RECOVERY OF SULPHUR FROM SULPHUR SLURRY BY USING HYBRID MEMBRANE-FILTER SYSTEM
FIELD OF THE INVENTION
The present invention relates to a process for separation of sulphur from sulphur slurry in a liquid redox type sulphur plant. The invention in particular relates to the removal of sulphur present at low to very low concentration in slurry by using integrated membrane - filter hybrid system. The invention relates to a process for concentrating sulphur in slurry from 0.1 %( wt) to 15 % (wt) by using porous membrane. The invention also relates to a process for removal of metal based catalyst solution from concentrated slurry coming out from porous membrane to make sulphur cake with 5-10%wt of water & at least 99.5 % wt purity on dry basis by using pressure filter.
BACKGROUND OF THE INVENTION:
Liquid Redox process is used for conversion of H2S present in gas stream to elemental sulphur by employing non toxic, chelated metal based catalyst. The catalyst is present in aqueous solution and the concentration of catalyst is in ppm level. Conversion of H2S in the gas containing H2S called acid gas to elementary sulphur is carried out in a bubble column process called absorber. H2S is absorbed in alkaline solution of catalyst and is ionized to hydrogen sulphide ion and sulphide ion. Sulphide ion converts to elemental sulphur in presence of chelated metal catalyst. During conversion of H2S, chelated metal catalyst is reduced and subsequently, oxidized by air. The regenerated catalyst is used for further oxidation of H2S. Sulphur formed remains in catalyst solution and is separated by different separation methods.
Typically reactions occurred in such liquid redox process can be summarized by the following equations
(Equation Removed)
M represents the polyvalent metals which form a stable complex cation in solution.
US patent 3068065 discloses use of chelated iron as trivalent metal for scrubbing acid gas to
remove H2S. Chelated iron uses EDTA-like chelating agents.
The process was improved by consideration of two different chelating agents instead of one.
US patent 4189462 claims use of two chelating agents gives better stability of chelated iron
used in the process of H2S removal.
Typical chelated complexes may be found in US Patent No 3580950, US Patent No 4401642,
US Patent No 4393037, US Patent No 4390516, US Patent No 4315403, US Patent No
4206194, US Patent No 4083945, US Patent No 3097925 and Indian patent No 187223 etc.
The process produces sulphur in solid state and the sulphur remains in solution. US patent
4374104 claims use of surfactant to enhance the separation of sulphur particles from catalyst
solution. Experience in large commercial plants has indicated that it is nearly impossible to
avoid the intrusion of oily and greasy material into plant and that even small quantities
selectively wet the sulphur particle and bring about agglomeration with entrapped air so that
separation by settling ,centrifuging is very difficult.
Different types of plant configuration and equipments are used to remove H2S from acid gas
by using chelated metal catalyst. US patent 4482524 claims the use of auto circulation system
which reduced the cost of unit significantly. Auto circulation equipment includes both the
absorber in which acid gas is distributed and regenerator in which air is distributed, in a
single vessel. Density difference of solution and flow of gas in absorber and regenerator
allow the circulation of solution between absorber and regenerator. Process with auto
circulation equipment is used to remove H2S from acid gas when treated acid gas is
discharged to atmosphere and its H2S level meets the pollution norms.
US patent 5126118 claims use of two stage plant configuration to remove H2S from acid gas.
In the first stage, acid is treated in the absorber with chelated metal catalyst solution. In the
absorber, sulphur is formed due to oxidation of H2S in presence of catalyst solution and remains in the solution. In the second stage, the catalyst is regenerated by air in oxidizer and catalyst solution is returned to absorber after separation of solid sulphur from the solution. This configuration is used when the treated acid gas has downstream application. The H2S level in treated acid gas depends on H2S concentration in the untreated acid gas and need of H2S concentration in the treated gas for downstream application.
Sulphur formed in the absorber remains in suspended phase in catalyst solution. These particles plug the nozzles of sparger which is used for sparging acid gas into catalyst solution uniformly. Plugging of nozzles affects the liquid redox process. Removal of suspended sulphur particle is necessary for better operation of liquid redox process. Different processes are used to separate sulphur from solution.
US 4784754 disclose sulphur along with catalyst solution called sulphur slurry is heated in a melter in order to get molten sulphur. Since the specific gravity of liquid sulphur is 1.8 to 2.0, liquid sulphur is separated from catalyst solution in a separator by gravity. In another process, sulphur is separated by filtration. Solid sulphur cake is discharged directly or sent to melter after making slurry with demineralized water. For improvement of sulphur separation from slurry, US patent 4784754 discloses a process which removes the froth or foam sulphur particles accumulated in the regenerated solution. Plurality of sulphur solution streams, minimum one from the top and one from the bottom of regenerator is recombined and sent to the melter or vacuum filter.
In the present melter system for sulphur separation from slurry, heating of solution containing sulphur is required to maintain temperature 130-140° C of the solution. Heating causes number of problems in the liquid redox process like thermal degradation of catalyst, catalyst loss and formation of black sulphur due to decomposition hydrocarbons. Handling of liquid sulphur is another problem of the melter system because of choking in melter, solidification of sulphur in pipes.
In the present filter system, separation of sulphur depends on concentration of sulphur in the slurry and proper washing of sulphur cake formed in the filter. Filter like vacuum filter, pressure filter or any other filter of this type works better if sulphur concentration in slurry is either moderate or high. If sulphur concentration in slurry is less than 1 %wt, direct use of vacuum filter or press filter is not suitable due to more filter surface area requirement and more filtration time requirement. For such type of slurry filtration, capital cost and operating cost are high for vacuum filter or press filter.
In view of all the drawbacks of prior art processes for separation of solid sulphur particles from catalyst solution of liquid redox process, there is still need to develop a process for efficient recovery of solid sulphur particles containing less than 1 %wt sulphur. The inventors of the present invention have found that the hybrid membrane-filter system can handle sulphur slurry having sulphur concentration as low as 0.1-5 %( wt). The invention of membrane-filter hybrid system also reduces slurry flow to pressure filter and consequently makes the separation process more economical. The present process gives high rate of sulphur removal efficiency and catalyst loss with sulphur is minimum. Clear filtrate obtained from this system improves liquid redox process performance by recycling useful catalyst free from sulphur.
Therefore the inventors of the present invention have felt the need of process and system whereby all the above said drawbacks can be overcome or minimized; hence the inventors have invented a process for removal of sulphur and a hybrid membrane filter system.
OBJECTS OF THE INVENTION:
The principal object of the present invention is to separate solid sulphur particles from slurry
with low concentration of sulphur particles obtained from a liquid redox type sulphur plant.
Further object of the invention is to provide sulphur of at least 99.5 wt% (on dry basis) purity.
Another object is to provide a membrane system for increase of sulphur particles population
in slurry and further increase of sulphur population in pressure filter.
Yet another object is to minimize loss of water soluble catalyst present in slurry.
Yet another object is to provide filtrate free from solid sulphur particles.
Still another object of the invention is to provide a low cost process for reducing sulphur
particle content in filtrate returned to process of the liquid redox sulphur plant.
SUMMARY OF THE INVENTION:
Accordingly the present invention relates to a process for separation of sulphur particle from slurry with low to very low sulphur concentration through membrane-filter hybrid system. Said process involves separation of sulphur from slurry obtained from liquid redox based sulphur plant. The liquid redox based sulphur plant comprises a process, air blower, heat exchanger and membrane-filter hybrid system. The plant processes acid gas coming from amine regenerator overhead of gas sweetening unit. The acid gas contains 0.1-1% H2S by volume. The liquid redox process uses metal based catalyst in aqueous solution and converts hydrogen sulphide present in the acid gas directly to elementary sulphur. The sulphur
produced is separated from the aqueous catalyst solution by membrane-filter hybrid system of the present invention.
In one embodiment the liquid redox based sulphur plant is provided with a membrane-filter hybrid system.
In another embodiment the colour and purity of sulphur separated from slurry is improved. In another embodiment population of sulphur particle in slurry have increased by membrane system.
In another embodiment loss of water soluble catalyst present in slurry have minimized. In another embodiment filtrate is free from solid sulphur particles
In another embodiment of the invention, membrane -filter hybrid system consists of cylindrical pressure vessel having porous membrane elements and vertical pressure filter. In another embodiment of the invention, ceramic porous membrane is used. In another embodiment of the invention, pore size of ceramic membrane is 0.1-1 micron. In another embodiment of the invention, concentration of slurry from 0.1 %-5 %( wt) to 10-15 %(wt) is achieved through a membrane system.
In yet another embodiment of the invention, vertical pressure filter having poly propylene filter cloth is used.
In another embodiment of the invention, pore size of cloth is 5-8 micron. In another embodiment of the invention, 10-15 %( wt) slurry is squeezed in vertical pressure filter and sulphur cake is formed.
In another embodiment of the invention, sulphur cake formed is having at least 99.5% purity on dry basis and water content is less than 10 %( wt).
In another embodiment of the invention, implementation of process for running liquid redox type sulphur plant takes minimum modification and minimum shutdown period.
BRIEF DESCRIPTION OF FIGURES:
Further aspects and advantages of the present invention will be readily understood from the following detailed description with reference to the accompanying figures. Reference numerals have been used to refer to identical or functionally similar elements. The figures together with a detailed description given below are incorporated in and thus form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present invention wherein: Figure 1 illustrates a schematic representation of experimental setup Figure 2A illustrates graphical representation of variation of sulphur concentration with time
Figure 2B illustrates graphical representation of variation of filtrate flow from membrane clarifier with time
Skilled artisans will appreciate that the elements in the figures are illustrated for simplicity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION:
While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of examples in the figures 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 as defined by the appended claims.
The present invention provides a process for separation of solid sulphur particles from the slurry through Membrane-filter hybrid system. The slurry herein means a heterogeneous mixture of solid sulphur particles and water soluble catalyst. The slurry is obtained from a liquid redox process. In the liquid redox process, H2S in the acid gas is converted to elementary sulphur in presence of metal liquid redox catalyst present in the aqueous solution. Sulphur formed due to liquid redox reaction of H2S is in the aqueous solution of the catalyst in suspended form. This process separates sulphur particles from the slurry without any heat requirement and gives complete separation of sulphur and the aqueous solution free from solid sulphur particles as filtrate.
Accordingly, the figures are showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
The term "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such set up or device. In other words, one or more elements in a system or system proceeded by "comprises...a" does not, without more constrains, preclude the existence of other elements or additional elements in the system or system.
The term "Use" here referred, is also meant to cover re-use, in context of the present
invention.
"Gauge (g) pressure" is zero referenced against ambient air pressure, so it is equal to absolute
pressure minus atmospheric pressure.
The process according to the present invention will now be described in more detail with
reference to the accompanying Figures 1-2.
The process is demonstrated by using a laboratory scale experimental setup as shown in
Figure 1. The setup comprises feed pump (101), membrane clarifier(102) , graduated feed
vessel(103) , graduated DM water vessel (104), graduated filtrate collection vessel-I(105) ,
pressure filter( 106) and graduated filtrate collection vessel-II (107). Experiment is done at
ambient temperature.
The slurry for experiment is prepared by mixing DM water with concentrated slurry. The
concentrated slurry is obtained from the liquid redox process of one commercial sulphur
plant. The sulphur plant is treating acid gas in the liquid redox process in presence of water
soluble catalyst and H2S in the acid gas is converted to elementary sulphur. The sulphur
particle concentration in the concentrated slurry is 16% by volume.
The slurry with different sulphur concentration (0.1%-5%wt) is prepared in the graduated
feed vessel (103). The slurry is pumped to membrane clarifier (102) at pressure by using feed
pump (101). The membrane clarifier consists of one ceramic porous membrane called as
membrane element. The membrane element is housed in a pressure vessel. The membrane
element comprise with 19 channel of diameter 30 mm each. The length of the membrane
element is 1016 mm.
The slurry passes through the channels of each membrane element at a constant flow of 60.8
LPM. The liquid of the slurry passes through the pores of the membrane and is accumulated
in the outside of the membrane element as filtrate. The slurry is concentrated due to removal
of liquid and is stored in the graduated feed vessel (103).
Continuous recycle of the slurry increase the concentration of sulphur particles in the slurry
and at end of the run, the concentration becomes 10%-30% by weight. The pressure
difference across the membrane is 1.68 kg/cm2. Even though thin range of sulphur
concentration can be achieved, but at this higher concentration membrane channels are prone
to blockage. Hence, the appropriate concentration of sulphur particles in the slurry should be
around 10-15 % by weight.
The clear filtrate from to membrane clarifier (102) is stored into graduate filtrate collection
vessel -1 (105) by gravity at the rate of 1.67 LPM. There is no sulphur particle in the filtrate.
The concentrated slurry obtained after several recycle of slurry through the membrane clarifier is sent to pressure filter (106) by gravity. The pressure filter consists of pump to spray the slurry over one plate filter. The filter cloth made of polypropylene is used in the plate filter. The pore size of the cloth is 5-8 microns. Feed slurry for pressure filter is prepared at different concentrations .The slurry is sprayed over the filter cloth and the liquid passed through the cloth. The solid sulphur cake is formed on the filter cloth. The clear solution from the other side of the filter cloth is collected in the graduated filtrate collection vessel -II (107) by gravity.
The solid sulphur cake so formed on the filter cloth is pressurized at 0.5-4 kg/cm2g by using inbuilt hydraulics pressure system. The solution is again collected in graduated filtrate collection vessel -II (107) by gravity. DM water is used to wash the cake and is sprayed over the cake by using a pump. The quantity of DM water to be used is -25% of the quantity of slurry treated in the pressure filter. The washed filtrate is collected again in graduated filtrate collection vessel -II (107). The cake is finally squeezed at a pressure of 15.31 kg/cm2g (16 bar) and is discharged from the cloth manually. The sulphur cake has thickness of 25-35 mm. The cake is analyzed which shows water content 8-10% by weight and purity of sulphur at least 99.5% by weight on dry basis.
DM water is also used to backwash the membrane of membrane clarifier (102). DM water in pumped into the shell side i.e., the outside of the membrane element at a pressure of 1-3 kg/cm2. The solid sulphur deposited on the membrane element is washed off and collected in the graduated feed vessel (103). The cleaned membrane is used for next set of experiment. The invention is now described by way of the following non-limiting example: Refer now to Figure 2A, which illustrates variation of concentration of concentrate slurry coming out from membrane clarifier with time. It shows requirement of time to reach a specific concentration of concentrate slurry (equibilirium stage). If the initial concentration of feed slurry to the membrane is very low, it takes more time to reach the equilibrium. (Table 1-4).
Refer now to Figure 2B which illustrates variation of filtrate flow from membrane clarifier with time. It shows varaition in filtrate flow from membrane clarifier is almost low and average filtrate flow is considered for design of membrane clarifier. Constant filtrate flow indicates smooth operation of membrane clarifier (Table 1-4) Example 1:
A new commercial sulphur plant based on liquid redox process is designed with membrane-filter hybrid system in the downstream of liquid redox process for separation of solid sulphur

particles from the slurry. The sulphur plant treats 9500 Nm3/hr acid gas containing 1000-1500 ppmv H2S in one process. The sulphur plant has two such processes to generate sulphur from H2S by using metal catalyst in the aqueous solution. The acid gas after treatment in the process is released to atmosphere and contains less than 10 ppmv H2S. The sulphur produced in the process is suspended in the solution. The solution containing 0.1 -5% (wt) sulphur is taken to the membrane-filter hybrid system from the process bottom. A pump is used to delivery slurry to the said system and a dedicated recycle pump is used to circulate the slurry through the membrane clarifier for concentrating the slurry to 10% by weight in respect to sulphur concentration. The clear solution of the slurry comes out from the filtrate side of the membrane clarifier and is stored in the filtrate vessel by gravity. The pressure difference across the membrane element is kept at 1.68 Kg/cm2.
The concentrated slurry is collected either in a slurry tank or fed to pressure filter directly at a pressure of 2 Kg/cm2g.
The high slurry flow is maintained in the channels of membrane filter so that the solid sulphur particle should not settle on the surface of the membrane. The flow in one membrane channel is generally maintained in 3-3.5LPM. The pressure drop along the membrane is 1-3 Kg/cm2.
The concentrated slurry from membrane clarifier is thereafter fed to the pressure filter which is operated in batch mode. The sulphur cake is formed on the filter cloth of the filter, washed and squeezed before discharging in the sulphur yard. DM water is used for washing and air is used to push the sulphur cake from the filter cloth to sulphur yard. Example 2:
(a) Feed : Sulphur slurry
Feed Concentration: 1.17 % wt sulphur Feed: 110.73 Kg
(b) Apparatus: Hybrid Membrane - Pressure filter system
Details of Membrane clarifier part of the apparatus
Membrane element: Porous ceramic membrane
No. of membrane element: 1
Pore size: 0.2 micron
No. of channels in a membrane element: 19
Channel diameter: 30 mm
Channel length: 1016 mm
Details of Pressure- filter part of the apparatus
Filter cloth: Poly propylene Pore size: 5-8 micron (c) Operating conditions (i) Membrane clarifier Feed pressure: upstream 1.88 Kg/cm2g Concentrate slurry pressure: downstream 0.75 Kg/cm2g Filtrate slurry pressure: 0 Kg/cm2g Temperature: 33-35°C (ii) Pressure filter
Feed pressure: upstream: 0 Kg/cm2g Ist squeezing pressure: 1.6 Kg/cm2g IInd squeezing pressure: 15.3 Kg/cm2g Temperature: 33-35° C RESULTS
Results of membrane clarifier (Table: 2):-
Filtrate collected: 99.32 Kg
Percent of sulphur in filtrate: nil
Concentrate sulphur slurry: 11.41 Kg
Concentration of sulphur in sulphur slurry: 11.36% wt.
Results of pressure filter (Table: 5), Set 1:-
Feed: 11.41 Kg
Sulphur % weight: 11.36%
Filtrate removed: 8.078Kg
After 1st squeezing: 1.664 Kg
DM water used: 1.5 Kg
Wash liquid removed: 0.6 Kg
After 2nd squeezing: 1.2 Kg
Wet cake quantity: 1.371Kg
Dry cake quantity: 1.3 Kg
Percentage of moisture: 5.17 % weight
Purity of sulphur: >99.5% weight
The process and the system disclosed in the present invention provides following
advantages:
• Trouble free operation: Membrane filter hybrid system makes the operation of sulphur plant smooth and trouble free by providing solid sulphur free filtrate recycle to the liquid redox process.
• Membrane filter hybrid system produces good quality sulphur which has purity at least 99.5% on dry basis and is easily deposable.
• Membrane filter hybrid system is applicable for separation of sulphur particle from slurry containing very low sulphur (0.1-5 % by weight).
• Membrane filter hybrid system has high sulphur removal efficiency (at least 99.5 %).
• Membrane filter hybrid system is an energy efficient process and does not need any heat energy.
• Membrane filter hybrid system can be skid mounted and needs less plant area.
• Membrane filter hybrid system can be operated either in batch or continuous mode.
• One membrane filter hybrid system can be integrated with number of sulphur plants and reduce the cost of the sulphur plant.
Table 1: Experimental Data for Slurry Concentration 0.83% by weight from Membrane Clarifier
Feed Solution at startup: 51.74 Kg Density of Filtrate: 1040 Kg/m3 Length of Membrane Element: 1016 mm

(Table Removed)
Table 2: Experimental Data for Slurry Concentration 1.17% by weight from Membrane Clarifier
Feed Solution at startup: 110.73 Kg Density of Filtrate: 1040 Kg/m3 Length of Membrane Element: 1016 mm

(Table Removed)
Table 3: Experimental Data for Slurry Concentration 5.58% by weight from Membrane Clarifier
Feed Solution at startup: 51.62 Kg Density of Filtrate: 1040 Kg/m3 Length of Membrane Element: 1016 mm

(Table Removed)
Table 4: Experimental Data for Slurry Concentration 13.32% by weight from Membrane Clarifier
Feed Solution at startup: 108 .08 Kg Density of Filtrate: 1040 Kg/m3 Length of Membrane Element: 1016 mm

(Table Removed)
Table 5: Experimental Data from Pressure Filter
Feed of each set of experiment is prepared separately.
(Table Removed)

We claim:
1. A process of removing sulphur particles and water soluble catalyst separately from
a slurry obtained from liquid redox process, comprising the steps of:
(a) treating the slurry obtained from liquid redox process having concentration of
0.1 to 5.0 wt% sulphur in a membrane clarifier to obtain a concentrated slurry
comprising 10-15 wt% sulphur and a filtrate comprising substantially pure water
soluble catalyst; and
(b) subjecting the concentrated slurry thus obtained in step (a) to a pressure
filtration process to obtain sulphur cake and a filtrate comprising substantially
pure water soluble catalyst, the said sulphur cake comprising at least 90 wt%
sulphur and less than 10 wt% of water, the sulphur having a purity of at least 99.5
wt% on dry basis.
2. The process as claimed in claim 1, wherein the filtrate obtained in steps (a) and (b) are available for use in a liquid redox process.
3. The process as claimed in claim 1, wherein the slurry comprises of heterogeneous mixture of sulphur and water soluble catalyst.
4. The process as claimed in claim 1, wherein the membrane clarifier consists of porous ceramic membrane.
5. The process as claimed in claim 4, wherein the porous ceramic membrane has pore size of 0.1-1.0 micron.
6. The process as claimed in claim 1, wherein pressure in the membrane clarifier ranges from 2-10 Kg/cm2g.
7. The process as claimed in claim 1, wherein pressure drop along the membrane clarifier varies from 1.0-2.0 Kg/cm2.
8. The process as claimed in claim 1, wherein the filtrate from the membrane clarifier is substantially devoid of sulphur particles.
9. The process as claimed in claim 1, wherein the cake comprises of 5-10 wt % water.
10. The process as claimed in claim 1 is carried out in continuous or batch operation.
11. The process as claimed in claim 1 is carried out at ambient temperature.
12. The process as claimed in claim 1, wherein the pressure filter consists of filter cloth; the said filter cloth is made of polypropylene.
13. The process as claimed in claim 12, wherein the pore size of the filter cloth is about 5-8 micron.
14. A hybrid separation system for removing sulphur particles and water soluble catalyst separately from a slurry obtained from liquid redox process, comprising:
(a) a membrane clarifier arranged so as to receive as input the slurry obtained
from liquid redox process having concentration of 0.1 to 5.0 wt% sulphur and
provide as a first output a concentrated slurry comprising 10-15 wt% sulphur and
as a second output a filtrate comprising substantially pure water soluble catalyst;
and
(b) a pressure filter arranged to receive the concentrated slurry from the
membrane clarifier and to provide sulphur cake as a first output and a filtrate
comprising substantially pure water soluble catalyst as a second output, the said
sulphur cake comprising at least 90 wt% sulphur and less than 10 wt% of water,
the sulphur having a purity of at least 99.5 wt% on dry basis.
15. The hybrid separation system as claimed in claim 14 wherein, the membrane clarifier consists of ceramic membrane.
16. The hybrid separation system as claimed in 15, wherein the porous ceramic membrane has pore size of 0.1-1.0 micron.
17. The hybrid separation system as claimed in claim 14, wherein the pressure filter consists of filter cloth; the said filter cloth is made of polypropylene.
18. The hybrid separation system as claimed in claim 17, wherein the pore size of the filter cloth is about 5-8 micron.
19. The hybrid separation system as claimed in claim 14 is operated at ambient temperature.
20. The hybrid separation system as claimed in claim 14 is operated in continuous or batch mode.