This application claims the priority of German application No. 199 36 816.3, filed Aug. 5, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for recovering oxygen at hyperbaric pressure by low-temperature air fractionation in a rectifying system which comprises at least one pressure column and one low-pressure column. Feed air is compressed to a first pressure which is about the same as the operating pressure of the pressure column. At least a first partial flow of the feed air is cooled at the first pressure in a main heat exchanger and passed into the pressure column. An oxygen flow is tapped from the low-pressure column; brought to a delivery pressure that is higher than the operating pressure of the low-pressure column; heated in the main heat exchanger; and discharged as product. The pressure of a process stream is work-expanded and the process stream is supplied to the low-pressure column. At least a portion of the mechanical energy, generated by the work-expansion of pressure, is used to drive a cold compressor.
A method and a corresponding apparatus are known from DE 2544340 A. The refrigeration performance of a pressure-expanding turbine in many cases is greater than the amount of refrigeration required by the plant. The excess energy is used to drive a cold compressor, which compresses an oxygen product from the low-pressure column in the gaseous state, before it is heated in the main heat exchanger.
It is an object of the present invention to make such a method energetically more advantageous.
This objective is accomplished due to the fact that the flow of oxygen from the low-pressure column in the liquid state is brought to the delivery pressure. The oxygen is evaporated by indirect heat exchange with a second partial flow of the feed air, which has been compressed to the first pressure. The second partial flow is brought upstream of the indirect heat exchange to a second pressure by the cold compressor.
Thus, according to the present invention, instead of the oxygen product flow itself, a partial air flow, which is used for the evaporation of the oxygen flow and is drawn off as a liquid under an elevated pressure, is brought to a higher pressure by the cold compressor driven by the expansion machine. In spite of this indirect transfer of the energy to the oxygen product flow, a larger effect can be achieved in this way. For the same loss of refrigeration, the pressure at which the oxygen is delivered is higher at the cold compressor than in the case of the previously known method.
The first pressure, to which the first and second partial flows of air are compressed jointly, is slightly above the operating pressure of the pressure column. The pressure difference preferably is such that the first partial flow of the air can overcome the flow resistance between the air compressor and the pressure column without pressure-changing measures and amounts, for example, to 0.1 to 0.5 bar.
The operating pressures at the head of the rectifying columns are, for example, (1) 2.5 to 10 bar, and preferably 4 to 7 bar in the pressure column, and (2) 1.05 to 4 bar, and preferably 1.1 to 1.5 bar in the low pressure column.
Preferably, an air compressor is used as the only externally driven machine for the method. This brings the total air to the first pressure, which at the same time represents the feed pressure of the expansion machine and the cold compressor. In this way, the oxygen product can be obtained at a delivery pressure, which is, for example, 0.5 to 4 bar, and preferably 1 to 3 bar, above the operating pressure of the low-pressure column. However, this is not associated with an energy consumption higher than that required for recovering the oxygen product at the pressure of the low-pressure column.
During the indirect heat exchange with the evaporating oxygen, the cold-compressed partial flow of air is condensed at least partially and preferably completely or essentially completely. The pressure on the condensate is subsequently relieved and the condensate is passed on to the pressure column and/or the low-pressure column.
The inventive method is particularly suitable for recovering impure oxygen with a purity of 80 to 99.5 mole % and preferably of 90 to 95 mole % at hyperbaric pressure.
For the method, nitrogen from the head of the pressure column, for example, or any other fraction from the pressure column can be supplied to the work-expansion. Preferably however, the process stream, which is subjected to the work-expansion, expansion, is formed by a third partial stream of the feed air, which is compressed to the first pressure.
Basically, it is possible to carry out the indirect heat exchange, through which the product oxygen is evaporated against the second partial stream of air that is condensing, in the main heat exchanger. Preferably however, a side condenser, separate from the main heat exchanger, is provided and is constructed as a cycling evaporator. Alternatively, it is also possible to use a counter-current heat exchanger or a falling film evaporator as a side condenser.
It is furthermore advantageous if a portion of the mechanical energy produced by the work-expansion is passed on to a braking device. The braking device may be formed, for example, by a braking fan and/or a braking generator and is outside of the cold box, which insulates the cold parts of the apparatus. By these means, energy can be emitted to the environment and the refrigeration, necessary for the method can be obtained without using a further expansion machine. Preferably, the expansion machine, the cold compressor, and the braking device are directly coupled mechanically, for example, over a common shaft.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.