Document 3eX9KJaxD57bQx3gd2ryQe42n

Manufacture and Uses of Active Carbon F. R. Houghton* f.r.i.c., and J. Wildmant The bulk of active carbon produced today is made from either coconut shells or coal-based materials, activated by selective oxidation with steam at elevated temperatures. This produces a product with the ability to effectively absorb ev6n trace quantities of either unwanted or valuable liquids and gases. This is its main field of application, where it plays a very important part in solvent recovery processes, water and effluent treatment, and the treatment of flue gas before discharge to the atmosphere. These are all applications which are likely to expand in the future due to stricter anti-pollution legislation. THE use of carbon dates back at least as far as the ancient Egyptians who used it for medicinal purposes. Its application ex panded somewhat in 1773 when Scheele discovered the phenomenon of gas adsorption on charcoal, and 12 years later Lowitz successfully demonstrated its decolorisation capacity. This technique was first used by the sugar industry, which soon switched over from wood chars to bone chars when it was discovered that the latter were superior and could be regenerated many times. Production of experimental carbons from a variety of basic materials followed the discovery of bone char, and liquid- phase operations continued to be the main area of application. Coconut shell as a raw material was first investigated about 1865 by Hunter, who reported its capacity for adsorbing vapour. Around the same time Lipsome produced a carbon for purifying potable water and Stenhouse ; suggested that carbon be used for ] ventilating sewers and also described the ' forerunner to the gas mask. \ At the turn of the twentieth century manufacturing processes for active carbon began to appear along with a number of patents. But the event that stimulated the development of the high-quality car bons used today was the 1914-18 War, in which chlorine gas was used by the German army, and for which an effective countermeasure had to be found quickly. Since then the chemical industry has continued research into the production, characterisation and uses of active carbon over an ever expanding field of application. Raw materials ' Materials used in the manufacture of . active carbons on an industrial scale include coconut shell, coals, wood and ' peat. Coconut shell contains about 76% volatile matter and moisture, which is removed largely at the source by partial carbonisation in order to minimise shipping costs. The cellulosic structure of shell ' determines the end product which, at normal yields of 30-40% on the carbonised basis, is a material with a very high internal surface, consisting of pores and capillaries of molecular dimensions. The ash content SEALED COVER normal procedure in some cases to grind the coal and reconstitute it into a form suitable for processing by the use of a binder such as pitch. Another method involves grinding the coals and utilising the volatile content as a binder by application of pressure. This second method allows for judicious blending of selected materials to control the swelling power of the coals and prevent coking and production of large pores which are unsuitable for adsorption purposes. Blend ing of coals also gives a degree of control over the structure and properties of the final product. Wood is a useful raw material which can be activated by either one of two methods --steam activation or chemical activation, depending upon the product required. The better product is normally produced - by chemical activation with phosphoric acid, which gives a char with a large surface consisting of pores of molecular dimensions suitable for adsorbing colour bodies from solution. The carbon is normally in a finely divided state and since it is produced from waste materials, such as sawdust, it is relatively cheap and can be used on a `throw-away' basis. Other techniques of chemical activation practised with wood and peat involve the use of zinc chloride and similar substances, the product being extruded, activated and subjected to various leaching processes to; remove the zinc salt.1 2 Fig. 1. Simplified diagram of an activated carbon retort - (Courtesy: Chemical Publishing. New York) of such carbons Is normally very low; the main ash constituents are dependent to some extent on the area from which the shells are obtained. `- ' Coal is also a basic material which is, in parts of Europe and the US, readily avail able and reasonably cheap. The final product naturally depends upon the type of coal used and the initial processing prior to carbonisation and activation. It Is Manufacturing processes Great developments have occurred in the methods of manufacture of active carbons since early days when normal procedure involved the combustion of stacks of wood in inadequate air supplies. Material produced in this way has a very limited surface area and is completely uncompetitive when compared with modern products. The bulk of present-day materials is produced from either coconut shells or coal-based materials, which are fed to a *F. R. Houghton Is R & 0 Msnager with Sutcliffe. Speakman & Co. Ltd. fJMMWman is a research chemist with Sutcliffe, Speakman 6 Co. Ltd. DSW 297547 STLCOPCB4067458  retort bn a continuous basis. The normal solvents from processes--such as syn activation process involves the use of thetic fibre and plastics manufacture-- steam at selected temperatures, selective where evaporation of residual solvents is oxidation of the material resulting in the practised. production of carbons with pores of An active carbon system is normally molecular dimensions. Shell carbon, being designed to meet the specific conditions of a cellulose base, produces a material applying at any particular plant. Since with a finer pore structure than is obtain conditions vary from place to place, it is able from coals, which require processing not really possible to design a standard prior to activation if a viable product is to unit which can be made on a production be obtained. line basis, unless a number of plants within Retorts designed to produce active a particular industry are to be served. carbons (see Fig. 1) usually operate in one Concentration of solvent normally dealt of three ways--vertically, horizontally or with can vary from a few ppm to as much using a series of hearths, depending upon as 1-5-2% by volume in the air stream, the method favoured by the particular according to the source. The effectiveness manufacturer. In all cases, the quality of of solvent recovery depends a great deal the product depends ultimately upon the upon its concentration and boiling point; residence time of the material in the the more dilute the concentration, the 'activation zone*. less efficient the process. Activation in the vertical retort, utilising Other factors must also be considered, steam, is controlled by the rate at which including the susceptibility of the solvent material is withdrawn from the discharge to reactions which can be catalysed by hopper. The activation zone covers only active carbon. These reactions could a minor part of the total space available and cause corrosion within the unit, or produce it is possible, although not common conditions conducive to ignition or practice when high quality is desirable, to explosion. These problems are common proceed from raw to finished product in to certain classes of solvents which are one pass. recovered effectively by active carbon. After withdrawal from the retorts, the They are overcome by careful selection of material is cooled and passed through a materials of construction and strict adher series of granulators and screens to give ence to operating conditions specified by a carbon of a known quality which is the manufacturers. available in a variety of grade sizes to suit Other problems are posed by substances most applications. which carbon cannot readily adsorb. The alternative process for manufacture, However, many of these can be solved by involving the use of chemical digestion of using the carbon as a base for a chemical sawdust, wood shavings or peat by substance which can either catalyse the phosphoric acid or zinc chloride, produces , decomposition of, or combine with, the a material with much larger pores, since the ! gas in question. This application includes process cannot be subjected to the same removing the contaminant from a radio measure of control as steam activation. active iodine source and the removal of The base materials normally contain noxious substances such as hydrogen large amounts of volatile matter and the sulphide by oxidation to elemental sulphur. advantage of chemical activation lies in Liquid effluents pose a different type of the production of highly adsorptive problem since the final product must either materials at much higher yields than be suitable for reuse or, if intended for would be possible using steam. disposal, must attain standards set by local Carbons produced in this manner, authorities or river boards. however, have one inherent failure--their Treatment may be carried out batch- lack of mechanical strength--and are not -i wise using powdered carbons, or, if suited to vapour-phase operation, or to I possible, by the use of a percolation liquid-phase operation in a granular form. ) technique utilising granular carbon on a ' cyclic basis. Trace amounts of sparingly- Grades of carbon / soluble solvents are normally recovered in A variety of grades and qualities of; this way but the technique is not applicable activated carbons are manufactured and to effluents containing solvents or other these can be classifed as follows: materials of a strongly polar nature. Coconut-shell-based carbons -- for , Colour bodies, on the other hand, are vapour-phase applications involving sol generally composed of molecules with a vents of small molecular dimensions. They relatively large cross-sectional area which can also be used for a few liquid-phase can be adsorbed by carbon, provided that operations. they are not strongly ionic. In general, Coal-based carbons--for catalyst-support systems used for this type of treatment applications, vapour-phase operation with incorporate powdered carbon containing medium- and relatively-high boiling sol a proportion of macropores on a 'throw vents, and for liquid-phase operation. away' basis; it is also possible to use Impregnated carbons--for mercury and suitable grades of granular active carbon, radioactive iodine removal and for Inclu regeneration being effected by high sion in cooker hoods to remove odour. temperatures in presence of steam. j Powdered carbon--for decolouring, water treatment and medicinal purposes. ` Water treatment t ! Gas/liquid recovery Many water supplies, suchas river' water, contain a proportion of sewage or Loss of reagent in a process directly industrial effluent and are unsuitable for affects the economics of the process and reuse without further treatment. This treat can also pose a pollution problem. There ment often involves dosage with powdered fore it Is essential to eliminate this loss, carbons in order to remove trace con and here active carbon is an invaluable taminants which give undesirable taste or tool, particulady for recovering valuable odour characteristics to the water. Fig. 2. Basic flow diagram of a gaseousphase adsorption plant using active carbon for solvent rbbovery Fig. 4. Flow diagram of a liquid-phase adsorption unit DSW 297548 STLCOPCB4067459 Soft drink manufacturers and other industries requiring water of extra high purity, practise chlorination/dechlorination techniques to remove traces of organic colour and taste. Active carbon is used in granular form to remove traces of chlorine from the treated water. The reaction takes place on the adsorbent, being of a catalytic nature, and the life expectancy of the carbon is normally very high. Attention is now being paid to reclama tion of water, particularly in the US where extensive research programmes are being carried out into the treatment of industrial effluents, river water and even sewage effluent, as a means of combating the increasing shortage of suitable water supplies. Basic research involving the use of active carbon is also being carried out now in the UK--on a much smaller scale than in the US--and results obtained so far are very encouraging and it is expected that greater interest will be generated as the need for new supplies increases. It has been demonstrated by Slack3 that the use of a coagulant, followed by active carbon in powder form, can produce an effluent from a sewage works of similar quality to the normal river intake of the district. The problem in this case is seasonal, however, and powdered material is used on a 'throw away' basis to minimise capital outlay. Further work is being carried out at Nottingham involving the purification of water from the River Trent using a granular carbon system. Treatment of this type results in the removal of organic material and con taminants, such as detergents, by adsorp tion on active carbon. The system can be operated on a cyclic basis, allowance being made for regeneration of the adsorbent by heat treatment at 800-900C in the presence of steam. Investigations are also proceeding into the treatment of raw sewage by physico chemical methods involving precipitation and carbon treatment. Again, the investigation is in its early stages but effluents with a very low BOD (3-5 ppm) have been produced, and it is highly probable that such treatment will, in the near future, be considered as an alternative to traditional biological methods. Treat ment of final effluent from an existing sewage works is quite feasible and reduces the detergent content to virtually nil whilst reducing the organic load, the net result being a higher grade effluent for disposal. This treatment could be of great significance in situations where one authority abstracts water from a point below the outfall of another authority's discharge. It is apparent that active carbon has a big part to play in future developments and that it may well form the basis of an entirely new process resulting in the production of high-grade waters. Solvent recovery processes There are two main types of recovery unit: a static system involving the use of an annular carbon bed; and a continuous unit in which the carbon is packed into boxes which rotate around a horizontal axis. Small units utilising the flat-bed technique have also been manufactured occasionally to satisfy specialised needs. Fig. 5 (right). Rotary bed plant for recovering 150 gallhr ofpetroleum solvent from a 20,000 cfm air flow. A , simplified diagram.of the plant Is shown above SHAM SOLVENT LAOEN AIH Fig. 6 (right). Packagedtype, fully-automatic recovery plant of the cylindrical, flat-bed design. The flow diagram is shown above Solvent recovery operations from pro cesses such as synthetic fibre spinning must be capable of dealing with a large air movement and solvent concentrations of the order of 1 % v/v. It is usual, therefore, to design plant to suit the prevailing conditions and, due to the volume of air involved, a great deal of carbon is normally required to adsorb the available solvent. Initial laboratory testing determines the capabilities of the adsorbent under the specified conditions and the ultimate number and size of adsorbers involved is determined by such characteristics as the volume of air available, velocity permis sible and resistance to airflow of the adsorbent, which in turn determines maximum bed depth of the adsorbent. The adsorbent is charged to a level which permits little or no solvent loss, a factor which can be regulated instru mentally; the solvent is stripped from the carbon by passing live steam through the bed in the reverse direction. The amount of solvent taken from the adsorbent is maintained at an economical level by controlling the steam input. Control of, the complete system is normally carried out on a fixed time basis, desorption and drying periods being regulated by the number of adsorbers involved and the time required to charge the adsorbent to its working capacity. Basic flow diagrams for static-bed type of plant are illustrated in Figs. 2 and 3. Very high efficiencies are obtained using this system, as have been consistently demonstrated by the monitor ing of effluent air from adsorbers on various sites, using a vary sensitive hydrocarbon analyser. DSW 297549 Ancillary equipment is dependent upon the solvent or solvent mixtures involved and includes distillation stages to effect separation. Or, in some cases, an aqueous phase containing a low concentration of a sparingly-soluble solvent may be stripped using a percolation technique, the basis of which is demonstrated in Fig. 4. The continuous unit is a much more compact plant, the carbon being packed into a series of boxes which form segments of a horizontal rotor. Each box traverses a full cycle which consists of adsorption, desorption and drying stages, a complete cycle representing one complete rotation. The efficiency of the carbon is determined experimentally and the speed of rotation is calculated from a knowledge of the available solvent and the capabilities of the adsorbent. This type of unit is suited to applications such as the recovery of hydrocarbon solvents in the printing industry and is popular in the US, where there is a demand for package-type units. An illustration is given in Fig. 5. Another package-type unit of flat-bed design, manufactured for specialised conditions and utilising titanium to resist severe corrosion, is illustrated in Fig. 6. Typical performance data which can be expected from Sutcliffe, Speakman carbons, operat ing with a variety of solvents, are given inTablel. Future developments Pollution of the environment is one of the big issues of today and active carbon will have an ever increasing part to play in the control, reduction or elimination of i conditions responsible. STLCOPCB4067460  Table 1. Some typical results of solvent adsorption by carbons, as experienced in solvent recovery practice Solvent Concen tration (% w/w) *Initial adsorp tion (% w/w) ] Cyclic adsorp tion (% w/w) Boiling point range rc/ i-S ' 1 a* ; ++ Methylene chloride.. .. ArkloneP .. ...... Acetone .. .. '' .. Tetrahydrofuran .. .. Hexane .. .. .. Ethyl acetate .. .. Trichlorethyiene .. .. n-Heptane .. .. .. Toluene .. .. .. Methyl isobutyl ketone .. Mixed rubber solvent .. ShellsolE........................... 3,3,5-trimethyl cyclohexyl acetate .. .. .. 10 0-5 10 0-5 0-48 0-5 0-6 012 0-4 0-2 0-3 0-3 0-1 28-3 44-9 20 3 22-0 21-3 27-6 44 6 ,, 22-4 23-3 220 26-7 35-6 36-4 17-3 20-8 12-6 90 8-2 13-6 19-9 5-9 9-6 9-0 10-3 11-9 7-9 1-4 40-5 1-4 47-6 2-3 56 2-0 66 3-5 68-7 2-1, 77-2 1-8 86-7 4-3 98 3-5 110 3-5 115-9 3-8 120-160 3-7 153-193 4-5 206 `Initial adsorption refers to the solvent adsorbed by new carbon to 'breakpoint', i.e. when solvent can be detected in the effluent air stream. The `slip* level at this point is normally of the order of 20 ppm. tCyclic adsorption refers to the solvent which can'be adsorbed and desorbed consistently at an economical steam consumption. (Steam ratio refers to tha total steam usad to desorb the solvent fioit the adsorbent and is an overall figure inclusive of condensate and moisture condensed on tha carbon tad. The use of carbon in water treatment is the subject of much research at present and it is likely that, within the next few years, new processes will be developed to produce reusable and even potable water from contaminated industrial and sewage effluents. Air treatment will also increase in importance and future trends may dictate that solvent recovery operations will become necessary to remove trace con taminants from effluent air systems in order to comply with legislation. Treat ment of flue gases to reduce oxides of . sulphur and other contaminants--which are at present vented to atmosphere to become someone else's problem--may be come the subject of clean air legislation. In this case active carbon may also have an important part to play in adsorbing the sulphur oxides which could then be recovered as elemental sulphur or sulphuric acid. Active carbon is also being used as a filter. In applications such as the filter hoods for removing cooking odours and in-some cigarettes, where active carbon in powdered or granular form is incorpor ated in the cigarette filter. Reducing atmospheric pollution by ' motor vehicles is also receiving a great deal of attention and a device has been developed which removes petrol vapours by adsorption with carbon. The adsorbed material is then desorbed from the carbon by hot gas when the engine is running and returned to the engine. Legislation is already in force in certain US states whereby all new motor vehicles must be fitted with this device, and the demand for suitable carbon will increase as the laws are extended. REFERENCES 1. Hauler J. W., 'Activated Carbon', Chemical Publishing, New York, 1963. 2. Smisek, M., and Cemy, S., 'Active Carbon', Elseviar Publishing, 1970. 3. Slack. J. G., Effluent 6 Weter Tnetment J.. 1969,9(5), 257. BIBLIOGRAPHY Maggs, F. A. P,, 'Manufacturing Carbon'. Pergamon, Oxford. Taft. R. A.. `Advanced Waste Treatment at Washing ton, DC'. May, 1969, Water Research Centre. Ohio. Cunent Status of Advanced Waste Treatment Processes, July 1970, Advanced Waste Treat ment Research Laboratory, Ohio. Active Carbon for Taste and Odour Control Water Research Association TP. 66. Nov. 1969. process control system The Protronic process control system of In the control room the panel instruments Hartmann 8- Braun AG--introduced last --comprising controllers, ratio, set-point year at the ACHEMA Exhibition--repre and manual stations, indicators, and sents a comprehensive automatic measur recorders with selection stations--are all ing and control system specially designed - of modular design with identical front for the chemical and petrochemical j dimensions, case depths and standardised industries where hazardous areas of j connections and terminal lineup. There operation exist. The circuits and instru- ' are two types of process controller; the B ments have been designed with special model with a band scale and control emphasis on explosion protection, signal deviation and the F series with a fixed scale interconnection techniques, computer and controlled variable indication. Within compatibility, assembly and wiring. the two types there are various possible The system uses intrinsically-safe two- configurations of controller. The simplest E wire transmitters for absolute and differen model has all functional groups mounted tial pressure, temperature and angle of permanently while in the 5 model the rotation. To fully utilise the two-wire input circuits and control amplifiers are technique, a number of transmitter supply designed as plug-in components. By units have been designed together with replacing these components a change in safety barriers which allow transition from the controller function and in the setting intrinsically-safe to non-intrinsically-safe ranges of the parameters can be accom circuits. The transmitters are not fed with plished easily. Also additional plug-in a supply voltage; instead they function units can be inserted such as output with a `live zero' receiving power from a limiters, start-up circuits, etc. continuously-flowing basic current of There are also controllers compatible 4 mA. The signal current is superimposed with process computers, such as controller on to this up to a value of 20 mA. The BM. This is similar to the type BS transmitter supply unit has a supply voltage controller except that, in addition, it limited to 20 V by Zener diodes, and, features motor-driven set-point setting, together with the safety barriers, conforms remote-control logic and a memory to all VOE regulations on intrinsically-safe . amplifier for supervisory control with a circuits. The low voltage of the supply process computer. If direct digital control unit allows a greater distance between the is employed an analogue standby controller transmitters and the control room--an is often required in case the computer important point on very large plants. breaks down, and this Is provided In the Protronic system by controller FS. Since its introduction, Hartmann 8 Braun's new control system has been accepted widely by the German chemical industry and a large number of systems have been successfully installed there. The company are now planning to market the system in other countries through their subsidiaries in the UK, Holland, France, Italy, Austria, Australia, the US. and Brazil. slurry consultants Pierson 8 Co. (Manchester) Ltd. have established a special department dealing with slurry processing consultancy as an extension to their existing activities in this field. Through their many international associates, Pierson have been able to build up a network of contacts, all dealing with different facets of slurry technology. The consultancy service has already had some success particularly in effluent processing. Through their associate companies Pierson are able to offer a complete package deal as far as actual equipment is concerned, ensuring that any advice given is guaranteed 100%. The consultancy work can be undertaken on an overall-fee or daily-rate basis. DSW 297550 STLCOPCB4067461