Method for Reducing Air Pollution

This invention concerns reduction of air pollution, in general. More specifically, it relates to a method for reducing air pollution in connection with generating nitrogen by cryogenic air separation. The nitrogen is for use in enhanced oil recovery and the like. In such a cryogenic operation, if the engines employed to drive the compressors make use of natural gas for fuel, the engines exhaust have high quantities of air pollutants.

Briefly, the invention is in connection with a cryogenic facility producing nitrogen for use in enhanced oil recovery and the like and having oxygen as a by-product. The said facility employs internal combustion engines for driving air and nitrogen compressors. The said engines use natural gas and air mixture for fuel. The invention concerns a method for reducing air pollution caused by the effluent from said facility. It comprises the capturing of said by-product oxygen, and the feeding of said captured oxygen into said fuel to reduce the NO.sub.x compound content of the exhaust from said internal combustion engines.

Again briefly, the invention relates to a cryogenic facility that produces nitrogen for enhanced oil recovery or the like. The said facility employs internal combustion engines, and said engines employ natural gas-air mixture for fuel. The engines create unacceptable levels of nitrogen oxides at the exhaust, and a method of reducing said levels of nitrogen oxides comprises the step of using by-product oxygen from said facility to enrich said internal combustion engines fuel whereby said levels of nitrogen oxides are reduced to an acceptable quantity.

The foregoing and other objects and benefits of the invention will be more fully set forth below in connection with the best mode contemplated by the inventor of carrying out the invention, and in connection with which there are illustration provided in the drawings, wherein:

FIG. 1 is a schematic block diagram illustrating the prior art; and

FIG. 2 is a similar schematic block diagram including the invention.

In enhanced oil recovery operations, the use of nitrogen as an injectant is often employed. In such case, the quantity of nitrogen required is large so that cryogenic air separation is an accepted form of technology to provide nitrogen in the quantities needed. However, it is economically attractive to fuel the cryogenic facility with its associated compression equipment, by burning natural gas which is readily available in the oil field. But, the natural gas in combination with air that is used in the combustion process yields toxic nitrogen oxides. These are classified as pollutants by the Environmental Protection Agency, and the EPA has emission restrictions which make it very difficult or impossible to obtain approval to install and operate the required facilities.

It may be noted that basically, all of the nitrogen contained in the aforementioned NO.sub.x emissions comes from the air supporting the combustion involved in the operation of the compressor engines which are being fueled by a mixture of natural gas and air. Thus, a simplified equation which illustrates the conditions may be expressed as follows:

It will be noted that elimination of the nitrogen component in the foregoing expression will eliminate the pollution. And, if only oxygen were added to the fuel that would be accomplished. However, it is also a fact that if there is only oxygen fed to the fuel mixture the engines would run too hot because of excessive combustion temperatures. Consequently, a feature of this invention is to make use of some of the oxygen from the cryogenic air separation to enrich the fuel mixture so that the requirement of reduction of pollutants may be met. It will be understood that the most feasible manner of determining how much oxygen to add is by making adjustment of the amount of oxygen added to the fuel mixture so as to minimize pollutants while not overheating the engines.

In order to take advantage of the availability of natural gas at the location where nitrogen is being generated for injection in one or more oil wells, the fuel for compressor engines used in the cryogenic facility is natural gas-air mixture. But, it is this fuel that creates unacceptable amount of NO.sub.x pollutants in the exhausts. However, if oxygen enriched air is used in the fuel mixture, the amount of NO.sub.x pollutants can be substantially reduced. So, by obtaining the oxygen employed in making the enrichment of the fuel mixture for the compressor engines as a by-product from the cryogenic facility, a means of pollutant control with economic benefit is obtained, since the oxygen would otherwise be vented to the atmosphere. An empirical determination of the optimum mixture of oxygen with the air of the fuel mixture to the engines may be made by experimentation for each engine under consideration. It may be noted that an optimum blend of oxygen and air would result in two improvements, i.e. (1) the reduction or elimination of NO.sub.x pollutants, and (2) the increased combustion efficiency with associated decrease in fuel consumption at the engines.

The drawings illustrate the prior art in FIG. 1. A cryogenic facility 11 is a type such as the unit described in Kirk-Othmer's "Encyclopedia of Chemical Technology", Third Edition, Volume 15. At page 935 there is a description of "Cryogenic Air Separation" which continues on page 936. Prior to this invention, if the cryogenic facility 11 was operated near a source of natural gas, that natural gas would be employed as the fuel for both air compressors 12 and nitrogen compressors 13. And, since the facility 11 would be operated only to generate a source of nitrogen for use in injection procedures for secondary recovery in oil wells, the oxygen which would be generated by the facility 11 as a by-product would be merely vented.

Because the compressor engines (not shown) which drive the air compressors 12 and the nitrogen compressors 13 need a mixture of air with the natural gas so as to provide a combustible mixture for the internal combustion engines, they would produce in the exhausts a quantity of pollutants in the form of NO.sub.x compounds which would be unacceptable so that clearance for building and operating such a unit could be unobtainable.

However, by applying this invention in accordance with the FIG. 2 diagram in the drawings, the pollution may be substantially reduced. Thus, the invention may be applied to a cryogenic facility 21 which is operated so as to capture oxygen developed as a by-product. This is indicated by a dashed line block 22 having the caption "CAPTURED OXYGEN" applied thereto. The captured oxygen is directed to fuel input lines 25 and 26 of air compressors 27 and nitrogen compressors 28, respectively. Such directed oxygen flows over paths 31 and 32 which are indicated by dashed lines in the FIG. 2 illustration. It will be understood that the amount of oxygen which is added to the fuel mixture inputs 25 and 26 is adjusted in each case so as to provide optimum conditions as described above. Therefore, the amount of exhaust pollution from schematically indicated stacks 35 and 36 from the air compressors 27 and the nitrogen compressors 28 respectively, is reduced to an optimum minimum while permitting the compressor engines to run more efficiently but without overheating.

It will be understood that the cryogenic facility 21 includes an air intake as indicated by an arrow 38 to the compressors 27. Then compressed air goes via an intake line 39 applied to the facility 21 where separation is carried out by distillation or by molecular sieve action and the nitrogen that is separated goes (as indicated) by an output line 40 that carries the caption N.sub.2. The nitrogen then goes to the nitrogen compressors 28 and the compressed nitrogen goes for injection as indicated by an ouput arrow 43.

A typical cryogenic facility for generating nitrogen to use in an oil field operation might run with an input of 45 million cubic feet per day, e.g. at air input (arrow 38), and an output of 25 million cubic feet per day of nitrogen, e.g. at nitrogen output (arrow 43). In such case there would be 20 million cubic feet per day of oxygen and nitrogen mixture without the capture of any of the oxygen.

While a particular embodiment of the invention has been described in considerable detail in accordance with the applicable statutes, this is not to be taken as in any way limiting the invention but merely as being descriptive thereof.