Removal of sulfides in spent caustic stream over active solid phase catalysts

Abstract

The present subject matter relates to the development of active catalyst composite based on supported transition metal oxides, especially, Cu, Co that are effective in the removal sulfides in the diluted spent caustic. The process for the reduction of sulfides in spent caustic comprises of reacting various organic and inorganic sulfides with molecular oxygen in the presence of active catalyst at various reaction temperatures ranging ambient to 200 C. and pressures between atmospheric pressure to 60 bars. The process also relates to complete scheme for the removal of sulfides in spent caustic.

Claims

1. A process for the removal of sulphides in spent caustic comprising: conducting wet air oxidation on the spent caustic in the presence of a catalyst composition, wherein the catalyst composition comprises a support material and a modifying agent, wherein the modifying agent comprises Co or Cu, and the support material is present in an amount from 2 wt % to 50 wt %; wherein the wet air oxidation is carried out at a temperature from 60 C. to 200 C. with a reaction duration between 1 hour to 8 hours; wherein the support material is a bulk oxide, metal phosphate, or a zeolite with varying Si/Al ratios between 20 to 280; wherein the bulk oxide is alumina, zirconia, titania, silica, niobia, or a combination thereof; wherein the zeolite is a faujazite-type zeolite which is a X type zeolite; wherein the metal phosphate is a Hydroxyapatite; wherein the catalyst composition has a surface area of 20 m.sup.2/g to 700 m.sup.2/g and a pore volume of 0.10 cc/g to 1.5 cc/g; wherein the process comprises steps of adsorption followed by neutralization; wherein the neutralization step adjusts pH between 5 to 7.5; and wherein the removal of sulphides is above 90% with respect to the spent caustic.

2. The process of claim 1, wherein the modifying agent is anchored, impregnated, exchanged, or contacted to a surface of the support material in or outside of pores of the support material.

3. The process of claim 1, wherein the Co or the Cu is present in an amount of up to 20.0 wt %.

4. The process of claim 1, wherein the process converts sulfidic content in the spent caustic or diluted spent caustic.

5. The process of claim 1, wherein the process further comprises adsorption of sulphides on different carbon forms.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1: illustrates proposed scheme for the removal of sulphides in spent caustic.

(2) FIG. 2: graph illustrating the effect of reaction temperature in the wet air oxidation of Na.sub.2S (3000 ppm) solution at various reaction temperatures and 6 bar zero air pressure over Cu/Al.sub.2O.sub.3 catalyst.

(3) FIG. 3: graph illustrating the effect of reaction duration in the wet air oxidation of Na.sub.2S (3000 ppm) solution at 150 C. and 6 bar zero air pressure over Cu/Al.sub.2O.sub.3 catalyst.

(4) FIG. 4: graph illustrating the effect of reaction temperature in the wet air oxidation of treated spent caustic at various reaction temperatures and 6 bar zero air pressure over Cu/Al.sub.2O.sub.3 catalyst.

(5) FIG. 5: graph illustrating the effect of reaction duration in the wet air oxidation of pretreated spent caustic solution at 150 C. and 6 bar zero air pressure over Cu/Al.sub.2O.sub.3 catalyst.

DETAILED DESCRIPTION

(6) The invention is described in detail in the following paragraphs by way of reference to various examples. However, such description is provided merely for illustrative purposes and should not be construed as limiting the scope of the invention.

(7) Catalytic Wet Air Oxidation

(8) The sulfidic content with initial sulphides content between 3000 to 8000 ppm is used as feed. The wet air oxidation is carried out at both atmospheric pressure and high pressures ranging from 1 to 60 bar and temperatures between 60 to 200 C. Typically 20 to 50 ml of spent caustic feed is loaded in the reactor and catalyst amounts from 10 ppm to 1 g of catalyst have been loaded. The catalytic experiments were conducted with duration of 1 to 8 h using zero air as an oxidant. The catalytic tests were conducted using PARR reactors. The product sample is collected and analysed using titration method.

(9) Sulfide Estimation

(10) The determination of sulfide in spent caustic was carried out by iodometric titration method. In a typical titration, take 1 ml of spent caustic in 100 ml jar and add 1 ml of zinc acetate (22%) and 1 ml of NaOH (6N). Make up the solution to 100 ml without any air bubbles and mixed by rotating back and forth vigorously about a transverse axis. Filter the cake and dissolve the cake in 100 mk DI water by adding 1:1 HCl of 2-3 ml. Added 0.025 N iodine solution to get the obvious yellow coloration and few starch solution drops added to get blue coloration. Titration was carried out using hypo solution of 0.025 N.

(11) Catalysts

(12) Zeolites are microporous crystalline aluminosilicate solids with well-defined channels and cavities having window diameters <10 nm. The aluminosilicate framework is negatively charged and is polyhedral by extra-framework cations. Advantage of zeolite framework is could accommodate molecules and ions. Therefore, zeolites have been widely used and studied as ion exchangers, sorbents, and catalysts in industrial processes. The extra-framework cations present in zeolites play a significant role in determining their adsorption and catalytic properties. Zeolite X is a synthetic aluminium-rich analogue of the naturally occurring mineral faujasite.

(13) The framework structure of zeolite X primarily contain Silicon and aluminium atoms alternate at the tetrahedral intersections, except that Si substitutes for Al at about 4% of the Al positions bonded with oxygen atoms. The zeolite X frame work consists of sodalite cavity or -cage as its principal building block. Typically, the -cages are connected tetrahedrally with six-rings via bridging oxygen yielding double six-rings (and, interconnected set of even larger cavities accessible in three dimensions through 12-ring windows. The Si and Al atoms occupy the vertices of these polyhedral and the oxygen atoms lie approximately midway between each pair of Si and Al atoms but are displaced from those points to give near-tetrahedral angles of Si and Al. Exchangeable cations that balance the negative charge of the alumina silicate framework are found within the zeolite cavities.

(14) Cation Exchange

(15) The sodium cations of the commercial zeolite X were re-placed with various alkali and alkaline earth metal cations by ion exchange with potassium, rubidium, caesium, magnesium, calcium, strontium, and barium salt solution at 353 K separately or in combination. The ion-exchange process was repeated several times to achieve the higher replacement of sodium ions with other alkali and alkaline earth metals. Cobalt cations were introduced into highly crystalline zeolite X by the cobalt ion exchange from aqueous solution.

(16) Preparation of Cobalt Zeolite-X

(17) 2 gm zeolite-X is taken in glass beaker and solution of 0.05M cobalt nitrate hexahydrate in 280 ml water with a ratio of 1:80. Mixed zeolite-X sample and heated it on heater for 4 h at 80 C. with constant stirring. Filter and wash the cake with hot distilled water. Kept the resultant solid for drying at a temperature 110 C. overnight and followed by calcination at 450 C. for 3 h.

(18) Preparation of Sodium Cobalt Zeolite-X

(19) Sodium chloride 1M solution in 20 ml water was prepared and mixed with 2 gm zeolite-X in water. Reflux heated for 4 hours at 80 C. with constant stirring. The resultant solution was filtered and kept the solid at 110 C. for overnight drying. 0.05 M in 320 ml water of cobalt nitrate hexa-hydrate solution, mix 4.65 gm of cobalt nitrate hexa-hydrate. Maintain the solid/liquid ratio 1:80. Mix the sodium zeolite-X with cobalt nitrate hexa-hydrate and heat it 4 hours at 80 C. under constant stirring. After filtering the solution, washed with hot water.

(20) Preparation of Potassium Cobalt Zeolite-X

(21) A solution of potassium nitrate 1M solution in 20 ml water was added to 2 gm zeolite-X. Heat for 4 hours at 80 C. under constant stirring. The filtered solid of potassium zeolite-X was washed hot water. The resultant solid is kept for drying at 110 C. overnight. A solution of 0.05M cobalt nitrate hexahydrate in 240 ml water with a ratio 1:80 was maintained.

(22) Other metal modified Zeolite-X such as Ba and Sr were synthesized the methods similar to the above.

(23) Preparation of Strontium Zeolite X

(24) About 0.5 gm of zeolite which has silica alumina ratio 84, 187, 272 and 408 was added to nearly 2 ml of distilled water. Mix the zeolite sample in water very well for about half an hour. Add 0.5% (by weight) of Sr in zeolite sample, the weight of Sr(NO.sub.3).sub.2 is 0.6231 g. Mix the solution of Sr(NO.sub.3).sub.2 and zeolite sample for 1 h. Heat the solution on heater at low temperature. The samples were calcined at 400 C. in furnace for 4 h.

(25) Other metals (Cs, Ba) modified zeolite X prepared in the similar method described above.

(26) Impregnation

(27) Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt from which the solvent is subsequently evaporated. The catalyst is prepared either by spraying the support with a solution of the metal compound or by adding the support material to a solution of a suitable metal salt, such that the required weight of the active component is incorporated into the support without the use of excess of solution. This is then followed by drying and subsequent decomposition of the salt at an elevated temperature, either by thermal decomposition or reduction. When used for the preparation of mixed metal catalysts, care has to be taken to confirm that a component in an impregnating solution of metal salts is not selectively adsorbed, resulting in an unexpectedly different and undesirable concentration of metals in a mixed-metal catalyst. This technique has been widely used for the preparation of small amounts of catalyst for basic studies.

(28) Hydroxyapatites as Novel Catalysts for the Removal of Sulphides

(29) CaHAP crystallizes with hexagonal P6.sub.3/m, symmetry with Ca.sup.2+ arranged in two non-equivalent sites, I and II, with Ca (I) ions aligned in columns whereas Ca(II) ions are in equilateral triangles centred on a screw axis surrounded with PO.sub.4.sup.3 tetrahedra. CaHAP exhibits both acid-base properties in its crystal lattice accompanied by important properties such as high adsorption capacity and ion-exchange capabilities.

(30) Synthesis of Calcium Hydroxyapatites:

(31) CaHAP Using NH.sub.4H.sub.2PO.sub.4 as Precursor

(32) A solution of calcium nitrate tetrahydrate (Ca(NO.sub.3).sub.2.4H.sub.2O) (6.6710.sup.2 mol) in 60 ml H.sub.2O was prepared and brought to pH 11-12 with NH.sub.4OH (4.98 N), addition and further diluted to 120 ml. A solution of ammonium dihydrogen phosphate (NH.sub.4H.sub.2PO.sub.4) (4.0010.sup.2 mol) in 100 ml of H.sub.2O was prepared and brought to pH 11-12 with NH.sub.4OH (4.98 N) and thereafter diluted to 160 ml. The calcium solution was vigorously stirred at room temperature, and the phosphate solution added drop wise over Ca. It takes 30 min to produce a milky, gelatinous precipitate which was stirred and boiled at 70 C. for 1 h. The precipitate was filtered, washed, dried at 80 C. overnight and lastly calcined at 500 C. for 3 h. The preparation reaction can be explained as follows:
6(NH.sub.4)H.sub.2PO.sub.4+10Ca(NO.sub.3).sub.2+14NH.sub.4OH.fwdarw.Ca.sub.10(PO.sub.4).sub.6(OH).sub.2+20NH.sub.4NO.sub.3+12H.sub.2O

(33) In order to study the effect of Ca to P ratio and the effect of metal addition to the hydroxyapatite framework the following list of catalysts have been synthesized using similar methods of calcium hydroxyapatite catalysts. As the acidic and basic properties of these materials changes with metal to phosphorous ratio, we have systematically varied the metal (Ca) to phosphorous ratio in the 1.1 to 2.16. Various metals modified CaHaP catalysts have been synthesized using Sr, Ba. Co (10 wt %) is impregnated on these supports. For comparison SrHaP support has been synthesized with Sr to P ratio of 1.1 to 2.16. SrHaP support structure has been confirmed using x-ray diffraction method. SrHaP has been further modified using Ba and Ca. Co (10 wt %) is impregnated on these supports.

(34) The following non-limiting examples illustrate in details about the invention. However, they are not intended to be limiting the scope of present invention in any way.

Example 1

(35) The refinery spent caustic feed without any pretreatment with sulfidic content of 3140 ppm is used for the experiment to remove sulfides. The reaction conditions are as follows: spent caustic: 50 ml, Catalyst: CoX zeolite and CoCaHAP, amount of catalyst: 50 mg, Oxidant: zero air, Temperature: 80 C. The results of various catalysts evaluated for removing sulfides has been presented in the Table 1. A maximum 58% conversion is achieved over CoX zeolite using 50 mg catalyst at reaction temperature of 80 C. at atmospheric pressure.

(36) TABLE-US-00001 TABLE 1 Reaction Amount of temperature, catalyst, % S Catalyst C. mg removal CoX 80 50 58 CoX 60 50 23 CoCaHAP 50 20 10 CoCaHAP 60 50 25 CoCaHAP 80 50 42 CoNaX 80 50 54 CoSrX 80 50 59

Example 2

(37) In order to study the effect of high temperature and pressure sulphides removal is conducted at 80 to 100 C. at 10 bar zero air pressure. The results are presented in the Table 2.

(38) TABLE-US-00002 TABLE 2 Spent Air Caustic Catalyst, Temperature Pressure Sulfides (mL) Catalyst mg ( C.) (bar) Conversion 30 CoX 50 80 10 61.25 30 CoX 50 100 10 76.25 30 CoX 50 120 10 92.5 30 CoX 50 120 10 91.25 30 CoX 100 120 10 68.75

Example 3

(39) The refinery spent caustic feed without any pretreatment with sulfidic content of 3140 ppm is used for the removal sulfides. The reaction conditions are as follows: spent caustic: 50 ml, amount of catalyst: 10-1000 mg, oxidant: zero air, Reaction temperature: 50-150 C., pressure atomspheric to 60 bars. The results of various catalysts evaluated for removing sulfides has been presented in the Table 3. A maximum 92% conversion is achieved over Co-CaHAP zeolite using 100 mg catalyst at reaction temperature of 150 C. at 60 bar pressure. The gas products were analyzed using the RGA and no significant amounts of H.sub.2S, SO.sub.2, and SO3 were observed.

(40) TABLE-US-00003 TABLE 3 Spent Sulphide Catalyst, caustic, Pressure, Temperature, Conversion Catalysts mg ml bar C. % CoX 100 25 60 120 66 CoX 100 25 30 120 43 CoX 100 25 15 120 37 CoCaHAp 100 25 60 150 94 CoCaHAp 100 25 30 150 66 CoCaHAp 100 25 15 150 57 CuX 100 25 60 150 70 CuX 100 25 30 150 57 CuX 100 25 15 150 43 25 60 150 31 25 30 150 14 25 15 120 13

Example 4

(41) In order to study the effect of the contaminants the simulated Na.sub.2S of 5000 ppm feed is prepared. This solution (50 ml) is used for the CWAO on Cu/Al.sub.2O.sub.3 catalyst of 50 mg at 150 C. and 6 bar air pressure over the 4 h reaction duration and a complete removal of sulfide is achieved. Similar result is also obtained over Co/CaHAP at high zero air pressures (above 20 bar). FIGS. 2 and 3 shows the results on Cu/Al.sub.2O.sub.3 catalyst treating 3000 ppm Na.sub.2S aqueous solution at 150 C. and 6 bar zero air pressure.

Example 5

(42) The refinery spent caustic with 3140 ppm of sulfides is pretreated with required quantities of H2SO4 and a reduction of 62% sulfides is observed. This followed with treating the above solution on activated carbon has further reduced the sulfides content up to 70%. The above stock solution is used for the reaction over Cu/X, Co/X, Co/CaHAP, Cu/Al.sub.2O.sub.3, Co/Al.sub.2O.sub.3 catalysts at 150 C. and 6 bar zero air pressure, above 98% total removal of the sulfides achieved.

Example 6

(43) The pretreated spent caustic with sulfidic contents with H.sub.2SO.sub.4 followed by the adsorption on activated carbon is used for carrying out catalytic wet air oxidation. The Cu/Al.sub.2O.sub.3 has showed more than 98% conversion at 120 C. FIG. 4 shows the CWAO treatment of refinery spent caustic using Cu/Al.sub.2O.sub.3 catalyst at various reaction temperatures and 6 bar pressure. The time on stream studies showed that the completed sulfides removal is achieved within 3 h reaction duration (FIG. 5).

Example 7

(44) Lower amounts of sulfides below 200 ppm have been adsorbed on the activated carbon and 10 to 20% sulfides were adsorbed. A process scheme is proposed in the FIG. 1. About 70-90% of sulfidic contents can be converted over the studied catalysts via wet air oxidation. In order to completely remove the sulfides it is proposed to carry out the adsorption over suitable materials such as carbons followed by low amounts of H.sub.2O.sub.2.

REFERENCES

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