Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-07-03
2002-01-08
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06336345
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method for the cryogenic separation of air, in which compressed and purified application air is cooled in a main heat exchanger and is supplied at least in part to a rectifying column, a first partial flow of the application air being removed from the main heat exchanger at an intermediate temperature and being supplied to a cold compression at this intermediate temperature.
A method and a device for the cryogenic separation of air are known, for example, from “Tieftemperaturtechnik”, 2nd Edition, 1985, Chapter 4 (Pages 281 to 337) by Hausen/Linde.
The invention is used in those cases in which a portion of the application air (“first partial flow”) is aftercompressed, for example, in order to be used for the evaporation of a liquid process flow. The liquid process flow may be a product flow (such as liquid oxygen, liquid nitrogen or liquid argon) from a rectifying column; the sump liquid or intermediate liquid of a rectifying column; or an external liquid which is taken, for example, from a storage tank. It is also possible to evaporate two or more such process flows against the aftercompressed partial air flow.
The “main heat exchanger” is preferably formed by a single heat exchanger block. In the case of larger systems, it may be useful to implement the main heat exchanger by several pipe trains which are connected in parallel with respect to the temperature course and which are formed by mutually separate structural elements. In principle, it is also conceivable that the main heat exchanger or each of these pipe trains is formed by two or more serially connected blocks.
In many cases, this aftercompression is carried out in a conventional manner in that the partial air flow is supplied to a corresponding machine approximately at ambient temperature. As an alternative, a cold compressor can be used for the aftercompression. In this case, “cold compression” is a compressing operation in which the gas is fed to the compression at a temperature which is clearly below the ambient temperature, generally below 250 K, preferably below 200 K.
Methods are known from International Patent Document WO 9528610 or European Patent Document EP 644388A, in which the cold compression is carried out at an intermediate temperature which is between the temperatures at the warm and cold end of the main heat exchanger. This intermediate temperature may particularly be at the point at which the curves of the flows to be warmed up and to be cooled come closest to one another in the heat exchange diagram (Q-T diagram) of the main heat exchanger (“theoretical pinch point”).
In the known methods, the partial air flow, which leads to the cold compression, is cooled in the main heat exchanger from the warm end to the intermediate temperature and, at the corresponding intermediate point of the main heat exchanger, is taken out directly from the cooling passages.
It is an object of the invention to provide the method of the initially mentioned type and a corresponding device which, with respect to energy, can be operated particularly advantageously.
This object is achieved in that the first partial flow is warmed up upstream of its removal in the main heat exchanger.
According to the invention, the partial air flow provided for the cold compression is therefore first cooled more than actually necessary in the main heat exchanger, thus beyond the intermediate temperature which corresponds approximately to the inlet temperature of the cold compression. Subsequently, it is warmed up again—also in the main heat exchanger—to the intermediate temperature. At first glance, this method of operation seems disadvantageous because, as a result of the cooling and reheating, which is unnecessary per se, additional exchange losses and therefore a higher energy consumption are to be expected. However, within the scope of the invention, it was found that, as a result, the heat transfer is improved in the cold part of the main heat exchanger (below the intermediate temperature).
The reason is that, in the cold part of the main heat exchanger, the flows to be warmed up and cooled off have a higher density than in the warm part. The heat exchanger passages, through which they flow, for constructive reasons, as a rule, have the same number and the same cross-sections. In the cold part, the passages are, as it were, operated with an underload of approximately 20%. Because of this fact, the flow conditions are not optimal in the cold part of the main heat exchanger. The invention achieves an improvement here, in that the partial air flow for the cold compression—which has to be subjected to a special treatment anyhow—supplements the flows which are to be cooled as well as the flows which are to be warmed up. It was found that the improvement of the heat transfer as a result of the flow conditions optimized within the scope of the invention in the cold part of the main heat exchanger overcompensates the expected additional exchange losses and, on the whole, results in a process which is particularly favorable with respect to energy. Also, the additional mass flow in the cold part of the main heat exchanger results in a steeper course of the curves of the flows to be warmed up and cooled down in the Q-T diagram and thus in an improvement at the point where these curves comes closest to one another (“theoretical pinch point”).
The first partial flow can be at least partially liquified downstream of the cold compression against an evaporating process flow. This heat exchange step can be carried out either in the main heat exchanger or in a separate condenser evaporator This method of operation will be particularly advantageous if the entire oxygen product or a large portion thereof is removed from the rectification as a liquid, is pressurized in liquid form and is finally evaporated against the cold-compressed partial air flow. In this case, just as much air is cold-compressed to ensure that the flow conditions in the cold part of the main heat exchanger are virtually optimal as a result of the reheating of this partial air flow according to the invention.
Preferably, the first partial flow is introduced into the cold end of the main heat exchanger before its warm-up. It is therefore first guided completely through the main heat exchanger and, when being warmed up, flows again through the entire cold part of the main heat exchanger, so that the entire cold part of the main heat exchanger benefits from the improved flow-through.
In this case, the cooling of the first partial flow can be carried out separately from or jointly with other portions of the application air For this purpose, a cooling air flow is cooled in the main heat exchanger, is taken out at the cold end of the main heat exchanger, and, at least partially, is fed again as a first partial flow to the cold end of the main heat exchanger.
In the case of the method according to the invention, it may be advantageous to separate liquid fractions before the rewarming of the first partial flow. For this purpose, after having been taken out of the cold end of the main heat exchanger, the cooling air flow is subjected to a phase separation, during which the first partial flow is formed at least by one part of the vapor phase taken out of the phase separation. Preferably, the entire vapor fraction from the phase separation is led to the cold compression, while the separated liquid is fed into the rectifying column or one of the rectifying columns, for example, into the pressure column of a two-column apparatus
Particularly in this case, it is advantageous for the cooling air flow to be expanded before it is subjected to the phase separation. However, also when a phase separation is absent, it may be useful to throttle off the cooling air flow before it is fed as a first partial flow to the cold end of the main heat exchanger.
In principle, the entire flow subjected to the cold compression can be formed by the first partial flow which is withdrawn from the main heat exchanger at the intermediate point. However, i
Crowell & Moring LLP
Doerrler William C.
Linde Aktiengesellschaft
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