Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Patent
1998-03-19
1999-11-23
Doerrler, William
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
62655, 95 54, C01B 2104
Patent
active
059879173
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and an apparatus for producing nitrogen at a nominal pressure in which air is precompressed in a precompression stage, then cooled, cleaned and reheated, whereafter separation of the nitrogen is effected in a separation stage and it is finally compressed to the nominal pressure in a final compression stage.
The main field of application of the invention is the production of high pressure nitrogen for injection moulding machines, in which the nitrogen is injected into the injection mould in order to produce hollow bodies. Such systems operate with the recirculation of nitrogen from the injection mould. The inlet of the final compression stage is thus at atmospheric pressure.
DE 4435702 A1 discloses part of such a system, namely the production of the nitrogen at the precompression pressure. This is of the order of 10 to 12 bar, determined by the separation stage. After depressurisation to atmospheric pressure, the nitrogen is brought in the final compression stage, which is not shown, to the order of 320 to 340 bar.
It is the object of the invention to simplify the production of nitrogen and substantially to reduce the necessary power requirement.
In order to solve this object the method referred to above is characterised in that the separation stage supplies the input pressure for the final compression stage, that the build-up of the inlet pressure of the final compression stage in the start-up phase is delayed by partial recirculation of the gases discharged from the separation stage to the inlet of the predepression stage and that the final compression stage is only switched on when the build-up of its inlet pressure is completed.
A substantial proportion of the compression work is thus saved by the fact that during continuous operation the pressure increase in the final compression stage does not begin at atmospheric pressure but at the outlet pressure of the separation stage. The two compression stages can be optimally matched to the attainment of the desired residual oxygen content.
The delayed switching on of the final compression stage facilitates its starting and avoids current peaks. Furthermore, a considerable energy saving results from the time delay.
Of significant importance is the fact that during the start-up phase before switching on the final compression stage the gas supplied from the separation stage is partially recirculated to the inlet of the precompression stage. This results in the desired residual oxygen content of the nitrogen being reached very rapidly. The recirculating gas is purified in the separation stage and thus increases the nitrogen content at the inlet of the precompression stage.
A further increase in the efficiency may be achieved by recirculating the nitrogen accumulating in the final compression stage as a result of internal leakage losses to the inlet of the precompression stage. The final compression stage thus operates without losses in this respect, the recirculated nitrogen again increasing the nitrogen content of the air at the inlet of the precompression stage.
The same applies to a further feature which is proposed, namely of performing the separation in a plurality of stages with gas recirculation to the inlet of the precompression stage, since the recycled gas is already enriched with nitrogen.
The advantages of the above features are produced regardless of the manner in which the air is cooled and reheated and the nitrogen is separated.
It is, however, very particularly advantageous to use the heat produced by the compression for reheating the air since this also contributes to the reduction of the energy requirement.
If, for instance, the compression is effected with oil cooling, the oil can give up its heat to the air before the latter is supplied to the separator.
Alternatively or additionally, there is the possibility in a further embodiment of the invention of using the heat produced from the cooling of the air for reheating the air.
The apparatus in accordance with the invention comprises an air precompressor, an air
REFERENCES:
patent: 5102432 (1992-04-01), Prasad
patent: 5582030 (1996-12-01), Dannohl
Doerrler William
Druckluff Dannohl GmbH
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