Platinum Metals Rev., 1963, 7, (3), 82
Platinum Oxidation Catalysts in the Control of Air Pollution
Platinum alloys supported on specially prepared alloy steel ribbons and formed into mat-like beds of various shapes and sizes have been developed for the flameless combustion of fumes from industrial processes. This article describes a number of typical installations employing catalytic combustion elements to eliminate objectionable vapours and so to avoid pollution of the atmosphere.
The commercial processing of a wide variety of materials in many different industrial operations often generates fumes and vapours that may be highly objectionable if they are allowed to be released to the air without further treatment. Sometimes these substances are present in industrial effluent vapours in concentrations sufficient that, in combination with other substances already present in air, they may irritate tissues of the eyes, nose or throat, or harm plant life. In other cases industrial fumes may partially condense inside ventilation ducts or stacks, or on the walls of adjacent structures, producing corrosion, creating fire hazards, or accumulating as deposits requiring periodic removal.
The chief trouble-makers in many of these industrial emissions are usually organic chemical compounds. Theoretically they can be incinerated to harmless carbon dioxide, or water vapour, but simple incineration is not often practical because of the expense involved and because the concentration of these air pollutants is generally very low compared with the often quite large volume of total effluent which also must be brought to incineration temperature.
A common offender is the food industry. Frying, roasting and rendering of food products often creates fumes and smokes having unpleasant odours and, less frequently, accompanied by eye-irritating fumes. The causative substances usually are present in only a few parts per million concentration.
The manufacturing of paints and varnishes, as well as operations using them, are often the target of public criticism because of the fumes released to the air. In cooking varnish, for example, not only are unpleasant odours sometimes emitted, but vapours are evolved that tend to form flammable deposits on the walls of the room and in ventilation ducts. Similarly, the drying and baking of surface coatings and cores used in foundries release vapours that are generally combustible and contain objectionable odours.
The chemical processing industry has come in for its share of blame as an emitter of air pollutants and some authorities consider it a significant and growing source of objectionable substances in the atmosphere. The expense would be staggering if it were required that the last traces of products be recovered from the waste gases of many chemical plants, yet even minute traces of chemicals can become large quantities when exhausted from continuous processes. The volume of exhaust gases is so high, while the proportion of combustible material is so low, that the contaminants are not present in flammable concentrations and cannot be consumed by ordinary means.
Catalysis now offers a solution to many of these problems, even to the disposal of sewage odours in trace amounts. It has long been known, for instance, that finely-divided platinum supported on a carrier will cause methanol vapours to combine with atmospheric oxygen at room temperature; ultimately, the temperature rise at the catalyst surface will ignite the methanol-air mixture. The oxidation of ammonia vapours to nitric acid in the presence of platinum alloy gauze as a catalyst is commonplace.
However, until 1940 catalysis was generally overlooked as a tool in preventing air pollution. Perhaps this is because of association with the relative complexity of reactors generally employed in chemical synthesis, necessary in order to achieve the desired rate and equilibrium conditions, and thus it was felt that adapting such devices for air pollution control would be too expensive to be practical. This, and the belief that deep beds of expensive catalyst might be required, are equally erroneous.
Platinum alloys supported on specially prepared alloy steel ribbons, formed into mat-like beds of various sizes and shapes, have been developed as oxidation catalysts for the flameless low temperature combustion of hydrocarbons and other combustibles in vented gaseous wastes. These catalysts, developed by the Catalytic Combustion Company, a Division of Universal Oil Products Company at Detroit, Michigan, promote the oxidation of industrial effluent vapours containing a wide range of hydrocarbons and other combustible compounds. Reaction rates are extremely high in the (usually) very thin beds of catalyst and small reaction chambers employed. In hundreds of systems used to remove air pollutants from the exhaust gases of industrial ovens, for instance, these units oxidise in excess of 97 per cent of combustible constituents.
The resulting products from catalytic combustion of these waste gases naturally will depend upon the nature of the original pollutants. In the case of hydrocarbons, the combustion products are harmless carbon dioxide and water vapour. Side reactions appear to be inhibited. This is demonstrated by the catalytic combustion of dimethylamine; analyses of the product gas show that oxides of nitrogen are absent. In general, no unusual products are known to have resulted from the catalytic combustion of other compounds when at least the stoichiometric oxygen requirement is met and the catalyst is operated above the minimum ignition temperature for the specific reactant.
Since most industrial operations involving the evolution of flammable vapours are conducted in such a manner as to maintain the concentrations of combustibles well below the flammable range, only simplified controls are generally required to maintain the catalyst bed temperature. This is because equilibrium concentrations of the products from normal combustion remain almost unchanged up to relatively high temperatures. The catalyst is resistant to the effects of high temperature (exceeding 1800°F) and so is the alloy steel support.
Catalyst bed temperatures obviously will change with the concentration of combustibles present and their nature. This does not, however, affect the catalytic combustion of these substances so long as the minimum catalytic ignition temperature (MCIT) is maintained. This, too, is a function of the combustibles present. On the other hand, suitable monitoring of the catalyst bed temperature may be used to warn of process upsets giving rise to undesirably high concentrations of combustibles—in fact, this feature is frequently used as a secondary control of the manufacturing process.
Wide variations of the heating value of the waste gas are well tolerated by the catalyst. This simplifies control requirements. It also permits the handling, without difficulty, of processes that release hydrocarbons on a cyclic basis. The sensible heat in the effluent gas from catalytic combustion may be recovered, in many instances, for use in heating make-up or process gas, or for space heating. The usual procedure is either to recycle some of the hot effluent gas to process, or to design suitable heat exchangers into the process system.
The catalyst configuration is characterised by a very large amount of available surface within a very small volume, a low thermal capacity, unusual mechanical strength and great resistance to thermal shock. The thermal expansion of the catalyst and the support are equal; this feature eliminates spalling and thus loss of catalyst. The pressure drop across the bed is negligible. The use of non-rigid materials and special construction allows the catalyst to “breathe” during wide temperature fluctuations but will not permit the bed position to shift. Therefore, destructive forces will not be set up between the catalyst and its support, or between the support and the element enclosure itself. This greatly contributes to the long life of the catalyst element.
These metallic-supported catalysts, as normally supplied, are non-specific for the range of most industrial emissions encountered. This is fortunate because waste gas streams often contain a mixture of several hydrocarbons, and perhaps also may include products of partial oxidation reactions, hydrogen, carbon monoxide, amines and nitrogen oxides. Concentrations may range from mere trace amounts to several per cent by volume.
The temperature required to initiate catalytic oxidation is dictated by the individual constituents of the fume stream. Most of the commonly encountered hydrocarbons and combustible organic vapours require catalyst surface temperatures in the range of 475 to 650°F to initiate catalytic oxidation. Some alcohols, paint solvents and light unsaturated hydrocarbons may oxidise at substantially lower catalyst surface temperatures; hydrocarbons evolved from the tar melting processes may require initiation temperatures of 750°F. Hydrogen, on the other hand, will undergo catalytic oxidation at ambient temperatures, whereas oxides of nitrogen must be mixed with supplementary fuel and brought to initiation temperatures in the range of 1100 to 1400°F.
Initiation temperatures — autogenous ignition temperatures—for the flame incineration of most hydrocarbons are rarely below 1200°F when concentrations are below the lower flammable limit. In accepted oven practice, incidentally, fire insurance underwriters generally require that the concentration of combustibles be kept at or below 25 per cent of the lower flammable limit.
Since the heat capacity of most waste gas streams may be considered as approximately the same as for air, or 0.0181 Btu per standard cubic foot, the release of 1 Btu in 1 cubic foot of fume will increase its temperature by approximately 55°F. If the waste gas stream is at 300°F, for example, 5.45 Btu per cubic foot must be supplied to bring the gas to a catalytic oxidation initiation temperature of 600°F. To bring this same gas to the minimum autogenous ignition temperature of 1200°F for flame combustion, however, will require 16.35 Btu per cubic foot—exactly three times as much.
The metallic-supported catalyst structure was specifically developed not only with the foregoing points in mind, but also to function reliably under the widely varying conditions commonly encountered in industrial air pollution control applications. The lightweight elements are fabricated as flat rectangular or square shapes, flat round shapes and cylindrical shapes, as required by various applications.
Standard element sizes are normally supplied for multiple mounting in simple gasketed frames. A typical bank of standard elements is shown in Fig. 1, each element measuring 18×24 inches; there it will be seen to resemble closely a conventional air-filter element. Hollow cylindrical bed elements for high pressure applications are shown in Fig. 2.
The construction of the catalyst elements is such as to lend itself readily to practically any desired configuration, because each element and its contained catalyst is self-supporting regardless of the mounting position or the direction of the gas flow. “Deep” beds, such as are used for purifying process gas streams in chemical plants are often 12 inches thick or more. These gas purification catalysts have operated for many thousands of hours in industrial installations.
A typical application of these catalytic elements in a paint baking oven is shown in sectional view in Fig. 3. This features not only the destruction of objectionable fumes and odours but also the return of heated, decontaminated air to the oven to supply a part of its heat requirements. In this application, the oven is divided into two zones; the first, in which most of the paint solvent is evaporated from the coated material, and the second, or baking zone, in which the coating is brought to dryness and cured. In the first zone, temperature is maintained only high enough to evaporate the solvents used in the paint. Relatively rich in combustibles, this low-temperature gas is brought to catalytic oxidation temperature of about 600°F by a preheat burner and passed through the catalyst elements. The heat release shown here is equivalent to 120,000 Btu per gallon of solvents in the fume stream. The hot decontaminated air is then mixed with air from the baking zone, which evolves little contaminants, and is returned to the oven system along with a controlled amount of fresh make-up air.
The basic design of a Catalytic Combustion unit as may be used for installations such as the varnish kettle shown in Fig. 4 is shown in Fig. 5. Another design of equipment for preventing polluted air from reaching the atmosphere is shown in Fig. 6. Here the fume is brought to reaction temperature and catalytically oxidised as before, but the hot gases are then heat exchanged against the incoming cool gases, also permitting most of the heat generated to be recovered. In an alternative application, the hot decontaminated effluent from the catalytic oxidiser can be exchanged against fresh ventilation air before exhausting to the stack and the fresh air used for space heating.
An interesting application of the cylindrical type of catalyst element shown in Fig. 2 in chemical plants is as an in-line purification device on tail-gases from a nitric acid unit. In this installation the process end-gas is mostly air, but is contaminated by a low concentration of foul-smelling acidic vapours and is at high pressure. An expanding turbine is used to recover as much of the energy in the tail-gas as possible. A Catalytic Combustion unit was installed ahead of the expander in order to remove these objectionable constituents before releasing the vapour to air. As a result, additional energy was made available, since heating the fume during catalytic combustion caused a predictable volumetric increase in the effluent ahead of the expander. The vapour released contained no objectionable constituents.
The future looks bright for the elimination of odorous fumes by catalytic combustion. Cities the world over are becoming more and more conscious of the air pollution problem as it affects human life as well as property values. And because catalytic oxidation permits economic disposal of these pollutants at low temperatures in relatively simple apparatus that permits in many instances useful recovery of the heat content of these streams, it promises to become the preferred method for this necessary measure.