7%51 ____________ ______ The invention relates to a method and apparatus for producing independent streams of nitrogen and carbon dioxide simultaneously. More particularly, the nitrogen is separated from air and the carbon dioxide is produced by combustion of a carbon-containing fluid with the oxygen by-product of the nitrogen separation step.
BACKGROUND OF THE INVENTION
__ _ _______________________ Combustion of a carbon-containing fuel produces a combustion products containing carbon dioxide, among other constituents. If the carbon dioxide is required in a highly concentrated form, the unwanted constituents are removed from the combustion products in subsequent steps, and the carbon dioxide is concentrated to the desired degree of purity.
It has hitherto been customary to burn the carbon-containing fluid by adding air. In this case, the combustion products or waste-gas also contains nitrogen, among other things. If the carbon dioxide is required free of nitrogen, this must be washed out of the waste gas. Washing operations, however, consume a considerable amount of energy and are also costly.
It is, therefore, an object of this invention to provide a method which permits inexpensive production of carbon dioxide and nitrogen.
According to the invention, this object is achieved by using substantially nitrogen-free carbon fluid containing and is burned with the addition of oxygen arising from the separation of nitrogen from the air.
According to the invention, the oxygen arising as a ~'f~
7~
1 by-product when nitrogen is produced by the fractionation of air, is used to burn the hydrocarbon-containing fluid.
In the case of the method according to the invention, nitrogen-free combustion products are produced by combusting a nitrogen-free fuel with oxygen. This eliminates the costly carbon dioxide/nitrogen separation.
~UMMARY OF THE INVENTION
_____________ _ _________ The method, according to this invention, produces in an energy efficient manner nitrogen independent of carbon dioxide, where both gases are produced simultaneously.
The process comprises separating nitrogen from air by fractionation to provide the nitrogen stream. A
susbstantially nitrogen-free carbon containing fluid is reacted with oxygen to give the carbon dioxide in the combustion products. The oxygen derived as a by-product from the nitrogen separation step, is used in the combustion process to provide the nitrogen free stream of carbon dioxide.
The apparatus, according to this invention for producing the nitrogen and carbon dioxide, comprises means for separating nitrogen from air to provide a nitrogen stream and a substantially nitrogen-free oxygen by-product stream. Means combusts a nitrogen-free carbon containing f~uid with the by-product oxygen to provide a substantially nitrogen-free combustion product containing carbon dioxide. Means introduces the fluid into the combustiondevice. Means delivers the oxygen by-product stream to the combustion device and means removes a portion of the combustion products and admixes it with the oxygen by-products stream to control the rate of ~4Q~2~
1 combustion.
BRIEF DESCRIPTION OF T~E DRAWINGS
____________.________._____________ Preferred embodiments of the invention are shown in the drawings wherein:
~ igure 1 is a schematic representation oE the apparatus in which the inventive process is carried out.
DETAILED DESCRIPTION OF THE PREFER~ED EMBODIMENTS
___________________________________________ The process, according to a preferred embodiment, provides a degree of control in the combustion of the carbon containing fluid with oxygen. Combustion taking place with the addition of pure oxygen is quite violent and hazardous from an explosive standpoint. This usually requires the use of high temperature, high pressure reaction vessels which are costly and complex in design.
To control the violent reaction, the oxygen, as a by-product of the air fractionation step is diluted by admixing the oxygen stream with the combustion products to provide a concentration of oxygen in the mixture which is approximately the same as that of oxygen in normal air.
This makes it possible to use conventional combustion vessels, yet still obtain a nitrogen free combustion product containing carbon dioxide.
The combustion step may be carried out at above atmospheric pressure. The so produced combustion products at the above atmospheric pressure may be depressurized by performing work and in so doing, driving a compressor to compress the portion of combustion gas which is admixed with the oxygen stream in controlling the rate of combustion.
The combustion gas may be cooled and passed to a ~l~LQ'7~
1 separator. The gases, as cooled, cause the unwanted volatile components to condense and are removed in the separator.
Further purification of the combustion gases may be achieved by passing the gases over an appropriate catalyst. An excess of oxygen is provided in the combustion vessel in order to prevent corrosion. A
catalyst is, therefore, used to separate the excess oxygen from the waste or combustion gases. Thus, it is sometimes preferred to introduce a gas containing hydrogen and/or methane to the combustion gases prior to contacting with the catalyst to remove the oxygen therefrom.
A turbine compressor may be used to compress the air prior to fractionation and to compress the produced nitrogen. The turbines may be steam operated, where pressurized steam is produced from the heat of combustion in the combustion vessel. The pressurized steam is fed to the turbines and provides at least a portion of the energy needed to compress the air in the nitrogen stream.
With reference to Figure 1, a preferred embodiment of the apparatus is schematically shown, where the method may be conducted for producing the streams of nitrogen and carbon dioxide. Air is provided at 1 at roughly 234,000 Nm3/h to an air compressor 2 where it is compressed to pressure of approximately 7 bars, as required in the fractionation process. Within the fractionator 3, the air is fractionated and separated by low temperature rectification into a stream of nitrogen 4 of the desired purity and of a flow rate of approximately 187,000 Nm /h.
Oxygen, which may be pure, is provided in line 5 at a 1 flow rate of approximately 45,800 Nm /h. The nitrogen is recompressed in compressor 6 to the necessary final pressure in the product discharge line.
A combustion vessel 7 may be operated as a charged combustion vessel at an excess pressure of between 2 and 3 atmospheres. This vessel is used for the combustion of a carbon containing fluid 8, preferrably butane or heavy oil, which is substantially free of nitrogen. Oxygen, as a by-product from the fractionation process, is fed through line 5 to the combustion vessel 7, at a pressure at which the combustion vessel is operated. Combustion is initiated by compressing air fed through line 9 into the combustion vessel.
Once combustion is underway, fluid 8, as it burns with the addition of oxygen from line 5, produces a waste gas or combustion products stream 14 containing the desired carbon dioxide. Depending upon the composition of the burned fuel 8 to be burned, the combustion products will contain additional components which may be, for example, water.
The heat produced by the combustion is used to produce steam in lines 10 and 11. This pressurized steam drives the stream turbines 12 and 13 which are coupled to the compressors 2 and 6 to aassist in the compr~ssion of the air and nitrogen in streams 1 and 4.
The combustion gases are depressurized in an expansion turbine 15. Such work may be used to drive a compressor which compresses a portion of the combustion gases as drawn off from line 19 and admixed with the pure oxygen in line 5 for the reasons already noted. In ~3~4Q~
1 instances where the combustion vessel 7 is operated at normal pressures, the expansion turbine 15 may not be needed.
The combustion gases 14 are cooled in condensor 16 until all the unwanted volatile components of the gas are condensed out. The condensate is removed in separator 17 and is drawn away through line 18. The gaseous fraction of the combustion gases 14, as it exits fromseparator 17 via line 19, is as explained divided into two flows.
The portion of combustion gases divided away at line 21 may be as high as 172,600 Nm3/h which, as explained, is compressed in the compressor coupled to the expansion turbine 15 prior to admixture with the oxygen stream 5.
The ratio between the two flows of gas must be such that, after admixture, the oxygen has approximately the same concentration as it would normally have in air so that the combustion proceeds in a normal controlled manner.In order to prevent corrosion within the combustion vessel, a slight excess of oxygen of approximately 1% over the amount needed for combustion may be used.
The remaining flow in line 19, approximately 23,000 Nm /h is compressed in compressor 12 to approximately 10 atmospheres. For further purification, the comb~stion gases containing mostly carbon dioxide is fed over a catalyst 23 for purpose of removing oxygen remaining in the gas. Several types of catalysts may be used as are well known by those skilled in the art, such as catalysts containing platinum or palladium. To improve the efficiency of the catalytic removal of the oxygen, the combustion gases may be admixed with hydrogen and/or ~4(~7~9 1 methane from line 25. Subsequently the purified carbon dioxide is then compressed for consumer usage.
If desired, the catalytic after cleaning of the combustion gases may be replaced by other appropriate methods such as carbon dioxide concentration - washing.
Although various preferred embodiments of the invention have ben described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope oE the appended claims.