FIELD OF THE INVENTION
This invention relates to an improved process for the production of phenolic resin based activated carbon spheres of high surface area. More particularly, this invention relates to an improved process for the production of activated carbon spheres used in NBC suits, gloves and filters for adsorbing toxic gases.
PRIOR ART
Activated carbon is a microcrystalline form of carbon with very high porosity and surface area. The desired pore structure and high surface area of an activated carbon product is attained by combining the right raw material and activation conditions (Balcy, 1992). Activated carbon adsorbs molecules from both liquid and gaseous phases depending upon the pore size distribution of the adsorbent (and also upon the geometry and size of the adsorbate molecule). In adsorption from the gas phase, mainly microporous carbon is used whereas mesoporous carbon is applied in liquid phase processes. Application of mesoporous activated carbons include; drinking water purification, waste-water treatment, sweetener decolorization, food and chemical processing. On the other hand, microporous carbons are used for solvent recovery, gasoline emission control, cigarette filters and industrial emission gas treatment (Benaddi, 2000). Activated carbon is available in many forms like Powder Activated Carbon, Granular Activated Carbon and Spherical Activated Carbon. Spherical activated carbon is the most useful form of activated carbon and is known for its high surface area together with high crushing strength.
Many authors (U.S. Patent 4371454 Hatsuo Saito et. al, U.S. Patent 4045368 Kunihiko Katori et. Al, 1977) have reported making pitch based activated carbon sphres. These patents state that spheres of activated carbon are obtained by mixing pitch with a Suitable aromatic solvent containing nitrogen compound, molding the resultant mixture in the form of spheres, removal of aromatic compound by extraction, infusibilizing the resultant spheres and thereafter activating these pitch spheres. However this process involves many critical steps like increasing the softening point of the pitch, shperisation of pitch followed by stabilization. Thus making the whole process expensive.
Zhichang Liu et. al (Carbon 37, p.663-667, 1999) have prepared activated carbon spheres from high softening point pitch (250°C) blending with 30 wt. % of naphthalene under nitrogen pressure of 0.5 Mpa. The blended pitch was pulverized into particles of 0.15 to 1.0 mm and then spheies were obtained using emulsion method. These pitch spheres were stabilized, carbonized and then activated.
U.S. Patent 4228037 (Hino Kuniaki et. al) disclosed a process of making spherical activated carbon with low dusting property and high mechanical strength The process involves the steps of increasing the softening point of pitch to 225 °C by distillation, lowering the softeaing point by blending with naphthalene and making spheres in an autoclave containing 0.5% by weight of polyvinyl alcohol at 1200 r.p.m., extraction of naphthalene with n-haxane, stabilization of pitch spheres to make them infusible. Stabilized pitch spheres were carbonized and then activated.
U.S. Patent 4371454 (Kaji Hissatsugu et. al) disclosed another process for preparing spherical carbon material or spherical activated carbon from high softening point pitch. High softening point (182°C) pitch was blended with naphthalene to lower the softening point, extruding the low softening point pitch (68°C) into string-like material of 1.5 nun in diameter, cutting the string into fragments, introducing the fragments into hot water to obtain spherical beads of required size and then extracting the beads with n-hexane. Thus obtained beads were infusibilized at 300°C and activated at 900°C.
U.S. Patent 20030092560 (Von Blucher et. al) disclosed a method, for producing spherical activated carbon by carbonization (pyrolysis, smoldering) and activation of polymer spherules based on styrene and divinylbenzene. Gel-like polymer spherules with sulphonic acid groups were wetted with oleimi 20% and later on fed into a rotary furnace fcr carbonization followed by activation. This process involves the evolution of sulphuric acid vapours during carbonization which may damage rotary kiln in addition to pollution hazards.
Qiong Cai et.al. (Carbon 42, p.775-783, 2004), have prepared phenolic resin based activated carbon spheres by polymer blending method. Polyvinyl butyral, novolac type phenolic resin and hexamethylenetetramine were blended by weight ratio of 11.0:100:124. The mixture was pulverized after evaporation of methanol under reduced pressure and formed into spheres by emulsion method. Phenolic resin based activated carbon spheres were obtained by carbonizing and activating the prepared spheres.
Arjun Singh et. al (812/DEL/2004) have disclosed a process for the preparation of phenolic resin spheres using phenol and formaldehyde as precursor by suspension polymerization technique. In this process, a monomer prepared from phenol and formaldehyde in the psesence of basic catalyst is dispersed into water for 1 - 2 hrs and stabilized by PVA for 20 -90 minutes at 90 - 98 °C with agitation rate in the range of 450 - 750 rpm. Thus stabilized monomer spheres were cross-linked by HMTA and then separated by filtration followed by washing and drying.
In the present invention, a polymeric synthetic precursor namely phenolic resin, rather than monomer prepared from the reaction of phenol and formaldehyde in presence of basic catalyst, is dispersed into water. Therefore, in the present invention the total time taken in making of phenolic resin spheres is much less than the time taken in other reported methods. These phenolic resin spheres are processed further to produce activated carbon spheres (ACS). The main aim of this study was to develop an improved process for the production of phenolic resin based activated carbon sphere (ACS), which are normally used in personal protecting cloths, masks, filters etc. Activated carbon spheres developed in the present inveaition have high adsorption capacity and high strength with low attrition loss.
SUMMARY OF THE INVENTION
According to this invention, an improved process for the production of phenolic resin based activated carbon spheres comprises the following steps.
1. Dispersion of phenolic resin by adding 2-4 times of water by weight of phenolic resin to the reaction vessel, stirring the mixture at 200 - 700 rpm, raising the temperature to 70 -100°C at the rate of 5 - 15 °C per minute followed by addition of 0.5 to I % of polyvinyl alcohol (PVA) by weight of phenolic resin as stabilizer.
2. Thermosetting of stabilized phenolic resin spheres obtained in step (1) by adding 0.5 to 7 % of hexamethylenetetraamine (HMTA) by weight of phenolic resin and maintaining the same temperature and stirring rate for further 2 - 5 hrs.
3. Cooling the mixture obtained from step (2), filtering and washing the phenolic resin spheres with water, methanol and acetone to remove protective colloid and other impurities.
4. Activation of phenolic resin spheres obtained from step (3) at a temperature from 800 to 1100°C for several hrs in the stream of carbon di oxide.
OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide an improved process for the production of phenolic resin based activated carbon spheres possessing high surface area.
Another objective of the present invention is to provide an improved one step process and hence more economical for the production of phenolic resin spheres.
Yet another objective of the present invention is to provide a process of conversion of total solid content of phenolic resin into spheres.
Yet further objective of the present invention is to provide a process for the production of phenolic resin spheres having spherical shape and high yield.
Yet further objective of the present invention is to provide a process for the production of phenolic resin spheres of required size varying from 0.01 mm to 2.5 mm in diameter, wherein size of the sphere can be controlled by changing reaction parameters i.e. agitation rate, viscosity ofthe resin etc.
Still further objective of the present invention is to provide a process for the production of phenolic resin spheres of uniform diameter as required.
Yet another objective of the present invention is to provide a simple, economical and cost effective process for the production of activated carbon spheres.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the process of preparation of phenolic resin spheres is carried out in a 1 -2 litre flat bottom four neck reaction vessel consisting of S.S. stirrer, a reflux Condenser and a thermometer. In this process heating mantle is used as a heating source. In the first step, phenolic resin is dispersed in water by the action of mechanical force produced by the stirrer and under the effect of interfacial tension, phenolic resin is converted into spheres. The stirring is carried out at 200 -700 RPM at room temperature.
In this invention, addition of stabilizer increases the viscosity of the mixture, which also reduces the probability of agglomerisation of spheres. In addition to that, it increase the interfacial tension between the spheres and reduces the difference in densities of two phases, thereby creating an additional hindrance to coalescence of the phenolic resin spheres. The process of the present invention comprises of following steps.
1. Conversion of phenolic resin into spheres:
In this step, a mixture of phenolic resin, 2 - 4 % of water by weight of phenolic resin and 0.5 - 5 % of PVA as stabilizer by weight of phenolic resin is taken in a four-neck flat bottom reaction vessel fitted with S.S. stirrer, a reflux condenser and a thermometer. The mixture is agitated at 200 - 700 RPM raising the temperature to 70 - 100°C at the rate of 5 - 15°C per minute. In this step, formation of phenolic resin spheres and their stabilization taikes place.
2. Thermosetting of stabilized phenolic resin spheres:
In this step, 0.5 - 7 % hexamethylenetetra amine (HMTA) by weight of phenolic resin is added to the reaction vessel containing stabilized phenolic resin spheres. The temperature and agitation rate is maintained to the level of step (1) for another 2-5 hrs. In this step, hardening of stabilized phenolic resin spheres takes place due to crosslinking.
3. Separation and drying of phenolic resin spheres:
Phenolic resin spheres obtained in step (2), are filtered and washed with water, methanol, acetone etc. to remove protective colloid and other impurities. Finally, the phenolic resin spheres thus obtained are dried in an oven at 110°C; these are used as precursor for the preparation of activated carbon spheres.
4. Activation of phenolic resin spheres:
In this step, activation of phenolic resin spheres obtained from step (3), is canied out at a temperature from 800 - 1100°C for several hrs in the stream of carbon di-oxide. In this step opening of micro pore takes place resulting in high porosity and high surface area.
The invention will now be illustrated with working examples.
Working example-1:
A 1000 ml flat bottom four-neck reaction vessel fitted with S.S. stirrer, reflux condenser and thermometer was charged with 100 gm phenolic resin of 65% solid content, 300 ml water and 5 gm of PVA. The temperature of the reaction vessel was raised to 90°0 at the rate of 5°C per minute with stirring rate 300 RPM. In the next step, 3 gm of HMTA was added to the reaction vessel and the stirring was continued for another 3.5 hrs maintaining the temperature range from 85 - 95°C. At the end of the reaction, the mixture was cooled to the room temperature and filtered. Finally, phenolic resin spheres thus obtainfd were washed vwth water, methanol and acetone. After drying of spheres, yield of convension of phenolic resin into spheres was determined gravimetrically.
Hie properties of phenolic resin sphere obtained are as follows.

Particle size Yield of spheres Particle Density Spherical shape

0.85-1.4 mm 64.8 % 1.29 g/cc > 98 %

Working example-2:
A 1000 ml flat bottom four-neck reaction vessel fitted with S.S. stirrer, reflux condenser and thermometer was charged with 50 gm phenolic resin of 65% solid conteat, 300 ml water aad 2 gm of PVA. The temperature of the reaction vessel was raised to 90°C at the rate of 10°C per minute with stirring rate 400 RPM. In the next step, 3.5 gm of HMTA was added to the reaction vessel and the stirring was continued for another 4.5 hrs maintaining the temperature range from 85 - 95°C. At the end of the reaction, the mixture was cooled to the room
temperature and filtered. Finally, phenolic resin spheres thus obtained wele washed with water, methanol and acetone. After drying of spheres, yield of conversion of phenolic resin into spheres was determined gravimetrically.
0.43-0.85 mm 64.5 % 1.29 g/cc > 98 %
The properties of phenolic resin sphere obtained are as follows.
Particle size
Yield of spheres
Particle Density
Spherical shape Working exampIe-3:
A; 1000 ml flat bottom four-neck reaction vessel fitted with S.S. stirrer, reflux condenser and thermometer was charged with 100 gm phenolic resin of 65% solid confent, 400 ml water and 1 gm of PVA. The temperature of the reaction vessel was raised to 90°C at the rate of 5°C per minute with stirring rate 600 RPM. In the next step, 5 gm of HMTA was added to the reaction vessel and the stirring was continued for another 4.0 hrs maintaining the temperature range from 85 - 95°C. At the end of the reaction, the mixture was cooled to the room temperature and filtered. Finally, phenolic resin spheres thus obtoined were washed with water, methanol and acetone. After drying of spheres, yield of conversion of phenolic resin into spheres was determined gravimetrically.
The properties of phenolic resin sphere obtained are as follows.

Particle size Yield of spheres Particle Density Spherical shape

0.21-0.43 mm 64.5 % 1.29 g/cc > 98 %

Activiation of phenolic resin spheres :
Thus obtained phenolic resin spheres were activated at 925±10°C with different activation conditions in presence of carbon di oxide to get activated carbon spheres (AGS). The average properties of thus obtained activated carbon spheres are given below.
Table Removed

CLAIMS:
What is claimed is:
1. A process for the preparation of phenolic resin based activated carbon spheres comprising
the steps of :-
(i) Dispersion of phenolic resin by adding 2-4 times of water by weight of phenolic resin to the reaction vessel, stirring the mixture at 200 - 700 rpm, raising the temperature to 70 - 100°C at the rate of 5 - 15 °C per minute followed by adding 0.5 to 5 % of polyvinyl alcohol (PVA) by weight of phenolic resin as stabilizer
(ii) Thermosetting of stabilizes phenolic resin spheres obtained in step (i) by adding 0.5 to 7 % of hexamethylenetetraamine (HMTA) by weight of phenolic resin and maintaining the same temperature and stirring rate for another 2 - 5 hrs
(iii) Cooling the mixture obtained from step (ii), filtering and washing the phenolic spheres with water, methanol and acetone to remove protective colloid and other impurities
iv) Activation of phenolic resin spheres obtained from step (iii) at a temperature from 800 to 1100°C for several hrs in the stream of carbon di-oxide.
2. A process of claim (1) wherein said water, PVA and HMTA are taken 2-4 times by weight, 0.5 - 5 % by weight and 0.5 - 7 % by weight of phenolic resin, respectively.
3. A process of claim (1) wherein said stabilizer is selected from polyvinyl acetate, carboxymethyl cellulose, polyvinyl alcohol and gelatin.
4. A process of claim (1) wherein, the particle size and particle size distribution of the said phenolic resin spheres is controlled by stirring rate, shape and size of the said stirrer and shape and size of the said reactor.
5. A process of claim (1) wherein said activating gas is selected from Steam, CO2, mixture of steam and CO2.
6. A process of claim (1) wherein the size of the said activated carbon sphere is between 0.01-2.5 mm.
7. A process of claim (1) to (5) wherein the said process results in time reiuction, cheaper and hence cost effective.
8. A process for the preparation of phenolic resin based activated carbon spheres (ACS) as described and exemplified herein.