The present disclosure relates generally to treatment of pollutants in an effluent stream
and more specifically, to a system for a multistage treatment of spent caustic effluent comprising
5 hydrocarbon compounds having a relatively high sulphide and high chemical oxygen demand
content.
BACKGROUND
Typically, caustic towers are used in petrochemical and refinery plants to remove acid
10 gases, hydrogen sulphide (H2S), carbon dioxide (CO2) from cracked ethylene gases and disulphides / mercaptans (R-SH) from the merox process of liquefied petroleum gas (LPG) /
kerosene streams. The effluent streams coming from the caustic towers are generally referred to
as spent caustic effluent. The spent caustic effluent is generally of hazardous, odorous, and/or
corrosive nature. As a result, it is pertinent to properly handle and dispose the spent caustic
15 stream to meet environmental regulations.
The spent caustic effluent includes sulphur compounds (sulphides), residual hydrocarbon
compounds such as phenols, cresylic acids and naphthenic acids, among other pollutants. The
compounds available in the spent caustic effluent contribute to a relatively high chemical oxygen
20 demand (COD). The presence of high COD compounds and the high sulphide compounds in the
spent caustic effluent makes the spent caustic effluent untreatable and unacceptable in a
commonly known biological water treatment plants. Moreover, the spent caustic streams may
include characteristics such as noxious odors, pH swings, foaming or poor settling of biological
solids that can create issues with conventional biological processes. Furthermore, the spent
25 caustic effluent includes contaminants that are not readily biodegradable.
Typically the spent caustic effluent is disposed of by high dilution with bio-treatment,
acid neutralization, deep well injection, incineration, wet air, catalytic/ humid peroxide oxidation
or other specialty processes. Among existing solutions, a wet air oxidation method is widely used
30 in the petrochemical and refinery plants, wherein organic matter present in the caustic spent
effluent is treated with oxidation in presence of liquid water. As a result, wet air oxidation
method oxidizes sulphide, mercaptans and reduces the COD of spent caustic effluent. After the
treatment, spent caustic effluent has a lower COD and sulphides subsequently, treated in the
biological treatment plants. However, in the wet air oxidation method, the oxidation reactions are
3
performed at elevated temperatures for example at approximately 200 – 300 degrees Celsius and
at high pressures such as 100 – 1100 psig which further requires a sophisticated and complex
pressurized systems. Consequently, the corresponding operating and capex requirement to treat
the caustic effluent becomes a non-viable treatment for the petrochemical and refinery plants. In
5 addition to the high operating cost, an efficiency of the wet air oxidation method remains
inconsistent and inefficient as the treated caustic spent effluent still contains high content of
residual hydrocarbons.
The wet air oxidation operating conditions generally practiced for various spent caustic
10 effluents treatments is shown in the below table:
Types of WAO Temperature
(
0C)
Pressure
(Psig)
Kind of Spent caustic
Low 110-120 25-100 Ethylene Spent Caustic: Sulphide
in Spent caustic
Mid
Temperature
200-220 300-600 LPG Merox spent caustic:
Complete treatment of sulphides
and mercaptanes, cresylic acids
and napthenic acids
High
temperature
240-260 700- 1100 Kerosene Merox Spent Caustic:
Complete treatment of sulphides
and mercaptane, cresylic acids
and napthenic acids
In view of the foregoing discussion, therefore, there exist a need for a system and a
method for efficiently and economically treating the spent caustic effluent in the petrochemical
15 and refinery plants at atmospheric temperature and pressure conditions.
SUMMARY
The present disclosure seeks to provide a system and a method for treating the spent
caustic effluent comprising naphthenic, cresylic, phenolic and sulfidic compounds using the
20 advanced oxidation technique generated by using hydrodynamic cavitation conditions in presence
of an externally added oxidizing agent.
4
The present disclosure discloses a system for multistage hydrodynamic cavitation
treatment of spent caustic effluent comprising hydrocarbon compounds having a relatively high
chemical oxygen demand and sulphide content. The system includes:
a first holding tank among the plurality of holding tanks adapted to store the untreated spent
5 caustic effluent therein and a first centrifugal pump mechanically coupled to the first holding
tank, wherein the first centrifugal pump is activated to maintain a predetermined flow rate within
a first stage of treatment of the untreated caustic spent effluent at atmospheric temperature and
pressure conditions.
a first venturi mechanically coupled to the first centrifugal pump, wherein the first venturi
10 comprises a first port mechanically coupled to an output of the first centrifugal pump, a second
port mechanically coupled to an inlet flow of an oxidizing agent and a third port mechanically
coupled to the first holding tank, wherein the spent caustic effluent establishes contact with the
inlet flow of the oxidizing agent at vena contracta of the first venturi, wherein spent caustic
effluent undergoes cavitation within an area near to the divergent section causing oxidation of
15 substantially large number of hydrocarbon compounds present in the spent caustic effluent, the
addition of oxidizing agent causes the synergistic production of oxidation conditions which
further causes the more oxidation of hydrocarbons compounds present in the spent caustic,
wherein cavitated spent caustic effluent is directed to the first holding tank, wherein the spent
caustic effluent is recycled through this stage for several times and maintained at this condition
20 for a predetermined first residence time;
a second holding tank mechanically coupled to an outlet port of the first holding tank for
receiving the cavitated spent caustic effluent which have gone through the first residence time
and a second centrifugal pump mechanically coupled to the second holding tank, wherein the
second centrifugal pump is activated to maintain a predetermined flow rate within a second stage
25 of treatment of the cavitated spent caustic effluent;
a second venturi mechanically coupled to the output of the second centrifugal pump, wherein
the second venturi causes re-cavitation and re-oxidation of the cavitated spent caustic effluent
with another inlet flow of an oxidizing agent to generate an outlet flow of the spent caustic
effluent from the second venturi, wherein the flow of spent caustic effluent at the outlet of the
30 second venturi has a relatively lower number of hydrocarbon compounds present than the flow of
the spent caustic effluent available at the outlet of the first holding tank.
In an embodiment, each of the first venturi and the second venturi has respective convergent
and divergent sections, wherein an angle of convergence of at least one of the first venturi and the
5
second venturi is within a range of 50-55 degrees and an angle of divergence of at least one of the
first venturi and the second venturi is within a range of 25-30 degrees.
In another embodiment, a first control valve to control the inlet flow of the oxidizing agent
5 and a second control valve to control the another inlet flow of the oxidizing agent, wherein the
inlet flow of the oxidizing agent in the first venturi is lower than or equal to an inlet flow of the
oxidizing agent of the second venturi.
In an embodiment, the oxidizing agent includes chlorine, chlorine dioxide, ozone and/or
10 hydrogen peroxide. In addition, a first rotameter is mechanically coupled to the first venturi to
monitor flow of the gaseous oxidizing agent towards the first venturi; and a second rotameter is
coupled to the second venturi to monitor flow of the oxidizing agent towards the second venturi.
In an embodiment, an inlet end of the first rotameter is coupled to a source of the oxidizing agent
and an inlet end of the additional rotameter is coupled to the same source of the oxidizing agent.
15 In an embodiment, a dosing pump is provided in case of liquid type oxidizing agent such as H2O2.
In an embodiment, a cavitation number (Cv) associated with cavitation process are
maintained in-between a range of 0.124-0.157.
20 In an embodiment, the multistage treatment process is at least a two-stage process and each
treatment stage is in series configuration with a preceding stage in the multistage treatment
process, wherein the incoming spent caustic effluent is contacted cross current contact with the
oxidizing agent at every stage.
25 The present disclosure discloses a method for multistage hydrodynamic cavitation treatment
of spent caustic effluent comprising hydrocarbon compounds having a relatively high chemical
oxygen demand and sulphide content. The method includes:
storing untreated spent caustic effluent released during merox process in a first holding tank;
activating a first centrifugal pump mechanically coupled to the first holding tank, wherein
30 the first centrifugal pump is activated to maintain a predetermined flow rate within a first stage
treatment of the untreated caustic spent effluent at a first pressure and at a first temperature;
directing an output of the first centrifugal pump towards a first venturi, wherein the a first
venturi mechanically coupled to the first centrifugal pump, wherein the first venturi comprises a
first port mechanically coupled to the output of the first centrifugal pump, a second port
6
mechanically coupled to an inlet flow of an oxidizing agent and a third port mechanically coupled
to an inlet within the first holding tank;
cavitating and oxidizing the spent caustic effluent with the inlet flow of the oxidizing agent
at vena contracta of the first venturi, wherein spent caustic effluent undergoes cavitation within
5 an area near to the divergent section causing oxidation of substantially large number of
hydrocarbon compounds present in the spent caustic effluent;
assimilating the cavitated spent caustic effluent within the first holding tank for a
predetermined first residence time;
receiving the cavitated spent caustic effluent from the first holding tank to a second holding
10 tank, wherein a second centrifugal pump is mechanically coupled to the second holding tank;
activating the second centrifugal pump to maintain a predetermined flow rate within a
second stage of treatment of the cavitated spent caustic effluent; and
directing an output of the second centrifugal pump towards a second venturi; wherein the
second venturi causes re-cavitation and re-oxidation of the cavitated spent caustic effluent with
15 another inlet flow of an oxidizing agent to generate an outlet flow from the second venturi,
wherein the flow of spent caustic effluent at the outlet of the second venturi has a relatively lower
number of hydrocarbon compounds present than the flow of the spent caustic effluent available at
the outlet of the first holding tank.
20 The present disclosure intends to achieve a plurality of objectives. As an example and
not as a limitation, an objective of the disclosure is to provide a system for treating the spent
caustic effluent with the external oxidizing agent in a manner such that the system is designed to
achieve the hydrodynamic cavitation within a venturi to build tailored cavitation conditions in
aqueous media for intensification of the oxidation reaction and sulphide/ COD reduction process
25 with the spent caustic effluent.
Another object of the present disclosure is to facilitate contacting of the oxidant with the
spent caustic effluent for the possible reduction of sulphide/ COD content of the spent caustic
effluent. Such contact between the oxidant and the spent caustic effluent treats the pollutant of
30 the spent caustic effluent and thereby, makes the treated spent caustic effluent amenable for
sending to biological treatment process for further COD reduction.
7
Yet another object of the present disclosure is to provide a means of tailoring the
cavitation conditions by alternating the constructional features of the venturi and the operating
conditions.
5 Another object of the present disclosure is to provide a means of controlling the
downstream turbulence by addition of liquid or gaseous oxidant to achieve a predetermined
cavitation to produce synergistically oxidation conditions to achieve a given oxidation chemical
transformation.
10 Yet another object of the present disclosure is to facilitate a multistage hydrodynamic
cavitation in a continuous manner in a series configuration of the multiple stages.
Another object of the present disclosure is to disclose operational design of cavitation
reactor and its venturi meter setups for the reduction of overall reactor volume/ reduction in
15 reaction time/ reduction in oxidant consumption to achieve commercial implementation of the
process at higher scale operation.
An object of the present disclosure is to design a hydrodynamic cavitation unit for
continuous treatment for a physicochemical transformation process.
20
Another object of the present disclosure is to provide the treated spent caustic effluent
with various oxidants in the alkaline pH range (13.0 to 7.0).
Another object of the present disclosure is to achieve the treated spent caustic effluent
25 having around 99% reduction in sulphide content and 80-85% reduction in COD content relative
to the untreated spent caustic effluent.
Additional aspects, advantages, features and objects of the present disclosure would be
made apparent from the drawings and the detailed description of the illustrative embodiments
30 construed in conjunction with the appended claims that follow.
It will be appreciated that numerous modifications and variations in addition to those
mentioned herein will occur to those skilled in the art. Accordingly, it is intended that the
8
invention not be limited to the disclosed embodiment, but it have the full scope permitted by the
language of the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
5 The summary above, as well as the following detailed description of illustrative
embodiments, is better understood when read in conjunction with the appended drawings. For the
purpose of illustrating the present disclosure, exemplary constructions of the disclosure are shown
in the drawings. However, the present disclosure is not limited to specific methods and
instrumentalities disclosed herein. Moreover, those in the art will understand that the drawings
10 are not to scale. Wherever possible, like elements have been indicated by identical numbers.
Embodiments of the present disclosure will now be described, by way of example only,
with reference to the following diagrams wherein:
Figure 1 is a schematic illustration of an exemplary block diagram of a system for
15 multistage hydrodynamic cavitation treatment of spent caustic effluent comprising hydrocarbon
compounds having a relatively high chemical oxygen demand and sulphide content in accordance
with an embodiment of the present disclosure;
Figure 2 is a schematic illustration of a venturi designed to facilitate hydrodynamic
20 cavitation of the spent caustic effluent in present of an oxidizing agent in accordance with an
embodiment of the present disclosure;
Figures 3A and 3B illustrate an exemplary single stage treatment system and a four stage
treatment system for treating the spent caustic effluent respectively in accordance with an
25 embodiment of the present disclosure;
Figures 4A and 4B illustrate exemplary samples of treated spent caustic effluent with
hydrogen peroxide at different intervals and corresponding characteristics of spent caustic
effluent in accordance with an embodiment of the present disclosure;
30
Figures 5A and 5B illustrate exemplary samples of treated spent caustic effluent with
chlorine at different intervals and corresponding characteristics of spent caustic effluent in
accordance with an embodiment of the present disclosure;
9
Figures 6A and 6B illustrate exemplary charts depicting degradation rate kinetics of COD
and sulphide content of the spent caustic effluent in accordance with an embodiment of the
present disclosure; and
5 Figure 7 illustrates a method for multistage treatment of spent caustic effluent comprising
hydrocarbon compounds having a relatively high chemical oxygen demand and sulphide content.
In the accompanying drawings, an underlined number is employed to represent an item
over which the underlined number is positioned or an item to which the underlined number is
10 adjacent. A non-underlined number relates to an item identified by a line linking the nonunderlined number to the item. When a number is non-underlined and accompanied by an
associated arrow, the non-underlined number is used to identify a general item at which the arrow
is pointing.
15 DETAILED DESCRIPTION
The following detailed description illustrates embodiments of the present disclosure and
ways in which they can be implemented. Although some modes of carrying out the present
disclosure have been disclosed, those skilled in the art would recognize that other embodiments
20 for carrying out or practicing the present disclosure are also possible.
Figure 1 is a schematic illustration of an exemplary block diagram of a system 100 for a
multistage hydrodynamic cavitation treatment of spent caustic effluent comprising hydrocarbon
compounds having a relatively high chemical oxygen demand and sulphide content in accordance
25 with an embodiment of the present disclosure. The treatment of the untreated spent caustic
effluent is achieved in multiple stages in a series configuration wherein the output of the first
stage of the treatment is fed to an input of a second stage and similarly, an output of the second
stage of the treatment is fed to an input of the third stage the treatment. In an embodiment, the
system 100 can be tailored to increase or decrease the multiple stages of the treatment depending
30 upon the requirements as well as presence of pollutants in the untreated spent caustic. Further,
the system 100 facilitates contact of the incoming spent caustic effluent in a form of cross current
contact with the oxidizing agent at every stage of the treatment.
10
The system 100 includes a first holding tank 102a, a second holding tank 102b and a third
holding tank 102c among the plurality of holding tanks (collectively referred herein to as 102).
The plurality of holding tanks 102 is adapted to store the spent caustic effluent therein during
respective stage of treatment. The spent caustic effluent is recycled for several times during each
5 stage of treatment for a predetermined residence time within respective holding tanks 102.
Further, each of the holding tanks 102 is mechanically coupled to respective centrifugal pump
104. For example, the first holding tank 102a is coupled to a centrifugal pump 104a, the second
holding tank 102b is coupled to a centrifugal pump 104b and a third holding tank 102c is coupled
to a centrifugal pump 104c.
10
In an embodiment, each of the centrifugal pumps 104 is activated to maintain a
predetermined flowrate within their respective stage of treatment at the substantially same
temperature and pressure conditions. As an example, and not as a limitation, the temperature and
pressure conditions are equivalent to atmospheric temperature and pressure conditions in the
15 surrounding environment. However, during the continuation of cavitation process, the dissipated
energy results into increase in the temperature of the spent caustic effluent. Since the system
disclosed herein can be operated at atmospheric temperature and pressure conditions, the system
100 may not require sophisticated temperature and pressure management units. As a result, an
overall cost of implementation of the system 100 is substantially reduced when compared to the
20 cost of implementation of the wet air oxidation system for treating the spent caustic effluent.
In an alternate embodiment, the first centrifugal pump 104a is activated to maintain a
predetermined flow rate within a first stage of treatment of the untreated caustic spent effluent at
a first pressure and at a first temperature. The second centrifugal pump 104b is activated to
25 maintain a predetermined flow rate within a second stage of treatment of the cavitated spent
caustic effluent at a second pressure and at a second temperature. The third centrifugal pump
104c is activated to maintain a predetermined flow rate within a third stage treatment of the
cavitated spent caustic effluent at a third pressure and at a third temperature. In such embodiment,
the pressure and temperature variations at each stage of the treatment are dependent of the level
30 pollutants present, and the treatment time of the effluent at the output of the respective stages of
the treatment of the spent caustic effluent.
In a first stage of treatment, the untreated spent caustic effluent is firstly stored in the first
holding tank 102a. The first centrifugal pump 104a is activated to direct the untreated spent
11
caustic effluent towards a first venturi 106a. The first venturi 106a is mechanically coupled to the
first centrifugal pump 104a and the first venturi comprises a first port 202a (convergent section)
mechanically coupled to an output of the first centrifugal pump 104a, a second port 204b (throat
section) is mechanically coupled to an inlet flow of an oxidizing agent and a third port 206a
5 (divergent section) is mechanically coupled to an inlet within the first holding tank 102a. The
untreated spent caustic effluent establishes contact with the inlet flow of the oxidizing agent at
vena contracta of the first venturi 106a wherein the untreated spent caustic effluent undergoes
cavitation within an area near to the divergent section causing oxidation of substantially large
number of hydrocarbon compounds present in the spent caustic effluent. Further the cavitated
10 spent caustic effluent is directed to the first holding tank 102a, the spent caustic is recycled
though this stage for several times and maintained at this condition for the predetermined
residence time. Subsequently, a first stage of treatment of the spent caustic effluent is achieved
and the output of the first stage of treatment is diverted towards a second stage of treatment.
15 In the second stage of treatment, the output of the first stage is sent to the second holding
tank 102b. The second centrifugal pump 104b is activated to direct the output of the first stage
towards a second venturi 106b. The second venturi 106b is mechanically coupled to the second
centrifugal pump 104b. The second venturi 106b comprises a first port 202b (convergent section)
mechanically coupled to an output of the second centrifugal pump 104b, a second port 204b
20 (throat section) mechanically coupled to an inlet flow of an oxidizing agent and a third port 206b
(divergent section) mechanically coupled to an inlet within the second holding tank 102b. The
output of the first stage establishes contact with another inlet flow of the oxidizing agent at vena
contracta of the second venturi 106b wherein an output of the first stage undergoes cavitation
within an area near to the divergent section of the second venturi106b causing oxidation of a
25 substantially large number of hydrocarbon compounds present in the spent caustic effluent. This
hydrodynamic cavitation in combination with the oxidation of the spent caustic effluent causes
the second stage of treatment of the spent caustic effluent. The second venturi 106b causes recavitation and re-oxidation of the spent caustic effluent obtained from the first stage of treatment
with another flow of the oxidizing agent to generate an outlet flow from the second venturi 106b.
30 The flow of spent caustic effluent at the outlet of the second holding tank has a relatively lower
number of hydrocarbon compounds and pollutants than the flow of the spent caustic effluent
available at the outlet of the first holding tank.
12
Further, the cavitated and oxidized spent caustic effluent is directed to the second
holding tank 102b, the spent caustic is recycled though this stage for several times and
maintained at this condition for the predetermined residence time. Subsequently, a second stage
of treatment of the spent caustic effluent is achieved and the output of the second stage of
5 treatment is diverted towards a third stage of treatment of the spent caustic effluent.
In the third stage of treatment, the output of the second stage is sent to the third holding
tank 102c. The third centrifugal pump 104c is activated to direct the output of the second stage
towards a third venturi 106c. The venturi 106c is mechanically coupled to the third centrifugal
10 pump 104c. The third venturi 106c comprises a first port 202c (divergent section) mechanically
coupled to an output of the third centrifugal pump 104c, a second port 204c (throat section)
mechanically coupled to another inlet flow of the oxidizing agent and a third port 206c (divergent
section) mechanically coupled to an inlet within the third holding tank 102c. The spent caustic
effluent obtained from the second stage of the treatment establishes contact with the another inlet
15 flow of the oxidizing agent at vena contracta of the third venturi 106c wherein an output from the
second stage of treatment undergoes cavitation within an area near to the vena contracta causing
oxidation of a substantially large number of hydrocarbon compounds present therein.
The third venturi 106c further causes re-cavitation and re-oxidation of the cavitated spent
20 caustic effluent obtained from the second stage of treatment with another flow of the oxidizing
agent to generate an outlet flow from the third venturi 106c. The flow of spent caustic effluent at
the outlet of the third holding tank 102c has a relatively lower number of hydrocarbon
compounds and pollutants than the flow of the spent caustic effluent available at the outlet of the
second holding tank 102b. This hydrodynamic cavitation in combination with the oxidation of
25 the spent caustic effluent causes a third stage of treatment of the spent caustic effluent. Further,
the cavitated spent caustic effluent is directed to the third holding tank 102c, the spent caustic is
recycled though this stage for several times and maintained at this condition for a predetermined
third residence time. The each stage the residence time can be varied depending on the
concentration of sulphide and COD content of the spent caustic. On completion of the residence
30 time, the treated spent caustic effluent is subsequently can be processed in the biological
treatment plants as the treated spent caustic effluent has a substantially reduced COD and the
sulphide contents therein.
13
In an embodiment, the system 100 includes a first control valve to control the inlet flow
of the oxidizing agent towards the first venturi 106a and a second control valve to control the
inlet flow of the oxidizing agent towards the second venturi 106b. In an embodiment, the inlet
flow of the oxidizing agent in the first venturi is lower than/ equal to an inlet flow of the
5 oxidizing agent of the second venturi 106b. Similarly, the inlet flow of the oxidizing agent in the
first venturi 160a and the second venturi 106b is lower than/ equal to an inlet flow of the
oxidizing agent of the third venturi 106c.
Although the present disclosure uses the same oxidizing agent as a supply for the
10 different stages of treatment, in alternate embodiments the first venturi 106a may use a first
oxidizing agent, the second venturi 106b may use a second oxidizing agent and the third Venturi
106c may use a third oxidizing agent for achieving oxidation of the spent caustic effluent at the
respective vena contracta. In an embodiment, the first oxidizing agent, the second oxidizing or
the third oxidizing agent is selected from a group of oxidizing agents including chlorine, chlorine
15 dioxide, ozone, hydrogen peroxide and a combination thereof.
In an embodiment, the system 100 includes a first rotameter or a dosing pump 108a
mechanically coupled to the first venturi106a to monitor flow of the oxidizing agent, a second
rotameter or a dosing pump108bcoupled to the second venturi106b to monitor flow of the
20 oxidizing agent and a third rotameter or a dosing pump108c coupled to the third venturi 106c to
monitor flow of the oxidizing agent. Inlets of the first rotameter or the dosing pump 108a, the
second rotameter or the dosing pump 108b and the third rotameter or the dosing pump108c are
coupled to an oxidizing agent source 110 or to respective individual oxidizing agent sources. In
an alternate embodiment, the inlet end of the first rotameter or the dosing pump108a is coupled to
25 a source of the first oxidizing agent, the inlet end of the second rotameter or the dosing pump
108b is coupled to a source of the second oxidizing agent, and the inlet end of the third rotameter
or the dosing pump 108c is coupled to a source of the third oxidizing agent, when each venturi is
operated using a respective different oxidizing agent.
30 The system 100 facilitates a continuous process for treating the spent caustic effluent
stream by using the hydrodynamic cavitation setup along with oxidants. Such combination of the
hydrodynamic cavitation with the oxidizing agents at synergistic oxidation conditions ensures
degradation/oxidation of sulphide/COD contributing compounds such as sulfidic, disulphidic,
naphthenic, cresylic, and mercaptane compounds. As discussed and illustrated, the continuous
14
process, in which the two or more hydrodynamic cavitation reactors (i.e. the venturi 106a, 106b
and 106c) are arranged in series, assists in achieving a plug flow model of standard chemical
engineering design. As a result, an overall total reactor volume or reduction in the oxidant
consumption or reduction in overall reaction time is achieved to obtain desired degree of sulphide
5 or COD content reduction in the spent caustic effluent.
Figure 2 is a schematic illustration of a venturi designed to facilitate hydrodynamic
cavitation of the spent caustic effluent in presence of an oxidizing agent in accordance with an
embodiment of the present disclosure. Each of the plurality of venturi devices such as the venturi
10 106a, the venturi 106b, and the venturi 106c are herein referred to as the venturi 106. The venturi
106 includes a converging section 202 and a diverging section 206. In an embodiment, an angle
of convergence of the venturi 202 is within a range of 50-55 degrees and an angle of divergence
of the venturi 206 is within a range of 25-30 degrees. In an embodiment, the each of the
centrifugal pump 104 coupled to the venturi 106 are operated to maintain a flow rate of 10 m3
/hr,
at a 4-6 kg/cm2
15 pressure head.
In an embodiment, the venturi 106 facilitates prevailing conditions which may generate
hydroxyl radicals during the hydrodynamic cavitation of the spent caustic effluent. The
cavitation can be described as the formation, growth and subsequent collapse of cavities. During
20 the collapse stage of cavities localized high temperature and pressure conditions are produced in
the divergent section of the venture. Although the reaction rate of the produced hydroxyl radicals
is several orders higher than atomic oxygen, however the number of hydroxyl radicals may not be
sufficient enough to oxidize the high COD/sulphide content of spent caustic effluent. The present
disclosure facilitates induction of external oxidizing agent at the vena contract region of the
25 venturi 106 through the second port 204. As an example, and not as a limitation, a reaction
mechanism for sulphide content or mercaptane content of the spent caustic effluent in shown in
the below equation.
R- CH3 or C2H5 or higher hydrocarbon,
15
R- aromatic ring or or
5 As a result, the externally induced oxidizing agent generates synergistic oxidizing
conditions. Such conditions can successfully oxidize/destroy the COD/hydrocarbons present in
the spent caustic effluent.
In an embodiment, the intensity of the cavitation prevailing within the venturi 106 is
10 related to global operating conditions through a cavitation number (Cv). The cavitation number
can be mathematically represented as
Cavitation number (Cv) [

⁄
]
Where P = the recovered pressure downstream of the venturi 106
15 Pv= the vapor pressure of the flowing spent caustic effluent at the prevailing temperature
and conditions. In an embodiment, the Pv is at - 4245 N/m2
at 30 °C.
= Density of the liquid
v = average velocity of liquid at the throat/ vena contracta.
20 The cavitation number at which the inception of cavitation occurs is known as cavitation
inception number (Cvi). Ideally, the cavitation inception occurs at Cvi =1, however the significant
cavitation effects at Cv value of less than 1 have been observed. Further the dynamic behavior of
the cavities plays a significant role in intensification of physical and chemical processes. The
performance of a hydrodynamic cavitation reactor for a specific type of transformation depends
25 on the cavitational conditions prevailing in the venturi 106.
In an embodiment, the flow rate of the spent caustic effluent through the venturi106 is
controlled with cavitation number (Cv), which is maintained in the range of 0.1 to 0.5 and more
particularly in the range of 0.12 to 0.15.In an embodiment, the cavitation number is maintained
30 within a range of 0.124-0.157 for 35–40 m/sec, venturi/orifice velocity at an inlet pressure P1 of
4-5 kg/cm2
.
Figures 3A and 3B illustrate an exemplary single stage treatment system and a four stage
treatment system for treating the spent caustic effluent respectively in accordance with an
16
embodiment of the present disclosure. As an example, and not as a limitation, all parts of the
reactor are made with high density polyethylene (HDPE). The holding tank 102 is provided with
inlet-outlet at the top/bottom of the tank for the circulation of the spent caustic effluent. All the
piping diameter system used is 1 inch HDPE pipes. The centrifugal pump 104 equipped with
5 motor capacity of 2.5 kW, is capable of pumping the spent caustic effluent at a flowrate of 10
m
3
/hr. The spent caustic effluent used for the experimentation is generated from the refinery
merox process. The measured COD of the untreated spent caustic is in the range of 28000-30000
ppm and sulphide content is in the range of 2500-3000 ppm (approximately). The merox process
spent caustic contaminants majorly consists of naphthenic and cresylic acids of high molecular
10 weight compounds. In both the exemplary single stage and the four stage treatment systems the
present disclosure facilitates contact of the oxidizing agent with the spent caustic effluent to
reduce sulphide/COD content of the spent caustic effluent. Subsequently, the treated spent
caustic effluent is amenable for biological treatment process for further COD reduction.
15 As illustrated, a hydrodynamic cavitation reactor comprises of a cavitation generator in
form of the venturi 106 adapted to receive the supply of the oxidizing agent. The venturi 106 is
designed with predetermined convergent and divergent sections for producing optimized
cavitation conditions. The convergent sections with an overall average angle of 50-55o
(upstream
section) with the central line and the divergent section with an overall average angle of 25-30o
20 (downstream section) with the central line. The cavitation number is in a range of Cv = 0.12 to
0.15, more particularly in the range of 0.1 to 0.25. A lower cavitation number indicates severe
cavitation conditions at the vena contracta region of the venturi 106. Further, the supply of the
external oxidizing agent at these operating conditions results into optimum conditions for
cavitation. The supply of the oxidizing agent is provided through 204 and controlled by an
25 external control valve mechanism, and the flow rate of the gaseous oxidizing agent is measured
by using the rotameter, and the liquid oxidizing agent is controlled by dosing pump. The amount
of oxidant addition is measured by weighing balance.
The cavitation is generated using the external centrifugal pump which is capable of
30 producing enough pressure required to pump the aqueous media at the predetermined flow rate to
produce the cavitation at the venturi throat. As illustrated in Figure 3B, the centrifugal
pumps/cavitation units with effluent storage tanks are arranged in a series configuration. Each
storage tank is provided with the enough residence time to achieve the desired degree of
cavitation which may enable the oxidation of sulphide/reduction in COD content of the effluent.
17
Further, the hydrodynamic cavitation units are arranged in a series model, to achieve the plug
flow model of standard chemical engineering design. As a result, an overall total reactor volume
or consumption of the oxidizing agent is substantially reduced to achieve the desire degree of
treatment of the spent caustic effluent. Furthermore, the series configuration of the holding tanks
5 leads to a plug flow model, and hence this may decrease the back mixing generally occurs in
CSTR model (continuous stirred tank reactor). Consequently, treatment time for the spent caustic
effluent and overall oxidant consumption rate is substantially reduced to achieve the desired
degree of reduction in sulphide/COD content of the spent caustic effluent.
10 EXPERIMENT 1
Figures 4A and 4B illustrate exemplary samples of treated spent caustic effluent with
hydrogen peroxide at different intervals and corresponding characteristics of spent caustic
effluent in accordance with an embodiment of the present disclosure.
15 In an embodiment, the experiments were carried out using a pilot scale hydrodynamic
cavitation setup such as illustrated in Figure 3A. During the experiment, a 50 L of the spent
caustic effluent is charged into the holding tank 102 and is treated under desired cavitation
conditions in presence of an oxidizing agent such as (50%) H2O2 at the venturi 106. The total
amount of hydrogen peroxide added is 1.2 kg over 3 hours of the experimentation. The treated
20 samples have been collected for every 30 minutes interval and analyzed for its COD, sulphide
and pH content. Figure 4A illustrates the samples and the respective colors whereas the Figure 4B
illustrates a Table-1 depicting various characteristics of the spent caustic effluent at with
increasing treatment time.
25 EXPERIMENT 2
Figure 5A and 5B illustrate exemplary samples of treated spent caustic effluent with
chlorine at different intervals and corresponding characteristics of spent caustic effluent in
accordance with an embodiment of the present disclosure.
30 In an embodiment, the experiments were carried out using a pilot scale hydrodynamic
cavitation setup such as illustrated in Figure 3A. During the experiment, a 50 L of the spent
caustic effluent is charged into the holding tank 102 and is treated under desired cavitation
conditions in presence of an oxidizing agent such as chlorine in the venturi 106. The total amount
of chlorine added is 1.5 kg over 2 hours of the experimentation. The treated samples have been
18
collected for every 30 minutes interval and analyzed for its COD, sulphide and pH content.
Figure 5A illustrates the samples and respective colors whereas the Figure 5B illustrates a Table2 depicting various characteristics of the spent caustic effluent with increasing treatment time.
5 Figure 6A and 6B illustrate exemplary charts depicting degradation rate kinetics of COD
and sulphide contents of the spent caustic effluent in accordance with an embodiment of the
present disclosure. As illustrated, the initial COD of 28280 ppm is decreased to 9050 and 1038
ppm with hydrogen peroxide and chlorine as an oxidant respectively. In other words, an
approximately of 60 percentage and 88 percentage of reduction in COD was observed when the
10 oxidizing agent was hydrogen peroxide and chlorine respectively. The initial sulphide content of
2890 ppm is decreased to 1038 and 24 ppm with hydrogen peroxide and chlorine as an oxidant
respectively. That is to say, an approximately 64% and 99% of reduction in sulphide content was
observed when the oxidizing agent was hydrogen peroxide and chlorine respectively. The
measured pH values of the treated effluent remained constant (around pH=13.2) in case of
15 hydrogen peroxide treatment process. The measured pH values during chlorine injection
experimentation is continuously decreasing from 13.2 to 6.8. The initial chloride content is 999
ppm, and it is increased to 16380 ppm for 2 hrs treatment.
The chlorine and hydrogen peroxides are causing the oxidation of hydrocarbons (organic
20 compounds) to lower or simple bio-degradable compounds due to presence of cavitation and
synergistic oxidizing conditions. The most noxious sulphide content of the spent caustic is
oxidized to simple higher oxidation states compounds such as sulphones, sulphoxides or even to
sulphonic acid or sulphonyl chlorides, resulting into decreased COD and negligible sulphide
content in the treated effluent. These compounds are easily treated in the aerobic treatment
25 facilities.
It has been observed that chlorine as an oxidant under simultaneous cavitation conditions
seems to be more effective in reducing the COD and sulphide content of spent caustic effluent
compared to hydrogen peroxide. Subsequently, the in situ neutralization of pH caused by chlorine
30 addition may improve the oil water separation (emulsion brakeage) efficiency, at API separator
section of ETP. However, the added chlorine gas increased the chloride content to 16380 ppm,
this may have a slight detrimental effect on biological methods. The treated spent caustic stream
is generally mixed with other effluent streams of higher volumes at refinery ETP. The dilution
effect may decrease the final chloride content to an acceptable level to the SBR/MBR of
19
biological treatment processes.
Figure 7 illustrates a method 700 for multistage treatment of spent caustic effluent
comprising hydrocarbon compounds having a relatively high chemical oxygen demand and
5 sulphide content. At step 702, the method 700 includes storing untreated spent caustic effluent
which is generated during the merox process in a first holding tank. At step 704, the method 700
includes activating a first centrifugal pump mechanically coupled to the first holding tank,
wherein the first centrifugal pump is activated to maintain a predetermined flow rate within a first
stage treatment of the untreated caustic spent effluent at atmospheric temperature and pressure
10 conditions.
At step 706, the method 700 includes directing an output of the first centrifugal pump
towards a first venturi, wherein the a first venturi mechanically coupled to the first centrifugal
pump, wherein the first venturi comprises a first port (convergent section) mechanically coupled
15 to the output of the first centrifugal pump, a second port (throat section) mechanically coupled to
an inlet flow of an oxidizing agent and a third port (divergent section) mechanically coupled to an
inlet within the first holding tank. At step 708, the method 700 includes cavitating and oxidizing
the spent caustic effluent with the inlet flow of the oxidizing agent at vena contracta of the first
venturi, wherein spent caustic effluent undergoes cavitation within an area near to the divergent
20 section causing oxidation of substantially large number of hydrocarbon compounds present in the
spent caustic effluent.
At step 710, the method 700 includes assimilating the cavitated spent caustic effluent
within the first holding tank and recycling it through this stage for several times with desired
25 predetermined residence time. At step 712, the method 700 includes receiving the cavitated spent
caustic effluent from the first holding tank to a second holding tank, wherein a second centrifugal
pump is mechanically coupled to the second holding tank.
At step 714, the method 700 includes activating the second centrifugal pump to maintain
30 a predetermined flow rate within a second stage of treatment of the cavitated spent caustic
effluent. At step 716, the method 700 includes directing an output of the second centrifugal pump
towards a second venturi; wherein the second venturi causes re-cavitation and re-oxidation of the
cavitated spent caustic effluent with another inlet flow of an oxidizing agent to generate an outlet
flow from the second venturi, wherein the flow of spent caustic effluent at the outlet of the
20
second venturi has a relatively lower number of hydrocarbon compounds present than the flow of
the spent caustic effluent available at the outlet of the first holding tank.
Among several advantages, the present disclosure facilitates customized system for
5 treating the spent caustic effluent in accordance with a level of pollutants present therein. For
example, the system and method disclosed herein enables creation of cavitation conditions in
aqueous media within the venturi and alteration of the flow of the oxidant and the spent caustic
effluent to achieve the desired output at atmospheric temperature conditions.
10 The present disclosure facilitates a more energy efficient way of producing cavitation by
ensuring that the untreated spent caustic effluent passes through a constriction of the venturi. As a
result, the present disclosure does not require any customized or specially made cavitation
reactors. The present disclosure in a significantly economic manner treats the spent caustic
effluent containing high COD and sulphide content.
15
The process has been converted from batch to continuous scale operation, with cross
current contact of oxidizing agent such as chlorine/ ClO2/ H2O2 addition to the system. Hence the
method and systems disclosed herein eliminate the back mixing, and achieve much faster
degradation kinetics in COD and sulphide content, or less oxidant consumption for the same
20 degradation rate kinetics can be achieved.
Modifications to embodiments of the present disclosure described in the foregoing are
possible without departing from the scope of the present disclosure as defined by the
accompanying claims. Expressions such as “including”, “comprising”, “incorporating”, “have”,
25 “is” used to describe and claim the present disclosure are intended to be construed in a nonexclusive manner, namely allowing for items, components or elements not explicitly described
also to be present. Reference to the singular is also to be construed to relate to the plural.

We claim:

1. A system for multistage treatment of spent caustic effluent comprising hydrocarbon
5 compounds having a relatively high chemical oxygen demand and sulphide content, wherein the
system comprising:
a first holding tank among the plurality of holding tanks adapted to store the untreated spent
caustic effluent therein and a first centrifugal pump mechanically coupled to the first holding
tank, wherein the first centrifugal pump is activated to maintain a predetermined flow rate within
10 a first stage of treatment of the untreated caustic spent effluent at atmospheric temperature and
pressure conditions;
a first venturi mechanically coupled to the first centrifugal pump, wherein the first venturi
comprises a first port mechanically coupled to an output of the first centrifugal pump, a second
port mechanically coupled to an inlet flow of an oxidizing agent and a third port mechanically
15 coupled to an inlet within the first holding tank, wherein the spent caustic effluent establishes
contact with the inlet flow of the oxidizing agent at vena contracta of the first venturi, wherein
spent caustic effluent undergoes cavitation within an area near to a divergent section causing the
oxidation of substantially large number of hydrocarbon compounds present within the spent
caustic effluent, wherein cavitated spent caustic effluent is directed to the first holding tank and is
20 recycled through therein for several times and maintained at this condition for a predetermined
residence time;
a second holding tank mechanically coupled to an outlet port of the first holding tank for
receiving the cavitated spent caustic effluent which have gone through the first residence time
and a second centrifugal pump mechanically coupled to the second holding tank, wherein the
25 second centrifugal pump is activated to maintain a predetermined flow rate within a second stage
of treatment of the cavitated spent caustic effluent;
a second venturi mechanically coupled to the output of the second centrifugal pump, wherein
the second venturi causes re-cavitation and re-oxidation of the cavitated spent caustic effluent
with another inlet flow of an oxidizing agent to generate an outlet flow of the spent caustic
30 effluent from the second venturi, wherein the flow of spent caustic effluent at the outlet of the
second venturi has a relatively lower number of hydrocarbon compounds present than the flow of
the spent caustic effluent available at the outlet of the first holding tank.
22
2. The system as claimed in claim 1, wherein each of the first venturi and the second venturi
has respective convergent and divergent sections, wherein an angle of convergence of at least one
of the first venturi and the second venturi is within a range of 50-55 degrees and an angle of
divergence of at least one of the first venturi and the second venturi is within a range of 25-30
5 degrees.
3. The system as claimed in claim 1, comprising a first control valve to control the inlet flow of
the oxidizing agent and a second control valve to control the another inlet flow of the oxidizing
agent, wherein the inlet flow of the oxidizing agent in the first venturi is lower than or equal to an
10 inlet flow of the oxidizing agent of the second venturi.
4. The system as claimed in claim 3, wherein the oxidizing agent includes chlorine, chlorine
dioxide, ozone and hydrogen peroxide.
15 5. The system as claimed in claim 3, further comprising:
a first rotameter mechanically coupled to the first venturi to monitor flow of the oxidizing
agent towards the first venturi; and
a second rotameter coupled to the second venturi to monitor flow of the oxidizing agent
towards the second venturi.
20
6. The system as claimed in claim 8, wherein an inlet end of the first rotameter is coupled to a
source of the oxidizing agent and an inlet end of the additional rotameter is coupled to the same
or a different source of the oxidizing agent.
25 7. The system as claimed in claim 1, wherein a cavitation number associated with cavitation
process within at least one of the first venturi and the second venturi is in-between a range of 0.1
to 0.5 and more particularly in the range of 0.124-0.157.
8. The system as claimed in claim 1, wherein the multistage treatment process is at least a two30 stage process and each treatment stage is in series configuration with a preceding stage in the
multistage treatment process, wherein the incoming spent caustic effluent is contacted cross
current contact with the oxidizing agent at every stage.
23
9. A method for multistage treatment of spent caustic effluent comprising hydrocarbon
compounds having a relatively high chemical oxygen demand and sulphide content, wherein the
method comprising:
5 storing untreated spent caustic effluent generated during merox process in a first holding
tank;
activating a first centrifugal pump mechanically coupled to the first holding tank, wherein
the first centrifugal pump is activated to maintain a predetermined flow rate within a first stage
treatment of the untreated caustic spent effluent at atmospheric temperature and pressure
10 conditions;
directing an output of the first centrifugal pump towards a first venturi, wherein the a first
venturi mechanically coupled to the first centrifugal pump, wherein the first venturi comprises a
first port mechanically coupled to the output of the first centrifugal pump, a second port
mechanically coupled to an inlet flow of an oxidizing agent and a third port mechanically coupled
15 to an inlet within the first holding tank;
cavitating and oxidizing the spent caustic effluent with the inlet flow of the oxidizing agent
at vena contracta of the first venturi, wherein spent caustic effluent undergoes cavitation within
an area near to a divergent section causing oxidation of substantially large number of
hydrocarbon compounds present within the spent caustic effluent;
20 assimilating the cavitated spent caustic effluent within the first holding tank, wherein the
spent caustic effluent is recycled through the stage for several times and maintained at this
condition for a predetermined residence time;
receiving the cavitated spent caustic effluent from the first holding tank to a second holding
tank, wherein a second centrifugal pump is mechanically coupled to the second holding tank;
25 activating the second centrifugal pump to maintain a predetermined flow rate within a
second stage of treatment of the cavitated spent caustic effluent; and
directing an output of the second centrifugal pump towards a second venturi; wherein the
second venturi causes re-cavitation and re-oxidation of the cavitated spent caustic effluent with
another inlet flow of an oxidizing agent to generate an outlet flow from the second venturi,
30 wherein the flow of spent caustic effluent at the outlet of the second venturi has a relatively lower
number of hydrocarbon compounds present than the flow of the spent caustic effluent available at
the outlet of the first holding tank.
24
10. The method as claimed in claim 9, wherein the multistage treatment process is at least a twostage process and each treatment stage is in series configuration with a preceding stage in the
multistage treatment process, wherein the incoming spent caustic effluent is contacted cross
current contact with the oxidizing agent at every stage.