FIELD OF THE INVENTION:
The present invention relates to a method of treatment of waste-water stream generated from
H2S rich hydrocarbon treating unit in refineries by solar light and getting a high value byproduct
hydrogen, particularly, the present invention relates to method of treating spent caustic stream
generated from H2S rich hydrocarbon stream treating unit. The present invention further relates
to an apparatus for treating spent caustic stream generated from H2S rich hydrocarbon treating
unit.
BACKGROUND OF THE INVENTION:
Sodium hydroxide i.e., NaOH (Caustic) is used as one of the most important primary reagent
to extract sulfidic components i.e. hydrogen sulphide, mercaptants and organic acid from the
hydrocarbon streams in the refineries and the petrochemical plants, further, caustic is used
because it is safe, effective, economical and environmentally acceptable.
Further, the H2S rich hydrocarbon stream contains H2S, Mercaptan Sulphur, SO2 etc. These are
normally treated with caustic to meet the required specifications in H2S treating unit. During
the process, caustic is converted to spent caustic which contains Na2S, NaSR (Sodium
mercaptides), NaOH & water. The spent caustic that come out of these hydrocarbon treating
system cannot be directly discharge to natural waterways as these are the contaminated streams
that may affect the drinking water quality and the biological chemical and oxygen demand
(COD and BOD) will increase if the spent caustic stream is directly discharge to waste-water
treatment plant (WWTP) therefore, this stream of spent caustic is sent to the Effluent treatment
plant (ETP) for treatment.
Conventionally, Wet Air Oxidation (WAO) process is used for the treatment of spent caustic
stream. This method is an oxidation process of organic and inorganic materials in aqueous form
by oxygen or air at elevated temperature and pressure. These reactions oxidize reduced sulfur
species to Na2SO4.
US Patent 7713399 discloses the process for treatment of spent aqueous caustic stream using a
membrane electrolyzer which is powered by a fuel cell to remove oxidizable sulfur containing
compounds & produce a safely dischargeable neutral aqueous stream, fresh caustic solution at
15% concentration by weight, and pure hydrogen gas.
3
US Patent application 20130008858 disclosesthe systems and methods for the demineralization
or the treatment of process streams, wherein, Wet Air Oxidation (WAO) process is involved.
Further, the treatment process described in this document use the electrodes in their treatment
process and involves down stream of oxidation unit, wherein, for oxidation unit generally air /
enriched oxygen or any oxygen atom carrying chemicals such as peroxide or permanganate are
used.
This document discloses the conventional process of treating the process stream. Further, no
suggestion regarding the reduction in the load of Effluent treatment plant has been disclosed.
The disadvantage in the prior arts is that the spent caustic to be discharged in the Effluent
treatment plant is not reduced to the level that lead to decrease the load of Effluent treatment
plant, further, the utilization of high amount of oxygen/air leads to increase load as well as
demands the utilization of external air/ enriched oxygen in Effluent treatment plant.
Although, the prior arts disclose various means for treating the spent caustic, however, none of
the prior art discloses a method and system that lead to reduce the load on Effluent treatment
plant (ETP) and wherein, the spent caustic after treatment can be recycled in closed loop to H2S
treatment method, so that the requirement of the reagents are reduced.
Industry need to evolve for a method and a system which is energy efficient and has reduced
utility requirement and which leads to produce highly efficient fuel.
In view of the above problems of the prior arts, the inventors of the present disclosure felt a
need to develop a method wherein energy and reagents utilization has to be regulated and the
load in the Effluent treatment plant by the method of the present invention as a whole has to be
reduced.
It is the aim of the inventor to provide a method and system for treating a sulfidic caustic stream
generated in refineries by solar light/Ultraviolet radiation and getting hydrogen as a byproduct
and to provide a method and apparatus that is efficient and has low operating costs as this
method utilize the solar light which is available in abundance.
4
Yet another aim of the present invention is to provide a method and system for treating spent
caustic stream generated from H2S rich hydrocarbon stream treating unit system for reducing
the consumption of fresh caustic and reducing the load to ETP.
Another aim of the present invention is to provide a method and system to that lead to
regeneration of spent caustic and recycling to H2S treating section, which was earlier send to
ETP. The present invention overcomes the shortcoming of the prior art.
SUMMARY OF THE INVENITON
The present invention provides a method and system for treating spent caustic stream generated
from H2S rich hydrocarbon stream treating unit and simultaneously reducing the load on the
Effluent treatment plant (ETP). In the present invention, the spent caustic stream is sent to
photolytic reactor where this waste-water stream gets partially treated in the presence of
ultraviolet radiation, wherein, H2 is produced as a byproduct which is a most valuable fuel. This
treated stream is further sent to ETP for final treatment.
In accordance with the present disclosure there is provided a method that leads to the production
of a highly valuable fuel, H2 which is the most promising zero emission non-fossil fuel energy
carrier & has lot of commendable properties such as highest energy content per unit mass, wide
flammability range, lowest flashpoint (-230oC), high octane number of 130, very high flame
speed with low ignition delay time.
In one aspect of the present invention the desulfurization of H2S rich stream is done with an
aqueous caustic (NaOH), a spent caustic stream is generated which mainly contains Na2S,
NaSR (Sodium mercaptides), NaOH & water. Conventionally this stream is pass to the ETP
where wet oxidation air method is used for its treatment, which oxidize organic & inorganic
material in the method stream by oxygen & reduce sulfur species to Na2SO4.
The reaction that govern the wet air oxidation process in ETP is:
10Na2S + 14O2 + 4H2O 6Na2SO4 + 4S + 8NaOH
According to the present invention, the spent caustic stream is routed to a photolytic reactor
placed at the downstream of spent caustic stream. This reactor is equipped with the UV bulbs
at the definite location. The spent caustic stream is radiated with ultraviolent radiation and lead
to obtain a high valuable fuel H2 and treated sulfidic stream, part of which is recycled back for
5
the regenerated caustic utilization & for the production of more H2 as per the following
reactions:
The reaction suggested for the present invention are as follows:
2Na2S + 2H2O Na2S2 + 2NaOH + H2
Na2S2 + H2S 2NaHS + S
2NaHS Na2S2 + H2
2NaOH + H2S Na2S + 2H2O
2Na2S + 2H2O Na2S2 + 2NaOH + H2
Another aspect of this invention discloses that Na2S2 & NaOH are generated as a byproduct
from the sulfidic effluent stream, which can be recycled back to the method for the further
utilization. Na2S2 further produce more H2 when react with H2S present in the method while
regenerated caustic (NaOH) is further utilized in the method.
The present invention a cyclic method of production of H2 and regeneration of caustic in the
presence of sunlight/ UV light.
Figure-2 depicts the same cyclic method of the present invention.
In an embodiment of the present invention, spent caustic stream is treated with ultraviolet
radiation and either intermittent after 2-4 cycles of recycling or continuously 20-50 wt % of
total regenerated stream produces a purged stream of treated sulfidic stream, this purged stream
is directed towards the Effluent treating plant for removal of undissociated components,
therefore the method of the present invention reduces the additional air/oxygen requirement in
the ETP and reduces the load of spent caustic disposal to ETP.
One aspect of the present invention discloses that the invention can be utilized in all refineries
and gas plant that generate sulfidic spent caustic.
Another aspect of the present invention discloses a composition of the spent caustic stream that
consists of Na2S, NaSR, NaOH and water. This spent caustic stream is fed into the apparatus
of the treating unit for further utilization.
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6
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects and advantages of the present invention will become apparent from
the following detailed description embodiments, taken in conjunction with drawings, wherein
Figure 1 is the schematic representation of the Wet Air Oxidation process, a conventional
method for the treatment of spent caustic stream.
Figure 2 is the schematic representation of sulfidic spent caustic treatment via photolytic
reactor setup and recycling of the same.
DETAILED DESCRIPTION OF THE INVENTION
While the disclosure is susceptible to various modifications and alternative forms, specific
aspects thereof have been shown by way of examples and will be described in detail below. It
should be understood, however that it is not intended to limit the invention to the particular
forms 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. The Applicants would
like to mention that the examples and comparative studies are mentioned to show only those
specific details that are pertinent to understanding the aspects of the present disclosure 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 present invention relates to a method and system where the conventional route of treatment
of spent caustic in ETP is bypassed and that stream is passed through the photolytic reactor for
the treatment of spent caustic & production of highly valuable fuel, H2.
One embodiment of the present invention relates to a method of treating spent caustic stream
generated from hydrogen sulfide (H2S) rich hydrocarbon stream treating unit comprises of:
a. providing a H2S rich hydrocarbon stream comprising of H2S, mercaptan sulphur,
SO2;
b. reacting the H2S rich hydrocarbon stream with an aqueous caustic stream to form
spent caustic stream in a H2S treating section;
c. radiating the spent caustic stream with ultraviolent radiation in photolytic reactor
and obtaining fuel H2 and treated sulfidic stream;
d. recycling treated sulfidic stream and obtaining the treated spent caustic solution.
7
In one embodiment of the present invention, the source of ultraviolet radiation is UV bulbs or
sunlight.
In one embodiment of the present invention, the method comprises purging the stream of treated
sulfidic stream.
In another embodiment of the present invention, the treated sulfidic stream is obtained either
intermittent after 2-4 cycles of recycling or continuously 20-50 wt% of total regenerated stream
to Effluent Treatment Plant (ETP) for removal of undissociated components.
In one embodiment of the present invention, the method comprises adding fresh caustic stream
to H2S treating section intermittently after 2-4 cycles of recycling treated sulfidic stream or
continuously an amount equal to purge stream.
In yet another embodiment of the present invention, the ultraviolet radiation has a UV-Visible
range of from 185 to 700 nm
In one embodiment of the present invention, the reaction in step (b) occurs at an ambient
temperature.
In other embodiment of the present invention, the method comprises recovering a hydrogen gas
stream from the treated sulfidic stream and regenerating an aqueous caustic.
Yet another embodiment of the present invention, the spent caustic comprises of Na2S, NaSR,
NaOH and water.
In another embodiment of the present invention, the spent caustic comprises of Na2S in an
amount of 3-8wt%, NaSR in an amount of 1-4 wt%, NaOH in an amount of 1-2 wt% and water.
In one of the embodiments of the present invention, the method comprises returning the
regenerated aqueous caustic to H2S treating unit.
8
In an embodiment of the present invention relates to the method, wherein, the method is devoid
of the utilization of an external oxygen/air.
On embodiment of the present invention relates to a system for treating spent caustic stream
generated from the H2S rich hydrocarbon stream treating unit comprises:
a. a H2S treating unit (B4) wherein the H2S treating unit comprises plurality of column
with trays and packaging;
b. a first inlet configured to introduce a hydrocarbon rich stream (S6) and a stream of an
aqueous caustic (S8) into the H2S treating unit (B4); wherein the H2S treatment is
configured to discharge streams of treated hydrocarbon (S7) and sulfidic spent caustic
(S9);
c. a photolytic reactor unit (B5) configured to receive sulfidic spent caustic stream (S9)
from a second inlet wherein the photolytic reactor unit (B5) comprises plurality of UV
bulbs to generate UV light which enhances the dissociation of sulfidic spent caustic
(S9) into an enriched fuel H2 (S11) and treated sulfidic stream (S12)
d. an outlet from the photolytic reactor for discharging part of the treated sulfidic stream
(S12) to the H2S treating unit (B4) to remove H2S from H2S rich stream,
e. an effluent treating unit (B7) in which a purged stream of treated sulfidic stream,
generated either intermittent after 2-4 cycles of recycling or continuously 20-50 wt %
of total regenerated stream is introduced for removal of undissociated components and
obtaining final treated stream (S15).
On embodiment of the present invention discloses that the process condition is carried out at
ambient temperature.
Embodiments are described with reference to the drawings. Figure-1 is a representative of
conventional ETP method for spent caustic. The conventional method for spent caustic
treatment is Wet Air Oxidation (WAO) in which a spent caustic stream (S1) is sent to the spent
caustic storage tank, further this stream (S2) is sent for the spent caustic oxidation in wet air
oxidation package. Oxidized spent caustic stream (S3) is sent to the flash mixing tank in which
dosing of H2O2, represented by stream S4 is done. The final obtained stream from flash mixing
tank (S5) is the treated spent caustic. Spent caustic storage tank, spent caustic oxidation (wet
air oxidation package) & flash mixing tank is represented by B1, B2 & B3 respectively.
9
Figure-2 is a representative of alternative method of sulfidic spent caustic treatment via
photolytic reactor setup. In the represented scheme, hydrocarbon H2S rich stream (S6) is sent
to the H2S rich hydrocarbon treating unit. Caustic wash is done for the removal of H2S from
S6, for that a stream of NaOH (S8) is added to the unit. Treated hydrocarbon stream (S7) from
the unit is sent for the further utilization, while sulfidic spent caustic stream (S9) generated from
the unit is sent to the photolytic reactor setup as shown in the figure-2. Conventionally the spent
caustic stream (S10) is routed directly to the ETP. The photolytic reactor setup utilizes sunlight/
UV light from UV bulbs to dissociate sulfidic spent caustic & generate A highly valuable fuel,
H2 (S11), treated sulfidic stream (S12). Part of this treated stream is recycled back for the
utilization of regenerated caustic (S13) & production of more H2. While a part of S12 is sent to
the ETP (S14) for final removal of undissociated components. A final treated stream (S15) is
obtained from ETP. H2S rich hydrocarbon treating unit, Photolytic reactor setup, UV bulbs &
ETP unit is represented by B4, B5, B6 & B7 respectively.
The claimed invention provides a dual benefit of reducing the load on ETP and generation of
highly valuable fuel, H2.
The present invention discloses that the sulfidic spent caustic stream generated from the treating
unit is sent to the photolytic reactor setup, wherein, the photolytic reactor setup utilize
ultraviolet light from UV bulbs to dissociate sulfidic spent caustic & generate a highly valuable
fuel, H2, and treated sulfidic stream. The part of this treated stream is recycled back for the
utilization of regenerated caustic & production of more H2. While a part of treated sulfidic
stream is sent to the ETP for final removal of undissociated components. A final treated stream
is obtained from ETP.
The present invention is an innovative technology for regeneration & utilization of an aqueous
caustic in the cyclic and continuous method.
Thus, the present invention provides dual benefits:
1. Reducing the load on Effluent Treatment Plant (ETP).
2. Production of valuable fuel, H2.
10
EXAMPLES:
The method was carried out in laboratory scale. The results of the same are illustrated in Tables
1 to 3.
In a typical H2S treating unit, fresh caustic is used as solvent for H2S removal, after H2S
treatment generated spent caustic is send to ETP for treatment and safe disposal. But after
envisaging the method that spent caustic can be recycle back to H2S treating method. The
present invention discloses complete method of H2S treating with spent caustic treatment, there
is huge reduction in requirement of fresh caustic in H2S treating section and also reduction of
spent caustic disposal load to ETP. Along with this Hydrogen is also generated in the method.
Table 1 shows the amount of spent caustic generated and the amount of hydrogen produced in
the photolytic treating unit.
Table 1: Result of the spent caustic treatment section
Spent Caustic Treatment section
Na2S + H2O + UV light → 1/2 Na2S2 + NaOH + 1/2 H2
Spent Caustic Generated
from H2S Treating
Section
Spent Caustic after
Treatment Hydrogen produced
Component Kg Kg Kg
NaOH 159.0 397.4 _
Na2S 464.9 0.0 _
NaSR 289.8 289.8 _
WATER 7439.5 7332.2 _
Na2S2 0.0 327.8 _
H2 0.0 _ 6.0
Total flow 8353.2 8347.2 6.0
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Table 2 shows the amount of spent caustic generated after recycling from the H2S treating
unit.
Table 2: Result of H2S Treating Section
Table 3 shows the amount of regenerated spent caustic, hydrogen gas and treated sulfidic
stream.
Table 3: Result of spent caustic treatment section
Spent Caustic Treatment section
NaHS + UV light → 1/2 Na2S2 + 1/2 H2
Na2S + H2O + UV light → 1/2 Na2S2 + NaOH + 1/2 H2
Feed to UV
photolysis
Regenerated
spent caustic
H2
produced
Sulfur
Produced
Component Kg Kg Kg Kg
H2S Treating Section
Na2S2 + H2S → S + 2NaHS
NaOH+ 1/2H2S → 1/2Na2S+H2O
Spent caustic after
treatment to H2S
Treating Section
Feed 2
(H2S) Spent Caustic
Component Kg Kg Kg
NaOH 397.4 _ 0.0
Na2S 0.0 _ 387.4
NaSR 289.8 _ 289.8
WATER 7332.2 _ 7511.1
Na2S2 327.8 _ _
H2 _ _ _
H2S _ 270.2 _
S _ _ 95.4
NaHS _ _ 333.8
Total flow 8347.2 270.2 8617.4
12
NaOH 0.0 198.7 _ _
Na2S 387.4 0.0 _ _
NaSR 289.8 289.8 _ _
WATER 7511.1 7421.7 _ _
Na2S2 _ 601.0 _ _
H2 _ _ 10.9
H2S _ 0.0 _ _
S 95.4 _ _ 95.4
NaHS 333.8 0.0 _ _
Total flow 8617.4 8511.1 10.9 95.4
From the Table above, it is observed that the total Hydrogen produced in the complete method
is 16.9 Kg in one cycle. In the disclosed requirement of fresh caustic is reduced as spent caustic
is being regenerated and components such as Na2S2 formed in disclosed method also reacts
with H2S and helped in H2S removal and also produced hydrogen.
BENEFITS OF THE PRESENT INVENTION:
• Regeneration of spent caustic and recycle to H2S treatment section which was earlier send to
ETP.
• Reduced consumption of fresh caustic
o Due to recycle;
o H2S reacting with other components generated during method which led to H2S
conversion to sulfur.
• Reduced load to Effluent Treatment Plant (ETP)
Therefore, the method and system of the present invention leads to the reduced consumption of
the fresh aqueous caustic as it is regenerated by the process and is recycled in a process. Further,
the spent caustic treated in the photolytic reactor leads to obtain the high valuable fuel H2, which
is the most promising zero emission non-fossil fuel energy carrier and has lot of commendable
properties such as highest energy content per unit mass, wide flammability range, lowest flashpoint
(-230ºC), high octane number of 130, very high flame speed with low ignition delay time.
Furthermore, only the purged stream as obtained after 2-4 cycles recycling has been sent to
Effluent Treatment Plant (ETP) unit, thereby reducing the load on Effluent Treatment Plant.

WE CLAIM:
1. A method of treating spent caustic stream generated from hydrogen sulfide (H2S) rich
hydrocarbon stream treating unit comprises of:
a. providing a H2S rich hydrocarbon stream comprising of H2S, mercaptan sulphur, SO2;
b. reacting the H2S rich hydrocarbon stream with an aqueous caustic stream to form
spent caustic stream in a H2S treating section;
c. radiating the spent caustic stream with ultraviolent radiation in photolytic reactor and
obtaining fuel H2 and treated sulfidic stream;
d. recycling treated sulfidic stream and obtaining the treated spent caustic solution.
2. The method as claimed in claim 1, wherein, source of ultraviolet radiation is UV bulbs or
sunlight.
3. The method as claimed in claim 1 wherein method comprises purging the stream of treated
sulfidic stream.
4. The method as claimed in claim 3, wherein treated sulfidic stream is obtained either
intermittent after 2-4 cycles of recycling or continuously 20-50 wt% of total regenerated stream
to Effluent Treatment Plant (ETP) for removal of undissociated components.
5. The method as claimed in claim 1, wherein method comprises adding fresh caustic stream to
H2S treating section intermittently after 2-4 cycles of recycling treated sulfidic stream or
continuously an amount equal to purge stream.
6. The method as claimed in claim 1, wherein ultraviolet radiation has a UV-Visible range of
from 185 to700 nm.
7. The method as claimed in claim 1, wherein reaction in step (b) occurs at an ambient
temperature.
8. The method as claimed in claim 1, wherein the method comprises recovering a hydrogen gas
stream from the treated sulfidic stream and regenerating an aqueous caustic.
9. The method as claimed in claim 1, wherein the spent caustic comprises of Na2S, NaSR, NaOH
and water.
10. The method as claimed in claim 1, wherein the method comprises returning the regenerated
aqueous caustic to H2S treating unit.
11. The method as claimed in claim 1, devoid utilization of an external oxygen/air.
12. A system for treating spent caustic stream generated from the H2S rich hydrocarbon stream
treating unit comprises:
14
a. a H2S treating unit (B4) wherein the H2S treating unit comprises plurality of column
with trays and packaging;
b. a first inlet configured to introduce a hydrocarbon rich stream (S6) and a stream of an
aqueous caustic (S8) into the H2S treating unit (B4); wherein the H2S treatment is
configured to discharge streams of treated hydrocarbon (S7) and sulfidic spent caustic
(S9);
c. a photolytic reactor unit (B5) configured to receive sulfidic spent caustic stream (S9)
from a second inlet wherein the photolytic reactor unit (B5) comprises plurality of
UV bulbs to generate UV light which enhances the dissociation of sulfidic spent
caustic (S9) into an enriched fuel H2 (S11) and treated sulfidic stream (S12)
d. an outlet from the photolytic reactor for discharging part of the treated sulfidic stream
(S12) to the H2S treating unit (B4) to remove H2S from H2S rich stream,
e. an effluent treating unit (B7) in which a purged stream of treated sulfidic stream,
generated either intermittent after 2-4 cycles of recycling or continuously 20-50 wt %
of total regenerated stream is introduced for removal of undissociated components
and obtaining final treated stream (S15).