DESC:Field of the invention:
The present invention relates to a method for manufacture of carbon nanotubes and a system therefor, and more particularly the present invention relates to a system and method for producing multiwall carbon nanotubes utilizing a multistage reactor.
Background of the invention:
Carbon nanotube (CNT) is unique material with high electrical conductivity, thermal conductivity, high tensile strength and light weight. Due to its wide variety of properties, it is being considered for large number of applications. This includes light weight polymer composites, energy storage including Lithium-ion batteries, EMF Shielding, tyre manufacturing, cement industries and other electronic industry applications include transistors, flat panel displays etc.
Carbon nanotubes fall into two classes: single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). Despite the obvious commonality, SWCNTs and MWCNTs have significantly different physical properties from each other because of their structural differences. The single-walled carbon nanotubes can be described as graphene sheets seamlessly rolled up to form hollow cylinders. Unlike a single-walled nanotube, a multi-walled carbon nanotube can be viewed as a concentric arrangement of SWCNTs, i.e. consisting of multiple layers of graphene rolled up seamlessly into a tube shape.
Multiwall carbon nanotube is having density of less than 0.15 g/cc. Multiwall carbon nanotube material is particularly interesting because it conducts electricity and is also thermal conductor. This is an ideal characteristic for battery and fuel cell electrodes.
To produce carbon nanotubes (CNTs), a high carbon source concentration and high carbon yield is essential. The commonly used precursors for carbon nanotubes are hydrocarbon gases such as acetylene, ethylene, methane and petroleum derived feedstocks etc.
MWCNTs are manufactured by various methods including arc discharge, laser ablation, chemical vapour deposition (CVD), catalytic CVD etc using various feedstocks. The feedstocks include petroleum feedstocks, carbon monoxide, methane, acetylene, biogas etc. The catalysts employed generally are transition metal oxides. The products are generally CNT and hydrogen if feedstock is based on hydrocarbon. The laser ablation and arc discharge methods normally require high amount of energy for CNTs formation. This energy is normally provided through laser or plasma arc discharge in order to reorganize the carbon atoms into CNTs. In these processes, the required temperature sometimes even exceed 3000°C which is advantageous for very fine crystallization of the CNTs. Therefore, the final product is always formed with good alignment of graphite. However, the essential needs of these systems are the continuous graphite target replacement and sensitive vacuum conditions. In contrast, the Chemical Vapor Deposition (CVD) has proven itself as a favored method for mass production of CNTs (Andrews et al., 1999; Colomer et al., 2000; Dasgupta et al., 2008). For large scale manufacturing of CNT, CVD and catalytic CVD are the methods used. With this technique, the carbon is obtained from hydrocarbon or other carbon bearing precursors and deposited onto a substrate in the presence of a catalyst. However, in chemical vapor deposition method, MWCNTs are often riddled with defects compared with SWCNTs, for which the quality is better controlled.
In the catalytic CVD method, fluidized beds are used for the production of multiwall CNT. However, the yield of MWCNT per gram of catalyst is lower because continuous injection of catalyst is required in the fluidized bed reactor demanding higher proportion of catalyst required. In a fixed bed reactor, the catalyst is loaded only once and therefore MWCNT grow over the catalyst for longer duration producing higher yield. In addition, the purity of MWCNT produced using fluidized bed reactors is also low. Since the catalyst is loaded continuously, the resulting MWCNT is riddled with lots of catalyst which has to be removed to produce high pure CNT, therefore it is challenging to get above 99%wt pure grade MWCNT that is required for electrode. On the contrary, in the fixed bed reactor, MWCNT produced is having less catalyst and therefore purity is higher.
In addition, there is a minimum threshold residence time required for the feedstock in MWCNT production. Controlling and maintaining higher residence time is challenging in fluidized bed reactors as compared to that in fixed bed. Thus, achieving higher residence time requires lower space velocity/flow rate for the feedstock which is challenging in fluidized reactor.
In the existing methods for producing MWCNT from hydrocarbon sources mainly catalytic chemical vapour deposition (CCVD) is used in a single stage fluidized or fixed bed reactor. In single stage fixed bed reactor, due to limited amount of catalyst and due to catalyst deactivation, the MWCNT yield is normally below 50% and therefore, the process generally is less efficient. Due to catalyst deactivation in fixed bed reactor, the quality of CNT produced deteriorates faster. The fixed bed reactor can be operated for limited period of time hence productivity is lost. In the multistage fluidized reactor, fluidization of catalyst and to maintain low space velocity to CNT growth is challenging, because fluidization and continuous injection requires minimum threshold velocity. Moreover, in the multistage fluidization reactor, continuous injection of catalyst at minimum space velocity results in low purity of CNT produced. This also leads to requirement of longer purification procedure to produce application ready CNT.
Accordingly, there exists a need to provide an economic, efficient, single step process for producing multiwalled carbon nanotubes that will overcome the above discussed drawbacks in the prior art.
Objects of the invention:
An object of the present invention is to provide a single step, high yield, efficient and economic process for producing multi wall carbon nanotube from hydrocarbon sources by employing a system utilizing a fixed bed reactor.
Another object of the present invention is to provide a process for producing multi wall carbon nanotube from hydrocarbon sources including petroleum gases such as, methane, ethane, propane, butane or higher order liquid feedstocks such as naphtha, kerosene or methane derived from biogas or any other hydrocarbon sources.
Still another object of the present invention is to provide a system and process for producing multi wall carbon nanotube from hydrocarbon sources with or without using carrier gases such as nitrogen, argon, helium, carbon-di-oxide etc.
Yet another object of the present invention is to provide a reactor that without any de-agglomerator or agitator for MWCNT agglomerate cutting.
Yet another object of the present invention is to provide a process for producing multi wall carbon nanotube having density less than 0.15g/cc and hydrogen gas.
Yet another object of the present invention is to produce MWCNT with high yield of more than 50wt% with density less than 0.15g/cc
Summary of the invention
The present invention provides a system and a method for manufacture of multiwall carbon nanotubes (MWCNT). The system comprises of a feedstock supplier, a multi-stage reactor and a membrane separator unit. The hydrocarbon feedstock is stored under pressure in the feedstock supplier from where it is supplied to the multi-stage reactor. The multi-stage reactor is having a base unit and a plurality of fixed-bed units arranged vertically on the base unit. Each fixed-bed unit is provided with a perforated base with a sliding gate. A catalytic fixed bed is held on the perforated base, wherein the catalytic fixed bed is a mixed transition metal oxide catalyst held on a catalyst support selected from titania, ceria, zirconia, alumina, silica and magnesia. The multi-stage reactor is fitted with an arrangement for heating the catalytic fixed bed. The perforated base enables the hydrocarbon feedstock and product gases to pass through the catalytic fixed bed to the next upper fixed-bed unit and the sliding gate enables the MWCNT produced in the process to evacuate to the next lower fixed-bed unit. The membrane separator unit is configured for separating the unreacted hydrocarbon feedstock and the product gases, received from the multi-stage reactor. The method of manufacture of MWCNT in a first step involves loading a fresh catalytic fixed bed on the perforated base of a plurality of fixed-bed units of a multi-stage reactor and activating the catalyst by heating the fresh catalytic fixed bed to a temperature ranging from 500ºC to 800ºC in a flow of hydrogen for a time period ranging from 4hrs to 12hrs. In next step, the method involves pre-heating the hydrocarbon feedstock and passing the pre-heated hydrocarbon feedstock through the plurality of fixed-bed units for a time period ranging from 6 hrs to 36 hrs while maintaining the temperature in a range from 500ºC to 900ºC. The hydrocarbon feedstock is passed through the multi-stage reactor with or without any carrier gas. The unreacted hydrocarbon feedstock passes through the perforated base and through the catalytic fixed bed of first fixed bed unit, forming MWCNT and hydrogen gas. The unreacted hydrocarbon feedstock and the product hydrogen gas pass to the next upper fixed-bed unit through a perforated base of each fixed bed and MWCNT produced in the process is evacuated to the next lower fixed-bed unit by opening a sliding gate provided on the perforated base. In next step, the method involves separating the unreacted hydrocarbon feedstock from hydrogen gas in the membrane separator unit. The unreacted hydrocarbon feedstock is supplied to the feedstock storage and recycled in the process, while the product MWCNT is evacuated from the plurality of fixed-bed units in the base unit.
Brief description of the drawings:
The objects and advantages of the present invention will become apparent when the disclosure is read in conjunction with the following figures, wherein
Figure 1 shows a system and a schematic process flow for producing multiwall carbon nanotube, in accordance with the present invention.
Detailed description of the embodiments:
The foregoing objects of the invention are accomplished and the problems and shortcomings associated with prior art techniques and approaches are overcome by the present invention described in the present embodiments.
The present invention deals with the conversion of hydrocarbon sources to multiwall carbon nanotube, including petroleum gases such as, methane, ethane, propane, butane or higher order liquid feedstocks such as naphtha, kerosene or methane derived from biogas or any other hydrocarbon sources. The present invention also deals with a process of conversion of hydrocarbon to multiwall carbon nanotube, with or without using carrier gases such as nitrogen, argon, helium, carbon-di-oxide etc. In fixed bed multistage reactor of the present invention, the issue of residence time is managed since space velocity of feedstock can be kept as low as possible. The yield of MWCNT produced is more due to multiple stages and each stage produces MWCNT. The MWCNT produced contains less catalyst since the MWCNT produced per gram of catalyst is higher. The present invention provides designing multistage fixed bed reactor, wherein the catalyst is loaded through perforated sliding gates in each stage and the solid MWCNT along with the catalyst is evacuated after completion of the reaction through bottom of the reactor after opening each sliding gate.
The conversion process involves catalytic chemical vapour deposition (CCVD) using a multistage reactor to maximize the yield of multiwall CNT and the bye-product is mainly hydrogen. The multistage reactor claimed in the present invention is fixed bed reactor. The yield of MWCNT is more than 50% because of the multistage reactor of the present invention. The process produces MWCNT and hydrogen. The MWCNT produced is having density of less than 0.15 g/cc, typically less than 0.10g/cc.
Referring to figure 1, a system (100) and a method for manufacture of multiwall carbon nanotube from hydrocarbon sources is disclosed. The present invention in one aspect provides a system (100) for manufacture of multiwall carbon nanotube from hydrocarbon sources
The system (100) comprises a feedstock supplier (10), a membrane separator unit (20) and a multi-stage reactor (30).
The feedstock storage (10) is for suitably storing the hydrocarbon feedstock. The hydrocarbon feedstock is selected from petroleum feedstocks and hydrocarbon sources such as methane, ethylene, propylene, acetylene, liquified petroleum gases (LPG) and biogas. The hydrocarbon feedstock is stored in high pressure tanks or cylinder hubs. The feedstock storage (10) is provided with an electrical heating arrangement (not shown) for pre heating the hydrocarbon feedstock. The hydrocarbon feedstock is pre-heated at a temperature ranging from 400ºC to 500ºC before entering the muti-stage reactor (30).
The multi-stage reactor (30) is a fixed bed reactor having a base unit (B) and a plurality of fixed-bed units (S1, S2, S3, …., Sn) arranged vertically on the base unit (B). The number of fixed-bed units (S1, S2, S3, …., Sn) generally ranges from 2 to 10. Each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, .., Sn) is provided with a perforated sliding gate (40). Each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, …., Sn) is loaded with a catalytic fixed bed (25) by opening the perforated sliding gate (40). Apart from holding the catalytic fixed bed thereon, the perforated sliding gate (40) enables fluid communication between the consecutive fixed bed units. The hydrocarbon feedstock along with product gases passes from the base unit (B) to a topmost fixed bed unit (Sn) through the perforations of the perforated sliding gate (40) of the plurality of fixed-bed units (S1, S2, S3, …., Sn). Since each stage is loaded with the catalyst (catalytic fixed bed), the reaction takes place at each stage to produce multiwall carbon nanotubes. After the reaction, the MWCNT is evacuated by opening each of the perforated sliding gate (40) and collected in the base unit (B). The hydrocarbon feedstock is fed to the multi-stage reactor (30) using control console suitably equipped with mass flow meters (not shown) during the reaction.
In an embodiment, the multi-stage reactor (30) is having cylindrical shape. In another embodiment, the multi-stage reactor (30) is having a conical frustum shape with diameter narrowing down towards a top thereof. In an embodiment, the diameter of the topmost fixed bed unit (Sn) is up to 50% lower than the first fixed bed unit (S1). Since the amount of feedstock hydrocarbon entering each stage upward decreases from bottom stage to top stage, the amount of catalyst required also decreases accordingly. The arrangement of the plurality of fixed bed units (S1, S2, S3, …., Sn) in conical frustum shape ensures effective utilization of the catalytic fixed bed.
The multi-stage reactor (30) is provided with a heating arrangement (not shown) for heating the catalytic fixed bed. The multi-stage reactor heating arrangement is an electrical heating arrangement or a molten salt heating arrangement.
In an embodiment, the catalytic fixed bed (25) contains a transition metal oxide catalyst supported on a catalyst support. The catalyst support is selected from titania, ceria, zirconia, alumina, silica and magnesia. The transition metal oxide catalyst is a combination of transition metal oxides selected from iron oxide, cobalt oxide, manganese oxide and Nickel oxide.
The membrane separator unit (20) receives the hydrogen gas produced in the multistage reactor (30) along with the unreacted hydrocarbon feedstock, from the topmost fixed bed unit (Sn). The hydrogen gas along with the unreacted hydrocarbon feedstock is passed through the membrane separator (20) for separating hydrogen gas from the unreacted hydrocarbon. The unreacted hydrocarbon is then recycled for the production of MWCNT.
Referring to figure 1, a schematic process flow diagram of a method for manufacture of multiwall carbon nanotube from hydrocarbon sources is disclosed. The present invention in another aspect provides a method for manufacture of multiwall carbon nanotubes from hydrocarbon sources. The method is carried out using a system (100) comprising a feedstock supplier (10), a membrane separator unit (20) and a multi-stage reactor (30). The multi-stage reactor (30) is a fixed bed reactor having a base unit (B) and a plurality of fixed-bed units (S1, S2, S3, …., Sn) arranged vertically on the base unit (B). The number of fixed-bed units (S1, S2, S3, …., Sn) generally ranges from 2 to 10. Each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, …., Sn) is provided with a perforated base (40) with a sliding gate for loading and unloading of a catalytic bed and for enabling fluid communication between the consecutive fixed bed units of the multi-stage reactor (30).
In first step, the method involves loading a fresh catalytic fixed bed (25) in each of the fixed bed units of the multi stage fixed bed reactor (30) by opening the sliding gate on the perforated base (40).
In an embodiment, the catalytic fixed bed (25) is a catalyst supported on a catalyst support. The catalyst is a mixture of transition metal oxides selected from iron oxide, cobalt oxide, manganese oxide and Nickel oxide. In an embodiment, the catalyst support is any one selected from titania, ceria, zirconia, alumina, silica and magnesia.
In next step, the method involves activating the catalyst by heating the fresh catalytic fixed bed (25) to a temperature ranging from 500ºC to 800ºC in a flow of hydrogen, for a time period ranging from 4hrs to 12hrs.
In next step, the method involves injecting a hydrocarbon feedstock from a feedstock supplier (10) to a base unit (B) of the multi-stage reactor (30) and passing the hydrocarbon feedstock through the multistage reactor (30) for a time period ranging from 6hrs to 36 hrs while maintaining the temperature in the multi-stage reactor (30) in a range from 500ºC to 900ºC. In an embodiment, the hydrocarbon feedstock is fed to the reactor with or without carrier gas selected from Nitrogen, carbon dioxide and inert gases like argon etc. The carrier gas has to be inert and commonly available (cheaper). Therefore, nitrogen is preferred in the case of naphtha as feedstock. However, for biogas as feedstock, since carbon dioxide is inherently present and inert as well therefore, the additional external carrier gas is not required.
The hydrocarbon feedstock received in the base unit (B) enters a first fixed bed unit (S1) to through the perforated base (40) thereof wherein a portion of the hydrocarbon feedstock in the presents of catalyst reacts to produce multiwall carbon nanotubes and product gases. The product gases normally contain hydrogen gas. The unreacted hydrocarbon feedstock and the product gases pass through the perforated base to the next fixed bed unit thereon.
In the next step the method involves receiving the mixture of the unreacted hydrocarbon feedstock and the product gases in a membrane separator unit (20) from the topmost fixed bed unit (Sn) and separating the unreacted hydrocarbon feedstock from the product gases (preferably hydrogen).
The unreacted hydrocarbon feedstock is directed to the feedstock supplier and recycled for production of MWCNT. The MWCNT formed in each of the fixed bed units is evacuated by opening the sliding gate of each of the fixed bed unit and collected in the base unit (B).
The MWCNT produced in the process is having density of 0.15g/cc and typically 0.10g/cc or less. The process yields MWCNT in the range from 50%wt to 90%wt. The process of manufacture of MWCNT can be continued for a prolonged period of time without the need to change the catalytic fixed bed. The multistage reactor (30) is more efficient and produces higher yield of MWCNT in the range from 50%wt to 90%wt. The MWCNT yield in a single stage fixed bed reactor of the prior art is less than 50%.
The multistage reactor (30) does not employ any de-agglomerator or agitator for MWCNT agglomerate cutting, thereby making the reactor mechanically less complex and less energy consuming.
The purity of MWCNT produced by the method of the present invention is over 80%. The MWCNT produced are required to be purified externally to produce more than 99%wt MWCNT. It is less challenging to purify and produce more than 99%wt MWCNT for electrodes suitable for batteries, fuel cell and electrolysers.
The process of production of MWCNT of the present invention is operated with or without any external carrier gas.
Experimental examples:
The process of preparation of multiwall carbon nanotube from hydrocarbon sources according to the present invention is further explained by way of illustrative example. The example is given by way of illustration and should not be construed to limit the scope of present invention.
Example 1
The multi-stage reactor (30) with three fixed bed units (S1, S2 and S3) was filled with 10 gm of Ni-Mn oxide catalyst supported on aluminum oxide, in each fixed bed unit. The metal oxide catalyst was activated by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The biogas having methane composition 60%wt and the rest being carbon dioxide, was fed to the reactor at 750ºC for 24hrs. During the reaction the product gas hydrogen and the unconverted methane was collected and analyzed. After the reaction time, the reactor was cooled, MWCNT was collected from the bottom of the reactor after opening the sliding gate, weighed and the conversion and the yield was calculated. The MWCNT was purified to remove the catalyst particles and the density was measured.
Yield of the MWCNT = 75.4 wt%
Density of the purified MWCNT = 0.05gm/cc.
Example 2
The multi-stage reactor (30) with five fixed bed units (S1, S2, S3, S4, S5) was filled with Ni-Mn oxide catalyst supported on aluminium oxide, 10gms in each fixed bed unit. The catalyst was activated by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The biogas having methane composition 60%wt and the rest being carbon dioxide, was fed to the reactor at 750ºC for 24hrs. During the reaction the product gas hydrogen and the unconverted methane was collected and analyzed. After the reaction time, the reactor was cooled, MWCNT was collected from the bottom of the reactor after opening the sliding gates, weighed and the conversion and the yield was calculated.
Yield of the MWCNT = 92wt%
Density of the purified MWCNT = 0.05gm/cc
Example 3
The multi-stage reactor (30) with three fixed bed units (S1, S2 and S3) was filled with Fe-Mn oxide catalyst supported on aluminium oxide, 10gms in each fixed bed unit. The metal oxide catalyst was activated by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The reactor was fed with naphtha along with nitrogen as carrier gas was fed to the reactor at 650ºC for 24hrs. During the reaction the product gas hydrogen and the unconverted naphtha was collected and analyzed. After the reaction time, the reactor was cooled, MWCNT was collected from the bottom of the reactor after opening the slide valves, weighed and the conversion and the yield was calculated. The MWCNT was purified to remove the catalyst particles and the density was measured.
Yield of the MWCNT = 72.5wt%
Density of the purified MWCNT = 0.14gm/cc.
Example 4
The multi-stage reactor (30) with five fixed bed units (S1, S2, S3, S4, S5) was filled with Fe-Mn oxide catalyst supported on aluminium oxide, 10gms in each fixed bed unit. The metal oxide catalyst was activated by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The reactor was fed with naphtha along with nitrogen as carrier gas was fed to the reactor at 650ºC for 24hrs. During the reaction the product gas hydrogen and the unconverted naphtha was collected and analyzed. After the reaction time, the reactor was cooled, MWCNT was collected from the bottom of the reactor after opening the slide valves, weighed and the conversion and the yield was calculated. The MWCNT was purified to remove the catalyst particles and the density was measured.
Yield of the MWCNT = 88.0wt%
Density of the purified MWCNT = 0.14gm/cc.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the scope of the claims of the present invention.
,CLAIMS:We claim
1. A system (100) for manufacture of multiwall carbon nanotubes (MWCNT), the system (100) comprising:
a feedstock supplier (10), the feedstock supplier (10) configured for storing a hydrocarbon feedstock under pressure, the feedstock supplier (10) fitted with a heating arrangement;
a multi-stage reactor (30), the multi-stage reactor (30) having a base unit (B) and a plurality of fixed-bed units (S1, S2, S3, …., Sn) arranged vertically on the base unit (B), each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, …., Sn) provided with a perforated base (40) with a sliding gate, the base unit (B) configured for receiving a controlled supply of the pre-heated hydrocarbon feedstock from the feedstock supplier (10), each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, …., Sn) holding a catalytic fixed bed (25) on the perforated base (40), wherein the perforated base (40) enables the hydrocarbon feedstock and product gases to pass there through to the next upper fixed-bed unit and the sliding gate enables the MWCNT produced in the process to evacuate to the next lower fixed-bed unit, the multi-stage reactor (30) fitted with a heating arrangement for heating the catalytic fixed bed of each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, …., Sn); and
a membrane separator unit (20), the membrane separator unit (20) receiving the mixture of the unreacted hydrocarbon feedstock and product gases from multi-stage reactor (30), the membrane separator unit (20) configured for separating the unreacted hydrocarbon feedstock from the product gases.
2. The system (100) as claimed in claim 1 wherein the number of fixed bed units from the plurality of fixed-bed units (S1, S2, S3, …., Sn) varies from 2 to 10.
3. The system (100) as claimed in claim 1 wherein the hydrocarbon feedstock is selected from petroleum feedstocks, methane, ethylene, propylene, acetylene, liquified petroleum gases (LPG) and biogas.
4. The system (100) as claimed in claim 1 wherein the hydrocarbon feedstock is biogas having methane composition in a range from 10%wt to 95%wt and carbon dioxide composition in a range from 5%wt to 90%wt
5. The system (100) as claimed in claim 1 wherein the catalytic fixed bed (25) contains a mixed transition metal oxide catalyst.
6. The system (100) as claimed in claim 1 wherein the catalytic fixed bed (25) contains a catalyst support selected from titania, ceria, zirconia, alumina, silica and magnesia.
7. A method for manufacture of multiwall carbon nanotubes (MWCNT), the method comprising the steps of:
loading a fresh catalytic fixed bed (25) on a perforated base (40) with a sliding gate, of a plurality of fixed-bed units (S1, S2, S3, …., Sn) of a multi-stage reactor (30),
activating a catalyst by heating the fresh catalytic fixed bed (25) to a temperature ranging from 500ºC to 800ºC in a flow of hydrogen for a time period ranging from 4hrs to 12hrs;
pre-heating a hydrocarbon feedstock in a feedstock supplier (10) to a temperature ranging from 400ºC to 500ºC and supplying the pre-heated hydrocarbon feedstock to a base unit (B) of the multi-stage reactor (30);
passing the preheated hydrocarbon feedstock through the plurality of fixed-bed units (S1, S2, S3, …., Sn) for a time period ranging from 6 hrs to 36 hrs while maintaining the temperature in a range from 500ºC to 900ºC thereby forming a MWCNT and product gases, wherein the unreacted hydrocarbon feedstock and the product gases pass to the next upper fixed-bed unit through a perforated base (40) of each fixed bed unit from the plurality of fixed-bed units (S1, S2, S3, …., Sn) and MWCNT produced in the process is evacuated to the next lower fixed-bed unit by opening a sliding gate provided on the perforated base (40);
receiving the mixture of the unreacted hydrocarbon feedstock and the product gases from the multi-stag reactor in a membrane separator unit (20) and separating the unreacted hydrocarbon feedstock from the product gases;
supplying the unreacted hydrocarbon feedstock back to the feedstock supplier (10); and
collecting the product MWCNT evacuated from the plurality of fixed-bed units (S1, S2, S3, …., Sn) in the base unit (B).
8. The method as claimed in claim 7 wherein the number of fixed bed units from the plurality of fixed-bed units (S1, S2, S3, …., Sn) varies from 2 to 10.
9. The method as claimed in claim 7 wherein the hydrocarbon feedstock is selected from petroleum feedstocks, methane, ethylene, propylene, acetylene, liquified petroleum gases (LPG) and biogas.
10. The method as claimed in claim 7 wherein the hydrocarbon feedstock is biogas having methane composition in a range from 10%wt to 95%wt and carbon dioxide composition in a range from 5%wt to 90%wt
11. The method as claimed in claim 7 wherein the hydrocarbon feedstock is fed to the multi-stage reactor (30) with or without a carrier gas.
12. The method as claimed in claim 11 wherein the carrier gas is a gas selected from nitrogen, carbon dioxide and an inert gas.
13. The method as claimed in claim 7 wherein the catalytic fixed bed (25) contains a mixed transition metal oxide catalyst.
14. The method as claimed in claim 13 wherein the mixed transition metal oxide catalyst is a selected from mixtures of iron oxide, cobalt oxide, manganese oxide and Nickel oxide.
15. The method as claimed in claim 7 wherein the catalytic fixed bed (25) contains a catalyst support selected from titania, ceria, zirconia, alumina, silica and magnesia.
16. The method as claimed in claim 7 wherein the unreacted hydrocarbon feedstock separated in the membrane separator unit (20) is fed to the feedstock supplier (10).