James G. Speight - Encyclopedia of Renewable Energy

Many mineral acid catalysts that are active in homogeneous catalysis can be made suitable for heterogeneous catalysis by supporting the catalyst on an inorganic oxide. Strongly acidic heterogeneous catalysts are prepared by supporting Brønsted acids such as trifluoro-sulfonic acid, sulfuric acid, phosphoric acid, and Lewis acids (such as such as boron trifluoride, BF 3, and antimony pentafluoride, SbF 5) on high surface area oxides such as silica, SiO 2, alumina, Al 2O 3, and zirconia, ZrO 2). In particular, supported phosphoric acid on silica is still widely used, and BF 3– γ -Al 2O 3and H 2SO 4–ZrO 2possess acidic sites that enable them to perform reactions that other solids are not strong enough acids to catalyze. Supported acids are difficult to characterize and are highly dependent on the methods and materials used in their preparation but do offer a suitable alternative for reactions that require strong and even super acidity.

See also: Catalysts.

Acid deposition is the scientific term used to describe acid rain (which includes including acid fog, acid sleet, and acid snow).

Acid deposition (acid rain) occurs when sulfur dioxide (SO 2) and, to a lesser extent, NO xemissions are transformed in the atmosphere and return to the earth as dry deposition or in rain, fog, or snow. Acid rain is another environmental problem that affects many industrialized area of the world resulting in damage crops, forests, wildlife populations, and causing respiratory and other illnesses in humans.

When atmospheric pollutants such as sulfur dioxide and nitrogen oxides mix with water vapors in the air, they are converted to sulfuric and nitric acids.

Sulfur dioxide with water in produce sulfurous acid:

In the gas phase, sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:

In the presence of water sulfur trioxide (SO 3) is converted rapidly to sulfuric acid:

Nitric acid is formed by the reaction of water with nitrogen dioxide and by the reaction of carbon dioxide with water:

Although carbonic acid is a weaker acid the nitric acid, sulfurous acid, and sulfuric acid, it does, however, contribute to acid rain which, in high concentrations, can cause damage to natural environments including forests and freshwater lakes.

Acid deposition can be classified as wet deposition such as acid rain, snow, sleet and fog or dry deposition such as deposition as particulate matter even less than PM 2.5. Effects of acid rain can be either chronic or episodic.

Chronic acidification is a long-term effect due to years of acid rain. Episodic acidification is due to heavy rain storms; it also occurs in spring as concentrated nitrate and sulphate in lower layer of snow pack get released when snow melts. A second method of acid deposition is known as dry deposition. Whilst wet deposition involves the precipitation of acids, dry deposition occurs when the acids are first transformed chemically into gases and salts, before falling under the influence of gravity back to Earth. Sulfur dioxide, for example, is deposited as a gas and as a salt.

The gases present in acid deposition are found to occur naturally in the environment. They are given off from a number of sources including volcanic eruptions and the rotting of vegetation. They become a problem when humans produce the gases in large amounts and at high concentrations by the burning of fossil fuels. It is arguable which gives off the most acid gases – fossil fuels or natural events such as eruption of volcanoes. The distances that pollutant gases travel means that acid deposition is an international or trans-boundary problem. This means that acid pollutants are not necessarily deposited in the same country where they were produced.

Acid rain falling over regions with alkaline soils or rocks is quickly neutralized but in areas with little acid-neutralizing capacity is the biosphere sensitive to acid rain. Over North America these areas include New England, eastern Canada, and mountainous regions, which have granitic bedrock and thin soils.

In areas where the biosphere is sensitive to acid rain, there has been ample evidence of the negative effects of acid rain on freshwater ecosystems. Elevated acidity in a lake or river is directly harmful to fish because it corrodes the organic gill material and attacks the calcium carbonate skeleton. In addition, the acidity dissolves toxic metals such as aluminum from the sediments. There is also ample evidence that acid rain is harmful to terrestrial vegetation, mostly because it leaches nutrients such as potassium and allows them to exit the ecosystem by runoff.

See also: Acid Rain.

Many gas streams, while ostensibly being hydrocarbon in nature, contain large amounts of acid gases such as hydrogen sulfide (H 2S) and carbon dioxide (CO 2) and acid gas is natural as or even process gas that contains significant amounts of hydrogen sulfide, carbon dioxide, or similar contaminants. The terms acid gas and sour gas are often (incorrectly) treated as synonyms.

A gas stream containing hydrogen sulfide or carbon dioxide is referred to as sour and a gas stream that is free from hydrogen sulfide is referred to as sweet . The corrosive nature of hydrogen sulfide and carbon dioxide in the presence of water (giving rise to an acidic aqueous solution) and because of the toxicity of hydrogen sulfide and the lack of heating value of carbon dioxide. However, because gas streams from a variety of renewable sources have a wide range of composition, including the concentration of the two acid gases, processes for the removal of acid gases vary and are subject to choice based upon the desired end-product.

In addition to hydrogen sulfide and carbon dioxide, a gas stream may contain other contaminants, such as mercaptan derivatives (such as methyl mercaptan, CH 3SH, and carbonyl sulfide, COS). The presence of these impurities may eliminate some of the sweetening processes since some processes remove large amounts of acid gas but not to a sufficiently low concentration. On the other hand, there are those processes that are not designed to remove (or are incapable of removing) large amounts of acid gases. However, these processes are also capable of removing the acid gas impurities to low levels when the acid gases are there in low to medium concentrations in the gas.

To sweeten (i.e., remove sulfur compounds from) the high acid content gas, it is first pre-scrubbed to remove entrained brine, hydrocarbons, and other constituents. The sour gas then enters an absorber, where lean amine solution chemically absorbs the acid gas components, as well as a small portion of hydrocarbons, rendering the gas ready for processing and sale. An outlet scrubber removes any residual amine, which is regenerated for recycling. Hydrocarbon contaminants entrained in the amine can be separated in a flash tank and used as fuel gas or sold. Process efficiency can be optimized by mixing different types of amine to increase absorption capacity, by increasing the amine concentration, or by varying the temperature of the lean amine absorption process.