DESC:
FIELD OF INVENTION
The present invention relates to a novel composition for selective separation of benzene from a mixture of hydrocarbons and other volatile organic compounds (VOC). Specifically, the present invention relates to oxidizing column used in prefilter tubes of gas monitoring devices comprising the novel composition.

BACKGROUND OF THE INVENTION
Benzene, which is known as a carcinogen, is one of the main components in petroleum products and other fuel oils. Benzene is a colorless liquid, which evaporates quickly when exposed to air. The concentration of benzene ranges from 0.1 ppb to 2.0 ppb in rural areas and from 6.0 ppb to 20 ppb in urban areas. Inhalation of benzene by humans causes short-term effects such as drowsiness, dizziness, headache, as well as eye, skin and respiratory tract irritation, and, at high levels may lead unconsciousness. The United States Environmental Protection Agency (EPA) states that the long-term effects from benzene inhalation exposure has caused various disorders in the blood, including reduced numbers of red blood cells and aplastic anemia. In the USA, the nationwide acceptable concentration of benzene content in gasoline is set about 0.62 % volume by Environmental Protection Agency. However, at crude oil, gasoline and other fuel oils work stations, it is found that concentration of benzene varies from 0.1% - 3%.

Therefore, the measurement of benzene concentration at work places such as gasoline filling station, chemical manufacturing sites, chemical filling stations and crude refineries is crucial to ensure safety of the workers handling such chemicals.
Generally, electronic gas monitors and other devices like detector tubes are used for the detection and monitoring of hydrocarbons and volatile organic compounds (VOC) in these working areas. Among them, photo-ionization detector (PID) based electronic instrument is popularly used for the detection of VOCs. To detect benzene selectively, the sample gas is first drawn though a PID prefilter tube and exposed to the PID instrument. The PID prefilter tube selectively oxidizes all hydrocarbons (VOC) except benzene present in the sample gas and allows benzene to pass to the PID instrument.

Conventionally, the PID prefilter tubes are available with hexavalent chromium as oxidizer in the oxidizing column. It oxidizes all VOCs except benzene and scrubs many of the VOCs present in the sample gas as well as other byproducts generated in the oxidation column. However, the PID prefilter tube containing hexavalent chromium in the oxidizing column, is considered hazardous and is one of the restricted substances by Restriction of Hazardous Substances (RoHS).

The adverse health effects associated with hexavalent chromium exposure include occupational asthma, eye irritation and damage, perforated eardrums, respiratory irritation, kidney damage, liver damage, pulmonary congestion and edema, upper abdominal pain, nose irritation and damage, respiratory cancer, skin irritation, and erosion and discoloration of the teeth. Some workers can also develop an allergic skin reaction, called allergic contact dermatitis. However, currently, there are no PID prefilter tubes available without hexavalent chromium to selectively separate benzene from the mixture of hydrocarbons and other VOCs.

Therefore, there is a need for a novel composition for selective separation of benzene from mixture of hydrocarbon gases and other VOCs.

OBJECTS OF PRESENT INVENTION
It is an object of the present invention to provide a composition for selective separation of benzene from the mixture of hydrocarbon gases and other VOCs.

It is another object of the present invention to provide a composition used as oxidizer in a prefilter tube of a gas monitoring device for detection of benzene.

It is another object of the present invention to provide a composition which is free of chromium.

Yet another object of the present invention is to provide a non-toxic composition for selective separation of benzene from the mixture of hydrocarbon gases and other VOCs.

Yet another object of the present invention is to provide a composition which selectively oxidizes all hydrocarbon gases and VOCs except benzene with high efficiency.

Yet another object of the present invention is to provide a prefilter tube for photoionization detectors comprising the composition for selective separation of benzene from a mixture of hydrocarbon gases and other VOCs.

It is another object of the present invention to provide a method for preparing the prefilter tube comprising the composition.

SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided a composition for selective separation of benzene, comprising of silica gel, cerium sulphate, selenium dioxide and concentrated sulfuric acid.

In an aspect the present invention provides a prefilter tube for photoionization detectors, comprising of:
a pre-column;
an oxidizing column comprising a composition for selective separation of benzene from a mixture of hydrocarbon gases and other Volatile Organic Compounds;
a post-column; and
a plug material,
wherein the composition comprises silica gel, cerium sulphate, selenium dioxide and concentrated sulfuric acid.

In another aspect the present invention provides a method of preparing the prefilter tube comprising the steps of:
placing a plug material at one end of a glass tube;
placing a post-column comprising inert material above the plug material;
placing an oxidizing column comprising the composition on top of the post column;
placing a plug material on top of the oxidizing column;
placing a star plug on top of the plug material; and
sealing both ends of the glass tube,
wherein sample gas pass from the one end of the glass tube having the star plug, and
wherein the composition is substantially free of chromium or its salts.

In another aspect the present invention provides a method of preparing the prefilter tube comprising the steps of:
placing a glass material at one end of a glass tube;
placing a plug material on top of the glass material;
placing a pre-column comprising inert material on top of the plug material;
placing an oxidizing column comprising the composition on top of the pre-column;
placing a post-column comprising inert material such as glass powder above the oxidizing column;
placing a plug material on top of the post-column; and
sealing both ends of the glass tube,
wherein the sample gas pass from the one end of the glass tube having the glass material, and
wherein the composition is substantially free of chromium or its salts.

In another aspect the present invention provides a method of determining benzene concentration using a PID instrument having the prefilter tube comprising the composition, comprising the steps of:
drawing sample gas through the prefilter tube comprising the composition;
oxidizing all the volatile organic gases present in the sample gas except benzene in the oxidizing column of the prefilter tube;
allowing the filtered sample gas to pass through the post column of the prefilter tube; and
measuring the amount of benzene present in the sample gas using the photoionization detector.

BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in connection with the drawings described hereinafter.
Figure 1 shows a schematic representation of the PID prefilter tube of one embodiment.
Figure 2 shows a schematic representation of the PID prefilter tube of another embodiment.

DETAILED DESCRIPTION OF THE INVENTION
It will be apparent to those skilled in the art that various changes and modifications may be made in the embodiments without departing from the spirit and scope of the invention. In addition, the embodiments are to be considered as illustrative and not restrictive of the scope of the invention. The materials and reagents used in the present invention are commercially available.

In accordance with the present invention there is provided a composition for selective separation of benzene from a mixture of hydrocarbon gases and other VOCs.

The composition of the present invention has several advantages over the conventional compositions containing chromium, which reduces the adverse impact on the environment and toxicity to the users due to the absence of hexavalent chromium.

Accordingly, in one aspect, the present invention provides a composition for oxidizing all hydrocarbon encountered in the working area where gasoline, petroleum products and fuel oil are used and to selectively separate benzene vapor present in the sample gas particularly for the measurement of its concentration with the help of a gas monitoring device.

Thus, in an aspect, the present invention provides a composition comprising silica gel, cerium sulphate, selenium dioxide and concentrated sulfuric acid.

The silica gel content in the composition of present invention ranges from 10 %w/w to 100 %w/w, preferably from 30 %w/w to 80 %w/w, more preferably from 50 %w/w to 60 %w/w. The cerium sulphate content in the composition of the present invention ranges from 0.01 %w/w to 2 %w/w, preferably from 0.05 %w/w to 1.5 %w/w, more preferably from 0.1 %w/w to 1 %w/w. The selenium dioxide content in the composition of the present invention ranges from 0.5 %w/w to 10 %w/w, preferably from 1 %w/w to 8 %w/w, more preferably from 1.5 %w/w to 5 %w/w. However, these specified amounts are exemplary and appropriate amounts may be arrived at without undue experimentation.

In a most preferred embodiment, the composition for selective separation of benzene comprises 50 – 60 %w/w silica gel, 0.1- 1 %w/w cerium sulphate, 1.5 – 5 %w/w selenium dioxide and 30 – 50 %w/w concentrated sulphuric acid.

In an embodiment, the composition of the present invention is substantially free of chromium or its salts. In a preferred embodiment, the composition of the present invention is substantially free of hexavalent chromium. As used herein "substantially free" means that the composition may contain trace amounts of chromium or its salts, but the amount of chromium or its salts present in the composition in such trace amounts do not have a material effect on the composition performance. Such substantially free compositions can also be devoid or absent of chromium or its salts.

In an embodiment, the composition of the present invention further comprises water as solvent. The amount of water present in the composition ranges from 0.1% to 15%, preferably from 0.3% to 12%, more preferably from 0.5% to 10%.

In an embodiment, the composition of the present invention is in the form selected from the group consisting of granule, powder and tablets.

In a most preferred embodiment, the composition of the present invention is in the form of granules.

The composition of the present invention enables oxidation of hydrocarbon gases and volatile organic compounds (VOCs) except benzene. This enables to selectively filter benzene from a mixture of gases containing hydrocarbons and VOCs.

The composition of the present invention is used to selectively separate benzene present in a mixture of gases containing hydrocarbons and VOCs particularly for the measurement of benzene concentration with the help of a gas monitoring device.

The gas monitoring devices include but not limited to photoionization detectors (PID).

In another aspect, the present invention provides a prefilter tube for photoionization detectors comprising the composition for selective separation of benzene from a mixture of hydrocarbon gases and other VOCs.

The prefilter tube comprises of plurality of columns. In a preferred embodiment, the prefilter tube comprises of a pre-column, an oxidizing column and a post-column. The pre-column and post-column comprise inert material such as glass powder which is washed and dehydrated. The glass powder comprised in the pre-column and post-column has mesh size of 30 - 60 (595 -250 micron). The glass powder is free of organic and inorganic contamination.

The oxidizing column enables selective oxidization of hydrocarbon gases and VOCs except benzene. The oxidizing column comprises the composition comprising silica gel, cerium sulphate, selenium dioxide and concentrated sulfuric acid. In a preferred embodiment, the oxidizing column comprises the composition comprising 50 – 60 %w/w silica gel, 0.1- 1 %w/w cerium sulphate, 1.5 – 5 %w/w selenium dioxide and 30 – 50 %w/w concentrated sulphuric acid.

In a preferred embodiment, the silica gel is in the form of powder. The silica gel in the form of powder is obtained by the following steps: washing silica gel with a mixture of hydrochloric acid and hydrogen peroxide, filtering the solution, heating the wet silica gel in an air-oven at 300oC for a period of two days, then cooling the silica gel to 65 ±5oC to obtain the silica gel with moisture content less than 1%. The silica gel powder so obtained has mesh size varying from 315 to 500 micron. The cerium (IV) sulphate oxidizes either completely or partially all the VOCs present in the sample gas. More preferably the oxidizing column comprises cerium sulphate as first reagent in the amount ranging from 1 to 5 mg cerium sulphate. The selenium dioxide enables oxidization of the partially oxidized VOCs to completely oxidized state. More preferably, the oxidizing column comprises selenium oxide as second reagent in the amount ranging from 10 – 50 mg. The concentrated sulphuric acid aids to complete oxidation of VOCs except benzene. More preferably, the oxidizing column comprises sulphuric acid in the amount ranging from 0.2 -1 ml.

In an embodiment, the prefilter tube further comprises a plug material to hold the columns in their position. The pre-column and the post-column separate the oxidizing column from the contact of plug material. In an embodiment, the plug material is made of material selected from the group consisting of porous ceramic plugs, glass wool, glass cloth, non-woven polypropylene wool and star plug made of stainless steel.

In another embodiment, the prefilter tube further comprises at least one glass material such as hollow glass tube to adjust the height of oxidizing column from the bottom end of the prefilter tube so that the oxidizing column is visible through a window provided in the PID instrument.

The prefilter tube is glass tube with both ends sealed and having length of 100 to 125 mm, inner diameter of 5.4 mm to 6.2 mm and outer diameter of 6.8 mm to 8.5 mm. The glass tube is made from materials selected from the group consisting of borosilicate glass, soda glass and Pyrex.

Referring to figure 1, the prefilter tube comprises of a glass tube (1), an oxidizing column (2), a post-column (3), plug material (4) at both ends of the oxidizing column (2), and a star plug (5). The star plug (5) is placed at one end of the glass tube (1) to hold the columns in place. The arrow mark in figure 1 indicates that flow direction of the sample gas.

Turning now to figure 2, the prefilter tube comprises of a glass tube (1), an oxidizing column (2), a pre-column (7), a post-column (3), plug material (4) at one end of the pre-column (7) and post-column (3), and a hollow glass tube (6). The hollow glass tube (6) is placed at one end of the glass tube (1) to adjust the position of oxidizing column (2) such that the oxidizing column (2) is visible through a window provided in the PID instrument. The hollow glass tube (6) has length of 10±2 mm. The arrow mark in figure 2 indicates that flow direction of the sample gas.

The photoionization detector comprises a shortwave length UV lamp which has the capacity to produce 10 eV energy filled with Krypton and fitted with calcium fluoride window to produce 125- 8000 nm wave length radiation. The radiation so produced is capable of ionizing benzene which has the ionization potential of 9.24 eV. All the hydrocarbons which have ionization potential lower than 10 eV, such as toluene and xylene, can be ionized by this radiation and the PID instrument shows reading corresponding to total VOCs present in the sample gas. In order to remove the interference of VOCs except benzene, these gases must be filtered before passing through the photoionization detector. For this purpose, the sample gas is passed through the prefilter tube before being exposed to the photoionization detector.

In one embodiment, the prefilter tube for photoionization detector comprises an oxidizing column. The oxidizing column comprises a composition comprising silica gel, cerium sulphate, selenium dioxide and concentrated sulfuric acid. The composition directly reacts with the sample gas and oxidizes all the VOCs present in the sample gas expect benzene while allowing the sample gas to pass through the prefilter tube. The oxidized gases are not detected by PID instrument while benzene is detected.

The oxidation reaction that takes place when the sample gas is passed through the prefilter tube is shown below.

In an embodiment, the oxidation capacity of the composition is adjusted such a way that the compound benzene should not either oxidized or adsorbed in the oxidizing column of the prefilter tube. In another embodiment, the oxidation capacity of the composition is adjusted such a way that the composition does not either react with or oxidize benzene and hence benzene passes through the prefilter tube as such when the sample gas is drawn through the prefilter tube. However, a negligible amount of benzene may be adsorbed in the oxidizing column.

In an embodiment, the sample gas comprising of VOCs directly reacts with the oxidizing column containing Ce4+ and turns the color of the oxidizing column from yellow to brown due to the formation of Ce3+ compound. This colored band length enables the user to access the usage of prefilter tube.

In another embodiment, the prefilter tube comprising the composition shows color change of the oxidizing column to measure how much of the prefilter tube is exhausted by reacting with the VOCs.

In a preferred embodiment, the volume of oxidizing column is from 0.4 to 1.5 CC and the volume of pre-column is from and post-column is each from 0.1 to 0.5 CC.
In an embodiment, the performance of oxidizing column with reference to ambient humidity is independent and this prefilter tube can be used for 20 % RH to 80 % RH at 20oC.

In an embodiment, the performance of oxidizing column with reference to temperature is independent and is operable in the temperature ranging from -20oC to 40oC.

In an embodiment, the composition of the present invention is useful for the measurement of benzene present in the range of 0.05 to 200 ppm.

In another aspect of the present invention, there is provided a method of preparing the prefilter tube comprising the composition.

In one embodiment, the method of preparing the prefilter tube comprises the steps of:

(i) placing a plug material at one end of a glass tube;
(ii) placing a post-column comprising inert material such as glass powder above the plug material;
(iii) placing an oxidizing column comprising the composition on top of the post column;
(iv) placing a plug material on top of the oxidizing column;
(v) placing a star plug on top of the plug material; and
(vi) sealing both ends of the glass tube,
wherein the sample gas pass from the one end of the glass tube having the star plug, and
wherein the composition is substantially free of chromium or its salts.

In another embodiment, the method of preparing the prefilter tube comprises the steps of:
(i) placing a glass material at one end of a glass tube;
(ii) placing a plug material on top of the glass material;
(iii) placing a pre-column comprising inert material such as glass powder on top of the plug material;
(iv) placing an oxidizing column comprising the composition on top of the pre-column;
(v) placing a post-column comprising inert material such as glass powder above the oxidizing column;
(vi) placing a plug material on top of the post-column; and
(vii) sealing both ends of the glass tube,
wherein the sample gas pass from the one end of the glass tube having the glass material, and
wherein the composition is substantially free of chromium or its salts.

In another aspect of the present invention, there is provided a method of determining benzene concentration using a PID instrument having the prefilter tube comprising the composition.

The method of determining benzene concentration using a PID instrument having the prefilter tube comprising the composition, comprises the steps of:
(i) drawing sample gas through the prefilter tube comprising the composition;
(ii) oxidizing all the volatile organic gases present in the sample gas except benzene in the oxidizing column of the prefilter tube;
(iii) allowing the filtered sample gas to pass through the post column of the prefilter tube; and
(iv) measuring the amount of benzene present in the sample gas using the photoionization detector.

The prefilter tube is placed on the PID instrument after cutting both the ends of the tube using a tip cutter. The PID instrument is operated as per the operational procedure given in the instrument manual.

The photoionization detector produces 10 eV energy and 125- 8000 nm wavelength radiation. Therefore, benzene having ionization potential of 9.24 eV is measured by the photoionization detector.

Examples:
Possible coexisting gases generally encountered in the fuel oil were studied using the PID instrument having the prefilter tube containing the composition of the present invention. Table 1 gives the response of the PID instrument tested at a chosen concentration of these gases. The test concentration chosen for different VOC are not necessarily the maximum allowable concentrations.
Sr. No. Compounds Concentration (ppmv) Apparent Benzene Response on PID instrument that was calibrated with benzene (ppmv)

01 Toluene 400 0.1
02 P-Xylene 200 0.0
03 Ethyl Benzene 200 0.0
04 Hydrogen Sulphide 150 0.0
05 Methane 25000 0.0
06 Propane 1000 0.0
07 Isobutane 100 0.0
08 Isobutylene 500 0.0
09 n – Pentane 1500 0.7
10 1,3 – Butadiene 300 0.0
11 n – Hexane 100 0.3
12 Cyclohexane 10 5.8
13 n – Octane 300 12.1
14 Ethanol 50 0.0
15 Isopropanol 100 0.0
16 Cyclohexanone 200 0.0
17 Tetrahydrofuran 100 0.0
18 Ethyl Acetate 100 0.0
19 Acrylonitrile 100 0.0
20 Acetone 100 0.1
21 Dichlorobenzene 50 8.9
22 Trichloroethylene 100 >50
23 Chlorobenzene 20 21.9

It is possible to easily modify or alter the prefilter tube according to the type of PID instrument. Various designs of the prefilter tube of the present invention can be adopted depending on the shape and the size of the PID instrument.

The composition of the present invention is non-toxic and enables selective separation of benzene from the mixture of hydrocarbon gases and other VOCs.

The composition of the present invention not only enables selective separation of benzene from the mixture of hydrocarbon gases and other VOCs but also enables effective and accurate measurement of benzene in a mixture of hydrocarbon gases and other VOCs.
,CLAIMS:
1. A composition for selective separation of benzene, comprising of:
a predetermined amount of silica gel;
a predetermined amount of cerium sulphate;
a predetermined amount of selenium dioxide and
a predetermined amount of concentrated sulfuric acid.

2. The composition as claimed in claim 1, wherein the amount of silica gel is from 10 %w/w to 100 %w/w.

3. The composition as claimed in claim 1, wherein the amount of cerium sulphate content is from 0.01 %w/w to 2 %w/w.

4. The composition as claimed in claim 1, wherein the amount of selenium dioxide content is from 0.5 %w/w to 10 %w/w.

5. The composition as claimed in claim 1, wherein the is substantially free of chromium or its salts.

6. A prefilter tube for photoionization detectors, comprising of:
a pre-column;
an oxidizing column comprising a composition for selective separation of benzene from a mixture of hydrocarbon gases and other Volatile Organic Compounds;
a post-column; and
a plug material,
wherein the composition comprises silica gel, cerium sulphate, selenium dioxide and concentrated sulfuric acid.

7. The prefilter tube as claimed in claim 6, wherein the pre-column and post-column comprise an inert material.

8. The prefilter tube as claimed in claim 7, wherein the inert material is glass powder which has mesh size of 30 – 60.

9. The prefilter tube as claimed in claim 6, wherein the plug material is positioned at both ends of the oxidizing column or at one end of the pre-column and post-column.

10. The prefilter tube as claimed in claim 6, further comprises at least one glass material to adjust the height of oxidizing column from the bottom end of the prefilter tube.

11. A method of preparing the prefilter tube comprising the steps of:
placing a plug material at one end of a glass tube;
placing a post-column comprising inert material above the plug material;
placing an oxidizing column comprising the composition on top of the post column;
placing a plug material on top of the oxidizing column;
placing a star plug on top of the plug material; and
sealing both ends of the glass tube,
wherein sample gas pass from the one end of the glass tube having the star plug, and
wherein the composition is substantially free of chromium or its salts.

12. A method of preparing the prefilter tube comprising the steps of:
placing a glass material at one end of a glass tube;
placing a plug material on top of the glass material;
placing a pre-column comprising inert material on top of the plug material;
placing an oxidizing column comprising the composition on top of the pre-column;
placing a post-column comprising inert material such as glass powder above the oxidizing column;
placing a plug material on top of the post-column; and
sealing both ends of the glass tube,
wherein the sample gas pass from the one end of the glass tube having the glass material, and
wherein the composition is substantially free of chromium or its salts.

13. A method of determining benzene concentration using a PID instrument having the prefilter tube comprising the composition, comprising the steps of:
drawing sample gas through the prefilter tube comprising the composition;
oxidizing all the volatile organic gases present in the sample gas except benzene in the oxidizing column of the prefilter tube;
allowing the filtered sample gas to pass through the post column of the prefilter tube; and
measuring the amount of benzene present in the sample gas using the photoionization detector.