Claims:We Claim:
1. A continuous process for catalytic removal of sulphides from spent caustic with a reactor, said process comprises the steps of:
(a) treating the spent caustic stream with sulphuric acid;
(b) routing the treated spent caustic stream of step (a) into the reactor;
(c) injecting the catalyst solution into spent caustic in the reactor;
(d) introducing air or a oxidising gas in the reactor;
(e) agitating the mixed feed of step (d); and
(f) obtaining a treated caustic stream with low sulphides.
2. The process as claimed in claim 1, wherein the reactor is selected from adiabatic fixed reactor, micro channel, monolith reactors, CSTR reactor, micro channel or rotating packed bed reactors.
3. The process as claimed in claim 1, wherein the process is performed at a temperature of 60-200 °C and 1 .0 -8.0 bar operating pressure, lower rotational speed 100 – 800 rpm of the reactor and residence time from 30 min – 4 h. (Please provide a range for the pressure with correct units)
4. The process as claimed in claim 1, wherein the spent caustic stream is selected from units of the refinery selected from LPG treating, merox with initial sulfidic content between 3000 to 8000 ppm.
5. The process as claimed in claim 1, wherein the catalyst loaded is in the range of
10 ppm to 1 gm.

6. The process as claimed in claim 2, wherein the monolith reactor is filled with monoliths selected from porous catalytic material or the catalytic material is washcoated in the channels of an inert monolithic support.
7. The process as claimed in claim 2, wherein the monolith reactor bears capillary hydraulic diameter ranging from 0.9 mm to 30 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s.
8. The process as claimed in claim 2, wherein the rotation packed bed reactor is washcoated with the homogeneous solution of catalyst while retaining the permeability for mass transfer.
9. The process as claimed in claim 9, wherein the rotation packed bed reactor has a rotational speed of 300-3000 rpm.
10. The process as claimed in claim 2, wherein the CSTR reactor is with low agitation in the range of 200-900 rpm.
11. The process as claimed in claim 2, wherein the CSTR reactor is designed for handling both homogeneous as well as heterogeneous catalyst and the heterogeneous catalyst is regenerated or reused.
12. The process as claimed in claim 1, wherein the process is a continuous fixed bed operation as well as one step batch process.
13. The process as claimed in claim 1, wherein the sulphur content in the spent caustic is having 1000 ppm sulfides and also phenols at the ETP.
14. The process as claimed in claim 1, wherein the sulphide content in the treated spent caustic is below 5 ppm.
15. A adiabatic fixed bed reactor for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor having a diameter of 0.8-3 m diameter with Tangent to tangent Length TL TL of 1.6-15 m with 2-4 nos. beds of inert balls on the top and bottom of the catalyst bed, wherein each bed is separated by perforated plate or wire mesh of 1mm – 8mm perforations arranged in triangular pitch.
16. The adiabatic fixed bed reactor as claimed in claim 15, wherein the process involves the steps of:
(a) removing oil and phenols from the spent caustic stream in the Activated Carbon Bed;
(b) pumping the treated spent caustic of step (a) at a pressure of 6 bar and heated in the steam heater to 60-120°C to fixed bed reactor from the bottom;
(c) stripping off the sulphides by a oxidizing stream such as air or pure oxygen which is compressed by a compressor at suitable pressure;
(d) transferring the treated spent caustic of step (c) from the top of the Reactor and is cooling to 30-45° C; and
(e) flashing the cooled spent caustic in the overhead Separator.
17. A rotating packed bed for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor is supported on the rotor having single or multiple rotating packed bed arranged in series.
18. The rotating packed bed for catalytic as claimed in claim 17, wherein rotor is a single or multiple beds of wire mesh or stacks of baffles made of SS of thickness 3-8 mm.
19. The rotating packed bed for catalytic as claimed in claim 17, wherein the baffles provide higher residence time and relatively better liquid dispersion at rotational speeds higher than 900 rpm.
20. The rotating packed bed for catalytic as claimed claim 19, wherein optionally the other plate one rotating and the other plate of the Rotor is bolted to the main housing of Rotating packed bed and perform as stationary bed, wherein the stator is welded with small baskets of mesh of SS-316.
21. The rotating packed bed for catalytic as claimed claim 20, wherein the baskets of wire mesh are used to contain the catalyst particles and wherein the perforations of the wire mesh depends on the size of the catalyst particles.
22. The continuous stirring tank reactor (CSTR) for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor impeller (a) for catalysis is welded with baskets of SS316 wire mesh for retaining the catalyst; and (b) for heterogeneous catalyst is having larger impeller of higher circulation sweep solids of bottom and suspended are used and the impeller blades are flat type of curved plates for better mixing.
23. The continuous stirring tank reactor (CSTR) as claimed in claim 22, wherein rotation of the reaction in the reactor is carried out in the range of 200-900 rpm with impeller tip velocity of 300-2000 ft./min and residence time allowed from 1-6 hrs for the reaction completion.
24. The monolith reactor for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor consist of 30 – 200 tubular packed beds of catalyst arranged in triangular or square pitch in vertical position.
25. The monolith reactor as claimed in claim 24, wherein monoliths that are either made of porous catalytic material or the catalytic material is (‘wash coated’) in the channels of an inert monolithic support.
26. The monolith reactor as claimed in claim 24, wherein the capillary hydraulic diameters ranged from 0.9 mm to 30 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s.
27. The monolith reactor as claimed in claim 24, wherein the treatment of sulphides in spent caustic is being done at moderate process conditions of 6-8 bar and the temperatures of 60-120°C.
, Description:FIELD OF INVENTION

The subject matter described in general relates to the development of process technology for the removal of sulfides in aqueous spent caustic streams using zero air or molecular oxygen as oxidant. In particular, the present invention relates to development of process design for the removal of sulfides in the aqueous spent caustic solutions and finding the active metals, metal oxides and support systems. The process developed is simple, efficient and energy intensive and a complete solution for treatment of sulphides and phenols in the aqueous spent caustic streams. The described invention also relates to the process design of the suitable pre-treatment method(s) for the removal sulphides in the aqueous spent caustic streams. This invention further relates to the process design of continuous fixed bed operation as well as one step batch process. The important object of this invention is the application of rotating packed bed for contacting gas-liquid with catalyst. the process is effectively carried out in monolith reactors of separated channels (straight, wavy or crimped) where Monolithic reactors are filled with monoliths that are either made of porous catalytic material or the catalytic material is (‘washcoated’) in the channels of an inert monolithic support.

BACKGROUND
Refinery spent caustic is recognised as a hazardous stream as it is highly corrosive, contains toxic compounds such as H2S or phenol and it imposes high levels of biological and chemical oxygen demand (BOD and COD) on natural water bodies. It also contains sulphides and mercaptans, which give rise to fouling or metallurgical damage in refinery equipment. Though there Spent Caustic is treated with addition of H2O2, the process is expensive and no further work available route for converting spent caustic into a valuable product. Hence, it has the least potential for reuse within the refinery and it is discarded at high cost.

The refineries are currently processing sour crude oils due to appealing economics that result in the generation of more H2S, mercaptanes, and other sulphur containing compounds in hydrocarbon streams. On the other hand, the removal of sulfur to achieve ultra-low sulphur levels in various hydrocarbon streams in meeting various environmental regulations is an important challenge. For this purpose, a dilute caustic stream is the cheap and widely used extractive reagent for the removal of sulphur containing compounds and referred as “spent caustic”. Since it majorly contains sulphide/mercaptans compounds that leads to fouling or metallurgical damage in refinery’s equipment, adequate effluent treatment procedures are required for its disposal. Spent caustic properties can vary from refinery to refinery and have the pH above 12 and sulphides concentration between 0.5 to 1.5 wt%. Spent caustic, depending on the source, contain phenols, mercaptane, amines and other organic compounds. Oxidation of sulfidic content in spent caustic to eliminate sulphides, mercaptans and combust toxic hydrocarbons/organic contaminants is practised. However, commercially available oxidation processes using peroxide treatment or ozonolysis routes are costly and pose various operational challenges. Therefore, there is a pressing need to develop an alternate, suitable, economical and environmental friendly process for the conversion of sulphidic/mercaptans compounds into water soluble sulphate salts by oxidation.

Specifically, the spent caustic solution contains sodium carbonate, sodium sulfides, mercaptanes, phenols and emulsified hydrocarbons. These are classified as hazardous waste, odorous and resistant to the biological treatment. Various sulphur containing compounds such as H2S, Na2S, NaHS, RSNa (sodium mercaptide) are present in the spent caustic. The spent caustic also consist of organic accompanies with other contaminants such as phenolics and naphthenic acids.

Removal of sulfides in spent caustic can be achieved by both physical and chemical methods. Wet air oxidation (WAO) is proposed to remove the organic pollutants at high temperatures (?200 °C) and high pressure (?150 bar) [1]. Wet air oxidation is an effective method for meeting the environmental regulations, however, the process is expensive due to severe process conditions, high cost of oxidants and safety is major concern. Fenton reagent (Fe2+/H2O2) can oxidize the refractory pollutants at relative low temperatures and pressures. However, this process consumes large quantities of H2O2 and high concentration of H2S that react with ferric ion result in the loss of catalyst efficiency.

In order to convert the sulfides in spent caustic various oxidation routes have been proposed viz H2O2 oxidation, oxidation using cobalt pthalocyanin and wet air oxidation. H2O2 treatment operates at ambient temperature and atmospheric pressure. This process removes sulphides and phenols by oxidation. On the other hand, the treatment with H2O2 is associated with high capital and functional costs. Stoichiometrically, 4 Kg of H2O2 is required to treat 1 Kg of sulphides. Oxidation using cobalt pthalocyanin homogeneous catalysts has the problems with separation of used catalysts and efficiency. On the other hand, wet air oxidation is a promising route to remove the sulfides and also reduces the hydrocarbons in the feed stream. In the wet air oxidation, the reactive sulfides are converted to soluble thiosulfate, sulfite and sulfates. The treated stream should be suitable for the biological treatment in the plants waste water. In order to operate the process under milder reaction conditions WAO in the presence of a suitable catalyst is proposed. Owing to the benefits of the catalytic wet air oxidation this invention describes the development of an efficient catalyst and optimal reaction conditions for the removal of sulphides below 5 ppm.

Recently, authors reported effective homogeneous catalyst based on cobalt pthalocyanin and its derivatives as a replacement for the H2O2 by the wet air oxidation route [7].

US 3023084 demonstrated wet air oxidation of sulfides in spent caustic at 204 °C and 35 bar pressure and steam is employed as the stripping gas. Sulfides in the feed treated at 3480 and 8960 ppm and the final sulfides in the product is 0 and 154 ppm respectively. The reaction temperature and pressures are of 138 °C and 126 °C, 2.57 and 1.37 bars.

US 3963611 discussed that the removal sulfides in spent caustic is achieved at temperature of 135 °C and pressure of 11 bars with residence time of 2.5 h with liquid feed rate of 178 lts/min. Prior to the reaction the pH is adjusted to below 9.6. 90% of sulfides conversion is achieved starting with 3780 ppm.

US 5082571 demonstrated that sulfide removal was attempted via wet oxidation route at 200 °C for sixty minutes. US 5246597 demonstrated that the method of reducing sulfide content in aqueous system. The reagents in this invention are H2O2 and ClO2. Combination of ClO2 and H2O2 resulted in reducing sulfides level from 100 to 10 ppm. NPRA report presented at San Antonio discussed wet air oxidation system for the treatment of spent caustic. It has stated that WAO characteristic is the formation of carboxylic acids and partially short chain organics in addition to CO2 and H2O. The reaction temperature and pressure are 260 °C and 90 bar. The recent paper published in Topics in catalysis 54(2011)579 discusses demerits of non-catalytic system and evaluated vanadium and copper catalysts and found that the most active catalyst is Cu/Silica and V/clinoptilolite and achieved the complete oxidation in 20 and 26 min, respectively. In further search for the efficient catalysts combinations of Co-Mn was recently studied at 200 °C. Cobalt based solid phase catalysts supported on Alumina, X-zeolite and Ca-hydroxyapatites were reported for the removal of sulphides in the spent caustic [8].

SUMMARY
Accordingly, the present invention provides a continuous process for catalytic removal of sulphides from spent caustic with a reactor, said process comprises the steps of:
(a) treating the spent caustic stream with sulphuric acid;
(b) routing the treated spent caustic stream of step (a) into a reactor;
(c) injecting the catalyst solution into spent caustic in a reactor;
(d) introducing air or a oxidising gas in the reactor;
(e) agitating the mixed feed of step (d); and
(f) obtaining a treated caustic stream with low sulphides.
In further another embodiment of the present invention, the adiabatic fixed bed reactor for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor having a diameter of 0.8-3 m diameter with Tangent to tangent Length TL TL of 1.6-15 m with 2-4 nos. beds of inert balls on the top and bottom of the catalyst bed, wherein each bed is separated by perforated plate or wire mesh of 1mm – 8mm perforations arranged in triangular pitch.
In further another embodiment of the present invention, the process with the adiabatic fixed bed reactor involves the steps of:
(a) removing oil and phenols from the spent caustic stream in the Activated Carbon Bed;
(b) pumping the treated spent caustic of step (a) at a pressure of 6 bar and heated in the steam heater to 60-120°C to fixed bed reactor from the bottom;
(c) stripping off the sulphides by a oxidizing stream such as air or pure oxygen which is compressed by a compressor at suitable pressure;
(d) transferring the treated spent caustic of step (c) from the top of the Reactor and is cooling to 30-45° C; and
(e) flashing the cooled spent caustic in the overhead Separator.
In one another embodiment of the present invention, a rotating packed bed for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor is supported on the rotor which may be single or multiple rotating packed bed arranged in series.
In further another embodiment of the present invention, the continuous stirring tank reactor (CSTR) for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor impeller (a) for heterogeneous catalysis is welded with baskets of SS316 wire mesh for retaining the catalyst; and (b) for heterogeneous catalyst is having larger impeller of higher circulation sweep solids of bottom and suspended may be used. The impeller blades may be flat type of curved plates for better mixing.
In another embodiment of the present invention, the treatment of sulphides in spent caustic is being done at moderate process conditions of 6-8 bar and the temperatures of 60-120°C.

BRIEF DESCRIPTION OF DRAWINGS
Figure 1: Process scheme for Fixed Bed catalytic treatment of sulphides in the aqueous spent caustic using Valve Swing Technology.
Figure 2: Process scheme for the homogeneous catalytic removal of sulphides in the aqueous spent caustic by rotating packed bed reactors.
Figure 3: Sulphide treatment in Monolith Reactors.
Figure 4: CSTR Reactors with innovative impeller design for Heterogeneous Catalytic process.
Figure 5: Small Scale Reactor and agitator designed with a SS mesh basket for supporting the catalyst (low RPM).
Figure 6: Rotating packed bed reactor.

DETAILED DESCRIPTION
Definitions
As used herein the term, “” in the context of the present invention means:
“Wet Air Oxidation”: Where the air or oxygen is used as an oxidant to remove
hydrocarbons or sulfides.
“Fixed bed reactors”: Where the catalyst is fixed in the reactor and the reactants will come in contact with catalyst and transform under feasible reaction conditions.
“Spent caustic”: The sulphur is removed from the hydrocarbon by the extraction with dilute NaOH. The used NaOH with sulphur is called spent caustic.
“Adiabatic reactors”: The reactors where there is no external heat loss.
“Monolith reactors”: The reactors are designed like monoliths where the catalyst is coated on the monoliths.
“Micro channels”: The reactors where chemical reactions occur inside numerous channels in micron range.
“Rotating bed reactors”: The reactors that rotate to improve the mass transfer co-efficient of reactants.
“CSTR reactor”: The reactor the reacts are mixed and stirred continuously.
“Wash coat”: The reactors where the catalyst is deposited on the wire mash bed by wash coating method, retaining its permeability.
The present invention provides a continuous process for catalytic removal of sulphides from spent caustic with a reactor, said process comprises the steps of:
(a) treating the spent caustic stream with sulphuric acid;
(b) routing the treated spent caustic stream of step (a) into a reactor;
(c) injecting the catalyst solution into spent caustic in a reactor;
(d) introducing air or a oxidising gas in the reactor;
(e) agitating the mixed feed of step (d); and
(f) obtaining a treated caustic stream with low sulphides.
In an embodiment of the present invention, the reactor is selected from adiabatic fixed reactor, micro channel, monolith reactors, CSTR reactor, micro channel or rotating packed bed reactors.
In another embodiment of the present invention, the process is performed at a temperature of 60-200 °C and 1 .0 -8.0 bar operating pressure, lower rotational speed 100 – 800 rpm of the reactor and residence time from 30 min – 4 h. (Please provide a range for the pressure with correct units)
In one another embodiment of the present invention, the spent caustic stream is selected from units of the refinery selected from LPG treating, merox with initial sulfidic content between 3000 to 8000 ppm.
In still another embodiment of the present invention, the catalyst loaded is in the range of
10 ppm to 1 gm.

In yet another embodiment of the present invention, the monolith reactor is filled with monoliths selected from porous catalytic material or the catalytic material is washcoated in the channels of an inert monolithic support.
In one another embodiment of the present invention, the monolith reactor bears capillary hydraulic diameter ranging from 0.9 mm to 30 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s.
In further another embodiment of the present invention, the rotation packed bed reactor is washcoated with the homogeneous solution of catalyst while retaining the permeability for mass transfer.
In still another embodiment of the present invention, the rotation packed bed reactor has a rotational speed of 300-3000 rpm.
In yet another embodiment of the present invention, the CSTR reactor is with low agitation in the range of 200-900 rpm.
In still another embodiment of the present invention, the CSTR reactor is designed for handling both homogeneous as well as heterogeneous catalyst and the heterogeneous catalyst is regenerated or reused.
In one another embodiment of the present invention, the process is a continuous fixed bed operation as well as one step batch process.
In yet another embodiment of the present invention, the sulphur content in the spent caustic is having 1000 ppm sulfides and also phenols at the ETP.
In another embodiment of the present invention, the sulphide content in the treated spent caustic is below 5 ppm.
In further another embodiment of the present invention, the adiabatic fixed bed reactor for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor having a diameter of 0.8-3 m diameter with Tangent to tangent Length TL TL of 1.6-15 m with 2-4 nos. beds of inert balls on the top and bottom of the catalyst bed, wherein each bed is separated by perforated plate or wire mesh of 1mm – 8mm perforations arranged in triangular pitch.
In one another embodiment of the present invention, the Reactor may be 10 % Torispherical dished shaped.
In further another embodiment of the present invention, the process with the adiabatic fixed bed reactor involves the steps of:
(a) removing oil and phenols from the spent caustic stream in the Activated Carbon Bed;
(b) pumping the treated spent caustic of step (a) at a pressure of 6 bar and heated in the steam heater to 60-120°C to fixed bed reactor from the bottom;
(c) stripping off the sulphides by a oxidizing stream such as air or pure oxygen which is compressed by a compressor at suitable pressure;
(d) transferring the treated spent caustic of step (c) from the top of the Reactor and is cooling to 30-45° C; and
(e) flashing the cooled spent caustic in the overhead Separator.
In one another embodiment of the present invention, a rotating packed bed for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor is supported on the rotor which may be single or multiple rotating packed bed arranged in series.
In still another embodiment of the present invention, in the rotating packed bed the rotor is a single or multiple beds of wire mesh or stacks of baffles made of SS of thickness 3-8 mm.
In yet another embodiment of the present invention, in the rotating packed bed the baffles provide higher residence time and relatively better liquid dispersion at rotational speeds higher than 900 rpm.
In still another embodiment of the present invention, optionally the other plate one rotating and the other plate of the Rotor may be bolted to the main housing of Rotating packed bed and may perform as stationary bed, wherein the stator may be welded with small baskets of mesh of SS-316.
In another embodiment of the present invention, the baskets of wire mesh may be used to contain the catalyst particles and wherein the perforations of the wire mesh depends on the size of the catalyst particles.
In further another embodiment of the present invention, the continuous stirring tank reactor (CSTR) for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor impeller (a) for heterogeneous catalysis is welded with baskets of SS316 wire mesh for retaining the catalyst; and (b) for heterogeneous catalyst is having larger impeller of higher circulation sweep solids of bottom and suspended may be used. The impeller blades may be flat type of curved plates for better mixing.
In another embodiment of the present invention, the rotation of the reaction in the reactor is carried out in the range of 200-900 rpm with impeller tip velocity of 300-2000 ft./min and residence time allowed from 1-6 hrs for the reaction completion.
In still another embodiment of the present invention, the monolith reactor for catalytic removal of sulphides in aqueous streams of spent caustic, said reactor consist of 30 – 200 tubular packed beds of catalyst arranged in triangular or square pitch in vertical position.
In another embodiment of the present invention, the monoliths that are either made of porous catalytic material or the catalytic material is (‘washcoated’) in the channels of an inert monolithic support.
In one another embodiment of the present invention, the capillary hydraulic diameters ranged from 0.9 mm to 30 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s.
In another embodiment of the present invention, the treatment of sulphides in spent caustic is being done at moderate process conditions of 6-8 bar and the temperatures of 60-120°C.

While the invention is susceptible to various modifications and/or alternative processes and/or compositions, specific embodiment thereof has been shown by way of example in the drawings, graphs and tables and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular processes and/or compositions disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention as defined by the appended claims.

The graphs, tables, figures and protocols have been represented where appropriate by conventional representations in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
The following description is of exemplary embodiments only and is not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that one or more processes or composition/s or systems or methods proceeded by “comprises... a” does not, without more constraints, preclude the existence of other processes, sub-processes, composition, sub-compositions, minor or major compositions or other elements or other structures or additional processes or compositions or additional elements or additional features or additional characteristics or additional attributes.
The terms, “alone or in combination” or any other variations thereof, are intended to describe and/or cover a non-exclusive inclusion, wherein the molecules or the oligonucleotides exist individually or together with any one or all of the other oligonucleotides.
In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings:
It must be noted that, as used in the specification/description and the appended claims and examples, the singular forms “a”, “an” and “the” may include plural referents unless the context clearly dictates otherwise.
Ranges may be expressed herein as from “about” one particular value, and or “to about” another particular value. When such a range is expressed, another aspect includes from the one particular value and or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about”, it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
The present invention relates to a process design for the effective removal by catalytic oxidation of sulphides in aqueous spent caustic. The process design includes the pre-treatment and oxidation steps. The pre-treatment steps involve the neutralization of spent caustic steam with acid such as sulphuric acid followed with adsorption on suitable adsorbents such as activated carbon. The invention related to the development of a simple catalytic wet air oxidation process for treating spent Caustic stream in refinery that can be used as such by completely replacing the existing H2O2 treatment or in combination to improve the economics as well as meeting stringent specifications. Other prior art discloses the separation/treatment of sulphides in spent caustic employing homogeneous catalyst such as copper sulphate or ferrous sulphate or only on a Fixed bed reactor.

The important object of the invention is the application of rotating packed bed for contacting gas-liquid with catalyst. The homogeneous solution of catalyst may be injected into spent caustic which diffuses through the unique liquid inlet distributor into the rotating packed to achieve better conversion of sulphides or the invention also relates to the unique design of the rotation packed bed which is coated with the homogeneous solution yet retaining its permeability a pre requisite for air-liquid-catalyst effective contact. The invention also relates to a process that may be effectively carried out in monolith reactors which are filled with monoliths that are either made of porous catalytic material or the catalytic material is (‘washcoated’) in the channels of an inert monolithic support. The Rotating packed bed is designed such that housing of the bed also supports liquid hold up volume of treated spent caustic stream that flows out of the bed. The effectiveness of the rotating Packed bed is studied at different temperatures, pressures and rotating speeds.

The invention also related to the treatment of sulphides in spent caustic that is being done at moderate process conditions of 60-200 °C and 6 bar operating pressure. This invention also relates to develop a simple fixed bed reactor system designed for treating spent caustic in refinery operating at moderate process conditions and with minimum utilities and with swing bed technology employing shut down valves (SDVs) for smooth automated operation. The process developed is simple, efficient and energy intensive and a complete solution for treatment of sulphides and phenols in the aqueous spent caustic streams. The described invention also relates to a process design of the suitable pre-treatment method(s) for the removal sulphides in the aqueous spent caustic streams. The various process parameters such as reaction temperatures, pressure, rpm, catalyst to feed ratio and residence time.

In one aspect the present invention provide a process for treating sulphides containing spent caustic by employing simple design of various configuration and designs of different reactors such as adiabatic reactors, monolith reactors, micro channel as well as rotating packed bed reactors. In another aspect the present invention provides a novel design of CSTR reactor with low agitation such as 100–500 rpm with effective internals for air or oxygen gas bubbling and reaction completion in reaction time of 30 min – 4 hrs. The CSTR design is unique as the system is designed for handling both homogeneous as well as heterogeneous catalyst and the heterogeneous catalyst may be regenerated or reused.

In another aspect the present invention provides a novel designed and constructed impeller fabricated with SS304L mesh baskets which may hold the heterogeneous catalyst. The catalyst of required size is agitated inside the SS basket welded to the 4–8 nos. of blades of the impeller. This process has the advantage as it does not require the separation of the catalyst from the treated stream or plugging of the filter which may be the case on case of suspended catalyst.

It is another aspect the present invention provides with different modes of the operation of the rotating packed bed. The present invention also provides for a design process which enables suitable pre-treatment method/treatment(s) for the removal sulphides in the aqueous spent caustic streams at various process parameters such as reaction temperatures, pressure, rpm, catalyst to feed ratio, time intervals and pressures.

In an additional aspect the present invention also provides removing mercaptanes, odorous compounds and phenols from a spent caustic stream.

The invention is to develop a simple process to meet the objective of designing a Catalytic Wet Air Oxidation Unit to treat sulfide content in spent caustic stream which has approximately 8000 ppm sulfides and also phenols at the ETP. The sulfides can be reduced to 50 -120 ppm. Still other objectives and advantages of our process will be realized upon reading the following description taken with the attached drawing.

The following are the advantaged of the invention- Invention of a simple catalytic process for treatment of refinery spent caustic at moderate process conditions, with various configuration and designs of reactors such as Adiabatic fixed reactor, monolith reactors where the reactors are filled with monoliths that are either made of porous catalytic material or the catalytic material is (‘washcoated’) in the channels of an inert monolithic support or the application of a rotating packed bed in which homogeneous solution of catalyst may be injected into spent caustic which diffuses through the unique liquid inlet distributor into the rotating packed or the invention also relates to the unique design of the rotation packed bed which is wash coated with the homogeneous solution of catalyst while retaining the permeability for mass transfer.

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

In the drawing the figure 1 represents, in diagrammatic form, an arrangement for practicing the process of the invention having a Fixed Bed Sulphide removal. Referring to the diagram a slip steam of refinery spent caustic having approximately (900-9000 ppm) sulphides are routed to Column of Activated Carbon Bed (C-001A/B) operating at atmospheric operation conditions and ambient temperature to treat the phenols. Prior to this the oil present in the spent caustic feed stream is removed by simple skimming in open oil Catcher (SP-05). The process design includes the pre-treatment and oxidation steps. The pre-treatment steps involve the neutralization of spent caustic steam with sulphuric acid followed with adsorption on activated carbon. The spent caustic stream is then routed to and then pumped to the Reactor at a pressure of (6 – 8 bar) via feed pump ( P-001 ). The feed Pump ( P -001) may also be incorporated before the Activated carbon bed ( C-001A/B) facilitating the operation of the Column at higher pressure of 6 – 8 bar pressure. The process may employ two number of Activated Carbon Bed Columns (C-001A/B) such that when Activated Carbon Bed Columns (C-001A) is in adsorption the other Activated Carbon Bed Column ( C-001B) may be in regeneration. Regeneration involves simple thermal regeneration of the bed at a temperature of 90 – 200 °C. The Reactor Feed Stream may be further heated to 60-200 °C in the Feed/Effluent Exchanger (E -001) followed by steam heater (E-002 ) as required by process conditions for effective adsorption process. The Reactors (R-001 A/B) are operated in swing bed mode with the application of suitable SDVs. The Reactor (R -001 A/B) is purged with compressed air at 6-8 bar pressure for effective oxidation of the sulphides. The rector effluents are condensed in the Overhead Cooler (E-003) to condense the heavier content before routing the Overhead Condensate Drum (V-001) in which an appreciable quantity of condensate separates from uncondensed gases. As previously indicated the invention is unique as it has advantages of operating the system with different configurations and design of reactors along with the application of rotating packed bed for sulphide removal.
The spent caustic stream after treating for oil removal and phenols in the Activated Carbon Bed is pumped at a pressure of 6 bar and heated in the steam heater to 60 – 120 °C. The heated spent caustic stream is then routed to the fixed bed reactor from the bottom. The spent caustic is stripped off the sulphides by a oxidizing stream such as air or pure oxygen which is compressed by a compressor at suitable pressure. The treated spent caustic leaves the fixed bed Reactor from the top of the Reactor and is cooled to 30 – 45 °C before flashing in the overhead Separator. The bottom liquid from the separator may then be routed to the subsequent treatments systems.
The reactor would be 0.8-3 m diameter and TL-TL of 1.6 – 15m and would have 2-4 nos. beds pf inert balls or ceramic balls on the top and bottom of the catalyst bed. Each bed is separated by perforated plate or wire mesh of 1mm – 8mm perforations arranged in triangular pitch. The Reactor may be 10 % Torispherical dished shaped.
Rotating packed bed: In the rotating packed bed as shown in Fig. 2 (H-001) the homogeneous solution of catalyst is injected into spent caustic and involves infusing liquid through specialised liquid inlet distributor into the rotating packed bed supported on a rotating shaft, to achieve better conversion of sulphides. Hence, superior mass transfer can be achieved for effectively contacting the air with the spent caustic stream in the rotating packed bed in counter current. The invention also relates to the unique design of the rotation packed bed (H -001) which is coated with the homogeneous solution yet retaining its permeability a pre requisite for air-liquid-catalyst effective contact. The rotating packed bed (H-001 ) unit may be designed to hold and ensure smooth flow of the treated caustic stream by designing a decanter at the bottom. The treated spent caustic stream flows through the eye of the rotor to the decanter before being routed to the Holding Tank (T 001).The Rotating packed bed supported on the rotor may be single or multiple rotating packed bed arranged in series to achieve outlet product specifications. The compressed Air stream is introduced at the outer surface of the packed bed such that it moves counter current to the feed spent caustic stream.
The Rotating Packed Bed (H -001) may be welded with pockets of wire mesh or metallic foam which would in turn hold the catalyst particles. However, in such a case the rotation of the shaft has to be restricted to around 3 00 rpm. The study of such a rotating packed bed (H-001) was found to be effective in treating the spent Caustic stream with 1000 ppm of sulphides.

In case of a Rotating Packed Bed where a homogenous catalyst is used. The heated liquid is mixed with homogeneous catalyst in a drum and then mixed feed and catalyst streams are routed through the Liquid distributor at the centre of the RPB. The liquid distributor is designed at the centre of the Rotor Eye. The liquid distributor may be a single tube with series of perforations, each series is situated at an angle of 90 ° apart. The liquid distributor may also be split into 4-6 nos different liquid distributors branching out of the main liquid distributor (“tree- arrangement” ).The air or oxidising gas is introduced at the outer periphery of the rotor bed. Due to high centrifugal force generate at rotational speed of 300 – 3000 rpm, gas- liquid mass transfer takes place and gas strips off the H2S from the spent caustic .The stripping at lower pressures is supported by the homogeneous catalyst.
The rotor may be a single or multiple beds of wire mesh or stacks of baffles made of SS of thickness 3–8 mm THK. The baffles provide higher residence time and relatively better liquid dispersion at rotational speeds higher than 900 rpm.
In the invention the homogeneous catalyst, instead of mixing with the feed may also be used to wash coat the wire mesh bed. The bed so prepared may then be used as the rotating bed.
Another design aspect may be the rotating bed may be split into 2 nos, one rotating and the other plate of the Rotor may be bolted to the main housing of RPB and may perform as stationary bed.The stator may be welded with small baskets of mesh of SS-316.The baskets of wire mesh may be used to contain the catalyst particles. The perforations of the wire mesh depend on the size of the catalyst particles.
The process may be effectively carried out in a design where in the Reactors (R -001A/B) are designed as monolith reactors of separated channels (straight, wavy or crimped). Monolithic reactors (R -001 A/B) are filled with monoliths that are either made of porous catalytic material or the catalytic material is (‘washcoated’) in the channels of an inert monolithic support. In both arrangements, the channel walls function as catalyst and the channels provide space for flow of gas and/or liquid. The capillary hydraulic diameters ranged from 0.9 mm to 30 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s, which is typical of that obtained in monolith reactors.

The homogeneous solution of catalyst is injected into spent caustic and involves infusing liquid through specialised liquid inlet distributor into the rotating packed bed supported on a rotating shaft, to achieve better conversion of sulphides. Hence, superior mass transfer can be achieved for effectively contacting the gas with the liquid in the rotating packed bed. The invention describes the unique design of the rotation packed bed which is coated with the homogeneous solution yet retaining its permeability a pre requisite for air-liquid-catalyst effective contact. Alternatively the process can be carried out on monolithic reactors where the active catalyst is wash coated on the monolith channels. In both arrangements, the channel walls function as catalyst and the channels provide space for flow of gas and/or liquid. The capillary hydraulic diameters ranged from 0.9 mm to 30 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s, which is typical of that obtained in monolith reactors. The invention also related to the treatment of sulphides in spent caustic that is being done at moderate process conditions of 60-200 °C and 6 bar operating pressure.
The described invention also relates to the process design of the suitable pre-treatment method(s) for the removal sulphides in the aqueous spent caustic streams. This invention further relates to the process design of continuous fixed bed operation as well as one step batch process. The various process parameters such as reaction temperatures, catalyst to feed ratio, stir speeds, time intervals and pressures. The synthesized catalysts based on Cobalt or Copper as active metals were found to be efficient for the removal sulfides in the refinery spent caustic.
The process is advantageous as invention also relates to the treatment of sulphides in spent caustic that is being done at moderate process conditions of 60-200 °C and atm-6 bar operating pressure, lower rotational speed 300 – 800 rpm of the rotor, residence time from 30 min – 4 h.
Adaptable to variation in feed, residual contaminants stay in aqueous phase, dissolved solids do not limit the application of wet air oxidation, effluent from WAO can be biodegradable, low operating cost, no further neutralization required. The above alternative route has several advantages compared to others in terms of separation and reusing the catalyst.

The spent caustic stream from different units of the refinery such as LPG treating, merox etc., with initial sulfidic content between 3000 to 8000 ppm was used as feed. The wet air oxidations were 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 600 ml of spent caustic feed is stripped off the free oil particles in the oil catcher which is of typical of refinery ETP plant. It is further routed to the clarifies. The oil free stream is then neutralized with H2SO4 at atmospheric pressure and ambient temperature. The oil free stream is then passed through a fixed bed of activated alumina or activated carbon for removing the organics by adsorption. It has been observed that the organics, particularly phenol have reduced from 350 ppm to below 25 ppm. The activated carbon or active alumina loaded in the column ranged from 1 to 10 g for the feed of 10 to 100 ml. The 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. Unlike other processes this is a simple, compact, energy intensive and economical technology developed for treating both phenols and sulphides in caustic stream in refinery and petrochemical industry. Or as in the case of RPB, the impeller may be welded with baskets of SS316 wire mesh for retaining the catalyst (heterogeneous) and stirring at different rpms. For heterogeneous catalyst larger impeller of higher circulation sweep solids of bottom and suspended may be used. The impeller blades may be flat type of curved plates for better mixing.

CSTR: The CSTR may be operated with homogeneous catalyst mixed with the spent caustic feed before loading CSTR Reactor. The stirring rate may vary from 200 – 900 rpm with impeller tip velocity of 300 -2000 ft/min. The residence time allowed from 1 – 6 hrs. for the reaction completion.
Or as in the case of RPB, the impeller may be welded with baskets of SS316 wire mesh for retaining the catalyst (heterogeneous) and stirring at different rpms. For heterogeneous catalyst larger impeller of higher circulation sweep solids of bottom and suspended may be used. The impeller blades may be flat type of curved plates for better mixing. CSTR operate at impeller tip velocity of 2 -8 m/sec and impeller design may pitched blade turbine type which promote axial flow or as in the case of homogeneous catalyst it is hub/disk mounted flat/curved blade impellers which provide shearing action and enhances liquid – liquid interfacial areas.
Micro channel reactors: The application uses a micro channel reactors with array of 10–30 array of channel with 2–8 wt % of catalytic coating designed to reduce the sulphide concentration from 800–8000 ppm at the inlet to 5–100 ppm, WHSV of 0.3-10 gm/litre, while operating in process conditions as 1 – 8 bar pressure at temperature of 40-120 °C.
Monolith Reactors: The Reactor consist of 30 – 200 tubular packed beds of catalyst arranged in triangular or square pitch in vertical position. The spent catalyst is routed from the top while air or oxygen gas is purged from the bottom of the tubes. The flow rate of gas is so maintained that the Reactor pressure is uniformly operated at 6–8 bar and the temperatures of 60–120 ° C. The feed may be pre heated if required. The Reactor consist of 30 – 200 tubular packed beds of catalyst arranged in triangular or square pitch in vertical position .The spent catalyst is routed from the top while air or oxygen gas is purged from the bottom of the tubes. The flow rate of gas is so maintained that the Reactor pressure is uniformly operated at 6 – 8 bar and the temperatures of 60 – 120 °C. The feed may be pre heated if required.
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.

Various solid phase catalysts in combination of active metal and suitable supports have been tested. The catalysts have been prepared by contacting the precursor of active element with support material by impregnation, ion exchange, anchoring or simple contacting. In particular examples the modified supports were also employed in the catalyst preparation. The modification is achieved by ion exchange of suitable elements.

Table 1: Feed characteristics
Effluent Feed at Battery Limit
Parameter Product
Maximum Temperature (°C) 40
Pressure (Kg/cm2) 1.0
Flow Rate (m3/h) 2.2
pH 12 – 14
Sulphides, ppm 8000
Combined naphthenic/cresylic
spent caustic feed rate

Table 2: Composition of feed:
Sl. No. Name of pollutants Value Units
1. Sulphides 8009 ppm
2. Phenols 3403 Ppm
3. Oil & Grease 3009 Ppm
4. BOD 26690 ppm
5. COD 66481 ppm
6. TDS 15046 ppm
7. TSS 463 ppm
8. Caustic 12338 ppm
9. pH 12 – 14
10 Operating Temperature 40 °C
11. Operating Pressure 1.0 kg/cm2

Table 3: Product Properties:
Sl. No. Parameter Value Units
1. Colour & Odour Clear & no unpleasant odour.
2. Temperature 5 °C more than the receiving water temperature. °C
3. Oil & Grease 10 ppm
4. pH 6 – 8.5
4. BOD 15 ppm
5. COD 250 ppm
6. Suspended Solids 20 ppm
7. Sulphides 0.5 ppm
8. Phenols 1.0 ppm

The above parameters are followed as per Andhra Pradesh Pollution Control Board (APPCB) specifications.

Process Products
Table 4: Treated Effluent At Battery Limit
Parameter Product
Maximum Temperature (°C) 45
Pressure (kg/cm2g) 5.0
Flow Rate (m3/h) 2.0
Recovery, % 95
Sulphides, ppm 10

Technical Specifications of the reactor:
Reactor Temperature, °C 120
Reactor Pressure, Kg/cm2g 6
Residence time , h 2
Catalyst loading, kg 100
Reactor Dimensions (mm X mm) 900X300

EXAMPLES
Example 1:
The example shows in diagrammatic form (Figure 1), an arrangement for practicing the process of the invention having a Fixed Bed Sulphide removal. Referring to the diagram a slip steam of refinery spent caustic having approximately (900-9000 ppm) sulphides are routed to Column of Activated Carbon Bed (C-001A/B) operating at atmospheric operation conditions and ambient temperature to treat the phenols. Prior to this the oil present in the spent caustic feed stream is removed by simple skimming in open oil Catcher (SP-05). The process design includes the pre-treatment and oxidation steps. The pre-treatment steps involve the neutralization of spent caustic steam with sulphuric acid followed with adsorption on activated carbon. The spent caustic stream is then routed to and then pumped to the Reactor at a pressure of (6 – 8 bar) via feed pump (P-001). The feed Pump (P -001) may also be incorporated before the Activated carbon bed (C-001A/B) facilitating the operation of the Column at higher pressure of 6 – 8 bar pressure. The process may employ two numbers of Activated Carbon Bed Columns (C-001A/B) such that when Activated Carbon Bed Columns (C-001A) is in adsorption the other Activated Carbon Bed Column (C-001B) may be in regeneration. Regeneration involves simple thermal regeneration of the bed at a temperature of 90 – 200 °C. The Reactor Feed Stream may be further heated to 60-200 °C in the Feed/Effluent Exchanger (E -001) followed by steam heater (E-002 ) as required by process conditions for effective adsorption process. The Reactors (R-001 A/B) are operated in swing bed mode with the application of suitable SDVs. The Reactor (R -001 A/B) is purged with compressed air at 6-8 bar pressure for effective oxidation of the sulphides. The rector effluents are condensed in the Overhead Cooler (E-003) to condense the heavier content before routing the Overhead Condensate Drum (V-001) in which an appreciable quantity of condensate separates from uncondensed gases. As previously indicated the invention is unique as it has advantages of operating the system with different configurations and design of reactors along with the application of rotating packed bed for sulphide removal.
The spent caustic stream after treating for oil removal and phenols in the Activated Carbon Bed is pumped at a pressure of 6 bar and heated in the steam heater to 60 – 120 °C. The heated spent caustic stream is then routed to the fixed bed reactor from the bottom. The spent caustic is stripped off the sulphides by a oxidizing stream such as air or pure oxygen which is compressed by a compressor at suitable pressure. The treated spent caustic leaves the fixed bed Reactor from the top of the Reactor and is cooled to 30 – 45 °C before flashing in the overhead Separator. The bottom liquid from the separator may then be routed to the subsequent treatments systems.
The reactor would be 0.8-3 m diameter and TL-TL of 1.6 – 15m and would have 2-4 nos. beds pf inert balls or ceramic balls on the top and bottom of the catalyst bed. Each bed is separated by perforated plate or wire mesh of 1mm – 8mm perforations arranged in triangular pitch. The Reactor may be 10 % Torispherical dished shaped.
The process of the present invention may also be performed with the above process where the Reactors (R-001 A/B) is micro channel, monolith reactors, CSTR reactor, micro channel or rotating packed bed reactors.
Example 2:
This example illustrates the process of example 1 with rotating packed bed:
In the rotating packed bed ( H -001) the homogeneous solution of catalyst is injected into spent caustic and involves infusing liquid through specialised liquid inlet distributor into the rotating packed bed supported on a rotating shaft, to achieve better conversion of sulphides.
Dimension of rotating packed bed reactor:
ROTOR DIAMETER: ½ incg- 5 INCH
PACKED BED DIAMTER: 2inch – 10 INCH
Nos of nozzles on Liquid Distributor: 8- 24 nos.
RPM : 300 - 1000 Power requirement : 1 – 10 Kw

Alternatively, the above invention can also be executed in the suitable micro channel or monolithic reactors where the active catalyst is coated on the channels.
Alternatively, the solid catalyst can be either suspended in the spent caustic solution or placed with the help of a porous (made of either stainless steel or cloth or plastic) container where the spent caustic solution is exposed to the solid catalyst in the reaction conditions.

Example 3:
A CSTR reactor is designed with low agitation such as 100 – 500 rpm with designed internals for air or oxygen gas bubbling and reaction completion in reaction time of 30 min – 4 hrs. The catalyst may be retained in the SS basket welded to the blades of the implelller while agitating or it may in a homogeneous form.
Dimension of Reactor:
Capacity: 0.5 - 10 KL
CSTR reactor DIAMETER: 0.4m – 8 m
Impleller Dia : 0.1 - 3 m
Width of baffle: 0.04 – 0.8 m
RPM : 100 - 500 Bulk velocity calc:0.1 - 2 m/sec
Impelller tip velocity: 0.05 – 1 m/sec