Process description (illustration)

• many technical processes suffer under the formation of waste air or gases, contaminated by hydrocarbons such as odors, solvents, chemical products or by-products, e.g.:
• food industry (odors, oils, grease, fat)
• painting processes (solvents)
• chemical and pharmaceutical industry (solvents, various organic compounds)
• technical processes (solvents, grease, waxes)

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The purification of such waste air and waste gases is possible at a low temperature level by means of thermal catalytic combustion of hydrocarbon compounds.

The catalytic combustion of hydrocarbon contaminated waste air and waste gases is realised through modern "state-of-the-art" technology. Hydrocarbons are burned on the catalysts with oxygen from air, forming carbon dioxide and water. In comparison to a simple thermal combustion, the catalyst reduces the required operation temperatures for this reaction substancially. This is the basic underlying thermodynamic principle of the catalytic process.
Technically one positive effect of the operation temperature reduction is that more simple and therefore cheaper construction materials are required. Chemically the formation of undesirable reaction products such as CO (carbon monoxide) and NOx nitrogen oxides) is avoided.
CO will be converted into CO2 on the catalyst at operation temperatures around 300 to 400 °C. At this temperature level NOx will be formed only at very low and uncritical levels. Low operation temperatures cause low energy consumption and therefore low operation costs, of course.
Catalysts are manufactured and offered on the market in diverse varieties with respect to their structural or chemophysical composition. They are selected by VOG Vormwald GmbH according to the given application:

• bulk catalysts (pellets, spherules) with noble metal impregnation (platinum, palladium)
• bulk catalysts from non-noble metals and mixtures, resp. their oxides (copper, chromium)
• monolithic catalysts with noble metals coating

The ceramic (pellets, sherules, monoliths) or metallic (monoliths) catalysts support only serves to stabilise the catalytic agent, which normally is coated onto the surface of the host as a thin layer.
The advantage of monolithic catalysts is the low pressure drop compared to pelletised catalysts, which in turn results in lower operational costs, due to lower power consumption of the blowers. Furthermore, sensitivity against dust particles in the gas stream, which can cause an inactivity of the catalytic function by surface coverage, is reduced on monoliths. 

For the design of a catalytic combustion plant it is of major importance to consider:

Kind and volume of the catalyst

In order to reach desirable conversion rates of hydrocarbons a certain catalyst volume is requested at a given waste air volume stream. This catalyst volume depends on the type of the catalyst itself as well as on the composition of the hydrocarbons and the operation temperature.

The operation temperature

To obtain desired conversion rates the lowest suitable operation temperature is requested. This temperature depends on the type of the catalyst but also on the chemical structure of the hydrocarbons. The basic operation temperature is maintained by the use of supporting energy, like electricity, natural gas, LPG or light oil, but also by the recovery of  combustion energy by means of a heat exchanger (plate/tube).

The catalytic process is - like any combustion process - an exothermic reaction and additional heat is produced. The amount of this exothermic heat is given in a simple approximation through concentration and chemical composition of the combustible agents.

In an operation plant the compensation of the overall heat losses by recovering the heat content of the cleaned air is a necessity, e.g. by preheating cold waste air with hot clean air in an economizer. Up to 90% of the heat content can be recovered. Starting at approximately 3g of hydrocarbons per Nm³ of waste air and therefore an exothermic temperature gain of approximately 90 °C all heat losses can be compensated by means of low-priced standard heat exchangers. The plant is running in self-sufficient mode. Any design of a complete process will always check the possibility of running the process autothermic. However, there are limits given due to technical and operational constraints.

The pressure drop

The pressure drop of a catalytic combustion plant and the associated power consumption of the attached waste air blower are determined by two major plant components: heat exchanger(s) and catalyst bed. The specific pressure drop of the catalyst is, amongst others, a function of his shape, as stated above.

Particles contaminating the waste air are removed by a filter.

For the recovery of the clean gas heat content normally pipe-shell heat exchangers are used. The recovery rate ranges between 60% and 80% and therefore reaches its optimum when correlated to the savings of energy and the size of the component (and the component costs).

Inside the combustion chamber the waste air is heated up to the required inlet temperature of the catalyst by means of a burner. Favourable combustion media are natural gas or LPG, as well as light oil. Smaller units also can be operated with electricity.

The catalyst is assembled in modules. Those modules allow a convenient control of the catalyst and facilitate an easy exchange of the catalyst, if necessary. In case of the use of monolithic catalysts, the monoliths have to be assembled to modules and stabilized by stainless steel frames to avoid damage of the monoliths during operation due to thermal stress as well as leaking gas streams.

Between the modules several thermoelements are installed for controlling the inlet temperature and the entire process temperature profile.

Downstream the catalyst the purified air again passes the economizer. In case the waste air still contains sufficient energy for secondary usage, a second cross bundle heat exchanger may be used to produce warm/hot water.

The operation control unit of the entire plant is located in a control cabinet. Process controlling is performed by microprocessor.

The purified air is released into the atmosphere by means of a stack.