The present invention relates to a rotary tube, in particular for a rotary tubular kiln (furnace) for the production of activated charcoal (=activated carbon) and to a rotary tubular kiln having such a rotary tube. The present invention relates, furthermore, to the use of this rotary tube or rotary tubular kiln for the production of activated charcoal.
Activated charcoal, because of its highly unspecific adsorptive properties, is the absorbent which is used the most. Statutory conditions, but also the increasing awareness of responsibility for the environment, lead to a growing demand for activated charcoal.
In this context, activated charcoal is used increasingly both in the civil and in the military sector. In the civil sector, activated charcoal is employed, for example, for the upgrading of gases, filter systems for air-conditioning, autofilters, etc., whilst, in the military sector, activated charcoal is employed in protective materials of all kinds (for example, respirators, protective covers and protective garments of all kinds, such as, for example, protective suits, etc.)
Activated charcoal is generally obtained by the carbonization (also designated synonymously as smouldering, pyrolysis or coking) and subsequent activation of suitable carbon-containing (i.e. carbonaceous) starting materials. In this context, those starting materials are preferred which lead to economically reasonable outputs. This is because the weight losses due to the removal of volatile constituents during carbonization and due to burn-up during activation are considerable. For further details regarding the production of activated charcoal, reference may be made, for example, to H. v. Kienle and E. Bäder, Aktivkohle und ihre industrielle Anwendung [Activated charcoal and its industrial use], Enke Verlag Stuttgart, 1980.
The quality of the activated charcoal produced, fine-pored or coarse-pored, solid or fragmentary, etc., depends on the carbon-containing starting material. Conventional starting materials are, for example, coconut shells, wood waste, peat, hard coal, pitches, but also particular plastics, such as, for example, sulphonated polymers, which play an important part, inter alia, in the production of activated charcoal in the form of granules or spherules.
Activated charcoal is used in various forms: powdered charcoal, splintered charcoal, granulated charcoal, formed charcoal and, since the end of the 1970s, also granular and spherical activated charcoal (what is known as “granular charcoal” and “spherical charcoal”). Granular, in particular spherical activated charcoal has, as compared with other forms of activated charcoal, such as powdered charcoal, splintered charcoal and the like, a series of advantages which makes it useful or even indispensable for specific applications: it is pourable, exceedingly abrasion-resistant and dust-free and very hard. Granular charcoal, in particular spherical charcoal, because of its special form, but also because of the extremely high abrasion resistance, is highly sought after for special areas of use, such as, for example, surface filter materials for protective suits against chemical toxins or filters for low pollutant concentrations in large air quantities.
The production of activated charcoal, in particular granular charcoal and spherical charcoal, is in most cases based on suitable polymers. Sulphonated polymers, in particular sulphonated styrene polymers cross-linked with divinylbenzene, are preferably used, in which case sulphonation can be achieved even in situ in the presence of sulphuric acid or fuming sulphuric acid. Suitable starting materials are, for example, ion exchanger resins or their precursors, which are mostly polystyrene resins cross-linked with divinylbenzene, the sulphonic acid groups already being present in the material in the case of finished ion exchangers and still having to be introduced by sulphonation in the case of ion exchanger precursors. The sulphonic acid groups perform a critical function, since they assume the role of a cross-linking agent in that they are removed during carbonization. However, in particular, the large quantities of sulphur dioxide released and the corrosion problems in the production equipment which are associated, inter alia, with these are disadvantageous and present difficulties.
The production of activated charcoal conventionally takes place in rotary tubular kilns. These have, for example, a feed point for charging the raw material at the kiln start and a discharge point for the final product at the kiln end.
In the conventional processes for the production of activated charcoal according to the prior art, both carbonization and subsequent activation are carried out in discontinuous production in a rotary tube.
In carbonization, which may be preceded by a pre-carbonization or pre-smouldering phase, the conversion of the carbon-containing starting material into carbon takes place, that is to say, in other words, the starting material is carbonized. During the carbonization of the abovementioned organic polymers based on styrene and divinylbenzene, which contain cross-linking functional chemical groups, leading in the event of their thermal decomposition to free radicals and therefore to crosslinkages, in particular sulphonic acid groups, the functional chemical groups, in particular sulphonic acid groups, are destroyed, at the same time as the removal of volatile constituents, such as, in particular, SO2, and free radicals are formed, which bring about high crosslinking, without there being any pyrolysis residue (=carbon). Suitable starting polymers of the above-mentioned type are, in particular, ion exchanger resins (for example, cation exchanger resins or acid ion exchanger resins, preferably with sulphonic acid groups, such as, for example, cation exchanger resins based on sulphonated styrene divinylbenzene copolymers) or their precursors (that is to say, the unsulphonated ion exchanger resins which still have to be sulphonated before or during carbonization by means of a suitable sulphonating agent, such as, for example, sulphuric acid and/or fuming sulphuric acid). In general, pyrolysis is carried out under an inert atmosphere (for example, nitrogen) or an at most slightly oxidizing atmosphere. It may likewise be advantageous, during carbonization, particularly at higher temperatures (for example, in the range of about 500° C. to 650° C.), to add a relatively small quantity of oxygen, particularly in the form of air (for example, 1 to 5%), to the inert atmosphere, in order to bring about an oxidation of the carbonized polymer skeleton and thereby facilitate subsequent activation.
On account of the acid reaction products (for example, SO2) removed during carbonization, this step in the process of producing the activated charcoal is extremely corrosive in terms of the kiln material and makes the most stringent demands as regards the corrosion resistance of the material of the rotary tubular kiln.
Carbonization is then followed by the activation of the carbonized starting material. The basic principle of activation is to break down part of the carbon generated during smouldering selectively and in a controlled manner under suitable conditions. This gives rise to numerous pores, splits and cracks, and the activated charcoal surface related to the unit of mass increases considerably. During activation, therefore, a controlled burn-up of the charcoal is carried out. Since carbon is broken down during activation, in this process a considerable loss of substance occurs in parts, which, under optimum conditions, is equivalent to a rise in porosity and signifies an increase in the inner surface (pore volume) of the activated charcoal. Activation therefore takes place under selectively or controlledly oxidizing conditions. Conventional activation gases are generally oxygen, in particular in the form of air, steam and/or carbon dioxide and also mixtures of these activation gases. Inert gases (for example, nitrogen) may be added, if appropriate, to the activation gases. In order to achieve a technically sufficiently high reaction rate, activation is generally carried out at relatively high temperatures, in particular in the temperature range of 700° C. to 1200° C., preferably 800° C. to 1100° C. This makes it necessary for the material of the rotary tubular kiln to satisfy high requirements as to temperature resistance.
Since the material of the rotary tubular kiln must therefore withstand both the highly corrosive conditions of the carbonization phase and the high-temperature conditions of the activation phase, only those materials are used for the production of the rotary tubular kiln which have good high-temperature corrosion resistance, that is to say, in particular steels which combine good resistance to chemically aggressive materials, in particular good corrosion resistance, and good high-temperature resistance in a single material.
Despite the high-temperature resistance of the materials, in particular steel, normally used for the rotary tube, the high operating temperatures in the production of activated charcoal, which may attain 1200° C. or even more, cause these materials or the steel to become relatively soft under these extreme temperatures and lose dimensional stability and consequently tend to a certain susceptibility with regard to mechanical deformations. In the production of activated charcoal, pressure differences and pressure fluctuations occur inherently in the method adopted: this is due particularly to the fact that, on the one hand, gaseous breakdown products are generated and, on the other hand, reaction or process gases are to be supplied and work is carried out under changing pressure conditions (for example, atmospheric pressure and reduced pressure or a vacuum), and in this case the pressure conditions cannot be kept constant for the entire duration of the method for the production of activated charcoal. The result of this, sometimes, is that the appreciable pressure differences and pressure fluctuations in the operating state may give rise to a deformation of the rotary tube. This may lead to damage to the rotary tube apparatus and to premature material fatigue, and, on the other hand, process management and process control consequently become appreciably more difficult.
One object of the present invention is, therefore, to make available an apparatus or a rotary tube which is suitable, in particular, for the production of activated charcoal, whilst the above-outlined disadvantages of the prior art are to be at least partially avoided or else at least mitigated.
To solve the problem outlined above, the present invention proposes a rotary tube according to the disclosure and claims. Further advantageous refinements are the subject-matter of the relevant subclaims.
A further subject of the present invention is a rotary tubular kiln (furnace) according to the disclosure and claims which comprise a rotary tube according to the present invention.
Finally, a further subject of the present invention is the use of the rotary tube or rotary tubular kiln according to the invention for the production of activated charcoal according to the disclosure and claims.
The subject of the present invention, according to a first aspect of the present invention, is therefore a rotary tube, in particular for a rotary tubular kiln for the production of activated charcoal, the rotary tube being provided on the outside with at least one reinforcing element for stabilizing the rotary tube in the operating state. A rotary tube with reinforcing elements is thus provided, which is dimensionally stable in the operating state, in particular under extreme temperature conditions, and has high resistance to deformations.
This is because the applicant surprisingly discovered that the mechanical stability or dimensional stability of the rotary tube can be considerably improved in the operating state, in particular even under extreme conditions (such as occur, for example, in the production of activated charcoal) when the rotary tube is provided on its outside or outer wall with at least one reinforcing element, preferably with a plurality of reinforcing elements.
A rotary tube is thereby provided which can better withstand mechanical deformations and is more resistant even to pronounced pressure differences and pressure fluctuations and is therefore dimensionally stable even under operating conditions. The rotary tube according to the invention consequently has an improved useful life with a reduced tendency to premature material fatigue. As a result of this, too, process management and process control are facilitated.
Further advantages, properties, aspects, particularities and features of the present invention may be gathered from the following description of the preferred exemplary embodiment illustrated in the drawings.