US2317770A

US2317770A - Refining mineral oils

Info

Publication number: US2317770A
Application number: US402188A
Authority: US
Inventors: Frederic A L Holloway; Durward O Wilkes
Original assignee: Standard Oil Development Co
Current assignee: Standard Oil Development Co
Priority date: 1941-07-12
Filing date: 1941-07-12
Publication date: 1943-04-27
Anticipated expiration: 1960-04-27

Description

Patented Apr. 27, 1943 UNITED STATES PATENT OFFICE REFINING MINERAL OILS Ware Application July 12, 1941, Serial No. 402,188

4 Claims.

The present invention relates to the refining of mineral oils. The invention more particularly relates to a process by which sulfur compounds such as hydrogen sulfide and mercaptan compounds may be eiiiciently and economically removed irom oils, particularly from petroleum oils. This invention is especially concerned with a process in which undesirable hydrogen sulde and mercaptan compounds are removed from feed oils containing the same by means of a selective solvent or reagent wherein the undesirable constituents are removed fromV the oil in an initial stage and in which the reagent is regenerated eciently in a secondary stage. In accordance with the present process, the mercaptan containing oil is contacted in a manner which is regulated with respect to the concentration and character of the particular mercaptan compounds present. The oil being treated just prior to removal of the same from the system is contacted with a relatively small amount of relatively mercaptan free solvent, while the feed oil introduced into the system is contacted with a relatively large quantity of partially mercaptan free solvent. The treated oil, substantially completely free of undesirable hydrogen sulde and mercaptans, is withdrawn fromvone end of the system, whilethe solvent containing the removed undesirable constituents is removed from the other end of the system and passed to a regeneration zone. The solvent is regenerated in a manner to remove from an intermediate point in the regeneration zone a partially regenerated solvent which is returned to the treating unit as described and to remove as fan end product in the regeneration zone a solvent relatively free of undesirable constituents which is utilized for contacting the feed oil prior to withdrawal of the feed oil from the solvent treating system.

It is well known in the art to utilizevarious solvents for the removal of sulfur compounds such as mercaptans frompetroleum oils and to regenerate the spent solvent by various processes. Solvents usually employed for the removal of mercaptan compounds from petroleum oils are various aqueous alkalis, as for example sodium hydroxide and potassium hydroxide solutions. In these processes the mercaptan containing oil is treated with a sufficient quantity of alkali to convert the mercaptan compounds to the corresponding mercaptides. The quantity of aqueous alkali utilized depends upon the particularv alkali being used, as well as upon the type of Vpetroleum oil being treated, and the character of the mercaptan compounds present. When employing caustic soda, it is a conventional practice to employ aqueous solutions having a sodium hydroxide concentration in Ythe range from about to'about 15%. AThe process for the removal of mercaptan compounds from feed cils is usually carried out in one or more mixing and settling Zones or stages, or in countercurrent tower treating operations. In this latter type of operation, the practice is to contact the oil and solvent throughout the phase contacting paths. After contacting the oil, the spent alkali solution containing the sulfur compounds as mercaptides is regenerated by various processes. For example, the mercaptides may beoxidized to form the corresponding disuldes by treating the same with an oxygen containing gas. These disulfide compounds are then removed from the oil by a subsequent step such as a decantation operation. The mercaptides may also be hydrolyzed to form the corresponding mercaptans which may be removed by volatilization.

In general, the solvent is regenerated or stripped of undesirable constituents by countercurrent action of an immiscible or partially miscible fluid, usually -by treating the solvent with inert gas or steam. The regenerated solvent is usually recycled to the extraction operation. These processes, however, have not been entirely satisfactory, due to the fact that the relatively higher molecular weight mercaptides are not readily oxidizable to disulfides. Furthermore, the relatively lower molecular weight mercaptides are not readily hydrolyzable to the corresponding mercaptans. A

In operations of this character, the oil being treated contains various mercaptans which` boil over a relatively wide range. For examplawhen treating petroleum oils boiling in the motor fuel boiling range, some of the mercaptans present comprise methyl mercaptan, ethyl mercaptan, the various propyl mercaptans, and the butyl mercaptans. The procedure thus employed is to contact the feed oil containing these-various mercaptans with the particular solvent being used in sufficient quantity to substantially completely remove `the least soluble mercaptan, which quantity is considerably in excess of that required for removal of the more soluble merk less soluble mercaptans from the feed oil.

We have now discovered a process by which it is possible efliciently and economically to remove hydrogen suliide and particularly undesirable mercaptan compounds from feed oils containingl the same and to handle the spent lsolvent in a manner by which the dissolved constituents are readily removed and the solvent regenerated whereby it may be recycled to the system. The process of our invention may be readily understood by reference to the attached drawing illustrating modifications of the same. Referring specically to Figure 1, for purposes of illustration it is assumed that the feed oil comprises a petroleum oil boiling in the motor fuel boiling range and contains mercaptan compounds boiling in the range of methyl mercaptan, ethyl mercaptan, propyl mercaptan, butyl mercaptan, and higher boiling mercaptans. The feed oil is introduced into contacting zone I by means of feed line 2. For purposes of illustration it is assumed that contacting zone I comprises a countercurrent treating tower containingv suitable distributing and contacting means. The feed oil flows upwardly through countercurrent treating tower I and contacts downowing solvent which is introduced by means of lines 3 and Il. For purposes of illustration it is assumed that the solvent comprises an aqueous solution of sodium hydroXide. The feed oil flowing upwardly through tower I initially contacts in the lower section partially regenerated solvent which is introduced into the system by means of line 4 and partially spent solvent from the upper section. The feed oil contacts relatively completely regenerated solvent in the upper section of tower I, which solvent is introduced by means of line 3. Conditions are adjusted with respect to solvent to oil ratios, as well as operating conditions such as temperature, so as to free the feed oil substantially completely of the various mercaptan compounds which are soluble in the sodium hydroxide solution. The treated feed oil, free of undesirable mercaptan compounds, is withdrawn from tower I by means of line 5 and handled in any manner desirable. The spent solvent solution containing dissolved therein the undesirable mercaptan constituents as mercaptides, is withdrawn from tower I by means of line 6, preferably heated in preheater '1, and introduced into the upper section of a solvent recovery zone 8. The spent solvent flows downwardly through solvent recovery zone 8, and contacts an upflowing stripping medium which is introduced by means of line 9. For purposes of description, it is assumed that` the stripping medium comprises open steam or an equivalent gas, or vapors generated by a reboiler. At an intermediate point in the solvent recovery zone a partially regenerated solvent stream is withdrawn by means of line II), cooled in cooler Il, and recycled to the intermediate point in tower I by means of line 4. A relatively small amount of the solvent flows through the lower section of regenerating Zone 3 and contacts the stripping steam under conditionsof higher steam to solvent ratio and consequently more severe stripping. The relatively small quantity of solvent which is substantially completely regenerated is withdrawn from solvent recovery zone 8 by means of line I2, cooled in cooler I3 and recycled to the upper section of tower I by means of line 3. 'llhe stripping steam and mercaptans removed from the solvent in solvent regeneration unit 8 are removed overhead by means of line I5 and handled in any manner desirable, usually by condensing the steam and heavy mercaptans and passing the non-condensed vapors to a burning line. When stripping steam is supplied by means of a closed coil reboiler, a partial reux of the condensed vapors is employed.

Referring specifically to Figure 2, feed oil, which for the purpose of description is taken to be a volatile petroleum naphtha such as that obtained by high pressure gas absorption and therefore containing considerable quantities of hydrogen sulde, is introduced into treating zone 20 by means of line 2l. For purposes of description it is assumed that treating zone 29 comprises a countercurrent treating tower containing suitable distributing and mixing means. The feed oil ows upwardly through treating Zone 20 and initially contacts in lower treating sections a partially regenerated solvent which for the purpose of illustration is assumed to be an aqueous solution of sodium hydroxide introduced by means of line 32 and partially spent solvent from the upper treating section. The treated oil finally contacts fresh solvent in the upper section of treating zone 20 which is introduced by means of line 22.

The oil substantially free of undesirable mercaptan compounds is withdrawn from the unit by means of line 23 and handled in any manner desirable. A major proportion of the spent sodium hydroxide solution is withdrawn from zone 20 :by means of line 24, passed through preheater 25 and introduced into the upper section of regeneration zone. 25.. The spent sodium hydroxide solution flows downwardly through regeneration zone 26 and contacts an upflowing stripping medium which is introduced by means of line 21, and which for purposes of description is assumed to be steam. The steam and volatilized mercaptans are removed from zone 26 by means of line 28 and handled in any manner desirable, usually by condensation of the steam and heavy mercaptans and passing the noncondensed -vapors through a burning line. The regenerated sodium hydroxide is withdrawn by means of line 29, passed to cooler 30, and introduced into zone 20 by means of line 32. VA portion of the spent caustic solution which is not withdrawn from treating Zone 20 by means of line'24 passes into the lower section where it may be diluted with water and contacts the incoming liquid under conditions to remove hydrogen sulfide substantially completely from the same. This portion of spent caustic iswithdrawn from the system by means-of line 3| and handled in any manner desirable, usually by disposal as a waste byproduct.

The process of the present invention may be widely varied. The process essentially comprises contacting a feed oil with a particular solvent in a manner that the ratio of solvent to oil is adjusted to remove the particular undesirable constituents depending upon their relative solubility in the solvent. Although the process may be adapted for the treatment of various mixtures by means of any desirable selective solvent, it is particularly applicable in the treatment of petroleum oils for the removal of mercaptan compounds when utilizing alkali metal hydroxide solutions. Y

The untreated feed oil is initially contacted with a relatively large volume of solvent, suicient in quantity to absorb and remove'substantially all of the `higher boiling or less soluble mercaptans such as the propyl and butyl mercaptans, and considerably in excess of the quantity required for removal of the less soluble mercaptans. The spent caustic from this operation enters the upper section of a regenerationtower, where it is stripped in a countercurrent manner Witha limited` amount of steam, just sufficient to nearly completely hydrolyze and vaporize the high boiling, less soluble mercaptans such as the propyl and butyl mercaptans. This quantity of steam is insufficient to remove all of the more soluble mercaptans such as the methyl and ethyl mercaptans, and as a result these lighter mercaptans are only partially stripped from the solvent in the upper regeneration section. At the approximate center of the regeneration tower, or bottom of the upper regeneration section, a major portion of the downowing solvent is withdrawn from the tower, cooled and returned to the approximate center of the treating tower, or top of the lower treating section, where it contacts additional feed oil for substantially complete removal of the heavy, less soluble mercaptans.

If the action described in the above paragraph were carried out alone, without the auxiliary action, to be described below, of the upper treating and lower regenerating section, it is evident that the incomplete stripping of light, more soluble mercaptans in the upper regenerating section would impair the absorption eiliciency of the recycled solvent, so that on incomplete absorption of light, more soluble mercaptans would take place on its subsequent reuse in the treating tower. During an intial starting period of such a process, the solvent would build up an increasing concentration of light mercaptans until the quantity of light mercaptans removed by the partial stripping with a limited quantity of steam in the regeneration section became just equal to the quantity of light mercaptans absorbed by the partially regenerated solvent in the treating section. Under these conditions, the mercaptan absorption and solvent regeneration process described would effect a nearly complete removal from the feed oil of heavy, less soluble mercaptans, but only a partial or fractional removal of light, more soluble mercaptans.

In order to make possible a nearly complete removal of the more soluble mercaptans without an increase in quantity of stripping medium, the actionV of the lower regenerating and upper stripping sections is necessary. In the lower regenerating section, the small fraction of partially regenerated solvent not withdrawn from the tower is allowed to continue to now down countercurrent to the same quantity of stripping medium used in the upper regenerating section. By this means, a considerably higher ratio of stripping medium to downowing solvent is effected in the lower regenerating section, using the same amount of stripping medium in both sections. The small volume of nearly completely regenerated solvent is removed from the bottom of the regenerating tower, cooledrand recycled to the top of the treating tower, where it is effective in nearly completely removing the considerable quantity4 of light mercaptans remaining in the feed oil after its initial countercurrent contact with the large volume of partially regenerated solvent.

In an operation such as described with respect to Figure 2, the untreated feed oil containing a considerable quantity of hydrogen sulfide as well as a mixture of mecaptans of wide variance in solubility characteristics is rst contacted with just sufficient quantity of caustic to neutralize and remove the comparatively acidichydrogen sulfide. This caustic is obtained as spent caustic from the upper mercaptan removal sections, and may thereforebe dilutedwith water to spring the mercaptans `fr'om'solution before neutralization of hydrogen sulfide. Since caustic spent with hydrogen sulfide is extremely diiicult to regenerate, it is in the usual case withdrawn from the treating u nt and disposed of as a waste product.l This lower section, then, serves for thei'initiall removal of hydrogen sulfide in a manner eco-Y nomical both from a standpoint of equipment cost and as will be seen below from the 'standf point of operating eiiciency.

The upflowing feed oil, after its initial washy for removal of hydrogen sulfide, next contacts caustic for removal in order of heavy mercaptans and light mercaptans, as describedin the process illustrated by Figure 1. An essential difference in this process from that of Figure l, however, is that the small volume of caustic fed to the top of the treating tower for nearly com plete removal of light mercaptans is supplied 4as fresh caustic to make up that disposed of as waste following neutralization of hydrogen sulfide,

whereas in the process illustrated in Figure l this stream of caustic is one nearly completely' regenerated on the lower stripping tower sec` tion.

The solvent may be introduced into thetreatA ing zone at any desired number of points; which will depend in general upon the characterfrcf the feed oil and the type of constituents it is desired to remove. The solvent which is introduced in to the treating zone at aA plurality of points is introduced in a, manner that the volume of 'flow of the solvent decreases in the direction of 'flow of the feed liquid. f'

When treating Apetroleum oil boiling inV the motorV fuel boiling range, and when introducingy the solvent at two points, it is preferred that the substantially completely regenerated solvent bey introduced at the top of the tower, and that the greater quantity of incompletely regenerated solvent be introduced at a point where the upflowing liquid is substantially free of propyl and butyl mercaptans and higher boiling mercaptans. This is generally' in the center section "of the tower.

The regenerated solvent may be segregated into any number of streams depending upon the' number of streams ofsolvent introduced into the treating system. -For example, when treating'a petroleum oil containing mercaptans boiling in the range from methyl to butyl and higher boil-1 ing mercaptans, and employing two points'of in traductionv of the solvent, it is preferred lthat a solvent stream be withdrawn from the regeneration tower at a point where the down owing solvent stream is free of propyl and butyl mer captans.

Any desirable temperature or pressure'may bev employed. It is preferred to employ tempera` tures in the range 'from about 75 F. to" l00 F. in the treating system and a pressure of about the vapor pressure of stocks treated. The temperatures employed in the regeneration zone should be suilicient so that the solvent is regenerated to the desired degree. For example, when regenerating an alkali metal hydroxide so-lutionj b e construed as ilimitingthesame' in. 'any man@v ner whatsoever:

Example 1 'Operations were conducted for the removal of various mercaptan compounds from petroleum oils boiling in the motor fuel boiling range, using as a reagent a 12 Baum sodium hydroxide solution. The minimum amount ofl caustic necessary to accomplish substantially complete removal of the individual mercaptan is as follows:

Minimum volume f 12 Baume caustic per 100 Compound volumes naphtha for complete removal of mercaptan Methyl mercaptan 0. l Ethyl mercaptan. 0. 5 Propyl mercaptan. v 2. 0 Butyl mercaptan 13. 0

From the above it is readily apparent that the i amount of sodium hydroxide solution used when operating to remove all mercaptans is excessive with respect to methyl, ethyl, and propyl mercaptans. This situation is aggravated when it is considered that in a commercial operation with ordinary treating towers it is necessary to use fromA 1.2 to 2.0 times the minimum values given here to insure nearly complete absorption.

Example 2 AOperations were conducted determining the minimum pounds of steam necessary per gallon of 12 Baume sodium hydroxide solution necessary substantially completely to remove the various mercaptans from the solution. The data secured are as follows:

Minimum pounds steam per gallon 12 Baum Compound caustic for complete stripping of individual mercaptan Methyl mercaptan 11.0 Ethyl mercaptan. 4. 0 Propyl mercaptan. 1. 0 Butyl mercaptan. l 02 Pounds steam per 100 gallons naphtha to strip C i 15% caustic treat of ompoum nearly all of the mercaptan (1.5Xminimum quantity) Methyl mercaptan 250 Ethyl mercaptan 92 Propyl mercaptan 22 Butyl mercaptan 4. 5

Itis evident from the above data that the steam required for removal of the low boiling mercaptans is relatively high. It is also evident that if these'mercaptans are not removed their concentration inthe causticwill build up until Example 3 In another operation a feed oil boiling in the motorfuel boiling range and containing mercaptans was treated with 15 gallons of caustic per 100 gallonsxof feed oil. 'Ihe spent caustic was then treated with 22 pounds of steam per 15 gallons of spent caustic, which was suflicient to strip the propyl and butyl mercaptans. 14 gallons per 100 gallons of feed were removed and recycled to an intermediate point in the absorber as described. The remaining one gallon of caustic per 100 gallons of oil feed was stripped with the same amount of steam used on the total 15 gallons, at a higher ratio of steam: to caustic, by which it was possible to effect nearly complete removal of methyl and ethyl mercaptans. Theone gallon of solvent per 100 gallons of feed, free of all mercaptans, was recycled to the upper section of the countercurrent treater, as described.

From the above data, it is apparent that the amount of steam required is 22 pounds as compared to 200 pounds when employing the conventional process described in Example 2.

What we claim as new and wish to protect by Letters Patent is:

1. Process for the removal of mercaptan compounds from petroleum oils, utilizing an alkaline solution, which comprises introducing the feed oil into c-ne end of an absorption Zone, introducing a substantially sulfur-free solvent into the other end of said absorption zone, introducing a partially regenerated solvent into an intermediate point ci said absorption Zone, countercurrently contacting the feed oil with said solvent, removing a treated oil from the treating zone, removing a spent solvent from the other end of said treating zone, and regenerating the same in a regeneration Zone in a manner to remove a n portion of the regenerated steam from an intermediate part of said regeneration zone, which stream is recycled to said intermediate point of said absorption zone, and removing from said regeneration zone a substantially sulfur-free solvent which is recycled to said absorption zone as clened.

2. Process as dened by claim 1 in which the ratio of solvent to oil throughout said absorption zfone is only slightly greater than the minimum requirement for the complete removal of the individual mercaptans.

3. Process for the removal of mercaptans from petroleum oils, which comprises countercurrently treating said petroleum oils with an alkaline solution in an absorption zone under conditions in which the ratio of solvent t0 oil decreases in the direction of flow of the oil, separating the spent alkaline solution and regenerating the same in a regeneration zone, employing a stripping iluid under conditions in whichy the ratio of stripping fluid to alkaline solution increases in the direction of flow of said alkaline solution.

4. Process as defined by claim 3 in which an intermediate stream is withdrawn from said regeneration zone and recycledto an intermediate point in said absorption zone, and in which a substantially completely regenerated alkaline solution stream is withdrawn from the lower section of said regeneration zone and recycled to the up- .per section of said absorption zone.

FREDERIC A. L. HOLLOWAY. DURWARD O. WILKES.