and *cost of chemicals while yielding relatively pure valuable by-products. '5

It gives a commercial butane free from H2S.

It yields a specific mercaptan in 'relatively pure form.

It provides ultimately for a purified cracked gasoline of superior qualities of octane rating'lO and color stability. Combined with these is the recovery of impurities in a valuable commercial form and a'high reduction in treating costs.

It must be understood that the preferred method of Fig. 3 has been applied to the treatment of 15 a cracked gasoline specifically for the purpose o f illustration and not of limitation, inasmuch as variations in temperature, pressure, tower construction an'd the like are permissible depending on the required split in the boiling ranges' of the raw cracked distillate, the character of the cracked gasoline and many other variables well known in the art.

For instance, in the -removal of HzS alone, the operation of flash tower 'I2 is not as important -25 as its chief function which is the stripping of the lightest hydrocarbons from the raw distillate so that the load on the primary tower 18 is reduced and thus permits condensation of overhead vapors therefrom at ordinarycooling water temperatures and comparatively low pressures. `This permits the use of a less costly primary tower '19 and cuts dov/nime use and cost of steam used in reboiler 80 and the cost of condensing the overhead vapors with cooling mediums. other than 35 water.

Again, while correct control and adjustment of the operations in primary tower 18 and secondary tower 92 are important for' the removal of HzS the gures given Yfor a 'particular cracked distillate may be varied to suit conditions. For inhigher pressures, the top and" bottom temperatures must be raised or, vice versa, if the pressure is reduced, the temperatures should be reduced.

A great many of such variables follow physical laws, the operation of which is well known to those skilled in the art, and the practice of the invention requires no other instructions to permit its use beyond those given. Ordinary laboratory controls to determine characteristics of a particular distillate Will govern correct renery control of the process.

The increase in octane number is another result of' the invention.

In the usual sulfuric acid treatment of cracked distillates for the removal Aof sulfur compounds complete mercaptan removal with acid but due to resultant large losses such treatment is usually uneconomical. Just 'suicient acid is therefore added to accomplish the desired degree of total sulfur removal and the mercaptans which remain after such -treatment are often from 10 85 to 50% of lthose present in the original stock. -These compounds must be converted to disulfides by a sweetening process such as plumbite treating, in order to improve the odor of the product and meet specications. By such usual treatments there -is often a' drop in octane value (as determined by the C. F. R. Motor method)v as high as 4 to 5 octane numbers under the original vstock with a correspondling derogatory `eiect on the lead Susceptibility of the gasoline. While such drop, or loss, is partly due to the formation of disulphldes during docfor treatment, yet octane number losses occur in such treatment over and above what is expected by the complete conversion of mercaptans to disulphides.

In consequence, it is highly desirable to not only eliminate sulphuric acid treatment with its high sulphonation, polymerization and volatility losses, but to eliminate or minimize the conversion of mercaptans by doctor treatment withits attendant octane losses.

This is accomplished largely inthe present invention wherein by selective isolation of given types of mercaptans, substantially complete removal of such mercaptans can be acocmplished by the use of caustic solutions.

The conditions governing such removal include the proper distillation range, fractionation, quantity andstrength of caustic solution, contact methods, and temperature.

While it is true in average commercial useof the process, not warranting the ultimate purity of complete mercaptan removal, that a small percentage of mercaptan remains in the gasoline after caustic treatment and provision is made for economically reducing this, yet the harmful effect of the remainder is markedly less than that in other known types of mercaptan removal pr'oc.

Thisis more particularly shown in the present invention wherein the light fraction, which contained 0.35% of mercaptan sulphur before the described caustic treatment, contained but 0.04% of sulphur after caustic treatment.

At the same time, the removal of mercaptan sulphur, as described, increased the octane number of the light fraction from 71.0 to 72.5, before and after caustic treatment respectively.

Thus, compared with the usual sulphuric acid treatment an increase in octane number is secured instead of a loss.`

The use of doctor solution hereinis principally for a safety factor because any slight traces of mercaptans might be corrosive. Further, the malodorous mercaptans are highly oiensive in commercial fuels.

Hence, while the mercaptans may not be elim-l inated to an ultimate degree in the system With- A out doctor treatment, the `use of a doctor treat` ment on largely mercaptan free distillate assures a minimum conversion of mercaptans to disul-l phides with accompanying loss of octane rating, while effecting removal of the main body of corrosive and bad smelling compounds.

The entire process, as described, as a wholecomprises a number of steps which, both alone and in combination in one form or another, give maximum results in desulphurization, minimum losses of valuable hydrocarbons,l high economy in treating costs, and improved recovery of valuable by-products. These steps may be summarized as follows:

Step No. 1

Comprises the fractionation, or separation, combined with stabilization of a raw total cracked distillate to yield a light fraction up to about 40% ofthe .volume of the raw distillate. The advantages of separately treating the light fraction-are; y

.(a) The concentrationof sulphur compoundsl in the form of mercaptans in the light fraction. (b) The thus concentrated mercaptans are the low boiling range mercaptans, or low molecular -the caustic solution.

Weight mercaptans, which are most soluble in caustic solutions.

(c) With this concentration ofI mercaptans in:

the light fraction, when the percentage of other remaining sulphur compounds is relatively low, with fairly complete mercaptan removal the oil is desulphurized to a very low figure.

(d) The low boiling mercaptans in the light fraction may bereadily distilled from caustic.

solutions and the remaining caustic solution is available for continued use.

(e) The low boiling point of the fraction yields organic acids 4of the fatty acid type, like `propionic andbutyric acids with a minimum contamination with phenols and naphthenic acids, which latterbcil at a much higher range.

Step No. 3 Comprises the cooling `of the light fraction to a temperature just above the freezing point of K Under average circumstances however, we prefer to treat at temperatures of 30-40 F. This step has the advantage of (a.) Giving the greatest rate of mercaptan removal with a minimum rate of caustic supply of a given concentration.

(b) Increases the rate of mercaptan removal when compared with higher temperatures.

y Step No. 4 Comprises the continuous treatment of the light fraction with caustic solution in a packed tower, which assures (a) Maximum removal of mercaptans with consequent greater desulphurzation in such -manner that the sulphur percentageis reduced to a iigure unattainable with sulphuric acid treatment without excessive losses.

.` Step No. 5

Comprises the regeneration of used caustic so- .lution which is possible due to the selective removal of I-IzS and 'strong organic acids and gives the advantages of (a.) Recovery of substantially pure mercap.

tans.

(b) Recovery of regenerated caustic solution suitable for re-use.

. Step No. 6 Comprises th optional treatment of the light `fraction after mercaptan removal with sulphuric acid and the separate treatment of the heavy fraction with sulphuric acid. The advantages are ,fraction is optional andis only necessary in certain cases Ato obtain additional treatment but in general a heavy acid'treatmentwith` large losses is eliminated.

countercurrent (ai The nnishing acid treatment of thel iight- (b) The separate acid treatment of the heavy fraction avoids treating the light fraction in nrany cases at all with acid.

` Y Step No. 7

Compriss the re-run distillation of both the light fraction and the heavy fraction separately,

' or together, when both have been acid treated to remove compounds polymerized by the acid.

10 If the light fraction'has not been acid treated the re-run distillation may be eliminated with consequent saving.

Step N0. 8

Comprises doctor, or other, sweetening when necessary. This step may be eliminated when certaiyn Vstocks are treated in which the mercaptan removal by the above described stepsis such that the sour sulphur content is low. The denition of the term total cracked distillate used herein indicates a distillate derived from cracking various stocks and isusually a tosomewhere between 350 F. and 437 F., but

variations from this range are permissible within the scope of the invention and may include distillates from other sources such, for instance, as

85' those made by polymerization 'of hydrocarbon gases as well as sulphur bearing distillates from other sources.

Among outstanding features-of the. invention are included two stages of caustic treatment, the first comprising caustic treatment at atmospheric or more4 elevated temperatures for the recovery of organic acids, the second comprising caustic treatmentat reduced temperatures below atmospheric', or at effective low temperatures, forthe removal of mercaptans.

In order to obtain these byeproducts in the most purified fo'rm, hydrogen sulphide' shoul'd be rst removed' as described so that the selective removal `of the other impurities may be readily accomplished, Y

We claim:-

1. A process of recovering mercaptans from hydrocarbon distillats which comprises treating a hydrocarbon distillate containing mercaptans, I-IzS, andother acidic compounds with a caustic soda -solution at a temperature between lthe freezing poiritof the solution and 300 F.

sufficiently high to selectively remove subztanf tially all said acidic compoundsof a greater acidity than said mercaptans so that said mercaptans remain dissolved in said distillate, treating the distillate. with further caustic soda solution'while at a lower temperature between the freezing i pointof the solution and about 60 F. sufficiently 65 low for said solution to absorblthe greater portion of said mercaptans, then freeing said mercaptans from said solution.

\ 2. A process of recovering products from petroleum oils'which comprises: treating a hydro- 70 carbon distillate containing aliphatic monocarboxylic acids and mercaptans with `a caustic alkalisolution while at a temperature between the freezing point of the solution and 300 F.

suiiicieritly high to selectively absorb substan- .75 tially Aall compounds having a greater acidity than said mercaptans and to leave said mercaptans dissolved in said distillate, recovering such alkaline solution, further treating the distillate with caustic alkali solution while at a temperature between just above the freezing point of '5 the solution and about 60 F. suicient to absorbl the greater portion of said mercaptans, then freeing said mercaptans from said solution. f 3.. A process of recovering products from pe- \troleum oils which comprises: treating a hydro- 10 .carbon distillatevcontaining aliphatic monocarboxylic acids and mercaptans with a caustic alkali solution while at a temperature between the freezing point of the solution and 300 F. sufficiently high to selectively absorb substantially all compounds having a greater acidity than said mercaptans and to leave said mercaptains dissolved' in said distillate, recovering such alkaline solution, further treating the distillate with caustic alkali solution while at a temperature between just above the freezing'point of the solution and about 60 suflcient to absorb the greater portion of, said mercaptans, then freeing said mercaptans from said solution, and returning said solution. freed from mercaptans to contact further mercaptan containing distillate.

4. A process of recovering products from petroleum ,oils which comprises: treatinga -hydro.- carbon distillate containing aliphatic monocar boxylic acids and mercaptans with a causticalkali solution while at a temperature between the freezing point of the solution and :300 \F. sufficiently high to selectively absorb substantially all compounds having a greater acidity than said mercaptans and to leave said mercaptans dissolved in said distillate, recovering such alkaline solution, further treating th' distillate with caustic alkali solution while at a lower temperalture between just above the freezing point `of.

the solution and about 60 F. suicient to absorb the greater portion of said mercaptans, then freeing said mercaptans from said solution.

5. A process of recovering products from petroleum oilsL which comprises: treating a hydrocarbon distillate containing aliphatic monocarboxylic acids and mercaptans with a caustic `'alkali solution while at a temperature between the freezing point of the solution and 300 F.

suiiicently high to selectively absorb substantially all compounds having 'a greater acidity than said mercaptans andto leave said mercap tans dissolved ir said distillate, recovering such alkaline solution, further treating the distillate With caustic alkali solution while at a temperature between just above the freezing point of the solution and about 60 F. suiiicient to absorb the greater portion of said mercaptans, then freeing said mercaptans from said solution, and maintaining a superatmospheric pressure during said treatment suilicent ,to hold said distillate in liquid form. Y

6. Aprocess of recovering products from petroleum oils which comprises: treating a cracked gasoline containing aliphatic monocarboxylic acids and mercaptans with a caustic alkalisolution while at a temperature between the freezing pointI of the solution and 300 F. suiiicientlyv high to selectively absorb substantially all coin- 4pounds having a greater acidity than said mercaptans and to leave said mercaptans dissolved 70 in said distiliate, recojering such alkali solution, further treating the distillate with caustic alkali solution while at a temperature between just above the freezing point of the solution and about F. suflicient to absorb the greater portion of 75 from said solution.

7. In a continuous process of recovering produ'cts from 'cracked gasoline distillates in which a continuously iiowing stream of the distillate containing aliphatic monocarboxylic acids and mercaptans is contacted in-a plurality of steps with caustic alkali solution to successively sellectively remove iirst the monocarboxylic acids at a temperature between just above'- the freezing point of the solution and 300 F. and then the mercaptans at a lower temperature between just above the freezing point ofv the 'solution and about 60 F., and said solution is independently moving HzSffrom a caustic soda solution while removed from the distillate after each treating step, that combination of steps which comprises:

continuously'removing said mercaptans from the mercaptans containingv solution and returning 'the mercaptan free solution back to the mercaptan treating step. v n

8. A process of recovering mercaptans from hydrocarbon distillates which comprises: re-

treating said distillate, substantially freed from HzS but containing mercaptans and other acidic compounds, with a caustic soda solution at a temperature between the freezing point-of the solution and 300 F. sufficiently high to selectively remove` substantially all said acidic compounds of a greater acidity than said mercaptans so that said mercaptans remain dissolved in said distillate, treating the distillate with further at a lower temperature between the freezing point of the solution and about 60 F. suiiciently low for said solution to absorb the greater portion of said mercaptans, then freeing said mercaptans from said solution.

9. The process according to claim 8 in which the HzS isiremoved from the hydrocarbon distillate by fractional distillation.

l0. A process of recovering mercaptans from hydrocarbon distillates which comprises: treating a hydrocarbon distillate, substantially free from HzS but containing mercaptans and other acidic compounds, with a caustic soda solution at a temperature between the freezing point of the solution and 300 F.- suiiiciently high to selectively remove substantially all said acidic compounds of a greater acidity than said mercaptans so that said mercaptans remain dissolved in said distillate, treating the distillate with further caustic sodasolution' while at a hydrocarbon distillate then 11. A process of recovering mercaptansy from hydrocarbon :distillates which comprises: treating' a hydrocarbon distillate containing mercaptans and other acidic compounds with a caustic soda solution at a temperature between the freez-l ing point of the solution and 300 highto selectively remove substantially all said l acidic compounds Aof a greater acidity than said mercaptans so that said mercaptans'remain dissolved in ,said distillate, treating the distillate with kfurther caustic soda solution while at a lower temperaturebetween the solution`and about 60 said solution to absorb the greater portion of saidmercaptans, then freeing saidlmercaptans .from said solution. y l12. In a process of recovering'vproducts from petroleum oils the combinationof steps which comprises: treating a hydrocarbon distillate con- I .taining aliphatic monocarboxylic acids and mer-- captans with a caustic alkali solution while at4 a temperature between the freezingpoint of the solution and 300 F.,sufciently high to selectively absorb compounds having a greater acidity than said vmercaptans and to leave said mercaptans dissolved in said distillate and separating 'such 'alkaline solutionffrom said distillate. 13. In a process of recovering mercaptans from petroleum oils in which a hydrocarbon distillate containing ali liatic monocarboxylic acids and mercaptans -is treated with an alkali solution to absorb said mercaptans and said alkali soluf tion is then subjected to distillation to recover said mercaptans and to regenerate said alkali solution, the method of preventing the accumulation of alkali salts of said aliphatick monocarboxylic acids in the regenerated alkali solution which comprises: rst treating said distillate with a caustic alkali solution whilev at a temperature between the tion and l30.0 F., sufticiently high to selectively absorb compounds having ajgreater acidity than saidv mercaptans and to dissolved in 'said distillate, and separating'the thus spent `caustic alkali solution' from said'dis- F. suiiicientl-y freezing point of the soluthe freezing point of A v F.` suiiiciently low for leave said mercaptans

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