Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Free carbon containing
Reexamination Certificate
1998-12-10
2001-05-29
Hendrickson, Stuart (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Free carbon containing
C502S180000
Reexamination Certificate
active
06239067
ABSTRACT:
This invention relates to a process for the production of activated carbon using water or supercritical carbon dioxide as activating agents for the carbonized starting material. The invention also relates to an apparatus for carrying out said process.
As is well known, activated carbon is a porous carbonaceous material, with a very developed internal surface, between 300 and 3000 m
2
/g, which has a great capacity to adsorb gases and substances dissolved in liquids.
With regard to the physical-chemical nature of activated carbon, there are two general properties which are responsible for the adsorptive nature of this material. These properties are: a) the pore size distribution and the surface area; and b) the chemical reactivity of the surface.
The first commercial activated carbons were prepared from wood and peat. In fact, any carbonaceous material can be employed to obtain activated carbons. Thus, wood, coals, lignites, peat, coconut, almond, walnut or hazelnut shells, and the stones of fruits (peach, plum, olive, etc.) are used. Out of these those favored by the manufacturers are wood, coal, lignite, peat and coconut shells.
The choice of the raw material is based upon criteria such as:
Quality of the activated carbon.
Low mineral matter content.
Availability and cost.
Degradation during storage.
Ease of activation.
According to the state of the art, the industrial scale manufacture of activated carbon is essentially comprised of two stages: a) Carbonization of the raw material; and b) Activation of the carbonized product.
The properties of the end product depend upon: a) the nature of the raw material; b) the activating agent; c) the time and temperature of the activation process. Out of these variables, the last two are also important at the time of obtaining an activated carbon which is easy to regenerate (ease in desorbing the adsorbed substances in order to return its original adsorptive properties to the activated carbon).
During the carbonization stage, the starting material is heated in an inert atmosphere (free of oxidants) at a high temperature, over 800° C. During this stage, the raw material is pyrolysed and a basic porous structure is developed. In this process, most of the noncarbonaceous elements, hydrogen, oxygen and nitrogen are eliminated in the form of gaseous products. The liberated atoms of elemental carbon rearrange themselves in ordered crystallographic formations known as elementary graphite crystals. The rearrangement of the crystals is irregular, therefore leaving free interstices between them. Apparently, as a result of the deposition and decomposition of tarry substances, these interstices begin to fill up, or at least become blocked by disordered carbon. In general, the product of carbonization known as char, is a virtually inactive material with a relatively low surface area (from 1 to 50 m
2
/g).
When the raw material used in obtaining activated carbon has thermoplastic properties (case of the bituminous coals, lignins and others), a prior oxidation step is required to decrease or eliminate the plastic behavior, if a precursor of activated carbon with a suitable porous development has to be generated during the carbonization step. The oxidation is carried out in the gas phase by means of oxidating atmospheres (oxygen, air, carbon dioxide) at temperatures of the order of 200° C., or otherwise in solution with conventional chemical agents. Oxidation, in the event of it being necessary is generally carried out before the carbonization step. Gentle oxidative treatments are common during the cooling stages after the carbonization and activation steps, aimed at the physical-chemical conditioning of the surface of the final product. During the activation step, the char is subjected to the controlled action of oxidizing gases, such as steam, carbon dioxide or some mixture of these gases. This controlled oxidation takes place at temperatures between 800 and 1.100° C. The oxidant agent essentially burns the most reactive parts of the carbonaceous skeleton. The extent of the burning will depend upon the nature of the gas employed, the temperature and the activation time. The basic reactions involved in this stage are:
C+H
2
O→CO+H
2
C+CO
2
→2CO
Being these reactions endothermic, the activation operation is controlled by means of an external input of energy. The use of carbon dioxide as an activating agent usually demands higher temperatures which increase the cost of the process.
The activation rate with steam decreases due to the presence of the H
2
generated, which remains firmly adsorbed to the surface. The same happens when carbon dioxide is used, as the CO generated also remains adsorbed on the surface.
It has now been discovered, surprisingly, that the drawbacks derived from the use of said activating agents in the gaseous state, according to the state of the art, can be solved if said agents are substituted by water or carbon dioxide in a supercritical state.
Therefore, and according to an aspect of the present invention, a process is provided for the manufacture of activated carbon characterised by the use, as activating agent for the carbonized material, of water or/and carbon dioxide in supercritical state (critical point of water: T=374° C. and P=215 bar; critical point for carbon dioxide: T=31° C. and P=72 bar).
The special properties of supercritical water and carbon dioxide make these two activating agents more efficient and effective than when they are used in gaseous state.
According to the invention, for temperatures and pressures above the critical point, the physical properties of water are completely different from those of water in the liquid or vapor phase state. In the vicinity of the critical point, the density and viscosity decrease remarkably, increasing the diffusivity of its molecules and the mobility of other chemical species dissolved in it. Consequently, an improvement in mass transfer processes is achieved, making supercritical water an excellent reaction medium. On the other hand, the dielectric constant decreases from 78 to 5, typical value for apolar compounds, thus making water a good solvent for gases and organic compounds. Consequently, supercritical water has great ease in penetrating the porous structure of the char, being able to extract and transport to the outside the molecules of the activation products. Supercritical CO
2
exhibits a similar behavior.
The difference between carrying out the activation with steam or CO
2
or with supercritical water or CO
2
according to the present invention, is defined by these properties. Among others, it is worth noting the following differences:
When working under supercritical conditions, the high pressure makes the number of water or CO
2
molecules in contact with the surface of the char to be greater than when steam or CO
2
in the gaseous state are used. In this manner, the activation rate is increased.
The great ease which the molecules of the supercritical fluid have to penetrate the porous structure allows the activation reaction to take place simultaneously in the whole of the inner surface of the char. In this manner, the activation rate is increased and a uniform development of the porous structure of the resulting activated carbon is achieved.
The solubilisation of the gases from the activation reactions, together with their great diffusivity in the supercritical fluid, turns the supercritical phase into a homogeneous phase which facilitates the exchange with the char surface and transport in the porous structure.
In activation with supercritical fluids, it is possible to easily control the final characteristics of the activated carbon by selecting the experimental conditions under which the process takes place, such as the pressure, temperature, density, flow, time, etc.
The activation temperature is lower, which implies an important economic saving. Thus, in most of chars, activation with supercritical water begins at about 450° C. and its rate increases rapidly with temperature.
Another advantage in th
Bryan Cave LLP
Hendrickson Stuart
Universidad de Salamanca
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