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Treatment Of Spent Caustic Refinery Effluents
RELATED APPLICATION This application claims priority to United States Patent Application No. 10/234,559, which was filed on September 4, 2002, and is incorporated herewith by reference.
BACKGROUND OF THE INVENTION This invention pertains to processes for treating spent caustic effluents, for instance, effluents from petroleum refineries, aluminum manufacturing, food processing or the like.
Many industrial operations generate dilute spent caustic effluents similar to petroleum refining processes. A typical dilute spent caustic effluent from such refineries may contain about 5% to 12% sodium hydroxide (w/w) with varying but significant quantities of organic compounds that include a spread of mercaptans, sulfidic oils, naphthenic acids, cresylic acids and derivatives. Included in the mixture are lesser quantities of other inorganic and organic compounds. In addition, a petroleum refinery effluent may contain approximately 5% to 20% by volume of oil along with the aqueous caustic solution. Traditionally such effluents have been considered essentially useless waste streams but have required significant processing before disposal to avoid contamination of the environment.
The patent to Helleur No. 4,079,585 describes a method and apparatus for removal and recovery of a constituent from industrial and municipal waste streams by evaporation and concentration of the constituent on account of intimate and turbulent contact between the waste stream liquid and hot gases produced by combustion in a scrubbing tower and states that submerged combustion techniques can be used to advantage in the process. According to that patent undesirable volatile pollutants could be removed without vaporizing them by addition of an alkali such as lime or caustic to retain acidic volatile pollutants reminiscent of sulfur dioxide, hydrogen sulfide, etc. in solution so that they can be disposed of in liquid form. The Helleur patent describes the method as being applicable to spent material from such industries because the oil industry and notes that, to avoid vaporization of selected combustible volatiles, the temperature of the combustion gases should be maintain below the flash point of the volatiles by cooling the combustion gas before contact with the liquid.
The Young et al. Patent No. 4,16,028 discloses a submerged combustion evaporator as the first stage in a process for concentration of constituents of industrial waste streams.
In the Ohkawa et al. Patent No. 3,966,594, treatment of waste water containing water-soluble organic substances in various ways is described and the submerged combustion method is stated to be industrially insufficient by way of concentration and combustion. Instead, that patent describes a process in which the waste water is treated with a water-insoluble organic solvent solution of an organic constituent. Based on the Anderson Patent No. 4,188,291 industrial waste water is processed by a submerged combustion evaporator and carbon dioxide within the combustion gases supplied to the waste water is sequestered by calcium hydroxide which has been added to supply calcium carbonate which is then separated from the waste stream. The spent caustic treatment process described within the DeRoeck et al.
Patent No. 5,244,576 introduces refinery gases containing carbon dioxide and hydrogen sulfide right into a sodium hydroxide solution to transform the carbon dioxide to sodium carbonate.
Within the Connally Patent No. 3,985,609 concentration of constituents in a liquid to be concentrated is effected by supplying the liquid to a submerged combustion evaporator.
The Echols Patent No. 5,934,207 describes evaporation of leachate by directing flames from a burner to which landfill gas is supplied into a hearth tube heater immersed in a tank containing leachate in order to heat and vaporize the liquid constituents in the leachate while disposing of the landfill gas. The Duesel Patent No. 5,342,482 discloses the usage of landfill gas as a fuel source for a submerged combustion gas evaporator in a meaning of oil refinery mean leachate evaporation process.
SUMMARY OF THE INVENTION
Accordingly, it’s an object of the present invention to provide a process for treating spent caustic refinery effluents which overcomes disadvantages of the prior art.
Another object of the invention is to offer a process for treating caustic effluents from a refinery to convert a caustic constituent to a useful commercial product.
An additional object of the invention is to supply a process for treating petroleum refinery effluents to provide a reusable oil product in addition to a converted caustic product. A further object to the invention is to provide a process for treating caustic effluents utilizing carbon dioxide in a combustion gas supplied to the effluent to transform a caustic constituent to a carbonate.
These and other objects of the invention are attained by supplying a caustic effluent to a submerged combustion gas evaporator to which combustion gases are supplied at a temperature and quantity selected to remove undesired vapor and gases while retaining a desired liquid constituent of the effluent, and separating the desired constituent from the remaining constituents of the effluent. In a selected embodiment of the invention the caustic effluent is a petroleum refinery effluent and carbon dioxide within the combustion gas supplied to the effluent converts a caustic constituent in the effluent to a carbonate comparable to sodium carbonate. In addition, oil is separated from the other effluent constituents, before, during or after evaporation, for reuse or further refining.
According to a different aspect of the invention, at the very least a few of the fuel used to produce combustion gas to the submerged combustion gas evaporator is landfill gas and the carbon dioxide content of the combustion gas is used to transform the caustic constituent of the effluent to a carbonate, thereby preventing dissemination of the carbon dioxide into the atmosphere.
In a representative embodiment of the invention the spent effluent supplied to the submerged combustion gas evaporator has an equivalent sodium hydroxide content of about 5% to 12% weight percent and an oil content of about 5% to 20%» by volume and the aqueous content of the spent effluent is reduced by evaporation as required to provide a desired carbonate concentration. The concentrated liquid may contain about 20%o to 30% sodium carbonate and the oil constituent is separated for reuse after concentration by a gravity separator.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will likely be apparent from a reading of the next description along side the accompanied drawings through which:
Fig. 1 is a schematic block diagram illustrating a representative embodiment of an arrangement for carrying out a process for treating caustic effluents in accordance with the invention;
Fig. 2 is a schematic sectional view illustrating a representative embodiment of a submerged combustion gas evaporator to be used in finishing up the strategy of the invention; and Fig. 3 is a schematic block diagram illustrating an additional representative embodiment of an arrangement for treating caustic effluents in accordance with the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Within the representative arrangement for carrying out the process of the invention schematically illustrated in Fig. 1, a submerged combustion gas evaporator 10, which is described in additional detail in reference to Fig 2, receives combustible gas from a landfill or other biogas source 11 or from another fuel source 12 providing natural gas, propane, butane or the like or providing a liquid fuel akin to waste petroleum oils or the like or from both. The evaporator 10 could also be operated on a batch, batch-continuous, or continuous basis and the fuel gas from the sources 11 and 12 could also be collected and stored to be used or be supplied continuously in accordance with the needs of the evaporator, hi one embodiment the system is located closely adjacent to a landfill from which landfill gases are conveniently available, but, if desired, landfill gases may be piped or transported from one or more landfills at remote locations to processing stations adjacent to a source of caustic effluent liquid similar to a refinery or other manufacturing facility.
Within the embodiment illustrated in Fig. 1 a caustic liquid from a source 14 similar to spent caustic effluent from a petroleum refinery, aluminum manufacturing plant or food processing facility is transmitted through a line 15 to a heat exchange unit 16 for preheating by hot exhaust gases after which piped through a line 17 to the inlet to the submerged combustion gas evaporator 10. The caustic in the spent effluent may be sodium hydroxide or every other alkaline material and the spent effluent could also be from a petroleum refinery, aluminum manufacturing plant, food processing facility or from various industrial operations similar to scrubbing. The sodium hydroxide equivalency of the caustic material within the spent effluent from the source 14 is preferably in a variety from about 1% to about 50%, desirably in a spread from about 1% to 15% and most desirably in a variety from about 2% to 12%. If the spent effluent is from a petroleum refinery and contains a considerable proportion of oil, the spent effluent could also be passed through a preliminary oil and water separator (not shown) to remove no less than a portion of the oil before it is delivered to the evaporator 10.
Following evaporation of vaporizable constituents from the spent effluent in the evaporator 10, concentrated liquid is removed through an outlet 18 and transmitted by a pump 19 through an oil separator 20 to remove oil from the aqueous constituents of the concentrated liquid which pass from the separator to an outlet 31. If necessary, some or all the aqueous constituents may be recycled through a line 21 back to the evaporator 10 for further processing after separation of the oil. The separated oil is removed from the separator through a line 22 on the market, further processing or recycle. The oil separator 20 could also be any conventional immiscible liquid separator akin to a gravity settling tank, an inclined plate separator or a centrifuge. Combustion air driven by a blower 23 and supplied through a line 24 is preheated by circulation through a heat exchange unit 25 through which the concentrated liquid passes from the outlet 18 and the preheated combustion air is supplied to the evaporator 10 through a line 26. The heat exchange within the unit 25 may be used to improve the efficiency of the burner. If desired some of the combustion air from the blower 23 may be diverted to an in-line air stripper (not shown) in the line 17 to remove volatile compounds from the spent effluent being supplied to the evaporator and supply them with the combustion air to the burner within the evaporator 10. Exhaust gases are withdrawn from the evaporator 10 through a line 27 after which passed through a demister 28 by which entrained liquid constituents are removed, after which the gas is passed through an exhaust purification unit 29 similar to an enclosed flare. In the purification unit 29 volatile organic compounds and other exhaust gas constituents are thermally oxidized or otherwise treated to render them odorless and harmless before being released into the atmosphere through a line 30 which passes through the heat exchange unit 16.
The arrangement of a preferred form of submerged combustion gas evaporator for use in the strategy of the invention is illustrated in Fig. 2. As shown in that figure the evaporator 10 has a housing 40 with a downwardly converging conical bottom 41 leading to a discharge valve 42 through which the concentrated aqueous caustic liquid and oil, if present, pass into the discharge line 18. Within the housing 40 liquids and entrained solids, if any, can settle and be directed by the conical housing portion 41 toward the discharge valve 42. At the top of the housing 40 a cover 47 is sealed with a sealing gasket 48 to the housing wall and a burner 49 is mounted to the cover. Combustion gases from the burner 49 are directed downwardly through a flame and combustion gas downcomer 51 into a distributor 52 having a series of mixing sparge pipes 53. The distributor pipes have orifices 54 through which the recent combustion gases are injected right into a pool 55 of the spent caustic effluent contained within the housing in order to evaporate liquid therefrom in an evaporation zone 45 by direct contact heat transfer, provide and distribute carbon dioxide to the spent effluent to effect conversion to carbonate and likewise drive off vaporizable constituents. Preferably, the operating pressure throughout the evaporator is within a range from about 50 inches negative to 100 inches positive of water.
The upper surface 56 of the caustic waste liquid pool within the housing is maintained at a desired level by controlling the supply of spent effluent admitted into the evaporator through the road 17 from the source 14. Preferably, the level of the upper surface 56 is in a range from about 5 inches to about 100 inches above that of the distributor 52 and the temperature of the pool of spent effluent 55 is in a range from about 100°F to about 250°F. If the evaporator is being operated on a continuous basis the spent effluent is supplied to the evaporator continuously at an appropriate rate in order to take care of the surface 56 on the indicated level inside the evaporator.
Then again, if the evaporator is being operated on a batch basis the effluent to be processed will likely be supplied continuously at an appropriate rate to as to keep up the surface 56 at the indicated level until the concentration of desired constituents in the effluent reaches a selected value. Thereafter the availability is terminated and the evaporator is either shut down and emptied or operation may continue until the surface of the effluent throughout the evaporator falls to a lower level (not shown) which is above the highest part of the gas distributor 52 at which time the evaporator will likely be shut down and the concentrated effluent might be discharged, after which the evaporator will be started again.
In a preferred embodiment the inlet temperature of the combustion gases within the distributor 52 is in a spread from about 600°F to 1800°F, the temperature and quantity being high enough to vaporize water and volatile constituents within the spent effluent so as to concentrate desired constituents akin to oil and/or a converted caustic at a desired rate.
When operated in a batch-continuous mode, the operation proceeds as in a continuous mode except that concentrated effluents are removed periodically from the discharge 42 without lowering the liquid level or shutting down the evaporator. As indicated in Fig. 1, fuel is supplied to the burner 49 through a line 13 and combustion air is supplied to the burner through a line 26 under sufficient pressure, preferably in a variety from about 5 inches to about 200 inches of water, to provide for efficient combustion and to force the combustion gases through the distributor 52 and convey the exhaust gases through the exhaust line 27.
It must be noted that, exclusive of process pumps, only a single moving member is required to carry out this process provided that a fuel supply is out there at adequate pressure, e. the blower 23 which supplies combustion air under pressure. Accordingly, the spent caustic effluent treatment technique of the invention not only removes odorous constituents from the effluent but in addition produces a commercially useful aqueous concentrate and separates a useful oil constituent in a simple and effective manner without the requiring complex moving parts which lead to difficulties embodied in the prior art spent caustic effluent handling systems.
Moreover, although spent caustic effluent is passed through the heat exchange units 16 and 25 in the arrangement disclosed in Fig. 1, the difficulties resulting from the prior art use of heat exchange evaporators aren’t encountered in those units since no evaporation is carried out in them and the consequent fouling of heat exchange surfaces is avoided. In simpler types of spent caustic effluent treatment systems in line with the invention, the heat recovery units 16 and 25 could also be omitted and, if the exhaust vapor meets environmental standards for direct discharge to the atmosphere, the exhaust treatment units 28 and 29 may even be omitted. Fig. 3 illustrates another embodiment which is identical to that of
Fig. 1 except that, as previously described, a source of hot gases spaced from the evaporator reminiscent of a hot gas generator 13′, which can for example be an internal combustion engine, supplies hot gases through the road 13 to the new gas distribution device 52 in a modified submerged hot gas evaporator 10′, the air supply line 26 being connected to the new gas generator 13′ rather than to the evaporator 10′. In this case the new gases from the new gas generator are supplied to the evaporator at a pressure in a range from about 60 inches negative to about 120 inches positive of water. In all other respects the caustic liquid treatment system of Fig. 3 is the same as that shown in Fig. 1 and the evaporator 10′ is similar because the evaporator 10 illustrated in Fig 2. In accordance with the invention, spent caustic effluent from petroleum refineries or the like will be concentrated efficiently and effectively without requiring a heat exchange evaporator of the sort utilized in conventional spent caustic effluent treatment systems having surfaces which could be fouled by the effluent residue and therefore require periodic cleaning or replacement. Typical oil refinery spent caustic effluents which could be processed by the current invention include sulfidic, cresylic and naphthenic effluents.
As described within the paper entitled “Effluent Caustic Treating Systems Using MERICONSM Technologies” by Merichem Chemicals and Refinery Services LLC, sulfidic caustic effluents are produced by fuel gas, liquefied petroleum gas (LPG) and gasoline treating processes. A typical refinery sulfidic caustic stream has the composition shown in Table 1 below. The usual contaminants are sodium sulfide and sodium mercaptide. These compounds cause high chemical and biological oxygen demand within the treatment process and produce odors and dangerous gases when neutralized.
Cresylic caustic effluents, which contain aromatic acid oils, are produced by caustic treating of cracked gasoline and cracked distillates. Cresylic caustic streams contain phenols, cresols and other organic acids that are generally present as water-soluble cresylates which is able to separate from the caustic as acid oils at a neutral pH. Cresylic caustic solutions produced from treating cracked gasolines generally come from two sources: 1) mercaptan extraction or oxidation systems using strong caustic; and 2) mercaptan oxidation systems using weak caustic. Table 2 below shows the characteristics of typical cresylic caustic effluents. Table 2
Concentrated cresylic caustic solutions have commercial value as chemical intermediates.
Naphthenic caustic solutions are generated from caustic treatment of kerosene and diesel cuts from a naphthenic crude slate. Table 3 below shows typical characteristics of naphthenate streams derived from kerosene and diesel stocks.
Petroleum refineries typically pay costs for shipping, treatment and disposal of raw industrial waste liquids including spent caustic. The refineries would find it useful to repurchase both the caustic liquid and oil for re-use in their process if they can be deodorized and the sodium hydroxide equivalency concentrated to acceptable values. In accordance with the present invention, petroleum refinery spent caustic can be processed to satisfy those requirements effectively and efficiently. hi a pilot test of the process of the invention, a big reduction in the extent of odor within the concentrated liquid was obtained in comparison with the feed material. The concentrated liquid was a two-phase mixture of oil and an aqueous phase that may very well be readily separated by gravity separation corresponding to decanting. After decanting, the aqueous phase assay showed that a major concentration of caustic was achieved. The assay also showed almost 100% conversion of the sodium hydroxide in the spent caustic effluent to sodium carbonate because of the reaction of carbon dioxide in the combustion gas with the hydroxide as discussed hereinafter. In this test the concentration of sodium carbonate within the concentrated aqueous phase was approximately 30% (w/w). h this regard, the direct combustion gas sparge that takes place within the submerged combustion gas evaporator during the process causes the formation of carbonic acid in the water that’s present within the evaporator during processing and carbonic acid then reacts with the caustic constituent, sodium hydroxide, to form sodium carbonate in the style described below. The dilute caustic feed material for the pilot test was an approximate
1:1 blend of two by-products of refinery operations known as the “sulfidic” and “cresylic” spent caustic streams. Assays of every of those streams and the material used as feed for the pilot test are presented in Table 4 below.
1 The full of all percentages is greater than one-hundred because some elements are double counted within various compounds.
The goal of the pilot test was to both deodorize and concentrate the sodium hydroxide content of a caustic material that was produced as a by-product in petroleum refining operations. Because the combustion gas supplied to the evaporator contained carbon dioxide, it was anticipated that some portion of the sodium hydroxide would be converted to sodium carbonate. It was not known how the mixture of oils carried throughout the caustic feed material would affect, or be affected by, the method.
The results of the pilot test showed that the sodium hydroxide had been converted almost entirely to sodium carbonate, and that an oil phase was produced that was immiscible with the aqueous meaning of oil refinery mean sodium carbonate phase. Decanting easily separated the 2 immiscible phases. Each of the phases produced within the experiment was judged to have significantly reduced levels of odor compared to the feed material, demonstrating that the target of deodorizing the dilute caustic feed during the method had been achieved. Table 5 below shows a comparison of selected constituents in the material used as feed to the pilot unit and the aqueous phase produced because of subjecting the fabric to the combined deodorization / evaporation process. Within the test, the volume of the feed material was reduced to approximately one-third of the unique spent caustic effluent liquid volume.
1 The full of all percentage for the feed sample is greater than one-hundred because some elements are double counted within various compounds.
The outcomes of two subsequent pilot tests were similar. In each test the oil that was recovered after the combined deodorization/evaporation process had concentrated the whole volume of feed material was between 10% and 15% of the whole volume of feed material processed. During the evaporation process the carbon dioxide contained within the combustion gas injected into the aqueous spent caustic liquid reacted with the water to produce carbonic acid in accordance with the following equation:
CO2 + H2O = H2CO3 (1) and the carbonic acid reacted with the sodium hydroxide to provide sodium carbonate and water in accordance with the following equation:
2NaOH + H2CO3 = Na2CO3 + 2H2O (2)
From the foregoing it is going to be seen that each mol of carbon dioxide within the combustion gas injected into the caustic liquid converts two mols of sodium hydroxide to at least one mol of sodium carbonate and one mol of water. The molecular weight of carbon dioxide is 44 and that of sodium hydroxide is 80, while the molecular weight of carbonic acid is 62, that of sodium carbonate is 106 and that of water is 18. Accordingly, 10,000 lbs. of a ten% sodium hydroxide solution produces 1,325 lbs. of sodium carbonate and 450 lbs. of water and consumes 550 lbs. or 4,488 standard cubic feet of carbon dioxide. Consequently, with a spent caustic feed that weighs 9.237 lbs. per gallon containing 10% of sodium hydroxide (w/w), each 10,000 gallons treated sequesters 1,451 cubic feet of carbon dioxide, or for a median of 30,000 gallons per day of spent caustic effluent treated, 2,781 tons of carbon dioxide per year are sequestered and therefore prevented from release to the atmosphere. Thus, the commercial value of the process includes the direct environmental and economic benefits, i^, potential revenue for managing spent caustic effluent, the potential sale of recovered product or products, the mitigation of greenhouse gas emissions with the potential for generating commercially valuable “carbon credits” in direct proportion to the amount of greenhouse gas that’s sequestered. If desired, caustic liquid which is a commercially available product rather than a spent caustic effluent may be supplied to the evaporator to sequester carbon dioxide.
The method could also be used to process caustic effluents with or without the recovery of products or solely for the purpose of sequestering carbon dioxide contained in the combustion gas supplied to the evaporator. Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled within the art. Accordingly, all such variations and modifications are included within the intended the scope of the invention.