GB2026997A - Production of Activated Carbon Suitable for the Treatment of Sewage - Google Patents

Abstract

Activated carbon is produced from a partially hydrolyzed vegetable waste material in a process including activation with at least one of the agents steam, carbon dioxide and flue gas, in which either one activating agent is dosed continuously or several activating agents are dosed alternately. A waste material autocatalytically hydrolyzed by an organic acid is used as the partially hydrolyzed waste material, said waste material is heated indirectly or directly between 150 and 400 DEG C at a heating- up rate of 10 to 40 DEG C/min, between 400 and 600 DEG C at a heating-up rate of 15 to 30 DEG C/min, then heat-treated at a temperature of 500 to 600 DEG C for a period not exceeding 30 minutes and finally activating the product with a residence time not exceeding 30 minutes.

Description

SPECIFICATION Process for the Production of Activated Carbon The invention relates to a process for the production of activated carbon having a high value of surface area and a regulated pore size.

The activated carbon of the invention .is used primarily for the purification of industrial sewages.

It is known in the art that activated carbon is used in a number of processes of the chemical industry for the removal of by-products or for the purification of sewages. Due to the increase of the number of problems of environmental protection the greatest part of the produced activated carbon is used today almost everywhere for these purposes. Therefore, during the production of activated carbon the aspects of this field of application must be taken into account in the first line.

When applying activated carbon for the purposes of purification certain compounds and groups of compounds, respectively, are adsorbed on its surface. This process is determined by the size of the molecule to be adsorbed and by the pore size of the activated carbon, respectively.

Thus, the proper choice of the pore size is of extraordinary importance; the efficiency of the purification process and its economy, respectively, depend on the pore size chosed or adjusted properly. Furthermore, the particle size of the activated carbon represents an important factor in the adsorption process. From the aspect of the particle size the requirements are different in the case of different technologies of purification.Thus, care must be taken to choose a suitable particle size e.g. when the activated carbon must remain suspended or floated in the liquid to be purified or when it must be sedimented within a defined period; the particle size is essential also in case of the so-called washing towers provided with a solid bed where particles must be used whose structure will not be more compact during operation.

Furthermore, it is known that the particle size of activated carbon bcan be regulated in two ways: either by mechanical treatment, e.g. by pressure, or by an adequate adjusting the parameters of the process of producing the activated carbon in a way that the original particle size of the starting material is maintained.

Accordingly, it is essential to keep the particle size unchanged during a possibly small number of operational phases, and, respectively, by a possibly low expenditure of labour and to regulate the particle size, depending on the later field of application.

The literature concerning the production of activated carbon is quite extensive. In the case of the most widespread process of production socalled chemical carbonizing agents, e.g. zinc chloride, oleum, sulphuric acid or phosphoric acid, are used. The process consists essentially in carrying out the carbonization of the starting material in the presence of one of the abovementioned carbonizing agents. Simultaneously with the carbonization also an activation is taking place. However, this method has a number of drawbacks, e.g. the yield is low, and during the formation of activated carbon appreciable changes take place in the starting material, thus it is disintegrated and crumbled.

The low yield is primarily due to the fact that the vegetable structure is destroyed by the action of the chemical carbonizing agent in a way that an appreciable part of the carbon content is lost in form of gaseous decomposition products.

Owing to the above mentioned deficiencies the activated carbon produced by chemical carbonization is too expensive for being applicable in appreciable amounts for the purposes of environmental protection, e.g. for sewage treatment.

The second big group of processes known for the production of activated carbon consists of processes based on thermal activation. However, the processes of this type which became known up to the present can be successfully carried out only when applying very high temperatures and long residence times.

In the following some actual examples are presented for both types of processes.

According to the process disclosed in the US Patent Specification No. 3,840,476 the activation of fluid coke is carried out in two steps. The carboniferous starting material is treated at first with an oxygenous gas at 300 to 4000C, then at 900 to 1 000C with a gas containing 50% steam. The activated carbon produced by this process is, as it follows from the specification, particularly suitable for binding the organic contaminations present in the sewages of petroleum refineries. The main drawback of this process is the length of the activation period (7 to 20 hours). As shown by our own measurements, the specific surface decreases appreciably with the length of the activation period.

In the US Patent Specification No. 3,755,193 the production of nitrogenous powdered carbon is described. An organic polymer is dissolved in a concentrated solution of an inorganic halide or thiocyanate and the obtained viscous liquid is carbonized at 700 to 1 2000C, altering the temperature meanwhile within said limits of temperature expediently at a heating rate of 2 6 C/min. The main drawback of this process is that the heating-up rate is rather low and thus the period of activation is long.

In the Hungarian Patent Specification No.

165,759 a process is described for the production of activated carbon according to which a mud suspension containing organic substances serves as starting material. In this process the mud is at first thickened, then pyrolyzed in a reducing atmosphere. The surface area of the obtained product is low, amounts to only about 32 m2/g.

According to the Swiss Patent Specifications No. 51 1,192 a afilter mass containing activated carbon is produced from a resinous wood material used as starting substance. The resinous sawdust is treated, based on its weight, with 3.6% by weight of sulphuric acid (diluted to 1 5 to 20 times of its original volume with water) and then subjected to dry distillation. To the formed carbonization product various additives, such as tar, pitch and sugar or molasses, are added, and the obtained mass is subjected to a heat treatment at 450 to 5000C. The main drawback of this method is that 100~400% of sugar or molasses, based on the weight of the carbonized material, must be added, making thus the process extraordinarily expensive.

In the Spanish Patent Specification No.

426,440 a process is described for the production of activated carbon according to which olive kernels and oil press cakes serve as starting material. The starting material is purified by flotation and subsequently carbonized at 700 to 9000C in a nitrogen atmosphere. Then the product is activated with zinc chloride. The main drawback of the process is that, on the one hand, high temperatures must be applied and, on the other hand, the used zinc chloride may lead to environmental pollution and requires corrosionresistant equipments.

In the Polish Patent Specification No. 72,948 a process carried out in a continuous operation on large scale is described for the production of charcoal. Lignocellulose-containing wood, sawdust, wood shavings or fruit kernels are used as starting material. The activation is carried out at 700 to 9000C, i.e. at a relatively high temperature, in a water-vapour atmosphere. A similar process is described in the German Patent Specification No. 737,332 where activation is carried out at 10000C.

In the German "Offenlegungsschrift" No.

20 52 507 a process is described for the production of activated carbon according to which a residue obtained at the acidic processing of xylose containing vegetable materials is used as starting material. In the description it is mentioned that acidic processing by means of a diluted mineral acid is carried out at an elevated temperature and that zinc chloride or phosphoric acid are used as chemical carbonization agents. The main drawback of this process is that the vegetable structure is extensively decomposed by the strong inorganic mineral acid used for the acidic processing, wherefore the advantages to be described later of the processing carried out by a relatively weak organic acid are lost.

Therefore the aim of the invention was to develop a process by means of which activated carbon of any desired pore size can be produced from a waste material of vegetable origin in an economical and simple way without the use of external acidic or alkaline reagents or salts, i.e.

without applying any chemical carbonization agents. Further aims were to prevent any essential change in the particle size of the starting material during the processing. to preserve as much as possible the total carbon content of the starting material in the residual activated carbon and to have a possibility of treating easily the in the form of volatile decomposition products escaping carbon-containing substances. Lastly, one of the aims was the possibility of carrying out the process in a continuous manner.

The invention is based on the recognition that vegetable wastes being autocatalytically partially hydrolysed with a relatively weak organic acid, primarily the residues of furfural production can be used as very favourable starting materials for vhe production of activated carbon. it is known e.g. from Hungarian Patent Specification No.

164,886 that during the steam extraction of vegetable wastes carried out between 50 and 2500C at 2 to 20 at a partial hydrolysis takes place which hydrolysis is autocatalyzed by organic acids such as acetic acid or formic acid present in the wastes and/or liberated by decomposition from the wastes. The wastes obtained in this way contain decomposition products derived from cellulose and lignin, e.g.

uronic acids, saccharides and gallates in an amount of about 6 to 15%, and these latter compounds are caramellizing already at a relatively low temperature (at about 150 C). The organic acids present in the partially hydrolyzed wastes catalyze the cracking of the waste material and thus increase the velocity of the reactions leading to the carbonization. In this way it is possible to carry out the carbonization without the use of any external carbonizing agent, moreover at lower temperatures and with a shorter residence time.

A further basis of the invention is the recognition that a system consisting of furfural, furfural resin intermediates and furfural resin, further being present in an amount of about 10 to 1 5%, based on the weight of the previously hydrolysed,starting material, does not participate in the above-mentioned caramelization and carbonization (thus does not decomposite) but it takes part in a cross-linking process along with the increase of the temperature. As already emphasized, the acid used for the previous hydrolysis is present in the wastes used as starting material, and due to the effect of this acid furfural resin is formed from the lastmentioned system.Owing to this the product cannot crumble during the carbonization process but rather the particles maintain their original particle size due to the formation of a resin skeleton.

Both the above-described favourable carbonization process and the formation of the resin skeleton take place, however, only in the case when the initial material is subjected to a heat treatment in the temperature range between 1 50 and 4000C at a heating-up rate of 10 to 400C/min. and in the temperature range between 400 and 6000C, wherein the so-called degassing (i.e. the complete removal of the volatile components) takes place, at a heating-up rate of 15 to 300C/ min. The carbonization carried out in the sense of the invention consists actually in this heat treatment conducted at a temperature not exceeding 6000C but being at least 5000C and for a period not exceeding 30 minutes by maintaining the just mentioned heating-up rates.

Just after the carbonization the carbonized material is activated in a way that it is treated at a temperature between 800 and 100000 and-in the sense of the invention-with a residence time not exceeding 30 minutes with at least one of the activating agents steam, carbon dioxide and flue gas, whereas an activating agent is dosed continuously or several activating agents are dosed alternately.

At the activation it is possible to adjust appropriately by an adequate choice of certain parameters, the average pore size and the pore volume and, respectively, the distribution of pore sizes, depending on the field of application of the activated carbon to be produced. The pore size of the activated carbon produced by the process according to the invention may range between 5 and 1000 A. In order to produce activated carbon having predominantly a pore size of 5-100 A the activation is carried out suitably at a temperature of 800 to 90000 and at a residence time of 3 to 5 minutes with steam or at a residence time of 20 to 30 minutes with carbon dioxide or flue gas.However, one may proceed also in a way that the said activating agents are mixed up with each other or applied alternatively.

For the production of activated carbon having predominantly a pore size of 100~1000 A, the activation is carried out suitably at a temperature of 900 to 100000 with a residence time of 20 to 30 minutes with carbon dioxide or flue gas or at a residence time of 7 to 10 minutes with steam.

The various activating agents can be applied also in this case as a mixture or alternately.

When performing the process according to the invention it is preferred to use dried waste materials as starting material. The particle size of this material is mostly in the range between 0.2 and 20 mm depending on the particle size of the initial material subjected to acidic hydrolysis.

Since the particle size of the initial material changes only barely in the process according to the invention, also the particle size of the produced activated carbon is essentially within this domain.

The waste materials obtained e.g. according to the processes described in the Hungarian Patent Specification No.164,886 or in the literature reference Escher-Wyss Mitteilungen, 1 969/2- 1970/1, pages 69 to 77, preferably the residues of the production of furfural and fodder yeast, are suitably used as starting material for the process of the invention. As an example the waste materials obtained by the acidic autocatalytic hydrolysis of wood shavings, maize cobs, maize straw, sawdust, rice husks, reed or reed chips may be mentioned.

However, moist waste materials, e.g. a material containing 5% moisture, may also serve as starting material. In this case the waste material is heated suitably to the initial temperature of heat treatment, i.e. to 1 500 C, at a heating-up rate of 300C/min in order to pre-dry the waste material. Heating is carried out suitably with natural gas or-directly or indirectly-with the flue gases obtained at the combustion of the decomposition products formed at the carbonization process.

During the carbonization a neutral gas, e.g.

nitrogen, carbon dioxide or another gas free of oxy.en or poor in oxygen (flue gas), can be lead in direct current or counter-current direction through the reactor containing the material to be carbonized. This operation is aimed at removing the decomposition products from the residual carbon skeleton by uniform rinsing.

Of the main advantages of the process according to the invention the following should be mentioned here: the activated carbon produced according to the invention possesses the actual, previously determined pore size, has a large specific surface and is obtained in an excellent yield. The process according to the invention can be carried out at low specific costs also in a continuous operation. The activated carbon produced according to the invention is thus particularly suitable for the purposes of environmental protection, and even its production does not cause any environmental pollution. The invention will be illustrated in detail by the following examples but is not limited by the same.

Example 1 100 g. of an air-dried residue obtained at the production of furfural from maize cobs are carbonized in a pyrolysis tube by raising the temperature to 20000 with a heating-up rate of 1 60C/min, between 200 and 40000 with a heating-up rate of 10 C/min and between 400 and 60000 with a heating-up rate of 1 60C/min.

Susequently the temperature is kept for 10 minutes at 60000. Meanwhile, nitrogen is led through the pyrolysis tube at a flow rate of 25 litres per hour.

The thus-formed condensable decomposition products are frozen out in a trap connected with the pyrolysis tube and cooled with a freezing mixture consisting of acetone and CO2. After the termination of the degassing the tube is allowed to cool in the applied gas stream. The product is then transferred into a tightly closed vessel. The yield is 36-41 g. (36--41%) of carbonized product.

The carbonized product is heated in the pyrolysis tube to 90000 at a heating-up rate of 16 to 200C/min and activated at this temperature for 9 to 10 minutes. Meanwhile water is introduced into the tube at said temperature at a feeding rate of 100 g/h, and the thus-formed water is transported through the material by using nitrogen at a flow rate of 25 litres/h as carrier gas.

After the completion of the activation process the tube is allowed to cool to room temperature on maintaining the nitrogen to cool to room temperature on maintaining the nitrogen stream.

The product is stored in a tightly closed vessel.

20.5 to 27 g of activated carbon are obtained, representing a yield of 57 to 66% based on the weight of the carbonized material and a yield of 20.5 to 27% based on the weight of the residue of furfural production which served as the starting material. Some parameters of the product as as follows: iodine number: 670 to 1000 mg. iodine/g. of sample. Methyleneblue number: 3 to 7 ml of methyleneblue/0.1 g. of sample.

Example 2 100 g. of an air-dried residue obtained at the production of furfural from wood chips are carbonized in a pyrolysis tube by raising the temperature to 20000 with a heating-up rate of 1 600/mien, between 200 and 40000 with a heating-up rate of 100cumin and between 400 and 55000 with a heating-up rate of 1 600/mien.

Subsequently, the material is carbonized for 10 minutes at 5500C whereas carbon dioxide is led as an inert gas through the pyrolysis tube at a flow rate of 25 litres/h. The condensable decomposition products are frozen out in a trap connected with the pyrolysis tube, said trap is packed with a freezing mixture consisting of acetone and CO2. After the termination of the degassing the tube is allowed to cool in the atmosphere used during the carbonization process. The product is stored in a tightly closed vessel. 36 to 37 g. (36~37%) of carbonized product are obtained.

10 g. of the carbonized product are heated in the pyrolysis tube to 850--9500C at a heatingup rate of 200C/min. Said temperature is maintained for 15 minutes. Meanwhile water is introduced into the tube at a feeding rate of 30 g./h and carbon dioxide at a flow rate of 25 litres/h. The water steam used as activating agent is transported through the tube by said carbon dioxide. After the termination of the activation process the tube is allowed to cool in a nitrogen atmosphere. The product is stored in a tightly closed vessel. 7.1 to 7.3 g. of activated carbon arf obtained, representing a yield of 71~73% based on the weight of the carbonized material and a yield of 26~27% based on the weight of the residue from the production of furfural. Some parameters of the product are as follows: Iodine number: 500 to 600 mg. iodine/g. of sample.

Methyleneblue number: 6 to 10 ml of methyleneblue/0.1 g. of sample.

Claims (5)

Claims

1. A process for the production of activated carbon from a partially hydrolysed vegetable waste material by carbonization and activation with at least one of the activating agents steam, carbon dioxide or flue gas, such that one activating agent is dosed continuously or several activating agents are dosed alternately, using as partially hydrolysed waste material a waste material autocatalytically hydrolysed with an organic acid, further by heating the waste material in an indirect or direct manner between 150 and 40000 at a heatingup rate of 10 to 400C/min, between 400 and 60006 at a heatingup rate of 15 to 300C/min, and by heat-treating the material at a temperature of 500 to 60000 for a period not exceeding 30 minutes, and finally by activating it with a residence time not exceeding 30 minutes.

2. A process as claimed in claim 1 ,wherein a heating-up rate of 10 to 200C/min between 150 to 40000 is maintained.

3. A process as claimed in any of the preceding claims, substantially as hereinbefore described with particular reference to any one of the Examples.

4. A process for the production of activated carbon which process comprises heating a partially hydrolysed waste material, to a temperature of 150 to 40000 at heating-up rate of 10 to 400C/min, the waste material being autocatalytically hydrolysed with an organic acid, further heating the waste material to a temperature of 400 to 60000 at a heating-up rate of 15 to 300C/min, heat treating the material at a temperature of 500 to 60000 for a period not exceeding 30 minutes, and activating the material with steam, carbon dioxide or flue gas, in a mixture thereof such that the activating agents are used alternately, for no longer than 30 minutes.

5. An activated carbon whenever produced by the process as claimed in any one of the preceding claims.