WO2007068778A1

WO2007068778A1 - Method for producing activated carbon from waste pet

Info

Publication number: WO2007068778A1
Application number: PCT/ES2006/000705
Authority: WO
Inventors: Francisco Javier LÓPEZ GARZÓN; Inmaculada FERNÁNDEZ MORALES; María DOMINGO GARCÍA; Manuel José PÉREZ MENDOZA; María del Camen ALMAZÁN ALMAZÁN
Original assignee: Universidad De Granada
Priority date: 2005-12-16
Filing date: 2006-12-15
Publication date: 2007-06-21
Also published as: ES2277565B1; ES2277565A1

Abstract

The invention relates to a method for producing activated carbon from waste PET, which prevents wastage of the volatile residues which are produced upon rupture of PET polymer chains during pyrolysis and which are used to obtain activated carbons. According to the invention, two types of activated carbon can be obtained from PET, namely one type using standard pyrolysis and activation and a second type using the volatiles that are generated in the pyrolysis process.

Description

Production process of activated carbon from PET waste

SECTOR OF THE TECHNIQUE The active carbons obtained according to the present invention have great applications in various industrial processes due to their excellent properties as adsorbents. Thus, they can be used in solvent recovery processes, air purification, gas separation, deodorization, desulfurization ... etc. They also have a wide range of applications as liquid phase adsorbents, which makes them useful in water treatments, solution bleaching and solute accumulation and recovery. Finally, they have applications in catalytic processes in which they can intervene as catalysts or catalyst supports.

STATE OF THE TECHNIQUE

The preparation of active carbons can be carried out from various precursors containing carbon of different origin. Thus, it is a usual procedure based on agricultural lignocellulosic precursors, as set out in Spanish patents 426440, 544081 and P200002114, which describe processes for obtaining olive and pomace bone, the first one, from cereal straw and almond peel, the second and using apple pulp, the third. Swiss patent 511192 collects the process starting from resinous wood, while Polish patent 72948 uses sawdust and fruit bone. Similarly, German patent 2052507 and Hungarian 482750 start from plant material for the preparation of activated carbons. Mineral carbons such as Coke or coal can be used, instead of the previous precursors, as described in the Spanish patent P9003143 and in the US patent 3840476. In all the aforementioned cases the procedure basically consists of two stages: firstly The pyrolysis, in an inert atmosphere at high temperature that produces carbonized, and secondly the activation of said carbonized. Through these procedures, and using the aforementioned precursors, active carbons with specific developed surfaces and high nanoporosity are obtained. However, this nanoporosity is polymodal: that is, the size of the nanopores they have in their structure is very varied, which is due to the fact that the structure of the starting precursor is very heterogeneous. It must be taken into account that this parameter (distribution of nanopores) is essential in the application of active carbons. Thus active carbons with a polymodal distribution and with a high surface area have a low selectivity in those processes in which these substances have application, such as adsorption and catalysis. For this reason, much of the profits of these active carbons are limited despite their high surface area.

An alternative to this drawback is the use of precursors with an ordered molecular structure. For this reason, synthetic polymers can be used for the preparation of activated carbons in a process similar to that used when the precursor is a lignocellulosic material. Synthesis of active carbons from this type of polymeric precursors are described, for example, in patents US 3755193 and US 3859421. However, these precursors have a high price, which makes the final product also very expensive. This inconvenience can be avoided by using waste polymers, which have a low market value. Among them, polyethylene terephthalate (PET) is a widely used polymer whose waste can be used for this purpose. The active carbons that are obtained from this polymer by pyrolysis and activation have high surfaces and unimodal distributions of nanopores. However, in the pyrolysis and activation of this polymer and, in general of any polymer, the yield in activated carbon is relatively small. Thus, when polyethylene terephthalate (PET) is subjected to pyrolysis, the yield in carbonaceous material is never greater than 22%. But in addition, as later, the carbonaceous material must be activated, the final yield in activated carbon is not more than 17% at best, and often does not reach 10%. The fact that the yield in the pyrolysis process is low is due to the fact that the polymer chains are broken giving rise to volatile residues constituted mainly by terephthalic acid and benzoic acid, which sublimate at temperatures well below the pyrolysis.

EXPLANATION OF THE INVENTION

Object of the invention The proposed invention manages to avoid wasting volatile residues, produced in the rupture of the polymer chains of PET during pyrolysis, which are used to obtain active carbons. In this invention two types of activated carbon are obtained from PET. The first one, 1F, by means of conventional pyrolysis and activation. The second type, 2F, is obtained by taking advantage of the volatiles that are generated in the pyrolysis process. With This achieves a double objective: the yield in active carbon from PET is considerably increased and active carbons are obtained from volatile residues which, given the homogeneous molecular structure, must produce an active carbon with a unimodal nanopoorosity.

Summary of the invention

To be able to take advantage of volatiles, its transformation into products that prevent sublimation during pyrolysis is required. This is achieved by transforming them into derived potassium salts whose stability is greater. This is because in the salts the proton (H ⁺ ) of the acid has been replaced by the K ⁺ cation. The latter has the same electric charge as the proton but is larger in size, so its polarizing character is smaller. This lower polarizing character of K ⁺ is what makes the stability of potassium salts greater than that of the fundamental constituent benzoic and terephthalic acids of volatiles. Therefore, while acids sublimate, their potassium salts do not and, therefore, can be transformed into coal. In this invention, in addition, the formation of potassium salts has been designed so that they are accompanied by an excess of KOH which acts as an activating agent. Thus, a mixture is obtained that we call KTf + KOH, which, by heating in a single stage, that is to say without subsequent activation, gives rise to activated carbon. This activated carbon has a unimodal nanoporous structure, since it comes from a homogeneous molecular substance, and a high surface area. Detailed description of the invention

The obtaining of the two types of active carbon is based on a device like the one shown in the figure. This consists of a tubular furnace (H) that has a gas inlet (E) and an outlet (S) connected to an externally cooled container (R), which contains a concentrated solution of KOH. This container in turn has an outlet to the air (A). The PET that is to be converted into active carbon is placed in the oven (H) which is heated by passing a flow of nitrogen (pyrolysis) to the desired temperature resulting in a carbonized which is subsequently activated with carbon dioxide at the same temperature of the pyrolysis giving rise to the first fraction, 1 F. During the pyrolysis process, volatile materials are produced which, upon reaching (.R), are transformed into the alkaline salts, KTf, remaining in solution. This solution is collected, once the pyrolysis is finished, and it is taken to dryness obtaining the KTf + KOH mixture, which also undergoes pyrolysis. This process is carried out in a tubular furnace, in nitrogen flow, heating at variable final temperatures. The potassium sai, KTf, of said mixture does not sublimate under the conditions of the pyrolysis but instead gives rise to a carbonaceous material, 2F. Since the potassium salt KTf contains the excess of KOH, the pyrolysis process is both activated. This fact is relevant because in a single stage the two processes are carried out: pyrolysis and activation. Depending on the heating conditions, the carbonaceous material obtained has relevant properties in its structure. Thus, high specific surfaces (> 1300 m ² / g) and developed porosities (volume of nanopores 0.59 cm ³ / g) and unimodal (0.7-0.9 nm) are achieved. The original part of the invention is the use of the KTf residue as a source of activated carbon. This produces the second fraction, which has been called 2F, which together with the classic process of pyrolysis and activation that allows obtaining the first fraction, 1 F, give rise to a yield in active carbon much higher than that obtained through pyrolysis and classic activation.

Preferred Embodiment

Typically, amounts between 5 and 50 g of waste PET are used which are placed in the tubular furnace (H) which has a temperature and heating rate programmer. The nitrogen stream that enters through (E) has a flow of between 20 and 200 cm ³ / min. The concentrated KOH solution found in the container (R) has a total hydroxide content that varies between 10% and 100% of the weight of PET in the oven (H). The heating rate ranges between 5 and 50 ° C / min. The final oven temperature is set between 700 and 95O ⁰ C and the residence time is close to one hour. Once this process is completed, the carbonaceous material located in the furnace (H) is collected and this is activated in a carbon dioxide stream under flow conditions, heating rate and final temperature similar to those described above, giving rise to the first fraction , 1 F. The container that has been connected to the pyrolysis furnace outlet contains the KTf + KOH mixture. This is brought to dryness by boiling, dried in an oven at 11O ⁰ C and subsequently introduced into the same oven (H) from which the container (R) is disconnected. The oven is connected to a stream of nitrogen at the inlet and the outlet goes directly to the atmosphere. The heating rate ranges between 5 and 50 ° C / min, the temperature The oven end is set between 650 and 800 ⁰ C and the residence time is set between 1 and 8 hours. In this way the second fraction of activated carbon 2F is achieved. The two fractions 1F and 2F do not mix, once obtained, since their properties are different.

Two illustrative examples of the yields obtained and the properties of activated carbons are set forth below. The results of the first of them are shown in the following table:

The meaning of the symbols is as follows: Tp = pyrolysis temperature. T _a = activation temperature. R = yield in active carbon. S _BET ⁼ surface area measured by nitrogen adsorption

These data were obtained after pyrolysis at 800 ⁰ C in the oven (H) for an approximate time of 1 hour. This process resulted in the coal oven which was operated at 800 ⁰ C resulting activated carbon was collected 1F. The activated carbon 2F, resulted from the use of the volatiles, which were collected in R, and its subsequent pyrolysis at 65O ⁰ C. The first relevant fact is that the total yield in activated carbon is more than double (17% + 23%) which is obtained if only pyrolysis and activation occurs by the traditional method (17%). It is also It is also important that the active carbon 2F obtained as a consequence of the invention requires a single heating (pyrolysis), instead of the two (pyrolysis + activation) that are required to obtain the 1F carbon, and at a much lower temperature (65O ⁰ C versus 800 ⁰ C). The surface of the coal that is obtained by this invention, 2F, is somewhat higher than that of the coal that results by the traditional method. The nanoporosity in both cases is unimodal, the pore size being similar in both cases.

The results of the second of the examples are shown in the following table:

These data were obtained after pyrolysis at 95O ⁰ C in the oven (H) for an approximate time of 1 hour. From this process the coal that resulted in the furnace was collected which was activated at 95O ⁰ C giving rise to activated carbon 1 F. The activated carbon 2F, resulted from the use of the volatiles, which were collected in R, and their subsequent pyrolysis at 800 ⁰ C. As in the first example, the total yield in activated carbon is more than double (13% + 16%) than that obtained if only pyrolysis and activation occurs using the traditional method (13%). Furthermore, it is also important that 2F activated carbon, obtained as a consequence of the invention, requires only one heating (pyrolysis), instead of the two (pyrolysis + activation) that are required to obtain 1F carbon, and at a very high temperature lower (800 ⁰ C versus 95 ⁰ C). They are obtained in this case two coals with much higher adsorption capacities than the first example and very similar pore sizes.

In summary, an active carbon production process is presented from PET waste in which PET waste is subjected to pyrolysis at a temperature between 800 ⁰ C and 1100 ⁰ C for a period ranging from 50 minutes to eight hours, depending on whether you want to obtain a little or very developed surface, obtaining a first fraction of activated carbon that is then activated. The Terephthalic Acid and Benzoic Acid resulting from the pyrosilis are captured by a basic hydrolysis process in which KOH is used (which acts as a carbon activating agent). This captured mixture is subsequently desiccated and subjected to another pyrolysis process at a temperature between 65O ⁰ C and 1100 ⁰ C and during a period that ranges between 50 minutes and eight hours (depending on whether a little surface is desired or very developed). This second pyrolysis results in a second fraction of activated carbon.

Through this process a higher percentage of activated carbon is obtained which is nanoporous (its pore size is between 0.5nm and 2nm) and has a high specific surface area that is between 648 and 1391 m ² / g

Description of the Figure

Figure 1.- The figure shows the scheme of the system for obtaining the active carbons consisting essentially of a tubular furnace (H) in which the PET is heated to the desired temperature. Said oven has an entrance of gases (E) through which nitrogen (N ₂ ) is introduced in order to maintain an inert atmosphere. In this way, at high temperature, the pyrolysis of PET is produced. Likewise, the oven has an outlet (S) through which volatile residues that originate in the pyrolysis of PET are expelled. These are collected in the container (R) containing the aqueous solution of KOH. The container (R) also has an outlet to the air (A) for the expulsion of gases.

Claims

1. Process of production of active carbon from PET waste, which comprises the following phases: a. Pyrolysis of PET waste, obtaining a first fraction of coal. b. Capture of Terephthalic Acid and Benzoic Acid resulting from pyrosilis by means of a basic hydrolysis process in which KOH is used. C. Drying of the mixture Terephthalate Potassium, Potassium Benzoate and potassium hydroxide used in the hydrolysis. d. Activation of the first fraction of coal obtained. and. Pyrolysis of the dried mixture, giving rise to a second fraction of activated carbon.

2. Production process of activated carbon from PET waste according to claim 1, characterized in that the potassium hydroxide used in the hydrolysis is a carbon activating agent.

3. Production process of activated carbon from PET waste according to any of the preceding claims characterized in that the pyrolysis of PET waste is carried out at a temperature between 800 ⁰ C and 1100 ⁰ C

4. Production process of activated carbon from PET waste according to any of the preceding claims characterized in that the pyrosilis of the dried mixture of Potassium Terephthalate, Potassium Benzoate and the KOH in the hydrolysis is carried out at a temperature between 65O ⁰ C and 1100 ⁰ C.

5. Production process of activated carbon from PET waste according to any of the preceding claims, characterized in that the pyrosilis time of the PET waste is between 50 minutes and 8 hours.

6. Production process of activated carbon from PET waste according to any of the preceding claims characterized in that the pyrosilis time of the dried mixture of Potassium Terephthalate, Potassium Benzoate and KOH is between 50 minutes and 8 hours.

7. Activated carbon obtained by the process according to previous claims.

8. Activated carbon obtained by the process according to claims 1 to 8 characterized by being nanoporous.

9. Activated carbon obtained by the process according to claims 1 to 8 characterized in that its pore size is between 0.5nm and 2nm

10. Activated carbon obtained by the process according to claims 1 to 8 characterized in that its pore size is between 0.5 and 2 nm and its specific surface area is between 648 and 1391 m ² / g