Title of Invention	"PROCESS FOR THE PREPARATION OF AN ADSORBENT FOR THE STORAGE OF FUEL GASES"
Abstract	The present invention relates to a process for the preparation of an adsorbent for the storage of fuel gases preferably natural gases and its blends at low pressure and ambient temperature, prepared from granular microporous carbon preferably from the coconut shell wherein granular microporous carbon is grinded into a fine powder and mixed with aqueous dispersion of a binder formulation to make a carbon-binding agent-water slurry, the aqueous dispersion is prepared by adding the ingredients of the said binder formulation to water and thoroughly stirring the mixture, the slurry is then dried by evaporating water by heating, the dried mixture is then consolidated to obtain dense and mechanically strong carbon monoliths of regular shapes and desired dimensions suited for the storage of gaseous fuels such as natural gas and Hythane.

Title of Invention

"PROCESS FOR THE PREPARATION OF AN ADSORBENT FOR THE STORAGE OF FUEL GASES"

Abstract

The present invention relates to a process for the preparation of an adsorbent for the storage of fuel gases preferably natural gases and its blends at low pressure and ambient temperature, prepared from granular microporous carbon preferably from the coconut shell wherein granular microporous carbon is grinded into a fine powder and mixed with aqueous dispersion of a binder formulation to make a carbon-binding agent-water slurry, the aqueous dispersion is prepared by adding the ingredients of the said binder formulation to water and thoroughly stirring the mixture, the slurry is then dried by evaporating water by heating, the dried mixture is then consolidated to obtain dense and mechanically strong carbon monoliths of regular shapes and desired dimensions suited for the storage of gaseous fuels such as natural gas and Hythane.

Full Text	The present invention relates to a process for preparing an adsorbent prepared from granular microporous carbon to be used in the storage of fuel gases at low pressure and ambient temperature and such adsorbents from granular microporous carbon preferably from the coconut shell. The object of the invention is to prepare an adsorbent for storing and adsorbing the fuel gases at a low pressure of about 400-600 psig at the ambient temperature resulting in the storage of the fuel as natural gas and its blends, in particular, Hythane , which is a mixture in the range of 5 to 20% hydrogen and 80-95% Natural gas by volume, of about 150 litres of gas per litre of adsorbent. Another object of the invention is to provide an environmentally friendly and easy-to-practice method of binding together adsorbent particles comprising microporous carbon powders and granules in order to yield compacted and mechanically strong monoliths of high packing density carbon to be utilised in storage container for efficient adsorptive storage and transportation of gaseous fuels such as natural gas and Hythane. PRIOR ART: Presently, the use of liquefied petroleum gas consisting mainly of propane and butane for the domestic purposes is widespread. Though the LPG is convenient to store as it liquefies at moderate pressure and ambient temperature. However, LPG has several drawbacks as of potential fire hazard. This is due to the fact that propane and butane vapours are heavier than air so that when spilled or leaked they will remain close to ground level thus creating a risk of fire or explosion. Moreover, LPG burning causes substantially larger emissions of CO2 on the basis of per unit of energy generated. The use of Natural gas for such purposes has also been tried because unlike that of LPG, there is an abundant supply of natural gas with world reserves estimated to last more than 100 years. Since natural gas is lighter than air, it is inherently safe because upon accidental leakage it would rapidly disperse. Natural gas also has the advantage of clean-burning characteristics with very low level emissions of toxic and regulated gases such as hydrocarbons, carbon monoxide and nitrogen oxides (NO and N02). Also it is the least carbon intensive of the fossil fuels and therefore produces the least amount of carbon dioxide per unit of energy generated. Octane rating of natural gas is about 130, substantially higher than octane rating of propane (105 - 110). The combination of an extensive resource base and environmental advantages makes natural gas a preferred fuel. For utilization of natural gas and/or Hythane as fuel in cooking appliances and in automobiles in places where pipeline transmission and distribution system of these gaseous fuels does not exist, it is required to provide an alternative cost-effective and convenient means of storage, transportation and distribution of these gaseous fuels in these places. The most commonly perceived way is to store natural gas and its blends under pressures between 3,000 to 5,000 psig. This method of high pressure storage requires use of very heavy and expensive storage cylinders and expensive fuelling equipment. The high pressure gas is also a safety hazard. In the case of an accident, high pressure natural gas and Hythane could explode due to ignition. Therefore, this method of high pressure Natural Gas storage is not acceptable for use in barbecue stoves and indoor burners. Another way of off-line (non-pipeline) transportation of natural gas and its blends is to supply it in liquid form in thermally-insulated tanks. However, this method of natural gas and Hythane storage and distribution is also not a viable option due to the prohibitively high costs involved in liquefaction and refrigeration, and extraordinary safety precautions that must be taken during the transportation and handling of such dangerous cryogenic liquids. To avoid such safety hazards , the solution founded is the use of adsorbents for storing such Natural Gas at low pressures. The adsorbents using monoliths of polyvinyl alcohol or other organic materials as binder are tried and found not to be sufficiently efficient for such adsorption. The use of microporous materials such as zeolites, molecular sieves, and activated carbons have also been known for long for their adsorptive affinity of gases and vapours. The adsorption capacity of an adsorbent generally increases with its increasing internal surface area and pore volume. However, with the activated carbon based adsorbent, it has generally been observed that higher the surface area, the lower is its packing density (that is, lesser weight of carbon can be packed into a fixed volume). This decrease of packing density is due partly to interparticle void spaces left while packing and partly to intraparticle voidage arising from the development of macropores within the carbon network. Thus, although there is usually an increase in adsorption capacity on a weight basis of adsorbent with increase in the surface area, little or no gain is generally made in the adsorption capacity on a volume basis. However, for adsorptive storage of gaseous fuels such as natural gas and Hythane, the volume of the container holding the adsorbent is an important limiting factor. Unfortunately, the microporous carbons, which can be made only in the form of granules or powders from the precursor materials by methods known to the art have low packing density. Thus, for applications of microporous carbons in the storage and transportation of natural gas and Hythane, it is necessary to consolidate them. The consolidation greatly reduces void spaces between carbon particles and possibly even macropore volume so that a larger quantity of adsorbent can be packed in a given storage volume. The compacted carbon so made is likely to exhibit higher adsorption capacity on the volume basis of adsorbent than is otherwise observed. The presently known art of consolidating the carbon particles into shaped material for use in the storage of natural gas is based mainly on the use of organic binding materials which suffers from major disadvantages. The polymer, polyvinylidene chloride (PVDC), which is often referred to by the Dow Chemical trade name Saran, has been used to bind KOH-activated coal carbon sold under trade name AX-21 to produce composite carbon pellets (Barton et al., Reports submitted to Ontario Ministry of Energy, 1984-85) This method has certain drawbacks. It requires the use of high percentages of polymer ranging from 30-90% by weight in the carbon-polymer mix. Furthermore, it is necessary to slowly heat the carbon-polymer composite pellets to a temperature of 700°C under vacuum in order to transform them into adsorbent. This heat treatment process liberates a large volume of corrosive HCI vapour, thus placing significant constraints on the materials of construction of the reactor. The heat treatment process can also generate vinyl chloride, a hazardous substance. Moreover, PVDC is not desirable to use as it is not a cost effective material and is also not easily available. The use of phenolic resins for carbon binding and compaction of activated carbons made from coals and scrap tires into shaped material for use in storage of natural gas is also known to the art (see J. Sun, T.A. Brady, M.S. Rood, C.M. Lehmann, Energy and Fuels, Vol. II, pp 316-322, 1997). However, this class of binders also has a number of limitations. It is necessary to use large quantities of binder (binder to carbon ratio >0.3) and to heat the carbon-binder mix to a high temperature (>500°C) under compressive pressure to achieve binding. Furthermore, phenolic resins being condensation products of phenol and formaldehyde can potentially cause emissions of harmful substances, for example, formaldehyde, during the course of pressure-temperature moulding of the carbon-resin binder mix. Another organic binder which has been used for compacting activated carbon powder derived from coal (sold under trade name AX-21) is polyvinyl alcohol (U.S. Patent No. 4,999,330). This polyvinyl alcohol method of binding requires heating and cooling under carefully controlled conditions, is cumbersome and time-consuming. Hence, the methods known in the art on fabrication of compacted carbon adsorbents for use in the storage of natural gas and other gaseous fuels are expensive, time-consuming, and are detrimental to health and the environment to quite an extent. To overcome the drawbacks existing in the problems on compacting of microporous carbons for use in the storage of natural gas, the subject invention has been developed. The novel method of compacting the carbon at low pressure and temperature disclosed in this invention is economically viable and environmentally safe to warrant its wide range of applications. The adsorbent of this invention consists of carbon monoliths of high packing density fabricated by binding and compacting microporous carbon particles, prepared from coconut shells, using an inorganic clay material as binder. In an another embodiment of the subject invention, the microporous carbon particles are prepared from the coal, coal coke, petroleum coke, wood and rice husks and the like for the storage, transportation and distribution of natural gas and Hythane in accordance with this invention. The use of non-toxic, non-hazardous, inexpensive and readily available inorganic materials as binding agents for binding together carbon particles to form monoliths of desired shapes and sizes is achieved by using a cationic clay of phyllosilicate family, either alone or as a mixture with sodium silicate or calcium hydroxide, as the binding agent. Preferred cationic clay of the said binding agent formulation is bentonite, Na-exchanged bentonite or pillared analogues of bentonite, preferably AI-, Fe-, Cr-, Zr- or Ti- pillared bentonite. In the said binder formulation, the clay content is 5% to 15% by weight, sodium silicate content is 0.5% to 1.5% by weight, and calcium hydroxide content in 0.5% to 1.5% by weight, all based on the weight of carbon in the monolith. The use of totally inorganic materials as binder in the subject invention eliminates the risk of emission of toxic and regulated gases or vapour during monolith production. The method of binding and compacting microporous carbon particles to yield high strength monoliths is achieved by mixing microporous carbon powder with water dispersion of the cationic clay binding agent referred to above, drying the slurry to desired consistency, and consolidating the carbon-binder mix so prepared either by compression under a modest pressure in the range of 8,000 psi to 16,000 psi in a die/pelleting press without having to apply heating as done conventionally, or by the extrusion process using a conventional extruder to yield dense, mechanically strong blocks of regular shape and desired dimension. The binding and compaction method, as disclosed above, causes nearly a two-fold reduction in bulk volume and a corresponding increase of packing density. As, the method does not require application of heat, it results in providing substantial processing cost savings. Since, the method does not involve heating and cooling of the carbon-binder mix during monolith fabrication, the carbon-binder mix are formed into monoliths in substantially little time as less as 15 minutes, thereby reducing the process time. Moreover, the monolith fabrication process is not associated with release of obnoxious gases or vapours of any kind and therefore is free from environmental hazards or occupational health and safety concerns. The method thus overcomes the difficulties associated with the methods known to the art on pelletization or briquetting of activated carbon powders for using them in natural gas storage. Accordingly, the subject invention relates to a process for the preparation of an adsorbent prepared from granular microporous carbon preferably from the coconut shell to be used in the storage of fuel gases preferably natural gas and its blends at low pressure and ambient temperature , comprising the steps of: a) grinding granular microporous carbon into a fine powder; b) preparing an aqueous dispersion of a binder formulation by adding the ingredients of the said binder formulation to water and thoroughly stirring the mixture, c) mixing said powdered microporous carbon powder of step (a) with the said aqueous dispersion of the binding agent to make a carbon-binding agent-water slurry; d) drying the said carbon-binding agent-water slurry by evaporating water by heating while stirring until moisture content of the resultant mix corresponds to the range from 80% to 100% by weight, based on the weight of carbon in the mix; e) consolidating the said dried carbon-binding agent mix at ambient temperature, to obtain dense and mechanically strong carbon monoliths of regular shapes and desired dimensions suited for the storage of gaseous fuels such as natural gas and Hythane; and f) drying the said carbon monoliths at a temperature of 60° C to 70° C and at a temperature of 120°C to 140°C in air atmosphere to obtain moisture free dried carbon monoliths. The embodiment of the invention resides in a process for the preparation of a granular microporous carbon prepared by carbonization and controlled partial oxidation of coconut shells, comprising:- crushing the said coconut shells to a particle size of 5-10mm, washing the said crushed coconut shells with water and drying the same at a temperature of 110°C carbonizing the said washed and dried crushed coconut shells at a temperature of 600-700°C in an inert atmosphere to obtain the char, crushing and screening the said char to obtain the mesh size of 8-30, oxidizing the said crushed and screened char at a temperature of 700-850°C to obtain granular microporous carbon having surface area of 1200-1800m2/g The said microporous carbon may also be derived from carbonaceous materials selected from wood, coal, various kinds of pitch, and petroleum tar. The said granular microporous carbon is obtained by chemical activation in an inert atmosphere with KOH, phosphoric acid or ZnCI2 at temperatures in the range 300-600°C or by carbonization at temperatures in the range 600-800°C , followed by controlled partial oxidation with CO2 or water vapour-N2 mixture at temperatures in the range 700-850°C. The consolidation of the said dried carbon-binding agent mix is obtained by compressing the said mixture, at ambient temperature, inside a suitable die at a pressure of 8,000 -16,000 psi using a suitable pelleting press or by extruding the said dried carbon-binding agent mix in a extruder. The binding agent is selected from clay mixed with or without sodium silicate or calcium hydroxide . The clay material is selected from the group consisting of cationic clays and pillared analogues of cationic clays selected from hydrous layer silicates of the called phyllosilicate family consisting of groups as smectite, vermiculite, serpentine-kaolin, talc-pyrophyllite, mica, chlorite and sepiolite-palygorskite, typical examples being bentonite, atapulgite, hectorite, beidellite, fuller's earth halloysite, illite kaolin, montmorillonite and mullite. The binding agent used is bentonite clay, Na- or K- exchanged bentonite clay, pillared bentonite clay (wherein pillaring species is a polycationic oxide of Al, Fe, Cr, Zr or Ti) and a combination thereof. The pillared analogues of cationic clays are materials in which small cations of the cationic clay are replaced by large polyoxocationic species. The present invention can better be understood with reference to the accompanying drawing, which is for illustrative purposes and should not in any way be construed to restict the scope of the invention keeping in view that certain modifications and improvements are possible without deviating from the scope off the invention. According to the present invention there is provided a process for preparation of an adsorbent prepared from granular microporous carbon as herein described preferably from the coconut shell to be used in the storage of fuel gases preferably natural gas and its blends at low pressure and ambient temperature, comprising the steps of: a) grinding granular microporous carbon into a fine powder; b) preparing an aqueous dispersion of a binder formulation as herein described by adding the ingredients of the said binder formulation to •Ingredients water and thoroughly stirring the mixture, c) mixing said powdered microporous carbon powder of step (a) with the said aqueous dispersion of the binding agent to make a carbon- binding agent-water slurry; d) drying the said carbon-binding agent-water slurry by evaporating water by heating while stirring until moisture content of the resultant mix corresponds to the range from 80% to 100% by weight, based on the weight of carbon in the mix; e) consolidating the said dried carbon-binding agent mix at ambient temperature, to obtain dense and mechanically strong carbon monoliths of regular shapes and desired dimensions suited for the storage of gaseous fuels such as natural gas and Hythane; and BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING Figure 1 depicts the block flow diagram showing various unit operations involved in the preparation of high density monolithic microporous carbon adsorbent from coconut shells for use in the storage, off-line transportation and distribution of natural gas and Hythane fuels. DETAILED DESCRIPTION OF THE INVENTION As illustrated in Figure 1, microporous carbon for binding and consolidation in accordance with the method disclosed in this invention is prepared from abundant and inexpensive coconut shells by the two-stage process of carbonization and controlled partial oxidation. The process for the preparation of activated carbon from coconut shells for use as guard bed; comprises: 1 pyrolysis/carbonization of dried coconut shell at 873 K under an inert atmosphere. 2 Activation of coconut char by partial gasification/controlled oxidation with CO2at 1073K. The coconut shells were cleaned of loose fibers and traces of kernel, crushed to particle size of about 10mm, washed with water and dried at a temperature of 110°C and then subjected to carbonization by heating to a temperature of 600°C to 700°C in an inert atmosphere. The char so produced is crushed, screened to 8-30 mesh size and then submitted to controlled partial oxidation with oxidising agents such as carbon dioxide and water vapour/nitrogen mixture at a temperature of 700°C to 850°C. The resultant product obtained is a granular microporous carbon having a surface area in the range 1200 -1800 m2/g. The process for the preparation of carbon monoliths comprises the steps of: a) crushing and milling the granular carbon into a fine powder by conventional means preferably in a ball mill; b) preparing an aqueous dispersion of the binder formulation by adding the ingredients of the formulation to water at the rate of 25 mL to 50 ml water per gram of binder and thoroughly agitating the mix; c) mixing dry or pre-wetted carbon powder of step (a) with the water dispersion of the binder of step (b) in appropriate proportions to make a slurry of carbon and binding agent; d) drying the carbon-binding agent slurry of step (c) to desired moisture content ranging from 80% to 100% by weight based on the weight of carbon used in step (c), by evaporating water while stirring in any conventional blending-drying equipment; e) consolidating carbon-binder mix of step (d) either by compression, at ambient temperature, inside a suitable die under a pressure of 8,000 to 16,000 psi using a suitable pelleting press or by extrusion process using a suitable extruder to provide high density, mechanically strong monoliths of desired shapes and sizes; and f) subjecting the monoliths to drying in steps at a temperature of 60°C to 70°C and at temperature of 120°C to 140°C in air atmosphere to provide dried monoliths free of moisture. the said binding agent used in step (b) is a clay material used alone or along with sodium silicate or calcium hydroxide, wherein the clay component is chosen from a group of cationic clays which are hydrous layer silicates of the so-called phyllosilicate family consisting of such groups as smectite, vermiculite, serpentine-kaolin, talc-pyrophylite, mica, brittle mica, chlorite and sepiolite-palygorskite, typical examples being bentonite, atapulgite, hectorite, beidellite, fuller's earth, halloysite, illite kaolin, montmorillonite and mullite or from a group consisting of pillared analogues of cationic clays which are materials in which small cations of cationic clay are replaced by large polyoxocationic species. The most preferred clays for use in the binding agent formulation include bentonite clay, sodium exchanged bentonite clay, pillared analogues of bentonite clay (such as AI-, Fe-, Cr-, Zr- or Ti- pillared bentonite clay) and combinations thereof. In the said binding agent formulation, the clay content is 5% to 15% by weight, sodium silicate content is 0.5% to 1.5% by weight, and calcium hydroxide content is 0.5% to 1.5% by weight, all based on the weight of carbon used in step (c). In step (d), the moisture content of the carbon-binder paste is controlled in the range from 80% to 100% by weight based on the weight of carbon used in the preparation. The packing density of the monoliths so obtained is nearly twice as much as that of the starting granular material. The binding agents used in the subject invention are thermally and chemically stable substances which at no stages of monolith fabrication process release obnoxious gases or vapours of any kind and therefore there is no cause for concern from the viewpoint of environmental pollution or occupational health and safety hazard. Another key feature of the method of this invention is that it does not suffer from the problem of pore plugging with binder. This is due to the fact that the major ingredient of binder formulation that is the clay component is applied as a dispersion in water rather than as a solution wherein size of the clay particulates are too large to penetrate into and occupy the micropores of carbon particles. The method of binding and compaction as disclosed above is also suitable for binding and consolidation of other microporous solid adsorbents such as zeolites, carbon molecular sieves, alumino-phosphates, silicoaluminophosphates, silicas and aluminas. The subject invention can better be understood with reference to the undermentioned examples, which should not be construed to restrict the scope of the invention. EXAMPLES: EXAMPLE 1 Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 8,000 psi while maintaining the die at ambient temperature. The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 600°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 700°C using carbon dioxide as the oxidizing agent. The binding agents used are bentonite clay mixed with sodium silicate. The said carbon-binder mix was prepared by thoroughly blending the fine powder of microporous carbon with the binding agent formulation dispersed in water, and then partially drying the blend (slurry) by evaporation of water under vigorous stirring until the moisture content of the resultant carbon-binder mix corresponded to the 80% by weight based on the weight of dry carbon powder used in the preparation. The binder dispersion was prepared by adding in the binder ingredients 25 ml water per gram of the binding agent and vigorously stirring the binder-water mixture. The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.78 g/ml. The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 44.5 grams and having a total geometric volume 56.8 ml was used. The natural gas storage capacity of the adsorbent was found to be 155 litres of natural gas per litre of adsorbent (155 v/v), as shown in Table 1. TABLE 1: (Table Removed) EXAMPLE 2 Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 10,000 psi while maintaining the die at ambient temperature. The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 650°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 750°C using water vapour - N2 mixture as oxidising agent. The binding agents used were bentonite clay mixed with calcium hydroxide. The said carbon-binder mix was prepared by the process as explained in example 1. The binder dispersion was prepared by adding in the binder ingredients 30 ml water per gram of the binding agent and vigorously stirring the binder-water mixture. The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of calcium hydroxide to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.79 g/ml. The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 36.6 grams and having a total geometric volume 54.9 ml was used. The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 2. TABLE 2: (Table Removed) EXAMPLE : 3 Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 12,000 psi while maintaining the die at ambient temperature. The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 660°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 720°C using water vapour - N2 mixture as oxidising agent. The binding agents used are Na-exchanged bentonite clay mixed with calcium hydroxide. The said carbon-binder mix was prepared by the process as explained in example 1. The binder dispersion was prepared by adding the binder ingredients 35 ml water per gram of the binding agent and vigorously stirring the binder-water mixture. The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of calcium hydroxide to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.79 g/ml. The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 43.4 grams and having a total geometric volume 54.7 ml was used. The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 3. TABLE 3: (Table Removed) EXAMPLE : 4 Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 11,000 psi while maintaining the die at ambient temperature. The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 670°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 735°C using water vapour - N2 mixture as the oxidizing agent. The binding agents used Al-pillared bentonite clay mixed with sodium silicate. The binder dispersion was prepared by adding the binder ingredients 40 ml water per gram of the binding agent and vigorously stirring the binder-water mixture. The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.78 g/ml. The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 4. TABLE 4: (Table Removed) 2.0 EXAMPLE : 5 Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 14,000 psi while maintaining the die at ambient temperature. The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 665°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 755°C using water vapour - N2 mixture as the oxidizing agent The binding agents used are bentonite clay mixed with sodium silicate. The binder dispersion was prepared by adding the binder ingredients 42 ml water per gram of the binding agent and vigorously stirring the binder-water mixture. The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.89 g/ml. The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 36.6 grams and having a total geometric volume 54.9 ml was used. The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 5. TABLE 5: (Table Removed) EXAMPLE : 6 Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 16,000 psi while maintaining the die at ambient temperature. The carbon monoliths are prepared from coconut shells as described in the foregoing examples. The binding agents used were bentonite clay mixed with sodium silicate. The binder dispersion was prepared by adding the binder ingredients 50 ml water per gram of the binding agent and vigorously stirring the binder-water mixture. The clay to carbon ratio in the said carbon mix binder is 0.05g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.005g/g. The packing density of monolith was found to be 0.87 g/ml. The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 36.6 grams and having a total geometric volume 54.9 ml was used. The natural gas storage capacity of the adsorbent was found to be 154 litres of natural gas per litre of adsorbent (154 v/v), as shown in Table 6. TABLE 6 (Table Removed) In the examples 1-6 , the packing densities of the cylindrical shaped monoliths were ranging from 0.67-0.78, which was determined from their physically measured dimensions and weights. A batch consisting of 5 to 7 monoliths were used in the measurement. The weights were recorded after degassing the monoliths under vacuum at a temperature of 130°C to 150°C. The mechanical strength of the monoliths was tested in an apparatus consisting of a screw-thread type hand press attached to an electronic balance. Monolithic pieces were subjected to a constant compressive pressure of 5 kg/cm2 (applied load divided by cross-sectional area of the monolith) and if found to remain intact it was concluded that the crushing strength of the particular monolith is at least 5 kg/cm2. Packing density of the granular samples was measured by filling a calibrated cylinder with a known sample weight and tapping the cylinder until a constant volume was obtained. The results are reported in Table A. The surface area of granular carbon samples was determined from nitrogen adsorption isotherm measured at liquid nitrogen temperature using BET equation. Packing density of the granular samples was measured by filling a calibrated cylinder with a known sample weight and tapping the cylinder until a constant volume was obtained. The results are reported in TABLE A (Table Removed) The binding and consolidation with the bentonite clay based binding agents result in a increase in the packing density of the microporous carbons prepared by using different oxidizing agents and having different surface areas, thus making these materials suitable for the storage, transportation and distribution of gaseous fuels such as natural gas and Hythane, as illustrated in the Table A The mechanical strength of the carbon monoliths was found to be greater than 5 kg/cm2 which is adequate for most applications. EXAMPLE :7 The polyvinyl alcohol based binder was used for fabrication of carbon monoliths, where Carbon used was a mixture of two coconut shell carbons (surface area ~ 1100 m2 and ~ 1600 m2/g) in the ratio 80 : 20. The binder was a mixture of 0.06 gram of polyvinyl alcohol (mol. wt. ~ 115,000), 0.004 gram of urea and 0.002 gram of ethylene glycol per gram of carbon. A slurry of carbon and binder ingredients in water was prepared, which was then partially evaporated to obtain a semi-dried mass with a moisture content of 40-50% by wt. (based on the total wt. of carbon used). Cylindrical shaped carbon monoliths of 20 mm diameter and 15-30 mm thickness were fabricated by pressing the semi-dried mass contained in a cylindrical die by means of a hydraulic press at a pressure of 50,000 psi while slowly heating the die to a temperature of 250°C and holding at that temperature for about 1 hour. The average packing density of dried (moisture free) monoliths was 0.84 g/mL. The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at ambient temperature (22°C - 25°C). A batch of pellets weighing 44.7 grams and having a total geometric volume of 53.0 ml was used in the test. The natural gas storage capacity of the adsorbent was measured. The natural gas storage capacity of the adsorbent was found to be 155 litres of gas per litre of adsorbent (155v/v), as shown in Table 7. TABLE 7 Natural Gas Storage by Monolithic Microporous Coconut Carbon Monoliths fabricated using Polyvinyl Alcohol based Binder at 500 psig Pressure and Ambient Temperature (Table Removed) EXAMPLE 8 Microporous carbon particles, prepared from coconut shells by carbonization and partial oxidation with water vapour - N2 mixture and having surface area of about 1240 m2/g, were binded together and consolidated into cylindrical monoliths having 20 mm diameter and 20-40 mm thickness using an admixture of bentonite clay and sodium silicate as binding agent in the amounts of 0.10 gram of bentonite clay and 0.010 gram of sodium silicate per gram of carbon. The average density of carbon monoliths was 0.79 g/mL. The monoliths were tested for the storage of Hythane fuel (composition : 90 volume percent natural gas and 10 volume percent hydrogen) under a pressure of about 550 psig and at ambient temperature 25°C. A batch of monoliths weighing 44.5 grams and having a total geometric volume 56.5 mL was used in the test. The fuel storage capacity of the adsorbent was measured. The Hythane storage capacity of the adsorbent was found to be 154 litres of Hythane per litre of adsorbent (154 v/v), as shown in Table 8. TABLE 8 Hythane Storage by Monolithic Microporous Coconut Carbon at 550 psig Pressure and Ambient Temperature. (Table Removed) Examples 1-6, which deal with the monolithic carbon adsorbent fabricated using clay-based binders according to the method of this invention, clearly illustrate that the concentrated storage of natural gas at low pressures can be achieved by the method and adsorbent materials of this invention, thus avoiding the high pressures needed for storage as compressed gas (CNG) or cryogenic temperatures needed for storage as liquefied natural gas (LNG). Example 7, deals with the monolithic carbon adsorbent monoliths fabricated using PVA-based binder. It is evident from the above mentioned table that PVA-based procedure is not only time-consuming, cumbersome and energy intensive but does not offer any advantages also. The natural gas storage capacity of the adsorbent fabricated using clay-based binders according to this invention, is similar to that of the adsorbent fabricated using PVA-based binder. On the other hand, the clay-based fabrication method has a number of advantages: it is simple and easy-to-practice ; fabrication process does not require costly heating ; and there is no risk of releasing obnoxious gases or vapours. Example 8 illustrates that the method and adsorbent material of this invention is also well suited for low pressure adsorptive storage of Hythane fuel. The adsorbent material of the present invention is placed in the containers used for storing the natural gas and Hythane. Storage containers containing such adsorbent can be fuelled in a central filling station. Once the natural gas or Hythane stored in a container is exhausted due to usage as fuel in the desired application, the container can be brought back to the filling station, refilled and transported back to the user. In this way, the same batch of adsorbent can be used for storage and distribution of natural gas or Hythane over and over again for years. While the invention has been described and illustrated herein by references to various specific materials, procedures and examples, it is understood that the invention is not restricted to the particular materials and procedures selected for that purpose. Numerous variations of such details can be employed, as will be appreciated by those skilled in the art without deviating from the scope of the invention. We claim: 1. A process for preparation of an adsorbent for the storage of fuel gases preferably natural gas and its blends at low pressure and ambient temperature, said adsorbent prepared from granular microporous carbon as herein described preferably from the coconut shell, comprising the steps of: a) grinding granular microporous carbon into a fine powder; b) preparing an aqueous dispersion of a binder as herein described by adding the ingredients of the said binder to water and thoroughly stirring the mixture, c) mixing said powdered microporous carbon powder of step (a) with the said aqueous dispersion of said binder to make a carbon-binder-water slurry; d) drying the said carbon-binder-water slurry by evaporating water by heating while stirring until moisture content of the resultant mix corresponds to the range from 80% to 100% by weight, based on the weight of carbon in the mix; e) consolidating the said dried carbon-binder mix at ambient temperature, to obtain dense and mechanically strong carbon monoliths of regular shapes and desired dimensions as herein described suited for the storage of gaseous fuels such as natural gas and Hythane; and f) drying the said carbon monoliths at a temperature of 60°C to 70°C and subsequently at a temperature of 120°C to 140°C in air atmosphere to obtain moisture free dried carbon monoliths. 2. A process as claimed in claim 1, wherein said microporous carbon is derived from carbonaceous materials selected from coconut shell, wood, coal, various kinds of pitch, and petroleum tar. 3. A process as claimed in claim 1, wherein the said consolidation of the said dried carbon-binder mix is prepared by compressing the said mixture, at ambient temperature, inside a suitable die at a pressure of 8,000 -16,000 psi using a suitable pelleting press. 4. A process as claimed in claim 1, wherein the said binder is selected from clay optionally mixed with sodium silicate or calcium hydroxide. 5. A process as claimed in claim 4, wherein the said clay is selected from the group consisting of cationic clays and pillared analogues of cationic clays. 6. A process as claimed in claim 5, wherein the said cationic clays is selected from hydrous layer silicates of the phyllosilicate family consisting of groups as smectite, vermiculite, serpentine-kaolin, talc- pyrophyllite, mica, chlorite and sepiolite-palygorskite, typical examples being bentonite, atapulgite, hectorite, beidellite, fuller's earth halloysite, illite kaolin, montmorillonite and mullite. 7. A process as claimed in claim 4, wherein the said binder is bentonite clay, Na- or K- exchanged bentonite clay, pillared bentonite clay (wherein pillaring species is a polycationic oxide of Al, Fe, Cr, Zr or Ti) and a combination thereof. 8. A process as claimed in claim 4, wherein the said binder is a mixture of bentonite clay and sodium silicate. 9. A process as claimed in claim 8, wherein the said binder is a mixture having 5% to 15% bentonite content by weight and 0.5% to 1.5% sodium silicate content by weight, both based on the weight of the carbon. 10. A process as claimed in claim 4, wherein the said binder is a mixture of bentonite clay and calcium hydroxide. 11. A process as claimed in claim 10, wherein the said binder is a mixture of Na-exchanged bentonite clay and calcium hydroxide. 12. A process as claimed in any of preceding claims, wherein the said binder is used at a ratio of 0.05 - 0.15 gram of bentonite clay per gram of microporous carbon powder. 13. An adsorbent prepared by the process as claimed in claim 1, applied in the storage of fuel gases preferably natural gas and its blends comprising monolithic microporous carbon adsorbent contained in a portable container with natural gas or Hythane under a pressure of about 400 psig to 600 psig at ambient temperature. 14. A process for preparation of an adsorbent for the storage of fuel gases preferably natural gas and its blends at low pressure and ambient temperature, prepared from granular microporous carbon preferably from the coconut shell, substantially as herein described with reference to the accompanying drawings.

Full Text

The present invention relates to a process for preparing an adsorbent prepared from granular microporous carbon to be used in the storage of fuel gases at low pressure and ambient temperature and such adsorbents from granular microporous carbon preferably from the coconut shell.
The object of the invention is to prepare an adsorbent for storing and adsorbing the fuel gases at a low pressure of about 400-600 psig at the ambient temperature resulting in the storage of the fuel as natural gas and its blends, in particular, Hythane , which is a mixture in the range of 5 to 20% hydrogen and 80-95% Natural gas by volume, of about 150 litres of gas per litre of adsorbent.
Another object of the invention is to provide an environmentally friendly and easy-to-practice method of binding together adsorbent particles comprising microporous carbon powders and granules in order to yield compacted and mechanically strong monoliths of high packing density carbon to be utilised in storage container for efficient adsorptive storage and transportation of gaseous fuels such as natural gas and Hythane.
PRIOR ART:
Presently, the use of liquefied petroleum gas consisting mainly of propane and butane for the domestic purposes is widespread. Though the LPG is convenient to store as it liquefies at moderate pressure and ambient temperature. However, LPG has several drawbacks as of potential fire hazard. This is due to the fact that propane and butane vapours are heavier than air so that when spilled or leaked they will remain close to ground level thus creating a risk of fire or explosion. Moreover, LPG burning causes substantially larger emissions of CO2 on the basis of per unit of energy generated.
The use of Natural gas for such purposes has also been tried because unlike that of LPG, there is an abundant supply of natural gas with world reserves estimated to last more than 100 years. Since natural gas is lighter than air, it is inherently safe because upon accidental leakage it would rapidly disperse. Natural gas also has the advantage of clean-burning characteristics with very low level emissions of toxic and regulated gases such as hydrocarbons, carbon monoxide and nitrogen oxides (NO and N02). Also it is the least carbon intensive of the fossil fuels and therefore produces the least amount of carbon dioxide per unit of energy generated. Octane rating of natural gas is about 130, substantially higher than octane rating of propane (105 - 110). The combination of an extensive resource base and environmental advantages makes natural gas a preferred fuel.
For utilization of natural gas and/or Hythane as fuel in cooking appliances and in automobiles in places where pipeline transmission and distribution system of these gaseous fuels does not exist, it is required to provide an alternative cost-effective and convenient means of storage, transportation and distribution of these gaseous fuels in these places. The most commonly perceived way is to store natural gas and its blends under pressures between 3,000 to 5,000 psig. This method of high pressure storage requires use of very heavy and expensive storage cylinders and expensive fuelling equipment. The high pressure gas is also a safety hazard. In the case of an accident, high pressure natural gas and Hythane could explode due to ignition. Therefore, this method of high pressure Natural Gas storage is not acceptable for use in barbecue stoves and indoor burners.
Another way of off-line (non-pipeline) transportation of natural gas and its blends is to supply it in liquid form in thermally-insulated tanks. However, this method of natural gas and Hythane storage and distribution is also not a viable option due to the prohibitively high costs involved in liquefaction and refrigeration, and extraordinary safety precautions that must be taken during the transportation and handling of such dangerous cryogenic liquids.
To avoid such safety hazards , the solution founded is the use of adsorbents for storing such Natural Gas at low pressures. The adsorbents using monoliths of polyvinyl alcohol or other organic materials as binder are tried and found not to be sufficiently efficient for such adsorption.
The use of microporous materials such as zeolites, molecular sieves, and activated carbons have also been known for long for their adsorptive affinity of gases and vapours. The adsorption capacity of an adsorbent generally increases with its increasing internal surface area and pore volume. However, with the activated carbon based adsorbent, it has generally been observed that higher the surface area, the lower is its packing density (that is, lesser weight of carbon can be packed into a fixed volume). This decrease of packing density is due partly to interparticle void spaces left while packing and partly to intraparticle voidage arising from the development of macropores within the carbon network. Thus, although there is usually an increase in adsorption capacity on a weight basis of adsorbent with increase in the surface area, little or no gain is generally made in the adsorption capacity on a volume basis. However, for adsorptive storage of gaseous fuels such as natural gas and Hythane, the volume of the container holding the adsorbent is an important limiting factor. Unfortunately, the microporous carbons, which can be made only in the form of granules or powders from the precursor materials by methods known to the art have low packing density. Thus, for applications of microporous carbons in the storage and transportation of natural gas and Hythane, it is necessary to consolidate them. The consolidation greatly reduces void spaces between carbon particles and possibly even macropore volume so that a larger quantity of adsorbent can be packed in a given storage volume. The compacted carbon so made is likely to exhibit higher adsorption capacity on the volume basis of adsorbent than is otherwise observed.
The presently known art of consolidating the carbon particles into shaped material for use in the storage of natural gas is based mainly on the use of organic binding materials which suffers from major disadvantages.
The polymer, polyvinylidene chloride (PVDC), which is often referred to by the Dow Chemical trade name Saran, has been used to bind KOH-activated coal carbon sold under trade name AX-21 to produce composite carbon pellets (Barton et al., Reports submitted to Ontario Ministry of Energy, 1984-85) This method has certain drawbacks. It requires the use of high percentages of polymer ranging from 30-90% by weight in the carbon-polymer mix. Furthermore, it is necessary to slowly heat the carbon-polymer composite pellets to a temperature of 700°C under vacuum in order to transform them into adsorbent. This heat treatment process liberates a large volume of corrosive HCI vapour, thus placing significant constraints on the materials of construction of the reactor. The heat treatment process can also generate vinyl chloride, a hazardous substance. Moreover, PVDC is not desirable to use as it is not a cost effective material and is also not easily available.
The use of phenolic resins for carbon binding and compaction of activated carbons made from coals and scrap tires into shaped material for use in storage of natural gas is also known to the art (see J. Sun, T.A. Brady, M.S. Rood, C.M. Lehmann, Energy and Fuels, Vol. II, pp 316-322, 1997). However, this class of binders also has a number of limitations. It is necessary to use large quantities of binder (binder to carbon ratio >0.3) and to heat the carbon-binder mix to a high temperature (>500°C) under compressive pressure to achieve binding. Furthermore, phenolic resins being condensation products of phenol and formaldehyde can potentially cause emissions of harmful substances, for example, formaldehyde, during the course of pressure-temperature moulding of the carbon-resin binder mix.
Another organic binder which has been used for compacting activated carbon powder derived from coal (sold under trade name AX-21) is polyvinyl alcohol
(U.S. Patent No. 4,999,330). This polyvinyl alcohol method of binding requires heating and cooling under carefully controlled conditions, is cumbersome and time-consuming.
Hence, the methods known in the art on fabrication of compacted carbon adsorbents for use in the storage of natural gas and other gaseous fuels are expensive, time-consuming, and are detrimental to health and the environment to quite an extent.
To overcome the drawbacks existing in the problems on compacting of microporous carbons for use in the storage of natural gas, the subject invention has been developed. The novel method of compacting the carbon at low pressure and temperature disclosed in this invention is economically viable and environmentally safe to warrant its wide range of applications.
The adsorbent of this invention consists of carbon monoliths of high packing density fabricated by binding and compacting microporous carbon particles, prepared from coconut shells, using an inorganic clay material as binder.
In an another embodiment of the subject invention, the microporous carbon particles are prepared from the coal, coal coke, petroleum coke, wood and rice husks and the like for the storage, transportation and distribution of natural gas and Hythane in accordance with this invention.
The use of non-toxic, non-hazardous, inexpensive and readily available inorganic materials as binding agents for binding together carbon particles to form monoliths of desired shapes and sizes is achieved by using a cationic clay of phyllosilicate family, either alone or as a mixture with sodium silicate or calcium hydroxide, as the binding agent. Preferred cationic clay of the said binding agent formulation is bentonite, Na-exchanged bentonite or pillared analogues of bentonite, preferably AI-, Fe-, Cr-, Zr- or Ti- pillared bentonite. In the said binder formulation, the clay content is 5% to 15% by weight,
sodium silicate content is 0.5% to 1.5% by weight, and calcium hydroxide content in 0.5% to 1.5% by weight, all based on the weight of carbon in the monolith. The use of totally inorganic materials as binder in the subject invention eliminates the risk of emission of toxic and regulated gases or vapour during monolith production.
The method of binding and compacting microporous carbon particles to yield high strength monoliths is achieved by mixing microporous carbon powder with water dispersion of the cationic clay binding agent referred to above, drying the slurry to desired consistency, and consolidating the carbon-binder mix so prepared either by compression under a modest pressure in the range of 8,000 psi to 16,000 psi in a die/pelleting press without having to apply heating as done conventionally, or by the extrusion process using a conventional extruder to yield dense, mechanically strong blocks of regular shape and desired dimension. The binding and compaction method, as disclosed above, causes nearly a two-fold reduction in bulk volume and a corresponding increase of packing density. As, the method does not require application of heat, it results in providing substantial processing cost savings. Since, the method does not involve heating and cooling of the carbon-binder mix during monolith fabrication, the carbon-binder mix are formed into monoliths in substantially little time as less as 15 minutes, thereby reducing the process time.
Moreover, the monolith fabrication process is not associated with release of obnoxious gases or vapours of any kind and therefore is free from environmental hazards or occupational health and safety concerns. The method thus overcomes the difficulties associated with the methods known to the art on pelletization or briquetting of activated carbon powders for using them in natural gas storage.
Accordingly, the subject invention relates to a process for the preparation of an adsorbent prepared from granular microporous carbon preferably
from the coconut shell to be used in the storage of fuel gases preferably natural gas and its blends at low pressure and ambient temperature , comprising the steps of:
a) grinding granular microporous carbon into a fine powder;
b) preparing an aqueous dispersion of a binder formulation by
adding the ingredients of the said binder formulation to water
and thoroughly stirring the mixture,
c) mixing said powdered microporous carbon powder of step (a)
with the said aqueous dispersion of the binding agent to make a
carbon-binding agent-water slurry;
d) drying the said carbon-binding agent-water slurry by evaporating
water by heating while stirring until moisture content of the
resultant mix corresponds to the range from 80% to 100% by
weight, based on the weight of carbon in the mix;
e) consolidating the said dried carbon-binding agent mix at
ambient temperature, to obtain dense and mechanically strong
carbon monoliths of regular shapes and desired dimensions
suited for the storage of gaseous fuels such as natural gas and
Hythane; and
f) drying the said carbon monoliths at a temperature of 60° C to
70° C and at a temperature of 120°C to 140°C in air atmosphere
to obtain moisture free dried carbon monoliths.
The embodiment of the invention resides in a process for the preparation of a granular microporous carbon prepared by carbonization and controlled partial oxidation of coconut shells, comprising:-
crushing the said coconut shells to a particle size of 5-10mm, washing the said crushed coconut shells with water and drying the same at a temperature of 110°C
carbonizing the said washed and dried crushed coconut shells at a temperature of 600-700°C in an inert atmosphere to obtain the char,
crushing and screening the said char to obtain the mesh size of 8-30, oxidizing the said crushed and screened char at a temperature of 700-850°C to obtain granular microporous carbon having surface area of 1200-1800m2/g
The said microporous carbon may also be derived from carbonaceous materials selected from wood, coal, various kinds of pitch, and petroleum tar.
The said granular microporous carbon is obtained by chemical activation in an inert atmosphere with KOH, phosphoric acid or ZnCI2 at temperatures in the range 300-600°C or by carbonization at temperatures in the range 600-800°C , followed by controlled partial oxidation with CO2 or water vapour-N2 mixture at temperatures in the range 700-850°C.
The consolidation of the said dried carbon-binding agent mix is obtained by compressing the said mixture, at ambient temperature, inside a suitable die at a pressure of 8,000 -16,000 psi using a suitable pelleting press or by extruding the said dried carbon-binding agent mix in a extruder.
The binding agent is selected from clay mixed with or without sodium silicate or calcium hydroxide .
The clay material is selected from the group consisting of cationic clays and pillared analogues of cationic clays selected from hydrous layer silicates of the called phyllosilicate family consisting of groups as smectite, vermiculite, serpentine-kaolin, talc-pyrophyllite, mica, chlorite and sepiolite-palygorskite, typical examples being bentonite, atapulgite, hectorite, beidellite, fuller's earth halloysite, illite kaolin, montmorillonite and mullite.
The binding agent used is bentonite clay, Na- or K- exchanged bentonite clay, pillared bentonite clay (wherein pillaring species is a polycationic oxide of Al, Fe, Cr, Zr or Ti) and a combination thereof. The pillared analogues of
cationic clays are materials in which small cations of the cationic clay are replaced by large polyoxocationic species.
The present invention can better be understood with reference to the accompanying drawing, which is for illustrative purposes and should not in any way be construed to restict the scope of the invention keeping in view that certain modifications and improvements are possible without deviating from the scope off the invention.
According to the present invention there is provided a process for preparation of an adsorbent prepared from granular microporous carbon as herein described preferably from the coconut shell to be used in the storage of fuel gases preferably natural gas and its blends at low pressure and ambient temperature, comprising the steps of:
a) grinding granular microporous carbon into a fine powder;
b) preparing an aqueous dispersion of a binder formulation as herein
described by adding the ingredients of the said binder formulation to
•Ingredients water and thoroughly stirring the mixture,
c) mixing said powdered microporous carbon powder of step (a) with the
said aqueous dispersion of the binding agent to make a carbon-
binding agent-water slurry;
d) drying the said carbon-binding agent-water slurry by evaporating water
by heating while stirring until moisture content of the resultant mix
corresponds to the range from 80% to 100% by weight, based on the
weight of carbon in the mix;
e) consolidating the said dried carbon-binding agent mix at ambient
temperature, to obtain dense and mechanically strong carbon
monoliths of regular shapes and desired dimensions suited for the
storage of gaseous fuels such as natural gas and Hythane; and
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 depicts the block flow diagram showing various unit operations involved in the preparation of high density monolithic microporous carbon adsorbent from coconut shells for use in the storage, off-line transportation and distribution of natural gas and Hythane fuels.
DETAILED DESCRIPTION OF THE INVENTION
As illustrated in Figure 1, microporous carbon for binding and consolidation in accordance with the method disclosed in this invention is prepared from abundant and inexpensive coconut shells by the two-stage process of carbonization and controlled partial oxidation.
The process for the preparation of activated carbon from coconut shells for use as guard bed; comprises:
1 pyrolysis/carbonization of dried coconut shell at 873 K under an inert
atmosphere.
2 Activation of coconut char by partial gasification/controlled oxidation
with CO2at 1073K.
The coconut shells were cleaned of loose fibers and traces of kernel, crushed to particle size of about 10mm, washed with water and dried at a temperature of 110°C and then subjected to carbonization by heating to a temperature of 600°C to 700°C in an inert atmosphere. The char so produced is crushed, screened to 8-30 mesh size and then submitted to controlled partial oxidation with oxidising agents such as carbon dioxide and water vapour/nitrogen mixture at a temperature of 700°C to 850°C. The resultant product obtained is a granular microporous carbon having a surface area in the range 1200 -1800 m2/g.
The process for the preparation of carbon monoliths comprises the steps of:
a) crushing and milling the granular carbon into a fine powder by
conventional means preferably in a ball mill;
b) preparing an aqueous dispersion of the binder formulation by
adding the ingredients of the formulation to water at the rate of
25 mL to 50 ml water per gram of binder and thoroughly
agitating the mix;
c) mixing dry or pre-wetted carbon powder of step (a) with the
water dispersion of the binder of step (b) in appropriate
proportions to make a slurry of carbon and binding agent;
d) drying the carbon-binding agent slurry of step (c) to desired
moisture content ranging from 80% to 100% by weight based on
the weight of carbon used in step (c), by evaporating water
while stirring in any conventional blending-drying equipment;
e) consolidating carbon-binder mix of step (d) either by
compression, at ambient temperature, inside a suitable die
under a pressure of 8,000 to 16,000 psi using a suitable
pelleting press or by extrusion process using a suitable extruder
to provide high density, mechanically strong monoliths of desired shapes and sizes; and
f) subjecting the monoliths to drying in steps at a temperature of 60°C to 70°C and at temperature of 120°C to 140°C in air atmosphere to provide dried monoliths free of moisture.
the said binding agent used in step (b) is a clay material used alone or along with sodium silicate or calcium hydroxide, wherein the clay component is chosen from a group of cationic clays which are hydrous layer silicates of the so-called phyllosilicate family consisting of such groups as smectite, vermiculite, serpentine-kaolin, talc-pyrophylite, mica, brittle mica, chlorite and sepiolite-palygorskite, typical examples being bentonite, atapulgite, hectorite, beidellite, fuller's earth, halloysite, illite kaolin, montmorillonite and mullite or from a group consisting of pillared analogues of cationic clays which are materials in which small cations of cationic clay are replaced by large polyoxocationic species. The most preferred clays for use in the binding agent formulation include bentonite clay, sodium exchanged bentonite clay, pillared analogues of bentonite clay (such as AI-, Fe-, Cr-, Zr- or Ti- pillared bentonite clay) and combinations thereof. In the said binding agent formulation, the clay content is 5% to 15% by weight, sodium silicate content is 0.5% to 1.5% by weight, and calcium hydroxide content is 0.5% to 1.5% by weight, all based on the weight of carbon used in step (c).
In step (d), the moisture content of the carbon-binder paste is controlled in the range from 80% to 100% by weight based on the weight of carbon used in the preparation.
The packing density of the monoliths so obtained is nearly twice as much as that of the starting granular material. The binding agents used in the subject invention are thermally and chemically stable substances which at no stages of monolith fabrication process release obnoxious gases or vapours of any
kind and therefore there is no cause for concern from the viewpoint of environmental pollution or occupational health and safety hazard.
Another key feature of the method of this invention is that it does not suffer from the problem of pore plugging with binder. This is due to the fact that the major ingredient of binder formulation that is the clay component is applied as a dispersion in water rather than as a solution wherein size of the clay particulates are too large to penetrate into and occupy the micropores of carbon particles.
The method of binding and compaction as disclosed above is also suitable for binding and consolidation of other microporous solid adsorbents such as zeolites, carbon molecular sieves, alumino-phosphates, silicoaluminophosphates, silicas and aluminas.
The subject invention can better be understood with reference to the undermentioned examples, which should not be construed to restrict the scope of the invention.
EXAMPLES: EXAMPLE 1
Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 8,000 psi while maintaining the die at ambient temperature.
The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 600°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 700°C using carbon dioxide as the oxidizing agent.
The binding agents used are bentonite clay mixed with sodium silicate. The said carbon-binder mix was prepared by thoroughly blending the fine powder
of microporous carbon with the binding agent formulation dispersed in water, and then partially drying the blend (slurry) by evaporation of water under vigorous stirring until the moisture content of the resultant carbon-binder mix corresponded to the 80% by weight based on the weight of dry carbon powder used in the preparation.
The binder dispersion was prepared by adding in the binder ingredients 25 ml water per gram of the binding agent and vigorously stirring the binder-water mixture.
The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.78 g/ml.
The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 44.5 grams and having a total geometric volume 56.8 ml was used.
The natural gas storage capacity of the adsorbent was found to be 155 litres of natural gas per litre of adsorbent (155 v/v), as shown in Table 1.
TABLE 1:

(Table Removed)
EXAMPLE 2
Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 10,000 psi while maintaining the die at ambient temperature.
The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 650°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 750°C using water vapour - N2 mixture as oxidising agent.
The binding agents used were bentonite clay mixed with calcium hydroxide. The said carbon-binder mix was prepared by the process as explained in example 1.
The binder dispersion was prepared by adding in the binder ingredients 30 ml water per gram of the binding agent and vigorously stirring the binder-water mixture.
The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of calcium hydroxide to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.79 g/ml.
The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 36.6 grams and having a total geometric volume 54.9 ml was used.
The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 2.
TABLE 2:

(Table Removed)
EXAMPLE : 3
Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 12,000 psi while maintaining the die at ambient temperature.
The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 660°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 720°C using water vapour - N2 mixture as oxidising agent.
The binding agents used are Na-exchanged bentonite clay mixed with calcium hydroxide. The said carbon-binder mix was prepared by the process as explained in example 1.
The binder dispersion was prepared by adding the binder ingredients 35 ml water per gram of the binding agent and vigorously stirring the binder-water mixture.
The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of calcium hydroxide to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.79 g/ml.
The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 43.4 grams and having a total geometric volume 54.7 ml was used.
The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 3.
TABLE 3:

(Table Removed)
EXAMPLE : 4
Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 11,000 psi while maintaining the die at ambient temperature.
The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 670°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 735°C using water vapour - N2 mixture as the oxidizing agent.
The binding agents used Al-pillared bentonite clay mixed with sodium silicate.
The binder dispersion was prepared by adding the binder ingredients 40 ml water per gram of the binding agent and vigorously stirring the binder-water mixture.
The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.78 g/ml.
The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 4. TABLE 4:

(Table Removed)
2.0
EXAMPLE : 5
Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 14,000 psi while maintaining the die at ambient temperature.
The carbon monoliths are prepared from coconut shells by first carbonizing the coconut shells by heating to a temperature at 665°C under inert atmosphere and then subjecting the char so produced to controlled partial oxidation at 755°C using water vapour - N2 mixture as the oxidizing agent
The binding agents used are bentonite clay mixed with sodium silicate.
The binder dispersion was prepared by adding the binder ingredients 42 ml water per gram of the binding agent and vigorously stirring the binder-water mixture.
The clay to carbon ratio in the said carbon mix binder is 0.10g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.010g/g. The packing density of monolith was found to be 0.89 g/ml.
The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 36.6 grams and having a total geometric volume 54.9 ml was used.
The natural gas storage capacity of the adsorbent was found to be 156 litres of natural gas per litre of adsorbent (156 v/v), as shown in Table 5.
TABLE 5:
(Table Removed)
EXAMPLE : 6
Cylindrical shaped microporous carbon monoliths were fabricated by pressing carbon-binder mix contained in a cylindrical die by means of a hydraulic press at pressures of 16,000 psi while maintaining the die at ambient temperature.
The carbon monoliths are prepared from coconut shells as described in the foregoing examples.
The binding agents used were bentonite clay mixed with sodium silicate.
The binder dispersion was prepared by adding the binder ingredients 50 ml water per gram of the binding agent and vigorously stirring the binder-water mixture.
The clay to carbon ratio in the said carbon mix binder is 0.05g/g, while the ratio of sodium silicate to carbon ratio in carbon-binder mix is 0.005g/g. The packing density of monolith was found to be 0.87 g/ml.
The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at temperature of 22 °C. A batch of monoliths weighing 36.6 grams and having a total geometric volume 54.9 ml was used.
The natural gas storage capacity of the adsorbent was found to be 154 litres of natural gas per litre of adsorbent (154 v/v), as shown in Table 6.
TABLE 6

(Table Removed)
In the examples 1-6 , the packing densities of the cylindrical shaped monoliths were ranging from 0.67-0.78, which was determined from their physically measured dimensions and weights. A batch consisting of 5 to 7 monoliths were used in the measurement. The weights were recorded after degassing the monoliths under vacuum at a temperature of 130°C to 150°C. The mechanical strength of the monoliths was tested in an apparatus consisting of a screw-thread type hand press attached to an electronic balance. Monolithic pieces were subjected to a constant compressive pressure of 5 kg/cm2 (applied load divided by cross-sectional area of the monolith) and if found to remain intact it was concluded that the crushing strength of the particular monolith is at least 5 kg/cm2. Packing density of the granular samples was measured by filling a calibrated cylinder with a known sample weight and tapping the cylinder until a constant volume was obtained. The results are reported in Table A. The surface area of granular carbon samples was determined from nitrogen adsorption isotherm measured at liquid nitrogen temperature using BET equation. Packing density of the granular samples was measured by filling a calibrated cylinder with a known sample weight and tapping the cylinder until a constant volume was obtained. The results are reported in TABLE A

(Table Removed)
The binding and consolidation with the bentonite clay based binding agents result in a increase in the packing density of the microporous carbons prepared by using different oxidizing agents and having different surface
areas, thus making these materials suitable for the storage, transportation and distribution of gaseous fuels such as natural gas and Hythane, as illustrated in the Table A The mechanical strength of the carbon monoliths was found to be greater than 5 kg/cm2 which is adequate for most applications.
EXAMPLE :7
The polyvinyl alcohol based binder was used for fabrication of carbon monoliths, where Carbon used was a mixture of two coconut shell carbons (surface area ~ 1100 m2 and ~ 1600 m2/g) in the ratio 80 : 20. The binder was a mixture of 0.06 gram of polyvinyl alcohol (mol. wt. ~ 115,000), 0.004 gram of urea and 0.002 gram of ethylene glycol per gram of carbon. A slurry of carbon and binder ingredients in water was prepared, which was then partially evaporated to obtain a semi-dried mass with a moisture content of 40-50% by wt. (based on the total wt. of carbon used). Cylindrical shaped carbon monoliths of 20 mm diameter and 15-30 mm thickness were fabricated by pressing the semi-dried mass contained in a cylindrical die by means of a hydraulic press at a pressure of 50,000 psi while slowly heating the die to a temperature of 250°C and holding at that temperature for about 1 hour. The average packing density of dried (moisture free) monoliths was 0.84 g/mL.
The monoliths were tested for the storage of natural gas fuel under a pressure of 500 psig and at ambient temperature (22°C - 25°C). A batch of pellets weighing 44.7 grams and having a total geometric volume of 53.0 ml was used in the test. The natural gas storage capacity of the adsorbent was measured.
The natural gas storage capacity of the adsorbent was found to be 155 litres of gas per litre of adsorbent (155v/v), as shown in Table 7.
TABLE 7
Natural Gas Storage by Monolithic Microporous Coconut Carbon Monoliths fabricated using Polyvinyl Alcohol based Binder at 500 psig Pressure and Ambient Temperature

(Table Removed)
EXAMPLE 8
Microporous carbon particles, prepared from coconut shells by carbonization and partial oxidation with water vapour - N2 mixture and having surface area of about 1240 m2/g, were binded together and consolidated into cylindrical monoliths having 20 mm diameter and 20-40 mm thickness using an admixture of bentonite clay and sodium silicate as binding agent in the
amounts of 0.10 gram of bentonite clay and 0.010 gram of sodium silicate per gram of carbon. The average density of carbon monoliths was 0.79 g/mL.
The monoliths were tested for the storage of Hythane fuel (composition : 90 volume percent natural gas and 10 volume percent hydrogen) under a pressure of about 550 psig and at ambient temperature 25°C. A batch of monoliths weighing 44.5 grams and having a total geometric volume 56.5 mL was used in the test. The fuel storage capacity of the adsorbent was measured.
The Hythane storage capacity of the adsorbent was found to be 154 litres of Hythane per litre of adsorbent (154 v/v), as shown in Table 8.
TABLE 8
Hythane Storage by Monolithic Microporous Coconut Carbon at 550 psig Pressure and Ambient Temperature.

(Table Removed)
Examples 1-6, which deal with the monolithic carbon adsorbent fabricated using clay-based binders according to the method of this invention, clearly illustrate that the concentrated storage of natural gas at low pressures can be achieved by the method and adsorbent materials of this invention, thus avoiding the high pressures needed for storage as compressed gas (CNG) or cryogenic temperatures needed for storage as liquefied natural gas (LNG).
Example 7, deals with the monolithic carbon adsorbent monoliths fabricated using PVA-based binder. It is evident from the above mentioned table that PVA-based procedure is not only time-consuming, cumbersome and energy intensive but does not offer any advantages also. The natural gas storage capacity of the adsorbent fabricated using clay-based binders according to this invention, is similar to that of the adsorbent fabricated using PVA-based binder. On the other hand, the clay-based fabrication method has a number of advantages: it is simple and easy-to-practice ; fabrication process does not require costly heating ; and there is no risk of releasing obnoxious gases or vapours. Example 8 illustrates that the method and adsorbent material of this invention is also well suited for low pressure adsorptive storage of Hythane fuel.
The adsorbent material of the present invention is placed in the containers used for storing the natural gas and Hythane. Storage containers containing such adsorbent can be fuelled in a central filling station. Once the natural gas or Hythane stored in a container is exhausted due to usage as fuel in the desired application, the container can be brought back to the filling station, refilled and transported back to the user. In this way, the same batch of
adsorbent can be used for storage and distribution of natural gas or Hythane over and over again for years.
While the invention has been described and illustrated herein by references to various specific materials, procedures and examples, it is understood that the invention is not restricted to the particular materials and procedures selected for that purpose. Numerous variations of such details can be employed, as will be appreciated by those skilled in the art without deviating from the scope of the invention.

We claim:
1. A process for preparation of an adsorbent for the storage of fuel gases
preferably natural gas and its blends at low pressure and ambient
temperature, said adsorbent prepared from granular microporous
carbon as herein described preferably from the coconut shell,
comprising the steps of:
a) grinding granular microporous carbon into a fine powder;
b) preparing an aqueous dispersion of a binder as herein
described by adding the ingredients of the said binder to water
and thoroughly stirring the mixture,
c) mixing said powdered microporous carbon powder of step (a)
with the said aqueous dispersion of said binder to make a
carbon-binder-water slurry;
d) drying the said carbon-binder-water slurry by evaporating water
by heating while stirring until moisture content of the resultant
mix corresponds to the range from 80% to 100% by weight,
based on the weight of carbon in the mix;
e) consolidating the said dried carbon-binder mix at ambient
temperature, to obtain dense and mechanically strong carbon
monoliths of regular shapes and desired dimensions as herein
described suited for the storage of gaseous fuels such as
natural gas and Hythane; and
f) drying the said carbon monoliths at a temperature of 60°C to
70°C and subsequently at a temperature of 120°C to 140°C in
air atmosphere to obtain moisture free dried carbon monoliths.
2. A process as claimed in claim 1, wherein said microporous carbon is
derived from carbonaceous materials selected from coconut shell,
wood, coal, various kinds of pitch, and petroleum tar.
3. A process as claimed in claim 1, wherein the said consolidation of the said dried carbon-binder mix is prepared by compressing the
said mixture, at ambient temperature, inside a suitable die at a pressure of 8,000 -16,000 psi using a suitable pelleting press.
4. A process as claimed in claim 1, wherein the said binder is selected
from clay optionally mixed with sodium silicate or calcium hydroxide.
5. A process as claimed in claim 4, wherein the said clay is selected from
the group consisting of cationic clays and pillared analogues of cationic
clays.
6. A process as claimed in claim 5, wherein the said cationic clays is
selected from hydrous layer silicates of the phyllosilicate family
consisting of groups as smectite, vermiculite, serpentine-kaolin, talc-
pyrophyllite, mica, chlorite and sepiolite-palygorskite, typical examples
being bentonite, atapulgite, hectorite, beidellite, fuller's earth halloysite,
illite kaolin, montmorillonite and mullite.
7. A process as claimed in claim 4, wherein the said binder is bentonite
clay, Na- or K- exchanged bentonite clay, pillared bentonite clay
(wherein pillaring species is a polycationic oxide of Al, Fe, Cr, Zr or Ti)
and a combination thereof.
8. A process as claimed in claim 4, wherein the said binder is a mixture
of bentonite clay and sodium silicate.
9. A process as claimed in claim 8, wherein the said binder is a mixture
having 5% to 15% bentonite content by weight and 0.5% to 1.5%
sodium silicate content by weight, both based on the weight of the
carbon.
10. A process as claimed in claim 4, wherein the said binder is a mixture of
bentonite clay and calcium hydroxide.
11. A process as claimed in claim 10, wherein the said binder is a mixture
of Na-exchanged bentonite clay and calcium hydroxide.
12. A process as claimed in any of preceding claims, wherein the said
binder is used at a ratio of 0.05 - 0.15 gram of bentonite clay per gram
of microporous carbon powder.

13. An adsorbent prepared by the process as claimed in claim 1, applied in
the storage of fuel gases preferably natural gas and its blends
comprising monolithic microporous carbon adsorbent contained in a
portable container with natural gas or Hythane under a pressure of
about 400 psig to 600 psig at ambient temperature.
14. A process for preparation of an adsorbent for the storage of fuel gases
preferably natural gas and its blends at low pressure and ambient
temperature, prepared from granular microporous carbon preferably
from the coconut shell, substantially as herein described with reference
to the accompanying drawings.

Documents:

557-del-2000-abstract.pdf

557-del-2000-claims.pdf

557-del-2000-correspondence-others.pdf

557-del-2000-correspondence-po.pdf

557-del-2000-description (complete).pdf

557-DEL-2000-Form-1-(07-03-2008).pdf

557-del-2000-form-1.pdf

557-del-2000-form-13.pdf

557-del-2000-form-19.pdf

557-del-2000-form-2.pdf

557-del-2000-form-26.pdf

557-del-2000-form-3.pdf

557-del-2000-gpa.pdf

557-del-2000-petition-138.pdf

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Patent Number

230927

Indian Patent Application Number

557/DEL/2000

PG Journal Number

13/2009

Publication Date

27-Mar-2009

Grant Date

28-Feb-2009

Date of Filing

06-Jun-2000

Name of Patentee

GAS AUTHORITY OF INDIA LIMITED

Applicant Address

GAIL BUILDING , 16. BHIKAJI CAMA PLACE, R.K. PURAM, NEW DELHI-110 066, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	RUPESH NARAIN PANDEY	34 OLD COLONY TRAIL, GUELPH, ONTARIO, CANADA, NIG 4A9 .
2	ASHUTOSH RASTOGI	GAS AUTHORITY OF INDIA LIMITED 16. BHIKAJI CAMA PLACE, R.K. PURAM, NEW DELHI-110 066, INDIA.
3	PARIVESH CHUGH	GAS AUTHORITY OF INDIA LIMITED, 16. BHIKAJI CAMA PLACE, R.K. PURAM, NEW DELHI-110 066, INDIA.
4	SHAMSUDDIN AHMED	247 COLE ROAD, GUELPH, ONTARIO, CANADA, N1G 3K1.
5	RAJ NARAIN PANDEY	34 OLD COLONY TRAIL, GUELPH,ONTAQRIO, CANADA, N1G 4A9 .

PCT International Classification Number

B65G 1/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA