Tannins and Allied Chemicals in Mercaptan Removal Processes J. HAPPEL AND S. P. CAULEY S o c o n y - l a c u u m Oil Company, Znc., Brooklyn 2 2 , V. 1 .
ilkali-soluble catalysts hawe been found with which a rapid economical regeneration of treating solutions, used for remowal of mercaptans froni gasoline, can be effected through air blowing. Representatiwe of the groups of ( onipounds w hich hawe been inwestigated and found to be rffectiwe for this purpose are tannins, gallic acid, hj droquinone, and pFrogallo1. The influence of temperature coucentrations and other variables upon oxidation rates of mercaptans is giwen. BIethods of using the cataljsts in mercaptan remopal processes are discussed. The use of catalj sts for regeneration of treating solutions presents a new approach to methods of mercaptan remowal froni gasolines and should result i n wider application of this method of gasoline treatment.
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V A J O R I T Y of the rcccntly developed gasoline sweetening processes are directed toward mercaptan removal rather than conversion t o disulfides. T h e benefits of this type of treatment, and various processes for accomplishing mercaptan removal, have been described at length in the literature and were r e v i e m d in. a previous paper b y the authors (1). Both conventional sweetening and mercaptan removal processes eliminate the objectionable odor in gasoline associated with mercaptans. T h e extraction processes have the added advantage t h a t improvements in octane number and lead susceptibility result 1,ecause of the reduction in sulfur content. Therefore, a considerable saving in refinery operating cost, results. In general the mercaptan removal processes depend upon two distinct operations which are equally important-that is, extraction of mercaptans from the oil being treated, and regeneration of the treating solution. Naphthas vary widely in mercaptan type and content, depending on the sulfur contents of the rrudee and the refinery processing steps involved in their production. Cracked gasolines usually contain a higher proportion of loi~-molecular-~veiglit mercaptans as compared 11-ith straightr u ~ inaphthas of the same boiling range. The former are inore readily removed by extraction. I n general, the extraction step utilizes aqueous solut,ions of alkali metal hydrosides to n.hich are added certain compounds which increase their ability t o remove mercaptans. Ordinary caustic soda solutions are not very effect ivo in removing the heavier mercaptans (above. butyl). Solutions which have accumulated limited amounts of natural organic acids are much more suitable. For esample, a 30 weight sodium hydroside solution which has picked u p 20 volume (r, of organic acids m a y be used t o advantage. S o t all gasolines contain enough acids t o cause them t o build up t o 20cG of t,hc caustic solution; then the outside addition of solutizers is necessary. Still higher extraction coefficients are obtainable by the the use of such materials a s the Shell K-2 solutizer solution, which is 6 L'V potassium hydroxide and 3 N potassium isobutyrste. The treating solutions are usually regenerated by steam stripping in a packed, or bubble plate, t o m r . T h e operations (regeneration aiid extraction) are interrelated in t h a t a reduction in efficiency of the former results in a n additional burden upon the latter. Although considerable progress has been made in the direction
of extraction of mercaptans from gasolines through the us(! of solubility promoters, the tcchnique of regenerating treating solutions has not been similarly improved. Gnfortunately, too, the agents which have increased the mercaptan extraction power of caustic solutions have adversely affected the ease of regencration by stripping. Shell has shoivn (6)t h a t somc' saving in steam, during regeneration, can be realized by dilution which also affects a good recovery of gasoline entrained in the treating solution. Regeneration bv stripping, however, is still a n expensive operation, and simplifying it should mat,eriallyincrease the applications of mercaptan removal processes to gasoline treating. I t has been known for some time t h a t caustic solutions containing mercaptides could be regenerated in the presence of gasoline or alone by reaction wit.h oxygen t o form disulfides ( 2 ) . I n the former case the disulfides produced remain in the gasoline, just as in the case of conventional svieetening processes. T h e rate of oxidation under normal conditions, however, is usually too slow for practical use. Some inyestigators ( 3 ) advocated t,he use of insoluble met,al salts for catalyzing this reaction, but none of these catalysts has found commercial use. A study \vas undertaken t o ascertain hoF the rate of this reaction could be more effectively increased. It was found in this work that a number of compounds are capable of promoting a rapid, economical regeneration of treating solutions containing mercaptans by air bloxing a t slightly above atmospheric tcmperatures. Disulfides, formed by the osidaticln of mercaptans, are only slightly soluble in the treating solution and may be separated by settling. If complete disulfide removals arc desired other means may also be employed. DESCRIPTION OF PROCESS
-1typical flow diagram showing the catalytic air regeneration system is shown in Figure 1. Estraction of mercaptans is conducted in the usual manner using countercurrent stages or a packed toiver. T h e fat solution is heated to 115-125" F. to reduce its viscosit,y and, if nccesaary, passes through a coalescer t o remove any entrained gasoline. The fat solution is fed to a packed tower or to other convenient apparatus for securing intimate contact with air, !There it is regenerated by air oxidation of mercaptides to disulfides. T h e solution is then transferred t o a settler where the disulfides are separated. The regenerated solution is retumed t o the estraction system. I n the case of t h e conventional solutizer process a steam stripper is substituted for the air regenerator and settler. Disulfides are soluble t o a small extent in the treating solution; a s a consequence, when the air regeneration method is used, a small amount of disulfides will be returned t o the gasoline being treated. This solubility varies from s e v t d tenths t o one per cent b y volume of the treating solution circulated, being higher in solutions containing t h e highest concentration of solubility promoters. Gasoline is also soluble in the treating solution t o about, the same extent, so t h a t t h e RSSR re-entry is less than the disulfide solubility. I n order t o reduce the amount of disulfides returncd t o gasoline, auxiliary xash syst,ems have been installed at some plants. Naphtha is generally used as the washing me1655
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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1 minute each. The initial mercaptan sulfur concentrations are 1.0 =t 0.10%. T h e air-tosolution ratio is 6 : l by volume. After each minutc~of regeneration the free space in the sepnrLitory funnel is purged with Ion- pressure air for a period of 20 seconds. This proccdrirc is conducted with a minimum of t h e between shalcings and is continued until t h o nwrcaptan concentration is reduced t o 0.1-0.2c{. The time required for regeneration is detcrrnincd approximately by a preliminary run.
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Vol. 39, No. 12
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Expcriencc has shown that t he ri.gc,ricmtion rates clwing a trpt n-ere essc~ntiallyciinstant, iii i lie rang(' of niercaptaii sulfur concentrations SOLUTION SETTLER uwtl, .-o t h a t the differc~ncein initial :tncl final conci.nt rat ions divided 11y the nuniber of reFigure 1. Typical Flow Diagram of Treating Process Csing Catal) tic gcilcrxtions may be talien as t h e regeneration Regeneration S j b t e m r;it(>. llcrcnptan sulfur contents of solutions \VI >re determined by pot elit iomct ric titration. of solutions, under t e s ~ for regeneration rates, ivcrc diuni and may bc periodically rejccrcd or rcrun to rcniovc t i i d s pvsdihle after the filial shaking operation. It was fides. Such naphtha ash streams may also be reforrileti liy rcymgnizcd that the laboratory mcthod used in contacting soluthermal or catalytic cracking; this results in conversion of thc tizcr solut ioiis with air would givc only coniparative results, major proportion of the disulfide to h>-drogen sulfide, n-hicli i q since i n plant operations q u i t e different contacting Inethods readily removed. e.oultl I)(,c~niployed. IIo\viLver, i t is believed that this method ' . rc.l)riitliicible results antl is thus useful in studying the other PROPERTIES O F REGENERATION C l T A L Y S T S ihlt,. involved. The types of catalysts which have becn found thus far to bc. r-r slion. t h c ~ ~ f f ( ~ofcdifferent ts variahles and catamost effective for regeneration of treating solutions are tannic nt~~~:ition r a t w The. h t a given i n the taljles on acid or tannins, phcnolie acids, and polyhydro9yhenzenca. .411 rates, although not a b d u t c , givc the relative cffccof these catalysts are solublc in the treating solution, insoluth t iv('iil+ lJl't li(' v:iiii)uu types of osid:ttion catalysts. Another in the gasoline, and relatively stable under the conditions of f:ic,tors I I O L sh01vn i n thtl tablci. is thc regeneration effected after all extraction and rcgencrarion. i,ont ncatiiig I J :iir ~ :in11 solutiun h a i c d. Experimental n.ork These catalysts are theinselves readily oxidized iii all nicrcaptnn sulfur coni:eiitration of a solution solutions, but it has been found that such osidation is i i i h i t i i t c d i i i - IYJ1l~t:itlt1.5-30 minuti.:: after regeneration a t atmoi;plicric by the presence of mercaptans. The concentration of niercnpt::ri> I 1~1111wi~ii t ui'i,,
