by W. R. Lucas The Atlantic Refining Co.
I INDUSTRIAL WASTES A
W O R K B O O K
F E A T U R E
E
C
Direct Oxygen Regeneration of Spent Petroleum Caustics This is generally the most economical method, as the cost of oxygen is almost all the direct expense of operation VÎAUSTIG treating has remained as
a general operation in petroleum refining despite the increased use of hydrogénation and reforming. T h e old and familiar problem of disposal or regeneration is still present and being aggravated by the trend to more stringent laws on stream and air pollution: Various techniques of regenerating spent caustics such as air oxidation, stripping with flue gas or steam, and electrolysis have been developed and are being used. However, each of these methods has its disadvantages. For example, air oxidation with its required air purging of the reactor may result in serious air pollution, A large reactor would be required for a given operation because of the oxygen dilution in the air. Stripping has the propensity to develop operational problems such as foaming and entrainment a t most inopportune times and usually cannot approach oxidation In degree of regeneration. Electrolysis is relatively expensive in most refinery locations and has a definite lack of flexibility. Oxygen Regeneration— High Industrial Potential
A process for the direct oxygen regeneration of spent refinery caustics has been developed by the Atlantic Refining Go. as a comprehensive solution to these difficulties and in anticipation of future legislation on stream and air pollution. T h e process is currently in use a t our Philadelphia refinery and is being installed at our Gulf Coast refinery. The process has been adapted to the regeneration of various caustic mixtures and the removal of sulfides and mercaptans from waste waters and crude cresylic acids. I t is potentially adaptable to a wide 84 A
W
I / E C
ORKBOOK
variety of industrial applications. The operation of the process on spent caustics from the treating of gasoline and light hydrocarbons is based on t h e conversion accomplished with oxygen in a gasdispersion reactor of special design under conditions of essentially ambient temperature and pressure. T h e generalized reactions occurring during regeneration a r e : 4 R S - + 0 2 + 2H 2 0 - * 2R2S2 + 4 O H ~ + 20 2 - * SOr-
(2)
Equation 1 represents the principal reaction and actually effects a complete regeneration of the caustic. Reaction 2 occurs because spent caustic from butane or gasoline treating sometimes contains sodium sulfide. A small purge of ' regenerated caustic would then be required to maintain t h e caustic strength and the sulfate impurity at a low level. T h e regeneration reaction is exothermic. However, the solution will have only a small temperature increase for relatively high sulfur contents. This effect is consistent with the degree of oxidation produced and the contaminant concentrations encountered. Mercaptan Size and Form
T h e effect of mercaptans molecular weight on regeneration was determined through preparation of various naphtha cuts having narrow boiling ranges from two crude oils, West Texas Permian a n d West Texas Devonian. T h e Ce cut had a boiling range covering the boiling points of both methyl and ethyl mercaptans, while the boiling range of the Οβ cut covered the mixture of isopropyl, w-propyl, a n d tert-
FEATURES
butyl mercaptans. A C 7 cut was made from West Texas Permian only; its boiling range covered both isobutyl and w-butyl mercaptans. These naphthas were caustic-washed with' 25 weight % caustics which were regenerated under constant operating conditions. T h e data ob tained on the caustics used to treat the cuts are shown in the figures on page 85 A. Process Flow
Spent caustic containing mer captans is charged directly to the regenerator, where it comes in con tact with oxygen (Figure 1). T h e oxidized caustic solution is then settled and most of the disulfides are removed as a supernatant oil layer. T o effect a final removal of disulfides, the caustic is washed with some raw plant stream such as gas oil, cycle stock, or furnace oil. Since the theoretical oxygen require ment is about V4 pound for each 1 pound of sulfur as mercaptide, the oxidation of a ton of sulfur each day would require slightly more than 4 standard cubic feet of oxygen per minute. Economics
The cost of oxygen comprises al most the entire direct expense of operating this regeneration process It would be approximately $30 to 40 for each ton of sulfur oxidized per day. T h e operating-labor costs are negligible, as the process requires little supervision and this is borrowed on a part-time basis from nearby units. If the recovered disulfides could be sold or used as a chemical raw material, the plant might make money. However, it is generally the most economical method for han dling spent petroleum caustics.
T h e Process V a r i a b l e s
Temperature
Contact Efficiency
O x y g e n Pressure
POOR EFFICIENCY
PSIG PRESSURE PSIG PRESSURE
RSH
RSH
RSH PRESSURE PSIG \MAXIMUM EFFICACY
PSIG PRESSURE
TIME (min.)
TIME (min.)
An increase in the oxygen pressure from 10 to 4 0 p.s.i.g. in a temperature range of 75 to 8 0 ° F. will multiply the regeneration rate by three to four times. The curves shown are not representative of optimum regener ation conditions, but only indicate the effect of this variable. Thus, within the range of pressures used, the re generation rate is approximately pro portional to.the oxygen pressure
Caustic R e g e n e r a t i o n vs.. Mercaptan Mol. Weight
RSH Content of Caustic (mgS/IOOcc)
CRUDE SOURCE-W.T PERMIAN PRES SURE M5psig CAUSTIC-25 wt.% TEMP' IOO°F
TIME (min.)
Small increases in the efficiency of contact between oxygen and caustic can increase the oxidation rate by as much as six times. The curve labeled "maximum efficiency" defines the max imum regeneration rate for the operat ing conditions used in the lab equip ment. The curve labeled "poor effi ciency" illustrates the degree to which the efficiency of contact may affect regeneration
TIME
(min.)
Over the range 8 0 ° to 100° F., temperature has a negligible effect on oxidation rate. Data above show only a small increase in regeneration rate will result by increasing tempera ture to 100° from 8 0 ° F. This effect is essentially constant over the range of oxygen pressure studied Caustic Regeneration vs. Mercaptan Mol. Weight
The r e g e n e r a t i o n r a t e d r o p s s h a r p l y CRUDE SOURCE ' W.T DEVONIAN as the m e r c a p t a n molecular w e i g h t 0 2 PRESSURE > 15 psig is r a i s e d f r o m the methyl t o the CAUSTIC ' 25 wt.% b u t y l r a n g e . The r e g e n e r a t i o n r e TEMR= IOO°F action a p p e a r s t o b e essentially RSH Content first-order f o r m e r c a p t a n s o f a n y of Caustic given molecular w e i g h t . G e n e r (mgS/IOOcc) a l l y , the reaction r a t e is d e c r e a s e d CUT as the molecular w e i g h t o f the ,CUT m e r c a p t a n is i n c r e a s e d . Methyl a n d ethyl m e r c a p t a n s h a v e r e a c tion rates f r o m 2 Ά t o 3 times as high as the b u t y l mercaptans. H o w e v e r , the molecular c o n f i g u r a TIME(min.) tion o f the m e r c a p t a n s is also a n The d a t a a b o v e w e r e o b t a i n e d f r o m important factor the r e g e n e r a t i o n o f caustics used t o t r e a t the W e s t Texas D e v o n i a n cuts. The differences in r e g e n e r a t i o n r a t e b e t w e e n c o r r e s p o n d i n g cuts f r o m these crudes might b e a t t r i b u t e d t o the FOUL isomeric f o r m o f the m e r c a p t a n present WASH OIL
•^ Figure 1 . System f o r regeneration by d i rect o x i d a t i o n
OXYGEN
SPENT CAUSTIC
REGENERATOR
SETTLER
WASH DRUM
WASH OIL
REGENERATED CAUSTIC
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly. VOL. 5 1 , NO. 2
•
FEBRUARY 1959
85 A
