DESC:Field of the invention:
The present invention generally relates to a method of production of carbon nanotubes and more particularly the present invention relates to a method of producing multiwall carbon nanotubes aerogel using biogas.
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 including 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 aerogel is having density of less than 0.10 g/cc, typically less than 0.08g/cc. Multiwall carbon nanotube aerogel material is particularly interesting because it is composed of a dispersion of MWCNTs which leave a honeycomb structure with controllable porosity. More-so, the aerogel has a large surface area, 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 feedstocks including gases such as acetylene, ethylene, methane, etc. The rapid depletion of petroleum-based resources and its related global warming problems have progressively accelerated the research and development of efficient and sustainable catalytic processes for full utilization of renewable resources such as biogas. Biogas, primarily constituting CH4 and CO2 can be a promising high carbon source for the production of MWCNTs.
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 exceed 1500°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. Further, in the catalytic CVD method, carrier gas is one of the most important parameters for the synthesis of CNT. It helps in feedstock or reactant dispersion in the catalytic reaction with the catalyst particles in the reactor which in turn improves the catalyst utilization. The carrier gases (nitrogen, helium, argon, or carbon dioxide gases) and their flow rates are found to have significant effect both the yields and the characteristics of the produced MWCNTs.
The fluidized bed reactors are commonly used for the manufacturing of MWCNT. Although, the fluidized bed process is the most promising route for large scale production of MWCNTs but the fluidization of nano sized entities is still a great challenge.
In the existing methods for producing MWCNT aerogel from biogas using catalytic chemical vapour deposition (CCVD), a carrier gas is normally required along with the carbon source to produce MWCNT aerogel, which further adds to the manufacturing cost. Moreover, existing methods produce MWCNT aerogel having density more than 0.10g/cc, implying having higher CNT consumption in the applications when they are used, compared to the use of MWCNT aerogel having density less than 0.08g/cc.
Accordingly, there is a need to provide an economic, efficient, single step process for producing multiwalled carbon nanotubes directly from biogas without using any carrier, 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 process for producing multi wall carbon nanotube aerogel from renewable source such as, biogas.
Another object of the present invention is to provide a process for producing multi wall carbon nanotube aerogel from mixture of methane and carbon-di-oxide without using any external carrier component.
Another objective of the present invention is to employ non-stoichiometric mixed metal oxides as catalyst to produce MWCNT aerogel. Further, to employ non-stochiometric stabilizers to stabilize the catalyst for improved performance at high temperatures.
Still another object of the present invention is to provide a single step, high yield, efficient and economic process for producing multi wall carbon nanotube aerogel from biogas using MWCNT reactor preferably a fluidized or fixed bed reactor.
Yet another object of the present invention is to provide a process for producing multi wall carbon nanotube aerogel having density less than 0.08g/cc and typically 0.05g/cc.
Yet another object of the present invention is efficient utilization of biowaste generated biogas for producing carbon nanotube aerogel and hydrogen gas.
Yet another object of the present invention is to produce structurally similar and uniformly aligned MWCNT aerogel
Summary of the invention
The present invention discloses a method of for producing multiwall carbon nanotube (MWCNT) aerogel having density less than 0.10g/cc and typically 0.05g/cc. The method utilizes an optimized biogas having methane composition from 10%wt to 95%wt and carbon dioxide composition in the range 5%wt to 90%wt is used as a feedstock in a fixed bed reactor, for obtaining MWCNT aerogel. The method involves loading the MWCNT reactor with a metal oxide catalyst supported on a catalyst support along with a non-stoichiometric catalyst stabilizer. The non-stoichiometric metal oxide catalyst is prepared in-situ by heating the reactor at 600ºC for 6hrs in the presence of hydrogen. The optimized biogas feedstock is then injected into the MWCNT reactor for catalytic chemical vapour deposition, at a temperature ranging from 550ºC to 900ºC for a time period ranging from 6hrs to 36hrs thereby forming MWCNT aerogel at a bottom of the reactor and a product gas mixture containing hydrogen and unreacted biogas. The MWCNT aerogel typically having density below 0.10 g/cc is evacuated from bottom of the MWCNT reactor, while the product gas mixture is passed through a membrane separator for separating hydrogen gas from the unreacted biogas. The unreacted biogas is then recycled for production of MWCNT aerogel.
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 schematic process flow diagram of the method for producing multiwall carbon nanotube aerogel from a biological waste, in accordance with the present invention;
Figure 2 shows schematic process flow diagram of the method for producing multiwall carbon nanotube aerogel from biogas feedstock in a MWCNT reactor, in accordance with the present invention; and
Figure 3a and 3b show a typical Field Emission Scanning Electron Microscopy (FESEM) picture of MWCNT aerogel, in accordance with the exemplary embodiment of 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 biogas consisting of intrinsically present methane and carbon dioxide that are produced directly from bio or agricultural residues. The carrier gas, carbon dioxide being intrinsically present in the biogas and the feedstock, no extrinsic addition/ external carrier is required as practiced in the prior art. Carrier gas plays important role in spatial distribution of hydrocarbon reactant in the process of producing CNT and in the spatial polymeric growth thereof. In the prior art, external/extrinsic carrier gases example, nitrogen or any inert gas is employed along with hydrocarbon reactant to produce CNT as described above. In the present invention, the intrinsically present carbon dioxide acts as a carrier gas thereby eliminating the need for extrinsic carrier gas and making the process more economical. The intrinsically present carbon dioxide helps in dispersion of the reactant methane, to produce CNT aerogel and promotes polymeric growth to longer carbon chain. The conversion process involves catalytic chemical vapour deposition (CCVD) using a catalytic reactor. The catalytic reactor can be either a fluid bed reactor or a fixed bed reactor. In a preferred embodiment, the catalytic reactor is a fixed bed reactor to maximize the yield of multiwall CNT aerogel and the bye-product is mainly hydrogen. The yield of MWCNT is more than 25%wt. The MWCNT aerogel produced is having density less than 0.10 g/cc, typically 0.05g/cc.
Referring to figure 1 and 2, a schematic process flow diagram of a method for producing multiwall carbon nanotube aerogel from biogas is disclosed.
The method of the present invention for producing multiwall carbon nanotube (MWCNT) aerogel requires an optimized purified biogas feedstock having a methane composition ranging from 10%wt to 95%wt and a carbon dioxide composition ranging from 5%wt to 90%wt. The biogas is obtained by subjecting renewable sources such as agricultural residue, vegetable kitchen waste to microbial fermentation. The biogas consists of methane in a proportion ranging from 30 wt% to 70wt%, carbon dioxide in a proportion ranging from 20 wt% to 70wt%, hydrogen sulphide in a proportion ranging from 0 to 1wt% and ammonia in a proportion ranging from 0 to 1wt%.
The biogas is purified by selectively removing hydrogen sulphide and ammonia. Hydrogen sulphide is selectively removed using amine column, while ammonia is selectively removed using reactive adsorption with diluted sulphuric acid to produce a purified biogas. The purified biogas specifically consists of methane and carbon dioxide. The purified biogas is then optimized by selectively controlling the methane composition from 10%wt to 95%wt and carbon dioxide composition in the range 5%wt to 90%wt using an alkanolamine column, by controlling the residence time and/or alkanolamine concentration. In an embodiment the alkanolamine is selected from monoethanol amine, diethanol amine and/or triethanol amine.
The optimized biogas having methane composition from 10%wt to 95%wt and carbon dioxide composition in the range 5%wt to 90%wt is used as a feedstock in a reactor, for obtaining MWCNT aerogel. In an embodiment the MWCNT reactor is a fixed bed reactor.
The step-by-step method of for producing multiwall carbon nanotube (MWCNT) aerogel is now explained.
In first step, the method involves loading the MWCNT reactor with a metal oxide catalyst supported on a catalyst support along with a non-stoichiometric catalyst stabilizer.
In next step the method involves preparing the non-stoichiometric metal oxide catalyst by heating the reactor at 600ºC for 6hrs in presence of hydrogen. In an embodiment the non-stoichiometric metal oxide catalyst is a mixed oxide of metals selected from Iron, Nickel, Cobalt, manganese, molybdenum and tungsten. In an embodiment, the catalyst support is selected from yttrium oxide, lanthanum oxide, alumina and magnesia. In an embodiment, the non-stoichiometric stabilizer is selected from phosphorous oxide, titania, chromia, strontium oxide and combinations thereof. The non-stoichiometric stabilizer provides stability to the non-stochiometric mixed metal oxide catalyst by participating in the redox reaction by exchanging electron and stabilizing and maintaining the catalytic activity for longer duration and produce higher CNT aerogel yield. Thus, the nonstoichiometric stabilizer is employed for stabilizing the catalyst to operate the reactor for longer duration without shutdown for CNT evacuation and thus maximizes the MWCNT aerogel yield.
In next step, the method involves injecting the optimized biogas feedstock into the MWCNT reactor for catalytic chemical vapour deposition, at a temperature ranging from 550ºC to 900ºC for a time period ranging from 6hrs to 36hrs thereby forming MWCNT aerogel and hydrogen, thereby forming a MWCNT aerogel at a bottom of the reactor and product gas mixture.
In next step, the method involves passing the product gas mixture containing hydrogen and unreacted biogas through a membrane separator for separating hydrogen gas from the unreacted biogas. The unreacted biogas is then recycled for production of MWCNT aerogel.
In final step, the method involves evacuating the MWCNT aerogel from bottom of the MWCNT reactor. The MWCNT aerogel is having density ranging from 0.02 to 0.10g/cc, typically 0.05g/cc. The process yields MWCNT aerogel in the range from 25%wt to 90%wt.
The process of preparation of multiwall carbon nanotube (MWCNT) aerogel from biogas 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.
Experimental examples
Experimental example 1
The reactor was filled with 10gm of Fe-Mn oxide with alumina support and strontium oxide stabilizer. The non stoichiometric metal oxide catalyst was prepared in-situ by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The biogas having methane composition varying between 40 and 60%wt, with the rest being carbon dioxide, was fed to the reactor at 750ºC for 8hrs. During the reaction the product gas hydrogen and the unconverted methane was collected and analyzed. After the reaction time, the reactor was cooled, CNT aerogel was collected, weighed and the conversion and the yield was calculated. The CNT aerogel was purified to remove the catalyst particles and the density was measured. Further the aerogel samples were subjected to Field Emission Scanning Electron Microscopy (FESEM) to find out the structure of MWCNT aerogel. Referring to Figures 3a and 3b, a structure of MWCNT aerogel as seen in FESEM is shown. In the subsequent experiments the biogas having varying composition of methane and carbon dioxide was used and the same experimental procedure as above was followed for each biogas composition. Table 1 represents the yield and density for each composition of biogas.
Table 1
Sr. no. Feed composition, %wt MWCNT aerogel yield, %wt MWCNT Aerogel density, g/cc
Methane Carbon dioxide
1 40 60 27.9 0.05
2 50 50 39.9 0.05
3 60 40 48.8 0.05
Experimental example 2
Experiment 2.1: The reactor was filled with 10gm of Ni-Mn oxide with alumina support without stabilizer. The non stoichiometric metal oxide catalyst was prepared in-situ by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The biogas having 60%methane composition with the rest being carbon dioxide, was fed to the reactor at 750ºC for 8hrs. During the reaction the product gas hydrogen and the unconverted methane was collected and analyzed. After the reaction time, the reactor was cooled, CNT aerogel was collected, weighed and the conversion and the yield was calculated. The CNT aerogel was purified to remove catalyst particles and the density was measured.
The experiment 2.1 was conducted without non-stoichiometric support by keeping the other reaction conditions same. The yield of MWCNT aerogel obtained was 43.0wt% with a density of 0.05g/cc indicating CNT aerogel formation, which was almost same as the yield and density of MWCNT aerogel obtained in experiment 2.1.
Experiment 2.2: The experiment 2.1 was conducted by increasing the reaction time to 16 hrs, and keeping all the other reaction parameters same. The CNT aerogel yield obtained was 42.5 wt% with a density of 0.14 g/cc indicating deterioration of CNT aerogel quality due to decrease in catalyst activity. This indicates that the stabilizer provides stability to the catalyst and therefore the plant can be operated for longer duration and hence more efficient.
Experimental example 3
Experiment 3.1: The reactor was filled with 10gm of Ni-Mn oxide catalyst with alumina support and strontium oxide stabilizer. The non stoichiometric metal oxide catalyst was prepared in-situ by heating the reactor at 600ºC for 6hrs in presence of hydrogen. The biogas having 60%methane composition with the rest being carbon dioxide, was fed to the reactor at 750ºC for 8hrs. During the reaction the product gas hydrogen and the unconverted methane was collected and analyzed. After the reaction time, the reactor was cooled, CNT aerogel was collected, weighed and the conversion and the yield has been calculated. The CNT aerogel was purified to remove catalyst particles and the density was measured. The yield of MWCNT aerogel obtained was 44.1wt% with a density of 0.05g/cc indicating CNT aerogel formation.
Experiment 3.2: The experiment 3.1 was conducted with reaction duration increased to 16hrs. The CNT aerogel yield obtained was 43.8wt% with a density of 0.05 g/cc indicating good quality CNT aerogel due to sustained catalyst activity due to non-stochiometric strontium oxide stabilizer.
Advantages of the invention:
The process of production of MWCNT aerogel of the present invention does not require any external carrier gas.
The process of the present invention gives more than 25% yield of the MWCNT aerogel having density less than 0.10 g/cc, typically 0.05g/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 method for producing multiwall carbon nanotube (MWCNT) aerogel from biogas, the method comprising the steps of:
loading a reactor with a metal oxide catalyst supported on a catalyst support along with a non-stoichiometric catalyst stabilizer;
heating the reactor at 550ºC to 650ºC for 6 to 7 hrs in presence of hydrogen thereby forming the non-stoichiometric metal oxide catalyst in-situ;
optimizing composition of a purified biogas feedstock containing methane and carbon dioxide, by selectively controlling the proportion of methane in a range from 10%wt to 95%wt and the proportion of carbon dioxide in a range from 5%wt to 90%wt;
injecting the optimized biogas feedstock into the reactor for catalytic chemical vapour deposition, at a temperature ranging from 550ºC to 900ºC for a time period ranging from 6hrs to 36hrs, thereby forming a MWCNT aerogel at a bottom of the reactor and product gas mixture;
passing the product gas mixture containing hydrogen and unreacted biogas through a membrane separator for separating hydrogen from the unreacted biogas and re-circulating the unreacted biogas in the reactor; and
evacuating the MWCNT aerogel having density ranging from 0.02 to 0.10g/cc, from bottom of the reactor
2. The method as claimed in claim 1, wherein the reactor is a fixed bed reactor.
3. The method as claimed in claim 1, wherein the metal oxide catalyst is a mixed oxide of metals selected from Iron, Nickel, Cobalt, manganese, molybdenum and tungsten.
4. The method as claimed in claim 1, wherein the catalyst support is a metal oxide selected from yttrium oxide, lanthanum oxide, alumina and magnesia.
5. The method as claimed in claim 1, wherein the non-stoichiometric stabilizer is a metal oxide selected from phosphorous oxide, titanium dioxide, chromium oxide, strontium oxide and combinations thereof.
6. The method as claimed in claim 1, wherein the purified biogas feedstock is optimized using an alkanolamine column, by controlling the residence time and/or alkanolamine concentration.
7. The method as claimed in claim 6, wherein the alkanolamine is selected from monoethanol amine, diethanol amine and/or triethanol amine.
8. The method as claimed in claim 6, wherein the purified biogas feedstock is obtained by subjecting a biogas mixture containing methane, Carbon dioxide, hydrogen sulphide and ammonia, to selectively remove hydrogen sulphide using amine column and ammonia using reactive adsorption with diluted sulphuric acid to produce a purified biogas.
Dated this 22nd day of March 2024