Description:FIELD OF INVENTION
The present invention relates to desulphurization of hydrocarbon fuel gases. More specifically, the present invention relates to a process for desulphurization of a hydrocarbon fuel gas by selective adsorption of mercaptans using an adsorbent system based on a nano-porous nitrogen enriched activated carbon framework. The process disclosed herein efficiently removes ethyl mercaptan from ethyl mercaptan additized liquified petroleum gas (LPG) and provides odor-free liquified petroleum gas (LPG).

BACKGROUND OF THE INVENTION
Liquefied petroleum gas (LPG) is one of the premium products of oil refineries, which is a high purity mixture of propane, isobutane, and n-butane. In natural vapor state, LPG is an odorless and colorless gas. These properties make LPG an eco-friendly alternative to environmentally hazardous propellant chlorofluorocarbons in aerosol Industry. LPG also finds application in Fuel Cell technology.

While LPG in natural vapor state is an odorless gas, LPG produced through crude oil processing contains malodorous sulfur-containing compounds such as H2S, methyl mercaptan, ethyl mercaptan, and carbonyl sulfide (COS) etc. as impurity. These impurities can be present naturally in petroleum gas or may be incorporated by decomposition of higher sulfur compounds during distillation and cracking operations. Lighter mercaptan like ethyl mercaptan is added to LPG during transport as a mandatory norm. These sulfur-containing compounds limit application of LPG in various industries. Hence, removal of added ethyl mercaptan from LPG at site of use in these industries is highly essential.

Various technologies such as chemical absorption, physical absorption, cryogenic distillation, and membrane system separation etc., have been explored for the removal of these sulfur-containing compounds from hydrocarbon fuel gases like LPG.

Among these, amine-based chemical adsorption is extensively explored. Campbell et. al. report that separation of sulphur-containing compounds like H2S, COS using amine based chemical adsorption technique majorly relies on the acidic nature of these compounds. The said method is not efficient for removal of lighter mercaptan like ethyl mercaptan because lighter mercaptans are not as acidic as hydrogen sulfide and have less affinity towards amines. Additional steps like drying or pre-treatment are required to reduce the mercaptan concentration to an appropriate level.

To separate lighter mercaptan, physical adsorbents such as unmodified and modified molecular sieves (13x type) and silica gels have been widely studied. Salem et. al. reports that 13x type molecular s sieves can be used for the separation of lighter mercaptans from a petroleum fraction if mercaptan content is lower than about 25 µg/g.
The zeolite-based physical adsorption technologies are also reported but zeolites as adsorbent fails to decrease the mercaptan concentrations to approx. nil or below ppm level when mercaptan concentration is higher than 25 µg/g. Further, such adsorption systems require additional treatment steps of zeolite adsorbent or modification of zeolites with metal ions to reduce mercaptan to a desired level.

US3816975 discloses desulfurization of an olefin-containing paraffin feedstock by selective adsorption using large pore zeolitic molecular sieves. The process requires contacting olefin with a sorbate-free adsorbent and desorbing olefin from the said adsorbent by displacement with water to avoid choking of zeolitic molecular sieves.

US3864452 discloses desulphurization of a gas stream by adsorption on crystalline molecular sieves, followed by the catalytic oxidation of the sulfur compounds on the molecular sieves to elemental sulfur. The process requires four high pressure adsorbing beds and two low pressure beds, The adsorption beds require regeneration by passing a substantially oxygen depleted gas through the column at above 825 ?. The process operates at high pressure and temperature.

US4795545 discloses a process for removal of sulfur compounds, oxygenates, and water from a light hydrocarbon feedstock. The process involves adsorption in two adsorption zones with two different adsorbents. The process is complex and costly due to the requirement of two types of adsorbents operating at different conditions.

US7311758 discloses desulfurization of natural gas using an adsorbent. The process requires de-acidization and dehydration before desulfurization. Further, a water-laden purge gas is required to remove mercaptans from the mercaptan-laden adsorbent. Therefore, the process is complex and requires many units.

Physical adsorption using activated carbon has been explored for removal of sulfur. The high porosity and surface area of activated carbon makes it suitable candidate for removal of sulfur from both gaseous and aqueous phases. Removal of H2S from varied streams have been explored. The H2S adsorption capacity of a typical coal-based activated carbon is only 0.01 to 0.02 g/cc, and the efficiency of H2S removal is often meager. Therefore, a large quantity of activated carbon is required for the removal.

WO2015038965A1 provides catalytic activated carbon structures and the methods of removing sulfur-containing compounds from fluid stream using such catalytic activated carbon structures. The catalytic activated carbon structures comprise activated carbon modified through incorporation of elemental nitrogen, organic nitrogen, metal/metal oxides etc., on the surface. However, the said method is restricted to removal of H2S only.

US4540842 discloses two-stage desulfurization of a pentane stream derived from fractionation of sulfur-containing natural gasoline. In the first zone, the said stream is contacted with molecular sieve adsorbent to remove dimethyl sulfide and in the second zone the output of first zone is contacted with activated carbon to remove carbon disulfide. The two-stage desulfurization is a complex process requiring two types of physical adsorbents requiring different operating conditions.

WO2003095594 discloses a method for the removal of gaseous organo-sulfur compounds from a fuel gas stream like natural gas, LPG etc., using an adsorbent comprising clay mineral and activated charcoal. LPG comprising lower concentration of ethyl mercaptan (2 ppmv) at gas flow velocity of 0.5 liters/min is used for desulphurization over an adsorbent bed at 40 ?.

Still, there is an urgent need for a simple desulphurization process wherein mercaptans, more specifically ethyl mercaptan can be removed effectively from a hydrocarbon fuel stream like mercaptan additized LPG at milder temperature and pressure.

OBJECTIVES OF THE INVENTION
The primary objective of the present invention is to provide a process for the selective adsorption and removal of organic thiols from non-desulfurized or mercaptan additized hydrocarbon fuel stream.

Another objective of the present invention is to provide a process for efficient desulphurization of a hydrocarbon fuel gas.

Another objective of the present invention is to provide a process for desulphurization of a hydrocarbon fuel gas with mercaptan content in a range of 5 ppm to 200 ppm.

Another objective of the present invention is to provide a process for desulphurization of a hydrocarbon fuel gas at ambient temperature and pressure.

Another objective of the present invention is to provide a process for desulphurization of a hydrocarbon fuel gas without any additional drying or pretreatment step.

Another objective of the present invention is to provide a process for removal of ethyl mercaptan from ethyl mercaptan additized liquified petroleum gas to obtain odor-free liquified petroleum gas.

SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts in a simplified format that are further described in the detailed description of the invention.
In a first aspect of the present invention, the present invention discloses a process desulphurization of a hydrocarbon fuel stream, comprising: passing the hydrocarbon fuel stream through an adsorption column comprising an arrangement of adsorbent beds packed with an adsorbent comprising an activated carbon framework enriched with nitrogen to obtain a desulphurized hydrocarbon fuel stream.
In one of the embodiments, the hydrocarbon fuel stream used in the process disclosed herein is selected from a group comprising of mercaptan additized liquefied petroleum gas, natural gas, methane, ethane, butane, propane, isobutane, and a combination thereof.

In one of the embodiments, the hydrocarbon fuel stream used in the process disclosed herein comprises a mercaptan selected from a group comprising of methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, iso-propyl mercaptan, n-butyl mercaptan, iso-butyl mercaptan, tert-butyl mercaptan, phenyl mercaptan, and a combination thereof.

In one of the embodiments, the hydrocarbon fuel stream used in the process disclosed herein comprises ethyl mercaptan as mercaptan.

In one of the embodiments, the hydrocarbon fuel stream used in the process disclosed herein comprises a mercaptan in a concentration range of 5 ppm to 200 ppm.

In one of the embodiments, the process disclosed herein reduces the concentration of mercaptans in the hydrocarbon fuel stream to less than 1 ppm, preferably ranging from 0.1 ppm to 1 ppm.

In one of the embodiments, the adsorbent comprising activated carbon framework enriched with nitrogen used in the process disclosed herein is enriched with nitrogen by doping 1% w/w to 11% w/w of nitrogen with respect to the total carbon content.

In one of the embodiments, the adsorbent comprising activated carbon framework enriched with nitrogen used in the process disclosed herein has pore volume in a range of 1.9 cc/g to 2.3 cc/g, pore radii in range of 1.4 nm to 1.10 nm, micropore area in a range of 400 m2/g to 1000 m2/g, and pore diameter in a range of 2.20 nm to 2.80 nm.

In one of the embodiments, the adsorbent comprising activated carbon framework enriched with nitrogen used in the process disclosed herein has specific surface area in a range of 1400 m2/g to 1700 m2/g.

In one of the embodiments, the process disclosed herein is carried out at a pressure in a range of 25 mbar to 1.5 bar and at a temperature in a range of 5 ? to 50 ?.

In one of the embodiments, the process disclosed herein involves passing of hydrocarbon fuel stream through an adsorbent column comprising an arrangement of adsorption beds at a flow rate in a range of 1 litres/minute to 3.0 litres/minute, preferably in a range of 1.2 litres/minute to 2.0 litres/minute.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates the general scheme of the desulphurization process disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION
Those skilled in the art will be aware that the process provided herein is subject to variations and modifications other than those specifically described. It is to be understood that the process provided herein includes all such variations and modifications. The process provided herein also includes all such steps of the process, features of the system, referred to or indicated in this specification, individually or collectively and all combinations of any or more of such steps or features.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of/consisting of”.

Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used herein interchangeably.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred method, and materials are now described.

The process provided herein is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally equivalent products and processes are clearly within the scope of the disclosure, as described herein.
In one aspect of the present invention, there is provided a process desulphurization of a hydrocarbon fuel stream, comprising: passing the hydrocarbon fuel stream through an adsorption column comprising an arrangement of adsorbent beds packed with an adsorbent comprising an activated carbon framework enriched with nitrogen to obtain a desulphurized hydrocarbon fuel stream.

The term “desulphurization” used herein denotes reducing content of mercaptans in a particular feedstream.

The term “desulphurized hydrocarbon fuel stream” used herein denotes a hydrocarbon fuel stream with reduced content of mercaptan.

The process disclosed herein is reducing content of mercaptan in a hydrocarbon fuel stream to less than 1 ppm, preferably ranging from 0.1 ppm to 1ppm.

The term “mercaptans” or “organic thiol” used herein denotes compounds that contain the characteristic functional group, -SH (sulfhydryl). Sulfhydryl compounds (R-SH) and disulfide compounds (RS-SR) are easily interconverted via oxidation/reduction reactions.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream comprising mercaptans selected from a group of aliphatic mercaptans, aromatic mercaptans, and a combination thereof.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream comprising a mercaptan selected from methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, iso-propyl mercaptan, n-butyl mercaptan, iso-butyl mercaptan, tert-butyl mercaptan, phenyl mercaptan, and a combination thereof.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream comprising ethyl mercaptan.

The process disclosed herein desulphurize a hydrocarbon fuel stream. The term “hydrocarbon fuel stream” or “fuel stream” used herein denotes a hydrocarbon stream intended to be used as fuel.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream selected from liquified petroleum gas, natural gas, methane ethane, butane propane, isobutane, and a combination thereof.

In one of the embodiments, the process disclosed herein desulphurize mercaptan-additized liquified petroleum gas (LPG).

The term “LPG” or “Liquefied petroleum gas” or “LP gas” used herein denotes a gaseous mixture comprising hydrocarbon gases majorly selected from propane, propylene, butylene, isobutane, and n-butane.

The term “ethyl mercaptan additized hydrocarbon fuel stream” or “ethyl mercaptan additized liquefied petroleum gas stream” used herein denotes a hydrocarbon fuel stream or liquefied petroleum gas stream containing 5 ppm to 200 ppm of ethyl mercaptan.

The concentration of mercaptans in the hydrocarbon fuel stream can vary and depends on the source from which the hydrocarbon fuel stream originates.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream containing 5 ppm to 200 ppm of mercaptans.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream containing 200 ppm of mercaptans.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream containing 181 ppm of mercaptans.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with nano-porous activated carbon framework enriched with nitrogen.

In the process disclosed herein, the hydrocarbon fuel stream is passed through an adsorbent column comprising an arrangement of adsorbent beds at a flow rate in a range of 1.0 litres/minute to 3.0 litres/minute.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the hydrocarbon fuel stream through an absorbent column comprising an arrangement of adsorbent beds at a flow rate in a range of 1.2 litres/minute to 3.0 litres/minute.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the hydrocarbon fuel stream through an absorbent column comprising an arrangement of adsorbent beds at a flow rate of 1.2 litres/minute.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the hydrocarbon fuel stream through an absorbent column comprising an arrangement of adsorbent beds at a flow rate of 2.0 litres/minute.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream comprising mercaptans by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with nano-porous activated carbon framework enriched with nitrogen. The process disclosed herein involves removal of mercaptans from the said fuel stream through selective adsorption of mercaptans over the adsorbent.

The term “adsorbent” used here denotes a material which is used to attract and collect the targeted contaminant molecule from a liquid or a gas by using a combination of physical and chemical forces.

The adsorbent used in process disclosed herein is nitrogen modified activated carbon framework.

The term “activated carbon framework” or “activated carbon” or “carbon material” used herein denotes a carbonaceous substance made from carbonaceous source materials, have high carbon content, low ash content, and significant volatiles matter.

The term “adsorbent column” used herein denotes a solid vertical structure which is used to fill the adsorbent.

The process provided by the present invention involves adsorption of mercaptans over an adsorbent. The said adsorbent is packed in plurality of adsorbent beds arranged one above the other in vertical orientation to constitute an adsorbent column. The height of adsorbent column depends on various factors such as the amount of adsorbent, quantity of LPG to pass through it, and the concentration of mercaptan in LPG. In most preferred embodiments, height of the adsorption column is at least four times the width of the adsorption column and the ratio between height of the adsorbent column and diameter of the adsorbent column (L/D) is in range of 4 to 12.

The adsorbent column used in the present invention may comprise at least one protective layer disposed between two consecutive adsorbent beds in the adsorbent column. Each protective layer comprises spheres composed of material selected from glass, quartz, ceramic, and a combination thereof. The spheres present in the protective layer have distribution of diameter in the range of 2 mm to 12 mm and the combined thickness of all the protective layers in the adsorption column is less than the diameter of the adsorption column.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the said fuel stream over an adsorbent column comprising an arrangement of adsorbent beds comprising activated carbon framework modified with 1% w/w to 11% w/w of nitrogen with respect to total carbon content of the said adsorbent.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the said fuel stream over an adsorbent column comprising an arrangement of adsorbent beds comprising activated carbon framework modified with 6.8% w/w to 7.1% w/w of nitrogen with respect to total carbon content of the said adsorbent.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the said fuel stream over an adsorbent column comprising an arrangement of adsorbent beds comprising activated carbon framework modified with 4.3% w/w to 5.0% w/w of nitrogen with respect to total carbon content of the said adsorbent.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing the said fuel stream over an adsorbent column comprising an arrangement of adsorbent beds packed with activated carbon framework modified with 9.8 % to 11.0 % of nitrogen with respect to total carbon content of the said adsorbent.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with nano-porous activated carbon framework enriched with nitrogen, enrichment of nitrogen in the nano-porous activated carbon is done using doping technique.

Any doping technique used in prior art can be used to effect enrichment of the nano-porous activated carbon.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein specific surface area of the adsorbent is in a range of 1400 m2/g to 1700 m2/g as determined by BET surface area analyser.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein specific surface area of the adsorbent is 1424 m2/g.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the specific surface area of the adsorbent is 1448 m2/g.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein pore volume of the adsorbent is in a range of 1.9 cc/g to 2.3 cc/g as determined by BET surface area analyser.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein micropore area of the adsorbent is in a range of 400 m2/g to 1000 m2/g as determined by BET surface area analyser.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework is enriched with nitrogen, wherein the micropore area of the adsorbent is 972 m2/g.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the micropore area of the adsorbent is 875 m2/g.

In one of the embodiments, the process disclosed herein desulphurize a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the micropore area of the adsorbent is 425 m2/g.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the adsorbent has pore diameter in the range of 2.2 nm to 2.8 nm as determined by BET surface area analyser.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the adsorbent has Barrett-Joyner-Halenda (BJH) pore radii in range of 1.4 nm to 1.10 nm.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the adsorbent has Barrett-Joyner-Halenda (BJH) pore radii of 1.5 nm.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an arrangement of adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein the adsorbent has Barrett-Joyner-Halenda (BJH) pore radii of 1.8 nm.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein adsorption occurs at ambient pressure in a range of 25 mbar to 1.5 bar.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein adsorption occurs at pressure of 25 mbar.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein adsorption occurs at ambient temperature in a range of 5 ? to 50 ?.

The process disclosed herein involves desulphurization of a hydrocarbon fuel stream by passing said fuel stream through an adsorbent column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen, wherein adsorption occurs at ambient temperature of 30 ?.

In another aspect of the present invention, the present invention discloses a process for desulphurization of ethyl mercaptan additized liquefied petroleum gas, comprising: passing the ethyl mercaptan additized liquefied petroleum gas through an adsorption column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen to obtain a desulphurized hydrocarbon fuel stream; wherein the ethyl mercaptan is having a concentration in a range of 5 ppm to 200 ppm; wherein the activated carbon framework is enriched with nitrogen by doping 1% w/w to 11% w/w of nitrogen with respect to the total carbon content.

In another aspect of the present invention, the present invention discloses a process for desulphurization of ethyl mercaptan additized liquefied petroleum gas, comprising: passing the ethyl mercaptan additized liquefied petroleum gas through an adsorption column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen to obtain a desulphurized hydrocarbon fuel stream; wherein the ethyl mercaptan is having a concentration in a range of 5 ppm to 200 ppm; wherein the activated carbon framework is enriched with nitrogen by doping 1% w/w to 11% w/w of nitrogen with respect to the total carbon content; wherein the process is carried out at a pressure in a range of 25 mbar to 1.5 bar and at temperature in a range of 5 ? to 50 ?.

In another aspect of the present invention, the present invention discloses a process for desulphurization of ethyl mercaptan additized liquefied petroleum gas, comprising: passing the ethyl mercaptan additized liquefied petroleum gas through an adsorption column comprising an arrangement of adsorbent beds packed with an adsorbent comprising nano-porous activated carbon framework enriched with nitrogen to obtain a desulphurized hydrocarbon fuel stream; wherein the ethyl mercaptan is having a concentration in a range of 5 ppm to 200 ppm; wherein the activated carbon framework is enriched with nitrogen by doping 1% w/w to 11% w/w of nitrogen with respect to the total carbon content; wherein the activated carbon framework has pore volume in a range of 1.9 cc/g to 2.3 cc/g, pore radii in range of 1.4 nm to 1.10 nm, micropore area in a range of 400 m2/g to 1000 m2/g, and pore diameter in a range of 2.2 nm to 2.8 nm.

EXAMPLES
Having described the basic aspects of the present invention, the following non-limiting examples illustrate specific embodiments thereof. Those skilled in the art will appreciate that many modifications may be made in the invention without changing the essence of invention.

A packed column adsorption bed having L/D ratio in range of 4 to 7 is loaded with 1 gram of nano-porous activated carbon adsorbent and adsorbent beds are subjected to a cycle of adsorption. The adsorption process is initially carried out to desulphurize 3 litres to 50 litres of LPG at 25mbar pressure and ambient temperature. A breakthrough test is carried out with ethanethiol rich hydrocarbon fuel stream having total sulphur concentrations of 27 ppm, 88 ppm and 104 ppm. To determine breakthrough adsorption concentration for the adsorption bed, 5 litres to 50 litres of LPG is pass through the adsorption bed and desulphurized hydrocarbon fuel stream is collected. The total content of sulphur in the desulphurized hydrocarbon fuel stream is analysed through gas chromatography GC-SCD analysis.

Four types of nano-porous activated carbon framework adsorbents having nitrogen concentration in a range of 1% w/w to 11% w/w with respect to total weight of carbon are tested during the process for their sulphur adsorption capacity. Example 1 to Example 4 provides the process and results obtained.

Example 1: Desulphurisation process using nano-porous activated carbon framework [52-200 ppm of mercaptan, 6.8% w/w N-doped]
An ethyl mercaptan rich LPG stream consisting of 52 ppm, 169.8 ppm and 200 ppm of ethyl mercaptan is passed through 1 g of catalytically active nano-porous activated carbon adsorbent having nitrogen content of 6.8% w/w with respect to total weight of the carbon material [NDC-1]. The flow rate of mercaptan rich gas is varied from 1.2 litres/minute to 2.0 litres/minute. The mercaptan containing LPG stream was passed through an adsorbent column having L/D ratio of 4.5. The LPG flow is terminated after 3L/5L of ethyl mercaptan rich LPG stream passes through the adsorbent column and ethyl mercaptan lean LPG stream was collected. The collected ethyl mercaptan lean LPG stream was analyzed for sulphur content using gas chromatography. The results are depicted in Table 1 below.
Table-1: Demonstrates the desulphurisation efficiency of NDC-1 [52-200 ppm of mercaptan]
S. No. Amount of fuel gas Adsorbent Flow rate Total mercaptan (in) ppm Total sulphur (in) ppm Total mercaptan (out) ppm Total sulphur (out) ppm
1 3L NDC1 1.2l/min 52.1 27 <1 <1
2 3L NDC1 1.2l/min 169.8 88 <1 <1
3 5L NDC1 2l/min 200.7 104 3.8 2

Example-2: Desulphurisation process using nano-porous activated carbon framework [181 ppm of mercaptan, 6.8% w/w N-doped]
An ethyl mercaptan rich LPG stream consisting of 181 ppm of ethyl mercaptan is passed through 1 g of catalytically active nano-porous activated carbon adsorbent having nitrogen content of 6.8% w/w with respect to total weight of the carbon material [NDC-1]. The ethyl mercaptan rich LPG stream was passed through an adsorbent column having L/D ratio of 6.0. The flow rate of ethyl mercaptan rich LPG stream is maintained at 2 litres/minute. Total up to 50L of ethyl mercaptan rich LPG stream is passed the column and then it was terminated and mercaptan lean LPG stream was collected. The collected mercaptan lean LPG stream was analyzed for sulphur content using gas chromatography. The results are depicted in Table 2 below.
Table-2: Demonstrates the desulphurisation efficiency of NDC-1 [181 ppm of ethyl mercaptan]
S. No. Amount of fuel gas Adsorbent Total mercaptan (in) ppm Total sulphur(in) ppm Total mercaptan(out) ppm Total sulphur(out) ppm
1 5L NDC1 181.4 94.0 3.8 2.0
2 10L NDC1 181.4 94.0 4.0 2.1
3 20L NDC1 181.4 94.0 5.0 2.6
4 30L NDC1 181.4 94.0 5.2 2.7
5 40L NDC1 181.4 94.0 8.4 4.4
6 50L NDC1 181.4 94 28.9 15

Example-3: Desulphurisation process using nano-porous activated carbon framework [200 ppm of ethyl mercaptan, 4.3% w/w to 11% w/w N-doped]
An ethyl mercaptan rich LPG stream consisting of 200 ppm of ethyl mercaptan is passed through 1 g of catalytically active nano-porous activated carbon adsorbent having nitrogen content in range of 4.3% w/w to 11% w/w with respect to total weight of the carbon material [NDC-1 to NDC-3] and activated carbon adsorbent [NDC-4]. The flow rate of ethyl mercaptan rich LPG stream is maintained at 2 litres/minute. Total up to 5L of ethyl mercaptan rich LPG stream is passed through the adsorbent column having L/D ratio in range of 4 to 7 and then it was terminated, and ethyl mercaptan lean LPG stream was collected. The collected ethyl mercaptan lean LPG stream was analyzed for sulphur content using gas chromatography. The results are depicted in Table 3 below.
Table-3: Demonstrates the desulphurisation efficiency of NDC-1 to NDC-4

S.
No. Amount of fuel gas Adsorbent % of Nitrogen (w/w) Total mercaptan (in) ppm Total sulphur (in) ppm Total mercaptan (out) ppm Total sulphur (out) ppm
1 5L NDC1 6.8-7.1 200.7 104.0 3.8 2.0
2 5L NDC2 4.3-5.0 200.7 104.0 12.7 6.6
3 5L NDC3 9.8-11.0 200.7 104.0 106.1 55.0
4 5L NDC4 0 200.7 104.0 198.8 103.0

Example-4: Breakthrough ethyl mercaptan adsorption capacity of adsorbent at different nitrogen enrichment percent
The breakthrough experiments for evaluating the dynamic performance of nitrogen enriched activated carbon for the separation and adsorption of ethyl mercaptan are conducted by passing 50 L of ethyl mercaptan rich LPG stream through an adsorbent column having L/D ratio in range of 4 to 7. Concentration of total sulphur is maintained at 104 ppm i.e., total 200 ppm of ethyl mercaptan. The total ethyl mercaptan adsorbed on activated carbon framework and nitrogen enriched activated carbon framework is estimated using XRF analysis. As discussed earlier the adsorption of mercaptans in nitrogen enriched activated carbon framework depends on the micropores induced due to the nitrogen enrichment of carbon framework, therefore breakthrough adsorption capacity follows the similar trend of micropore area as shown in Table-4 below:
Table-4: Breakthrough ethyl mercaptan adsorption capacity of adsorbents at different nitrogen enrichment percent
S. No. Adsorbent % of Nitrogen (w/w) Surface area (m2/g) Micropore area (m2/g) BJH pore radii (nm) Ethyl mercaptan adsorption breakthrough concentration (ppm)
1 NDC1 6.8-7.1 1700 973 1.5 10868
2 NDC2 4.3-5.0 1448 875 1.5 7054
3 NDC3 9.8-11.0 1424 425 1.8 6524
4 NDC4 0 782 0 9.3 741

The above findings reveal that the present invention provides an efficient process for desulphurization of a hydrocarbon fuel gas to desulphurized odor-less hydrocarbon fuel gas with mercaptan content as low as 1 ppm mercaptan at ambient temperature and pressure.
It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the constructions set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense. The invention has been described with reference to preferred and alternate embodiments. Modifications and alterations will become apparent to those skilled in the art upon reading and understanding the detailed discussion of the invention provided herein. This invention is intended to include all such modifications and alterations insofar as they come within the scope of the present invention. These and other modifications of the preferred embodiments as well as other embodiments of the invention will be obvious from the disclosure herein, whereby the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.
Finally, to the extent necessary to understand or complete the disclosure of the present invention, all publications, patents, and patent applications mentioned herein are expressly incorporated by reference therein to the same extent as though each were individually so incorporated. , Claims:1. A process for desulphurization of a hydrocarbon fuel stream, comprising:
passing the hydrocarbon fuel stream through an adsorption column comprising an arrangement of adsorbent beds packed with an adsorbent comprising an activated carbon framework enriched with nitrogen to obtain a desulphurized hydrocarbon fuel stream.
2. The process as claimed in claim 1, wherein the hydrocarbon fuel stream is selected from a group comprising of mercaptan additized liquefied petroleum gas, natural gas, methane ethane, butane, propane, isobutane, and a combination thereof.
3. The process as claimed in claim 2, wherein the hydrocarbon fuel stream comprises a mercaptan selected from a group comprising of methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, iso-propyl mercaptan, n-butyl mercaptan, iso-butyl mercaptan, tert-butyl mercaptan, phenyl mercaptan, and a combination thereof.
4. The process as claimed in claim 3, wherein the mercaptan is ethyl mercaptan and wherein the mercaptan is having a concentration in a range of 5 ppm to 200 ppm.
5. The process as claimed in claim 1, wherein the process reduces concentration of mercaptan in the hydrocarbon fuel stream to less than 1 ppm, preferably ranging from 0.1 ppm to 1ppm.
6. The process as claimed in claim 1, wherein the activated carbon framework is enriched with nitrogen by doping 1% w/w to 11% w/w of nitrogen with respect to the total carbon content.
7. The process as claimed in claim 1, wherein the activated carbon framework has pore volume in a range of 1.9 cc/g to 2.3 cc/g, pore radii in range of 1.4 nm to 1.10 nm, micropore area in a range of 400 m2/g to 1000 m2/g, and pore diameter in a range of 2.2 nm to 2.8 nm.
8. The process as claimed in claim 1, wherein the activated carbon framework has specific surface area in a range of 1400 m2/g to 1700 m2/g.
9. The process as claimed in claim 1, wherein the process is carried out at a pressure in a range of 25 mbar to 1.5 bar and at a temperature in a range of 5 ? to 50 ?.
10. The process as claimed in claim 1, wherein the hydrocarbon fuel stream is passed over the adsorption bed at a flow rate in a range of 1 liters/minute to 3.0 liters/minute, preferably in a range of 1.2 litres/minute to 2.0 litres/minute.