DESC:FIELD
The present disclosure relates to the field of petroleum refinery stream, typically to the treatment of petroleum refinery spent caustic stream.
DEFINITIONS
As used in the present disclosure, the following words and phrases are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used to indicate otherwise.
Coalescer system refers to a system adapted for separating or removing hydrocarbon oil from refinery spent caustic stream.
Spent Caustic stream refers to a waste caustic solution that has become exhausted and is no longer useful. Generally, the spent caustic stream comprises sodium hydroxide or potassium hydroxide, water, and contaminants.
Refinery spent caustic stream refers to the spent caustic stream generated in different refinery processes such as merox processing of gasoline; merox processing of kerosene/jet fuel; and caustic scrubbing/merox processing of LPG.
BACKGROUND
In petroleum refineries, hydrocarbon product streams are passed through a plurality of treatment units containing caustic, particularly sodium hydroxide, for removing acid components such as hydrogen sulphide, carbonyl sulphide, mercaptans, naphthenic acids, phenolic components and the like from the hydrocarbon product streams. These treatments results in generating a significant amount of waste caustic stream comprising the above mentioned contaminants, and is termed as refinery spent caustic stream.
The refinery spent caustic stream is hazardous and corrosive in nature; and has the foul odour. Therefore, the disposal of the refinery spent caustic stream is a challenge. Since refinery spent caustic stream contains phenolic and naphthenic constituent, therefore regular process of refinery effluent treatment is not useful; as it upsets the chemical and biological processing. There is a need to process the refinery spent caustic stream separately so as to overcome the above difficulties.
Conventionally, chemical oxidation or wet air oxidation is used for treating refinery spent caustic stream. However, following are the drawbacks associated with the use of conventional methods:
• difficulty in treating refinery spent caustic stream;
• requires a higher temperature and pressure for removing phenolic constituents or mixtures from the refinery spent caustic stream;
• release of foul smell or odour; and
• incapability of removing phenolic constituents or mixtures from the refinery spent caustic stream.
There is, therefore, felt a need for an effective process to treat spent caustic stream, particularly refinery spent caustic stream. Further, there is felt a need for a process that overcomes the above mentioned drawbacks.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide a process for treating refinery spent caustic stream.
Another object of the present disclosure is to provide a process for treating refinery spent caustic stream at ambient temperature and pressure.
Still another object of the present disclosure is to provide a process for effectively removing sulfur containing contaminants, naphthenic contaminants, and phenolic contaminants from the refinery spent caustic stream.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
In an aspect, the present disclosure provides a process for treatment of refinery spent caustic stream. The refinery spent caustic stream comprises an aqueous solution of alkali metal hydroxide, hydrocarbon component, a naphthenic component, a sulphur component and a phenolic component.
The alkali metal hydroxide present in the refinery spent caustic stream is at least one selected from the group consisting of sodium hydroxide, and potassium hydroxide.
The process of the present disclosure comprises the following steps:
The refinery spent caustic stream is introduced to a coalescer unit and the components of the stream are allowed to settle to obtain a first biphasic mixture comprising an upper oil layer and a lower caustic layer. The upper oil layer comprises the hydrocarbon component dispersed in the refinery spent caustic stream. The upper oil layer is separated from the first biphasic mixture to obtain a substantially oil free caustic layer.
The oil free caustic layer is introduced in an acidification tank and subjected to controlled acidification using an acid to pH in the range of 5 to 6 at a temperature in the range of 60 ºC to 70 ºC to obtain a second biphasic mixture comprising an organic layer and an aqueous layer. The organic layer comprises the naphthenic component. The organic layer is separated from the second biphasic mixture to obtain the aqueous layer comprising a salt solution. The acid used for acidification is concentrated sulphuric acid.
The separated salt solution is selectively oxidized with a first oxidant to obtain a resultant salt solution comprising a substantially oxidized sulphur component and a phenolic component.
In one embodiment, the first oxidant is hydrogen peroxide and the amount of the first oxidant is in the range of 1 g/lit to 50 g/lit.
The resultant salt solution is further oxidized with a second oxidant at an elevated temperature to obtain a treated refinery product containing the substantially oxidized sulphur component and a substantially oxidized phenolic component in water.
The temperature is in the range of 20 ºC to 100 ºC.
In one embodiment, the second oxidant is a nano-catalyst comprising at least one metal selected from the group consisting of iron, zinc, copper, silver, and oxides thereof. Preferably, the nano-catalyst is iron oxide.
The amount of the nano-catalyst used for oxidation is in the range of 0.0005 g/lit to 0.5 g/lit of the salt solution.
The nano-catalyst is characterized by:
• the particle size in the range of 50 nm to 100 nm,
• the average surface area the range of 30 m2/g to 40 m2/g, and
• the pore size in the range of 60 Å o 70 Å.
The process of the present disclosure is capable of reducing the amount of the hydrocarbon component, the unoxidized sulfur component, the naphthenic component, and the unoxidized phenolic component in a refinery spent caustic stream by 90 wt% to 100 wt%.
DETAILED DESCRIPTION
Refinery spent caustic streams resulting from the removal of the contaminants from the petroleum feed streams produced in refineries are highly alkaline, odorous and toxic, and contain sulfidic, phenolic and naphthenic salts. Environmental concerns and legislation dictate that such refinery spent caustic stream may no longer be directly disposed. Conventional methods such as a chemical oxidation method and/or wet air oxidation method can be used for treating refinery spent caustic stream. However, these methods are associated with drawbacks such as emission of foul smell or odour; and incapability of removing phenolic constituents or mixtures from the refinery spent caustic stream.
The present disclosure, therefore, envisages a process for treating a refinery spent caustic stream that overcomes the drawbacks of conventional methods.
In an aspect, the present disclosure provides a process for treatment of the refinery spent caustic stream comprising an aqueous solution of alkali metal hydroxide, hydrocarbon component, a naphthenic component, a sulphur component and a phenolic component. The process is described in detail herein below:
Initially, the refinery spent caustic stream is introduced in a coalescer unit and the components of the stream are allowed to settle to obtain a first biphasic mixture comprising an upper oil layer and a lower caustic layer. The upper oil layer is separated from the first biphasic mixture to obtain a substantially oil free caustic layer.
In accordance with the embodiments of the present disclosure, the refinery spent caustic stream is allowed to settle for a time period in the range of 12 hours to 72 hours.
In accordance with an exemplary embodiment of the present disclosure, the refinery spent caustic stream is allowed to settle for 24 hours.
The oil free caustic layer is then introduced in an acidification tank and subjected to controlled acidification using acid to the pH in the range of 5 to 6 to obtain a second biphasic mixture comprising an organic layer and an aqueous layer. The organic layer comprises the naphthenic component. The second biphasic mixture obtained after acidification is allowed to settle, and the organic layer is separated to obtain the aqueous layer comprising a salt solution.
In accordance with the embodiments of the present disclosure, the acid is concentrated sulphuric acid.
In accordance with the embodiments of the present disclosure, the addition of sulphuric acid is controlled in such a way that the temperature is maintained in the range of 60 ºC to 70 ºC.
The naphthenic component in the refinery spent caustic stream is present in a salt form. After the step of acidification using acid, the naphthenic component is converted to naphthenic acid, which is separated in organic layer. Similarly other acid impurities present in the refinery spent caustic stream are separated in organic layer.
The separated salt solution is selectively oxidized using a first oxidant to obtain a resultant salt solution comprising a substantially oxidized sulphur component and a phenolic component. The unoxidized sulphur compounds such as mercaptans and sulfides present in the salt solution get oxidized by oxidant.
In accordance with the embodiments of the present disclosure, the first oxidant used is hydrogen peroxide (H2O2).
In accordance with the embodiments of the present disclosure, the amount of the first oxidant is in the range of 1 g/lit to 50 g/lit.
In accordance with exemplary embodiments of the present disclosure, the amount of the first oxidant is in the range of 5 g/lit to 15 g/lit.
The resultant salt solution is further oxidized with a second oxidant at a predetermined temperature to obtain a treated refinery product containing the substantially oxidized sulphur component and a substantially oxidized phenolic component in water.
In accordance with the embodiments of the present disclosure, the second oxidant is a nano-catalyst comprising at least one metal selected from the group consisting of iron, zinc, copper, silver, and oxides thereof.
In accordance with an exemplary embodiment of the present disclosure, the nano-catalyst is iron oxide.
In accordance with the embodiments of the present disclosure, the nano-catalyst is characterized by the particle size in the range of 50 nm to 100 nm, the average surface area the range of 30 m2/g to 40 m2/g, and the pore size in the range of 60 Å o 70 Å.
In accordance with the embodiments of the present disclosure, the amount of the second oxidant is in the range of 0.0005 g/lit to 0.5 g/lit of the salt solution.
In accordance with exemplary embodiments of the present disclosure, the amount of the second oxidant is in the range of 0.01 g/lit to 0.1 g/lit of the resultant mixture.
In accordance with the embodiments of the present disclosure, the predetermined temperature is in the range of 20 ºC to 100 ºC.
In accordance with exemplary embodiments of the present disclosure, the predetermined temperature is in the range of 30 ºC to 70 ºC.
In accordance with the process of the present disclosure, the first oxidant is added before the addition of the second oxidant, due to which mercaptans such as methyl mercaptans, ethyl mercaptans; and sulfides such as hydrogen sulfide, sodium sulfide, carbonyl sulfide get oxidized by the oxidant. In the next step the second oxidant, which is a nano-catalyst is added, wherein oxidation of the phenolic components and other impurities takes place.
A major drawback associated with the conventional methods is poisoning or deactivation of the catalyst used for oxidation of the phenolic components. The poisoning or deactivation of the catalyst which occurs due to the presence of unoxidized sulphur components such as sulfides or mercaptans.
To overcome this poisoning or deactivation of the catalyst, the process of the present disclosure converts the sulfides and/ or mercaptans to corresponding oxides. These oxides do not poison or deactivate metal catalyst. Therefore, the process of the present disclosure performs oxidation in two steps, first with oxidant only, which oxidizes the easily oxidizable sulfides and/ or mercaptans to corresponding oxides; followed by oxidation with oxidant in the presence of the nano-catalyst, wherein oxidation of difficult to oxidize phenolic component takes place. The addition of oxidant in the first step helps in avoiding deactivation of catalyst, which in turn helps in carrying out the oxidation using low amounts of catalysts.
The process of the present disclosure effectively reduces the amount of the hydrocarbon component, the unoxidized sulphur component, the naphthenic component, and the unoxidized phenolic component in the refinery spent caustic stream by 90 wt% to 100 wt%. The process of the present disclosure effectively reduces more than 90% of the unoxidized sulphur component at 30 °C using hydrogen peroxide in an amount in the range of 5 g/lit to 15 g/lit and also in absence of catalyst. Further, more than 90% of the unoxidized phenolic component is reduced at 50 °C using hydrogen peroxide in an amount in the range of 5 g/lit to 15 g/lit and iron oxide in an amount of 0.1 g/lit.
Compared to the conventional methods, the method of the present disclosure can be carried out an ambient temperature and pressure. Thus, the process of the present disclosure is cost effective. The present disclosure provides an economical and eco-friendly process to reduce the amount of the unoxidized sulphur component, the naphthenic component, and the unoxidized phenolic component in the refinery spent caustic stream using lower concentrations of the oxidant and the catalyst. Therefore, the process of the present disclosure is eco-friendly.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and are not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
EXPERIMENTAL DETAILS
Experiments
Refinery spent caustic streams generated from different refinery units namely LPG merox unit, naphtha washed caustic, kerosene merox unit, merox extractor caustic, CDU III spent caustic and heavy naphtha washed caustic were used for the experimentation. The chemical composition of these streams is given below in Table 1 to 3.
Table 1: Chemical composition of a sulfidic spent caustic stream
Components
Naphtha wash Caustic LPG Prewash Caustic Merox Extractor Caustic
H2S
(ppm) 14900 64000 1100
RSH
(ppm) 31280 157000 12100
Acid oil
(Vol %) Nil 0.5 Nil
Phenol
(ppm) 1290 60 50
Thiosulfate (ppm) Nil 800 Nil
COD
(ppm) 130000 34000 20000
Strength (%) 14.8 12.5 6.6
COD: Chemical Oxygen Demand
Table 2: Chemical composition of a phenolic spent caustic stream
Components
Kerosene Merox Caustic Naphtha wash Caustic
H2S
(ppm) NIL Nil
RSH
(ppm) 20 600
Acid oil (Vol %) 1.2 5.0
Phenol
(ppm) 11800 3120
Thiosulfate (ppm) NIL 110
COD (ppm) 126000 180880
Strength (%) 8.4 3.3
COD: Chemical Oxygen Demand
Table 3: Chemical composition of a naphthenic spent caustic stream
Components
CDU III Spent caustic Extractor caustic MEROX 2
H2S
(ppm) 439 Nil
RSH
(ppm) 1385 2400
Acid oil (Vol %) 22% 9.2
Phenol
(ppm) 5310 520
Thiosulfate (ppm) 1200 Nil
COD (ppm) 633300 320500
Strength (%) 7.2 6.1
COD: Chemical Oxygen Demand
Refinery spent caustic streams consisting of sulfidic spent caustic stream and phenolic spent caustic stream is treated as per the process of the present disclosure given below.
General procedure:
The refinery spent caustic stream was introduced in a coalescer unit, wherein the components of the refinery spent caustic stream were allowed to settle for a time period of 24 hours to obtain a first biphasic mixture comprising an upper oil layer and a lower caustic layer. The upper oil layer was separated from the first biphasic mixture to obtain an oil free caustic layer.
The oil free caustic layer was then introduced in an acidification tank and the contents of the caustic layer were subjected to controlled acidification using concentrated sulphuric acid (H2SO4) to pH of 5 by slow addition of under stirring at a speed of 50 rpm to obtain a biphasic mixture comprising an organic layer and an aqueous layer. The addition of sulphuric acid was controlled in such a way that the temperature of a resultant mixture was in the range of at 60 ºC to 70 ºC. The organic layer comprised the naphthenic component and other acidic contaminants. The organic layer was separated from the aqueous layer by skimming to obtain the aqueous layer comprising a salt solution.
The separated salt solution was selectively oxidized with a predetermined amount of hydrogen peroxide (H2O2) under stirring at a speed of 100 rpm to obtain a resultant salt solution comprising a substantially oxidized sulphur component and the phenolic component. Hydrogen peroxide effectively oxidizes approximately 85 wt% to 90 wt% of sulfidic contaminants and mercaptan contaminants present in the salt solution.
The resultant salt solution was further oxidized using a predetermined amount of iron oxide nano-catalyst at a predetermined temperature under stirring at a speed of 100 rpm for 180 minutes to obtain a treated solution of the substantially oxidized sulphur component and a substantially oxidized phenolic component in water.
Experiment 1 to 9:
Experiments 1 to 9 were carried out by following the general procedure given above. The amount of the hydrogen peroxide was varied in the range of 5 g/lit to 15 g/lit and the amount of iron oxide was varied in the range of 0.01 g/lit to 0.1 g/lit. The temperature of the step of oxidation was maintained at 30 °C.
The reduction in amount of naphthenic component, the unoxidized sulphur component (sulfides, mercaptans) and the unoxidized phenolic component present in the treated stream is summarized in Table 4 given below:
Table 4: The percentage reduction in unoxidized sulphur component and unoxidized phenolic component at different oxidant and catalyst concentration at 30°C after 180 min.
Ex. No. Catalyst Concentration
g/lit Oxidant Concentration
g/lit Phenol Reduction
wt % Sulfide Reduction
wt % Mercaptan Reduction
wt %
1 0.01 5 24.05 97.65 97.53
2 10 31.01 99.41 98.63
3 15 37.97 100.00 100.00
4 0.05 5 29.81 100.00 100.00
5 10 39.24 100.00 100.00
6 15 46.84 100.00 100.00
7 0.1 5 61.39 100.00 100.00
8 10 74.05 100.00 100.00
9 15 82.66 100.00 100.00

From Table 4, it is evident that the unoxidized sulphur component is effectively reduced with hydrogen peroxide at 30 °C and at very low concentration of nano-catalyst (iron oxide). Further, more than 60% of the unoxidized phenolic component is also reduced at 0.1 g/lit concentration of iron oxide at 30 °C.
Experiment 10 to 18:
Experiments 10 to 18 were carried out by following the general procedure given above. The amount of the hydrogen peroxide was varied in the range of 5 g/lit to 15 g/lit and the amount of iron oxide was varied in the range of 0.01 g/lit to 0.1 g/lit. The temperature of the step of oxidation was maintained at 50 °C.
The reduction in the unoxidized sulphur component and the unoxidized phenolic component present in the refinery spent caustic stream is summarized in Table 5 given below:
Table 5: The percentage reduction in unoxidized phenolic component and unoxidized sulphur component at different oxidant and catalyst concentration at 50°C after 180min.
Ex. No. Catalyst Concentration
g/lit Oxidant Concentration
g/lit Phenol Reduction
wt % Sulfide Reduction
wt % Mercaptan Reduction
wt %
10 0.01 5 25.32 100.00 100.00
11 10 36.58 100.00 100.00
12 15 40.51 100.00 100.00
13 0.05 5 35.44 100.00 100.00
14 10 41.77 100.00 100.00
15 15 42.78 100.00 100.00
16 0.1 5 67.09 100.00 100.00
17 10 82.28 100.00 100.00
18 15 89.87 100.00 100.00

From Table 5, it is evident that the unoxidized sulphur component is effectively reduced with hydrogen peroxide at ambient temperature and at very low concentrations of hydrogen peroxide as well as a nano-catalyst (iron oxide). Further, more than 65% of the unoxidized phenolic component is also reduced at 0.1 g/lit concentration of iron oxide at 50 °C.
Experiment 19 to 27:
Experiments 19 to 27 were carried out by following the general procedure given above. The amount of the hydrogen peroxide was varied in the range of 5 g/lit to 15 g/lit and the amount of iron oxide was varied in the range of 0.01 g/lit to 0.1 g/lit. The temperature of the step of oxidation was maintained at 70 °C.
The reduction in unoxidized sulphur component and unoxidized phenolic component present in the refinery spent caustic stream is summarized in Table 6 given below:
Table 6: The percentage reduction in unoxidized phenolic component and unoxidized sulphur component at different oxidant and catalyst concentration at 70°C after 180min.
Ex.
No. Catalyst Concentration
g/lit Oxidant Concentration
g/lit Phenol Reduction
wt % Sulfide Reduction
wt % Mercaptan Reduction
wt %
19 0.01 5 36.08 100.00 100.00
20 10 42.41 100.00 100.00
21 15 46.84 100.00 100.00
22 0.05 5 88.61 100.00 100.00
23 10 93.04 100.00 100.00
24 15 93.54 100.00 100.00
25 0.1 5 94.18 100.00 100.00
26 10 96.58 100.00 100.00
27 15 99.11 100.00 100.00

From Table 6, it is evident that the unoxidized sulphur component is effectively reduced with hydrogen peroxide at 70 °C and at very low concentrations of hydrogen peroxide as well as a nano-catalyst (iron oxide). Further, more than 90% of the unoxidized phenolic component is also reduced at 0.1 g/lit concentration of iron oxide at 70 °C.
From the data tabulated in Table 4, Table 5 and Table 6, it is evident that the sulfide component and the mercaptan component are effectively oxidized at low temperature.
Further, the process of the present disclosure effectively reduces more than 90% of the unoxidized phenolic component at 70 °C.
Therefore, the process of the present application effectively reduces the amount of hydrocarbon component, the odorous unoxidized sulfur components, the naphthenic components, and the unoxidized phenolic components in the refinery spent caustic stream by 90 wt% to 100 wt%.
TECHNICAL ADVANCES AND ECONOMICAL SIGNIFICANCE
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process that:
• removes odorous sulphur containing contaminants, naphthenic contaminants, and phenolic contaminants effectively from refinery spent caustic streams; and
• reduces release of foul smell.
The disclosure has been described with reference to the accompanying embodiments which do not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein.
The foregoing description of the specific embodiments so fully revealed the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
,CLAIMS:We claim:
1. A process for treatment of refinery spent caustic stream comprising an aqueous solution of alkali metal hydroxide, a hydrocarbon component, a naphthenic component, a sulphur component and a phenolic component, said process comprising the following steps:
a) introducing the stream to a coalescer unit, and allowing the components of the stream to settle to obtain a first biphasic mixture comprising an upper oil layer and a lower caustic layer, wherein the upper oil layer comprises the hydrocarbon component;
b) separating the upper oil layer from the first biphasic mixture to obtain a substantially oil free caustic layer;
c) introducing the caustic layer to an acidification tank and subjecting the components of the caustic layer to controlled acidification to pH in the range of 5 to 6 using an acid to obtain a second biphasic mixture comprising an organic layer and an aqueous layer, wherein the organic layer comprises the naphthenic component;
d) separating the organic layer from the second biphasic mixture to obtain an aqueous layer comprising a salt solution;
e) selectively oxidizing the separated salt solution with a first oxidant to obtain a resultant salt solution comprising a substantially oxidized sulphur component and the phenolic component; and
f) further oxidizing the resultant salt solution with a second oxidant at an elevated temperature to obtain a treated refinery product containing the substantially oxidized sulphur component and a substantially oxidized phenolic component.
2. The process as claimed in claim 1, wherein said alkali metal hydroxide present in said refinery spent caustic stream is at least one selected from the group consisting of sodium hydroxide, and potassium hydroxide.
3. The process as claimed in claim 1, wherein said acid is concentrated sulphuric acid.
4. The process as claimed in claim 1, wherein said process step of acidification is carried out at a temperature in the range of 60 ºC to 70 ºC.
5. The process as claimed in claim 1, wherein said first oxidant is hydrogen peroxide.
6. The process as claimed in claim 1, wherein the amount of said first oxidant is in the range of 1 g/lit to 50 g/lit.
7. The process as claimed in claim 1, wherein said second oxidant is a nano-catalyst comprising at least one metal selected from the group consisting of iron, zinc, copper, silver, and oxide thereof.
8. The process as claimed in claim 7, wherein said nano-catalyst is iron oxide.
9. The process as claimed in claim 1, wherein the amount of said second oxidant is in the range of 0.0005 g/lit to 0.5 g/lit of the salt solution obtained in step (f).
10. The process as claimed in claim 1, wherein said the temperature in process step (f) is in the range of 20 ºC to 100 ºC.
11. The process as claimed in claim 7, wherein said nano-catalyst is characterized by:
• the particle size in the range of 50 nm to 100 nm,
• the average surface area the range of 30 m2/g to 40 m2/g, and
• the pore size in the range of 60 Å o 70 Å.
12. The process as claimed in claim 1, wherein the amount of the hydrocarbon component, the unoxidized sulfur component, the naphthenic component, and the unoxidized phenolic component in the refinery spent caustic stream is reduced by 90 wt% to 100 wt%.