Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-04-16
2001-02-20
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C095S009000, C095S223000, C095S230000, C422S168000, C422S169000, C423S235000, C423S237000, C423S24000R
Reexamination Certificate
active
06190629
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low maintenance scrubbers for the removal of water soluble and/or alkaline gases. The invention further relates to high efficiency acid scrubbers for the removal of hazardous components from an exhaust gas stream. The invention also relates to the removal of alkaline gases by reaction with an organic acid. The invention still further relates to methods of removing hazardous gases by their reaction with non-hazardous materials.
2. Background of the Related Art
In numerous chemical processes, waste gases pass into an exhaust stream. Many waste gas components, including by-products and excess reactants, are toxic and/or hazardous, and must be removed before the remaining exhaust is released into the environment. Two examples of hazardous gases which enter an exhaust stream are ammonia (NH
3
) and hydrochloric acid (HCl). These gases are both used for the hydride vapor-phase epitaxy (HVPE) growth of gallium nitride (GaN) and related semiconductors. Typically the ratio of ammonia (NH
3
) and hydrochloric acid (HCl) is very larger; on the order of NH
3
/HCl=500. Further, only a fraction of the total quantity of each of these gases used during HVPE actually participate in a reaction, with the bulk passing into the exhaust stream. Because these waste gases are toxic and corrosive, they cannot be released into the atmosphere, and therefore must be separated from the non-toxic components of the exhaust stream and because ammonia (NH
3
) is the largest component of the waste gas efficient removal of ammonia (NH
3
) is one of the most crucial features of a scrubber system.
Fortunately, both ammonia and HCl are highly soluble in water. Thus, a simple method for removing them is to pass the exhaust stream through a volume of water. However, as the water becomes saturated with a waste gas, the efficiency with which the gas is absorbed decreases, allowing more and more of the waste to pass through the water over time. Saturation of a volume of water with dissolved waste gases can occur relatively rapidly, so that the water must be replaced frequently. A considerable amount of maintenance is therefore required, and the overall efficiency of such a process for waste gas removal is low.
Prior art methods for improving the removal of ammonia gas from an exhaust stream have involved the addition of inorganic acids, such as hydrochloric acid or nitric acid, to the water. This greatly increases the quantity of alkaline gases that can be absorbed, since the dissolved gas will react with the acids to produce inert salts. In order to provide high solubility of alkaline gases, high concentrations of acid are needed.
However, concentrated inorganic acids are hazardous to personnel, as well as being highly corrosive to system components and other equipment. In addition, hazardous by-products which require costly and difficult disposal, are produced. Furthermore, there is an inherent disadvantage in using mineral acids in that the quantity of these acids that can be added to the water is limited. As a result, relatively frequent replacement of the mineral acid/water mixture is required, leading to overall inefficiency of the scrubbing process.
However, solid acids, both inorganic and organic, including various carboxylic acids, have been used for the removal of gases from air. For example, U.S. Pat. No. 1,586,327 to Perrott, et al. discloses the purification of air to render the air suitable for breathing by providing a filter material composed of a granular mixture of an inert filler and an acid. Such a filter material may be packed into a canister of a gas mask for removing ammonia from air to be inhaled.
U.S. Pat. No. 5,626,820 to Kinkead, et al. discloses a chemical air filter for use in the air handling system of a clean room. The filter includes a pleated web of non-woven fibrous material, and having activated carbon particles distributed throughout the web. In one example of the '820, the use of citric acid in combination with a charcoal filter is disclosed.
SUMMARY OF THE INVENTION
The present invention provides a method for removing water soluble, hazardous waste gases from an exhaust stream by passage through a scrubber mixture, wherein the method requires minimal maintenance and poses little hazard to personnel. The scrubber mixture of the invention is preferably prepared from water and a non-toxic, organic acid. The exhaust stream is passed through the scrubber mixture to remove water-soluble and/or alkaline gases (e.g., ammonia). The use of a solid acid improves the capacity and efficiency of the scrubber, greatly increasing the quantity of gas that can be dissolved in a given volume of scrubber mixture, as compared with water or an aqueous solution alone. The increased capacity of the scrubber mixture also reduces the amount of maintenance required for the scrubber. An exemplary acid for use in practice of the invention is citric acid, which has the advantages of being non-toxic, relatively non-irritating to the skin, and does not significantly attack laboratory equipment. Therefore, scrubbing operations are significantly less hazardous as compared to the prior art which uses inorganic mineral acids. In addition, citric acid is inexpensive, providing a low-cost scrubber system. Furthermore, the by-product of the reaction of citric acid with ammonia, ammonium citrate, is benign, as is the case for the by-products of reactions of citric acid with many other waste gases.
According to one aspect of the invention, the use of a solid acid as a component of the scrubber mixture provides for increased capacity of a given volume of scrubber mixture. A solid acid may be added to the scrubber in (large) quantities, greatly exceeding the amount that is soluble in a given volume of water of the scrubber mixture. Undissolved acid, present in a scrubber vessel as a solid deposit, acts as a convenient source of acid for replenishment of the liquid component of the scrubber mixture. Thus, as alkaline waste gas passes through the scrubber mixture and reacts with the dissolved acid, some of the solid portion of the acid dissolves, thereby maintaining the concentration of dissolved acid. According to the invention, the lifetime of the scrubber mixture is limited only by the total amount of solid acid present in the scrubber. The system of the invention thus represents an improvement over the prior art use of inorganic acids, in which the lifetime of the scrubber is limited, more severely, by the quantity of acid that is soluble in the water.
In another aspect of the invention, a method is provided for indicating that the mixture in a scrubber must be replaced. A pH indicator such as phenolphthalein may be added to the acid/water mixture described above. When the scrubber is operational, the mixture is acidic, and the solution of phenolphthalein is colorless. However, if the acid becomes depleted, the liquid component of the scrubber mixture turns a vivid purple-red color. The inclusion of a pH indicator in the scrubber mixture provides a simple, inexpensive, and safe method for determining the status of the scrubber.
In another aspect of the invention, a method is provided for using a single scrubber for removing unequal quantities of acidic and alkaline water-soluble gases from a single exhaust stream. In certain processes, both acidic and alkaline gases will enter the exhaust; for example, during HVPE deposition of gallium nitride, hydrochloric acid and ammonia gases are used during different stages of the process. Overall, a much greater quantity of ammonia than hydrochloric acid enters the exhaust stream. By constantly passing the exhaust stream through the acid/water scrubber mixture as described above, both gases can be efficiently removed in a single process. When ammonia is present in the exhaust, it will react with the acid in solution making the solution less acidic. When the hydrochloric acid is present in the exhaust, it will dissolve efficiently in the water, thereby increasing the acidity of the liqu
Miller David J.
Solomon Glenn S.
Ueda Tetsuzo
CBL Technologies, Inc.
Dunn Tom
Lumen Intellectual Property Services
Medina Maribel
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